Irradiated CD4⁺ T cells can provide help for effector CD8⁺ T cells. (A) Mice bearing 10-day pulmonary metastases were conditioned with TBI then received live CD4⁺ T cells (10⁷), CD 8⁺ T cells (2.5 × 10⁶), irradiated CD4⁺ T cells (10⁷), the combination of CD8⁺ (2.5 × 10⁶) and irradiated CD4⁺ T cells (10⁷), or CD8⁺ T cells (2.5 × 10⁶) that were culture activated in vitro in the presence of irradiated CD4⁺ T cells. Mice treated with live CD4⁺ T cells or the combination of irradiated CD4⁺ combined with CD8⁺ T cells each had superior response (P < .01) compared with untreated, CD8⁺ T cells alone, irradiated CD4⁺ T cells, or CD8⁺ cocultured with irradiated CD4⁺ T cells. (B) Serum was collected from 2 mice from each of the indicated groups 24 hours after T-cell transfer and ELISA for IFN-γ was performed.