Involvement of IL-17A-producing TCR γδ T cells in late protective immunity against pulmonary 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 and in mice. However, the involvement of IL-17A in protective immunity was not clearly demonstrated in the chronic stage of -infected mice.

Methods: We analyzed role of IL-17A in host defense against chronically infected 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 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 infection.