The inflammation-associated protein TSG-6 cross-links hyaluronan via hyaluronan-induced TSG-6 oligomers

Tumor necrosis factor-stimulated gene-6 (TSG-6) is a hyalu-ronan (HA)-binding protein that plays important roles ininflammation and ovulation. TSG-6-mediated cross-linking ofHA has been proposed as a functional mechanism (e.g.for regu-lating leukocyte adhesion), but direct evidence for cross-linkingis lacking, and we know very little about its impact on HA ultra-structure. Here we used films of polymeric and oligomeric HAchains, end-grafted to a solid support, and a combination ofsurface-sensitive biophysical techniques to quantify the bindingof TSG-6 into HA films and to correlate binding to morpholog-ical changes. We find that full-length TSG-6 binds with pro-nounced positive cooperativity and demonstrate that it cancross-link HA at physiologically relevant concentrations. Ourdata indicate that cooperative binding of full-length TSG-6arises from HA-induced protein oligomerization and that theTSG-6 oligomers act as cross-linkers. In contrast, the HA-bind-ing domain of TSG-6 (the Link module) alone binds withoutpositive cooperativity and weaker than the full-length protein.Both the Link module and full-length TSG-6 condensed andrigidified HA films, and the degree of condensation scaled withthe affinity between the TSG-6 constructs and HA. We proposethat condensation is the result of protein-mediated HA cross-linking. Our findings firmly establish that TSG-6 is a potent HAcross-linking agent and might hence have important implica-tions for the mechanistic understanding of the biological func-tion of TSG-6 (e.g.in inflammation).