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Background: Myositis autoantibodies have traditionally been divided into myositis-specific (MSAs) and myositis-associated autoantibodies (MAAs). The MSAs are nearly exclusively found in PM or DM, whereas the MAAs are found in patients with myositis-CTD overlap conditions. While an individual patient may have multiple MAAs, it has generally been regarded that the MSAs are mutually exclusive. To test this assumption, we screened a large cohort of adult PM and DM patients for MAAs and MSAs and analysed the data for the presence of coexisting autoantibodies

Methods: A total of 1637 adult myositis PM or DM patients recruited to either the UKMyoNet or EuMyonet cohorts were investigated for MAAs/MSAs by radiolabelled immunoprecipitation using K562 cell extracts. Patients immunoprecipitating a band at approximately 140 kDa where further investigated by NXP2 or MDA5 ELISAs to confirm the presence of these autoantibodies.

Results: Autoantibodies (MAAs: PMScl, snRNP, Ku, u3RNP, Ro60, La, RNA Pol I/II/III, AMA and Topo isomerase; MSAs: Jo-1, PL12, Pl7, EJ, KS, Zo, OJ, SRP, Mi-2, TIF1, SAE, NXP2 and MDA5) were found in 1009 (61.6%) patients recruited to the study. Of the autoantibody-positive patients, 856 (84.7%) had a single MSA or MAA, 131 (13.0%) had two MSAs/MAAs and 23 (2.3%) had three or more MSAs/MAAs.

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