Evolution of the receptor binding phenotype of influenza A (H5) viruses

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.

Enhanced human receptor binding by H5 haemagglutinins

Mutant H5N1 influenza viruses have been isolated from humans that have increased human receptor avidity. We have compared the receptor binding properties of these mutants with those of wild-type viruses, and determined the structures of their haemagglutinins in complex with receptor analogues. Mutants from Vietnam bind tighter to human receptor by acquiring basic residues near the receptor binding site. They bind more weakly to avian receptor because they lack specific interactions between Asn-186 and Gln-226. In contrast, a double mutant, Δ133/Ile155Thr, isolated in Egypt has greater avidity for human receptor while retaining wild-type avidity for avian receptor. Despite these increases in human receptor binding, none of the mutants prefers human receptor, unlike aerosol transmissible H5N1 viruses. Nevertheless, mutants with high avidity for both human and avian receptors may be intermediates in the evolution of H5N1 viruses that could infect both humans and poultry.

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H5N1 influenza virus binding.

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Haemagglutinin receptor specificity using biolayer interferometry.

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Haemagglutinin receptor complex crystal structure determination.

Receptor binding properties of the influenza virus hemagglutinin as a determinant of host range

Host cell attachment by influenza A viruses is mediated by the hemagglutinin glycoprotein (HA), and the recognition of specific types of sialic acid -containing glycan receptors constitutes one of the major determinants of viral host range and transmission properties. Structural studies have elucidated some of the viral determinants involved in receptor recognition of avian-like and human-like receptors for various subtypes of influenza A viruses, and these provide clues relating to the mechanisms by which viruses evolve to adapt to human hosts. We discuss structural aspects of receptor binding by influenza HA, as well as the biological implications of functional interplay involving HA binding, NA sialidase functions, the effects of antigenic drift, and the inhibitory properties of natural glycans present on mucosal surfaces.

Receptor binding and transmission studies of H5N1 influenza virus in mammals

The H5N1 influenza A virus that is currently circulating in Asia, Africa and Europe has resulted in persistent outbreaks in poultry with sporadic transmission to humans. Thus far, it is believed that H5N1 does not possess sufficient ability for human-to-human transmission and subsequent pandemic infection. Both receptor binding specificity and virus infectivity are key factors in determining whether influenza A virus becomes pandemic. The use of human viral isolates in various studies has helped to illustrate the changes in receptor binding specificity and virulence as a result of adaptation in humans. In this review, we highlight the important amino acids and domains of viral proteins related to receptor binding specificity that have been reported for humans and avians using mammalian models. Thus, this review will consolidate findings from studies that have shed light on the receptor binding and transmission characteristics of the H5N1 influenza virus, with the goal of improving our ability to predict the transmission efficiency or pandemic potential of new viral strains.

H5N1 receptor specificity as a factor in pandemic risk

The high pathogenicity of H5N1 viruses in sporadic infections of humans has raised concerns for its potential to acquire the ability to transmit between humans and emerge as a highly pathogenic pandemic virus. Because avian and human influenza viruses differ in their specificity for recognition of their host cell receptors, receptor specificity represents one barrier for efficient transmission of avian viruses in human hosts. Over the last century, each influenza virus pandemic has coincided with the emergence of virus with an immunologically distinct hemagglutinin exhibiting a 'human-type' receptor specificity, distinct from that of viruses with the same hemagglutinin circulating in zoonotic species. Recent studies suggest that it is possible for H5N1 to acquire human type receptor specificity, but this has not occurred in nature. This review covers what is known about the molecular basis for the switch between avian and human-type receptor specificity for influenza viruses that have successfully adapted to man, the potential for H5N1 to evolve to human-type receptor specificity and its relevance to pandemic risk.

Receptor binding and pH stability - how influenza A virus hemagglutinin affects host-specific virus infection

Influenza A virus strains adopt different host specificities mainly depending on their hemagglutinin (HA) protein. Via HA, the virus binds sialic acid receptors of the host cell and, upon endocytic uptake, HA triggers fusion between the viral envelope bilayer and the endosomal membrane by a low pH-induced conformational change leading to the release of the viral genome into the host cell cytoplasm. Both functions are crucial for viral infection enabling the genesis of new progeny virus. Adaptation to different hosts in vitro was shown to require mutations within HA altering the receptor binding and/or fusion behavior of the respective virus strain. Human adapted influenza virus strains (H1N1, H3N2, H2N2) as well as recent avian influenza virus strains (H5, H7 and H9 subtypes) which gained the ability to infect humans mostly contained mutations in the receptor binding site (RBS) of HA enabling increased binding affinity of these viruses to human type (α-2,6 linked sialic acid) receptors. Thus, the receptor binding specificity seems to be the major requirement for successful adaptation to the human host; however, the RBS is not the only determinant of host specificity. Increased binding to a certain cell type does not always correlate with infection efficiency. Furthermore, viruses carrying mutations in the RBS often resulted in reduced viral fitness and were still unable to transmit between mammals. Recently, the pH stability of HA was reported to affect the transmissibility of influenza viruses. This review summarizes recent findings on the adaptation of influenza A viruses to the human host and related amino acid substitutions resulting in altered receptor binding specificity and/or modulated fusion pH of HA. Furthermore, the role of these properties (receptor specificity and pH stability of HA) for adaptation to and transmissibility in the human host is discussed. This article is part of a Special Issue entitled: Viral Membrane Proteins -- Channels for Cellular Networking.

Characteristics of human infection with avian influenza viruses and development of new antiviral agents

Since 1997, several epizootic avian influenza viruses (AIVs) have been transmitted to humans, causing diseases and even deaths. The recent emergence of severe human infections with AIV (H7N9) in China has raised concerns about efficient interpersonal viral transmission, polygenic traits in viral pathogenicity and the management of newly emerging strains. The symptoms associated with viral infection are different in various AI strains: H5N1 and newly emerged H7N9 induce severe pneumonia and related complications in patients, while some H7 and H9 subtypes cause only conjunctivitis or mild respiratory symptoms. The virulence and tissue tropism of viruses as well as the host responses contribute to the pathogenesis of human AIV infection. Several preventive and therapeutic approaches have been proposed to combat AIV infection, including antiviral drugs such as M2 inhibitors, neuraminidase inhibitors, RNA polymerase inhibitors, attachment inhibitors and signal-transduction inhibitors etc. In this article, we summarize the recent progress in researches on the epidemiology, clinical features, pathogenicity determinants, and available or potential antivirals of AIV.

Novel avian-origin influenza A (H7N9) virus attaches to epithelium in both upper and lower respiratory tract of humans

Influenza A viruses from animal reservoirs have the capacity to adapt to humans and cause influenza pandemics. The occurrence of an influenza pandemic requires efficient virus transmission among humans, which is associated with virus attachment to the upper respiratory tract. Pandemic severity depends on virus ability to cause pneumonia, which is associated with virus attachment to the lower respiratory tract. Recently, a novel avian-origin H7N9 influenza A virus with unknown pandemic potential emerged in humans. We determined the pattern of attachment of two genetically engineered viruses containing the hemagglutinin of either influenza virus A/Shanghai/1/13 or A/Anhui/1/13 to formalin-fixed human respiratory tract tissues using histochemical analysis. Our results show that the emerging H7N9 virus attached moderately or abundantly to both upper and lower respiratory tract, a pattern not seen before for avian influenza A viruses. With the caveat that virus attachment is only the first step in the virus replication cycle, these results suggest that the emerging H7N9 virus has the potential both to transmit efficiently among humans and to cause severe pneumonia.

Antigenicity and transmissibility of a novel clade 2.3.2.1 avian influenza H5N1 virus

A genetic variant of the H5N1 influenza virus, termed subclade 2.3.2.1, was first identified in Bulgaria in 2010 and has subsequently been found in Vietnam and Laos. Several cases of human infections with this virus have been identified. Thus, it is important to understand the antigenic properties and transmissibility of this variant. Our results showed that, although it is phylogenetically closely related to other previously characterized clade 2.3 viruses, this novel 2.3.2.1 variant exhibited distinct antigenic properties and showed little cross-reactivity to sera raised against other H5N1 viruses. Like other H5N1 viruses, this variant bound preferentially to avian-type receptors, but contained substitutions at positions 190 and 158 of the haemagglutinin (HA) protein that have been postulated to facilitate HA binding to human-type receptors and to enhance viral transmissibility among mammals, respectively. However, this virus did not appear to have acquired the capacity for airborne transmission between ferrets. These findings highlight the challenges in selecting vaccine candidates for H5N1 influenza because these viruses continue to evolve rapidly in the field. It is important to note that some variants have obtained mutations that may gain transmissibility between model animals, and close surveillance of H5N1 viruses in poultry is warranted.

Sialic Acid Receptors of Viruses

Sialic acid linked to glycoproteins and gangliosides is used by many viruses as a receptor for cell entry. These viruses include important human and animal pathogens, such as influenza, parainfluenza, mumps, corona, noro, rota, and DNA tumor viruses. Attachment to sialic acid is mediated by receptor binding proteins that are constituents of viral envelopes or exposed at the surface of non-enveloped viruses. Some of these viruses are also equipped with a neuraminidase or a sialyl-O-acetyl-esterase. These receptor-destroying enzymes promote virus release from infected cells and neutralize sialic acid-containing soluble proteins interfering with cell surface binding of the virus. Variations in the receptor specificity are important determinants for host range, tissue tropism, pathogenicity, and transmissibility of these viruses.

The short stalk length of highly pathogenic avian influenza H5N1 virus neuraminidase limits transmission of pandemic H1N1 virus in ferrets

H5N1 influenza viruses pose a pandemic threat but have not acquired the ability to support sustained transmission between mammals in nature. The restrictions to transmissibility of avian influenza viruses in mammals are multigenic, and overcoming them requires adaptations in hemagglutinin (HA) and PB2 genes. Here we propose that a further restriction to mammalian transmission of the majority of highly pathogenic avian influenza (HPAI) H5N1 viruses may be the short stalk length of the neuraminidase (NA) protein. This genetic feature is selected for when influenza viruses adapt to chickens. In our study, a recombinant virus with seven gene segments from a human isolate of the 2009 H1N1 pandemic combined with the NA gene from a typical chicken-adapted H5N1 virus with a short stalk did not support transmission by respiratory droplet between ferrets. This virus was also compromised in multicycle replication in cultures of human airway epithelial cells at 32°C. These defects correlated with a reduction in the ability of virus with a short-stalk NA to penetrate mucus and deaggregate virions. The deficiency in transmission and in cleavage of tethered substrates was overcome by increasing the stalk length of the NA protein. These observations suggest that H5N1 viruses that acquire a long-stalk NA through reassortment might be more likely to support transmission between humans. Phylogenetic analysis showed that reassortment with long-stalk NA occurred sporadically and as recently as 2011. However, all identified H5N1 viruses with a long-stalk NA lacked other mammalian adapting features and were thus several genetic steps away from becoming transmissible between humans.

Viral and host factors required for avian H5N1 influenza A virus replication in mammalian cells

Following the initial and sporadic emergence into humans of highly pathogenic avian H5N1 influenza A viruses in Hong Kong in 1997, we have come to realize the potential for avian influenza A viruses to be transmitted directly from birds to humans. Understanding the basic viral and cellular mechanisms that contribute to infection of mammalian species with avian influenza viruses is essential for developing prevention and control measures against possible future human pandemics. Multiple physical and functional cellular barriers can restrict influenza A virus infection in a new host species, including the cell membrane, the nuclear envelope, the nuclear environment, and innate antiviral responses. In this review, we summarize current knowledge on viral and host factors required for avian H5N1 influenza A viruses to successfully establish infections in mammalian cells. We focus on the molecular mechanisms underpinning mammalian host restrictions, as well as the adaptive mutations that are necessary for an avian influenza virus to overcome them. It is likely that many more viral and host determinants remain to be discovered, and future research in this area should provide novel and translational insights into the biology of influenza virus-host interactions.

Intranasal immunization of baculovirus displayed hemagglutinin confers complete protection against mouse adapted highly pathogenic H7N7 reassortant influenza virus

Background

Avian influenza A H7N7 virus poses a pandemic threat to human health because of its ability for direct transmission from domestic poultry to humans and from human to human. The wide zoonotic potential of H7N7 combined with an antiviral immunity inhibition similar to pandemic 1918 H1N1 and 2009 H1N1 influenza viruses is disconcerting and increases the risk of a putative H7N7 pandemic in the future, underlining the urgent need for vaccine development against this virus.

Methodology/Principal Findings

In this study, we developed a recombinant vaccine by expressing the H7N7-HA protein on the surface of baculovirus (Bac-HA). The protective efficacy of the live Bac-HA vaccine construct was evaluated in a mouse model by challenging mice immunized intranasally (i.n.) or subcutaneously (s.c.) with high pathogenic mouse adapted H7N7 reassorted strain. Although s.c. injection of live Bac-HA induced higher specific IgG than i.n. immunization, the later resulted in an elevated neutralization titer. Interestingly, 100% protection from the lethal viral challenge was only observed for the mice immunized intranasally with live Bac-HA, whereas no protection was achieved in any other s.c. or i.n. immunized mice groups. In addition, we also observed higher mucosal IgA as well as increased IFN-γ and IL-4 responses in the splenocytes of the surviving mice coupled with a reduced viral titer and diminished histopathological signs in the lungs.

Conclusion

Our results indicated that protection from high pathogenic H7N7 (NL/219/03) virus requires both mucosal and systemic immune responses in mice. The balance between Th1 and Th2 cytokines is also required for the protection against the H7N7 pathogen. Intranasal administration of live Bac-HA induced all these immune responses and protected the mice from lethal viral challenge. Therefore, live Bac-HA is an effective vaccine candidate against H7N7 viral infections.

Structural determinants for naturally evolving H5N1 hemagglutinin to switch its receptor specificity

Of the factors governing human-to-human transmission of the highly pathogenic avian-adapted H5N1 virus, the most critical is the acquisition of mutations on the viral hemagglutinin (HA) to "quantitatively switch" its binding from avian to human glycan receptors. Here, we describe a structural framework that outlines a necessary set of H5 HA receptor-binding site (RBS) features required for the H5 HA to quantitatively switch its preference to human receptors. We show here that the same RBS HA mutations that lead to aerosol transmission of A/Vietnam/1203/04 and A/Indonesia/5/05 viruses, when introduced in currently circulating H5N1, do not lead to a quantitative switch in receptor preference. We demonstrate that HAs from circulating clades require as few as a single base pair mutation to quantitatively switch their binding to human receptors. The mutations identified by this study can be used to monitor the emergence of strains having human-to-human transmission potential.

Mutations in haemagglutinin that affect receptor binding and pH stability increase replication of a PR8 influenza virus with H5 HA in the upper respiratory tract of ferrets and may contribute to transmissibility

The H5N1 influenza A viruses have circulated widely in the avian population for 10 years with only sporadic infection of humans observed and no sustained human to human transmission. Vaccination against potential pandemic strains is one strategy in planning for future influenza pandemics; however, the success of live attenuated vaccines for H5N1 has been limited, due to poor replication in the human upper respiratory tract (URT). Mutations that increase the ability of H5N1 viruses to replicate in the URT will aid immunogenicity of these vaccines and provide information about humanizing adaptations in H5N1 strains that may signal transmissibility. As well as mediating receptor interactions, the haemagglutinin (HA) protein of influenza facilitates fusion of the viral membrane and genome entry into the host cell; this process is pH dependent. We have shown in this study that the pH at which a panel of avian influenza HA proteins, including H5, mediate fusion is higher than that for human influenza HA proteins, and that mutations in the H5 HA can reduce the pH of fusion. Coupled with receptor switching mutations, increasing the pH stability of the H5 HA resulted in increased viral shedding of H5N1 from the nasal cavity of ferrets and contact transmission to a co-housed animal. Ferret serum antibodies induced by infection with any of the mutated H5 HA viruses neutralized HA pseudotyped lentiviruses bearing homologous or heterologous H5 HAs, suggesting that this strategy to increase nasal replication of a vaccine virus would not compromise vaccine efficacy.

Highly pathogenic H5N1 influenza A virus strains provoke heterogeneous IFN-α/β responses that distinctively affect viral propagation in human cells

The fatal transmissions of highly pathogenic avian influenza A viruses (IAV) of the H5N1 subtype to humans and high titer replication in the respiratory tract indicate that these pathogens can overcome the bird-to-human species barrier. While type I interferons (IFN-α/β) are well described to contribute to the species barrier of many zoonotic viruses, current data to the role of these antiviral cytokines during human H5N1 IAV infections is limited and contradictory. We hypothesized an important role for the IFN system in limiting productive infection of avian H5N1 strains in human cells. Hence, we examined IFN-α/β gene activation by different avian and human H5N1 isolates, if the IFN-α/β response restricts H5N1 growth and whether the different strains were equally capable to regulate the IFN-α/β system via their IFN-antagonistic NS1 proteins. Two human H5N1 isolates and a seasonal H3N2 strain propagated efficiently in human respiratory cells and induced little IFN-β, whereas three purely avian H5N1 strains were attenuated for replication and provoked higher IFN secretion. Replication of avian viruses was significantly enhanced on interferon-deficient cells, and exogenous IFN potently limited the growth of all strains in human cells. Moreover, IFN-α/β activation by all strains depended on retinoic acid-inducible gene I excluding principal differences in receptor activation between the different viruses. Interestingly, all H5N1 NS1 proteins suppressed IFN-α/β induction comparably well to the NS1 of seasonal IAV. Thus, our study shows that H5N1 strains are heterogeneous in their capacity to activate human cells in an NS1-independent manner. Our findings also suggest that H5N1 viruses need to acquire adaptive changes to circumvent strong IFN-α/β activation in human host cells. Since no single amino acid polymorphism could be associated with a respective high- or low induction phenotype we propose that the necessary adaptations to overcome the human IFN-α/β barrier involve mutations in multiple H5N1 genes.

Emergence of influenza viruses with zoonotic potential: open issues which need to be addressed. A review

The real and perceived impact of influenza infections in animals has changed dramatically over the last 10 years, due mainly to the better understanding of the public health implications of avian and swine influenza viruses. On a number of occasions in the last decade avian-to-human transmissions of H5, H7 and H9 virus subtypes have occurred, and the first influenza pandemic of the new millennium occurred as a result of the emergence and spread of a virus from pigs. Although the mechanisms that allow influenza viruses to jump from one host species to another are not fully understood, several genetic signatures linked to the crossing of species barriers have been identified. This has led to a re-evaluation of the importance of understanding these viruses in the animal reservoir, to the extent that millions of euros have been invested in surveillance, research and capacity building worldwide. This has resulted in an enhanced collaboration with our medical counterparts, leading to many discoveries that will contribute to an understanding of the complex mechanisms that lead to the emergence of a pandemic virus.

Systemic distribution of different low pathogenic avian influenza (LPAI) viruses in chicken

Background

Since we were able to isolate viable virus from brain and lung of H7N1 low pathogenic avian influenza virus (LPAIV) infected chickens, we here examined the distribution of different LPAIV strains in chickens by measuring the viral AI RNA load in multiple organs. Subtypes of H5 (H5N1, H5N2), H7 (H7N1, H7N7) and H9 (H9N2), of chicken (H5N2, H7N1, H7N7, H9N2), or mallard (H5N1) origin were tested. The actual presence of viable virus was evaluated with virus isolation in organs of H7N7 inoculated chickens.

Findings

Viral RNA was found by PCR in lung, brain, intestine, peripheral blood mononuclear cells, heart, liver, kidney and spleen from chickens infected with chicken isolated LPAIV H5N2, H7N1, H7N7 or H9N2. H7N7 virus could be isolated from lung, ileum, heart, liver, kidney and spleen, but not from brain, which was in agreement with the data from the PCR. Infection with mallard isolated H5N1 LPAIV resulted in viral RNA detection in lung and peripheral blood mononuclear cells only.

Conclusion

We speculate that chicken isolated LPAI viruses are spreading systemically in chicken, independently of the strain.

Cross talk between animal and human influenza viruses

Although outbreaks of highly pathogenic avian influenza in wild and domestic birds have been posing the threat of a new influenza pandemic for the past decade, the first pandemic of the twenty-first century came from swine viruses. This fact emphasizes the complexity of influenza viral ecology and the difficulty of predicting influenza viral dynamics. Complete control of influenza viruses seems impossible. However, we must minimize the impact of animal and human influenza outbreaks by learning lessons from past experiences and recognizing the current status. Here, we review the most recent influenza virology data in the veterinary field, including aspects of zoonotic agents and recent studies that assess the pandemic potential of H5N1 highly pathogenic avian influenza viruses.

A review of influenza haemagglutinin receptor binding as it relates to pandemic properties

Haemagglutinin is a determinant of many viral properties, and successful adaptation to a human-like form is thought to be an important step toward pandemic influenza emergence. The availability of structurally distinct sialic acid linked receptors in the sites of human and avian influenza infection are generally held to account for the differences observed, but the relevance of other selection pressures has not been elucidated. There is evidence for genetic and structural constraints of haemagglutinin playing a role in restricting haemagglutinin adaptation, and also for differences in the selection pressure to alter binding, specifically when considering virus replication within host compared to transmission between hosts. Understanding which characteristics underlie such adaptations in humans is now possible in greater detail by using glycan arrays. However, results from these assays must also interpreted in context of an as yet still to be determined detailed knowledge of the structural diversity of sialic acids in the human respiratory tract. A clearer understanding of the evolutionary benefits conveyed by different haemagglutinin properties would have substantial impact and would affect the risk we allocate to viral propagation in different species, such as swine and poultry. Relevant to the H5N1 threat, current evidence also suggests that mortality associated with any emergent pandemic from current strains may be reduced if haemagglutinin specificity changes, further emphasising the importance of understanding how and if selection pressures in the human will cause such an alteration.

Experimental adaptation of an influenza H5 HA confers respiratory droplet transmission to a reassortant H5 HA/H1N1 virus in ferrets

Highly pathogenic avian H5N1 influenza A viruses occasionally infect humans, but currently do not transmit efficiently among humans. The viral haemagglutinin (HA) protein is a known host range determinant since it mediates virus binding to host-specific cellular receptors^1–3. Here, we therefore assessed the molecular changes in HA that would allow an H5 HA-possessing virus to transmit among mammals. We identified a reassortant virus with H5 HA possessing four mutations in a 2009 pandemic H1N1 virus backbone capable of droplet transmission in a ferret model. The transmissible H5 reassortant virus preferentially recognized human-type receptors, replicated efficiently in ferrets, caused lung lesions and weight loss, but it was not highly pathogenic and did not cause mortality. These results suggest that H5 HA can convert to an HA that supports efficient viral transmission in mammals. However, we do not know whether the four mutations in the H5 HA identified in this study would render a wholly avian H5N1 virus transmissible. The genetic origin of the remaining seven viral genes may also critically contribute to transmissibility in mammals. Nevertheless, as H5N1 viruses continue to evolve and infect humans, receptor-binding variants of H5N1 viruses with pandemic potential, including avian-human reassortant viruses as tested here, may emerge. Our findings emphasize the need for pandemic preparedness for H5 HA-possessing viruses and will help individuals conducting surveillance in regions with circulating H5N1 viruses to recognize key residues that predict the pandemic potential of isolates, which will inform the development, production, and distribution of effective countermeasures.

Adaptive pathways of zoonotic influenza viruses: from exposure to establishment in humans

Human influenza viruses have their ultimate origin in avian reservoirs and may adapt, either directly or after passage through another mammalian species, to circulate independently in the human population. Three sets of barriers must be crossed by a zoonotic influenza virus before it can become a human virus: animal-to-human transmission barriers; virus-cell interaction barriers; and human-to-human transmission barriers. Adaptive changes allowing zoonotic influenza viruses to cross these barriers have been studied extensively, generating key knowledge for improved pandemic preparedness. Most of these adaptive changes link acquired genetic alterations of the virus to specific adaptation mechanisms that can be screened for, both genetically and phenotypically, as part of zoonotic influenza virus surveillance programs. Human-to-human transmission barriers are only sporadically crossed by zoonotic influenza viruses, eventually triggering a worldwide influenza outbreak or pandemic. This is the most devastating consequence of influenza virus cross-species transmission. Progress has been made in identifying some of the determinants of influenza virus transmissibility. However, interdisciplinary research is needed to further characterize these ultimate barriers to the development of influenza pandemics, at both the level of the individual host and that of the population.

The role of receptor binding specificity in interspecies transmission of influenza viruses

Influenza A virus infection begins with the binding of the hemagglutinin (HA) glycoprotein to sialic acid-containing receptors on the surface of the target cell. Avian influenza viruses, including avian H5N1, H7, and H9N2 viruses, can occasionally cross the species barrier and infect humans; however, these viruses do not spread efficiently from person to person, perhaps, partly, owing to differences in the receptor-binding specificities of human and avian influenza viruses. The HAs of avian influenza viruses must adapt to receptors in humans to acquire efficient human-to-human transmissibility. In this review, we discuss the receptor binding specificity of influenza A viruses and its role in interspecies transmission.

Pathogenesis of influenza virus infections: the good, the bad and the ugly

The clinical outcome of different influenza virus infections ranges from subclinical upper respiratory tract disease to fatal lower respiratory tract disease. An important determinant in the pathogenesis of these diseases is the tissue tropism of the influenza virus. Furthermore, virulence is often correlated with virus replication and is regulated by multiple virus genes. Host defense against virus infection consists of both innate and adaptive immune responses. However, excessive or dysbalanced immune response may result in lung tissue damage, reduced respiratory capacity, and severe disease or even death. By interdisciplinary efforts to better understand the intricate interaction between virus, tissue, and immune response, we may be able to find new ways to improve the outcome of influenza virus infections.

Receptor-binding profiles of H7 subtype influenza viruses in different host species

Influenza viruses of gallinaceous poultry and wild aquatic birds usually have distinguishable receptor-binding properties. Here we used a panel of synthetic sialylglycopolymers and solid-phase receptor-binding assays to characterize receptor-binding profiles of about 70 H7 influenza viruses isolated from aquatic birds, land-based poultry, and horses in Eurasia and America. Unlike typical duck influenza viruses with non-H7 hemagglutinin (HA), all avian H7 influenza viruses, irrespective of the host species, displayed a poultry-virus-like binding specificity, i.e., preferential binding to sulfated oligosaccharides Neu5Acα2-3Galβ1-4(6-O-HSO(3))GlcNAc and Neu5Acα2-3Galβ1-4(Fucα1-3)(6-O-HSO(3))GlcNAc. This phenotype correlated with the unique amino acid sequence of the amino acid 185 to 189 loop of H7 HA and seemed to be dependent on ionic interactions between the sulfate group of the receptor and Lys193 and on the lack of sterical clashes between the fucose residue and Gln222. Many North American and Eurasian H7 influenza viruses displayed weak but detectable binding to the human-type receptor moiety Neu5Acα2-6Galβ1-4GlcNAc, highlighting the potential of H7 influenza viruses for avian-to-human transmission. Equine H7 influenza viruses differed from other viruses by preferential binding to the N-glycolyl form of sialic acid. Our data suggest that the receptor-binding site of contemporary H7 influenza viruses in aquatic and terrestrial birds was formed after the introduction of their common precursor from ducks to a new host, presumably, gallinaceous poultry. The uniformity of the receptor-binding profile of H7 influenza viruses in various wild and domestic birds indicates that there is no strong receptor-mediated host range restriction in birds on viruses with this HA subtype. This notion agrees with repeated interspecies transmission of H7 influenza viruses from aquatic birds to poultry.

In vitro evolution of H5N1 avian influenza virus toward human-type receptor specificity

Acquisition of α2-6 sialoside receptor specificity by α2-3 specific highly-pathogenic avian influenza viruses (H5N1) is thought to be a prerequisite for efficient transmission in humans. By in vitro selection for binding α2-6 sialosides, we identified four variant viruses with amino acid substitutions in the hemagglutinin (S227N, D187G, E190G, and Q196R) that revealed modestly increased α2-6 and minimally decreased α2-3 binding by glycan array analysis. However, a mutant virus combining Q196R with mutations from previous pandemic viruses (Q226L and G228S) revealed predominantly α2-6 binding. Unlike the wild type H5N1, this mutant virus was transmitted by direct contact in the ferret model although not by airborne respiratory droplets. However, a reassortant virus with the mutant hemagglutinin, a human N2 neuraminidase and internal genes from an H5N1 virus was partially transmitted via respiratory droplets. The complex changes required for airborne transmissibility in ferrets suggest that extensive evolution is needed for H5N1 transmissibility in humans.

Solid-phase assays of receptor-binding specificity

Influenza virus attachment to sialic acid-containing molecules on the cell surface initiates the infection. The spectrum of functional receptors on target cells and decoy receptors on cells and epithelial mucus varies substantially between animal species leading to variations in the receptor-binding specificity of viruses circulating in these species. Analysis of the receptor specificity of different animal and human influenza viruses can give insight into factors and mechanisms that determine viral host range, tissue and cell tropism, replication efficiency, and pathogenesis. Knowledge of viral receptor specificity may also be useful for the development of more efficient influenza vaccines and anti-influenza drugs.A majority of known receptor specificity assays measure influenza virus binding to sialic acid-containing natural and synthetic compounds (receptor analogues). Here, we describe protocols of two solid-phase enzyme-linked receptor-binding assays which are technically similar to standard ELISA. Each assay determines binding of the virus immobilized in the wells of 96-well plate to receptor analogues in solution. In the direct binding assay, the virus binds to either synthetic biotinylated sialylglycopolymers or to peroxidase-labeled sialylglycoprotein fetuin (Fet-HRP); the apparent association constants of the virus-receptor complexes are calculated from the Scatchard plots of the binding data. In the fetuin-binding inhibition assay, the virus is incubated with a mixture of unlabeled receptor analogue and standard preparation of Fet-HRP; the association constant for analogue is calculated based on the level of its competition with Fet-HRP.

Biased mutational pattern and quasispecies hypothesis in H5N1 virus

Like other RNA viruses, influenza viruses are subject to high mutation rates. Carrying segmented RNA genomes, their genetic variability is even higher. We aimed at analyzing the mutational events occurring during the infection of chickens by the Highly Pathogenic Avian Influenza (HPAI) H5N1 virus. We therefore studied the different sequences of two surface proteins, hemagglutinin (HA) and neuraminidase (NA), as well as two internal proteins, PB2 and NS. Three organs (lung, spleen, brain) were obtained from a chicken, experimentally infected with a lethal dose of HPAI H5N1 virus. Cloning these PCR fragments enabled us to investigate the mutations undergone by the virus after several replicative cycles. The first outcome is the presence of a strong mutational bias, resembling host-driven ADAR1 adenosine deamination, which is responsible for 81% of all mutations. Whereas the frequency of RNA dependent RNA polymerase-related mutations is compatible with the survival of the virus, the ADAR1-like activity usually strongly increases the mutation frequency into a level of “error catastrophe” in theory incompatible with virus survival. Nevertheless, the virus was successfully infective. HPAI H5N1 virus displayed traits in agreement with the quasispecies theory. The role of this quasispecies structure in successful infection and the superposition with the ADAR1-like response is discussed.

Analysis of influenza virus hemagglutinin receptor binding mutants with limited receptor recognition properties and conditional replication characteristics

To examine the range of selective processes that potentially operate when poorly binding influenza viruses adapt to replicate more efficiently in alternative environments, we passaged a virus containing an attenuating mutation in the hemagglutinin (HA) receptor binding site in mice and characterized the resulting mutants with respect to the structural locations of mutations selected, the replication phenotypes of the viruses, and their binding properties on glycan microarrays. The initial attenuated virus had a tyrosine-to-phenylalanine mutation at HA1 position 98 (Y98F), located in the receptor binding pocket, but viruses that were selected contained second-site pseudoreversion mutations in various structural locations that revealed a range of molecular mechanisms for modulating receptor binding that go beyond the scope that is generally mapped using receptor specificity mutants. A comparison of virus titers in the mouse respiratory tract versus MDCK cells in culture showed that the mutants displayed distinctive replication properties depending on the system, but all were less attenuated in mice than the Y98F virus. An analysis of receptor binding properties confirmed that the initial Y98F virus bound poorly to several different species of erythrocytes, while all mutants reacquired various degrees of hemagglutination activity. Interestingly, both the Y98F virus and pseudoreversion mutants were shown to bind very inefficiently to standard glycan microarrays containing an abundance of binding substrates for most influenza viruses that have been characterized to date, provided by the Consortium for Functional Glycomics. The viruses were also examined on a recently developed microarray containing glycans terminating in sialic acid derivatives, and limited binding to a potentially interesting subset of glycans was revealed. The results are discussed with respect to mechanisms for HA-mediated receptor binding, as well as regarding the species of molecules that may act as receptors for influenza virus on host cell surfaces.

Genetic drift evolution under vaccination pressure among H5N1 Egyptian isolates

BACKGROUND

The highly pathogenic H5N1 is a major avian pathogen that intensively affects the poultry industry in Egypt even in spite of the adoption of vaccination strategy. Antigenic drift is among the strategies the influenza virus uses to escape the immune system that might develop due to the pressure of extensive vaccination. H5N1 mutates in an intensified manner and is considered a potential candidate for the possible next pandemic with all the catastrophic consequences such an eventuality will entail.

METHODS

H5N1 was isolated from the pooled organ samples of four different affected flocks in specific pathogen free embryonated chicken eggs (SPF-ECE). A reverse transcriptase polymerase chain reaction (RT-PCR) was performed to the haemagglutingin and neuraminidase. Sequencing of the full length haemagglutingin was performed. Sequence analyses of the isolated strains were performed and compared to all available H5N1 from Egyptian human and avian strains in the flu database. Changes in the different amino acid that may be related to virus virulence, receptor affinity and epitope configuration were assigned and matched with all available Egyptian strains in the flu database.

RESULTS

One out of the four strains was found to be related to the B2 Egyptian lineage, 2 were related to A1 lineage and the 4th was related to A2 lineage. Comparing data obtained from the current study by other available Egyptian H5N1 sequences remarkably demonstrates that amino acid changes in the immune escape variants are remarkably restricted to a limited number of locations on the HA molecule during antigenic drift. Molecular diversity in the HA gene, in relevance to different epitopes, were not found to follow a regular trend, suggesting abrupt cumulative sequence mutations. However a number of amino acids were found to be subjected to high mutation pressure.

CONCLUSION

The current data provides a comprehensive view of HA gene evolution among H5N1 subtype viruses in Egypt. Egyptian H5N1-AIVs are constantly undergoing genetic changes and reveal a complex pattern of drifts. These findings raise the concerns about the value of using influenza vaccines in correlation with the development of antigenic drift in influenza epidemics.

Effect of receptor binding domain mutations on receptor binding and transmissibility of avian influenza H5N1 viruses

Although H5N1 influenza viruses have been responsible for hundreds of human infections, these avian influenza viruses have not fully adapted to the human host. The lack of sustained transmission in humans may be due, in part, to their avian-like receptor preference. Here, we have introduced receptor binding domain mutations within the hemagglutinin (HA) gene of two H5N1 viruses and evaluated changes in receptor binding specificity by glycan microarray analysis. The impact of these mutations on replication efficiency was assessed in vitro and in vivo. Although certain mutations switched the receptor binding preference of the H5 HA, the rescued mutant viruses displayed reduced replication in vitro and delayed peak virus shedding in ferrets. An improvement in transmission efficiency was not observed with any of the mutants compared to the parental viruses, indicating that alternative molecular changes are required for H5N1 viruses to fully adapt to humans and to acquire pandemic capability.

Effects of receptor binding specificity of avian influenza virus on the human innate immune response

Humans infected by the highly pathogenic H5N1 avian influenza viruses (HPAIV) present unusually high concentrations in serum of proinflammatory cytokines and chemokines, which are believed to contribute to the high pathogenicity of these viruses. The hemagglutinins (HAs) of avian influenza viruses preferentially bind to sialic acids attached through α2,3 linkages (SAα2,3) to the terminal galactose of carbohydrates on the host cell surface, while the HAs from human strains bind to α2,6-linked SA (SAα2,6). To evaluate the role of the viral receptor specificity in promoting innate immune responses in humans, we generated recombinant influenza viruses, one bearing the HA and neuraminidase (NA) genes from the A/Vietnam/1203/2004 H5N1 HPAIV in an influenza A/Puerto Rico/8/1934 (A/PR/8/34) backbone with specificity for SAα2,3 and the other a mutant virus (with Q226L and G228S in the HA) with preferential receptor specificity for SAα2,6. Viruses with preferential affinity for SAα2,3 induced higher levels of proinflammatory cytokines and interferon (IFN)-inducible genes in primary human dendritic cells (DCs) than viruses with SAα2,6 binding specificity, and these differences were independent of viral replication, as shown by infections with UV-inactivated viruses. Moreover, human primary macrophages and respiratory epithelial cells showed higher expression of proinflammatory genes after infection with the virus with SAα2,3 affinity than after infection with the virus with SAα2,6 affinity. These data indicate that binding to SAα2,3 by H5N1 HPAIV may be sensed by human cells differently than binding to SAα2,6, inducing an exacerbated innate proinflammatory response in infected individuals.

Neuraminidase inhibitor sensitivity and receptor-binding specificity of Cambodian clade 1 highly pathogenic H5N1 influenza virus

The evolution of the highly pathogenic H5N1 influenza virus produces genetic variations that can lead to changes in antiviral susceptibility and in receptor-binding specificity. In countries where the highly pathogenic H5N1 virus is endemic or causes regular epidemics, the surveillance of these changes is important for assessing the pandemic risk. In Cambodia between 2004 and 2010, there have been 26 outbreaks of highly pathogenic H5N1 influenza virus in poultry and 10 reported human cases, 8 of which were fatal. We have observed naturally occurring mutations in hemagglutinin (HA) and neuraminidase (NA) of Cambodian H5N1 viruses that were predicted to alter sensitivity to neuraminidase inhibitors (NAIs) and/or receptor-binding specificity. We tested H5N1 viruses isolated from poultry and humans between 2004 and 2010 for sensitivity to the NAIs oseltamivir (Tamiflu) and zanamivir (Relenza). All viruses were sensitive to both inhibitors; however, we identified a virus with a mildly decreased sensitivity to zanamivir and have predicted that a V149A mutation is responsible. We also identified a virus with a hemagglutinin A134V mutation, present in a subpopulation amplified directly from a human sample. Using reverse genetics, we verified that this mutation is adaptative for human α2,6-linked sialidase receptors. The importance of an ongoing surveillance of H5N1 antigenic variance and genetic drift that may alter receptor binding and sensitivities of H5N1 viruses to NAIs cannot be underestimated while avian influenza remains a pandemic threat.

Influenza A (H5N1) viruses from pigs, Indonesia

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

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

Virulence determinants of pandemic influenza viruses

Influenza A viruses cause recurrent, seasonal epidemics and occasional global pandemics with devastating levels of morbidity and mortality. The ability of influenza A viruses to adapt to various hosts and undergo reassortment events ensures constant generation of new strains with unpredictable degrees of pathogenicity, transmissibility, and pandemic potential. Currently, the combination of factors that drives the emergence of pandemic influenza is unclear, making it impossible to foresee the details of a future outbreak. Identification and characterization of influenza A virus virulence determinants may provide insight into genotypic signatures of pathogenicity as well as a more thorough understanding of the factors that give rise to pandemics.

Characterization of influenza virus sialic acid receptors in minor poultry species

It is commonly accepted that avian influenza viruses (AIVs) bind to terminal α2,3 sialic acid (SA) residues whereas human influenza viruses bind to α2,6 SA residues. By a series of amino acid changes on the HA surface protein, AIVs can switch receptor specificity and recognize α2,6 SA positive cells, including human respiratory epithelial cells. Animal species, like pigs and Japanese quail, that contain both α2,3 and α2,6 SA become ideal environments for receptor switching. Here, we describe the SA patterns and distributions in 6 common minor domestic poultry species: Peking duck, Toulouse geese, Chinese ring-neck pheasant, white midget turkey, bobwhite quail, and pearl guinea fowl. Lectins specific to α2,3 and α2,6 SA (Maakia amurensis agglutinin and Sambuca nigra agglutinin, respectively) were used to detect SA by an alkaline phosphotase-based method and a fluorescent-based method. Differences in SA moieties and their ability to bind influenza viruses were visualized by fluorescent labeling of 4 different H3N2 influenza viruses known to be specific for one receptor or the other. The geese and ducks showed α2,3 SA throughout the respiratory tract and marginal α2,6 SA only in the colon. The four other avian species showed both α2,3 and α2,6 SA in the respiratory tract and the intestines. Furthermore, the turkey respiratory tract showed a positive correlation between age and α2,6 SA levels. The fact that these birds have both avian and human flu receptors, combined with their common presence in backyard farms and live bird markets worldwide, mark them as potential mixing bowl species and necessitates improved surveillance and additional research about the role of these birds in influenza host switching.

Influenza A virus transmission: contributing factors and clinical implications

Efficient human-to-human transmission is a necessary property for the generation of a pandemic influenza virus. To date, only influenza A viruses within the H1-H3 subtypes have achieved this capacity. However, sporadic cases of severe disease in individuals following infection with avian influenza A viruses over the past decade, and the emergence of a pandemic H1N1 swine-origin virus in 2009, underscore the need to better understand how influenza viruses acquire the ability to transmit efficiently. In this review, we discuss the biological constraints and molecular features known to affect virus transmissibility to and among humans. Factors influencing the behaviour of aerosols in the environment are described, and the mammalian models used to study virus transmission are presented. Recent progress in understanding the molecular determinants that confer efficient transmission has identified crucial roles for the haemagglutinin and polymerase proteins; nevertheless, influenza virus transmission remains a polygenic trait that is not completely understood. The clinical implications of this research, including methods currently under investigation to mitigate influenza virus human-to-human transmission, are discussed. A better understanding of the viral determinants necessary for efficient transmission will allow us to identify avian influenza viruses with pandemic potential.

Rapid evolution of low-pathogenic H9N2 avian influenza viruses following poultry vaccination programmes

To investigate whether currently circulating H9N2 avian influenza viruses (AIVs) in domestic poultry have evolved in Korean poultry since 2007, genetic and serological comparisons were conducted of H9N2 isolates from poultry slaughterhouses from January 2008 to December 2009. The isolation rate was relatively low in 2008 but increased gradually from January 2009 onwards. Genetic and phylogenetic analyses revealed that reassortant viruses had emerged, generating at least five novel genotypes, mostly containing segments of a previously prevalent domestic H9N2 virus lineage (Ck/Korea/04116/04-like). It was noteworthy that the N2 genes of some H9N2 isolates (genotypes D, E and F) were derived from those of H3N2-like viruses commonly isolated among domestic ducks in live-poultry markets. Animal challenge studies demonstrated that the pathogenicity of Ck/Korea/SH0906/09 (genotype B) and Ck/Korea/SH0912/09 (genotype F) in domestic avian species was altered due to reassortment. Furthermore, serological analysis revealed that the isolates were antigenically distinct from previous Korean H9N2 viruses including Ck/Korea/01310/01. Such antigenic diversity was illustrated further in experiments using H9N2-immunized chickens, which could not inhibit the replication and transmission of challenge viruses from each genotype. These results suggest that H9N2 viruses from domestic poultry have undergone substantial evolution since 2007 by immune selection as a result of vaccinal and natural immunity, coupled with reassortment. Taken together, this study demonstrates that periodical updating of vaccine strains, based on continuous surveillance data, is an important issue in order to provide sufficient protectivity against AIV infections.

A polymorphism in the hemagglutinin of the human isolate of a highly pathogenic H5N1 influenza virus determines organ tropism in mice

We characterized a human H5N1 virus isolate (KAN-1) encoding a hemagglutinin (HA) with a K-to-E substitution at amino acid position 222 that was previously described to be selected in the lung of the infected patient. In mice, the growth of the HA(222E)-encoding virus was mainly confined to the lung, but reversion to 222K allowed virus to spread to the brain. The HA(222E) variant showed an overall reduced binding affinity compared to that of HA(222K) for synthetic Neu5Ac2-3Gal-terminated receptor analogues, except for one analogue [Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)(6-HSO(3))GlcNAcbeta, Su-SLe(x)]. Our results suggest that human-derived mutations in HA of H5N1 viruses can affect viral replication efficiency and organ tropism.

Isolation and characterization of highly pathogenic avian influenza virus subtype H5N1 from donkeys

Background

The highly pathogenic H5N1 is a major avian pathogen that crosses species barriers and seriously affects humans as well as some mammals. It mutates in an intensified manner and is considered a potential candidate for the possible next pandemic with all the catastrophic consequences.

Methods

Nasal swabs were collected from donkeys suffered from respiratory distress. The virus was isolated from the pooled nasal swabs in specific pathogen free embryonated chicken eggs (SPF-ECE). Reverse transcriptase polymerase chain reaction (RT-PCR) and sequencing of both haemagglutingin and neuraminidase were performed. H5 seroconversion was screened using haemagglutination inhibition (HI) assay on 105 donkey serum samples.

Results

We demonstrated that H5N1 jumped from poultry to another mammalian host; donkeys. Phylogenetic analysis showed that the virus clustered within the lineage of H5N1 from Egypt, closely related to 2009 isolates. It harboured few genetic changes compared to the closely related viruses from avian and humans. The neuraminidase lacks oseltamivir resistant mutations. Interestingly, HI screening for antibodies to H5 haemagglutinins in donkeys revealed high exposure rate.

Conclusions

These findings extend the host range of the H5N1 influenza virus, possess implications for influenza virus epidemiology and highlight the need for the systematic surveillance of H5N1 in animals in the vicinity of backyard poultry units especially in endemic areas.

Inhibition of H5N1 highly pathogenic influenza virus by suppressing a specific sialyltransferase

Avian influenza viruses preferentially use alpha2,3-linked sialic acid as a receptor for binding and entry into target cells. The sialic acid is the terminal residue of various types of glycan. There are two major types of alpha2,3-linked sialic acid differing in the penultimate bond: Neu5Acalpha2-3Galbeta1-3GalNAc and Neu5Acalpha2-3Galbeta1-4GlcNAc. In the human airway, while Neu5Acalpha2-3Galbeta1-3GalNAc is present only in alveolar epithelial cells, the Neu5Acalpha2-3Galbeta1-4GlcNAc is expressed in both the upper and lower airway. Previous data showed preferential binding of hemagglutinin from H5N1 highly pathogenic influenza virus to Neu5Acalpha2-3Galbeta1-4GlcNAc. We further show here that suppression of this sialic acid by siRNA against a sialyltransferase, ST3GAL4, can inhibit H5N1 avian influenza virus infection and that this gene is abundantly expressed in human pharynx, trachea and bronchus. These data suggest that the ST3GAL4 gene is responsible for biosynthesis of the viral receptor and may play a crucial role in infection of H5N1 avian influenza virus in humans.

[Interspecies transmission, adaptation to humans and pathogenicity of animal influenza viruses]

The emergence in 2009 of a novel A(H1N1)v influenza virus of swine origin and the regular occurrence since 2003 of human cases of infection with A(H5N1) avian influenza viruses underline the zoonotic and pandemic potential of type A influenza viruses. Influenza viruses from the wild aquatic birds reservoir usually do not replicate efficiently in humans. Domestic poultry and swine can act as intermediate hosts for the acquisition of determinants that increase the potential of transmission and adaptation to humans, through the accumulation of mutations or by genetic reassortment. The rapid evolution of influenza viruses following interspecies transmission probably results from the selection of genetic variations that favor optimal interactions between viral proteins and cellular factors, leading to an increased multiplication potential and a better escape to the host antiviral response. Whereas influenza viruses usually cause asymptomatic infections in wild aquatic birds, they may be highly pathogenic in other species. Molecular determinants of host-specificity and pathogenesis have been identified in most viral genes, notably in genes that encode viral surface glycoproteins, proteins involved in the viral genome replication, and proteins that counteract the host immune response. However, our knowledge of these numerous and interdependant determinants remains incomplete, and the molecular mechanisms involved are still to be understood.

A mutation in H5 haemagglutinin that conferred human receptor recognition is not maintained stably during duck passage

A/Hong Kong/213/97 (HK213; H5N1), isolated from a human, binds to both avian- and human-type receptors, due to a haemagglutinin (HA) mutation probably acquired during adaptation to humans. Duck passage of this virus conferred lethality in ducks. Sequence analyses of the duck-passaged virus revealed that its HA gene reverted back to one recognizing only avian-type receptors, and consequently it bound human tissue to a lesser extent. This finding suggests that viruses with human-type receptor specificity are unlikely to be maintained in waterfowl, unlike those with the human-type PB2 mutation, such as H5N1 viruses of the Qinghai Lake lineage.