Characterisation and specificity of glomerular basement membrane antigens identified by sera of patients with anti-GBM nephritis.

The sera of 21 patients positive for antibodies against GBM in indirect immunofluorescence tests were examined by immunoblotting. We demonstrated antibodies against 50, 48, 43 and 29 kD molecular weight peptides in 20 of 21 sera using collagenase-digested GBM, in 19 of 21 using trypsin-digested GBM, and in 10 of 21 using elastase-digested GBM. Although the spectrum of molecular weights of the antigenic proteins was similar in all three digests, they differed with respect to preservation of antigenicity upon reduction with mercaptoethanol. Many of the sera of patients and controls reacted with proteins unrelated to GBM, e.g. albumin and prealbumin. Furthermore, some control sera reacted with one single peptide of the above-mentioned specific GBM peptides. Our results suggest that the highly purified 29 kD peptide of the collagenase digest or the 50 kD peptide of the trypsin digest provide the best antigens to develop a screening test for antibodies against GBM. However, serum antibodies against these antigens will not be absolutely specific for anti-GBM antibody-mediated nephritis, as shown by the immunoblot experiments.