CD4 and HIV-1 infection: A striking difference between the two viral subtypes is that the level of immune activation is lower in HIV-2 compared to HIV-1 infection, although expression patterns and prognostic values for immune activation markers were found to be similar when patients with HIV-1 or HIV-2 infection were matched for CD4+ T-cell depletion levels (35, 36) These observations were paralleled by insights from simian immunodeficiency virus (SIV) infection in sooty mangabeys (SMs) and African green monkeys (AGMs).