Introduction Programmed death-1 (PD-1) is an immune checkpoint receptor and that regulates T cell activity by inhibiting immune responses and promoting self-tolerance. Programmed Death Ligand 1 (PD-L1), the natural receptor for PD-1, predominantly expressed on tumor cells, binds PD-1 to activate downstream signaling pathways and suppresses T cell activation, cytokine secretion, and induces apoptosis; Critically, the PD-1/PD-L1 axis is a key mechanism enabling tumor immune evasion (1).
