FIGURE 5.

CD28 costimulation is required both during the priming and recall phase of VV-specific memory CD8⁺ T cells to maximize the recall response. A, CD28 costimulation during the primary response is required for an optimal VV-specific recall response. In brief, 1.5 × 10⁶ purified CD8⁺ T cells from spleens of VV-WR-infected B6 or CD80/CD86^−/− mice (number of B8R-specific memory CD8⁺ T cells determined by tetramer staining) were transferred into naive B6-CD45.1⁺ mice. One day posttransfer, mice were challenged with 2 × 10⁶ PFU of VV-WR i.p. and expansion of B8R-specific CD8⁺ T cells in the spleen was enumerated by tetramer staining 5 days later. B, CD28 costimulation during challenge maximizes the recall response. In brief, 4 × 10⁶ purified CD8⁺ T cells from spleens of VV-WR-infected B6-CD45.1⁺ mice (number of B8R-specific memory CD8⁺ T cells determined by tetramer staining) were transferred into naive B6 or CD80/CD86^−/− mice. One day posttransfer, mice were challenged with 2 × 10⁶ PFU of VV-WR i.p. and expansion of B8R-specific CD8⁺ T cells in the spleen was enumerated by tetramer staining 5 days later. Experimental schemes are shown above, and representative data from two individual experiments are shown. ○, B6→B6; ●, CD80/CD86^−/− →B6 (A) or B6→CD80/CD86^−/− (B). Each set of connected circles represent individual mice, and numbers indicate average fold expansion of transferred memory cells after 5 days in each group, calculated as written in Materials and Methods. *, p < 0.05.