Programmed cell death-1 (PD-1) is a protein expressed on T cells, B cells, natural killer cells, activated monocytes, and dendritic cells that helps regulate the body’s immune response.1,2 PD-1 is not expressed on resting T cells, and its expression is induced when T cells are activated.2 Known as a “checkpoint protein,” PD-1 binds to its ligands, PD-L1 and PD-L2, which are expressed on a number of cells, including normal and malignant cells to deactivate the T cell, reduce cytokine production, and reduce proliferation of T cells.1,3 This balance is critical to prevent T cells from attacking normal cells in the body.3
Cancer cells exploit this pathway by over-expressing PD-L1 to evade the body’s immune response. PD-L1 is highly expressed on multiple types of cancer cells, including both solid and hematologic malignancies.1 Both PD-1 and PD-L1 are established targets for anticancer therapy.3
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1. Chen DS, Irving BA, Hodi FS. Clin Cancer Res. 2012;18(24):6580-6587. 2. Keir ME, Butte MJ, Freeman GJ, Sharpe AH. Annu Rev Immunol. 2008;26:677-704. 3. American Cancer Society. https://www.cancer.org/treatment/treatments-and-side-effects/treatment-types/immunotherapy/immune-checkpoint-inhibitors.html. Accessed 4/19/2019.