Isolation and partial characterization of antigens from basement membranes and streptococcal cell membrane (SCM) employing anti-SCM monoclonal antibody

Monoclonal antibodies (mAb) against streptococcal cell membrane (SCM) antigen were used to identify specific cross-reactive peptides prepared by trypsin digestion of purified glomerular basement membrane (GBM) and lung basement membrane (LBM). Anti-SCM mAb-coupled HPLC columns were used to affinity isolate soluble LBM, GBM, and SCM antigens which then were sized by HPLC. Alternatively, SCM, GBM, and LBM digests were subjected to an initial separation by HPLC into component polypeptides, followed by affinity purification and ELISA of these fractions using anti-SCM mAb. Comparison of the antigenic reactivities by ELISA of the sized polypeptides on a nanomolar basis permitted the estimation of their individual relative epitope densities. The results for SCM antigens showed increasing epitope density with increasing molecular size, which suggests that intact SCM consists of repeating epitopes. Low mol. wt GBM polypeptides in nanogram amounts inhibited mAb binding to SCM, indicating that these small GBM polypeptides may similarly contain more than a single cross-reactive epitope. The identification of these cross-reactive epitopes in LBM and GBM has important implications for the etiology of post-streptococcal sequelae.