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Influenza A Virus Entry: Implications in Virulence and Future Therapeutics

Influenza A Virus Entry: Implications in Virulence and Future Therapeutics HindawiPublishingCorporation AdvancesinVirology Volume2013,ArticleID121924,9pages http://dx.doi.org/10.1155/2013/121924 ReviewArticle In�uen�a�VirusEntry�Implicationsin VirulenceandFutureTherapeutics 1 2 EmilyRumschlag-Booms andLijunRong DepartmentofBiology,NortheasternIllinoisUniversity,Chicago,Chicago,IL60625,USA DepartmentofMicrobiologyandImmunology,CollegeofMedicine,UniversityofIllinoisatChicago,IL60612,USA CorrespondenceshouldbeaddressedtoEmilyRumschlag-Booms;e-booms@neiu.edu Received9August2012;Revised9December2012;Accepted23December2012 AcademicEditor:HectorAguilar-Carreno Copyright©2013E.Rumschlag-BoomsandL.Rong. is is an open access article distributed under the Creative Commons AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkis properlycited. In�uenza A viruses have broad host tropism, being able to infect a range of hosts from wild fowl to swine to humans. is broad tropism makes highly pathogenic in�uenza A strains, such as H5N1, potentially dangerous to humans if they gain the ability to jumpfromananimalreservoirtohumans.Howin�uenzaAvirusesareabletojumpthespeciesbarrierisincompletelyunderstood duetothecomplexgeneticnatureoftheviralsurfaceglycoprotein,hemagglutinin,whichmediatesentry,combinedwiththevirus’s ability to use various receptor linkages. Current therapeutics against in�uenza A include those that target the uncoating process aerentryaswellasthosethatpreventviralbudding.Whiletherearetherapeuticsindevelopmentthattargetentry,currentlythere arenoneclinicallyavailable.Wereviewherethegeneticsofin�uenzaAvirusesthatcontributetoentrytropism,howthesegenetic alterationsmaycontributetoreceptorusageandspeciestropism,aswellashownoveltherapeuticscanbedevelopedthattargetthe majorsurfaceglycoprotein,hemagglutinin. 1.Introduction estimated by the World Health Organization. Infections are characterized by upper respiratory distress along with high In�uenza viruses belong to the Orthomyxoviridae family, fever, myalgia, headache and severe malaise, nonproductive which consists of several genera. e �rst includes both cough, sore throat, and rhinitis. Severe illness and death are in�uenza A and B viruses, while another is comprised of mainly associated with the young, elderly, and those with in�uenza C virus [1]. ese classi�cations are based on compromisedimmunesystems[2]. the distinct antigenic nature of the internal nucleoprotein In�uenzaviruseshaveravagedhumanandpoultrypopu- and matrix proteins of each virus. Infection with in�uenza lationsaroundtheworldforcenturies,causingseriousillness subtypesBandCismostlyrestrictedtohumans[2,3],while anddeath,majoreconomicloss,inadditiontoinstillingfear subtype A is able to infect a wide range of hosts including asthenextpotentialdeadlypandemic.Duringthetwentieth butnotlimitedtohumans,swine,horses,domesticandwild century, this virus caused three major pandemics, which birds, fowl, and dogs [4–8]. is broad spectrum of hosts resulted in an estimated 20–50 million deaths combined plays apivotalroleinthe abilityofthe virus toreassort, worldwide [9–11]. In the twenty-�rst century, 2009 Pan- mutate,andspread,allofwhichcontributetotheever-present demic H1N1 was caused by a reassorted swine strain. e globalthreatofin�uenza. reassortment included in�uenza viruses of human, avian, In�uenza A virus poses the most serious hazard of the and two swine strains [12]. e resultant reassorted swine three subtypes, causing global economic losses as well as strain then jumped to humans, spreading around the world severe health concerns. In�uenza A virus is the causative withinafewweeks[12, 13]. e initial result of this event agentofsevererespiratoryillnessinfectingnearly15%ofthe was more than 22 million reported cases, 13,000 deaths, the blocking of countries’ borders, and the closing of numerous world’s population with upwards of 250,00–500,000 deaths 2 AdvancesinVirology schools [14]. A recent study suggests the actual impact may in the RNA polymerase, in�uenza constantly accumulates bemorethan10timestheinitialestimates[15].Whileweare mutationswithinitsgenomeduringreplication.esemuta- currentlyinthepostpandemicphase,thisH1N1strainisthe tions may be silent or they may alter the virulence and pathogenicityofthevirus.Forinstance,ifahighlypathogenic currentlycirculatingendemicin�uenzastrainamonghuman populations.Upwardsof20–40%oftheworld’spopulationis avianvirusacquiresthenecessarymutationsthatfacilitateits thoughttohaveimmunologicalprotectionforthetimebeing, ability to efficiently enter and replicate in humans, then the astheyhavealreadybeenexposedtothevirus. viruscanbecomeaseriousthreattohumans. In�uenza viruses possess several unique characteristics, e viral surface is studded with two major surface many of which potentiate the menace posed by this virus. spikeglycoproteins,hemagglutinin(HA)andneuraminidase Onesuchfeatureisthesegmentednatureoftheviralgenome (NA), which differ greatly in genetic variation [8]. In addi- [16]. e virus carries eight negative-sense RNA segments. tion, an essential ion channel protein, M2, exists on the Due to the segmented nature of the viral RNA, if a host cell virion surface. HA and NA exist on the virion surface in is infected with two viruses of different in�uenza strains, a ratio of approximately 4/5:1, with an estimated 400–600 the gene segments of one virus can recombine with those totalspikes.HAisresponsibleformediatingentryintotarget of another virus during replication. is reassortment event cells via the host cell receptor, sialic acid (SA). NA plays a is referred to as antigenic shi. e newly formed virus can major role during the budding process by releasing progeny be especially dangerous if a human adapted strain acquires virions from the host cell. To date, 16 subtypes of HA and 9 gene(s)whichtransformitintoahighlypathogenicstrain,or subtypesofNAhavebeenidenti�ed[34,35].esesubtypes if a highly pathogenic strain acquires the necessary gene(s) have been mainly identi�ed amongst different avian species, to infect and spread amongst humans. Either scenario is as birds are the natural reservoirs of in�uenza virus [7, 8]. predicted to raise serious threats worldwide, as was the case Since entry is the �rst requirement for infection, it is crucial in1957and1968[17,18]. that we understand its role in host tropism, pathogenesis, emajorin�uenzapandemicsinthetwentiethcentury, as well as the role of differences between HA subtypes along with the 2009 Pandemic H1N1, are thought to have and species-speci�c viruses. Furthermore, HA has garnered arisen via antigenic shi. e pandemic of 1957, better recent attention as a target for broad-spectrum neutralizing known as “Asian In�uenza” H2N2 virus, was originated in antibodies[36,37]. Southern China and spread rapidly to the United States and HA has been shown to be an important determinant Europe causing more than 1 million deaths worldwide [19]. for in�uenza virus virulence and pathogenesis. Genomic Sequenceanalysisalongwithbiochemicalstudiessuggestthat studies of the 1957 (H2N2) and 1968 (H3N2) pandemics thisparticularviruswasoriginatedfromthereassortmentor revealed that a major contribution to virulence was due to genetic mixing of an avian virus with that of a human virus theexchangeoftheHAsegmentsbetweenhumanandavian [19–22]. While the recombinant virus was not particularly strains[24].Sequencecomparisonofthe1918(H1N1)virus virulent, the high level of mortality associated with it is to other in�uenza A viruses from various species reveals attributed to the immunological naivety of the infected that the entire 1918 virus is more closely related to avian populations. A similar scenario was seen with the pandemic in�uenza A viruses than with any other species, namely of 1968, the “Hong �ong In�uenza.” e HA gene of this humans,suggestingthataccumulatedmutationsintheavian virus wasof theH3 subtypeandoriginatedfromanavian HA gene allowed it to better adapt to the human host. e source along with the PB1 viral polymerase protein [21, 23– critical role of mutations within the avian virus genome 25].esetwoaviangenesegmentsreassortedwithahuman underlies the importance of studying mutations within the virus,creatinganewviruswithgreatervirulenceandtheabil- H5N1 virus genome that may be critical to sustain infection itytoinfecthumans.Furthermore,humanpopulationswere in and among humans although no sustained human-to- immunologicallynaïvetothisrecombinantvirus,makingthe humantransmissionhasbeenreportedyet[38,39]. health impact that much greater. Much devastation and loss HA exists on the virion surface as a trimer of HA are attributed to pandemics arising from antigenic shi and and HA subunits linked by disul�de bonding. is surface it is antigenic shi that is predicted to be the likely cause glycoprotein is �rst synthesized as a single polypeptide of the next pandemic [26]. Furthermore, evidence points to (HA ) of approximately 550 amino acids, which is highly antigenic shi as the perpetrator of the most severe of the N-glycosylated. HA assembles into a trimer in the rough in�uenza pandemics. It is believed that antigenic shi was endoplasmic reticulum (ER) before passing through the responsible for the �rst and most severe pandemic of the Golgicomplexonitswaytothecellmembrane.Forthevirus 20th century in 1918, killing an estimated 50 million people to be infectious, the HA precursor protein must be cleaved worldwide [27–32]. Recent sequence analysis of this H1N1 intoitssubunits,HA andHA [40,41].IfHAisnotcleaved, virus, referred to as the “Spanish Flu”, strongly suggests that 1 2 the virus was directly transmitted to humans from an avian fusion of the viral envelope with the endosomal membrane cannot occur, thus the genomic contents cannot be released source[32]. withinthetargetcell.Atthestructurallevel,cleavageofHA While antigenic shi is a powerful means of acquiring is important because it reveals the hydrophobic N-terminus geneticchange,antigenicdriresultsinmoresubtlechanges of HA , the fusion peptide, which is inserted into the host in the genome. Antigenic dri in in�uenza viruses refers to membraneduringHA-mediatedviral/hostmembranefusion residue substitutions in the virus’ coding sequence via point and viral entry. Upon endocytosis, the acidic pH (5–5.5) mutations [33]. Due to the lack of a proofreading function AdvancesinVirology 3 ∘ ∘ environment triggers HA to undergo the irreversible [42– agglutinateat4 C[57].Ashiintemperatureto37 Cwould 45] conformational changes necessary for fusion to occur, causethevirustoelute,whileadditionofnewin�uenzavirus allowing the viral and host membranes to fuse, releasing nolongercausedagglutination.isphenomenonsuggested the viral genomic contents into the cytoplasm to begin that,inadditiontobindingasurfacemoleculeontheerythro- replication[46]. cytes, the virus carries a receptor-destroying enzyme. It was Twoknown classesofproteases areinvolved inHA laterdiscovered thatthecellularcomponentremovedbythe cleavage [47–49]. e �rst known protease class recognizes viruswasSAandthattreatmentoferythrocyteswithpuri�ed asinglearginineatthecleavagesite.HAwithsuchacleavage sialidase from Vibrio cholerae prevented agglutination [58– site is processed at the cellular surface during the budding 60].is�ndingwasthe�rstdemonstrationthatSAactsasa process or on released viral particles by secretory proteases, receptorforin�uenzaAviruses. such as tryptase Clara, a trypsin-like protease found in the SA encompasses a large family of sugar molecules. e alveolar �uid of rat lungs, plasmin, and bacterial proteases mostprevalentmemberofthisfamilyisN-acetylneuraminic [50]. is particular set of trypsin-like enzymes is found acid (NeuAc). It primarily exists as a six-carbon ring with either in specialized cells or within speci�c organs, thus several unique components extending from the ring. e virusescarryingsuchanHAhavemorerestrictedactivation, most important feature of SA, with regards to in�uenza, is infection capability, and therefore limited replication and the manner in which the free sugar is attached to the host spread [40, 41, 50]. Due to this restriction, trypsin-like cell surface. Host cells carry various surface glycoproteins activatedvirusesaregenerallythoughttobelesspathogenic. and glycolipids, many of which are highly modi�ed. ese It is interesting to note that while low pathogenicity is gen- surface proteins that are modi�ed with a terminal SA play a erally associated with a virus whose HA has the trypsin-like crucialroleinin�uenzaentry,servingastheviralattachment cleavage site, the most highly pathogenic human in�uenza and entry receptor. SA can be attached to the underlying virus was restricted to trypsin-like enzyme cleavage [51]. glycocalyxinoneofthreemainlinkagepatterns,either𝛼𝛼2,3, eenzyme-limited,restrictedsitesofreplicationcorrespond 𝛼𝛼2,6,or 𝛼𝛼 2,8[61].Whileotherlinkagesexist,thesethreeare with sites of natural infection for humans and birds, that themostprevalentinmammaliancells[62]. being limited to the upper-respiratory tract (humans) or In addition to viral entry, SA plays an equally important gastrointestinaltracts(birds)[20]. roleindetermininghosttropism.In�uenzatropismishighly e other variant of HA contains a polybasic consensus in�uencedbythelinkageofSA,withavianandhumanviruses sequence cleavage site, R-X-K/R-R, which is recognized preferentially utilizing different linkages. Avian viruses have by the subtilisin-like endoproteases, furin, and PC5/6 [52, been classi�ed as predominantly 𝛼𝛼2,3 speci�c, while human 53]. is protease is expressed in the trans-Golgi network, viruses tend to favor the 𝛼𝛼2,6 linkage [63– 66]. ese pref- therefore the HA is activated during the exocytic route erences have been established from studies examining SA during virus maturation [53, 54].Asthisproteaseisnearly distributionwithinaspeci�chost,bindingofthevirustoSA ubiquitously expressed, a virus with this cleavage site can at the protein level, as well as studies analyzing replication replicate throughout a host and cause extensive systemic efficiencyinthepresenceofthesespeci�clinkages.Increased spread. Due to this characteristic, viruses with the polybasic prevalenceofthe𝛼𝛼2,3-linkedSAintheaviangastrointestinal site are generally considered highly pathogenic strains, such tract correlates with the ability of the virus to enter and as the H5 and H7 strains [53, 55]. A comparative analysis replicate here, as it is the site of natural infection for birds at the entry level between a low pathogenic H5N2 strain [23,61].Asthenaturalcarrierofthevirus,avianpopulations andahighlypathogenicH5N1strainrevealedthatthemajor usually display few disease symptoms [7]. Due to the high restrictiontoentrywasthecleavesitesequence.Replacement prevalence of receptors within the gastrointestinal tract, the ofthemonobasiccleavagesiteintothepolybasiccleavagesite virusreplicatesmostefficientlythereandisexcretedthrough on HA enhanced the HA-mediated entry [56]. However, the wasteproducts.Ontheotherhand,humanin�uenzaviruses worsthumanin�uenzaepidemicrecorded,the1918Spanish predominantly utilize SA in 𝛼𝛼2,6 conformation which is Flu, did not have the polybasic cleavage site as previously highlyprevalentintheupperrespiratorytract[23,65,67,68]. discussed. Instead, it had a single arginine, highlighting the SA linkages within the human respiratory tract are present complicated nature of in�uenza viruses and their virulence onnasalmucosalepithelialcells,paranasalsinuses,pharynx, andpathogenicity[51]. trachea, and bronchi, all carrying 𝛼𝛼 2,6 SA while 𝛼𝛼 2,3 SA is In addition to the aforementioned two classes of pro- found on nonciliated cuboidal bronchi and alveolus, as well teases, the following proteolytic enzymes are implicated as type II cells within the alveolar wall [69].Basedonthese in HA activation: type II transmembrane serine proteases data,itisthelowlevelsof𝛼𝛼2,3linkagesintheuppertractthat (TTSPs) TMPRSS2, TMPRSS4, and human airway trypsin- have been postulated as being a block for efficient infection like protease (HAT) [47]. While less is known about these andreplicationof𝛼𝛼2,3preferringvirusesinhumans.Onthe proteases, their activity may be additionally associated with other hand, �ow cytometry studies using lectins speci�c for viralpathogenicityandtropism. 𝛼𝛼2,3and 𝛼𝛼2,6SAshowedthatbothlinkageswerepresenton In�uenza entry tropism is mainly determined by the human bronchial epithelial cells with 𝛼𝛼2,6 SA being the vast binding preference of HA to its receptor, SA. SA has long majority[70,71],suggestinganotherlayerinthecomplexity been believed to be the sole receptor for the in�uenza virus. ofin�uenzatropismbeyondSApreference.Inaddition,itstill It was discovered nearly 70 years ago that upon addition remains unclear if all subtypes of in�uenza virus target the of in�uenza virus to chicken erythrocytes, the cells would samesubsetofrespiratorycellsand/orhavethesameaffinity 4 AdvancesinVirology and avidity for 𝛼𝛼2,3 and 𝛼𝛼2,6 SA linkages. Besides terminal Interestingly,the1918H1N1virusHAcarriedglutamine sialic-acid linkages, speci�city is also in�uenced by internal 226andglycine228correspondingtotheavian𝛼𝛼2,3receptor, linkages along with modi�cation of inner oligosaccharides howeverthevirusisabletobindthe 𝛼𝛼2,6 receptor, demon- strating that changes at these positions are not necessary for including sulfation, fucosylation, and sialylation [72, 73]. Overcomingthisbindingrestrictionmaybeonestepneeded alteredreceptorbinding[30].Furtheranalysisofthe1918HA foravianin�uenzatomoreefficientlyspreadfromhumanto revealedthatasinglechangefromasparaginetoglutamateat human. position 190 was responsible for the altered binding pheno- type [30]. In addition, a change from asparagine to glycine e linkage of SA and its in�uence on in�uenza entry at 225 in combination with the change at 190 increased has been extensively characterized to determine its role in respiratory transmission of the virus in the ferret model, tropism; however in early 2008 an additional level of com- furtherhighlightingtheimportanceoftheRBDresidues. plexitywasrevealed.Chandrasekaranetal.reportedthatthe More extensive studies focusing on the H3 subtype crucial determinant for in�uenza tropism is the structure of revealed that human viruses with 𝛼𝛼2,6 preference have the underlying glycocalyx, not the terminal SA linkage [74]. leucine at 226 and serine at 228, while avian viruses with Using a series of analyses, it was reported that avian viruses 𝛼𝛼2,3 preference have glutamine at 226 and glycine at 228 prefer SA in a cone-like topology. is shape is adopted by [63, 82]. Residues 193 and 218 have also been implicated as SAs, both 𝛼𝛼2,3 and 𝛼𝛼 2,6, with short underlying glycan(s) important determinants for receptor preference in the H3 and allows HA to contact Neu5Ac and galactose sugars in a subtype,howeverspeci�cresiduechangeshavenotbeenfully trisaccharidemotif.Humanvirusesarereportedtopreferan established[83]. umbrella-like glycan topology, which is unique to 𝛼𝛼2,6 SAs Similarly, studies focusing on seasonally circulating H1 withlongunderlyingglycans.isreportalsoconcludedthat virusesfoundavianandhumanvirusesdifferattwopositions 𝛼𝛼2,6 alone is insufficient for human transmission, as avian in their binding preference. A proline at 186 and a glycine virusescanutilize𝛼𝛼2,6SAonashortsugarchain,suggesting at 225 correspond with 𝛼𝛼2,3 type binding, while serine 186 that the virus must adopt the ability to utilize 𝛼𝛼2,6 SA on and asparagine 225 are favored by 𝛼𝛼2,6 type binding [ 25]. a long sugar chain. It was concluded that it is the glycan Interestingly, HA proteins of both avian and human viruses composition, and thus the SA topology that may in�uence have glutamine at 226 and glycine at 228. e discrepancies in�uenzatropism,notjusttheSAlinkagepresent. highlighted by the H3 and H1 studies in combination with e region of HA that is responsible for binding SA the studies on the H1N1 Spanish Flu illustrate the complex is referred to as the receptor-binding domain (RBD). is natureofreceptorbinding,thatis,notallin�uenzaAviruses pocket-shaped depression is located at the membrane-distal behave in a similar way, not even among avian strains nor tip of each monomer within the trimeric HA structure and humanstrains. is comprised of the 190 helix (residues 190–198), the 130 Since the initial H5N1 in�uenza outbreak that began in loop(residues135–138),andthe220loop(residues221–228) China in 1997, several studies have focused on the RBD of basedonH3numbering[25].Severalkeyconservedresidues this particular viral strain. ese studies include the use of includingtyrosine98,tryptophan153,andhistidine183form sialoglycoconjugates, crystal HA structures, and simulated thebaseofthebindingpocketandarecrucialformaintaining computer-based binding assays [55, 69, 72, 74, 84–87]. thestructuralbasisofthebindingpocketaswellasinforming Residues that have been implicated in human receptor-type interactions with SA [75]. Sequence analysis from several binding include asparagine 227, asparagine 159, lysine 182, strainsofin�uenzaalongwithstructuralmodelinghasgiven and arginine 192. It is proposed that residues 159, 182, and great insight into the residues that play a pivotal role in SA 192 in�uence binding by stabilizing the HA binding pocket bindingpreference[25,76]. andmaintainingstructuralintegrity,astheseresiduesarenot Structural studies of HA suggest that avian and human indirectcontactwithSA.Aswitchofglutaminetoleucineat in�uenzavirusesappeartobedistinctintheirRBDsatposi- position226andaswitchofglycinetoserineatposition228 tions226and228[77,78].AvianHAstendtohaveglutamine equatesashifromavianreceptortohumanreceptorspeci- and glycine at the respective positions while human HAs �city,asseenintheH3virusaswell[86,88].Furtherstudies carry leucine and serine [78–81].eavian HAwiththese speci�cally targeting the HA of the A/Vietnam/1203/2004 residuesformsanarrowbindingpocketforthe𝛼𝛼2,3receptor H5N1virusdemonstratetheimportanceofmutationE190D while the change in residues for human HAs results in a which reduced the binding to 2,3 linkages, as well as the broader pocket for the 𝛼𝛼2,6 receptor. Neither position 226 doublemutantQ226L/G228S[86].InadditiontheH5N1HA nor 228 plays a direct role in binding, rather they seem to surface residue tyrosine 161 has recently been implicated in in�uencethecontourofthepocket[70,82].esedifferences altering glycoconjugate recognition and cell-type dependent in pocket shape correlate well with the glycan topology entry. Substitution of tyrosine 161 to alanine switched the studies. Additionally, it was shown that a lab-adapted strain binding preference from N-acetylneuraminic acid to N- whichprefers𝛼𝛼2,6bindingwasabletoswitchpreferencesto glycolylneuraminic acid [87]. Itisimportanttonotethat 𝛼𝛼2,3 when grown in the presence of 𝛼𝛼-2 macroglobulin, a differentstrainsoftheH5N1virusfromdifferenttimepoints 𝛼𝛼2,6glycoproteinfoundinhighconcentrationsinhorsesera duringthevirusoutbreakwereusedinthesestudies. [65]. Based on these and other studies, it seems that residue Tobetterunderstandtheroleofnaturallyacquiredmuta- 226 and 228 have an important indirect role in SA binding tions and their ability to potentiate sustained transmission, compatibilityandthereforepreference. two recent studies showed that as few as four to �ve amino AdvancesinVirology 5 acid substitutions along with gene reassortment may be viruses,orpuri�edviralprotein(s)thatillicitastrongneutral- sufficientforrespiratorydroplettransmissionbetweenferrets izing antibody response [96]. Of these vaccines, most target [89, 90]. Herfst et al. identi�ed (based on H5 numbering) the globular head region of HA, containing the receptor- binding domain, thus preventing attachment of the virus glutamine to leucine at 222 and glycine to serine at 224 as critical residues to alter sialic acid speci�city from the to susceptible cells. ese neutralizing antibodies are rarely avian 𝛼𝛼2,3 SA to the human 𝛼𝛼2,6 SA. Two additional HA immunoresponsive to an alternate in�uenza strain, oen substitutions, threonine to alanine at 156 and histidine to losing their potency as their corresponding strain acquires tyrosine at 103 play a role in disrupting N-linked glyco- mutations during circulation. Furthermore, due to the virus sylation and monomer interaction, respectively. Lastly, a ability to constantly acquire genetic changes, it is difficult switchfromglutamatetolysineat627inthepolymerasePB2 to predict what the circulating strain(s) for the upcoming subunit was identi�ed, which is a common change seen in year will be. A mismatch of vaccine strain with circulating mammalian adapted in�uenza strains. Imai et al. identi�ed strain(s)willofferlittletonoprotection. four substitutions, all within HA. Similar to the results of In addition to vaccines, anti-�u therapeutics have been Herfst et al., it was found that substitutions of asparagine to developedwhichcanbedividedintotwoclasses,anti-NAand anti-M2 [97]. e �rst class of inhibitors speci�cally targets lysine at 220 and glutamine to leucine at 222 can alter SA speci�city from the avian 𝛼𝛼2,3 SA to the human 𝛼𝛼2,6 SA. the NA protein of the virus, halting the spread of progeny virions[98].Duringthebuddingprocessofin�uenza,newly Again,adisruptioninN-linkedglycosylationviaasparagine produced progeny virions are tethered to the host cell toaspartateatposition154wasidenti�edalongwithachange surface via HA proteins interacting with SA molecules. NA in the stalk region corresponding to threonine to isoleucine functions in recognizing this HA-SA interaction and cleaves at315. theSAmoiety,releasingtheviralparticle[99].ecurrently WhilethelinkageofSAandtheresidueswithintheRBD approved therapeutics for in�uenza infection include NA plays a signi�cant role in viral entry, the glycoconjugate to inhibitors which block this step, thus preventing release and which SA is attached appears to be an additional important furtherspreadofthevirus,bothwithintheinfectedhostand factor. Chu and Whittaker determined that cells de�cient in consequentlytoothers.Includedinthiscategoryofantivirals theGnT1genelackterminalN-linkedglycosylation,render- are two of the most commonly used therapies, Zanamivir ing them de�cient in in�uenza A viral entry [91]. e GnT1 (tradenameRelenza)andoseltamivirphosphate(tradename gene encodes the enzyme N-acetylglucosaminyltransferase, Tami�u) for the treatment and prevention of in�uenza A whichisinvolvedinmodi�cationofN-linkedglycansinthe and B viruses. Zanamivir and oseltamivir phosphate are Golgi apparatus. ese cells lack N-glycoproteins, but still competitive inhibitors for the active site of the NA enzyme possesssurface𝛼𝛼2,3SAand 𝛼𝛼2,6SA.Whileconsideringthis [100]. While Zanamivir and oseltamivir phosphate can be phenotype, this mutant Chinese Hamster Ovary (CHO) cell highly effective in both treating in�uenza and in outbreak linehasthecapacitytobindHAandexpressessufficientlevels control, in the 2008-2009 �u season, nearly 100% of H1N1 ofbothSAlinkagesonthecellsurfaceforattachment.Inthis samplestestedbytheCenterforDiseaseControl(CDC)were mutantCHOcellline,completeentrywasblocked,asvirions shown to be resistant to oseltamivir phosphate [101, 102]. werenotendocytosed.eseresultssuggestaspeci�crolefor is high level of resistance highlights the need to develop N-linked glycoproteins in in�uenza A entry, perhaps acting newantiviralsagainstin�uenza. asacofactorinmediatingentry. Another class of in�uenza inhibitors, the adamantanes, is those which block the M2 ion channel on the virion ere is evidence suggesting that SA is not necessary surface. e M2 ion channel is embedded within the virion for infection by in�uenza. e HA of a human H1N1 str- lipidbilayerandfacilitateshydrogeniontransport,ultimately ainwasshowntobindglycoconjugatesotherthanSA[92].In leading to virion uncoating and replication during the entry addition, Stray et al. demonstrated the ability of desialylated process [103]. e adamantane derivatives, amantadine and MadinDarby canine kidney (MDCK) cells to be infected by rimantadine,wereapprovedforthetreatmentandprevention in�uenza A virus [93]. A study by Nicholls et al. demon- of in�uenza A only, as only in�uenza A class viruses have stratedtheabilityoftheH5N1virustoinfectupperandlower the M2 protein [103]. e CDC and WHO report that respiratory tract cells in the presence or absence of 𝛼𝛼2,3 SA greater than 99% of circulating strains are resistant to M2 whichisbelievedtobetheentryreceptorforthisvirus[94].In inhibitorsandhaverecommendedtheirusetobediscontin- addition, the levels of SA on susceptible and nonsusceptible ued.ereforetheseinhibitorsareonlyusedwhenaspeci�c cells do not correlate with either 𝛼𝛼2,3 SA or 𝛼𝛼2,6 SA for nonresistant strain is thought to be the causative infectious an H5N1 strain [95]. Taken together, it is possible that the agent [101]. ese antivirals, along with the NA inhibitors, barrier to efficient human infection and human-to-human provideatreatmentoptionaerinfection,howeversincenot transmission of the H5N1 virus is not due to SA linkages, all in�uenza A viruses respond to these treatments, these but rather it is due to inefficient use or expression of a yet drugs may be ineffective. In addition, resistance to these unidenti�edentrymediator. treatmentsoverthecourseofanoutbreakorin�uenzaseason To �ght the spread of in�uenza, prophylactic therapeu- underliestheurgencytodevelopnewantiviraltherapies. tics and vaccines continue to be vital methods of control. Since there is a lack of clinically available inhibitor(s) Vaccines are the primary means of controlling the spread of which targets the major viral surface protein, HA, recent thevirusandarebasedoninactivatedviruses,liveattenuated clinicaltrialsarepushingforwardtoexpandtherepertoireof 6 AdvancesinVirology in�uenzatherapeutics,includingdrugsthattargetHA,which [6] A. D. M. E. Osterhaus, B. E. E. Martina, G. F. Rimmelzwaan, T. M. Bestebroer, and R. A. M. Fouchier, “In�uenza B virus in isthetargetofmostneutralizingantibodies[104].esenew seals,”Science,vol.288,no.5468,pp.1051–1053,2000. drugs include cyanovirin-N and thiazolides. Cyanovirin-N targets the high-mannose oligosaccharides, neutralizing the [7] R. G. Webster, M. Yakhno, V. S. Hinshaw et al., “Intestinal viral particle [104, 105]. e thiazolides work to block the in�uenza: replication and characterization of in�uenza viruses maturation of HA posttranslationally [104, 106]. Recently, inducks,”Virology,vol.84,no.2,pp.268–278,1978. several groups have identi�ed broad-spectrum neutralizing [8] R.G.Webster,W.J.Bean,O.T.Gorman,T.M.Chambers,and antibodiesthatareprotectiveagainstgroup1(whichincludes Y. Kawaoka, “Evolution and ecology of in�uenza A viruses,” HA subtypes 1, 2, 5, 6, 8, 9, 11, 12, 13, and 16) and group MicrobiologicalReviews,vol.56,no.1,pp.152–179,1992. 2 (which includes HA subtypes 3, 4, 7, 10, 14, and 15) [9] N. P. Johnson and J. Mueller, “Updating the accounts: global in�uenza A viruses [36, 107–109]. ese broad-spectrum mortality of the 1918–1920 “Spanish” in�uenza pandemic,” neutralizingantibodies(referredtoasCR6261,F10,andF16) Bulletin of the History of Medicine, vol. 76, no. 1, pp. 105–115, are further distinct in that they target the stem region of the HA molecule. e proposed model is that by targeting [10] A. W. Crosby, America’s Forgotten Pandemic, Cambridge Uni- the stem region, more speci�cally the fusion peptide, these versityPress,2003. antibodiesareabletopreventexposureofthefusionpeptide [11] K. D. Patterson and G. F. Pyle, “e geography and mortality under acid conditions. is trapping mechanism prevents of the 1918 in�uenza pandemic,” Bulletin of the History of the viral membrane from fusing with the host endosomal Medicine,vol.65,no.1,pp.4–21,1991. membrane, locking the genetic contents within the viral [12] G.J.D.Smith,D.Vijaykrishna,J.Bahletal.,“Originsandevo- particle. An additional stem-directed neutralizing antibody, lutionarygenomicsofthe2009swine-originH1N1in�uenzaA CR8020,isactiveagainstgroup2in�uenzaAviruses.While epidemic,”Nature,vol.459,no.7250,pp.1122–1125,2009. not as broad in activity as the aforementioned antibodies, [13] M. Nelson, M. Gramer, A. Vincent, and E. C. Holmes, “Global CR8020 could be utilized in conjunction with a group 1 transmission of in�uenza viruses from humans to swine,” neutralizing antibody, providing a one-two punch against Journal of General Virology, vol. 93, part 10, pp. 2195–2203, bothHAsubunits. In�uenzaviruseswillcontinuetocirculateamonganimal [14] CDC H1N1 �u. Center for Disease Control and Prevention and human populations, acquiring and exchanging genetic Website. components as they do. Due to the constant change in their genetic pro�les, in�uenza viruses will continue to pose a [15] F. S. Dawood, A. D. Iuliano, C. Reed, M. I. Meltzer, D. K. Shay serious threat, from both an economic and public health et al., “Estimated global mortality associated with the �rst 12 pointofview.Continuedstudyandsurveillanceofin�uenza monthsof2009pandemicin�uenzaAH1N1viruscirculation: amodellingstudy,”eLancetInfectiousDiseases,vol.12,no.9, viruses is further highlighted by our inability to maintain pp.687–695,2012. effective in�uenza vaccines and prophylactic therapeutics. e prospective of new antivirals, especially those that pro- [16] R.LambandR.Krug,“Orthomyxoviridae:thevirusesandtheir vide broad-spectrum protection, provide renewed efforts in replication,”inField’sVirology,pp.1487–1532,2001. ourabilitytocontrolin�uenzainfectionandspread. [17] P.Palese,“In�uenza:oldandnewthreats,”NatureMedicine,vol. 10,no.12,pp.S82–S87,2004. [18] E. D. Kilbourne, “In�uenza pandemics: can we prepare for the References unpredictable?” Viral Immunology, vol. 17, no. 3, pp. 350–357, [1] F.A.Murphy,“Virustaxonomy,”inVirology,B.N.Fields,D.M. 2004. Knipe, and P. M. Howley, Eds., pp. 15–57, Lippincott-Raven, [19] E. D. Kilbourne, “In�uenza pandemics of the 20th century,” Philadelphia,Pa,USA,1996. EmergingInfectiousDiseases,vol.12,no.1,pp.9–14,2006. [2] D.M.Fleming,M.Zambon,andA.I.M.Bartelds,“Population [20] M. C. Zambon, “e pathogenesis of in�uenza in humans,” estimates of persons presenting to general practitioners with ReviewsinMedicalVirology,vol.11,no.4,pp.227–241,2001. in�uenza-like illness, 1987–96: a study of the demography of [21] Y.-C. Hsieh, T. Z. Wu, D. P. Liu et al., “In�uenza pandemics: in�uenza-like illness in sentinel practice networks in England past, present and future,” Journal of the Formosan Medical andWales, and in eNetherlands,” Epidemiology & Infection, Association,vol.105,no.1,pp.1–6,2006. vol.124,no.2,pp.245–253,2000. [22] T. Horimoto and Y. Kawaoka, “In�uenza: lessons from past [3] K. M. Sullivan and A. S. Monto, “Acute respiratory illness in pandemics, warnings from current incidents,” Nature Reviews the community. Frequency of illness and the agents involved,” Microbiology,vol.3,no.8,pp.591–600,2005. Epidemiology&Infection,vol.110,no.1,pp.145–160,1999. [23] T. Ito, J. N. S. S. Couceiro, S. Kelm et al., “Molecular basis for [4] G.Yuanji,J.Fengen,W.Pingetal.,“Isolationofin�uenzaCvirus the generation in pigs of in�uenza A viruses with pandemic from pigs and experimental infection of pigs with in�uenza C potential,” Journal of Virology, vol. 72, no. 9, pp. 7367–7373, virus,” Journal of General Virology, vol. 64, no. 1, pp. 177–182, [24] Y. Kawaoka, S. Krauss, and R. G. Webster, “Avian-to-human [5] A. Hay, “e virus genome and its replication,” in Textbook of transmissionofthePB1geneofin�uenzaAvirusesinthe1957 In�uen�a, K. G. Nicholson, R. G. Webster, and A. J. Hay, Eds., and 1968 pandemics,” Journal of Virology,vol.63,no.11,pp. pp.43–53,1998. 4603–4608,1989. AdvancesinVirology 7 [25] J. J. Skehel and D. C. Wiley, “Receptor binding and membrane of the hemagglutinin polypeptide,” Virology, vol. 68, no. 2, pp. fusion in virus entry: the in�uenza hemagglutinin,”Annual 440–454,1975. ReviewofBiochemistry,vol.69,pp.531–569,2000. [42] F. X. Bosch, W. Garten, H.-D. Klenk, and R. Rott, “Proteolytic [26] N. M. Ferguson, C. Fraser, C. A. Donnelly, A. C. Ghani, and cleavage of in�uenza virus hemagglutinins: primary structure R. M. Anderson, “Public health risk from the avian H5N1 of the connecting peptide between HA1 and HA2 determines in�uenza epidemic,”Science, vol. 304, no. 5673, pp. 968–969, proteolytic cleavability and pathogenicity of avian in�uenza viruses,”Virology,vol.113,no.2,pp.725–735,1981. [27] C.F.BaslerandP.V.Aguilar,“Progressinidentifyingvirulence [43] R. T. C. Huang, K. Wahn, H.-D. Klenk, and R. Rott, “Fusion determinantsofthe1918H1N1andtheSoutheastAsianH5N1 between cell membrane and liposomes containing the gly- in�uenza A viruses,”Antiviral Research, vol. 79, no. 3, pp. coproteins of in�uenza virus,”Virology, vol. 104, no. 2, pp. 166–178,2008. 294–302,1980. [28] R.B.Belshe,“eoriginsofpandemicin�uenza�lessonsfrom [44] T. Maeda and S.-I. Ohnishi, “Activation of in�uenza virus by the1918virus,”eNewEnglandJournalofMedicine,vol.353, acidicmediacauseshemolysisandfusionoferythrocytes,”FEBS no.21,pp.2209–2211,2005. Letters,vol.122,no.2,pp.283–287,1980. [29] L. Glaser, J. Stevens, D. Zamarin et al., “A single amino acid [45] J. White, K. Matlin, and A. Helenius, “Cell fusion by Semliki substitution in 1918 in�uenza virus hemagglutinin changes Forest, in�uenza, and vesicular stomatitis viruses,”e Journal receptorbindingspeci�city,”JournalofVirology,vol.79,no.17, ofCellBiology,vol.89,no.3,pp.674–679,1981. pp.11533–11536,2005. [46] P. A. Bullough, F. M. Hughson, J. J. Skehel, and D. C. Wiley, [30] A. H. Reid, T. G. Fanning, J. V. Hultin, and J. K. Taubenberger, “Structureofin�uenzahaemagglutininatthepHofmembrane “Origin and evolution of the 1918 “Spanish” in�uenza virus fusion,”Nature,vol.371,no.6492,pp.37–43,1994. hemagglutinin gene,” Proceedings of the National Academy of [47] S. Bertram, I. Glowacka, I. Steffen, A. Kühl, and S. Pöhlmann, Sciences of the United States of America, vol. 96, no. 4, pp. “Novel insights into proteolytic cleavage of in�uenza virus 1651–1656,1999. hemagglutinin,” Reviews in Medical Virology, vol. 20, no. 5, pp. [31] T.M.Tumpey,C.F.Basler,P.V.Aguilaretal.,“Characterization 298–310,2010. of the reconstructed 1918 Spanish in�uenza pandemic virus,” [48] E. Böttcher, T. Matrosovich, M. Beyerle, H.-D. Klenk, W. Science,vol.310,no.5745,pp.77–80,2005. Garten, and M. Matrosovich, “Proteolytic activation of [32] S. J. Gamblin, L. F. Haire, R. J. Russell et al., “e structure in�uenzavirusesbyserineproteasesTMPRSS2andHATfrom and receptor binding properties of the 1918 in�uenza hemag- human airway epithelium,” Journal of Virology, vol. 80, no. 19, glutinin,”Science,vol.303,no.5665,pp.1838–1842,2004. pp.9896–9898,2006. [33] I. A. Wilson and N. J. Cox, “Structural basis of immune [49] C.Chaipan,D.Kobasa,S.Bertrametal.,“Proteolyticactivation recognition of in�uenza virus hemagglutinin,”Annual Review ofthe1918in�uenzavirushemagglutinin,”JournalofVirology, ofImmunology,vol.8,pp.737–787,1990. vol.83,no.7,pp.3200–3211,2009. [34] R. A. M. Fouchier, V. Munster, A. Wallensten et al., “Charac- [50] H.Kido,Y.Yokogoshi,K.Sakaietal.,“Isolationandcharacter- terization of a novel in�uenza A virus hemagglutinin subtype izationofanoveltrypsin-likeproteasefoundinratbronchiolar (H16) obtained from black-headed gulls,” Journal of Virology, epithelial Clara cells. A possible activator of the viral fusion vol.79,no.5,pp.2814–2822,2005. glycoprotein,” e Journal of Biological Chemistry, vol. 267, no. [35] D. Kaye and C. R. Pringle, “Avian in�uenza viruses and their 19,pp.13573–13579,1992. implicationforhumanhealth,”ClinicalInfectiousDiseases,vol. [51] T. M. Tumpey, A. García-Sastre, J. K. Taubenberger et al., 40,no.1,pp.108–112,2005. “Pathogenicity of in�uenza viruses with genes from the 1918 [36] D. Corti, J. Voss, S. J. Gamblin, G. Codoni, A. Macagno et al., pandemic virus: functional roles of alveolar macrophages and “Neutralizingantibodyselectedfromplasmacellsthatbindsto neutrophilsinlimitingvirusreplicationandmortalityinmice,” group1andgroup2in�uenzaAhemagglutinins,”Science,vol. JournalofVirology,vol.79,no.23,pp.14933–14944,2005. 333,no.6044,pp.850–856,2011. [52] G. omas, “Furin at the cutting edge: from protein traffic [37] P. Leyssen, E. de Clercq, and J. Neyts, “Molecular strategies to to embryogenesis and disease,” Nature Reviews Molecular Cell inhibit the replication of RNA viruses,” Antiviral Research, vol. Biology,vol.3,no.10,pp.753–766,2002. 78,no.1,pp.9–25,2008. [53] A.Stieneke-Grober,M.Vey,H.Anglikeretal.,“In�uenzavirus [38] T. Y. Aditama, G. Samaan, R. Kusriastuti, O. D. Sampurno, W. hemagglutinin with multibasic cleavage site is activated by Purbaetal.,“Avianin�uenzaH5N1transmissioninhouseholds, furin, a subtilisin-like endoprotease,” e EMBO Journal, vol. Indonesia,”PLoSONE,vol.7,no.1,ArticleIDe29971,2012. 11,no.7,pp.2407–2414,1992. [39] Y. Yang, M. E. Halloran, J. D. Sugimoto, and I. M. Longini Jr., [54] D. J. Krysan, N. C. Rockwell, and R. S. Fuller, “Quantitative “Detecting human-to-human transmission of avian in�uenza characterizationoffurinspeci�city:energeticsofsubstratedis- A (H5N1),” Emerging Infectious Diseases, vol. 13, no. 9, pp. crimination using an internally consistent set of hexapeptidyl 1348–1353,2007. methylcoumarinamides,” e Journal of Biological Chemistry, [40] H.-D. Klenk, R. Rott, M. Orlich, and J. Blodorn, “Activation of vol.274,no.33,pp.23229–23234,1999. in�uenza A viruses by trypsin treatment,”Virology, vol. 68, no. [55] D. J. Hulse, R. G. Webster, R. J. Russell, and D. R. Perez, 2,pp.426–439,1975. “Molecular determinants within the surface proteins involved [41] S. G. Lazarowitz and P. W. Choppin, “Enhancement of the in the pathogenicity of H5N1 in�uenza viruses in chickens,” infectivity of in�uenca A and B viruses by proteolytic cleavage JournalofVirology,vol.78,no.18,pp.9954–9964,2004. 8 AdvancesinVirology [56] E.Rumschlag-Booms,Y.Guo,J.Wang,M.Caffrey,andL.Rong, [71] M. N. Matrosovich, T. Y. Matrosovich, T. Gray, N. A. Roberts, “Comparative analysis between a low pathogenic and a high and H.-D. Klenk, “Human and avian in�uenza viruses target pathogenic in�uenza H5 hemagglutinin in cell entry,” Virology different cell types in cultures of human airway epithelium,” Journal,vol.6,article76,2009. Proceedings of the National Academy of Sciences of the United StatesofAmerica,vol.101,no.13,pp.4620–4624,2004. [57] G.K.Hirst,“Adsorptionofin�uenzahemagglutininsandvirus [72] J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. by red blood cells,” e Journal of Experimental Medicine, vol. Paulson, and I. A. Wilson, “Structure and receptor speci�city 76,no.2,pp.195–209,1942. of the hemagglutinin from an H5N1 in�uenza virus,” Science, [58] F.BurnetandJ.D.Stone,“ereceptor-destroyingenzymeofV. vol.312,no.5772,pp.404–410,2006. cholerae,”AustralianJournalofExperimentalBiology&Medical [73] J.Stevens,O.Blixt,L.Glaseretal.,“Glycanmicroarrayanalysis Science,vol.25,no.3,pp.227–233,1947. of the hemagglutinins from modern and pandemic in�uenza [59] A. Gottschalk, “Neuraminidase: the speci�c enzyme of virusesrevealsdifferentreceptorspeci�cities,”JournalofMolec- in�uenza virus and Vibrio cholerae,” Biochimica et Biophysica ularBiology,vol.355,no.5,pp.1143–1155,2006. Acta,vol.23,pp.645–646,1957. [74] A. Chandrasekaran, A. Srinivasan, R. Raman et al., “Gly- [60] A. Gottschalk, e Chemistry and Biology of Sialic Acids and can topology determines human adaptation of avian H5N1 Related Substances, University Press, Cambridge, Mass, USA, virus hemagglutinin,” Nature Biotechnology,vol.26,no.1, pp. 107–113,2008. [61] Y. Suzuki, “Sialobiology of in�uenza molecular mechanism [75] N. K. Sauter, J. E. Hanson, G. D. Glick et al., “Binding of of host range variation of in�uenza viruses,” Biological and in�uenza virus hemagglutinin to analogs of its cell-surface PharmaceuticalBulletin,vol.28,no.3,pp.399–408,2005. receptor, sialic acid: analysis by proton nuclear magnetic res- onance spectroscopy and X-ray crystallography,” Biochemistry, [62] C. F.Brewer and T. K. Dam, “Essentials of Glycobiology, Edited vol.31,no.40,pp.9609–9621,1992. by A. Varki, R. Cummings, J. Esko, H. Freeze, G. Hart, and J. Marth, Cold Spring Harbor Laboratory Press, Cold Spring [76] T.Suzuki,A.Portner,R.A.Scroggsetal.,“Receptorspeci�cities Harbor, New York, 1999, 653 pp.,” Carbohydrate Research, vol. ofhumanrespiroviruses,”JournalofVirology,vol.75,no.10,pp. 325,no.3,pp.233–234,2000. 4604–4613,2001. [77] S. Chutinimitkul,S. Herfst,J.Steelet al.,“Virulence-associated [63] M.N.Matrosovich,A.S.Gambaryan,S.Tenebergetal.,“Avian substitution D222G in the hemagglutinin of 2009 pandemic in�uenza A viruses differ from human viruses by recognition in�uenza A(H1N1) virus affects receptor binding,” Journal of of sialyloligosaccharides and gangliosides and by a higher Virology,vol.84,no.22,pp.11802–11813,2010. conservation of the HA receptor-binding site,” Virology, vol. 233,no.1,pp.224–234,1997. [78] A. Vines, K. Wells, M. Matrosovich, M. R. Castrucci, T. Ito, and Y. Kawaoka, “e role of in�uenza A virus hemagglutinin [64] C. R. Parrish and Y. Kawaoka, “e origins of new pandemic residues 226 and 228 in receptor speci�city and host range viruses:theacquisitionofnewhostrangesbycanineparvovirus restriction,” Journal of Virology, vol. 72, no. 9, pp. 7626–7631, andin�uenzaAviruses,”AnnualReviewofMicrobiology,vol.59, pp.553–586,2005. [79] R. J. Connor, Y. Kawaoka, R. G. Webster, and J. C. Paulson, [65] G. N. Rogers, T. J. Pritchett, J. L. Lane, and J. C. Paulson, “Receptor speci�city in human, avian, and equine H2 and H3 “Differential sensitivity of human, avian, and equine in�uenza in�uenza virus isolates,” Virology, vol. 205, no. 1, pp. 17–23, A viruses to a glycoprotein inhibitor of infection: selection of receptorspeci�cvariants,”Virology,vol.131,no.2,pp.394–408, 1983. [80] C.W.Naeve,V.S.Hinshaw,andR.G.Webster,“Mutationsinthe hemagglutinin receptor-binding site can change the biological [66] G. N. Rogers and J. C. Paulson, “Receptor determinants of propertiesofanin�uenzavirus,”JournalofVirology,vol.51,no. human and animal in�uenza virus isolates: differences in 2,pp.567–569,1984. receptor speci�city of the H3 hemagglutinin based on species [81] E. Nobusawa, T. Aoyama, H. Kato, Y. Suzuki, Y. Tateno, and oforigin,”Virology,vol.127,no.2,pp.361–373,1983. K. Nakajima, “Comparison of complete amino acid sequences [67] G. N. Rogers, J. C. Paulson, R. S. Daniels et al., “Single amino andreceptor-bindingpropertiesamong13serotypesofhemag- acidsubstitutionsinin�uenzahaemagglutininchangereceptor glutinins of in�uenza A viruses,” Virology, vol. 182, no. 2, pp. bindingspeci�city,”Nature,vol.304,no.5921,pp.76–78,1983. 475–485,1991. [68] J. N. S. S. Couceiro, J. C. Paulson, and L. G. Baum, “In�uenza [82] C. T. Hardy, S. A. Young, R. G. Webster, C. W. Naeve, and R. J. virus strains selectively recognize sialyloligosaccharides on Owens, “Egg �uids and cells of the chorioallantoic membrane human respiratory epithelium; the role of the host cell in of embryonated chicken eggs can select different variants of selectionofhemagglutininreceptorspeci�city,”VirusResearch, in�uenza A (H3N2) viruses,” Virology, vol. 211, no. 1, pp. vol.29,no.2,pp.155–165,1993. 302–306,1995. [69] K. Shinya, M. Ebina, S. Yamada, M. Ono, N. Kasai, and Y. [83] P.S.Daniels,S.Jeffries,P.Yatesetal.,“ereceptor-bindingand Kawaoka, “Avian �u: in�uenza virus receptors in the human membrane-fusionpropertiesofin�uenzavirusvariantsselected airway,”Nature,vol.440,no.7083,pp.435–436,2006. usinganti-haemagglutininmonoclonalantibodies,”eEMBO Journal,vol.6,no.5,pp.1459–1465,1987. [70] M. Matrosovich, N. Zhou, Y. Kawaoka, and R. Webster, “e surface glycoproteins of H5 in�uenza viruses isolated from [84] E. C. J. Claas, J. C. de Jong, R. van Beek, G. F. Rimmelz- humans, chickens, and wild aquatic birds have distinguishable waan, and A. D. M. E. Osterhaus, “Human in�uenza virus properties,” Journal of Virology, vol. 73, no. 2, pp. 1146–1155, A/HongKong/156/97(H5N1)infection,”Vaccine,vol.16,no.9- 1999. 10,pp.977–978,1998. AdvancesinVirology 9 [85] J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. [101] H. T. Nguyen, A. M. Fry, and L. V. Gubareva, “Neuraminidase Paulson, and I. A. Wilson, “Structure and receptor speci�city inhibitor resistance in in�uenza viruses and laboratory testing of the hemagglutinin from an H5N1 in�uenza virus,” Science, methods,”Antiviralerapy,vol.17,pp.159–173,2012. vol.312,no.5772,pp.404–410,2006. [102] N.J.Dharan,L.V.Gubareva,J.J.Meyeretal.,“Infectionswith oseltamivir-resistant in�uenza A�H1N1� virus in the �nited [86] G. Ayora-Talavera, H. Shelton, M. A. Scull et al., “Mutations in H5N1 in�uenza virus hemagglutinin that confer binding to States,” JournaloftheAmericanMedicalAssociation, vol. 301, human tracheal airway epithelium,” PLoS ONE, vol. 4, no. 11, no.10,pp.1034–1041,2009. ArticleIDe7836,2009. [103] S.D.Cady,W.Luo,F.Hu,andM.Hong,“Structureandfunction ofthein�uenzaAM2protonchannel,”Biochemistry,vol.48,no. [87] M. Wang, D. M. Tscherne, C. McCullough, M. Caffrey, A. Garcia-Sastre et al., “Residue Y161 of in�uenza virus hemag- 31,pp.7356–7364,2009. glutininisinvolvedinviralrecognitionofsialylatedcomplexes [104] D.A.Boltz,J.R.Aldridge,R.G.Webster,andE.A.Govorkova, from different hosts,” Journal of Virology, vol. 86, no. 8, pp. “Drugsindevelopmentforin�uenza,”Drugs,vol.70,no.11,pp. 4455–4462,2012. 1349–1362,2010. [88] S. Yamada, Y. Suzuki, T. Suzuki et al., “Haemagglutinin muta- [105] B.R.O’Keefe,D.F.Smee,J.A. Turpin et al.,“Potentanti- tionsresponsibleforthebindingofH5N1in�uenzaAvirusesto in�uenza activity of cyanovirin-N and interactions with viral human-typereceptors,”Nature,vol.444,no.7117,pp.378–382, hemagglutinin,” Antimicrobial Agents and Chemotherapy, vol. 47,no.8,pp.2518–2525,2003. [89] M. Imai, T. Watanabe, M. Hatta, S. C. Das, M. Ozawa et [106] J. F. Rossignol, S. La Frazia, L. Chiappa, A. Ciucci, and M. G. al., “Experimental adaptation of an in�uenza H5 HA confers Santoro, “iazolides, a new class of anti-in�uenza molecules respiratorydroplettransmissiontoareassortantH5HA/H1N1 targeting viral hemagglutinin at the post-translational level,” virusinferrets,”Nature,vol.486,pp.420–428,2012. e Journal of Biological Chemistry, vol. 284, no. 43, pp. 29798–29808,2009. [90] S.Herfst,E.J.A.Schrauwen,M.Linster,S.Chutinimitkul,E.de Wit et al., “Airborne transmission of in�uenza A/H5N1 virus [107] J. Sui, W. C. Hwang, S. Perez et al., “Structural and functional between ferrets,” Science, vol. 336, no. 6088, pp. 1534–1541, bases for broad-spectrum neutralization of avian and human in�uenza A viruses,” Nature Structural and Molecular Biology, vol.16,no.3,pp.265–273,2009. [91] V. C. Chu and G. R. Whittaker, “In�uenza virus entry and infection require host cell N-linked glycoprotein,” Proceedings [108] D. C. Ekiert and I. A. Wilson, “Broadly neutralizingantibodies of the National Academy of Sciences of the United States of against in�uenza virus and prospects for universal therapies,” America,vol.101,no.52,pp.18153–18158,2004. CurrentOpinioninVirology,vol.2,no.2,pp.134–141,2012. [92] E. M. Rapoport, L. V. Mochalova, H.-J. Gabius, J. Romanova, [109] M. rosby, E. van den Brink, M. Jongeneelen et al., “Hetero- and N. V. Bovin, “Search for additional in�uenza virus to subtypic neutralizing monoclonal antibodies cross-protective cell interactions,” Glycoconjugate Journal, vol. 23, no. 1-2, pp. againstH5N1andH1N1recoveredfromhumanIgM memory 115–125,2006. Bcells,”PLoSONE,vol.3,no.12,ArticleIDe3942,2008. [93] S. J. Stray, R. D. Cummings, and G. M. Air, “In�uenza virus infection of desialylated cells,” Glycobiology,vol.10,no.7, pp. 649–658,2000. [94] J. M. Nicholls, M. C. W. Chan, W. Y. Chan et al., “Tropism of avian in�uenza A �H5N1� in the upper and lower respiratory tract,”NatureMedicine,vol.13,no.2,pp.147–149,2007. [95] Y.Guo,E.Rumschlag-Booms,J.Wangetal.,“Analysisofhema- gglutinin-mediated entry tropism of H5N1 avian in�uenza,” VirologyJournal,vol.6,article39,2009. [96] R. Rappuoli and P. R. Dormitzer, “In�uenza� options to improvepandemicpreparation,”Science,vol.336,no.6088,pp. 1531–1533,2012. [97] J. Cinatl, M. Michaelis, and H. W. Doerr, “e threat of avian in�uenza A �H5N1�. Part III� antiviral therapy,”Medical Microbiology and Immunology, vol. 196, no. 4, pp. 203–212, [98] A.Moscona,“Neuraminidaseinhibitorsforin�uenza,”eNew England Journal of Medicine, vol. 353, no. 13, pp. 1363–1373, [99] J. S. Rossman and R. A. Lamb, “In�uenza virus assembly and budding,”Virology,vol.411,no.2,pp.229–236,2011. [100] J.Magano,“Syntheticapproachestotheneuraminidaseinhibit- ors zanamivir �Relenza� and oseltamivir phosphate �Tami�u� for the treatment of in�uenza,” Chemical Reviews, vol. 109, no. 9,pp.4398–4438,2009. 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Influenza A Virus Entry: Implications in Virulence and Future Therapeutics

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HindawiPublishingCorporation AdvancesinVirology Volume2013,ArticleID121924,9pages http://dx.doi.org/10.1155/2013/121924 ReviewArticle In�uen�a�VirusEntry�Implicationsin VirulenceandFutureTherapeutics 1 2 EmilyRumschlag-Booms andLijunRong DepartmentofBiology,NortheasternIllinoisUniversity,Chicago,Chicago,IL60625,USA DepartmentofMicrobiologyandImmunology,CollegeofMedicine,UniversityofIllinoisatChicago,IL60612,USA CorrespondenceshouldbeaddressedtoEmilyRumschlag-Booms;e-booms@neiu.edu Received9August2012;Revised9December2012;Accepted23December2012 AcademicEditor:HectorAguilar-Carreno Copyright©2013E.Rumschlag-BoomsandL.Rong. is is an open access article distributed under the Creative Commons AttributionLicense,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkis properlycited. In�uenza A viruses have broad host tropism, being able to infect a range of hosts from wild fowl to swine to humans. is broad tropism makes highly pathogenic in�uenza A strains, such as H5N1, potentially dangerous to humans if they gain the ability to jumpfromananimalreservoirtohumans.Howin�uenzaAvirusesareabletojumpthespeciesbarrierisincompletelyunderstood duetothecomplexgeneticnatureoftheviralsurfaceglycoprotein,hemagglutinin,whichmediatesentry,combinedwiththevirus’s ability to use various receptor linkages. Current therapeutics against in�uenza A include those that target the uncoating process aerentryaswellasthosethatpreventviralbudding.Whiletherearetherapeuticsindevelopmentthattargetentry,currentlythere arenoneclinicallyavailable.Wereviewherethegeneticsofin�uenzaAvirusesthatcontributetoentrytropism,howthesegenetic alterationsmaycontributetoreceptorusageandspeciestropism,aswellashownoveltherapeuticscanbedevelopedthattargetthe majorsurfaceglycoprotein,hemagglutinin. 1.Introduction estimated by the World Health Organization. Infections are characterized by upper respiratory distress along with high In�uenza viruses belong to the Orthomyxoviridae family, fever, myalgia, headache and severe malaise, nonproductive which consists of several genera. e �rst includes both cough, sore throat, and rhinitis. Severe illness and death are in�uenza A and B viruses, while another is comprised of mainly associated with the young, elderly, and those with in�uenza C virus [1]. ese classi�cations are based on compromisedimmunesystems[2]. the distinct antigenic nature of the internal nucleoprotein In�uenzaviruseshaveravagedhumanandpoultrypopu- and matrix proteins of each virus. Infection with in�uenza lationsaroundtheworldforcenturies,causingseriousillness subtypesBandCismostlyrestrictedtohumans[2,3],while anddeath,majoreconomicloss,inadditiontoinstillingfear subtype A is able to infect a wide range of hosts including asthenextpotentialdeadlypandemic.Duringthetwentieth butnotlimitedtohumans,swine,horses,domesticandwild century, this virus caused three major pandemics, which birds, fowl, and dogs [4–8]. is broad spectrum of hosts resulted in an estimated 20–50 million deaths combined plays apivotalroleinthe abilityofthe virus toreassort, worldwide [9–11]. In the twenty-�rst century, 2009 Pan- mutate,andspread,allofwhichcontributetotheever-present demic H1N1 was caused by a reassorted swine strain. e globalthreatofin�uenza. reassortment included in�uenza viruses of human, avian, In�uenza A virus poses the most serious hazard of the and two swine strains [12]. e resultant reassorted swine three subtypes, causing global economic losses as well as strain then jumped to humans, spreading around the world severe health concerns. In�uenza A virus is the causative withinafewweeks[12, 13]. e initial result of this event agentofsevererespiratoryillnessinfectingnearly15%ofthe was more than 22 million reported cases, 13,000 deaths, the blocking of countries’ borders, and the closing of numerous world’s population with upwards of 250,00–500,000 deaths 2 AdvancesinVirology schools [14]. A recent study suggests the actual impact may in the RNA polymerase, in�uenza constantly accumulates bemorethan10timestheinitialestimates[15].Whileweare mutationswithinitsgenomeduringreplication.esemuta- currentlyinthepostpandemicphase,thisH1N1strainisthe tions may be silent or they may alter the virulence and pathogenicityofthevirus.Forinstance,ifahighlypathogenic currentlycirculatingendemicin�uenzastrainamonghuman populations.Upwardsof20–40%oftheworld’spopulationis avianvirusacquiresthenecessarymutationsthatfacilitateits thoughttohaveimmunologicalprotectionforthetimebeing, ability to efficiently enter and replicate in humans, then the astheyhavealreadybeenexposedtothevirus. viruscanbecomeaseriousthreattohumans. In�uenza viruses possess several unique characteristics, e viral surface is studded with two major surface many of which potentiate the menace posed by this virus. spikeglycoproteins,hemagglutinin(HA)andneuraminidase Onesuchfeatureisthesegmentednatureoftheviralgenome (NA), which differ greatly in genetic variation [8]. In addi- [16]. e virus carries eight negative-sense RNA segments. tion, an essential ion channel protein, M2, exists on the Due to the segmented nature of the viral RNA, if a host cell virion surface. HA and NA exist on the virion surface in is infected with two viruses of different in�uenza strains, a ratio of approximately 4/5:1, with an estimated 400–600 the gene segments of one virus can recombine with those totalspikes.HAisresponsibleformediatingentryintotarget of another virus during replication. is reassortment event cells via the host cell receptor, sialic acid (SA). NA plays a is referred to as antigenic shi. e newly formed virus can major role during the budding process by releasing progeny be especially dangerous if a human adapted strain acquires virions from the host cell. To date, 16 subtypes of HA and 9 gene(s)whichtransformitintoahighlypathogenicstrain,or subtypesofNAhavebeenidenti�ed[34,35].esesubtypes if a highly pathogenic strain acquires the necessary gene(s) have been mainly identi�ed amongst different avian species, to infect and spread amongst humans. Either scenario is as birds are the natural reservoirs of in�uenza virus [7, 8]. predicted to raise serious threats worldwide, as was the case Since entry is the �rst requirement for infection, it is crucial in1957and1968[17,18]. that we understand its role in host tropism, pathogenesis, emajorin�uenzapandemicsinthetwentiethcentury, as well as the role of differences between HA subtypes along with the 2009 Pandemic H1N1, are thought to have and species-speci�c viruses. Furthermore, HA has garnered arisen via antigenic shi. e pandemic of 1957, better recent attention as a target for broad-spectrum neutralizing known as “Asian In�uenza” H2N2 virus, was originated in antibodies[36,37]. Southern China and spread rapidly to the United States and HA has been shown to be an important determinant Europe causing more than 1 million deaths worldwide [19]. for in�uenza virus virulence and pathogenesis. Genomic Sequenceanalysisalongwithbiochemicalstudiessuggestthat studies of the 1957 (H2N2) and 1968 (H3N2) pandemics thisparticularviruswasoriginatedfromthereassortmentor revealed that a major contribution to virulence was due to genetic mixing of an avian virus with that of a human virus theexchangeoftheHAsegmentsbetweenhumanandavian [19–22]. While the recombinant virus was not particularly strains[24].Sequencecomparisonofthe1918(H1N1)virus virulent, the high level of mortality associated with it is to other in�uenza A viruses from various species reveals attributed to the immunological naivety of the infected that the entire 1918 virus is more closely related to avian populations. A similar scenario was seen with the pandemic in�uenza A viruses than with any other species, namely of 1968, the “Hong �ong In�uenza.” e HA gene of this humans,suggestingthataccumulatedmutationsintheavian virus wasof theH3 subtypeandoriginatedfromanavian HA gene allowed it to better adapt to the human host. e source along with the PB1 viral polymerase protein [21, 23– critical role of mutations within the avian virus genome 25].esetwoaviangenesegmentsreassortedwithahuman underlies the importance of studying mutations within the virus,creatinganewviruswithgreatervirulenceandtheabil- H5N1 virus genome that may be critical to sustain infection itytoinfecthumans.Furthermore,humanpopulationswere in and among humans although no sustained human-to- immunologicallynaïvetothisrecombinantvirus,makingthe humantransmissionhasbeenreportedyet[38,39]. health impact that much greater. Much devastation and loss HA exists on the virion surface as a trimer of HA are attributed to pandemics arising from antigenic shi and and HA subunits linked by disul�de bonding. is surface it is antigenic shi that is predicted to be the likely cause glycoprotein is �rst synthesized as a single polypeptide of the next pandemic [26]. Furthermore, evidence points to (HA ) of approximately 550 amino acids, which is highly antigenic shi as the perpetrator of the most severe of the N-glycosylated. HA assembles into a trimer in the rough in�uenza pandemics. It is believed that antigenic shi was endoplasmic reticulum (ER) before passing through the responsible for the �rst and most severe pandemic of the Golgicomplexonitswaytothecellmembrane.Forthevirus 20th century in 1918, killing an estimated 50 million people to be infectious, the HA precursor protein must be cleaved worldwide [27–32]. Recent sequence analysis of this H1N1 intoitssubunits,HA andHA [40,41].IfHAisnotcleaved, virus, referred to as the “Spanish Flu”, strongly suggests that 1 2 the virus was directly transmitted to humans from an avian fusion of the viral envelope with the endosomal membrane cannot occur, thus the genomic contents cannot be released source[32]. withinthetargetcell.Atthestructurallevel,cleavageofHA While antigenic shi is a powerful means of acquiring is important because it reveals the hydrophobic N-terminus geneticchange,antigenicdriresultsinmoresubtlechanges of HA , the fusion peptide, which is inserted into the host in the genome. Antigenic dri in in�uenza viruses refers to membraneduringHA-mediatedviral/hostmembranefusion residue substitutions in the virus’ coding sequence via point and viral entry. Upon endocytosis, the acidic pH (5–5.5) mutations [33]. Due to the lack of a proofreading function AdvancesinVirology 3 ∘ ∘ environment triggers HA to undergo the irreversible [42– agglutinateat4 C[57].Ashiintemperatureto37 Cwould 45] conformational changes necessary for fusion to occur, causethevirustoelute,whileadditionofnewin�uenzavirus allowing the viral and host membranes to fuse, releasing nolongercausedagglutination.isphenomenonsuggested the viral genomic contents into the cytoplasm to begin that,inadditiontobindingasurfacemoleculeontheerythro- replication[46]. cytes, the virus carries a receptor-destroying enzyme. It was Twoknown classesofproteases areinvolved inHA laterdiscovered thatthecellularcomponentremovedbythe cleavage [47–49]. e �rst known protease class recognizes viruswasSAandthattreatmentoferythrocyteswithpuri�ed asinglearginineatthecleavagesite.HAwithsuchacleavage sialidase from Vibrio cholerae prevented agglutination [58– site is processed at the cellular surface during the budding 60].is�ndingwasthe�rstdemonstrationthatSAactsasa process or on released viral particles by secretory proteases, receptorforin�uenzaAviruses. such as tryptase Clara, a trypsin-like protease found in the SA encompasses a large family of sugar molecules. e alveolar �uid of rat lungs, plasmin, and bacterial proteases mostprevalentmemberofthisfamilyisN-acetylneuraminic [50]. is particular set of trypsin-like enzymes is found acid (NeuAc). It primarily exists as a six-carbon ring with either in specialized cells or within speci�c organs, thus several unique components extending from the ring. e virusescarryingsuchanHAhavemorerestrictedactivation, most important feature of SA, with regards to in�uenza, is infection capability, and therefore limited replication and the manner in which the free sugar is attached to the host spread [40, 41, 50]. Due to this restriction, trypsin-like cell surface. Host cells carry various surface glycoproteins activatedvirusesaregenerallythoughttobelesspathogenic. and glycolipids, many of which are highly modi�ed. ese It is interesting to note that while low pathogenicity is gen- surface proteins that are modi�ed with a terminal SA play a erally associated with a virus whose HA has the trypsin-like crucialroleinin�uenzaentry,servingastheviralattachment cleavage site, the most highly pathogenic human in�uenza and entry receptor. SA can be attached to the underlying virus was restricted to trypsin-like enzyme cleavage [51]. glycocalyxinoneofthreemainlinkagepatterns,either𝛼𝛼2,3, eenzyme-limited,restrictedsitesofreplicationcorrespond 𝛼𝛼2,6,or 𝛼𝛼 2,8[61].Whileotherlinkagesexist,thesethreeare with sites of natural infection for humans and birds, that themostprevalentinmammaliancells[62]. being limited to the upper-respiratory tract (humans) or In addition to viral entry, SA plays an equally important gastrointestinaltracts(birds)[20]. roleindetermininghosttropism.In�uenzatropismishighly e other variant of HA contains a polybasic consensus in�uencedbythelinkageofSA,withavianandhumanviruses sequence cleavage site, R-X-K/R-R, which is recognized preferentially utilizing different linkages. Avian viruses have by the subtilisin-like endoproteases, furin, and PC5/6 [52, been classi�ed as predominantly 𝛼𝛼2,3 speci�c, while human 53]. is protease is expressed in the trans-Golgi network, viruses tend to favor the 𝛼𝛼2,6 linkage [63– 66]. ese pref- therefore the HA is activated during the exocytic route erences have been established from studies examining SA during virus maturation [53, 54].Asthisproteaseisnearly distributionwithinaspeci�chost,bindingofthevirustoSA ubiquitously expressed, a virus with this cleavage site can at the protein level, as well as studies analyzing replication replicate throughout a host and cause extensive systemic efficiencyinthepresenceofthesespeci�clinkages.Increased spread. Due to this characteristic, viruses with the polybasic prevalenceofthe𝛼𝛼2,3-linkedSAintheaviangastrointestinal site are generally considered highly pathogenic strains, such tract correlates with the ability of the virus to enter and as the H5 and H7 strains [53, 55]. A comparative analysis replicate here, as it is the site of natural infection for birds at the entry level between a low pathogenic H5N2 strain [23,61].Asthenaturalcarrierofthevirus,avianpopulations andahighlypathogenicH5N1strainrevealedthatthemajor usually display few disease symptoms [7]. Due to the high restrictiontoentrywasthecleavesitesequence.Replacement prevalence of receptors within the gastrointestinal tract, the ofthemonobasiccleavagesiteintothepolybasiccleavagesite virusreplicatesmostefficientlythereandisexcretedthrough on HA enhanced the HA-mediated entry [56]. However, the wasteproducts.Ontheotherhand,humanin�uenzaviruses worsthumanin�uenzaepidemicrecorded,the1918Spanish predominantly utilize SA in 𝛼𝛼2,6 conformation which is Flu, did not have the polybasic cleavage site as previously highlyprevalentintheupperrespiratorytract[23,65,67,68]. discussed. Instead, it had a single arginine, highlighting the SA linkages within the human respiratory tract are present complicated nature of in�uenza viruses and their virulence onnasalmucosalepithelialcells,paranasalsinuses,pharynx, andpathogenicity[51]. trachea, and bronchi, all carrying 𝛼𝛼 2,6 SA while 𝛼𝛼 2,3 SA is In addition to the aforementioned two classes of pro- found on nonciliated cuboidal bronchi and alveolus, as well teases, the following proteolytic enzymes are implicated as type II cells within the alveolar wall [69].Basedonthese in HA activation: type II transmembrane serine proteases data,itisthelowlevelsof𝛼𝛼2,3linkagesintheuppertractthat (TTSPs) TMPRSS2, TMPRSS4, and human airway trypsin- have been postulated as being a block for efficient infection like protease (HAT) [47]. While less is known about these andreplicationof𝛼𝛼2,3preferringvirusesinhumans.Onthe proteases, their activity may be additionally associated with other hand, �ow cytometry studies using lectins speci�c for viralpathogenicityandtropism. 𝛼𝛼2,3and 𝛼𝛼2,6SAshowedthatbothlinkageswerepresenton In�uenza entry tropism is mainly determined by the human bronchial epithelial cells with 𝛼𝛼2,6 SA being the vast binding preference of HA to its receptor, SA. SA has long majority[70,71],suggestinganotherlayerinthecomplexity been believed to be the sole receptor for the in�uenza virus. ofin�uenzatropismbeyondSApreference.Inaddition,itstill It was discovered nearly 70 years ago that upon addition remains unclear if all subtypes of in�uenza virus target the of in�uenza virus to chicken erythrocytes, the cells would samesubsetofrespiratorycellsand/orhavethesameaffinity 4 AdvancesinVirology and avidity for 𝛼𝛼2,3 and 𝛼𝛼2,6 SA linkages. Besides terminal Interestingly,the1918H1N1virusHAcarriedglutamine sialic-acid linkages, speci�city is also in�uenced by internal 226andglycine228correspondingtotheavian𝛼𝛼2,3receptor, linkages along with modi�cation of inner oligosaccharides howeverthevirusisabletobindthe 𝛼𝛼2,6 receptor, demon- strating that changes at these positions are not necessary for including sulfation, fucosylation, and sialylation [72, 73]. Overcomingthisbindingrestrictionmaybeonestepneeded alteredreceptorbinding[30].Furtheranalysisofthe1918HA foravianin�uenzatomoreefficientlyspreadfromhumanto revealedthatasinglechangefromasparaginetoglutamateat human. position 190 was responsible for the altered binding pheno- type [30]. In addition, a change from asparagine to glycine e linkage of SA and its in�uence on in�uenza entry at 225 in combination with the change at 190 increased has been extensively characterized to determine its role in respiratory transmission of the virus in the ferret model, tropism; however in early 2008 an additional level of com- furtherhighlightingtheimportanceoftheRBDresidues. plexitywasrevealed.Chandrasekaranetal.reportedthatthe More extensive studies focusing on the H3 subtype crucial determinant for in�uenza tropism is the structure of revealed that human viruses with 𝛼𝛼2,6 preference have the underlying glycocalyx, not the terminal SA linkage [74]. leucine at 226 and serine at 228, while avian viruses with Using a series of analyses, it was reported that avian viruses 𝛼𝛼2,3 preference have glutamine at 226 and glycine at 228 prefer SA in a cone-like topology. is shape is adopted by [63, 82]. Residues 193 and 218 have also been implicated as SAs, both 𝛼𝛼2,3 and 𝛼𝛼 2,6, with short underlying glycan(s) important determinants for receptor preference in the H3 and allows HA to contact Neu5Ac and galactose sugars in a subtype,howeverspeci�cresiduechangeshavenotbeenfully trisaccharidemotif.Humanvirusesarereportedtopreferan established[83]. umbrella-like glycan topology, which is unique to 𝛼𝛼2,6 SAs Similarly, studies focusing on seasonally circulating H1 withlongunderlyingglycans.isreportalsoconcludedthat virusesfoundavianandhumanvirusesdifferattwopositions 𝛼𝛼2,6 alone is insufficient for human transmission, as avian in their binding preference. A proline at 186 and a glycine virusescanutilize𝛼𝛼2,6SAonashortsugarchain,suggesting at 225 correspond with 𝛼𝛼2,3 type binding, while serine 186 that the virus must adopt the ability to utilize 𝛼𝛼2,6 SA on and asparagine 225 are favored by 𝛼𝛼2,6 type binding [ 25]. a long sugar chain. It was concluded that it is the glycan Interestingly, HA proteins of both avian and human viruses composition, and thus the SA topology that may in�uence have glutamine at 226 and glycine at 228. e discrepancies in�uenzatropism,notjusttheSAlinkagepresent. highlighted by the H3 and H1 studies in combination with e region of HA that is responsible for binding SA the studies on the H1N1 Spanish Flu illustrate the complex is referred to as the receptor-binding domain (RBD). is natureofreceptorbinding,thatis,notallin�uenzaAviruses pocket-shaped depression is located at the membrane-distal behave in a similar way, not even among avian strains nor tip of each monomer within the trimeric HA structure and humanstrains. is comprised of the 190 helix (residues 190–198), the 130 Since the initial H5N1 in�uenza outbreak that began in loop(residues135–138),andthe220loop(residues221–228) China in 1997, several studies have focused on the RBD of basedonH3numbering[25].Severalkeyconservedresidues this particular viral strain. ese studies include the use of includingtyrosine98,tryptophan153,andhistidine183form sialoglycoconjugates, crystal HA structures, and simulated thebaseofthebindingpocketandarecrucialformaintaining computer-based binding assays [55, 69, 72, 74, 84–87]. thestructuralbasisofthebindingpocketaswellasinforming Residues that have been implicated in human receptor-type interactions with SA [75]. Sequence analysis from several binding include asparagine 227, asparagine 159, lysine 182, strainsofin�uenzaalongwithstructuralmodelinghasgiven and arginine 192. It is proposed that residues 159, 182, and great insight into the residues that play a pivotal role in SA 192 in�uence binding by stabilizing the HA binding pocket bindingpreference[25,76]. andmaintainingstructuralintegrity,astheseresiduesarenot Structural studies of HA suggest that avian and human indirectcontactwithSA.Aswitchofglutaminetoleucineat in�uenzavirusesappeartobedistinctintheirRBDsatposi- position226andaswitchofglycinetoserineatposition228 tions226and228[77,78].AvianHAstendtohaveglutamine equatesashifromavianreceptortohumanreceptorspeci- and glycine at the respective positions while human HAs �city,asseenintheH3virusaswell[86,88].Furtherstudies carry leucine and serine [78–81].eavian HAwiththese speci�cally targeting the HA of the A/Vietnam/1203/2004 residuesformsanarrowbindingpocketforthe𝛼𝛼2,3receptor H5N1virusdemonstratetheimportanceofmutationE190D while the change in residues for human HAs results in a which reduced the binding to 2,3 linkages, as well as the broader pocket for the 𝛼𝛼2,6 receptor. Neither position 226 doublemutantQ226L/G228S[86].InadditiontheH5N1HA nor 228 plays a direct role in binding, rather they seem to surface residue tyrosine 161 has recently been implicated in in�uencethecontourofthepocket[70,82].esedifferences altering glycoconjugate recognition and cell-type dependent in pocket shape correlate well with the glycan topology entry. Substitution of tyrosine 161 to alanine switched the studies. Additionally, it was shown that a lab-adapted strain binding preference from N-acetylneuraminic acid to N- whichprefers𝛼𝛼2,6bindingwasabletoswitchpreferencesto glycolylneuraminic acid [87]. Itisimportanttonotethat 𝛼𝛼2,3 when grown in the presence of 𝛼𝛼-2 macroglobulin, a differentstrainsoftheH5N1virusfromdifferenttimepoints 𝛼𝛼2,6glycoproteinfoundinhighconcentrationsinhorsesera duringthevirusoutbreakwereusedinthesestudies. [65]. Based on these and other studies, it seems that residue Tobetterunderstandtheroleofnaturallyacquiredmuta- 226 and 228 have an important indirect role in SA binding tions and their ability to potentiate sustained transmission, compatibilityandthereforepreference. two recent studies showed that as few as four to �ve amino AdvancesinVirology 5 acid substitutions along with gene reassortment may be viruses,orpuri�edviralprotein(s)thatillicitastrongneutral- sufficientforrespiratorydroplettransmissionbetweenferrets izing antibody response [96]. Of these vaccines, most target [89, 90]. Herfst et al. identi�ed (based on H5 numbering) the globular head region of HA, containing the receptor- binding domain, thus preventing attachment of the virus glutamine to leucine at 222 and glycine to serine at 224 as critical residues to alter sialic acid speci�city from the to susceptible cells. ese neutralizing antibodies are rarely avian 𝛼𝛼2,3 SA to the human 𝛼𝛼2,6 SA. Two additional HA immunoresponsive to an alternate in�uenza strain, oen substitutions, threonine to alanine at 156 and histidine to losing their potency as their corresponding strain acquires tyrosine at 103 play a role in disrupting N-linked glyco- mutations during circulation. Furthermore, due to the virus sylation and monomer interaction, respectively. Lastly, a ability to constantly acquire genetic changes, it is difficult switchfromglutamatetolysineat627inthepolymerasePB2 to predict what the circulating strain(s) for the upcoming subunit was identi�ed, which is a common change seen in year will be. A mismatch of vaccine strain with circulating mammalian adapted in�uenza strains. Imai et al. identi�ed strain(s)willofferlittletonoprotection. four substitutions, all within HA. Similar to the results of In addition to vaccines, anti-�u therapeutics have been Herfst et al., it was found that substitutions of asparagine to developedwhichcanbedividedintotwoclasses,anti-NAand anti-M2 [97]. e �rst class of inhibitors speci�cally targets lysine at 220 and glutamine to leucine at 222 can alter SA speci�city from the avian 𝛼𝛼2,3 SA to the human 𝛼𝛼2,6 SA. the NA protein of the virus, halting the spread of progeny virions[98].Duringthebuddingprocessofin�uenza,newly Again,adisruptioninN-linkedglycosylationviaasparagine produced progeny virions are tethered to the host cell toaspartateatposition154wasidenti�edalongwithachange surface via HA proteins interacting with SA molecules. NA in the stalk region corresponding to threonine to isoleucine functions in recognizing this HA-SA interaction and cleaves at315. theSAmoiety,releasingtheviralparticle[99].ecurrently WhilethelinkageofSAandtheresidueswithintheRBD approved therapeutics for in�uenza infection include NA plays a signi�cant role in viral entry, the glycoconjugate to inhibitors which block this step, thus preventing release and which SA is attached appears to be an additional important furtherspreadofthevirus,bothwithintheinfectedhostand factor. Chu and Whittaker determined that cells de�cient in consequentlytoothers.Includedinthiscategoryofantivirals theGnT1genelackterminalN-linkedglycosylation,render- are two of the most commonly used therapies, Zanamivir ing them de�cient in in�uenza A viral entry [91]. e GnT1 (tradenameRelenza)andoseltamivirphosphate(tradename gene encodes the enzyme N-acetylglucosaminyltransferase, Tami�u) for the treatment and prevention of in�uenza A whichisinvolvedinmodi�cationofN-linkedglycansinthe and B viruses. Zanamivir and oseltamivir phosphate are Golgi apparatus. ese cells lack N-glycoproteins, but still competitive inhibitors for the active site of the NA enzyme possesssurface𝛼𝛼2,3SAand 𝛼𝛼2,6SA.Whileconsideringthis [100]. While Zanamivir and oseltamivir phosphate can be phenotype, this mutant Chinese Hamster Ovary (CHO) cell highly effective in both treating in�uenza and in outbreak linehasthecapacitytobindHAandexpressessufficientlevels control, in the 2008-2009 �u season, nearly 100% of H1N1 ofbothSAlinkagesonthecellsurfaceforattachment.Inthis samplestestedbytheCenterforDiseaseControl(CDC)were mutantCHOcellline,completeentrywasblocked,asvirions shown to be resistant to oseltamivir phosphate [101, 102]. werenotendocytosed.eseresultssuggestaspeci�crolefor is high level of resistance highlights the need to develop N-linked glycoproteins in in�uenza A entry, perhaps acting newantiviralsagainstin�uenza. asacofactorinmediatingentry. Another class of in�uenza inhibitors, the adamantanes, is those which block the M2 ion channel on the virion ere is evidence suggesting that SA is not necessary surface. e M2 ion channel is embedded within the virion for infection by in�uenza. e HA of a human H1N1 str- lipidbilayerandfacilitateshydrogeniontransport,ultimately ainwasshowntobindglycoconjugatesotherthanSA[92].In leading to virion uncoating and replication during the entry addition, Stray et al. demonstrated the ability of desialylated process [103]. e adamantane derivatives, amantadine and MadinDarby canine kidney (MDCK) cells to be infected by rimantadine,wereapprovedforthetreatmentandprevention in�uenza A virus [93]. A study by Nicholls et al. demon- of in�uenza A only, as only in�uenza A class viruses have stratedtheabilityoftheH5N1virustoinfectupperandlower the M2 protein [103]. e CDC and WHO report that respiratory tract cells in the presence or absence of 𝛼𝛼2,3 SA greater than 99% of circulating strains are resistant to M2 whichisbelievedtobetheentryreceptorforthisvirus[94].In inhibitorsandhaverecommendedtheirusetobediscontin- addition, the levels of SA on susceptible and nonsusceptible ued.ereforetheseinhibitorsareonlyusedwhenaspeci�c cells do not correlate with either 𝛼𝛼2,3 SA or 𝛼𝛼2,6 SA for nonresistant strain is thought to be the causative infectious an H5N1 strain [95]. Taken together, it is possible that the agent [101]. ese antivirals, along with the NA inhibitors, barrier to efficient human infection and human-to-human provideatreatmentoptionaerinfection,howeversincenot transmission of the H5N1 virus is not due to SA linkages, all in�uenza A viruses respond to these treatments, these but rather it is due to inefficient use or expression of a yet drugs may be ineffective. In addition, resistance to these unidenti�edentrymediator. treatmentsoverthecourseofanoutbreakorin�uenzaseason To �ght the spread of in�uenza, prophylactic therapeu- underliestheurgencytodevelopnewantiviraltherapies. tics and vaccines continue to be vital methods of control. Since there is a lack of clinically available inhibitor(s) Vaccines are the primary means of controlling the spread of which targets the major viral surface protein, HA, recent thevirusandarebasedoninactivatedviruses,liveattenuated clinicaltrialsarepushingforwardtoexpandtherepertoireof 6 AdvancesinVirology in�uenzatherapeutics,includingdrugsthattargetHA,which [6] A. D. M. E. Osterhaus, B. E. E. Martina, G. F. Rimmelzwaan, T. M. Bestebroer, and R. A. M. Fouchier, “In�uenza B virus in isthetargetofmostneutralizingantibodies[104].esenew seals,”Science,vol.288,no.5468,pp.1051–1053,2000. drugs include cyanovirin-N and thiazolides. Cyanovirin-N targets the high-mannose oligosaccharides, neutralizing the [7] R. G. Webster, M. Yakhno, V. S. Hinshaw et al., “Intestinal viral particle [104, 105]. e thiazolides work to block the in�uenza: replication and characterization of in�uenza viruses maturation of HA posttranslationally [104, 106]. Recently, inducks,”Virology,vol.84,no.2,pp.268–278,1978. several groups have identi�ed broad-spectrum neutralizing [8] R.G.Webster,W.J.Bean,O.T.Gorman,T.M.Chambers,and antibodiesthatareprotectiveagainstgroup1(whichincludes Y. Kawaoka, “Evolution and ecology of in�uenza A viruses,” HA subtypes 1, 2, 5, 6, 8, 9, 11, 12, 13, and 16) and group MicrobiologicalReviews,vol.56,no.1,pp.152–179,1992. 2 (which includes HA subtypes 3, 4, 7, 10, 14, and 15) [9] N. P. Johnson and J. Mueller, “Updating the accounts: global in�uenza A viruses [36, 107–109]. ese broad-spectrum mortality of the 1918–1920 “Spanish” in�uenza pandemic,” neutralizingantibodies(referredtoasCR6261,F10,andF16) Bulletin of the History of Medicine, vol. 76, no. 1, pp. 105–115, are further distinct in that they target the stem region of the HA molecule. e proposed model is that by targeting [10] A. W. Crosby, America’s Forgotten Pandemic, Cambridge Uni- the stem region, more speci�cally the fusion peptide, these versityPress,2003. antibodiesareabletopreventexposureofthefusionpeptide [11] K. D. Patterson and G. F. Pyle, “e geography and mortality under acid conditions. is trapping mechanism prevents of the 1918 in�uenza pandemic,” Bulletin of the History of the viral membrane from fusing with the host endosomal Medicine,vol.65,no.1,pp.4–21,1991. membrane, locking the genetic contents within the viral [12] G.J.D.Smith,D.Vijaykrishna,J.Bahletal.,“Originsandevo- particle. An additional stem-directed neutralizing antibody, lutionarygenomicsofthe2009swine-originH1N1in�uenzaA CR8020,isactiveagainstgroup2in�uenzaAviruses.While epidemic,”Nature,vol.459,no.7250,pp.1122–1125,2009. not as broad in activity as the aforementioned antibodies, [13] M. Nelson, M. Gramer, A. Vincent, and E. C. Holmes, “Global CR8020 could be utilized in conjunction with a group 1 transmission of in�uenza viruses from humans to swine,” neutralizing antibody, providing a one-two punch against Journal of General Virology, vol. 93, part 10, pp. 2195–2203, bothHAsubunits. In�uenzaviruseswillcontinuetocirculateamonganimal [14] CDC H1N1 �u. Center for Disease Control and Prevention and human populations, acquiring and exchanging genetic Website. components as they do. Due to the constant change in their genetic pro�les, in�uenza viruses will continue to pose a [15] F. S. Dawood, A. D. Iuliano, C. Reed, M. I. Meltzer, D. K. Shay serious threat, from both an economic and public health et al., “Estimated global mortality associated with the �rst 12 pointofview.Continuedstudyandsurveillanceofin�uenza monthsof2009pandemicin�uenzaAH1N1viruscirculation: amodellingstudy,”eLancetInfectiousDiseases,vol.12,no.9, viruses is further highlighted by our inability to maintain pp.687–695,2012. effective in�uenza vaccines and prophylactic therapeutics. e prospective of new antivirals, especially those that pro- [16] R.LambandR.Krug,“Orthomyxoviridae:thevirusesandtheir vide broad-spectrum protection, provide renewed efforts in replication,”inField’sVirology,pp.1487–1532,2001. ourabilitytocontrolin�uenzainfectionandspread. [17] P.Palese,“In�uenza:oldandnewthreats,”NatureMedicine,vol. 10,no.12,pp.S82–S87,2004. [18] E. D. Kilbourne, “In�uenza pandemics: can we prepare for the References unpredictable?” Viral Immunology, vol. 17, no. 3, pp. 350–357, [1] F.A.Murphy,“Virustaxonomy,”inVirology,B.N.Fields,D.M. 2004. Knipe, and P. M. Howley, Eds., pp. 15–57, Lippincott-Raven, [19] E. D. Kilbourne, “In�uenza pandemics of the 20th century,” Philadelphia,Pa,USA,1996. EmergingInfectiousDiseases,vol.12,no.1,pp.9–14,2006. [2] D.M.Fleming,M.Zambon,andA.I.M.Bartelds,“Population [20] M. C. Zambon, “e pathogenesis of in�uenza in humans,” estimates of persons presenting to general practitioners with ReviewsinMedicalVirology,vol.11,no.4,pp.227–241,2001. in�uenza-like illness, 1987–96: a study of the demography of [21] Y.-C. Hsieh, T. Z. Wu, D. P. Liu et al., “In�uenza pandemics: in�uenza-like illness in sentinel practice networks in England past, present and future,” Journal of the Formosan Medical andWales, and in eNetherlands,” Epidemiology & Infection, Association,vol.105,no.1,pp.1–6,2006. vol.124,no.2,pp.245–253,2000. [22] T. Horimoto and Y. Kawaoka, “In�uenza: lessons from past [3] K. M. Sullivan and A. S. Monto, “Acute respiratory illness in pandemics, warnings from current incidents,” Nature Reviews the community. Frequency of illness and the agents involved,” Microbiology,vol.3,no.8,pp.591–600,2005. Epidemiology&Infection,vol.110,no.1,pp.145–160,1999. [23] T. Ito, J. N. S. S. Couceiro, S. Kelm et al., “Molecular basis for [4] G.Yuanji,J.Fengen,W.Pingetal.,“Isolationofin�uenzaCvirus the generation in pigs of in�uenza A viruses with pandemic from pigs and experimental infection of pigs with in�uenza C potential,” Journal of Virology, vol. 72, no. 9, pp. 7367–7373, virus,” Journal of General Virology, vol. 64, no. 1, pp. 177–182, [24] Y. Kawaoka, S. Krauss, and R. G. Webster, “Avian-to-human [5] A. Hay, “e virus genome and its replication,” in Textbook of transmissionofthePB1geneofin�uenzaAvirusesinthe1957 In�uen�a, K. G. Nicholson, R. G. Webster, and A. J. Hay, Eds., and 1968 pandemics,” Journal of Virology,vol.63,no.11,pp. pp.43–53,1998. 4603–4608,1989. AdvancesinVirology 7 [25] J. J. Skehel and D. C. Wiley, “Receptor binding and membrane of the hemagglutinin polypeptide,” Virology, vol. 68, no. 2, pp. fusion in virus entry: the in�uenza hemagglutinin,”Annual 440–454,1975. ReviewofBiochemistry,vol.69,pp.531–569,2000. [42] F. X. Bosch, W. Garten, H.-D. Klenk, and R. Rott, “Proteolytic [26] N. M. Ferguson, C. Fraser, C. A. Donnelly, A. C. Ghani, and cleavage of in�uenza virus hemagglutinins: primary structure R. M. Anderson, “Public health risk from the avian H5N1 of the connecting peptide between HA1 and HA2 determines in�uenza epidemic,”Science, vol. 304, no. 5673, pp. 968–969, proteolytic cleavability and pathogenicity of avian in�uenza viruses,”Virology,vol.113,no.2,pp.725–735,1981. [27] C.F.BaslerandP.V.Aguilar,“Progressinidentifyingvirulence [43] R. T. C. Huang, K. Wahn, H.-D. Klenk, and R. Rott, “Fusion determinantsofthe1918H1N1andtheSoutheastAsianH5N1 between cell membrane and liposomes containing the gly- in�uenza A viruses,”Antiviral Research, vol. 79, no. 3, pp. coproteins of in�uenza virus,”Virology, vol. 104, no. 2, pp. 166–178,2008. 294–302,1980. [28] R.B.Belshe,“eoriginsofpandemicin�uenza�lessonsfrom [44] T. Maeda and S.-I. Ohnishi, “Activation of in�uenza virus by the1918virus,”eNewEnglandJournalofMedicine,vol.353, acidicmediacauseshemolysisandfusionoferythrocytes,”FEBS no.21,pp.2209–2211,2005. Letters,vol.122,no.2,pp.283–287,1980. [29] L. Glaser, J. Stevens, D. Zamarin et al., “A single amino acid [45] J. White, K. Matlin, and A. Helenius, “Cell fusion by Semliki substitution in 1918 in�uenza virus hemagglutinin changes Forest, in�uenza, and vesicular stomatitis viruses,”e Journal receptorbindingspeci�city,”JournalofVirology,vol.79,no.17, ofCellBiology,vol.89,no.3,pp.674–679,1981. pp.11533–11536,2005. [46] P. A. Bullough, F. M. Hughson, J. J. Skehel, and D. C. Wiley, [30] A. H. Reid, T. G. Fanning, J. V. Hultin, and J. K. Taubenberger, “Structureofin�uenzahaemagglutininatthepHofmembrane “Origin and evolution of the 1918 “Spanish” in�uenza virus fusion,”Nature,vol.371,no.6492,pp.37–43,1994. hemagglutinin gene,” Proceedings of the National Academy of [47] S. Bertram, I. Glowacka, I. Steffen, A. Kühl, and S. Pöhlmann, Sciences of the United States of America, vol. 96, no. 4, pp. “Novel insights into proteolytic cleavage of in�uenza virus 1651–1656,1999. hemagglutinin,” Reviews in Medical Virology, vol. 20, no. 5, pp. [31] T.M.Tumpey,C.F.Basler,P.V.Aguilaretal.,“Characterization 298–310,2010. of the reconstructed 1918 Spanish in�uenza pandemic virus,” [48] E. Böttcher, T. Matrosovich, M. Beyerle, H.-D. Klenk, W. Science,vol.310,no.5745,pp.77–80,2005. Garten, and M. Matrosovich, “Proteolytic activation of [32] S. J. Gamblin, L. F. Haire, R. J. Russell et al., “e structure in�uenzavirusesbyserineproteasesTMPRSS2andHATfrom and receptor binding properties of the 1918 in�uenza hemag- human airway epithelium,” Journal of Virology, vol. 80, no. 19, glutinin,”Science,vol.303,no.5665,pp.1838–1842,2004. pp.9896–9898,2006. [33] I. A. Wilson and N. J. Cox, “Structural basis of immune [49] C.Chaipan,D.Kobasa,S.Bertrametal.,“Proteolyticactivation recognition of in�uenza virus hemagglutinin,”Annual Review ofthe1918in�uenzavirushemagglutinin,”JournalofVirology, ofImmunology,vol.8,pp.737–787,1990. vol.83,no.7,pp.3200–3211,2009. [34] R. A. M. Fouchier, V. Munster, A. Wallensten et al., “Charac- [50] H.Kido,Y.Yokogoshi,K.Sakaietal.,“Isolationandcharacter- terization of a novel in�uenza A virus hemagglutinin subtype izationofanoveltrypsin-likeproteasefoundinratbronchiolar (H16) obtained from black-headed gulls,” Journal of Virology, epithelial Clara cells. A possible activator of the viral fusion vol.79,no.5,pp.2814–2822,2005. glycoprotein,” e Journal of Biological Chemistry, vol. 267, no. [35] D. Kaye and C. R. Pringle, “Avian in�uenza viruses and their 19,pp.13573–13579,1992. implicationforhumanhealth,”ClinicalInfectiousDiseases,vol. [51] T. M. Tumpey, A. García-Sastre, J. K. Taubenberger et al., 40,no.1,pp.108–112,2005. “Pathogenicity of in�uenza viruses with genes from the 1918 [36] D. Corti, J. Voss, S. J. Gamblin, G. Codoni, A. Macagno et al., pandemic virus: functional roles of alveolar macrophages and “Neutralizingantibodyselectedfromplasmacellsthatbindsto neutrophilsinlimitingvirusreplicationandmortalityinmice,” group1andgroup2in�uenzaAhemagglutinins,”Science,vol. JournalofVirology,vol.79,no.23,pp.14933–14944,2005. 333,no.6044,pp.850–856,2011. [52] G. omas, “Furin at the cutting edge: from protein traffic [37] P. Leyssen, E. de Clercq, and J. Neyts, “Molecular strategies to to embryogenesis and disease,” Nature Reviews Molecular Cell inhibit the replication of RNA viruses,” Antiviral Research, vol. Biology,vol.3,no.10,pp.753–766,2002. 78,no.1,pp.9–25,2008. [53] A.Stieneke-Grober,M.Vey,H.Anglikeretal.,“In�uenzavirus [38] T. Y. Aditama, G. Samaan, R. Kusriastuti, O. D. Sampurno, W. hemagglutinin with multibasic cleavage site is activated by Purbaetal.,“Avianin�uenzaH5N1transmissioninhouseholds, furin, a subtilisin-like endoprotease,” e EMBO Journal, vol. Indonesia,”PLoSONE,vol.7,no.1,ArticleIDe29971,2012. 11,no.7,pp.2407–2414,1992. [39] Y. Yang, M. E. Halloran, J. D. Sugimoto, and I. M. Longini Jr., [54] D. J. Krysan, N. C. Rockwell, and R. S. Fuller, “Quantitative “Detecting human-to-human transmission of avian in�uenza characterizationoffurinspeci�city:energeticsofsubstratedis- A (H5N1),” Emerging Infectious Diseases, vol. 13, no. 9, pp. crimination using an internally consistent set of hexapeptidyl 1348–1353,2007. methylcoumarinamides,” e Journal of Biological Chemistry, [40] H.-D. Klenk, R. Rott, M. Orlich, and J. Blodorn, “Activation of vol.274,no.33,pp.23229–23234,1999. in�uenza A viruses by trypsin treatment,”Virology, vol. 68, no. [55] D. J. Hulse, R. G. Webster, R. J. Russell, and D. R. Perez, 2,pp.426–439,1975. “Molecular determinants within the surface proteins involved [41] S. G. Lazarowitz and P. W. Choppin, “Enhancement of the in the pathogenicity of H5N1 in�uenza viruses in chickens,” infectivity of in�uenca A and B viruses by proteolytic cleavage JournalofVirology,vol.78,no.18,pp.9954–9964,2004. 8 AdvancesinVirology [56] E.Rumschlag-Booms,Y.Guo,J.Wang,M.Caffrey,andL.Rong, [71] M. N. Matrosovich, T. Y. Matrosovich, T. Gray, N. A. Roberts, “Comparative analysis between a low pathogenic and a high and H.-D. Klenk, “Human and avian in�uenza viruses target pathogenic in�uenza H5 hemagglutinin in cell entry,” Virology different cell types in cultures of human airway epithelium,” Journal,vol.6,article76,2009. Proceedings of the National Academy of Sciences of the United StatesofAmerica,vol.101,no.13,pp.4620–4624,2004. [57] G.K.Hirst,“Adsorptionofin�uenzahemagglutininsandvirus [72] J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. by red blood cells,” e Journal of Experimental Medicine, vol. Paulson, and I. A. Wilson, “Structure and receptor speci�city 76,no.2,pp.195–209,1942. of the hemagglutinin from an H5N1 in�uenza virus,” Science, [58] F.BurnetandJ.D.Stone,“ereceptor-destroyingenzymeofV. vol.312,no.5772,pp.404–410,2006. cholerae,”AustralianJournalofExperimentalBiology&Medical [73] J.Stevens,O.Blixt,L.Glaseretal.,“Glycanmicroarrayanalysis Science,vol.25,no.3,pp.227–233,1947. of the hemagglutinins from modern and pandemic in�uenza [59] A. Gottschalk, “Neuraminidase: the speci�c enzyme of virusesrevealsdifferentreceptorspeci�cities,”JournalofMolec- in�uenza virus and Vibrio cholerae,” Biochimica et Biophysica ularBiology,vol.355,no.5,pp.1143–1155,2006. Acta,vol.23,pp.645–646,1957. [74] A. Chandrasekaran, A. Srinivasan, R. Raman et al., “Gly- [60] A. Gottschalk, e Chemistry and Biology of Sialic Acids and can topology determines human adaptation of avian H5N1 Related Substances, University Press, Cambridge, Mass, USA, virus hemagglutinin,” Nature Biotechnology,vol.26,no.1, pp. 107–113,2008. [61] Y. Suzuki, “Sialobiology of in�uenza molecular mechanism [75] N. K. Sauter, J. E. Hanson, G. D. Glick et al., “Binding of of host range variation of in�uenza viruses,” Biological and in�uenza virus hemagglutinin to analogs of its cell-surface PharmaceuticalBulletin,vol.28,no.3,pp.399–408,2005. receptor, sialic acid: analysis by proton nuclear magnetic res- onance spectroscopy and X-ray crystallography,” Biochemistry, [62] C. F.Brewer and T. K. Dam, “Essentials of Glycobiology, Edited vol.31,no.40,pp.9609–9621,1992. by A. Varki, R. Cummings, J. Esko, H. Freeze, G. Hart, and J. Marth, Cold Spring Harbor Laboratory Press, Cold Spring [76] T.Suzuki,A.Portner,R.A.Scroggsetal.,“Receptorspeci�cities Harbor, New York, 1999, 653 pp.,” Carbohydrate Research, vol. ofhumanrespiroviruses,”JournalofVirology,vol.75,no.10,pp. 325,no.3,pp.233–234,2000. 4604–4613,2001. [77] S. Chutinimitkul,S. Herfst,J.Steelet al.,“Virulence-associated [63] M.N.Matrosovich,A.S.Gambaryan,S.Tenebergetal.,“Avian substitution D222G in the hemagglutinin of 2009 pandemic in�uenza A viruses differ from human viruses by recognition in�uenza A(H1N1) virus affects receptor binding,” Journal of of sialyloligosaccharides and gangliosides and by a higher Virology,vol.84,no.22,pp.11802–11813,2010. conservation of the HA receptor-binding site,” Virology, vol. 233,no.1,pp.224–234,1997. [78] A. Vines, K. Wells, M. Matrosovich, M. R. Castrucci, T. Ito, and Y. Kawaoka, “e role of in�uenza A virus hemagglutinin [64] C. R. Parrish and Y. Kawaoka, “e origins of new pandemic residues 226 and 228 in receptor speci�city and host range viruses:theacquisitionofnewhostrangesbycanineparvovirus restriction,” Journal of Virology, vol. 72, no. 9, pp. 7626–7631, andin�uenzaAviruses,”AnnualReviewofMicrobiology,vol.59, pp.553–586,2005. [79] R. J. Connor, Y. Kawaoka, R. G. Webster, and J. C. Paulson, [65] G. N. Rogers, T. J. Pritchett, J. L. Lane, and J. C. Paulson, “Receptor speci�city in human, avian, and equine H2 and H3 “Differential sensitivity of human, avian, and equine in�uenza in�uenza virus isolates,” Virology, vol. 205, no. 1, pp. 17–23, A viruses to a glycoprotein inhibitor of infection: selection of receptorspeci�cvariants,”Virology,vol.131,no.2,pp.394–408, 1983. [80] C.W.Naeve,V.S.Hinshaw,andR.G.Webster,“Mutationsinthe hemagglutinin receptor-binding site can change the biological [66] G. N. Rogers and J. C. Paulson, “Receptor determinants of propertiesofanin�uenzavirus,”JournalofVirology,vol.51,no. human and animal in�uenza virus isolates: differences in 2,pp.567–569,1984. receptor speci�city of the H3 hemagglutinin based on species [81] E. Nobusawa, T. Aoyama, H. Kato, Y. Suzuki, Y. Tateno, and oforigin,”Virology,vol.127,no.2,pp.361–373,1983. K. Nakajima, “Comparison of complete amino acid sequences [67] G. N. Rogers, J. C. Paulson, R. S. Daniels et al., “Single amino andreceptor-bindingpropertiesamong13serotypesofhemag- acidsubstitutionsinin�uenzahaemagglutininchangereceptor glutinins of in�uenza A viruses,” Virology, vol. 182, no. 2, pp. bindingspeci�city,”Nature,vol.304,no.5921,pp.76–78,1983. 475–485,1991. [68] J. N. S. S. Couceiro, J. C. Paulson, and L. G. Baum, “In�uenza [82] C. T. Hardy, S. A. Young, R. G. Webster, C. W. Naeve, and R. J. virus strains selectively recognize sialyloligosaccharides on Owens, “Egg �uids and cells of the chorioallantoic membrane human respiratory epithelium; the role of the host cell in of embryonated chicken eggs can select different variants of selectionofhemagglutininreceptorspeci�city,”VirusResearch, in�uenza A (H3N2) viruses,” Virology, vol. 211, no. 1, pp. vol.29,no.2,pp.155–165,1993. 302–306,1995. [69] K. Shinya, M. Ebina, S. Yamada, M. Ono, N. Kasai, and Y. [83] P.S.Daniels,S.Jeffries,P.Yatesetal.,“ereceptor-bindingand Kawaoka, “Avian �u: in�uenza virus receptors in the human membrane-fusionpropertiesofin�uenzavirusvariantsselected airway,”Nature,vol.440,no.7083,pp.435–436,2006. usinganti-haemagglutininmonoclonalantibodies,”eEMBO Journal,vol.6,no.5,pp.1459–1465,1987. [70] M. Matrosovich, N. Zhou, Y. Kawaoka, and R. Webster, “e surface glycoproteins of H5 in�uenza viruses isolated from [84] E. C. J. Claas, J. C. de Jong, R. van Beek, G. F. Rimmelz- humans, chickens, and wild aquatic birds have distinguishable waan, and A. D. M. E. Osterhaus, “Human in�uenza virus properties,” Journal of Virology, vol. 73, no. 2, pp. 1146–1155, A/HongKong/156/97(H5N1)infection,”Vaccine,vol.16,no.9- 1999. 10,pp.977–978,1998. AdvancesinVirology 9 [85] J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. [101] H. T. Nguyen, A. M. Fry, and L. V. Gubareva, “Neuraminidase Paulson, and I. A. Wilson, “Structure and receptor speci�city inhibitor resistance in in�uenza viruses and laboratory testing of the hemagglutinin from an H5N1 in�uenza virus,” Science, methods,”Antiviralerapy,vol.17,pp.159–173,2012. vol.312,no.5772,pp.404–410,2006. [102] N.J.Dharan,L.V.Gubareva,J.J.Meyeretal.,“Infectionswith oseltamivir-resistant in�uenza A�H1N1� virus in the �nited [86] G. Ayora-Talavera, H. Shelton, M. A. Scull et al., “Mutations in H5N1 in�uenza virus hemagglutinin that confer binding to States,” JournaloftheAmericanMedicalAssociation, vol. 301, human tracheal airway epithelium,” PLoS ONE, vol. 4, no. 11, no.10,pp.1034–1041,2009. ArticleIDe7836,2009. [103] S.D.Cady,W.Luo,F.Hu,andM.Hong,“Structureandfunction ofthein�uenzaAM2protonchannel,”Biochemistry,vol.48,no. [87] M. Wang, D. M. Tscherne, C. McCullough, M. Caffrey, A. Garcia-Sastre et al., “Residue Y161 of in�uenza virus hemag- 31,pp.7356–7364,2009. glutininisinvolvedinviralrecognitionofsialylatedcomplexes [104] D.A.Boltz,J.R.Aldridge,R.G.Webster,andE.A.Govorkova, from different hosts,” Journal of Virology, vol. 86, no. 8, pp. “Drugsindevelopmentforin�uenza,”Drugs,vol.70,no.11,pp. 4455–4462,2012. 1349–1362,2010. [88] S. Yamada, Y. Suzuki, T. Suzuki et al., “Haemagglutinin muta- [105] B.R.O’Keefe,D.F.Smee,J.A. Turpin et al.,“Potentanti- tionsresponsibleforthebindingofH5N1in�uenzaAvirusesto in�uenza activity of cyanovirin-N and interactions with viral human-typereceptors,”Nature,vol.444,no.7117,pp.378–382, hemagglutinin,” Antimicrobial Agents and Chemotherapy, vol. 47,no.8,pp.2518–2525,2003. [89] M. Imai, T. Watanabe, M. Hatta, S. C. Das, M. Ozawa et [106] J. F. Rossignol, S. La Frazia, L. Chiappa, A. Ciucci, and M. G. al., “Experimental adaptation of an in�uenza H5 HA confers Santoro, “iazolides, a new class of anti-in�uenza molecules respiratorydroplettransmissiontoareassortantH5HA/H1N1 targeting viral hemagglutinin at the post-translational level,” virusinferrets,”Nature,vol.486,pp.420–428,2012. e Journal of Biological Chemistry, vol. 284, no. 43, pp. 29798–29808,2009. [90] S.Herfst,E.J.A.Schrauwen,M.Linster,S.Chutinimitkul,E.de Wit et al., “Airborne transmission of in�uenza A/H5N1 virus [107] J. Sui, W. C. Hwang, S. Perez et al., “Structural and functional between ferrets,” Science, vol. 336, no. 6088, pp. 1534–1541, bases for broad-spectrum neutralization of avian and human in�uenza A viruses,” Nature Structural and Molecular Biology, vol.16,no.3,pp.265–273,2009. [91] V. C. Chu and G. R. Whittaker, “In�uenza virus entry and infection require host cell N-linked glycoprotein,” Proceedings [108] D. C. Ekiert and I. A. Wilson, “Broadly neutralizingantibodies of the National Academy of Sciences of the United States of against in�uenza virus and prospects for universal therapies,” America,vol.101,no.52,pp.18153–18158,2004. CurrentOpinioninVirology,vol.2,no.2,pp.134–141,2012. [92] E. M. Rapoport, L. V. Mochalova, H.-J. Gabius, J. Romanova, [109] M. rosby, E. van den Brink, M. Jongeneelen et al., “Hetero- and N. V. Bovin, “Search for additional in�uenza virus to subtypic neutralizing monoclonal antibodies cross-protective cell interactions,” Glycoconjugate Journal, vol. 23, no. 1-2, pp. againstH5N1andH1N1recoveredfromhumanIgM memory 115–125,2006. Bcells,”PLoSONE,vol.3,no.12,ArticleIDe3942,2008. [93] S. J. Stray, R. D. Cummings, and G. M. Air, “In�uenza virus infection of desialylated cells,” Glycobiology,vol.10,no.7, pp. 649–658,2000. [94] J. M. Nicholls, M. C. W. Chan, W. Y. Chan et al., “Tropism of avian in�uenza A �H5N1� in the upper and lower respiratory tract,”NatureMedicine,vol.13,no.2,pp.147–149,2007. [95] Y.Guo,E.Rumschlag-Booms,J.Wangetal.,“Analysisofhema- gglutinin-mediated entry tropism of H5N1 avian in�uenza,” VirologyJournal,vol.6,article39,2009. [96] R. Rappuoli and P. R. Dormitzer, “In�uenza� options to improvepandemicpreparation,”Science,vol.336,no.6088,pp. 1531–1533,2012. [97] J. Cinatl, M. Michaelis, and H. W. Doerr, “e threat of avian in�uenza A �H5N1�. Part III� antiviral therapy,”Medical Microbiology and Immunology, vol. 196, no. 4, pp. 203–212, [98] A.Moscona,“Neuraminidaseinhibitorsforin�uenza,”eNew England Journal of Medicine, vol. 353, no. 13, pp. 1363–1373, [99] J. S. Rossman and R. A. Lamb, “In�uenza virus assembly and budding,”Virology,vol.411,no.2,pp.229–236,2011. [100] J.Magano,“Syntheticapproachestotheneuraminidaseinhibit- ors zanamivir �Relenza� and oseltamivir phosphate �Tami�u� for the treatment of in�uenza,” Chemical Reviews, vol. 109, no. 9,pp.4398–4438,2009. 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