CD4 and neoplasm: Tumor cells can secrete exosomes carrying PD-L1, inhibit the activation of CD8+T-cells in tumor microenvironment or peripheral circulation, inhibit the proliferation of CD4+T-cells, upregulate the immunosuppressive function of Treg cells, downregulate the expression level of NKG2D in NK cells to inhibit immune killing, and finally promote immune evasion (Chen et al., 2018).