ABSTRACT
Polymyositis (PM), dermatomyositis (DM) and sporadic inclusion body myositis (s-IBM) are three distinct idiopathic inflammatory myopathies, characterized by a common histological endomysial inflammation and distinct immune-mediated mechanisms1. In PM and s-IBM, sensitized CD8+ cytotoxic T cells recognize as yet unidentified muscle antigens. The cytotoxic cells surround, invade and destroy non-necrotic muscle fibers that express class I major histocompatibility complex (MHC) molecules1,2. DM differs from the other two diseases clinically, because of the characteristic rash that accompanies or often precedes the muscle weakness, and immunopathologically, with the presence of an intramuscular microangiopathy, mediated by the complement C5b-C9 membranolytic attack complex, which leads to the destruction of endothelial cells, loss of capillaries, muscle ischemia, muscle fiber necrosis and perifascicular atrophy1,3.
