Antenatal membranous glomerulonephritis with vascular injury induced by anti-neutral endopeptidase antibodies: toward new concepts in the pathogenesis of glomerular diseases

Membranous nephropathy: pathogenesis and treatments

Membranous nephropathy (MN), an autoimmune disease, can manifest at any age and is among the most common causes of nephrotic syndrome in adults. In 80% of cases, the specific etiology of MN remains unknown, while the remaining cases are linked to drug use or underlying conditions like systemic lupus erythematosus, hepatitis B virus, or malignancy. Although about one-third of patients may achieve spontaneous complete or partial remission with conservative management, another third face an elevated risk of disease progression, potentially leading to end-stage renal disease within 10 years. The identification of phospholipase A2 receptor as the primary target antigen in MN has brought about a significant shift in disease management and monitoring. This review explores recent advancements in the pathophysiology of MN, encompassing pathogenesis, clinical presentations, diagnostic criteria, treatment options, and prognosis, with a focus on emerging developments in pathogenesis and therapeutic strategies aimed at halting disease progression. By synthesizing the latest research findings and clinical insights, this review seeks to contribute to the ongoing efforts to enhance our understanding and management of this challenging autoimmune disorder.

Update on New Antigens in the Pathogenesis of Membranous Nephropathy

Previously, membranous nephropathies were divided into primary and secondary categories when the exact mechanism or pathogenetic factor were unknown. Approximately 70% accounted for primary membranous nephropathies. The remaining 30% were called secondary because they developed due to well-known diseases such as autoimmune diseases, tumours, infections, or drug assumptions. The discoveries of the M-type phospholipase A2 receptor and of thrombospondin type 1 domain containing 7A as causative antigens in a part of the so-called primary membranous nephropathies opened new knowledge on the effective causes of a large part of these diseases. The availability of novel techniques such as laser micro-dissection and tandem mass spectrometry, as well as immunochemistry with antibodies directed against novel proteins, allowed the confirmation of new antigens involved. The use of confocal microscopy and Western blot allowed detection of the new antigen on glomerular membrane, and the same antigen and relative antibodies have been detected in serum samples. Through these techniques, four new antigens were first detected, including neural epidermal growth factor 1 and semaphorin 3B in the so-called primary membranous nephropathy, and exostosin 1 and 2 and neural cell adhesion molecule 1 in lupus membranous nephropathy. The aim of this study is to describe the characteristics of the new antigens discovered and their association with other diseases. In addition, new antigens are on the horizon, and the story of primary membranous nephropathy is still to be completely written and understood.

Course monitoring of membranous nephropathy: Both autoantibodies and podocytes require multidimensional attention

A variety of podocyte antigens have been identified in human membranous nephropathy (MN), which is divided into various antigen-dominated subtypes, confirming the concept that MN is the common pattern of glomerular injury in multiple autoimmune responses. The detection of autoantibodies has been widely used, which promoted the clinical practice of MN toward personalized precision medicine. However, given the potential risks of immunosuppressive therapy, more autoantibodies and biomarkers need to be identified to predict the prognosis and therapeutic response of MN more accurately. In this review, we attempted to summarize the autoantigens/autoantibodies and autoimmune mechanisms that can predict disease states based on the current understanding of MN pathogenesis, especially the podocyte injury manifestations. In conclusion, both the autoimmune response and podocyte injury require multidimensional attention in the disease course of MN.

Immunological Pathogenesis of Membranous Nephropathy: Focus on PLA2R1 and Its Role

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.

Immunology of membranous nephropathy

Accounting for about 20 to 50% of cases of primary nephrotic syndrome, membranous nephropathy (MN) is the leading cause of nephrotic syndrome in adults. A rat model created nearly 60 years ago to research the primary MN disorder, Heymann nephritis, has provided us with a plethora of important information. Recently, our knowledge about MN has dramatically progressed. Heymann nephritis and human MN are now known to share a high degree of similarity in pathogenesis. This review summarizes our current understanding of MN pathogenesis while focusing particularly on the immunological aspects.

Treatment of primary membranous nephropathy: where are we now?

In the last 10 years, basic science and clinical research have made important contributions to the understanding and management of primary membranous nephropathy (MN). The identification of antibodies directed against the M-type phospholipase A₂ receptor (PLA2R) and thrombospondin type-1 domain-containing 7A protein have added a new perspective on diagnosis, monitoring the immunological activity, predicting prognosis and guiding therapy in patients with primary MN. Mounting evidence suggests that quantification and follow-up of antiPLA2R Abs levels can help in assessing prognosis and evaluate the response to treatment. The kidney disease improving global outcomes guidelines published in 2012 have not been updated. New data on the use of rituximab suggest it should be considered as a potential initial therapy in the treatment of patients with primary MN.

Anti-neutral endopeptidase, natriuretic peptides disarrangement, and proteinuria onset in membranous nephropathy

Neutral endopeptidase (NEP) is the first podocytic antigen responsible for human membranous nephropathy (MN). Besides the prevailing pathogenetic mechanism of immune complex, NEP is also involved in the metabolism of natriuretic peptides (NP). The identification of anti-NEP antibodies in human MN suggests that the decreased circulating NEP may down-regulate the NP catabolism. In this context, we hypothesize that NP disarrangement secondary to anti-NEP antibodies may account, in part, for the onset of proteinuria in MN. Whereas the pathways for the onset of proteinuria caused by elevated NP level are still obscure. The data presented in this review focus on those which support this hypothesis with regards to evidence from the glomerular haemodynamic changes, endothelial permeability, glomerular basement membrane disruption, and podocyte detachment.

Identification and characterization of a new autoimmune protein in membranous nephropathy by immunoscreening of a renal cDNA library

Membranous Nephropathy (MN) represents a large amount of Nephrotic Syndromes in the adult population and its definitive diagnosis is currently carried out through biopsy. An autoimmune condition has been demonstrated in idiopathic MN (iMN) in which some kidney structures are targeted by patient autoantibodies. Some candidate antigens have been described and other likely involved target proteins responsible for the disease are not known yet. In this work our aim is to identify these proteins by screening a lambda-phage library with patients' sera. We enrolled four groups of patients: two MN groups of 12 full iMN patients; one control group of 15 patients suffering from other renal diseases; one control group of 15 healthy individuals. A commercial cDNA phagemide library was screened using the above described sera, in order to detect positive signals due to antigen-antibody bond. We detected one phagemide clone expressing a protein which was shown to be targeted by the antibodies of the iMN sera only. Control sera were negative. The sequence analysis of cDNA matched the Synaptonemal Complex protein 65 (SC65) coding sequence. Further proteomic analyses were carried out to validate our results. We provide evidence of an involvement of SC65 protein as an autoimmune target in iMN. Considering the invasiveness and the resulting risk coming from renal biopsy, our ongoing aim is to set a procedure able to diagnose affected patients through a little- or non-invasive method such as blood sampling rather than biopsy.

Fetomaternal alloimmunity as a cause of liver disease

Fetomaternal alloimmune disease has traditionally been associated with haematological disease such as fetomaternal alloimmune thrombocytopaenia and Rh haemolytic anaemia, but is now known to also be organ specific. Alloimmune membranous glomerulonephritis (AMG) is one of the most well understood organ-specific alloimmune diseases. Neonatal haemochromatosis (NH) is a rare condition characterised by early liver failure in infants, with evidence suggesting that it is also alloimmune. Both AMG and NH appear to involve the passive transfer of alloantibodies to the fetus, which bind a specific alloantigen, fix complement and activate the terminal complement cascade. Although differences between AMG and NH are known, and evidence of the presence of antigen-specific alloantibodies in NH is still missing, we will use AMG as an example of fetomaternal organ specific alloimmune disease, and critically compare this to other emerging evidence that indicates that NH is also alloimmune.

The anti-fibrotic effect of mycophenolic acid-induced neutral endopeptidase

Mycophenolic acid (MPA) appears to have anti-fibrotic effects, but the molecular mechanisms underlying this are unknown. We prospectively studied 35 stable kidney transplant recipients maintained on cyclosporine and azathioprine. We converted 20 patients from azathioprine to enteric-coated mycophenolate sodium (EC-MPS) and continued the remaining 15 patients on azathioprine. Exploratory mRNA expression profiling, performed on five randomly selected EC-MPS patients, revealed significant upregulation of neutral endopeptidase (NEP), which is an enzyme that degrades angiotensin II. We confirmed these microarray data by measuring levels of NEP expression in all subjects; in addition, we found that NEP gene expression correlated inversely with proteinuria. In an additional 33 patients, glomerular and tubular NEP protein levels from renal graft biopsies were significantly higher among the 13 patients receiving cyclosporine + EC-MPS than among the 12 patients receiving cyclosporine + azathioprine or 8 patients receiving cyclosporine alone. Glomerular NEP expression inversely correlated with glomerulosclerosis and proteinuria, and tubular NEP expression inversely correlated with interstitial fibrosis. Incubation of human proximal tubular cells with MPA increased NEP gene expression in a dose- and time-dependent manner. Moreover, MPA reduced angiotensin II-induced expression of the profibrotic factor plasminogen activator inhibitor-1, and a specific NEP inhibitor completely reversed this effect. Taken together, our data suggest that MPA directly induces expression of neutral endopeptidase, which may reduce proteinuria and slow the progression of renal damage in kidney transplant recipients.

Anti-erythropoietin antibodies followed by endogenous erythropoietin production in a dialysis patient

We present a case of a young girl with end-stage renal disease secondary to anti-glomerular basement membrane disease who was receiving maintenance peritoneal dialysis and developed pure red cell aplasia secondary to anti-erythropoietin (EPO) antibodies. This occurred 13 months after the initiation of EPO alfa therapy for anemia. Initially, the patient required intermittent red blood cell transfusions. After immunosuppressive therapy had been initiated with corticosteroids and cyclosporine, the EPO antibody levels decreased precipitously, associated with an increased level of endogenous EPO production. For the following 6 months, the patient maintained adequate (>10 g/dL) hemoglobin levels and did not require red cell transfusions.

Familial membranous nephropathy: an X-linked genetic susceptibility?

BACKGROUND

Membranous nephropathy (MN) is the most common histological diagnosis in adults with nephrotic syndrome and a leading cause of end-stage kidney failure from glomerulonephritis. Little is known about the underlying aetiology, although anti-glomerular antibodies have been implicated. No specific underlying genetic defect has yet been identified.

METHODS

In a family with four members in three generations affected by primary MN, the serum of affected members and their mothers were assessed for anti-glomerular antibodies.

RESULTS

All four affected are male and connected through the maternal line, indicative of X-linked inheritance. Age of onset of nephrotic syndrome varied between 1 and 67 years of age, suggesting that a potential underlying gene may confer a genetic predisposition to MN, but other factors, genetic or environmental, are necessary to trigger the disease. Serologic studies revealed antibodies against glomerular and peritubular endothelial cells in the mother of the youngest patient.

CONCLUSIONS

We have identified the largest reported family with a potential X-linked susceptibility to MN. Foeto-maternal alloimmunization may have triggered the disease in the youngest individual. Considering that the majority of patients with sporadic MN are male, identification of an X-linked predisposing factor may have implications well beyond this family and could provide a target for treatment.

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.

Pure red-cell aplasia caused by the antibody to recombinant erythropoietin, epoetin-beta, in a Japanese patient with chronic renal failure

A 68-year-old male with chronic renal failure and anemia received recombinant human erythropoietin (rHuEPO), epoetin beta, for approximately 1 year. Although the agent was initially effective for improving anemia, anemia refractory to EPO administration appeared and then worsened later, and pure red-cell aplasia (PRCA) was diagnosed. Anti-EPO antibody was detected by radioimmunoprecipitation (RIP) assay in the patient's serum. The antibody inhibited the proliferation of EPO-dependent cell line in a dose-dependent manner neutralizing EPO activity. The antibody also reacted with the other epoetin alfa products. The antibody did not recognize the carbohydrate moieties or denatured epoetin beta. The result suggested that the antibody recognized the conformational epitope of epoetin beta peptide molecule. Withdrawal of EPO and administration of cyclosporine decreased the titers of antibody; however, erythroid progenitor has not yet regenerated although the requirement for red blood cell transfusion is decreasing.