Gut-CNS axis and the microbiome in MS

A collection of presentations formed this interesting session on the latest findings in the gut/central nervous system axis and the microbiome in MS.

Emanuelle Waubant began the session by discussing the Microbiome in Adult and Pediatric MS.

She shared findings across a range of 12 studies, covering a sample size of 300 in total, mostly of relapsing-remitting MS patients, many of whom were on disease-modifying therapies. Just one study was in paediatric MS.

She noted a range of factors affecting a person's gut microbial profile, including

• age / disease duration
• geographical origin
• diet
• supplements / vitamins or probiotics
• medications
• genetic make-up

Although much interesting data was drawn across the 12 studies, there is not enough consistency in findings across studies, and there is insufficient significance in the microbial diversity of patients with MS versus control groups.

Noting the complexity of working across such a huge range of bacteria in the gut, Emanuelle shared studies now assessing overall metabolic sequences or groups of bacteria, and looking for patterns in over or under representation.

She discussed studies looking to predict metabolites in the blood from stool samples, as metabolites can indicate immune response.

• Some have found a correlation between microbial abundance and immune gene expression (Jangi, 2016).
• However the overall conclusion was that there is a great deal of variation across studies and causality has not been demonstrated yet.

Some studies have found links between certain bacteria over or under-representation and a higher relapse rate, and Emanuelle shared some of her studies that have followed this line of enquiry.

Emanuelle pointed out the complexity of assessing bacteria in the gut, with hundreds of bacteria forms present, and bacterial clusters made up of many - up to 30 types - of individual bacteria.

Her work looked at clusters of bacteria and bacterial pathways rather than individual bacteria. Key findings included:

• Out of 10 clusters, one was associated with subsequent relapse.
• In this were four of the bacteria also found to correlate with higher relapse risk in other studies.
• Nine bacterial pathways out of 309 pathways were strongly associated with a hazard of subsequent relapse.
• Four of those nine are associated with time to relapse and linked to methane production in people with MS.

She shared that causality has not yet been established between microbiome and disease progression, but that it is important to identify key bacterial pathways to find potential intervention targets.

Current intervention studies include:

• probiotic supplementation
• microbiome transplant, based on an EAE model (EAE referring to a form of inflammatory, demyelinating disease replicating MS in mice).

She concluded by highlighting that more studies are needed to establish causality and elucidate the biological pathways involved.

Stephanie Kurten then presented a non-traditional view of MS. She queried whether MS could originate outside of the central nervous system, in her research looking at the enteric nervous system (ENS) as a potential autoimmune target in MS.

She shared evidence that microbiota derived from MS patients triggered spontaneous encephalomyelitis in mice (Berer et al, 2017), and suggested that the gut itself, with its own immune system, links to the sympathetic and parasympathetic branches of the autonomic nervous system and its links to the microbiome, might have a role in the onset of MS.

Stephanie outlined further evidence for this theory noting:

• that the enteric nervous system contains more neurons than the spinal cord including sensory and motor neurons, interneurones and interstitial pacemaker cells
• that studies have found significant numbers of people with MS to have gastrointestinal dysfunction (Levinthal, 2013; Almeida, 2019)
• contribution of the ENS to Parkinson's and ALS is also suggested

Current research suggests that antibodies are built up against the generic nervous system itself (Wunsch, 2017). Stephanie is involved in a study looking at excitatory and inhibiting neurons in the gut.

• The working hypothesis is that when an immune response is triggered in MP4 EAE mice, MBP/PLP specific antibodies target the enteric gla, and the expression of MBP/PLP causes secondary neuronal degeneration.
• At present it is not clear whether the enteric nervous system pathology comes before or after MS onset.

Next, Ali Mirza presented on the functional potential of the pediatric MS microbiome using metagenomic analysis of stool samples.

The sample was of individuals who had no previous disease-modifying therapy and no exposure to interferon-beta.

• 20 MS cases under 21 who were matched to controls
• 80% were female and mean age was 16 at stool sample and 13 at MS onset

The study had three aims; to assess:

• functional diversity
• relative abundance and presence of gene ontology
• relative abundance and presence of metabolic pathways

The findings included:

• No overall difference in functional diversity across the two groups
• Strong differences found between the MS and control group in the relative abundance of five particular bacterial pathways
• Relative to controls, the MS cases exhibited higher:
  • Archaea-related methanogenesis
  • vitamin B production
  • viral activity
  • heavy metal metabolism
  • L-glutamate degradation
• Relative to controls, the MS cases exhibited lower:
  • homolactic fermentation
  • bacterial carbohydrate degradation

Stephanie Tankou presented on the effect of vancomycin on intestinal permeability during experimental autoimmune encephalomyelitis (EAE).

The research found that those mice with dysbiosis who were induced with EAE had increased pro-inflammatory cytokines, leading to increased PAR2 expression and activation, increased MLCK activation and increased intestinal permeability, in turn leading to increased neuroinflammation

By contrast, those mice who were treated with vancomycin two weeks prior to EAE induction, as a means of protecting their microbiome, had contrasting results, with a decrease in immune activation, PAR2 and MLCK activation, a decreased intestinal permeability and a corresponding decreased level of neuroinflammation (see figure 1 for visual explanation).

Finally, Zahra Moinfar outlined her team's findings that gut dysbiosis in neuromyelitis optica promotes CNS autoimmunity.

She highlighted that microbiota dysbiosis can be seen in neuro optic myelitis (NMO), and that after infecting the mice with EAE, they found suggestions that gut microbiota influences lymphocytes.

Further research found that:

• NMO gut microbiota enhances EAE susceptibility
• A reduction in Tregs may contribute to increased EAE susceptibility
• Different mechanisms may be involved in EAE enhancement in NMO and the healthy controls, for example:
  • CD4+Foxp3+Helios+ Tregs decreased in NMO but not the healthy controls in the gut and spleen
  • CD4+Foxp3+CD25+ Tregs decreased in both NMO and health control groups in the gut, spleen and CNS.