Plasmacytoid dendritic cells: key players in the initiation and regulation of immune responses.

Plasmacytoid dendritic cells (pDCs) are a rare population of circulating cells that selectively express Toll-like receptors (TLR)7 and TLR9 and have the capacity to produce large amounts of type I interferons (IFNs) in response to viruses or host-derived nucleic acid-containing complexes. Through the production of type I IFNs, pDCs initiate protective immunity by activating classical DCs, T cells, natural killer cells, and B cells. Upon activation, pDCs also differentiate into mature DCs and may contribute to the contraction of T-cell responses. In this review, we describe how pDCs are key players in the initiation and regulation of immune responses and discuss their emerging role in the pathogenesis of human diseases.