Protective effects of CD8+ T cells from plasmid NP DNA vaccinated mice. Balb/c mice were immunized intramuscularly with 200 μg of influenza A/PR/8/34 (H1N1) NP DNA vaccine, 2–3 weeks apart, three times, and then challenged with 1LD50 of influenza A/HK/68 (H3N2) virus. Three months later, spleen cells from survivors were stimulated with Vac-NP, Vac-gp120 or A/HK/68 virus for 5 days. Unstimulated cells from naïve mice served as negative controls. CD8+ T cells were enriched using MACS and injected interperitoneally into naïve mice (1 × 107 cells per mouse). Four hours later, they were challenged with 100 MID50 of A/HK/68 virus (in 10 μl intranasally). Lung and nasal wash virus titers were determined after 8 days. * p < 0.05 to 0.001, Vac-NP versus Vac-gp120. Values are GMT log10/ml of lung homogenate or nasal wash ± SEM. (4–6 mice per group).