Abstract
The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been identified as the prime target for vaccine development. The spike protein mediates both binding to host cells and membrane fusion and is also so far the only known viral target of neutralizing antibodies. Coronavirus spike proteins are large trimers that are relatively unstable, a feature that might be enhanced by the presence of a polybasic cleavage site in SARS-CoV-2 spike. Exchange of K986 and V987 for prolines has been shown to stabilize the trimers of SARS-CoV-1 and the Middle East respiratory syndrome coronavirus spike proteins. Here, we test multiple versions of a soluble spike protein for their immunogenicity and protective effect against SARS-CoV-2 challenge in a mouse model that transiently expresses human angiotensin-converting enzyme 2 via adenovirus transduction. Variants tested include spike proteins with a deleted polybasic cleavage site, proline mutations, or a combination thereof, besides the wild-type protein. While all versions of the protein were able to induce neutralizing antibodies, only the antigen with both a deleted cleavage site and the K986P and V987P (PP) mutations completely protected from challenge in this mouse model.IMPORTANCE A vaccine for SARS-CoV-2 is urgently needed. A better understanding of antigen design and attributes that vaccine candidates need to have to induce protective immunity is of high importance. The data presented here validate the choice of antigens that contain the PP mutations and suggest that deletion of the polybasic cleavage site may lead to a further-optimized design.
Highlights
The sequence based on the S gene of SARS-CoV-2 strain Wuhan-1 was initially codon optimized for mammalian cell expression
All constructs were recognized in a similar manner by monoclonal antibody (MAb) CR3022 [16, 17], an antibody that binds to the receptor binding domain (RBD) (Fig. 1F)
Since we did not observe cleavage of the majority of the protein when it was purified, even with the polybasic cleavage site present, we speculate that removal of the site might make the protein more stable in vivo postvaccination
Summary
The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been identified as the prime target for vaccine development. Exchange of K986 and V987 for prolines has been shown to stabilize the trimers of SARS-CoV-1 and the Middle East respiratory syndrome coronavirus spike proteins. We test multiple versions of a soluble spike protein for their immunogenicity and protective effect against SARS-CoV-2 challenge in a mouse model that transiently expresses human angiotensin-converting enzyme 2 via adenovirus transduction. Variants tested include spike proteins with a deleted polybasic cleavage site, proline mutations, or a combination thereof, besides the wild-type protein. While all versions of the protein were able to induce neutralizing antibodies, only the antigen with both a deleted cleavage site and the K986P and V987P (PP) mutations completely protected from challenge in this mouse model
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