Autophagy activation by interferon-γ via the p38 mitogen-activated protein kinase signalling pathway is involved in macrophage bactericidal activity.

Macrophages are involved in many essential immune functions. Their role in cell-autonomous innate immunity is reinforced by interferon-γ (IFN-γ), which is mainly secreted by proliferating type 1 T helper cells and natural killer cells. Previously, we showed that IFN-γ activates autophagy via p38 mitogen-activated protein kinase (p38 MAPK), but the biological importance of this signalling pathway has not been clear. Here, we found that macrophage bactericidal activity increased by 4 hr after IFN-γ stimulation. Inducible nitric oxide synthase (NOS2) is a major downstream effector of the Janus kinase-signal transducer and activator of transcription 1 signalling pathway that contributes to macrophage bactericidal activity via nitric oxide (NO) generation. However, no NO generation was observed after 4 hr of IFN-γ stimulation, and macrophage bactericidal activity at early stages after IFN-γ stimulation was not affected by the NOS inhibitors, NG-methyl-l-arginine acetate salt and diphenyleneiodonium chloride. These results suggest that an NOS2-independent signalling pathway is involved in IFN-γ-mediated bactericidal activity. We also found that this macrophage activity was attenuated by the addition of the p38 MAPK inhibitors, PD 169316, SB 202190, and SB 203580, or by the expression of short hairpin RNA against p38α or the essential factors for autophagy, Atg5 and Atg7. Collectively, our results suggest that the IFN-γ-mediated autophagy via p38 MAPK, without the involvement of NOS2, also contributes to the ability of macrophages to kill intracellular bacteria. These observations provide direct evidence that p38 MAPK-mediated autophagy can support IFN-γ-mediated cell-autonomous innate immunity.