TSG6 transfers heavy chains onto HA, allowing for the formation of aptly named “heavy-chain HA” (HC:HA) that extends out into the extracellular environment to interact with circulating cells. We explored the role of TSG6 in inflammatory bowel disease (IBD), focusing on its mediation of HC:HA formation during disease.

When we measured TSG6 protein and mRNA levels in IBD patient tissue samples, we found both levels significantly higher compared to what we’d find in healthy patients. Does this mean that IBD patient tissue also has increased HC:HA? Staining and western blotting of heavy chains in patient colon tissue confirm this. In fact, IBD patients had higher HA production in general, measured both by the increase in the hyaluronan synthase proteins and in the HA purified in the colon tissues. TSG6 expression in colon samples was positively correlated to the amount of HA in those same samples, meaning that TSG6 increased in patient tissue, so did HA. As more TSG6 appeared to be implicated in worsened IBD symptoms, we therefore anticipated that mice with the TSG6 knocked out from their genome with have improved colitis symptoms. 

This was far from the outcome. Upon measuring the disease progression in our mice, we found our TSG6 knockout (KO) mice showed increased susceptibility to 7-day DSS colitis compared to our wild type mice. We had expected the exact opposite outcome, which led us to explore which of the KO mice’s immune system had changed. We measured the levels of cytokines circulating in our diseased KO mice’s blood and found increased levels of pro-inflammatory cytokines, as well as decreased levels of anti-inflammatory cytokines. We moved from examining the blood to examining colon tissue, focusing first on HA expression. Unlike in our human patient samples, our TSG6 KO mice had no difference in hyaluronan synthase expression compared to our WT mice. However, the HA levels in colon tissue were drastically lowered, meaning that the loss of TSG6 is not affecting HA at the initial production steps, but does alter how HA is incorporated into colon tissue.

We show that TSG6 activity— transferring heavy chains on to HA on the cell surface— allows leukocytes to adhere to. When we block TSG6 activity with our NG4 antibody, we lose not only lose a significant amount of  HC:HA cables, but also leukocytes binding to the epithelial cells. Taking this a step further, we activated monocytes (which would be their circulating state during disease) and then allowed them to bind to various states of HA. While the binding of activated monocytes to HA did result in an increase in pro-inflammatory signals, activated monocytes binding to HC:HA cables released fewer inflammatory chemicals. Therefore, the HC:HA cables, which are formed thanks to TSG6 activity, have an anti-inflammatory effect on monocytes that bind to it, which may explain why deleting TSG6 in mice worsens disease.  

 While we have shown that the activity of TSG6 represses the immune response by forming HC:HA, there is still much left to be discovered. Future studies will need to explore why increased TSG6 correlates with worsened disease in IBD patients, though our results suggest that the long-lasting, chronic inflammation of IBD eventually overcomes the protection provided by TSG6 activity.