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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., (© 2021 by the American Association for the Study of Liver Diseases.)

Bioconjugation is an important chemical biology research focus, especially in the development of methods to produce pharmaceutical bioconjugates and antibody-drug conjugates (ADCs). In this report, an enzyme-catalyzed conjugation method combined with a chemical reaction was used to modify a native antibody under mild reaction conditions. Our investigation revealed that lipoic-acid ligase (LplA) modifies native IgG1 with biased site-specificity. An intact mass analysis revealed that 98.3% of IgG1 was modified by LplA and possessed at least one molecule of octanocic acid. The average number of modifications per antibody was calculated to be 4.6. Peptide mapping analysis revealed that the modified residues were K225, K249 and K363 in the Fc region, and K30, K76 and K136 in the heavy chain and K39/K42, K169, K188 and K190 in the light chain of the Fab region. Careful evaluation including solvent exposed amino acid analysis suggested that these conjugate sites were not only solvent exposed but also biased by the site-specificity of LplA. Furthermore, antibody fragment conjugation may be able to take advantage of this enzymatic approach. This feasibility study serves as a demonstration for preparing enzymatically modified antibodies with conjugation site analysis., (Copyright © 2021. Published by Elsevier Ltd.)

Αlpha-lipoic acid is a naturally occurring antioxidant in human body and has been widely used as an antioxidant clinically. Accumulating evidences suggested that α -lipoic acid might have immunomodulatory effects on both adaptive and innate immune systems. This review focuses on the evidences and potential targets involved in the immunomodulatory effects of α -lipoic acid. It highlights the fact that α -lipoic acid may have beneficial effects in autoimmune diseases once the immunomodulatory effects can be confirmed by further investigation.

Objectives: Insulin autoimmune syndrome (IAS) or Hirata disease is a rare cause of autoimmune hypoglycemia with apparent high insulin levels and anti-insulin autoantibodies and was first described by Hirata in Japan in 1970. IAS cases are usually related to exposure to sulfhydryl-containing drugs, which stimulate the production of insulin autoantibodies. Among sulfhydryl-containing compounds, alpha lipoic acid (ALA) has recently emerged as a cause of IAS. After the first observations of ALA-induced IAS were reported in Japan in 2006, an increasing number of cases related to ALA administration have been described. An Italian group recently reported on six cases of IAS of which one was associated with HLA-DRB1*04:06 and the remaining five with HLA-DRB1*04:03. This suggests that the latter is potentially involved in the genetic susceptibility of people of European descent to IAS., Methods: Here, we describe two new cases of IAS in women that were triggered by ALA., Results: Both cases are associated with HLA-DRB1*04:03 and confirm the evidence that HLA-DRB1*04:03 rather than HLA-DRB1*04:06 is specifically related to IAS susceptibility in Europeans., Conclusions: Case reports of ALA-induced hypoglycemic episodes highlight the need for greater care in prescribing ALA supplementation as well as the identification of specific and personalized therapeutic targets., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

A major problem in autoimmunity has been identification of the earliest events that lead to breach of tolerance. Although there have been major advances in dissecting effector pathways and the multilineage immune responses to mitochondrial self-antigens in primary biliary cholangitis, the critical links between environmental factors and tolerance remain elusive. We hypothesized that environmental xenobiotic modification of the E2 subunit of the pyruvate dehydrogenase (PDC-E2) inner lipoyl domain can lead to loss of tolerance to genetically susceptible hosts. Previously we demonstrated that serum anti-PDC-E2 autoantibodies cross-react with the chemical xenobiotics 2-octynoic acid and 6,8-bis (acetylthio) octanoic acid and further that there is a high frequency of PDC-E2-specific peripheral plasmablasts. Herein we generated 104 recombinant monoclonal antibodies (mAbs) based on paired heavy-chain and light-chain variable regions of individual plasmablasts derived from primary biliary cholangitis patients. We identified 32 mAbs reactive with native PDC-E2, including 20 specific for PDC-E2 and 12 cross-reactive with both PDC-E2 and 2-octynoic acid and 6,8-bis (acetylthio) octanoic acid. A lower frequency of replacement somatic hypermutations, indicating a lower level of affinity maturation, was observed in the complementarity-determining regions of the cross-reactive mAbs in comparison to mAbs exclusively recognizing PDC-E2 or those for irrelevant antigens. In particular, when the highly mutated heavy-chain gene of a cross-reactive mAb was reverted to the germline sequence, the PDC-E2 reactivity was reduced dramatically, whereas the xenobiotic reactivity was retained. Importantly, cross-reactive mAbs also recognized lipoic acid, a mitochondrial fatty acid that is covalently bound to PDC-E2., Conclusion: Our data reflect that chemically modified lipoic acid or lipoic acid itself, through molecular mimicry, is the initial target that leads to the development of primary biliary cholangitis. (Hepatology 2017;66:885-895)., (© 2017 by the American Association for the Study of Liver Diseases.)

Data from genome wide association studies and geoepidemiological studies established that a combination of genetic predisposition and environmental stimulation is required for the loss of tolerance in primary biliary cholangitis (PBC). The serologic hallmark of PBC are the presence of high titer anti-mitochondrial autoantibodies (AMA) that recognize the lipoyl domain of the mitochondrial pyruvate dehydrogenase E2 (PDC-E2) subunit. Extensive efforts have been directed to investigate the molecular basis of AMA. Recently, experimental data has pointed to the thesis that the breaking of tolerance to PDC-E2 is a pivotal event in the initial etiology of PBC, including environmental xenobiotics including those commonly found in cosmetics and food additives, suggesting that chemical modification of the PDC-E2 epitope may render its vulnerable to become a neo-antigen and trigger an immune response in genetically susceptible hosts. Here, we will discuss the natural history, genetics and immunobiology of PBC and structural constraints of PDC-E2 in AMA recognition which makes it vulnerable to chemical modification.

Environmental stimulation is a major factor in the initiation and perpetuation of autoimmune diseases. We have addressed this issue and focused on primary biliary cirrhosis (PBC), an autoimmune disease of the liver. Immunologically, PBC is distinguished by immune mediated destruction of the intra hepatic bile ducts and the presence of high titer antimitochondrial autoantibodies (AMA) directed against a highly specific epitope within the lipoic acid binding domain of the pyruvate dehydrogenase E2 subunit (PDC-E2). We submit that the uniqueness of AMA epitope specificity and the conformational changes of the PDC-E2 lipoyl domain during physiological acyl transfer could be the lynchpin to the etiology of PBC and postulate that chemical xenobiotics modification of the lipoyl domain of PDC-E2 is sufficient to break self-tolerance, with subsequent production of AMA in patients with PBC. Indeed, using quantitative structure activity relationship (QSAR) analysis on a peptide-xenobiotic conjugate microarray platform, we have demonstrated that when the lipoyl domain of PDC-E2 was modified with specific synthetic small molecule lipoyl mimics, the ensuing structures displayed highly specific reactivity to PBC sera, at levels often higher than the native PDC-E2 molecule. Hereby, we discuss our recent QSAR analysis data on specific AMA reactivity against a focused panel of lipoic acid mimic in which the lipoyl di-sulfide bond are modified. Furthermore, data on the immunological characterization of antigen and Ig isotype specificities against one such lipoic acid mimic; 6,8-bis(acetylthio)octanoic acid (SAc), when compared with rPDC-E2, strongly support a xenobiotic etiology in PBC. This observation is of particular significance in that approximately one third of patients who have taken excessive acetaminophen (APAP) developed AMA with same specificity as patients with PBC, suggesting that the lipoic domain are a target of APAP electrophilic metabolites such as NAPQI. We submit that in genetically susceptible hosts, electrophilic modification of lipoic acid in PDC-E2 by acetaminophen or similar drugs can facilitate loss of tolerance and lead to the development of PBC., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Our laboratory has hypothesized that xenobiotic modification of the native lipoyl moiety of the major mitochondrial autoantigen, the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), may lead to loss of self-tolerance in primary biliary cirrhosis (PBC). This thesis is based on the finding of readily detectable levels of immunoreactivity of PBC sera against extensive panels of protein microarrays containing mimics of the inner lipoyl domain of PDC-E2 and subsequent quantitative structure-activity relationships (QSARs). Importantly, we have demonstrated that murine immunization with one such mimic, 2-octynoic acid coupled to bovine serum albumin (BSA), induces anti-mitochondrial antibodies (AMAs) and cholangitis. Based upon these data, we have focused on covalent modifications of the lipoic acid disulfide ring and subsequent analysis of such xenobiotics coupled to a 15mer of PDC-E2 for immunoreactivity against a broad panel of sera from patients with PBC and controls. Our results demonstrate that AMA-positive PBC sera demonstrate marked reactivity against 6,8-bis(acetylthio)octanoic acid, implying that chemical modification of the lipoyl ring, i.e. disruption of the S-S disulfide, renders lipoic acid to its reduced form that will promote xenobiotic modification. This observation is particularly significant in light of the function of the lipoyl moiety in electron transport of which the catalytic disulfide constantly opens and closes and, thus, raises the intriguing thesis that common electrophilic agents, i.e. acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDs), may lead to xenobiotic modification in genetically susceptible individuals that results in the generation of AMAs and ultimately clinical PBC., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Senescence is the result of an imbalance between free radical production and antioxidant defenses, with concomitant oxidative stress and age-dependent functional decline. This process is especially evident in the immune cells, which use free radicals in their functions and suffer a senescent deterioration probably linked to oxygen stress. We hypothesize that oxidative damage and antioxidant imbalance may play a critical role in the immune dysfunction in aging. In the present study, we investigated this hypothesis in aged rats by treatment with alpha-lipoic acid (α-LA). We studied the effect of α-LA on immune function by examining immunomodulating factors in the plasma. Then we examined oxidative damage and antioxidant defense systems in the plasma. We found out that immune dysfunction in aged animals is associated with increased oxidative damage and decreased antioxidant status and treatment with α-LA effectively elevated immune function, decreased oxidative insult and enhanced antioxidant status. These results suggest that α-LA may be effective in improving immune function in aging through decreasing oxidative damage and revitalizing antioxidants in blood., (Crown Copyright © 2011. Published by Elsevier Inc. All rights reserved.)

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease in which an immune-mediated injury targets the small intrahepatic bile ducts. PBC is further characterized by highly specific serum antimitochondrial autoantibodies (AMAs) and autoreactive T cells, a striking female predominance, a strong genetic susceptibility and a plethora of candidate environmental factors to trigger the disease onset. For these reasons, PBC appears ideal to represent the developments of the clonal selection theory over the past decades. First, a sufficiently potent autoimmunogenic stimulus in PBC would require the coexistence of numerous pre-existing conditions (mostly genetic, as recently illustrated by genome-wide association studies and animal models) to perpetuate the destruction of the biliary epithelium by the immune system via the persistence of forbidden clones. Second, the proposed modifications of mitochondrial autoantigens caused by infectious agents and/or xenobiotics well illustrate the possibility that peculiar changes in the antigen structure and flexibility may contribute to tolerance breakdown. Third, the unique apoptotic features shown for cholangiocytes are the ideal setting for the development of mitochondrial autoantigen presentation to the immune system through macrophages and AMA; thus, turning the non-traditional mitochondrial antigen into a traditional one. This article will review the current knowledge on PBC etiology and pathogenesis in light of the clonal selection theory developments.

Background: Abnormal regulation of the inflammatory response is an important component of diseases such as diabetes, Alzheimer's disease and multiple sclerosis (MS). Lipoic acid (LA) has been shown to have antioxidant and anti-inflammatory properties and is being pursued as a therapy for these diseases. We first reported that LA stimulates cAMP production via activation of G-protein coupled receptors and adenylyl cyclases. LA also suppressed NK cell activation and cytotoxicity. In this study we present evidence supporting the hypothesis that the anti-inflammatory properties of LA are mediated by the cAMP/PKA signaling cascade. Additionally, we show that LA oral administration elevates cAMP levels in MS subjects., Methodology/principal Findings: We determined the effects of LA on IL-6, IL-17 and IL-10 secretion using ELISAs. Treatment with 50 µg/ml and 100 µg/ml LA significantly reduced IL-6 levels by 19 and 34%, respectively, in T cell enriched PBMCs. IL-17 levels were also reduced by 35 and 50%, respectively. Though not significant, LA appeared to have a biphasic effect on IL-10 production. Thymidine incorporation studies showed LA inhibited T cell proliferation by 90%. T-cell activation was reduced by 50% as measured by IL-2 secretion. Western blot analysis showed that LA treatment increased phosphorylation of Lck, a downstream effector of protein kinase A. Pretreatment with a peptide inhibitor of PKA, PKI, blocked LA inhibition of IL-2 and IFN gamma production, indicating that PKA mediates these responses. Oral administration of 1200 mg LA to MS subjects resulted in increased cAMP levels in PBMCs four hours after ingestion. Average cAMP levels in 20 subjects were 43% higher than baseline., Conclusions/significance: Oral administration of LA in vivo resulted in significant increases in cAMP concentration. The anti-inflammatory effects of LA are mediated in part by the cAMP/PKA signaling cascade. These novel findings enhance our understanding of the mechanisms of action of LA.

Novosphingobium aromaticivorans, a unique ubiquitous bacterium that metabolizes xenobiotics and activates environmental estrogens, has been suggested as a pathogenic factor in the development of primary biliary cirrhosis (PBC). To define the molecular basis of PBC sera reactivity, we investigated the characteristic of the bacterial antigens involved. We cloned and sequenced four genes from N. aromaticivorans coding for immunoreactive proteins, arbitrarily named Novo 1 through Novo 4. We subsequently analyzed these proteins for their homology to known mitochondrial proteins and defined their reactivity using monoclonal antibodies (mAbs), rabbit anti-lipoic acid antibody, and PBC/control sera. Moreover, we studied their phylogenetic relation with the known PBC autoantigens. Novo proteins have an extraordinary degree of amino acid homology with all of the major human mitochondrial autoantigens PDC-E2 (Novo 1 and 2), OGDC-E2 (Novo 3), and BCOADC-E2 (Novo 4). Moreover, Novo 1-4 contain a lipoylated domain, are recognized by AMA-positive sera, and react with specific mAbs to mitochondrial antigens. Interestingly, the phylogenetic relation of the proteins emphasizes the conservation of the lipoylated domain. In conclusion, our data provide a high degree of confidence that N. aromaticivorans may potentiate the breakdown of self tolerance in genetically susceptible individuals.

Previous work has demonstrated that immunization of rabbits with the xenobiotic 6-bromohexanoate coupled to BSA breaks tolerance and induces autoantibodies to mitochondria in rabbits. Such immunized rabbits develop high-titer Abs to pyruvate dehydrogenase complex (PDC)-E2, the major autoantigen of primary biliary cirrhosis. In efforts to map the fine specificity of these autoantibodies, rabbits were immunized biweekly with 6-bromohexanoate-BSA and screened for reactivity using a unique xenobiotic-peptide-agarose microarray platform with an emphasis on identifying potential structures that mimic the molecular image formed by the association of lipoic acid with the immunodominant PDC-E2 peptide. Essentially, a total of 23 xenobiotics and lipoic acid were coupled to the 12-mer peptide backbones, PDC, a mutant PDC, and albumin. As expected, we succeeded in breaking tolerance using this small organic molecule coupled to BSA. However, unlike multiple experimental methods of breaking tolerance, we report in this study that, following continued immunization, the rabbits recover tolerance. With repeated immunization, the response to the rPDC-E2 protein increased with a gradual reduction in autoantibodies against the lipoic acid-peptide, i.e., the primary tolerance-breaking autoantigen. Detailed analysis of this system may provide strategies on how to restore tolerance in patients with autoimmune disease.

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.

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., (Copyright 2000 Academic Press.)

PBC is characterized by T cell-mediated destruction of the biliary epithelial cells lining the small intrahepatic bile ducts. The E2 and E3 binding protein (E3BP (protein X)) components of pyruvate dehydrogenase complex (PDC) are disease-specific autoantigens in PBC. Attempts to localize the T cell autoepitopes within PDC-E2 have, however, generated contradictory results. One study has suggested the presence of T cell epitopes throughout PDC-E2, whilst another has identified a single dominant 14 amino acid T cell epitope (p163) spanning the lipoic acid binding lysine residue in the inner lipoyl domain (ILD) of PDC-E2. The aim of the current study was to determine the prevalence of T cell responses to p163 and PDC-E2 ILD, and the role played by lipoylation of these antigens in their immunogenicity, in a UK PBC population. We found that the majority of the PBC patients showing a 6-day peripheral blood T cell proliferative response to native human PDC also responded, in a MHC class II-restricted fashion, to biochemically purified PDC-E2 and E3BP (which co-purify) (9/10 positive (SI > 2.76), mean SI 5.74 +/- 5.04 (PDC-E2/E3BP) versus 6.67 +/- 3.84 (PDC), P = NS), implying that the important PBC-specific T cell epitopes are contained within the PDC-E2 or E3BP components of PDC. Only a minority of patients responsive to PDC, however, responded to either lipoylated recombinant PDC-E2 ILD (4/10 positive, mean SI 1.98 +/- 1.24, P < 0.005 versus PDC response) or lipoylated p163 (4/12 positive, mean SI 1.90 +/- 1.58, P < 0.001). The lipoylation state did not affect the T cell response to either ILD or p163. Our findings suggest that in some UK patients with PBC there are immunodominant T cell autoepitopes within PDC-E2/E3BP which are outside the ILD of PDC-E2.

Previous research has established that 4-hydroxy-2-nonenal (HNE), a highly toxic product of lipid peroxidation, is a potent inhibitor of mitochondrial respiration. HNE exerts its effects on respiration by inhibiting alpha-ketoglutarate dehydrogenase (KGDH). Because of the central role of KGDH in metabolism and emerging evidence that free radicals contribute to mitochondrial dysfunction associated with numerous diseases, it is of great interest to further characterize the mechanism of inhibition. In the present study, treatment of rat heart mitochondria with HNE resulted in the selective inhibition of KGDH and pyruvate dehydrogenase (PDH), while other NADH-linked dehydrogenases and electron chain complexes were unaffected. KGDH and PDH are structurally and catalytically similar multienzyme complexes, suggesting a common mode of inhibition. To determine the mechanism of inhibition, the effects of HNE on purified KGDH and PDH were examined. These studies revealed that inactivation by HNE was greatly enhanced in the presence of substrates that reduce the sulfur atoms of lipoic acid covalently bound to the E2 subunits of KGDH and PDH. In addition, loss of enzyme activity induced by HNE correlated closely with a decrease in the availability of lipoic acid sulfhydryl groups. Use of anti-lipoic acid antibodies indicated that HNE modified lipoic acid in both purified enzyme preparations and mitochondria and that this modification was dependent upon the presence of substrates. These results therefore identify a potential mechanism whereby free radical production and subsequent lipid peroxidation lead to specific modification of KGDH and PDH and inhibition of NADH-linked mitochondrial respiration.

In the last decade, the cloning and biochemical identification of mitochondrial autoantigens in primary biliary cirrhosis (PBC) as members of the 2-oxoacid dehydrogenase complex has greatly advanced the detection of antimitochondrial antibodies (AMA) and the understanding of the immunobiology of the disease. Here, we discuss the methods of detecting AMA and its isotypes, methods of epitope mapping, and using these methods in PBC liver immunohistochemistry and Ig gene usage. Increasing evidence, including the specific association of AMA with PBC, the unique similar but noncross-reactive conformational epitope of the lipoyl domains of the mitochondrial autoantigens, the specific binding of anti-PDC-E2 monoclonal antibodies and human combinatorial antibodies derived from PBC patients to the apical area of bile duct epithelial cells in PBC livers, and Ig gene usage of AMA, suggests that AMA is not an epiphenomenon of the disease but plays a significant role in the pathogenesis of PBC.

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.

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.

Approximately 95% of patients with primary biliary cirrhosis have antimitochondrial antibodies against the E2 component of the pyruvate dehydrogenase complex (E2p). Immunodominant sites on E2p have been localized to the inner lipoyl domain, which serves as a covalent attachment site for the essential cofactor, lipoic acid. However, it is not clear whether the presence of lipoic acid is necessary for autoimmune recognition of human E2p. To facilitate further studies on the inner lipoyl domain and to assess the importance of lipoic acid in antibody binding, we used the previously cloned human E2p cDNA in the construction and high-level expression in Escherichia coli of a subgene encoding the domain. Purification and analysis of the gene product revealed that both lipoylated and unlipoylated forms of the intact domain are generated. Immunoblotting, enzyme-linked immunosorbent assay inhibition experiments and antibody affinity measurements using isolated lipoylated and unlipoylated domains demonstrated that the presence of the lipoyl residue is crucial for effective recognition by primary biliary cirrhosis patients' autoantibodies, which have a higher relative affinity for the lipoylated form. Contrary to some previous suggestions, these results indicate that antibodies in primary biliary cirrhosis patients' sera bind most effectively to a unique peptide-cofactor conformation in the lipoyl domain of the human E2p polypeptide. Moreover, the availability of large amounts of human lipoyl domain will permit further studies into the role of the antigen (if any) in disease pathogenesis.