PD-1

null Programmed cell death protein 1 (PD-1, CD279) is an inhibitory receptor of the CD28 superfamily induced on activated, exhausted and antigen-experienced T cells, B cells, NK cells and myeloid cells. PD-1 engages two ligands, PD-L1 (B7-H1/CD274), inducibly expressed on hematopoietic and parenchymal cells, and PD-L2 (B7-DC/CD273), restricted to dendritic cells and macrophages. Ligand binding phosphorylates the PD-1 cytoplasmic ITIM and ITSM motifs, recruiting SHP-2 phosphatase that attenuates TCR and CD28 proximal signaling, blunting proliferation, cytokine output, cytotoxicity and survival. This pathway physiologically limits autoimmunity and tissue damage during chronic antigen exposure, but is co-opted by tumors and chronic infections to sustain T-cell exhaustion. PD-1 blockade with pembrolizumab, nivolumab, cemiplimab and related antibodies has revolutionized oncology across dozens of cancer types; PD-1 is also implicated in primary biliary cholangitis and lupus autoimmunity.