A region on the human thyrotropin receptor which can induce antibodies that inhibit thyrotropin-mediated activation of in vitro thyroid cell function also contains a highly immunogenic epitope.

Autoantibodies to the thyrotropin receptor (TSHr) bind to the extracellular domain of the TSHr (ETSHr) and either stimulate or inhibit thyroid cell function and/or growth. In order to investigate the regulation and the specificity of the immune response to the TSHr, our laboratory recently produced recombinant human ETSHr protein by using the baculovirus expression system. In the present study, we used the recombinant ETSHr protein, a panel of overlapping synthetic peptides derived from the TSHr, and polyclonal rabbit antibodies produced against recombinant ETSHr and synthetic peptides to define a highly immunogenic region (aa 352-388) of the TSHr. Moreover, we used competitive inhibition studies to identify a dominant epitope (aa 367-372) within this region to which ETSHr antibodies react. This immunodominant epitope lies within a region unique to the TSHr when compared to the other glycoprotein hormone receptors. These data, together with the earlier observation that antibodies against aa region 357-372 can inhibit thyrotropin (TSH)-mediated activation of thyroid cells in culture, show that aa 367-372 represents, an immunodominant epitope within a functionally important region which is unique to the TSHr. Therefore, this region may play an important role in the induction or modulation of the specific immune response against the TSHr.