LitCovid-sentences  

PMC:7105881 / 2136-60070 JSONTXT 14 Projects

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Id Subject Object Predicate Lexical cue
T15 0-12 Sentence denotes Introduction
T16 13-130 Sentence denotes Coronaviruses (CoVs) belong to the subfamily Othocoronavirinae, in the family Coronaviridae of the order Nidovirales.
T17 131-444 Sentence denotes According to the 10th Report on Virus Taxonomy from the International Committee on Taxonomy of Viruses (ICTV), the Othocoronavirinae is comprised of four genera, including alphacoronavirus (alpha-CoV), betacoronavirus (beta-CoV), gammacoronavirus (gamma-CoV), and deltacoronavirus (delta-CoV) (King et al., 2018).
T18 445-690 Sentence denotes Alpha- and beta-CoVs can infect mammals, including but not limited to bats, pigs, cats, mice, and humans (Kusanagi et al., 1992; Li et al., 2005b; Poon et al., 2005; Drexler et al., 2014; Pedersen, 2014; Kudelova et al., 2015; Cui et al., 2019).
T19 691-824 Sentence denotes Gamma- and delta-CoVs usually infect birds, while some of them could infect mammals (Woo et al., 2009a, 2012, 2014; Ma et al., 2015).
T20 825-935 Sentence denotes Since the late sixties, CoVs have been recognized as one of the viral sources responsible for the common cold.
T21 936-1617 Sentence denotes Among all CoVs identified so far, seven have the ability to infect humans, including human coronavirus 229E (HCoV-229E) and human coronavirus NL63 (HCoV-NL63), which belong to alpha-CoVs (Hamre and Procknow, 1966; Chiu et al., 2005), as well as human coronavirus OC43 (HCoV-OC43), human coronavirus HKU1 (HCoV-HKU1), severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the newly emerged coronavirus (2019-nCoV), which are known to be beta-CoVs (Drosten et al., 2003; Ksiazek et al., 2003; Vabret et al., 2003; Woo et al., 2005; Zaki et al., 2012; Du et al., 2016b; Zhang et al., 2020; Zhu et al., 2020) (Figure 1).
T22 1618-1743 Sentence denotes FIGURE 1 Phylogenetic tree of coronaviruses (CoVs) based on the nucleotide sequences of RNA dependent RNA polymerase (RdRp).
T23 1744-1845 Sentence denotes The Tree, with 1,000 bootstrap values, was constructed by the maximum likelihood method using MEGA 6.
T24 1846-1949 Sentence denotes The four main phylogenetic clusters correspond to genera alpha-CoV, beta-CoV, gamma-CoV, and delta-CoV.
T25 1950-1994 Sentence denotes Each CoV genus contains different subgenera.
T26 1995-2035 Sentence denotes The letters in blue indicate human CoVs.
T27 2036-2181 Sentence denotes Four human CoVs, including HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1, have been identified in humans, but without causing severe infections.
T28 2182-2284 Sentence denotes HCoV-229E was isolated from nasal secretions of medical students with minor upper respiratory disease.
T29 2285-2384 Sentence denotes This virus was an original isolate, and was first reported in the 1960s (Hamre and Procknow, 1966).
T30 2385-2610 Sentence denotes In addition to HCoV-229E, several studies have reported the recovery of HCoV-OC43 from patients with upper respiratory tract illness (Tyrrell and Bynoe, 1965; Hamre et al., 1967; McIntosh et al., 1967; Kapikian et al., 1969).
T31 2611-2838 Sentence denotes In 2004, HCoV-NL63 was isolated from clinical species of infants suffering from pneumonia or bronchiolitis, and characterized for its ability to infect human respiratory tract (Fouchier et al., 2004; van der Hoek et al., 2004).
T32 2839-2975 Sentence denotes The subsequent study in 2005 identified a new member of CoVs, named HCoV-HKU1, from a 71-year-old man with pneumonia (Woo et al., 2005).
T33 2976-3191 Sentence denotes Generally, these four viruses are the most common pathogens causing mild upper respiratory infection or asymptomatic infection, and count for about 30% of all colds (Myint, 1994; Lau et al., 2006; Kim et al., 2017).
T34 3192-3348 Sentence denotes In the serological surveillance on healthy adults, HCoV-229E, HCoV-NL63, and HCoV-OC43 demonstrated more than 90% seropositive with the immunological assay.
T35 3349-3466 Sentence denotes It appears common for these CoVs to infect children (Mourez et al., 2007; Shao et al., 2007; Severance et al., 2008).
T36 3467-3665 Sentence denotes In contrast to the above three human CoVs, HCoV-HKU1 has around 50% seropositive in healthy individuals and a relatively low exposure rate in children (Lehmann et al., 2008; Severance et al., 2008).
T37 3666-3806 Sentence denotes Although the prevalence of various CoVs is different, the incidence among these viruses shows no significant difference (Woo et al., 2009b).
T38 3807-4016 Sentence denotes The afore-mentioned four CoVs have been detected in 2.1–17.9% of clinical specimens (Esper et al., 2006; Lau et al., 2006; Gerna et al., 2007; Regamey et al., 2008; Matoba et al., 2015; Killerby et al., 2018).
T39 4017-4230 Sentence denotes These viruses have also been associated with lower respiratory tract illness in children, elders, and immunodeficient individuals (Falsey et al., 2002; Fouchier et al., 2004; Woo et al., 2005; Gerna et al., 2006).
T40 4231-4406 Sentence denotes HCoV-229E and HCoV-OC43 may lead to central nervous system infection since viral RNAs are detected in the brain of some patients (Arbour et al., 2000; Desforges et al., 2014).
T41 4407-4656 Sentence denotes Unlike the above four human CoVs, SARS-CoV, MERS-CoV, and 2019-nCoV have caused severe pneumonia and/or failure of other organs, even death, among infected populations (Nicholls et al., 2003; Zhong et al., 2003; Zaki et al., 2012; Zhu et al., 2020).
T42 4657-4868 Sentence denotes The epidemic outbreak of SARS-CoV began in the Guangdong Province of China in November 2002, and spread through human-to-human transmission to other parts of the world within a few months (Ksiazek et al., 2003).
T43 4869-5033 Sentence denotes From November 2002 to August 2003, SARS-CoV infected more than 8,098 people in 29 counties, resulting in over 774 deaths with ∼10% fatality rate (Du et al., 2009a).
T44 5034-5143 Sentence denotes Palm civets serving as a potential intermediate host of this virus were traced immediately (Tu et al., 2004).
T45 5144-5246 Sentence denotes Chinese horseshoe bats (Rhinolophus sinicus) are the natural reservoir of SARS-CoV (Li et al., 2005b).
T46 5247-5440 Sentence denotes Various bat SARS-related CoVs (SARSr-CoV) have been identified in Yunnan, China, several of which can infect human cells, and have been further characterized (Ge et al., 2013; Hu et al., 2017).
T47 5441-5520 Sentence denotes These discoveries indicate the threat of re-emergence of SARS-CoV or SARSr-CoV.
T48 5521-5701 Sentence denotes A decade later, another highly pathogenic human CoV, MERS-CoV, emerged, and the first patient with MERS-CoV infection was reported in Saudi Arabia in June 2012 (Zaki et al., 2012).
T49 5702-5874 Sentence denotes By December 26, 2019, a total of 2,494 laboratory-confirmed cases of MERS, including 858 associated deaths in 27 countries (fatality rate 34.4%), were reported to the WHO1.
T50 5875-6073 Sentence denotes Globally, the majority (about 80%) of human cases have been reported in Saudi Arabia, where people get infected through direct contact with infected dromedary camels or persons2 (Zaki et al., 2012).
T51 6074-6275 Sentence denotes Isolation of MERS-CoV and detection of neutralizing antibodies from dromedary camels suggest that these camels are potentially an important intermediate host (Reusken et al., 2013; Azhar et al., 2014).
T52 6276-6359 Sentence denotes Similar to SARS-CoV, MERS-CoV is also an emerging zoonotic virus (Li and Du, 2019).
T53 6360-6598 Sentence denotes Bats habituate several CoVs phylogenetically related to MERS-CoV, and some of them are identical to MERS-CoVs, suggesting that MERS-CoV may originate from bats (Annan et al., 2013; Lelli et al., 2013; Lau et al., 2018; Luo et al., 2018a).
T54 6599-6834 Sentence denotes Different from SARS-CoV, which has not caused infections in humans since 2004 (Du et al., 2009a), the transmission of MERS-CoV has not been interrupted, and the infected human cases continue increasing1 (Mobaraki and Ahmadzadeh, 2019).
T55 6835-6897 Sentence denotes Currently, human-to-human transmission of MERS-CoV is limited.
T56 6898-6992 Sentence denotes A new CoV, 2019-nCoV, has caught worldwide attention (Liu and Saif, 2020; Zhang et al., 2020).
T57 6993-7557 Sentence denotes It was first identified in Wuhan, China in December 2019, from patients with pneumonia (Zhu et al., 2020), and has infected more than 70000 people globally, including 2,009 deaths (∼2.7% fatality rate), as of February 19, 2020, particularly in China, and the other parts of the world, including Australia, Japan, Malaysia, Singapore, South Korea, Viet Nam, Cambodia, Philippines, Thailand, Nepal, Sri Lanka, India, United States, Canada, France, Finland, Germany, Italy, Russian Federation, Spain, Sweden, United Kingdom, Belgium, Egypt, and United Arab Emirates3.
T58 7558-7766 Sentence denotes Different from MERS-CoV but similar to SARS-CoV, 2019-nCoV can cause human-to-human transmission, and its intermediate host that leads to the current human infection and outbreak is still under investigation.
T59 7768-7863 Sentence denotes Genome of Emerging Human Coronaviruses, as Well as Structure and Function of Their Key Proteins
T60 7864-7951 Sentence denotes The human CoVs are enveloped viruses with a positive-sense, single-stranded RNA genome.
T61 7952-7983 Sentence denotes They are 80–160 nm in diameter.
T62 7984-8164 Sentence denotes Like other CoVs, human CoVs contain the largest viral genome [27–32 kilobase pairs (kb)] among the RNA viruses, and they share similar genome organization (Fehr and Perlman, 2015).
T63 8165-8485 Sentence denotes Two large overlapping open reading frames (ORFs), ORF 1a and ORF 1b, occupy two-thirds of the genome at the 5′-terminus, and a third of the genome at the 3′-terminus encodes four common structural proteins in the gene order of spike (S), envelope (E), membrane (M), and nucleocapsid (N) (5′–3′) (Fehr and Perlman, 2015).
T64 8486-8825 Sentence denotes The large ORF 1ab is a replicase gene encoding polyproteins 1a (pp1a) and pp1b/1ab, which can be cleaved into 15–16 non-structural proteins (nsp2-nsp16 or nsp1-nsp16) by 3C-like proteinase (3CLpro, nsp5) and papain-like proteinase (PLpro, nsp3) (Bailey-Elkin et al., 2014; Fehr and Perlman, 2015; Tomar et al., 2015; Snijder et al., 2016).
T65 8826-9011 Sentence denotes In addition to the genes encoding the above structural proteins, the genes encoding accessory proteins have also been detected in the 3′ region between S–E–M–N (Fehr and Perlman, 2015).
T66 9012-9251 Sentence denotes Some beta-CoVs, such as HCoV-OC43 and HCoV-HKU1, contain hemagglutinin-esterase (HE) gene located between ORF 1ab and S gene encoding an additional structural protein, HE (De Groot et al., 2011; Desforges et al., 2013; Huang et al., 2015).
T67 9252-9522 Sentence denotes Similar to other human CoVs, SARS-CoV possesses a ∼29-kb genome, which encodes pp1a and pp1ab, four main structural proteins (S, E, M, and N), and eight accessory proteins, such as 3a, 3b, 6, 7a, 7b, 8a, 8b, and 9b (Figure 2A) (Marra et al., 2003; Snijder et al., 2003).
T68 9523-9727 Sentence denotes The MERS-CoV genome is about 30 kb in length and encodes pp1a, pp1ab, four structural proteins (S, E, M, and N), and five accessory proteins (3, 4a, 4b, 5, and 8b) (Figure 2A) (van Boheemen et al., 2012).
T69 9728-9902 Sentence denotes The genomic RNA of SARS-CoV and MERS-CoV is packed inside capsid formed by the N protein, while the M, E, and S proteins form the envelope surrounding the capsid (Figure 2B).
T70 9903-9981 Sentence denotes Accessory genes may incorporate into virions at low levels (Liu et al., 2014).
T71 9982-10096 Sentence denotes Nevertheless, neither SARS-CoV nor MERS-CoV appears to contain the HE gene (Rota et al., 2003; Zaki et al., 2012).
T72 10097-10248 Sentence denotes FIGURE 2 Schematic structure of SARS-CoV, MERS-CoV, and 2019-nCoV. (A) Schematic diagram of genomic organization of SARS-CoV, MERS-CoV, and 2019-nCoV.
T73 10249-10312 Sentence denotes The genomic regions or open-reading frames (ORFs) are compared.
T74 10313-10662 Sentence denotes Structural proteins, including spike (S), envelope (E), membrane (M) and nucleocapsid (N) proteins, as well as non-structural proteins translated from ORF 1a and ORF 1b and accessory proteins, including 3a, 3b, 6, 7a, 7b, 8a, 8b, and 9b (for SARS-CoV), 3, 4a, 4b, 5, and 8b (for MERS-CoV), and 3a, 6, 7a, 7b, 8, and 10 (for 2019-nCoV) are indicated.
T75 10663-10750 Sentence denotes 5′-UTR and 3′-UTR, untranslated regions at the N- and C-terminal regions, respectively.
T76 10751-10875 Sentence denotes Kb, kilobase pair. (B) Schematic structure of virion of SARS-CoV, MERS-CoV, and 2019-nCoV and its major structural proteins.
T77 10876-10961 Sentence denotes Several proteins of human CoVs are important for viral infection and/or pathogenesis.
T78 10962-11066 Sentence denotes For example, most nsps participate in viral RNA replication and/or transcription (Snijder et al., 2016).
T79 11067-11229 Sentence denotes The accessory proteins interact with host cells, potentially helping the viruses to evade the immune system and increase their virulence (Menachery et al., 2017).
T80 11230-11410 Sentence denotes The HE protein assists in the attachment of virus–host cells, thus playing a key role in the production of infectious virions, as in the case of HCoV-OC43 (Desforges et al., 2013).
T81 11411-11532 Sentence denotes The M and E proteins are responsible for virus assembly or promote virulence (Scobey et al., 2013; DeDiego et al., 2014).
T82 11533-11726 Sentence denotes Different from the above proteins, the S protein of human CoVs mediates viral entry into host cells and subsequent membrane fusion, enabling viral infection (Du et al., 2009a; Lu et al., 2014).
T83 11727-11884 Sentence denotes The S protein is a class I viral protein, which can be cleaved into two functional subunits, an amino-terminal S1 subunit and a carboxyl-terminal S2 subunit.
T84 11885-12054 Sentence denotes The S1 subunit is responsible for virus–host cell receptor binding, whereas the S2 subunit is involved in virus–host membrane fusion (Li et al., 2005a; Lu et al., 2014).
T85 12055-12147 Sentence denotes The S1 contains two major domains, an N-terminal domain (NTD) and a C-terminal domain (CTD).
T86 12148-12391 Sentence denotes In general, NTDs mediate sugar binding (Li, 2016; Li et al., 2017; Ou et al., 2017; Hulswit et al., 2019; Tortorici et al., 2019), whereas CTDs facilitate protein receptor recognition (Wong et al., 2004; Hofmann et al., 2006; Lu et al., 2013).
T87 12392-12539 Sentence denotes The NTDs and CTDs of the S1 subunit can bind host receptors or function as receptor-binding domains (RBDs) (Lu et al., 2013; Hulswit et al., 2019).
T88 12540-12635 Sentence denotes The entry of human CoVs relies on the interaction between viral and cellular membrane proteins.
T89 12636-12746 Sentence denotes Recognition of S1 subunit with a receptor and/or sugar on the cell surface initiates the infection (Li, 2015).
T90 12747-12922 Sentence denotes After the initial recognition and binding, the S protein undergoes conformational changes, followed by membrane fusion through the S2 region (Li, 2015, 2016; Du et al., 2017).
T91 12923-13041 Sentence denotes Consequently, the viral genetic materials are delivered into the host cell through the fusion core (Du et al., 2009a).
T92 13042-13263 Sentence denotes Similar to HCoV-NL63 (Hofmann et al., 2005; Li, 2015), SARS-CoV recognizes, through the RBD in the CTD region of its S1 subunit, angiotensin-converting enzyme 2 (ACE2) as the receptor on the target cell (Li et al., 2003).
T93 13264-13356 Sentence denotes The RBD (CTD) in two states (standing or lying) has been observed in the trimeric S protein.
T94 13357-13495 Sentence denotes ACE2 binds to standing RBD, specifically in the receptor-binding motif (RBM), keeping the RBD in the “standing” state (Yuan et al., 2017).
T95 13496-13616 Sentence denotes In addition to human ACE2, SARS-CoV S protein could also bind to palm civet and mouse ACE2s (Li et al., 2004; Li, 2008).
T96 13617-13743 Sentence denotes Mutations in the RBD of S1 subunit are required for cross-species transmission of SARS-CoV (Li et al., 2005c; Li, 2008, 2016).
T97 13744-13905 Sentence denotes Several bat SARSr-CoVs have been identified, and these CoVs can utilize human ACE2 as their receptor to bind the target cells (Ge et al., 2013; Hu et al., 2017).
T98 13906-14018 Sentence denotes The structure of SARS-CoV S trimer and RBD binding to the ACE2 receptor is shown in Supplementary Figures S1A,B.
T99 14019-14122 Sentence denotes MERS-CoV RBD shares a structure similar to that of the homology domain of SARS-CoV (Wang et al., 2013).
T100 14123-14258 Sentence denotes However, antibodies induced by SARS-CoV RBD have no cross-reactivity and/or cross-neutralizing activity to MERS-CoV (Du et al., 2013b).
T101 14259-14430 Sentence denotes Moreover, MERS-CoV utilizes dipeptidyl peptidase 4 (DPP4) as a receptor through the RBD (CTD) region (Raj et al., 2013), which is distinct from the SARS-CoV receptor ACE2.
T102 14431-14612 Sentence denotes Although the core regions of SARS-CoV and MERS-CoV RBDs are similar, their RBM regions are significantly different, which explains why they recognize different receptors (Li, 2015).
T103 14613-14695 Sentence denotes MERS-CoV S trimer also maintains a structure similar to that of SARS-CoV S trimer.
T104 14696-14852 Sentence denotes Both standing and lying states can be detected in the MERS-CoV RBDs, whereas DPP4 only binds to the standing RBD (Pallesen et al., 2017; Yuan et al., 2017).
T105 14853-14943 Sentence denotes MERS-CoV is clustered with Ty-BatCoV HKU4 and Pi-BatCoV HKU5 in the subgenus Merbecovirus.
T106 14944-15040 Sentence denotes Ty-BatCoV HKU4, but not Pi-BatCoV HKU5, could use human DPP4 as a receptor (Yang et al., 2014b).
T107 15041-15199 Sentence denotes Recently, some MERS-related CoVs (MERSr-CoVs) have been discovered from bats that can enter human DPP4-expressing cells (Lau et al., 2018; Luo et al., 2018a).
T108 15200-15337 Sentence denotes These findings suggest that the emergence of MERSr-CoV may threaten human health owing to their potential for cross-species transmission.
T109 15338-15461 Sentence denotes The MERS-CoV S protein and RBD and their complexes, along with the DPP4 receptor, are shown in Supplementary Figures S1C,D.
T110 15462-15642 Sentence denotes Recent studies have found that the new human CoV, 2019-nCoV, which belongs to the species of SARSr-CoV, shares high sequence identify (about 79.5%) to SARS-CoV (Zhou et al., 2020).
T111 15643-15817 Sentence denotes The genome of 2019-nCoV encodes pp1ab (translated from ORF 1ab), four structural proteins (S, E, M, and N), and six accessory proteins (3a, 6, 7a, 7b, 8, and 10) (Figure 2A).
T112 15818-15886 Sentence denotes Same as SARS-CoV and MERS-CoV, 2019-nCoV appears to have no HE gene.
T113 15887-15978 Sentence denotes The virion of 2019-nCoV consists of similar structure as SARS-CoV and MERS-CoV (Figure 2B).
T114 15979-16084 Sentence denotes Like SARS-CoV, 2019-nCoV also uses ACE2 as its cellular receptor to enter host cells (Zhou et al., 2020).
T115 16085-16197 Sentence denotes Currently, the structure of 2019-nCoV RBD and/or its binding with the viral receptor has not yet been available.
T116 16199-16267 Sentence denotes Overview of Vaccines Against Emerging Pathogenic Human Coronaviruses
T117 16268-16559 Sentence denotes Unlike the four low pathogenic human CoVs, including HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1, which cause mild to no pathogenesis in humans, SARS-CoV, MERS-CoV, and 2019-CoV are three highly pathogenic human CoVs (Channappanavar and Perlman, 2017; Cui et al., 2019; Zhu et al., 2020).
T118 16560-16888 Sentence denotes With the increasing numbers of 2019-nCoV and MERS-CoV infections and continuous threat of re-emergence of SARS-CoV, as well as the potential of SARS- and MERS-related CoVs to cause human infections, it is critical to develop vaccines with strong efficacy and safety targeting these viruses to prevent their infections in humans.
T119 16889-17031 Sentence denotes Since the vaccines against 2019-nCoV have not been available, the rest of the review will focus on the vaccines against SARS-CoV and MERS-CoV.
T120 17032-17253 Sentence denotes Although a variety of vaccines have been developed against SARS-CoV and MERS-CoV, most of them are in the preclinical studies, and only several have been tested in clinical trials4 ,5 (Du et al., 2016b; Cho et al., 2018).
T121 17254-17526 Sentence denotes Nevertheless, no vaccines have been approved for the prevention of SARS and MERS in humans, demonstrating the need to develop effective and safe vaccines to control current MERS-CoV infection, or to be stockpiled for potential use against re-emerged SARS-CoV or SARSr-CoV.
T122 17527-17646 Sentence denotes Particularly, effective and safe vaccines are urgently needed to prevent and control the current outbreak of 2019-nCoV.
T123 17647-17971 Sentence denotes Most SARS-CoV and MERS-CoV vaccines developed thus far are based on the inactivated or live attenuated viruses, DNAs, proteins, nanoparticles, viral vectors, including virus-like particles (VLPs) (Zeng et al., 2004; Jiang et al., 2005; Liu et al., 2005; Du et al., 2009a, 2016b; Pimentel et al., 2009; Al-Amri et al., 2017).
T124 17972-18033 Sentence denotes Each vaccine type has different advantages and disadvantages.
T125 18034-18167 Sentence denotes For instance, inactivated and live-attenuated virus-based vaccines are vaccine types developed using the most traditional approaches.
T126 18168-18398 Sentence denotes Although they generally induce highly potent immune responses and/or protection, the possibility for incomplete inactivation of viruses or recovering virulence exists, resulting in significant safety concerns (Zhang et al., 2014).
T127 18399-18549 Sentence denotes Also, these traditional vaccines may induce the antibody-dependent enhancement (ADE) effect, as in the case of SARS-CoV infection (Luo et al., 2018b).
T128 18550-18812 Sentence denotes Similarly, some viral-vectored vaccines can elicit specific antibody and cellular immune responses with neutralizing activity and protection, but they might also induce anti-vector immunity or present pre-existing immunity, causing some harmful immune responses.
T129 18813-19191 Sentence denotes Instead, DNA and nanoparticle vaccines maintain strong safety profile; however, the immunogenicity of these vaccines is usually lower than that of virus- or viral vector-based vaccines, often requiring optimization of sequences, components, or immunization routes, inclusion of appropriate adjuvants, or application of combinational immunization approaches (Zhang et al., 2014).
T130 19193-19239 Sentence denotes Subunit Vaccines Against Sars-CoV and Mers-CoV
T131 19240-19336 Sentence denotes Subunit vaccines are vaccines developed based on the synthetic peptides or recombinant proteins.
T132 19337-19680 Sentence denotes Unlike inactivated or live-attenuated virus and some viral vectored vaccines, this vaccine type mainly contains specific viral antigenic fragments, but without including any components of infectious viruses, eliminating the concerns of incomplete inactivation, virulence recovery, or pre-existing immunity (Du et al., 2008; Deng et al., 2012).
T133 19681-19848 Sentence denotes Similar to DNA or VLP-based vaccines, subunit vaccines are generally safe without causing potential harmful immune responses, making them promising vaccine candidates.
T134 19849-20015 Sentence denotes Moreover, subunit vaccines may target specific, well-defined neutralizing epitopes with improved immunogenicity and/or efficacy (Du et al., 2008; Zhang et al., 2014).
T135 20016-20153 Sentence denotes A number of subunit vaccines against SARS-CoV and MERS-CoV have been developed, and these are described in detail in the next paragraphs.
T136 20154-20255 Sentence denotes The targets used for the development of SARS-CoV and MERS-CoV subunit vaccines are also be discussed.
T137 20257-20332 Sentence denotes Potential Targets for Development of SARS-CoV and MERS-CoV Subunit Vaccines
T138 20333-20431 Sentence denotes The S protein of SARS-CoV and MERS-CoV plays a vital role in receptor binding and membrane fusion.
T139 20432-20729 Sentence denotes Thus, the S protein, but not other structural proteins, is the major antigen to induce protective neutralizing antibodies to block viruses from binding their respective receptor and thus inhibit viral infection (Bisht et al., 2004; Buchholz et al., 2004; Bukreyev et al., 2004; Yang et al., 2004).
T140 20730-20850 Sentence denotes As a result, the S protein is also a major target for the development of subunit vaccines against SARS-CoV and MERS-CoV.
T141 20851-21123 Sentence denotes Both full-length S protein and its antigenic fragments, including S1 subunit, NTD, RBD, and S2 subunit, can serve as important targets for the development of subunit vaccines (Guo et al., 2005; Mou et al., 2013; Wang et al., 2015; Jiaming et al., 2017; Zhou et al., 2018).
T142 21124-21546 Sentence denotes Although subunit vaccines based on the full-length S protein may elicit potent immune responses and/or protection, studies have found that antibodies induced by some of these vaccines mediate enhancement of viral infection in vitro, as in the case of SARS-CoV (Kam et al., 2007; Jaume et al., 2012), raising safety concerns for the development of full-length S protein-based subunit vaccines against SARS-CoV and MERS-CoV.
T143 21547-21675 Sentence denotes In contrast, RBD-based subunit vaccines comprise the major critical neutralizing domain (Du and Jiang, 2015; Zhou et al., 2019).
T144 21676-21802 Sentence denotes Therefore, these vaccines may generate potent neutralizing antibodies with strong protective immunity against viral infection.
T145 21803-22013 Sentence denotes S1 subunit, for example, is much shorter than the full-length S protein, but it is no less able to induce strong immune responses and/or protection against viral infection (Li et al., 2013; Adney et al., 2019).
T146 22014-22103 Sentence denotes Thus, this fragment can be used as an alternative target for subunit vaccine development.
T147 22104-22443 Sentence denotes Despite their ability to induce immune responses and/or neutralizing antibodies, NTD and S2 as the targets of subunit vaccines are less immunogenic, eliciting significantly lower antibody titers, cellular immune responses, and/or protection than the other regions, such as full-length, S1, and RBD (Guo et al., 2005; Jiaming et al., 2017).
T148 22444-22659 Sentence denotes Therefore, in terms of safety and efficacy, the RBD and/or S1 of S protein could be applied as critical targets for the development of subunit vaccine candidates against SARS-CoV, MERS-CoV, SARSr-CoV, and MERSr-CoV.
T149 22660-22926 Sentence denotes Because of its conserved amino acid sequences and high homology among different virus strains (Elshabrawy et al., 2012; Zhou et al., 2018), the S2 subunit has potential to be used as a target for the development of universal vaccines against divergent virus strains.
T150 22927-23070 Sentence denotes In addition to the S protein, the N protein of SARS-CoV and MERS-CoV may serve as an additional target for the development of subunit vaccines.
T151 23071-23325 Sentence denotes Unlike S protein, the N protein has no ability to elicit neutralizing antibodies to block virus-receptor interaction and neutralize viral infection, but it may induce specific antibody and cellular immune responses (Liu et al., 2006; Zheng et al., 2009).
T152 23326-23565 Sentence denotes Several immunodominant B-cell and T-cell epitopes have been identified in the N protein of SARS-CoV and MERS-CoV, some of which are conserved in mice, non-human primates, and humans (Liu et al., 2006; Chan et al., 2011; Veit et al., 2018).
T153 23566-23676 Sentence denotes Other proteins, such as M protein, can be used as potential targets of SARS-CoV and MERS-CoV subunit vaccines.
T154 23677-23907 Sentence denotes Notably, SARS-CoV M protein-derived peptides have immunogenicity to induce high-titer antibody responses in the immunized animals (He et al., 2005b), suggesting the potential for utilizing this protein to develop subunit vaccines.
T155 23909-23942 Sentence denotes Subunit Vaccines Against SARS-CoV
T156 23943-24158 Sentence denotes Numerous subunit vaccines against SARS-CoV have been developed since the outbreak of SARS, the majority of which use the S protein and/or its antigenic fragments, in particular, RBD, as the vaccine target (Table 1).
T157 24159-24203 Sentence denotes TABLE 1 Subunit Vaccines against SARS-CoVa.
T158 24204-24341 Sentence denotes Name Antigenicity and functionality Adjuvant Route Animal models Antibody response Cellular immune response Protection References
T159 24342-24410 Sentence denotes Subunit vaccines based on SARS-CoV full-length or trimeric S protein
T160 24411-24503 Sentence denotes FL-S and EC-S proteins Bind to SARS-CoV S1, NTD, RBD, and S2-specific mAbs MPL + TDM S.C.
T161 24505-24559 Sentence denotes BALB/c mice Elicit SARS-CoV S-specific Abs (IgG, > 1:
T162 24560-24677 Sentence denotes 2 × 105), neutralizing (> 1:2.4 × 104) pseudotyped SARS-CoV (Tor2, GD03, and SZ3 strains) N/A N/A He et al., 2006a
T163 24678-24750 Sentence denotes S andS-foldon proteins N/A TiterMax Gold; Alum Hydro+MPL S.C. or I.M.
T164 24752-25046 Sentence denotes BALB/c mice Elicit SARS-CoV S-specific Abs (IgG, > 1:104) in mice, neutralizing (∼2.4 × 102 for S; ∼1:7 × 102 for S-foldon) live SARS-CoV (Urbani strain) N/A Protect vaccinated mice from challenge of SARS-CoV (Urbani strain, 105 TCID50) with undetectable viral load in lungs Li et al., 2013
T165 25047-25094 Sentence denotes triSpike protein N/A Alum hydro I.P. or S.C.
T166 25096-25477 Sentence denotes BALB/c mice; Hamsters Elicits SARS-CoV S-specific mucosal and serum Abs (IgA and IgG) in mice and hamsters, blocking S-ACE2 receptor binding and neutralizing live SARS-CoV (HKU-39849 strain); induces ADE N/A Protects vaccinated hamsters from challenge of SARS-CoV (Urbani strain, 103 TCID50) with undetectable or reduced viral load in lungs Kam et al., 2007; Jaume et al., 2012
T167 25478-25524 Sentence denotes Subunit vaccines based on SARS-CoV RBD protein
T168 25525-25568 Sentence denotes RBD-Fc protein N/A Freund’s I.D. or I.M.
T169 25570-25718 Sentence denotes BALB/c mice; Rabbits Elicits SARS-CoV S/RBD-specific Abs (IgG) in mice and rabbits, neutralizing pseudotyped (rabbits: ≥ 7.3 × 104) and live (mice:
T170 25719-25966 Sentence denotes 1:4 × 103; rabbits: > 1:1.5 × 104) SARS-CoV (BJ01 strain) N/A Protects majority (4/5) of vaccinated mice from challenge of SARS-CoV (BJ01 strain, 106 TCID50), with one mouse showing mild alveolar damage in lungs He et al., 2004; Du et al., 2007
T171 25967-26119 Sentence denotes RBD193-CHO; RBD219-CHO proteins Binds to SARS-CoV RBD-specific mAbs (neutralizing 24H8, 31H12, 35B5, 33G4, 19B2; non-neutralizing 17H9) Freund’s S.C.
T172 26121-26688 Sentence denotes BALB/c mice Elicit SARS-CoV RBD-specific Abs, neutralizing pseudotyped (< 1:104 for RBD193-CHO; 1:5.8 × 104 for RBD219-CHO) and live (< 1:103 for RBD193-CHO; 1:103 for RBD219-CHO) SARS-CoV (GZ50 strain) Induce SARS-CoV RBD-specific cellular immune responses (IFN-γ, IL-2, IL-4, IL-10) in mice Protect all (for RBD219-CHO) or majority (3/5, for RBD219-CHO) of vaccinated mice from challenge of SARS-CoV (GZ50 strain, 100 TCID50 for RBD193-CHO; 5 × 105 TCID50 for RBD219-CHO) with undetectable viral RNA or no, to reduced, viral load in lungs Du et al., 2009c, 2010
T173 26689-26821 Sentence denotes RBD-293T protein Binds to SARS-CoV RBD-specific mAbs (neutralizing 24H8, 31H12, 35B5, 33G4, 19B2; non-neutralizing 17H9) SAS S.C.
T174 26823-27119 Sentence denotes BALB/c mice Elicits SARS-CoV RBD-specific Abs (IgG), neutralizing pseudotyped (1:6.9 × 105) and live (1:1.6 × 103) SARS-CoV (GZ50 strain) N/A Protects all vaccinated mice from challenge of SARS-CoV (GZ50 strain, 100 TCID50) with undetectable viral RNA and viral load in lungs Du et al., 2009b
T175 27120-27165 Sentence denotes S318-510 protein N/A Alum; Alum + CpG S.C.
T176 27167-27245 Sentence denotes 129S6/SvEv mice Elicits SARS-CoV-specific Abs (IgG, IgG1, and IgG2a) in mice.
T177 27246-27400 Sentence denotes Reduces neutralization after removing glycosylation Induces SARS-CoV S-specific cellular immune responses (IFN-γ) in mice N/A Zakhartchouk et al., 2007
T178 27401-27463 Sentence denotes Subunit vaccines based on non-RBD SARS-CoV S protein fragments
T179 27464-27541 Sentence denotes S1 and S1-foldon proteins N/A TiterMax Gold; Alum Hydro + MPL S.C. or I.M.
T180 27543-27834 Sentence denotes BALB/c mice Elicit SARS-CoV S-specific Abs (IgG, > 1:104) in mice, neutralizing (1:1.7 × 102 for S1; 1:90 for S1-foldon) live SARS-CoV (Urbani strain) N/A Protect vaccinated mice from challenge of SARS-CoV (Urbani strain, 105 TCID50) with undetectable viral load in lungs Li et al., 2013
T181 27835-27866 Sentence denotes S2 protein N/A Freund’s S.C.
T182 27868-28074 Sentence denotes BALB/c mice Elicits SARS-CoV S2-specific Abs (IgG, 1:1.6 × 103) in mice with no neutralizing activity Induces SARS-CoV S2-specific cellular immune responses (IFN-γ and IL-4) in mice N/A Guo et al., 2005
T183 28075-28150 Sentence denotes Subunit vaccines based on SARS-CoV non-S structural proteins (i.e. N and M)
T184 28151-28182 Sentence denotes rN protein N/A Freund’s I.P.
T185 28184-28385 Sentence denotes BALB/c mice Elicits SARS-CoV N-specific Abs (IgG (1:1.8 × 103), IgG1, and IgG2a) in mice Induces cellular immune responses with up-regulated IFN-γ and IL-10 cytokines in mice N/A Zheng et al., 2009
T186 28386-28434 Sentence denotes rN protein N/A Montanide + CpG; Freund’s S.C.
T187 28436-28589 Sentence denotes BALB/c mice Elicits SARS-CoV N-specific Abs (IgG) in mice Induces SARS-CoV N-specific cellular immune responses (IFN- γ) in mice N/A Liu et al., 2006
T188 28590-28696 Sentence denotes M1-31 and M132-161 peptides Bind to sera from SARS patients or immunized mice and rabbits Freund’s I.D.
T189 28698-28799 Sentence denotes BALB/c mice; NZW rabbits Induce SARS-CoV M-specific Abs (IgG) in rabbits N/A N/A He et al., 2005b
T190 28800-29432 Sentence denotes aAbs, antibodies; ADE, antibody-dependent enhancement; Alum hydro, aluminum hydroxide; CHO, Chinese hamster ovary; CpG, cysteine-phosphate-guanine; I.D., intradermal; I.M., intramuscular; IFN-γ, interferon gamma; IL-2, interleukin 2; IL-4, interleukin 4; IL-10, Interleukin 10; I.P., intraperitoneal; mAbs, monoclonal antibodies; Montanide, Montanide ISA-51; MPL + TDM, monophosphoryl lipid A and trehalose dicorynomycolate; N/A, not reported; NTD, N-terminal domain; NZW rabbits, New Zealand White rabbits; RBD, receptor-binding domain; SAS, Sigma adjuvant system; S.C., subcutaneous; TCID50, median tissue culture infectious dose.
T191 29434-29490 Sentence denotes SARS-CoV Subunit Vaccines Based on Full-Length S Protein
T192 29491-29695 Sentence denotes Subunit vaccines based on SARS-CoV S protein, including full-length or trimeric S protein, are immunogenic with protection against SARS-CoV infection (He et al., 2006a; Kam et al., 2007; Li et al., 2013).
T193 29696-30072 Sentence denotes Either insect cell-expressed full-length (FL-S) or extracellular domain (EC-S) SARS-CoV S protein developed high-titer S-specific antibodies with neutralizing activity against pseudotyped SARS-CoV expressing S protein of representative SARS-CoV human and palm civet strains (Tor2, GD03, and SZ3) isolated during the 2002 and 2003 or 2003 and 2004 outbreaks (He et al., 2006a).
T194 30073-30367 Sentence denotes In addition, full-length S-ectodomain proteins fused with or without a foldon trimeric motif (S or S-foldon) could elicit specific antibody responses and neutralizing antibodies, protecting immunized mice against SARS-CoV challenge with undetectable virus titers in the lungs (Li et al., 2013).
T195 30368-30585 Sentence denotes Moreover, a subunit vaccine (triSpike) based on a full-length S protein trimer induced specific serum and mucosal antibody responses and efficient neutralizing antibodies against SARS-CoV infection (Kam et al., 2007).
T196 30586-31016 Sentence denotes Nevertheless, this vaccine also resulted in Fcγ receptor II (FcγRII)-dependent and ACE2-independent ADE, particularly in human monocytic or lymphoblastic cell lines infected with pseudotyped SARS-CoV expressing viral S protein, or in Raji B cells (B-cell lymphoma line) infected with live SARS-CoV (Kam et al., 2007; Jaume et al., 2012), raising significant concerns over the use of full-length S protein as a SARS vaccine target.
T197 31018-31056 Sentence denotes SARS-CoV Subunit Vaccines Based on RBD
T198 31057-31304 Sentence denotes SARS-CoV RBD contains multiple conformation-dependent epitopes capable of eliciting high-titer neutralizing antibodies; thus, it is a major target for the development of SARS vaccines (He et al., 2004, 2005a; Jiang et al., 2012; Zhu et al., 2013).
T199 31305-31379 Sentence denotes Subunit vaccines based on the SARS-CoV RBD have been extensively explored.
T200 31380-31701 Sentence denotes Studies have found that a fusion protein containing RBD and the fragment crystallizable (Fc) region of human IgG1 (RBD-Fc) elicited highly potent neutralizing antibodies against SARS-CoV in the immunized rabbits and mice, which strongly blocked the binding between S1 protein and SARS-CoV receptor ACE2 (He et al., 2004).
T201 31702-31951 Sentence denotes This RBD protein induced long-term, high-level SARS-CoV S-specific antibodies and neutralizing antibodies that could be maintained for 12 months after immunization, protecting most of the vaccinated mice against SARS-CoV infection (Du et al., 2007).
T202 31952-32368 Sentence denotes In addition, recombinant RBDs (residues 318–510 or 318–536) stably or transiently expressed in Chinese hamster ovary (CHO) cells bound strongly to RBD-specific monoclonal antibodies (mAbs), elicited high-titer anti-SARS-CoV neutralizing antibodies, and protected most, or all, of the SARS-CoV-challenged mice, with undetectable viral RNA and undetectable or significantly reduced viral load (Du et al., 2009c, 2010).
T203 32369-32521 Sentence denotes Significantly, a 293T cell-expressed RBD protein maintains excellent conformation and good antigenicity to bind SARS-CoV RBD-specific neutralizing mAbs.
T204 32522-32659 Sentence denotes It elicited highly potent neutralizing antibodies that completely protected immunized mice against SARS-CoV challenge (Du et al., 2009b).
T205 32660-32957 Sentence denotes Particularly, RBDs from the S proteins of Tor2, GD03, and SZ3, representative strains of SARS-CoV isolated from human 2002–2003, 2003–2004, and palm civet strains, can induce high-titer cross-neutralizing antibodies against pseudotyped SARS-CoV expressing respective S proteins (He et al., 2006c).
T206 32958-33157 Sentence denotes Different from the full-length S protein-based SARS subunit vaccines, no obvious pathogenic effects have been identified in the RBD-based SARS subunit vaccines (Kam et al., 2007; Jaume et al., 2012).
T207 33159-33221 Sentence denotes SARS-CoV Subunit Vaccines Based on Non-RBD S Protein Fragments
T208 33222-33422 Sentence denotes SARS subunit vaccines based on S protein fragments (S1 and S2), other than the RBD, have shown immunogenicity and/or protective efficacy against SARS-CoV infection (Guo et al., 2005; Li et al., 2013).
T209 33423-33630 Sentence denotes For example, recombinant S1 proteins fused with or without foldon elicited specific antibodies with neutralizing activity that protected immunized mice against high-dose SARS-CoV challenge (Li et al., 2013).
T210 33631-34179 Sentence denotes Although some studies have demonstrated that recombinant SARS-CoV S2 (residues 681–980) protein elicits specific non-neutralizing antibody response in mice (Guo et al., 2005), others have indicated that mAbs targeting highly conserved heptad repeat 1 (HR1) and HR2 domains of SARS-CoV S protein have broad neutralizing activity against pseudotyped SARS-CoV expressing S protein of divergent strains (Elshabrawy et al., 2012), indicating the potential of utilizing the S2 region as a broad-spectrum anti-SARS-CoV vaccine target (Zheng et al., 2009).
T211 34181-34241 Sentence denotes SARS-CoV Subunit Vaccines Based on Non-S Structural Proteins
T212 34242-34388 Sentence denotes Subunit vaccines based on the N and M proteins of SARS-CoV have shown immunogenicity in vaccinated animals (Liu et al., 2006; Zheng et al., 2009).
T213 34389-34833 Sentence denotes Studies have revealed that a plant-expressed SARS-CoV N protein conjugated with Freund’s adjuvant elicited specific IgG antibodies, including IgG1 and IgG2a subtypes, and cellular immune responses in mice, whereas another E. coli-expressed N protein conjugated with Montanide ISA-51 and cysteine-phosphate-guanine (CpG) adjuvants induced specific IgG antibodies toward a Th1 (IgG2a)-type response in mice (Liu et al., 2006; Zheng et al., 2009).
T214 34834-35027 Sentence denotes Although N-specific antibodies have been detected in convalescent-phase SARS patient and immunized rabbit sera, they have no neutralizing activity against SARS-CoV infection (Qiu et al., 2005).
T215 35028-35274 Sentence denotes In addition, immunodominant M protein peptides (M1-31 and M132-161) identified using convalescent-phase sera of SARS patients and immunized mouse and rabbit sera have immunogenicity to elicit specific IgG antibodies in rabbits (He et al., 2005b).
T216 35275-35452 Sentence denotes In spite of their immunogenicity, it appears that these N- and M-based SARS subunit vaccines have not been investigated for their protective efficacy against SARS-CoV infection.
T217 35453-35571 Sentence denotes Thus, it is unclear whether these non-S structural protein-based SARS subunit vaccines can prevent SARS-CoV infection.
T218 35573-35626 Sentence denotes Potential Factors Affecting SARS-CoV Subunit Vaccines
T219 35627-35781 Sentence denotes A number of factors may affect the expression of proteins to be used as SARS subunit vaccines; apart from their immunogenicity and/or protective efficacy.
T220 35782-35947 Sentence denotes Understanding of these factors is important to generate subunit vaccines with good quality, high immunogenicity, and excellent protection against SARS-CoV infection.
T221 35948-36054 Sentence denotes The expression of recombinant protein-based SARS subunit vaccines may be changed by the following factors.
T222 36055-36354 Sentence denotes First, addition of an intron splicing enhancer to the truncated SARS-CoV S protein fragments results in better enhancement of protein expression in mammalian cells than the exon splicing enhancers, and different cells may result in different fold increase of protein expression (Chang et al., 2006).
T223 36355-36575 Sentence denotes Second, inclusion of a post-transcriptional gene silencing suppressor p19 protein from tomato bushy stunt virus to a SARS-CoV N protein may significantly increase its transient expression in tobacco (Zheng et al., 2009).
T224 36576-36871 Sentence denotes The following factors may affect the immunogenicity and protective efficacy of protein-based SARS subunit vaccines, including same proteins expressed in different expression systems, and same proteins with various lengths, amino acid mutations, or deletions (He et al., 2006b; Du et al., 2009b).
T225 36872-37084 Sentence denotes For example, RBD proteins containing different lengths (193-mer: RBD193-CHO or 219-mer: RBD219-CHO) elicited different immune responses and protective efficacy against SARS-CoV challenge (Du et al., 2009c, 2010).
T226 37085-37453 Sentence denotes A recombinant SARS-CoV RBD (RBD-293T) protein expressed in mammalian cell system was able to induce stronger neutralizing antibody response than those expressed in insect cells (RBD-Sf9) and E. coli (RBD-Ec) (Du et al., 2009b), suggesting that RBD purified from mammalian cells has preference for further development due to its ability to maintain native conformation.
T227 37454-37739 Sentence denotes Notably, a single mutation (R441A) in the RBD of SARS-CoV disrupted its major neutralizing epitopes and affinity to bind viral receptor ACE2, thus abolishing the vaccine’s immunogenicity, and hence, its ability to induce neutralizing antibodies in immunized animals (He et al., 2006b).
T228 37740-37943 Sentence denotes Additionally, deletion of a particular amino acid by changing a glycosylation site in the SARS-CoV RBD (RBD219-N1) also resulted in the alteration of subunit vaccine’s immunogenicity (Chen et al., 2014).
T229 37944-38113 Sentence denotes Other factors that potentially affect the immunogenicity of SARS subunit vaccines include immunization routes and adjuvants (Zakhartchouk et al., 2007; Li et al., 2013).
T230 38114-38314 Sentence denotes Significantly high-titer antibodies were induced by monomeric or trimeric SARS-CoV S and S1 proteins through the intramuscular (I.M.) route compared to the subcutaneous (S.C.) route (Li et al., 2013).
T231 38315-38534 Sentence denotes Moreover, a SARS-CoV RBD subunit vaccine conjugated with Alum plus CpG adjuvants elicited a higher level of IgG2a antibody and interferon gamma (IFN-γ) secretion than the RBD with Alum alone (Zakhartchouk et al., 2007).
T232 38536-38569 Sentence denotes Subunit Vaccines Against MERS-CoV
T233 38570-38750 Sentence denotes Subunit vaccines against MERS-CoV have been developed extensively, almost all of which are based on the S protein, including full-length S timer, NTD, S1, and S2, particularly RBD.
T234 38751-38914 Sentence denotes These subunit vaccines, including their antigenicity, functionality, immunogenicity, and protective efficacy in different animal models, are summarized in Table 2.
T235 38915-38959 Sentence denotes TABLE 2 Subunit Vaccines against MERS-CoVa.
T236 38960-39106 Sentence denotes Name Functionality and antigenicity Adjuvant Route Animal models Antibody response Cellular immune response Protective efficacy References
T237 39107-39163 Sentence denotes Subunit vaccines based on MERS-CoV full-length S protein
T238 39164-39307 Sentence denotes MERS S-2P protein Binds to DPP4 receptor and MERS-CoV S-NTD, RBD, and S2-specific neutralizing mAbs (G2, D12, and G4, respectively) SAS I.M.
T239 39309-39424 Sentence denotes BALB/c mice Elicits neutralizing Abs in mice, neutralizing 7 pseudotyped MERS-CoV N/A N/A Pallesen et al., 2017
T240 39425-39471 Sentence denotes Subunit vaccines based on MERS-CoV RBD protein
T241 39472-39577 Sentence denotes rRBD (S367-606) protein N/A Alum Hydro + CpG or poly(I:C); IFA + CpG (mouse); Alum (NHPs) I.M. or S.C.
T242 39579-40083 Sentence denotes BALB/c mice; NHPs Elicits MERS-CoV RBD-specific Abs in mice (IgG, IgG1, IgG2a, and IgG2b) and NHPs (IgG), neutralizing pseudotyped (mouse: < 1:5 × 102) and live (NHPs: < 1:5 × 102) MERS-CoV (EMC2012 strain) Induces MERS-CoV RBD-specific cellular immune responses (IFN-γ, TNF-α, IL-2, IL-4, IL-6, and IL-10) in mice and/or monkeys Partially protects vaccinated NHPs from challenge of MERS-CoV (EMC2012 strain, 6.5 × 107 TCID50) with alleviated pneumonia and decreased viral load Lan et al., 2014, 2015
T243 40084-40169 Sentence denotes RBD (S377-662)-Fc protein Binds to DPP4 receptor Poly(I:C); Montanide I.N. or S.C.
T244 40171-40460 Sentence denotes BALB/c mice Elicits MERS-CoV S1- and RBD-specific Abs (IgA, IgG (> 1:104), IgG1, IgG2a, and IgG3) in mice, neutralizing (≥ 1:2.4 × 102) live MERS-CoV (EMC2012 strain) Induces MERS-CoV S1-specific cellular immune responses (IFN-γ and IL-2) in mice N/A Du et al., 2013c; Ma et al., 2014b
T245 40461-40627 Sentence denotes RBD (S377-588)-Fc protein Binds to DPP4 receptor and MERS-CoV RBD specific neutralizing mAbs (Mersmab1, m336, m337, and m338) Montanide; MF59; AddaVax I.M. or S.C.
T246 40629-41294 Sentence denotes BALB/c mice; hDPP4-Tg mice; Rabbits Elicits MERS-CoV S1 and RBD-specific Abs in mice (IgG (> 1:105), IgG1, and IgG2a) and rabbits (IgG), neutralizing 17 pseudotyped (≥ 1:104) and 2 live (≥ 1:103) MERS-CoV (EMC2012 and London1-2012 strains) Induces MERS-CoV S1-specific cellular immune responses (IFN-γ and IL-2) in mice Protects vaccinated Ad5/hDPP4-transduced BALB/c mice and majority (4/6) of vaccinated hDPP4-Tg mice from MERS-CoV (EMC2012 strain, 105 PFU for BALB/c; 103–4 TCID50 for Tg) challenge, without immunological toxicity or eosinophilic immune enhancement Du et al., 2013a; Ma et al., 2014b; Tang et al., 2015; Zhang et al., 2016; Nyon et al., 2018
T247 41295-41436 Sentence denotes RBD-Fd protein Binds to DPP4 receptor and MERS-CoV RBD-specific neutralizing mAbs (Mersmab1, m336, m337, and m338) MF59; Alum I.M. or S.C.
T248 41438-41763 Sentence denotes BALB/c mice; hDPP4-Tg mice Elicits MERS-CoV S1-specific Abs (IgG (> 1:105), IgG1, and IgG2a) in mice, neutralizing at least 9 pseudotyped (> 1:104) and live (> 1:103) MERS-CoV (EMC2012 strain) N/A Protects majority (5/6) of vaccinated hDPP4-Tg mice from challenge of MERS-CoV (EMC2012 strain, 104 TCID50) Tai et al., 2016
T249 41764-41912 Sentence denotes RBD (T579N) protein Binds to receptor DPP4 and MERS-CoV RBD-specific neutralizing mAbs (hMS-1, m336, m337, and m338) Montanide; Alum I.M. or S.C.
T250 41914-42145 Sentence denotes BALB/c mice; hDPP4-Tg mice Elicits neutralizing Abs (> 1:3 × 103) in mice against live MERS-CoV (EMC2012 strain) N/A Protects all vaccinated hDPP4-Tg mice from challenge of MERS-CoV (EMC2012 strain, 104 TCID50) Du et al., 2016a
T251 42146-42208 Sentence denotes Subunit vaccines based on non-RBD MERS-CoV S protein fragments
T252 42209-42246 Sentence denotes S1 protein N/A Ribi; Alum pho I.M.
T253 42248-42548 Sentence denotes BALB/c mice; NHPs Elicits MERS-CoV S1-specific Abs in mice (IgG and IgG1) and NHPs (IgG), neutralizing 8 pseudotyped and live MERS-CoV (JordanN3 strain) N/A Protects vaccinated NHPs from challenge of MERS-CoV (JordanN3 strain, 5 × 106 PFU) with reduced abnormalities on chest CT Wang et al., 2015
T254 42549-42583 Sentence denotes S1 protein N/A Advax + SAS I.M.
T255 42585-42972 Sentence denotes Dromedary camels; Alpacas Elicits neutralizing Abs in dromedary camels (≥ 1:80) and alpacas (≥ 1:6.4 × 102) against live MERS-CoV (EMC2012 strain) N/A Protects vaccinated dromedary camels and alpacas from challenge of MERS-CoV (EMC2012 strain, 107 TCID50) with reduced and delayed viral shedding in the upper airways (in camels) or complete protection (in alpacas) Adney et al., 2019
T256 42973-43012 Sentence denotes rNTD protein N/A Alum pho + CpG I.M.
T257 43014-43461 Sentence denotes BALB/c mice; Ad5-hDPP4 mice Elicits MERS-CoV S-NTD-specific Abs (IgG, ≥ 1:104) in mice, neutralizing pseudotyped and live (1:40) MERS-CoV (EMC2012 strain) Induces MERS-CoV S-NTD-specific cellular immune responses (IFN-γ, IL-2, IL-6, IL-10, and IL-17A) in mice Protects vaccinated Ad5-hDPP4-transduced mice from challenge of MERS-CoV (EMC2012 strain, 105 PFU) with reduced lung abnormalities and respiratory tract pathology Jiaming et al., 2017
T258 43462-43653 Sentence denotes SP3 peptide (aa736-761) N/A Freund’s N/A BALB/c mice; NZW rabbits Elicits MERS-CoV S-specific Abs (IgG, 1:104) in rabbits, neutralizing pseudotyped MERS-CoV N/A N/A Yang et al., 2014a
T259 43654-44331 Sentence denotes aaa, amino acid; Abs, antibodies; Ad5, adenovirus serotype 5; Ad5-hDPP4 mice, Ad5-hDPP4-transuced mice; Alum hydro, aluminum hydroxide; Alum pho, Aluminum phosphate; hDPP4, human dipeptidyl peptidase 4; hDPP4-Tg mice, transgenic mice expressing MERS-CoV receptor human DPP4; IFA, incomplete Freund’s adjuvant; I.M., intramuscular; I.N., intranasal; mAbs, monoclonal antibodies; Montanide, Montanide ISA51; N/A, not reported; NHPs, non-human primates; NZW, rabbits, New Zealand White rabbits; PFU, plaque-forming unit; rRBD, recombinant RBD; SAS, Sigma Adjuvant System; S.C., subcutaneous; TCID50, median tissue culture infectious dose; TNF-α, tumor necrosis factor (TNF)-alpha.
T260 44333-44389 Sentence denotes MERS-CoV Subunit Vaccines Based on Full-Length S Protein
T261 44390-44643 Sentence denotes Subunit vaccines based on the full-length S protein cover both RBD and non-RBD neutralizing epitopes, some of which may be located in the conserved S2 subunit; thus this type of subunit vaccines are expected to induce high-titer neutralizing antibodies.
T262 44644-44955 Sentence denotes Although several MERS-CoV full-length S protein-based vaccines have been reported in other vaccine types, including viral vectors and DNAs (Wang et al., 2015; Wang C. et al., 2017; Haagmans et al., 2016; Zhou et al., 2018), only a few subunit vaccines have been developed that rely on the full-length S protein.
T263 44956-45170 Sentence denotes For example, a recombinant MERS-CoV S protein trimer (MERS S-2P) in prefusion conformation binds to the DPP4 receptor, as well as to the MERS-CoV NTD, RBD, and S2-specific neutralizing mAbs (Pallesen et al., 2017).
T264 45171-45375 Sentence denotes Whereas this protein induces neutralizing antibodies in mice against divergent pseudotyped MERS-CoV in vitro, its in vivo protective activity against MERS-CoV infection is unknown (Pallesen et al., 2017).
T265 45376-45608 Sentence denotes Therefore, more studies are needed to elucidate the potential for the development of MERS-CoV full-length S-based subunit vaccines, including understanding their protective efficacy and identifying possible harmful immune responses.
T266 45610-45648 Sentence denotes MERS-CoV Subunit Vaccines Based on RBD
T267 45649-45865 Sentence denotes Numerous MERS-CoV RBD-based subunit vaccines have been developed and extensively evaluated in available animal models since the emergence of MERS-CoV (Table 2) (Du et al., 2013c; Tai et al., 2017; Zhou et al., 2018).
T268 45866-46109 Sentence denotes In general, these subunit vaccines have strong immunogenicity and are capable of inducing high neutralizing antibodies and/or protection against MERS-CoV infection (Ma et al., 2014b; Zhang et al., 2016; Tai et al., 2017; Wang Y. et al., 2017).
T269 46110-46237 Sentence denotes Most subunit vaccines based on the MERS-CoV RBD have been described in detail in a previous review article (Zhou et al., 2019).
T270 46238-46421 Sentence denotes In this section, we will briefly introduce these RBD-targeting MERS vaccines, and compare their functionality, antigenicity, immunogenicity, and protection against MERS-CoV infection.
T271 46422-46631 Sentence denotes Co-crystallographic analyses of MERS-CoV RBD and/or RBD/DPP4 complexes have confirmed that the RBD is attributed to residues 367–588 (Chen et al., 2013) or 367–606 (Lu et al., 2013) in the MERS-CoV S1 subunit.
T272 46632-46856 Sentence denotes Indeed, a recombinant MERS-CoV RBD (rRBD) fragment (residues 367–606) elicits RBD-specific antibody and cellular immune responses and neutralizing antibodies in mice and/or non-human primates (NHPs) (Lan et al., 2014, 2015).
T273 46857-47028 Sentence denotes However, it only partially protects NHPs from MERS-CoV infection by alleviating pneumonia and clinical manifestations, as well as decreasing viral load (Lan et al., 2015).
T274 47029-47337 Sentence denotes In addition, an RBD protein fragment containing MERS-CoV S residues 377–622 fused with the Fc tag of human IgG can induce MERS-CoV S1- and/or RBD-specific humoral and cellular immune responses in the immunized mice with neutralizing activity against MERS-CoV infection (Du et al., 2013c; Jiang et al., 2013).
T275 47338-47686 Sentence denotes However, after comparing several versions of MERS-CoV RBD fragments with different lengths, it was found that a truncated RBD (residues 377–588) had the highest DPP4-binding affinity and induced the highest-titer IgG antibodies and neutralizing antibodies against MERS-CoV, identifying its role as a critical neutralizing domain (Ma et al., 2014b).
T276 47687-48111 Sentence denotes Subsequently, several MERS-CoV subunit vaccines have been designed based on the identified critical neutralizing domain of RBD fragment, including those expressed in a stable CHO cell line (S377-588-Fc), fusing with a trimeric motif foldon (RBD-Fd), or containing single or multiple mutations in the RBD of representative human and camel strains from the 2012–2015 MERS outbreaks (Tai et al., 2016, 2017; Nyon et al., 2018).
T277 48112-48447 Sentence denotes These RBD proteins maintain good conformation, functionality, antigenicity, and immunogenicity, with ability to bind the DPP4 receptor and RBD-specific neutralizing mAbs and to elicit robust neutralizing antibodies cross-neutralizing multiple strains of MERS pseudoviruses and live MERS-CoV (Tai et al., 2016, 2017; Nyon et al., 2018).
T278 48448-49118 Sentence denotes It is noted that the wild-type MERS-CoV RBD proteins consisting of the identified critical neutralizing domain confer partial protection of hDPP4-transgenic (hDPP4-Tg) mice from MERS-CoV infection without causing immunological toxicity or eosinophilic immune enhancement (Tai et al., 2016; Wang Y. et al., 2017; Nyon et al., 2018); nevertheless, a structurally designed mutant version of such RBD protein with a non-neutralizing epitope masked (T579N) preserves intact conformation and significantly improves overall neutralizing activity and protective efficacy, resulting in the full protection of hDPP4-Tg mice against high-dose MERS-CoV challenge (Du et al., 2016a).
T279 49119-49328 Sentence denotes The above studies indicate that protein lengths to be chosen as MERS-CoV subunit vaccines and/or structure-based vaccine design can impact on the immunogenicity and/or protection of RBD-based subunit vaccines.
T280 49330-49392 Sentence denotes MERS-CoV Subunit Vaccines Based on Non-RBD S Protein Fragments
T281 49393-49500 Sentence denotes MERS vaccines targeting non-RBD regions of S protein have been developed and investigated in mice and NHPs.
T282 49501-49797 Sentence denotes It has been shown that a MERS-CoV S1 protein formulated with Ribi (for mice) or aluminum phosphate (for NHPs) adjuvant elicited robust neutralizing antibodies in mice and NHPs against divergent strains of pseudotyped and live MERS-CoV, protecting NHPs from MERS-CoV infection (Wang et al., 2015).
T283 49798-50257 Sentence denotes In addition, MERS-CoV S1 protein adjuvanted with Advax and Sigma Adjuvant System induced low-titer neutralizing antibodies in dromedary camels with reduced and delayed viral shedding after MERS-CoV challenge, but high-titer neutralizing antibodies in alpacas with complete protection of viral shedding from viral infection, indicating that protection of MERS-CoV infection is positively correlated with serum neutralizing antibody titers (Adney et al., 2019).
T284 50258-50478 Sentence denotes Moreover, immunization with a recombinant MERS-CoV NTD protein (rNTD) can induce neutralizing antibodies and cell-mediated responses, protecting Ad-hDPP4-transduced mice against MERS-CoV challenge (Jiaming et al., 2017).
T285 50479-50706 Sentence denotes Notably, specific antibodies with neutralizing activity have been elicited by a S2 peptide sequence (residues 736–761) of MERS-CoV in rabbits (Yang et al., 2014a), but the protective efficacy of this peptide vaccine is unknown.
T286 50707-50849 Sentence denotes The above reports demonstrate the potential for the development of MERS subunit vaccines based on the non-RBD fragments of MERS-CoV S protein.
T287 50851-50911 Sentence denotes MERS-CoV Subunit Vaccines Based on Non-S Structural Proteins
T288 50912-51114 Sentence denotes Unlike SARS subunit vaccines which have been designed based on viral N and M proteins, it appears that very few subunit vaccines have been developed based on the non-S structural protein(s) of MERS-CoV.
T289 51115-51380 Sentence denotes One study reports the induction of specific antibodies by MERS-CoV N peptides (Yang et al., 2014a), and another report shows that N protein is used for development of vaccines based on viral vector Vaccinia virus, modified Vaccinia Ankara (MVA) (Veit et al., 2018).
T290 51381-51636 Sentence denotes This may be potentially a consequence of the weak immunogenicity and/or protective efficacy of non-S structural proteins, further confirming the role of MERS-CoV S protein as the key target for the development of MERS vaccines, including subunit vaccines.
T291 51638-51691 Sentence denotes Potential Factors Affecting MERS-CoV Subunit Vaccines
T292 51692-51972 Sentence denotes Similar to SARS-CoV subunit vaccines, the immunogenicity and/or protection of MERS-CoV subunit vaccines may also be affected by a number of factors, such as antigen sequences, fragment lengths, adjuvants, vaccination pathways, antigen doses, immunization doses and intervals used.
T293 51973-52401 Sentence denotes As described above, MERS-CoV subunit vaccines containing different antigens or fragment lengths, particularly those based on the RBD, have apparently variable immunogenicity and/or protective efficacy, and a critical neutralizing domain that contains an RBD fragment corresponding to residues 377–588 of S protein elicits the highest neutralizing antibodies among several fragments tested (Ma et al., 2014b; Zhang et al., 2015).
T294 52402-52649 Sentence denotes Adjuvants play an essential role in enhancing host immune responses to MERS-CoV subunit vaccines, including those based on the RBD, and different adjuvants can promote host immune responses to variant levels (Lan et al., 2014; Zhang et al., 2016).
T295 52650-52873 Sentence denotes For example, while a MERS-CoV RBD subunit vaccine (S377-588 protein fused with Fc) alone induced detectable neutralizing antibody and T-cell responses in immunized mice, inclusion of an adjuvant enhanced its immunogenicity.
T296 52874-53191 Sentence denotes Particularly, among the adjuvants (Freund’s, aluminum, Monophosphoryl lipid A, Montanide ISA51 and MF59) conjugated with this RBD protein, MF59 could best potentiate the protein to induce the highest-titer anti-S antibodies and neutralizing antibodies, protecting mice against MERS-CoV infection (Zhang et al., 2016).
T297 53192-53467 Sentence denotes Moreover, a recombinant RBD (rRBD) protein plus alum and CpG adjuvants elicited the highest neutralizing antibodies against pseudotyped MERS-CoV infection, whereas the strongest T-cell responses were induced by this protein plus Freund’s and CpG adjuvants (Lan et al., 2014).
T298 53468-53644 Sentence denotes Vaccination pathways are important in inducing efficient immune responses, and different immunization routes may elicit different immune responses to the same protein antigens.
T299 53645-53961 Sentence denotes For example, immunization of mice with a MERS-CoV subunit vaccine (RBD-Fc) via the intranasal route induced higher levels of cellular immune responses and stronger local mucosal neutralizing antibody responses against MERS-CoV infection than those induced by the same vaccine via the S.C. pathway (Ma et al., 2014a).
T300 53962-54289 Sentence denotes In addition, while Freund’s and CpG-adjuvanted rRBD protein elicited higher-level systematic and local IFN-γ-producing T cells via the S.C. route, this protein adjuvanted with Alum and CpG induced higher-level tumor necrosis factor-alpha (TNF-α) and interleukin 4 (IL-4)-secreting T cells via the I.M. route (Lan et al., 2014).
T301 54290-54413 Sentence denotes Antigen dosage, immunization doses, and intervals may significantly affect the immunogenicity of MERS-CoV subunit vaccines.
T302 54414-54688 Sentence denotes Notably, a MERS-CoV RBD (S377-588-Fc) subunit vaccine immunized at 1 μg elicited strong humoral and cellular immune responses and neutralizing antibodies in mice although the one immunized at 5 and 20 μg elicited a higher level of S1-specific antibodies (Tang et al., 2015).
T303 54689-54936 Sentence denotes In addition, among the regimens at one dose and two doses at 1-, 2-, and 3-week intervals, 2 doses of this protein boosted at 4 weeks resulted in the highest antibodies and neutralizing antibodies against MERS-CoV infection (Wang Y. et al., 2017).
T304 54938-55025 Sentence denotes Potential Challenges and Future Perspectives for Sars-CoV and Mers-CoV Subunit Vaccines
T305 55026-55199 Sentence denotes Compared with other vaccine types such as inactivated virus and viral-vectored vaccines, SARS and MERS subunit vaccines are much safer and do not cause obvious side effects.
T306 55200-55465 Sentence denotes However, these subunit vaccines may face some important challenges, mostly arising from their relatively low immunogenicity, which must be combined with appropriate adjuvants or optimized for suitable protein sequences, fragment lengths, and immunization schedules.
T307 55466-55598 Sentence denotes In addition, structure and epitope-based vaccine design has become a promising strategy to improve the efficacy of subunit vaccines.
T308 55599-55787 Sentence denotes This is evidenced by a structurally designed MERS-CoV RBD-based protein which has significantly improved neutralizing activity and protection against MERS-CoV infection (Du et al., 2016a).
T309 55788-56035 Sentence denotes It is prospected that more structure-guided novel strategies will be developed to improve the overall immunogenicity and efficacy of subunit vaccines against emerging pathogenic human coronaviruses, including those targeting SARS-CoV and MERS-CoV.
T310 56036-56233 Sentence denotes Although a large number of SARS and MERS subunit vaccines have been developed with potent immunogenicity and/or protection in available animal models, virtually all remain in the preclinical stage.
T311 56234-56423 Sentence denotes It is thus expected that one or several of these promising subunit vaccines can be further processed into clinical trials to confirm their immunogenicity against viral infections in humans.
T312 56425-56507 Sentence denotes Rapid Development of Subunit Vaccines Against the New Pathogenic Human Coronavirus
T313 56508-56622 Sentence denotes Currently, the newly identified 2019-nCoV is spreading to infect people, resulting in significant global concerns.
T314 56623-56732 Sentence denotes It is critical to rapidly design and develop effective vaccines to prevent infection of this new coronavirus.
T315 56733-57060 Sentence denotes Since S protein and its fragments, such as RBD, of SARS-CoV, and MERS-CoV are prime targets for developing subunit vaccines against these two highly pathogenic human CoVs, it is expected that similar regions of 2019-nCoV can also be used as key targets for developing vaccines against this new coronavirus (Jiang et al., 2020).
T316 57061-57219 Sentence denotes Similarly, other regions of 2019-nCoV, including S1 and S2 subunits of S protein and N protein, can be applied as alternative targets for vaccine development.
T317 57220-57491 Sentence denotes Taken together, the approaches and strategies in the development of subunit vaccines against SARS and MERS described in this review will provide important information for the rapid design and development of safe and effective subunit vaccines against 2019-nCoV infection.
T318 57493-57513 Sentence denotes Author Contributions
T319 57514-57581 Sentence denotes SJ and LD conceived the idea and revised and edited the manuscript.
T320 57582-57641 Sentence denotes NW and LD collected information and drafted the manuscript.
T321 57642-57679 Sentence denotes JS performed the structural analysis.
T322 57680-57739 Sentence denotes All authors read and made final approval of the manuscript.
T323 57741-57761 Sentence denotes Conflict of Interest
T324 57762-57934 Sentence denotes The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.