Biliary atresia: is lack of innate immune response tolerance key to pathogenesis?

Regulation of bile duct epithelial injury by hepatic CD71+ erythroid cells

Extramedullary hematopoietic cells are present in the liver of normal neonates in the first few days of life and persist in infants with biliary atresia. Based on a previous report that liver genes are enriched by erythroid pathways, we examined the liver gene expression pattern at diagnosis and found the top 5 enriched pathways are related to erythrocyte pathobiology in children who survived with the native liver beyond 2 years of age. Using immunostaining, anti-CD71 antibodies identified CD71+ erythroid cells among extramedullary hematopoietic cells in the livers at the time of diagnosis. In mechanistic experiments, the preemptive antibody depletion of hepatic CD71+ erythroid cells in neonatal mice rendered them resistant to rhesus rotavirus-induced (RRV-induced) biliary atresia. The depletion of CD71+ erythroid cells increased the number of effector lymphocytes and delayed the RRV infection of livers and extrahepatic bile ducts. In coculture experiments, CD71+ erythroid cells suppressed the activation of hepatic mononuclear cells. These data uncover an immunoregulatory role for CD71+ erythroid cells in the neonatal liver.

Autophagy and senescence in fibrosing cholangiopathies

Fibrosing cholangiopathy such as primary sclerosing cholangitis (PSC) and biliary atresia (BA) is characterized by biliary epithelial injuries and concentric fibrous obliteration of the biliary tree together with inflammatory cell infiltration. In these diseases, inappropriate innate immunity is reported to contribute more to bile duct pathology as compared with various aspects of "classical" autoimmune diseases. Primary biliary cirrhosis (PBC) is characterized by chronic cholangitis with bile duct loss and classical autoimmune features. Cellular senescence of cholangiocytes and a senescence-associated secretory phenotype lead to the production of proinflammatory cytokines and chemokines that may modify the milieu of the bile duct and then trigger fibroinflammatory responses in PSC and PBC. Furthermore, deregulated autophagy might be involved in cholangiocyte senescence and possibly in the autoimmune process in PBC, and the deregulated innate immunity against enteric microbes or their products that is associated with cholangiocyte senescence might result in the fibrosing cholangitis that develops in PBC and PSC. In BA, innate immunity against double-stranded RNA viruses might be involved in cholangiocyte apoptosis and also in the development of the epithelial-mesenchymal transition of cholangiocytes that results in fibrous obliteration of bile ducts. These recent advances in the understanding of immune-mediated biliary diseases represent a paradigm shift: the cholangiocyte is no longer viewed merely as a passive victim of injury; it is now also considered to function as a potential effector in bile duct pathology.

Clues to the etiology of bile duct injury in biliary atresia

Biliary atresia (BA) is an infantile obstructive cholangiopathy of unknown etiology with suboptimal therapy, which is responsible for 40 to 50% of all pediatric liver transplants. Although the etiology of bile duct injury in BA in unknown, it is postulated that a pre- or perinatal viral infection initiates cholangiocyte apoptosis and release of antigens that trigger a Th1 immune response that leads to further bile duct injury, inflammation, and obstructive fibrosis. Humoral immunity and activation of the innate immune system may also play key roles in this process. Moreover, recent investigations from the murine BA model and human data suggest that regulatory T cells and genetic susceptibility factors may orchestrate autoimmune mechanisms. What controls the coordination of these events, why the disease only occurs in the first few months of life, and why a minority of infants with perinatal viral infections develop BA are remaining questions to be answered.

Biliary atresia: cellular dynamics and immune dysregulation

The cause of biliary atresia is unknown; in the past few decades, the majority of investigations related to its pathogenesis have centered on viral infections and immunity. The acquired or perinatal form of biliary atresia entails a progressive inflammatory injury of bile ducts, leading to fibrosis and obliteration of both the extrahepatic and intrahepatic bile ducts. Theories of pathogenesis include viral infection, chronic inflammatory or autoimmune-mediated bile duct injury, and abnormalities in bile duct development. This review will focus solely on human studies pertaining to a potential viral trigger of bile duct injury at diagnosis and provide insight into the interplay of the innate and adaptive immune responses in the pathogenesis of disease.