title: Immunogenetic analysis reveals that epitope shifting occurs during B-cell affinity maturation in primary biliary cirrhosis.
abstract: Primary biliary cirrhosis (PBC) is a liver disease characterized by serum autoantibodies against the pyruvate dehydrogenase complex (PDC) located in the inner mitochondrial membrane. The predominant target in PDC has previously been localized to the inner lipoyl domain (ILD) of the E2 subunit. The etiology of PBC is unknown, although molecular mimicry with bacterial PDC has been proposed. Here, we have investigated the etiology of PBC and nature of the autoimmune response by analyzing the structure of a human monoclonal antibody with ILD specificity. Mutants of the monoclonal antibody, which was originally isolated from a patient with PBC, were expressed as Fab by phage display, and tested for reactivity against recombinant domains of the E2 subunit. Fab in which the V(H)-encoded portions were reverted to germline lost reactivity against the ILD alone, but recognized a different epitope in a didomain construct encompassing the ILD, hinge region and E1/E3 binding domain. The complete V(H) and V(L )germline revertant was unreactive with the human ILD and didomain, the Escherichia coli didomain, and whole PDC. We hypothesize that the IgM on the surface of the naïve B-cell first recognizes an as yet unidentified antigen, and that accumulation of somatic mutations results in an intermolecular epitope shift directed towards an epitope involving the E1/E3 binding domain. Further mutations result in the specificity being redirected to the ILD. These findings also suggest that bacterial molecular mimicry is not involved in initiating disease.
authors.affiliation: Molecular Immunology Group, Southampton University Hospitals Trust, Tenovus Laboratory, Southampton, SO16 6YD, UK. kp1@soton.ac.uk
authors.forename: K N
authors.initials: KN
authors.lastname: Potter
authors.forename: R K
authors.initials: RK
authors.lastname: Thomson
authors.forename: A
authors.initials: A
authors.lastname: Hamblin
authors.forename: S D
authors.initials: SD
authors.lastname: Richards
authors.forename: J G
authors.initials: JG
authors.lastname: Lindsay
authors.forename: F K
authors.initials: FK
authors.lastname: Stevenson
mesh_terms: D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D000918:Antibody Specificity / Q000276:immunology; D001402:B-Lymphocytes / Q000166:cytology / Q000276:immunology* / Q000378:metabolism*; D002454:Cell Differentiation / Q000235:genetics; D022801:Complementarity Determining Regions / Q000235:genetics; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000235:genetics* / Q000276:immunology; D015322:Gene Rearrangement, B-Lymphocyte / Q000235:genetics*; D015326:Gene Rearrangement, B-Lymphocyte, Heavy Chain / Q000235:genetics; D005803:Genes, Immunoglobulin / Q000235:genetics; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D007143:Immunoglobulin Heavy Chains / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D007147:Immunoglobulin Light Chains / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D008105:Liver Cirrhosis, Biliary / Q000235:genetics* / Q000276:immunology*; D008213:Lymphocyte Activation / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D011768:Pyruvate Dehydrogenase Complex / Q000737:chemistry / Q000276:immunology; D016415:Sequence Alignment
chemical_list: D000911:Antibodies, Monoclonal; D022801:Complementarity Determining Regions; D000939:Epitopes; D007140:Immunoglobulin Fab Fragments; D007143:Immunoglobulin Heavy Chains; D007147:Immunoglobulin Light Chains; D021122:Protein Subunits; D011768:Pyruvate Dehydrogenase Complex