Mitochondrial antibodies in primary biliary cirrhosis: species and nonspecies specific determinants of M2 antigen

New insights into the pathogenesis of primary biliary cholangitis asymptomatic stage

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

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

BACKGROUND AND AIMS

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

APPROACH AND RESULTS

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

CONCLUSIONS

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

Autoantibodies in Systemic Lupus Erythematosus Target Mitochondrial RNA

The mitochondrion supplies energy to the cell and regulates apoptosis. Unlike other mammalian organelles, mitochondria are formed by binary fission and cannot be directly produced by the cell. They contain numerous copies of a compact circular genome that encodes RNA molecules and proteins involved in mitochondrial oxidative phosphorylation. Whereas, mitochondrial DNA (mtDNA) activates the innate immune system if present in the cytosol or the extracellular milieu, it is also the target of circulating autoantibodies in systemic lupus erythematosus (SLE). However, it is not known whether mitochondrial RNA is also recognized by autoantibodies in SLE. In the present study, we evaluated the presence of autoantibodies targeting mitochondrial RNA (AmtRNA) in SLE. We quantified AmtRNA in an inducible model of murine SLE. The AmtRNA were also determined in SLE patients and healthy volunteers. AmtRNA titers were measured in both our induced model of murine SLE and in human SLE, and biostatistical analyses were performed to determine whether the presence and/or levels of AmtRNA were associated with clinical features expressed by SLE patients. Both IgG and IgM classes of AmtRNA were increased in SLE patients (n = 86) compared to healthy controls (n = 30) (p < 0.0001 and p = 0.0493, respectively). AmtRNA IgG levels correlated with anti-mtDNA-IgG titers (r_s = 0.54, p < 0.0001) as well as with both IgG and IgM against β-2-glycoprotein I (anti-β₂GPI; r_s = 0.22, p = 0.05), and AmtRNA-IgG antibodies were present at higher levels when patients were positive for autoantibodies to double-stranded-genomic DNA (p < 0.0001). AmtRNA-IgG were able to specifically discriminate SLE patients from healthy controls, and were negatively associated with plaque formation (p = 0.04) and lupus nephritis (p = 0.03). Conversely, AmtRNA-IgM titers correlated with those of anti-β₂GPI-IgM (r_s = 0.48, p < 0.0001). AmtRNA-IgM were higher when patients were positive for anticardiolipin antibodies (aCL-IgG: p = 0.01; aCL-IgM: p = 0.002), but AmtRNA-IgM were not associated with any of the clinical manifestations assessed. These findings identify mtRNA as a novel mitochondrial antigen target in SLE, and support the concept that mitochondria may provide an important source of circulating autoantigens in SLE.

The human immune system recognizes neopeptides derived from mitochondrial DNA deletions

Mutations in mitochondrial (mt) DNA accumulate with age and can result in the generation of neopeptides. Immune surveillance of such neopeptides may allow suboptimal mitochondria to be eliminated, thereby avoiding mt-related diseases, but may also contribute to autoimmunity in susceptible individuals. To date, the direct recognition of neo-mtpeptides by the adaptive immune system has not been demonstrated. In this study we used bioinformatics approaches to predict MHC binding of neopeptides identified from known deletions in mtDNA. Six such peptides were confirmed experimentally to bind to HLA-A*02. Pre-existing human CD4(+) and CD8(+) T cells from healthy donors were shown to recognize and respond to these neopeptides. One remarkably promiscuous immunodominant peptide (P9) could be presented by diverse MHC molecules to CD4(+) and/or CD8(+) T cells from 75% of the healthy donors tested. The common soil microbe, Bacillus pumilus, encodes a 9-mer that differs by one amino acid from P9. Similarly, the ATP synthase F0 subunit 6 from normal human mitochondria encodes a 9-mer with a single amino acid difference from P9 with 89% homology to P9. T cells expanded from human PBMCs using the B. pumilus or self-mt peptide bound to P9/HLA-A2 tetramers, arguing for cross-reactivity between T cells with specificity for self and foreign homologs of the altered mt peptide. These findings provide proof of principal that the immune system can recognize peptides arising from spontaneous somatic mutations and that such responses might be primed by foreign peptides and/or be cross-reactive with self.

Molecular diagnostics of primary biliary cirrhosis

BACKGROUND

Primary biliary cirrhosis (PBC) is an autoimmune liver disease of unknown etiology characterized by the presence of antimitochondrial antibodies (AMA) in 90 - 95% of patients. AMA are directed against members of 2-oxo-acid dehydrogenase complex, including mainly the E2 subunit of pyruvate dehydrogenase, the E2 subunit of branched chain 2-oxo-acid dehydrogenase complex and the E2 subunit of the oxoglutarate dehydrogenase complex. Apart from AMA, PBC is characterized by the presence of PBC-specific antinuclear antibodies (ANA). The molecular targets of these PBC-specific ANA have been characterized as gp210, lamin B receptor, nucleoporin 62, sp100 and promyelocytic leukemia proteins.

OBJECTIVE

To discuss the molecular diagnostics of PBC in the context of AMA and PBC-specific ANA detection by the use of conventional and 'new' novel technologies.

METHODS

Critical analysis of all published data regarding PBC serology between 1985 and 2007 was performed in order to suggest a diagnostic algorithm for the serological diagnosis of PBC.

RESULTS/CONCLUSIONS

AMA are first detected by indirect immunofluorescence (IIF) on frozen sections of rat liver, kidney and stomach substrates. However, because IIF is time-consuming, labor-intensive and observer-dependent, molecular-based assays such as immunoblot and enzyme-linked immunosorbent assays have been developed with high sensitivity and specificity. Similarly, molecular-based assays have also been developed for the detection of PBC-specific ANA. The latter investigation seems to be of outmost importance because these autoantibodies can be used as a positive tool in the diagnosis of AMA-negative PBC while at the same time identifying a subgroup of PBC patients with more advanced disease. New test systems for the detection of PBC-specific antibodies based on the xMultiple Analyte Profiling Luminex methodology seems to be the future in molecular diagnostics of PBC as it was expected first to decrease the cost and second to speed up an accurate serological profile, although they may decrease further the proportion of AMA-negative PBC cases.

Analysis of the clinical relevance of antimitochondrial antibodies to the β- and γ-subunits of the F1F0-ATPase in patients with primary biliary cirrhosis

Background

In a recent study we showed that in patients with primary biliary cirrhosis (PBC) being positive or negative for anti-M2 antibodies reacting with the 2-oxoacid-dehydrogenase complex (ODC) also antibodies to the beta- and gamma-subunits of F₁F₀-ATPase (anti-β, anti-γ) occur. This is a mitochondrial enzyme but parts are also expressed on plasma membranes of endothelial cells. Here we wanted to analyse in more detail their clinical relevance.

Methods

Fifty-nine untreated and histologically defined PBC patients who had been followed for at least five years were included into the study (51 anti-M2 positive, 8 anti-M2 negative). Twenty-three of them were treated in the follow up with ursodeoxycholic acid (UDCA), eight received during a trial methotrexate (MTX). In 13 patients orthotopic liver transplantation (OLT) had to be performed. Serum samples before and during therapy were available. Patients were analysed with respect to laboratory parameters, disease activity and histological stages.

Patients’ sera were tested by ELISA for IgG- and IgM-antibodies against the beta- and gamma-subunits which had been recombinant expressed in E.coli and highly purified by electro-elution from SDS-gels after electrophoresis.

Results

Fifty-nine percent of the anti-M2 positive and 50% of the anti-M2 negative PBC patients had anti-β- and/or anti-γ-antibodies. There were no differences between anti-β- and/or anti-γ-antibody positive or negative patients with respect to biochemical parameters, immunoglobulins, histological stages or disease activity. Antibody reactivity significantly decreased during UDCA and MTX-treatment and also after OLT.

Conclusions

Antibodies to the β- and γ-subunits of F₁F₀-ATPase occur in anti-M2 positive and –negative PBC but do not have any relevance with respect to clinical activity or prognosis. However, in contrast to the anti-M2 antibodies they decrease during UDCA and immunosuppressive therapy.

Identification of new autoantigens for primary biliary cirrhosis using human proteome microarrays

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease of unknown etiology and is considered to be an autoimmune disease. Autoantibodies are important tools for accurate diagnosis of PBC. Here, we employed serum profiling analysis using a human proteome microarray composed of about 17,000 full-length unique proteins and identified 23 proteins that correlated with PBC. To validate these results, we fabricated a PBC-focused microarray with 21 of these newly identified candidates and nine additional known PBC antigens. By screening the PBC microarrays with additional cohorts of 191 PBC patients and 321 controls (43 autoimmune hepatitis, 55 hepatitis B virus, 31 hepatitis C virus, 48 rheumatoid arthritis, 45 systematic lupus erythematosus, 49 systemic sclerosis, and 50 healthy), six proteins were confirmed as novel PBC autoantigens with high sensitivities and specificities, including hexokinase-1 (isoforms I and II), Kelch-like protein 7, Kelch-like protein 12, zinc finger and BTB domain-containing protein 2, and eukaryotic translation initiation factor 2C, subunit 1. To facilitate clinical diagnosis, we developed ELISA for Kelch-like protein 12 and zinc finger and BTB domain-containing protein 2 and tested large cohorts (297 PBC and 637 control sera) to confirm the sensitivities and specificities observed in the microarray-based assays. In conclusion, our research showed that a strategy using high content protein microarray combined with a smaller but more focused protein microarray can effectively identify and validate novel PBC-specific autoantigens and has the capacity to be translated to clinical diagnosis by means of an ELISA-based method.

Relevance of the inner mitochondrial membrane enzyme F1F0-ATPase as an autoantigen in autoimmune liver disorders

BACKGROUND AND AIMS

Recently, a non-M2-related mitochondrial 60 kDa protein found to be recognized by antimitochondrial antibody (AMA) negative sera from patients with primary biliary cirrhosis (PBC) has been shown to contain parts of the five F(1)-ATPase subunits α, β, γ, δ and ε. In this study, we examined whether this enzyme is, indeed, a target antigen in PBC.

METHODS

Analysed were 60 AMA-positive/anti-M2-negative and 103 anti-M2-positive PBC patients, 46 patients with autoimmune hepatitis (AIH), 35 patients with primary sclerosing cholangitis (PSC), 110 patients with viral hepatitis, 40 patients with inflammatory bowel diseases (IBD), 33 patients with connective tissue diseases (systemic lupus erythematosus, mixed connective tissue disease, Sjögren disease, systemic sclerosis) and 25 blood donors. The F(1)-ATPase-subunits α-δ were recombinantly expressed in Escherichia coli, purified and applied to ELISA and Western blotting.

RESULTS

In all, 40 of the 60 AMA-positive/anti-M2-negative (67%) and 44 (43%) of the 103 anti-M2-positive PBC-sera reacted with at least one of the F(1)-subunits α-δ. The β- and γ-subunits were preferentially recognized. However, also up to 57% of patients with AIH and 34% of patients with PSC had anti-β- or γ-antibodies, while patients with viral hepatitis had these antibodies in up to 13%. Patients with IBD had anti-β and anti-γ-antibodies in up to 20 and 5% respectively. None of the patients with connective tissue diseases had antibodies to the β- and only 6% to the γ-subunit. Sera from healthy blood donors were negative.

CONCLUSIONS

Antibodies to the β- and γ-subunits of F(1)-ATPase are further AMAs in PBC but occur also in other autoimmune liver disorders; they may be, therefore, indicators for a general autoimmune process of the liver.

Disease-specific autoantibodies in primary biliary cirrhosis

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

Primary biliary cirrhosis: What do autoantibodies tell us?

Primary biliary cirrhosis (PBC) is a chronic, progressive, cholestatic, organ-specific autoimmune disease of unknown etiology. It predominantly affects middle-aged women, and is characterized by autoimmune-mediated destruction of small- and medium-size intrahepatic bile ducts, portal inflammation and progressive scarring, which without proper treatment can ultimately lead to fibrosis and hepatic failure. Serum autoantibodies are crucial tools for differential diagnosis of PBC. While it is currently accepted that antimitochondrial antibodies are the most important serological markers of PBC, during the last five decades more than sixty autoantibodies have been explored in these patients, some of which had previously been thought to be specific for other autoimmune diseases.

Catalytic domain of PDC-E2 contains epitopes recognized by antimitochondrial antibodies in primary biliary cirrhosis

AIM: To search for further immunodominant peptides of the pyruvate dehydrogenase complex E2-component (PDC-E2) recognized by antimitochondrial antibodies (AMA) in primary biliary cirrhosis (PBC).

METHODS: Sera from 95 patients with PBC were tested by enzyme-linked immunosorbent assay against 33 synthetic overlapping peptides (25 amino acids; aa) covering the entire length of the E2-subunit of PDC-E2. Furthermore, the inner lipoyl peptide 167-184 was used in an unlipoylated and a lipoylated form as well as coupled to ovalbumin. Sera from 11 AMA negative/ANA positive PBC patients, 63 patients with other liver disorders and 22 healthy blood donors served as controls.

RESULTS: Of the 95 PBC-sera, 74% reacted with the peptide 475-499 and 58% with the peptide 407-431 located within the catalytic domain of PDC-E2. Patients with other disorders or healthy controls were positive in only up to 18%. Antibodies to the unlipoylated and lipoylated peptide 167-184 within the inner lipoyl domain were found in only 5% and 11% of the PBC sera, respectively; using ovalbumin-coupled peptides, the incidence increased up to 57% (unlipoylated form).

CONCLUSION: Peptides within the catalytic site of PDC-E2 rather than the previously reported lipoyl binding peptide 167-184 may represent major immunodominant epitopes recognized by AMA in PBC.

Antimitochondrial antibodies and other antibodies in primary biliary cirrhosis: diagnostic and prognostic value

Antimitochondrial antibodies (AMA) are the serologic cornerstone in the diagnosis of primary biliary cirrhosis (PBC), even if they are not detectable in a proportion of patients, notwithstanding the most sensitive and sophisticated technologies used. To fill in the serologic gap in AMA-negative PBC, there is sound evidence to consider antinuclear antibody (ANA) patterns, such as anti-multiple nuclear dots and anti-membranous/rim-like, as PBC-specific surrogate hallmarks of the disease, and their detection can be considered virtually diagnostic. Furthermore, particular ANA specificities, such as anti-gp210, anti-p62, anticentromere antibodies, and anti-dsDNA, may provide additional diagnostic and prognostic information.

Demonstration of autoantibodies to recombinant human sulphite oxidase in patients with chronic liver disorders and analysis of their clinical relevance

It has been shown previously that sera from patients with cholestatic liver diseases react with sulphite oxidase (SO) prepared from chicken liver. In order to analyse this reactivity and the clinical relevance of anti-SO antibodies in more detail, we produced human recombinant SO. Human recombinant SO (60 kDa) was expressed in Escherichia coli and applied to enzyme-linked immunosorbent assay and Western blot. Sera from patients with autoimmune liver disorders [primary biliary cirrhosis (PBC) n = 96; autoimmune hepatitis (AIH) n = 77; primary sclerosing cholangitis (PSC) n = 39], and from patients with other hepatic (n = 154) and non-hepatic chronic inflammatory disorders (n = 113) were investigated. Highest incidence and activities of IgG-anti-SO antibodies were observed in PSC patients. Nine of 16 untreated (56%) and four of 23 PSC patients treated with ursodeoxycholic acid (UDCA) (17%) were positive. Antibody activity decreased significantly during UDCA treatment. Five per cent of PBC and 9% of AIH patients, but also 15% of patients with alcoholic liver disease, were IgG anti-SO-positive. In patients with viral hepatitis and non-hepatic disorders they could be hardly detected. Anti-SO antibodies are further anti-mitochondrial antibodies in chronic liver diseases. They occur predominantly in PSC, and UDCA treatment seams to decrease antibody activity. Whether these antibodies are primary or secondary phenomena and whether they are related to the aetiology or pathogenesis, at least in a subgroup of patients with chronic liver diseases, has still to be evaluated.

Abnormal intestinal permeability in primary biliary cirrhosis

Antimitochondrial antibodies (AMAs) found in patients with primary biliary cirrhosis (PBC) cross-react with bacterial proteins and hence molecular mimicry has been proposed as a mechanism for AMA development. Alterations in gastrointestinal permeability would provide a potential route for increased exposure of gut flora to the immune system. In this study we aimed to compare the measured gastrointestinal permeability in patients with PBC to that in patients with liver disease (hepatitis C) and healthy control populations. Subjects drank a mixture of sucrose, lactulose, and mannitol dissolved in water. Eight-hour urinary excretion of the sugars was measured to assess intestinal permeability. Antiendomysial antibody testing was performed to exclude subclinical celiac disease. Eighty-six patients with PBC were evaluated and compared to 69 hepatitis C patients and 155 healthy controls. The mean urinary excretion of sucrose in the PBC patients (133.89 +/- 72.56 mg) was significantly higher than that in hepatitis C patients (101.07+/-63.35) or healthy controls (89.46+/-41.76) (P=0.0001), suggesting abnormal gastric or proximal small intestinal permeability. Sucrose excretion was not increased among patients with hepatitis C compared to healthy controls. The ratio of lactulose:mannitol excretion, reflecting small bowel permeability, was also elevated in the PBC group (0.017+/-0.012) compared to healthy controls (0.012+/-0.007) (P=0.0001) but was equal to that found among patients with hepatitis C (0.016+/-0.011) (P=NS). We conclude that the permeability of both the stomach and the small bowel is increased in patients with PBC, however, it is unclear if it is a cause, consequence, or manifestation of the disease.

Applicability of the IAIHG scoring system to the diagnosis of antimitochondrial/anti-M2 seropositive variant form of autoimmune hepatitis

BACKGROUND AND AIMS

According to the International Autoimmune Hepatitis Group (IAIHG) criteria, circulating antimitochondrial antibodies (AMA) do not support the diagnosis of autoimmune hepatitis (AIH). The aims of this study were to characterize a subset of patients with AIH who have AMA and antiM2 seropositivity, and to assess the applicability of the revised scoring system of the IAIHG in the diagnosis of this variant form of AIH.

METHODS

Eighteen patients with AMA-AIH were enrolled and compared with 206 classical AIH and 85 primary biliary cirrhosis (PBC) controls. Human leukocyte antigen (HLA) class II alleles were determined by polymerase chain reaction (PCR) amplification with sequence-specific primers, and biopsies were blindly reevaluated.

RESULTS

The patients with AMA-AIH were, on average, older than patients with classical AIH and had an hepatocellular pattern of elevated liver enzymes, hypergammaglobulinemia and lower levels of cholesterol, when compared with PBC controls. There were no histological signs of PBC or overlapping forms in any AMA-AIH biopsies. The majority of patients with AMA-AIH carried HLA antigens associated with classical AIH (DRB1*03, n = 5; DRB1*04, n = 7, and DRB1*13, n = 6). Pretreatment scores classified all AMA-AIH patients with probable (n = 17) or improbable (n = 1) AIH. After treatment, only 28% of AMA-AIH patients reached scores for definite diagnosis, compared with 90.1% of AIH-1 and 96.4 AIH-2. In the AMA-AIH group, only patients who relapsed after immunosuppressive drug withdrawal could be classified with definite AIH.

CONCLUSIONS

AMA-AIH shares common features with classical AIH. The diagnosis of AMA-AIH may be swayed by the IAIHG criteria, rendering questionable the applicability of the revised scoring system to this variant form of AIH.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Autoimmune tests in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is characterized by an immune mediated, irreversible destruction of the small intrahepatic bile ducts leading to progressive liver cirrhosis and frequently to liver failure. The course of the disease is variable and an early diagnosis is desirable to identify individuals with rapidly progressing disease, to initiate adequate therapeutic measures and to evaluate the necessity of liver transplantation. Serological tests represent the single most important diagnostic feature of PBC because liver histology, biochemistry, or clinical syndrome alone are not reliable in this respect. The molecular definition of the autoantigen targets of antimitochondrial antibodies (AMA) has resulted in the development of reproducible and effective serological testing strategies. AMA directed against the ketoacid dehydrogenase complex are highly disease-specific but not directed against liver-specific target structures. Despite a high disease specificity, their usefulness for predicting the course of disease, the timing of liver transplantation, or disease recurrence after transplantation is limited. The realization that about 5% of patients with PBC do not display AMA has led to the identification of PBC-specific antinuclear autoantibodies directed against the nuclear pore complex and other targets. The overlap of PBC with autoimmune hepatitis and primary sclerosing cholangitis represents a diagnostic challenge in which autoantibody determinations play a central role and contribute to the administration of suitable treatment options.

Primary biliary cirrhosis and primary sclerosing cholangitis

Primary biliary cirrhosis and primary sclerosing cholangitis are the most common chronic cholestatic liver diseases in adults that lead to biliary cirrhosis and its inherent complications such as portal hypertension and liver failure. Although important advances in the understanding of the pathogenesis of these conditions have been accomplished in the last two decades, much work is needed to uncover the interaction of genetic and immunologic mechanisms involved in their pathogenesis. Ursodeoxycholic acid at dosage of 13 to 15 mg/kg/d is the only agent that can currently be recommended in the treatment of PBC. No medical therapy aimed at disrupting disease progression is available for patients with primary sclerosing cholangitis, although several agents with different properties are currently under evaluation. Liver transplantation is the treatment of choice for patients with primary biliary cirrhosis and primary sclerosing cholangitis with end-stage liver disease.

Antimitochondrial antibody profiles in primary biliary cirrhosis distinguish at early stages between a benign and a progressive course: a prospective study on 200 patients followed for 10 years

In recent retrospective studies, it was shown that subtypes of antimitochondrial antibodies (AMA) can help to discriminate between a benign [only anti-M9 and/or anti-M2 positive by enzyme-linked immunosorbent assay (ELISA)] and a rather progressive course (anti-M2, -M4 and/or -M8 positive). According to different constellations of these AMA subspecificities in ELISA and complement fixation test (CFT), four AMA profiles (A-D) were defined. In 1984 we started a prospective study based on 200 PBC patients with known AMA profiles in order to correlate the antibody pattern with the clinical outcome. Progression was defined primarily as the necessity of liver transplantation and death due to hepatic failure or variceal bleeding. At entry, 18 (9%) of the 200 patients had AMA profile A (only anti-M9), 57 (29%) profile B (only anti-M2 with or without anti-M9), 74 (37%) profile C (anti-M2 in association with anti-M4/-M8 by ELISA), and 51 (26%) profile D (anti-M2/-M4/-M8 by ELISA and CFT). At the beginning of the study, 177 patients had PBC stage I/II. During the observation period of ten years, ten patients died and in 18 orthotopic liver transplantation (OLT) was performed; all these patients belonged to profile C/D. Furthermore, 44% of the patients with profile C and 31% of the patients with profile D progressed to late stages, as defined by histology and clinical manifestations such as portal hypertension and increase of bilirubin, while only one of the patients with profile B and none of the profile A-patients developed late stage PBC. A significant increase of bilirubin was observed only in C/D-patients. AMA profiles did not change during the follow-up. In conclusion, AMA profiles discriminate between a benign and a progressive course of PBC already at early stages.

Mitochondrial antigen/antibody systems in primary biliary cirrhosis: revisited

Methods for the evaluation of the four antimitochondrial antibody subtypes in primary biliary cirrhosis - anti-M2, -M4, -M8, -M9 - are described. The importance of the application of different preparations for the demonstration of complement fixing antibodies and the detection of antibodies by ELISA or Western blotting is emphasized. Complement fixing antigens can be prepared by discontinuous isopynic sucrose density gradient centrifugation using mitochondrial subfractions derived with from beef heart (M2), rat liver (M4), or pig kidney (M8). Anti-M9 antibodies do not fix complement. For ELISA, the pyruvate dehydrogenase or the ATPase-associated antigen fraction (M2), the sulfite oxidase fraction (M4), and the chromatographically purified M8-fraction should be used. The same antigen fractions are suitable for Western blotting, but anti-M4 and anti-M8 by ELISA and Western blotting a purified fraction prepared from rat liver has to be applied. Correlating antimitochondrial antibody-subtypes with clinical condition and the natural course, there is convincing evidence that especially the presence of complement fixing antibodies against the subtypes M2, M4, and M8 is a reliable indicator for a more active course. Patients expressing only anti-M9 (without anti-M2) have biochemically all the typical features also found in classical anti-M2 positive primary biliary cirrhosis patients, but seem not to advance to late stages. Since these antimitochondrial antibody-subtypes are present even in very early stages stages without changing their pattern during the course, antimitochondrial antibody-profiles can also be taken as early prognostic parameters. The evaluation of the immunological activity by antimitochondrial antibody-subtype testing may further facilitate the decision whether therapy with ursodeoxycholic acid should be combined with steroids and/or immunosuppressive agents. The role of mitochondrial autoantigens in the induction of this chronic destructive bile duct process is also discussed. The concept is put forward that not bile ducts but naive(?) B-cells expose the different mitochondrial antigens, thereby stimulating autoreactive T-cells to provide a second signal for antibody production. The degree of breakage of tolerance to the different mitochondrial epitopes may be one crucial factor which determines the diversity of antimitochondrial antibody-subtypes in patients with primary biliary cirrhosis.

Abnormal expression of PDC-E2 on the apical surface of biliary epithelial cells in patients with antimitochondrial antibody-negative primary biliary cirrhosis

The presence of antimitochondrial antibodies (AMA) is a major criterion for the diagnosis of primary biliary cirrhosis (PBC). Although it is not clear that AMA are involved in the pathogenesis of the disease, the study of these autoantibodies has enabled much information to be accumulated about the specificity of this response. The autoantigens have been identified as components of a functionally related enzyme family, the 2-oxo-acid-dehydrogenase complex. Within this complex, pyruvate dehydrogenase E2 subunit (PDC-E2) has been determined to be the immunodominant autoantigen. Using a panel of mouse monoclonal antibodies and human combinatorial autoantibodies, it has been demonstrated that patients with PBC, but not controls, have an abnormal expression of either PDC-E2 or a cross-reacting molecule in the apical region of biliary epithelium. Others have shown a similar reaction using rabbit sera directed to PDC-E2. Our previous studies have concentrated on AMA-positive patients. In this study, the presence of PDC-E2, class II, immunoglobulin (Ig) A, and B7/BB1 in the bile duct epithelial cells of AMA-positive as well as AMA-negative patients is addressed. Most patients with AMA-negative PBC (seven of nine) react in a fashion similar to AMA-positive patients with intense staining of the apical region of the bile duct epithelial cells of "PDC-E2," increased IgA expression, and little major histocompatibility complex (MHC) class II staining in the early-stage patients. Interestingly, the two AMA-negative patients that did not express PDC-E2 on the apical side of their biliary epithelium had anticentromere antibodies and Sjögren's syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial antigens, molecular mimicry and autoimmune disease

The immune system is normally tolerant to mitochondrial self-antigens, but responsive against bacteria. Low-titre anti-mitochondrial antibodies (AMA) might be involved in this discrimination. Tolerance is broken in diseases characterised by high titre AMA. Some of these AMA, against cardiolipin, cross-react with DNA. The best studied AMA are those characterising primary biliary cirrhosis (PBC). These are directed against E2 subunits of the oxo-acid dehydrogenase complexes, and also against subunits E1 alpha, E1 beta and X of the pyruvate dehydrogenase complex. AMA of PBC patients also react with bacterial E2s. Reactivities are primarily peptide-specific but with cross-reactivity between mitochondrial and microbial antigens and between E2s of respective complexes. Immunodominant epitopes, for anti E2 AMA, include the conserved sequence flanking the site of lipoyl attachment. It is proposed that the initial stimulus for antibody production is chronic urinary tract infection. AMA themselves are not pathogenic, but CD4+ T-cells would be primed, recognising the lipoyl domain epitope in association with class II HLA. Inappropriate expression of class II antigens on bile duct epithelia, (as found in PBC), might lead to presentation of a particular fragment of HLA-DR alpha, known to be a major MHC presented self-peptide in the mouse. That sequence strongly mimics the lipoyl domain and might be recognised by primed T-cells, initiating the autoimmune cascade. In the mouse, a peptide of ND1 of Complex I is presented in association with class I MHC. Cells exhibiting somatic mutation of such a peptide might thus be subject to attack by CD8+ T-cells. If such peptides were presented by class II HLA, autoimmune diseases might arise, related to mimicry between such peptides and microbial sequences and/or self-antigens. These considerations might apply in Leber's disease and in age-related pathology.

Characterisation of anticytoplasmic antibodies and their clinical associations

OBJECTIVES

To characterise the cytoplasmic staining patterns identified by indirect immunofluorescence (IF) of human epithelial (HEp-2) cells and the antigens recognised using additional serological techniques. To define the disease associations of anticytoplasmic antibodies.

METHODS

Sera from 1173 patients were screened for cytoplasmic IF staining on HEp-2 cells and the patterns characterised. The presence of antimitochondrial antibodies (AMA) was evaluated by a sensitive anti-pyruvate dehydrogenase complex enzyme linked immunosorbent assay (ELISA) (IgG) and by immunoblotting. Detection of antibodies to extractable nuclear antigens (ENA) was performed by double immunodiffusion and the presence of anti-ribosomal P antibodies was determined by immunoblotting.

RESULTS

Cytoplasmic IF staining was demonstrated in 75 sera (6.4%). Six different patterns were recognised: coarse granular filamentous speckles (AMA, n = 9); condensed large speckles (anti-golgi apparatus antibodies, n = 3); cytoskeletal (n = 9); centriolar (n = 4); diffuse coarse speckles (n = 33); and fine speckles (n = 17). Of the nine sera with an AMA pattern, the presence of these antibodies was confirmed in seven by the ELISA (n = 6) and on immunoblotting (n = 7). One of the seven patients had primary biliary cirrhosis, and two had scleroderma. Two patients with anti-golgi antibodies had rheumatoid arthritis and two with anticentriolar antibodies had scleroderma. Of 33 sera that had cytoplasmic staining and were ANA negative, three were positive for anti-Ro and two were positive for anti-Jo-1 antibodies.

CONCLUSIONS

In general, defined cytoplasmic IF patterns have no specific disease associations. However, the finding of cytoplasmic fluorescence should not be ignored, as it may indicate the presence of antibodies to ENA in the absence of nuclear staining.

Prediction of prognosis of primary biliary cirrhosis in Japan

The clinical profile of primary biliary cirrhosis in Japan was clarified on the basis of data on 1066 patients attending 212 hospitals and institutions in this country. Six hundred and twelve patients (57.4%) were asymptomatic. The majority of the patients were middle-aged women. Pruritus was the most frequent initial symptom of symptomatic primary biliary cirrhosis. Antimitochondrial antibodies were positive in 877 patients (82.5%). Sjögren's syndrome was the most common associated autoimmune disease. Liver biopsy was performed in 753 patients at the time of diagnosis, and histological staging by Scheuer's classification indicated that 307 (43.7%) patients were in stage I and 222 (31.6%) were in stage II. The most frequent causes of death were hepatic failure and/or gastrointestinal bleeding, which affected 166 (78.3%) of the 212 patients who died. Statistical analysis using Cox's regression method revealed that the patient's age and the serum bilirubin, albumin, and total cholesterol concentrations were significant predictors of the prognosis. A prognostic index was also calculated that could be used to predict the duration of survival for patients with primary biliary cirrhosis.

Human combinatorial autoantibodies and mouse monoclonal antibodies to PDC-E2 produce abnormal apical staining of salivary glands in patients with coexistent primary biliary cirrhosis and Sjögren's syndrome

An increase in the incidence of Sjögren's syndrome in patients with primary biliary cirrhosis has been noted. Indeed, primary biliary cirrhosis has been described as a ductal disease with involvement not only of the biliary tract but of epithelial ductal cells in other organs. We have previously reported the development of a panel of mouse monoclonal antibodies directed at PDC-E2, the major autoantigen of primary biliary cirrhosis. One such antibody, C355.1, but none of the other monoclonal antibodies, reacted not only with mitochondria but also with the apical region of biliary epithelium of patients with primary biliary cirrhosis but not in similar specimens from patients with other liver disease or normal human liver. In addition, we have reported the development of human combinatorial antibodies specific for PDC-E2; these reagents also reacted uniquely with the biliary epithelium of patients with primary biliary cirrhosis. In this paper, we have performed a similar study and have compared the staining of monoclonal antibody C355.1 and a human combinatorial antibody, SP4, with control monoclonal antibodies with respect to their reactivity of salivary glands in 9 patients with primary biliary cirrhosis associated with Sjögren's syndrome, 11 patients with Sjögren's syndrome alone and 7 control patients. Interestingly, the apical region of the salivary gland epithelial cells of approximately 50% of patients with coexisting primary biliary cirrhosis and Sjögren's syndrome had a staining pattern similar to that seen in primary biliary cirrhosis biliary epithelium. In contrast, we did not observe this reactivity in any patient with Sjögren's syndrome alone or in any control patient.(ABSTRACT TRUNCATED AT 250 WORDS)

Naturally occurring antibodies reacting with lipoic acid: screening method, characterization and biochemical interest

The development of a covalent enzyme-linked immunoassay (CELIA) using lipoic acid covalently bound to modified polystyrene microplates has permitted the detection, in the sera of normal BALB/c mice, of natural antibodies reacting with lipoic acid (LA). Hybridomas producing monoclonal anti-LA antibodies were obtained from splenocytes of non-immune BALB/c mice. Two of them, of IgM isotype, recognized LA but failed to react with dihydrolipoic acid (DHLA, the reduced form of LA), suggesting that the integrity of the dithiolane ring was of importance for antibody recognition. They did not give positive reactions with other disulfide linked biological molecules such as oxidized glutathione or cystine. Anti-LA antibodies, coated on polystyrene microplates, were used for the detection of free LA in a competitive assay based on peroxidase-LA conjugate.

Antimitochondrial antibody profiles in patients with primary biliary cirrhosis before orthotopic liver transplantation and titres of antimitochondrial antibody-subtypes after transplantation

Four antimitochondrial antibody profiles (A-D) have been defined in primary biliary cirrhosis according to the presence of antibodies to M2, M4, M8, and M9 in ELISA and the complement fixation test: A: anti-M9 positive in ELISA and western blot, B: anti-M9 and/or anti-M2 positive in ELISA, C: anti-M2, -M4 and/or -M8 positive in ELISA, D: anti-M2, -M4, and/or -M8 positive in ELISA and complement fixation test. These profiles predict the outcome of primary biliary cirrhosis in the early stages and reflect differences in the natural course of the disease (benign versus progressive). In this study sera from 29 patients with advanced primary biliary cirrhosis who had received liver transplant were retested before and after orthotopic liver transplantation. Twenty-eight were antimitochondrial antibody/anti-M2 positive, and one patient had only antibodies to nuclear dots in the immunofluorescence test on cell cultures. When the antimitochondrial antibody-profiles in these 28 anti-M2 positive patients were analysed, it became evident that 26 of them belonged to subgroup C or D before orthotopic liver transplantation. Two patients had profile B; one had high titres of antinuclear and smooth muscle antibodies indicating an overlap syndrome between primary biliary cirrhosis and autoimmune chronic active hepatitis. The other patient had antibodies to nuclear dots in association with anti-M2. None of the patients had profile A. Antibody titres were studied after orthotopic liver transplantation in 23 of the 28 patients who survived for 1 to 13 years.(ABSTRACT TRUNCATED AT 250 WORDS)

Is primary biliary cirrhosis an autoimmune disease?

This review summarizes the experimental and clinical support for an autoimmune origin of primary biliary cirrhosis (PBC). Direct proof is lacking, but indications in favour of an immunologic destructive mechanism include the demonstration of antibodies and T cell clones with specificity for mitochondrial autoantigens, and the lymphocytic infiltration/destruction of small bile ducts similar to that of graft-vs-host disease and rejection. There is a weak association with other autoimmune diseases, but no clear HLA linkage. Spontaneous animal models for PBC are lacking, and immunization of animals with purified autoantigen does not result in typical disease. Anti-mitochondrial antibodies (AMAs) of M2 type are diagnostic of PBC, and are mainly directed against a functional, restricted epitope on the E2 subunit of the pyruvate dehydrogenase complex (PDC). PDC-E2 shows several similarities to other classical autoantigens. The pathogenic role of AMA remains elusive. Recent studies have shown that AMAs detect an antigenic epitope expressed on the luminal surface of biliary epithelium in PBC liver. The initial triggering event might represent a microbial infection (molecular mimicry), or an aberrant surface expression of a true autoepitope.

Immunoreactivity to M2 proteins in antimitochondrial antibody-negative patients with primary biliary cirrhosis

Although antimitochondrial auto-antibodies are characteristically present in the serum of patients with primary biliary cirrhosis (PBC), there is a discrepancy between the positivity for antimitochondrial antibody (AMA) and that for anti-M2 auto-antibody. In an attempt to explain the discrepancy, this study investigates the relationship between the AMA titre, determined by indirect immunofluorescence, and immunoreactivity to four inner mitochondrial membrane proteins (M2 proteins) with molecular weights of 70, 50, 47, and 40 kDa in 129 patients with PBC. Antimitochondrial antibody positivity was identified in 114 (88%) of 129 patients with clinically and histologically confirmed PBC. There were no significant differences between the AMA-negative and AMA-positive groups in clinical characteristics or histologically determined disease stage. Immunoblot analysis showed that all patients had anti-M2 auto-antibodies to one or more of the four M2 proteins. Nine (60%) of the 15 AMA-negative patients had antibodies to only one M2 protein (either 70 or 47 kDa). In contrast, 34 (53%) of the 64 patients with high AMA titres (> or = 1:320) had antibodies to all four M2 proteins. There was a significant rank correlation between the AMA titre and the number of antibodies to M2 proteins (P < 0.01). These findings indicate that the AMA titre is not influenced by the immunogenicity of M2 proteins but by the number of M2 proteins that elicit an antibody response and that decreased immunoreactivity to M2 proteins may induce AMA negativity in PBC serum samples.

Myopathy with mitochondrial alterations in patients with primary biliary cirrhosis and antimitochondrial antibodies

OBJECTIVE

To describe a syndrome of severe progressive myopathy, cardiomyopathy, and gastrointestinal dysmotility in 2 patients with asymptomatic primary biliary cirrhosis (PBC) and circulating antimitochondrial autoantibodies, and to review pertinent literature concerning this syndrome.

METHODS

Clinical, electrophysiologic, serologic, and pathologic studies of the 2 affected patients were conducted.

RESULTS

Skeletal muscle involvement was manifested by progressive weakness of the proximal muscles, marked diaphragmatic dysfunction with consequent hypoventilation and respiratory failure, and moderately elevated levels of muscle-associated enzymes. Serum from both patients contained antimitochondrial antibodies that reacted with components of the mitochondrial keto acid dehydrogenase enzyme complex. Results of electromyography were consistent with a myopathic process. The microscopic and ultrastructural changes in the skeletal muscles were distinct from those of typical myositis, and were notable for striking subsarcolemmal aggregation of abnormal mitochondria in the absence of significant inflammation.

CONCLUSION

Severe skeletal muscle, cardiac, and gastrointestinal pathology with abnormalities of the muscle mitochondria develops in a subset of patients with mild PBC and antimitochondrial antibodies. The pathogenesis of this syndrome is unclear, but may be related to the presence of the antimitochondrial autoantibodies.

Antibody to two forms of dihydrolipoamide acetyltransferase (PDC-E2) in primary biliary cirrhosis

Dihydrolipoamide acetyltransferase, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), is the major autoantigen in primary biliary cirrhosis. By immunoblotting with sera from patients with primary biliary cirrhosis, we observed a double band, of molecular weight 70 and 74 kD for PDC-E2, when a preparation of bovine heart mitochondria was not boiled prior to electrophoresis. This double band could also be detected using antisera raised in rats or rabbits against intact PDC or PDC-E2, but not in antisera raised against a synthetic decamer representing the lipoic acid binding sequence of PDC-E2; the latter reacted only with the 74 kD component. Antibody eluted from either the 70 or 74 kD component reacted with both 70 and 74 kD components. By ELISA, sera from patients with primary biliary cirrhosis reacted more strongly with a non-boiled than a boiled PDC-E2, whereas immune animal sera reacted equally with both preparations. Thus, according to whether preparations of PDC are boiled or not, two conformationally alternative forms of the PDC-E2 protein can be revealed by immunoblotting. The two forms in non-boiled preparations migrate at molecular weights corresponding to 70 and 74 kD.

Sera from patients with tuberculosis recognize the M2a-epitope (E2-subunit of pyruvate dehydrogenase) specific for primary biliary cirrhosis

Anti-M2 antibodies in primary biliary cirrhosis (PBC) have been shown to react with the alpha-ketoacid dehydrogenase complex of the inner mitochondrial membrane consisting of six epitopes (E2 subunit of the pyruvate dehydrogenase complex (PDC), 70 kD; protein X of the PDC, 56 kD; alpha-ketoglutarate dehydrogenase complex, 52 kD; branched-chain alpha-ketoacid dehydrogenase, 52 kD; E1 alpha subunit of PDC, 45 kD; and E1 beta-subunit of PDC, 36 kD). These epitopes are also present in the M2 fraction which is a chloroform extract from beef heart mitochondria. The E2 subunit of the PDC at 70 kD (M2a), especially, is a major target epitope which is recognized by about 85% of all PBC sera. However, analysing sera from 28 patients with active pulmonary tuberculosis it became evident that 12 (43%) also recognized the PDC-E2 subunit (M2a), as shown by Western blotting using the M2 fraction, the purified PDC, and the recombinant PDC-E2. In contrast, only two of 82 patients with other bacterial and viral infections including 25 patients with Escherichia coli infections reacted with the PBC-specific epitope at 70 kD. Naturally occurring mitochondrial antibodies (NOMA) were present in 54% of the patients with tuberculosis and in 50% of patients with other infectious disorders. They recognized either a determinant at 65 kD (epsilon) or at 60/55 kD (zeta/eta). None of the sera from 100 blood donors had anti-M2 but 14 had NOMA. Testing anti-M2 and NOMA-positive marker sera by Western blotting against membrane fractions derived from mycobacteria and E. coli it could be shown that--like mammalian mitochondria--they contain both the PBC-specific M2 antigen as well as the non-PBC-specific naturally occurring mitochondrial antigen system (NOMAg). The observation that PBC-specific antibodies were preferentially induced in patients suffering from a mycobacterial infection may provide some new clues to the still unknown etiology of PBC.

Distribution of the PBC-specific- (M2) and the naturally-occurring mitochondrial antigen- (NOMAg) systems in plants

In previous studies it was demonstrated that antibodies in sera from patients with primary biliary cirrhosis (PBC) and their relatives can recognize two different antigen systems in the ATPase fraction prepared from beef heart mitochondria, namely the PBC-related M2- and the naturally occurring mitochondrial antigen (NOMAg)-related epitopes. Since separation of these two antigen systems could not be achieved using mammalian mitochondria, mitochondria from a wide spectrum of plants were analysed with respect to the presence of mitochondrial antigens. Mitochondria from 29 species of plants were prepared and tested by ELISA and Western blot using marker sera from patients with PBC reacting in the Western blot with M2a,b,c,d (alpha-ketoacid-dehydrogenase complex) and NOMAg-specific sera recognizing the three major epitopes epsilon, zeta, and eta at 65, 61 and 58 kD. Naturally occurring mitochondrial antibody (NOMA)-positive marker sera reacted in the ELISA with mitochondria from all plants, and the zeta/eta positive sera gave also a positive reaction at 61/58 kD in the Western blot while the epsilon epitope could not be visualized by this method. In contrast, the M2 antigen was detected preferentially in lower plants such as algae, fungi, and ferns. Analysing these data with respect to the evolution of proteins one would have to assume that the M2 antigen was lost in most higher plants or underwent some structural alterations. Furthermore, considering the fact that the M2- and the NOMAg-related epitopes could be only partially separated, i.e. there were no plant mitochondria showing only M2 but no NOMAg, one could speculate that anti-M2 antibodies are derived from the pool of naturally occurring antibodies.

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.

Specific reactivity of recombinant human PDC-E1 alpha in primary biliary cirrhosis

The mitochondrial autoantigens recognized by autoantibodies in patients with primary biliary cirrhosis have been identified as components of related multi-enzyme complexes, including acyltransferases of the pyruvate dehydrogenase complex (PDC), the branched-chain alpha-keto acid dehydrogenase complex (BCODH), the alpha-ketoglutarate dehydrogenase complex (OGDC), protein X and pyruvate dehydrogenase (PDC) E1 alpha and E1 beta. The major autoantigens, PDC-E2, BCODH-E2 and OGDC-E2, share some sequence homology; the epitopes on these antigens appear to be close to, or identical with, the lipoic acid binding site. Furthermore, all three antigens share some structural homology. In contrast, antibodies to PDC-E1 alpha are present in lower titers, and have been more difficult to detect. PDC-E1 alpha also differs from the three major autoantigens in that it lacks any covalently bound lipoic acid. PDC-E1 alpha cannot be purified in large quantities and becomes unstable in the absence of PDC-E1 beta. To address these problems, we have subcloned recombinant human PDC-E1 alpha to pGEX, pGEX is a vector which produces a recombinant polypeptide fused to glutathione S-transferase. The resultant E1 alpha fusion protein is stable and has a low background in immunoassays. Using the recombinant protein, we have developed an ELISA that allows rapid and reproducible quantification of antibodies to human PDC-E1 alpha. Finally, we demonstrate that a major epitope on PDC-E1 alpha is within a 300 amino acid region that contains the enzyme functional sites, namely the phosphorylation site and the TPP binding site.

An automated microassay for enzyme inhibitory effects of M2 antibodies in primary biliary cirrhosis

In primary biliary cirrhosis (PBC), autoantibodies are produced to the M2 group of mitochondrial antigens, of which a major constituent is the E2 subunit of the pyruvate dehydrogenase complex (PDC). These antibodies, in addition to conventional reactivities with PDC, characteristically inhibit the catalytic function of PDC in vitro, as judged by a macroinhibition assay based on spectrophotometry. We describe a microinhibition assay adapted for microtitre plates and for an automated readout of results by an ELISA plate reader. We show that this microassay has a sensitivity similar to that of the macroassay. In a study of 83 sera, from PBC and other diseases, the inhibitory microassay proved specific for PBC. This automated inhibitory microassay for PBC-sera could become a primary laboratory procedure for the diagnosis of PBC, particularly because it may identify antibodies to the actual autoepitope on the PDC-E2.

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.

Plasma membrane association of primary biliary cirrhosis mitochondrial marker antigen M2

Antibody reactivity against the 'mitochondrial M2 antigen' was determined in sera from 10 patients with primary biliary cirrhosis (PBC), using Western blotting after SDS-PAGE separation of rat liver mitochondria (RLM) and plasma membrane proteins. The molecular weights of the major M2 antigens in rat liver mitochondria were 67 and 50 kD. Two of the 10 PBC patients did not react to any of these major antigens, eight reacted to the 67-kD and four of those also to the 50-kD antigen. The 67- and 50-kD antigens were present in both plasma membrane and RLM and had affinity to concanavalin A. Antibody reactivity against the 67-kD antigen could be detected in both IgG and IgA as well as in the IgM class. The reactive IgG subclasses to both types of antigen preparations were mainly of the G1 and G3 isotypes. This reactivity was always stronger with antigens from the plasma membrane preparations. Sera from two patients with high antibody titres against mitochondria also reacted with IgG2 against the 50-kD antigen from plasma membrane, but not to the corresponding antigen in mitochondria. Reactivity of antibodies in PBC sera to the periphery of viable hepatocytes and radioactive surface labelling of the 50-kD component are both consistent with a plasma membrane localization of M2. Serum from healthy controls and several patients with different diseases did not contain antibodies reactive against any of the antigens described. We suggest that antigens, partly identical to the mitochondrial M2, are located in the plasma membrane compartment. The PBC pathogenetical consequences of these findings are discussed.

Antimitochondrial antibody profiles determined at early stages of primary biliary cirrhosis differentiate between a benign and a progressive course of the disease. A retrospective analysis of 76 patients over 6-18 years

Seventy-six patients with clinically and histologically defined primary biliary cirrhosis, who were in stage I/II at the time of first diagnosis and could be followed for 6-18 years (mean 10 years), were analysed with respect to their clinical course. Forty-four of the 76 patients remained in stage I/II during the observation period (group 1), while 32 progressed to late stages (group 2). Sera of these patients were retested for anti-M2, anti-M4, anti-M8 and anti-M9 antibodies by ELISA and the complement fixation test (CFT). Four different AMA-profiles A-D could be differentiated in these patients: profile A, only anti-M9 positive by ELISA; B, anti-M9 and/or anti-M2 positive by ELISA; C, anti-M2, anti-M4 and/or anti-M8 positive by ELISA; and D, anti-M2, anti-M4 and/or anti-M8 positive by ELISA and CFT. When the natural courses of patients and the four different AMA-profiles were compared retrospectively it became evident that 97% of group 2 patients already had the AMA-profile C/D at entry into the study. Of the patients in group 1 70% had the AMA-profile A/B. Increases in bilirubin levels during the course of the disease were observed preferentially in patients of group 2 (profile C/D). We conclude that the determination of AMA-profiles in PBC patients is a sensitive approach for the early prediction of the outcome of the disease.