CD273 is a single-pass type 1 membrane protein belonging to the immunoglobulin receptor superfamily, and is a ligand for CD279 (PD-1). CD273 is highly expressed in the tissues of the heart, liver, lung, placenta and pancreas. It is expressed in dendritic cells, and its expression can be induced on inflammatory macrophages when activated by interleukin 4 or Th2 cells. CD273 functions to inhibit T-cell proliferation by blocking cell cycle progression and cytokine production, and is particularly important in the maintenance of self tolerance.