CD4 and infection: Although we did not find more GP66-specific CD4 T cells (Fig. 2A), GITRL tg mice did develop a rapid response of CD25+ FoxP3- and T-bet+ CD4+ T cells early after infection and displayed a strong increase in CD4+ T cells expressing CD40L and producing IL-2 and IFNγ upon restimulation with viral peptide (Fig. 3).