Multivalent, but not divalent, antigen receptor cross-linkers synergize with CD40 ligand for induction of Ig synthesis and class switching in normal murine B cells. A redefinition of the TI-2 vs T cell-dependent antigen dichotomy.

A number of previous studies have suggested that cross-linkage of the B cell Ag receptor may be critical for induction of humoral immune responses to T cell-dependent (TD) Ags in vivo. Previous work also indicated a critical role, in these responses, for CD40-mediated signaling mediated by binding of the inducible T cell membrane protein, CD40 ligand (CD40L). Data in this manuscript demonstrate that concentrations of bivalent anti-IgD or anti-IgM Ab as high as 30 micrograms/ml induced little if any enhancement of CD40-dependent Ig secretion by resting murine B cells. In contrast, concentrations as low as 3 pg/ml of multivalent, dextran-conjugated, anti-IgD (alpha delta-dex) or anti-IgM (alpha mu-dex) were strongly synergistic with CD40L for induction of B cell proliferation, viable cell outgrowth, Ig isotype switching, and maturation to Ig secretion. As many as 30% of the B cells became membrane IgG1+ after stimulation with CD40L, anti-Ig-dextran, and IL-4 + IL-5, with a concomitant three- to fivefold increase in numbers of viable cells as compared with control cultures. High Ig secretory responses were obtained in response to the combined actions of CD40L and alpha delta-dex or alpha mu-dex, utilizing concentrations of B cell activator that when acting alone induced only modest Ig secretion. Surprisingly, although we previously demonstrated that alpha delta-dex selectively and strongly suppressed IgE production by T cell-activated B cells, it strikingly augmented IgE expression by CD40L-activated B cells. These data suggest 1) a key role for Ag receptor cross-linkage in CD40-dependent induction of humoral immune responses, 2) that to achieve a membrane Ig-dependent enhancing effect in the presence of activated T cells, TD Ags must be displayed to the B cell as a multivalent array of epitopes, 3) that picomolar concentrations of Ag can mediate this effect, and 4) that at least for induction of IgE responses, B cell stimulation via CD40L or via activated T cells may lead to a qualitatively different pathway of activation.