Accelerated Disassembly of IgE:Receptor Complexes by a Disruptive Macromolecular Inhibitor

IgE antibodies bind the high affinity IgE Fc receptor (FcεRI), found primarily on mast cells and basophils, and trigger inflammatory cascades of the allergic response(1,2). Inhibitors of IgE:FcεRI binding have been identified and an anti-IgE therapeutic antibody (omalizumab) is used to treat severe allergic asthma(3,4). However, preformed IgE:FcεRI complexes that prime cells prior to allergen exposure dissociate extremely slowly(5) and cannot be disrupted by strictly competitive inhibitors. IgE-Fc conformational flexibility indicated that inhibition could be mediated by allosteric or other non-classical mechanisms(6–8). Here we demonstrate that an engineered protein inhibitor, DARPin E2_79(9–11), acts through a non-classical inhibition mechanism, not only blocking IgE:FcεRI interactions, but actively stimulating the dissociation of preformed ligand-receptor complexes. The structure of the E2_79:IgE-Fc(3-4) complex predicts the presence of two non-equivalent E2_79 sites in the asymmetric IgE:FcεRI complex, with Site 1 distant from the receptor and Site 2 exhibiting partial steric overlap. While the structure is suggestive of an allosteric inhibition mechanism, mutational studies and quantitative kinetic modeling indicate that E2_79 acts through a facilitated dissociation mechanism at Site 2 alone. These results demonstrate that high affinity IgE:FcεRI complexes can be actively dissociated to block the allergic response and suggest that protein:protein complexes may be more generally amenable to active disruption by macromolecular inhibitors.