Novel aspects of complement in kidney injury

Transcription

1 & 2010 International Society of Nephrology Novel aspects of complement in kidney injury Mark B. Vieyra 1 and Peter S. Heeger 1 1 Renal Division, Mount Sinai School of Medicine, New York, New York, USA Complement activation is integral to the development and progression of multiple forms of kidney disease. The liver is the principal source of serum complement, but various kidney cell types and bone marrow derived immune cells can produce a full array of complement proteins. Locally produced and activated complement yields cleavage products that function as vital intermediaries, amplifying inflammation in ischemia reperfusion injury and transplant rejection, among other pathological states. Additional new studies indicate that during cognate T-cell antigen presenting cell interactions, both cell types produce alternative pathway complement components. The resultant activation products have an essential role in T-cell activation, expansion, and differentiation, which in turn has a profound impact on the development of immune-mediated kidney disease. The recognition of an expanded role for kidney cell derived and immune cell produced complement as pathogenic to the kidney supports the need for future studies to test the efficacy of complement inhibitors in the prevention and/or treatment of selected kidney diseases. Kidney International (2010) 77, ; doi: /ki ; published online 16 December 2009 KEYWORDS: complement; ischemia reperfusion; kidney injury; transplantation Correspondence: Peter S. Heeger, Mount Sinai School of Medicine, One Gustave Levy Place, Annenberg Building Box 1243, New York, New York 10029, USA. peter.heeger@mssm.edu Received 6 August 2009; revised 14 October 2009; accepted 27 October 2009; published online 16 December 2009 Serum complement, traditionally considered a component of the innate immune system required for host defense against invading pathogens, is an established mediator of various forms of kidney disease. Over the last decade, a number of experimental observations, including the fact that wild-type (WT) mice with intact serum complement activity do not reject C3-deficient kidney allografts, 1 have provided new mechanistic insights into the physiological andpathogeniceffectsofcomplement in vivo. These paradigm-shifting findings indicate that tissue- and immune cell-derived complement, in addition to serum complement, can exert profound effects on the development of tissue inflammation, as well as on the strength and cytokine profile of adaptive T-cell immune responses. In this, we summarize several of these observations with a focus on proximal complement activation products (rather than C6-9), describe how these novel mechanisms apply to kidney disease, and provide a perspective on how they could ultimately influence strategies aimed at preventing or treating kidney injury. OVERVIEW OF THE COMPLEMENT CASCADE Complement activation is initiated through classical, alternative, or mannose-binding lectin pathways that converge at the production of the C3 convertase, a central amplification step within the cascade (Figure 1). Subsequent cleavage of C3 and then of C5 initiates formation of the membrane attack complex and yields soluble and surface-bound split products, including C3a, C3b, ic3b, C3dg, and C5a, which serve as chemoattractants, activators of innate immune cells (for example, macrophages), and opsonins, among other functions (Figure 1). The current concept is that complement activation must be regulated to prevent bystander damage to host cells in vivo. This regulation is accomplished through secretion and expression of both soluble and membrane-bound complement regulatory proteins (Figure 1). Soluble regulatory proteins include Factor H, which together with Factor I, inactivates C3 convertases. Decay-accelerating factor (DAF or CD55) is one of the several membrane-bound, complement regulatory proteins, and its function is to accelerate the decay of cell surface-assembled, C3 convertases (Figure 1). Disassociation/inactivation of the C3 convertase limits amplification of complement activation and as a consequence, restricts production of the aforementioned cleavage products. 2 SERUM COMPLEMENT AND KIDNEY DISEASE Serum complement participates in the phenotypic expression of diverse forms of kidney disease. The reader is referred Kidney International (2010) 77,

2 M Vieyra and PS Heeger: Complement and kidney injury Classical pathway Lectin pathway Alternative pathway C1q C1r, C1s, C4, and C2 MBL MASPs, C4, and C2 FD, FB C3 C3(H 2 O) or C3b C4b2a DAF/CD55 MCP/CD46/crry Factor H C4b2a C3bBb C3a Vasodilation Smooth muscle cell contraction Chemoattractant for basophils, eosinophils Mast cell activation Phagocyte activation Release of granule-based enzymes Generation of oxygen radicals T-cell activation and survival C3 Amplification step C3b Opsonin MCP/CD46/crry + factor I Factor H + factor I ic3b C3dg Opsonin B-cell activation C5 C4b2aC3b or C3bBbC3b (C5 convertase) C5a C5b C5b-9 Vasodilation, endothelial cell activation Chemoattractant for neutrophils, eosinophils, monocytes, T cells Phagocyte activation Release of granule-based enzymes Generation of oxygen radicals T-cell activation and survival Protectin CD59 Figure 1 Schematic overview of the complement cascade illustrating the three activation pathways (classical, mannose-binding lectin (MBL), and alternative), the common pathway, the membrane attack complex (C5b-9), and the site of action of the cell surface-expressed complement regulators (red) and the soluble complement regulator Factor H (blue). Representative functions of select cleavage products are listed in boxes. elsewhere for a review of this topic, which includes how antibody-initiated, classical pathway activation of serum complement affects the pathogenesis of several glomerulonephritides. 3 Serum complement activated through the classical pathway is also the source of C4d deposits, characteristic of antibody-mediated kidney transplant rejection (reviewed in Nangaku and Couser 3 and Collins et al. 4 ). As another example of how serum complement can contribute to renal disease, loss-of-function mutations of Factor H (or membrane cofactor protein, see Figure 1) can result in unregulated serum, alternative pathway, complement activation, and atypical forms of hemolytic uremic syndrome (reviewed in Kavanagh et al. 5 ). Serum complement can also influence the strength of adaptive humoral immunity, 6 which could potentially influence the phenotypic expression of autoantibodymediated kidney disease. The mechanism underlying this effect involves the C3b cleavage product C3dg that binds to B cell-expressed complement receptor 2 (CD21), facilitating antigen presentation to B cells and lowering the threshold for B-cell activation, together enhancing pathogen-specific antibody production (reviewed in Carroll 6 ). KIDNEY-DERIVED COMPLEMENT IN THE PATHOGENESIS OF RENAL DISEASE Although traditional thinking focuses on the pathogenic effects of liver-derived serum complement as a principal pathogenic mediator of kidney injury, it is now abundantly clear that complement components are produced by parenchymal tissues other than the liver. The non-liverderived complement can be released and activated locally under various circumstances. Evidence indicates that this local production of complement can function as a danger signal to initiate and amplify kidney inflammation and repair. In groundbreaking studies that highlighted the importance of non-liver, tissue-derived complement production, Sacks and colleagues 1 demonstrated that WT mice with intact serum complement activity do not reject allogeneic C3-deficient kidneys. Mechanisms of this surprising observation include an effect of complement on the development of pathogenic T-cell immunity (discussed below), and the fact that kidney-derived complement is a key mediator of ischemia reperfusion (IR) injury. 7,8 C3 is synthesized by tubular, mesangial, and endothelial cells and is rapidly upregulated after IR. 9 C3- or Factor B-deficient mice are resistant to renal IR injury, underscoring the key role for complement in this process. 7,10 In contrast, C4 deficiency is not protective, together indicating that kidney IR injury requires complement activation through the alternative pathway. 7,10 In further support of a key role for complement in this process are data that verify that postischemic injury is exacerbated in animals deficient in the complement regulator DAF (in which restraint on activation of locally produced complement is lifted). 8 Other studies show that murine C3-deficient kidneys are protected from post-ischemic damage after transplantation into syngeneic, murine recipients with normal serum complement activity. 7 WT (C3 þ ) kidneys develop IR injury upon transplantation into syngeneic C3-deficient recipients 496 Kidney International (2010) 77,

3 M Vieyra and PS Heeger: Complement and kidney injury (no serum C3). These data provide formal proof that kidneyderived C3, not serum C3, drives the expression of IR injury in this model. Mechanistic analyses show that the complement-split products C5a and C3a are important intermediaries in the development of kidney IR injury. These cleavage products bind in a paracrine manner to their receptors ( and ) expressed on renal tubular, endothelial, and innate immune cells (for example, neutrophils, macrophages). Signaling through the 7-transmembrane spanning, G-protein-coupled and stimulate upregulation and release of chemokines, which facilitates and amplifies leukocyte infiltration into the kidney, augments local inflammation through cytokine production, and ultimately results in tissue injury and renal dysfunction. 8,11 Small interfering RNA-induced downregulation of the 12 or administration of a antagonist 13 can prevent/limit IR injury in rodents, supporting a pathogenic role for C5a interactions in the pathogenesis of this disease. 14 The previously described fact that an extravascular pool of kidney-generated complement is pathogenic in kidney IR injury further increased the possibility that blocking local complement activation might be useful therapeutically. As proof of this concept, overexpression of a molecularly engineered, lipid-tailed, complement regulatory protein on kidney graft endothelium limited the extent of IR injury after transplantation. 15 These novel insights and intriguing therapeutic results support the notion that regulation or inhibition of local (kidney-derived) complement activation could be useful to prevent and possibly treat renal IR injury in humans. Kidney-derived complement also affects other tubulointerstitial diseases. 1,7,8,16 C3-deficient kidneys are resistant to adriamycin-induced tubular damage and progressive renal failure, even when transplanted into recipients with normal circulating complement activity. 16 Addition of C3a to cultured renal tubular cells decreases e-cadherin and upregulates a-smooth muscle actin and type I collagen production, consistent with an epithelial-to-mesenchymal transition associated with progressive renal fibrosis. 17 In -deficient mice, adriamycin induces less injury with lower expression of interstitial type 1 collagen and less a-smooth muscle actin. Taken together, these data support the view that local complement release and activation can trigger acute kidney inflammation and, in select circumstances, participate in the development of chronic tubulointerstitial injury in rodent models. The extent to which complement activation contributes to the pathogenesis of progressive kidney injury in humans is not known and will require further investigation. COMPLEMENT AND ADAPTIVE CELLULAR IMMUNITY The recent insight that immune cells, including T cells and antigen presenting cells (APCs), produce complement has implications for transplantation and autoimmune disease (Figure 2). This locally synthesized and activated complement has a key physiological role in immune cell survival and function. a b DAF DAF CD80/ CD86 CD28 IL-12, IL23, others CD80/ CD86 CD28 Apoptosis + MHC TCR C3, fd, fb CD3 MHC TCR CD3 Bcl2 Fas + C3, fd, fb Proliferation CD4/CD8 CD4/ CD8 CD40 CD40L (CD154) Studies by Heeger et al. and Medof et al showed that cognate T-cell APC interactions that result in T-cell activation are associated with the upregulation and release of alternative pathway complement components (C3, Factor B, Factor D) by both cell types. Simultaneous with the induced complement release, cell surface DAF expression is markedly but transiently downregulated on both the T cell and the APC, lifting restraint on activation of the locally released complement (Figure 2). The resultant cleavage products, C3a and C5a, bind to and, expressed on T cells, and augment the strength of the induced T-cell response. 24 Unexpectedly, this research group further documented that complement release is a downstream consequence of costimulatory signals transmitted by CD28/B7 and CD40/ CD40 ligand interactions, two second signals that are AKT AKTp AKT AKTp PI3Kγ PI3Kγ C3a C5a APC T cell APC T cell CD40 CD40L (CD154) Figure 2 Schematic depiction of how complement modulates T-cell immunity. Cognate T-cell APC interactions result in the upregulation and release of alternative pathway complement components by both partners and in the downregulation of (a) cell-surface DAF, which permits local complement activation resulting in the production of (b) C3a and C5a. These split products bind to their G-protein-coupled receptors expressed on T cells, signaling through PI3Kg and AKT to induce proliferation and inhibit apoptosis, while simultaneously activating APCs to upregulate B7 and innate cytokine (for example, IL-12) production. APC, antigen presenting cell; DAF, decay-accelerating factor; IL-12, interleukin-12; PI3Kg, phosphoinositide 3-kinase-g. Kidney International (2010) 77,

4 M Vieyra and PS Heeger: Complement and kidney injury believed to be required for T-cell activation. In the absence of and, T-cell APC interactions do not result in T-cell activation, despite the CD28/B7 ligation. Moreover, addition of recombinant C5a augments T-cell proliferation even in the absence of CD28/B7 signaling. Taken together, these findings indicate that /-transmitted signals are requisite downstream components underlying costimulation. 24 Further evidence that complement physiologically regulates T cells is derived from experiments showing that the ligation of C3a and C5a interactions is biochemically linked to known intracellular signaling pathways involved in T-cell activation (Figure 2). 20,24 and signaling in T cells activates phosphoinositide 3-kinase-g, and induces phosphorylation of the central intracellular signaling molecule AKT. Phosphorylation of AKT upregulates the anti-apoptotic gene Bcl2 and downregulates surface expression of the proapoptotic molecule Fas. 20 Taken together, these effects enhance T-cell proliferation and diminish T-cell apoptosis, explaining the complementmediated expansion of the effector T-cell repertoire after antigenic stimulation. Intriguingly, the evidence also indicates that and signaling is required for T-cell homeostasis, as T cells deficient in both receptors spontaneously undergo accelerated cell death in vitro and in vivo. 24 The immune cell-derived and locally produced C3a and C5a also bind to / on APCs (including dendritic cells and macrophages, Figure 2). / ligation activates APCs, inducing the release of innate cytokines (for example, interleukin-12, interleukin-23) and upregulating APC costimulatory molecules (for example, B7), further amplifying the immune response and modulating the phenotype toward interferon-g-producing Th1 immunity. 8,18 25 Importantly, in vivo studies conducted using bone marrow chimeric animals demonstrated that all of these effects on T-cell immunity are mediated by immune cell-derived complement and are independent of serum complement. 19,20,22 Complement-dependent effects on alloreactive T-cell immunity regulate the phenotypic expression of immunemediated injury in animal models. In addition to the aforementioned observation that WT mice do not reject allogeneic C3-deficient kidneys, 1 WT mice reject DAFdeficient heart allografts (enhances local complement activation) with accelerated kinetics compared with WT grafts. 22 The accelerated rejection of DAF-deficient heart transplants is associated with augmented anti-donor T-cell reactivity and is notable in animals devoid of B cells, confirming that local complement activation accelerates graft rejection through a T cell-dependent mechanism. Additional data show that immune cell-derived and donor graft-derived complement, but not serum complement, regulate expansion of both alloreactive CD4 þ and CD8 þ T cells. 22,23 Endothelial cell-driven expansion of alloreactive T cells in vitro and in vivo is also regulated by the locally produced complement (produced by both T cells and endothelial cells). 23 Further confirming a key role for C5a interactions as pathogenic in transplant rejection are data indicating that blockade prolongs kidney transplant survival in rodents. 14 This improved outcome is associated with an abrogation of intragraft mononuclear cell infiltration and a diminution in T-cell alloreactivity. 14 Taken together, these results indicate that complement is a physiologically important regulator of alloreactive T-cell immunity. The findings support the need for testing complement blockade as an adjunctive therapeutic approach to prolong transplant survival in humans. The effects of immune cell-derived complement extend beyond transplantation. In this regard, complement positively regulates antiviral immunity and autoimmunity in rodent models. Experimental allergic encephalomyelitis is one model of T cell-mediated autoimmunity that mimics aspects of human multiple sclerosis. 21,24 DAF-deficient mice develop more severe experimental allergic encephalomyelitis than do WT animals, and the phenotype is associated with stronger autoreactive T-cell immunity. These effects on T cells are - and -dependent, as mice deficient in either or both of these receptors develop less autoimmunity and are resistant to experimental allergic encephalomyelitis, regardless of DAF expression. 24 In separate work directly relevant to the pathogenesis of autoimmune renal disease, Bao et al. 26 described a new model of focal and segmental glomerulosclerosis in DAF knockout mice. In this model, focal and segmental glomerulosclerosis was caused by enhanced T-cell immunity as a result of the absence of DAF on T cells, and was not a direct or indirect effect of complement on antibody-initiated glomerular injury. These combined data obtained from numerous disease models verify that the regulation of T-cell responses by immune cell-derived complement can have important pathophysiological consequences. RELEVANCE TO HUMAN DISEASE Selected studies have suggested that immune cell-derived and/or graft-derived complement contributes to human transplant rejection. The quantity of RNA message for alternative pathway complement components and complement receptors, including and, is higher in human transplant tissue with histological evidence of rejection compared with non-injured control tissue. 14,27 Gene expression profiling of human kidney transplants reveals higher expression of several complement genes in deceased donor grafts with longer ischemic times, and interestingly, the complement gene upregulation correlates inversely with early and late renal function. 28 In another intriguing report, donor kidney expression of a specific polymorphic variant of C3 is associated with worse posttransplantion outcomes. 29 The precise mechanism through which this mutation alters allograft injury in human transplant recipients remains unclear, and an independent study of a disparate and larger patient population could not verify these initial findings Kidney International (2010) 77,

5 M Vieyra and PS Heeger: Complement and kidney injury CONCLUSIONS Although the complement system was originally discovered as a serum component that complemented antibodies in the killing of bacteria, it is clear that complement has a multitude of other functions. The complexities of the contributions of the complement to kidney disease extend beyond the established role of the membrane attack complex as a mediator of glomerulonephritis. One must understand the various activation pathways (such as classical pathway activation in antibodymediated injury, alternative pathway activation as pathogenic in kidney IR injury), and comprehend the expression and function of complement regulatory proteins (for example, Factor H mutations and hemolytic uremic syndrome) and the source of the complement (serum or non-liver derived). Emerging data indicate that in response to injury, kidneyderived complement can function as a danger signal that initiates inflammation and repair, and if left unchecked can contribute to the development of chronic organ damage. Local complement activation also physiologically regulates immune cell survival and proliferation, modulating the strength and phenotype of adaptive T-cell immune responses involved in autoimmune glomerular disease and alloimmune transplant rejection. Consideration of the cellular source and local function of complement has potential therapeutic implications. Antibodies capable of blocking complement activation, including an anti-c5 antibody approved for use in humans with paroxysmal nocturnal hemoglobinuria, 31 may be most beneficial as inhibitors of circulating serum complement (for example, decreasing posttransplant graft injury caused by alloantibody-initiated complement activation). Small molecule receptor inhibitors, which are in development for human use, may better penetrate tissues to restrain downstream consequences of the released cleavage products (for example, C3a and C5a), and thereby restrain the influence of complement over immune cells or the kidney parenchyma. Local treatment of donor organs with cell surface-expressed complement inhibitors to prevent delayed graft function post transplant represents one approach 15 that could theoretically benefit transplant recipients without the requirement for systemic drug administration. The recognition of the diversity through which complement participates in renal injury supports the need for continued design and testing of complement inhibitors in the clinic. DISCLOSURE The authors declared no competing interests. ACKNOWLEDGMENTS The study was supported by National Institutes of Health grants AI/DK43578 and AI awarded to PSH. REFERENCES 1. Pratt JR, Basheer SA, Sacks SH. 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