MC-derived IL-6 and CD40-CD40L interactions are involved in B-cell proliferation up-regulation. Naive (B) or activated B cells (Bact) were labeled with CFSE and cocultured as indicated. Proliferation index of naive B (A) and activated B cells (B) cultured alone, with nonsensitized MCs (MC), with activated MCs (MC/IgE-Ag), and with activated IL-6^−/− MCs and in the presence of anti–IL-6R or isotype control antibody (Ctrl) were analyzed by flow cytometry. Proliferation indexes are from 3 independent experiments with naive B cells and activated B cells, respectively. Proliferation indexes from naive B and activated B cells alone were fixed to 1. *P < .05. Date are the average ± SD (C) The mean fluorescence intensity of CD40 expression on naive B cells (B) and on activated B cells (Bact) at baseline and in coculture with nonsensitized MCs (MC) or activated MCs (MC/IgE-Ag) is shown. (D) Proliferation indexes of activated B cells alone, cocultured with activated MCs without and with blocking CD40L Ab or control Ab, are calculated. Proliferation index of activated B cells alone was fixed to 1. Results are from 3 independent experiments using at least 3 different cultures of MCs. *P < .05; ** P < .02.