Role of hemagglutinin cleavage for the pathogenicity of influenza virus

N-linked glycosylation of the hemagglutinin protein influences virulence and antigenicity of the 1918 pandemic and seasonal H1N1 influenza A viruses

The hemagglutinin (HA) protein is a major virulence determinant for the 1918 pandemic influenza virus; however, it encodes no known virulence-associated determinants. In comparison to seasonal influenza viruses of lesser virulence, the 1918 H1N1 virus has fewer glycosylation sequons on the HA globular head region. Using site-directed mutagenesis, we found that a 1918 HA recombinant virus, of high virulence, could be significantly attenuated in mice by adding two additional glycosylation sites (asparagine [Asn] 71 and Asn 286) on the side of the HA head. The 1918 HA recombinant virus was further attenuated by introducing two additional glycosylation sites on the top of the HA head at Asn 142 and Asn 172. In a reciprocal experimental approach, deletion of HA glycosylation sites (Asn 142 and Asn 177, but not Asn 71 and Asn 104) from a seasonal influenza H1N1 virus, A/Solomon Islands/2006 (SI/06), led to increased virulence in mice. The addition of glycosylation sites to 1918 HA and removal of glycosylation sites from SI/06 HA imposed constraints on the theoretical structure surrounding the glycan receptor binding sites, which in turn led to distinct glycan receptor binding properties. The modification of glycosylation sites for the 1918 and SI/06 viruses also caused changes in viral antigenicity based on cross-reactive hemagglutinin inhibition antibody titers with antisera from mice infected with wild-type or glycan mutant viruses. These results demonstrate that glycosylation patterns of the 1918 and seasonal H1N1 viruses directly contribute to differences in virulence and are partially responsible for their distinct antigenicity.

Enhanced protective efficacy of H5 subtype influenza vaccine with modification of the multibasic cleavage site of hemagglutinin in retroviral pseudotypes

Traditionally, the multibasic cleavage site (MBCS) of surface protein H5-hemagglutinin (HA) is converted to a monobasic one so as to weaken the virulence of recombinant H5N1 influenza viruses and to produce inactivated and live attenuated vaccines. Whether such modification benefits new candidate vaccines has not been adequately investigated. We previously used retroviral vectors to generate wtH5N1 pseudotypes containing the wild-type HA (wtH5) from A/swine/Anhui/ca/2004 (H5N1) virus. Here, we generated mtH5N1 pseudotypes, which contained a mutant-type HA (mtH5) with a modified monobasic cleavage site. Groups of mice were subcutaneously injected with the two types of influenza pseudotypes. Compared to the group immunized with wtH5N1 pseudotypes, the inoculation of mtH5N1 pseudotypes induced significantly higher levels of HA specific IgG and IFN-γ in immunized mice, and enhanced protection against the challenge of mouse-adapted avian influenza virus A/Chicken/Henan/12/2004 (H5N1). This study suggests modification of the H5-hemagglutinin MBCS in retroviral pseudotypes enhances protection efficacy in mice and this information may be helpful for development of vaccines from mammalian cells to fight against H5N1 influenza viruses.

Plasmin-mediated activation of pandemic H1N1 influenza virus hemagglutinin is independent of the viral neuraminidase

Influenza virus is well recognized to modulate host tropism and pathogenesis based on mutations in the proteolytic cleavage site of the viral hemagglutinin (HA), which activates HA and exposes the fusion peptide for membrane fusion. Instead of the conventional trypsin-mediated cleavage event, modification of the cleavage site allows extended use of host cell proteases and enhanced spread in vivo. For H1N1 influenza viruses, the mouse-adapted A/WSN/33 strain is known to replicate in the brain based on recruitment of plasminogen by the viral neuraminidase (NA), as well as a Ser-Tyr substitution at the P2 position of the HA cleavage site. Here, we show that an equivalent Ser-Tyr substitution has occurred in the HA of naturally occurring human H1N1 influenza viruses. We characterize one of these viruses (A/Beijing/718/2009), as well as the prototype A/California/04/2009 with a Ser-Tyr substitution in the cleavage site, and show that these HAs are preferentially cleaved by plasmin. Importantly, cleavage activation by plasmin/plasminogen was independent of the viral NA, suggesting a novel mechanism for HA cleavage activation. We show that the viral HA itself can recruit plasminogen for HA cleavage. We further show that cellular factors, as well as streptokinase from bacteria commonly coinfecting the respiratory tract of influenza patients, can be a source of activated plasminogen for plasmin-mediated cleavage of influenza virus HAs that contain a Ser-Tyr substitution in the cleavage site.

Cleavage activation of human-adapted influenza virus subtypes by kallikrein-related peptidases 5 and 12

A critical step in the influenza virus replication cycle is the cleavage activation of the HA precursor. Cleavage activation of influenza HA enables fusion with the host endosome, allowing for release of the viral genome into the host cell. To date, studies have determined that HA activation is driven by trypsin-like host cell proteases, as well as yet to be identified bacterial proteases. Although the number of host proteases that can activate HA is growing, there is still uncertainty regarding which secreted proteases are able to support multicycle replication of influenza. In this study, we have determined that the kallikrein-related peptidases 5 and 12 are secreted from the human respiratory tract and have the ability to cleave and activate HA from the H1, H2, and H3 subtypes. Each peptidase appears to have a preference for particular influenza subtypes, with kallikrein 5 cleaving the H1 and H3 subtypes most efficiently and kallikrein 12 cleaving the H1 and H2 subtypes most efficiently. Cleavage analysis using HA cleavage site peptide mimics revealed that the amino acids neighboring the arginine cleavage site affect cleavage efficiency. Additionally, the thrombolytic zymogens plasminogen, urokinase, and plasma kallikrein have all been shown to cleave and activate influenza but are found circulating mainly as inactive precursors. Kallikrein 5 and kallikrein 12 were examined for their ability to activate the thrombolytic zymogens, and both resulted in activation of each zymogen, with kallikrein 12 being a more potent activator. Activation of the thrombolytic zymogens may therefore allow for both direct and indirect activation of the HA of human-adapted influenza viruses by kallikrein 5 and kallikrein 12.

Viral envelope glycoprotein processing by proprotein convertases

The proprotein convertases (PCs) are a family of nine mammalian enzymes that play key roles in the maintenance of cell homeostasis by activating or inactivating proteins via limited proteolysis under temporal and spatial control. A wide range of pathogens, including major human pathogenic viruses can hijack cellular PCs for their own purposes. In particular, productive infection with many enveloped viruses critically depends on the processing of their fusion-active viral envelope glycoproteins by cellular PCs. Based on their crucial role in virus-host interaction, PCs can be important determinants for viral pathogenesis and represent promising targets of therapeutic antiviral intervention. In the present review we will cover basic aspects and recent developments of PC-mediated maturation of viral envelope glycoproteins of selected medically important viruses. The molecular mechanisms underlying the recognition of PCs by viral glycoproteins will be described, including recent findings demonstrating differential PC-recognition of viral and cellular substrates. We will further discuss a possible scenario how viruses during co-evolution with their hosts adapted their glycoproteins to modulate the activity of cellular PCs for their own benefit and discuss the consequences for virus-host interaction and pathogenesis. Particular attention will be given to past and current efforts to evaluate cellular PCs as targets for antiviral therapeutic intervention, with emphasis on emerging highly pathogenic viruses for which no efficacious drugs or vaccines are currently available.

Genetic variation and phylogenetic analysis of hemagglutinin genes of H9 avian influenza viruses isolated in China during 2010-2012

Genetic variation and phylogenetic relationships of H9 avian influenza viruses (AIVs) were analyzed based on hemagglutinin (HA) gene sequences of 84 Chinese H9 reference viruses recently available in GenBank, 3 widely used vaccine strains and 29 novel isolates. The novel isolates were obtained from vaccinated poultry flocks in 11 provinces of China during 2010 to 2012. The nucleotide homologies of HA genes of these isolates ranged from 87.8-99.8%, and from 89.8-93.2% as compared with the vaccine strains. Among the 29 novel isolates and the 84 reference viruses, 69.9% of the them belonged to the lineage h9.4.2.5 and had the dominant PSRSSR↓GLF motifs in the HA cleavage sites, while 27.4% of the them belonged to the newly emerging lineage h9.4.2.6 and had the dominant PARSSR↓GLF motifs, no consecutive basic amino acids insertion, showing the characteristic feature of low-pathogenic AIV. All the lineage h9.4.2.5 viruses and 75% of the lineage h9.4.2.6 viruses had the substitution Q226L (in H3 numbering). Additional potential glycosylation site at residues 313-315 (NCS) were found merely in all the lineage h9.4.2.5 viruses. Our results demonstrated that lineage h9.4.2.5 was more dominant than other lineages as it harbored more viruses that widely distributed in China in recent years. New lineage h9.4.2.6 previously existed mainly in South China had emerged in North China. Updated vaccine and increased veterinary biosecurity on poultry farms and trade markets are needed to prevent and control avian influenza.

Isolation of recombinant phage antibodies targeting the hemagglutinin cleavage site of highly pathogenic avian influenza virus

Highly pathogenic avian influenza (HPAI) H5N1 viruses, which have emerged in poultry and other wildlife worldwide, contain a characteristic multi-basic cleavage site (CS) in the hemagglutinin protein (HA). Because this arginine-rich CS is unique among influenza virus subtypes, antibodies against this site have the potential to specifically diagnose pathogenic H5N1. By immunizing mice with the CS peptide and screening a phage display library, we isolated four antibody Fab fragment clones that specifically bind the antigen peptide and several HPAI H5N1 HA proteins in different clades. The soluble Fab fragments expressed in Escherichia coli bound the CS peptide and the H5N1 HA protein with nanomolar affinity. In an immunofluorescence assay, these Fab fragments stained cells infected with HPAI H5N1 but not those infected with a less virulent strain. Lastly, all the Fab clones could detect the CS peptide and H5N1 HA protein by open sandwich ELISA. Thus, these recombinant Fab fragments will be useful novel reagents for the rapid and specific detection of HPAI H5N1 virus.

Matriptase proteolytically activates influenza virus and promotes multicycle replication in the human airway epithelium

Influenza viruses do not encode any proteases and must rely on host proteases for the proteolytic activation of their surface hemagglutinin proteins in order to fuse with the infected host cells. Recent progress in the understanding of human proteases responsible for influenza virus hemagglutinin activation has led to the identification of members of the type II transmembrane serine proteases TMPRSS2 and TMPRSS4 and human airway trypsin-like protease; however, none has proved to be the sole enzyme responsible for hemagglutinin cleavage. In this study, we identify and characterize matriptase as an influenza virus-activating protease capable of supporting multicycle viral replication in the human respiratory epithelium. Using confocal microscopy, we found matriptase to colocalize with hemagglutinin at the apical surface of human epithelial cells and within endosomes, and we showed that the soluble form of the protease was able to specifically cleave hemagglutinins from H1 virus, but not from H2 and H3 viruses, in a broad pH range. We showed that small interfering RNA (siRNA) knockdown of matriptase in human bronchial epithelial cells significantly blocked influenza virus replication in these cells. Lastly, we provide a selective, slow, tight-binding inhibitor of matriptase that significantly reduces viral replication (by 1.5 log) of H1N1 influenza virus, including the 2009 pandemic virus. Our study establishes a three-pronged model for the action of matriptase: activation of incoming viruses in the extracellular space in its shed form, upon viral attachment or exit in its membrane-bound and/or shed forms at the apical surface of epithelial cells, and within endosomes by its membrane-bound form where viral fusion takes place.

H5N1 pathogenesis studies in mammalian models

H5N1 influenza viruses are capable of causing severe disease and death in humans, and represent a potential pandemic subtype should they acquire a transmissible phenotype. Due to the expanding host and geographic range of this virus subtype, there is an urgent need to better understand the contribution of both virus and host responses following H5N1 virus infection to prevent and control human disease. The use of mammalian models, notably the mouse and ferret, has enabled the detailed study of both complex virus-host interactions as well as the contribution of individual viral proteins and point mutations which influence virulence. In this review, we describe the behavior of H5N1 viruses which exhibit high and low virulence in numerous mammalian species, and highlight the contribution of inoculation route to virus pathogenicity. The involvement of host responses as studied in both inbred and outbred mammalian models is discussed. The roles of individual viral gene products and molecular determinants which modulate the severity of H5N1 disease in vivo are presented. This research contributes not only to our understanding of influenza virus pathogenesis, but also identifies novel preventative and therapeutic targets to mitigate the disease burden caused by avian influenza viruses.

Protease activation mutants elicit protective immunity against highly pathogenic avian influenza viruses of subtype H7 in chickens and mice

Protease activation mutants of the highly pathogenic avian influenza virus A/FPV/Rostock/34 (H7N1) have been generated that are fully dependent on the presence of trypsin for growth in cell culture. Unlike wild-type virus, the mutants do not induce systemic infection in chicken embryos and show low pathogenicity in both chicken embryos and adult chickens. Inactivated vaccines prepared from the mutants protected chickens and mice very efficiently against infection with highly pathogenic wild-type virus in a cross-reactive manner. The potential of these mutants to be used as veterinary and prepandemic vaccines will be discussed.

Influenza HA subtypes demonstrate divergent phenotypes for cleavage activation and pH of fusion: implications for host range and adaptation

The influenza A virus (IAV) HA protein must be activated by host cells proteases in order to prime the molecule for fusion. Consequently, the availability of activating proteases and the susceptibility of HA to protease activity represents key factors in facilitating virus infection. As such, understanding the intricacies of HA cleavage by various proteases is necessary to derive insights into the emergence of pandemic viruses. To examine these properties, we generated a panel of HAs that are representative of the 16 HA subtypes that circulate in aquatic birds, as well as HAs representative of the subtypes that have infected the human population over the last century. We examined the susceptibility of the panel of HA proteins to trypsin, as well as human airway trypsin-like protease (HAT) and transmembrane protease, serine 2 (TMPRSS2). Additionally, we examined the pH at which these HAs mediated membrane fusion, as this property is related to the stability of the HA molecule and influences the capacity of influenza viruses to remain infectious in natural environments. Our results show that cleavage efficiency can vary significantly for individual HAs, depending on the protease, and that some HA subtypes display stringent selectivity for specific proteases as activators of fusion function. Additionally, we found that the pH of fusion varies by 0.7 pH units among the subtypes, and notably, we observed that the pH of fusion for most HAs from human isolates was lower than that observed from avian isolates of the same subtype. Overall, these data provide the first broad-spectrum analysis of cleavage-activation and membrane fusion characteristics for all of the IAV HA subtypes, and also show that there are substantial differences between the subtypes that may influence transmission among hosts and establishment in new species.

IAV is associated with significant morbidity and mortality, and represents a challenging public health threat that affects social and economic welfare each year, particularly during IAV pandemics. Although we know that all human strains derive, either directly or via intermediate hosts, from avian viral sources, we know very little about the phenotypic characteristics of the 16 HA subtypes that circulate in aquatic birds and have potential to infect mammals. HA membrane fusion properties, in conjunction with the characteristics for protease activation of HA, a requirement for fusion, are critical factors involved in the ecology and transmission of IAVs, and need to be understood if we are to derive explanations for how pandemic viruses emerge in humans. We examined the cleavage-activation and membrane fusion characteristics for the 16 HA subtypes by transiently expressing HA proteins in cells. Our findings show that the cleavability of the HAs vary considerably between subtypes and depending on the protease. Additionally, analysis of the pH of fusion for each subtype showed that HA stability varied significantly among the subtypes, as well as within subtypes from viruses isolated from different species. Overall, these data have implications for host range, potential for adaptation, and persistence in natural environments.

A single amino acid at the hemagglutinin cleavage site contributes to the pathogenicity but not the transmission of Egyptian highly pathogenic H5N1 influenza virus in chickens

Highly pathogenic avian influenza (HPAI) virus H5N1 has been enzootic in Egypt since 2008. Virus-associated mortality (but not the number of cases) in humans and poultry seems to have decreased over time, but the reason for this remains unknown. We investigated the role of a single amino acid substitution in the hemagglutinin cleavage site on virus pathogenicity and transmission in chickens. The R325G substitution significantly reduced pathogenicity without altering the transmission efficiency of HPAI H5N1 virus.

Biological properties of H5 hemagglutinin expressed by vaccinia virus vector and its immunological reactivity with human sera

A recombinant vaccinia virus harboring the full length hemagglutinin (HA) gene derived from a highly pathogenic avian influenza A/Thailand/1(KAN-1)/2004 (H5N1) virus (rVac-H5 HA virus) was constructed. The immunogenicity of the expressed HA protein was characterized using goat antiserum, mouse monoclonal antibody, and human sera. The expressed HA protein localized both in the cytoplasm and on the cytoplasmic membrane of the thymidine kinase negative cells infected with the rVac-H5 HA virus, as determined by immunofluorescence assay. Western blot analysis demonstrated that the rVac-H5 HA protein was post-translationally processed by proteolytic cleavage of the HA0 precursor into HA1 and HA2 domains; and all of these HA forms were immunogenic in BALB/c mice. The molecular weight (MW) of each HA domain was the same as the wild-type H5 HA produced in Madin-Darby canine kidney cells infected with the H5N1 virus, but was higher than that expressed by a baculovirus-insect cell system. Sera from all H5N1 survivors reacted to HA0, HA1, and HA2 domains; whereas sera from H5N1-uninfected subjects reacted to the HA2 domain only, but not to HA0 or HA1, indicating that some cross-subtypic immunity exists in the general population. There was a lot-to-lot variation of the recombinant HA produced in the baculovirus-insect cell system that might affect the detection rate of antibody directed against certain HA domains.

Influenza A virus entry inhibitors targeting the hemagglutinin

Influenza A virus (IAV) has caused seasonal influenza epidemics and influenza pandemics, which resulted in serious threat to public health and socioeconomic impacts. Until now, only 5 drugs belong to two categories are used for prophylaxis and treatment of IAV infection. Hemagglutinin (HA), the envelope glycoprotein of IAV, plays a critical role in viral binding, fusion and entry. Therefore, HA is an attractive target for developing anti‑IAV drugs to block the entry step of IAV infection. Here we reviewed the recent progress in the study of conformational changes of HA during viral fusion process and the development of HA-based IAV entry inhibitors, which may provide a new choice for controlling future influenza pandemics.

The pathobiology of two Indonesian H5N1 avian influenza viruses representing different clade 2.1 sublineages in chickens and ducks

To determine the pathobiology of Indonesian H5N1 highly pathogenic avian influenza, two viruses representing clades 2.1.1 and 2.1.3 were inoculated into broiler chickens and Pekin ducks via the eyes, nostrils and oropharynx. In chickens, both viruses produced fulminant disease; tissue tropism was broad but predominantly endothelial and viral loads in tissues were high. Except for one case of meningoencephalitis, the infection in ducks was sub-clinical, leading only to seroconversion. In these ducks, virus and viral antigen occurred in lower amounts, mainly in the respiratory tract (airsac and sinuses), prior to day 7 after inoculation. During clinical disease, chickens shed high virus titres orally and cloacally. Ducks intermittently shed low virus titres from the oral route for up to 8 days post-inoculation. We discuss the significance of the data for understanding the pathogenesis and pathobiology of Indonesian H5N1 in chickens and ducks.

Matriptase, HAT, and TMPRSS2 activate the hemagglutinin of H9N2 influenza A viruses

Influenza A viruses of the subtype H9N2 circulate worldwide and have become highly prevalent in poultry in many countries. Moreover, they are occasionally transmitted to humans, raising concern about their pandemic potential. Influenza virus infectivity requires cleavage of the surface glycoprotein hemagglutinin (HA) at a distinct cleavage site by host cell proteases. H9N2 viruses vary remarkably in the amino acid sequence at the cleavage site, and many isolates from Asia and the Middle East possess the multibasic motifs R-S-S-R and R-S-R-R, but are not activated by furin. Here, we investigated proteolytic activation of the early H9N2 isolate A/turkey/Wisconsin/1/66 (H9-Wisc) and two recent Asian isolates, A/quail/Shantou/782/00 (H9-782) and A/quail/Shantou/2061/00 (H9-2061), containing mono-, di-, and tribasic HA cleavage sites, respectively. All H9N2 isolates were activated by human proteases TMPRSS2 (transmembrane protease, serine S1 member 2) and HAT (human airway trypsin-like protease). Interestingly, H9-782 and H9-2061 were also activated by matriptase, a protease widely expressed in most epithelia with high expression levels in the kidney. Nephrotropism of H9N2 viruses has been observed in chickens, and here we found that H9-782 and H9-2061 were proteolytically activated in canine kidney (MDCK-II) and chicken embryo kidney (CEK) cells, whereas H9-Wisc was not. Virus activation was inhibited by peptide-mimetic inhibitors of matriptase, strongly suggesting that matriptase is responsible for HA cleavage in these kidney cells. Our data demonstrate that H9N2 viruses with R-S-S-R or R-S-R-R cleavage sites are activated by matriptase in addition to HAT and TMPRSS2 and, therefore, can be activated in a wide range of tissues what may affect virus spread, tissue tropism and pathogenicity.

A single residue substitution in the receptor-binding domain of H5N1 hemagglutinin is critical for packaging into pseudotyped lentiviral particles

Background

Serological studies for influenza infection and vaccine response often involve microneutralization and hemagglutination inhibition assays to evaluate neutralizing antibodies against human and avian influenza viruses, including H5N1. We have previously characterized lentiviral particles pseudotyped with H5-HA (H5pp) and validated an H5pp-based assay as a safe alternative for high-throughput serological studies in BSL-2 facilities. Here we show that H5-HAs from different clades do not always give rise to efficient production of H5pp and the underlying mechanisms are addressed.

Methodology/Findings

We have carried out mutational analysis to delineate the molecular determinants responsible for efficient packaging of HA from A/Cambodia/40808/2005 (H5Cam) and A/Anhui/1/2005 (H5Anh) into H5pp. Our results demonstrate that a single A134V mutation in the 130-loop of the receptor binding domain is sufficient to render H5Anh the ability to generate H5Anh-pp efficiently, whereas the reverse V134A mutation greatly hampers production of H5Cam-pp. Although protein expression in total cell lysates is similar for H5Anh and H5Cam, cell surface expression of H5Cam is detected at a significantly higher level than that of H5Anh. We further demonstrate by several independent lines of evidence that the behaviour of H5Anh can be explained by a stronger binding to sialic acid receptors implicating residue 134.

Conclusions

We have identified a single A134V mutation as the molecular determinant in H5-HA for efficient incorporation into H5pp envelope and delineated the underlying mechanism. The reduced binding to sialic acid receptors as a result of the A134V mutation not only exerts a critical influence in pseudotyping efficiency of H5-HA, but has also an impact at the whole virus level. Because A134V substitution has been reported as a naturally occurring mutation in human host, our results may have implications for the understanding of human host adaptation of avian influenza H5N1 viruses.

Live attenuated influenza viruses produced in a suspension process with avian AGE1.CR.pIX cells

BACKGROUND

Current influenza vaccines are trivalent or quadrivalent inactivated split or subunit vaccines administered intramuscularly, or live attenuated influenza vaccines (LAIV) adapted to replicate at temperatures below body temperature and administered intranasally. Both vaccines are considered safe and efficient, but due to differences in specific properties may complement each other to ensure reliable vaccine coverage. By now, licensed LAIV are produced in embryonated chicken eggs. In the near future influenza vaccines for human use will also be available from adherent MDCK or Vero cell cultures, but a scalable suspension process may facilitate production and supply with vaccines.

RESULTS

We evaluated the production of cold-adapted human influenza virus strains in the duck suspension cell line AGE1.CR.pIX using a chemically-defined medium. One cold-adapted A (H1N1) and one cold-adapted B virus strain was tested, as well as the reference strain A/PR/8/34 (H1N1). It is shown that a medium exchange is not required for infection and that maximum virus titers are obtained for 1 × 10⁻⁶ trypsin units per cell. 1 L bioreactor cultivations showed that 4 × 10⁶ cells/mL can be infected without a cell density effect achieving titers of 1 × 10⁸ virions/mL after 24 h.

CONCLUSIONS

Overall, this study demonstrates that AGE1.CR.pIX cells support replication of LAIV strains in a chemically-defined medium using a simple process without medium exchanges. Moreover, the process is fast with peak titers obtained 24 h post infection and easily scalable to industrial volumes as neither microcarriers nor medium replacements are required.

Proteomic analysis of the lungs of mice infected with different pathotypes of H5N1 avian influenza viruses

The virulence of influenza virus is determined by viral and host factors. Data on the genetic basis of the virulence of H5N1 influenza viruses have increased over the past decade; however, the contributions of host factors to the outcomes of H5N1 infection remain largely unknown. Here, we tested two chicken H5N1 viruses in mice and found that A/chicken/VN1214/2007 was nonlethal in mice and only replicated in the lung, whereas A/chicken/VN1180/2006 was highly lethal and replicated systemically in mice. To investigate the host response against these two different virus infections, we performed proteomic analysis by using 2D DIGE on the lung tissues of mice collected on days 1 and 3 postinoculation with different viruses or PBS as a control. Thirty-nine differentially expressed (DE) proteins related to "immune and stimulus response," "macromolecular biosynthesis and metabolism," and "cellular component and cytoskeleton" were identified in the virus-inoculated groups. Moreover, 13 DE proteins were identified between the two virus-inoculated groups, implying that these proteins may play important roles in the different outcomes of infection with these two viruses. Our data provide important information regarding the host response to mild and lethal H5N1 influenza virus infection.

Hemagglutinin activating host cell proteases provide promising drug targets for the treatment of influenza A and B virus infections

Cleavage of the influenza virus hemagglutinin (HA) by host cell proteases is crucial for infectivity and spread of the virus. Some years ago, we identified TMPRSS2 and HAT from human airways as activating proteases of influenza A viruses containing a monobasic HA cleavage site. Therefore, these proteases are considered as potential drug targets. In this report, first we show that HA of influenza B virus is activated by TMPRSS2 and HAT, too. We further demonstrate that benzylsulfonyl-d-arginine-proline-4-amidinobenzylamide (BAPA), which is a potent inhibitor of HAT and TMPRSS2, efficiently suppresses virus propagation in TMPRSS2-expressing human airway epithelial cells by inhibition of HA cleavage. BAPA treatment reduced virus titers of different influenza A and B viruses more than 1000-fold and delayed virus propagation by 24-48 h at non-cytotoxic concentrations. A combination of BAPA with the neuraminidase (NA) inhibitor oseltamivir carboxylate efficiently blocked influenza virus replication in airway epithelial cells at remarkable lower concentrations for each compound than treatment with either inhibitor alone. Our studies provide a novel and potent approach for influenza chemotherapy that should be considered for influenza treatment.

Structure of the cleavage-activated prefusion form of the parainfluenza virus 5 fusion protein

The paramyxovirus parainfluenza virus 5 (PIV5) enters cells by fusion of the viral envelope with the plasma membrane through the concerted action of the fusion (F) protein and the receptor binding protein hemagglutinin-neuraminidase. The F protein folds initially to form a trimeric metastable prefusion form that is triggered to undergo large-scale irreversible conformational changes to form the trimeric postfusion conformation. It is thought that F refolding couples the energy released with membrane fusion. The F protein is synthesized as a precursor (F0) that must be cleaved by a host protease to form a biologically active molecule, F1,F2. Cleavage of F protein is a prerequisite for fusion and virus infectivity. Cleavage creates a new N terminus on F1 that contains a hydrophobic region, known as the FP, which intercalates target membranes during F protein refolding. The crystal structure of the soluble ectodomain of the uncleaved form of PIV5 F is known; here we report the crystal structure of the cleavage-activated prefusion form of PIV5 F. The structure shows minimal movement of the residues adjacent to the protease cleavage site. Most of the hydrophobic FP residues are buried in the uncleaved F protein, and only F103 at the newly created N terminus becomes more solvent-accessible after cleavage. The conformational freedom of the charged arginine residues that compose the protease recognition site increases on cleavage of F protein.

Acquisition of a novel eleven amino acid insertion directly N-terminal to a tetrabasic cleavage site confers intracellular cleavage of an H7N7 influenza virus hemagglutinin

A critical feature of highly pathogenic avian influenza viruses (H5N1 and H7N7) is the efficient intracellular cleavage of the hemagglutinin (HA) protein. H7N7 viruses also exist in equine species, and a unique feature of the equine H7N7 HA is the presence of an eleven amino acid insertion directly N-terminal to a tetrabasic cleavage site. Here, we show that three histidine residues within the unique insertion of the equine H7N7 HA are essential for intracellular cleavage. An asparagine residue within the insertion-derived glycosylation site was also found to be essential for intracellular cleavage. The presence of the histidine residues also appear to be involved in triggering fusion, since mutation of the histidine residues resulted in a destabilizing effect. Importantly, the addition of a tetrabasic site and the eleven amino acid insertion conferred efficient intracellular cleavage to the HA of an H7N3 low pathogenicity avian influenza virus. Our studies show that acquisition of the eleven amino acid insertion offers an alternative mechanism for intracellular cleavage of influenza HA.

Analysis and visualization of H7 influenza using genomic, evolutionary and geographic information in a modular web service

We have reported previously on use of a web-based application, Supramap (http://supramap.org) for the study of biogeographic, genotypic, and phenotypic evolution. Using Supramap we have developed maps of the spread of drug-resistant influenza and host shifts in H1N1 and H5N1 influenza and coronaviruses such as SARS. Here we report on another zoonotic pathogen, H7 influenza, and provide an update on the implementation of Supramap as a web service. We find that the emergence of pathogenic strains of H7 is labile with many transitions from high to low pathogenicity, and from low to high pathogenicity. We use Supramap to put these events in a temporal and geospatial context. We identify several lineages of H7 influenza with biomarkers of high pathogenicity in regions that have not been reported in the scientific literature. The original implementation of Supramap was built with tightly coupled client and server software. Now we have decoupled the components to provide a modular web service for POY (http://poyws.org) that can be consumed by a data provider to create a novel application. To demonstrate the web service, we have produced an application, Geogenes (http://geogenes.org). Unlike in Supramap, in which the user is required to create and upload data files, in Geogenes the user works from a graphical interface to query an underlying dataset. Geogenes demonstrates how the web service can provide underlying processing for any sequence and metadata database. © The Willi Hennig Society 2012.

Targeting the host or the virus: current and novel concepts for antiviral approaches against influenza virus infection

Influenza epidemics and pandemics are constant threats to human health. The application of antiviral drugs provides an immediate and direct control of influenza virus infection. At present, the major strategy for managing patients with influenza is through targeting conserved viral proteins critical for viral replication. Two classes of conventional antiviral drugs, the M2 ion channel blockers and the neuraminidase inhibitors, are frequently used. In recent years, increasing levels of resistance to both drug classes has become a major public health concern, highlighting the urgent need for the development of alternative treatments. Novel classes of antiviral compounds or biomolecules targeting viral replication mechanism are under development, using approaches including high-throughput small-molecule screening platforms and structure-based designs. In response to influenza virus infection, host cellular mechanisms are triggered to defend against the invaders. At the same time, viruses as obligate intracellular pathogens have evolved to exploit cellular responses in support of their efficient replication, including antagonizing the host type I interferon response as well as activation of specific cellular pathways at different stages of the replication cycle. Numerous studies have highlighted the possibility of targeting virus-host interactions and host cellular mechanisms to develop new treatment regimens. This review aims to give an overview of current and novel concepts targeting the virus and the host for managing influenza.

Impact of chicken-origin cells on adaptation of a low pathogenic influenza virus

Understanding the growth dynamics of influenza viruses is an essential step in virus replication and cell-adaptation. The aim of this study was to elucidate the growth kinetic of a low pathogenic avian influenza H9N2 subtype in chicken embryo fibroblast (CEF) and chicken tracheal epithelial (CTE) cells during consecutive passages. An egg-adapted H9N2 virus was seeded into both cell culture systems. The amount of infectious virus released into the cell culture supernatants at interval times post-infection were titered and plaque assayed. The results as well as cell viability results indicate that the infectivity of the influenza virus was different among these primary cells. The egg-adapted H9N2 virus featured higher infectivity in CTE than in CEF cells. After serial passages and plaque purifications of the virus, a CTE cell-adapted strain was generated which carried amino acid substitutions within the HA stem region. The strain showed faster replication kinetics in cell culture resulting in an increase in virus titer. Overall, the present study provides the impact of cell type, multiplicity of infection, cellular protease roles in virus infectivity and finally molecular characterization during H9N2 virus adaptation procedure.

Distinct gene loci control the host response to influenza H1N1 virus infection in a time-dependent manner

Background

There is strong but mostly circumstantial evidence that genetic factors modulate the severity of influenza infection in humans. Using genetically diverse but fully inbred strains of mice it has been shown that host sequence variants have a strong influence on the severity of influenza A disease progression. In particular, C57BL/6J, the most widely used mouse strain in biomedical research, is comparatively resistant. In contrast, DBA/2J is highly susceptible.

Results

To map regions of the genome responsible for differences in influenza susceptibility, we infected a family of 53 BXD-type lines derived from a cross between C57BL/6J and DBA/2J strains with influenza A virus (PR8, H1N1). We monitored body weight, survival, and mean time to death for 13 days after infection. Qivr5 (quantitative trait for influenza virus resistance on chromosome 5) was the largest and most significant QTL for weight loss. The effect of Qivr5 was detectable on day 2 post infection, but was most pronounced on days 5 and 6. Survival rate mapped to Qivr5, but additionally revealed a second significant locus on chromosome 19 (Qivr19). Analysis of mean time to death affirmed both Qivr5 and Qivr19. In addition, we observed several regions of the genome with suggestive linkage. There are potentially complex combinatorial interactions of the parental alleles among loci. Analysis of multiple gene expression data sets and sequence variants in these strains highlights about 30 strong candidate genes across all loci that may control influenza A susceptibility and resistance.

Conclusions

We have mapped influenza susceptibility loci to chromosomes 2, 5, 16, 17, and 19. Body weight and survival loci have a time-dependent profile that presumably reflects the temporal dynamic of the response to infection. We highlight candidate genes in the respective intervals and review their possible biological function during infection.

Evolution and adaptation of hemagglutinin gene of human H5N1 influenza virus

The H5N1 HPAI virus has brought heavy loss to poultry industry. Although, there exists limited human-to-human transmission, it poses potential serious risks to public health. HA is responsible for receptor-binding and membrane-fusion and contains the host receptor-binding sites and major epitopes for neutralizing antibodies. To investigate molecular adaption of HPAI H5N1 viruses, we performed a phylogenetic analysis of HA sequences with 240 HPAI virus strains isolated from human. The topology of the tree reveals overall clustering of strains in four major clusters based on geographic location, and shows antigenic diversity of HA of human H5N1 isolates co-circulating in Asia, Africa, and Europe. The four clusters possess distinct features within the cleavage site and glycosylation sites, respectively. We identified six sites apparently evolving under positive selection, five of which persist in the population. Three positively selected sites are found to be located either within or flanking the receptor-binding sites, suggesting that selection at these sites may increase the affinity to human-type receptor. Furthermore, some sites are also associated with glycosylation and antigenic changes. In addition, two sites are found to be selected differentially in the two clusters. The analyses provide us deep insight into the adaptive evolution of human H5N1 viruses, show us several candidate mutations that could cause a pandemic, and suggest that efficiency measures should be taken to deal with potential risks.

Updated values for molecular diagnosis for highly pathogenic avian influenza virus

Highly pathogenic avian influenza (HPAI) viruses of the H5N1 strain pose a pandemic threat. H5N1 strain virus is extremely lethal and contagious for poultry. Even though mortality is 59% in infected humans, these viruses do not spread efficiently between humans. In 1997, an outbreak of H5N1 strain with human cases occurred in Hong Kong. This event highlighted the need for rapid identification and subtyping of influenza A viruses (IAV), not only to facilitate surveillance of the pandemic potential of avian IAV, but also to improve the control and treatment of infected patients. Molecular diagnosis has played a key role in the detection and typing of IAV in recent years, spurred by rapid advances in technologies for detection and characterization of viral RNAs and proteins. Such technologies, which include immunochromatography, quantitative real-time PCR, super high-speed real-time PCR, and isothermal DNA amplification, are expected to contribute to faster and easier diagnosis and typing of IAV.

Theoretical investigation on the binding specificity of sialyldisaccharides with hemagglutinins of influenza A virus by molecular dynamics simulations

Recognition of cell-surface sialyldisaccharides by influenza A hemagglutinin (HA) triggers the infection process of influenza. The changes in glycosidic torsional linkage and the receptor conformations may alter the binding specificity of HAs to the sialylglycans. In this study, 10-ns molecular dynamics simulations were carried out to examine the structural and dynamic behavior of the HAs bound with sialyldisaccharides Neu5Acα(2-3)Gal (N23G) and Neu5Acα(2-6)Gal (N26G). The analysis of the glycosidic torsional angles and the pair interaction energy between the receptor and the interacting residues of the binding site reveal that N23G has two binding modes for H1 and H5 and a single binding mode for H3 and H9. For N26G, H1 and H3 has two binding modes, and H5 and H9 has a single binding mode. The direct and water-mediated hydrogen bonding interactions between the receptors and HAs play dominant roles in the structural stabilization of the complexes. It is concluded from pair interaction energy and Molecular Mechanic-Poisson-Boltzmann Surface Area calculations that N26G is a better receptor for H1 when compared with N23G. N23G is a better receptor for H5 when compared with N26G. However, H3 and H9 can recognize N23G and N26G in equal binding specificity due to the marginal energy difference (≈2.5 kcal/mol). The order of binding specificity of N23G is H3 > H5 > H9 > H1 and N26G is H1 > H3 > H5 > H9, respectively. The proposed conformational models will be helpful in designing inhibitors for influenza virus.

Cleavage activation of the human-adapted influenza virus subtypes by matriptase reveals both subtype and strain specificities

Cleavage activation of the hemagglutinin (HA) precursor is an essential step in the influenza virus replication cycle that is driven by host cell proteases. HA cleavage activation is required for virus-endosome membrane fusion and the subsequent release of the influenza virus genome into the cytoplasm. Previous studies have determined that HA cleavage is most likely driven by either membrane-bound or extracellular trypsin-like proteases that reside in the respiratory tract. However, there is still uncertainty regarding which proteases are critical for HA cleavage in vivo. Therefore, further investigation of HA cleavage activation is needed in order to gain insight into the critical proteases involved. Matriptase is a member of the type II transmembrane serine protease family that is highly expressed in a membrane-bound form throughout the respiratory tract. One feature of matriptase is that, once activated, the catalytic domain is secreted into the extracellular space and so serves as a functional extracellular protease. In this study, we have determined that the secreted, catalytic domain of matriptase has the ability to cleave and activate HA from the influenza virus H1 subtype but not the H2 and H3 subtypes. Furthermore, matriptase selectively cleaved the HA of particular strains within the H1 subtype, revealing both subtype and H1 strain specificity. Matriptase was also found to activate thrombolytic zymogens that have been shown to cleave and activate the influenza virus HA. Our data demonstrate that matriptase has the ability to cleave HA directly or indirectly by activating HA-cleaving zymogens.

Mechanisms of coronavirus cell entry mediated by the viral spike protein

Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm. To deliver their nucleocapsid into the host cell, they rely on the fusion of their envelope with the host cell membrane. The spike glycoprotein (S) mediates virus entry and is a primary determinant of cell tropism and pathogenesis. It is classified as a class I fusion protein, and is responsible for binding to the receptor on the host cell as well as mediating the fusion of host and viral membranes—A process driven by major conformational changes of the S protein. This review discusses coronavirus entry mechanisms focusing on the different triggers used by coronaviruses to initiate the conformational change of the S protein: receptor binding, low pH exposure and proteolytic activation. We also highlight commonalities between coronavirus S proteins and other class I viral fusion proteins, as well as distinctive features that confer distinct tropism, pathogenicity and host interspecies transmission characteristics to coronaviruses.

Differentiated swine airway epithelial cell cultures for the investigation of influenza A virus infection and replication

Please cite this paper as: Bateman et al. (2013) Differentiated swine airway epithelial cell cultures for the investigation of influenza A virus infection and replication. Influenza and Other Respiratory Viruses 7(2) 139–150.

Background  Differentiated human airway epithelial cell cultures have been utilized to investigate cystic fibrosis, wound healing, and characteristics of viral infections. These cultures, grown at an air–liquid interface (ALI) in media with defined hormones and growth factors, recapitulate many aspects of the in vivo respiratory tract and allow for experimental studies at the cellular level.

Objectives  To optimize growth conditions for differentiated swine airway epithelial cultures and to use these cultures to examine influenza virus infection and replication.

Methods  Primary swine respiratory epithelial cells were grown at an air–liquid interface with varying amounts of retinoic acid and epidermal growth factor. Cells grown with optimized concentrations of these factors for 4 weeks differentiated into multilayer epithelial cell cultures resembling the lining of the swine respiratory tract. Influenza virus infection and replication were examined in these cultures.

Results/Conclusions  Retinoic acid promoted ciliogenesis, whereas epidermal growth factor controlled the thickness of the pseudoepithelium. The optimal concentrations for differentiated swine cell cultures were 1·5 ng/ml epidermal growth factor and 100 nm retinoic acid. Influenza A viruses infected and productively replicated in these cultures in the absence of exogenous trypsin, suggesting that the cultures express a protease capable of activating influenza virus hemagglutinin. Differences in virus infection and replication characteristics found previously in pigs in vivo were recapitulated in the swine cultures. This system could be a useful tool for a range of applications, including investigating influenza virus species specificity, defining cell tropism of influenza viruses in the swine respiratory epithelium, and studying other swine respiratory diseases.

Adaptive pathways of zoonotic influenza viruses: from exposure to establishment in humans

Human influenza viruses have their ultimate origin in avian reservoirs and may adapt, either directly or after passage through another mammalian species, to circulate independently in the human population. Three sets of barriers must be crossed by a zoonotic influenza virus before it can become a human virus: animal-to-human transmission barriers; virus-cell interaction barriers; and human-to-human transmission barriers. Adaptive changes allowing zoonotic influenza viruses to cross these barriers have been studied extensively, generating key knowledge for improved pandemic preparedness. Most of these adaptive changes link acquired genetic alterations of the virus to specific adaptation mechanisms that can be screened for, both genetically and phenotypically, as part of zoonotic influenza virus surveillance programs. Human-to-human transmission barriers are only sporadically crossed by zoonotic influenza viruses, eventually triggering a worldwide influenza outbreak or pandemic. This is the most devastating consequence of influenza virus cross-species transmission. Progress has been made in identifying some of the determinants of influenza virus transmissibility. However, interdisciplinary research is needed to further characterize these ultimate barriers to the development of influenza pandemics, at both the level of the individual host and that of the population.

H5N1 influenza virus-like particles produced by transient expression in mammalian cells induce humoral and cellular immune responses in mice

Vaccination is an effective way to protect from influenza virus infection. Among the new candidates of influenza vaccines, influenza virus-like particles (VLPs) seem to be promising. Here, we generated 2 types of H5N1 influenza VLPs by co-expressing influenza virus Env (envelope protein) and murine leukemia virus (MLV) Gag–Pol. VLPs generated by co-transfection of pHCMV-wtH5 or pHCMV-mtH5 with pSV-Mo-MLVgagpol and pHCMV-N1 were named as wtH5N1 VLPs or mtH5N1 VLPs. The plasmid of pHCMV-wtH5 encoded the wild-type hemagglutinin (HA) (wtH5) from A/swine/Anhui/ca/2004 (H5N1) with a multibasic cleavage site, while pHCMV-mtH5 encoded the modified mutant-type (mtH5) with a monobasic cleavage site. Influenza virus HA VLPs were characterized and equal amounts of them were used to immunize mice subcutaneously, intraperitoneally, or intramuscularly. The levels of HA-specific IgG1, IFN-γ, and neutralization antibodies were significantly induced in mice immunized with wtH5N1 VLPs or mtH5N1 VLPs via all 3 routes, while HA-specific IgG2a was barely detectable. IL-4 secretion was detected in mice subcutaneously immunized with wtH5N1 VLPs or mtH5N1 VLPs, or intramuscularly immunized with mtH5N1 VLPs. Our results indicated that both H5N1 influenza VLPs could induce specific humoral and cellular immune responses in immunized mice. In conclusion, our study provides helpful information for designing new candidate vaccines against H5N1 influenza viruses.

Genetic evolution of the H9N2 avian influenza virus in Korean poultry farms

We performed whole genome sequencing of 22 H9N2 avian influenza viruses (AIV) isolated from domestic laying hens on farms between 2005 and 2008, and compared the sequences with viruses previously reported in Asia. A previous study revealed that two antigenically distinct sublineages were established within the MS96 lineage by antigenic drift since the first H9N2 AIV outbreak in South Korea. We designated them as the 01310-like lineage and the 116/04-like lineage. Since late 2004, most identified isolates in Korea have belonged to the 116/04-like lineage, however, in this study we found that six among twenty-two isolates were belonged to 01310-like lineage, indicating that the genetic divergence is still occurring after 2004. Furthermore, it is noteworthy that five isolates among the defined 01310-like lineage had a 24 amino acid deletion in the neuraminidase stalk region, which were not in any other H9N2 isolates previously reported. The internal genes analysis demonstrated extensive reassortment events among isolates from poultry farms, live bird markets, and wild birds, and multiple new genotypes were identified. We identified several features of gene evolution in H9N2 AIV suggesting that the long-term H9N2 AIV surveillance study should be continued in South Korea.

Systemic virus distribution and host responses in brain and intestine of chickens infected with low pathogenic or high pathogenic avian influenza virus

BACKGROUND

Avian influenza virus (AIV) is classified into two pathotypes, low pathogenic (LP) and high pathogenic (HP), based on virulence in chickens.Differences in pathogenicity between HPAIV and LPAIV might eventually be related to specific characteristics of strains, tissue tropism and host responses.

METHODS

To study differences in disease development between HPAIV and LPAIV, we examined the first appearance and eventual load of viral RNA in multiple organs as well as host responses in brain and intestine of chickens infected with two closely related H7N1 HPAIV or LPAIV strains.

RESULTS

Both H7N1 HPAIV and LPAIV spread systemically in chickens after a combined intranasal/intratracheal inoculation. In brain, large differences in viral RNA load and host gene expression were found between H7N1 HPAIV and LPAIV infected chickens. Chicken embryo brain cell culture studies revealed that both HPAIV and LPAIV could infect cultivated embryonic brain cells, but in accordance with the absence of the necessary proteases, replication of LPAIV was limited. Furthermore, TUNEL assay indicated apoptosis in brain of HPAIV infected chickens only. In intestine, where endoproteases that cleave HA of LPAIV are available, we found minimal differences in the amount of viral RNA and a large overlap in the transcriptional responses between HPAIV and LPAIV infected chickens. Interestingly, brain and ileum differed clearly in the cellular pathways that were regulated upon an AI infection.

CONCLUSIONS

Although both H7N1 HPAIV and LPAIV RNA was detected in a broad range of tissues beyond the respiratory and gastrointestinal tract, our observations indicate that differences in pathogenicity and mortality between HPAIV and LPAIV could originate from differences in virus replication and the resulting host responses in vital organs like the brain.

The role of cell tropism for the pathogenesis of influenza in humans

Seasonal, pandemic and avian influenza viruses are able to infect humans, but the disease outcome often differs, ranging from mild upper respiratory tract disease to fatal pneumonia. The cell tropism of influenza viruses is thought to be an important determinant of these factors. Therefore, this review focuses on the factors that, together, determine the cell tropism of influenza viruses. These include: the receptor specificity of the viral hemagglutinin and the distribution of these receptors in the respiratory tract; the presence of inhibitory factors in the fluid lining the respiratory mucosa; and the requirement for host cell proteases that can cleave the precursor hemagglutinin of influenza viruses. Finally, we will discuss how the route of inoculation influences the cell types infected by influenza viruses and associated pathogenesis.

Genetics, evolution, and the zoonotic capacity of European Swine influenza viruses

The European swine influenza virus lineage differs genetically from the classical swine influenza viruses and the triple reassortants found in North America and Asia. The avian-like swine H1N1 viruses emerged in 1979 after an avian-to-swine transmission and spread to all major European pig-producing countries. Reassortment of these viruses with seasonal H3N2 viruses led to human-like swine H3N2 viruses which appeared in 1984. Finally, human-like swine H1N2 viruses emerged in 1994. These are triple reassortants comprising genes of avian-like H1N1, seasonal H1N1, and seasonal H3N2 viruses. All three subtypes established persistent infection chains and became prevalent in the European pig population. They successively replaced the circulating classical swine H1N1 viruses of that time and gave rise to a number of reassortant viruses including the pandemic (H1N1) 2009 virus. All three European lineages have the capacity to infect humans but zoonotic infections are benign.

Influenza viruses: from birds to humans

Avian influenza viruses are the precursors of human influenza A viruses. They may be transmitted directly from avian reservoirs, or infect other mammalian species before subsequent transmission to their human host. So far, avian influenza viruses have caused sporadic-yet increasingly more frequently recognized-cases of infection in humans. They have to adapt to and circulate efficiently in human populations, before they may trigger a worldwide human influenza outbreak or pandemic. Cross-species transmission of avian influenza viruses from their reservoir hosts-wild waterbirds-to terrestrial poultry and to humans is based on different modes of transmission and results in distinctive pathogenetic manifestations, which are reviewed in this paper.

The multibasic cleavage site of the hemagglutinin of highly pathogenic A/Vietnam/1203/2004 (H5N1) avian influenza virus acts as a virulence factor in a host-specific manner in mammals

Highly pathogenic avian influenza (HPAI) viruses of the H5 and H7 subtypes typically possess multiple basic amino acids around the cleavage site (MBS) of their hemagglutinin (HA) protein, a recognized virulence motif in poultry. To determine the importance of the H5 HA MBS as a virulence factor in mammals, recombinant wild-type HPAI A/Vietnam/1203/2004 (H5N1) viruses that possessed (H5N1) or lacked (ΔH5N1) the H5 HA MBS were generated and evaluated for their virulence in BALB/c mice, ferrets, and African green monkeys (AGMs) (Chlorocebus aethiops). The presence of the H5 HA MBS was associated with lethality, significantly higher virus titers in the respiratory tract, virus dissemination to extrapulmonary organs, lymphopenia, significantly elevated levels of proinflammatory cytokines and chemokines, and inflammation in the lungs of mice and ferrets. In AGMs, neither H5N1 nor ΔH5N1 virus was lethal and neither caused clinical symptoms. The H5 HA MBS was associated with mild enhancement of replication and delayed virus clearance. Thus, the contribution of H5 HA MBS to the virulence of the HPAI H5N1 virus varies among mammalian hosts and is most significant in mice and ferrets and less remarkable in nonhuman primates.

Polyvalent DNA vaccines expressing HA antigens of H5N1 influenza viruses with an optimized leader sequence elicit cross-protective antibody responses

Highly pathogenic avian influenza A (HPAI) H5N1 viruses are circulating among poultry populations in parts of Asia, Africa, and the Middle East, and have caused human infections with a high mortality rate. H5 subtype hemagglutinin (HA) has evolved into phylogenetically distinct clades and subclades based on viruses isolated from various avian species. Since 1997, humans have been infected by HPAI H5N1 viruses from several clades. It is, therefore, important to develop strategies to produce protective antibody responses against H5N1 viruses from multiple clades or antigenic groups. In the current study, we optimized the signal peptide design of DNA vaccines expressing HA antigens from H5N1 viruses. Cross reactivity analysis using sera from immunized rabbits showed that antibody responses elicited by a polyvalent formulation, including HA antigens from different clades, was able to elicit broad protective antibody responses against multiple key representative H5N1 viruses across different clades. Data presented in this report support the development of a polyvalent DNA vaccine strategy against the threat of a potential H5N1 influenza pandemic.

Identification of host genes linked with the survivability of chickens infected with recombinant viruses possessing H5N1 surface antigens from a highly pathogenic avian influenza virus

Seventeen recombinant viruses were generated by a reverse genetic technique to elucidate the pathogenicity of highly pathogenic avian influenza viruses (HPAIVs) in chickens. The recombinant viruses generated possessed hemagglutinin (HA) and neuraminidase (NA) genes from an HPAIV. Other segments were combinations of the genes from an HPAIV and two low-pathogenic avian influenza viruses (LPAIVs) derived from chicken (LP) and wild bird (WB). Exchange of whole internal genes from an HPAIV with those of an LPAIV resulted in a significant extension of the survival time following intranasal infection of the chickens with the recombinants. Survival analysis demonstrated that the exchange of a gene segment affected survivability of the chickens with statistical significance. The analysis revealed three groups of recombinants with various gene constellations that depended upon the survivability of the infected chickens. Recombinants where the PA gene was exchanged from LP to WB in the LP gene background, LP (W/PA), did not kill any chickens. LP (W/PA) replicated less efficiently both in vitro and in vivo, suggesting that the intrinsic replication ability of LP (W/PA) affects pathogenicity; however, such a correlation was not seen for the other recombinants. Microarray analysis of the infected chicken lungs indicated that the expression of 7 genes, CD274, RNF19B, OASL, ZC3HAV1 [corrected] , PLA2G6, GCH1, and USP18, correlated with the survivability of the chickens infected (P < 0.01). Further analysis of the functions of these genes in chickens would aid in the understanding of host gene responses following fatal infections by HPAIVs.

Human pulmonary microvascular endothelial cells support productive replication of highly pathogenic avian influenza viruses: possible involvement in the pathogenesis of human H5N1 virus infection

Highly pathogenic avian influenza (HPAI) H5N1 viruses continue to cause sporadic human infections with a high fatality rate. Respiratory failure due to acute respiratory distress syndrome (ARDS) is a complication among hospitalized patients. Since progressive pulmonary endothelial damage is the hallmark of ARDS, we investigated host responses following HPAI virus infection of human pulmonary microvascular endothelial cells. Evaluation of these cells for the presence of receptors preferred by influenza virus demonstrated that avian-like (α2-3-linked) receptors were more abundant than human-like (α2-6-linked) receptors. To test the permissiveness of pulmonary endothelial cells to virus infection, we compared the replication of selected seasonal, pandemic (2009 H1N1 and 1918), and potentially pandemic (H5N1) influenza virus strains. We observed that these cells support productive replication only of HPAI H5N1 viruses, which preferentially enter through and are released from the apical surface of polarized human endothelial monolayers. Furthermore, A/Thailand/16/2004 and A/Vietnam/1203/2004 (VN/1203) H5N1 viruses, which exhibit heightened virulence in mammalian models, replicated to higher titers than less virulent H5N1 strains. VN/1203 infection caused a significant decrease in endothelial cell proliferation compared to other subtype viruses. VN/1203 virus was also found to be a potent inducer of cytokines and adhesion molecules known to regulate inflammation during acute lung injury. Deletion of the H5 hemagglutinin (HA) multibasic cleavage site did not affect virus infectivity but resulted in decreased virus replication in endothelial cells. Our results highlight remarkable tropism and infectivity of the H5N1 viruses for human pulmonary endothelial cells, resulting in the potent induction of host inflammatory responses.

Coexpression of avian influenza virus H5 and N1 by recombinant Newcastle disease virus and the impact on immune response in chickens

To analyze the contribution of neuraminidase (NA) toward protection against avian influenza virus (AIV) infection, three different recombinant Newcastle disease viruses (NDVs) expressing hemagglutinin (HA) or NA, or both, of highly pathogenic avian influenza virus (HPAIV) were generated. The lentogenic NDV Clone 30 was used as backbone for the insertion of HA of HPAIV strain A/chicken/Vietnam/P41/05 (H5N1) and NA of HPAIV strain A/duck/Vietnam/TG24-01/05 (H5N1). The HA was inserted between the genes encoding NDV phosphoprotein (P) and matrixprotein (M), and the NA was inserted between the fusion (F) and hemagglutinin-neuraminidase protein (HN) genes, resulting in NDVH5VmPMN1FHN. Two additional recombinants were constructed carrying the HA gene between the NDV P and M genes (NDVH5VmPM) or the NA between F and HN (NDVN1FHN). All recombinants replicated well and stably expressed the HA gene, the NA gene, or both. Chickens immunized with NDVH5VmPMN1FHN or NDVH5VmPM were protected against two different HPAIV H5N1 and also against HPAIV H5N2. In contrast, immunization of chickens with NDVN1FHN induced NDV- and AIV N1-specific antibodies but did not protect the animals against a lethal dose of HPAIV H5N1. Furthermore, expression of AIV N1, in addition to AIV H5 by NDV, did not increase protection against HPAIV H5N1.