Pathogenesis of myasthenia gravis. Acetylcholine receptor-related antigenic determinants in tumor-free thymuses and thymic epithelial tumors.

The authors describe an immunohistologic study of acetylcholine receptor (AChR)-related antigenic determinants in tumor-free thymuses of myasthenia gravis (MG) patients (13 cases) and nonmyasthenic controls (10 cases) and in thymic epithelial tumors of patients with MG (8 cases) and without MG (6 cases). Monoclonal antibodies (MAbs) to the cytoplasmic part and to the extracellular main immunogenic region (MIR) of the alpha subunit of AChRs were used. Their intrathymic binding sites were defined by double-immunostaining, and compared with alpha-bungarotoxin (alpha-Bgt) labeling demonstrated by fluorescence microscopy. Tumor-free thymuses of MG patients and control patients contained cytoplasmic AChR epitopes and alpha-Bgt binding sites on myoid cells and some epithelial cells. Only myoid cells also expressed extracellular MIR epitopes, suggesting that they bear complete AChRs, and are important targets for the autoimmune attack in tumor-free MG thymus. Evidence that AChR-related antigenic determinants of epithelial cells are also significant for MG is provided by our findings in thymic epithelial tumors. All eight tumors with MG but only two out of six tumors without MG showed cytoplasmic AChR epitopes and alpha-Bgt binding sites on neoplastic epithelial cells. Myoid cells and MIR epitopes did not occur in the neoplasms, but in some tumor-free thymic remnants beside thymomas. It is assumed that nonneoplastic and neoplastic thymic epithelial cells contain only incomplete AChRs or AChR-like molecules. The different expression of AChR epitopes in thymic epithelial tumors and tumor-free thymuses might explain some of the heterogeneous region specificities of anti-AChR antibodies in sera of MG patients with and without thymoma.