Sometimes journal articles are just getting published ahead of print when researchers submitted a paper for reviewing. Whilst in this case this does not cause any serious problems - normally a note at the end of the paper or an addition in the discussion section is sufficient to acknowledge the publication- this causes a problem if publishing a blog entry. In this case a paper describing the immunogenicity of MERS-CoV receptor binding fragments was published ahead of publication just one day after the post describing the immunogenicity of recombinant adenovirus expressing either the full length or the S1 subunit ((r)Ad5.MERS-S and (r)Ad5.MERS-S respectively) was published on this blog.
|Coronavirus S protein -full length (top) and S1 subdomains (bottom)|
As described before, the S protein of all Coronaviruses plays an important role in the entry of Coronavirus particles into the host cell, which is mediated by a receptor-binding domain (RBD) located within the S1 subunit with the RBD itself being subdivided into two core subdomains flanking the receptor-binding subdomain that interacts with the N-terminal domain of DPP4. Accordingly previous data suggest that a truncated fragment of the MERS-CoV S1 containing the RBD fused with human IgG Fc fragment (S377-588-Fc) not only prevents MERS-CoV infection of cell lines but also elicits a high antibody titer in rabbits and mice infected with MERS-CoV akin to an fusion protein generated based on the SARS S1 subunit. In the present study, five human fusion proteins representing fragments of the viral S protein containing not only the RBD but also the N terminal core subdomain as well truncations of the C terminal core subdomain and/or additional N-terminal residues were investigated for their antigenic potential.
Co-immunoprecipitation of purified fusion proteins with either recombinant -soluble- hDPP4 receptor or hDPP4 expressing Huh7 cells confirmed that all proteins bind the receptor when compared to Fc alone, although the S358-588-Fc, S367-606-Fc, and S377-588-Fc fragments show the highest receptor binding affinity.
In order to investigate if the fragments are immunogenic, mice and rabbits were vaccinated with the fragments and an adjuvant at day 0 and boosted twice at 21 days intervals. The titer of neutralizing and MERS-CoV S antibodies was then determined 10 days after the last booster as well (in the case of the S377-588-Fc fragment) at days 0, 10, 31 and 136 post initial vaccination. All five fragments of MERS-CoV S tested induced a robust antibody response, although the IgG response induced by the S367-588-Fc and S377-588-Fc fragments was stronger when compared to levels induced by S350-588-Fc, fragments, suggesting that the presence of an extended N- or C-terminus might mask the RBD and thus reduces immunogenicity. Interestingly however, despite the lower induction of lower S1 specific IgG titers, the S350-588-Fc fragment does induce high levels of IgG1 S1 specific titers that are comparable to those induced by S367-588-Fc and S377-588-Fc. In addition, the presence of an extended N- or C-terminus prevents the induction of high IgG2a titers since the S377-588-Fc fragment produces the IgG2a highest titers when compared to the other fragments.
In conclusion, the expression of a subdomain of the MERS-CoV S1 protein is sufficient to elicit a strong immunogenic response. Unfortunately, the study did not include full-length MERS-CoV S nor full-length MERS-CoV S1. Based on the results however, it may feasible to assume that the expression of the S377-588 subdomain might induce a stronger response. Future studies however need to address if the infection of mice and rabbits with an adenoviral construct akin to the one used before can induce the formation of neutralizing antibodies as well.
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