High level expression of hepatitis B virus surface antigen in stably transfected Drosophila Schneider-2 cells

Two transfer vector systems have been constructed for the generation of Drosophila melanogaster Schneider-2 (DS-2) cells transfected stably and used to express the small surface antigen of hepatitis B virus (HBsAg). One system is based on the cotransfection of an expression vector for the S gene under the control of an inducible Drosophila metallothionein (Mtn) promotor and a resistance plasmid which carries a selectable marker dihydrofolate reductase (dhfr) gene under the control of a Drosophila actin 5C distal promoter. The second system is based on the transfection of a single plasmid, which includes both expression units. Both vector systems were suitable for the generation of stably transfected DS-2 cell-lines secreting high levels of HBsAg. The quantities of HBsAg expression from polyclonal DS-2 cells correlated strictly with the concentration of the transfected S gene expression vector. Clonal cell-lines selected from the most efficient HBsAg producing polyclonal cell-populations were examined in more detail. All of the transfected S genes were found to be integrated and the copy numbers per genome varied extremely between 10 and 240. Furthermore, the levels of secreted HBsAg varied greatly between different clones and in best they reached up to 7 microg/ml under serum-free cell culture conditions. Thus, DS-2 cells transfected stably provide an alternative source for the production of HBsAg particles for diagnostic purposes and vaccine development.