Heterogeneity of nonimmune immunoglobulin Fc reactivity among gram-positive cocci: description of three major types of receptors for human immunoglobulin G

Two hundred and thirty strains of various gram-positive cocci were tested for quantitative, nonimmune binding of radiolabeled human polyclonal immunoglobulin G (IgG). The majority of coagulase-positive staphylococci and streptococci belonging to serogroups C and G showed a high uptake of IgG. The binding of immunoglobulin to group A streptococci was considerably less, with a number of strains completely negative. None of the pneumococcal or the group B or D streptococcal strains displayed any binding capacity. Heterogeneity of the IgG reactivity of various reactive strains was studied in an inhibition assay using 10 different animal serum pools. Three different inhibition patterns were seen, each of them revealing a striking degree of homogeneity within single bacterial species. Staphylococcus aureus and group A streptococci, respectively, constituted two homogeneous groups which differed markedly from each other and from C and G streptococci. No differences were observed between group C and G streptococci. Based on the profound differences between these homogeneous groups, three major types of Fc receptors could be defined. Type I and II Fc receptors were found on S. aureus and on group A streptococci, respectively. Fc receptor type III represented the immunoglobulin-binding structure of both group C and G streptococci.