Reinvigorating exhausted HIV-specific T cells via PD-1-PD-1 ligand blockade.

The programmed death (PD)-1-PD-1 ligand (PD-L) pathway, which is part of the B7-CD28 family, consists of the PD-1 receptor and its two ligands PD-L1 and PD-L2. Engagement of PD-1 by its ligands inhibits immune responses, and recent work has shown that PD-1 is highly expressed on exhausted T cells during chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. Blockade of this pathway reinvigorates the exhausted T cells, allowing them to expand and produce effector cytokines, raising the issue of whether this pathway has been exploited by a variety of viruses during chronic infection. New studies now extend these observations to HIV infection and human disease.