Graves’ IgG stimulation of iodide uptake in FRTL-5 rat thyroid cells: A clinical assay complementing FRTL-5 assays measuring adenylate cyclase and growth-stimulating antibodies in autoimmune thyroid disease

Abstract

With optimal conditions and cells maintained in the absence of thyrotropin (TSH) for 7–10 days, IgG preparations from ∼ 90% of patients with active Graves’ disease can exhibit statistically significant stimulation of cAMP levels in rat FRTL-5 thyroid cells as compared to normal controls. FRTL-5 cells maintained in the absence of TSH for 7–10 days lose their ability to take up iodide. Iodide uptake returns upon readdition of TSH over a 60-hour period via a cAMP-mediated process; thus TSH can be replaced by dibutyryl cAMP or other agents which increase cAMP levels, for example, thyroid-stimulating autoantibodies (TSAbs) from Graves’ sera. TSAb stimulation of iodide uptake requires the continued presence of TSAb over at least the first 24 hours of a 48-hour reversal period; TSH, in contrast, can be withdrawn after 5 hours and will still achieve maximal effects at 36–48 hours. Iodide uptake, measured as a 30-minute pulse at 48 hours, appears, however, to be faster with TSAb than TSH. With optimized conditions (cells depleted of TSH > 7–10 days; 3-isobytyl-1-methyl xanthine, 0.005 mM; TSAb addition for the entire 48-hour assay period; and a 30-minute pulse of 10 µM ¹²⁵I-sodium iodide at 37 C), TSAb stimulation is concentration-dependent with a half-maximal activity at ∼ 10-fold lower concentrations than in the cAMP stimulation assay. In a series of 24 patients with Graves’ disease, IgGs with positive values in the cAMP assay were positive in the iodide uptake assay. Activity coefficients expressed as percent of basal in both assays correlate extremely well (r = 0.85). Using these two assays and an FRTL-5 assay measuring growth antibodies (by their ability to increase thymidine uptake), TSAbs were found in 100% of patients with active Graves’ disease and could broadly be grouped into three categories: IgGs with high stimulatory cAMP/iodide uptake activity but low to moderate effects on thymidine uptake; IgGs with moderate effects in all assays; and IgGs with low or undetectable cAMP/iodide stimulatory activity, but high thymidine uptake activity. The value of the FRTL-5 rat thyroid cells assays system for measuring Graves’ autoantibodies appears, thus, to be superior to all current systems, including human cells and slices.