Specific binding of bovine, ovine, caprine and equine IgG subclasses to defined types of immunoglobulin receptors in gram-positive cocci

One hundred and twenty bacterial strains were tested for non-immune binding of radiolabelled bovine, ovine, caprine and equine immunoglobulins. Bacteria possessing previously defined IgG receptors interacted in a well defined manner with purified IgG subclass immunoglobulins. Human group C and G streptococci carrying IgG receptors type III were capable of binding all IgG subclasses in the four mammalian species studied. Protein A-containing staphylococci demonstrated a restricted specificity with binding of bovine IgG1, ovine IgG1, caprine IgG1 and IgG2 as well as equine IgG(ab). Group A streptococci which can bind human IgG did not show specific reactivity. A new type of binding unrelated to the regular Fc-mediated binding was observed with equine IgG(T).The differences in specificity for IgG subclasses suggest that structures with binding capacity to streptococcal type III Fc receptors are different from staphylococcal protein A reactive sites. Inhibition experiments performed with purified immunoglobulins showed that individual IgG subclasses differed greatly in their inhibiting capacity reflecting differences in avidity.The high avidity and the broad, unrestricted immunoglobulin G reactivity of streptococcal IgG receptor type III indicate that human group C and G streptococci may provide a valuable tool for solid phase absorption of immunoglobulins from several mammalian species.