The self-amplifying cascade of messenger and effector molecules of the complement system serves as a powerful danger-sensing system that protects the host from a hostile microbial environment, while maintaining proper tissue and organ function through effective clearance of altered or dying cells. As an important effector arm of innate immunity, it also plays important roles in the regulation of adaptive immunity. Innate and adaptive immune responses have been identified as crucial players in the pathogenesis of arterial hypertension and hypertensive end organ damage. In line with this view, complement activation may drive the pathology of hypertension and hypertensive injury through its impact on innate and adaptive immune responses. It is well known that complement activation can cause tissue inflammation and injury and complement-inhibitory drugs are effective treatments for several inflammatory diseases. In addition to these proinflammatory properties, complement cleavage fragments of C3 and C5 can exert anti-inflammatory effects that dampen the inflammatory response to injury. Recent experimental data strongly support a role for complement in arterial hypertension. The remarkably similar clinical and histopathological features of malignant nephrosclerosis and atypical hemolytic uremic syndrome, which is driven by complement activation, suggest a role for complement also in the development of malignant nephrosclerosis. Herein, we will review canonical and noncanonical pathways of complement activation as the framework to understand the multiple roles of complement in arterial hypertension and hypertensive end organ damage.
- arterial hypertension
hypertension affects 30% of the population and 70% of the elderly. In spite of this high prevalence and decades of research, the etiology of most cases of hypertension remains undefined (18). Recent data suggest that hypertension and hypertensive end organ damage are not only mediated by hemodynamic injury but also by innate and adaptive immune responses (5, 31). In arterial hypertension, activation and infiltration of leukocytes are found in several tissues including the aortic adventitia and the kidney. Important advances have been made in recent years on the role of immune mechanisms in arterial hypertension as reviewed recently (6, 22, 27, 31). However, the mechanisms causing inflammation in the hypertensive setting remain elusive. Furthermore, a pathogenic role of complement has been demonstrated in autoimmune diseases. Complement deficiencies have been associated with an increased risk to develop autoimmune disease, and complement inhibition is an effective treatment for several inflammatory diseases (3). Complement may also play a role in the pathogenesis of hypertensive renal injury in malignant nephrosclerosis, which shares clinical similarities with atypical hemolytic uremic syndrome (aHUS). Atypical HUS is caused by the lack of complement inhibitors and can be cured by complement blockade (20). However, data on the role of complement in experimental arterial hypertension are scarce (25, 30, 34, 35). Herein, we will review canonical and noncanonical pathways of complement activation. A better understanding of such pathways will serve as a framework to gain detailed insights into the multiple roles of complement in arterial hypertension and its role as a potential therapeutic target.
Complement and Complosome
The complement system is an ancient part of innate immunity. It is central to the detection and destruction of invading microbes (12, 36). Liver-derived plasma complement is essential for the protection from pathogens (28). However, complement components can also be produced by tissue-resident and migratory/immune cells, including T cells and antigen-presenting cells (13). The complement components produced by immune cells and their cleavage products that activate several groups of defined cognate receptors on such cells can bridge innate and adaptive immunity. Dendritic cells in the kidney can be identified by expression of myosin heavy chain (MHC) II and CD11c, a component of complement receptor 4. More than 90% of renal dendritic cells also express the CD11b, a component of complement receptor 3. Complement receptor 3 and 4 are important in binding and phagocytosis of complement-opsonized pathogens including parasites. Complement receptor 3 also regulates cytokine responses, leukocyte trafficking, and synapse formation (21).
This property of shaping adaptive immune responses makes complement a potential regulator of arterial hypertension. Canonical complement activation is initiated by one or more of the three pathways 1) the classical pathway, 2) the mannose-binding lectin (MBL) pathway, and 3) the alternative pathway (Fig. 1A) (reviewed in Refs. 13, 26). The proteins C2, C4, C3, and C5 are proteolytically cleaved when activated. The resultant protein fragments bind to defined receptors on tissue cells or enter the circulation, eliciting local and systemic responses. As such, complement cleavage fragments of C3 and C5 mediate local inflammatory reactions, the recruitment and activation of phagocytes, and the removal of apoptotic cells and immune complexes (9). Assembly of C5b-9 in membranes of target cells induces cell lysis. Many of the downstream effects of complement are mediated by C3a, C5a, C3b and derivatives thereof. The anaphylatoxins C3a and C5a are small polypeptides released during complement activation that bind to their cognate seven transmembrane-spanning receptors C3aR1, C5aR1, and C5aR2 (11, 26). Binding of C3a to C3aR and C5a to C5aR1 leads to cell activation through the recruitment of G-protein subtypes, which induces an increase in intracellular calcium concentration as well as other signal transduction events. Binding of C5a to C5aR1 preferentially mediates proinflammatory responses. In contrast, C5aR2 is uncoupled from G proteins due to the lack of intracellular amino acid motifs. The function of C5aR2 varies with cell type: C5aR2 can act either as a nonsignaling decoy receptor antagonizing C5aR1 or as an active transducer of pro- or anti-inflammatory signals (15). Evaluation of the role of anaphylatoxin receptors is further complicated by “biased agonism.” Biased agonism describes the ligand-dependent selectivity for certain signal transduction pathways in one and the same receptor and has been described for C5aR. The ligand 17-kDa protein (Skp), present in the periplasm of Gram-negative bacteria as a 17-kDa molecular chaperone, can bind and activate C5aR. Skp is a chemoattractant for neutrophils but is unable to induce superoxide generation from neutrophils and granule enzyme release. In contrast, C5a can be a chemoattractant for neutrophils and a secretagogue, eliciting neutrophil degranulation and superoxide generation responses. The inability of Skp to act as a secretagogue is due to the presence of glutamine 103 as the amino acid residue equivalent to leucine 72 present in the COOH terminal of C5a that interacts with the C5a receptor. If the glutamine 103 in Skp is substituted with leucine 72, then it confers to Skp the ability to function as a secretagogue. Thus structural differences in the C5aR ligands due to their amino acid sequence can influence the type of response in C5aR-mediated functions in neutrophils (8, 23).
Recent data suggest a role for the inflammasome in arterial hypertension (14). The inflammasome and complement are traditionally viewed as quintessential components of innate immunity. Recent studies from the Kemper laboratory have additionally highlighted an unanticipated direct role for noncanonical, intracellularly generated C3a and C5a in human T cells (16) as shown in Fig. 1B. The intracellular C3a-C3aR axis maintains homeostatic T-cell survival, thus exerting a primarily anti-inflammatory function. Only if shuttled to the cell surface, the C3a-C3aR complex induces proinflammatory cytokine production. In contrast, the intracellular C5a-C5aR1 axis is proinflammatory by triggering T-cell receptor activation. Intracellular binding of C5a to C5aR1 causes reactive oxygen species production and NLRP3 assembly. The inflammasome promotes TH1 and decreases TH17 induction. In contrast, if C5a is secreted, it can bind to C5aR2 on the surface of the cell, which negatively controls the intracellular signaling pathway. The cross talk between intracellularly activated complement components, which are considered as “complosome” and the inflammasome, plays a fundamental role in TH1 induction and regulation (2). It has previously been shown that the T-cell number in C3aR−/−, C5aR1−/−, and C3aR/C5aR1−/− double-deficient mice is significantly reduced (24). When these authors adoptively transferred wild-type or C3aR/C5aR1-double-deficient T cells into SCID mice, they found that the number of C3aR/C5aR1-double-deficient T cells was significantly lower than that of transferred wild-type T cells. These data clearly point toward a reduced viability of T cells that lack proper C3aR and C5aR1 signaling.
Complement activation must be physiologically restrained to limit damage of host cells. Complement regulation occurs at multiple steps through distinct mechanisms. It is now clear that many, if not all cells of the body, can produce complement factors. The production of local complement can be induced and/or increased by proinflammatory cytokines, which suggests that extrahepatic complement production has evolved to respond to environmental cues that signal the requirement or presence of an immune response. In turn, increases in local complement can modulate cytokine production by immune cells indicating the existence of bidirectional feedback loops (13). Complement activation also has anti-inflammatory effects in some contexts. Congenital deficiency of complement proteins is associated with an increased risk for autoimmunity (4). Several lines of evidence suggest that complement activation facilitates the removal of injured cells and debris. By promoting the rapid disposal of intracellular molecules that are released during cell injury, the complement system reduces the immunogenicity of these targets and maintains proper tissue and organ function (17).
Anaphylatoxins in Hypertension
As discussed above, complement activation leads to generation of cleavage fragments like the anaphylatoxins C5a and C3a. The role of C5a in hypertension has recently been examined. Zhang et al. (34, 35) reported increased levels of C5a in humans with high blood pressure. Infusion of ANG II-causing arterial hypertension led to increased systemic anaphylatoxin generation in mice. C5aR1-deficient mice exhibited markedly reduced cardiac remodeling and inflammation after ANG II infusion. Pharmacological inhibition of C5a production by an anti-C5 monoclonal antibody or C5aR1 targeting with an inhibitor (PMX53) recapitulated the effects of C5aR1 deficiency (34, 35). Bone marrow chimera experiments revealed that C5aR1 expression on bone marrow-derived cells was critical in mediating ANG II-induced cardiac injury and remodeling. Treatment of DOCA salt hypertensive rats with the C5aR1 antagonist PMX53 attenuated endothelial dysfunction and decreased hypertensive cardiac injury. PMX53 treatment resulted in less ventricular collagen deposition and hypertrophy as compared with untreated hypertensive rats. C5aR1 antagonism did not change systolic blood pressure (7).
A blood pressure-independent nephroprotective effect of C5aR1 deficiency was also found in an accelerated model of hypertension (30). Moreover, using a C5aR1 reporter mouse, expression of C5aR1 in the kidney was shown on dendritic cells as well as in monocytes/macrophages and granulocytes. No expression was detected on resident cells in the heart or the aorta, whereas weak expression occurred in the kidney on podocytes and parietal cells. However, in contrast to the work of Zhang et al., cardiac injury was accelerated in C5aR1-deficient mice with significantly increased cardiac fibrosis and heart weight in C5aR1-deficient mice after ANG II infusion. The reason for the difference between cardiac and renal injury is unclear; however, preliminary data suggest counterregulatory functions of C5aR1 and C5aR2 in hypertensive injury. A similar counterregulatory function of both receptors has recently been described in experimental ANCA vasculitis (33). Since the abundance and proportion of C5aR1 and C5aR2 differs in various tissues, the consequences of C5aR1 deficiency may also be different between tissues. These observations indicate that the C5a:C5aR1 axis drives end organ damage in the kidney, while its role in cardiac injury remains unclear.
In C3aR-deficient mice, Zhang et al. (34) observed a trend for increased cardiac injury when compared with wild-type mice after ANG II infusion. Similarly, preliminary data from our laboratory suggest an enhanced renal injury in C3aR deficient mice after ANG II infusion.
Complement and Hypertension
Hypertension stimulates structural arterial remodeling, which is characterized by vascular smooth muscle cell (VSMC) hyperplasia and infiltration of inflammatory cells. Recent findings demonstrate a key role for complement C1-induced activation of β-catenin signaling in this process (25). C1q activates β-catenin signaling by binding to Frz and cutting Lrp5/6 with C1r/s, which is independent of wnt ligands in aorta as well as skeletal muscle (19, 25). ANG II infusion raised blood pressure, promoted arterial remodeling characterized by VSMC proliferation, and upregulated the expression of Wnt/β-catenin target genes. Pharmacologic or genetic inhibition of β-catenin signaling suppressed VSMC proliferation without lowering blood pressure. ANG II infusion recruited macrophages into the aorta, and these macrophages secreted the complement component C1q. Depletion of macrophages, administration of a C1 inhibitor, or genetic ablation of C1q suppressed ANG II-induced activation of β-catenin signaling and VSMC proliferation, identifying macrophage-secreted complement C1 as an inducer of β-catenin signaling and VSMC proliferation in hypertensive arterial remodeling (1, 25).
Role of Complement in Malignant Nephrosclerosis
Patients presenting with severe hypertension in combination with hypertensive retinopathy, hypertensive encephalopathy, and often renal injury with mild signs of thrombotic microangiopathy are labeled to have “malignant hypertension.” The pathogenesis of this renal injury is unclear. Malignant nephrosclerosis, which is also called stenosing arteriosclerosis, is an acute renal failure in the setting of malignant hypertension. It is a form of thrombotic microangiopathy defined by progressive narrowing of interlobular arterial branches and afferent arterioles with intima fibrosis and onion-like appearance of intimal scarring, generally in the absence of elastosis. In contrast, the structural changes of so-called benign nephrosclerosis, which is also called hyaline arteriolosclerosis, are characterized by intimal fibroelastosis in arteries including interlobular caliber vessels and subendothelial hyalinosis in afferent arterioles and prearterioles. The mechanisms causing transition from high blood pressure to malignant hypertension with malignant nephrosclerosis and renal failure are completely unknown. Furthermore, it is unknown why some patients with severe hypertension develop malignant nephrosclerosis and others, with similar levels of blood pressure elevation, do not. Malignant nephrosclerosis is hallmarked by complement deposition, inflammation, and signs of thrombotic microangiopathy with a renal pathology resembling certain histological features of aHUS (32). A crucial involvement of the complement system in pathogenesis of thrombotic microangiopathy has been highlighted by numerous studies in the recent years, and it has been shown that aHUS can be cured by therapeutic complement inhibition using the C5 cleavage inhibitor eculizumab (20). The shared clinical and histopathological features between malignant nephrosclerosis and aHUS suggest a role for complement also in the development of malignant nephrosclerosis and point to complement inhibition as a potential therapeutic strategy in malignant hypertension. One may speculate that overactivity of complement factors or lack of complement inhibitors defines the subset of hypertensive patients that develop malignant hypertension and nephrosclerosis in response to arterial hypertension. Presently, there are no human data supporting that targeting the complement cascade is beneficial in patients with arterial hypertension or cardiovascular disease. If future work shows that patients with subtile genetic or acquired changes in the complement cascade are more prone to develop malignant nephrosclerosis, complement inhibition could indeed be an option. As mentioned above eculizumab binds C5 and inhibits conversion of C5a into C5a and C5b. Since our data suggest that binding of C3a to C3aR has protective and binding of C5a to C5aR1 has deleterious effects in hypertensive renal injury, eculizumab would be expected to be beneficial by leaving C3a intact and concurrently inhibiting the generation of C5a. Most likely more selective antagonism (or in the case of C5aR2 agonism) of certain complement components or receptors will be clinically helpful.
Why should evolution favor complement as a regulator of blood pressure? Blood pressure control and host defense are essential mechanisms of homeostasis. Infection can cause hypotension via fluid loss during fever, tachypnea, and diarrhea. Septicemia induces inflammation-related vascular fluid losses. Thus the risk of hypotension related to inflammation might have favored selection of mechanisms that link immune and complement activation to blood pressure increases for short-term survival benefits. Such an evolutionary force may explain why important antimicrobial effectors could have direct hypertensive effects by promoting vasoconstriction or sodium retention (31).
Summary and Conclusion
Synthesis of all of the available evidence has led us recently to propose the following hypothesis as a mechanism by which complement may link inflammation and hypertension (Fig. 2) (31). Small increases in prohypertensive factors like ANG II, aldosterone, or salt retention activate the innate immune system. This process includes activation of the complement cascade, release of “damage-associated molecular patterns” (DAMPs) that activate Toll-like receptors, and activation of the inflammasome. These factors can directly induce hypertensive end organ damage and further activate the circulating complement system. Moreover, activation of monocytes/macrophages and granulocytes can also directly aggravate hypertensive injury. Hypertensive tissue injury leads to altered or dying cells with release and formation of cryptic or neoantigens that are recognized by dendritic cells. The dendritic cells then activate T and B cells that migrate into the heart, aorta, and kidney. Complement and complement receptors could play important roles in all phases of this scenario by shaping the inflammatory response. Complement receptors on monocytes/macrophages and granulocytes can fine tune their activation and migration. Dendritic cells can be identified by expression of CD11c, a component of complement receptor 4. More than 90% of renal DCs also express the CD11b, a component of complement receptor 3. Also C3aR and C5aR are expressed on dendritic cells (28) suggesting that anaphylatoxins play a major role in dendritic cell function. In addition, anaphylatoxins and their receptors on the surface and within in T cells control their differentiation, activation, and survival.
The complement system has long been viewed as a complex network mainly serving innate immune functions. A growing body of evidence has accumulated demonstrating that the functions of complement go beyond innate immunity. New discoveries in inflammatory diseases refine our appreciation of the close interdependency of “ancient” complement and “modern” adaptive immune mechanisms. Complement activation can cause autoimmunity, tissue inflammation and injury, and complement-inhibitory drugs are effective treatments for several inflammatory diseases. New discoveries are made in the complement system with undiminished pace, and arterial hypertension is a new field in complement research (29). The major reason to treat hypertension is to prevent end organ damage. While blood pressure lowering is clearly important to reach this goal, the prevention of local inflammation that accompanies hypertensive end organ damage should also to be addressed. Since complement has pro- as well as anti-inflammatory functions, complete inhibition of the canonical complement cascade may result in unwanted effects in arterial hypertension. In contrast, specific targeting of defined complement receptor pathways such as the anaphylatoxin receptors may significantly improve the protection of end organs from hypertensive damage. In case of the C5aR2 receptor, agonism could be beneficial (10). Thus the goal of this review was not only to highlight some of the “hot areas” of discovery and surprise in complement research like the complosome but also specifically raise further awareness for the complexity of the connections between complement, innate, as well as adaptive immunity, and arterial hypertension.
The work of U. Wenzel, M. Bode, and H. Ehmke was supported by German Research Foundation Grants We 1688/17-1 (to U. Wenzel) and Deutsche Forschungsgemeinschaft SFB 1192. The work of J. Köhl was supported by German Research Foundation Grants KO1245/1 and IRTG 1911.
No conflicts of interest, financial or otherwise, are declared by the author(s).
U.O.W., M.B., J.K., and H.E. prepared figures; U.O.W., M.B., J.K., and H.E. drafted manuscript; U.O.W., M.B., J.K., and H.E. edited and revised manuscript; U.O.W., M.B., J.K., and H.E. approved final version of manuscript.
- Copyright © 2017 the American Physiological Society