Mechanisms of pathogenicity of influenza A (H5N1) viruses in mice

H5N1 pathogenesis studies in mammalian models

H5N1 influenza viruses are capable of causing severe disease and death in humans, and represent a potential pandemic subtype should they acquire a transmissible phenotype. Due to the expanding host and geographic range of this virus subtype, there is an urgent need to better understand the contribution of both virus and host responses following H5N1 virus infection to prevent and control human disease. The use of mammalian models, notably the mouse and ferret, has enabled the detailed study of both complex virus-host interactions as well as the contribution of individual viral proteins and point mutations which influence virulence. In this review, we describe the behavior of H5N1 viruses which exhibit high and low virulence in numerous mammalian species, and highlight the contribution of inoculation route to virus pathogenicity. The involvement of host responses as studied in both inbred and outbred mammalian models is discussed. The roles of individual viral gene products and molecular determinants which modulate the severity of H5N1 disease in vivo are presented. This research contributes not only to our understanding of influenza virus pathogenesis, but also identifies novel preventative and therapeutic targets to mitigate the disease burden caused by avian influenza viruses.

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.

The virulence of 1997 H5N1 influenza viruses in the mouse model is increased by correcting a defect in their NS1 proteins

The NS1 protein of human influenza A viruses binds the 30-kDa subunit of the cleavage and polyadenylation specificity factor (CPSF30), a protein required for 3' end processing of cellular pre-mRNAs, thereby inhibiting production of beta interferon (IFN-β) mRNA. The NS1 proteins of pathogenic 1997 H5N1 viruses contain the CPSF30-binding site but lack the consensus amino acids at positions 103 and 106, F and M, respectively, that are required for the stabilization of CPSF30 binding, resulting in nonoptimal CPSF30 binding in infected cells. Here we have demonstrated that strengthening CPSF30 binding, by changing positions 103 and 106 in the 1997 H5N1 NS1 protein to the consensus amino acids, results in a remarkable 300-fold increase in the lethality of the virus in mice. Unexpectedly, this increase in virulence is not associated with increased lung pathology but rather is characterized by faster systemic spread of the virus, particularly to the brain, where increased replication and severe pathology occur. This increased spread is associated with increased cytokine and chemokine levels in extrapulmonary tissues. We conclude that strengthening CPSF30 binding by the NS1 protein of 1997 H5N1 viruses enhances virulence in mice by increasing the systemic spread of the virus from the lungs, particularly to the brain.

Neuroinflammation resulting from covert brain invasion by common viruses - a potential role in local and global neurodegeneration

Neurodegenerative diseases are a horrendous burden for their victims, their families, and society as a whole. For half a century scientists have pursued the hypothesis that these diseases involve a chronic viral infection in the brain. However, efforts to consistently detect a specific virus in brains of patients with such diseases as Alzheimer's or multiple sclerosis have generally failed. Neuropathologists have become increasingly aware that most patients with neurodegenerative diseases demonstrate marked deterioration of the brain olfactory bulb in addition to brain targets that define the specific disease. In fact, the loss of the sense of smell may precede overt neurological symptoms by many years. This realization that the olfactory bulb is a common target in neurodegenerative diseases suggests the possibility that microbes and/or toxins in inhaled air may play a role in their pathogenesis. With regard to inhaled viruses, neuropathologists have focused on those viruses that infect and kill neurons. However, a recent study shows that a respiratory virus with no neurotropic properties can rapidly invade the mouse olfactory bulb from the nasal cavity. Available data suggest that this strain of influenza is passively transported to the bulb via the olfactory nerves (mechanism unknown), and is taken up by glial cells in the outer layers of the bulb. The infected glial cells appear to be activated by the virus, secrete proinflammatory cytokines, and block further spread of virus within the brain. At the time that influenza symptoms become apparent (15 h post-infection), but not prior to symptom onset (10 h post-infection), proinflammatory cytokine-expressing neurons are increased in olfactory cortical pathways and hypothalamus as well as in the olfactory bulb. The mice go on to die of pneumonitis with severe acute phase and respiratory disease symptoms but no classical neurological symptoms. While much remains to be learned about this intranasal influenza-brain invasion model, it suggests the hypothesis that common viruses encountered in our daily life may initiate neuroinflammation via olfactory neural networks. The numerous viruses that we inhale during a lifetime might cause the death of only a few neurons per infection, but this minor damage would accumulate over time and contribute to age-related brain shrinkage and/or neurodegenerative diseases. Elderly individuals with a strong innate inflammatory system, or ongoing systemic inflammation (or both), might be most susceptible to these outcomes. The evidence for the hypothesis that common respiratory viruses may contribute to neurodegenerative processes is developed in the accompanying article.

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

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

Multiorgan distribution of human influenza A virus strains observed in a mouse model

Multiorgan spread and pathogenesis of influenza infection with three human influenza A viruses was studied in mice. Mouse-adapted viruses A/Dunedin/4/73(H3N2), A/Mississippi/1/85(H3N2), and A/PR/8/34(H1N1) differed considerably in virulence (p.f.u./LD(50)): 79,000 p.f.u. for Dunedin, 5,000 p.f.u. for Mississippi, and 65 p.f.u. for PR/8, which qualified Dunedin as low virulent, Mississippi as intermediate, and PR/8 as highly virulent. All three viruses were detected in lungs, heart, and thymus by cultivation and RT-PCR. Moreover, vRNA of all viruses was found in liver and spleen, of Dunedin and PR/8 also in kidneys and that of Dunedin and Mississippi in blood. Only vRNA of Dunedin was demonstrated in brain. Lung damage accompanied by histopathological changes and thymus reduction were most extensive after infection with the highly virulent virus PR/8. We assume that the ability to spread to multiple organs may be a more common property of influenza viruses in mammalian hosts than previously believed.

Novel linear DNA vaccines induce protective immune responses against lethal infection with influenza virus type A/H5N1

Vaccine development for possible influenza pandemics has been challenging. Conventional vaccines such as inactivated and live attenuated virus preparations are limited in terms of production speed and capacity. DNA vaccination has emerged as a potential alternative to conventional vaccines against influenza pandemics. In this study, we use a novel, cell-free DNA manufacturing process (synDNA) to produce prototype linear DNA vaccines against the influenza virus type A/H5N1. This synDNA process does not require bacterial fermentation, so it avoids the use of antibiotic resistance genes and other nucleic acid sequences unrelated to the antigen gene expression in the actual therapeutic DNA construct. The efficacy of various vaccines expressing the hemagglutinin and neuraminidase proteins (H5N1 synDNA), hemagglutinin alone (H5 synDNA) or neuraminidase alone (N1 synDNA) was evaluated in mice. Two of the constructs (H5 synDNA and H5N1 synDNA) induced a robust protective immune response with up to 93% of treated mice surviving a lethal challenge of a virulent influenza A/Vietnam/1203/04 H5N1 isolate. In combination with a potent biological activity and simplified production footprint, these characteristics make DNA vaccines prepared with our synDNA process highly suitable as alternatives to other vaccine preparations.

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

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

Subclinical infection with avian influenza A (H5N1) virus in cats

Infection without disease may occur under natural conditions after contact with infected birds.

Avian influenza A virus subtype H5N1 was transmitted to domestic cats by close contact with infected birds. Virus-specific nucleic acids were detected in pharyngeal swabs from 3 of 40 randomly sampled cats from a group of 194 animals (day 8 after contact with an infected swan). All cats were transferred to a quarantine station and monitored for clinical signs, virus shedding, and antibody production until day 50. Despite unfamiliar handling, social distress and the presence of other viral and nonviral pathogens that caused illness and poor health and compromised the immune systems, none of the cats developed clinical signs of influenza. There was no evidence of horizontal transmission to other cats because only 2 cats developed antibodies against H5N1 virus.

Characterization of an H5N1 avian influenza virus from Taiwan

In 2003, an avian influenza (AI) virus of H5N1 subtype (A/Duck/China/E319-2/03; Dk/CHN/E319-2/03) was isolated from a smuggled duck in Kinmen Island of Taiwan. Phylogenetic analysis and pairwise comparison of nucleotide and amino acid sequences revealed that the virus displayed high similarity to the H5N1 viruses circulating in Asia during 2004 and 2005. The hemagglutinin (HA) protein of the virus contained multiple basic amino acid residues (-RERRRKR-) adjacent to the cleavage site between the HA1 and HA2 domains, showing the highly pathogenic (HP) characteristics. The HP phenotype was confirmed by experimental infection of chickens, which led up to 100% mortality within 24-72h postinfection. The virus replicated equally well in the majority of organs of the infected chickens with titers ranging from 10(7.5) to 10(4.7) 50% embryo lethal dose (ELD50) per gram of tissue. In a mouse model the virus exhibits low pathogenic characteristics with a lethal infection observed only after applying high inoculating dose (>or=10(7.6) ELD50) of the virus. The infectious virus particles were recovered only from the pulmonary system including trachea and lungs. Our study suggests that ducks infected with H5N1 AIV of HPAI pathotype showing no disease signs can carry the virus silently and that bird smuggling represent a serious risk for H5N1 HPAI transmission.

Live, attenuated influenza A H5N1 candidate vaccines provide broad cross-protection in mice and ferrets

BACKGROUND

Recent outbreaks of highly pathogenic influenza A H5N1 viruses in humans and avian species that began in Asia and have spread to other continents underscore an urgent need to develop vaccines that would protect the human population in the event of a pandemic.

METHODS AND FINDINGS

Live, attenuated candidate vaccines possessing genes encoding a modified H5 hemagglutinin (HA) and a wild-type (wt) N1 neuraminidase from influenza A H5N1 viruses isolated in Hong Kong and Vietnam in 1997, 2003, and 2004, and remaining gene segments derived from the cold-adapted (ca) influenza A vaccine donor strain, influenza A/Ann Arbor/6/60 ca (H2N2), were generated by reverse genetics. The H5N1 ca vaccine viruses required trypsin for efficient growth in vitro, as predicted by the modification engineered in the gene encoding the HA, and possessed the temperature-sensitive and attenuation phenotypes specified by the internal protein genes of the ca vaccine donor strain. More importantly, the candidate vaccines were immunogenic in mice. Four weeks after receiving a single dose of 10(6) 50% tissue culture infectious doses of intranasally administered vaccines, mice were fully protected from lethality following challenge with homologous and antigenically distinct heterologous wt H5N1 viruses from different genetic sublineages (clades 1, 2, and 3) that were isolated in Asia between 1997 and 2005. Four weeks after receiving two doses of the vaccines, mice and ferrets were fully protected against pulmonary replication of homologous and heterologous wt H5N1 viruses.

CONCLUSIONS

The promising findings in these preclinical studies of safety, immunogenicity, and efficacy of the H5N1 ca vaccines against antigenically diverse H5N1 vaccines provide support for their careful evaluation in Phase 1 clinical trials in humans.

Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts

The ongoing outbreak of avian influenza A virus (subtype H5N1) infection in Asia is of great concern because of the high human case fatality rate and the threat of a new influenza pandemic. Case reports in humans and felids suggest that this virus may have a different tissue tropism from other influenza viruses, which are normally restricted to the respiratory tract in mammals. To study its pathogenesis in a mammalian host, domestic cats were inoculated with H5N1 virus intratracheally (n = 3), by feeding on virus-infected chicks (n = 3), or by horizontal transmission (n = 2) and examined by virological and pathological assays. In all cats, virus replicated not only in the respiratory tract but also in multiple extra-respiratory tissues. Virus antigen expression in these tissues was associated with severe necrosis and inflammation 7 days after inoculation. In cats fed on virus-infected chicks only, virus-associated ganglioneuritis also occurred in the submucosal and myenteric plexi of the small intestine, suggesting direct infection from the intestinal lumen. All cats excreted virus not only via the respiratory tract but also via the digestive tract. This study in cats demonstrates that H5N1 virus infection causes systemic disease and spreads by potentially novel routes within and between mammalian hosts.