CD4 and AIDS: Indeed, studies in Simian immunodeficiency virus (SIV)mac-infected rhesus macaques (RM; a non-natural host for SIV that progress to AIDS) showed that the levels of infection and depletion of CM cells dictate the tempo of progression to AIDS [32], and that vaccine-induced preservation of CD4 CM T-cells predicts survival from SIV challenge [33,34].