More about the science
The strategy of using a live bug like Salmonella to stimulate a protective immune response has been around awhile. But such microbes have typically had to be weakened or attenuated before safe use, disabling some of their virulence in order to prevent a full-blown occurrence of disease in the vaccine recipient.
Salmonella turns out to be a superb choice as an antigen delivery system. Other infectious bacteria like Shigella, Vibrio cholera and pathogenic E. coli, all of which have been explored as vaccine candidates, only invade cells in the intestinal tract, failing to reach the liver and spleen, which are important workhorses for mounting an immune response.
Salmonella however, can spread throughout lymph tissues, spleen and liver, provoking system-wide immunity. Nevertheless, Salmonella vaccine strains produced through deletion mutations present many of the drawbacks of other attenuated forms, including reduced survival rate in the body’s inhospitable environment, and depressed virulence. Now, our team has led the development of two new vaccine candidates, labeled x9088 and x9558, under grants from the NIH and the Bill and Melinda Gates Foundation. These novel strains belong to a family known as recombinant attenuated Salmonella vaccines or RASVs. The critical component boosting their effectiveness is a delayed mechanism of attenuation. Salmonella’s notorious virulence is essentially short-circuited, but only after it has stimulated a robust systemic immune response to pneumococcal surface protein A (PspA), a vital bacterial pneumonia antigen.
This feat is accomplished through genetic trickery to tame S. typhimurium, producing altered bacterial strains requiring mannose and/or arabinose—sugars available in the lab, but absent in the human body. After roughly 7 cell divisions, the bacterium exhausts its stores of specialized sugar. Unable to sustain the integrity of its cell wall, it bursts. By this method, Salmonella can be placed on a self-destruct timer, one that may be sensitively tuned to achieve maximum immunogenicity following colonization of host tissues.
In comparison with attenuated Salmonella produced through deletion mutation, our RASV delayed attenuation strains provoked significantly greater anti-PspA immune response (measured in serum antibody levels) as well as conferring greater protection from Streptococcus pneumoniae infection. The safety aspect of self-destruct vaccines also makes them highly attractive.
Indeed, in critical proof-of-concept mouse studies, we have shown that a 20 percent higher protection rate can be achieved even in the presence of a 10-fold increase in the challenge dose of pneumonia pathogen. Vaccine strain x9088 displayed a heightened ability to colonize the liver and spleen. In x9558, recombinant manipulation was used to delay not only virulence attenuation but also the onset of PspA synthesis—kick-started only in vivo, in an arabinose-free environment. Ultimately, to produce the vaccine, we will need to add the antigens for all 91 variant strains of S. pneumoniae will need to be incorporated to provide comprehensive immune defense from the disease.
