Objective. To determine the involvement of interleukin-1 (IL-1), tumor necrosis factor (TNF), and IL-6 in the cartilage pathology of murine antigen-induced arthritis (AIA) and zymosan-induced arthritis (ZIA).

Methods. Arthritis was induced by intraarticular injection of zymosan in naive mice or by subcutaneous injection of methylated bovine serum albumin in sensitized animals. Mini-osmotic pumps releasing human recombinant IL-1 receptor antagonist (IL-1ra) protein were implanted intraperitoneally 2 days before arthritis induction, and neutralizing antibodies directed against murine IL-1α, IL-1β, TNFα, or IL-6 were administered 1 day before. Proteoglycan (PG) synthesis and degradation were assessed in patellar cartilage.

Results. Murine IL-1α and IL-1β injected intraarticularly at doses of 0.1–100 ng suppressed chondrocyte PG synthesis. The highest dose of TNF tested (100 ng) decreased PG synthesis marginally. In contrast, the maximum dose of IL-6 (1 μg) stimulated PG synthesis 2 days after injection. Treatment of AIA with neutralizing monoclonal antibodies against either TNFα or IL-6 did not reduce either the PG degradation or the suppression of its synthesis. However, treatment with anti-IL-1 (α + β) polyclonal antibodies totally prevented PG suppression, although the initial breakdown of PG was unaffected. This effect was confirmed when IL-1ra was administered in high doses. Moreover, treatment of ZIA with anti-IL-1 (α + β), but not with anti-TNF, resulted in normal PG synthesis, confirming the key role played by IL-1 in the inhibition of PG synthesis. Treatment of AIA with anti-IL-1 did not affect inflammation during the acute phase, but a significant reduction of ongoing inflammation was noted at day 7, and there was a marked reduction in the loss of cartilage PG.

Conclusion. The suppression of PG synthesis in both ZIA and AIA in mice is due to the combined local action of IL-1 (α + β), and neither IL-6 nor TNF is involved. Moreover, the normalization of PG synthesis brought about by blocking of IL-1 ameliorates the cartilage damage associated with AIA.