2019 Recent Advances in the Vaccine Development Against Middle East Respiratory Syndrome-Coronavirus

2019·0.39 MB·English

fmicb-10-01781 August2,2019 Time:16:46 #1
REVIEW
published:02August2019
doi:10.3389/fmicb.2019.01781
Recent Advances in the Vaccine
Development Against Middle East
Respiratory Syndrome-Coronavirus
CheanYeahYong1,2,HuiKianOng3,SweeKeongYeap4,KokLianHo3and
WenSiangTan1,2*
1DepartmentofMicrobiology,FacultyofBiotechnologyandBiomolecularSciences,UniversitiPutraMalaysia,Serdang,
Malaysia,2LaboratoryofVaccinesandImmunotherapeutics,InstituteofBioscience,UniversitiPutraMalaysia,Serdang,
Malaysia,3DepartmentofPathology,FacultyofMedicineandHealthSciences,UniversitiPutraMalaysia,Serdang,Malaysia,
4ChinaASEANCollegeofMarineSciences,XiamenUniversityMalaysia,Sepang,Malaysia
Middle East respiratory syndrome (MERS) is a deadly viral respiratory disease caused
by MERS-coronavirus (MERS-CoV) infection. To date, there is no specific treatment
proven effective against this viral disease. In addition, no vaccine has been licensed
to prevent MERS-CoV infection thus far. Therefore, our current review focuses on
the most recent studies in search of an effective MERS vaccine. Overall, vaccine
candidates against MERS-CoV are mainly based upon the viral spike (S) protein,
Editedby:
due to its vital role in the viral infectivity, although several studies focused on other
LuLu,
FudanUniversity,China viral proteins such as the nucleocapsid (N) protein, envelope (E) protein, and non-
Reviewedby: structuralprotein16(NSP16)havealsobeenreported.Ingeneral,thepotentialvaccine
JincunZhao,
candidates can be classified into six types: viral vector-based vaccine, DNA vaccine,
GuangzhouMedicalUniversity,China
VincentMunster, subunit vaccine, nanoparticle-based vaccine, inactivated-whole virus vaccine and live-
NationalInstitutesofHealth(NIH), attenuated vaccine, which are discussed in detail. Besides, the immune responses
UnitedStates
andpotentialantibodydependentenhancementofMERS-CoVinfectionareextensively
JasperFukWooChan,
TheUniversityofHongKong, reviewed.Inaddition,animalmodelsusedtostudyMERS-CoVandevaluatethevaccine
HongKong
candidatesarediscussedintensively.
*Correspondence:
WenSiangTan Keywords: Middle East respiratory syndrome, coronavirus, animal model, vaccine, antibody dependent
[email protected] enhancement
Specialtysection:
INTRODUCTION
Thisarticlewassubmittedto
Virology,
asectionofthejournal Camel flu, or more commonly known as the Middle East respiratory syndrome (MERS), is a
FrontiersinMicrobiology respiratorydiseasecausedbyMERS-coronavirus(MERS-CoV).MERS-CoVwasfirstidentifiedin
Received:10May2019 SaudiArabiain2012(Zakietal.,2012).AsofFebruary2019,27countriesworldwidehavereported
Accepted:18July2019 cases of MERS-CoV infection, with 2,374 reported viral infection and 823 associated deaths,
Published:02August2019 which corresponds to ∼35% fatality in identified cases (World Health Organization [WHO],
Citation: 2019b), although the actual fatality rate of the viral infection is most likely below 35% due to
YongCY,OngHK,YeapSK, someunidentified,mild,orasymptomaticcases.MajorityofthesecasesoccurredinSaudiArabia,
HoKLandTanWS(2019)Recent amountingto1,983ofreportedcases,with745associateddeathsor∼37.5%fatality(WorldHealth
AdvancesintheVaccine
Organization[WHO],2019a).
DevelopmentAgainstMiddleEast
MajorityoftheidentifiedMERS-CoVcasesarenosocomiallyacquiredviadirectclosecontact
RespiratorySyndrome-Coronavirus.
Front.Microbiol.10:1781. with infected patients (Chowell et al., 2015; Cauchemez et al., 2016), whereas cases of zoonotic
doi:10.3389/fmicb.2019.01781 transmissionfromdromedarycamelstohumanswerereportedprimarilyinSaudiArabia,where
FrontiersinMicrobiology|www.frontiersin.org 1 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #2
Yongetal. AdvancesinMERSVaccine
human-camel interaction is frequent (Gossner et al., 2016). candidates were able to elicit systemic antibody responses,
Hitherto, no specific treatments and vaccines are available for producing high titer of serum IgG upon immunization, but
MERS-CoV infections. Although MERS-CoV is currently not manyfailedtogeneratesufficientmucosalimmunityunlessthe
listed as a potential pandemic threat, a recent outbreak in vaccines were administered via a mucosal or intranasal route.
South Korea which demonstrated virus emergence in second Activation of mucosal immunity is heavily dependent on the
and third generation contacts, has immediately raised concern routeofimmunization,andthisisacommonchallengeinvaccine
that multiple mutations of MERS-CoV might cause enhanced development for many respiratory pathogens (Ma et al., 2014a;
human-to-human transmission (Wang et al., 2015b; Oh et al., Guo et al., 2015). Pre-existing neutralizing mucosal antibodies
2018).Recently,MERS-CoVwasaddedtotheNIAID’spathogen are important as a first line of defenses against MERS-CoV
priority list as Category C Priority Pathogens due to its infection (Guo et al., 2015). All neutralizing antibodies elicited
potential applications in biological warfare (Du et al., 2016b). by vaccines based on S protein could bind to the receptor
Preventive measures against MERS-CoV infection, particularly binding domain (RBD) of the protein thereby inhibiting viral
vaccinedevelopment,arecrucialtoavoiddeadlyandunexpected internalization and membrane fusion (Du et al., 2017). Little is
futurepandemics. known about the memory B-cell responses against MERS-CoV,
MiddleEastrespiratorysyndrome-coronavirus,thecausative apartfromarecentstudywhichdemonstratedthepersistenceof
agent of MERS, is a positive sense, single-stranded RNA anti-MERS-CoVantibodiesinMERSsurvivorsupto34months
Betacoronavirus which belongs to the family of Coronaviridae. (Payne et al., 2016). On the other hand, antibody responses
Its viral genome is about 30 kb in length, flanked by a 5’- against another closely related coronavirus, SARS-CoV, were
terminal cap and 3’-poly(A) tail (van Boheemen et al., 2012; not persistent, whereby a 6-year follow-up study did not detect
Scobey et al., 2013). MERS-CoV genome contains at least 10 memory B-cell responses in SARS survivors (Tang et al., 2011).
open reading frames (ORFs), which encodes for 4 structural ItislikelythatsomeoftheB-cellsdifferentiateintoMERS-CoV-
proteins:spike(S)protein,envelope(E)protein,membrane(M) specific memory B-cells following infection or vaccination, but
protein, nucleocapsid (N) protein, 16 non-structural proteins the longevity and protective efficacy of these memory B-cells
(NSP1-NSP16),and5accessoryproteins(ORF3,ORF4a,ORF4b, againstMERS-CoVinfectionorre-challengeremainunresolved
ORF5, and ORF8b) (van Boheemen et al., 2012; Du et al., questions(Duetal.,2016b;PerlmanandVijay,2016).
2017). Of all these viral proteins, S and N proteins are of T-cell responses elicited by MERS-CoV vaccines also play
particularinterestinthedevelopmentofvaccinesagainstMERS- important roles in protection against MERS. This is supported
CoV, although other proteins such as E protein and NSP16 are bythefactthatviralclearancewasimpossibleinT-celldeficient
potentialimmunogensasliveattenuatedvaccines(Almazanetal., mice, but was possible in mice lacking B-cells (Zhao et al.,
2013;Menacheryetal.,2017). 2014).AlthoughT-cellsaredemonstratedtobeacriticaleffector
in acute viral clearance, protection for subsequent MERS-CoV
infectionislargelymediatedbyhumoralimmunity(Zhaoetal.,
CRITERIA FOR AN EFFECTIVE
2014). Several animal studies also demonstrated activation of
MERS-CoV VACCINE T-cell responses following immunization with a MERS-CoV
vaccine candidate, resulting in the elevated secretion of Th1
Two viral proteins of MERS-CoV, S and N proteins, were and Th2 cytokines (Lan et al., 2014; Ma et al., 2014a; Malczyk
demonstratedtobehighlyimmunogenicandcapableofeliciting et al., 2015; Muthumani et al., 2015). It is also noteworthy to
T-cell responses. However, only S protein was shown to induce mention that adjuvants could be co-administered with MERS-
neutralizingantibodies,thecriticaleffectorsagainstMERS-CoV CoV vaccines to tailor and possibly enhance the immune
(Agnihothram et al., 2014). Notably, N protein had also been responses elicited by the vaccines. One study has indicated
proposed to be a potential protective immunogen for both that co-administration of the MERS-CoV vaccine based on
neutralizing antibodies and T-cell immune responses through the S protein with Alum in mice resulted in a Th2 biased
insilicoapproaches(Shietal.,2015).Despitetheprediction,no immunity,whereasamorerobustTh1andTh2mixedimmune
biologicaldatahavebeenpresentedthusfar.AnotherpotentialB response was produced when an additional adjuvant, cysteine-
cellepitopeoftheMERS-CoVEproteinwasidentifiedrecently phosphate-guanine (CpG) oligodeoxynucleotides (ODN) was
using in silico methods, yet similarly, no biological data were included in the formulation (Lan et al., 2014). To date, no
presented (Xie et al., 2018). Therefore, most of the MERS-CoV detail investigation on MERS-CoV vaccine-induced memory
vaccine candidates are still based on the full length or part T-cellresponsesisreported.However,MERS-CoVinfectionwas
oftheSprotein. shown to induce memory CD4+ and CD8+ T-cells responses
Ideally,aneffectiveMERS-CoVvaccineisrequiredtoinduce in MERS survivors, at least up to 24 months (Zhao et al.,
bothrobusthumoralandcell-mediatedimmunities,particularly 2017).Thereislittleunderstandingaboutthebiologicalfunction
antibodyresponsesarecrucialforthesurvivalofthevaccinated of memory CD4+ T-cells but they are likely to contribute to
hosts(Duetal.,2016b).Previousstudiesindicatedthatthelevel directvirusinhibitionviacytokineproduction,particularlyIFN-
of serum neutralizing antibodies correlated positively with the γ, and enhance the effector functions of CD8+ T-cells and
reduction of lung pathogenesis, which increased the survival of B-cells (MacLeod et al., 2010). Although subsequent MERS-
animals challenged with MERS-CoV (Zhao et al., 2015; Zhang CoV infection is generally antibody mediated, memory CD8+
etal.,2016).Ingeneral,mostofthepotentialMERS-CoVvaccine T cells are believed to facilitate virus clearance by eliminating
FrontiersinMicrobiology|www.frontiersin.org 2 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #3
Yongetal. AdvancesinMERSVaccine
infectedcells(KaechandAhmed,2001;Zhaoetal.,2017).MERS 2005). Many potential vaccines against MERS-CoV were also
survivorswholaterdemonstratedstrongvirus-specificmemory mainly focused on the same full-length S protein, raising a
CD8+T-cellresponseswerealsoshowntoexperiencemitigated safety concern on the practical application of these vaccines
morbidity during the hospitalization period (Zhao et al., 2017). (Duetal.,2016b).
Similarly,theimportanceofT-cellresponsesagainstSARS-CoV To date, no ADE has been observed in MERS-CoV. Indeed,
was also highlighted in many studies (Channappanavar et al., the ADE of SARS-CoV infection in human cells was only
2014;ChuH.etal.,2014;Zhaoetal.,2016).Interestingly,unlike discovered 8 years after the virus was first identified in 2003
SARS-CoV,MERS-CoVcaninfectboththeCD4+andCD8+T (Yip et al., 2011). Jaume et al. (2012) demonstrated that non-
cells in human, resulting in the downregulation of hDPP4, and neutralizing antibodies induced by the full-length S protein of
induced intrinsic and extrinsic caspase-dependent apoptosis in SARS-CoV facilitated the viral entry into host cells via a FcγR-
Tcells,whichmayleadtosevereimmunopathology(Chuetal., dependent pathway. Our understanding about MERS-CoV is
2016).Inaddition,Chuetal.(2016)demonstratedthecapability relatively lesser compared to SARS-CoV, mainly due to the fact
ofMERS-CoVininfectingtheTcellsofcommonmarmosets. that the former was discovered less than 7 years, thus it is
It is critical for a potential MERS-CoV vaccine to induce unsurprisingthattheADEofMERS-CoVhasyettobereported
robust humoral and cell-mediated immunities. Although the (Du et al., 2016b). Nevertheless, by employing appropriate
protection against MERS-CoV is mainly mediated by humoral strategiesandmethods,theADEofMERS-CoVinfectioncould
immunity, T-cell responses are crucial for acute viral clearance. berevealedinthefuture.
MucosalrouteisrecommendedforMERS-CoVvaccinedelivery Twoapproacheshavebeensuggestedtomitigatetheadverse
to induce the mucosal immunity in addition to the systemic effects of ADE. The first approach involves shielding the
responses.Persistenceofthevirus-specificantibodiesinducedby non-neutralizing epitopes of the S proteins by glycosylation,
MERS-CoV vaccine is not thoroughly studied but represents a whereas the second approach, namely immunofocusing, aims
majorchallenge.AneffectiveMERS-CoVvaccineisalsorequired to direct the adaptive immune responses to target only the
to induce immunological memory to provide a long-lived critical neutralizing epitope to elicit a more robust protective
protectionwhichinturnreducestheneedofboosters,andinthe immunity (Du et al., 2016a; Okba et al., 2017). A supporting
longrunwillbringdownthecostofvaccinations.Lastly,different evidence for the latter is that a MERS-CoV vaccine candidate
adjuvants may also be used to improve the immunogenicity of based on a shorter S1 domain induced slightly stronger
MERS-CoV vaccines but would require detail studies on the neutralizing activity than that based on the full-length S
interactionsbetweenthemtoensureoptimalvaccineefficacyand protein. In addition, a vaccine candidate based on the even
safety.Sofar,threepotentialMERS-CoVvaccines:aDNAvaccine shorterRBDinducedthehighestneutralizingimmuneresponses
and two viral vector-based vaccines have advanced into clinical (Okbaetal.,2017).
trials(NationalInstitutesofHealth[NIH],2016,2018b,c).
CURRENT ANIMAL MODELS
POTENTIAL ANTIBODY DEPENDENT EMPLOYED FOR EVALUATION OF
ENHANCEMENT (ADE) OF MERS-CoV MERS-CoV VACCINES
INFECTION
Animal models available for evaluation of MERS-CoV vaccines
Antibodydependentenhancement(ADE)isaconditionwhereby are highly limited, thus representing a huge challenge for
non-neutralizingantibodiesareproducedfollowinganinfection vaccine development. MERS-CoV infects the human (Zaki
oravaccination,whichenhancetheinfectivityofthesubsequent et al., 2012), non-human primates-rhesus macaques (de Wit
infection (Kuzmina et al., 2018). ADE of viral infections have et al., 2013; Munster et al., 2013) and marmosets (Falzarano
beenreportedfordenguevirus,humanimmunodeficiencyvirus, et al., 2014), and dromedary camels (Alagaili et al., 2014;
influenza virus, other alpha and flaviviruses, SARS-CoV, and Chu D.K. et al., 2014; Memish et al., 2014). The first animal
Ebolavirus(Dutryetal.,2011;Kuzminaetal.,2018).Thus,ADE model adopted for the development of MERS-CoV vaccine
isacriticalissuethatshouldbeconsideredseriouslyindesigning was rhesus macaques (de Wit et al., 2013; Munster et al.,
aMERS-CoVvaccine. 2013). They demonstrated clinical symptoms of MERS-CoV
Attributed to the taxonomic and structural similarities infection including an increase in respiratory rate and body
between SARS-CoV and MERS-CoV, the processes involved temperature,hunchedposture,piloerection,cough,andreduced
in development of new vaccines against these two viruses, to foodintake.Radiographicimaginganalysisalsorevealedvarying
a large extent, are similar. Vaccine candidates against SARS- degree of pulmonary diseases following infection. Although
CoVwereinitiallydevelopedbasedonthefull-lengthSprotein. the viral RNA of MERS-CoV was detected in most of the
However, these vaccines were later demonstrated to induce respiratory tissues, but viral tropism was restricted primarily
non-neutralizing antibodies which did not prevent MERS-CoV to the lower respiratory tract. Rhesus macaques infected
infection, andtheimmunized animalswere notprotected from with MERS-CoV experienced transient, mild to moderate
the viral challenge instead they experienced adverse effects disease severity (van Doremalen and Munster, 2015; Du
like enhanced hepatitis, increased morbidity, and stronger et al., 2016b). It is noteworthy that the pathological changes
inflammatory responses (Weingartl et al., 2004; Czub et al., induced in rhesus macaques infected by MERS-CoV were
FrontiersinMicrobiology|www.frontiersin.org 3 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #4
Yongetal. AdvancesinMERSVaccine
the results of the host inflammatory responses triggered viral clearance in these infected mice was observed at day-
by the virus instead of the direct viral cytolytic activity 8 post-infection, failing to recapitulate severe human diseases
(Prescottetal.,2018). (Zhaoetal.,2014). Later, a more established transgenic mouse
The common marmoset is another frequently used animal model expressing hDPP4 globally was developed, and it was
model to evaluate MERS-CoV vaccines (Falzarano et al., the first lethal animal model available to evaluate MERS-
2014). Similar to rhesus macaques, humoral and cell-mediated CoV vaccines. Mortality was noted in these mice within days
immunitiescouldbedetectedintheseanimalsfollowingMERS- post-infection, and virus dissemination to other organs was
CoVvaccination.ThecommonmarmosetsinfectedwithMERS- observed with exceptionally high titer detected in the lung
CoV developed moderate to severe acute pneumonia and and brain (Agrawal et al., 2015). Recently, a transgenic mouse
increased viral load in the respiratory tract in addition to model was produced by replacing the full-length mouse DPP4
other clinical symptoms experienced by rhesus macaques (van gene with the human equivalent. However, these transgenic
Doremalen and Munster, 2015; Yu et al., 2017). Intriguingly, mice did not demonstrate any sign of diseases following the
thecommonmarmosetalsodemonstratedsignsofrenaldamage MERS-CoV infection, and no virus dissemination to other
as in human cases following MERS-CoV infection, and the organs was observed (Pascal et al., 2015). CRISPR/Cas9 was
viral RNA could be detected in other non-respiratory organs also previously employed to sensitize the mice to MERS-
contrary to rhesus macaques (van Doremalen and Munster, CoV infection by substituting two amino acids at positions
2015; Yeung et al., 2016). Falzarano et al. (2014) also reported 288 and 230 of the mouse DPP4. Although these genetically
that the common marmoset could serve as a partially lethal engineered mice allowed viral replication in the lungs, they
animal model. Similarly, Chan et al. (2015) demonstrated did not experience apparent morbidity following infection by
that marmosets challenged with MERS-CoV developed severe the wild-type MERS-CoV. Severe diseases were observed only
diseases, leading to fatality. Thereafter, marmosets have been when the mice were infected by mouse-adapted MERS-CoV
successfullyusedasamoderateandseveremodeltostudyMERS- generated via 15 serial lung passages (Cockrell et al., 2016).
CoV (Baseler et al., 2016; Yeung et al., 2016; Chen et al., 2017; As mouse DPP4 is vital to normal glucose homeostasis and
vanDoremalenetal.,2017;Yuetal.,2017;deWitetal.,2019). immunity, altering the mouse DPP4 could have unforeseen
The dromedary camels serve as a natural reservoir for complicationstothemousemodel(Fanetal.,2018).Therefore,
MERS-CoV, and are responsible for zoonotic transmission anothertransgenicmousemodelhasbeenintroduced,inwhich
of the virus to humans. Mild clinical symptoms such as the hDPP4 gene was inserted into the genome of C57BL/6-
increase in body temperature and rhinorrhea were observed mouseatRosa26locususingtheCRISPR/Cas9technology.This
in the dromedary camels infected with MERS-CoV (Adney mouse model, namely R26-hDPP4, when infected by MERS-
et al., 2014). Interesting, MERS-CoV tropism in dromedary CoV at low dose, developed severe lung diseases related to
camels is limited to the upper respiratory tract, and is less acuterespiratorysymptoms(ARDS)andcentralnervoussystem
apparent in the lower respiratory tract, contrary to rhesus (CNS).Inaddition,theR26-hDPP4isalsosusceptibletoinfection
macaques (Adney et al., 2014). The viral RNAs of MERS- by a MERS-CoV pseudovirus, serving as an alternative to test
CoV are detectable in the respiratory tract, lymph node and MERS-CoV vaccines in the absence of BSL-3 facility (Fan
the excreted breath of the infected dromedary camels. Viral et al., 2018). All of the animal models described above are
shedding from the upper respiratory tract of the dromedary summarizedinTable1.
camels may explain the efficiency of virus transmission among Apartfromthemousemodel,rabbitswerealsoreportedtobe
the camels, and from camels to humans (Adney et al., asymptomaticallyinfectedbyMERS-CoV.Byextensiveresearch,
2014). The dromedary camels immunized with MERSV-CoV theseanimalscouldrepresentanotherpotentialanimalmodelto
vaccines were also shown to activate both the B-cell and T-cell evaluate MERS-CoV vaccines (Haagmans et al., 2015). Smaller
responses (Muthumani et al., 2015; Haagmans et al., 2016; animal models are more economically available to vaccine
Adneyetal.,2019). evaluations in addition to the ease of animal manipulation and
Although camels are the natural reservoirs of MERS-CoV, readilyavailablemethodsintestingvaccineefficacy.
whilstmacaquesandmarmosetsarecloselyrelatedtothehuman,
the handling of these large mammals is laborious and costly.
CURRENT MERS-CoV VACCINE
The lack of small animal models for the initial screening of
potential vaccine candidates greatly hampers the development PLATFORMS
of MERS-CoV vaccines. Unlike SARS-CoV, MERS-CoV does
not readily infect smaller rodents such as mice or hamsters As of now, SARS-CoV and MERS-CoV are the only
duetothesubstantialdifferencesintheviralbindingreceptors, coronaviruses known to cause severe diseases in human.
dipeptidyl peptidase 4 (DPP4) (Goldstein and Weiss, 2017). Development of SARS vaccines was mainly focused on the S
Nevertheless,considerableamountofeffortshavebeendevoted protein of SARS-CoV (Bukreyev et al., 2004; Weingartl et al.,
to produce MERS-CoV-permissive small rodents for evaluation 2004; Yang et al., 2004; Czub et al., 2005; Kam et al., 2007; Lin
of MERS-CoV vaccines. Mice transduced by a viral vector to et al., 2007; Fett et al., 2013). To date, no vaccine has been
express human DPP4 (hDPP4) were shown to be susceptible licensed to prevent MERS-CoV infection. Although several
to MERS-CoV infection, manifested by the development of vaccine candidates are currently in clinical trials, many still
pneumoniaandhistopathologicalchangesinthelungs.However, remained in the pre-clinical stage. Current approaches for the
FrontiersinMicrobiology|www.frontiersin.org 4 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #5
Yongetal. AdvancesinMERSVaccine
TABLE1|AnimalmodelsusedforvaccinedevelopmentagainstMiddleEastrespiratorysyndrome-coronavirus.
Animalmodels Results Advantages/Limitations References
Rhesusmacaques Theanimalsmanifestedclinicalsignswithin24hfollowing Munsteretal.,2013
infection,includingincreaseinrespiratoryrateandbody
temperature,hunchedposture,piloerection,cough,reducedfood
intakeandvaryingdegreeofpneumonia.Nomortalitywas
observedintheinfectedanimalsthroughoutthestudy.The
increaseinwhitebloodcellcountswasearlyandtransient,and
viral-loadwasreportedtobehigherinlowerrespiratorytractand
decreasedovertime.
Rhesusmacaques Theanimalsexperiencedearlyincreaseinneutrophilatday-1 – Geneticallyclosertohuman deWitetal.,2013
post-infection(P.I.),andrestoredatday-3P.I.Developmentof – Donotrecapitulateseverediseasesin
pneumoniaintheanimalswasrapidaftertheinfectionbut human
short-lived.Nomortalityorvirusdisseminationtoother – Expensivemodelduetohighhusbandry
non-respiratorytissuewasobservedintheinfectedanimals. requirement
Infectionisrestrictedprimarilyatlowerrespiratorytract.
Rhesusmacaques Therectaltemperatureoftheanimalsincreasedat1to2daysP.I. Yuetal.,2017
andrestoredthereafter.Extensivelunglesionsandvaryingdegree
ofinflammationwereobservedinthelungsoftheanimals
collectedatday-3P.I.Otherpathologicalchangesoftheinfected
lungsincludeinterstitialpneumonia,pulmonaryedema,
hemorrhaging,degenerationandnecrosisofpneumocytesand
bronchialepithelialcells.Nosignofdamageinother
non-respiratoryorganswasobserved.
Immunosuppressed Theimmunosuppressedanimalsdevelopedrapidpneumoniabut – PotentialmodelformimickingMERS-CoV Prescottetal.,2018
rhesusmacaques lessseverethanthenon-immunosuppressedmonkey.Higherviral infectioninimmunocompromisedpatients
load,viralshedding,andvirusdisseminationtoother – Expensivemodelduetohighhusbandry
non-respiratoryorganswereobservedintheimmunosuppressed requirement
animalsfollowinginfection. – Additionaltreatmentisrequiredtoproduce
immunosuppressedanimals
Common Mostoftheinfectedanimalsdevelopedprogressivesevere – Severe,partiallylethalanimalmodel Falzaranoetal.,2014
marmosets pneumoniacharacterizedbyinterstitialinfiltration.Someanimals – Abletomanifestrenaldamage
wereeuthanizedbecauseofdiseasesseverity.Extensivelung – Expensivemodelduetohighhusbandry
lesionswereobservedinalltheinfectedanimalsatdifferent requirement
necropsiestimepoints.ViralRNAcouldbedetectedinblood,
respiratoryorgansandothernon-respiratoryorgansincluding
kidney,suggestingvirusdissemination.
Common Infectedanimalsdevelopedseverepneumoniaatday-3P.I. Yuetal.,2017
marmosets characterizedbyexudativepathologicalchangeswithwidespread
pulmonaryedema,hemorrhaging,andhugenumberof
inflammatorycells.
Common Invitroanalysisusinglungandkidneycellsshowedthat Yeungetal.,2016
marmosets hyperexpressionofSmad7orFGF2inducedbyMERS-CoVledto
animmenseapoptoticresponse.Commonmarmosetsinfected
withMERS-CoVdemonstratedacuterespiratorydistressand
disseminatedinfectioninkidneysandotherorgans.
Dromedarycamels TheanimalsinfectedexperimentallywithMERS-CoVdeveloped – Potentialmodelforpathogenesisstudiesof Adneyetal.,2014
mildsymptomssuchasincreaseinbodytemperatureand MERS-CoVandtransmissiontohuman
rhinorrhea.Symptomsoftheinfectedanimalslastedlessthan – Donotrecapitulateseverediseasesin
2weeks.Sheddingofinfectiousviruswasdetectedinlessthan human
7daysP.I.butviralRNAremaineddetectableupto35daysP.I.in – Expensivemodelduetohighhusbandry
thenasalswabs.ViralRNA,butnotinfectiousvirus,wasdetected requirement
intheexhaledbreathoftheinfectedanimalsatday-3and-5P.I.
TheInfectionwasrestrictedtoupper-respiratorytract.
hDPP4-transduced MicetransducedwithadenoviralvectortoexpresshDPP4in – Easeofmanipulation Zhaoetal.,2014
mice lungsweresusceptibletoMERS-CoVinfection.Followingthe – Lowhusbandryrequirement
infection,micedevelopedinterstitialpneumoniainadditionto – Readilyavailablemethodsintestingvaccine
reducedweightgaininyoungmiceandweightlossinagedmice. efficacy
Nomortalitywasobservedinallinfectedanimals,andvirus – Donotrecapitulateseverediseasesin
clearancewasdetectedatday-6to-8P.I.ExpressionofhDPP4 human
intheanimals’lungslastedfor17to22daysaftertransduction.
(Continued)
FrontiersinMicrobiology|www.frontiersin.org 5 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #6
Yongetal. AdvancesinMERSVaccine
TABLE1|Continued
Animalmodels Results Advantages/Limitations References
Transgenicmice Followingtheinfection,thetransgenicanimals – Lethalanimalmodel Agrawaletal.,2015
expressinghDPP4 developedseverepneumonia,and100%mortality – Easeofmanipulation
globally wasdetectedatday-6P.I.Virusdisseminationto – Lowhusbandryrequirement
othernon-respiratoryorganswasdetectedwith – Readilyavailablemethodsintesting
significantlyhighviralRNAinthebrainsandlungs. vaccineefficacy
NoviralRNAcouldbedetectedinthekidneyorthe – Lackphysiologicalexpressionpattern
liveroftheinfectedmice. becauseallmousecellsexpresshDPP4
hDPP4-humanized HumanizedmicecanbeinfectedwithMERS-CoV – Easeofmanipulation Pascaletal.,2015
transgenicmice butdonotdemonstrateclinicalsignofdiseases. – Lowhusbandryrequirement
Pathologicalchangesincludingperi-bronchiolar – Readilyavailablemethodsintesting
inflammation,interstitialinfiltration,andminimal vaccineefficacy
peri-vascularinflammationwereobservedat2to – Correctphysiologicalexpressionpattern
4daysP.I.ViralRNAwasdetectedinthelungs,and – Littletonoclinicalsign
novirusdisseminationtootherorganswas
observed
CRISPR/Cas9- Micegenomewasmodifiedtoincorporatehuman – Severe,partiallylethalanimalmodel Cockrelletal.,2016
engineered codonsataminoacidpositions288and330inthe (challengedwithmouse-adapted
mice mouseDPP4genecausingthemtobecome MERS-CoVonly)
susceptibletoMERS-CoVinfection.Theinfected – Easeofmanipulation
micedidnotdemonstrateanysignofdiseasesbut – Lowhusbandryrequirement
supportedviralreplicationinthelungs.Inflammation – Readilyavailablemethodsintesting
oftheinfectedlungswasmoderate.Severedisease vaccineefficacy
couldbeinducedinthesemicebyinfectingthem
withmouse-adaptedMERS-CoV.
hDPP4-knockin hDPP4-knockinmiceweresusceptibleto – Lethalanimalmodel Fanetal.,2018
miceusing MERS-CoVinfection.Themiceexperienceddrastic – Easeofmanipulation
CRISPR/Cas9 weightlossabovethetypicaleuthanization – Lowhusbandryrequirement
endpoint(20%)byday-5P.I.Lesionsandvirusload – Readilyavailablemethodsintesting
weredetectedinthebrainsandthelungsofthe vaccineefficacy
micebutnotinthekidneysorlivers. – Lackphysiologicalexpressionpattern
becauseallmousecellsexpresshDPP4
MERS-CoV,MiddleEastrespiratorysyndrome-coronavirus;DPP4,dipeptidylpeptidase4;hDPP4,humandipeptidylpeptidase4;P.I.,post-infection.
development of MERS-CoV vaccines are mostly referred to increase the immunogenicity of displayed epitopes, particularly
the methods used for the development of SARS-CoV vaccines thoseofsmallerones(Murataetal.,2003;Quanetal.,2008).Live
during the past two decades, which include: viral vector-based attenuatedvaccinesarecomposedofliveviruses,whichhavebeen
vaccine, DNA vaccine, subunit vaccine, virus-like particles modifiedtoremoveorreducetheirvirulence.Thistypeofvaccine
(VLPs)-based vaccine, inactivated whole-virus (IWV) vaccine is often very immunogenic, whereby a single administration
andliveattenuatedvaccine. withoutanadjuvantissufficienttoinduceprotectiveimmunity.
In general, IWV vaccine is the most rapid approach for However, the risk of reversion to a virulent virus has limited
vaccineproductionfollowinganewoutbreak.However,theuse its usage as MERS vaccine. Viral vector-based vaccine is one
of IWV as a vaccine in MERS was reported to be associated of the most popular approaches in developing MERS vaccines.
with hypersensitivity-type lung immunopathologic reaction in Two out of the three candidate vaccines which have entered
the mouse model (Agrawal et al., 2015), thereby limiting its the clinical phase are viral vector vaccines. This approach
potential. Subunit vaccine is by far the most popular method utilizes well-studied virus replication system to display MERS-
in the development of MERS vaccine, mostly focusing on the CoV antigen, thereby inducing protective immunity against
recombinant RBD of the S protein produced in heterologous MERS-CoV. Another candidate vaccine currently in phase I/II
expression systems. Subunit vaccines, however, are often clinical trial is a DNA vaccine. Unlike other types of vaccines,
administeredalongwithadjuvantstoboosttheimmunogenicity DNA vaccine production does not involve virus replication,
oftherecombinantantigens.NanoparticlessuchasVLPs-based protein expression and purification, therefore reduce the cost
vaccines are similar to subunit vaccines, in which only specific of production. However, administration of DNA vaccines often
viralproteinsareexpressed.Unlikesubunitvaccines,VLPs-based requires an external device such as electroporator or gene
vaccinesarecomprisedofrecombinantviralproteinscapableof gun, which eventually increases the cost of immunization.
self-assemblingintolargerparticlesresemblingviruses.Although Table 2 summarizes the vaccine candidates against MERS-
theimmunogenicityofVLPs-basedvaccinescouldbeenhanced CoV infection, which are further discussed intensively in the
byadjuvants,theVLPsthemselvescanserveasadjuvantswhich followingsections.
FrontiersinMicrobiology|www.frontiersin.org 6 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #7
Yongetal. AdvancesinMERSVaccine
TABLE2|PotentialvaccinecandidatesagainstMiddleEastrespiratorysyndrome-coronavirus.
Vaccinetype Vectorandantigen Administrationroute Results References
Viral-vector rAd5encodingS1protein IM ImmunizationwithrAd5constructsexpressing Hashemetal.,2019
CD40-targetedS1fusionprotein(rAd5-S1/F/CD40L)
offeredcompleteprotectiontohDPP4transgenic
miceagainstMERS-CoVchallengeandprevented
pulmonaryperivascularhemorrhage.
rAd5orrAd41encodingS IMorIG IGadministrationofrAd5-SorrAd41-Selicited Guoetal.,2015
protein antigen-specificIgGandneutralizingantibodyin
serum,butT-cellresponseswerenotdetected.
AsingleIMinjectionofAd5-SorAd41-Sinduced
systemichumoralresponseinadditiontothe
functionalantigen-specificT-cellresponsesinthe
spleenandpulmonarylymphocytesofthemice,
whichpersistedforseveralmonths.
rAd5encodingSproteinor IMandlaterboostedwithIN Immunizedmicedemonstratedantibodyresponses Kimetal.,2014
S1 againstspikeprotein,whichneutralizedMERS-CoV
invitro.Strongerneutralizingantibodyresponses
wereobservedinthemicevaccinatedwiththe
vectorencodingtheshorterS1proteinthanthe
full-lengthSprotein.
ChAdOx1encodingS IMorIN Singledoseintranasalorintramuscularimmunization Munsteretal.,2017
protein∗ protectedtransgenicBALB/cmiceagainstlethal
viruschallenge.Immunogenicityandefficacywere
comparablebetweenimmunizationroutes.
ChAdOx1encodingS IM SingledoseimmunizationwithChAdOx1MERS Alharbietal.,2017
protein∗ vaccinewithtPAinduced5logsofneutralizing
antibodiesinBALB/cmice.
MVAencodingSprotein IM ImmunizationwithMVAMERSvaccinecontaining Alharbietal.,2017
tPAregulatedbyF11promoterinduced4.7logsof
neutralizingantibodiesinBALB/cmice.
MVAencodingSprotein∗∗ IMorSC Bothimmunizationroutesinducedneutralizing Volzetal.,2015
antibodiesandCD8+T-cellresponsesinmice.
VaccinatedmicewereprotectedagainstMERS-CoV
challengeinfectionaftertransductionwiththehDPP4
receptor.
MVAencodingSprotein∗∗ IM Neutralizingantibodyresponseswereinducedin Songetal.,2013
immunizedmice.
MVAencodingthe IMorIP CD8+T-cellresponsewaselicitedintheimmunized Veitetal.,2018
N-protein miceinbothimmunizationroutes.
NDVencodingSprotein IM RecombinantNDVexpressingMERS-CoVSprotein Liuetal.,2017
inducedneutralizingantibodiesinBALB/cmiceand
Bactriancamels.
Viral-vectorand rAd5andMERS-CoVS IM Heterologousprime-boostvaccinationwithrAd5-S Jungetal.,2018
nanoparticle nanoparticle proteinandalum-adjuvantedrecombinantSprotein
inducedbothTh1andTh2immuneresponsesin
SPFBALB/cmice.
DNA DNAencodingSprotein∗∗∗ IMfollowedbyEP TheDNAvaccinewasimmunogenicinmice,camels Muthumanietal.,2015
andrhesusmacaques.Whentheimmunized
macaqueswerechallengedwithMERS-CoV,
characteristicclinicalsymptomsincluding
pneumoniawerereduced.
DNAencodingSorS1 IM DNAencodingS1proteinelicitedstrongerantibody Al-Amrietal.,2017
protein andcellularimmuneresponsesinmicethanthat
encodingtheSprotein.BothDNAsencodingS1and
Sproteinsinducedneutralizingantibodiesthat
cross-reactedwithMERS-CoVstrainsofhumanand
camelorigins.
DNAencodingS1protein IM ImmunizationwithDNAencodingS1proteinand Chietal.,2017
passivetransferofimmuneserafromthevaccinated
miceprotectedhDPP4-transduced-micefrom
MERS-CoVinfection.
(Continued)
FrontiersinMicrobiology|www.frontiersin.org 7 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #8
Yongetal. AdvancesinMERSVaccine
TABLE2|Continued
Vaccinetype Vectorandantigen Administrationroute Results References
Subunit MERS-CoVS1protein SC Adjuvanted(MF59)MERS-CoVS1protein Wangetal.,2017c
protectedhDPP4transgenicmiceagainstlethal
MERS-CoVchallenge,wheretheprotection
correlatedwellwiththeneutralizingantibodytiter.
MERS-CoVS1protein IM Immunizationwithadjuvanated(AdvaxHCXL Adneyetal.,2019
adjuvantandSigmaAdjuvantSystem)S1protein
reducedanddelayedvirussheddingintheupper
respiratorytractofdromedarycamelsandprovided
completeprotectioninalpacaagainstMERS-CoV
challenge.
MERS-CoVSproteintrimer IM RecombinantprefusiontrimericMERS-CoVS Pallesenetal.,2017
onFd proteininducedhightiterofneutralizingantibodies
inBALB/cJmice.
RBDtrimeronFd SCorIM Adjuvanted(alum)RBD-Fdinducedneutralizing Taietal.,2016
antibodiesinBALB/cmiceandprotected(83%)
hDPP4transgenicmiceagainstlethalMERS-CoV
challenge.
RBDfusedtoFc SC AdjuvantedRBD-Fcinducedhightiterof Maetal.,2014b
neutralizingantibodiesinBALB/cmiceand
NewZealandwhiterabbits.
RBDfusedtoFc SC MiceimmunizedwiththevaccineandMontanide Duetal.,2013
ISA51adjuvantproducedneutralizingantibodies
whichinhibitedbindingoftheRBDtoDPP4
receptor,neutralizingMERS-CoVinfection.
RBDfusedtoFc INorSC Micevaccinatedwithbothimmunizationroutesin Maetal.,2014a
thepresenceofadjuvants(MontanideISA51
adjuvantforSCandPoly(I:C)forIN)elicited
systemichumoralimmuneresponses.Stronger
systemiccellularimmuneresponsesandlocal
mucosalimmuneresponseswereobservedinmice
immunizedviaINroute.
RBDfusedtoFc IM hCD26/DPP4transgenicmiceimmunizedwiththe Nyonetal.,2018
vaccineinthepresenceofadjuvant,AddaVax
elicitedneutralizingantibodiesandwereprotected
againstMERS-CoVinfection.
RBDfusedtoFc SC Miceimmunizedwiththevaccinealoneproduced Zhangetal.,2016
detectableneutralizingantibodiesandcellular
immuneresponses.Immunogenicityofthevaccine
improvedwhentheadjuvantssuchasFreund’s
adjuvant,alum,monophosphoryllipidA,Montanide
ISA51orMF59wasincludedintheformulation.
MF59wasdemonstratedtobesuperiorin
enhancingthevaccineimmunogenicityand
protectionagainstviralchallenge.
RecombinantRBD IMorSC Whenthesubunitvaccinewasadministered Lanetal.,2014
togetherwithcombinationofalumandCpGODN,
optimizedRBD-specifichumoralandcellular
immunitywereelicted.RobustRBD-specific
antibodyandT-cellresponseswereinducedinmice
immunizedwiththevaccineincombinationwithIFA
andCpGODN,butlowlevelofneutralizing
antibodieswereinduced.
RecombinantRBD IM Rhesusmacaquesimmunizedwiththesubunit Lanetal.,2015
vaccineandalumadjuvantproducedneutralizing
antibodiesandexperiencedmitigatedclinical
symptomswhenchallengedwithMERS-CoV.
rNTDofSprotein IM ImmunizationwithrNTDofMERS-CoVSprotein Jiamingetal.,2017
adjuvantedwithaluminducedneutralizing
antibodiesandreducedtherespiratorytract
pathologyofBALB/cmicechallengedwith
MERS-CoV.
(Continued)
FrontiersinMicrobiology|www.frontiersin.org 8 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #9
Yongetal. AdvancesinMERSVaccine
TABLE2|Continued
Vaccinetype Vectorandantigen Administrationroute Results References
VLPs MERS-CoVVLPs IM Co-administrationoftheVLPs-basedvaccine Wangetal.,2017b
andalumactivatedRBDspecifichumoraland
cellularimmuneresponsesinrhesusmacaques.
Sproteinnanoparticles IM Sproteinproducedinthebaculovirusinsect Colemanetal.,2014
cellsexpressionsystemassembledinto
nanoparticlesofapproximately25nm.Mice
immunizedwiththesenanoparticlesinthe
presenceofalumproducedhightiterof
neutralizingantibody.
Sproteinnanoparticles IM ThevaccinetogetherwithMatrixM1adjuvant Colemanetal.,2017
activatedSproteinspecifichumoralimmune
responses,andprotectedhDPP4transduced
miceagainstviralchallenge.
CPVVLPdisplayingRBD IM Immunizationofthemicewiththechimeric Wangetal.,2017a
VLPsdisplayingRBDinthepresenceof
adjuvants[alumorPoly(I:C)]elicitedneutralizing
antibodyresponsesaswellascellularimmune
responses.
InfluenzaAVLPdisplaying IM Immunizationofthemicewiththechimeric Lanetal.,2018
Sprotein VLPsdisplayingRBDinthepresenceofa
combinationofadjuvants(alumandCpGODN)
elicitedneutralizingantibodyresponses.
Nanoparticle FerritindisplayingRBD IM Immunizationwithchaperna-mediatedferritin Kimetal.,2018
nanoparticledisplayingMERS-CoVRBD
adjuvantedwithMF59inducedRBD-specific
antibodiesinBALB/cmicewhichinhibitedRBD
bindingtohDPP4receptorprotein.
Inactivatedwhole MERS-CoV IM Miceimmunizedwiththeinactivatedvaccinein Dengetal.,2018
-virus thepresenceofacombinationofadjuvants
(alumandCpGODN)elicitedneutralizing
activitybutnotcell-mediatedimmunity.This
vaccinealsoprotectedhDPP4transducedmice
againstMERS-CoVchallenge.
MERS-CoV IM Gammaradiation-inactivatedMERS-CoV Agrawaletal.,2016
inducedneutralizingantibodiesandreduced
viralloadinhDPP4transgenicmicebutmay
causehypersensitivity-typelung
immunopathologicreactionuponMERS-CoV
challenge.
ChimericRABVdisplaying IM Theinactivatedvaccineinducedhightiterof Wirblichetal.,2017
S1protein neutralizingantibodiesinmice,andprotected
hDPP4-transduced-miceagainstMERS-CoV
infection.
Live-attenuated MERS-CoVmutant - ThemutantwasproducedbydeletingtheE Almazanetal.,2013
geneoftheMERS-CoV.Thismutantlacked
infectivitybutwassingle-cyclereplicative.
MERS-CoVmutant IN AttenuatedMERS-CoVthroughmutationof Menacheryetal.,2017
NSP16(D130A)protectedCRISPR-Cas9-
targeted288–330+/+C57BL/6micefrom
mouse-adaptedMERS-CoVchallenge.
MVexpressingfull-lengthor IP TherecombinantMVwasreplication Malczyketal.,2015
truncated,solublevariantof competent.ImmunizationofthetypeIinterferon
Sprotein receptor-deficient(IFNAR−/−)CD46Gemice
withtherecombinantMVinducedbothMVand
Sproteinspecificneutralizingantibodiesaswell
ascellularimmuneresponses.Therecombinant
MVprotectedhDPP4-transduced-miceagainst
viralchallenge.
MVexpressingNprotein IP RecombinantMVexpressingMERS-CoVN Bodmeretal.,2018
proteininducedN-specificTcellresponsesin
IFNAR−/−-CD46Gemice.
(Continued)
FrontiersinMicrobiology|www.frontiersin.org 9 August2019|Volume10|Article1781
fmicb-10-01781 August2,2019 Time:16:46 #10
Yongetal. AdvancesinMERSVaccine
TABLE2|Continued
Vaccinetype Vectorandantigen Administrationroute Results Reference
RecombinantVSV INorIM RecombinantVSVwasproducedbyreplacingthe Liuetal.,2018
expressingSprotein glycoproteinofVSVwiththeSproteinof
MERS-CoV.Therecombinantvirusinduced
neutralizingantibodiesandTcellresponsesin
rhesusmacaquesafterasingleintramuscularor
intranasalimmunizationdose.
MERS-CoV,MiddleEastrespiratorysyndrome-coronavirus;rAd5,recombinanthumanadenovirustype-5;rAd41,recombinanthumanadenovirustype-41;MVA,modified
vacciniavirusAnkara;ChAdOx1,chimpanzeeadenovirus,OxfordUniversity#1;NDV,Newcastlediseasevirus;MV,measlesvirus;CPV,canineparvovirus;RABV,rabies
virus;VLP,virus-likeparticle;NSP,non-structuralprotein;Sprotein,spikeprotein;S1protein,spikeproteinreceptorbindingsubunit;RBD,receptor-bindingdomaininS1;
Nprotein,nucleocapsidprotein;rNTD,recombinantN-terminaldomain;Fd,foldontrimerizationmotif;Fc,FcregionofhumanIgG;tPA,leadersequenceofthehuman
tissueplasminogenactivatorgene;IFNAR−/−-CD46Gemice,geneticallymodifiedmicedeficientoftypeIIFNreceptorandtransgenicallyexpressinghumanCD46;SPF,
specific-pathogen-free;IM,intramuscular;IN,intranasal;IP,intraperitoneal;SC,subcutaneous;IG,intragastric;EP,electroporation.Vectorsandantigensmarkedwith“∗”
haveenteredphaseIclinicaltrial(∗MERS001;∗∗MVA-MERS-S;∗∗∗GLS-5300).
VIRAL VECTOR-BASED VACCINE (ChAdOx1) represents an attractive alternative to the human
adenoviralvectorduetoitsgoodsafetyprofileandlackofpre-
The first viral vector-based vaccine was reported by Moss et al. existingimmunityinhumanpopulation(Dicksetal.,2012),and
(1984) who developed a potential hepatitis B vaccine using the has since been employed in the vaccine development against
vaccinia viral vector. Unlike subunit or inactivated vaccines, MERS-CoV infection. The recombinant ChAdOx1 encoding
whichgenerallyfunctionasextracellularantigens,aviralvector full-length S protein (ChAdOx1 MERS) was shown to be
works by carrying a DNA encoding immunogenic components immunogenic in mice, and lethal virus challenge using hDPP4
into host cells, followed by intracellular antigen expression, transgenicmousemodelfurtherdemonstrateditshighprotective
therebyactivatingabroadspectrumcell-mediatedimmunityin efficacyagainstMERS-CoV(Alharbietal.,2017;Munsteretal.,
addition to the humoral immune responses. Majority of the 2017). It is noteworthy that the immunogenicity of S protein
viral-vectorbasedvaccinesdonotrequireadjuvantforoptimum could be improved by insertion of a gene encoding the signal
efficacy(Uraetal.,2014).Adenovirusandmodifiedvacciniavirus peptide of human tissue plasminogen activator (tPA) upstream
Ankara(MVA)arethetwomostcommonviralvectorsusedinthe of the S gene of MERS-CoV, in both ChAdOx1 and MVA
developmentofMERS-CoVvaccines. vectors (Alharbi et al., 2017). Currently, a candidate MERS-
Mice immunized intramuscularly with the recombinant CoVvaccineknownasMERS001,whichcontainstheChAdOx1
humanadenoviral(type5or41)vectorencodingthefull-length encodingtheSproteinofMERS-CoVisatphaseIclinicaltrial.
Sproteinwereshowntoinducesystemicneutralizingantibodies The trial is estimated to be completed by December 2019, in
and mucosal T-cells immunity. Intriguingly, no mucosal T-cell which the safety and immunogenicity of MERS001 at different
responsewasdetectedwhenthevaccinewasadministeredviaan dosage are being studied in healthy adult volunteers recruited
intragastricroute,contrarytopreviousfindingswhichsuggested and sponsored by the University of Oxford, United Kingdom
theimportanceofmucosalvaccinationinactivatingthemucosal (NationalInstitutesofHealth[NIH],2018b).
immunity(Guoetal.,2015).Arecombinanthumanadenovirus Recombinant MVA encoding the full-length S protein
type 5 (rAd5) vector encoding the shorter S1 extracellular represents another potential MERS-CoV vaccine candidate due
domain of the S protein was reported to elicit slightly stronger to its good safety profile, decent immunogenicity, and high
neutralizing antibody responses than that encoding the full- protective efficacy against MERS-CoV (Song et al., 2013; Volz
length, suggesting the effect of immunofocusing (Kim et al., et al., 2015; Alharbi et al., 2017). Another candidate vaccine
2014). A recent study by Hashem et al. (2019) demonstrated currently in phase I clinical trial is MVA-MERS-S. The trial is
thatrAd5constructsexpressingCD40-targetedS1fusionprotein being performed by the University Medical Center Hamburg-
(rAd5-S1/F/CD40L) offered a complete protection to hDPP4 Eppendorf, Germany, in which the safety and immunogenicity
transgenic mice against MERS-CoV challenge, and prevented of MVA-MERS-S in healthy adult volunteers are being assessed
pulmonary perivascular hemorrhage. Additionally, Jung et al. (National Institutes of Health [NIH], 2018c). Apart from the
(2018) showed that heterologous prime-boost vaccination with S protein, the highly conserved N protein of MERS-CoV was
rAd5-S protein and alum-adjuvanted recombinant S protein inserted into MVA, and inoculated into mice. Although the
nanoparticlesuccessfullyinducedboththeTh1andTh2immune recombinantMVAencodingtheN-proteinelicitedCD8+T-cell
responsesinspecific-pathogen-freeBALB/cmice. responseintheimmunizedmice,itsprotectiveefficacywasnot
Pre-existing immunity against human adenovirus in human investigated(Veitetal.,2018).
population is widespread, hampering its clinical application Apart from adenovirus and MVA, Newcastle disease virus
as a vector for vaccine development (Fausther-Bovendo and (NDV) was also used as a viral-vector for displaying MERS-
Kobinger,2014).Recentdevelopmentsofnewadenovirusvectors CoV S protein. The NDV-based vaccine candidate induced
for vaccine antigen delivery focus on the serotype to which neutralizing antibodies in BALB/c mice and Bactrian camels
human population is less exposed. Chimpanzee adenovirus (Liu et al., 2017). Although viral vector-based vaccines are able
FrontiersinMicrobiology|www.frontiersin.org 10 August2019|Volume10|Article1781