Background: A major mitochondrial autoantigen recognized by sera of patients with primary biliary cirrhosis (PBC) is dihydrolipoamide acetyltransferase (E2) of the pyruvate dehydrogenase complex (PDH). The α subunit of pyruvate decarboxylase (E1α) of PDH is also recognized in some E2-reactive PBC sera, suggesting that the occurrence of autoimmunity against E1α is subsequent to that against E2.
Methods: To investigate the mechanism inducing autoimmunity against E1α, we surveyed immunoreactive sequences of E1α by ELISA with synthesized oligopeptides, and determined minimum amino acid residues for each determinant.
Results: The major determinants of E1α appeared to reside in its N-terminal region, apparently forming ‘nested epitopes’, and all E1α-reactive PBC sera tested recognized these regions. Minor epitopes were also found scattered throughout the entire sequence. The reactivities of these minor epitopes to individual PBC sera were proportional to those of the major epitopes. All the epitopes were located in hydrophilic regions of E1α, and many of them were out of the known functional domains (TPP-binding domain, subunit interaction site, and phosphorylation sites) whose structures are phylogenically well conserved. Furthermore, the sequences of many epitopes appeared to be specific to humans.
Conclusion: These observations suggest that determinant spreading might underlie the autoimmunity against E1α.