Autoantibody-triggered podocyte membrane budding drives autoimmune kidney disease.

Membranous glomerulonephritis is a major cause of nephrotic syndrome and chronic renal insufficiency. This condition may be associated with a wide spectrum of infections, cancers, autoimmune diseases, and drugs, although primary forms of the disease remain the most common. Histologically, membranous

Membranous nephropathy (MN) is the leading cause of primary nephrotic syndrome in white adults and a major cause of nephrotic syndrome across global populations. In MN, circulating antibodies permeate the glomerular basement membrane and, in the subepithelial space, form immune complexes with antigens on podocyte membranes. Recently, the M-type phospholipase A2 receptor (PLA2R) was identified as the specific podocyte antigen responsible for eliciting immune complex formation with circulating antibodies. Anti-PLA2R antibodies are detected in 60%–75% of idiopathic MN cases across many ethnicities.1,2 Additional podocyte autoantigens—mitochondrial SOD 2, aldose reductase, α-enolase, and neutral endopeptidase3,4—have likewise emerged as potential targets of MN-specific autoantibodies, potentially filling in the missing gaps in PLA2R antibody-negative disease. These breakthroughs have established MN as a disease of autoantibodies and, in many ways, challenge the continued use of the term idiopathic MN.5 Nonetheless, we still do not know why, exactly, such autoantibodies develop in MN. The identified podocyte antigens are endogenously expressed; only the autoantibodies against such antigens are detected in patients with MN. Previous case reports of familial forms of MN have suggested a genetic predisposition to disease.6 In a recent genomewide association study (GWAS) in three European populations (French, Dutch, and British), Stanescu et al. described associations of MN with the HLA locus on chromosome 6p21 and the PLA2R1 locus (encoding PLA2R) on chromosome 2q24.7 The association with HLA was significant in all three patient samples, whereas the association with PLA2R1 was significant in the Dutch and British samples (as well as in joint analysis of all three populations). Strikingly, whereas the risk of disease was relatively modest in individuals with risk alleles at any one locus, the odds ratio for MN was an astronomical 78.5 (95% confidence interval [95% CI], 34.6 to 178.2) in individuals homozygous for risk alleles at both loci, indicative of strong genetic interaction. This GWAS was thus unusual because the effect sizes imparted by the combined risk alleles were very large, suggesting a potential role for genetics for noninvasive screening or risk stratification of MN. This study also provided an independent line of evidence implicating PLA2R1 in the pathogenesis of disease, suggesting that sequence variants within PLA2R1 may alter expression or function of PLA2R, potentially unmasking it as an autoantigen that, in conjunction with the right MHC haplotype, results in activation of T cells and stimulation of autoantibody production. Limitations of this GWAS included the relatively small sample size, which precluded precise localization of the risk alleles within each locus. Particularly, the origin of the signal within the MHC locus remained unclear,8 because this region has a very complicated structure, and class I and class II response loci may each contain multiple independent haplotypes with opposing effects on risk of disease. These findings thus required follow-up in larger cohorts and validation beyond European populations. In this issue of JASN, Lv and colleagues genotyped 1112 Chinese patients with MN and 1020 healthy controls for the top single-nucleotide polymorphism (SNPs) in the European GWAS (three SNPs within the PLA2R1 locus and three SNPs within HLA genes).9 All three SNPs within PLA2R1 were highly associated with MN, and the strongest signal emerged from the same SNP (rs4664308) identified in the European GWAS. The HLA-DQA1 SNP (rs2187668) also showed association with MN, whereas two other HLA-located SNPs showed no such association with disease. Thus, this study robustly replicated the genetic signal demonstrated in a GWAS of European cohorts. However, in this Chinese population, the odds ratio for MN associated with homozygosity for both risk alleles was 9.9 (95% CI, 1.1 to 91.9), which is much lower than the odds ratio described for Europeans. Interestingly, the odds ratio rose to 11.1 (95% CI, 6.5 to 19.2) when looking at patients homozygous for the PLA2R1 risk allele but either homozygous or heterozygous for the HLA-DQA1 risk allele. Similar findings have been reported in replication studies from Korea10 and Taiwan11; the lower odds ratio in Asians suggests true differences in effect size between different ethnicities but may also reflect convergence to the mean. Because these SNPs are thought to represent "tag-SNPs" for the true causal alleles, differences in linkage disequilibrium structure between ethnicities may also account for this discrepancy in odds ratios. It is expected that fine mapping of these loci and more detailed analysis of haplotype structures across different populations will clarify the origin of the signals within each locus. Among the most intriguing aspects of the report from Lv et al. is the subanalysis done in 71 patients with MN, subdivided into low risk versus high risk according to their PLA2R1 and HLA-DQA1 genotypes, looking at two increasingly important phenotypes of MN: whether anti-PLA2R antibodies are detectable in serum, and whether PLA2R can be detected in glomerular deposits by immunofluorescence.12 None of the 19 MN patients homozygous for low-risk PLA2R1 and HLA-DQA1 genotypes had detectable anti-PLA2R antibodies, whereas 36 of the 52 (65%) remaining patients with one or both high-risk genotypes demonstrated antibody positivity. Likewise, PLA2R staining of glomeruli was enhanced in none of the 19 patients with both low-risk genotypes compared with 36 of the 50 (65%) of the remaining patients with one or both high-risk genotypes and adequate biopsy tissue for analysis. Interestingly, the predictive power of genotypes was mostly attributable to the PLA2R1 locus: 73% of individuals homozygous for risk alleles at this locus alone were antibody positive. Similar to most risk alleles underlying complex traits, the PLA2R1 risk variants are located in noncoding regions, and the identity of the causal mutation is not known. One can speculate that the causal variant(s) exert a regulatory role, somehow altering the expression level or localization of the encoded protein. These results, linking genotype to phenotype, concur with a recent study by the investigators of the European GWAS in which the PLA2R1 gene was sequenced in 60 patients with PLA2R-related MN (by serology and/or histopathology) and six common sequence variants were significantly associated with disease.13 Lv et al. also report an association between risk genotypes of PLA2R1 and histopathologic stage of MN, although this classification scheme has not been found, in two recent MN cohorts, to correlate with disease outcomes.14,15 The last decade has witnessed remarkable progress in unraveling the genetic factors that influence the pathogenesis of virtually every glomerular disease. MN does not stand alone by coming increasingly close to shedding the qualifier, "idiopathic." Other glomerular diseases, such as FSGS,16 IgA nephropathy,17 and ANCA-associated vasculitis,18 have demonstrated a genetic predisposition to disease in studies similar to those by Stanescu et al. and Lv et al. However, the large odds ratio conferred by joint homozygosity for MN risk alleles suggests that genetic profiling may have some utility as a screening test for MN or for defining epidemiologic risk in the population. For example, based on published genotype frequencies in controls, as many as 0.14% of Asians and 0.54% of Europeans are homozygous for risk alleles at both loci and are therefore at highest risk for MN.7,9 Identification and prospective follow-up of these individuals in existing epidemiologic cohorts may help define the natural history of disease. Moreover, the PLA2R1 and HLA genotypes may serve as a method for defining subtypes of MN by pathogenetic mechanism, thereby enabling better analysis of disease progression, remission, and response to therapy. For example, genotypes could potentially explain why some patients with primary MN have spontaneous remissions whereas others display a frequently relapsing phenotype, and why recurrent MN does or does not occur in recipients of kidney transplants. This sort of risk stratification is desperately needed in MN, a disease with the broadest spectrum of natural history, ranging from spontaneous remission to unrelenting progression toward ESRD. Recent advances have moved us away from proteinuria and toward autoantibody titers in the search for a disease-specific risk assessment tool,19 but this may just be an intermediate step as we get closer to decoding the genes that explain why such autoantibodies develop. Disclosure None.

First Identification of an Antigen in Autoimmune Idiopathic Membranous Nephropathy: Toward Targeted Therapy?

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.

Thrombospondin type-1 domain-containing 7A-related membranous nephropathy associated with glomerular AL amyloidosis

Membranous nephropathy: a review on the pathogenesis, diagnosis, and treatment.

B Cell Suppression in Primary Glomerular Disease

Circulating Antipodocyte Antibodies in Membranous Nephropathy: Pathophysiologic and Clinical Relevance

Background and Aims Membranous glomerulonephritis (MGN) is a major cause of nephrotic syndrome (NS) and nephrotic-range proteinuria. To better understand the population presenting with NS due to MGN, we analyzed our NS cohort over a 13-year period. Method We conducted a retrospective observational study from 2010 to 2022, including patients with inaugural nephrotic syndrome and first biopsy-confirmed MGN. Data were collected anonymously from electronic health records, covering patient demographics, comorbidities, laboratory findings at diagnosis (serum albumin, serum creatinine, 24-h proteinuria, and hematuria), pathological features, and clinical outcomes (mortality, dialysis requirement three years post-diagnosis, immunosuppressive treatments, remission/relapse rates, and yearly eGFR progression). The primary goal was to characterize adult population presenting with inaugural NS caused by MGN. Data analysis was conducted using SPSS® v28.0.1.0. Results 48 adults with inaugural presenting nephrotic syndrome and first biopsy-proven MGN diagnosis were retrieved from the primary analysis. Thirty-two (66.7%) were classified as primary—17 (35.4%) PLA2Rab positive, 12 (25%) PLA2Rab status unknown and 3 (6.3%) PLA2Rab negative. Autoimmune disease was the most common cause of secondary MGN (14.6%), followed by neoplasia (8.3%), infection (6.3%) and drugs (4.2%). Thirty-three (68.8%) patients were males, 47 (97.1%) were Caucasian, and the median age at diagnosis was 56 years. Seven (14.6%) patients had previous CKD, 7 (14.6%) were diabetic, and 3 (6.3%) had previous neoplasia of which 1 (2.1%) was active. Median serum creatinine, serum albumin, and 24 h-proteinuria at diagnosis were 0.96 mg/dL, 23.7 g/L and 7.78 g, respectively. (Micro)hematuria was observed in 22 (45.8%) patients. The most common pathological findings were GBM thickening (median 99.8% of glomeruli), tubular necrosis (75% of biopsies), and interstitial inflammatory infiltrate (35.4% of biopsies). Regarding first line immunosuppression used, the modified Ponticelli regimen was the most common (31.3% of patients)- mainly driven by the first years of analysis, followed by the rituximab-based regimen (10.4%), corticosteroid only (10.4%) and calcineurin inhibitors (8.3%). Fifteen (31.3%) patients underwent partial remission, and 22 (45.8%) complete remission—achieved with a median of 18 months. Fifteen (31.3%) patients relapsed, with a median of 1 relapse. The rituximab- based regimen was the most frequent second-line immunosuppressant (22.9%). Three years post-diagnosis, 6 (12.5%) patients were chronically hemodialyzed, 4 died (8.3%) and, for the remaining, the median eGFR was 66 ml/min/1.73m2. Conclusion In our MGN-NS population, most patients were male, presented with severe hypoalbuminemia and without renal dysfunction. The overall prognosis was good. The modified Ponticelli regimen was the most common first-line induction immunosuppressive scheme, surpassed by Rituximab-based regimens in the latter years.

Pathophysiological advances in membranous nephropathy: time for a shift in patient's care

Membranous nephropathy (MN) is the major cause of nephrotic syndrome with special pathological features, caused by the formation of immune complexes in the space between podocytes and the glomerular basement membrane. In idiopathic membranous nephropathy (IMN) the immune complexes are formed by circulating antibodies binding mainly to one of two naturally-expressed podocyte antigens: the M-type receptor for secretory phospholipase A2 (PLA2R1) and the Thrombospondin type-1 domain-containing 7A (THSD7A). Formation of antibodies against PLA2R1 is much more common, accounting for 70–80% of IMN. However, the mechanism of anti-podocyte antibody production in IMN is still unclear. In this review, we emphasize that the exposure of PLA2R1 is critical for triggering the pathogenesis of PLA2R1-associated MN, and propose the potential association between inflammation, pollution and PLA2R1. Our review aims to clarify the current research of these precipitating factors in a way that may suggest future directions for discovering the pathogenesis of MN, leading to additional therapeutic targets and strategies for the prevention and early treatment of MN.

Membranous nephropathy is an autoimmune disease that results in an accumulation of antigen-antibody (IgG) immune complexes along the subepithelial region of the glomerular basement membrane and is the most common cause of nephrotic syndrome in adults. The diagnosis of membranous nephropathy is based on the presence of granular IgG on immunofluorescence microscopy and subepithelial electron dense deposits along the glomerular basement membrane on electron microscopy. Prior to 2009, the target antigen within the immune complexes was unknown. However, in the past 15 years, and in particular the past 5 years, several target antigens have been identified. These target antigens include PLA2R, THSD7A, EXT1 and EXT2, NELL1, SEMA3B, NCAM1, CNTN1, HTRA1, FAT1, PCDH7, NTNG1, PCSK6, NDNF and MPO. Several rare putative antigens have also been reported. These findings have transformed our understanding of membranous nephropathy from that of an idiopathic disease, which results from an autoimmune response to an unknown target antigen, to a disease in which a target antigen can be identified in ~80% of cases. Improved understanding of the distinctive clinical association, pathology and prognostic findings of each target antigen will have implications for clinical evaluation and therapeutic targeting in patients with membranous nephropathy.

Membranous nephropathy (MN) is a common cause of nephrotic syndrome in adults. MN is characterized by subepithelial accumulation of immune complexes along the glomerular basement membrane. The immune complexes are composed of immunoglobulin G and a target antigen. PLA2R is the target antigen in approximately 60% of MN cases, and MN is traditionally classified as PLA2R-positive or PLA2R-negative MN. Over the last 7years, additional target antigens have been identified, which have specific disease associations, distinctive clinical and pathologic findings, and therapeutic implications. The newly discovered target antigens include NELL1, EXT1/EXT2, NCAM1, SEMA3B, PCDH7, FAT1, CNTN1, NTNG1, PCSK6 and NDNF. To group all these antigens into a generic 'PLA2R-negative' MN group is imprecise and un-informative. We propose a logical approach for detection of the target antigen which includes (i) currently available serology-based testing to detect anti-PLA2R and anti-THSD7A antibodies; and (ii) kidney biopsy testing to detect the target antigens. Determination of the antigen on kidney biopsy can be done by immunohistochemistry or immunofluorescence studies. Alternatively, laser capture microdissection (LCM) of glomeruli followed by mass spectrometry (MS) can be used to identify a target antigen. LCM/MS has the advantage of being a one-stop test and is particularly useful for detection of rare target antigens. At the current time, while it is possible to detect the newer antigens by immunohistochemistry/immunofluorescence/LCM/MS, serology-based tests to detect serum antibodies to the new antigens are not yet available. It is critical that serology-based tests should be developed not just for accurate diagnosis, but as a guide for treatment. We review the current methodology and propose an algorithm for diagnosis and detection of target antigens in MN that may shape the current practice in the future. Membranous nephropathy (MN) results from accumulation of subepithelial immune complexes along the glomerular basement membrane.PLA2R is the most common target antigen, but newly discovered target antigens have filled the void of PLA2R-negative MN.MN associated with the newly discovered target antigens have distinctive clinical and pathologic findings, treatment and prognostic implications. These include NELL1, EXT1/EXT2, NCAM1, PCDH7, SEMA3B, CNTN1, FAT1, NDNF and PCSK6.Immunohistochemistry/immunofluorescence methodology is currently in use for detecting target antigens in kidney biopsy tissue, although we anticipate laser capture microdissection of glomeruli followed by mass spectrometry will become available soon.Serologic testing is currently available for only detecting antibodies to PLA2R and THSD7A. It is critical that serologic tests become available for detecting antibodies to the newly discovered antigens.

Membranous nephropathy (MN) is relatively major cause of nephrotic syndrome in adults which is recognized as an organ-specific autoimmune disease. The etiology of most cases is idiopathic, whereas the secondary MN is caused by systemic autoimmune diseases, infections, medications and malignancies. The idiopathic disease is developed by the formation of sub-epithelial immune complex deposits most likely due to binding the circulating auto-antibodies to intrinsic antigen on podocytes. The major auto antibody is the anti-phospholipase A2 receptor(anti-PLA2R), however, it is not enough sensitive. Several attempts for diagnostic biomarker identification by modern analytical technologies have been devoted recently. This article reviews the biomarker candidates for primary type of MN that are detected by different approaches on human subjects.

Membranous nephropathy (MN) is a rare but potentially severe autoimmune disease and a major cause of nephrotic syndrome in adults. Traditional treatments for patients with MN include steroids with alkylating agents such as cyclophosphamide or calcineurin inhibitors such as cyclosporine, which have an undesirable side effect profile. Newer therapies like rituximab, although superior to cyclosporine in maintaining disease remission, do not only affect pathogenic B or plasma cells, but also inhibit the production of protective antibodies and therefore the ability to fend off foreign organisms and to respond to vaccination. These are undesired effects of general B or plasma cell-targeted treatments. The discovery of several autoantigens in patients with MN offers the great opportunity for more specific treatment approaches. Indeed, such treatments were recently developed for other autoimmune diseases and tested in different preclinical models, and some are about to jump to clinical practice. As such treatments have enormous potential to enhance specificity, efficacy and compatibility also for MN, we will discuss two promising strategies in this perspective: The elimination of pathogenic antibodies through endogenous degradation systems and the depletion of pathogenic B cells through chimeric autoantibody receptor T cells.