Non-immune F(ab')2- and Fc-mediated interactions of mammalian immunoglobulins with S. aureus and group C and G streptococci.

The distribution among mammalian species of non-immune F(ab')2- and Fc-mediated immunoglobulin interactions with surface proteins of S. aureus (protein A) and of group C and G streptococci was studied. Serum samples from 48 mammalian species representing 15 orders were first tested for their capacity to inhibit streptococcal F(ab')2-mediated binding; 26 of these sera were also tested for streptococcal IgG Fc-mediated binding. Analogous inhibition experiments were then carried out with staphylococci. All mammalian species studied inhibited both types of immunoglobulin binding to streptococci, viz the serum samples contained both F(ab')2- and Fc-reactive immunoglobulins. The reactivity was equal to that of human serum in 26 out of 47 mammalian sera. Seven sera showed a low degree of inhibition compared to human serum. The inhibiting capacities of the two streptococcal non-immune interactions showed a direct correlation (r = 0.91, p less than 0.0001 for the r-value) for individual species. The inhibition patterns observed with S. aureus differed from the profiles recorded with the streptococcal strains, suggesting that these organisms interact with separate sites on the immunoglobulin molecules. Isolated F(ab')2-binding was recorded in 5 out of 24 sera, and Fc-binding alone was noted in 7 sera. Taken together, the present studies demonstrate that mammalian immunoglobulins possess F(ab')2- and Fc-binding sites for protein A and for receptors on group C and G streptococci. The F(ab')2-mediated binding to streptococci is associated with Fc-reactivity, in contrast to protein A which may interact exclusively with a complementary structure in either the F(ab')2- or the Fc-portion of the immunoglobulins.