University of Groningen
Complement-mediated kidney diseases
Poppelaars, Felix; Thurman, Joshua M.
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Molecular Immunology
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10.1016/j.molimm.2020.10.015
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Poppelaars, F., & Thurman, J. M. (2020). Complement-mediated kidney diseases. Molecular Immunology,
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Molecular Immunology 128 (2020) 175–187
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Complement-mediated kidney diseases
Felix Poppelaars
a,*
, Joshua M. Thurman
baDivision of Nephrology, Department of Internal Medicine, University Medical Center Groningen, University of Groningen, AA53, Postbus 196, 9700 AD, Groningen, the Netherlands
bDivision of Nephrology and Hypertension, University of Colorado Anschutz Medical Campus, Aurora, CO, USA
A R T I C L E I N F O Keywords: Complement Kidney Glomerulonephritis Therapeutics A B S T R A C T
It has long been known that the complement cascade is activated in various forms of glomerulonephritis. In many of these diseases, immune-complexes deposit in the glomeruli and activate the classical pathway. Researchers have also identified additional mechanisms by which complement is activated in the kidney, including diseases in which the alternative and lectin pathways are activated. The kidney appears to be particularly susceptible to activation of the alternative pathway, and this pathway has been implicated as a primary driver of atypical hemolytic uremic syndrome, C3 glomerulopathy, anti-neutrophil cytoplasmic antibody-associated vasculitis, as well as some forms of immune-complex glomerulonephritis. In this paper we review the shared and distinct mechanisms by which complement is activated in these different diseases. We also review the opportunities for using therapeutic complement inhibitors to treat kidney diseases.
1. Introduction
The complement cascade was first described at the end of the 19th century as a system that mediates the downstream effects of antibodies, “complementing” their function. These early studies are nicely sum-marized in a book by Paul Ehrlich (Ehrlich, 1906). The bacteriolytic and hemolytic properties of serum that were explored in these early exper-iments form the basis of the clinical complement assays that are still used today. Although appreciated for its bactericidal role, it was soon understood that activation of the complement system could also cause inflammation and injury to the host. In 1915 it was observed that serum complement levels decrease in patients with post-streptococcal glomerulonephritis (Gunn, 1915). Mid-century studies extended this observation, revealing that complement levels were also decreased in systemic lupus erythematosus and in other forms of glomerulonephritis (Fischel and Gajdusek, 1952; Lange et al., 1951; Williams and D.H., 1958).
Animal models of hypersensitivity correlated changes in serum complement levels with the immune response to foreign antigens. Serum complement levels fell in parallel with serum antigen, suggesting that
the consumption of complement proteins occurred as the antigen was deposited in tissues (Schwab et al., 1950). Depletion of B cells with cytotoxic agents prevented the fall in serum levels of antigen and com-plement, further supporting the idea that the change in complement levels reflected consumption of complement proteins by antibody-antigen complexes (Schwab et al., 1950). The development of immunofluorescence methods confirmed that C3 is deposited in the glomeruli of patients with lupus and other forms of hypo-complementemic glomerulonephritis, helping to validate this concept in human patients (Lachmann et al., 1962).
The concept of complement deposits in the glomeruli, and decreased levels of complement proteins in serum as a common denominator in glomerulonephritis was reinforced in numerous subsequent studies (Durante et al., 1976; Schur and Sandson, 1968; Verroust et al., 1974). To this day, clinicians monitor serum C3 and C4 levels in patients with glomerulonephritis, and kidney biopsies are routinely stained for C3 and C1q deposits. These analyses support the connection between immune-complexes (ICs), complement activation in glomeruli, and consumption of plasma complement proteins. Nevertheless, some inconsistent findings have emerged. For example, complement is
Abbreviations: MPGN, membranoproliferative glomerulonephritis; IC, immune-complex; aHUS, atypical hemolytic uremic syndrome; ANCA, anti-neutrophil cytoplasmic antibody; FHR, factor H related protein; SCR, short consensus repeat; C3G, C3 glomerulopathy; MGRS, monoclonal gammopathy of renal significance; PGNMID, proliferative glomerulonephritis with monoclonal IgG deposits; MBL, mannose binding lectin; IgAN, IgA nephropathy; ESRD, end-stage renal disease; ANCA, antineutrophil cytoplasmic antibody; MPO, myeloperoxidase; PR3, proteinase 3.
* Corresponding author.
E-mail address: f.poppelaars@umcg.nl (F. Poppelaars).
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activated in the glomeruli in diseases that are not associated with low serum levels of complement proteins (West et al., 1973). Furthermore, it was shown that C3 continued to be consumed in two patients with membranoproliferative glomerulonephritis (MPGN) even after the pa-tients underwent removal of both kidneys (Vallota et al., 1971). Although MPGN is a renal limited disease, the findings demonstrated that C3 was consumed outside of the kidneys in these patients. Thus, a fall in systemic complement levels does not necessarily reflect activation within the kidney, and activation within the kidney does not necessarily cause systemic hypocomplementemia.
Recent discoveries in basic complement biology have shed new light on the pathogenesis of kidney disease. New complement activating molecules have been identified, such as the additional initiators of the lectin pathway. These discoveries have prompted investigators to study these new molecules in kidney disease using genetic and protein anal-ysis, as well as elegant animal models. Complement research has also led
to new treatments for kidney diseases, including the use of eculizumab (a monoclonal antibody against C5) for atypical hemolytic uremic syn-drome [aHUS, (Legendre et al., 2013)] and avacopan (a C5a receptor inhibitor) for anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (Jayne et al., 2017). Neither ANCA-associated vasculitis nor aHUS are IC diseases, and it is interesting that the first kidney diseases treated with complement inhibitors fall outside the original paradigm of complement as a mediator of antibody-induced disease.
Basic research into complement biology has helped understand the pathogenesis of kidney disease, but the reverse is also true. Detailed molecular studies of patients with various kidney diseases have provided insight into the basic biology of the complement system. For example, it is now known that variations in the structure and levels of the factor H related proteins (FHRs) are associated with kidney disease. Initially, the FHR proteins were assumed to be complement regulators (Eberhardt et al., 2013; Heinen et al., 2009), but since they lack the domains of
Table 1
Mechanisms of complement activation in immune-complex-mediated kidney disease.
Trigger for Complement Activation Kidney biopsy findings
Animal models Knock-out or
treatment Effect
Cryoglobulinemia Renal deposition of cryoglobulins (Ig, either monoclonal or mixed, that precipitate in-vitro below 37 ◦C).
Glom. deposition of IgM and C3. Similar or lesser amounts of IgG and C1q. Often C4(d). Possibly LP involvement (i.e. MBL & MASP-1).
C1q deficiency in mice Partly protective C3, CFB, C5 deficiency
in mice Protective CD59 deficiency in
mice No effect
Overexpression of
Crry in mice Not protective
Anti-Glomerular Basement Membrane (GBM) Disease
Circulating IgG auto-antibodies against type IV collagen, resulting in IC formation on the GBM in the kidney.
Linear IgG deposits along the GBM. Co-deposition of C1q and C3 are common. C4d and C5b-9 can be present. Potential AP involvement (factor B & properdin).
C1q deficiency in mice Not protective C1q/CFB/C2
deficiency in mice Partly protective C4 deficiency in mice Partly protective C3, CR3 deficiency in
mice Protective
Anti-CR3 mAb in rats Protective Overexpression of
Crry in mice Protective
Lupus Nephritis
Loss of tolerance to self-antigens and exaggerated B-cell and T-cell responses leading to autoantibody production and subsequent glomerular deposition of IC.
Glom. co-localization of IgG, IgM, IgA with the C1q, C4 and C3 (‘full house pattern’). C5b-9 is generally present. Presence of the AP, namely factor B has been shown.
C1q, C4, C4/C3
deficiency in mice Harmful C3, C3aR, CFH
deficiency in mice Harmful CFD, CFB deficiency in mice Protective MASP-1&3 deficiency in mice Protective C5aR1 deficiency in mice Protective Crry-Ig treatment in mice Protective CR2-fH, CR2-Crry, sCR2 treatment in mice Protective siRNA against CFB in mice Protective
C5aR antagonist, anti-
C5 mAb in mice Partly protective
Membranous Nephropathy
Circulating IgG4 auto-antibodies against M-type phospholipase A2 receptor, leading to IgG4 deposition on the podocytes.
IgG4 deposits in the subepithelial space of the glomerular capillary wall. C4d, C3 and C5b-9 are seen in the majority of cases, while C1q is absent. MBL deposits are seen in half of the cases.
C6 deficiency in rats Protective Anti-Crry Abs in rats Harmful
IgA Nephropathy Glomerular deposition of circulating ICs formed by autoantibodies binding to galactose-deficient IgA1.
Mesangial deposition of IgA1. Deposits of C3, properdin and C5b-9 are generally present, while C1q is typically absent. In 1/
3 – ¼, LP involvement (i.e. MBL, Ficolin-2). Likewise, C4d is present in 1/
3 of cases C3aR deficiency in mice Protective C5aR1 deficiency in mice Protective
Abbreviations: Ab, antibody; AP, alternative pathway; C3aR, C3a receptor; C5aR1, C5a receptor1; CD59, membrane attack complex-inhibitory protein; CFB, com-plement factor B; CFD, comcom-plement factor D; CFH, comcom-plement factor H; CR2, comcom-plement receptor 2; CR3, comcom-plement receptor 3; Crry, comcom-plement receptor 1 (CR1)–related gene y; fH, factor H; GBM, glomerular basement membrane; IC, immune complex; Ig, Immunoglobulin; LP, lectin pathway; mAb, monoclonal antibody; MBL, mannose-binding lectin; MASP-1, MBL-associated serine protease 1; MASP-3, MBL-associated serine protease 3; siRNA, small interfering RNA; sCR2, soluble CR2.
Factor H that are responsible for the complement inhibiting activity (CCPs1–4) this was called into question (Timmann et al., 1991). Instead, molecular studies of patients with C3 glomerulopathy (C3G) revealed that multiple CFHR variants are associated with the disease (Chen et al., 2014; Gale et al., 2010; Malik et al., 2012; Tortajada et al., 2013), and these clinical observations motivated experiments examining the ability of the FHRs to antagonize factor H on target surfaces (Goicoechea de Jorge et al., 2013).
2. Complement and immune-complex-mediated glomerulonephritis
Several autoimmune diseases, including SLE and cryoglobulinemia, are associated with IC glomerulonephritis (Table 1). So, why is the glomerulus such a common site of IC deposition? This may be related to the fact that the glomerular basement membrane (GBM) is negatively charged, and fenestrations in glomerular endothelial cells expose the GBM to plasma proteins. As a result, the size and charge of antigens or ICs may favor binding to the GBM. Consequently, immune responses to antigens can start elsewhere in the body and then migrate to the glomerulus due to physicochemical interactions with the capillary wall (Fig. 1).
Antibodies may also bind to antigens expressed within the glomer-ulus. In anti-GBM disease, for example, antibodies bind to a specific epitope within type IV collagen of the GBM (Pedchenko et al., 2010). On the other hand, the epitope can also be generated by conformational changes in the protein. In membranous nephropathy, antibodies react with epitopes contained in intrinsic glomerular proteins, most commonly the M-type phospholipase A2 receptor (PLA2R) or the thrombospondin type-1 domain-containing 7A (THSD7A) (Beck et al., 2009; Tomas et al., 2014). Epitope spreading can also occur, resulting in generation of antibodies reactive to additional epitopes in PLA2R (Seitz-Polski et al., 2016). In SLE, several intrinsic renal antigens have
been identified, including α-enolase, annexin A1, and annexin A2
(Bruschi et al., 2015; Yung et al., 2010). Other proteins that are ordi-narily sequestered can also become exposed during inflammation. For example, neutrophil extracellular traps (NETs) are released by stimu-lated neutrophils, and contain high concentrations of many of the an-tigens targeted by SLE autoantibodies (Gupta and Kaplan, 2016). NETs activate the complement system, which can mediate their inflammatory effect and also inhibit NET degradation (Leffler et al., 2012). Thus, NETs can deposit a rich source of antigen within the kidney, and also provide an additional mechanism for activating complement.
In additional to IC-mediated inflammation, clonal proliferation of B cells and plasma cells can produce immunoglobulin or immunoglobulin fragments that deposit within the kidney and cause disease (Leung et al., 2019). When the burden of immunoglobulin-producing cells is not suf-ficient to be termed malignant but still causes kidney damage, the dis-ease is termed a monoclonal gammopathy of renal significance (MGRS) (N. Leung et al., 2012). Free light chains are toxic to the renal tubules and can cause glomerular injury but do not activate complement. In contrast, heavy chain deposition is associated with co-deposition of complement proteins and hypocomplementemia if the heavy chain is of an IgG1 or IgG3 isotype (Bridoux et al., 2017). Similarly, proliferative glomerulonephritis with monoclonal IgG deposits (PGNMID) is a form of glomerulonephritis associated with IgG deposition, glomerular C3 de-posits and systemic hypocomplementemia (Nasr et al., 2009). These diseases are not autoimmune per se, but they are examples of antibody-induced complement activation within the kidney.
The treatment of IC-mediated glomerulonephritis primarily focuses on suppressing the adaptive immune system. In lupus nephritis, for example, the standard treatments include corticosteroids in combina-tion with mycophenolate mofetil or cyclophosphamide (Ginzler et al., 2005). The protocol for membranous disease is similar, although ritux-imab (a B cell depleting drug) is also now a first line agent (Fervenza et al., 2019). What, then, is the role for therapeutic complement inhi-bition in these diseases? The cytotoxic medications used to suppress autoimmunity can take several weeks to work. Furthermore, they are not always effective. Complement inhibitors, therefore, might be useful for rapidly suppressing kidney damage by preexisting ICs, and for sup-pressing antibody-mediated inflammation in patients who do not respond to immunosuppressive treatment.
3. Immune complex–mediated disease with non-classical pathway-activating immunoglobulin
The interaction between immunoglobulins and the complement system goes well beyond the classical pathway. Early reports demon-strated antibody-mediated lysis of virus-infected cells by serum depleted of, or deficient in, C2 or C4, whereas this ability was not retained in Factor-B-depleted serum (Joseph et al., 1975). Moreover, these findings were confirmed using purified components of the alternative pathway, excluding the contribution of other pathways via a bypass mechanism (Patrick Sissons et al., 1979). Lysis by the alternative pathway was only seen when IgG or Fab was bound to these cells, and these antibodies did not initiate complement activation by themselves (Sissons et al., 1980). Later studies revealed that fixed IgG permits C3b to covalently bind to the cell-surface and combine with Factor B without being inhibited by Factor H (Reiter and Fishelson, 1989). In addition to the alternative pathway, activation of the lectin pathway can also be triggered by an-tibodies or ICs (Malhotra et al., 1995; Roos et al., 2001; Zhang et al., 2006). Several studies have demonstrated that mannose binding lectin (MBL) can bind to immunoglobulins and initiate complement activation (Arnold et al., 2006). Others have reported that ficolins can also activate complement in an antibody-dependent fashion, namely binding of Ficolin-3 to IgM or IgG (Man-Kupisinska et al., 2016).
Recent work has expanded our understanding of the molecular mechanisms that determine the complement binding properties of im-munoglobulins. Polymerization of immunoglobulins was one of the first Fig. 1. Immune complex-mediated glomerulonephritis. The glomerular
capillary wall is composed of three layers: a fenestrated endothelial layer, a glomerular basement membrane (GBM), and an epithelial layer (podocytes). A) Immune complexes can form in the capillary wall when: 1) circulating antigen becomes trapped in the capillary wall, 2) pre-formed circulating immune complexes become trapped in the capillary wall, or 3) circulating immuno-globulin binds to antigens expressed within the capillary wall. B) Immune complexes can deposit: 1) in the subendothelial space, 2) within the GBM, and 3) in the subepithelial space. They can also form within the mesangium (not shown). The location of the immune complexes affects the histologic and clinical patterns of injury.
identified molecular events that governs complement activation (Hiemstra et al., 1987). The polymeric structure of immunoglobulins increases the avidity for complement factors (Diebolder et al., 2014;
Hiemstra et al., 1987). Glycosylation is another important characteristic of antibodies that has a profound impact on their complement-activating ability. For example, non-glycosylated IgG lost the ability to activate complement (Nose and Wigzell, 1983). More recent studies found that sialylation and carbamylation impaired antibody-mediated complement activation (Koro et al., 2014; Quast et al., 2015). What, then, determines which complement pathway is initiated in IC–mediated diseases? Not only does the immunoglobulin isotype and subclass result in distinct complement binding-properties, but changes in glycosylation also im-pacts their affinity to complement proteins. As such, IgG and IgA gly-coforms that lack galactose become accessible for MBL binding, and thus selectively activate the lectin pathway (Malhotra et al., 1995; Terai et al., 2006). Considered together, these findings not only show that complement binding is dependent upon the conformation of the Fc domain and that glycosylation is an important contributor to the ability of antibodies to activate complement, but they also reveal a connection between changes in glycosylation patterns and the pathway of com-plement that is initiated.
3.1. IgA nephropathy
IgA nephropathy (IgAN) is the most common cause of glomerulo-nephritis worldwide, and up to 40 % of cases progress to end-stage renal disease (ESRD) within 20 years after diagnosis (Barbour et al., 2019). The pathogenesis of IgAN is believed to have a multi-hit mechanism, consisting of (1) elevated levels of galactose-deficient IgA1, (2) pro-duction of autoantibodies specific for the altered IgA1, (3) subsequent formation of circulating ICs, and (4) glomerular deposition of these ICs leading to renal inflammation and injury (Suzuki et al., 2011).
The first reports of IgAN implicated the alternative pathway. More specifically, deposition of C3, properdin, and C5b-9 was found in the glomerular mesangium, whereas IgG and C1q were not detected (Evans et al., 1973; Rauterberg et al., 1987). Consistent with this, IgA has been shown to activate the complement system through the alternative pathway in vitro (Hiemstra et al., 1987). Later reports demonstrated glomerular deposition of MBL and MASP-1 in a subset of patients with IgAN, suggesting the involvement of the lectin pathway (Endo et al., 1998). Mesangial deposition of ficolin-2 and C4d has also been reported (Faria et al., 2015; Hisano et al., 2001; Roos et al., 2006). An important breakthrough came when Roos et al. demonstrated that glomerular deposition of MBL was associated with greater disease severity and a worse prognosis (Roos et al., 2006). In accordance, other investigators have found an association of mesangial C4d deposition with worse clinical outcomes in IgAN patients, including faster disease progression and lower renal survival (Espinosa et al., 2014; Faria et al., 2015;
Segarra et al., 2018). A recent study demonstrated that arteriolar C4d deposition in IgAN is also associated with faster disease progression, indicating that complement activation in IgAN is not limited to the glomeruli (Faria et al., 2020).
Although plasma C3 levels are usually normal in IgAN, plasma levels of C3 activation fragments are elevated in some patients and associated with circulating IgA-ICs, renal histology and progression (Tanaka et al., 1991; Wyatt and Julian, 1988; Zwirner et al., 1997). Recently, an elegant study by Medjeral-Thomas et al. showed decreased plasma levels of MASP-3 in IgAN patients (Medjeral-Thomas et al., 2018). Further-more, low plasma MASP-3 levels were associated with faster disease progression. These findings warrant further research since MASP-3, an alternative splice product of the MASP1 gene, was shown to activate pro-factor D into factor D, linking the lectin with the alternative pathway (Dobo et al., 2016).
Large genome–wide association studies identified the gene deletion of complement factor H–related genes 1 and 3 (CFHR1/3) as a protective locus for IgAN (Kiryluk et al., 2014). This gene deletion was later shown
to be associated with increased plasma levels of factor H, decreased plasma C3a levels, and reduced glomerular staining for C3 in IgAN (Zhu et al., 2015). Subsequent studies have further examined the role of the FHR proteins in the pathophysiology of IgAN. Plasma levels of FHR-1 and the FHR-1/Factor H ratio are increased in IgAN and were shown to be associated with progressive disease (Medjeral-Thomas et al., 2017;
Tortajada et al., 2017). Furthermore, elevated plasma FHR-5 is associ-ated with histologic markers of disease severity in IgAN ( Medjeral-Th-omas et al., 2017). In addition to plasma levels, glomerular staining for FHR-5 has also been correlated with progressive IgAN as well as glomerular complement activation (Medjeral-Thomas et al., 2018). These findings are in line with the hypothesis that factor H regulates complement activation and thereby ameliorates the severity of IgAN, while FHR-1 and FHR-5 antagonize factor H and thereby contribute to IgAN development and disease progression (Thurman and Laskowski, 2017).
3.2. Membranous nephropathy
Membranous nephropathy (MN) is a common cause of the nephrotic syndrome in adults and is characterized by IC deposits in the sub-epithelial space of the glomerular capillary wall. Up to 40 % of MN cases progress to ESRD over a period of 5–10 years (Couser, 2017). In approximately 25 % of cases, MN is the result of a systemic disease, such as neoplasia, infection, or medications, in which case it is termed “sec-ondary MN.” In contrast, primary MN is a kidney–specific autoimmune disease induced by autoantibodies that recognize proteins expressed on the surface of podocytes (Couser, 2017). A landmark paper by Beck et al. identified M-type phospholipase A2 receptor (PLA2R) as the main podocyte target antigen for autoantibodies in primary MN (Beck et al., 2009).
The contribution of complement activation to the pathogenesis of MN was first suggested by the finding that depletion of C3 prevented proteinuria in an animal model (Salant et al., 1980). A similar role for complement in human MN is suggested by the fact that glomerular complement deposits are commonly detected in these patients. Initial studies reported C3 deposition in half of the patients with MN and detection of C3 associated with greater proteinuria (Doi et al., 1984). Using contemporary techniques, C3 deposition is present in 70–100 % of patients with MN (Endo et al., 2004; Zhang et al., 2012).
Deficiency of C6 prevented renal injury and delayed the onset of proteinuria in a rat model, indicating a prominent role for the membrane attack complex (Groggel et al., 1983). The importance of the membrane attack complex in human MN is supported by the consistent finding of C5b-9 in the subepithelial IC deposits (Papagianni et al., 2002). Furthermore, urinary excretion of sC5b-9 correlates with disease activity in both experimental MN and patients with MN (Pruchno et al., 1989;
Schulze et al., 1991). Studies have also revealed that inhibition of complement regulators is necessary for development of disease in models of MN (Quigg et al., 1995; Schiller et al., 1998). This indicates that MN may not solely depend on antibody-mediated complement activation, but also requires defective complement regulation.
While MN is an IC-mediated disease, the majority of the IgG in the glomeruli and anti-PLA2R antibodies in the serum of patients with MN is IgG4 (Beck et al., 2009), which does not bind C1q or activate the clas-sical pathway (Bindon et al., 1988). Interestingly, C4d is seen in the majority of primary MN cases (Kusunoki et al., 1989) ( Espinosa--Hernandez et al., 2012; Val-Bernal et al., 2011), but C1q is usually ab-sent (Ma et al., 2013). This pattern suggests that activation occurs via the lectin pathway. Indeed, MBL colocalizes with IgG4 in biopsy specimens from children with MN (Segawa et al., 2010). Similarly glomerular MBL deposits were detected in almost half of adult MN cases, and the in-tensity correlated with the inin-tensity of IgG4 staining (Hayashi et al., 2018). Furthermore, MN patients with glomerular MBL may have a less favorable clinical outcome. Nevertheless, Bally et al. described five MN patients who had complete MBL deficiency, indicating that complement
activation can also occur through the alternative pathway and that the lectin pathway is not always required (Bally et al., 2016).
The mechanism by which IgG4 activates the complement system remains a speculative issue, but it may be due to altered glycosylation of the immunoglobulin. Alternatively, IgG autoantibodies may undergo a subclass switch from IgG1 to IgG4 during disease progression. It is possible, therefore, that it is IgG1 that initially activates the complement system even though IgG1 is less prominent than IgG4 by the time of biopsy (Huang et al., 2013).
Altogether, these findings demonstrate a role for the lectin pathway in glomerular complement activation in IC–mediated kidney diseases. Furthermore, observational studies suggest a significant contribution of the lectin pathway to the progression of kidney diseases. This has broadened our understanding of the mechanisms of complement acti-vation in IC–mediated kidney diseases and has led to the development of new therapeutics to inhibit the lectin pathway in renal diseases. More-over, these findings also highlight that other pathways, besides the classical pathway, can be involved in IC-mediated diseases.
4. Complement in non-immune-complex glomerulonephritis
Although complement was initially described in its relation to anti-bodies, it rapidly became clear that complement could also act without the presence of antibodies. Indeed, evolutionary insight into the origin of the complement system suggests that C3 and factor B were the initial components to evolve, and that this ancestral alternative pathway emerged in multicellular animals long before the appearance of immu-noglobulins (Elvington et al., 2016). Research has also revealed that uncontrolled alternative pathway activation is central to the pathogen-esis of several kidney diseases that are not associated with IC deposits, including C3G and aHUS (Noris and Remuzzi, 2015) (Table 2).
Activation of the complement system can take place on surfaces (solid-phase), as well as in plasma (fluid-phase). The alternative pathway is constitutively active in the fluid-phase as a result of the hydrolysis of C3, a process called ‘tickover’ (Mathern and Heeger, 2015). The hydrolyzed C3, denoted as C3(H2O), can form a
C3-convertase with factor B and cleave plasma C3. C3b generated by tickover can then bind to nearby surfaces and initiate solid-phase alternative pathway activation. Another crucial element of the com-plement system is the amplification loop (Lachmann, 2009). Regardless of the initiation pathway, the C3b generated by complement activation can interact with factor B to form additional C3-convertase, creating a positive feedback loop. Regulation of the alternative pathway and amplification loop is therefore essential to constrain complement acti-vation and to prevent unintended host tissue injury. Control of the complement system is maintained by a group of complement regulatory proteins in the circulation (fluid-phase regulators) as well as on cell surfaces (solid-phase regulators) (Zipfel and Skerka, 2009). However, this control can be overwhelmed through congenital or acquired drivers such as: (1) reduced expression or diminished function of complement regulators due to genetic deficiency or autoantibodies, (2) excessive complement activity through ‘gain-of-function’ mutations or autoanti-bodies stabilizing the C3-convertase (7).
4.1. C3 glomerulopathy
C3G is a rare complement-mediated glomerular disease. Before C3G was formally defined, most cases were probably classified under the histologic pattern of membranoproliferative glomerulonephritis (MPGN). The diagnosis is now based on detection of glomerular C3 deposition with absent or scanty immunoglobulin deposition (Pickering et al., 2013). C3G is further subdivided in dense deposit disease (DDD) and C3 glomerulonephritis (C3GN) based on the pattern of electron dense deposits within the glomerular basement membrane by electron microscopy. C3G is caused by dysregulation of the alternative pathway C3-convertase via congenital and/or acquired complement
abnormalities, and up to 80 % of the C3G cases have autoantibodies and/or mutations affecting the alternative pathway (Servais et al., 2012). However, the presence of these complement abnormalities in unaffected relatives suggests that it may be a ‘two-hit’ disease or that additional drivers are needed.
The link between alternative pathway dysregulation and C3G is older than is often appreciated. More than 25 years ago, Høgåsen et al. described a porcine model of DDD and went on to show that this was caused by a hereditary deficiency of factor H (Jansen et al., 1993). These findings were later confirmed in factor H knock-out mice, establishing that uncontrolled activation of C3 results in a C3G-like phenotype (Pickering et al., 2002). Murine models of the disorder have provided important insights, although these models do not fully reflect the eti-ology of C3G. Abnormalities in complement genes are only detected in 25 % of C3G cases, while acquired drivers of complement dysregulation are present in most cases (Goodship et al., 2012; Marinozzi et al., 2017a,
b; Sethi et al., 2011). The mechanisms behind the mutations are pri-marily either gain-of-function mutations in the complement activating components (C3 and CFB) or loss of function mutations in the comple-ment regulators (MCP, CFH, CFI) (Pickering et al., 2013; Servais et al., 2012). Variants (i.e. mutations, deletions, duplications and rearrange-ments) in complement factor H-related (CFHR) genes have also been associated with C3G (Zipfel et al., 2020), although the functional con-sequences of genetic modifications in FHR proteins found in C3G are still not yet fully understood.
Acquired complement abnormalities in C3G are caused by autoan-tibodies targeting the components or regulators of the C3-convertase (Servais et al., 2012). The most well-known autoantibody is C3 nephritic factor (C3NeF), an antibody that recognizes the neo-epitope of the C3-convertase. C3NeF stabilizes the C3-convertase and thereby ex-tends its half-life from seconds to almost an hour and consequently prolongs the C3 cleaving activity (Daha et al., 1976). C3NeF is found in 70–80 % of patients with DDD and 40–50 % of patients with C3GN (Bomback and Appel, 2012; Pickering et al., 2013). Other autoanti-bodies in C3G include antiautoanti-bodies that bind to native factor B (stabilizing the alternative pathway C3-convertase via factor B) or anti-factor H antibodies (blocking regulation of the alternative pathway C3-convertase by factor H) (Blanc et al., 2015; Marinozzi et al., 2017a,b;
Servais et al., 2012). Although the functional effect of C3NeF is similar to the mutations in complement genes associated with this disease, it is still uncertain as to whether C3NeF causes C3G or is an epiphenomenon. 4.2. Post-infectious glomerulonephritis
Post-infectious glomerulonephritis shares key clinical and patho-logical features with C3G. Post-infectious GN is a common cause of GN that develops within weeks after exposure to an infectious pathogen and typically presents as an acute nephritic syndrome. The pathological hallmark of post-infectious GN consists of proliferative GN and sub-epithelial deposits (or ‘humps’) on electron microscopy, together with bright C3 staining on immunofluorescence with or without IgG deposi-tion (Kambham, 2012). Considerable overlap therefore exists with his-topathological findings in C3GN (Pickering et al., 2013). The onset of C3GN can also be triggered by infections and C3GN is frequently pre-ceded by an upper respiratory tract infection (Medjeral-Thomas et al., 2014; Vernon et al., 2012), but post-infectious GN is sometimes diag-nosed based on biopsy findings without any evidence of a preceding infection. Discriminating between C3GN and post-infectious GN, therefore, can be a diagnostic challenge.
The clinical course of post-infectious GN is generally self-limited and resolves within weeks. In a minority of cases, however, post-infectious GN can persist and even lead to ESRD (Hoy et al., 2012). Although post-infectious GN was initially seen as an IC–mediated disease, recent data show that alternative pathway dysregulation might underlie the disease in some cases (Sethi et al., 2013). The glomerular deposits can contain immunoglobulins, C4d and C3 (Sethi et al., 2015; Verroust et al.,
Table 2
Mechanisms of complement activation in non-immune-complex-mediated kidney disease.
Trigger for Complement Activation Kidney biopsy findings
Animal models Knock-out or
treatment Effect
C3 Glomerulopathy (C3G)
An environmental trigger combined with acquired (autoantibodies) and/or inherited (genetic mutations) complement abnormalities resulting into dysregulation of the alternative pathway C3-convertase.
C3G is divided by electron microscopy in DDD or C3GN based on the presence of dense osmiophilic deposits within the lamina densa of the GBM. The pathological hallmark is glomerular C3 deposition with absent or scanty immunoglobulin deposition.
CFH deficiency
in mice DDD-like phenotype CFB, CFI deficiency in CFH-/- mice Protective Properdin deficiency in CFH-/- mice Lethal MASP-1&3, CFD deficiency in
CFH-/- mice Not protective CR3 deficiency in CFH-/- mice Harmful C5, C5aR1 deficiency in CFH-/- mice Protective sCR1, Anti-C5 mAb in CFH-/- mice Protective mFH, MFHR1, CR2-fH in CFH-/- mice Protective Post-infectious glomerulonephritis
An infectious trigger combined with acquired and/or genetic alternative pathway dysregulation.
Subepithelial deposits on electron microscopy with bright C3 staining with or without IgG deposition. Glom. C4d deposition is only seen in half of the cases, while C1q staining is only seen in a minority.
Not assessed
Atypical Hemolytic Uremic Syndrome (aHUS)
Alternative pathway dysregulation due to inherited (genetic mutations) and/or acquired (autoantibodies) drivers combined with environmental triggers.
Thrombotic microangiopathy (TMA) without Ig deposition.
CFHΔ16-20
deficiency in mice TMA/ aHUS Properdin, C5
deficiency in
CFHΔ16-20-/- mice Protective C5aR deficiency in
CFHΔ16-20-/- mice Partly protective C6, C9 deficiency in
CFHΔ16-20-/- mice Partly protective Anti-Properdin mAb in CFHΔ16-20-/- mice Protective Anti-C5 mAb in CFHΔ16-20-/- mice Protective Gain of function C3 mutation (C3KI) in mice TMA/ aHUS C5 deficiency in
C3KI mutated mice Protective Anti-C5 mAb in C3KI
mutated mice Protective
Anti-neutrophil cytoplasmic autoantibody (ANCA)- associated
glomerulonephritis
Unknown initial trigger(s), but neutrophils, ANCA and complement form a positive feedback loop.
Few or no IC deposits (so-called “pauci-immune”). Glom. deposition of C3, factor B, properdin, and C5b-9 can be found in ANCA-associated glomerulonephritis. MASP-2, C4, Properdin deficiency in mice Not protective CFB, C5, C5aR1
deficiency in mice Protective C5aR2 deficiency in
mice Harmful
C3aR, C6 deficiency
in mice Not protective C3 depletion by CVF
treatment in mice Protective Anti-C5 mAb, C5aR
antagonist in mice Protective Abbreviations: ANCA, Anti-neutrophil cytoplasmic autoantibody; aHUS, atypical hemolytic uremic syndrome; AP, alternative pathway; C3aR, C3a receptor; C3G, C3 Glomerulopathy; C3GN, C3 Glomerulonephritis; C3KI, Homozygous C3 p.D1115 N; CVF, cobra venom factor; C5aR1, C5a receptor 1; C5aR2, C5a receptor 2; CFB, complement factor B; CFD, complement factor D; CFH, complement factor H; CFI, complement factor I; CR2, complement receptor 2; CR3, complement receptor 3; DDD, dense deposit disease; fH, factor H; GBM, glomerular basement membrane; IC, immune complex; Ig, Immunoglobulin; MASP-1, mannose-binding lectin- associated serine protease 1; MASP-2, mannose-binding lectin-associated serine protease 2; MASP-3, mannose-binding lectin-associated serine protease 3; mFH, mini factor H; MFHR1, fusion protein of factor H and related protein 1; sCR1, soluble complement receptor 1; TMA, thrombotic microangiopathy.
1974). However, glomerular C4d staining is only seen in approximately half of post-infectious GN cases, while C1q staining is only seen in a minority of cases and is often low-intensity (Sethi et al., 2013; Verroust et al., 1974). Moreover, C3 staining is usually positive at a greater in-tensity than IgG (Khalighi et al., 2016; Sethi et al., 2013). Altogether, these findings support the notion that, at least in some cases, the alter-native pathway is activated in post-infectious GN. Involvement of the alternative pathway is further suggested by the low C3 plasma levels with normal plasma levels of C4 in a substantial number of patients combined (Angioi et al., 2016). Moreover, transient expression of C3NeF and complement regulatory gene mutations have been detected in pa-tients with post-infectious GN (Angioi et al., 2016; Sethi et al., 2013).
Clinical guidelines acknowledge the similarity of post-infectious GN and C3GN, but it is unclear if these two entities are clinically distinct or represent a continuum. The term “atypical post-infectious GN” has been proposed to describe the subset of patients in whom the alternative pathway is activated and where the disease course is not self-limited, but persists and can even cause end-stage kidney disease (Sethi et al., 2013). Some have even suggested that atypical post-infectious GN falls in the spectrum of C3G (Al-Ghaithi et al., 2016; Angioi et al., 2016), although others contend that these patients are misdiagnosed as post-infectious GN and should be re-classified as C3GN (Sandhu et al., 2012). A C3G consensus report recommended labeling these cases as ‘GN with domi-nant C3 (infection related)’ and following their progress in order to allow further differentiation (Pickering et al., 2013).
4.3. Atypical hemolytic uremic syndrome
Several different complement-mediated kidney diseases share the
same pathophysiological feature of a “hyperinflammatory complement phenotype” (Lachmann, 2009). The broad outlines are clear, uncon-trolled complement activation mediates renal injury and dysfunction. What, then, determines whether a patient with alternative pathway dysregulation develops one disease but not the other? A study by Pick-ering et al. revealed distinct genotype–phenotype correlations in factor H that lead to either aHUS or C3G (Pickering et al., 2007). Factor H regulates complement activation on tissue surfaces (the solid-phase), as well as in blood (the fluid-phase). Factor H contains several functional domains. The N-terminus (SCRs 1–4) is crucial for decay accelerating activity and co-factor activity, whereas an internal region (SCRs 6–8) and the C-terminus (SCRs 19–20) are essential for cell surface recogni-tion. The majority of Factor H mutations (and autoantibodies to Factor H) linked to aHUS affect the SCR 19–20 region (Gurjar et al., 2018). These mutations impair the ability of factor H to control complement activation on surfaces, while regulation can still take place in plasma (Fig. 2). C3G, on the other hand, is associated with defective function of factor H in the fluid-phase (Pickering et al., 2013).
In support of this paradigm, animal models demonstrated that ho-mozygous factor H deficiency triggers excessive fluid-phase complement activation resulting into the development of C3G. Loss of the SCR 19–20 region of factor H, on the other hand, causes spontaneous thrombotic microangiopathy (Pickering et al., 2007). In reality, however, the causes of aHUS and C3G are closely intertwined. Complement mutations and/or autoantibodies associated with C3G have been identified in pa-tients with aHUS and vice versa. Moreover, selected individuals have developed both diseases at different times, indicating that the boundary between C3G and aHUS may be subtle (Goodship et al., 2017).
Fig. 2. Complement regulation by factor H.
Factor H contains 20 repeating structural units referred to as short consensus repeats (SCRs). The first four SCRs comprise the complement inhibitory region of the protein whereas SCRs 6- 8 and 19-20 mediate binding of the protein to surfaces (e.g. cell membranes and extracellular matrix). Defects in factor H expression or SCRs 1-4 hamper control of the alternative pathway in the fluid phase, and associate with C3 glo-merulopathy (C3G). Defects in the binding re-gions hamper control of the alternative pathway on surfaces, and are associated with atypical hemolytic uremic syndrome (aHUS).
4.4. Anti-neutrophil antibody associated vasculitis
Vasculitis refers to inflammation of the vessel wall that results in structural and functional damage. This inflammatory process can affect different types and sizes of blood vessels in various organs. Vasculitis is classified based on clinicopathological features (Singh et al., 2006), one of which is the presence of antineutrophil cytoplasmic antibodies (ANCA). The ANCA-associated vasculitides (AAV) are a heterogeneous group of diseases characterized by vascular inflammation of small ves-sels with few or no IC deposits (so-called “pauci-immune”) (Kallenberg, 2014). A major complication in ANCA vasculitis is pauci-immune glomerulonephritis in which the glomerular capillaries are affected (Jennette and Nachman, 2017).
The ANCA are directed against myeloperoxidase (MPO) or protein-ase 3 (PR3), proteins stored within neutrophil granules (Jennette and Nachman, 2017). Although the pathogenic role of ANCA remains controversial, in vitro and animal studies demonstrate that these auto-antibodies trigger the formation of small vessel vasculitis (Xiao et al., 2002). A current hypothesis for the pathogenesis of ANCA vasculitis involves primed neutrophils that express increased levels of MPO or PR3 on their surface (Nakazawa et al., 2019). These antigens are then recognized by ANCA, causing neutrophil activation, degranulation as well as transmigration, and subsequent vascular injury. Signaling of C5a through the C5a receptor 1 (C5aR1) appears to be critical to this process (Min Chen et al., 2017).
Traditionally, ANCA vasculitis was not thought to be complement- mediated because serum levels of C3 and C4 are rarely low and com-plement deposition in biopsies is generally less pronounced than in IC diseases. Interest in the complement system in ANCA vasculitis was initially sparked by an elegant series of animal experiments. Depletion of C3 by pretreatment with cobra venom factor was protective in a model of MPO-ANCA vasculitis and glomerulonephritis (Xiao et al., 2007). Animal models implicated alternative pathway activation, as Factor B deficient mice were protected whereas C4 deficient mice were not (Xiao et al., 2007) although protection was not seen with properdin deficiency (Freeley et al., 2016). MASP-2 was not required for disease, excluding a role for the lectin pathway via a C4 bypass mechanism (Freeley et al., 2016; Gaya da Costa et al., 2018). The detrimental role of C5a in this model was demonstrated by the fact that C5 deficiency was protective, whereas C6 deficiency was not (Schreiber et al., 2009; Xiao et al., 2014). Furthermore, ANCA-induced glomerulonephritis was shown to be attenuated in C5aR1-deficient mice, while deficiency of C5aR2 resulted in more severe disease (Xiao et al., 2014). Studies also demonstrated that treatment with an anti-C5 antibody or C5aR antagonist is protective in a murine model of the disease (Huugen et al., 2007; Xiao et al., 2014). C3a, unlike C5a, does not seem to form a key effector molecule in complement-mediated inflammation and injury in MPO-ANCA vascu-litis (Dick et al., 2018).
In patients with active ANCA vasculitis, urinary, and plasma levels of C3a, Bb, C5a, and sC5b-9 are significantly higher compared to those in remission (Gou et al., 2013a). In contrast, plasma levels of properdin are reduced during active disease compared to remission, while plasma C4d is not different between patients with active disease and in remission. Moreover, different complement components and activation products have also been linked to outcome (Gou et al., 2013a; Manenti et al., 2015). Multiple studies have found a certain degree of complement deposition in renal biopsies from patients with ANCA-associated glomerulonephritis, and this has associated with a greater degree of proteinuria and worse renal function (Chen et al., 2009; Gou et al., 2013b; Haas and Eustace, 2004; Xing et al., 2009). Glomerular deposi-tion of properdin, factor B, C3, and C5b-9 can be found in ANCA-associated glomerulonephritis.
In conclusion, the alternative pathway appears to be activated by distinct mechanisms in the glomerular diseases as discussed above. Furthermore, complement activation in ANCA-associated glomerulo-nephritis may occur outside of the kidney. Yet the common involvement
of this complement pathway in multiple different glomerular diseases suggests a susceptibility of the glomerulus to complement mediated injury that may be shared by a wide variety of diseases.
5. Complement therapeutics in kidney disease
Given the important role that complement activation plays in the pathogenesis of numerous kidney diseases, many of the new anti- complement drugs that are in clinical development are being tested in these diseases. There are new drugs that selectively block the alternative pathway, while others block complement activation at the level of C3, and some specifically block C5a or the C5a receptor (Fig. 3). The mo-lecular approach also varies among these agents. There are monoclonal antibodies, but also small molecules that can be given orally or subcu-taneous and siRNA agents that can suppress C5 production by the liver for prolonged periods. In the future, this assortment of new drugs may provide clinicians with the tools to specifically block the main effector molecule of the complement system that is pathogenic in a particular disease, and for physicians to choose a delivery method for patient convenience as well as for the optimal pharmacokinetics and pharma-codynamics of the complement inhibitor.
5.1. Complement inhibition in atypical hemolytic uremic syndrome Eculizumab is a monoclonal antibody to C5 that blocks the genera-tion of C5a and the C5b-9. The Food and Drug Administragenera-tion (FDA) approved eculizumab for the treatment of aHUS in 2011, and it is now the standard of care for this disease (Goodship et al., 2017). A longer acting form of the drug, ravulizumab, allows the dosing interval to be increased to every eight weeks. Ravulizumab was effective in a phase 3 study in aHUS (Rondeau et al., 2020). The two drugs were not compared head to head, but they seem to be comparable based on historical data. Of note, most of the patients had an improvement in kidney function, and 58.6 % of the patients who required dialysis at the beginning of the study were able to discontinue renal replacement therapy. Ravulizumab was recently approved by the FDA for aHUS. Nomacopan (another C5 inhibitor) and OMS721 (a MASP2 inhibitor) had initiated clinical trials in aHUS. Both trials appear to currently be on hold, although a trial of nomacopan in hematopoietic stem cell transplantation-associated TMA is planned.
5.2. Complement inhibition for immune-complex glomerulonephritis Eculizumab has been used off-label for treatment of proliferative lupus nephritis (Coppo et al., 2015; Pickering et al., 2015) and IgAN (Ring et al., 2015; Rosenblad et al., 2014). Effects of eculizumab treat-ment in IgAN were evaluated in two case reports with variable results, while preliminary data of three cases of patients with biopsy-proven lupus nephritis demonstrated that eculizumab treatment results in normalization of complement parameters and improvement of renal function. Eculizumab was also tested in a clinical trial of MN, although the results were not published. No significant reduction in proteinuria was seen after treatment with eculizumab over a 16-week period compared to untreated patients (Cunningham and Quigg, 2005). How-ever, it was unclear whether the negative results could be attributed to the short treatment duration and the low dose of eculizumab. There are not any ongoing clinical trials of eculizumab in glomerulonephritis. An siRNA molecules that knocks down production of C5 is being evaluated in a Phase 2 trial of patients with IgAN (NCT03841448).
Because eculizumab blocks complement at the level of C5, it does not prevent generation of C3 fragments. Drugs that block complement activation higher in the cascade could, therefore, be more effective for certain kidney diseases, and these C3 complement inhibitors are currently in phase 2 and 3 clinical trials of various types of glomerulo-nephritis. For example, a clinical trial of APL-2 (a small molecule C3 inhibitor) is enrolling patients with IgAN, lupus nephritis, MN and C3G
(NCT03453619). Another C3 inhibitor (AMY-101) is also being tested in C3G, because these two drugs target the C3 protein directly, they should block complement activation regardless of the initiation pathway. OMS721 (a MASP2 inhibitor) also has a trial underway that is enrolling patients with IgAN, lupus nephritis, and MN and C3G (NCT02682407). Considering the lectin pathway is activated in a subset of patients with these diseases, it’s not clear whether this strategy will be equally effective in all patients.
Several alternative pathway inhibitors are also being evaluated in IC mediated kidney disease. Novartis has a small molecule factor B inhib-itor that is being separately tested in C3G and IgAN (NCT03373461 and NCT03832114). An oral factor D inhibitor (ALXN2040) was also being tested in C3G and IC MPGN, although press reports indicate that this trial was recently stopped.
5.3. Complement inhibition for C3G
The prognosis of C3G remains poor with almost half of the patients progressing to end-stage renal disease (Medjeral-Thomas et al., 2014). There is still no curative treatment available for C3G, and the current approach is aimed at blood pressure control and reducing proteinuria (Servais et al., 2012). Furthermore, evidence to support the use of plasma therapy/exchange in C3G is lacking (Smith et al., 2019). The complement system seems an ideal target for the treatment of C3G and preclinical studies have shown promising results with complement in-hibitors in the disease (Wang et al., 2018; Zhang et al., 2013). Several case series have reported outcomes in patients treated with eculizumab (Bomback et al., 2012; Gurkan et al., 2013; Le Quintrec et al., 2018,
2015; Oosterveld et al., 2015; Ruggenenti et al., 2019; Welte et al., 2018). Although C5 inhibition may be beneficial in some patients, particularly those with inflammatory lesions on biopsy (Le Quintrec et al., 2018), the results have been mixed. Several of the newer agents are currently being tested in C3G as discussed above, including alter-native pathway inhibitors (small molecule inhibitors of factor B and factor D). A recent study reported the unexpected finding that factor D deficiency did not protect mice from glomerular disease in a model of C3G (Zhang et al., 2020), indicating that factor D might not be a good target. The logical target for intervention in C3G seems to be inhibition of C3 activation and animal studies have demonstrated promising
results. As these drugs block all activation of C3 via all three pathways, they should be effecdtive in C3G and in IC glomerulonephritis. Soluble CR1 (sCR1, a C3 inhibitor) treatment in a DDD-like mice model pre-vented renal deposition of C3 activation fragments and normalized systemic C3 levels (Wang et al., 2018). Correspondingly, short-term treatment of sCR1 in a pediatric patient with DDD and end stage renal disease resulted in a rise of serum C3 and decrease of sC5b-9 levels. Results of trials with C3 inhibitors (APL-2 and AMY-101) are therefore awaited with great interest. A MASP-2 inhibitor (OMS721) is also being tested in C3G (Zipfel et al., 2019). Although MASP-3 activates pro-factor D (a component of the alternative pathway), it is not clear that MASP-2 inhibition will block complement activation in C3G. Moreover, defi-ciency of MASP-1/3 did not reduce complement activation in an experimental model of C3G (Ruseva et al., 2014).
5.4. Complement inhibition in ANCA vasculitis
Based on the promising pre-clinical results, avacopan was tested in patients with ANCA vasculitis. As C5a appears to be the effector mole-cule in this complement-mediated disease, selective C5a blockade may be as effective as other complement inhibitory drugs but with fewer side effects or risks. A phase II study of the orally available human C5aR antagonist was performed in patients with newly diagnosed or relapsing ANCA vasculitis and the results were encouraging (Jayne et al., 2017). C5aR inhibition with avacopan could successfully reduce or even replace high-dose glucocorticoids in the treatment of ANCA vasculitis. A phase III trial of avacopan in patients with ANCA vasculitis is currently underway (Merkel et al., 2020).
6. Conclusions
The kidney was one of the first organs identified as a target of complement-mediated inflammation, and over the last 100 years clini-cians and researchers have accumulated a large body of compelling evidence implicating the complement system in a variety of different kidney diseases. Complement activation is central to the pathogenesis of IC-mediated glomerulonephritis, as initially understood. It is now clear, however, that complement plays an important role in diseases in which the ICs contain non-classical pathway activating immunoglobulin (IgAN Fig. 3. New Anti-Complement Drugs Being Tested in Glomerular Diseases. Drugs that
block the complement cascade at various places have been developed, and several of these are undergoing clinical trials in patients with glomerular disease. Drugs that block specific activation pathways may be well-suited for some of the diseases, and drugs that block specific activation fragments may prevent kid-ney injury with fewer systemic side-effects than drugs that block all activation. The convertases for each pathway (activating enzymes) are shown in green. The pro-inflammatory frag-ments are shown in red boxes. Drugs that are in kidney disease trials are shown in blue boxes (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
and membranous disease) and diseases in which minimal glomerular immunoglobulin is seen (AAV and aHUS).
It is noteworthy to mention that the complement system is activated in diseases caused by so many disparate processes, raising the question as to whether there are factors that can unify all of these findings. Are there common mechanisms that underly all these diseases, or is com-plement activation in each disease a distinct process that need to be understood on its own? Perhaps a more economical explanation is that specific characteristics of the kidney make it uniquely susceptible to complement-mediated inflammation. These are important questions to ask, especially as many new drugs for targeting the complement cascade at different points in the cascade are under development. Hence, it is quite possible that the most effective therapeutic strategy will be different for each type of complement-mediated disease. Thus, one of the challenges in bringing the new complement inhibitors into the clinic will be to utilize what is known about the biology of each disease to choose the optimal approach.
Author statement
FP and JT wrote the manuscript.
Declaration of Competing Interest
JMT receives royalties from Alexion Pharmaceuticals, Inc. and is a consultant for Q32 Bio, Inc., a company developing complement in-hibitors. He also holds stock and will receive royalty income from Q32 Bio, Inc.
Acknowledgements
This work was supported by National Institutes of Health Grants DK113586, DK076690, CA225840, and the Department of Defense Grant LR180050 (JMT).
References
Al-Ghaithi, B., Chanchlani, R., Riedl, M., Thorner, P., Licht, C., 2016. C3 Glomerulopathy and post-infectious glomerulonephritis define a disease spectrum. Pediatr. Nephrol. 31, 2079–2086.
Angioi, A., Fervenza, F.C., Sethi, S., Zhang, Y., Smith, R.J., Murray, D., Van Praet, J., Pani, A., De Vriese, A.S., 2016. Diagnosis of complement alternative pathway disorders. Kidney Int. 89, 278–288.
Arnold, J.N., Dwek, R.A., Rudd, P.M., Sim, R.B., 2006. Mannan binding lectin and its interaction with immunoglobulins in health and in disease. Immunol. Lett. 106, 103–110.
Bally, S., Debiec, H., Ponard, D., Dijoud, F., Rendu, J., Faure, J., Ronco, P., Dumestre- Perard, C., 2016. Phospholipase A2 receptor-related membranous nephropathy and mannan-binding lectin deficiency. J. Am. Soc. Nephrol. 27, 3539–3544. Barbour, S.J., Coppo, R., Zhang, H., Liu, Z.H., Suzuki, Y., Matsuzaki, K., Katafuchi, R.,
Er, L., Espino-Hernandez, G., Kim, S.J., Reich, H.N., Feehally, J., Cattran, D.C., International Ig, A.N.N, 2019. Evaluating a new international risk-prediction tool in IgA nephropathy. JAMA Intern. Med. 179, 942–952.
Beck Jr., L.H., Bonegio, R.G., Lambeau, G., Beck, D.M., Powell, D.W., Cummins, T.D., Klein, J.B., Salant, D.J., 2009. M-type phospholipase A2 receptor as target antigen in idiopathic membranous nephropathy. N. Engl. J. Med. 361, 11–21.
Bindon, C.I., Hale, G., Bruggemann, M., Waldmann, H., 1988. Human monoclonal IgG isotypes differ in complement activating function at the level of C4 as well as C1q. J. Exp. Med. 168, 127–142.
Blanc, C., Togarsimalemath, S.K., Chauvet, S., Le Quintrec, M., Moulin, B., Buchler, M., Jokiranta, T.S., Roumenina, L.T., Fremeaux-Bacchi, V., Dragon-Durey, M.A., 2015. Anti-factor H autoantibodies in C3 glomerulopathies and in atypical hemolytic uremic syndrome: one target, two diseases. J. Immunol. 194, 5129–5138. Bomback, A.S., Appel, G.B., 2012. Pathogenesis of the C3 glomerulopathies and
reclassification of MPGN. Nat. Rev. Nephrol. 8, 634–642.
Bomback, A.S., Smith, R.J., Barile, G.R., Zhang, Y., Heher, E.C., Herlitz, L., Stokes, M.B., Markowitz, G.S., D’Agati, V.D., Canetta, P.A., Radhakrishnan, J., Appel, G.B., 2012. Eculizumab for dense deposit disease and C3 glomerulonephritis. Clin. J. Am. Soc. Nephrol.: CJASN 7, 748–756.
Bridoux, F., Javaugue, V., Bender, S., Leroy, F., Aucouturier, P., Debiais-Delpech, C., Goujon, J.M., Quellard, N., Bonaud, A., Clavel, M., Trouillas, P., Di Meo, F., Gombert, J.M., Fermand, J.P., Jaccard, A., Cogne, M., Touchard, G., Sirac, C., 2017. Unravelling the immunopathological mechanisms of heavy chain deposition disease with implications for clinical management. Kidney Int. 91, 423–434.
Bruschi, M., Galetti, M., Sinico, R.A., Moroni, G., Bonanni, A., Radice, A., Tincani, A., Pratesi, F., Migliorini, P., Murtas, C., Franceschini, F., Trezzi, B., Brunini, F., Gatti, R., Tardanico, R., Barbano, G., Piaggio, G., Messa, P., Ravani, P., Scolari, F., Candiano, G., Martini, A., Allegri, L., Ghiggeri, G.M., 2015. Glomerular autoimmune multicomponents of human lupus nephritis in vivo (2): planted antigens. J. Am. Soc. Nephrol. 26, 1905–1924.
Chen, M., Xing, G.Q., Yu, F., Liu, G., Zhao, M.H., 2009. Complement deposition in renal histopathology of patients with ANCA-associated pauci-immune glomerulonephritis. Nephrol. Dial. Transplant. 24, 1247–1252.
Chen, Q., Wiesener, M., Eberhardt, H.U., Hartmann, A., Uzonyi, B., Kirschfink, M., Amann, K., Buettner, M., Goodship, T., Hugo, C., Skerka, C., Zipfel, P.F., 2014. Complement factor H-related hybrid protein deregulates complement in dense deposit disease. J. Clin. Invest. 124, 145–155.
Chen, M., Jayne, D.R.W., Zhao, M.-H., 2017. Complement in ANCA-associated vasculitis: mechanisms and implications for management. Nat. Rev. Nephrol. 13, 359–367. Coppo, R., Peruzzi, L., Amore, A., Martino, S., Vergano, L., Lastauka, I., Schieppati, A.,
Noris, M., Tovo, P.A., Remuzzi, G., 2015. Dramatic effects of eculizumab in a child with diffuse proliferative lupus nephritis resistant to conventional therapy. Pediatr. Nephrol. 30, 167–172.
Couser, W.G., 2017. Primary membranous nephropathy. Clin. J. Am. Soc. Nephrol.: CJASN 12, 983–997.
Cunningham, Patrick N., Quigg, Richard J., 2005. Contrasting roles of complement activation and its regulation in membranous nephropathy. JASN 16 (May (5)), 1214–1222. https://doi.org/10.1681/ASN.2005010096.
Daha, M.R., Fearon, D.T., Austen, K.F., 1976. C3 nephritic factor (C3NeF): stabilization of fluid phase and cell-bound alternative pathway convertase. J. Immunol. 116, 1–7. Dick, J., Gan, P.Y., Kitching, A.R., Holdsworth, S.R., 2018. The C3aR promotes
macrophage infiltration and regulates ANCA production but does not affect glomerular injury in experimental anti-myeloperoxidase glomerulonephritis. PLoS One 13, e0190655.
Diebolder, C.A., Beurskens, F.J., de Jong, R.N., Koning, R.I., Strumane, K., Lindorfer, M. A., Voorhorst, M., Ugurlar, D., Rosati, S., Heck, A.J., van de Winkel, J.G., Wilson, I. A., Koster, A.J., Taylor, R.P., Saphire, E.O., Burton, D.R., Schuurman, J., Gros, P., Parren, P.W., 2014. Complement is activated by IgG hexamers assembled at the cell surface. Science 343, 1260–1263.
Dobo, J., Szakacs, D., Oroszlan, G., Kortvely, E., Kiss, B., Boros, E., Szasz, R., Zavodszky, P., Gal, P., Pal, G., 2016. MASP-3 is the exclusive pro-factor D activator in resting blood: the lectin and the alternative complement pathways are fundamentally linked. Sci. Rep. 6, 31877.
Doi, T., Mayumi, M., Kanatsu, K., Suehiro, F., Hamashima, Y., 1984. Distribution of IgG subclasses in membranous nephropathy. Clin. Exp. Immunol. 58, 57–62. Durante, A., Rovati, C., di Belgiojoso, G.B., Minetti, L., 1976. Serum and glomerular
complement in 205 cases of glomerulonephritis. In: Proceedings of the European Dialysis and Transplant Association. European Dialysis and Transplant Association, pp. 189–196, 12.
Eberhardt, H.U., Buhlmann, D., Hortschansky, P., Chen, Q., Bohm, S., Kemper, M.J., Wallich, R., Hartmann, A., Hallstrom, T., Zipfel, P.F., Skerka, C., 2013. Human factor H-related protein 2 (CFHR2) regulates complement activation. PLoS One 8, e78617. Ehrlich, P., 1906. Collected Studies on Immunity. John Wiley & Sons, New York. Elvington, M., Liszewski, M.K., Atkinson, J.P., 2016. Evolution of the complement system: from defense of the single cell to guardian of the intravascular space. Immunol. Rev. 274, 9–15.
Endo, M., Ohi, H., Ohsawa, I., Fujita, T., Matsushita, M., Fujita, T., 1998. Glomerular deposition of mannose-binding lectin (MBL) indicates a novel mechanism of complement activation in IgA nephropathy. Nephrol. Dial Transplant. 13 (Aug (8)), 1984–1990. https://doi.org/10.1093/ndt/13.8.1984. PMID: 9719152.
Endo, M., Fuke, Y., Tamano, M., Hidaka, M., Ohsawa, I., Fujita, T., Ohi, H., 2004. Glomerular deposition and urinary excretion of complement factor H in idiopathic membranous nephropathy. Nephron Clin. Pract. 97, c147–153.
Espinosa, M., Ortega, R., Sanchez, M., Segarra, A., Salcedo, M.T., Gonzalez, F., Camacho, R., Valdivia, M.A., Cabrera, R., Lopez, K., Pinedo, F., Gutierrez, E., Valera, A., Leon, M., Cobo, M.A., Rodriguez, R., Ballarin, J., Arce, Y., Garcia, B., Munoz, M.D., Praga, M., Spanish Group for Study of Glomerular, D, 2014. Association of C4d deposition with clinical outcomes in IgA nephropathy. Clin. J. Am. Soc. Nephrol.: CJASN 9, 897–904.
Espinosa-Hernandez, M., Ortega-Salas, R., Lopez-Andreu, M., Gomez-Carrasco, J.M., Perez-Saez, M.J., Perez-Seoane, C., Aljama-Garcia, P., 2012. C4d as a diagnostic tool in membranous nephropathy. Nefrologia 32, 295–299.
Evans, D.J., Williams, D.G., Peters, D.K., Sissons, J.G., Boulton-Jones, J.M., Ogg, C.S., Cameron, J.S., Hoffbrand, B.I., 1973. Glomerular deposition of properdin in Henoch- Schonlein syndrome and idiopathic focal nephritis. Br. Med. J. 3, 326–328. Faria, B., Henriques, C., Matos, A.C., Daha, M.R., Pestana, M., Seelen, M., 2015.
Combined C4d and CD3 immunostaining predicts immunoglobulin (Ig)A nephropathy progression. Clin. Exp. Immunol. 179, 354–361.
Faria, B., Canao, P., Cai, Q., Henriques, C., Matos, A.C., Poppelaars, F., Gaya da Costa, M., Daha, M.R., Silva, R., Pestana, M., Seelen, M.A., 2020. Arteriolar C4d in IgA nephropathy: a cohort study. Am. J. Kidney Dis.
Fervenza, F.C., Appel, G.B., Barbour, S.J., Rovin, B.H., Lafayette, R.A., Aslam, N., Jefferson, J.A., Gipson, P.E., Rizk, D.V., Sedor, J.R., Simon, J.F., McCarthy, E.T., Brenchley, P., Sethi, S., Avila-Casado, C., Beanlands, H., Lieske, J.C., Philibert, D., Li, T., Thomas, L.F., Green, D.F., Juncos, L.A., Beara-Lasic, L., Blumenthal, S.S., Sussman, A.N., Erickson, S.B., Hladunewich, M., Canetta, P.A., Hebert, L.A., Leung, N., Radhakrishnan, J., Reich, H.N., Parikh, S.V., Gipson, D.S., Lee, D.K., da Costa, B.R., Juni, P., Cattran, D.C., Investigators, M., 2019. Rituximab or cyclosporine in the treatment of membranous nephropathy. N. Engl. J. Med. 381, 36–46.
