Programmed cell death 1 ligand 2 (PD-L2; CD273; B7-DC) is type I transmembrane glycoprotein composed of IgC- and IgV-type extracellular domains and it is a member of the B7 gene family. (1) PD-L2 can bind to programmed cell death protein 1 (PD-1) resulting in down-regulation of T cell responses. (2) Compare to PD-L1, another ligand of PD-1, expression of PD-L2 is much more restricted. (3) It is inducibly expressed on the surface of macrophages, dendritic cells, bone marrow derived mast cells and certain B cell populations. (3) (4) (5) The PD-L2 expression is mainly induced by Interleukin 4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM CSF) and to lesser extent by Interferon-γ (IFN-γ). (5) As PD-L1 and PD-L2 can inhibit effector T cell proliferation and cytokine production, the PD-L:PD-1 pathway is an appealing therapeutic target. (2) Blocking the PD-1 pathway enhance anti-tumor immunity, while stimulating this pathway may be useful for down-regulating ongoing immune responses in transplant rejection and autoimmune and allergic diseases. (2)

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