CD4 and infection: Notably, splenic Ag-experienced CD4+ T cells purified on day 60 postinfection also produced large amounts of IL-2 very rapidly upon PMA/ionomycin restimulation (4 h) (Fig. 4F), most likely facilitating their rapid expansion during the early phases of secondary infection, as shown specifically for the CD4+YFP+GFP− subset (Fig. 4A).