B7 homolog 1 (B7-H1), also known as programmed death-ligand 1 (PD-L1) and cluster of differentiation 274 (CD274), is a 40kDa type I transmembrane protein that has been speculated to play a major role in suppressing the immune system during particular events such as pregnancy, tissue allografts, autoimmune disease and other disease states such as hepatitis. The binding of B7-H1 to its receptor transmits an inhibitory signal which reduces the proliferation of these CD8 T cells at the lymph nodes.
Upregulation of B7-H1 may allow cancers to evade the host immune system. An analysis of 196 tumor specimens from patients with renal cell carcinoma found that high tumor expression of B7-H1 was associated with increased tumor aggressiveness and a 4.5-fold increased risk of death. In addition, B7-H1 acts as a positive costimulatory molecule in intracellular infection. In humans, B7-H1 was found to have altered expression in pediatric patients with Systemic lupus erythematosus (SLE).