Residue 627 of PB2 is a determinant of cold sensitivity in RNA replication of avian influenza viruses

Replication and transcription activities of ribonucleoprotein complexes reconstituted from avian H5N1, H1N1pdm09 and H3N2 influenza A viruses

Avian influenza viruses pose a serious pandemic threat to humans. Better knowledge on cross-species adaptation is important. This study examined the replication and transcription efficiency of ribonucleoprotein complexes reconstituted by plasmid co-transfection between H5N1, H1N1pdm09 and H3N2 influenza A viruses, and to identify mutations in the RNA polymerase subunit that affect human adaptation. Viral RNA polymerase subunits PB1, PB2, PA and NP derived from influenza viruses were co-expressed with pPolI-vNP-Luc in human cells, and with its function evaluated by luciferase reporter assay. A quantitative RT-PCR was used to measure vRNA, cRNA, and mRNA levels for assessing the replication and transcription efficiency. Mutations in polymerase subunit were created to identify signature of increased human adaptability. H5N1 ribonucleoprotein complexes incorporated with PB2 derived from H1N1pdm09 and H3N2 viruses increased the polymerase activity in human cells. Furthermore, single amino acid substitutions at PB2 of H5N1 could affect polymerase activity in a temperature-dependent manner. By using a highly sensitive quantitative reverse transcription-polymerase chain reaction, an obvious enhancement in replication and transcription activities of ribonucleoproteins was observed by the introduction of lysine at residue 627 in the H5N1 PB2 subunit. Although less strongly in polymerase activity, E158G mutation appeared to alter the accumulation of H5N1 RNA levels in a temperature-dependent manner, suggesting a temperature-dependent mechanism in regulating transcription and replication exists. H5N1 viruses can adapt to humans either by acquisition of PB2 from circulating human-adapted viruses through reassortment, or by mutations at critical sites in PB2. This information may help to predict the pandemic potential of newly emerged influenza strains, and provide a scientific basis for stepping up surveillance measures and vaccine production.

Adaptive mutations in the H5N1 polymerase complex

Adaptation of the viral polymerase to host factors plays an important role in interspecies transmission of H5N1 viruses. Several adaptive mutations have been identified that, in general, determine not only host range, but also pathogenicity and transmissibility of the virus. The available evidence indicates that most of these mutations are found in the PB2 subunit of the polymerase. Particularly prominent mutations are located in the C-terminal domain of PB2 involving the amino acid exchanges E627K and D701N. Both mutations, that are also responsible for the adaptation of other avian viruses to mammalian hosts, have been described in human H5N1 isolates. In animal models, it could be demonstrated that they enhance pathogenicity in mice and induce contact transmission in guinea pigs. Mutation E627K has also been identified as a determinant of air-borne H5N1 transmission in ferrets. We are only beginning to understand the underlying mechanisms at the molecular level. Thus, mutation D701N promotes importin-α mediated nuclear transport in mammalian cells. Mutation E627K also enhances the replication rate in an importin-α dependent fashion in mammalian cells, yet without affecting nuclear entry of PB2. Numerous other adaptive mutations, some of which compensate for the lack of PB2 E627K, have been observed in PB2 as well as in the polymerase subunit PB1, the nucleoprotein NP, and the nuclear export protein NEP (NS2).

H5N1 hybrid viruses bearing 2009/H1N1 virus genes transmit in guinea pigs by respiratory droplet

In the past, avian influenza viruses have crossed species barriers to trigger human pandemics by reassorting with mammal-infective viruses in intermediate livestock hosts. H5N1 viruses are able to infect pigs, and some of them have affinity for the mammalian type α-2,6-linked sialic acid airway receptor. Using reverse genetics, we systematically created 127 reassortant viruses between a duck isolate of H5N1, specifically retaining its hemagglutinin (HA) gene throughout, and a highly transmissible, human-infective H1N1 virus. We tested the virulence of the reassortants in mice as a correlate for virulence in humans and tested transmissibility in guinea pigs, which have both avian and mammalian types of airway receptor. Transmission studies showed that the H1N1 virus genes encoding acidic polymerase and nonstructural protein made the H5N1 virus transmissible by respiratory droplet between guinea pigs without killing them. Further experiments implicated other H1N1 genes in the enhancement of mammal-to-mammal transmission, including those that encode nucleoprotein, neuraminidase, and matrix, as well as mutations in H5 HA that improve affinity for humanlike airway receptors. Hence, avian H5N1 subtype viruses do have the potential to acquire mammalian transmissibility by reassortment in current agricultural scenarios.

Adaptation of avian influenza A virus polymerase in mammals to overcome the host species barrier

Avian influenza A viruses, such as the highly pathogenic avian H5N1 viruses, sporadically enter the human population but often do not transmit between individuals. In rare cases, however, they establish a new lineage in humans. In addition to well-characterized barriers to cell entry, one major hurdle which avian viruses must overcome is their poor polymerase activity in human cells. There is compelling evidence that these viruses overcome this obstacle by acquiring adaptive mutations in the polymerase subunits PB1, PB2, and PA and the nucleoprotein (NP) as well as in the novel polymerase cofactor nuclear export protein (NEP). Recent findings suggest that synthesis of the viral genome may represent the major defect of avian polymerases in human cells. While the precise mechanisms remain to be unveiled, it appears that a broad spectrum of polymerase adaptive mutations can act collectively to overcome this defect. Thus, identification and monitoring of emerging adaptive mutations that further increase polymerase activity in human cells are critical to estimate the pandemic potential of avian viruses.

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.

Highly pathogenic H5N1 influenza A virus strains provoke heterogeneous IFN-α/β responses that distinctively affect viral propagation in human cells

The fatal transmissions of highly pathogenic avian influenza A viruses (IAV) of the H5N1 subtype to humans and high titer replication in the respiratory tract indicate that these pathogens can overcome the bird-to-human species barrier. While type I interferons (IFN-α/β) are well described to contribute to the species barrier of many zoonotic viruses, current data to the role of these antiviral cytokines during human H5N1 IAV infections is limited and contradictory. We hypothesized an important role for the IFN system in limiting productive infection of avian H5N1 strains in human cells. Hence, we examined IFN-α/β gene activation by different avian and human H5N1 isolates, if the IFN-α/β response restricts H5N1 growth and whether the different strains were equally capable to regulate the IFN-α/β system via their IFN-antagonistic NS1 proteins. Two human H5N1 isolates and a seasonal H3N2 strain propagated efficiently in human respiratory cells and induced little IFN-β, whereas three purely avian H5N1 strains were attenuated for replication and provoked higher IFN secretion. Replication of avian viruses was significantly enhanced on interferon-deficient cells, and exogenous IFN potently limited the growth of all strains in human cells. Moreover, IFN-α/β activation by all strains depended on retinoic acid-inducible gene I excluding principal differences in receptor activation between the different viruses. Interestingly, all H5N1 NS1 proteins suppressed IFN-α/β induction comparably well to the NS1 of seasonal IAV. Thus, our study shows that H5N1 strains are heterogeneous in their capacity to activate human cells in an NS1-independent manner. Our findings also suggest that H5N1 viruses need to acquire adaptive changes to circumvent strong IFN-α/β activation in human host cells. Since no single amino acid polymorphism could be associated with a respective high- or low induction phenotype we propose that the necessary adaptations to overcome the human IFN-α/β barrier involve mutations in multiple H5N1 genes.

Emergence of influenza viruses with zoonotic potential: open issues which need to be addressed. A review

The real and perceived impact of influenza infections in animals has changed dramatically over the last 10 years, due mainly to the better understanding of the public health implications of avian and swine influenza viruses. On a number of occasions in the last decade avian-to-human transmissions of H5, H7 and H9 virus subtypes have occurred, and the first influenza pandemic of the new millennium occurred as a result of the emergence and spread of a virus from pigs. Although the mechanisms that allow influenza viruses to jump from one host species to another are not fully understood, several genetic signatures linked to the crossing of species barriers have been identified. This has led to a re-evaluation of the importance of understanding these viruses in the animal reservoir, to the extent that millions of euros have been invested in surveillance, research and capacity building worldwide. This has resulted in an enhanced collaboration with our medical counterparts, leading to many discoveries that will contribute to an understanding of the complex mechanisms that lead to the emergence of a pandemic virus.

Investigation of influenza virus polymerase activity in pig cells

Reassortant influenza viruses with combinations of avian, human, and/or swine genomic segments have been detected frequently in pigs. As a consequence, pigs have been accused of being a "mixing vessel" for influenza viruses. This implies that pig cells support transcription and replication of avian influenza viruses, in contrast to human cells, in which most avian influenza virus polymerases display limited activity. Although influenza virus polymerase activity has been studied in human and avian cells for many years by use of a minigenome assay, similar investigations in pig cells have not been reported. We developed the first minigenome assay for pig cells and compared the activities of polymerases of avian or human influenza virus origin in pig, human, and avian cells. We also investigated in pig cells the consequences of some known mammalian host range determinants that enhance influenza virus polymerase activity in human cells, such as PB2 mutations E627K, D701N, G590S/Q591R, and T271A. The two typical avian influenza virus polymerases used in this study were poorly active in pig cells, similar to what is seen in human cells, and mutations that adapt the avian influenza virus polymerase for human cells also increased activity in pig cells. In contrast, a different pattern was observed in avian cells. Finally, highly pathogenic avian influenza virus H5N1 polymerase activity was tested because this subtype has been reported to replicate only poorly in pigs. H5N1 polymerase was active in swine cells, suggesting that other barriers restrict these viruses from becoming endemic in pigs.

Tropism and infectivity of influenza virus, including highly pathogenic avian H5N1 virus, in ferret tracheal differentiated primary epithelial cell cultures

Tropism and adaptation of influenza viruses to new hosts is partly dependent on the distribution of the sialic acid (SA) receptors to which the viral hemagglutinin (HA) binds. Ferrets have been established as a valuable in vivo model of influenza virus pathogenesis and transmission because of similarities to humans in the distribution of HA receptors and in clinical signs of infection. In this study, we developed a ferret tracheal differentiated primary epithelial cell culture model that consisted of a layered epithelium structure with ciliated and nonciliated cells on its apical surface. We found that human-like (α2,6-linked) receptors predominated on ciliated cells, whereas avian-like (α2,3-linked) receptors, which were less abundant, were presented on nonciliated cells. When we compared the tropism and infectivity of three human (H1 and H3) and two avian (H1 and H5) influenza viruses, we observed that the human influenza viruses primarily infected ciliated cells and replicated efficiently, whereas a highly pathogenic avian H5N1 virus (A/Vietnam/1203/2004) replicated efficiently within nonciliated cells despite a low initial infection rate. Furthermore, compared to other influenza viruses tested, VN/1203 virus replicated more efficiently in cells isolated from the lower trachea and at a higher temperature (37°C) compared to a lower temperature (33°C). VN/1203 virus infection also induced higher levels of immune mediator genes and cell death, and virus was recovered from the basolateral side of the cell monolayer. This ferret tracheal differentiated primary epithelial cell culture system provides a valuable in vitro model for studying cellular tropism, infectivity, and the pathogenesis of influenza viruses.

Myxovirus resistance gene A (MxA) expression suppresses influenza A virus replication in alpha interferon-treated primate cells

Alpha interferon (IFN-α) production is triggered when influenza virus RNA is detected by appropriate pattern recognition receptors in the host cell. IFN-α induces the expression of more than 300 interferon-stimulated genes (ISGs), and this blunts influenza virus replication. The human ISG MxA can inhibit influenza A virus replication in mouse cells by interfering with a step in the virus replication cycle after primary transcription of the negative-strand RNA genome to mRNA (J. Pavlovic, O. Haller, and P. Staeheli, J. Virol. 66:2564-2569, 1992). To determine the role of MxA in blocking human influenza A virus replication in primate cells, we manipulated MxA expression in rhesus kidney epithelial cells (LLC-MK(2)) and human lung carcinoma cells (A549). We found that IFN-α treatment prior to influenza virus infection suppressed virus replication and induced the expression of many ISGs, including MxA. However, IFN-α-mediated suppression of virus replication was abolished by small interfering RNA (siRNA) knockdown of MxA expression in IFN-treated cells. In addition, influenza virus replication was suppressed in Vero cells stably transfected with MxA. A strand-specific reverse transcription-PCR (RT-PCR) assay showed that positive-strand influenza virus mRNA and negative-strand genomic RNA (gRNA) accumulated to high levels at 8 h after infection in control Vero cells containing the empty vector. However, in Vero cells stably transfected with MxA positive-strand influenza virus mRNA, complementary positive-strand influenza virus genome RNA (cRNA) and influenza virus gRNA were drastically suppressed. Thus, in primate cells, MxA inhibits human seasonal influenza virus replication at a step prior to primary transcription of gRNA into mRNA. Taken together, these results demonstrate that MxA mediates control of influenza virus replication in primate cells treated with IFN-α.

Complete genomic sequence of an H5N1 influenza virus from a parrot in southern China

An H5N1 avian influenza virus (AIV) designated A/Parrot/Guangdong/C99/2005 (H5N1) was first isolated from a sick parrot in Guangdong in southern China in 2005. The complete genome of this strain was analyzed. Genome sequence analysis showed that all 8 gene segments of the virus nucleotide had 99.0% homology to A/chicken/Henan/12/2004 (H5N1). Phylogenetic analysis demonstrated that all 8 gene segments of the virus were derived from the Eurasian lineage. The availability of genome sequences is useful to investigate the host range and genetic evolution of the H5N1 avian influenza virus in Southern China.

Full genome sequence of a recombinant H5N1 influenza virus from a condor in southern China

In this study, we report the first genomic information on an H5N1 avian influenza virus (AIV) isolated from a condor in Guangdong Province in southern China in 2003. Full genome sequencing and phylogenetic analyses show that it is a recombinant virus containing genome segments derived from the Eurasia and North America gene pools. This will be useful for analyses of the evolution of H5N1 AIV in southern China.

A comparative study of short linear motif compositions of the influenza A virus ribonucleoproteins

Protein-protein interactions through short linear motifs (SLiMs) are an emerging concept that is different from interactions between globular domains. The SLiMs encode a functional interaction interface in a short (three to ten residues) poorly conserved sequence. This characteristic makes them much more likely to arise/disappear spontaneously via mutations, and they may be more evolutionarily labile than globular domains. The diversity of SLiM composition may provide functional diversity for a viral protein from different viral strains. This study is designed to determine the different SLiM compositions of ribonucleoproteins (RNPs) from influenza A viruses (IAVs) from different hosts and with different levels of virulence.

The 96 consensus sequences (regular expressions) of SLiMs from the ELM server were used to conduct a comprehensive analysis of the 52,513 IAV RNP sequences. The SLiM compositions of RNPs from IAVs from different hosts and with different levels of virulence were compared. The SLiM compositions of 845 RNPs from highly virulent/pandemic IAVs were also analyzed. In total, 292 highly conserved SLiMs were found in RNPs regardless of the IAV host range. These SLiMs may be basic motifs that are essential for the normal functions of RNPs. Moreover, several SLiMs that are rare in seasonal IAV RNPs but are present in RNPs from highly virulent/pandemic IAVs were identified.

The SLiMs identified in this study provide a useful resource for experimental virologists to study the interactions between IAV RNPs and host intracellular proteins. Moreover, the SLiM compositions of IAV RNPs also provide insights into signal transduction pathways and protein interaction networks with which IAV RNPs might be involved. Information about SLiMs might be useful for the development of anti-IAV drugs.

Influenza polymerase activity correlates with the strength of interaction between nucleoprotein and PB2 through the host-specific residue K/E627

The ribonucleoprotein (RNP) complex is the essential transcription-replication machinery of the influenza virus. It is composed of the trimeric polymerase (PA, PB1 and PB2), nucleoprotein (NP) and RNA. Elucidating the molecular mechanisms of RNP assembly is central to our understanding of the control of viral transcription and replication and the dependence of these processes on the host cell. In this report, we show, by RNP reconstitution assays and co-immunoprecipitation, that the interaction between NP and polymerase is crucial for the function of the RNP. The functional association of NP and polymerase involves the C-terminal ‘627’ domain of PB2 and it requires NP arginine-150 and either lysine-627 or arginine-630 of PB2. Using surface plasmon resonance, we demonstrate that the interaction between NP and PB2 takes place without the involvement of RNA. At 33, 37 and 41°C in mammalian cells, more positive charges at aa. 627 and 630 of PB2 lead to stronger NP-polymerase interaction, which directly correlates with the higher RNP activity. In conclusion, our study provides new information on the NP-PB2 interaction and shows that the strength of NP-polymerase interaction and the resulting RNP activity are promoted by the positive charges at aa. 627 and 630 of PB2.

Adaptive mutations in NEP compensate for defective H5N1 RNA replication in cultured human cells

Infection of mammals by avian influenza viruses requires adaptive mutations to achieve high-level replication in the new host. However, the basic mechanism underlying this adaptation process is still unknown. Here we show that avian polymerases, lacking the human signature PB2-E627K, are incapable of generating usable complementary RNA templates in cultured human cells and therefore require adaptation. Characterization of the highly pathogenic human H5N1 isolate A/Thailand/1(KAN-1)/2004 that retained the avian PB2-E627 reveals that the defect in RNA replication is only partially compensated by mutations in the polymerase. Instead, mutations in the nuclear export protein are required for efficient polymerase activity. We demonstrate that adaptive mutations in nuclear export proteins of several human isolates enhance the polymerase activity of avian polymerases in human cultured cells. In conclusion, when crossing the species barrier, avian influenza viruses acquire adaptive mutations in nuclear export protein to escape restricted viral genome replication in mammalian cells.

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.

Avian influenza A virus PB2 promotes interferon type I inducing properties of a swine strain in porcine dendritic cells

The 2009 influenza A virus (IAV) pandemic resulted from reassortment of avian, human and swine strains probably in pigs. To elucidate the role of viral genes in host adaptation regarding innate immune responses, we focussed on the effect of genes from an avian H5N1 and a porcine H1N1 IAV on infectivity and activation of porcine GM-CSF-induced dendritic cells (DC). The highest interferon type I responses were achieved by the porcine virus reassortant containing the avian polymerase gene PB2. This finding was not due to differential tropism since all viruses infected DC equally. All viruses equally induced MHC class II, but porcine H1N1 expressing the avian viral PB2 induced more prominent nuclear NF-κB translocation compared to its parent IAV. The enhanced activation of DC may be detrimental or beneficial. An over-stimulation of innate responses could result in either pronounced tissue damage or increased resistance against IAV reassortants carrying avian PB2.

Isolation and phylogenetic analysis of avian-origin European H1N1 swine influenza viruses in Jiangsu, China

Isolates of the A(H1N1)pdm2009 virus were first identified in asymptomatic swine in Jiangsu province, China in January 2010, indicating that the virus has retro-infected swine after circulating through humans in mainland China. The purpose of this study was to determine whether the avian-origin European H1N1 swine influenza virus (SIV) and the A(H1N1)pdm2009 virus are cocirculating in swine in Jiangsu province of China. From May 2010 to May 2011, 1,030 nasal swab samples were collected from healthy swine in Jiangsu province of China and were tested for influenza A H1N1 using reverse transcription-PCR. Fragments of the complete genomes of viruses from the samples that were positive for influenza A H1N1 were sequenced and analysed. A total of 32 avian-origin European H1N1 SIVs were isolated, and no A(H1N1)pdm2009 viruses were identified; full-length genomes of 18 strains were sequenced. The eight gene segments of some of the isolated H1N1 viruses have 99.1-99.8% sequence identity with the human A/Jiangsu/ALS1/2011(H1N1) isolates in the same region. Our study indicates that the avian-origin European H1N1 SIVs remain endemic in swine and have retro-infected humans after circulating through swine, which may present a risk factor for public health.

Molecular evolution of the H6 subtype influenza A viruses from poultry in eastern China from 2002 to 2010

Background

Although extensive data demonstrates that the majority of H6 duck isolates belonged to a single H6N2 virus lineage with a single gene constellation in southern China from 2000 to 2005, the prevalence of H6N2 virus in poultry in Eastern China is largely unknown.

Results

Epidemiology revealed that H6N2 viruses were the most frequently detected influenza subtypes in live bird markets from 2002 to 2008 in Eastern China, but from 2009 onwards, they were replaced with novel H6N6 viruses. We phylogenetically and antigenically analyzed 42 H6 viruses isolated mainly in domestic ducks from 2002 to 2010 in Eastern China. Surprisingly, none of these isolates grouped with the previously described H6N2 viruses which belonged to a single H6N2 virus lineage with a single gene constellation in domestic ducks in southern China from 2000 to 2005. Two distinct hemagglutinin lineages were identified and they all underwent frequent reassortment with multiple virus subtypes from the natural gene pool, but few reassortants were persistent or prevalent.

Conclusions

Five subtypes of H6 influenza viruses (H6N1, H6N2, H6N5, H6N6 and H6N8) cocirculated in Eastern China, which form a significant part of the natural influenza virus reservoir in domestic ducks, and significant viral reassortment is still ongoing in this species.

Host immune-related gene responses against highly pathogenic avian influenza virus infection in vitro differ among chicken cell lines established from different organs

Highly pathogenic avian influenza virus (HPAIV) induces acute disease in chickens causing high mortality and morbidity and is a major threat to poultry industries in Southeast Asian countries. The mechanisms of disease manifestation and host innate immune responses against HAPIV in chickens are not well understood. In this study, we examined virus replication and host gene expressions in four chicken cell lines in vitro to elucidate the impact of host innate immune responses against viral replication. It was demonstrated that viral replication efficiencies were different depending on the cell line. The viral replication appeared to be affected by the basal expression of IFN related genes. The expression of immune-related genes against the viral infection also varied in a cell line dependent manner. In non-immune derived cell lines, but not in immune derived cell lines, the expression of the CCL5 and CCL20 genes were induced by HPAIV infection. Reverse genetics HPAIV, with internal genes from avirulent avian influenza, reduced virus replication and affected immune-related gene expression in a cell line dependent manner. These results suggest the possibility that differential immune responses in different cell types in local tissues could modulate the consequences of HPAIV infection in chickens.

Biochemical impact of the host adaptation-associated PB2 E627K mutation on the temperature-dependent RNA synthesis kinetics of influenza A virus polymerase complex

Most avian influenza A viruses, which preferentially replicate at the high temperatures found in the digestive tract of birds, have a glutamic acid at residue 627 of the viral RNA polymerase PB2 subunit (Glu-627), whereas the human viruses, which optimally replicate at the low temperatures observed in the human respiratory tract, have a lysine (Lys-627). The mechanism of action for this mutation is still not understood, although interaction with host factors has been proposed to play a major role. In this study, we explored an alternative, yet related, hypothesis that this PB2 mutation may alter the temperature-dependent enzymatic polymerase activity of the viral polymerase. First, the avian polymerase protein, which was purified from baculovirus expression system, indeed remained significantly active at higher temperatures (i.e. 37 and 42 °C), whereas the human E627K mutant drastically lost activity at these high temperatures. Second, our steady-state kinetics data revealed that the human E627K mutant polymerase is catalytically more active than the avian Glu-627 polymerase at 34 °C. Importantly, the E627K mutation elevates apparent K(cat) at low temperatures with little effect on K(m), suggesting that the E627K mutation alters the biochemical steps involved in enzyme catalysis rather than the interaction with the incoming NTP. Third, this temperature-dependent kinetic impact of the human E627K mutation was also observed with different RNA templates, with different primers and also in the presence of nucleoprotein. In conclusion, our study suggests that the amino acid sequence variations at residue 627 of PB2 subunit can directly alter the enzyme kinetics of influenza polymerase.

Isolation and phylogenetic analysis of pandemic H1N1/09 influenza virus from swine in Jiangsu province of China

To investigate whether the 2009 pandemic H1N1 influenza A virus was still being transmitted in swine, a total of 1029 nasal swab samples from healthy swine were collected from January to May 2010 in Jiangsu province of China. Eight H1N1 influenza viruses were isolated and identified, and their full length genomes were sequenced. We found that all eight of the H1N1 viruses shared higher than 98.0% sequence identity with the 2009 pandemic virus A/Jiangsu/1/2009 (JS1). In addition, some of these viruses had D225G (3/8) mutations in the receptor binding sites of the hemagglutinin (HA) protein, indicating enhancement of their binding affinity to the sialic α2, 3Gal receptor. In conclusion, the 2009 pandemic H1N1 influenza A virus has retro-infected swine from humans in mainland China, and significant viral evolution is still ongoing in this species.

The contribution of animal models to the understanding of the host range and virulence of influenza A viruses

Since ferrets were first used in 1933 during the initial isolation of influenza A viruses, animal models have been critical for influenza research. The following review discusses the contribution of mice, ferrets, and non-human primates to the study of influenza virus host range and pathogenicity.

Virulence and genetic compatibility of polymerase reassortant viruses derived from the pandemic (H1N1) 2009 influenza virus and circulating influenza A viruses

Gene mutations and reassortment are key mechanisms by which influenza A virus acquires virulence factors. To evaluate the role of the viral polymerase replication machinery in producing virulent pandemic (H1N1) 2009 influenza viruses, we generated various polymerase point mutants (PB2, 627K/701N; PB1, expression of PB1-F2 protein; and PA, 97I) and reassortant viruses with various sources of influenza viruses by reverse genetics. Although the point mutations produced no significant change in pathogenicity, reassortment between the pandemic A/California/04/09 (CA04, H1N1) and current human and animal influenza viruses produced variants possessing a broad spectrum of pathogenicity in the mouse model. Although most polymerase reassortants had attenuated pathogenicity (including those containing seasonal human H3N2 and high-pathogenicity H5N1 virus segments) compared to that of the parental CA04 (H1N1) virus, some recombinants had significantly enhanced virulence. Unexpectedly, one of the five highly virulent reassortants contained a A/Swine/Korea/JNS06/04(H3N2)-like PB2 gene with no known virulence factors; the other four had mammalian-passaged avian-like genes encoding PB2 featuring 627K, PA featuring 97I, or both. Overall, the reassorted polymerase complexes were only moderately compatible for virus rescue, probably because of disrupted molecular interactions involving viral or host proteins. Although we observed close cooperation between PB2 and PB1 from similar virus origins, we found that PA appears to be crucial in maintaining viral gene functions in the context of the CA04 (H1N1) virus. These observations provide helpful insights into the pathogenic potential of reassortant influenza viruses composed of the pandemic (H1N1) 2009 influenza virus and prevailing human or animal influenza viruses that could emerge in the future.

Influence of PB2 host-range determinants on the intranuclear mobility of the influenza A virus polymerase

Avian influenza A viruses often do not propagate efficiently in mammalian cells. The viral polymerase protein PB2 is important for this host restriction, with amino-acid polymorphisms at residue 627 and other positions acting as ‘signatures’ of avian- or human-adapted viruses. Restriction is hypothesized to result from differential interactions (either positive or inhibitory) with unidentified cellular factors. We applied fluorescence recovery after photobleaching (FRAP) to investigate the mobility of the viral polymerase in the cell nucleus using A/PR/8/34 and A/Turkey/England/50-92/91 as model strains. As expected, transcriptional activity of a polymerase with the avian PB2 protein was strongly dependent on the identity of residue 627 in human but not avian cells, and this correlated with significantly slower diffusion of the inactive polymerase in human but not avian nuclei. In contrast, the activity and mobility of the PR8 polymerase was affected much less by residue 627. Sequence comparison followed by mutagenic analyses identified residues at known host-range-specific positions 271, 588 and 701 as well as a novel determinant at position 636 as contributors to host-specific activity of both PR8 and Turkey PB2 proteins. Furthermore, the correlation between poor transcriptional activity and slow diffusional mobility was maintained. However, activity did not obligatorily correlate with predicted surface charge of the 627 domain. Overall, our data support the hypothesis of a host nuclear factor that interacts with the viral polymerase and modulates its activity. While we cannot distinguish between positive and inhibitory effects, the data have implications for how such factors might operate.

Molecular evolution of the six internal genes of H5N1 equine influenza A virus

Phylogenetic and evolutionary patterns of the six internal genes of an equine H5N1 influenza A virus isolated in Egypt on 2009 were analyzed using direct sequencing. All of the internal genes of the equine H5N1 strain showed a genetic pattern potentially related to Eurasian lineages. Variable dendrogram topologies revealed an absence of reassortment in the equine strain while confirming its close relatedness to other Egyptian H5N1 strains from human and avian species. The equine strain is characterized by a variety of amino acid substitutions in six internal proteins compared to the available Egyptian H5N1 strains. Interestingly, the equine strain displayed amino acids in the PB2, PA, M2 and NS2 proteins that are unique among the available H5N1 sequences in the flu database, and their potential effect on virulence needs to be further investigated.

Different incubation temperatures affect viral polymerase activity and yields of low-pathogenic avian influenza viruses in embryonated chicken eggs

Various incubation conditions (35°C-38°C, 2-7 days) have been used in surveillance studies of the prevalence of avian influenza viruses in wild birds. Here, we studied viral polymerase activity and virus growth kinetics of low-pathogenic avian influenza viruses (LPAIVs) isolated from field samples [A/duck/Hong Kong/365/1978 (H4N6) and A/duck/Nanchang/2-0480/2000 (H9N2)] during incubation at different temperatures (35°C, 37°C, and 39°C) in the allantoic cavity of 10-day-old embryonated chicken eggs (ECE). The higher incubation temperatures (37°C and 39°C) resulted in a significantly higher rate of virus growth, which is most likely a result of increased viral polymerase activity (20%-60%), than was observed at 35°C, and as much as a 100% greater virus yield (as measured by hemagglutination assay) was observed two days after inoculation. Our findings revealed that the optimal activity of the viral polymerase complex, resulting in the highest yield of LPAIV field isolates, could be obtained by incubation for two days in ECE at 37°C and 39°C.

Virulence determinants of pandemic influenza viruses

Influenza A viruses cause recurrent, seasonal epidemics and occasional global pandemics with devastating levels of morbidity and mortality. The ability of influenza A viruses to adapt to various hosts and undergo reassortment events ensures constant generation of new strains with unpredictable degrees of pathogenicity, transmissibility, and pandemic potential. Currently, the combination of factors that drives the emergence of pandemic influenza is unclear, making it impossible to foresee the details of a future outbreak. Identification and characterization of influenza A virus virulence determinants may provide insight into genotypic signatures of pathogenicity as well as a more thorough understanding of the factors that give rise to pandemics.

Influenza A virus transmission: contributing factors and clinical implications

Efficient human-to-human transmission is a necessary property for the generation of a pandemic influenza virus. To date, only influenza A viruses within the H1-H3 subtypes have achieved this capacity. However, sporadic cases of severe disease in individuals following infection with avian influenza A viruses over the past decade, and the emergence of a pandemic H1N1 swine-origin virus in 2009, underscore the need to better understand how influenza viruses acquire the ability to transmit efficiently. In this review, we discuss the biological constraints and molecular features known to affect virus transmissibility to and among humans. Factors influencing the behaviour of aerosols in the environment are described, and the mammalian models used to study virus transmission are presented. Recent progress in understanding the molecular determinants that confer efficient transmission has identified crucial roles for the haemagglutinin and polymerase proteins; nevertheless, influenza virus transmission remains a polygenic trait that is not completely understood. The clinical implications of this research, including methods currently under investigation to mitigate influenza virus human-to-human transmission, are discussed. A better understanding of the viral determinants necessary for efficient transmission will allow us to identify avian influenza viruses with pandemic potential.

Artificial hybrids of influenza A virus RNA polymerase reveal PA subunit modulates its thermal sensitivity

BACKGROUND

Influenza A virus can infect a variety of different hosts and therefore has to adapt to different host temperatures for its efficient viral replication. Influenza virus codes for an RNA polymerase of 3 subunits: PB1, PB2 and PA. It is well known that the PB2 subunit is involved in temperature sensitivity, such as cold adaptation. On the other hand the role of the PA subunit in thermal sensitivity is still poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

To test which polymerase subunit(s) were involved in thermal stress we reconstituted artificial hybrids of influenza RNA polymerase in ribonucleoprotein (RNP) complexes and measured steady-state levels of mRNA, cRNA and vRNA at different temperatures. The PA subunit was involved in modulating RNP activity under thermal stress. Residue 114 of the PA subunit was an important determinant of this activity.

CONCLUSIONS/SIGNIFICANCE

These findings suggested that influenza A virus may acquire an RNA polymerase adapted to different body temperatures of the host by reassortment of the RNA polymerase genes.

The 2009 pandemic H1N1 and triple-reassortant swine H1N1 influenza viruses replicate efficiently but elicit an attenuated inflammatory response in polarized human bronchial epithelial cells

The pandemic H1N1 virus of 2009 (2009 H1N1) produced a spectrum of disease ranging from mild illness to severe illness and death. Respiratory symptoms were frequently associated with virus infection, with relatively high rate of gastrointestinal symptoms reported. To better understand 2009 H1N1 virus pathogenesis in humans, we studied virus and host responses following infection of two cell types: polarized bronchial and pharyngeal epithelial cells, which exhibit many features of the human airway epithelium, and colon epithelial cells to serve as a human intestinal cell model. Selected 2009 H1N1 viruses were compared to both seasonal H1N1 and triple-reassortant swine H1N1 influenza viruses that have circulated among North American pigs since before the 2009 pandemic. All H1N1 viruses replicated productively in airway cells; however, in contrast to seasonal H1N1 virus infection, infection with the 2009 H1N1 and triple-reassortant swine H1N1 viruses resulted in an attenuated inflammatory response, a weaker interferon response, and reduced cell death. Additionally, the H1N1 viruses of swine origin replicated less efficiently at the temperature of the human proximal airways (33°C). We also observed that the 2009 H1N1 viruses replicated to significantly higher titers than seasonal H1N1 virus in polarized colon epithelial cells. These studies reveal that in comparison to seasonal influenza virus, H1N1 viruses of swine origin poorly activate multiple aspects of the human innate response, which may contribute to the virulence of these viruses. In addition, their less efficient replication at human upper airway temperatures has implications for the understanding of pandemic H1N1 virus adaptation to humans.

Evidence for avian and human host cell factors that affect the activity of influenza virus polymerase

Typical avian influenza A viruses do not replicate efficiently in humans. The molecular basis of host range restriction and adaptation of avian influenza A viruses to a new host species is still not completely understood. Genetic determinants of host range adaptation have been found on the polymerase complex (PB1, PB2, and PA) as well as on the nucleoprotein (NP). These four viral proteins constitute the minimal set for transcription and replication of influenza viral RNA. It is widely documented that in human cells, avian-derived influenza A viral polymerase is poorly active, but despite extensive study, the reason for this blockade is not known. We monitored the activity of influenza A viral polymerases in heterokaryons formed between avian (DF1) and human (293T) cells. We have discovered that a positive factor present in avian cells enhances the activity of the avian influenza virus polymerase. We found no evidence for the existence of an inhibitory factor for avian virus polymerase in human cells, and we suggest, instead, that the restriction of avian influenza virus polymerases in human cells is the consequence of the absence or the low expression of a compatible positive cofactor. Finally, our results strongly suggest that the well-known adaptative mutation E627K on viral protein PB2 facilitates the ability of a human positive factor to enhance replication of influenza virus in human cells.

Evolution of canine and equine influenza (H3N8) viruses co-circulating between 2005 and 2008

Influenza virus, subtype H3N8, was transmitted from horses to greyhound dogs in 2004 and subsequently spread to pet dog populations. The co-circulation of H3N8 viruses in dogs and horses makes bi-directional virus transmission between these animal species possible. To understand the dynamics of viral transmission, we performed virologic surveillance in dogs and horses between 2005 and 2008 in the United States. The genomes of influenza A H3N8 viruses isolated from 36 dogs and horses were sequenced to determine their origin and evolution. Phylogenetic analyses revealed that H3N8 influenza viruses from horses and dogs were monophyletic and distinct. There was no evidence of canine influenza virus infection in horses with respiratory disease or new introductions of equine influenza viruses into dogs in the United States. Analysis of a limited number of equine influenza viruses suggested substantial separation in the transmission of viruses causing clinically apparent influenza in dogs and horses.

Genetic characterization of 2008 reassortant influenza A virus (H5N1), Thailand

In January and November 2008, outbreaks of avian influenza have been reported in 4 provinces of Thailand. Eight Influenza A H5N1 viruses were recovered from these 2008 AI outbreaks and comprehensively characterized and analyzed for nucleotide identity, genetic relatedness, virulence determinants, and possible sites of reassortment. The results show that the 2008 H5N1 viruses displayed genetic drift characteristics (less than 3% genetic differences), as commonly found in influenza A viruses. Based on phylogenetic analysis, clade 1 viruses in Thailand were divided into 3 distinct branches (subclades 1, 1.1 and 1.2). Six out of 8 H5N1 isolates have been identified as reassorted H5N1 viruses, while other isolates belong to an original H5N1 clade. These viruses have undergone inter-lineage reassortment between subclades 1.1 and 1.2 and thus represent new reassorted 2008 H5N1 viruses. The reassorted viruses have acquired gene segments from H5N1, subclade 1.1 (PA, HA, NP and M) and subclade 1.2 (PB2, PB1, NA and NS) in Thailand. Bootscan analysis of concatenated whole genome sequences of the 2008 H5N1 viruses supported the reassortment sites between subclade 1.1 and 1.2 viruses. Based on estimating of the time of the most recent common ancestors of the 2008 H5N1 viruses, the potential point of genetic reassortment of the viruses could be traced back to 2006. Genetic analysis of the 2008 H5N1 viruses has shown that most virulence determinants in all 8 genes of the viruses have remained unchanged. In summary, two predominant H5N1 lineages were circulating in 2008. The original CUK2-like lineage mainly circulated in central Thailand and the reassorted lineage (subclades 1.1 and 1.2) predominantly circulated in lower-north Thailand. To prevent new reassortment, emphasis should be put on prevention of H5N1 viruses circulating in high risk areas. In addition, surveillance and whole genome sequencing of H5N1 viruses should be routinely performed for monitoring the genetic drift of the virus and new reassorted strains, especially in light of potential reassortment between avian and mammalian H5N1 viruses.

A 27-amino-acid deletion in the neuraminidase stalk supports replication of an avian H2N2 influenza A virus in the respiratory tract of chickens

The events and mechanisms that lead to interspecies transmission of, and host adaptation to, influenza A virus are unknown; however, both surface and internal proteins have been implicated. Our previous report highlighted the role that Japanese quail play as an intermediate host, expanding the host range of a mallard H2N2 virus, A/mallard/Potsdam/178-4/83 (H2N2), through viral adaptation. This quail-adapted virus supported transmission in quail and increased its host range to replicate and be transmitted efficiently in chickens. Here we report that of the six amino acid changes in the quail-adapted virus, a single change in the hemagglutinin (HA) was crucial for transmission in quail, while the changes in the polymerase genes favored replication at lower temperatures than those for the wild-type mallard virus. Reverse genetic analysis indicated that all adaptive mutations were necessary for transmission in chickens, further implicating quail in extending this virus to terrestrial poultry. Adaptation of the quail-adapted virus in chickens resulted in the alteration of viral tropism from intestinal shedding to shedding and transmission via the respiratory tract. Sequence analysis indicated that this chicken-adapted virus maintained all quail-adaptive mutations, as well as an additional change in the HA and, most notably, a 27-amino-acid deletion in the stalk region of neuraminidase (NA), a genotypic marker of influenza virus adaptation to chickens. This stalk deletion was shown to be responsible for the change in virus tropism from the intestine to the respiratory tract.

Avian influenza: our current understanding

Avian influenza (AI) virus is one of the most important diseases of the poultry industry around the world. The virus has a broad host range in birds and mammals, although the natural reservoir is wild birds where it typically causes an asymptomatic to mild infection. The virus in poultry can cause a range of clinical diseases and is defined either as low pathogenic AI (LPAI) or highly pathogenic AI (HPAI) depending on the type of disease it causes in chickens. Viruses that replicate primarily on mucosal surfaces and cause mild disease with low mortality are termed LPAI. Viruses that replicate on mucosal surfaces and systemically and cause severe disease with a mortality rate of 75% or greater in experimentally infected chickens are referred to as HPAI. A virus that is highly pathogenic in chickens may infect but result in a completely different disease and replication pattern in other host species. Outbreaks of HPAI have been relatively uncommon around the world in the last 50 years and have had limited spread within a country or region with one major exception, Asian lineage H5N1 that was first identified in 1996. This lineage of virus has spread to over 60 countries and has become endemic in poultry in at least four countries. AI virus also represents a public health threat, with some infected humans having severe disease and with a high case fatality rate. AI remains a difficult disease to control because of the highly infectious nature of the virus and the interface of domestic and wild animals. A better understanding of the disease and its transmission is important for control.

Influenza A virus polymerase: structural insights into replication and host adaptation mechanisms

The heterotrimeric RNA-dependent RNA polymerase of influenza viruses catalyzes RNA replication and transcription activities in infected cell nuclei. The nucleotide polymerization activity is common to both replication and transcription processes, with an additional cap-snatching function being employed during transcription to steal short 5'-capped RNA primers from host mRNAs. Cap-binding, endonuclease, and polymerase activities have long been studied biochemically, but structural studies on the polymerase and its subunits have been hindered by difficulties in producing sufficient quantities of material. Recently, because of heightened effort and advances in expression and crystallization technologies, a series of high resolution structures of individual domains have been determined. These shed light on intrinsic activities of the polymerase, including cap snatching, subunit association, and nucleocytoplasmic transport, and open up the possibility of structure-guided development of new polymerase inhibitors. Furthermore, the activity of influenza polymerase is highly host- and cell type-specific, being dependent on the identity of a few key amino acid positions in the different subunits, especially in the C-terminal region of PB2. New structures demonstrate the surface exposure of these residues, consistent with ideas that they might modulate interactions with host-specific factors that enhance or restrict activity. Recent proteomic and genome-wide interactome and RNA interference screens have suggested the identities of some of these potential regulators of polymerase function.

Substitution of lysine at 627 position in PB2 protein does not change virulence of the 2009 pandemic H1N1 virus in mice

A lysine at the 627 position (627K) of PB2 protein of influenza virus has been recognized as a determinant for host adaptation and a virulent element for some influenza viruses. While seasonal influenza viruses exclusively contained 627K, the pandemic (H1N1) 2009 possessed a glutamic acid (627E), even after circulation in humans for more than 6months. To explore the potential role of E627K substitution in PB2 in the pandemic (H1N1) 2009 virus, we compared pathogenicity and growth properties between a recombinant virus containing 627K PB2 gene and the parental A/California/4/2009 strain containing 627E. Our results showed that substitution of 627K in PB2 gene does not confer higher virulence and growth rate for the pandemic (H1N1) 2009 virus in mice and cell culture respectively, suggesting 627K is not required for human adaptation of the pandemic (H1N1) 2009 virus.

Evolution of highly pathogenic avian H5N1 influenza viruses and the emergence of dominant variants

Highly pathogenic avian H5N1 viruses have circulated in South-east Asia for more than a decade and have now spread to more than 60 countries. The evolution of these viruses is characterized by frequent reassortment of the so-called 'internal' genes, creating novel genotypes. Additionally, over time, the surface glycoprotein, haemagglutinin (HA), which is the primary target of the adaptive immune response, has evolved by point mutation into 20 genetically and potentially antigenically distinct clades. To investigate the evolution of avian H5N1 influenza viruses, we undertook a high-resolution analysis of the reassortment of internal genes and evolution of HA of 651 avian H5N1 viruses from 2000 to 2008. Our analysis suggested: (i) all current H5N1 genotypes were derived from a single, clearly defined sequence of initial reassortment events; (ii) reassortment of just three of the internal genes had the most importance in avian H5N1 virus evolution; (iii) HA and the constellation of internal genes may be jointly important in the emergence of dominant variants. Further, our analysis led to the identification of evolutionarily significant molecular changes in the internal genes that may be significant for the emergence of these dominant variants.

[Interspecies transmission, adaptation to humans and pathogenicity of animal influenza viruses]

The emergence in 2009 of a novel A(H1N1)v influenza virus of swine origin and the regular occurrence since 2003 of human cases of infection with A(H5N1) avian influenza viruses underline the zoonotic and pandemic potential of type A influenza viruses. Influenza viruses from the wild aquatic birds reservoir usually do not replicate efficiently in humans. Domestic poultry and swine can act as intermediate hosts for the acquisition of determinants that increase the potential of transmission and adaptation to humans, through the accumulation of mutations or by genetic reassortment. The rapid evolution of influenza viruses following interspecies transmission probably results from the selection of genetic variations that favor optimal interactions between viral proteins and cellular factors, leading to an increased multiplication potential and a better escape to the host antiviral response. Whereas influenza viruses usually cause asymptomatic infections in wild aquatic birds, they may be highly pathogenic in other species. Molecular determinants of host-specificity and pathogenesis have been identified in most viral genes, notably in genes that encode viral surface glycoproteins, proteins involved in the viral genome replication, and proteins that counteract the host immune response. However, our knowledge of these numerous and interdependant determinants remains incomplete, and the molecular mechanisms involved are still to be understood.

PB2 residue 271 plays a key role in enhanced polymerase activity of influenza A viruses in mammalian host cells

The direct infection of humans with highly pathogenic avian H5N1 influenza viruses has suggested viral mutation as one mechanism for the emergence of novel human influenza A viruses. Although the polymerase complex is known to be a key component in host adaptation, mutations that enhance the polymerase activity of avian viruses in mammalian hosts are not fully characterized. The genomic comparison of influenza A virus isolates has identified highly conserved residues in influenza proteins that are specific to either human or avian viruses, including 10 residues in PB2. We characterized the activity of avian polymerase complexes containing avian-to-human mutations at these conserved PB2 residues and found that, in addition to the E627K mutation, the PB2 mutation T271A enhances polymerase activity in human cells. We confirmed the effects of the T271A mutation using recombinant WSN viruses containing avian NP and polymerase genes with wild-type (WT) or mutant PB2. The 271A virus showed enhanced growth compared to that of the WT in mammalian cells in vitro. The 271A mutant did not increase viral pathogenicity significantly in mice compared to that of the 627K mutant, but it did enhance the lung virus titer. Also, cell infiltration was more evident in lungs of 271A-infected mice than in those of the WT. Interestingly, the avian-derived PB2 of the 2009 pandemic H1N1 influenza virus has 271A. The characterization of the polymerase activity of A/California/04/2009 (H1N1) and corresponding PB2 mutants indicates that the high polymerase activity of the pandemic strain in mammalian cells is, in part, dependent on 271A. Our results clearly indicate the contribution of PB2 amino acid 271 to enhanced polymerase activity and viral growth in mammalian hosts.

Reassortment between avian H5N1 and human H3N2 influenza viruses creates hybrid viruses with substantial virulence

The spread of avian H5N1 influenza viruses around the globe has become a worldwide public health concern. To evaluate the pathogenic potential of reassortant viruses between currently cocirculating avian H5N1 and human H3N2 influenza viruses, we generated all the 254 combinations of reassortant viruses between A/chicken/South Kalimantan/UT6028/06 (SK06, H5N1) and A/Tokyo/Ut-Sk-1/07 (Tok07, H3N2) influenza viruses by reverse genetics. We found that the presence of Tok07 PB2 protein in the ribonucleoprotein (RNP) complex allowed efficient viral RNA transcription in a minigenome assay and that RNP activity played an essential role in the viability and replicative ability of the reassortant viruses. When the pathogenicity of 75 reassortant H5 viruses was tested in mice, 22 were more pathogenic than the parental SK06 virus, and three were extremely virulent. Strikingly, all 22 of these viruses obtained their PB2 segment from Tok07 virus. Further analysis showed that Tok07 PB1 alone lacked the ability to enhance the pathogenicity of the reassortant viruses but could do so by cooperating with Tok07 PB2. Our data demonstrate that reassortment between an avian H5N1 virus with low pathogenicity in mice and a human virus could result in highly pathogenic viruses and that the human virus PB2 segment functions in the background of an avian H5N1 virus, enhancing its virulence. Our findings highlight the importance of surveillance programs to monitor the emergence of human H5 reassortant viruses, especially those containing a PB2 segment of human origin.

Molecular determinants of adaptation of highly pathogenic avian influenza H7N7 viruses to efficient replication in the human host

Two viruses isolated during the highly pathogenic avian influenza (HPAI) H7N7 virus outbreak in The Netherlands in 2003, one isolated from a person with conjunctivitis and one from a person who died as the result of infection, were used for an in vitro study of influenza A virus pathogenicity factors. The two HPAI H7N7 viruses differed in 15 amino acid positions in five gene segments. Assays were designed to investigate the role of each of these substitutions in attachment and entry, transcription and genome replication, and virus production and release as determined by hemagglutinin (HA), polymerase proteins, nonstructural protein 1 (NS1), and neuraminidase (NA). These in vitro studies confirmed the roles of the E627K substitution in basic polymerase 2 (PB2) and the A143T substitution in HA in pathogenicity observed in a mouse model previously. However, the in vitro studies identified a contribution of acidic polymerase (PA) and NA to the efficient replication in human cells of the fatal case virus, despite the fact that these are rarely marked as determinants of pathogenicity in in vivo studies. With the exception of PB2 E627K, all substitutions contributing to enhanced replication of the fatal case virus in vitro were present in poultry viruses prior to transmission to the human fatal case, indicating that viruses with enhanced replication efficiency in the mammalian host can be generated in poultry. Thus, detailed in vitro analyses of mutations facilitating replication of avian influenza viruses in mammalian cells are important to assess the zoonotic risk posed by these viruses and, in addition, highlight the value of in vitro studies to complement animal models.

H5N1 influenza viruses: outbreaks and biological properties

All known subtypes of influenza A viruses are maintained in wild waterfowl, the natural reservoir of these viruses. Influenza A viruses are isolated from a variety of animal species with varying morbidity and mortality rates. More importantly, influenza A viruses cause respiratory disease in humans with potentially fatal outcome. Local or global outbreaks in humans are typically characterized by excess hospitalizations and deaths. In 1997, highly pathogenic avian influenza viruses of the H5N1 subtype emerged in Hong Kong that transmitted to humans, resulting in the first documented cases of human death by avian influenza virus infection. A new outbreak started in July 2003 in poultry in Vietnam, Indonesia, and Thailand, and highly pathogenic avian H5N1 influenza viruses have since spread throughout Asia and into Europe and Africa. These viruses continue to infect humans with a high mortality rate and cause worldwide concern of a looming pandemic. Moreover, H5N1 virus outbreaks have had devastating effects on the poultry industries throughout Asia. Since H5N1 virus outbreaks appear to originate from Southern China, we here examine H5N1 influenza viruses in China, with an emphasis on their biological properties.

Resurrected pandemic influenza viruses

Influenza viruses continue to pose a major global public health problem. There is a need to better understand the pathogenicity and transmission of pandemic influenza viruses so that we may develop improved methods for their prevention and control. Reconstruction of the 1918 virus and studies elucidating the exceptional virulence and transmissibility of the virus are providing exciting new insights into this devastating pandemic strain. The primary approach has been to reconstruct and analyze recombinant viruses, in which genes of the 1918 virus are replaced with genes of contemporary influenza viruses of lesser virulence. This review highlights the current status of the field and discusses the molecular determinants of the 1918 pandemic virus that may have contributed to its virulence and spread. Identifying the exact genes responsible for the high virulence of the 1918 virus will be an important step toward understanding virulent influenza strains and will allow the world to better prepare for and respond to future influenza pandemics.

Temperature sensitivity on growth and/or replication of H1N1, H1N2 and H3N2 influenza A viruses isolated from pigs and birds in mammalian cells

Influenza A viruses have been isolated from a wide range of animal species, aquatic birds being the reservoir for their genetic diversity. Avian influenza viruses can be transmitted to humans, directly or indirectly through an intermediate host like pig. This study aimed to define in vitro conditions that could prove useful to evaluate the potential of influenza viruses to adapt to a different host. Growth of H1N1, H1N2 and H3N2 influenza viruses belonging to different lineages isolated from birds or pigs prior to 2005 was tested on MDCK or NPTr cell lines in the presence or absence of exogenous trypsin. Virus multiplication was compared at 33, 37 and 40 degrees C, the infection site temperatures in human, swine and avian hosts, respectively. Temperature sensitivity of PB2-, NP- and M-RNA replication was also tested by quantitative real-time PCR. Multiplication of avian viruses was cold-sensitive, whatever cell type. By contrast, temperature sensitivity of swine viruses was found to depend on the virus and the host cell: for an H1N1 swine isolate from 1982, multiplication was cold-sensitive on NPTr cells and undetectable at 40 degrees C. From genetic analyses, it appears that temperature sensitivity could involve other residues than PB2 residue 627 and could affect other steps of the replication cycle than replication.

Use of animal models to understand the pandemic potential of highly pathogenic avian influenza viruses

It has been 40 years since the last influenza pandemic and it is generally considered that another could occur at any time. Recent introductions of influenza A viruses from avian sources into the human population have raised concerns that these viruses may be a source of a future pandemic strain. Therefore, there is a need to better understand the pathogenicity of avian influenza viruses for mammalian species so that we may be better able to predict the pandemic potential of such viruses and develop improved methods for their prevention and control. In this review, we describe the virulence of H5 and H7 avian influenza viruses in the mouse and ferret models. The use of these models is providing exciting new insights into the contribution of virus and host responses toward avian influenza viruses, virus tropism, and virus transmissibility. Identifying the role of individual viral gene products and mapping the molecular determinants that influence the severity of disease observed following avian influenza virus infection is dependent on the use of reliable animal models. As avian influenza viruses continue to cause human disease and death, animal pathogenesis studies identify avenues of investigation for novel preventative and therapeutic agents that could be effective in the event of a future pandemic.