TSG-6 Protein, a Negative Regulator of Inflammatory Arthritis, Forms a Ternary Complex with Murine Mast Cell Tryptases and Heparin

TSG-6 (TNF-α-stimulated gene/protein 6), a hyaluronan (HA)-binding protein, has been implicated in the negative regulation of inflammatory tissue destruction. However, little is known about the tissue/cell-specific expression of TSG-6 in inflammatory processes, due to the lack of appropriate reagents for the detection of this protein in vivo. Here, we report on the development of a highly sensitive detection system and its use in cartilage proteoglycan (aggrecan)-induced arthritis, an autoimmune murine model of rheumatoid arthritis. We found significant correlation between serum concentrations of TSG-6 and arthritis severity throughout the disease process, making TSG-6 a better biomarker of inflammation than any of the other arthritis-related cytokines measured in this study. TSG-6 was present in arthritic joint tissue extracts together with the heavy chains of inter-α-inhibitor (IαI). Whereas TSG-6 was broadly detectable in arthritic synovial tissue, the highest level of TSG-6 was co-localized with tryptases in the heparin-containing secretory granules of mast cells. In vitro, TSG-6 formed complexes with the tryptases murine mast cell protease-6 and -7 via either heparin or HA. In vivo TSG-6-tryptase association could also be detected in arthritic joint extracts by co-immunoprecipitation. TSG-6 has been reported to suppress inflammatory tissue destruction by enhancing the serine protease-inhibitory activity of IαI against plasmin. TSG-6 achieves this by transferring heavy chains from IαI to HA, thus liberating the active bikunin subunit of IαI. Because bikunin is also present in mast cell granules, we propose that TSG-6 can promote inhibition of tryptase activity via a mechanism similar to inhibition of plasmin.