Mycobacteria Induce IFN-γ Production in Human Dendritic Cells via Triggering of TLR2

Abstract IFN-γ is of central importance for the induction of robust cell-mediated immunity and for the activation of APC. Recent studies using experimental murine systems have now suggested a fundamental role for APC-derived IFN-γ during infection with intracellular pathogens. It is currently unknown whether human dendritic cells (DC) can respond to bacterial stimulation with production of IFN-γ. To test this question, we used human monocyte-derived DC stimulated by Mycobacterium bovis bacillus Calmette-Guérin as a model system. We demonstrate production of IFN-γ mRNA and protein on the single cell level. IFN-γ in DC cultures was not simply produced by contaminating lymphocytes because production of DC-IFN-γ could also be demonstrated in highly purified DC cultures containing virtually no T, B, and NK cells. TLR2 was identified as a key receptor involved in triggering production of DC-IFN-γ. Interestingly, DC-IFN-γ seems to participate in an autocrine DC activation loop, and production of DC-IFN-γ could be enhanced by costimulation of DC with IL-12/IL-15/IL-18. In conclusion, we have demonstrated production of IFN-γ by human DC on the single cell level, identified TLR2 as a pattern recognition receptor involved in this process, and elucidated some of the functional consequences of autocrine IFN-γ production by human DC.