Gut Microbiota Regulates K/BxN Autoimmune Arthritis through Follicular Helper T but Not Th17 Cells

Effects of PACAP Deficiency on Immune Dysfunction and Peyer's Patch Integrity in Adult Mice

PACAP (pituitary adenylate cyclase activating polypeptide) is a widespread neuropeptide with cytoprotective and anti-inflammatory effects. It plays a role in innate and adaptive immunity, but data are limited about gut-associated lymphoid tissue. We aimed to reveal differences in Peyer's patches between wild-type (WT) and PACAP-deficient (KO) mice. Peyer's patch morphology from young (3-months-old) and aging (12-15-months-old) mice was examined, along with flow cytometry to assess immune cell populations, expression of checkpoint molecules (PD-1, PD-L1, TIM-3, Gal-9) and functional markers (CD69, granzyme B, perforin) in CD3+, CD4+, and CD8+ T cells. We found slight differences between aging, but not in young, WT, and KO mice. In WT mice, aging reduced CD8+ T cell numbers frequency and altered checkpoint molecule expression (higher TIM-3, granzyme B; lower Gal-9, CD69). CD4+ T cell frequency was higher with similar checkpoint alterations, indicating a regulatory shift. In PACAP KO mice, aging did not change cell population frequencies but led to higher TIM-3, granzyme B and lower PD-1, PD-L1, Gal-9, and CD69 expression in CD4+ and CD8+ T cells, with reduced overall T cell activity. Thus, PACAP deficiency impacts immune dysfunction by altering checkpoint molecules and T cell functionality, particularly in CD8+ T cells, suggesting complex immune responses by PACAP, highlighting its role in intestinal homeostasis and potential implications for inflammatory bowel diseases.

Systemic autoimmune abnormalities alter the morphology of mucosa-associated lymphoid tissues in the rectum of MRL/MpJ-Faslpr/lpr mice

Systemic autoimmune diseases (ADs) might affect the morphology and function of gut-associated lymphoid tissue (LTs) indirectly; however, their exact relationship remains unclear. Therefore, we investigated mouse LTs in the anorectal canal and morphologically compared them between MRL/MpJ-Fas+/+ and MRL/MpJ-Faslpr/lpr mice. LT aggregations, also known as rectal mucosa-associated lymphoid tissues (RMALTs), were exclusively seen in the lamina propria and submucosa of the rectum. The mean size and number of the LT aggregations both significantly increased in MRL/MpJ-Faslpr/lpr mice compared to those in MRL/MpJ-Fas+/+ mice. The distance from the anorectal junction to the first LT aggregate was significantly shorter in MRL/MpJ-Faslpr/lpr mice than that in MRL/MpJ-Fas+/+ mice. Immunostaining revealed that the RMALTs included CD3+, CD4+, and CD8+ T cells; B220+ B cells; IBA1+ macrophages; Ki67+ proliferative cells; and PNAd+ high-endothelial venules (HEVs). The numbers of macrophages, proliferative cells, CD4+ T cells, CD8+ T cells, and HEVs were significantly increased in MRL/MpJ-Faslpr/lpr mice compared to those in MRL/MpJ mice. Furthermore, the gene expression levels of chemokines (Cxcl9 and Cxcl13) and their corresponding receptors (Cxcr3 and Cxcr5) were significantly higher in MRL/MpJ-Faslpr/lpr mice than those in MRL/MpJ-Fas+/+ mice. Although the morphology of rectal epithelium was comparable between the strains, M cell number was significantly higher in MRL/MpJ-Faslpr/lpr mice than in MRL/MpJ-Fas+/+ mice. Thus, ADs could alter RMALT morphology, and quantitative changes in T-cell subsets, proliferative cells, macrophages, HEVs, chemokine expression, and M cells could affect their cell composition and development.

Impaired immune tolerance mediated by reduced Tfr cells in rheumatoid arthritis linked to gut microbiota dysbiosis and altered metabolites

BACKGROUND

Patients with rheumatoid arthritis (RA) showed impaired immune tolerance characterized by reduced follicular regulatory T (Tfr) cells, and they also exhibited altered gut microbiotas and their metabolites in RA. However, the association of gut microbiotas and their metabolites with the immune tolerance mediated by Tfr cells in RA remains unclear.

METHODS

Peripheral blood and stool samples were collected from 32 new-onset RA patients and 17 healthy controls (HCs) in the Second Hospital of Shanxi Medical University between January 2022 and June 2022. The peripheral blood was used to detect the circulating regulatory T (Treg), helper T(Th)17, Tfr, and follicular helper T (Tfh) cells by modified flow cytometry. The stool samples were used to analyze the gut microbiotas and their metabolites via 16S rDNA sequencing and metabolomic profiling. We aimed to characterize the gut microbiotas and their metabolites in RA and identified their association with Tfr cell-mediated immune tolerance.

RESULTS

The new-onset RA demonstrated reduced Treg and Tfr cells, associated with the disease activity and autoantibodies. There were significant differences in gut microbiotas between the two groups as the results of β diversity analysis (P = 0.039) including 21 differential gut microbiotas from the phylum to genus levels. In which, Ruminococcus 2 was associated with the disease activity and autoantibodies of RA, and it was identified as the potential biomarker of RA [area under curve (AUC) = 0.782, 95% confidence interval (CI) = 0.636-0.929, P = 0.001]. Eleven differential metabolites were identified and participated in four main pathways related to RA. Arachidonic acid might be the potential biomarker of RA (AUC = 0.724, 95% CI = 0.595-0.909, P = 0.038), and it was the core metabolite as the positive association with six gut microbiotas enriched in RA. The reduced Tfr cells were associated with the altered gut microbiotas and their metabolites including the Ruminococcus 2, the arachidonic acid involved in the biosynthesis of unsaturated fatty acid pathway and the 3-methyldioxyindole involved in the tryptophan metabolism pathway.

CONCLUSION

The breakdown of immune tolerance mediated by reduced Tfr cells was associated with the altered gut microbiotas and their metabolites implying the possible mechanism of RA pathogenesis from the perspective of microecology-metabolism-immune.

Fecal microbiota transplantation regulates TFH/TFR cell imbalance via TLR/MyD88 pathway in experimental autoimmune hepatitis

Objective

Autoimmune hepatitis (AIH) is a chronic immune-mediated inflammatory liver disease. Intestinal flora disturbance in AIH is closely related to TFH/TFR cell imbalances. As a new method of microbial therapy, the role of fecal microbiota transplantation (FMT) in AIH remains elusive. Here, we attempted to verify the functional role and molecular mechanism of FMT in AIH.

Methods

An experimental autoimmune hepatitis (EAH) mouse model was established to mimic the characteristics of AIH. H&E staining was used to detect histological features in mouse liver tissues. Serological tests were employed to identify several liver function biomarkers. Flow cytometry was utilized to examine the status of TFH/TFR cell subsets. Western blotting was used to evaluate TLR pathway-associated protein abundance. RT‒qPCR was applied to evaluate Treg cell markers and inflammation marker levels in mouse liver tissues.

Results

There was significant liver inflammation and dysregulated TFR/TFH cells with elevated levels of liver inflammation-associated biomarkers in EAH mice. Interestingly, transferring therapeutic FMT into EAH mice dramatically reduced liver injury and improved the imbalance between splenic TFR and TFH cells. FMT treatment also reduced elevated contents of serum alanine transaminase (ALT), aspartate aminotransferase (AST), and total bilirubin (TBIL) in EAH mice. Furthermore, therapeutic FMT reversed the increased levels of IL-21 while promoting IL-10 and TGF-β cytokines. Mechanistically, FMT regulated TFH cell response in EAH mice in a TLR4/11/MyD88 pathway-dependent manner.

Conclusion

Our findings demonstrated that liver injury and dysregulation between TFR and TFH cells in EAH might be reversed by therapeutic FMT via the TLR4/11-MyD88 signaling pathway.

Influence of Helicobacter pylori infection on risk of rheumatoid arthritis: a nationwide population-based study

The relationship between Helicobacter pylori infection and rheumatoid arthritis has been investigated, but the results remain controversial. This study aims to determine the association between the two diseases via a 17-year retrospective cohort study. Using the National Health Insurance Research Database, a nationwide population based in Taiwan, we identified 97,533 individuals with H. pylori infection and matched controls between 2000 and 2017 using propensity score matching at a 1:1 ratio. The adjusted hazard ratio of rheumatoid arthritis was determined by multiple Cox regression. The incidence rate of rheumatoid arthritis was 1.28 per 10,000 person-months in the H. pylori cohort, with a higher risk compared to the control group. In the < 30 years old subgroup, the risk was highest, especially in women < 30 years old with H. pylori infection. Patients with < 1 year follow-up showed 1.58 times higher susceptibility to rheumatoid arthritis. Individuals with follow-ups of 1-5 years and over 5 years demonstrated 1.43 and 1.44 times higher risks of rheumatoid arthritis, respectively. Our study showed H. pylori infection was associated with the development of rheumatoid arthritis. Clinicians should note higher risk, especially < 30 years old. More research needed to understand underlying mechanism.

Modifiable risk factors linked to the development of rheumatoid arthritis: evidence, immunological mechanisms and prevention

Rheumatoid Arthritis (RA) is a common autoimmune disease that targets the synovial joints leading to arthritis. Although the etiology of RA remains largely unknown, it is clear that numerous modifiable risk factors confer increased risk to developing RA. Of these risk factors, cigarette smoking, nutrition, obesity, occupational exposures and periodontal disease all incrementally increase RA risk. However, the precise immunological mechanisms by which these risk factors lead to RA are not well understood. Basic and translational studies have provided key insights into the relationship between inflammation, antibody production and the influence in other key cellular events such as T cell polarization in RA risk. Improving our general understanding of the mechanisms which lead to RA will help identify targets for prevention trials, which are underway in at-risk populations. Herein, we review the modifiable risk factors that are linked to RA development and describe immune mechanisms that may be involved. We highlight the few studies that have sought to understand if modification of these risk factors reduces RA risk. Finally, we speculate that modification of risk factors may be an appealing avenue for prevention for some at-risk individuals, specifically those who prefer lifestyle interventions due to safety and economic reasons.

Assessment of Intestinal Permeability and Inflammation Bio-Markers in Patients with Rheumatoid Arthritis

Increased intestinal permeability and inflammation, both fueled by dysbiosis, appear to contribute to rheumatoid arthritis (RA) pathogenesis. This single-center pilot study aimed to investigate zonulin, a marker of intestinal permeability, and calprotectin, a marker of intestinal inflammation, measured in serum and fecal samples of RA patients using commercially available kits. We also analyzed plasma lipopolysaccharide (LPS) levels, a marker of intestinal permeability and inflammation. Furthermore, univariate, and multivariate regression analyses were carried out to determine whether or not there were associations of zonulin and calprotectin with LPS, BMI, gender, age, RA-specific parameters, fiber intake, and short-chain fatty acids in the gut. Serum zonulin levels were more likely to be abnormal with a longer disease duration and fecal zonulin levels were inversely associated with age. A strong association between fecal and serum calprotectin and between fecal calprotectin and LPS were found in males, but not in females, independent of other biomarkers, suggesting that fecal calprotectin may be a more specific biomarker than serum calprotectin is of intestinal inflammation in RA. Since this was a proof-of-principle study without a healthy control group, further research is needed to validate fecal and serum zonulin as valid biomarkers of RA in comparison with other promising biomarkers.

Innate and adaptive immune abnormalities underlying autoimmune diseases: the genetic connections

With the exception of an extremely small number of cases caused by single gene mutations, most autoimmune diseases result from the complex interplay between environmental and genetic factors. In a nutshell, etiology of the common autoimmune disorders is unknown in spite of progress elucidating certain effector cells and molecules responsible for pathologies associated with inflammatory and tissue damage. In recent years, population genetics approaches have greatly enriched our knowledge regarding genetic susceptibility of autoimmunity, providing us with a window of opportunities to comprehensively re-examine autoimmunity-associated genes and possible pathways. In this review, we aim to discuss etiology and pathogenesis of common autoimmune disorders from the perspective of human genetics. An overview of the genetic basis of autoimmunity is followed by 3 chapters detailing susceptibility genes involved in innate immunity, adaptive immunity and inflammatory cell death processes respectively. With such attempts, we hope to expand the scope of thinking and bring attention to lesser appreciated molecules and pathways as important contributors of autoimmunity beyond the 'usual suspects' of a limited subset of validated therapeutic targets.

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.

Crosstalk between Gut Microbiota and Host Immunity: Impact on Inflammation and Immunotherapy

Gut microbes and their metabolites are actively involved in the development and regulation of host immunity, which can influence disease susceptibility. Herein, we review the most recent research advancements in the gut microbiota-immune axis. We discuss in detail how the gut microbiota is a tipping point for neonatal immune development as indicated by newly uncovered phenomenon, such as maternal imprinting, in utero intestinal metabolome, and weaning reaction. We describe how the gut microbiota shapes both innate and adaptive immunity with emphasis on the metabolites short-chain fatty acids and secondary bile acids. We also comprehensively delineate how disruption in the microbiota-immune axis results in immune-mediated diseases, such as gastrointestinal infections, inflammatory bowel diseases, cardiometabolic disorders (e.g., cardiovascular diseases, diabetes, and hypertension), autoimmunity (e.g., rheumatoid arthritis), hypersensitivity (e.g., asthma and allergies), psychological disorders (e.g., anxiety), and cancer (e.g., colorectal and hepatic). We further encompass the role of fecal microbiota transplantation, probiotics, prebiotics, and dietary polyphenols in reshaping the gut microbiota and their therapeutic potential. Continuing, we examine how the gut microbiota modulates immune therapies, including immune checkpoint inhibitors, JAK inhibitors, and anti-TNF therapies. We lastly mention the current challenges in metagenomics, germ-free models, and microbiota recapitulation to a achieve fundamental understanding for how gut microbiota regulates immunity. Altogether, this review proposes improving immunotherapy efficacy from the perspective of microbiome-targeted interventions.

Serum Levels of IFABP2 and Differences in Lactobacillus and Porphyromonas gingivalis Abundance on Gut Microbiota Are Associated with Poor Therapeutic Response in Rheumatoid Arthritis: A Pilot Study

Intestinal dysbiosis is related to the physiopathology and clinical manifestation of rheumatoid arthritis (RA) and the response to pharmacologic treatment. The objectives of this study were (1) to analyze the effect of conventional synthetic disease modifying anti-rheumatic drugs (csDMARDs) on the abundance of gut microbiota's bacteria; (2) to evaluate the relationship between the differences in microbial abundance with the serum levels of intestinal fatty-acid binding protein 2 (IFABP2), cytokines, and the response phenotype to csDMARDs therapy in RA. A cross-sectional study was conducted on 23 women diagnosed with RA. The abundance of bacteria in gut microbiota was determined with qPCR. The ELISA technique determined serum levels of IFABP2, TNF-α, IL-10, and IL-17A. We found that the accumulated dose of methotrexate or prednisone is negatively associated with the abundance of Lactobacillus but positively associated with the abundance of Bacteroides fragilis. The Lactobacillus/Porphyromonas gingivalis ratio was associated with the Disease Activity Score-28 for RA with Erythrocyte Sedimentation Rate (DAS28-ESR) (r = 0.778, p = 0.030) and with the levels of IL-17A (r = 0.785, p = 0.027) in the group treated with csDMARD. Moreover, a relation between the serum levels of IFABP2 and TNF-α (r = 0.593, p = 0.035) was observed in the group treated with csDMARD. The serum levels of IFABP2 were higher in patients with secondary non-response to csDMARDs therapy. In conclusion, our results suggest that the ratios of gut microbiota's bacteria and intestinal permeability seems to establish the preamble for therapeutic secondary non-response in RA.

Gut dysbiosis contributes to chlamydial induction of hydrosalpinx in the upper genital tract

Chlamydia trachomatis is one of the most common sexually infections that cause infertility, and its genital infection induces tubal adhesion and hydrosalpinx. Intravaginal Chlamydia muridarum infection in mice can induce hydrosalpinx in the upper genital tract and it has been used for studying C. trachomatis pathogenicity. DBA2/J strain mice were known to be resistant to the chlamydial induction of hydrosalpinx. In this study, we took advantage of this feature of DBA2/J mice to evaluate the role of antibiotic induced dysbiosis in chlamydial pathogenicity. Antibiotics (vancomycin and gentamicin) were orally administrated to induce dysbiosis in the gut of DBA2/J mice. The mice with or without antibiotic treatment were evaluated for gut and genital dysbiosis and then intravaginally challenged by C. muridarum. Chlamydial burden was tested and genital pathologies were evaluated. We found that oral antibiotics significantly enhanced chlamydial induction of genital hydrosalpinx. And the antibiotic treatment induced severe dysbiosis in the GI tract, including significantly reduced fecal DNA and increased ratios of firmicutes over bacteroidetes. The oral antibiotic did not alter chlamydial infection or microbiota in the mouse genital tracts. Our study showed that the oral antibiotics-enhanced hydrosalpinx correlated with dysbiosis in gut, providing the evidence for associating gut microbiome with chlamydial genital pathogenicity.

Antibiotic-induced gut dysbiosis and autoimmune disease: A systematic review of preclinical studies

Antibiotic-induced gut dysbiosis is believed to be associated with the onset and development of autoimmune diseases. To evaluate microbiota's variations triggered by antibiotic therapy and its outcomes on autoimmune diseases, preclinical studies regarding these subjects were included in this review. The studies were selected on PubMed, Scopus and Web of Science from 2011 to 2021 by three researchers that extracted study data and risk of bias, which were verified by a further 3 independent researchers. The team assessed the strength of evidence across studies. Of the eligible studies, 17 showed an improvement of the studied disease after antibiotic therapy and 10 had a negative effect on the course of the condition. The ameliorating factors of the studied diseases were mostly seen when using an antibiotic cocktail. Male animals had a good outcome after therapy and, for all genders, the increase in IL-10 and Treg cells was often shown to ameliorate disease after the antibiotic intervention. Firmicutes, Proteobacteria and Bacteroidetes appeared altered after the antibiotic intervention, leading to amelioration or worsening of the condition depending on the autoimmune disease. We identified that the number of autoimmune conditions approached leads to specific conclusions regarding the interventions, making it difficult to achieve an overall conclusion. Overall, even though pre-clinical studies must be translated to the human model, the studied aspects of gender, age, lineage and disease model substantially impact the outcomes that make for many intricacies that were not-established in the study of antibiotic-induced gut dysbiosis and autoimmunity.

Lactobacillus improves the effects of prednisone on autoimmune hepatitis via gut microbiota-mediated follicular helper T cells

Background

Autoimmune hepatitis (AIH) is a chronic, immune-mediated liver dysfunction. The gut microbiota and T follicular helper (Tfh) cells play critical roles in the immunopathogenesis and progression of AIH. We aimed to investigate the effect of gut microbiota combined with prednisone therapy on Tfh cell response in AIH.

Methods

Samples from AIH patients and mouse model of experimental autoimmune hepatitis (EAH) were analyzed using real-time quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting, flow cytometry, and hematoxylin–eosin staining to determine the role of gut microbiota on AIH.

Results

Lactobacillus significantly increased the levels of Bacteroides fragilis, Clostridium, Clostridium leptum, Bifidobacterium, and Lactobacillus and significantly enhanced the suppressive effects of prednisone on the levels of AIH clinical indexes in AIH patients. Lactobacillus exerts the same prptective effects as prednisone in EAH mice and enhanced the effects of prednisone. Lactobacillus also reinforced the inhibitory effects of prednisone on the levels of serum IL-21 and the proportions of Tfh cells in peripheral blood mononuclear cells. Mechanistically, prednisone and Lactobacillus regulated Tfh cell response in EAH mice in an MyD88/NF-κB pathway-dependent manner.

Conclusion

Our results suggested a therapeutic potential of Lactobacillus in the prednisone-combined treatment of AIH.

Gut microbiome and autoimmune disorders

Autoimmune diseases have long been known to share a common pathogenesis involving a dysregulated immune system with failure to recognize self from non-self antigens. This immune dysregulation is now increasingly understood to be induced by environmental triggers in genetically predisposed individuals. Although several external environmental triggers have been defined in different autoimmune diseases, much attention is being paid to the role of the internal micro-environment occupied by the microbiome which was once termed "the forgotten organ". In this regard, the gut microbiome, serving as an intermediary between some of those external environmental effectors and the immune system helps programming of the immune system to be tolerant to innocent external and self antigens. However, in the presence of perturbed gut microbiota (dysbiosis), the immune system could be erroneously directed in favor of pro-inflammatory pathways to instigate different autoimmune processes. An accumulating body of evidence, including both experimental and human studies (observational and interventional) points to a role of gut microbiome in different autoimmune diseases. Such evidence could provide a rationale for gut microbiome manipulation with therapeutic and even preventative intents in patients with established or predisposed to autoimmune diseases respectively. Perturbations of the gut microbiome have been delineated in some immune mediated diseases, IBD in particular. However, such patterns of disturbance (microbiome signatures) and related pathogenetic roles of the gut microbiome are context dependent and cannot be generalized in the same exact way to other autoimmune disorders and the contribution of gut microbiome to different disease phenotypes has to be precisely defined. In this review, we revise the evidence for a role of gut microbiome in various autoimmune diseases and possible mechanisms mediating such a role.

Crosstalk between immune cells and bone cells or chondrocytes

The term "osteoimmunology" was coined to denote the bridge between the immune system and the skeletal system. Osteoimmunology is interdisciplinary, and a full understanding and development of this "bridge" will provide an in-depth understanding of the switch between body health and disease development. B lymphocytes can promote the maturation and differentiation of osteoclasts, and osteoclasts have a negative feedback effect on B lymphocytes. Different subtypes of T lymphocytes regulate osteoclasts in different directions. T lymphocytes have a two-way regulatory effect on osteoblasts, while B lymphocytes have minimal regulatory effects on osteoblasts. In contrast, osteoblasts can promote the differentiation and maturation of T lymphocytes and B lymphocytes. Different immune cells have different effects on chondrocytes; some cooperate with each other, while some antagonize each other. In a healthy adult body, bone resorption and bone formation are in a dynamic balance under the action of multiple mechanisms. In this review, we summarize the interactions and key signaling molecular mechanisms between each type of cell in the immune system and the skeletal system.

Mechanism of Immunoregulatory Properties of Vasoactive Intestinal Peptide in the K/BxN Mice Model of Autoimmune Arthritis

The K/BxN mouse model of rheumatoid arthritis (RA) closely resembles the human disease. In this model, arthritis results from activation of autoreactive KRN T cells recognizing the glycolytic enzyme glucose-6-phosphate isomerase (GPI) autoantigen, which provides help to GPI-specific B cells, resulting in the production of pathogenic anti-GPI antibodies that ultimately leads to arthritis symptoms from 4 weeks of age. Vasoactive intestinal peptide (VIP) is a neuropeptide broadly distributed in the central and peripheral nervous system that is also expressed in lymphocytes and other immune cell types. VIP is a modulator of innate and adaptive immunity, showing anti-inflammatory and immunoregulatory properties. Basically, this neuropeptide promotes a shift in the Th1/Th2 balance and enhances dedifferentiation of T regulatory cells (Treg). It has demonstrated its therapeutic effects on the collagen-induced arthritis (CIA) mouse model of RA. In the present hypothesis and theory article, we propose that the immunoregulatory properties of VIP may be due likely to the inhibition of T cell plasticity toward non-classic Th1 cells and an enhanced follicular regulatory T cells (Tfr) activity. The consequences of these regulatory properties are the reduction of systemic pathogenic antibody titers.

Modification of Gut Microbiota in Inflammatory Arthritis: Highlights and Future Challenges

PURPOSE OF REVIEW

This Review evaluates the available information on the modification of the microbiota by diet, prebiotics, probiotics, or drugs and its association with the severity of arthritis in animals and humans and highlights how this modulation could have therapeutic applications in RA.

RECENT FINDINGS

The gut microbiota and microbiota-derived metabolites play a role in developing rheumatoid arthritis (RA) in animals and humans, making the intestinal microbiota an exciting novel approach to suppress autoimmunity. Studies in animal models of RA show that it is possible to modify the intestinal microbiota with drugs, natural products, diet, probiotics, and prebiotics. Furthermore, these changes showed beneficial effects on symptom relief in animal models of RA and that these effects were associated with modulation of the immune response. Therapies that modify the gut microbiota would significantly impact the preclinical stage of arthritis, based on the fact that dysbiosis occurs before clinical arthritis. The effects of interventions to modulate the microbiota could not reverse arthritis. Furthermore, the therapies modulating therapies in controlling symptoms were limited once arthritis developed. The results obtained in the study of acarbose, probiotics, and prebiotics suggest that these interventions may decrease the disease's incidence rather than treat or cure it.

Association between Tfh and PGA in children with Henoch-Schönlein purpura

Objective

The aim of this study was to investigate the roles of follicular helper CD4⁺ T cells (Tfh) and serum anti-α-1,4-d-polygalacturonic acid (PGA) antibody in the pathogenesis of Henoch-Schönlein purpura (HSP).

Methods

ELISA was performed to determine serum PGA-IgA and PGA-IgG. Flow cytometry was utilized to determine the peripheral CD4⁺ CXCR5⁺ and CD4⁺ CXCR5⁺ ICOS⁺ Tfh cells. Real-time PCR was conducted to determine the expression of Bcl-6 gene. Then the change of Tfh cells was analyzed, together with the association with the anti-PGA antibody as well as the roles in the pathogenesis of HSP.

Results

Compared with the cases with acute respiratory infection and elective surgery, the proportion of CD4⁺ CXCR5⁺ and CD4⁺ CXCR5⁺ ICOS⁺ Tfh cells in the HSP group showed significant elevation (P < 0.001). A significant correlation was noticed between PGA-IgA and CD4⁺ CXCR5⁺ Tfh cells (r = 0.380 and P = 0.042) and CD4⁺ CXCR5⁺ ICOS⁺ Tfh cells (r = 0.906 and P < 0.001). The expression of Bcl-6 in the HSP group showed no statistical difference compared with that in the acute respiratory infection and the surgery control (P < 0.05).

Conclusion

Increased activity of Tfh cells, which is closely related to mucosal immunity, may be a major contributor in the elevation of PGA-IgA, and Tfh cells and PGA-IgA are closely related to the occurrence of HSP.

Intestinal barrier dysfunction plays an integral role in arthritis pathology and can be targeted to ameliorate disease

Background

Evidence suggests an important role for gut-microbiota dysbiosis in the development of rheumatoid arthritis (RA). The link between changes in gut bacteria and the development of joint inflammation is missing. Here, we address whether there are changes to the gut environment and how they contribute to arthritis pathogenesis.

Methods

We analyzed changes in markers of gut permeability, damage, and inflammation in peripheral blood and serum of RA patients. Serum, intestines, and lymphoid organs isolated from K/BxN mice with spontaneous arthritis or from wild-type, genetically modified interleukin (IL)-10R^−/−or claudin-8^−/−mice with induced arthritis were analyzed by immunofluorescence/histology, ELISA, and flow cytometry.

Findings

RA patients display increased levels of serum markers of gut permeability and damage and cellular gut-homing markers, both parameters positively correlating with disease severity. Arthritic mice display increased gut permeability from early stages of disease, as well as bacterial translocation, inflammatory gut damage, increases in interferon γ (IFNγ)⁺and decreases in IL-10⁺intestinal-infiltrating leukocyte frequency, and reduced intestinal epithelial IL-10R expression. Mechanistically, both arthritogenic bacteria and leukocytes are required to disrupt gut-barrier integrity. We show that exposing intestinal organoids to IFNγ reduces IL-10R expression by epithelial cells and that mice lacking epithelial IL-10R display increased intestinal permeability and exacerbated arthritis. Claudin-8^−/−mice with constitutively increased gut permeability also develop worse joint disease. Treatment of mice with AT-1001, a molecule that prevents development of gut permeability, ameliorates arthritis.

Conclusions

We suggest that breakdown of gut-barrier integrity contributes to arthritis development and propose restoration of gut-barrier homeostasis as a new therapeutic approach for RA.

Funding

Funded by Versus Arthritis (21140 and 21257) and UKRI/MRC (MR/T000910/1).

Serum gut-permeability markers LPB, LPS, and I-FABP are increased in RA

Mice with arthritis have increased gut permeability and intestinal inflammation

Both bacteria and leukocytes are needed to disrupt gut-barrier integrity

Prevention of gut-barrier dysfunction in arthritis ameliorates joint inflammation

Rheumatoid arthritis is an autoimmune disorder characterized by chronic joint inflammation. Accumulating evidence suggests that changes in the composition of the bacteria residing in the gut could be responsible for joint inflammation. Currently, it is unclear how bacteria or their products instruct cells of the immune system to become harmful and induce arthritis. Researchers at University College London have shown that, in arthritis, there is profound damage to the gut lining, which fails to work properly as a barrier, as well as an accumulation in the gut of white blood cells that cause inflammation. The authors show that, in arthritis, bacteria cross the prohibited border of the intestinal lining and that repairing gut permeability defects with specific drugs inhibits joint inflammation.

Intragastric Administration of Casein Leads to Nigrostriatal Disease Progressed Accompanied with Persistent Nigrostriatal-Intestinal Inflammation Activited and Intestinal Microbiota-Metabolic Disorders Induced in MPTP Mouse Model of Parkinson's Disease

Gut microbial dysbiosis and alteration of gut microbiota composition in Parkinson's disease (PD) have been increasingly reported, no recognized therapies are available to halt or slow progression of PD and more evidence is still needed to illustrate its causative impact on gut microbiota and PD and mechanisms for targeted mitigation. Epidemiological evidence supported an association between milk intake and a higher incidence of Parkinson's disease (PD), questions have been raised about prospective associations between dietary factors and the incidence of PD. Here, we investigated the significance of casein in the development of PD. The mice were given casein (6.75 g/kg i.g.) for 21 days after MPTP (25 mg/kg i.p. × 5 days) treatment, the motor function, dopaminergic neurons, inflammation, gut microbiota and fecal metabolites were observed. The experimental results revealed that the mice with casein gavage after MPTP treatment showed a persisted dyskinesia, the content of dopamine in striatum and the expression of TH in midbrain and ileum were decreased, the expression of Iba-1, CD4, IL-22 in midbrain and ileum increased continuously with persisted intestinal histopathology and intestinal barrier injury. Decreased intestinal bile secretion in addition with abnormal digestion and metabolism of carbohydrate, lipids and proteins were found, whereas these pathological status for the MPTP mice without casein intake had recovered after 24 days, no significant differences were observed with regard to only treated with casein. Our study demonstrates that intestinal pathologic injury, intestinal dysbacteriosis and metabolism changes promoted by casein in MPTP mice ultimately exacerbated the lesions to dopaminergic neurons.

Toll-Like Receptors, Infections, and Rheumatoid Arthritis

Toll-like receptors (TLR) that belong to the group of protein recognition receptor (PPR) provide an innate immune response following the sensing of conserved pathogen-associated microbial patterns (PAMPs) and changes in danger-associated molecular patterns (DAMPs) that are generated as a consequence of cellular injury. Analysis of the TLR pathway has moreover offered new insights into the pathogenesis of rheumatoid arthritis (RA). Indeed, a dysfunctional TLR-mediated response characterizes RA patients and participates in establishment of a chronic inflammatory state. Such an inappropriate TLR response has been attributed (i) to the report of important alterations in the microbiota and abnormal responses to infectious agents as part of RA; (ii) to the abnormal presence of TLR-ligands in the serum and synovial fluid of RA patients; (iii) to the overexpression of TLR molecules; (iv) to the production of a large panel of pro-inflammatory cytokines downstream of the TLR pathway; and (v) to genetic variants and epigenetic factors in susceptible RA patients promoting a hyper TLR response. As a consequence, the development of promising therapeutic strategies targeting TLRs for the treatment and prevention of RA is emerging.

Gut Microbiome and Metabolome Were Altered and Strongly Associated With Platelet Count in Adult Patients With Primary Immune Thrombocytopenia

Gut microbiota has been implicated in the pathogenesis of many autoimmune diseases. This is still an area of active research given that the role of gut microbiota on the primary immune thrombocytopenia (ITP) remains unclear. In this study, fecal samples of 30 untreated adult primary ITP patients and 29 healthy controls (HCs) were used to investigate the gut microbial community and metabolite profiles. Our results show that fecal bacteria such as Blautia, Streptococcus, and Lactobacillus are enriched, whereas bacteria such as Bacteroides are depleted in ITP patients. Notably, fecal metabolites such as fatty acyls and glycerophospholipids are enriched and strongly correlate with discrepant gut microbiota. Furthermore, combinations of Weissella and Streptococcus anginosus, or Cer (t18:0/16:0), Cer (d18:1/17:0), and 13-hydroxyoctadecanoic acid could provide good diagnostic markers for ITP. Moreover, a strong negative correlation was found between platelet count and altered gut microbiota such as S. anginosus and gut metabolites such as Cer (t18:0/16:0) in ITP. In conclusion, dysbiosis of both gut microbiota and metabolome develops in ITP patients compared to HCs. Several ITP-altered gut bacteria and metabolites can be diagnostic biomarkers for ITP, and are highly correlated with platelet count, suggesting that they may also play a role in ITP pathogenesis.

Microbiota and metabolites in rheumatic diseases

As a gigantic community in the human body, the microbiota exerts pleiotropic roles in human health and disease ranging from digestion and absorption of nutrients from food, defense against infection of pathogens, to regulation of immune system development and immune homeostasis. Recent advances in "omics" studies and bioinformatics analyses have broadened our insights of the microbiota composition of the inner and other surfaces of the body and their interactions with the host. Apart from the direct contact of microbes at the mucosal barrier, metabolites produced or metabolized by the gut microbes can serve as important immune regulators or initiators in a wide variety of diseases, including gastrointestinal diseases, metabolic disorders and systemic rheumatic diseases. This review focuses on the most recent understanding of how the microbiota and metabolites shape rheumatic diseases. Studies that explore the mechanistic interplay between microbes, metabolites and the host could thereby provide clues for novel methods in the diagnosis, therapy, and prevention of rheumatic diseases.

T and B-cell signaling in activated PI3K delta syndrome: From immunodeficiency to autoimmunity

Phosphatidylinositol 3 kinases (PI3K) are a family of lipid kinases that are activated by a variety of cell-surface receptors, and regulate a wide range of downstream readouts affecting cellular metabolism, growth, survival, differentiation, adhesion, and migration. The importance of these lipid kinases in lymphocyte signaling has recently been highlighted by genetic analyses, including the recognition that both activating and inactivating mutations of the catalytic subunit of PI3Kδ, p110δ, lead to human primary immunodeficiencies. In this article, we discuss how studies on the human genetic disorder "Activated PI3K-delta syndrome" and mouse models of this disease (Pik3cd^E1020K/+ mice) have provided fundamental insight into pathways regulated by PI3Kδ in T and B cells and their contribution to lymphocyte function and disease, including responses to commensal bacteria and the development of autoimmunity and tumors. We highlight critical roles of PI3Kδ in T follicular helper cells and the orchestration of the germinal center reaction, as well as in CD8⁺ T-cell function. We further  present data demonstrating the ability of the AKT-resistant FOXO1^AAA mutant to rescue IgG1 class switching defects in Pik3cd^E1020K/+ B cells, as well as data supporting a role for PI3Kδ in promoting multiple T-helper effector cell lineages.

Autoantibodies and B Cells: The ABC of rheumatoid arthritis pathophysiology

Rheumatoid arthritis (RA) is an autoimmune disease characterized by joint inflammation. In the last few decades, new insights into RA‐specific autoantibodies and B cells have greatly expanded our understanding of the disease. The best‐known autoantibodies in RA—rheumatoid factor (RF) and anti‐citrullinated protein antibodies (ACPA)—are present long before disease onset, and both responses show signs of maturation around the time of the first manifestation of arthritis. A very intriguing characteristic of ACPA is their remarkably high abundance of variable domain glycans. Since these glycans may convey an important selection advantage of citrulline‐reactive B cells, they may be the key to understanding the evolution of the autoimmune response. Recently discovered autoantibodies targeting other posttranslational modifications, such as anti‐carbamylated and anti‐acetylated protein antibodies, appear to be closely related to ACPA, which makes it possible to unite them under the term of anti‐modified protein antibodies (AMPA). Despite the many insights gained about these autoantibodies, it is unclear whether they are pathogenic or play a causal role in disease development. Autoreactive B cells from which the autoantibodies originate have also received attention as perhaps more likely disease culprits. The development of autoreactive B cells in RA largely depends on the interaction with T cells in which HLA “shared epitope” and HLA DERAA may play an important role. Recent technological advances made it possible to identify and characterize citrulline‐reactive B cells and acquire ACPA monoclonal antibodies, which are providing valuable insights and help to understand the nature of the autoimmune response underlying RA. In this review, we summarize what is currently known about the role of autoantibodies and autoreactive B cells in RA and we discuss the most prominent hypotheses aiming to explain the origins and the evolution of autoimmunity in RA.

Individuals at risk of seropositive rheumatoid arthritis: the evolving story

The aetiology of the autoimmune disease rheumatoid arthritis (RA) involves a complex interplay between genetic and environmental factors that initiate many years before the onset of clinical symptoms. These interactions likely include both protective and susceptibility factors which together determine the risk of developing RA. More than 100 susceptibility loci have been linked to RA. The strongest association is with HLA-DRB1 alleles encoding antigen presenting molecules containing a unique sequence in the peptide-binding grove called the 'shared epitope'. Female sex, infections during childhood, lifestyle habits (e.g. smoking and diet) and distinct microbial agents, amongst many others, are interacting risk factors thought to contribute to RA pathogenesis by dysregulating the immune system in individuals with genetic susceptibility. Interestingly, patients with RA develop autoantibodies many years before the clinical onset of disease, providing strong evidence that the lack of tolerance to arthritogenic antigens is amongst the earliest events in the initiation of seropositive RA. Here, we will discuss the clinical and mechanistic evidence surrounding the role of different environmental and genetic factors in the phases leading to the production of autoantibodies and the initiation of symptomatic RA. Understanding this complexity is critical in order to develop tools to identify drivers of disease initiation and propagation and to develop preventive therapeutics.

The Roles of Peyer's Patches and Microfold Cells in the Gut Immune System: Relevance to Autoimmune Diseases

Microfold (M) cells are located in the epithelium covering mucosa-associated lymphoid tissues, such as the Peyer's patches (PPs) of the small intestine. M cells actively transport luminal antigens to the underlying lymphoid follicles to initiate an immune response. The molecular machinery of M-cell differentiation and function has been vigorously investigated over the last decade. Studies have shed light on the role of M cells in the mucosal immune system and have revealed that antigen uptake by M cells contributes to not only mucosal but also systemic immune responses. However, M-cell studies usually focus on infectious diseases; the contribution of M cells to autoimmune diseases has remained largely unexplored. Accumulating evidence suggests that dysbiosis of the intestinal microbiota is implicated in multiple systemic diseases, including autoimmune diseases. This implies that the uptake of microorganisms by M cells in PPs may play a role in the pathogenesis of autoimmune diseases. We provide an outline of the current understanding of M-cell biology and subsequently discuss the potential contribution of M cells and PPs to the induction of systemic autoimmunity, beyond the mucosal immune response.

Microbiota-Dependent Involvement of Th17 Cells in Murine Models of Inflammatory Arthritis

Objective

Intestinal microbiota are associated with the development of inflammatory arthritis. The aim of this study was to dissect intestinal mucosal immune responses in the preclinical phase of arthritis and determine whether the presence of Th17 cells, beyond involvement of the cytokine interleukin‐17 (IL‐17), is required for arthritis development, and whether the involvement of Th17 cells in arthritis depends on the composition of the host microbiota.

Methods

Mucosal T cell production of IL‐17, interferon‐γ, tumor necrosis factor α (TNFα), IL‐22, and granulocyte–macrophage colony‐stimulating factor (GM‐CSF) was analyzed by flow cytometry and Luminex assay before arthritis onset in mice immunized to develop collagen‐induced arthritis (CIA). Pathogenic features of arthritis in mice with CIA and mice with antigen‐induced arthritis were compared between Th17 cell–deficient (CD4‐Cre⁺Rorc^flox/flox) and Th17 cell–sufficient (CD4‐Cre⁻Rorc^flox/flox) mice. In addition, the impact of intestinal microbiota on the Th17 cell dependence of CIA was assessed.

Results

Lamina propria CD4 T cells were activated before the onset of arthritis in mice with CIA, with marked up‐regulation of several cytokines, including IL‐17A, TNFα, and GM‐CSF. CD4‐Cre⁺Rorc^flox/flox mice showed a specific reduction in intestinal mucosal levels of Th17 cells and partially reduced levels of IL‐17–producing CD8 T cells. However, total levels of IL‐17A, mostly produced by γδ T cells and neutrophils, were unaffected. The severity of arthritis was significantly reduced in Th17 cell–deficient mice, suggesting that Th17 cells have additional, IL‐17A–independent roles in inflammatory arthritis. Accordingly, antigen‐stimulated T cells from Th17 cell–deficient mice produced less IL‐17A, IL‐17F, and GM‐CSF. Importantly, the dependence of CIA on the involvement of Th17 cells was mitigated in the presence of an alternative microbiome.

Conclusion

These data from murine models suggest that activation of mucosal immunity precedes the development of arthritis, and also that Th17 cells have a microbiota‐dependent role in arthritis. Therefore, a microbiome‐guided stratification of patients might improve the efficacy of Th17‐targeted therapies.

Feline gut microbiota composition in association with feline coronavirus infection: A pilot study

Feline coronaviruses (FCoV) colonize the intestinal tract, however, due to not fully understood mutations, they can spread systemically and cause feline infectious peritonitis (FIP). Recent studies on human medicine report that gut microbiota is involved in the development of systemic disorders and could influence the immune response to viral diseases. The aim of this study was to provide preliminary data on the fecal microbiota composition in healthy cats compared to FCoV-infected cats, with and without FIP. Cats were equally grouped as healthy FCoV-negative, healthy FCoV-positive or FIP affected (total n = 15). Fecal sample were evaluated for the microbiota composition. A total of 3,231,916 sequences were analyzed. The samples' alpha-diversity curves did not reach a proper plateau and, for the beta-diversity, the samples seemed not to group perfectly by category, even if the healthy FCoV-positive group showed a hybrid microbial composition between FCoV-negative and FIP groups. Although there were no taxa significantly linked to the different conditions, some peculiar patterns were recognized: Firmicutes was always the most represented phylum, followed by Bacteroidetes and Actinobacteria. In FCoV-positive cats, the Firmicutes and Bacteroidetes were respectively over- and under-represented, compared to the other groups. Among FIP cats, three subjects shared a similar microbiome, one cat showed a different microbial profile and the other one had the lowest number of diverse phyla. Despite the limited number of animals, some differences in the fecal microbiome between the groups were observed, suggesting to further investigate the possible correlation between gut microbiota and FCoV infection in cats.

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Mutated form of enteric feline coronaviruses (FCoV) may spread systemically inducing feline infectious peritonitis (FIP).

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Gut microbiota is known to be influenced by viral diseases in people and in some animal species.

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Fecal microbiota was evaluated in 15 cats divided in three groups as healthy, positive to FCoV and affected by FIP.

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Some microbiota peculiarities have been observed associated with the presence of coronavirus and/or disease.

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Feline gut microbiota composition may be associated with FCoV infection and thus need to be further investigated.

A cross talk between dysbiosis and gut-associated immune system governs the development of inflammatory arthropathies

BACKGROUND

Emerging evidence suggests that dysbiosis, imbalanced gut microbial community, might be a key player in the development of various diseases, including inflammatory arthropathies, such as rheumatoid arthritis, spondyloarthritis (mainly, ankylosing spondylitis and psoriatic arthritis), and osteoarthritis. Yet, the underlying mechanisms and corresponding interactions remain poorly understood.

METHODS

We conducted a critical and extensive literature review to explore the association between dysbiosis and the development of inflammatory arthropathies. We also reviewed the literature to assess the perspectives that ameliorate inflammatory arthropathies by manipulating the microbiota with probiotics, prebiotics or fecal microbiota transplantation.

RESULTS

Some bacterial species (e.g. Prevotella, Citrobacter rodentium, Collinsella aerofaciens, Segmented filamentous bacteria) participate in the creation of the pro-inflammatory immune status, presumably via epitope mimicry, modification of self-antigens, enhanced cell apoptosis mechanisms, and destruction of tight junction proteins and intestinal barrier integrity, all leading to the development and maintainance of inflammatory arthropathies. Whether dysbiosis is an epiphenomenon or is an active driver of these disorders remains unclear, yet, recent observations clearly suggest that dysbiosis precedes and triggers their development implying a causative relationship between dysbiosis and inflammatory arthropathies. The underlying mechanisms include dysbiosis-mediated changes in the functional activity of the intestinal immune cell subsets, such as innate lymphoid cells, mucosa-associated invariant T cells, invariant natural killer T cells, T-follicular helper and T-regulatory cells. In turn, disturbed functionality of the gut-associated immune system is shown to promote the overgrowth of many bacteria, thus establishing a detrimental vicious circle of actively maintaining arthritis.

CONCLUSIONS

Analysis of the data described in the review supports the notion that a close, dynamic and tightly regulated cross talk between dysbiosis and the gut-associated immune system governs the development of inflammatory arthropathies.

Multiple hit infection and autoimmunity: the dysbiotic microbiota-ACPA connection in rheumatoid arthritis

PURPOSE OF REVIEW

This review highlights the most recent data obtained in this field and provides clues toward the better understanding of the close interplay between microbiota and host, leading to autoimmune diseases.

RECENT FINDINGS

A well-described model of microbiota/host interaction of relevance to autoimmunity is linking anti-citrullinated peptide antibody positive rheumatoid arthritis and alterations of microbiota largely concentrating on Porphyromonas gingivalis and more recently of Aggregatibacter actinomycetemcomitans and Prevotella copri.

SUMMARY

The perception of the classical link between microbial infection and development of autoimmune disease has evolved to the more recent concept of the connection between the microbiome/dysbiosis and breaking of immunological tolerance.

Microbe-metabolite-host axis, two-way action in the pathogenesis and treatment of human autoimmunity

The role of microorganism in human diseases cannot be ignored. These microorganisms have evolved together with humans and worked together with body's mechanism to maintain immune and metabolic function. Emerging evidence shows that gut microbe and their metabolites open up new doors for the study of human response mechanism. The complexity and interdependence of these microbe-metabolite-host interactions are rapidly being elucidated. There are various changes of microbial levels in models or in patients of various autoimmune diseases (AIDs). In addition, the relevant metabolites involved in mechanism mainly include short-chain fatty acids (SCFAs), bile acids (BAs), and polysaccharide A (PSA). Meanwhile, the interaction between microbes and host genes is also a factor that must be considered. It has been demonstrated that human microbes are involved in the development of a variety of AIDs, including organ-specific AIDs and systemic AIDs. At the same time, microbes or related products can be used to remodel body's response to alleviate or cure diseases. This review summarizes the latest research of microbes and their related metabolites in AIDs. More importantly, it highlights novel and potential therapeutics, including fecal microbial transplantation, probiotics, prebiotics, and synbiotics. Nonetheless, exact mechanisms still remain elusive, and future research will focus on finding a specific strain that can act as a biomarker of an autoimmune disease.

The microbiome in systemic autoimmune disease: mechanistic insights from recent studies

PURPOSE OF REVIEW

The resident bacterial communities and the host immune system have coevolved for millennia. However, recent changes in modern societies have disrupted this coevolutionary homeostasis and contributed to a rise in immune-mediated conditions. The purpose of this review is to provide an overview of recently elucidated mechanisms of how certain taxa within the bacterial microbiome propagate autoimmunity.

RECENT FINDINGS

Interactions between the bacterial microbiome with innate and adaptive immune cells propagate autoreactivity, chronic inflammation, and tissue damage in susceptible hosts. These interactions contribute to autoimmune diseases such as rheumatoid arthritis or systemic lupus erythematosus, which are the focus of this review. Recent findings suggest that autoimmune manifestations in genetically susceptible individuals can arise through cross-reactivity with commensal orthologs of autoantigens or commensal-mediated posttranslational modification of autoantigens. Physiologic responses to gut, oral, or skin commensal bacteria can thus be misdirected toward such autoantigens in susceptible hosts. In addition, recent studies highlight that a breach of the gut barrier and translocation of commensal bacteria to non-gut organs can trigger several autoimmune pathways that can be prevented by commensal vaccination or dietary interventions.

SUMMARY

Complex host-microbiota interactions contribute to systemic autoimmunity outside the gut. On a molecular level, posttranslational modification of, and cross-reactivity with, autoantigens represent mechanisms of how the microbiota mediates autoimmunity. On a cellular level, translocation of live gut bacteria across a dysfunctional gut barrier allows for direct interactions with immune and tissue cells, instigating autoimmunity systemically.

Data-driven multiple-level analysis of gut-microbiome-immune-joint interactions in rheumatoid arthritis

Background

Rheumatoid arthritis (RA) is the most common autoimmune disease and affects about 1% of the population. The cause of RA remains largely unknown and could result from a complex interaction between genes and environment factors. Recent studies suggested that gut microbiota and their collective metabolic outputs exert profound effects on the host immune system and are implicated in RA. However, which and how gut microbial metabolites interact with host genetics in contributing to RA pathogenesis remains unknown. In this study, we present a data-driven study to understand how gut microbial metabolites contribute to RA at the genetic, functional and phenotypic levels.

Results

We used publicly available disease genetics, chemical genetics, human metabolome, genetic signaling pathways, mouse genome-wide mutation phenotypes, and mouse phenotype ontology data. We identified RA-associated microbial metabolites and prioritized them based on their genetic, functional and phenotypic relevance to RA. We evaluated the prioritization methods using short-chain fatty acids (SCFAs), which were previously shown to be involved in RA etiology. We validate the algorithms by showing that SCFAs are highly associated with RA at genetic, functional and phenotypic levels: SCFAs ranked at top 3.52% based on shared genes with RA, top 5.69% based on shared genetic pathways, and top 16.94% based on shared phenotypes. Based on the genetic-level analysis, human gut microbial metabolites directly interact with many RA-associated genes (as many as 18.1% of all 166 RA genes). Based on the functional-level analysis, human gut microbial metabolites participate in many RA-associated genetic pathways (as many as 71.4% of 311 genetic pathways significantly enriched for RA), including immune system pathways. Based on the phenotypic-level analysis, gut microbial metabolites affect many RA-related phenotypes (as many as 51.3% of 978 phenotypes significantly enriched for RA), including many immune system phenotypes.

Conclusions

Our study demonstrates strong gut-microbiome-immune-joint interactions in RA, which converged on both genetic, functional and phenotypic levels.

Inhibition of Glycolysis Reduces Disease Severity in an Autoimmune Model of Rheumatoid Arthritis

The K/BxN mouse is a spontaneous model of arthritis driven by T cell receptor transgenic CD4⁺ T cells from the KRN strain that are activated by glucose-6-phosphate isomerase (GPI) peptides presented by the H-2^g7 allele from the NOD strain. It is a model of autoimmune seropositive arthritis because the production of anti-GPI IgG is necessary and sufficient for joint pathology. The production of high levels of anti-GPI IgG requires on the expansion of CD4⁺ follicular helper T (Tfh) cells. The metabolic requirements of this expansion have never been characterized. Based on the therapeutic effects of the combination of metformin and 2-deoxyglucose (2DG) in lupus models that normalized the expansion of effector CD4⁺ T cells. We showed that the CD4⁺ T cells and to a lesser extent, the B cells from K/BxN mice are more metabolically active than the KRN controls. Accordingly, preventive inhibition of glycolysis with 2DG significantly reduced joint inflammation and the activation of both adaptive and innate immune cells, as well as the production of pathogenic autoantibodies. However, contrary to the lupus-prone mice, the addition of metformin had little beneficial effect, suggesting that glycolysis is the major driver of immune activation in this model. We propose that K/BxN mice are another model in which autoreactive Tfh cells are highly glycolytic and that their function can be limited by inhibiting glucose metabolism.

Rheumatoid arthritis and the mucosal origins hypothesis: protection turns to destruction

Individuals at high risk of developing seropositive rheumatoid arthritis (RA) can be identified for translational research and disease prevention studies through the presence of highly informative and predictive patterns of RA-related autoantibodies, especially anti-citrullinated protein antibodies (ACPAs), in the serum. In serologically positive individuals without arthritis, designated ACPA positive at risk, the presence of mucosal inflammatory processes associated with the presence of local ACPA production has been demonstrated. In other at-risk populations, local RA-related autoantibody production is present even in the absence of serum autoantibodies. Additionally, a proportion of at-risk individuals exhibit local mucosal ACPA production in the lung, as well as radiographic small-airway disease, sputum hypercellularity and increased neutrophil extracellular trap formation. Other mucosal sites in at-risk individuals also exhibit autoantibody production, inflammation and/or evidence of dysbiosis. As the proportion of individuals who exhibit such localized inflammation-associated ACPA production is substantially higher than the likelihood of an individual developing future RA, this finding raises the hypothesis that mucosal ACPAs have biologically relevant protective roles. Identifying the mechanisms that drive both the generation and loss of externally focused mucosal ACPA production and promote systemic autoantibody expression and ultimately arthritis development should provide insights into new therapeutic approaches to prevent RA.

Modulation of Inflammatory Arthritis in Mice by Gut Microbiota Through Mucosal Inflammation and Autoantibody Generation

OBJECTIVE

Observations of microbial dysbiosis in patients with rheumatoid arthritis (RA) have raised interest in studying microbial-mucosal interactions as a potential trigger of RA. Using the murine collagen-induced arthritis (CIA) model, we undertook this study to test our hypothesis that microbiota modulate immune responses leading to autoimmune arthritis.

METHODS

CIA was induced by immunization of mice with type II collagen (CII) in adjuvant on days 0 and 21, with arthritis appearing on days 23 and 24. Intestinal microbiota were profiled by 16S ribosomal RNA sequencing every 7 days during the course of CIA, and intestinal mucosal changes were evaluated on days 14 and 35. Then, microbiota were depleted either early (7 days before immunization) or late (day 21 after immunization) by administration of broad-spectrum antibiotics. Disease severity, autoantibody and systemic cytokine production, and intestinal mucosal responses were monitored in the setting of microbial reduction.

RESULTS

Significant dysbiosis and mucosal inflammation occurred early in CIA, prior to visible arthritis, and continued to evolve during the course of disease. Depletion of the microbiota prior to the induction of CIA resulted in an ~40% reduction in disease severity and in significantly reduced levels of serum inflammatory cytokines and anti-CII antibodies. In intestinal tissue, production of interleukin-17A (IL-17A) and IL-22 was delayed. Unexpectedly, microbial depletion during the late phase of CIA resulted in a >50% decrease in disease severity. Anti-CII antibodies were mildly reduced but were significantly impaired in their ability to activate complement, likely due to altered glycosylation profiles.

CONCLUSION

These data support a model in which intestinal dysbiosis triggers mucosal immune responses that stimulate T and B cells that are key for the development of inflammatory arthritis.

The Anti-Inflammatory Mediator, Vasoactive Intestinal Peptide, Modulates the Differentiation and Function of Th Subsets in Rheumatoid Arthritis

Genetic background, epigenetic modifications, and environmental factors trigger autoimmune response in rheumatoid arthritis (RA). Several pathogenic infections have been related to the onset of RA and may cause an inadequate immunological tolerance towards critical self-antigens leading to chronic joint inflammation and an imbalance between different T helper (Th) subsets. Vasoactive intestinal peptide (VIP) is a mediator that modulates all the stages comprised between the arrival of pathogens and Th cell differentiation in RA through its known anti-inflammatory and immunomodulatory actions. This “neuroimmunopeptide” modulates the pathogenic activity of diverse cell subpopulations involved in RA as lymphocytes, fibroblast-like synoviocytes (FLS), or macrophages. In addition, VIP decreases the expression of pattern recognition receptor (PRR) such as toll-like receptors (TLRs) in FLS from RA patients. These receptors act as sensors of pathogen-associated molecular pattern (PAMP) and damage-associated molecular pattern (DAMP) connecting the innate and adaptive immune system. Moreover, VIP modulates the imbalance between Th subsets in RA, decreasing pathogenic Th1 and Th17 subsets and favoring Th2 or Treg profile during the differentiation/polarization of naïve or memory Th cells. Finally, VIP regulates the plasticity between theses subsets. In this review, we provide an overview of VIP effects on the aforementioned features of RA pathology.

T Follicular Helper Cells in Autoimmune Disorders

T follicular helper (Tfh) cells are a distinct subset of CD4⁺ T lymphocytes, specialized in B cell help and in regulation of antibody responses. They are required for the generation of germinal center reactions, where selection of high affinity antibody producing B cells and development of memory B cells occur. Owing to the fundamental role of Tfh cells in adaptive immunity, the stringent control of their production and function is critically important, both for the induction of an optimal humoral response against thymus-dependent antigens but also for the prevention of self-reactivity. Indeed, deregulation of Tfh activities can contribute to a pathogenic autoantibody production and can play an important role in the promotion of autoimmune diseases. In the present review, we briefly introduce the molecular factors involved in Tfh cell formation in the context of a normal immune response, as well as markers associated with their identification (transcription factor, surface marker expression, and cytokine production). We then consider in detail the role of Tfh cells in the pathogenesis of a broad range of autoimmune diseases, with a special focus on systemic lupus erythematosus and rheumatoid arthritis, as well as on the other autoimmune/inflammatory disorders. We summarize the observed alterations in Tfh numbers, activation state, and circulating subset distribution during autoimmune and some other inflammatory disorders. In addition, central role of interleukin-21, major cytokine produced by Tfh cells, is discussed, as well as the involvement of follicular regulatory T cells, which share characteristics with both Tfh and regulatory T cells.

Signal Transducer and Activator of Transcription 3 Hyperactivation Associates With Follicular Helper T Cell Differentiation and Disease Activity in Rheumatoid Arthritis

Follicular helper T (Tfh) cells are the specialized CD4⁺ T cell subset that supports B cells to produce high-affinity antibodies and generate humoral memory. Not only is the function of Tfh cells instrumental to mount protect antibodies but also to support autoantibody production and promote systemic inflammation in autoimmune diseases. However, it remains unclear how the activation of Tfh cells is driven in autoimmune diseases. Here, we report that in patients with rheumatoid arthritis (RA), excessive generation of CXCR5⁺PD-1⁺ memory Tfh cells was observed and the frequency of memory Tfh cells correlated with disease activity score calculator for RA (DAS28). The differentiation of Tfh cells is dependent on signal transducer and activator of transcription 3 (STAT3), the key transcription factor downstream of cytokine signal pathways. A drastic increase of phosphorylated STAT3 (pSTAT3) in CD4⁺ T cells were detected in RA patients who also produced larger amounts of STAT3-stimulating cytokines, including IL-6, IL-21, IL-10, and leptin than those of healthy controls. Importantly, the phosphorylation status of STAT3 in CD4⁺ T cells positively correlated with the plasma concentration of IL-6 and the frequency of memory Tfh cells. This study reveals an IL-6-pSTAT3-Tfh immunoregulatory axis in the pathogenesis of RA and reinforces its candidature as biomarkers and targets for diagnosis and therapy.

Review: Transcriptional Regulation of CD4+ T Cell Differentiation in Experimentally Induced Arthritis and Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by chronic inflammation of the joint synovium and infiltration by activated inflammatory cells. CD4+ T cells form a large proportion of the inflammatory cells invading the synovial tissue, and are involved in the RA pathologic process. In general, CD4+ T cells differentiate into various T helper cell subsets and acquire the functional properties to respond to specific pathogens, and also mediate some autoimmune disorders such as RA. Because the differentiation of T helper cell subsets is determined by the expression of specific transcription factors in response to the cytokine environment, these transcription factors are considered to have a role in the pathology of RA. Treg cells control an excess of T cell–mediated immune response, and the transcription factor FoxP3 is critical for the differentiation and function of Treg cells. Treg cell dysfunction can result in the development of systemic autoimmunity. In this review, we summarize how the expression of transcription factors modulates T helper cell immune responses and the development of autoimmune diseases, especially in RA. Understanding the role of transcription factors in the pathogenesis of autoimmunity may lead to novel therapeutic strategies to control the differentiation and function of both T helper cells and Treg cells.

Chlamydia Spreading from the Genital Tract to the Gastrointestinal Tract - A Two-Hit Hypothesis

Chlamydia trachomatis, a leading bacterial cause of sexually transmitted infection-induced infertility, is frequently detected in the gastrointestinal tract. Chlamydia muridarum, a model pathogen for investigating C. trachomatis pathogenesis, readily spreads from the mouse genital tract to the gastrointestinal tract, establishing long-lasting colonization. C. muridarum mutants, despite their ability to activate acute oviduct inflammation, are attenuated in inducing tubal fibrosis and are no longer able to colonize the gastrointestinal tract, suggesting that the spread of C. muridarum to the gastrointestinal tract may contribute to its pathogenicity in the upper genital tract. However, gastrointestinal C. muridarum cannot directly autoinoculate the genital tract. Both antigen-specific CD8⁺ T cells and profibrotic cytokines, such as TNFα and IL-13, are essential for C. muridarum to induce tubal fibrosis; this may be induced by the gastrointestinal C. muridarum, as a second hit, to transmucosally convert tubal repairing - initiated by C. muridarum infection of tubal epithelial cells (serving as the first hit) - into pathogenic fibrosis. Testing the two-hit mouse model should both add new knowledge to the growing list of mechanisms by which gastrointestinal microbes contribute to pathologies in extragastrointestinal tissues and provide information for investigating the potential role of gastrointestinal C. trachomatis in human chlamydial pathogenesis.

Alteration of the intestinal microbiome characterizes preclinical inflammatory arthritis in mice and its modulation attenuates established arthritis

Perturbations of the intestinal microbiome have been observed in patients with new-onset and chronic autoimmune inflammatory arthritis. However, it is currently unknown whether these alterations precede the development of arthritis or are rather a consequence of disease. Modulation of intestinal microbiota by oral antibiotics or germ-free condition can prevent arthritis in mice. Yet, the therapeutic potential of modulation of the microbiota after the onset of arthritis is not well characterized. We here show that the intestinal microbial community undergoes marked changes in the preclinical phase of collagen induced arthritis (CIA). The abundance of the phylum Bacteroidetes, specifically families S24-7 and Bacteroidaceae was reduced, whereas Firmicutes and Proteobacteria, such as Ruminococcaceae, Lachnospiraceae and Desulfovibrinocaceae, were expanded during the immune-priming phase of arthritis. In addition, we found that the abundance of lamina propria Th17, but not Th1, cells is highly correlated with the severity of arthritis. Elimination of the intestinal microbiota during established arthritis specifically reduced intestinal Th17 cells and attenuated arthritis. These effects were associated with reduced serum amyloid A expression in ileum and synovial tissue. Our observations suggest that intestinal microbiota perturbations precede arthritis, and that modulation of the intestinal microbiota after the onset of arthritis may offer therapeutic opportunities.