Objective: We have explored the recently described 64 kDa extraocular muscle antigen that is associated with autoantibodies in the serum of patients with severe Grave's ophthalmopathy. The localization of the antigen and the specificity of autoantibodies for both eye muscle antigens and ophthalmopathy patients were investigated.
Design: Western blotting and immunoprecipitation of metabolically labelled antigen from eye muscle and control tissues with sera from ophthalmopathy, Graves' without ophthalmopathy, and normals were used.
Patients: Sera from normals (n = 9), patients with recent onset Graves' ophthalmopathy (n = 23), and patients with Graves' disease without ophthalmopathy (n = 8) were utilized.
Measurements: Immunoblots using detergent phase separated (amphiphilic) antigen preparations from fetal eye muscle, skeletal muscle and control tissues were quantitated. Metabolically labelled eye muscle and skeletal muscle antigens were immunoprecipitated using patient and control IgG.
Results: In the eye muscle detergent phase, immunoreactivity around 64 kDa was detected in 30% of the patients with ophthalmopathy (n = 23) as well as 38% of patients with Graves' disease and no ophthalmopathy (n = 8) and in 30% of normal sera (n = 9). There was significantly more of this anti-64 kDa reactivity in sera from the ophthalmopathy patients compared with the normals (P less than 0.01). 64 kDa reactivity to detergent phase antigens prepared from human thyroid, skeletal muscle, brain, and liver was also observed with these positive sera indicating the polyreactivity of the IgG interactions to conserved antigens in this region.
Conclusions: We conclude that IgG antibodies binding to a recurrent 64 kDa antigen are present in many normal human sera, with increased concentrations detectable in sera from Graves' ophthalmopathy patients. Such 'specificity-crossover' with similar molecular weight transmembrane antigens is likely to be caused by natural autoantibodies reacting with recurrent autoepitopes rather than a factor aetiological in the disease process.