Induction of autoimmune cholangitis in non-obese diabetic (NOD).1101 mice following a chemical xenobiotic immunization

Incorporating the Molecular Mimicry of Environmental Antigens into the Causality of Autoimmune Hepatitis

Molecular mimicry between foreign and self-antigens has been implicated as a cause of autoimmune hepatitis in experimental models and cross-reacting antibodies in patients. This review describes the experimental and clinical evidence for molecular mimicry as a cause of autoimmune hepatitis, indicates the limitations and uncertainties of this premise, and encourages investigations that assess diverse environmental antigens as sources of disease-relevant molecular mimics. Pertinent articles were identified in PubMed using multiple search phrases. Several pathogens have linear or conformational epitopes that mimic the self-antigens of autoimmune hepatitis. The occurrence of an acute immune-mediated hepatitis after vaccination for severe acute respiratory syndrome (SARS)-associated coronavirus 2 (SARS-CoV-2) has suggested that vaccine-induced peptides may mimic disease-relevant tissue antigens. The intestinal microbiome is an under-evaluated source of gut-derived antigens that could also engage in molecular mimicry. Chaperone molecules may enhance the pathogenicity of molecular mimics, and they warrant investigation. Molecular mimics of immune dominant epitopes within cytochrome P450 IID6, the autoantigen most closely associated with autoimmune hepatitis, should be sought in diverse environmental antigens and assessed for pathogenicity. Avoidance strategies, dietary adjustments, vaccine improvement, and targeted manipulation of the intestinal microbiota may emerge as therapeutic possibilities. In conclusion, molecular mimicry may be a missing causality of autoimmune hepatitis. Molecular mimics of key immune dominant epitopes of disease-specific antigens must be sought in diverse environmental antigens. The ubiquity of molecular mimicry compels rigorous assessments of peptide mimics for immunogenicity and pathogenicity in experimental models. Molecular mimicry may complement epigenetic modifications as causative mechanisms of autoimmune hepatitis.

Liver immunity, autoimmunity, and inborn errors of immunity

The liver is the front line organ of the immune system. The liver contains the largest collection of phagocytic cells in the body that detect both pathogens that enter through the gut and endogenously produced antigens. This is possible by the highly developed differentiation capacity of the liver immune system between self-antigens or non-self-antigens, such as food antigens or pathogens. As an immune active organ, the liver functions as a gatekeeping barrier from the outside world, and it can create a rapid and strong immune response, under unfavorable conditions. However, the liver's assumed immune status is anti-inflammatory or immuno-tolerant. Dynamic interactions between the numerous populations of immune cells in the liver are key for maintaining the delicate balance between immune screening and immune tolerance. The anatomical structure of the liver can facilitate the preparation of lymphocytes, modulate the immune response against hepatotropic pathogens, and contribute to some of its unique immunological properties, particularly its capacity to induce antigen-specific tolerance. Since liver sinusoidal endothelial cell is fenestrated and lacks a basement membrane, circulating lymphocytes can closely contact with antigens, displayed by endothelial cells, Kupffer cells, and dendritic cells while passing through the sinusoids. Loss of immune tolerance, leading to an autoaggressive immune response in the liver, if not controlled, can lead to the induction of autoimmune or autoinflammatory diseases. This review mentions the unique features of liver immunity, and dysregulated immune responses in patients with autoimmune liver diseases who have a close association with inborn errors of immunity have also been the emphases.

Linking Human Betaretrovirus with Autoimmunity and Liver Disease in Patients with Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is an autoimmune liver disease characterized by the production of diagnostic antimitochondrial antibodies (AMA) reactive to the pyruvate dehydrogenase complex. A human betaretrovirus (HBRV) resembling mouse mammary tumor virus has been characterized in patients with PBC. However, linking the viral infection with the disease is not a straight-forward process because PBC is a complex multifactorial disease influenced by genetic, hormonal, autoimmune, environmental, and other factors. Currently, PBC is assumed to have an autoimmune etiology, but the evidence is lacking to support this conjecture. In this review, we describe different approaches connecting HBRV with PBC. Initially, we used co-cultivation of HBRV with biliary epithelial cells to trigger the PBC-specific phenotype with cell surface expression of cryptic mitochondrial autoantigens linked with antimitochondrial antibody expression. Subsequently, we have derived layers of proof to support the role of betaretrovirus infection in mouse models of autoimmune biliary disease with spontaneous AMA production and in patients with PBC. Using Hill’s criteria, we provide an overview of how betaretrovirus infection may trigger autoimmunity and propagate biliary disease. Ultimately, the demonstration that disease can be cured with antiviral therapy may sway the argument toward an infectious disease etiology in an analogous fashion that was used to link H. pylori with peptic ulcer disease.

Animal Models of Autoimmune Liver Diseases: a Comprehensive Review

Autoimmune liver diseases (AILDs) are potentially life-threatening chronic liver diseases which include autoimmune hepatitis, primary biliary cholangitis, primary sclerosing cholangitis, and recently characterized IgG4-related sclerosing cholangitis. They are caused by immune attack on hepatocytes or bile ducts, with different mechanisms and clinical manifestations. The etiologies of AILDs include a susceptible genetic background, environment insults, infections, and changes of commensal microbiota, but remain complicated. Understanding of the underlying mechanisms of AILDs is mandatory for early diagnosis and intervention, which is of great importance for better prognosis. Thus, animal models are developed to mimic the pathogenesis, find biomarkers for early diagnosis, and for therapeutic attempts of AILDs. However, no animal models can fully recapitulate features of certain AILD, especially the late stages of diseases. Certain limitations include different living condition, cell composition, and time frame of disease development and resolution. Moreover, there is no IgG4 in rodents which exists in human. Nevertheless, the understanding and therapy of AILDs have been greatly advanced by the development and mechanistic investigation of animal models. This review will provide a comprehensive overview of traditional and new animal models that recapitulate different features and etiologies of distinct AILDs.

Examining pathogenic concepts of autoimmune hepatitis for cues to future investigations and interventions

BACKGROUND

Multiple pathogenic mechanisms have been implicated in autoimmune hepatitis, but they have not fully explained susceptibility, triggering events, and maintenance or escalation of the disease. Furthermore, they have not identified a critical defect that can be targeted. The goals of this review are to examine the diverse pathogenic mechanisms that have been considered in autoimmune hepatitis, indicate investigational opportunities to validate their contribution, and suggest interventions that might evolve to modify their impact. English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed. Genetic and epigenetic factors can affect susceptibility by influencing the expression of immune regulatory genes. Thymic dysfunction, possibly related to deficient production of programmed cell death protein-1, can allow autoreactive T cells to escape deletion, and alterations in the intestinal microbiome may help overcome immune tolerance and affect gender bias. Environmental factors may trigger the disease or induce epigenetic changes in gene function. Molecular mimicry, epitope spread, bystander activation, neo-antigen production, lymphocytic polyspecificity, and disturbances in immune inhibitory mechanisms may maintain or escalate the disease. Interventions that modify epigenetic effects on gene expression, alter intestinal dysbiosis, eliminate deleterious environmental factors, and target critical pathogenic mechanisms are therapeutic possibilities that might reduce risk, individualize management, and improve outcome. In conclusion, diverse pathogenic mechanisms have been implicated in autoimmune hepatitis, and they may identify a critical factor or sequence that can be validated and used to direct future management and preventive strategies.

Industrial, Biocide, and Cosmetic Chemical Inducers of Cholestasis

A frequent side effect of many drugs includes the occurrence of cholestatic liver toxicity. Over the past couple of decades, drug-induced cholestasis has gained considerable attention, resulting in a plethora of data regarding its prevalence and mechanistic basis. Likewise, several food additives and dietary supplements have been reported to cause cholestatic liver insults in the past few years. The induction of cholestatic hepatotoxicity by other types of chemicals, in particular synthetic compounds, such as industrial chemicals, biocides, and cosmetic ingredients, has been much less documented. Such information can be found in occasional clinical case reports of accidental intake or suicide attempts as well as in basic and translational study reports on mechanisms or testing of new therapeutics in cholestatic animal models. This paper focuses on such nonpharmaceutical and nondietary synthetic chemical inducers of cholestatic liver injury, in particular alpha-naphthylisocyanate, 3,5-diethoxycarbonyl-1,4-dihydrocollidine, methylenedianiline, paraquat, tartrazine, triclosan, 2-octynoic acid, and 2-nonynoic acid. Most of these cholestatic compounds act by similar mechanisms. This could open perspectives for the prediction of cholestatic potential of chemicals.

Preclinical insights into cholangiopathies: disease modeling and emerging therapeutic targets

INTRODUCTION

The common predominant clinical features of cholangiopathies such as primary sclerosing cholangitis (PSC), primary biliary cholangitis (PBC), and biliary atresia (BA) are biliary damage/senescence and liver fibrosis. Curative therapies are lacking, and liver transplantation is the only option. An understanding of the mechanisms and pathogenesis is needed to develop novel therapies. Previous studies have developed various disease-based research models and have identified candidate therapeutic targets. Areas covered: This review summarizes recent studies performed in preclinical models of cholangiopathies and the current understanding of the pathophysiology representing potential targets for novel therapies. A literature search was conducted in PubMed using the combination of the searched term 'cholangiopathies' with one or two keywords including 'model', 'cholangiocyte', 'animal', or 'fibrosis'. Papers published within five years were obtained. Expert opinion: Access to appropriate research models is a key challenge in cholangiopathy research; establishing more appropriate models for PBC is an important goal. Several preclinical studies have demonstrated promising results and have led to novel therapeutic approaches, especially for PSC. Further studies on the pathophysiology of PBC and BA are necessary to identify candidate targets. Innovative therapeutic approaches such as stem cell transplantation have been introduced, and those therapies could be applied to PSC, PBC, and BA.

Diagnosis and Management of Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is a chronic, cholestatic, autoimmune disease with a variable progressive course. PBC can cause debilitating symptoms including fatigue and pruritus and, if left untreated, is associated with a high risk of cirrhosis and related complications, liver failure, and death. Recent changes to the PBC landscape include a name change, updated guidelines for diagnosis and treatment as well as new treatment options that have recently become available. Practicing clinicians face many unanswered questions when managing PBC. To assist these healthcare providers in managing patients with PBC, the American College of Gastroenterology (ACG) Institute for Clinical Research & Education, in collaboration with the Chronic Liver Disease Foundation (CLDF), organized a panel of experts to evaluate and summarize the most current and relevant peer-reviewed literature regarding PBC. This, combined with the extensive experience and clinical expertise of this expert panel, led to the formation of this clinical guidance on the diagnosis and management of PBC.

Identification of a xenobiotic as a potential environmental trigger in primary biliary cholangitis

BACKGROUND & AIMS

Primary biliary cholangitis (PBC) is an autoimmune-associated chronic liver disease triggered by environmental factors, such as exposure to xenobiotics, which leads to a loss of tolerance to the lipoic acid-conjugated regions of the mitochondrial pyruvate dehydrogenase complex, typically to the E2 component. We aimed to identify xenobiotics that might be involved in the environmental triggering of PBC.

METHODS

Urban landfill and control soil samples from a region with high PBC incidence were screened for xenobiotic activities using analytical, cell-based xenobiotic receptor activation assays and toxicity screens.

RESULTS

A variety of potential xenobiotic classes were ubiquitously present, as identified by their interaction with xenobiotic receptors - aryl hydrocarbon receptor, androgen receptor and peroxisome proliferator activated receptor alpha - in cell-based screens. In contrast, xenoestrogens were present at higher levels in soil extracts from around an urban landfill. Furthermore, two landfill sampling sites contained a chemical(s) that inhibited mitochondrial oxidative phosphorylation and induced the apoptosis of a hepatic progenitor cell. The mitochondrial effect was also demonstrated in human liver cholangiocytes from three separate donors. The chemical was identified as the ionic liquid [3-methyl-1-octyl-1H-imidazol-3-ium]+ (M8OI) and the toxic effects were recapitulated using authentic pure chemical. A carboxylate-containing human hepatocyte metabolite of M8OI, bearing structural similarity to lipoic acid, was also enzymatically incorporated into the E2 component of the pyruvate dehydrogenase complex via the exogenous lipoylation pathway in vitro.

CONCLUSIONS

These results identify, for the first time, a xenobiotic in the environment that may be related to and/or be a component of an environmental trigger for PBC. Therefore, further study in experimental animal models is warranted, to determine the risk of exposure to these ionic liquids.

LAY SUMMARY

Primary biliary cholangitis is a liver disease in which most patients have antibodies to mitochondrial proteins containing lipoic acid binding site(s). This paper identified a man-made chemical present in soils around a waste site. It was then shown that this chemical was metabolized into a product with structural similarity to lipoic acid, which was capable of replacing lipoic acid in mitochondrial proteins.

Pathophysiology of primary biliary cholangitis

Primary biliary cholangitis is a prototypical autoimmune disease characterized by an overwhelming female predominance, a distinct clinical phenotype, and disease specific anti-mitochondrial antibodies targeted against a well-defined auto-antigen. In a genetically susceptible host, multi-lineage loss of tolerance to the E2 component of the 2-oxo-dehydrogenase pathway and dysregulated immune pathways directed at biliary epithelial cells leads to cholestasis, progressive biliary fibrosis, and cirrhosis in a subset of patients. Several key insights have shed light on the complex pathogenesis of disease. First, characteristic anti-mitochondrial antibodies (AMAs) target lipoic acid containing immunodominant epitopes, particularly pyruvate dehydrogenase complex (PDC-E2), on the inner mitochondrial membrane of BECs. Next, breakdown of the protective apical bicarbonate rich umbrella may sensitize BECs to aberrant apoptotic pathways leaving the antigenic PDC-E2 epitope immunologically tact within an apoptotic bleb. A multi-lineage immune response ensues characterized by an imbalance between effector and regulatory activity resulting in progressive and self-perpetuating biliary injury. Genome wide studies shed light on important pathways involved in disease, key among them being IL-12. Epigenetic mechanisms and microRNAs may play help shed light on the missing heritability and female preponderance of disease. Taken together, these findings have dramatically advanced our understanding of disease and may lead to important therapeutic advances.

Epigenetics in the Primary Biliary Cholangitis and Primary Sclerosing Cholangitis

Epigenomics, the study of modifications to genetic material that do not alter the underlying DNA sequence, is generating increasing interest as a means to help clarify disease pathogenesis and outcomes. Although genome-wide association studies have identified several potential candidate genes that may be implicated in primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), it is estimated that these genes explain less than 20% of the heritability of these diseases. Thus, to date, the origins of “missing heritability” for PBC and PSC remain elusive. The epigenome may provide a potentially elegant solution to this phenomenon, as it can be modified by both internal and external exposures (coined the “exposome”). This may explain differences in disease presentation, treatment response, and rates of progression between individuals. Epigenetic changes may also provide a framework for discovering potential biomarkers for diagnosis and screening of PBC and PSC. Importantly, because the epigenome is modifiable, it may also highlight novel pathways for therapeutic discovery and interventions of these diseases.

The induction of autoimmune hepatitis in the human leucocyte antigen-DR4 non-obese diabetic mice autoimmune hepatitis mouse model

Autoimmune hepatitis (AIH) is a chronic liver disease characterized by progressive inflammation, female preponderance and seropositivity for autoantibodies such as anti-smooth muscle actin and/or anti-nuclear, anti-liver kidney microsomal type 1 (anti-LKM1) and anti-liver cytosol type 1 (anti-LC1) in more than 80% of cases. AIH is linked strongly to several major histocompatibility complex (MHC) alleles, including human leucocyte antigen (HLA)-DR3, -DR7 and -DR13. HLA-DR4 has the second strongest association with adult AIH, after HLA-DR3. We investigated the role of HLA-DR4 in the development of AIH by immunization of HLA-DR4 (DR4) transgenic non-obese diabetic (NOD) mice with DNA coding for human CYP2D6/FTCD fusion autoantigen. Immunization of DR4 mice leads to sustained mild liver injury, as assessed biochemically by elevated alanine aminotransferase, histologically by interface hepatitis, plasma cell infiltration and mild fibrosis and immunologically by the development of anti-LKM1/anti-LC1 antibodies. In addition, livers from DR4 mice had fewer regulatory T cells (T_regs ), which had decreased programmed death (PD)-1 expression. Splenic T_regs from these mice also showed impaired inhibitory capacity. Furthermore, DR4 expression enhanced the activation status of CD8⁺ T cells, macrophages and dendritic cells in naive DR4 mice compared to naive wild-type (WT) NOD mice. Our results demonstrate that HLA-DR4 is a susceptibility factor for the development of AIH. Impaired suppressive function of T_regs and reduced PD-1 expression may result in spontaneous activation of key immune cell subsets, such as antigen-presenting cells and CD8⁺ T effectors, facilitating the induction of AIH and persistent liver damage.

Endogenous interleukin-22 protects against inflammatory bowel disease but not autoimmune cholangitis in dominant negative form of transforming growth factor beta receptor type II mice

During chronic inflammation, interleukin (IL)-22 expression is up-regulated in both CD4 and CD8 T cells, exerting a protective role in infections. However, in autoimmunity, IL-22 appears to have either a protective or a pathogenic role in a variety of murine models of autoimmunity and, by extrapolation, in humans. It is not clear whether IL-22 itself mediates inflammation or is a by-product of inflammation. We have taken advantage of the dominant negative form of transforming growth factor beta receptor type II (dnTGF-βRII) mice that develop both inflammatory bowel disease and autoimmune cholangitis and studied the role and the biological function of IL-22 by generating IL-22(-/-) dnTGF-βRII mice. Our data suggest that the influence of IL-22 on autoimmunity is determined in part by the local microenvironment. In particular, IL-22 deficiency exacerbates tissue injury in inflammatory bowel disease, but has no influence on either the hepatocytes or cholangiocytes in the same model. These data take on particular significance in the previously defined effects of IL-17A, IL-12p40 and IL-23p19 deficiency and emphasize that, in colitis, there is a dominant role of IL-23/T helper type 17 (Th17) signalling. Furthermore, the levels of IL-22 are IL-23-dependent. The use of cytokine therapy in patients with autoimmune disease has significant potential, but must take into account the overlapping and often promiscuous effects that can theoretically exacerbate inflammation.

A contemporary perspective on the molecular characteristics of mitochondrial autoantigens and diagnosis in primary biliary cholangitis

Primary biliary cholangitis (PBC) is an autoimmune hepatobiliary disease characterized by immune mediated destruction of the intrahepatic small bile ducts and the presence of antimitochondrial antibodies (AMAs). The mitochondrial autoantigens have been identified as the E2 subunits of the 2-oxo-acid dehydrogenase complex, including the E2 subunits of pyruvate dehydrogenase, branched-chain 2-oxo acid dehydrogenase complex, oxoglutarate dehydrogenase complex, E3 binding protein and PDC E1 alpha subunit. The AMA epitope is mapped within the E2 lipoic acid binding domain, which is particularly important for oxidative phosphorylation. In addition, lipoic acid, which serves as a swinging arm to capture electrons, is particularly susceptible to an electrophilic attack and may provide clues to the etiology of PBC. This review emphasizes the molecular characteristics of AMAs, including detection, immunochemistry and the putative role in disease. These data have significance not only specifically for PBC, but generically for autoimmunity.

Adaptive immunity in the liver

The anatomical architecture of the human liver and the diversity of its immune components endow the liver with its physiological function of immune competence. Adaptive immunity is a major arm of the immune system that is organized in a highly specialized and systematic manner, thus providing long-lasting protection with immunological memory. Adaptive immunity consists of humoral immunity and cellular immunity. Cellular immunity is known to have a crucial role in controlling infection, cancer and autoimmune disorders in the liver. In this article, we will focus on hepatic virus infections, hepatocellular carcinoma and autoimmune disorders as examples to illustrate the current understanding of the contribution of T cells to cellular immunity in these maladies. Cellular immune suppression is primarily responsible for chronic viral infections and cancer. However, an uncontrolled auto-reactive immune response accounts for autoimmunity. Consequently, these immune abnormalities are ascribed to the quantitative and functional changes in adaptive immune cells and their subsets, innate immunocytes, chemokines, cytokines and various surface receptors on immune cells. A greater understanding of the complex orchestration of the hepatic adaptive immune regulators during homeostasis and immune competence are much needed to identify relevant targets for clinical intervention to treat immunological disorders in the liver.

It is time to change primary biliary cirrhosis (PBC): New nomenclature from "cirrhosis" to "cholangitis", and upcoming treatment based on unveiling pathology

Primary biliary cirrhosis (PBC) is a chronic, organ-specific, autoimmune liver disease characterized by progressive cholestasis, eventually leading to cirrhosis. Several lines of evidence have revealed a crucial role of adaptive as well as innate immune responses in the etiopathogenesis of PBC, and more recently, the biology of bile duct cells and genome-wide association studies (GWAS) demonstrated several key molecules and pathways in this enigmatic disease. Although ursodeoxycholic acid (UDCA) has been the only approved drug for PBC with clinical evidences for improvement of long-term outcomes, a substantial population have suboptimal responses to UDCA, resulting in unfavorable outcomes. In this regard, second-line treatment for patients refractory to UDCA is strongly awaited. In Japan, bezafibrate (BF) has been frequently used for this purpose, yet recent clinical trials failed to clearly demonstrate clinical efficacy of BF. Novel pharmacotherapies targeted to key molecules and pathways in PBC are upcoming. Finally, we sincerely call on all members of the Japan Society of Hepatology to use from this moment on the name "primary biliary cholangitis" for the disease known by its abbreviation PBC, in keeping with a very recent global agreement.

Xenobiotics and loss of tolerance in primary biliary cholangitis

Data from genome wide association studies and geoepidemiological studies established that a combination of genetic predisposition and environmental stimulation is required for the loss of tolerance in primary biliary cholangitis (PBC). The serologic hallmark of PBC are the presence of high titer anti-mitochondrial autoantibodies (AMA) that recognize the lipoyl domain of the mitochondrial pyruvate dehydrogenase E2 (PDC-E2) subunit. Extensive efforts have been directed to investigate the molecular basis of AMA. Recently, experimental data has pointed to the thesis that the breaking of tolerance to PDC-E2 is a pivotal event in the initial etiology of PBC, including environmental xenobiotics including those commonly found in cosmetics and food additives, suggesting that chemical modification of the PDC-E2 epitope may render its vulnerable to become a neo-antigen and trigger an immune response in genetically susceptible hosts. Here, we will discuss the natural history, genetics and immunobiology of PBC and structural constraints of PDC-E2 in AMA recognition which makes it vulnerable to chemical modification.

Animal models of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is characterized histologically by the presence of chronic non-suppurative destructive cholangitis of the small interlobular bile duct, leading to chronic progressive cholestasis. Most PBC patients are asymptomatic and have a reasonable prognosis, but a few develop esophageal varices or jaundice, rapidly leading to liver failure within a short period. As multiple factors appear to be involved in the onset of PBC, its clinical course may be complicated. Therefore, the use of an animal model would be valuable for clarifying the pathogenesis of PBC. Here, we review recent data of selected PBC models, particularly spontaneous models, xenobiotic immunized models, and infection-triggered models. There are a number of spontaneous models: the NOD.c3c4, dominant-negative TGF-β receptor II, IL-2Rα-/-, Scurfy, and Ae2a,b-/- mice. These animal models manifest distinct clinical and immunological features similar, but also often different, from those of human PBC. It is clear that a combination of genetic predisposition, environmental factors, and immunological dysfunction contribute to the pathogenesis of PBC. The diverse clinical course and complexity of the immunological mechanisms of PBC cannot be fully recapitulated solely any single animal model. The challenge remains to develop a progressive PBC disease model that exhibits fibrosis, and ultimately hepatic failure.

Genome-Wide Association Studies in Primary Biliary Cirrhosis

Genome-wide association studies (GWASs) have been a significant technological advance in our ability to evaluate the genetic architecture of complex diseases such as primary biliary cirrhosis (PBC). To date, six large-scale studies have been performed that have identified 27 risk loci in addition to human leukocyte antigen (HLA) associated with PBC. The identified risk variants emphasize important disease concepts; namely, that disturbances in immunoregulatory pathways are important in the pathogenesis of PBC and that such perturbations are shared among a diverse number of autoimmune diseases-suggesting the risk architecture may confer a generalized propensity to autoimmunity not necessarily specific to PBC. Furthermore, the impact of non-HLA risk variants, particularly in genes involved with interleukin-12 signaling, and ethnic variation in conferring susceptibility to PBC have been highlighted. Although GWASs have been a critical stepping stone in understanding common genetic variation contributing to PBC, limitations pertaining to power, sample availability, and strong linkage disequilibrium across genes have left us with an incomplete understanding of the genetic underpinnings of disease pathogenesis. Future efforts to gain insight into this missing heritability, the genetic variation that contributes to important disease outcomes, and the functional consequences of associated variants will be critical if practical clinical translation is to be realized.

The immunogenetics of primary biliary cirrhosis: A comprehensive review

Primary biliary cirrhosis (PBC), a classic autoimmune liver disease, is characterised by a progressive T cell predominant lymphocytic cholangitis, and a serologic pattern of reactivity in the form of specific anti-mitochondrial antibodies (AMA). CD4+ T cells are particularly implicated by PBC's cytokine signature, the presence of CD4+ T cells specific to mitochondrial auto-antigens, the expression of MHC II on injured biliary epithelial cells, and PBC's coincidence with other similar T cell mediated autoimmune conditions. CD4+ T cells are also central to current animal models of PBC, and their transfer typically also transfers disease. The importance of genetic risk to developing PBC is evidenced by a much higher concordance rate in monozygotic than dizygotic twins, increased AMA rates in asymptomatic relatives, and disproportionate rates of disease in siblings of PBC patients, PBC family members and certain genetically defined populations. Recently, high-throughput genetic studies have greatly expanded our understanding of the gene variants underpinning risk for PBC development, so linking genetics and immunology. Here we summarize genetic association data that has emerged from large scale genome-wide association studies and discuss the evidence for the potential functional significance of the individual genes and pathways identified; we particularly highlight associations in the IL-12-STAT4-Th1 pathway. HLA associations and epigenetic effects are specifically considered and individual variants are linked to clinical phenotypes where data exist. We also consider why there is a gap between calculated genetic risk and clinical data: so-called missing heritability, and how immunogenetic observations are being translated to novel therapies. Ultimately whilst genetic risk factors will only account for a proportion of disease risk, ongoing efforts to refine associations and understand biologic links to disease pathways are hoped to drive more rational therapy for patients.

Primary biliary cirrhosis: From bench to bedside

Primary biliary cirrhosis (PBC) is a chronic non-suppurative destructive intrahepatic cholangitis leading to cirrhosis after a protractive non cirrhotic stage. The etiology and pathogenesis are largely unknown and autoimmne mechanisms have been implicated to explain the pathological lesions. Many epitopes and autoantigens have been reported as crucial in the pathophysiology of the disease and T and B cells abnormalities have been described, the exact pathways leading to the destruction of small intrahepatic ductules are mostly speculative. In this review we examined the various epidemiologal and geoepidemiological data as well as the complex pathogenetic aspects of this disease, focusing on recent in vivo and in vitro studies in this field. Initiation and progression of PBC is believed to be a multifactorial process with strong infuences from the patient’s genetic background and by various environmental factors. The role of innate and adaptive immunity, including cytokines, chemokines, macrophages and the involvement of apoptosis and reactive oxygen species are outlined in detailed. The current pathogenetic aspects are presented and a novel pathogenetic theory unifying the accumulated clinical information with in vitro and in vivo data is formulated. A review of clinical manifestations and immunological and pathological diagnosis was presented. Treatment modalities, including the multiple mechanisms of action of ursodeoxycholate were finally discussed.

Environmental factors in primary biliary cirrhosis

The etiology of the autoimmune liver disease primary biliary cirrhosis (PBC) remains largely unresolved, owing in large part to the complexity of interaction between environmental and genetic contributors underlying disease development. Observations of disease clustering, differences in geographical prevalence, and seasonality of diagnosis rates suggest the environmental component to PBC is strong, and epidemiological studies have consistently found cigarette smoking and history of urinary tract infection to be associated with PBC. Current evidence implicates molecular mimicry as a primary mechanism driving loss of tolerance and subsequent autoimmunity in PBC, yet other environmentally influenced disease processes are likely to be involved in pathogenesis. In this review, the authors provide an overview of current findings and touch on potential mechanisms behind the environmental component of PBC.

Autoimmune features in metabolic liver disease: a single-center experience and review of the literature

Non-alcoholic steatohepatitis (NASH) is the progressive phenotype of non-alcoholic fatty liver disease associated with the metabolic syndrome. The existence of autoimmune features in NASH has been reported, but its significance remains unclear. We herein report the autoantibody profile of 54 patients with histologically proven NASH and further determined the development of autoimmunity in three different murine NASH models (monosodium glutamate, CDAA (choline-deficient L-amino acid-defined), and TSOD (Tsumura Suzuki, Obese Diabetes)) at 48 weeks of age. Forty-eight percent (26/54) of NASH cases were positive for antinuclear (ANA) or antimitochondrial antibody and manifested histological signs of overlap with autoimmune hepatitis and primary biliary cirrhosis, respectively. These patients were significantly older (60 ± 10 versus 50 ± 16 years), more frequently women (81 % versus 43 %), and with more severe portal inflammatory infiltrate compared with patients without autoimmunity. In one third of mice, regardless of the model, we observed a marked lymphoid infiltrate with non-suppurative cholangitis, and several cases were ANA-positive, but none AMA-positive. Our data suggest that autoimmunity may share some pathogenetic traits with the chronic inflammation of NASH, possibly related to advanced age.

E. coli Induced Experimental Model of Primary Biliary Cirrhosis: At Last

Recurrent urinary tract infections (UTI) have been considered potential triggers of primary biliary cirrhosis (PBC), an autoimmune cholestatic liver disease characterised by progressive destruction of intrahepatic bile ducts. Additional support for the link made between PBC and UTI was based on early observations of recurrent episodes of bacteriuria in female patients with PBC. A series of large epidemiological studies demonstrated a strong correlation between recurrent UTI and PBC, initiating a series of studies investigating the role of Escherichia coli (E. coli, the most prevalent organism isolated in women with UTI) as a trigger of PBC. Immunological evidence of B- and T-cell cross-reactive responses implicating PBC-specific autoantigens and E. coli mimics have been clearly demonstrated, adding support to the notion that E. coli is a potential infectious inducer of PBC in susceptible individuals. One of the major limitations in proving the E. coli/PBC association was the lack of reliable E. coli-infected animal models of PBC. This review provides an overview of the evidence linking this infectious agent with PBC and discusses the pros and cons of a recently developed E. coli-infected animal model of PBC.

Liver immunology

The liver is the largest organ in the body and is generally regarded by nonimmunologists as having little or no lymphoid function. However, such is far from accurate. This review highlights the importance of the liver as a lymphoid organ. Firstly, we discuss experimental data surrounding the role of liver as a lymphoid organ. The liver facilitates tolerance rather than immunoreactivity, which protects the host from antigenic overload of dietary components and drugs derived from the gut and it is instrumental to fetal immune tolerance. Loss of liver tolerance leads to autoaggressive phenomena, which if not controlled by regulatory lymphoid populations, may lead to the induction of autoimmune liver diseases. Liver-related lymphoid subpopulations also act as critical antigen-presenting cells. The study of the immunological properties of liver and delineation of the microenvironment of the intrahepatic milieu in normal and diseased livers provides a platform to understand the hierarchy of a series of detrimental events that lead to immune-mediated destruction of the liver and the rejection of liver allografts. The majority of emphasis within this review will be on the normal mononuclear cell composition of the liver. However, within this context, we will discuss selected, but not all, immune-mediated liver disease and attempt to place these data in the context of human autoimmunity.

Chronic cholestatic liver diseases: clues from histopathology for pathogenesis

Chronic cholestatic liver diseases include fibrosing cholangiopathies such as primary biliary cirrhosis or primary sclerosing cholangitis. These and related cholangiopathies clearly display pathologies associated with (auto)immunologic processes. As the cholangiocyte's apical membrane is exposed to the toxic actions of the bile fluid, the interaction of bile with cholangiocytes and the biliary tree in general must be considered to completely understand the pathogenesis of cholangiopathies. While the molecular processes involved in the hepatocellular formation of bile are well understood in both normal and pathophysiologic conditions, those in the bile ducts of normal liver and in livers with cholangiopathies lag behind. This survey highlights key mechanisms known to date that are important for the formation of bile by hepatocytes and its modification by the biliary tree. It also delineates the clinical pathophysiologic findings for cholangiopathies and puts them in perspective with current experimental models to reveal the pathogenesis of cholangiopathies and develop novel therapeutic approaches.

The genetics of complex cholestatic disorders

Cholestatic liver diseases are caused by a range of hepatobiliary insults and involve complex interactions among environmental and genetic factors. Little is known about the pathogenic mechanisms of specific cholestatic diseases, which has limited our ability to manage patients with these disorders. However, recent genome-wide studies have provided insight into the pathogenesis of gallstones, primary biliary cirrhosis, and primary sclerosing cholangitis. A lithogenic variant in the gene that encodes the hepatobiliary transporter ABCG8 has been identified as a risk factor for gallstone disease; this variant has been associated with altered cholesterol excretion and metabolism. Other variants of genes encoding transporters that affect the composition of bile have been associated with cholestasis, namely ABCB11, which encodes the bile salt export pump, and ABCB4, which encodes hepatocanalicular phosphatidylcholine floppase. In contrast, studies have associated primary biliary cirrhosis and primary sclerosing cholangitis with genes encoding major histocompatibility complex proteins and identified loci associated with microbial sensing and immune regulatory pathways outside this region, such as genes encoding IL12, STAT4, IRF5, IL2 and its receptor (IL2R), CD28, and CD80. These discoveries have raised interest in the development of reagents that target these gene products. We review recent findings from genetic studies of patients with cholestatic liver disease. Future characterization of genetic variants in animal models, stratification of risk alleles by clinical course, and identification of interacting environmental factors will increase our understanding of these complex cholestatic diseases.

The immunobiology and pathophysiology of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an autoimmune disease characterized by clinical homogeneity among patients, an overwhelming female predominance, production of a multilineage immune response to mitochondrial autoantigens, inflammation of small bile ducts, and in some patients the development of fibrosis and cirrhosis. The targets in this disease are small bile ducts, and the prototypic serologic response includes antimitochondrial antibodies (AMAs). Several key observations have greatly advanced our understanding of PBC. First, the multilineage immune response, including AMAs, is directed at the E2 component of the 2-oxo-dehydrogenase pathway, particularly PDC-E2. Second, such autoantibodies may be identified years before the clinical diagnosis of disease. Third, the autoreactive T cell precursor frequency for both CD4 and CD8 cells is significantly higher in liver and regional lymph node than in blood, so the multilineage antimitochondrial response may be required for the development of this disease. Fourth, the apotope of biliary cells contains intact PDC-E2; this apotope, in a setting that includes granulocyte macrophage colony-stimulating factor-stimulated macrophages and AMAs, produces an intense proinflammatory response. Fifth, several mouse models of PBC highlight the importance of loss of tolerance to PDC-E2 as well as a critical role for the interleukin (IL)-12 signaling pathway. Finally, genome-wide association studies suggest an important role for the IL-12 pathway in disease susceptibility. Taken together, these findings have resulted in a better understanding of the mechanism for selective biliary cell destruction and have also suggested unique pathways for therapeutic intervention.

Animal models of chronic liver diseases

Chronic liver diseases are frequent and potentially life threatening for humans. The underlying etiologies are diverse, ranging from viral infections, autoimmune disorders, and intoxications (including alcohol abuse) to imbalanced diets. Although at early stages of disease the liver regenerates in the absence of the insult, advanced stages cannot be healed and may require organ transplantation. A better understanding of underlying mechanisms is mandatory for the design of new drugs to be used in clinic. Therefore, rodent models are being developed to mimic human liver disease. However, no model to date can completely recapitulate the "corresponding" human disorder. Limiting factors are the time frame required in humans to establish a certain liver disease and the fact that rodents possess a distinct immune system compared with humans and have different metabolic rates affecting liver homeostasis. These features account for the difficulties in developing adequate rodent models for studying disease progression and for testing new pharmaceuticals to be translated into the clinic. Nevertheless, traditional and new promising animal models that mimic certain attributes of chronic liver diseases are established and being used to deepen our understanding in the underlying mechanisms of distinct liver diseases. This review aims at providing a comprehensive overview of recent advances in animal models recapitulating different features and etiologies of human liver diseases.

Therapeutic effect of cytotoxic T lymphocyte antigen 4/immunoglobulin on a murine model of primary biliary cirrhosis

Collectively, the data in both humans and murine models of human primary biliary cirrhosis (PBC) suggest that activated T cells, particularly CD8 T cells, play a critical role in biliary cell destruction. Under physiological conditions, T-cell activation involves two critical signals that involve the major histocompatibility complex and a set of costimulatory molecules, which include a receptor on T cells termed cytotoxic T lymphocyte antigen 4 (CTLA-4). Germane to the studies reported herein, signaling by CTLA-4 has the potential to modulate costimulation and induce inhibitory signals. In this study, we have taken advantage of our well-defined murine model of PBC, in which mice are immunized with 2-octynoic acid coupled to bovine serum albumin (2OA-BSA), leading to the production of high-titer antimitochondrial autoantibodies (AMAs) and portal cellular infiltrates. To investigate the potential of CTLA-4-Ig (immunoglobulin) as an immunotherapeutic agent, we treated mice both before and after induction of autoimmune cholangitis. First, we demonstrate that CTLA-4-Ig treatment, begun 1 day before 2OA-BSA immunization, completely inhibits the manifestations of cholangitis, including AMA production, intrahepatic T-cell infiltrates, and bile duct damage. However, and more critically, treatment with CTLA-4-Ig, initiated after the development of autoimmune cholangitis in previously immunized mice, also resulted in significant therapeutic benefit, including reduced intrahepatic T-cell infiltrates and biliary cell damage, although AMA levels were not altered.

CONCLUSION

These data suggest that an optimized regimen with CTLA-4-Ig has the potential to serve as an investigative therapeutic tool in patients with PBC.

Primary biliary cirrhosis: a multi-faced interactive disease involving genetics, environment and the immune response

Primary biliary cirrhosis (PBC) is considered a model autoimmune disease based on several features, including the presence of a highly directed and very specific immune response to mitochondrial autoantigens, a female predominance, a targeted destruction of the biliary epithelium, and homogeneity between patients. It is essentially a chronic progressive cholestatic liver disease characterized by immune-mediated destruction of small- and medium-sized intrahepatic bile ducts. There is considerable variation in the incidence and prevalence of the disease between regions of the world, although such differences likely reflect not only a true disparity in disease but also differences in awareness; for example, in the United States, PBC is often detected in an asymptomatic stage based on multi-phasic clinical testing. There has been considerable progress at defining the immune response in this disease, including quantitation of autoreactive T cells against PDC-E2, the major mitochondrial autoantigen. The overwhelming data suggests that patients develop PBC based on a genetic predisposition and loss of tolerance to one or more environmental agents. In this review, we will present an updated overview of PBC and place it in the context of autoimmunity.

Etiopathogenesis of primary biliary cirrhosis: an overview of recent developments

Substantial advancements in the field of primary biliary cirrhosis (PBC) research have broadened our understanding of this enigmatic disease. Genome-wide studies have identified several new candidate genes involved in the immunoregulatory process, particularly those responsible for antigen presentation and lymphocyte signaling. Examples include the HLA class-II region and genes implicated in IL12-JAK/STAT signaling, and the NF-κB and TNF signaling pathways. Environmental triggers appear to disrupt the pre-existing, unstable immune tolerance in genetically susceptible individuals, and molecular mimics of the PBC-specific autoantigen (PDC) may be derived from microbes or xenobiotic compounds, which modify native proteins, making them immunogenic. Although the vast majority of patients with PBC are AMA-positive, a variety of disease-specific antinuclear antibodies have been recognized in conferring a worse clinical outcome. There has also been a revived interest in the role of antibody-secreting B cells in murine models suggesting that depletion of these cells paradoxically exacerbates cholangiopathy. Biliary specificity in PBC is most likely driven by the uniqueness of cholangiocyte apoptosis in which the PDC-E2 autoantigen undergoes differential glutathiolation. Cholangiocytes also possess the ability to phagocytose neighboring apoptotic cells, present intact immunoreactive antigen, and undergo attack from autoantibodies, the innate immune system, and autoreactive lymphocytes. Cellular senescence and a lack of functioning T-regulatory cells are proposed mechanisms by which this multi-lineage process is thought to be enhanced. This review summarizes these key advances as the true complexities of the disease process begin to be unraveled.

Pathological features of new animal models for primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by immune mediated biliary damage and frequent appearance of autoantibodies against mitochondrial enzymes. There is almost no useful animal model that is globally recognized and routinely used, however, several unique animal models manifested the characteristic clinical and pathological features of human PBC within the last 5 years. Herein, we compare the pathological features of previously reported and newly introduced novel animal models of PBC. Knowledge and understanding of the strengths and the limitations of each animal model have led to the development of promising therapies and novel tools to characterize these clinical conditions. Moreover, suitability of the model for the intended purpose should be confirmed by further research and analysis.

The multi-hit hypothesis of primary biliary cirrhosis: polyinosinic-polycytidylic acid (poly I:C) and murine autoimmune cholangitis

A void in understanding primary biliary cirrhosis (PBC) is the absence of appropriate animal models. Our laboratory has studied a murine model of autoimmune cholangitis induced following immunization with 2-octynoic acid (2OA), an antigen identified following extensive quantitative structural activity relationship (QSAR) analysis, using human autoantibodies and three-dimensional analysis of the mitochondrial autoantigen, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2). Mice immunized with 2OA coupled to bovine serum albumin (BSA) develop anti-mitochondrial antibodies (AMAs) of the identical specificity as humans with PBC, and in addition develop inflammatory portal cell infiltrates in liver. However, the natural history of disease is less severe than in humans and does not include fibrosis. Data from human and autoimmune murine models suggest that environmental and/or infectious agents can exacerbate autoimmune reactions, and a model of PBC has been described in which polyinosinic-polycytidylic acid (poly I:C), a viral RNA mimetic and Toll-like receptor 3 (TLR-3) agonist induces low-titre AMAs and in mild portal infiltrates. We took advantage of our established model to determine whether immunization with 2OA-BSA coupled with poly I:C alters the disease process. Indeed, the addition of poly I:C produces a profound exacerbation of autoimmune cholangitis, including a significant increase in CD8(+) infiltrating T cells, as well as a marked increase of proinflammatory cytokines. In addition, mice have evidence of fibrosis. These findings lend support to the concept that besides breakdown of self-tolerance, there is a requirement of a second 'hit' during the breakdown process that leads to disease which more faithfully mimics human PBC.

Electrophile-modified lipoic derivatives of PDC-E2 elicits anti-mitochondrial antibody reactivity

Our laboratory has hypothesized that xenobiotic modification of the native lipoyl moiety of the major mitochondrial autoantigen, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), may lead to loss of self-tolerance in primary biliary cirrhosis (PBC). This thesis is based on the finding of readily detectable levels of immunoreactivity of PBC sera against extensive panels of protein microarrays containing mimics of the inner lipoyl domain of PDC-E2 and subsequent quantitative structure-activity relationships (QSARs). Importantly, we have demonstrated that murine immunization with one such mimic, 2-octynoic acid coupled to bovine serum albumin (BSA), induces anti-mitochondrial antibodies (AMAs) and cholangitis. Based upon these data, we have focused on covalent modifications of the lipoic acid disulfide ring and subsequent analysis of such xenobiotics coupled to a 15mer of PDC-E2 for immunoreactivity against a broad panel of sera from patients with PBC and controls. Our results demonstrate that AMA-positive PBC sera demonstrate marked reactivity against 6,8-bis(acetylthio)octanoic acid, implying that chemical modification of the lipoyl ring, i.e. disruption of the S-S disulfide, renders lipoic acid to its reduced form that will promote xenobiotic modification. This observation is particularly significant in light of the function of the lipoyl moiety in electron transport of which the catalytic disulfide constantly opens and closes and, thus, raises the intriguing thesis that common electrophilic agents, i.e. acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDs), may lead to xenobiotic modification in genetically susceptible individuals that results in the generation of AMAs and ultimately clinical PBC.

Primary biliary cirrhosis

Primary biliary cirrhosis is a chronic liver disease characterised by intrahepatic bile-duct destruction, cholestasis, and, in some cases, cirrhosis. Evidence supporting the autoimmune nature of this disorder includes the appearance of highly specific antimitochondrial antibodies (AMAs) and autoreactive T cells. Concordance rates in monozygotic twins, familial prevalence, and genetic associations underscore the importance of genetic factors, whereas findings of epidemiological studies and murine models suggest a possible role for exogenous chemicals and infectious agents through molecular mimicry. The incidence of primary biliary cirrhosis has increased over recent decades, possibly attributable to augmented testing of liver biochemistry rather than a rise in disease incidence. AMAs remain the hallmark of diagnosis in most cases and allow detection of asymptomatic patients. Symptomatic individuals usually present with either pruritus or fatigue and, more rarely, with either jaundice or complications of cirrhosis. The prognosis of primary biliary cirrhosis has improved because of early diagnosis and use of ursodeoxycholic acid, the only established medical treatment for this disorder. Although not a cure, treatment can slow disease progression and delay the need for liver transplantation. However, some patients do not respond adequately to ursodeoxycholic acid and might need alternative therapeutic approaches.

B cell depletion therapy exacerbates murine primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is considered a model autoimmune disease due to the clinical homogeneity of patients and the classic hallmark of antimitochondrial antibodies (AMAs). Indeed, the presence of AMAs represents the most highly directed and specific autoantibody in autoimmune diseases. However, the contribution of B cells to the pathogenesis of PBC is unclear. Therefore, although AMAs appear to interact with the biliary cell apotope and contribute to biliary pathology, there is no correlation of disease severity and titer of AMAs. The recent development of well-characterized monoclonal antibodies specific for the B cell populations, anti-CD20 and anti-CD79, and the development of a well-defined xenobiotic-induced model of autoimmune cholangitis prompted us to use these reagents and the model to address the contribution of B cells in the pathogenesis of murine PBC. Prior to the induction of autoimmune cholangitis, mice were treated with either anti-CD20, anti-CD79, or isotype-matched control monoclonal antibody and followed for B cell development, the appearance of AMAs, liver pathology, and cytokine production. Results of the studies reported herein show that the in vivo depletion of B cells using either anti-CD20 or anti-CD79 led to the development of a more severe form of cholangitis than observed in control mice, which is in contrast with results from several other autoimmune models that have documented an important therapeutic role of B cell-specific depletion. Anti-CD20/CD79-treated mice had increased liver T cell infiltrates and higher levels of proinflammatory cytokines.

CONCLUSION

Our results reflect a novel disease-protective role of B cells in PBC and suggest that B cell depletion therapy in humans with PBC should be approached with caution.

Primary biliary cirrhosis and autoimmune hepatitis: apotopes and epitopes

Autoimmune liver diseases (ALDs) represent a wide spectrum of chronic inflammatory diseases that are characterized by an immune-mediated attack against either hepatocytes (in the case of autoimmune hepatitis types 1 and 2, AIH-1, 2) or cholangiocytes (in primary biliary cirrhosis, PBC). PBC is considered a model autoimmune disease due to the homogeneity of patients, the high specificity of antimitochondrial antibodies (AMAs), and the specificity of biliary epithelial cell (BEC) destruction. It ensues from a multi-lineage loss of tolerance to the E2 component of the pyruvate dehydrogenase complex (PDC-E2). One of the major unanswered questions in the pathogenesis of PBC is the specificity of small intrahepatic bile duct attack while PDC-E2 is present in mitochondria of nucleated cells. Recent findings suggest that the apoptosis of BECs may be of considerable importance for understanding PBC, and that they are more than simply an innocent victim of an immune attack. Rather, they attract immune attack by virtue of the unique biochemical mechanisms by which they handle PDC-E2. The role of apoptotic cells in AIH is not well defined, but advances in the study of autoreactive T cells stem mostly from AIH type 2, where the main autoantigen (CYP2D6) is known, enabling the characterization of antigen-specific immune responses. This review article is intended to provide a critical overview of current evidence on tissue specificity in ALDs, as well as the characteristics of the relevant epitopes and apotopes and their biological and clinical significance.

Pathogenesis of cholestatic liver disease and therapeutic approaches

Cholestatic liver disorders are caused by genetic defects, mechanical aberrations, toxins, or dysregulations in the immune system that damage the bile ducts and cause accumulation of bile and liver tissue damage. They have common clinical manifestations and pathogenic features that include the responses of cholangiocytes and hepatocytes to injury. We review the features of bile acid transport, tissue repair and regulation, apoptosis, vascular supply, immune regulation, and cholangiocytes that are associated with cholestatic liver disorders. We now have a greater understanding of the physiology of cholangiocytes at the cellular and molecular levels, as well as genetic factors, repair pathways, and autoimmunity mechanisms involved in the pathogenesis of disease. These discoveries will hopefully lead to new therapeutic approaches for patients with cholestatic liver disease.

The autoimmunity of primary biliary cirrhosis and the clonal selection theory

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease in which an immune-mediated injury targets the small intrahepatic bile ducts. PBC is further characterized by highly specific serum antimitochondrial autoantibodies (AMA) and autoreactive T cells, a striking female predominance, a strong genetic susceptibility, and a plethora of candidate environmental factors to trigger the disease onset. For these reasons PBC appears ideal to represent the developments of the clonal selection theory over the past decades. First, a sufficiently potent autoimmunogenic stimulus in PBC would require the coexistence of numerous pre-existing conditions (mostly genetic, as recently illustrated by genome-wide association studies and animal models) to perpetuate the destruction of the biliary epithelium by the immune system via the persistence of forbidden clones. Second, the proposed modifications of mitochondrial autoantigens caused by infectious agents and/or xenobiotics well illustrate the possibility that peculiar changes in the antigen structure and flexibility may contribute to tolerance breakdown. Third, the unique apoptotic features demonstrated for cholangiocytes are the ideal setting for the development of mitochondrial autoantigen presentation to the immune system through macrophages and AMA thus turning the non traditional mitochondrial antigen into a traditional one. This article will review the current knowledge on PBC etiology and pathogenesis in light of the clonal selection theory developments.

Immune-mediated bile duct injury: The case of primary biliary cirrhosis

Autoimmune cholangitis would be the appropriate name to define the immune-mediated bile duct injury following the breakdown of tolerance to mitochondrial proteins and the appearance of serum autoantibodies and autoreactive T cells. Nevertheless, the condition is universally named primary biliary cirrhosis (PBC). The disease etiology and pathogenesis remain largely unknown despite the proposed lines of evidence. One twin study and numerous epidemiology reports suggest that both a susceptible genetic background and environmental factors determine disease onset while a recent genome-wide association study proposed highly significant associations with several common genetic polymorphisms in subgroups of patients. Specific infectious agents and chemicals may contribute to the disease onset and perpetuation in a genetically susceptible host, possibly through molecular mimicry. Importantly, several murine models have been proposed and include strains in which PBC is genetically determined or induced by immunization with chemicals and bacteria. From a pathogenetic standpoint, new exciting data have demonstrated the unique apoptotic features of bile duct cells that allow the mitochondrial autoantigens to be taken up in their intact form within apoptotic blebs. We are convinced that the application of the most recent molecular techniques will soon provide developments in PBC etiology and pathogenesis with likely implications in diagnostics and therapeutics.

Toxicology of autoimmune diseases

Susceptibility to most autoimmune diseases is dependent on polygenic inheritance, environmental factors, and poorly defined stochastic events. One of the significant challenges facing autoimmune disease research is in identifying the specific events that trigger loss of tolerance and autoimmunity. Although many intrinsic factors, including age, sex, and genetics, contribute to autoimmunity, extrinsic factors such as drugs, chemicals, microbes, or other environmental factors can also act as important initiators. This review explores how certain extrinsic factors, namely, drugs and chemicals, can promote the development of autoimmunity, focusing on a few better characterized agents that, in most instances, have been shown to produce autoimmune manifestations in human populations. Mechanisms of autoimmune disease induction are discussed in terms of research obtained using specific animal models. Although a number of different pathways have been delineated for drug/chemical-induced autoimmunity, some similarities do exist, and a working model is proposed.

Primary biliary cirrhosis: a 2010 update

Primary biliary cirrhosis (PBC) is a chronic inflammatory autoimmune disease that mainly targets the cholangiocytes of the interlobular bile ducts in the liver. The condition primarily affects middle-aged women. Without treatment, PBC generally progresses to cirrhosis and eventually liver failure over a period of 10-20 years. PBC is a rare disease with prevalence of less than 1/2000. PBC is thought to result from a combination of multiple genetic factors and superimposed environmental triggers. The contribution of the genetic predisposition is evidenced by the familial clustering. Several risk factors, including exposure to infectious agents and chemical xenobiotics, have been suggested. Ursodeoxycholic acid (UDCA) is currently the only FDA-approved medical treatment for PBC. When administered at doses of 13-15 mg/kg/day, a majority of patients with PBC have a normal life expectancy without additional therapeutic measures. One out of three patients does not adequately respond to UDCA therapy and may need additional medical therapy and/or liver transplantation. This review summarises current knowledge on the epidemiology, ethiopathogenesis, clinical, and therapeutic aspects of PBC.

Murine models of autoimmune cholangitis

PURPOSE OF REVIEW

Primary biliary cirrhosis (PBC) is a human autoimmune liver disease whose molecular pathogenesis is poorly understood because of the difficulty in accessing human tissue and the absence of appropriate animal models. Recently, several unique murine models of human PBC have been discovered. These models have great potential for illustrating the cause and the cellular events that lead to biliary-specific damage. The purpose of this review is to summarize recent progress in these models.

RECENT FINDINGS

The murine models of autoimmune cholangitis include the transforming growth factor beta receptor II (TGF-betaRII) dominant-negative (dnTGF-betaRII), IL-2 receptor alpha deleted (IL-2Ralpha-/-), scurfy, nonobese diabetic (NOD) c3c4, and Ae2 gene-disrupted (Ae2a,b-/-) mice. Recently, we have also established a successful murine model following the immunization with a chemical mimicry of the lipoyl-lysine residue of the E2 component of PDC-E2.

SUMMARY

These emerging murine models have greatly enabled researchers to address the pathogenesis of human PBC and to elucidate pathogenic factors. These models will ultimately lead to new therapeutic options for human PBC.

Experimental evidence on the immunopathogenesis of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease for which an autoimmune pathogenesis is supported by clinical and experimental data, including the presence of autoantibodies and autoreactive T cells. The etiology remains to be determined, yet data suggest that both a susceptible genetic background and unknown environmental factors determine disease onset. Multiple infectious and chemical candidates have been proposed to trigger the disease in a genetically susceptible host, mostly by molecular mimicry. Most recently, several murine models have been reported, including genetically determined models as well as models induced by immunization with xenobiotics and bacteria.