Th17 cytokines in recall responses against Francisella tularensis in humans.

To determine whether cytokines and T-cell subsets other than Th1 cells contribute to secondary immune responses against Francisella species, we investigated production of Th17-associated cytokines IL-17 and IL-22 in a recall response to Francisella tularensis. Peripheral blood mononuclear cells (PBMCs) from volunteers previously immunized with the F. tularensis live vaccine strain (LVS) were stimulated in vitro with bacterial lysates of LVS or a nonpathogenic type A B38 strain. Gene expression analysis by real-time PCR showed that IL-17 and IL-22 transcripts were induced in immune PBMCs at a significantly higher level than in cells from nonvaccinated volunteers stimulated with LVS or B38 antigens at 24 h. In addition, we detected both cell-associated and secreted IL-22 at 24 h after stimulation and IL-17 at 72 h post-stimulation. Intracellular IL-22 and IL-17 were observed in memory CD4+ cells and less in memory CD8+ cells. These findings suggest that Th17 responses in addition to the Th1 response may play an important role in adaptive immunity against Francisella.