The occurrence of cryoproteins in synovial fluid; the association of a complement-fixing activity in rheumatoid synovial fluid with cold-precipitable protein.

A significant portion of a complement-fixing activity found in the synovial fluid of patients with rheumatoid arthritis, and previously implicated as a possible cause of the low synovial fluid complement levels in these patients behaves as a high solubility cryoprotein. Analysis of rheumatoid synovial fluid cryoproteins has revealed mixed immunoglobulins, bound complement components, fibrinogen, DNA, and rheumatoid factor. Sorbitol density gradient studies on whole synovial fluid before and after removal of this activity has shown that the complement-fixing activity migrates in the 19S and heavier regions and that portion is removed with cryoprecipitation. Cryoproteins found in nonrheumatoid synovial fluid are generally devoid of complement-fixing activity and predominantly contain fibrinogen. DNA and IgG are also present, with IgG occurring significantly less frequently than in rheumatoid cryoproteins. These findings are discussed in relationship to recent studies demonstrating the presence of complement-fixing antibody to denatured DNA in rheumatoid cryoproteins.