Antigenic phenotypes common to rat oval cells, primary hepatocellular carcinomas and developing bile ducts

The shared expression of monoclonal antibody-defined antigens by oval cells and by bile ducts, neoplastic nodules and primary hepatocellular carcinomas (PHC) has provided support for the ability of oval cells to undergo differentiation along ductular or hepatocyte lineages and/or to progress to hepatocellular carcinoma. With the aim of obtaining additional insight into this process, we have combined serial section and double labeling immunofluorescence analysis to determine if phenotypes expressed in vitro by four rat oval cell lines and the H5D.61 hepatocellular carcinoma cell line and in situ by ethionine-induced primary hepatocellular carcinomas reproduce antigenic patterns occurring during normal liver development. Analysis using monoclonal antibodies specific for the oval cell antigens OV6 and OC.2 and hepatocyte markers HBD.1 and H.4 defined subpopulations in four oval cell lines and neoplastic hepatocytes in PHC and H5D.61 with OC.2-/OV6+ and OC.2+/OV6+ phenotypes. Cells with an OC2+/OV6- phenotype were rarely observed in cell lines or primary tumors. In contrast, areas composed of OV6+/H.4+ cells were frequently found in PHC. Examination of fetal and neonatal rat livers demonstrated the stage-specific appearance of three of these phenotypes during liver development. The OC.2+/OV6- phenotype appeared transiently prior to embryonic day (ED) 18 in a subpopulation of HBD.1+ hepatoblasts. OV6 expression was first detected at ED18 on developing bile ducts that were negative for OC.2. These newly formed ducts rapidly acquired OC.2, starting with ducts in the hilar region and spreading outward towards the periphery. This OC.2 expression gradient persisted in the newborn rat liver but became more skewed towards doubly positive cells, with OC.2-/OV6+ cells being found primarily in the periphery. Hepatocytes expressing both OV6 and H.4 were not observed in fetal liver but appeared in neonatal liver in close proximity to OV6+ interlobular ducts. From these findings, it was concluded that oval cells and PHC display phenotypes representing normal stages in liver development, suggesting that oval cells and cells within ethionine-induced PHC are capable of initiating but are unable to complete pathways of hepatocytic or biliary differentiation.