PD-1 is involved in immune tolerance by suppressing activated immune cells via interaction with its ligands. Two known ligands of PD-1 are PD-L1 and PD-L2. PD-L1 is a 290 aa transmembrane glycoprotein. PD-L2 (B7DC) may lead to local cytokine production that is beneficial to the tumor cells. PD-L1 and PD-L2 play different roles in the immune regulatory process. PD-L1 inhibits T-cell function in peripheral tissues, whereas PD-L2 suppresses immune T-cell activation in lymphoid organs.