Different modes of IgE binding to CD23 revealed with major birch allergen, Bet v 1-specific monoclonal IgE.

We investigated the binding of IgE and different types of allergen-IgE complexes to CD23-expressing human B cells. We performed the experiments using chimeric Bip 1 (CB1), a chimeric humanized IgE specific for the major birch allergen, Bet v 1, together with monomeric and oligomeric forms of recombinant Bet v 1 (rBet v 1), and Bet v 1-specific IgG antibodies. In this model IgE binding to CD23 was independent of variations in antibody affinities towards monomeric and oligomeric Bet v 1 as demonstrated by plasmon surface resonance. CB1 alone or in the form of small immune complexes consisting of one molecule of CB1 plus allergen, showed comparable binding to CD23 on B cells. Using anti-IgE antibody probes discriminating CD23-bound from CD23-unbound IgE, it is demonstrated that in large immune complexes obtained with oligomeric Bet v 1 or by super-crosslinking of small immune complexes with Bet v 1-specific IgG, anti-IgE staining of B cells increased. This increase of staining was due to the presence of IgE antibodies in the immune complexes that were not directly engaged in CD23 binding, and thus available for IgE detection. Our study thus reveals that CD23 can bind in a comparable manner to free IgE and IgE-allergen complexes of different size and composition, which may also include allergen-specific IgG. The interplay of free IgE with IgE-allergen immune complexes of different sizes and composition with CD23 binding represents a mechanism for the modulation of CD23-mediated immune responses such as IgE-facilitated allergen presentation in allergic diseases.