Background: Sporadic inclusion body myositis (sIBM) is a subgroup of the idiopathic inflammatory myopathies (IIMs) and is characterized by both degenerative and autoimmune features. Unlike other IIMs, myositis-specific autoantibodies had not been found in sIBM patients until recently.

Objectives: We aimed to establish the prevalence and clinical associations of anti-cN-1A in a large Danish cohort with connective tissue diseases (CTD).

Methods: In a cross-sectional study design, a total of 568 participants (183 IIMs (55 sIBM, 128 non-sIBM: dermatomyositis (DM), polymyositis (PM), immune-mediated necrotizing myopathy (IMNM)), 164 systemic lupus erythematosus (SLE), 121 systemic sclerosis (SSc), and 100 blood donors (BD)) were tested for the presence of: a) cN-1A autoantibodies using a commercial Anti-cN-1A ELISA and b) myositis specific and associated autoantibodies (anti-Jo-1, anti-PL-7, anti-PL-12, anti-OJ, anti-EJ, anti-SRP, anti-Mi-2α/β, anti-MDA5, anti-TIF1γ, anti-NPX2, anti-SAE1, anti-PM-Scl75, anti-PM-Scl100, anti-Ro52, anti-Ku) using a commercial lineblot kit. The patients were classified according to the ACR classification criteria for IIMs (2017), SLE (1997) and SSc (1980), respectively. Clinical features were compared between anti-cN-1A positive and anti-cN-1A negative sIBM patients using two-sample t-test and Mann-Whitney test for continuous normally and non-normally distributed data and Chi-square test for categorical data, as appropriate.

Results: cN-1A antibodies were predominantly found in IIM patients. In the sIBM cohort, 24 patients (43.6%) were anti-cN-1A positive vs. 25 (19.5%) in the non-sIBM myositis cohort and 17 (10%) in the SLE cohort. None of the participants in the SSc or the BD cohorts had presence of anti-cN-1A. Anti-cN-1A positivity had a sensitivity of 43.6% and a specificity of 91.8% for sIBM. The positive and negative predictive values were 36.4% and 93.8%, respectively.

There was no significant difference in gender, age at study entry, age at symptom onset, duration of symptoms or max creatine kinase (CK) levels during disease course between the anti-cN-1A positive and negative sIBM patients. Dysphagia was present in 19 (79%) of the anti-cN-1A positive and in 17 (55%) of the anti-cN-1A negative sIBM patients (P = 0.06).

Conclusion: Antibodies against cN-1A are the first and so far the only serological marker for sIBM. Our data showed that cN-1A autoantibodies are specific for sIBM and further corroborate the potential diagnostic role of cN-1A autoantibodies in this distinct subgroup of myositis.

Disclosure of Interests: Louise Pyndt Diederichsen: None declared, Sine Søndergaard Korsholm: None declared, Line Vinderslev Iversen: None declared, Christoffer Tandrup Nielsen: None declared, Marie-Louise From Hermansen: None declared, Søren Jacobsen: None declared, Nanna Witting: None declared, Markus E. Krogager: None declared, Tina Friis Grant/research support from: Anti cN-1A ELISA kits and EUROLINE Autoimmune Inflammatory Myopathies 16 AG kits have been provided for a project free of charge from Euroimmun.

DOI: 10.1136/annrheumdis-2019-eular.7264