Effect of in vivo treatment with STI-571 on the functional responses of antigen-specific CD4⁺ T cells to immunization. BALB/c mice were given STI-571 (12.5 mg/kg per day) or vehicle alone intraperitoneally for 10 days. On day 10, all the animals received 2.5 × 10⁶ anti-HA TCR⁺ transgenic CD4⁺ T cells intravenously. Then 9 days later, half of the mice in each group were immunized subcutaneously with 1 × 10⁷ pfu vacc-HA. Animals were killed 6 days later, and T cells were purified from their spleens as indicated in “Materials and methods.” T cells were then cultured with media alone or HA-peptide (12.5 μg/mL) plus fresh splenocytes for 48 hours. Supernatants were collected and assayed for IL-2 (A) and IFN-gamma production (B) by ELISA. Data represent mean ± SE of triplicate cultures from 3 mice in each group and are representative of 3 independent experiments with similar results. Values for T cells cultured without HA-peptide were below the limit of detection (NS indicates not statistically significant)