Identification of Thyroglobulin in Orbital Tissues of Patients with Thyroid-Associated Ophthalmopathy

Abstract

Thyroid-associated ophthalmopathy (TAO) is thought to have an autoimmune pathogenesis because of its association with autoimmune thyroid disease, in particular with Graves' disease. Nevertheless, the nature of the autoimmune reaction is unclear, and a target orbital autoantigen has not been conclusively identified. A widely discussed hypothesis is that antigens constitutively shared by the thyroid and orbital tissues are targets of an autoimmune reaction. It has been also postulated that a thyroid-soluble antigen, namely thyroglobulin (Tg), is transported to orbital tissues through the lymphatics, where it accumulates and elicits autoimmune damages in susceptible individuals. Here we have investigated whether Tg is present in orbital tissues from patients with TAO. Retrobulbar tissue specimens were obtained from three patients with Graves' disease and TAO who underwent decompressive orbitotomy, and at autopsy from two patients with no thyroid or eye disease. All patients with TAO had been previously treated with radioiodine to control Graves' hyperthyroidism. Western blot analysis with a monoclonal anti-Tg antibody showed the presence of intact Tg, both in soluble and membrane-associated fractions of orbital tissue extracts from the patients with TAO, in amounts estimated to range from ~320 to ~900 pg/μg of tissue protein. In contrast, Tg was not detected in orbital tissue extracts from patients with no thyroid or eye disease. Tg was also demonstrated in orbital tissue extracts from two of three patients with TAO by enzyme-linked immunosorbent assay (ELISA), in amounts estimated to range from ~450 to ~1000 pg/μg of protein. In addition, Tg in orbital tissue extracts from patients with TAO was immunoprecipitated by a rabbit anti-Tg antibody, suggesting that it retained its native conformation. An anti-thyroxine (T4) antibody captured in solid-phase Tg from orbital tissue extracts, showing that it contained thyroid hormone residues and had therefore originated in the thyroid. Tg-anti-Tg immune complexes were not found in orbital tissues, suggesting that if an autoimmune reaction to Tg occurs in TAO, it is likely to be cell mediated.