IgE-binding factors: their possible role in the regulation of IgE synthesis.

B cell-derived IgE-BFs (sCD23) are cleavage fragments of surface Fc epsilon R II. Their production is increased by IL4 and suppressed by IFN-gamma and IFN-alpha. IgE-BFs are likely to play a role in the regulation of human IgE synthesis as shown by the following two observations: i. MabER specifically blocks both the spontaneous IgE by synthesis by atopic B cells and the IL4-induced IgE synthesis by normal lymphocytes, ii. purified IgE-BFs enhance the IL4-induced and the spontaneous IgE synthesis. Soluble fragments of Fc epsilon R II also display BCGF-like activity although the exact structure of these fragments is not yet identified. The cDNA coding for Fc epsilon R II has been cloned and functionally expressed. The predicted amino acid sequence reveals no homology between human and rodent IgE-BFs indicating that they are unrelated molecules.