Highly feasible immunoprotective multicistronic SARS-CoV-2 vaccine candidate blending novel eukaryotic expression and Salmonella bactofection.

Introduction: The emergence of SARS-CoV-2 variants has raised concerns on future vaccine efficacy as most vaccines target only the spike protein. Hence, vaccines targeting multiple SARS-CoV-2 proteins will offer broader protection and improve our preparedness to combat the pandemic.

Objectives: The study aimed to develop a novel vaccine strategy by combining a eukaryotic vector expressing multiple SARS-CoV-2 genes and Salmonella -mediated in vivo DNA delivery.

Methods: The eukaryotic vector was designed to function as a DNA-launched RNA replicon in a self-replicating and self-amplifying mRNA mechanism. By exploiting the self-cleaving peptide, P2A, we fused four SARS-CoV-2 targets, including receptor-binding domain (RBD), heptad repeat domain (HR), membrane protein (M) and epitopes of nsp13, in a single open reading frame. Western blot and immunofluorescence assays were used to determine protein expression. In mice, the vaccine's safety and immunogenicity were investigated.

Results: Western blot analysis revealed co-expression all four proteins from the vaccine construct, confirming the efficiency of Salmonella -mediated gene delivery and protein expression. The vaccine candidate was safe and elicited robust antigen-specific antibody titers in mice, and a recall response from splenocytes revealed induction of strong cell-mediated immunity. Flow cytometry demonstrated an increase in sub-populations of CD4+ and CD8+ T cells with the highest CD4+ and CD8+ T cells recorded for HR and RBD, respectively. Overall, humoral and cellular immune response data suggested the induction of both Th1 and Th2 immunity with polarization towards an antiviral Th1 response. We recorded a potent SARS-CoV-2 neutralizing antibody titers in the immunized mice sera.

Conclusions: The Salmonella bactofection ensured optimum in vivo gene delivery, and through a P2A-enabled efficient multicistronic expression, the vaccine candidate elicited potent anti-SARS-CoV-2 immune responses. These findings provide important insight into development of an effective multivalent vaccine to combat SARS-CoV-2 and its variants.