Cartilage-specific constitutive expression of TSG-6 protein (product of tumor necrosis factor alpha-stimulated gene 6) provides a chondroprotective, but not antiinflammatory, effect in antigen-induced arthritis.

Objective: To study the chondroprotective effect of constitutively expressed TSG-6 protein (tumor necrosis factor alpha-induced protein 6; Tnfip6) in cartilage, using antigen-induced arthritis (AIA) in mice.

Methods: Transgenic mice constitutively expressing TSG-6 protein in cartilage were generated. Cartilage-specific constitutive expression of TSG-6 protein was confirmed by in situ hybridization, Western blot analysis, and immunohistochemistry. Control and transgenic mice were immunized with methylated bovine serum albumin (mBSA), and arthritis was induced by the intraarticular injection of mBSA. Mice were monitored up to day 35 after the challenge, and knee joint sections were examined for loss of cartilage proteoglycan (aggrecan) using Safranin O staining and antibodies to neoepitopes generated by various metalloproteinases (MPs). The loss of aggrecan in Safranin O-stained sections was quantified by morphometric methods.

Results: Tsg6/tnfip6 transgenic mice constitutively expressed tsg6/tnfip6 messenger RNA and corresponding TSG-6 protein in cartilage from embryonic life through adulthood, without any phenotypic abnormalities. These mice were used for AIA studies. Intraarticular injection of mBSA uniformly induced severe inflammation both in control (wild-type and an irrelevant transgenic line) mice and in tsg6/tnfip6 transgenic mice. In contrast to the mBSA-injected knee joints of control animals that were heavily damaged from day 5, the cartilage of transgenic mice that constitutively expressed TSG-6 protein remained intact for at least 1 week, and this was followed by a relatively reduced loss of aggrecan. Concomitant with the loss of aggrecan, MP-generated neoepitopes accumulated in unprotected joints. By day 35, the proteoglycan content returned to nearly normal levels in tsg6/tnfip6 transgenic mice, whereas it remained low in MP-damaged knee cartilage of control mice.

Conclusion: TSG-6 protein is known to form a complex with inter-alpha-inhibitor (IalphaI), a potent serine protease inhibitor, which may be immobilized via the hyaluronan (HA)-binding domain of TSG-6 protein in the HA-rich extracellular matrix of cartilage. Thus, the local accumulation of TSG-6 protein and TSG-6 protein-bound IalphaI in tsg6/tnfip6 transgenic mice may inhibit serine proteases and subsequent activation of MPs. It is suggested that this mechanism might protect cartilage from extensive degradation even in the presence of acute inflammation.