Rheumatoid arthritis is a chronic inflammatory disorder that primarily affects the joints and results in the destruction of cartilage and subchondral bone by the inflamed synovium. Investigating the time course of cartilage destruction is of importance to better understand whether structural damage is reversible and when therapeutic intervention may be most effective.
To study the time course and mechanisms of inflammatory destruction of articular cartilage, we used the human TNFα transgenic (hTNFtg) mouse model of rheumatoid arthritis. hTNFtg mice were sacrificed from weeks 2 to 14 in 2-week intervals, and sections of the hind paws were analyzed for histopathological changes. Specifically, the cartilage area and thickness were determined by histomorphometry, and proteoglycan loss was assessed by toluidin-blue staining. Inflammation was quantified by measuring the area of hyperplastic synovial tissue. The length of adhesion zones between pannus and cartilage were taken as measures for synovial attachment. To study the effects of proteoglycan loss on the attachment of synoviocytes in vitro, synovial fibroblasts were seeded onto isolated and IL-1-treated (2 ng/ml and 10 ng/ml, 24 hours) murine femoral heads, and the attachment was quantified by light microscopy.
In hTNFtg mice, synovial inflammation increased over time (0.025 mm2 at week 2 versus 0.65 mm2 at week 14; P < 0.05), with the most prominent increase between weeks 4 and 6 (0.029 mm2 and 0.78 mm2, respectively; P < 0.05). Loss of cartilage was seen only between weeks 2 and 4. After week 4, the cartilage area and thickness remained stable, arguing for a dissociation between cartilage degradation and pannus formation. As seen in toluidin-blue staining, a loss of proteoglycans occurred over time, which started at week 4 and peaked at week 8. There was a prominent attachment of pannus tissue to articular cartilage. Attachment started after week 4, peaked at week 10 and showed a decrease thereafter. This observation was supported by in vitro attachment data showing that early moderate loss of proteoglycans (as induced by 1 ng/ml IL-1) strongly enhanced attachment of synovial fibroblasts while an almost complete loss of proteoglycans (as induced by 10 ng/ml IL-1) did not facilitate fibroblast attachment.