Tissue Damage Caused by Impaired Phagocytosis of Dead Cells: A Previously Unrecognized Adverse Effect Contributing to the Pathogenesis of γδ T Cells in Legionella Pneumonia.

IL-17 plays a critical role in the immunological control of various infectious diseases; its function has been investigated in the removal of both extracellular and intracellular bacteria. Our group previously revealed the importance of IL-17 in neutrophil migration following Legionella infection by using IL-17AF knockout mice; however, aside from neutrophil infiltration, alternative causes for the reduced survival of these mice have not been characterized. In this study, we found that γδ T cells in IL-17AF knockout mice were markedly increased and produced the cytotoxic substances granzyme B and perforin. Moreover, the elimination of γδ T cells from these mice, via an anti-TCRδ Ab, caused a substantial reduction in the level of lactate dehydrogenase in bronchoalveolar lavage fluid, indicating that γδ T cells contribute to lung tissue damage. Moreover, although cells lysed by cytotoxic substances are typically eliminated by phagocytic cells, in IL-17AF knockout mice, lung homeostasis was not maintained because of a decrease in phagocytic cells that impaired the clearance of dead cells. Our results indicate that increased γδ T cells in IL-17AF knockout mice help eliminate Legionella by releasing cytotoxic substances and lysing infected cells; however, this results in tissue damage due to insufficient removal of dead cells by phagocytic cells. This study enhances our understanding of the protective response against Legionella and provides insights into γδ T cell-mediated protective immunity against various infectious diseases.