Fig. 3

Screening and evaluation of the T-RBD or TF-RBD mRNA vaccine candidates. a Immune procedures for evaluating the T-RBD or TF-RBD mRNA vaccine candidates. Groups of 6- to 8-week-old female naive C57BL/6 mice (n = 10) were vaccinated with two doses (1.5 μg, low; 15 μg, high) of the mRNA LNPs at 2-week intervals. Blood collection and spleen extraction were performed at the time points shown after immunization. b ELISA revealed the RBD-specific IgG titer of SARS-CoV-2. Serum samples were collected after each immunization event and used for the detection of IgG to evaluate the humoral response dynamics. A placebo formulation was given as the control. The red arrows indicate the immunization time point. c, d Neutralization assays of live SARS-CoV-2 in (c) and the SARS-CoV-2 pseudovirus in (d). e Protein-specific T cell ELISPOT assay results. Spleen cells (2 × 10⁵) were stimulated with the S RBD protein at a final concentration of 10 μg/mL. f, g Protein-specific ICS assays. Spleens were harvested at 28 days after booster immunization, and the cells were stimulated with the S RBD protein at a final concentration of 10 μg/mL. The proportions of CD4⁺ protein-specific T cells in (f) and CD8⁺ protein-specific T cells in (g) are shown. The data are shown as the mean ± standard error of the mean (SEM). The dotted horizontal lines indicate the limits of quantification for ELISA and NT₅₀ titers. P values were determined by one-way ANOVA (ns, p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001). See also Figures S1–S4