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The phospholipase A2 receptor 1 (PLA2R1) is the major target antigen in patients with membranous nephropathy (MN), an antibody-mediated autoimmune glomerular disease. Investigation of MN pathogenesis has been hampered by the lack of reliable animal models. Here, we overcome this issue by generating a transgenic mouse line expressing a chimeric PLA2R1 (chPLA2R1) consisting of three human PLA2R1 domains (cysteine-rich, fibronectin type-II and CTLD1) and seven murine PLA2R1 domains (CTLD2-8) specifically in podocytes. Mice expressing the chPLA2R1 were healthy at birth and showed no major glomerular alterations when compared to mice with a wild-type PLA2R1 status. Upon active immunization with human PLA2R1 (hPLA2R1), chPLA2R1-positive mice developed anti-hPLA2R1 antibodies, a nephrotic syndrome, and all major histological features of MN, including granular deposition of mouse IgG and complement components in immunofluorescence and subepithelial electron-dense deposits and podocyte foot process effacement in electron microscopy. In order to investigate the role of the complement system in this model, we further crossed chPLA2R1-positive mice with mice lacking the central complement component C3 (C3 -/- mice). Upon immunization with hPLA2R1, chPLA2R1-positive C3 -/- mice had substantially less severe albuminuria and nephrotic syndrome when compared to chPLA2R1-positive mice with a wild-type C3 status. In conclusion, we introduce a novel active immunization model of PLA2R1-associated MN and demonstrate a pathogenic role of the complement system in this model., (Copyright © 2023 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Little is known about the mechanistic significance of the ubiquitin proteasome system (UPS) in a kidney autoimmune environment. In membranous nephropathy (MN), autoantibodies target podocytes of the glomerular filter resulting in proteinuria. Converging biochemical, structural, mouse pathomechanistic, and clinical information we report that the deubiquitinase Ubiquitin C-terminal hydrolase L1 (UCH-L1) is induced by oxidative stress in podocytes and is directly involved in proteasome substrate accumulation. Mechanistically, this toxic gain-of-function is mediated by non-functional UCH-L1, which interacts with and thereby impairs proteasomes. In experimental MN, UCH-L1 becomes non-functional and MN patients with poor outcome exhibit autoantibodies with preferential reactivity to non-functional UCH-L1. Podocyte-specific deletion of UCH-L1 protects from experimental MN, whereas overexpression of non-functional UCH-L1 impairs podocyte proteostasis and drives injury in mice. In conclusion, the UPS is pathomechanistically linked to podocyte disease by aberrant proteasomal interactions of non-functional UCH-L1., (© 2023. The Author(s).)

Significance Statement: Membranous nephropathy (MN) is an autoimmune kidney disease characterized by immune deposits in the glomerular basement membrane. Circulating anti-phospholipase A 2 receptor 1 (PLA 2 R1) antibodies are detectable in 70%-80% of patients with MN, but experimental evidence of pathogenicity has been lacking. This study demonstrates the pathogenicity of human anti-PLA 2 R1 antibodies in minipigs, a model for MN that intrinsically expresses PLA 2 R1 on podocytes. After passive transfer of human anti-PLA 2 R1 antibody-containing plasma from patients with PLA 2 R1-associated MN to minipigs, antibodies were detected in the minipig glomeruli, but not in response to plasma from healthy controls. The minipigs developed histomorphological characteristics of MN, local complement activation in the glomeruli, and low-level proteinuria within 7 days, showing that human anti-PLA 2 R1 antibodies are pathogenic., Background: Primary membranous nephropathy (MN) is an autoimmune kidney disease in which immune complexes are deposited beneath the epithelium in the glomeruli. The condition introduces a high risk for end-stage kidney disease. Seventy percent to 80% of patients with MN have circulating antibodies against phospholipase A 2 receptor 1 (PLA 2 R1), and levels correlate with treatment response and prognosis. However, experimental evidence that human anti-PLA 2 R1 antibodies induce MN has been elusive., Methods: In passive transfer experiments, minipigs received plasma or purified IgG from patients with PLA 2 R1-associated MN or from healthy controls. Anti-PLA 2 R1 antibodies and proteinuria were monitored using Western blot, ELISA, and Coomassie staining. Kidney tissues were analyzed using immunohistochemistry, immunofluorescence, electron microscopy, and proteomic analyses., Results: Minipigs, like humans, express PLA 2 R1 on podocytes. Human anti-PLA 2 R1 antibodies bound to minipig PLA 2 R1 in vitro and in vivo . Passive transfer of human anti-PLA 2 R1 antibodies from patients with PLA 2 R1-associated MN to minipigs led to histological characteristics of human early-stage MN, activation of components of the complement cascade, and low levels of proteinuria. We observed development of an autologous, later phase of disease., Conclusions: A translational approach from humans to minipigs showed that human anti-PLA 2 R1 antibodies are pathogenic in MN, although in the heterologous phase of disease only low-level proteinuria developed., (Copyright © 2023 by the American Society of Nephrology.)

Membranous nephropathy (MN) is an antibody-mediated autoimmune disease characterized by glomerular immune complexes containing complement components. However, both the initiation pathways and the pathogenic significance of complement activation in MN are poorly understood. Here, we show that components from all three complement pathways (alternative, classical and lectin) are found in renal biopsies from patients with MN. Proximity ligation assays to directly visualize complement assembly in the tissue reveal dominant activation via the classical pathway, with a close correlation to the degree of glomerular C1q-binding IgG subclasses. In an antigen-specific autoimmune mouse model of MN, glomerular damage and proteinuria are reduced in complement-deficient mice compared with wild-type littermates. Severe disease with progressive ascites, accompanied by extensive loss of the integral podocyte slit diaphragm proteins, nephrin and neph1, only occur in wild-type animals. Finally, targeted silencing of C3 using RNA interference after the onset of proteinuria significantly attenuates disease. Our study shows that, in MN, complement is primarily activated via the classical pathway and targeting complement components such as C3 may represent a promising therapeutic strategy., (© 2023. The Author(s).)

Competing Interests: Declaration of interests TW received honoraria for lectures from Bayer, GlaxoSmithKline and Novartis, and from Retrophin, USA for advisory board activities. PFZ received consulting fees from Eleva and Novartis, and honoraria from Alexion and Bayer. JK holds shares from MoKi Analytics. AM received speaker honoraria from, Chiesi and Biomerieux and holds shares in Moter Diagnostics and MoKi Analytics. EH received fees from Morphosys, Planegg, Germany and Novartis, Basel, Switzerland for advisory board activities. RAKS received fees from Morphosys, Planegg, Germany for advisory board activities. DS received honoraria for lectures and advisory board activities from Alexion, GlaxoSmithKline, Travere Therapeutics, and Astellas. All other authors declare no competing interests.

Background: Primary membranous nephropathy (MN) is caused by circulating autoantibodies binding to antigens on the podocyte surface. PLA 2 R1 is the main target antigen in 70%-80% of cases, but the pathogenesis is unresolved in 10%-15% of patients., Methods: We used native western blotting to identify IgG4 autoantibodies, which bind an antigen endogenously expressed on podocyte membranes, in the serum of the index patient with MN. These IgG4 autoantibodies were used to immunoprecipitate the target antigen, and mass spectrometry was used to identify Netrin G1 (NTNG1). Using native western blot and ELISA, NTNG1 autoantibodies were analyzed in cohorts of 888 patients with MN or other glomerular diseases., Results: NTNG1 was identified as a novel target antigen in MN. It is a membrane protein expressed in healthy podocytes. Immunohistochemistry confirmed granular NTNG1 positivity in subepithelial glomerular immune deposits. In prospective and retrospective MN cohorts, we identified three patients with NTNG1-associated MN who showed IgG4-dominant circulating NTNG1 autoantibodies, enhanced NTNG1 expression in the kidney, and glomerular IgG4 deposits. No NTNG1 autoantibodies were identified in 561 PLA 2 R1 autoantibodies-positive patients, 27 THSD7A autoantibodies-positive patients, and 77 patients with other glomerular diseases. In two patients with available follow-up of 2 and 4 years, both NTNG1 autoantibodies and proteinuria persisted., Conclusions: NTNG1 expands the repertoire of target antigens in patients with MN. The clinical role of NTNG1 autoantibodies remains to be defined., (Copyright © 2022 by the American Society of Nephrology.)

Membranous nephropathy (MN) is an autoimmune disease caused by autoantibodies against the podocyte antigens phospholipase A 2 receptor 1 (PLA 2 R1) and thrombospondin type 1 domain containing protein 7A (THSD7A) in 80% and 2-3% of patients, respectively. THSD7A antibodies are considered to be pathogenic and highly specific for MN patients. Using an indirect immunofluorescence test (IIFT) we detected THSD7A-antibodies (titre 1:10) in the serum of a patient with high proteinuria who, however, in the kidney biopsy was diagnosed with diabetic nephropathy and MN was excluded as a possible cause of proteinuria. Different immunofluorescence assays and Western blot techniques using recombinant THSD7A (rTHSD7A) or THSD7A from different human tissues revealed that the circulating THSD7A-autoantibodies were only of the IgG3 subclass. The patient serum reacted exclusively with rTHSD7A and only when the antigen was present in reducing Western blot conditions, or on formaldehyde-fixed cells for the IIFT. Our findings show for the first time the existence of circulating THSD7A-antibodies recognizing denatured/reduced rTHSD7A, which do not react with glomerular THSD7A in vivo and are thus presumptively non-pathogenic. As a consequence, kidney biopsy or Western blot analyses of THSD7A under non-reducing conditions should be performed to confirm the diagnosis of THSD7A-associated MN, especially in cases with low THSD7A-antibody levels in the IIFT., (© 2021. The Author(s).)

The phospholipase A2 receptor 1 (PLA2R1) is the major autoantigen in patients suffering from membranous nephropathy. To date, the lack of endogenous glomerular expression of PLA2R1 in mice and rats has impeded the establishment of PLA2R1-dependent animal models of this disease. Here, we generated a transgenic mouse line expressing murine full-length PLA2R1 in podocytes. Furthermore, expression of murine PLA2R1 did not result in any morphological disturbance as high-resolution confocal microscopy demonstrated an intact nephrin distribution with normal foot processes. Transfer of rabbit anti-mPLA2R1 antibodies to these mice induced nephrotic range proteinuria, hypercholesterolemia, and histomorphological signs of membranous nephropathy. Immunohistochemical and immunofluorescence analyses revealed enhanced staining for murine PLA2R1 in the presence of unaffected staining for murine thrombospondin type-1 domain-containing 7A in the diseased mice, resembling what is classically found in patients with PLA2R1-associated membranous nephropathy Thus, our mouse model of membranous nephropathy will allow investigation of PLA2R1-specific pathomechanisms and may help to develop and assess antigen-specific treatments in vivo., (Copyright © 2019 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Background: Membranous nephropathy (MN) is an autoimmune disease induced by circulating antibodies against the podocyte protein phospholipase A2 receptor 1 (PLA2R1-ab) in 80% of patients and represents the leading cause of nephrotic syndrome in adults. PLA2R1-ab levels correlate with disease activity and treatment response. However, their predictive role for long-term renal outcome is not clear., Methods: The aim of this prospective observational multicenter study was to investigate the predictive role of PLA2R1-ab levels at the time of diagnosis for long-term outcome in a cohort of 243 patients with newly diagnosed biopsy-proven PLA2R1-associated MN. Statistical analyses included Cox proportional hazard models. The primary study endpoint was defined prior to data collection as doubling of serum creatinine or development of end-stage renal disease., Results: During the median follow-up time of 48 months, 36 (15%) patients reached the study endpoint. Independent predictors for reaching the study endpoint were baseline PLA2R1-ab levels (HR = 1.36, 95%CI 1.11-1.66, p = 0.01), percentage of tubular atrophy and interstitial fibrosis (HR = 1.32, 95%CI 1.03-1.68, p = 0.03), PLA2R1-ab relapse during follow-up (HR = 3.22, 95%CI 1.36-7.60, p = 0.01), and relapse of proteinuria (HR = 2.60, 95%CI 1.17-5.79, p = 0.02). Fifty-four (22%) patients received no immunosuppressive treatment during the study, in 41 (76%) of them PLA2R1-ab spontaneously disappeared during follow-up, 29 (54%) patients had a complete remission of proteinuria, and 19 (35%) had a partial remission. Patients not treated with immunosuppression were more often females and had lower PLA2R1-ab levels, proteinuria, and serum creatinine at baseline compared to patients receiving immunosuppression. However, no conclusion on the efficacy of immunosuppressive therapies can be made, since this was not a randomized controlled study and treatment decisions were not made per-protocol., Conclusions: PLA2R1-ab levels are, in addition to pre-existing renal damage, predictive factors for long-term outcome and should therefore be considered when deciding the treatment of patients with MN., Competing Interests: The authors have declared that no competing interests exist.

Background: Membranous nephropathy (MN) is a common cause of nephrotic syndrome in adults, but is less frequent in children. Antibodies against four antigens leading to MN have been described in children: phospholipase A 2 receptor 1 (PLA 2 R1), thrombospondin type-1 domain-containing 7A (THSD7A), neutral endopeptidase (NEP), and cationic bovine serum albumin (BSA)., Methods: Twelve children with MN were included in this study. Sera of all patients were analyzed for antibodies against PLA 2 R1, THSD7A, NEP, and BSA. All sera were also analyzed using Western blot with human glomerular extracts (HGE) under non reducing conditions. In 5 cases renal biopsies were analyzed for PLA 2 R1, THSD7A, NEP, BSA, and all IgG subclasses., Results: Six patients were PLA 2 R1-antibody-positive, whereas THSD7A, NEP, and BSA antibodies were not found in any of our 12 patients. All sera were analyzed by Western blot using human glomerular extracts; however, no further potential antigens were found. Five kidney biopsies from 2 PLA 2 R1-antibody-positive and 3 PLA 2 R1-antibody-negative patients were available for additional analyses, confirming the diagnosis of PLA 2 R1-associated MN in 2 cases, whereas none of the biopsies revealed enhanced staining for THSD7A, NEP or BSA. IgG2 and IgG4 stainings were positive in both patients with PLA 2 R1-associated MN and negative in the other biopsies. During follow-up (median 24 months), 4 children with PLA 2 R1-associated MN went into remission, preceded by decline of PLA 2 R1 antibodies. Five of the 6 PLA 2 R1-antibody-negative children went into remission., Conclusions: In children with MN, PLA 2 R1-associated MN appears to be common, whereas MN associated with THSD7A, NEP or BSA was not encountered. PLA 2 R1 antibody levels are closely associated with disease activity, whereas PLA 2 R1-antibody-negative patients often have a good prognosis. However, the pathophysiology of MN in a considerable number of children remains unclear.

Since the discovery of the phospholipase A 2 receptor 1 (PLA 2 R1) and thrombospondin type-1 domain-containing 7A (THSD7A) as endogenous antigens involved in the development of membranous nephropathy (MN) in over 80% of adult patients, substantial progress in the diagnosis, prognosis, and therapy of MN has been made. In most cases of patients with MN, it is now possible to specifically define the responsible pathogenic mechanisms of disease and make a diagnosis even without a renal biopsy. Moreover, the presence of antibodies in the blood and the detection of the antigens in renal biopsies allow the definite diagnosis without the morphologic uncertainties, which now still apply for only about 20% of all renal biopsies showing MN. The discovery that the expression of THSD7A in malignant tumors might serve as the site of primary antigen recognition for the immune system to start MN might lead to a better understanding of not only tumor-associated MN, which accounts for up to 10% of all patients with MN, but also of the pathomechanisms relevant for MN development in general.

Thrombospondin type 1 domain-containing 7A (THSD7A) is a target antigen identified in adult membranous nephropathy (MN) along with the major antigen phospholipase A 2 receptor 1 (PLA 2 R1). The prevalence of THSD7A-Ab-positive patients is unknown, and it is unclear whether the clinical presentation differs between patients positive for PLA 2 R1-Ab or THSD7A-Ab. We screened serum samples of 1276 patients with MN from three different cohorts for the presence of THSD7A-Ab by Western blot analysis and a newly developed indirect immunofluorescence test (IFT). Compared with Western blot analysis, the IFT had a 92% sensitivity and a 100% specificity. The prevalence of THSD7A-associated MN in a prospective cohort of 345 patients with MN was 2.6%, and most were women. In this cohort, the percentage of patients with THSD7A-associated MN and malignant disease significantly exceeded that of patients with PLA 2 R1-associated MN and malignant disease. In all cohorts, we identified 40 patients with THSD7A-associated MN, eight of whom developed a malignancy within a median time of 3 months from diagnosis of MN. In one patient with THSD7A-associated MN and metastases of an endometrial carcinoma, immunohistochemistry showed THSD7A expression on the metastatic cells and within follicular dendritic cells of the metastasis-infiltrated lymph node. We conclude that the IFT allows sensitive and specific measurement of circulating THSD7A-Ab in patients with MN. Patients with THSD7A-associated MN differ in their clinical characteristics from patients with PLA 2 R1-associated MN, and more intensive screening for the presence of malignancies may be warranted in those with THSD7A-associated MN., (Copyright © 2017 by the American Society of Nephrology.)