In the human inflammatory myopathies (polymyositis and dermatomyositis), the early, widespread appearance of MHC class I on the surface of muscle cells and the occurrence of certain myositisspecific autoantibodies are striking features. We have used a controllable muscle-specific promoter system to up-regulate MHC class I in the skeletal muscles of young mice. These mice develop clinical, biochemical, histological, and immunological features very similar to human myositis. The disease is inflammatory, limited to skeletal muscles, self-sustaining, more severe in females, and often accompanied by autoantibodies, including, in some mice, autoantibodies to histidyl-tRNA synthetase, the most common specificity found in the spontaneous human disease, anti-Jo-1. This model suggests that an autoimmune disease may unfold in a highly specific pattern as the consequence of an apparently nonspecific event-the sustained up-regulation of MHC class I in a tissue-and that the specificity of the autoantibodies derives not from the specificity of the stimulus, but from the context, location, and probably the duration of the stimulus. This model further suggests that the presumed order of events as an autoimmune disease develops needs to be reconsidered.

SUMMARYMuscle is an attractive target for gene therapy and for immunization with DNA vaccines and is also the target of immunological injury in myositis. It is important therefore to understand the immunologic capabilities of muscle cells themselves. In this study, we show that proinflammatory stimuli induce the expression of other cytokines such as IL-6, transforming growth factor-beta (TGF-b), and granulocytemacrophage colony-stimulating factor (GM-CSF) by muscle cells themselves, as well as the upregulation of human leucocyte antigen (HLA) class I, class II and intercellular adhesion molecule-1 (ICAM-1). Thus, muscle cells have an inherent ability to express and respond to a variety of cytokines and chemokines. The levels of HLA class I, class II and ICAM-1 in inflamed muscle may be affected by the secreted products of the stimulation.