Background

Idiopathic membranous nephropathy, a common form of the nephrotic syndrome, is an antibody-mediated autoimmune glomerular disease. Serologic diagnosis has been elusive because the target antigen is unknown.

Methods

We performed Western blotting of protein extracts from normal human glomeruli with serum samples from patients with idiopathic or secondary membranous nephropathy or other proteinuric or autoimmune diseases and from normal controls.

We used mass spectrometry to analyze the reactive protein bands and confirmed the identity and location of the target antigen with a monospecific antibody.

Results

Serum samples from 26 of 37 patients (70%) with idiopathic but not secondary membranous nephropathy specifically identified a 185-kD glycoprotein in nonreduced glomerular extract.

Mass spectrometry of the reactive protein band detected the M-type phospholipase A₂ receptor (PLA₂R).

Reactive serum specimens recognized recombinant PLA₂R and bound the same 185-kD glomerular protein as did the monospecific anti-PLA₂R antibody.

Anti-PLA₂R autoantibodies in serum samples from patients with membranous nephropathy were mainly IgG4, the predominant immunoglobulin subclass in glomerular deposits.

PLA₂R was expressed in podocytes in normal human glomeruli and colocalized with IgG4 in immune deposits in glomeruli of patients with membranous nephropathy.

IgG eluted from such deposits in patients with idiopathic membranous nephropathy, but not in those with lupus membranous or IgA nephropathy, recognized PLA₂R.

Conclusions

A majority of patients with idiopathic membranous nephropathy have antibodies against a conformation-dependent epitope in PLA₂R. PLA₂R is present in normal podocytes and in immune deposits in patients with idiopathic membranous nephropathy, indicating that PLA₂R is a major antigen in this disease.

Idiopathic membranous nephropathy, a common cause of the nephrotic syndrome in adults, is an organ-specific autoimmune disease. Despite extensive investigation, a target antigen has been elusive.

Studies of membranous nephropathy in a rat model (Heymann's nephritis) established that the subepithelial immune deposits that characterize the disease are formed in situ, as a result of capping and shedding of the target antigen, megalin, from the basal surface of podocytes when it forms a complex with circulating antimegalin antibodies.^{1,2,3,4,5,6,7,8}

Although megalin is not expressed on human podocytes, we hypothesized that a similar process, albeit with an unknown antigen, is operative in human membranous nephropathy.

Additional evidence from the work of Debiec et al.^9,10 supports the in situ formation of glomerular immune deposits in patients with membranous nephropathy.

They described an alloimmune form of membranous nephropathy in neonates born to mothers with a deficiency of neutral endopeptidase, a protein expressed on podocytes, to which the mothers had been sensitized during previous pregnancies.

To identify the target antigen in patients with idiopathic membranous nephropathy, we used circulating antibodies from adults with this disease to detect normal glomerular proteins by using Western blotting.

Subsequent analysis with the use of mass spectrometry and confirmation with the use of protein-specific reagents allowed for the identification and characterization of the predominant protein detected by these circulating antibodies

NEJM Volume 361:11-21July 2, 2009 Number 1

Laurence H. Beck, Jr., M.D., Ph.D., Ramon G.B. Bonegio, M.D., Gérard Lambeau, Ph.D., David M. Beck, B.A., David W. Powell, Ph.D., Timothy D. Cummins, M.S., Jon B. Klein, M.D., Ph.D., and David J. Salant, M.D.

SEE FULLTEXT

http://content.nejm.org/cgi/content/full/361/1/11

http://www.e-medicum.com/noticiasDelDia/verNoticia.php?noticia=82894