The link between immunity and hypertension in the kidney and heart.

Macrophages, originating from monocytes found in the circulation, express a variety of markers and serve a great deal of roles in innate immunity as well as mediate adaptive immune response by cross-talking with B and T cells ( 117 ). These cells are recruited to injured or infected cells via chemokine signaling (e.g., CCL2,CXCL1, CX3CL1) from a number of other cells including fibroblasts, endothelial cells, and epithelial cells ( 118 ). Of note, cardiac-resident macrophages appear to play a more protective role against fibrosis than monocyte-derived macrophages that infiltrate the heart and promote hypertrophy, but further research is needed to clarify the distinction in responses between monocyte derived macrophages and cardiac resident ( 119 ). There are traditionally understood to be two main macrophage phenotypes: M1 and M2 ( 120 ). M1 macrophages are considered “traditionally activated” and become active upon exposure to IFN-γ and TLR stimulation ( 120 ). These macrophages are considered pro-inflammatory and secrete inflammatory cytokines such as IL-1β, TNF, and IL-12 ( 121 ). In a neonatal injured heart model, inhibition of cyclooxygenase-2 resulted in increased M1 macrophage recruitment at the wound site which may contribute to the treatment induced suppressed cardiac hypertrophy and fibrosis ( 122 ); however, resolution-phase macrophages can exhibit M1 markers yet express a unique inflammatory phenotype ( 123 ), making the role of M1 macrophages in cardiac fibrosis unclear. Depletion of macrophages (both phenotypes) has been shown to reduce cardiac remodeling in the Dahl salt-sensitive rat ( 124 ), but did not identify the key contributing phenotype. In contrast to M1 macrophages, M2 macrophages are anti-inflammatory and pro-fibrotic in function; as such, the M2 phenotype may especially contribute to the fibrogenesis in hypertensive hearts. M2 macrophages are “alternatively activated”(activated by IL-13, CSF-1, IL-4, IL-10, and TGF-β) ( 121 , 125 , 126 ). These cytokines, commonly produced by Th2 T cells, have been shown to be directly linked to the production of collagens and the onset of fibrosis ( 126 ), potentially through their stimulation of M2 macrophages. Furthermore Angiotensin II and cations have been implicated in the transition of fibroblasts to myofibroblasts capable of collagen production and deposition ( 127 ). The M2 macrophage, alone, is not capable of depositing the collagen seen in fibrosis; rather, these cells are able to promote the transition of the homeostatic fibroblasts into fibrosis-inducing myofibroblasts ( 128 ). Macrophages, once activated at the site of injury, can recruit more macrophages and circulating fibroblasts, leading to increased inflammation and fibrosis ( 127 ). M2 macrophages induce myofibroblast differentiation through a number of mechanisms such as the production of TGF-1β. A recent study by Murray et al., implicated IL-13, TGF-β1, and CCL2 axis in the hyper-stimulation of myofibroblasts in a model of idiopathic pulmonary fibrosis with a usual interstitial pneumonia pathology (IPF/UIP) ( 129 ). IL-13, TGF-β1, and CCl2 together appeared to result in a synergistic effect in the activation of myofibroblasts. Though these results have not been explored in the heart specifically, they indicate the M2 macrophage can exacerbate organ fibrosis, which may be relevant in hypertension and heart failure through the same myofibroblastic axis. TGF-β plays a critical role in the onset and progression of fibrosis through myofibroblast differentiation ( 130 , 131 ). TGF-β signals through the SMAD pathway inside of the homeostatic fibroblasts and induces the assembly of SMADs 2 and 3 into SMAD 4. SMAD 4 and its R-Smad counterparts then form a complex in the nucleus and modify the gene expression of the fibroblast into a myofibroblast ( 132 ). As such, TGF-β and its downstream effects are potential targets for the ablation of fibrosis in hypertension and heart failure. When translated, TGF-β is associated with its negative regulator, the latency associated protein (LAP) ( 133 ). TGF-β function can be arbitrated through multiple mechanisms, including through interaction with αVβ6, an inflammation-associated integrin expressed on epithelial cells ( 134 ). Munger et al., demonstrated that, though there are multiple methods for TGF-β to be activated, αVβ6 interaction is sufficient to initiate TGF-β signaling ( 135 – 138 ). IL-10 is an anti-inflammatory cytokine produce by a myriad of cells including macrophages, CD4 + and CD8 + T cells, B cells, other monocytic cells ( 139 ). IL-10 targets M2 macrophages and activates them to a collagen-producing, pro-fibrotic phenotype ( 140 ). Pro-fibrotic macrophages play a critical role in the pathological progression of hypertension to diastolic dysfunction and heart failure ( 52 ). Furthermore, it has been shown that IL-10 deletion in a murine heart failure model attenuates erroneous fibrotic response and reduces mortality ( 52 ), suggesting IL-10 signaling cascades may be relevant therapeutic targets to prevent the progression of hypertension into heart failure. Macrophages, their differentiation, and their signaling molecules present potentially valuable targets for preventing the onset of fibrosis during hypertension and heart failure (outlined in Figure 2 ). However, the progression of fibrosis in hypertension and heart failure is not yet fully understood; as such, other pro-inflammatory immune cells may be contributing to the pathology. In a pressure overload mouse model of heart failure through transverse aortic constriction, Wang et al., found that depletion of CD11C + cells blunted left ventricular fibrosis and hypertrophy, implicating dendritic cells in contributing to this pro-inflammatory state ( 141 ), but CD11C + macrophages may be relevant to this model ( 142 ). In the DOCA + Salt model of hypertension, depletion of CD11C + cells attenuated progression into cardiac hypertrophy and fibrosis, further highlighting the role of dendritic cells ( 143 ). Natural killer (NK) cells, on the other hand, may play a protective role via attenuating cardiac fibrosis ( 144 ). Additional research is needed to clarify the role of other innate immune cells in hypertension induced cardiac fibrosis. Macrophages and their mechanistic role in fibrosis. M2 macrophages can interact with αVβ6, thereby releasing a TGF-β inhibitor protein, and allowing them to interact with fibroblasts in the heart. TGF-β leads to myofibroblast differentiation from fibroblasts via the SMAD2/3 pathway. It is these myofibroblasts that are thought to account for much of the fibrotic remodeling in many diseases, including diastolic dysfunction and heart failure.

Clinically, elevated immune cell activation/function of CD3 + /CD4 + ( 191 ), CD3 + ,CD8 + ( 99 ), and monocytes ( 192 ) have been described in hypertension. Upregulation of the cytokines IL-17 and IFNγ ( 21 ), IL-6 and TNFα ( 44 ), and IL-18 ( 18 ), has been correlated with clinical manifestation of essential hypertension, among other cytokines ( 193 ). To this end, targeting the immune system to reduce cytokine production and immune cell activation and infiltration may reduce blood pressure and subsequent tissue damage. This excellent review by Murray et al., describes the effects of immunosuppressants on blood pressure in animal models ( 193 ); however, clinical trials have been complicated through the nephrotoxic (and other tissue) effects of several immunosuppressants -such as calcineurin inhibitors- resulting in blood pressure elevation driven by tissue damage and sodium transporter upregulation ( 194 – 196 ). Our lab has recently identified the critical role tubular PD-L1 plays in mediating CD8 + T-distal convoluted tubular interaction and hypertension; by knocking down PD-L1 in mice using renal tubule specific nanoparticles ( 197 , 198 ) we were able to blunt blood pressure elevation in the DOCA + salt or adoptive transfer models of hypertension, identifying another potential therapeutic target ( 10 ). Further studies are needed to address the potential effectiveness of alternative immunosuppressant therapies in reducing blood pressure. Several promising pre-clinical studies have been conducted indicating the feasibility of using FDA-approved nanodrug platforms to deliver tissue specific immunosuppressant therapy (minimizing off-target toxicities), but further validation and studies are required before use in a clinical setting ( 199 ). Regardless of current difficulties, targeted immunosuppression to reduce inflammation and immune-mediated organ dysfunction in hypertension continues to be of scientific interest.

The immune system plays a role in the pathogenesis of hypertension through several inflammatory signaling mechanisms involving cells from both the innate and adaptive immune system; however, the signaling molecules and pathways governing these interactions have proven to be complex and not yet fully understood. Within both the heart ( 109 ) and kidney ( 200 ), current data suggests higher pressure can, alone, drive immune infiltration and subsequent inflammation within the invaded organ leading to dysfunction. Immunodeficient mice (Rag2 −/− ) exhibited reduced pressure-driven cardiac remodeling ( 109 ), and the immunosuppressant tacrolimus reduced T cell infiltration, renal damage, and blood pressure elevation in the Dahl salt-sensitive rat on high salt diet ( 95 ). The immune system not only contributes to blood pressure elevation ( 35 ) but also mediates organ dysfunction and dysregulation initiated by elevated blood pressure ( 94 ). This inextricable connection between immunity, hypertension, inflammation, and organ dysfunction lends high priority to targeting the immune system to lower blood pressure or, at least, reduce inflammation due to hypertension ( 47 ). Even current FDA approved anti-hypertensives may mediate some of their beneficial effects through immune modulation, but further studies are needed for confirmation ( 157 ). Targeting the immune system to lower blood pressure and reduce organ damage has proven complicated ( 201 , 202 ), but organ specific immune targeting using nanotechnology appears to be a promising solution to reduce toxicities ( 155 , 199 ). Recent identification of single nucleotide polymorphisms in SH2B adaptor protein 3 contributing to T cell involvement in hypertension and renal damage ( 203 ) further complicates therapeutic targeting as genetic mutations may predispose certain immune cells to promote inflammation, supporting consideration of individual patient genetic predispositions when identifying driving factors of hypertension to design future treatment plans, as is becoming increasingly debated ( 204 ). As contributing immune players and inflammation-mediating molecules are characterized, novel pathways can be identified and targeted therapeutically to lower blood pressure and attenuate hypertension-mediated organ dysfunction. Recent studies have highlighted several such immunity-associated relevant receptors or cytokines and confirmed their relevance in several animal models of hypertension.

T-helper cells—a subset of CD4 + T cells—are further subdivided into Th1, Th17, Th2, Th3, and Tr1. These cells serve a variety of functions in response to both foreign antigens and injury. Decades of literature, especially recent studies, have identified CD4 + T cells in the development and progression of cardiomyopathy ( 145 ). Laroumanie et al., recently implicated CD4 + T cells in the progression of hypotrophy to heart failure in a myocardial infarction murine model ( 109 ). Utilizing multiple mice strains including, RAG2-KO, CD8 + -KO, and MHCII-KO, the authors found that CD4 + T cells appear to be the primary drivers of cardiac fibrosis after TAC-induced heart failure. Furthermore, adoptive transfer of T cells from CD8 + -KO mice (only CD4 + T cells transferred) to RAG2-KO (immune incompetent) mice resulted in an increase of fibrosis in the recipients ( 109 ). In the same study, to determine a mechanism by which CD4 + T cells increase fibrosis, Lysyl oxidase (LOX), an amine oxidase that influences cross-linking between elastin and collagen, expression was investigated LOX is known to play a role in ECM development and fibrotic disease ( 146 , 147 ). The researchers found no significant difference between pro-LOX production between WT and RAGII-KO mice; however, they showed that the ratio of mature LOX to pro-LOX increased in WT mice—hinting at some interaction from the CD4 + T cells that might induce the maturation of LOX in heart failure thereby driving elastin and collagen cross-linking. While preventing the aberrant increase in LOX maturation may prevent the progression of hypotrophy to heart failure, the production of LOX from pro-LOX is not fully understood and may be dependent on a number of factors, complicating the development of therapies ( 109 ). If pro-inflammatory T cells are contributing to the development of hypertension and subsequent cardiac fibrosis, it would likely follow that regulatory T cells may play a protective effect. Indeed, Wang et al., found that increased T reg presence (induced by IL2/JES6-1 treatment) blunted left ventricular hypertrophy in the transverse aortic constriction model of heart failure ( 148 ); as such, Tregs may play a protective role in hypertension induced cardiac fibrosis, but further research is needed to outline the protective mechanisms involved. The extent of the role of either CD4 + or CD8 + T cells in the progression of hypertension and cardiovascular disease is not yet clear. As previously mentioned, Laroumanie et al., provided evidence pointing toward the role of CD4 + T cells in the progression of heart failure ( 109 ); however, in tandem with their CD4 + counterparts, other evidence supports that CD8 + T cells may also play a critical role in cardiovascular disease. In their 2019 study on immune cell activity post myocardial infarction, IIatovskaya et al., examined CD8 + T cell-specific effects by comparing the progression of scar formation after TAC in WT or CD8 + -deficient tm1Mak mice. When CD8 + T cells were ablated, TAC mice exhibited abnormal scar formation ( 149 ). Additionally, they found a significant increase of mature LOX when CD8 + T cells were present, in contrast to the results found in the study by Laroumanie et al. ( 109 ) Regardless of immune cell source, it is evident that LOX production plays some causal role in the development of cardiac-straining scar tissue ( 109 , 149 ). Reactive oxygen species (ROS) are a hallmark of many diseases, including cancer, heart disease, and other organ diseases ( 150 ). Several potential mechanisms are available for a T cell to induce ROS release or production, such as the rupture of phagocytes that can sequester ROS ( 151 ). Excessive ROS production due to T cell activity can negatively affect contractility within the heart, among other dysfunctions. One such mechanism, as depicted in Figure 3 , has been identified to involve Myosin II, which relies on myosin phosphatase to be dephosphorylated, leading to the relaxation of the muscle fibers. Myosin phosphatase is activated by protein kinase G, which, previously, had been activated by secondary messenger cGMP. This cGMP is produced by soluble Guanylate Cyclase (sGC) in the heart. sGC is activated by endogenous NO, which is produced by eNOS. eNOS is known to be inhibited by increased levels of intracellular ROS; as such, downstream contractility regulated by eNOS will be negatively impacted by increased ROS ( 152 ). By this mechanism, T cell activity in the failing heart might indirectly lead to decreased cardiomyocyte relaxation, which could further exacerbate cardiac stress and the progression to heart failure. Therapies for eNOS inactivity exist in which sGC is stimulated by small molecules to be more reactive to decreased levels of NO ( 153 ). Further research is needed to determine whether the ablation of T cells might have a protective role on this function of the cardiomyocytes through reducing inflammatory ROS production ( 153 , 154 ). The outcome of ROS production on cardiomyocyte function. Soluble Guanylate Cyclase (sGC) drives Myosin II dephosphorylation and smooth muscle relaxation via production of the secondary messenger cGMP. sGC is activated by intercellular NO produced by endothelial nitric oxide synthase (eNOS). Reactive oxygen species inhibit eNOS activity, thereby affecting cardiomyocyte function further downstream. sGC enhancers target sGC at an allosteric site to increase its sensitivity to NO, thereby rescuing function despite lower NO availability.

Classically understood to activate as the “first line” in host defense without developing memory, innate immunity is comprised of dendritic cells, natural killer cells, neutrophils, macrophages, and other granulocytes ( 1 ). Although these cells typically function in infection clearance and homeostasis, a myriad of inflammatory responses have been identified when any of these immune cells become dysregulated such as in respiratory diseases ( 2 ) and aging ( 3 ). In the context of hypertension, monocytes, neutrophils, natural killer cells, and dendritic cells have been identified as present within the kidney and contributing to blood pressure elevation through several inflammatory mechanisms ( 4 , 5 ). In contrast to innate immunity, adaptive immune cells (T and B lymphocytes) are classically understood to take longer for recruitment and activation through antigen-specific interactions, resulting in formation of memory and enabling more rapid and robust response following re-challenge with the infectious agent. Further sub-categories of T cells have been identified—each fulfilling a unique and critical biological niche, including CD4 + , CD8 + , and CD4 − /CD8 − γδ T cells, within which further sub-classes have been identified ( 6 ). T cells, in particular, have been demonstrated to play a critical role in the development of hypertension ( 7 ), but B cell involvement cannot be ruled out ( 8 ). Certain cytokines, proteins that regulate the immune cellular function, have been directly linked to pro-inflammatory immune responses; however, the specific response can vary between target cell, surrounding tissue, timing, and cytokine concentration ( 9 ). In hypertension models, many cytokines characterized classically as “pro-inflammatory” have been found to arbitrate at least some of the animal model's blood pressure elevation or organ damage, including IFNγ ( 10 – 12 ), TNFα ( 13 ), RANTES ( 14 ), IL-1 ( 15 , 16 ), IL-6 ( 17 ), IL-17 ( 12 ), IL-18 ( 18 ), and, potentially, IL-4 ( 18 – 20 ). IFNγ and IL-17, especially, have been confirmed to be elevated in clinical hypertension ( 21 ). We further demonstrated that IFNγ is critical for mediating CD8 + T cell infiltration and interaction within the kidney, driving increased sodium retention ( 10 ). Adoptive transfer of primary CD8 + T cells from hypertensive mice is able to induce salt sensitivity hypertension(SSH) in WT mice, however, adoptive transfer of primary CD8 + T cells from IFNγ KO hypertensive mice failed to induce SSHTN in WT mice ( 22 ). Collectively these data evidence the critical role of IFNγ in mediating immune-driven hypertension. For a thorough discussion of other cytokines within the kidney contributing to hypertension, we refer the reader to this review by Drs. Wen and Crowley ( 23 ). Cardiovascular disease is the leading global killer, and hypertension is the primary contributing cause to this premature mortality ( 24 ). Of note, untreated or unsuccessfully managed hypertension has been demonstrated to result in organ dysfunction and damage—particularly within the heart and kidney ( 25 ). Either successful reduction of blood pressure or attenuation of hypertension induced organ dysfunction are key targets to prevent this premature mortality. Reducing blood pressure globally has proven challenging: not only due to resource scarcity -be that diet or medical- in certain areas ( 24 ) but also due to the clinical findings that the origin of elevated blood pressure is unknown in 90%–95% of cases (“essential hypertension”) ( 26 ) and up to 20%–30% of patient cases are resistant to current treatments (“resistant hypertension”) ( 27 ). Although patient compliance to drug regimens complicates treatment of this disease, true diagnosis of “resistant hypertension” requires assurance of adherence to administered therapeutics; as such, other confounding factors are clearly involved ( 28 ). Furthermore, the gut microbiota may further contribute to treatment resistance through direct enhancement of drug metabolism thereby reducing therapeutic effect ( 29 , 30 ). In light of recent studies, hypertension is presently understood as an inflammatory disease, involving immune cell invasion into organ tissue resulting in a myriad of cytokine expression changes, eventually driving endothelial dysfunction and organ damage ( 31 – 34 ). Through a combination of depletion and adoptive transfer studies, researchers were able to identify cells from both the innate and adaptive immunity that contributed to the development of hypertension ( 35 ) and subsequent tissue inflammation, especially the contributive roles of macrophages ( 36 ) and T cells ( 7 ). Other pre-clinical studies contributed to our current understanding that dendritic cells ( 37 – 40 ) and neutrophils ( 41 , 42 ), are also participating in the pathogenesis of hypertension. Such pre-clinical studies have been corroborated clinically through the association of pro-inflammatory biomarkers with increased risk of treatment resistant hypertension in patients with chronic kidney disease ( 43 ), an independent association between several pro-inflammatory markers and higher blood pressure in healthy patients ( 44 ), and more than 50 years of observations of a myriad of immune cells accumulating in the blood vessels and kidneys of hypertensive patients ( 45 ). The key regulatory role of the kidney in the cardiovascular system has been well established ( 46 ); as such, identifying which immune cells are infiltrating the tissue and elucidating the signaling mechanisms involved are critical in improving treatment of this non-communicable disease. Indeed, recent advances have highlighted several new molecular targets involving the immune system that may serve as new targets for the treatment of resistant hypertension ( 10 , 28 , 47 ), but many pathways have not yet been elucidated. Additionally, unsuccessful management of hypertension has long been associated with progression to left ventricular hypertrophy and diastolic dysfunction within the heart ( 48 – 51 ), and the immune system has been implicated in this progression—particularly cardiac macrophages ( 52 ). Based on the critical functions of the kidney and heart in the cardiovascular system, this review will focus on the currently understood link between specific immune cells and hypertension through their interactions with -and within- the kidneys and heart, highlighting “key players” mediating these interactions.

Chronic hypertension has been identified as the leading cause of congestive heart failure ( 108 ). The progression of hypertension into cardiac dysfunction, and that of cardiac dysfunction into heart failure, is a multi-faceted progression which often involves morphological and molecular biological changes in the cardiomyocytes, alterations within the surrounding microenvironment, and physical changes to the heart. In addition to playing a role in the perturbation of blood pressure, often through salt-sensitive inappropriate sodium handling ( 109 ), decades of research have shown that immune cells are partially culprit in the onset of hypertension, the progression of cardiac dysfunction to heart failure and, possibly, even the progression of hypertension to cardiac dysfunction ( 41 ). Either hypervolemia or increased peripheral resistance increase the pumping force requirements of the heart, especially within the left ventricle ( 110 ), to overcome the pressure in the aorta to successfully deliver blood to the rest of the body. This increase in pressure within the ventricle causes excessive stretching, resulting in cardiomyocyte injury and subsequent cardiac hypertrophy—a process mediated by the immune system ( 111 ). Furthermore, immune regulation of T cells, macrophages, fibroblasts, and dendritic cells all play a role in the worsening of cardiac function in hypertension and heart failure; as such, immunological targets appear to be promising targets to increase heart health and function in treatment of heart disease ( 112 ). Collagen deposition in cardiac fibrosis plays an important role in the maintenance of the heart. Fibrosis functions in both maintaining homeostasis—especially within the interstitium—and in repairing physical injury that occurs. Fibrotic remodeling is a complex function that involves the secretion and deposition of connective molecules, most importantly Collagens I and III, into the interstitial and perivascular spaces. However, excessive deposition of these connective molecules can impair cardiac function. The increased deposition of molecules within the extracellular matrix (ECM) stiffens the heart chambers, thereby inhibiting the heart from expanding and contracting efficiently, and, ultimately, resulting in increased cardiac stress to maintain homeostatic output. As this fibrotic deposition progresses, cardiac output becomes sufficiently reduced to cause damage to other organs and damage to cardiac cells due to impaired circulation and increased pressure, a pathology so common that 50% of heart failure patients present with this increased cardiac load due to fibrosis ( 52 , 113 ). Therefore, preventing fibrosis in heart failure remains an attractive target to help preserve cardiac function. Matrix Metalloproteinases (MMPs) are a family of proteins that help regulate ECM deposition and degradation, many of which are produced by macrophages and other immune cells ( 114 ). In particular, MMP-9 has been implicated in a wide range of functions, including the degradation of ECM ( 115 ). During cardiac dysfunction, when the fibrotic response is excessive, MMP9 is produced to chaperone the correct degradation and disposition of molecules for proper ECM development and scar formation. However, it has been noted that the expression of MMP9 is reduced in patients with congestive heart failure ( 116 ), potentially indicating an impaired ability to degrade excess collagen and further highlighting the importance of extracellular matrix homeostasis.