Mechanisms of complement-mediated clearance of bacteria from the murine lung.

Complement factors enhance host defense against bacterial challenges by attracting phagocytic cells to the site of the inoculum and by opsonizing bacteria for phagocytic ingestion. The relative contribution of these 2 mechanisms to in vivo clearance of bacteria from the lung has not been described. Hypocomplementemic and normal animals were challenged with various bacteria. Clearance of bacteria was studied by quantitative lung culture. Phagocytic response was determined by bronchoalveolar lavage. Staphylococci were cleared by macrophages without regard to the complement status of the host. Hypocomplementemic animals cleared pneumococci less efficiently than did control animals. This defect correlated with decreased neutrophil recruitment. Pseudomonas was not cleared in hypocomplementemic animals, but there was no difference in the number or type of phagocytes. This implies that an opsonic rather thn a chemotactic defect was responsible. These data suggest that the mechanism of complement-mediated defense against bacterial challenge varies with the type of pathogen present.