Characterization of H7N9 influenza A viruses isolated from humans

Avian influenza H7N9/13 and H7N7/13: a comparative virulence study in chickens, pigeons, and ferrets

Human influenza cases caused by a novel avian H7N9 virus in China emphasize the zoonotic potential of that subtype. We compared the infectivity and pathogenicity of the novel H7N9 virus with those of a recent European avian H7N7 strain in chickens, pigeons, and ferrets. Neither virus induced signs of disease despite substantial replication in inoculated chickens and rapid transmission to contact chickens. Evidence of the replication of both viruses in pigeons, albeit at lower levels of RNA excretion, was also detected. No clear-cut differences between the two H7 isolates emerged regarding replication and antibody development in avian hosts. In ferrets, in contrast, greater replication of the avian H7N9 virus than of the H7N7 strain was observed with significant differences in viral presence, e.g., in nasal wash, lung, and cerebellum samples. Importantly, both viruses showed the potential to spread to the mammal brain. We conclude that efficient asymptomatic viral replication and shedding, as shown in chickens, facilitate the spread of H7 viruses that may harbor zoonotic potential. Biosafety measures are required for the handling of poultry infected with avian influenza viruses of the H7 subtype, independently of their pathogenicity for gallinaceous poultry.

IMPORTANCE

This study is important to the field since it provides data about the behavior of the novel H7N9 avian influenza virus in chickens, pigeons, and ferrets in comparison with that of a recent low-pathogenicity H7N7 strain isolated from poultry. We clearly show that chickens, but not pigeons, are highly permissive hosts of both H7 viruses, allowing high-titer replication and virus shedding without any relevant clinical signs. In the ferret model, the potential of both viruses to infect mammals could be demonstrated, including infection of the brain. However, the replication efficiency of the H7N9 virus in ferrets was higher than that of the H7N7 strain. In conclusion, valuable data for the risk analysis of low-pathogenicity avian influenza viruses of the H7 subtype are provided that could also be used for the risk assessment of zoonotic potentials and necessary biosafety measures.

In silico structural homology modelling and docking for assessment of pandemic potential of a novel H7N9 influenza virus and its ability to be neutralized by existing anti-hemagglutinin antibodies

The unpredictable nature of pandemic influenza and difficulties in early prediction of pandemic potential of new isolates present a major challenge for health planners. Vaccine manufacturers, in particular, are reluctant to commit resources to development of a new vaccine until after a pandemic is declared. We hypothesized that a structural bioinformatics approach utilising homology-based molecular modelling and docking approaches would assist prediction of pandemic potential of new influenza strains alongside more traditional laboratory and sequence-based methods. The newly emerged Chinese A/Hangzhou/1/2013 (H7N9) influenza virus provided a real-life opportunity to test this hypothesis. We used sequence data and a homology-based approach to construct a 3D-structural model of H7-Hangzhou hemagglutinin (HA) protein. This model was then used to perform docking to human and avian sialic acid receptors to assess respective binding affinities. The model was also used to perform docking simulations with known neutralizing antibodies to assess their ability to neutralize the newly emerged virus. The model predicted H7N9 could bind to human sialic acid receptors thereby indicating pandemic potential. The model also confirmed that existing antibodies against the HA head region are unable to neutralise H7N9 whereas antibodies, e.g. Cr9114, targeting the HA stalk region should bind with high affinity to H7N9. This indicates that existing stalk antibodies initially raised against H5N1 or other influenza A viruses could be therapeutically beneficial in prevention and/or treatment of H7N9 infections. The subsequent publication of the H7N9 HA crystal structure confirmed the accuracy of our in-silico structural model. Antibody docking studies performed using the H7N9 HA crystal structure supported the model's prediction that existing stalk antibodies could cross-neutralise the H7N9 virus. This study demonstrates the value of using in-silico structural modelling approaches to complement physical studies in characterization of new influenza viruses.

Identification, characterization, and natural selection of mutations driving airborne transmission of A/H5N1 virus

Recently, A/H5N1 influenza viruses were shown to acquire airborne transmissibility between ferrets upon targeted mutagenesis and virus passage. The critical genetic changes in airborne A/Indonesia/5/05 were not yet identified. Here, five substitutions proved to be sufficient to determine this airborne transmission phenotype. Substitutions in PB1 and PB2 collectively caused enhanced transcription and virus replication. One substitution increased HA thermostability and lowered the pH of membrane fusion. Two substitutions independently changed HA binding preference from α2,3-linked to α2,6-linked sialic acid receptors. The loss of a glycosylation site in HA enhanced overall binding to receptors. The acquired substitutions emerged early during ferret passage as minor variants and became dominant rapidly. Identification of substitutions that are essential for airborne transmission of avian influenza viruses between ferrets and their associated phenotypes advances our fundamental understanding of virus transmission and will increase the value of future surveillance programs and public health risk assessments.

Considerations for the rapid deployment of vaccines against H7N9 influenza

The threat of an outbreak of avian-origin influenza H7N9 and the devastating consequences that a pandemic could have on global population health and economies has mobilized programs of constant surveillance and the implementation of preemptive plans. Central to these plans is the production of prepandemic vaccines that can be rapidly deployed to minimize disease severity and deaths resulting from such an occurrence. In this article, we review current H7N9 vaccine strategies in place and the available technologies and options that can help accelerate vaccine production and increase dose-sparing capabilities to provide enough vaccines to cover the population. We also present possible means of reducing disease impact during the critical period after an outbreak occurs before a strain matched vaccine becomes available and consider the use of existing stockpiles and seed strains of phylogenetically related subtypes, alternate vaccination regimes and vaccine forms that induce cross-reactive immunity.

Profiles of acute cytokine and antibody responses in patients infected with avian influenza A H7N9

The influenza A H7N9 virus outbreak in Eastern China in the spring of 2013 represented a novel, emerging avian influenza transmission to humans. While clinical and microbiological features of H7N9 infection have been reported in the literature, the current study investigated acute cytokine and antibody responses in acute H7N9 infection. Between March 27, 2013 and April 23, 2013, six patients with confirmed H7N9 influenza infection were admitted to Drum Tower Hospital, Nanjing, China. Acute phase serum cytokine profiles were determined using a high-throughput multiplex assay. Daily H7 hemagglutinin (HA)-specific IgG, IgM, and IgA responses were monitored by ELISA. Neutralizing antibodies specific for H7N9 viruses were determined against a pseudotyped virus expressing the novel H7 subtype HA antigen. Five cytokines (IL-6, IP-10, IL-10, IFNγ, and TNFα) were significantly elevated in H7N9-infected patients when compared to healthy volunteers. Serum H7 HA-specific IgG, as well as IgM and IgA responses, were detected within 8 days of disease onset and increased in a similar pattern during acute infection. Neutralizing antibodies developed shortly after the appearance of binding antibody responses and showed similar kinetics as a fraction of the total H7 HA-specific IgG responses. H7N9 infection resulted in hallmark serum cytokine increases, which correlated with fever and disease persistence. The novel finding of simultaneous development of IgG, IgM, and IgA responses in acute H7N9 infection points to the potential for live influenza viruses to elicit fast and potent protective antibodies to limit the infection.

H7N9 and other pathogenic avian influenza viruses elicit a three-pronged transcriptomic signature that is reminiscent of 1918 influenza virus and is associated with lethal outcome in mice

Modulating the host response is a promising approach to treating influenza, caused by a virus whose pathogenesis is determined in part by the reaction it elicits within the host. Though the pathogenicity of emerging H7N9 influenza virus in several animal models has been reported, these studies have not included a detailed characterization of the host response following infection. Therefore, we characterized the transcriptomic response of BALB/c mice infected with H7N9 (A/Anhui/01/2013) virus and compared it to the responses induced by H5N1 (A/Vietnam/1203/2004), H7N7 (A/Netherlands/219/2003), and pandemic 2009 H1N1 (A/Mexico/4482/2009) influenza viruses. We found that responses to the H7 subtype viruses were intermediate to those elicited by H5N1 and pdm09H1N1 early in infection but that they evolved to resemble the H5N1 response as infection progressed. H5N1, H7N7, and H7N9 viruses were pathogenic in mice, and this pathogenicity correlated with increased transcription of cytokine response genes and decreased transcription of lipid metabolism and coagulation signaling genes. This three-pronged transcriptomic signature was observed in mice infected with pathogenic H1N1 strains such as the 1918 virus, indicating that it may be predictive of pathogenicity across multiple influenza virus strains. Finally, we used host transcriptomic profiling to computationally predict drugs that reverse the host response to H7N9 infection, and we identified six FDA-approved drugs that could potentially be repurposed to treat H7N9 and other pathogenic influenza viruses.

IMPORTANCE

Emerging avian influenza viruses are of global concern because the human population is immunologically naive to them. Current influenza drugs target viral molecules, but the high mutation rate of influenza viruses eventually leads to the development of antiviral resistance. As the host evolves far more slowly than the virus, and influenza pathogenesis is determined in part by the host response, targeting the host response is a promising approach to treating influenza. Here we characterize the host transcriptomic response to emerging H7N9 influenza virus and compare it with the responses to H7N7, H5N1, and pdm09H1N1. All three avian viruses were pathogenic in mice and elicited a transcriptomic signature that also occurs in response to the legendary 1918 influenza virus. Our work identifies host responses that could be targeted to treat severe H7N9 influenza and identifies six FDA-approved drugs that could potentially be repurposed as H7N9 influenza therapeutics.

The R292K mutation that confers resistance to neuraminidase inhibitors leads to competitive fitness loss of A/Shanghai/1/2013 (H7N9) influenza virus in ferrets

BACKGROUND

Neuraminidase (NA) inhibitors are the only licensed therapeutic option for human zoonotic H7N9 infections. An NA-R292K mutation that confers broad-spectrum resistance to NA inhibitors has been documented in H7N9 patients after treatment.

METHODS

We evaluated the transmission potential of a human influenza A H7N9 isolate with a NA-R292K mutation in the ferret model followed by genotyping assay to monitor its competitive fitness in vivo.

RESULTS

Plaque-purified A/Shanghai/1/2013 wild-type and NA-R292K viruses transmitted at comparable efficiency to direct or respiratory droplet contact ferrets. In ferrets inoculated with the plaque-purified A/Shanghai/1/2013 NA-R292K virus with dominant K292 (94%), the resistant K292 genotype was outgrown by the wild-type R292 genotype during the course of infection. Transmission of the resistant K292 genotype was detected in 3/4 direct contact and 3/4 respiratory droplet contact ferrets at early time points but was gradually replaced by the wild-type genotype. In the respiratory tissues of inoculated or infected ferrets, the wild-type R292 genotype dominated in the nasal turbinate, whereas the resistant K292 genotype was more frequently detected in the lungs.

CONCLUSIONS

The NA inhibitor-resistant H7N9 virus with the NA-R292K mutation may transmit among ferrets but showed compromised fitness in vivo while in competition with the wild-type virus.

Zoonotic infections with avian influenza A viruses and vaccine preparedness: a game of "mix and match"

Various direct avian-to-human transmissions of influenza A virus subtypes upon exposure to infected poultry have been previously observed in the past decades. Although some of these strains caused lethal infections, the lack of sustained person-to-person transmission has been the major factor that prevented these viruses from causing new pandemics. In 2013, three (A/H7N9, A/H6N1, and A/H10N8) novel avian influenza viruses (AIVs) yet again breached the animal-human host species barrier in Asia. Notably, roughly 20% of the A/H7N9-infected patients succumbed to the zoonotic infection whereas two of three A/H10N8 human infections were also lethal. Thus, these events revived the concerns of potential pandemic threats by AIVs in the horizon. This article reviews the various human incursions with AIV variants and provides insight on how continued circulation of these viruses poses perpetual challenge to global public health. As the world anticipates for the next human pandemic, constant vigilance for newly emerging viruses in nature is highly encouraged. With the various numbers of AIVs demonstrating their capacity to breach the animal-human host interface and apparent limitations of current antivirals, there is a need to broaden the selection of pre-pandemic vaccine candidate viruses and development of novel alternative therapeutic strategies.

Insights into the human glycan receptor conformation of 1918 pandemic hemagglutinin-glycan complexes derived from nuclear magnetic resonance and molecular dynamics studies

The glycan receptor binding and specificity of influenza A viral hemagglutinin (HA) are critical for virus infection and transmission in humans. However, ambiguities in the interpretation of the receptor binding specificity of hemagglutinin from human- and avian-adapted viruses have prevented an understanding of its relationship with aerosol transmissibility, an exclusive property of human-adapted viruses. A previous conformational study, which we performed, indicated that human and avian receptors sample distinct conformations in solution. On the basis of detailed nuclear magnetic resonance (NMR) studies provided herein, we offer evidence of the distinct structural constraints imposed by hemagglutinin receptor binding sites on the glycan conformational space upon binding. The hemagglutinin from the SC18 virus, which has efficient aerosol transmissibility in humans (human-adapted), imposed the most stringent constraints on the conformational space of the human glycan receptor (LSTc), compared to single (NY18) or double (AV18) amino acid HA mutants, a property correlating to the ligand–HA binding strength. This relationship was also observed for the avian-adapted HA, where the high affinity binding partner, AV18, imposed the most stringent conformational constraints on the avian receptor, compared to those imposed by NY18. In particular, it is interesting to observe how different HAs when binding to human or avian glycosidic receptors impose significantly different conformational states, in terms of the states sampled by the glycosidic backbone and/or the entire molecule shape (linear or bent), when compared to the corresponding unbound glycans. Significantly, we delineate a “characteristic NMR signature” for the human adapted hemagglutinin (SC18) binding to human glycan receptors. Therefore, the conformational space constraints imposed by the hemagglutinin receptor binding site provide a characteristic signature that could be a useful tool for the surveillance of human adaptation of other (such as H7N9 and H5N1) deadly influenza viruses.

An H7N1 influenza virus vaccine induces broadly reactive antibody responses against H7N9 in humans

Emerging H7N9 influenza virus infections in Asia have once more spurred the development of effective prepandemic H7 vaccines. However, many vaccines based on avian influenza viruses--including H7--are poorly immunogenic, as measured by traditional correlates of protection. Here we reevaluated sera from an H7N1 human vaccine trial performed in 2006. We examined cross-reactive antibody responses to divergent H7 strains, including H7N9, dissected the antibody response into head- and stalk-reactive antibodies, and tested the in vivo potency of these human sera in a passive-transfer H7N9 challenge experiment with mice. Although only a low percentage of vaccinees induced neutralizing antibody responses against the homologous vaccine strain and also H7N9, we detected strong cross-reactivity to divergent H7 hemagglutinins (HAs) in a large proportion of the cohort with a quantitative enzyme-linked immunosorbent assay. Furthermore, H7N1 vaccination induced antibodies to both the head and stalk domains of the HA, which is in sharp contrast to seasonal inactivated vaccines. Finally, we were able to show that both neutralizing and nonneutralizing antibodies improved in vivo virus clearance in a passive-transfer H7N9 challenge mouse model.

Molecular mechanism of the airborne transmissibility of H9N2 avian influenza A viruses in chickens

H9N2 avian influenza virus has been prevalent in poultry in many parts of the world since the 1990s and occasionally crosses the host barrier, transmitting to mammals, including humans. In recent years, these viruses have contributed genes to H5N1 and H7N9 influenza viruses, threatening public health. To explore the molecular mechanism for the airborne transmission of H9N2 virus, we compared two genetically close strains isolated from chickens in 2001, A/chicken/Shanghai/7/2001(SH7) and A/chicken/Shanghai/14/2001 (SH14). SH7 is airborne transmissible between chickens, whereas SH14 is not. We used reverse genetics and gene swapping to derive recombinant SH7 (rSH7), rSH14, and a panel of reassortant viruses. Among the reassortant viruses, we identified segments HA and PA as governing the airborne transmission among chickens. In addition, the NP and NS genes also contributed to a lesser extent. Furthermore, the mutational analyses showed the transmissibility phenotype predominantly mapped to the HA and PA genes, with HA-K363 and PA-L672 being important for airborne transmissibility among chickens. In addition, the viral infectivity and acid stability are related to the airborne transmissibility. Importantly, airborne transmission studies of 18 arbitrarily chosen H9N2 viruses from our collections confirmed the importance of both 363K in HA and 672L in PA in determining their levels of transmissibility. Our finding elucidates the genetic contributions to H9N2 transmissibility in chickens and highlights the importance of their prevalence in poultry.

IMPORTANCE

Our study investigates the airborne transmissibility of H9N2 viruses in chickens and the subsequent epidemic. H9N2 virus is the donor for several prevalent reassortant influenza viruses, such as H7N9/2013 and the H5N1 viruses. Poultry as the reservoir hosts of influenza virus is closely associated with human society. Airborne transmission is an efficient pathway for influenza virus transmission among flocks and individuals. Exploring the mechanism of the airborne transmission of the H9N2 virus in chickens could provide essential data regarding prevention and control of influenza endemics and pandemics.

Low infectivity of a novel avian-origin H7N9 influenza virus in pigs

We studied the pathogenesis and transmissibility of a novel avian-origin H7N9 influenza virus in pigs. When pigs were infected with H7N9 influenza virus, they did not show any clear clinical signs (such as sneezing, fever and loss of body weight), and they shed viruses through their noses for 2 days after infection. No transmission occurred between infected and naïve pigs. Pigs suffered from mild pneumonia, which was accompanied by the induction of inflammatory cytokines and chemokines such as IL-8 and CCL1. Taken together, our results suggest that pigs may not play an active role in transmitting H7N9 influenza virus to mammals.

Anti-influenza viral effects of honey in vitro: potent high activity of manuka honey

BACKGROUND AND AIMS

Influenza viruses are a serious threat to human health and cause thousands of deaths annually. Thus, there is an urgent requirement for the development of novel anti-influenza virus drugs. Therefore, the aim of this study was to evaluate the anti-influenza viral activity of honey from various sources.

METHODS

Antiviral activities of honey samples were evaluated using MDCK cells. To elucidate the possible mechanism of action of honey, plaque inhibition assays were used. Synergistic effects of honey with known anti-influenza virus drugs such as zanamivir or oseltamivir were tested.

RESULTS

Manuka honey efficiently inhibited influenza virus replication (IC50 = 3.6 ± 1.2 mg/mL; CC50 = 82.3 ± 2.2 mg/mL; selective index = 22.9), which is related to its virucidal effects. In the presence of 3.13 mg/mL manuka honey, the IC50 of zanamivir or oseltamivir was reduced to nearly 1/1000th of their single use.

CONCLUSIONS

Our results showed that honey, in general, and particularly manuka honey, has potent inhibitory activity against the influenza virus, demonstrating a potential medicinal value.

Molecular basis of a pandemic of avian-type influenza virus

Despite heroic efforts to prevent the emergence of an influenza pandemic, avian influenza A virus has prevailed by crossing the species barriers to infect humans worldwide, occasionally with morbidity and mortality at unprecedented levels, and the virus later usually continues circulation in humans as a seasonal influenza virus, resulting in health-social-economic problems each year. Here, we review current knowledge of influenza viruses, their life cycle, interspecies transmission, and past pandemics and discuss the molecular basis of pandemic acquisition, notably of hemagglutinin (lectin) acting as a key contributor to change in host specificity in viral infection.

Analysis of influenza virus receptor specificity using glycan-functionalized gold nanoparticles

Recent cases of human infection with avian influenza H5N1 and H7N9 viruses underscore an urgent need for techniques that can rapidly assess their potential threat to the humans. Determination of the receptor-binding property of influenza virus is crucial to direct viral control and prevention measures. Current methods to perform this analysis are dependent on immunoanalytical strategies that use unstable biological components and complex procedures. We have developed a facile colorimetric assay to determine the interaction of the viral hemagglutinin (HA) protein with host glycan receptors using glycan-functionalized gold nanoparticles (gGNPs). This method is based on the color and absorbance changes of gold probes when the solution is simply mixed with HAs or intact viruses. The resulting sensitivity and selectivity has enabled HA/virus binding to various glycan structures to be differentiated visually and rapidly. Using this system, we have screened, in parallel, the receptor specificity of eight representative human and avian viral HAs and three whole viruses including an emerging H7N9 strain. Our results reveal the detailed receptor-binding profiles of H7N9 virus and its HA and show that they effectively bind to human-type receptors. This gGNP-based assay represents a strategy that would be helpful for developing simple and sensitive systems to probe glycan-mediated biological processes.

Continuing reassortment leads to the genetic diversity of influenza virus H7N9 in Guangdong, China

On 30 March 2013, a novel avian influenza A H7N9 virus causing severe human respiratory infections was identified in China. Preliminary sequence analyses have shown that the virus is a reassortant of H7N9 and H9N2 avian influenza viruses. In this study, we conducted enhanced surveillance for H7N9 virus in Guangdong, China, from April to August 2013. We isolated two H7N9 viral strains from environmental samples associated with poultry markets and one from a clinical patient. Sequence analyses showed that the Guangdong H7N9 virus isolated from April to May shared high sequence similarity with other strains from eastern China. The A/Guangdong/1/2013 (H7N9) virus isolated from the Guangdong patient on 10 August 2013 was divergent from previously sequenced H7N9 viruses and more closely related to local circulating H9N2 viruses in the NS and NP genes. Phylogenetic analyses revealed that four internal genes of the A/Guangdong/1/2013 (H7N9) virus-the NS, NP, PB1, and PB2 genes-were in clusters different from those for H7N9 viruses identified previously in other provinces of China. The discovery presented here suggests that continuing reassortment led to the emergence of the A/Guangdong/1/2013 (H7N9) virus as a novel H7N9 virus in Guangdong, China, and that viral adaptation to avian and human hosts must be assessed.

IMPORTANCE

In this study, we isolated and characterized the avian influenza A H7N9 virus in Guangdong, China, from April to August 2013. We show that the viruses isolated from Guangdong environmental samples and chickens from April to May 2013 were highly similar to other H7N9 strains found in eastern China. The H7N9 virus isolated from the clinical patient in Guangdong in August 2013 was divergent from previously identified H7N9 viruses, with the NS and NP genes originating from recent H9N2 viruses circulating in the province. This study provides direct evidence that continuing reassortment occurred and led to the emergence of a novel H7N9 influenza virus in Guangdong, China. These results also shed light on how the H7N9 virus evolved, which is critically important for future monitoring and tracing of viral transmission.

Understanding the undelaying mechanism of HA-subtyping in the level of physic-chemical characteristics of protein

The evolution of the influenza A virus to increase its host range is a major concern worldwide. Molecular mechanisms of increasing host range are largely unknown. Influenza surface proteins play determining roles in reorganization of host-sialic acid receptors and host range. In an attempt to uncover the physic-chemical attributes which govern HA subtyping, we performed a large scale functional analysis of over 7000 sequences of 16 different HA subtypes. Large number (896) of physic-chemical protein characteristics were calculated for each HA sequence. Then, 10 different attribute weighting algorithms were used to find the key characteristics distinguishing HA subtypes. Furthermore, to discover machine leaning models which can predict HA subtypes, various Decision Tree, Support Vector Machine, Naïve Bayes, and Neural Network models were trained on calculated protein characteristics dataset as well as 10 trimmed datasets generated by attribute weighting algorithms. The prediction accuracies of the machine learning methods were evaluated by 10-fold cross validation. The results highlighted the frequency of Gln (selected by 80% of attribute weighting algorithms), percentage/frequency of Tyr, percentage of Cys, and frequencies of Try and Glu (selected by 70% of attribute weighting algorithms) as the key features that are associated with HA subtyping. Random Forest tree induction algorithm and RBF kernel function of SVM (scaled by grid search) showed high accuracy of 98% in clustering and predicting HA subtypes based on protein attributes. Decision tree models were successful in monitoring the short mutation/reassortment paths by which influenza virus can gain the key protein structure of another HA subtype and increase its host range in a short period of time with less energy consumption. Extracting and mining a large number of amino acid attributes of HA subtypes of influenza A virus through supervised algorithms represent a new avenue for understanding and predicting possible future structure of influenza pandemics.

H7N9: A killer in the making or a false alarm?

Influenza virus remains one of the most important disease-causing viruses owing to its high adaptability and even higher contagious nature. Thus, it poses a constant threat of pandemic, engulfing a large population within the smallest possible time interval. A similar threat was anticipated with the identification of the novel H7N9 virus in China on 30 March 2013. Detection of transmission of the virus between humans has caused a stir with the identification of family clusters along with sporadic infections all across China. In this review we analyze the potential of the novel H7N9 virus as a probable cause of a pandemic and the possible consequences thereof.

Analysis of recombinant H7N9 wild-type and mutant viruses in pigs shows that the Q226L mutation in HA is important for transmission

The fact that there have been more than 300 human infections with a novel avian H7N9 virus in China indicates that this emerging strain has pandemic potential. Furthermore, many of the H7N9 viruses circulating in animal reservoirs contain putative mammalian signatures in the HA and PB2 genes that are believed to be important in the adaptation of other avian strains to humans. To date, the definitive roles of these mammalian-signature substitutions in transmission and pathogenesis of H7N9 viruses remain unclear. To address this we analyzed the biological characteristics, pathogenicity, and transmissibility of A/Anhui/1/2013 (H7N9) virus and variants in vitro and in vivo using a synthetically created wild-type virus (rAnhui-WT) and two mutants (rAnhui-HA-226Q and rAnhui-PB2-627E). All three viruses replicated in lungs of intratracheally inoculated pigs, yet nasal shedding was limited. The rAnhui-WT and rAnhui-PB2-627E viruses were transmitted to contact animals. In contrast, the rAnhui-HA-226Q virus was not transmitted to sentinel pigs. Deep sequencing of viruses from the lungs of infected pigs identified substitutions arising in the viral population (e.g., PB2-T271A, PB2-D701N, HA-V195I, and PB2-E627K reversion) that may enhance viral replication in pigs. Collectively, the results demonstrate that critical mutations (i.e., HA-Q226L) enable the H7N9 viruses to be transmitted in a mammalian host and suggest that the myriad H7N9 genotypes circulating in avian species in China and closely related strains (e.g., H7N7) have the potential for further adaptation to human or other mammalian hosts (e.g., pigs), leading to strains capable of sustained human-to-human transmission. Importance: The genomes of the zoonotic avian H7N9 viruses emerging in China have mutations in critical genes (PB2-E627K and HA-Q226L) that may be important in their pandemic potential. This study shows that (i) HA-226L of zoonotic H7N9 strains is critical for binding the α-2,6-linked receptor and enables transmission in pigs; (ii) wild-type A/Anhui/1/2013 (H7N9) shows modest replication, virulence, and transmissibility in pigs, suggesting that it is not well adapted to the mammalian host; and (iii) both wild-type and variant H7N9 viruses rapidly develop additional mammalian-signature mutations in pigs, indicating that they represent an important potential intermediate host. This is the first study analyzing the phenotypic effects of specific mutations within the HA and PB2 genes of the novel H7N9 viruses created by reverse genetics in an important mammalian host model. Finally, this study illustrates that loss-of-function mutations can be used to effectively identify residues critical to zoonosis/transmission.

Novel avian influenza A (H7N9) virus induces impaired interferon responses in human dendritic cells

In March 2013 a new avian influenza A(H7N9) virus emerged in China and infected humans with a case fatality rate of over 30%. Like the highly pathogenic H5N1 virus, H7N9 virus is causing severe respiratory distress syndrome in most patients. Based on genetic analysis this avian influenza A virus shows to some extent adaptation to mammalian host. In the present study, we analyzed the activation of innate immune responses by this novel H7N9 influenza A virus and compared these responses to those induced by the avian H5N1 and seasonal H3N2 viruses in human monocyte-derived dendritic cells (moDCs). We observed that in H7N9 virus-infected cells, interferon (IFN) responses were weak although the virus replicated as well as the H5N1 and H3N2 viruses in moDCs. H7N9 virus-induced expression of pro-inflammatory cytokines remained at a significantly lower level as compared to H5N1 virus-induced “cytokine storm” seen in human moDCs. However, the H7N9 virus was extremely sensitive to the antiviral effects of IFN-α and IFN-β in pretreated cells. Our data indicates that different highly pathogenic avian viruses may show considerable differences in their ability to induce host antiviral responses in human primary cell models such as moDCs. The unexpected appearance of the novel H7N9 virus clearly emphasizes the importance of the global influenza surveillance system. It is, however, equally important to systematically characterize in normal human cells the replication capacity of the new viruses and their ability to induce and respond to natural antiviral substances such as IFNs.

Avian influenza A viruses: from zoonosis to pandemic

Zoonotic influenza A viruses originating from the animal reservoir pose a threat for humans, as they have the ability to trigger pandemics upon adaptation to and invasion of an immunologically naive population. Of particular concern are the H5N1 viruses that continue to circulate in poultry in numerous countries in Europe, Asia and Africa, and the recently emerged H7N9 viruses in China, due to their relatively high number of human fatalities and pandemic potential. To start a pandemic, zoonotic influenza A viruses should not only acquire the ability to attach to, enter and replicate in the critical target cells in the respiratory tract of the new host, but also efficiently spread between humans by aerosol or respiratory droplet transmission. Here, we discuss the latest advances on the genetic and phenotypic determinants required for avian influenza A viruses to adapt to and transmit between mammals.

Potency of an inactivated influenza vaccine prepared from A/duck/Mongolia/119/2008 (H7N9) against the challenge with A/Anhui/1/2013 (H7N9)

H7N9 influenza virus infection in humans was reported in China on March 31, 2013. Humans are immunologically naïve to the H7N9 subtype, for which the seasonal influenza vaccine is not effective. Thus, the development of an H7N9 influenza virus vaccine is an urgent issue. To prepare for the emergence of an influenza pandemic, we have established a library comprising more than 1300 influenza virus strains with 144 different combinations of 16 HA and 9 NA subtypes. An H7N9 virus strain isolated from a 35-year-old woman, A/Anhui/1/2013 (H7N9), was found to be antigenically similar to H7N9 influenza viruses isolated from migratory ducks. In the present study, the potency of an inactivated whole virus particle vaccine prepared from an H7N9 low pathogenic avian influenza virus, A/duck/Mongolia/119/2008 (H7N9), selected from the library, was assessed by a challenge with A/Anhui/1/2013 (H7N9). The results indicate that the test vaccine was potent enough to induce sufficient immunity to reduce the impact of disease caused by the challenge with A/Anhui/1/2013 (H7N9) in mice. The present results indicate that an inactivated whole virus particle vaccine prepared from an influenza virus strain stored in the library could be useful as a vaccine strain in case of an influenza pandemic.

Development of a reverse transcription loop-mediated isothermal amplification assay for the rapid diagnosis of avian influenza A (H7N9) virus infection

A genetic diagnosis system for detecting avian influenza A (H7N9) virus infection using reverse transcription-loop-mediated isothermal amplification (RT-LAMP) technology was developed. The RT-LAMP assay showed no cross-reactivity with seasonal influenza A (H3N2 and H1N1pdm09) or influenza B viruses circulating in humans or with avian influenza A (H5N1) viruses. The sensitivity of the RT-LAMP assay was 42.47 copies/reaction. Considering the high specificity and sensitivity of the assay for detecting the avian influenza A (H7N9) virus and that the reaction was completed within 30 min, the RT-LAMP assay developed in this study is a promising rapid diagnostic tool for avian influenza A (H7N9) virus infection.

The planning, execution, and evaluation of a mass prophylaxis full-scale exercise in cook county, IL

Increasing threats of bioterrorism and the emergence of novel disease agents, including the recent international outbreaks of H7N9 influenza and MERS-CoV, have stressed the importance and highlighted the need for public health preparedness at local, regional, and national levels. To test plans that were developed for mass prophylaxis scenarios, in April 2013 the Cook Country Department of Public Health (CCDPH) and the Triple Community (TripCom) Medical Reserve Corps (MRC) executed a full-scale mass prophylaxis exercise in response to a simulated anthrax bioterrorism attack. The exercise took place over 2 days and included the TripCom Point-of-Dispensing (POD) Management Team, volunteers from the TripCom MRC, and neighboring public health departments and MRCs. Individuals from the community volunteered as actors during the exercise, while local municipal, police, and fire personnel coordinated their responses to create the most realistic simulation possible. The exercise was designed to test the capacity of TripCom and CCDPH to implement plans for organizing municipal staff and volunteers to efficiently distribute prophylaxis to the community. Based on results from POD clinic flow, accuracy of prophylaxis distribution, and observations from evaluators, the exercise was successful in demonstrating areas that were operationally efficient as well as identifying areas that can be improved on. These include improvements to the just-in-time training for POD staff, the health screening and consent forms handed out to patients, the physical setup of the POD, and the command structure and communication for the management of POD operations. This article demonstrates the need for full-scale exercises and identifies gaps in POD planning that can be integrated into future plans, exercises, and emergency response.

Animal models for influenza virus pathogenesis, transmission, and immunology

In humans, infection with an influenza A or B virus manifests typically as an acute and self-limited upper respiratory tract illness characterized by fever, cough, sore throat, and malaise. However, influenza can present along a broad spectrum of disease, ranging from sub-clinical or even asymptomatic infection to a severe primary viral pneumonia requiring advanced medical supportive care. Disease severity depends upon the virulence of the influenza virus strain and the immune competence and previous influenza exposures of the patient. Animal models are used in influenza research not only to elucidate the viral and host factors that affect influenza disease outcomes in and spread among susceptible hosts, but also to evaluate interventions designed to prevent or reduce influenza morbidity and mortality in man. This review will focus on the three animal models currently used most frequently in influenza virus research - mice, ferrets, and guinea pigs - and discuss the advantages and disadvantages of each.

Airborne transmission of highly pathogenic H7N1 influenza virus in ferrets

Avian H7 influenza viruses are recognized as potential pandemic viruses, as personnel often become infected during poultry outbreaks. H7 infections in humans typically cause mild conjunctivitis; however, the H7N9 outbreak in the spring of 2013 has resulted in severe respiratory disease. To date, no H7 viruses have acquired the ability for sustained transmission among humans. Airborne transmission is considered a requirement for the emergence of pandemic influenza, and advanced knowledge of the molecular changes or signature required for transmission would allow early identification of pandemic vaccine seed stocks, screening and stockpiling of antiviral compounds, and eradication efforts focused on flocks harboring threatening viruses. Thus, we sought to determine if a highly pathogenic influenza A H7N1 (A/H7N1) virus with no history of human infection could become capable of airborne transmission among ferrets. We show that after 10 serial passages, A/H7N1 developed the ability to be transmitted to cohoused and airborne contact ferrets. Four amino acid mutations (PB2 T81I, NP V284M, and M1 R95K and Q211K) in the internal genes and a minimal amino acid mutation (K/R313R) in the stalk region of the hemagglutinin protein were associated with airborne transmission. Furthermore, transmission was not associated with loss of virulence. These findings highlight the importance of the internal genes in host adaptation and suggest that natural isolates carrying these mutations be further evaluated. Our results demonstrate that a highly pathogenic avian H7 virus can become capable of airborne transmission in a mammalian host, and they support ongoing surveillance and pandemic H7 vaccine development.

IMPORTANCE

The major findings of this report are that a highly pathogenic strain of H7N1 avian influenza virus can be adapted to become capable of airborne transmission in mammals without mutations altering receptor specificity. Changes in receptor specificity have been shown to play a role in the ability of avian influenza viruses to cross the species barrier, and these changes are assumed to be essential. The work reported here challenges this paradigm, at least for the influenza viruses of the H7 subtype, which have recently become the focus of major attention, as they have crossed to humans.

Divergent H7 immunogens offer protection from H7N9 virus challenge

The emergence of avian H7N9 viruses in humans in China has renewed concerns about influenza pandemics emerging from Asia. Vaccines are still the best countermeasure against emerging influenza virus infections, but the process from the identification of vaccine seed strains to the distribution of the final product can take several months. In the case of the 2009 H1N1 pandemic, a vaccine was not available before the first pandemic wave hit and therefore came too late to reduce influenza morbidity. H7 vaccines based on divergent isolates of the Eurasian and North American lineages have been tested in clinical trials, and seed strains and reagents are already available and can potentially be used initially to curtail influenza-induced disease until a more appropriately matched H7N9 vaccine is ready. In a challenge experiment in the mouse model, we assessed the efficacy of both inactivated virus and recombinant hemagglutinin vaccines made from seed strains that are divergent from H7N9 from each of the two major H7 lineages. Furthermore, we analyzed the cross-reactive responses of sera from human subjects vaccinated with heterologous North American and Eurasian lineage H7 vaccines to H7N9. Vaccinations with inactivated virus and recombinant hemagglutinin protein preparations from both lineages raised hemagglutination-inhibiting antibodies against H7N9 viruses and protected mice from stringent viral challenges. Similar cross-reactivity was observed in sera of human subjects from a clinical trial with a divergent H7 vaccine. Existing H7 vaccine candidates based on divergent strains could be used as a first line of defense against an H7N9 pandemic. In addition, this also suggests that H7N9 vaccines that are currently under development might be stockpiled and used for divergent avian H7 strains that emerge in the future.

IMPORTANCE

Sporadic human infections with H7N9 viruses started being reported in China in the early spring of 2013. Despite a significant drop in the number of infections during the summer months of 2013, an increased number of cases has already been reported for the 2013-2014 winter season. The high case fatality rate, the ability to bind to receptors in the human upper respiratory tract in combination with several family clusters, and the emergence of neuraminidase inhibitor-resistant variants that show no loss of pathogenicity and the ability to transmit in animal models have raised concerns about a potential pandemic and have spurred efforts to produce vaccine candidates. Here we show that antigen preparations from divergent H7 strains are able to induce protective immunity against H7N9 infection.

Novel avian-origin influenza A (H7N9) virus attachment to the respiratory tract of five animal models

We determined the pattern of attachment of the avian-origin H7N9 influenza viruses A/Anhui/1/2013 and A/Shanghai/1/2013 to the respiratory tract in ferrets, macaques, mice, pigs, and guinea pigs and compared it to that in humans. The H7N9 attachment pattern in macaques, mice, and to a lesser extent pigs and guinea pigs resembled that in humans more closely than the attachment pattern in ferrets. This information contributes to our knowledge of the different animal models for influenza.

Role of receptor binding specificity in influenza A virus transmission and pathogenesis

The recent emergence of a novel avian A/H7N9 influenza virus in poultry and humans in China, as well as laboratory studies on adaptation and transmission of avian A/H5N1 influenza viruses, has shed new light on influenza virus adaptation to mammals. One of the biological traits required for animal influenza viruses to cross the species barrier that received considerable attention in animal model studies, in vitro assays, and structural analyses is receptor binding specificity. Sialylated glycans present on the apical surface of host cells can function as receptors for the influenza virus hemagglutinin (HA) protein. Avian and human influenza viruses typically have a different sialic acid (SA)-binding preference and only few amino acid changes in the HA protein can cause a switch from avian to human receptor specificity. Recent experiments using glycan arrays, virus histochemistry, animal models, and structural analyses of HA have added a wealth of knowledge on receptor binding specificity. Here, we review recent data on the interaction between influenza virus HA and SA receptors of the host, and the impact on virus host range, pathogenesis, and transmission. Remaining challenges and future research priorities are also discussed.

Rapid strategy for screening by pyrosequencing of influenza virus reassortants--candidates for live attenuated vaccines

BACKGROUND

Live attenuated influenza vaccine viruses (LAIVs) can be generated by classical reassortment of gene segments between a cold adapted, temperature sensitive and attenuated Master Donor Virus (MDV) and a seasonal wild-type (wt) virus. The vaccine candidates contain hemagglutinin (HA) and neuraminidase (NA) genes derived from the circulating wt viruses and the remaining six genes derived from the MDV strains. Rapid, efficient selection of the viruses with 6∶2 genome compositions from the large number of genetically different viruses generated during reassortment is essential for the biannual production schedule of vaccine viruses.

METHODOLOGY/PRINCIPAL FINDINGS

This manuscript describes a new approach for the genotypic analysis of LAIV reassortant virus clones based on pyrosequencing. LAIV candidate viruses were created by classical reassortment of seasonal influenza A (H3N2) (A/Victoria/361/2011, A/Ohio/02/2012, A/Texas/50/2012) or influenza A (H7N9) (A/Anhui/1/2013) wt viruses with the MDV A/Leningrad/134/17/57(H2N2). Using strain-specific pyrosequencing assays, mixed gene variations were detected in the allantoic progenies during the cloning procedure. The pyrosequencing analysis also allowed for estimation of the relative abundance of segment variants in mixed populations. This semi-quantitative approach was used for selecting specific clones for the subsequent cloning procedures.

CONCLUSIONS/SIGNIFICANCE

The present study demonstrates that pyrosequencing analysis is a useful technique for rapid and reliable genotyping of reassortants and intermediate clones during the preparation of LAIV candidates, and can expedite the selection of vaccine virus candidates.

Identifying areas with a high risk of human infection with the avian influenza A (H7N9) virus in East Asia

OBJECTIVES

The rapid emergence, spread, and disease severity of avian influenza A (H7N9) in China has prompted concerns about a possible pandemic and regional spread in the coming months. The objective of this study was to predict the risk of future human infections with H7N9 in China and neighboring countries by assessing the association between H7N9 cases at sentinel hospitals and putative agricultural, climatic, and demographic risk factors.

METHODS

This cross-sectional study used the locations of H7N9 cases and negative cases from China's influenza-like illness surveillance network. After identifying H7N9 risk factors with logistic regression, we used Geographic Information Systems (GIS) to construct predictive maps of H7N9 risk across Asia.

RESULTS

Live bird market density was associated with human H7N9 infections reported in China from March-May 2013. Based on these cases, our model accurately predicted the virus' spread into Guangxi autonomous region in February 2014. Outside China, we find there is a high risk that the virus will spread to northern Vietnam, due to the import of poultry from China.

CONCLUSIONS

Our risk map can focus efforts to improve surveillance in poultry and humans, which may facilitate early identification and treatment of human cases.

The host protease TMPRSS2 plays a major role in in vivo replication of emerging H7N9 and seasonal influenza viruses

Proteolytic cleavage of the hemagglutinin (HA) protein is essential for influenza A virus (IAV) to acquire infectivity. This process is mediated by a host cell protease(s) in vivo. The type II transmembrane serine protease TMPRSS2 is expressed in the respiratory tract and is capable of activating a variety of respiratory viruses, including low-pathogenic (LP) IAVs possessing a single arginine residue at the cleavage site. Here we show that TMPRSS2 plays an essential role in the proteolytic activation of LP IAVs, including a recently emerged H7N9 subtype, in vivo. We generated TMPRSS2 knockout (KO) mice. The TMPRSS2 KO mice showed normal reproduction, development, and growth phenotypes. In TMPRSS2 KO mice infected with LP IAVs, cleavage of HA was severely impaired, and consequently, the majority of LP IAV progeny particles failed to gain infectivity, while the viruses were fully activated proteolytically in TMPRSS2+/+ wild-type (WT) mice. Accordingly, in contrast to WT mice, TMPRSS2 KO mice were highly tolerant of challenge infection by LP IAVs (H1N1, H3N2, and H7N9) with ≥1,000 50% lethal doses (LD50) for WT mice. On the other hand, a high-pathogenic H5N1 subtype IAV possessing a multibasic cleavage site was successfully activated in the lungs of TMPRSS2 KO mice and killed these mice, as observed for WT mice. Our results demonstrate that recently emerged H7N9 as well as seasonal IAVs mainly use the specific protease TMPRSS2 for HA cleavage in vivo and, thus, that TMPRSS2 expression is essential for IAV replication in vivo.

IMPORTANCE

Influenza A virus (IAV) is a leading pathogen that infects and kills many humans every year. We clarified that the infectivity and pathogenicity of IAVs, including a recently emerged H7N9 subtype, are determined primarily by a host protease, TMPRSS2. Our data showed that TMPRSS2 is the key host protease that activates IAVs in vivo through proteolytic cleavage of their HA proteins. Hence, TMPRSS2 is a good target for the development of anti-IAV drugs. Such drugs could also be effective for many other respiratory viruses, including the recently emerged Middle East respiratory syndrome (MERS) coronavirus, because they are also activated by TMPRSS2 in vitro. Consequently, the present paper could have a large impact on the battle against respiratory virus infections and contribute greatly to human health.

Comprehensive sequence analysis of HA protein of H7 subtype avian influenza viruses: an emphasis on mutations in novel H7N9 viruses

ABSTRACT: 

Aim: H7 avian influenza viruses pose a major public health threat raising concerns regarding their pandemic potential, especially after the recent outbreak in China of H7N9 subtype viruses. The objective was to gain insight into the geographical and host-wise distribution of H7 subtype viruses, and to understand molecular determinants responsible for their adaptation in humans. Materials & methods: In this study we carried out a global comprehensive analysis of 1749 HA sequences belonging to the H7 subtype available in the Global Initiative on Sharing All Influenza Data (GISAID) EpiFlu™ database. We also analyzed full-genome sequences of the 27 influenza strains belonging to the H7N9 subtype isolated recently from China. Results: Most of the H7 strains were from North America (749) followed by in Europe (659) and Asia (284). The majority of the sequences belonged to the H7N7 subtype (524) followed by H7N3 (440) and H7N2 (411), while 107 belonged to H7N9. Comparison of HA sequences of H7 viruses isolated from humans showed the presence of mutations and determinants that could have played a pivotal role in avian-to-human transmission and adaptability in humans. Mutational analysis of all the segments of the recent H7N9 viruses isolated from humans in China revealed that these viruses possessed several characteristic features of mammalian influenza viruses. Conclusion: H7 viruses, irrespective of being of high or low pathogenicity have a propensity to adapt to humans causing mild to severe infections. These viruses have signature mutations that have been associated with interspecies transmission and human adaptability, raising concerns regarding their pandemic potential.

Virulence-affecting amino acid changes in the PA protein of H7N9 influenza A viruses

Novel avian-origin influenza A(H7N9) viruses were first reported to infect humans in March 2013. To date, 143 human cases, including 45 deaths, have been recorded. By using sequence comparisons and phylogenetic and ancestral inference analyses, we identified several distinct amino acids in the A(H7N9) polymerase PA protein, some of which may be mammalian adapting. Mutant viruses possessing some of these amino acid changes, singly or in combination, were assessed for their polymerase activities and growth kinetics in mammalian and avian cells and for their virulence in mice. We identified several mutants that were slightly more virulent in mice than the wild-type A(H7N9) virus, A/Anhui/1/2013. These mutants also exhibited increased polymerase activity in human cells but not in avian cells. Our findings indicate that the PA protein of A(H7N9) viruses has several amino acid substitutions that are attenuating in mammals.

IMPORTANCE

Novel avian-origin influenza A(H7N9) viruses emerged in the spring of 2013. By using computational analyses of A(H7N9) viral sequences, we identified several amino acid changes in the polymerase PA protein, which we then assessed for their effects on viral replication in cultured cells and mice. We found that the PA proteins of A(H7N9) viruses possess several amino acid substitutions that cause attenuation in mammals.

Characterization of the sialic acid binding activity of influenza A viruses using soluble variants of the H7 and H9 hemagglutinins

Binding of influenza viruses to target cells is mediated by the viral surface protein hemagglutinin. To determine the presence of binding sites for influenza A viruses on cells and tissues, soluble hemagglutinins of the H7 and H9 subtype were generated by connecting the hemagglutinin ectodomain to the Fc portion of human immunoglobulin G (H7Fc and H9Fc). Both chimeric proteins bound to different cells and tissues in a sialic acid-dependent manner. Pronounced differences were observed between H7Fc and H9Fc, in the binding both to different mammalian and avian cultured cells and to cryosections of the respiratory epithelium of different virus host species (turkey, chicken and pig). Binding of the soluble hemagglutinins was similar to the binding of virus particles, but showed differences in the binding pattern when compared to two sialic acid-specific plant lectins. These findings were substantiated by a comparative glycan array analysis revealing a very narrow recognition of sialoglycoconjugates by the plant lectins that does not reflect the glycan structures preferentially recognized by H7Fc and H9Fc. Thus, soluble hemagglutinins may serve as sialic acid-specific lectins and are a more reliable indicator of the presence of binding sites for influenza virus HA than the commonly used plant lectins.

Influenza reverse genetics: dissecting immunity and pathogenesis

Reverse genetics systems allow artificial generation of non-segmented and segmented negative-sense RNA viruses, like influenza viruses, entirely from cloned cDNA. Since the introduction of reverse genetics systems over a decade ago, the ability to generate ‘designer’ influenza viruses in the laboratory has advanced both basic and applied research, providing a powerful tool to investigate and characterise host–pathogen interactions and advance the development of novel therapeutic strategies. The list of applications for reverse genetics has expanded vastly in recent years. In this review, we discuss the development and implications of this technique, including the recent controversy surrounding the generation of a transmissible H5N1 influenza virus. We will focus on research involving the identification of viral protein function, development of live-attenuated influenza virus vaccines, host–pathogen interactions, immunity and the generation of recombinant influenza virus vaccine vectors for the prevention and treatment of infectious diseases and cancer.

Identification of amino acid changes that may have been critical for the genesis of A(H7N9) influenza viruses

Novel influenza A viruses of the H7N9 subtype [A(H7N9)] emerged in the spring of 2013 in China and had infected 163 people as of 10 January 2014; 50 of them died of the severe respiratory infection caused by these viruses. Phylogenetic studies have indicated that the novel A(H7N9) viruses emerged from reassortment of H7, N9, and H9N2 viruses. Inspections of protein sequences from A(H7N9) viruses and their immediate predecessors revealed several amino acid changes in A(H7N9) viruses that may have facilitated transmission and replication in the novel host. Since mutations that occurred more ancestrally may also have contributed to the genesis of A(H7N9) viruses, we inferred historical evolutionary events leading to the novel viruses. We identified a number of amino acid changes on the evolutionary path to A(H7N9) viruses, including substitutions that may be associated with host range, replicative ability, and/or host responses to infection. The biological significance of these amino acid changes can be tested in future studies.

IMPORTANCE

The novel influenza A viruses of the H7N9 subtype [A(H7N9)], which first emerged in the spring of 2013, cause severe respiratory infections in humans. Here, we performed a comprehensive evolutionary analysis of the progenitors of A(H7N9) viruses to identify amino acid changes that may have been critical for the emergence of A(H7N9) viruses and their ability to infect humans. We provide a list of potentially important amino acid changes that can be tested for their significance for the influenza virus host range, replicative ability, and/or host responses to infection.

Host adaptation and transmission of influenza A viruses in mammals

A wide range of influenza A viruses of pigs and birds have infected humans in the last decade, sometimes with severe clinical consequences. Each of these so-called zoonotic infections provides an opportunity for virus adaptation to the new host. Fortunately, most of these human infections do not yield viruses with the ability of sustained human-to-human transmission. However, animal influenza viruses have acquired the ability of sustained transmission between humans to cause pandemics on rare occasions in the past, and therefore, influenza virus zoonoses continue to represent threats to public health. Numerous recent studies have shed new light on the mechanisms of adaptation and transmission of avian and swine influenza A viruses in mammals. In particular, several studies provided insights into the genetic and phenotypic traits of influenza A viruses that may determine airborne transmission. Here, we summarize recent studies on molecular determinants of virulence and adaptation of animal influenza A virus and discuss the phenotypic traits associated with airborne transmission of newly emerging influenza A viruses. Increased understanding of the determinants and mechanisms of virulence and transmission may aid in assessing the risks posed by animal influenza viruses to human health, and preparedness for such risks.

TMPRSS2 is a host factor that is essential for pneumotropism and pathogenicity of H7N9 influenza A virus in mice

Cleavage of the hemagglutinin (HA) by host proteases is essential for the infectivity of influenza viruses. Here, we analyzed the role of the serine protease TMPRSS2, which activates HA in the human respiratory tract, in pathogenesis in a mouse model. Replication of the human H7N9 isolate A/Anhui/1/13 and of human H1N1 and H3N2 viruses was compared in TMPRSS2 knockout (TMPRSS2(-/-)) and wild-type (WT) mice. Knockout of TMPRSS2 expression inhibited H7N9 influenza virus replication in explants of murine tracheas, bronchi, and lungs. H1N1 virus replication was also strongly suppressed in airway explants of TMPRSS2(-/-) mice, while H3N2 virus replication was only marginally affected. H7N9 and H1N1 viruses were apathogenic in TMPRSS2(-/-) mice, whereas WT mice developed severe disease with mortality rates of 100% and 20%, respectively. In contrast, all H3N2 infected TMPRSS2(-/-) and WT mice succumbed to lethal infection. Cleavage analysis showed that H7 and H1 are efficiently activated by TMPRSS2, whereas H3 is less susceptible to the protease. Our data demonstrate that TMPRSS2 is a host factor that is essential for pneumotropism and pathogenicity of H7N9 and H1N1 influenza virus in mice. In contrast, replication of H3N2 virus appears to depend on another, not yet identified protease, supporting the concept that human influenza viruses differ in protease specificity.

IMPORTANCE

Cleavage of the hemagglutinin (HA) by host proteases is essential for the infectivity of influenza virus, but little is known about its relevance for pathogenesis in mammals. Here, we show that knockout mice that do not express the HA-activating protease TMPRSS2 are resistant to pulmonary disease with lethal outcome when infected with influenza A viruses of subtypes H7N9 and H1N1, whereas they are not protected from lethal H3N2 virus infection. These findings demonstrate that human influenza viruses differ in protease specificity, and that expression of the appropriate protease in respiratory tissues is essential for pneumotropism and pathogenicity. Our observations also demonstrate that HA-activating proteases and in particular TMPRSS2 are promising targets for influenza therapy.

Epidemiology of human infections with avian influenza A(H7N9) virus in China

BACKGROUND

The first identified cases of avian influenza A(H7N9) virus infection in humans occurred in China during February and March 2013. We analyzed data obtained from field investigations to describe the epidemiologic characteristics of H7N9 cases in China identified as of December 1, 2013.

METHODS

Field investigations were conducted for each confirmed case of H7N9 virus infection. A patient was considered to have a confirmed case if the presence of the H7N9 virus was verified by means of real-time reverse-transcriptase-polymerase-chain-reaction assay (RT-PCR), viral isolation, or serologic testing. Information on demographic characteristics, exposure history, and illness timelines was obtained from patients with confirmed cases. Close contacts were monitored for 7 days for symptoms of illness. Throat swabs were obtained from contacts in whom symptoms developed and were tested for the presence of the H7N9 virus by means of real-time RT-PCR.

RESULTS

Among 139 persons with confirmed H7N9 virus infection, the median age was 61 years (range, 2 to 91), 71% were male, and 73% were urban residents. Confirmed cases occurred in 12 areas of China. Nine persons were poultry workers, and of 131 persons with available data, 82% had a history of exposure to live animals, including chickens (82%). A total of 137 persons (99%) were hospitalized, 125 (90%) had pneumonia or respiratory failure, and 65 of 103 with available data (63%) were admitted to an intensive care unit. A total of 47 persons (34%) died in the hospital after a median duration of illness of 21 days, 88 were discharged from the hospital, and 2 remain hospitalized in critical condition; 2 patients were not admitted to a hospital. In four family clusters, human-to-human transmission of H7N9 virus could not be ruled out. Excluding secondary cases in clusters, 2675 close contacts of case patients completed the monitoring period; respiratory symptoms developed in 28 of them (1%); all tested negative for H7N9 virus.

CONCLUSIONS

Most persons with confirmed H7N9 virus infection had severe lower respiratory tract illness, were epidemiologically unrelated, and had a history of recent exposure to poultry. However, limited, nonsustained human-to-human H7N9 virus transmission could not be ruled out in four families.

H6 influenza viruses pose a potential threat to human health

Influenza viruses of the H6 subtype have been isolated from wild and domestic aquatic and terrestrial avian species throughout the world since their first detection in a turkey in Massachusetts in 1965. Since 1997, H6 viruses with different neuraminidase (NA) subtypes have been detected frequently in the live poultry markets of southern China. Although sequence information has been gathered over the last few years, the H6 viruses have not been fully biologically characterized. To investigate the potential risk posed by H6 viruses to humans, here we assessed the receptor-binding preference, replication, and transmissibility in mammals of a series of H6 viruses isolated from live poultry markets in southern China from 2008 to 2011. Among the 257 H6 strains tested, 87 viruses recognized the human type receptor. Genome sequence analysis of 38 representative H6 viruses revealed 30 different genotypes, indicating that these viruses are actively circulating and reassorting in nature. Thirty-seven of 38 viruses tested in mice replicated efficiently in the lungs and some caused mild disease; none, however, were lethal. We also tested the direct contact transmission of 10 H6 viruses in guinea pigs and found that 5 viruses did not transmit to the contact animals, 3 viruses transmitted to one of the three contact animals, and 2 viruses transmitted to all three contact animals. Our study demonstrates that the H6 avian influenza viruses pose a clear threat to human health and emphasizes the need for continued surveillance and evaluation of the H6 influenza viruses circulating in nature.

IMPORTANCE

Avian influenza viruses continue to present a challenge to human health. Research and pandemic preparedness have largely focused on the H5 and H7 subtype influenza viruses in recent years. Influenza viruses of the H6 subtype have been isolated from wild and domestic aquatic and terrestrial avian species throughout the world since their first detection in the United States in 1965. Since 1997, H6 viruses have been detected frequently in the live poultry markets of southern China; however, the biological characterization of these viruses is very limited. Here, we assessed the receptor-binding preference, replication, and transmissibility in mammals of a series of H6 viruses isolated from live poultry markets in southern China and found that 34% of the viruses are able to bind human type receptors and that some of them are able to transmit efficiently to contact animals. Our study demonstrates that the H6 viruses pose a clear threat to human health.

Transcriptomic characterization of the novel avian-origin influenza A (H7N9) virus: specific host response and responses intermediate between avian (H5N1 and H7N7) and human (H3N2) viruses and implications for treatment options

A novel avian-origin H7N9 influenza A virus (IAV) emerged in China in 2013, causing mild to lethal human respiratory infections. H7N9 originated with multiple reassortment events between avian viruses and carries genetic markers of human adaptation. Determining whether H7N9 induces a host response closer to that with human or avian IAV is important in order to better characterize this emerging virus. Here we compared the human lung epithelial cell response to infection with A/Anhui/01/13 (H7N9) or highly pathogenic avian-origin H5N1, H7N7, or human seasonal H3N2 IAV. The transcriptomic response to H7N9 was highly specific to this strain but was more similar to the response to human H3N2 than to that to other avian IAVs. H7N9 and H3N2 both elicited responses related to eicosanoid signaling and chromatin modification, whereas H7N9 specifically induced genes regulating the cell cycle and transcription. Among avian IAVs, the response to H7N9 was closest to that elicited by H5N1 virus. Host responses common to H7N9 and the other avian viruses included the lack of induction of the antigen presentation pathway and reduced proinflammatory cytokine induction compared to that with H3N2. Repression of these responses could have an important impact on the immunogenicity and virulence of H7N9 in humans. Finally, using a genome-based drug repurposing approach, we identified several drugs predicted to regulate the host response to H7N9 that may act as potential antivirals, including several kinase inhibitors, as well as FDA-approved drugs, such as troglitazone and minocycline. Importantly, we validated that minocycline inhibited H7N9 replication in vitro, suggesting that our computational approach holds promise for identifying novel antivirals.

IMPORTANCE

Whether H7N9 will be the next pandemic influenza virus or will persist and sporadically infect humans from its avian reservoir, similar to H5N1, is not known yet. High-throughput profiling of the host response to infection allows rapid characterization of virus-host interactions and generates many hypotheses that will accelerate understanding and responsiveness to this potential threat. We show that the cellular response to H7N9 virus is closer to that induced by H3N2 than to that induced by H5N1, reflecting the potential of this new virus for adaptation to humans. Importantly, dissecting the host response to H7N9 may guide host-directed antiviral development.

Human cytotoxic T lymphocytes directed to seasonal influenza A viruses cross-react with the newly emerging H7N9 virus

In February 2013, zoonotic transmission of a novel influenza A virus of the H7N9 subtype was reported in China. Although at present no sustained human-to-human transmission has been reported, a pandemic outbreak of this H7N9 virus is feared. Since neutralizing antibodies to the hemagglutinin (HA) globular head domain of the virus are virtually absent in the human population, there is interest in identifying other correlates of protection, such as cross-reactive CD8(+) T cells (cytotoxic T lymphocytes [CTLs]) elicited during seasonal influenza A virus infections. These virus-specific CD8(+) T cells are known to recognize conserved internal proteins of influenza A viruses predominantly, but it is unknown to what extent they cross-react with the newly emerging H7N9 virus. Here, we assessed the cross-reactivity of seasonal H3N2 and H1N1 and pandemic H1N1 influenza A virus-specific polyclonal CD8(+) T cells, obtained from HLA-typed study subjects, with the novel H7N9 virus. The cross-reactivity of CD8(+) T cells to H7N9 variants of known influenza A virus epitopes and H7N9 virus-infected cells was determined by their gamma interferon (IFN-γ) response and lytic activity. It was concluded that, apart from recognition of individual H7N9 variant epitopes, CD8(+) T cells to seasonal influenza viruses display considerable cross-reactivity with the novel H7N9 virus. The presence of these cross-reactive CD8(+) T cells may afford some protection against infection with the new virus.

Novel H7N9 influenza virus shows low infectious dose, high growth rate, and efficient contact transmission in the guinea pig model

The zoonotic outbreak of H7N9 subtype avian influenza virus that occurred in eastern China in the spring of 2013 resulted in 135 confirmed human cases, 44 of which were lethal. Sequencing of the viral genome revealed a number of molecular signatures associated with virulence or transmission in mammals. We report here that, in the guinea pig model, a human isolate of novel H7N9 influenza virus, A/Anhui/1/2013 (An/13), is highly dissimilar to an H7N1 avian isolate and instead behaves similarly to a human seasonal strain in several respects. An/13 was found to have a low 50% infectious dose, grow to high titers in the upper respiratory tract, and transmit efficiently among cocaged guinea pigs. The pH of fusion of the hemagglutinin (HA) and the binding of virus to fixed guinea pig tissues were also examined. The An/13 HA displayed a relatively elevated pH of fusion characteristic of many avian strains, and An/13 resembled avian viruses in terms of attachment to tissues. One important difference was seen between An/13 and both the H3N2 human and the H7N1 avian viruses: when inoculated intranasally at a high dose, only the An/13 virus led to productive infection of the lower respiratory tract of guinea pigs. In sum, An/13 was found to retain fusion and attachment properties of an avian influenza virus but displayed robust growth and contact transmission in the guinea pig model atypical of avian strains and indicative of mammalian adaptation.

Comparison of patients hospitalized with influenza A subtypes H7N9, H5N1, and 2009 pandemic H1N1

Hospitalization with H7N9 virus infection is associated with older age and chronic heart disease, and patients have a longer duration of hospitalization than patients with H5N1 or pH1N1. This suggests that host factors are an important contributor to H7N9 severity.

Background. Influenza A(H7N9) viruses isolated from humans show features suggesting partial adaptation to mammals. To provide insights into the pathogenesis of H7N9 virus infection, we compared risk factors, clinical presentation, and progression of patients hospitalized with H7N9, H5N1, and 2009 pandemic H1N1 (pH1N1) virus infections.

Methods. We compared individual-level data from patients hospitalized with infection by H7N9 (n = 123), H5N1 (n = 119; 43 China, 76 Vietnam), and pH1N1 (n = 3486) viruses. We assessed risk factors for hospitalization after adjustment for age- and sex-specific prevalence of risk factors in the general Chinese population.

Results. The median age of patients with H7N9 virus infection was older than other patient groups (63 years; P < .001) and a higher proportion was male (71%; P < .02). After adjustment for age and sex, chronic heart disease was associated with an increased risk of hospitalization with H7N9 (relative risk, 9.68; 95% confidence interval, 5.24–17.9). H7N9 patients had similar patterns of leukopenia, thrombocytopenia, and elevated alanine aminotransferase, creatinine kinase, C-reactive protein, and lactate dehydrogenase to those seen in H5N1 patients, which were all significantly different from pH1N1 patients (P < .005). H7N9 patients had a longer duration of hospitalization than either H5N1 or pH1N1 patients (P < .001), and the median time from onset to death was 18 days for H7N9 (P = .002) vs 11 days for H5N1 and 15 days for pH1N1 (P = .154).

Conclusions. The identification of known risk factors for severe seasonal influenza and the more protracted clinical course compared with that of H5N1 suggests that host factors are an important contributor to H7N9 severity.