Diverse Myeloid and Lymphoid Cell Subpopulations Produce Gamma Interferon during Early Innate Immune Responses toFrancisella tularensisLive Vaccine Strain

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Abstract
Francisella tularensis, a small gram-negative intracellular bacterium responsible for causing tularemia, is highly pathogenic and classified as a category A agent of bioterrorism. As for other intracellular pathogens, successful protective immune responses toFrancisella tularensisrequire rapid and efficient induction of gamma interferon (IFN-γ) production. Studies using intracellular bacteria such asListeria monocytogenesas well asFrancisellasuggest that natural killer (NK) and T cells are important sources of IFN-γ. However, comprehensive characterization of specific sources of IFN-γ produced duringFrancisellainfection in vivo remains incomplete, and depletion of NK cells before infection of mice with theF. tularensislive vaccine strain (LVS) has little impact on the course or outcome of infection. In this study, we determined the cell subpopulations that respond quickly to intradermalF. tularensisLVS infection of mice by producing IFN-γ within hours to a few days. Splenic and liver lymphocytes were obtained from LVS-infected mice and analyzed for IFN-γ mRNA by reverse transcription-PCR, for intracellular cytokine expression by multiparameter flow cytometry, and for ex vivo production of IFN-γ protein by enzyme-linked immunosorbent assay. Cells producing IFN-γ were readily detectable by day 3 after infection, and numbers progressively increased through days 5 to 7. Importantly, the cell types responsible for IFN-γ production were much more varied than expected: these included not only NK cells and T cells, which might be predicted, but also other cells, including dendritic cells (DCs), “NK DCs,” NK T cells, and neutrophils. Most importantly, since RAG-1 knockout mice appeared to exhibit a frequency of IFN-γ-producing cells comparable to that of intact wild-type mice, early IFN-γ production by innate immune cells does not depend on the presence of T or B cells.