Vaccines have been instrumental in the public health response to the SARS-CoV-2 pandemic. The existing approved vaccines induce neutralizing antibodies and are very effective at preventing symptomatic infection. However, current vaccines do not elicit sterilizing and mucosal immunity necessary to avoid breakthrough infections.
Testing an attenuated version of SARS-CoV-2
In a recent publication, Ye et al (2) set out to make an attenuated version of SARS-CoV-2 and test its utility as a vaccine. The authors created a recombinant strain with a point mutation that inactivates NSP16, a protein needed to cap viral RNAs and prevent them from being detected by the immune system. The mutation, D130A, was known to inactive the analogous enzyme in other coronaviruses. The mutant SARS-CoV-2 strain was characterized to determine if (1) the virus was viable, (2) the ability of the virus to replicate was attenuated, and (3) the virus could be a candidate for vaccine development.
The answer to all three questions was “yes”. The virus was viable in in vitro assays, but its growth was attenuated both in vitro and in vivo. Hamsters infected with the mutant virus had significantly lower viral loads in the upper and lower respiratory tracts than hamsters infected with the wildtype virus, and infection with the attenuated virus did not kill transgenic mice as wildtype virus did.
Use of the mutant strain as a live vaccine was tested in hamsters using a single dose regimen. Hamsters were inoculated intranasally with wildtype or mutant virus, monitored for 28 days, then challenged with wildtype virus on day 29. On day 28, the presence of neutralizing antibodies was assessed using a live virus neutralization assay that detected bound virus using a fluorescently-labeled antibody. The fluorescent signal was assessed and quantified using the Azure Sapphire Biomolecular Imager.
Exposure to the mutant strain was found to be as effective as the wildtype virus at preventing SARS-CoV-2 infection. Viral titers in the lung and respiratory tract were so low the vaccination may have elicited sterilizing immunity. Indeed, hamsters co-housed with infected but vaccinated hamsters never became infected themselves. Promisingly, spike protein-specific IgA was detected in the serum and bronchoalveolar lavage fluid of vaccinated mice, suggesting mucosal immunity was stimulated.
Ye et al conclude the results provide strong justification to pursue further developing the strain as a potential vaccine.
How COVID infections begin
COVID infections usually begin in the upper respiratory tract mucosa. Therefore, stimulating mucosal immunity may be most effective at preventing infection. Mucosal immunity is independent of the systemic immune system and is mediated primarily by secretory IgA antibodies. Injectable vaccines such as those used for flu and pneumonia primarily induce serum IgG, which may protect the lower respiratory tract and prevent systemic spread. Inducing upper airway mucosal protection may require nasal immunization. Additionally, in some experiments inducing a mucosal cellular immune response has required immunizing with whole, wild-type or attenuated viruses or bacteria.(1)
Live attenuated vaccines use a weakened form of the infectious agent (virus or bacterium). The agent can replicate enough to stimulate a strong immune response but not so much as to cause disease. Live attenuated vaccines result in a strong and long-lasting immune response, are used to protect against several agents, and were crucial to the eradication of smallpox and the near eradication of poliovirus.
In addition to fluorescent imaging of tissue culture plates, the Sapphire provides densitometry, phosphor, multichannel fluorescence, near-infrared, chemiluminescence, and white light imaging of blots, gels, tissues, and more. Learn more about the Sapphire Imager and how Azure can support your research by clicking here.
More research done with the Azure Sapphire:
- Holmgren J, Czerkinsky C. Mucosal immunity and vaccines. Nature Med. 2005;11:S45-S53.
- Ye ZW, Ong CP, Tang K, et al. Intranasal administration of a single dose of a candidate live attenuated vaccine derived from an NSP16-deficient SARS-CoV-2 strain confers sterilizing immunity in animals. Cell Mol Immunol. 2022;Mar 29;1-14.