In 2003, severe acute respiratory syndrome coronavirus (SARS-CoV) was identified as the etiological agent of severe acute respiratory syndrome, a disease characterized by severe pneumonia that sometimes results in death. SARS-CoV is a zoonotic virus that crossed the species barrier, most likely originating from bats or from other species including civets, raccoon dogs, domestic cats, swine, and rodents. A SARS-CoV vaccine should confer long-term protection, especially in vulnerable senescent populations, against both the 2003 epidemic strains and zoonotic strains that may yet emerge from animal reservoirs. We report the comprehensive investigation of SARS vaccine efficacy in young and senescent mice following homologous and heterologous challenge.
Methods and Findings
Using Venezuelan equine encephalitis virus replicon particles (VRP) expressing the 2003 epidemic Urbani SARS-CoV strain spike (S) glycoprotein (VRP-S) or the nucleocapsid (N) protein from the same strain (VRP-N), we demonstrate that VRP-S, but not VRP-N vaccines provide complete short- and long-term protection against homologous strain challenge in young and senescent mice. To test VRP vaccine efficacy against a heterologous SARS-CoV, we used phylogenetic analyses, synthetic biology, and reverse genetics to construct a chimeric virus (icGDO3-S) encoding a synthetic S glycoprotein gene of the most genetically divergent human strain, GDO3, which clusters among the zoonotic SARS-CoV. icGD03-S replicated efficiently in human airway epithelial cells and in the lungs of young and senescent mice, and was highly resistant to neutralization with antisera directed against the Urbani strain. Although VRP-S vaccines provided complete short-term protection against heterologous icGD03-S challenge in young mice, only limited protection was seen in vaccinated senescent animals. VRP-N vaccines not only failed to protect from homologous or heterologous challenge, but resulted in enhanced immunopathology with eosinophilic infiltrates within the lungs of SARS-CoV–challenged mice. VRP-N–induced pathology presented at day 4, peaked around day 7, and persisted through day 14, and was likely mediated by cellular immune responses.
This study identifies gaps and challenges in vaccine design for controlling future SARS-CoV zoonosis, especially in vulnerable elderly populations. The availability of a SARS-CoV virus bearing heterologous S glycoproteins provides a robust challenge inoculum for evaluating vaccine efficacy against zoonotic strains, the most likely source of future outbreaks.
Experiments in mice suggest challenges in vaccine design for controlling future SARS-CoV zoonosis, especially in vulnerable elderly populations.
Severe acute respiratory syndrome (SARS) is a flu-like illness and was first recognized in China in 2002, after which the disease rapidly spread around the world. SARS was associated with high death rates, much higher than those for flu. Around 10% of people recognized as being infected with SARS died, and the death rate approached 50% among elderly people. The virus causing SARS was identified as a member of the coronavirus family; it is generally thought that this virus “jumped” to humans from bats, which harbor related viruses. Although SARS was declared eradicated by the World Health Organization in May 2005, there is still the possibility that similar viruses will again cross the species barrier and infect humans, with potentially serious consequences. As a result, many groups are working to develop vaccines that will protect against SARS infection.
Why Was This Study Done?
A SARS vaccine should be effective in people of all ages, including the elderly who are more likely to get seriously ill or die if they become infected. In addition, potential vaccines should protect against different variants of the virus, because there are different types of the virus that could potentially cross the species barrier from animals to humans. Of the different proteins that make up the SARS coronavirus, the spike glycoprotein is thought to elicit an immune response in humans that can protect against future infection. The researchers therefore examined vaccine candidates based on this particular protein (termed SARS-CoV S), as well as a second one called SARS-CoV N, in mice. Specifically, they tested whether the vaccines would protect against SARS infection in both young and older mice, and whether they would protect against infection by different strains of the SARS virus.
What Did the Researchers Do and Find?
The researchers created vaccines based on SARS-CoV S and SARS-CoV N by taking the genes coding for those proteins and inserting them into another type of virus particle that acted as a delivery vehicle. They injected mice with these vaccines and then tested whether the mice generated an immune response against the specific SARS proteins, which they did. The next step was to work out whether mice injected with the vaccines would be protected against later infection with SARS-CoV. The researchers found that mice injected with vaccine based on SARS-CoV S were protected against later infection with a standard SARS-CoV strain, both in the short term (eight weeks after vaccination) and the long term (54 weeks after vaccination). However, the vaccine based on SARS-CoV N did not seem to result in protection, and, worryingly, caused pathological changes in the lungs of mice following virus challenge. To find out if their candidate vaccines would protect against different strains of SARS, the researchers made a synthetic test virus that contained a mixture of genetic material from different natural variants of the virus. This test virus was used to “challenge” mice that had been immunized with the two different vaccines. The researchers found that the vaccine based on SARS-CoV S protected against infection by the test virus when mice were vaccinated young, but it failed to efficiently protect when administered to older mice.
What Do These Findings Mean?
The findings confirm others suggesting that vaccines based on the SARS-CoV S protein are more effective than those based on SARS-CoV N. They also suggest that the former can provide long-term protection in animals vaccinated young against closely related viruses. However, protection against more distantly related viruses remains a challenge, especially when vaccinating older animals. The differences seen between young and older mice suggest that older mice might provide a useful model for animal testing of candidate vaccines for diseases like SARS, flu, and West Nile virus that pose a particular threat to elderly people. Overall, these results provide useful lessons toward future SARS vaccine development in animals. The synthetic virus strain generated here, and others like it, are likely to be useful tools for such future studies.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030525.
• The World Health Organization provides guidance, archives, and other information resources on SARS
• Information from the US Centers for Disease Control on SARS
• Wikipedia (an internet encyclopedia anyone can edit) has an entry on SARS
• Collected resources from MedLinePlus about SARS