Growth of H5N1 influenza A viruses in the upper respiratory tracts of mice

Co-infection subverts mucosal immunity in the upper respiratory tract

Polymicrobial interactions on mucosal surfaces can influence inflammation, immunity, and disease outcome. Here, we review how host responses to colonization in the upper respiratory tract with the bacterial pathogen Streptococcus pneumoniae can be altered by co-infection. Recent advances provide a mechanistic understanding of how mucosal immunity can be subverted at distinct immunological time-points during pneumococcal colonization by other pathogens such as Haemophilus influenzae, influenza type A and Staphylococcus aureus. These examples use animal models of co-infection to highlight how otherwise effective host responses can be rendered ineffective by co-infection, and vice versa. The complex microbial ecology of mucosal sites must be considered to fully understand how immune responses in a natural setting influence the outcome of host-pathogen interactions.

The Mouse as Model System to Study Host-Pathogen Interactions in Influenza A Infections

The mouse is one of the most important mammalian model systems for studying host-pathogen-interactions during influenza A virus infections and for assessing the virulence of newly emerging influenza viruses. Here, we provide the basic protocols for infecting mice with influenza virus and studying the main pathological changes associated with disease. Critical parameters, e.g., virus variants and subtypes or mouse strains, are discussed. Curr. Protoc. Mouse Biol. 2:177-205 © 2012 by John Wiley & Sons, Inc.

Low pathogenic avian influenza isolates from wild birds replicate and transmit via contact in ferrets without prior adaptation

Direct transmission of avian influenza viruses to mammals has become an increasingly investigated topic during the past decade; however, isolates that have been primarily investigated are typically ones originating from human or poultry outbreaks. Currently there is minimal comparative information on the behavior of the innumerable viruses that exist in the natural wild bird host. We have previously demonstrated the capacity of numerous North American avian influenza viruses isolated from wild birds to infect and induce lesions in the respiratory tract of mice. In this study, two isolates from shorebirds that were previously examined in mice (H1N9 and H6N1 subtypes) are further examined through experimental inoculations in the ferret with analysis of viral shedding, histopathology, and antigen localization via immunohistochemistry to elucidate pathogenicity and transmission of these viruses. Using sequence analysis and glycan binding analysis, we show that these avian viruses have the typical avian influenza binding pattern, with affinity for cell glycoproteins/glycolipids having terminal sialic acid (SA) residues with α 2,3 linkage [Neu5Ac(α2,3)Gal]. Despite the lack of α2,6 linked SA binding, these AIVs productively infected both the upper and lower respiratory tract of ferrets, resulting in nasal viral shedding and pulmonary lesions with minimal morbidity. Moreover, we show that one of the viruses is able to transmit to ferrets via direct contact, despite its binding affinity for α 2,3 linked SA residues. These results demonstrate that avian influenza viruses, which are endemic in aquatic birds, can potentially infect humans and other mammals without adaptation. Finally this work highlights the need for additional study of the wild bird subset of influenza viruses in regard to surveillance, transmission, and potential for reassortment, as they have zoonotic potential.

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.

Integrated clinical, pathologic, virologic, and transcriptomic analysis of H5N1 influenza virus-induced viral pneumonia in the rhesus macaque

Viral pneumonia has been frequently reported during early stages of influenza virus pandemics and in many human cases of highly pathogenic avian influenza (HPAI) H5N1 virus infection. To better understand the pathogenesis of this disease, we produced nonlethal viral pneumonia in rhesus macaques by using an HPAI H5N1 virus (A/Anhui/2/2005; referred to as Anhui/2). Infected macaques were monitored for 14 days, and tissue samples were collected at 6 time points for virologic, histopathologic, and transcriptomic analyses. Anhui/2 efficiently replicated in the lung from 12 h to 3 days postinfection (p.i.) and caused temporal but severe pneumonia that began to resolve by day 14. Lung transcriptional changes were first observed at 6 h, and increased expression of vascular permeability regulators and neutrophil chemoattractants correlated with increased serum leakage and neutrophil infiltration in situ. Additional inflammatory, antiviral, and apoptotic genes were upregulated from 12 h, concurrent with viral antigen detection and increasing immune cell populations. A shift toward upregulation of acquired immunity was apparent after day 6. Expression levels of established immune cell molecular markers revealed remarkable similarity with pathological findings, indicating early and robust neutrophil infiltration, a slight delay in macrophage accumulation, and abundant late populations of T lymphocytes. We also characterized the putative mechanisms regulating a unique, pneumonia-associated biphasic fever pattern. Thus, this study is the first to use a comprehensive and integrative approach to delineate specific molecular mechanisms regulating influenza virus-induced pneumonia in nonhuman primates, an important first step toward better management of human influenza virus disease.

Two years after pandemic influenza A/2009/H1N1: what have we learned?

The world had been anticipating another influenza pandemic since the last one in 1968. The pandemic influenza A H1N1 2009 virus (A/2009/H1N1) finally arrived, causing the first pandemic influenza of the new millennium, which has affected over 214 countries and caused over 18,449 deaths. Because of the persistent threat from the A/H5N1 virus since 1997 and the outbreak of the severe acute respiratory syndrome (SARS) coronavirus in 2003, medical and scientific communities have been more prepared in mindset and infrastructure. This preparedness has allowed for rapid and effective research on the epidemiological, clinical, pathological, immunological, virological, and other basic scientific aspects of the disease, with impacts on its control. A PubMed search using the keywords "pandemic influenza virus H1N1 2009" yielded over 2,500 publications, which markedly exceeded the number published on previous pandemics. Only representative works with relevance to clinical microbiology and infectious diseases are reviewed in this article. A significant increase in the understanding of this virus and the disease within such a short amount of time has allowed for the timely development of diagnostic tests, treatments, and preventive measures. These findings could prove useful for future randomized controlled clinical trials and the epidemiological control of future pandemics.

Mutations in polymerase genes enhanced the virulence of 2009 pandemic H1N1 influenza virus in mice

Influenza A virus can infect a wide variety of animal species with illness ranging from mild to severe, and is a continual cause for concern. Genetic mutations that occur either naturally or during viral adaptation in a poorly susceptible host are key mechanisms underlying the evolution and virulence of influenza A virus. Here, the variants containing PA-A36T or PB2-H357N observed in the mouse-adapted descendants of 2009 pandemic H1N1 virus (pH1N1), A/Sichuan/1/2009 (SC), were characterized. Both mutations enhanced polymerase activity in mammalian cells. These effects were confirmed using recombinant SC virus containing polymerase genes with wild type (WT) or mutant PA or PB2. The PA-A36T mutant showed enhanced growth property compared to the WT in both human A549 cells and porcine PK15 cells in vitro, without significant effect on viral propagation in murine LA-4 cells and pathogenicity in mice; however, it did enhance the lung virus titer. PB2-H357N variant demonstrated growth ability comparable to the WT in A549 cells, but replicated well in PK15, LA-4 cells and in mice with an enhanced pathogenic phenotype. Despite such mutations are rare in nature, they could be observed in avian H5 and H7 subtype viruses which were currently recognized to pose potential threat to human. Our findings indicated that pH1N1 may adapt well in mammals when acquiring these mutations. Therefore, future molecular epidemiological surveillance should include scrutiny of both markers because of their potential impact on pathogenesis.

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.

Role of position 627 of PB2 and the multibasic cleavage site of the hemagglutinin in the virulence of H5N1 avian influenza virus in chickens and ducks

Highly pathogenic H5N1 avian influenza viruses have caused major disease outbreaks in domestic and free-living birds with transmission to humans resulting in 59% mortality amongst 564 cases. The mutation of the amino acid at position 627 of the viral polymerase basic-2 protein (PB2) from glutamic acid (E) in avian isolates to lysine (K) in human isolates is frequently found, but it is not known if this change affects the fitness and pathogenicity of the virus in birds. We show here that horizontal transmission of A/Vietnam/1203/2004 H5N1 (VN/1203) virus in chickens and ducks was not affected by the change of K to E at PB2-627. All chickens died between 21 to 48 hours post infection (pi), while 70% of the ducks survived infection. Virus replication was detected in chickens within 12 hours pi and reached peak titers in spleen, lung and brain between 18 to 24 hours for both viruses. Viral antigen in chickens was predominantly in the endothelium, while in ducks it was present in multiple cell types, including neurons, myocardium, skeletal muscle and connective tissues. Virus replicated to a high titer in chicken thrombocytes and caused upregulation of TLR3 and several cell adhesion molecules, which may explain the rapid virus dissemination and location of viral antigen in endothelium. Virus replication in ducks reached peak values between 2 and 4 days pi in spleen, lung and brain tissues and in contrast to infection in chickens, thrombocytes were not involved. In addition, infection of chickens with low pathogenic VN/1203 caused neuropathology, with E at position PB2-627 causing significantly higher infection rates than K, indicating that it enhances virulence in chickens.

Influenza viruses: an introduction

We provide a brief introduction into the genome organization, life cycle, pathogenicity, and host range of influenza A viruses. We also briefly summarize influenza pandemics and currently available measures to control influenza virus outbreaks, including vaccines and antiviral compounds to influenza viruses.

Apoptosis, cytokine and chemokine induction by non-structural 1 (NS1) proteins encoded by different influenza subtypes

Background

Influenza pandemic remains a serious threat to human health. Viruses of avian origin, H5N1, H7N7 and H9N2, have repeatedly crossed the species barrier to infect humans. Recently, a novel strain originated from swine has evolved to a pandemic. This study aims at improving our understanding on the pathogenic mechanism of influenza viruses, in particular the role of non-structural (NS1) protein in inducing pro-inflammatory and apoptotic responses.

Methods

Human lung epithelial cells (NCI-H292) was used as an in-vitro model to study cytokine/chemokine production and apoptosis induced by transfection of NS1 mRNA encoded by seven infleunza subtypes (seasonal and pandemic H1, H2, H3, H5, H7, and H9), respectively.

Results

The results showed that CXCL-10/IP10 was most prominently induced (> 1000 folds) and IL-6 was slightly induced (< 10 folds) by all subtypes. A subtype-dependent pattern was observed for CCL-2/MCP-1, CCL3/MIP-1α, CCL-5/RANTES and CXCL-9/MIG; where induction by H5N1 was much higher than all other subtypes examined. All subtypes induced a similar temporal profile of apoptosis following transfection. The level of apoptosis induced by H5N1 was remarkably higher than all others. The cytokine/chemokine and apoptosis inducing ability of the 2009 pandemic H1N1 was similar to previous seasonal strains.

Conclusions

In conclusion, the NS1 protein encoded by H5N1 carries a remarkably different property as compared to other avian and human subtypes, and is one of the keys to its high pathogenicity. NCI-H292 cells system proves to be a good in-vitro model to delineate the property of NS1 proteins.

The changing nature of avian influenza A virus (H5N1)

Highly pathogenic avian influenza A virus subtype H5N1 has been endemic in some bird species since its emergence in 1996 and its ecology, genetics and antigenic properties have continued to evolve. This has allowed diverse virus strains to emerge in endemic areas with altered receptor specificity, including a new H5 sublineage with enhanced binding affinity to the human-type receptor. The pandemic potential of H5N1 viruses is alarming and may be increasing. We review here the complex dynamics and changing nature of the H5N1 virus that may contribute to the emergence of pandemic strains.

Reassortment and mutation of the avian influenza virus polymerase PA subunit overcome species barriers

The emergence of new pandemic influenza A viruses requires overcoming barriers to cross-species transmission as viruses move from animal reservoirs into humans. This complicated process is driven by both individual gene mutations and genome reassortments. The viral polymerase complex, composed of the proteins PB1, PB2, and PA, is a major factor controlling host adaptation, and reassortment events involving polymerase gene segments occurred with past pandemic viruses. Here we investigate the ability of polymerase reassortment to restore the activity of an avian influenza virus polymerase that is normally impaired in human cells. Our data show that the substitution of human-origin PA subunits into an avian influenza virus polymerase alleviates restriction in human cells and increases polymerase activity in vitro. Reassortants with 2009 pandemic H1N1 PA proteins were the most active. Mutational analyses demonstrated that the majority of the enhancing activity in human PA results from a threonine-to-serine change at residue 552. Reassortant viruses with avian polymerases and human PA subunits, or simply the T552S mutation, displayed faster replication kinetics in culture and increased pathogenicity in mice compared to those containing a wholly avian polymerase complex. Thus, the acquisition of a human PA subunit, or the signature T552S mutation, is a potential mechanism to overcome the species-specific restriction of avian polymerases and increase virus replication. Our data suggest that the human, avian, swine, and 2009 H1N1-like viruses that are currently cocirculating in pig populations set the stage for PA reassortments with the potential to generate novel viruses that could possess expanded tropism and enhanced pathogenicity.

Conserved host response to highly pathogenic avian influenza virus infection in human cell culture, mouse and macaque model systems

Background

Understanding host response to influenza virus infection will facilitate development of better diagnoses and therapeutic interventions. Several different experimental models have been used as a proxy for human infection, including cell cultures derived from human cells, mice, and non-human primates. Each of these systems has been studied extensively in isolation, but little effort has been directed toward systematically characterizing the conservation of host response on a global level beyond known immune signaling cascades.

Results

In the present study, we employed a multivariate modeling approach to characterize and compare the transcriptional regulatory networks between these three model systems after infection with a highly pathogenic avian influenza virus of the H5N1 subtype. Using this approach we identified functions and pathways that display similar behavior and/or regulation including the well-studied impact on the interferon response and the inflammasome. Our results also suggest a primary response role for airway epithelial cells in initiating hypercytokinemia, which is thought to contribute to the pathogenesis of H5N1 viruses. We further demonstrate that we can use a transcriptional regulatory model from the human cell culture data to make highly accurate predictions about the behavior of important components of the innate immune system in tissues from whole organisms.

Conclusions

This is the first demonstration of a global regulatory network modeling conserved host response between in vitro and in vivo models.

In vitro and in vivo efficacy of fluorodeoxycytidine analogs against highly pathogenic avian influenza H5N1, seasonal, and pandemic H1N1 virus infections

Various fluorodeoxyribonucleosides were evaluated for their antiviral activities against influenza virus infections in vitro and in vivo. Among the most potent inhibitors was 2'-deoxy-2'-fluorocytidine (2'-FdC). It inhibited various strains of low and highly pathogenic avian influenza H5N1 viruses, pandemic H1N1 viruses, an oseltamivir-resistant pandemic H1N1 virus, and seasonal influenza viruses (H3N2, H1N1, influenza B) in MDCK cells, with the 90% inhibitory concentrations ranging from 0.13 to 4.6 μM, as determined by a virus yield reduction assay. 2'-FdC was then tested for efficacy in BALB/c mice infected with a lethal dose of highly pathogenic influenza A/Vietnam/1203/2004 H5N1 virus. 2'FdC (60 mg/kg/d) administered intraperitoneally (i.p.) twice a day beginning 24 h after virus exposure significantly promoted survival (80% survival) of infected mice (p=0.0001). Equally efficacious were the treatment regimens in which mice were treated with 2'-FdC at 30 or 60 mg/kg/day (bid X 8) beginning 24 h before virus exposure. At these doses, 70-80% of the mice were protected from death due to virus infection (p=0.0005, p=0.0001; respectively). The lungs harvested from treated mice at day four of the infection displayed little surface pathology or histopathology, lung weights were lower, and the 60 mg/kg dose reduced lung virus titers, although not significantly compared to the placebo controls. All doses were well tolerated in uninfected mice. 2'-FdC could also be administered as late as 72 h post virus exposure and still significantly protect 60% mice from the lethal effects of the H5N1 virus infection (p=0.019). Other fluorodeoxyribonucleosides tested in the H5N1 mouse model, 2'-deoxy-5-fluorocytidine and 2'-deoxy-2',2'-difluorocytidine, were very toxic at higher doses and not inhibitory at lower doses. Finally, 2'-FdC, which was active in the H5N1 mouse model, was also active in a pandemic H1N1 influenza A infection model in mice. When given at 30 mg/kg/d (bid X 5) beginning 24h before virus exposure), 2'-FdC also significantly enhanced survival of H1N1-infected mice (50%, p=0.038) similar to the results obtained in the H5N1 infection model using a similar dosing regimen (50%, p<0.05). Given the demonstrated in vitro and in vivo inhibition of avian influenza virus replication, 2'FdC may qualify as a lead compound for the development of agents treating influenza virus infections.

Synergistic stimulation of type I interferons during influenza virus coinfection promotes Streptococcus pneumoniae colonization in mice

Pneumococcal infection of the respiratory tract is often secondary to recent influenza virus infection and accounts for much of the morbidity and mortality during seasonal and pandemic influenza. Here, we show that coinfection of the upper respiratory tract of mice with influenza virus and pneumococcus leads to synergistic stimulation of type I IFNs and that this impairs the recruitment of macrophages, which are required for pneumococcal clearance, due to decreased production of the chemokine CCL2. Type I IFN expression was induced by pneumococcal colonization alone. Colonization followed by influenza coinfection led to a synergistic type I IFN response, resulting in increased density of colonizing bacteria and susceptibility to invasive infection. This enhanced type I IFN response inhibited production of the chemokine CCL2, which promotes the recruitment of macrophages and bacterial clearance. Stimulation of CCL2 by macrophages upon pneumococcal infection alone required the pattern recognition receptor Nod2 and expression of the pore-forming toxin pneumolysin. Indeed, the increased colonization associated with concurrent influenza virus infection was not observed in mice lacking Nod2 or the type I IFN receptor, or in mice challenged with pneumococci lacking pneumolysin. We therefore propose that the synergistic stimulation of type I IFN production during concurrent influenza virus and pneumococcal infection leads to increased bacterial colonization and suggest that this may contribute to the higher rates of disease associated with coinfection in humans.

Reversion of PB2-627E to -627K during replication of an H5N1 Clade 2.2 virus in mammalian hosts depends on the origin of the nucleoprotein

H5N1 highly pathogenic avian influenza viruses (HPAIV) of clade 2.2 spread from Southeast Asia to Europe. Intriguingly, in contrast to all common avian strains specifying glutamic acid at position 627 of the PB2 protein (PB2-627E), they carry a lysine at this position (PB2-627K), which is normally found only in human strains. To analyze the impact of this mutation on the host range of HPAIV H5N1, we altered PB2-627K to PB2-627E in the European isolate A/Swan/Germany/R65/2006 (R65). In contrast to the parental R65, multicycle growth and polymerase activity of the resulting mutant R65-PB2(K627E) were considerably impaired in mammalian but not in avian cells. Correspondingly, the 50% lethal dose (LD₅₀) in mice was increased by three orders of magnitude, whereas virulence in chicken remained unchanged, resulting in 100% lethality, as was found for the parental R65. Strikingly, R65-PB2(K627E) reverted to PB2-627K after only one passage in mice but did not revert in chickens. To investigate whether additional R65 genes influence reversion, we passaged R65-PB2(K627E) reassortants containing genes from A/Hong Kong/156/97 (H5N1) (carrying PB2-627E), in avian and mammalian cells. Reversion to PB2-627K in mammalian cells required the presence of the R65 nucleoprotein (NP). This finding corresponds to results of others that during replication of avian strains in mammalian cells, PB2-627K restores an impaired PB2-NP association. Since this mutation is apparently not detrimental for virus prevalence in birds, it has not been eliminated. However, the prompt reversion to PB2-627K in MDCK cells and mice suggests that the clade 2.2 H5N1 HPAIV may have had a history of intermediate mammalian hosts.

The pathogenesis of influenza virus infections: the contributions of virus and host factors

Influenza viruses cause acute respiratory inflammation in humans and symptoms such as high fever, body aches, and fatigue. Usually these symptoms improve after several days; however, the 2009 pandemic H1N1 influenza virus [influenza A(H1N1) 2009] is more pathogenic than seasonal influenza viruses and the pathogenicity of highly pathogenic H5N1 viruses is still higher. The 1918 influenza pandemic virus caused severe pneumonia, resulting in an estimated 50 million deaths worldwide. Several virulence factors have been identified in these virus strains, but host factors are also responsible for the pathogenesis of infections caused by virulent viruses. Here, we review the contributions of both virus and host factors to the pathogenesis of these viral infections.

Influenza A viruses: new research developments

Influenza A viruses are zoonotic pathogens that continuously circulate and change in several animal hosts, including birds, pigs, horses and humans. The emergence of novel virus strains that are capable of causing human epidemics or pandemics is a serious possibility. Here, we discuss the value of surveillance and characterization of naturally occurring influenza viruses, and review the impact that new developments in the laboratory have had on our understanding of the host tropism and virulence of viruses. We also revise the lessons that have been learnt from the pandemic viruses of the past 100 years.

Amino acid residues 253 and 591 of the PB2 protein of avian influenza virus A H9N2 contribute to mammalian pathogenesis

We investigated the tropism, host responses, and virulence of two variants of A/Quail/Hong Kong/G1/1997 (H9N2) (H9N2/G1) with D253N and Q591K in the PB2 protein in primary human macrophages and bronchial epithelium in vitro and in mice in vivo. Virus with PB2 D253N and Q591K had greater polymerase activity in minireplicon assays, induced more tumor necrosis factor alpha (TNF-α) in human macrophages, replicated better in differentiated normal human bronchial epithelial (NHBE) cells, and was more pathogenic for mice. Taken together, our studies help define the viral genetic determinants that contribute to pathogenicity of H9N2 viruses.

Detection of expression of influenza virus receptors in tissues of BALB/c mice by histochemistry

Infection of host cells with the influenza virus is mediated by specific interactions between the viral hemagglutinin and its cell receptor, oligosaccharides containing sialic acid (SA) residues. Avian and human influenza viruses preferentially bind to α-2, 3-linked and α-2, 6-linked sialic acids, respectively. Therefore, differential expression of these receptors may be crucial to influenza virus infection. To date, the distribution of these two receptors has never been investigated in the tissues of BALB/c mice, which is the routine animal model for influenza research. Here, the expression pattern of alpha-2,3 and alpha-2,6 sialic acid-linked receptors in various organs (respiratory tract, gastrointestinal tract, brain, cerebellum, spleen, liver, kidney and heart) of BALB/c mice were determined. Histochemical staining of mouse tissue sections was performed by using biotinylated Maackia amurensis lectin II (MAAII), and Sambucus nigra agglutinin (SNA) were performed to detect the alpha-2,3 and alpha-2,6 sialic acid-linked receptors, respectively. The results showed that the alpha-2,3 and alpha-2,6 sialic acid-linked receptors were both expressed on trachea, lung, cerebellum, spleen, liver and kidney. Only the epithelial cells of cecum, rectum and blood vessels in the heart express the alpha-2,6 sialic acid-linked receptors. The distribution patterns of the two receptors may explain why this model animal can be infected by the AIV and HuIV and the pathological changes when infection occurred. These data can account for the multiple organ involvement observed in influenza infection and should assist investigators in interpreting results obtained when analyzing AIV or HuIV in the mouse model of disease.

Genomic polymorphism of the pandemic A (H1N1) influenza viruses correlates with viral replication, virulence, and pathogenicity in vitro and in vivo

The novel pandemic A (H1N1) virus was first identified in Mexico in April 2009 and quickly spread worldwide. Like all influenzas, the H1N1 strain-specific properties of replication, virulence, and pathogenicity are a result of the particular genomic sequence and concerted expression of multiple genes. Thus, specific mutations may support increased virulence and may be useful as biomarkers of potential threat to human health. We performed comparative genomic analysis of ten strains of the 2009 pandemic A (H1N1) influenza viruses to determine whether genotypes associated with clinical phenotypes, which ranged from mild to severe illness and up to lethal. Virus replication capacity was tested for each strain in vitro using cultured epithelial cells, while virulence and pathogenicity were investigated in vivo using the BALB/c mouse model. The results indicated that A/Sichuan/1/2009 strain had significantly higher replication ability and virulence than the other strains, and five unique non-synonymous mutations were identified in important gene-encoding sequences. These mutations led to amino acid substitutions in HA (L32I), PA (A343T), PB1 (K353R and T566A), and PB2 (T471M), and may be critical molecular determinants for replication, virulence, and pathogenicity. Our results suggested that the replication capacity in vitro and virulence in vivo of the 2009 pandemic A (H1N1) viruses were not associated with the clinical phenotypes. This study offers new insights into the transmission and evolution of the 2009 pandemic A (H1N1) virus.

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.

Acquisition of human-type receptor binding specificity by new H5N1 influenza virus sublineages during their emergence in birds in Egypt

Highly pathogenic avian influenza A virus subtype H5N1 is currently widespread in Asia, Europe, and Africa, with 60% mortality in humans. In particular, since 2009 Egypt has unexpectedly had the highest number of human cases of H5N1 virus infection, with more than 50% of the cases worldwide, but the basis for this high incidence has not been elucidated. A change in receptor binding affinity of the viral hemagglutinin (HA) from α2,3- to α2,6-linked sialic acid (SA) is thought to be necessary for H5N1 virus to become pandemic. In this study, we conducted a phylogenetic analysis of H5N1 viruses isolated between 2006 and 2009 in Egypt. The phylogenetic results showed that recent human isolates clustered disproportionally into several new H5 sublineages suggesting that their HAs have changed their receptor specificity. Using reverse genetics, we found that these H5 sublineages have acquired an enhanced binding affinity for α2,6 SA in combination with residual affinity for α2,3 SA, and identified the amino acid mutations that produced this new receptor specificity. Recombinant H5N1 viruses with a single mutation at HA residue 192 or a double mutation at HA residues 129 and 151 had increased attachment to and infectivity in the human lower respiratory tract but not in the larynx. These findings correlated with enhanced virulence of the mutant viruses in mice. Interestingly, these H5 viruses, with increased affinity to α2,6 SA, emerged during viral diversification in bird populations and subsequently spread to humans. Our findings suggested that emergence of new H5 sublineages with α2,6 SA specificity caused a subsequent increase in human H5N1 influenza virus infections in Egypt, and provided data for understanding the virus's pandemic potential.

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.

Single HA2 mutation increases the infectivity and immunogenicity of a live attenuated H5N1 intranasal influenza vaccine candidate lacking NS1

BACKGROUND

H5N1 influenza vaccines, including live intranasal, appear to be relatively less immunogenic compared to seasonal analogs. The main influenza virus surface glycoprotein hemagglutinin (HA) of highly pathogenic avian influenza viruses (HPAIV) was shown to be more susceptible to acidic pH treatment than that of human or low pathogenic avian influenza viruses. The acidification machinery of the human nasal passageway in response to different irritation factors starts to release protons acidifying the mucosal surface (down to pH of 5.2). We hypothesized that the sensitivity of H5 HA to the acidic environment might be the reason for the low infectivity and immunogenicity of intranasal H5N1 vaccines for mammals.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate that original human influenza viruses infect primary human nasal epithelial cells at acidic pH (down to 5.4), whereas H5N1 HPAIVs lose infectivity at pH ≤ 5.6. The HA of A/Vietnam/1203/04 was modified by introducing the single substitution HA2 58K→I, decreasing the pH of the HA conformational change. The H5N1 reassortants containing the indicated mutation displayed an increased resistance to acidic pH and high temperature treatment compared to those lacking modification. The mutation ensured a higher viral uptake as shown by immunohistochemistry in the respiratory tract of mice and 25 times lower mouse infectious dose₅₀. Moreover, the reassortants keeping 58K→I mutation designed as a live attenuated vaccine candidate lacking an NS1 gene induced superior systemic and local antibody response after the intranasal immunization of mice.

CONCLUSION/SIGNIFICANCE

Our finding suggests that an efficient intranasal vaccination with a live attenuated H5N1 virus may require a certain level of pH and temperature stability of HA in order to achieve an optimal virus uptake by the nasal epithelial cells and induce a sufficient immune response. The pH of the activation of the H5 HA protein may play a substantial role in the infectivity of HPAIVs for mammals.

An overview of influenza A virus receptors

Influenza A viruses are spherical particles that attach to cells through bonds between hemagglutinin and specific cellular receptors. Numerous studies performed have recently revealed that Sialic acid (SA) is a crucial component of influenza A virus receptors. This brief review summarizes recent advances in our understanding of influenza A virus receptors. The introduction describes the classification of influenza A virus receptors and the review continues with a survey of the distribution of SA in different tissue and host. This is followed by research applications of influenza A virus receptors, and explanation of why receptor studies are so important on a world-wide scale.

Toll-like receptor pre-stimulation protects mice against lethal infection with highly pathogenic influenza viruses

Since the beginning of the 20th century, humans have experienced four influenza pandemics, including the devastating 1918 'Spanish influenza'. Moreover, H5N1 highly pathogenic avian influenza (HPAI) viruses are currently spreading worldwide, although they are not yet efficiently transmitted among humans. While the threat of a global pandemic involving a highly pathogenic influenza virus strain looms large, our mechanisms to address such a catastrophe remain limited. Here, we show that pre-stimulation of Toll-like receptors (TLRs) 2 and 4 increased resistance against influenza viruses known to induce high pathogenicity in animal models. Our data emphasize the complexity of the host response against different influenza viruses, and suggest that TLR agonists might be utilized to protect against lethality associated with highly pathogenic influenza virus infection in humans.

Effect of an asparagine-to-serine mutation at position 294 in neuraminidase on the pathogenicity of highly pathogenic H5N1 influenza A virus

Like the histidine-to-tyrosine substitution at position 274 in neuraminidase (NA H274Y), an asparagine-to-serine mutation at position 294 in this protein (NA N294S) confers oseltamivir resistance to highly pathogenic H5N1 influenza A viruses. However, unlike viruses with the NA H274Y mutation, the properties of viruses possessing NA N294S are not well understood. Here, we assessed the effect of the NA N294S substitution on the replication and pathogenicity of human H5N1 viruses and on the efficacy of the NA inhibitors oseltamivir and zanamivir in mouse and ferret models. Although NA N294S-possessing H5N1 viruses were attenuated in mice and ferrets compared to their oseltamivir-sensitive counterparts, one of the infected ferrets died from systemic infection, demonstrating the potential lethality in ferrets of oseltamivir-resistant H5N1 viruses with the NA N294S substitution. The efficacy of oseltamivir, but not that of zanamivir, against an NA N294S-possessing virus was substantially impaired both in ferrets and in vitro. These results demonstrate the considerable pathogenicity of NA N294S substitution-possessing H5N1 viruses and underscore the importance of monitoring the emergence of the NA N294S mutation in circulating H5N1 viruses.

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.

Influenza A (H5N1) viruses from pigs, Indonesia

TOC summary: Pigs may serve as intermediate hosts in which this avian virus can adapt to mammals.

Pigs have long been considered potential intermediate hosts in which avian influenza viruses can adapt to humans. To determine whether this potential exists for pigs in Indonesia, we conducted surveillance during 2005–2009. We found that 52 pigs in 4 provinces were infected during 2005–2007 but not 2008–2009. Phylogenetic analysis showed that the viruses had been introduced into the pig population in Indonesia on at least 3 occasions. One isolate had acquired the ability to recognize a human-type receptor. No infected pig had influenza-like symptoms, indicating that influenza A (H5N1) viruses can replicate undetected for prolonged periods, facilitating avian virus adaptation to mammalian hosts. Our data suggest that pigs are at risk for infection during outbreaks of influenza virus A (H5N1) and can serve as intermediate hosts in which this avian virus can adapt to mammals.

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.

Development of adenoviral vector-based mucosal vaccine against influenza

The recent pandemic threat of the influenza virus makes the increased safety and efficiency of vaccination against the pathogen a most important issue. It has been well established that for maximum protective effect, the vaccination should mimic natural infection. Therefore, recent efforts to develop a new influenza vaccine have focused on intranasal immunization strategies. Intranasal immunization is capable of inducing secretory IgA and serum IgG responses to provide a double defense against mucosal pathogens. On the other hand, it is desirable that a live pathogen is not present in the vaccine. In addition, for optimal induction of the immune responses via the nasal route, efficient and safe mucosal adjuvants are also required. This is possible to attain using an adenoviral vector for vaccine development. Adenoviral vectors are capable of delivering and protecting the antigen encoding sequence. They also possess a natural mechanism for penetrating into the nasal mucous membrane and are capable of activating the innate immune response. This review describes the basic prerequisites for the involvement of recombinant adenoviruses for mucosal (nasal) vaccine development against the influenza virus.

A single-amino-acid substitution in the HA protein changes the replication and pathogenicity of the 2009 pandemic A (H1N1) influenza viruses in vitro and in vivo

Background

The novel pandemic A (H1N1) virus was first identified in Mexico in April 2009 and since then it spread world wide over a short period of time. Although the virus infection is generally associated with mild disease and a relatively low mortality, it is projected that mutations in specific regions of the viral genome, especially within the receptor binding domain of the hemagglutinin (HA) protein could result in more virulent virus stains, leading to a more severe pandemic.

Results

Here, we found that a single amino acid substitution of Asp-to-Gly at position 222 in the HA protein of the A (H1N1) virus occurred after two passage propagation in the allantoic cavities of chicken embryonated eggs, and this single residue variation dramatically increased the viral replication ability in MDCK cells and pathogenicity in BALB/c mice.

Conclusions

A substitution of Asp-to-Gly at position 222 in the HA protein was prone to occur under positive selection pressures, and this single amino acid mutation could dramatically increase the virus replication ability in vitro and pathogenicity in vivo. Our finding offers a better understanding of the transmission and evolution of the 2009 pandemic A (H1N1) virus and brings attention to further potentially severe influenza pandemic that may result from cross-host evolution of the influenza viruses.

Pathogenicity of swine influenza viruses possessing an avian or swine-origin PB2 polymerase gene evaluated in mouse and pig models

PB2 627K is a determinant of influenza host range and contributes to the pathogenicity of human-, avian-, and mouse-adapted influenza viruses in the mouse model. Here we used mouse and pig models to analyze the contribution of a swine-origin and avian-origin PB2 carrying either 627K or 627E in the background of the classical swine H1N1 (A/Swine/Iowa/15/30; 1930) virus. The results showed PB2 627K is crucial for virulence in the mouse model, independent of whether PB2 is derived from an avian or swine influenza virus (SIV). In the pig model, PB2 627E decreases pathogenicity of the classical 1930 SIV when it contains the swine-origin PB2, but not when it possesses the avian-origin PB2. Our study suggests the pathogenicity of SIVs with different PB2 genes and mutation of codon 627 in mice does not correlate with the pathogenicity of the same SIVs in the natural host, the pig.

PB2 residue 158 is a pathogenic determinant of pandemic H1N1 and H5 influenza a viruses in mice

Influenza A viruses are human and animal pathogens that cause morbidity and mortality, which range from mild to severe. The 2009 H1N1 pandemic was caused by the emergence of a reassortant H1N1 subtype (H1N1pdm) influenza A virus containing gene segments that originally circulated in human, avian, and swine virus reservoirs. The molecular determinants of replication and pathogenesis of H1N1pdm viruses in humans and other mammals are poorly understood. Therefore, we set out to elucidate viral determinants critical to the pathogenesis of this novel reassortant using a mouse model. We found that a glutamate-to-glycine substitution at residue 158 of the PB2 gene (PB2-E158G) increased the morbidity and mortality of the parental H1N1pdm virus. Results from mini-genome replication assays in human cells and virus titration in mouse tissues demonstrated that PB2-E158G is a pathogenic determinant, because it significantly increases viral replication rates. The virus load in PB2-E158G-infected mouse lungs was 1,300-fold higher than that of the wild-type virus. Our data also show that PB2-E158G had a much stronger influence on the RNA replication and pathogenesis of H1N1pdm viruses than PB2-E627K, which is a known pathogenic determinant. Remarkably, PB2-E158G substitutions also altered the pathotypes of two avian H5 viruses in mice, indicating that this residue impacts genetically divergent influenza A viruses and suggesting that this region of PB2 could be a new antiviral target. Collectively, the data presented in this study demonstrate that PB2-E158G is a novel pathogenic determinant of influenza A viruses in the mouse model. We speculate that PB2-E158G may be important in the adaptation of avian PB2 genes to other mammals, and BLAST sequence analysis identified a naturally occurring human H1N1pdm isolate that has this substitution. Therefore, future surveillance efforts should include scrutiny of this region of PB2 because of its potential impact on pathogenesis.

Pathogenicity of highly pathogenic avian H5N1 influenza A viruses isolated from humans between 2003 and 2008 in northern Vietnam

Vietnam is one of the countries most affected by highly pathogenic H5N1 influenza A viruses. To evaluate the potential pathogenicity in mammals of H5N1 viruses isolated from humans in Vietnam, we determined the sequences of all eight genes of 22 human isolates collected between 2003 and 2008 and compared their virulence in mice. The isolates were classified into clade 1 and clade 2.3.4 and differed in pathogenicity for mice. Whilst lysine at position 627 of PB2 (PB2-627K) is a critical virulence determinant for clade 2.3.4 viruses, asparagine at position 701 of PB2 and other unknown virulence determinants appear to be involved in the high pathogenicity of clade 1 viruses, warranting further studies to determine the factors responsible for the high virulence of H5N1 viruses in mammals.

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.

Ferrets develop fatal influenza after inhaling small particle aerosols of highly pathogenic avian influenza virus A/Vietnam/1203/2004 (H5N1)

BACKGROUND

There is limited knowledge about the potential routes for H5N1 influenza virus transmission to and between humans, and it is not clear whether humans can be infected through inhalation of aerosolized H5N1 virus particles. Ferrets are often used as a animal model for humans in influenza pathogenicity and transmissibility studies. In this manuscript, a nose-only bioaerosol inhalation exposure system that was recently developed and validated was used in an inhalation exposure study of aerosolized A/Vietnam/1203/2004 (H5N1) virus in ferrets. The clinical spectrum of influenza resulting from exposure to A/Vietnam/1203/2004 (H5N1) through intranasal verses inhalation routes was analyzed.

RESULTS

Ferrets were successfully infected through intranasal instillation or through inhalation of small particle aerosols with four different doses of Influenza virus A/Vietnam/1203/2004 (H5N1). The animals developed severe influenza encephalomyelitis following intranasal or inhalation exposure to 10¹, 10², 10³, or 10⁴ infectious virus particles per ferret.

CONCLUSIONS

Aerosolized Influenza virus A/Vietnam/1203/2004 (H5N1) is highly infectious and lethal in ferrets. Clinical signs appeared earlier in animals infected through inhalation of aerosolized virus compared to those infected through intranasal instillation.

The PB2 subunit of the influenza virus RNA polymerase affects virulence by interacting with the mitochondrial antiviral signaling protein and inhibiting expression of beta interferon

The PB2 subunit of the influenza virus RNA polymerase is a major virulence determinant of influenza viruses. However, the molecular mechanisms involved remain unknown. It was previously shown that the PB2 protein, in addition to its nuclear localization, also accumulates in the mitochondria. Here, we demonstrate that the PB2 protein interacts with the mitochondrial antiviral signaling protein, MAVS (also known as IPS-1, VISA, or Cardif), and inhibits MAVS-mediated beta interferon (IFN-beta) expression. In addition, we show that PB2 proteins of influenza viruses differ in their abilities to associate with the mitochondria. In particular, the PB2 proteins of seasonal human influenza viruses localize to the mitochondria while PB2 proteins of avian influenza viruses are nonmitochondrial. This difference in localization is caused by a single amino acid polymorphism in the PB2 mitochondrial targeting signal. In order to address the functional significance of the mitochondrial localization of the PB2 protein in vivo, we have generated two recombinant human influenza viruses encoding either mitochondrial or nonmitochondrial PB2 proteins. We found that the difference in the mitochondrial localization of the PB2 proteins does not affect the growth of these viruses in cell culture. However, the virus encoding the nonmitochondrial PB2 protein induces higher levels of IFN-beta and, in an animal model, is attenuated compared to the isogenic virus encoding a mitochondrial PB2. Overall this study implicates the PB2 protein in the regulation of host antiviral innate immune pathways and suggests an important role for the mitochondrial association of the PB2 protein in determining virulence.

Distinct propagation efficiencies of H5N1 influenza virus Thai isolates in newly established murine respiratory region-derived cell clones

Inbred mice have been widely used for the study of influenza viruses as a mammalian model, while suitable cell lines derived from murine tissue have been limited. Here, we established several immortalized cell clones from respiratory regions of inbred mice (C57BL/6 and BALB/c) by transformation using simian virus 40 large T antigen expression vector. Twenty-five cell clones from C57/BL and BALB/c, designated as MRDC/C and MRDC/B series, respectively, showed different susceptibility to Thai isolates of influenza A virus H5N1. Two murine cell clones, C6 and B7 which were extensively studied expressed both SAα2,3 and SAα2,6 sialic acid receptors. Interestingly, the 6 Thai patient-derived H5N1 isolates examined showed varied virus propagation efficiency in murine cell clones, although there were only slight differences in their propagation in MDCK and A549 cell lines. The results indicate that the murine cell clones are useful for examining the propagation efficiency of H5N1 viruses in vitro.

PB2 and hemagglutinin mutations are major determinants of host range and virulence in mouse-adapted influenza A virus

Serial mouse lung passage of a human influenza A virus, A/Hong Kong/1/68 (H3N2) (HK-wt), produced a mouse-adapted variant, MA, with nine mutations that was >10(3.8)-fold more virulent. In this study, we demonstrate that MA mutations of the PB2 (D701N) and hemagglutinin (HA) (G218W in HA1 and T156N in HA2) genes were the most adaptive genetic determinants for increased growth and virulence in the mouse model. Recombinant viruses expressing each of the mutated MA genome segments on the HK-wt backbone showed significantly increased disease severity, whereas only the mouse-adapted PB2 gene increased virulence, as determined by the 50% lethal dose ([LD(50)] >10(1.4)-fold). The converse comparisons of recombinant MA viruses expressing each of the HK-wt genome segments showed the greatest decrease in virulence due to the HA gene (10(2)-fold), with lesser decreases due to the M1, NS1, NA, and PB1 genes (10(0.3)- to 10(0.8)-fold), and undetectable effects on the LD(50) for the PB2 and NP genes. The HK PB2 gene did, however, attenuate MA infection, as measured by weight loss and time to death. Replication of adaptive mutations in vivo and in vitro showed both viral gene backbone and host range effects. Minigenome transcription assays showed that PB1 and PB2 mutations increased polymerase activity and that the PB2 D701N mutation was comparable in effect to the mammalian adaptive PB2 E627K mutation. Our results demonstrate that host range and virulence are controlled by multiple genes, with major roles for mutations in PB2 and HA.

Pandemic influenza A (H1N1) virus infection and avian influenza A (H5N1) virus infection: a comparative analysis

The 2009 H1N1 and H5N1 influenza viruses are newly (re-) emerged influenza A viruses (2009 A(H1N1) and A(H5N1), respectively) that have recently posed tremendous health threats in many regions worldwide. With the 2009 outbreak of H1N1 influenza A, the world witnessed the first influenza pandemic of the 21st century. The disease has rapidly spread across the entire globe, and has resulted in hundreds of thousands of cases with confirmed infection. Although characterized by high transmissibility, the virulence and fatality of the 2009 A(H1N1) influenza virus have thus far remained relatively low. The reverse holds true for A(H5N1) influenza; at a fatality rate that exceeds 60%, it is known to cause severe damage to the human respiratory system, but is not presently capable of efficient transmission from human to human. Apart from the clear differences between the two types of influenza, there are some significant similarities that warrant attention. In particular, the more severe and fatal 2009 A(H1N1) influenza cases have shown symptoms similar to those reported in cases of A(H5N1) influenza. Histopathological findings for these cases, to the extent available, also appear to have similarities for both diseases in terms of damage and severity. Here we review important recent publications in this area, and we discuss some of the key commonalities and contrasts between the two influenza A types in terms of their biology, origins, clinical features, pathology and pathogenesis, and receptors and transmissibility.

Animal Models for Influenza Virus Pathogenesis and Transmission

Influenza virus infection of humans results in a respiratory disease that ranges in severity from sub-clinical infection to primary viral pneumonia that can result in death. The clinical effects of infection vary with the exposure history, age and immune status of the host, and also the virulence of the influenza strain. In humans, the virus is transmitted through either aerosol or contact-based transfer of infectious respiratory secretions. As is evidenced by most zoonotic influenza virus infections, not all strains that can infect humans are able to transmit from person-to-person. Animal models of influenza are essential to research efforts aimed at understanding the viral and host factors that contribute to the disease and transmission outcomes of influenza virus infection in humans. These models furthermore allow the pre-clinical testing of antiviral drugs and vaccines aimed at reducing morbidity and mortality in the population through amelioration of the virulence or transmissibility of influenza viruses. Mice, ferrets, guinea pigs, cotton rats, hamsters and macaques have all been used to study influenza viruses and therapeutics targeting them. Each model presents unique advantages and disadvantages, which will be discussed herein.

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 strain-dependent pathogenesis in fatal H1N1 and H5N1 subtype infections of mice

Future treatments may involve customizing treatment to the virus pathotype.

To determine if fatal infections caused by different highly virulent influenza A viruses share the same pathogenesis, we compared 2 different influenza A virus subtypes, H1N1 and H5N1. The subtypes, which had shown no pathogenicity in laboratory mice, were forced to evolve by serial passaging. Although both adapted viruses evoked diffuse alveolar damage and showed a similar 50% mouse lethal dose and the same peak lung concentration, each had a distinct pathologic signature and caused a different course of acute respiratory distress syndrome. In the absence of any virus labeling, a histologist could readily distinguish infections caused by these 2 viruses. The different histologic features described in this study here refute the hypothesis of a single, universal cytokine storm underlying all fatal influenza diseases. Research is thus crucially needed to identify sets of virulence markers and to examine whether treatment should be tailored to the influenza virus pathotype.

[Virological characteristics of pandemic (H1N1) 2009 influenza virus]

In the spring of 2009, a novel swine-origin H1N1 virus, whose antigenicity is quite different from those of seasonal human H1N1 strains, emerged in Mexico and readily transmitted and spread among humans, resulting in the first influenza pandemic in the 21st century. This novel H1N1 virus was shown to be a triple reassortant comprising genes derived from avian, human, and swine viruses. Here, we review our current knowledge of this pandemic influenza virus and discuss future aspects of the pandemic.