Effect of macrophage activation on killing of Listeria monocytogenes. Roles of reactive oxygen or nitrogen intermediates, rate of phagocytosis, and retention of bacteria in endosomes.

The role of macrophage activation in the killing of L. monocytogenes is unclear. Some studies suggest that activation for enhanced production of reactive oxygen and nitrogen intermediates may not be of central importance. Recent data have indicated an important role for interferon-gamma (IFN-gamma) induced retention of L. monocytogenes in endosomes. Data from the present study indicate that proteose peptone-elicited macrophages from DBA2/J, CD-1, and C3H/HeN mice are listericidal. Activation of these cells in vitro for 20 h by IFN-gamma (20 or 500 U/ml) increased H2O2 or nitrite production, but did not increase the number of L. monocytogenes killed during a subsequent 6-h or 7-h culture. Incubation of macrophages with IFN-gamma plus lipopolysaccharide (LPS) caused greater activation and increased the number of Listeria killed during a 6-h or 7-h culture. However, this seems primarily attributable to enhanced phagocytosis. Proteose peptone-elicited macrophages were significantly more effective than resident macrophages in preventing the escape of L. monocytogenes from endosomes into the cytoplasm. This capability was not significantly enhanced by IFN-gamma in vitro, but was enhanced by IFN-gamma plus LPS. This correlates well with the effects of these activation stimuli on killing of L. monocytogenes by proteose peptone-elicited macrophages. These results indicate that enhanced retention of L. monocytogenes in endosomes is induced by proteose peptone elicitation and that further macrophage activation in vitro by IFN-gamma does not improve listericidal activity.