DUAL MECHANISM OF PERTURBATION OF THYROTROPIN-MEDIATED ACTIVATION OF THYROID-CELLS BY ANTIBODIES TO THE THYROTROPIN RECEPTOR (TSHR) AND TSHR-DERIVED PEPTIDES

To further define the epitopes with which anti-TSH receptor (anti-TSHR ) antibodies react and mediate their biological effects, we used antib odies against the extracellular domain of TSHR (ETSHR) protein and nin e peptides derived from the ETSHR. Peptides were chosen based on their predicted immunogenicity as well as their uniqueness to the TSHR. Ant ipeptide antibodies showed varying degrees of reactivity against ETSHR , with antipeptide-2-(352-366) and -3A-(357-372) showing relatively st ronger reactivity with the receptor. Antibodies were tested for their ability to stimulate thyroid cells and were found to be ineffective in causing both cAMP release and iodide uptake. However, anti-3A and ant i-ETSHR showed blocking TSHR antibody (TSHRAb) activities of 76.9% and 79.7%, respectively, which were significantly different (P < 0.005) c ompared to that of preimmune serum. Anti-2 and -91 (AA 32-46) also sho wed blocking TSHRAb activities of 37.5% and 35.6%, respectively (P < 0 .05). Antisera were also tested for their ability to block TSH binding to thyroid membranes in a RRA. Anti-ETSHR, but not any of the antipep tide antibodies, displayed TSH binding inhibitory immunoglobulin activ ity. These findings suggest that there might be different mechanisms t hat mediate blocking TSHR antibody activity. One mechanism involves th e inhibition of TSH binding to the receptor, and the other probably in volves a step subsequent to TSH binding.