Phenotypic heterogeneity within clonogenic ductal cell populations isolated from normal adult rat liver

Oval cells represent a heterogeneous population composed of ductal cells, transitional cells with characteristics of both hepatocytes and bile ductal cells, and bipotential stem cells capable of differentiation along a biliary or hepatocytic lineage. In an attempt to define markers that would distinguish between individual cell types within the oval cell population, a number of investigators have utilized hybridoma technology to produce cell type-specific monoclonal antibodies. Several of these have proved to be of value in delineating lineage relationships during fetal development and carcinogenesis in the adult liver. Most recently, monoclonal antibodies specific for OC2 and OC3, two oval cell antigens identified in our laboratory, have been used in combination with magnetic beads or a fluorescence-activated cell sorter to isolate antigenically defined subpopulations from adult and fetal rat liver. Using OC2-positive fetal liver cells as an immunogen, we have produced a monoclonal antibody identifying a bile ductal antigen, designated BD1, that is differentially expressed by oval cells and normal ductal cells. This antigen shows a heterogeneous pattern of reactivity that defines three distinct cell populations in regenerating rat liver: a BD1-negative, [3H]thymidine-labeled cell population thought to contain hepatic stem cells; a BD1-positive, thymidine-negative population of terminally differentiated ductal cells; and a BD1-positive, [3H]thymidine-positive population of mature ductal cells. Analysis of BD1 expression in vitro on continuous lines of bile duct epithelial cells (BDEC) demonstrated that BD1 was rapidly increased in late G1 and lost during G2/M. High passage cultures of BDEC and primary cultures of oval cells expressed low or undetectable levels of BD1 and high passage BDEC failed to express BD1 when arrested in late G1. Taken together, these results suggested that oval cells and high passage BDEC might share a subtle defect in cell cycle regulation marked by an inability to upregulate the expression of BD1.