Gut Microbial Antigenic Mimicry in Autoimmunity

The role of gut microbiota in different murine models of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by immune system dysfunction that can lead to serious health issues and mortality. Recent investigations highlight the role of gut microbiota alterations in modulating inflammation and disease severity in SLE. This review specifically summaries the variations in gut microbiota composition across various murine models of lupus. By focusing on these differences, we aim to elucidate the intricate relationship between gut microbiota dysbiosis and the development and progression of SLE in preclinical settings.

Type 17 immunity: novel insights into intestinal homeostasis and autoimmune pathogenesis driven by gut-primed T cells

The IL-23 signaling pathway in both innate and adaptive immune cells is vital for orchestrating type 17 immunity, which is marked by the secretion of signature cytokines such as IL-17, IL-22, and GM-CSF. These proinflammatory mediators play indispensable roles in maintaining intestinal immune equilibrium and mucosal host defense; however, their involvement has also been implicated in the pathogenesis of chronic inflammatory disorders, such as inflammatory bowel diseases and autoimmunity. However, the implications of type 17 immunity across diverse inflammation models are complex. This review provides a comprehensive overview of the multifaceted roles of these cytokines in maintaining gut homeostasis and in perturbing gut barrier integrity, leading to acute and chronic inflammation in various models of gut infection and colitis. Additionally, this review focuses on type 17 immunity interconnecting multiple organs in autoimmune conditions, with a particular emphasis on the pathogenesis of autoimmune arthritis and neuroinflammation driven by T cells primed within the gut microenvironment.

Biologic therapy for ulcerative colitis associated with immune thrombocytopenia

Ulcerative colitis (UC), a subtype of inflammatory bowel disease, occasionally manifests with extraintestinal manifestations. We present a 51-year-old male with refractory UC and immune thrombocytopenia (ITP) resistant to conventional treatments. The introduction of biologics, ustekinumab or adalimumab, resulted in clinical remission of colitis and improvements in platelet count. This case underscores the efficacy of biologics in managing refractory UC associated with ITP, emphasizing their potential to control intestinal inflammation and address concurrent thrombocytopenia, potentially avoiding surgical intervention.

Risk of inflammatory bowel disease following hospitalisation with infectious mononucleosis: nationwide cohort study from Denmark

Infectious mononucleosis (IM) is suspected to be associated with inflammatory bowel disease (IBD) development. Using a Danish nationwide cohort of people developing severe IM and their age-, sex-, and socioeconomic (SES) index-matched counterparts, we investigated the subsequent risk of IBD, Crohn's disease (CD), or ulcerative colitis (UC) development from 1977 to 2021. Among 39,684 severe IM patients we find a sex-, age-, and SES index-adjusted HR for IBD of 1.35 (95% CI: 1.22-1.49). This significantly increased risk was seen for both CD (HR: 1.56; 95% CI: 1.34-1.83) and to a lesser extent UC (HR: 1.23; 95% CI: 1.08-1.40) and remains following negative control matching with a cohort diagnosed with Chlamydia trachomatis infection (HR: 1.39; 95% CI: 1.01-1.91). Those with severe IM at 0-9 years had a particularly increased risk for CD (HR: 1.77; 95% CI: 1.26-2.49). Here we show an increased risk for IBD diagnosis following IM hospitalisation, indicating an association between severe EBV disease and later IBD development. Further exploration of the potential factors contributing to IBD susceptibility following EBV disease is warranted.

Life history of a brain autoreactive T cell: From thymus through intestine to blood-brain barrier and brain lesion

Brain antigen-specific autoreactive T cells seem to play a key role in inducing inflammation in the central nervous system (CNS), a characteristic feature of human multiple sclerosis (MS). These T cells are generated within the thymus, where they escape negative selection and become integrated into the peripheral immune repertoire of immune cells. Typically, these autoreactive T cells rest in the periphery without attacking the CNS. When autoimmune T cells enter gut-associated lymphatic tissue (GALT), they may be stimulated by the microbiota and its metabolites. After activation, the cells migrate into the CNS through the blood‒brain barrier, become reactivated upon interacting with local antigen-presenting cells, and induce inflammatory lesions within the brain parenchyma. This review describes how microbiota influence autoreactive T cells during their life, starting in the thymus, migrating through the periphery and inducing inflammation in their target organ, the CNS.

Unravelling the Oral-Gut Axis: Interconnection Between Periodontitis and Inflammatory Bowel Disease, Current Challenges, and Future Perspective

As the opposite ends of the orodigestive tract, the oral cavity and the intestine share anatomical, microbial, and immunological ties that have bidirectional health implications. A growing body of evidence suggests an interconnection between oral pathologies and inflammatory bowel disease [IBD], implying a shift from the traditional concept of independent diseases to a complex, reciprocal cycle. This review outlines the evidence supporting an 'oral-gut' axis, marked by a higher prevalence of periodontitis and other oral conditions in IBD patients and vice versa. We present an in-depth examination of the interconnection between oral pathologies and IBD, highlighting the shared microbiological and immunological pathways, and proposing a 'multi-hit' hypothesis in the pathogenesis of periodontitis-mediated intestinal inflammation. Furthermore, the review underscores the critical need for a collaborative approach between dentists and gastroenterologists to provide holistic oral-systemic healthcare.

Interplay between the Chaperone System and Gut Microbiota Dysbiosis in Systemic Lupus Erythematosus Pathogenesis: Is Molecular Mimicry the Missing Link between Those Two Factors?

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease characterized by self-immune tolerance breakdown and the production of autoantibodies, causing the deposition of immune complexes and triggering inflammation and immune-mediated damage. SLE pathogenesis involves genetic predisposition and a combination of environmental factors. Clinical manifestations are variable, making an early diagnosis challenging. Heat shock proteins (Hsps), belonging to the chaperone system, interact with the immune system, acting as pro-inflammatory factors, autoantigens, as well as immune tolerance promoters. Increased levels of some Hsps and the production of autoantibodies against them are correlated with SLE onset and progression. The production of these autoantibodies has been attributed to molecular mimicry, occurring upon viral and bacterial infections, since they are evolutionary highly conserved. Gut microbiota dysbiosis has been associated with the occurrence and severity of SLE. Numerous findings suggest that proteins and metabolites of commensal bacteria can mimic autoantigens, inducing autoimmunity, because of molecular mimicry. Here, we propose that shared epitopes between human Hsps and those of gut commensal bacteria cause the production of anti-Hsp autoantibodies that cross-react with human molecules, contributing to SLE pathogenesis. Thus, the involvement of the chaperone system, gut microbiota dysbiosis, and molecular mimicry in SLE ought to be coordinately studied.

Good girl goes bad: Understanding how gut commensals cause disease

This review examines the complex connection between commensal microbiota and the development of opportunistic infections. Several underlying conditions, such as metabolic diseases and weakened immune systems, increase the vulnerability of patients to opportunistic infections. The increasing antibiotic resistance adds significant complexity to the management of infectious diseases. Although commensals have long been considered beneficial, recent research contradicts this notion by uncovering chronic illnesses linked to atypical pathogens or commensal bacteria. This review examines conditions in which commensal bacteria, which are usually beneficial, contribute to developing diseases. Commensals' support for opportunistic infections can be categorized based on factors such as colonization fitness, pathoadaptive mutation, and evasion of host immune response. Individuals with weakened immune systems are especially susceptible, highlighting the importance of mucosal host-microbiota interaction in promoting infection when conditions are inappropriate. Dysregulation of gut microbial homeostasis, immunological modulation, and microbial interactions are caused by several factors that contribute to the development of chronic illnesses. Knowledge about these mechanisms is essential for developing preventive measures, particularly for susceptible populations, and emphasizes the importance of maintaining a balanced gut microbiota in reducing the impact of opportunistic infections.

The gut microbiome in systemic lupus erythematosus: lessons from rheumatic fever

For more than a century, certain bacterial infections that can breach the skin and mucosal barriers have been implicated as common triggers of autoimmune syndromes, especially post-infection autoimmune diseases that include rheumatic fever and post-streptococcal glomerulonephritis. However, only in the past few years has the importance of imbalances within our own commensal microbiota communities, and within the gut, in the absence of infection, in promoting autoimmune pathogenesis become fully appreciated. A diversity of species and mechanisms have been implicated, including disruption of the gut barrier. Emerging data suggest that expansions (or blooms) of pathobiont species are involved in autoimmune pathogenesis and stimulate clonal expansion of T cells and B cells that recognize microbial antigens. This Review discusses the relationship between the gut microbiome and the immune system, and the potential consequence of disrupting the community balance in terms of autoimmune development, focusing on systemic lupus erythematosus. Notably, inter-relationships between expansions of certain members within gut microbiota communities and concurrent autoimmune responses bear features reminiscent of classical post-infection autoimmune disease. From such insights, new therapeutic opportunities are being considered to restore the balance within microbiota communities or re-establishing the gut-barrier integrity to reinforce immune homeostasis in the host.

Sex and Gender in Ageing and Longevity: Highlights From an International Course

Gender medicine is a multidisciplinary science and represents an important perspective for pathophysiological and clinical studies in the third millennium. Here, it is provided an overview of the topics discussed in a recent course on the Role of Sex and Gender in Ageing and Longevity. The paper highlights three themes discussed in the course, i.e., the interaction of gender/sex with, i) the pathophysiology of age-related diseases; ii), the role of genetics and epigenetics in ageing and longevity and, iii) the immune responses of older people to pathogens, vaccines, autoantigens, and allergens. Although largely unexplored, it is clear that sex and gender are modulators of disease biology and treatment outcomes. It is becoming evident that men and women should no longer be considered as subgroups, but as biologically distinct groups of patients deserving consideration for specific therapeutic approaches.

Immunologic derangement caused by intestinal dysbiosis and stress is the intrinsic basis of reactive arthritis

Reactive arthritis (ReA) is defined as arthritis resulting from infections in other body parts, such as the gastrointestinal and urogenital tracts. The primary clinical manifestations involve acute-onset and self-limiting asymmetric large joint inflammation in the lower limbs. Although bacterial or chlamydia infections have long been recognized as playing a pivotal role in its pathogenesis, recent studies suggest that antibiotic treatment may perpetuate rather than eradicate chlamydia within the host, indicating an involvement of other mechanisms in Reactive arthritis. Reactive arthritis is currently believed to be associated with infection, genetic marker (HLA-B27), and immunologic derangement. As an autoimmune disease, increasing attention has been given to understanding the role of the immune system in Reactive arthritis. This review focuses on elucidating how the immune system mediates reactive arthritis and explores the roles of intestinal dysbiosis-induced immune disorders and stress-related factors in autoimmune diseases, providing novel insights into understanding reactive arthritis.

When inflammatory stressors dramatically change, disease phenotypes may transform between autoimmune hematopoietic failure and myeloid neoplasms

Aplastic anemia (AA) and hypoplastic myelodysplastic syndrome are paradigms of autoimmune hematopoietic failure (AHF). Myelodysplastic syndrome and acute myeloid leukemia are unequivocal myeloid neoplasms (MNs). Currently, AA is also known to be a clonal hematological disease. Genetic aberrations typically observed in MNs are detected in approximately one-third of AA patients. In AA patients harboring MN-related genetic aberrations, a poor response to immunosuppressive therapy (IST) and an increased risk of transformation to MNs occurring either naturally or after IST are predicted. Approximately 10%-15% of patients with severe AA transform the disease phenotype to MNs following IST, and in some patients, leukemic transformation emerges during or shortly after IST. Phenotypic transformations between AHF and MNs can occur reciprocally. A fraction of advanced MN patients experience an aplastic crisis during which leukemic blasts are repressed. The switch that shapes the disease phenotype is a change in the strength of extramedullary inflammation. Both AHF and MNs have an immune-active bone marrow (BM) environment (BME). In AHF patients, an inflamed BME can be evoked by infiltrated immune cells targeting neoplastic molecules, which contributes to the BM-specific autoimmune impairment. Autoimmune responses in AHF may represent an antileukemic mechanism, and inflammatory stressors strengthen antileukemic immunity, at least in a significant proportion of patients who have MN-related genetic aberrations. During active inflammatory episodes, normal and leukemic hematopoieses are suppressed, which leads to the occurrence of aplastic cytopenia and leukemic cell regression. The successful treatment of underlying infections mitigates inflammatory stress-related antileukemic activities and promotes the penetration of leukemic hematopoiesis. The effect of IST is similar to that of treating underlying infections. Investigating inflammatory stress-powered antileukemic immunity is highly important in theoretical studies and clinical practice, especially given the wide application of immune-activating agents and immune checkpoint inhibitors in the treatment of hematological neoplasms.

Microbiota-dependent indole production stimulates the development of collagen-induced arthritis in mice

Altered tryptophan catabolism has been identified in inflammatory diseases like rheumatoid arthritis (RA) and spondyloarthritis (SpA), but the causal mechanisms linking tryptophan metabolites to disease are unknown. Using the collagen-induced arthritis (CIA) model, we identified alterations in tryptophan metabolism, and specifically indole, that correlated with disease. We demonstrated that both bacteria and dietary tryptophan were required for disease and that indole supplementation was sufficient to induce disease in their absence. When mice with CIA on a low-tryptophan diet were supplemented with indole, we observed significant increases in serum IL-6, TNF, and IL-1β; splenic RORγt+CD4+ T cells and ex vivo collagen-stimulated IL-17 production; and a pattern of anti-collagen antibody isotype switching and glycosylation that corresponded with increased complement fixation. IL-23 neutralization reduced disease severity in indole-induced CIA. Finally, exposure of human colonic lymphocytes to indole increased the expression of genes involved in IL-17 signaling and plasma cell activation. Altogether, we propose a mechanism by which intestinal dysbiosis during inflammatory arthritis results in altered tryptophan catabolism, leading to indole stimulation of arthritis development. Blockade of indole generation may present a unique therapeutic pathway for RA and SpA.

Microbial amyloids in neurodegenerative amyloid diseases

Human-disease associated amyloidogenic proteins are not unique in their ability to form amyloid fibrillar structures. Numerous microbes produce amyloidogenic proteins that have distinct functions for their physiology in their amyloid form, rather than solely detrimental. Emerging data indicate associations between various microbial organisms, including those which produce functional amyloids, with neurodegenerative diseases. Here, we review some of the evidence suggesting that microbial amyloids impact amyloid disease in host organisms. Experimental data are building a foundation for continued lines of enquiry and suggest that that direct or indirect interactions between microbial and host amyloids may be a contributor to amyloid pathologies. Inhibiting microbial amyloids or their interactions with the host may therefore represent a tangible target to limit various amyloid pathologies.

Targeting gut microbiota: new therapeutic opportunities in multiple sclerosis

Multiple sclerosis (MS) causes long-lasting, multifocal damage to the central nervous system. The complex background of MS is associated with autoimmune inflammation and neurodegeneration processes, and is potentially affected by many contributing factors, including altered composition and function of the gut microbiota. In this review, current experimental and clinical evidence is presented for the characteristics of gut dysbiosis found in MS, as well as for its relevant links with the course of the disease and the dysregulated immune response and metabolic pathways involved in MS pathology. Furthermore, therapeutic implications of these investigations are discussed, with a range of pharmacological, dietary and other interventions targeted at the gut microbiome and thus intended to have beneficial effects on the course of MS.

Vaccines for immune tolerance against autoimmune disease

The high prevalence and rising incidence of autoimmune diseases have become a prominent public health issue. Autoimmune disorders result from the immune system erroneously attacking the body's own healthy cells and tissues, causing persistent inflammation, tissue injury, and impaired organ function. Existing treatments primarily rely on broad immunosuppression, leaving patients vulnerable to infections and necessitating lifelong treatments. To address these unmet needs, an emerging frontier of vaccine development aims to restore immune equilibrium by inducing immune tolerance to autoantigens, offering a potential avenue for a cure rather than mere symptom management. We discuss this burgeoning field of vaccine development against inflammation and autoimmune diseases, with a focus on common autoimmune disorders, including multiple sclerosis, type 1 diabetes, rheumatoid arthritis, inflammatory bowel disease, and systemic lupus erythematosus. Vaccine-based strategies provide a new pathway for the future of autoimmune disease therapeutics, heralding a new era in the battle against inflammation and autoimmunity.

Cross-reactivity and sequence similarity between microbial transglutaminase and human tissue antigens

Microbial transglutaminase (mTG) is a bacterial survival factor, frequently used as a food additive to glue processed nutrients. As a result, new immunogenic epitopes are generated that might drive autoimmunity. Presently, its contribution to autoimmunity through epitope similarity and cross-reactivity was investigated. Emboss Matcher was used to perform sequence alignment between mTG and various antigens implicated in many autoimmune diseases. Monoclonal and polyclonal antibodies made specifically against mTG were applied to 77 different human tissue antigens using ELISA. Six antigens were detected to share significant homology with mTG immunogenic sequences, representing major targets of common autoimmune conditions. Polyclonal antibody to mTG reacted significantly with 17 out of 77 tissue antigens. This reaction was most pronounced with mitochondrial M2, ANA, and extractable nuclear antigens. The results indicate that sequence similarity and cross-reactivity between mTG and various tissue antigens are possible, supporting the relationship between mTG and the development of autoimmune disorders 150W.

Microbiota-dependent indole production is required for the development of collagen-induced arthritis

Altered tryptophan catabolism has been identified in inflammatory diseases like rheumatoid arthritis (RA) and spondyloarthritis (SpA), but the causal mechanisms linking tryptophan metabolites to disease are unknown. Using the collagen-induced arthritis (CIA) model we identify alterations in tryptophan metabolism, and specifically indole, that correlate with disease. We demonstrate that both bacteria and dietary tryptophan are required for disease, and indole supplementation is sufficient to induce disease in their absence. When mice with CIA on a low-tryptophan diet were supplemented with indole, we observed significant increases in serum IL-6, TNF, and IL-1β; splenic RORγt+CD4+ T cells and ex vivo collagen-stimulated IL-17 production; and a pattern of anti-collagen antibody isotype switching and glycosylation that corresponded with increased complement fixation. IL-23 neutralization reduced disease severity in indole-induced CIA. Finally, exposure of human colon lymphocytes to indole increased expression of genes involved in IL-17 signaling and plasma cell activation. Altogether, we propose a mechanism by which intestinal dysbiosis during inflammatory arthritis results in altered tryptophan catabolism, leading to indole stimulation of arthritis development. Blockade of indole generation may present a novel therapeutic pathway for RA and SpA.

Advances in the implications of the gut microbiota on the treatment efficacy of disease-modifying anti-rheumatic drugs in rheumatoid arthritis

Alterations in the composition or function of the gut microbiota are associated with the etiology of human diseases. Drug-microbiota interactions can affect drug bioavailability, effectiveness, and toxicity through various routes. For instance, the direct effect of microbial enzymes on drugs can either boost or diminish their efficacy. Thus, considering its wide range of metabolic capabilities, the gut microbiota is a promising target for pharmacological modulation. Furthermore, drugs can alter the microbiota and the mechanisms by which they interact with their host. Individual variances in microbial profiles can also contribute to the different host responses to various drugs. However, the influence of interactions between the gut microbiota and drugs on treatment efficacy remains poorly elucidated. In this review, we will discuss the impact of microbiota dysbiosis in the pathogenesis of rheumatoid arthritis (RA), and we will attempt to elucidate the crosstalk between the gut microbiota and disease-modifying anti-rheumatic drugs (DMARDs), with an emphasis on how drug-microbiota interactions affect the treatment efficacy in RA. We speculate that improved knowledge of these critical interactions will facilitate the development of novel therapeutic options that use microbial markers for predicting or optimizing treatment outcomes.

Two putative glutamate decarboxylases of Streptococcus pneumoniae as possible antigens for the production of anti-GAD65 antibodies leading to type 1 diabetes mellitus

Type 1 diabetes mellitus (T1DM) has been increasing in prevalence in the last decades and has become a global burden. Autoantibodies against human glutamate decarboxylase (GAD65) are among the first to be detected at the onset of T1DM. Diverse viruses have been proposed to be involved in the triggering of T1DM because of molecular mimicry, i.e., similarity between parts of some viral proteins and one or more epitopes of GAD65. However, the possibility that bacterial proteins might also be responsible for GAD65 mimicry has been seldom investigated. To date, many genomes of Streptococcus pneumoniae (the pneumococcus), a prominent human pathogen particularly prevalent among children and the elderly, have been sequenced. A dataset of more than 9000 pneumococcal genomes was mined and two different (albeit related) genes (gadA and gadB), presumably encoding two glutamate decarboxylases similar to GAD65, were found. The various gadASpn alleles were present only in serotype 3 pneumococci belonging to the global lineage GPSC83, although some homologs have also been discovered in two subspecies of Streptococcus constellatus (pharyngis and viborgensis), an isolate of the group B streptococci, and several strains of Lactobacillus delbrueckii. Besides, gadBSpn alleles are present in > 10% of the isolates in our dataset and represent 16 GPSCs with 123 sequence types and 20 different serotypes. Sequence analyses indicated that gadA- and gadB-like genes have been mobilized among different bacteria either by prophage(s) or by integrative and conjugative element(s), respectively. Substantial similarities appear to exist between the putative pneumococcal glutamate decarboxylases and well-known epitopes of GAD65. In this sense, the use of broader pneumococcal conjugate vaccines such as PCV20 would prevent the majority of serotypes expressing those genes that might potentially contribute to T1DM. These results deserve upcoming studies on the possible involvement of S. pneumoniae in the etiopathogenesis and clinical onset of T1DM.

Gut microbiota: a newly identified environmental factor in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease that predominantly affects women of childbearing age and is characterized by the damage to multiple target organs. The pathogenesis of SLE is complex, and its etiology mainly involves genetic and environmental factors. At present, there is still a lack of effective means to cure SLE. In recent years, growing evidence has shown that gut microbiota, as an environmental factor, triggers autoimmunity through potential mechanisms including translocation and molecular mimicry, leads to immune dysregulation, and contributes to the development of SLE. Dietary intervention, drug therapy, probiotics supplement, fecal microbiome transplantation and other ways to modulate gut microbiota appear to be a potential treatment for SLE. In this review, the dysbiosis of gut microbiota in SLE, potential mechanisms linking gut microbiota and SLE, and immune dysregulation associated with gut microbiota in SLE are summarized.

The prevalence of IgG antibodies against milk and milk antigens in patients with multiple sclerosis

Introduction

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS). The pathophysiology of MS is complex and is said to be influenced by multiple environmental determinants, including diet. We and others have previously demonstrated how consumption of bovine milk can aggravate disease severity in MS patients, which can be explained by molecular mimicry between milk antigens and those expressed within the CNS. In this study we set out to identify alternatives to drinking cow milk which might be less detrimental to MS patients who have a genetic predisposition towards developing antibody titers against bovine milk antigens that cross-react with CNS antigens.

Methods

To this end, we screened 35 patients with MS and 20 healthy controls for their IgG reactivity against an array of animal-sourced milk, plant-based alternatives as well as individual antigens from bovine milk.

Results

We demonstrate that MS patients have a significantly higher IgG response to animal-sourced milk, especially cow milk, in comparison to healthy donors. We also show that the reactivity to cow milk in MS patients can be attributed to reactivity against different bovine milk antigens. Finally, our correlation data indicate the co-existence of antibodies to individual bovine milk antigens and their corresponding cross-reactive CNS antigens.

Discussion

Taken together, we suggest screening of blood from MS patients for antibodies against different types of milk and milk antigens in order to establish a personalized diet regimen.

SARS-CoV-2 Gut-Targeted Epitopes: Sequence Similarity and Cross-Reactivity Join Together for Molecular Mimicry

The gastrointestinal tract can be heavily infected by SARS-CoV-2. Being an auto-immunogenic virus, SARS-CoV-2 represents an environmental factor that might play a role in gut-associated autoimmune diseases. However, molecular mimicry between the virus and the intestinal epitopes is under-investigated. The present study aims to elucidate sequence similarity between viral antigens and human enteric sequences, based on known cross-reactivity. SARS-CoV-2 epitopes that cross-react with human gut antigens were explored, and sequence alignment was performed against self-antigens implicated in enteric autoimmune conditions. Experimental SARS-CoV-2 epitopes were aggregated from the Immune Epitope Database (IEDB), while enteric antigens were obtained from the UniProt Knowledgebase. A Pairwise Local Alignment tool, EMBOSS Matcher, was employed for the similarity search. Sequence similarity and targeted cross-reactivity were depicted between 10 pairs of immunoreactive epitopes. Similar pairs were found in four viral proteins and seven enteric antigens related to ulcerative colitis, primary biliary cholangitis, celiac disease, and autoimmune hepatitis. Antibodies made against the viral proteins that were cross-reactive with human gut antigens are involved in several essential cellular functions. The relationship and contribution of those intestinal cross-reactive epitopes to SARS-CoV-2 or its potential contribution to gut auto-immuno-genesis are discussed.

Visiting Molecular Mimicry Once More: Pathogenicity, Virulence, and Autoimmunity

The concept of molecular mimicry describes situations in which antigen sharing between parasites and hosts could benefit pathogen evasion from host immune responses. However, antigen sharing can generate host responses to parasite-derived self-like peptides, triggering autoimmunity. Since its conception, molecular mimicry and the consequent potential cross-reactivity following infections have been repeatedly described in humans, raising increasing interest among immunologists. Here, we reviewed this concept focusing on the challenge of maintaining host immune tolerance to self-components in parasitic diseases. We focused on the studies that used genomics and bioinformatics to estimate the extent of antigen sharing between proteomes of different organisms. In addition, we comparatively analyzed human and murine proteomes for peptide sharing with proteomes of pathogenic and non-pathogenic organisms. We conclude that, although the amount of antigenic sharing between hosts and both pathogenic and non-pathogenic parasites and bacteria is massive, the degree of this antigen sharing is not related to pathogenicity or virulence. In addition, because the development of autoimmunity in response to infections by microorganisms endowed with cross-reacting antigens is rare, we conclude that molecular mimicry by itself is not a sufficient factor to disrupt intact self-tolerance mechanisms.

Short chain fatty acids, a possible treatment option for autoimmune diseases

Gut microbiota can interact with the immune system through its metabolites. Short-chain fatty acids (SCFAs), as one of the most abundant metabolites of the resident gut microbiota play an important role in this crosstalk. SCFAs (acetate, propionate, and butyrate) regulate nearly every type of immune cell in the gut's immune cell repertoire regarding their development and function. SCFAs work through several pathways to impose protection towards colonic health and against local or systemic inflammation. Additionally, SCFAs play a role in the regulation of immune or non-immune pathways that can slow the development of autoimmunity either systematically or in situ. The present study aims to summarize the current knowledge on the immunomodulatory roles of SCFAs and the association between the SCFAs and autoimmune disorders such as celiac disease (CD), inflammatory bowel disease (IBD), rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (SLE), type 1 diabetes (T1D) and other immune-mediated diseases, uncovering a brand-new therapeutic possibility to prevent or treat autoimmunity.

Alterations of the intestinal mucus layer correlate with dysbiosis and immune dysregulation in human Type 1 Diabetes

BACKGROUND

In preclinical models of Type 1 Diabetes (T1D) the integrity of the gut barrier (GB) is instrumental to avoid dysregulated crosstalk between the commensal microbiota and immune cells and to prevent autoimmunity. The GB is composed of the intestinal epithelial barrier (IEB) and of the mucus layer containing mucins and antimicrobial peptides (AMPs) that are crucial to maintain immune tolerance. In preclinical models of T1D the alterations of the GB primarily affect the mucus layer. In human T1D increased gut permeability and IEB damage have been demonstrated but the integrity of the mucus layer was never assessed.

METHODS

We evaluated GB integrity by measuring serological markers of IEB damage (serological levels of zonulin) and bacterial translocation such as lipopolysaccharide binding protein (LBP) and myeloid differentiation protein 2 (MD2), and mRNA expression of tight junction proteins, mucins and AMPs in intestinal tissue of T1D patients and healthy controls (HC). Simultaneously, we performed immunological profiling on intestinal tissue and 16S rRNA analysis on the mucus-associated gut microbiota (MAGM).

FINDINGS

Our data show a GB damage with mucus layer alterations and reduced mRNA expression of several mucins (MUC2, MUC12, MUC13, MUC15, MUC20, MUC21) and AMPs (HD4 and HD5) in T1D patients. Mucus layer alterations correlated with reduced relative abundance of short chain fatty acids (SCFA)-producing bacteria such as Bifidobacterium dentium, Clostridium butyricum and Roseburia intestinalis that regulate mucin expression and intestinal immune homeostasis. In T1D patients we also found intestinal immune dysregulation with higher percentages of effector T cells such as T helper (Th) 1, Th17 and TNF-α⁺ T cells.

INTERPRETATION

Our data show that mucus layer alterations are present in T1D subjects and associated with dysbiosis and immune dysregulation.

FUNDING

Research Grants from the Juvenile Diabetes Foundation (Grant 1-INO-2018-640-A-N to MF and 2-SRA-2019-680-S-B to JD) and from the Italian Ministry of Health (Grant RF19-12370721 to MF).

Lupus Nephritis and Dysbiosis

Lupus nephritis (LN) is one of the most common and serious complications of systemic lupus erythematosus (SLE). The risk factors for developing LN by SLE patients are not fully understood. They are considered to be a mix of genetic and environmental variables, one of them being dysbiosis, proposed recently to interfere with autoimmunity. As of yet, the relations between the human microbiome, its genetic determinants, individual variability and clinical consequences remain to be established. One of the major obstacles in studying them is the magnitude of confounders, such as diet, drugs, infections or antibiotics use. They also make comparison between the studies extremely complicated. We reviewed the available evidence for the interplay between microbiome, dysbiosis and mechanisms triggering the autoimmune responses and potentially contributing to LN development. One such mechanism is the stimulation of autoimmune responses by bacterial metabolites that can mimic autoantigens and cause antibody production. These mimicking microbial antigens seem to be a promising target for future interventions.

The gut microbiota promotes distal tissue regeneration via RORγ+ regulatory T cell emissaries

Specific microbial signals induce the differentiation of a distinct pool of RORγ+ regulatory T (Treg) cells crucial for intestinal homeostasis. We discovered highly analogous populations of microbiota-dependent Treg cells that promoted tissue regeneration at extra-gut sites, notably acutely injured skeletal muscle and fatty liver. Inflammatory meditators elicited by tissue damage combined with MHC-class-II-dependent T cell activation to drive the accumulation of gut-derived RORγ+ Treg cells in injured muscle, wherein they regulated the dynamics and tenor of early inflammation and helped balance the proliferation vs. differentiation of local stem cells. Reining in IL-17A-producing T cells was a major mechanism underlying the rheostatic functions of RORγ+ Treg cells in compromised tissues. Our findings highlight the importance of gut-trained Treg cell emissaries in controlling the response to sterile injury of non-mucosal tissues.

Bacterial Metabolites: A Link between Gut Microbiota and Dermatological Diseases

Dysbiosis has been identified in many dermatological conditions (e.g., psoriasis, atopic dermatitis, systemic lupus erythematosus). One of the ways by which the microbiota affect homeostasis is through microbiota-derived molecules (metabolites). There are three main groups of metabolites: short-chain fatty acids (SCFAs), tryptophan metabolites, and amine derivatives including trimethylamine N-oxide (TMAO). Each group has its own uptake and specific receptors through which these metabolites can exert their systemic function. This review provides up-to-date knowledge about the impact that these groups of gut microbiota metabolites may have in dermatological conditions. Special attention is paid to the effect of microbial metabolites on the immune system, including changes in the profile of the immune cells and cytokine disbalance, which are characteristic of several dermatological diseases, especially psoriasis and atopic dermatitis. Targeting the production of microbiota metabolites may serve as a novel therapeutic approach in several immune-mediated dermatological diseases.

Gut-joint axis: Gut dysbiosis can contribute to the onset of rheumatoid arthritis via multiple pathways

Rheumatoid Arthritis (RA) is an autoimmune disease characterized by loss of immune tolerance and chronic inflammation. It is pathogenesis complex and includes interaction between genetic and environmental factors. Current evidence supports the hypothesis that gut dysbiosis may play the role of environmental triggers of arthritis in animals and humans. Progress in the understanding of the gut microbiome and RA. has been remarkable in the last decade. In vitro and in vivo experiments revealed that gut dysbiosis could shape the immune system and cause persistent immune inflammatory responses. Furthermore, gut dysbiosis could induce alterations in intestinal permeability, which have been found to predate arthritis onset. In contrast, metabolites derived from the intestinal microbiota have an immunomodulatory and anti-inflammatory effect. However, the precise underlying mechanisms by which gut dysbiosis induces the development of arthritis remain elusive. This review aimed to highlight the mechanisms by which gut dysbiosis could contribute to the pathogenesis of RA. The overall data showed that gut dysbiosis could contribute to RA pathogenesis by multiple pathways, including alterations in gut barrier function, molecular mimicry, gut dysbiosis influences the activation and the differentiation of innate and acquired immune cells, cross-talk between gut microbiota-derived metabolites and immune cells, and alterations in the microenvironment. The relative weight of each of these mechanisms in RA pathogenesis remains uncertain. Recent studies showed a substantial role for gut microbiota-derived metabolites pathway, especially butyrate, in the RA pathogenesis.

MHC Class II Presentation in Autoimmunity

Antigen presentation by major histocompatibility complex class II (MHC-II) molecules is crucial for eliciting an efficient immune response by CD4+ T cells and maintaining self-antigen tolerance. Some MHC-II alleles are known to be positively or negatively associated with the risk of the development of different autoimmune diseases (ADs), including those characterized by the emergence of autoreactive T cells. Apparently, the MHC-II presentation of self-antigens contributes to the autoimmune T cell response, initiated through a breakdown of central tolerance to self-antigens in the thymus. The appearance of autoreactive T cell might be the result of (i) the unusual interaction between T cell receptors (TCRs) and self-antigens presented on MHC-II; (ii) the posttranslational modifications (PTMs) of self-antigens; (iii) direct loading of the self-antigen to classical MHC-II without additional nonclassical MHC assistance; (iv) the proinflammatory environment effect on MHC-II expression and antigen presentation; and (v) molecular mimicry between foreign and self-antigens. The peculiarities of the processes involved in the MHC-II-mediated presentation may have crucial importance in the elucidation of the mechanisms of triggering and developing ADs as well as for clarification on the protective effect of MHC-II alleles that are negatively associated with ADs.

Gut microbiome in type 1 diabetes: the immunological perspective

INTRODUCTION

Type 1 diabetes (T1D) is a prevalent, and yet uncurable, autoimmune disease targeting insulin-producing pancreatic β-cells. Despite a known genetic component in T1D onset, genetics alone cannot explain the alarming worldwide rise in T1D incidence, which is attributed to a growing impact of environmental factors, including perturbations of the gut microbiome.

AREAS COVERED

Intestinal commensal bacteria plays a crucial role in host physiology in health and disease by regulating endocrine and immune functions. An aberrant gut microbiome structure and metabolic function have been documented prior and during T1D onset. In this review, we summarize and discuss the current studies depicting the taxonomic profile and role of the gut microbial communities in murine models of T1D, diabetic patients and human interventional trials.

EXPERT OPINION

Compelling evidence have shown that the intestinal microbiota is instrumental in driving differentiation and functions of immune cells. Therefore, any alterations in the intestinal microbiome composition or microbial metabolite production, particularly early in life, may impact disease susceptibility and amplify inflammatory responses and hence accelerate the course of T1D pathogenesis.

Epstein-Barr virus infection, B-cell dysfunction and other risk factors converge in gut-associated lymphoid tissue to drive the immunopathogenesis of multiple sclerosis: a hypothesis

Multiple sclerosis is associated with Epstein-Barr virus (EBV) infection, B-cell dysfunction, gut dysbiosis, and environmental and genetic risk factors, including female sex. A disease model incorporating all these factors remains elusive. Here, we hypothesise that EBV-infected memory B cells (MBCs) migrate to gut-associated lymphoid tissue (GALT) through EBV-induced expression of LPAM-1, where they are subsequently activated by gut microbes and/or their products resulting in EBV reactivation and compartmentalised anti-EBV immune responses. These responses involve marginal zone (MZ) B cells that activate CD4⁺ T-cell responses, via HLA-DRB1, which promote downstream B-cell differentiation towards CD11c⁺/T-bet⁺ MBCs, as well as conventional MBCs. Intrinsic expression of low-affinity B-cell receptors (BCRs) by MZ B cells and CD11c⁺/T-bet⁺ MBCs promotes polyreactive BCR/antibody responses against EBV proteins (e.g. EBNA-1) that cross-react with central nervous system (CNS) autoantigens (e.g. GlialCAM). EBV protein/autoantigen-specific CD11c⁺/T-bet⁺ MBCs migrate to the meningeal immune system and CNS, facilitated by their expression of CXCR3, and induce cytotoxic CD8⁺ T-cell responses against CNS autoantigens amplified by BAFF, released from EBV-infected MBCs. An increased abundance of circulating IgA⁺ MBCs, observed in MS patients, might also reflect GALT-derived immune responses, including disease-enhancing IgA antibody responses against EBV and gut microbiota-specific regulatory IgA⁺ plasma cells. Female sex increases MZ B-cell and CD11c⁺/T-bet⁺ MBC activity while environmental risk factors affect gut dysbiosis. Thus, EBV infection, B-cell dysfunction and other risk factors converge in GALT to generate aberrant B-cell responses that drive pathogenic T-cell responses in the CNS.