The presence of anti-protein A antibodies in patients with rheumatoid arthritis.

Sixteen patients with rheumatoid arthritis (RA) were examined for the presence of anti-protein A antibodies. The F(ab')2 preparations from five RA patients showed significant binding to IgG-free protein A on ELISA. The protein A binding was further examined by immunoblotting. The F(ab')2 preparations of high protein A-binding protein gave a specific reaction with IgG-free protein A on nitrocellulose paper. This demonstrates the presence of anti-protein A antibodies in patients with RA. Those RA patients with anti-protein A antibodies had more active disease as judged by the Lansbury's activity index. The level of serum rheumatoid factor (RAHA) was significantly higher in patients with anti-protein A antibodies than in those without anti-protein A antibodies.