A radioimmunoassay for detection of anti-glomerular basement membrane (GBM) antibody was set up with a 70,000 mol wt GBM antigen, labeled with Iodine-125I and containing both types of oligosaccharidic chains present in the whole membrane.
Separation of free radioactive antigens from antigens bound to immunoglobulins was obtained by precipitation with polyethylene glycol (mol wt, 6,000), at a final concentration of 20%. In the presence of normal human or rabbit sera, less than 20% of labeled antigens were precipitated. In the presence of rabbit anti-human GBM antibodies, up to 82% of GBM antigens were precipitated, while in the presence of sera or of kidney eluate from a patient with Goodpasture's syndrome, the precipitation of GBM antigens reached 43%. The avidity of rabbit anti-GBM antibodies for human GBM antigens is higher than that of human anti-GBM antibodies. In the case of Goodpasture's syndrome, the binding of anti-GBM antibodies to labeled antigens was inhibited more efficiently by the disaccharide-containing glycopeptide than by the heteropolysaccharide-containing glycopeptide purified from whole GBM.
Anti-GBM antibodies were searched for in the serum of 300 normal blood donors, of 120 patients with glomerulonephritis (GN) and granular deposits, and of 14 patients with GN and linear deposits of immunoglobulins. After correction for the “nonspecific” precipitation, the average percentage (±1 SD) of labeled antigens precipitated in the serum of normal blood donors was 0.3±3.2%; 12 patients with GN and linear deposits exhibited high circulating anti-GBM antibody titers, while 8% of the patients with GN and granular deposits presented significant, albeit lower, anti-GBM activity in their sera.