Correlation of pathogenicity and gene constellation of an influenza A virus (fowl plague). I. Exchange of a single gene

Emergence of highly pathogenic H5N2 and H7N1 influenza A viruses from low pathogenic precursors by serial passage in ovo

Highly pathogenic (HPAI) strains emerge from their low pathogenic (LPAI) precursors and cause severe disease in poultry with enormous economic losses, and zoonotic potential. Understanding the mechanisms involved in HPAI emergence is thus an important goal for risk assessments. In this study ostrich-origin H5N2 and H7N1 LPAI progenitor viruses were serially passaged seventeen times in 14-day old embryonated chicken eggs and Ion Torrent ultra-deep sequencing was used to monitor the incremental changes in the consensus genome sequences. Both virus strains increased in virulence with successive passages, but the H7N1 virus attained a virulent phenotype sooner. Mutations V63M, E228V and D272G in the HA protein, Q357K in the nucleoprotein (NP) and H155P in the neuraminidase protein correlated with the increased pathogenicity of the H5N2 virus; whereas R584H and L589I substitutions in the polymerase B2 protein, A146T and Q220E in HA plus D231N in the matrix 1 protein correlated with increased pathogenicity of the H7N1 virus in embryos. Enzymatic cleavage of HA protein is the critical virulence determinant, and HA cleavage site motifs containing multibasic amino acids were detected at the sub-consensus level. The motifs PQERRR/GLF and PQRERR/GLF were first detected in passages 11 and 15 respectively of the H5N2 virus, and in the H7N1 virus the motifs PELPKGKK/GLF and PELPKRR/GLF were detected as early as passage 7. Most significantly, a 13 nucleotide insert of unknown origin was identified at passage 6 of the H5N2 virus, and at passage 17 a 42 nucleotide insert derived from the influenza NP gene was identified. This is the first report of non-homologous recombination at the HA cleavage site in an H5 subtype virus. This study provides insights into how HPAI viruses emerge from low pathogenic precursors and demonstrated the pathogenic potential of H5N2 and H7N1 strains that have not yet been implicated in HPAI outbreaks.

A nonpathogenic duck-origin H9N2 influenza A virus adapts to high pathogenicity in mice

H9N2 influenza viruses continue to circulate in wild birds and poultry in Eurasian countries and have repeatedly infected mammals, including pigs and humans, posing a significant threat to public health. To understand the adaptation of H9N2 influenza viruses to mammals, we serially passaged a nonpathogenic duck-origin H9N2 influenza virus, A/duck/Jiangsu/1/2008 (DK1), in mouse lungs. Increased virulence was detectable after five sequential passages, and a highly pathogenic mouse-adapted strain (DK1-MA) with a 50% mouse lethal dose of 10(2.37) 50% egg infectious dose was obtained after 18 passages. DK1-MA grew faster and reached significantly higher titers than DK1 in mouse lungs and could sporadically spread to other organs. Moreover, DK1-MA induced a greater magnitude of pulmonary edema and higher levels of inflammatory cellular infiltration in bronchoalveolar lavage fluids than DK1 did. Genomic sequence alignment revealed eight amino acid substitutions (HA-L80F, HA-N193D, NA-A27T, PB2-F404L, PA-D3V, PA-S225R, NP-V105M, M1-A166V) in six viral proteins of DK1-MA compared with DK1 virus. Except for HA-L80F, the other seven substitutions were all located in known functional regions involved in interaction of viral proteins or interaction between the virus and host factors. Taken together, our results suggest that multiple amino acid substitutions may be involved in the adaptation of H9N2 avian influenza virus to mice, resulting in lethal infection, enhanced viral replication, severe pulmonary edema, and excessive inflammatory cellular infiltration in lungs. These observations provide helpful insights into the pathogenic potential of H9N2 avian influenza viruses that could pose threats to human health in the future.

Avian influenza virus h3 hemagglutinin may enable high fitness of novel human virus reassortants

Reassortment of influenza A virus genes enables antigenic shift resulting in the emergence of pandemic viruses with novel hemagglutinins (HA) acquired from avian strains. Here, we investigated whether historic and contemporary avian strains with different replication capacity in human cells can donate their hemagglutinin to a pandemic human virus. We performed double-infections with two avian H3 strains as HA donors and a human acceptor strain, and determined gene compositions and replication of HA reassortants in mammalian cells. To enforce selection for the avian virus HA, we generated a strictly elastase-dependent HA cleavage site mutant from A/Hong Kong/1/68 (H3N2) (Hk68-Ela). This mutant was used for co-infections of human cells with A/Duck/Ukraine/1/63 (H3N8) (DkUkr63) or the more recent A/Mallard/Germany/Wv64-67/05 (H3N2) (MallGer05) in the absence of elastase but presence of trypsin. Among 21 plaques analyzed from each assay, we found 12 HA reassortants with DkUkr63 (4 genotypes) and 14 with MallGer05 (10 genotypes) that replicated in human cells comparable to the parental human virus. Although DkUkr63 replicated in mammalian cells at a reduced level compared to MallGer05 and Hk68, it transmitted its HA to the human virus, indicating that lower replication efficiency of an avian virus in a mammalian host may not constrain the emergence of viable HA reassortants. The finding that HA and HA/NA reassortants replicated efficiently like the human virus suggests that further HA adaptation remains a relevant barrier for emergence of novel HA reassortants.

Multiple amino acid substitutions are involved in the adaptation of H9N2 avian influenza virus to mice

To explore adaptation of avian influenza virus to mice we previously performed serial lung-to-lung passages of the influenza A/Chicken/Jiangsu/7/2002 (H9N2) strain, resulting in the isolation of a variant influenza strain lethal for mice. We now report that virulence correlates with improved growth characteristics on mammalian cells and extended tissue tropism in vivo. Sequencing of the complete genomes of the wild-type and mouse-adapted viruses revealed 25 amino acid substitutions. Some were found to reiterate known substitutions in human and swine H9N2 influenza isolates. Functions affected include nuclear localization signals and sites of protein and RNA interaction, while others are known determinants of pathogenicity and host specificity such as the viral polymerase PB2 E627K substitution. These observations suggest that enhanced growth characteristics and modified cell tropism may contribute to increased virulence in mice. We conclude that multiple amino acid substitutions are likely to be involved in the adaptation of H9N2 avian influenza virus to mice.

Chapter 7 Orthomyxovirus infections

The earth is a unity for influenza A virus in a manner not yet found for probably any other parasite and epidemics occur in all inhabited parts of the globe regardless of latitude, longitude, altitude, climate, rainfall, temperature, humidity, race and sex. Influenza A is the classic pandemic virus infection of man and influenza B virus also can cause sharp outbreaks, resulting in significant mortality. An overwhelming amount of data has accumulated on the biochemistry, cell biology, and epidemiology of influenza, but prospects of control of epidemics in the near future are dim. Meanwhile, a holding operation can be achieved using inactivated vaccine and rimantadine (100 mg/daily) in special risk groups in the population until new more effective vaccines and broad spectrum antivirals (active against influenza A and B virus) are developed. Research work is centered on biotechnology to produce immunogenic peptides and proteins and more logical searches for antivirals using amino acid sequence data and also virus specific enzymes such as the virion transcriptase as targets.

Genetic compatibility and virulence of reassortants derived from contemporary avian H5N1 and human H3N2 influenza A viruses

The segmented structure of the influenza virus genome plays a pivotal role in its adaptation to new hosts and the emergence of pandemics. Despite concerns about the pandemic threat posed by highly pathogenic avian influenza H5N1 viruses, little is known about the biological properties of H5N1 viruses that may emerge following reassortment with contemporary human influenza viruses. In this study, we used reverse genetics to generate the 63 possible virus reassortants derived from H5N1 and H3N2 viruses, containing the H5N1 surface protein genes, and analyzed their viability, replication efficiency, and mouse virulence. Specific constellations of avian–human viral genes proved deleterious for viral replication in cell culture, possibly due to disruption of molecular interaction networks. In particular, striking phenotypes were noted with heterologous polymerase subunits, as well as NP and M, or NS. However, nearly one-half of the reassortants replicated with high efficiency in vitro, revealing a high degree of compatibility between avian and human virus genes. Thirteen reassortants displayed virulent phenotypes in mice and may pose the greatest threat for mammalian hosts. Interestingly, one of the most pathogenic reassortants contained avian PB1, resembling the 1957 and 1968 pandemic viruses. Our results reveal the broad spectrum of phenotypes associated with H5N1/H3N2 reassortment and a possible role for the avian PB1 in the emergence of pandemic influenza. These observations have important implications for risk assessment of H5N1 reassortant viruses detected in surveillance programs.

The influenza pandemics of 1957 and 1968 were caused by hybrid viruses consisting of a mixture of human and avian influenza genes. The introduction of avian genes resulted in a sudden change of the virus surface antigens, allowing its worldwide spread due to lack of immunity in the population. The highly pathogenic avian influenza H5N1 virus has continued its spread in domestic and wild birds in Asia, Europe, and Africa. Although H5N1 infection in humans is rare and person-to-person transmission is very inefficient, the steady accumulation of human cases has raised concern over the possible reassortment between H5N1 and human seasonal influenza resulting in a virus with new surface antigens and pandemic potential. In this study, we used recombinant DNA technology to generate a systematic collection of hybrid viruses (with genes from human and avian viruses) bearing H5N1 surface antigens and analyzed their properties in cell culture and in mice. The H5N1 hybrid viruses revealed a broad range of viability and multiplication capacity in cell cultures. In addition, several H5N1 hybrid viruses were highly virulent in mice. Results from this systematic analysis provide important insight to support risk assessment of reassortant H5N1 avian influenza viruses.

Inefficient transmission of H5N1 influenza viruses in a ferret contact model

The abilities to infect and transmit efficiently among humans are essential for a novel influenza A virus to cause a pandemic. To evaluate the pandemic potential of widely disseminated H5N1 influenza viruses, a ferret contact model using experimental groups comprised of one inoculated ferret and two contact ferrets was used to study the transmissibility of four human H5N1 viruses isolated from 2003 to 2006. The effects of viral pathogenicity and receptor binding specificity (affinity to synthetic sialosaccharides with alpha2,3 or alpha2,6 linkages) on transmissibility were assessed. A/Vietnam/1203/04 and A/Vietnam/JP36-2/05 viruses, which possess "avian-like" alpha2,3-linked sialic acid (SA) receptor specificity, caused neurological symptoms and death in ferrets inoculated with 10(3) 50% tissue culture infectious doses. A/Hong Kong/213/03 and A/Turkey/65-596/06 viruses, which show binding affinity for "human-like" alpha2,6-linked SA receptors in addition to their affinity for alpha2,3-linked SA receptors, caused mild clinical symptoms and were not lethal to the ferrets. No transmission of A/Vietnam/1203/04 or A/Turkey/65-596/06 virus was detected. One contact ferret developed neutralizing antibodies to A/Hong Kong/213/03 but did not exhibit any clinical signs or detectable virus shedding. In two groups, one of two naïve contact ferrets had detectable virus after 6 to 8 days when housed together with the A/Vietnam/JP36-2/05 virus-inoculated ferrets. Infected contact ferrets showed severe clinical signs, although little or no virus was detected in nasal washes. This limited virus shedding explained the absence of secondary transmission from the infected contact ferret to the other naïve ferret that were housed together. Our results suggest that despite their receptor binding affinity, circulating H5N1 viruses retain molecular determinants that restrict their spread among mammalian species.

Intersegmental recombination between the haemagglutinin and matrix genes was responsible for the emergence of a highly pathogenic H7N3 avian influenza virus in British Columbia

In February 2004 a highly pathogenic avian influenza (HPAI) outbreak erupted in British Columbia. Investigations indicated that the responsible HPAI H7N3 virus emerged suddenly from a low pathogenic precursor. Analysis of the haemagglutinin (HA) genes of the low and high pathogenic viruses isolated from the index farm revealed the only difference to be a 21 nt insert at the HA cleavage site of the highly pathogenic avian influenza virus. It was deduced that this insert most probably arose as a result of non-homologous recombination between the HA and matrix genes of the same virus. Over the course of the outbreak, a total of 37 isolates with, and 3 isolates without inserts were characterized. The events described here appear very similar to those which occurred in Chile in 2002 where the virulence shift of another H7N3 virus was attributed to non-homologous recombination between the HA and nucleoprotein genes.

Genetics of influenza viruses

Influenza A viruses contain genomes composed of eight separate segments of negative-sense RNA. Circulating human strains are notorious for their tendency to accumulate mutations from one year to the next and cause recurrent epidemics. However, the segmented nature of the genome also allows for the exchange of entire genes between different viral strains. The ability to manipulate influenza gene segments in various combinations in the laboratory has contributed to its being one of the best characterized viruses, and studies on influenza have provided key contributions toward the understanding of various aspects of virology in general. However, the genetic plasticity of influenza viruses also has serious potential implications regarding vaccine design, pathogenicity, and the capacity for novel viruses to emerge from natural reservoirs and cause global pandemics.

Cooperation between the hemagglutinin of avian viruses and the matrix protein of human influenza A viruses

To analyze the compatibility of avian influenza A virus hemagglutinins (HAs) and human influenza A virus matrix (M) proteins M1 and M2, we doubly infected Madin-Darby canine kidney cells with amantadine (1-aminoadamantane hydrochloride)-resistant human viruses and amantadine-sensitive avian strains. By using antisera against the human virus HAs and amantadine, we selected reassortants containing the human virus M gene and the avian virus HA gene. In our system, high virus yields and large, well-defined plaques indicated that the avian HAs and the human M gene products could cooperate effectively; low virus yields and small, turbid plaques indicated that cooperation was poor. The M gene products are among the primary components that determine the species specificities of influenza A viruses. Therefore, our system also indicated whether the avian HA genes effectively reassorted into the genome and replaced the HA gene of the prevailing human influenza A viruses. Most of the avian HAs that we tested efficiently cooperated with the M gene products of the early human A/PR/8/34 (H1N1) virus; however, the avian HAs did not effectively cooperate with the most recently isolated human virus that we tested, A/Nanchang/933/95 (H3N2). Cooperation between the avian HAs and the M proteins of the human A/Singapore/57 (H2N2) virus was moderate. These results suggest that the currently prevailing human influenza A viruses might have lost their ability to undergo antigenic shift and therefore are unable to form new pandemic viruses that contain an avian HA, a finding that is of great interest for pandemic planning.

Role of hemagglutinin cleavage for the pathogenicity of influenza virus

Although human epidemics of influenza occur on nearly an annual basis and result in a significant number of "excess deaths," the viruses responsible are not generally considered highly pathogenic. On occasion, however, an outbreak occurs that demonstrates the potential lethality of influenza viruses. The human pandemic of 1918 spread worldwide and killed millions, and the limited human outbreak of highly pathogenic avian viruses in Hong Kong at the end of 1997 is a warning that this could happen again. In avian species such as chickens and turkeys, several outbreaks of highly pathogenic influenza viruses have been documented. Although the reason for the lethality of the human 1918 viruses remains unclear, the pathogenicity of the avian viruses, including those that caused the human 1997 outbreak, relates primarily to properties of the hemagglutinin glycoprotein (HA). Cleavage of the HA precursor molecule HA0 is required to activate virus infectivity, and the distribution of activating proteases in the host is one of the determinants of tropism and, as such, pathogenicity. The HAs of mammalian and nonpathogenic avian viruses are cleaved extracellularly, which limits their spread in hosts to tissues where the appropriate proteases are encountered. On the other hand, the HAs of pathogenic viruses are cleaved intracellularly by ubiquitously occurring proteases and therefore have the capacity to infect various cell types and cause systemic infections. The x-ray crystal structure of HA0 has been solved recently and shows that the cleavage site forms a loop that extends from the surface of the molecule, and it is the composition and structure of the cleavage loop region that dictate the range of proteases that can potentially activate infectivity. Here influenza virus pathogenicity is discussed, with an emphasis on the role of HA0 cleavage as a determining factor.

Molecular epidemiology of influenza

The genome of the influenza A viruses comprises eight single-stranded RNA segments, and this property makes genetic reassortment after double infection of a host with two different influenza A strains possible. Nature takes advantage of genetic reassortment during antigenic shift creating new pandemic strains. After concurrent infection of a host with both avian and human strains, the hemagglutinin gene of the human virus may be replaced by the allelic gene of the avian virus. This reassortment leads to a human virus strain that has avian hemagglutinin molecules on its surface, significant because the human population lacks neutralizing antibodies to this new glycoprotein. The Hong Kong pandemic of 1968 resulted from just such an event.

Virulence of influenza A virus for mouse lung

The experimental infection of mouse lung with influenza A virus has proven to be an invaluable model for studying the mechanisms of viral adaptation and virulence. These investigations have identified critical roles for the haemagglutinin (HA) and matrix (M) genes of the virus in determining virulence for mouse lung. For the HA gene, the loss of glycosylation sites from the encoded polypeptide or changes which may affect the pH of HA-mediated endosome fusion have been observed following adaptation. These alterations also have the potential to impact on receptor specificity, beta inhibitor sensitivity and activation cleavage which may act in concert to account for the increased virulence of adapted strains. For the M gene, two specific changes in the M1 protein have been identified in strains adapted to, or virulent for, mouse lung. These changes are likely to affect pH-dependent association/dissociation of M1 with the viral ribonucleoprotein, and control virulence as well as growth. The role of other genes in mouse lung virulence remains unknown.

Virulence-associated sequence duplication at the hemagglutinin cleavage site of avian influenza viruses

Recent highly pathogenic (HP) field isolates of avian influenza (AI) virus from Mexico all possess an insertion of at least two basic amino acids (arg-lys) at the cleavage site of the hemagglutinin (HA) glycoprotein. One HP isolate has additional information which yields a 4 amino acid insert (arg-lys-arg-lys). We present here the nucleotide sequence of the HA gene of this unique isolate and compare it to recent H5N2 and other avian influenza isolates. The complete HA nucleotide sequence of the isolate and phylogenetic relationship suggest that it was derived in direct succession from a non-pathogenic strain isolated about 1 month earlier. The unique insertion sequence is a direct duplication of part of the purine-rich region preceding the arginine codon at the HA cleavage site. This evidence along with other data in this report provide compelling support for a proposed model explaining the mechanism of spontaneous, virulence-related insertions in type A influenza viruses.

Molecular evolution of influenza viruses

There are two different mechanisms by which influenza viruses might evolve: (1) Because the RNA genome of influenza viruses is segmented, new strains can suddenly be produced by reassortment, as happens, for example, during antigenic shift, creating new pandemic strains. (2) New viruses evolve relatively slowly by stepwise mutation and selection, for example, during antigenic or genetic drift. Influenza A viruses were found in various vertebrate species, where they form reservoirs that do not easily mix. While human influenza A viruses do not spread in birds and vice versa, the species barrier to pigs is relatively low, so that pigs might function as "mixing vessels" for the creation of new pandemic reassortants in Southeast Asia, where the probability is greatest for double infection of pigs by human and avian influenza viruses. Phylogenetic studies revealed that about 100 years ago, an avian influenza A virus had crossed the species barrier, presumably first to pigs, and from there to humans, forming the new stable human and classical swine lineages. In 1979, again, an avian virus showed up in the North European swine population, forming another stable swine lineage. The North European swine isolates from 1979 until about 1985 were genetically extremely unstable. A hypothesis is put forward stating that a mutator mutation is necessary to enable influenza virus to cross the species barrier by providing the new host with sufficient variants from which it can select the best fitting ones. As long as the mutator mutation is still present, such a virus should be able to cross the species barrier a second time, as happened about 100 years ago. Although the most recent swine isolates from northern Germany are again genetically stable, we nevertheless should be on the lookout to see if a North European swine virus shows up in the human population in the near future.

The critical cut-off temperature of avian influenza viruses

We have measured the pathogenicity for 6-week-old chicks of infection by H7 avian influenza viruses. One virus, strain S3 from A/FPV/Rostock/34(H7N1) showed a temperature sensitive phenotype at 41.5 degrees C and reduced pathogenicity. By analysis of reassortants made between virus S3 and A/FPV/Dobson/27(H7N7), a fully pathogenic virus, two conclusions arise. (1) The critical cut-off temperature for avian influenza virus in 6-week-old chicks is 41.5 degrees. (2) RNA segment 1 of virus S3 is responsible for the lack of pathogenicity in reassortant viruses. Nucleotide sequencing of RNA segment 1 from S3 and its parent, A/FPV/Rostock/34 has revealed a single mutation at nucleotide 1561. This results in a substitution of isoleucine for leucine at amino acid position 512 in the cap-binding protein, PB2.

Studies on the genetic basis of human influenza A virus adaptation to mice: degrees of virulence of reassortants with defined genetic content

A highly virulent mouse-adapted variant of influenza virus A/Aichi/2/68 (H3N2) was crossed either with the original A/USSR/90/77 (H1N1) influenza virus strain or with its mouse-adapted, moderately mouse virulent variant. The reassortants were characterized with respect to their genetic content and pneumovirulence for mice. The reassortants fell into three categories: avirulent, highly virulent (resembling in this respect the parent A/Aichi/2/68 virus) and moderately virulent (resembling the mouse-adapted A/USSR/90/77 parent virus). The analysis of the parental origin of the genes of 6 reassortants allowed to suggest that changes in the HA gene and in a polymerase gene (most likely, PB1) were necessary for the acquisition of virulence by the A/USSR/90/77 virus in the course of adaptation to mice, whereas the changes in two other polymerase genes as well as in the genes NA and NS were not involved. The low degree of pathogenicity characteristic of the mouse-adapted A/USSR/90/77 virus was determined by gene(s) other than HA.

Difference in growth behavior of human, swine, equine, and avian influenza viruses at a high temperature

Growth characteristics of a wide range of influenza A viruses from different mammals and bird species were examined in an established line of canine kidney (MDCK) cells at an ordinary (37 degrees C) and a high temperature (42 degrees C). Although all viruses employed in the present study possessed a capability of replicating at 37 degrees C, virus growth at 42 degrees C showed considerable variation and reflected differences in the natural hosts of the isolates. All reference strains and isolates from bird species grew well in the MDCK cells maintained at 42 degrees C, but human viruses did not, showing an asymmetrical growth behavior. In contrast to this, growth of swine and equine viruses showed growth characteristics intermediate between human and avian viruses. Of the two swine viruses examined, replication of one strain occurred equally well at both temperatures and another failed to grow at 42 degrees C. Similarly, two of the three equine viruses tested belonging to H3N8 antigenic subtypes grew at 42 degrees C. However, the results obtained from comparison of plaque sizes and growth curves indicated that the replication of the above swine and equine viruses was restricted under a stringent temperature when compared to avian viruses. The detailed analysis of cloned viruses revealed that some of the swine and equine viruses contained two variants which are readily distinguished by growth behavior at 42 degrees C. Genome analysis of parental and virus clones by oligonucleotide mapping and migration profiles of RNA segments did not detect any differences among the above variants exhibiting the asymmetrical growth characteristics at 42 degrees C.

The structure of serotype H10 hemagglutinin of influenza A virus: comparison of an apathogenic avian and a mammalian strain pathogenic for mink

The primary structure of the hemagglutinin of the apathogenic avian influenza virus A/chick/Germany/N/49 (H10N7) and of the serologically related strain A/mink/Sweden/84 (H10N4) pathogenic for mink has been elucidated by nucleotide sequence analysis, and the carbohydrates attached to the polypeptide have been determined. The H10 hemagglutinin has 65, 52, 46, 45, and 44% amino acid sequence homology with serotypes H7, H3, H1, H2, and H5, respectively. H10 and H7 hemagglutinins are also most closely related in their glycosylation patterns. There is a high sequence homology between both H10 strains supporting the concept that the mink virus has obtained its hemagglutinin from an avian strain. The sequence homology includes the cleavage site which consists of a single arginine as is the case with most other hemagglutinins exhibiting low susceptibility to proteolytic activation. The similarity in hemagglutinin structure between both H10 strains is discussed in light of the distinct differences in the pathogenicity of both viruses.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

Molecular aspects of the epidemiology of virus disease

With regard to molecular epidemiology, influenza A viruses belong to the best-studied virus systems. At least two large reservoirs of influenza A viruses have been built up in nature, one in humans and another one in water fowls. The latter one is very heterogenous, consisting of viruses belonging to 13 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes in almost all possible combinations. The segmented structure of the influenza virus genome allows the creation of new influenza strains by reassortment. By replacement of the HA gene of human strains new pandemic viruses can be generated (antigenic shift). The particular structure of the HA enables the human influenza A-viruses to create variants which can escape the immune response of the host (antigenic drift). The nucleoprotein is responsible for keeping those two large reservoirs apart. Mixing of genes of viruses from these two reservoirs seems to happen predominantly by double infection of pigs, which apparently are tolerant for infection by either human or avian influenza viruses. The molecular mechanisms described for influenza viruses can be explained by the particular structure of their genome and their components and cannot be generalized. Each virus has developed its own strategy to multiply and to spread.

Molecular analyses of the hemagglutinin genes of H5 influenza viruses: origin of a virulent turkey strain

Comparative sequence analysis of the hemagglutinin (HA) genes of a highly virulent H5N8 virus isolated from turkeys in Ireland in 1983 and a virus of the same subtype detected simultaneously in healthy ducks showed only four amino acid differences between these strains. Partial sequencing of six of the other genes and antigenic similarity of the neuraminidases established the overall genetic similarity of these two viruses. Comparison of the complete sequence of two H5 gene sequences and partial sequences of other virulent and avirulent H5 viruses provides evidence for at least two different lineages of H5 influenza virus in the world, one in Europe and the other in North America, with virulent and avirulent members in each group. In vivo studies in domestic ducks showed that all of the H5 viruses that are virulent in chickens and turkeys replicate in the internal organs of ducks but did not produce any disease signs. Additionally, both viruses isolated from turkeys and ducks in Ireland were detected in the blood. These studies provide the first conclusive evidence for the possibility that fully virulent influenza viruses in domestic poultry can arise directly from viruses in wild aquatic birds. Studies on the cleavability of the HA of virulent and avirulent H5 viruses showed that the principles established for H7 viruses (F. X. Bosch, M. Orlich, H. D. Klenk, and R. Rott, 1979, Virology 95, 197-207; F. X. Bosch, W. Garten, H. D. Klenk, and R. Rott, 1981, Virology 113, 725-735) also apply to the H5 subtype. These are (1) only the HAs of virulent influenza viruses were cleaved in tissue culture in the absence of trypsin and (2) virulent H5 influenza viruses contain a series of basic amino acids at the cleavage site of the HA, whereas avirulent strains contain only a single arginine with the exception of the avirulent Chicken/Pennsylvania virus. Thus, a series of basic amino acids at the cleavage site probably forms a recognition site for the enzyme(s) responsible for cleavage.

Glycosylation affects cleavage of an H5N2 influenza virus hemagglutinin and regulates virulence

Based on nucleotide sequence analysis of the hemagglutinin (HA) gene from the virulent and avirulent A/chicken/Pennsylvania/83 influenza viruses, it was previously postulated that acquisition of virulence was associated with a point mutation that resulted in loss of a glycosylation site. Since there are two potential glycosylation sites in this region of the HA molecule and since all Asn-Xaa-Thr/Ser sequences in the HAs of different strains are not necessarily glycosylated, the question remained open as to whether either one of these sites was glycosylated. We now provide direct evidence that a site-specific glycosylation affects cleavage of the influenza virus HA and thus virulence. We have identified the glycosylation sites on the HA1 subunit from the virulent and avirulent strains by direct structural analysis of the isolated proteins. Our results show that the only difference in glycosylation between the HA1s of the virulent and avirulent strains is the lack of an asparagine-linked carbohydrate on the virulent HA1 polypeptide at residue 11. Further, we show that the HA1s of both the avirulent and virulent viruses are not glycosylated at one potential site, while all other sites contain carbohydrate. Amino acid sequence analysis of the HA1 of an avirulent revertant of the virulent strain confirmed these findings.

The nucleoprotein as a possible major factor in determining host specificity of influenza H3N2 viruses

In an attempt to assess the importance of the nucleoprotein (NP) in the determination of host specificity, a series of experiments was performed on influenza A viruses of the H3N2 subtype. We have examined rescue of mutants of A/FPV/Rostock/34 with temperature-sensitive (ts) lesions in the nucleoprotein (NP) gene by double infection of chick embryo cells with H3N2 strains isolated from different species. The ts mutants could be rescued by all avian H3N2 strains but not by any of the human H3N2 isolates. Only two of the swine H3N2 strains tested were able to rescue our mutants. The NP gene of these two swine isolates resembled the NP gene of the avian strains genetically in the hybridization test. However, their NPs reacted differently with a set of monoclonal antibodies when compared with NPs of avian H3N2 strains. Concerning multiplication in ducks they behaved like the other swine and human strains. The phosphopeptide fingerprints of all swine isolates tested were alike and were different from those of human or avian origin. Our observations are compatible with the idea that human H3N2 strains might not be able to cross the species barrier to birds directly, and possibly also not the other way around, without prior reassortment in pigs, which seem to have a broader host range concerning the compatibility of the NP gene in reassortants.

The neuraminidases of the virulent and avirulent A/Chicken/Pennsylvania/83 (H5N2) influenza A viruses: sequence and antigenic analyses

To define the sequence changes that occurred in an avian influenza virus neuraminidase (NA) during the evolution of virulence, we have studied the NA of the virulent and avirulent A/Chick/Penn/83 (H5N2) influenza viruses. A comparison of the deduced amino acid sequence from these viruses shows that the virulent strain, which evolved from the avirulent by the accumulation of point mutations (Bean et al., 1985), acquired four amino acid changes in the NA: one in the transmembrane segment, one in the stalk, and two in the head. A comparison of the deduced amino acid sequences with those of the human N2 NAs indicates a 20-amino acid deletion in the stalk of the Chick/Penn/83 NA. Antigenic analysis of the NAs from the avirulent and virulent Chick/Penn/83 virus shows they are antigenically very closely related, but can be distinguished with two monoclonal antibodies at a site which probably involves at least one of the amino acid changes in the NA head. Antigenic analysis also shows the Chick/Penn/83 NAs are closely related to the NAs of other N2 avian influenza viruses isolated between 1965 and 1984, supporting previous studies which indicate a relative antigenic stability of the NA among avian N2 influenza viruses. The Chick/Penn/83 NAs are the first N2 NA genes of an avian virus to be sequenced. These NAs are antigenically closely related to the 1957 human N2 NAs, and show a high degree of amino acid sequence homology with the prototype 1957 human N2 NA. These data give further support to the view that the 1957 human H2N2 viruses were at least partially derived from an avian source.

Is virulence of H5N2 influenza viruses in chickens associated with loss of carbohydrate from the hemagglutinin?

The A/Chick/Penn/83 (H5N2) influenza virus that appeared in chickens in Pennsylvania in April 1983 and subsequently became virulent in October 1983, was examined for plaque-forming ability and cleavability of the hemagglutinin (HA) molecule. The avirulent virus produced plaques and cleaved the HA only in the presence of trypsin. In contrast, the virulent virus produced plaques and cleaved the HA precursor into HA1 and HA2 in the presence or absence of trypsin. The apparent molecular weight of the HA1 from the avirulent virus was higher than that from the virulent virus, but when the viruses were grown in the presence of tunicamycin, the molecular weights of HA were indistinguishable. Two of nine monoclonal antibodies to the HA of the avirulent virus indicate that there is at least one epitope on the HA that is different between the virulent and avirulent viruses. The amino acid sequences of the HAs from the two viruses were compared by sequencing their respective HA gene. The nucleotide sequence coding for the processed HA polypeptide contained 1641 nucleotides specifying a protein of 547 amino acids. The amino acid sequences of the virulent and avirulent viruses were indistinguishable through the connecting peptide region, indicating that the difference in cleavability of the H5 HA is not directly attributed to the amino acid sequence of the connecting peptide. Four of seven nucleotide changes resulted in amino acid changes at residues 13, 69, and 123 of HA1 and at residue 501 of the HA2 polypeptide. Since there were no deletions or insertions in the amino acid sequence of the virulent or avirulent viruses, the possibility exists that the difference in molecular weight is due to loss of a carbohydrate side chain in the virulent strain. The amino acid change in the virulent strain at residue 13 is the only mutation that could affect a glycosylation site and this is in the vicinity of the connecting peptide. It is postulated that the loss of this carbohydrate may permit access of an enzyme that recognizes the basic amino acid sequences and results in cleavage activation of the HA in the virulent virus.

Mouse neurotropic recombinants of influenza A viruses

Recombinants with known gene constellations between fowl plague virus (FPV) and various prototype influenza virus strains have been examined for neurovirulence in suckling mice. Strongly neurotropic recombinants were obtained from crosses between FPV and the strains virus N, Hong Kong, and PR8, but not between FPV and equi 2 or swine viruses. All highly neurotropic recombinants had RNA segment 4 (HA) derived from FPV and RNA segment 2 (Ptra gene) from the other prototype strain. The derivation of two other RNA segments of the polymerase complex, namely RNA segments 3 (Pol 2) and 5 (NP) and also segment 8 (NS) can modulate these properties. For example, if in recombinants between FPV and virus N in addition to RNA segment 2 also RNA segments 3 and/or 8 are derived from virus N, neurovirulence is further enhanced, while replacement of RNA segment 5 of FPV by the corresponding segment of virus N decreases or abolishes neurovirulence. The derivation of the other genes does not seem to be relevant for neurovirulence in the crosses mentioned above. Of the prototype strains tested, the turkey England (t. Engl.) strain is the only one which was highly neurotropic for suckling mice. Recombinants between FPV and t. Engl. which have kept the HA gene of t. Engl. were still neurotropic, while those with the HA gene of FPV were completely avirulent. The results obtained demonstrated that 1. the creation of influenza virus recombinants neurotropic for mice is not a rare event; 2. one of the parents should multiply well in mouse lungs; 3. the presence of a cleavable hemagglutinin is necessary, but not sufficient. In the pair FPV/turkey England the hemagglutinin of turkey England seems to determine neurovirulence.

Plaquing characteristics of influenza A virus recombinants of defined genetic composition. Brief report

The plaque size and morphology of twenty-two influenza A virus recombinants representing seven distinct families were analyzed on MDCK cells. By examination of the genetic composition of the recombinants no relationship could be established between any gene, including those coding for the surface antigens, and the plaque size and morphology.

Characterization of virus-like particles produced by an influenza A virus

The influenza strain 413 1,1 segregated as a stable recombinant during passage of the isolate 19/N which was obtained after double infection of chick embryo fibroblasts by virus N and the fowl plague virus (FPV) mutant ts 19. Its gene constellation was determined by molecular hybridization. Upon infection of chick embryo cells by this recombinant strain, two particle populations of high (H) and low (L) buoyant densities were produced. By biological and biochemical parameters, the H-population (delta = 1.22 g/cm3) cannot be distinguished from standard infectious influenza virus. In contrast, the noninfectious L-particles (delta = 1.14 g/cm3) lack all virus-specific glycoproteins (HA, NA) as well as the matrix protein M and are visualized by electron microscopy as spikeless particles. Significant changes in the quantitative composition of the phospholipid bilayer are evident as compared to the H-particles. In addition to the previously characterized eight genes both populations contain a variety of smaller RNA fragments which hybridize with complementary RNA and presumably represent degradation products of full-length genes.

Genotypic characterization and clinical evaluation of an influenza A/Texas/1/77 (H3N2)-like recombinant: RIT 4199

The RIT 4199 (H3N2) strain was obtained by recombination of PR/8/34 (H0N1) and A/Alaska/5/77 (H3N2) isolates. Its genetic composition was determined by RNA-RNA hybridization and by identification by gel electrophoresis of the double-standard RNA formed. The RIT 4199 was inoculated intranasally to 27 seronegative volunteers at a dose of 10(7.3)EID50. The results show that it is suitable for live vaccine strain.

The influenza virus: Antigenic composition and immune response

The architecture and chemical composition of the influenza virus particle is described with particular reference to the protein constituents and their genetic control. The dominant role in infection of the surface proteins - haemagglutinins and neuraminidases - acting as antigens and undergoing variation in time known as antigenic drift and shift is explained. The immuno-diffusion technique has illuminated the interrelationships of the haemagglutinins of influenza A viruses recovered over long periods of time. The H0 and H1 haemagglutinins are now regarded as a single sub-type with H2 and H3 representing the haemagglutinins of the 1957 and 1968 sub-types. Animal influenza viruses of pigs, horses and birds are described. A relation to human influenza strains has been shown to exist in certain instances as is the capacity of some human strains to pass to the animal kingdom.

Susceptibility of influenza A viruses to amantadine is influenced by the gene coding for M protein

Influenza A virus recombinants derived from "resistant" and "sensitive" parental viruses were examined for susceptibility to inhibition by amantadine. Correlation of gene constellation and amantadine susceptibility revealed that the gene coding for M protein influences sensitivity or resistance to amantadine. All recombinants which derived an M protein from an amantadine-resistant parent were found to be resistant to amantadine. All amantadine-sensitive recombinants derived an M gene from the amantadine-sensitive parent. However, a few amantadine-resistant recombinants which derived an M gene from the sensitive parent were also isolated, suggesting that the expression of amantadine sensitivity in these recombinants may be influenced by other genes.