Francisella tularensis is one of the most infectious human pathogens known. Although much has been learned about the immune response of mice using an attenuated live vaccine strain (LVS) derived from F. tularensis subspecies holarctica (Type B), little is known about the responses of human monocyte-derived immature dendritic cells (DC). Here, we show that optimal phagocytosis of LVS by DC is dependent on serum opsonization. We demonstrate that complement factor C3-derived opsonins and the major complement receptors expressed by DC, the integrins CR3 (CD11b/CD18) and CR4 (CD11c/CD18), play a critical role in this adhesion-mediated phagocytosis. LVS induced proinflammatory cytokine production and up-regulation of costimulatory surface proteins (CD40, CD86, and MHC Class II) on DC but resisted killing. Once taken up, LVS grew intracellularly, resulting in DC death. DC maturation and cytokine production were induced by direct contact/phagocytosis of LVS or interaction with soluble products of the bacteria, and enhanced activation was seen when LVS was pretreated with serum. Sonicated LVS and supernatants from LVS cultures were potent activators of DC, but LVS LPS failed to activate DC maturation or cytokine production. Serum-treated LVS rapidly induced (within 6 h) a number of cytokines including IL-10, a potent suppressor of macrophage functions and down-regulator of Th1-like responses and the Th1 response inducer IL-12. These results suggest that the simultaneous production of an activating (IL-12, IL-1beta, and TNF-alpha) and a suppressing (IL-10) cytokine profile could contribute to the immunopathogenesis of tularemia.

Although activation of CD4+ T cells mediates pathogenesis in Leishmania amazonensis (La)-infected mice, these susceptible mice do not develop a polarized Th2 response, suggesting a unique mechanism of disease susceptibility. To understand how Th cell activities are regulated, we examined the frequency and phenotypes of regulatory T (Treg) cells. At 1–3 wk of infection, relatively high percentages of CD4+CD25+CD86+ T cells, as well as high levels of FoxP3, TGF-β1, and IL-10RI transcripts, were detected in the skin and draining lymph nodes, indicating local accumulation of Treg cells. Lesion-derived, IL-10-producing CD4+CD25+ cells effectively suppressed proliferation and cytokine (IL-2 and IFN-γ) production of CD4+CD25− effector cells. Adoptive transfer of lesion-derived CD4+CD25+ cells to syngeneic, naive C57BL/6 mice before infection significantly reduced disease development. To further validate the beneficial role of Treg cells in La infection, we adoptively transferred CD25+ T cell-depleted splenocytes (derived from naive mice) into RAG1−/− mice. This transfer rendered RAG1−/− mice more susceptible to La infection than the mice receiving control splenocytes. The beneficial effect of Treg cells was transitory and correlated with decreased activation of IFN-γ-producing effector T cells. This study uncovers an intriguing role of Treg cells in restraining pathogenic responses during nonhealing Leishmania infection and emphasizes a balance between Treg and Th1-like effector cells in determining the outcome of New World cutaneous leishmaniasis.