Diet and gut microbes drive fragile T cells behind immunotherapy side effects

Scientists at the University of Pittsburgh have identified a key mechanism behind one of the most frustrating problems in modern cancer care: the severe side effects that force many patients to abandon immunotherapy. In a study published in Cell, the team showed that a population of unstable immune cells, pushed into a fragile state by diet and gut bacteria, drives the inflammatory damage known as immune-related adverse events. The findings, demonstrated in a new mouse model and corroborated with patient samples, point toward potential ways to prevent these toxicities without weakening the anti-cancer power of treatment.

Immune checkpoint inhibitors have transformed the outlook for many cancers by releasing the molecular brakes on T cells, the immune system’s primary tumor-killing warriors. But the same liberation that lets T cells attack tumors can also unleash them on healthy tissue. Depending on the specific checkpoint inhibitor used, between 20 and 40 percent of patients experience immune-related adverse events, and up to 25 percent of those receiving these therapies suffer severe episodes that end treatment and lead to hospitalization. Currently, there are no treatments for these adverse events approved by the U.S. Food and Drug Administration, leaving clinicians with few options beyond blunt immunosuppression and, often, permanent discontinuation of therapy.

“Immune related adverse events are a huge problem and can greatly impact quality of life for patients. In many cases, it impacts their ability to stay on immunotherapy or benefit from treatment,” said senior author Abigail Overacre-Delgoffe, Ph.D., assistant professor in the Department of Immunology at Pitt’s School of Medicine and a member of the Tumor Microenvironment Center at the UPMC Hillman Cancer Center.

One of the field’s central obstacles has been the absence of a faithful animal model. The side effects vary enormously from patient to patient, making them difficult to study in humans, and existing mouse models have failed to reproduce the complexity of human immune-related inflammation. “Most existing mouse models relied on genetically modified mice or experimental conditions that didn’t really capture the variability we see in patients,” said lead author Dipyaman Patra, Ph.D., a postdoctoral associate in Overacre-Delgoffe’s lab. “This got us thinking about whether common lifestyle factors, particularly diet and the gut microbiome, could influence the host immune system even before immunotherapy begins.”

That question grew out of Overacre-Delgoffe’s earlier work showing that diet and the gut microbiome can influence how cancer patients respond to immunotherapy. Her team’s answer was strikingly simple in concept: instead of engineering mice with genetic knockouts, they fed ordinary mice a Western-style diet, low in fiber and high in fat and processed carbohydrates, beginning two weeks before immune checkpoint inhibitor treatment and continuing throughout therapy. The diet alone, combined with the drug treatment, was enough to make the mice susceptible to immune-related adverse events that mirrored many of the immune and microbiome features seen in affected patients.

“We were able to generate the first of its kind preclinical mouse model of adverse events that didn’t require full gene knockouts,” said Overacre-Delgoffe. “By combining this novel preclinical model alongside patient data, we were able to identify a very new mechanism of adverse events that hopefully will result in new targeted therapies, allowing more cancer patients to stay on treatment and out of the hospital.”

The mechanism the team uncovered centers on regulatory T cells, the immune system’s peacekeepers. These cells normally suppress excessive immune responses and prevent healthy tissue from being attacked. Under stress, however, regulatory T cells can become unstable, a phenomenon known as fragility, in which they lose their suppressive function. The mice on the Western-style diet showed clear signs of this fragility during immunotherapy. Critically, when the researchers examined regulatory T cells collected from immunotherapy patients, they found the same fragility in patients who had experienced severe immune-related adverse events, but not in patients who had avoided the side effect, linking the animal finding directly to human disease.

Restoring the stability of these cells dramatically reduced the severity of the adverse events in mice, suggesting that regulatory T cell fragility is a major underlying cause of the toxicity rather than a byproduct of it. The researchers achieved this restoration in several ways: by blocking a specific immune signaling protein on regulatory T cells, by switching the mice from the Western-style diet to a high-fiber diet, and by supplying metabolite supplementation. Each intervention calmed the inflammation, offering multiple potential entry points for future therapies.

The microbiome experiments added a vivid demonstration of how powerfully diet shapes this process. Mice fed the Western-style diet developed gut microbial communities resembling those seen in humans with immune-related inflammation. When the researchers transferred fecal material from Western diet-fed mice into mice eating a regular high-fiber diet, the recipients developed gut inflammation. Conversely, fecal transfers from male mice, which are less susceptible to severe adverse events, into female mice reduced the severity of their inflammation. Most strikingly, fecal microbiome transfers from human patients consuming a Western-style diet into germ-free mice increased the severity of immune-related adverse events in those mice, evidence that diet and the gut microbiome are genuinely responsible for these toxic side effects rather than merely correlated with them.

Beyond the mechanistic insights, the new model gives researchers a platform for answering questions that have long resisted study: why some patients develop severe adverse events while others sail through treatment, and which biological markers might predict susceptibility before therapy begins. “We now have some potential targets that we’re going after that seem to limit immune-related adverse events without negatively impacting immunotherapy, which would be the first of its kind and is quite exciting,” Overacre-Delgoffe said. Future work will pursue tissue-specific drug targets that limit or eliminate the side effect without disrupting the anti-tumor immune response, and metabolite supplementation stands out as a promising intervention that could reduce susceptibility without interfering with the microbiome. The study’s additional authors include Dipyaman Patra, Katelyn Wolfgang, Chenxian Ye, Lora Rigatti, Kelsey Ertwine, Kristin M. Morder, Jessica Jana, Ishita Baral, Drew Hurd, Kaitlin Bossong, Steven Mullett, Yingtong Dou, Stacy Gelhaus, Dario Vignali, Diwakar Davar, Anthony Cillo, and Greg Delgoffe, all of Pitt, UPMC Hillman Cancer Center, or both. The research was supported by the National Institutes of Health, the Damon Runyon Cancer Research Foundation, the Gateway Foundation for Cancer Research, the Cancer Research Institute, the Sy Holzer Endowed Immunotherapy Fund, The Pittsburgh Foundation, and UPMC Hillman Cancer Center SPORE programs.

Subject of Research: Diet- and microbiome-driven regulatory T cell fragility as a cause of immune checkpoint inhibitor adverse events

Article Title: Study identifies key drivers of microbiome-linked cancer immunotherapy side effect

Article References: Study identifies key drivers of microbiome-linked cancer immunotherapy side effect. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: cancer immunotherapy, immune checkpoint inhibitors, immune-related adverse events, regulatory T cells, gut microbiome, Western diet, University of Pittsburgh, Cell journal, preclinical mouse model, fecal microbiome transfer, T cell fragility, metabolite supplementation