Idiopathic Inflammatory Myopathies: A Review of Immunopathological Features and Current Models of Pathogenesis

The idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of chronic systemic disorders characterized by muscle inflammation and progressive muscle weakness. The major clinical variants are dermatomyositis (DM) including a distinct juvenile (JDM) subtype, polymyositis (PM), and inclusion body myositis (IBM) (Engel & Hohlfeld, 2004). IBM is divided into sporadic IBM (sIBM), the most common muscle disease starting after age 50 years, occurring mainly in men and leading to severe disability, and hereditary inclusion body myopathy, characterized by pathologic alterations resembling those of sIBM except for a lack of muscle inflammation (hence “myopathy” instead of “myositis”) (Askanas & Engel, 1998). DM may occur in children or adults and is considered a humorally-mediated microangiopathy, while PM occurs mainly after the second decade of life and is a T cell-mediated disease characterized by cytotoxic attack against non-necrotic muscle fibers (Dalakas, 2011c). For all IIM forms, both target antigens and triggering factors for autoimmune response remain unknown. A growing body of evidence suggests that genetically susceptible individuals probably develop an idiopathic inflammatory myopathy in response to particular environmental stimuli (Feldman et al., 2008; Needham & Mastaglia, 2007; O’Hanlon et al., 2006; O’Hanlon & Miller, 2009; Rider et al., 2010; Sarkar et al., 2005; Vegosen et al., 2007).