Angiogenesis in rheumatoid Arthritis: Pathological characterization, pathogenic mechanisms, and nano-targeted therapeutic strategies.

Angiogenesis is essential for processes such as embryonic development, tissue repair, and organ regeneration. These processes are tightly regulated, localized, and typically transient, lasting from days to months. As depicted in Fig. 1 , angiogenesis involves a series of six meticulously orchestrated steps under normal physiological conditions [ 15 ]. However, these angiogenic steps become dysregulated in various pathological states, including RA, atherosclerosis, psoriasis, endometriosis, and obesity [ 16 ]. Fig. 1 Steps of Angiogenesis under Physiological Conditions. (A) The steps of angiogenesis under physiological conditions include the following: (Step.1) ECs receive pro-angiogenic signals, accelerating activation. (Step.2) Once activated, ECs dissociate from their parent vessels and secrete proteolytic enzymes, such as MMPs, that degrade the basement membrane, detach pericytes, and increase vascular permeability. (Step.3) Tip cells navigate the environment and invade the target tissue, guiding the sprouting process and transforming into more proliferative phenotypes, called stem cell. (Step.4) During sprouting angiogenesis, ECs proliferate and migrate, forming lumens through intercellular junctions. (Step.5) The DLL4/Notch-1 pathway is activated, increasing the expression of VEGFR1 while inhibiting VEGFR2 expression, facilitating vessel fusion and pruning. (Step.6) Tip cells also secrete PDGF-β, which binds to PDGFR-β on pericytes, leading to pericyte recruitment and BM deposition, thus forming stable and mature vessels. (B) Stromal cells in the surrounding tissue release various pro-angiogenic growth factors such as VEGF, FGF, HGF, and Ang, which bind to receptors on ECs like VEGFR, FGFR, c-Met, and Tie, initiating pro-angiogenic signaling and activating ECs. (C) Under the stimulation of VEGF, tip cells secrete DLL4, which interacts with Notch receptors in stem cells. Notch is cleaved by γ-secretase, releasing the Notch intracellular domain (NICD), which inhibits VEGFR2 expression while upregulating VEGFR1 expression, promoting vessel fusion and pruning. Endothelial cell: EC, VEGF: Vascular endothelial growth factor, FGF: Fibroblast growth factor, HGF: Hepatocyte growth factor, PDGF: Platelet-derived growth factor, VEGFR: Vascular endothelial growth factor receptor, FGFR: Fibroblast growth factor receptor, PDGFR: Platelet-derived growth factor receptor, c-Met: Cellular-mesenchymal to epithelial transition factor, Tie: Tyrosine kinase receptors with immunoglobulin and EGF homology domains, MMP: Matrix metalloproteinase, Ang: Angiopoietin. Fig. 1 Steps of Angiogenesis under Physiological Conditions. (A) The steps of angiogenesis under physiological conditions include the following: (Step.1) ECs receive pro-angiogenic signals, accelerating activation. (Step.2) Once activated, ECs dissociate from their parent vessels and secrete proteolytic enzymes, such as MMPs, that degrade the basement membrane, detach pericytes, and increase vascular permeability. (Step.3) Tip cells navigate the environment and invade the target tissue, guiding the sprouting process and transforming into more proliferative phenotypes, called stem cell. (Step.4) During sprouting angiogenesis, ECs proliferate and migrate, forming lumens through intercellular junctions. (Step.5) The DLL4/Notch-1 pathway is activated, increasing the expression of VEGFR1 while inhibiting VEGFR2 expression, facilitating vessel fusion and pruning. (Step.6) Tip cells also secrete PDGF-β, which binds to PDGFR-β on pericytes, leading to pericyte recruitment and BM deposition, thus forming stable and mature vessels. (B) Stromal cells in the surrounding tissue release various pro-angiogenic growth factors such as VEGF, FGF, HGF, and Ang, which bind to receptors on ECs like VEGFR, FGFR, c-Met, and Tie, initiating pro-angiogenic signaling and activating ECs. (C) Under the stimulation of VEGF, tip cells secrete DLL4, which interacts with Notch receptors in stem cells. Notch is cleaved by γ-secretase, releasing the Notch intracellular domain (NICD), which inhibits VEGFR2 expression while upregulating VEGFR1 expression, promoting vessel fusion and pruning. Endothelial cell: EC, VEGF: Vascular endothelial growth factor, FGF: Fibroblast growth factor, HGF: Hepatocyte growth factor, PDGF: Platelet-derived growth factor, VEGFR: Vascular endothelial growth factor receptor, FGFR: Fibroblast growth factor receptor, PDGFR: Platelet-derived growth factor receptor, c-Met: Cellular-mesenchymal to epithelial transition factor, Tie: Tyrosine kinase receptors with immunoglobulin and EGF homology domains, MMP: Matrix metalloproteinase, Ang: Angiopoietin. The etiology of RA is complex and not yet fully understood. Available studies suggest that both genetic and non-genetic factors (female gender and environmental influences) contribute to its development [ 17 ]. In genetically susceptible individuals, exposure to environmental risk factors may initiate asymptomatic synovitis [ 18 ]. Over time, this asymptomatic synovitis can progress to pannus formation, characterized by synovial hyperplasia, neovascularization, and inflammatory cell infiltration. This aggressive pannus invades and erodes cartilage and bone, resulting in severe clinical symptoms such as joint tenderness, stiffness, swelling, and reduced range of motion. Ultimately, these irreversible clinical manifestations lead to significant disability in affected patients ( Fig. 2 A). Fig. 2 The Critical Role of Pathological Angiogenesis in the Progression of RA. (A) Susceptible individuals exposed non-genetic risk factors exhibited asymptomatic synovitis. As the disease advances, the aggressive pannus invades and erodes cartilage and joints, resulting in clinical symptoms such as joint tenderness, stiffness, swelling, and diminished range of motion. (B) The vascular layer of normal synovium contains only a few macrophages and collagen fibers (left). In RA synovial vascular layer, angiogenesis can be divided into an early phase characterized by intense inflammation (middle) and a later phase marked by significant angiogenesis (right). (C) Mechanism of invasion of pannus into adjacent bone and cartilage. T cells and macrophages migrate from the pannus and produce inflammatory cytokines such as TNF-α, IL-1, IL-6, and IL-17. Inflammatory cytokines-stimulated fibroblast-like synoviocytes not only activates osteoclasts by inducing RANKL, but also inhibits osteoblast differentiation by producing DKK1 and sclerostin. RANKL: receptor activator of nuclear factor κB ligand. DKK1: Dickkopf-related protein 1. Fig. 2 The Critical Role of Pathological Angiogenesis in the Progression of RA. (A) Susceptible individuals exposed non-genetic risk factors exhibited asymptomatic synovitis. As the disease advances, the aggressive pannus invades and erodes cartilage and joints, resulting in clinical symptoms such as joint tenderness, stiffness, swelling, and diminished range of motion. (B) The vascular layer of normal synovium contains only a few macrophages and collagen fibers (left). In RA synovial vascular layer, angiogenesis can be divided into an early phase characterized by intense inflammation (middle) and a later phase marked by significant angiogenesis (right). (C) Mechanism of invasion of pannus into adjacent bone and cartilage. T cells and macrophages migrate from the pannus and produce inflammatory cytokines such as TNF-α, IL-1, IL-6, and IL-17. Inflammatory cytokines-stimulated fibroblast-like synoviocytes not only activates osteoclasts by inducing RANKL, but also inhibits osteoblast differentiation by producing DKK1 and sclerostin. RANKL: receptor activator of nuclear factor κB ligand. DKK1: Dickkopf-related protein 1. The synovium, a delicate inner membrane made up of loose connective tissue, lines the inner surfaces of joint capsules, tendon sheaths, and bursae. It consists of a thin cellular layer and a vascular layer. Under normal physiological conditions, the vascular layer is largely devoid of inflammatory cells, with only a few macrophages and collagen fibers present [ 19 ]. This sparse vascular layer provides oxygen and nutrients to the cellular layer and the avascular joint cartilage [ 20 ]. However, in RA joints, characterized by a highly inflammatory and hypoxic environment, synovial pannus formation occurs in two phases: an initial phase of intense inflammation in the vascular layer, followed by significant angiogenesis [ 21 ]. During the phase of intense inflammation, there is a marked proliferation of macrophage-derived type A synovial cells and fibroblast-derived type B synovial cells in the cellular layer. These cells produce and release large quantities of inflammatory cytokines and pro-angiogenic factors, triggering angiogenesis. Tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) are key cytokines in the synovial tissue that regulate the inflammatory response. They can directly interact with ECs to induce neovascularization [ 20 , 22 ]. This pre-angiogenic phase of inflammation is not unique to RA; it is also seen in other conditions with angiogenic potential, such as cancer, diabetes, and other chronic inflammatory diseases [ 20 ]. More critically, the subsequent phase of significant angiogenesis is a major contributor to the increased disability and mortality rates in RA patients. During this phase, the vascular layer, infiltrated by inflammatory cells such as CD4 + T cells, B cells, macrophages, and neutrophils, promotes ECs to sprout from existing blood vessels, proliferate, and migrate to the inflamed site. This process leads to the formation of an invasive and bone-destructive pannus ( Fig. 2 B). Unlike physiological angiogenesis, this pannus plays a key role in sustaining and advancing RA. Furthermore, the growing pannus continuously supplies oxygen and nutrients, facilitating immune cell migration into the inflamed synovium, where they produce high levels of pro-inflammatory cytokines and chemokines, perpetuating synovitis and synovial proliferation [ 5 ]. As synovial proliferation increases, the distance between new and existing blood vessels grows, raising the demand for oxygen and nutrients, which exacerbates synovial hypoxia. The newly formed pannus further amplifies the inflammatory and hypoxic microenvironment, driving the cyclical and progressive nature of pathological angiogenesis in RA [ 23 ]. Angiogenesis plays a crucial role in the formation of the inflammatory pannus; without it, leukocyte ingress would be impeded [ 24 ]. This tumor-like pannus, regulated by various mechanisms, invades adjacent bone and cartilage, leading directly to joint destruction and deformity. In RA joints, T cells and macrophages migrate from the pannus into the inflamed synovium, supplying oxygen and nutrients. These cells produce inflammatory cytokines, including TNF-α, IL-1, IL-6, and IL-17, which directly activate osteoclasts, resulting in bone destruction. Additionally, IL-17 from T cells indirectly promotes the differentiation of osteoclast precursors by inducing receptor activators of nuclear factor κB ligand (RANKL) in synovial fibroblasts, further stimulating osteoclast activity [ 25 ]. Concurrently, macrophages and neutrophils release inflammatory cytokines that stimulate fibroblast-like synoviocytes (FLS) to produce Dickkopf-related protein 1 (DKK1) and sclerostin, inhibiting the differentiation of osteoblasts from osteocytes ( Fig. 2 C) [ 26 ]. If left untreated, this invasive pannus can extend to nearby bones, cartilage, tendons, and ligaments, with activated osteoclasts and chondrocytes causing additional joint destruction and deformity. This progression can lead to long-term disability, chronic pain, and premature death. The significant angiogenesis observed in RA has been a key factor in the global increase in RA-related disability and mortality rates over the past decade [ 7 ]. Compared to normal vasculature, RA-induced angiogenesis exhibits five distinct features: abnormal vascular morphology, aberrant EPC and EC phenotypes, partial disruption of the vascular barrier, heightened vascular permeability, and insufficient vascular perfusion. The healthy synovium is a delicate structure composed of a cellular layer consisting of macrophage-like synoviocytes and FLS [ 18 ]. Adjacent to the cellular layer, the vascular layer exhibits a meticulously organized arrangement of sparsely distributed blood vessels interspersed with macrophages and mast cells [ 27 ]. These vessels, located approximately 35 μm beneath the synovial surface, consistently have a diameter of 8–9 μm [ 28 ]. The density of these vessels varies depending on the anatomical location, tissue type, and depth within the synovium. Various physical, spatial, and fluidic factors may restrict the penetration depth of the vessels, hindering their access to the cartilage surface [ 29 ]. The dense microvascular network within the synovial vascular layer is tailored to meet the metabolic requirements of FLS and the avascular joint cartilage under normal physiological conditions. Macroscopic examination reveals an increased number and abnormal morphology of blood vessels in the synovium of RA joints. Fig. 3 A(a-b) displays power Doppler ultrasonography (PDUS) images of a healthy joint, illustrating normal vessels within the quadriceps fat pad [ 30 ]. Additional arthroscopic findings reveal thin, sparse vessels in the synovial vascular layer juxtaposed with the avascular joint cartilage ( Fig. 3 A(c-d)) [ 27 ]. In contrast, anteroposterior radiographs of RA-affected joints exhibit narrowed, symmetric joint spaces with subchondral cysts (indicated by arrows) in bilateral knee joints ( Fig. 3 B(a)). PDUS imaging unveils intra-articular synovial hypertrophy (asterisks), areas of active inflammation (red), and erosion of bone by pannus (circled) ( Fig. 3 B(b)) [ 31 ]. The arthroscopic evaluation demonstrates congested and increased blood vessels in cases of mild synovitis ( Fig. 3 B(c)). Severe synovitis progression is associated with pronounced angiogenesis, presenting vessels that are congested and hypertrophic, and may exhibit rod-shaped, pale, swollen features, even forming villi resembling embryonic capillary formations ( Fig. 3 B(d)) [ 32 , 33 ]. Vascular patterns in RA joints are classified as linear, tortuous, or mixed ( Fig. 3 B(e-f)), resembling the vascular architecture seen in tumors [ 34 ]. A systematic arthroscopy-based assessment of synovial vascular morphology in 100 RA patients disclosed that 49 % exhibited linear patterns, 28 % had mixed patterns, and 23 % showed tortuous patterns. The diagnostic sensitivity and specificity for linear patterns were reported as 77 % and 70 %, respectively [ 35 , 36 ]. Another study investigating RA synovial vascular patterns revealed that 41 % displayed linear morphology, 33 % exhibited mixed patterns, and 26 % portrayed tortuous patterns. Notably, patients with linear and mixed patterns demonstrated a higher prevalence of rheumatoid factor (RF) positivity and an increased risk of developing RA compared to those with tortuous patterns. Depictions of VEGF expression indicated greater abundance in linear and tortuous vessels than in mixed patterns, with particularly elevated TGF-β expression in linear vessels [ 37 ]. Concurrently, researchers examined synovial vascular patterns in psoriatic arthritis (PsA), another common chronic inflammatory condition. Reports suggest that both RA and PsA manifest similar symptoms like joint pain, swelling, and potential joint damage and functional impairment. Linear branching vessels typify RA, whereas PsA is characterized by tortuous and dense vessel formations. These distinctions might be linked to varying cytokine expression patterns and immune cell infiltration in the synovium. Higher levels of IL-1β, IL-2, IL-10, and IFN-γ are noted in PsA synovium [ 38 ], contrasting with RA, where increased infiltration of immune cells such as B and T cells is observed [ 39 ]. Additionally, investigations unveiled that RA synovium can exhibit expansive, cystic morphologies and develop arteriovenous fistulas [ 40 ]. Fig. 3 Synovial Vascular Morphology in Healthy and RA patients under Macroscopic Examination. (A) In the healthy joint (a), PDUS images show a physiological vessel (arrow) in the quadriceps fat pad (b). Adapted with permission from Refs. [ 27 , 30 , 47 ], copyright 2022. Adapted with permission from Ref. [ 30 ], copyright 2018. Adapted with permission from Ref. [ 27 ], copyright 2014. Under arthroscopy, sparse blood vessels and avascular joint cartilage can be macroscopically observed (c). Thin veins, arterioles, and capillaries in the normal synovial vascular layer form a closed quadrangular array (d). Adapted with permission from Ref. [ 48 ], copyright 2019. (B) In RA joint, anteroposterior radiograph results show a narrow symmetric joint space with subchondral cysts (arrow) in bilateral knee joints (a). Adapted with permission from Ref. [ 49 ], copyright 2017. Intra-articular synovial hypertrophy (asterisks), areas of active inflammation (red), and erosion of bone by pannus (circled) could be seen on PDUS (b). Adapted with permission from Ref. [ 31 ], copyright 2023. The main feature of RA synovium is increased number of blood vessels, eroding articular cartilage (c). Congested and increased blood vessels are observed in mild synovitis (d, left). As the condition advances to severe synovitis, there is significant angiogenesis (d, middle). Blood vessels become congested, hypertrophic, rod-shaped, pale, swollen even forming villi (d, right). Adapted with permission from Ref. [ 33 ], copyright 2016. The vascular morphology patterns in RA (e) can be categorized into three types: linear (f, left), tortuous (f, middle), and mixed (f, right). Adapted with permission from Ref. [ 35 ], copyright 2003. Fig. 3 Synovial Vascular Morphology in Healthy and RA patients under Macroscopic Examination. (A) In the healthy joint (a), PDUS images show a physiological vessel (arrow) in the quadriceps fat pad (b). Adapted with permission from Refs. [ 27 , 30 , 47 ], copyright 2022. Adapted with permission from Ref. [ 30 ], copyright 2018. Adapted with permission from Ref. [ 27 ], copyright 2014. Under arthroscopy, sparse blood vessels and avascular joint cartilage can be macroscopically observed (c). Thin veins, arterioles, and capillaries in the normal synovial vascular layer form a closed quadrangular array (d). Adapted with permission from Ref. [ 48 ], copyright 2019. (B) In RA joint, anteroposterior radiograph results show a narrow symmetric joint space with subchondral cysts (arrow) in bilateral knee joints (a). Adapted with permission from Ref. [ 49 ], copyright 2017. Intra-articular synovial hypertrophy (asterisks), areas of active inflammation (red), and erosion of bone by pannus (circled) could be seen on PDUS (b). Adapted with permission from Ref. [ 31 ], copyright 2023. The main feature of RA synovium is increased number of blood vessels, eroding articular cartilage (c). Congested and increased blood vessels are observed in mild synovitis (d, left). As the condition advances to severe synovitis, there is significant angiogenesis (d, middle). Blood vessels become congested, hypertrophic, rod-shaped, pale, swollen even forming villi (d, right). Adapted with permission from Ref. [ 33 ], copyright 2016. The vascular morphology patterns in RA (e) can be categorized into three types: linear (f, left), tortuous (f, middle), and mixed (f, right). Adapted with permission from Ref. [ 35 ], copyright 2003. Histological assessment of microvascular density (MVD) in RA is well-established, typically conducted through immunofluorescence or immunohistochemistry to evaluate endothelial marker expression like CD31, CD34, and von Willebrand factor (vWF) in synovial tissues. The expression levels of these markers are notably elevated in the synovium of RA patients compared to healthy synovium, likely attributed to increased fractional area, proliferation, and survival rates of ECs [ 20 , [41] , [42] , [43] ]. MVD correlates closely with the severity of synovitis, mononuclear cell infiltration, and counts of joint tenderness/swelling. It serves as a significant biomarker for predicting the activity and progression of early RA [ [44] , [45] ]. This increase in vascular density in the RA synovium is believed to facilitate nutrient and oxygen transport and promote the migration of inflammatory and immune cells, leading to escalated production of pro-inflammatory cytokines and chemokine networks [ 20 ]. Notably, angiogenesis in RA often exhibits three or more interconnecting branches, which contrasts with the usual two branches seen in normal vessels. The branching patterns in angiogenesis are likely regulated by hypoxia, where reduced oxygen levels in the synovial fluid promote the formation of additional branches [ 46 ]. The endothelium, a continuous and quiescent cellular monolayer, uniformly lines the inner surfaces of all vascular structures in the body, including capillaries, arterioles, arteries, veins, and lymphatic vessels [ 50 ]. It plays crucial roles in vascular homeostasis, acting as a semipermeable barrier that regulates substance exchange and modulates innate immune responses, controlling vascular tone through a delicate balance of vasodilators and vasoconstrictors, orchestrating angiogenesis by secreting pro-angiogenic factors that affect EC proliferation, migration, and tubulogenesis, and facilitating the delivery of oxygen and essential nutrients, thus playing a vital role in metabolic processes like glucose, lipid, and amino acid metabolism [ 51 ]. In RA angiogenesis, ECs display an abnormal phenotype characterized by the loss of cellular polarity, detachment from the basement membrane (BM), and cell stacking, resulting in luminal protrusions. This altered phenotype enlarges intercellular junctions, forming trans-endothelial channels that enhance leukocyte infiltration into the synovium and increase vascular permeability [ 52 ]. Notably, early-stage RA patients exhibit elevated CD146 levels in synovial fluid, indicating increased EC activation [ 53 ]. These phenotypic variations lead to endothelial damage, triggering excessive pro-angiogenic factor production, reduced release of angiogenic inhibitors, and subsequently causing an imbalance in angiogenesis [ 23 ]. EPCs, a distinctive stem cell subtype known for their strong proliferative abilities, play a crucial role in neovascularization by differentiating into mature ECs. EPCs possess a unique capacity to home in on sites of endothelial damage, aiding in repair and regeneration, ultimately maturing into functional ECs. They are notably mobilized and amassed at the tips of burgeoning vessels, forming cellular cords and undergoing in situ differentiation. These differentiated EPCs integrate into ongoing neovascularization through mechanisms like vacuolization and subsequent vacuole fusion [ 54 ]. Despite their inherent beneficial functions, EPCs can be co-opted to contribute to pathological angiogenesis, significantly impacting the progression of RA. In RA, augmented levels of EPCs are detected in synovial tissue, contrasting with reduced EPC counts in circulation. This disparity is attributed to the migration of EPCs from bone marrow niches to inflamed sites, exacerbating pathological angiogenesis [ 55 ]. Research links a decline in circulating EPC levels to increased RA disease activity, highlighted by metrics like Disease Activity Score 28 (DAS28), erythrocyte sedimentation rate, and serum RF levels. A specific study involving 126 RA patients revealed that lower circulating EPC counts correlated with enhanced bone erosion [ 56 ]. Experimental models showcase the proliferation of EPC markers in inflamed synovial vasculature, notably peaking around three weeks post-induction in collagen-induced arthritis (CIA) mice [ 57 ]. Adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1) and very late activation antigen-4 (VLA-4) are critical for EPC adhesion to ECs and FLSs, guiding targeted recruitment to inflamed joints. The hypoxic synovial environment in arthritic joints triggers EPC mobilization through CXCL13-CXCR5 receptor interactions, activating PLC, MEK, and AP-1 signaling pathways that promote EPC homing, angiogenesis, joint swelling, and cartilage erosion [ 7 ]. Moreover, the increased presence of EPCs in RA synovial tissues facilitates aggregation at vessel formation fronts via VEGFR2 expression [ 58 ]. Additionally, angiogenic growth factor receptors on EPCs interact with corresponding ligands to stimulate ECs to release proteases that enhance EC migration and proliferation ( Fig. 4 A) [ 59 ]. Fig. 4 Abnormalities in EPC Phenotype and Vascular Barriers in RA Synovium. (A) In RA, EPCs home to inflamed synovium, primarily through the CXCL13/CXCR5 axis. Then, this axis activates the downstream PLC/MEK signaling pathway, promoting VEGF release. Additionally, the VCAM-1/VLA4 interaction enhances the adhesion of EPCs to ECs, mediating the selective recruitment of EPCs to inflamed joint tissues. (B) In healthy, normal vessels are characterized by pericyte coverage, mature and homogenized BM barriers, and ECs arranged in a squared pattern (left). In the hypoxic and highly inflammatory RA microenvironment, in certain newly formed vessels display activated ECs, detachment of pericytes, and disrupted vascular barriers, resulting in immature vessels (right). EPC: Endothelial progenitor cell. EC: Endothelial cell. BM: Basilar membrane. RAM: Rheumatoid arthritis microenvironment. Fig. 4 Abnormalities in EPC Phenotype and Vascular Barriers in RA Synovium. (A) In RA, EPCs home to inflamed synovium, primarily through the CXCL13/CXCR5 axis. Then, this axis activates the downstream PLC/MEK signaling pathway, promoting VEGF release. Additionally, the VCAM-1/VLA4 interaction enhances the adhesion of EPCs to ECs, mediating the selective recruitment of EPCs to inflamed joint tissues. (B) In healthy, normal vessels are characterized by pericyte coverage, mature and homogenized BM barriers, and ECs arranged in a squared pattern (left). In the hypoxic and highly inflammatory RA microenvironment, in certain newly formed vessels display activated ECs, detachment of pericytes, and disrupted vascular barriers, resulting in immature vessels (right). EPC: Endothelial progenitor cell. EC: Endothelial cell. BM: Basilar membrane. RAM: Rheumatoid arthritis microenvironment. Activated ECs synthesize and express soluble E-selectin (sE-selectin) and P-selectin. During states of abnormal inflammation, these selectins can quickly shed from the surface of activated ECs, promoting the adhesion of circulating leukocytes to vascular ECs [ 60 ]. sE-selectin, a prominent member of the endothelial selectin family involved in RA-associated angiogenesis, demonstrates significantly increased expression levels in RA serum, synovial fluid, and vascular ECs [ 61 ]. Research indicates that deficiency in P-selectin can improve the clinical and histological symptoms in antibody-induced arthritis (AIA) mice by disrupting leukocyte-EC interactions [ 62 ]. The essential architecture of human capillaries comprises pericytes, ECs, and the BM. The maintenance of pericyte coverage stability and their connections with ECs are pivotal for preserving the integrity of the vascular barrier [ 63 ]. Three-dimensional electron microscopy investigations demonstrate that pericytes project claw-like protrusions and columnar extensions toward ECs, bridging intercellular gaps and embedding within the shared BM [ 64 ]. Jiang et al. highlighted that disruptions in pericyte detachment can occur through a pericyte-to-fibroblast transition [ 64 ]. This transition empowers pericytes to function as active stromal cells in response to mechanical cues from the surrounding microenvironment, a crucial aspect in cancer invasion and metastasis mechanisms [ 65 , 66 ]. The cellular arrangement fosters a “peg-and-socket” interaction between pericytes and ECs, vital for upholding vascular barrier integrity. The crosstalk between pericytes and ECs is governed by various angiogenic factors, including PDGF/PDGFRβ, Ang1/Tie-2, CXCL12/CXCR4, and endothelial-produced hepatocyte growth factor (HB-EGF)/EGF receptors (ErbB). While the first two factors will be elaborated on later, the focus here is on the latter pair. The CXCL12/CXCR4 axis, a recently recognized pathway, is implicated in pericyte recruitment and migration during tumor angiogenesis. CXCL12 collaborates with stem cell factor and IL-3 to regulate pericyte recruitment during the formation and maturation of endothelial tubes [ 67 ]. HB-EGF binds to ErbB 1 and 4 on pericytes, governing their recruitment and migration, ultimately leading to BM matrix assembly and EC vessel diameter restriction [ 68 , 69 ]. HB-EGF also serves as a potent stimulator of angiogenesis in multiple myeloma and hepatocellular carcinoma [ 70 , 71 ]. In addition, pericytes establish connections with ECs through adhesins and adhesion plaques, enhancing intercellular communication [ 72 ]. Neural cell adhesion molecule (NCAM), a specific adhesion glycoprotein, exhibits variable expression in 25–50 % of blood vessels in inflamed synovial joints. The partial presence of NCAM in certain vessels underscores incomplete interactions between pericytes and ECs, which contribute to vascular instability [ 73 ]. In the RA microenvironment, hypoxia and inflammation collaboratively induce substantial changes in the phenotype, morphology, and function of vascular pericytes and ECs. Particularly, pericytes exhibit distinct abnormal morphologies and weakened connections with ECs, resulting in reduced density and maturity. These modifications lead to incomplete pericyte coverage of the BM, creating increased intercellular gaps and subsequently heightened vascular wall permeability ( Fig. 4 B) [ 32 , 74 ]. Elevated levels of CXCR4 and CXCL12 in RA serum and synovial fluid, positively correlated with erythrocyte sedimentation rate, C-reactive protein, RF, and DAS28, further support this observation [ 75 ]. Acknowledging the critical role of the SDF-1/CXCR4 axis in guiding mesenchymal stem cells (MSCs) towards injury sites, Gan et al. engineered biomimetic nanoparticles that target CXCR4 receptors to enhance drug delivery to RA-inflamed joints abundant in SDF-1 concentrations [ 76 ]. Recent studies have identified that in RA-enriched environments, HB-EGF-producing inflammatory macrophages release specific mediators like IL-1, HB-EGF, and epiregulin. These cells, dubbed “pro-invasive macrophages,” promote the invasiveness of FLS [ 77 ]. Elevated plasma HB-EGF levels in RA patients correlate positively with the presence of carotid plaques and pulse wave velocity, indicating subclinical atherosclerosis [ 78 ]. Vascular maturity serves as a crucial determinant of vascular barrier function [ 64 , 79 ]. Elena and colleagues utilized dual staining of ECs (CD31) and pericytes/α-smooth muscle actin (α-SMA) in synovial tissues from RA patients to evaluate vascular maturity. Their findings revealed certain RA synovial tissues lacked adequate endothelial coverage, displaying immature vessels evident at the disease onset. As RA progresses, particularly in patients with high disease activity, severity, or significant arthritic cell infiltration, there is a marked increase in the prevalence of these immature vessels [ 73 , 80 ]. Another study indicated the presence of immature vessels within the synovium of patients with inflammatory arthritis, characterized by a sparse distribution of ECs and pericytes [ 81 ]. Aberrant EC phenotypes promote the expansion of intercellular connections, consequently increasing vascular permeability. The endocytosis of the endothelium-specific adhesion molecule VE-cadherin plays a pivotal role in this mechanism [ 82 , 83 ]. Phosphorylation of tyrosine at the Y685 site on VE-cadherin serves as an indicator of endothelial dysfunction, often initiated by TNF stimulation [ 84 ]. Additionally, VE-cadherin can undergo cleavage by metalloproteinases, tissue plasminogen activators, and elastase released by activated leukocytes, resulting in elevated levels of soluble VE-cadherin. Investigations in pathological vascular conditions reveal an intricate correlation between VE-cadherin levels and angiogenic activity in diseases such as ovarian cancer, multiple myeloma, and rheumatism [ 84 ]. Noteworthy is the heightened expression of VE-cadherin observed in the synovium in conditions like juvenile idiopathic arthritis and RA. Further studies suggest that IL-6 and VEGF stimulation act as primary instigators of VE-cadherin cleavage in RA synovium. Incubation with IL-6 induces morphological instability in RA synovial explants by mediating VE-cadherin endocytosis, leading to a gradual decrease in its surface expression and weakening of intercellular cohesion [ 85 ]. Furthermore, Kosuke and colleagues illustrated that VEGF stimulates VE-cadherin expression in non-ECs, contributing to RA advancement. VEGF upregulates VE-cadherin in RSFLs in a time- and dose-dependent manner. The interaction between VEGFR2 and VEGF activates the ERK/MAPK and PI3K/AKT/mTOR pathways, which subsequently enhance VE-cadherin expression in RA FLS [ 86 ]. In RA joints, heightened vascular permeability, along with decreased lymphatic contractility, further increases interstitial fluid pressure (IFP). Under normal circumstances, tissue IFP is typically equivalent to atmospheric pressure [ 87 ]. However, during cartilage loading and synovitis episodes, IFP tends to rise [ 88 ]. The dynamics of IFP in RA are notably significant, impacted by both the synovial vascular and lymphatic systems. Large molecules above 40 kDa are commonly cleared by the lymphatic system, while those under 10 kDa are predominantly cleared through the vascular pathway [ 89 ]. In TNF transgenic RA mice, an initial compensatory escalation in lymphatic drainage is observed at arthritis onset. Nevertheless, this lymphatic compensation proves insufficient in severe synovitis, leading to an accelerated loss of lymphatic contractility and lymph node collapse [ 90 , 91 ]. This phenomenon primarily arises from immune cell-mediated modulation of lymphatic contraction via nitric oxide (NO) signaling induced by cell movement. Immune cells synthesize inducible nitric oxide synthase (iNOS), and endothelial nitric oxide synthase (eNOS) activated by shear stress, promoting NO production in lymphatic ECs (LEC) that impacts lymphatic muscle cells (LMC) [ 91 ]. The invasive pannus introduces a multitude of immune and inflammatory cells into the inflamed joint, releasing cytokines, chemokines, and catabolic factors that impair LECs and LMCs. These factors further stimulate NO production catalyzed by iNOS and eNOS in LECs, leading to LMC relaxation, lymphatic contractile dysfunction, or even collapse ( Fig. 5 A). This collapse is part of broader pathological alterations, including increased synovial proliferation, lymphatic damage, loss of contractility, and reduced draining lymph node volume [ 91 ]. Additionally, activated macrophages within these lymph nodes express CXCL13, a potent B-cell chemokine. Elevated CXCL13 levels in the lymph node sinuses prompt migration, accumulation, and obstruction of lymphatics by IgM + CD23 + CD21hiCD1dhi B cells [ 91 ]. Studies suggest that RA patients exhibit larger and more abundant popliteal lymph nodes (PLNs), positively correlating with knee synovial volumes [ 92 ]. In collapsed PLNs, reduced lymph velocity, intra-nodal pressure, and lymph pump pressure have been observed, aligning with decreased joint drainage during arthritis flare-ups [ 93 ]. The combination of heightened vascular permeability and hindered lymphatic reflux heightens IFP in the RA synovium, impeding the transportation of oxygen and other soluble molecules, and exacerbating local tissue perfusion deficits [ 94 ]. Fig. 5 Mechanisms Underlying Loss of Lymphatic Contractility and Insufficient Vascular Perfusion in RA. (A) In healthy conditions, lymphatic endothelial cells (LEC) and lymphatic muscle cells (LMC) form cohesive units that facilitate efficient lymph transport (left). RA joint exhibits no lymphatic contraction. Mechanically, the accumulation of macrophages and B cells promotes NO secretion by damaged LECs. This secretion keeps the LMC in a diastolic state, further culminating in decreased lymph flow (right). (B) Causes of reduced blood in RA synovium: Chaotic blood flow (due to the increased microvascular density and depth, and altered vascular morphology), and vascular compression (due to the thickening of FLS layer, increased synovial effusion, and the rigidity of joint movement). Causes of hypoxic microenvironment in RA synovium: Increased oxygen consumption (due to FLS proliferation and enhanced metabolism), inadequate oxygen supply (due to the increased distance between FLS and blood vessels, as well as compression by synovial effusion), and increased expression of vasoconstrictive substances. FLS: Fibroblast-like synoviocytes. Ang-1: Angiopoietin-1. NO: Nitric oxide. iNOS: Inducible nitric oxide synthase. eNOS: Endothelial nitric oxide synthase. Fig. 5 Mechanisms Underlying Loss of Lymphatic Contractility and Insufficient Vascular Perfusion in RA. (A) In healthy conditions, lymphatic endothelial cells (LEC) and lymphatic muscle cells (LMC) form cohesive units that facilitate efficient lymph transport (left). RA joint exhibits no lymphatic contraction. Mechanically, the accumulation of macrophages and B cells promotes NO secretion by damaged LECs. This secretion keeps the LMC in a diastolic state, further culminating in decreased lymph flow (right). (B) Causes of reduced blood in RA synovium: Chaotic blood flow (due to the increased microvascular density and depth, and altered vascular morphology), and vascular compression (due to the thickening of FLS layer, increased synovial effusion, and the rigidity of joint movement). Causes of hypoxic microenvironment in RA synovium: Increased oxygen consumption (due to FLS proliferation and enhanced metabolism), inadequate oxygen supply (due to the increased distance between FLS and blood vessels, as well as compression by synovial effusion), and increased expression of vasoconstrictive substances. FLS: Fibroblast-like synoviocytes. Ang-1: Angiopoietin-1. NO: Nitric oxide. iNOS: Inducible nitric oxide synthase. eNOS: Endothelial nitric oxide synthase. Despite the abundance of blood vessels within the RA synovium, the morphological and functional abnormalities of these vessels result in significant disruptions in blood flow and increased intra-articular pressure, which exacerbates the hypoxic microenvironment within synovial tissues. Hypoxia causes hypoxia-inducible factor (HIF)-1 to not be degraded, leading to a significant increase in HIF-1 levels. HIF-1 plays a pivotal role in RA angiogenesis by directly binding to hypoxia response elements (HREs) located within the promoter region of VEGF. The interplay among the hypoxic microenvironment, VEGF, and abnormal vasculature establishes a vicious cycle that critically contributes to the uncontrolled pathological angiogenesis observed in RA. Within inflamed RA joints, the heterogeneous vascular morphology and augmented vascular density contribute to irregular blood vessels, manifesting as linear, tortuous, and mixed formations, ultimately resulting in inadequate blood flow or stasis. Anomalous vascular structures are frequently linked to unstable vascular functions, including barrier disruption and heightened permeability. Despite continuous angiogenesis, the newly formed vasculature may fail to adequately compensate for the damage, resulting in diminished blood flow [ 95 ]. Moreover, neovascularization facilitates the infiltration of immune and inflammatory cells into the inflamed joints, promoting FLS proliferation and synovial layer thickening [ 96 , 97 ]. The FLS-rich synovial effusion fills the joint cavity, exerting physical compression on the vessels, further impeding blood flow [ 98 ]. In advanced RA cases, the already rigid joints intermittently compress or even collapse synovial vessels during movement. Although reduced local blood flow in the RA joint can worsen the hypoxic microenvironment, increased cellular oxygen consumption further exacerbates this condition from a metabolic perspective. Studies since 1970 have consistently shown that oxygen tension in the synovial fluid of RA patients is lower than in healthy individuals. High-sensitivity microelectrode measurements indicate that the median oxygen partial pressure in RA synovial tissue is 2–4 %, compared to 9–12 % in healthy tissues [ 99 ]. The cellular response to hypoxia includes the activation of oxygen-sensitive adaptive transcriptional reactions, predominantly regulated by the HIF pathway. HIF, a heterodimeric transcription factor composed of HIF-1α and HIF-1β subunits, regulates HIF-dependent gene expression. Under hypoxic stress, HIF-α accumulates in the cytoplasm, translocates to the nucleus, dimerizes with HIF-β, interacts with HIF coactivators, and binds to HREs in target genes to initiate transcription [ 20 , 99 ]. Studies have shown an inverse correlation between synovial oxygen levels and markers of synovitis as well as disease activity, contrasting with the positive correlation observed for HIF-1α. [ 100 ]. The primary causes of hypoxia in the synovial tissue of RA patients include ( Fig. 5 B): 1) Enhanced Oxygen Consumption: The inflammatory environment in RA promotes the proliferation of FLS, thickening the synovial tissue and increasing metabolic demands, which leads to higher oxygen consumption. 2) Inadequate Oxygen Supply: As FLS multiply, the spatial distance between these cells and adjacent blood vessels increases, making it difficult to meet the tissue's heightened oxygen requirements. Despite increased angiogenesis in RA, the rate of new vessel formation is insufficient to support the elevated metabolic activity associated with synovial growth [ 101 ]. 3) Increased Expression of Vasoconstrictive Agents: An imbalance in the factors that regulate vasodilation and vasoconstriction in RA also plays a crucial role. Recent studies have reported reduced vascular dilation responses in AIA rats, along with decreased eNOS activity and tetrahydrobiopterin expression [ 102 ]. Additionally, plasma levels of homoarginine, a precursor for NO synthesis linked to vasodilation, are significantly lower in RA patients compared to healthy individuals [ 103 ]. At the same time, vasoconstrictive agents such as angiotensin II are overexpressed in the stromal cells of RA synovial tissue, mediated by angiotensin-converting enzymes that induce local vasoconstriction [ 104 ]. This pronounced perfusion deficit in RA synovial tissue leads to the expression of various pro-angiogenic factors, with VEGF being a key gene containing HREs and a potent stimulator of angiogenesis. Furthermore, the perfusion deficit promotes a metabolic shift towards glycolysis in FLS. The acidic, hypoxic environment stimulates the generation of reactive oxygen species (ROS), which further drive angiogenesis and perpetuate the cycle of increased vascularization [ 105 ].

Fang Zhao: Writing – original draft, Investigation. Zeyu Hu: Investigation. Gejing Li: Investigation. Min Liu: Investigation. Qiong Huang: Supervision. Kelong Ai: Writing – review & editing, Investigation, Conceptualization. Xiong Cai: Supervision, Conceptualization.

Pathological angiogenesis in RA is marked by the formation of structurally abnormal and functionally impaired blood vessels. This process initiates a cascade of detrimental effects, including increased glycolysis, oxidative stress, synovitis, and bone destruction, which collectively worsen the disease progression. In the RA synovium, FLS and ECs enhance their metabolic functions to support angiogenesis. Recent studies have shown that these cells exhibit robust glycolytic activity, even in the presence of sufficient oxygen and functional mitochondria for oxidative glucose metabolism. This phenomenon, known as the Warburg effect, allows significant lactate production under aerobic conditions [ 106 ]. Despite being less efficient for ATP production, glycolysis accounts for approximately 85 % of ATP generation in FLS and ECs [ 107 ]. In RA, the synovial microvasculature is highly dysregulated, leading to inefficient oxygen delivery and a severely hypoxic microenvironment [ 36 ]. FLS and ECs adapt their metabolic profiles to generate the essential building blocks required for proliferation, activation, and invasiveness. This metabolic shift enables them to produce sufficient energy to promote synovial proliferation and angiogenesis [ 108 ]. In the RA synovium, FLS undergo metabolic conversion primarily through the following pathways ( Fig. 6 ): 1) Increased glycolysis in RA leads to rapid ATP production, providing the energy needed to activate the Rho/ROCK signaling pathway. This allows FLS to respond quickly to pro-angiogenic stimuli, resulting in increased cytokine release and upregulation of key signaling molecules such as VEGF, FGFs, MMP-9, and MMP-2, thus accelerating angiogenesis and leukocyte infiltration independently of HIF-1α [ 105 , 109 ]. 2) Glycolysis, being oxygen-independent, facilitates the redistribution of oxygen to surrounding cells [ 110 ]. 3) In the hypoxic environment of RA, elevated glycolytic activity leads to increased pyruvate production, which is converted into lactate by cytosolic lactate dehydrogenase. This lactate enhances the invasive behavior of RA FLS and promotes bFGF expression, subsequently inducing MMP-2 in chondrocytes and activating Rho GTPases. These actions culminate in increased cell migration and dysfunctional angiogenesis [ 108 ]. Furthermore, the metabolic shift toward glycolysis in inflamed RA joints induces a phenotypic transformation in FLS and alters the expression of the tumor suppressor gene p53. This alteration impairs DNA repair, disrupts apoptotic processes, and enhances the survival and proliferation of FLS [ 100 ]. Fig. 6 Impact of Glycolysis in RA Angiogenesis. FLS in RA efficiently produce ATP via glycolysis, providing essential energy that facilitates the activation of the Rho/ROCK signaling pathway. This pathway plays a pivotal role in upregulating the secretion of various pro-angiogenic factors, including VEGF, FGF, and MMPs. Moreover, lactate, a metabolic byproduct of glycolysis, further stimulates the expression of endogenous FGF, thus expediting the process of angiogenesis (left). In ECs located in RA synovium, the glycolytic process is likely influenced by the enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase isozyme 3 (PFKFB3). PFKFB3 enhances the conversion of fructose-6-phosphate to fructose-2,6-bisphosphate, leading to increased lactate production. This, in turn, promotes the phosphorylation of Akt, facilitating pathological angiogenesis (right). Fig. 6 Impact of Glycolysis in RA Angiogenesis. FLS in RA efficiently produce ATP via glycolysis, providing essential energy that facilitates the activation of the Rho/ROCK signaling pathway. This pathway plays a pivotal role in upregulating the secretion of various pro-angiogenic factors, including VEGF, FGF, and MMPs. Moreover, lactate, a metabolic byproduct of glycolysis, further stimulates the expression of endogenous FGF, thus expediting the process of angiogenesis (left). In ECs located in RA synovium, the glycolytic process is likely influenced by the enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase isozyme 3 (PFKFB3). PFKFB3 enhances the conversion of fructose-6-phosphate to fructose-2,6-bisphosphate, leading to increased lactate production. This, in turn, promotes the phosphorylation of Akt, facilitating pathological angiogenesis (right). The glycolytic activity in ECs and FLS is regulated by phosphofructokinase-2/fructose-2,6-bisphosphatase isozyme 3 (PFKFB3). As a key rate-limiting enzyme, PFKFB3 is the most effective allosteric activator of phosphofructokinase-1, catalyzing the conversion of fructose-6-phosphate to fructose-2,6-bisphosphate. This conversion is essential for angiogenesis, as it facilitates Akt phosphorylation in ECs through glycolytic intermediates like lactate. Both knockdown and pharmacological inhibition of PFKFB3 in ECs significantly reduce pathological angiogenesis [ 111 , 112 ]. Recent studies indicate that targeting PFKFB3 in ECs enhances BM deposition and pericyte coverage, leading to enlarged lumens, smoother surfaces of ECs, and improved tumor vessel stability. This modulation also reduces the expression of vascular cell adhesion molecules, including VCAM-1, E-selectin, and intracellular adhesion molecule-1 (ICAM-1), on ECs. Inhibiting PFKFB3 in both ECs and pericytes not only decreases glycolysis but also improves pericyte coverage and endothelial stability, facilitating vascular normalization [ 113 ]. PFKFB3 is predominantly located in the nuclei of synovial FLS, partially translocating to the cytoplasm during active glycolysis. Treatments with PFKFB3 inhibitors or siRNA targeting suppress the expression of inflammatory and chemotactic factors, thereby inhibiting glucose uptake and lactate secretion by RA FLS. Such interventions also reduce inflammation in CIA mouse models [ 114 ]. In initial CD4 + T cells from RA patients lacking phosphofructokinase, glucose is increasingly diverted to the pentose phosphate pathway, leading to reductive stress and potentially accelerating the loss of apoptotic T cells [ 115 ]. Interestingly, PFKFB3 also has important non-glycolytic functions within the nucleus. It catalyzes the activation of nuclear fructose-2,6-bisphosphate, which activates Cdk1 to phosphorylate p27 at Thr-187. This action reduces p27 levels and accelerates the cell cycle during the G1/S transition, protecting FLS from apoptosis [ 115 ]. Given its significant role in promoting pro-angiogenic factor secretion in RA FLS, further exploration of PFKFB3 is warranted due to its implications for disease progression and therapeutic strategies. In synovial tissues of RA patients, severe hypoxia and inflammation, compounded by mitochondrial damage, lead to an accelerated accumulation of ROS, including superoxide (O 2 ·- ), hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals (OH − ). These changes disrupt redox homeostasis and significantly contribute to processes such as synovial hyperplasia, pannus formation, and the degradation of bone and cartilage, all of which drive the progression of RA. The detailed mechanisms underlying these observations are illustrated in Fig. 7 . Fig. 7 The Relationship between Oxidative Stress and Angiogenesis in RA. (A) In the hypoxic microenvironment of RA synovium, NOX and XDH catalyze the conversion of O 2 to ROS. ROS further promotes the formation of lipid peroxidation products such as MDA and 4-HNE, which in turn enhance the expression of VEGF. (B) Sites of ROS production within mitochondria: ROS is generated in the mitochondrial matrix, including complexes I and III and various metabolic enzymes. ROS generation also occurs at complex III in the mitochondrial intermembrane space. (C) Excessive ROS can induce cell death, triggering the cGAS/STING signaling pathway and producing a cascade of inflammatory cytokines. (D) Highly infiltrated inflammatory cells in RA joints, such as neutrophils and macrophages, secrete various pro-inflammatory cytokines. These cells can produce excessive ROS via various oxidases (NOX, MPO, XO, and iNOS) in the synovium, thereby triggering angiogenesis. NOX: NADPH oxidase; XDH: xanthine dehydrogenase; SOD: superoxide dismutase; MDA: malondialdehyde; 4-HNE: 4-hydroxynonenal; HRE: hypoxia response element; OMM: Outer mitochondrial membrane; IMS: intermembrane space; IMM: inner mitochondrial membrane; OGDH: 2-oxoglutarate dehydrogenase; PDH: pyruvate dehydrogenase; GPDH: glycerol-3-phosphate dehydrogenase; FQR: flavin-containing quinone oxidoreductase; mtDNA: mitochondrial DNA; PTPC: permeability transition pore complex; MPO: myeloperoxidase; XO: xanthine oxidase. Fig. 7 The Relationship between Oxidative Stress and Angiogenesis in RA. (A) In the hypoxic microenvironment of RA synovium, NOX and XDH catalyze the conversion of O 2 to ROS. ROS further promotes the formation of lipid peroxidation products such as MDA and 4-HNE, which in turn enhance the expression of VEGF. (B) Sites of ROS production within mitochondria: ROS is generated in the mitochondrial matrix, including complexes I and III and various metabolic enzymes. ROS generation also occurs at complex III in the mitochondrial intermembrane space. (C) Excessive ROS can induce cell death, triggering the cGAS/STING signaling pathway and producing a cascade of inflammatory cytokines. (D) Highly infiltrated inflammatory cells in RA joints, such as neutrophils and macrophages, secrete various pro-inflammatory cytokines. These cells can produce excessive ROS via various oxidases (NOX, MPO, XO, and iNOS) in the synovium, thereby triggering angiogenesis. NOX: NADPH oxidase; XDH: xanthine dehydrogenase; SOD: superoxide dismutase; MDA: malondialdehyde; 4-HNE: 4-hydroxynonenal; HRE: hypoxia response element; OMM: Outer mitochondrial membrane; IMS: intermembrane space; IMM: inner mitochondrial membrane; OGDH: 2-oxoglutarate dehydrogenase; PDH: pyruvate dehydrogenase; GPDH: glycerol-3-phosphate dehydrogenase; FQR: flavin-containing quinone oxidoreductase; mtDNA: mitochondrial DNA; PTPC: permeability transition pore complex; MPO: myeloperoxidase; XO: xanthine oxidase. In the RA synovial environment, hypoxia promotes angiogenesis, either directly or through ROS generation [ 116 ]. ROS induces lipid peroxidation, leading to the formation of secondary compounds like malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), which stimulate FLS to release pro-angiogenic factors. Additionally, increased intra-articular pressure and heightened oxygen demand from immune cells and FLS exacerbate hypoxic conditions within the joint. Xanthine dehydrogenase (XDH), an enzyme known to promote oxidative stress, converts to xanthine oxidase (XO) under hypoxic conditions. XDH enhances electron transfer to oxygen, resulting in the production of O 2 ·- and H 2 O 2. [ 52 ] This accumulation further stabilizes HIF-α, which partners with HIF-β to upregulate the expression of VEGF through HREs. Moreover, hypoxic conditions stimulate the activity of NADPH oxidase (NOX), which catalyzes the formation of superoxide radicals. These radicals are subsequently converted to hydrogen peroxide by superoxide dismutase (SOD), further driving angiogenesis in RA, as depicted in Fig. 7 A. The mitochondrial electron transport chain is a major source of ROS through its facilitation of electron transfer to molecular oxygen. Within the mitochondrial matrix, Complex I generates O 2 ·- , while Complex III releases it in both the matrix and intermembrane space [ 117 ]. Other metabolic enzymes contributing to ROS generation include 2-oxoglutarate dehydrogenase, pyruvate dehydrogenase, glycerol-3-phosphate dehydrogenase, and flavin-containing quinone oxidoreductase ( Fig. 7 B) [ 118 ]. In the mitochondrial matrix, manganese superoxide dismutase (Mn-SOD) converts O 2 ·- into H 2 O 2 , a process also occurring in the intermembrane space and cytosol via copper and zinc-containing SOD (Cu/Zn-SOD). Additionally, cardiolipin (CL) in the inner mitochondrial membrane is particularly vulnerable to oxidative damage [ 119 ]. CL can redistribute from the inner to the outer mitochondrial membrane, resulting in the accumulation of oxidized CL, which may initiate the formation of mitochondrial permeability transition pores (mPTP). The opening of mPTP can lead to further ROS production, a phenomenon known as ‘ROS-induced ROS release’, thereby exacerbating oxidative stress [ 120 ]. Simultaneously, mitochondrial aerobic respiration, peroxisomes, and NOX contribute to the production of O 2 ·- , promoting the secretion of VEGF in ECs, smooth muscle cells (SMCs), and macrophages [ 121 ]. Excessive generation of ROS activates damage-associated molecular pattern molecules, including lipids, proteins, DNA, and RNA, leading to various modes of cellular death. This process promotes the release of pro-inflammatory cytokines. Notably, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway has become a critical focus in inflammation research. Excessive ROS facilitates the translocation of mitochondrial DNA (mtDNA) from mitochondria to the cytoplasm via the permeability transition pore complex. mtDNA acts as a potent activator of cGAS, triggering STING signaling and leading to the synthesis of cytokines such as TNF, IL-6, and IL-1 ( Fig. 7 C) [ 121 ]. In CIA mice, activation of the cGAS/STING pathway is associated with increased chemokine expression and toe joint swelling [ 120 ]. Moreover, suppression or genetic deletion of STING reduces retinal neovascularization and vascular permeability in models of oxygen-induced retinopathy, highlighting its role in angiogenesis [ 122 ]. In the RA synovium, various immune cells, including macrophages, T cells, B cells, dendritic cells (DCs), and neutrophils, as well as FLS, produce numerous inflammatory cytokines such as TNF-α, IL-1, and IL-6. These cytokines activate several oxidases, including NOX, myeloperoxidase (MPO), XO, and iNOS ( Fig. 7 D). MPO in neutrophils catalyzes H 2 O 2 to form the more potent oxidant hypochlorous acid. Increased iNOS activity in macrophages under inflammatory conditions converts O 2 and arginine into NO and L-citrulline, thereby enhancing the production of the powerful oxidant peroxynitrite and depleting antioxidants [ 52 ]. Furthermore, excessive generation of ROS leads to increased mitochondrial ROS (mtROS) in ECs, which in turn enhances VEGF expression. Elevated levels of mtROS, even if transient, can activate eNOS in ECs, promoting cell proliferation and angiogenic sprouting [ 123 ]. Additionally, VEGF stimulates Rho-related GTPase 1, triggering NOX activation and further increasing ROS production. This cascade initiates autophosphorylation of VEGFR2 and activates downstream signaling pathways, including Akt, p38, and ERK [ 116 ]. Notably, NOX4 has been shown to enhance ROS production upon stimulation, which coincides with increased expression of VCAM-1 and VEGF in FLS, further promoting angiogenesis as well as the proliferation and migration of FLS [ 124 ]. In RA, angiogenesis and synovitis are intricately linked, with each process reinforcing the other. Increased vascular density and depth enhance the delivery of nutrients and oxygen to the hyperplastic synovium, facilitating immune cell infiltration and the production of pro-inflammatory cytokines and chemokines [ 20 ]. Chen et al. highlighted the crucial roles of Notch-1 and Notch-3 in hypoxia-induced RA FLS invasion and angiogenesis, elucidating a functional link between HIF-1α, Notch-1, and Notch-3 signaling in the angiogenesis and invasive behavior of RA FLS in co-culture with human umbilical vein endothelial cells (HUVEC) [ 125 ]. Further research has shown that the vascular endothelial system in the synovium promotes the pro-inflammatory phenotype of FLS, particularly through the JAG1/DLL4-Notch3 pathway [ 126 ]. The proximity of FLS to blood vessels correlates with an increased expression of the pro-inflammatory marker Thy1 (CD90) [ 29 ]. Recently, Anja and colleagues identified the synovial fluid protein Syntenin-1 and its receptor Syndecan-1, expressed on ECs and FLS in RA synovial tissue, as novel inducers of RA pannus. These molecules integrate the inflammatory, angiogenic, and metabolic networks of ECs and FLS through VEGFR and Notch1, involving HIF-1α and RAPTOR [ 127 ]. Following significant phenotypic changes, FLS secrete a range of pro-inflammatory cytokines and angiogenic factors that facilitate cell migration, invasion, and pannus formation [ 114 ]. Ji-Won and colleagues observed elevated levels of pro-angiogenic cytokines, such as placental growth factor (PlGF) and VEGF, in the synovial fluid and serum of RA patients, with levels correlating with the severity of synovitis [ 128 ]. This phenomenon largely arises from the lack of an organized BM in FLS, impairing its function as a traditional barrier between synovial tissue and fluid, thus allowing the diffusion of compounds through its permeable structures [ 129 ]. Additionally, the uncontrolled secretion of pro-inflammatory factors by FLS disrupts the balance between pro-inflammatory and anti-inflammatory factors, directly stimulating the proliferation of vascular ECs and promoting pannus formation. Moreover, fibroblast activation protein-alpha (FAP-α), which is abundantly overexpressed in RA, significantly influences the proliferative capabilities of FLS. In oncology, FAP-α enhances the expression of pro-angiogenic factors while reducing anti-angiogenic molecules through paracrine effects [ 130 ]. The specific mechanism of FAP-α in RA angiogenesis remains to be fully elucidated. In summary, FLS and ECs, as integral components of the pannus, undergo phenotypic changes in response to inflammatory stimuli, accelerating the progression of RA. In healthy individuals, a dynamic balance exists between osteoclast-mediated bone resorption and osteoblastic bone formation. However, in RA, chronic angiogenesis intensifies osteoclast activity, leading to significant erosion of adjacent bone and cartilage [ 131 ]. Within inflamed synovial tissues, FLS and immune cells—such as macrophages, T cells, and B cells—elaborate pro-inflammatory cytokines transported through neovascularization. Notably, FLS and macrophages produce receptor activators of RANKL and macrophage colony-stimulating factor (M-CSF), both crucial for osteoclast differentiation [ 132 ]. Remarkably, Th17 cells, following the loss of Foxp3 expression and IL-17 knockout, can independently initiate osteoclastogenesis via RANKL expression [ 131 , 132 ]. The secretion of IL-17 by Th17 cells in inflammatory conditions stimulates FLS to enhance RANKL expression, activating NF-κB and AP-1 signaling pathways and exacerbating bone destruction [ 131 ]. Concurrently, B cells inhibit osteoblast differentiation in RA by producing CCL3 and TNF, contributing to bone loss and erosion within the bone marrow microenvironment [ 133 ]. Moreover, M-CSF not only promotes the proliferation and survival of osteoclasts but also triggers inflammatory responses by activating Th17 cells, innate lymphoid cells, and stromal cells in the synovial tissue of RA patients [ 134 ]. Another significant contributor to angiogenic bone destruction in RA is the elevated level of MMPs in ECs, which exceeds that in osteoclasts by 40-fold [ 135 ]. As RA progresses, activated FLS secrete MMPs, leading to cartilage degradation. MMPs exacerbate bone destruction through several mechanisms: 1) directly cleaving type II collagen and other matrix proteins, thus accelerating joint damage; 2) modulating inflammatory cytokines and chemokines that stimulate FLS and chondrocytes to produce additional MMPs, creating a vicious cycle of degradation; and 3) disrupting the extracellular matrix, enhancing cell migration and invasion into the joint [ 136 ]. Furthermore, ROS significantly amplifies RA-induced bone damage by increasing MMP activity and inducing post-translational modifications of cartilage structural proteins, including glycation, carbonylation, lipid oxidation, and nitration [ 120 ]. These modifications increase susceptibility to degradation, accelerating the destruction of the cartilage matrix and promoting osteoblast apoptosis [ 52 ].

In summary, angiogenesis in RA involves multiple various stromal cells and various soluble factors. Bioactive nanomedicines with unique therapeutic properties can directly or indirectly suppress angiogenesis by modulating these core factors, thereby exerting therapeutic effects in RA. This review provides a comprehensive summary of two complex therapeutic strategies for RA angiogenesis: targeting activated stromal cells, and targeting the specific elimination of pro-angiogenic mediators. Nanoagents targeted activated stromal cells can suppress the secretion of pro-angiogenic mediators by targeting FLS, neutrophil, macrophage and ECs, while also serving as drug delivery vehicles that enhance the bioavailability of angiogenesis inhibitors. Additionally, anti-angiogenic nanoagents can specifically eliminate pro-angiogenic mediators or competitively inhibit the binding of these mediators to their respective receptors, including VEGF, PDGF, and selectins. This review highlights the exceptional bioactivity and delivery capabilities of anti-angiogenic nanoagents, emphasizing their ability to effectively inhibit angiogenesis in RA. By utilizing the unique properties of nanomaterials, these nanoagents address the major limitations of current RA treatments, which are often characterized by poor efficacy and significant side effects. Furthermore, the multi-targeted nature of these nanoagents positions them as highly promising therapeutic agents in RA management. Their potential to overcome the constraints of palliative treatments opens new avenues for more effective and targeted approaches in the treatment of RA. Clinically, RA management primarily relies on surgery and drug therapy to eliminate excess immune complexes, relieve pain, and suppress inflammation. However, surgery often carries the risk of secondary joint cavity damage. For instance, synovectomy which can completely stop synovial fluid production, further impairing joint mobility and function [ 262 ]. By contrast, drug therapy is less invasive and more adaptable, offering an increasingly diverse repertoire for clinical use. Currently, traditional clinical drug therapy for RA primarily focuses on single-factor anti-inflammatory therapies, such as NSAIDs, GCs, DMARDs, and monoclonal antibody therapies. However, their use is significantly limited due to concerns over long-term safety, high costs, poor specificity, low bioavailability and suboptimal efficacy. Specifically, traditional DMARDs are linked to various adverse effects, including leukopenia, elevated liver enzymes, and nausea. Additionally, GCs carry significant risks such as osteoporosis, infections, hyperglycemia, and hypertension [ 9 ]. Furthermore, researches have been established that DMARDs do not alter the vascular patterns in RA patients [ 35 ]. Given the pronounced side effects of conventional anti-inflammatory drugs, which provide only temporary relief for RA symptoms, there is an urgent need to identify novel treatment targets for RA. Therefore, anti-angiogenic strategies are emerging as a promising new approach for the clinical treatment of RA ( Table 4 ). As of May 21, 2024, a comprehensive search of ongoing clinical trials for RA was conducted using resources from the U.S. National Library of Medicine and the European Clinical Trials Register, yielding 3063 and 849 trials respectively ( Table 5 ). However, less than 0.08 % of these trials involve FDA-approved angiogenesis inhibitors for RA treatment. This notably low percentage is primarily due to the poor bioavailability and severe adverse reactions associated with these inhibitors. For example, pazopanib, used clinically for frequent invasive administration, poses an increased risk of infection and soft tissue damage, ultimately decreasing patient compliance [ 153 ]. Additionally, in a prospective trial involving patients with RA, Everolimus was associated with serious adverse events impacting the gastrointestinal tract, skin, and nervous system, resulting in drug discontinuation for 16.4 % and 10.0 % of participants, respectively. Compared to the placebo group, patients treated with Everolimus exhibited more frequent alterations in hematological markers, liver function tests, and lipid profiles [ 263 ]. Table 4 FDA-approved anti-angiogenic agents for the treatment of arthritis. Table 4 Category Drug First approved Clinical use Targets Application in arthritis Ref Model Effect Monoclonal antibody Bevacizumab 02/2024 Cervical cancer, colorectal cancer (CRC), glioblastoma, non-squamous-non-small-cell lung cancer (non-squamous-NSCLC), ovanancancer, renal cell carcinoma VEGF-A CIA rat induced RA model Decreased the arthritis index and synovial pathological injury index, inhibited levels of VEGF. [ 264 ] Improved the symptoms of RA rats [ 265 ] Ramucirumab 04/2014 Adenocarcinoma, CRC, NSCLC VEGFR-2 – – [ 15 ] Ranibizumab 06/2006 Wet age-related macular degeneration (Wet AMD), diabetic retinopathy VEGF-A AIA rat induced RA model Decreased the angiogenic, inflammatory cytokines, and apoptotic markers. [ 266 ] Olaratumab 10/2016 Soft tissue sarcoma (STS) PDGFR-α – – [ 15 ] Aflibercept 11/2011 Wet AMD, CRC, diabetic macular edema VEGF-A, VEGF-B, PlGF – – [ 15 ] Tyrosine kinase inhibitors Inlyta 01/2012 Renal cell carcinoma (RCC) VEGFR-1/-2/3, c-Kit, PDGFR-α, PDGFR-β [ 15 ] Cometriq 11/2012 Medullary thyroid cancer (MTC), RCC, hepatocellular carcinoma (HCC) VEGFR-2, c-Met, c-Kit, Ret, Flt-3, Tie-2, AXL, RON [ 15 ] Pazopanib 10/2009 RCC, STS VEGFR-1/-2/3, c-Kit, PDGFR-α/-β Partial medial meniscectomy (PMM) induced OA model Relieved OA pain and inhibited cartilage degeneration. [ 267 ] PMM or sodium monoiodoacetate induced OA model Mitigated the synovial blood vessels, osteopathic formation, and cartilage degradation. [ 153 ] Regorafenib 09/2012 CRC, Castrointestinal stromal tumor, Liver cancer VEGFR-2, TIE2 VEGFR-2, VEGFR-3, EGFR, Ret [ 15 ] Vandetanib 07/2011 MTC PMM induced OA model Reduced pain and cartilage degeneration. [ 267 ] Destabilization of medial meniscus induced OA model Reduced OARSI grading, subchondral bone thickening, and endothelial cell numbers in synovium. [ 268 ] Votrient 10/2009 Advanced RCC, advanced STS VEGF-A, PDGFR-α/-β Sunitinib 01/2006 Gastrointestinal stromal tumor (GISTs), Pancreatic cancer, RCC PDGFR, VEGFR CIA mice induced RA model Improved the pathological score, and a decrease in the synovial microvascular density. [ 269 ] Regorafenib 09/2012 CRC, GISTs, HCC VEGFR-1/-2/-3, c-Kit, PDGFR-β, Ret, Raf-1, bRaf, FGFR-1, Tie-2 Human cartilage proteoglycan induced arthritis model Reduced disease severity. [ 270 ] Sorafenib 12/2005 RCC, HCC, differentiated thyroid cancer, thyroid cancer VEGFR-1/-2/-3, c-Kit, Flt-3, PDGFR-β, Raf, Ret CIA rat induced RA model Decreased tissue VEGF and VEGF receptor levels, perisynovial inflammation. [ 271 ] AIA rat induced RA model Suppressed paw swelling, synovial hyperplasia, and inflammatory infiltration, reduced MVD and VEGFR-2 and FGFR-1 expression in synovial tissues. [ 272 ] AIA rat induced RA model Promoted FLS apoptosis. [ 273 ] RAFLS Inhibited FLS proliferation. [ 274 ] mTOR inhibitors Temsirolimus 05/2007 RCC mTOR RAFLS Reduced the level of IL-17. [ 275 ] Everolimus 03/2009 RCC, subependymal giant cell astrocytoma, pancreas neuroendocrine tumor ovariectomized rat induced osteoporosis model. Inhibited proliferation of osteoclast precursors and inhibit bone resorption. [ 276 ] patients with active RA Improved pain and swollen joint counts. [ 263 ] Sirolimus 05/2015 Lymphangioleiomyomatosis, malignant perivascular epithelioid cell tumour patients with active RA Restored the balance of Th17/Treg cells and reduced the probability of disease flare-ups. [ 277 ] Upregulated Treg cells. [ 278 ] Table 5 Ongoing clinical trials for FDA-approved anti-angiogenic agents in RA treatment (Data source: https://www,clinicaltrials.gov & https://www.chictr.org.cn/index.html ). Table 5 Study Title FDA approved anti-angiogenic agent Condition or disease Starting time Status Number of participants ldentifier Intra-articular bevacizumab for recurrent hemarthroses at target joints with chronic hemophilic synovitis Bevacizumab Synovitis February 01, 2014 Phase I Terminated 5 NCT02060305 Observational cohort study of bevacizumab intravenous injection in the treatment of early and middle stage of knee osteoarthritis Knee osteoarthritis July 26, 2021 Not yet recruiting 36 ChiCTR2100049278 Study evaluating Temsirolimus in active rheumatoid arthritis on concomitant methotrexate therapy Temsirolimus RA November 2003 Phase II Terminated – NCT00076206 Sirolimus for autoimmune disease of blood cells RA 11/2006 Phase II Completed 30 NCT00392951 The efficacy and safety of Sirolimus in refractory rheumatoid arthritis: a multi-center randomized controlled trial in China Sirolimus RA November 2016 Phase IV Recruiting 300 ChiCTR-IPR-17011566 Efficacy and safety of Sirolimus in new-onset rheumatoid arthritis: a prospective, double-blinded, randomized, placebo-controlled, monocentric study in China RA July 15, 2019 Not yet recruiting 60 ChiCTR1900024261 FDA-approved anti-angiogenic agents for the treatment of arthritis. Ongoing clinical trials for FDA-approved anti-angiogenic agents in RA treatment (Data source: https://www,clinicaltrials.gov & https://www.chictr.org.cn/index.html ). For decades, RA has been primarily defined as an immune-mediated inflammatory disease. However, the differences in the concentrations of inflammatory and anti-inflammatory factors in synovial fluid and serum between healthy individuals and RA patients are not always statistically significant due to substantial biological heterogeneity among individuals [ 239 , 279 ]. This suggests that additional biological processes may contribute to RA pathogenesis. Recent studies have identified significantly elevated VEGF expression in RA patients, underscoring the pivotal role of angiogenesis in RA onset and progression [ 128 ]. Consequently, angiogenesis has emerged as a promising therapeutic target for future RA treatment. However, the clinical application of current angiogenesis inhibitors approved by FDA remains challenging due to their low bioavailability, short half-life, and considerable adverse effects. The advent of nanoagents offers a potential solution to these challenges. Anti-angiogenic nanoagents have demonstrated promising therapeutic outcomes in preclinical arthritis models. Notably, pazopanib, an FDA-approved TKI targeting VEGF receptor 1 and 2, has exhibited enhanced therapeutic efficacy when formulated within nanoparticle-based drug delivery systems. Given these advantages, anti-angiogenic nanoagents hold immense potential for RA treatment, particularly in achieving targeted drug delivery, minimizing adverse effects, and enhancing therapeutic efficacy. Furthermore, the integration of vascular imaging systems with nanomaterials presents a promising avenue for comprehensive RA diagnosis and treatment, paving the way for more precise and personalized therapeutic strategies in the future. This approach has already been validated in the clinical diagnosis and treatment of cancer, age-related macular degeneration, and other angiogenesis-related diseases [ 280 ]. Targeting synovial angiogenesis markers has become a valuable non-invasive tool, providing more opportunities for early assessment and monitoring of RA disease activity [ 22 ]. Therefore, the development of nanomaterials holds significant promise in the field of RA diagnosis and treatment. The integration of nanotechnology with personalized medicine approaches could further optimize therapeutic efficacy, offering tailored solutions for RA patients. We believe that the advancements in nanotechnology heralds significant potential for enhancing anti-angiogenic therapies in RA. With continuous research and development, we are poised at the threshold of a transformative era in RA treatment, converting historical challenges into future opportunities.

Based on the aforementioned angiogenesis pathological mechanisms in RA, RA-associated angiogenesis involves a complex interaction of multiple cell types and cytokines. Among these, the aberrant accumulation of ROS and hypoxia serves as the central driving force. The elevated levels of ROS and the persistent hypoxic environment in RA synovial tissue significantly influence the biological behavior of local cells and promote the occurrence of angiogenesis. Specifically, FLS, neutrophils, macrophages and ECs are highly sensitive to this environment and become activated, leading to abnormal morphological and functional changes. Activated stromal cells, in turn, secrete angiogenic mediators, such as VEGF and PDGF ( Fig. 1 B). Owing to their unique bioactivity, enhanced targeting capabilities, improved bioavailability, and prolonged circulation time, nanomaterials have become an emerging trend in RA treatment. By employing passive or active targeting strategies, nanomaterials can increase drug accumulation at inflamed sites while minimizing systemic side effects, thereby significantly improving patient compliance. Moreover, the tunable particle size of nanomaterials enables optimized accumulation and controlled release at sites of inflammation. By precisely adjusting nanoparticle size, prolonged systemic circulation can be achieved, facilitating sustained drug release at RA-affected joints and further enhancing therapeutic efficacy. Bioactive nanomaterials, with their unique biological activities, have the potential to directly or indirectly inhibit angiogenesis by modulating cells and cytokines, thereby exerting therapeutic effects on RA. These nanomedicines can be broadly categorized into four main strategies: hypoxia-relieving bioactive nanomaterials, ROS-scavenging bioactive nanomaterials, biomimetic nanomedicines, and nanomedicine targeted delivery systems. Hypoxia plays a pivotal role in promoting angiogenesis, primarily due to the direct interaction of HIF with HREs located in the VEGF promoter region. This interaction is crucial for driving angiogenesis in RA. Recently, some special nanomaterials are capable of increasing O 2 levels either through in situ O 2 generation or by delivering exogenous O 2 , offering a promising approach for the treatment of RA. Nanoparticle-mediated in situ production of oxygen for the treatment of RA is mainly mediated by catalase (CAT). [ 197 ]. The concentration of H 2 O 2 in RA tissues (as high as 1.0 mM) is approximately 100 times higher than in healthy tissues. This phenomenon is considered a primary factor contributing to DNA damage and served as a critical driver of increased oxidative stress, inflammation, angiogenesis, and tissue damage in RA [ 116 , 198 ]. H 2 O 2 is hydrolyzed into H 2 O and O 2 under the catalytic action of catalase (CAT) and CAT-like nanomaterials (such as metals and metal oxides) (Equation (1) ). The resulting O 2 can be utilized to alleviate tissue hypoxia. Furthermore, CAT also exhibits catalytic activity to scavenge ROS, protecting cell membranes from damage [ 199 ]. The activity of catalase is significantly reduced in RA patients compared to healthy. This phenomenon contributes to the elevated levels of H 2 O 2 and hypoxia in RA tissues [ 200 ]. Therefore, increasing CAT levels in RA tissues has emerged as an important strategy to slow disease progression. However, the application of free CAT is limited by its poor stability, protease-induced degradation, short half-life, and low cellular penetration. To overcome these limitations, loading CAT onto inorganic or polymeric nanostructures has proven to be an effective approach to enhance its stability in vivo and mitigate the hypoxic microenvironment [ 199 ]. Recently, researchers have developed CAT enzyme-like nanomaterials (nanozymes), including metals and metal oxides. Metal nanozymes, such as platinum (Pt) and rhodium (Rh), exhibit significant CAT enzyme-like activity. Due to their high stability and excellent biocompatibility, these nanoparticles have been employed to decompose H 2 O 2 and generate O 2 , providing therapeutic benefits for alleviating hypoxia and treating RA [ 201 , 202 ]. Notably, Rh nanoparticles exert anti-RA therapeutic effects by utilizing both POD and CAT activities, which promote apoptosis of FLS and inhibit angiogenesis [ 202 ]. Furthermore, metal oxide nanozymes, including MnO 2 , CeO 2 , and iron oxides, have also been developed as potential treatments for RA [ 203 , 204 ]. Yang et al. synthesized goethite, a typical hydroxy iron oxide, where the tetrahedrally coordinated Fe can form a composite catalytic center with adjacent hydroxyl groups, promoting H 2 O 2 decomposition and O 2 production [ 205 ]. Goethite exhibits high CAT activity and catalyze the conversion of excessive H 2 O 2 to O 2 in RA animal models, thereby providing antioxidant and oxygenation effects, which help alleviate inflammation. Equation 1 H 2 O 2 C A T C A T − l i k e n a n o e n z y m e → O 2 + H 2 O In recent years, nanomaterials with high oxygen binding capacity have been extensively explored for direct exogenous O 2 delivery to RA tissues to alleviate hypoxic microenvironment The primary O 2 -delivering nanomaterials can be classified into the following categories: 1) perfluorocarbon (PFC), a class of synthetic compounds with unique physicochemical properties, are characterized by low boiling points, transparent liquid states, and chemical inertness. Approved by the FDA as artificial blood substitutes, PFCs are clinically utilized to deliver O 2 to ischemic tissues and enhance oxygenation. PFC emulsion, with their nanoscale dimensions, can traverse pathologically obstructed vasculature, and exhibits an impressive oxygen dissolution concentration, reaching around 40 %–50 %, which surpasses water's capacity by 20 times and exceeds plasma by 2 times [ 206 ]. Oxygen solubility in PFC-based nanomaterials is governed by partial pressure gradients, enabling passive diffusion of dissolved O 2 into hypoxic regions [ 199 ]. PFC-based nanomaterials were engineered to transfer O 2 to synovial tissues, counteracting hypoxia in RA pathogenesis [ 207 ]. 2) Hemoglobin (Hb), a primary component of erythrocytes, possesses intrinsic reversible oxygen-binding capacity for tissue oxygenation. Synthetic Hb analogs have been engineered as erythrocyte substitutes to deliver O 2 to hypoxic sites. However, clinical applications of free Hb are constrained by its short circulatory half-life and instability. To address these limitations, advanced nano-encapsulation and nano-conjugation strategies have been developed to enhance Hb stability and prolong systemic circulation [ 199 , 208 ]. 3) Metal-Organic Framework (MOF) are porous coordination polymers formed through self-assembly of inorganic nodes (metal ions/clusters) and organic ligands. MOF exhibit exceptional compositional diversity, ultrahigh surface areas, tunable pore architectures, high porosity, and multifunctionality, rendering them promising candidates for gas storage and separation. Recently, MOF-based nanomaterials have garnered significant attention for oxygen delivery for RA [ 198 , 209 ]. In the pathogenesis of RA, excessive ROS generation is a common trigger and significant promoter in the process of angiogenesis. ROS induces lipid peroxidation, leading to the formation of secondary compounds such as MDA and 4-HNE, which stimulate FLS to release pro-angiogenic factors. Additionally, ROS accumulation and mitochondrial damage contribute to cartilage destruction and accelerated joint deformation in RA patients [ 210 ]. Currently, many kinds of nanodrugs with ROS-scavenging bioactive efficacy have been developed to treat RA. Specially, these nanodrugs with scavenging ROS mainly in the following two aspects: the first category is nano-ROS sacrificers, which effectively scavenges excessive ROS through the redox reaction of variable valence elements. The other is antioxidant nanozymes, which have regenerative active centers that can mimic the catalytic function of endogenous antioxidant enzymes (SOD and CAT) to continuously degrade ROS. In recent years, various nano ROS sacrificers have been widely used in the treatment of RA by rapidly reacting with excess ROS and sacrificing their own activity, thereby protecting endogenous biomolecules from oxidative damage. These sacrificers can be categorized into three types: DNA origami, transition metal compounds, and inorganic compounds, all of which act as active reductants to exhibits biological activity in scavenging high levels of ROS in RA tissues. Firstly, DNA origami typically consists of multiple short DNA strands self-assembled into three-dimensional structures through specific pairing rules. The DNA molecules inherently possess numerous phosphate groups and nitrogenous bases, which exhibit electrophilic properties and can react with ROS, such as hydroxyl radicals and superoxide anions [ 211 ]. The stable structure and chemical activity of DNA origami are partially disrupted following ROS interaction [ 212 , 213 ]. Secondly, some transition metal compounds enable redox reactions with ROS to eliminate ROS. Currently, the transition metal nanocomplexes adopted in the treatment of RA mainly include Mn 3 O 4 nanoparticles and molybdenum-based polyoxometalates (POMs) [ 214 ]. These nanomaterials exploit their unique redox cycling properties to effectively scavenge excessive ROS in inflammatory microenvironments, thereby mitigating oxidative damage and inflammatory responses in joint tissues [ 215 ]. Mn 3 O 4 nanoparticles utilize the redox cycle between Mn 2+ and Mn 3+ to consume O 2 ·- and H 2 O 2 , facilitating the repolarization of pro-inflammatory M1 macrophages to anti-inflammatory M2 macrophages. Specifically, Mn 3 O 4 contains Mn 2+ and Mn 3+ in different oxidation states. Mn 3+ is reduced to Mn 2+ by accepting electrons, while oxygen atoms in Mn 3 O 4 combine with electrons to form MnO 2 (Equation (2) ). The reduced Mn 2+ can react with O 2 ·- and catalyze H 2 O 2 decomposition, which further releases electrons into Mn 3+ , scavenging the ROS while generating oxygen (Equation (3) ) [ 216 ]. Similarly, POMs scavenge excess ROS, especially H 2 O 2 and O 2 ·- , by redox cycling between Mo 6+ /Mo 5+ . In this process, Mo 6+ is reduced to Mo 5+ , which releases electrons and catalyzes the reaction with O 2 ·- and the decomposition of H 2 O 2 . Conversely, Mo 5+ can be oxidized to Mo 6+ , which continues to keep the POM cluster active by accepting electrons [ 214 ]. Equation 2 M n 3 O 4 + e − = 2 M n 2 + + M n O 2 Equation 3 M n 2 + + O 2 · − = M n 3 + + O 2 ↑ M n 2 + + H 2 O 2 = M n 3 + + H 2 O + O 2 ↑ Several inorganic elements exhibited significant antioxidant activity by utilizing their various oxidation states. Selenium, carbon, silicon and phosphorus are essential elements required by the human body, and nanoparticles based on these elements have shown excellent efficacy in RA treatment with low side effects. Selenium nanoparticles (SeNPs) rely on the unique chemical properties of selenium, particularly the high reactivity of selenium atoms, enabling them to rapidly scavenge excess ROS. Specifically, selenium atoms can react with O 2 ·- or ·OH, as demonstrated by the reactions: Se + O 2 ·- → SeO + O 2 or Se + ·OH → SeO + H 2 O. This process leads to the formation of stable products, such as SeO, through electron transfer [ 217 ]. Fullerene C 60 , a crystalline carbon-based nanomaterial, has shown remarkable ROS-scavenging capacity. The electron-affinity conjugated double bond structure of fullerene C 60 enables it to effectively quench ROS by accepting unpaired electrons. Additionally, the unique chemical properties of C 60 allow it to absorb protons and acquire a positive charge, thereby modulating the mitochondrial membrane potential to inhibit the generation of O 2 ·- . Specifically, after carrying protons, fullerene C 60 enters between the inner and outer mitochondrial membranes, where excess protons alter the potential difference between the two sides of the membrane. In turn, mitochondrial ROS production is highly dependent on the transmembrane potential difference (the difference in proton concentration between the inner and outer mitochondria), and a slight decrease in transmembrane potential can significantly inhibit ROS generation [ 218 , 219 ]. Silicon-based nanomaterials have been widely applied in the treatment of bone-related diseases, CaSi 2 nanosheets. CaSi 2 acts as an ROS sacrificial agent and undergoes hydrolysis to form antioxidant hydrogen (H 2 ), alkaline Ca(OH) 2 , and amorphous silica (Equation (4) ). The released H 2 directly reacts with hydroxyl radicals in a non-enzymatic reduction reaction and reduces the oxidative damage of ONOO − (Equation (5) ), clearing free radicals in RA tissues and modulating macrophage polarization. The production of Ca(OH) 2 neutralizes the acidic microenvironment in RA and inhibits osteoclast activity [ 220 ]. Equation 4 C a S i + 6 H 2 O = C a ( O H ) 2 + 2 S i O 2 + 5 H 2 ↑ Equation 5 H 2 + 2 · OH = 2 H 2 O H 2 + · ONO O − = N O 2 − + H 2 O Various nanozymes have been identified with antioxidant activity similar to endogenous antioxidant enzymes, including CAT-like nanozymes, SOD-like nanozymes, and glutathione peroxidase (GPx)-like nanozymes. CeO 2 is an antioxidant nanoenzyme characterized by unique redox properties and the development and transfer of surface oxygen vacancies [ 221 ]. Ce exists in two oxidation states: Ce 3+ and Ce 4+ . When the Ce 3+ /Ce 4+ ratio is high, CeO 2 exhibits SOD-like activity. Specifically, CeO 2 particles react with O 2 · - to form oxygen vacancies and reduce Ce 4+ to Ce 3+ , generating H 2 O 2 (Equation (6) ). When the Ce 3+ /Ce 4+ ratio is low, CeO 2 tends to exhibit CAT-like activity by reacting with H 2 O 2 , breaking it down into H 2 O and O 2 , while Ce 3+ is oxidized to Ce 4+ (Equation (7) ). In the acidic microenvironment of RA tissues, excessive hydrogen ions (H + ) hinder the conversion of Ce 4+ to Ce 3+ , thereby limiting the antioxidant efficacy of CeO 2 . Recently, Fu et al. developed a MgAl-Layered Double Hydroxide (LDH)-loaded CeO 2 nanoplatform (LDH-CeO 2 ) to effectively scavenge excess ROS in RA. The MgAl-LDH is mildly alkaline, effectively neutralizing the acidic microenvironment in RA tissues to enhance CeO 2 's antioxidant activity. The enhanced antioxidant activity of CeO 2 further triggers significant M2 polarization of macrophages to reprogram the inflammatory microenvironment in RA [ 222 ]. Prussian blue (PB) is a multifunctional nanozyme that mimics the enzyme activity of natural antioxidants, such as SOD, POD, and CAT. Specifically, PB exhibits SOD-like activity by accepting electrons from superoxide anions, and converting them into molecular oxygen. Then, PB can be oxidized by O 2 ·- , ultimately converting O 2 ·- into H 2 O 2 . This transforms O 2 ·- into less reactive species, such as O 2 and H 2 O 2 , thereby effectively mitigating the impact of O 2 ·- . In this process, PB catalyzes the dismutation reaction of O 2 ·- , closely resembling the function of SOD enzymes [ 223 ]. The CAT-like mechanism of PB involves accepting electrons from electron-donor substrates to reduce PB, which then donates electrons to H 2 O 2 , facilitating its oxidation [ 223 ]. Based on this, our group has developed a multifunctional nanodrug delivery system (M@PB@SIN NPs) for RA treatment. PB serves as a carrier for the anti-inflammatory drug sinomenine (SIN). M@PB@SIN NPs synergistically increase the loading rate of SIN by utilizing the hollow structure of PB and the interaction between SIN and Fe (III) in PB, which in turn promotes its anti-inflammatory effects. Moreover, PB exhibits antioxidant enzyme activity by scavenging excess ROS in macrophages, exerting its antioxidant effects in synovial tissues of AIA model rats. The hybrid membrane composed of erythrocyte and macrophage cell membranes imparts biomimetic properties to M@PB@SIN NPs, enabling specific targeting to inflamed joints and facilitating effective RA treatment [ 224 ]. Equation 6 O 2 · − + C e 4 + = C e 3 + + O 2 ↑ O 2 · − + C e 3 + + 2 H + = C e 4 + + H 2 O 2 Equation 7 H 2 O 2 + C e 3 + + 2 H + = C e 4 + + · O H + H 2 O · O H + H 2 O 2 = H O 2 − + H 2 O H O 2 − + C e 4 + = C e 3 + + H + + O 2 ↑ In RA, various stromal cells such as FLS, neutrophils, macrophages, and ECs play pivotal roles in inflammatory responses, local immune regulation, and angiogenesis. Biomimetic nanomedicines use natural cell structures as carriers, giving the drugs they carry some endogenous immune escape properties and lesion targeting, thereby significantly reducing the clearance of the drugs they carry and greatly increasing the circulation time and bioavailability of the drugs they carry [ 225 ]. Biomimetic nanomedicines based on special cell membrane artifacts and extracellular vesicles have become a highly innovative and promising strategy in the field of RA therapy due to their ability to reconfigure complex structures and mimic cellular functions. Cell membrane camouflaged nanodrugs are composed of a nanoparticle core encapsulated by a cell membrane bilayer. This configuration inherits the unique properties of the source cells, enabling nanodrugs to penetrate the bloodstream and accumulate in the inflamed synovial tissues of RA joints, thus delivering high concentrations of the loaded drugs. In RA treatment, cell membrane-camouflaged nanomedicines have been developed from macrophages, FLS, neutrophils, and MSCs. Activated macrophages play a crucial role in the early stages of RA by rapidly recruiting immune cells from the bone marrow via chemokines and inflammatory cytokines, subsequently adhering to the vascular endothelium to promote disease progression [ 226 ]. Coated with red cells and macrophages membrane to enhance the system's biomimetic properties, allowing the prolonged circulation and efficient accumulation of polydatin at the inflammatory site of RA [ 227 ]. FLS is likewise a key effector cell in the pathogenesis of RA. RAFLS membrane-coated nanodrugs not only reduced the clearance rate and extended the circulation time of hematin, but also effectively targeted FLS to inhibit synovial hyperplasia [ 228 ]. Neutrophils, as the first line of defense in innate immunity, are crucial in combating pathogens. By coating with neutrophil membranes, nanomedicines can inherit neutrophil membrane proteins and functional capabilities, thereby gaining the ability to migrate to the inflamed synovial tissues [ 229 ]. Exosomes are extracellular vesicles secreted by various cells under both normal and pathological conditions. They play critical roles in immune responses, antigen presentation, cell migration, and cell differentiation. Increasing evidence supports the significant involvement of exosomes in the pathogenesis of RA [ 226 ]. Exosomes are primarily composed of proteins and lipids, including fusion proteins, transport proteins, phospholipases, cholesterol, and sphingolipids. Exosomes have a variety of markers on their surface, such as transmembrane proteins (CD 9, CD 81 and CD 63) and heat shock proteins (HSP60, HSP70, and HSP90). Exosomes also carry a wide range of molecular cargo, including proteins, DNA, RNA, mRNA, and microRNAs (miRNAs), which mediate intercellular signaling [ 230 ]. Furthermore, exosomes possess inherent targeting properties and biocompatibility, enabling precise drug delivery without triggering immune rejection. This characteristic makes exosomes an innovative strategy for RA treatment, offering multiple regulatory mechanisms and precise intervention [ 231 ]. MSC-derived exosomes, typically 50–150 nm in size without causing microinfarcts, exhibit stable characteristics that allow for easy storage and transport. Due to the inherent migratory properties of MSCs towards inflamed tissues, MSC-derived exosomes have emerged as unique biomimetic nanoplatforms for targeted drug delivery in chronic inflammatory diseases [ 232 ]. MSC-derived exosomes regulate angiogenesis more than other exosomes because they contain various mRNAs, miRNAs, and proteins from the parent MSCs. MiRNA-150-5p, found in MSC-derived exosomes, has been shown to regulate angiogenesis [ 233 ]. MSCs in RA joints secrete a variety of chemokines (CXCL12, MIP-1a, CXCL8, and PDGF), anti-inflammatory cytokines (TGF-β, IL-4, IL-10), and immune-modulatory factors (indoleamine 2,3-dioxygenase and hepatocyte growth factor), which suppress the proliferation and activation of effector T-cells while promoting the generation of regulatory T-cells (Tregs), thereby balancing immune responses and inhibiting inflammation [ 204 , 234 ]. Stella et al. isolated MSC-derived exosomes and microparticles via differential ultracentrifugation, demonstrating that exosomes were more effective than microparticles in reducing CD4 + IFN-γ + T lymphocytes and inducing Tregs, thereby suppressing T lymphocyte proliferation. Furthermore, MSC-derived exosomes showed enhanced antigen-specific anti-inflammatory effects in a CIA model, potentially due to the reduced differentiation of plasma blasts and Breg cells [ 235 ]. These findings underscore the superior immunoregulatory potential of MSC-derived exosomes in RA treatment. Eun et al. explored exosomes derived from immortalized human adipose-derived MSCs (RA-iMSCs-Exo) in RA therapy. RA-iMSCs-Exo enhanced the expression of immunomodulatory factors (TGF-β1, PGE2, IL-1Ra) and anti-inflammatory cytokines (IL-4, IL-10) while reducing pro-inflammatory cytokines (TNF-α, IL-1β, IL-6). RA-iMSCs-Exo also promoted Treg cell conversion from T-cells and M2 macrophage polarization, improving cartilage damage in CIA mice [ 236 ]. Currently, small molecule drugs used for the treatment of RA are primarily administered via intravenous injection or intra-articular injection. However, these approaches have several limitations, including low targeting efficiency, poor bioavailability, and significant side effects. One of the most critical challenges is the rapid leakage of these drugs from the joint spaces or microvasculature, which significantly impairs their ability to reach and accumulate at the targeted site. Targeted nanomedicine delivery systems not only enable sustained drug release to reduce the frequency of administration, but also significantly improve bioavailability of drugs [ 237 ]. Notably, nanomedicines are generally larger than small molecule drugs, making them less likely to leak through joint gaps or microvasculature, thus prolonging their local retention time. Targeted nanomedicine delivery systems can generally be classified into two categories: passive targeting and active targeting medicines. Passive targeting depends on the size and shape of nanomedicines, which dictate their distribution and accumulation within RA tissues. Specifically, passive targeting typically leverages the EPR effect to treat conditions associated with altered tissue characteristics. In contrast, active targeting systems involve modifying the surface of nanomedicines with specific ligands (peptides, proteins, or antibodies) to recognize and bind receptors that are specifically expressed on the cell membrane of target cells. For effective therapy, active targeting systems require that the relevant receptors on the target tissues be uniquely or over-expressed, thereby facilitating preferential drug accumulation in the diseased tissue and forming a selective therapeutic system [ 225 ]. Currently, surface-modified ligands for RA-targeted nanomedicines include hyaluronic acid (HA) and Arg-Gly-Asp (RGD) peptides. HA is a natural glycosaminoglycan widely found in connective tissues, and mucous tissues. In synovial fluid, the high concentration of HA helps lubricate the joint and reduces friction during bone movement, effectively preventing joint wear [ 237 ]. Due to its excellent biocompatibility, biodegradability, and specific binding affinity with the CD44 receptor, HA has been extensively studied in drug delivery systems and tissue engineering applications. The CD44 receptor is overexpressed on T cells, activated macrophages, and FLS in RA [ 238 ]. Our team previously developed a novel dual-targeting nanocomposite (HA-M@P@HF). In this system, HA serves as a ligand for CD44, enabling dual targeting of dysregulated macrophages and RAFLS in RA synovium. Halofuginone (HF) is encapsulated within PLGA to enhance its anti-inflammatory activity and inhibit synovial proliferation. The hybrid membrane of HA-M@P@HF further extends the circulation time of HF in the bloodstream and facilitates its targeted delivery. HA-M@P@HF can modulate immune inflammation and synovial hyperplasia, primarily through the synergistic effect of M1-to-M2 macrophage repolarization and RAFLS apoptosis [ 239 ]. The RGD peptide interacts with integrin receptors on ECs with high affinity, making it a specialized ligand for targeted drug delivery in ECs. For instance, Koning et al. constructed a RGD medicated polyethylene glycol (PEG) liposome. RGD-PEG liposomes evaluated the uptake of HUVECs. Compared to PEG liposomes, RGD-PEG liposomes accumulated three times more in the inflammation sites of LPS-induced arthritis rat model. Furthermore, RGD-PEG liposomes exhibited superior efficacy in inhibiting disease progression, significantly reducing the peak severity of arthritis [ 238 ].

In response to the heightened inflammation and hypoxic microenvironment in RA, angiogenesis is markedly enhanced. This phenomenon is primarily driven by the activation of various stromal cells—including immune and inflammatory cells (FLS, neutrophils, and macrophages) and ECs, which undergo significant morphological and functional alterations. These changes further promote the secretion of pro-angiogenic mediators, such as VEGF, PDGF, and selectin. As illustrated in Fig. 1 B, the existence of pro-angiogenic mediators serves as a critical trigger that initiates the angiogenic process. The newly formed blood vessels facilitate the infiltration of large numbers of immune and inflammatory cells into the synovial tissue, thereby exacerbating the progression of synovitis. Invasive angiogenesis further contributes to joint and cartilage damage, driving the progression of RA. Therefore, nanomedicines designed based on two key mechanisms—targeting activated stromal cells and targeting specific removal pro-angiogenic mediators—holds promise for effectively inhibiting RA-associated angiogenesis at its source ( Fig. 9 ). Fig. 9 Anti-Angiogenic Nanoagents for treating RA. Fig. 9 Anti-Angiogenic Nanoagents for treating RA. In RA joints, immune and inflammatory cells, such as FLS, macrophages and neutrophils, were activated and secreted pro-angiogenic mediators, binding to the corresponding receptors on ECs, thereby initiating the angiogenesis signaling cascade ( Table 1 ). Based on this mechanism, researchers have developed various nanomaterials targeting two key cellular systems: immune and inflammatory cells, as well as ECs, which are the key cellular players in the angiogenic process. These nanomaterials not only enable the targeted delivery of anti-angiogenic drugs through the activation of stromal cells, but also directly inhibit the secretion of pro-angiogenic mediators and suppress EC migration. These dual actions contribute to an effective anti-angiogenic nanoagent for RA treatment ( Table 2 ). Table 2 Anti-angiogenic nanoagents targeting activated stromal cells in treating RA. Table 2 Type Nanomaterials Drug-loaded Size/z-pot Model Administration of model Effects Ref Nanodrugs for FLS Synovial MSC --- 100 nm/--- CIA mice model Intra-articular (i.a.) injection Reduced synovial VEGF and ameliorated arthritis severity. [ 240 ] MSC --- --- CIA mice model Intra-peritoneal (i.p.) injection Reduced hind paw thickness, the clinical arthritic scores, inhibited synoviocyte hyperplasia. [ 233 ] Graphene oxide quantum dot combining with hyaluronic acid-inserted hybrid membrane Sinomenine hydrochloride 115.00 ± 3.86 nm/-9.23 ± 0.23 mV CIA and AIA rat model Intravenous (i.v.) injection Inhibited macrophage polarization, synovial hyperplasia, cartilage and bone erosion. [ 241 ] Nanodrugs for neutrophil Nanoparticle modified with mannose MTX 188.17± 1.71 nm/1.22±0.18 mV CIA rat model i.a. injection Exerted anti-inflammatory, antiangiogenic, and analgesic properties. [ 242 ] Neutrophil membrane-coated nanoparticles --- 70-110 nm/-15—30 mV CIA mice model i.a. injection Neutralized proinflammatory cytokines, and inhibited joint damage. [ 244 ] Nanodrugs for macrophage Mannosylated liposomes Morin 132.5 ± 5.2 nm/ -54.8 ± 0.67mV AIA mice model i.v. injection Inhibited inflammation, cartilage degradation, cellular infiltration and pannus formation [ 245 ] Arachyl alcohol modificated and thioether bonds responsed nanoassemblies Darutigenol 68.45 ±0.9322 nm/-22.8 ± 2.084 mV CIA mice model i.v. injection Decreased joint score and joint thickness, and bone degradation [ 246 ] Ceria nanoparticle-immobilized MSC nanovesicle --- 100nm/-23mV CIA mice model i.a. injection Immunomodulation, and inhibited inflammation [ 247 ] Nanodrugs for ECs RGD-Modified Polymeric Micelles MTX and Nimesulide 60.20 ± 3.21 nm/1.60 ± 1.98mV CFA mice model i.v. injection Suppressed angiogenesis of chick embryos, promoted retention of micelles in arthritic joints. [ 142 ] Modifying selenium nanoparticles with PEG, RGD and Ru. --- 50 nm/5.3 mV CIA mice model i.v. injection Promoted apoptosis of HUVECs, inhibited the growth of new vessels and the levels of inflammatory cytokine. [ 248 ] αvβ3-integrin-targeted perfluorocarbon nanocarriers Fumagillin prodrug 252 nm/-18 mV KRN mice model i.v. injection Suppressed inflammatory cytokine production. [ 207 ] Poly-lactic acid, poly-caprolactone, and PEG coated with synovial homing peptide. Methotrexate (MTX) ≈170 nm/-8 mV AIA and CIA mice model i.v. injection Targeted and killed CD34 + cells, prevented neo-angiogenesis and synovial inflammation. [ 249 ] Anti-angiogenic nanoagents targeting activated stromal cells in treating RA. FLS not only serve as a critical source of synovial proliferation and inflammation but also act as the primary effector cells in the secretion of pro-angiogenic mediators within the RA joint. Given the central role of FLS in RA angiogenesis, MSC-derived exosomes, which possess potent immunomodulatory properties, have been shown to inhibit synovial hyperplasia and angiogenesis, thus reducing joint destruction. Recently, Zhang et al. utilized differential ultracentrifugation to isolate exosomes from RA patient-derived synovial MSCs (SMSC-Exos) ( Fig. 10 A). SMSC-Exos effectively fused with RAFLS, demonstrating superior targeting capabilities. SMSC-Exos significantly reduced the expression of VEGF mRNA and protein in RAFLS, while also decreasing the VEGF concentration in the cell supernatant, effectively blocking angiogenesis within RA joints. Additionally, SMSC-Exos promoted cartilage repair and prevented chondrocyte apoptosis by inhibiting the circEDIL3/miR-485-3p/PIAS3/STAT3/VEGF signaling axis ( Fig. 10 B) [ 240 ]. Although MSC-derived exosomes can target RA-FLS and inhibit VEGF secretion to block angiogenesis, the specific components involved in this process have yet to be fully elucidated. Chen and colleagues discovered that the expression of mRNA-150-5p in RAFLS was lower compared to healthy. MSC-derived exosomes, which contain various mRNAs and miRNAs from the parent MSCs, were secreted and enter the circulatory system, thereby mediating the transfer of mRNA between cells. Given that mRNA-150-5p plays a crucial role in regulating angiogenesis, they constructed MSC-derived exosomes expressing mRNA-150-5p (Exo-150) to treat RA by specifically targeting RAFLS. Co-incubation of Exo-150 with RAFLS revealed efficient uptake by RAFLS. Exo-150 downregulated the expression of MMP14 and VEGF in RA-FLS, inhibited cell migration and invasion, and effectively blocked angiogenesis, leading to a significant reduction in clinical arthritis scores in CIA mice ( Fig. 10 C) [ 233 ]. Furthermore, Lin et al. developed a multifunctional nanotherapeutic system for anti-RA treatment. This system utilized a hybrid membrane composed of RAFLS and erythrocyte membranes, imparting biomimetic properties that enhanced immune evasion and targeting specificity towards RA-FLS. The system exhibited clear fluorescent signals in the inflamed joints of the affected rats ( Fig. 10 D). HA-modified nanoparticles further improved targeting to inflamed RA joints. The nanotherapeutic system internalized by RAFLS via endocytosis, successfully escaped from lysosomes by 6 h. Released SIN inhibited RAFLS proliferation and inflammatory cytokine secretion, exerting anti-inflammatory effects. Additionally, graphene oxide quantum dots (GOQD) cleared ROS, thereby inhibiting angiogenesis in the synovial tissue of an AIA rat model and ultimately reducing the synovitis score [ 241 ]. Overall, these nanostrategies targeting FLS effectively inhibit the secretion of VEGF and block angiogenesis. This approach offers novel strategies for the precise treatment of RA, contributing to better therapeutic outcomes in RA patients. Fig. 10 (A) Schematic illustration of the mechanism of SMSCs-Exos in RA. (B) Representative micro-CT photograph, clinical scores and histological assessment scores. Adapted with permission from Ref. [ 240 ], copyright 2021. (C) MMP14 and VEGF mRNA (a) and protein (b) expression, and image of tube formation assay, MMP14 (c), VEGF, and CD31 (d) immunohistochemistry in RA FLS and CIA mice with Exo-150 treatment. Adapted with permission from Ref. [ 233 ], copyright 2018. (D) Fluorescent distribution of HA@RFM@GP@SIN NPs at various time points in the arthritis site. Adapted with permission from Ref. [ 241 ], copyright 2024. Fig. 10 (A) Schematic illustration of the mechanism of SMSCs-Exos in RA. (B) Representative micro-CT photograph, clinical scores and histological assessment scores. Adapted with permission from Ref. [ 240 ], copyright 2021. (C) MMP14 and VEGF mRNA (a) and protein (b) expression, and image of tube formation assay, MMP14 (c), VEGF, and CD31 (d) immunohistochemistry in RA FLS and CIA mice with Exo-150 treatment. Adapted with permission from Ref. [ 233 ], copyright 2018. (D) Fluorescent distribution of HA@RFM@GP@SIN NPs at various time points in the arthritis site. Adapted with permission from Ref. [ 241 ], copyright 2024. During the pathogenesis of RA, neutrophils are among the first immune cells to arrive at the affected joints. Due to their propensity for dissolution in response to severe inflammatory stimuli, neutrophils are often utilized as carriers for the delivery of anti-angiogenic drugs. MTX, the first-line clinical medication for RA, not only exerts anti-inflammatory and analgesic effects but also inhibits angiogenesis by reducing the secretion of VCAM-1 and ICAM-1 by neutrophils [ 243 ]. However, the poor water solubility, significant side effects, and narrow therapeutic time window of MTX greatly limit its efficacy. Lyu et al. developed a mannosylated MTX-modified nanoparticle formulation (MTX-M-NPs) ( Fig. 11 A). Upon intravenous injection, the mannitol interacts with the mannose receptors on the surface of neutrophils in the vascular system of RA synovial tissue, facilitating internalization and targeted delivery to neutrophils ( Fig. 11 B). Under the highly inflammatory environment of RA joints, MTX-M-NPs undergo dissolution, resulting in the specific release of MTX within the newly formed blood vessels of the synovial tissue. MTX exerts its anti-angiogenic effects by reducing neutrophil secretion of ICAM-1 and VCAM-1 ( Fig. 11 C). Furthermore, MTX-M-NPs increase the plasma and joint concentrations of MTX in CIA rat model, extending the drug's circulation time and allowing MTX to continuously exert its inherent anti-inflammatory properties. Notably, MTX-M-NPs not only reduce the hepatic toxicity of free MTX, but also alleviate joint swelling and bone erosion in CIA rats [ 242 ]. Additionally, Zhang et al. developed neutrophil membrane-coated nanoparticles (N-NPs). Utilizing the key antigens and functional proteins naturally enriched on the extracellular membrane of neutrophils, N-NPs targeted neutrophils and exert anti-RA effects. The LFA-1 integrin on the neutrophil membrane specifically interacts with the overexpressed ICAM-1 on HUVECs, competitively inhibiting HUVEC activation and thereby impeding angiogenesis. By retaining the specific receptors from the source neutrophils membrane, N-NPs can selectively bind to inflammatory factors and neutralize pro-inflammatory cytokines, thereby exerting anti-synovial inflammation effects. N-NPs demonstrated significant prophylactic and therapeutic efficacy in preventing joint cartilage damage in both CIA rat models and transgenic mouse models of human arthritis ( Fig. 11 D) [ 244 ]. Fig. 11 (A) Schematic illustration of treatment of RA by MTX-M-NPs to target neutrophils. (B) TEM microscopy results (a) and characterization (b) of NPs, MTX-NPs and MTX-M-NPs, and pharmacokinetics (c) of free MTX, MTX-NPs and MTX-M-NPs. (C) Morphology of neutrophils after incubation with MTX-M-NPs and effect of different formulations on neovascularization in the CAM assays. Adapted with permission from Ref. [ 242 ], copyright 2021. (D) Representative images of H&E staining and safranin-O staining on knee sections from CIA mice treated with N-NPs, CIA, anti-IL-1β antibody or anti-TNF-α antibody. Values of paw volume recorded every other day for a total of 60 days. Adapted with permission from Ref. [ 244 ], copyright 2018. Fig. 11 (A) Schematic illustration of treatment of RA by MTX-M-NPs to target neutrophils. (B) TEM microscopy results (a) and characterization (b) of NPs, MTX-NPs and MTX-M-NPs, and pharmacokinetics (c) of free MTX, MTX-NPs and MTX-M-NPs. (C) Morphology of neutrophils after incubation with MTX-M-NPs and effect of different formulations on neovascularization in the CAM assays. Adapted with permission from Ref. [ 242 ], copyright 2021. (D) Representative images of H&E staining and safranin-O staining on knee sections from CIA mice treated with N-NPs, CIA, anti-IL-1β antibody or anti-TNF-α antibody. Values of paw volume recorded every other day for a total of 60 days. Adapted with permission from Ref. [ 244 ], copyright 2018. The abundance of macrophages in the synovial tissue is positively correlated with the severity of angiogenesis in RA. Activated macrophages exhibit overexpression of various specific receptors on their surface, providing a unique opportunity for targeted drug delivery. Morin (3,5,7,2′,4′-pentahydroxyflavone), a common flavonoid compound known for its anti-inflammatory properties, has recently been shown to exert anti-angiogenic effects by inhibiting VEGF-induced migration and tube formation in HUVECs. However, its poor water solubility, low bioavailability, and potential toxicity at high doses severely limit its clinical application. To address these limitations, Farhas et al. developed mannose-modified Morin liposomes (ML-Morin) for RA treatment. ML-Morin delivers Morin by binding modified mannitol to the mannitol receptor overexpressed on the surface of RA macrophages to target and be internalized by macrophages, further downregulating macrophage secretion of VEGF and exerting antiangiogenic effects ( Fig. 12 A). Morin encapsulated by liposomal extends its circulation time, facilitates sustained release in the RA joint, further enhancing Morin's solubility and bioavailability. In addition, ML-Morin significantly reduces the release of inflammatory cytokines (TNF-α, IL-1β, and IL-6) from macrophages in the AIA rat model and improves cartilage degeneration ( Fig. 12 B) [ 245 ]. Darutigenol (DL), in addition to its known anti-inflammatory and anti-rheumatic activities, has recently been shown to inhibit angiogenesis, although the exact mechanism remains unclear. Yan et al. designed a sulfur ether bond-crosslinked DL prodrug nanoplatform (DL-S-AA MPEG NPs) for effective RA treatment. The modification of fatty alcohols not only promotes the molecular self-assembly of DL, but also enables targeting of RA macrophages through specific receptor interactions, which in turn affect their immunoregulatory function. In response to the high ROS levels in macrophages, the sulfur ether bonds are cleaved, promoting the release of DL, which then exerts its anti-angiogenic effect. DL-S-AA MPEG NPs are spherical and have a small particle size, which facilitates passive transport to inflammatory sites through extravasation and subsequent inflammation cell-mediated sequestration (ELVIS) ( Fig. 12 C). DL-S-AA MPEG NPs are capable of inhibiting M1 polarization (CD86) and promoting M2 polarization (CD206) ( Fig. 12 D). Furthermore, DL-S-AA MPEG NPs inhibit the activation of the CXCL12/CXCR7 system, reducing the expression of MMP9 and further inhibiting joint cartilage destruction, thus contributing to anti-angiogenesis [ 246 ]. Additionally, MSCs can target macrophages to deliver immune-modulatory cytokines that alter their characteristics. Koo et al. developed cerium oxide nanoparticle (Ce NPs)-immobilized MSC nanovesicles (Ce-MSCNV) for RA combination therapy ( Fig. 12 E). Ce NPs act as antioxidants by cycling between two reversible ionic states, Ce 3+ and Ce 4+ , and removing excess ROS produced in RA-damaged knee joints. Furthermore, MSCNVs deliver immune-modulatory cytokines to macrophages, reducing the expression of M1-specific markers (IL-1β, IL-6, NOS2, and TNF-α) and increasing M2-specific markers (Arg1 and Mrc1). This leads to a decrease in the secretion of pro-angiogenic and pro-inflammatory cytokines, thus exerting anti-angiogenic effects. MSCNVs also induce tolerance in dendritic cells, promoting the generation of Treg cells and thereby facilitating immune modulation. These two key elements of Ce-MSCNVs (Ce NPs and MSCNVs) work in concert to create a synergistic effect and exhibit anti-chondrogenic degradation, anti-synovitis and anti-angiogenic properties in a CIA mouse model ( Fig. 12 F) [ 204 ]. Fig. 12 (A) Confocal microscopy images showing ML-Morin uptake by synovial macrophages (a) and VEGF expression in synovial macrophages incubated with ML-Morin (b–c). (B) Effects of ML-Morin on the ankle joint of AIA model rats. Adapted with permission from Ref. [ 245 ], copyright 2017. (C) Particle size distribution and TEM images of DL-AA NPs. (D) Characterization of NPs, MTX-NPs, and MTX-M-NPs and drug concentrations in blood. Adapted with permission from Ref. [ 246 ], copyright 2024. (E) Super-resolution microscopy images and fluorescence intensities of co-localization of DiI-labelled MSCNVs and Cy5-labelled Ce. (F) Histological and immunohistochemical evaluation of the knee joints 45 days after immunization. Adapted with permission from Ref. [ 204 ], copyright 2018. Fig. 12 (A) Confocal microscopy images showing ML-Morin uptake by synovial macrophages (a) and VEGF expression in synovial macrophages incubated with ML-Morin (b–c). (B) Effects of ML-Morin on the ankle joint of AIA model rats. Adapted with permission from Ref. [ 245 ], copyright 2017. (C) Particle size distribution and TEM images of DL-AA NPs. (D) Characterization of NPs, MTX-NPs, and MTX-M-NPs and drug concentrations in blood. Adapted with permission from Ref. [ 246 ], copyright 2024. (E) Super-resolution microscopy images and fluorescence intensities of co-localization of DiI-labelled MSCNVs and Cy5-labelled Ce. (F) Histological and immunohistochemical evaluation of the knee joints 45 days after immunization. Adapted with permission from Ref. [ 204 ], copyright 2018. RGD peptides, composed of amino acid sequences, not only targets ECs for the delivery of anti-angiogenic drugs but also inhibits EC proliferation, invasion, and migration by competitively binding to integrin receptors on the EC surface [ 250 , 251 ]. Geng et al. further investigated the potential mechanisms by which the RGD peptide acts as an integrin αVβ3 inhibitor to suppress angiogenesis. They demonstrated that cyclic RGD peptide (c(RGDyk)) binds to integrin αVβ3 on the surface of ECs, inhibiting VEGFR2 activation and further suppressing EC migration through the negative regulation of the FAK/ERK1/2 signaling pathway, thereby inhibiting angiogenesis ( Fig. 13 A) [ 252 ]. Building on this, Wang and colleagues developed an RGD-modified micelle formulation (R-M/N-PMs) loaded with low doses of MTX and nimesulide (NIM). The polymeric micelles, known to improve the solubility of hydrophobic drugs, were selected as an ideal delivery vehicle for the frontline RA therapeutic agents. RGD modification enhances the targeting of RA synovial vasculature by allowing R-M/N-PMs to bind and be internalized by ECs. This leads to the suppression of EC migration and angiogenesis. Furthermore, the RGD modification increased the retention of MTX and NIM in the inflamed joints and improved their therapeutic efficacy. R-M/N-PMs reduced the levels of inflammatory cytokines in the serum and alleviated joint swelling and bone erosion ( Fig. 13 B) [ 142 ]. Ruthenium complexes have been shown to exhibit anti-angiogenic effects in cancer [ 253 ], but their role in RA angiogenesis remains unexplored. Liu and colleagues developed peptide-conjugated selenium nanoparticles (Se@RuNPs) to target angiogenesis in RA. RGD modification bind to integrin αVβ3 on the EC membrane, thereby enhancing Se@RuNPs uptake specifically by newly formed blood vessels in RA-inflamed tissue. Ruthenium complexes stimulate the production of NO through the induction of nitric oxide synthase, promoting EC apoptosis and inhibiting neovascularization in the synovial tissue. SeNPs, as the nanoparticle carrier, also provide antioxidant properties to further suppress angiogenesis. Additionally, Se@RuNPs enhance autophagy flux by increasing AMPK phosphorylation and inhibiting mTOR phosphorylation, while suppressing NF-κB activity, leading to reduced levels of inflammatory cytokines. Se@RuNPs exhibited in significant protection against joint swelling and erythema in CIA mouse models ( Fig. 13 C) [ 248 ]. Fumonisin, a fungal toxin produced by Aspergillus fumigatus, exhibits significant antiangiogenic activity. However, its instability limits clinical application. Zhou developed a fumonisin prodrug loaded and perfluorocarbon (PFC)-based nanoparticle system (Fum-PD NP). The fumonisin prodrug is linked via lipid bonds, overcoming the photoinstability of the natural fumonisin. The PFC nanoparticles not only serve as a carrier for the anti-angiogenic drug but also facilitate oxygen delivery to the RA microenvironment. The RGD peptides modification enables Fum-PD NP to specifically target and bind neovascular ECs in the inflamed joints. Upon undergoing phospholipase cleavage at the SN-2 position, the active fumonisin is released, exerting its anti-angiogenic effects ( Fig. 13 D). In the KRN arthritis mouse model, Fum-PD NP significantly reduced the expression of CD31 and decreased neovascularization. Fum-PD NP also increased oxygen tension in the synovial tissue. Compared to PFC or free fumonisin, Fum-PD NP exhibited stronger effects in inhibiting the progression of arthritis. Fum-PD NP enhanced endothelial NOS expression, leading to NO production and the subsequent activation of the AMPK/mTOR pathway, which promotes autophagy. Increased autophagy flux leads to IKK degradation, thereby inhibiting the subsequent inflammatory response [ 207 ]. Building on the anti-angiogenic mechanisms of RGD peptides, Colombo et al. developed joint-specific synovial-homing peptides (tBNPs) similar in composition to RGD peptides, which consist of a 9-amino acid sequence (CKSTHDRLC). The major difference between BNPs and RGD peptides is that tBNPs bind only to endothelial progenitor cells in inflamed synovial tissue rather than to other normal tissues. After binding to endothelial progenitor cells, tBNPs promote alterations in cellular functions, accelerating cell death and preventing the progression of angiogenesis in RA synovial tissue. When loaded with MTX, tBNPs further increase the local concentration of MTX in the inflamed synovium and inhibit the progression of synovial inflammation in RA ( Fig. 13 E). Furthermore, MTX-loaded tBNPs significantly reduced the systemic toxicity of MTX [ 249 ]. Fig. 13 (A) Schematic diagram of the mechanism by which c(RGDyk) suppressed integrin αVβ3 to inhibit angiogenesis. Adapted with permission from Ref. [ 252 ], copyright 2024. (B) Representative photographs of hind legs were taken on the 27th day. Adapted with permission from Ref. [ 142 ], copyright 2019. (C) The exact mechanism of Se@RuNPs inducing NO to recruit immune cells and regulate inflammatory response. Adapted with permission from Ref. [ 248 ], copyright 2021. (D) Schematic representation of Fum-PD NP delivery mechanism. Adapted with permission from Ref. [ 207 ], copyright 2014. (E) Schematic representation showing that tBNPs-MTX target endothelial progenitor CD34 + cells. Adapted with permission from Ref. [ 249 ], copyright 2019. Fig. 13 (A) Schematic diagram of the mechanism by which c(RGDyk) suppressed integrin αVβ3 to inhibit angiogenesis. Adapted with permission from Ref. [ 252 ], copyright 2024. (B) Representative photographs of hind legs were taken on the 27th day. Adapted with permission from Ref. [ 142 ], copyright 2019. (C) The exact mechanism of Se@RuNPs inducing NO to recruit immune cells and regulate inflammatory response. Adapted with permission from Ref. [ 248 ], copyright 2021. (D) Schematic representation of Fum-PD NP delivery mechanism. Adapted with permission from Ref. [ 207 ], copyright 2014. (E) Schematic representation showing that tBNPs-MTX target endothelial progenitor CD34 + cells. Adapted with permission from Ref. [ 249 ], copyright 2019. As illustrated in Fig. 1 , pro-angiogenic mediators secreted by various stromal cells serve as the primary triggers that initiate physiologic and pathological angiogenesis. Monoclonal antibodies specifically designed to target pro-angiogenic growth factors and/or their receptors represent the first class of angiogenesis inhibitors approved by the FDA for clinical cancer therapy. Currently, several anti-angiogenic nanoagents have been developed for the removal of vascular endothelial growth factors, such as gold nanoparticles (AuNPs), Rh nanoenzymes, manganese ferrate nanoparticles, polydopamine nanoparticles, selenium-doped carbon quantum dots, and macrophage-derived microvesicles. These nanoagents combine with pro-angiogenic mediators by competitively inhibiting their own binding capacity to special removed them, in turn inhibit angiogenesis. These nanoparticles can also exert a combined anti-angiogenic effect by loading angiogenesis inhibitors to maximize penetration into the inflamed vessels of RA. In the field of RA therapy, anti-angiogenic nanoagents have been explored for several years. The next sections detail targeted therapeutic strategies for direct specific removal of proangiogenic mediators, including VEGF, PDGF, and selectins ( Table 3 ). Table 3 Anti-angiogenic nanoagents targeting specific removal pro-angiogenic mediators in treating RA. Table 3 Type Nanomaterials Drug-loaded Size/z-pot Model Administration of model Effects Ref Nanodrugs for VEGF Hyaluronate-gold nanoparticle/Tocilizumab – 64.83 nm/−25.65 ± 3.65 mV CIA mice model – Reduced the expression of VEGF and IL-6. [ 254 ] Spafloxacin doped and HSA loaded concave-cubic rhodium nanozyme – 10 nm/19.8 mV CIA mice model i.v. injection Relieved the hypoxia of the joint to resist angiogenesis. [ 202 ] Manganese ferrite and ceria nanoparticle-anchored mesoporous silica nanoparticles MTX 80.8 ± 2.2 nm/--- AIA rat i.a. injection Produced O 2 , and caused the polarization of proinflammatory M1 macrophages to the anti-inflammatory M2 phenotype. [ 255 ] Nanotechnology- based formulation Pazopanib 200–1000 nm/--- PMM induced OA model i.a. injection Relieved pain by suppressing sensory neuronal ingrowth into the knee synovium and neuronal plasticity. [ 153 ] Nanodrugs for PDGF Methoxypolyethylene glycol amine (mPEGNH2) modified polydopamine nanoparticles – ∼200 nm/−5.1 ± 1.53 mV Anterior cruciate ligament transection (ACLT)-induced OA mice i.a. injection inhibited subchondral bone resorption, angiogenesis, and cartilage degradation [ 256 ] Hyaluronic acid modified with aldehyde and methacrylic anhydride Selenium-doped carbon quantum dots grafted with triphenylphosphine 185 μm/+10.88 mV ACLT-induced OA mice i.a. injection inhibited osteoclastogenesis and H-vessel invasion [ 257 ] Nanodrugs for selectin LMWH-d-α-tocopheryl succinate nanoparticles MTX 130nm/--- CIA mice model i.v. injection Inhibited early recruitment of neutrophils, and reduced MMP-9 secretion. [ 181 ] Macrophage-derived microvesicle-coated nanoparticle Tacrolimus 130 ± 14 nm/−25.5 ± 3.4 mV CIA mice model i.v. injection Decreased paw swelling, and prevented bone erosion. [ 258 ] Sialic acid-modified tetra malonic acid derivative of C70 fullerene – 93.78 ± 0.90 nm/-17.80 ± 0.40 mV CIA mice model i.v. injection Eliminated intracellular ROS and suppressed the differentiation of macrophages and osteoclasts. [ 259 ] Anti-angiogenic nanoagents targeting specific removal pro-angiogenic mediators in treating RA. AuNPs have garnered significant attention due to their excellent biocompatibility, minimal toxicity, and large surface area. AuNPs alter the structure and function of VEGF by forming strong covalent Au-S bonds with the sulfhydryl group in the cysteine in VEGF, as well as electrostatic and weak coordination interactions with the amino group provided by the lysine residue in the VEGF. This affects VEGFR binding ability and downstream VEGF/VEGFR2 signaling pathway, which has shown anti-angiogenic effects in cancer and RA [ 260 , 261 ]. Building on this, Hwiwon Lee designed a HA-AuNP/Tocilizumab nano-complex (H-A/T NP). Tocilizumab is an FDA-approved drug for moderate-to-severe active RA patient since 2010 by reducing the inflammatory response. AuNPs not only function as drug carriers for Tocilizumab, but also exert direct anti-angiogenic effects via the VEGF/VEGFR pathway. The HA modification enhances active targeting to RA lesions. H-A/T NP can inhibit HUVEC proliferation and inflammatory responses by simultaneously targeting both VEGF and IL-6. In CIA mouse model, H-A/T NP demonstrated significant anti-angiogenic effects and alleviated cartilage and bone damage [ 254 ]. HIF directly binds to HREs within the VEGF promoter region, establishing a positive feedback loop essential for angiogenesis in RA [ 260 ]. The biological activity of nanomedicines to alleviate the hypoxic microenvironment can inhibit VEGF transcription and thus exert anti-angiogenic effects at the RA joints through in situ O 2 generation or by delivering exogenous O 2 . Li and colleagues developed an ultrasound-sensitive nanoparticle system (Rh/SPX-HSA) incorporating spaquinone (SPX) and human serum albumin (HSA)-loaded rhodium nanocatalysts for RA treatment. Rhodium nanocatalysts display inherent CAT and POD activities, generating O 2 and ·OH. The O 2 generated in situ inhibits HIF-1α expression, thereby alleviating the hypoxic microenvironment in the joints and reducing VEGF transcription to achieve anti-angiogenic effects. Rh/SPX-HSA significantly decreased HIF-1α and VEGF expression in the CIA mouse model. The HSA modification allows active targeting of inflamed joints. Under ultrasound exposure, SPX can be released, and induces excessive ROS production. This results in mitochondrial dysfunction and subsequent inhibition of FLS proliferation. Moreover, in situ oxygen supply generated by Rh further enhances SPX-O 2 interactions, activating mitochondrial caspase cascades and promoting FLS apoptosis. As a result, Rh/SPX-HSA notably reduced synovial hyperplasia and cartilage damage in the CIA mouse model [ 201 ]. Furthermore, Jonghoon et al. developed a mesoporous silica nanoparticle system (MFC-MSN) anchored with manganese ferrite and CeO 2 nanoparticles, which synergistically produce O 2 and scavenge ROS for RA therapy. The manganese ferrite nanoparticles alleviate the hypoxic microenvironment in RA joints by catalyzing the Fenton reaction, downregulating hypoxia markers such as HIF-1α, and thereby inhibiting VEGF transcription to suppress angiogenesis. The MFC-MSN system demonstrated substantial inhibition of angiogenesis in the knee joints of AIA model rats. Additionally, CeO 2 nanoparticles, with their two reversible oxidation states (Ce 3+ and Ce 4+ ), act as effective ROS scavengers, counteracting oxidative stress in RA. The hydroxyl radicals produced by the manganese ferrite nanoparticles during the Fenton reaction are further converted by CeO 2 into O 2 , enhancing the synergistic effect of H 2 O 2 decomposition and O 2 generation. This synergistic effect induces M2 macrophage polarization, exerting anti-inflammatory effects in the AIA rat model [ 255 ]. Elevated levels of VEGF not only directly promote angiogenesis but also play a critical role in joint pain in arthritis. This is primarily due to the differential roles of VEGF receptors: VEGFR1 is involved in pain signaling, while VEGFR2 is critical for angiogenesis and bone erosion. Pazopanib, an FDA-approved tyrosine kinase inhibitor targeting both VEGFR1 and VEGFR2, has shown efficacy in RA treatment. However, its low bioavailability and frequent dosing regimen hinder its clinical application. Ma et al. utilized a rapid nanoprecipitation method to encapsulate pazopanib within polymeric nanoparticles, synthesizing a sustained-release formulation (Nano-PAZII) for OA treatment ( Fig. 14 A) [ 153 ]. Polymeric nanoparticles not only achieve pH-dependent release of pazopanib due to their good biodegradability and biocompatibility, but also enhance the bioavailability of pazopanib by targeting pazopanib to the inflamed synovium ( Fig. 14 B). Nano-PAZII reduced synovial angiogenesis by targeting the VEGFR2/FLK1 pathway, and also protected cartilage by decreasing the expression of catabolic enzymes and inflammatory proteins in the cartilage ( Fig. 14 C). Moreover, Nano-PAZII alleviated OA pain by inhibiting the VEGFR1/FLT1 pathway. Importantly, Nano-PAZII exhibited good safety profiles, without the addictive properties typically associated with opioid analgesics. Fig. 14 (A) Schematic illustration of a single IA injection of Nano-PAZII for long-term OA treatment. (B) The amount of pazopanib was quantified using high-performance liquid chromatography and in vitro release of pazopanib from the PEG-b-PCL nanoparticles was fitted with Higuchi model with R 2  = 0.9672, first-order model with R 2  = 0.9731, and zero-order kinetics model with R 2  = 0.9956. (C) HE staining and quantitative results for the presence of CD31 + vessels in the synovium. Adapted with permission from Ref. [ 153 ], copyright 2024. Fig. 14 (A) Schematic illustration of a single IA injection of Nano-PAZII for long-term OA treatment. (B) The amount of pazopanib was quantified using high-performance liquid chromatography and in vitro release of pazopanib from the PEG-b-PCL nanoparticles was fitted with Higuchi model with R 2  = 0.9672, first-order model with R 2  = 0.9731, and zero-order kinetics model with R 2  = 0.9956. (C) HE staining and quantitative results for the presence of CD31 + vessels in the synovium. Adapted with permission from Ref. [ 153 ], copyright 2024. PDGF secreted by osteoclasts plays a pivotal role in angiogenesis in both OA and RA. Based on this, Wu and colleagues synthesized PEG-modified polydopamine nanoparticles (PDA-PEG NPs) for early OA treatment. Polydopamine nanoparticles not only act as antioxidants to scavenge ROS to inhibit osteoclastogenesis, but also suppress angiogenesis by downregulating PDGF-BB expression in osteoclasts. The PEG modification of PDA-PEG NPs allows for active targeting of inflamed areas in OA. PDA-PEG NPs demonstrated low toxicity both in vitro and in vivo and effectively mitigated OA progression induced by anterior cruciate ligament transection [ 256 ]. Additionally, Zuo and colleagues developed a dynamic composite of selenium-doped carbon quantum dots (SC) and hyaluronic acid-modified hydrogels (AHAMHs) to create hydrogel microspheres (SCT-HA). AHAMHs are widely used in cartilage regeneration due to their excellent biocompatibility. By modifying the surface of SC with triphenylphosphine, SCT-HA targeted osteoclast mitochondria and subsequently, exhibited in situ ROS scavenging capabilities in osteoclasts. In the mildly acidic OA microenvironment, the Schiff base bonds of SCT-HA undergo hydrolysis, releasing triphenylphosphine-modified SC, which subsequently inhibits PDGF-BB levels in osteoclasts, thereby preventing the progression of angiogenesis. Micro-CT angiography revealed that SCT-HA significantly suppressed the increase in the number and volume of blood vessels in the subchondral bone of OA joints. Furthermore, SCT-HA modulated the initiation and progression of abnormal subchondral bone remodeling, thereby inhibiting cartilage degeneration [ 257 ]. This suggests that developing PDGF-targeting nanomedicines holds significant potential for treating angiogenesis in RA. P-selectin is expressed on the surface of activated ECs and enhances adhesion to the vascular surface by binding to P-selectin glycoprotein ligand-1 (PSGL-1) on the surface of inflammatory cells such as neutrophils and macrophages. Low-molecular-weight heparin (LMWH) exhibits a high affinity for P-selectin and can competitively bind to P-selectin, preventing other pro-angiogenic cells from adhering to ECs, thereby inhibiting angiogenesis. Based on this, Li and colleagues developed an LMWH-D-α-tocopherol succinate (TOS) micelle nanoparticle (LT NPs) for RA therapy. LMWH targets inflamed ECs in the joints by competitively inhibiting the binding of P-selectin to PSGL-1, which in turn reduces EC/neutrophil adhesion and suppresses angiogenesis ( Fig. 15 A and C). Under the acidic conditions of RA, the ester bonds in LT NPs are cleaved, then releasing TOS. TOS decreases MMP-9 levels in the RA joints, thereby inhibiting FLS migration, invasion, and inflammatory responses, ultimately preventing cartilage degradation in RA. LT NPs also suppress the early recruitment of neutrophils, contributing to the prevention of inflammation flare-ups ( Fig. 15 B). LT NPs demonstrated excellent stability in normal tissues and blood, without increasing the risk of bleeding or hemolysis-related side effects [ 181 ]. Additionally, Li and colleagues developed macrophage-derived microvesicles (MNPs) loaded with tacrolimus for targeted RA treatment ( Fig. 15 D). MNPs are produced by interactions between the macrophage cytoskeleton and the cell membrane that occur in response to cytochalasin B stimulation. MNPs preserve the presence of CD44 and Mac-1 on the surface of RA macrophages, enabling them to bind to P-selectin and ICAM-1 on ECs, respectively, thereby targeting the inflammatory sites of the pannus and inhibiting its progression ( Fig. 15 E). Upon reaching the RA pannus, MNPs release tacrolimus, resulting in improved inflammatory responses in CIA mice [ 258 ]. Fig. 15 (A) Distribution of NPs in paws and major organs. (B) Flow cytometry analysis of neutrophils in the paws of CIA mice treated with different formulations and quantitative analysis of fluorescent-labelled neutrophils attached to HUVECs after separate incubation with different formulations. (C) Representative CLSM images of the joints of CIA mice. Adapted with permission from Ref. [ 181 ], copyright 2021. (D) Schematic illustration of MNP targeting sites of RA. MNP could target sites of RA through ICAM-1 or P-selectin adhesion. (E) Western blot of CD44 and Mac-1 in MMV or MNP. Adapted with permission from Ref. [ 258 ], copyright 2019. (F) Molecular docking simulations of sLex and STMF in complex with E-selectin. (G) IF analysis of E-selectin and Cy7 at joints of RA mice. Adapted with permission from Ref. [ 259 ], copyright 2024. Fig. 15 (A) Distribution of NPs in paws and major organs. (B) Flow cytometry analysis of neutrophils in the paws of CIA mice treated with different formulations and quantitative analysis of fluorescent-labelled neutrophils attached to HUVECs after separate incubation with different formulations. (C) Representative CLSM images of the joints of CIA mice. Adapted with permission from Ref. [ 181 ], copyright 2021. (D) Schematic illustration of MNP targeting sites of RA. MNP could target sites of RA through ICAM-1 or P-selectin adhesion. (E) Western blot of CD44 and Mac-1 in MMV or MNP. Adapted with permission from Ref. [ 258 ], copyright 2019. (F) Molecular docking simulations of sLex and STMF in complex with E-selectin. (G) IF analysis of E-selectin and Cy7 at joints of RA mice. Adapted with permission from Ref. [ 259 ], copyright 2024. E-selectin can dose-dependently promote the migration of ECs incubated with RA synovial fluid. Sialyl Lewis X (sLex), a tetrasaccharide structure expressed on the surface of neutrophils and monocytes, is a natural ligand for E-selectin. Synthetic sLex analogs can competitively bind to E-selectin, preventing the adhesion of other pro-angiogenic cells to ECs, thereby inhibiting angiogenesis ( Fig. 15 F). Based on this, Liu and colleagues developed an sLex-modified C70 fullerene complex (STMF) for RA treatment. sLex specifically binds to E-selectin on activated ECs, allowing STMF to target RA joints in mice and block the angiogenesis process ( Fig. 15 F). C70 fullerene acts as a ROS scavenger to eliminate ROS in macrophages, exerting an antioxidative effect. By scavenging intracellular ROS, STMF inhibits the differentiation of macrophages into the M1 phenotype and reduces osteoclastogenesis, ultimately repairing bone erosion ( Fig. 15 G) [ 259 ].

Growth factors (VEGF and PDGF), cytokines (TNF-α, IL-6), chemokines (CXC and CC subfamilies), cell adhesion molecules (integrins and selectins), protein hydrolases, and mTOR signaling molecules are important pro-angiogenic mediators that act on ECs in pathological angiogenesis of RA. These mediators are primarily secreted by immune cells such as DCs, macrophages, and T cells, as well as inflammatory cells like FLS and neutrophils. As shown as Table 1 , DCs directly affect ECs by secreting growth factors such as VEGF-A and FGF2, along with chemokines like IL-8/CXCL8, Groα/CXCL1, ENA-78/CXCL5, CTAP-II/CXCL7, MCP-1/CCL2, and SLC/CCL21. Macrophages release numerous chemokines and adhesion molecules such as ICAM-1, ICAM-3, VCAM, and αvβ3, along with various cytokines and growth factors that interact with corresponding receptors on ECs. FLS are key contributors as well, secreting MMPs (MMP-1, MMP-13, MMP-3, MMP-9), growth factors (VEGF, FGF, HGF, PDGF, TGF-β), and angiopoietin-2, along with integrin αvβ3 to initiate angiogenesis. Table 1 Types of pro-angiogenic factors secreted by immune and inflammatory cells in RA. Table 1 Cell type Categories of secreted pro-angiogenic factors Corresponding receptor on EC Regulating signaling pathway Ref. Dendritic cells Image 1 Growth factor VEGF-A, FGF2 VEGFR, FGFR Activation of transcription factors CREB, HIF-1a and STAT3 [ 5 ] ET-1 ETR [ 5 ] Chemokine IL-8/CXCL8, Groa/CXCL1, ENA-78/CXCL5, CTAP-II/CXCL7 CXCR1, CXCR2 [ 5 ] MCP-1/CCL2, SLC/CCL21 CCR2, CCR7 [ 5 ] Activation of ERK1/2 pathway [ 5 ] Macrophage Image 2 Chemokine SDF-1/CXCL12, Groa/CXCL1, ENA-78/CXCL5, CTAP-II/CXCL7 CXCR4, CXCR2 [ 137 ] MEC/CCL28, SLC/CCL21, MCP-1/CCL2, MIP-1a/CCL3, FKN/CX3CL1 CCR10, CCR7, CCR2, CCR1, CX3CR1 [ 137 ] Cytokine IL-1, IL-6, IL-8, IL-8, IL-18, MIF, TNF-α [ 59 ] Growth factor VEGF, bFGF VEGFR, FGFR [ 59 ] Adhesion molecule ICAM-1, -3 VCAM, αvβ3 [ 59 , 138 ] CD4 + T cell Image 3 Il-17 IL-17RC Activation of PI3K pathway [ 139 ] Cytokine PlGF, IFN-γ, TNF-α, IL-2 VEGFR1 [ 5 ] Myeloid-derived suppressor cell Image 4 Growth factor VEGF VEGFR Activation of JAK2/STAT3 pathway [ 5 , 140 ] MMPs MMP9 [ 141 ] Fibroblast-like synoviocyte Image 5 Cytokine TNF-α, IL-18, IL-6, IL-8, IL-1β, MIF [ 59 ] Growth factor VEGF, bFGF, HGF, PDGF, TGF-β VEGFR, FGFR, HGFR, PDGFR [ 59 ] Angiogenin Ang2 Tie2 [ 59 ] Chemokine Groa/CXCL1, ENA-78/CXCL5, IL-8/CXCL8, IP-10/CXCL10, CXCL12 CXCR2, CXCR1, CXCR3, CXCR4 [ 59 ] MCP-1/CCL2, RANTES/CCL5, SLC/CCL21, FKN/CX3CL1 CCR2, CCR5 CCR7, CX3CR1 [ 59 ] MMPs MMP1 MMP13 MMP3 MMP9 [ 59 ] Adhesion molecule E-selectin, ICAM-1, -3 VCAM1, TMS/AMS, JAM-C, Cadherin. αvβ3 [ 59 , 142 ] Neutrophil Image 6 Growth factor VEGF VEGFR [ 143 , 144 ] Cytokine IL-17A IL-17R [ 143 ] PSGL-1 P-selectin [ 145 ] Activation of β -catenin pathway [ 146 ] Leukocyte Image 7 Sialyl Lewis x/CD15s E-selectin [ 147 ] Types of pro-angiogenic factors secreted by immune and inflammatory cells in RA. Among these factors, growth factors (VEGF, PDGF), adhesion molecules (αvβ3 integrin, P- and E-selectins), vascular homeostasis factors, and proteolytic enzymes trigger downstream signaling cascades essential for angiogenesis ( Fig. 8 ). Biomaterials have been developed to directly combine with these pro-angiogenic mediators to exert anti-angiogenic effects. Fig. 8 Schematic diagram showing crosstalk of signaling pathways mediated by pro-angiogenesis factors during RA angiogenesis. Fig. 8 Schematic diagram showing crosstalk of signaling pathways mediated by pro-angiogenesis factors during RA angiogenesis. The VEGF family comprises seven members: VEGF-A, VEGF-B, VEGF-C, VEGF-D, PlGF, and non-human genome-encoded VEGF-E and svVEGF [ 148 ]. VEGF-A is crucial for regulating both physiological and pathological angiogenesis [ 149 ]. Vaccines targeting VEGF-A have been developed, demonstrating efficacy in protecting CIA mice against inflammation and joint destruction [ 150 ]. VEGFR-1, also known as FLT-1, serves a dual function in vascular biology; it facilitates angiogenesis by promoting monocyte migration and EPC recruitment, while also modulating excessive angiogenic activity through competitive inhibition of excess VEGF-A [ 15 ]. VEGFR-1 variations have been identified as a novel genetic risk factor for RA severity in angiogenesis [ 151 ]. Immunohistochemical analysis of RA synovial tissue has revealed VEGF-A expression in various cells near the synovial surface, encompassing macrophage-derived type A synovial cells and fibroblast-derived type B synovial cells, and SMCs. VEGFR-1 is expressed in the ECs of deep synovial vascular layer, with VEGF-A and VEGFR-1 positive cells separated by 5–50 μm [ 152 ]. In osteoarthritis research, it has been discovered that the VEGF-A/VEGFR-1 pathway may influence joint pain transmission beyond angiogenesis by regulating nociceptive synaptic transmission in the dorsal horn of the spinal cord [ 153 ]. HIFs regulate the expression of VEGF family members and their receptors, thereby establishing a signaling cascade essential for the angiogenesis positive feedback loop in RA [ 154 ]. In severely affected RA joints, elevated leukocyte activity and metabolic needs intensify the hypoxic conditions. This exacerbation results in HIF-1α accumulation in the cytoplasm, which subsequently translocate to the nucleus to form a complex with HIF-β. Such translocation prompts macrophages and FLS to enhance the expression and secretion of VEGF, thus facilitating angiogenesis [ 59 ]. Elevated levels of VEGF further regulate angiogenesis through the following pathways. Yan Wang and colleagues identified a significant association of the VEGF/SphK1/S1P pathway with angiogenesis in RA. They note that targeted inhibition using SphK1-specific siRNA curtails the VEGF/SphK1/S1P cascade, significantly reducing the FLS's stimulatory impact on ECs [ 155 ]. Further research indicates that the activation of SphK1 translocation through the VEGFR2/PKC/ERK1/2 pathway is crucial for VEGF-driven angiogenesis [ 156 ]. The VEGF/PI3K/AKT signaling pathway has been identified as pivotal in the mediation of inflammatory cell infiltration and angiogenesis in CIA rats synovial tissue [ 157 ]. Cheng and colleagues found that JAK/STAT signaling pathway as a contributor to the VEGF-induced cell migration and tube formation in EA.hy926 cells under inflammatory conditions [ 158 ] Prophylactic administration with anti-VEGF antibodies has demonstrated improvement in mitigating inflammation and angiogenesis in CIA mice [ 159 ]. Like VEGF, PDGF consists of four homodimers—PDGF-AA, PDGF-BB, PDGF-C, PDGF-D—and one heterodimer, PDGF-AB, each with unique biological functions [ 160 ]. Under normal conditions, PDGF ligands typically display low to undetectable expression levels in vascular systems. PDGF orchestrates a variety of downstream signaling pathways through interactions with its cell membrane receptors, PDGFR-α and PDGFR-β [ 161 ]. Specifically, PDGF-AA, acting through PDGFR-α, phosphorylates STAT3 (Y705) and inactivates tumor suppressor Rb1 to accelerate the deterioration and angiogenesis of glioma stem cells [ 15 ]. Lin and his team found that the regulatory influence of endothelial PDGFR‐β on H‐type vessels in osteoporosis model is mediated by the PDGFRβ‐P21‐activated kinase 1‐Notch1 intracellular domain signaling cascade [ 162 ]. Moreover, renowned PDGF-BB/PDGFR-β pathway is crucial for recruiting pericytes, fostering vascular reconstruction, and activating matrix cells vital for wound healing processes. This pathway also supports the growth and proliferation of ECs, thereby promoting angiogenesis and vascular maturation [ 15 ]. In RA patients, plasma cytokine profiling has revealed a marked upregulation of PDGF-BB, positively correlated with the proliferation of FLS and their acquisition of an aggressive phenotype [ 163 , 164 ]. The administration of the PDGFR-specific inhibitor, imatinib, has effectively mitigated inflammation in AIA rats, by disrupting the Akt signaling pathway and suppressing PDGF-dependent FLS growth [ 165 ]. PDGF, located in the subchondral bone spaces, vascular channels, and within chondrocytes, significantly contributes to angiogenesis in both OA and RA. Compared to OA, RA exhibits significantly higher levels of lymphocytic infiltration and PDGF expression [ 166 ]. Although RA angiogenesis involves partial vascular barrier disruption, elevated PDGF expression has been detected in both the synovial tissue and fluid of RA patients [ 167 ]. In the synovial environment of RA patients, there is a complex interplay between various cytokines and growth factors. Stimulated by TGF-β or TNF, PDGF enhances the synthesis and secretion of pro-inflammatory factors such as IL-6, IL-8, MIP-1α, and MMP-3, which is the main explanation [ 168 ]. Furthermore, the autocrine action of PDGF-BB leads to direct phosphorylation of PDGFR, triggering the PI3K/AKT signaling pathway. This activation is instrumental in fostering the development of invasive synovial structures and contributing to the degradation of the matrix in deteriorating cartilage [ 169 ]. New therapeutic agents targeting PDGF for RA are currently in development. Imatinib mesylate—originally designated for chronic myeloid leukemia—has demonstrated efficacy in reducing RA FLS activation by disrupting PDGFR-mediated signaling [ 170 ]. However, the considerable toxicity associated with imatinib mandates continued research into alternative PDGF-targeted therapies for RA. Adhesion molecules are broadly categorized into five categories: integrins, selectins, cadherins, members of the immunoglobulin superfamily, and other adhesion molecules. Adhesion molecules, expressed on the cell surface, mediate interactions between cells and extracellular matrix (ECM). In the study of RA angiogenesis, integrins and selectins have received the most extensive investigation. Integrins, which are composed of 18 α and 8 β subunits, form a diverse array of receptors that are essential for cell communication with the ECM. Hormones, cytokines, inflammatory mediators, ROS, and endotoxins can activate integrins, enhancing their ability to bind to ECM proteins with RGD (Arginine-Glycine-Aspartic acid) sequence [ 171 ]. Integrins profoundly influence the pathological processes of RA by regulating immune cells migration, activation and ECM interaction. Serum integrin subunit β2 (ITGB2) expression is reportedly upregulated in RA, positively correlated with RF and DAS28 [ 172 ]. Lymphocyte function-associated antigen-1 (LFA-1), serving as the α subunit in combination with ITGB2, plays a critical role in mediating leukocyte adhesion and migration by interacting with ICAM-1. This interaction accelerates angiogenesis and inflammation, leading to cartilage deformation and destruction [ 173 ]. The αvβ3 integrin, prevalent in cancer cells, ECs, and pericytes, plays a crucial role in angiogenesis. αvβ3 integrin synergizes with VEGF to activate angiogenesis in ECs via VEGFR2 phosphorylation [ 174 ]. The αvβ3 integrin is instrumental in upregulation of MMP-2, MMP-9 and urokinase plasminogen activator, which promoting the progress of angiogenesis in RA [ 175 ]. Importantly, the inhibition of αvβ3 integrin and downstream signal molecules can repress the cell migration and proliferation in FLS and trigger ECs apoptosis, thereby reducing RA-associated inflammation [ 176 ]. Recent studies have identified increased αvβ3 integrin expression in osteoclasts and macrophages in RA synovial tissue. Deng and colleagues have developed an innovative strategy that leverages the αvβ3 integrin and RGD interaction in joint tissues. This strategy has been effective in alleviating swelling in the ankles and feet, restoring the functional equilibrium of bone, and reversing bone erosion in the joints of late-stage AIA rats, highlighting the potential of αvβ3 integrin as a therapeutic target in RA management [ 138 ]. In states of abnormal inflammation, these selectins can rapidly shed from the surface of activated ECs and mediate the adhesion of circulating leukocytes to vascular ECs [ 60 ]. Compared to healthy controls, RA patients exhibit significantly higher concentrations E-selectin and P-selectin, but not L-selectin [ 177 ]. The phenomenon is explained by the fact that elevated E− and P-selection levels reflects a state of endothelial activation and dysregulation in RA synovium [ 178 ]. E-selectin gene therapy significantly augments angiogenic responses and recruits EPCs in hindlimb ischemia models [ 179 ]. E-selectin acts as a biomarker for evaluating the severity of endothelial dysfunction and vascular inflammatory diseases, promoting RA synovial fluid-induced endothelial migration in a dose-dependent manner [ 180 ]. Conversely, reducing E-selectin levels can diminish this migration [ 59 ]. Furthermore, the presence of the E-selectin ligand CD15s enables FLS to intact with ECs within flowing blood vessels similarly to leukocytes, facilitating their exit from the bloodstream and migrate to distant sites [ 147 ]. Additionally, studies have shown that P-selectin deficiency can mitigate the clinical and histological symptoms of AIA mice by interfering with leukocyte-EC interactions [ 62 ]. P-selectin glycoprotein ligand-1 on neutrophils or monocytes accelerates the binding to P-selectin on translocated and inflamed ECs or activated platelets [ 181 ]. Angiopoietin derived from vascular cells, including angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2), interact with the EC surface receptor Tie2, serving as critical regulators of ECs survival and vascular stability [ 148 ]. Specifically, the Ang-1/Tie2 interaction activates the PI3K-Akt pathway, facilitating EC chemotaxis. The PI3K-Akt pathway, once activated, also promotes the phosphorylation of Forkhead box O1, inhibiting its nuclear migration and consequently suppressing the transcription of apoptosis-related genes. Moreover, Ang-1/Tie2 interaction activates the A20-binding inhibitor (ABIN) of NF-κB, subsequently inhibiting NF-κB and reducing the expression of inflammatory markers such as ICAM-1, VCAM-1, and E-selectin. The Ang-1/Tie2 interaction also stabilizes the actin cytoskeleton through GTPase activation, thereby enhancing endothelial integrity [ 182 ]. The collaboration between Ang-1 and Tie2 is essential not only for maintaining the dormant EC phenotype but also for facilitating the maturation and stabilization of blood vessels under physiological states, including embryonic development [ 183 ]. Ang-2, a highly homologous endogenous antagonist of Ang-1, is uniquely stored within Weibel-Palade bodies, specialized organelles in ECs. These storage granules serve a critical role in the rapid release of stored factors upon various stimuli. Under physiological conditions, Ang-2 levels are minimal but increase in response to pathological states such as tumor growth, wound healing, and inflammatory responses, promoting neutrophil migration through exocytotic release into the ECM [ 15 , 184 ]. Ang-2 competes with Ang-1 for Tie2 binding, leading to Tie2 inactivation and promoting pericyte detachment, which disrupts endothelial cohesion, consequently driving EC activation and angiogenesis [ 185 ]. Additionally, Ang-2 can trigger hypertensive responses, endothelial dysfunction, and vascular hypertrophy in animal models of inflammatory arthritis. Locally synthesized Ang-2 also impacts synovial vasculature by acting on angiotensin receptors, thus regulating synovial hyperplasia [ 186 ]. Significantly, the Ang2/Tie2 axis has been documented within RA synovial vessels, where it enhances the expression of MMP-2 and MMP-9 under TLR2 stimulation, contributing to vascular instability and the advancement of synovitis in RA. Inhibition of Tie2 partially mitigates these effects [ 187 ]. Biochemical research has shown that elevated levels of Ang-1 and Ang-2 in both synovial fluid and serum of RA patients. This elevation represents a compensatory mechanism to counterbalance the effects of increased vascular instability induced by Ang-2 [ 188 , 189 ]. Relative to OA patients, Malte and colleagues observed a significant increase in the Ang-1/Ang-2 ratio and Tie2 levels in the synovial tissue and serum of RA patients, although the serum levels of Ang-2 show no significant difference [ 190 ]. Daitaro et al. have posited that serum Ang-1 levels may indicate persistent arthritis through its role in new vessel formation, whereas serum Ang-2 levels might reflect the dynamic state of angiogenesis [ 191 ]. In the sterile environment of normal synovial fluid, proteolytic activity o is tightly regulated by inhibitors to prevent potential damage. However, specific stimuli can active MMPs to disrupt and remodel the ECM and BM by degrading key components such as collagen, elastin, laminin, and fibronectin [ 192 ]. Additionally, in conjunction with other host proteases and glycosidases, MMPs contribute to the degradation of proteoglycans, indirectly influencing joint remodeling through the signaling via cell surface receptors or the proteolytic processing of cytokines and receptor molecules [ 193 ]. Neutrophils, as the primary leukocytes infiltrating the synovial space, significantly contribute to MMPs production in RA [ 193 ]. MMP-1, MMP-2 and MMP-9 are synthesized by macrophage-derived type A synovial cells and fibroblast-derived type B synovial cells in RA inflammatory conditions [ 193 ]. Although B cells and T cells produce these MMPs in response to specific stimuli, their production is relatively modest. Osteoclasts also produce MMPs, playing a role in bone resorption. A recent discovery by Zhu highlighted a synergistic interaction between MMP-9 and MMP-14 in osteoclasts, disrupting a signaling pathway through the cleavage of the low-density lipoprotein receptor-related protein-1 ligand Galectin 3 in mice models [ 193 ]. Pathologically, MMP-2 and MMP-9 modulate PDGF signaling, catalyze the degradation of type I collagen, regulate perivascular SMCs, and mobilize VEGF and other factors essential for vascular remodeling [ 136 ]. Building on this, Li et al. developed a targeted peptide against MMP-2/9, effectively reducing pro-inflammatory factor secretion and ameliorating RA symptoms [ 194 ]. MMP-9 also facilitates the transition from vascular quiescence to active angiogenesis. MMP-7 and MMP-12 proteolyzed by specific collagen chains and plasminogen, can foster the production of endogenous angiogenesis inhibitors such as angiostatin [ 136 ]. Among the MMPs, MMP-1 and MMP-13 limit collagen degradation rates, playing a dual role in ECM destruction in RA [ 195 ]. MMP-1 is prevalent, effectively degrading interstitial collagen. MMP-13, uniquely expressed by chondrocytes, specializes in degrading type II collagen, essential for cartilage degradation. Moreover, MMP-13 has emerged as a potential biomarker for differentiating between RA patients with moderate or low disease activity [ 196 ].

Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by the proliferation of synovial tissue, pannus formation, and irreversible damage to joint cartilage and bone. It leads to increased disability and premature death, affecting individuals regardless of age or gender [ 1 ]. In 2020, approximately 17.6 million people worldwide were affected by RA, and projections suggest that this number will rise to 31.7 million by 2050, marking an 80.2 % increase since 2020. More than 17 % of diagnosed RA patients experience progressive joint destruction and deformities, contributing to a substantial portion of the global disability burden (0.1 %) due to inadequate treatment. The relationship between chronic inflammation in RA and an elevated risk of cardiovascular disease is well-documented [ 2 ]. RA patients face twice the risk of myocardial infarction and a 50 % higher mortality risk compared to the general population [ 3 , 4 ], highlighting the significant disability and mortality associated with RA and the pressing need for more effective, targeted treatments. Pathological angiogenesis in RA is characterized by several distinct features, including abnormal vascular morphology, altered phenotypes of endothelial progenitor cells (EPCs) and endothelial cells (ECs), partially disrupted vascular barriers, increased vascular permeability, and insufficient vascular perfusion. Persistent pathological angiogenesis is closely linked to chronic synovitis in RA [ 5 ]. This aberrant angiogenesis promotes immune cell migration and the release of pro-inflammatory mediators [ 6 ]. Furthermore, it can extend into surrounding tissues, leading to joint damage, deformities, and an increased risk of disability and premature death [ 7 ]. Understanding the regulatory mechanisms of pro-angiogenic factors is crucial for influencing angiogenic behavior and treatment outcomes, underscoring the potential for novel anti-angiogenic therapies in the management of RA. Despite significant advances in the treatment of RA, effective therapeutic strategies remain limited. For example, prolonged use of disease-modifying antirheumatic drugs (DMARDs) can exacerbate side effects and increase the risk of liver and kidney damage [ 8 ]. Nonsteroidal anti-inflammatory drugs (NSAIDs) can cause adverse effects, including anemia, liver function abnormalities, fatigue, nausea, and central nervous system (CNS) issues. Additionally, glucocorticoids (GCs) are associated with significant side effects, such as osteoporosis, increased susceptibility to infections, hyperglycemia, and hypertension. Even advanced biological therapies carry risks, particularly heightened vulnerability to infections and elevated cholesterol levels [ 9 ]. Given the limited efficacy and substantial side effects of traditional anti-inflammatory medications, there is an urgent need to explore new therapeutic targets for comprehensive RA management. Nanoagents, characterized by their high surface-to-volume ratio and small size, enhance drug solubility and prolong circulation time [ 10 ]. They accumulate in inflamed joints through the enhanced permeability and retention (EPR) effect and can penetrate vascular cells via energy-dependent endocytosis, remaining within the joint cavity [ 11 , 12 ]. Recent advancements in anti-angiogenic nanoagents have shown potential in treating cancer, retinal diseases, and osteoarthritis (OA) [ 11 , 13 , 14 ]. However, no comprehensive reviews have yet examined the progress and fundamental mechanisms of anti-angiogenic nanoagents specifically in RA. Therefore, this paper aims to provide a comprehensive review of the pathological mechanisms of angiogenesis in RA, along with the application of anti-angiogenic nanoagents for RA treatment ( Scheme 1 ). We anticipate that this discussion will enhance the understanding of angiogenesis in the progression of RA and broaden the therapeutic horizons of anti-angiogenic strategies. Scheme 1 Schematic diagram of anti-angiogenic nanoagents constructed by targeting activated stromal cells (fibroblast-like synoviocytes, neutrophils, macrophages, and endothelial cells) and specific removal pro-angiogenic mediators (vascular endothelial growth factor, platelet-derived growth factor, and selectin) for treatment of RA. Scheme 1 Schematic diagram of anti-angiogenic nanoagents constructed by targeting activated stromal cells (fibroblast-like synoviocytes, neutrophils, macrophages, and endothelial cells) and specific removal pro-angiogenic mediators (vascular endothelial growth factor, platelet-derived growth factor, and selectin) for treatment of RA.

The authors declare no conflict of interest, financial or otherwise.