Monoclonal antibodies and synthetic peptides define the active site of Fc(ε)RI and a potential receptor antagonist

Defining the structure of the human high-affinity receptor for IgE, Fc(ε)RI, is crucial to understand the receptor:ligand interaction, and to develop drugs to prevent IgE-dependent allergic diseases. To this end, a series of four anti-Fc(ε)RI monoclonal antibodies (mAbs), including three new mAbs, 47, 54, and 3B4, were used in conjunction with synthetic Fc(ε)RI peptides to define functional regions of the Fc IgE-binding site and identify an antagonist of IgE binding. The spatial orientation of the epitopes detected by these antibodies and their relationship to the IgE-binding region of Fc(ε)RI was defined by a homology model based on the closely related Fc(γ)RIIa. Using recombinant soluble Fc(ε)RI-α as well as Fc(ε)RI-α expressed on the cell surface, a series of direct and competitive binding experiments indicated that the mAbs detected nonoverlapping epitopes. One antibody (15-1), previously thought to be located close to the IgE-binding site, was precisely mapped to a single loop within the IgE-binding site by both mutagenesis and overlapping synthetic peptides encompassing the entire extracellular domain. A synthetic peptide (ε)RI-11, containing the amino acids 101-120 and the mAb 15-1 epitope, inhibited IgE binding and may form the basis for the development of a useful receptor-based therapy.