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Plasmacytoid dendritic cells (pDC) constitute a distinct population of DC in the peripheral and secondary lymphoid organs. After activation with the ligands for Toll-like receptors (TLRs), particularly TLR7 and TLR9, pDC secrete large amounts of type I interferons and differentiate into mature DC, expressing co-stimulatory molecules and other cytokines. New immunoregulatory molecules on human pDC were sought and their regulation by TLR ligands assessed by pDC gene expression profiling. We established that both the CD300a and CD300c cell surface molecules are expressed by pDC. Because CD300 molecules play important roles in regulating immune responses, we investigated their contribution to pDC function in more detail. First, their down-regulation by CpG-ODN was confirmed by real time PCR and RT-PCR for CD300a and CD300c. Using the monoclonal antibody CMRF-35, to detect both CD300a and CD300c, we demonstrated that both TLR7 and TLR9 ligands down-regulated cell surface CD300a and CD300c. We showed that exogenous IFN-a down-regulated CD300a/c expression in pDC suggesting that the TLR ligands induced down-regulation of CD300a/c might be an indirect effect. In subsequent experiments, we added neutralizing antibody to IFN-a and found that it abolished the CpG-ODN induced CD300a/c down-regulation. The effect of CD300a and CD300c activation on the function of pDC was investigated by cross-linking CMRF-35 on pDC. This had no effect on CD80, CD83 and CD86 expression, but decreased MHC-II expression. Significant reductions in pDC TNF-a and IL6 production occurred after CMRF-35 cross-linking, however, in marked contrast, CpG-ODN induced IFN was increased in these experiments. Thus the immune regulators CD300a and CD300c regulate the cytokine production in pDC, whilst the pDC released cytokines regulated the CD300a/c expression. This suggests that CD300 molecules may play a pivotal role in fine tuning pDC driven immune responses and may contribute to the pDC biology of allogeneic bone marrow transplantation.

CD300A AND CD300C ON PLASMACYTOID DENDRITIC CELLS ARE DOWN-REGULATED BY TLR7 AND TLR9 LIGAND INDUCED TYPE I INTERFERON