Antibodies to human glomerular basement membrane: modified methodology for detection in human serum

A new radioimmunoassay for detection of circulating antibodies to human glomerular basement membrane (GBM) is described. GBM was isolated, purified, and an antigenic component extracted by 8 M urea incubation. The 8 M urea extract was radiolabelled, separated on Sephadex G-150, and employed as GBM antigen for detection of binding in a double antibody radioimmunoassay (RIA). Three groups of sera were studied: (a) 40 normal subjects bound a mean of 0.5% +/- 3.4% (s.d.), (b) 12 patients with glomerulonephritis with non-linear immune deposits in direct immunofluorescent staining of kidneys bound a mean of 0.2% +/- 4.4% (s.d.), (c) 14 patients with Goodpasture's syndrome or glomerulonephritis with linear deposits in direct immunofluorescent staining of kidneys bound a mean of 23% +/- 9.1% (s.d.). Serial dilutions of sera from patients with Goodpasture's syndrome showed a progressive reduction in binding while binding of positive sera was inhibited by increasing quantities of unlabelled GBM. The GBM antigen extracted with 8 M urea is convenient to prepare and conditions for performance of the RIA are reproducible. Hence, this may represent a valuable on-site aid for diagnosis and management of patients with anti-GBM mediated glomerulonephritis.