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GENETICS OF SUSCEPTIBILITY TO MYASTHENIA GRAVIS

Myasthenia gravis is a disease characterized by extreme muscular weakness and flaccid paralysis. Many lines of evidence indicate that this condition arises as a consequence of an autoimmune response directed against the nicotinic acetylcholine receptor (AChR) l of the neuromuscular junction (1-3). Mice immunized with AChR purified from the electrogenic organ of Torpedo californica form anti-AChR antibodies and often develop muscular weakness and flaccid paralysis which closely resembles human myasthenia gravis. This condition, termed experimental myasthenia gravis (EMG), is strain dependent in that the frequency of weakness and paralysis is much greater in some strains than in others (4, 5). Mice of the C57BL/6, SJL, and AKR strains exhibit high susceptibility with 50-70% of the animals immunized developing muscular weakness and paralysis. Other strains such as BALB/c, C 3 H / H e , and SWR display low susceptibility with 0-15% of the animals immunized developing muscular weakness. This strain-dependent difference in susceptibility might reflect differences in the immune system or differences in the neuromuscular junction. The observation (5) that nonimmunized mice from highand low-susceptibility strains were indistinguishable with respect to their sensitivity toward the cholinergic antagonist d-tubocurarine suggested that the strain dependence was a consequence of something other than a difference at the neuromuscular junction. In this paper, we examine the possibility that susceptibility to muscular weakness after immunization with T. californica AChR is determined by regions of the mouse genome known to regulate immune responsiveness. In the mouse, two loci have been identified that participate in determining an individual's ability to respond to any given antigen: the major histocompatibility complex (I-I-2) and the region that contains the structure genes for the constant region of immunoglobulin heavy chains (IgCH) (6-9). The H-2 region is located on chromosome 17, and codes for the major histocompatibility antigens, of the mouse (10-