Fig. 3

RBD-PP can elicit anti-SARS-CoV-2 neutralizing antibodies. Mice were vaccinated with the different vaccines on days 0, 14, and 28 of the experiment. Serum was collected 7 days after each vaccination. A Detection of anti-RBD IgG titers in mouse serum using ELISA (n = 3). B The left panel shows S1 and RBD proteins expressed in HEK293 cells. The right panel shows the results of a Western blot using diluted anti-RBD-PP and anti-ClyA serum (1:1000 dilution). C The time-course curve of the anti-RBD IgG titer elicited with different vaccines (n = 3). D Detection of anti-RBD IgG1 and (E) IgG2a titers in mouse serum using ELISA (n = 3). F Antiserum collected after three immunizations with different vaccines to block the binding of S1 and ACE2, denoted by the OD450 (n = 10). G The neutralizing activity of the immune serum was evaluated against SARS-CoV-2 wild-type (WT) virus, and the 50% neutralization titer (NT₅₀) was determined (n = 3). H The ratio of the NT₅₀ titer to the endpoint titer after three immunizations with 50 µg doses of vaccines (n = 3). I The neutralizing activity of the immune serum was evaluated against the SARS-CoV-2 B.1.617.2 (Delta) variant, and the NT₅₀ was determined (n = 3). J Serum after three immunizations with 50 µg of RBD-PP and comparison of the NT₅₀ between SARS-CoV-2 wild-type and SARS-CoV-2 B.1.617.2 (Delta) variant (n = 3). The dotted horizontal lines represent the lower limit of detection of the assay. Data are shown as the mean ± SD. Statistical significance was calculated via one-way ANOVA (A, D, E, G, I), two-way ANOVA (C) or Student’s t test (F, H, J) to obtain p values: ***p < 0.001, **p < 0.01 and *p < 0.05