Immune Mechanisms in the Pathophysiology of Hypertension

Abstract

Hypertension is the leading risk factor for morbidity and mortality worldwide. Despite current anti-hypertensive therapies, most hypertensive individuals fail to achieve adequate blood pressure control. Moreover, even with adequate control, a significant residual risk of cardiovascular events and associated organ damage remains. This suggests that current treatment modalities are not addressing a key element of the underlying pathology. Emerging evidence implicate immune cells as key mediators in the development and progression of hypertension. In this review, we discuss our current understanding of the diverse roles of innate and adaptive immune cells in hypertension, highlighting key findings from human and rodent studies. We explore mechanisms by which these immune cells promote hypertensive pathophysiology, shedding light on their multifaceted involvement. In addition, we highlight recent advances in our understanding of autoimmunity, HIV, and immune checkpoints, which provide valuable insight into mechanisms of chronic and dysregulated inflammation in hypertension.

Introduction

Nearly half of the adult U.S. population has hypertension.^1–3 Despite its pervasiveness, the etiology of hypertension remains poorly defined. Emerging evidence suggests that dysregulated inflammation plays a central role in hypertension and its associated damage to organs such as the kidney and vasculature. While immune cells and cytokines have been associated with hypertension in humans for decades, the recent development of immunological tools and animal models have allowed study of their roles in various experimental models. Emerging data from humans and rodent models have demonstrated that immune cells are not innocent bystanders but are key mediators of hypertension. While there are currently no anti-hypertensive drugs that directly target immune cells, recent innovations in immunotherapies and deepening understanding of the immune system offer opportunities for the development of new treatment approaches to not only lower blood pressure but prevent target organ damage. To avoid global immunosuppression, identification of key strategic targets and a comprehensive understanding of how immune mechanisms drive the pathophysiology of hypertension will be vital. In this review, we discuss the roles of innate and adaptive immune cells in hypertension, highlighting key evidence of their involvement in human and murine hypertension. In addition to the mechanisms by which they promote hypertension, we will discuss important properties of immune cells that contribute to hypertension pathogenesis such as memory and plasticity. Additionally, we review recent advancements in the fields of autoimmunity, HIV, and immune checkpoint therapies that have provided insight into mechanisms of immune activation and chronic inflammation in hypertension.

Innate Immune Cells in Hypertension

Innate immune cells, including monocytes, macrophages, dendritic cells, and neutrophils, are the primary sentinels and first responders of the immune system. They provide rapid, non-specific responses to infection or tissue damage.⁴ Innate immune cells also play key roles in activating adaptive immune cells and thus leading to more specific responses.⁴ Below, we discuss recent evidence from animal models and human studies that highlight key roles for a variety of innate immune cells in the pathogenesis of hypertension (Figure 1).

Figure 1:

Multiple hypertensive stimuli activate innate immune cells to promote inflammation leading to blood pressure elevations and end-organ damage. Innate immune cells, including monocytes, macrophages, dendritic cells (DCs), and neutrophils, are the first responders of the immune system and provide rapid, non-specific responses to tissue damage. With initial blood pressure elevations, factors such as increased endothelial cell stretch, high sodium concentrations, sympathetic nervous system (SNS) activation, and angiotensin II promote innate immune cell activation. As a result, monocyte/macrophages and DCs exhibit increased reactive oxygen species (ROS), isolevuglandin (IsoLG) adduct formation, and release of inflammatory cytokines that damage the surrounding tissue. Macrophages have diverse roles in hypertension. Renal macrophages have been found to promote juxtaglomerular (JG) production of renin. In response to osmotic stress, dermal macrophages secrete VEGF-C, promoting lymphangiogenesis. In DCs, IsoLG adducts can form neoantigens in association with increased surface expression of co-stimulatory molecules CD80 and CD86, suggesting an augmented ability to prime the adaptive immune response. Potential roles for neutrophils and mast cells in hypertension are less well defined. Neutrophils isolated from hypertensive patients display elevated superoxide anion release and increased neutrophil extracellular trap formation (NETosis). NETs have also been observed at sites of hypertensive end-organ damage in rodent models of hypertension. Elevated immunoglobulin E (IgE) in hypertension can also activate mast cells leading to enhanced secretion of interleukin-6 (IL-6).

Monocytes and Macrophages

Circulating monocytes and tissue-resident macrophages have diverse roles in the pathogenesis of hypertension. Following vascular damage resulting from elevated blood pressure in hypertension, monocytes and other inflammatory cells are recruited to target organs including the heart, kidney, and vasculature.^5–9 Upon tissue infiltration, monocytes can differentiate into macrophages and dendritic cells (DCs) or become activated with minimal phenotypic change. Monocyte-derived cells display remarkable diversity, allowing them to orchestrate intricate immune responses and contribute to tissue homeostasis. In response to cellular injury and stress “danger signals” or damage-associated molecular patterns (DAMPs), macrophages play important roles in clearing and repairing damaged tissue. Coordination of these pro-inflammatory and pro-resolution phases is tightly regulated. During hypertension, this balance seems to be dysregulated, resulting in prolonged and disproportionately pro-inflammatory responses. As a result, activated macrophages release inflammatory cytokines and reactive oxygen species (ROS) that damage the surrounding tissue.⁴ Monocytes also interact with activated platelets by forming prothrombotic monocyte-platelet aggregates (MPAs) which are found to be elevated in hypertensive patients.^10,11 Binding of activated platelets, which can respond to shear stress and hormones from the renin-angiotensin-aldosterone system, have been found to promote monocyte maturation into proinflammatory phenotypes and cross-presenting DCs.^12–14

Macrophages have been implicated in promoting blood pressure (BP) elevation and hypertensive end-organ damage in both murine and human studies. In humans, increased macrophage infiltration in the kidneys correlates with increased blood pressure, renal injury, and renal fibrosis.¹⁵ Mice deficient in macrophages were found to be protected from BP elevation and vascular damage as evident by their blunted levels of vascular remodeling, vascular superoxide production and preserved endothelial function.^16,17 Likewise, deletion of monocytes completely prevents the hypertensive response and vascular dysfunction caused by the pro-hypertensive peptide angiotensin II (Ang II).¹⁶ Monocytes have been implicated in modulating endothelial levels of tetrahydrobiopterin, a critical co-factor for the nitric oxide synthases, thus promoting vascular dysfunction in experimental hypertension.¹⁸ More recent work by Oh et al demonstrated that macrophages experiencing endoplasmic reticulum stress directly promote juxtaglomerular cells to secrete renin.¹⁹ In mice with myeloid-specific knockout of vitamin D receptor, macrophages are more susceptible to sustained ER stress. Renal infiltration of these macrophages, through their increased release of miR-106b-5p, stimulate juxtaglomerular cells to secrete renin, causing these mice to develop renin-dependent hypertension.¹⁹

Circulating monocytes from hypertensive patients have an augmented, pro-inflammatory phenotype.^20,21 Monocytes from hypertensive humans exhibit increased isolevuglandin (IsoLG) protein adduct formation that drive production of cytokines like interleukin-1 beta (IL-1β), interleukin-23 (IL-23), and interleukin-6 (IL-6).²² These changes likely stem from various factors such as high salt concentrations and crosstalk with endothelial cells (ECs) activated in response to hypertensive stretch of the vasculature. In response to stretch from increased hemodynamic pressure, ECs undergo transcriptional changes, which alter their interactions with immune cells. Using uniaxial mechanical stretch to model this phenomenon, Loperena et al demonstrated that hypertensive stretch of human aortic ECs activates monocytes and promotes their switch to an antigen presenting phenotype.²³ Human monocytes exposed to these stretched ECs had increased expression of pro-inflammatory cytokines including IL-6, IL-1β, IL-23, CCL4, and TNFα as well as an enhanced ability to drive proliferation of autologous T cells.²³ Macrophages can also be altered by exposure to high salt and osmotic stress, which can be encountered when these cells enter the renal medulla and deeper renal cortex. Macrophages exposed to high concentrations of sodium displayed enhanced production of inflammatory cytokines.²⁴ Adding a layer of complexity to their roles in hypertension, dermal macrophages have been demonstrated to be protective in conditions of high sodium. Machnik et al have reported that during high-salt loading, sodium concentrations in the skin exceed the storage capacity of glycosaminoglycans (GAGs), leading to hypertonicity.²⁵ They demonstrated that in response to this osmotic stress, dermal macrophages secrete vascular endothelial growth factor C (VEGF-C).²⁵ VEGF-C-mediated lymphangiogenesis enables drainage of sodium back into the circulation for removal by the kidneys. This work suggests a new paradigm by which macrophages and salt storage in the skin buffer BP elevation during hypertension. However, subsequent findings have challenged this role of dermal VEGF-C and lymphangiogenesis in controlling BP.²⁶ Thus, more studies are needed to determine whether and how skin macrophages buffer BP in response to high sodium.

Finally, myeloid-derived suppressor cells (MDSCs), a heterogeneous population of immature and immunosuppressive myeloid cells, accumulate in hypertension and potentially serve a counter-regulatory mechanism. In multiple murine models of hypertension, Shah et al report the accumulation of splenic and renal CD11b⁺Gr1⁺ myeloid cells after hypertension induction.²⁷ Adoptive transfer of MDSCs blunted Ang II-induced blood pressure elevations, suggesting a protective role of MDSCs in hypertension.²⁷ While both granulocytic and monocytic MDSCs were increased in hypertension, granulocytic MDSC displayed the most predominant increase.²⁷ Collectively, these studies demonstrate that monocytes and macrophages are not only diverse but likely play variable roles in hypertension that warrant further investigation.

Dendritic Cells

Dendritic cells (DCs) are professional antigen presenting cells that act as a link between the innate and adaptive immune responses. In the immature state, DCs survey peripheral tissues for evidence of pathogens, tissue damage, and resultant antigens. DCs potently activate the adaptive response by presenting antigen in the context of co-stimulation molecules and polarizing cytokines. In DCs isolated from hypertensive mice, costimulatory molecules such as CD86 are upregulated and their blockade prevents hypertension development.²⁸ This requirement for a co-stimulatory signal suggests that the inflammation underlying hypertension is antigen-mediated, rather than a nonspecific inflammatory event.

A likely source of antigens in hypertension is the aberrant post-translational modification of self-proteins. In 2014, our group discovered that oxidative lipid peroxidation of arachidonic acid leads to formation of immunogenic isolevuglandin (IsoLG)-protein adducts resulting from IsoLG covalent modification of lysine residues.²² In hypertensive humans and murine models, IsoLG-protein adducts accumulate in DCs. IsoLG-adducted peptides are presented in the context of both class I and class II major histocompatibility complexes (MHC) and promote renal and vascular dysfunction leading to BP elevation.²² In humans, IsoLG-adducts within DCs are associated with increased surface expression of co-stimulatory molecules CD80 and CD86 and increased production of pro-inflammatory cytokines. IsoLG-positive DCs produce higher levels of cytokines IL-6, IL-1β, and IL-23, which in turn drive differentiation of naïve T cells into pro-inflammatory Th17 cells. Adoptive transfer of IsoLG⁺ DCs sensitized mice to subpressor dose of Ang II and promoted T cell proliferation and cytokine production. Interestingly, DC IsoLG formation can be induced by several factors including sodium entry through the epithelial sodium channel (ENaC), sympathetic stimuli, and interaction with ECs.^9,22,29 DCs sense increased sodium levels via serum/glucocorticoid kinase 1 (SGK1) leading to activation of NADPH-oxidase and accumulation of IsoLGs.³⁰ In addition to high interstitial sodium, DCs form IsoLG protein-adducts in response to norepinephrine in a dose-dependent manner. In murine Ang II-induced hypertension, IsoLG formation in DCs was prevented by renal denervation.⁹

Our group also demonstrated that DCs form IsoLG adducts as a result of interactions with human aortic endothelial cells undergoing hypertensive stretch. In response to increased stretch as in hypertension, ECs secrete GAS6 which promotes transition of CD14⁺ classical monocytes into a subpopulation of DCs expressing Axl and sialic acid-binding Ig-like lectin-6 (SIGLEC-6) and higher IsoLG formation. In line with this, hypertensive humans have a higher frequency of these Axl⁺ SIGLEC-6⁺ DCs in the circulation, and mice lacking Axl exhibit blunted hypertensive responses to Ang II.³¹ Taken together, these findings demonstrate a key role of DCs, at least in part through IsoLG formation, in promoting adaptive immune cell activation and the development of hypertension.

Neutrophils

Although less well studied than other immune cells in hypertension, recent evidence supports a role for neutrophils in hypertension. Neutrophils comprise the largest proportion of circulating leukocytes and are typically the initial responders to tissue damage or infection. They mediate their antimicrobial and pro-inflammatory roles by releasing ROS and neutrophil extracellular traps (NETs).³² Analysis of a Japanese population demonstrated a positive correlation between elevated neutrophil counts and the incidence of hypertension.³³ In addition, in a cross-sectional study using the large-scale population within UK Biobank, Siedlinski et al demonstrated that an increased neutrophil count is associated with increased risk of developing hypertension.³⁴

Neutrophils isolated from hypertensive patients display elevated superoxide anion release compared to normotensive controls.³⁵ This superoxide anion release can occur in the context of neutrophil extracelluar trap formation (NETosis), which is a process whereby a network of chromatin fibers decorated with microbial peptides and proteases is released. Not only are NETs toxic to the endothelium but they also contain tissue factor and can initiate coagulation and vascular damage.³⁶ In addition, NETosis involves neutrophils externalizing autoantigens and releasing immunostimulatory cytokines such as IL-17A. NETs also activate the inflammasome in adjacent macrophages, leading to release of IL-1β and IL-18.³⁷ These cytokines promote further NETosis, creating a feed-forward loop of inflammation that results in organ damage. Consistent with this, NETs have been observed at sites of hypertensive end-organ damage, including the fibrotic renal interstitium and aneurysmal aorta biopsies of hypertensive humans.³⁸ Chrysanthopoulou et al showed that exposure of neutrophils to Ang II stimulates NETosis in a ROS-dependent manner.³⁸ A more recent study demonstrated an increase in NETosis in renal neutrophils in hypertensive mice and showed that IsoLGs play a key role in NET formation.³⁹ Treatment with IsoLG scavenger 2-HOBA resulted in nearly complete abrogation of NETosis both in vitro and in the kidneys and aortas of hypertensive mice.³⁹ These findings suggest a role for neutrophils in the pathogenesis of hypertension; however, whether they play a causal role will require further studies.

Mast Cells

To date, there is limited evidence that suggest mast cells play a potential role in hypertension. Immunoglobulin E (IgE), which promotes mast cell activation, is increased in hypertensive humans and mice. IgE activation of mast cells promotes Ang II-induced hypertension via increased secretion of IL-6.⁴⁰ Given that B and T cells of the adaptive response are needed to produce IgE, the findings of Ge et al suggest that mast cells may play a secondary role in propagating BP elevation in hypertension as part of an innate response after adaptive immune cell activation. This highlights the dynamic coordination between adaptive and innate immune responses underlying hypertension.

Adaptive Immune Cells in Hypertension

Adaptive immune cells mediate more specific, albeit delayed, responses to infections or tissue damage. The notion that T cells are involved in hypertension gained broad acceptance following a seminal study by Guzik and colleagues. In this study, they demonstrated that Rag1^−/− mice, which lack T and B lymphocytes, are resistant to Ang II and DOCA-salt induced hypertension. Interestingly, adoptive transfer of T cells, but not B cells, restored hypertensive responses.⁵ Subsequent research by various groups, using various models of immunodeficient mice, have further supported the role of T cells in hypertension. In Dahl salt-sensitive rats, Mattson et al demonstrated that Rag1 or CD247 deficiency exhibited protection against BP elevation, glomerular damage, and albuminuria with high-salt feeding.^41,42 Similarly, studies by Crowley et al showed that mice with severe combined immunodeficiency (SCID) have reduced cardiac hypertrophy and albuminuria following Ang II infusion.⁴³ While these studies established a key pathogenic role for T cells in hypertension, the impracticality of global T cell ablation as a therapeutic approach calls for a more granular understanding of the distinct roles of different T cell subsets.

T cells can be classified based on their surface markers, lineage determining transcription factors, cytokine profile, maturation state, and metabolic profile. The most well-known and simplified classifications are between CD8⁺ cytotoxic T cells, which directly kill infected and damaged cells, and CD4⁺ helper T cells, which release cytokines to coordinate and augment immune responses. Below, we discuss the different roles of CD8⁺ T cells and CD4⁺ T cell subsets in the pathogenesis of hypertension (Figure 2).

Figure 2: Multiple stimuli including innate immune cells activate T cells to promote hypertension development and related end-organ damage.

Increases in a variety of factors such as angiotensin II (Ang II), sympathetic outflow, reactive oxygen species (ROS), and interstitial sodium lead to immune cell activation in hypertension. This includes activation of adaptive immune cells, either directly or indirectly through antigen presenting cells (APCs). Cells of the adaptive immune system secrete factors including cytokines that promote hypertension and its associated end organ damage. CD8⁺ T cells primarily secrete interferon gamma (IFNγ) while CD4⁺ T cell subsets secrete characteristic cytokines including IFNγ by Th1 cells, interleukin-17A (IL-17A) by Th17 cells, and interleukin-21 (IL-21) by T follicular helper (Tfh) cells. These cytokines promote vascular dysfunction and remodeling and impair renal natriuretic function leading to blood pressure elevations and end organ damage in hypertension. In the vasculature, these cytokines inhibit nitric oxide production in endothelial cells, stimulates collagen synthesis by adventitial cells and promotes hypertrophic remodeling of mesenteric arteries by stimulating vascular smooth muscle cells (VSMC). In the kidney, the cytokines promote renal injury and alter sodium transporters. Tregs produce cytokines such as interleukin-10 (IL-10) and function to promote peripheral tolerance and limit hypertension development. IL-21 augments T cell production of IL-17A and IFNγ and limits Treg function, skewing the Th17/Treg axis towards a pro-inflammatory phenotype. Together, IL-17A, IFNγ, and IL-21 promote blood pressure elevation and end organ damage observed in hypertension. Gamma delta (γδ) T cells, and potentially innate lymphoid cells as well (ILCs), can also release cytokines such as IFNγ and IL-17A to enhance hypertension development.

CD8⁺ T cells

Activated CD8⁺ T cells release cytokines and mediate contact-dependent cytotoxicity. Trott et al showed that CD8^−/− mice are protected from Ang II- induced hypertension. Notably, analysis of the TCR repertoire in mice with Ang II-induced hypertension demonstrated a significant increase in clonality among CD8⁺ T cells in the kidneys, but not in blood vessels or the spleen. In addition, CD8^−/− mice demonstrated less renal microvascular rarefaction and a preserved capacity to secrete a sodium load in response to Ang II.⁴⁴

A putative mechanism by which CD8⁺ T cells are activated in hypertension is via the mineralocorticoid receptor (MR), a major regulator of renal sodium handling and BP. Sun et al demonstrated that stimulation of the MR on CD8⁺ T cells promotes IFNγ production via interactions with transcription factors NFAT1 and AP-1. T cell specific deletion of MR caused a significant decrease in BP and vascular responses to Ang II suggesting the MR plays an important role in promoting T cell IFNγ production in hypertension.¹³³ Interestingly, Youn et al showed that, compared to age- and sex- matched controls, humans with hypertension have an increase in pro-inflammatory memory CD8⁺ T cells expressing markers of immunosenescence.¹³⁴ Immunosenescence is characterized by gain of CD57 and/or a loss of CD28 expression. CD28 is progressively and irreversibly downregulated with repeated antigen stimulation and is prominent with human age-related and autoimmune pathologies.⁴⁵ Loss of CD28 increases the propensity for inappropriate activation of CD8⁺ T cells independent of the T cell receptor and co-stimulation signals. Yuon et al observed that these immunosenescent CD8⁺ T cells have increased expression of perforin, granzyme B, IFNγ, and TNFα.¹³⁴ While it is not clear how perforin and granzyme B contribute to hypertension, their roles in promoting apoptosis could lead to the release of damage associated molecular patterns (DAMPs) and autoantigens. Notably, T cell loss of CD28 during aging occurs in humans and primates, but not mice, suggesting that additional human studies are needed to better understand this phenomenon.

CD4⁺ T cells and their cytokines

CD4⁺ T cells have a diverse and broad range of effector functions. Upon recognition of MHC II-peptide complexes, naïve CD4⁺ T cells undergo clonal expansion and differentiation. CD4⁺ T cells are context-dependent such that they can differentiate into pro- or anti-inflammatory subsets depending on the cytokine milieu of the microenvironment in which they are primed. There are multiple subsets of CD4⁺ T cells including: Th1, Th2, Th17, T follicular helper cells (Tfh), and regulatory T cells (Tregs). Each subset mediates their effector functions largely by releasing cytokines. Th1 cells are characterized by their production of IFNγ and respond to intracellular pathogens; Th2 cells produce IL-4, IL-5, and IL-13 and respond to extracellular pathogens; Th17 cells produce IL-17A and IL-21 and are involved in autoimmunity and respond to extracellular pathogens; Tfh cells produce IL-21 and function to promote class switching and affinity maturation in B cells; and Tregs produce IL-10 and function to promote peripheral tolerance and regulating the immune response to limit collateral damage. Hypertension is generalized as a skewing of CD4⁺ T cells towards pro-inflammatory subsets and away from the anti-inflammatory Treg subset. The notion that Th1, Tfh, and Th17 subtypes are pathogenic in hypertension is supported by findings that the primary cytokines released by these cells (IL-17A, IFNγ, and IL-21, respectively) are elevated in hypertensive mouse models and in humans with hypertension.^46–48 As discussed below, these cytokines are not only released by CD4⁺ T cells, but also CD8⁺ T cells and innate lymphoid cells (ILCs), and clearly play a role in the genesis of hypertension. They alter vascular function and remodeling and also impair natriuretic functions of the kidney by modulating expression of sodium transporters, either through release of IFNγ and/or via direct contact with renal tubular cells.^49,50 Below, we discuss these mechanisms further.

IL-17A

A key role for IL-17A in hypertension and vascular dysfunction was first demonstrated by Madhur and colleagues. In mice, they demonstrated Ang II infusion not only increases T cell production of IL-17A, but also found that IL-17A^−/− mice exhibit blunted elevations in BP, preserved endothelium-dependent vasodilation, decreased superoxide production, and decreased aortic T cell infiltration in response to Ang II infusion.⁵¹ Underscoring the involvement of IL-17A in human hypertension, circulating CD4⁺ T cells of hypertensive patients had markedly increased production of IL-17A which correlated with increases in systolic BP in pre-hypertensive individuals.^47,52

IL-17A has pleiotropic mechanisms by which it promotes elevations in BP and end-organ damage. In the vasculature, IL-17A acts on both endothelial cells (ECs), vascular smooth muscle cells (VSMCs) and adventitial cells to drive vascular dysfunction. In ECs, IL-17A inhibits production of nitric oxide which reduces vasodilation.⁵³ IL-17A stimulates collagen synthesis by adventitial cells, leading to aortic stiffening and loss of compliance which can elevate central and systolic BP.⁵⁴ Orejudo et al discovered infusion of IL-17A induced inward hypertrophic remodeling and stiffness of mesenteric arteries by modulating VSMC phenotype.⁵⁵ In the kidneys, IL-17A plays a major role in renal injury both at the glomerular and tubular levels. Administration of an IL-17A neutralizing antibody reduced murine renal fibrosis and prevented hypertension-induced albuminuria, an indicator of glomerular damage.⁵⁶ Systemic infusion of IL-17A also induces tubular renal damage as evidenced by increased expression of neutrophil gelatinase-associated lipocalin (NGAL) and increased renal inflammation.⁵⁶ In addition, IL-17A increases sodium transporter expression in renal tubular cells, leading to inappropriate increases in salt and water reabsorption resulting in increases in intravascular volume and BP.⁵⁷ Interestingly, this effect appears to be biphasic, depending on the duration of Ang II infusion. During the early stages, IL-17A increases NHE3 expression in the proximal tubule. After 4 weeks of Ang II, IL-17 stimulates activation of distal sodium transporters ENaC and the sodium-chloride symporter NCC.⁵⁷ Moreover, IL-17A also promotes renal immune infiltration by inducing chemokine production, thereby creating a positive feedback-loop of renal inflammation.⁵⁶

IFNγ

IFNγ is a pleiotropic cytokine that is canonically expressed by CD4⁺ Th1 cells but can also be expressed by CD8⁺ T, natural killer (NK), and type 1 innate lymphoid cells. IFNγ functions include activating macrophages, upregulating expression of MHC-I, and enhancing the production of nitric oxide.⁵⁸ It is able to mediate these diverse effects by activating a variety of transcription factors to carry out a complex pattern of cell-specific gene regulation. Many of its effects also occur via synergy with other cytokines such as TNFα.⁵⁸ Given these diverse effects, it is perhaps unsurprising that the role of IFNγ in hypertension appears to be context dependent. While IFNγ deficiency in mice abrogates cardiac hypertrophy in Ang II-induced hypertension, hypertrophy is exacerbated with IFNγ deficiency in other murine models of hypertension.^6,59,60

In the kidney, IFNγ promotes renal inflammation and injury and increases expression of sodium transporters. For example, Marko et al demonstrated that mice with IFNγ receptor deficiency have reduced renal inflammation and tubulointerstitial damage in response to Ang II.⁶ In addition, Kamat et al demonstrated that mice deficient in IFNγ exhibited improved natriuretic responses to a saline challenge after Ang II infusion.⁴⁹ Profiling of renal transporters revealed that IFNγ deficiency abrogates increases in phosphorylation of NKCC and sodium chloride cotransporter (NCC) in the distal tubule in response to Ang II, potentially explaining the enhanced handling of a sodium load.⁴⁹ Another potential explanation is that renal tubule cells, in response to IFNγ, upregulate expression of programmed death ligand 1 (PD-L1) which increases the avidity by which they interact with CD8⁺ T cells. As a result, tubule cells increase NCC expression in a contact-dependent manner.⁶¹

Several studies of IFNγ deficiency in murine models of hypertension have demonstrated a role for IFNγ in elevating BP,^62,63 while other studies have demonstrated no effect on BP.^6,64 This discrepancy may stem from several factors including dosage of Ang II and experiment duration. At very high doses of Ang II, blood pressure may plateau, potentially masking differences between experimental groups. Another potential explanation is the use of different mouse strains as various backgrounds are known to significantly alter the direction of CD4⁺ T cell polarization.⁶⁵ C57BL/6 mice used by groups such as Kamat et al are Th1-dominant.^49,66 In contrast, experiments by Garcia et al were performed on Balb/c mice which are Th2-dominant.⁵⁹ Th1 and Th2 cells have an antagonistic relationship whereby IFNγ inhibits Th2 responses.⁵⁸ Given this, further studies are needed to determine whether targeting IFNγ would lead to a dysregulated, aberrant Th2 response that may impact hypertension development or other conditions. Ultimately, these studies underscore the complexity of IFNγ effects in hypertension that warrants further investigation.

IL-21

For B cells to secrete antibodies, naïve B cells must mature and differentiate into secreting plasma cells. To facilitate this process, Tfh cells secrete IL-21 to promote class switching and affinity maturation. Due to this requirement of IL-21 for B cell maturation, increased immunoglobulins observed in hypertension may be reflective of increased IL-21 signaling. Elevations in serum immunoglobulins (IgGs), albeit modest, are observed in hypertensive individuals.⁶⁷ While this suggests a role for B cells in hypertension, adoptive transfer of B cells into Rag1^−/− mice was not sufficient to recapitulate hypertension and IgG transfer and B cell deficiency did not alter Ang II-induced hypertension.^5,68 Since other studies have suggested a pro-hypertensive role for B cells,⁶⁹ further studies are needed to definitively determine their role.

IL-21, which is produced by both Tfh and Th17 subsets, has roles in hypertension that extend beyond B cell maturation. Dale et al demonstrated that circulating CD4⁺ T cells produce higher levels of IL-21 in both murine and human hypertension.⁴⁶ This increased production of IL-21 not only correlated with a higher systolic BP but also greater IL-17A production. In response to Ang II, IL-21^−/− mice not only had lower BP and reduced vascular dysfunction but also blunted CD4⁺ T cell production of IL-17A and IFNγ. This could be explained by effects of IL-21 to influence T cell activation by inhibiting Foxp3 and enhancing IL-17A expression, thereby skewing the Th17/Treg axis towards a pro-inflammatory phenotype.^70,71 Thus, the role of IL-21 in determining the balance of pro- and anti-inflammatory CD4⁺ T cell fate positions it as an attractive therapeutic target.

Regulatory T cells (Tregs)

Tregs are a specialized subset of CD4⁺ T cells that primarily function to promote peripheral tolerance and limit inflammation. These cells are defined by their expression of the transcription factor forkhead box protein 3 (FOXP3), which not only marks Treg identity but also mediates their immunosuppressive function.⁷² Lack of FOXP3 in humans and mice results in development of autoimmune-like pathology in various tissues.⁷³ Given the role of inflammation in hypertension, the ability of Tregs to suppress adaptive and innate immune responses suggests that Tregs may be protective in hypertension. Indeed, studies using adoptive transfer of Tregs have shown reduced end-organ damage in Ang II-induced hypertension including reduced cardiac hypertrophy and fibrosis.⁷⁴ Barhoumi et al demonstrated that adoptive transfer of Tregs improved vascular relaxation, arterial stiffness, and immune infiltration in the aortic adventitia and periadventitial fat.⁷⁵ Interestingly, Tregs may protect against hypertensive renal damage in a sexually dimorphic manner. Tipton and colleagues demonstrated that, compared to their male counterparts, female spontaneously hypertensive rats (SHR) have enhanced renal Treg infiltration and treatment with the Treg cytokine IL-10 reduces BP in male but not female rats.^76,77 These studies underscore a protective role of Tregs in hypertensive end-organ injury. In contrast, studies on the effect of Tregs on BP has produced mixed results, with some studies demonstrating reductions in BP after adoptive transfer and others showing no effect.^{44,74,75,78–82} This may be attributed to several factors including technical differences in timing and dose of Tregs transferred and phenotypic properties of the Tregs including whether they were cultured prior to transfer. Interestingly, Tregs can exhibit lineage instability in inflammatory settings whereby they lose their Foxp3 expression and adopt a pro-inflammatory exTreg phenotype that resembles conventional effector T cells.⁸³ Notably, exposure to high salt (HS) environments have been found to promote Treg/Th17 imbalance via upregulation of SGK1. SGK1 signaling promotes Th17 differentiation while antagonizing Treg stability.^84,85

Given the dynamic adaptability and heterogeneity of Tregs, their role in hypertension may be subset- and tissue- specific. Our recent deep phenotyping of immune cell changes in human hypertension revealed a decrease in total circulating Tregs, along with a selective decrease in a subset of Tregs expressing the chemokine receptor CCR10.⁸⁶ Similarly, others have found that humans with hypertension have a significant decrease in a subset of circulating Tregs expressing Helios, a marker of Treg stability.^84,85 Further underscoring the heterogeneous role of Tregs in hypertension, Fabbiano et al demonstrated that CD39⁺ Tregs blunt Ang II-induced elevations in mean arterial pressure and promote neutrophil apoptosis by hydrolyzing ATP.⁸⁷ This layer of immune regulation is intriguing as recent work by Shokoples et al demonstrate that ATP may act as a DAMP during hypertension.⁸⁸ While ATP is usually quickly converted to immunosuppressive adenosine by ectonucleotidases CD39 and CD73, excessive accumulation of extracellular ATP during cellular injury promotes binding to its low affinity purinergic receptor P2RX7.⁸⁹ P2RX7 signaling has been found to activate the NLRP3 inflammasome, amplify Ca²⁺ influx at immunological synapses during T cell activation, and may be a key metabolic switch that promotes generation of memory CD8⁺ T cells.⁸⁹ Plasma ATP levels have been observed to be elevated in hypertensive mice and in a population of hypertensive Chinese patients.⁹⁰ Shokoples et al demonstrated that disruption of P2RX7 either by genetic inactivation or antagonism attenuated Ang II-induced BP elevation, vascular injury and reduced infiltration of activated T cells in the thoracic aorta. Importantly, they demonstrated that P2RX7 signalling is required for the development of effector memory cells during hypertension.⁸⁸ Interestingly, ATP-P2RX7 signalling on Tregs has been found to promote loss of FOXP3 and conversion into Th17-like cells in a model of inflammatory bowel disease.⁹¹ Collectively, this suggests extracellular ATP levels and P2RX7 signaling may be a key immune modulatory node between tolerance and dysregulated inflammation. Ultimately, more granular profiling of changes in Treg subsets, particularly in relevant tissues in humans, combined with further mechanistic studies will help refine their role in hypertension which will be key to unlocking their therapeutic potential in hypertension.

T Cell Memory

A key feature of the adaptive response is its ability to establish immunological memory following initial antigen exposure. Memory T cells have lower activation thresholds and are less co-stimulation dependent.⁹² This allows for the mounting of faster and more robust responses against subsequent exposures. Prior studies have demonstrated a role for memory T cells in hypertension in mice.^22,93,94 This may be particularly relevant to the residual cardiovascular risk observed in hypertensive patients even with adequate blood pressure control and may explain the increased long-term risk for cardiovascular disease in women who have experienced pre-eclampsia (Box 1).⁹³

Box 1: Residual risk and T cell memory.

Current therapeutic approaches for hypertension are designed to lower BP to a target of less than 130/80 mm Hg based on recent guidelines.² However, even with adequate control of BP, a residual 1.5-fold increased risk of cardiovascular events remains.¹³⁰ This elevated risk is not related to antihypertensive drug class used, nor can it be explained by the presence of pre-existing subclinical end-organ damage.^129,130 Thus, the etiology of this residual risk remains largely unexplained.

A plausible explanation for this residual risk based on human and animal studies involves immune cell memory. In animal models of hypertension, a single hypertensive stimulus gives rise to memory T cells that persist and exhibit greater activation in response to a second stimulus.^47,93 These activated memory T cells infiltrate target organs like the kidneys and vasculature and release cytokines that promote dysfunction and contribute to further BP elevation.^93,131 Hence, antigen reactive T cells may be “primed” with initial BP elevation and on repeated challenges perpetuate chronic elevations in BP and end-organ damage in hypertension. Indeed, patients with pre-eclampsia who have initial elevations in BP during pregnancy, despite normalization of BP after delivery have elevated post-pregnancy risk of chronic hypertension and cardiovascular events.¹³² Further studies are needed to determine whether these observations in pre-eclampsia and residual risk in primary hypertension are due to persistence of memory T cells formed during initial BP elevation.¹³²

γδ T cells

In contrast to conventional αβ T cells, γδ T cells have T cell receptors (TCRs) consisting of γ and δ subunits. These cells are considered unconventional T cells because they exhibit features of both innate and adaptive immune cells. For example, γδ T cells are enriched at epithelial and mucosal barriers and respond rapidly to pathogenic insults in these tissues.⁹⁵ They also exhibit features of antigen presentation, cytotoxicity, and the production of pro-inflammatory cytokines. These cells are also not activated by traditional MHC-dependent peptide antigen presentation. Caillon et al demonstrated that γδ T cells promote BP elevation and mesenteric artery vascular dysfunction in mice with Ang II-induced hypertension.⁹⁶ In addition, γδ T cells accumulate in the mesenteric arteries of hypertensive mice and adoptive transfer of these cells promotes BP elevation in response to subpressor doses of Ang II.⁹⁷ In humans, microarray profiling of blood samples revealed a positive correlation between γδ T cells and systolic BP.⁹⁶

Given that γδ T cells are not MHC-restricted, the exact mechanisms by which these cells are activated in hypertension remains unclear. Emerging evidence suggests that γδ T cells can detect stress-related self-antigens and metabolites via butyrophilins.^95,98,99 Further studies will be needed to determine whether butyrophillins activate γδ T cells in response to vascular stress in hypertension. A role for butyrophilins is supported by a study of 5,959 Japanese individuals which demonstrated that a genetic polymorphism of the butyrophilin subfamily 2 member A1 gene (BTN2A1) is associated with increased hypertension risk.¹⁰⁰ Ultimately, although a role for γδ T cells in hypertension has been clearly demonstrated in murine models, more work is needed to determine mechanisms by which they are activated and their precise role in hypertension.

Innate Lymphoid Cells (ILCs)

Innate lymphoid cells (ILCs), as implied by their name, exhibit features of both innate immune cells and conventional T lymphocytes. Multiple subsets of ILCs have been identified based on distinct transcription factor and cytokine profiles including natural killer (NK) cells, ILC1s, ILC2s and ILC3s. These cells are considered the innate counterparts of CD8⁺ T cells, Th1, Th2 and Th17 subsets, respectively.¹⁰¹ Unlike conventional T cells, ILCs lack antigen-specific receptors, but have receptors that detect changes in the microenvironment, such as metabolites and cytokines released during tissue damage.¹⁰¹ Thus, ILCs not only secrete similar cytokines to those of conventional T cells but they respond faster and with a lower activation threshold. Interestingly, ILCs can also form immunological memory and modulate conventional T cells, albeit through unknown mechanisms.^96,102 Little is known about the role of ILCs in hypertension, but given their unique ability to carry out adaptive effector functions such as release of IL-17A and IFNγ but with a lower threshold for activation, their role in hypertension warrants further investigation.

Taken together, findings from animal models and correlative evidence in humans suggests a key role for a variety of innate and adaptive immune cells in the pathogenesis of hypertension. These cells coordinate responses to vascular damage resulting from initial BP elevation, and subsequently promote inflammation in target organs such as the vasculature, kidney, central nervous system, and heart. Dysfunction of these target organs, in turn, exacerbates BP elevation, creating a vicious cycle with immune cells playing a central role (Figure 3). Studies specifically targeting immune cells in human hypertension will be key to translating the plethora of findings from animal models into potential new therapies.

Figure 3: Immune cells play a central role in driving blood pressure elevations and end-organ damage that feeds forward to promote further increases in blood pressure.

Dysfunction of a variety of organs including the central nervous system (CNS), kidneys, vasculature, and heart leads to blood pressure elevations in hypertension. In addition, activation of immune cells can promote blood pressure elevations, at least in part through infiltration and damage of these hypertension-related organs. Blood pressure elevations in hypertension also directly promotes end-organ damage in the kidneys, heart, and vasculature leading to further immune cell infiltration and activation in a vicious cycle. Together, these processes suggest a central role for immune cells in the pathogenesis of hypertension that may be amenable to therapeutic targeting.

HIV

Due to advances in antiretroviral therapy (ART), people infected with HIV are living longer. As such, we are now seeing the non-infectious consequences of HIV driven by chronic inflammation. One notable consequence is the elevated risk of hypertension and cardiovascular disease. A 2018 meta-analysis demonstrated that people living with HIV are twice as likely to develop cardiovascular disease compared to uninfected, age matched adults.¹⁰³ As discussed, CD4⁺ T cells are critical in the pathophysiology of hypertension. HIV infection causes CD4⁺ T cell counts to plummet while ART causes them to rise rapidly. In a cross-sectional study, Peck et al demonstrated the adverse effects of immune reconstitution on hypertension. Notably, increases in BP paralleled that of ART-mediated increases in CD4⁺ T cells. Patients with the highest incidence of hypertension were those who had undergone ART but had the lowest CD4⁺ T cell counts before initiating treatment. This suggests that ART-mediated CD4⁺ T cell reconstitution likely leads to activation and BP elevation.¹⁰⁴ In line with this, patients living with HIV but who had not started ART had significantly lower blood pressure than HIV-uninfected adults.¹⁰⁵ It can be difficult however to precisely delineate the relationship between HIV infection and blood pressure changes given effects of immune activation and sepsis-related cytokine release from secondary infections, depletion of the intestinal mucosal barrier, and direct effects of HIV-related proteins.¹⁰⁶ While more mechanistic studies are needed, immune reconstitution in HIV provides a unique model that supports a role for CD4⁺ T cells in driving hypertension development.

Chronic inflammation in HIV also promotes immune cell dysfunction. From two independent cohorts, Wang et al demonstrated that HIV⁺ patients who were not on ART, exhibited higher frequency of memory NK cells. These memory NK cells transcriptionally resembled memory T cells and phenotypically displayed enhanced cytotoxic capacity and production of IFNγ.¹⁰⁷ Formation of NK memory can occur in response to a specific antigen or non-antigen-specific inflammatory cytokine milieu.¹⁰⁸ Intriguingly, formation of NK memory cells is in part mediated by IL-18 and engagement of the NLRP3 inflammasome--- both of which have been implicated in hypertension and HIV.^109–112 Whether memory NK cells contribute to potentiation of inflammatory responses in hypertension warrants further investigation. In T cells, repetitive antigen stimulation and chronic inflammation can lead to loss of co-stimulatory molecule CD28.¹¹³ CD28⁻ T cells are characterized by enhanced production of effector molecules, senescent phenotype, and lower dependency on TCR signaling.¹¹³ Notably, CD28⁻CD8⁺ T cells are found to be enriched in both HIV⁺ and hypertensive patients.^114–116 A recurring theme throughout this review is the feed-forward loop of hypertensive stimuli, immune activation, and target organ damage. This iterative cycle may contribute to formation and accumulation of CD28⁻CD8⁺ T cells, further exacerbating inflammation. While more work is needed to elucidate the link between HIV and hypertension, HIV and ART provide a valuable model of chronic inflammation and immune reconstitution that can be leveraged to understand the contribution of inflammation to hypertension.

Immune Checkpoint Inhibitors (ICI)

The increasing use of immune checkpoint inhibitors (ICI) for the treatment of cancer provides unique insight into mechanisms of immune activation. Current FDA-approved ICIs primarily inhibit cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed death 1 (PD-1). These immune checkpoints function to dampen immune responses and promote T cell exhaustion to maintain self-tolerance.¹¹⁷ Blockade of these immune checkpoints results in reinvigorated immune responses against tumor antigens but can also cause immune-related adverse events including cardiovascular toxicity and renal injury.¹¹⁸ A recent study by our group demonstrated that patients with melanoma who underwent dual immune checkpoint inhibitor therapy targeting PD-1 and CTLA-4 had a significant increase in systolic BP after a 2-year follow-up.¹¹⁹ These patients experienced increased blood pressure even when accounting for minimal changes in glomerular filtration rate (GFR) using a multivariable linear regression analysis. These patients had normal GFR on average at baseline and after ICI therapy, so it remains likely that ICI therapy induces both blood pressure elevation and kidney injury to synergistically promote hypertension. This finding not only suggests the need for close monitoring of BP in patients with cancer receiving dual ICI therapy but also underscore the effects of augmented T cell activation on blood pressure. These findings highlight an opportunity to leverage the depth of understanding of immune responses in oncology for the development of new therapies to limit inflammation in cardiovascular disease.

Autoimmune diseases

The integral role of T cells, oxidative stress, immune infiltration of target organs, and cytokine-mediated inflammation in hypertension mimics the pathophysiology of several autoimmune diseases. Indeed, clinical associations between autoimmune disease and hypertension have been repeatedly observed. For example, Panoulas et al reported that 70.5% of patients with rheumatoid arthritis (RA) have hypertension¹²⁰ and in a meta-analysis hypertension is 1.5-fold more common in those with moderate to severe psoriasis.¹¹⁸ Similarly, systemic lupus erythematosus (SLE) patients are 52 times more likely to experience cardiovascular events than those without SLE, with hypertension being the greatest risk factor.¹²¹ Furthermore, our group recently reported increased IsoLG adduct formation within circulating monocytes of patients with SLE and in multiple SLE murine models.¹²² Treatment with 2-HOBA, an IsoLG scavenger, attenuated hypertension and renal injury as well as lupus autoantibody levels, underscoring oxidative stress and inflammation as common mechanisms linking hypertension to SLE. In a NZBWF1 murine model of SLE, development of hypertension parallels autoantibody levels despite low plasma levels of renin.^123,124 Treatment with a superoxide scavenger and a NADPH oxidase inhibitor reduced blood pressure and renal injury without reducing levels of autoantibodies. While it is clear inflammation and oxidative stress are important mediators of autoimmune disease and hypertension, more work is needed to understand the mechanistic links between these conditions.

Collectively, these studies in HIV, autoimmunity, and ICI therapy highlight key themes of immune activation and dysregulated inflammation that underlie hypertension pathogenesis. Observed BP alterations in these conditions of immune dysregulation provide insight into potential new anti-inflammatory treatments for hypertension. First is the potential repurposing of current anti-inflammatory treatments in autoimmune disease for hypertension. This approach capitalizes on clinical trials to demonstrate safety and anti-inflammatory efficacy of these drugs. Drugs such as mycophenolate mofetil (MMF) have shown benefit in reducing blood pressure in limited numbers of patients with autoimmune diseases such as SLE, psoriasis, and rheumatoid arthritis.^125,126 However, such approaches for the treatment of hypertension would be limited by their global immunosuppressive effects. Hence, more specific and targeted approaches are needed. Recent evidence indicates that targeted biologic therapies such as TNF alpha inhibitors in patients with RA can reduce BP, though this has not been universally observed.¹²⁷ As such, further research and development is needed to create more specific immune targeted therapies tailored for hypertension. Rigorous measurement of BP and reporting of incident hypertension in future studies of anti-inflammatory and immune-based therapies for autoimmune and other inflammatory conditions will also be key for further determining pathogenic immune mechanisms in human hypertension. Such insight can lead to potential new immunotherapies for hypertension and randomized clinical trials for the treatment of hypertension and its associated clinical sequalae. This is needed as significant deficits remain in the treatment of hypertension, including challenges with medication adherence, medication intolerance, and residual cardiovascular risk even with adequate control of blood pressure.^3,128,129 Development of new therapies that co-opt inherent features of the immune system such as memory, specificity, and amelioration of target organ damage have the potential to bring about a new paradigm for hypertension treatment.

Concluding Remarks

Hypertension is a disease of chronic and dysregulated inflammation. Ideal therapies would reduce blood pressure and limit hypertensive end-organ damage without triggering global immunosuppression. One strategy that would enable specificity and limit-off target effects is to identify the hypertension-specific antigens that promote inflammation. An alternative strategy would be to modulate immune cell function and phenotype. We have discussed several immune cell types that contribute to hypertensive inflammation, blood pressure elevation, and end organ damage, each of which could be a therapeutic target. However, to effectively respond to pathogens, immune cells coordinate with each other as an integrated system and are context-dependent by design. This integrated coordination is also true in hypertension. In lieu of a multi-targeted approach, there may be benefit in targeting central regulatory nodes of the inflammatory response. The challenge is to identify these key antigens and regulatory nodes such that the inflammation driving the pathophysiology of hypertension can be therapeutically targeted without compromising natural immune defenses.

Key Points.

• Emerging evidence from human and rodent studies suggests an important role for both innate and adaptive immune cells in the pathogenesis of hypertension.

• Innate immune cell release of inflammatory cytokines, production of reactive oxygen species, and neoantigen presentation results in vascular damage, renal injury, inflammation, and elevated blood pressure.

• There is a shift of adaptive immune cells towards pro-inflammatory roles in hypertension resulting in enhanced cytotoxicity and inflammatory cytokine production leading to impaired vascular and renal natriuretic functions.

• Immune cells coordinate as an integrated system, suggesting particular benefit to targeting regulatory nodes of the inflammatory response to treat hypertension.

• Current evidence supports a model whereby immune activation and resultant hypertension-related organ dysfunction act synergistically to amplify blood pressure and promote end-organ damage in hypertension.

• Recent advances in our understanding of autoimmunity, HIV, and immune checkpoint inhibitor therapies provide key insight into effects of immune activation and chronic inflammation in the development of hypertension.