In vivo clearance of tumor. Mice were inoculated with 10⁶ A31 lymphoma cells intravenously on day 0 and treated on day 10 with EBRT (5Gy), anti-Id mAb (100 μg intravenously), or both together. Animals were culled 5 days later, spleens were removed and homogenized, and the relative number of tumor, CD8⁺, or CD4⁺ T cells were calculated. (A) Shown is the percentage of tumor cells present in the spleen of a representative mouse following each treatment, as measured by flow cytometry using PE-labeled anti-CD19 versus FITC-labeled anti-Id on a FACS Calibur. (B) Histogram illustrating the total number of tumor (Id⁺) cells, CD8⁺ T cells, and CD4⁺ T cells present in the spleens of mice following each treatment. The average number of cells found in each group are shown, with error bars indicating the SEM (n = 3). Data represent 1 of 3 independent experiments. Combination therapy results in a significantly increased depletion of tumor cells compared with controls (P < .01) or single-modality treated (P < .01) cohorts. However, CD8⁺ and CD4⁺ cell numbers are also reduced following combination therapy treatment compared with single treatment groups, indicating that clearance of tumor is not dependent on an enhanced T-cell response.