M2 Autoantigens Research Articles

Prevention of primary biliary cirrhosis through immune tolerance reestablishment in a mouse model

Objective To investigate a new therapeutic pathway for primary biliary cirrhosis (PBC) by immune tolerance reestablishment in a PBC mouse model. Methods Spleenic cells from naive mice were incubated with M2 in the presence of ECDI and the ceils were injected into caudal vein of the mice which would be used for development of PBC model. Spleenic cells incubated with bovine serum albumin (BSA) were injected as controls. 16 weeks later, anti-mitoehondrial antibody (AMA) , alkaline phosphatase(AKP) and portal inflammation were assayed for evaluating the prevention effect. Results AMA positive rate in tolerance group was lower than that in BSA and PBC groups ( P = 0. 007, P = 0. 003 ). The difference between BSA and PBC was not significantly. Serum AKP levels in tolerance, BSA and PBC group were (80.5 ±9.8) U/L, (93.8 ±15.7) U/L and (92.5 ±17.7) U/L, separately. The level in tolerance group was lower than that in BSA and PBC groups (P =0.0095, P =0.029). The rates of portal areas with cell infiltration were 42. 67% ± 12. 30% , 57. 07% ± 11. 35% and 51. 53% ± 9. 96% , separately. The number of infiltrated portal tracts in tolerance group was less than that in PBC group (P = 0.039) and BSA group (P = 0. 0024). Conclusion PBC was prevented to some extent by reestablishing immune tolerance to M2 autoantigen. This provides clues for finding a better treatment proposal. Key words: Primary biliary cirrhosis; Model; Immune tolerance

Inhibition of enzyme function by human autoantibodies to an autoantigen pyruvate dehydrogenase E2; different epitope for spontaneous human and induced rabbit autoantibodies

Antibodies to the mitochondrial autoantigen M2, characteristic of the autoimmune liver disease primary biliary cirrhosis (PBC), react with the E2 subunit of the pyruvate dehydrogenase enzyme (PDH-E2). We examined the effect of disease sera on the enzyme activation catalysed by the PDH complex. Inhibition of enzyme activity was observed in 19 of 24 sera of patients with PBC with a level of greater than 90% inhibition in 14 at a serum dilution of 1/50. The onset of inhibition by serum was rapid, within the time of mixing, and the inhibitory activity was shown to reside in the immunoglobulin fraction of the serum. The immunoglobulin fraction of control sera from patients with other liver diseases (n = 26) and healthy persons (n = 8) failed to produce inhibitory activity. In addition sera from four rabbits, intensively immunized with a recombinant human M2 autoantigen, gave anti-M2 reactions by fluorescence, ELISA and immunoblotting, but did not inhibit the activity of PDH. The failure of experimentally induced M2 antibodies in rabbits to inhibit is interesting in view of the reactivity of the natural M2 autoantibodies of PBC with the highly conserved site on the enzyme which carries the essential lipoic acid cofactor.

Autoantibodies to M2 mitochondrial autoantigens in normal human sera by immunofluorescence and novel assays.

Primary biliary cirrhosis (PBC) is characterized by the presence of antimitochondrial antibodies (anti-M2), directed against the E2 subunits of the 2-oxo-acid dehydrogenase complexes (2-OADC), chiefly pyruvate dehydrogenase complex (PDC-E2). We present here a detailed study, based on a large panel of normal sera, of the specificity of tests for anti-M2 by immunofluorescence and for anti-PDC by other assays for the diagnosis of PBC. The assays for anti-PDC included immunoblotting with bovine heart mitochondria, ELISA using recombinant PDC-E2 and an enzyme inhibition assay using purified porcine PDC. The positivity rates for normal sera were 0 (0/170), 2 (4/201), 1.5 (3/198) and 0% (0/186) for immunofluorescence, immunoblotting, ELISA and the enzyme inhibition assay, respectively. The seven positive reactions detected either by immunoblotting (n = 4) or ELISA (n = 3) were negative by the other three assays and in no instance did biochemical indices give any indication of chronic liver disease. Thus, as judged by reactivity with normal sera, the specificity of a positive test for the antibody to the major M2 autoantigen (PDC-E2) is 100% for immunofluorescence and the enzyme inhibition assay, 98% for immunoblotting and 98.5% for ELISA.

Immunoreactivity of antimitochondrial autoantibodies in Japanese patients with primary biliary cirrhosis.

The incidence and prevalence of primary biliary cirrhosis show wide geographic differences. The frequency of this disease in Japan is lower than in Northern Europe. To elucidate the immunoreactivity of serum with enzymes of the 2-oxo-acid dehydrogenase complex (2-OADC) and the M2 mitochondrial antigenic complex in Japanese patients, we examined sera from 107 patients with primary biliary cirrhosis from three geographically different regions of Japan. The sera were assayed by immunofluorescence on frozen tissue sections, immunoblotting on bovine heart mitochondria and recombinant E2 subunit of branched chain oxo-acid dehydrogenase complex (BCOADC-E2), ELISA using recombinant E2 subunit of human pyruvate dehydrogenase complex (PDC-E2) and purified porcine 2-oxoglutarate dehydrogenase complex (OGDC), and enzyme inhibition assay using procine PDC and OGDC. Of the 107 sera, 95 (88%) reacted by immunofluorescence, 102 (95%) by immunoblotting with at least one of the M2 autoantigens, although only 78 (73%) reacted with PDC-E2; 72 (67%) by ELISA with PDC-E2; and 81 (76%) with PDC by the enzyme inhibition assay. Thus, the frequency of reactivity with PDC-E2 by all assays was lower for Japanese than the reported frequency for Caucasian patients with primary biliary cirrhosis, whereas the frequency of reactivity by immunoblotting and ELISA against 2-OADC enzymes other than PDC was relatively higher. The relative frequency of reactivity of autoantibodies to the M2 autoantigens was similar for the three different regions of Japan. The different autoantibody profiles for Japanese and Caucasian patients with primary biliary cirrhosis point to immunogenetic and environmental determinants of this disease, which should provide new insights into its autoimmune origins.

The kaleidoscope of autoimmunity.

The M-2 autoantigen in primary biliary cirrhosis was summarized by E. Gershwin (UCD, USA). Primary biliary cirrhosis (PBC) is thought to result from autoimmune mediated destruction of intrahepatic bile ducts with progressive inflammatory scarring that is followed by liver failure. The association of PBC with high titer autoantibodies to mitochondriaI antigens has been long recognized. In 1985, three groups used immunoblotting to show that some five mitochondrial autoantigens could be identified with molecular weights ranging from 74 to 36 kd. In 1987, a cDNA for the 74 kd mitochondrial autoantigen was cloned and sequenced leading to identification of the PBC autoantigen as the mitochondrial pyruvate dehydrogenase complex (PDC)’. The M2 autoantigens of PBC have been identified with the functionally related enzyme family, of which PDC is the most prominent antigenic component. Each of the 2-0x0-acid-dehydrogenase complexes (2-OADC) are located on the mammalian inner mitochondrial membrane and include PDC, the 2-oxoglutarate dehydrogenase complex (OGDC), and the branchedchain 2-0x0-acid dehydrogenase (BCOADC). Each of the three enzymes consist of three subunits, E l , E2, and E3; all are nuclear-encoded proteins that are separately imported into mitochondria by a leader sequence for assembly into high molecular weight multimers of the inner membrane. Cloned antigens have been reliably employed in assays for presence of autoantibodies*; the use of cloned recombinant antigens should replace that of traditional immunofluorescence for AMA assay. Finally, there is increasing evidence that PDC-E2 is located on the cell membrane of biliary epithelial cells. A similar rapid progress was achieved deciphering the enigma of the enzymes proteinase-3 and myeloproxidase as target autoantigens in Wegener’s granulomatosis. Allan Wiik (Copenhagen, Denmark) depicted the major revelations in the field: Since 1939 Wegener’s granulomatosis (WG) has been considered a clinical and histopathologic entity characterized by generalized or local small vessel vasculitis with epithelioid and giant cell granulomas commonly involving upper airways, lungs and kidneys. The diagnosis largely relied on biopsy findings in inflammed tissues, showing focal necrotizing vasculitis with granulomas and glomerulitis. In 1985 autoantibodies directed to granule content of neutrophils were found in the large majority of patients with active WG but only in a minority of WG patients with inactive disease-’. The antineutrophil cytoplasmic antibodies (ANCA), now caIled classical ANCA (c-ANCA) as detected by indirect immunofluorescence (IIF) technique were found to be very specific for this vasculitic entity, and the titers of c-ANCA correlated with disease activity. ANCA’s targeting the azurophil granule enzyme, myeloperoxidase in patients with different forms of focal necrotizing glomerulitis were reported, however, these antibodies were found to be rare in WG patients. Studies from several groups indicated that the c-ANCA recognize a 29 kD serine protease present in azurophil granules most likely identical to the leucocyte proteinase-3. N-terminus amino acid sequence data indicated close identity between proteinase-3, myeloblastin, azurophil granule protein-7 and neutrohpil protease-4 described by other investigators not working with a~toimmunity*-~. c-ANCA’s are mainly contained in the IgG class, especially in the subclasses 1 and 4, which may indicate antibody production through prolonged antigen driven mechanisms. IgM c-ANCA have been found in some WG patients manifesting pronounced plumonary haemorrhage as a result of necrotizing capillaritis. As patients with WG have been successfully treated some investigators find that the patients produce anti-idiotypic antibodies to c-ANCA. Such anti-idiotypic antibodies seem to be common even in healthy blood donors, and intravenous y-globulin

Clarification of the identity of the major M2 autoantigen in primary biliary cirrhosis

1. In primary biliary cirrhosis, the major M2 autoantigen, reacting with antimitochondrial antibodies in sera from greater than 90% of patients, has been identified as the E2 component of the pyruvate dehydrogenase complex. However, two recent reports suggest that alternative polypeptides may be major autoantigens. 2. The evidence that a 75 kDa subunit of complex I of the respiratory chain is a major autoantigen (Frostell, Mendel-Hartvig, Nelson, Totterman, Bjorkland & Ragan, Scand. J. Immunol. 1988; 28, 157-65) is refuted. The findings of Frostell et al. can be explained by contamination of complex I with the pyruvate dehydrogenase complex, evidence for which is presented here. 3. Inspection of the partial amino acid sequence of an unidentified mitochondrial autoantigen (Muno, Kominami, Ishii, Usui, Saituku, Sakakibara & Namihisa, Hepatology 1990; 11, 16-23) shows that it is the E1 beta-subunit of the pyruvate dehydrogenase complex, previously identified as a major autoantigen, and not a 'new' alternative major autoantigen. 4. These findings substantiate previous work showing that the mitochondrial M2 autoantigens identified so far in primary biliary cirrhosis are all polypeptide components of the pyruvate dehydrogenase complex or the other related 2-oxo acid dehydrogenase complexes.

Autoantibodies in primary biliary cirrhosis: Analysis of reactivity against eukaryotic and prokaryotic 2-oxo acid dehydrogenase complexes

Six components of the mammalian 2-oxo acid dehydrogenase complexes have previously been identified as M2 autoantigens in primary biliary cirrhosis. In this report, we present data showing that both polypeptide-specific and cross-reacting antibodies are present in patients' sera. Antibodies reacting with E2 of the pyruvate dehydrogenase complex cross-react with protein X but not with any other mammalian antigen. The main immunogenic region on protein X has been localized to within its single lipoyl domain. Polypeptide-specific antibodies bind to E1 alpha and E1 beta of the pyruvate dehydrogenase complex. Antibodies reacting with the E2 polypeptides of the 2-oxoglutarate dehydrogenase complex and branched-chain 2-oxo acid dehydrogenase complex show some cross-reactivity but do not recognize any of the antigens of the pyruvate dehydrogenase complex. Antibodies against the E2 component of the mammalian pyruvate dehydrogenase complex cross-react effectively with the corresponding protein from yeast but not with E2 from Escherichia coli. Antibody titer against mammalian antigens is significantly higher than against the bacterial antigens, arguing against a bacterial origin for primary biliary cirrhosis.

Primary biliary cirrhosis: Considerations on pathogenesis based on identification of the M2 autoantigens

Primary biliary cirrhosis: Considerations on pathogenesis based on identification of the M2 autoantigens

The Lipoate‐Containing Domain of PDC E2 Contains the Main Immunogenic Region of the 70‐kDa M2 Autoantigen in Primary Biliary Cirrhosis

The Lipoate‐Containing Domain of PDC E2 Contains the Main Immunogenic Region of the 70‐kDa M2 Autoantigen in Primary Biliary Cirrhosis

Molecular aspects of the M2 autoantigens in primary biliary cirrhosis: What a difference a year makes

Molecular aspects of the M2 autoantigens in primary biliary cirrhosis: What a difference a year makes

Primary biliary cirrhosis: current knowledge, perspectives, and future directions.

These perspectives from the first International Symposium on Primary Biliary Cirrhosis review recent advances and single out some areas for further enquiry. The latter include frequency and type of associated autoimmune diseases, the existence of clinical subsets of PBC, immunohistochemical analysis of lymphoid infiltrates in the liver, effects of immunosuppressive and other treatment regimens, and models for predicting the optimal time for liver transplantation. The M2 autoantigens have been identified as mitochondrial 2-oxo-acid dehydrogenase enzymes. These include pyruvate dehydrogenase (70-74 kd antigen) and branched chain 2-oxo-acid dehydrogenase and 2-oxo-acid glutaric dehydrogenase (45-52 kd antigens). Each of these enzymes has three subunits, E1 to E3. For PDH, an autoepitope has been identified as a decapeptide containing the attachment site of lipoic acid, an essential cofactor for enzyme activity. Current questions include the degree to which antibodies to PDH, and related enzymes, account for the mitochondrial reactivity defined by immunofluorescence or other procedures, the cell-surface expression of M2 autoantigens, and the significance of the occurrence of nonmitochondrial (such as centromeric) autoantibodies in PBC. The unknown T lymphocyte contribution to the autoimmune response in PBC may involve inducer and effector components. A postulated T-cell autoepitope may be presented, in association with MHC class I or class II molecules, on the surface of biliary epithelial cells. T cell lines from PBC livers removed during transplantation could provide data on the T-lymphocyte contribution to the pathogenesis of PBC.

Chronic experimental autoimmune encephalomyelitis in the guinea pig. Presence of anti-M2 antibodies in central nervous system tissue and the possible role of M2 autoantigen in the induction of the disease

Experimental autoimmune encephalomyelitis (EAE) can be transferred adoptively with T cells sensitized to the basic protein of myelin (BP). However, in the guinea pig, the chronic form of EAE has not been found to be inducible with BP alone, nor has it been adoptively transferred. An antibody response to the central nervous system (CNS) myelin autoantigens was looked for in serum and target CNS tissue in S13 guinea pigs with isologous CNS tissue-induced chronic EAE. Antibody activity was estimated by an immunoenzymatic technique and by autoradiography, using immunoprecipitated and electrophoresed relevant radiolabelled antigens. In serum, IgG antibody response to BP and M2 reached its maximum level 30 to 40 d after immunization and then declined progressively until it became undetectable. On the other hand, while anti-BP antibodies were seldom detected in CNS tissue acid extract, anti-M2 IgG antibodies were always present in CNS tissue of chronic EAE animals, and the amount of these antibodies was related to the severity of symptoms and lesions. No antibody response to proteolipid or to galactocerebroside was detected in serum or CNS tissue. BP-immunized controls showed no chronic EAE and no response to M2 in their serum or CNS tissue. Inasmuch as M2 has been shown to be a glycoprotein of CNS myelin, and anti-M2 antibodies to have a demyelinating property, the latter would be responsible for CNS tissue demyelination in chronic EAE. A shared role of BP and M2 in the induction of chronic EAE in the guinea pig is suggested.

Identification and analysis of the major M2 autoantigens in primary biliary cirrhosis.

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial antibodies in the serum. It is possible that the PBC-specific immunoreactive trypsin-sensitive antigens on the inner mitochondrial membrane, termed M2, are important in the pathogenesis of this autoimmune disease. We have previously shown that a major M2"a" antigen is the E2 component of the pyruvate dehydrogenase multienzyme complex located within mitochondria. Analysis of the primary structure of the E2 components of all three 2-oxo acid dehydrogenase complexes reveals a high degree of homology with a similar highly segmented structure including lipoyl domains, E3-binding domains, C-terminal catalytic domains, and interdomain linker sequences. Immunoblotting of PBC patients' sera against purified E2 protein from 2-oxoglutarate dehydrogenase complex and branched-chain 2-oxo acid dehydrogenase complex reveals that these polypeptides are also autoantigens in this disease. Sera from 29 of 40 (72.5%) PBC patients gave a positive response against bovine 2-oxoglutarate dehydrogenase complex E2 and from 25 of 40 (62.5%) PBC patients gave a positive response against bovine branched-chain 2-oxo acid dehydrogenase complex E2. All 40 PBC patients (100%) have autoantibodies directed against at least one of the E2 components of the family of 2-oxo acid dehydrogenase complexes. Identification of these M2 mitochondrial autoantigens and detailed knowledge of their structure will allow important questions concerning this autoimmune disease to be addressed.

PRIMARY BILIARY CIRRHOSIS: IDENTIFICATION OF TWO MAJOR M2 MITOCHONDRIAL AUTOANTIGENS

Primary biliary cirrhosis (PBC) is characterised by the presence of antimitochondrial antibodies. The PBC-specific, immunoreactive, trypsin-sensitive antigens on the inner mitochondrial membrane (M2) have hitherto not been identified. A major 70 kD M2 autoantigen is the E2 component (lipoate acetyltransferase) of the pyruvate dehydrogenase enzyme complex located within mitochondria. This has been confirmed by immunoblotting of PBC patients' sera against purified E2 protein: sera from 38/40 (95%) patients with established clinical, biochemical, and histological features of PBC (18 stage II/III, 22 stage IV) reacted positively with E2; whilst no sera from 39 controls (27 non-PBC chronic liver disease, 12 healthy normal women) gave a positive response. Immunoblotting showed that a second subunit of the pyruvate dehydrogenase complex, a 50 kD polypeptide of unknown function (component X), is also an M2 autoantigen. Identification of these M2 mitochondrial antigens should facilitate the development of a specific serological test for PBC and the study of autoimmunising epitopes.