TPP-1, a peptide inhibitor, targets the interaction between programmed cell death protein 1 (PD-1) and its ligand (PD-L1), binding specifically to PD-L1 with a dissociation constant (Kd) of 94.67 nM. At a concentration of 20 µM, TPP-1 effectively counteracts the PD-L1-induced suppression of IFN-γ production in anti-CD3-stimulated isolated human CD4+ T cells. Furthermore, TPP-1 demonstrates significant efficacy in reducing tumor volume in a mouse xenograft model of HL-60 leukemia. This model involved HL-60 cells co-cultured with anti-CD3-activated isolated human CD4+ T cells, with TPP-1 administration at a dosage of 4 mg/kg showing notable antitumor activity.