Frequencies and numbers of IE1-specific CD8+ T cells in the spleens and livers of mice lacking IFN-γ or Prf. BALB/c mice (wild type) or BALB/c mice lacking either IFN-γ (IFN-γ−/−) or Prf (Prf−/−) were infected with 102 PFU mCMV. (A) Frequencies of IE1-specific CD8+ T cells at days 6, 10, and 18 after infection were determined by FACS staining with TCRβ-APC, CD8α-APC-Cy7, and H2-Ld IE tetramer-PE. Numbers in the density plots indicate the mean frequency of IE1+ cells in the CD8 T-cell pool. (B) Numbers of IE1-specific CD8+ T cells in the spleen and liver at days 6, 10, and 18 after infection; numbers represent the means ± SE of six mice from two independent experiments for all times, except for day 6. *, P < 0.02; **, P < 0.005; ***, P < 0.0005.