IL-21 induces IgE secretion by human naive cord blood B cells and synergizes with IL-4 to induce high-level production of IgE. (A-D) CB B cells were cultured at (A-C) 10 × 10³ cells/200 μL per well; (D) 5 × 10² to 50 × 10³/well with (A,B) CD40L alone (■) or with IL-4 (100 U/mL; ▴), IL-21 (50 ng/mL; ▾), or both IL-4 and IL-21 (●). (C) CD40L and increasing concentrations of IL-21 (0-50 ng/mL) only (■) or together with 100 U/mL IL-4 (▴). Culture supernatants were collected after 10 to 14 days (A,C,D) or at the indicated times (B) and the amount of secreted IgE determined by Igϵ heavy chain-specific enzyme-linked immunosorbent assay. The results in panel A represent the mean plus or minus SEM of data from 8 independent experiments; those results in panels B, C, and D are representative of multiple experiments performed using B cells from different CB donors. (E,F) CB B cells were labeled with CFSE and then cultured with CD40L alone (■) or together with IL-4 (▴), IL-21 (▾), or IL-4 plus IL-21 (●). After 5 days, the cells were resorted into different division intervals based on dilution of CFSE (E; representing the division profile of cells stimulated with CD40L/IL-21), and equal numbers of cells then recultured for a further 2 days. After this time, secretion of (F) IgE was determined.