Programmed death protein 1 (PD-1) is also known as CD279 and PDCD1，is a type I membrane protein and is a member of the extended CD28/CTLA-4 family of T cell regulators. PDCD1 is expressed on the surface of activated T cells，B cells，macrophages，myeloid cells and a subset of thymocytes. PD-1 has two ligands，PD-L1 and PD-L2，which are members of the B7 family. PD-L1 is expressed on almost all murine tumor cell lines，including PA1 myeloma，P815 mastocytoma，and B16 melanoma upon treatment with IFN-γ. PD-L2 expression is more restricted and is expressed mainly by DCs and a few tumor lines. PD1 inhibits the T-cell proliferation and production of related cytokines including IL-1，IL-4，IL-10 and IFN-γ by suppressing the activation and transduction of PI3K/AKT pathway. In addition，coligation of PD1 inhibits BCR-mediating signal by dephosphorylating key signal transducer. In vitro，treatment of anti-CD3 stimulated T cells with PD-L1-Ig results in reduced T cell proliferation and IFN-γ secretion. Monoclonal antibodies targeting PD-1 that boost the immune system are being developed for the treatment of cancer. This protein is suitable for use in protein studies such as protein structure analysis and protein-protein interactions. It can also be used as an immunogen，as a protein standard，or in cell biology research applications.
