Antigenic relationship between oval cells and a subpopulation of hepatic foci, nodules, and carcinomas induced by the "resistant hepatocyte" model system

Although proliferation of small ductular-like cells, designated oval cells, is often observed during the early stages of chemically induced hepatocarcinogenesis, their role during the carcinogenic process remains controversial. To investigate the possibility that oval cells may give rise to preneoplastic lesions that ultimately progress to hepatocellular carcinomas, we have carried out phenotypic analysis with a panel of monoclonal antibodies to determine if there is an antigenic relationship between oval cells and hepatic foci, nodules, and tumors induced by the resistant hepatocyte model system. In this model, rats are given a single dose (200 mg/kg) of diethylnitrosamine, followed by a brief exposure to 2-acetylaminofluorene and a partial hepatectomy. We found that approximately 10% of the early focal lesions observed 28 days after diethylnitrosamine expressed either one or both of the oval cell antigens designated OC.2 and OV-6. By 28 weeks after diethylnitrosamine, 16 of 16 hepatic nodules heterogeneously expressed OV-6 whereas 5-10% of the persistent nodules contained scattered small hepatocyte-like cells that expressed OC.2. Examination of resistant hepatocyte-induced primary hepatocellular carcinomas with an expanded panel of monoclonal antibodies demonstrated that most cells comprising 29 of 29 tumors expressed OV-6 and that 15-20% of the OV-6-positive tumors contained subpopulations of cells also expressing 3 additional oval cell antigens, OC.2, OC.3, and OV-1. All of the tumors examined expressed normal levels of the hepatocyte antigens, H.1 and HBD.1, and had dramatically reduced levels of H.2, H.4, and cell CAM 105 but showed elevated levels of the transferrin receptor, gamma-glutamyltranspeptidase, and the normal hepatocyte antigen, H.5. In conclusion, our findings demonstrate an antigenic relationship between oval cells and a subpopulation of hepatic foci, nodules, and tumors in the resistant hepatocyte model, suggesting that at least some primary tumors may be derived from oval cells in this model system.