PD-L1 is a type I transmembrane protein of 290 amino acids, and it is a member of the B7 family. Mouse PD-L1 has 70% amino acid identity to its human orthologue. Binding of PD-L1 to its receptor PD-1 leads to the inhibition of T cell receptor–mediated lymphocyte proliferation and cytokine secretion. PD-L1 induces IL-10 production in T cells stimulated with low levels of anti-CD3. PD-L1/PD-1 interaction suppresses immune responses against autoantigens and tumors and plays an important role in the maintenance of peripheral immune tolerance. Disruption of the PD-L1 gene leads to up-regulated T cell responses and the generation of self-reactive T cells. Antibodies against PD-1 or PD-L1 leads to increased antitumor immunity. PDL1 has an important role in conferring fetomaternal tolerance in an allogeneic pregnancy model; antibodies against PD-L1 lead to a breakdown in maternal tolerance to the fetus. PD-L1 shares its receptor with PD-L2 (CD273, B7-DC). PD-L2 has a more limited expression than PD-L1, being expressed on activated macrophages and dendritic cells. PD-L1 is expressed in many tumors, and the interaction with its receptor activates signaling pathways that inhibit T-cell activity and therefore the antitumor immune response. Antibodies targeting PD1 or PD-L1 block the PD1 pathway and reactivate T cell activity.