CD4 and infection: Previous studies of FIV in cats have shown that activated (CD4+ CD25+) and resting (CD4+ CD25–) CD4+ T-cells from peripheral blood can be latently infected ex vivo and that FIV replication can be reactivated by the application of ConA or IL-2 (Joshi et al., 2005, 2004), mirroring the crucial role of IL-2 in productive infection with HIV-1 (Oswald-Richter et al., 2004).