4 Discussion
In this study, we described new findings on intratracheal administration
of mucosal adjuvant SF-10 derived from PS for COVID-19 vaccines. The
main findings include i) S1-SF-10-IT vaccine induced S1-specific IgG in
serum to levels similar to those found with S1-AS03-IM, together with
induction of higher S-IgA and IgG levels in the respiratory mucosa than
S1-AS03-IM, which plays an important role in protection against
SARS-CoV-2 infection. ii) S1-SF-10-IT induced S1-specific IgG and IgA
ASCs and S1-responsive IFN-γ, IL-4 and IL-17A CSCs in both the spleen
and lungs.
Respiratory mucosal immunity plays an important front-line role against
respiratory viral infections. Especially, S-IgA in the respiratory
mucosa protects against invasion of viruses into mucosal cells,
resulting in prevention of infection and transmission of pathogens to
other individuals.12, 25 Our results demonstrated that
S1-SF-10-IT vaccine induced S1-specific IgG and IgA ASCs in the spleen
and lungs (Figure 3) and S1-specific antibodies in the serum and BALF
(Figure 1, 2A-D), which efficiently inhibited S1/ACE2 binding (Figure
2E-H). The S1-AS03-IM induced S1-specific IgG in the serum and BALF
(Figure 2A, B), but not S1-specific IgA in serum and BALF (Figure 2C,
D), and S1-specific IgA ASCs in the spleen and lungs (Figure 3B, D). The
antibodies-related inhibitory effects of S1/ACE2 binding in BALF induced
by S1-AS03-IM were also weaker than intratracheal application of
S1-SF-10-IT (Figure 2F, H).
In addition, the role of mucosal IgG induced by S1-SF-10-IT cannot be
ignored; S1-SF-10-IT induced significantly higher BALF IgG levels
compared with S1-AS03-IM regardless of the time of administration
(Figure 2B). Based on the above results, our findings suggest that
vaccination with S1-SF-10-IT provides efficient protection against
respiratory infection.
In addition to protective humoral immunity, T cell-mediated immunity is
important for protection of SARS-CoV-2 infection. It was reported
recently that SARS-CoV-2 antigen responsive IFN-γ CSCs in blood plays an
important role in the early phase of SARS-CoV-2 infection and
acceleration of virus clearance.26 Also, IFN-γ
producing lung tissue-resident memory T cells (TRMs) conduct immune
surveillance for pathogens that could invade the tissues and robustly
protect against site-specific infection by viruses, including SARS-CoV-2
in the respiratory tract, thus preventing infection.27S1-AS03-IM induced S1-responsive IFN-γ CSCs in the spleen, but not in
the lungs. In contrast, S1-SF-10-IT induced S1-responsive IFN-γ CSCs in
not only the spleen but also in the lungs. Additionally, S1-SF-10-IT
vaccine induced S1-responsive granzyme β expression in splenic
CD8+ T cells, a marker of cellular immunity
(Supplementary Figure 1). Based on this likely scenario, we suggest that
the use of the intratracheal route for S1-SF-10 vaccine delivery offers
the advantage of preventing respiratory infection, such as SARS-CoV-2,
better than intramuscularly delivered vaccine.
Although S1-SF-10-IT induced significantly higher splenic S1-responsive
IL-17A CSCs relative to S1-AS03-IM, splenic S1-responsive IL-4 CSCs
induced by S1-SF-10-IT were lower than those by S1-AS03-IM (Figure 4E).
These results indicate that S1-SF-10-IT tends to induce mild Th2 type
immunity and marked Th17 type immunity, compared with intramuscularly
injected vaccine. Since IL-17A is secreted by Th17 and involved in
production and migration to mucosal sites of IgA,28,
29 these results imply that IL-17A induced by S1-SF-10-IT vaccine is
mediated at least in part by marked IgA induction in serum and
respiratory tract mucosa. Although the functional properties of
vaccine-induced Th17 TRM in the lungs remain to be examined in detail,
Th17 TRM could mediate the induction of respiratory IgA following
vaccination with S1-SF-10-IT.28, 29 On the other hand,
others indicated that Th17 induced by SARS-CoV-2 infection can induce a
cytokine storm.30, 31 Since there are two types of
Th17; the first is non-pathogenic Th17, which produces IL-10, has
anti-inflammatory activity, while the second is the pathogenic Th17,
which does not induce IL-10,31, 32 further studies are
needed to determine whether lung Th17 induced by S1-SF-10-IT
immunization are non-pathogenic type capable of inducing IL-17A and
IL-10 to protect against SARS-CoV-2 infection.
Our study has certain limitations. We investigated protective immunity
against SARS-CoV-2 infection using by S1/ACE2 binding inhibition assayin vitro . To confirm the protective immunity induced by mucosal
administration of S1-SF-10 against mortality, severity and transmission
by SARS-CoV-2 infection in vivo , we need to conduct protection
experiments in SARS-CoV-2-infected animals.
In conclusion, we have demonstrated in the present study that SF-10,
which mimics human PS, is a suitable and effective mucosal adjuvant for
COVID-19 vaccines and can be used for intratracheal delivery of vaccines
to initiate rapid and potent local respiratory mucosal and systemic
immunity. Intratracheal administration of COVID-19 vaccines using SF-10
are expected to provide effective protection against respiratory
infection of SARS-CoV-2.