Autoepitope mapping and reactivity of autoantibodies to the dihydrolipoamide dehydrogenase-binding protein (E3BP) and the glycine cleavage proteins in primary biliary cirrhosis

Clinical performance of AMA-M2, anti-gp210 and anti-sp100 antibody levels in primary biliary cholangitis: When detected by multiplex bead-based flow fluorescent immunoassay

BACKGROUND AND AIM

Primary biliary cholangitis (PBC) is a chronic autoimmune cholangiopathy, characterized by the presence of some autoantibodies in the serum. This study aimed to evaluate the clinical significance of AMA-M2, anti-gp210 and anti-sp100 antibody levels detected by multiplex bead-based flow fluorescent immunoassay (MBFFI) in PBC.

METHODS

This study cohort included 238 PBC patients, 81 autoimmune hepatitis (AIH) patients, 62 systemic lupus erythematosus (SLE) patients, and 118 healthy controls. Serum AMA-M2, anti-gp210 and anti-sp100 antibody were detected by MBFFI and immunoblotting assay (IBT). The relationship between three antibody levels and cirrhosis, liver function, cholestasis markers and therapeutic effect to ursodesoxycholic acid (UDCA) was evaluated in PBC.

RESULTS

MBFFI were presented good coincidence rate (87.39%-95.38%) with IBT. The level of AMA-M2, anti-gp210 and anti-sp100 antibodies in PBC patients were higher than other disease group and healthy controls (p ＜ .01). When compared with the healthy controls group, the AUC of AMA-M2, anti-gp210 and anti-sp100 antibodies were 0.9245, 0.7619, and 0.6789, respectively. In addition, gp210 antibody levels have diagnostic value in patients with liver cirrhosis (AUC: 0.7567). We found that when combine detect these three antibodies, the sensitivity was higher than individually detection. High level of serum anti-gp210 antibody could be related to worse liver function and more severe cholestasis in PBC patients. Moreover, serum antibody levels may decrease or remained flat in patients who responded well to UDCA.

CONCLUSION

The detection of AMA-M2, anti-gp210 and anti-sp100 antibody levels by MBFFI showed good performance in the diagnosis of PBC. Serum anti-gp210 antibody level is related to cirrhosis, poor liver function and severe cholestasis in PBC.

Roles of Dihydrolipoamide Dehydrogenase in Health and Disease

Significance: Dihydrolipoamide dehydrogenase (DLDH) is a flavin-dependent disulfide oxidoreductase. The active form of DLDH is a stable homodimer, and its deficiencies have been linked to numerous metabolic disorders. A better understanding of redox and nonredox features of DLDH may reveal druggable targets for disease interventions or preventions. Recent Advances: In this article, the authors review the different roles of DLDH in selected pathological conditions, including its deficiency in humans, its role in stroke and neuroprotection, skin photoaging, Alzheimer's disease, and DLDH as a nondehydrogenating protein, and construction of genetically modified DLDH animal models for further studying the role of DLDH in specific pathological conditions. DLDH is also vulnerable to oxidative modifications in pathological conditions. Critical Issues: Novel animal models need to be constructed using gene knockdown techniques to investigate the redox- and nonredox roles of DLDH in related metabolic diseases. Specific small-molecule DLDH inhibitors need to be discovered. The relationship between modifications of specific amino acid residues in DLDH and given pathological conditions is an interesting area that remains to be comprehensively evaluated. Future Directions: Cell-specific or tissue-specific knockdown of DLDH creating specific pathological conditions will provide more insights into the mechanisms, whereby DLDH may have therapeutic values under a variety of pathological conditions. Antioxid. Redox Signal. 39, 794-806.

Current understanding of primary biliary cholangitis

Primary biliary cholangitis (PBC) causes chronic and persistent cholestasis in the liver, eventually resulting in cirrhosis and hepatic failure without appropriate treatment. PBC mainly develops in middle-aged women, but it is also common in young women and men. PBC is considered a model of autoimmune disease because of the presence of disease-specific autoantibodies, that is, antimitochondrial antibodies (AMAs), intense infiltration of mononuclear cells into the bile ducts, and a high prevalence of autoimmune diseases such as comorbidities. Histologically, PBC is characterized by degeneration and necrosis of intrahepatic biliary epithelial cells surrounded by a dense infiltration of mononuclear cells, coined as chronic non-suppurative destructive cholangitis, which leads to destructive changes and the disappearance of small- or medium-sized bile ducts. Since 1990, early diagnosis with the detection of AMAs and introduction of ursodeoxycholic acid as first-line treatment has greatly altered the clinical course of PBC, and liver transplantation-free survival of patients with PBC is now comparable to that of the general population.

Review article: pathophysiology and management of primary biliary cholangitis

BACKGROUND

Primary biliary cholangitis (PBC), an immune-mediated disease characterised by destruction of intrahepatic bile ducts, results in progressive damage to the biliary tree, cholestasis and ultimately advanced liver disease. In the last decade, advances in practice have improved clinical care, driven novel therapeutic options and improved risk stratification tools.

AIMS

To provide an overview of the disease characteristics of PBC and review a patient-centred management approach for the clinical team caring for those with PBC.

METHODS

We reviewed the current literature and guidelines on PBC with a focus on management and therapies.

RESULTS

A confident diagnosis of PBC is usually made based on serum liver tests and immune serology. Management of PBC should focus on three main 'process' pillars: (a) treat and risk-stratify through use of biochemical and prognostic criteria; (b) manage concurrent symptoms and other associated diseases; and (c) stage disease, monitor progression and prevent complications. With ongoing complexities in management, including a newly licensed therapy (obeticholic acid) and alternative non-licensed treatments and ongoing clinical trials, discussion with PBC expert centres is encouraged.

CONCLUSIONS

PBC is a dynamic disease wherein current treatment goals have become appropriately ambitious. Goals of care should prioritise prevention of end-stage liver disease and amelioration of patient symptom burden for all.

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.

Evolution of our understanding of PBC

The discovery of mitochondrial autoantigens recognized by antimitochondrial antibodies (AMAs) in 1987 marked the dawn of a new era in primary biliary cholangitis (PBC) research. Since then, there has been substantial progress in our understanding of PBC partly bestowed by the development of innovative technologies in molecular biology, immunology, and genetics. Here, we review this evolutionary progress in understanding PBC. We now recognize that the epitopes of AMAs, CD4⁺, and CD8⁺ T cells are all mapped to the same region of the inner lipoyl domain of pyruvate dehydrogenase complex E2 subunit (PDC-E2), and that intrahepatic biliary epithelial cells (BECs) are exclusively targeted in PBC. BECs express PDC-E2 on apotopes in an immunologically intact form during apoptosis, but not other epithelial cells, which could explain the tissue specificity of PBC. In addition, genetic factors, environmental triggers, and epigenetic modifications play crucial roles in the development of PBC. Intact lipoylated PDC-E2, presumably after modification with xenobiotics such as 2-octynamide or 2-nonyamide that are abundantly present in the environment, is endocytosed by antigen-presenting cells and are presented to CD4⁺ or CD8⁺ T cells. An immune complex consisting of PDC-E2 and anti-PDC-E2 autoantibodies cross-present autoantigens in a more efficient manner. Finally, an adenylate uridine-rich element (ARE) Del -/- mouse model has been established, which presents a disease modeling human PBC, including female dominance as one of its most important features, and can be used to dissect the immunopathology of PBC. Expanding our knowledge of the pathology from a very early stage of the disease will provide the key to cure PBC.

Environmental basis of primary biliary cholangitis

Autoimmunity is a consequence of both genetic and environmental factors, occurring in genetically susceptible hosts with environmental triggers. While genome-wide association studies have revealed a number of susceptible genes contributing to etiology, the environmental triggers remain poorly understood. Primary biliary cholangitis, formally known as primary biliary cirrhosis, is considered a model autoimmune disease for which our group has extensively evaluated environmental factors involved in its etiology. Bacterial infection and xenobiotics have been proposed as candidate environmental factors that may explain tolerance breakdown and production of primary biliary cholangitis-specific antimitochondrial autoantibodies. Large-scale case-control studies have consistently detected an association of primary biliary cholangitis with urinary tract infections caused by Escherichia coli, as E. coli PDC-E2 is molecularly similar to human PDC-E2, the immunodominant target of AMAs. Another bacterium of interest is Novosphingobium aromaticivorans, a ubiquitous xenobiotic-metabolizing bacterium that produces lipoylated proteins, which are highly reactive with sera from primary biliary cholangitis patients. Regarding xenobiotics, case-control studies have suggested that frequent use of nail polish is associated with an increased susceptibility to primary biliary cholangitis. We found that 2-octynamide, the conjugate derived from 2-octynoic acid present in cosmetics, lipsticks, and some chewing gums, was unique in both its quantitative structure-activity relationship analysis and reactivity with primary biliary cholangitis sera. 2-nonyamide is another xenobiotic that also has the optimal chemical structure for xenobiotic modification of the PDC-E2 epitope, as demonstrated by the enhanced epitope recognition with AMA-positive PBC sera. Moreover, we found that C57BL/6 mice immunized with 2-octynoic acid-BSA possess many of the features characteristic to primary biliary cholangitis. Impact statement Autoimmunity is believed to develop in genetically susceptible hosts with triggers from the environment. Researchers have recently demonstrated that bacteria and xenobiotics commonly present in our environment are potential triggers of tolerance breakdown against autoantigens and autoimmunity, particularly in primary biliary cholangitis (PBC). The link between xenobiotics and PBC has been further confirmed with the establishment of PBC model mice by immunizing mice with xenobiotics.

Toward solving the etiological mystery of primary biliary cholangitis

Primary biliary cholangitis (PBC) is considered a model autoimmune disease due to its signature anti‐mitochondrial antibody (AMA) autoantibody, female predominance, and relatively specific portal infiltration and cholestasis. The identification and cloning of the major mitochondrial autoantigens recognized by AMA have served as an immunologic platform to identify the earliest events involved in loss of tolerance. Despite the relatively high concordance rate in identical twins, genome‐wide association studies have not proven clinically useful and have led to suggestions of epigenetic events. To understand the natural history and etiology of PBC, several murine models have been developed, including spontaneous models, models induced by chemical xenobiotic immunization, and by “designer” mice with altered interferon metabolism. Herein, we describe five such models, including 1) NOD.c3c4 mice, 2) dominant negative form of transforming growth factor receptor type II mice, 3) interleukin‐2R α^−/− mice, 4) adenylate‐uridylate‐rich element Del^−/− mice, and 5) 2‐octynoic acid‐conjugated bovine serum albumin immunized mice. Individually there is no perfect murine model, but collectively the models point to loss of tolerance to PDC‐E2, the major mitochondrial autoantigen, as the earliest event that occurs before clinical disease is manifest. Although there is no direct association of AMA titer and PBC disease progression, it is noteworthy that the triad of PBC monocytes, biliary apotopes, and AMA leads to an intense proinflammatory cytokine burst. Further, the recurrence of PBC after liver transplantation indicates that, due to major histocompatibility complex restriction, disease activity must include not only adaptive immunity but also innate immune mechanisms. We postulate that successful treatment of PBC may require a personalized approach with therapies designed for different stages of disease. (Hepatology Communications 2017;1:275–287)

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.

Advances in pharmacotherapy for primary biliary cirrhosis

INTRODUCTION

Primary biliary cirrhosis (PBC) is a chronic autoimmune liver disease mostly seen in middle-aged women characterized by progressive nonsuppurative destruction of small bile ducts resulting in intrahepatic cholestasis, parenchymal injury and ultimately end-stage liver disease. Despite major breakthroughs in our understanding of PBC, there remains only one FDA-approved agent for treatment: ursodeoxycholic acid (UDCA) to which one-third of patients are unresponsive.

AREAS COVERED

Biochemical response to treatment with UDCA is associated with excellent survival rates in PBC patients. However, there is a need for alternative treatments for nonresponders. Results from human epidemiological and genetic studies as well as preclinical studies in PBC animal models have provided a strong impetus for the development of new therapeutic agents. In this review, we discuss the recent advances in translational research in PBC focusing on promising therapeutic approaches, namely immune-based targeted therapies and agents targeting the synthesis and circulation of bile acids.

EXPERT OPINION

We are in a new era for the development of novel therapies for PBC. Data on fibrates, budesonide and obeticholic acid offer encouragement for nonresponders to UDCA.

Escherichia coli infection induces autoimmune cholangitis and anti-mitochondrial antibodies in non-obese diabetic (NOD).B6 (Idd10/Idd18) mice

Several epidemiological studies have demonstrated that patients with primary biliary cirrhosis (PBC) have a higher incidence of urinary tract infections (UTI) and there is significant homology of the immunodominant mitochondrial autoantigen, the E2 component of the pyruvate dehydrogenase complex (PDC-E2), between mammals and bacteria. Previous work has demonstrated that non-obese diabetic (NOD).B6 Idd10/Idd18 infected with Novosphingobium aromaticivorans developed liver lesions similar to human PBC. It was postulated that the biliary disease was dependent upon the presence of the unique N. aro glycosphingolipids in activating natural killer T (NK T) cells. To address this issue, we infected NOD.B6 Idd10/Idd18 mice with either Escherichia coli, N. aro or use of a phosphate-buffered saline (PBS) vehicle control and serially followed animals for the appearance of liver pathology and anti-mitochondrial autoantibodies (AMA). Of striking importance, the biliary disease of E. coli-infected mice was more severe than N. Aro-infected mice and the titre of AMA was higher in E. coli-infected mice. Furthermore, the immunopathology did not correlate with the ability of bacterial extracts to produce antigen-dependent activation of NK T cells. Our data suggest that the unique glycosphingolipids of N. aro are not required for the development of autoimmune cholangitis. Importantly, the data highlight the clinical significance of E. coli infection in a genetically susceptible host, and we suggest that the appearance of autoimmune cholangitis is dependent upon molecular mimicry. These data highlight that breach of tolerance to PDC-E2 is probably the first event in the natural history of PBC in genetically susceptible hosts.

Tracing environmental markers of autoimmunity: introducing the infectome

We recently introduced the concept of the infectome as a means of studying all infectious factors which contribute to the development of autoimmune disease. It forms the infectious part of the exposome, which collates all environmental factors contributing to the development of disease and studies the sum total of burden which leads to the loss of adaptive mechanisms in the body. These studies complement genome-wide association studies, which establish the genetic predisposition to disease. The infectome is a component which spans the whole life and may begin at the earliest stages right up to the time when the first symptoms manifest, and may thus contribute to the understanding of the pathogenesis of autoimmunity at the prodromal/asymptomatic stages. We provide practical examples and research tools as to how we can investigate disease-specific infectomes, using laboratory approaches employed from projects studying the “immunome” and “microbiome”. It is envisioned that an understanding of the infectome and the environmental factors that affect it will allow for earlier patient-specific intervention by clinicians, through the possible treatment of infectious agents as well as other compounding factors, and hence slowing or preventing disease development.

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.

Environment and primary biliary cirrhosis: electrophilic drugs and the induction of AMA

Environmental stimulation is a major factor in the initiation and perpetuation of autoimmune diseases. We have addressed this issue and focused on primary biliary cirrhosis (PBC), an autoimmune disease of the liver. Immunologically, PBC is distinguished by immune mediated destruction of the intra hepatic bile ducts and the presence of high titer antimitochondrial autoantibodies (AMA) directed against a highly specific epitope within the lipoic acid binding domain of the pyruvate dehydrogenase E2 subunit (PDC-E2). We submit that the uniqueness of AMA epitope specificity and the conformational changes of the PDC-E2 lipoyl domain during physiological acyl transfer could be the lynchpin to the etiology of PBC and postulate that chemical xenobiotics modification of the lipoyl domain of PDC-E2 is sufficient to break self-tolerance, with subsequent production of AMA in patients with PBC. Indeed, using quantitative structure activity relationship (QSAR) analysis on a peptide-xenobiotic conjugate microarray platform, we have demonstrated that when the lipoyl domain of PDC-E2 was modified with specific synthetic small molecule lipoyl mimics, the ensuing structures displayed highly specific reactivity to PBC sera, at levels often higher than the native PDC-E2 molecule. Hereby, we discuss our recent QSAR analysis data on specific AMA reactivity against a focused panel of lipoic acid mimic in which the lipoyl di-sulfide bond are modified. Furthermore, data on the immunological characterization of antigen and Ig isotype specificities against one such lipoic acid mimic; 6,8-bis(acetylthio)octanoic acid (SAc), when compared with rPDC-E2, strongly support a xenobiotic etiology in PBC. This observation is of particular significance in that approximately one third of patients who have taken excessive acetaminophen (APAP) developed AMA with same specificity as patients with PBC, suggesting that the lipoic domain are a target of APAP electrophilic metabolites such as NAPQI. We submit that in genetically susceptible hosts, electrophilic modification of lipoic acid in PDC-E2 by acetaminophen or similar drugs can facilitate loss of tolerance and lead to the development of PBC.

Infectome: a platform to trace infectious triggers of autoimmunity

The "exposome" is a term recently used to describe all environmental factors, both exogenous and endogenous, which we are exposed to in a lifetime. It represents an important tool in the study of autoimmunity, complementing classical immunological research tools and cutting-edge genome wide association studies (GWAS). Recently, environmental wide association studies (EWAS) investigated the effect of environment in the development of diseases. Environmental triggers are largely subdivided into infectious and non-infectious agents. In this review, we introduce the concept of the "infectome", which is the part of the exposome referring to the collection of an individual's exposures to infectious agents. The infectome directly relates to geoepidemiological, serological and molecular evidence of the co-occurrence of several infectious agents associated with autoimmune diseases that may provide hints for the triggering factors responsible for the pathogenesis of autoimmunity. We discuss the implications that the investigation of the infectome may have for the understanding of microbial/host interactions in autoimmune diseases with long, pre-clinical phases. It may also contribute to the concept of the human body as a superorganism where the microbiome is part of the whole organism, as can be seen with mitochondria which existed as microbes prior to becoming organelles in eukaryotic cells of multicellular organisms over time. A similar argument can now be made in regard to normal intestinal flora, living in symbiosis within the host. We also provide practical examples as to how we can characterise and measure the totality of a disease-specific infectome, based on the experimental approaches employed from the "immunome" and "microbiome" projects.

The X-factor in primary biliary cirrhosis: monosomy X and xenobiotics

Primary biliary cirrhosis (PBC) is a chronic, cholestatic, autoimmune liver disease characterised by the destruction of small- and medium-sized bile ducts. The serological hallmark of PBC includes antimitochondrial antibodies (AMA). The disease has a striking female predominance, and primarily affects women of middle-age. First-degree relatives, and in particular female relatives, are known to have an increased risk of developing the disease. Several studies have attempted to explain the female predominance of PBC, and autoimmune diseases in general. Two components that are of interest in PBC include monosomy X and xenobiotics. Monosomy X has been noted to be prevalent in the peripheral blood mononuclear cells of PBC patients. Xenobiotics, which are exogenous chemicals not normally found within the body, have been implicated in the modification of, and loss of, tolerance to AMA. Several cosmetics are known to contain these xenobiotics, which is of interest given the information provided in regards to known risk factors for PBC development. This review will focus on X monosomy and xenobiotics, which appear to constitute the X-factor of PBC.

Popular and unpopular infectious agents linked to primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a progressive cholestatic liver disease characterized by the autoimmune destruction of the biliary epithelial cells of the small and medium-size bile ducts. The disease affects middle aged women and usually affects more than one member within a family. The pathognomonic serological hallmark of the disease is the presence of circulating anti-mitochondrial antibodies, and disease-specific anti-nuclear antibodies. Susceptibility genes and environmental risk factors such as infections and smoking have been reported as important for the development of the disease. Among the environmental agents, infectious triggers are the best studied. Most of the work published so far has investigated the role of infections caused by Novosphingobium aromaticivorans and Escherichia coli. This review will discuss the popular and unpopular infectious agents causatively linked to PBC. It will also examine reports investigating the epidemiological aspects of the disease and their direct or indirect implications to bacterial-induced PBC.

Predicting and preventing autoimmunity: the case of anti-mitochondrial antibodies

To be able to predict who will develop autoimmune disease would allow for early treatment which may dramatically alter the course of the disease. In some cases, it may also lead to prevention of the disease development. The prediction of disease development is based on the analysis of risk factors which have been associated with the disease in question. These factors include genetic susceptibility, as well as immunological and environmental factors. One autoimmune disease that may serve as a model for disease prediction is primary biliary cirrhosis (PBC), an autoimmune liver disease affecting the small- and medium-sized bile ducts. PBC could be an ideal model due to recent advances in elucidating its genetic associations. As well, a variety of immunological and environmental risk factors have been well established. Indeed, the presence of PBC-specific antimitochondrial antibodies and/or antinuclear antibodies has been shown to be predictor of disease development and possibly prognosis. This review will examine the current evidence which suggests that we may potentially be able to predict the development of PBC in some individuals. These concepts may also be applied to autoimmune diseases in general.

Epstein-barr virus as a trigger of autoimmune liver diseases

The pathogenesis of autoimmune diseases includes a combination of genetic factors and environmental exposures including infectious agents. Infectious triggers are commonly indicated as being involved in the induction of autoimmune disease, with Epstein-Barr virus (EBV) being implicated in several autoimmune disorders. EBV is appealing in the pathogenesis of autoimmune disease, due to its high prevalence worldwide, its persistency throughout life in the host's B lymphocytes, and its ability to alter the host's immune response and to inhibit apoptosis. However, the evidence in support of EBV in the pathogenesis varies among diseases. Autoimmune liver diseases (AiLDs), including autoimmune hepatitis (AIH), primary biliary cirrhosis (PBC), and primary sclerosing cholangitis (PSC), have a potential causative link with EBV. The data surrounding EBV and AiLD are scarce. The lack of evidence surrounding EBV in AiLD may also be reflective of the rarity of these conditions. EBV infection has also been linked to other autoimmune conditions, which are often found to be concomitant with AiLD. This paper will critically examine the literature surrounding the link between EBV infection and AiLD development. The current evidence is far from being conclusive of the theory of a link between EBV and AiLD.

Primary biliary cirrhosis and liver transplantation

Primary biliary cirrhosis (PBC) is an immune-mediated chronic progressive inflammatory liver disease, predominantly affecting middle-aged women, characterized by the presence of antimitochondrial antibodies (AMAs), which can lead to liver failure. Genetic contributions, environmental factors including chemical and infectious xenobiotics, autoimmunity and loss of tolerance have been aggressively investigated in the pathogenesis of PBC, however, the actual impact of these factors is still controversial. Survival of PBC patients has been largely improved with the widespread use of ursodeoxycholic acid (UDCA), however, one third of patients still do not respond to the treatment and proceed to liver cirrhosis, requiring liver transplantation as a last resort for cure. The outcome of liver transplantation is excellent with 5- and 10-year survival rates around 80% and 70%, respectively, while along with long survival, the recurrence of the disease has become an important outcome after liver transplantation. Prevalence rates of recurrent PBC rage widely between 1% and 35%, and seem to increase with longer follow-up. Center-specific issues, especially the use of protocol biopsy, affect the variety of incidence, yet, recurrence itself does not affect patient and graft survival at present, and retransplantation due to recurrent disease is extremely rare. With a longer follow-up, recurrent disease could have an impact on patient and graft survival.

Hair dyes as a risk for autoimmunity: from systemic lupus erythematosus to primary biliary cirrhosis

Environmental and genetic factors appear to be involved in the pathogenesis of primary biliary cirrhosis (PBC), a chronic cholestatic liver disease characterized by immune-mediated destruction of the small and medium sized intrahepatic bile ducts. Environmental factors include exposure to various infectious, xenobiotic and chemical compounds. These exposures may occur occupationally, through water or air contamination, pharmacological administration or by elective exposure, to name a few. Hair dyes are compounds that have been implicated in the development of several autoimmune diseases, including systemic lupus erythematosus (SLE) and PBC. So far, only epidemiological studies have addressed the role of hair dyes in PBC, with limited results. Hair dyes in SLE have been examined, and have recently demonstrated an association, both epidemiologically and immunologically. This follows a series of negative studies, which may not have taken into account several features of hair dye use. This review will examine the literature surrounding hair dye use and SLE, and compare this to data surrounding PBC. Treating physicians should be prepared for questions surrounding the need to take precautions against repeated hair dye use and this topic is discussed further.

Towards systemic sclerosis and away from primary biliary cirrhosis: the case of PTPN22

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by immune-mediated destruction of the small and medium size intrahepatic bile ducts. PBC patients often have concomitant autoimmune diseases, which are most often autoimmune thyroid disease, as well as Sicca syndrome. Occasionally, some PBC patients will also have systemic sclerosis of the limited cutaneous type (lcSSc). Conversely, up to one-fourth of SSc patients are positive for antimitochondrial antibody, the serologic hallmark of PBC. It is also common for SSc patients to have concomitant autoimmune disease, which may include PBC in rare cases. This has led to speculation of shared environmental and/or genetic factors, which lead to the development of PBC in SSc patients and vice versa. Recent genetic studies have revealed associations with several genes in both SSc and PBC. PTPN22 is one gene that has been associated with SSc, but not with PBC. It may be argued that some SSc patients with a particular genotype, which shares genes found in both conditions may develop PBC. Likewise, particular genes such as PTPN22 may infer susceptibility to SSc alone. The presence of PTPN22 may also contribute to the development of SSc in PBC patients. The lack of a large number of overlapping genes may, in part, explain the relative rarity of PBC with SSc and vice versa. This review will examine the literature surrounding the genetic associations of PBC and SSc, and the role of PTPN22 in particular.

Role of the anion exchanger 2 in the pathogenesis and treatment of primary biliary cirrhosis

The essential anion exchanger (AE) involved in biliary bicarbonate secretion is AE2/SLC4A2, a membrane protein which has also been recognized to be relevant for the regulation of the intracellular pH (pH(i)) in several cell types. Previously, we reported that the expression of AE2 mRNA is diminished in liver biopsies and peripheral blood mononuclear cells from patients with primary biliary cirrhosis (PBC). Immunohistochemical studies indicated that the expression of the AE2 protein is decreased in the bile ducts and hepatocytes in PBC livers. Moreover, we found that bile duct cells isolated from PBC patients and cultured for a few passages exhibit defective Na(+)-independent Cl(-)/HCO(3)(-) exchange. Interestingly, positron emission tomography studies have shown that PBC patients, even at early stages of the disease, fail to secrete bicarbonate to bile in response to secretin, a defect that can be partially reversed after several months of treatment with ursodeoxycholic acid. Altogether, these findings sustain our hypothesis that dysfunctions related to AE2 might have a role in the pathogenesis of PBC. Inadequate AE2 function in lymphocytes may disturb pH(i) regulation in these cells and alter immune homeostasis leading to autoimmunity. On the other hand, reduced AE2 in cholangiocytes could cause cholestasis and oxidative stress of bile duct cells. Cholangiocyte changes, together with altered immune homeostasis, could favor the development of antimitochondrial antibodies and the autoimmune attack on biliary ducts. Our recent findings that Ae2(a,b)-deficient mice indeed display most of these features strongly support the notion that AE2 abnormalities may be involved in the pathogenesis of PBC.

Disease-specific autoantibodies in primary biliary cirrhosis

Anti-mitochondrial autoantibodies (AMA) are specific markers of primary biliary liver cirrhosis (PBC), a cholestatic autoimmune disease which is characterised by a progressive destruction of the biliary epithelial cells followed by fibrosis, cirrhosis and liver failure. The prevalence of AMA in PBC is more than 90% and they can precede long before the clinical symptoms. AMA are conventionally detected by indirect immunofluorescence (IIF) using rodent liver, kidney, and stomach sections as substrates. Additionally, different PBC-specific anti-nuclear autoantibodies (ANA) can be observed in 30% of patients presenting with multiple nuclear dot or nuclear membrane staining patterns, which preferentially are identified using HEp-2 cells as substrate. The identification of the major PBC-specific mitochondrial and nuclear targets has allowed the generation of monospecific antigenic targets which are increasingly used in solid-phase assays for routine detection of AMA and ANA in mono- or multiparametric screen test systems. In the present paper, we give an overview of the diagnostic significance of autoantibodies in PBC, discuss the competencies of different techniques used for their determination and propose an effective diagnostic strategy.

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.

Primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an immune-mediated chronic cholestatic liver disease with a slowly progressive course. Without treatment, most patients eventually develop fibrosis and cirrhosis of the liver and may need liver transplantation in the late stage of disease. PBC primarily affects women (female preponderance 9–10:1) with a prevalence of up to 1 in 1,000 women over 40 years of age. Common symptoms of the disease are fatigue and pruritus, but most patients are asymptomatic at first presentation. The diagnosis is based on sustained elevation of serum markers of cholestasis, i.e., alkaline phosphatase and gamma-glutamyl transferase, and the presence of serum antimitochondrial antibodies directed against the E2 subunit of the pyruvate dehydrogenase complex. Histologically, PBC is characterized by florid bile duct lesions with damage to biliary epithelial cells, an often dense portal inflammatory infiltrate and progressive loss of small intrahepatic bile ducts. Although the insight into pathogenetic aspects of PBC has grown enormously during the recent decade and numerous genetic, environmental, and infectious factors have been disclosed which may contribute to the development of PBC, the precise pathogenesis remains enigmatic. Ursodeoxycholic acid (UDCA) is currently the only FDA-approved medical treatment for PBC. When administered at adequate doses of 13–15 mg/kg/day, up to two out of three patients with PBC may have a normal life expectancy without additional therapeutic measures. The mode of action of UDCA is still under discussion, but stimulation of impaired hepatocellular and cholangiocellular secretion, detoxification of bile, and antiapoptotic effects may represent key mechanisms. One out of three patients does not adequately respond to UDCA therapy and may need additional medical therapy and/or liver transplantation. This review summarizes current knowledge on the clinical, diagnostic, pathogenetic, and therapeutic aspects of PBC.

Immune-mediated liver diseases: programmed cell death ligands and circulating apoptotic markers

Primary biliary cirrhosis, primary sclerosing cholangitis and autoimmune hepatitis are the three major immune-mediated liver diseases. The etiologies of primary biliary cirrhosis, primary sclerosing cholangitis and autoimmune hepatitis are largely unknown, but seem to be influenced by genetic and environmental factors. Autoantibodies can be found in nearly all patients with primary sclerosing cholangitis and autoimmune hepatitis, and in the vast majority of patients with primary sclerosing cholangitis. In addition, autoimmune hepatitis is associated with high concentrations of serum globulins. Enhanced liver cell death by apoptosis has been described in all of these liver diseases, although the precise mechanisms remain unclear. In general, apoptosis can be initiated via an extrinsic pathway that is triggered by engagement of death receptors on the cell surface, or via an intrinsic pathway that is induced by mitochondrial injury and is influenced by members of the Bcl-2 family. In both pathways, effector caspases are finally activated that cleave and degrade cell structures, resulting in the release of apoptotic products into the circulation. New diagnostic tests can detect these apoptotic markers and programmed cell death ligands such as Fas and Fas-ligands, nucleosomes, caspases, cytokeratin fragments, macrophage migration inhibitory factor, soluble intracellular adhesion molecule, natural killer cells group 2D and programmed death ligands. Several of these markers have been found to be altered in tissue and/or blood of immune-mediated liver diseases, some also in nonimmune-mediated liver diseases. Beyond their potential usefulness as additional diagnostic markers, they may be valuable for the estimation of disease severity and therapy monitoring. This review summarizes current knowledge on apoptotic mechanisms, death receptor ligands and circulating apoptotic markers in immune-mediated liver diseases.

Bacteria and primary biliary cirrhosis

Infectious agents have been postulated to play a pathogenic role in the loss of immunological tolerance and the induction of primary biliary cirrhosis, an immune-mediated cholestatic liver disease characterized by progressive destruction of the small intrahepatic bile ducts and subsequent cirrhosis and liver failure. This review discusses emerging issues implicating infectious agents such as Escherichia coli, mycobacteria, chlamydia, helicobacter species, lactobacilli, Novosphingobium aromaticivorans, and betaretroviruses in the pathogenesis of primary biliary cirrhosis. We also review the immunopathological mechanisms responsible for the induction of the disease with special emphasis on the role of molecular mimicry and microbial/self immunological cross-reactivity.

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

Our laboratory has suggested that loss of tolerance to pyruvate dehydrogenase (PDC-E2) leads to an anti-mitochondrial antibody response and autoimmune cholangitis, similar to human primary biliary cirrhosis (PBC). We have suggested that this loss of tolerance can be induced either via chemical xenobiotic immunization or exposure to select bacteria. Our work has also highlighted the importance of genetic susceptibility. Using the non-obese diabetic (NOD) congenic strain 1101 (hereafter referred to as NOD.1101 mice), which has chromosome 3 regions from B6 introgressed onto a NOD background, we exposed animals to 2-octynoic acid (2OA) coupled to bovine serum albumin (BSA). 2OA has been demonstrated previously by a quantitative structural activity relationship to react as well as or better than lipoic acid to anti-mitochondrial antibodies. We demonstrate herein that NOD.1101 mice immunized with 2OA-BSA, but not with BSA alone, develop high titre anti-mitochondrial antibodies and histological features, including portal infiltrates enriched in CD8(+) cells and liver granulomas, similar to human PBC. We believe this model will allow the rigorous dissection of early immunogenetic cause of biliary damage.

The causes of primary biliary cirrhosis: Convenient and inconvenient truths

The most difficult issue in autoimmunity remains etiology. Although data exist on effector mechanisms in many autoimmune diseases, the underlying cause or causes are still generically ascribed to genetics and environmental influences. Primary biliary cirrhosis (PBC) is considered a model autoimmune disease because of its signature antimitochondrial autoantibody (AMA), the homogeneity of clinical characteristics, and the specificity of biliary epithelial cell (BEC) pathology. Twenty years ago, we reported the cloning and identification of the E2 component of pyruvate dehydrogenase (PDC-E2) as the immunodominant autoantigen of PBC, allowing for vigorous dissection of T and B lymphocyte responses against PDC-E2 and development of several valid experimental models. There has also been considerable study of the biology of BECs, which has included the unique properties of apoptosis in which there is exposure of PDC-E2 to the effector processes of the immune system. In this review, we present these data in the context of our proposal that the proximal cause of PBC is autoimmunity directed against well-identified mitochondrially located autoantigens in individuals with inherited deficits of immune tolerance. We present these data under the umbrella of convenient truths that support this thesis as well as some inconvenient truths that are not readily accommodated by current theory.

CONCLUSION

We emphasize that the potential initiator of PBC includes inter alia particular environmental xenobiotics; pathogenesis is aided and abetted by genetic weaknesses in mechanisms of immune regulation; and subsequent multilineage immunopathology impacts upon uniquely susceptible BECs to culminate clinically in the chronic autoimmune cholangiolitis of PBC.

The clinical significance of IgA antimitochondrial antibodies in sera and saliva in primary biliary cirrhosis

It still remains unclear how antimitochondrial autoantibodies (AMA) are involved with immunopathogenesis of primary biliary cirrhosis (PBC). We have suggested the potential role of IgA-AMA in damage to epithelial cells in PBC. In the current study, we investigated whether IgA-AMA were detectable in sera and saliva of PBC patients, to examine the association between detectable IgA-type autoantibodies in sera or saliva and progression of liver diseases. Fifty-three patients with PBC were enrolled, and IgA-AMA in sera and saliva were sought by immunoblotting using pork heart mitochondria as antigens. The progression of PBC was determined as Scheuer's classification consisting of four histological stages. We found IgA-AMA, IgA-anti-PDC-E2, and IgA-anti-E3BP in 43/53 (81%), 37/53 (70%), and 35/53 (66%) sera of patients with PBC, but none of controls. The progression of PBC was statistically associated with presence of IgA-anti-PDC-E2 (P = 0.0124), but neither with IgA-AMA (P = 0.1296) nor anti-IgA-E3BP (P = 0.5973). In saliva, detectable IgA-AMA, IgA-anti-PDC-E2, and IgA-anti-E3BP were noted in 12/26 (46%), 6/26 (23%), and 11/26 (42%), respectively. Detection of IgA-anti-PDC-E2 was strongly associated with progression of PBC (P = 0.0002), whereas detection of IgA-AMA and IgA-anti-E3BP were not associated (P = 0.2145 and P = 0.5118). The current findings suggest that the presence of IgA-anti-PDC-E2 in sera or saliva might be associated with progression of PBC, although a prospective study with PBC patients with detectable IgA-anti-PDC-E2 at early stages will be required to conclude the contribution of IgA-anti-PDC-E2 to the progression of PBC.

The X and why of xenobiotics in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic autoimmune liver disease characterized by inflammation and destruction of intrahepatic biliary epithelial cells, ultimately leading to liver failure. The serological hallmark of PBC is the presence of high-titer antimitochondrial antibodies (AMA) against the inner lipoyl domain of E2 subunits of 2-oxo-acid dehydrogenase complexes, in particular the E2 component of the pyruvate dehydrogenase complex (PDC-E2). The initiating events triggering the autoimmune response are not yet identified but the hypothesis of molecular mimicry is a widely proposed mechanism for the development of autoimmunity in PBC. Several candidates, including bacteria and viruses, have been suggested as causative agents, but also environmental factors, such as chemical xenobiotics, have been implicated in the pathogenesis of primary biliary cirrhosis. In this review, we will discuss our current knowledge of the immunoreactivity of xenobiotically modified PDC peptide antigens. In addition, we will provide a working hypothesis how xenobiotic modification of antigens might occur that ultimately leads to the breaking of self-tolerance and the induction of PBC.

Serum reactivity against bacterial pyruvate dehydrogenase: increasing the specificity of anti-mitochondrial antibodies for the diagnosis of primary biliary cirrhosis

Antimitochondrial antibodies (AMA) are the serum hallmark of primary biliary cirrhosis (PBC). However, AMA-positivity can be found in non-PBC sera when lower dilutions are used, thus raising issues about the specificity and sensitivity of the test. AMA reacts primarily with the lipoylated domains of pyruvate dehydrogenase-E2 (PDC-E2) which is highly conserved across species, including bacteria. We studied 77 serum samples, including 24 from patients with anti-PDC-E2-positive PBC and 53 controls (16 with autoimmune hepatitis (AIH), 10 with primary sclerosing cholangitis (PSC), and 27 healthy individuals) for their reactivities at serial dilutions (1:10, 1:20 and 1:40) against Escherichia coli DH5 alpha lysate overexpressing human PDC-E2 using immunoblotting (IB). A murine anti-human PDC-E2 monoclonal antibody (mAB) was used as control. We further studied positive sera using adsorption with a synthetic E. coli peptide sharing similarity with human PDC-E2. Finally, we verified whether a unique buffer for E. coli preparation could reduce non-specific serum reactivity. Results demonstrated that 100% of anti-PDC-E2-positive PBC and up to 38% of control sera at 1:10 dilution recognized E. coli PDC-E2 at IB while dilution tests indicated that the overall potency of PBC reactivity was 100-fold higher compared to controls. In fact, a subgroup (20-38%) of non-PBC sera were positive at low titers but lost the reactivity when absorbed with the synthetic E. coli peptide. Finally, our unique buffer reduced the reactivity of non-PBC sera as measured by ELISA. In conclusion, we demonstrated that weak cross-reactivity with E. coli PDC-E2 occurs in non-PBC sera at lower dilutions and that such reactivity is not due to AMA-positivity. The use of a specific buffer might avoid the risk of false positive AMA determinations when E. coli-expressed recombinant antigens are used.

B cells in autoimmune diseases: insights from analyses of immunoglobulin variable (Ig V) gene usage

The role of B cells in autoimmune diseases has not been fully elucidated. It is also unclear whether breaking of B cell tolerance in patients with autoimmune diseases is due to underlying defects in the molecular mechanisms involved in the arrangement of antibody genes or deficiencies in the subsequent selective influences that shape the antibody repertoire. Analysis of immunoglobulin (Ig) variable (V) gene usage is beginning to provide answers to some of these questions. Such analyses have identified some differences in the basic Ig V gene repertoire of patients with autoimmune diseases compared to healthy controls, even though none of these differences can be considered major. Defects in positive and negative selection, mutational targeting and, in some cases, receptor editing have also been detected. In addition, analysis of Ig V gene usage in target organs and tissues of patients with autoimmune diseases has clearly demonstrated that there is a highly compartmentalized clonal expansion of B cells driven by a limited number of antigens in these tissues. Great progress has been made in the structural and functional characterization of disease-associated antibodies, largely because of the development of the combinatorial library technique. Use of antibodies generated by this technique offers great promise in identifying B cell epitopes on known target antigens and in gaining greater insights into the pathogenic role of B cells in both B and T cell mediated autoimmune diseases.

Identification of 2-nonynoic acid, a cosmetic component, as a potential trigger of primary biliary cirrhosis

Antimitochondrial antibodies (AMA) are unique among autoimmune serologic reactants because of their extremely high association with the index disease primary biliary cirrhosis (PBC). This autoantibody response is specifically directed only to the lipoyl domain of the mitochondrial 2-oxo-acid dehydrogenase complexes, which prompted us to search for environmental mimotopes in the form of xenobiotics and led to our identification of 2-octynoic acid as a high-affinity reactant for AMA. To focus on the chemical characteristics requisite for binding of AMA to the xenobiotic-modified self-peptide, quantitative structure-activity relationship (QSAR) studies were performed using a panel of alkynoic compounds, including examination of the length of the carbon chain and the location of the triple bond in the identified mimotope. Analyses of octynamides that varied in the position of the triple bond demonstrated that only the 2-octynamide reacted strongly with PBC sera. Furthermore, among 2-alkynamides with varying carbon chain length, 2-octyn-, 2-nonyn- (particularly) and 2-decynamide exhibited the highest reactivity. Thus, an optimal chemical structure of the xenobiotically modified epitope recognized by AMA-positive PBC sera is provided by 2-nonynoic acid. The methyl ester of this compound is ranked 2,324th out of 12,945 compounds to which there is occupational exposure, with an 80% female prevalence due to its use in cosmetic products. Our findings illustrate an unusual polyreactivity of anti-PDC-E2 and support the idea of epitope mimicry in the genesis of this autoantibody and perhaps of PBC itself.

PDC-E3BP is not a dominant T-cell autoantigen in primary biliary cirrhosis

BACKGROUND

Autoantibody responses reactive with the E2 and E3BP components of pyruvate dehydrogenase complex (PDC), which characterise primary biliary cirrhosis (PBC) crossreact, precluding the identification, from serological studies, of the antigen to which the principal breakdown of tolerance occurs. Although autoreactive T-cell responses to PDC-E2 have been well characterised it is, at present, unclear whether T-cell tolerance breakdown also occurs to PDC-E3BP. The aims of this study were to characterise autoreactive T-cell responses to PDC-E3BP in PBC and potential factors regulating their expression.

METHODS

Peripheral blood T-cell proliferative responses to purified recombinant human PDC-E2 and PDC-E3BP at a range of concentrations were characterised in PBC patients and control subjects.

RESULTS

T-cell proliferative responses to both E2 and E3BP were absent from control subjects (median peak stimulation index (SI) to PDC-E2 1.2 [range 0.3-1.9], 0/10 positive (SI>2.32), median peak SI to PDC-E3BP 1.1 [0.7-2.1]], 0/10 positive). Significant responses to PDC-E2 were seen in the majority of patients (median peak SI 11.4 [0.4-24.4], 17/20 (85%) positive) but to PDC-E3BP in only a minority (median peak SI 1-9 [0.6-9.95], 8/20 (40%) positive). Where responses to PDC-E3BP were seen they were universally secondary to responses to PDC-E2.

CONCLUSIONS

Despite the presence of antibodies reactive with PDC-E3BP in the majority of PBC patients this self-protein is not a dominant T-cell autoantigen in PBC.

Mitochondrial antigens as targets of cellular and humoral auto-immunity in primary biliary cirrhosis

Several factors point toward an auto-immune pathogenesis for primary biliary cirrhosis(PBC), mostly based on the presence of serum auto-antibodies to mitochondrial antigens(AMAs) and autoreactive T cells (both helper and cytotoxic). Interestingly, epitopes recognized by AMA and T-cell clones are located within overlapping areas of the antigens. Moreover,a role for an imbalance in cytokine pattern and for natural-killer lymphocytes has also been proposed. Despite several experimental reports, no clear evidence is available regarding the interaction of these factors leading to bile duct destruction. This article reviews the current reports regarding the auto-immune reaction against mitochondrial auto-antigens in PBC.

Chemical xenobiotics and mitochondrial autoantigens in primary biliary cirrhosis: identification of antibodies against a common environmental, cosmetic, and food additive, 2-octynoic acid

Emerging evidence has suggested environmental factors as causative agents in the pathogenesis of primary biliary cirrhosis (PBC). We have hypothesized that in PBC the lipoyl domain of the immunodominant E2 component of pyruvate dehydrogenase (PDC-E2) is replaced by a chemical xenobiotic mimic, which is sufficient to break self-tolerance. To address this hypothesis, based upon our quantitative structure-activity relationship data, a total of 107 potential xenobiotic mimics were coupled to the lysine residue of the immunodominant 15 amino acid peptide of the PDC-E2 inner lipoyl domain and spotted on microarray slides. Sera from patients with PBC (n = 47), primary sclerosing cholangitis (n = 15), and healthy volunteers (n = 20) were assayed for Ig reactivity. PBC sera were subsequently absorbed with native lipoylated PDC-E2 peptide or a xenobiotically modified PDC-E2 peptide, and the remaining reactivity analyzed. Of the 107 xenobiotics, 33 had a significantly higher IgG reactivity against PBC sera compared with control sera. In addition, 9 of those 33 compounds were more reactive than the native lipoylated peptide. Following absorption, 8 of the 9 compounds demonstrated cross-reactivity with lipoic acid. One compound, 2-octynoic acid, was unique in both its quantitative structure-activity relationship analysis and reactivity. PBC patient sera demonstrated high Ig reactivity against 2-octynoic acid-PDC-E2 peptide. Not only does 2-octynoic acid have the potential to modify PDC-E2 in vivo but importantly it was/is widely used in the environment including perfumes, lipstick, and many common food flavorings.

The Western immunoblotting pattern of anti-mitochondrial antibodies is independent of the clinical expression of primary biliary cirrhosis

Anti-mitochondrial antibodies are the serological markers of primary biliary cirrhosis. We analysed the detailed anti-mitochondrial antibodies patterns to see whether the immunological specificities detected at the time of the diagnosis correlate with the histological, clinical and immunological expression of the disease. One hundred and thirty primary biliary cirrhosis patients were studied at the time of presentation/diagnosis. Anti-mitochondrial antibodies reactivity was dissected and evaluated by Western immunoblotting with bovine heart submitochondrial particles as antigenic source. Six different Western immunoblotting patterns have been identified with the following hierarchy: pattern A (anti-PDC-E2+anti-E3BP, 38.5%), pattern B (anti-PDC-E2+anti-E3BP+anti-OGDC-E2, 20.8%), pattern C (anti-PDC-E2+anti-E3BP+anti-BCOADC-E2+anti-OGDC-E2, 13.1%), pattern D (anti-PDC-E2+anti-E3BP+anti-BCOADC-E2, 6.9%), pattern E (anti-BCOADC-E, 6.1%) and pattern F (anti-mitochondrial antibodies negative primary biliary cirrhosis, 14.6%). The different patterns were neither associated with peculiar clinical, biochemical, histological and immunological features nor with the Mayo Risk Score. The anti-mitochondrial antibodies pattern at presentation is independent of the stage of the liver disease; therefore, the Western immunoblotting characterisation of anti-mitochondrial antibodies does not seem to be helpful in identifying the clinical, biochemical or histological expression of primary biliary cirrhosis at the time of the diagnosis.

Sidechain biology and the immunogenicity of PDC-E2, the major autoantigen of primary biliary cirrhosis

The E2 component of mitochondrial pyruvate dehydrogenase complex (PDC-E2) is the immunodominant autoantigen of primary biliary cirrhosis. Whereas lipoylation of PDC-E2 is essential for enzymatic activity and predominates under normal conditions, other biochemical systems exist that also target the lysine residue, including acylation of fatty acids or xenobiotics and ubiquitinylation. More importantly, the immunogenicity can be affected by derivatization of the lysine residue, as the recognition of lipoylated PDC-E2 by patient autoantibodies is enhanced compared with octanoylated PDC-E2. Furthermore, our laboratory has shown that various xenobiotic modifications of a peptide representing the immunodominant region of PDC-E2 are immunoreactive against patient sera. The only purported regulatory system that prevents the accumulation of potentially autoreactive PDC-E2 is glutathionylation, in which the lysine-lipoic acid moiety is further modified with glutathione during apoptosis. Interestingly, this system is found in several cell lines, including HeLa, Jurkat, and Caco-2 cells, but not in cholangiocytes and salivary gland epithelial cells, both of which are targets for destruction in primary biliary cirrhosis. Hence, the failure of this or other regulatory system(s) may overwhelm the immune system with immunogenic PDC-E2 that can initiate the breakdown of tolerance in a genetically susceptible individual. In this review the authors survey the data available on the biochemical life of PDC-E2, with particular emphasis on the lysine residue and its known interactions with machinery involved in various posttranslational modifications.

Lack of immunological or molecular evidence for a role of mouse mammary tumor retrovirus in primary biliary cirrhosis

BACKGROUND & AIMS

Recent observations, including a pilot clinical trial, have suggested that a human mouse mammary tumor virus (MMTV) causes primary biliary cirrhosis (PBC). We attempted to confirm such data.

METHODS

We obtained sera from 101 patients (53 with PBC and 48 controls), fixed liver sections from 10 patients (8 PBC and 2 controls), fresh liver specimens (6 PBC and 6 controls), and fresh peripheral blood lymphocytes (PBLs) (10 PBC and 10 controls). We studied sera for reactivities against 3 different strains of MMTV virions, MMTV(C3H), MMTV(FM), and MMTV(LA), including goat polyclonal antibodies against MMTV virions, gp52, and p27 as positive controls. We stained liver specimens using polyclonal antibodies against MMTV and gp52 and further examined tissue samples and PBLs for specific MMTV genome sequences.

RESULTS

By Western blot analysis, no detectable reactivity in any of the PBC sera against any of the 3 MMTV strains or MMTV gp52 or p27 was observed. However, viral proteins were recognized by our control positive polyclonal antibodies. We note that 13%-60% of PBC sera presented low reactivity against 2 proteins of approximately 57 and 74 kilodaltons. Such reactivity is related to the trace amounts of mitochondrial antigens in the virus preparations derived from murine mammary tumor tissue. No detectable immunohistochemical or molecular evidence for MMTV was found in the liver specimens or PBLs.

CONCLUSIONS

We were unable to recapitulate the data on this specific retroviral etiology of PBC and suggest that such data could be the result of contamination.

'True' antimitochondrial antibody-negative primary biliary cirrhosis, low sensitivity of the routine assays, or both?

Anti-mitochondrial antibody (AMA) is considered the serological hallmark of primary biliary cirrhosis (PBC), but may be missing in a proportion of these patients. We assessed sensitivity and specificity of the currently available techniques for AMA detection in a large series of PBC patients and controls, and analysed their clinical and immunological features according to the AMA status. By indirect immunofluorescence on rat tissue sections and HEp-2 cells, Western immunoblot with bovine submitochondrial particles, and two ELISAs with AMA-specific recombinant proteins, we evaluated the presence of AMA in 127 PBC patients, 166 patients with type 1 autoimmune hepatitis and 100 with non alcoholic fatty liver disease. In PBC patients Western immunoblot detects AMA significantly more often than indirect immunofluorescence on HEp-2 cells (85%versus 72%, P = 0.02) or rodent tissue sections (71%, P = 0.01); both ELISAs are only slightly less sensitive than Western immunoblot (81% and 78%). Ten patients with non alcoholic fatty liver disease were AMA-positive by indirect immunofluorescence, but none recognized AMA-specific epitopes in Western immunoblot or in ELISAs. Twelve patients with type 1 autoimmune hepatitis were AMA-positive by indirect immunofluorescence, but only 6 (3.6%) reacted by Western immunoblot and ELISAs. Western immunoblot or ELISA should be regarded as first-line assay for the detection of AMA. Up to 15% of PBC patients are consistently AMA-negative, yet they share the same clinical, biochemical and histological features of AMA-positive PBC. Detection of AMA in type 1 autoimmune hepatitis might identify a subset of patients at risk of developing a hepatitic/cholestatic syndrome.

Antimitochondrial and other autoantibodies

Primary biliary cirrhosis (PBC) is characterized by the presence of high-titer disease specific autoantibodies directed against mitochondrial antigens (AMA) of the inner mitochondrial membrane, that are members of the 2-oxo acid complex. Among numerous other autoantibodies found in PBC the focus of ongoing studies is on the PBC-specific anti-nuclear antibodies, that are of diagnostic and clinical relevance since they can be used as a 'positive tool' in the diagnosis of AMA-negative PBC while at the same time identifying a subgroup of patients with more advanced liver disease.

Immunization with a xenobiotic 6-bromohexanoate bovine serum albumin conjugate induces antimitochondrial antibodies

The E2 subunit of pyruvate dehydrogenase complex (PDC-E2) is the major autoantigen recognized by antimitochondrial Abs (AMA) in primary biliary cirrhosis (PBC). Recently, we replaced the lipoic acid moiety of PDC-E2 with a battery of synthetic structures designed to mimic a xenobiotically modified lipoyl hapten on a 12-aa peptide that was found within the immunodominant autoepitope of PDC-E2 and demonstrated that AMA in PBC reacted against several organic modified mimotopes as well as, or sometimes significantly better than, the native lipoyl domain. Based on this data, we immunized rabbits with one such xenobiotic organic compound, 6-bromohexanoate, coupled to BSA. One hundred percent of immunized rabbits developed AMA that have each and every characteristic of human AMAs with reactivity against PDC-E2, E2 subunit of branched chain 2-oxo-acid dehydrogenase, and E2 subunit of 2-oxoglutarate dehydrogenase complex. The rabbit AMA also inhibited enzymatic function of PDC-E2 and, importantly, binds to peptide sequences not present in the xenobiotic carrier immunogen. In contrast, BSA-immunized controls did not produce such activity. Our observation that animals immunized with a xenobiotic BSA complex produce autoantibodies that react not only with the xenobiotic, but also with mitochondrial autoantigens recognized by autoimmune PBC sera, suggests that environmental xenobiotic agents can be a risk factor for the induction of PBC.

Antimitochondrial antibodies in primary biliary cirrhosis: the role of xenobiotics

Primary biliary cirrhosis is an enigmatic autoimmune disease of women characterized by antimitochondrial antibodies and destruction of intrahepatic bile ducts. The etiology of PBC is unknown, but we present data herein that the disease may be induced by xenobiotic (i.e. chemicals) exposure. In particular, we postulate that halogenated compounds will bind to the autoantigen, break tolerance, and lead to an intense mucosal response.

Investigation of a mechanism for accelerated breakdown of immune tolerance to the primary biliary cirrhosis-associated autoantigen, pyruvate dehydrogenase complex

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by autoreactive T- and B-cell responses to the highly conserved enzyme pyruvate dehydrogenase complex (PDC). In this study we have examined the breakdown of T-cell tolerance to self-PDC using a mouse model. Female SJL/J mice were sensitized intraperitoneally with foreign-PDC (bovine) and/or self-PDC (murine) in complete Freund's adjuvant, and serum, spleen, and liver tissue was taken 8 weeks later. Animals sensitized with foreign-PDC produced IgG antibodies that were reactive with both foreign and self-PDC, but splenic T cells from these animals only responded to stimulation with foreign PDC. Sensitization with self-PDC elicited neither antibodies nor reactive T cells. Significantly, cosensitization with mixed self-PDC and foreign-PDC resulted in a full breakdown of self-tolerance, with generation of both antibody and T-cell responses to self-PDC of the type seen exclusively in human PBC patients. Mild bile duct lesions deficient in CD8(+) T cells were seen 8 weeks after sensitization with either foreign or self-PDC. However, after sensitization with mixed self-PDC and foreign-PDC, these lesions were significantly larger and heavily infiltrated by CD8(+) T cells. Liver-infiltrating T cells derived from the self-PDC and foreign-PDC cosensitized but not from control animals showed reactivity with self-PDC, suggesting a possible role for autoreactive PDC-specific T-cell responses in the pathogenesis of the observed histologic changes. It is likely that B-cell cross-reactivity between foreign and self-PDC enhances the potential for breakdown of T-cell self-tolerance by allowing efficient presentation of self-antigens in the inoculum. This model may provide a useful system for investigating the etiology and treatment of PBC.