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.

Recent evidence suggests that platelets are not only involved in haemostatic processes but also modulate immune responses. As antigen-presenting cells (APC) are of crucial importance for the regulation of immunity, in this study we wanted to define the role of platelet factor 4 (PF-4) as one of the major platelet-derived chemokines on the transition of monocytes into APCs. Our experiments show that within 3 days PF-4 in conjunction with IL-4 induces a rapid differentiation of monocytes into APC. These PFAPC (PF-4/IL-4 differentiated APC) display unique phenotypical and functional characteristics setting them apart from macrophages and conventional dendritic cells. Functional studies revealed that PFAPC preferentially stimulated proliferation of lymphocytes and lytic NK activity while they induced only moderate cytokine responses. Beyond day 3 of differentiation, PFAPC became less immunostimulatory and maintained their capacity to phagocytose particulate material even after LPS-induced maturation. These experiments uncover a previously unknown role for the platelet-derived CXC-chemokine PF-4 in differentiation of human APC. Our data further support the newly discovered function of platelets in immunomodulation and provide new evidence for a rapid transition of monocytes into APC under the influence of inflammatory stimuli.