Autoantibodies to Su/Argonaute 2 in Japanese patients with inflammatory myopathy

Anti-Su antibodies are found in 5–20% of cases of various systemic autoimmune rheumatic diseases and in 5–10% of dermatomyositis (DM)/polymyositis (PM) patients. In 2006, the 100 kDa Su antigen was identified as argonaute2 (Ago2), and it was found to play a major role in RNA interference. However, immunoprecipitation (IP) remains the main method for detecting anti-Su and the clinical significance of the antibodies is uncertain. Sera from patients with DM/PM (n = 224) were screened by an ELISA that uses recombinant biotinylated Ago2 protein. Some serum samples were tested by IP and by indirect immunofluorescence (IIF) analysis. Seventeen (7.5%, 17/224) sera from DM/PM were positive in ELISA. Of the 33 IP-tested sera (17 ELISA-positive and 16 ELISA-negative with high background), 13 were found to be anti-Ago2/Su-positive in IP and ELISA. Only one IP-positive serum was judged to be ELISA-negative. Among the 13 patients with anti-Su/Ago2, 7 cases also had myositis-specific autoantibodies. Six sera that were positive by both IP and ELISA showed the GW body pattern in IIF. Interestingly, one ELISA-positive serum with an inconclusive result in IP also showed the GW body pattern. Our novel ELISA appears to be useful for screening anti-Su/Ago2 antibodies (sensitivity: 93%, specificity: 79%).