CD4 and HIV infectious disease: Memory CD4+ T cells expressing high levels of α4β7 are highly susceptible to HIV infection in DC-T cell mixtures in vitro [23, 100]; the proportion of these cells correlates with mucosal SIV susceptibility in vivo [83]; and antibody blockade of α4β7 significantly reduces mucosal SIV transmission [101] and therapeutically reduces plasma and gastrointestinal SHIV viral load [102].