: Experimental autoimmune myasthenia gravis (EAMG) is induced in susceptible strains of mice by immunization with acetylcholine receptor (AchR). Muscle weakness is thought to be entirely mediated by high-affinity IgG autoantibodies, with little participation from cell-mediated effector mechanisms. Recently, strong evidence has been provided that EAMG depends on the production of Th1 cytokines, especially IL-12 and interferon-gamma (IFN-gamma). IFN-gamma has been shown to have powerful direct influences on muscle cells in culture, among them upregulation of MHC class II and cell adhesion molecules. The applicant proposes that Th1 cytokines, predominantly IFN-gamma, have a direct action on muscle in vivo, causing increased susceptibility to immune-mediated damage and interference with muscle function, regeneration and repair. Three specific aims are proposed to test this hypothesis. In Aim 1, the investigator will define the role of key Th1 and Th2 cytokines in EAMG, by administration of recombinant cytokines, neutralization of cytokine activity by monoclonal antibodies (mAbs) and studies in cytokine gene knockout mice. The investigator will be particularly interested in whether Th1 and Th2 cytokines are antagonistic or synergistic. In Aim 2, the applicant will purify antibodies made under the influence of various cytokines and assess their pathogenicity by adoptive transfer. The direct effect of cytokines will be measured, individually and in combinations, with and without co-application of autoantibodies to muscle cells grown in culture. These changes in cultured muscle cells will then be correlated with muscle tissue findings in vivo. In Aim 3, a dominant-negative IFN-gamma receptor transgene under the control of tissue specific promoters will be used to confer specific tissue resistance to the actions of IFN-gamma. This hypothesis may show that muscle unresponsiveness to IFN-gamma may confer resistance to EAMG. In addition to providing significant advances in understanding the immunopathogenic basis of MG, such a result could also lead to novel therapy.