U.S. researchers have hit on a new approach to developing a vaccine against serotype 3 Streptococcus pneumoniae based on generating an live attenuated version of the bacterium that carries a synthetic pneumolysin gene. The scientists claim that while the method they used for reducing gene expression has previously been applied to viral pathogens, their prototype S. pneumoniae vaccine is the first in which gene customization has been used successfully to control virulence in bacteria.
They hope their studies could lead to the development of pneumococcal vaccines based on weakened bacterial strains, and are already investigating whether the approach can be used to reduce the expression of other pneumococcal virulence genes.
The researchers were led by Liise-anne Pirofski, M.D., professor of medicine and of microbiology and immunology at Albert Einstein College of Medicine’s Division of Infectious Diseases, and colleague Jacque Mitrani, Ph.D., chair in biomedical research. Their results are published in the Journal of Infectious Diseases in a paper titled “Designed Reduction of Streptococcus pneumoniae Pathogenicity via Synthetic Changes in Virulence Factor Codon-pair Bias.”
The pediatric vaccine that has been used to protect against pneumococcal disease over the last decade does not protect against S. pneumoniae serotype 3, which has emerged as a cause of serious pneumonia in both children and adults, Dr. Pirofski notes. In order to generate a vaccine that might protect against serotype 3, the researchers engineered a live, attenuated bacterial strain that carried a synthetic version of the pneumolysin toxin gene they hoped would reduce the amount of toxin generated.
“Our idea was to design a live vaccine that would stimulate the immune system sufficiently to ward off disease but wouldn’t lead to the severely damaging inflammatory response that this strain can cause,” explains co-author J. Robert Coleman, Ph.D. “The novelty of this approach lies in the fact that the gene’s expression would be reduced, but not eliminated. Previous approaches to genetic regulation of virulence relied on knocking out genes, which eliminates their expression completely.”
The engineered bacterium did, in fact, trigger much lower inflammatory responses when injected into mice than the unattenuated serotype 3 strain. Importantly, 4 of 5 mice receiving the attenuated strain also survived a subsequent challenge from the unattenuated serotype 3 strain, which was lethal in all five unvaccinated control animals.
The authors point out that recoding genes rather than completely eliminating them retains wild-type bacterial epitopes, and this appears to have an advantage over knockout strategies. If the pneumolysin gene had been knocked out completely, vaccinated animals would not generate antipneumolysin antibodies, which can confer protection against S. pneumoniae in mice, they note.
Indeed, the researchers confirmed that expression of some pneumolysin was apparently needed for bacterial clearance. Wild-type bacteria in which pneumolysin production was unchecked, and knockout bacteria that produced no pneumolysin, were both lethal to mice.
The researchers admit that the effect of pneumolysin expression on virulence could be serotype-specific. However, they conclude, “the data herein suggest that modulation of virulence gene expression warrants further consideration as an approach to the design of a live-attenuated and/or universal S. pneumoniae vaccine.”