Cytotoxic T cells work to detect and eliminate infected cells and tumor cells from the body. To limit damage to surrounding tissue, normal cells differentiate themselves by expressing a protein signal called PD-L1 (programmed death ligand 1). This PD-L1 signal is a stop sign designed to avoid elimination of normal cells by cytotoxic T cells. T cells detect the PD-L1 signal through a receptor called PD-1 (programmed death receptor 1). Some tumors can also express the PD-L1 signal to mimic normal cells and escape elimination. Anti-PD-1 therapy works by blocking the PD-1/PD-L1 interaction. PD-L1 expression may be associated with patient benefit from anti-PD-1 therapy.

Reference
Sacher AG, Gandhi L. Biomarkers for the clinical use of PD-1/PD-L1 inhibitors in non-small-cell lung cancer. JAMA Oncol. 2016

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