RELATIONSHIPS BETWEEN LOCAL INFLAMMATION, INTERLEUKIN-6 CONCENTRATION AND THE ACUTE PHASE PROTEIN RESPONSE IN ARTHRITIS PATIENTS

Interleukin-6 (IL-6) concentrations in knee joint synovial fluids and paired plasma samples of arthritis patients were examined with respect to each other and parameters of the inflammatory response. Synovial fluid and plasma IL-6 concentrations were significantly higher in patients with inflammatory arthritis than those detected in patients with osteoarthritis (P < 0.001). The IL-6 concentrations in synovial fluids were considerably higher than, but significantly correlated with (r = 0.65; P < 0.001), those of plasma. Furthermore, synovial fluid IL-6 concentrations in bilaterally inflamed knees were significantly correlated (r = 0.79; P < 0.001) and there was a significant correlation with the extent of inflammatory cell infiltrate (r = 0.75; P < 0.001). In unselected rheumatoid arthritis patients there was only a weak correlation between IL-6 and plasma C-reactive protein (CRP) concentration, and no correlation between IL-6 and erythrocyte sedimentation rate (ESR). However, both ESR and CRP concentration were highly correlated with plasma IL-6 concentration in patients with other inflammatory arthritides, particularly psoriatic and HLA B27 positive spondyloarthritis (r = 0.72-0.94; P < 0.005). These relationships suggest that IL-6 production in inflamed knee joints can be a significant determinant of acute phase protein responses in arthritis patients, although the situation in patients with rheumatoid arthritis is more complex and may be influenced by other disease-related factors.