Ultrastructural analysis of hepatitis B surface antigen production in vitro

Recently several continuous cell lines (among these PLC/PRF/5 cells) producing hepatitis B surface antigen (HBsAg) were established from human hepatocellular carcinomas. The cultured cells provide the first opportunity to study HBsAg synthesis and secretion in vitro. HBsAg, but not HBcAg, was localized by the fluorescent antibody technique in the cytoplasm and on the surface of the cultured cells. Under the electron microscope, th PLC/PRF/5 cells displayed morphologic characteristics of both hepatocytes and hepatocellular carcinoma cells. However, 22-nm. spherical or filamentous HBsAg particles were not seen in the cells, although spherical HBsAg particles were observed in the supernatant culture media. Therefore, the indirect immunoperoxidase technique was used to demonstrate HBsAg at the ultrastructural level. Electron-dense reaction product was detected along the nuclear envelope, on rough-surfaced endoplasmic reticulum, and in cisternae of endoplasmic reticulum. These findings suggest that HBsAg is synthesized on rough-surfaced endoplasmic reticulum and transferred into endoplasmic cisternae for processing and secretion. This mode of HBsAg production is identical with that observed in hepatocytes of patients infected with hepatitis B virus. The absence of detectable intracellular HBsAg particles suggests that the cultured cells secrete the particles very rapidly or that they may have a defect in intracisternal packaging of HBsAg into particles.