Objectives The diagnosis of inclusion body myositis (IBM) can be challenging as it can be difficult to clinically distinguish from other forms of myositis, particularly polymyositis (PM). Recent studies have shown frequent presence of autoantibodies directed against cytosolic 5′-nucleotidase 1A (cN-1A) in patients with IBM. We therefore, examined the autoantigenicity and disease specificity of major epitopes of cN-1A in patients with sporadic IBM compared with healthy and disease controls.

Methods Serum samples obtained from patients with IBM (n=238), PM and dermatomyositis (DM) (n=185), other autoimmune diseases (n=246), other neuromuscular diseases (n=93) and healthy controls (n=35) were analysed for the presence of autoantibodies using immunodominant cN-1A peptide ELISAs.

Results Autoantibodies directed against major epitopes of cN-1A were frequent in patients with IBM (37%) but not in PM, DM or non-autoimmune neuromuscular diseases (<5%). Anti-cN-1A reactivity was also observed in some other autoimmune diseases, particularly Sjögren's syndrome (SjS; 36%) and systemic lupus erythematosus (SLE; 20%).

Conclusions In summary, we found frequent anti-cN-1A autoantibodies in sera from patients with IBM. Heterogeneity in reactivity with the three immunodominant epitopes indicates that serological assays should not be limited to a distinct epitope region. The similar reactivities observed for SjS and SLE demonstrate the need to further investigate whether distinct IBM-specific epitopes exist.