Myositis autoantibodies and clinical phenotypes.

Autoantibodies are powerful diagnostic tools in idiopathic inflammatory myopathies, especially for confirming the diagnosis and contributing to the definition of disease subsets. They are present in over 80 % of patients with immuno-mediated myositis and directed towards ubiquitously expressed intracellular complexes. Most of these autoantibodies are reported also in other autoimmune diseases, while some are considered myositis-specific. Myositis autoantibodies are traditionally categorized in two groups, based on their diagnostic accuracy: myositis-specific antibodies (MSA) and myositis-associated antibodies (MAA), the latter mostly occurring in myositis-overlap syndromes. Besides the so-called traditional MSA, including anti-synthetases, anti-SRP and anti-Mi-2 antibodies, additional newly conceived immune targets have been recently identified, mostly in patients with severe forms of dermatomyositis or necrotizing myopathy. They mainly encompass enzymatic proteins essentially involved in the regulation of gene transcription or post-translational modifications, i.e., TIF1-γ, NXP-2, MDA5, SAE and HMGCR. Among the MAA, anti-PM/Scl and anti-Ku characterize an overlap polydermatomyositis/systemic sclerosis syndrome with severe interstitial lung involvement.