Detection of binding and blocking autoantibodies to the human sodium-iodide symporter in patients with autoimmune thyroid disease

The sodium iodide symporter (NIS) is a novel autoantigen in autoimmune thyr oid disease (ATD). A recent study has described the development of a bioass ay for human (h) NIS antibody detection, but this will not detect antibodie s that bind the symporter without modulating its activity. Therefore, the e stablishment of a binding assay is of importance to determine the overall p revalence of hNIS antibodies in ATD patients. An in vitro transcription and translation system was used to produce [S-35]-labeled hNIS. The radiolabel ed ligand reacted specifically in immunoprecipitation experiments with rabb it antiserum raised against a peptide fragment of hNIS. Subsequently, the r eactivity of control and ATD sera to translated [S-35]hNIS was determined u sing RIAs. A significant difference in the frequency of hNIS antibody-positive sera wa s found when patients with either Graves' disease (GD) or autoimmune hypoth yroidism (AH) were compared with normal controls (P = 0.01 and P = 0.03, re spectively). Of 49 GD and 29 AH sera tested, 11 (22%) and 7 (24%), respecti vely, were found to contain hNIS antibodies. Differences were also signific ant when the antibody-binding indices of the control group of sera were com pared with those of both the GD and the AH patient sera (P < 0.0001 and P = 0.001, respectively). In contrast, sera from 10 patients with Addison's di sease and 10 patients with vitiligo (without associated ATD) were all negat ive for antibody reactivity to the symporter. No differences were detected when the antibody binding indices of either the Addison's disease or the vi tiligo sera were compared with those of the normal sera group (P = 0.9 and P = 0.6, respectively). Eight of the 11 (73%) GD and 3 of the 7 (43%) AH sera, which were positive for hNIS antibodies in the immunoprecipitation assay, were also found to in hibit; iodide uptake in hNIS-transfected CHO-K1 cells, suggesting the exist ence of antibodies in some serum samples that bind to the symporter without ; modulating its function. Overall, a significant correlation was found bet ween the iodide uptake inhibition and the binding assays for hNIS antibody detection (r = 0.49, P < 0.0001). In summary, we have developed a specific and quantitative assay for the det ection of hNIS binding antibodies in sera of patients with ATD. This system offers the advantage of studying antibody reactivity against conformationa l epitopes and will be useful in understanding the role of NIS autoreactivi ty in the pathogenesis of ATD.