Characterization of autoantibodies in primary membranous nephropathy and their clinical significance

How to Choose the Right Treatment for Membranous Nephropathy

Membranous nephropathy is an autoimmune disease affecting the glomeruli and is one of the most common causes of nephrotic syndrome. In the absence of any therapy, 35% of patients develop end-stage renal disease. The discovery of autoantibodies such as phospholipase A2 receptor 1, antithrombospondin and neural epidermal growth factor-like 1 protein has greatly helped us to understand the pathogenesis and enable the diagnosis of this disease and to guide its treatment. Depending on the complications of nephrotic syndrome, patients with this disease receive supportive treatment with diuretics, ACE inhibitors or angiotensin-receptor blockers, lipid-lowering agents and anticoagulants. After assessing the risk of progression of end-stage renal disease, patients receive immunosuppressive therapy with various drugs such as cyclophosphamide, steroids, calcineurin inhibitors or rituximab. Since immunosuppressive drugs can cause life-threatening side effects and up to 30% of patients do not respond to therapy, new therapeutic approaches with drugs such as adrenocorticotropic hormone, belimumab, anti-plasma cell antibodies or complement-guided drugs are currently being tested. However, special attention needs to be paid to the choice of therapy in secondary forms or in specific clinical contexts such as membranous disease in children, pregnant women and patients undergoing kidney transplantation.

Netrin G1 Is a Novel Target Antigen in Primary Membranous Nephropathy

BACKGROUND

Primary membranous nephropathy (MN) is caused by circulating autoantibodies binding to antigens on the podocyte surface. PLA2R1 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 PLA2R1 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.

Update nephrotisches Syndrom – neue pathophysiologische Konzepte 2022

Pathogenese der MGN Die Identifikation der für die Entstehung der MGN verantwortlichen Autoantikörper ermöglicht es, die Diagnose und Therapie an die molekulare Pathogenese der Erkrankung anzupassen. Die zugrunde liegenden Mechanismen der PLA2R1-Antikörper-Entstehung sind noch unklar. Umweltfaktoren wie Luftverschmutzung sowie genetische Faktoren spielen möglicherweise eine wichtige Rolle. Bei der THSD7A-assoziierten MGN können THSD7A-exprimierende Tumore zur Entstehung der THSD7A-Antikörper beitragen.

Klinische Anwendung Eine antigenspezifische Diagnose ist hilfreich, um die anschließende Differenzialdiagnostik der MGN, z. B. eine Tumorsuche, zu individualisieren. PLA2R1-Antikörper sind sehr spezifisch für die Diagnose einer MGN und ermöglichen eine personalisierte Entscheidung für oder gegen eine Nierenbiopsie zur Diagnosesicherung, abhängig von der klinischen und laborchemischen Präsentation der Patienten. Die PLA2R1-Antikörper ermöglichen eine stärkere Anpassung der Therapie an die Pathogenese und der immunologischen Aktivität der MGN. Der PLA2R1-Antikörperspiegel wird sowohl zur Risikoeinteilung als auch für Therapie-Entscheidungen genutzt.

PLA2R1-Antikörperbestimmung PLA2R1-Antikörper können gegen verschiede Bindungsstellen am PLA2R1 gerichtet sein. Aktuell ist im klinischen Alltag die Bestimmung der Gesamt-PLA2R1-Antikörperspiegel entscheidend für die Prognose und Therapieüberwachung.

Neue potenzielle Antigene der MGN In den letzten Jahren wurde eine Vielzahl weiterer potenzieller Antigene bei Patienten mit MGN beschrieben. Ihre pathophysiologische Rolle ist noch nicht geklärt. Je nach beschriebenem Antigen wurden Assoziationen der MGN mit verschiedenen Erkrankungen berichtet.

Pathogenese der MCD und FSGS bleibt ungeklärt. Eine T-Zell-Störung wurde seit Langem als mögliche Ursache angenommen, während in den letzten Jahren auch für B-Zellen und Autoantikörper zunehmend eine pathogenetische Rolle postuliert wird.

Diagnose und Therapie Für die Diagnose einer MCD und FSGS bei Erwachsenen ist die Nierenbiopsie obligat. In der Therapie der primären MCD und FSGS bleibt auch in den neuen KDIGO-Therapie-Empfehlungen die Steroidtherapie die 1. Wahl. In der Zweitlinientherapie werden vor allem Calcineurin-Inhibitoren eingesetzt. Als weitere Therapieoptionen gelten Cyclophosphamid, MMF und Rituximab.

Membranous nephropathy: new pathogenic mechanisms and their clinical implications

Membranous nephropathy (MN) is characterized histomorphologically by the presence of immune deposits in the subepithelial space of the glomerular filtration barrier; its clinical hallmarks are nephrotic range proteinuria with oedema. In patients with primary MN, autoimmunity is driven by circulating autoantibodies that bind to one or more antigens on the surface of glomerular podocytes. Compared with other autoimmune kidney diseases, the understanding of the pathogenesis of MN has substantially improved in the past decade, thanks to the discovery of pathogenic circulating autoantibodies against phospholipase A₂ receptor 1 (PLA₂R1) and thrombospondin type 1 domain-containing protein 7A (THSD7A). The subsequent identification of more proteins associated with MN, some of which are also endogenous podocyte antigens, might further advance the clinical characterization of MN, including its diagnosis, treatment and prognosis. Insights from studies in patients with MN, combined with the development of novel in vivo and in vitro experimental models, have potential to improve the management of patients with MN. Characterizing the interaction between autoimmunity and local glomerular lesions provides an opportunity to develop more specific, pathogenesis-based treatments.

Characterization of THSD7A-antibodies not binding to glomerular THSD7A in a patient with diabetes mellitus but no membranous nephropathy

Membranous nephropathy (MN) is an autoimmune disease caused by autoantibodies against the podocyte antigens phospholipase A₂ receptor 1 (PLA₂R1) 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.

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.