Direct characterization of target podocyte antigens and auto-antibodies in human membranous glomerulonephritis: Alfa-enolase and borderline antigens

The identification of glomerular auto-antigens in idiopathic human membranous glomerulonephritis (MGN) is a crucial step towards the definition of the mechanisms of the disease. Recent 'in vivo' studies demonstrated a heterogeneous composition of glomerular immune-deposits in MGN biopsies only a part of which have been characterized. We studied with a proteomical approach IgGs eluted from laser capture microdissected glomeruli of 8 MGN patients and showed the existence of other three immune proteins in MGN glomeruli (alpha-enolase, elongation factor 2 and Glycyl Aminoacyl-tRNA Synthetase). One of these, i.e. alpha-enolase, fulfilled all criteria for being considered an auto-antigen. Specific IgG(1) and IgG(4) reacting with podocyte alpha-enolase were, in fact, eluted from microdissected glomeruli and Confocal- and Immuno Electron-Microscopy showed co-localization of alpha-enolase with IgG(4) and C5b-9 in immune-deposits. Serum levels of anti a-enolase IgG4 were determined in 131 MGN patients and were found elevated in 25% of cases. Overall, our data demonstrate that glomerular alpha-enolase is a target antigen of autoimmunity in human MGN. Circulating anti alpha-enolase auto-antibodies can be detected in sera of a significant quota of MGN patients. Like other auto-antigens, alpha-enolase may be implicated in the pathogenesis of human MGN.