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Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease associated with autoimmune disorders. The aetiology is unknown, although it has been suggested that the disease may be related to infectious agents. Previous studies revealed that sera from patients with PBC react against Mycobacterium gordonae. This specific reactivity, characterized by a recognition of two membrane polypeptides of 70-65 and 55 kD, cross-react with the two major mitochondrial autoantigens of PBC. As the most immunogenic components of mycobacteria are the heat shock proteins (hsp), which have been associated with autoimmunity, this study has been undertaken to characterize whether the reacting polypeptides in PBC are hsp from M. gordonae. Cultures of M. gordonae were incubated at 37 degrees C and 46 degrees C before sonication, protein extraction and separation by SDS-PAGE. Exposure of M. gordonae to heat shock treatment resulted in membrane protein overexpression, similar to the 70-65-kD polypeptide recognized by the sera from patients with PBC. Immunoprecipitation assays with a monoclonal antibody directed against the Hsp65 kD of mycobacteria and with sera from patients with PBC revealed similar reacting profiles characterized by the precipitation of the overexpressed 65-kD polypeptide from M. gordonae. Competitive immunoblotting showed that binding of the monoclonal antibody to the Hsp65 kD protein was prevented by preincubation with sera from patients with PBC, but not with sera from healthy subjects. Furthermore, monoclonal antibody to the Hsp65 kD protein recognized the main mitochondrial autoantigens of PBC (PDH-E2 and BCKDH-E2). These data indicate the existence of cross-reacting epitopes contained on M. gordonae Hsp65 kD and the main mitochondrial antigens in patients with PBC.