CD4 and infection: As to the cellular mechanism underlying this massive CD4+ T cell depletion, another study in SIV-infected rhesus macaques found that SIV exploits a large resident population of CD4+ memory T cells to produce high levels of virus that both directly, through lytic infection, and indirectly, through Fas-mediated apoptosis of infected and uninfected cells, deplete the majority of CD4+ T cells in GALT within the first 3 weeks of infection [21].