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.