4.2 Spike protein (S)
The S protein is the most promising antigen for production of SARS-CoV-2 vaccine. It is a surface protein directly recognized by host immune system (Wrapp et al., 2020). It mediates binding with ACE2 receptor in human lungs which is an essential step for virus fusion and entry into host cell to establish subsequent pathogenicity in lungs (Wrapp et al., 2020; Lan et al., 2020). Also, the S protein based vaccines developed against SARS-CoV and MERS-CoV were found to be effective to a large extent (Zhou et al., 2018; Zhang et al., 2020). The S protein in SARS-CoV-2 is cleaved into amino-terminal S1 and carboxy-terminal S2 subunits which is not the case with SARS-CoV (Saif, 2020). The S1 subunit is further composed of two domains; N-terminal domain (NTD) and C-terminal domain (CTD). RBD is located in the CTD and S2 subunit contains the basic elements required for membrane fusion (Zhang et al., 2020). The full length S protein and its antigenic components have the potential to serve as important candidates for vaccine development against COVID-19 (Jiaming et al., 2017; Zhou et al., 2018). Though, the comparison of full length S protein sequences of SARS-CoV-2 and SARS-CoV revealed maximum variability in S1 subunit of S protein, it is still a promising target for vaccine development against COVID-19 (Yu et al., 2020; Morse et al., 2020).
The native S proteins exist as a trimeric form on the surface of virus. However, when its ectodomain or S1 is expressed as a recombinant protein in eukaryotic systems, the protein exists predominantly in a monomeric form (Li et al., 2013; Kim et al., 2020). Recreating the trimeric structure has been shown to increase immunogenicity which depicts the importance of mimicking the native trimeric structure (Li et al., 2013; Krammer et al., 2012; Kim et al., 2020). Using its patented Trimer-Tag© technology, Clover Biopharmaceuticals produced a COVID-19 S-Trimer subunit vaccine by a rapid mammalian cell-culture based expression system which resembles the native trimeric viral spike. Having extensive commercial-scale GMP biomanufacturing capabilities in China, Clover is expected to rapidly scale-up and produce large-quantities of this new coronavirus vaccine having GSK2 adjuvant (Liu et al., 2017; Glaxo Smit Kline, 2020; Zhang et al., 2020). Based on their experience with MERS and SARS, Novavax developed their COVID-19 candidate vaccine targeting the S protein of SARS-CoV-2 by using their recombinant nanoparticle vaccine technology along with their proprietary adjuvant Matrix-MTM (Novavax, 2020). After single shot immunization high immunogenicity was exhibited in animal models with significant display of S protein-specific antibodies, antibodies blocking the binding of S protein to the receptor, and wild-type VN antibodies. First human trial is expected by mid-May.
A SARS coronavirus vaccine based on a recombinant full-length trimeric S protein exhibited native antigenicity, specific binding to soluble ACE2 receptor, adequate protection against challenge infection in animal model (Morse et al., 2020). The vaccines based on full-length S protein are supposed to present the correct conformation of the protein, provide more epitopes, and exhibit higher immunogenicity (Zhang et al., 2020) but in case of SARS-CoV some vaccines mediate enhancement of viral infection which has raised safety concerns over full-length S protein based vaccines (Jaume et al., 2012; Wang et al., 2020). Furthermore, the S protein based subunit vaccines have been reported to produce higher VN antibody titers and better protection than live-attenuated SARS-CoV, full-length S protein, and DNA-based S protein vaccines (Buchholz et al., 2004; Morse et al., 2020). However, mRNA vaccines put forward by Moderna are composed of mRNAs which encode for full-length S, S1, or S2 proteins from SARS-CoV and MERS-CoV and are carried in cationic lipid nanoparticles (Morse et al., 2020). Higher VN antibody titers were observed in mice vaccinated with mRNA encoding full-length S protein compared to mRNA encoding S protein or S2 subunit and in New Zealand white rabbits full-length S protein reduced more than 90% of viral load in lungs along with significant induction of VN antibody against MERS-CoV.
RBD of S protein comprises of major critical neutralizing domain which directly interacts with the ACE2 receptor of host cells and immunization with RBD based vaccine blocks this interaction by inducing specific VN antibodies (Zhang et al., 2020; Zhou et al., 2019). Since, these vaccines exert strong protective immunity against viral infection most of the COVID-19 subunit vaccines under process are based on RBD antigen (Zhang et al., 2020; Wang et al., 2020). As a candidate for vaccine against SARS-CoV it has been demonstrated that RBD, consisting of multiple conformational neutralizing epitopes, induced higher VN antibody titer (Zhu et al., 2013). RBD of S protein is considered more suitable because it is relatively more conserved than the S1 subunit and contains multiple conformational neutralizing epitopes (Jiang et al., 2005; Zhang et al., 2020). However, S1 subunit itself induces strong immune response and protection against CoV infection (Li et al., 2013; Adney et al., 2019; Zhang et al., 2020). On the other hand, S2 subunit protein has conserved amino acid sequence and high homology among different virus strains which makes it a potential target for development of a broad spectrum vaccine effective against divergent virus strains (Elshabrawy et al., 2012; Zhou et al., 2018; Wang et al., 2020). Though, S2 subunit as well as NTD have the potential to elicit VN antibodies, they are less immunogenic, exhibit lower antibody titers, cellular immune responses, and protection compared to other antigenic determinants (Jiaming et al., 2017). Thus, based on the above facts RBD and S1 protein are the critical vaccine candidates for SARS-CoV as well as SARS-CoV-2.