CD28 and HIV infectious disease: The role for CD28 on activated T cells is particularly pertinent as CD28 expression is lost on a proportion of activated T cells with age, in HIV infection, and in the number of immune disorders in primates (but not rodents) (Weng et al., 2009; Aberg, 2012; Broux et al., 2012), and the functional consequence of this loss of CD28 is unknown.