Immunogenetic analysis reveals that epitope shifting occurs during B-cell affinity maturation in primary biliary cirrhosis

• Published in: Journal of molecular biology 2001-02, Vol.306 (1), p.37-46
• Main Authors: Potter, Kathleen N, Thomson, Richard K, Hamblin, Angela, Richards, Susan D, Lindsay, J.Gordon, Stevenson, Freda K
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Antibodies, Monoclonal - chemistry; Antibodies, Monoclonal - genetics; Antibodies, Monoclonal - immunology; Antibody Specificity - immunology; autoantibodies; autoimmune disease; B-Lymphocytes - cytology; B-Lymphocytes - immunology; B-Lymphocytes - metabolism; Cell Differentiation - genetics; Complementarity Determining Regions - genetics; Enzyme-Linked Immunosorbent Assay; epitope spreading; Epitopes - genetics; Epitopes - immunology; Gene Rearrangement, B-Lymphocyte - genetics; Gene Rearrangement, B-Lymphocyte, Heavy Chain - genetics; Genes, Immunoglobulin - genetics; germline revertant; Humans; Immunoglobulin Fab Fragments - chemistry; Immunoglobulin Fab Fragments - genetics; Immunoglobulin Fab Fragments - immunology; Immunoglobulin Heavy Chains - chemistry; Immunoglobulin Heavy Chains - genetics; Immunoglobulin Heavy Chains - immunology; Immunoglobulin Light Chains - chemistry; Immunoglobulin Light Chains - genetics; Immunoglobulin Light Chains - immunology; Liver Cirrhosis, Biliary - genetics; Liver Cirrhosis, Biliary - immunology; Lymphocyte Activation - genetics; Molecular Sequence Data; Mutation - genetics; Protein Structure, Tertiary; Protein Subunits; Pyruvate Dehydrogenase Complex - chemistry; Pyruvate Dehydrogenase Complex - immunology; Sequence Alignment; V-genes
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1006/jmbi.2000.4210

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 VH-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 VH and VLgermline 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 naı̈ve 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.