Update on pathology of inflammatory myopathy

Inflammatory myopathy is classified into primary autoimmune myositis and secondary myositis due to various factors, such as drugs and autoimmune connective tissue diseases. Autoimmune myositis mainly consists of dermatomyositis, antisynthetase syndrome-associated myositis, immune-mediated necrotizing myopathy, and inclusion body myositis. This review aims to provide insights into muscle pathology for clinical practice and an understanding of pathophysiology in inflammatory myopathy by summarizing current knowledge about the pathology of each subform of autoimmune myositis and some secondary myositis. Dermatomyositis is associated with type I interferon upregulation. Expression of myxovirus resistance protein A (a type I interferon-induced protein) in myofibers is utilized as a sensitive diagnostic marker. Antisynthetase syndrome-associated myositis is morphologically characterized by perifascicular necrosis. A recent study suggests the presence of a characteristic immunological micromilieu suitable for plasma cells in the skeletal muscle tissue. Immune-mediated necrotizing myopathy features an active necrotic and regenerating process. In inclusion body myositis, inflammatory cellular infiltration and rimmed vacuoles reflecting autophagy disruption are observed. The lymphocytes invading myofibers are composed of a highly differentiated T-cell population, which is considered a potential therapeutic target.