All mice were maintained in specific pathogen-free barrier facilities and used according to protocols approved by the Immune Disease Institute and the Harvard Medical School Animal Care and Use Committees. For stimulation, purified CD8+ T cells were cultured at 106 cells/ml (10 ml) in T25 flasks coated with 1 μg/ml each of anti-CD3 (clone 2C11) and anti-CD28 (clone 37.51) by pretreatment with 300 μg/ml goat anti–hamster IgG.