Rabbit antibodies toward extracellular loops of the membrane spanning region of human thyrotropin receptor possess thyroid stimulating activities.

We have synthesized three different peptides, E1 (amino acid residues 478-497), E2 (amino acid residues 561-580) and E3 (amino acid residues 649-652), corresponding to the first, the second and the third extracellular loops of the membrane spanning region of human thyrotropin receptor (TSH-R), respectively. We have produced rabbit antibodies toward these peptides and evaluated their thyroid stimulating antibody (TSAb) and TSH-binding inhibitor immunoglobulin (TBII) activities. Although only slight TSAb activity was observed in E1 antibodies, E2 and E3 antibodies possessed strong TSAb activities, the values of which were 1118% and 910%, respectively. None of these antibody had TBII activities. These results suggest that antibodies against the extracellular loops of the TSH-R can stimulate cAMP formation in thyroid cells and that these regions may be one of the candidates for the epitope against autoantibodies from patients with Graves' disease.