Human plasmacytoid dendritic cells support Th17 cell effector function in response to TLR7 ligation.

Signals involved in the commitment of Th17 differentiation are of substantial interest for our understanding of antimicrobial defense mechanisms and autoimmune disorders. Various ways in which myeloid dendritic cells modulate Th17 differentiation have been identified. However, although plasmacytoid dendritic cells (PDCs) are regarded as important players in antiviral/antimicrobial host defense and autoimmune diseases, a putative modulatory role of PDCs in Th17 differentiation has not yet been elucidated in detail. We demonstrated that PDCs are capable of promoting Th17 differentiation in response to TLR7 stimulation. Further, both the differentiation of Th17 cells from naive T cells and the amplification of Th17 effector functions of memory T cells are promoted by PDCs after TLR7 activation. Our data are of strong clinical relevance because TLR7 activation in PDCs might represent one of the missing links between innate and adaptive immune mechanisms and contribute to the amplification of Th17-driven autoimmune disorders as well as viral host defense.