The ORT-VAC system involves the use of modified bacterial strains in which an essential bacterial gene, dapD, has been engineered
to be under the control of the lac operator. In the absence of an inducer such as lactose, the LacI repressor protein binds
to the operator site, blocking transcription of dapD. Lack of the dapD protein eventually causes cell death. However, if these
bacterial cells are transformed with a high copy number plasmid containing the lac operator and for the development of vaccines,
the antigen of interest, the lac operator sequences in the plasmids titrate LacI away from the chromosomal lac operator. The
presence of plasmid thus enables dapD to be expressed again, so cells can survive for as long as plasmids are maintained (see
The strains that have been most extensively evaluated in preclinical and early phase clinical studies are attenuated mutants
of Salmonella enterica serovars Typhi and Typhimurium.3 Researchers have tested strains expressing in excess of 50 different bacterial, viral, and protozoan antigens. Cobra used
a plasmid expressing the F1 antigen of Yersinia pestis to convey immunity against bubonic plague in mice in an ORT-VAC Salmonella Typhimurium strain, in conjunction with the Defence Science and Technology Laboratory.12 This provided single-dose protection against a challenge with Y. pestis. The ORT-VAC strain was also able to maintain a high copy number plasmid that was rapidly lost from a control strain. This
demonstrates the potential of the plasmid system to provide a method of increasing the vaccine dose, and therefore, creating
a more potent vaccine.
The Demise of the Needle?
The benefits of needle-free delivery of vaccines are clear. Not only are the public health problems from use of sharps reduced,
but vaccination programs that make use of these new systems are also likely to be easier to implement, and thus be more effective.
Much effort is focused on developing novel mechanisms for delivering vaccines in both solid and liquid form through the skin.
However, interest is also growing in oral vaccine technologies. In particular, using live bacteria to deliver vaccines orally
has a number of added advantages, including the relatively simple downstream processing and the increase in immunogenicity
afforded by the vector doubling up as an adjuvant. To take this approach forward, new technologies have been developed to
overcome maintenance issues of plasmids and studies with a range of bacterial species and antigens have already shown promising
results. There are still hurdles to overcome, such as maintaining the balance between strain attenuation and efficacy, and
dealing with the issue of immunity against the vector itself in subsequent vaccinations. Nonetheless, perhaps the end of the
needle is nigh?
ROCKY CRANENBURGH, PHD, is head of research at Cobra BioManufacturing plc, Keele, UK, +44 0 1782 714181, email@example.com
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Opin Mol Ther. 2005;7(1):62–72.
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spp. For human vaccination against heterologous pathogens. FEMS Immunol Med Microbiol. 2000;27:35–364.
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system for heterologous antigens. J Drug Target. 2003;11(8–10):471–479.