Lung CD4^-CD8^- double-negative T cells are prominent producers of IL-17A and IFN-γ during primary respiratory murine infection with Francisella tularensis live vaccine strain

For several intracellular infections, pulmonary vaccination provides measurably better protection against pulmonary challenge. The unique factors that contribute to pulmonary immune responses are not well characterized. In this study, we show that CD4^- CD8^- double negative (DN) T cells are a major responding T cell subset in the lungs of mice during pulmonary Francisella tularensis live vaccine strain (LVS) infection. DN T cells were a minor (<2%) subset in spleens and lungs of mice during sublethal intradermal infection with LVS. In contrast, they were a major responding T cell subset in lungs during pulmonary LVS infection, producing large quantities of IFN-γ and IL-17A. The numbers of IL-17A⁺ DN T cells in the lungs exceeded that of CD4⁺ and CD8⁺ T cells on day 7 postinfection; by day 14 postinfection, all three IL-17A-producing T cell subsets were present in equivalent numbers. CD4⁺, CD8⁺, and DN T cell production of IL-17A was not observed in the spleens of pulmonary-infected mice or the lungs and spleens of intradermally infected mice. Correspondingly, IL-17A knockout mice were more susceptible to respiratory than intradermal LVS infection, with delayed clearance 1-3 wk postinfection. Finally, in vitro treatment of LVS-infected macrophages and alveolar type II epithelial cells with IFN-γ and IL-17A affected significantly greater LVS growth control than treatment with either cytokine alone. The data presented in this study demonstrate that DN cells contribute to production of IL-17A and IFN-γ in the lungs during inhalational Francisella infection and that these cytokines additively activate host cells to control LVS intracellular growth.