To compare the alterations in proteoglycan metabolism in antigen induced arthritis and polycation induced arthritis and to determine the involvement of interleukin-1 (IL-1) in the cartilage degradation that occurs in these models of rheumatoid arthritis (RA).
The time course for loss of proteoglycan into the synovial fluid (SF) and inhibition of proteoglycan synthesis, as well as depletion of articular cartilage proteoglycan content, was compared in rabbit antigen arthritis and polycation arthritis. The ability of recombinant IL-1 receptor antagonist IL-1ra to block the acute cartilage loss at 24 h in these models was investigated, compared to its ability to block the cartilage breakdown induced by direct administration of IL-1 in rabbits.
Initial loss of cartilage proteoglycan was accompanied by release of high levels of glycosaminoglycan (GAG) into the SF and decrease in proteoglycan synthetic rates in both antigen and polycation induced arthritis SF GAG rapidly returned to control levels, while proteoglycan synthesis and cartilage proteoglycan content remained depressed, suggesting that the inhibition in proteoglycan synthesis prevented recovery to normal levels. GAG loss from the cartilage into the SF in response to IL-1 injection, as well as other effects of IL-1 challenge, was blocked in a dose dependent manner by IL-1ra administered either intraarticularly (ED50 = 160 ng) or intravenously (iv) (ED50 = 0.09mg/kg). In the antigen induced arthritis model, IL-1ra (20 mg/kg, iv -2h) inhibited GAG release by 40%, whereas in polycation induced arthritis no inhibition was observed even with repeated administration of high doses of inhibitor.
These studies suggest that sustained depression of proteoglycan synthesis may be responsible for the chronic depletion of articular cartilage proteoglycan in the antigen and the polycation model of RA. However, while IL-1 may play a role in the initial breakdown of articular cartilage in antigen induced arthritis, it does not appear to be involved in polycation induced arthritis in the rabbit.