Toll-like receptor expression reveals CpG DNA as a unique microbial stimulus for plasmacytoid dendritic cells which synergizes with CD40 ligand to induce high amounts of IL-12

Human plasmacytoid dendritic cells (DC) (PDC, CD123(+)) and myeloid DC (MDC , CD11c(+)) may be able to discriminate between distinct classes of microbi al molecules based on a different pattern of Toll-like receptor (TLR) expre ssion. TLR1-TLR9 were examined in purified PDC and MDC. TLR9, which is crit ically involved in the recognition of CpG motifs in mice, was present in PD C but not in MDC. TLR4, which is required for the response to LIPS, was sel ectively expressed on MDC. Consistent with TLR expression, PDC were suscept ible to stimulation by CpG oligodeoxynucleoticle (ODN) but not by LIPS, whi le MDC responded to LIPS but not to CpG ODN. In PDC, CpG ODN supported surv ival, activation (CD80, CD86, CD40, MHC class II), chemokine production (IL -8, IP-10) and maturation (CD83). CD40 ligand (CD40L) and CpG ODN synergize d to activate PDC and to stimulate the production of IFN-alpha and IL-12 in cluding bioactive IL-12 p70. Previous incubation of PDC with IL-3 decreased the amount of CpG-induced IFN-alpha and shifted the cytokine response in f avor of IL-12. CpG ODN-activated PDC showed an increased ability to stimula te proliferation of naive allogeneic CD4 T cells, but Th1 polarization of d eveloping T cells required simultaneous activation of PDC by CD40 ligation and CpG ODN. CpG ODN-stimulated PDC expressed CCR7, which mediates homing t o lymph nodes. In conclusion, our studies reveal that IL-12 p70 production by PDC is under strict control of two signals, an adequate exogenous microb ial stimulus such as CpG ODN, and CD40L provided endogenously by activated T cells. Thus, CpG ODN acts as an enhancer of T cell help, while T cell-con trolled restriction to foreign antigens is maintained.