OWithout the trillions of bacteria in the gut, muscles might not be able to rebuild themselves after injury. According to a study published on February 22 in Immunity, T cells that normally reside in the mouse colon play a crucial role in tissue regeneration and depend on gut microbes to do so. Without these helpful microbes, the study suggests inflammation could spiral out of control, preventing healing and causing fibrosis.
“The main message of the article is that the microbiota influences your immune system and your general health in a far greater way than we previously appreciated,” says Bola Hanna, an immunologist at Harvard Medical School. Hanna studies regulatory T cells, a class of immune cells found in tissues throughout the body. He describes regulatory T cells as the ‘peacekeepers’ of the immune system, as they keep other immune cells in check, ensuring that inflammation doesn’t get out of control.
“To discover that immune cell populations that are altered [in the gut] . . . having systemic effects and influencing physiological and pathophysiological processes occurring elsewhere is obviously of major interest,” says Alexander Rudensky, an immunologist at Memorial Sloan Kettering Cancer Center who was not involved in the study. “This opens the door to exploring other aspects of physiology that may be affected by cells generated in the gut.”
Hanna says the study started spontaneously, as he looked in depth at the role of regulatory T cells in wound healing. Following an injury, regulatory T cells flock to the damaged muscle. Their number peaks four days later, after which the tissue changes from an inflammatory to an anti-inflammatory state. Regulatory T cells are thought to mediate this transition, which is a crucial step for wound healing.
When the study began, Hanna was profiling T cells at an injury site using single-cell RNA sequencing. This analysis identified several types of T cells, but one stood out for Hanna: he observed some that expressed a transcription factor called RORγ, a hallmark of the regulatory T cells that reside in the colon.
T cells in the colon have many roles, including making sure other immune cells don’t attack the helpful microbes that live there. They also play a role in metabolism and digestion.
Intrigued by these early findings, Hanna set out to show that these cells had in fact traveled from the gut. He and his colleagues used a technique called optical tagging to track these cells as they moved. They genetically engineered mice to express a special type of green fluorescent protein throughout their bodies called Kaede, which turns red in response to light. Then, using a laser, the researchers zapped the immune cells in the intestines of the mice. “Now we were able to track these colonic regulatory T cells and see if they could leave the colon and go somewhere else,” says Hanna.
The cells didn’t just go to the muscle; they traveled to other areas of the body, including organs, the team observed. And after injury, labeled cells appeared in the damaged tissue, suggesting colon cells had indeed traveled to the muscle.
The researchers used T-cell receptor (TCR) sequencing, a method of profiling T-cell receptors, to show that the same T-cell populations existed in the gut and muscle of the healer mice. TCRs are markers on the surface of mature T cells that bind to a specific antigen and are unique to a population of cells from the same parent cell, also called a clonal population. The results showed that regulatory T cells in gut and muscle during healing belonged to the same clonal populations, again suggesting that these regulatory T cells originated in gut and traveled to muscle.
However, the role played by these regulatory T cells in muscle repair was unclear. In another experiment, the researchers used genetically modified mice that lacked RORγ+ regulatory T cells; they found that these mice healed more slowly and developed fibrosis. Using flow cytometry, they found higher levels of IL-17, an inflammatory cytokine, in the wounds of these post-injury mice compared to normal mice. Unregulated levels of IL-17 have been related delay wound healing.
The team then linked these T cells directly to gut microbes. They found that mice that grew up in a germ-free environment or were given antibiotics couldn’t heal wounds as well as normal mice and suffered from fibrosis. Indeed, in the gut, regulatory T cells activate in response to antigens produced by food and bacteria, but otherwise remain in a naïve, quiescent state. Without a microbial community, mice lack these regulatory T cells in the colon and in injured muscles.
Hanna calls the discovery that gut microbes play a central role in wound healing “really cool.”
The team also explored the role of gut-derived regulatory T cells in other forms of healing, finding that these microbiota-activated cells not only help muscles heal after injury; they also work to heal liver damage. In mouse models of nonalcoholic fatty liver disease, regulatory T cells helped slow inflammation and their function was microbiome-dependent.
Hanna points out that “this work raises a question about the use of antibiotic treatment, because colonic regulatory T cells are highly dependent on the microbiota. We may need to be judicious about whether or not to use antibiotics in the event of tissue damage. This could be particularly relevant for patients right after surgery or those with severe wounds, as both groups often receive antibiotics. Additionally, IL-17 is linked to autoimmune disorders and increased tumorigenesis, meaning the gut could also play a role in other inflammatory disorders.
“I’m personally intrigued by the beauty of this cross-communication that we have in our bodies,” says Hanna. “They affect us in ways beyond our comprehension.”