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.