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.
Andreas Kupz, Ulrike Zedler, Manuela Stäber, Carolina Perdomo, Anca Dorhoi, Roland Brosch, Stefan H.E. Kaufmann
(A–G) Percentage of IFN-γ+ cells among total lung CD3–NK1.1+ (A, C, and E) and CD3+CD8+ (B, D, and F) cells and serum IL-18 levels (G) of straight WT or Il18–/– (A and B) and CD11cDTR BM chimeras (C and D) as well as DTX- and PBS-treated CD11cDTR mixed BM chimeras (E and F) 24 hours after injection of 1 × 108 CFU Mtb H37Rv. (H) Percentage of GFP+ cells among total viable splenocytes 24 hours after i.v. or i.t. injection of 1 × 108 CFU Mtb H37Rv or Mtb H37Rv–GFP into naive B6 mice. (I) Uptake of Mtb H37Rv–GFP by individual splenic DC subsets (CD11c+MHC-II+CD4+, CD11c+MHC-II+CD8+, or CD11c+MHC-II+CD4–CD8–) 24 hours after i.v. or i.t. delivery of 1 × 108 CFU. Results are presented as individual data points (A–G) or as pooled data (mean ± SEM) and representative FACS plots (H and I) of 8 to 10 (A–G) or 4 to 6 (H and I) mice per group from 2 (H and I) or 3 (A–G) pooled, independent experiments. *P < 0.05, **P < 0.01, ***P < 0.001, by 1-way ANOVA. Mtb-GFP, Mtb H37Rv–GFP; PI, propidium iodide.