The Intestinal Barrier in Multiple Sclerosis: Implications for Pathophysiology and Therapeutics

Carlos R. Camara-Lemarroy; Luanne Metz; Jonathan B. Meddings; Keith A. Sharkey; V. Wee Yong

Disclosures

Brain. 2018;141(7):1900-1916. 

In This Article

Intestinal Barrier Homeostasis, the Microbiome and Neuroinflammation: Possible Mechanisms Linking These Entities

The interactions between the microbiome and the intestinal barrier, particularly the contribution of the microbiome in maintaining barrier homeostasis, could be central in accounting for its regulation of neuroinflammation (Figure 2). Several studies have established that there are alterations in the gut microbiome of patients with multiple sclerosis, which has further fuelled the interest in the brain-gut-microbiome connection in multiple sclerosis research.

Figure 2.

An altered intestinal barrier leads to immune changes in the gut and the CNS. (1) Multiple sclerosis-associated microbiota and immune derangements lead to an altered barrier and increased permeability. (2) Microbiota diversity is reduced, as is production of SCFA's, and some bacteria translocate to the lamina propria. (3) LPS produced by bacteria cause low-grade inflammation and endotoxaemia, and loss of SCFA signalling alters lymphocyte phenotypes. (4) LPS, microbial-associated molecular patterns (MAMPs) and reduced SCFAs alter the blood–brain barrier. (5) LPS and activated lymphocytes reach the CNS, where in absence of normal SCFA concentrations, microglia and astrocyte neuroimmune responses are affected. A = astrocytes; BBB = blood–brain barrier; M = microglia; TLR = Toll-like receptors.

Early studies showed that, when compared to controls, patients with relapsing-remitting multiple sclerosis have an abundance of Anaerostipes, Faecalibacterium, Pseudomonas, Mycoplasma, Haemophilus, Blautia, and Dorea and a relative decrease of Bacteroides, Prevotella, Parabacteroides and Adlercreutzia (Cantarel et al., 2015; Miyake et al., 2015; Chen et al., 2016). In paediatric multiple sclerosis, patients have higher levels of members of Desulfovibrionaceae and depletion in Lachnospiraceae and Ruminococcaceae (Tremlett et al., 2016a). However, a clear and consistent 'multiple sclerosis microbiome phenotype' has not been described, and a myriad of different species have been implicated. For example, studies have found a significant depletion in Clostridial species (Rumah et al., 2013; Miyake et al., 2015), Butyricimonas (Jangi et al., 2016), Roseburia (Swidsinski et al., 2017) and increases in Streptococcus (Cosorich et al., 2017), Methanobrevibacter, Akkermansia and Coprococcus (Cantarel et al., 2015; Jangi et al., 2016). Multicentre studies aiming at defining a 'core microbiome' are underway (Pröbstel and Baranzini, 2018). Furthermore, some of these changes in the microbiome have been associated with immunological derangements, such as differences in the expression of genes involved in interferon and nuclear factor kappa-B (NF-κB) signalling (Jangi et al., 2016), and numbers of pro-inflammatory T helper 17 (Th17) cells in the intestine (Cosorich et al., 2017). At least one study found that differences in the microbiota could predict relapse risk in paediatric multiple sclerosis patients (Tremlett et al., 2016b).

Insights into how the microbiome could alter neuroinflammatory responses (reviewed in Colpitts and Kasper, 2017; Wekerle, 2017) have been illuminated by studies in germ-free mice where the microbiome regulates the shift back-and-forth of immune cells from pro- to anti-inflammatory phenotypes (Berer et al., 2011). Mice maintained under germ-free conditions have an attenuated form of experimental autoimmune encephalomyelitis (EAE), an inflammatory model of multiple sclerosis, and show lower levels of IL-17 in both the gut and the CNS, while also showing an increase in regulatory T cells (Tregs) peripherally (Lee et al., 2011). Colonization with segmented filamentous bacteria in germ-free mice leads to increased production of IL-17 and development of severe EAE. In contrast, other gut commensals such as P. histicola are able to suppress EAE severity, by decreasing pro-inflammatory Th1 and Th17 cells, and increasing Tregs and suppressive macrophages (Mangalam et al., 2017). B. fragilis, another common commensal strain, can also suppress EAE by expanding Tregs expressing the ectonucleotidase CD39, allowing for increased migration of this regulatory cell type into the CNS (Wang et al., 2014). Microbiota abundant in patients with multiple sclerosis induce the differentiation in vitro of human peripheral blood mononuclear cells into Th1 cells while reducing Treg numbers; conversely, microbiota that are decreased in patients with multiple sclerosis stimulate anti-inflammatory IL-10-expressing T cells and FoxP3+ Tregs (Cekanaviciute et al., 2017). Microbiota from patients with multiple sclerosis transplanted to mice prone to develop spontaneous EAE increases their susceptibility to EAE (Berer et al., 2017). Interestingly, multiple sclerosis patient-derived microbiota transplantation did not lead to changes in tight junction protein expression in the mouse recipient gut, but splenic lymphocytes had impaired IL-10 production (Berer et al., 2017).

An altered microbiome also leads to changes in some bacteria-associated products known to influence neuroimmune responses. Short chain fatty acids (SCFAs) such as butyrate, propionate and acetate are produced by bacterial fermentation of dietary carbohydrate and fibre. They play important roles in maintaining intestinal homeostasis, such as mediating sodium transport, serving as the principal energy source of intestinal epithelial cells and modulating gene transcription via inhibition of histone deacetylase activity (Kiela and Ghishan, 2016). Although not focusing on the concentration of SCFAs, CSF metabolomics studies from patients with multiple sclerosis have shown significant differences when compared to controls. SCFAs such as acetate are reduced (Simone et al., 1996; Kim et al., 2017), while others such as formate (Kim et al., 2017) have been found to be elevated in patients CSF. In studies evaluating metabolites in urine, propionate metabolism has also been found to be altered in patients with multiple sclerosis (Gebregiworgis et al., 2016).

In experimental models, eradication of the gut microbiota, or even just limiting the intestinal microbiome diversity, leads to impaired microglia structure and immune function, a process regulated by SCFAs (Erny et al., 2015, 2017). Astrocytes may also be influenced by SCFAs and the microbiome. Dietary tryptophan is metabolized by the gut microbiota into aryl hydrocarbon receptor agonists such as indoxyl-3-sulfate and indole-3-propionic acid, which can modulate astrocyte inflammatory function trough limiting NF-κB activation in a suppressor of cytokine signalling 2-dependent manner (Rothhammer et al., 2016). SCFAs also reduce T cell proliferation and cytokine production in the gut (D'Souza et al., 2017; Wan Saudi and Sjöblom, 2017). In EAE models, the administration of SCFAs led to amelioration of disease severity in association with a reduction of Th1 cells and an increase in Tregs (Mizuno et al., 2017). Interestingly, an altered microbiota may also alter innate immune responses in the gut favourable for systemic autoimmunity. For example, some types of intraepithelial lymphocytes may act as Tregs that suppress the pathogenic response to the immunizing antigen in EAE (Tang et al., 2007). CD4(+) intraepithelial lymphocytes obtained from transgenic mice prone to develop spontaneous EAE can infiltrate the CNS and ameliorate EAE severity in wild-type mice on transfer, showing regulatory properties (Kadowaki et al., 2016). These same cells proliferate in response to gut-derived antigens, aryl hydrocarbon receptor ligands and microbiota.

SCFAs could also modulate blood–brain barrier permeability. It is well known that SCFAs enhance intestinal epithelial cell barrier function by increasing the expression of tight junction proteins (D'Souza et al., 2017; Wan Saudi and Sjöblom, 2017). Butyrate has also been shown to increase the expression of occludin and zona occludens-1, thus restoring blood–brain barrier permeability in models of traumatic brain injury (Li et al., 2016). In germ-free mice exhibiting an altered blood–brain barrier, butyrate administration led to increased occludin expression and preserved blood–brain barrier permeability (Braniste et al., 2014). Overall, changes in SCFA-producing bacteria in the gut, and the influx of SCFAs into the blood stream, could thus have a distal effect in microglia and astrocyte functions, as well as in modifying blood–brain barrier permeability and the entrance of immune cells into the CNS (Figure 2).

Besides the above-discussed mechanisms suggesting bystander activation, another possible immunopathogenic link between multiple sclerosis and the gut microbiota is that of molecular mimicry. CNS-specific, self-reactive lymphocytes might be cross-activated by both gut microbiota antigens and myelin (Berer and Krishnamoorthy, 2014). Although there is no conclusive evidence for these mechanisms, commonly found pathogenic and non-pathogenic gut bacteria such as Bacteroides spp. and Enterococcus faecalis possess potential myelin basic protein encephalitogenic mimics (Westall, 2006).

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