Radiation does not sensitize lymphoma to direct anti-CD40 cytotoxicity. (A) The πBCL₁ lymphoma cells either left untreated or treated with 5 Gy external beam irradiation were cultured for 72 hours in 24-well plates coated with mAb (5 μg/mL) with specificity as indicated. Cells were then harvested, stained with PI to detect DNA content, and analyzed by flow cytometry. Gated regions indicate the percentage of sub-G₁ fragmented DNA, indicative of apoptosis. There was a clear additive effect between irradiation and mAb to the B-cell receptor (BCR) but not between irradiation and anti-CD40 mAb. (B) Clonogenic survival of πBCL₁ treated with external beam radiation (0 to 5 Gy) in the presence of control (□) or anti-CD40 mAb (as before; ▵). While there was a clear difference in survival over the range of radiation doses, addition of anti-CD40 mAb did not significantly affect clonogenic survival.