Role of Lipoylation of the Immunodominant Epitope of Pyruvate Dehydrogenase Complex: Toward a Peptide-Based Diagnostic Assay for Primary Biliary Cirrhosis

Primary biliary cirrhosis is an immune-mediated chronic liver disease whose diagnosis relies on the detection of serum antimitochondrial antibodies directed against a complex set of proteins, among which pyruvate dehydrogenase complex is considered the main autoantigen. We studied the immunological role of the lipoyl domain of this protein using synthetic lipoylated peptides, showing that the lipoyl chain chirality does not affect autoantibody recognition and, most importantly, confirming that both lipoylated and unlipoylated peptides are able to recognize specific autoantibodies in patients sera. In fact, 74% of patients sera recognize at least one of the tested peptides but very few positive sera recognized exclusively the lipoylated peptide, suggesting that the lipoamide moiety plays a marginal role within the autoreactive epitope. These results are supported by a conformational analysis showing that the lipoyl moiety of pyruvate dehydrogenase complex appears to be involved in hydrophobic interactions, which may limit its exposition and thus its contribution to the complex antigenic epitope. A preliminary analysis of the specificity of the two most active peptides indicates that they could be part of a panel of synthetic antigens collectively able to mimic in a simple immunoenzymatic assay the complex positivity pattern detected in immunofluorescence.