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.     

Isolation and characterization of avian influenza viruses, including highly pathogenic H5N1, from poultry in live bird markets in Hanoi, Vietnam, in 2001

Since 1997, outbreaks of highly pathogenic (HP) H5N1 and circulation of H9N2 viruses among domestic poultry in Asia have posed a threat to public health. To better understand the extent of transmission of avian influenza viruses (AIV) to humans in Asia, we conducted a cross-sectional virologic study in live bird markets (LBM) in Hanoi, Vietnam, in October 2001. Specimens from 189 birds and 18 environmental samples were collected at 10 LBM. Four influenza A viruses of the H4N6 (n = 1), H5N2 (n = 1), and H9N3 (n = 2) subtypes were isolated from healthy ducks for an isolation frequency of over 30% from this species. Two H5N1 viruses were isolated from healthy geese. The hemagglutinin (HA) genes of these H5N1 viruses possessed multiple basic amino acid motifs at the cleavage site, were HP for experimentally infected chickens, and were thus characterized as HP AIV. These HA genes shared high amino acid identities with genes of other H5N1 viruses isolated in Asia during this period, but they were genetically distinct from those of H5N1 viruses isolated from poultry and humans in Vietnam during the early 2004 outbreaks. These viruses were not highly virulent for experimentally infected ducks, mice, or ferrets. These results establish that HP H5N1 viruses with properties similar to viruses isolated in Hong Kong and mainland China circulated in Vietnam as early as 2001, suggest a common source for H5N1 viruses circulating in these Asian countries, and provide a framework to better understand the recent widespread emergence of HP H5N1 viruses in Asia.     

Avian influenza (H5N1) viruses isolated from humans in Asia in 2004 exhibit increased virulence in mammals

The spread of highly pathogenic avian influenza H5N1 viruses across Asia in 2003 and 2004 devastated domestic poultry populations and resulted in the largest and most lethal H5N1 virus outbreak in humans to date. To better understand the potential of H5N1 viruses isolated during this epizootic event to cause disease in mammals, we used the mouse and ferret models to evaluate the relative virulence of selected 2003 and 2004 H5N1 viruses representing multiple genetic and geographical groups and compared them to earlier H5N1 strains isolated from humans. Four of five human isolates tested were highly lethal for both mice and ferrets and exhibited a substantially greater level of virulence in ferrets than other H5N1 viruses isolated from humans since 1997. One human isolate and all four avian isolates tested were found to be of low virulence in either animal. The highly virulent viruses replicated to high titers in the mouse and ferret respiratory tracts and spread to multiple organs, including the brain. Rapid disease progression and high lethality rates in ferrets distinguished the highly virulent 2004 H5N1 viruses from the 1997 H5N1 viruses. A pair of viruses isolated from the same patient differed by eight amino acids, including a Lys/Glu disparity at 627 of PB2, previously identified as an H5N1 virulence factor in mice. The virus possessing Glu at 627 of PB2 exhibited only a modest decrease in virulence in mice and was highly virulent in ferrets, indicating that for this virus pair, the K627E PB2 difference did not have a prevailing effect on virulence in mice or ferrets. Our results demonstrate the general equivalence of mouse and ferret models for assessment of the virulence of 2003 and 2004 H5N1 viruses. However, the apparent enhancement of virulence of these viruses in humans in 2004 was better reflected in the ferret.     

Lethality to ferrets of H5N1 influenza viruses isolated from humans and poultry in 2004

The 2004 outbreaks of H5N1 influenza viruses in Vietnam and Thailand were highly lethal to humans and to poultry; therefore, newly emerging avian influenza A viruses pose a continued threat, not only to avian species but also to humans. We studied the pathogenicity of four human and nine avian H5N1/04 influenza viruses in ferrets (an excellent model for influenza studies). All four human isolates were fatal to intranasally inoculated ferrets. The human isolate A/Vietnam/1203/04 (H5N1) was the most pathogenic isolate; the severity of disease was associated with a broad tissue tropism and high virus titers in multiple organs, including the brain. High fever, weight loss, anorexia, extreme lethargy, and diarrhea were observed. Two avian H5N1/04 isolates were as pathogenic as the human viruses, causing lethal systemic infections in ferrets. Seven of nine H5N1/04 viruses isolated from avian species caused mild infections, with virus replication restricted to the upper respiratory tract. All chicken isolates were nonlethal to ferrets. A sequence analysis revealed polybasic amino acids in the hemagglutinin connecting peptides of all H5N1/04 viruses, indicating that multiple molecular differences in other genes are important for a high level of virulence. Interestingly, the human A/Vietnam/1203/04 isolate had a lysine substitution at position 627 of PB2 and had one to eight amino acid changes in all gene products except that of the M1 gene, unlike the A/chicken/Vietnam/C58/04 and A/quail/Vietnam/36/04 viruses. Our results indicate that viruses that are lethal to mammals are circulating among birds in Asia and suggest that pathogenicity in ferrets, and perhaps humans, reflects a complex combination of different residues rather than a single amino acid difference.     

Clue to the molecular mechanism of virulence of highly pathogenic H5N1 avian influenza viruses isolated in 2004

Highly pathogenic avian H5N1 influenza A viruses have spread throughout Asia since 2003. These viruses are highly lethal to birds and humans. Of the 74 confirmed human cases, 49 were fatal (as of Mar 30, 2005), raising concerns of a possible pandemic by these viruses. Despite the well-established pathogenicity of these viruses, the molecular mechanism for expressing such high virulence remains elusive. Thus, we examined the pathogenicity of the H5N1 viruses isolated in Vietnam in 2003-2004 using animal models (mouse, duck, and ferret). Viruses from humans were generally more pathogenic in mice and ferrets than those from birds. Indeed, one human isolate was even lethal to ferrets. The human isolate possessing Lys at amino acid position 627 of PB2 was more virulent than that possessing Glu at this position, underscoring the importance of Lys at this position 627 of PB2 for efficient growth in mammals.     

Oligomeric Recombinant H5 HA1 Vaccine Produced in Bacteria Protects Ferrets from Homologous and Heterologous Wild-Type H5N1 Influenza Challenge and Controls Viral Loads Better than Subunit H5N1 Vaccine by Eliciting High-Affinity Antibodies

Recombinant hemagglutinin from influenza viruses with pandemic potential can be produced rapidly in various cell substrates. In this study, we compared the functionality and immunogenicity of bacterially produced oligomeric or monomeric HA1 proteins from H5N1 (A/Vietnam/1203/04) with those of the egg-based licensed subunit H5N1 (SU-H5N1) vaccine in ferrets challenged with homologous or heterologous H5N1 highly pathogenic influenza strains. Ferrets were vaccinated twice with the oligomeric or monomeric rHA1 or with SU-H5N1 (Sanofi Pasteur) emulsified with Titermax adjuvant and were challenged with wild-type homologous (A/Vietnam/1203/04; clade 1) or heterologous (A/Whooperswan/Mongolia/244/2005; clade 2.2) virus. Only the oligomeric rHA1 (not the monomeric rHA1) immunogen and the SU-H5N1 vaccine provided protection against the lethality and morbidity of homologous and heterologous highly pathogenic H5N1. Oligomeric rHA1 generated more cross-neutralizing antibodies and higher levels of serum antibody binding to HA1, with stronger avidity and a better IgG/IgM ratio, than monomeric HA1 and SU-H5N1 vaccines, as determined by surface plasmon resonance (SPR). Importantly, viral loads after heterologous H5N1 challenge were more efficiently controlled in ferrets vaccinated with the oligomeric rHA1 immunogen than in SU-H5N1-vaccinated ferrets. The reduction of viral loads in the nasal washes correlated strongly with higher-avidity antibodies to oligomeric rHA1 derived from H5N1 clade 1 and clade 2.2 viruses, as measured by SPR. This is the first study to show the role of antibody avidity for the HA1 globular head domain in reduction of viral loads in the upper respiratory tract, which could significantly reduce viral transmission.     

Evaluation of the efficacy of a pre-pandemic H5N1 vaccine (MG1109) in mouse and ferret models

The threat of a highly pathogenic avian influenza (HPAI) H5N1 virus causing the next pandemic remains a major concern. In this study, we evaluated the immunogenicity and efficacy of an inactivated whole-virus H5N1 pre-pandemic vaccine (MG1109) formulated by Green Cross Co., Ltd containing the hemagglutinin (HA) and neuraminidase (NA) genes of the clade 1 A/Vietnam/1194/04 virus in the backbone of A/Puerto Rico/8/34 (RgVietNam/04xPR8/34). Administration of the MG1109 vaccine (2-doses) in mice and ferrets elicited high HI and SN titers in a dose-dependent manner against the homologous (RgVietNam/04xPR8/34) and various heterologous H5N1 strains, (RgKor/W149/06xPR8/34, RgCambodia/04xPR8/34, RgGuangxi/05xPR8/34), including a heterosubtypic H5N2 (A/Aquatic bird/orea/W81/05) virus. However, efficient cross-reactivity was not observed against heterosubtypic H9N2 (A/Ck/Korea/H0802/08) and H1N1 (PR/8/34) viruses. Mice immunized with 1.9 μg HA/dose of MG1109 were completely protected from lethal challenge with heterologous wild-type HPAI H5N1 A/EM/Korea/W149/06 (clade 2.2) and mouse-adapted H5N2 viruses. Furthermore, ferrets administered at least 3.8 μg HA/dose efficiently suppressed virus growth in the upper respiratory tract and lungs. Vaccinated mice and ferrets also demonstrated attenuation of clinical disease signs and limited virus spread to other organs. Thus, this vaccine provided immunogenic responses in mouse and ferret models even against challenge with heterologous HPAI H5N1 and H5N2 viruses. Since the specific strain of HPAI H5N1 virus that would potentially cause the next outbreak is unknown, pre-pandemic vaccine preparation that could provide cross-protection against various H5 strains could be a useful approach in the selection of promising candidate vaccines in the future.     

Lethal H5N1 influenza viruses escape host anti-viral cytokine responses

The H5N1 influenza viruses transmitted to humans in 1997 were highly virulent, but the mechanism of their virulence in humans is largely unknown. Here we show that lethal H5N1 influenza viruses, unlike other human, avian and swine influenza viruses, are resistant to the antiviral effects of interferons and tumor necrosis factor alpha. The nonstructural (NS) gene of H5N1 viruses is associated with this resistance. Pigs infected with recombinant human H1N1 influenza virus that carried the H5N1 NS gene experienced significantly greater and more prolonged viremia, fever and weight loss than did pigs infected with wild-type human H1N1 influenza virus. These effects required the presence of glutamic acid at position 92 of the NS1 molecule. These findings may explain the mechanism of the high virulence of H5N1 influenza viruses in humans.     

Preclinical Evaluation of a Replication-Deficient Intranasal ΔNS1 H5N1 Influenza Vaccine

Background

We developed a novel intranasal influenza vaccine approach that is based on the construction of replication-deficient vaccine viruses that lack the entire NS1 gene (ΔNS1 virus). We previously showed that these viruses undergo abortive replication in the respiratory tract of animals. The local release of type I interferons and other cytokines and chemokines in the upper respiratory tract may have a “self-adjuvant effect”, in turn increasing vaccine immunogenicity. As a result, ΔNS1 viruses elicit strong B- and T- cell mediated immune responses.

Methodology/Principal Findings

We applied this technology to the development of a pandemic H5N1 vaccine candidate. The vaccine virus was constructed by reverse genetics in Vero cells, as a 5∶3 reassortant, encoding four proteins HA, NA, M1, and M2 of the A/Vietnam/1203/04 virus while the remaining genes were derived from IVR-116. The HA cleavage site was modified in a trypsin dependent manner, serving as the second attenuation factor in addition to the deleted NS1 gene. The vaccine candidate was able to grow in the Vero cells that were cultivated in a serum free medium to titers exceeding 8 log₁₀ TCID_50/ml. The vaccine virus was replication deficient in interferon competent cells and did not lead to viral shedding in the vaccinated animals. The studies performed in three animal models confirmed the safety and immunogenicity of the vaccine. Intranasal immunization protected ferrets and mice from being infected with influenza H5 viruses of different clades. In a primate model (Macaca mulatta), one dose of vaccine delivered intranasally was sufficient for the induction of antibodies against homologous A/Vietnam/1203/04 and heterologous A/Indonesia/5/05 H5N1 strains.

Conclusion/Significance

Our findings show that intranasal immunization with the replication deficient H5N1 ΔNS1 vaccine candidate is sufficient to induce a protective immune response against H5N1 viruses. This approach might be attractive as an alternative to conventional influenza vaccines. Clinical evaluation of ΔNS1 pandemic and seasonal influenza vaccine candidates are currently in progress.     

Evolution of highly pathogenic H5N1 avian influenza viruses in Vietnam between 2001 and 2007

Highly pathogenic avian influenza (HPAI) H5N1 viruses have caused dramatic economic losses to the poultry industry of Vietnam and continue to pose a serious threat to public health. As of June 2008, Vietnam had reported nearly one third of worldwide laboratory confirmed human H5N1 infections. To better understand the emergence, spread and evolution of H5N1 in Vietnam we studied over 300 H5N1 avian influenza viruses isolated from Vietnam since their first detection in 2001. Our phylogenetic analyses indicated that six genetically distinct H5N1 viruses were introduced into Vietnam during the past seven years. The H5N1 lineage that evolved following the introduction in 2003 of the A/duck/Hong Kong/821/2002-like viruses, with clade 1 hemagglutinin (HA), continued to predominate in southern Vietnam as of May 2007. A virus with a clade 2.3.4 HA newly introduced into northern Vietnam in 2007, reassorted with pre-existing clade 1 viruses, resulting in the emergence of novel genotypes with neuraminidase (NA) and/or internal gene segments from clade 1 viruses. A total of nine distinct genotypes have been present in Vietnam since 2001, including five that were circulating in 2007. At least four of these genotypes appear to have originated in Vietnam and represent novel H5N1 viruses not reported elsewhere. Geographic and temporal analyses of H5N1 infection dynamics in poultry suggest that the majority of viruses containing new genes were first detected in northern Vietnam and subsequently spread to southern Vietnam after reassorting with pre-existing local viruses in northern Vietnam. Although the routes of entry and spread of H5N1 in Vietnam remain speculative, enhanced poultry import controls and virologic surveillance efforts may help curb the entry and spread of new HPAI viral genes.     

Detection and Characterization of Clade 1 Reassortant H5N1 Viruses Isolated from Human Cases in Vietnam during 2013

Highly pathogenic avian influenza (HPAI) H5N1 is endemic in Vietnamese poultry and has caused sporadic human infection in Vietnam since 2003. Human infections with HPAI H5N1 are of concern due to a high mortality rate and the potential for the emergence of pandemic viruses with sustained human-to-human transmission. Viruses isolated from humans in southern Vietnam have been classified as clade 1 with a single genome constellation (VN3) since their earliest detection in 2003. This is consistent with detection of this clade/genotype in poultry viruses endemic to the Mekong River Delta and surrounding regions. Comparison of H5N1 viruses detected in humans from southern Vietnamese provinces during 2012 and 2013 revealed the emergence of a 2013 reassortant virus with clade 1.1.2 hemagglutinin (HA) and neuraminidase (NA) surface protein genes but internal genes derived from clade 2.3.2.1a viruses (A/Hubei/1/2010-like; VN12). Closer analysis revealed mutations in multiple genes of this novel genotype (referred to as VN49) previously associated with increased virulence in animal models and other markers of adaptation to mammalian hosts. Despite the changes identified between the 2012 and 2013 genotypes analyzed, their virulence in a ferret model was similar. Antigenically, the 2013 viruses were less cross-reactive with ferret antiserum produced to the clade 1 progenitor virus, A/Vietnam/1203/2004, but reacted with antiserum produced against a new clade 1.1.2 WHO candidate vaccine virus (A/Cambodia/W0526301/2012) with comparable hemagglutination inhibition titers as the homologous antigen. Together, these results indicate changes to both surface and internal protein genes of H5N1 viruses circulating in southern Vietnam compared to 2012 and earlier viruses.     

Influenza H5 hemagglutinin DNA primes the antibody response elicited by the live attenuated influenza A/Vietnam/1203/2004 vaccine in ferrets

Priming immunization plays a key role in protecting individuals or populations to influenza viruses that are novel to humans. To identify the most promising vaccine priming strategy, we have evaluated different prime-boost regimens using inactivated, DNA and live attenuated vaccines in ferrets. Live attenuated influenza A/Vietnam/1203/2004 (H5N1) candidate vaccine (LAIV, VN04 ca) primed ferrets efficiently while inactivated H5N1 vaccine could not prime the immune response in seronegative ferrets unless an adjuvant was used. However, the H5 HA DNA vaccine alone was as successful as an adjuvanted inactivated VN04 vaccine in priming the immune response to VN04 ca virus. The serum antibody titers of ferrets primed with H5 HA DNA followed by intranasal vaccination of VN04 ca virus were comparable to that induced by two doses of VN04 ca virus. Both LAIV-LAIV and DNA-LAIV vaccine regimens could induce antibody responses that cross-neutralized antigenically distinct H5N1 virus isolates including A/HongKong/213/2003 (HK03) and prevented nasal infection of HK03 vaccine virus. Thus, H5 HA DNA vaccination may offer an alternative option for pandemic preparedness.     

Pathogenicity and Transmissibility of North American Triple Reassortant Swine Influenza A Viruses in Ferrets

North American triple reassortant swine (TRS) influenza A viruses have caused sporadic human infections since 2005, but human-to-human transmission has not been documented. These viruses have six gene segments (PB2, PB1, PA, HA, NP, and NS) closely related to those of the 2009 H1N1 pandemic viruses. Therefore, understanding of these viruses'' pathogenicity and transmissibility may help to identify determinants of virulence of the 2009 H1N1 pandemic viruses and to elucidate potential human health threats posed by the TRS viruses. Here we evaluated in a ferret model the pathogenicity and transmissibility of three groups of North American TRS viruses containing swine-like and/or human-like HA and NA gene segments. The study was designed only to detect informative and significant patterns in the transmissibility and pathogenicity of these three groups of viruses. We observed that irrespective of their HA and NA lineages, the TRS viruses were moderately pathogenic in ferrets and grew efficiently in both the upper and lower respiratory tracts. All North American TRS viruses studied were transmitted between ferrets via direct contact. However, their transmissibility by respiratory droplets was related to their HA and NA lineages: TRS viruses with human-like HA and NA were transmitted most efficiently, those with swine-like HA and NA were transmitted minimally or not transmitted, and those with swine-like HA and human-like NA (N2) showed intermediate transmissibility. We conclude that the lineages of HA and NA may play a crucial role in the respiratory droplet transmissibility of these viruses. These findings have important implications for pandemic planning and warrant confirmation.     

Epitope mapping of the hemagglutinin molecule of a highly pathogenic H5N1 influenza virus by using monoclonal antibodies

We mapped the hemagglutinin (HA) antigenic epitopes of a highly pathogenic H5N1 influenza virus on the three-dimensional HA structure by characterizing escape mutants of a recombinant virus containing A/Vietnam/1203/04 (H5N1) ΔHA and neuraminidase genes in the genetic background of A/Puerto Rico/8/34 (H1N1) virus. The mutants were selected with a panel of eight anti-HA monoclonal antibodies (MAbs), seven to A/Vietnam/1203/04 (H5N1) virus and one to A/Chicken/Pennsylvania/8125/83 (H5N2) virus, and the mutants’ HA genes were sequenced. The amino acid changes suggested three MAb groups: four MAbs reacted with the complex epitope comprising parts of the antigenic site B of H3 HA and site Sa of H1 HA, two MAbs reacted with the epitope corresponding to the antigenic site A in H3 HA, and two MAbs displayed unusual behavior: each recognized amino acid changes at two widely separate antigenic sites. Five changes were detected in amino acid residues not previously reported as changed in H5 escape mutants, and four others had substitutions not previously described. The HA antigenic structure differs substantially between A/Vietnam/1203/04 (H5N1) virus and the low-pathogenic A/Mallard/Pennsylvania/10218/84 (H5N2) virus we previously characterized (N. V. Kaverin et al., J. Gen. Virol. 83:2497-2505, 2002). The hemagglutination inhibition reactions of the MAbs with recent highly pathogenic H5N1 viruses were consistent with the antigenic-site amino acid changes but not with clades and subclades based on H5 phylogenetic analysis. These results provide information on the recognition sites of the MAbs widely used to study H5N1 viruses and demonstrate the involvement of the HA antigenic sites in the evolution of highly pathogenic H5N1 viruses, findings that can be critical for characterizing pathogenesis and vaccine design.     

Permissible variation in the 3' non-coding region of the haemagglutinin genome segment of the H5N1 candidate influenza vaccine virus NIBRG-14 [corrected

The candidate H5N1 vaccine virus NIBRG-14 was created in response to a call from the World Health Organisation in 2004 to prepare candidate vaccine viruses (CVVs) to combat the threat of an H5N1 pandemic. NIBRG-14 was created by reverse genetics and is composed of the neuraminidase (NA) and modified haemagglutinin (HA) genes from A/Vietnam/1194/2004 and the internal genes of PR8, a high growing laboratory adapted influenza A(H1N1) strain. Due to time constraints, the non-coding regions (NCRs) of A/Vietnam/1194/2004 HA were not determined prior to creating NIBRG-14. Consequently, the sequence of the primers used to clone the modified A/Vietnam/1194/2004 HA was based upon previous experience of cloning H5N1 viruses. We report here that the HA 3' NCR sequence of NIBRG-14 is different to that of the parental wild type virus A/Vietnam/1194/2004; however this does not appear to impact on its growth or antigen yield. We introduced additional small changes into the 3'NCR of NIBRG-14; these had only minor effects on viral growth and antigen content. These findings may serve to assure the influenza vaccine community that generation of CVVs using best-guess NCR sequences, based on sequence alignments, are likely to produce robust viruses.     

Reverse genetics provides direct evidence for a correlation of hemagglutinin cleavability and virulence of an avian influenza A virus.

To obtain direct evidence for a relationship between hemagglutinin (HA) cleavability and the virulence of avian influenza A viruses, we generated a series of HA cleavage mutants from a virulent virus, A/turkey/Ontario/7732/66 (H5N9), by reverse genetics. A transfectant virus containing the wild-type HA with R-R-R-K-K-R at the cleavage site, which was readily cleaved by endogenous proteases in chicken embryo fibroblasts (CEF), was highly virulent in intramuscularly or intranasally/orally inoculated chickens. By contrast, a mutant containing the HA with an avirulent-like sequence (R-E-T-R) at the cleavage site, which was not cleaved by the proteases in CEF, was avirulent in chickens, indicating that a genetic alteration confined to the HA cleavage site can affect cleavability and virulence. Mutant viruses with HA cleavage site sequences of T-R-R-K-K-R or T-T-R-K-K-R were as virulent as viruses with the wild-type HA, whereas a mutant with a two-amino-acid deletion but retention of four consecutive basic residues (R-K-K-R) was as avirulent as a virus with the avirulent-type HA. Interestingly, although a mutant containing an HA with R-R-R-K-T-R, which has reduced cleavability in CEF, was as virulent as viruses with high HA cleavability when given intramuscularly, it was less virulent when given intranasally/orally. We conclude that the degree of HA cleavability in CEF predicts the virulence of avian influenza viruses.     

Molecular changes in the polymerase genes (PA and PB1) associated with high pathogenicity of H5N1 influenza virus in mallard ducks

The highly pathogenic (HP) influenza viruses H5 and H7 are usually nonpathogenic in mallard ducks. However, the currently circulating HP H5N1 viruses acquired a different phenotype and are able to cause mortality in mallards. To establish the molecular basis of this phenotype, we cloned the human A/Vietnam/1203/04 (H5N1) influenza virus isolate that is highly pathogenic in ferrets, mice, and mallards and found it to be a heterogeneous mixture. Large-plaque isolates were highly pathogenic to ducks, mice, and ferrets, whereas small-plaque isolates were nonpathogenic in these species. Sequence analysis of the entire genome revealed that the small-plaque and the large-plaque isolates differed in the coding of five amino acids. There were two differences in the hemagglutinin (HA) gene (K52T and A544V), one in the PA gene (T515A), and two in the PB1 gene (K207R and Y436H). We inserted the amino acid changes into the wild-type reverse genetic virus construct to assess their effects on pathogenicity in vivo. The HA gene mutations and the PB1 gene K207R mutation did not alter the HP phenotype of the large-plaque virus, whereas constructs with the PA (T515A) and PB1 (Y436H) gene mutations were nonpathogenic in orally inoculated ducks. The PB1 (Y436H) construct was not efficiently transmitted in ducks, whereas the PA (T515A) construct replicated as well as the wild-type virus did and was transmitted efficiently. These results show that the PA and PB1 genes of HP H5N1 influenza viruses are associated with lethality in ducks. The mechanisms of lethality and the perpetuation of this lethal phenotype in ducks in nature remain to be determined.     

Increased pathogenicity of a reassortant 2009 pandemic H1N1 influenza virus containing an H5N1 hemagglutinin

A novel H1N1 influenza virus emerged in 2009 (pH1N1) to become the first influenza pandemic of the 21st century. This virus is now cocirculating with highly pathogenic H5N1 avian influenza viruses in many parts of the world, raising concerns that a reassortment event may lead to highly pathogenic influenza strains with the capacity to infect humans more readily and cause severe disease. To investigate the virulence of pH1N1-H5N1 reassortant viruses, we created pH1N1 (A/California/04/2009) viruses expressing individual genes from an avian H5N1 influenza strain (A/Hong Kong/483/1997). Using several in vitro models of virus replication, we observed increased replication for a reassortant CA/09 virus expressing the hemagglutinin (HA) gene of HK/483 (CA/09-483HA) relative to that of either parental CA/09 virus or reassortant CA/09 expressing other HK/483 genes. This increased replication correlated with enhanced pathogenicity in infected mice similar to that of the parental HK/483 strain. The serial passage of the CA/09 parental virus and the CA/09-483HA virus through primary human lung epithelial cells resulted in increased pathogenicity, suggesting that these viruses easily adapt to humans and become more virulent. In contrast, serial passage attenuated the parental HK/483 virus in vitro and resulted in slightly reduced morbidity in vivo, suggesting that sustained replication in humans attenuates H5N1 avian influenza viruses. Taken together, these data suggest that reassortment between cocirculating human pH1N1 and avian H5N1 influenza strains will result in a virus with the potential for increased pathogenicity in mammals.     

Novel Genotypes of H9N2 Influenza A Viruses Isolated from Poultry in Pakistan Containing NS Genes Similar to Highly Pathogenic H7N3 and H5N1 Viruses

The impact of avian influenza caused by H9N2 viruses in Pakistan is now significantly more severe than in previous years. Since all gene segments contribute towards the virulence of avian influenza virus, it was imperative to investigate the molecular features and genetic relationships of H9N2 viruses prevalent in this region. Analysis of the gene sequences of all eight RNA segments from 12 viruses isolated between 2005 and 2008 was undertaken. The hemagglutinin (HA) sequences of all isolates were closely related to H9N2 viruses isolated from Iran between 2004 and 2007 and contained leucine instead of glutamine at position 226 in the receptor binding pocket, a recognised marker for the recognition of sialic acids linked α2–6 to galactose. The neuraminidase (NA) of two isolates contained a unique five residue deletion in the stalk (from residues 80 to 84), a possible indication of greater adaptation of these viruses to the chicken host. The HA, NA, nucleoprotein (NP), and matrix (M) genes showed close identity with H9N2 viruses isolated during 1999 in Pakistan and clustered in the A/Quail/Hong Kong/G1/97 virus lineage. In contrast, the polymerase genes clustered with H9N2 viruses from India, Iran and Dubai. The NS gene segment showed greater genetic diversity and shared a high level of similarity with NS genes from either H5 or H7 subtypes rather than with established H9N2 Eurasian lineages. These results indicate that during recent years the H9N2 viruses have undergone extensive genetic reassortment which has led to the generation of H9N2 viruses of novel genotypes in the Indian sub-continent. The novel genotypes of H9N2 viruses may play a role in the increased problems observed by H9N2 to poultry and reinforce the continued need to monitor H9N2 infections for their zoonotic potential.     

Homologous recombination as an evolutionary force in the avian influenza A virus

Avian influenza A viruses (AIVs), including the H5N1, H9N2,and H7N7 subtypes, have been directly transmitted to humans,raising concerns over the possibility of a new influenza pandemic.To prevent a future avian influenza pandemic, it is very importantto fully understand the molecular basis driving the change inAIV virulence and host tropism. Although virulent variants ofother viruses have been generated by homologous recombination,the occurrence of homologous recombination within AIV segmentsis controversial and far from proven. This study reports threecirculating H9N2 AIVs with similar mosaic PA genes descendedfrom H9N2 and H5N1. Additionally, many homologous recombinantsare also found deposited in GenBank. Recombination events canoccur in PB2, PB1, PA, HA, and NP segments and between lineagesof the same/different serotype. These results collectively demonstratethat intragenic recombination plays a role in driving the evolutionof AIVs, potentially resulting in effects on AIV virulence andhost tropism changes.