Circulating antipodocyte antibodies in membranous nephropathy: pathophysiologic and clinical relevance

Thrombospondin Type 1 Domain-Containing 7A Localizes to the Slit Diaphragm and Stabilizes Membrane Dynamics of Fully Differentiated Podocytes

BACKGROUND

About 3%-5% of adults with membranous nephropathy have autoantibodies directed against thrombospondin type 1 domain-containing 7A (THSD7A), a podocyte-expressed transmembrane protein. However, the temporal and spatial expression of THSD7A and its biologic function for podocytes are unknown, information that is needed to understand the effects of THSD7A autoantibodies in this disease.

METHODS

Using a variety of microscopic techniques, we analyzed THSD7A localization in postnatal, adult, and autoantibody-injected mice as well as in human podocytes. We also analyzed THSD7A function in human podocytes using confocal microscopy; Western blotting; and adhesion and migration assays.

RESULTS

We found that THSD7A expression begins on glomerular vascularization with slit diaphragm formation in development. THSD7A localizes to the basal aspect of foot processes, closely following the meanders of the slit diaphragm in human and mice. Autoantibodies binding to THSD7A localize to the slit diaphragm. In human podocytes, THSD7A expression is accentuated at filopodia and thin arborized protrusions, an expression pattern associated with decreased membrane activity of cytoskeletal regulators. We also found that, phenotypically, THSD7A expression in human podocytes is associated not only with increases in cell size, enhanced adhesion, and reduced detachment from collagen type IV-coated plates but also, with decreased ability to migrate.

CONCLUSIONS

Our findings suggest that THSD7A functions as a foot process protein involved in the stabilization of the slit diaphragm of mature podocytes and that autoantibodies to THSD7A, on the basis of their localization, might structurally and functionally alter the slit diaphragm's permeability to protein.

Anti-PLA2R Antibodies in Chinese Patients with Membranous Nephropathy

BACKROUND ~This study used two standardized methods to evaluate anti-PLA2R antibody in serum of primary membranous nephropathy (PMN) among Chinese patients to determine  Anti-PLA2R antibody distribution and whether immunological reactivity reflected by antibody titer correlates with kidney function parameters. MATERIAL AND METHOD ~Overall, 82 subjects with biopsy-proven primary membranous nephropathy (PMN) , 22 cases with secondary membranous nephropathy (SMN), 40 non-MN patients with established glomerulonephritis, 20 healthy volunteers were recruited from the Division of Nephrology, Nanfang Hospital, China. Anti-PLA2R antibody in the serum of each patient was evaluated by both recombinant cell-based indirect immunofluorescence assay (RC-IFA) and enzyme linked immunosorbent assay (ELISA). Kidney function was assessed by proteinuria for 24 hours, serum albumin, blood urea nitrogen (BUN), serum creatine, serum cystatin C. We assessed the correlation between anti-PLA2R antibody levels and clinical parameter in the PMN patients. RESULTS ~ Fifty-three patients with PMN (64.6%) were positive for anti-PLA2R antibody. The level of antibody determined by RC-IFA ranged from 1:10 to 1:1000 and 0 to 1423 RU/ml by ELISA. The two anti-PLA2R test systems correlated very well with each other and reached an agreement of 95.7% for PMN patients. The level of antibody detected by ELISA in patients with PMN also significantly correlated with proteinuria and nephritic-range proteinuria (> 3.5g/day) . CONCLUSIONS ~Anti-PLA2R antibody is sensitive and extremely specific for diagnosis of Chinese patients with primary membranous nephropathy. Concentration of autoantibody against PLA2R is an ideal marker for monitoring the activity of immunological disease.

The accuracy of the anti-phospholipase A2 receptor antibody in the diagnosis of idiopathic membranous nephropathy: a comparison of different cutoff values as measured by the ELISA method

AIM

Recent studies have revealed that anti-phospholipase A2 receptor antibodies (aPLA2R-ABs) may play a role in the diagnosis of idiopathic membranous nephropathy (IMN). We will investigate the application of an aPLA2R-AB with different cutoff values for diagnosing IMN.

METHODS

From August 2014 to December 2014, patients with proteinuria greater than 1 g/day were screened at the First Affiliated Hospital of Zhengzhou University. Patients were divided into the IMN and non-IMN groups based on the results of renal biopsy. The sensitivity, specificity, and receiver operating characteristic (ROC) curve of aPLA2R-AB for diagnosing IMN were analyzed.

RESULTS

A total of 229 patients (113 males, average age of 45.3 ± 15.8 years) were enrolled in this study, and 118 patients were diagnosed with IMN. The sensitivity/specificity of aPLA2R-AB in the diagnosis of IMN was 65.3/97.3 %, 60.2/97.3 %, and 45.8/97.3 % for the different cutoff values (14, 20, and 40 RU/ml). The area of the ROC curve was 0.87.

CONCLUSION

Anti-PLA2R-AB with a cutoff value of 14 RU/ml based on the ELISA method plays an important role in the diagnosis of IMN.

Immunology of membranous nephropathy: from animal models to humans

Membranous nephropathy (MN), the leading cause of nephrotic syndrome in adults, is characterized by the deposition of subepithelial immune deposits that consist mainly of immunoglobulin (Ig)G and complement. Most of the cases are primary or idiopathic (iMN), while only approximately 25% of the cases are secondary to some known disease such as systemic lupus erythematosus, hepatitis B, drugs and malignancies. Most of our knowledge on the pathogenesis of iMN has relied upon old experimental models (i.e. Heymann nephritis) that have shown that immune deposits are formed in situ by the reaction of autoantibodies against the respective podocyte antigen. Recent findings indicate that podocyte proteins also act as an autoantigen in human iMN. The M-type phospholipase A2 receptor (PLA2R) has been identified as the main target antigen, as it can be found in approximately 70% of iMN patients but only rarely in other glomerulonephritides. Podocytes damage in the experimental model of Heymann nephritis is complement-mediated. In humans, the presence of complement within the subepithelial deposits is well established, but IgG4, which does not activate complement by classical or alternative pathways, represents the predominant subclass of IgG anti-PLA2R. Some evidence suggests that IgG4 anti-PLA2R autoantibodies can bind mannan-binding lectin (MBL) and activate the lectin complement pathway. A genetic background for iMN has been demonstrated by genome-wide association studies that have shown highly significant associations of the PLA2R1 and the human leucocyte antigen (HLA)-DQA1 loci with iMN. In addition to their diagnostic value, anti-PLA2R antibodies may be useful to monitor disease activity and predict response to treatment.

Immunopathogenesis of membranous nephropathy: an update

Membranous nephropathy (MN) is a non-inflammatory organ-specific autoimmune disease which affects the kidney glomerulus, resulting in the formation of immune deposits on the outer aspect of the glomerular basement membrane, complement-mediated proteinuria, and severe renal failure in 30% of patients. In the last 10 years, substantial advances have been made in the understanding of the molecular bases of MN, with the identification of several antigens and predisposing genes in children and adults. These ground-breaking findings already have a major impact on diagnosis and monitoring and to some extent on therapies. However, there is evidence that the disease is more complex and involves a variety of antigen-antibody systems and genes involved in immune response, progression, recovery, and protective mechanisms. We herein review these recent findings which open new perspectives of research. Understanding the complex pathogenesis of MN will offer many opportunities for future therapeutic interventions and will hopefully have a major impact on patient care. New insights into the molecular mechanisms of MN may also enlighten the pathogenesis of organ-specific autoimmune diseases.

[Pathophysiology of extramembranous glomerulopathies. Fifty years of progress, from laboratory to patient]

Membranous nephropathy (MN) is a kidney disease characterized by deposition of immune complexes and complement on the outer aspect of the glomerular capillary wall. It is responsible for a loss of serum proteins in the urine and kidney failure. During the last ten years, considerable progress has occurred in the understanding of the molecular bases of the disease with the description of three distinct mechanisms in humans. In the neonatal allo-immune form, antibodies are directed against neutral endopeptidase (NEP), a podocyte antigen absent in the mothers who become immunized against this antigen expressed by placenta cells during pregnancy. NEP was the first podocyte antigen to be identified in MN. Most adult forms of MN are autoimmune diseases without identified etiology (primary MN), linked to the production of antibodies raised against another podocyte antigen, the type-M phospholipase A2 receptor (PLA2R1). Anti-PLA2R1 antibodies are detected in 70 to 80% of patients before any immunosuppressive treatment, and only occasionally in secondary forms of MN, variants of PLAR1 and HLA-DQA1 genes are very significantly associated with occurrence of primary MN in Caucasians. The third mechanism is characterized by immunization against a foreign protein, cationic bovine serum albumin (BSA), which is involved in rare forms of MN during early childhood. This finding points to a possible role of food and environmental antigens in membranous nephropathy.