Background
Anti-TNF therapies are effective at preventing inflammation and structural damage in rheumatoid arthritis (RA). However, the role of TNF in cartilage destruction in RA is not well understood. Therefore, we studied the effects of TNF on cartilage and compared TNF production by different cell types involved in joint pathology.
Methods
Primary human chondrocytes and cartilage explants were cultured with recombinant TNF. Bovine cartilage was co-cultured with activated human peripheral blood mononuclear cells (PBMCs) or fibroblast-like synoviocytes (FLS). Expression of cytokines and metalloproteinases (MMPs) was assessed by qPCR and MSD, and proteoglycan depletion from cartilage was assessed using histomorphometry and colorimetric detection in tissue culture supernatants. D2E7 was used to block TNF both in vitro and in vivo in a human TNF transgenic (hTNF-Tg) mouse model of arthritis.
Results
TNF elicited strong pro-inflammatory and catabolic effects on isolated human chondrocytes and cartilage explants leading to upregulation of IL-6 and MMPs, as well as proteoglycan depletion from bovine cartilage explants. In an effort to identify cellular sources of TNF, we challenged chondrocytes, FLS and PBMCs with inflammatory stimuli present in RA joints and found that PBMC that were used to model inflammatory cell infiltration produced significantly higher levels of TNF. Moreover, co-culture with activated PBMCs resulted in proteoglycan depletion from bovine cartilage explants. In sharp contrast with stromal cells, TNF failed to induce high amounts of IL-6 and MMPs in PBMCs, suggesting that different cell populations play distinct roles in the triggering and propagation of joint destruction. TNF blockade protected cartilage from damage both in co-culture systems and in a hTNF-Tg mouse model of arthritis.
Conclusions
Our data demonstrate that TNF directly triggers a catabolic program in human chondrocytes leading to cartilage damage and further suggest that neutralization of TNF produced by immune cells infiltrating the inflamed joints, decreases catabolic activity of chondrocytes and fibroblasts, which, in turn, contributes to the cartilage protective effects of anti-TNF biologics in arthritis.