IL-21R Y510 mediates maximal proliferative response to IL-21. (A) Ba/F3 cells not transfected or stably transfected with the indicated constructs were treated with 0 to 100 ng/mL IL-21 for 3 days, and thymidine incorporation was determined. A representative experiment of 3 performed is shown. (B-C) Ba/F3 cells stably (B) or transiently (C) expressing IL-21R constructs were stained with DDAO-SE and cultured for 2 days in medium containing 10 or 100 ng/mL IL-21 or 1 ng/mL IL-3. Percent DDAO-SE dilution is indicated. Ba/F3 indicates parental Ba/F3 cells; pRV, Ba/F3 cells transiently transfected with empty vector. (D) IL-21R–deficient splenic B cells transiently transduced with the indicated constructs were stained with DDAO-SE and cultured for 4 days in medium containing 1 μg/mL anti-CD40 with or without 100 ng/mL IL-21 and analyzed. Percent DDAO-SE dilution is indicated. Statistical analyses are comparisons to parental Ba/F3 cells (B), Ba/F3 cells transduced with pRV (C), and IL-21R–deficient splenic B cells transduced with pRV (D).