CD4 and infection: The essential role of T-cell proliferation coupled with IL-2-producing capacity was identified in HIV-2 studies describing maintenance of virus-specific CD4 T-cell help, and polyfunctionality of both CD4 and CD8 T cells as hallmarks of non-progressive infection (Alatrakchi et al., 2006; Duvall et al., 2006, 2008), providing additional support for the role of proliferative IL-2-producing T cells in viral control.