IL-21 treatment of CLL cells enhances direct cytotoxicity mediated by antibodies and fludarabine. (A) IL-21 mediates tyrosine phosphorylation of STAT1^Y701 and STAT3^Y705 in C19⁺ B-CLL cells. CD19⁺ B-CLL cells were stimulated with media or IL-21 (100 ng/mL) for 24 hours. Cells were lysed in appropriate buffers and analyzed by Western blotting using mAb specific for indicated STAT proteins and GAPDH. Shown are the results of a representative responding and nonresponding patient. (B) IL-21 enhances additively antibody and fludarabine-mediated cytotoxicity. CD19⁺ B-CLL cells were treated with media or IL-21 at 100 ng/mL concentration for 18 hours followed by incubation with cross-linker alone (media), trastuzumab with cross-linker (Trastuzumab), rituximab with cross-linker (Rituximab), alemtuzumab with cross-linker (Alemtuzumab), and fludarabine. At 48 hours, direct cytotoxicity by antibody exposure and fludarabine was analyzed by annexin V/PI staining. Shown here are the results from 7 consecutive experiments. Error bars are SD among samples. (C) IL-21 enhances fludarabine-induced cytotoxicity. Dose kinetics. CD19⁺ B-CLL cells were treated with IL-21 at 100 ng/mL concentration of IL-21 or media alone for 18 hours, and the pretreated cells were incubated with fludarabine at 0.1, 1, 2.5, and 5 μM concentration. At 48 hours, direct cytotoxicity by fludarabine was analyzed by annexin V/PI staining. Shown here are the results from 7 consecutive experiments. Error bars are SD between samples.