Extracellular stimulation by serum proteins required for maximal intracellular killing of microorganisms by mouse peritoneal macrophages.

Intracellular killing of catalase-positive Staphylococcus aureus by resident mouse peritoneal macrophages was very low in the absence of serum but maximal in the presence of fresh normal serum. A large proportion of catalase-negative Streptococcus pyogenes were killed in the absence of extracellular serum, and maximal killing was reached only when serum was present extracellularly. Further investigations revealed that stimulation of intracellular killing by extracellular serum is dependent on the interaction of immunoglobulin G and Fc receptors and of complement component C3b with C3b receptors in the macrophage membrane.