Early activation of NK cells after lung infection with the intracellular bacterium, Francisella tularensis LVS

Francisella tularensis is a gram-negative intracellular bacterium that has been classified as a Category A biothreat because of its ability to induce deadly pneumonic tularemia when inhaled. In the present study, an experimental model of F. tularensis LVS intranasal infection was used to study the immune cells involved in cytokine secretion in the lungs after infection. Dramatic increases in the numbers of cells secreting IFN-γ were observed 72 h after intranasal infection of BALB/c and C57BL/6 mice with sublethal (1000 CFU) or lethal (10,000 CFU) doses of F. tularensis LVS and the cells primarily responsible for this IFN-γ expression were identified as CD11b+ DX5+ NK cells. The findings were further confirmed in C57BL/6 mice showing that cells responsible for IFN-γ secretion in the lungs were CD11b+ DX5+ NK1.1+. NK cell depletion studies showed a decrease in the percentage of IFN-γ secreting cells, due not only to a diminished proportion of IFN-γ secreting NK cells, but also to a reduced percentage of T cells secreting IFN-γ. The results indicate that IFN-γ is secreted in response to respiratory infection with F. tularensis LVS, and that NK cells are the early responders responsible for IFN-γ secretion.