CD4 and neoplasm: The inactivation of PD-1-expressing CD8+ T cells by PD-L1 inhibits tumor-infiltrating CD4+/CD8+ T cells (CD4+/CD8+ TILs) and leads to a decrease in cytokines including tumor necrosis factor-alpha (TNF-α), interferon gamma (IFN-γ), and interleukin 2 (IL-2), allowing cancer cells to escape the immune reaction [104,105,106,107].