Stable HBV surface antigen expression by Vero cell clones after transfection

After transfection of Vero cells with the hepatitis B surface (HBs) antigen gene and the dominant selective marker amino-glycoside 3' phosphotransferase (APH3'), a late transient HBs expression is observed during the first weeks after transfection. We isolated 300 G418-resistant cell clones transformed by plasmids carrying the two genes. Only 6% of them were found to express the HBs antigen stably (versus 65% in murine cells). The HBs antigen expression is maximal after cells have reached confluency, and this level of antigen expression remains stable for months when cells are fed twice weekly without trypsinization. One clone - GAR 1412 - has been studied in greater detail. The cells (10(6] are not tumorigenic for nude mice, and the excreted HBs antigen, which is under the form 22 nm spherical particles, is immunogenic in Balb/C mice.