Immunoreactivity and cross-reactivity of human and bovine retinal S-antigen

The reactions of human and bovine retinal S-antigen (S-ag) with polyclonal rabbit antibodies were compared in ELISA, immunoblotting and inhibition ELISA. Titrations in ELISA with plastic-adsorbed S-ags revealed that the great majority (70-100%) of human S-ag epitopes and anti-human S-ag antibodies were cross-reactive. In contrast, the cross-reactivity of bovine S-ag (25%) and anti-bovine S-ag antibodies (15-20%) indicated an important contribution of species-specific epitopes to the antigenicity of bovine S-ag. Immunoblotting of S-ag fragments after treatment with chymotrypsin confirmed these differences and also demonstrated different chymotrypsin-induced cleavage patterns of human and bovine S-ag. Thus, in assays involving partial denaturation and/or degradation of the antigens, human S-ag showed little, and bovine S-ag a marked species-specific immunoreactivity. In inhibition ELISA however, in which S-ags in solution could be studied, species-specific reactions strongly predominated for both S-ags. Anti-human S-ag and anti-bovine S-ag antibodies could be absorbed with nM concentrations of fluid phase human and bovine S-ag, respectively, whereas in both cases the cross-reacting antigen had no detectable inhibitory potential. The epitopic structures of human and bovine S-ag in solution may thus be largely different.