Epitope-dependent synergism and antagonism between CD40 antibodies and soluble CD40 ligand for the regulation of CD23 expression and IgE synthesis inhuman B cells

Background: The induction of IgE synthesis in naive B cells requires two T- cell-derived signals: one delivered through CD40 and the other via interleu kin-4 (IL-4). The natural counterstructure to CD40 is the CD40 ligand (CD40 L). We have asked about the interplay between CD40L and CD40 mAb that recog nize distinct epitopes in delivering signals for regulating IL-4-dependent IgE synthesis and the expression of CD23, the low-affinity IgE receptor, in resting B cells. Methods: After culture of purified human tonsillar B cells with CD40 agonis ts and IL-4, surface CD23 was determined by flow cytometric analysis. CD23 levels in cell lysates and supernatants were quantified by ELISA, as were t hose of secreted IgE. Results: With regard to both induction of CD23 and IgE production, soluble CD40L trimer (sCD40LT) showed synergistic interaction with two mAb to CD40 which bind to epitopes located outside the ligand binding site (EA5 and 5C3 ), but not with a mAb (G28-5) which effectively competes for CD40L binding to CD40. Each of the two noncompeting mAb to CD40 was able to cooperate str ongly with sCD40LT in promoting high-level induction of CD23 even in the ab sence of IL-4, an effect mirrored in the promotion of strong homotypic clus tering and high-rate DNA synthesis. G28-5, uniquely, induced a downregulati on in IL-4-induced CD23 expression with time, a change that was accompanied by an increase in the amount of soluble CD23 detected. While the two nonco mpeting mAb consistently synergized with sCD40LT for the promotion of IL-4- dependent IgE synthesis, sCD40LT and G28-5 (which, by itself, was the most potent of the CD40 mAb at inducing IL-4-dependent IgE production) exhibited mutual antagonism in this regard, the level of which could be quite profou nd. Conclusions: This study demonstrates that appropriate targeting of CD40 can modulate IgE synthesis either positively or negatively.