The human biliary epithelial cell plasma membrane antigen in primary biliary cirrhosis: Pyruvate dehydrogenase X?
Abstract
BACKGROUND & AIMS: Patients with primary biliary cirrhosis (PBC) have autoantibodies that react with components of mitochondrial multienzyme complexes. In addition to binding to mitochondria, patients' autoantibodies to the assumed major autoantigen pyruvate dehydrogenase complex (PDC) dihydrolipoamide acetyltransferase (E2) bind to the plasma membrane of biliary epithelial cells (BECs) specifically in PBC. The aim of this study was to characterize BEC plasma membrane antigens recognized by patients' autoantibodies in PBC.
METHODS: Antigens prepared from intracellular and plasma membrane-enriched fractions of BECs purified from PBC and control liver were immunoblotted with anti- PDC.
RESULTS: In the intracellular fraction, anti-PDC recognized BEC protein bands corresponding to the molecular weight value of E2 and X components of human heart PDC on Western blots. No difference was observed between PDC-E2 in BECs from PBC and controls. However, in PBC but not controls, a 50-kilodalton antigen was detected in the plasma membrane-enriched fraction. This antigen comigrated with component X of purified human heart PDC and was recognized by antibodies specific for PDC-X.
CONCLUSIONS: The data suggest that PDC-X or a cross-reactive 50- kilodalton antigen is the BEC plasma membrane antigen recognized by patients' autoantibodies in PBC. Furthermore, this antigen, rather than PDC-E2, may be a major B-cell target antigen in PBC.
(Gastroenterology 1997 Nov;113(5):1727-33)
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Primary Biliary Cirrhosis
2013, The Autoimmune Diseases: Fifth EditionPrimary biliary cirrhosis (PBC) affects primarily middle-aged women and is characterized by chronic cholestasis and the presence in serum of high-titer antimitochondrial autoantibodies (AMA). The disease leads to the destruction of the intrahepatic bile ducts and progressive fibrosis. The diagnosis is made when two out of three criteria are met, i.e., positive serum AMA, alkaline phosphatase elevation, and compatible liver histology. Like other autoimmune diseases, the onset of PBC seems to depend on the cumulative effects of some susceptibility genes and the effects of environmental agents, infectious or chemical. Genome-wide association studies strongly point to the genes of the IL-12 pathway, among others. The association of AMA, identified as autoantibody directed against the E2 subunit of the mitochondrial pyruvate complex (PDC-E2), with biliary ductular lesions, is enigmatic, but it is suspected that PDC-E2 during degradation undergoes faulty apoptosis with production of a disease-inducing apotope. The presence in nearly 50% of cases of atypical PBC-related antinuclear antibodies remains a mystery. Treatment with the now-approved ursodeoxycholic acid is surprisingly effective. Several informative transgene-based mouse models have been developed and confirm the nexus between AMA reactivity and cholangiocyte destruction, and experimental models are available supporting the participation in pathogenesis of environmental xenobiotics. Possible tracks towards an understanding of the “cause” of PBC can be visualized.
Primary Biliary Cirrhosis
2006, The Autoimmune Diseases, Fourth EditionThis chapter discusses clinical, pathologic, and epidemiologic features of Primary biliary cirrhosis (PBC). The diagnosis of PBC is based on three objective criteria. This triad consists of detectable serum antimitochondrial antibodies (AMA), increased enzymes indicating cholestasis for longer than 6 months, and a compatible or diagnostic liver histology. The chapter also describes the histology, autoimmune features, and epidemiology of PBC. It is noted that though genetics should be regarded as the major determinant in susceptibility to PBC, several other factors have been proposed, including a history of previous infections, comorbidity with other autoimmune diseases, and lifestyle factors, such as smoking and a high-fat diet. Furthermore, it is crucial to determine the pathogenic role of AMA in the bile duct damage of PBC. Once again, the development of an animal model appears to be the only way to provide a clear demonstration of such pathogenic mechanism. Finally, from a clinical standpoint, new clinical trials are needed to identify novel therapies in the long-term treatment of PBC.
Pathogenesis of primary biliary cirrhosis
2003, Clinics in Liver DiseaseAnti-mitochondrial autoantibodies
2002, Clinical and Applied Immunology ReviewsAnti-mitochondrial antibodies (AMA) are a serological hallmark of primary biliary cirrhosis (PBC). The major autoantibody targets are located in the inner mitochondrial membrane, are encoded by nuclear genes and are components of the 2-oxo acid dehydrogenase complexes. Greater than 90% of PBC patients react with one or more of these autoantigens. The major epitopes have been mapped to the lipoic acid binding domain. The apparent hepatic targets of immune destruction, the apical biliary epithelial cells, express proteins that mimic these epitopes. Nevertheless, the role of AMA in biliary pathology is not clear and observed abnormalities may be due to T-cells that respond to similar epitopes. AMAs are often accompanied by autoantibodies to other intracellular components such as the nuclear pore complex, centromeres/kinetochores, and other nuclear antigens. The origin or inciting agent(s) of AMA is not known but epidemiological and immunological evidence implicates environmental agents.
Ductular morphogenesis and functional polarization of normal human biliary epithelial cells in three-dimensional culture
2001, Journal of HepatologyBackground/Aims: The understanding of the physiology and function of human biliary epithelial cells (hBEC) has been improved by studies in monolayer culture systems. The aim was to develop a polarized model to elucidate the mechanisms of ductular morphogenesis and functional differentiation of hBEC.
Methods: The morphological, phenotypic and functional properties of hBEC cultured as three-dimensional aggregates in collagen gel were assessed in medium supplemented with (or without) human hepatocyte growth factor (hHGF) and foetal bovine serum.
Results: In the absence of added mitogens and serum, cells maintained as morphologically polarized aggregates, organized around a central lumen, were positive for phenotypic markers of biliary epithelium and negative for markers of other cell types. Functional markers, gamma-glutamyl-transferase, anion exchanger-2, responses to gamma interferon and forskolin induced secretion, were preserved. hHGF increased both the size and number of aggregates and induced hBEC to invade the gel and lumena forming anastomosing networks of cells.
Conclusions: Collagen gel culture in the absence of added growth factors and serum provides a model for analysis of the polarized functions of hBEC. The formation of poorly organized cords of cells in response to hHGF suggests that collagen gel culture may provide a model for the investigation of atypical ductular morphogenesis of the human biliary tract.
The 2-oxo-acid dehydrogenase complexes and, in particular, the E2 component of the pyruvate dehydrogenase complex (PDC) are the target of antimitochondrial antibodies (AMA). More than 95% of primary biliary cirrhosis (PBC) patients have detectable levels of autoantibodies to PDC-E2 and in general these react with a region of the molecule that contains the prosthetic group lipoic acid (LA). LA is vital to the function of the enzyme, although there is conflicting evidence as to whether its presence is required for PDC-E2 recognition by AMA. Some, but not all, monoclonal antibodies (mAbs) to PDC-E2 produce an intense staining pattern at the apical surface of bile duct epithelial cells (BEC) in patients with PBC, and it has been argued that the molecule at the apical surface of PBC bile duct cells is a modified form of PDC-E2 or a cross-reactive molecule, acting as a molecular mimic. Herein, we characterize the epitopes recognized by 4 anti–PDC-E2 mAbs that give apical staining patterns (3 mouse and 1 human). In particular, by using a combination of recombinant antigens, competitive inhibition assays, and a unique peptide-on-bead assay, we determined that these apically staining mAbs recognize 3 or 4 distinct epitopes on PDC-E2. More importantly, this suggests that a portion spanning the entire inner lipoyl domain of PDC-E2 can be found at the BEC apical surface. In addition, competition assays with patient sera and a PDC-E2–specific mAb showed significant epitope overlap with only 1 of the 3 mouse mAbs and showed a differential response to the peptide bound to beads. These findings further highlight the heterogeneous response of patient autoantibodies to the inner lipoyl domain of PDC-E2. (HEPATOLOGY 2001;33:792-801.)