Figure 4.

Immune response and protection in the immunized K18-hACE2-transgenic mice (n = 5). (A, B) The endpoint titers of serum IgG specific to Wuhan-Hu-1 RBD protein and S2M2-30 peptide. The values on the Y-axis represent the reciprocal of the highest serum dilution showing a positive result. The dashed lines represent the mean + 2 × SD of the Preimmune group. It is noteworthy that no significant difference between the S2M2-30-KLH and Preimmune groups in panel (A). (C) Copy numbers of viral RdRp gene in mice lung homogenate samples were compared on the 2^nd day post intranasal infection with B.1.1.7/Alpha (UK). Copy numbers of all viral RdRp genes were normalized to β-actin housekeeping gene expression. (D–F) Microneutralization assays. (D) Wild-type HKU-001a, (E) B.1.617.2/Delta, and (F) Omicron BQ.1.1 strains were utilized to assess serum neutralizing activity. The dashed lines in panels (D–F) represent the mean - 2 × SD of the negative control group in which Preimmune serum was used for testing. All data are presented as mean ± SEM. P values less than 0.05 are considered statistically significant, and the ‘ns’ indicates no significance. The *, **, and **** represent P values less than 0.05, 0.01, and 0.001, respectively.