PDCD1 and neoplasm: Cancer immunotherapy using ICIs has become an effective therapy for tumor treatment; however, one of the reasons for its poor efficacy is the presence of immunosuppressive mechanisms in TME that diminish the effector function of CD8 tumor-infiltrating lymphocytes (TILs) [25], and T-cell up-regulation of PD-1 has become a major marker of T-cell dysfunction [26].