The role of exosomes in immunopathology and potential therapeutic implications.

The precise regulation of exosome biogenesis and cargo sorting ensures the efficient packaging of specific biomolecules, such as proteins, nucleic acids, and lipids, which endows exosomes with diverse biological functions. As key mediators of intercellular communication, exosomes can transfer functional cargo to recipient cells, modulating their behavior and facilitating dynamic interactions within the tumor and disease microenvironments. Exosomes can mediate communication between ‌homotypic (same-type) and heterotypic (different-type) cells. Remarkably, exosomes derived from cells in one organ can enter the peripheral circulation and reach distant organs, where they are internalized by recipient cells, enabling cross-organ communication. Therefore, we systematically review the immunopathological roles of exosomes in diseases across systems, focusing on three dimensions: homotypic cell interactions, heterotypic cell interactions, and cross-organ communication (Fig.  3 ). Table  1 lists the functions of exosomes in immunopathology. Fig. 3 Schematic of exosome-mediated information communication between different types of cells. Exosomes mediate communication between different cells through homotypic, heterotypic, and cross-organ manners during tumor suppression and immunopathology of eight systems Table 1 Exosomal sources, effector molecules, and functions in immunopathology Source Cargo Acceptor cell Function Ref. Tumor cells miR-145 Macrophages Induce macrophage polarization toward an M2 phenotype [ 110 ] Fleur Nagri protozoa — Promoting the production of the cytokine IL-8 [ 138 ] Nociceptive neurons miR-21 Driving a pro-inflammatory phenotype [ 140 ] Macrophages — Stimulating the pro-inflammatory response [ 150 ] Epithelial cells — Facilitate M1 macrophage polarization [ 161 – 163 ] Visceral AT — Promoting M1 macrophage Polarization [ 166 , 167 ] Ectopic endometrial stromal cells miR-146a-5p Facilitating M2 macrophage polarization [ 193 ] Uterine cavity miR-210-3p Inducing M2 macrophage polarization [ 196 ] Uterine cavity — Decreasing the proportion of CD80 + macrophages [ 197 ] Syncytiotrophoblasts 5’-tRNA fragments Causing inflammation in macrophages [ 201 ] Trophoblasts — Promoting macrophage to M1 polarization [ 202 ] Trophoblastic cells miR-141 Inducing the formation of M1 macrophages [ 203 ] Renal tubular epithelial cells Ccl2 mRNA Activating macrophages [ 209 ] Tubular epithelial cells miR-23a Promoting tubulointerstitial inflammation [ 211 ] Tubular epithelial cell miR-19b-3p Promoting M1 macrophage activation [ 212 ] Fibroblast-like synoviocytes — Promoting M1 macrophage polarization [ 223 ] Osteoarthritic chondrocytes — Increasing the production of mature IL-1β in macrophages [ 224 ] RA fibroblast-like synoviocytes — Stimulate macrophage migration [ 225 ] Serum miR-6089 Activating macrophage [ 226 ] Synoviocytes CDO1 Promoting M1 polarization [ 227 ] Adipocytes miR-34a Inhibiting M2 macrophage polarization [ 240 ] Adipocytes miR-1224 Inhibits M2 macrophage polarization [ 241 ] Cardiomyocytes miR-9-5p Promotes M1 polarization [ 174 ] Alveolar epithelial cells miR-92a-3p Alveolar macrophages Inducing pulmonary inflammation [ 144 ] Lung miR-6238 Modulating neutrophil infiltration into the lung [ 145 ] Tumor cells TGF-β1 and NKG2D ligands NK cell Reduce NKG2D expression [ 112 ] Tumor cells — Induce NK cell exhaustion [ 113 ] Endothelial cells — Neutrophils Driving neutrophil reverse transendothelial migration to disseminate inflammation [ 143 ] Escherichia coli IL-17A Inducing neutrophilic inflammation [ 159 ] Cardiomyocyte miR-9-5p Inducing neutrophils to the N1 phenotype [ 182 ] Macrophages miR-146a Promoting NET formation [ 190 ] Endothelial cells — Monocytes Promoting monocytes into pro-inflammatory macrophages [ 142 ] Nasal lavage fluid/ respiratory tract/Alveolar macrophages — Monocytes NKs Neutrophils Promoting inflammation via TLR signaling [ 151 – 153 ] Tumor cells — DCs Impairing their differentiation and maturation [ 105 ] Tumor cells Tumor antigens Induced antitumor-specific CD8 + T cell responses [ 123 ] Ascites Mart1 Triggered tumor-specific T-cell responses [ 124 ] Alveolar epithelial cells — Facilitating activation of monocyte-derived DCs [ 154 ] Tumor cells — Immature DCs Hindering their maturation through TGF-β1 [ 107 ] Tumor cells — Myeloid precursors Inhibiting the differentiation of myeloid precursors into DC [ 106 ] Tumor tissue — T cells Through decreased T-cell receptor signaling [ 115 ] B cells — Impairing CD8 + T cell responses [ 120 ] Plasma samples from NPC patients — Inhibiting T cell proliferation [ 116 ] B cells MHC class II molecules Activating T cells [ 15 ] β cells IA-2 GAD65 Enhancing antigen presentation, T cell activation [ 232 ] Trophoblastic cells miR-141 Jurkat T cells Reducing Jurkat T cell proliferation [ 205 ] M1 macrophages — T H cells Worsening experimental autoimmune neuritis by enhancing T H 1 and T H 17 responses [ 139 ] T lymphocytes miR-142-3p Treg cells Impairing Treg cell function [ 237 ] Tumor cells — Promoting Treg cell expansion [ 117 , 118 ] Primary islet cells — T and B cells Activate T and B cells [ 235 ] Tumor cells — B cells Producing pro-tumorigenic IgG [ 119 ] Astrocyte Components Neurons Damaging neurons [ 129 ] Macrophages miR-146a Microglial Hindering Aβ clearance [ 130 ] Macrophages miR-155-5p Glial Inducing inflammatory responses [ 133 ] Activated monocytes Activated macrophages Neurotoxic cargo Neurons Astrocytes Promoting cellular damage [ 134 ] Microglia α-Syn Neuron Neuronal damage [ 135 ] Fleur Nagri protozoa — Glial Microglial Triggering a pro-inflammatory immune response [ 137 ] Microglia Viral RNA Bystander CNS cells Promoting an inflammatory immune response [ 141 ] Tumor cells — Pleural mesothelial cells Enhancing DC infiltration into lung tumors [ 128 ] Trophoblasts DDIT4 Endothelial cells Triggering the inflammatory response [ 204 ] Macrophages miR-21-3p Vascular smooth muscle cells Enhancing vascular smooth muscle cell migration and proliferation [ 186 ] Macrophages CD5L Vascular smooth muscle cells Mediating lipid metabolism-induced inflammatory damage [ 188 ] Macrophages Circ_100696 Vascular smooth muscle cells Enhancing vascular smooth muscle cell proliferation and migration [ 189 ] Liver — Skeletal muscle and the pancreas Enhancing glucose effectiveness and insulin secretion [ 178 ] Peritoneal macrophages LncRNA CHL1-AS1 Endometrial stromal cells Promoting cell proliferation, migration, and invasion and inhibiting apoptosis, [ 198 ] Macrophages — Renal cell, Neutrophils Pro-inflammatory effect and enhancing renal cell IL-8 production and the migration of neutrophils [ 214 , 215 ] Macrophage miR-155 Mesenchymal stem cells Inhibiting the osteogenic differentiation of mesenchymal stem cells [ 219 ] Neoplastic mast cells miR-30a miR-23a Osteoblast Blocking osteoblast differentiation [ 221 ] β cells — β cell promote a pro-inflammatory islet transcriptome [ 229 ] T lymphocytes miR-142-3p miR-142-5p miR-155 β cells Promoting apoptosis [ 244 ] Macrophages miR-210-3p, Adipocytes Influencing insulin sensitivity [ 243 ] Alveolar epithelial cells Tenascin-C — Exacerbating ALI [ 148 ] Macrophages miR-223 — Inducing intestinal barrier dysfunction [ 168 ] Neutrophils PAD4 — Impairing intestinal barrier integrity [ 170 ] Macrophages miR-16-5p — Promoting the progression of AS [ 187 ] Macrophages miR-27-3p — Activating inflammation [ 242 ] Schematic of exosome-mediated information communication between different types of cells. Exosomes mediate communication between different cells through homotypic, heterotypic, and cross-organ manners during tumor suppression and immunopathology of eight systems Exosomal sources, effector molecules, and functions in immunopathology Monocytes NKs Neutrophils IA-2 GAD65 Activated monocytes Activated macrophages Neurons Astrocytes Glial Microglial Renal cell, Neutrophils miR-30a miR-23a miR-142-3p miR-142-5p miR-155

Lötvall J. first discovered that exosomes naturally carry miRNAs and that the selective packaging of specific RNAs into exosomes is affected by various sorting mechanisms [ 16 , 98 ]. Two articles showed that SNF8 (ESCRT-II subunit) and hnRNPA2B1 mediate the sorting of circular RNAs (circRNAs), including circRHOBTB3 and circNEIL3, respectively [ 99 , 100 ]. In addition, KRAS, hnRNPA2B1, Alyref, Fus, Lupus La protein, and Y-box protein I (YBP1) have been reported to mediate RNA sorting into exosomes [ 101 – 104 ]. The specific motifs in miRNAs are essential for their exosomal sorting. Sumoylated hnRNPA2B1 specifically binds exosomal miRNAs by recognizing GGAG motifs and controlling their loading into exosomes. Alyref and Fus are involved in the export of miRNAs with CGGGAG into exosomes. Since miRNAs are sorted into exosomes via RBPs, exosomal selectivity is determined by RBPs. If RBPs are selectively sorted to exosomes, then miRNAs should also be identified. Although few studies have investigated how RBPs are sorted into exosomes, RBP Ago2 is likely selectively sorted into exosomes because it can directly interact with Alix [ 96 ].

Research on exosomes has been conducted over the past five decades [ 10 ]. In 1967, Wolf discovered that minute particulate material could be obtained by ultracentrifugation in fresh plasma from which platelets had been removed. He named those particles ‘platelet dust' [ 5 ]. Crawford published additional images of these vesicles containing lipids and contractile proteins, describing them as ‘microparticles’ in 1971 [ 11 ]. Two seminal studies published by Harding et al. and Pan et al. in the 1980s marked the expansion of exosome research. These two researchers independently discovered a novel intracellular sorting and trafficking pathway called the exosomal secretory pathway [ 12 , 13 ]. In brief, they reported that reticulocytes are capable of secreting small vesicles ~50 nm in size during the maturation process. These small vesicles help reticulocytes export transferrin into the extracellular space [ 12 , 13 ]. A few years later, Johnstone et al. coined the term ‘exosome’ to describe these vesicles, marking a pivotal moment in exosome research [ 14 ]. These early studies provided a solid foundation for the subsequent surge of interest over the following years. Since then, with the development of techniques such as molecular biology, nanotechnology, and flow cytometry, the biological functions of exosomes have been gradually explored in greater depth, and research into their roles in health and disease has expanded significantly. Raposo et al. discovered that B lymphocytes can secrete exosome particles containing MHC-II molecules in 1996. These molecules can stimulate the immune response of CD4 + T cells [ 15 ]. This discovery established the foundation for further research into exosome biology within the context of immunology. Since Lötvall and colleagues reported that exosomes can mediate the transfer of messenger RNA (mRNA) and microRNA (miRNA) between cells in 2007, the ability of exosomes to mediate intercellular communication by transferring a wide range of molecular cargos, including proteins, lipids, mRNAs, miRNAs, and other noncoding RNAs, has attracted increasing attention [ 16 ]. These cargos influence the behavior and function of target cells, making exosomes essential messengers in various biological processes. Rothman, Schekman, and Sudhof won the 2013 Nobel Prize in Physiology or Medicine for discovering the regulatory mechanism of intracellular vesicle transport, which elevated exosome research to a new echelon [ 17 – 19 ]. Since then, exosome research has been utilized in various fields, including stem cells, immunity, miRNA, targeted drug delivery, disease diagnosis, and therapy.

Recent advances have led to the development of several innovative strategies for exosome clearance. One such approach is modified nanoparticle with targeting binders (MONOTAB), a plug-and-play monofunctional degradation platform that can drag extracellular targets into lysosomes for degradation. Owing to the high levels of phosphatidylserine on the outer leaflet of exosomes, researchers have used Annexin V-coated nanoparticles to capture exosomes. MONOTAB subsequently mediates the degradation of exosomes by lysosomes through the inherent lysosome-targeting ability of these nanoparticles [ 259 ]. Orme et al. pioneered the use of therapeutic plasma exchange (TPE) to remove soluble PD-L1- and PD-L1-positive exosomes from the circulation in patients with malignant melanoma. Therapeutic plasma exchange (TPE) is a procedure in which blood is passed through an apheresis machine that separates plasma from cellular components. The removed plasma is discarded and replaced with a colloid solution, such as albumin. TPE effectively clears plasma-restricted substances, such as large antibodies and exosomes, that are too large for rapid diffusion. On average, each session removes ~65–70% of these noncellular, intravascular components [ 260 ]. Moreover, our group reported that proton pump inhibitor-induced macropinocytosis facilitates the clearance of immunosuppressive exosomes from tumor cells, thereby enhancing antitumor immunity [ 261 ].

Exosomes are critical in the immunopathology of the endocrine system. Studies have shown that exosomes can accelerate endocrine system diseases by modulating innate and adaptive immune cells.

Obesity is a complex metabolic condition characterized by excessive fat accumulation, leading to a chronic low-grade inflammatory state. Obesity disrupts insulin sensitivity and promotes adipose inflammation, with exosomes playing a critical role in these processes. Exosomes influence both macrophage polarization and T-cell function in obesity. Exosomes are emerging as crucial mediators of heterotypic cell communication in obesity, influencing immune responses and insulin sensitivity. For example, adipocyte-secreted exosomal miRNA-34a inhibits M2 macrophage polarization, promoting obesity-induced adipose inflammation [ 240 ]. Exosomal miR-1224, derived from adipocytes, also inhibits M2 macrophage polarization by targeting MSI2, further contributing to obesity-induced AT inflammation [ 241 ]. Additionally, miR-27-3p, which is highly expressed in exosomes from M1 macrophages, impairs mitophagy through the miR-27-3p‒Miro1 axis, which activates inflammation and drives the development of insulin resistance [ 242 ]. The AT microenvironment encourages AT macrophages to release exosomes containing miR-210-3p, which can be transferred to neighboring cells, influencing insulin sensitivity [ 243 ]. Furthermore, human T lymphocytes release exosomes containing miRNAs such as miR-142-3p, miR-142-5p, and miR-155. These exosomal miRNAs can be transferred in an active form to β cells, promoting apoptosis during autoimmune attack [ 244 ].

The biogenesis, sorting, and release of different types of exosomes involve a series of finely regulated processes. i) Early endosomes (EEs) form the initial stage of exosome formation; ii) then, EEs mature into late endosomes (LEs) along with inward buds to form multivesicular bodies (MVBs) containing many intraluminal vesicles (ILVs); and iii) finally, the MVBs are transported to and fuse with the PM, leading to the release of exosomes into the extracellular space. The successful completion of these three processes is conducive to exosome biogenesis. iv) If MVBs fuse with lysosomes, they are degraded, inhibiting exosome formation (Fig.  1a ). Currently, two main types of exosome biogenesis exist: endosomal sorting complex required for transport (ESCRT)-dependent and ESCRT-independent pathways [ 20 – 22 ]. This classification is based primarily on the second step of exosome generation, namely, the generation of ILVs, which also includes how cargos are sorted into ILVs. Here, we classify exosome biogenesis into the four steps mentioned above and summarize the known mechanisms responsible for the regulation of each step. Fig. 1 Biogenesis of EVs. a Four critical steps of exosome biogenesis. b Regulators of EE formation involving exosome origin. c Regulators of ILV formation. d Regulators of MVB and PM fusion. e Regulators of MVB and lysosome fusion Biogenesis of EVs. a Four critical steps of exosome biogenesis. b Regulators of EE formation involving exosome origin. c Regulators of ILV formation. d Regulators of MVB and PM fusion. e Regulators of MVB and lysosome fusion

The interventions mentioned above lack specificity for individual molecules, which may impact normal cellular functions. A better approach to reduce the number of pathological exosomes is to elucidate the mechanisms involved in cargo sorting into exosomes, as this could lead to the development of novel therapeutic strategies for disease treatment. Circ-0034880-enriched TEXs facilitate strong interactions between primary tumor cells and protumor macrophages, promoting the formation of the premetastatic niche and colorectal cancer liver metastasis. Rb1 effectively inhibits circ-0034880 loading in TEXs, reversing this phenomenon [ 256 ]. Exosomal miR-21 derived from tubular epithelial cells may accelerate the progression of renal fibrosis through the miR-21/PTEN/Akt pathway. Inhibition of epithelial exosomal miR-21 abolishes fibroblast activation in vitro [ 257 ]. Late endosomal/lysosomal adaptor and MAPK and MTOR activator 1 (LAMTOR1) facilitate PD-L1 lysosomal degradation by interacting with HRS, thereby reducing exosomal PD-L1. Using LAMTOR1 to design peptides enhances the efficacy of immunotherapy in lung cancer [ 258 ]. Our group recently discovered that tumor cell-derived MFGE8 specifically promotes PD-L1 sorting onto exosomes through the integrin signaling pathway. MFGE8-neutralizing antibodies can negate anti-PD-1 therapy resistance by preventing PD-L1 sorting onto exosomes [ 92 ].

Reducing exosome secretion involves targeting critical pathways and molecules that regulate their production. Inhibition of the migration of tumor cells can be achieved by preventing the intercellular exchange of M2 macrophage-derived exosomes related to caveolin-1 [ 245 ]. Rab5 is a small GTPase critical for EE trafficking. Rab5 knockdown reduces the production of exosomes and promotes macrophage polarization toward an antitumor phenotype, suggesting that Rab5 could serve as a potential therapeutic target for triple-negative breast cancer [ 35 ]. Knocking down the ESCRT-0 subunit HRS significantly reduces the expression of exosomes derived from head and neck squamous cell carcinoma and enhances the activation of CD8 + T cells [ 246 ]. GW4869, an nSMase2 inhibitor, inhibited 4T1 tumor growth in BALB/c mice by inhibiting TEX production [ 247 ]. The syndecan-syntenin-Alix pathway, regulated by phospholipase D2 (PLD2), promotes MVB formation. When human prostate cancer bone metastasis-derived C4-2B cell lines are treated with halopemide, a paninhibitor of PLD, the exosomes secreted by C4-2B cells lose their ability to stimulate osteoblasts, thereby inhibiting bone metastasis [ 248 ]. Cholesterol is another key player in MVB formation. The cholesterol synthesis inhibitor simvastatin reduces exosome release from cardiomyocytes, attenuating angiotensin II (AngII)-induced cardiac fibrosis, highlighting its potential role in modulating exosome release and mitigating cardiac fibrosis [ 249 ]. Furthermore, we revealed that neddylated Coro1a is a novel exosome biogenesis regulator that facilitates Rab7-mediated lysosomal targeting. Knockout of Coro1a or enhancement of Coro1a neddylation can effectively decrease TEX production, facilitating antitumor activation [ 40 ]. Lysosomal function can effectively influence exosome biogenesis [ 83 ]. PTEN can suppress tumor metastasis by enhancing lysosome acidification and reducing exosome release through a TFEB-dependent mechanism [ 250 ]. Rab27a benefits the PM transport of MVBs and the subsequent fusion of MVBs and the PM. Rab27a has been widely used to regulate exosome production. We found that TEXs are responsible for resistance to anti-PD-L1 therapy and that Rab27a knockout-mediated inhibition of TEX production can increase the sensitivity of tumors to anti-PD-L1 therapy [ 251 ]. The SNARE complex is proposed to mediate the fusion of MVBs with the PM. Deletion of VAMP-7 in 4T1 cells inhibits exosome secretion, significantly reducing tumor growth and lung metastasis [ 252 ].

Integrins and endocytic pathways both play crucial roles in promoting the absorption of exosomes by target cells. Blocking the absorption of pathological exosomes by target cells can help mitigate disease progression. Integrins such as αvβ3 are essential for exosome uptake, and inhibiting their activity markedly reduces exosome absorption [ 253 ]. Target cell receptors such as ICAM-1 and LFA-1 commonly bind exosomes. Blocking ICAM-1 on TEXs decreases their interaction with CD8 + T cells and mitigates the PD-L1-mediated immunosuppressive effects of TEXs [ 254 ]. Inhibiting endocytic pathways can also reduce the uptake of exosomes by cells. Moreover, chemical endocytosis inhibitors that target heparan sulfate proteoglycans, actin, tyrosine kinase, dynamin-2, sodium/proton exchangers, or PI3Ks have been shown to significantly decrease the internalization of BMSC-derived exosomes by multiple myeloma cells [ 255 ].

Exosomes are known to carry a variety of substances, including proteins, nucleic acids, lipids, and metabolites [ 84 ]. In addition to cell-specific cargos, exosomes possess several common markers. For example, proteins such as TSG101, HSP70, CD9, CD63, and CD81 are popularly used as specific biomarkers for exosome analysis. Exosomal proteins strongly affect exosome functions. Tetraspanins, such as CD9, CD63, CD81, and CD82, assist in cell penetration, invasion, and fusion processes [ 73 ]. HSP70 and HSP90 are involved in antigen presentation and binding [ 85 ]. MVB formation-associated proteins, such as Alix, lipid raft markers, and TSG101, participate in the biogenesis and release of exosomes [ 86 ]. Additionally, nucleic acids within exosomes play a critical role in cellular communication, with mRNAs or miRNAs incorporated into the membrane during exosome formation and capable of altering gene expression in recipient cells. Among these nucleic acids, miRNAs are the most abundant and influence various biological processes, including exocytosis, hematopoiesis, angiogenesis, and intercellular communication [ 87 – 89 ]. During the endosome inward budding process, many components in the cytoplasm can be passively sorted into exosomes. However, in the ESCRT-dependent exosome formation pathway, ESCRT-0 recognizes and selectively sorts ubiquitinated proteins into exosomes. In addition, many miRNAs can be sorted into exosomes by coupling with RNA-binding proteins (RBPs). Whether miRNA sorting is selective is determined by the exosomal selectivity of RBPs. Here, we introduce the selective sorting mechanism of proteins and miRNAs (Fig.  2 ). Fig. 2 Regulation of exosome cargo sorting. Proteins are sorted into exosomes through ESCRT-dependent mechanisms, such as ubiquitinated GPCR143 and PD-L1, which are sorted to exosomes after being recognized by HRS. Tetraspanis and Rab22a-NeoF1 can be recognized by ESCRT complexes and are thus sorted into exosomes. ESCRT-independent pathways involve LAMP2A-dependent exosomal sorting of proteins with the KFERQ motif. Alix-associated Ago2 or TfR are sorted to exosomes via an ESCRT-independent pathway. Proteins, including SNF8, KRAS, Lupus La, and YBP1, are involved in miRNA sorting into exosomes. RBPs hnRNPA2B1, Alyref, and Fus facilitate the exosomal sorting of miRNAs by recognizing specific motifs in miRNAs Regulation of exosome cargo sorting. Proteins are sorted into exosomes through ESCRT-dependent mechanisms, such as ubiquitinated GPCR143 and PD-L1, which are sorted to exosomes after being recognized by HRS. Tetraspanis and Rab22a-NeoF1 can be recognized by ESCRT complexes and are thus sorted into exosomes. ESCRT-independent pathways involve LAMP2A-dependent exosomal sorting of proteins with the KFERQ motif. Alix-associated Ago2 or TfR are sorted to exosomes via an ESCRT-independent pathway. Proteins, including SNF8, KRAS, Lupus La, and YBP1, are involved in miRNA sorting into exosomes. RBPs hnRNPA2B1, Alyref, and Fus facilitate the exosomal sorting of miRNAs by recognizing specific motifs in miRNAs

Given their involvement in the pathological process of various diseases, exosomes are promising intervention targets for preventing and treating related diseases. We believe that four strategies exist to achieve this goal. First, reducing the generation of pathological exosomes, which blocks the pathogenic effect of exosomes from the source; second, inhibiting the uptake of exosomes by target cells, thereby weakening the pathological functions of exosomes; third, preventing pathogenic effector cargos from being sorted into exosomes, thereby downregulating the pathogenicity of exosomes; and fourth, eliminating existing pathogenic exosomes (Fig.  4 ). Fig. 4 Strategies to mitigate the pathological effects of exosomes. Four strategies to prevent exosomal pathological effects. i) Reducing the generation of pathological exosomes; ii) inhibiting exosome uptake by target cells; iii) preventing the sorting of pathogenic cargos into exosomes; and iv) clearing existing pathological exosomes Strategies to mitigate the pathological effects of exosomes. Four strategies to prevent exosomal pathological effects. i) Reducing the generation of pathological exosomes; ii) inhibiting exosome uptake by target cells; iii) preventing the sorting of pathogenic cargos into exosomes; and iv) clearing existing pathological exosomes

Extracellular vesicles (EVs) mainly consist of exosomes and ectosomes. Exosomes are produced via the endosome pathway, while ectosomes are released by outward budding of the plasma membrane (PM). The diameter of ectosomes varies greatly, ranging from tens of nanometers to several micrometres. The diameter of exosomes is relatively small, ranging from 30 to 150 nm [ 1 ]. According to the MISEV2023 guidelines, the direct use of secretion pathway-based terminology such as “exosomes” and “microvesicles” is no longer recommended, as their biogenesis is often difficult to trace and lacks universal applicability. Instead, the guidelines propose classifying EVs into small EVs (sEVs) (diameter <200 nm, corresponding to traditional exosomes and a subset of microvesicles 200 nm, encompassing traditionally defined microvesicles >200 nm and apoptotic bodies) [ 2 ]. While most studies using classical ultracentrifugation methods primarily isolate sEVs [ 3 ] and since current research on the biogenesis of exosomes is relatively well established [ 4 ], this article specifically focuses on recent advances in exosome biogenesis—the term “exosomes” is retained here. However, owing to the lack of specific markers, the EVs isolated in the cited literature are not exclusively composed of exosomes, as they may contain vesicles from multiple biogenetic pathways. Almost all types of cells can release exosomes. Exosomes are considered ‘platelet dust,’ a byproduct of the body’s natural waste disposal processes and maintain homeostasis for a long period of time [ 5 ]. However, they are now widely recognized as critical cellular signaling and intercellular communication tools. Exosomes share a similar molecular composition and structural configuration with their parental cells, effectively serving as functional extensions. Owing to their high degree of topological similarity to cells, exosomes can traverse multiple cell types and physiological barriers, such as the blood‒brain, intestinal, placental, etc. [ 6 – 8 ]. Remarkable accessibility positions exosomes as vital mediators of intercellular communication, bridging central and peripheral communication and cross-organ interactions. The cargos carried by exosomes include membrane-bound/soluble proteins, nucleic acids, lipids, and metabolites, reflecting their parent cells’ origin and functional specificity. On the one hand, exosomes protect their cargos from degradation and inactivation, ensuring their integrity during delivery to target cells. On the other hand, exosomes facilitate the entry of their cargo and functional integration in recipient cells. These distinctive features position exosomes as pivotal modulators of cell phenotypes and functions, particularly in regulating and reprogramming immune cells under pathological conditions [ 9 ]. In this review, we summarize the origin, cargo sorting, and role of exosomes in modulating immune cells across different systems and tumor immunity, with a particular focus on their functions during pathological processes. In addition, we summarize potential strategies for eliminating the pathological effects of exosomes.

Cytokines are indispensable mediators of information exchange between different cells and are essential for regulating various physiological and pathological processes. Like cytokines, exosomes play a pivotal role in information exchange between cells. In addition, unlike cytokines, which are involved mainly in the functional regulation of neighboring cells, exosomes can cross-organly communicate and play a unique role in cross-organ functional regulation. Therefore, the role of exosomes in mediating intercellular communication is likely no less critical than that of cytokines; thus, their functional importance should not be underestimated. Elucidating the cross-organ regulatory function of exosomes, especially the cross-organ distribution pattern of exosomes in disease states, will expand our understanding of the pathogenesis of related diseases, thereby facilitating the identification of new therapeutic targets. However, current exosome research faces numerous challenges due to ‌technical bottlenecks and a limited understanding‌. The stochastic nature of cargo loading into exosomes and the significant size variation of vesicles formed during MVB budding result in substantial heterogeneity‌, even among exosomes derived from the same cell type. This heterogeneity is the root cause of the lack of specific exosome biomarkers. In addition, while exosomes from different cell sources exhibit distinct functions, the functional implications of heterogeneity within exosomes of the same cellular origin remain unclear. The emergence of single-vesicle analysis techniques enables the identification of functional molecules on individual vesicles, offering a potential solution to these challenges. In the study of exosome biodistribution, current approaches rely primarily on labeling exosomes and tracking their postreintroduction distribution in vivo‌. However, this method likely differs significantly from the ‌native distribution of endogenously produced exosomes‌, posing a major challenge for real-time, precise tracking of endogenous exosomes. Additionally, the absence of ‌core universal biomarkers‌ makes absolute isolation of exosomes technically unachievable. Consequently, experimental results are inevitably confounded by contamination from other EVs and nonvesicular components‌. Moreover, many studies use exosomes isolated from heterogeneous tissues or organs, making it impossible to define their ‌cell source‌s. Notably, exosomes from distinct cell sources may perform similar functions if they carry shared effector molecules. Furthermore, in the disease treatment strategy of targeting exosomes, inhibiting the secretion of exosomes is likely to destroy the critical physiological functions of exosomes because exosomes secreted by normal cells are inevitably inhibited. Therefore, discovering how target cells uniquely take up exosomes in disease states and how pathogenic effector molecules are sorted explicitly into exosomes is more likely to lead to the development of disease-specific therapeutic targets. In summary, despite enormous challenges, exosomes have undoubtedly become a promising target for treating human diseases.