Introduction: Autoantibodies in patients with polymyositis/dermatomyositis (PM/DM) are associated with unique subsets, clinical course and outcome. Anti-MJ antibodies, which recognize the nuclear protein NXP-2/MORC3, are reported in ~25% of juvenile DM. Prevalence and clinical significance of anti-MJ antibodies in adult Italian PM/DM patients were studied. Methods: Sera from 58 consecutive adult Italian PM/DM patients were analyzed by immunoprecipitation of ³⁵S-labeled K562 cells extract, ELISA (anti-MJ, Jo-1), Western blot and indirect immunofluorescence. Clinical associations were analyzed using information from medical charts. Results: Anti-MJ antibodies were the most prevalent specificity (17%) found mainly in DM (30%, 8 cases) vs 8% of PM (2 cases, P = 0.02). Comparing 10 anti-MJ (+) vs 48 anti-MJ (-) cases, DM was more common (P = 0.03), and age at onset was younger in anti-MJ (+) (P = 0.0006). In anti-MJ (+), heliotrope rash (P = 0.01) and calcinosis (P = 0.09) were more frequent. None of them had heart or lung involvement, or malignancy. Myopathy in anti-MJ (+) patients responded well to therapy and none of them had elevated CPK at last visit (0% vs 25% in anti-MJ (-)). Only 60% of anti-MJ (+) showed immunofluorescent nuclear dots staining, despite PML localization of NXP-2/MORC3. Conclusions: Anti-MJ antibodies are the most frequent specificity in our cohort of adult Italian PM/DM. Anti-MJ (+) were associated with young onset DM, calcinosis, no internal organ involvement and good response of myopathy to therapy. Anti-MJ reported in juvenile DM is also found in adult PM/DM, and could be a new useful biomarker.

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.