Myasthenia gravis. Identification of skeletal and heart muscle antigens not related to the acetylcholine receptor.

Myasthenia gravis (MG) is a neuromuscular disease thought to have an autoimmune etiology. The acetylcholine receptor has been considered the primary site of antibody binding; however, other muscle components may be involved in the pathogenesis of myasthenia gravis. This study describes a hypertonic sucrose extract of skeletal muscle (Muscle-HSE) that reacts with antibodies in myasthenic sera. The active component in Muscle-HSE is not the acetylcholine receptor as demonstrated by the inability of this extract to bind [125I]alpha-bungarotoxin. Muscle-HSE does, however, contain two distinct antigenic components reactive with MG sera. One antigen reacted with 70% (14/20) of myasthenic sera in the passive hemagglutination assay. This antigen was detected in the HSE of both skeletal muscle and heart, and was unaffected by treatment with Triton X-100. The second antigen reacted with 10% (2/20) of MG sera in the complement fixation assay, was unique to skeletal muscle, and was inactivated by Triton X-100.