Thymomatous epithelial cells and skeletal muscle share a common epitope defined by a monoclonal antibody.

Monoclonal antibodies have been raised against thymomatous epithelial cells with the use of fragments of a human thymoma as source of antigen. These monoclonals, which do not react with cultured epithelial cells or sections from normal thymus (except for some cells of Hassall's corpuscles), label a large number of cells in thymoma sections. In addition, they recognize cross-striations from skeletal muscle cells. Immunoblotting studies reveal that the proteins recognized on thymoma and muscle are in the same molecular weight range, suggesting that these proteins, which share a common epitope, are identical. These findings indicate that the production of circulating anti-cross-striational autoantibodies observed in most patients with thymoma could derive from immunization against a protein abnormally expressed by neoplastic epithelial cells.