ABSTRACT
Background: Currently available COVID-19 vaccination regimens in the US deliver either a homologous spike (S) mRNA prime-boost or a prime-only S DNA adenovirus-vectored antigen to elicit humoral and cell-mediated responses to confer protection against SAR-CoV-2 infection. Alternatively, heterologous vaccination using two different platforms has the potential to enhance and expand immune protection. Addition of a second SARS-CoV-2 antigen, the nucleocapsid (N) protein that is less subject to mutation and elicits vigorous T-cell responses, may also be advantageous. We report immunological responses to homologous and heterologous prime-boost vaccination regimens with a human DNA adenovirus serotype 5 S plus N (AdS+N) and/or a self-amplifying S-only mRNA vaccine (AAAH) delivered with a nanostructured lipid carrier (NLC). Methods: CD-1 mice received homologous or heterologous prime-boost combinations of AdS+N and AAAH. Priming doses were administered on Day 0, booster doses were delivered on Day 21, and mice were euthanized for blood and organ collection on Day 35. Serum was analyzed for anti-S (both wild type and variant) and anti-N IgG subtypes by ELISA. Spleen-resident CD4+ and CD8+ T cells were tested for IFN-γ, TNF-α, and IL-2 production in response to S-WT, S Delta variant and N protein overlapping peptides by intracellular cytokine staining (ICS). Splenocyte cytokine secretion upon stimulation with S-WT/N peptides was also assessed by IFN-γ and IL-4 ELISpot. Serum neutralization of the original Wuhan strain, Delta, and B.1.351 variants was assessed by a pseudovirus neutralization assay. Results: The highest humoral and T-cell responses were seen with the heterologous AAAH prime-AdS+N boost regimen, with a significant increase in T-cell responses relative to homologous vaccination. S protein-binding IgG was similar between wild type and Delta variant S proteins, with a strong/clear Th1/Th2 bias, and T cells responded to S wild type and S Delta peptides with similar levels of cytokine expression. Sera from AAAH prime-AdS+N boost mice showed the ability to neutralize Wuhan D614G, Delta, and B.1.351 (South Africa) variant pseudoviruses at high levels. Conclusion: Heterologous vaccination with the AAAH RNA vaccine prime and an AdS+N DNA boost may provide substantially improved humoral and cell-based immunity against SARS-CoV-2 variants by leveraging the advantages of each vaccine platform technology and by inclusion of immune responses to N.