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.
OBJECTIVE
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.
Csepregi
Antal
A
Department of Gastroenterology, Hepatology and Endocrinology, Hannover Medical School, Carl-Neuberg-Street 1, 30625 Hannover, Germany.
Obermayer-Straub
Petra
P
Kneip
Susanne
S
Kayser
Anne
A
Loges
Stephanie
S
Schmidt
Eleonore
E
Nemesánszky
Elemér
E
Szalay
Ferenc
F
Manns
Michael P
MP
Strassburg
Christian P
CP
D000217:Acyltransferases / Q000737:chemistry / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D001323:Autoantibodies / Q000276:immunology*; D001324:Autoantigens / Q000276:immunology*; D001402:B-Lymphocytes / Q000276:immunology*; D018076:DNA, Complementary / Q000235:genetics; D018604:Epitope Mapping; D018985:Epitopes, B-Lymphocyte / Q000276:immunology*; D019693:Hepatitis, Autoimmune / Q000097:blood / Q000150:complications / Q000201:enzymology / Q000276:immunology*; D006801:Humans; D008105:Liver Cirrhosis, Biliary / Q000097:blood / Q000150:complications / Q000201:enzymology / Q000276:immunology*; D011869:Radioligand Assay; D012680:Sensitivity and Specificity
D001323:Autoantibodies; D001324:Autoantigens; D018076:DNA, Complementary; D018985:Epitopes, B-Lymphocyte; D000217:Acyltransferases; C033588:dihydrolipoamide acyltransferase