Selective accumulation of pro-inflammatory T cells in the intestine contributes to the resistance to autoimmune demyelinating disease

Gut-tropic T cells and extra-intestinal autoimmune diseases

Gut-tropic T cells primarily originate from gut-associated lymphoid tissue (GALT), and gut-tropic integrins mediate the trafficking of the T cells to the gastrointestinal tract, where their interplay with local hormones dictates the residence of the immune cells in both normal and compromised gastrointestinal tissues. Targeting gut-tropic integrins is an effective therapy for inflammatory bowel disease (IBD). Gut-tropic T cells are further capable of entering the peripheral circulatory system and relocating to multiple organs. There is mounting evidence indicating a correlation between gut-tropic T cells and extra-intestinal autoimmune disorders. This review aims to systematically discuss the origin, migration, and residence of gut-tropic T cells and their association with extra-intestinal autoimmune-related diseases. These discoveries are expected to offer new understandings into the development of a range of autoimmune disorders, as well as innovative approaches for preventing and treating the diseases.

Systemic LPS Administration Stimulates the Activation of Non-Neuronal Cells in an Experimental Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by deficiency of the survival motor neuron (SMN) protein. Although SMA is a genetic disease, environmental factors contribute to disease progression. Common pathogen components such as lipopolysaccharides (LPS) are considered significant contributors to inflammation and have been associated with muscle atrophy, which is considered a hallmark of SMA. In this study, we used the SMNΔ7 experimental mouse model of SMA to scrutinize the effect of systemic LPS administration, a strong pro-inflammatory stimulus, on disease outcome. Systemic LPS administration promoted a reduction in SMN expression levels in CNS, peripheral lymphoid organs, and skeletal muscles. Moreover, peripheral tissues were more vulnerable to LPS-induced damage compared to CNS tissues. Furthermore, systemic LPS administration resulted in a profound increase in microglia and astrocytes with reactive phenotypes in the CNS of SMNΔ7 mice. In conclusion, we hereby show for the first time that systemic LPS administration, although it may not precipitate alterations in terms of deficits of motor functions in a mouse model of SMA, it may, however, lead to a reduction in the SMN protein expression levels in the skeletal muscles and the CNS, thus promoting synapse damage and glial cells' reactive phenotype.

Increased regulatory activity of intestinal innate lymphoid cells type 3 (ILC3) prevents experimental autoimmune encephalomyelitis severity

Experimental autoimmune encephalomyelitis (EAE) induced in inbred rodents, i.e., genetically identical animals kept under identical environmental conditions, shows variable clinical outcomes. We investigated such variations of EAE in Dark Agouti rats immunized with spinal cord homogenate and identified four groups: lethal, severe, moderate, and mild, at day 28 post immunization. Higher numbers of CD4+ T cells, helper T cells type 1 (Th1) and 17 (Th17) in particular, were detected in the spinal cord of the severe group in comparison with the moderate group. In addition, increased proportion of Th1 and Th17 cells, and heightened levels of interferon (IFN)-γ and interleukin (IL)-6 were detected in the small intestine lamina propria of the severe group. A selective agonist of free fatty acid receptor type 2 (Ffar2) applied orally in the inductive phase of EAE shifted the distribution of the disease outcomes towards milder forms. This effect was paralleled with potentiation of intestinal innate lymphoid cells type 3 (ILC3) regulatory properties, and diminished Th1 and Th17 cell response in the lymph nodes draining the site of immunization. Our results suggest that different clinical outcomes in DA rats are under determinative influence of intestinal ILC3 activity during the inductive phase of EAE.

The Role of Diet and Interventions on Multiple Sclerosis: A Review

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) characterized by inflammation and neurodegeneration. The most prominent clinical features include visual loss and sensorimotor symptoms and mainly affects those of young age. Some of the factors affecting its pathogenesis are genetic and/or environmental including viruses, smoking, obesity, and nutrition. Current research provides evidence that diet may influence MS onset, course, and quality of life of the patients. In this review, we address the role of nutrition on MS pathogenesis as well as dietary interventions that show promising beneficial results with respect to MS activity and progression. Investigation with large prospective clinical studies is required in order to thoroughly evaluate the role of diet in MS.

IL-17 and CCR9+α4β7- Th17 Cells Promote Salivary Gland Inflammation, Dysfunction, and Cell Death in Sjögren's Syndrome

Previous studies have evaluated the roles of T and B cells in the pathogenesis of Sjögren's syndrome (SS); however, their relationships with age-dependent and metabolic abnormalities remain unclear. We examined the impacts of changes associated with aging or metabolic abnormalities on populations of T and B cells and SS disease severity. We detected increased populations of IL-17-producing T and B cells, which regulate inflammation, in the salivary glands of NOD/ShiLtJ mice. Inflammation-induced human submandibular gland cell death, determined based on p-MLKL and RIPK3 expression levels, was significantly increased by IL-17 treatment. Among IL-17-expressing cells in the salivary gland, peripheral blood, and spleen, the α4β7 (gut-homing integrin)-negative population was significantly increased in aged NOD/ShiLtJ mice. The α4β7-positive population markedly increased in the intestines of aged NOD/ShiLtJ mice following retinoic acid (RA) treatment. A significant increase in α4β7-negative IL-17-expressing cells in salivary glands may be involved in the onset and progression of SS. These results suggest the potential therapeutic utility of RA in SS treatment.

Sepsis and multiple sclerosis: Causative links and outcomes

Sepsis is a life-threatening condition characterized by an acute cytokine storm followed by prolonged dysfunction of the immune system in the survivors. Post-septic lymphopenia and functional deficits of the remaining immune cells lead to increased susceptibility to secondary infections and other morbid conditions causing late death in the patients. This state of post-septic immunoparalysis may also influence disorders stemming from inappropriate or overactive immune responses, such as autoimmune and immunoinflammatory diseases, including multiple sclerosis. In addition, ongoing autoimmunity likely influences the susceptibility to and outcome of sepsis. This review article addresses the bidirectional relationship between sepsis and multiple sclerosis, with a focus on the immunologic mechanisms of the interaction and potential directions for future studies.

Targeting the gut to treat multiple sclerosis

The gut-brain axis (GBA) refers to the complex interactions between the gut microbiota and the nervous, immune, and endocrine systems, together linking brain and gut functions. Perturbations of the GBA have been reported in people with multiple sclerosis (pwMS), suggesting a possible role in disease pathogenesis and making it a potential therapeutic target. While research in the area is still in its infancy, a number of studies revealed that pwMS are more likely to exhibit altered microbiota, altered levels of short chain fatty acids and secondary bile products, and increased intestinal permeability. However, specific microbes and metabolites identified across studies and cohorts vary greatly. Small clinical and preclinical trials in pwMS and mouse models, in which microbial composition was manipulated through the use of antibiotics, fecal microbiota transplantation, and probiotic supplements, have provided promising outcomes in preventing CNS inflammation. However, results are not always consistent, and large-scale randomized controlled trials are lacking. Herein, we give an overview of how the GBA could contribute to MS pathogenesis, examine the different approaches tested to modulate the GBA, and discuss how they may impact neuroinflammation and demyelination in the CNS.

ILC3, a Central Innate Immune Component of the Gut-Brain Axis in Multiple Sclerosis

Gut immune cells have been increasingly appreciated as important players in the central nervous system (CNS) autoimmunity in animal models of multiple sclerosis (MS). Among the gut immune cells, innate lymphoid cell type 3 (ILC3) is of special interest in MS research, as they represent the innate cell counterpart of the major pathogenic cell population in MS, i.e. T helper (Th)17 cells. Importantly, these cells have been shown to stimulate regulatory T cells (Treg) and to counteract pathogenic Th17 cells in animal models of autoimmune diseases. Besides, they are also well known for their ability to stabilize the intestinal barrier and to shape the immune response to the gut microbiota. Thus, proper maintenance of the intestinal barrier and the establishment of the regulatory milieu in the gut performed by ILC3 may prevent activation of CNS antigen-specific Th17 cells by the molecular mimicry. Recent findings on the role of ILC3 in the gut-CNS axis and their relevance for MS pathogenesis will be discussed in this paper. Possibilities of ILC3 functional modulation for the benefit of MS patients will be addressed, as well.

The Nerves to Conduct a Multiple Sclerosis Crime Investigation

Multiple sclerosis (MS) is a chronic inflammatory neurodegenerative autoimmune disease characterized by the aberrant infiltration of immune cells into the central nervous system (CNS) and by the loss of myelin. Sclerotic lesions and various inhibitory factors hamper the remyelination processes within the CNS. MS patients typically experience gradual cognitive and physical disabilities as the disease progresses. The etiology of MS is still unclear and emerging evidence suggests that microbiome composition could play a much more significant role in disease pathogenesis than was initially thought. Initially believed to be isolated to the gut microenvironment, we now know that the microbiome plays a much broader role in various tissues and is essential in the development of the immune system. Here, we present some of the unexpected roles that the microbiome plays in MS and discuss approaches for the development of next-generation treatment strategies.

The trimebutine effect on Helicobacter pylori-related gastrointestinal tract and brain disorders: A hypothesis

The localization of bacterial components and/or metabolites in the central nervous system may elicit neuroinflammation and/or neurodegeneration. Helicobacter pylori (a non-commensal symbiotic gastrointestinal pathogen) infection and its related metabolic syndrome have been implicated in the pathogenesis of gastrointestinal tract and central nervous system disorders, thus medications affecting the nervous system - gastrointestinal tract may shape the potential of Helicobacter pylori infection to trigger these pathologies. Helicobacter pylori associated metabolic syndrome, by impairing gut motility and promoting bacterial overgrowth and translocation, might lead to brain pathologies. Trimebutine maleate is a prokinetic drug that hastens gastric emptying, by inducing the release of gastrointestinal agents such as motilin and gastrin. Likewise, it appears to protect against inflammatory signal pathways, involved in inflammatory disorders including brain pathologies. Trimebutine maleate also acts as an antimicrobial agent and exerts opioid agonist effect. This study aimed to investigate a hypothesis regarding the recent advances in exploring the potential role of gastrointestinal tract microbiota dysbiosis-related metabolic syndrome and Helicobacter pylori in the pathogenesis of gastrointestinal tract and brain diseases. We hereby proposed a possible neuroprotective role for trimebutine maleate by altering the dynamics of the gut-brain axis interaction, thus suggesting an additional effect of trimebutine maleate on Helicobacter pylori eradication regimens against these pathologies.

The role of faecal microbiota transplantation: looking beyond Clostridioides difficile infection

Faecal microbiota transplantation (FMT) is the transfer of screened and minimally processed faecal material from a 'healthy' donor to 'diseased' recipient. It has an established role, and is recommended as a therapeutic strategy, in the management of recurrent Clostridioides difficile infection (CDI). Recognition that gut dysbiosis is associated with, and may contribute to, numerous disease states has led to interest in exploiting FMT to 'correct' this microbial imbalance. Conditions for which it is proposed to be beneficial include inflammatory bowel disease, irritable bowel syndrome, liver disease and hepatic encephalopathy, neuropsychiatric conditions such as depression and anxiety, systemic inflammatory states like sepsis, and even coronavirus disease 2019. To understand what role, if any, FMT may play in the management of these conditions, it is important to consider the potential risks and benefits of the therapy. Regardless, there are several barriers to its more widespread adoption, which include incompletely understood mechanism of action (especially outside of CDI), inability to standardise treatment, disagreement on its active ingredients and how it should be regulated, and lack of long-term outcome and safety data. Whilst the transfer of faecal material from one individual to another to treat ailments or improve health has a history dating back thousands of years, there are fewer than 10 randomised controlled trials supporting its use. Moving forward, it will be imperative to gather as much data from FMT donors and recipients over as long a timeframe as possible, and for trials to be conducted with rigorous methodology, including appropriate control groups, in order to best understand the utility of FMT for indications beyond CDI. This review discusses the history of FMT, its appreciable mechanisms of action with reference to CDI, indications for FMT with an emerging evidence base above and beyond CDI, and future perspectives on the field.

Blocking integrin α4β7-mediated CD4 T cell recruitment to the intestine and liver protects mice from western diet-induced non-alcoholic steatohepatitis

BACKGROUND & AIMS

The heterodimeric integrin receptor α₄β₇ regulates CD4 T cell recruitment to inflamed tissues, but its role in the pathogenesis of non-alcoholic steatohepatitis (NASH) is unknown. Herein, we examined the role of α₄β₇-mediated recruitment of CD4 T cells to the intestine and liver in NASH.

METHODS

Male littermate F11r^+/+ (control) and junctional adhesion molecule A knockout F11r^-/- mice were fed a normal diet or a western diet (WD) for 8 weeks. Liver and intestinal tissues were analyzed by histology, quantitative reverse transcription PCR (qRT-PCR), 16s rRNA sequencing and flow cytometry. Colonic mucosa-associated microbiota were analyzed using 16s rRNA sequencing. Liver biopsies from patients with NASH were analyzed by confocal imaging and qRT-PCR.

RESULTS

WD-fed knockout mice developed NASH and had increased hepatic and intestinal α₄β₇⁺ CD4 T cells relative to control mice who developed mild hepatic steatosis. The increase in α₄β₇⁺ CD4 T cells was associated with markedly higher expression of the α₄β₇ ligand mucosal addressin cell adhesion molecule 1 (MAdCAM-1) in the colonic mucosa and livers of WD-fed knockout mice. Elevated MAdCAM-1 expression correlated with increased mucosa-associated Proteobacteria in the WD-fed knockout mice. Antibiotics reduced MAdCAM-1 expression indicating that the diet-altered microbiota promoted colonic and hepatic MAdCAM-1 expression. α₄β₇ blockade in WD-fed knockout mice significantly decreased α₄β₇⁺ CD4 T cell recruitment to the intestine and liver, attenuated hepatic inflammation and fibrosis, and improved metabolic indices. MAdCAM-1 blockade also reduced hepatic inflammation and fibrosis in WD-fed knockout mice. Hepatic MAdCAM-1 expression was elevated in patients with NASH and correlated with higher expression of α₄ and β₇ integrins.

CONCLUSIONS

These findings establish α₄β₇/MAdCAM-1 as a critical axis regulating NASH development through colonic and hepatic CD4 T cell recruitment.

LAY SUMMARY

Non-alcoholic steatohepatitis (NASH) is an advanced and progressive form of non-alcoholic fatty liver disease (NAFLD), and despite its growing incidence no therapies currently exist to halt NAFLD progression. Herein, we show that blocking integrin receptor α₄β₇-mediated recruitment of CD4 T cells to the intestine and liver not only attenuates hepatic inflammation and fibrosis, but also improves metabolic derangements associated with NASH. These findings provide evidence for the potential therapeutic application of α₄β₇ antibody in the treatment of human NASH.

Environmental enrichment alleviates the deleterious effects of stress in experimental autoimmune encephalomyelitis

Background

Clinical observations support the hypothesis that stressful events increase relapse occurrence in multiple sclerosis patients, while stress-reduction strategies can modulate this effect. However, a direct cause-effect relationship between stress level and relapse cannot be firmly established from these data.

Objectives

The purpose of this work was to address whether modulation of stress could interfere with symptom relapse in an animal model of multiple sclerosis with relapsing-remitting course.

Methods

Mice bred in standard or enriched environment were subjected to repeated acute stress during the remission phase of relapsing-remitting PLP-induced experimental autoimmune encephalomyelitis.

Results

We report that repeated acute stress induced a twofold increase in relapse incidence in experimental autoimmune encephalomyelitis. On the other hand, environmental enrichment reduced relapse incidence and severity, and reversed the effects of repeated acute stress.

Conclusion

These data provide the platform for further studies on the biological processes that link stress and multiple sclerosis relapses in a suitable animal model.

Microbiome in Multiple Sclerosis; Where Are We, What We Know and Do Not Know

An increase of multiple sclerosis (MS) incidence has been reported during the last decade, and this may be connected to environmental factors. This review article aims to encapsulate the current advances targeting the study of the gut-brain axis, which mediates the communication between the central nervous system and the gut microbiome. Clinical data arising from many research studies, which have assessed the effects of administered disease-modifying treatments in MS patients to the gut microbiome, are also recapitulated.

Fecal Microbiota Transplantation in Neurological Disorders

Background: Several studies suggested an important role of the gut microbiota in the pathophysiology of neurological disorders, implying that alteration of the gut microbiota might serve as a treatment strategy. Fecal microbiota transplantation (FMT) is currently the most effective gut microbiota intervention and an accepted treatment for recurrent Clostridioides difficile infections. To evaluate indications of FMT for patients with neurological disorders, we summarized the available literature on FMT. In addition, we provide suggestions for future directions.

Methods: In July 2019, five main databases were searched for studies and case descriptions on FMT in neurological disorders in humans or animal models. In addition, the ClinicalTrials.gov website was consulted for registered planned and ongoing trials.

Results: Of 541 identified studies, 34 were included in the analysis. Clinical trials with FMT have been performed in patients with autism spectrum disorder and showed beneficial effects on neurological symptoms. For multiple sclerosis and Parkinson's disease, several animal studies suggested a positive effect of FMT, supported by some human case reports. For epilepsy, Tourette syndrome, and diabetic neuropathy some studies suggested a beneficial effect of FMT, but evidence was restricted to case reports and limited numbers of animal studies. For stroke, Alzheimer's disease and Guillain-Barré syndrome only studies with animal models were identified. These studies suggested a potential beneficial effect of healthy donor FMT. In contrast, one study with an animal model for stroke showed increased mortality after FMT. For Guillain-Barré only one study was identified. Whether positive findings from animal studies can be confirmed in the treatment of human diseases awaits to be seen. Several trials with FMT as treatment for the above mentioned neurological disorders are planned or ongoing, as well as for amyotrophic lateral sclerosis.

Conclusions: Preliminary literature suggests that FMT may be a promising treatment option for several neurological disorders. However, available evidence is still scanty and some contrasting results were observed. A limited number of studies in humans have been performed or are ongoing, while for some disorders only animal experiments have been conducted. Large double-blinded randomized controlled trials are needed to further elucidate the effect of FMT in neurological disorders.

A Unique Role of T Helper 17 Cells in Different Treatment Stages of Multiple Myeloma

BACKGROUND

T helper 17 (Th17) cells are a subset of CD4-positive T cells, which secrete interleukin 17 and specifically express the retinoic acid receptor-related orphan receptors γt gene. Recently, Studies have shown that the level of Th17 cells in peripheral blood of newly diagnosed multiple myeloma (MM) patients is significantly higher than that of healthy persons. Th17 cells play an important role in the immune microenvironment of MM and interact with tumor cells, osteoclasts, and osteoblasts. Th17 cells might be a potential therapeutic target for MM patients.

PATIENTS AND METHODS

In this study, we further tracked the levels of Th17 cells in peripheral blood of 56 patients with MM from newly diagnosed to partial remission to complete remission to relapse and 11 healthy donors.

RESULTS

The level of Th17 cells increased further when the disease reached partial remission, decreased to normal level when it reached complete remission, and increased again when the disease recurred. In addition, we also found that in newly diagnosed MM patients, Th17 cell levels fluctuated greatly; not all patients were upregulated, and patients with normal Th17 cell levels had the highest chance of complete remission.

CONCLUSION

Th17 cells contribute to the stratification of different treatment stages of MM patients. The level of Th17 cells in patients with newly diagnosed MM is associated with the treatment outcome of complete remission.

The intestinal barrier in multiple sclerosis: implications for pathophysiology and therapeutics

Biological barriers are essential for the maintenance of homeostasis in health and disease. Breakdown of the intestinal barrier is an essential aspect of the pathophysiology of gastrointestinal inflammatory diseases, such as inflammatory bowel disease. A wealth of recent studies has shown that the intestinal microbiome, part of the brain-gut axis, could play a role in the pathophysiology of multiple sclerosis. However, an essential component of this axis, the intestinal barrier, has received much less attention. In this review, we describe the intestinal barrier as the physical and functional zone of interaction between the luminal microbiome and the host. Besides its essential role in the regulation of homeostatic processes, the intestinal barrier contains the gut mucosal immune system, a guardian of the integrity of the intestinal tract and the whole organism. Gastrointestinal disorders with intestinal barrier breakdown show evidence of CNS demyelination, and content of the intestinal microbiome entering into the circulation can impact the functions of CNS microglia. We highlight currently available studies suggesting that there is intestinal barrier dysfunction in multiple sclerosis. Finally, we address the mechanisms by which commonly used disease-modifying drugs in multiple sclerosis could alter the intestinal barrier and the microbiome, and we discuss the potential of barrier-stabilizing strategies, including probiotics and stabilization of tight junctions, as novel therapeutic avenues in multiple sclerosis.

Oral neonatal antibiotic treatment perturbs gut microbiota and aggravates central nervous system autoimmunity in Dark Agouti rats

Gut microbiota dysbiosis has been considered the essential element in the pathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). Antibiotics were administered orally to Dark Agouti (DA) rats early in their life with the aim of perturbing gut microbiota and investigating the effects of such intervention on the course of EAE. As a result, the diversity of the gut microbiota was reduced under the influence of antibiotics. Mainly, Firmicutes and Actinobacteria were replaced by Proteobacteria and Bacteroidetes, while decreased proportions of Clostridia and Bacilli classes were accompanied by an increase in Gamma-Proteobacteria in antibiotic-treated animals. Interestingly, a notable decrease in the Helicobacteraceae, Spirochaetaceae and Turicibacteriaceae was scored in antibiotic-treated groups. Also, levels of short chain fatty acids were reduced in the faeces of antibiotic-treated rats. Consequently, aggravation of EAE, paralleled with stronger immune response in lymph nodes draining the site of immunization, and increased inflammation within the CNS, were observed in antibiotic-treated DA rats. Thus, the alteration of gut microbiota leads to an escalation of CNS-directed autoimmunity in DA rats. The results of this study indicate that antibiotic use in early life may have subsequent unfavourable effects on the regulation of the immune system.

Microbial monotherapy with Prevotella histicola for patients with multiple sclerosis

Introduction: The gut microbiome helps to maintain a person's healthy state while perturbations in its function often leading to the development of inflammatory diseases including multiple sclerosis (MS). Consequently, gut-commensals which restore homeostasis have the potential to become novel therapeutic options for treating MS. MS patients have presented gut dysbiosis with a reduction in bacteria belonging to the Prevotella genus. Notably, increased levels of Prevotella are observed when disease-modifying therapies are used. Additionally, Prevotella histicola, an anaerobic bacterium derived from the human, can suppress disease in mice with experimental autoimmune encephalomyelitis, a preclinical MS model. Areas covered: This review compares MS microbiome studies from different geographical regions to identify common gut bacteria. Literature on the potential use of P. histicola as a therapy for MS and the next steps for developing microbial monotherapies in MS is also discussed. Expert commentary: Recent findings presenting an inverse correlation between Prevotella and MS disease severity and ability of P. histicola to suppress disease in preclinical models suggest that P. histicola might provide an additional treatment option for MS patients. However, rigorous testing in well-designed control trials should be performed to determine the safety and efficacy P. histicola in MS patients.

Experimental Autoimmune Encephalomyelitis (EAE) as Animal Models of Multiple Sclerosis (MS)

Multiple sclerosis (MS) is a multifocal demyelinating disease of the central nervous system (CNS) leading to the progressive destruction of the myelin sheath surrounding axons. It can present with variable clinical and pathological manifestations, which might reflect the involvement of distinct pathogenic processes. Although the mechanisms leading to the development of the disease are not fully understood, numerous evidences indicate that MS is an autoimmune disease, the initiation and progression of which are dependent on an autoimmune response against myelin antigens. In addition, genetic susceptibility and environmental triggers likely contribute to the initiation of the disease. At this time, there is no cure for MS, but several disease-modifying therapies (DMTs) are available to control and slow down disease progression. A good number of these DMTs were identified and tested using animal models of MS referred to as experimental autoimmune encephalomyelitis (EAE). In this review, we will recapitulate the characteristics of EAE models and discuss how they help shed light on MS pathogenesis and help test new treatments for MS patients.

Influence of the Gut Microbiome on Autoimmunity in the Central Nervous System

Autoimmune disorders of the CNS have complex pathogeneses that are not well understood. In multiple sclerosis and neuromyelitis optica spectrum disorders, T cells destroy CNS tissue, resulting in severe disabilities. Mounting evidence suggests that reducing inflammation in the CNS may start with modulation of the gut microbiome. The lymphoid tissues of the gut are specialized for the induction of regulatory cells, which are directly responsible for the suppression of CNS-damaging autoreactive T cells. Whether cause or effect, the onset of dysbiosis in the gut of patients with multiple sclerosis and neuromyelitis optica provides evidence of communication along the gut-brain axis. Thus, current and future therapeutic interventions directed at microbiome modulation are of considerable appeal.

The gut-brain connection: triggering of brain autoimmune disease by commensal gut bacteria

In a transgenic model of spontaneous experimental autoimmune encephalomyelitis, autoimmune attack against the CNS requires the presence of an intact commensal gut flora. Extending this observation to human autoimmune disease, such as multiple sclerosis, we postulate that the pathogenic reaction requires the coincidence of at least three factors: a permissive genetic disposition, a pro-inflammatory intestinal microbial profile, and the accumulation of autoreactive T cells in the gut-associated lymphatic tissue. This concept may offer new approaches to diagnostic markers and non-invasive therapies.

Mechanisms of T cell organotropism

Protective immunity relies upon T cell differentiation and subsequent migration to target tissues. Similarly, immune homeostasis requires the localization of regulatory T cells (Tregs) to the sites where immunity takes place. While naïve T lymphocytes recirculate predominantly in secondary lymphoid tissue, primed T cells and activated Tregs must traffic to the antigen rich non-lymphoid tissue to exert effector and regulatory responses, respectively. Following priming in draining lymph nodes, T cells acquire the 'homing receptors' to facilitate their access to specific tissues and organs. An additional level of topographic specificity is provided by T cells receptor recognition of antigen displayed by the endothelium. Furthermore, co-stimulatory signals (such as those induced by CD28) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response. We here review the molecular mechanisms supporting trafficking of both effector and regulatory T cells to specific antigen-rich tissues.

Gut-associated lymphoid tissue, gut microbes and susceptibility to experimental autoimmune encephalomyelitis

Gut microbiota and gut-associated lymphoid tissue have been increasingly appreciated as important players in pathogenesis of various autoimmune diseases, including multiple sclerosis. Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis that can be induced with an injection of spinal cord homogenate emulsified in complete Freund's adjuvant in Dark Agouti (DA) rats, but not in Albino Oxford (AO) rats. In this study, mesenteric lymph nodes (MLN), Peyer's patches (PP) and gut microbiota were analysed in these two rat strains. There was higher proportion of CD4(+) T cells and regulatory T cells in non-immunised DA rats in comparison to AO rats. Also, DA rat MLN and PP cells were higher producers of pro-inflammatory cytokines interferon-γ and interleukin-17. Finally, microbial analyses showed that uncultivated species of Turicibacter and Atopostipes genus were exclusively present in AO rats, in faeces and intestinal tissue, respectively. Thus, it is clear that in comparison of an EAE-susceptible with an EAE-resistant strain of rats, various discrepancies at the level of gut associated lymphoid tissue, as well as at the level of gut microbiota can be observed. Future studies should determine if the differences have functional significance for EAE pathogenesis.

Correlation of Gut Microbiota Composition with Resistance to Experimental Autoimmune Encephalomyelitis in Rats

Multiple sclerosis is a chronic inflammatory disease of the central nervous system (CNS). It is widely accepted that autoimmune response against the antigens of the CNS is the essential pathogenic force in the disease. It has recently become increasingly appreciated that activated encephalitogenic cells tend to migrate toward gut associated lymphoid tissues (GALTs) and that interrupted balance between regulatory and inflammatory immunity within the GALT might have decisive role in the initiation and propagation of the CNS autoimmunity. Gut microbiota composition and function has the major impact on the balance in the GALT. Thus, our aim was to perform analyses of gut microbiota in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. Albino Oxford (AO) rats that are highly resistant to EAE induction and Dark Agouti (DA) rats that develop EAE after mild immunization were compared for gut microbiota composition in different phases after EAE induction. Microbial analyses of the genus Lactobacillus and related lactic acid bacteria showed higher diversity of Lactobacillus spp. in EAE-resistant AO rats, while some members of Firmicutes and Proteobacteria (Undibacterium oligocarboniphilum) were detected only in feces of DA rats at the peak of the disease (between 13 and 16 days after induction). Interestingly, in contrast to our previous study where Turicibacter sp. was found exclusively in non-immunized AO, but not in DA rats, in this study it was detected in DA rats that remained healthy 16 days after induction, as well as in four of 12 DA rats at the peak of the disease. Similar observation was obtained for the members of Lachnospiraceae. Further, production of a typical regulatory cytokine interleukin-10 was compared in GALT cells of AO and DA rats, and higher production was observed in DA rats. Our data contribute to the idea that gut microbiota and GALT considerably influence multiple sclerosis pathogenesis.

Autoimmune host-microbiota interactions at barrier sites and beyond

The microbiota is considered to be an important factor influencing the pathogenesis of autoimmunity at both barrier sites and internal organs. Impinging on innate and adaptive immunity, commensals exert protective or detrimental effects on various autoimmune animal models. Human microbiome studies of autoimmunity remain largely descriptive, but suggest a role for dysbiosis in autoimmune disease. Humanized gnotobiotic approaches have advanced our understanding of immune-commensal interactions, but little is known about the mechanisms in autoimmunity. We propose that, similarly to infectious agents, the microbiota mediates autoimmunity via bystander activation, epitope spread, and, particularly under homeostatic conditions, via crossreactivity. This review presents an overview of the current literature concluding with outstanding questions in this field.

From classic to spontaneous and humanized models of multiple sclerosis: impact on understanding pathogenesis and drug development

Multiple sclerosis (MS), a demyelinating disease of the central nervous system (CNS), presents as a complex disease with variable clinical and pathological manifestations, involving different pathogenic pathways. Animal models, particularly experimental autoimmune encephalomyelitis (EAE), have been key to deciphering the pathophysiology of MS, although no single model can recapitulate the complexity and diversity of MS, or can, to date, integrate the diverse pathogenic pathways. Since the first EAE model was introduced decades ago, multiple classic (induced), spontaneous, and humanized EAE models have been developed, each recapitulating particular aspects of MS pathogenesis. The advances in technologies of genetic ablation and transgenesis in mice of C57BL/6J background and the development of myelin-oligodendrocyte glycoprotein (MOG)-induced EAE in C57BL/6J mice yielded several spontaneous and humanized EAE models, and resulted in a plethora of EAE models in which the role of specific genes or cell populations could be precisely interrogated, towards modeling specific pathways of MS pathogenesis/regulation in MS. Collectively, the numerous studies on the different EAE models contributed immensely to our basic understanding of cellular and molecular pathways in MS pathogenesis as well as to the development of therapeutic agents: several drugs available today as disease modifying treatments were developed from direct studies on EAE models, and many others were tested or validated in EAE. In this review, we discuss the contribution of major classic, spontaneous, and humanized EAE models to our understanding of MS pathophysiology and to insights leading to devising current and future therapies for this disease.