CD4 and cancer: Programmed death-ligand1 (PD-L1)/programmed cell death 1 (PD-1) expression in the tumor microenvironment suppresses cancer immunosurveillance profoundly [68]. One study showed that S-protein exposure increases surface expression of PD-L1 on a wide range of immune cell types and tumor cells, and PD-1 on T cells, which suppresses the activity of CD4+ and CD8+ T cells against cancer cells [69].