Mycobacterium bovis BCG-Specific Th17 Cells Confer Partial Protection against Mycobacterium tuberculosis Infection in the Absence of Gamma Interferon

Protective immunity against tuberculosis (TB) requires the integrated response of a network of lymphocytes. Both gamma interferon (IFN-gamma)- and interleukin 17 (IL-17)-secreting CD4(+) T cells have been identified in subjects with latent TB infection and during experimental Mycobacterium tuberculosis infection, but the contribution of Th17 cells to protective immunity is unclear. To examine their protective effects in vivo, we transferred mycobacterium-specific IL-17- and IFN-gamma-secreting CD4(+) T cells isolated from M. tuberculosis BCG-immunized IL-12p40(-/-) and IFN-gamma(-/-) or wild-type mice, respectively, into M. tuberculosis-infected IL-12p40(-/-) or RAG(-/-) mice. In the absence of IL-12 and IL-23, neither IL-17-secreting (Th17) nor IFN-gamma-secreting (Th1) BCG-specific T cells expanded or provided protection against M. tuberculosis. In RAG(-/-) recipients with an intact IL-12/IL-23 axis, both Th17 and Th1 cells were activated and induced significant protection against M. tuberculosis. The reduction in the bacterial load following transfer of IFN-gamma(-/-) Th17 cells was associated with significant prolongation of survival compared to recipients of naive IFN-gamma(-/-) T cells. This effect was at the cost of an increased inflammatory infiltrate characterized by an excess of neutrophils. Therefore, Th17 cells can provide IFN-gamma-independent protection against M. tuberculosis, and this effect may contribute to the early control of M. tuberculosis infection.