IFN-γ is a critical mediator of host defense against Mycobacterium tuberculosis (Mtb) infection. Antigen-specific CD4+ T cells have long been regarded as the main producer of IFN-γ in tuberculosis (TB), and CD4+ T cell immunity is the main target of current TB vaccine candidates. However, given the recent failures of such a TB vaccine candidate in clinical trials, strategies to harness CD4-independent mechanisms of protection should be included in future vaccine design. Here, we have reported that noncognate IFN-γ production by Mtb antigen–independent memory CD8+ T cells and NK cells is protective during Mtb infection and evaluated the mechanistic regulation of IFN-γ production by these cells in vivo. Transfer of arenavirus- or protein-specific CD8+ T cells or NK cells reduced the mortality and morbidity rates of mice highly susceptible to TB in an IFN-γ–dependent manner. Secretion of IFN-γ by these cell populations required IL-18, sensing of mycobacterial viability, Mtb protein 6-kDa early secretory antigenic target–mediated (ESAT-6–mediated) cytosolic contact, and activation of NLR family pyrin domain–containing protein 3 (NLRP3) inflammasomes in CD11c+ cell subsets. Neutralization of IL-18 abrogated protection in susceptible recipient mice that had received noncognate cells. Moreover, improved Mycobacteriumbovis bacillus Calmette-Guérin (BCG) vaccine–induced protection was lost in the absence of ESAT-6–dependent cytosolic contact. Our findings provide a comprehensive mechanistic framework for antigen-independent IFN-γ secretion in response to Mtb with critical implications for future intervention strategies against TB.
Authors
Andreas Kupz, Ulrike Zedler, Manuela Stäber, Carolina Perdomo, Anca Dorhoi, Roland Brosch, Stefan H.E. Kaufmann
(A) Percentage of IFN-γ+ cells among total viable splenic CD3+CD8+, CD3+CD4+, CD3+CD4–CD8– (DN) T cells and CD3–NK1.1+ cells 2 hours after B6 mice were injected with 1 × 108 CFU of either Stm (as a positive control), BCG, HKBCG, Mtb H37Rv, iMtb, or PBS. (B) Percentage of IFN-γ+ cells among total viable splenic CD3+CD8+, CD3+CD4+, CD3+CD4–CD8– (DN) T cells and CD3–NK1.1+ cells at different time points after B6 mice were injected with 1 × 108 CFU Mtb H37Rv. (C) Serum IL-18 concentrations at different time points after injection of B6 mice with 1 × 108 CFU Stm, BCG, Mtb H37Rv, or iMtb H37Rv. (D) Percentage of IFN-γ+ cells among total viable CD3–NK1.1+ in spleen and lung 24 hours after injection of different doses of Mtb H37Rv, BCG, or iMtb H37Rv. (E and F) Percentage of IFN-γ+ CD3–NK1.1+ cells (E) and recoverable CFU (F) from either spleen, lung, or draining LN 24 hours after injection of 1 × 108 CFU Mtb H37Rv via the i.v., i.d., i.t., or s.c. route. Results are presented as pooled data (mean ± SEM) (A–F) and representative FACS plots (E) of 5 to 9 (A), 5 to 10 (B and C), 5 (D), or 7 to 10 (E and F) mice per group from at least 2 to 3 pooled, independent experiments. Dotted lines indicate the mean percentage of the smallest reliably detectable IFN-γ+ response by CD3–NK1.1+ cells (E) and the respective mean recoverable CFU (F) 24 hours after Mtb exposure.