MX1 and infection: Mx1GFP × tgMx1CreRosa26lsl-tdT mice were infected with L.m.-gp66 and assessed for history of IFN signaling (i.e., Mx1 expression) in antigen-specific CD4+ T cells identified using LLO:I-Ab or gp66:I-Ab tetramers on day 7 after infection (Fig. 3 j).