Comparative immunoreactivity of anti-trifluoroacetyl (TFA) antibody and anti-lipoic acid antibody in primary biliary cirrhosis: searching for a mimic

Abstract

Previous studies documenting the existence of cross-reactivity between the lipoated (but not unlipoated) forms of the inner lipoyl domain (E2L2) of PDC-E2 [the major autoantigen in Primary biliary cirrhosis (PBC)] and trifluoroacetylated (TFA) proteins, led us to hypothesize that PBC may be due to an initial insult with an environmental agent that cross-reacts with TFA. Therefore, we performed a comparative study of the reactivity of rabbit anti-TFA antibody and anti-lipoic acid (LA) antibody against the mitochondrial autoantigens of human PBC and various TFA and LA conjugated proteins. Whereas both anti-TFA and anti-LA reacted with PDC-E2, the wild-type lipoated form of E2L2, OGDC-E2, E3-BP and LA-KLH, neither reacted with BCOADC-E2 or the non-lipoated form of E2L2. Of interest was that while anti-TFA reacted with PDC-E2, TFA-RSA and LA-KLH, it failed to inhibit PDC-E2 enzyme function. In contrast, anti-LA demonstrated cytoplasmic and mitochondrial staining, and inhibited PDC enzyme activity. Hence, although considerable cross reactivity exists between anti-TFA and anti-LA, the molecular nature of the interaction is clearly different. One of 14 PBC sera reacted weakly with TFA-albumin, whereas four of 14 PBC sera reacted with LA-KLH. Immunohistochemically, both anti-TFA and anti-LA antibodies reacted focally with periportal hepatocytes and bile ducts in both PBC and controls. However, anti-LA produced much stronger focalized staining of the bile ducts of diseased liver. This study suggests that while anti-TFA antibody recognizes lipoic acid-linked enzymes and proteins, the epitope recognized differs from that of anti-LA antibody and PBC autoantibodies. It is unlikely that a response to TFA is the triggering event in PBC. Anti-LA antibodies share a higher degree of similarity to PBC sera providing suggestive evidence that anti-LA antibodies or anti-LA like antibodies (mimotopes) may help define the initiator of the autoimmune response.