TNF-α and IFN-γ stimulate a macrophage precursor cell line to kill Listeria monocytogenes in a nitric oxide-independent manner

Macrophages are important effector cells for resolving infection with the facultative intracellular bacterium Listeria monocytogenes. However, not all macrophages have the ability to kill this organism. Certain factors, such as cytokines, are apparently required for induction of macrophage bactericidal activity. In vivo studies have shown that both TNF-α and IFN-γ play important roles in resistance against Listeria. Yet whether they act directly on macrophages has been difficult to determine, because homogeneous populations of cells that can be induced to express microbicidal activity have not been available. Instead, bactericidal macrophages are typically found in heterogeneous exudates, such as those elicited by inflammatory agents. In this study we show that sequential stimulation with TNF-α and IFN-γ induces the nonphagocytic, nonbactericidal mouse macrophage precursor hybrid cell line W1C3 to phagocytose and kill Listeria efficiently. This provides the first direct evidence that TNF-α and IFN-γ are both necessary and sufficient to induce macrophages to kill Listeria, and that they act directly on macrophages. Data presented here also show that TNF-α and IFN- γ induced the macrophages to produce large amounts of reactive nitrogen intermediates (RNI), but complete inhibition of RNI generation did not decrease bactericidal activity. This indicates that induction of listericidal activity in these cells does not require generation of RNI. Taken together, these findings suggest that TNF-α and IFN-γ act in synergy directly on at least some macrophages to induce them to express listericidal activity in a RNI-independent manner.