Pathogenesis of Hong Kong H5N1 influenza virus NS gene reassortants in mice: the role of cytokines and B- and T-cell responses

Confronting potential influenza A (H5N1) pandemic with better vaccines

Better understanding of host-virus interaction is essential to produce effective vaccines against influenza (H5N1) viruses.

Influenza A (H5N1) viruses are strong candidates for causing the next influenza pandemic if they acquire the ability for efficient human-to-human transmission. A major public health goal is to make efficacious vaccines against these viruses by using novel approaches, including cell-culture system, reverse genetics, and adjuvant development. Important consideration for the strategy includes preparation of vaccines from a currently circulating strain to induce broad-spectrum immunity toward newly emerged human H5 strains. This strategy would be a good solution early in a pandemic until an antigenically matched and approved vaccine is produced. The concept of therapeutic vaccines (e.g., antidisease vaccine) directed at diminishing the cytokine storm frequently seen in subtype H5N1–infected persons is underscored. Better understanding of host–virus interaction is essential to identify tools to produce effective vaccines against influenza (H5N1).

Serum levels of interleukin 6 in recently hospitalized tick-borne encephalitis patients correlate with age, but not with disease outcome

Infection with many encephalitic viruses is associated with the induction of the proinflammatory cytokine interleukin (IL)-6. In some situations, induction of high levels of this cytokine is associated with a protective response, but in others it can be linked to tissue damage and disease. In the studies reported here, levels of serum IL-6 and virus-specific antibodies were measured on admission to hospital and correlated with clinical outcomes. Only some patients demonstrated raised levels of serum IL-6, and there was no correlation between high levels of this cytokine and either gender or the severity of clinical disease. A statistically significant association between raised IL-6 and age was observed, with all individuals below the age of 26 showing normal levels of serum IL-6, regardless of clinical presentation. Furthermore, not all patients had detectable levels of virus-specific serum immunoglobulin G (IgG) antibodies, but an inverse and statistically significant correlation between raised IL-6 levels and IgG titre was observed. Consequently, serum levels of IL-6 cannot be used as a reliable indicator of disease outcome.

Virulence of H5N1 avian influenza virus enhanced by a 15-nucleotide deletion in the viral nonstructural gene

More and more H5N1 subtype avian influenza viruses possessing a 15-nucleotide (15-nt) deletion in the viral nonstructural protein (NS) gene from position 263 to 277 have emerged since 2000. In order to investigate the biological significance of this deletion, two pairs of H5N1 reassortants designated as rWSN-SD versus rWSN-mSD and rWSN-YZ versus rWSN-mYZ were generated by reverse genetics technique. These recombinant viruses shared the same inner genes of PB1, PB2, PA, NP, and M from strain A/WSN/33(H1N1) and outer genes of HA and NA from strain A/Duck/Shandong/093/2004 (H5N1) (A/D/SD/04), whereas they bore different NS gene. Recombinant rWSN-SD carried the full sequence NS gene from A/D/SD/04 in the natural state without deletion, whereas rWSN-mSD carried the same NS gene, but with an artificial 15-nt deletion from position 263 to 277. On the other hand, rWSN-YZ contained the NS gene in the natural state with a deletion from A/Duck/Yangzhou/232/2004 (H5N1) (A/D/YZ/04), while rWSN-mYZ bore the same NS gene but with an artificial insertion of 15-nt in site 263-277. All the four reassortants grew well in embryonated chicken eggs with similar mean death time (MDT) and viral titer of EID50 or HA. However, the virulence of these reassortant viruses in chickens and mice was different. Reassortant viruses with deletion in their NS gene (rWSN-mSD and rWSN-YZ) had much higher intraveneous pathogenicity index (IVPI) in chickens and lower MLD50 in mice than their counterparts without the deletion (rWSN-SD and rWSN-mYZ). Furthermore, rWSN-mSD and rWSN-YZ caused significantly more deaths in infected chickens and higher virus titers in tissues of inoculated mice than did rWSN-SD and rWSN-mYZ respectively. Sequence analysis also showed that H5N1 viruses carrying the 15-nt deletion in the NS gene invariably had the D92E shift in their NS1 protein. The results indicated that the 15-nucleotide deletion of NS gene from site 263 to 277 associated with D92E shift in NS1 protein contributes to the virulence increase of H5N1 viruses in chickens and mice.

Characterization of the complete genome of influenza A (H5N1) virus isolated during the 2006 outbreak in poultry in India

An outbreak of highly pathogenic avian influenza A (H5N1) virus in poultry was reported from Nandurbar and Jalgaon districts of Maharashtra and adjoining areas of Uchhal in Gujarat and Burhanpur in Madhya Pradesh in India from January to April, 2006. In the present study, the full genome of two previously uncharacterized strains of H5N1 viruses isolated at the National Institute of Virology (NIV), Pune, from post-mortem tissues of chicken collected from Navapur, Nandurbar district during the outbreak, has been presented. All the genes belong to clade 2.2 of the Z genotype and are close to the 2006 isolates from Iran, Afghanistan, Mongolia, Italy, and Krasnodar. In a study reported earlier, based on the partial gene sequences of HA, the authors (Pattnaik et al.) hypothesized that the viruses in Jalgaon and Navapur, causing outbreaks 12 days apart, were introduced at different times from different sources. However, our Navapur isolates are closer to the isolate reported from Jalgaon than that from Navapur. Molecular markers suggest that the isolates are sensitive to both drugs Oseltamivir and Amantadine. Amino acid residues responsible for pathogenesis, glycosylation, and receptor binding have also been discussed. The relationship between the Indian viruses and those in the East Africa/West-Asia flyway of migratory birds and the position of Nandurbar in this route suggests that the viruses in India may have been introduced through migratory birds although the role of trade as a possible route of introduction of the virus cannot be ruled out.

Emerging influenza

In 1918 the Spanish influenza pandemic, caused by an avian H1N1 virus, resulted in over 50 million deaths worldwide. Several outbreaks of H7 influenza A viruses have resulted in human cases, including one fatal case. Since 1997, the outbreaks of highly pathogenic avian influenza (HPAI) of the H5N1 subtype have affected a wide variety of mammals in addition to poultry and wild birds. Here, we give an overview of the current knowledge of the determinants of pathogenicity of these three subtypes of avian influenza A virus in mammals. Common mechanisms for acquisition of virulence and replication of these avian influenza viruses in mammals are becoming apparent. Therefore, monitoring these and additional genetic changes upon zoonotic infections is important. Identification of genetic changes responsible for transmission between mammals will be an important task for the near future.

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.

Avian influenza: should China be alarmed?

Avian influenza has emerged as one of the primary public health concern of the 21st century. Influenza strain H5N1 is capable of incidentally infecting humans and other mammals. Since their reemergence in 2003, highly pathogenic avian influenza A (H5N1) viruses have been transmitted from poultry to humans (by direct or indirect contact with infected birds) in several provinces of Mainland China, which has resulted in 22 cases of human infection and has created repercussions for the Chinese economy. People have been concerned whether a new pandemic will occur in the future. The eradication of pathogenic avian influenza viruses appears to be the most effective way to prevent an influenza pandemic. This paper will examine the features of H5N1, including incidence, infection, immunity, clinical management, prevention and control, and therapy in Mainland China.

Experimental infection of H5N1 HPAI in BALB/c mice

Background

In 2005 huge epizooty of H5N1 HPAI occurred in Russia. It had been clear that territory of Russia becoming endemic for H5N1 HPAI. In 2006 several outbreaks have occurred. To develop new vaccines and antiviral therapies, animal models had to be investigated. We choose highly pathogenic strain for these studies.

Results

A/duck/Tuva/01/06 belongs to Quinghai-like group viruses. Molecular markers – cleavage site, K627 in PB2 characterize this virus as highly pathogenic. This data was confirmed by direct pathogenic tests: IVPI = 3.0, MLD₅₀ = 1,4Log10EID₅₀. Also molecular analysis showed sensivity of the virus to adamantanes and neuraminidase inhibitors. Serological analysis showed wide cross-reactivity of this virus with sera produced to H5N1 HPAI viruses isolated earlier in South-East Asia. Mean time to death of infected animals was 8,19+/-0,18 days. First time acute delayed hemorrhagic syndrome was observed in mice lethal model. Hypercytokinemia was determined by elevated sera levels of IFN-gamma, IL-6, IL-10.

Conclusion

Assuming all obtained data we can conclude that basic model parameters were characterized and virus A/duck/Tuva/01/06 can be used to evaluate anti-influenza vaccines and therapeutics.

The RNA binding domain of influenza A virus NS1 protein affects secretion of tumor necrosis factor alpha, interleukin-6, and interferon in primary murine tracheal epithelial cells

Primary differentiated respiratory epithelial cell cultures closely model the in vivo environment and allow for studies of innate immune responses generated specifically by epithelial cells, the primary cell type infected by human influenza A virus strains. We used primary murine tracheal epithelial cell (mTEC) cultures to investigate antiviral and cytokine responses to influenza A virus infection, focusing on the contribution of the RNA binding domain of the NS1 protein. rWSN NS1 R38A replication is attenuated in mTEC cultures; however, viral antigen is detected predominantly in ciliated cells, similar to wild-type virus. NS1 and NS1 R38A proteins display a primarily cytoplasmic localization in infected mTEC cultures. Increased production of tumor necrosis factor alpha, interleukin-6, and beta interferon is observed during rWSN NS1 R38A infection, and cytokines are secreted in a directional manner. Cytokine pretreatment of mTEC cultures and Vero cells suggest that rWSN NS1 R38A is more sensitive to the presence of antiviral/inflammatory cytokines than wild-type virus. Our results demonstrate that the RNA binding domain is a critical regulator of both cytokine production and cytokine sensitivity during influenza A virus infection of primary tracheal epithelial cells.

Influenza (H5N1) viruses in poultry, Russian Federation, 2005-2006

We studied 7 influenza (H5N1) viruses isolated from poultry in western Siberia and the European part of the Russian Federation during July 2005-February 2006. Full genome sequences showed high homology to Qinghai-like influenza (H5N1) viruses. Phylogenetic analysis not only showed a close genetic relationship between the H5N1 strains isolated from poultry and wild migratory waterfowls but also suggested genetic reassortment among the analyzed isolates. Analysis of deduced amino acid sequences of the M2 and neuraminidase proteins showed that all isolates are potentially sensitive to currently available antiviral drugs. Pathogenicity testing showed that all studied viruses were highly pathogenic in chickens; for 3 isolates tested in mice and 2 tested in ferrets, pathogenicity was heterogeneous. Pathogenicity in mammalian models was generally correlated with Lys at residue 627 of polymerase basic protein 2.

Genetic analysis of nonstructural genes (NS1 and NS2) of H9N2 and H5N1 viruses recently isolated in Israel

The avian influenza virus subtype H9N2 affects wild birds, domestic poultry, swine, and humans; it has circulated amongst domestic poultry in Israel during the last 6 years. The H5N1 virus was recorded in Israel for the first time in March 2006. Nonstructural (NS) genes and NS proteins are important in the life cycle of the avian influenza viruses. In the present study, NS genes of 21 examples of H9N2 and of two examples of H5N1 avian influenza viruses, isolated in Israel during 2000-2006, were completely sequenced and phylogenetically analyzed. All the H9N2 isolates fell into a single group that, in turn, was subdivided into three subgroups in accordance with the time of isolation; their NS1 and NS2 proteins possessed 230 and 121 amino acids, respectively. The NS1 protein of the H5N1 isolates had five amino acid deletions, which was typical of highly pathogenic H5N1 viruses isolated in various countries during 2005-2006. Comparative analysis showed that the NS proteins of the H9N2 Israeli isolates contained few amino acid sequences associated with high pathogenicity or human host specificity.

Avian influenza virus (H5N1): a threat to human health

Pandemic influenza virus has its origins in avian influenza viruses. The highly pathogenic avian influenza virus subtype H5N1 is already panzootic in poultry, with attendant economic consequences. It continues to cross species barriers to infect humans and other mammals, often with fatal outcomes. Therefore, H5N1 virus has rightly received attention as a potential pandemic threat. However, it is noted that the pandemics of 1957 and 1968 did not arise from highly pathogenic influenza viruses, and the next pandemic may well arise from a low-pathogenicity virus. The rationale for particular concern about an H5N1 pandemic is not its inevitability but its potential severity. An H5N1 pandemic is an event of low probability but one of high human health impact and poses a predicament for public health. Here, we review the ecology and evolution of highly pathogenic avian influenza H5N1 viruses, assess the pandemic risk, and address aspects of human H5N1 disease in relation to its epidemiology, clinical presentation, pathogenesis, diagnosis, and management.

Model answers or trivial pursuits? The role of mathematical models in influenza pandemic preparedness planning

The panzootic of H5N1 influenza in birds has raised concerns that the virus will mutate to spread more readily in people, leading to a human pandemic. Mathematical models have been used to interpret past pandemics and outbreaks, and to thus model possible future pandemic scenarios and interventions. We review historical influenza outbreak and transmission data, and discuss the way in which modellers have used such sources to inform model structure and assumptions. We suggest that urban attack rates in the 1918-1919 pandemic were constrained by prior immunity, that R(0) for influenza is higher than often assumed, and that control of any future pandemic could be difficult in the absence of significant prior immunity. In future, modelling assumptions, parameter estimates and conclusions should be tested against as many relevant data sets as possible. To this end, we encourage researchers to access FluWeb, an on-line influenza database of historical pandemics and outbreaks.

Structure and function of the NS1 protein of influenza A virus

The avian influenza A virus currently prevailing in Asia causes fatal pneumonia and multiple organ failure in birds and humans. Despite intensive research, understanding of the characteristics of influenza A virus that determine its virulence is incomplete. NS1A protein, a non-structural protein of influenza A virus, was reported to contribute to its pathogenicity and virulence. NS1A protein is a multifunctional protein that plays a significant role in resisting the host antiviral response during the influenza infection. This review briefly outlines the current knowledge on the structure and function of the NS1A protein.

A question of self-preservation: immunopathology in influenza virus infection

Influenza A viruses that circulate normally in the human population cause a debilitating, though generally transient, illness that is sometimes fatal, particularly in the elderly. Severe complications arising from pandemic influenza or the highly pathogenic avian H5N1 viruses are often associated with rapid, massive inflammatory cell infiltration, acute respiratory distress, reactive hemophagocytosis and multiple organ involvement. Histological and pathological indicators strongly suggest a key role for an excessive host response in mediating at least some of this pathology. Here, we review the current literature on how various effector arms of the immune system can act deleteriously to initiate or exacerbate pathological damage in this viral pneumonia. Generally, the same immunological factors mediating tissue damage during the anti-influenza immune response are also critical for efficient elimination of virus, thereby posing a significant challenge in the design of harmless yet effective therapeutic strategies for tackling influenza virus.

Neuraminidase Inhibitors and their Role in Avian and Pandemic Influenza

Continuing occurrences of human infections with avian influenza A (H5N1) viruses have ignited increasing fears that the next influenza pandemic is imminent. Fortunately, options for antiviral prophylaxis and treatment have been improved dramatically since the previous pandemics by the availability of neuraminidase inhibitors such as zanamivir and oseltamivir. However, although the prophylactic and therapeutic efficacy of these drugs is well established for uncomplicated seasonal human influenza, clinical effectiveness seems limited for human H5N1 infections despite in vitro susceptibility and efficacy in animal studies. Factors which might contribute to this apparently limited efficacy include suboptimal dosing or routes of administration, suboptimal timing of treatment and the inability of antiviral drugs to interfere with immunopathology, and the development of drug resistance. Efforts to optimize the use of neuraminidase inhibitor treatment in H5N1 disease are urgently needed and might eventually aid in the judicious use of stockpiled neuraminidase inhibitors in the event of a pandemic.

Molecular Pathogenesis of H5N1 Influenza Virus Infections

Highly pathogenic H5N1 influenza viruses have become endemic in poultry populations throughout Southeast Asia and continue to infect humans with a greater than 50% case fatality rate. So far, human-to-human transmission of these viruses has been limited. Here, we discuss the molecular features of H5N1 influenza viruses that might affect their pathogenicity, and explain the current lack of efficient human-to-human transmission. Such knowledge is critical in evaluating the pandemic risk these viruses pose.

Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia

Avian influenza A (H5N1) viruses cause severe disease in humans, but the basis for their virulence remains unclear. In vitro and animal studies indicate that high and disseminated viral replication is important for disease pathogenesis. Laboratory experiments suggest that virus-induced cytokine dysregulation may contribute to disease severity. To assess the relevance of these findings for human disease, we performed virological and immunological studies in 18 individuals with H5N1 and 8 individuals infected with human influenza virus subtypes. Influenza H5N1 infection in humans is characterized by high pharyngeal virus loads and frequent detection of viral RNA in rectum and blood. Viral RNA in blood was present only in fatal H5N1 cases and was associated with higher pharyngeal viral loads. We observed low peripheral blood T-lymphocyte counts and high chemokine and cytokine levels in H5N1-infected individuals, particularly in those who died, and these correlated with pharyngeal viral loads. Genetic characterization of H5N1 viruses revealed mutations in the viral polymerase complex associated with mammalian adaptation and virulence. Our observations indicate that high viral load, and the resulting intense inflammatory responses, are central to influenza H5N1 pathogenesis. The focus of clinical management should be on preventing this intense cytokine response, by early diagnosis and effective antiviral treatment.

Pandemic influenza: a potential role for statins in treatment and prophylaxis

The next influenza pandemic may be imminent. Because antiviral agents and vaccines will be unavailable to people in most countries, we need to determine whether other agents could offer clinical benefits. Influenza is associated with an increase in acute cardiovascular diseases, and influenza viruses induce proinflammatory cytokines. Statins are cardioprotective and have anti-inflammatory and immunomodulatory effects, and they thus might benefit patients with influenza. This hypothesis should be evaluated by using administrative databases to search for reduced rates of hospitalization and death due to influenza-related conditions among people taking statins. These studies should be followed by laboratory studies of statins in animal and cell-based models of influenza virus infection and, later, by clinical trials. Positive results from such studies would provide physicians in all countries with something to offer patients for treatment and prophylaxis of pandemic influenza. Generic statins will be widely distributed and inexpensive. They might be the only agents that could alter the course of a global pandemic.

Differential expression of chemokines and their receptors in adult and neonatal macrophages infected with human or avian influenza viruses

In 1997, avian influenza virus H5N1 was transmitted directly from chicken to human and resulted in a severe disease that had a higher mortality rate in adults than in children. The characteristic mononuclear leukocyte infiltration in the lung and the high inflammatory response in H5N1 infection prompted us to compare the chemokine responses between influenza virus–infected adult and neonatal monocyte-derived macrophages (MDMs). The effects of avian influenza virus A/Hong Kong/483/97 (H5N1) (H5N1/97), its precursor A/Quail/Hong Kong/G1/97 (H9N2) (H9N2/G1), and human influenza virus A/Hong Kong/54/98 (H1N1) (H1N1/98) were compared. Significantly higher expression of CCL2, CCL3, CCL5, and CXCL10 was induced by avian influenza viruses than by human influenza virus. Moreover, the increase in CCL3 expression in H5N1/97-infected adult MDMs was significantly higher than that in neonatal MDMs. Enhanced expression of CCR1 and CCR5 was found in avian virus–infected adult MDMs. The strong induction of chemokines and their receptors by avian influenza viruses, particularly in adult MDMs, may account for the severity of H5N1 disease

X-ray structure of influenza virus NS1 effector domain

The nonstructural protein NS1 of influenza virus is an antagonist of host immune responses and is implicated in virulence. It has two domains, an N-terminal double-stranded RNA-binding domain (RBD) and an effector domain crucial for RBD function, for nuclear export and for sequestering messenger RNA-processing proteins. Here we present the crystallographic structure of the effector domain, which has a novel fold and suggests mechanisms for increased virulence in H5N1 strains.

Influenza virus NS1 protein protects against lymphohematopoietic pathogenesis in an in vivo mouse model

Destruction of peripheral lymphocytes and detrimental alterations in hematopoietic precursors are associated with influenza virus infection in birds and humans. A prominent feature among H5N1 influenza-virus-infected patients with a severe or fatal outcome was found to be lymphopenia and reactive hemophagocytosis. We show here that NS1 protein from human H5N1 influenza isolate A/HK/156/97 reduces both systemic and pulmonary pro-inflammatory cytokines in an in vivo mouse model and protects against bone marrow lymphocyte depletion, an effect which has been shown to be mediated by TNFalpha. These data suggest that the outcome of disease-associated lymphohematopoietic pathogenesis with a pathogenic influenza A virus may depend on the balance between the virus-replication-induced generation of pro-inflammatory cytokines which are a crucial component of the host's anti-viral defense and the ability of the NS1 protein, with or without the interaction of other virus proteins, to counteract such cytokine-mediated adverse effects.

Influenza and the challenge for immunology

The continued westward dissemination of H5N1 influenza A viruses in avian populations and the nearly 50% mortality rate of humans infected with H5N1 are a source of great international concern. A mutant H5N1 virus with the capability to spread rapidly between humans could cause a global catastrophe. Governments have reacted by developing national response plans, stockpiling antiviral drugs and speeding up the development and approval of vaccines. Here we summarize what is known about the interaction between influenza A viruses and the mammalian host response, specifically emphasizing issues that might be of interest to the broader immunology community.

Large-scale sequence analysis of avian influenza isolates

The spread of H5N1 avian influenza viruses (AIVs) from China to Europe has raised global concern about their potential to infect humans and cause a pandemic. In spite of their substantial threat to human health, remarkably little AIV whole-genome information is available. We report here a preliminary analysis of the first large-scale sequencing of AIVs, including 2196 AIV genes and 169 complete genomes. We combine this new information with public AIV data to identify new gene alleles, persistent genotypes, compensatory mutations, and a potential virulence determinant.

Public health risk from avian influenza viruses

Since 1997, avian influenza (AI) virus infections in poultry have taken on new significance, with increasing numbers of cases involving bird-to-human transmission and the resulting production of clinically severe and fatal human infections. Such human infections have been sporadic and are caused by H7N7 and H5N1 high-pathogenicity (HP) and H9N2 low-pathogenicity (LP) AI viruses in Europe and Asia. These infections have raised the level of concern by human health agencies for the potential reassortment of influenza virus genes and generation of the next human pandemic influenza A virus. The presence of endemic infections by H5N1 HPAI viruses in poultry in several Asian countries indicates that these viruses will continue to contaminate the environment and be an exposure risk with human transmission and infection. Furthermore, the reports of mammalian infections with H5N1 AI viruses and, in particular, mammal-to-mammal transmission in humans and tigers are unprecedented. However, the subsequent risk for generating a pandemic human strain is unknown. More international funding from both human and animal health agencies for diagnosis or detection and control of AI in Asia is needed. Additional funding for research is needed to understand why and how these AI viruses infect humans and what pandemic risks they pose.