Intranasal Boosting with Spike Fc-RBD of Wild-Type SARS-CoV-2 Induces Neutralizing Antibodies against Omicron Subvariants and Reduces Viral Load in the Nasal Turbinate of Mice
, Shuofeng Yuan 1,4, Kin-Hang Kok 1,4, Kelvin Kai-Wang To 1,4,5, Honglin Chen 1,4,5, Zhiwei Chen 1,4,5, Dong-Yan Jin 4,5,6, Kwok-Yung Yuen 1,3,4,5 and Jasper Fuk-Woo Chan 1,3,4,5,*
Round 1
Reviewer 1 Report
The study discovered that intranasal boosting with spike Fc-RBD of wild type sars-cov-2 after priming with intramuscular vaccination with inactivated vaccine can induce neutralizing antibody against omicron subvariants. I think the finding is interesting. I have some comments about the study.
1. NAb titer issue. In Figure 2G and H and Figure 3B and C. We can see the NAb titer (Ancestral strain) vs NAb titer (BA.5.2 or XBB.1 strain) had 10 fold or 100 fold difference. How do author justify such difference in titer?
2. Authors may have to provide IC50 titler of the antibody targeting BA.5 and/or XBB.1 for readers to compare.
3. Can author discuss possible reason for Fc-RBD being better than RBD in discussion section?
4. It appears that CVx3 (im) is sufficient for preserving survival rate of infected mice. However, RdRp expression in Nasal turbinate and lung has no difference between CVx3 (i.m.) and PBS. Can author explain this observation in discussion section?
Author Response
REVIEWER 1
The study discovered that intranasal boosting with spike Fc-RBD of wild type sars-cov-2 after priming with intramuscular vaccination with inactivated vaccine can induce neutralizing antibody against omicron subvariants. I think the finding is interesting. I have some comments about the study.
- NAb titer issue. In Figure 2G and H and Figure 3B and C. We can see the NAb titer (Ancestral strain) vs NAb titer (BA.5.2 or XBB.1 strain) had 10 fold or 100 fold difference. How do author justify such difference in titer?
RESPONSE 1: Thank you for the comment. The mice in Figures 2G and 2H, as well as those in Figures 3B and 3C received the CoronaVac vaccine which is using the wild-type SARS-CoV-2 strain. Therefore, expectedly, the NAb titer raised against the ancestral / wild-type strain is markedly higher than that raised against the Omicron strains as Omicron is highly immunoevasive.
- Authors may have to provide IC50titer of the antibody targeting BA.5 and/or XBB.1 for readers to compare.
RESPONSE 2: Thank you for the comment. We have now added the NT50 titers for BA.5.2 and/or XBB.1 in the text as advised (lines 261-264, lines 285-286, and lines 329-330).
- Can author discuss possible reason for Fc-RBD being better than RBD in discussion section?
RESPONSE 3: Thank you for the comment. Thank you for the comment. We have added a more thorough discussion on this in the Discussion section (lines 344-365).
- It appears that CVx3 (im) is sufficient for preserving survival rate of infected mice. However, RdRp expression in Nasal turbinate and lung has no difference between CVx3 (i.m.) and PBS. Can author explain this observation in discussion section?
RESPONSE 4: Thank you for the comment. The nasal turbinate and lung samples were collected at the sacrifice of the mice on day four post-infection, at which stage none of the mice in the PBS or CVx3 (i.m.) group died. While the viral loads (which tend to be at a low level for mice challenged with Omicron BA.5.2 instead of wild-type SARS-CoV-2 as in Figures 3D and 3E even in the absence of vaccines) of the CVx3 (i.m.) and PBS groups were similar at day four post-infection, it is possible that the PBS-treated mice that died between days 5 and 6 post-infection developed immunopathology-related death (protected by CVx3 (i.m.) vaccination).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors combined priming with inactivated whole virus and boosting with different kinds of RBD or S-related immunogens in mice. Afterward, they characterized the antigenic properties and neutralizing activities against parental and omicron sub-variants of mice serum. Viral loads in lung and nasal turbinate were also determined and differentiated. The results are straightforward and descriptive. A minor issue is that the IC50 titer needs to be defined/described in the manuscript. Is it the NT50? It is better to use the term consistently.
Author Response
The authors combined priming with inactivated whole virus and boosting with different kinds of RBD or S-related immunogens in mice. Afterward, they characterized the antigenic properties and neutralizing activities against parental and omicron sub-variants of mice serum. Viral loads in lung and nasal turbinate were also determined and differentiated. The results are straightforward and descriptive. A minor issue is that the IC50 titer needs to be defined/described in the manuscript. Is it the NT50? It is better to use the term consistently.
RESPONSE: Thank you for the comment. We agree with the reviewer that it would be clearer for us to use the term NT50 instead of IC50. We have now revised all the figures and text using NT50 instead of IC50 (Figures 2G, 2H, 3B, 3C, and 4B to 4G).
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Fine.
