Inhibition of alpha-ketoglutarate dehydrogenase activity by a distinct population of autoantibodies recognizing dihydrolipoamide succinyltransferase in primary biliary cirrhosis

Autoantibodies in Primary Biliary Cholangitis

Primary biliary cholangitis (PBC) is a chronic immune-mediated liver disease characterized by a lymphocytic cholangitis, with subsequent cholestasis, progressive liver fibrosis, and ultimately complications arising from end-stage liver disease. Testing for autoantibodies is important in the diagnosis of PBC, as well as stratifying prognosis. This review focuses on the role of autoantibodies in the diagnosis of PBC, as well as the relationship between autoantibodies with pathophysiology and prognostication, along with a discussion regarding novel and other related disease autoantibodies.

An Autoantigen Atlas From Human Lung HFL1 Cells Offers Clues to Neurological and Diverse Autoimmune Manifestations of COVID-19

COVID-19 is accompanied by a myriad of both transient and long-lasting autoimmune responses. Dermatan sulfate (DS), a glycosaminoglycan crucial for wound healing, has unique affinity for autoantigens (autoAgs) from apoptotic cells. DS-autoAg complexes are capable of stimulating autoreactive B cells and autoantibody production. We used DS-affinity proteomics to define the autoantigen-ome of lung fibroblasts and bioinformatics analyses to study the relationship between autoantigenic proteins and COVID-induced alterations. Using DS-affinity, we identified an autoantigen-ome of 408 proteins from human HFL1 cells, at least 231 of which are known autoAgs. Comparing with available COVID data, 352 proteins of the autoantigen-ome have thus far been found to be altered at protein or RNA levels in SARS-CoV-2 infection, 210 of which are known autoAgs. The COVID-altered proteins are significantly associated with RNA metabolism, translation, vesicles and vesicle transport, cell death, supramolecular fibrils, cytoskeleton, extracellular matrix, and interleukin signaling. They offer clues to neurological problems, fibrosis, smooth muscle dysfunction, and thrombosis. In particular, 150 altered proteins are related to the nervous system, including axon, myelin sheath, neuron projection, neuronal cell body, and olfactory bulb. An association with the melanosome is also identified. The findings from our study illustrate a connection between COVID infection and autoimmunity. The vast number of COVID-altered proteins with high intrinsic propensity to become autoAgs offers an explanation for the diverse autoimmune complications in COVID patients. The variety of autoAgs related to mRNA metabolism, translation, and vesicles suggests a need for long-term monitoring of autoimmunity in COVID. The COVID autoantigen atlas we are establishing provides a detailed molecular map for further investigation of autoimmune sequelae of the pandemic, such as "long COVID" syndrome.

Summary Sentence

An autoantigen-ome by dermatan sulfate affinity from human lung HFL1 cells may explain neurological and autoimmune manifestations of COVID-19.

Paradoxical sex-specific patterns of autoantibody response to SARS-CoV-2 infection

Background

Pronounced sex differences in the susceptibility and response to SARS-CoV-2 infection remain poorly understood. Emerging evidence has highlighted the potential importance of autoimmune activation in modulating the acute response and recovery trajectories following SARS-CoV-2 exposure. Given that immune-inflammatory activity can be sex-biased in the setting of severe COVID-19 illness, the aim of the study was to examine sex-specific autoimmune reactivity to SARS-CoV-2 in the absence of extreme clinical disease.

Methods

In this study, we assessed autoantibody (AAB) reactivity to 91 autoantigens previously linked to a range of classic autoimmune diseases in a cohort of 177 participants (65% women, 35% men, mean age of 35) with confirmed evidence of prior SARS-CoV-2 infection based on presence of antibody to the nucleocapsid protein of SARS-CoV-2. Data were compared to 53 pre-pandemic healthy controls (49% women, 51% men). For each participant, socio-demographic data, serological analyses, SARS-CoV-2 infection status and COVID-19 related symptoms were collected by  an electronic survey of questions. The symptoms burden score was constructed based on the total number of reported symptoms (N = 21) experienced within 6 months prior to the blood draw, wherein a greater number of symptoms corresponded to a higher score and assigned as more severe burden.

Results

In multivariable analyses, we observed sex-specific patterns of autoreactivity associated with the presence or absence (as well as timing and clustering of symptoms) associated with prior COVID-19 illness. Whereas the overall AAB response was more prominent in women following asymptomatic infection, the breadth and extent of AAB reactivity was more prominent in men following at least mildly symptomatic infection. Notably, the observed reactivity included distinct antigens with molecular homology with SARS-CoV-2.

Conclusion

Our results reveal that prior SARS-CoV-2 infection, even in the absence of severe clinical disease, can lead to a broad AAB response that exhibits sex-specific patterns of prevalence and antigen selectivity. Further understanding of the nature of triggered AAB activation among men and women exposed to SARS-CoV-2 will be essential for developing effective interventions against immune-mediated sequelae of COVID-19.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-03184-8.

A Master Autoantigen-ome Links Alternative Splicing, Female Predilection, and COVID-19 to Autoimmune Diseases

Chronic and debilitating autoimmune sequelae pose a grave concern for the post-COVID-19 pandemic era. Based on our discovery that the glycosaminoglycan dermatan sulfate (DS) displays peculiar affinity to apoptotic cells and autoantigens (autoAgs) and that DS-autoAg complexes cooperatively stimulate autoreactive B1 cell responses, we compiled a database of 751 candidate autoAgs from six human cell types. At least 657 of these have been found to be affected by SARS-CoV-2 infection based on currently available multi-omic COVID data, and at least 400 are confirmed targets of autoantibodies in a wide array of autoimmune diseases and cancer. The autoantigen-ome is significantly associated with various processes in viral infections, such as translation, protein processing, and vesicle transport. Interestingly, the coding genes of autoAgs predominantly contain multiple exons with many possible alternative splicing variants, short transcripts, and short UTR lengths. These observations and the finding that numerous autoAgs involved in RNA-splicing showed altered expression in viral infections suggest that viruses exploit alternative splicing to reprogram host cell machinery to ensure viral replication and survival. While each cell type gives rise to a unique pool of autoAgs, 39 common autoAgs associated with cell stress and apoptosis were identified from all six cell types, with several being known markers of systemic autoimmune diseases. In particular, the common autoAg UBA1 that catalyzes the first step in ubiquitination is encoded by an X-chromosome escape gene. Given its essential function in apoptotic cell clearance and that X-inactivation escape tends to increase with aging, UBA1 dysfunction can therefore predispose aging women to autoimmune disorders. In summary, we propose a model of how viral infections lead to extensive molecular alterations and host cell death, autoimmune responses facilitated by autoAg-DS complexes, and ultimately autoimmune diseases. Overall, this master autoantigen-ome provides a molecular guide for investigating the myriad of autoimmune sequalae to COVID-19 and clues to the rare but reported adverse effects of the currently available COVID vaccines.

An Autoantigen Atlas from Human Lung HFL1 Cells Offers Clues to Neurological and Diverse Autoimmune Manifestations of COVID-19

COVID-19 is accompanied by a myriad of both transient and long-lasting autoimmune responses. Dermatan sulfate (DS), a glycosaminoglycan crucial for wound healing, has unique affinity for autoantigens (autoAgs) from apoptotic cells. DS-autoAg complexes are capable of stimulating autoreactive B cells and autoantibody production. Using DS affinity, we identified an autoantigenome of 408 proteins from human fetal lung fibroblast HFL11 cells, at least 231 of which are known autoAgs. Comparing with available COVID data, 352 proteins of the autoantigenome have thus far been found to be altered at protein or RNA levels in SARS-Cov-2 infection, 210 of which are known autoAgs. The COVID-altered proteins are significantly associated with RNA metabolism, translation, vesicles and vesicle transport, cell death, supramolecular fibrils, cytoskeleton, extracellular matrix, and interleukin signaling. They offer clues to neurological problems, fibrosis, smooth muscle dysfunction, and thrombosis. In particular, 150 altered proteins are related to the nervous system, including axon, myelin sheath, neuron projection, neuronal cell body, and olfactory bulb. An association with the melanosome is also identified. The findings from our study illustrate a strong connection between viral infection and autoimmunity. The vast number of COVID-altered proteins with propensity to become autoAgs offers an explanation for the diverse autoimmune complications in COVID patients. The variety of autoAgs related to mRNA metabolism, translation, and vesicles raises concerns about potential adverse effects of mRNA vaccines. The COVID autoantigen atlas we are establishing provides a detailed molecular map for further investigation of autoimmune sequelae of the pandemic.

A comprehensive autoantigen-ome of autoimmune liver diseases identified from dermatan sulfate affinity enrichment of liver tissue proteins

BACKGROUND

Autoimmune diseases result from aberrant immune attacks by the body itself. It is mysterious how autoantigens, a large cohort of seemingly unconnected molecules expressed in different parts of the body, can induce similar autoimmune responses. We have previously found that dermatan sulfate (DS) can form complexes with molecules of apoptotic cells and stimulate autoreactive CD5+ B cells to produce autoantibodies. Hence, autoantigenic molecules share a unique biochemical property in their affinity to DS. This study sought to further test this uniform principle of autoantigenicity.

RESULTS

Proteomes were extracted from freshly collected mouse livers. They were loaded onto columns packed with DS-Sepharose resins. Proteins were eluted with step gradients of increasing salt strength. Proteins that bound to DS with weak, moderate, or strong affinity were eluted with 0.4, 0.6, and 1.0 M NaCl, respectively. After desalting, trypsin digestion, and gel electrophoresis, proteins were sequenced by mass spectrometry. To validate whether these proteins have been previously identified as autoantigens, an extensive literature search was conducted using the protein name or its alternative names as keywords. Of the 41 proteins identified from the strong DS-affinity fraction, 33 (80%) were verified autoantigens. Of the 46 proteins with moderate DS-affinity, 27 (59%) were verified autoantigens. Of the 125 proteins with weak DS-affinity, 44 (35%) were known autoantigens. Strikingly, these autoantigens fell into the classical autoantibody categories of autoimmune liver diseases: ANA (anti-nuclear autoantibodies), SMA (anti-smooth muscle autoantibodies), AMA (anti-mitochondrial autoantibodies), and LKM (liver-kidney microsomal autoantigens).

CONCLUSIONS

This study of DS-affinity enrichment of liver proteins establishes a comprehensive autoantigen-ome for autoimmune liver diseases, yielding 104 verified and 108 potential autoantigens. The liver autoantigen-ome sheds light on the molecular origins of autoimmune liver diseases and further supports the notion of a unifying biochemical principle of autoantigenicity.

A repertoire of 124 potential autoantigens for autoimmune kidney diseases identified by dermatan sulfate affinity enrichment of kidney tissue proteins

Autoantigens are the molecular targets in autoimmune diseases. They are a cohort of seemingly unrelated self-molecules present in different parts of the body, yet they can trigger a similar chain of autoimmune responses such as autoantibody production. We previously reported that dermatan sulfate (DS) can bind self-molecules of dying cells to stimulate autoreactive CD5+ B cells to produce autoantibodies. The formation of autoantigen-DS complexes converts the normally non-antigenic self-molecules to none-self antigens, and thus DS-affinity represents a common underlying biochemical property for autoantigens. This study sought to apply this property to identify potential autoantigens in the kidney. Total proteins were extracted from mouse kidney tissues and loaded onto DS-Sepharose resins. Proteins without affinity were washed off the resins, whereas those with increasing DS-affinity were eluted with step gradients of increasing salt strength. Fractions with strong and moderate DS-affinity were sequenced by mass spectrometry and yielded 25 and 99 proteins, respectively. An extensive literature search was conducted to validate whether these had been previously reported as autoantigens. Of the 124 proteins, 79 were reported autoantigens, and 19 out of 25 of the strong-DS-binding ones were well-known autoantigens. Moreover, these proteins largely fell into the two most common autoantibody categories in autoimmune kidney diseases, including 40 ANA (anti-nuclear autoantibodies) and 25 GBM (glomerular basement membrane) autoantigens. In summary, this study compiles a large repertoire of potential autoantigens for autoimmune kidney diseases. This autoantigen-ome sheds light on the molecular etiology of autoimmunity and further supports our hypothesis DS-autoantigen complexes as a unifying principle of autoantigenicity.

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.

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.

Role for mycobacterial infection in pathogenesis of primary biliary cirrhosis?

Primary biliary cirrhosis (PBC) is a progressive cholestatic liver disease characterized by the immune-mediated destruction of biliary epithelial cells in small intrahepatic bile ducts. The disease is characterized by circulating antimitochondrial antibodies (AMAs) as well as disease-specific antinuclear antibodies, cholestatic liver function tests, and characteristic histological features, including granulomas. A variety of organisms are involved in granuloma formation, of which mycobacteria are the most commonly associated. This has led to the hypothesis that mycobacteria may be involved in the pathogenesis of PBC, along with other infectious agents. Additionally, AMAs are found in a subgroup of patients with mycobacterial infections, such as leprosy and pulmonary tuberculosis. Antibodies against species-specific mycobacterial proteins have been reported in patients with PBC, but it is not clear whether these antibodies are specific for the disease. In addition, data in support of the involvement of the role of molecular mimicry between mycobacterial and human mitochondrial antigens as triggers of cross-reactive immune responses leading to the loss of immunological tolerance, and the induction of pathological features have been published. Thus, antibodies against mycobacterial heat shock protein appear to cross-recognize AMA-specific autoantigens, but it is not clear whether these autoantibodies are mycobacterium-species-specific, and whether they are pathogenic or incidental. The view that mycobacteria are infectious triggers of PBC is intriguing, but the data provided so far are not conclusive.

Comparative analysis of portal cell infiltrates in antimitochondrial autoantibody-positive versus antimitochondrial autoantibody-negative primary biliary cirrhosis

Substantial evidence supports dysregulated B-cell immune responses in patients with primary biliary cirrhosis (PBC), including the presence of serum antimitochondrial antibodies (AMAs). However, recent reports from murine models of PBC suggest that B cells may also provide regulatory function, and indeed the absence of B cells in such models leads to exacerbation of disease. The vast majority of patients with PBC have readily detectable AMAs, but a minority (<5%) are AMA negative (AMA(-)), even with recombinant diagnostic technology. This issue prompted us to examine the nature of B-cell infiltrates surrounding the portal areas in AMA-positive (AMA(+)) and AMA(-) patients, because they display indistinguishable clinical features. Of importance was the finding that the degree of bile duct damage around the portal areas was significantly milder in AMA(+) PBC than those observed in AMA(-) PBC patients. The portal areas from AMA(-) patients had a significant increase of cluster of differentiation (CD)5(+) cells infiltrating the ductal regions, and the levels of B-cell infiltrates were worse in the early phase of bile duct damage. The frequency of positive portal areas and the magnitude of CD5(+) and CD20(+) cellular infiltrates within areas of ductal invasion is associated with the first evidence of damage of biliary duct epithelia, but becomes reduced in the ductopenia stage, with the exception of CD5(+) cells, which remain sustained and predominate over CD20(+) cells.

CONCLUSION

Our data suggest a putative role of B-cell autoimmunity in regulating the portal destruction characteristic of PBC.

B-cell depletion with anti-CD20 ameliorates autoimmune cholangitis but exacerbates colitis in transforming growth factor-beta receptor II dominant negative mice

The treatment of primary biliary cirrhosis (PBC) with conventional immunosuppressive drugs has been relatively disappointing and there have been few efforts in defining a role for the newer biological agents useful in rheumatoid arthritis and other systemic autoimmune diseases. In this study we took advantage of transforming growth factor-beta (TGF-beta) receptor II dominant negative (dnTGF-betaRII) mice, a mouse model of autoimmune cholangitis, to address the therapeutic efficacy of B-cell depletion using anti-CD20. Mice were treated at either 4-6 weeks of age or beginning at 20-22 weeks of age with intraperitoneal injections of anti-CD20 every 2 weeks. We quantitated B-cell levels in all mice as well as antimitochondrial antibodies (AMA), serum and hepatic levels of proinflammatory cytokines, and histopathology of liver and colon. In mice whose treatment was initiated at 4-6 weeks of age, anti-CD20 therapy demonstrated a significantly lower incidence of liver inflammation associated with reduced numbers of activated hepatic CD8(+) T cells. However, colon inflammation was exacerbated. In contrast, in mice treated at 20-22 weeks of age, anti-CD20 therapy had relatively little effect on either liver or colon disease. As expected, all treated animals had reduced levels of B cells, absence of AMA, and increased levels in sera of tumor necrosis factor alpha (TNF-alpha), interleukin 6 (IL-6), and chemokine (C-C motif) ligand (CCL2) (monocyte chemoattractant protein 1 [MCP-1]).

CONCLUSION

These data suggest potential usage of anti-CD20 in early PBC resistant to other modalities, but raise a cautionary note regarding the use of anti-CD20 in inflammatory bowel disease.

B cells suppress the inflammatory response in a mouse model of primary biliary cirrhosis

BACKGROUND & AIMS

Mice that express a dominant-negative form of transforming growth factor-beta receptor restricted to T cells (dnTGF-betaRII) develop antimitochondrial antibodies and liver inflammation similar to human primary biliary cirrhosis.

METHODS

To address the role of B cells in this model of primary biliary cirrhosis, we bred B cell-deficient mice (Igmu(-/-)) with dnTGF-betaRII mice, creating Igmu(-/-)dnTGF-betaRII mice, and compared the resulting disease phenotype with that of dnTGF-betaRII mice (controls). We also performed adoptive transfer of dnTGF-betaRII CD8(+) splenocytes, with or without B cells, to 8-week-old female Rag-1(-/-) mice to assess the role of B cells in the inflammatory response.

RESULTS

The B cell-deficient Igmu(-/-)dnTGF-betaRII mice unexpectedly developed a more severe form of cholangitis than controls (dnTGF-betaRII mice) and had a significantly greater frequency of activated CD4(+) and CD8(+) T cells in the liver. They also had reduced frequency of Foxp3(+) regulatory T cells in the hepatic CD4(+) T-cell population and natural killer (NK) T cells (NK1.1(+) CD3(+)) in hepatic inflammatory cell infiltrates. The Igmu(-/-)dnTGF-betaRII mice had increased levels of proinflammatory cytokines (tumor necrosis factor-alpha and interleukin-6) and developed a more severe form of colitis than controls. Adoptive transfer of CD8(+) splenocytes from dnTGF-betaRII mice and peritoneal cavity-derived, but not spleen-derived, CD19(+) B cells into Rag-1(-/-) mice resulted in decreased amounts of liver inflammation and bile duct damage, compared with Rag-1(-/-) mice in which only CD8(+) splenocytes were transferred.

CONCLUSION

B cells have a suppressive effect on the inflammatory response in the dnTGF-betaRII model of primary biliary cirrhosis.

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

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

Pathogenesis of primary biliary cirrhosis: A unifying model

Primary biliary cirrhosis (PBC) is a disease of unknown etiology leading to progressive destruction of small intrahepatic bile ducts and eventually to liver cirrhosis and failure. It is characterised by female predominance and serum auto-antibodies to mitochondrial antigens targeting the E2 components of the 2-oxoacid dehydrogenase complex. Although they are associated with disease pathogenesis, no concrete evidence has been presented so far. Epidemiological data indicate that a geographical clustering of cases and possible environmental factors are implicated in pathogenesis. A number of genetic factors play a role in determining disease susceptibility or progression, although no definitive conclusion has been reached so far. A key factor to immune pathogenesis is considered to be the breakdown of immune tolerance, either through molecular mimicry or through the so called determinant density model. In this review, the available data regarding the pathogenesis of primary biliary cirrhosis are described and discussed. A new unifying hypothesis based on early endothelin overproduction in primary biliary cirrhosis (PBC) is presented and discussed.

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

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

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

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

Antimitochondrial and other autoantibodies

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

The immunology of primary biliary cirrhosis: the end of the beginning?

The chronic liver disease primary biliary cirrhosis (PBC) is characterised by autoreactive B-cell and T-cell responses directed against mitochondrial antigens. In recent years these responses have been extensively characterised and the principal PBC associated autoantigen identified as pyruvate dehydrogenase complex (PDC). The identification of anti-PDC responses (present in over 95% of PDC patients) has given rise to important questions pertinent to our understanding of the pathogenesis of PBC. What specific role to anti-PDC responses play in target cell damage? How and why does immune tolerance break down to as highly conserved and ubiquitously expressed self-antigen as PDC? Why does breakdown in tolerance to an antigen present in all nucleated cells result in damage restricted to the intra-hepatic bile ducts? In attempting to answer these key questions we have, in this review, proposed a unifying hypothesis for the pathogenesis of PBC.

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

The anti-mitochondrial antibody response in primary biliary cirrhosis (PBC) is primarily directed at E2 components of PDC, OGDC, and BCOADC, and E3BP. Previous work has shown that the immunodominant autoreactive T- cell epitope is the PDC-E2 163-176 peptide, restricted by HLA DR53. To address molecular mimicry and cross-recognition among mitochondrial autoantigens, we analyzed reactivity, including agonism and antagonism assays, to a series of single amino acid-substituted peptides using cloned T-cell lines in PBC and controls. Interestingly, fine specificities were unique for every single T-cell clone, but the clones could be categorized into two distinct groups based on recognition motifs of the T-cell receptor (TCR) ligand: group A (170)ExDK(173) and group B (168)EIExD(172). (170)E is the most critical TCR contact residue for both groups of cloned T-cell lines, whereas (173)K and (168)E are the critical TCR contact residues for group A and group B cloned T-cell lines, respectively. More importantly, some group A-cloned T-cell lines cross-reacted to human E3BP 34-47, human OGDC-E2 100-113, and several peptides derived from various microbial proteins carrying an ExDK motif, whereas group B-cloned T-cell lines reacted only to E3BP 34-47 carrying an EIExD motif. Furthermore, an RGxG motif was exclusively found in the complementarity-determining region (CDR3) of the TCR Vbeta in the group B-cloned T-cell lines, while G, S, and/or R were frequently found in the CDR3 of the TCR Vbeta in the group A-cloned T-cell lines. These data provide a framework for understanding molecular mimicry among mitochondrial antigens.

Autoantigens in primary biliary cirrhosis

The automimmune liver disease primary biliary cirrhosis (PBC) is characterised by serum autoantibodies directed at mitochondrial and nuclear antigens (seen in most patients and a subset of patients, respectively). The antimitochondrial antibodies (AMA) characteristic of PBC are directed at members of the 2-oxoacid dehydrogenase components of multienzyme complexes; in particular, the E2 and E3 binding protein (E3BP) components of the pyruvate dehydrogenase complex (PDC). The presence of autoantibodies reactive with PDC-E2 and/or E3BP is strongly predictive of the presence of PBC. Therefore, the detection of these antibodies plays a very important role in the diagnosis of PBC. Originally demonstrated using immunofluorescence approaches, AMA can now be detected by the use of commercially available enzyme linked immunosorbent assays (ELISAs). Although the ELISA based approaches have advantages in terms of laboratory practicality, they are slightly less sensitive for the diagnosis of PBC than immunofluorescence (occasional patients with PBC show reactivity with PDC related antigens not present in the antigen preparations available for use with ELISA). Therefore, immunofluorescence should continue to be available as a complementary diagnostic test for use in occasional patients. In a subset of patients with PBC, autoantibodies are directed at increasingly well characterised nuclear antigens. Antinuclear antibody (ANA) positive patients are typically AMA negative. There are no significant differences in disease phenotype between AMA positive and AMA negative groups. At present, the clinical detection of ANA is mostly by Hep2 immunofluorescence, although ELISA kits for individual nuclear antigens are increasingly becoming available.

The pyruvate dehydrogenase complex as a target autoantigen in primary biliary cirrhosis

Mitochondrial autoantigens and their B and T cell autoepitopes have been well defined in primary biliary cirrhosis (PBC). However, the relationships of the antimitochondrial antibodies and the mechanisms of bile duct destruction in PBC remain an enigma. The serological hallmark of PBC remains the presence of antibodies to mitochondria, particularly to the E2 component of the pyruvate dehydrogenase complex (PDC-E2). However, several mechanisms may now be proposed which may explain the immune-mediated bile duct damage in PBC. These include the possible role of T cell-mediated cytotoxicity as well as the interaction between the IgA class of antimitochondrial antibodies and the mitochondrial autoantigens. A prominent feature in this discussion is the highly directed and specific immune response to the mitochondrial antigens, including PDC-E2 as well as other members of the 2-oxo-acid dehydrogenase complexes. Ultimately, the mechanisms that lead to this immune reaction should provide data on other questions in PBC, including the reasons for female predominance, the absence of PBC in children and the relative ineffectiveness of immunosuppressive agents.

Comparative immunoreactivity of anti-trifluoroacetyl (TFA) antibody and anti-lipoic acid antibody in primary biliary cirrhosis: searching for a mimic

Previous studies documenting the existence of cross-reactivity between the lipoated (but not unlipoated) forms of the inner lipoyl domain (E2L2) of PDC-E2 [the major autoantigen in Primary biliary cirrhosis (PBC)] and trifluoroacetylated (TFA) proteins, led us to hypothesize that PBC may be due to an initial insult with an environmental agent that cross-reacts with TFA. Therefore, we performed a comparative study of the reactivity of rabbit anti-TFA antibody and anti-lipoic acid (LA) antibody against the mitochondrial autoantigens of human PBC and various TFA and LA conjugated proteins. Whereas both anti-TFA and anti-LA reacted with PDC-E2, the wild-type lipoated form of E2L2, OGDC-E2, E3-BP and LA-KLH, neither reacted with BCOADC-E2 or the non-lipoated form of E2L2. Of interest was that while anti-TFA reacted with PDC-E2, TFA-RSA and LA-KLH, it failed to inhibit PDC-E2 enzyme function. In contrast, anti-LA demonstrated cytoplasmic and mitochondrial staining, and inhibited PDC enzyme activity. Hence, although considerable cross reactivity exists between anti-TFA and anti-LA, the molecular nature of the interaction is clearly different. One of 14 PBC sera reacted weakly with TFA-albumin, whereas four of 14 PBC sera reacted with LA-KLH. Immunohistochemically, both anti-TFA and anti-LA antibodies reacted focally with periportal hepatocytes and bile ducts in both PBC and controls. However, anti-LA produced much stronger focalized staining of the bile ducts of diseased liver. This study suggests that while anti-TFA antibody recognizes lipoic acid-linked enzymes and proteins, the epitope recognized differs from that of anti-LA antibody and PBC autoantibodies. It is unlikely that a response to TFA is the triggering event in PBC. Anti-LA antibodies share a higher degree of similarity to PBC sera providing suggestive evidence that anti-LA antibodies or anti-LA like antibodies (mimotopes) may help define the initiator of the autoimmune response.

Is there a serological difference between men and women with primary biliary cirrhosis?

OBJECTIVE

Primary biliary cirrhosis (PBC) is an autoimmune disease affecting small intrahepatic bile ducts of the liver, causing destruction of the epithelium that results in eventual fibrosis and scarring. We still lack a complete epidemiological description of this disease, although interesting geographic differences in prevalence have been described. One consistent feature has been the relative scarcity of men with PBC. In fact, published ratios of women to men range from 3:1 to as high as 22:1. Thus far, the only clinical difference reported between men and women with PBC is a putative higher risk of hepatocarcinoma in men. Previous serological studies have shown that about 95% of all patients possess antimitochondrial antibodies to members of the highly conserved 2-oxo-acid dehydrogenase family of proteins, namely pyruvate dehydrogenase complex E2 (PDC-E2), branched-chain 2-oxo-acid dehydrogenase complex E2 (BCOADC-E2), and 2-oxo glutarate dehydrogenase complex E2 (OGDC-E2). However, there has been no information as to whether there is a difference in serological response between men and women. Using the serological hallmark of antimitochondrial antibodies (AMAs) and taking advantage of the availability of recombinant mitochondrial autoantigens, investigations were performed to determine if there were any serological differences between men and women with PBC.

METHODS

Sera were collected from 88 patients with PBC, of whom 46 were men and 42 were women. Using a combination of immunoblotting and enzyme-linked immunoabsorbent assay (ELISA) against beef heart mitochondria (BHM), recombinant PDC-E2, BCOADC-E2, and OGDC-E2, we determined the relative autoantibody reactivities of our study population.

RESULTS

Both men and women with PBC produced high titer antimitochondrial antibodies. The frequency of reactivity was similar in both groups and included, in descending order, PDC-E2, E3BP (Protein X), BCOADC-E2, and finally OGDC-E2. More importantly, antigenic specificity was nearly identical regardless of gender.

CONCLUSIONS

AMAs are the serological hallmark of PBC in both men and women, and there is no significant difference in reactivity between the two groups of patients.

In situ nucleic acid detection of PDC-E2, BCOADC-E2, OGDC-E2, PDC-E1alpha, BCOADC-E1alpha, OGDC-E1, and the E3 binding protein (protein X) in primary biliary cirrhosis

The characteristic serological feature of primary biliary cirrhosis (PBC) is the presence of antimitochondrial antibodies (AMAs), and the major proteins recognized by AMAs are subunits of the 2-oxo acid dehydrogenase complexes (2-OADC), including the E2 components of the pyruvate dehydrogenase complex (PDC), the 2-oxo-glutarate dehydrogenase complex (OGDC), the branched-chain 2-oxoacid dehydrogenase complex (BCOADC), the E3 binding protein (E3BP or protein X) and the E1a component of mammalian PDC. Previous work has postulated that either E3BP, or a molecule cross-reactive with the PDC-E2 molecule, is uniquely expressed on the surface of biliary epithelial cells in PBC. To address this issue, we performed in situ hybridization for all of the major 2-OADC components at the mRNA level, including PDC-E2, BCOADC-E2, OGDC-E2, PDC-E1a, BCOADC-E1a, OGDC-E1, and E3BP using 13 PBC and 9 control livers using 7 mitochondrial antisense probes. In both PBC and controls, the expression of all 2-OADC component mRNA studied herein were found in hepatocytes and infiltrating mononuclear cells, without significant differences. Interestingly, however, despite published data on immunohistochemical staining, interlobular bile ducts including the injured bile ducts in PBC were generally negative or only faintly positive, with the exception of 1 bile duct in 1 of 13 cases of PBC and 1 of 9 control liver specimens. Moreover, confocal microscopic examination and image analysis revealed that the mRNA signal intensity of each of the 2-OADC components in the bile ducts of PBC was relatively lower in comparison with control liver diseases. These data suggest that continuous enhanced synthesis of the 2-OADC components is not likely to be occurring in the biliary epithelial cells in PBC, and that an increase of PDC-E2 or E3BP immunoreactivity in PBC is caused by exogenous imported or cross-reactive molecules.

Identification and precursor frequency analysis of a common T cell epitope motif in mitochondrial autoantigens in primary biliary cirrhosis

The immunodominant antimitochondrial antibody response in patients with primary biliary cirrhosis (PBC) is directed against the E2 component of the pyruvate dehydrogenase complex (PDC-E2). Based on our earlier observations regarding peripheral blood mononuclear cell (PBMC) T cell epitopes, we reasoned that a comparative analysis of the precursor frequencies of PDC-E2 163-176-specific T cells isolated from PBMC, regional hepatic lymph nodes, and from the liver of PBC patients would provide insight regarding the role of T cells in PBC. Results showed a disease-specific 100-150-fold increase in the precursor frequency of PDC-E2 163-176-specific T cells in the hilar lymph nodes and liver when compared with PBMC from PBC patients. Interestingly, autoreactive T cells and autoantibodies from PBC patients both recognize the same dominant epitope. In addition, we demonstrated cross-reactivity of PDC-E2 peptide 163-176-specific T cell clones with PDC-E2 peptide 36-49 and OGDC-E2 peptide 100-113 thereby identifying a common T cell epitope "motif" ExETDK. The peptide 163-176-specific T cell clones also reacted with purified native PDC-E2, suggesting that this epitope is not a cryptic determinant. These data provide evidence for a major role for PDC-E2 peptide 163-176 and/or peptides bearing a similar motif in the pathogenesis of PBC.

Monoclonal Antibodies to Mitochondrial E2 Components Define Autoepitopes in Primary Biliary Cirrhosis

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by the presence of antimitochondrial Abs (AMA). The autoantigens recognized by AMA are the E2 components of the pyruvate dehydrogenase complex (PDC-E2), the branched chain 2-oxoacid dehydrogenase complex E (BCOADC-E2), and the 2-oxoglutarate dehydrogenase complex E (OGDC-E2). Previous studies using murine monoclonal and human combinatorial Abs to PDC-E2 have demonstrated an intense linear staining pattern in the apical region of biliary epithelial cells (BEC) in PBC but not control liver. We therefore examined whether mAbs to the other mitochondrial autoantigens BCOADC-E2 and OGDC-E2 demonstrated disease-specific patterns of reactivity. Using an expressed recombinant “trihybrid” protein containing the lipoyl domains of PDC-E2, OGDC-E2, and BCOADC-E2, we immunized BALB/c mice to produce 35 mAbs specific for one or more of the above mitochondrial autoantigens. Seven of these mAbs uniquely stained the apical region of BEC in PBC. Of these seven, one was reactive to PDC-E2, two recognized BCOADC-E2, three were reactive to OGDC-E2, and one recognized all three Ags. Our current data demonstrate that, similar to our previous studies regarding PDC-E2, mAbs to BCOADC-E2 and OGDC-E2, or a molecule that cross-reacts with the inner lipoyl domain of all three enzymes, also show a uniquely intense staining pattern in the apical region of BEC in patients with PBC when compared with diseased controls. The abundance of such disease-specific determinants in the target cells of PBC raises interesting possibilities regarding the role of these autoantigens in the pathogenesis of this disease.

Random phage mimotopes recognized by monoclonal antibodies against the pyruvate dehydrogenase complex-E2 (PDC-E2)

Dihydrolipoamide acetyltransferase, the E2 component of the pyruvate dehydrogenase complex (PDC-E2), is the autoantigen most commonly recognized by autoantibodies in primary biliary cirrhosis (PBC). We identified a peptide mimotope(s) of PDC-E2 by screening a phage-epitope library expressing random dodecapeptides in the pIII coat protein of fd phage using C355.1, a murine monoclonal antibody (mAb) that recognizes a conformation-dependent epitope in the inner lipoyl domain of PDC-E2 and uniquely stains the apical region of bile duct epithelium (BDE) only in patients with PBC. Eight different sequences were identified in 36 phage clones. WMSYPDRTLRTS was present in 29 clones; WESYPFRVGTSL, APKTYVSVSGMV, LTYVSLQGRQGH, LDYVPLKHRHRH, AALWGVKVRHVS, KVLNRIMAGVRH and GNVALVSSRVNA were singly represented. Three common amino acid motifs (W-SYP, TYVS, and VRH) were shared among all peptide sequences. Competitive inhibition of the immunohistochemical staining of PBC BDE was performed by incubating the peptides WMSYPDRTLRTS, WESYPDRTLRTS, APKTYVSVSGMV, and AALWGVKVRHVS with either C355.1 or a second PDC-E2-specific mAb, C150.1. Both mAbs were originally generated to PDC-E2 but map to distinct regions of PDC-E2. Two of the peptides, although selected by reaction with C355.1, strongly inhibited the staining of BDE by C150.1, whereas the peptide APKTYVSVSGMV consistently inhibited the staining of C355.1 on biliary duct epithelium more strongly than the typical mitochondrial staining of hepatocytes. Rabbit sera raised against the peptide WMSYPDRTLRTS stained BDE of livers and isolated bile duct epithelial cells of PBC patients more intensively than controls. The rabbit sera stained all size ducts in normals, but only small/medium-sized ductules in PBC livers. These studies provide evidence that the antigen present in BDE is a molecular mimic of PDC-E2, and not PDC-E2 itself.

Humoral and cellular immune responses to dihydrolipoamide dehydrogenase (E3): lack of specificity for primary biliary cirrhosis

Immune responses to dihydrolipoamide dehydrogenase, the E3 subunit which is a common component of 2-oxoacid dehydrogenase complexes, have been suggested to be associated with the etiology of primary biliary cirrhosis (PBC). However, since an antibody to E3 could be detected in Caucasian patients with PBC, but was not specific for the disease, the proposal is not evident at the antibody level. We have identified the antibody also in Japanese patients with PBC by immunoblotting with sera at a 1:100 dilution and have assessed cellular immune responses to E3 by proliferation assay of peripheral blood lymphocytes. Anti-E3 antibody was detected more frequently in 25 of 43 PBC (58.1%) than in normal controls (p < 0.01) and in chronic liver diseases (p < 0.05), but the antibody was not specific for PBC as reported in Caucasian PBC. Anti-E3 antibody-positive sera of PBC patients or normal controls and their IgG fraction did not inhibit the enzyme activity of E3. Lymphocyte blastogenesis to E3 in PBC was significantly greater than normal controls (p < 0.05), but was not significant as compared with chronic liver disease or non-hepatic autoimmune diseases. Thus, these data do not support the hypothesis that the immune response to the E3 subunit is associated with etiology of PBC.

Primary biliary cirrhosis: the molecule and the mimic

Our understanding of the immunobiology of PBC has dramatically changed with the application of molecular biology to clinical medicine. Because of the molecular characterization and identification of the mitochondrial autoantigens, it is now possible to define explicitly mitochondrial autoantigens and examine recognition sites at the primary sequence level. In addition, the expression of cloned antigens has facilitated the development of more reliable assays for mitochondrial autoantibodies. The use of cloned recombinant antigens should, one day, replace the traditional AMA immunofluorescence for diagnostic assays. Possible genetic and environmental factors associated with risk for PBC can also be investigated. It is now also possible to begin the task to defining the role of T cells in the immunopathology of PBC and exploring the issue of whether specific immunotherapy is feasible. There is increasing evidence that PDC-E2 or a similar molecule is located on the cell membrane of biliary epithelial cells. The mechanism for this expression remains to be studied. The explosion of data in PBC is an example of the application of new techniques to investigate old problems. This has occurred because of networking between laboratories in many countries and the generous exchange of sera and donation of livers removed at transplantation. Unfortunately, there is no animal model for PBC; if an animal model was found it would have major importance. Finally, we emphasize the need to study patients early in the course of disease in order to define the events that initiate pathology.

Molecular mimicry in halothane hepatitis: biochemical and structural characterization of lipoylated autoantigens

Exposure of human individuals to halothane causes, in about 20% of all cases, a mild transient form of hepatotoxicity. A small subset of exposed individuals, however, develops a potentially severe and life-threatening form of hepatic damage, coined halothane hepatitis. Halothane hepatitis is thought to have an immunological basis. Sera of afflicted individuals contain a wide variety of autoantibodies against hepatic proteins, in both trifluoroacetylated form (CF3CO-proteins) and, at least in part, in native form. CF3CO-proteins are elicited in the course of oxidative biotransformation of halothane, and include the trifluoroacetylated forms of protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78, and ERp99. Current evidence suggests that CF3CO-proteins arise in all halothane-exposed individuals; however, the vast majority of individuals appear to immunochemically tolerate CF3CO-proteins. The lack of immunological responsiveness of these individuals towards CF3CO-proteins might be due to tolerance, induced through the occurrence of structures in the repertoire of self-determinants, which immunochemically and structurally mimic CF3CO-proteins very closely. In fact, lipoic acid, the prosthetic group of the constitutively expressed E2 subunits of the family of mammalian 2-oxoacid dehydrogenase complexes and of protein X, was shown by immunochemical and molecular modelling analysis to be a perfect structural mimic of N6-trifluoroacetyl-L-lysine (CF3 CO-Lys), the major haptenic group of CF3CO-proteins. As a consequence of molecular mimicry, autoantibodies in patients' sera not only recognize CF3CO-proteins, but also the E2 subunit proteins of the 2-oxoacid dehydrogenase complexes and protein X, as autoantigens associated with halothane hepatitis. Furthermore, a fraction of patients with halothane hepatitis exhibit irregularities in the hepatic expression levels of these native, not trifluoroacetylated autoantigens. Collectively, these data suggest that molecular mimicry of CF3CO-Lys by lipoic acid, or the impairment thereof, might play a role in the susceptibility of individuals for the development of halothane hepatitis.

Peripheral B lymphocyte repertoire to mitochondrial antigen in primary biliary cirrhosis--positive correlation between the disease activity and the frequency of circulating B lymphocytes specific for pyruvate dehydrogenase complex

B lymphocytes committed to the production of IgG antibodies (Abs) to mitochondrial antigen such as pyruvate dehydrogenase complex(PDC) were quantitated in the peripheral blood of patients with primary biliary cirrhosis(PBC) using Epstein-Barr virus as a polyclonal activator of human B lymphocytes. B lymphocytes committed to the production of IgG Abs to PDC were found in high frequency in patients with PBC(0.54 +/- 0.16%, mean value +/- SE, of total IgG-producing B lymphocytes) in contrast to type C chronic hepatitis and healthy subjects in which they were less than 0.01%. The frequency of these B lymphocytes specific for PDC increased in parallel to the progression of the Scheuer's stage from I to II (stage I: 0.35 +/- 0.23%, stage II: 1.04 +/- 0.32%), but decreased with further progression to stage IV (stage III: 0.39 +/- 0.21%, stage IV: 0.07 +/- 0.06%). In addition, B lymphocytes specific for PDC decreased in the peripheral blood during the administration of cyclosporin A; this was accompanied by an improvement of lymphocyte infiltration severity in the liver. These data indicate that B lymphocytes specific for PDC are present in the peripheral blood of patients with PBC and their frequency reflects the degree of the lymphocyte infiltration in the liver.

Identification of the dihydrolipoamide acetyltransferase subunit of the human pyruvate dehydrogenase complex as an autoantigen in halothane hepatitis. Molecular mimicry of trifluoroacetyl-lysine by lipoic acid

Trifluoroacetylated (CF3CO-) proteins, elicited upon exposure of animals or humans to halothane, were recognized by anti-CF3CO antibody, monospecific for the hapten derivative N6-trifluoroacetyl-L-lysine. Anti-CF3CO antibodies cross-reacted with the dihydrolipoamide acetyltransferase (E2 subunit) of pyruvate dehydrogenase, indicating that epitopes on the E2 subunit of pyruvate dehydrogenase molecularly mimic those on CF3CO-proteins. Lipoic acid, the prosthetic group of the E2 subunit of pyruvate dehydrogenase was essential in this process, in that only the lipoylated form of the recombinantly expressed inner lipoyl domain of the human E2 subunit of pyruvate dehydrogenase, but not the unlipolyated form, was recognized by anti-CF3CO antibody. Furthermore, based on a high degree of structural relatedness, both CF3CO-Lys and (6RS)-lipoic acid, as well as the lipoylated peptide ETDK(lipoyl)ATIG specifically inhibited the recognition by anti-CF3CO antibody of the E2 subunit of pyruvate dehydrogenase, of trifluoroacetylated rabbit serum albumin and of human liver CF3CO-proteins. In sera of patients with halothane hepatitis, autoantibodies with properties identical to those of anti-CF3CO antibody were identified which could not discriminate between CF3CO-proteins and the E2 subunit of pyruvate dehydrogenase. These data suggest that the E2 subunit pyruvate of dehydrogenase is an autoantigen in halothane hepatitis and that molecular mimicry of CF3CO-proteins by the E2 subunit of pyruvate dehydrogenase is due to the similar structures of CF3CO-Lys and lipoic acid.

Immunoblotting as a confirmatory test for antimitochondrial antibodies in primary biliary cirrhosis

Primary biliary cirrhosis is characterised by the presence of antimitochondrial antibodies which are directed against components of mitochondrial dehydrogenase complexes. The specificity of antimitochondrial antibodies for primary biliary cirrhosis as detected by immunoblotting was investigated. Commercially available preparations of pyruvate and oxo-glutarate dehydrogenases and beef-heart mitochondria were used as source of antigens. Sera from 47 primary biliary cirrhosis patients (46 of whom were antimitochondrial antibody positive by immunofluorescence), 16 non-primary biliary cirrhosis patients (antimitochondrial antibody positive by immunofluorescence), 23 liver-kidney microsomal antibody positive chronic active hepatitis patients, and 32 patients with connective tissue diseases were examined. Of the 47 subjects with primary biliary cirrhosis, 43 (91%) and 13 (28%) tested positive by immunoblotting for pyruvate and oxo-glutarate dehydrogenase, respectively. Only three primary biliary cirrhosis patients were negative for both antigens, including the only one shown to be antimitochondrial antibody negative by immunofluorescence. The other two patients were positive by immunoblotting with beef-heart mitochondria. In contrast, only three of 16 (19%) non-primary biliary cirrhosis patients who were antimitochondrial antibody positive by immunofluorescence tested positive by immunoblotting (for both pyruvate dehydrogenase and beef-heart mitochondria). None of the 23 liver-kidney microsomal antibody positive and the 32 patients with rheumatic diseases were positive by immunoblotting with any antigen. Our data show that immunoblotting with commercially available oxo-acid dehydrogenases is a reproducible method for the detection of antimitochondrial antibodies highly specific for primary biliary cirrhosis.

Mechanisms of halothane toxicity: novel insights

Exposure of individuals to halothane causes, in 20% of patients, a mild form of hepatotoxicity. In contrast, a very small subset of individuals only develops halothane hepatitis, which is thought to have an immunological basis. Sera of halothane hepatitis patients contain antibodies directed against some discrete liver trifluoroacetyl (TFA)-protein adducts, which arise upon oxidative biotransformation of halothane and include protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78 and ERp99. No immune response occurs in the majority of human individuals, although evidence suggests that TFA-protein adducts arise in all halothane-exposed individuals. The lack of immunological responsiveness of individuals might be due to tolerance, induced by a presumed repertoire of self-peptides that molecularly mimic TFA-protein adducts. Thus, constitutively expressed proteins of 52 and 64 kDa have been identified that confer molecular mimicry of TFA-protein adducts. The 64 kDa protein corresponds to the E2 subunit of the mitochondrial pyruvate dehydrogenase complex. Lipoic acid, the prosthetic group of the E2 subunit, is involved in the molecular mimicry process. A fraction of halothane hepatitis patients exhibit irregularities in the expression levels of the 52 kDa protein and the E2 subunit protein. Molecular mimicry of TFA-protein adducts by the 52 kDa protein and the E2 subunit protein might play a role in the susceptibility of individuals to development of halothane hepatitis.

Antimitochondrial antibodies in kindreds of patients with primary biliary cirrhosis: antimitochondrial antibodies are unique to clinical disease and are absent in asymptomatic family members

The 2-oxo-acid dehydrogenase family of enzymes have been identified as the major mitochondrial autoantigens of primary biliary cirrhosis. Using immunoblotting, enzyme-linked immunosorbent assay and enzyme inhibition with both purified mitochondrial proteins and recombinant autoantigens, we have studied family members and spouses of patients with primary biliary cirrhosis for the presence of antimitochondrial antibodies. Antimitochondrial antibodies and other common autoantigens were also tested for by indirect immunofluorescence. This study included 27 index patients with primary biliary cirrhosis, 15 spouses and 48 first- and second-degree relatives. Overall, 7 relatives (11%) were positive for autoantibodies to nuclear and cytoplasmic antigens by indirect immunofluorescence against mouse liver and stomach sections. However, with immunofluorescence, the reactivity strictly paralleled that of antimitochondrial antibodies in only one of these (1:640)--a sibling with mild pruritus and a liver biopsy specimen diagnostic of primary biliary cirrhosis despite normal levels of serum alkaline phosphatase. In addition, one of the mothers, who had a history of sarcoidosis, was positive by immunoblotting for antibodies to the E2 subunit of the pyruvate dehydrogenase complex and protein X. All other relatives were negative for all of the assays. Antibodies to neither the 2-oxo-acid dehydrogenase enzymes nor the recently proposed family of naturally occurring mitochondrial antibodies were found in spouses or healthy relatives. Three other first-degree relatives suffered from liver disease: two died (one from primary biliary cirrhosis and the other from an unknown type of liver disease) and one (a sibling with primary biliary cirrhosis) was unavailable for testing. Our results are consistent with a familial predisposition to primary biliary cirrhosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of peptide-specific and cross-reactive epitopes in proteins reacting with antimitochondrial antibodies of primary biliary cirrhosis

Recently the main targets of antimitochondrial antibodies (AMA) of primary biliary cirrhosis have been identified as parts of three related mitochondrial multienzyme complexes, namely pyruvate dehydrogenase (PDH), branched chain alpha-ketoacid dehydrogenase (BKDH) and alpha-ketoglutarate dehydrogenase (alpha-KGDH). Usually AMA-positive PBC serum samples show reactivity to more than one of these, raising the question whether they are exclusively different antibodies or are, at least in part, the result of cross-reactive specificities. With Western immunoblotting, four antigens with molecular masses of 74, 52, 51 and 43 kDa, are recognized by PBC sera. In this study, using affinity purified antibodies from mitochondrial proteins immobilized on nitrocellulose blots, we demonstrate the presence of peptide-specific and cross-reactive epitopes in some targets. In particular, at least three different epitopes present in the 74-kDa protein (presumed to by PDH-E2) are also present in the 51-kDa protein (probably PDH-X), and two in the 52-kDa peptide (possibly BCKDH-E2). Moreover, the 43-kDa mitochondrial protein (the identity of which is more problematic) has three epitopes. One of these is also present in the 74-, 52- and 51-kDa proteins, a second in the 74- and 51-kDa, and a third seems to be peptide-specific. These results show that different sera with the same immunoblotting pattern of reactivity can have antibodies with different antigenic specificities and, conversely, that the same specificity can be responsible for more than one band.

Use of designer recombinant mitochondrial antigens in the diagnosis of primary biliary cirrhosis

The appearance of autoantibodies against mitochondria in patients with primary biliary cirrhosis has been known for more than 25 yr. In the past, based on the biochemical complexity of the mitochondrion and the use of crude extracts for immunodiagnosis, a degree of nonspecificity in assaying for antibodies to mitochondria has been present. This problem has been largely circumvented by the cloning of the mitochondrial antigens and the identification of the E2 subunits of the pyruvate dehydrogenase complex and the branched chain 2-oxo-acid dehydrogenase complex as the major and immunodominant autoantigens of primary biliary cirrhosis. More than 90% of patients with primary biliary cirrhosis have been shown to react with one or both of these enzymes using either recombinant antigen or purified native protein. Approximately 10% of patients recognize only E2 subunits of branched chain 2-oxo-acid dehydrogenase complex and not pyruvate dehydrogenase complex. Such patients would be missed by diagnostic assay that has a low sensitivity to antibodies against E2 subunits of branched chain 2-oxo-acid dehydrogenase complex. The use of recombinant and biochemically pure antigens has permitted structural and conformational analysis of epitope mapping. We have taken advantage of the antigenic mapping studies of both primary biliary cirrhosis and branched chain 2-oxo-acid dehydrogenase complex E2 subunits and designed a molecule that expresses the immunodominant epitopes of both. Using this dual-headed molecule that coexpresses the epitope of two different antigens, we report herein a sensitive and reproducible assay for antibodies to mitochondria in patients with primary biliary cirrhosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory autoantibody to a conformational epitope of the pyruvate dehydrogenase complex, the major autoantigen in primary biliary cirrhosis

The mitochondrial autoantibodies present in primary biliary cirrhosis (PBC) react with the 2-oxoacid dehydrogenase enzymes that include the pyruvate dehydrogenase complex (PDC). All epitopes so far demonstrable, including the inner lipoyl domain of PDC-E2, have been revealed by immunoblotting. To identify other epitopes, advantage was taken of the capacity of PBC sera to inhibit in vitro the catalytic function of the PDC enzyme. PBC sera were analyzed by affinity chromatography, using columns containing either recombinant PDC-E2 or intact PDC. Fractions that bound to the column (B) and nonbinding effluent fractions (NB) were tested by immunoblotting and ELISA and for their capacity to inhibit enzyme function. After separation on the PDC-E2 column the B fractions were reactive with PDC-E2 and intact PDC, whereas the NB fractions did not react by immunoblotting or ELISA with PDC-E2 but did react strongly by ELISA with PDC and did strongly inhibit the enzyme function. After separation of sera on the PDC column, the B fractions reacted more strongly with PDC than PDC-E2 by ELISA and strongly inhibited the enzyme function, whereas the NB fractions were nonreactive. Thus we describe a hitherto undetected population of autoantibodies in PBC sera that react only with intact PDC but not with the recombinant PDC-E2 subunit that contains the lipoyl epitope, are demonstrable by ELISA but not by immunoblotting, and notably, inhibit enzyme function. These nonblotting inhibitory autoantibodies in PBC are presumed to react with an exclusively conformational determinant perhaps presented by the tertiary structure of the entire enzyme complex.

Primary biliary cirrhosis: paradigm or paradox for autoimmunity

Primary biliary cirrhosis has been classified as a model autoimmune disease based on striking defects in immune regulation and the presence of autoantibodies to mitochondria. Until recently the significance and definition of mitochondrial autoreactivity was unknown. Since 1987, there has been a vast improvement in the understanding and definition of the biochemical and molecular target autoantigens. The cloning of complementary DNAs for mitochondrial antigens has led to the identification of three enzymes of the 2-oxo-acid dehydrogenase family as the targets of the autoantibodies to mitochondria in patients with primary biliary cirrhosis. The major reactive autoantigen is the E2 subunit of pyruvate dehydrogenase. Immunodominant sites on pyruvate dehydrogenase E2 (autoepitopes) have been mapped and have been shown to be the site of attachment of the functionally important lipoic acid prosthetic group. The autoepitope for the other enzymes probably occupies an equivalent site on the enzyme. The availability and definition of these mitochondrial autoepitopes have allowed specific questions to be addressed relating to the processing and targeting of these autoantigens as well as further studies on mechanisms of immunopathology. Similarly, the availability of well-defined autoantigens could contribute to the development of valid animal models in addition to the already described reproduction of the biliary ductular lesions by transfer of peripheral blood lymphocytes from patients with primary biliary cirrhosis into severe combined immunodeficient mice. Such models will facilitate specific study of the role of major histocompatibility complex expression and the characterization of T-cell reactivity. Thus, primary biliary cirrhosis is a key example of significant progress in autoimmunity being made by use of recombinant DNA technology.