ARTICLE IN BRIEF
Investigators reported changes in both pro- and anti-inflammatory epigenetic factors in the gut microbiome of subjects with multiple sclerosis that may contribute to disease pathogenesis.
PHILADELPHIA—Investigators have identified changes in both pro- and anti-inflammatory microorganisms in the gut of multiple sclerosis (MS) patients, compared with healthy controls. In addition, they found that disease-modifying therapy was associated with a change in the prevalence of those same microorganisms.
These findings, reported here in April at the AAN Annual Meeting, add to a growing body of studies suggesting that the gut “microbiome” may play a role in modulating the immune system in MS, perhaps even contributing to the cause or course of the disease. [See “Research on the Human Microbiome.”]
The precedent for an effect from the gut on autoimmunity is strong, according to Sushrut Jangi, MD, lead author and post-doctoral fellow in the lab of Howard Weiner, MD, at Brigham and Women's Hospital in Boston.
Inflammatory bowel disease is an autoimmune disease caused by changes in the microbiome, “so it made sense to think about whether microorganisms in the intestine might influence the immune system beyond the gut,” said Dr. Jangi. “It is actually an old idea that early microbe exposure may influence how the immune system develops.”
In addition, recent work in the mouse model of MS shows that genetically predisposed mice raised in a germ-free environment never develop the disease. “It means you need microbes to have autoimmunity.”
To determine whether there are differences in the gut microbiome in MS, and if changes occur with treatment, Dr. Jangi and colleagues analyzed stool samples from 61 MS patients and 43 healthy controls. They performed two measures of overall richness and diversity, and used two different high-throughput sequencing tools to identify and quantify bacterial and Archaeal families present in each sample. [Archaea microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells.] Individual families were identified by differences in sequence of the 16S hypervariable region of the ribosomal RNA gene, a standard tool for exploring microbial diversity.
They found that while there were no overall differences in richness or diversity, there were significant differences in the microorganisms present between groups. Specifically, they found that MS patients had an increased number of Archaea species, particularly of the family Methanobrevibacteriaceae, which normally comprises about 10 percent of the gut population. Their mean relative abundance was 6.6-fold higher in MS patients compared with controls.
Dr. Jangi pointed out that the change was not seen in all patients: some had no Methanobrevibacter, and some had levels hundreds of times that of normal.
The cell wall and lipid membranes of these organisms make them strongly immunogenic, “which is consistent with a role in the induction of local and systemic inflammatory processes in the host,” Dr. Jangi said.
The authors also found two organisms with anti-inflammatory properties that were lower in MS patients than in controls: the Butyricimonas genus from the Bacteroidetes phylum, and the Lachnospiraceae family from the Firmicutes phylum. Both of these produce butyrate, which has been shown to reduce inflammatory signaling. Both bacteria are also low in inflammatory bowel disease and rheumatoid arthritis.
The levels of both these bacteria were increased in patients taking MS treatment, compared with those who were not. The reduction in levels of these bacteria “may be contributing to overactivity of the immune system in the gut,” Dr. Jangi said, “so it makes sense that untreated patients would have less of these.”
For the future, the group plans to expand their sample to include other racial groups (their sample was overwhelmingly Caucasian) and other regions of the country. A colleague is also introducing Methanobrevibacter into a mouse model of MS — experimental autoimmune encephalomyelitis — to determine whether it can trigger disease.
“It is possible they have a causative role, but the more likely possibility is that they act as adjuvants, stimulating the immune system,” Dr. Jangi said. “They may also be simply a product of having MS. These are early results, and there is a lot of work to do. The work requires interdisciplinary cooperation between neurologists and gastrointestinal specialists to do it right.”
Emmanuelle Waubant, MD, PhD, FAAN, a professor of medicine at the University of California, San Francisco, has studied the gut microbiome in children with MS. She noted that several research groups have seen fairly large differences in a diverse group of microorganisms between MS patients and healthy controls, suggesting there may be more to find in the samples Dr. Jangi analyzed, she said. “In our data, we see a huge influence of race,” as well as many more species that vary between groups.
Whether the MS-related changes found in these studies are causative, or indeed whether causation goes the other way, remains an important and still open question. But the idea of a connection between the gut flora and MS is steadily gaining ground in the field, she said. Nonetheless, she added, it is still difficult to get work in this field published; so much of the knowledge that is accumulating is shared at conferences rather than in journals.
Ellen Mowry, MD, an assistant professor of neurology at Johns Hopkins University, said “this is the tip of the iceberg” in terms of plumbing the changes in the microbiome in MS. “Considering disease-modifying therapies is important because most of our patients are treated — and with a variety of agents — and it is imperative to take that into consideration.”
She noted that in a study she performed, treatment with glatiramer acetate was associated with a different microbiome signature than the one in untreated patients. “The question is whether the treatment itself caused a change in the microbial signature,” or whether that change pre-existed in patients whose disease made them candidates for therapy. “We don't really know yet.”
Longitudinal studies of MS patients, beginning shortly after diagnosis, may offer the best opportunity to answer these questions, Dr. Mowry said.
EXPERTS COMMENT ON THE MICROBIOME & MS
RESEARCH ON THE HUMAN MICROBIOME
Increasingly, researchers have become interested in the role of the microbiome — the aggregate of microorganisms, including bacteria, fungi, and archaea, that reside on the surface and in deep layers of skin, saliva and oral mucosa, as well as gastrointestinal tracts — in auto-immune diseases such as diabetes, rheumatoid arthritis, multiple sclerosis, fibromyalgia, and some cancers. Most of the microorganisms associated with humans appear not to be harmful, but some research suggests that there are differences in how they may function in different people.
In 2008, the National Institutes of Health launched the Human Microbiome Project to support research and to develop tools and datasets for studying the role of these microorganisms in human health and disease. Among its goals, some of which have been achieved, the Human Microbiome Project hopes to establish a resource repository for microorganisms; develop a reference set of microbial genome sequences; develop new technologies for computational analysis; and to study the ethical, legal, and social implications of human microbiome research. More than 200 peer-review papers have been funded by the Human Microbiome Project to date.