Autoantibody to alanyl-tRNA synthetase in patients with idiopathic pulmonary fibrosis

Background and objectives: The pathogenesis of IPF is unknown and it is hypothesized that immunological responses are involved. The purpose of this study was to detect autoantibodies in IPF patients and to identify the relevant antigens. Methods: Sera from 37 healthy subjects and 22 IPF patients who had no clinical symptoms of collagen vascular disease were examined for immunostaining of A549 human type II cells and human lung tissue. Immunoprecipitation and proteome analysis were performed to identify the antigen. Results: Fifty per cent of the patient sera and none of the control sera exhibited positive staining. Sera from 10 of the 22 IPF patients showed positive immunohistochemistry and immunoprecipitated a 110-kDa protein from the A549 cell lysate. Sera from only two of 41 patients with collagen vascular disease showed positive immunoreactivity. Proteome analysis using tandem mass spectrometry revealed that the protein was alanyl-tRNA synthetase. Transfection of cDNA of this enzyme into CHO-K1 cells conferred positive staining on these cells with the patients' IgG. The 135-kDa fusion protein consisting of 108-kDa enzyme protein and 27-kDa YFP from the cell lysate of the transfected cells was immunoprecipitated by the patient IgG. In addition, sera from IPF patients significantly inhibited the enzyme activity of alanyl-tRNA synthetase. Conclusion: A significant number of IPF patients possess circulating autoantibodies against alanyl-tRNA synthetase, suggesting the involvement of an autoimmune background in the pathogenesis of IPF.