Actions of Immune Cells in the Hypertensive Kidney

Abstract

Purpose of Review

Inflammatory processes play a critical role in the pathogenesis of hypertension. Innate and adaptive immune responses participate in blood pressure (BP) elevation and end-organ damage. In this review, we discuss recent studies illustrating mechanisms through which immune cells and cytokines regulate BP via their actions in the kidney.

Recent findings

Cells of the innate immune system, including monocytes, neutrophils, and dendritic cells (DCs), can all promote BP elevation via effects on kidney function. These innate immune cells can directly impact oxidative stress and cytokine generation in the kidney and/or present antigens to lymphocytes for the engagement of the adaptive immune system. Once activated by DCs, effector memory T cells accumulate in the hypertensive kidney and facilitate renal salt and water retention. Individual subsets of activated T cells can secrete tumor necrosis factor-alpha (TNF-α), interleukin-17a (IL-17a), and interferon-gamma (IFN-γ), each of which has augmented the elevation of blood pressure in hypertensive models by enhancing renal sodium transport. B cells, regulate blood pressure via vasopressin receptor 2 (V₂R)-dependent effects on fluid transport in the kidney.

Summary

Immune cells of the innate and adaptive immune systems drive sodium retention and blood pressure elevation in part by altering renal solute transport.

Introduction

Hypertension is the leading risk factor for the global burden of disease, and the number of people with high blood pressure exceeds 1 billion [1]. Despite pharmacological and non-pharmacological therapies that target the systemic vascular response, cardiac output, and sympathetic outflow, blood pressure in up to half of these patients remains poorly controlled [2]. Thus, novel therapies may be required to address biologically resistant hypertension.

Animal models and clinical studies indicate that immune cells promote the pathogenesis of hypertension via direct actions in the kidney, vasculature, skin, and nervous system [3–9]. While the etiology of salt-sensitive hypertension is complex, experimental models have shown that inappropriate immune activation plays an essential role in salt-induced blood pressure elevation [10–13]. Other groups and our own have tried to explore immune-mediated mechanisms that may underpin salt-sensitive hypertension. Within the innate immune system, myeloid cell populations, including the monocytes, neutrophils, and dendritic cells, can all promote blood pressure elevation by modulating the functions of the cardiovascular system and the kidney [14–16]. Lysozyme M-positive monocytes mediate angiotensin II-induced vascular dysfunction, and hypertension [14], whereas cyclooxygenase-2 in hematopoietic cells protects against salt-sensitive hypertension.

In contrast to the detrimental effect of pro-hypertensive macrophages in the kidney and vasculature during hypertension, macrophages in the skin may regulate sodium storage and maintain blood pressure homeostasis to limit salt-induced hypertension[17]. Another myeloid subset, the dendritic cell (DC), links innate and adaptive immune responses in hypertension by presenting specific antigens to T lymphocytes that, in turn, mediate blood pressure elevation [16]. Within the adaptive immune system, subsets of activated B and T lymphocytes can each increase blood pressure [18, 19]. T cells augment hypertension by promoting oxidative stress and sodium reabsorption in the kidney [20, 11]. A small subset of innate‐like T cells called “γδ T cells” participate in the development of hypertension by priming both innate and adaptive immune cells [21]. Renal natural killer (NK) cells are newly identified players involved in hypertensive pregnancy [22]. By contrast, T regulatory cells (Tregs) protect against hypertension and associated target organ damage [23, 24].

Different immune cell populations release inflammatory mediators that serve a range of functions during hypertension. Accordingly, in hypertensive animals, plasma levels of inflammatory mediators are elevated [25], whereas genetic deletion and/or pharmacological inhibition of TNF-α[26], IL-17a[27], IFN-γ[28], or IL-6[29] can limit the chronic hypertensive response. Similarly, patients with high blood pressure have increased circulating levels of several inflammatory cytokines, including IL-6, IL-1β, and TNF-α [30–33]. Over 120 immune cell subsets have been characterized in humans [34], and cells of the innate immune system, including monocytes, macrophage, dendritic cells, and neutrophils, produce pro-hypertensive cytokines including TNF-α, IL-6, and IFNs[35]. In turn, the innate immune system engages and activates cells of the adaptive immune system after receiving a hypertensive stimulus by processing and presenting antigens to naïve T lymphocytes. These T cells then differentiate into various subsets depending on the surrounding microenvironment. CD8⁺ T cells express IFN-γ, TNF-α, and IL-12[36], whereas CD4⁺ T helper cells, including Th1 cells produce IFN-γ and TNF-α [37]. By contrast, Th2 cells release IL-4, IL-5 and IL-13[38]. Th17 cells generate IL-17 and IFN-γ [39] whereas Tregs secrete the immunosuppressive cytokines IL-10 and TGF-β [40].

Through these cytokines and the generation of reactive oxygen species, immune cells can modulate the functions of different cardiovascular control centers. In the kidney, inflammatory cells instigate cellular injury or repair, increase local levels of oxidant stress, and elaborate cytokines to alter renal blood flow and sodium transporter expression or activity. In this review, we will focus on a set of experimental data illustrating how cells of the innate and adaptive immune systems modulate the hypertensive response through direct actions in the kidney.

Innate immune Cells in Hypertension

Neutrophils

Neutrophils are the most abundant cell type in the human blood and the first line of defense in the innate immune system. The number of neutrophils is increased in the aorta after 7 days of angiotensin (Ang) II-induced hypertension [14]. However, the adoptive transfer of neutrophils does not restore the hypertensive response in mice depleted of myeloid cells [14]. Nevertheless, T regulatory (Treg) cells promote direct apoptosis of neutrophils, which attenuates hypertension during activation of the renin-angiotensin system (RAS), suggesting that neutrophils in this context may foster BP elevation [41]. Within a Japanese population, elevated neutrophil levels correlated significantly with the risk of developing hypertension [42], and increased neutrophil to lymphocyte ratios predicted an increased incidence of hypertension [43]. Although the numbers of neutrophils in the kidney are increased during hypertension [44], we are not aware of direct evidence showing that neutrophils in the kidney regulate blood pressure.

Monocytes

Monocytes can release pro-inflammatory cytokines such as IL-6, IL-1β, and TNF-α[45], and their numbers are markedly increased in the target organs during hypertension [46–49]. Dahl-S rats develop significant hypertension and elevations in total circulating numbers of monocytes compared to Dahl salt-resistant (Dahl-R) rats with high salt diet treatment [50] whereas immunosuppressive therapy reduces renal monocyte infiltration and arterial pressure in the Dahl S animals [51]. Monocytes contribute to blood pressure elevation in several experimental models, including a high salt diet [52], deoxycorticosterone acetate (DOCA)-salt [53], aldosterone-salt [54, 55], and Ang II [14] induced hypertension. Adoptive transfer of CD11b⁺ Gr-1⁺ monocytes into LysM-positive cell-depleted mice reestablishes the hypertensive response during chronic Ang II infusion by instigating oxidative stress in the vasculature [14]. In human studies, a high salt diet increases circulating numbers of monocytes [56], and endothelial stretch activates monocytes to produce pro-hypertensive cytokines [57]. Thus, monocytes may act in both the systemic and renal vasculature to promote blood pressure elevation.

NK cells

Natural killer (NK) cells function similarly to cytotoxic T cells in the vertebrate adaptive immune response, but their role in hypertension is less well-documented. Kossmann et al. provided the first evidence that NK cells contribute to vascular dysfunction during RAS activation [58]. Recently, renal NK cells were found to be activated in hypertensive pregnancy [22]. Chronic infusion of IL-17 can induce NK cell activation in pregnant rats[59]. On the other hand, in Ang II-induced hypertension, CD1d mutant mice that lack functional NK cells have accelerated cardiac remodeling, and administration of an NK cell activator attenuates Ang II-induced cardiac remodeling[60]. Thus, the role of NK cells in hypertension remains an open debate warranting further study.

Macrophages

Macrophages exhibit complex effects on blood pressure, several of which occur in the kidney. Macrophages accumulate in the kidney to influence the susceptibility to hypertension and/or associated target organ damage in various pre-clinical models. In the Ang II hypertension model, CCR2 deficiency limits the accumulation of macrophages in the kidney, local oxidative stress, and renal damage [61]. Similarly, treatment with liposome-encapsulated clodronate to deplete macrophages reduces renal oxidative stress and inflammation and lowers blood pressures during Ang II infusion [62]. In the experimental model of nicotine-induced hypertension, the accumulation of CD161a⁺/CD68⁺ macrophages in the renal medulla is associated with increased renal expression of NKCC2, whereas ablation of these CD161a⁺ myeloid cells reverses the hypertensive phenotype and limits NKCC2 induction [63]. Thus, macrophage subsets may confer salt sensitivity. Conversely, high salt conditions may induce a macrophage phenotype [64, 65] that elevates blood pressure and promotes renal damage. Within macrophages, activation of the NLRP3 inflammasome triggers the maturation of IL-1β, and inflammasome activity is required for a full hypertensive response in preclinical studies [66, 67]. Moreover, we recently found that IL-1 receptor activation suppresses the renal accumulation of Ly6C⁺/Ly6G⁻ macrophages that release nitric oxide, inhibiting NKCC2 activation and consequent renal salt retention [68]. These studies point to a role for IL-1 receptor activation in hypertension. Nevertheless, blockade of IL-1 with canakinumab did not reduce blood pressure among human hypertensive patients with atherosclerosis in the recent CANTOS clinical trial [69]. Thus, it is not clear that IL-1 receptor stimulation plays an essential role in human hypertension.

COX-2-dependent prostaglandins facilitate natriuresis that should protect against salt sensitivity. In this regard, mice lacking COX-2 in bone marrow-derived cells or the PGE₂ type 4 (EP₄) receptor in macrophages exhibit increased blood pressure, accumulation of T cells in the kidney, enhanced phosphorylation of the renal sodium chloride cotransporter (NCC), and impaired renal sodium clearance [70]. Thus, some macrophage populations can promote hypertension through their generation of pro-hypertensive cytokines and reactive oxygen species, whereas others can constrain blood pressure elevation through the generation of nitric oxide and/or expression of COX-2.

Dendritic Cells

Dendritic cells (DCs), marked by co-expression of CD11c and MHC class II, are the most potent antigen-presenting cells in the body. A series of experiments in mice indicate that DCs engage the adaptive immune system to promote hypertension by presenting specific antigens to pro-hypertensive T cells that express receptors specific for those antigens. First, hypertensive stimuli upregulate co-stimulatory molecules on the DC surface that potentiate the DC’s capacity to activate T cells. For example, chronic Ang II infusion increases the expression of the costimulatory molecule CD86 on CD11c⁺ DCs, and mice lacking CD86 have fewer activated T cells and a blunted hypertensive response [71]. Upon maturation, DCs express CD70 as a surface marker, and activation of T cells by mature DCs can be detected by the emergence of an effector memory T cell (T_EM) phenotype. Induction of hypertension by treatment with L-NAME/high salt increases the expression of CD70 on DCs and enhances the formation of renal T_EM’s. Inversely, deficiency of CD70 limits the accumulation of T_EM in the kidney and attenuates the hypertensive response [72].

Regarding a putative antigen, hypertensive stimuli generate oxidant stress within DCs with consequent modification of isolevuglandins that are then presented as peptides to pro-hypertensive T cells [16]. Exposure of DCs to these isolevuglandin-modified peptides augments cytokine production by the DC and upregulates expression of the costimulatory molecules CD80 and CD86 on the DC surface [16]. Through the activation of T cells, DCs promote vascular dysfunction and sodium retention in the initial stages of hypertension. In turn, excess sodium enters the DC through sodium transporters and activates the DC to further provoke T cell elaboration of the pro-hypertensive cytokines IL-17a and IFN-γ that exaggerate hypertension [73]. Inversely, the ablation of myeloid CD11c⁺ cells prevents the development of hypertension in response to Ang II infusion plus a high-salt diet [74]. Thus, DCs can promote hypertension through effects on T cells in several experimental models.

To explore the contribution of the classical DC subset whose differentiation depends on Fms-like tyrosine kinase 3 ligand (FLT3L), we subjected FLT3L-deficient mice and controls to Ang II-induced hypertension. These studies revealed that classical DCs mediate hypertension by promoting renal oxidative stress and cytokine production by renal T cells.[75]. These changes in the kidney were associated with the upregulated expression of tubular sodium channels with consequent fluid retention and blood pressure elevation [75]. Thus, classical DCs make a distinct contribution to volume expansion following RAS activation.

Given the importance of DCs in linking innate and adaptive immune responses during hypertension, identifying signals within the DC that may regulate pro-hypertensive T cell activation could offer novel targets for blood pressure control. In this regard, our group reported that the ubiquitin-editing protein, A20, in CD11c⁺ myeloid cell protects against Ang II-induced hypertension by inhibiting DC-mediated T cell activation [76]. Accordingly, mice with heterozygous deletion of A20 in CD11c-expressing myeloid cells have spontaneous DC activation and enhanced accumulation of CD40⁺ DCs in the hypertensive kidney [76]. In an elegant set of experiments, the Kirabo group established that the salt sensing kinase, serum/glucocorticoid kinase1 (SGK1) in CD11c⁺ antigen-presenting cells (APCs) is integral to salt-sensitive hypertension [77]. Mice lacking SGK1 in CD11c⁺ cells have fewer renal inflammatory cells, blunted DC activation, and less blood pressure elevation in the high salt/L-NAME model [77]. Thus, targeting signaling cascades in DCs may represent a potent strategy for disrupting T cell-dependent hypertension.

Adaptive immune Cells in Hypertension

T Lymphocytes

The role of T lymphocytes in hypertension has received considerable scrutiny. More than 40 years ago, investigators established that nude mice have a blunted elevation of blood pressure with DOCA-salt treatment, whereas grafting thymus from wild-type mice into nude mice restores the hypertensive response [78]. Whereas T cells were known to infiltrate the hypertensive kidney, Guzik et al. provided the first conclusive evidence of the role of T cells in the pathogenesis of Ang II-induced hypertension. In their studies, mice lacking functional T and B cells do not develop hypertension during Ang II infusion, whereas adoptive transfer of T but not B cells restores the hypertensive responses[19]. This finding was confirmed and further investigated in transgenic rats during salt-induced hypertension [79]. Our group subsequently reported that SCID mice, which are deficient of lymphocytes, are partially protected from hypertension due to preserved diuresis and natriuresis [11]. Although both CD4⁺ and CD8⁺ T cells accumulate in the kidney of hypertensive mice, the CD8⁺ subset provides the dominant contribution to blood pressure elevation [80]. Moreover, CD8⁺ T cells interact directly with distal convoluted tubule cells during DOCA-salt treatment, leading to the up-regulation and activation of the Na-Cl co-transporter (NCC), with consequent blood pressure elevation [81]. Following T cell activation, CD8⁺ T_EM, rather than CD4⁺ T_EM promote hypertension by releasing pro-hypertensive cytokines such as TNF-α and IFN-γ [76]. Induction of these cytokines is associated with enhanced sodium transporter expression and renal vasoconstriction, with resulting reductions in renal blood flow and augmented fluid retention [76].

Individual subsets of CD4⁺ T cells, such as Th1, Th17, and Treg cells, have divergent effects on blood pressure regulation. Animals lacking the Th1 cell-promoting transcription factor T-bet have a preserved hypertensive response but are less susceptible to hypertensive kidney injury, implicating Th1 cells in end-organ damage [82, 83]. By contrast, Th17 cells that produce IL-17 have direct effects in the kidney to promote blood pressure elevation. For example, high salt media drives the differentiation of naïve T cells toward the Th17 phenotype via an SGK1-dependent pathway [84, 85], and mice deficient of SGK1 solely in T cells have a blunted hypertensive response [86]. Moreover, genetic deletion or pharmacologic blockade of IL-17a significantly protects against Ang II-induced hypertension and limits the expression of sodium transporters in the kidney [28, 87]. Consistent with their immunosuppressive effects, Tregs mitigate hypertension and/or end-organ damage [88–90]. For example, deficiency of Tregs exaggerates Ang II-dependent hypertension, whereas repeated adoptive transfer of Tregs attenuates blood pressure elevation in this model [91, 89]. Tregs also interact with neutrophils and the complement system to afford protection from hypertension. In this regard, the attenuated hypertensive response noted in animals lacking the complement receptors C3aR and C5aR is abolished by Treg depletion [92]. Moreover, IL-10 produced by Tregs ameliorates hypertension by mitigating vascular oxidative stress [93]. Thus, Tregs prevent hypertension via actions affecting multiple arms of the immune system. Similar to Th17 cells, γδ T cells produce pro-inflammatory cytokines, including IFN-γ, TNF-α, and IL-17.[94]. Accordingly, γδ T cells augment the hypertensive response despite their small numbers in the periphery [95]. γδ T cells also activate other T cell subsets and are increased in hypertensive patients [95]. Thus, the effects of individual T cell subsets accrue from the cytokines they elaborate and effects on other immune system components, oxidant stress, vascular function, and renal sodium handling.

B Lymphocytes

B cells play an essential role in hypertension via producing antibody, particularly in the setting of autoimmunity [96]. For example, anti-CD20 therapy that depletes B cells protects against Ang II-dependent hypertension in mice [97], and B cell-deficient mice are similarly protected [18, 98]. Mice with genetic deficiency of the B cell-activating factor receptor (BAFF-R^−/− mice) or transcription factor Myb (c-myb^h/h mice), both of which lack mature B cells, are resistant to hypertension. Inversely, adoptive transfer of wild-type B cells into BAFF-R^−/− mice restores their capacity to develop hypertension [18]. After a hypertensive stimulus, c-myb^h/h animals have reduced renal vasopressin receptor 2 (V₂R) and increased urine output and sodium excretion compared to controls, indicating that B cells can interfere with blood pressure homeostasis through effects on salt and water handling via regulation of the V₂R [98]. These actions of B cells appear to require costimulatory signals from T cells, as the adoptive transfer of B cells alone does not restore the hypertensive response in lymphocyte-deficient animals [19]. The actions of B cells to promote hypertension may also require their differentiation into plasma cells as plasma cell depletion is protective in autoimmune hypertension [99]. Hence, the role of B cells in the regulation of hypertension seems to depend on actions of IgG and alterations in kidney function.

Conclusions

Immune cells impact the severity of hypertension by altering blood pressure-regulating functions in the heart, blood vessels, kidneys, and nervous system. Various immune cell subsets infiltrate the kidney during hypertension, and specific depletion of these subsets or their functions can partially protect against the development of hypertension in experimental models. Hypertensive stimuli such as AngII and high salt-diet activate both the innate and adaptive immune systems. In turn, activated inflammatory cells work cooperatively to provoke blood pressure elevation. In this review, we have discussed the direct renal actions of these immune cells via their elaboration of vasoactive mediators as summarized in Figure 1.

Figure1. Mediators induced or elaborated by immune cells in the hypertensive kidney.

Innate immune cells: monocytes, macrophages, and dendritic cells produce cytokines and ROS, which promote hypertension. Adaptive immune cells: CD8⁺ T cells, Th1, Th17, γδ T cells, and B cells produce factors that contribute to hypertension whereas Tregs inhibit blood pressure elevation.

Translation of these findings is urgently needed for the treatment of biologically resistant hypertension. In this regard, medications developed for autoimmune disease may afford protection in hypertensive patients by diminishing levels of inflammatory cytokines such as TNF-α, IL-1, and IL-6. For example, an IL-1 monoclonal antibody limits cardiovascular mortality [100]. Nevertheless, treatment with this antibody did not reduce blood pressure in hypertensive patients [69, 101]. These findings raise the possibility that disrupting inflammatory functions could limit target organ damage accruing from hypertension through blood pressure-independent mechanisms. Thus, mitigating the complications of hypertension may be a reasonable goal for immunomodulators, even where they do not impact blood pressure. Meanwhile, additional preclinical studies should continue to identify specific immune cell populations and signaling cascades that can be targeted for protection from hypertension without untoward off-target immunosuppressive effects.

Key Points.

• Innate and adaptive immune cells contribute to the development of hypertension by modulating renal function.

• Specific immune cell subsets play varying roles in regulating blood pressure.

• Pro-hypertensive cytokines augment hypertensive responses, whereas some anti-inflammatory cytokines limit blood pressure elevation.