CD4 and infection: The central role of gut epithelial barrier in the disease progression following SIV infection has been investigated using African green monkeys and the sooty mangabey (Gordon et al., 2007; Pandrea et al., 2007; Silvestri et al., 2007; Klatt et al., 2012) Despite high viral loads there is no disease progression to AIDS in these animals although the infection induces a rapid loss of intestinal CD4+ T cells, which is similar to what has been observed in HIV-infected humans and SIV-infected macaques.