This study was initiated to explore the contribution of endogenous cytokines to CD40-induced B cell proliferation and differentiation. First, both CD40 and Ag receptor (AgR) cross-linking induced purified tonsillar human B lymphocytes to secrete the same pattern of cytokines, including IL-1 beta, IL-6, IL-10, granulocyte-macrophage-CSF, and TNF-alpha, whereas IL-1 alpha, IL-2, IL-3, IL-4, IL-5, IL-7, granulocyte (G)-CSF, or IFN-gamma were not detected. Second, cotriggering of CD40 and AgR resulted in additive secretion of both IL-6 and IL-10. Addition of IL-4 to CD40-activated B cells increased IL-6 levels but decreased IL-10 levels. In contrast, exogenous IL-10 diminished IL-6 levels. Neutralization of IL-6 and IL-10 using blocking Abs did not alter CD40-induced B cell growth. In contrast, IL-6 neutralization markedly inhibited the IL-4-induced IgE secretion (57 +/- 10%) as well as the IgG and IgM production resulting from AgR and CD40 cotriggering (49 +/- 16.5 and 29.5 +/- 4.5%, respectively). Blocking IL-10 inhibited the IgA secretion (25 +/- 2.7%) obtained after CD40 activation and the production of IgG, IgA, and IgM (24.1 +/- 5.6, 25 +/- 8, and 42 +/- 6.5%, respectively) by B lymphocytes undergoing dual ligation of CD40 Ag and AgR. Simultaneous neutralization of both endogenous IL-6 and IL-10 resulted in an increased inhibition of Ig secretion for B cells cotriggered by CD40 Ag and AgR (65-75%). Thus, endogenously produced IL-6 and IL-10 are involved in the differentiation of CD40-activated B cell.