(a) WT, B7.1−/−, and B7.2−/− 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. As indicated, groups of WT and B7.2−/− mice were treated with agonistic CD40 mAb (αCD40) at the time of VACV infection and were injected i.p on days 0, 1, 2, and 3, with 150 µg of either control IgG or blocking anti-B7.1 in PBS. Top: Representative plots of tetramer staining, gating on CD8+ cells. Percentages of activated B8R-tetramer positive CD8 T cells (CD8+CD44+B8R+) are indicated. Bottom: Total numbers of B8R-tetramer positive CD8+CD44+ T cells per spleen are shown. Results are mean number ± SEM (n=4 mice/group) from one experiment. Statistical significance was determined by Student’s t test. Similar results were obtained in 2 separate experiments.