CD4 and HIV infectious disease: The mechanisms underlying the elevated percentage of IL-10+ and TGF-β+ NK cells during HIV infection remain unclear; however, it was reported that the HIV Env and Tat peptides are responsible for IL-10 production by CD4+ T cells [32–34], and the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway was reported as involved in this process [35, 36].