Salmonella-mediated oral DNA vaccination using stabilized eukaryotic expression plasmids

The use of Salmonella for the delivery of plasmid-encoded heterologous antigens to eukaryotic host cells has proven successful in experimental systems, but its general applicability is still hampered by a severe instability of transformants carrying these expression plasmids. To overcome the problem of plasmid instability, new low copy number expression plasmids were constructed using different replicons. Comparative studies between transformants of the high copy number plasmid pCMVbeta and the different low copy number plasmids that contain the pMB1, p15A or pSC101 replicons on the pCMVbeta backbone, revealed a dramatic increase in plasmid stability both in vitro and in vivo. Analysis of the resulting immune responses against antigens encoded by these vectors indicated that the increased stability resulted in a strong and reproducible induction of both antigen-specific CD4(+) and CD8(+) T-cell and antibody responses even after a single application. In addition, protective immunity was induced against Listeria monocytogenes using listeriolysin as antigen, regardless of the copy number of the delivery plasmid employed. Finally, Salmonella expressing two independent antigens on compatible low copy number plasmids elicited robust responses to either antigen that is as effective as Salmonella transformed with each plasmid singly adding further versatility to this delivery system.