ABOVE: Regulatory T cell (red) interacting with an antigen-presenting cell (blue) NIAID

Without the trillions of bacteria in the gut, muscles might not be able to knit themselves back together after an injury. According to a study published February 22 in Immunity, T cells that normally reside in the mouse colon play a crucial role in tissue regeneration—and rely on gut microbes to do so. Without these helpful microbes, the study suggests, inflammation could get out of control, preventing healing and causing fibrosis.

“The main message of the paper is that the microbiota is influencing your immune system and your general health in a way larger way than we appreciated before,” 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 because they rein in other immune cells, ensuring inflammation doesn’t get out of control.

“To find that immune cell populations that are modified [in the gut] . . . have systemic effects and influence physiological and pathophysiological processes that occur elsewhere is obviously of major interest,” says Alexander Rudensky, an immunologist at Memorial Sloan Kettering Cancer Center who was not involved in the study. “It sets the stage to explore further other aspects of physiology that can be affected by the cells generated in the intestine.”

Hanna says that the study started spontaneously, as he was taking an in-depth look at the role of regulatory T cells in wound healing. Following an injury, regulatory T cells flock to damaged muscle. Their numbers peak four days later, after which the tissue goes from an inflammatory state to an anti-inflammatory state. It’s thought that regulatory T cells mediate this transition, which is a crucial step for wound healing.

When the study first started, Hanna was profiling the T cells at an injury site using single-cell RNA sequencing. This analysis pinpointed several types of T cells, but one stood out to Hanna: he observed ones that expressed a transcription factor called RORγ, a hallmark of regulatory T cells that reside in the colon.

Colon T cells have many roles, including ensuring that the other immune cells don’t attack the helpful microbes living there. They also play a role in metabolism and digestion.

Intrigued by these initial findings, Hanna set out to show that these cells had, in fact, journeyed 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 the body called Kaede, which becomes red in response to light. Then, using a laser, the researchers zapped immune cells in the mice’s guts. “Now, we were able to track these colonic regulatory T cells and see if they could leave the colon and go to other places,” says Hanna.

The cells didn’t just go to the muscle; they traveled to other areas of the body, including the organs, the team observed. And after an injury, tagged cells appeared in the damaged tissue, suggesting that cells from the colon 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 populations of T cells existed in the gut and muscle of healing 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 the regulatory T cells in the gut and the muscle during healing belonged to the same clonal populations, again suggesting that these regulatory T cells had originated in the gut and traveled to the muscle.

However, it wasn’t clear what role these regulatory T cells played in repairing muscle. In another experiment, the researchers used genetically engineered mice that lack 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 mice following injury in comparison to normal mice. Unregulated levels of IL-17 have been linked to delayed wound healing.

The team then directly tied these T cells to gut microbes. They found that mice that had either grown up in a germ-free environment or received antibiotics couldn’t heal from injury as well as normal mice and suffered from fibrosis. That’s because, 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 finding that gut microbes play a pivotal 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 don’t just help muscles heal after injury; they also work to heal the liver from damage. In mouse models of nonalcoholic fatty liver disease, regulatory T cells helped slow inflammation, and their functioning was dependent on the microbiome.

Hanna says that “this work raises a question about the use of antibiotic treatment, since colon regulatory T cells are highly dependent on microbiota. We might have to be judicious about whether we use antibiotics in the case of tissue injury.” This might be especially relevant to patients just after surgery or those with severe wounds, since both groups often receive antibiotics. In addition, 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 body,” says Hanna. “They are impacting us in ways beyond our understanding.”