PD-1 and its ligands in T-cell immunity

The past year has seen significant advances in our understanding of the critical roles of negative immunoregulatory signals delivered by the programmed death 1 (PD-1)–PD-1 ligand (PD-L) pathway in regulating T-cell activation and tolerance. Emerging evidence indicates that PD-Ls play an essential role on dendritic cells (DCs), both directly during DC–T cell interactions and indirectly through signaling into the DC. Recent studies point to a novel role for PD-L1 in maintaining tissue tolerance. Finally, PD-1 has recently been shown to be highly expressed on exhausted T cells during chronic viral infection, and blockade of PD-1 or PD-L1 can revive exhausted T cells, enabling them to proliferate and produce effector cytokines.