Primary Biliary Cirrhosis: Indication for a Single Mitochondrial Antigenic Epitope Detected by Patient Autoantibodies and a Novel Monoclonal Antibody

A monoclonal antibody specific for the major primary biliary cirrhosis (PBC)‐associated mitochondrial antigen (subunit I of NADH‐ubiquinone reductase) was produced and used to study the binding sites recognized by anti‐mitochondrial autoantibodies (AMA) in PBC sera. Immunization of mice with purified beef heart mitochondrial inner membranes resulted in one monoclonal antibody which reacted with mitochondrial proteins. This antibody (PBC‐MoAb), which was of the IgG2b subclass with kappa light chains, exhibited a pattern of immunofluorescence reactivity with rat kidney, human thyroid, and cultured human epithelial cells (Hep‐2) similar to that obtained with sera from PBC patients. Similar binding patterns between PBC‐MoAb and AMA were also found in western blot analysis using mitochondria as antigen. Both types of antibodies revealed a major antigen of 75 kDa, a minor antigen of 60 kDa, and a third antigen (70 kDa), which was detected only in samples that had not been boiled prior to electrophoresis. Furthermore, optimal binding of the PBC‐MoAb and AMA to the 75 and 70 kDa antigens required reduction of the antigen with mercaptoethanol prior to electrophoresis. Competition ELISA experiments were conducted to compare the epitopes recognized by PBC‐MoAb and AMA. Of 28 PBC sera tested, 27 inhibited the binding of PBC‐MoAb to mitochondrial inner membranes by almost 100% and one serum inhibited binding by 50%, indicating that most PBC sera contain autoantibodies reactive with the same or a closely related antibody binding site as the PBC‐MoAb. PBC‐MoAb inhibited AMA binding to the inner membrane by more than 80% in 10 sera, 60–80% in 11 sera, and 40–59% in seven sera, with an average inhibition of 71%. Our observations strongly indicate that anti‐mitochondrial autoantibody binding sites are restricted to a highly immunogenic epitope on the major PBC‐specific antigen (NADH‐ubiquinone reductase subunit I), and that the anti‐mitochondrial monoclonal antibody obtained has a specificity identical with the human PBC‐specific M2 type anti‐mitochondrial autoantibody.