Nephrotic Syndrome
A New Specific Test for Idiopathic Membranous Nephropathy

By: Hanna Debiec and Pierre Ronco
INSERM Unit 702 and
Division of Nephrology, Tenon Hospital, Paris, France

August 9, 2011 -- Circulating anti-phospholipase A2 receptor (PLA2R) antibodies have been identified as an immunological marker of disease activity in patients with idiopathic membranous nephropathy. A simple assay has been developed to detect anti-PLA2R antibodies in serum. Now, we can monitor the response to drugs and adapt therapy in daily clinical practice.

However, the antibodies have been found in some patients with hepatitis B, in patients with cancer-associated membranous nephropathy and in one patient with sarcoidosis-associated membranous nephropathy, although coincidental occurrence of the two disease processes cannot be excluded.

Cumulative data from serum samples obtained from patients in the USA, Europe and China have shown that 70–80% of patients with idiopathic membranous nephropathy have anti-PLA2R antibodies that are reactive with native glomerular extract using a western blot immunoassay. This finding is 20–30% more than in the cohort of patients described by Hoxha and co-workers.4 This discrepancy does not seem to be associated with the differ- ent sensitivities of the methods,8 but rather with the immunosuppressive treatment that some patients in Hoxha et al.’s study were receiving at the time of serum collection.

Why were anti-PLA2R antibodies not detected in all patients with idiopathic membranous nephropathy irrespective of the test used? Three possible explanations exist for this discrepancy. Firstly, idiopathic membranous nephropathy is not a uniform disease.

Membranous nephropathy, an antibody-mediated glomerular disease, is the major cause of nephrotic syndrome in adults.1 The formation of subepithelial immune deposits and complement activation are responsible for functional impairment of the glomeru- lar capillary wall, which causes proteinuria. The heterogeneity of membranous nephro- pathy and a lack of reliable biomarkers mean that treatment of patients with this disease is controversial and challenging. In 2009, M-type phospholipase A2 receptor (PLA2R), a transmembrane protein located on podocytes, was identified as a major target antigen involved in the development of idiopathic membranous nephropathy (also known as primary membranous glomerulonephritis) in adults. This dis- covery has opened a new era for the diag- nosis of membranous nephropathy and for monitoring patients’ responses to treatment. A widely available and simple assay for sero- logical monitoring of anti-PLA2R antibodies in patients was urgently needed.

In Nephrology Dialysis Transplantation, Hoxha and co-workers4 present a new indirect immunofluorescence test (IIFT) that enables the easy and specific detection of anti-PLA2R antibodies in serum. This method is suitable for qualitative and quantitative determination of the presence of human IgG antibodies against PLA2R. The detection of circulating anti-PLA2R anti- bodies was performed on a slide containing a mosaic of two different biochips in one incubation field. The first biochip is coated with human embryonic kidney (HEK)293 cells expressing the PLA2R protein, the second biochip contains nontransfected HEK293 cells as the substrate (Figure 1). Using IIFT, anti-PLA2R antibodies were found in 52% of patients with biopsy-proven idiopathic membranous nephropathy (n = 100), but in none of those with secondary membranous nephropathy (n = 17). The antibodies were also not detected in any of the healthy control individuals (n = 153) or in the patients with glomerular injuries not attributable to membranous nephropathy (n = 90). The presence of anti-PLA2R anti- bodies in patients with secondary membranous nephropathy remains controversial, and might have different target antigens that have not been identified yet. The genetic susceptibility to developing membranous nephropathy was investigated in a genome- wide association study published in 2011. Only two alleles were found to account for the genetic risk, HLA-DQA1 and PLA2R1. Interestingly, the association with HLA- DQA1 was stronger than with PLA2R1, which suggests that the HLA-DQA1 allele might facilitate autoantibody development targeting not only PLA2R but also other antigens. Secondly, the antibody levels fluctuate with disease activity. Thirdly, some patients who were negative for PLA2R might have been misclassified as idiopathic when they actually had a secondary form of membranous nephropathy.

A simple serological diagnosis of idiopathic membranous nephropathy is now possible. Although the exact mechanisms and role of PLA2R in the pathogenesis of idiopathic membranous nephropathy are currently unknown, the presence of anti- PLA2R antibodies is highly specific for idiopathic membranous nephropathy, which should no longer be considered an idiopathic disease. In this regard, the IIFT developed by Hoxha and co-workers is of major clinical interest. This simple test, which is now commercially available (Euroimmun AG, Lübeck, Germany), makes the dosage of anti-PLA R antibodies immediately to the drug and limit the incidence of adverse events, including the production of antichimeric antibodies or the develop- ment of infectious complications and malignancies. However, it is too early to measure the impact, if any, of the IIFT on the indications for a kidney biopsy. But in a time of worldwide financial constraint, one certainly should consider that a positive test in a patient argues against a secondary form of the disease, thus limiting investigations that are both costly and tiring for the patients.

Acknowledgments: The authors would like to acknowledge the support of grant ANR-07-Physio-016-01.

Also see: BUSM Researchers Develop Blood Test to Detect Membraneous Nephropathy