Sporadic inclusion body myositis (sIBM) is usually classified as an idiopathic inflammatory myopathies. Although the diagnosis of sIBM is sometimes challenging, recent studies have shown that the autoantibodies against cytosolic 5'-nucleotidase 1A (NT5C1A) are the possible diagnostic biomarker for sIBM. Few reports have shown the frequencies of anti-NT5C1A antibodies in systemic autoimmune rheumatic diseases (SARDs) using large cohorts of SARDs.
Serum samples obtained from 314 patients including dermatomyositis (DM) (n=144), systemic lupus erythematosus (SLE) (n=50), systemic sclerosis (SSc) (n=50), Sjögren's syndrome (SS) (n=50), polymyositis (PM) (n=10) and mixed connective tissue disease (n=10), and healthy controls (n=42) in addition to 10 patients with typical sIBM were analysed for the presence of autoantibodies using full-length recombinant NT5C1A ELISA.
Japanese patients with DM (11%), PM (10%), SLE (6%), SSc (8%) or SS (4%) had anti-NT5C1A antibodies at lower frequencies than patients with sIBM. Interestingly, 4 of 17 DM/PM patients with anti-NT5C1A antibodies were found to have no other myositis-specific/associated autoantibodies.
There is a wide heterogeneity of anti-NT5C1A antibody immunoreactivity. Some populations of SARDs are positive for anti-NT5C1A are also positive for anti-NT5C1A. However, the anti-NT5C1A frequencies in the patients with SARDs are low also in Japanese.