Multiple sclerosis (MS), a clinically common autoimmune disease, affects millions of individuals worldwide; many of these individuals reside in northern geographic locations. For these individuals the quality of life slowly diminishes as the disease progresses; the host immune system attacks the insulating structure known as the myelin sheath that surrounds the axon shaft of neurons in the spinal cord [26
]. When the sheath is degraded, the ability for electrical signals to be sent through the axon is reduced, thus resulting in paralysis and other symptoms. The most prevalent form of MS is relapsing–remitting MS (RRMS) that affects 85% of the total patient population; RRMS is initially diagnosed as a syndrome of neuronal dysfunction with a repeating series of relapses and remissions that follow over time. Approximately 70% of RRMS patients develop secondary-progressive MS (SPMS), which causes a steady and progressive neurological impairment [27
]. The precise reason as to why immune cells destroy myelin remains unclear [27
]. In MS, the myelin sheath that surrounds the axons of neuronal cells is degraded by host immune cell populations. Although the exact etiology is debated, there are hallmark events that undeniably occur.
Genetic variation is attributed to about one-third of the disease risk [29
]. Environmental conditions as well as lifestyle choices play an additional factor in disease risk. Lacking a predominant exogenous risk factor, there is ambiguity as to whether MS starts in the periphery or in the central nervous system (CNS) [26
]. In peripheral models of MS, pathogenic T cells are activated and subsequently released to the draining lymph nodes [26
]. From the draining lymph nodes, these pathogenic T cells can enter circulation and gain access to the central nervous system by trafficking with activated B cells and monocytes [26
]. Intrinsic models propose that the rise of autoreactive lymphocytes is secondary to intrinsic CNS damage [26
]. Additionally, autoreactive B cells can be found in the meninges, parenchyma, and cerebrospinal fluid [26
]. These autoreactive B cells can secrete antibodies which tend to increase with age in MS patients [30
]. There might be unknown autoantigens, making the mechanisms behind autoreactive B cell pathology speculative [26
]; however, next-generation sequencing has provided evidence that antigen-experienced B cells potentially go through maturation prior to entering the CNS [31
]. Inflammation is a primary result from autoreactive lymphocytes. These responses cause axonal damage and could potentially trigger a self-sustaining chronic neurodegenerative process [26
]. As a result, resident CNS cells such as the microglia and astrocytes additionally secrete inflammatory molecules, further exacerbating neurodegeneration [32
]. The extent to which all these cell populations work in tandem to cause disease highlights the need to prevent the initial onset of inflammation and neurodegeneration.
Compounding the magnitude of autoreactive immune cell populations, defective Treg cells have also been noted in MS [33
]. Defective Tregs could contribute to the production of autoreactive lymphocytes and additionally exacerbate the effects of preexisting autoreactive lymphocytes. Studies show that Tregs are lower in numbers and have reduced functionality in patients with MS [34
]. Ultimately, it is the occurrence of defective Treg cells that can help explain why autoreactive immune cells arise. The degradation of the myelin sheath by host immune cells may be mediated by the T helper 17 (Th17) cells [35
]. The imbalance between effector T cell and Tregs leads to the pro-inflammatory states which characterize MS. The increased levels of Th17 cells secrete pro-inflammatory cytokines and chemokines that recruit immune cells for the degradation events. Reduced and dysfunctional Tregs will fail to keep the exacerbated Th17 in check resulting in myelin degradation.
Gut microbial imbalances tend to shift towards a pro-inflammatory state that have profound effects on the intestinal physiology of the individual. Additionally, dysbiosis has been associated with intestinal barrier disruptions. When the integrity of these tight junction protein complexes diminishes there is an increase in intestinal permeability; the bacterial antigens can pass out of the intestinal lumen and travel to other locations in the body. As a result, levels of antigens, like the endotoxin lipopolysaccharide, can increase in the blood circulation which could have systemic inflammatory effects [36
]. Systemic translocation of bacterial antigens can have a profound effect on CNS immunity and impact the integrity of the blood–brain barrier [37
]. This process can result in the ultimate passage of autoreactive lymphocytes into the CNS and have direct access to the myelin sheath.
3.1. The Gut Microbiome and Multiple Sclerosis
As previously discussed, autoimmunity has been shown to be impacted by the gut microbiome. However, it has also been theorized that disease itself can shape the structure and function of the gut microbiome. What this implies is that there is a bi-directional relationship between diseased states and the structure and function of the gut microbiome. This then raises more questions: What comes first, the disease or the aberrant gut microbiome? The proposed multifactorial and multidirectional association between the gut microbiome and the CNS of MS is described in Figure 1
In some instances of autoimmunity, the aberrant gut microbiome precedes the onset of disease. There is an increasing interest in determining how disease itself shapes the gut microbiome. Risk factors that have been associated with autoimmunity also impact the gut microbiome [38
]. Additionally, autoimmunity can directly impact how the immune system responds to the gut microbiota. In the case of inflammatory bowel diseases, the immune system targets resident microbiota, thus altering the overall structure of the gut microbiome [39
]. Targeting non-pathobionts and clearing them from the intestines could have profound impacts on the immune system of the host. If a population of bacteria that promotes anti-inflammatory responses is eliminated it is possible that unchecked systemic inflammation could occur, thus further exacerbating the initial autoimmune disease.
The hypothesis that the gut microbiome is an environmental modulator of CNS inflammatory demyelination was first tested in experimental autoimmune encephalomyelitis (EAE), the most widely used animal model to study MS. The treatment with a broad-spectrum antibiotic intervention affected the balance between inflammation and inflammatory regulation in EAE by modulating intestinal microbiota and, as a consequence, Treg cell populations [40
]. Additionally, Yokote et al. reported similar findings but noted that the alterations in intestinal microbiota impacted the natural killer T (NKT) cell populations [41
]. These experiments analyzed the impacts antibiotic intervention could have on modulating EAE in SJL as well as C57 mice. Utilizing antibiotics to therapeutically target the gut microbiome has been proposed for models of diabetes [42
] as well as ulcerative colitis [43
]. A study conducted by Nakamura et al. set out to test the efficacy of an antibiotic cocktail on an experimental autoimmune uveitis (EAU) model. Autoimmune uveitis has both a genetic and environmental culmination that impacts disease susceptibility and is characterized by a distinct increase in Th17 cell populations with a decrease in Treg cell populations [44
]. Utilizing an antibiotic cocktail consisting of ampicillin, vancomycin, neomycin, and metronidazole, the researchers noted that clinical scores of EAU were significantly reduced compared to the control group when the antibiotics were administered orally. The bacterial phyla Firmicutes and Bacteroidetes as well as the class of Alphaproteobacteria were reduced, while there was an increase in the class of Gammaproteobacteria. More importantly, the utilization of antibiotics significantly increased the expression of Foxp3+
Treg cell populations with a reduction in IL-17 producing Th17 cells [44
]. Taking this all into consideration, the treatment of broad spectrum antibiotics conferred protection against the EAU pathology.
More recently, a study performed in non-obese diabetic mice showed that early treatment of disease with antibiotics delayed the onset of EAE, reduced severity and the progression of disease. The protection was observed when mice were treated between days 0 and 14. The treatment of EAE mice with same antibiotics at later days (30–44 and 70–84) did not affect disease progression and severity [45
]. In this later study, we hypothesized that the interaction between the gut microbiome and CNS disease is bidirectional. We compared the gut microbiota composition of non-obese diabetic (NOD) EAE mice on days 0, 14, 30 and 58. We compared the gut microbiome of NOD mice induced with EAE. In our study, approximately 70% of mice exhibited disease progression and developed a severe form of EAE. When averaging the clinical scores of the remaining mice, the resulting pattern showed a continuation of mild disease throughout the duration of the experiment. We found that the mice which developed a severe secondary form of EAE harbored a dysbiotic gut microbiome when compared to the healthy control mice, and that the differences were observed at early stages of disease [45
]. It might be relevant to note that only early intervention with antibiotics affected the progression of disease, while only early stages of disease showed effects with respect to the composition of the microbiota. The recent findings suggest then that the interaction between the gut microbiota and neuroinflammation is reciprocal. However, the experimental evidence summarized later in this review strongly indicates that changes in the composition of the gut microbiota significantly affect neuroinflammation and the disease course, which opens new therapeutic avenues to explore.
The effects of the lack of microbiota on health and disease have been explored using experimental GF animals. GF mice colonies are generated and maintained under constant sterile conditions. Significant anatomical and immunological effects result from the lack of exposure to microbes and microbial antigens mice: these mice show reduced numbers Peyer’s patches and lymphoid follicles in the gut associated-lymphoid tissues. These secondary lymphoid tissues are also smaller and harbor a reduced number of T cells than conventional, specific pathogen-free (SPF) mice. In the context of immune function these animals show biased responses characterized by reduced frequencies of pro-inflammatory Th17 cell subsets [46
]. The altered immune system of GF mice influences their susceptibility to experimental autoimmune diseases. GF mice are less susceptible to glucose intolerance than mice housed conventionally [47
]. Reduced susceptibility to disease has also been observed in models of inflammatory bowel disease [48
], rheumatoid arthritis (RA) [49
], and spontaneous [50
] and actively induced EAE [51
], among others. It is important to note, however, that same gut-associated alterations constitute a barrier for appropriate development of the immune system and brain function, which could be considered by many an experimental limitation for models designed to study complex, multifactorial diseases such as multiple sclerosis.
Significant changes in specific microbial taxa have been observed in the intestinal microbiome of MS patients when compared to healthy controls [52
], evidence that might support the hypothesis that the gut microbiome can play a role in the development of MS. However, very little is known whether the disease affects the composition of the gut microbiome. Understanding how MS pathology affects gut microbiota can give insight to novel therapeutic approaches to impact disease progression. Similarly, dysbiosis drives disease progression in the inflammatory bowel disease model, therefore it is possible that dysbiosis also promotes inflammation in the MS model.
The effects of the gut microbiota on other neurological diseases are also being extensively evaluated, as evidenced by the dramatic increase observed in the number of published works in the recent years [55
]. Changes in the gut microbiota composition have also been observed in patients suffering from diseases such as neuromyelitis optica [56
] and Parkinson’s disease [57
]. Experimental models of neurological diseases such as autism spectrum disorders [60
] and behavioral disorders [63
] further suggest the influence of the gut microbiome on these pathologies. Other neurological diseases are also being currently evaluated in the context of the microbiome, such as Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis [55
It is hypothesized that there is a bi-directional relationship between the gut microbiota and MS [68
]. The composition of the gut microbiome might shape MS pathology at the same time MS disease progression also could alter the gut microbiome. However, two recent works clearly suggest that the changes in microbiota composition drive neuroinflammatory effects rather than the opposite. These recent studies support the premise that changes observed in the gut microbiome of MS patients are correlated with functional mechanisms that might regulate disease. Fecal transplantation of the gut microbiome can influence the progression of EAE. When fecal material from discordant monozygotic twins was transplanted into mice, there was a profound impact on spontaneous EAE disease incidence [69
]. The MS gut microbiome had the ability to increase the likelihood of spontaneous EAE induction in mice as opposed to the healthy twin. Moreover, the stool from MS patients also increased the severity of EAE in mice [70
]. These recent works clearly support the proposed concept that although interactions may be bidirectional the composition of the gut microbiota affect the progression of CNS inflammatory demyelination. Therefore, it is clear that targeting the gut microbiome might have profound impacts on MS pathology. Table 1
summarizes the colonization studies transferring MS microbiota into GF mice, as well as approaches based on monocolonization, multi-species colonization, and the treatment with immunomodulatory compounds purified from gut symbionts, discussed next.