Majority of thyroid peroxidase autoantibodies in patients with autoimmune thyroid disease are directed to a single TPO domain.

Murine monoclonal antibodies (mAb) produced against native human thyroid peroxidase (TPO) are powerful tools for analyzing the autoantibody (Aab) epitopes on TPO. Binding sites of thirteen mAbs cover all or most antigenic regions on TPO. We determined the competition between Aabs from 75 AITD patients and 13 mAbs in binding to TPO. Autoantibodies recognize predominantly the TPO area close or identical to mAb#9 epitope. All sera tested inhibited this mAb binding by 92.9 +/- 14.8 (mean +/- SD), range from 69-100%. AITD patients' sera with low Aabs titer up to 1/2,000 inhibited mAb#9 binding to TP0 by 85 +/- 11.5% (mean +/- SD) and did not influence remaining mAbs binding to TPO. With elevated Aab levels the inhibition of other mAbs binding was higher, but never exceeded 35%. The amount of Aabs yielding 50% inhibition of mAbs binding was lowest for mAb#9. In order to obtain this degree of inhibition for other mAbs 5 to 25 times more Aabs were needed. Our results demonstrate that the majority of autoantibodies in sera of patients with AITD recognize a single immunodominant region on the TPO mapped by mAb#9. They account for about 80-90% of serum TPO autoantibodies. The autoimmune response to other regions on TPO molecule is directed to several other epitopes, but represents quantitatively a minority of autoantibodies. This response intensifies with increasing Aabs level in the serum.