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Frequency of IgG and IgM autoantibodies to four specific M2 mitochondrial autoantigens in primary biliary cirrhosis

We have previously identified four of the M2 antigens in primary biliary cirrhosis as the E2 components (dihydrolipoamide acyltransferases) of pyruvate dehy‐drogenase complex, branched‐chain 2‐oxo acid dehy‐drogenase complex and 2‐oxoglutarate dehydrogenase complex and the protein X component of pyruvate dehydrogenase complex (approximate molecular masses: 74, 50, 50 and 52 kD, respectively). In the present study, we have examined by immunoblotting the frequency of IgG and IgM autoantibodies to these four proteins in 129 patients with primary biliary cirrhosis (36 histological Stage I, 42 Stage II/III, 51 Stage IV) and 77 controls (49 non–primary biliary cirrhosis chronic liver disease, 16 primary Sjögren's syndrome, 12 healthy normal women). One hundred twenty‐seven of 129 (98%) primary biliary cirrhosis patients had antibodies against at least one of the four M2 polypeptides, compared to 2/77 controls (both had autoimmune chronic active hepatitis and were antimitochondrial antibody positive by indirect immunofluorescence). One hundred twenty‐one of 129 (94%) primary biliary cirrhosis sera reacted with the E2 component and protein X of pyruvate dehydrogenase complex, 69/129 (53%) primary biliary cirrhosis sera reacted with E2 of branched‐chain 2‐oxo acid dehydrogenase complex and 113/129 (88%) reacted with E2 of 2‐oxoglutarate dehydrogenase complex. Primary biliary cirrhosis patients with histological Stage I disease had a lower incidence of autoantibodies to each M2 protein, compared to more advanced disease (IgG, p < 0.05) but only 2/36 Stage I patients had no anti‐M2 antibodies. There was no correlation between the presence of IgG or IgM antibodies to the M2 polypeptides and established prognostic markers in primary biliary cirrhosis (serum bilirubin and albumin levels). The specificity of autoantibodies to the E2 components of all three 2‐oxo acid dehydrogenase complexes for primary biliary cirrhosis suggests that a common mechanism or immunological defect underlies the development of this autoimmune disease.