Blocking of antibody complement-dependent effector functions by streptococcal IgG Fc-receptor and staphylococcal protein A.

Using haemolysis in gel, two bacterial IgG-binding substances, an Fc-receptor isolated from group A streptococci type M15, and protein A from Staphylococcus aureus, were shown to inhibit complement-mediated lysis of sheep erythrocytes sensitized with rabbit IgG. When the crude alkaline extracts of ten types of group A streptococci were tested to see whether streptococcal components other than Fc-binding material might affect lysis, the degree of inhibition was found to be correlated with Fc-binding activity. In no case was the lysis of IgM-coated cells inhibited. Opsonophagocytosis experiments showed that both purified streptococcal Fc-receptor and protein A impaired antibody complement-dependent killing by human polymorphonuclear leukocytes of each of two strains of group B streptococci (lacking IgG Fc-receptors). Furthermore, the impairment was ascribable to interference with the fixation of complement to the antibodies, as demonstrated in pre-opsonization experiments with one of the strains. Our results suggest that blocking of the binding of complement to IgG is an important virulence mechanism in Fc-receptor-bearing streptococci and staphylococci.