Combination therapy results in expansion of CD8 T cells in tumor-bearing animals. Tumor-bearing (10⁶ A31 cells, intravenously, day 0) CBA mice were treated on day 10 with TBI and anti-CD40 mAb (1 mg intravenously) according to the standard protocol. Naive (nontumor-bearing) mice were treated at the equivalent time point. Animals were killed 5 days after treatment, spleens removed, and the presence of tumor and CD8 T cells determined by 2-color flow cytometry using FITC-labeled anti-Id versus PE-labeled anti-CD19 and using FITC-labeled anti-CD3 versus PE-labeled anti-CD8, respectively (A). The actual number of CD8 T cells present in the spleen following each treatment was then calculated (B). Shown is the mean for triplicate animals. Error bars represent SEM. Similar results were obtained in at least 3 separate experiments. A 10-fold expansion of CD8 T cells in tumor-bearing mice over naive animals was observed following combination therapy with 5 Gy TBI. This expansion did not occur at higher (8 Gy) or lower doses (2 Gy; data not shown).