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Delineation of the Discontinuous-Conformational Epitope of a Monoclonal Antibody Displaying Full in Vitro and in Vivo Thyrotropin Activity

Institut de Recherche Interdisciplinaire en Biologie Humaine et Moléculaire (S.C., J.V.D., G.V.), Faculté de Médecine, University of Brussels, B-1070 Brussels, Belgium; Institute of Endocrine Sciences (M.B.), University of Milan, Istituto Auxologico Italiano Istituto di Ricovero e Cura a Carattere Scientifico (IRCCS) and Ospedale Maggiore di Milano IRCCS, 20122 Milan, Italy; Research Department (N.G.M.), B.R.A.H.M.S. AG, Biotechnology Center Hennigsdorf, 16761 Berlin, Germany; Biochemie-Zentrum-Heidelberg (V.P.), University of Heidelberg, 69120 Heidelberg, Germany; Department of Medicine and Pediatrics (S.R.), University of Chicago, Chicago, Illinois 60637; and Department of Genetics (G.V.), Erasme Hospital, University of Brussels, B-1070 Brussels, Belgium

Address all correspondence and requests for reprints to: G. Vassart, Institut de Recherche Interdisciplinaire en Biologie Humaine et Moléculaire, ULB, 808 Lennik Street, B-1070 Brussels, Belgium. E-mail: gvassart{at}ulb.ac.be.

An experimental murine model of Graves’ disease was used to produce monoclonal antibodies (mAbs) with thyroid stimulating activity. Two of these, IRI-SAb2 and IRI-SAb3, showed particularly high potency (in the low nanomolar range) and efficacy. IRI-SAb2 behaved as a full agonist of the human TSH receptor (TSHr), even when tested in physiological salt concentrations. Both IRI-SAb2 and IRI-SAb3 were displaced from the TSHr by autoantibodies from patients with Graves’ disease or harboring thyroid-blocking antibodies, but not from control subjects or patients with Hashimoto thyroiditis. The epitopes of IRI-SAb2 and IRI-SAb3 were precisely mapped, at the amino acid level, to the amino-terminal portion of the concave portion of the horseshoe structure of TSHr ectodomain. They overlap closely with each other and, surprisingly, with the epitope of a mAb with blocking activity. When injected iv in mice, both mAbs caused biological and histological signs of hyperthyroidism. Unexpectedly, they also triggered an inflammatory response in the thyroid glands. Delineation of the conformational epitopes of these stimulating antibodies opens the way to the identification of the molecular mechanisms implicated in the activation of the TSHr.

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