Abstract: IntroductionProgrammed death-1 (PD-1) is an inhibitory T-cell receptor expressed by activated T-lymphocytes,and its two major ligands are PD-L1 (B7-H1,CD274) and PD-L2 (B7-DC,CD273).Under normal conditions,PD-L1 is mainly expressed on T cells,B cells,dendritic cells,and macrophages,whereas PD-L2 is expressed selectively on activated dendritic cells,macrophages,marrow-originated mast cells,and peritoneum B1 cells.