Researchers at the RIKEN Center for Integrative Medical Sciences (IMS) have discovered that a particular combination of microorganisms in the gut can worsen symptoms of multiple sclerosis in a mouse model. Their study found that specific gut bacteria, including a newly identified strain, enhance the activity of immune cells that attack the body’s brain and spinal cord. Reporting in Nature, the team suggests their findings could ultimately help scientists to develop strategies against multiple sclerosis. “These data suggest that the synergistic effects that result from the presence of these microorganisms should be considered in the pathogenicity of multiple sclerosis, and that further study of these microorganisms may lead to preventive strategies for this disease,” they concluded in their published paper, which is titled, “Gut microorganisms act together to exacerbate inflammation in spinal cords.”
Multiple sclerosis is an autoimmune disease in which the immune system attacks the myelin that covers the nerve cells of the brain and spinal cord. Demyelination impacts on how quickly neurons can communicate with each other and with muscles, causing a variety of symptoms, including numbness, weak muscles, tremors, and difficulty walking. Gut microorganisms have been reported to affect symptoms of multiple sclerosis, but how bacteria in the intestines can affect myelin of the brain and spinal cord remained a mystery. “Accumulating evidence indicates that gut microorganisms have a pathogenic role in autoimmune diseases, including in multiple sclerosis,” the team noted. “Studies of experimental autoimmune encephalomyelitis (an animal model of multiple sclerosis), as well as human studies, have implicated gut microorganisms in the development or severity of multiple sclerosis. However, it remains unclear how gut microorganisms act on the inflammation of extra-intestinal tissues such as the spinal cord.”
Researchers led by Hiroshi Ohno, PhD, at RIKEN IMS, aimed to try and understand this connection, using a mouse that develops experimental autoimmune encephalomyelitis (EAE), as a model for multiple sclerosis. These animals experience demyelination of the spinal cord, which results from autoimmune attack by CD4+ T cells that produce the cytokine IL-17A.
The team’s initial experiments showed that giving these mice the antibiotic ampicillin (but not other antibiotics on their own) reduced demyelination and attenuated the symptoms of EAE. Ampicillin treatment also prevented the activation of these specific T cells. “Mice that were orally treated with ampicillin were protected from demyelination of the spinal cord, and from infiltration of cells (including CD4+ T cells that produce IFNγ and IL-17A) into the spinal cord,” they wrote. Ohno further explained, “We found that treatment with ampicillin, and only ampicillin, selectively reduced activity of T cells that attack an important protein called myelin oligodendrocyte glycoprotein [MOG], which helps myelin stick to neurons.”
The observations were confirmed by taking immune cells from the animals’ small intestines and other regions and measuring cytokine production in the presence of a MOG protein fragment, MOG35-55. The results showed that cytokine production was only reduced by ampicillin, and only when the T cells came from the small intestine. The team realized from their results that microorganisms in the small intestine were acting to activate MOG-specific T cells, which could then attack myelin. “These observations support the hypothesis that MOG-specific T cells are activated by microorganisms in the small intestine,” they wrote.
The next step was to figure out which bacteria were responsible. Because ampicillin was the only antibiotic that reduced symptoms in the mouse model, the researchers looked for microbiota that were almost completely deleted specifically in the ampicillin-treated mice. They found only one such bacteria, a new strain called OTU0002, which genome sequencing indicated was probably a newly isolated bacterium of the Erysipelotrichaceae family. Interestingly, the team found bacteria of the same family that are closely related to OTU0002, in other mammals, including humans.
To test the hypothesis that OTU0002 was the culprit in their mouse model, they examined mice that lacked all bacteria except OTU0002. They found that the EAE symptoms in these mice were more severe than those in completely germ-free mice. This indicated that the newly discovered gut bacterium was responsible for the worsening symptoms. “But, there was a problem,” commented first author Eiji Miyauchi, PhD. “Symptoms in the OTU0002-only mice were not as bad as those in the regular model mice. This means that the original effect must involve more than one microorganism.”
Previous research had found that antigens from gut microbiota cross-react with, and activate autoreactive T cells in a condition known as autoimmune uveitis. The RIKEN IMS scientists reasoned that a different bacterium might be cross-reacting with MOG-specific T cells, mimicking the location on MOG that the T cells recognized. “We, therefore, hypothesized that antigens from bacteria in the small intestine that cross-react with MOG-specific T cells may have an essential role in the development of EAE,” they wrote.
Shotgun genome sequencing revealed that a protein expressed by Lactobacillus reuteri does resemble a region of MOG, and when tested, it weakly activated MOG-specific T cells. Experiments also showed that EAE symptoms in mice co-colonized with L. reuteri and OTU0002 were more severe than those in OTU0002-only mice, and were just as severe as those in the original model mice, indicating that these two bacteria worked together produced far more devastating results.
“Our study emphasizes the necessity of considering the synergic effects of gut microorganisms on autoimmune diseases, and suggests that the expansion and education of autoreactive T cells by microorganisms in the small intestine may have a critical role during the development of EAE,” they noted. “Other studies have focused on fecal microbes, or a single microbe, in patients with multiple sclerosis or in model mice,” said Miyauchi. “Our data emphasize the necessity of considering the synergistic effects of intestinal microbes on autoimmune diseases and give hope to people looking for effective treatments for multiple sclerosis.” However, Miyauchi continued, further studies will be required. “… because gut microbes and T cell binding locations on myelin differ between mouse and human, further studies using human microbes and autoreactive T cells are now needed.”
As the researchers concluded, “As patients with multiple sclerosis show increased TH17 cells in the small intestine, it may be worth investigating the microbiota of the small intestine—which can be captured only by analyses of endoscopic samples or biopsies, and not from fecal samples—for further studies in humans.”