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

Primary biliary cirrhosis (PBC) is a liver disease characterized by serum a utoantibodies against the pyruvate dehydrogenase complex (PDC) located in t he inner mitochondrial membrane. The predominant target in PDC has previous ly been localized to the inner lipoyl domain (ILD) of the E2 subunit. The e tiology of PBC is unknown, although molecular mimicry with bacterial PDC ha s 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 w as originally isolated from a patient with PBC, were expressed as Fab by ph age display, and tested for reactivity against recombinant domains of the E 2 subunit. Fab in which the V-H-encoded portions were reverted to germline lost reactivity against the ILD alone, but recognized a different epitope i n 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 th e human ILD and didomain, the Escherichia coli didomain, and whole PDC. We hypothesize that the IgM on the surface of the naive B-cell first recognize s an as yet unidentified antigen, and that accumulation of somatic mutation s results in an intermolecular epitope shift directed towards an epitope in volving the E1/E3 binding domain. Further mutations result in the specifici ty being redirected to the ILD. These findings also suggest that bacterial molecular mimicry is not involved in initiating disease. (C) 2001 Academic Press.