Effect of animal age and chronicity of interleukin‐1 exposure on cartilage proteoglycan depletion in vivo

Abstract Rheumatoid arthritis and osteoarthritis are characterized by an early depletion of cartilage proteoglycans, which leads to a decrease in cartilage compressibility and, eventually, to a loss of joint function. Interleukin‐1, which is thought to have a role in mediating this loss of proteoglycans in arthritis, induces an acute depletion of proteoglycans from articular cartilage following intra‐articular injection in rabbits. As the structure and metabolism of proteoglycans are known to change with age, my laboratory investigated the effect of age on depletion and recovery of proteoglycans in response to interleukin‐1 in the rabbit. Loss of cartilage proteoglycans induced by interleukin‐1 was less servere in immature animals, increased until the age of sexual maturity, and then remained constant. The rate of recovery and compensatory overshoot in the rate of proteoglycan synthesis following challenge with interleukin‐1 was more rapid in immature animals and may have been responsible for the quicker return of the cartilage proteoglycan content to control levels in younger animals. With multiple exposures to interleukin‐1 at time intervals too short for recovery to occur, smaller amounts of interleukin‐1 induced loss of proteoglycans, and the proteoglycan content and the rate of synthesis remained depressed longer after treatment had stopped. The decreased ability of mature cartilage to replace proteoglycans rapidly after exposure to cytokines would increase the probability of subsequent inflammatory episodes before recovery is complete; this may result in increased susceptibility of adult cartilage to proteoglycan depletion.