Self-amplifying RNA (saRNA) vaccines can be administered at lower doses than non-replicating mRNA vaccines to produce similar levels of antigen expression. As well as reducing the risk of side effects and any potential adverse events, lower dosing offers advantages for manufacturing. To investigate the immune response elicited by saRNA vaccination, Belgian researchers generated various saRNA vaccines against SARS-CoV-2, which they used to immunize mice. Their findings, published in Molecular Therapy, show that saRNA vaccination induces high neutralizing antibody titers and triggers cell-mediated immunity. In addition, a series of experiments performed in a validated SARS-CoV-2 hamster model suggested that basing saRNA vaccines on conserved antigens could protect against novel strains. This study supports the development of multiple antigen saRNA vaccines for emerging SARS-CoV-2 variants and other viruses.
Amplified antigen expression boosts the humoral immune response
The saRNA molecules used in this study encoded either the SARS-CoV-2 Spike protein receptor-binding domain (S-RBD) or the nucleocapsid (N) protein. They were produced using linearized plasmid DNA as a template for in vitro transcription with New England Biolabs’ HiScribe T7 High Yield RNA Synthesis Kit, followed by co-transcriptional capping using TriLink’s CleanCap® technology. To facilitate intracellular delivery, the saRNAs were formulated into lipid nanoparticles, either individually (S-RBD-saRNA and N-saRNA vaccines) or in combination (ZIP1642) before immunizing mice.
ELISA-based analysis of murine sera showed S-RBD-specific and N-specific IgG antibodies to be produced in response to the S-RBD-saRNA and N-saRNA vaccines, respectively. Antibodies to both antigens were detected at equivalent titers following vaccination with ZIP1642. The S1-RBD antibodies were subsequently proven to neutralize three different SARS-CoV-2 variants, with the neutralization titers being considerably higher than those observed in the sera of convalescent COVID-19 patients. These findings suggest amplified antigen expression to boost the humoral immune response.
Dual-antigen saRNA vaccination activates SARS-CoV-2-specific cell-mediated immunity
Cell-mediated immunity promises to enhance protection against SARS-CoV-2 by compensating for diminishing antibody levels over time. To characterize the cellular immune response to the dual-antigen vaccine, McCafferty et al. used flow cytometry to interrogate the T cell populations of immunized mice. ZIP1642 was shown to expand both S-RBD-specific and N-specific CD3+CD4+ and CD3+CD8+ T cell populations and cause a Th1-shifted cytokine response, verifying its potential to activate cell-mediated immunity specific to SARS-CoV-2.
saRNA vaccines exert a protective effect
To establish whether the potent humoral and cellular immune responses engendered by vaccination with saRNAs could protect against known SARS-CoV-2 variants, McCafferty et al. performed a series of experiments in a validated SARS-CoV-2 hamster model. Their data show low doses of ZIP1642 to be protective against the WA1/2020 strain, with immunohistochemical analysis of lung tissue demonstrating reduced virus-induced lung pathology. They also found that vaccination with N-saRNA lowered the viral load in animals challenged with B.1.351. These observations support the use of more conserved antigens for saRNA development, which could ultimately mitigate the risk posed by emerging strains.
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Article reference: McCafferty S, Haque AKMA, Vandierendonck A, et al., A dual-antigen self-amplifying RNA SARS-CoV-2 vaccine induces potent humoral and cellular immune responses and protects against SARS-CoV-2 variants through T cell-mediated immunity, Mol Ther. 2022 Apr 20; S1525-0016(22)00243-X.