The quest for the autoantibody immunodominant region on thyroid peroxidase: guided mutagenesis based on a hypothetical three-dimensional model.

Thyroid peroxidase (TPO), a cell surface glycoprotein, is the major autoantigen in autoimmune thyroiditis in humans. The molecular cloning and expression of Ig genes from thyroid-infiltrating B cells has generated a large repertoire of human TPO Fab that have been used to map an immunodominant region on TPO. However, the topological site of this region, consisting of a cluster of highly conformational epitopes, remains unknown. Using the recently elucidated three-dimensional structure of myeloperoxidase as a model, we stably expressed on the surface of eukaryotic cells eight "guided" mutants of the TPO molecule. The sites of these mutations were strategically located to alter the surface contour of the molecule with minimal disruption to its core structure. Remarkably, in the present study (in contrast to previous unguided TPO mutagenesis studies), all eight TPO mutants retained recognition by the TPO Fab. These results support the validity of the model used for mutagenesis. Although not identifying the immunodominant region on TPO in thyroid autoimmunity, our data provide evidence against the involvement of certain topological segments and may help to narrow the search for this region. The most open region remaining as a candidate location is the antero-inferior portion of the molecule.