Heterogeneous response of antimitochondrial autoantibodies and bile duct apical staining monoclonal antibodies to pyruvate dehydrogenase complex E2: The molecule versus the mimic

The 2-oxo-acid dehydrogenase complexes and, in particular, the E2 component of the pyruvate dehydrogenase complex (PDC) are the target of antimitochon drial antibodies (AMA), More than 95% of primary biliary cirrhosis (PBC) pa tients have detectable levels of autoantibodies to PDC-E2 and in general th ese react with a region of the molecule that contains the prosthetic group lipoic acid (LA). LA is vital to the function of the enzyme, although there is conflicting evidence as to whether its presence is required for PDC-E2 recognition by AMA. Some, but not all, monoclonal antibodies (mAbs) to PDC- E2 produce an intense staining pattern at the apical surface of bile duct e pithelial cells (BEC) in patients with PBC, and it has been argued that the molecule at the apical surface of PBC bile duct cells is a modified form o f PDC-E2 or a cross-reactive molecule, acting as a molecular mimic. Herein, we characterize the epitopes recognized by 4 anti-PDC-E2. mAbs that give a pical staining patterns (3 mouse and 1 human), in particular, by using a co mbination of recombinant antigens, competitive inhibition assays, and a uni que peptide-on-bead assay, we determined that these apically staining mAbs recognize 3 or 4 distinct epitopes on PDC-E2, More importantly, this sugges ts that a portion spanning the entire inner lipoyl domain of PDC-E2 can be found at the BEC apical surface. In addition, competition assays with patie nt sera and a PDC-E2-specific mAb showed significant epitope overlap with o nly 1 of the 3 mouse mAbs and showed a differential response to the peptide bound to beads. These findings further highlight the heterogeneous respons e of patient autoantibodies to the inner lipoyl domain of PDC-E2.