Background Specific autoantibodies in patients with polymyositis/dermatomyositis (PM/DM) are associated with unique subsets, and they are useful in monitoring clinical course and predicting outcome.

Objectives to analyze the prevalence of myositis specific antibodies (MSA) and their clinical significance in a cohort of adult PM/DM patients.

Methods Autoantibodies in sera from 58 consecutive adult PM/DM patients (74% female, mean age 43±17, mean follow up 55 months) were analyzed by immunoprecipitation of ³⁵S-labeled K562 cell extracts, ELISA (anti-MJ, Ro52, La, Jo-1), Western Blot and Indirect Immunofluorescence (IIF). Clinical association of antibody specificity was analyzed using information from charts and database.

Results Autoantibody prevalence in our cohort is shown in table 1. Anti-MJ antibodies are the most prevalent specificity (10/58;17%) in our PM/DM patients, followed by anti-Jo-1 (10%),-p155/140 (5%),-SRP (5%),-EJ (4%), and anti-Mi2,-SMN complex,-OJ with one case each. Anti-MJ was found in 30% of DM and 8% of PM (P=0.02). Among 10 cases of anti-MJ, 8 were DM and 2 were PM. When clinical features of 10 cases of anti-MJ (+) vs 48 cases anti-MJ(-) were compared, DM is more common (P=0.03) and no overlap syndrome patients were found in anti-MJ(+) group (0% vs 13%). Age of onset (25.5 vs 46.1 years) and age at initial visit (37.6 vs 54.6 years) were younger in anti-MJ(+) group (P=0.002), and 2 anti-MJ(+) were pediatric onset DM. In anti-MJ(+) patients, heliotrope rash (P=0.01) and calcinosis (P=0.057) were common, however, none of them had heart involvement (0% vs 27%, P=0.03), interstitial lung disease (0% vs 33%, P=0.048), or cancer (0% vs 8%). Myopathy in anti-MJ (+) patients was well responsive to steroid therapy and elevated CPK in the last visit was not seen (0% vs 25%). 4 patients had cancer, two were negative one was anti-Mi2 and one –OJ positive.

Conclusions Anti-MJ antibodies have been described in pediatric population with DM (1,2) but are detected also in adult PM/DM, and they are the most frequent specificity in our cohort, found in 17% of PM/DM (30% in DM and 4% in PM). Anti-MJ(+) patients have DM of young onset, severe calcinosis, no internal organ involvement and good response of myopathy to steroid. No association between MSA and cancer was found. Anti-MJ will be a useful new addition of MSA to help clinical monitoring of patients with adult PM/DM.

1. Oddis CV et al. Arthritis Rheum 1997;40:S139.

2. Gunawardena H et al. Arthritis Rheum 2009;60:1807-14.

Disclosure of Interest None Declared