IL-21 induces apoptosis of CLL B cells, as detected by annexin V/PI 2-color fluorescence or by analysis of mitochondrial depolarization using DiOC6 staining. IL-21 (40 ng/mL) induces a time-dependent apoptosis of a CD40-activated (72 hours) CLL B-cell population from a representative patient (no. 13) evaluated as 2-color fluorescence analysis by PI and annexin V staining (A) or as mitochondrial depolarization by DiOC6 staining (B). (C) Summary of IL-21-induced apoptosis in either resting or CD40-activated CLL B cells studied is shown. Data are expressed as increments of apoptotic-cell percentages (by PI and annexin V staining) over spontaneous apoptotic rates using a 40-ng/mL concentration of rhIL-21 and a 72-hour stimulation. (D) CLL B cells from 4 representative patients were cocultured with CD40L-transfected cells for different time intervals before treatment with IL-21 (40 ng/mL for 72 hours) and evaluation of apoptosis as in panel A. (E) Induction of apoptosis by IL-21 is dose dependent at concentrations ranging from 20 to 80 ng/mL. Data are expressed as in panel C.