Crystal structure of IgE bound to its B-cell receptor CD23 reveals a mechanism of reciprocal allosteric inhibition with high affinity receptor FcεRI

Abstract

The role of IgE in allergic disease mechanisms is performed principally through its interactions with two receptors, FcεRI on mast cells and basophils, and CD23 (FcεRII) on B cells. The former mediates allergic hypersensitivity, the latter regulates IgE levels, and both receptors, also expressed on antigen-presenting cells, contribute to allergen uptake and presentation to the immune system. We have solved the crystal structure of the soluble lectin-like "head" domain of CD23 (derCD23) bound to a subfragment of IgE-Fc consisting of the dimer of Cε3 and Cε4 domains (Fcε3-4). One CD23 head binds to each heavy chain at the interface between the two domains, explaining the known 2:1 stoichiometry and suggesting mechanisms for cross-linking membrane-bound trimeric CD23 by IgE, or membrane IgE by soluble trimeric forms of CD23, both of which may contribute to the regulation of IgE synthesis by B cells. The two symmetrically located binding sites are distant from the single FcεRI binding site, which lies at the opposite ends of the Cε3 domains. Structural comparisons with both free IgE-Fc and its FcεRI complex reveal not only that the conformational changes in IgE-Fc required for CD23 binding are incompatible with FcεRI binding, but also that the converse is true. The two binding sites are allosterically linked. We demonstrate experimentally the reciprocal inhibition of CD23 and FcεRI binding in solution and suggest that the mutual exclusion of receptor binding allows IgE to function independently through its two receptors.

Fig. 1.

Structure of the derCD23–Fcε3-4 complex. The two molecules of derCD23 (light and dark blue Cα traces with surfaces) bind one to each heavy chain between the Cε3 (dark red and green) and Cε4 domains (light red and green). The carbohydrate is shown in all-atom representation (red and yellow, without surfaces) and can be seen behind the (red) Cε3 domain. The adjacent N and C termini of each derCD23 molecule, the former being the connection to the “stalk,” the latter to the “tail” region, can be seen at the extreme left and right of the figure. [The complex shown here comprises chains A (red), B (green), G (light blue), and H (dark blue)].

Fig. 2.

Salt bridges and hydrogen bonds at the derCD23–Fcε3-4 interface. The H-bonds associated with the four salt bridges are shown in red, additional H-bonds present in all six independent interactions are shown in green, and a further H-bond present in five of six molecules is shown in yellow.

Fig. 3.

Composite images of the derCD23 and sFcεRIα complexes with Fcε3-4 to show the mutual incompatibility of their binding modes. (A) derCD23–Fcε3-4 and sFcεRIα–Fcε3-4 (Protein Data Bank ID code 1F6A) complexes superposed on their (Cε4)₂ domain pairs. The receptors are shown as surfaces (derCD23, light and dark blue; sFcεRIα, red), and the Fcε3-4 structures are shown as Cα traces (in corresponding colors). The closed (derCD23-binding) and open (sFcεRIα-binding) conformations of the Cε3 domains may be seen. (B) Steric clashes between the sFcεRIα structure (red Cα trace) and both chains of the derCD23-Fcε3-4 complex (blue) are indicated (orange surfaces). (C) Steric clashes of both derCD23 molecules (blue Cα traces) with the sFcεRIα–Fcε3-4 complex (red) are indicated (green surfaces).

Fig. 4.

sFcεRIα displaces derCD23 and vice versa in a FRET inhibition assay. (A) Competition of derCD23 binding to IgE-Fc (blue) and Fcε3-4 (red) by unlabeled sFcεRIα. (B) Competition of sFcεRIα binding to IgE-Fc (blue) and Fcε3-4 (red) by unlabeled derCD23. All data: n = 3; error bars ± SEM.