Demonstration of PDC-E1 subunits as major antigens in the complement-fixing fraction M4 and re-evaluation of PDC-E1-specific antibodies in PBC patients.
BACKGROUND
Primary biliary cirrhosis (PBC) is characterized by the presence of antimitochondrial antibodies (AMA). Autoantibodies specific for the mitochondrial M4 antigen can be detected by a complement fixation test (CFT) but not by immunoblotting. The aim of this study was to elucidate the identity of the M4 antigen.


METHODS
M4 proteins were purified by affinity chromatography using IgG fractions of PBC marker sera being CFT positive (n=5) or negative (n=5) and identified by Western blotting, silver staining and sequence analysis. Further, a cohort of 57 PBC patients was tested for the reactivity to M4 and pyruvate dehydrogenase complex (PDC).


RESULTS
Two AMA patterns of the marker sera were visualized: CFT-positive sera were defined as PDC-E2(+)/E1(+) and the CFT-negative sera as PDC-E2(+)/E1(-). The major proteins in the M4 fraction could be related to the PDC-E1 subunits. A clear-cut association between anti-M4 reactivity in the CFT and the reactivity to both PDC subunits could also be documented in the cohort of 57 PBC patients showing anti-PDC-E1alpha and E1beta antibodies at a frequency of 74% and 67%.


CONCLUSIONS
CFT reactivity against M4 antigens could be preferentially identified as a reaction against PDC-E1. As PDC-E1 subunits as compared with PDC-E2 lack lipoyl-binding sites, they probably have to be considered as an independent and important target.
Berg
Christoph P
CP
Department of Internal Medicine I, Medical Clinic, University of Tübingen, Tübingen, Germany. peter.a.berg@uni-tuebingen.de
Stein
Gerburg M
GM
Klein
Reinhild
R
Pascu
Maria
M
Berg
Thomas
T
Kammer
Winfried
W
Priemer
Martin
M
Nordheim
Alfred
A
Schulze-Osthoff
Klaus
K
Gregor
Michael
M
Wesselborg
Sebastian
S
Berg
Peter A
PA
D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001323:Autoantibodies / Q000097:blood / Q000276:immunology; D001324:Autoantigens / Q000276:immunology*; D015331:Cohort Studies; D003168:Complement Fixation Tests; D005260:Female; D006801:Humans; D016056:Immunodominant Epitopes / Q000276:immunology; D008105:Liver Cirrhosis, Biliary / Q000097:blood / Q000276:immunology*; D008297:Male; D008875:Middle Aged; D008928:Mitochondria / Q000276:immunology; D030481:Pyruvate Dehydrogenase (Lipoamide) / Q000276:immunology*; D011768:Pyruvate Dehydrogenase Complex / Q000276:immunology; D051381:Rats
D001323:Autoantibodies; D001324:Autoantigens; D016056:Immunodominant Epitopes; D011768:Pyruvate Dehydrogenase Complex; D030481:Pyruvate Dehydrogenase (Lipoamide)