Programmed death 1 ligand 1 (PD-L1) also known as cluster of differentiation (CD274) or B7 homolog 1 (B7-H1)，is a member of the growing B7 family of immune molecules and is involved in the regulation of cellular and humoral immune responses. B7-H1 is a cell surface immunoglobulin superfamily with two Ig-like domains within the extracellular region and a short cytoplasmic domain. PD-L1 is highly expressed in the heart，skeletal muscle，placenta and lung and weakly expressed in the thymus，spleen，kidney and liver. PD-L1 is expressed on activated T-cells，B-cells，dendritic cells，keratinocytes and monocytes. PD-L1 is up-regulated on T- and B-cells，dendritic cells，keratinocytes and monocytes after LPS and IFNG activation and up-regulated in B-cells activated by surface Ig cross-linking. PD-L1 is involved in the costimulatory signal，essential for T-cell proliferation and production of IL10 and IFNG，in an IL2-dependent and a PDCD1-independent manner.
