Structure of intact IgE and the mechanism of ligelizumab revealed by electron microscopy

Background IgE is the central antibody isotype in TH2‐biased immunity and allergic diseases. The structure of intact IgE and the impact of IgE‐targeting molecules on IgE however remain elusive. In order to obtain insights into IgE biology and the clinical impact, we aimed for structure determination of IgE and the complex of IgE with the anti‐IgE antibody ligelizumab. Methods Structures of two distinct intact IgE with specificity for cross‐reactive carbohydrate determinants and Der p 2 as well as complexes of ligelizumab‐Fab with IgE and IgE Fc were assessed by negative stain electron microscopy and solution scattering. Inhibition of IgE binding and displacement of receptor‐bound IgE were assessed using cellular assays, basophil activation testing and ELIFAB assays. Results Our data reveal that the investigated IgE molecules share an overall rigid conformation. In contrast to the IgE Fc fragment, the IgE Fc in intact IgE is significantly less asymmetrically bent. The proximal and the distal Fabs are rigidly tethered to the Fc. Binding of ligelizumab to IgE in a 2:1 stoichiometry induces an extended and twofold symmetrical conformation of IgE, which retains a rigid Fab‐Fc architecture. Analyses of effector cell activation revealed that ligelizumab inhibits IgE binding without displacing receptor‐bound IgE. Together with an interference of CD23 binding, the data underline a functional activity similar to omalizumab. Conclusions Our data reveal the first structures of intact IgE suggesting that the IgE Fab is fixed relative to the Fc. Furthermore, we provide a structural rationale for the inhibitory mechanism of ligelizumab. Electron microscopy and solution scattering reveal that the structure of entire IgE is highly rigid with a spatial organization differing from current models. The Fab arms are rigidly tethered to the Fc and lack flexibility. Ligelizumab traps IgE in an extended conformation retaining the Fab arm assembly. The localization of the epitope enables inhibition of binding to both IgE receptors.