Site-directed mutagenesis of lysine within the immunodominant autoepitope of PDC-E2

A Promising Tool in Serological Diagnosis: Current Research Progress of Antigenic Epitopes in Infectious Diseases

Infectious diseases, caused by various pathogens in the clinic, threaten the safety of human life, are harmful to physical and mental health, and also increase economic burdens on society. Infections are a complex mechanism of interaction between pathogenic microorganisms and their host. Identification of the causative agent of the infection is vital for the diagnosis and treatment of diseases. Etiological laboratory diagnostic tests are therefore essential to identify pathogens. However, due to its rapidity and automation, the serological diagnostic test is among the methods of great significance for the diagnosis of infections with the basis of detecting antigens or antibodies in body fluids clinically. Epitopes, as a special chemical group that determines the specificity of antigens and the basic unit of inducing immune responses, play an important role in the study of immune responses. Identifying the epitopes of a pathogen may contribute to the development of a vaccine to prevent disease, the diagnosis of the corresponding disease, and the determination of different stages of the disease. Moreover, both the preparation of neutralizing antibodies based on useful epitopes and the assembly of several associated epitopes can be used in the treatment of disease. Epitopes can be divided into B cell epitopes and T cell epitopes; B cell epitopes stimulate the body to produce antibodies and are therefore commonly used as targets for the design of serological diagnostic experiments. Meanwhile, epitopes can fall into two possible categories: linear and conformational. This article reviews the role of B cell epitopes in the clinical diagnosis of infectious diseases.

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.

Ongoing activation of autoantigen-specific B cells in primary biliary cirrhosis

The serologic hallmark of primary biliary cirrhosis (PBC), the antimitochondrial response to the E2 component of the pyruvate dehydrogenase complex (PDC-E2), has unique features, including continuous high titers of immunoglobulin M (IgM) and IgG reactivity throughout all stages of disease, capable not only of target enzyme inhibition, but also crossreactive with chemical xenobiotics that share molecular homology with the inner lipoyl domain of PDC-E2; such chemicals have been proposed as potential etiological agents. We used flow cytometry and enzyme-linked immunospot assay (ELISPOT) to examine B-cell subsets in 59 subjects, including 28 with PBC, 13 with primary sclerosing cholangitis (PSC), and 18 healthy controls. Strikingly, in PBC, although there were no significant differences in B-cell phenotype subpopulations, 10% of the total IgG and IgA plasmablast population and 23% of the IgM plasmablast population were uniquely reactive with PDC-E2, detected in the CXCR7+ CCR10low plasmablast population. In contrast, plasmablast reactivity to a control antigen, tetanus toxoid, was minimal and similar in all groups. Additionally, we isolated plasmablast-derived polyclonal antibodies and compared reactivity with plasma-derived antibodies and noted a distinct noncirculating tissue source of xenobiotic crossreacting antibodies. The high levels of autoantigen specific peripheral plasmablasts indicate recent activation of naive or memory B cells and a continuous and robust activation. The presence of CXCR7+ CCR10low PDC-E2-specific ASCs suggests a mechanistic basis for the migration of circulating antigen specific plasmablasts to the mucosal epithelial ligands CXCL12 and CCL28.

CONCLUSION

Our findings suggest a sustained rigorous B-cell response in PBC, likely activated and perpetuated by cognate autoantigen.

Anti-CD40 ligand monoclonal antibody delays the progression of murine autoimmune cholangitis

While there have been significant advances in our understanding of the autoimmune responses and the molecular nature of the target autoantigens in primary biliary cirrhosis (PBC), unfortunately these data have yet to be translated into new therapeutic agents. We have taken advantage of a unique murine model of autoimmune cholangitis in which mice expressing a dominant negative form of transforming growth factor β receptor II (dnTGFβRII), under the control of the CD4 promoter, develop an intense autoimmune cholangitis associated with serological features similar to human PBC. CD40-CD40 ligand (CD40L) is a major receptor-ligand pair that provides key signals between cells of the adaptive immune system, prompting us to determine the therapeutic potential of treating autoimmune cholangitis with anti-CD40L antibody (anti-CD40L; MR-1). Four-week-old dnTGFβRII mice were injected intraperitoneally with either anti-CD40L or control immunoglobulin (Ig)G at days 0, 2, 4 and 7 and then weekly until 12 or 24 weeks of age and monitored for the progress of serological and histological features of PBC, including rigorous definition of liver cellular infiltrates and cytokine production. Administration of anti-CD40L reduced liver inflammation significantly to 12 weeks of age. In addition, anti-CD40L initially lowered the levels of anti-mitochondrial autoantibodies (AMA), but these reductions were not sustained. These data indicate that anti-CD40L delays autoimmune cholangitis, but the effect wanes over time. Further dissection of the mechanisms involved, and defining the events that lead to the reduction in therapeutic effectiveness will be critical to determining whether such efforts can be applied to PBC.

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

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

Urinary tract infection as a risk factor for autoimmune liver disease: from bench to bedside

Autoimmune liver diseases include autoimmune hepatitis (AIH), primary biliary cirrhosis (PBC), and primary sclerosing cholangitis. A variety of environmental and genetic risk factors have been associated with these conditions. Recurrent urinary tract infections (rUTI) have been strongly associated with PBC, and to a lesser extent with AIH. These observations were initially based on the observation of significant bacteriuria in female patients with PBC. Larger epidemiological studies demonstrated that there was indeed a strong correlation between recurrent UTI and PBC. AIH has not been linked to recurrent UTI in epidemiological studies; however treatment of UTI with nitrofurantoin can induce AIH. As Escherichia coli is the most prevalent organism isolated in women with UTI, it has been suggested that molecular mimicry between microbial and human PDC-E2 (the main autoantigenic target in PBC) epitopes may explain the link between UTI and PBC. Multiple studies have demonstrated molecular mimicry and immunological cross-reactivity involving microbial and self-antigen mimics. This review will examine the literature surrounding UTI and autoimmune liver disease. This will include case reports and epidemiological studies, as well as experimental data.

Bacteria and primary biliary cirrhosis

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

Primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic and slowly progressive cholestatic liver disease of autoimmune etiology characterized by injury of the intrahepatic bile ducts that may eventually lead to liver failure. Affected individuals are usually in their fifth to seventh decades of life at time of diagnosis, and 90% are women. Annual incidence is estimated between 0.7 and 49 cases per million-population and prevalence between 6.7 and 940 cases per million-population (depending on age and sex). The majority of patients are asymptomatic at diagnosis, however, some patients present with symptoms of fatigue and/or pruritus. Patients may even present with ascites, hepatic encephalopathy and/or esophageal variceal hemorrhage. PBC is associated with other autoimmune diseases such as Sjogren's syndrome, scleroderma, Raynaud's phenomenon and CREST syndrome and is regarded as an organ specific autoimmune disease. Genetic susceptibility as a predisposing factor for PBC has been suggested. Environmental factors may have potential causative role (infection, chemicals, smoking). Diagnosis is based on a combination of clinical features, abnormal liver biochemical pattern in a cholestatic picture persisting for more than six months and presence of detectable antimitochondrial antibodies (AMA) in serum. All AMA negative patients with cholestatic liver disease should be carefully evaluated with cholangiography and liver biopsy. Ursodeoxycholic acid (UDCA) is the only currently known medication that can slow the disease progression. Patients, particularly those who start UDCA treatment at early-stage disease and who respond in terms of improvement of the liver biochemistry, have a good prognosis. Liver transplantation is usually an option for patients with liver failure and the outcome is 70% survival at 7 years. Recently, animal models have been discovered that may provide a new insight into the pathogenesis of this disease and facilitate appreciation for novel treatment in PBC.

A new level of architectural complexity in the human pyruvate dehydrogenase complex

Mammalian pyruvate dehydrogenase multienzyme complex (PDC) is a key metabolic assembly comprising a 60-meric pentagonal dodecahedral E2 (dihydrolipoamide acetyltransferase) core attached to which are 30 pyruvate decarboxylase E1 heterotetramers and 6 dihydrolipoamide dehydrogenase E3 homodimers at maximal occupancy. Stable E3 integration is mediated by an accessory E3-binding protein (E3BP) located on each of the 12 E2 icosahedral faces. Here, we present evidence for a novel subunit organization in which E3 and E3BP form subcomplexes with a 1:2 stoichiometry implying the existence of a network of E3 "cross-bridges" linking pairs of E3BPs across the surface of the E2 core assembly. We have also determined a low resolution structure for a truncated E3BP/E3 subcomplex using small angle x-ray scattering showing one of the E3BP lipoyl domains docked into the E3 active site. This new level of architectural complexity in mammalian PDC contrasts with the recently published crystal structure of human E3 complexed with its cognate subunit binding domain and provides important new insights into subunit organization, its catalytic mechanism and regulation by the intrinsic PDC kinase.

Autoreactivity to lipoate and a conjugated form of lipoate in primary biliary cirrhosis

BACKGROUND & AIMS

Although considerable effort has been directed toward the mapping of peptide epitopes by autoantibodies, the role of nonprotein molecules has been less well studied. The immunodominant autoantigen in primary biliary cirrhosis (PBC), E2 components of pyruvate dehydrogenase complexes (PDC-E2), has a lipoate molecule bonded to the domain to which autoantibodies are directed.

METHODS

We examined sera from patients with PBC (n = 105), primary sclerosing cholangitis (n = 70), and rheumatoid arthritis (n = 28) as well as healthy volunteers (n = 43) for reactivity against lipoic acid. The lipoic acid hapten specificity of the reactive antibodies in PBC sera was determined following incubation of aliquots of the sera with human serum albumin (HSA), lipoylated HSA (HSA-LA), PDC-E2, lipoylated PDC-E2, polyethylene glycol (PEG), lipoylated PEG, free lipoic acid, and synthetic molecular mimics of lipoic acid.

RESULTS

Anti-lipoic acid specific antibodies were detected in 81% (79 of 97) of antimitochondrial antibody (AMA)-positive patients with PBC but not in controls. Two previously unreported specificities in AMA-positive sera that recognize free lipoic acid and a carrier-conjugated form of lipoic acid were also identified.

CONCLUSIONS

We hypothesize that conjugated form(s) of native or xenobiotic lipoic acid mimics contribute to the initiation and perpetuation of autoimmunity by at first breaking self-tolerance and participating in subsequent determinant spreading. The variability in the immunoreactive carrier/lipoate conjugates provides an experimental framework on which potential mechanisms for the breakdown of self-tolerance following exposure to xenobiotics can be investigated. The data have implications for patients taking lipoic acid as a dietary supplement.

Pathogenesis of primary biliary cirrhosis

Primary biliary cirrhosis is an enigmatic autoimmune disease that predominantly affects women. The serologic signatures of PBC are high titer antimitochondrial antibodies that are directed at the inner lipoyl domains of the 2-oxo-dehydrogenase enzymes, particularly PDC-E2. Of note, is that the antibody response and the CD4 and CD8 response, are all directed at a similar epitope, the inner lipoyl domain. This unique immunologic response suggests that modification of the inner lipoyl domain is associated with the immunogenetic basis of disease.

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.

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.

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.

Immunoreactivity of porcine heart dihydrolipoamide acetyl- and succinyl-transferases (PDC-E2, OGDC-E2) with primary biliary cirrhosis sera: characterization of the autoantigenic region and effects of enzymatic delipoylation and relipoylation

Analysis of the primary structure of the lipoyl domain of the dihydrolipoamide acetyltransferase (PDC-E2) component of the porcine pyruvate dehydrogenase complex (PDC) reveals a high degree of homology with M2 antigen and human PDC-E2. The porcine PDC-E2 and the dihydrolipoamide succinyltransferase (OGDC-E2) component of the porcine 2-oxoglutarate dehydrogenase complex (OGDC) were identified as mitochondrial autoantigen with sera from patients with primary biliary cirrhosis (PBC). Immunodominant regions (autoepitopes) on the porcine-PDC-E2 component have been mapped to two regions around Lys-46 (outer lipoyl domain) and Lys-173 (inner lipoyl domain), which contained covalently bound lipoic acid prosthetic group. When these lipoyl domains were cleaved at Asp-45 or Asp-172 with endoproteinase Asp-N, the autoantigenicities of the two domains completely disappeared; this suggested the requirement of Asp-45 or Asp-172 residues for the immunoreaction with PBC sera. In addition, a single 14-amino acid epitope peptide histidine-substituted at Asp-172 did not exhibit competitive inhibition of autoantigen binding. Fragmentation of lipoyl domain of the porcine PDC-E2 by limited proteolysis and BrCN-cleavage after alkylation resulted in rapid loss of autoantigenicity. Enzymatic delipoylation and relipoylation of the complexed and free PDC-E2 and OGDC-E2 components did not influence immunoreactivity with PBC sera.

Persistence of autoantibodies against recombinant mitochondrial and nuclear pore proteins after orthotopic liver transplantation for primary biliary cirrhosis

Primary biliary cirrhosis is an autoimmune disease characterized by high titer autoantibodies predominantly against mitochondrial antigens PDC-E2, BCOADC-E2 and OGDC-E2. Currently orthotopic liver transplant (OLT) is the major form of treatment for end-stage primary biliary cirrhosis (PBC), but it is still unclear whether the autoimmune response continues post-transplantation. In this study we took advantage of a well-defined collection of sera collected serially before and after liver transplantation. We assayed these sera for quantitative and isotype-specific titers of antibodies against a set of recombinant mitochondrial autoantigens. We also studied reactivity to gp210. Serum samples were taken before transplantation and at intervals of 6 months, 1, 2, and 3 years after OLT. Before OLT 24/35 patients were AMA-positive, including seven out of the 35 to PDC-E2 alone, eight to both PDC-E2 and OGDC-E2, six to both PDC-E2 and BCOADC-E2, two to BCOADC-E2 alone and one to OGDC-E2. Following OLT, the frequency of sera that responded to PDC-E2 alone increased from seven to 12/35. Similarly, reactivity to BCOADC-E2 slightly increased from two to four out of 35. However, there was an overall decrease in sera that responded to more than one antigen. Neither Ig isotype nor subclass of the autoimmune response changed following OLT. Findings with gp210 were similar, in that reactivity to gp210 was found in nine out of 35 patients pre-OLT; following OLT the frequency decreased to seven out of 35 patients. Overall, the titers of AMAs decline slightly during the first year post-OLT, but are equivalent to pre-OLT values by 6 months. Moreover, the antibody subclass/ isotype remained unchanged. These data suggest that the removal of a diseased PBC liver has little, if any, impact on the serological characteristics of PBC. Moreover, it provides information regarding the natural history of PBC, particularly on the long latency time for disease development.

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.

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.

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.

Combinatorial autoantibodies to dihydrolipoamide acetyltransferase, the major autoantigen of primary biliary cirrhosis

mRNA from a regional lymph node of a patient with primary biliary cirrhosis (PBC) was used to construct a combinatorial immunoglobulin library in the lambda phage vector system. Six human monoclonal IgG Fab clones (LC1-LC6) specific for the major autoantigen of PBC--dihydrolipoamide acetyltransferase, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2)--were isolated, appearing at a frequency of 0.01% in the combinatorial immunoglobulin library. These Fab clones recognize human PDC-E2 with high affinity (Ka = 10(-7)-10(-9) M-1). Using both immunoblotting and ELISA, LC1-LC6 showed little cross-reactivity to any of the other autoantigens commonly recognized by PBC sera or to other antigens commonly recognized by PBC sera or to other antigens such as histone, calf thymus DNA, and bovine serum albumin. The Fab monoclonal antibodies show a typical anti-mitochondrial staining pattern in HEp-2 cells but react strongly with the luminal aspect of biliary epithelial cells of patients with PBC. Our results demonstrate that a recombinant combinatorial immunoglobulin library can be used to isolate high-affinity Fabs against a specific autoantigen. Such reagents will facilitate the analysis of immunoglobulin gene structure, idiotype, and antigen-antibody interactions in autoimmune disease.

Expression and lipoylation in Escherichia coli of the inner lipoyl domain of the E2 component of the human pyruvate dehydrogenase complex

The dihydrolipoamide acetyltransferase subunit (E2p) of mammalian pyruvate dehydrogenase complex has two highly conserved lipoyl domains each modified with a lipoyl cofactor bound in amide linkage to a specific lysine residue. A sub-gene encoding the inner lipoyl domain of human E2p has been over-expressed in Escherichia coli. Two forms of the domain have been purified, corresponding to lipoylated and non-lipoylated species. The apo-domain can be lipoylated in vitro with partially purified E. coli lipoate protein ligase, and the lipoylated domain can be reductively acetylated by human E1p (pyruvate dehydrogenase). Availability of the two forms will now allow detailed biochemical and structural studies of the human lipoyl domains.

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.

Characterization and epitope mapping of human monoclonal antibodies to PDC-E2, the immunodominant autoantigen of primary biliary cirrhosis

Further to define the epitopes of PDC-E2, the major autoantigen in primary biliary cirrhosis (PBC), we have developed and characterized five human monoclonal antibodies. These antibodies were derived by fusing a regional hepatic lymph node from a patient with PBC with the mouse human heterohybrid cell line F3B6. Previous studies of epitope mapping of PDC-E2 have relied on whole sera and have suggested that the immunodominant epitope lies within the inner lipoyl domain of the molecule. However, selective absorption studies using whole sera and a series of overlapping recombinant peptides of PDC-E2 have suggested that the epitope may also include a large conformational component. Moreover, several laboratories have suggested that autoantibodies against the 2-oxo acids dehydrogenase autoantigens are cross-reactive. The five monoclonal antibodies generated included three IgG2a and two IgM antibodies and were studied for antigen specificity using recombinant PDC-E2, recombinant BCKD-E2, histone, dsDNA, IgG (Fc), collagen and a recombinant irrelevant liver specific control, the F alloantigen. The antibodies were also used to probe blots of human, bovine, mouse and rat mitochondria. Finally, fine specificity was studied by selective ELISA and absorption against overlapping expressing fragments of PDC-E2. All five monoclonals, but none of the other mitochondrial autoantigens were specific for PDC-E2. In fact, although affinity purified antibodies to PDC-E2 from patients with PBC cross-reacted with protein X, the human monoclonals did not, suggesting that protein X contains an epitope distinct from that found on PDC-E2. Additionally, all three IgG2 monoclonals recognized distinct epitopes within the inner lipoyl domain of PDC-E2.

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)

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.