Surface IL-21R expression on resting and CD40-activated CLL B cells. Purified CLL B cells, either resting or stimulated by coculture with CD40L-transfected murine fibroblasts, were stained with PE-labeled anti-IL-21R or with an irrelevant PE-labeled Ig as control. (A) The percentage of IL-21R⁺ resting or CD40-activated B cells from all 20 patients with CLL. The cut-off value, arbitrarely defined as 15%, is indicated by a horizontal line. (B) Time-dependent induction of IL-21R surface expression in 2 representative patients.