Development of plant-produced subunit vaccines to elicit the immunogenicity against SARS-CoV-2 variant viruses

Abstract

Since the emergence of the coronavirus pandemic in 2019 (COVID-19), the development of effective vaccines to combat the infection has accelerated worldwide. While the severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) with mutations in the receptor-binding domain (RBD) with high transmissibility, enhanced infectivity, and immune escape from vaccination is also predominantly emerging. Effective vaccines against variant of concern (VOC) and optimized booster vaccination strategies are thus highly required. Here, the gene encoding seven different RBD (ancestral (Wuhan), Alpha, Beta, Gamma, Kappa, Delta, and Epsilon variants) fused with the fragment crystallizable region (Fc) of human IgG1 (RBD-Fc) was constructed and cloned into the plant expression vector and produced in Nicotiana benthamiana by transient expression. Further, the immunogenicity of plant-produced variant RBD-Fc subunit vaccines was tested in cynomolgus monkeys. First, monkeys were intramuscularly immunized with prime-booster and 4-month booster dose with the Wuhan RBD-Fc vaccine (Baiya SARS-CoV-2 Vax 1) on day 0, 21, and 133. Second, each group of monkeys were intramuscularly injected thrice with different variant RBD-Fc vaccines on day 0, 21, and 42. Then, the blood samples were collected for evaluating the antigen-specific and neutralizing antibodies against SARS-CoV-2 variants. Baiya SARS-CoV-2 Vax 1 immunized monkeys elicited significantly high levels of anti-RBD antibodies. Interestingly, the sera collected from 4-month booster dose monkeys showed cross-neutralizing antibody response against the SARS-CoV-2 variants; Alpha, Beta, Gamma, Delta, and Omicron. The three doses of variant RBD-Fc vaccines also elicited high levels of antigen-specific and neutralizing antibodies, especially in Delta and Epsilon vaccine groups. However, the high neutralizing activity of immunized variant vaccine sera was demonstrated specifically against the homologous variants. These findings suggested that the long-term booster dose might be helpful for protecting against further SARS-CoV-2 variants and the variant-specific vaccine might be applied as a booster or cocktail vaccine to induce broadly neutralizing antibodies. These results revealed the potential for using the plant-produced protein subunit vaccines in the fight against SARS-CoV-2.