Usefulness of cytoplasmic 5'- nucleotidase 1A autoantibodies for diagnosing patients with inclusion body myositis

To clarify the diagnostic usefulness of autoantibodies against cytoplasmic 5' - nucleotidase 1A (NT5C1A) for detecting inclusion body myositis (IBM). Recently, autoantibodies against NT5C1A were known to be positive in some IBM patients. Diagnosing IBM is a key to avoid unnecessary long-term steroids. It is important to assess the diagnostic usefulness of anti-NT5C1A autoantibody for differentiating IBM from other myositis. We recruited 80 patients who were diagnosed as IBM during the period from 1993 to February 2017. We analyzed the serum samples of them with ELISA, using full-length human NT5c1A as antigen. We also assess the titer among patients with polymyositis (PM, n = 20), dermatomyositis (DM, n = 56), necrotizing autoimmune myopathy (NAM, n = 28), anti-synthetase syndrome (ASS, n = 20) other myositis (n = 80), connective tissue disease without myositis (SjS (n = 19), RA (n = 4), and SLE (n = 7)), multiple sclerosis (MS, n = 19), and amyotrophic lateral sclerosis (ALS, n = 29). Cut-off value was set at three standard deviations above the mean of healthy volunteers (HVs, n = 50) serums. NT5C1A autoantibodies were detected in IBM (27/80, 34%), PM (5/20, 25%), DM (1/56, 2%), NAM (1/28, 4%), ASS (3/20, 15%), other myositis (4/80, 5%), SjS (8/19, 42%), MS (4/19, 21%), ALS (1/29, 3%) and HVs (1/50, 2%). Among myositis patients, the sensitivity and specificity of anti-NT5C1A autoantibody seropositivity for being IBM were 34% and 93%, respectively. The autoantibody was also detected in other immune-mediated disease without myositis.