Cytokine inter-relationships and their association with disease activity in arthritis

In order to investigate the relationships between cytokine production and arthritic disease we have determined the concentrations of immunoreactive interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-6, tumour necrosis factor-alpha (TNF-alpha), interferon-alpha (IFN-alpha), IFN-gamma, and soluble IL-2-receptor (sIL-2R), as well as bioactive IL-1 and IL-6, in synovial fluids (SF) and plasma of patients with a variety of arthritides. Careful assay revealed only minimal concentrations of IL-1, particularly its biologically active form, in SF. No IL-1 was detectable in the plasma of patients that had IL-1 in their SF. Concentrations of both immunoreactive IL-1 beta and TNF-alpha in SF of rheumatoid arthritis (RA) patients were significantly higher than those in SF from patients with other inflammatory arthritides or osteoarthritis (OA). IL-6 and sIL-2R concentrations in both SF and plasma were higher in RA patients than in OA patients, and were significantly correlated. Approximately half of the SF from patients with all arthropathies contained detectable IFN-alpha, whilst IFN-Y was present in less than 10%. There were significant associations between IL-6, sIL-2R, IL-1 beta, TNF-alpha and IFN-alpha. The concentration of these cytokines, where detectable, was also related to leukocyte counts in the SF, as well as to parameters assessing local and systemic disease activity. Although IL-6 was the cytokine most clearly related to other cytokines, and to parameters assessing disease activity, the relationship between general articular disease activity and IL-6 was only evident in patients with arthropathies other than rheumatoid arthritis.