Journal of Hepatology
Volume 39, Issue 4 , Pages 639-648, October 2003

Pathogenesis of primary biliary cirrhosis

Centre for Liver Research, University of Newcastle, School of Clinical Medical Sciences, 4th Floor William Leech Building, Medical School, Framlington Place, Newcastle-upon-Tyne NE2 4HH, UK

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1. Introduction 

Primary biliary cirrhosis (PBC) was first described by Addison and Gull in 1857. Previously regarded as a rare condition, recent systematic epidemiological studies have suggested that it may in fact affect as many as 30 per 100,000 of the population in high prevalence areas [1], rising to 1 in 700 in women over the age of 40 (the most affected demographic group). Recent higher prevalence rates reflect, in part, increased awareness of the disease and increased availability and application of serological diagnostic tools [2]. The condition is characterised by damage to, and destruction of, the biliary epithelial cells (BEC) lining the small intra-hepatic bile ducts. Bile duct loss is progressive and results, over time, in the development of fibrosis and biliary cirrhosis. Tissue damage is largely restricted to the liver, associated extra-hepatic abnormality being essentially limited to the salivary and lachrymal glands (secondary Sjogrens syndrome).

The direction taken in the study of the pathogenesis of PBC has, over the last 20 years, been heavily influenced by the observation that patients typically have high titres of serum autoantibodies directed against highly conserved, mitochondrially expressed, autoantigens (most classically pyruvate dehydrogenase complex (PDC)). One of the paradoxes PBC presents us, however, is that it is a disease with a very limited tissue distribution which is seemingly characterised by autoreactive immune responses directed at ubiquitous self-antigens. This has led some to doubt whether autoreactive immune responses in fact play any meaningful role in the pathogenesis of PBC. The aim of this review is to summarise the current state of our understanding of the pathogenesis of PBC.

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2. The population and molecular epidemiology of PBC 

Recent studies have suggested that PBC clusters both within families and geographical areas. Numerous early case reports and series identified familial cases of PBC, hinting at the possibility of increased disease prevalence within families. That this is indeed the case has been confirmed in a recent comprehensive, geographically-based, cohort study. In this study from the North-East of England the λs (sibling relative risk) for PBC was estimated at 10.5 suggesting a familial risk similar to that seen in the more classical autoimmune diseases [3]. One interpretation of this observation would be that there is a genetic component to PBC susceptibility. Families typically, however, share environment as well as genes and it is impossible to exclude, from these studies alone, an environmental explanation for familial disease risk. The classical approach to dissecting genetic contribution to complex disease is through the study of concordance rates in twins. Until recently no twin data (other than a single case report) were available for PBC. A recent study, published to date in abstract form only, has suggested a concordance rate of 75% for PBC in monozygotic twin pairs [4]. If confirmed this observation would suggest a significant genetic contribution to disease susceptibility.

If there is a significant genetic contribution to the pathogenesis of PBC the responsible loci remain unidentified. Linkage-based approaches have not, as yet, been applied to PBC, largely because the necessary informative families are not frequently encountered. Several large-scale international collaborative projects are underway to collect such informative families. In the absence of the appropriate families in which to use linkage-based approaches studies of the genetic basis of PBC performed to date have exclusively used the candidate gene association approach. This approach, although able to identify relatively weakly associated susceptibility loci with the potential to be missed when linkage-based approaches are used, is prone to error, particularly when small patient cohorts and/or inappropriate control groups are used. This potential for error would be one explanation for the lack of reproducibility of many genetic associations described in PBC. The only convincingly reproducible association identified to date is with DRB1*08 (DRB1*0801 in Northern European populations and DRB1*0803 in Japanese population). This association is statistically robust, but accounts for only relatively little of the putative genetic component to disease susceptibility. The current state of knowledge regarding genetic factor contributing to PBC susceptibility has been extensively reviewed elsewhere recently [5].

Evidence to support environmental exposure as a risk factor for PBC came from early studies highlighting micro-geographical variation in disease prevalence in Sheffield in Northern England, with supply of water from a specific source being highlighted as being one associated feature [6]. Geographical clustering within a UK health administration area has recently been confirmed using formal cluster analysis [7]. Just as familial clustering cannot exclude an environmental aetiological factor, however, geographical clustering cannot exclude a genetic explanation given the stability of the North-Eastern population studied (the vast majority of patients in this study remaining resident in the area in which they were born). Some geographical clustering in this study resulted, therefore, from co-affected family members living close to each other. The degree of the clustering seen would suggest that this effect cannot, however, be the sole explanation. This study would therefore support some form of environmental risk factor. The obvious candidates would be an as yet unidentified infectious agent and chemical environmental exposure (in its broadest sense incorporating food, water and air borne factors). It is intriguing to note that the areas identified as having high disease incidence in both the Triger and the Prince studies were ones with high levels of previous, but now discontinued, coal mining activity.

Attempts have been undertaken to identify putative specific environmental triggers using case control studies assessing history of exposure to envisaged risk factors. Comprehensive studies have been performed in both North-Eastern England and the USA [8], [9]. Neither identified significant disease associated exposure. One problem inherent in the design of such studies in PBC is, however, the fact that the disease has a prolonged clinical prodrome and the environmental exposure event of interest may have occurred many years before presentation, let alone participation in the case-control study.

The consensus from these epidemiological and genetic studies in PBC is, perhaps unsurprisingly, that the disease has a complex aetiology with both genetic and environmental susceptibility factors playing roles.

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3. The mechanism of biliary epithelial cell damage in PBC 

The histological changes seen in the liver in PBC are highly consistent. What clues do they give us regarding the aetiology of the disease? There is broad consensus that the key lesion in PBC is BEC damage and loss, with the development of fibrosis and its sequelae being a more generic secondary process shared with other forms of biliary disease. Although the issue of the downstream events leading to biliary cirrhosis is beyond the scope of this review, it should be borne in mind that such events are an important potential target for therapeutic intervention, and may represent sites where agents such as ursodeoxycholic acid have their principal effects.

Bile duct damage occurs in portal tracts in the context of a lymphocyte rich mononuclear cell infiltrate. T-cells predominate, with CD8+ cells particularly prominent in the peri-ductular areas [10]. The cellular infiltrate also includes significant numbers of eosinophils (especially in early disease). Heterogeneous expression of ‘Th1’ (interferon-γ) and ‘Th2’ (interleukin, IL-4 and IL-5) cytokine patterns has been reported [11], [12].

In PBC both BEC and salivary epithelial cells appear to undergo a phenotypic change, demonstrating apical surface upregulation of either PDC or an antigen cross-reactive with it [13]. PDC is normally restricted in its expression to the inner mitochondrial membrane (following translation in the endoplasmic reticulum and targeted active uptake by the mitochondria). No other situation in which cell surface expression of PDC occurs has as yet been described. This surface expression of PDC appears to start early in the disease process occurring before the well characterised upregulation of class II MHC and ICAM-1 [14]. It does not appear to occur as a simple consequence of increased PDC component transcription as elevation in PDC component mRNA levels is not seen in affected bile duct cells [15].

There is strong evidence to support the view that the actual mechanism of BEC loss is apoptosis. Markers suggestive of the presence of ongoing apoptosis within affected portal tracts include DNA fragmentation and Fas, Lewis Y antigen, bax and bcl-x expression [16], [17], [18], [19], [20]. Down-regulation of the inhibitor of apoptosis bcl-2 has also been reported [21]. The presence of granzyme B and Fas-ligand expressing mononuclear cells on immuno-histochemistry [22], [23], [24], and the demonstration of cytotoxic effector cell function in vitro for CD8+ cells isolated from PBC, are suggestive that the pro-apoptotic signal to the BEC is delivered by cytotoxic effector cells [25] (Fig. 1).

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  • Fig. 1. 

    A model for immune mediated BEC injury in PBC. In this model the final pathway leading to BEC loss is apoptosis triggered by cytotoxic T-cell (or other immune effector cell) pathways. Potential pro-apoptotic signals include Fas-ligand and granzyme B. Cytotoxic T-cell activity is supported in by CD4+ T-cell ‘help’ mediated in part by localised cytokine secretion. CD4+ T-cell activation is induced by professional APC. Much interest has recently been focused on the context of antigen presentation and, in particular, importance of activation of APCs via by pathogen-derived factors via the highly conserved family of TLR. This model does not imply the identity of the MHC class I and II restricted epitopes which could, based on the situ evidence of effector cell function alone, be derived from infectious agent and self antigens alike.

If cytotoxic effector cells are delivering the final pro-apoptosis signal to BEC why might such a signal be being delivered? In essence there are two potential scenarios.

1.An appropriate response to infected (viral or otherwise) or otherwise harmfully modified BEC.

2.An inappropriate response to normal or harmlessly modified BEC (e.g. autoimmune response).

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4. Models for the pathogenesis of PBC 

4.1. An appropriate response to infected cells (PBC as an infectious disease) 

Given the epidemiological evidence supporting an environmental factor in the pathogenesis of PBC infectious agents are obvious candidates as disease triggers/causative agents. Infectious agents could, in theory, contribute to the pathogenesis of PBC in one of two ways; as direct pathogenetic factors (infecting BEC and directly inducing apoptosis or acting as a stimulus for immune mediated clearance of infected cells) or as triggers for autoimmunity (a scenario which will be discussed in more detail below). Evidence has been cited to support a role for both bacteria and, more latterly, chlamydial species and retroviruses in the pathogenesis of PBC.

There is some evidence to suggest that PBC patients suffer from an increased incidence of bacterial infections. A high incidence of urinary tract infection has been particularly highlighted. An increased incidence of urinary tract infection prior to the diagnosis of PBC (and at a stage, therefore, where an aetiological role would be possible), was not, however, reported in either of the population case-control studies [8], [9]. More recent studies implicating mycobacterial infection in disease pathogenesis have, similarly, proved not to be reproducible. Moreover, the case control epidemiological studies again failed to identify an increased prevalence of previous clinical mycobacterial infection in PBC patients.

In situ studies of liver tissue do, however, provide some support for the bacterial infection hypothesis. Scavenger cells surrounding damaged inter-lobular bile ducts have been shown to contain lipoteichoic acid from gram positive bacteria [26]. Moreover, features of the host response seen in PBC, such as the presence of MCP-2 and MCP-3 expressing mononuclear cells in the portal tract infiltrate and around the periphery of the archetypal epithelioid granulomata have also been interpreted as suggesting a role for localised bacterial infection [27]. It is impossible, however, to exclude the possibility that features associated with localised bacterial infection reflect secondary infection in damaged tissues of debilitated patients rather than a primary process.

More recently data have been presented (to date in abstract form only) supporting infection with Chlamydia pneumoniae as a potential factor in the pathogenesis of PBC [28]. In this study chlamydial antigen was found in all of 25 PBC liver samples but only nine of 105 normal and other liver disease control liver samples. Independent replication of this observation is awaited.

An alternative model for the pathogenesis of PBC invokes retroviral infection as a key aetiological process. PBC patients appear to have an increased prevalence of serum antibodies reactive with retroviral antigens [29]. Moreover, viral particles have been identified in biliary epithelium by electron microscopy, and exogenous retroviral nucleotide sequences cloned from biological material from PBC patients [30]. Co-culture of BEC from normal liver with tissue extracts derived from the draining lymph nodes of the liver from advanced disease PBC patients induces upregulation of a cell surface antigen showing cross-reactivity with PDC (the surface expression phenotype characteristic of BEC in PBC) [31]. Finally, when the supernatant from these cultures is added to fresh cultures of normal liver BEC surface expression of PDC cross-reactive antigen is once again seen [32]. Retroviral sequences could also be isolated from these “passaging” experiment cultures [30]. Taken together, these observations have been interpreted as indicating a retroviral aetiology for PBC [33].

Whilst potentially highly significant some caution should be applied when addressing the retroviral model and the data used to support it. At a general level, the role played by retroviral infection in the pathogenesis of autoimmunity has been much debated and several aetiological agents identified in different autoimmune diseases. These retroviral associations have, however, largely not stood up to later scrutiny and the whole question of the role played by retroviruses in autoimmune disease remains, accordingly, deeply controversial [34]. At a specific level contamination of cells in culture by animal retroviruses is a well recognised laboratory problem [35], [36]. The similarity between the betaretrovirus isolated from biological material from PBC patients and mouse mammary tumour virus (MMTV; a widely used laboratory agent) should be noted in this regard [30].

Given the potential importance of the retroviral model confirmatory studies by independent laboratories are badly needed. Until such studies have been performed, and any aetiological role for retroviral infection in PBC confirmed, this author believes that proposed studies of potentially toxic anti-retroviral therapy in PBC are premature.

4.2. An inappropriate response to normal cells (PBC as an autoimmune disease) 

Autoantigens constitute the other putative targets for cytotoxic effector responses directed at BEC in PBC. Whilst observations such as the increased incidence of autoimmune disease patients [5] are supportive of autoimmunity as a pathogenetic process in PBC, the key evidence to support the autoimmune model is the almost universal presence of autoreactive responses in PBC patients. Paradoxically, for a disease in which the role played by autoreactive responses has been questioned, PBC is in many ways a landmark disease for our understanding of autoreactivity representing, as it does, one of the first diseases in which autoreactive immune responses were identified, and one of the first diseases in which the identity of the antigen against which these autoreactive responses are directed was established. The presence of high titre serum autoantibodies in PBC was first described in 1958 [37]. In a series of subsequent landmark studies the non-organ, non-species specific antigens against with which these antibodies are reactive were identified first as being contained within mitochondria [38], second as being expressed on the inner mitochondrial membrane [39], and finally, following the cloning of a mitochondrial expressed antigen [40] as being members of the 2-oxo-acid dehydrogenase family of multi-enzyme complexes (in particular PDC) [41], [42].

Autoantibodies reactive with mitochondrial antigens (anti-mitochondrial antibodies (AMA)) remain one of the key diagnostic tools for the disease, the presence of these antibodies having both 95% specificity and sensitivity for the diagnosis of PBC [43]. The strength of the association between AMA and the PBC disease process is indicated by the fact that the majority of individuals who are AMA positive in the absence of either symptoms suggestive of PBC or cholestatic liver biochemistry still have features suggestive of early PBC on liver biopsy and, when followed up over a number of years, go on to develop typical disease [44].

Characterisation of autoreactive responses in PBC started with the autoantibody responses exemplified by AMA. More recent studies have addressed the question of autoreactive T-cell responses. The current focus is on mechanisms whereby tolerance breakdown occurs, and the nature and extent of the role played by autoreactive responses in disease pathogenesis.

4.3. Autoantibody responses in PBC 

The best characterised autoantibody responses in PBC are directed at PDC and the other members of the 2-oxo-acid dehydrogenase family of multi-enzyme complexes (oxo-glutarate dehydrogenase complex (OGDC) and branch-chain oxo-acid dehydrogenase complex (BCOADC)). The autoantibody responses seen to this family of complexes, the members of which all play key roles in cellular metabolism, have been comprehensively reviewed elsewhere [45]. The complexes share a common multimeric structure. PDC, for example, is built around a core containing the dihydrolipoamide acetyl transferase (E2) component tightly bound to the E3-binding protein (E3BP) component. The other key components pyruvate dehydrogenase (E1) and dihydrolipoamide dehydrogenase (E3) attach to the 30 edges and 12 faces respectively of the icosahedral PDC-E2/E3BP core. OGDC and BCOADC possess a similar structure (with the exception that they both lack E3BP). The multi-domain structure of all the complexes means that they are large in size (of the order of ribosomes or small viruses).

95% of PBC patients have serum autoantibodies reactive by immunoblotting and ELISA with human PDC-E2 [46]. Antibodies reactive with E3BP are present in all patients showing reactivity with PDC-E2 [41] as a result of significant if not total cross-reactivity between PDC-E2 and E3BP [47]. Antibodies reactive with the E1α and E1β subunits of PDC (which are not cross-reactive with PDC-E2 or E3BP) are seen at lower frequency (40 and 10% of PBC patients, respectively).

Anti-PDC-E2 antibodies are not restricted to the serum, secretory IgA anti-PDC-E2 also having been identified in the bile, saliva and urine [48], [49], [50]. This observation has been interpreted as indicating at least some mucosal targeting of the autoreactive immune response in PBC. These secretory anti-PDC antibodies retain the ability to inhibit PDC enzyme function characteristic of their serum counterparts.

Antibodies are seen to the E2 components of OGDC and BCOADC in approximately 90 and 50% of PBC patients, respectively [51], [52], [53]. Antibodies reactive with BCOADC E1α have recently been identified using highly purified human BCOADC as the antigen source [54]. Antibodies reactive with the E3 subunit common to PDC, OGDC and BCOADC and to the E1a subunit of OGDC have not, as yet, been identified.

The E2 components of all the complexes and E3BP (where present) fold into several distinct domains which are linked by flexible regions of polypeptide rich in alanine and proline residues – such flexibility is essential for the catalytic function of the complex. Both E2 and E3BP have a central core region, responsible for binding to other E2/E3BP polypeptides and which, in the case of E2, contains the residues essential for catalytic activity. The core domain is then linked to a binding domain, which in the case of E2 is thought to be responsible for binding to E1 (and possibly E3), whilst the corresponding region in E3BP is assumed to bind E3 only, although this has not been demonstrated directly. At the amino terminal of the polypeptides are compact domains containing covalently-attached lipoic acid co-factor(s) [55]. PDC-E2 contains two tandemly-repeated lipoyl domains, whilst E3BP has a single lipoyl domain. The lipoyl domains are exposed on the surface of the E2 core, which may partly explain their antigenicity (see below). Each domain contains a single lipoyl residue covalently attached to a lysine residue in a DKA sequence motif. The three-dimensional structure of the inner lipoyl domain of human PDC-E2 has recently been determined and it has been demonstrated that the lipoyl-lysine residue lies on an exposed β-turn in the structure [56].

Attempts to map the B-cell epitopes within PDC-E2 using different experimental approaches (trypsinised whole PDC-E2, truncated recombinant polypeptide and sequence-specific peptides) have generated largely consistent results. The dominant B-cell epitope within PDC-E2 is conformational in nature and spans the inner lipoic acid binding domain [57], [58], [59], [60], [61]. Significantly lower (100-fold) titre responses are seen to the outer lipoyl domain. Auto-antibody responses have not been reported to the other domains of PDC-E2. The lipoic acid cofactor attached to the inner lipoyl domain of PDC-E2 itself appears to form a key part of the epitope, the AMA response to lipoylated recombinant PDC-E2 being of significantly higher titre and affinity than that seen to un-lipoylated antigen [62]. Fewer data are available regarding B-cell epitope localisation within E3BP. Although a significant response is seen to the lipoic acid binding domain, which shows significant homology with the inner lipoyl domain of PDC-E2 [63], some patients also demonstrate a response to the non-homologous ‘catalytic’ domain [47]. As is the case with PDC-E2 the lipoic acid co-factor appears to form part of the PDC-E3BP autoepitope [47]. The dominant epitopes within OGDC-E2 and BCOADC-E2 appear, as is the case with PDC-E2, to be conformational in nature and to be contained within the lipoic acid binding domain [64], [65].

In addition to the intra-subunit conformational auto-epitopes described above comparison of antibody responses using immuno-blotting and enzyme function inhibition assays suggest the presence of some AMA which can block PDC function in vitro but which show no reactivity in immunoblot against PDC-E2 [66]. These non-blotting, inhibitory AMA are presumed to react exclusively with conformational determinant(s) presented by the tertiary structure of the entire multienzyme complex. These data could be interpreted as suggesting that the source of the antigenic drive is the intact multi-enzyme complex rather than a ‘mimicking’ protein, and point to loss of immune tolerance as a fundamental process in the pathogenesis of PBC.

Antibodies reactive with the 2-oxo acid dehydrogenase complexes are not the only autoantibodies seen in PBC. Anti-nuclear autoantibodies (ANA) are also seen, albeit it at a much lower frequency (only being present in about a third of PBC patients). They are, however, seen more frequently and at much higher titres in the small subgroup anti-PDC negative PBC patients [67], [68], [69]. Autoantibodies specific for the proteins of the nuclear pore complex (gp210, p62) may be associated with more active or severe disease [70]. This may also be true for the autoantibodies that react with proteins (Sp100 and PML) forming the antigenic targets that produce the multiple nuclear dot pattern revealed by direct immunofluorescence [68]. Another subset of autoantibodies previously reported to be reactive with carbonic anhydrase II [71] have more recently been described as a non-specific marker of autoimmunity rather than being associated with AMA-negative PBC [72].

4.4. Autoreactive T-cell responses in PBC 

CD4+ T-cells (T-helper cells) reactive with PDC-E2 are present in the peripheral blood mononuclear cell (PBMC) pool in the majority of PBC patients, but almost entirely absent from controls [73], [74], [75], [76]. These cells have subsequently been shown to be reactive with native human antigen [77]. PDC-E2 specific T-cells are present within the mononuclear cell infiltrates seen in PBC patient derived liver tissue [74], [76]. Comparison of the autoreactive T-cell response between early and late stage disease patients, and between the PBMC and liver infiltrating T-cell pools suggests that the greatest magnitude response is seen within the liver and in the earliest disease stages [75], [78]. Epitope mapping studies utilising over-lapping peptide libraries have identified a dominant HLA DR4*0101 restricted T-cell epitope spanning residues 163–176 of PDC-E2 (encompassing the lipoic acid binding residue of the inner lipoyl domain) [76], [79], [80]. Other, as yet uncharacterised epitopes showing different MHC restriction may be present elsewhere within PDC-E2 [81], although the restricted T-cell receptor variable region usage reported would imply that the number of epitopes recognised is likely to be limited [82]. Data regarding T-cell responses to E3BP remain limited. A single pilot study has suggested no reactivity [83] with the attendant implication, taking T-cell and B-cell data together, that the original breakdown of immune tolerance is to PDC-E2 with the induction of antibody responses to this antigen which subsequently cross-react with E3BP. The question of CD4+ T-cell responses to the other component subunits of PDC, to OGDC and BCOADC and to the nuclear autoantigens implicated in PBC remains to be addressed.

HLA-A2 restricted CD8+ T-cells reactive with self-PDC-derived epitopes have recently been identified [84], [85]. As appears to be the case with CD4+ T-cells the dominant autoepitope spans the lipoic acid binding residue of the inner lipoyl domain of PDC-E2 (residues 159–167), and PDC-E2 specific autoreactive cells are enriched in the liver (as opposed to peripheral blood) compartment and in the tissue of patients with early disease [25]. Critically, CD8+ T-cells isolated from the livers of PBC patients show cytotoxic activity against PDC-E2 aa159–167 pulsed autologous cells suggesting that they have at least the potential to be responsible for BEC cytotoxicity in vivo.

Study of other potential cytotoxic effector cells in the liver has so far been limited. Natural killer T (NKT) cells, which co-express killer and T-cell surface markers have generated much recent interest as potential regulatory and effector cells. They are of particular interest in the context of PBC given the enrichment of NKT cells seen normally in the liver [86]. Enrichment of functional NKT cells, quantified by the use of tetramer technology, has been reported in the liver in PBC patients [87]. Whether this enrichment is a cause or a result of the disease process remains unclear at present. This is an area where further study is warranted.

4.5. The role played by autoreactive responses in disease pathogenesis 

There are at present no data to support a direct pathogenetic role for AMA. In the setting of human bacterial infection (both urinary infection and acute mycobacterial infection) self-limiting anti-PDC antibody responses can be induced in the apparent absence of other features of PBC. Moreover, there are no convincing data to support an association between the titre of AMA and nature and extent of bile duct damage. Furthermore, anti-PDC antibodies can be both actively induced in, and passively transferred into, experimental animals without the apparent induction of bile duct damage [88], [89], [90], [91].

The evidence to support a pathogenetic role for self-PDC specific T-cells is much stronger although still indirect. The presence of PDC-E2 reactive CD8+ cells with cytotoxic potential in the liver at the time that BEC are apoptosing constitutes the strongest evidence for a causal role for T-cell responses [25]. Further evidence comes from murine modelling studies in which SJL/J mice induced to break T-cell tolerance to self-PDC exhibit portal tract inflammatory responses and bile duct damage [92], [93]. The extent to which the histological changes seen in this model relate to those seen human PBC requires, however, further clarification [94].

4.6. Mechanisms of tolerance breakdown 

Significant progress has been made recently in our understanding of the mechanism of breakdown of the normal state of tolerance to self-PDC. Observations from both human infectious disease settings (where anti-PDC antibody responses can be expressed in the apparent absence of breakdown of T-cell tolerance and the development of liver disease [95], [96]) and murine modelling studies (where induction of anti-PDC antibody responses is easily achieved by sensitisation with cross-reactive non-self-PDC but induction of T-tolerance requires specific manipulation [93], [97]) suggest a hierarchy of tolerance breakdown. The key pathogenetic step for the development of PBC appears to be breakdown of T-cell tolerance, a concept supported by the evidence outlined above implicating autoreactive cytotoxic T-cells as the probable effector cells responsible for BEC killing. In contrast, breakdown of B-cell self-tolerance appears less tightly restricted and to have fewer (or indeed no) direct pathological sequelae. This hierarchy of tolerance in all probability reflects the high degree of structural conservation of PDC and related complexes over evolution (dictated by structure-function correlates). The degree to which structure has to be conserved to retain function may mean that conservation of conformational B-cell epitopes across evolution is inevitable. The data suggesting that lipoic acid represents both a key immunological and functional component of the structure of PDC-E2 would certainly support this view.

Functional tolerance of self-PDC exists under normal conditions, however, suggesting that effective mechanisms must exist to control breakdown of tolerance to this key self-antigen. If the tolerance hierarchy model is correct it would suggest that the key level for exertion of tolerance is at the level of CD4+ cells reactive with self-PDC derived epitopes (with maintenance of B-cell tolerance exerted, to a significant extent, by regulation of T-cell mediated ‘help’). The mechanism leading to the seemingly key pathogenetic step of breakdown in CD4+ T-cell tolerance to self-PDC remains, at present, unclear. Two contrasting models can be proposed (Fig. 2).

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  • Fig. 2. 

    Models for the breakdown of CD4+ T-cell tolerance to self-PDC. (a) Molecular mimicry model: In this model exposure to a pathogen (viral, bacterial or other) derived epitope showing sequence homology with a self-PDC sequence results in a T-cell response showing cross-reactivity with the self-antigen. A key factor in such pathways could be the parallel action of pathogen-derived factors on APC TLRs altering the context of antigen presentation and promoting active immunity as opposed to tolerance. (b) Determinant density model: In this model potentially self-PDC reactive T-cells survive negative selection in the thymus because their TCR shows only low affinity for self-peptide/MHC. In the periphery the low TCR affinity means that expose to sporadic self-PDC derived peptides generated as ‘noise’ in the antigen processing pathways is insufficient to result in T-cell activation. Enrichment of APC presentation of self-PDC derived epitopes could, however, give sufficient low affinity presentation events to surpass a triggering threshold and induce CD4+ T-cell activation. Self-PDC enrichment could result from uptake by dendritic cells of self-PDC/anti-PDC complexes via the Fc receptor or of self-PDC by surface Ig of PDC-specific activated B-cells. In this model the B-cell response to PDC, although not pathogenetic in its own right plays a key role in promoting T-cell self-tolerance breakdown.

The first, and simplest model is molecular mimicry in which a T-cell response occurs to a pathogen-derived epitope which is cross-reactive with a PDC-derived self-epitope (Fig. 2a). The context of pathogen associated inflammation mediated by toll-like receptor (TLR)/TLR-ligand interaction would play a key role in permitting the activation of normally ‘forbidden’ self-reactive pre-cursors (and would provide a further ‘role’ for the bacterial factors identified as being present in the liver tissue of PBC patients). Several studies have indeed identified peptides derived from the sequences of pathogen proteins which show cross-reactivity with the previously identified PDC-E2 163–176 HLA DR*0401 restricted epitope [80]. The weakness of this model is that it is as yet unclear whether any of these potentially cross-reactive epitopes are actually generated during natural infection with the parental pathogen.

The second model is based on the concept of determinant density (Fig. 2b). It has now been clearly established that potentially autoreactive T-cells can survive during normal thymic selection [98]. These cells typically, however, have T-cell receptors (TCR) demonstrating only low affinity with MHC/self-peptide (low enough to avoid triggering of negative selection in the thymus). This low affinity for self has the effect that casual exposure to low levels of self-epitope generated as ‘noise’ during normal antigen processing and presentation in the periphery is insufficient to result in activation. A significant increase in uptake, processing and presentation of self could generate, however, sufficient low affinity TCR/MHC/peptide interaction events to surpass the threshold for T-cell activation. One mechanism for such boosted presentation of relevance to PBC could be the actions of anti-PDC antibodies (through Fc-mediated uptake of PDC/anti-PDC complexes by dendritic cells) and their secreting B-cells (through surface Ig uptake of self-PDC). Dendritic cells and B-cells (in their activated form [99]) can function as professional antigen presenting cells able to promote the primary T-cell response needed for the expression of T-cell autoreactivity. There is evidence to suggest that such antibody augmented T-cell tolerance breakdown can occur in vivo. In human studies the generation of CD8+ T-cells reactive with aa159–167 of PDC-E2 following stimulation in vitro with human PDC-E2 was achieved at a 100-fold lower concentration of antigen when the antigen was complexed with a monoclonal human anti-PDC-E2 antibody prior to pulsing [84]. One interpretation of this observation is that antibody/antigen complexes are taken up more effectively by dendritic cells increasing the efficiency of antigen presentation. Furthermore, in murine modelling studies SJL mice which are normally fully tolerant of self-PDC, and which generate antibody but not T-cell responses reactive with self-PDC within the 10 weeks following sensitisation with foreign PDC break T-cell tolerance to self following co-sensitisation with self- and foreign-PDC. This observation would be compatible with either antigen/antibody complex uptake, again, by antigen presenting cells, or uptake, processing and presentation of self-PDC by activated PDC-specific B-cells expressing cross-reactive surface anti-PDC.

Two key questions remain to be answered in this model. The first is what triggers the antibody response cross-reactive with self-PDC. The second is what determines, from what would appear to be a very large pool of people at risk of developing anti-PDC antibody responses at some stage of their life, who goes on to develop antibody driven T-cell tolerance breakdown; the key check-point for disease development? The first of these questions appears to be relatively straightforward to answer. It is likely that many stimuli can generate antibody responses cross-reactive with self-PDC. The most obvious stimuli are again bacterial or viral proteins (both homologous bacterial PDC and other bacterial or viral molecules showing sequence or structural homology with PDC (a further iteration of molecular mimicry). Other postulated candidate stimuli include self-PDC modified either by cleavage or modification in cells undergoing apoptosis [100] and xenobiotically modified self-PDC [101]. There is no clear answer to the second question at present. Potential factors could be the source and concentration of the self-antigen needed to form the dendritic cell-directed immune complexes or to be taken up and presented by B-cells (perhaps being released by cells undergoing necrosis or apoptosis) and the state of activation of the targeted antigen presenting cells (TLR mediated activation of antigen presenting cells (APCs) apparently being able to increase the efficacy of presentation and promote tolerance breakdown [97]). Another area which remains to be addressed is the role (if any) normally played by the increasingly well recognised population of regulatory T-cells (such as CD4+ CD25+ cells) in the control of autoreactive responses to PDC and the extent to which failure of such regulation contributes to the expression of autoreactivity in the disease state.

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5. Conclusions 

With the demonstration of the presence of self-PDC-specific cytotoxic T-cells able to induce target cell apoptosis in affected portal tracts, at a time when the biliary epithelial cells are undergoing apoptosis the weight of evidence points strongly to PBC indeed being an autoimmune disease. The data point to AMA playing more of a role in the induction of T-cell tolerance breakdown than in target cell damage. A role for infectious organisms (including postulated viral agents) has yet to be proven. Such agents may play a key role in inducing tolerance breakdown. Anyone so bold as to think that PBC has been ‘solved’ should reflect, however, on the questions which remain to be answered. Why does the disease exhibit such a marked sex bias? Why does breakdown of tolerance to the universal antigen PDC result in targeted damage? How and why does the disease recur following liver transplant? Can our knowledge of the immune processes occurring in the disease be translated into targeted approaches to therapy? Answering these questions will not only allow us to understand PBC but may represent an exciting opportunity to shed light on the workings of the normal immune system and its failure in autoimmune disease.

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References 

  1. James OFW, Bhopal R, Howel D, Gray J, Burt AD, Metcalf JV. Primary biliary cirrhosis once rare, now common in the UK?. Hepatology. 1999;30:390–394
  2. Metcalf JV, Howel D, James OFW, Bhopal RS. Primary biliary cirrhosis:epidemiology helping the clinician. Br Med J. 1996;312:1181–1182
  3. Jones DE, Watt FE, Metcalf JV, Bassendine MF, James OF. Familial primary biliary cirrhosis reassessed: a geographically-based population study. J Hepatol. 1999;30:402–407
  4. Selmi C, Mayo MJ, Bach N, Gish RG, Wright HI, Ishibashi H, et al.  Occurrence of primary biliary cirrhosis in monozygotic twins. Hepatology. 2002;36:387A
  5. Jones DEJ, Donaldson PT. Genetic factors in the pathogenesis of primary biliary cirrhosis. Clin Liver Dis. 2003; (in press)
  6. Triger DR. Primary biliary cirrhosis: an epidemiological study. Br Med J. 1980;281:772–775
  7. Prince MI, Chetwynd A, Diggle P, Jarner M, Metcalf JV, James OFW. The geographical distribution of primary biliary cirrhosis in a well-defined cohort. Hepatology. 2001;34:1083–1088
  8. Howel D, Fischbacher CM, Bhopal RS, Gray J, Metcalf JV, James OFW. An exploratory population-based case-control study of primary biliary cirrhosis. Hepatology. 2000;31:1055–1060
  9. Parikh-Patel A, Gold EB, Wormann H, Krivy KE, Gershwin ME. Risk factors for primary biliary cirrhosis in a cohort of patients from the United States. Hepatology. 2001;33:16–21
  10. Bjorkland A, Loof L, Mendel-Hartvig I, Totterman TH. Primary biliary cirrhosis: high proportion of B-cells in blood and liver tissue produce anti-mitochondrial antibodies of several Ig classes. J Immunol. 1994;153:2750–2757
  11. Harada K, Van de Water J, Leung PS, Coppel RL, Ansari AA, Nakanuma Y, et al.  In situ nucleic acid hybridisation of cytokines in primary biliary cirrhosis: predominance of the Th1 subset. Hepatology. 1997;25:791–796
  12. Shackel NA, McGuinness PH, Abbott CA, Gorrell MD, McCaughan GW. Identification of novel molecules and pathogenic pathways in primary biliary cirrhosis: cDNA array analysis of intrahepatic differential gene expression. Gut. 2001;49:565–576
  13. Joplin R, Gershwin ME. Ductular expression of autoantigens in primary biliary cirrhosis. Semin Liver Dis. 1997;17:97–103
  14. Tsuneyama K, Van de Water J, Leung PS, Cha S, Nakanuma Y, Kaplan M, et al.  Abnormal expression of the E2 component of the pyruvate dehydrogenase complex on the luminal surface of biliary epithelium occurs before major histocompatability complex class II and BB1/B7 expression. Hepatology. 1995;21:1031–1037
  15. Harada K, Van de Water J, Leung PS, Coppel RL, Nakanuma Y, Gershwin ME. In situ nucleic acid hybridization of pyruvate dehydrogenase complex-E2 in primary biliary cirrhosis: pyruvate dehydrogenase complex-E2 messenger RNA is expressed in hepatocytes but not in biliary epithelium. Hepatology. 1997;25:27–32
  16. Kuroki T, Seki S, Kawakita N, Nakatani K, Hisa T, Kitada T, et al.  Expression of antigens related to apoptosis and cell proliferation in chronic non-suppurative destructive cholangitis in primary biliary cirrhosis. Virchows Arch. 1996;429:119–129
  17. Koga H, Sakisaka S, Ohishi M, Sata M, Tanikawa K. Nuclear DNA fragmentation and expression of Bcl-2 in primary biliary cirrhosis. Hepatology. 1997;25:1077–1084
  18. Graham AM, Dollinger MM, Howie SE, Harrison DJ. Bile duct cells in primary biliary cirrhosis are ‘primed’ for apoptosis. Eur J Gastroenterol Hepatol. 1998;10:540–541
  19. Strater J, Moller P. Pathogenesis of primary biliary cirrhosis: CD95-induced apoptosis at last?. Eur J Gastroenterol Hepatol. 1998;10:539–541
  20. Tinmouth J, Lee M, Wanless IR, Tsui FWL, Inman R, Heathcote EJ. Apoptosis of biliary epithelial cells in primary biliary cirrhosis and primary sclerosing cholangitis. Liver. 2002;22:228–234
  21. Iwata M, Harada K, Kono N, Kaneko S, Kobayashi K, Nakanuma Y. Expression of Bcl-2 familial proteins is reduced in small bile duct lesions of primary biliary cirrhosis. Hum Pathol. 2000;31:179–184
  22. Harada K, Ozaki S, Gershwin ME, Nakanuma Y. Enhanced apoptosis relates to bile duct loss in primary biliary cirrhosis. Hepatology. 1997;26:1399–1405
  23. Iwata M, Harada K, Hiramatsu K, Tsuneyama K, Kaneko S, Kobayashi K, et al.  Fas ligand expressing mononuclear cells around intrahepatic bile ducts co-express CD68 in primary biliary cirrhosis. Liver. 2000;21:129–135
  24. Fox CK, Furtwaengler A, Nepomuceno RR, Martinez OM, Krams SM. Apoptotic pathways in primary biliary cirrhosis and autoimmune hepatitis. Liver. 2001;21:272–279
  25. Kita H, Matsumura S, He X, Ansari A, Lian Z, Van de Water J, et al.  Quantitative and functional analysis of PDC-E2-specific autoreactive cytotoxic T lymphocytes in primary biliary cirrhosis. J Clin Invest. 2002;109:1231–1240
  26. Tsuneyama K, Harada K, Kono N, Hiramatsu K, Zen Y, Sudo Y, et al.  Scavenger cells with gram-positive bacterial lipoteichoic acid infiltrate around the damaged interlobular bile ducts of primary biliary cirrhosis. J Hepatol. 2001;35:156–163
  27. Tsuneyama K, Harada K, Yasoshima M, Hiramatsu K, Mackay CR, Mackay IR, et al.  Monocyte chemotactic protein-1, -2, and -3 are distinctively expressed in portal tracts and granulomata in primary biliary cirrhosis: implications foe pathogenesis. J Pathol. 2001;193:102–109
  28. Abdulkarim AS, Petrovic LM, Kim WR, Angulo P, Keach JC, Lindor KD. Primary biliary cirrhosis: an infectious disease caused by chlamydia pneumoniae. Gastroenterology. 2002;122:A641
  29. Mason AL, Xu L, Guo L, Munoz S, Jaspen JB, Bryer-Ash M, et al.  Detection of retroviral antibodies in primary biliary cirrhosis and other idiopathic liver disorders. Lancet. 1998;351
  30. Xu L, Shen Z, Guo L, Fodera B, Keogh A, Joplin R, et al.  Does a betaretrovirus infection trigger primary biliary cirrhosis?. Proc Natl Acad Sci (USA). 2003;100:8454–8459
  31. Sadamoto T, Joplin R, Keogh A, Mason A, Carman W, Neuberger J. Expression of pyruvate-dehydrogenase complex PDC-E2 on biliary epithelial cells induced by lymph nodes from primary biliary cirrhosis. Lancet. 1998;352:1595–1596
  32. Keogh AC, Sadamoto T, Joplin R, Carman W, Mason AL, Xu L, et al.  Expression of pyruvate dehydrogenase complex PDC-E2 on biliary epithelial cells induced by incubation with lymph nodes from primary biliary cirrhosis. Hepatology. 1999;30:147A
  33. Mason AL, Xu L, Guo L, Garry RF. Retroviruses in autoimmune liver disease: genetic or environmental agents. Arch Immunol Ther Exp. 1999;47:289–297
  34. Perrona H, Seigneurin JM. Human retroviral sequences associated with extracellular particles in autoimmune diseases: epiphenomenon or possible role in aetiopathogenesis. Microbes Infect. 1999;1:309–322
  35. De Boer EC, Teppenema JS, Steerenberg PA, De Jong WH. Retrovirus type C in the mouse bladder carcinoma line MBT-2. J Urol. 2000;163:1999–2001
  36. Popovic M, Kalyanaraman VS, Reitz MS, Sarngadharan MG. Identification of the RPMI 8226 retrovirus and its dissemination as a significant contaminant of some widely used human and marmoset cell lines. Int J Cancer. 1982;30:93–99
  37. Mackay IR. Primary biliary cirrhosis showing a high titre of autoantibody: report of a case study. N Engl J Med. 1958;258:185–188
  38. Walker JG, Doniach D, Roitt IM, Sherlock S. Serological tests in the diagnosis of primary biliary cirrhosis. Lancet. 1965;i:827–831
  39. Berg PA, Muscatello U, Horne RW, Roitt IM, Doniach D. Mitochondrial antibodies in primary biliary cirrhosis II. The complement fixing antigen as a component of mitochondrial inner membranes. Br J Exp Pathol. 1969;50:200–208
  40. Gershwin ME, Mackay IR, Sturgess A, Coppel RL. Identification and specificity of a cDNA encoding 70 kD mitochondrial antigen recognized in primary biliary cirrhosis. J Immunol. 1987;138:3525–3531
  41. Yeaman SJ, Fussey SP, Danner DJ, James OFW, Mutimer DJ, Bassendine MF. Primary biliary cirrhosis: identification of two major M2 mitochondrial autoantigens. Lancet. 1988;i:1067–1070
  42. Van de Water J, Fregeau D, Davis P, Ansari A, Danner D, Leung P, et al.  Autoantibodies of primary biliary cirrhosis recognise dihydrolipoamide acetyltransferase and inhibit enzyme function. J Immunol. 1988;141:2321–2324
  43. Jones DEJ. Autoantigens in primary biliary cirrhosis. J Clin Path. 2000;53:1–8
  44. Metcalf JV, Mitchinson HC, Palmer JM, Jones DEJ, Bassendine MF, James OFW. Natural history of early primary biliary cirrhosis. Lancet. 1996;348:1399–1402
  45. Yeaman SJ, Kirby JA, Jones DEJ. Autoreactive responses to pyruvate dehydrogenase complex in the pathogenesis of primary biliary cirrhosis. Immunol Rev. 2000;174:238–249
  46. Palmer JM, Bassendine MF, James OFW, Yeaman SJ. Human pyruvate dehydrogenase complex as an autoantigen in primary biliary cirrhosis. Clin Sci. 1993;85:289–293
  47. Palmer JM, Jones DEJ, Quinn J, McHugh A, Yeaman SJ. Characterisation of the autoantibody responses to recombinant E3 binding protein (protein X) of pyruvate dehydrogenase in primary biliary cirrhosis. Hepatology. 1999;30:21–26
  48. Palmer JM, Doshi M, Kirby J, Yeaman SJ, Bassendine MF, Jones DEJ. Secretory autoantibodies in primary biliary cirrhosis. Clin Exp Immunol. 2000;122:1–7
  49. Tanaka A, Nalbandian G, Leung PSC, Benson GD, Munoz S, Findor JA, et al.  Mucosal immunity and primary biliary cirrhosis: presence of anti-mitochondrial antibodies in urine. Hepatology. 2000;32:910–915
  50. Nishio A, Van de Water J, Leung PS, Joplin R, Neuberger JM, Lake J, et al.  Comparative studies of antimitochondrial autoantibodies in sera and bile in primary biliary cirrhosis. Hepatology. 1997;25:1085–1089
  51. Fussey SPM, Guest JR, James OFW, Bassendine MF, Yeaman SJ. Identification and analysis of the major M2 autoantigens in primary biliary cirrhosis. Proc Natl Acad Sci (USA). 1988;85:8654–8658
  52. Surh CD, Danner DJ, Ahmed A, Coppel RC, Mackay IR, Dickson ER, et al.  Reactivity of primary biliary cirrhosis sera with a human fetal liver cDNA clone of branched-chain a-keto acid dehydrogenase dihydrolipoamide acyltransferase, the 52 kDa mitochondrial autoantigen. Hepatology. 1989;9:63–68
  53. Mutimer DJ, Fussey SP, Yeaman SJ, Kelly PJ, James OFW, Bassendine MF. Frequency of IgG and IgM autoantibodies to four specific M2 mitochondrial autoantigens in primary biliary cirrhosis. Hepatology. 1989;10:403–407
  54. Mori T, Ono K, Hakozaki M, Kasukawa R, Kochi H. Autoantibodies of sera from patients with primary biliary cirrhosis recognize the alpha subunit of the decarboxylase component of human branched-chain 2-oxox acid dehydrogenase complex. J Hepatol. 2001;34:799–804
  55. Reed LJ, Hackert ML. Structure-function relationships in dihydrolipoamide acyltransferases. J Biol Chem. 1990;265:8971–8974
  56. Howard MJ, Fuller C, Broadhurst RW, Perham RN, Tang JG, Quinn J, et al.  Three dimensional structure of the major autoantigen in primary biliary cirrhosis. Gastroenterology. 1998;115:139–146
  57. Tuaillon N, Andre C, Briand JP, Penner E, Muller S. A lipoyl synthetic octapeptide of dihydrolipoamide acetyltransferase specifically recognised by anti-M2 autoantibodies in primary biliary cirrhosis. J Immunol. 1992;148:445–450
  58. Surh CD, Coppel R, Gerswin ME. Structural requirement for autoreacitiviy on human pyruvate dehydrogenase-E2, the major autoantigen of primary biliary cirrhosis. J Immunol. 1990;144:3367–3374
  59. Fussey SP, Bassendine MF, James OFW, Yeaman SJ. Characterisation of the reactivity of autoantibodies in primary biliary cirrhosis. FEBS Lett. 1989;246:49–53
  60. Fussey SPM, Ali ST, Guest JR, James OFW, Bassendine MF, Yeaman SJ. Reactivity of primary biliary cirrhosis sera with Escherichia coli dihydrolipoamide acetyltransferase (E2p): characterisation of the main immunogenic region. Proc Natl Acad Sci (USA). 1990;87:3987–3991
  61. Fussey SPM, Bassendine MF, James OFW, Yeaman SJ. The lipoate-containing domain of PDC-E2 contains the main immunogenic region of the 70 kD M2 autoantigen in primary biliary cirrhosis. Ann N Y Acad Sci. 1989;573:444–446
  62. Quinn J, Diamond AG, Palmer JM, James OFW, Bassendine MF, Yeaman SJ. Lipoylated and unlipoylated domains of human PDC-E2 as autoantigens in primary biliary cirrhosis: significance of lipoate attachment. Hepatology. 1993;18:1384–1391
  63. Harris RA, Bowker-Kinley MM, Wu P, Jeng J, Popov KM. Dihydrolipoamide dehydrogenase-binding protein of the human pyruvate dehydrogenase complex. J Biol Chem. 1997;272:19746–19751
  64. Leung PS, Chuang DT, Wynn RM, Cha S, Danner D, Ansari A, et al.  Autoantibodies to BCOADC-E2 in patients with primary biliary cirrhosis recognize a conformational epitope. Hepatology. 1995;22:505–513
  65. Moteki S, Leung PS, Dickson ER, Van Thiel DH, Galpersin C, Buch T, et al.  Epitope mapping and reactivity of autoantibodies to the E2 component of 2-oxoglutarate dehydrogenase complex in primary biliary cirrhosis using recombinant 2oxoglutarate dehydrogenase complex. Hepatology. 1996;23:436–444
  66. Rowley MJ, McNeilage JL, Armstrong JM, Mackay IR. Inhibitory autoantibody to a conformational epitope of the pyruvate dehydrogenase complex. Clin Immunol Immunopathol. 1991;60:356–370
  67. Michieletti P, Wanless I, Katz A, Scheuer P, Yeaman S, Bassendine M, et al.  Antimitochondrial antibody negative primary biliary cirrhosis: a distinct syndrome of autoimmune cholangitis. Gut. 1994;35:260–265
  68. Szostecki C, Guldner HH, Will H. Autoantibodies against ‘nuclear dots’ in primary biliary cirrhosis. Semin Liver Dis. 1997;17:71–78
  69. Courvalin J-C, Worman HJ. Nuclear envelope protein autoantibodies in primary biliary cirrhosis. Semin Liver Dis. 1997;17:79–90
  70. Zuchner D, Sternsdorf T, Szostecki C, Heathcote E, Cauch-Dudek K, Will H. Prevalence, kinetics and therapeutic modulation of autoantibodies against Sp100 and promyelocytic leukaemia protein in a large cohort of patients with primary biliary cirrhosis. Hepatology. 1997;26:1123–1130
  71. Gordon S, Quattrociocchi-Longe T, Khan B, Kodali V, Chen J, Silverman A, et al.  Antibodies to carbonic anhydrase in patients with immune cholangiopathies. Gastroenterology. 1995;108:1802–1809
  72. Comay D, Cauch-Dudek K, Hemphill D, Diamandis E, Wanless I, Heathcote EJ. Are antibodies to carbonic anhydrase II specific for anti-mitochondrial antibody-negative primary biliary cirrhosis. Dig Dis Sci. 2000;45:2018–2021
  73. Van de Water J, Ansari AA, Surh CD, Coppel R, Roche T, Donkovsky HB, et al.  Evidence for the targeting by 2-oxo-dehydrogenase enzymes in the T-cell response of primary biliary cirrhosis. J Immunol. 1991;146:89–94
  74. Van de Water J, Ansari A, Prindiville T, Coppel RL, Ricalton N, Kotzin BL, et al.  Heterogeneity of autoreactive T-cell clones specific for the E2 component of the pyruvate dehydrogenase complex in primary biliary cirrhosis. J Exp Med. 1995;181:723–733
  75. Jones DEJ, Palmer JM, Yeaman SJ, James OFW, Bassendine MF, Diamond AG. T-cell responses to the components of pyruvate dehydrogenase complex in primary biliary cirrhosis. Hepatology. 1995;21:995–1002
  76. Shimoda S, Nakamura M, Ishibashi H, Hayashida K, Niho Y. HLA DRB4 0101-resticted immunodominant T-cell autoepitope of pyruvate dehydrogenase complex in primary biliary cirrhosis: evidence of molecular mimicry in human autoimmune disease. J Exp Med. 1995;181:1835–1845
  77. Jones DEJ, Palmer JM, Yeaman SJ, Bassendine MF, Diamond AG. T-cell responses to natural human proteins in primary biliary cirrhosis. Clin Exp Immunol. 1997;116:562–568
  78. Shimoda S, Van de Water J, Ansari A, Nakamura M, Ishibashi H, Coppel RL, et al.  Identification and precursor frequency analysis of a common T-cell epitope motif in mitochondrial autoantigens in primary biliary cirrhosis. J Clin Invest. 1998;102:1831–1840
  79. Shigematsu H, Shimoda S, Nakamura M, Matsushita S, Nishimura Y, Sakamoto N, et al.  Fine specificity of T cells reactive to human PDC-E2 163–176 peptide, the immunodominant autoantigen in primary biliary cirrhosis: implications for molecular mimicry and cross-recognition among mitochondrial autoantigens. Hepatology. 2000;32:901–909
  80. Shimoda S, Nakamura M, Shigematsu H, Tanimoto H, Gushima T, Gershwin ME, et al.  Mimicry peptides of human PDC-E2 163–176 peptide, the immunodominant T-cell epitope of primary biliary cirrhosis. Hepatology. 2000;31:1212–1216
  81. Palmer JM, Diamond AG, Yeaman SJ, Bassendine MF, Jones DEJ. T-cell responses to the putative autoepitope in primary biliary cirrhosis. Clin Exp Immunol. 1999;116:133–139
  82. Pingel S, Arenz M, Meyer zum Buschenfelde KH, Lohr HF. Pyruvate dehydrogenase specific T-cells in primary biliary cirrhosis show restricted antigen recognition sites. Liver. 2002;22:308–316
  83. Robe AJ, Palmer JM, Yeaman SJ, Jones DEJ. The E3BP (protein X) component of pyruvate dehydrogenase complex (PDC) is not a T-cell autoantigen in primary biliary cirrhosis (PBC). J Hepatol. 2000;32:128
  84. Kita H, Lian Z, Van de Water J, He X, Matsumura S, Kaplan M, et al.  Identification of HLA-A2-restricted CD8+ cytotoxic T-cell responses in primary biliary cirrhosis: T-cell activation is augmented by immune complexes cross-presented by dendritic cells. J Exp Med. 2002;195:113–123
  85. Matsumura S, Kita H, He XS, Ansari AA, Lian ZX, Van de Water J, et al.  Comprehensive mapping of HLA-A0201-restricted CD8 T-cell epitopes on PDC-E2 in primary biliary cirrhosis. Hepatology. 2002;36:1125–1134
  86. Seki S, Habu Y, Kawamura T, Takeda K, Dobashi H, Ohkawa T, et al.  The liver as a crucial organ in the first line of host defense: the roles of Kupffer cells, natural killer (NK) cells and NK1.1 Ag+ T cells in helper 1 immune responses. Immunol Rev. 2000;174:35–46
  87. Kita H, Naidenko OV, Kronenberg M, Ansari AA, Rogers P, He XS, et al.  Quantitiation and phenotypic analysis of natural killer T-cells in primary biliary cirrhosis using a human CD1d tetramer. Gastroenterology. 2002;123:1031–1042
  88. Krams SM, Surh CD, Coppel RL, Ansari AA, Reubner B, Gershwin ME. Immunization of experimental animals with dihydrolipoamide acetyltransferase, as a purified recombinant polypeptide, generates mitochondrial antibodies but not primary biliary cirrhosis. Hepatology. 1989;9:411–416
  89. Jones DEJ, Palmer JM, Kirby JA, De Cruz DJ, McCaughan GW, Sedgewick JD, et al.  Experimental autoimmune cholangitis: a mouse model of immune-mediated cholangiopathy. Liver. 2000;20:351–356
  90. Robe AJ, Palmer JM, Kirby J, Jones DEJ. No role for autoreactive antibody in the breakdown of T-cell tolerance to pyruvate dehydrogenase complex, the autoantigen in primary biliary cirrhosis. Hepatology. 2001;34:121A
  91. Leung PS, Quan C, Park O, Van de Water J, Kurth MJ, Nantz MH, et al.  Immunization with a xenobiotic breaks tolerance to mitochondrial autoantigens. Hepatology. 2002;36:295A
  92. Jones DEJ, Palmer JM, Yeaman SJ, Kirby JA, Bassendine MF. Breakdown of tolerance to pyruvate dehydrogenase complex in experimental autoimmune cholangitis a murine model of primary biliary cirrhosis. Hepatology. 1999;30:65–70
  93. Jones DEJ, Palmer JM, Bennett K, Robe AJ, Yeaman SJ, Robertson H, et al.  Investigation of a mechanism for accelerated breakdown of immune-tolerance to the primary biliary cirrhosis associated autoantigen, pyruvate dehydrogenase complex. Lab Invest. 2002;82:211–219
  94. Sasaki M, Long SA, Van de Water J, He X-S, Shultz L, Coppel RL, et al.  The SJL/J mouse is not a model for PBC. Hepatology. 2002;35:1284
  95. Butler P, Hamilton-Miller J, Baum H, Burroughs AK. Detection of M2 antibodies in patients with recurrent urinary tract infection using an ELISA and purified PBC specific antigens. Evidence for a molecular mimicry mechanism in the pathogenesis of primary biliary cirrhosis. Biochem Mol Biol Int. 1995;35:473–485
  96. Klein R, Wiebel M, Engelhart S, Berg PA. Sera from patients with tuberculosis recognise the M2a-epitope (E2-subunit of pyruvate dehydrogenase complex) specific for primary biliary cirrhosis. Clin Exp Immunol. 1993;92:308–316
  97. Jones DEJ, Palmer JM, Burt AD, Robe AJ, Kirby JA. Bacterial motif DNA as an adjuvant for the breakdown of immune self-tolerance to pyruvate dehydrogenase complex. Hepatology. 2002;36:679–686
  98. Liu GY, Fairchild PJ, Smith RM, Prowle JR, Kioussis D, Wraith DC. Low avidity recognition of self-antigen by T cells permits escape form central tolerance. Immunity. 1995;3:407–415
  99. Constant SL. B lymphocytes as antigen-presenting cells for CD4+ T-cell priming in vivo. J Immunol. 1999;162:5696–5703
  100. Odin JA, Huebert RC, Casciola-Rosen L, LaRusso NF, Rosen A. Bcl-2-dependent oxidation of pyruvate dehydrogenase-E2, a primary biliary cirrhosis autoantigen, during apoptosis. J Clin Invest. 2001;108:223–232
  101. Long SA, Quan C, Van de Water J, Nantz MH, Kurth MJ, Barsky D, et al.  Immunoreactivity of organic mimeotopes of the E2 component of pyruvate dehydrogenase: connecting xenobiotics with primary biliary cirrhosis. J Immunol. 2001;167:2956–2963

PII: S0168-8278(03)00270-8

doi:10.1016/S0168-8278(03)00270-8

Journal of Hepatology
Volume 39, Issue 4 , Pages 639-648, October 2003

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