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

Autoimmune Markers in Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease. The most common antibody associated with PBC is the anti-mitochondrial antibody (AMA), present in 90% to 95% of patients. For patients who are AMA-negative, novel biomarkers, such as antinuclear antibody-specific antibodies Sp100 and gp210 and anti-kelch-like-12 and anti-hexokinase-1 antibodies, may further aid in the diagnosis of PBC. Several laboratory methods, including immunofluorescence, enzyme-linked immunosorbent assay, immunoblotting, and bead-based assays, exist to evaluate for the presence of antibodies. This article describes various methods used to evaluate antibodies as well as describe the antibodies present in PBC.

New insights into the pathogenesis of primary biliary cholangitis asymptomatic stage

Primary biliary cholangitis (PBC) is a chronic cholestatic progressive liver disease and one of the most important progressive cholangiopathies in adults. Damage to cholangiocytes triggers the development of intrahepatic cholestasis, which progresses to cirrhosis in the terminal stage of the disease. Accumulating data indicate that damage to biliary epithelial cells [(BECs), cholangiocytes] is most likely associated with the intracellular accumulation of bile acids, which have potent detergent properties and damaging effects on cell membranes. The mechanisms underlying uncontrolled bile acid intake into BECs in PBC are associated with pH change in the bile duct lumen, which is controlled by the bicarbonate (HCO3-) buffer system "biliary HCO3- umbrella". The impaired production and entry of HCO3- from BECs into the bile duct lumen is due to epigenetic changes in expression of the X-linked microRNA 506. Based on the growing body of knowledge on the molecular mechanisms of cholangiocyte damage in patients with PBC, we propose a hypothesis explaining the pathogenesis of the first morphologic (ductulopenia), immunologic (antimitochondrial autoantibodies) and clinical (weakness, malaise, rapid fatigue) signs of the disease in the asymptomatic stage. This review focuses on the consideration of these mechanisms.

Immunologic Responses and the Pathophysiology of Primary Biliary Cholangitis

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

Differential immune response to xenobiotic-modified self-molecule in simple and connective tissue disease-associated primary biliary cholangitis

BACKGROUND AND AIMS

Our previous studies demonstrated that 2-octynoic acid (2OA) might alter the conformational structure of the inner lipoic acid (LA) binding domain (ILD) in the E2 subunit of pyruvate dehydrogenase complex (PDC-E2), leading to the loss of immune tolerance in simple primary biliary cholangitis (S-PBC). Here, we further explore if this etiological mechanism also accounts for connective tissue disease-associated PBC (CTD-PBC).

METHODS

Intein-mediated protein ligation was used to prepare ILD, LA-ILD and 2OA-ILD, and their reactivity with serum samples from 124 S-PBC and 132 CTD-PBC patients was examined. The antibodies to LA, 2OA, LA-ILD and 2OA-ILD, the isotypes of antibodies to LA, 2OA and ILD, were comparatively detected between the two patient groups by enzyme-linked immunosorbent assay and immunoblotting.

RESULTS

Both the percentage and reactivity of antibody to 2OA in S-PBC were significantly higher than in CTD-PBC. Antibodies to 2OA and to LA between the two groups separately shared the same characteristics. Remarkably, coexistence of the antibodies to LA-ILD and to 2OA, and coexistence of the antibodies to LA and to 2OA in S-PBC were both significantly more frequent than in CTD-PBC, whereas the percentage of anti-LA antibody without anti-2OA antibody in S-PBC was markedly lower than in CTD-PBC. Moreover, the isotype of antibody to LA was predominantly IgG in CTD-PBC, whilst this isotype was mainly IgM in S-PBC.

CONCLUSION

Xenobiotic 2OA might play less important pathogenic role in CTD-PBC than in S-PBC, suggesting that different underlying mechanisms are involved in their immune intolerance to PDC-E2.

E. coli and the etiology of human PBC: Antimitochondrial antibodies and spreading determinants

BACKGROUND AND AIMS

The increased frequency of urinary tract infections in patients with primary biliary cholangitis (PBC) and the cross-reactivity between the lipoyl domains (LD) of human pyruvate dehydrogenase complex (hPDC-E2) and Escherichia coli PDC-E2 (ePDC-E2) have long suggested a role of E. coli in causality of PBC. This issue, however, has remained speculative. We hypothesized that by generating specific constructs of human and E. coli PDC-E2, we would be able to assess the specificity of autoantibody responses and define whether exposure to E. coli in susceptible hosts is the basis for the antimitochondrial antibody (AMA) response.

APPROACH AND RESULTS

Importantly, the reactivity of hPDC-E2 LD (hPDC-E2LD) affinity-purified antibodies against hPDC-E2LD could only be removed by prior absorption with hPDC-E2LD and not ePDC-E2, suggesting the presence of unique human PDC-E2 epitopes distinct from E. coli PDC-E2. To identify the autoepitope(s) present in hPDC-E2LD, a more detailed study using a variety of PDC-E2 constructs was tested, including the effect of lipoic acid (LA) on ePDC-E2 conformation and AMA recognition. Individual recombinant ePDCE2 LD domains LD1, LD2 and LD3 did not react with either AMA or antibodies to LA (anti-LA), but in contrast, anti-LA was readily reactive against purified recombinant LD1, LD2, and LD3 expressed in tandem (LP); such reactivity increased when LP was precultured with LA. Moreover, when the three LD (LD1, LD2, LD3) domains were expressed in tandem in pET28a or when LD1 was expressed in another plasmid pGEX, they were lipoylated and reactive to PBC sera.

CONCLUSIONS

In conclusion, our data are consistent with an exposure to E. coli that elicits specific antibody to ePDC-E2 resulting in determinant spreading and the classic autoantibody to hPDC-E2LD. We argue this is the first step to development of human PBC.

A Case of Autoimmune Hepatitis/Primary Biliary Cholangitis Overlap Syndrome during Treatment with Brodalumab for Generalized Pustular Psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by accelerated tumor necrosis factor-α (TNF-α) /interleukin (IL) -23/IL-17 axis, epidermal hyperproliferation, and dysregulated differentiation. Psoriasis is occasionally associated with autoimmune liver diseases such as autoimmune hepatitis (AIH) or primary biliary cholangitis (PBC), caused by autoimmunity against hepatocyte- or cholangiocyte-specific autoantigens, respectively. Overlap syndrome is a condition in which patients have features of both AIH and PBC. It has been reported that AIH, PBC, or the overlap syndrome can be triggered by certain drug therapies. A 65-year-old Japanese man developed increased serum levels of aspartate and alanine aminotransferases, and positive anti-nuclear and anti-mitochondrial M2 antibodies, along with neutropenia, at 4 weeks of treatment with an anti-IL-17 receptor A antibody brodalumab for generalized pustular psoriasis. Histological evaluation of the liver revealed interface hepatitis and non-suppurative destructive cholangitis, which is compatible with the overlap syndrome of AIH and PBC. This is the first case of AIH/PBC overlap syndrome during treatment with brodalumab for generalized pustular psoriasis. The relationship between brodalumab and AIH/PBC overlap syndrome should be further elucidated. The risk of autoimmune liver diseases in patients with psoriasis treated with brodalumab should be carefully considered.

Investigation of immune complexes formed by mitochondrial antigens containing a new lipoylated site in sera of primary biliary cholangitis patients

Primary biliary cholangitis (PBC) is characterized by the presence of serum anti-mitochondrial autoantibodies (AMAs). To date, four antigens among the 2-oxo-acid dehydrogenase complex family, which commonly have lipoyl domains as an epitope, have been identified as AMA-corresponding antigens (AMA-antigens). It has recently been reported that AMAs react more strongly with certain chemically modified mimics than with the native lipoyl domains in AMA-antigens. Moreover, high concentrations of circulating immune complexes (ICs) in PBC patients have been reported. However, the existence of ICs formed by AMAs and their antigens has not been reported to date. We hypothesized that AMAs and their antigens formed ICs in PBC sera, and analyzed sera of PBC and four autoimmune diseases (Sjögren's syndrome, systemic lupus erythematosus, systemic scleroderma, and rheumatoid arthritis) using immune complexome analysis, in which ICs are separated from serum and are identified by nano-liquid chromatography-tandem mass spectrometry. To correctly assign MS/MS spectra to peptide sequences, we used a protein-search algorithm that including lipoylation and certain xenobiotic modifications. We found three AMA-antigens, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), the E2 subunit of the 2-oxo-glutarate dehydrogenase complex (OGDC-E2) and dihydrolipoamide dehydrogenase binding protein (E3BP), by detecting peptides containing lipoylation and xenobiotic modifications from PBC sera. Although the lipoylated sites of these peptides were different from the well-known sites, abnormal lipoylation and xenobiotic modification may lead to production of AMAs and the formation ICs. Further investigation of the lipoylated sites, xenobiotic modifications, and IC formation will lead to deepen our understanding of PBC pathogenesis.

Chemical Probes Reveal Sirt2's New Function as a Robust "Eraser" of Lysine Lipoylation

Lysine lipoylation, a highly conserved lysine post-translational modification, plays a critical role in regulating cell metabolism. The catalytic activity of a number of vital metabolic proteins, such as pyruvate dehydrogenase (PDH), depends on lysine lipoylation. Despite its important roles, the detailed biological regulatory mechanism of lysine lipoylation remains largely unexplored. Herein we designed a powerful affinity-based probe, KPlip, to interrogate the interactions of lipoylated peptide/proteins under native cellular environment. Large-scale chemical proteomics analysis revealed a number of binding proteins of KPlip, including sirtuin 2 (Sirt2), an NAD⁺-dependent protein deacylase. To explore the potential activity of Sirt2 toward lysine lipoylation, we designed a single-step fluorogenic probe, KTlip, which reports delipoylation activity in a continuous manner. The results showed that Sirt2 led to significant delipoylation of KTlip, displaying up to a 60-fold fluorescence increase in the assay. Further kinetic experiments with different peptide substrates revealed that Sirt2 can catalyze the delipoylation of peptide (DLAT-PDH, K259) with a remarkable catalytic efficiency (k_cat/K_m) of 3.26 × 10³ s^-1 M^-1. The activity is about 400-fold higher than that of Sirt4, the only mammalian enzyme with known delipoylation activity. Furthermore, overexpression and silencing experiments demonstrated that Sirt2 regulates the lipoylation level and the activity of endogenous PDH, thus unequivocally confirming that PDH is a genuine physiological substrate of Sirt2. Using our chemical probes, we have successfully established the relationship between Sirt2 and lysine lipoylation in living cells for the first time. We envision that such chemical probes will serve as useful tools for delineating the roles of lysine lipoylation in biology and diseases.

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.

Autoreactive monoclonal antibodies from patients with primary biliary cholangitis recognize environmental xenobiotics

A major problem in autoimmunity has been identification of the earliest events that lead to breach of tolerance. Although there have been major advances in dissecting effector pathways and the multilineage immune responses to mitochondrial self-antigens in primary biliary cholangitis, the critical links between environmental factors and tolerance remain elusive. We hypothesized that environmental xenobiotic modification of the E2 subunit of the pyruvate dehydrogenase (PDC-E2) inner lipoyl domain can lead to loss of tolerance to genetically susceptible hosts. Previously we demonstrated that serum anti-PDC-E2 autoantibodies cross-react with the chemical xenobiotics 2-octynoic acid and 6,8-bis (acetylthio) octanoic acid and further that there is a high frequency of PDC-E2-specific peripheral plasmablasts. Herein we generated 104 recombinant monoclonal antibodies (mAbs) based on paired heavy-chain and light-chain variable regions of individual plasmablasts derived from primary biliary cholangitis patients. We identified 32 mAbs reactive with native PDC-E2, including 20 specific for PDC-E2 and 12 cross-reactive with both PDC-E2 and 2-octynoic acid and 6,8-bis (acetylthio) octanoic acid. A lower frequency of replacement somatic hypermutations, indicating a lower level of affinity maturation, was observed in the complementarity-determining regions of the cross-reactive mAbs in comparison to mAbs exclusively recognizing PDC-E2 or those for irrelevant antigens. In particular, when the highly mutated heavy-chain gene of a cross-reactive mAb was reverted to the germline sequence, the PDC-E2 reactivity was reduced dramatically, whereas the xenobiotic reactivity was retained. Importantly, cross-reactive mAbs also recognized lipoic acid, a mitochondrial fatty acid that is covalently bound to PDC-E2.

CONCLUSION

Our data reflect that chemically modified lipoic acid or lipoic acid itself, through molecular mimicry, is the initial target that leads to the development of primary biliary cholangitis. (Hepatology 2017;66:885-895).

The fingerprint of antimitochondrial antibodies and the etiology of primary biliary cholangitis

The identification of environmental factors that lead to loss of tolerance has been coined the holy grail of autoimmunity. Our work has focused on the reactivity of antimitochondrial autoantibodies (AMA) to chemical xenobiotics and has hypothesized that a modified peptide within PDC-E2, the major mitochondrial autoantigen, will have been immunologically recognized at the time of loss of tolerance. Herein, we successfully applied intein technology to construct a PDC-E2 protein fragment containing amino acid residues 177-314 of PDC-E2 by joining a recombinant peptide spanning residues 177-252 (PDC-228) with a 62-residue synthetic peptide from 253 to 314 (PP), which encompasses PDC-E2 inner lipoyl domain (ILD). We named this intein-constructed fragment PPL. Importantly, PPL, as well as lipoic acid conjugated PPL (LA-PPL) and xenobiotic 2-octynoic acid conjugated PPL (2OA-PPL), are recognized by AMA. Of great importance, AMA has specificity for the 2OA-modified PDC-E2 ILD peptide backbone distinct from antibodies that react with native lipoylated PDC-E2 peptide. Interestingly, this unique AMA subfraction is of the immunoglobulin M isotype and more dominant in early-stage primary biliary cholangitis (PBC), suggesting that exposure to 2OA-PPL-like compounds occurs early in the generation of AMA. To understand the structural basis of this differential recognition, we analyzed PPL, LA-PPL, and 2OA-PPL using electron paramagnetic resonance spectroscopy, with confirmations by enzyme-linked immunosorbent assay, immunoblotting, and affinity antibody analysis. We demonstrate that the conformation of PDC-E2 ILD is altered when conjugated with 2OA, compared to conjugation with lipoic acid.

CONCLUSION

A molecular understanding of the conformation of xenobiotic-modified PDC-E2 is critical for understanding xenobiotic modification and loss of tolerance in PBC with widespread implications for a role of environmental chemicals in the induction of autoimmunity. (Hepatology 2017;65:1670-1682).

Environmental Basis of Autoimmunity

The three common themes that underlie the induction and perpetuation of autoimmunity are genetic predisposition, environmental factors, and immune regulation. Environmental factors have gained much attention for their role in triggering autoimmunity, with increasing evidence of their influence as demonstrated by epidemiological studies, laboratory research, and animal studies. Environmental factors known to trigger and perpetuate autoimmunity include infections, gut microbiota, as well as physical and environmental agents. To address these issues, we will review major potential mechanisms that underlie autoimmunity including molecular mimicry, epitope spreading, bystander activation, polyclonal activation of B and T cells, infections, and autoinflammatory activation of innate immunity. The association of the gut microbiota on autoimmunity will be particularly highlighted by their interaction with pharmaceutical agents that may lead to organ-specific autoimmunity. Nonetheless, and we will emphasize this point, the precise mechanism of environmental influence on disease pathogenesis remains elusive.

Primary Biliary Cholangitis: Medical and Specialty Pharmacy Management Update

BACKGROUND

Chronic liver disease and cirrhosis are a leading cause of morbidity and mortality in the United States. Primary biliary cholangitis (PBC), previously known as primary biliary cirrhosis and which has been designated an orphan condition, is a chronic autoimmune disease resulting in the destruction of the small bile ducts in the liver. Without effective treatment, disease progression frequently leads to liver failure and death. Until May 2016, the only FDA-approved treatment for PBC was ursodiol (UDCA), an oral hydrophilic bile acid, which can slow progression of liver damage due to PBC. However, 1 out of 3 patients taking UDCA has an inadequate biochemical response, leading to increased risk of disease progression, liver transplantation, and mortality. Given this unmet clinical need, new therapies are in development for the treatment of PBC. To provide pharmacists with an overview of the latest research on the pathophysiology of PBC and potential new treatment options and to highlight medical and specialty pharmacy approaches to managing access to drugs to treat orphan diseases such as PBC, a 2-hour satellite symposium was presented in conjunction with the 2015 Academy of Managed Care Pharmacy (AMCP) Nexus meeting. Although obeticholic acid was approved by the FDA for the treatment of PBC in May 2016, this development occurred after the symposium presentation. The symposium was supported by an independent educational grant from Intercept Pharmaceuticals and was managed by Analysis Group. Robert Navarro, PharmD, moderated the CPE-accredited symposium titled "Medical and Specialty Pharmacy Management Update on Primary Biliary Cirrhosis." Expert panelists included Christopher L. Bowlus, MD; James T. Kenney, RPh, MBA; and Gary Rice, RPh, MS, MBA, CSP.

OBJECTIVE

To summarize the educational satellite symposium presentations and discussions.

SUMMARY

Autoimmune liver diseases, including PBC, are responsible for 15% of all liver transplants performed and an equal percentage of deaths related to liver disease. UDCA is the only FDA-approved therapy for treatment of PBC and is considered the standard of care. Nevertheless, many patients do not respond to UDCA, creating the need for new therapeutic options to improve clinical outcomes for PBC patients with inadequate response to treatment. While several agents are being studied in combination with UDCA, monotherapy with the novel agent obeticholic acid, a farnesoid X receptor agonist, has also shown promising results. Health plans are anticipated to assign any newly introduced therapy for the treatment of PBC to specialty pharmacy given its orphan disease status. This assignment enables the health plan to receive disease education, which is particularly important when new drugs are indicated for orphan diseases, and assistance with designing appropriate prior authorization criteria. The clinical value of any new therapeutic options that will inform formulary decisions and prior authorization criteria will be assessed based on evidence of efficacy, safety, and tolerability, among other factors, such as the potential to reduce or delay medical resource utilization (e.g., liver transplant). Key considerations for prior authorization of a new therapy will be determining which PBC patients are appropriate candidates for the new therapy and developing criteria for that determination. These are likely to include clinical diagnostic criteria and degree of response to prior treatment with UDCA. Initially, any new therapy would likely be positioned as noncovered until appropriate prior authorization criteria are established.

CONCLUSIONS

PBC is a chronic liver disease with significant morbidity and mortality, as well as a significant burden on the health care system if the disease progresses to the point at which a liver transplant is needed. Although UDCA, the current standard of care, has improved outcomes for many patients, others have an inadequate response to this treatment. This symposium discussed these issues and also addressed the overall treatment paradigm for orphan drug therapies, key implications for patient management, and the role of specialty pharmacy management and any associated needs both in general and specifically for new therapeutic options for PBC.

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.

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.

Anti-kelch-like 12 and anti-hexokinase 1: novel autoantibodies in primary biliary cirrhosis

BACKGROUND & AIMS

Using high-density human recombinant protein microarrays, we identified two potential biomarkers, kelch-like 12 (KLHL12) and hexokinase-1 (HK1), in primary biliary cirrhosis (PBC). The objective of this study was to determine the diagnostic value of anti-KLHL12/HK1 autoantibodies in PBC. Initial discovery used sera from 22 patients with PBC and 62 non-PBC controls. KLHL12 and HK1 proteins were then analysed for immunoglobulin reactivity by immunoblot and enzyme-linked immunosorbent assay (ELISA) in two independent cohorts of PBC and disease/healthy control patients.

METHODS

Serum samples from 100 patients with PBC and 165 non-PBC disease controls were analysed by immunoblot and samples from 366 patients with PBC, 174 disease controls, and 80 healthy donors were tested by ELISA.

RESULTS

Anti-KLHL12 and anti-HK1 antibodies were each detected more frequently in PBC compared with non-PBC disease controls (P < 0.001). Not only are both markers highly specific for PBC (≥95%) but they also yielded higher sensitivity than anti-gp210 and anti-sp100 antibodies. Combining anti-HK1 and anti-KLHL12 with available markers (MIT3, gp210 and sp100), increased the diagnostic sensitivity for PBC. Most importantly, anti-KLHL12 and anti-HK1 antibodies were present in 10-35% of anti-mitochondrial antibody (AMA)-negative PBC patients and adding these two biomarkers to conventional PBC assays dramatically improved the serological sensitivity in AMA-negative PBC from 55% to 75% in immunoblot and 48.3% to 68.5% in ELISA.

CONCLUSIONS

The addition of tests for highly specific anti-KLHL12 and anti-HK1 antibodies to AMA and ANA serological assays significantly improves efficacy in the clinical detection and diagnosis of PBC, especially for AMA-negative subjects.

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.

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.

Antimitochondrial antibody recognition and structural integrity of the inner lipoyl domain of the E2 subunit of pyruvate dehydrogenase complex

Antimitochondrial autoantibodies (AMAs), the serological hallmark of primary biliary cirrhosis, are directed against the lipoyl domain of the E2 subunit of pyruvate dehydrogenase (PDC-E2). However, comprehensive analysis of the amino acid residues of PDC-E2 lipoyl β-sheet with AMA specificity is lacking. In this study, we postulated that specific residues within the lipoyl domain are critical to AMA recognition by maintaining conformational integrity. We systematically replaced each of 19 residue peptides of the inner lipoyl domain with alanine and analyzed these mutants for reactivities against 60 primary biliary cirrhosis and 103 control sera. Based on these data, we then constructed mutants with two, three, or four replacements and, in addition, probed the structure of the substituted domains using thiol-specific spin labeling and electron paramagnetic resonance (EPR) of a (5)Ile→Ala and (12)Ile→Ala double mutant. Single alanine replacement at (5)Ile, (12)Ile, and (15)Glu significantly reduced AMA recognition. In addition, mutants with two, three, or four replacements at (5)Ile, (12)Ile, and (15)Glu reduced AMA reactivity even further. Indeed, EPR reveals a highly flexible structure within the (5)Ile and (12)Ile double-alanine mutant. Autoreactivity is largely focused on specific residues in the PDC-E2 lipoyl domain critical in maintaining the lipoyl loop conformation necessary for AMA recognition. Collectively, the AMA binding studies and EPR analysis demonstrate the necessity of the lipoyl β-sheet structural conformation in anti-PDC-E2 recognition.

Human intrahepatic biliary epithelial cells engulf blebs from their apoptotic peers

The phagocytic clearance of apoptotic cells is critical for tissue homeostasis; a number of non-professional phagocytic cells, including epithelial cells, can both take up and process apoptotic bodies, including the release of anti-inflammatory mediators. These observations are particularly important in the case of human intrahepatic biliary cells (HiBEC), because such cells are themselves a target of destruction in primary biliary cirrhosis, the human autoimmune disease. To address the apoptotic ability of HiBECs, we have focused on their ability to phagocytize apoptotic blebs from autologous HiBECs. In this study we report that HiBEC cells demonstrate phagocytic function from autologous HiBEC peers accompanied by up-regulation of the chemokines CCL2 [monocyte chemotactic protein-1 (MCP-1)] and CXCL8 [interleukin (IL)-8]. In particular, HiBEC cells express the phagocytosis-related receptor phosphatidylserine receptors (PSR), implying that HiBECs function through the 'eat-me' signal phosphatidylserine expressed by apoptotic cells. Indeed, although HiBEC cells acquire antigen-presenting cell (APC) function, they do not change the expression of classic APC function surface markers after engulfment of blebs, both with and without the presence of Toll-like receptor (TLR) stimulation. These results are important not only for understanding of the normal physiological function of HiBECs, but also explain the inflammatory potential and reduced clearance of HiBEC cells following the inflammatory cascade in primary biliary cirrhosis.

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.

Antimitochondrial antibody heterogeneity and the xenobiotic etiology of primary biliary cirrhosis

Antimitochondrial antibodies (AMAs) directed against the lipoyl domain of the E2 subunit of pyruvate dehydrogenase (PDC-E2) are detected in 95% of patients with primary biliary cirrhosis (PBC) and are present before the onset of clinical disease. The recent demonstration that AMAs recognize xenobiotic modified PDC-E2 with higher titers than native PDC-E2 raises the possibility that the earliest events involved in loss of tolerance are related to xenobiotic modification. We hypothesized that reactivity to such xenobiotics would be predominantly immunoglobulin M (IgM) and using sera from a large cohort of PBC patients and controls (n = 516), we examined in detail sera reactivity against either 6,8-bis(acetylthio) octanoic acid (SAc)-conjugated bovine serum albumin (BSA), recombinant PDC-E2 (rPDC-E2) or BSA alone. Further, we also defined the relative specificity to the SAc moiety using inhibition enzyme-linked immunosorbent assay (ELISA); SAc conjugate and rPDC-E2-specific affinity-purified antibodies were also examined for antigen specificity, isotype, and crossreactivity. Reactivity to SAc conjugates is predominantly IgM; such reactivity reflects a footprint of previous xenobiotic exposure. Indeed, this observation is supported by both direct binding, crossreactivity, and inhibition studies. In both early and late-stage PBC, the predominant Ig isotype to SAc is IgM, with titers higher with advanced stage disease. We also note that there was a higher level of IgM reactivity to SAc than to rPDC-E2 in early-stage versus late-stage PBC. Interestingly, this finding is particularly significant in light of the structural similarity between SAc and the reduced form of lipoic acid, a step which is similar to the normal physiological oxidation of lipoic acid.

CONCLUSION

Specific modifications of the disulfide bond within the lipoic-acid-conjugated PDC-E2 moiety, i.e., by an electrophilic agent renders PDC-E2 immunogenic in a genetically susceptible host.

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.

Autoimmunity in 2011

The mechanisms leading to the onset and perpetuation of systemic and tissue-specific autoimmune diseases are complex, and numerous hypotheses have been proposed or confirmed over the past 12 months. It is particularly of note that the number of articles published during 2011 in the major immunology and autoimmunity journals increased by 3 % compared to the previous year. The present article is dedicated to a brief review of the reported data and, albeit not comprehensive of all articles, is aimed at identifying common and future themes. First, clinical researchers were particularly dedicated to defining refractory forms of diseases and to discuss the use and switch of therapeutic monoclonal antibodies in everyday practice. Second, following the plethora of genome-wide association studies reported in most multifactorial diseases, it became clear that genomics cannot fully explain the individual susceptibility and additional environmental or epigenetic factors are necessary. Both these components were widely investigated, both in organ-specific (i.e., type 1 diabetes) and systemic (i.e., systemic lupus erythematosus) diseases. Third, a large number of 2011 works published in the autoimmunity area are dedicated to dissect pathogenetic mechanisms of tolerance breakdown in general or in specific conditions. While our understanding of T regulatory and Th17 cells has significantly increased in 2011, it is of note that most of the proposed lines of evidence identify potential targets for future treatments and should not be overlooked.

Serum autoantibodies for the diagnosis and management of autoimmune liver diseases

The spectrum of autoimmune liver diseases (AILD) includes primary biliary cirrhosis, primary sclerosing cholangitis and autoimmune hepatitis. The immunological mechanisms triggering the initiation and perpetuation of AILD remains unknown, while autoantigens are now recognized in most cases, and are generally nontraditional in their widespread distribution. Sensitive and specific methods for the detection of serum autoantibodies in patients affected by AILD represent a challenge for researchers and clinicians who desire to obtain an early and certain diagnosis as well as markers of disease control. To this regard, the use and interpretation of serum autoantibodies in AILD may be seen as paradigmatic for the large gaps in our knowledge based on the lack of true population-based studies. The present review article will critically discuss the available evidence on the use of autoantibody findings in the diagnosis or management of autoimmune liver disease.

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.

Recent advances in understanding the pathogenesis of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic progressive cholestatic autoimmune liver disease characterized by the destruction of small intrahepatic bile ducts and the presence of highly specific serum antimitochondrial antibodies (AMAs). The human leukocyte antigen (HLA) gene has been proved to have strongest association with PBC susceptibility, and non-HLA genes, including IL12A, IL12RB2, STAT4, IRF5-TNPO3, ORMDL3/IKZF3, MMEL1, SPIB, CTLA-4, FCRL3 and A2BP1, are also closely associated with PBC susceptibility. Four AMAs including anti-M2, anti-M4, anti-M8 and anti-M9, and antinuclear antibodies (ANAs), such as antinuclear dot antibodies (SP100, PML, NDP52, SP140), antinuclear pore antibodies (gp210, p62), antinuclear envelope antibodies (Lamin and Lamin B receptor), and anti-centromere antibodies, may also be involved in the pathogenesis of PBC. The imbalance between Th17 cells and regulatory T lymphocytes (Treg) may also play an important role in the pathogenesis of PBC. In addition, senescence, autophagy, apoptosis of biliary epithelial cells (BECs), and environmental factors, such as Epstein-Barr virus (EBV) infection and smoking, may also contribute to the pathogenesis of PBC. Understanding of the mechanisms responsible for the pathogenesis of PBC has important implications for the treatment of PBC.

Xenobiotics and autoimmunity: does acetaminophen cause primary biliary cirrhosis?

The serologic hallmark of primary biliary cirrhosis (PBC) is the presence of antimitochondrial autoantibodies (AMAs) directed against the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2). The PBC-related autoepitope of PDC-E2 contains lipoic acid, and previous work has demonstrated that mimics of lipoic acid following immunization of mice lead to a PBC-like disease. Furthermore, approximately one-third of patients who have ingested excessive amounts of acetaminophen (paracetamol) develop AMA of the same specificity as patients with PBC. Quantitative structure-activity relationship (QSAR) data indicates that acetaminophen metabolites are particularly immunoreactive with AMA, and we submit that in genetically susceptible hosts, electrophilic modification of lipoic acid in PDC-E2 by acetaminophen or similar drugs can facilitate a loss of tolerance and lead to the development of PBC.

Xenobiotic-induced liver injury and fibrosis

INTRODUCTION

Many different drugs and xenobiotics (chemical compounds foreign to an organism) can injure the bile duct epithelium and cause inflammatory bile duct diseases (cholangiopathies) ranging from transient cholestasis to vanishing bile duct syndrome, sclerosing cholangitis with development of biliary fibrosis and cirrhosis. Animal models of xenobiotic-induced liver injury have provided major mechanistic insights into the molecular mechanisms of xenobiotic-induced cholangiopathies and biliary fibrosis including primary biliary cirrhosis and primary sclerosing cholangitis.

AREAS COVERED

In this review, the authors discuss the basic principles of xenobiotic-induced liver and bile duct injury and biliary fibrosis with emphasis on animal models. A PubMed search was performed using the search terms "xenobiotic," "liver injury," "cholestasis," and "biliary fibrosis." Reference lists of retrieved articles were also searched for relevant literature.

EXPERT OPINION

Xenobiotic-induced cholangiopathies are underestimated and frequently overlooked medical conditions due to their often transient nature. However, biliary disease may progress to vanishing bile duct syndrome, biliary fibrosis, and cirrhosis. Moreover, xenobiotics may prime the liver for subsequent liver disease by other agents and may also contribute to the development of hepatobiliary cancer though interaction with resident stem cells.