UNIQUE AUTOANTIBODY EPITOPES IN AN IMMUNODOMINANT REGION OF THYROID PEROXIDASE

To define the autoantibody epitopes in amino acids 513-633 of thyroid peroxidase (TPO), a region frequently recognized in thyroiditis, cDNA sequences coding for peptide fragments of this region were amplified a nd ligated into pMalcRI and pGEX vectors for expression as recombinant fusion proteins, Western blots and enzyme-linked immunosorbent assay were then used to examine the reactivity in sera from 45 Hashimoto's a nd 47 Graves' disease patients. Two autoantibody epitopes within TPO a mino acids 589-633 were identified; 16 of 35 patients reactive to TPO5 13-633 recognized the epitope of TPO592-613, while 6 patients recogniz ed the epitope of TPO607-633. Eleven other patients with thyroiditis a nd two with Graves' disease recognized only the whole 589-633 fragment , and this response accounted for the Hashimoto's disease specificity. An amino acid sequence comparison of TPO592-613 with analogous region s of other peroxidase enzymes revealed significant differences in this area, and the substitution of even a single amino acid in one of the epitopes markedly decreased the binding affinity of autoantibodies. Ad ditionally, the exclusive recognition by patients of only one of the e pitopes within this region suggests a genetic restriction of the autoa ntibody response.