PHENOTYPIC HETEROGENEITY WITHIN CLONOGENIC DUCTAL CELL-POPULATIONS ISOLATED FROM NORMAL ADULT-RAT LIVER

Oval cells represent a heterogeneous population composed of ductal cel ls, transitional cells with characteristics of both hepatocytes and bi le ductal cells, and bipotential stem cells capable of differentiation along a biliary or hepatocytic lineage. In an attempt to define marke rs that would distinguish between individual cell types within the ova l cell population, a number of investigators have utilized hybridoma t echnology to produce cell type-specific monoclonal antibodies. Several of these have proved to be of value in delineating lineage relationsh ips during fetal development and carcinogenesis in the adult liver. Mo st recently, monoclonal antibodies specific for OC2 and OC3, two oval cell antigens identified in our laboratory, have been used in combinat ion with magnetic beads or a fluorescence-activated cell sorter to iso late antigenically defined subpopulations from adult and fetal rat liv er. Using OC2-positive fetal liver cells as an immunogen, we have prod uced a monoclonal antibody identifying a bile ductal antigen, designat ed BD1, that is differentially expressed by oval cells and normal duct al cells. This antigen shows a heterogeneous pattern of reactivity tha t defines three distinct cell populations in regenerating rate liver: a BD1-negative, [H-3]thymidine-positive population thought to contain hepatic stem cells; a BD1-positive, [H-3]thymidine-positive population of mature ductal cells. Analysis of BD1 expression in vitro on contin ous lines of bile duct epithelial cells (BDEC) demonstrated BD1 was ra pidly increased in late G(1) and lost during G(2)/M. High passage cult ures of BDEC and primary cultures of oval cells expressed low or undet ectable levels of BD1 and high passage BDEC failed to express BD1 when arrested in late G(1). Taken together, these results suggested that o val cells and high passage BDEC might share a subtle defect in cell cy cle regulation marked by an inability to upregulate the expression of BD1.