Groups of C57BL/6 wild type or CD28/CTLA-4-deficient (CD28/CTLA-4−/−) mice were infected i.p with VACV-WR (2 × 105 PFU/mouse). Seven days post-infection splenocytes were harvested and stained for CD8, CD44, B8R-tetramer. Representative plots of tetramer staining, gating on CD8 cells. Percentages of activated B8R-tetramer positive CD8 T cells (CD8+CD44+B8R+) are indicated. (b) At day 7, splenocytes were stimulated with VACV peptides as indicated and CD8 T cell priming assessed by intracellular IFN-γ staining. Representative plots of IFN-γ staining in gated CD8 T cells. Percent positive indicated. Results are mean number ± SEM (n=4 mice/group) from one experiment. *, p < 0.05 (wt mice vs knockout) as determined by Student’s t test. Similar results were obtained in 2 separate experiments.