Neutrophil Fc receptor participation in phagocytosis of type III group B streptococci.

Human peripheral blood neutrophils bear receptors for immunoglobulin G, FcRII, and FcRIII that differ structurally and functionally. We investigated the role of FcRII and FcRIII in the phagocytosis of group B streptococci (GBS) by measuring neutrophil uptake of radiolabeled type III GBS. The mean uptake of GBS opsonized with human serum containing complement and specific antibody was 76%, but when this serum was heated, the mean uptake was only 22%. A monoclonal antibody to FcRIII, Leu-11b, inhibited in a dose-dependent manner uptake of GBS opsonized with heated or intact serum to maxima of 40 and 30%, respectively. Conversely, a monoclonal antibody to FcRII, IV.3, inhibited by 77% the uptake of GBS opsonized with heated serum but had no effect when GBS was opsonized with intact serum. Leu-11b and IV.3 had an additive inhibitory effect with heated but not with intact serum. Neither monoclonal antibody inhibited the uptake of GBS opsonized with hypogammaglobulinemic serum. Therefore, FcRII is the primary mediator of the phagocytosis of GBS opsonized by antibody alone, whereas FcRIII plays a lesser role. Surprisingly, FcRII is not necessary for phagocytosis when complement is also present. FcRIII participates, to a limited extent, in phagocytosis of GBS opsonized with antibody whether or not complement is present. These findings suggest that the function of FcRII in triggering phagocytosis may be particularly important in host defense against type III GBS in the setting of complement deficiency of young infants.