Immunization with a Xenobiotic 6-Bromohexanoate Bovine Serum Albumin Conjugate Induces Antimitochondrial Antibodies

Abstract

The E2 subunit of pyruvate dehydrogenase complex (PDC-E2) is the major autoantigen recognized by antimitochondrial Abs (AMA) in primary biliary cirrhosis (PBC). Recently, we replaced the lipoic acid moiety of PDC-E2 with a battery of synthetic structures designed to mimic a xenobiotically modified lipoyl hapten on a 12-aa peptide that was found within the immunodominant autoepitope of PDC-E2 and demonstrated that AMA in PBC reacted against several organic modified mimotopes as well as, or sometimes significantly better than, the native lipoyl domain. Based on this data, we immunized rabbits with one such xenobiotic organic compound, 6-bromohexanoate, coupled to BSA. One hundred percent of immunized rabbits developed AMA that have each and every characteristic of human AMAs with reactivity against PDC-E2, E2 subunit of branched chain 2-oxo-acid dehydrogenase, and E2 subunit of 2-oxoglutarate dehydrogenase complex. The rabbit AMA also inhibited enzymatic function of PDC-E2 and, importantly, binds to peptide sequences not present in the xenobiotic carrier immunogen. In contrast, BSA-immunized controls did not produce such activity. Our observation that animals immunized with a xenobiotic BSA complex produce autoantibodies that react not only with the xenobiotic, but also with mitochondrial autoantigens recognized by autoimmune PBC sera, suggests that environmental xenobiotic agents can be a risk factor for the induction of PBC.

Induction of Primary Biliary Cirrhosis in Guinea Pigs following Chemical Xenobiotic Immunization

“…Similarly, s.c. immunization of SJL/J mice with biotinylated murine PDC produced Ab and T cell reactivities to unmodified and modified murine PDC (25). Although mild inflammation in the portal tract was seen in immunized SJL/J mice, both the rabbit and the SJL mice model fall short of any evidence of the bile duct injury, cholestasis, or fibrosis that are characteristic of PBC livers (9). Therefore, it is likely that other factors such as architecture of the biliary system, inflammatory signals, and detoxification pathways are important in the development of PBC.…”

“…The breaking of tolerance to mitochondrial autoantigens can be accomplished by immunization of xenobiotic small molecules that are not of mitochondrial origin (9). Although it appears that AMA can be induced by either self or foreign 2-OADC as well as xenobiotic agents, the successful generation of PBC liver pathology may require a combination of physiological, genetic, and environmental factors.…”

“…A pyramid scheme consisting of immunizing rabbits with a progressively decreasing number of mixtures of the reactive xenobiotic was used to identify the optimal structure that would lead to breaking of immune tolerance to the native OADC mitochondrial proteins in vivo. This work led to the identification of a BSA conjugate of 6BH (6BH-BSA) as producing a xenobiotic agent capable of inducing of AMA (9). The chemical structure of 6BH and its hypothetical alignment with the DKA amino acid residues of the PDC-E2 lipoyl domain is shown in Fig.…”

“…Screening of these compounds for reactivity with AMA-positive PBC sera led to the identification of a number of organic chemicals that were recognized by AMA with higher relative affinity than that for parent lipoylated-PDC-E2 (3,4,8). Furthermore, immunization of rabbits with one of these xenobiotics, 6-bromohexanoate, conjugated to BSA (6BH-BSA) led to the induction of AMA with the same antigenic specificities and capacity to inhibit the activity of PDC enzymes that is displayed by sera from patients with PBC (9). These data suggest that the immune process in PBC can be triggered by exposure to PDC-E2 that had been modified by xenobiotics.…”

Antimitochondrial antibodies in acute liver failure: Implications for primary biliary cirrhosis

“…[4][5][6] Interestingly, although immunization of experimental animals with xenobiotic modified E2 subunits induces true self-reacting AMA, it does not appear to induce clinical liver disease. 7 Fulminant hepatic failure, also known as acute liver failure (ALF), is a clinical condition that reflects both oxidant stresses and hepatic damage. ALF is characterized by severe and sudden liver cell dysfunction leading to coagulopathy and hepatic encephalopathy in previously healthy persons with no known underlying liver disease.…”