Involvement of IL‐17A‐producing TCR γδ T cells in late protective immunity against pulmonary Mycobacterium tuberculosis infection

INTRODUCTION Interleukin (IL)-17A is a cytokine originally reported to induce neutrophil-mediated inflammation and anti-microbial activity. The CD4+ T cells, which produce IL-17A, have been well characterized as Th17 cells. On the other hand, IL-17A-producing TCR γδ+ T cells have been reported to participate in the immune response at an early stage of infection with Listeria monocytogenes and Mycobacterium bovis in mice. However, the involvement of IL-17A in protective immunity was not clearly demonstrated in the chronic stage of M. tuberculosis-infected mice. METHODS We analyzed role of IL-17A in host defense against chronically infected M. tuberculosis using IL-17A KO mice. RESULTS We found that TCR γδ+ T cells are a primary source of IL-17A, but that mycobacterial antigen-specific Th17 cells were hardly detected even at the chronic stage of M. tuberculosis infection. IL-17A-deficient mice showed a decreased survival rate, and increased bacterial burden in the lungs after the infection when compared to the wild-type mice. Furthermore, a histological analysis showed an impaired granuloma formation in the infected lungs of IL-17A-deficient mice, which was considered to be due to a decrease of IFN-γ and TNF at the chronic stage. CONCLUSION Our data suggest that the IL-17A-producing TCR γδ+ T cells, rather than the Th17 cells, in the infected lungs are an indispensable source of protective immunity against M. tuberculosis infection.