The results of studies in mice by researchers at Georgia State University suggest that targeted immunization against bacterial flagellin, the major structural protein of bacterial flagella, could represent a new strategy to protect against chronic inflammatory diseases of the gastrointestinal tract. The scientists, headed by a team at the Institute for Biomedical Sciences and the Neuroscience Institute, found that repeated injection of flagellin into experimental mice was associated with increased production of anti-flagellin antibodies, and led to beneficial changes to intestinal microbiota, in terms of reduced numbers of flagellated bacteria and their ability to cause inflammation. Immunized animals were also protected against developing colitis and diet-induced obesity.

The findings, published in Nature Communications, could feasibly lead to the development of a new way to vaccinate against diseases associated with chronic inflammation of the digestive tract, such as inflammatory bowel diseases (IBD), as well as protect against obesity and metabolic syndrome, the researchers suggested. “The administration of flagellin, and perhaps other bacterial antigens, has the potential to vaccinate against an array of diseases associated with, and driven by gut inflammation,” said Benoit Chassaing, PhD, senior author of the study and assistant professor in the Neuroscience Institute and the Institute for Biomedical Sciences at Georgia State, and team leader at the National Institute for Health and Medical Research and the Universite de Paris in Paris, France.

“This work is a proof of concept and demonstrates that targeted training of the immune system can protect against an array of chronic inflammatory diseases.” Chassaing and colleagues reported their findings in a paper titled, “Flagellin-elicited adaptive immunity suppresses flagellated microbiota and vaccinates against chronic inflammatory diseases.”

Previous studies have shown changes to intestinal microbiota are associated with inflammatory bowel diseases, such as ulcerative colitis and Crohn’s disease, and diseases characterized by low-grade inflammation of the intestinal tract, such as obesity and metabolic syndrome. “An array of chronic inflammatory diseases are associated with dysbiosis in the intestinal microbiota and a breakdown in the normally mutually beneficial host–microbiota relationship,” the authors explained. Germ-free mice receiving microbiota transplants from IBD patients or animals with colitis will also develop intestinal inflammation, indicating that microbiota dysbiosis can play an active role in driving such inflammatory diseases.

The Georgia State University researchers had previously discovered that a common feature of microbiotas associated with inflammation is an increased level of flagellin expression by specific microbiota species, and this can drive bacteria to penetrate the intestinal mucosa and disrupt homeostasis. “Elevated levels of flagellin might reflect enriched levels of motile bacteria that have high ability to penetrate the mucus layer that serves to protect the host against microbial onslaught,” the team suggested. “The link between elevated microbiota flagellin levels and intestinal inflammation is thought to involve flagellin’s ability to activate pro-inflammatory gene expression via TLR5 and the NLRC4 inflammasome.”

Interestingly, they pointed out, the coating of gut bacteria by flagellin-specific IgA—which occurs normally in homeostasis—acts to suppress levels of flagellated bacteria and protects against microbiota encroachment that is thought to play a role in promoting IBD and metabolic syndrome. “Adaptive immunity to flagellin is an established feature of IBD, especially Crohn’s disease, in that IBD patients have elevations in serum antibodies (IgG and IgA) and a greater frequency of flagellin-specific CD4 T cells,” the scientists noted. They hypothesized that “boosting levels of mucosal flagellin-specific IgA might help keep flagellated bacteria in check and, consequently protect against development of chronic gut inflammation.”

To test this hypothesis the team repeatedly immunized mice with flagellin to elicit an adaptive immune response and trigger the production of anti-flagellin antibodies. The experiments showed that this targeted immunization against bacterial flagellin elicited the production of systemic and mucosal anti-flagellin antibodies, and was associated with lower levels of fecal flagellin. Immunized mice also exhibited beneficially altered intestinal microbiota, which was linked with a lower proinflammatory state. Encouragingly, the flagellin injections protected animals against subsequent immune dysregulation-induced colitis, and also protected animals against diet-induced obesity.

Recent prior work by the Chassaing team, and by others, had shown that microbiota encroachment was a feature of metabolic syndrome in humans. “We suspected that one factor that might contribute to such microbiota encroachment is relatively low amount of flagellin-specific mucosal IgA relative to the amount of flagellated bacteria present,” they wrote. To investigate this they measured levels of flagellin and flagellin-specific IgA in fecal samples that had been previously isolated from human subjects ranging in body mass index from healthy to obese. As suspected, analyses indicated that anti-flagellin IgA levels were inversely proportional to flagellin in the feces. So, levels of fecal flagellin were comparatively higher in overweight individuals than they were in normal weight subjects, and even higher in obese subjects. In contrast, levels of flagellin-specific IgA were lower in fecal samples from obese subjects, than they were in samples from normal weight subjects.

“These results are in accord with the possibility that insufficient immune responses to flagellin contribute to low-grade inflammation thought to promote this disorder and, consequently, that immunization with flagellin may be able to prevent or ameliorate this disorder,” they wrote. Experiments in the mice showed that when fed on an obesogenic high-fat diet, flagellin-immunized mice gained less weight and put on less fat than non-immunized control animals. “Moreover, flagellin immunization was associated with decreased intestinal inflammation,” the investigators wrote.

“These results support the notion that natural acquisition of flagellin-specific IgA protects against chronic inflammatory diseases, thus leading to the suggestion that eliciting anti-flagellin antibodies via immunization might be a means to vaccinate against such diseases,” the authors concluded. “… elicitation of flagellin-specific IgA via exogenous administration of flagellin has potential to protect against some chronic inflammatory diseases, including colitis and obesity.” Chassaing acknowledged that “ … significant work is now needed to test other antigens, other immunization routes and additional inflammatory models, as well as the human relevance of these findings.” Nevertheless, added co-author Andrew Gewirtz, PhD, a professor in Georgia State’s Institute for Biomedical Sciences, “If the approach proves translatable to humans, its impact on public health would be enormous.”