Proteins with epitopes of the acetylcholine receptor in epithelial cell cultures of thymomas in myasthenia gravis

Thymomas from 12 patients with myasthenia gravis (MG) were investigated for the presence of epitopes of the alpha-subunit of the nicotinic acetylcholine receptor (AchR) using monoclonal antibodies (MAb) reacting against the AchR. In all but two of the tumors epitopes corresponding to antigenic determinants located on the cytoplasmic side of the AchR were identified. From eight thymomas cell lines were established that have been kept in culture for up to 6 months. The cultured cells expressed the same AchR-epitopes as did the primary tumors. During early passages the percentage of epithelial cells positive for the AchR epitopes approximately mirrored the percentage of positive cells in the original tumors. With passaging the relative number of positive cells usually declined but in some cultures an increase was observed. Three cell lines that showed extensive staining with an MAb against the AchR were radiolabeled to characterize the antigen. From protein extracts of these three cell lines proteins of 45 kd and 156 kd molecular weight (MW) were precipitated. These proteins are different from other proteins described in the context of both thymomas and MG. The negative reactivity with MAb against other epitopes of the alpha-subunit, especially against the main immunogenic region (MIR), speaks in favor of membrane-associated proteins of only limited crossreactivity to the AchR. A previous study found an almost exclusive occurrence of these AchR-epitopes in thymomas associated with MG, but not in other thymomas of similar histologic type. The expression of the proteins described here could therefore play a role in the triggering of the autoimmune process against the AchR of the motor endplate in MG patients.