Interleukin-6 serum levels correlate with footpad swelling in adjuvant-induced arthritic Lewis rats treated with cyclosporin A or indomethacin.

To investigate the role of interleukin 6 (IL-6) in adjuvant-induced arthritis, serum from adjuvant-immunized Lewis rats treated with cyclosporin, indomethacin, or saline was evaluated for IL-6 activity. Inflammation was quantitated by measuring paw volume. We found that an increase in serum IL-6 activity parallels the kinetics of paw edema development in adjuvant-immunized rats. Daily treatment with 5 mg cyclosporin A/kg prevented the increase in paw volume and held serum IL-6 activity to levels observed in untreated (normal) rats. Daily treatment with 1 mg indomethacin/kg resulted in a 50% reduction in serum IL-6 levels and a significant decrease (approximately 50%) in paw volume on Day 17 compared to saline-treated rats. Linear regression analysis confirmed the positive correlation between mean paw volume and mean serum IL-6 activity (R2 = 0.783, P less than 0.01 on Day 17) in normal, arthritic, and cyclosporin A- or indomethacin-treated groups. These results are consistent with a role for IL-6 in the pathology of arthritis and suggest that serum IL-6 activity may be a useful parameter for monitoring disease activity.