PD-1 and its ligand PD-L1 is an important immune regulatory pathway. The interaction between PD-1 and PD-L1 has been reported to negatively regulate the proliferation and cytokine production of T-cells.

PD-L1 expression has been reported on many different tumor cells; accordingly, cancers use it to evade the host immune system. PD-L1 expression has been correlated with poor prognosis in cancer. PD-L1 expression on tumor cells has been further correlated with increased tumor aggressiveness and increased risk of death for a number of solid cancers, including ovarian cancer and pancreatic cancer. In addition, surface expression of PD-L1 on cancer cells has been described in several hematological cancers.

PD-L1-reactive CTLs are readily isolated from the peripheral blood of cancer patients and to a lesser extent from the blood of healthy donors.

Thus, PD-L1 may serve as a widely applicable target for immunotherapeutic strategies.