Plasmacytoid dendritic cells (PDC) are the main type I interferon (IFN-I)producers and play a central role in innate and adaptive immunity. CD303 (BDCA-2) is a type-II c-type lectin specifically expressed by human PDC. CD303 signaling induces tyrosine phosphorylation and src-kinase dependent calcium influx as was shown previously. Crosslinking CD303 results in the inhibition of IFN-I production in stimulated PDC. In this study it is demonstrated that PDC express a signalosome similar to the BCR signalosome, consisting of Lyn, Syk, Btk, Slp65 (Blnk) and PLCγ2. CD303 associates with the signaling adapter FcR γ-chain. Triggering CD303 leads to tyrosine phosphorylation of Syk, Slp65, PLCγ2 and cytoskeletal proteins. Analogous to BCR signaling, CD303 signaling is likely linked with its internalization by clathrin-mediated-endocytosis. The inhibition of IFN-I production by stimulated PDC can be induced by phorbol ester treatment, thus mimicking CD303 signaling and suggesting an involvement of protein kinase C members. Furthermore, CD303 signaling leads to reduced levels of transcripts for IFN-I genes and IFN-I responsive genes, indicating that the inhibition of IFNI production by stimulated PDC is at least partially regulated at the transcriptional level. In contrast to BCR signaling, CD303 signaling appears to extenuate the activation of the NF-kB pathway in PDC, implying the inhibition of IFN-I production is likely linked to early CD303 signaling events. These results support a possible therapeutic value of an anti-CD303 monoclonal antibody strategy, since the production of IFN-I by PDC is considered to be a major pathophysiological factor in systemic lupus erythematosus patients.