IL-21 treatment of T cells does not enhance direct cytotoxicity of fludarabine. CD3⁺ T cells from CLL patients were treated with IL-21 at 100 ng/mL concentration or media alone and at indicated concentration of fludarabine ranging from 0.08 to 20μM. At 72 hours, direct cytotoxicity by fludarabine was analyzed by annexin V/PI staining. Shown here is the result from 5 consecutive experiments. Error bars are SD between samples.