Characterization of a lipoyl domain-independent B-cell autoepitope on the human branched-chain acyltransferase in primary biliary cirrhosis and overlap syndrome with autoimmune hepatitis.
BACKGROUND AND AIMS: Antimitochondrial antibodies (AMA) which recognize pyruvate acetyltransferase (PDC-E2) represent a highly diagnostic feature of primary biliary cirrhosis ( PBC). The analysis of immunofluorescence (IF)-AMA-positive sera in PBC patients indicates a conformational epitope located within the lipoyl binding domain of bovine branched-chain acyltransferase (BCKADC-E2) alone or in combination with AMA directed against PDC-E2 the significance of which is presently unclear. In the present study, immunoreactivities and disease associations of AMA against BCKADC-E2 were analyzed. B-cell autoepitopes on BCKADC-E2 were mapped by immunoprecipitation assay. METHODS: Sera of 96 IF-AMA-positive patients with serological evidence of anti-BCKADC-E2 alone (n = 26), anti-PDC-E2 alone (n = 15), and both anti-BCKADC-E2 and anti-PDC-E2 (n = 55) were analyzed by Western blot and ELISA in addition to an analysis of B cell autoepitopes on BCKADC-E2 by immunoprecipitation using in vitro translated, unmodified human proteins. Ninety-four patients without IF-AMA [blood donors (n = 30), rheumatoid arthritis (n = 40), autoimmune hepatitis (AIH)(n = 10) and primary sclerosing cholangitis (PSC) (n = 14) served as controls. RESULTS: Eighty of 81 (99%) sera positive for BCKADC-E2 recognized the full length, mature protein, while only 2/10 AIH sera and none of the other controls showed reactivity. Of the 68 PBC sera 58 (85%) recognized the N-terminus consisting of aa 1-144 representing the lipoyl domain. Surprisingly, C-terminal sequences (aa 143-421) were recognized by 46 out of 68 sera (68%). Three PBC sera reacted with the C-terminus only. Only 1/7 serum from patients with an "overlap syndrome of PBC and AIH" was reactive with C-terminal sequences. CONCLUSIONS: Our analysis of BCKADC-E2-positive PBC sera identified a novel B cell epitope on the C-terminal part of the human protein. Our data indicate that a distinct subset of AMA recognize sequence(s) on BCKADC-E2 which located outside of the lipoyl binding domain. The absence of immunoreactivity against C-terminal sequences may serve as a marker differentiating patients with PBC and overlap syndrome of PBC with AIH.[1]References
- Characterization of a lipoyl domain-independent B-cell autoepitope on the human branched-chain acyltransferase in primary biliary cirrhosis and overlap syndrome with autoimmune hepatitis. Csepregi, A., Obermayer-Straub, P., Kneip, S., Kayser, A., Loges, S., Schmidt, E., Nemesánszky, E., Szalay, F., Manns, M.P., Strassburg, C.P. Clin. Dev. Immunol. (2003) [Pubmed]
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