Skin patch delivery of a SARS-CoV-2 spike DNA vaccine produces broad neutralising antibody responses
Since the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), in December 2019, a recorded 775 million cases and an estimated 7.1 million deaths have occurred globally. Whilst the pandemic has transitioned to endemicity, this does not alleviate the severity of disease burden, reduce resurgences of seasonal infections, or lessen the threat of novel immune escape variants. To ameliorate both symptoms and transmissibility of emerging SARS-CoV-2 variants, continual prophylactic and therapeutic research and development is vital. During the pandemic, multiple vaccines were rapidly developed as various modalities, such as protein subunit; inactivated virus; viral vector; nanoparticle; DNA; and mRNA. Although differing vaccines yielded variable protective efficacy, the cumulative effect of herd immunity successfully minimised waves of SARS-CoV-2 infection. However, recent research indicates waning population immunity when measuring neutralisation from sera of vaccinated individuals. These observations are attributed to the continual mutations of the spike glycoprotein, which result in enhanced infectivity and immune evasion. This is concerning as the spike glycoprotein is the primary target of vaccine-induced neutralising antibodies. Furthermore, other challenges exist with current vaccines available on the market. Particularly, mRNA vaccines require ultra-low storage temperature to retain vaccine stability. These limitations hinder global transport and distribution, which must be addressed to accomplish vaccine equity in resource-limited nations.
To overcome programmatic and logistical barriers to mass vaccination campaigns, alternative vaccine delivery methods should be explored. One such vaccine delivery system is the high-density microarray patch (HD-MAP). The HD-MAP is a square one-by-one centimetre polymer patch containing 5000 micro-projections/cm2, each 250 μM length. The vaccine is coated onto the tips of micro-projections and applied to the skin at 20 m/s using a spring-loaded applicator. This application’s velocity enables the micro-projections to penetrate the stratum corneum and deposit the vaccine into the (epi)dermal layers of skin, which are rich in immune cells. As a function of the dynamic application coupled with the co-localisation of vaccine with immune cells, HD-MAP vaccine delivery has routinely been shown to result in enhanced immune responses. Beyond the enhanced vaccine immune responses observed, microarray patches alleviate many of the challenges associated with vaccine distribution. These improvements include a more stable vaccine profile, fewer required doses, elimination of cold-chain transport requirements, and easier application. The administration of HD-MAPs does not require specialised medical personnel, promoting accessibility, convenience, and the potential for self-administration.
mRNA vaccines have been effective against COVID-19, though their instability outside of cold-chain storage and the continuous evolution of the virus pose challenges. As a potential alternative, we have investigated a stable DNA vaccine targeting the Delta variant. DNA vaccines have been shown to be immunogenic, eliciting both robust cellular and antibody responses whilst avoiding stimulation of vector-specific antibodies. Other advantages of DNA plasmids include the adaptability of the genetic sequence, providing the potential to quickly protect against mutating viral variants; and the possibility of prime-boost and multiple booster vaccination regimes. We conducted dose-match comparative studies and assessed immune cell responses from the Delta 6P DNA vaccine administered via HD-MAP and intramuscular (IM) injection in mice.
The Delta 6P plasmid encodes for the SARS-CoV-2 Delta HexaPro spike protein, consisting of six proline substitutions and a mutated furin cleavage site. Previous studies have found that the addition of six prolines greatly increases the stabilisation of the spike protein in its prefusion conformation and enhances expression efficiency compared to previous generations. The notable effects of HexaPro as a vaccine immunogen include, but are not limited to, the induction of robust CD4+ T helper 1 (Th1) cells and significantly increased antibody responses. Furthermore, studies have identified that the SARS-CoV-2 HexaPro spike glycoprotein provides complete protection against viral challenge after a single dose. Additionally, delivery with the HD-MAP resulted in significantly improved immunological cellular responses when compared to intradermal needle and syringe.
We propose Delta 6P as a potential DNA vaccine for SARS-CoV-2 with HD-MAP delivery as an alternative for electroporation and liquid jet injectors. The immunogenicity of Delta 6P delivered by the HD-MAP or IM was assessed in animal models. Whilst similar antibody levels were induced compared to the IM injection groups, we observed significantly higher antibody levels in the HD-MAP groups when evaluated against the RBD of the SARS-CoV-2 Delta variant. The vaccine-elicited T cell response was evaluated and compared between HD-MAP and IM cohorts in vivo relative to the activity of T helper (Th) cells and cytotoxic T cells, and in vitro using a conventional intracellular cytokine staining (ICS) analysis. Comparative analysis showed that IM vaccination elicited higher Delta spike-specific T helper cell response, but not cytotoxic T cells. The ICS analysis showed that spike-specific CD4+ T and CD8+ T cell responses were largely comparable, but the number of polyfunctional CD4+ T cells was higher in the HD-MAP group. Collectively, the data suggest that the spike-specific CD4+ and CD8+ T cell response was largely similar between HD-MAP and IM groups.
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