CD4 and HIV infectious disease: Blood and gut mucosal CD4+ T-cell depletion is a hallmark of HIV infection that affects both the small and large intestine [63], owing to viral infection via the CD4 receptor, the depletion of Th17 cells [64], chronic immune activation [65], and an alteration in cell types responsible for maintaining IL-17-producing cells, such as gut-resident APCs [66].