CD4 and neoplasm: For example, in the tumor microenvironment (TME), IFN-γ secreted by CD4+ and CD8+ T cells as an immune surveillance mechanism could upregulate PD-L1/PD-L2 in cancer cells, thereby contributing to tumor immune evasion via the IFN-γ-STAT1-PD-L1/L2 axis in melanoma [13].