Immune checkpoint proteins: a focus on PD-1/PD-L1

Programmed cell death 1 receptor (PD-1) and its ligand programmed cell death ligand 1 (PD-L1) are immune checkpoint proteins found on the cell surface of T cells. Under physiological conditions their interaction results in T cell immune suppression, but cancer cells can hijack this pathway to escape immune detection. However, this can be reversed by blocking PD-1’s interaction with PD-L1. This has resulted in an intense research effort to investigate the underlying mechanisms and the clinical utility of inhibitors targeting these checkpoint proteins as therapeutic anticancer drugs. In this Collection we highlight a selection of recent papers focused on the cellular, molecular, diagnostic and clinical effects of these crucial proteins and their inhibitors.