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
Rag1–/– mice were infected with 200 CFU Mtb H37Rv via the aerosol route and treated weekly with either anti–IL-18 Ab or control IgG or were left untreated. As an additional control, naive Rag1–/– mice were treated with anti–IL-18 Ab. Long-term survival (A), CFU in lung and spleen (B and C), a cytokine profile (F), as well as cell infiltrates (D and E) and iNOS expression in lung tissue (D) were assessed 34 days p.i. Results are presented as individual data points (B, C, and E), pooled data (mean ± SEM) (F), and representative images (D) of 6 (A) or 5 (B–F) mice per group from 2 pooled independent experiments. *P < 0.05, **P < 0.01, and ***P < 0.001, by log-rank (Mantel-Cox) test (A), 1-way ANOVA (B, C, and E), and 1-way ANOVA for each cytokine (F). Original magnification, ×25 (Giemsa), ×50 (TB-Fluor), ×100 (iNOS). G-CSF, granulocyte-CSF; GM-CSF, granulocyte macrophage–CSF.