Enhanced immunogenicity of a synthetic DNA vaccine expressing consensus SARS-CoV-2 Spike protein using needle-free immunization

The ongoing global pandemic of Coronavirus Disease 2019 (COVID-19) calls for an urgent development of effective and safe prophylactic and therapeutic measures. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) glycoprotein is a major immunogenic and protective protein, and plays a crucial role in viral pathogenesis. In this study, we successfully constructed a synthetic codon-optimized DNA-based vaccine as a countermeasure against SARS-CoV-2; denoted as VIU-1005. The design was based on the synthesis of codon-optimized coding sequence for optimal mammalian expression of a consensus full-length S glycoprotein. The successful construction of the vaccine was confirmed by restriction digestion and sequencing, and the protein expression of the S protein was confirmed by western blot and immunofluorescence staining in mammalian cells. The immunogenicity of the vaccine was tested in two mouse models (BALB/c and C57BL/6J). Th1-skewed systemic S-specific IgG antibodies and neutralizing antibodies (nAbs) were significantly induced in both models four weeks post three injections with 100 μg of the VIU-1005 vaccine via intramuscular needle injection but not intradermal or subcutaneous routes. Importantly, such immunization induced long-lasting IgG response in mice that lasted for at least 6 months. Interestingly, using a needle-free system, we showed an enhanced immunogenicity of VIU-1005 in which lower doses such as 25-50 μg or less number of doses were able to elicit significantly high levels of Th1-biased systemic S-specific IgG antibodies and nAbs via intramuscular immunization compared to needle immunization. Compared to the intradermal needle injection which failed to induce any significant immune response, intradermal needle-free immunization elicited robust Th1-biased humoral response similar to that observed with intramuscular immunization. Furthermore, immunization with VIU-1005 induced potent S-specific cellular response as demonstrated by the significantly high levels of IFN-γ, TNF and IL-2 cytokines production in memory CD8+ and CD4+ T cells in BALB/c mice. Together, our results demonstrate that the synthetic VIU-1005 candidate DNA vaccine is highly immunogenic and capable of inducing long-lasting and Th1-skewed immune response in mice. Furthermore, we show that the use of needle-free system could enhance the immunogenicity and minimize doses needed to induce protective immunity in mice, supporting further preclinical and clinical testing of this candidate vaccine. ### Competing Interest Statement AMH, MAA, TSA, KAA, SSA and AA are inventors on a US patent application related to this work. AMH and AA work as scientific consultants in SaudiVax ltd and receive fees for consulting. SA and MH are employees of SaudiVax ltd and receives salary and benefits. All other authors report there are no competing interests.