Use of ATPase-associated antigen (M2) for detection of antimitochondrial antibodies in primary biliary cirrhosis by fluorometric immunoassay.

An indirect binding assay, the fluorometric immunoassay (FIAX), was established for the detection of anti-M2 antibodies which are specific markers for primary biliary cirrhosis (PBC). Submitochondrial particles (SMP) from beef heart and rat liver and the ATPase-associated antigen (M2) were used. The antigens were fixed to a cellulose acetate surface, SMP at a concentration of 2 mg/ml, ATPase at a concentration of 0.2 mg/ml. Sera were used at 1:60 and 1:120 and bound antimitochondrial antibodies (AMA) were demonstrated by fluorescent isothiocyanate labelled monospecific anti-human IgG, IgM and IgA antibodies. The fluorescent signals were proportional to the AMA titre in the serum samples and were measured in a fluorometer (FIAX 100). Of 94 patients with PBC, 92 had AMA against SMP from beef heart compared with 76 in the complement fixation test (CFT) and 84 in the immunofluorescence test (IFL). Ninety reacted with the ATPase-associated M2 antigen. Sera from patients known to have AMA of different specificities (anti-M1, anti-M3, anti-M5, anti-M6) reacted with SMP from beef heart and/or rat liver but not with M2.