Studies into the occurrence of soluble antigen-antibody complexes in disease. VI. Further characterization of the biological and physical properties of the rheumatoid biologically active factor (RBAF)

The rheumatoid biologically active factor (RBAF) was characterized further with respect to its biological and physical characteristics. The histamine-releasing activity of the RBAF in the guinea-pig lung was influenced in the same manner as soluble immune complexes when the lungs were being perfused at 20°C or 45°C or when the perfusate lacked calcium or magnesium or contained N-ethylmaleimide, phenol, theophylline, adrenaline or succinate. The RBAF was consistently associated with complement-fixing activity and RBAF-positive synovial fluid showed a lower total haemolytic complement level than RBAF-negative fluid. However, RBAF activity was not lost following absorption with anti-human beta 1C globulin. There was a higher frequency of free DNA and/or single-stranded DNA in RBAF-positive than negative synovial fluid. RBAF-positive synovial fluid was more active than RBAF-negative fluid in neutrophil chemotaxis when examined at 1:10 but not when undiluted. Mixed IgG–IgM cryoprecipitates failed to show RBAF activity and aggregates seen on analytical ultracentrifugation of rheumatoid synovial fluid did not correspond with the RBAF. The RBAF was stable to freezing and thawing but was labile to acid pH and to heating at 56°C. It was concluded that the RBAF is likely to be a soluble immune complex consisting of IgG and a second constituent which is labile to acid and heat.