The role of C3 in mediating binding and ingestion of group B streptococcus serotype III by murine macrophages

To understand how complement effects phagocytosis of type III group B streptococcus, we assessed the specific role of C3 in mediating binding and ingestion of these bacteria by macrophages. Phagocytosis of bacteria by resident mouse peritoneal macrophages was measured under conditions in which C3 deposition on bacteria was inhibited or after blockade of C3-ligands or of complement receptor type three (CR3) with specific antibodies. C3 depletion, incubation with F(ab')2 fragments of antibody to C3, or blockade of CR3 completely inhibited the binding of bacteria that was seen in the presence of nonimmune serum. Immune serum increased the number of associated organisms 6-fold compared to that seen with nonimmune serum. With this serum, 82% of organisms were ingested. C3 depletion or CR3 blockage had a modest effect, but this interaction could be ablated completely only after Fc receptors were blocked. Using varied concentrations of an IgG2a MAb against type III capsular antigen, it was possible to show that small amounts of antibody incapable of mediating bacterial binding by itself directed an interaction that also depended upon C3. Phagocytosis of group B streptococci by macrophages in the presence of little or no antibody requires complement and C3 opsonization specifically. C3-dependent binding may be important in determining mononuclear phagocyte-dependent clearance of these pathogens from blood, particularly in patients with little or no type-specific serum antibody.