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This study aims to characterize myositis-specific antibodies in a well-defined cohort of patients with idiopathic inflammatory myopathy and to determine their association with cancer. Sera from 40 patients with polymyositis, dermatomyositis, and controls were tested by protein and RNA immunoprecipitation to detect autoantibodies, and immunoprecipitation-Western blot was used for anti-MJ/NXP-2, anti-MDA5, and anti-TIF1γ/α identification. Medical records were re-evaluated with specific focus on cancer. Anti-MJ/NXP-2 and anti-TIF1γ/α were the most common antibodies in dermatomyositis. In six dermatomyositis cases, we found five solid forms of cancer and one Hodgkin's lymphoma in long-term remission. Among patients with cancer-associated dermatomyositis, three were positive for anti-TIF1γ/α, two for anti-Mi-2, and one for anti-MJ/NXP-2. The strongest positivity of anti-TIF1γ was seen in two active forms of cancer, and this antibody was either negative or positive at low titers in the absence of cancer or in the 7-year remission Hodgkin's lymphoma. Four out of twenty (20 %) patients with polymyositis had solid cancer, but no specific association with autoantibodies was identified; further, none of the four cases of antisynthetase syndrome had a history of cancer. No serum myositis-associated autoantibody was observed in control sera, resulting in positive predictive value 75 %, negative predictive value 78.5 %, sensitivity 50 %, specificity 92 %, and area under the ROC curve 0.7083 for the risk of paraneoplastic DM in anti-TIF1γ/α (+) patients. Myositis-specific autoantibodies can be identified thanks to the use of immunoprecipitation, and their association with cancer is particularly clear for anti-TIF1γ/α in dermatomyositis. This association should be evaluated in a prospective study by immunoprecipitation in clinical practice.

The pathogenesis of idiopathic inflammatory myositis (IIMs, including polymyositis and dermatomyositis) remains largely enigmatic, despite advances in the study of the role played by innate immunity, adaptive immunity, genetic predisposition, and environmental factors in an orchestrated response. Several factors are involved in the inflammatory state that characterizes the different forms of IIMs which share features and mechanisms but are clearly different with respect to the involved sites and characteristics of the inflammation. Cellular and non-cellular mechanisms of both the immune and non-immune systems have been identified as key regulators of inflammation in polymyositis/dermatomyositis, particularly at different stages of disease, leading to the fibrotic state that characterizes the end stage. Among these, a special role is played by an interferon signature and complement cascade with different mechanisms in polymyositis and dermatomyositis; these differences can be identified also histologically in muscle biopsies. Numerous cellular components of the adaptive and innate immune response are present in the site of tissue inflammation, and the complexity of idiopathic inflammatory myositis is further supported by the involvement of non-immune mechanisms such as hypoxia and autophagy. The aim of this comprehensive review is to describe the major pathogenic mechanisms involved in the onset of idiopathic inflammatory myositis and to report on the major working hypothesis with therapeutic implications.