3.2 Intratracheal mucosal immunization with S1-SF-10 induces rapid and effective protective humoral immunity against SARS-CoV-2 compared with intramuscular immunization with S1-AS03 in S1/ACE2 binding inhibitor screening assay
Next, we compared the effectiveness of S1-SF-10-IT with that of intramuscular S1-AS03 (a potent intramuscular adjuvant and clinically used candidate of COVID-19 vaccine adjuvant reported)22, 23 (Figure 2A-D). The serum levels of S1-specific IgG were significantly higher in the S1-AS03-IM group compared with the S1-IM group, and similar to those in the S1-SF-10-IT group after double and triple vaccinations (Figure 2A). However, the BALF levels of S1-specific IgG were significantly lower in the S1-AS03-IM group than in the S1-SF-10-IT group (Figure 2B). Serum and BALF S1-specific IgA were also induced in S1-SF-10-IT group, but not S1-IM group (Figure 2C, D).
Next, we applied the S1/ACE2 Inhibitor Screening Colorimetric Assay24 to determine the protective immunity against SARS-CoV-2 of IgG in serum and S-IgA in BALF induced by S1-SF-10-IT vaccine. The S1:ACE2 binding inhibition (BI) (%) of serum and BALF of mice immunized with S1-IM was <10% at any concentration of specimens after double and triple vaccinations. The BI of mice that received double vaccination with S1-AS03-IM was approximately 20% at the highest concentration of serum and BALF. In contrast, the BIs of mice immunized with S1-SF-10-IT were approximately 40% in serum and 80% in BALF at the highest concentration. After triple vaccinations, the BI levels of S1-AS03-IM and S1-SF-10-IT were 80-90% at the highest concentrations of serum and BALF, respectively. Although the BI values of BALF after triple vaccinations with S1-SF-10-IT were approximately 50% even at the lowest concentrations, those of mice immunized with S1-AS03-IM were very low at <2%. These results indicate that S1-SF-10-IT rapidly and effectively induces protective immunity against SARS-CoV-2 systemically in serum and in the respiratory mucosa compared with S1-AS03-IM.