Inhibition of complement regulation is key to the pathogenesis of active Heymann nephritis

The complement system in the pathogenesis and progression of kidney diseases: What doesn't kill you makes you older

The Complement System is an evolutionarily conserved component of immunity that plays a key role in host defense against infections and tissue homeostasis. However, the dysfunction of the Complement System can result in tissue damage and inflammation, thereby contributing to the development and progression of various renal diseases, ranging from atypical Hemolytic Uremic Syndrome to glomerulonephritis. Therapeutic interventions targeting the complement system have demonstrated promising results in both preclinical and clinical studies. Currently, several complement inhibitors are being developed for the treatment of complement-mediated renal diseases. This review aims to summarize the most recent insights into complement activation and therapeutic inhibition in renal diseases. Furthermore, it offers potential directions for the future rational use of complement inhibitor drugs in the context of renal diseases.

Complement activation and effector pathways in membranous nephropathy

Complement activation has long been recognized as a central feature of membranous nephropathy (MN). Evidence for its role has been derived from the detection of complement products in biopsy tissue and urine from patients with MN and from mechanistic studies primarily based on the passive Heymann nephritis model. Only recently, more detailed insights into the exact mechanisms of complement activation and effector pathways have been gained from patient data, animal models, and in vitro models based on specific target antigens relevant to the human disease. These data are of clinical relevance, as they parallel the recent development of numerous specific complement therapeutics for clinical use. Despite efficient B-cell depletion, many patients with MN achieve only partial remission of proteinuria, which may be explained by the persistence of subepithelial immune complexes and ongoing complement-mediated podocyte injury. Targeting complement, therefore, represents an attractive adjunct treatment for MN, but it will need to be tailored to the specific complement pathways relevant to MN. This review summarizes the different lines of evidence for a central role of complement in MN and for the relevance of distinct complement activation and effector pathways, with a focus on recent developments.

Membranous nephropathy: Clearer pathology and mechanisms identify potential strategies for treatment

Primary membranous nephropathy (PMN) is one of the common causes of adult-onset nephrotic syndrome and is characterized by autoantibodies against podocyte antigens causing in situ immune complex deposition. Much of our understanding of the disease mechanisms underpinning this kidney-limited autoimmune disease originally came from studies of Heymann nephritis, a rat model of PMN, where autoantibodies against megalin produced a similar disease phenotype though megalin is not implicated in human disease. In PMN, the major target antigen was identified to be M-type phospholipase A2 receptor 1 (PLA2R) in 2009. Further utilization of mass spectrometry on immunoprecipitated glomerular extracts and laser micro dissected glomeruli has allowed the rapid discovery of other antigens (thrombospondin type-1 domain-containing protein 7A, neural epidermal growth factor-like 1 protein, semaphorin 3B, protocadherin 7, high temperature requirement A serine peptidase 1, netrin G1) targeted by autoantibodies in PMN. Despite these major advances in our understanding of the pathophysiology of PMN, treatments remain non-specific, often ineffective, or toxic. In this review, we summarize our current understanding of the immune mechanisms driving PMN from animal models and clinical studies, and the implications on the development of future targeted therapeutic strategies.

Mechanisms of Primary Membranous Nephropathy

Membranous nephropathy (MN) is an autoimmune disease of the kidney glomerulus and one of the leading causes of nephrotic syndrome. The disease exhibits heterogenous outcomes with approximately 30% of cases progressing to end-stage renal disease. The clinical management of MN has steadily advanced owing to the identification of autoantibodies to the phospholipase A2 receptor (PLA2R) in 2009 and thrombospondin domain-containing 7A (THSD7A) in 2014 on the podocyte surface. Approximately 50–80% and 3–5% of primary MN (PMN) cases are associated with either anti-PLA2R or anti-THSD7A antibodies, respectively. The presence of these autoantibodies is used for MN diagnosis; antibody levels correlate with disease severity and possess significant biomarker values in monitoring disease progression and treatment response. Importantly, both autoantibodies are causative to MN. Additionally, evidence is emerging that NELL-1 is associated with 5–10% of PMN cases that are PLA2R- and THSD7A-negative, which moves us one step closer to mapping out the full spectrum of PMN antigens. Recent developments suggest exostosin 1 (EXT1), EXT2, NELL-1, and contactin 1 (CNTN1) are associated with MN. Genetic factors and other mechanisms are in place to regulate these factors and may contribute to MN pathogenesis. This review will discuss recent developments over the past 5 years.

Regulation of Complement Activation by Heme Oxygenase-1 (HO-1) in Kidney Injury

Heme oxygenase is a cytoprotective enzyme with strong antioxidant and anti-apoptotic properties. Its cytoprotective role is mainly attributed to its enzymatic activity, which involves the degradation of heme to biliverdin with simultaneous release of carbon monoxide (CO). Recent studies uncovered a new cytoprotective role for heme oxygenase-1 (HO-1) by identifying a regulatory role on the complement control protein decay-accelerating factor. This is a key complement regulatory protein preventing dysregulation or overactivation of complement cascades that can cause kidney injury. Cell-specific targeting of HO-1 induction may, therefore, be a novel approach to attenuate complement-dependent forms of kidney disease.

Molecular Pathogenesis of Membranous Nephropathy

Membranous nephropathy is a noninflammatory autoimmune disease of the kidney glomerulus, characterized by the formation of immune deposits, complement-mediated proteinuria, and risk of renal failure. Considerable advances in understanding the molecular pathogenesis have occurred with the identification of several antigens [neutral endopeptidase, phospholipase A2 receptor (PLA₂R), thrombospondin domain-containing 7A (THSD7A)] in cases arising from the neonatal period to adulthood and the characterization of antibody-binding domains (that is, epitopes). Immunization against PLA2R occurs in 70% to 80% of adult cases. The development of highly specific and sensitive assays of circulating antibodies has induced a paradigm shift in diagnosis and treatment monitoring. In addition, several interacting loci in HLA-DQ, HLA-DR, and PLA2R1, as well as classical human leukocyte antigen (HLA)-D alleles have been identified as being risk factors, depending on a patient's ethnicity. Additionally, mechanisms of antibody pathogenicity and pathways of complement activation are now better understood. Further research is mandatory for designing new therapeutic strategies, including the identifying triggering events, the molecular bases of remission and progression, and the T cell epitopes involved.

Unmet challenges in membranous nephropathy

PURPOSE OF REVIEW

Despite major advances in since the discovery of the phospholipase A2 receptor (PLA2R) as the major autoantigen on podocytes in primary membranous nephropathy, there are still several unanswered questions as highlighted here.

RECENT FINDINGS

A substantial body of literature, included in more than 680 articles since 2009, has documented genetic susceptibility to primary membranous nephropathy involving PLA2R1 and class II MHC alleles, the clinical value of anti-PLA2R assays, the significance of epitope spreading of the anti-PLA2R response, discovery of thrombospondin type I domain-containing 7A (THSD7A) as a minor antigen in primary membranous nephropathy, and the ability to transfer disease into mice by infusion of anti-THSD7A sera. However, the normal physiology and pathophysiology of PLA2R and THSD7A in podocytes is still unknown and the genetic influence on disease susceptibility is unexplained. We still do not know what causes loss of self-tolerance to PLA2R and THSD7A or how the autoantibodies, which are predominantly of the IgG4 subclass, cause podocyte injury and proteinuria. Complement deposits are prominent in membranous nephropathy but we are still uncertain how the complement system is activated and whether or not it plays a role in podocyte damage. Notwithstanding the advances over the past decade, our treatments have not changed substantially.

SUMMARY

This review identifies opportunities to extend the advances that have been made to better understand the immunopathogenesis and genetic basis of primary membranous nephropathy and apply the knowledge to design more specific therapies.

The Modified Vaccination Technique

In addition to active and passive immunizations, there is a third method of immunization, the modified vaccination technique, which is based on injecting a combination of target antigens and antibodies against this antigen. The vaccine is essentially comprised of immune complexes with pre-determined immune-inducing components. When such an immune complex (target antigen × antibody against the target antigen) with a slight antigen excess is administered, it evokes a corrective immune response by the production of the same antibody with the same specificity against the target antigen that is present in the immune complex (pre-determined immune response).

Phospholipase A2 Receptor-Related Membranous Nephropathy and Mannan-Binding Lectin Deficiency

Most patients with idiopathic membranous nephropathy (IMN) have IgG4 autoantibodies against phospholipase A2 receptor (PLA2R). C3 and C5b-9 are found in immune deposits of IMN kidney biopsy specimens, but the pathway of complement activation in IMN remains elusive. We report the case of a patient who developed IMN with intense staining for PLA2R, IgG4, C3, C5b-9, factor B, and properdin and very weak staining for C1q, C4d, and IgG1. Measurement of mannan binding lectin (MBL) antigenic level and activity revealed MBL deficiency. Genotyping revealed a heterozygous (A/C) polymorphism in codon 57 of MBL2 exon 1 associated with homozygous and heterozygous variations in the promoter region at -550 (L/L) and -221 (X/Y), respectively, suggesting that the patient harbored the LXA/LYC haplotypes linked to MBL deficiency. Genetic sequencing in 77 consecutive patients with IMN identified four patients with MBL2 promoter and coding region variations associated with MBL deficiency and the same complement pattern in immune deposits as the index patient. In contrast, patients with wild-type MBL2 had immune deposits with intense Cd4 staining. Thus, IMN can develop in patients with complete MBL deficiency, with complement activated mainly by the alternative pathway, whereas the lectin pathway is also activated in those with wild-type MBL2.

Alternative Pathway Dysregulation and the Conundrum of Complement Activation by IgG4 Immune Complexes in Membranous Nephropathy

Membranous nephropathy (MN), a major cause of nephrotic syndrome, is a non-inflammatory immune kidney disease mediated by IgG antibodies that form glomerular subepithelial immune complexes. In primary MN, autoantibodies target proteins expressed on the podocyte surface, often phospholipase A2 receptor (PLA2R1). Pathology is driven by complement activation, leading to podocyte injury and proteinuria. This article overviews the mechanisms of complement activation and regulation in MN, addressing the paradox that anti-PLA2R1 and other antibodies causing primary MN are predominantly (but not exclusively) IgG4, an IgG subclass that does not fix complement. Besides immune complexes, alterations of the glomerular basement membrane (GBM) in MN may lead to impaired regulation of the alternative pathway (AP). The AP amplifies complement activation on surfaces insufficiently protected by complement regulatory proteins. Whereas podocytes are protected by cell-bound regulators, the GBM must recruit plasma factor H, which inhibits the AP on host surfaces carrying certain polyanions, such as heparan sulfate (HS) chains. Because HS chains present in the normal GBM are lost in MN, we posit that the local complement regulation by factor H may be impaired as a result. Thus, the loss of GBM HS in MN creates a micro-environment that promotes local amplification of complement activation, which in turn may be initiated via the classical or lectin pathways by subsets of IgG in immune complexes. A detailed understanding of the mechanisms of complement activation and dysregulation in MN is important for designing more effective therapies.

Complement regulation in renal disease models

Activation of the complement system is tightly regulated by plasma and cell-associated complement regulatory proteins (CRPs), such as factor H (fH), decay-accelerating factor, and membrane cofactor protein. Animal models of disease have provided considerable insights into the important roles for CRPs in the kidney. Mice deficient in fH have excessive fluid phase C3 activation and inactivation, leading to deposition of inactivated C3b in glomerular capillary walls (GCW), comparable with dense deposit disease. In contrast, when fH lacks C-terminal surface targeting regions, local activation on the GCW leads to a disease reminiscent of thrombotic microangiopathy. The uniquely rodent protein, CR1-related y (Crry), has features analogous to human membrane cofactor protein. Defective Crry leads to unrestricted alternative pathway activation in the tubulointerstitium, resulting in pathologic features ranging from thrombotic microangiopathy (TMA), acute kidney injury, and tubulointerstitium nephritis. In the presence of initiators of the classic or lectin pathways, commonly in the form of immune complexes in human glomerular diseases, complement regulation is stressed, with the potential for recruitment of the spontaneously active alternative pathway. The threshold for this activation is set by CRPs; pathology is more likely when complement regulation is defective. Within the endocapillary region of the GCW, fH is key, while decay-accelerating factor and Crry are protective on mesangial cells and podocytes. Arguably, acquired alterations in these CRPs is a more common event, extending from pathologic states of cellular injury or production of inhibitory antibodies, to physiological fine tuning of the adaptive immune response.

Membranous nephropathy: from models to man

As recently as 2002, most cases of primary membranous nephropathy (MN), a relatively common cause of nephrotic syndrome in adults, were considered idiopathic. We now recognize that MN is an organ-specific autoimmune disease in which circulating autoantibodies bind to an intrinsic antigen on glomerular podocytes and form deposits of immune complexes in situ in the glomerular capillary walls. Here we define the clinical and pathological features of MN and describe the experimental models that enabled the discovery of the major target antigen, the M-type phospholipase A2 receptor 1 (PLA2R). We review the pathophysiology of experimental MN and compare and contrast it with the human disease. We discuss the diagnostic value of serological testing for anti-PLA2R and tissue staining for the redistributed antigen, and their utility for differentiating between primary and secondary MN, and between recurrent MN after kidney transplant and de novo MN. We end with consideration of how knowledge of the antigen might direct future therapeutic strategies.

The role of complement in membranous nephropathy

Membranous nephropathy (MN) describes a histopathologic pattern of injury marked by glomerular subepithelial immune deposits and collectively represents one of the most common causes of adult nephrotic syndrome. Studies in Heymann nephritis, an experimental model of MN, have established a paradigm in which these deposits locally activate complement to cause podocyte injury, culminating in cytoskeletal reorganization, loss of slit diaphragms, and proteinuria. There is much circumstantial evidence for a prominent role of complement in human MN because C3 and C5b-9 are found consistently within immune deposits. Secondary MN often shows the additional presence of C1q, implicating the classic pathway of complement activation. Primary MN, however, is IgG4-predominant and IgG4 is considered incapable of binding C1q and activating the complement pathway. Recent studies have identified the M-type phospholipase A2 receptor (PLA2R) as the major target antigen in primary MN. Early evidence hints that IgG4 anti-PLA2R autoantibodies can bind mannan-binding lectin and activate the lectin complement pathway. The identification of anti-PLA2R antibodies as likely participants in the pathogenesis of disease will allow focused investigation into the role of complement in MN. Definitive therapy for MN is immunosuppression, although future therapeutic agents that specifically target complement activation may represent an effective temporizing measure to forestall further glomerular injury.

CD8+ regulatory T cells induced by T cell vaccination protect against autoimmune nephritis

Autoreactive T cells play a pivotal role in the pathogenesis of autoimmune kidney disease. T cell vaccination (TCV) may limit autoimmune disease and induce CD8+ regulatory T cells (Tregs). We used Heymann nephritis (HN), a rat model of human membranous nephritis, to study the effects of TCV on autoimmune kidney disease. We harvested CD4+ T cells from renal tubular antigen (Fx1A) -immunized rats and activated these cells in vitro to express the MHC Class Ib molecule Qa-1. Vaccination of Lewis rats with these autoreactive Fx1A-induced T cells protected against HN, whereas control-primed T cells did not. Rats that underwent TCV had lower levels of proteinuria and serum creatinine and significantly less glomerulosclerosis, tubular damage, and interstitial infiltrates. Furthermore, these rats expressed less IFN-γ and IL-6 in splenocytes, whereas the numbers of Tregs and the expression of Foxp3 were unchanged. In vitro cytotoxicity assays showed CD8+ T cell-mediated elimination of Qa-1-expressing CD4+ T cells. In vivo, TCV abrogated the increase in Qa-1-expressing CXCR5+ TFH cells observed in HN compared with controls. Taken together, these results suggest that TCV protects against autoimmune kidney disease by targeting Qa-1-expressing autoreactive CD4+ cells.

The pathology and clinical features of early recurrent membranous glomerulonephritis

We assessed the earliest manifestations of recurrent membranous glomerulonephritis (MGN) in renal allografts. Clinical, laboratory and pathologic data were reviewed in 21 patients at the initial biopsy within 4 months post-transplant with evidence of MGN and on follow-up biopsies, compared to a biopsy control group of eight transplants without recurrent MGN. The mean time of first biopsy with pathologic changes was 2.7 months. In each earliest biopsy, immunofluorescence (IF) showed granular glomerular basement membrane (GBM) staining for C4d, IgG, kappa and lambda. IF for C3 was negative or showed trace staining in 16/21. On each MGN biopsy positive by IF, 14/19 showed absence of deposits or rare tiny subepithelial deposits by electron microscopy (EM). At the earliest biopsy, the mean proteinuria was 1.1 g/day; 16 patients had <1 g/day proteinuria. Follow-up was available in all patients (mean 35 months posttransplant). A total of 13 patients developed >1 g/day proteinuria; 12 were treated with: rituximab (n = 8), ACEI and increased prednisone dose (n = 2), ACEI or ARB only (n = 2). All patients showed reduction in proteinuria after treatment. A total of 11/16 patients showed progression of disease by EM on follow-up biopsy. Recognition of early allograft biopsy features aids in diagnosis of recurrent MGN before patients develop significant proteinuria.

Membranous nephropathy: recent travels and new roads ahead

Insights from experimental studies have been recently translated into substantial advances in understanding the pathogenesis of human membranous nephropathy (MN). These include identification of neutral endopeptidase (NEP) as the target antigen in alloimmune MN resulting from fetomaternal immunization in NEP-deficient mothers, and our demonstration that a high proportion of patients with idiopathic MN (IMN) have circulating antibodies to the M-type phospholipase A2 receptor (PLA2R), a transmembrane protein located on podocytes. Here we highlight the studies that led to these discoveries and our current knowledge about the possible role of anti-PLA2R autoantibodies in the pathogenesis of IMN. Given that the sensitivity and specificity of anti-PLA2R for IMN are >75 and 100%, respectively, we foresee that a widely available assay for anti-PLA2R will prove to be valuable for diagnosing IMN, distinguishing it from secondary MN, and evaluating response to therapy. We suggest reasons why 25% of patients with IMN have tested negative for anti-PLA2R, and propose possible explanations for the presence of complement deposits in IMN despite the fact that immunoglobulin G4 (IgG4), the predominant anti-PLA2R IgG subclass, is incapable of activating the classical complement pathway. Finally, we point out avenues to be explored, including the events that induce production of anti-PLA2R, their ability to cause podocyte injury, the role of complement, and the nature of the antibodies in secondary forms of MN.

The Many Effects of Complement C3- and C5-Binding Proteins in Renal Injury

The complement system is an important component of the innate immune system and a modulator of adaptive immunity. The entire complement system is focused on C3 and C5. Thus, there are proteins that activate C3 and C5, those that regulate this activation, and those that transduce the effects of C3 and C5 activation products; each can affect the kidney in renal injury. The normal kidney has the inherent capacity to protect itself from complement activation through cellular expression of decay-accelerating factor, membrane cofactor protein (in human beings), and Crry (in rodents). In addition, plasma factor H protects vascular spaces in the kidney. Although the main function of these proteins is to limit complement activation, there is now considerable evidence that they can transduce signals on engagement in immune cells. The G-protein-coupled 7-span transmembrane receptors for C3a and C5a, and the integral membrane complement receptors (CR) for C3b, iC3b, and C3dg, are expressed outside the kidney, particularly in cells of hematopoietic and immune lineage. These are important in renal injury through their infiltration of the kidney and/or by affecting kidney-directed immune responses. There is mounting evidence that intrinsic glomerular and tubular cell C3aR and C5aR expression and activation also can affect renal injury. CR1 on podocytes and the beta2 integrins CR3 and CR4 in kidney dendritic cells have functions that remain poorly defined. Cells of the kidney also have the capacity to produce and activate their own complement proteins. Thus, intrinsic renal cells express decay-accelerating factor, membrane cofactor protein, Crry, C3aR, C5aR, CR1, CR3, and CR4. These can be engaged by C3 and C5 activation products derived from systemic and local pools in renal injury. Given their capacity to provide signals that influence kidney cellular behavior, their activation can have substantial effects in renal injury. Defining these in a cell- and disease-specific fashion is an exciting challenge for future research.

Nested N-terminal megalin fragments induce high-titer autoantibody and attenuated Heymann nephritis

It was shown previously that an N-terminal fragment (nM60) that encompasses amino acid residues 1 to 563 of megalin could induce active Heymann nephritis (AHN) as efficiently as the native protein. For delineation of a minimal structure within this fragment that is sufficient to induce AHN, smaller protein fragments that encompass residues 1 to 236 (L6), 1 to 195 (L5), 1 to 156 (L4), and 1 to 120 (L3), representing successive C-terminal truncations within ligand-binding repeats of nM60, were cloned and produced in a baculovirus insect cell expression system. Protein fragments L4, L5, and L6 clearly were glycosylated. All four fragments stimulated proliferation of megalin-sensitized lymph node cells and induced high-titer anti-megalin autoantibodies in Lewis rats. A full-blown disease, as assessed by severity of proteinuria, was observed in rats that were immunized with L6 and L5, whereas animals that were immunized with L4 and L3 developed only mild disease. The proteinuria levels correlated with staining for complement (C3, C5b-9) and IgG1 isotype in glomerular immune deposits. The results suggest that one or more molecular determinants on the region that comprises amino acid residues 157 to 236 contribute to the induction of a full-blown form of AHN. Study of the structure, conformation, and posttranslational modifications of these determinants could provide greater insight into the molecular correlates of immunopathogenesis in this disease model.

Differential capacity of anti-RAP and anti-megalin antibodies to produce progressive passive Heymann nephritis — implications for the pathogenesis of idiopathic human membranous glomerulonephritis

Passive Heymann nephritis (PHN) induced with heterologous antisera has been described according to various criteria, which may or may not include induction of chronic disease and proteinuria. Characteristics of the glomerular immune deposits determined by the antigenic specificities of the antisera presumably account for differences in disease outcome. In this study, the clinical and immunohistological features in the model produced with monospecific antisera were compared against megalin or receptor associated protein (RAP), two proteins that have been implicated as target antigens in PHN. Rats injected with either anti-megalin or anti-RAP antiserum developed typical glomerular immune deposits of PHN when examined after 7 days. Although the deposits stained for complement, none of the animals had abnormal proteinuria in this time frame. Over a longer time course (7-16 weeks), immune deposits persisted and proteinuria increased to pathological levels in all animals injected with anti-megalin serum. By contrast, immune deposits had cleared from the kidneys of rats injected with anti-RAP antiserum when examined at 7-8 weeks post-injection and the proteinuria levels observed up to 13 weeks remained in the normal range. Additional doses of anti-RAP antiserum given 4 and 17 days after the first injection did not prolong the duration of glomerular immune deposits. These results demonstrate a clear divergence in pathogenic potential of antisera generated against the two renal antigens, which suggest differences in the immune deposits linked to a soluble antigen that is non-covalently bound to the podocyte membrane versus those linked to an integral membrane antigen. These observations could provide clues to the nature of the unknown glomerular autoantigen of idiopathic membranous glomerulonephritis in humans.

Mechanisms of immune-deposit formation and the mediation of immune renal injury

The passive trapping of preformed immune complexes is responsible for some forms of glomerulonephritis that are associated with mesangial or subendothelial deposits. The biochemical characteristics of circulating antigens play important roles in determining the biologic activity of immune complexes in these cases. Examples of circulating immune complex diseases include the classic acute and chronic serum sickness models in rabbits, and human lupus nephritis. Immune deposits also form "in situ". In situ immune deposit formation may occur at subepithelial, subendothelial, and mesangial sites. In situ immune-complex formation has been most frequently studied in the Heymann nephritis models of membranous nephropathy with subepithelial immune deposits. While the autoantigenic target in Heymann nephritis has been identified as megalin, the pathogenic antigenic target in human membranous nephropathy had been unknown until the recent identification of neutral endopeptidase as one target. It is likely that there is no universal antigen in human membranous nephropathy. Immune complexes can damage glomerular structures by attracting circulating inflammatory cells or activating resident glomerular cells to release vasoactive substances, cytokines, and activators of coagulation. However, the principal mediator of immune complex-mediated glomerular injury is the complement system, especially C5b-9 membrane attack complex formation. C5b-9 inserts in sublytic quantities into the membranes of glomerular cells, where it produces cell activation, converting normal cells into resident inflammatory effector cells that cause injury. Excessive activation of the complement system is normally prevented by a series of circulating and cell-bound complement regulatory proteins. Genetic deficiencies or mutations of these proteins can lead to the spontaneous development of glomerular disease. The identification of specific antigens in human disease may lead to the development of fundamental therapies. Particularly promising future therapeutic approaches include selective immunosuppression and interference in complement activation and C5b-9-mediated cell injury.

Experimental membranous nephropathy redux

Membranous nephropathy (MN) is a common cause of nephrotic syndrome in adults. Active and passive Heymann nephritis (HN) in rats are valuable experimental models because their features so closely resemble human MN. In HN, subepithelial immune deposits form in situ as a result of circulating antibodies. Complement activation leads to assembly of C5b-9 on glomerular epithelial cell (GEC) plasma membranes and is essential for sublethal GEC injury and the onset of proteinuria. This review revisits HN and focuses on areas of substantial progress in recent years. The response of the GEC to sublethal C5b-9 attack is not simply due to disruption of the plasma membrane but is due to the activation of specific signaling pathways. These include activation of protein kinases, phospholipases, cyclooxygenases, transcription factors, growth factors, NADPH oxidase, stress proteins, proteinases, and others. Ultimately, these signals impact on cell metabolic pathways and the structure/function of lipids and key proteins in the cytoskeleton and slit-diaphragm. Some signals affect GEC adversely. Thus C5b-9 induces partial dissolution of the actin cytoskeleton. There is a decline in nephrin expression, reduction in F-actin-bound nephrin, and loss of slit-diaphragm integrity. Other signals, such as endoplasmic reticulum stress, may limit complement-induced injury, or promote recovery. The extent of complement activation and GEC injury is dependent, in part, on complement-regulatory proteins, which act at early or late steps within the complement cascade. Identification of key steps in complement activation, the cellular signaling pathways, and the targets will facilitate therapeutic intervention in reversing GEC injury in human MN.

Contrasting roles of complement activation and its regulation in membranous nephropathy

The complement system is involved in defense against microorganisms, the processing of immune complexes and apoptotic debris, and the development of an appropriate immune response. Along with these physiologic effects, complement activation has the potential to result in tissue pathology. To limit this, various complement regulatory proteins (CRP) are present on host cells, including the glomerular podocyte. Experimental data from the Heymann nephritis (HN) rat model of human membranous nephropathy (MN) have shown that IgG antibodies in subepithelial immune deposits initiate complement activation and C5b-9-mediated damage of the overlying podocyte. Although IgG can activate the classical pathway, there also is evidence that alternative pathway activation occurs in MN, which could occur because of absent, dysfunctional, or inhibited podocyte CRP. Related to this are experimental data in HN showing the presence of antibodies that bind and inhibit podocyte CRP; although such antibodies have not been documented in human MN, a decrease in CR1 quantity on the podocyte has been observed. A s a result of a relative lack of CRP and the exposure of activating complement proteins to tubular cells, alternative complement pathway activation and C5b-9-mediated tubular injury can occur in MN and other proteinuric diseases. Overall, in a disease such as MN, the balance between complement regulation and activation is tipped toward its being activated. Therefore, a number of therapeutic approaches have been developed to counteract this, including recombinant forms of endogenous CRP and complement-inhibitory monoclonal antibodies. There is good reason to be optimistic that approaches to block complement activation will become viable therapy for human MN in the future.

Presence of immunoglobulin M antibodies around the glomerular capillaries and in the mesangium of normal and passive Heymann nephritis rats

Summary Diffuse distribution of small, faintly staining, beaded deposits of rat immunoglobulin M (IgM) around the glomerular capillary blood vessels, and a more intensely staining larger deposition in the mesangium, were observed on the kidney sections of normal rats. As glomerular-fixed nephritogenic antigens are known to be present on the epithelial aspect of the glomerular basement membrane (GBM), especially at the soles of foot processes and at the slit pores, it was assumed that the IgM antibodies were directed against these antigens. Investigation by immunofluorescent antibody double-staining techniques of rat kidney sections obtained from normal and rabbit anti-FX1A-injected rats stained for the nephritogenic antigen showed that a number of antigenic sites in the glomeruli and in the mesangium shared antibody hits by heterologous rabbit IgG and autologous rat IgM antibodies. Most sites in the glomeruli stained specifically for rat IgM or rabbit IgG, but preferentially for the latter. The intensely fluorescent mesangial deposits stained mainly for rat IgM, indicating that at these sites the antigenic material was virtually saturated, while areas at the entry to the mesangial space also stained for rabbit IgG, indicating that at these locations free nephritogenic epitopes were still available for reaction with the anti-FX1A antibody. Western blot analysis have shown that the rabbit anti-rat FX1A IgG and the rat anti-rat KF3 IgM antibodies are directed against the same renal tubular-derived antigen with a molecular weight of 70,000. These experimental findings collectively demonstrate that the heterologous IgG and autologous IgM antibodies are directed against the same nephritogenic antigen, which is found in the glomeruli, the mesangium and the proximal convoluted tubules. Thus, the IgM autoantibody has a possible physiological role but, in addition, there is evidence of active immunophagocytic events, manifested in a rapid and continuous entrapment and expulsion of macromolecules after their processing by the mesangial cells of normal and passive Heymann nephritis rats.

Down-regulation of pathogenic autoantibody response in a slowly progressive Heymann nephritis kidney disease model

In the present article, we describe an antigen-specific down-regulation of a pathogenic autoantibody (aab)-mediated disease process in an experimental autoimmune kidney disease in rats called slowly progressive Heymann nephritis (SPHN). This autoimmune disease is initiated and maintained by pathogenic immunoglobulin G (IgG) autoantibodies (aabs), which cause an immune-complex (IC) glomerulonephritis associated with proteinuria. We achieved down-regulated pathogenic aab response in SPHN rats by injections of an IC containing the native nephritogenic antigen and specific high-titred nonpathogenic IgM aabs, in antigen excess. The injected IC increased the level of circulating nonpathogenic IgM aabs; the increased levels of specific IgM aabs in turn facilitated the removal of the injected altered nephritogenic and liberated autoantigens from the renal tubules and greatly diminished the production of pathogenic aabs and the build up of immune deposits in the glomeruli. While animals treated early had advantages over rats whose kidney disease was well established before treatment; animals treated late into the disease still manifested noticeable improvements in similar areas, i.e. with lessened proteinuria, kidney lesion reduction and a decreased pathogenic aab response. At the end of the experiment at 29 weeks, 80% of all the treated rats had insignificantly low levels of circulating IgG aabs, indicating cessation of pathogenic aab production and corresponding termination of the disease process. In contrast, most untreated rats with the kidney disease still had high levels of circulating pathogenic aabs at the end of the experiment, which maintained disease progression.

Production of a new model of slowly progressive Heymann nephritis

A slowly progressive autoimmune kidney disease was induced in Sprague Dawley rats by subcutaneous injection of a chemically modified kidney antigen (rKF3), incorporated into Alum and Distemper complex vaccine, followed by subcutaneous injections of an aqueous preparation of the same antigen. Pathogenic autoantibodies developed, which reacted with fixed glomerular nephritogenic antigen. Subsequently, immunopathological events lead to chronic progressive immune complex glomerulonephritis and proteinuria. The slowly developing disease was morphologically and functionally similar to Heymann nephritis (HN). The damage observed in the kidneys of experimental animals at 8 weeks and at the end of the experiment was examined by direct fluorescent antibody test, histology and electron microscopy. The changes were similar to the typical lesions found in HN rat kidneys, but less severe. Animals became proteinuric from 17 weeks onward (instead of the usual 4-8 weeks). By the end of the experiment, at 8 months, 100% of the rats were proteinuric. This new experimental model of autoimmune kidney disease, which is not complicated by intraperitoneal deposition and retention of Freund's complete adjuvant and renal tubular antigens, allowed us to investigate the pathogenesis of the disease processes from a different aspect, and promises to be a useful and improved model for the investigation of future treatment options.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Decay-accelerating factor expression in the rat kidney is restricted to the apical surface of podocytes

BACKGROUND

Decay-accelerating factor (DAF) has inhibitory activity toward complement C3 and C5 convertases. DAF is present in human glomeruli and on cultured human glomerular visceral epithelial cells (GEC). We studied the distribution and function of rat DAF.

METHODS

Function-neutralizing antibodies (Abs) were raised against DAF. The distribution of DAF in vivo was determined by immunoelectron microscopy. Functional studies were performed in cultured GEC and following IV injection of anti-DAF Abs into rats.

RESULTS

DAF was present exclusively on the apical surfaces of GEC, and was not present on the basal surfaces of GEC, nor other glomerular or kidney cells. DAF was functionally active on cultured GEC, and served to limit complement activation in concert with CD59, an inhibitor of C5b-9 formation. Upon injection into normal rats, anti-DAF F(ab')2 Abs bound to GEC in vivo, yet there was no evidence for complement activation and animals did not develop abnormal albuminuria. Anti-megalin complement-activating IgG Abs were "planted" on GEC, which activated complement as evidenced by the presence of C3d on GEC. Attempts to inhibit DAF function with anti-DAF Abs did not affect the quantity of complement activation by these anti-megalin Abs, nor did it lead to development of abnormal albuminuria. In contrast, in the puromycin aminonucleoside model of GEC injury and proteinuria, anti-DAF Abs slowed the recovery from renal failure that occurs in this model.

CONCLUSION

In cultured rat GEC, DAF is an effective complement regulator. In vivo, DAF is present on GEC apical surfaces. Yet, it appears that DAF is not essential to prevent complement activation from occurring under normal circumstances and in those cases in which complement-activating Abs are present on the basal surfaces of GEC in vivo. However, in proteinuric conditions, DAF appears to be protective to GEC.

A complement-dependent model of thrombotic thrombocytopenic purpura induced by antibodies reactive with endothelial cells

Thrombotic thrombocytopenic purpura (TTP) is an immunologically mediated disease characterized by thrombocytopenia, hemolytic anemia, and pathologic changes in various organs, including the kidney, which are secondary to widespread thromboses. Central to TTP is platelet activation, which may occur from a variety of mechanisms, including endothelial cell activation or injury. In this study, injection of K6/1, a monoclonal antibody with widespread reactivity toward endothelia, led to dose-dependent thrombocytopenia in rats. This was magnified if animals were preimmunized with mouse IgG, thereby resulting in an accelerated autologous phase of injury. In this setting, significant anemia also resulted. Rats injected with K6/1 developed renal injury, consisting of tubular damage and glomerular thrombi. Thrombocytopenia and renal morphological abnormalities were eliminated if animals were complement depleted with cobra venom factor prior to K6/1 injection and worsened when the activity of the ubiquitous complement regulator Crry was inhibited with function-neutralizing antibodies. Therefore, we have developed a complement-dependent model of TTP in rats by injecting monoclonal antibodies reactive with endothelial cells. Antibody-directed complement activation leads to stimulation of platelets, through direct interactions with complement fragments and/or indirectly through endothelial cell activation or injury, with the subsequent development of TTP.

Il-4 therapy prevents the development of proteinuria in active Heymann nephritis by inhibition of Tc1 cells

The role of IL-4, a key Th2 cytokine, in promoting or inhibiting active Heymann nephritis (HN) was examined. HN is induced by immunization with Fx1A in CFA, and proteinuria in HN is associated with subepithelial IgG and C3 deposition and infiltration of CD8(+) T-cytotoxic 1 (Tc1) cells and macrophages into glomeruli, as well as induction of Abs to Crry. Treatment with rIL-4 from the time of Fx1A/CFA immunization stimulated an earlier IgG1 response to Fx1A, induced anti-Crry Abs, and up-regulated IL-4 mRNA in lymphoid tissue, but did not alter proteinuria. Treatment with MRCOx-81, an IL-4-blocking mAb, resulted in greater proteinuria, which suggests endogenous IL-4 regulated the autoimmune response. Delay of rIL-4 treatment until 4 wk post-Fx1A/CFA immunization and just before the onset of proteinuria prevented the development of proteinuria and reduced Tc1 cell infiltrate in glomeruli. Delayed treatment with IL-4 had no effect on titer or isotype of Abs to Fx1A or on Ig, C3, and C9 accumulation in glomeruli. Treatment with rIL-13, a cytokine that alters macrophage function such as rIL-4, but has no direct effect on T or B cell function, reduced glomerular macrophage infiltrate, but did not prevent proteinuria or CD8+ T cell infiltrate. Anti-Crry Abs were paradoxically only induced with rIL-4 therapy, not in HN controls with proteinuria. It was concluded that the rIL-4 effect was probably by inhibition of Tc1 cells, which normally mediate the glomerular injury that results in proteinuria.

Glomerular complement regulation is overwhelmed in passive Heymann nephritis

BACKGROUND

An injection of anti-Fx1A antibodies in rats leads to passive Heymann nephritis (PHN), a model of membranous nephropathy. Fx1A is a crude extract of renal cortex that contains megalin as a principal component. However, when rats are given anti-megalin antibodies, abnormal proteinuria does not occur. Because of the established complement dependence of PHN, we hypothesized that antibodies neutralizing complement regulatory proteins in the rat glomerulus also were required to induce PHN. Two likely targets are Crry and CD59, proteins abundant on the rat podocyte and contained within Fx1A that inhibit the C3 convertase and C5b-9 assembly, respectively.

METHODS

Rats were injected with anti-megalin monoclonal antibodies, followed by anti-Crry and/or anti-CD59 F(ab')(2) antibodies five days later. In a second group of experiments, rats were injected with anti-Fx1A or anti-Fx1A immunodepleted of reactivity against Crry and/or CD59.

RESULTS

In the setting of podocyte-associated anti-megalin monoclonal antibodies, simultaneous neutralization of Crry and CD59 function led to the development of significant proteinuria (11.0 +/- 2.1 mg/day, P < 0.001 vs. all other groups). In contrast, animals that had neither or only one of these complement regulators inhibited had normal urinary protein excretion (< or =6 mg/day). In animals given anti-Fx1A depleted of anti-Crry and/or anti-CD59, all groups developed typical PHN, characterized by heavy proteinuria and extensive glomerular deposition of C3 and C5b-9.

CONCLUSION

Crry and CD59 play an important role in restraining complement-mediated injury following subepithelial immune complex deposition; however, in PHN, their regulatory capacity is overwhelmed.

A protein with characteristics of factor H is present on rodent platelets and functions as the immune adherence receptor

Complement-coated particles interact with specific immune adherence receptors (IAR). In primates, this function is served by complement receptor 1 (CR1) on erythrocytes. In contrast, rodent platelets bear IAR distinct from CR1, the identity of which was studied here. A 150-kDa C3b-binding protein was isolated from rat platelets, which had immunochemical and biochemical identity to plasma factor H. Immunofluorescence microscopy and flow cytometry demonstrated that factor H was present on the surface of rat and mouse platelets, which could be removed by treatment with neuraminidase. Sheep erythrocytes bearing C3b underwent immune adherence with rat and mouse platelets, which was blocked with anti-factor H F(ab')(2) antibodies, but not with antibodies binding to the complement regulator, Crry, on the platelet surface. By reverse transcription-polymerase chain reaction using rat platelet RNA and primers designed from mouse factor H, a 472-base pair product was generated that was identical in sequence to that produced from rat liver RNA. The translated protein product was 85% similar to mouse liver factor H. The 3'-nucleotide sequence from platelets predicted a soluble factor H protein. By Northern analysis, liver and platelets had identically sized factor H mRNA. Thus, rat and mouse platelets have a membrane protein with characteristics of factor H that is linked via sialic acid residues and functions as the IAR. Whether platelet factor H is acquired by passive adsorption from sera and/or is produced by platelets remains to be determined.

Crry/p65, a membrane complement regulatory protein, has costimulatory properties on mouse T cells

It is known that certain type I membrane molecules (complement receptors type 1 and 2) belonging to the regulators of complement activation (RCA) family are involved in the regulation of B lymphocyte activation. In contrast, only GPI-anchored RCA molecules (CD55) have been described to be involved in T lymphocyte activation. In this study, we describe a novel function for the mouse RCA type I membrane protein Crry/p65 as a costimulatory molecule in CD4+ T cell activation. This is shown by increased anti-CD3-induced proliferation of CD4+ spleen T lymphocytes in the presence of the Crry/p65-specific mAb P3D2. Furthermore, Ab-induced coligation of Crry/p65 and CD3 favors IL-4 rather than IFN-gamma secretion in these cells. Crry/p65 signaling was also observed regardless of additional Ca2+, protein kinase C, or CD28-mediated costimuli. Analysis of intracellular intermediaries shows that Crry/p65-CD3 coligation enhances certain TCR/CD3-mediated signals, producing increased early tyrosine phosphorylation of many substrates and enhanced activation of the mitogen-activated protein kinase, extracellular signal-related kinase. These data fit well with the association of Crry/p65 with the tyrosine kinase Lck found in T cell lysates. The epitope recognized by the mAb P3D2 interferes with the protective role of Crry/p65 on C3 deposition. The relationship between protective function and costimulation by Crry/p65 is discussed. Our results support a multifunctional role for Crry/p65 in T cells and suggest new links between the natural and adaptive immune responses.

Production and functional analysis of rat CD59 and chimeric CD59-Crry as active soluble proteins in Pichia pastoris

Crry (CR1-related gene/protein) is a rodent complement regulator that inhibits C3 convertases. CD59 is a conserved protein inhibitor active towards C8 and C9. We have previously produced rat Crry as a recombinant soluble (rs) protein in Pichia pastoris. In this study we produced functionally active rat rsCD59 and a chimeric rsCD59-Crry protein in P. pastoris. The GPI anchor addition site of rat CD59 (Asn-79) was replaced either by a stop codon to produce rsCD59, or with the sequence of the first five short consensus repeats of Crry to produce rsCD59-Crry. Proteins were generated by fermentation and purified by affinity chromatography on an anti-CD59 column. In a standard classical pathway haemolysis assay, all three rs proteins had inhibitory activity, with 50% inhibition at 0.5 microM (rsCrry and rsCD59-Crry) and 4.4 microM (rsCD59). In an assay examining inhibition of C5b-9, in which C5b-7 was first formed, followed by purified C8 and C9, rsCD59 and rsCD59-Crry were active with 50% inhibition at 0.8 microM (rsCD59-Crry) and 1.3 microM (rsCD59). The degree of inhibition was independent of whether the C8 and C9 were of rat or human origin. Therefore, we have produced rsCD59 and rsCD59-Crry in P. pastoris. The rsCD59 retains its inhibitory activity towards C5b-9, while rsCD59-Crry appears to have the combined activities of Crry and CD59. In a haemolytic assay, the inclusion of CD59 to Crry is of no additional benefit to Crry, which may illustrate the overall importance of the C3 convertase step. Yet, inclusion of Crry to CD59 increases the potency of CD59 towards C5b-9.

A small N-terminal 60-kD fragment of gp600 (megalin), the major autoantigen of active Heymann nephritis, can induce a full-blown disease

Active Heymann nephritis of rat, an autoimmune glomerular disease, is an immunohistological, ultrastructural, and clinical model of human membranous glomerulonephritis. Both diseases in their full-blown form are characterized by (1) the formation of large, subepithelial glomerular immune deposits, which stain for IgG, C3, and membrane attack (C5b-9) components of complement and (2) the excretion of large amounts of protein in the urine (proteinuria). The target autoantigen of active Heymann nephritis is a large transmembrane renal glycoprotein with a molecular weight of approximately 600 kD, variously named gp600, gp330, LRP-2, or "megalin." This study was performed to identify the region in this enormously large glycoprotein that would produce full-blown active Heymann nephritis. A stable, small (60-kD) proteolytic fragment of gp600 was isolated and localized to the N-terminal end of the molecule using Western blot, sequencing, and amino acid analyses. Based on its primary structure, this fragment contains approximately 60 cysteine residues, the cross-linking of which to each other probably explains its stability. Immunization of rats with this fragment induced a full-blown disease that was comparable to the disease induced by a preparation containing the whole protein. These results indicate that this small fragment, retaining the natural disulfide bonds and probably its overall structure, contains those B and T cell epitopes that are sufficient to produce this organ-specific autoimmune disease.

Crry, a complement regulatory protein, modulates renal interstitial disease induced by proteinuria

Crry, a complement regulatory protein, modulates renal interstitial disease induced by proteinuria.

BACKGROUND

Recent studies have suggested a role for urinary complement components in mediating tubulointerstitial damage, which is known to have a good correlation with progression of chronic renal diseases. Although accumulating evidence suggests that complement regulatory proteins play an important protective role in glomeruli, their role in renal tubules remains unclear. In order to establish the role of a complement regulatory protein, Crry, in renal tubular injury, we employed a molecular biological approach to block the expression of Crry in tubules of animals with proteinuria induced with puromycin aminonucleoside nephritis (PAN). Methods and Results. Two different antisense oligodeoxynucleotides (ODNs) against Crry were designed and applied to cultured rat mesangial cells in vitro in order to establish their efficacy. Antisense ODN treatment resulted in decreased expression of Crry protein associated with increased sensitivity to complement attack in cell lysis assays compared with control ODN treatment or no treatment (44.7, 1.50, and 1.34%, respectively). Antisense ODNs did not affect the expression of Thy1 as a control, confirming the specificity of our ODNs. In vivo, we performed selective right renal artery perfusion to administer antisense ODNs to the kidney and showed prominent uptake of ODNs by proximal tubular cells. Reduced expression of Crry protein was demonstrated in proximal tubular cells in antisense ODNs-treated kidneys. Normal rats treated with the antisense ODNs did not show any pathological changes. However, in PAN, rats with massive proteinuria showed increased deposition of C3 and C5b-9 in tubules in antisense-treated kidneys, and histological assessment revealed more severe tubulointerstitial injury in antisense-treated animals compared with controls.

CONCLUSION

These results establish a pathogenic role for complement in leading to tubulointerstitial injury during proteinuria and, to our knowledge for the first time, show a protective role of a complement regulatory protein, Crry, in renal interstitial disease.

Targeting the complement system

Interest has blossomed in the development of complement inhibitors, in parallel with a growth in our understanding of the biology of the complement cascade. The first generation of designed inhibitors was based on naturally occurring complement receptors and regulatory molecules. These agents provided useful tools for exploring the role of complement in experimental models of disease, but may have limited therapeutic application in humans because of their short half-lives, limited bioavailability and possible antigenicity. More recently, humanized antibodies and synthetic molecules that block the activation of complement have been developed, which look as though they may overcome some of these difficulties. The possibility for precision inhibition of a limited part of the complement cascade, or for inhibition confined to a single organ, may offer effective therapeutic results, while avoiding the disadvantages of nonselective complement blockade. This review examines the recent evidence that complement inhibition will reduce tissue damage resulting from organ transplantation, ischaemia-reperfusion injury, cancer, glomerulonephritis and the use of extracorporeal circuits.