Programmed cell death protein 1 (PDCD1), also known as PD1, is an immune-checkpoint receptor mainly expressed by T cells that negatively regulates immune response []. It interacts with its ligands PD-L1 and PD-L2 to inhibit T cell effector functions in an antigen-specific manner []. The PD1-mediated inhibitory pathway is exploited by some tumours to attenuate anti-tumour immunity and escape destruction by the immune system [].
