The presentation of a vaccine antigen to the immune cells is critical to obtain an efficient immune response. It is known that nanostructures that are made of protein subunits assembled into a highly repetitive and crystalline arrangement are excellent inducers of the antibody response. Therefore, the fusion of vaccine antigens to those nanostructures is likely to improve the quality and the quantity of the antibody response directed to the vaccine. Based on this concept, we have developed a nanoparticle (nano) that harbors a flexuous rod-shape structure of 14x80nm that is made of a coat protein (CP) of a plant virus, the papaya mosaic virus (PapMV) and an ssRNA of 1500 nucleotides. The PapMV nano alone is a strong inducer of the innate antiviral response through the activation of the toll-like receptor (TLR) 7 and 8.
To confirm the efficacy of the PapMV nano in vaccine development, we have chosen to fuse the receptor binding domain (RBD) of the SARS-CoV-2 to the PapMV nano using a bacterial transpeptidase called sortase A (SrtA). Through the recognition of a donor and an acceptor peptide that are fused on the surface of the PapMV nano and to the C-terminus of the RBD, the SrtA attached covalently to the surface of the PapMV nano to the RBD antigen and generated the RBD-PapMV vaccine candidate.
This vaccine was shown to significantly enhance the quality and the quantities of antibodies directed to the RBD. The combination of the optimized antigenic presentation of the RBD to the immune system at the surface of the PapMV nano and the adjuvant property provided by the activation of the TLR7/8 was responsible for the quality of the immune response that we have recorded. The anti-RBD antibodies, generated in vaccinated animals, neutralized SARS-CoV-2 infection in vitro against the ancestral, Delta and Omicron variants. We have also recorded a strong T-cell response against the RBD antigen that can contribute to the efficacy of the vaccine. Finally, immunization of mice susceptible to the infection by SARS-CoV-2 (K18-hACE2 transgenic mice) with the RBD-PapMV vaccine induced complete protection to the ancestral SARS-CoV-2 infectious challenge. Immunization was sufficient to significantly reduce the viral load 2 and 4 days post-challenged in vaccinated and challenged animals. Sterile immunity was obtained when 2 immunizations at 21 days intervals were administered to the animals. The induction of the broad neutralization against SARS-CoV-2 variants induced by the RBD-PapMV vaccine demonstrates the potential of the PapMV vaccine platform in the development of efficient vaccines against viral respiratory infections.