PDCD1 and neoplasm: According to previous studies, the role of TCM in tumor immunotherapy is mainly associated with the positive regulation of natural killer (NK) cells, CD8/CD4 T cells, dendritic cells, M2 macrophages, IL-2, tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), the negative regulation of regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), cancer-associated fibroblasts, PD-1/PD-L1, transforming growth factor-β, and TNF-β.