Expression of programmed-death receptor ligands 1 and 2 may contribute to the poor stimulatory potential of murine immature dendritic cells

Recent data have revealed that Ag presentation by immature dendritic cells (imDCs) plays a role in establishing and maintaining T-cell tolerance, but the mechanism remains unclear. PD-L1 and PD-L2, ligands for programmed-death receptor 1 (PD-1), members of the expanding B7 family, were highlighted for their inhibitory role in T-cell responses. Here, we show that blockade of PD-1 ligands on imDCs resulted in enhanced T-cell proliferation, which is perhaps due to the enhancement of IL-2 production from DC-stimulated T cells. PD-1 ligands blockade on mDCs did not show a significant stimulatory effect as markedly as imDCs. The inhibitory effects of PD-1 ligands would be dependent on maturation status of DCs, where attenuated positive costimulatory molecules provided the opportunity for PD-1 ligands to exert their strong capacity. Our data are consistent with the hypothesis that imDCs have an inhibitory bias, and indicate that PD-L1 and PD-L2 contribute to the poor stimulatory capacity of imDCs.