Abstract
The SARS-CoV-2 pandemic has necessitated the rapid development of prophylactic vaccines. Two mRNA vaccines have been approved for emergency use by the FDA and have demonstrated extraordinary effectiveness. The success of these mRNA vaccines establishes the speed of development and therapeutic potential of mRNA. These authorized vaccines encode full-length versions of the SARS-CoV-2 spike protein. They are formulated with lipid nanoparticle (LNP) delivery vehicles that have inherent immunostimulatory properties. Different vaccination strategies and alternative mRNA delivery vehicles would be desirable to ensure flexibility of future generations of SARS-CoV-2 vaccines and the development of mRNA vaccines in general. Here, we report on the development of an alternative mRNA vaccine approach using a delivery vehicle called charge-altering releasable transporters (CARTs). Using these inherently nonimmunogenic vehicles, we can tailor the vaccine immunogenicity by inclusion of coformulated adjuvants such as oligodeoxynucleotides with CpG motifs (CpG-ODN). Mice vaccinated with the mRNA-CART vaccine developed therapeutically relevant levels of receptor binding domain (RBD)-specific neutralizing antibodies in both the circulation and in the lung bronchial fluids. In addition, vaccination elicited strong and long-lasting RBD-specific TH1 T cell responses including CD4+ and CD8+ T cell memory.
Highlights
Coronavirus pandemics have been a growing concern for more than a decade, and several attempts have been made to develop vaccines against SARS-CoV-1 and the Middle Eastern Respiratory Syndrome (MERS).[1]
We present an alternative 3-component mRNA vaccine utilizing mRNA encoding the receptor binding domain (RBD) of the SARS-CoV-2 spike protein formulated with a highly efficient, nontoxic, polyethylene glycol (PEG)-free mRNA delivery platform called charge-altering releasable transporters (CARTs)[14−16] and a TLR9 agonist (CpG) as a coformulated adjuvant[16,17] (Figure 1A)
RBD-expressing pseudovirus particles containing a zsGreen and firefly luciferase vectors were coincubated with titrated concentrations of heat-inactivated mouse serum. (B) The pseudovirus particle−serum mix was added to wells containing ACE-2-overexpressing 293F cells
Summary
Coronavirus pandemics have been a growing concern for more than a decade, and several attempts have been made to develop vaccines against SARS-CoV-1 and the Middle Eastern Respiratory Syndrome (MERS).[1]. The currently approved mRNA vaccines[5,6] generated by in vitro transcription use chemically modified nucleotides incorporated in mRNAs encoding the full viral spike protein, usually containing 2 structural epitope mutations, formulated in lipid nanoparticles (LNPs) and are administered intramuscularly. Despite their extraordinary success, the underlying science that contributes to the most effective, safe, and scalable vaccine against COVID-19 continues to evolve. The chemistry of the delivery vehicle is important as the ionizable lipids that are a component of LNPs act as adjuvants but can induce adverse events,[10] and the use of polyethylene glycol (PEG) in the LNP formulations can contribute to allergic reactions.[11,12]
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