The rest of the authors declare that they have no conflicts of interest

The rest of the authors declare that they have no conflicts of interest.. the CD23 surface, both binding and inhibition assays and bad stain electron microscopy were performed. Results A hitherto unfamiliar IgE-binding site was mapped within the stalk region of CD23, and the nonCN-glycosylated monomeric version NP118809 of CD23 was superior in IgE binding compared with glycosylated CD23. Furthermore, we shown that a restorative anti-IgE antibody, omalizumab, which inhibits IgE binding to FcRI, also inhibited IgE binding to NP118809 CD23. Conclusion Our results provide a fresh model for the CD23-IgE connection. We show the stalk region of CD23 is usually crucially involved in IgE binding and that the interaction can be blocked by the therapeutic anti-IgE antibody omalizumab. Keywords: CD23, allergy, IgE, low-affinity IgE receptor, B cell, allergen IgE-associated allergy is the most frequent immunologically mediated hypersensitivity disease, affecting more than 25% of the population worldwide.1 IgE is the least abundant immunoglobulin class and therefore exerts its pathologic effects mainly through interaction with 2 cellular receptors: the high-affinity receptor FcRI and the low-affinity receptor CD23 (FcRII).2 The molecular and structural interaction between IgE and FcRI has been investigated in great detail3 and represents an important target for therapeutic strategies for allergy.4C6 The interaction between IgE and the low-affinity receptor CD23 is less well understood than that between IgE and the PIAS1 high-affinity receptor FcRI, although CD23 plays several important functions in allergic inflammation. CD23 is a key molecule in IgE-facilitated allergen presentation and subsequent activation of allergen-specific T cells,7 a mechanism that can be blocked NP118809 by allergen-specific IgG after specific immunotherapy.8 Furthermore, CD23 was suggested to be important in the regulation of IgE production,9 and there is growing evidence that CD23 mediates transport of allergens NP118809 through respiratory and gut epithelial barriers.10,11 Thus far, only parts of the 3-dimensional structure (ie, parts of the head domain name) of CD23 have been analyzed by using crystallography12 and nuclear magnetic resonance analysis,13 and results were not in full agreement. By means of site-directed mutagenesis of the head domain name and subsequent expression of the mutants in cells, evidence for involvement of the head domain name in IgE binding was provided.14 These findings were supported by structural studies that have analyzed the interaction between an incomplete recombinant CD23 head domain name expressed in and a recombinant C3-C4Ccomprising subfragment of the Fc a part of human IgE.15 One study observed enhanced IgE binding of CD23 variants containing the stalk region compared with CD23 variants representing only the head domain name,16 and another study observed enhanced IgE binding of a CD23 variant containing a single amino acid exchange in the stalk region.17 Both attributed the enhanced IgE binding to the fact that this stalk region can contribute to IgE binding through accurate formation of CD23 trimers because the current assumption is that CD23 occurs in trimeric form around the cell surface. In this study we expressed 4 monomeric folded CD23 variants in insect cells, the first comprising the full extracellular portion of CD23, including the stalk and head domain name; the second identical with the first, apart from a NP118809 single amino acid exchange in the stalk region abolishing the only N-linked glycosylation site of CD23; a third representing the full head domain name; and a fourth consisting of a truncated head domain name. We were able to directly map hitherto unknown IgE-binding sites around the stalk region of CD23 and could show that a nonCN-glycosylated monomeric version of CD23 bound IgE much stronger than glycosylated CD23. Furthermore, we demonstrate that omalizumab, a therapeutic antibody.