IgE class switching is critically dependent upon the nature of the B cell activator, in addition to the presence of IL-4.

Cross-linkage of membrane IgD on resting murine B cells, by anti-IgD mAb conjugated to dextran (alpha delta-dex), induces high levels of proliferation, and in the presence of IL-2 or IL-5, Ig secretion in vitro. The structural and functional similarities between alpha delta-dex and TNP-Ficoll for B cell responses led us to propose that alpha delta-dex could provide a model system for studying B cell activation induced by T cell-independent, type II Ag. In this report, we study the effects of Ig class switch and differentiation factors on Ig isotype production by murine B cells activated by alpha delta-dex, and directly compare these to responses obtained after activation by LPS. We show that an IL-4-containing CD4+ T cell supernatant (Th2 SN) stimulates large increases in IgG1 and IgE production by LPS-activated B cells, but fails to stimulate detectable levels of IgE by alpha delta-dex-activated cells, despite inducing high levels of secreted IgM and IgG1. This is correlated with undetectable steady state levels of both germ-line and rearranged (productive) IgE-specific RNA in B cells stimulated with alpha delta-dex + Th2 SN. Alpha delta-dex is selective in its failure to costimulate IgE production in that IFN-gamma-containing T cell supernatant (Th1 SN) and transforming growth factor-beta-supplemented Th2 SN selectively stimulate a large IgG2a and IgA secretory response, respectively. Anti-IgD conjugated to Sepharose beads, in distinct contrast to dextran, costimulates a strong IgE response. These findings underscore the importance of the specific B cell activator, in addition to IL-4, in the regulation of IgE production.