PDCD1 and cancer: PD-1 binds two ligands: programmed cell death-ligand 1 (PD-L1), which is expressed on activated T cells, B cells, dendritic cells, macrophages, and cancer cells, and programmed cell death-ligand 2 (PD-L2), expressed on activated macrophages, dendritic cells and, to a limited extent, cancer cells.