Expression of hepatitis B virus surface antigen containing the pre-S region in mammalian cell culture system

The surface antigen of hepatitis B virus (HBV) has been expressed in a mouse cell line using metallothionein-bovine papilloma virus vectors. Four different recombinant plasmids were used and it was found that the expression level varies significantly from one plasmid to another. Removing the HBV polyadenylation signal from the plasmid drastically reduced the expression level. Providing even a heterologous polyadenylation signal improved the expression level from the reduced one by at least tenfold. The steady-state level of cytoplasmic HBV-specific RNA was much higher when the polyadenylation signal was present. One of the constructs gave a very high expression level and has been characterized further. It was possible to detect any plasmid in the episomal form in the producing clones and all the HBV DNA was found to be integrated in the mouse chromosome at more than one site. The copy number of the plasmid varied greatly between different clones and in the best one it is approximately 150 copies per haploid genome. Cells can be grown continuously in the presence of cadmium chloride without splitting for more than two months and the excreted surface antigen can be harvested during this period. Western blot analysis showed that the antigen contains the pre-S region.