Several reports exist on delivery of naked DNA into mammalian cells that could induce protein expression within. Thus it is no wonder that vaccination could be experimented with the delivery of heterologous DNA into mammalian cells. One form of DNA vaccination is the use of plasmid DNA to immunise against an array of diseases.
Basic components of a DNA vaccine vector
The DNA vaccines are usually bacterial plasmid vectors that contain the trans gene (the gene from another source of organism that is to be inserted into the host) that is controlled by an eukaryotic promoter. The promoter regulates the expression of the trans gene in mammalian cells.
The choice of vector is very crucial in determining the efficiency of the DNA vaccine. Basically the plasmid vectors is required to have a eukaryotic promoter, a cloning site for insertion of the heterologous gene, a polyadenylation sequence, selectable marker and a bacterial origin of replication. Efficient expression of the gene in mammalian cells may require a strong promoter. Some of the most widely used strong promoters include cytomegalovirus (CMV), simian virus 40 (SV40) promoters. A downstream cloning site is also found for insertion of the gene of interest and to ensure that it lies adjacent to the promoter. The gene to be inserted usually accompanies a polyadenylation site for mRNA stability. Selectable markers usually include antibiotic resistant genes for screening and selection purposes. Last but not the least an Escherichia Coli ColE1 origin of replication for amplification of the plasmid containing the gene of interest within the bacteria. ColE1 ori region provides a high copy number thus having high yields of plasmid DNA during purification.
Several modifications could be made to this very basic construct and used for efficient gene delivery and expression.