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

Disease-modifying Therapies and the Intestinal Barrier

An interesting aspect of the above mentioned findings is that the microbiome can also be altered by whatever immunomodulatory therapy the multiple sclerosis patient is receiving (Cantarel et al., 2015; Tremlett et al., 2016b). The question of whether gut dysbiosis precedes the development of multiple sclerosis or follows the immune alterations (innate, acquired or drug-induced) is also a matter of debate (Ochoa-Repáraz et al., 2017). Disease-modifying therapies are medications that have improved the clinical course of relapsing-remitting multiple sclerosis. While their principal mechanisms are thought to be immune-modulating, their possible effects over the intestinal barrier that may contribute to therapeutic efficacy have not been explicitly evaluated. Below we summarize evidence suggesting that disease-modifying therapies could modulate the intestinal barrier, the gut microbiome and the interaction between the two (Figure 3). However, the evidence is indirect, and whether this actually plays a meaningful role in clinical response remains to be established.

Figure 3.

Disease-modifying therapies can modulate the intestinal barrier. Different disease-modifying therapies in clinical use may beneficially modulate intestinal barrier function through a variety of mechanisms. (1) Oral disease-modifying therapies have antimicrobial properties, while minocycline is a tetracycline antibiotic. Dimethyl fumarate acts as a Michael acceptor and can deplete bacterial nucleophilic thiols. (2) Glatiramer acetate has been shown to increase syndecan, the most abundant heparan sulphate proteoglycan in the gastrointestinal tract. (3) Fingolimod, dimethyl fumarate and minocycline increase tight junction expression. Dimethyl fumarate increases zona occludens-1 (ZO-1) in a heme-oxygenase-1 (HO-1) dependent pathway, while S1P signalling increases E-cadherin (E-CN). (4) Most disease-modifying therapies modulate lymphocyte (LYM) populations and functions in non-neurological tissues, such as in the lamina propria. Whether any of these effects have a mechanistic relevance for their therapeutic action is unknown.

Interferons

There is evidence suggesting that endogenous interferons could affect the intestinal barrier. Type I interferons, including IFNα and IFNβ, are an integral part of the innate host immune response to gut microbiota, and they modulate bilateral interactions between epithelial cells and commensal flora (Giles and Stagg, 2017). For example, IFNβ has shown stabilizing properties in biological barriers (such as the intestinal, blood–brain and blood–lung barriers), partly through the upregulation of tight junction proteins in endothelial cell layers (Kraus et al., 2004; LeMessurier et al., 2013; Long et al., 2014). The commensal microbiota also stimulates dendritic cell IFNβ production, which increases the proliferation of Tregs in the intestine, a process itself inhibited by intestinal epithelial cell apoptosis (Nakahashi-Oda et al., 2016). Type I interferons also inhibit the continuous proliferation of the intestinal epithelium by activating the p53 pathway and inducing epithelial cell apoptosis (Katlinskaya et al., 2016), and mice lacking type I interferon receptor on Paneth cells show an altered microbiota (Tschurtschenthaler et al., 2014).

Glatiramer Acetate

Various studies have shown that glatiramer acetate reduces colonic injury in animal models of colitis, through reduction of TNFα signalling, elevation of regulatory T cells and increase in anti-inflammatory mediators such as IL-10 and TGFβ (Aharoni et al., 2005, 2007). In one such study, glatiramer acetate attenuated colitis severity and prevented the destabilization of the intestinal epithelial barrier (Yablecovitch et al., 2011). There is also evidence suggesting that patients with multiple sclerosis treated with glatiramer acetate have different microbiota composition. In a small study, glatiramer-treated patients had stool taxonomic units (evaluated by hybridization of 16S rRNA to a DNA microarray) of Bacteroidaceae, Faecalibacterium, Ruminococcus, Lactobacillaceae, Clostridium, and other Clostridiales that were significantly different than those of untreated patients (Cantarel et al., 2015).

Natalizumab

Dysregulated recruitment of leucocytes into the intestine is one of the components of the immune response responsible for barrier breakdown in IBD (Danese et al., 2005; Fiorino et al., 2010). Integrins are expressed on intestinal lymphocytes and are essential in their homing to intestinal lymphoid tissues and trafficking through the intestinal mucosa (Hamann et al., 1994; Tanaka et al., 1995; Miura et al., 1996; Farstad et al., 1997; Bradley et al., 1998; Fujimori et al., 2002). Natalizumab, which blocks the activity of integrins (both α4β1 and α4β7), has shown effectiveness in reducing the severity of IBD (Fiorino et al., 2010; Bamias et al., 2013). However, its association with JC virus-related CNS complications has led to the development of specific α4β7-antibodies such as vedolizumab, now routinely used in the treatment of IBD (Zundler et al., 2017).

Nonetheless, the effects of natalizumab on integrins and lymphocyte trafficking in the gut suggests it could modulate the inflammatory response in this site in multiple sclerosis. A potential role for intestinal lymphocytes and integrins in multiple sclerosis pathophysiology has been suggested by results from mouse EAE models. Th17 cells, prominent drivers of EAE, are controlled and redirected in the small intestine. Th17 cells, which are normally pro-inflammatory, acquire a regulatory phenotype in the intestine and are ultimately eliminated through the intestinal lumen (Esplugues et al., 2011). In EAE, there is infiltration of proinflammatory Th1/Th17 cells and reduction of Tregs in the gut, in association with functional and morphological changes (Nouri et al., 2014). Furthermore, mice lacking integrin α show a loss of Th17 cells in the intestine and resistance against EAE (Acharya et al., 2010; Melton et al., 2010). In spontaneously EAE resistant B10.S mice, blocking α4β7 integrin leads to peripheral availability of Th17 cells and increased severity of EAE (Berer et al., 2014). In patients with multiple sclerosis, natalizumab treatment reduces the populations of integrin α-4-positive Th1, Th17 and Tregs differentially, while affecting the immune function of residual integrin α-4-positive T cells (Kimura et al., 2016). The gut might act as a checking point, a reservoir and an activation site for Th17 and other T cells, a process regulated in part by intestinal integrins. Natalizumab and its non-selective integrin blockade could lead to changes in the way lymphocytes interact with the intestinal tissue. Considering the abovementioned findings, it is possible that natalizumab's therapeutic properties in multiple sclerosis could depend, at least in part, on these intestinal effects, besides those seen in blood–brain barrier, integrins and lymphocyte trafficking.

Fingolimod

Another drug that acts through the regulation of leucocyte trafficking is fingolimod, a functional antagonist of the sphingosine 1-phosphate receptor (S1P). S1P1 receptors are highly expressed on lymphocyte membranes and are critical for T and B cell egress from secondary lymphoid organs. S1P can affect the intestinal barrier by modulating tight junction proteins (Greenspon et al., 2011; Pászti-Gere et al., 2016), particularly under inflammatory conditions (Dong et al., 2015). For instance, fingolimod reduces endothelial barrier dysfunction in blood vessels and lung epithelium in experimental models of sepsis and haemorrhagic shock (Lundblad et al., 2013; Bonitz et al., 2014). Fingolimod also sequesters and alters the activation of lymphocytes in intestinal tissues (Chiba et al., 1998; Yanagawa et al., 1998; Henning et al., 2001; Halin et al., 2005; Sugito et al., 2005; Daniel et al., 2007), an effect thought to be mechanistically relevant in multiple sclerosis therapeutics. In the mouse EAE model, development of EAE was associated with increased accumulation of T cells in Peyer's patches, a process increased by fingolimod (Spirin et al., 2014). Fingolimod can also directly affect the microbiota. Both sphingosine and fingolimod inhibit C. perfringens growth and endotoxin production in vitro, suggesting an intrinsic antibacterial property (Rumah et al., 2017).

Dimethyl Fumarate

Dimethyl fumarate (DMF) is derived from the simple organic acid fumaric acid, and it acts as an immunomodulator by promoting T cell apoptosis, shifting to a Th2 response and acting as an antioxidant. There is limited but interesting evidence suggesting DMF could beneficially affect both the intestinal barrier and the gut microbiota. DMF alleviates experimentally induced colitis and reduces the Th1 response in mouse models and protects human intestinal epithelial cells against oxidative barrier dysfunction by preserving zona occludens-1 and occludin expression in vitro (Casili et al., 2016). DMF also preserves intestinal mucosa morphology after mycotoxin exposure and decreases intestinal permeability by strengthening tight junctions (Ma et al., 2017). In this model, DMF also led to increased microbiome diversity, with more abundance of bacteria producing SCFAs, such as Gemella, Roseburia, Bacillus and Bacteroides. DMF can also directly reduce C. perfringens growth and exhibits anti-mildew and antibacterial properties (Ma et al., 2017; Rumah et al., 2017).

Alemtuzumab

Alemtuzumab is an anti-CD52 antibody that causes depletion of mainly lymphocytes and is highly effective in the clinical management of multiple sclerosis (Hartung et al., 2015). Despite its specific mechanism of action, there is evidence suggesting it has detrimental effects over the integrity of the intestinal barrier and might alter the gut microbiome.

In mice, anti-CD52 antibodies induce increased intestinal barrier permeability (Qu et al., 2009) and lead to reductions in epithelial cell populations and to altered tight junction ultrastructure (Shen et al., 2013, 2015). In macaques, alemtuzumab-induced intestinal barrier disruption is associated with epithelial cell apoptosis as well as with increased circulating levels of d-lactate and endotoxin, indirect markers of intestinal barrier breakdown and bacterial translocation (Li et al., 2011; Qu et al., 2015). Lymphocyte depletion with alemtuzumab treatment in macaque models also resulted in dramatic changes in the gut microbiota (Li et al., 2010). Lactobacillales, Enterobacteriales, Clostridiales, and the genus Prevotella and Faecalibacterium were primarily responsible for the variations of the gut microbiota after lymphocyte depletion (Li et al., 2013). The diversity of fungal microbiota was similarly affected (Li et al., 2014). Despite this preclinical evidence, alemtuzumab-induced intestinal barrier disruption is infrequent in clinical practice. However, a case of spontaneous pancolitis was described in a patient with multiple sclerosis treated with alemtuzumab recently (Vijiaratnam et al., 2016), and historically, the use of alemtuzumab in haematological malignancies has been associated with the development of diarrhoea and opportunistic intestinal infections (Goteri et al., 2006; Ronchetti et al., 2014).

Teriflunomide

Teriflunomide selectively and reversibly inhibits dihydroorotate dehydrogenase, leading to a reduction in the number of activated lymphocytes that enter the CNS (Miller, 2015). Teriflunomide could alter the microbiome and the host response to enteral pathogens. Treatment of porcine intestinal epithelial cells with teriflunomide led to reduced capacity to fight bacterial infection through suppression of STAT-6 signalling (Yi et al., 2016). Teriflunomide could also directly inhibit C. perfringens growth in vitro (Rumah et al., 2017). Animals treated with teriflunomide in a mouse model of EAE had fewer antigen-presenting cells in Peyer's patches as well as an increase in gut-specific CD39(+) Treg cells that could protect against EAE when used in an adoptive transfer regimen (Ochoa-Repáraz et al., 2016).

Minocycline

Minocycline is a second-generation tetracycline that was first introduced over half a century ago. Besides its antibiotic effects, it also has anti-inflammatory, immune-modulating and anti-apoptotic properties, all of which have been proposed as possible pathways towards neuroprotection (Yong et al., 2004; Giuliani et al., 2005). A recent randomized, double-blind, placebo controlled trial showed that oral minocycline could delay the appearance of a new demyelinating events in patients with clinically isolated syndrome, as well as reduce the appearance of T2 lesions in the brain (Metz et al., 2017).

Minocycline's immune-modulating and anti-inflammatory properties have also been observed in intestinal tissues. In a chemically-induced colitis model in mice, minocycline reduced intestinal inflammation, mucosal injury, restored microbiota and preserved tight junction protein expression (Huang et al., 2009; Garrido-Mesa et al., 2011a, b). As an antibiotic, minocycline also alters the gut microbiome. A recent study evaluated the effects of various commonly used antibiotics, including minocycline, on the salivary and gut microbiome in 66 healthy adults. Antibiotic exposure led to reductions in health-associated butyrate-producing species as well as proliferation of potentially resistant strains in the gut microbiome, although the changes were more robust after amoxicillin and ciprofloxacin administration (Zaura et al., 2015). Other studies have shown that some gut commensals such as Bifidobacteria and E. coli are susceptible to minocycline (Moubareck et al., 2005; Kirchner et al., 2014). Minocycline thus presents an intriguing option in dual modulation of the intestinal barrier function. It could have protective anti-inflammatory properties while also altering the composition of the gut microbiome.

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