Self-amplifying RNA SARS-CoV-2 lipid nanoparticle vaccine candidate induces high neutralizing antibody titers in mice

Abstract

The spread of the SARS-CoV-2 into a global pandemic within a few months of onset motivates the development of a rapidly scalable vaccine. Here, we present a self-amplifying RNA encoding the SARS-CoV-2 spike protein encapsulated within a lipid nanoparticle (LNP) as a vaccine. We observe remarkably high and dose-dependent SARS-CoV-2 specific antibody titers in mouse sera, as well as robust neutralization of both a pseudo-virus and wild-type virus. Upon further characterization we find that the neutralization is proportional to the quantity of specific IgG and of higher magnitude than recovered COVID-19 patients. saRNA LNP immunizations induce a Th1-biased response in mice, and there is no antibody-dependent enhancement (ADE) observed. Finally, we observe high cellular responses, as characterized by IFN-γ production, upon re-stimulation with SARS-CoV-2 peptides. These data provide insight into the vaccine design and evaluation of immunogenicity to enable rapid translation to the clinic.

Fig. 1. Antibody quantification and neutralization of a SARS-CoV-2 saRNA vaccinated mice compared to COVID-19 recovered patients.

a Schematic of vaccination of BALB/c mice with saRNA encoding pre-fusion stabilized spike protein in LNP, b SARS-CoV-2 specific IgG responses in mice vaccinated with doses of LNP-formulated saRNA ranging from 0.01–10 μg of saRNA with n = 7 biologically independent animals and COVID-19 recovered patients with n = 9 biologically independent samples, c SARS-CoV-2 pseudotyped virus neutralization of sera from BALB/c mice vaccinated with doses of LNP-formulated saRNA ranging from 0.01–10 μg of saRNA with n = 7 biologically independent animals and COVID-19 recovered patients with n = 9 biologically independent samples, d Correlation between SARS-CoV-2-specific IgG and SARS-CoV-2 neutralization IC₅₀ for vaccinated mice (n = 7 biologically independent animals) and recovered COVID-19 patients (n = 9 biologically independent samples), e SARS-CoV-2 viral neutralization of sera from BALB/c mice vaccinated with doses of LNP-formulated saRNA ranging from 0.01 to 10 µg of saRNA with n = 7 biologically independent animals, f Correlation between SARS-CoV-2-specific IgG and SARS-CoV-2 wild type viral neutralization titers for vaccinated mice (n = 7 biologically independent animals). Electroporated pDNA (DNA + EP) was used as a positive control while saRNA encoding the rabies glycoprotein (RABV) in pABOL was used as a negative control (RABV control). * indicates significance of p < 0.05 using a two-way ANOVA adjusted for multiple comparisons. Line and error bars indicated mean ± SD. Components of this figure were created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License; https://creativecommons.org/licenses/by/3.0/.

Fig. 2. Th1/Th2 skew in response to SARS-CoV-2 saRNA LNP vaccine.

a IgG1 and IgG2a responses in mice vaccinated with doses of LNP-formulated saRNA ranging from 0.01–10 μg of saRNA with n = 7 biologically independent animals, b Th1/Th2 skewing responses in mice vaccinated with doses of LNP-formulated saRNA ranging from 0.01–10 μg of saRNA with n = 7 biologically independent animals, and 10 μg of electroporated pDNA (EP pDNA) with n = 8 biologically independent animals. The asterisk (*) indicates significance of p < 0.05 as determined by a Kruskal–Wallis test. Line and error bars indicated mean ± SD.

Fig. 3. Cellular and secreted cytokine responses to a SARS-CoV-2 saRNA LNP vaccine.

a Quantification of IFN-γ splenocytes upon restimulation with SARS-CoV-2 peptides, expressed as spot forming units (SFU) per 10⁶ cells with n = 7 biologically independent animals. Electroporated pDNA (EP pDNA) was used as a positive control while saRNA encoding the rabies glycoprotein (RABV) in pABOL was used as a negative control (RABV control). b–g Cytokine profile in sera of mice 4 h after vaccination with SARS-CoV-2 LNP vaccine with n = 7 biologically independent animals. Remaining cytokines can be found in Supplementary Fig. 5. The asterisk (*) indicates significance of p < 0.05 as determined by a Kruskal–Wallis test. Line and error bars indicated mean ± SD.