Changes in joint cartilage aggrecan after knee injury and in osteoarthritis

To determine the concentrations of aggrecan fragments in synovial fluid from patients with knee joint injury, osteoarthritis (OA), or acute pyrophosphate arthritis (PPA; pseudogout), and to test their relative reactivity with the 846 epitope, a putative marker of cartilage aggrecan synthesis.

Samples of knee joint fluid from 385 patients and 9 healthy-knee volunteers were obtained in a cross-sectional study. Study groups were acute PPA/ pseudogout (n = 60), anterior cruciate ligament (ACL) rupture (n = 159), meniscus lesion (n = 129), and primary knee OA (n = 37). The 846 epitope on aggrecan was assayed by competitive solution-phase radioimmunoassay. Aggrecan fragments were assayed by enzyme-linked immunosorbent assay using a monoclonal antibody (1-F21). Cartilage oligomeric matrix protein (COMP), C-propeptide of type II collagen (CPII), bone sialoprotein, matrix metalloproteinases 1 and 3, and tissue inhibitor of metalloproteinases 1 were previously quantified by immunoassays.

Reactivity of the 846 epitope was increased in all study groups compared with the reference group, and was highest in patients with primary OA. The median levels (in microg fetal aggrecan equivalents/ml) of the epitope were 0.28 (range 0.24-0.47) in the reference group, 0.48 (range 0.26-1.32) in PPA/pseudogout, 0.61 (range 0.12-2.87) in ACL rupture, 0.53 (range 0.22-3.02) in meniscus lesion, and 0.68 (range 0.31-4.31) in primary OA. The 846 epitope reactivity per microg aggrecan fragments in the joint fluid was higher in late-stage OA than in early-stage OA. Epitope 846 reactivity correlated positively with several markers of matrix turnover, particularly with COMP (r(s) = 0.421) and CPII (r(s) = 0.307).

The observed differences in 846 epitope reactivity in synovial fluid, and its concentration in relation to aggrecan and other markers of matrix turnover, were consistent with marked ongoing changes in aggrecan turnover after joint injury and in the development of OA. OA is thus a disease characterized by dynamic changes in tissue macromolecule turnover, which is reflected by measurable changes in aggrecan epitopes in the synovial fluid.