Negative role of inducible PD-1 on survival of activated dendritic cells.

PD-1 expressed on activated DCs suppresses T cell activation via decreasing DC survival. PD-1 is a well-established negative regulator of T cell responses by inhibiting proliferation and cytokine production of T cells via interaction with its ligands, B7-H1 (PD-L1) and B7-DC (PD-L2), expressed on non-T cells. Recently, PD-1 was found to be expressed in innate cells, including activated DCs, and plays roles in suppressing production of inflammatory cytokines. In this study, we demonstrate that PD-1 KO DCs exhibited prolonged longevity compared with WT DCs in the dLNs after transfer of DCs into hind footpads. Interestingly, upon LPS stimulation, WT DCs increased the expression of PD-1 and started to undergo apoptosis. DCs, in spleen of LPS-injected PD-1 KO mice, were more resistant to LPS-mediated apoptosis in vivo than WT controls. Moreover, treatment of blocking anti-PD-1 mAb during DC maturation resulted in enhanced DC survival, suggesting that PD-1:PD-L interactions are involved in DC apoptosis. As a result, PD-1-deficient DCs augmented T cell responses in terms of antigen-specific IFN- production and proliferation of CD4 and CD8 T cells to a greater degree than WT DCs. Moreover, PD-1 KO DCs exhibited increased MAPK1 and CD40-CD40L signaling, suggesting a possible mechanism for enhanced DC survival in the absence of PD-1 expression. Taken together, our findings further extend the function of PD-1, which plays an important role in apoptosis of activated DCs and provides important implications for PD-1-mediated immune regulation.