ABSTRACT
Objective:To investigate the efficacy of a SARS-CoV-2 recombinant protein vaccine as a booster dose.Methods:A new immunogen, namely RBD-sc-trimer, was designed by tandem repeating of single receptor binding domain (RBD) of SARS-CoV-2 spike (S) protein to mimic the trimeric form of RBD presented by the virus. The RBD-sc-trimer protein was expressed as a His-tagged fusion protein using a baculovirus expression system and purified by nickel affinity column. The purified protein was identified by Western blot. Its in vitro binding activity to human angiotensin converting enzyme 2 (hACE2) was analyzed by ELISA. The immunogenicity of RBD-sc-trimer as well as RBD proteins of other forms including RBD dimer (RBD-Fc), RBD monomer (RBD) and S protein trimer (S trimer) as a booster dose was evaluated in BALB/c mice. Results:In terms of both binding and neutralizing antibodies against SARS-CoV-2, RBD-sc-trimer showed an immunogenicity that was superior to that of RBD-Fc and RBD and close to the level of S trimer. The antibody response induced by RBD-sc-trimer was characterized as Th1-biased. Moreover, it displayed a stronger cross-neutralization activity against SARS-CoV-2 Beta, Delta and Omicron variants. The titer of neutralizing antibody against Omicron induced by RBD-sc-trimer only decreased by 9.1 folds relative to the prototype strain, while the antibody response induced by RBD-Fc and S trimer decreased by 68.4 and 70.8 folds, respectively.Conclusions:The recombinant protein, RBD-sc-trimer, which was capable of eliciting stronger humoral response in mice as a booster dose and showed the superiority in raising cross-reactive antibodies against SARS-CoV-2 variants over non-trimeric RBD forms, should be considered as an optimal immunogen for the development of more effective SARS-CoV-2 vaccines.