A new study published in PLOS Pathogens shows how to modify an effective live attenuated Severe Acute Respiratory Syndrome (SARS) vaccine to make it genetically stable.
An attenuated vaccine is developed by reducing the harmful quality of a pathogen, but still keeping it alive. Luis Enjuanes, from the National Center for Biotechnology in Madrid, Spain, and his research team had previously brought a SARS coronavirus (SARS-CoV) into use lacking the envelope (or E) gene as a promising vaccine candidate.
‘To prevent such compensation and reversal to virulence, scientists initiated small deletions in the E gene that did not eliminate its PDZ binding motif. Such mutants stayed reduced in virulence but appear to no longer be ready for the incorporation of novel protein domains into the virus genome.’
AdvertisementThe vaccine is known as SARS-CoV-ΔE, and scientists had demonstrated that it was attenuated in different animal models, intimating that the E protein is imperative for the virus' ability to cause disease. They had also showed that the vaccine protected mice against challenges with virulent SARS-CoV that is lethal to unvaccinated mice, proving that it is an effective vaccine.
In the current study, scientists addressed the concern of stability of the vaccine candidate. In order to do this, the investigators propagated the SARS-CoV-ΔE virus for some generations in cell lines and mice. They found that the virus accumulated mutations and reverts to a virulent phenotype.
Focusing on a collection of mutants, they were able to tell the molecular basis of the reversion. The analysis says that the E protein carries a motif called a PDZ binding motif or PBM that controls cellular pathways, which is significant for viral replication, dissemination in the host, and pathogenesis. And all the reverted viruses had incorporated into the genome a functional PBM, seemingly to compensate for the removal of this motif with deletion of the E protein.
To prevent such compensation and reversal to virulence, the scientists initiated small deletions in the E gene that did not eliminate its PBM, instead of destroying the entire gene. Such mutants stayed reduced in virulence but appear to no longer be ready for the incorporation into the virus genome and so avoid the reversion to the strong form.
In order to create an extra safeguard, the scientists took mutations into another SARS-CoV gene known as nsp1. This gene is usually found at a distant site from that of the E gene in the viral genome. This position makes it less likely that a single mutational event can restore both the nsp1 gene and the E gene to their un-attenuated sequences and thereby reinstate virulence. This is the reason the scientists picked up nsp1 as a second attenuation target.
The scientists came to know that small deletions within the nsp1 gene alone brought up an attenuated virus that could not cause disease but defended vaccinated mice against challenge with the virulent parental virus. And when they examined the new vaccine that includes small attenuating mutations in both the E and nsp1 genes, they saw that it maintained its attenuation after prolonged propagation in both inside a living organism and a test tube and provided full protection against the virulent original SARS-CoV.
The scientists conclude that "understanding the molecular mechanisms leading to pathogenicity and the in vivo evaluation of vaccine genetic stability contributed to a rational design of a promising SARS-CoV vaccine." They also suggest that "understanding how an attenuated SARS-CoV reverted to virulence could also be useful for vaccine development against other relevant coronaviruses, such as the MERS-CoV."
SARS is the first emerging epidemic of this century. It is too early to get to a conclusion that how SARS will evolve in the future, but the world has learned several encouraging lessons.
References:1. Jose M. Jimenez-Guardeņo, Jose A. Regla-Nava, Jose L. Nieto-Torres, Marta L. DeDiego, Carlos Castaņo-Rodriguez, Raul Fernandez-Delgado, Stanley Perlman, Luis Enjuanes. Identification of the Mechanisms Causing Reversion to Virulence in an Attenuated SARS-CoV for the Design of a Genetically Stable Vaccine. PLOS Pathogens, 2015; 11 (10): e1005215 DOI: 10.1371/journal.ppat.1005215