Isotype and Immunoglobulin Subclass Distribution of eye Muscle Membrane Reactive Antibodies in the Serum of Patients with Thyroid-Associated Ophthalmopathy as Detected in Western Blotting

Abstract

We have determined the immunoglobulin (Ig) class (isotype) and IgG subclass of autoantibodies in the serum of patients with thyroid-associated ophthalmopathy (TAO) or autoimmune thyroid disorders without evident ophthalmopathy reactive in Western blotting with antigens of 55, 64, 75 and 95 kDa in pig eye muscle membrane (PEMM). The 22 sera studied were shown, previously, to contain IgG antibodies reactive with one or more of the four antigens. The majority of sera antibodies reactive with PEMM antigens were of two or more IgG subclasses. of the IgG subclass specificities IgG₁ and IgG₂ subclass antibodies were, overall, the most common. We were unable to demonstrate IgG subclass restriction for antibodies reactive with the 95 or 55 kDa antigens in PEMM, antibody activity being equally distributed in all four subclasses tested. While most of the sera which recognized a 64 kDa antigen did so with an IgG₄ antibody, all other subclasses were also represented. on the other hand all 13 sera reactive with a 75 kDa antigen did so using Ig of the IgG, subclass and 12 of these used the IgG, subclass as well, IgG, and IgG, subclasses being represented in only 3 and 4 sera, respectively. There were no differences, in respect to Ig class or IgG subclass distribution of eye muscle reactive antibodies between patients with Graves’ hyperthyroidism with ophthalmopathy and those with Hashimoto's thyroiditis, and eye disease. Control sera from five normal subjects and three patients with non-autoimmune thyroid disorders did not contain antibodies reactive with these PEMM antigens of any Ig class or IgG subclass.

IgG₃ and IgG₁ antibodies fix complement and can mediate cytotoxicity, while those of the IgG₄ subclass do not. Antibodies to eye muscle membrane antigens thus have the potential to mediate cytotoxic damage of eye muscle cells, which is consistent with the hypothesis that one or more of these antibodies may be pathogenic in TAO.

Key Words:

• Cytotoxicity
• eye muscle antibodies
• thyroid-associated ophthalmopathy
• Western blotting
• 64 kDa protein