Listericidal and nonlistericidal mouse macrophages differ in complement receptor type 3-mediated phagocytosis of L. monocytogenes and in preventing escape of the bacteria into the cytoplasm.
Listeria monocytogenes is a facultative intracellular bacterium that escapes phagocytic vesicles and replicates in the cytoplasm, where it becomes coated with F-actin. Macrophages, important anti-Listeria effector cells, are heterogeneous in their ability to kill Listeria. Complement receptor type 3 (CR3) mediates most phagocytosis of Listeria by listericidal macrophages. Experiments described here tested whether nonlistericidal macrophages also phagocytosed Listeria through CR3 and whether the ability of Listeria to escape into the cytoplasm correlated with lack of listericidal activity. We show here that CR3 mediated an average of 66% of the phagocytosis of serum-opsonized Listeria by listericidal peptone-elicited macrophages but only 35% by nonlistericidal thioglycolate-elicited macrophages. In thioglycolate-elicited macrophages, most Listeria were cytoplasmic and actin coated, whereas in peptone-elicited macrophages most were retained in the phagosome. These results indicate that listericidal and nonlistericidal macrophages phagocytose Listeria through different receptors and that nonlistericidal macrophages allow Listeria to escape into the cytoplasm.