PL120131 is a PD-1/PD-L1 inhibitory peptide that can interfere with the interaction of PD-1/PD-L1 by binding to PD-1. PL120131 is capable of inhibiting the PD-1 -mediated apoptotic signaling pathway, protecting Jurkat cells and primary lymphocytes from apoptosis. PL120131 aids in the antitumor activity of cytotoxic T lymphocytes (CTLs).