PDCD1 and neoplasm: Previous studies have demonstrated that the expression level of PD-1 on cytotoxic T cells in tumor tissues is greater than that of PD-1 on cytotoxic T cells in peripheral blood (Xu et al., 2021, Prodi et al., 2024), suggesting that the payload cytokine would preferentially be delivered to tumor-infiltrated cytotoxic T cells by using an antibody against PD-1 as a directional molecule, which would decrease the risk of inducing systemic immune activation.