Figure 2.

A strong immune response was elicited by RBD-mFc intranasal immunization. (A) Schematic of the BALB/c immunization strategy. Four mice from each group were immunized with different vaccines by different routes at the times indicated (days). Serum was collected every two weeks and assessed for specific antibody response to RBD. (B) Overall immune response of the four immunized groups. The data show the reciprocal endpoint dilution titers, with each data point representing the mean of four animals. Mice immunized with RBD-mFc protein developed a more rapid and efficient response after prime and boost vaccination compared to RBD. Serum antibody responses were analyzed 14 days after the 2^nd boost immunization. RBD-specific IgG (C) and IgA (D) were assessed by ELISA. Intranasal immunization with RBD-mFc induced a stronger immune effect than immunization in the other two groups. Analysis of IgG subclasses of the RBD-specific antibody response (E, F). (G) The neutralizing antibody response to SARS-CoV-2 was determined by PRNT and represented as the reciprocal half-maximal inhibition concentration (IC50). All RBD protein-immunized sera could efficiently inhibit authentic SARS-CoV-2 infection, whereas sera from the PBS control showed no neutralization activity The data in (B–G) represent the mean ± SD. Significant differences were determined by one-way ANOVA with Tukey’s multiple comparisons test. ns, not significant; *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001.