Fig. 2.

Immune responses in vaccinated BALB/c mice. (A) Experiment schedule. BALB/c mice were immunized twice intramuscularly at Day 0 and Day 21. Vaccine components were 5 μg SΔTM or RBD, which adjuvanted with 50 μg Alum and/or 50 μg CpG. Alum/CpG only groups were as control. On Day 35, Day 144 (only SΔTM groups) and Day 201 (only SΔTM groups), blood was collected to perform serological assays (SAs) (N = 10). On Day 35, 5 mice from each SΔTM groups were sacrificed to conduct intracellular staining (ICS) assay (N = 5). On Day 201, 5 mice from each SΔTM groups were sacrificed to conduct B cell ELISPOT (N = 5). (B) Binding antibody titers and (C) pseudovirus neutralizing antibody titers of sera collected at Day 35 from SΔTM or RBD vaccinated mice. (D) Neutralizing antibody titers of sera collected at Day 35 from SΔTM vaccinated mice detected by live virus-based neutralization assay. (E) IgG isotyping of sera collected at Day 35 from SΔTM vaccinated mice. (F) Antigen specific CD4⁺ T cell responses and (G) CD8⁺ T cell responses in SΔTM vaccinated mice were determined by ICS at Day 35. Dotted lines represent the limit of detection (LOD). Each dot represents an individual mouse. Numbers on the top of each bar represent geometric mean titers. Statistical analysis was performed using t-test with Welch’s correction. *P < 0.05; ****P < 0.0001.