Current concepts in the pathogenesis of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an organ-specific autoimmune disease that predominantly affects women and is characterized by chronic, progressive destruction of small intrahepatic bile duct with portal inflammation and ultimately fibrosis, leading to liver failure in the absence of treatment. The serologic hallmark of PBC is the presence of autoantibodies to mitochondria, especially to the E2 component of the pyruvate dehydrogenase complex (PDC). Current theories on the pathogenesis of PBC favor the hypothesis that the disease develops as a result of an inappropriate immune response following stimulation by an environmental or infectious agent. Like other better characterized autoimmune diseases, there appears to be a genetic susceptibility and a triggering event that initiates the autoimmune attack on bile duct cells. DRB1*0801 and DRB1*0803 are the major susceptibility alleles among Northern European and Japanese populations, respectively. The generation of immune responsiveness to self-antigen can result in pathogenic autoimmune damage of the intrahepatic biliary epithelial cells mediated by both humoral and cellular immune responses. The pathogenetic mechanism is believed to be caused by a defect in immunologic tolerance, resulting in the activation and expansion of self-antigen specific T and B lymphocyte clones and the production of circulating autoantibodies in addition to a myriad of cytokines and other inflammatory mediators. Human and animal studies have suggested that the induction of an antibody response reactive with self-antigen may result from a number of different priming events. Among the events demonstrated to induce an antibody response cross-reactive with self-PDC are exposures to bacterial PDC or retroviral proteins or xenobiotics or microchimerism. The diversity of the potential events giving rise to antibody responses cross-reactive with PDC, which could promote subsequent T-cell tolerance breakdown, suggests the intriguing possibility that PBC could represent a condition with a common final pathway but with multiple triggers able to induce a B-cell response cross-reactive with self-PDC. There are important questions about the pathogenesis of PBC which remain unanswered.