Myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG) are caused by antibody-mediated autoimmune responses to nicotinic acetylcholine receptors (AChRs) which impair neuromuscular transmission. At least half of the autoantibodies in human and canine MG and EAMG are directed at the main immunogenic region (MIR) on muscle AChR a1 subunits, as shown by competition for binding between rat mAbs and autoantibodies.
The first aim of the proposal is to determine the structure of the MIR. We have: 1) discovered that rats with EAMG and humans with MG recognize different epitopes within the MIR, 2) determined why the antigenicity of the MIR depends on its native conformation, and 3) have identified MIR epitopes recognized by rat mAbs. The proposed studies will map the adjacent epitopes recognized in human and canine MG. Chimeras will be made in which candidate MIR sequences of human muscle a1 subunits are placed in human neuronal a7 AChRs. Chimeras and mutagenesis studies will precisely map the epitopes recognized in MG. Chimeras of human a1 in Aplesia acetylcholine binding protein will be made which contain the MIR epitopes recognized in MG and EAMG. These will be used to test the pathogenicity of autoantibodies to each epitope and to determine the crystal structure of complexes with Fabs to the epitopes. This will define the structures of the most pathologically significant epitopes in one of the best characterized autoimmune diseases. These studies should also help reveal normal functional roles of the MIR.
The second aim of this proposal is to develop a specific immunosuppressive therapy for EAMG. We have discovered that ongoing EAMG can be inhibited by intraperitoneal injection of rats with bacterially-expressed human AChR subunits. Antisera from successfully treated rats are much less potent at passively transferring EAMG than are control EAMG sera. It is our hypothesis that treatment acts not only on regulatory T cells but also shifts the specificities of autoantibodies produced by B cells away from the MIR and other pathologically significant extracellular epitopes. Treatment shifts autoantibody specificities toward pathologically irrelevant epitopes on the cytoplasmic surface of the AChR. We will test this hypothesis and investigate specific immunosuppressive therapy using constructs containing only cytoplasmic domains. Because this approach would avoid introducing subunit sequences that could provoke pathological autoantibodies and uses antigens from all of the subunits of the human AChR, it may provide a safe and robust strategy for antigen-specific immunotherapy of MG. Our long term goal is to test an optimized therapy on canine MG as a model for human studies.
