Fig. 4

Evaluation of the immune protection provided by the TF-RBD mRNA vaccine candidate during challenge in vivo. a Immunization and challenge procedures for the 1.5 and 15 μg TF-RBD mRNA vaccine candidates in mice. Naive 6- to 8-week-old female naive C57BL/6 mice (n = 5) were administered two doses of the vaccines via the intramuscular route. The mice were infected with Ad5-ACE2 and subsequently challenged with live SARS-CoV-2. Blood collection was performed at the indicated time points after immunization (14 days after prime immunization and 14 days after booster immunization). b The anti-RBD-specific IgG antibody titers in the vaccine and placebo groups were determined by ELISA. c The viral RNA loads (copies/g) in the mouse lungs were measured. d The body weights of the mice were monitored and recorded for 5 consecutive days after the challenge. The mice were euthanized after observation. e Hematoxylin and eosin (H&E) staining to assess pathological changes in the lungs of mice at 5 dpi. The black arrows indicate the infiltrating inflame cells in the alveolar space (scale bar, left 5 × 200 μm, right 20 × 50 μm). f Fluorescence immunohistochemistry (FIHC) staining images of mouse lungs at 5 dpi. The red arrows show the sections incubated with the anti-SARS-CoV-2 S protein antibody, and the blue fluorescence indicates the cell nuclei locations as determined by DAPI staining (scale bar, left 10 × 100 μm, right 40 × 20 μm). The dotted horizontal lines indicate the limits of quantification for the ELISA and viral RNA load data. The data are shown as the mean ± SEM. P values were determined by one-way ANOVA (ns, p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001)