Heterologous Vaccination with SARS-CoV-2 Spike saRNA Prime followed by DNA Dual-Antigen Boost Induces Robust Antibody and T-Cell Immunogenicity against both Wild Type and Delta Spike as well as Nucleocapsid Antigens

Abstract

ABSTRACTWe assessed if immune responses are enhanced in CD-1 mice by heterologous vaccination with two different nucleic acid-based COVID-19 vaccines: a next-generation human adenovirus serotype 5 (hAd5)-vectored dual-antigen spike (S) and nucleocapsid (N) vaccine (AdS+N) and a self-amplifying and -adjuvanted S RNA vaccine (SASA S) delivered by a nanostructured lipid carrier. The AdS+N vaccine encodes S modified with a fusion motif to increase cell-surface expression. The N antigen is modified with an Enhanced T-cell Stimulation Domain (N-ETSD) to direct N to the endosomal/lysosomal compartment to increase the potential for MHC class I and II stimulation. The S sequence in the SASA S vaccine comprises the D614G mutation, two prolines to stabilize S in the prefusion conformation, and 3 glutamines in the furin cleavage region to increase cross-reactivity across variants. CD-1 mice received vaccination by prime > boost homologous and heterologous combinations. Humoral responses to S were the highest with any regimen including the SASA S vaccine, and IgG against wild type S1 and Delta (B.1.617.2) variant S1 was generated at similar levels. An AdS+N boost of an SASA S prime enhanced both CD4+ and CD8+ T-cell responses to both S wild type and S Delta peptides relative to all other vaccine regimens. Sera from mice receiving SASA S homologous or heterologous vaccination were found to be highly neutralizing of all pseudovirus tested: Wuhan, Delta, and Beta strain pseudoviruses. The findings here support the clinical testing of heterologous vaccination by an SASA S > AdS+N regimen to provide increased protection against COVID-19 and SARS-CoV-2 variants.