CD4 and HIV-1 infection: Earlier studies have revealed the central role of IL-2 and IFNγ as survival and proliferative factors (70, 71) in HIV-1 infection; as disease progresses, the frequency of IL-2-producing CD4+ T-cells is found to decline (42), which in turn relates to the reduced renewal capacity and increased susceptibility of these populations of cells to apoptosis (70).