A molecule that balances the immune system in the gut

A healthy gut requires a molecule called gp96 to train the immune system to tolerate food and normal microbes, report researchers at the Medical University of South Carolina in the May 19 issue of Scientific Reports. The study emphasizes the importance of gp96 in maintaining a balanced immune system in the gut. 

A healthy gut depends on a balance of inflammatory and tolerant T cells, which make up part of the adaptive immune system. In patients with colitis, inflammatory T cells in the lower intestines mistake the molecular structures of food or healthy gut bacteria for dangerous pathogens that must be destroyed. To determine how this happens, Bei Liu, M.D., associate professor in the MUSC Department of Microbiology and Immunology, considered the idea that these patients’ adaptive immune systems might be poorly trained.

T cells are trained by professional antigen-presenting cells (pAPCs) in the gut. pAPCs express toll-like receptors on their surfaces that recognize and trap molecular patterns called antigens on bacteria, food and our own cells. A pAPC that has trapped a specific antigen will travel to a lymph node and display that antigen to a naive, untrained T cell. The T cell then differentiates to a mature state and travels throughout the body to locate its antigen. 

Types of pAPCs 

There are two major types of pAPCs in the gut.

• One type of pAPC trains tolerant T cells to accept harmless antigens.

• Another type of pAPC trains inflammatory T cells to attack harmful antigens on microbes or molecules that may enter the gut.

Liu and her team found that without gp96 — a molecule inside most cells that helps toll-like receptors and integrins fold and function properly — pAPCs in the gut were more inflammatory. 

First, the group generated a new genetic knockout mouse, which is a modified mouse in which an existing gene has been inactivated. Without gp96, pAPCs were less able to travel to lymph nodes, for which they need integrins, and less able to respond to bacterial antigens, for which they need functioning toll-like receptors. The model showed inflammatory pAPCs greatly outnumbered tolerant ones in the lymph nodes and the colon. Because pAPCs train T cells according to their kind, the shift in their cell type also caused a shift in the types of T cells that left the lymphatic system and traveled back to the gut. More inflammatory-type T cells and less tolerant-type T cells were found in the colon of mice without gp96 as compared to wild-type mice. 

This was good proof that gp96 is needed to maintain the proper balance of T cell populations, but Liu and her group wanted to connect that finding to a biological function relevant to humans. 

Researchers also examined oral tolerance, a state in which the immune system is trained to remain neutral to food and harmless bacteria. The loss of oral tolerance may be an early change in people who develop food allergies and inflammatory bowel disease. They fed chicken ovalbumin to wild-type and gp96 knockout mice. Through the transfer of the specific T cell approach, these mice then received donor T cells that had not been exposed to ovalbumin and therefore did not recognize it as a tolerable antigen. The idea was to see if those naive donor T cells would receive antigen-specific training from their pAPCs and then become tolerant to ovalbumin. Donor T cells adopted a tolerant state in wild-type mice, but not in gp96 knockout mice. Their pAPCs were unable to properly train their immune T cells to tolerate ovalbumin.  

Given this finding, it was likely that mice without gp96 would develop signs of inflammatory bowel disease as they grew, unable to adapt to a normal diet. As expected, gp96 knockout mice grew normally at first. At about 24 weeks, however, seventy percent of them developed spontaneous colitis, while none of the wild-type mice did. Mice without gp96 also had higher levels of gut immunoglobulin A, which is associated with chronic inflammation. 

This is the first study to describe the roles of the molecular chaperone gp96 in maintaining gut homeostasis. Although this study offered proof that gp96 is required to prevent colitis, further study is needed to connect the loss of gp96 to the development of colitis in human patients. Yet this ubiquitous molecule, tasked with folding a number of proteins that immune cells need to function, may have a fundamental responsibility to maintain proper immune balance in the gut. 

Liu says it’s an important area of study given the prevalence of inflammatory bowel disease (IBD), including Crohn’s disease and ulcerative colitis, which affects an estimated 3.1 million adults. Symptoms can be debilitating and sometimes lead to life-threatening complications. A 2016 Centers for Disease Control and Prevention study found it takes a greater toll on Americans than previously believed.

Next steps for researchers

Next, Liu is building on these findings to determine the role of protein gp96 in IBD development in human patients, as well as to determine if there are certain microbes in the gut that promote inflammation and inflammation-induced colon cancer in human.

“IBD affects so many people and can cause a poor quality of life. However, the exact cause of this disease is still unknown. Knowing what may be causing it from this study, will have a potential therapeutic value for treatment of human IBD,” says Liu.