Host range restriction and pathogenicity in the context of influenza pandemic

Molecular events leading to the creation of a pandemic influenza virus

Influenza A virus is a potent pathogen of annual respiratory illness with huge potential of causing occasional pandemics of catastrophic consequences. In April 2009, a novel, swine-origin influenza A H1N1/09 virus was identified in Mexico which continued to spread globally. This unique virus emerged from an avian, human, Eurasian swine viral strain and a North American swine strain belonging to the lineage of the 1930 swine virus. Till date H1N1/09 pandemic has been relatively mild and lacks the previously described molecular markers of influenza A pathogenicity and transmissibility. In this review, we will discuss the molecular and antigenic determinants of this virus and its designation as a low pathogenic strain, which carries the potential to develop into a devastating strain with subsequent mutations and reassortments.

The NS segment of an H5N1 highly pathogenic avian influenza virus (HPAIV) is sufficient to alter replication efficiency, cell tropism, and host range of an H7N1 HPAIV

A reassortant avian influenza virus (designated FPV NS GD), carrying the NS-segment of the highly pathogenic avian influenza virus (HPAIV) strain A/Goose/Guangdong/1/96 (GD; H5N1) in the genetic background of the HPAIV strain A/FPV/Rostock/34 (FPV; H7N1), was rescued by reverse genetics. Remarkably, in contrast to the recombinant wild-type FPV (rFPV), the reassortant virus was able to replicate more efficiently in different human cell lines and primary mouse epithelia cells without prior adaptation. Moreover, FPV NS GD caused disease and death in experimentally infected mice and was detected in mouse lungs; in contrast, rFPV was not able to replicate in mice effectively. These results indicated an altered host range and increased virulence. Furthermore FPV NS GD showed pronounced pathogenicity in chicken embryos. In an attempt to define the molecular basis for the apparent differences, we determined that NS1 proteins of the H5N1 and H7N1 strains bound the antiviral kinase PKR and the F2F3 domain of cleavage and polyadenylation specificity factor 30 (CPSF30) with comparable efficiencies in vitro. However, FPV NS GD infection resulted in (i) increased expression of NS1, (ii) faster and stronger PKR inhibition, and (iii) stronger beta interferon promoter inhibition than rFPV. Taken together, the results shed further light on the importance of the NS segment of an H5N1 strain for viral replication, molecular pathogenicity, and host range of HPAIVs and the possible consequences of a reassortment between naturally occurring H7 and H5 type HPAIVs.

Adaptive strategies of the influenza virus polymerase for replication in humans

Transmission of influenza viruses into the human population requires surmounting barriers to cross-species infection. Changes in the influenza polymerase overcome one such barrier. Viruses isolated from birds generally contain polymerases with the avian-signature glutamic acid at amino acid 627 in the PB2 subunit. These polymerases display restricted activity in human cells. An adaptive change in this residue from glutamic acid to the human-signature lysine confers high levels of polymerase activity in human cells. This mutation permits escape from a species-specific restriction factor that targets polymerases from avian viruses. A 2009 swine-origin H1N1 influenza A virus recently established a pandemic infection in humans, even though the virus encodes a PB2 with the restrictive glutamic acid at amino acid 627. We show here that the 2009 H1N1 virus has acquired second-site suppressor mutations in its PB2 polymerase subunit that convey enhanced polymerase activity in human cells. Introduction of this polymorphism into the PB2 subunit of a primary avian isolate also increased polymerase activity and viral replication in human and porcine cells. An alternate adaptive strategy has also been identified, whereby introduction of a human PA subunit into an avian polymerase overcomes restriction in human cells. These data reveal a strategy used by the 2009 H1N1 influenza A virus and identify other pathways by which avian and swine-origin viruses may evolve to enhance replication, and potentially pathogenesis, in humans.

A genetically engineered waterfowl influenza virus with a deletion in the stalk of the neuraminidase has increased virulence for chickens

A deletion of about 20 amino acids in the stalk of the neuraminidase (NA) is frequently detected upon transmission of influenza A viruses from waterfowl to domestic poultry. Using reverse genetics, a recombinant virus derived from a wild duck influenza virus isolate, A/Mallard/Marquenterre/Z237/83 (MZ), and an NA stalk deletion variant (MZ-delNA) were produced. Compared to the wild type, the MZ-delNA virus showed a moderate growth advantage on avian cultured cells. In 4-week-old chickens inoculated intratracheally with the MZ-delNA virus, viral replication in the lungs, liver, and kidneys was enhanced and interstitial pneumonia lesions were more severe than with the wild-type virus. The MZ-delNA-inoculated chickens showed significantly increased levels of mRNAs encoding interleukin-6 (IL-6), transforming growth factor-beta4 (TGF-beta4), and CCL5 in the lungs and a higher frequency of apoptotic cells in the liver than did their MZ-inoculated counterparts. Molecular mechanisms possibly underlying the growth advantage of the MZ-delNA virus were explored. The measured enzymatic activities toward a small substrate were similar for the wild-type and deleted NA, but the MZ-delNA virus eluted from chicken erythrocytes at reduced rates. Pseudoviral particles expressing the MZ hemagglutinin in combination with the MZ-NA or MZ-delNA protein were produced from avian cultured cells with similar efficiencies, suggesting that the deletion in the NA stalk does not enhance the release of progeny virions and probably affects an earlier step of the viral cycle. Overall, our data indicate that a shortened NA stalk is a strong determinant of adaptation and virulence of waterfowl influenza viruses in chickens.

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.

Hemagglutinin-dependent tropism of H5N1 avian influenza virus for human endothelial cells

Although current H5N1 highly pathogenic avian influenza viruses (HPAIV) are inefficiently transmitted to humans, infected individuals can suffer from severe disease, often progressing rapidly to acute respiratory distress syndrome and multiorgan failure. This is in contrast with the situation with human influenza viruses, which in immunocompetent individuals usually cause only a respiratory disease which is less aggressive than that observed with avian H5N1 viruses. While the biological basis of inefficient transmission is well documented, the mechanisms by which the H5N1 viruses cause fatal disease remain unclear. In the present study, we demonstrate that human pulmonary microvascular endothelial cells (hPMEC) had a clearly higher susceptibility to infection by H5N1 HPAIV than to infection by human influenza viruses. This was measurable by de novo intracellular nucleoprotein production and virus replication. It was also related to a relatively higher binding capacity to cellular receptors. After infection of hPMEC, cell activation markers E-selectin and P-selectin were upregulated, and the proinflammatory cytokines interleukin-6 and beta interferon were secreted. H5N1 virus infection was also associated with an elevated rate of cell death. Reverse genetics analyses demonstrated a major role for the viral hemagglutinin in this cell tropism. Overall, avian H5N1 viruses have a particular receptor specificity targeting endothelial cells that is different from human influenza viruses, and this H5N1 receptor specificity could contribute to disease pathogenesis.

Attenuated strains of influenza A viruses do not induce degradation of RNA polymerase II

We have previously shown that infection with laboratory-passaged strains of influenza virus causes both specific degradation of the largest subunit of the RNA polymerase II complex (RNAP II) and inhibition of host cell transcription. When infection with natural human and avian isolates belonging to different antigenic subtypes was examined, we observed that all of these viruses efficiently induce the proteolytic process. To evaluate whether this process is a general feature of nonattenuated viruses, we studied the behavior of the influenza virus strains A/PR8/8/34 (PR8) and the cold-adapted A/Ann Arbor/6/60 (AA), which are currently used as the donor strains for vaccine seeds due to their attenuated phenotype. We have observed that upon infection with these strains, degradation of the RNAP II does not occur. Moreover, by runoff experiments we observe that PR8 has a reduced ability to inhibit cellular mRNA transcription. In addition, a hypervirulent PR8 (hvPR8) variant that multiplies much faster than standard PR8 (lvPR8) in infected cells and is more virulent in mice than the parental PR8 virus, efficiently induces RNAP II degradation. Studies with reassortant viruses containing defined genome segments of both hvPR8 and lvPR8 indicate that PA and PB2 subunits individually contribute to the ability of influenza virus to degrade the RNAP II. In addition, recently it has been reported that the inclusion of PA or PB2 from hvPR8 in lvPR8 recombinant viruses, highly increases their pathogenicity. Together, the data indicate that the capacity of the influenza virus to degrade RNAP II and inhibit the host cell transcription machinery is a feature of influenza A viruses that might contribute to their virulence.

Superior efficacy of a recombinant flagellin:H5N1 HA globular head vaccine is determined by the placement of the globular head within flagellin

Transmission of highly pathogenic avian influenza (HPAI) between birds and humans is an ongoing threat that holds potential for the emergence of a pandemic influenza strain. A major barrier to an effective vaccine against avian influenza has been the generally poor immunopotency of many of the HPAI strains coupled with the manufacturing constraints employing conventional methodologies. Fusion of flagellin, a toll-like receptor-5 ligand, to vaccine antigens has been shown to enhance the immune response to the fused antigen in preclinical studies. Here, we have evaluated the immunogenicity and efficacy of a panel of flagellin-based hemagglutinin (HA) globular head fusion vaccines in inbred mice. The HA globular head of these vaccines is derived from the A/Vietnam/1203/04 (VN04; H5N1) HA molecule. We find that replacement of domain D3 of flagellin with the VN04 HA globular head creates a highly effective vaccine that elicits protective HAI titers which protect mice against disease and death in a lethal challenge model.

Novel swine-origin influenza A virus in humans: another pandemic knocking at the door

Influenza A viruses represent a continuous pandemic threat. In April 2009, a novel influenza A virus, the so-called swine-origin influenza A (H1N1) virus (S-OIV), was identified in Mexico. Although S-OIV originates from triple-reassortant swine influenza A (H1) that has been circulating in North American pig herds since the end of the 1990s, S-OIV is readily transmitted between humans but is not epidemic in pigs. After its discovery, S-OIV rapidly spread throughout the world within few weeks. In this review, we sum up the current situation and put it into the context of the current state of knowledge of influenza and influenza pandemics. Some indications suggest that a pandemic may be mild but even "mild" pandemics can result in millions of deaths. However, no reasonable forecasts how this pandemic may develop can be made at this time. Despite stockpiling by many countries and WHO, antiviral drugs will be limited in case of pandemic and resistances may emerge. Effective vaccines are regarded to be crucial for the control of influenza pandemics. However, production capacities are restricted and development/production of a S-OIV vaccine will interfere with manufacturing of seasonal influenza vaccines. The authors are convinced that S-OIV should be taken seriously as pandemic threat and underestimation of the menace by S-OIV to be by far more dangerous than its overestimation.

Infection and replication of avian influenza H5N1 virus in an infected human

The highly pathogenic avian influenza H5N1 viruses usually cause severe diseases and high mortality in infected humans. However, the tissue tropism and underlying pathogenesis of H5N1 virus infection in humans have not been clearly elucidated yet. In this study, an autopsy was conducted to better understand H5N1 virus distributions in tissues of infected humans, and whether H5N1 virus can replicate in extrapulmonary tissues. We found that the lungs had the higher viral load than the spleen, whereas no detectable viruses in tissues of heart, liver, kidney, large intestine, small intestine, or brain. Specifically, the viral load was higher in the left lung (7.1 log10 copies per ml) in relation to the right lung (5.7 log10 copies per ml), resulting in more severe pathological damage in the left lung, and lung tissues contained both positive- and negative-stranded viral RNA. However, there existed a low level of H5N1 viruses in the spleen (3.8 log10 copies per ml), with the absence of positive-stranded viral RNA. Our results indicate that replication of H5N1 viruses mainly occurs in the lungs, and the degree of lung damage is highly correlated with the viral load in the lungs. The low-load viruses in the spleen might be introduced through blood circulation or other ways.

April 2009: an outbreak of swine-origin influenza A(H1N1) virus with evidence for human-to-human transmission

A swine-origin influenza A(H1N1) virus is currently responsible for an outbreak of infections in the human population, with laboratory-confirmed cases reported in several countries and clear evidence for human-to-human transmission. We provide a description of the outbreak at the end of April 2009, and a brief review of the zoonotic potential of swine influenza viruses.

Highly pathogenic H5N1 avian influenza virus: cause of the next pandemic?

Since 1997, when human infections with a highly pathogenic (HP) avian influenza A virus (AIV) subtype H5N1 - previously infecting only birds - were identified in a Hong Kong outbreak, global attention has focused on the potential for this virus to cause the next pandemic. From December 2003, an unprecedented H5N1 epizootic in poultry and migrating wild birds has spread across Asia and into Europe, the Middle East, and Africa. Humans in close contact with sick poultry and on rare occasion with other infected humans, have become infected. As of early March 2007, 12 countries have reported 167 deaths among 277 laboratory-confirmed human infections to WHO. WHO has declared the world to be in Phase 3 of a Pandemic Alert Period. This paper reviews the evolution of HP AIV H5N1, molecular changes that enable AIVs to infect and replicate in human cells and spread efficiently from person-to-person, and strategies to prevent the emergence of a pandemic virus.

How to find natural reservoir hosts from endemic prevalence in a multi-host population: a case study of influenza in waterfowl

The transmission dynamics of infectious diseases critically depend on reservoir hosts, which can sustain the pathogen (or maintain the transmission) in the population even in the absence of other hosts. Although a theoretical foundation of the transmission dynamics in a multi-host population has been established, no quantitative methods exist for the identification of natural reservoir hosts. For a host to maintain the transmission alone, the host-specific reproduction number (U), interpreted as the average number of secondary transmissions caused by a single primary case in the host(s) of interest in the absence of all other hosts, must be greater than unity. If the host-excluded reproduction number (Q), representing the average number of secondary transmissions per single primary case in other hosts in the absence of the host(s) of interest, is below unity, transmission cannot be maintained in the multi-host population in the absence of the focal host(s). The present study proposes a simple method for the identification of reservoir host(s) from observed endemic prevalence data across a range of host species. As an example, we analyze an aggregated surveillance dataset of influenza A virus in wild birds among which dabbling ducks exhibit higher prevalence compared to other bird species. Since the heterogeneous contact patterns between different host species are not directly observable, we test four different contact structures to account for the uncertainty. Meeting the requirements of U>1 and Q<1 for all four different contact structures, mallards and other dabbling ducks most likely constitute the reservoir community which plays a predominant role in maintaining the transmission of influenza A virus in the water bird population. We further discuss epidemiological issues which are concerned with the interpretation of influenza prevalence data, identifying key features to be fully clarified in the future.

Sequencing and mutational analysis of the non-coding regions of influenza A virus

The genome of influenza A virus consists of eight negative-stranded RNA segments which contain one or two coding regions flanked by the 3' and 5' non-coding regions (NCRs). Despite the importance of NCRs in replication and pathogenesis of influenza virus, sequencing of influenza virus genome has mainly been focused on coding regions of the individual genes and very limited NCR sequences are available. In this study, we sequenced the NCRs of seven influenza A virus strains of different host origin and varying pathogenicity using two recently developed methods [de Wit, E., Bestebroer, T.M., Spronken, M.I., Rimmelzwaan, G.F., Osterhaus, A.D., Fouchier, R.A., 2007. Rapid sequencing of the non-coding regions of influenza A virus. J. Virol. Methods 139, 85-89; Szymkowiak, C., Kwan, W.S., Su, Q., Toner, T.J., Shaw, A.R., Youil, R., 2003. Rapid method for the characterization of 3' and 5' UTRs of influenza viruses. J. Virol. Methods 107, 15-20]. In addition to sequence and length variation present in the segment-specific NCRs among different influenza strains, we also observed sequence variations at the fourth nucleotide of 3' NCR of polymerase genes. To evaluate the role of sequence change in the NCRs in reporter gene expression, we introduced mutations at the NCRs of two polymerase gene segments, PB1 and PA, and created the green fluorescent protein (GFP) reporter plasmids. By measuring the GFP expression level, we confirmed that single or two mutations introduced at the 3' and 5' NCRs of PB1 and PA gene could alter the protein expression levels. Our study reaffirms the importance of NCRs in influenza virus replication and further analysis of their roles will lead to better understanding of influenza pathogenesis.

Distinct glycan topology for avian and human sialopentasaccharide receptor analogues upon binding different hemagglutinins: a molecular dynamics perspective

Hemagglutinin (HA) binds to sialylated glycans exposed on the host cell surface in the initial stage of avian influenza virus infection. It has been previously hypothesized that glycan topology plays a critical role in the human adaptation of avian flu viruses, such as the potentially pandemic H5N1. Comparative molecular dynamics studies are complementary to experimental techniques, including glycan microarray, to understand the mechanism of species-specificity switch better. The examined systems comprise explicitly solvated trimeric forms of avian H3, H5, and swine H9 in complex with avian and human glycan receptor analogues--LSTa (alpha-2,3-linked lactoseries tetrasaccharide a) and LSTc (alpha-2,6-linked lactoseries tetrasaccharide c), respectively. The glycans adopted distinct topological profiles with inducible torsional angles when bound to different HAs. The corresponding receptor binding domain amino acid contact profiles were also distinct. Avian H5 was able to accommodate LSTc in a tightly "folded umbrella"-like topology through interactions with all five sugar residues. After considering conformational entropy, the relative binding free-energy changes, calculated using the molecular mechanics-generalized Born surface area technique, were in agreement with previous experimental findings and provided insights on electrostatic, van der Waals, desolvation, and entropic contributions to HA-glycan interactions. The topology profile and the relative abundance of free glycan receptors may influence receptor binding kinetics. Glycan composition and topological changes upon binding different HAs may be important determinants in species-specificity switch.

Swine influenza viruses a North American perspective

Influenza is a zoonotic viral disease that represents a health and economic threat to both humans and animals worldwide. Swine influenza (SI) was first recognized clinically in pigs in the Midwestern U.S., in 1918, coinciding with the human influenza pandemic known as the Spanish flu. Since that time SI has remained of importance to the swine industry throughout the world. In this review, the epidemiology of swine influenza virus (SIV) infection in North American pigs is described in detail. The first 80 years of SI remained relatively static, whereas the last decade has become dynamic with the establishment of many emerging subtypes. With the increasing number of novel subtypes and genetic variants, the control of SI has become increasingly difficult and innovative strategies to combat this economically important zoonotic disease are critical. Therefore, protective immune responses against influenza virus infections as well as new paradigms of vaccine development in pigs are discussed in the review. It is expected that the dynamic evolutionary changes of SIVs in North American pigs will continue, making currently available prophylactic approaches of limited use to control the spread and economic losses associated with this important swine pathogen.

Influenza virus pathogenicity is determined by caspase cleavage motifs located in the viral proteins

Almost all influenza virus proteins are found to contain caspase cleavage motifs. Two caspase cleavage consensus sequences, EXD downward arrowY and D/EXXD downward arrowY (caspase motifs) were identified in N- and C-terminal regions of influenza virus proteins nucleocapsid NP (positions D(16) and D(497)) and ionic channel M2 (positions D(23) and D(87)). Using reverse genetics with the highly-virulent avian influenza virus A/FPV/Rostock (H7N1), as a vector precursor, these NP and M2 caspase motifs were artificially altered by site-directed mutagenesis and pathogenicity of the generated caspase mutant viruses was tested in chickens. Three main groups of virus mutants were identified. The first group of mutants was characterized by high replication in cells and low virulence in chickens. These virus mutants possessed the altered N-terminal NP and C-terminal M2 caspase motifs. The second group of virus mutants, possessing the altered N-terminal caspase motif of M2, was characterized by attenuated replication in cultured cells and reduced pathogenic properties in chickens. Third, mutations generated in the C-terminus of NP were lethal and restricted virus rescue by reverse genetics, implying a critical role of this caspase site in virus replication. Thus, these data suggested that, (i) caspase motifs in virus proteins play a significant role in virus pathogenicity; (ii) the lack of direct correlation between replication potential and pathogenicity, observed in caspase mutants of the first virus group, implied that virus caspase motifs could affect immunopathogenesis during the infection process, rather than simply controlling virus production in target cells in the chicken host.

Pandemic influenza as a current threat

Pandemics of influenza emerge from the aquatic bird reservoir, adapt to humans, modify their severity, and cause seasonal influenza. The catastrophic Spanish H1N1 virus may have obtained all of its eight gene segments from the avian reservoir, whereas the Asian H2N2 and the Hong Kong H3N2 pandemics emerged by reassortment between the circulating human virus and an avian H2 or H3 donor. Of the 16 hemagglutinin subtypes, the H2, H5, H6, H7, and H9 viruses are considered to have pandemic potential. While this chapter focuses on the evolution of the Asian highly pathogenic (HP) H5N1 influenza virus, other subtypes are also considered. The unique features of the HP H5N1 viruses that have devastated the domestic poultry of Eurasia are discussed. Although they transmit poorly to humans, they continue to kill more than 60% of infected persons. It is unknown whether HP H5N1 will acquire human pandemic status; if it does not, another subtype eventually will do so, for a future influenza pandemic is inevitable.

Host restriction of avian influenza viruses at the level of the ribonucleoproteins

Although transmission of avian influenza viruses to mammals, particularly humans, has been repeatedly documented, adaptation and sustained transmission in the new host is a rare event that in the case of humans may result in pandemics. Host restriction involves multiple genetic determinants. Among the known determinants of host range, key determinants have been identified on the genes coding for the nucleoprotein and polymerase proteins that, together with the viral RNA segments, form the ribonucleoproteins (RNPs). The RNP genes form host-specific lineages and harbor host-associated genetic signatures. The functional significance of these determinants has been studied by reassortment and reverse genetics experiments, underlining the influence of the global genetic context. In some instances the molecular mechanisms have been approached, pointing to the importance of the polymerase activity and interaction with cellular host factors. Better knowledge of determinants of host restriction will allow monitoring of the pandemic potential of avian influenza viruses.

Avian Influenza A virus polymerase association with nucleoprotein, but not polymerase assembly, is impaired in human cells during the course of infection

Strong determinants of the host range of influenza A viruses have been identified on the polymerase complex formed by the PB1, PB2, and PA subunits and on the nucleoprotein (NP). In the present study, molecular mechanisms that may involve these four core proteins and contribute to the restriction of avian influenza virus multiplication in human cells have been investigated. The efficiencies with which the polymerase complexes of a human and an avian influenza virus isolate assemble and interact with the viral NP and cellular RNA polymerase II proteins were compared in mammalian and in avian infected cells. To this end, recombinant influenza viruses expressing either human or avian-derived core proteins with a PB2 protein fused to the One-Strep purification tag at the N or C terminus were generated. Copurification experiments performed on infected cell extracts indicate that the avian-derived polymerase is assembled and interacts physically with the cellular RNA polymerase II at least as efficiently as does the human-derived polymerase in human as well as in avian cells. Restricted growth of the avian isolate in human cells correlates with low levels of the core proteins in infected cell extracts and with poor association of the NP with the polymerase compared to what is observed for the human isolate. The NP-polymerase association is restored by a Glu-to-Lys substitution at residue 627 of PB2. Overall, our data point to viral and cellular factors regulating the NP-polymerase interaction as key determinants of influenza A virus host range. Recombinant viruses expressing a tagged polymerase should prove useful for further studies of the molecular interactions between viral polymerase and host factors during the infection cycle.

Analysis of N-glycans in embryonated chicken egg chorioallantoic and amniotic cells responsible for binding and adaptation of human and avian influenza viruses

The initial step essential in influenza virus infection is specific binding of viral hemagglutinin to host cell-surface glycan receptors. Influenza A virus specificity for the host is mediated by viral envelope hemagglutinin, that binds to receptors containing glycans with terminal sialic acids. Human viruses preferentially bind to alpha2-->6 linked sialic acids on receptors of host cells, whereas avian viruses are specific for the alpha2-->3 linkage on the target cells. Human influenza virus isolates more efficiently infect amniotic membrane (AM) cells than chorioallantoic membrane (CAM) cells. N-glycans were isolated from AM and CAM cells of 10-day-old chicken embryonated eggs and their structures were analyzed by multi-dimensional HPLC mapping and MALDI-TOF-MS techniques. Terminal N-acetylneuraminic acid contents in the two cell types were similar. However, molar percents of alpha2-->3 linkage preferentially bound by avian influenza virus were 27.2 in CAM cells and 15.4 in AM cells, whereas those of alpha2-->6 linkage favored by human influenza virus were 8.3 (CAM) and 14.2 (AM). Molar percents of sulfated glycans, recognized by human influenza virus, in CAM and AM cells were 3.8 and 12.7, respectively. These results have revealed structures and molar percents of N-glycans in CAM and AM cells important in determining human and avian influenza virus infection and viral adaptation.

Characterization of H5N1 influenza A viruses isolated from domestic green-winged teal

Two avian influenza virus strains, A/domestic green-winged teal/Hunan/67/2005 (H5N1) (D-GWT/67) and A/domestic green-winged teal/Hunan/79/2005 (H5N1) (D-GWT/79), were isolated from healthy domestic green-winged teals (Anas crecca) in Hunan Province, South China. Genomic analysis showed that both isolates were reassortants. The hemagglutinin (HA) genes of the two isolates were closely related to that of an H5N1 strain isolated from tree sparrow (A/tree sparrow/Henan/1/04). The neuraminidase (NA) genes and the internal protein genes of both isolates were closely related to those from A/chicken/Shantou/4231/2003-like (H5N1) viruses, with exception of the matrix (M) gene of D-GWT/79, which was closely related to that of the H7N3 strain A/mallard/Netherlands/12/2000 isolated from wild mallard duck. The virulence of the two isolates was examined in chickens, ducks, and mice. Both strains were found to be highly pathogenic in chickens and ducks, but showed low pathogenicity in mice. These findings contribute to the realization that domestic green-winged teals carrying the H5N1 virus may play an important role in transmitting the virus among birds.

Compatibility among polymerase subunit proteins is a restricting factor in reassortment between equine H7N7 and human H3N2 influenza viruses

Reassortment is an important driving force for influenza virus evolution, and a better understanding of the factors that affect this process could improve our ability to respond to future influenza pandemics and epidemics. To identify factors that restrict the generation of reassortant viruses, we cotransfected human embryonic kidney cells with plasmids for the synthesis of viral RNAs of both A/equine/Prague/1/56 (Prague; H7N7) and A/Yokohama/2017/03 (Yokohama; H3N2) viruses together with the supporting protein expression plasmids. Of the possible 256 genotypes, we identified 29 genotypes in 120 randomly plaque-picked reassortants examined. Analyses of these reassortants suggested that the formation of functional ribonucleoprotein (RNP) complexes was a restricting factor, a finding that correlated with the activities of RNP complexes composed of different combinations of the proteins from the two viruses, as measured in a minigenome assay. For at least one nonfunctional RNP complex (i.e., Prague PB2, Prague PB1, Yokohama PA, and Prague NP), the lack of activity was due to the inability of the three polymerase subunit proteins to form a heterotrimer. Adaptation of viruses possessing a gene encoding a chimera of the PA proteins of the two viruses and the remaining genes from Prague virus resulted in compensatory mutations in the PB2 and/or PA protein. These results indicate substantial incompatibility among the gene products of the two test viruses, a critical role for the RNP complex in the generation of reassortant viruses, and a functional interaction of PB2 and PA.

Recent avian H5N1 viruses exhibit increased propensity for acquiring human receptor specificity

Adaptation of avian influenza viruses for replication and transmission in the human host is believed to require mutations in the hemagglutinin glycoprotein (HA) which enable binding to human alpha2-6 sialosides and concomitant reduction in affinity for avian alpha2-3 linked sialosides. Here, we show by glycan microarray analyses that the two mutations responsible for such specificity changes in 1957 H2N2 and 1968 H3N2 pandemic viruses, when inserted into recombinant HAs or intact viruses of some recent avian H5N1 isolates (clade 2.2), impart such attributes. This propensity to adapt to human receptors is primarily dependent on arginine at position 193 within the receptor-binding site, as well as loss of a vicinal glycosylation site. Widespread occurrence of these susceptible H5N1 clade 2.2 influenza strains has already occurred in Europe, the Middle East, and Africa. Thus, these avian strains should be considered high-risk, because of their significantly lower threshold for acquiring human receptor specificity and, therefore, warrant increased surveillance and further study.

Multivalent HA DNA vaccination protects against highly pathogenic H5N1 avian influenza infection in chickens and mice

BACKGROUND

Sustained outbreaks of highly pathogenic avian influenza (HPAI) H5N1 in avian species increase the risk of reassortment and adaptation to humans. The ability to contain its spread in chickens would reduce this threat and help maintain the capacity for egg-based vaccine production. While vaccines offer the potential to control avian disease, a major concern of current vaccines is their potency and inability to protect against evolving avian influenza viruses.

METHODOLOGY / PRINCIPAL FINDINGS

The ability of DNA vaccines encoding hemagglutinin (HA) proteins from different HPAI H5N1 serotypes was evaluated for its ability to elicit neutralizing antibodies and to protect against homologous and heterologous HPAI H5N1 strain challenge in mice and chickens after DNA immunization by needle and syringe or with a pressure injection device. These vaccines elicited antibodies that neutralized multiple strains of HPAI H5N1 when given in combinations containing up to 10 HAs. The response was dose-dependent, and breadth was determined by the choice of the influenza virus HA in the vaccine. Monovalent and trivalent HA vaccines were tested first in mice and conferred protection against lethal H5N1 A/Vietnam/1203/2004 challenge 68 weeks after vaccination. In chickens, protection was observed against heterologous strains of HPAI H5N1 after vaccination with a trivalent H5 serotype DNA vaccine with doses as low as 5 microg DNA given twice either by intramuscular needle injection or with a needle-free device.

CONCLUSIONS/SIGNIFICANCE

DNA vaccines offer a generic approach to influenza virus immunization applicable to multiple animal species. In addition, the ability to substitute plasmids encoding different strains enables rapid adaptation of the vaccine to newly evolving field isolates.

Differential host gene expression in cells infected with highly pathogenic H5N1 avian influenza viruses

In order to understand the molecular mechanisms by which different strains of avian influenza viruses overcome host response in birds, we used a complete chicken genome microarray to compare early gene expression levels in chicken embryo fibroblasts (CEF) infected with two avian influenza viruses (AIV), A/CK/Hong Kong/220/97 and A/Egret/Hong Kong/757.2/02, with different replication characteristics. Gene ontology revealed that the genes with altered expression are involved in many vital functional classes including protein metabolism, translation, transcription, host defense/immune response, ubiquitination and the cell cycle. Among the immune-related genes, MEK2, MHC class I, PDCD10 and Bcl-3 were selected for further expression analysis at 24 hpi using semi-quantitive RT-PCR. Infection of CEF with A/Egret/Hong Kong/757.2/02 resulted in a marked repression of MEK2 and MHC class I gene expression levels. Infection of CEF with A/CK/Hong Kong/220/97 induced an increase of MEK2 and a decrease in PDCD10 and Bcl-3 expression levels. The expression levels of alpha interferon (IFN-alpha), myxovirus resistance 1 (Mx1) and interleukin-8 (IL-8) were also analyzed at 24 hpi, showing higher expression levels of all of these genes after infection with A/CK/Hong Kong/220/97 compared to A/Egret/Hong Kong/757.2/02. In addition, comparison of the NS1 sequences of the viruses revealed amino acid differences that may explain in part the differences in IFN-alpha expression observed. Microarray gene expression analysis has proven to be a useful tool on providing important insights into how different AIVs affect host gene expression and how AIVs may use different strategies to evade host response and replicate in host cells.

CCR2+ monocyte-derived dendritic cells and exudate macrophages produce influenza-induced pulmonary immune pathology and mortality

Infection with pathogenic influenza virus induces severe pulmonary immune pathology, but the specific cell types that cause this have not been determined. We characterized inflammatory cell types in mice that overexpress MCP-1 (CCL2) in the lungs, then examined those cells during influenza infection of wild-type (WT) mice. Lungs of both naive surfactant protein C-MCP mice and influenza-infected WT mice contain increased numbers of CCR2(+) monocytes, monocyte-derived DC (moDC), and exudate macrophages (exMACs). Adoptively transferred Gr-1(+) monocytes give rise to both moDC and exMACs in influenza-infected lungs. MoDC, the most common inflammatory cell type in infected lungs, induce robust naive T cell proliferation and produce NO synthase 2 (NOS2), whereas exMACs produce high levels of TNF-alpha and NOS2 and stimulate the proliferation of memory T cells. Relative to WT mice, influenza-infected CCR2-deficient mice display marked reductions in the accumulation of monocyte-derived inflammatory cells, cells producing NOS2, the expression of costimulatory molecules, markers of lung injury, weight loss, and mortality. We conclude that CCR2(+) monocyte-derived cells are the predominant cause of immune pathology during influenza infection and that such pathology is markedly abrogated in the absence of CCR2.

Identification of human-to-human transmissibility factors in PB2 proteins of influenza A by large-scale mutual information analysis

Background

The identification of mutations that confer unique properties to a pathogen, such as host range, is of fundamental importance in the fight against disease. This paper describes a novel method for identifying amino acid sites that distinguish specific sets of protein sequences, by comparative analysis of matched alignments. The use of mutual information to identify distinctive residues responsible for functional variants makes this approach highly suitable for analyzing large sets of sequences. To support mutual information analysis, we developed the AVANA software, which utilizes sequence annotations to select sets for comparison, according to user-specified criteria. The method presented was applied to an analysis of influenza A PB2 protein sequences, with the objective of identifying the components of adaptation to human-to-human transmission, and reconstructing the mutation history of these components.

Results

We compared over 3,000 PB2 protein sequences of human-transmissible and avian isolates, to produce a catalogue of sites involved in adaptation to human-to-human transmission. This analysis identified 17 characteristic sites, five of which have been present in human-transmissible strains since the 1918 Spanish flu pandemic. Sixteen of these sites are located in functional domains, suggesting they may play functional roles in host-range specificity. The catalogue of characteristic sites was used to derive sequence signatures from historical isolates. These signatures, arranged in chronological order, reveal an evolutionary timeline for the adaptation of the PB2 protein to human hosts.

Conclusion

By providing the most complete elucidation to date of the functional components participating in PB2 protein adaptation to humans, this study demonstrates that mutual information is a powerful tool for comparative characterization of sequence sets. In addition to confirming previously reported findings, several novel characteristic sites within PB2 are reported. Sequence signatures generated using the characteristic sites catalogue characterize concisely the adaptation characteristics of individual isolates. Evolutionary timelines derived from signatures of early human influenza isolates suggest that characteristic variants emerged rapidly, and remained remarkably stable through subsequent pandemics. In addition, the signatures of human-infecting H5N1 isolates suggest that this avian subtype has low pandemic potential at present, although it presents more human adaptation components than most avian subtypes.

Ecology of avian influenza virus in birds

Avian influenza A virus (an orthomyxovirus) is a zoonotic pathogen with a natural reservoir entirely in birds. The influenza virus genome is an 8-segment single-stranded RNA with high potential for in situ recombination. Two segments code for the hemagglutinin (H) and neuraminidase (N) antigens used for host-cell entry. At present, 16 H and 9 N subtypes are known, for a total of 144 possible different influenza subtypes, each with potentially different host susceptibility. With >10,000 species of birds found in nearly every terrestrial and aquatic habitat, there are few places on earth where birds cannot be found. The avian immune system differs from that of humans in several important features, including asynchronous B and T lymphocyte systems and a polymorphic multigene immune complex, but little is known about the immunogenetics of pathogenic response. Postbreeding dispersal and migration and a naturally high degree of environmental vagility mean that wild birds have the potential to be vectors that transmit highly pathogenic variants great distances from the original sources of infection.

Evaluation of a vectored equine herpesvirus type 1 (EHV-1) vaccine expressing H3 haemagglutinin in the protection of dogs against canine influenza

In 2004, canine influenza virus (CIV) was identified as a respiratory pathogen of dogs for the first time and found to be closely related to H3N8 equine influenza virus (EIV). We generated a recombinant vectored vaccine that expresses H3 of a recent isolate of EIV using equine herpesvirus type 1 (EHV-1) as the delivery vehicle. This EHV-1 vectored vaccine exhibited robust and stable EIV H3 expression and induced a strong influenza virus-specific response in both mice and dogs upon intranasal or subcutaneous administration. Furthermore, upon challenge with the recent CIV isolate A/canine/PA/10915-07, protection of vaccinated dogs could be demonstrated by a significant reduction in clinical sings, and, more importantly, by a significant reduction in virus shedding. We concluded that the EHV-1/H3 recombinant vector can be a valuable alternative for protection of dogs against clinical disease induced by CIV and can significantly reduce virus spread.

Source of high pathogenicity of an avian influenza virus H5N1: why H5 is better cleaved by furin

The origin of the high pathogenicity of an emerging avian influenza H5N1 due to the -RRRKK- insertion at the cleavage loop of the hemagglutinin H5, was studied using the molecular dynamics technique, in comparison with those of the noninserted H5 and H3 bound to the furin (FR) active site. The cleavage loop of the highly pathogenic H5 was found to bind strongly to the FR cavity, serving as a conformation suitable for the proteolytic reaction. With this configuration, the appropriate interatomic distances were found for all three reaction centers of the enzyme-substrate complex: the arrangement of the catalytic triad, attachment of the catalytic Ser(368) to the reactive S1-Arg, and formation of the oxyanion hole. Experimentally, the--RRRKK--insertion was also found to increase in cleavage of hemagglutinin by FR. The simulated data provide a clear answer to the question of why inserted H5 is better cleaved by FR than the other subtypes, explaining the high pathogenicity of avian influenza H5N1.

Quantitative biochemical rationale for differences in transmissibility of 1918 pandemic influenza A viruses

The human adaptation of influenza A viruses is critically governed by the binding specificity of the viral surface hemagglutinin (HA) to long (chain length) alpha2-6 sialylated glycan (alpha2-6) receptors on the human upper respiratory tissues. A recent study demonstrated that whereas the 1918 H1N1 pandemic virus, A/South Carolina/1/1918 (SC18), with alpha2-6 binding preference transmitted efficiently, a single amino acid mutation on HA resulted in a mixed alpha2-3 sialylated glycan (alpha2-3)/alpha2-6 binding virus (NY18) that transmitted inefficiently. To define the biochemical basis for the observed differences in virus transmission, in this study, we have developed an approach to quantify the multivalent HA-glycan interactions. Analysis of the molecular HA-glycan contacts showed subtle changes resulting from the single amino acid variations between SC18 and NY18. The effect of these changes on glycan binding is amplified by multivalency, resulting in quantitative differences in their long alpha2-6 glycan binding affinities. Furthermore, these differences are also reflected in the markedly distinct binding pattern of SC18 and NY18 HA to the physiological glycans present in human upper respiratory tissues. Thus, the dramatic lower binding affinity of NY18 to long alpha2-6 glycans, as against a mixed alpha2-3/6 binding, correlates with its inefficient transmission. In summary, this study establishes a quantitative biochemical correlate for influenza A virus transmission.

Injectable peramivir mitigates disease and promotes survival in ferrets and mice infected with the highly virulent influenza virus, A/Vietnam/1203/04 (H5N1)

The post-exposure therapeutic efficacy of injectable peramivir against highly pathogenic avian influenza type A H5N1 was evaluated in mice and in ferrets. Seventy to eighty percent of the H5N1-infected peramivir-treated mice, and 70% in the oseltamivir treated mice survived the 15-day study period, as compared to 36% in control (vehicle) group. Ferrets were infected intranasally with H5N1 followed by treatment with multiple doses of peramivir. In two of three trials, a statistically significant increase in survival over a 16-18 day period resulted from peramivir treatment, with improved survival of 40-64% in comparison to mock-treated or untreated animals. Injected peramivir mitigates virus-induced disease, reduces infectious virus titers in the lungs and brains and promotes survival in ferrets infected intranasally with this highly neurovirulent isolate. A single intramuscular peramivir injection protected mice against severe disease outcomes following infection with highly pathogenic avian influenza and multi-dose treatment was efficacious in ferrets.

Lessons from the past: familial aggregation analysis of fatal pandemic influenza (Spanish flu) in Iceland in 1918

The pandemic influenza of 1918 (Spanish flu) killed 21-50 million people globally, including in Iceland, where the characteristics and spread of the epidemic were well documented. It has been postulated that genetic host factors may have contributed to this high mortality. We identified 455 individuals who died of the Spanish flu in Iceland during a 6-week period during the winter of 1918, representing >92% of all fatal domestic cases mentioned by historical accounts. The highest case fatality proportion was 2.8%, and peak excess mortality was 162/100,000/week. Fatality proportions were highest among infants, young adults, and the elderly. A genealogical database was used to study relatedness and relative risk (RR) of the fatal influenza victims and relatives of their unaffected mates. The significance of these RR computations was assessed by drawing samples randomly from the genealogical database matched for age, sex, and geographical distribution. Familial aggregation of fatalities was seen, with RRs for death ranging from 3.75 for first-degree relatives (P < 0.0001) to 1.82 (P = 0.005), 1.12 (P = 0.252), and 1.47 (P = 0.0001) for second- to fourth-degree relatives of fatal influenza victims, respectively. The RRs within the families of unaffected mates of fatal influenza victims were 2.95 (P < 0.0001), 1.27 (P = 0.267), 1.35 (P = 0.04), and 1.42 (P = 0.001), for first- to fourth-degree relatives, respectively. In conclusion, the risk of death from the Spanish flu was similar within families of patients who succumbed to the illness and within families of their mates who survived. Our data do not provide conclusive evidence for the role of genetic factors in susceptibility to the Spanish flu.

Influenza A virus strains differ in sensitivity to the antiviral action of Mx-GTPase

Interferon-mediated host responses are of great importance for controlling influenza A virus infections. It is well established that the interferon-induced Mx proteins possess powerful antiviral activities toward most influenza viruses. Here we analyzed a range of influenza A virus strains for their sensitivities to murine Mx1 and human MxA proteins and found remarkable differences. Virus strains of avian origin were highly sensitive to Mx1, whereas strains of human origin showed much weaker responses. Artificial reassortments of the viral components in a minireplicon system identified the viral nucleoprotein as the main target structure of Mx1. Interestingly, the recently reconstructed 1918 H1N1 "Spanish flu" virus was much less sensitive than the highly pathogenic avian H5N1 strain A/Vietnam/1203/04 when tested in a minireplicon system. Importantly, the human 1918 virus-based minireplicon system was almost insensitive to inhibition by human MxA, whereas the avian influenza A virus H5N1-derived system was well controlled by MxA. These findings suggest that Mx proteins provide a formidable hurdle that hinders influenza A viruses of avian origin from crossing the species barrier to humans. They further imply that the observed insensitivity of the 1918 virus-based replicon to the antiviral activity of human MxA is a hitherto unrecognized characteristic of the "Spanish flu" virus that may contribute to the high virulence of this unusual pandemic strain.

Glycan topology determines human adaptation of avian H5N1 virus hemagglutinin

A switch in specificity of avian influenza A viruses' hemagglutinin (HA) from avian-like (alpha2-3 sialylated glycans) to human-like (alpha2-6 sialylated glycans) receptors is believed to be associated with their adaptation to infect humans. We show that a characteristic structural topology--and not the alpha2-6 linkage itself--enables specific binding of HA to alpha2-6 sialylated glycans and that recognition of this topology may be critical for adaptation of HA to bind glycans in the upper respiratory tract of humans. An integrated biochemical, analytical and data mining approach demonstrates that HAs from the human-adapted H1N1 and H3N2 viruses, but not H5N1 (bird flu) viruses, specifically bind to long alpha2-6 sialylated glycans with this topology. This could explain why H5N1 viruses have not yet gained a foothold in the human population. Our findings will enable the development of additional strategies for effective surveillance and potential therapeutic interventions for H5N1 and possibly other influenza A viruses.

Hepadnaviruses have a narrow host range — do they?

Host range describes the range of species that a virus can infect to productively propagate itself. Productive infection requires compatibility between virus and host molecules. Thus host range may be restricted by lack of appropriate permissivity factors;alternatively, hosts may actively counteract infection using restriction factors. Incompatibility between virus and host can manifest on the level of individual cells,of tissues or organs,and of the entire organism. All hepatitis B viruses are hepatotropic,but individual viruses infect the livers of only selected mammalian (orthohepadnaviruses) and avian (avihepadnaviruses) hosts. Hence a narrow host range is thought to be a salient feature of hepadnaviruses. Here we briefly review general mechanisms of host range restriction,and summarise older as well as recent data pertaining to hepadnaviral host range. Clearly,the term species-specific is inadequate for many hepadnaviruses because they can infect different species from one genus,and even species from different genera. For a few others,only a single species,or genus,has been identified that supports efficient infection;however,this could as well relate to the restricted number of experimentally addressable test species. Together with the uncertainty about quantitative phylogenetic relationships between species,still largely based on morphological rather than molecular criteria,this leaves the term narrow open to interpretation. Finally,few if any of the host molecules enabling productive infection by a hepadnavirus have unambiguously been identified,the role of restriction factors has not yet been assessed,and even on the virus side the so-called host determining regions in the PreS domains of the large envelope proteins appear to be relevant only under specialised experimental conditions. Hence this important aspect of hepadnavirus biology is still far from being understood.

Avian and 1918 Spanish influenza a virus NS1 proteins bind to Crk/CrkL Src homology 3 domains to activate host cell signaling

NS1 (nonstructural protein 1) is an important virulence factor of the influenza A virus. We observed that NS1 proteins of the 1918 pandemic virus (A/Brevig Mission/1/18) and many avian influenza A viruses contain a consensus Src homology 3 (SH3) domain-binding motif. Screening of a comprehensive human SH3 phage library revealed the N-terminal SH3 of Crk and CrkL as the preferred binding partners. Studies with recombinant proteins confirmed avid binding of NS1 proteins of the 1918 virus and a representative avian H7N3 strain to Crk/CrkL SH3 but not to other SH3 domains tested, including p85alpha and p85beta. Endogenous CrkL readily co-precipitated NS1 from cells infected with the H7N3 virus. In transfected cells association with CrkL was observed for NS1 of the 1918 and H7N3 viruses but not A/Udorn/72 or A/WSN/33 NS1 lacking this sequence motif. SH3 binding was dispensable for suppression of interferon-induced gene expression by NS1 but was associated with enhanced phosphatidylinositol 3-kinase signaling, as evidenced by increased Akt phosphorylation. Thus, the Spanish Flu virus resembles avian influenza A viruses in its ability to recruit Crk/CrkL to modulate host cell signaling.

Identification of H2N3 influenza A viruses from swine in the United States

Although viruses of each of the 16 influenza A HA subtypes are potential human pathogens, only viruses of the H1, H2, and H3 subtype are known to have been successfully established in humans. H2 influenza viruses have been absent from human circulation since 1968, and as such they pose a substantial human pandemic risk. In this report, we isolate and characterize genetically similar avian/swine virus reassortant H2N3 influenza A viruses isolated from diseased swine from two farms in the United States. These viruses contained leucine at position 226 of the H2 protein, which has been associated with increased binding affinity to the mammalian alpha2,6Gal-linked sialic acid virus receptor. Correspondingly, the H2N3 viruses were able to cause disease in experimentally infected swine and mice without prior adaptation. In addition, the swine H2N3 virus was infectious and highly transmissible in swine and ferrets. Taken together, these findings suggest that the H2N3 virus has undergone some adaptation to the mammalian host and that their spread should be very closely monitored.

The role of the N-terminal caspase cleavage site in the nucleoprotein of influenza A virus in vitro and in vivo

The N-terminal caspase cleavage in the nucleoprotein (NP) of influenza A virus is correlated with the host origin of the virus, thus could be a molecular determinant for host range. We studied how mutations targeting the NP cleavage motif of human and avian influenza viruses affect virus replication in vitro and in vivo. The "avian-like" D16-->G substitution in the NP, which makes this protein resistant to cleavage, did not significantly affect the human A/Puerto Rico/8/34 (H1N1) virus replication in vitro but decreased the lethality of this virus in mice by 68-fold. Gene incompatibility contributed to the attenuated phenotype of the reassortant A/Puerto Rico/8/34 virus with avian NP derived from A/Teal/Hong Kong/w312/97 (H6N1) virus in vitro and in vivo. Insertion of the "human-like" G16-->D mutation into avian NP, which resulted in susceptibility to caspase cleavage, did not rescue virulence, but made the reassortant virus even more attenuated. Introducing the human-like G16-->D substitution into the NP of highly pathogenic A/Vietnam/1203/04 (H5N1) virus decreased lethality in mice. We confirmed that position 16, which associated with the N-terminal caspase cleavage of the NP, is important for optimal virus fitness in vitro and in vivo. An avian-like mutation at position 16 in the NP of human virus as well as a human-like substitution at this residue in avian NP both resulted in virus attenuation.

Up to new tricks - a review of cross-species transmission of influenza A viruses

Influenza is a highly contagious disease that has burdened both humans and animals since ancient times. In humans, the most dramatic consequences of influenza are associated with periodically occurring pandemics. Pandemics require the emergence of an antigenically novel virus to which the majority of the population lacks protective immunity. Historically, influenza A viruses from animals have contributed to the generation of human pandemic viruses and they may do so again in the future. It is, therefore, critical to understand the epidemiological and molecular mechanisms that allow influenza A viruses to cross species barriers. This review summarizes the current knowledge of influenza ecology, and the viral factors that are thought to determine influenza A virus species specificity.

Differential polymerase activity in avian and mammalian cells determines host range of influenza virus

As recently shown, mutations in the polymerase genes causing increased polymerase activity in mammalian cells are responsible for the adaptation of the highly pathogenic avian influenza virus SC35 (H7N7) to mice (G. Gabriel et al., Proc. Natl. Acad. Sci. USA 102:18590-18595, 2005). We have now compared mRNA, cRNA, and viral RNA levels of SC35 and its mouse-adapted variant SC35M in avian and mammalian cells. The increase in levels of transcription and replication of SC35M in mammalian cells was linked to a decrease in avian cells. Thus, the efficiency of the viral polymerase is a determinant of both host specificity and pathogenicity.

Detecting emerging transmissibility of avian influenza virus in human households

Accumulating infections of highly pathogenic H5N1 avian influenza in humans underlines the need to track the ability of these viruses to spread among humans. A human-transmissible avian influenza virus is expected to cause clusters of infections in humans living in close contact. Therefore, epidemiological analysis of infection clusters in human households is of key importance. Infection clusters may arise from transmission events from (i) the animal reservoir, (ii) humans who were infected by animals (primary human-to-human transmission), or (iii) humans who were infected by humans (secondary human-to-human transmission). Here we propose a method of analysing household infection data to detect changes in the transmissibility of avian influenza viruses in humans at an early stage. The method is applied to an outbreak of H7N7 avian influenza virus in The Netherlands that was the cause of more than 30 human-to-human transmission events. The analyses indicate that secondary human-to-human transmission is plausible for the Dutch household infection data. Based on the estimates of the within-household transmission parameters, we evaluate the effectiveness of antiviral prophylaxis, and conclude that it is unlikely that all household infections can be prevented with current antiviral drugs. We discuss the applicability of our method for the detection of emerging human-to-human transmission of avian influenza viruses in particular, and for the analysis of within-household infection data in general.

Recent outbreaks of emerging diseases such as SARS and H5N1 avian influenza have underlined the fact that animal pathogens may acquire the ability to spread efficiently in humans. Monitoring the transmissibility of pathogens from the animal reservoir in humans is key for early detection of epidemic spread, and for effective control. Here we have used data from a small but well-defined study of H7N7 avian influenza virus transmission in human households to estimate the transmissibility of H7N7 avian influenza in humans living in close contact. The analyses make use of household final size data (i.e., the number of individuals in the household who are ultimately infected), which, for many pathogens, are easily collected. For the H7N7 data, the analyses indicate that the transmission chain in humans may well have extended beyond the first generation of infections in humans, and that less than half of the household infections could have been prevented by antiviral prophylaxis. Our method of analysis provides a rapid and generally applicable tool that can be used to monitor emerging human transmissibility of pathogens from the animal reservoir.

The critically ill avian influenza A (H5N1) patient

OBJECTIVE

This review examines perspectives of human infection with avian influenza A H5N1 (AI H5N1), specifically focusing on the presentation, diagnosis, and management of those critically ill with AI H5N1.

DATA SOURCE

PubMed (1966-2006), PubMed "related articles," publications and Web sites of the World Health Organization and the Centers for Disease Control and Prevention, personal files, abstract proceedings, and reference lists.

STUDY SELECTION

We reviewed English-language publications pertaining to clinical presentation, diagnosis, and management of AI H5N1 and infection control expressly relating to the intensive care setting.

DATA SYNTHESIS

The majority of reported patients with AI H5N1 are critically ill and require intensive care management. These patients progress rapidly to severe acute respiratory distress syndrome. Multiorgan failure occurs in a large proportion. Because of the nonspecific clinical, laboratory, and radiologic features, it is critical to seek a history of exposure to poultry or wild birds in suspected cases. Reverse transcription polymerase chain reaction performed on nasopharyngeal aspirate is the most reliable method for the laboratory diagnosis of AI H5N1. Treatment includes starting neuraminidase inhibitor oseltamivir as early as possible in addition to the standard supportive management. Aerosol generating procedures should be minimized to avoid nosocomial transmission. Strict infection control procedures are paramount to staff safety, although human-to-human transmission is rare as of this time.

CONCLUSIONS

Many patients with AI H5N1 are critically ill either at presentation or shortly thereafter. Intensivists and intensive care units are therefore at the front line for this new cause of severe lung injury. Practitioners must be familiar with the nonspecific presentation of AI H5N1 and its diagnostic and therapeutic options. Although treating the infected patient with AI H5N1 is a priority, safeguarding healthcare workers and other patients must be considered of equal priority.