The immunobiology of mucosal-associated invariant T cell (MAIT) function in primary biliary cholangitis: Regulation by cholic acid-induced Interleukin-7. J Autoimmun. 2018;90:64-75. [PMID: 29429758 DOI: 10.1016/j.jaut.2018.01.007]

Probiotics-regulated lithocholic acid suppressed B-cell differentiation in neuromyelitis optica spectrum disorder

Intestinal microbes play a crucial role in gut health and the immune-mediated central nervous system through the "gut-brain" axis. However, probiotic safety and efficacy in Neuromyelitis optica spectrum disorder (NMOSD) are not well-explored. A pilot clinic trial for NMOSD with probiotic intervention revealed alterations in the microbiota (increased Anaerostipes, Bacteroides; decreased Granulicatella, Streptococcus, Rothia). Metabolite analysis showed elevated 2-methylbutyric and isobutyric acids, reduced lithocholic acid (LCA), and glycodeoxycholic acid (GDCA). Immune markers Interleukin (IL-7), vascular endothelial growth factor (VEGF-A), and B lymphocyte chemoattractant (BLC) decreased, while plasma cells and transitional B cells increased post-probiotics, suggesting potential immunomodulatory effects on NMOSD.

Innate-like T cells in liver disease

Mammalian innate-like T cells (ILTCs), including mucosal-associated invariant T (MAIT), natural killer T (NKT), and γδ T cells, are abundant tissue-resident lymphocytes that have recently emerged as orchestrators of hepatic inflammation, tissue repair, and immune homeostasis. This review explores the involvement of different ILTC subsets in liver diseases. We explore the mechanisms underlying the pro- and anti-inflammatory effector functions of ILTCs in a context-dependent manner. We highlight latest findings regarding the dynamic interplay between ILTC functional subsets and other immune and parenchymal cells which may inform candidate immunomodulatory strategies to achieve improved clinical outcomes in liver diseases. We present new insights into how distinct gene expression programs in hepatic ILTCs are induced, maintained, and reprogrammed in a context- and disease stage-dependent manner.

Regulation of MAIT cells through host-derived antigens

Mucosal-associated invariant T (MAIT) cells are a major subset of innate-like T cells that function at the interface between innate and acquired immunity. MAIT cells recognize vitamin B2-related metabolites produced by microbes, through semi-invariant T cell receptor (TCR) and contribute to protective immunity. These foreign-derived antigens are presented by a monomorphic antigen presenting molecule, MHC class I-related molecule 1 (MR1). MR1 contains a malleable ligand-binding pocket, allowing for the recognition of compounds with various structures. However, interactions between MR1 and self-derived antigens are not fully understood. Recently, bile acid metabolites were identified as host-derived ligands for MAIT cells. In this review, we will highlight recent findings regarding the recognition of self-antigens by MAIT cells.

Mucosal-associated invariant T cells promote ductular reaction through amphiregulin in biliary atresia

BACKGROUND

Biliary atresia (BA) is a neonatal fibro-inflammatory cholangiopathy with ductular reaction as a key pathogenic feature predicting poor survival. Mucosal-associated invariant T (MAIT) cells are enriched in human liver and display multiple roles in liver diseases. We aimed to investigate the function of MAIT cells in BA.

METHODS

First, we analyzed correlations between liver MAIT cell and clinical parameters (survival, alanine transaminase, bilirubin, histological inflammation and fibrosis) in two public cohorts of patients with BA (US and China). Kaplan-Meier survival analysis and spearman correlation analysis were employed for survival data and other clinical parameters, respectively. Next, we obtained liver samples or peripheral blood from BA and control patients for bulk RNA sequencing, flow cytometry analysis, immunostaning and functional experiments of MAIT cells. Finally, we established two in vitro co-culture systems, one is the rhesus rotavirus (RRV) infected co-culture system to model immune dysfunction of human BA which was validated by single cell RNA sequencing and the other is a multicellular system composed of biliary organoids, LX-2 and MAIT cells to evaluate the role of MAIT cells on ductular reaction.

FINDINGS

Liver MAIT cells in BA were positively associated with low survival and ductular reaction. Moreover, liver MAIT cells were activated, exhibited a wound healing signature and highly expressed growth factor Amphiregulin (AREG) in a T cell receptor (TCR)-dependent manner. Antagonism of AREG abrogated the proliferative effect of BA MAIT cells on both cholangiocytes and biliary organoids. A RRV infected co-culture system, recapitulated immune dysfunction of human BA, disclosed that RRV-primed MAIT cells promoted cholangiocyte proliferation via AREG, and further induced inflammation and fibrosis in the multicellular system.

INTERPRETATION

MAIT cells exhibit a wound healing signature depending on TCR signaling and promote ductular reaction via AREG, which is associated with advanced fibrosis and predictive of low survival in BA.

FUNDING

This work was funded by National Natural Science Foundation of China grant (82001589 and 92168108), National Key R&D Program of China (2023YFA1801600) and by Basic and Applied Basic Research Foundation of Guangdong (2020A1515110921).

Multi-omics analysis reveals the associations between altered gut microbiota, metabolites, and cytokines during pregnancy

For embryo implantation and fetal development, the maternal immune system undergoes dramatic changes. The mechanisms involved in inducing alterations of maternal immunity have not been fully clarified. Gut microbiome and metabolites were thought to influence the host immune response. During normal pregnancy, notable changes occur in the gut microbiota and metabolites. However, the relationship of these alterations to immune function during pregnancy remains unclear. In this study, we examined gut microbiota, fecal metabolites, plasma metabolites, and cytokines in pregnant women and non-pregnant women. Our findings revealed that, in comparison to non-pregnant women, pregnant women exhibit a significant increase in the relative abundance of Actinobacteriota and notable differences in metabolic pathways related to bile acid secretion. Furthermore, there was a marked reduction in pro-inflammatory cytokines levels in pregnant women. Correlation analyses indicated that these alterations in cytokines may be linked to specific gut bacteria and metabolites. Bacteria within the same microbial modules exhibited consistent effects on cytokines, suggesting that gut bacteria may function as functional groups. Mediation analysis further identified that certain bacteria might influence cytokines through metabolites, such as bile acids and arachidonic acid. Our findings propose potential biological connections between bacteria, metabolites, and immunity, which require further validation in future studies.IMPORTANCEA great number of studies have focused on diseases induced by intestinal microecological disorders and immune imbalances. However, the understanding of how intestinal microbiota interacts with immunity during normal pregnancy, which is fundamental to studying pathological pregnancies related to intestinal microbiota disturbances, has not been well elucidated. Our study employed multi-omics analysis to discover that changes in gut microbiota and metabolites during pregnancy can impact immune function. In addition, we identified several metabolites that may mediate the effect of gut microbes on plasma cytokines. Our study offered new insights into our understanding of the connections between the gut microbiome, metabolome, and the immune system during pregnancy.

Circulating cytokines and alcoholic liver disease: a two-sample bidirectional Mendelian randomization study

BACKGROUND

Increased inflammation in the liver during ethanol exposure is a major feature of alcoholic liver disease (ALD). An important contributing component to the development of ALD is the inflammatory response brought on by immunological response, however the connection between individual circulating cytokines and ALD is still unclear. To ascertain the causation, we conducted a two-sample bidirectional Mendelian randomization research.

METHODS

We extracted 41 cytokines and growth factors of 8293 Europeans and ALD cases of the same ethnicity (1416 cases and 217,376 controls) from the Genome-Wide Association Studies (GWAS) database for two-sample bidirectional MR analysis.

RESULTS

Our analyses suggest that higher interleukin-7 (IL-7) levels are associated with an increased risk of ALD (p = 0.028, OR = 1.191,95% CI = 1.019-1.392), while tumor necrosis factor related apoptosis inducing ligand (TRAIL) is a protective factor for ALD (p = 0.032, OR = 0.863, 95% CI = 0.754-0.988) which can reduce the risk of disease occurrence. In addition, genetically predicted ALD does not affect the expression of circulating cytokines regulators.

CONCLUSIONS

Our study supports that cytokines play a pivotal role in the pathogenesis of ALD. To determine the mechanisms and pathways of action of these biomarkers, further basic research is required to ensure their clinical suitability for preventing and treating ALD.

Sulfated bile acid is a host-derived ligand for MAIT cells

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that recognize bacterial riboflavin-based metabolites as activating antigens. Although MAIT cells are found in tissues, it is unknown whether any host tissue-derived antigens exist. Here, we report that a sulfated bile acid, cholic acid 7-sulfate (CA7S), binds the nonclassical MHC class I protein MR1 and is recognized by MAIT cells. CA7S is a host-derived metabolite whose levels were reduced by more than 98% in germ-free mice. Deletion of the sulfotransferase 2a family of enzymes (Sult2a1-8) responsible for CA7S synthesis reduced the number of thymic MAIT cells in mice. Moreover, recognition of CA7S induced MAIT cell survival and the expression of a homeostatic gene signature. By contrast, recognition of a previously described foreign antigen, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), drove MAIT cell proliferation and the expression of inflammatory genes. Thus, CA7S is an endogenous antigen for MAIT cells, which promotes their development and function.

MAIT cells in immune-mediated tissue injury and repair

Mucosal-associated invariant T (MAIT) cells are T cells that express a semi-invariant αβ T-cell receptor (TCR), recognizing non-peptide antigens, such as microbial-derived vitamin B2 metabolites, presented by the nonpolymorphic MHC class I related-1 molecule. Like NKT cells and γδT cells, MAIT cells belong to the group of innate-like T cells that combine properties of the innate and adaptive immune systems. They account for up to 10% of the blood T-cell population in humans and are particularly abundant at mucosal sites. Beyond the emerging role of MAIT cells in antibacterial and antiviral defenses, increasing evidence suggests additional functions in noninfectious settings, including immune-mediated inflammatory diseases and tissue repair. Here, we discuss recent advances in the understanding of MAIT cell functions in sterile tissue inflammation, with a particular focus on autoimmunity, chronic inflammatory diseases, and tissue repair.

Tissue-resident and innate-like T cells in patients with advanced chronic liver disease

Chronic liver disease results from the orchestrated interplay of components of innate and adaptive immunity in response to liver tissue damage. Recruitment, positioning, and activation of immune cells can contribute to hepatic cell death, inflammation, and fibrogenesis. With disease progression and increasing portal pressure, repeated translocation of bacterial components from the intestinal lumen through the epithelial and vascular barriers leads to persistent mucosal, hepatic, and systemic inflammation which contributes to tissue damage, immune dysfunction, and microbial infection. It is increasingly recognised that innate-like and adaptive T-cell subsets located in the liver, mucosal surfaces, and body cavities play a critical role in the progression of advanced liver disease and inflammatory complications of cirrhosis. Mucosal-associated invariant T cells, natural killer T cells, γδ T cells, and tissue-resident memory T cells in the gut, liver, and ascitic fluid share certain characteristic features, which include that they recognise microbial products, tissue alarmins, cytokines, and stress ligands in tissues, and perform effector functions in chronic liver disease. This review highlights recent advances in the comprehension of human tissue-resident and unconventional T-cell populations and discusses the mechanisms by which they contribute to inflammation, fibrosis, immunosuppression, and antimicrobial surveillance in patients with cirrhosis. Understanding the complex interactions of immune cells in different compartments and their contribution to disease progression will provide further insights for effective diagnostic interventions and novel immunomodulatory strategies in patients with advanced chronic liver disease.

Pyroptotic MAITs link microbial translocation with severity of alcohol-associated liver disease

BACKGROUND

Mucosal-associated invariant T cells (MAITs) are markedly reduced in patients with alcohol-associated liver disease (ALD); however, the potential mechanism underlying MAITs' loss remains elusive. Hence, we aimed to explore what induced MAITs' loss and its clinical significance.

METHODS

The characteristics of pyroptotic MAITs were evaluated in a cohort of patients with ALD, including 41 patients with alcohol-associated liver cirrhosis (ALC) and 21 patients with ALC complicated with severe alcoholic hepatitis (ALC + SAH).

RESULTS

In patients with ALD, blood MAITs were significantly decreased, hyperactivated, and displayed enhanced cell death through pyroptosis. The frequencies of pyroptotic MAITs increased with disease severity in patients with ALC and patients with ALC + SAH. These frequencies were negatively associated with the frequencies of MAITs and positively correlated with the levels of MAITs' activation, plasma levels of intestinal fatty acid-binding protein (a marker of intestinal enterocyte damage), soluble CD14, lipopolysaccharide-binding protein, and peptidoglycan recognition proteins (surrogate markers of microbial translocation). Pyroptotic MAITs were also found in the liver of patients with ALD. Interestingly, MAITs underwent further activation and pyroptosis in vitro under stimulation by Escherichia coli or direct bilirubin. Notably, blocking IL-18 signaling reduced the activation and frequencies of pyroptotic MAITs.

CONCLUSIONS

The loss of MAITs in patients with ALD is, at least in part, due to cell death from pyroptosis and is associated with the severity of ALD. Such increased pyroptosis may be affected by dysregulated inflammatory responses to intestinal microbial translocation or direct bilirubin.

Mucosal-Associated Invariant T Cells in the Digestive System: Defender or Destroyer?

Mucosal-associated invariant T (MAIT) cells are a subset of innate T lymphocytes that express the semi-invariant T cell receptor and recognize riboflavin metabolites via the major histocompatibility complex class I-related protein. Given the abundance of MAIT cells in the human body, their role in human diseases has been increasingly studied in recent years. MAIT cells may serve as targets for clinical therapy. Specifically, this review discusses how MAIT cells are altered in gastric, esophageal, intestinal, and hepatobiliary diseases and describes their protective or pathogenic roles. A greater understanding of MAIT cells will provide a more favorable therapeutic approach for digestive diseases in the clinical field.

MAIT cells ameliorate liver fibrosis by enhancing the cytotoxicity of NK cells in cholestatic murine models

BACKGROUND AND AIMS

Mucosal-associated invariant T (MAIT) cells are innate-like lymphocytes that display a critical role in various liver diseases. However, the role of MAIT cells in cholestatic liver fibrogenesis remains obscure. Our study aims to assess the contribution of MAIT cells and underlying mechanisms during this process.

METHODS

Cholestatic murine models using MAIT cell-deficient (MR1^- /^- ) and wild-type (WT) mice were established by feeding a 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-enriched diet or bile duct ligation (BDL). Liver samples were collected to determine the severity of fibrosis. Lymphocytes of the liver were isolated for analysing the phenotype and function of MAIT cells. Cell co-culture experiments were performed to investigate the cross-talk between MAIT and NK cells.

RESULTS

Liver MAIT cells were more activated with increased cytokines in cholestatic mice models than in control mice, although their frequency was decreased. MAIT cell deficiency led to severe liver inflammation and fibrosis with more activated HSCs in cholestatic mice. In addition, MR1^- /^- mice had an increased frequency of NK cells with higher expression of stimulatory receptors relative to WT mice. Paradoxically, activated MAIT cells significantly promoted the anti-fibrotic ability of NK cells by enhancing their cytotoxicity against HSCs in co-culture experiments. Importantly, this effect depended on direct cell-cell contact and TNF-α produced by MAIT cells.

CONCLUSION

Our findings indicate that MAIT cells ameliorate cholestatic liver fibrosis by enhancing the cytotoxicity of NK cells against HSCs. An in-depth understanding of the MAIT cell-mediated regulatory effect will provide more valuable immunotherapy strategies to treat liver fibrosis.

Immunologic Responses and the Pathophysiology of Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is an autoimmune liver disease with a female predisposition and selective destruction of intrahepatic small bile ducts leading to nonsuppurative destructive cholangitis. It is characterized by seropositivity of antimitochondrial antibodies or PBC-specific antinuclear antibodies, progressive cholestasis, and typical liver histologic manifestations. Destruction of the protective bicarbonate-rich umbrella is attributed to the decreased expression of membrane transporters in biliary epithelial cells (BECs), leading to the accumulation of hydrophobic bile acids and sensitizing BECs to apoptosis. A recent X-wide association study reveals a novel risk locus on the X chromosome, which reiterates the importance of Treg cells.

Primary Biliary Cholangitis and Primary Sclerosing Cholangitis: Current Knowledge of Pathogenesis and Therapeutics

Cholangiopathies encompass various biliary diseases affecting the biliary epithelium, resulting in cholestasis, inflammation, fibrosis, and ultimately liver cirrhosis. Primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are the most important progressive cholangiopathies in adults. Much research has broadened the scope of disease biology to genetic risk, epigenetic changes, dysregulated mucosal immunity, altered biliary epithelial cell function, and dysbiosis, all of which interact and arise in the context of ill-defined environmental triggers. An in-depth understanding of the molecular pathogenesis of these cholestatic diseases will help clinicians better prevent and treat diseases. In this review, we focus on the main underlying mechanisms of disease initiation and progression, and novel targeted therapeutics beyond currently approved treatments.

MAIT cells in liver inflammation and fibrosis

Mucosal-associated invariant T cells or MAIT cells are an abundant cell type in humans and especially so in the liver. MAIT cells are a subset of T lymphocytes that sit at a bridge between innate and adaptive immunity, so-called innate-like or "unconventional" T cells. The specificity of their antigen receptor (T cell receptor or TCR) is for the conserved major histocompatibility complex (MHC)-related molecule MR1, which presents a modified bacterial metabolite from the vitamin B2 biosynthesis pathway - this allows them to respond in the presence of many bacteria or yeast. MAIT cells also possess an array of cytokine receptors, which allows triggering independently of the TCR. The combination of such signals drives their functionality - this means they can respond to a range of stimuli and likely play a role not only in infection or inflammation, but also under homeostatic conditions.In this review, we will look at the question of what MAIT cells are doing in the normal liver and how they behave in the setting of disease. These questions are of relevance because MAIT cells are such a distinctive cell type enriched in the liver under normal conditions, and their modulation could be of therapeutic benefit. The recent discovery that they appear to be involved in liver fibrosis is particularly of interest in this context.

Bile from Patients with Primary Sclerosing Cholangitis Contains Mucosal-Associated Invariant T-Cell Antigens

Primary sclerosing cholangitis (PSC) is associated with altered microbiota of the gut and bile. Mucosal-associated invariant T (MAIT) cells, enriched in human liver, uniquely recognize microbial-derived metabolites. This study aimed to determine whether bile from patients with PSC contains antigens activating MAIT cells. Bile was collected at the time of liver transplantation from patients with PSC (n = 28). The bile samples were either directly incubated with peripheral blood mononuclear cells from healthy donors or with antigen-presenting cells followed by co-culture with peripheral blood mononuclear cells. MAIT cell activation was assessed by flow cytometry. An anti-MR1 antibody was used to determine whether the activation was major histocompatibility complex class I-related protein (MR1) restricted. Biliary microbiota profiles were generated using 16S rRNA amplicon sequencing, and the abundance of the bacterial gene ribD was predicted. Eight of 28 bile samples could activate MAIT cells. This activation was partly MR1-dependent in five of eight bile samples. Microbial DNA was detected in 15 of 28 bile samples, including the five bile samples leading to MR1-dependent activation. A higher abundance of the ribD gene expression in the group of bile samples that could activate MAIT cells was predicted on the basis of the 16S sequencing. In co-culture experiments, cholangiocytes could take up and present biliary antigens to MAIT cells. These findings suggest a pathophysiological pathway in PSC connecting the immune system and the microbiome.

Changes in the phenotype and function of mucosal-associated invariant T cells in neutrophilic asthma

Asthma is a chronic heterogeneous inflammatory disease. Most neutrophilic asthma (NA) cases are severe asthma involving many inflammatory cells and mediators, although the specific pathogenesis is not clear. Mucosal-associated invariant T (MAIT) cells as innate-like T lymphocytes play an important role in the immune response in asthma by producing cytokines. In this study, we evaluated the phenotype and function of circulating MAIT cells in patients with NA and inflammatory-related cytokines in plasma and induced sputum supernatants using flow cytometry. The results showed that the frequency of circulating MAIT cells in asthma patients, particularly NA patients, decreased significantly, and CD8⁺ MAIT and MAIT Temra cells also decreased significantly. Increased expression of CD69 and PD-1 on MAIT cells indicated excessive activation and depletion, leading to the decrease in MAIT cells. Levels of IL-17A and TNF-α secreted by MAIT cells of NA patients increased, whereas IFN-γ levels decreased, indicating that MAIT cells in NA are biased to the Th17 subtype. MAIT cells were also negatively correlated with clinical parameters, indicating that these cells are related to asthma severity. Pro-inflammatory cytokines in plasma and sputum supernatant increased to varying degrees, whereas IL-10 declined, corresponding with asthma severity. We speculate that increased IL-17A and TNF-α synergistically stimulated respiratory epithelial cells to secrete IL-6 and IL-8, thereby recruiting neutrophils to inflammatory sites and aggravating asthma symptoms. Therefore, MAIT cells could serve as a potential therapeutic target in NA immunity, thus providing a new strategy for the treatment of asthma.

Persistent deficiency of mucosa-associated invariant T (MAIT) cells during alcohol-related liver disease

BACKGROUND

Alcohol-related liver disease (ALD) is a major cause of chronic liver diseases. Inflammatory response is a basic pathological feature of ALD. Mucosal-associated invariant T(MAIT) cells are a novel population of innate immune cells, which may be depleted in various inflammatory diseases. However, the changes of MAIT cell in ALD remains unclear.

RESULTS

In this study, the levels of MAIT cell were significantly decreased in patients with alcoholic fatty liver disease, alcoholic cirrhosis, and mixed cirrhosis (alcoholic + viral). Furthermore, the reduction of circulating MAIT cells was correlated with liver function in patients with cirrhosis. Functional changes among circulating MAIT cells in patients with alcoholic cirrhosis, including increased production of IL-17A and perforin, and reduced production of TNF-α. Plasma cytokine and chemokine levels were quantified using multiple immunoassays and ELISA. Serum levels of chemokine IL-8 were correlated with MAIT cell frequency in patients with alcoholic cirrhosis. Moreover, no differences were observed in the expression of CCR6, CXCR6, and PD-1 in circulating MAIT cells of patients with alcoholic cirrhosis. The MAIT cells in patients with alcoholic cirrhosis were prone to apoptosis, which was promoted by IL-12, IL-18, and IL-8.

CONCLUSIONS

Our findings indicate persistent MAIT cell loss during alcohol-related liver disease and suggest that MAIT cells can be promising indicator and therapeutic targets in ALD.

Incorporating mucosal-associated invariant T cells into the pathogenesis of chronic liver disease

Mucosal-associated invariant T (MAIT) cells have been described in liver and non-liver diseases, and they have been ascribed antimicrobial, immune regulatory, protective, and pathogenic roles. The goals of this review are to describe their biological properties, indicate their involvement in chronic liver disease, and encourage investigations that clarify their actions and therapeutic implications. English abstracts were identified in PubMed by multiple search terms, and bibliographies were developed. MAIT cells are activated by restricted non-peptides of limited diversity and by multiple inflammatory cytokines. Diverse pro-inflammatory, anti-inflammatory, and immune regulatory cytokines are released; infected cells are eliminated; and memory cells emerge. Circulating MAIT cells are hyper-activated, immune exhausted, dysfunctional, and depleted in chronic liver disease. This phenotype lacks disease-specificity, and it does not predict the biological effects. MAIT cells have presumed protective actions in chronic viral hepatitis, alcoholic hepatitis, non-alcoholic fatty liver disease, primary sclerosing cholangitis, and decompensated cirrhosis. They have pathogenic and pro-fibrotic actions in autoimmune hepatitis and mixed actions in primary biliary cholangitis. Local factors in the hepatic microenvironment (cytokines, bile acids, gut-derived bacterial antigens, and metabolic by-products) may modulate their response in individual diseases. Investigational manipulations of function are warranted to establish an association with disease severity and outcome. In conclusion, MAIT cells constitute a disease-nonspecific, immune response to chronic liver inflammation and infection. Their pathological role has been deduced from their deficiencies during active liver disease, and future investigations must clarify this role, link it to outcome, and explore therapeutic interventions.

Exercise training partially rescues impaired mucosal associated invariant t-cell mobilization in breast cancer survivors compared to healthy older women

Exercise may attenuate immunosenescence with aging that appears to be accelerated following breast cancer treatment, although limited data on specific cell types exists and acute and chronic exercise have been investigated independently in older adults.

PURPOSE

To determine the mucosal associated invariant T (MAIT) cell response to acute exercise before (PRE) and after (POST) 16 weeks of exercise training in breast cancer survivors (BCS) and healthy older women (CON).

METHODS

Age-matched BCS and CON performed 45 min of intermittent cycling at 60% peak power output wattage. Blood samples were obtained at rest, immediately (0 h) and 1 h after exercise to determine MAIT cell counts, frequency, and intracellular cytokine expression.

RESULTS

At PRE, MAIT cell counts were greater in CON (137%) than BCS at 0 h (46%, p < 0.001), with increased MAIT cell frequency in CON but not BCS. TNFα⁺ and IFNγ⁺ MAIT cell counts increased at 0 h by ~120% in CON (p < 0.001), while BCS counts and frequencies were unchanged. Similar deficits were observed in CD3⁺ and CD3⁺ CD8⁺ cells. At POST, exercise-induced mobilization and egress of MAIT cell counts and frequency showed trends towards improvement in BCS that approached levels in CON. Independent of group, TNFα frequency trended to improve (p = 0.053).

CONCLUSIONS

MAIT mobilization in older BCS following acute exercise was attenuated; however, exercise training may partially rescue these initial deficits, including greater sensitivity to mitogenic stimulation. Using acute exercise before and after interventions provides a unique approach to identify age- and cancer-related immuno-dysfunction that is less apparent at rest.

Clinical Management of Primary Biliary Cholangitis-Strategies and Evolving Trends

PBC is a chronic progressive autoimmune disorder involving the destruction of intrahepatic small bile ducts, cholestasis, fibrosis, and ultimately cirrhosis if left untreated. It is largely driven by the autoimmune response, but bile acids and the intestinal microbiota are implicated in disease progression as well. The only drugs licensed for PBC are UDCA and OCA. UDCA as a first-line and OCA as a second-line therapy are safe and effective, but the lack of response in a significant portion of patients and inadequate control of symptoms such as fatigue and pruritus remain as concerns. Liver transplantation is an end-stage therapy for many patients refractory to UDCA, which gives excellent survival rates but also moderate to high recurrence rates. The limited options for FDA-approved PBC therapies necessitate the development of alternative approaches. Currently, a wide variety of experimental drugs exist targeting immunological and physiological aspects of PBC to suppress inflammation. Immunological therapies include drugs targeting immune molecules in the B cell and T cell response, and specific cytokines and chemokines implicated in inflammation. Drugs targeting bile acids are also noteworthy as bile acids can perpetuate hepatic inflammation and lead to fibrosis over time. These include FXR agonists, ASBT inhibitors, and PPAR agonists such as bezafibrate and fenofibrate. Nonetheless, many of these drugs can only delay disease progression and fail to enhance patients' quality of life. Nanomedicine shows great potential for treatment of autoimmune diseases, as it provides a new approach that focuses on tolerance induction rather than immunosuppression. Tolerogenic nanoparticles carrying immune-modifying agents can be engineered to safely and effectively target the antigen-specific immune response in autoimmune diseases. These may work well with PBC especially, given the anatomical features and immunological specificity of the disease. Nanobiological therapy is thus an area of highly promising research for future treatment of PBC.

The pathophysiological function of non-gastrointestinal farnesoid X receptor

Farnesoid X receptor (FXR) influences bile acid homeostasis and the progression of various diseases. While the roles of hepatic and intestinal FXR in enterohepatic transport of bile acids and metabolic diseases were reviewed previously, the pathophysiological functions of FXR in non-gastrointestinal cells and tissues have received little attention. Thus, the roles of FXR in the liver, immune system, nervous system, cardiovascular system, kidney, and pancreas beyond the gastrointestinal system are reviewed herein. Gain of FXR function studies in non-gastrointestinal tissues reveal that FXR signaling improves various experimentally-induced metabolic and immune diseases, including non-alcoholic fatty liver disease, type 2 diabetes, primary biliary cholangitis, sepsis, autoimmune diseases, multiple sclerosis, and diabetic nephropathy, while loss of FXR promotes regulatory T cells production, protects the brain against ischemic injury, atherosclerosis, and inhibits pancreatic tumor progression. The downstream pathways regulated by FXR are diverse and tissue/cell-specific, and FXR has both ligand-dependent and ligand-independent activities, all of which may explain why activation and inhibition of FXR signaling could produce paradoxical or even opposite effects in some experimental disease models. FXR signaling is frequently compromised by diseases, especially during the progressive stage, and rescuing FXR expression may provide a promising strategy for boosting the therapeutic effect of FXR agonists. Tissue/cell-specific modulation of non-gastrointestinal FXR could influence the treatment of various diseases. This review provides a guide for drug discovery and clinical use of FXR modulators.

Bile Acids Impair Vaccine Response in Children With Biliary Atresia

Background

Vaccination is the best way to protect children under 5 years from death or disability. Children with biliary atresia (BA), which is the most common pediatric cholestatic end-stage liver disease (PELD), are more vulnerable to infectious diseases. However, the vaccination coverage and factors modulating vaccine responses in children with BA are largely unknown.

Methods

In this study, 288 children (median age: 7 months) diagnosed with BA before liver transplantation were enrolled for the evaluation of vaccination status and the factors affecting the immune response to the hepatitis B (HBV) vaccine. Moreover, 49 BA children (median age: 4 months) were enrolled for flow cytometric analysis of CD4⁺ T cells and CD19⁺ B cell subsets and correlations with serum bile acid levels.

Results

Generally, these children had very low routine vaccination rates for the meningococcal serogroup AC (Men AC) (41.2%), measles-mumps-rubella (MMR) (31.3%), poliomyelitis (Polio) (25.3%), hepatitis A (HAV) (25.0%), Japanese encephalitis (JE) (15.0%), diphtheria-tetanus-pertussis (DTP) (14.2%), meningococcal serogroup A (Men A) (13.5%) and varicella (VAR) (10.8%) vaccines, but not for the HBV (96.2%) and bacillus Calmette-Guérin (BCG) (84.7%) vaccines. Remarkably, 19.8% (57/288) of the patients had HBV infection. Out of 220 patients vaccinated for HBV, 113 (51.4%), 85 (38.6%) and 22 (10%) had one, two or three doses of the HBV vaccine, respectively. Furthermore, logistic regression analysis revealed that the bile acid level was an independent factor associated with poor HBV vaccine response (p = 0.03; OR = 0.394; 95% CI = 0.170-0.969). Immunophenotyping showed that bile acids were only negatively correlated with the CD19⁺CD27⁺IgG⁺ post-class-switched memory B cell ratio (p = 0.01).

Conclusion

This study reveals the overall vaccination rates of routine vaccines in Chinese BA children are very low and the poor HBV vaccine responses are associated with bile acids, possibly via the inhibition of CD19⁺CD27⁺IgG⁺ post-class-switched memory B cell response.

Clinical Trial Registration

http://www.chictr.org.cn, identifier ChiCTR1800019165.

CXCL12-CXCR4-Mediated Chemotaxis Supports Accumulation of Mucosal-Associated Invariant T Cells Into the Liver of Patients With PBC

Objectives: To explore the potential role of CD3⁺CD8⁺CD161^high TCRVα7.2⁺ mucosal-associated invariant T (MAIT) cells in the pathogenesis of primary biliary cholangitis (PBC).

Methods: We enrolled 55 patients with PBC, 69 healthy controls (HCs), and 8 patients with hepatic hemangioma. Circulating MAIT cells and their chemokine receptor profiles and cytokine production were quantified using flow cytometry. Liver-resident MAIT cells were examined by immunofluorescence staining. CXCL12-mediated chemotaxis of MAIT cells was measured using a transwell migration assay. Plasma interleukin (IL)-18 was measured using ELISA, and cytokine production in IL-18-stimulated MAIT cells was detected using flow cytometry.

Result: Peripheral MAIT cells were found to be significantly lower in patients with PBC (3.0 ± 3.2% vs. 9.4 ± 8.0%, p < 0.01) and negatively correlated with alkaline phosphatase (ALP) levels (r = −0.3209, p < 0.05). Liver immunofluorescence staining suggested that MAIT cells might accumulate in PBC liver. MAIT cells from patients with PBC expressed higher levels of CXCR4 (84.8 ± 18.0% vs. 58.7 ± 11.4%, p < 0.01), and the expression of CXCL12 was higher in PBC liver. CXCL12 promoted MAIT cell chemotaxis (70.4 ± 6.8% vs. 52.2 ± 3.5%, p < 0.01), which was attenuated by CXCR4 antagonist. MAIT cells from PBC produced significantly more interferon-γ (IFN-γ) (88.3 ± 4.2% vs. 64.2 ± 10.1%, p < 0.01), tumor necrosis factor-α (TNF-α) (93.0 ± 1.1% vs. 80.1 ± 5.3%, p < 0.01), Granzyme B (89.3 ± 3.3% vs. 72.1 ± 7.0%, p < 0.01), and perforin (46.8 ± 6.6% vs. 34.8 ± 7.7%, p < 0.05). MAIT cells from PBC expressed higher levels of IL18-Rα (83.8 ± 10.2% vs. 58.3 ± 8.7%, p < 0.01). Plasma IL-18 was more abundant in patients with PBC (286.8 ± 75.7 pg/ml vs. 132.9 ± 78.1 pg/ml, p < 0.01). IL-18 promoted IFN-γ production in MAIT cells (74.9 ± 6.6% vs. 54.7 ± 6.7%, p < 0.01), which was partially attenuated by blocking IL-18R (68.6 ± 8.3% vs. 43.5 ± 4.2%, p < 0.01).

Conclusion: Mucosal-associated invariant T cells from patients with PBC accumulated in the liver via CXCL12-CXCR4-mediated chemotaxis, produced pro-inflammatory cytokines, and contributed to portal inflammation, which was potentially mediated by elevated IL-18. Targeting MAIT cells might be a therapeutic approach for PBC.

Pathophysiological Roles of Mucosal-Associated Invariant T Cells in the Context of Gut Microbiota-Liver Axis

Mucosal-associated invariant T (MAIT) cells are a subset of T lymphocytes expressing a semi-invariant T-cell receptor (TCR) present as TCR Vα7.2-Jα33 in humans and TCR Vα19-Jα33 in mice. They are activated by ligands produced during microbial biosynthesis of riboflavin that is presented by major histocompatibility complex class I-related (MR1) molecules on antigen-presenting cells. MAIT cells also possess interleukin (IL)-12 and IL-18 receptors and can be activated by the respective cytokines released from microbially stimulated antigen-presenting cells. Therefore, MAIT cells can be involved in bacterial and viral defenses and are a significant part of the human immune system. They are particularly abundant in the liver, an organ serving as the second firewall of gut microbes next to the intestinal barrier. Therefore, the immune functions of MAIT cells are greatly impacted by changes in the gut-microbiota and play important roles in the gut-liver pathogenesis axis. In this review, we discuss the nature and mechanisms of MAIT cell activation and their dynamics during different types of liver pathogenesis conditions. We also share our perspectives on important aspects that should be explored further to reveal the exact roles that MAIT cells play in liver pathogenesis in the context of the gut microbiota.

Gut microbiome, liver immunology, and liver diseases

The gut microbiota is a complex and plastic consortium of microorganisms that are intricately connected with human physiology. The liver is a central immunological organ that is particularly enriched in innate immune cells and constantly exposed to circulating nutrients and endotoxins derived from the gut microbiota. The delicate interaction between the gut and liver prevents accidental immune activation against otherwise harmless antigens. Work on the interplay between the gut microbiota and liver has assisted in understanding the pathophysiology of various liver diseases. Of immense importance is the step from high-throughput sequencing (correlation) to mechanistic studies (causality) and therapeutic intervention. Here, we review the gut microbiota, liver immunology, and the interaction between the gut and liver. In addition, the impairment in the gut-liver axis found in various liver diseases is reviewed here, with an emphasis on alcohol-associated liver disease (ALD), nonalcoholic fatty liver disease (NAFLD), and autoimmune liver disease (AILD). On the basis of growing evidence from these preclinical studies, we propose that the gut-liver axis paves the way for targeted therapeutic modalities for liver diseases.

Bile Acids as a New Type of Steroid Hormones Regulating Nonspecific Energy Expenditure of the Body (Review)

The review is devoted to the systematization, classification, and generalization of the results of modern scientific research on the role of bile acids as a new class of steroid hormones. The paper presents the evidence for bile acid participation in the regulation of the body energy metabolism, body weight control, as well as the pathogenesis of obesity, diabetes mellitus, insulin resistance, and cardiovascular diseases. Particular attention is paid to the role of bile acids in the control of nonspecific energy expenditure of the body. The applied aspects of using the novel data about the membrane and intracellular receptors responsible for the development of hormonal regulatory effects of bile acids are analyzed. According to the authors, the modern data on the role of bile acids in the regulation of body functions allow a deeper understanding of the pathogenesis of body weight disorders and associated cardiovascular diseases. The review demonstrates promising directions in the search for specific methods of prevention and correction of these pathological conditions.

Antigen Recognition by MR1-Reactive T Cells; MAIT Cells, Metabolites, and Remaining Mysteries

Mucosal-associated Invariant T (MAIT) cells recognize vitamin B-based antigens presented by the non-polymorphic MHC class I related-1 molecule (MR1). Both MAIT T cell receptors (TCR) and MR1 are highly conserved among mammals, suggesting an important, and conserved, immune function. For many years, the antigens they recognize were unknown. The discovery that MR1 presents vitamin B-based small molecule ligands resulted in a rapid expansion of research in this area, which has yielded information on the role of MAIT cells in immune protection, autoimmune disease and recently in homeostasis and cancer. More recently, we have begun to appreciate the diverse nature of the small molecule ligands that can bind MR1, with several less potent antigens and small molecule drugs that can bind MR1 being identified. Complementary structural information has revealed the complex nature of interactions defining antigen recognition. Additionally, we now view MAIT cells (defined here as MR1-riboflavin-Ag reactive, TRAV1-2+ cells) as one subset of a broader family of MR1-reactive T cells (MR1T cells). Despite these advances, we still lack a complete understanding of how MR1 ligands are generated, presented and recognized in vivo. The biological relevance of these MR1 ligands and the function of MR1T cells in infection and disease warrants further investigation with new tools and approaches.

Mucosal-associated invariant T cells in hepatitis B virus-related liver failure

BACKGROUND

Liver failure has high mortality and poor prognosis, and establishing new reliable markers for predicting its prognosis is necessary. Mucosal-associated invariant T (MAIT) cells are a novel population of innate-like lymphocytes involved in inflammatory liver disease, and their potential role in liver failure remains unclear.

AIM

To investigate alteration of circulating MAIT cells and assess its prognostic value in patients with hepatitis B virus (HBV)-related liver failure.

METHODS

We recruited 55 patients with HBV-related liver failure, 48 patients with chronic hepatitis B and 40 healthy controls (HCs) from Nantong Third People’s Hospital Affiliated to Nantong University. Peripheral blood mononuclear cells were isolated, and the percentage and number of circulating MAIT cells were detected by flow cytometry. Plasma levels of interleukin (IL)-7, IL-12p70, IL-18 and interferon-α were measured by Luminex assay.

RESULTS

Circulating MAIT cells were significantly decreased in HBV-related liver failure patients (percentage: 2.00 ± 1.22 vs 5.19 ± 1.27%, P < 0.0001; number: 5.47 ± 4.93 vs 84.43 ± 19.59, P < 0.0001) compared with HCs. More importantly, there was a significant reduction of MAIT cells in patients with middle/late-stage compared with early-stage liver failure. Circulating MAIT cells partially recovered after disease improvement, both in percentage (4.01 ± 1.21 vs 2.04 ± 0.95%, P < 0.0001) and in cell count (17.24 ± 8.56 vs 7.41 ± 4.99, P < 0.0001). The proportion (2.29 ± 1.01 vs 1.58 ± 1.38%, P < 0.05) and number (7.30 ± 5.70 vs 2.94 ± 1.47, P < 0.001) of circulating MAIT cells were significantly higher in the survival group than in the dead/liver transplantation group, and the Kaplan–Meier curve showed that lower expression of circulating MAIT cells (both percentage and cell count) predicted poor overall survival (P < 0.01). Also, the levels of IL-12 (20.26 ± 5.42 pg/mL vs 17.76 ± 2.79 pg/mL, P = 0.01) and IL-18 (1470.05 ± 1525.38 pg/mL vs 362.99 ± 109.64 pg/mL, P < 0.0001) were dramatically increased in HBV-related liver failure patients compared with HCs.

CONCLUSION

Circulating MAIT cells may play an important role in the process of HBV-related liver failure and can be an important prognostic marker.

Single-cell mass cytometry on peripheral blood identifies immune cell subsets associated with primary biliary cholangitis

The relationship between primary biliary cholangitis (PBC), a chronic cholestatic autoimmune liver disease, and the peripheral immune system remains to be fully understood. Herein, we performed the first mass cytometry (CyTOF)-based, immunophenotyping analysis of the peripheral immune system in PBC at single-cell resolution. CyTOF was performed on peripheral blood mononuclear cells (PBMCs) from PBC patients (n = 33) and age-/sex-matched healthy controls (n = 33) to obtain immune cell abundance and marker expression profiles. Hierarchical clustering methods were applied to identify immune cell types and subsets significantly associated with PBC. Subsets of gamma-delta T cells (CD3⁺TCRgd⁺), CD8⁺ T cells (CD3⁺CD8⁺CD161⁺PD1⁺), and memory B cells (CD3⁻CD19⁺CD20⁺CD24⁺CD27⁺) were found to have lower abundance in PBC than in control. In contrast, higher abundance of subsets of monocytes and naïve B cells were observed in PBC compared to control. Furthermore, several naïve B cell (CD3⁻CD19⁺CD20⁺CD24⁻CD27⁻) subsets were significantly higher in PBC patients with cirrhosis (indicative of late-stage disease) than in those without cirrhosis. Alternatively, subsets of memory B cells were lower in abundance in cirrhotic relative to non-cirrhotic PBC patients. Future immunophenotyping investigations could lead to better understanding of PBC pathogenesis and progression, and also to the discovery of novel biomarkers and treatment strategies.

Liver fibrogenesis: un update on established and emerging basic concepts

Liver fibrogenesis is defined as a dynamic and highly integrated process occurring during chronic injury to liver parenchyma that can result in excess deposition of extracellular matrix (ECM) components (i.e., liver fibrosis). Liver fibrogenesis, together with chronic inflammatory response, is then primarily involved in the progression of chronic liver diseases (CLD) irrespective of the specific etiology. In the present review we will first offer a synthetic and updated overview of major basic concepts in relation to the role of myofibroblasts (MFs), macrophages and other hepatic cell populations involved in CLD to then offer an overview of established and emerging issues and mechanisms that have been proposed to favor and/or promote CLD progression. A special focus will be dedicated to selected issues that include emerging features in the field of cholangiopathies, the emerging role of genetic and epigenetic factors as well as of hypoxia, hypoxia-inducible factors (HIFs) and related mediators.

Immune system and cholangiocytes: A puzzling affair in primary biliary cholangitis

Primary biliary cholangitis (PBC) is a cholestatic liver disease characterized by the destruction of the small and medium bile ducts. Its pathogenesis is still unknown. Despite the genome wide association study findings, the therapies targeting the cytokines pathway, tested so far, have failed. The concept of the biliary epithelium as a key player of the PBC pathogenesis has emerged over the last few years. It is now well accepted that the biliary epithelial cells (BECs) actively participate to the genesis of the damage. The chronic stimulation of BECs via microbes and bile changes the cell phenotype toward an active state, which, across the production of proinflammatory mediators, can recruit, retain, and activate immune cells. The consequent immune system activation can in turn damage BECs. Thus, the crosstalk between both innate and adaptive immune cells and the biliary epithelium creates a paracrine loop responsible for the disease progression. In this review, we summarize the evidence provided in literature about the role of BECs and the immune system in the pathogenesis of PBC. We also dissect the relationship between the immune system and the BECs, focusing on the unanswered questions and the future potential directions of the translational research and the cellular therapy in this area.

Mucosal-Associated Invariant T cell in liver diseases

Mucosal-associated invariant T cells (MAIT cells) are a new population of innate immune cells, which are abundant in the liver and play complex roles in various liver diseases. In this review, we summarize MAIT cells in the liver diseases in recent studies, figure out the role of MAIT cells in various liver disease, including Alcoholic liver disease, Non-alcoholic liver disease, Autoimmune liver diseases, Viral hepatitis and Liver Cancer. Briefly, MAIT cells are involved in anti-bacteria responses in the alcoholic liver diseases. Besides, the activated MAIT cells promote the liver inflammation by secreting inflammatory cytokines and produce regulatory cytokines, which induces anti-inflammatory macrophage polarization. MAIT cells participate in the liver fibrosis via enhancing hepatic stellate cell activation. In viral hepatitis, MAIT cells exhibit a flawed and exhausted phenotype, which results in little effect on controlling the virus and bacteria. In liver cancer, MAIT cells indicate the disease progression and the outcome of therapy. In summary, MAIT cells are attractive biomarkers and therapeutic targets for liver disease.

Recurrence of disease following organ transplantation in autoimmune liver disease and systemic lupus erythematosus

Disease recurrence after organ transplantation associated with graft failure is a major clinical challenge in autoimmune diseases. Primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC) and autoimmune Hepatitis (AIH) are the three most common (autoimmune liver diseases) ALD for which liver transplantation (LT) is the most effective treatment option for patients with end-stage diseases. Although the 5- and 10-year survival rates of post-LT patients are remarkable (80-84% and 71-79% in PBC, 73-87% and 58-83% in PSC, 76-79% and 67-77% respectively in AIH patients), post-LT disease recurrence is not uncommon. Here, we summarize literature findings on disease recurrence of these ALD with emphasis on the incidence, risk factors and impact on long-term outcome. We noted that the incidence of disease recurrence varies between studies, which ranges from 53% to 10.9% in PBC, 8.2% to 44.7% in PSC and 7% to 42% in AIH. The variations are likely due to differences in study design, such as sample size, duration of studies and follow up time. This is further compounded by the lack of precise clinical diagnosis criteria and biomarkers of disease recurrence in these ALD, variation in post-LT treatment protocols to prevent disease recurrence and a multitude of risk factors associated with these ALD. While recurrence of PBC and AIH does not significantly impact long term outcome including overall survival, recurrent PSC patients often require another LT. Renal transplantation, like LT, is the treatment of choice in patients with end-stage lupus nephritis. While calcineurin inhibitor (CNI) and immunosuppressive drugs have improved the survival rate, post-transplant recurrence of lupus nephritis from surveillance-biopsy proven lupus nephritis range from 30% to 44%. On the other hand, recurrence of post-transplant lupus nephritis from registry survey analysis were only 1.1% to 2.4%. In general, risk factors associated with an increased frequency of post-transplant recurrence of autoimmune diseases are not clearly defined. Large scale multi-center studies are needed to further define guidelines for the diagnosis and clinical management to minimize disease recurrence and improve outcomes of post-transplant patients.

No difference in TCRβ repertoire of CD4+ naive T cell between patients with primary biliary cholangitis and healthy control subjects

Primary biliary cholangitis (PBC) is considered as a model of organ-specific autoimmune disease based on the serological findings of anti-mitochondrial antibodies (AMA), infiltrates of T cells, and selective destruction of epithelial cells in the liver. T-cell-mediated autoimmune mechanisms are considered to be involved in the pathogenesis of primary biliary cholangitis (PBC). In this context, we used a combination of multiplex-PCR, Illumina sequencing and IMGT/HighV-QUEST for a standardized analysis of the T cell receptor β-chain (TCRβ) repertoire of CD4^＋naive T cells in PBC patients compared with healthy volunteers. Nonfunctional TCRs were used to study the pre-selection TCR repertoire, as they are not subject to functional selection (positive and negative selection). Functional TCRs were used to study the post-selection TCR repertoire. The results showed that there was not significant difference between PBC patients and healthy volunteers in TCRβ diversity, CDR3 length distributions, degree of sequence sharing, and usage frequency of TRBV and TRBJ segments, no matter in Pre-selection or Post-selection repertoires. In conclusion, early events in thymic T cell development and repertoire generation are not abnormality in PBC patients. The breakdown of self-tolerance to autoantigen may be derived from other immunological dysregulation or environmental agents.

The challenges of primary biliary cholangitis: What is new and what needs to be done

Primary Biliary Cholangitis (PBC) is an uncommon, chronic, cholangiopathy of autoimmune origin and unknown etiology characterized by positive anti-mitochondrial autoantibodies (AMA), female preponderance and progression to cirrhosis if left untreated. The diagnosis is based on AMA- or PBC-specific anti-nuclear antibody (ANA)-positivity in the presence of a cholestatic biochemical profile, histologic confirmation being mandatory only in seronegative cases. First-line treatment is ursodeoxycholic acid (UDCA), which is effective in preventing disease progression in about two thirds of the patients. The only approved second-line treatment is obeticholic acid. This article summarizes the most relevant conclusions of a meeting held in Lugano, Switzerland, from September 23rd-25th 2018, gathering basic and clinical scientists with various background from around the world to discuss the latest advances in PBC research. The meeting was dedicated to Ian Mackay, pioneer in the field of autoimmune liver diseases. The role of liver histology needs to be reconsidered: liver pathology consistent with PBC in AMA-positive individuals without biochemical cholestasis is increasingly reported, raising the question as to whether biochemical cholestasis is a reliable disease marker for both clinical practice and trials. The urgent need for new biomarkers, including more accurate markers of cholestasis, was also widely discussed during the meeting. Moreover, new insights in interactions of bile acids with biliary epithelia in PBC provide solid evidence of a role for impaired epithelial protection against potentially toxic hydrophobic bile acids, raising the fundamental question as to whether this bile acid-induced epithelial damage is the cause or the consequence of the autoimmune attack to the biliary epithelium. Strategies are needed to identify difficult-to-treat patients at an early disease stage, when new therapeutic approaches targeting immunologic pathways, in addition to bile acid-based therapies, may be effective. In conclusion, using interdisciplinary approaches, groundbreaking advances can be expected before long in respect to our understanding of the etiopathogenesis of PBC, with the ultimate aim of improving its treatment.

The biology and functional importance of MAIT cells

In recent years, a population of unconventional T cells called 'mucosal-associated invariant T cells' (MAIT cells) has captured the attention of immunologists and clinicians due to their abundance in humans, their involvement in a broad range of infectious and non-infectious diseases and their unusual specificity for microbial riboflavin-derivative antigens presented by the major histocompatibility complex (MHC) class I-like protein MR1. MAIT cells use a limited T cell antigen receptor (TCR) repertoire with public antigen specificities that are conserved across species. They can be activated by TCR-dependent and TCR-independent mechanisms and exhibit rapid, innate-like effector responses. Here we review evidence showing that MAIT cells are a key component of the immune system and discuss their basic biology, development, role in disease and immunotherapeutic potential.

Cholangiocyte pathobiology

Cholangiocytes, the epithelial cells lining the intrahepatic and extrahepatic bile ducts, are highly specialized cells residing in a complex anatomic niche where they participate in bile production and homeostasis. Cholangiocytes are damaged in a variety of human diseases termed cholangiopathies, often causing advanced liver failure. The regulation of cholangiocyte transport properties is increasingly understood, as is their anatomical and functional heterogeneity along the biliary tract. Furthermore, cholangiocytes are pivotal in liver regeneration, especially when hepatocyte regeneration is compromised. The role of cholangiocytes in innate and adaptive immune responses, a critical subject relevant to immune-mediated cholangiopathies, is also emerging. Finally, reactive ductular cells are present in many cholestatic and other liver diseases. In chronic disease states, this repair response contributes to liver inflammation, fibrosis and carcinogenesis and is a subject of intense investigation. This Review highlights advances in cholangiocyte research, especially their role in development and liver regeneration, their functional and biochemical heterogeneity, their activation and involvement in inflammation and fibrosis and their engagement with the immune system. We aim to focus further attention on cholangiocyte pathobiology and the search for new disease-modifying therapies targeting the cholangiopathies.

Immunological abnormalities in patients with primary biliary cholangitis

Primary biliary cholangitis (PBC), an autoimmune liver disease occurring predominantly in women, is characterized by high titers of serum anti-mitochondrial antibodies (AMAs) and progressive intrahepatic cholestasis. The immune system plays a critical role in PBC pathogenesis and a variety of immune cell subsets have been shown to infiltrate the portal tract areas of patients with PBC. Amongst the participating immune cells, CD4 T cells are important cytokine-producing cells that foster an inflammatory microenvironment. Specifically, these cells orchestrate activation of other immune cells, including autoreactive effector CD8 T cells that cause biliary epithelial cell (BEC) injury and B cells that produce large quantities of AMAs. Meanwhile, other immune cells, including dendritic cells (DCs), natural killer (NK) cells, NKT cells, monocytes, and macrophages are also important in PBC pathogenesis. Activation of these cells initiates and perpetuates bile duct damage in PBC patients, leading to intrahepatic cholestasis, hepatic damage, liver fibrosis, and eventually cirrhosis or even liver failure. Taken together, the body of accumulated clinical and experimental evidence has enhanced our understanding of the immunopathogenesis of PBC and suggests that immunotherapy may be a promising treatment option. Herein, we summarize current knowledge regarding immunological abnormalities of PBC patients, with emphasis on underlying pathogenic mechanisms. The differential immune response which occurs over decades of disease activity suggests that different therapies may be needed at different stages of disease.

The genetics of primary biliary cholangitis

PURPOSE OF REVIEW

Primary biliary cholangitis (PBC) is a female predominant chronic autoimmune disease of the intrahepatic bile ducts and with a long latent period. It is crucial to understand how genetics contribute to the disease.

RECENT FINDINGS

Geo-epidemiological studies in PBC have provided evidence of familial risk; case-control studies and genome wide association studies have identified various human leukocyte antigen (HLA) and non-HLA alleles that are associated with PBC. However, these alleles are non-PBC specific and most of the identified non-HLA loci were also found to be susceptible genes in other autoimmune diseases and different between study populations.

SUMMARY

Patients with PBC are often asymptomatic and often left undiagnosed. There are no known HLA and non-HLA alleles specific for PBC. Global effort and novel approaches such as epigenetics directed at identification of genetic risk factors will greatly facilitate accurate and timely diagnosis, which will improve prognosis and increase treatment options.

Pathogen infections and primary biliary cholangitis

Primary biliary cholangitis (PBC) is a multi-factorial disease caused by the interaction of both genetic predisposition and environmental triggers. Bacterial infection has been investigated most intensively, both epidemiologically and experimentally, as a prime environmental aetiology in PBC. The association of recurrent history of urinary tract infection (UTI) with PBC has been frequently confirmed by several large-scale, case-control studies, despite variation in geographic area or case-finding methods. Escherichia coli is a predominant pathogen in most cases with UTI. Animal studies and molecular mimicry analysis between the human and E. coli E2 subunit of the 2-oxo-acid dehydrogenase complexes demonstrated that E. coli infection is a key factor in breaking immunological tolerance against the mitochondria, resulting in the production of anti-mitochondrial autoantibodies (AMA), the disease-specific autoantibodies of PBC. Novosphingobium aromaticivorans, a ubiquitous xenobiotic-metabolizing bacterium, is another candidate which may be involved in the aetiology of PBC. Meanwhile, improved environmental hygiene and increased prevalence of PBC, especially in males, may argue against the aetiological role of bacterial infection in PBC. Multiple mechanisms can result in the loss of tolerance to mitochondrial autoantigens in PBC; nonetheless, bacterial infection is probably one of the dominant pathways, especially in female patients. Notably, there is a rising prevalence of male patients with PBC. With increasing exposure to environmental xenobiotics in both genders, studies directed towards identifying the environmental culprit with systematically designed case-control studies are much needed to further determine the environmental factors and role of bacterial infections in PBC.

MHC Molecules, T cell Receptors, Natural Killer Cell Receptors, and Viral Immunoevasins-Key Elements of Adaptive and Innate Immunity

Molecules encoded by the Major Histocompatibility Complex (MHC) bind self or foreign peptides and display these at the cell surface for recognition by receptors on T lymphocytes (designated T cell receptors-TCR) or on natural killer (NK) cells. These ligand/receptor interactions govern T cell and NK cell development as well as activation of T memory and effector cells. Such cells participate in immunological processes that regulate immunity to various pathogens, resistance and susceptibility to cancer, and autoimmunity. The past few decades have witnessed the accumulation of a huge knowledge base of the molecular structures of MHC molecules bound to numerous peptides, of TCRs with specificity for many different peptide/MHC (pMHC) complexes, of NK cell receptors (NKR), of MHC-like viral immunoevasins, and of pMHC/TCR and pMHC/NKR complexes. This chapter reviews the structural principles that govern peptide/MHC (pMHC), pMHC/TCR, and pMHC/NKR interactions, for both MHC class I (MHC-I) and MHC class II (MHC-II) molecules. In addition, we discuss the structures of several representative MHC-like molecules. These include host molecules that have distinct biological functions, as well as virus-encoded molecules that contribute to the evasion of the immune response.

Endogenous IL-10 maintains immune tolerance but IL-10 gene transfer exacerbates autoimmune cholangitis

The immunomodulatory effect of IL-10 as an immunosuppressive and anti-inflammatory cytokine is well known. Taking advantage of our established mouse model of autoimmune cholangitis using 2-octynoic acid conjugated ovalbumin (2-OA-OVA) induction, we compared liver pathology, immune cell populations and antimitochondrial antibodies between IL-10 knockout and wild type mice immunized with 2-OA-OVA. At 10 weeks post immunization, portal inflammation and fibrosis were more severe in 2-OA-OVA immunized IL-10 knockout mice than in wild type mice. This was accompanied by significant higher levels of collagen I and III expression, T, NK and NKT subsets in liver and IgG anti-mitochondrial autoantibodies (AMAs) compared to 2-OA-OVA immunized wild type mice, suggesting that endogenous IL-10 is necessary for the maintenance of immune tolerance in primary biliary cholangitis (PBC). Further, we investigated whether administration of exogenous IL-10 could prevent PBC by administration of IL-10 expressing recombinant adeno-associated virus (AAV-IL-10) either 3 days before or 3 weeks after the establishment of liver pathology. Interestingly, administration of AAV-IL-10 resulted in increased liver inflammation and fibrosis, accompanied by increases in IFN-γ in liver CD4⁺ T cell, granzyme B, FasL, and CD107a in liver CD8⁺ T and NKT cells, and granzyme B and FasL in liver NK cells of AAV-IL-10 administered mice compared with control mice. Furthermore, administration of AAV-IL-10 significantly increased levels of proinflammatory cytokines and chemokines (IFN-γ, TNF-α, CXCL9 and CXCL10) and collagen I and III production in naïve mice, together with increase in immune cell infiltration and collagen deposition in the liver, suggesting a role of IL-10 in fibrosis. In conclusion, our data demonstrate that endogenous IL-10 is critical in the maintenance of immune tolerance but exogenous administration of IL-10 exacerbates liver inflammation and fibrosis. Furthermore, the distinctive presence of inflammatory immune cell populations and collagen expression in AAV-IL-10 treated naïve mice cautions against the clinical use of exogenous IL-10 in patients with autoimmune cholangitis.

Mucosal-Associated Invariant T Cells Improve Nonalcoholic Fatty Liver Disease Through Regulating Macrophage Polarization

Mucosal-associated invariant T (MAIT) cells, a novel population of innate-like lymphocytes, have been involved in various inflammatory and autoimmune diseases. However, their role in the development of nonalcoholic fatty liver disease (NAFLD) remains unclear. In this study, we investigated the alterations of phenotype and immunological function of MAIT cells in NAFLD. Analysis of PBMCs in 60 patients with NAFLD and 48 healthy controls (HC) revealed that circulating MAIT cell frequency decreased in NAFLD, especially in the patients with higher serum levels of γ-glutamyl transferase or total triglyceride. Functional alterations of circulating MAIT cells were also detected in NAFLD patients, such as the increased production of IL-4 whereas the decreased production of IFN-γ and TNF-α. Furthermore, elevated expression of CXCR6 was observed in circulating MAIT cells of patients. Meanwhile, we found an increased number of MAIT cells in the livers of NAFLD, and the number was even greater in patients with higher NAFLD activity score. Moreover, activated MAIT cells induced monocytes/macrophages differentiation into M2 phenotype in vitro. Additionally, MAIT cells were enriched and displayed Th2 type cytokines profile in livers of wild type mice fed with methionine and choline deficient diet (MCD). Notably, mice deficient of MAIT cells exhibited more severe hepatic steatosis and inflammation upon MCD, accompanied with more CD11c+ proinflammatory macrophages (M1) and less CD206+ anti-inflammatory macrophages (M2) in livers. Our results indicate that MAIT cells protect against inflammation in NAFLD through producing regulatory cytokines and inducing anti-inflammatory macrophage polarization, which may provide novel therapeutic strategies for NAFLD.