CD4⁺ T-cell expansion and in vivo activation are heightened in Ctx⁻ mice after LCMV infection. (A) LCMV-specific CD4⁺ T cells were quantitated in WT and prf^−/− mice after LCMV infection by MHC-peptide tetramer (IAb-Gp61) staining of spleen cells. CD4⁺/CD16⁻ cells from uninfected and day 8 LCMV-infected WT or prf^−/− mice are shown in the example dot plots, with the percentage of IAb-Gp61⁺ of CD4⁺ cells listed. *P < .001. (B) Splenic CD4⁺ T cells spontaneously producing IFN-γ in vivo were quantitated 8 days after LCMV infection in WT and prf^−/− mice, as described in Figure 3 (n = 8/group). *P < .01.