Epitope mapping of TSH receptor-blocking antibodies in Graves' disease that appear during pregnancy

Spontaneous remission of Graves' disease during pregnancy is thought to be due to a reduction of thyroid-stimulating antibody activity. We suspected, however, that a broader change in TSH receptor antibody characteristics mig ht play an important role in modulating disease activity during pregnancy. We measured TSH binding inhibitory Ig, thyroid-stimulating antibody, and th yroid stimulating-blocking antibody activities in 13 pregnant Graves' disea se patients at first, second, and third trimesters and 4 months postpartum. To measure and epitope-map thyroid-stimulating antibody and thyroid stimul ating-blocking antibody activities, we used CHO cells transfected with wild -type human TSH receptor or with several TSH receptor-LH/hCG receptor chime ras: Mc1+2, Mc2, and Mc4. These chimeric cells have their respective TSH re ceptor residues 9-165, 90-165, and 261-370 substituted with equivalent resi dues of the LH/hCG receptor. Overall thyroid-stimulating antibody decreased , whereas thyroid stimulating-blocking antibody increased progressively dur ing pregnancy. TSH binding inhibitory Ig fluctuated in individual patients, but overall the activities remained statistically un-changed. Thyroid stim ulating-blocking antibody appeared in subjects who were either negative for thyroid-stimulating antibody or whose thyroid-stimulating antibody activit y increased or decreased during pregnancy. Epitope mapping showed that the thyroid-stimulating antibodies were mainly directed against residues 9-165 of the N-terminus of the TSH receptor extracellular domain. All thyroid sti mulating-blocking antibodies had blocking activities against residues 261-3 70 of the C-terminus of the ectodomain. However, the majority of the thyroi d stimulating-blocking antibodies had a hybrid conformational epitope direc ted against N-terminal residues 9-89 or 90-165 as well. Despite a change in the activity level, we did not observe any change in the epitope of either the stimulatory or blocking Abs as pregnancy advanced. In conclusion, a ch ange in the specificity of TSH receptor antibody from stimulatory to blocki ng activity was observed during pregnancy, and the appearance of thyroid st imulating-blocking antibody may contribute to the remission of Graves' dise ase during pregnancy.