The severity of immune-mediated intestinal diseases such as graft-versus-host disease (GVHD) or inflammatory bowel diseases (IBD) is known to be associated with alterations in the gut microbiome, but what leads to such disruption in the microbial community has remained a mystery. Researchers at Baylor College of Medicine, the University of Michigan, and collaborating institutions working with animal models of GVHD have now discovered that alterations in the gut microbiome are connected to an increase in oxygen levels in the intestine that follows immune-mediated intestinal damage. The research also showed that pharmacologically reducing intestinal oxygen levels alleviated the microbial imbalance and reduced severity of the intestinal disease.

“There is a lot of data showing that microbes change in many diseases, but we do not understand how that happens,” said leading author Pavan Reddy, PhD, professor and director of Baylor’s Dan L. Duncan Comprehensive Cancer Center, who was at the University of Michigan during the development of this project. “This study is one of the first to provide an explanation and a potential solution for the imbalance in the gut microbiome that exacerbates GVHD and possibly other inflammatory intestinal conditions.” Reddy and colleagues reported on their study in Immunity, in a paper titled, “Ambient oxygen levels regulate intestinal dysbiosis and GVHD severity after allogeneic stem cell transplantation,” in which they concluded, “… targeting ambient intestinal oxygen levels may represent a novel, non-immunosuppressive strategy to mitigate T cell-driven intestinal diseases.”

The composition of the gut microbiome directly contributes to human health and diseases, the authors commented. “… a healthy gut is inhabited by a diverse community of mostly obligate anaerobic bacteria (eubiosis) that is influenced by the host genetics, diet, and immunity.” A breakdown of the balance in this diverse community of microorganisms—dysbiosis—is often characterized by a shift in microbial community structure, and is associated with diseases including immune-mediated intestinal diseases such as GVHD, the team continued. However, they noted, “While a strong correlation exists between disease severity and dysbiosis, the mechanisms that lead to dysbiosis are unknown.

Gastrointestinal GVHD (GI GVHD) is a potentially life-threatening complication of bone marrow transplantation (allogeneic hematopoietic stem cell transplantation; allo-HSCT). “It is the complication that can prevent us from using this therapy that has proven to be effective to treat many blood cancers and inherited blood diseases,” Reddy said. However, as the authors pointed out, while a reduction in intestinal microbial diversity, and the loss of obligate anaerobes relative to other bacteria is associated with increased GVHD mortality, the role of the microbiome in GVHD, and the mechanisms that underpin the development of dysbiosis, remains unclear. It’s not well understood whether dysbiosis is caused by or is a consequence of severe GVHD, or whether dysbiosis before or after HSCT directly increases, or negatively regulates, GVHD severity. “… whether dysbiosis is a cause or consequence, an amplifier or a mollifier, and the mechanisms remain elusive,” the team stated. “The idea is to understand what makes GVHD worse so we can effectively control it,” Reddy further commented.

Through their studies in mouse models of GI GVHD, Reddy and colleagues discovered that the damage immune cells cause to intestinal cells prevents these cells from fully using oxygen to conduct their normal functions. “… pre-transplant dysbiosis by itself is not pathogenic, but rather post-HCT dysbiosis is a consequence of the changes in intestinal luminal oxygen level from the tissue injury caused by allogeneic donor T cells after allo-HCT,” they noted. Consequently, all the oxygen that is not being used by intestinal cells oozes into the intestine, changing the environment for the resident microbes. “… we show in multiple specific pathogen-free (SPF), gnotobiotic, and germ-free murine models of GI GVHD that the initiation of the intestinal damage by the pathogenic T cells altered ambient oxygen levels in the GI tract and caused dysbiosis,” the scientists wrote.

“Most of the ‘good microbes’ we have in the intestine grow in oxygen-poor environments—oxygen is toxic to them,” Reddy noted. “They are called anaerobic (without oxygen) bacteria.” When oxygen levels in the intestine increase, these microbes tend to disappear, and oxygen-loving microbes tend to grow. An increase in oxygen level provides an explanation for the microbiome changes in the context of these inflammatory diseases.”

The findings suggested that restoring the normal environment by reducing the oxygen level in the intestine could help re-establish the balance of the microbial community and lead to attenuation of GVHD. “Indeed, we discovered that reducing the intestinal oxygen level actually made a difference in the progression of GVHD in the animal models,” Reddy said. “We found that a commonly used drug to reduce iron overload, an iron chelator, mitigated the microbial imbalance and reduced the severity of GVHD.”

Iron (Fe) chelators have been used for many years to treat conditions in which excess iron causes tissue damage, such as hemochromatosis. Iron chelators are compounds that bind to iron, pulling it out and removing it from the body. “We discovered that iron chelators also can act as oxygen sinks,” Reddy said. “In our animal models, iron chelators removed iron from the intestine and that facilitated the restoration of an oxygen-poor environment that gave anaerobic bacteria an opportunity to bloom. Importantly, this reduced the severity of GVHD.” The team explained in their paper, “… targeting excess O2 with iron chelation promoted physiologic hypoxia, mitigated dysbiosis, and attenuated GVHD severity, thus suggesting that Fe chelation with available oral drugs may be a novel strategy to clinically mitigate GVHD severity.”

The researchers’ next steps include conducting studies to determine whether iron chelation can help control the severity of GVHD in patients who have received a bone marrow transplant. “… our data provide a mechanism for induction of dysbiosis and demonstrate that promotion of eubiosis by regulating ambient oxygen reduces the severity of intestinal damage after allo-HCT,” they wrote. “Thus, targeting ambient intestinal oxygen levels may represent a novel, non-immunosuppressive strategy to mitigate T cell-driven intestinal diseases.” Reddy added. “The study also is relevant to more common inflammatory bowel diseases, including Crohn’s disease and ulcerative colitis.”

Another advantage of iron chelation would be that it may reduce or avoid the use of immune suppressor medications that are usually used to control GVHD. Suppressing the immune system may control GVHD, but also favors infections, which can be life-threatening. “If iron chelation helps control the condition in patients, it would be a novel non-immunosuppressive approach to treat GVHD with seemingly little side effects,” Reddy said.

The authors acknowledged limitations of their study, and the need for further research. Nevertheless, they concluded, “our data thus provide a novel role for tissue O2 as a mechanism for dysbiosis and Fe chelation as a therapeutic target for amelioration of non-infectious immune-mediated intestinal diseases.”