Direct expression of hepatitis B surface antigen in monkey cells from an SV40 vector

We have developed an SV40-based vector that can be used for the efficient direct expression of foreign genes in permissive monkey cells. The vector lacks the coding sequences for the major SV40 late protein (VP-1) and possesses unique Eco RI and Bam HI restriction sites so that DNA fragments containing the coding sequences of foreign genes bounded by these two restriction sites can be conveniently inserted and directly expressed under the control of the VP-1 transcriptional unit. We have inserted into this vector the gene encoding the surface antigen of hepatitis B virus and observed the synthesis of this protein in monkey cells infected with this vector at a level comparable with that of VP-1. Furthermore, we have shown that the HBsAg synthesized is assembled into and secreted as a complex structure (22-nm particle) indistinguishable from that formed naturally during human infection. These observations enable us to conclude that HBsAg is the only component encoded by HBV that is required for the secretion and assembly of the 22-nm particle and that this process can occur without the involvement of a potential signal peptide suggested by the DNA sequence which precedes the coding region of mature HBsAg.