CD4 and HIV-1 infection: Although CD4+ T regulatory cells (Tregs) are believed to play an important role in HIV pathogenesis [60], as illustrated by their involvement in the immune response in the apathogenic model of African Green Monkeys to SIV infection [61] as well as in pathogenic SIV infection of macaques by FoxP3+ CD8+ T cells [62], no specific studies have been designed to address their potential role in R5 vs. CXCR4-dependent HIV-1 infection.