Identification of antigenic domains on the human sodium-iodide symporter which are recognized by autoantibodies from patients with autoimmune thyroiddisease

The sodium-iodide symporter (NIS) is a novel autoantigen in autoimmune thyr oid disease. In the present study we have characterized the antigenic domai ns on the human symporter which are recognized by autoantibodies from patie nts with either Graves' disease (GD) or autoimmune hypothyroidism (AH). Del etion derivatives of complementary DNA (cDNA) encoding the Na+/I- symporter were constructed using polymerase chain reaction (PCR) amplification. Thes e deletion constructs were translated in vitro with the concomitant incorpo ration of [S-35]methionine into the protein products. The reactivity of sev en GD and six AH sera, which were known to contain symporter-binding antibo dies, to each of the radiolabelled modified symporters was then determined in immunoprecipitation experiments. Analyses of the results obtained in the radiobinding assays suggest the existence of multiple antibody binding sit es on human NIS (hNIS), including regions between amino acids (aa) 1-134, 1 91-286, 290-411, 411-520 and 520-588. Computer prediction of the potential B cell epitopes on the symporter revealed that, apart from aa 134-191, all the epitope domains identified overlapped, at least in part, with areas pre dicted to be highly antigenic. Interestingly, the antigenic domains represe nted by aa 191-286, 290-411 and 411-520 include regions of the polypeptide which form putative extracellular domains in the secondary structure model of the rat symporter. No correlation between the recognition of specific ep itopes on the human symporter and the type of autoimmune thyroid disease wa s demonstrated.