Assessment of adjuvantation strategy of lipid squalene nanoparticles for enhancing the immunogenicity of a SARS-CoV-2 spike subunit protein against COVID-19

Lipid squalene nanoparticles (SQ@NPs) play a crucial role in the immunogenicity of the SARS-CoV-2 spike subunit protein against COVID-19. In a mouse model, we found that one dose of the S-protein with SQ@NPs works as well as two doses of the S-protein alone without compromising safety aspects following i.m. injection. These results pave the way for optimal vaccine formulations against COVID-19. [Display omitted] •The adjuvant efficacy of lipid squalene nanoparticles (SQ@NPs) was assessed in mice.•One dose of S-protein with SQ@NPs works as well as two doses of S-protein alone.•SQ@NPs rephrase the activation of T cells.•The SQ@NP-adjuvanted vaccine is considerably tolerated via i.m. injection in mice. Vaccination is regarded as the most effective intervention for controlling the coronavirus disease 2019 (COVID-19) pandemic. The objective of this study is to provide comprehensive information on lipid squalene nanoparticle (SQ@NP)-adjuvanted COVID-19 vaccines regarding modulating immune response and enhancing vaccine efficacy. After being adjuvanted with SQ@NP, the SARS-CoV-2 spike (S) subunit protein was intramuscularly (i.m.) administered to mice. Serum samples investigated by ELISA and virus neutralizing assay showed that a single-dose SQ@NP-adjuvanted S-protein vaccine can induce antigen-specific IgG and protective antibodies comparable with those induced by two doses of nonadjuvanted protein vaccine. When the mice received a boosting vaccine injection, anamnestic response was observed in the groups of adjuvanted vaccine. Furthermore, the secretion of cytokines in splenocytes, such as interferon (IFN)-γ, interleukin (IL)-5 and IL-10, was significantly enhanced after adjuvantation of S-protein vaccine with SQ@NP; however, this was not the case for the vaccine adjuvanted with conventional aluminum mineral salts. Histological examination of injection sites showed that the SQ@NP-adjuvanted vaccine was considerably well tolerated following i.m. injection in mice. These results pave the way for the performance tuning of optimal vaccine formulations against COVID-19.