Differing epitope selection of experimentally-induced and natural antibodies to a disease-specific autoantigen, the E2 subunit of pyruvate dehydrogenase complex (PDC-E2)

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Abstract
Naturally-occurring autoantibodies to a family of mltochondrlal enzymes, the 2-oxoacld dehydrogenase complexes (2-OADC), characterize the human liver disease primary biliary cirrhosis. The Immunodominant epitope for these autoantibodies is associated with the llpoyi-binding domain of the E2 subunit of the enzymes. The reactivity of these disease-associated autoantibodies was compared with that of antibodies raised in rats and rabbits, by Immunization with various preparations derived from the 2-OADC enzymes, using Immunization protocols that have successfully Induced various organ-specific autoimmune diseases in animals. The immunogens Included the intact pyruvate dehydrogenase complex (PDC) from bovine heart, human recombinant PDC-E2, and short synthetic peptides representing the Immunodominant lipoic acid binding sequences of the 2-OADC enzymes. The techniques for antibody analysis Included Immunofluorescence, Immunoblotting on mitochondrial extracts, ELISAs using entire PDC, PDC-E2, or synthetic peptides, epitope mapping by peptide scanning on overlapping octameric peptides representing the human PDC-E2 sequence, affinity purification on PDC-E2, and Inhibition in vitro by sera of the catalytic function of PDC. Experimental Immunization did not elicit any evidence of autoimmune disease. Moreover, the experimentally-Induced antibodies in striking contrast to the natural autoantibodies showed preferential reactivity with PDC-E2 rather than with intact PDC, failed to Inhibit in vitro the catalytic function of PDC, and, on peptide scanning, reacted with discrete epitopes, but at sites other than the llpoyl-binding region of PDC-E2. Our data indicate that ‘multisystem’ autoimmune diseases Including primary biliary cirrhosis may not be ellcitable experimentally because a critical disease-relevant autoepltope is not engaged by the immune system.

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