Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis
Immune complexes were precipitated with polyethylene glycol (PEG) from sera of patients with rheumatoid arthritis (RA), psoriatic arthritis and systemic lupus erythematosus. Precipitates were tested for their capacity to consume complement and for the presence of fibronectin (Fn) and specific autoantibodies rheumatoid factor (RF, anti-DNA). The results showed enrichment of autoantibody activity in the precipitates. In RA, RF was especially enriched in 2.5% PEG precipitates, while IgM anti-DNA activity was more evident in 3.5% precipitates. IgG anti-DNA antibody was detected only in 3.5% precipitates from lupus sera. Complement consumption activity of precipitates was mainly related to IgM autoantibodies. There was a strong correlation between the presence of RF activity and Fn in the PEG precipitates. Nephelometric studies revealed direct interaction between the Fn and IgM RF or heat aggregated gammaglobulin. It may be possible to monitor the serum levels of immune complex material using PEG precipitation.