To study the proteins associated with Sa antigen, a target of the anti-Sa antibodies specific for rheumatoid arthritis, and to elucidate the nature of these proteins.
Sa antigen was extracted from fresh human placental tissue by anion exchange chromatography and subjected to SDS-PAGE electrophoresis. Serum samples were collected from 155 patients with connective tissue diseases, including rheumatoid arthritis (71 cases), ankylosing spondylitis (11 cases), psoriatic arthritis (7 cases), reactive arthritis (7 cases), juvenile idiopathic arthritis (4 cases), osteoarthritis (5 cases), polymyalgia rheumatica (6 cases), gout (6 cases), systemic lupus erythematous (7 cases), Sjogren's syndrome (10 cases), adult onset Still's disease (3 cases) and Sa antibodies were detected by immunoblotting. The gel bands corresponding to the stained bands were excised, trypsin-digested in gel, and analyzed by LC-ESI-MS/MS. Once identified, the protein was recombinate and expressed in Escherichia coli, and the antibodies were detected by immunoblotting. Then the protein was citrullinated to detect the antibodies again.
Immunoblotting showed anti-Sa antibodies, band (s) with apparent molecular weight of 50 000 (and) 55 000, in 34 of the 71 patients of rheumatoid arthritis and 4 of the 84 patients of other rheumatic diseases, with a sensitivity rate of 47.9% and a specificity rate of 95.2%. The target protein was identified as vimentin. The positive rate of anti-vimentin antibody was statistically different between the RA patients and the patients with other rheumatic diseases (P = 0.005), with a sensitivity rate of 36.6% and a specificity rate of 83.3%, respectively. But there was no obvious correlation between anti-vimentin antibody and anti-Sa antibodies (Kappa = 0.316). The positive rate of anti-citrullinated vimentin antibody was significantly higher in the RA patients than in the patients with other rheumatic diseases (P < 0.01), with a sensitivity rate of 49.3%. There was a high correlation between anti-citrullinated vimentin antibody and anti-Sa antibodies (Kappa = 0.746), albeit a low specificity rate (86.9%).
Citrullinated vimentin is closely correlated with Sa antigen.