Differences between thyrotropin receptor antibody bioactivity and inhibition of I-125-bovine thyrotropin binding

Thyrotropin (TSHR) receptor antibodies that bind to the TSHR without stimul ating the TSHR have been identified with a direct binding assay. Moreover, TSHR antibodies that exhibit thyroid epithelial cell stimulation without in hibition of I-125-bovine thyrotropin (bTSH) binding and vice versa have bee n described. These data suggest that stimulation or blocking of the TSHR by stimulating (TSAB) or blocking (TSBAB) TSHR antibodies could be possible w ithout detectable bTSH-displacement activity. However, to date, possible di fferences between TSAB or TSBAB activity and inhibition of I-125-bTSH bindi ng have not been systematically investigated. Therefore we compared inhibit ion of I-125-bTSH binding and TSAB or TSBAB activity of sera from 113 patie nts with Graves' disease treated with antithyroid drugs. To exclude the dif ferent assay conditions of previous investigations as possible confounding factors, we determined TSAB or TSBAB and inhibition of I-125-bTSH binding ( TBIIW) with the same Chinese hamster ovary (CHO) cells expressing the human TSHR. Furthermore inhibition of I-125-bTSH binding was also determined as thyrotropin-binding inhibitory immunoglobulin (TBII) with solubilized porci ne thyroid membranes (TRAK, Brahms, Berlin Germany) and the highly sensitiv e recombinant human TSH receptor assay (hTRAK, Brahms, Berlin Germany). Onl y 78% (54/69) of TSAB-positive and 78% (21/27) of TSBAB-positive sera detec ted with JP26 cells exhibit inhibition of I-125-bTSH binding measured as TB II or TBIIW. Furthermore, 59% (10/17) of sera without TSAB and TSBAB activi ty revealed inhibition of I-125-bTSH binding measured as TBII or TBIIW. We found significant differences between TSHR bioactivities (TSAB or TSBAB) an d inhibition of I-125-bTSH binding. Moreover, there was no agreement betwee n the detectable TSHR bioactivities (TSAB or TSBAB) and their detectable in hibition of I-125-bTSH binding. Therefore, it is very likely that TSH displ acement by TSHR antibodies and stimulation or blocking of the TSHR by TSHR antibodies are different functions that do not need to occur together.