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.