DNA vaccines are circular, double-stranded plasmid DNA (pDNA) molecules, which are capable of initiating the expression of protein antigens of interest when introduced into cells. For this purpose, the pDNA contains an eukaryotic expression cassette consisting of a transcriptional promoter, a protein coding sequence derived from the target antigen gene, and a transcriptional terminator (Fig. 73.1). Although many different promoters have been investigated, none have been shown to be clearly superior to the constitutive CMV IE promoter. DNA vaccines can consist of single genes on one plasmid, multiple genes on one plasmid, multiple plasmids, or a combination of the above. In biscistronic or tricistronic constructs, internal ribosomal entry sites (IRES) or equivalent sequences, dual or triple promoters, or cleavable linkage regions in fusion proteins can be used for expression of multiple genes. Upon transfer into cells, the pDNA enters the nucleus and transcribes a messenger RNA (mRNA) encoding the antigen of interest. The antigen can be identical to the wild-type protein of the pathogen, or can be genetically modified to improve immunogenicity and/or reduce toxicity to the host. The pDNA may also contain an antibiotic resistance gene and a bacterial origin of replication for growth and propagation in E. coli. Constructs using selection elements for bacterial replication other than antibiotic elements have also been utilized.
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