Current insights in polymyositis and dermatomyositis

Polymyositis (PM) and dermatomyositis (DM) are characterized by immune‐mediated muscle damage. Early immunohistochemical studies of PM and DM muscle biopsy samples suggested that PM is mediated by cytotoxic CD8 T lymphocytes, whereas DM is a vasculopathy mediated by antibodies and complement deposition to the intramuscular microvessels. This classic view is now challenged by recent advances in immunology and molecular biology, as well as by clinical observation of the two diseases. Clonal expansion of cytotoxic CD8 T cells was found in the affected muscles and in peripheral blood from PM and DM patients. Autoantigens recognized by these clones are yet to be identified. Patient sera contain various autoantibodies. Although they are not directed to endothelial antigens, detection of specific antibodies helps when classifying the patients. Infiltrating cells, as well as muscle cells, produce cytokines and chemokines that could potentiate inflammation. Myofibers overexpress major histocompatibility complex class I and class II molecules that render themselves vulnerable to immune attacks while they also express molecules that protect themselves from apoptotic cell death. Corticosteroid administration is still a gold standard for PM/DM treatment. A major life‐threatening complication is an interstitial lung disease, the acute form of which should not be treated with corticosteroids alone. Future studies should be carried out to further elucidate their ethiopathology and to develop more effective and specific treatment. (Clin. Exp. Neuroimmunol. doi: 10.1111/j.1759‐1961.2009.00002.x, January 2010)