Muramyl dipeptide-induced enhancement of phagocytosis of antibiotic pretreated Escherichia coli by macrophages.

Treatment of mice with muramyl dipeptide, a known immunoadjuvant, resulted in marked augmentation of the phagocytic activity of peritoneal macrophages incubated in vitro with Escherichia coli. Even greater phagocytosis occurred when the E. coli were pretreated for 2 hr with subinhibitory concentrations of the semisynthetic penicillins cyclacillin or ampicillin, but not penicillin G to which they were resistant. The antibiotic-pretreated E. coli were more rapidly ingested by the macrophages derived from MDP-treated mice as compared to similar cells from normal mice. Optimum augmentation of phagocytosis of untreated or antibiotic-pretreated E. coli occurred 2 to 3 days after administration of MDP to the mice. Similar augmentation of phagocytosis occurred by treating cultures of peritoneal macrophages from normal mice in vitro with MDP prior to incubation with the antibiotic-pretreated bacteria. These results indicate that macrophages from MDP stimulated mice interact with antibiotic-pretreated bacteria to a greater extent than with untreated E. coli, resulting in increased phagocytosis and killing of the bacteria.