Molecular basis for the generation in pigs of influenza A viruses with pandemic potential

Host range restriction and pathogenicity in the context of influenza pandemic

Influenza A viruses cause pandemics at random intervals. Pandemics are caused by viruses that contain a hemagglutinin (HA) surface glycoprotein to which human populations are immunologically naive. Such an HA can be introduced into the human population through reassortment between human and avian virus strains or through the direct transfer of an avian influenza virus to humans. The factors that determine the interspecies transmission and pathogenicity of influenza viruses are still poorly understood; however, the HA protein plays an important role in overcoming the interspecies barrier and in virulence in avian influenza viruses. Recently, the RNA polymerase (PB2) protein has also been recognized as a critical factor in host range restriction, while the nonstructural (NS1) protein affects the initial host immune responses. We summarize current knowledge of viral factors that determine host range restriction and pathogenicity of influenza A viruses.

Inhibition of influenza-virus-induced cytopathy by sialylglycoconjugates

The anti-viral activity of gangliosides such as SPG (sialylparagloboside), GD1a, GM3, and GM4 was assessed by inhibition of the cytopathy of MDCK cells due to infection with the influenza virus A/PR/8/34. The inhibitory effect was in the following sequence: SPG>GD1a>GM3>GM4. The IC50 of SPG and GD1a was 7 and 70 microM, respectively, indicating that they are more effective than the representative inhibitor amantadine. Although 3'-sialyllactose (3'-SL) and 3'-sialyllactosamine (3'-SLN), which are identical to the terminal trisaccharides of GM3 and SPG, respectively, did not show any inhibitory effect, introduction of an amino group to the reducing end of 3'-SL following amidation with lauroyl chloride gave the inhibitory potency, which was comparable to that of GM3. These results suggest that the viral hemagglutinin recognizes exogenous sialyloligosaccharides rather than inherent sialyloligosaccharides expressed on MDCK cells, since introduction of the hydrophobic moiety to oligosaccharides might cause micelle formation.

Amantadine resistance among porcine H1N1, H1N2, and H3N2 influenza A viruses isolated in Germany between 1981 and 2001

This study was designed to gain insight into amantadine susceptibility of porcine influenza A viruses isolated in Germany between 1981 and 2001. The 12 studied H1N1, H1N2, and H3N2 porcine influenza virus strains were isolated in chicken eggs and passaged once in MDCK cells. Plaque reduction assays were applied to examine virus susceptibility to amantadine. Genotyping was used to confirm drug resistance. In the results of these antiviral studies, only 3 of the 12 isolates were shown to be amantadine-susceptible. All resistant strains contained the AA substitutions G16E, S31N, and R77Q in the membrane protein 2 (M2). Additionally, L27A was detected in two H1N1 strains. S31N and/or L27A are well-known amino acid substitutions in M2 that confer amantadine resistance. The role of the pig as an intermediate host of avian and human influenza A viruses, the possible involvement of genetic reassortment, and the high incidence of naturally amantadine-resistant porcine influenza A viruses suggest a real risk of emergence of amantadine resistant human viruses. Therefore, drug susceptibility monitoring appears to be warranted for effective application of those drugs.

[Christopher Columbus flu. A hypothesis for an ecological catastrophe]

When Christopher Columbus and his men embarked on the second Colombian expedition to the New World (1493), the crew suffered from fever, respiratory symptoms and malaise. It is generally accepted that the disease was influenza. Pigs, horses and hens acquired in Gomera (Canary Islands) traveled in the same ship. The pigs may well have been the origin of the flu and the intermediary hosts for genetic recombination of other viral subtypes. The Caribbean archipelago had a large population of birds, the natural reservoir of the avian influenza virus. In this ecological scenario there was a concurrence of several biological elements that had never before coexisted in the New World: pigs, horses, the influenza virus and humans. We propose that birds are likely to have played an important role in the epidemiology of the flu occurring on the second Colombian trip, which caused a fatal demographic catastrophe, with an estimated mortality of 90% among the natives.

A recombinant pseudorabies virus encoding the HA gene from H3N2 subtype swine influenza virus protects mice from virulent challenge

The hemagglutinin (HA) gene of A/Swine/Inner Mogolian/547/2001 (H3N2) swine influenza virus (SIV) was recombined into the genome of pseudorabies virus (PRV) Bartha-K61 vaccine strain, generating a recombinant PRV expressing the HA gene, designated as rPRV-HA. One group of 15 mice was inoculated intranasally (i.n.) with 10(5.0) PFU of rPRV-HA, and another two control groups of mice (15 mice per group) were mock-inoculated or inoculated with Bartha-K61. Mice inoculated with rPRV-HA developed hemagglutination inhibition antibodies 3 weeks post-inoculation. Twenty-eight days post-inoculation, all mice were challenged i.n. with 10(5.0) TCID50 of A/Swine/Heilongjiang/74/2000 (H3N2). No challenge virus was isolated from vaccinated mice, and mild pathological lesions were observed only in lungs following challenge. The results demonstrate that the recombinant rPRV-HA expressing the HA gene from H3N2 SIV can protect mice from heterologous virulent challenge, and may represent a candidate vaccine against SIV.

The origin of human pathogens: evaluating the role of agriculture and domestic animals in the evolution of human disease

Many significant diseases of human civilization are thought to have arisen concurrently with the advent of agriculture in human society. It has been hypothesised that the food produced by farming increased population sizes to allow the maintenance of virulent pathogens, i.e. civilization pathogens, while domestic animals provided sources of disease to humans. To determine the relationship between pathogens in humans and domestic animals, I examined phylogenetic data for several human pathogens that are commonly evolutionarily linked to domestic animals: measles, pertussis, smallpox, tuberculosis, taenid worms, and falciparal malaria. The majority are civilization pathogens, although I have included others whose evolutionary origins have traditionally been ascribed to domestic animals. The strongest evidence for a domestic-animal origin exists for measles and pertussis, although the data do not exclude a non-domestic origin. As for the other pathogens, the evidence currently available makes it difficult to determine if the domestic-origin hypothesis is supported or refuted; in fact, intriguing data for tuberculosis and taenid worms suggests that transmission may occur as easily from humans to domestic animals. These findings do not abrogate the importance of agriculture in disease transmission; rather, if anything, they suggest an alternative, more complex series of effects than previously elucidated. Rather than domestication, the broader force for human pathogen evolution could be ecological change, namely anthropogenic modification of the environment. This is supported by evidence that many current emerging infectious diseases are associated with human modification of the environment. Agriculture may have changed the transmission ecology of pre-existing human pathogens, increased the success of pre-existing pathogen vectors, resulted in novel interactions between humans and wildlife, and, through the domestication of animals, provided a stable conduit for human infection by wildlife diseases.

Emerging microbiological food safety issues related to meat

Avian influenza viruses and antibiotic-resistant pathogens have become topics of current public health interest. This paper will focus on the significance of these pathogens to the meat industry as well as other emerging microbiological food safety topics likely to impact the meat industry. These include surveillance of foodborne pathogens, microbial source tracking, risk assessment, and human populations at increased risk of infection by foodborne microbes. These emerging issues will likely lead to even greater challenges to producing microbiologically safe meat products than the industry has ever experienced. However, accompanying such challenges will be innovative solutions that provide even greater public health protection to meat-containing foods.

Restricted infectivity of a human-Lineage H3N2 influenza A virus in pigs is hemagglutinin and neuraminidase gene dependent

Influenza A viruses cause pandemics at sporadic intervals. Pandemic viruses can potentially be introduced into the human population through in toto transfer of an avian influenza virus or through reassortment between avian and human strains. Pigs are believed to play a central role in the creation of pandemic viruses through reassortment because of their susceptibility to infection with both avian and human influenza viruses. However, we recently found that a human-lineage H3N2 influenza virus was highly restricted in its ability to infect pigs after intranasal inoculation. We hypothesized that this restricted infectivity phenotype was controlled by the hemagglutinin (HA) and neuraminidase (NA). To test this, we infected pigs with reverse genetics-created HA plus NA reassortant viruses. Specifically, introduction of the HA and NA genes of a contemporary H3N2 swine virus into the genetic background of the wholly human virus resulted in a significant increase in virus shedding and pathogenicity. These data indicate that the HA/NA can play important roles in controlling human influenza virus infectivity in pigs. The results further support the premise that a barrier exists to human influenza virus infection in pigs, which may limit the role of pigs in pandemic virus creation through reassortment of human and avian influenza viruses.

Cross-species transfer of viruses: implications for the use of viral vectors in biomedical research, gene therapy and as live-virus vaccines

All living organisms are continuously exposed to a plethora of viruses. In general, viruses tend to be restricted to the natural host species which they infect. From time to time viruses cross the host-range barrier expanding their host range. However, in very rare cases cross-species transfer is followed by the establishment and persistence of a virus in the new host species, which may result in disease. Recent examples of viruses that have crossed the species barrier from animal reservoirs to humans are hantavirus, haemorrhagic fever viruses, arboviruses, Nipah and Hendra viruses, avian influenza virus (AI), monkeypox virus, and the SARS-associated coronavirus (SARS-CoV). The opportunities for cross-species transfer of mammalian viruses have increased in recent years due to increased contact between humans and animal reservoirs. However, it is difficult to predict when such events will take place since the viral adaptation that is needed to accomplish this is multifactorial and stochastic. Against this background the intensified use of viruses and their genetically modified variants as viral gene transfer vectors for biomedical research, experimental gene therapy and for live-vector vaccines is a cause for concern. This review addresses a number of potential risk factors and their implications for activities with viral vectors from the perspective of cross-species transfer of viruses in nature, with emphasis on the occurrence of host-range mutants resulting from either cell culture or tropism engineering. The issues are raised with the intention to assist in risk assessments for activities with vector viruses.

Reasons for the increase in emerging and re-emerging viral infectious diseases

In the past two decades, humans have faced many new viral infectious agents in emerging and re-emerging infectious diseases (EIDs). Many factors contribute to the appearance of EIDs. These factors are complex but can be classified into three different categories: virus factors, human factors, and ecological factors. The factors contributing to the cause of such viral infectious diseases will be systematically reviewed in this article.

Avian influenza virus infections in humans

Seroepidemiologic and virologic studies since 1889 suggested that human influenza pandemics were caused by H1, H2, and H3 subtypes of influenza A viruses. If not for the 1997 avian A/H5N1 outbreak in Hong Kong of China, subtype H2 is the likely candidate for the next pandemic. However, unlike previous poultry outbreaks of highly pathogenic avian influenza due to H5 that were controlled by depopulation with or without vaccination, the presently circulating A/H5N1 genotype Z virus has since been spreading from Southern China to other parts of the world. Migratory birds and, less likely, bird trafficking are believed to be globalizing the avian influenza A/H5N1 epidemic in poultry. More than 200 human cases of avian influenza virus infection due to A/H5, A/H7, and A/H9 subtypes mainly as a result of poultry-to-human transmission have been reported with a > 50% case fatality rate for A/H5N1 infections. A mutant or reassortant virus capable of efficient human-to-human transmission could trigger another influenza pandemic. The recent isolation of this virus in extrapulmonary sites of human diseases suggests that the high fatality of this infection may be more than just the result of a cytokine storm triggered by the pulmonary disease. The emergence of resistance to adamantanes (amantadine and rimantadine) and recently oseltamivir while H5N1 vaccines are still at the developmental stage of phase I clinical trial are causes for grave concern. Moreover, the to-be pandemic strain may have little cross immunogenicity to the presently tested vaccine strain. The relative importance and usefulness of airborne, droplet, or contact precautions in infection control are still uncertain. Laboratory-acquired avian influenza H7N7 has been reported, and the laboratory strains of human influenza H2N2 could also be the cause of another pandemic. The control of this impending disaster requires more research in addition to national and international preparedness at various levels. The epidemiology, virology, clinical features, laboratory diagnosis, management, and hospital infection control measures are reviewed from a clinical perspective.

Avian Flu to Human Influenza

Influenza A viral infection causes substantial annual morbidity and mortality worldwide, particularly for infants, the elderly, and the immunocompromised. The virus mainly replicates in the respiratory tract and is spread by respiratory secretions. A growing concern is the recent identification of H5N1 strains of avian influenza A in Asia that were previously thought to infect only wild birds and poultry, but have now infected humans, cats, pigs, and other mammals, often with fatal results, in an ongoing outbreak. A human pandemic with H5N1 virus could potentially be catastrophic because most human populations have negligible antibody-mediated immunity to the H5 surface protein and this viral subtype is highly virulent. Whether an H5N1 influenza pandemic will occur is likely to hinge on whether the viral strains involved in the current outbreak acquire additional mutations that facilitate efficient human-to-human transfer of infection. Although there is no historical precedent for an H5N1 avian strain causing widespread human-to-human transmission, some type of influenza A pandemic is very likely in the near future. The possibility of an H5N1 influenza pandemic has highlighted the many current limitations of treatment with antiviral agents and of vaccine production and immunogenicity. Future vaccine strategies that may include more robust induction of T-cell responses, such as cytotoxic T lymphocytes, may provide better protection than is offered by current vaccines, which rely solely or mainly on antibody neutralization of infection.

The origins of new pandemic viruses: the acquisition of new host ranges by canine parvovirus and influenza A viruses

Transfer of viruses between hosts to create a new self-sustaining epidemic is rare; however, those new viruses can cause severe outbreaks. Examples of such viruses include three pandemic human influenza A viruses and canine parvovirus in dogs. In each case one virus made the original transfer and spread worldwide, and then further adaptation resulted in the emergence of variants worldwide. For the influenza viruses several changes were required for growth and spread between humans, and the emergence of human H2N2 and H3N2 strains in 1957 and 1968 involved the acquisition of three or two new genomic segments, respectively. Adaptation to humans involved several viral genes including the hemagglutinin, the neuraminidase, and the replication proteins. The canine adaptation of the parvoviruses involved capsid protein changes altering the recognition of the host transferrin receptors, allowing canine transferrin receptor binding and its use as a receptor for cell infection.

Quail carry sialic acid receptors compatible with binding of avian and human influenza viruses

There is growing evidence that some terrestrial avian species may play a role in the genesis of influenza viruses with pandemic potential. In the present investigation, we examined whether quail, a widespread-farmed poultry, possess the proper characteristics for serving as an intermediate host for the zoonotic transmission of influenza viruses. Using a lectin-based staining based on specific agglutinins, we found that, in addition to the presence of sialic acid alpha2,3-galactose (SAalpha2,3-gal) linked receptors, there are abundant sialic acid alpha2,6-galactose (SAalpha2,6-gal) linked receptors in quail trachea and intestine. The presence of abundant SAalpha2,6-gal-linked receptors explains, at least in part, the circulation of avian influenza viruses with human-like receptor specificity in quail. In quail trachea, SAalpha2,3-gal linked receptors are present primarily in non-ciliated cells, while SAalpha2,6-gal linked receptors are localized predominantly on the surface of ciliated cells. In quail intestine, both types of receptors were found on epithelial cells as well as in crypts. In a solid-phase overlay binding assay, both avian and human influenza viruses bind to plasma membranes prepared from epithelial cells of quail trachea and intestine, strongly suggesting that these receptors are functional for binding of influenza viruses from different species. Together with previous observations, these results are consistent with the notion that quail could provide an environment for the spread of reassortants between avian and human influenza viruses, thus acting as a potential intermediate host.

Receptor-binding properties of swine influenza viruses isolated and propagated in MDCK cells

To study the receptor specificities of H1 and H3 influenza viruses isolated recently from pigs, we employed the analogues of natural receptors, namely sialyloligosaccharides conjugated with polyacrylamide in biotinylated and label free forms. All Madin-Darby canine kidney (MDCK) cell-propagated viruses with human H3 or classical swine H1 hemagglutinins bound only to Neu5Acalpha2-6Galbeta1-bearing polymers, and not to Neu5Acalpha2-3Galbeta1-bearing polymers. This receptor-binding pattern is typical for human influenza viruses and it differs from the previously described receptor-binding specificity of egg-adapted swine influenza viruses. Swine virus isolates with avian-like H1 and H3 hemagglutinins displayed distinct receptor specificity by binding to both Neu5Acalpha2-6Gal- and Neu5Acalpha2-3Gal-containing receptors. These viruses, as well as egg-adapted swine and turkey viruses with a classical swine HA, differed from the related duck viruses by increased affinity to sulfated sialyloligosaccaride, Su-SiaLe(x). Except for avian-like H3 viruses, none of the studied swine viruses bound to Neu5Gc-containing sialoglycopolymers, suggesting that binding to these sialic acid species abundantly expressed in pigs may not be essential for virus replication in this host.

Glycan microarray analysis of the hemagglutinins from modern and pandemic influenza viruses reveals different receptor specificities

Influenza A virus specificity for the host is mediated by the viral surface glycoprotein hemagglutinin (HA), which binds to receptors containing glycans with terminal sialic acids. Avian viruses preferentially bind to alpha2-3-linked sialic acids on receptors of intestinal epithelial cells, whereas human viruses are specific for the alpha2-6 linkage on epithelial cells of the lungs and upper respiratory tract. To define the receptor preferences of a number of human and avian H1 and H3 viruses, including the 1918 H1N1 pandemic strains, their hemagglutinins were analyzed using a recently described glycan array. The array, which contains 200 carbohydrates and glycoproteins, not only revealed clear differentiation of receptor preferences for alpha2-3 and/or alpha2-6 sialic acid linkage, but could also detect fine differences in HA specificity, such as preferences for fucosylation, sulfation and sialylation at positions 2 (Gal) and 3 (GlcNAc, GalNAc) of the terminal trisaccharide. For the two 1918 HA variants, the South Carolina (SC) HA (with Asp190, Asp225) bound exclusively alpha2-6 receptors, while the New York (NY) variant, which differed only by one residue (Gly225), had mixed alpha2-6/alpha2-3 specificity, especially for sulfated oligosaccharides. Only one mutation of the NY variant (Asp190Glu) was sufficient to revert the HA receptor preference to that of classical avian strains. Thus, the species barrier, as defined by the receptor specificity preferences of 1918 human viruses compared to likely avian virus progenitors, can be circumvented by changes at only two positions in the HA receptor binding site. The glycan array thus provides highly detailed profiles of influenza receptor specificity that can be used to map the evolution of new human pathogenic strains, such as the H5N1 avian influenza.

A live attenuated vaccine for Lassa fever made by reassortment of Lassa and Mopeia viruses

Lassa virus (LASV) and Mopeia virus (MOPV) are closely related Old World arenaviruses that can exchange genomic segments (reassort) during coinfection. Clone ML29, selected from a library of MOPV/LASV (MOP/LAS) reassortants, encodes the major antigens (nucleocapsid and glycoprotein) of LASV and the RNA polymerase and zinc-binding protein of MOPV. Replication of ML29 was attenuated in guinea pigs and nonhuman primates. In murine adoptive-transfer experiments, as little as 150 PFU of ML29 induced protective cell-mediated immunity. All strain 13 guinea pigs vaccinated with clone ML29 survived at least 70 days after LASV challenge without either disease signs or histological lesions. Rhesus macaques inoculated with clone ML29 developed primary virus-specific T cells capable of secreting gamma interferon in response to homologous MOP/LAS and heterologous MOPV and lymphocytic choriomeningitis virus. Detailed examination of two rhesus macaques infected with this MOPV/LAS reassortant revealed no histological lesions or disease signs. Thus, ML29 is a promising attenuated vaccine candidate for Lassa fever.

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

Receptor specificity of influenza A/H5 viruses including human 2003-04 isolates was studied. All but two isolates preserved high affinity to Sia2-3Gal (avian-like) receptors. However, two isolates (February, 2003, Hong Kong) demonstrated decreased affinity to Sia2-3Gal and moderate affinity to a Sia2-6Gal (human-like) receptors. These two viruses had a unique Ser227-Asn change in the hemagglutinin molecule. Thus, a single amino acid substitution can significantly alter receptor specificity of avian H5N1 viruses, providing them with an ability to bind to receptors optimal for human influenza viruses. Asian 2003-04 H5 isolates from chickens and humans demonstrated highest affinity to the sulfated trisaccharide Neu5Acalpha2-3Galbeta1-4(6-HSO3)GlcNAcbeta (Su-3'SLN) receptor but, in contrast to 1997 isolates, had increased affinity to fucosylated Su-3'SLN. American poultry H5 viruses also had increased affinity to Su-3'SLN. These data demonstrate that the genetic evolution of avian influenza A(H5N1) viruses is accompanied during adaptation to poultry by the evolution of their receptor specificity.

The virosome concept for influenza vaccines

There is a need for more efficacious inactivated influenza vaccines, since current formulations show suboptimal immunogenicity in at-risk populations, like the elderly. More effective vaccines are also urgently needed for an improved influenza pandemic preparedness. In this context, there is considerable interest in virosomes. Virosomes are virus-like particles, consisting of reconstituted influenza virus envelopes, lacking the genetic material of the native virus. Virosomes are produced from influenza virus through a detergent solubilization and removal procedure. Properly reconstituted virosomes retain the cell binding and membrane fusion properties of the native virus, mediated by the viral envelope glycoprotein haemagglutinin. These functional characteristics of virosomes form the basis for their enhanced immunogenicity. First, the repetitive arrangement of haemagglutinin molecules on the virosomal surface mediates a cooperative interaction of the antigen with Ig receptors on B lymphocytes, stimulating strong antibody responses. In addition, virosomes interact efficiently with antigen-presenting cells, such as dendritic cells, resulting in activation of T lymphocytes. In a murine model system, virosomes, as compared to conventional subunit vaccine, which consists of isolated influenza envelope glycoproteins, induce a more balanced T helper 1 versus T helper 2 response, virosomes in particular eliciting stronger T helper 1 responses than subunit vaccine. Also, as a result of fusion of the virosomes with the endosomal membrane, part of the virosomal antigen gains access to the major histocompatibility class I presentation pathway, thus priming cytotoxic T lymphocyte activity. Finally, virosomes represent an excellent platform for inclusion of lipophilic adjuvants for further stimulation of vaccine immunogenicity. By virtue of these characteristics, virosomes represent a promising novel class of inactivated influenza vaccines, which not only induce high virus-neutralizing antibody titres, but also prime the cellular arm of the immune system.

Avian influenza A (H5N1)

Since their reemergence in 2003, highly pathogenic avian influenza A (H5N1) viruses have reached endemic levels among poultry in several southeast Asian countries and have caused a still increasing number of more than 100 reported human infections with high mortality. These developments have ignited global fears of an imminent influenza pandemic. The current knowledge of the virology, clinical spectrum, diagnosis and treatment of human influenza H5N1 virus infections is reviewed herein.

A single amino acid substitution in 1918 influenza virus hemagglutinin changes receptor binding specificity

The receptor binding specificity of influenza viruses may be important for host restriction of human and avian viruses. Here, we show that the hemagglutinin (HA) of the virus that caused the 1918 influenza pandemic has strain-specific differences in its receptor binding specificity. The A/South Carolina/1/18 HA preferentially binds the alpha2,6 sialic acid (human) cellular receptor, whereas the A/New York/1/18 HA, which differs by only one amino acid, binds both the alpha2,6 and the alpha2,3 sialic acid (avian) cellular receptors. Compared to the conserved consensus sequence in the receptor binding site of avian HAs, only a single amino acid at position 190 was changed in the A/New York/1/18 HA. Mutation of this single amino acid back to the avian consensus resulted in a preference for the avian receptor.

Enhanced expression of an alpha2,6-linked sialic acid on MDCK cells improves isolation of human influenza viruses and evaluation of their sensitivity to a neuraminidase inhibitor

The extensive use of neuraminidase (NA) inhibitors to treat influenza virus infections mandates close monitoring for resistant variants. Cultured cells do not provide a reliable means of evaluating the susceptibility of human influenza virus isolates to NA inhibitors. That is, the growth of such viruses in cell lines (e.g., Madin-Darby canine kidney [MDCK] cells) is not inhibited by these drugs, even though their sialidase activity is drug-sensitive. Matrosovich et al. (J. Virol. 77:8418-8425, 2003) showed that an MDCK cell line overexpressing the human beta-galactoside alpha2,6-sialyltransferase I (ST6Gal I) gene has the potential to assess the sensitivity of human influenza virus isolates to NA inhibitors, based on studies with a limited number of viruses. Here, we asked whether clinical isolates of influenza virus are universally sensitive to an NA inhibitor (oseltamivir) in an MDCK cell line expressing the ST6Gal I gene. The sensitivity of viruses to oseltamivir correlated with the sensitivity of viral sialidase to the compound, demonstrating the potential utility of this modified cell line for detecting NA inhibitor-resistant viruses. Moreover, in ST6Gal I-overexpressing cells, the growth of human influenza viruses was up to 2 logs higher than in MDCK cells. We conclude that the human ST6Gal I-expressing MDCK cell line is useful not only for evaluating their sensitivity to NA inhibitors, but also for isolation of influenza viruses from clinical samples.

Sialobiology of influenza: molecular mechanism of host range variation of influenza viruses

The gene pool of influenza A viruses in aquatic birds provides all of the genetic diversity required for human and lower animals. Host range selection of the receptor binding specificity of the influenza virus hemagglutinin occurs during maintenance of the virus in different host cells that express different receptor sialo-sugar chains. In this paper, functional roles of the hemagglutinin and neuraminidase spikes of influenza viruses are described in the relation to 1) host range of influenza viruses, 2) receptor binding specificity of human and other animal influenza viruses, 3) recognition of sialyl sugar chains by Spanish influenza virus hemagglutinin, 4) highly pathogenic and potentially pandemic H5N1, H9N2, and H7N7 avian influenza viruses and molecular mechanism of host range variation of influenza viruses, 5) role of the neuraminidase spike for the host range of influenza viruses, and 6) Development of anti-influenza drugs.

Influenza: lessons from past pandemics, warnings from current incidents

Recent outbreaks of highly pathogenic avian influenza A virus infections (H5 and H7 subtypes) in poultry and in humans (through direct contact with infected birds) have had important economic repercussions and have raised concerns that a new influenza pandemic will occur in the near future. The eradication of pathogenic avian influenza viruses seems to be the most effective way to prevent influenza pandemics, although this strategy has not proven successful so far. Here, we review the molecular factors that contribute to the emergence of pandemic strains.

Improvement of influenza A/Fujian/411/02 (H3N2) virus growth in embryonated chicken eggs by balancing the hemagglutinin and neuraminidase activities, using reverse genetics

The H3N2 influenza A/Fujian/411/02-like virus strains that circulated during the 2003-2004 influenza season caused influenza epidemics. Most of the A/Fujian/411/02 virus lineages did not replicate well in embryonated chicken eggs and had to be isolated originally by cell culture. The molecular basis for the poor replication of A/Fujian/411/02 virus was examined in this study by the reverse genetics technology. Two antigenically related strains that replicated well in embryonated chicken eggs, A/Sendai-H/F4962/02 and A/Wyoming/03/03, were compared with the prototype A/Fujian/411/02 virus. A/Sendai differed from A/Fujian by three amino acids in the neuraminidase (NA), whereas A/Wyoming differed from A/Fujian by five amino acids in the hemagglutinin (HA). The HA and NA segments of these three viruses were reassorted with cold-adapted A/Ann Arbor/6/60, the master donor virus for the live attenuated type A influenza vaccines (FluMist). The HA and NA residues differed between these three H3N2 viruses evaluated for their impact on virus replication in MDCK cells and in embryonated chicken eggs. It was determined that replication of A/Fujian/411/02 in eggs could be improved by either changing minimum of two HA residues (G186V and V226I) to increase the HA receptor-binding ability or by changing a minimum of two NA residues (E119Q and Q136K) to lower the NA enzymatic activity. Alternatively, recombinant A/Fujian/411/02 virus could be adapted to grow in eggs by two amino acid substitutions in the HA molecule (H183L and V226A), which also resulted in the increased HA receptor-binding activity. Thus, the balance between the HA and NA activities is critical for influenza virus replication in a different host system. The HA or NA changes that increased A/Fujian/411/02 virus replication in embryonated chicken eggs were found to have no significant impact on antigenicity of these recombinant viruses. This study demonstrated that the reverse genetics technology could be used to improve the manufacture of the influenza vaccines.

Influenza virus neuraminidase contributes to secondary bacterial pneumonia

Secondary bacterial pneumonia is a common cause of death during influenza epidemics. We hypothesized that virus-specific factors could contribute to differences in annual excess mortality. Recombinant influenza viruses with neuraminidases from representative strains from the past 50 years were created and characterized. The specific level of their neuraminidase activity correlated with their ability to support secondary bacterial pneumonia. Recombinant viruses with neuraminidases from 1957 and 1997 influenza strains had the highest level of activity, whereas a virus with the neuraminidase from a 1968 strain had the lowest level of activity. The high level of activity of the neuraminidase from the 1957 strain, compared with that of other neuraminidases, more strongly supported the adherence of Streptococcus pneumoniae and the development of secondary bacterial pneumonia in a mouse model. These data lend support to our hypothesis that the influenza virus neuraminidase contributes to secondary bacterial pneumonia and subsequent excess mortality.

Standardization of a duplex RT-PCR for the detection of Influenza A and Newcastle disease viruses in migratory birds

Influenza A and Newcastle disease viruses are pathogens of social and economical importance known to be disseminated throughout the world by migratory birds. Many efforts have been made to control the introduction of these viruses into new environments, and complete world surveillance has yet to be achieved. Virus isolation and immunofluorescence techniques are time consuming, have inherent limited sensitivity and present a lack of host cells permissive universally to all Influenza A viruses. In this paper, the use of a duplex RT-PCR is described capable of sorting out any NDV and Influenza A virus strain simultaneously in oral and cloacal swab specimens. This method includes fluorescent detection of amplicons that provide accurate analysis of many DNA fragments within one base discrimination. Reference viruses were used for standardization of the assay and samples from wild-type viruses were screened, with four positive results for Influenza A detected in migratory birds captured in the state of Sao Paulo. This screening test can be considered a first step for further studies of these viruses circulating in avian species in Brazil, and hopefully will contribute to broaden the sample spectrum from wild birds, leading to a better understanding of these viruses and their participation in the southeastern region of the country.

Receptor specificity of influenza viruses from birds and mammals: new data on involvement of the inner fragments of the carbohydrate chain

We studied receptor-binding properties of influenza virus isolates from birds and mammals using polymeric conjugates of sialooligosaccharides terminated with common Neu5Ac alpha2-3Gal beta fragment but differing by the structure of the inner part of carbohydrate chain. Viruses isolated from distinct avian species differed by their recognition of the inner part of oligosaccharide receptor. Duck viruses displayed high affinity for receptors having beta1-3 rather than beta1-4 linkage between Neu5Ac alpha2-3Gal-disaccharide and penultimate N-acetylhexosamine residue. Fucose and sulfate substituents at this residue had negative and low effect, respectively, on saccharide binding to duck viruses. By contrast, gull viruses preferentially bound to receptors bearing fucose at N-acetylglucosamine residue, whereas chicken and mammalian viruses demonstrated increased affinity for oligosaccharides that harbored sulfo group at position 6 of (beta1-4)-linked GlcNAc. These data suggest that although all avian influenza viruses preferentially bind to Neu5Ac alpha2-3Gal-terminated receptors, the fine receptor specificity of the viruses varies depending on the avian species. Further studies are required to determine whether observed host-dependent differences in the receptor specificity of avian viruses can affect their ability to infect humans.

Use of real-time reverse transcriptase polymerase chain reaction assay and cell culture methods for detection of swine influenza A viruses

OBJECTIVE

To evaluate sensitivity and specificity of a real-time reverse transcriptase polymerase chain reaction (RT-PCR) assay performed on pooled nasal swab specimens, compared with virus isolation performed on individual nasal swab specimens by use of 2 cell culture lines for detection of swine influenza A viruses.

SAMPLE POPULATION

900 nasal swab specimens obtained from pigs at an abattoir and 62 nasal swab specimens submitted for diagnostic testing.

PROCEDURES

Primers were chosen to amplify a conserved portion of the influenza virus matrix gene. Assay sensitivity was initially determined by testing serial dilutions of various subtypes of swine influenza viruses. Sensitivity and specificity were confirmed by use of nasal swab specimens with or without addition of known concentrations of influenza virus and further validated by testing nasal swab specimens obtained through an abattoir surveillance program or submitted for diagnostic testing. Aliquots of specimens were pooled in sets of 10, and results of real-time RT-PCR assays were compared with results of virus isolation of individual specimens in Madin Darby canine kidney (MDCK) and mink lung (Mv1Lu) cells.

RESULTS

Real-time RT-PCR assay was highly specific (100%) and sensitive (88% to 100%). Among the 16 viruses isolated, 3 grew only in Mv1Lu cells and 3 grew only in MDCK cells.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicate that real-time RT-PCR assay is a fast and accurate test for screening numerous nasal swab specimens for swine influenza virus. Some viruses were isolated in only MDCK or Mv1Lu cells, indicating that use of >1 cell line may be required to isolate a broad range of influenza A viruses.

H3N2 influenza virus transmission from swine to turkeys, United States

Swinelike H3N2 influenza viruses were isolated from two geographically distinct turkey farms in the United States.

In 1998, a novel H3N2 reassortant virus emerged in the United States swine population. We report the interspecies transmission of this virus to turkeys in two geographically distant farms in the United States in 2003. This event is of concern, considering the reassortment capacity of this virus and the susceptibility of turkey to infection by avian influenza viruses. Two H3N2 isolates, A/turkey/NC/16108/03 and A/turkey/MN/764/03, had 98.0% to 99.9% nucleotide sequence identity to each other in all eight gene segments. All protein components of the turkey isolates had 97% to 98% sequence identity to swine H3N2 viruses, thus demonstrating interspecies transmission from pigs to turkeys. The turkey isolates were better adapted to avian hosts than were their closest swine counterparts, which suggests that the viruses had already begun to evolve in the new host. The isolation of swine-like H3N2 influenza viruses from turkeys raises new concerns for the generation of novel viruses that could affect humans.

Pathogenesis of Swine Influenza Virus Subtype H1N2 Infection in Pigs

The purpose of this study was to elucidate pathogenesis and viral distribution in pigs infected with swine influenza virus subtype H1N2, over a period of 10 days, by morphometric analysis and in-situ hybridization. Fifteen colostrum-deprived pigs aged 3 weeks were inoculated intranasally with virus. Pneumonia was severe at 1 day post-inoculation (dpi), moderate at 3 and 5 dpi, and mild at 7 and 10 dpi. The pulmonary lesion score was correlated with the score of cells positive by in-situ hybridization for swine influenza virus (r(s)= 0.9114, P< 0.05). The distribution of swine influenza virus varied according to the duration of infection. At 1 and 3 dpi, hybridization signals were detected mainly in the bronchial and bronchiolar epithelial cells, but they were detected mainly in the pneumocytes and macrophages (alveolar and interstitial) at 7 and 10 dpi. The results confirmed that swine influenza virus subtype H1N2, isolated in Korea, is a virulent pathogen causing severe pneumonia.

No apoptotic deaths and different levels of inductions of inflammatory cytokines in alveolar macrophages infected with influenza viruses

Influenza viruses are reported to infect mainly the respiratory tract epithelium of hosts. Our studies in a pig model show that influenza A viruses infect alveolar macrophages that constitutively reside in the respiratory tract, without causing apoptosis. Tumor necrosis factor alpha was the inflammatory cytokine most highly induced in these macrophages. In vivo, alveolar macrophages infected with human H3N2 influenza virus showed greater expression of tumor necrosis factor alpha than did alveolar macrophages infected with human H1N1 influenza virus. Induction of specific inflammatory cytokine such as TNF-alpha is a polygenic trait that involves the HA and NA genes. Markedly elevated expression of tumor necrosis factor alpha may be responsible for the high mortality rate caused by H3N2 influenza virus infection in elderly patients.

Avian influenza viruses and their implication for human health

Widespread outbreaks of avian influenza in domestic fowl throughout eastern Asia have reawakened concern that avian influenza viruses may again cross species barriers to infect the human population and thereby initiate a new influenza pandemic. Simultaneous infection of humans (or swine) by avian influenza viruses in the presence of human influenza viruses could theoretically generate novel influenza viruses with pandemic potential as a result of reassortment of genome subunits between avian and mammalian influenza viruses. These hybrid viruses would have the potential to express surface antigens from avian viruses to which the human population has no preexisting immunity. This article reviews current knowledge of the routes of transmission of avian influenza A viruses to humans, places the risk of appearance of a new pandemic influenza virus in perspective, and describes the recently observed epidemiology and clinical syndromes of avian influenza in humans.

Molecular constraints to interspecies transmission of viral pathogens

The successful replication of a viral pathogen in a host is a complex process involving many interactions. These interactions develop from the coevolution of pathogen and host and often lead to a species specificity of the virus that can make interspecies transmissions difficult. Nevertheless, viruses do sporadically cross species barriers into other host populations, including humans. In zoonotic infections, many of these interspecies transfer events are dead end, where transmission is confined only to the animal-to-human route but sometimes viruses adapt to enable spread from human to human. A pathogen must overcome many hurdles to replicate successfully in a foreign host. The viral pathogen must enter the host cell, replicate with the assistance of host factors, evade inhibitory host products, exit the first cell and move on to the next, and possibly leave the initial host and transmit to another. Each of these stages may require adaptive changes in the pathogen. Although the factors that influence each stage of the replication and transmission of most agents have not been resolved, the genomics of both hosts and pathogens are now at hand and we have begun to understand some of the molecular changes that enable some viruses to adapt to a new host.

Characterization of avian H3N3 and H1N1 influenza A viruses isolated from pigs in Canada

H3N3 and H1N1 influenza A viruses were isolated from Canadian pigs in 2001 and 2002. These viruses are phylogenetically related to waterfowl viruses and antigenically distinct from reference swine influenza viruses. The isolation of these viruses reemphasizes the potential for interspecies transmission of influenza viruses from waterfowl to pigs in North America.

Isolation and identification of swine influenza recombinant A/Swine/Shandong/1/2003(H9N2) virus

Ten influenza virus isolates were obtained from infected pigs from different places in Shandong province showing clinical symptoms from October 2002 to January 2003. All 10 isolates were identified in China's National Influenza Research Center as influenza A virus of H9N2 subtype. The complete genome of one isolate, designated A/Swine/Shandong/1/2003(H9N2), was sequenced and compared with sequences available in GenBank. The results of analyses indicated that the sequence of A/Swine/Shandong/1/2003(H9N2) was similar to those of several chicken influenza viruses and duck influenza viruses recently prevalent in South China. According to phylogenetic analysis of the complete gene sequences, A/Swine/Shandong/1/2003(H9N2) possibly originated from the reassortment of chicken influenza viruses and duck influenza viruses. It was found that the amino acid sequence at the HA cleavage site in Sw/SD/1/2003 is R-S-L-R-G, differing clearly from that of other H9N2 subtype isolates of swine influenza and avian influenza, which is R-S-S-R-G.

Antiviral effect of Saccharomyces cerevisiae beta-glucan to swine influenza virus by increased production of interferon-gamma and nitric oxide

The aim of these experiments was to investigate the potential antiviral effect of Saccharomyces cerevisiae beta-glucan on the pneumonia induced by swine influenza virus (SIV). Forty colostrum-deprived 5-day-old piglets were randomly divided into four groups of 10. The 20 pigs in groups 1 and 2 were administered Saccharomyces cerevisiae beta-glucan orally (50 mg/day/pig; En-Bio Technology Co., Ltd) for 3 days before SIV infection and those in groups 3 and 4 were given culture medium/diluent alone. Groups 1 and 3 were inoculated intranasally with 3 ml of tissue culture fluid containing 2 x 10(6) tissue culture infective doses 50% (TCID(50))/ml of SIV and those in groups 2 and 4 were exposed in the same manner to uninfected cell culture supernatant. The microscopic lung lesions induced by SIV infection (group 1 pigs) were significantly more severe than those induced by infection in animals pre-administered beta-glucan (group 3) (P < 0.05). Significantly more SIV nucleic acid was detected in the lungs of pigs experimentally infected with SIV only (group 1) at 5, 7 and 10 days post-inoculation (dpi) compared with lungs from pigs pre-administered beta-glucan and infected with SIV (group 3) (P < 0.05). The concentrations of interferon-gamma (IFN-gamma) and nitric oxide (NO) in bronchoalveolar lavage fluid from pigs pre-administered beta-glucan and infected with SIV (group 3) were significantly higher than for any other group at 7 and 10 dpi for IFN-gamma, and at 5, 7 and 10 dpi for NO (P < 0.05). Saccharomyces cerevisiae beta-glucan reduced the pulmonary lesion score and viral replication rate in SIV-infected pigs. These findings support the potential application of beta-glucan as prophylactic/treatment agent in influenza virus infection.

The index influenza A virus subtype H5N1 isolated from a human in 1997 differs in its receptor-binding properties from a virulent avian influenza virus

To gain insight into the events that occur when avian influenza viruses are transmitted to humans, the receptor-binding properties of the index H5N1 influenza virus isolated from a human in 1997 and the A/turkey/Ontario/7732/66 (H5N9) virus were compared, by using a haemadsorption assay. Cells expressing the haemagglutinin (HA) of the human isolate were adsorbed by both chicken red blood cells (RBCs) and human RBCs; those expressing the avian virus HA were only adsorbed by chicken RBCs. These results indicate that human and avian influenza virus H5 HAs differ in their recognition of sialyloligosaccharides on the RBCs of different animal species. Mutational analyses indicated that differences in both the oligosaccharide chains and in the amino acid sequences around the HA receptor-binding site were responsible for this difference in receptor binding. These data further support the concept that alteration in receptor recognition is important for replication of avian viruses in humans.

Human and avian influenza viruses target different cell types in cultures of human airway epithelium

The recent human infections caused by H5N1, H9N2, and H7N7 avian influenza viruses highlighted the continuous threat of new pathogenic influenza viruses emerging from a natural reservoir in birds. It is generally believed that replication of avian influenza viruses in humans is restricted by a poor fit of these viruses to cellular receptors and extracellular inhibitors in the human respiratory tract. However, detailed mechanisms of this restriction remain obscure. Here, using cultures of differentiated human airway epithelial cells, we demonstrated that influenza viruses enter the airway epithelium through specific target cells and that there were striking differences in this respect between human and avian viruses. During the course of a single-cycle infection, human viruses preferentially infected nonciliated cells, whereas avian viruses as well as the egg-adapted human virus variant with an avian virus-like receptor specificity mainly infected ciliated cells. This pattern correlated with the predominant localization of receptors for human viruses (2-6-linked sialic acids) on nonciliated cells and of receptors for avian viruses (2-3-linked sialic acids) on ciliated cells. These findings suggest that although avian influenza viruses can infect human airway epithelium, their replication may be limited by a nonoptimal cellular tropism. Our data throw light on the mechanisms of generation of pandemic viruses from their avian progenitors and open avenues for cell level-oriented studies on the replication and pathogenicity of influenza virus in humans.

The Structure and Receptor Binding Properties of the 1918 Influenza Hemagglutinin

The 1918 influenza pandemic resulted in about 20 million deaths. This enormous impact, coupled with renewed interest in emerging infections, makes characterization of the virus involved a priority. Receptor binding, the initial event in virus infection, is a major determinant of virus transmissibility that, for influenza viruses, is mediated by the hemagglutinin (HA) membrane glycoprotein. We have determined the crystal structures of the HA from the 1918 virus and two closely related HAs in complex with receptor analogs. They explain how the 1918 HA, while retaining receptor binding site amino acids characteristic of an avian precursor HA, is able to bind human receptors and how, as a consequence, the virus was able to spread in the human population.

Evaluation of transmission of swine influenza type A subtype H1N2 virus in seropositive pigs

OBJECTIVE

To examine clinical signs, virus infection and shedding, and transmission of swine influenza virus (SIV) subtype H1N2 among seropositive pigs.

ANIMALS

Eighteen 3-week-old pigs with maternal antibodies against SIV subtypes H1N1, H3N2, and H1N2.

PROCEDURE

Ten pigs (principal) were inoculated intranasally with subtype H1N2 and 2 groups of contact pigs (n = 4) each were mixed with principal pigs on day 7 (group 1) or 28 (group 2). Two principal pigs each were necropsied on days 4, 14, 21, 28, and 42 days after inoculation. Four pigs in each contact group were necropsied 35 and 14 days after contact. Virus excretion was evaluated after inoculation or contact. Lung lesions and the presence of SIV in various tissues were examined.

RESULTS

Mild coughing and increased rectal temperature were observed in principal pigs but not in contact pigs. Nasal virus shedding was detected in all principal pigs from day 2 for 3 to 5 days, in group 1 pigs from day 2 for 4 to 9 days after contact, and in group 2 pigs from day 4 for 2 to 6 days after contact. Trachea, lung, and lymph node specimens from infected pigs contained virus. Antibody titers against all 3 subtypes in all pigs gradually decreased.

CONCLUSIONS AND CLINICAL RELEVANCE

Protection from viral infection and shedding was not observed in pigs with maternal antibodies, but clinical disease did not develop. Vaccination programs and good management practices should be considered for control of SIV subtype H1N2 infection on swine farms.

Interactions of influenza A virus with sialic acids present on porcine surfactant protein D

Pigs can be infected with both human and avian influenza A virus (IAV) strains and are therefore considered to be important intermediates in the emergence of new IAV strains due to mixing of viral genes derived from human, avian, or porcine influenza viruses. These reassortant strains may have potential to cause pandemic influenza outbreaks in humans. The innate immune response against IAV plays a significant role in containment of IAV in the airways. We studied the interactions of IAV with porcine surfactant protein D (pSP-D), an important component of this first line defense system. Hemagglutination inhibition analysis shows that the distinct interactions of pSP-D with IAV mediated by the N-linked carbohydrate moiety in the carbohydrate recognition domain of pSP-D depend on the terminal sialic acids (SAs) present on this carbohydrate. Analysis by both lectin staining and by cleavage with linkage-specific sialidases shows that the carbohydrate of pSP-D is exclusively sialylated with alpha(2,6)-linked SAs, in contrast to surfactant protein A, which contains both alpha(2,3)- and alpha(2,6)-linked SAs on its N-linked carbohydrate. Enzymatic modification of the SA-linkages present on pSP-D demonstrates that the type of SA-linkage is important for its hemagglutination-inhibitory activity, and correlates with receptor-binding specificity of the IAV strains. The SAs present on pSP-D appear especially important for interactions with poorly glycosylated IAV strains. It remains to be elucidated to what extent the unique sialylation profile of pSP-D is involved in host range control of IAV in pigs, and whether it facilitates adaptation of avian or human IAV strains that can contribute to the production of reassortant strains in pigs.

Differences between influenza virus receptors on target cells of duck and chicken and receptor specificity of the 1997 H5N1 chicken and human influenza viruses from Hong Kong

To study whether influenza virus receptors in chickens differ from those in other species, we compared the binding of lectins and influenza viruses with known receptor specificity to cell membranes and gangliosides from epithelial tissues of ducks, chickens, and African green monkeys. We found that chicken cells contained Neu5Ac alpha(2-6)Gal-terminated receptors recognized by Sambucus nigra lectin and by human viruses. This finding explains how some recent H9N2 viruses replicate in chickens despite their human virus-like receptor specificity. Duck virus bound to gangliosides with short sugar chains that were abundant in duck intestine. Human and chicken viruses did not bind to these gangliosides and bound more strongly than duck virus to gangliosides with long sugar chains that were found in chicken intestinal and monkey lung tissues. Chicken and duck viruses also differed by their ability to recognize the structure of the third sugar moiety in Sia2-3Gal-terminated receptors. Chicken viruses preferentially bound to Neu5Ac alpha(2-3)Gal beta(1-4)GlcNAc-containing synthetic sialylglycopolymer, whereas duck viruses displayed a higher affinity for Neu5Ac alpha(2-3)Gal beta(1-3)GalNAc-containing polymer. Our data indicate that sialyloligosaccharide receptors in different avian species are not identical and provide a potential explanation for the differences between the hemagglutinin and neuraminidase proteins of duck and chicken viruses.

Human-specific regulation of alpha 2-6-linked sialic acids

Many microbial pathogens and toxins recognize animal cells via cell surface sialic acids (Sias) that are alpha 2-3- or alpha 2-8-linked to the underlying glycan chain. Human influenza A/B viruses are unusual in preferring alpha 2-6-linked Sias, undergoing a switch from alpha 2-3 linkage preference during adaptation from animals to humans. This correlates with the expression of alpha 2-6-linked Sias on ciliated human airway epithelial target cells and of alpha 2-3-linked Sias on secreted soluble airway mucins, which are unable to inhibit virus binding. Given several known differences in Sia biology between humans and apes, we asked whether this pattern of airway epithelial Sia linkages is also human-specific. Indeed, we show that since the last common ancestor with apes, humans underwent a concerted bidirectional switch in alpha 2-6-linked Sia expression between airway epithelial cell surfaces and secreted mucins. This can explain why the chimpanzee appears relatively resistant to experimental infection with human Influenza viruses. Other tissues showed additional examples of human-specific increases or decreases in alpha 2-6-linked Sia expression and only one example of a change specific to certain great apes. Furthermore, while human and great ape leukocytes both express alpha 2-6-linked Sias, only human erythrocytes have markedly up-regulated expression. These cell type-specific changes in alpha 2-6-Sia expression during human evolution represent another example of a human-specific change in Sia biology. Because the data set involves multiple great apes, we can also conclude that Sia linkage expression patterns can be conserved during millions of years of evolution within some vertebrate taxa while undergoing sudden major changes in other closely related ones.

The Impact of Avian Influenza Viruses on Public Health

In the late 1990s, H5N1 and H9N2 avian influenza viruses caused respiratory infections in humans in Hong Kong. Exposure to domestic poultry in live-bird markets was significantly associated with human H5N1 disease. Seroepidemiologic studies conducted among contacts of H5N1-infected persons determined that human-to-human transmission of the avian H5N1 viruses occurred but was rare. The relatively high rates of H5 and H9 antibody seroprevalence among Hong Kong poultry workers in 1997 highlight the potential for avian viruses to transmit to humans, particularly those with occupational exposure. Such transmission increases the likelihood of reassortment between a currently circulating human virus and an avian virus and thus the creation of a strain with pandemic potential.

Porcine pulmonary collectins show distinct interactions with influenza A viruses: role of the N-linked oligosaccharides in the carbohydrate recognition domain

Influenza A virus (IAV) infections are a major cause of respiratory disease of humans and animals. Pigs can serve as important intermediate hosts for transmission of avian IAV strains to humans, and for the generation of reassortant strains; this may result in the appearance of new pandemic IAV strains in humans. We have studied the role of the porcine lung collectins surfactant proteins D and A (pSP-D and pSP-A), two important components of the innate immune response against IAV. Hemagglutination inhibition assays revealed that both pSP-D and pSP-A display substantially greater inhibitory activity against IAV strains isolated from human, swine, and horse, than lung collectins from other animal species. The more potent activity of pSP-D results from interactions mediated by the asparagine-linked oligosaccharide located in the carbohydrate recognition domain of pSP-D, which is absent in SP-Ds from other species characterized to date. Presence of this sialylated oligosaccharide moiety enhances the anti-influenza activity of pSP-D, as demonstrated by assays of viral aggregation, inhibition of infectivity, and neutrophil response to IAV. The greater hemagglutination inhibitory activity of pSP-A is due to porcine-specific structural features of the conserved asparagine-linked oligosaccharide in the carbohydrate recognition domain of SP-A. A more efficient lung collectin-mediated immune response against IAV in pigs may play a role in providing conditions by which pigs can act as "mixing vessel" hosts that can lead to the production of reassortant, pandemic strains of IAV.