Cells in the gut send secret messages to the immune system, and a study by researchers at La Jolla Institute for Immunology (LJI) has now managed to glimpse at what they’re saying. The research revealed how epithelial cells lining the intestines communicate with resident patrolling T cells by expressing a protein called HVEM, which prompts the T cells to survive longer and move more to stop potential infections.

“The research shows how barrier cells in the intestine, structural elements of the tissue, and resident immune cells communicate to provide host defense,” said LJI professor and CSO Mitchell Kronenberg, PhD. Going forward, Kronenberg and colleagues are interested in investigating the role of HVEM in maintaining a healthy population of gut microbes. Kronenberg said there are signs that a lack of HVEM can sway the composition of the gut microbiome even in the absence of pathogenic bacteria.

Kronenberg is senior author of the team’s published study in Science Immunology, which is titled, “Epithelial HVEM maintains intraepithelial T cell survival and contributes to host protection.”

Intestinal intraepithelial lymphocytes (IELs) are one of the largest populations of lymphocytes in the body, the authors explained. “They are found above the basement membrane within the intestinal epithelium, and they interact extensively with intestinal epithelial cells (IECs) by actively patrolling the basement membrane and by migration into the lateral intercellular space.” IELs are believed to play a crucial role in maintaining the intestinal barrier integrity, and for wound repair, and protection against pathogens. IELs include innate lymphoid cells (ILCs) “ … but are mostly T lymphocytes, referred to as intraepithelial T cells (IETs),” the authors continued. “IETs are migratory cells located above the basement membrane, which they patrol extensively.”

One might picture how the cells are lining up like a busy queue outside a nightclub. The epithelial cells are tightly squashed up together, while the T cell security guards circulate around the line, patrolling for signs of trouble. “These T cells move around the epithelial cells as if they are truly patrolling,” said Kronenberg.

However, the authors noted, while the IETs stay in close contact with IECs, “how intestinal epithelial cells and basement membrane influence IET survival and function, at steady state or after infection, is unclear.” What keeps these T cells in the epithelium to do their job? “We’ve got some insight on what gets T cells to the gut, but we need to understand what keeps them there,” said Kronenberg. In fact, a lot of immune cells reside long-term in specific tissues. By understanding the signals that keep T cells in certain tissues, Kronenberg hopes to shed light on conditions like inflammatory bowel disease, where far too many inflammatory T cells gather in the bowel.

Through their new study, the researchers found that important signals in the gut are sent through the basement membrane, a thin layer of proteins beneath the epithelium. In the nightclub scene analogy, the basement membrane would be the sidewalk on which everyone stands. The experiments showed that epithelial cells receive signals through HVEM proteins on their surface that stimulate synthesis of basement membrane proteins. The findings demonstrated that without HVEM, the epithelial cells couldn’t do their job because they produced less collagen and other structural components needed to maintain a healthy basement membrane.

The results indicated that T cells detect the basement membrane via adhesion molecules they express on their surface, called integrins. The interaction of the T cell integrins with the basement membrane proteins promotes messages that allow the T cells to survive and patrol in the epithelium.  It is as if the epithelial cells have written messages on the sidewalk: “Stay here,” “Patrol here,” “Do your job.”  The team further explained, “Ligand binding to epithelial HVEM at steady state stimulated the synthesis of extracellular membrane proteins, a key one being collagen IV. Collagen IV affected cell survival by binding to ß1 integrins expressed by the IETs.” But without a sufficient basement membrane, T cells could not survive as well or go on patrol.

Using a mouse model, the researchers showed that removing HVEM expression selectively in the gut epithelial cells dealt a major blow to gut health. Without HVEM the patrolling T cells weren’t able to survive as well, they didn’t move as much, and were less effective at staving off an infection. When challenged with an invasive strain of Salmonella that causes gastroenteritis, the T cells couldn’t prevent the infection from taking over the intestines and spread to the liver and spleen.

“Intravital microscopy showed that the patrolling movement of IETs was reduced without epithelial HVEM,” the scientist wrote. “As likely consequences of decreased number and movement, protective responses to Salmonella enterica were reduced in mice lacking either epithelial HVEM, HVEM ligands, or ß1 integrins …”

The results indicated that HVEM from epithelial cells laid the groundwork for T cells to guard the gut—it was the very reason they survived in the epithelium—communicating with the T cells indirectly through the basement membrane. “Here, we showed that HVEM expressed by small intestine (SI) epithelial cells was involved in the homeostasis of natural IETs, the patrolling function of IETs at steady state, and the response to pathogenic bacteria,” the team further noted. “Therefore, the data reveal how epithelial responses influencing the basement membrane, a structural element of tissue, are integrated to regulate tissue-resident T cells in the intestine at steady state and after infection.”

The experiments were spearheaded by study first authors Goo-Young Seo, PhD, instructor at LJI, and Daisuke Takahashi, PhD, formerly of LJI and now at Keio University in Tokyo. The team worked closely with the laboratory of LJI professor Hilde Cheroutre, PhD, the LJI Microscopy Core, the LJI Flow Cytometry Core, and employed intra-vital imaging RNA  sequencing techniques to investigate HVEM’s role in the gut.