Expression of recombinant SARS-CoV-2 proteins in plants

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) responsible for the COVID19 pandemic is a continuing threat to global health and prosperity. There is an urgent need to develop innovative vaccine technologies and diagnostic tests to combat the virus. Antigenic proteins encoded by SARS-CoV-2 can be expressed as recombinant proteins in heterologous expression systems. These recombinant antigenic proteins can be used to develop vaccines and diagnostic kits to identify people with antibodies against the virus. To combat the global pandemic, a low cost, high yield and scalable expression system is required to ensure that the SARS-CoV-2 recombinant proteins can be made in large amounts across the world. This affordability issue is particularly important in developing countries. Plant based expression systems provide the ideal vehicle for expressing SARS-CoV-2 recombinant proteins. This project will use a leaf-based expression system that enables extraordinarily high yields of SARS-CoV-2 recombinant proteins to be made with a low carbon footprint. This project will identify SARS-CoV-2 antigens that are suitable for development of a plant-based vaccine.

Antigens, derived from in silico screening of the SARS-CoV-2 genome, will be incorporated and displayed on virus like particles (VLPs). These VLPs are formed by self-assembly of hundreds of copies of the single viral protein. In effect VLPs are the outer coating of the virus without the infectious nucleic acid. Each VLP will display multiple SARS-CoV-2 antigens, enhancing their potential to be highly immunogenic. Expression in leaf chloroplasts enables extraordinarily high yields of recombinant proteins. The protein sequences for SARS-CoV-2 VLPs will be back translated into DNA using a chloroplast codon usage table in silico. The resulting synthesized genes will be inserted into a transformation vector and transformed into chloroplasts. Chloroplast transformants expressing SARS-CoV-2 VLPs will be characterised by PCR and protein blot analyses. These techniques are used to confirm targeted transgene integration and concentration of SARS-CoV-2 VLPs. The SARS-CoV-2 VLPs expressed in leaves will be purified and their structures examined by electron microscopy. Purified SARS-CoV-2 VLPs will then be tested using antibodies and anti-sera to determine correct folding and incorporation of the antigens. The project combines expertise in plant expression systems (Day), vaccine development (Derrick) and visualisation of virus-like particles (Roseman).