PD-L1 is a type I transmembrane glycoprotein which serves as one of the ligands for CD279 (PD1). Ther interactions between CD279 and its ligand can regulate the balance between the stimulatory and inhibitory signals required in response to microbes. PD-L1 is also partially responsible for costimulation of T cell proliferation although conflicting data suggests that it may also inhibit T cell proliferation and cytokine production.