Sequence of the hemagglutinin gene from influenza virus A/Seal/Mass/1/80

Antigenic Architecture of the H7N2 Influenza Virus Hemagglutinin Belonging to the North American Lineage

The North American low pathogenic H7N2 avian influenza A viruses, which lack the 220-loop in the hemagglutinin (HA), possess dual receptor specificity for avian- and human-like receptors. The purpose of this work was to determine which amino acid substitutions in HA affect viral antigenic and phenotypic properties that may be important for virus evolution. By obtaining escape mutants under the immune pressure of treatment with monoclonal antibodies, antigenically important amino acids were determined to be at positions 125, 135, 157, 160, 198, 200, and 275 (H3 numbering). These positions, except 125 and 275, surround the receptor binding site. The substitutions A135S and A135T led to the appearance of an N-glycosylation site at 133N, which reduced affinity for the avian-like receptor analog and weakened binding with tested monoclonal antibodies. Additionally, the A135S substitution is associated with the adaptation of avian viruses to mammals (cat, human, or mouse). The mutation A160V decreased virulence in mice and increased affinity for the human-type receptor analog. Conversely, substitution G198E, in combination with 157N or 160E, displayed reduced affinity for the human-type receptor analog.

Molecular Analysis of the Avian H7 Influenza Viruses Circulating in South Korea during 2018-2019: Evolutionary Significance and Associated Zoonotic Threats

Avian influenza virus (AIV) subtypes H5 and H7, possessing the ability to mutate spontaneously from low pathogenic (LP) to highly pathogenic (HP) variants, are major concerns for enormous socio-economic losses in the poultry industry, as well as for fatal human infections. Through antigenic drift and shift, genetic reassortments of the genotypes pose serious threats of increased virulence and pathogenicity leading to potential pandemics. In this study, we isolated the H7-subtype AIVs circulating in the Republic of Korea during 2018–2019, and perform detailed molecular analysis to study their circulation, evolution, and possible emergence as a zoonotic threat. Phylogenetic and nucleotide sequence analyses of these isolates revealed their distribution into two distinct clusters, with the HA gene sharing the highest nucleotide identity with either the A/common teal/Shanghai/CM1216/2017, isolated from wild birds in Shanghai, China, or the A/duck/Shimane/2014, isolated from Japan. Mutations were found in HA (S138A (H3 numbering)), M1 (N30D and T215A), NS1 (P42S), PB2 (L89V), and PA (H266R and F277S) proteins—the mutations had previously been reported to be related to mammalian adaptation and changes in the virulence of AIVs. Taken together, the results firmly put forth the demand for routine surveillance of AIVs in wild birds to prevent possible pandemics arising from reassortant AIVs.

Characterization of the low-pathogenic H7N7 avian influenza virus in Shanghai, China

H7N7 avian influenza virus (AIV) can divided into low-pathogenic AIV and high-pathogenic AIV groups. It has been shown to infect humans and animals. Its prevalence state in wild birds in China remains largely unclear. In this study, a new strain of H7N7 AIV, designated CM1216, isolated from wild birds in Shanghai, China, was characterized. Phylogenetic and nucleotide sequence analyses of CM1216 revealed that HA, NA, PB1, NP, and M genes shared the highest nucleotide identity with the Japan H7 subtype AIV circulated in 2019; the PB2 and PA genes shared the highest nucleotide identity with the Korea H7 subtype AIV circulated in wild birds in 2018, while NS gene of CM1216 was 98.93% identical to that of the duck AIV circulating in Bangladesh, and they all belong to the Eurasian lineage. A Bayesian phylogenetic reconstruction of the 2 surface genes of CM1216 showed that multiple reassortments might have occurred in 2015. Mutations were found in HA (A135 T, T136S, and T160 A [H3 numbering]), M1 (N30D and T215 A), NS1 (P42S and D97 E), PB2 (R389 K), and PA (N383D) proteins; these mutations have been shown to be related to mammalian adaptation and changes in virulence of AIVs. Infection studies demonstrated that CM1216 could infect mice and cause symptoms characteristic of influenza virus infection and proliferate in the lungs without prior adaption. This study demonstrates the need for routine surveillance of AIVs in wild birds and detection of their evolution to become a virus with high pathogenicity and ability to infect humans.

A Brief Introduction to Influenza A Virus in Marine Mammals

Influenza A infection has been detected in marine mammals going back to 1975, with additional unconfirmed outbreaks as far back as 1931. Over the past forty years, infectious virus has been recovered on ten separate occasions from both pinnipeds (harbor seal, elephant seal, and Caspian seal) and cetaceans (striped whale and pilot whale). Recovered viruses have spanned a range of subtypes (H1, H3, H4, H7, H10, and H13) and, in all but H1N1, show strong evidence for deriving directly from avian sources. To date, there have been five unusual mortality events directly attributed to influenza A virus; these have primarily occurred in harbor seals in the Northeastern United States, with the most recent occurring in harbor seals in the North Sea.There are numerous additional reports wherein influenza A virus has indirectly been identified in marine mammals; these include serosurveillance efforts that have detected influenza A- and B-specific antibodies in marine mammals spanning the globe and the detection of viral RNA in both active and opportunistic surveillance in the Northwest Atlantic. For viral detection and recovery, nasal, rectal, and conjunctival swabs have been employed in pinnipeds, while blowhole, nasal, and rectal swabs have been employed in cetaceans. In the case of deceased animals, virus has also been detected in tissue. Surveillance has historically been somewhat limited, relying largely upon opportunistic sampling of stranded or bycaught animals and primarily occurring in response to a mortality event. A handful of active surveillance projects have shown that influenza may be more endemic in marine mammals than previously appreciated, though live virus is difficult to recover. Surveillance efforts are hindered by permitting and logistical challenges, the absence of reagents and methodology optimized for nonavian wild hosts, and low concentration of virus recovered from asymptomatic animals. Despite these challenges, a growing body of evidence suggests that marine mammals are an important wild reservoir of influenza and may contribute to mammalian adaptation of avian variants.

Influenza Virus Infection of Marine Mammals

Interspecies transmission may play a key role in the evolution and ecology of influenza A viruses. The importance of marine mammals as hosts or carriers of potential zoonotic pathogens such as highly pathogenic H5 and H7 influenza viruses is not well understood. The fact that influenza viruses are some of the few zoonotic pathogens known to have caused infection in marine mammals, evidence for direct transmission of influenza A virus H7N7 subtype from seals to man, transmission of pandemic H1N1 influenza viruses to seals and also limited evidence for long-term persistence of influenza B viruses in seal populations without significant genetic change, makes monitoring of influenza viruses in marine mammal populations worth being performed. In addition, such monitoring studies could be a great tool to better understand the ecology of influenza viruses in nature.

Sequencing and mutational analysis of the non-coding regions of influenza A virus

The genome of influenza A virus consists of eight negative-stranded RNA segments which contain one or two coding regions flanked by the 3' and 5' non-coding regions (NCRs). Despite the importance of NCRs in replication and pathogenesis of influenza virus, sequencing of influenza virus genome has mainly been focused on coding regions of the individual genes and very limited NCR sequences are available. In this study, we sequenced the NCRs of seven influenza A virus strains of different host origin and varying pathogenicity using two recently developed methods [de Wit, E., Bestebroer, T.M., Spronken, M.I., Rimmelzwaan, G.F., Osterhaus, A.D., Fouchier, R.A., 2007. Rapid sequencing of the non-coding regions of influenza A virus. J. Virol. Methods 139, 85-89; Szymkowiak, C., Kwan, W.S., Su, Q., Toner, T.J., Shaw, A.R., Youil, R., 2003. Rapid method for the characterization of 3' and 5' UTRs of influenza viruses. J. Virol. Methods 107, 15-20]. In addition to sequence and length variation present in the segment-specific NCRs among different influenza strains, we also observed sequence variations at the fourth nucleotide of 3' NCR of polymerase genes. To evaluate the role of sequence change in the NCRs in reporter gene expression, we introduced mutations at the NCRs of two polymerase gene segments, PB1 and PA, and created the green fluorescent protein (GFP) reporter plasmids. By measuring the GFP expression level, we confirmed that single or two mutations introduced at the 3' and 5' NCRs of PB1 and PA gene could alter the protein expression levels. Our study reaffirms the importance of NCRs in influenza virus replication and further analysis of their roles will lead to better understanding of influenza pathogenesis.

Panorama phylogenetic diversity and distribution of Type A influenza virus

Background

Type A influenza virus is one of important pathogens of various animals, including humans, pigs, horses, marine mammals and birds. Currently, the viral type has been classified into 16 hemagglutinin and 9 neuraminidase subtypes, but the phylogenetic diversity and distribution within the viral type largely remain unclear from the whole view.

Methodology/Principal Findings

The panorama phylogenetic trees of influenza A viruses were calculated with representative sequences selected from approximately 23000 candidates available in GenBank using web servers in NCBI and the software MEGA 4.0. Lineages and sublineages were classified according to genetic distances, topology of the phylogenetic trees and distributions of the viruses in hosts, regions and time.

Conclusions/Significance

Here, two panorama phylogenetic trees of type A influenza virus covering all the 16 hemagglutinin subtypes and 9 neuraminidase subtypes, respectively, were generated. The trees provided us whole views and some novel information to recognize influenza A viruses including that some subtypes of avian influenza viruses are more complicated than Eurasian and North American lineages as we thought in the past. They also provide us a framework to generalize the history and explore the future of the viral circulation and evolution in different kinds of hosts. In addition, a simple and comprehensive nomenclature system for the dozens of lineages and sublineages identified within the viral type was proposed, which if universally accepted, will facilitate communications on the viral evolution, ecology and epidemiology.

Characterization of An Avian Influenza Virus of Subtype H7N2 Isolated from Chickens in Northern China

An H7N2 avian influenza virus was isolated from chickens during routine surveillance in northern China in 2002. To understand the origin of this virus, we completely sequenced its genome. The PB1, PA, HA, and M genes of this virus were highly homologous with those of the wild bird virus A/Africa starling/Eng-Q/983/79 (H7N1). The NP and NS genes were closely related to those of two other wild bird viruses isolated 30 years ago. The closest relatives of the PB2 and NA genes of the virus were those of the A/swine/Germany/2/81 (H1NI) and A/Leningrad/134/57 (H2N2), respectively. Animal inoculation tests showed that the virus cannot replicate efficiently in chickens. However, after intranasal inoculation, the virus induced 20% weight loss and replicated well in the lungs of mice. The virus was also recovered from the hearts and brains of the mice. These results suggest that the influenza virus isolated in chickens in northern China in 2002 originated in wild birds and may pose a threat for both avian species and mammalian hosts.

A critical step in the folding of influenza virus HA determined with a novel folding assay

Most principles of protein folding emerged from refolding studies in vitro on small, soluble proteins, because large ones tend to misfold and aggregate. We developed a folding assay allowing the study of large proteins in detergent such that the extent of cellular assistance required for proper folding can be determined. We identified a critical step in the in vivo folding pathway of influenza virus hemagglutinin (HA). Only the formation of the first few disulfides in the top domain of HA required the intact endoplasmic reticulum. After that, HA proceeded to fold efficiently in a very dilute solution, despite its size and complexity. This study paves the way for detailed structural analyses during the folding of complex proteins.

Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome

Highly pathogenic avian influenza A viruses of subtypes H5 and H7 are the causative agents of fowl plague in poultry. Influenza A viruses of subtype H5N1 also caused severe respiratory disease in humans in Hong Kong in 1997 and 2003, including at least seven fatal cases, posing a serious human pandemic threat. Between the end of February and the end of May 2003, a fowl plague outbreak occurred in The Netherlands. A highly pathogenic avian influenza A virus of subtype H7N7, closely related to low pathogenic virus isolates obtained from wild ducks, was isolated from chickens. The same virus was detected subsequently in 86 humans who handled affected poultry and in three of their family members. Of these 89 patients, 78 presented with conjunctivitis, 5 presented with conjunctivitis and influenza-like illness, 2 presented with influenza-like illness, and 4 did not fit the case definitions. Influenza-like illnesses were generally mild, but a fatal case of pneumonia in combination with acute respiratory distress syndrome occurred also. Most virus isolates obtained from humans, including probable secondary cases, had not accumulated significant mutations. However, the virus isolated from the fatal case displayed 14 amino acid substitutions, some of which may be associated with enhanced disease in this case. Because H7N7 viruses have caused disease in mammals, including horses, seals, and humans, on several occasions in the past, they may be unusual in their zoonotic potential and, thus, form a pandemic threat to humans.

Serological evidence of transmission of human influenza A and B viruses to Caspian seals (Phoca caspica)

Seroepidemiological surveillance of influenza in Caspian seals (Phoca caspica) was conducted. Antibodies to influenza A virus were detected in 54% (7/13), 57% (4/7), 40% (6/15) and 26% (11/42) of the serum samples collected in 1993, 1997, 1998 and 2000 by enzyme-linked immunosorbent assay (ELISA). In an hemagglutination-inhibition (HI) test using H1-H15 reference influenza A viruses as antigens, more than half of the examined ELISA-positive sera reacted with an H3N2 prototype strain A/Aichi/2/68. These sera were then examined by HI test with a series of naturally occurring antigenic variants of human H3N2 virus, and H3 viruses of swine, duck, and equine origin. The sera reacted strongly with the A/Bangkok/1/79 (H3N2) strain, which was prevalent in humans in 1979-1981. The present results indicate that human A/Bangkok/1/79-like virus was transmitted to Caspian seals probably in the early 1980s, and was circulated in the population. Antibodies to influenza B virus were detected by ELISA in 14% (1/7) and 10% (4/42) serum samples collected from Caspian seals in 1997 and 2000, respectively. Our findings indicate that seal might be a reservoir of both influenza A and B viruses originated from humans.

Isolation and characterization of H4N6 avian influenza viruses from pigs with pneumonia in Canada

In October 1999, H4N6 influenza A viruses were isolated from pigs with pneumonia on a commercial swine farm in Canada. Phylogenetic analyses of the sequences of all eight viral RNA segments demonstrated that these are wholly avian influenza viruses of the North American lineage. To our knowledge, this is the first report of interspecies transmission of an avian H4 influenza virus to domestic pigs under natural conditions.

Vaccinating chickens against avian influenza with fowlpox recombinants expressing the H7 haemagglutinin

OBJECTIVE

To evaluate the vaccine efficacy of a fowlpox virus recombinant expressing the H7 haemagglutinin of avian influenza virus in poultry.

PROCEDURE

Specific-pathogen-free poultry were vaccinated with fowlpox recombinants expressing H7 or H1 haemagglutinins of influenza virus. Chickens were vaccinated at 2 or 7 days of age and challenged with virulent Australian avian influenza virus at 10 and 21 days later, respectively. Morbidity and mortality, body weight change and the development of immune responses to influenza haemagglutinin and nucleoprotein were recorded.

RESULTS

Vaccination of poultry with fowlpox H7 avian influenza virus recombinants induced protective immune responses. All chickens vaccinated at 7 days of age and challenged 21 days later were protected from death. Few clinical signs of infection developed. In contrast, unvaccinated or chickens vaccinated with a non-recombinant fowlpox or a fowlpox expressing the H1 haemagglutinin of human influenza were highly susceptible to avian influenza. All those chickens died within 72 h of challenge. In younger chickens, vaccinated at 2 days of age and challenged 10 days later the protection was lower with 80% of chickens protected from death. Chickens surviving vaccination and challenge had high antibody responses to haemagglutinin and primary antibody responses to nucleoprotein suggesting that although vaccination protected substantially against disease it failed to completely prevent replication of the challenge avian influenza virus.

CONCLUSION

Vaccination of chickens with fowlpox virus expressing the avian influenza H7 haemagglutinin provided good protection against experimental challenge with virulent avian influenza of H7 type. Although eradication will remain the method of first choice for control of avian influenza, in the circumstances of a continuing and widespread outbreak the availability of vaccines based upon fowlpox recombinants provides an additional method for disease control.

Baculovirus-derived hemagglutinin vaccines protect against lethal influenza infections by avian H5 and H7 subtypes

Baculoviruses were engineered to express hemagglutinin (HA) genes of recent avian influenza (AI) isolates of the H5 and H7 subtypes. The proteins were expressed as either intact (H7) or slightly truncated versions (H5). In both cases purified HA proteins from insect cell cultures retained hemagglutination activity and formed rosettes in solution, indicating proper folding. Although immunogenic in this form, these proteins were more effective when administered subcutaneously in a water-in-oil emulsion. One or two-day-old specific pathogen free (SPF) White Rock chickens, free of maternal AI antibodies, responded with variable serum HI titers, but in some cases the titers were comparable to those achieved using whole virus preparations. Vaccination of three-week-old chickens with 1.0 microg of protein per bird generated a more consistent serum antibody response with an average geometric mean titer (GMT) of 121 (H5) and 293 (H7) at 21 days postvaccination. When challenged with highly pathogenic strains of the corresponding AI subtypes, the vaccinated birds were completely protected against lethal infection and in some cases exhibited reduced or no cloacal shedding at 3 days postinfection. Vaccine protocols employing these recombinant HA proteins will not elicit an immune response against internal AI proteins and thus will not interfere with epidemiological surveys of natural influenza infections in the field.

Thermolysin activation mutants with changes in the fusogenic region of an influenza virus hemagglutinin

Influenza virus A/seal/Mass/1/80 (H7N7) mutants were obtained; the hemagglutinins (HAs) of the mutants were not activated by trypsin, as in the wild-type virus, but by thermolysin. The mutants grew efficiently under multiple replication cycle conditions and formed plaques in chicken embryo cells only when thermolysin was added to the culture medium. They exhibited hemolytic activity and induced protective immunity in chickens after an asymptomatic course of infection. Nucleotide sequencing of the HA gene and direct amino acid sequencing showed that insertion of a single leucine into the fusion peptide of the HA2 chain close to the cleavage site and a shift of the cleavage site toward the C terminus by one amino acid were responsible for the changes in the biological properties of the thermolysin activation mutants. Revertants could be obtained when trypsin or trypsin-like endoproteases were present in the virus-producing system.

Modification of infectious bursal disease virus antigen VP2 for cell surface location fails to enhance immunogenicity

The host protective antigen gene VP2 of infectious bursal disease virus (IBDV) was genetically modified and expressed by recombinant fowlpox viruses (rFPV). To achieve cell surface localization, VP2 was expressed as a hybrid protein with signal sequence and membrane anchors of influenza virus hemagglutinin or neuraminidase. Native VP2 was expressed as VP2 alone or as self-processing VP2-VP4-VP3 polyprotein for coexpression of IBDV structural proteins. VP2 hybrid protein containing the carboxy-terminal membrane anchor sequence of influenza virus hemagglutinin was located on the cell surface and was N-glycosylated. The expression of VP2 fused to the N-terminal signal/anchor sequence of influenza virus neuraminidase led to cell lysis and the VP2 protein remained mainly unglycosylated. Cell surface localization of VP2 reduced immunogenicity (antibody induction) and abolished protection in poultry in comparison with the native VP2 expressed by FPV as VP2 alone or as the self-processing VP2-VP4-VP3. Vaccination of poultry with rFPV expressing native VP2 protein alone provided better protection from IBDV infection than VP2 derived from the VP2-VP4-VP3 polyprotein.

DNA vaccines: protective immunizations by parenteral, mucosal, and gene-gun inoculations

Plasmid DNAs expressing influenza virus hemagglutinin glycoproteins have been tested for their ability to raise protective immunity against lethal influenza challenges of the same subtype. In trials using two inoculations of from 50 to 300 micrograms of purified DNA in saline, 67-95% of test mice and 25-63% of test chickens have been protected against a lethal influenza challenge. Parenteral routes of inoculation that achieved good protection included intramuscular and intravenous injections. Successful mucosal routes of vaccination included DNA drops administered to the nares or trachea. By far the most efficient DNA immunizations were achieved by using a gene gun to deliver DNA-coated gold beads to the epidermis. In mice, 95% protection was achieved by two immunizations with beads loaded with as little as 0.4 micrograms of DNA. The breadth of routes supporting successful DNA immunizations, coupled with the very small amounts of DNA required for gene-gun immunizations, highlight the potential of this remarkably simple technique for the development of subunit vaccines.

Protection against a lethal influenza virus challenge by immunization with a haemagglutinin-expressing plasmid DNA

Direct DNA inoculations have been used to demonstrate that in vivo transfections can be used to elicit protective immune responses. The direct inoculation of an H7 haemagglutinin-expressing DNA protected chickens against lethal challenge with an H7N7 influenza virus. Three-week-old chickens were vaccinated by inoculating 100 micrograms of plasma DNA by each of three routes (intravenous, intraperitoneal and subcutaneous). One month later, chickens were boosted with 100 micrograms of DNA by each of the three routes. At 1-2 weeks postboost, chickens were challenged via the nares with 100 lethal doses of an H7N7 virus. Low to undetectable levels of H7-specific antibodies were present postvaccination and boost. High titres of H7-specific antibodies appeared within 1 week of challenge. In a series of four experiments, 50% (28/56) of the DNA-vaccinated and < 2% (1/67) of the control chickens survived the challenge. This exceptionally simple method of immunization holds high promise for the development of subunit vaccines.

Hemagglutinin activation of pathogenic avian influenza viruses of serotype H7 requires the protease recognition motif R-X-K/R-R

The hemagglutinin of influenza virus A/FPV/Rostock/34 (H7) was altered at its multibasic cleavage site by site-directed mutagenesis and assayed for proteolytic activation after expression in CV-1 cells. The results indicated that the cellular protease responsible for activation recognizes the tetrapeptide motif R-X-K/R-R that must be presented in the correct sequence position. Studies on plaque variants of influenza virus A/fowl/Victoria/75 (H7N7) showed that alteration of the consensus sequence resulted in a loss of pathogenicity for chickens.

Subtype H7 influenza viruses: comparative antigenic and molecular analysis of the HA-, M-, and NS-genes

Antigenic analysis of the haemagglutinin and matrix protein with corresponding sets of monoclonal antibodies as well as sequence analysis of HA-, M-, and NS-genes were carried out to establish antigenic and genetic relationships between four fowl plague virus (FPV) strains of H7 subtype. The data obtained revealed close genetic relatedness between the oldest known influenza A virus, A/chicken/Brescia/1902 (H7N7), and two FPV strains, A/FPV/Dobson (H7N7) and A/FPV/Weybridge (H7N7). These three strains apparently differ in all genes investigated from the A/FPV/Rostock isolate.

Sequence analysis of the equine H7 influenza virus haemagglutinin gene

The nucleotide sequences of ten haemagglutinin genes of representative H7N7 equine influenza viruses isolated between 1956 and 1977 have been determined by primer extension sequencing. Their nucleotide and deduced amino acid sequences demonstrate a high degree of homology. These equine viruses can be divided into two distinct subgroups, the prototype-like, and a group comprising the early American isolates and the remaining equine viruses. The equine H7 haemagglutinins form a quite distinct group compared to H7 haemagglutinins isolated from other species. Each of these equine H7 haemagglutinins possess a tetrabasic amino acid cleavage site separating the HA1 and HA2 domains but, in addition, all ten contain a nine amino acid insertion prior to the tetrabasic sequence. The haemagglutinin glycoproteins of all ten viruses are capable of cleavage activation in virus infected primary chicken embryo fibroblast cells.

Reassortants with equine 1 (H7N7) influenza virus hemagglutinin in an avian influenza virus genetic background are pathogenic in chickens

Reassortants possessing the hemagglutinin (HA) gene from A/Equine/London/1416/73 (H7N7) [Eq/Lond] and five or more genes from A/Chicken/Pennsylvania/1370/83 (H5N2) [Ck/Penn] were lethal in chickens. This result demonstrates that horses can maintain influenza viruses whose HAs are capable of promoting virulence. Thus, reassortment of equine and avian influenza virus genes could generate viruses that might be lethal in domestic poultry.

Glycosylation of the hemagglutinin-neuraminidase glycoprotein of human parainfluenza virus type 1 affects its functional but not its antigenic properties

The hemagglutinin-neuraminidase (HN) glycoprotein of human parainfluenza virus type 1 (hPIV-1) has been shown to be similar in predicted protein sequence and structure to those of Sendai virus, but it is more highly glycosylated. Because glycosylation can modify protein structure and function, we investigated the effect of glycosylation on the antigenic structure and biological function of the HN of hPIV-1. Antigenic and functional analyses were carried out with purified hPIV-1 virions treated with Endoglycosidase F, which removes carbohydrate moieties, because treatment of hPIV-1-infected LLC-MK2 cells with an inhibitor of glycosylation resulted in virions which were deficient in both HN and F surface glycoproteins. No change in the antigenic structure of the HN of hPIV-1 was detected after carbohydrate removal; epitope recognition by a panel of 7 hPIV-1 HN monoclonal antibodies (MAbs) was unchanged compared to untreated virions. Moreover, there was no change in the cross-reactivity of 8 of 10 Sendai virus HN MAbs, and only a slight change in the remaining 2. Nor did carbohydrate removal appear to affect hemagglutinating or neuraminidase activities; hemagglutination titers with chicken erythrocytes (cRBC) were unchanged, and in vitro neuraminidase activity with a small substrate (N-acetylneuraminlactose) showed only a 20% reduction. However, elution of deglycosylated hPIV-1 from agglutinated cRBC as a result of neuraminidase activity was reduced by 80%. These results suggest that the enzymatic activity of hPIV-1 HN was not directly affected by carbohydrate removal but that the reduction in elution was due to a change in the interaction of the HN with the host receptor. This was further supported by a 2- to 16-fold reduction in the ability of all 7 hPIV-1 HN MAbs to inhibit hemagglutination of deglycosylated hPIV-1 virus. Such a change in HN-host receptor interaction was found to involve a change in receptor specificity because deglycosylated virus was able to fully agglutinate cRBC stripped of receptors required by the native, glycosylated virus. We propose the following model for our results: deglycosylation of the HN of hPIV-1 causes the hemagglutinating portion of the molecule to recognize a new receptor which is not susceptible to enzymatic cleavage by the neuraminidase.

Generation of seal influenza virus variants pathogenic for chickens, because of hemagglutinin cleavage site changes

Influenza virus A/seal/Mass/1/80 (H7N7) was adapted to grow in MDCK cells and chicken embryo cells (CEC) in the absence of exogenous protease. The biological properties of the virus variants obtained coincided with intracellular activation of the hemagglutinin (HA) by posttranslational proteolytic cleavage and depended on the cell type used for adaptation. MDCK cell-adapted variants contained point mutations in regions of the HA more distant from the cleavage site. It is proposed that these mutations are probably responsible, through an unknown mechanism, for enhanced cleavability of HA in MDCK cells. Such virus variants were apathogenic in chickens. CEC-adapted variants, on the other hand, contained an insertion of basic amino acids at the HA cleavage site, in addition to scattered point mutations. The insertions converted the cleavage sites in the variant virus HAs so that they came to resemble the cleavage site found in highly pathogenic avian influenza viruses. CEC variants with such cleavage site modifications were highly pathogenic for chickens. The lethal outcome of the infection in chickens demonstrated for the first time that an influenza virus derived from a mammalian species can be modified during adaptation to a new cell type to such an extent that the resulting virus variant becomes pathogenic for an avian species.

Bovine and mouse serum beta inhibitors of influenza A viruses are mannose-binding lectins

Normal bovine and mouse sera contain a component, termed beta inhibitor, that inhibits the infectivity and hemagglutinating activity of influenza A viruses of the H1 and H3 subtypes. To investigate the nature of the interaction of beta inhibitors with influenza A viruses we isolated a mutant of the virus Mem71H-BelN (H3N1) that could grow in the presence of bovine serum. The mutant virus was resistant to hemagglutination inhibition by mouse serum as well as by bovine serum and had undergone changes in the receptor-binding and the antigenic properties of its hemagglutinin (HA) molecule. Sequence analysis of the HA genes of parent and mutant viruses revealed a single nucleotide change in the mutant, resulting in the substitution Thr----Asn at residue 167 of the HA1 chain of HA. This change leads to loss of the potential glycosylation site Asn-165-Val-166-Thr-167 at the tip of the HA spike, which in viruses of the H3 subtype is known to bear a high-mannose (type II) carbohydrate side chain N-linked to Asn-165. The association of beta inhibitor resistance with loss of this carbohydrate side chain suggested that beta inhibitors may be lectins. In support of this hypothesis, treatment of the beta inhibitor-sensitive parent virus Mem71H-BelN with periodate converted it to the resistant state. Furthermore, the inhibitory activity of both bovine and mouse sera for the parental virus was abrogated by D-mannose. We conclude that the beta inhibitors in bovine and mouse sera are mannose-binding lectins that inhibit hemagglutination and neutralize virus infectivity by binding to carbohydrate at the tip of the HA spike, blocking access of cell-surface receptors to the receptor-binding site on HA.

The hemagglutinin-neuraminidase glycoproteins of human parainfluenza virus type 1 and Sendai virus have high structure-function similarity with limited antigenic cross-reactivity

Human parainfluenza virus type 1 (hPIV-1) is closely related to Sendai virus on the basis of cross-reactivity of antisera. We examined this association further by using monoclonal antibodies to the Sendai virus hemagglutinin-neuraminidase (HN) glycoprotein to determine the relationship between overall protein structure and the hemagglutination and neuraminidase functions. Of 10 monoclonal antibodies representing four nonoverlapping antigenic sites on the HN of Sendai virus, only 4 from two sites cross-reacted with hPIV-1, indicating a limited conservation of epitopes. One of these four inhibited the hemagglutinating activity of hPIV-1 comparably to Sendai virus, but none appreciably inhibited the neuraminidase activity of hPIV-1. The ability of some of these monoclonal antibodies to inhibit only hemagglutinating or neuraminidase activity of either virus provided evidence for two separate active sites on the HN molecule. To determine the overall structural relationship of the HNs of hPIV-1 and Sendai virus, we cloned and sequenced the HN gene of hPIV-1. The HN clone was made from genomic RNA and was identified by hybrid-arrested in vitro translation of mRNA. The predicted HN protein sequence of hPIV-1 was identical in length to that of Sendai virus and had a shared identity of 72%. There was a marked conservation of structural elements (cysteines, prolines, and glycines), which would predict a similar molecular conformation. However, there were 10 potential glycosylation sites on the HN of hPIV-1, compared with 5 on Sendai virus. Some of these sites may be responsible for the inability of the Sendai virus monoclonal antibodies to cross-react. The results of our study support a close structure-function relationship between hPIV-1 and Sendai virus but suggest limited antigenic cross-reactivity.

Single-amino-acid substitution in an antigenic site of influenza virus hemagglutinin can alter the specificity of binding to cell membrane-associated gangliosides

Antigenic variants of influenza virus A/Mem/1/71-Bel/42 (H3N1) selected with monoclonal antibodies and having single substitutions in their hemagglutinins were examined for their ability to hemagglutinate and hemolyse erythrocytes coated with different gangliosides. The majority of variants, including one with a substitution near the receptor-binding site (Asn-133----Lys), did not differ from the parent in specificity for receptor molecules. However, a substitution in HA1 at residue 205 (Ser----Tyr), which is distant from the receptor-binding site in antigenic site D, affected hemagglutination and hemolysis of erythrocytes coated with sialyl-paraglobosides. The variant preferentially recognized N-acetylneuraminic acid-alpha 2,6-galactose linkages to sialylparaglobosides, whereas the parent and other variants preferentially recognized N-acetylneuraminic acid-alpha 2,3-galactose linkages. In the trimeric hemagglutinin molecule, residue 205 is located across the subunit interface from the receptor-binding site. The bulky hydrophobic tyrosine in the variant may cause a conformational change in the receptor-binding pocket on the neighboring subunit and influence receptor binding.

Antigenic and molecular characterization of subtype H13 hemagglutinin of influenza virus

Influenza A viruses with subtype H13 hemagglutinin display an unusual host range. Although common in shorebirds, they are very rare or absent in wild ducks; additionally, H13 viruses have been isolated from a whale. To study the molecular basis for this host range, we have determined the complete nucleotide sequences of the hemagglutinin genes of three H13 influenza viruses from different species or geographical areas: A/gull/Maryland/77, A/gull/Astrachan (USSR)/84, and A/pilot whale/Maine/84. Based on the deduced amino acid sequences, H13 hemagglutinin shares the basic structure of other type A hemagglutinin subtypes such as H3, but has clearly diverged from other completely sequenced subtypes. Unique features of H13 hemagglutinin include the occurrence, near the receptor binding pocket, of residues Arg/Lys-227 and Trp-229 (H3 numbering); the significance of these are unknown. The sequence of the HA1-HA2 cleavage site resembles those of avirulent avian influenza viruses. The whale H13 hemagglutinin is similar to those from gulls, supporting the hypothesis that influenza viruses from avian sources can enter marine mammal populations but are probably not permanently maintained there. Antigenic analysis using a panel of monoclonal antibodies suggests that, like other subtypes, H13 viruses are heterogeneous, with different antigenic variants predominating in the eastern versus the western hemispheres.

Laboratory characterization of a swine influenza virus isolated from a fatal case of human influenza

A swine influenza virus-like type A (H1N1) virus, designated A/Wisconsin/3523/88, was isolated in September 1988 from a Wisconsin woman who had died with primary viral pneumonia. Antigenic analyses with hemagglutinin-specific monoclonal antibodies and postinfection ferret serum indicated that the hemagglutinin of A/Wisconsin/3523/88 was antigenically closely related to viruses currently circulating in swine. Genetic analysis of the A/Wisconsin/3523/88 virus by RNA fingerprinting and partial RNA sequence analysis of seven of the eight segments indicated that the genome of the human isolate was similar to that of enzootic swine viruses. These laboratory data supported the epidemiologic findings that this human infection occurred by transmission of an enzootic swine influenza virus and that the virus showed no major genetic changes potentially related to increased pathogenesis.

Analysis of the relationship between cleavability of a paramyxovirus fusion protein and length of the connecting peptide

The relationship between the length of the connecting peptide in a paramyxovirus F0 protein and cleavage of F0 into the F1 and F2 subunits has been examined by constructing a series of mutant F proteins via site-directed mutagenesis of a cDNA clone encoding the simian virus 5 F protein. The mutant F proteins had one to five arginine residues deleted from the connecting peptide. The minimum number of arginine residues required for cleavage-activation of the simian virus 5 F0 protein by host cell proteases was found to be four. F proteins with two or three arginine residues in the connecting peptide were not cleaved by host cell proteases but could be cleaved by exogenously added trypsin. The mutant F protein possessing a connecting peptide consisting of one arginine residue was not cleaved by trypsin. The altered F proteins were all transported to the infected-cell plasma membrane as shown by cell surface immunofluorescence or cell surface trypsinization. However, the only mutant F protein found to be biologically active as detected by syncytium formation was the F protein which has four arginine residues at the cleavage site. The results presented here suggest that in the paramyxovirus F protein the number of basic amino acid residues in the connecting peptide is important for cleavage of the precursor protein by host cell proteases but is not the only structural feature involved. In addition, the data indicate that cleavage of F0 into F1 and F2 does not necessarily result in biological activity and that the connecting peptide may affect the local conformation of the F polypeptide.

Complete nucleotide sequence of the neuraminidase gene of the human influenza virus A/Chile/1/83 (H1N1). Brief report

The complete nucleotide sequence of the neuraminidase (NA) gene of influenza virus A/Chile/1/83 (H1N1) has been determined after reverse transcription and cloning into the plasmid pAT 153/PvuII/8. The gene is 1461 nucleotides long and codes for a protein of 470 amino acids. The overall nucleotide and predicted amino acid sequence of the A/Chile/1/83 NA exhibits a high homology with other N1 neuraminidases. Hyper-variable regions concerning A to G exchanges are discussed.

Evolution of influenza polymerase: nucleotide sequence of the PB2 gene of A/Chile/1/83 (H1 N1)

The complete nucleotide sequence of the PB2 gene of influenza virus A/Chile/1/83 (H1 N1) is presented. Sequence comparison between A/Chile PB2 protein and the known PB2 sequences of the influenza strains A/WSN/33 (H1 N1), A/PR/8/34 (H1 N1), A/NT/60/68 (H3 N2), A/Kiev/59/79 (H1 N1), A/FPV/Rostock/34 (H7 N1), and B/Ann Arbor/1/66 indicates extensive amino acid homology for the influenza A virus PB2 proteins. Small clusters of basic amino acids are conserved in all PB2 proteins including the influenza B PB2 protein which has only 39% sequence homology overall to the PB2 polypeptides of type A influenza viruses. The evolutionary rate of 5.7 x 10(-3) nucleotide substitutions per site per year and 0.25% amino acid changes per year between the A/Chile/1/83 and A/NT/60/68 PB2 appears to be higher than that calculated earlier for A/NT, A/PR/8 and A/WSN. An unusually high degree of sequence change between A/Chile/1/83 and A/Kiev/59/79 PB2 polymerase was revealed and this is discussed in terms of its probable origin.

Retrovirus-expressed hemagglutinin protects against lethal influenza virus infections

An influenza virus hemagglutinin gene, H7, has been expressed in a replication-competent Schmidt-Ruppin Rous sarcoma virus-derived vector. This virus, P1/H7, expressed a glycosylated precursor of the H7 protein which was processed to a mature form and transported to the cell surface. The expressed H7 glycoprotein could not be detected in P1/H7 virus particles. A P1/H7 stock which expressed 5 to 10% of the level of H7 observed in influenza virus-infected chicken embryo fibroblasts was used to immunize 1-month-old chickens. This immunization resulted in low or undetectable levels of hemagglutination-inhibiting and neutralizing antibody. Despite the low serum response, challenge with a highly pathogenic H7N7 virus revealed complete protection against lethal infection.

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.

Formation of mixed hemagglutinin trimers in the course of double infection with influenza viruses belonging to different subtypes

Chick embryo primary cultured cells were infected with influenza viruses belonging to H1, H2, H3, H5 or H7 subtypes of hemagglutinin. The cells were subjected to a single or a double infection, labelled with 14C-amino acids from 2 to 6 hours postinfection, lysed with a mixture of ionic and non-ionic detergents, and the lysates were clarified by low-speed centrifugation. The clarified lysates contained 14C-labelled hemagglutinin mostly in the form of 9S trimers, as shown by velocity sedimentation in sucrose gradients with polyacrylamide gel electrophoresis (PAGE) analysis of the gradient fractions. The lysates were immunoprecipitated with antihemagglutinin antibodies specific for one of the co-infecting viruses. The immunoprecipitates were analysed by PAGE. Cells infected separately with each virus and mixed before lysis were used as a control sample in every experiment. In the lysates of cells doubly infected with H2 and H5 influenza viruses the analysis revealed the presence of structures containing HA monomers of both viruses, whereas no such structures were revealed in the lysate of a mixture of separately infected cells. Mixed structures (most likely HA trimers containing monomers of the two co-infecting viruses) were also found in the lysates of cells doubly infected with strains belonging to H1 and H2 subtypes. No such structures were revealed when the cells were co-infected with viruses belonging to H1 and H3 subtypes or H3 and H7 subtypes. The results suggest an extensive formation of mixed HA trimers in the course of double infection with viruses belonging to closely related subtypes, whereas the formation of mixed trimers by more distantly related HA monomers does not occur or is very scarce. The identity of the mixed structures as HA trimers was confirmed by immunoprecipitation experiments with 9S structures.

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 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.

Host cell-mediated selection of a mutant influenza A virus that has lost a complex oligosaccharide from the tip of the hemagglutinin

During serial passage in Madin-Darby bovine kidney (MDBK) cells, a substrain of influenza virus A/WSN is lost from the population and is replaced by a mutant virus with altered host cell binding properties. This selection does not occur during growth in chicken embryo fibroblasts (CEF). It occurs during growth in MDBK cells because the parental virus produced by these cells has a dramatically reduced affinity for cellular receptors [Crecelius, D.M., Deom, C. M. & Schulze, I. T. (1984) Virology 139, 164-177]. We have now compared the hemagglutinin (HA) subunits, HA1 and HA2, of the parent and mutant viruses by NaDodSO4/PAGE and have found that when the viruses are grown in either host cell the HA1 subunit of the mutant is smaller than that of the parent virus. The nonglycosylated HAs, made in the presence of tunicamycin, have the same apparent molecular weight, indicating that the HA1 subunit of the mutant virus contains less carbohydrate than that of the parent. This reduction in carbohydrate content was observed with 11 independently derived mutants that had been selected by growth in MDBK cells. The nucleotide sequence of the HA gene of the parent and mutant viruses indicates that there are five potential glycosylation sites on the parent HA1 subunit and four on the mutant and that the mutation responsible for this difference is a single base change that eliminates the glycosylation site at amino acid 125 of the parent HA1 subunit. Treatment of the parent and mutant HAs from both cell sources with endo-beta-N-acetylglucosaminidases F and H showed that the HA1 of the parent virus has four complex and one high-mannose oligosaccharides, whereas that of the mutant virus has three complex and one high-mannose oligosaccharides. Thus, all of the potential sites on both HA1 subunits are glycosylated. We conclude that the oligosaccharide attached to amino acid 125 of the parent HA by MDBK cells can reduce the affinity of the virus for cellular receptors and that the mutant virus has a higher affinity than the parent because the mutant HA is not glycosylated at that site. Since amino acid 125 of the parent HA is glycosylated by both CEF and MDBK cells, we further conclude that the host-determined structure of the oligosaccharide at that site affects the affinity of the parent virus for cellular receptors and, thereby, determines whether the mutant virus will have a growth advantage.

Spin-labeling of influenza virus hemagglutinin permits analysis of the conformational change at low pH and its inhibition by antibody

To study the conformational changes in the hemagglutinin (HA) molecule of A/seal/Mass/1/80 (H7N7) (Seal) influenza virus at low pH, a spin-labeling method was used. This method also permits study of antibody interaction with the HA. A synthetic nitroxide compound was used for spin-labeling of tyrosine residues of the isolated HA molecule. Electron spin resonance (ESR) spectra of the spin-labeled HA at various pH values indicated that a conformational transition occurred under acidic conditions, and around pH 5.8 the HA molecule has maximal flexibility. Since virus-induced hemolysis occurs optimally at pH 5.8-5.9, the HA molecule in the maximally flexible conformation is considered to mediate membrane fusion. The ESR spectra of the antibody-bound HA at various pH values revealed that monoclonal antibodies to different regions on the molecule may inhibit the conformational change by different modes. One antibody inhibited the changes in the HA that resulted in flexibility, while the other did not. These results support the assumption that monoclonal antibodies, which failed to inhibit hemagglutination of the virus yet neutralized viral infectivity, inhibited the fusion step in the viral replication process by interfering with a low pH-induced conformational change in the HA molecule (Kida, H., Webster, R.G. and Yanagawa, R. (1983) Arch. Virol. 76, 91-99).

Antigenic structure of the influenza B virus hemagglutinin: nucleotide sequence analysis of antigenic variants selected with monoclonal antibodies

We report here the complete nucleotide sequence of the hemagglutinin (HA) gene of influenza B virus B/Oregon/5/80 and, through comparative sequence analysis, identify amino acid substitutions in the HA1 polypeptide responsible for the antigenic alterations in laboratory-selected antigenic variants of this virus. The complete nucleotide sequence of the B/Oregon/5/80 HA gene was established by a combination of chemical sequencing of a full-length cDNA clone and dideoxy sequencing of the virion RNA. The nucleotide sequence is very similar to previously reported influenza B virus HA gene sequences and differs at only nine nucleotide positions from the B/Singapore/222/79 HA gene (Verhoeyen et al., Nucleic Acids Res. 11:4703-4712, 1983). The nucleotide sequences of the HA1 portions of the HA genes of 18 laboratory-selected antigenic variants were determined by the dideoxy method. Comparison of the deduced amino acid sequences of the parental and variant HA1 polypeptides revealed 16 different amino acid substitutions at nine positions. All amino acid substitutions resulted from single-point mutations, and no double mutants were detected, demonstrating that as in the influenza A viruses, single amino acid substitutions are sufficient to alter the antigenicity of the HA molecule. Many of the amino acid substitutions in the variants occurred at positions also observed to change in natural drift strains. The substitutions appear to identify at least two immunodominant regions which correspond to proposed antigenic sites A and B on the influenza A virus H3 HA.

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.

Mutations in the hemagglutinin receptor-binding site can change the biological properties of an influenza virus

Avian influenza virus reassortants containing human influenza virus hemagglutinins do not replicate in ducks. Two mutations in the receptor-binding site of a human hemagglutinin at residues 226 and 228 allowed replication in ducks. The mutations resulted in a receptor-binding-site sequence identical to the known avian influenza virus sequences.

Sequence of the neuraminidase gene of an avian influenza A virus (A/parrot/ulster/73, H7N1)

The complete sequence of the neuraminidase (NA) gene of the influenza A strain A/parrot/ Ulster /73 ( H7N1 ) has been determined after reverse transcribing and cloning it into the pBR322 plasmid, followed by subcloning into M13 vectors and sequencing with dideoxynucleotide chain terminators. The gene consists of 1458 nucleotides and codes for a protein of 469 amino acids. The neuraminidase has seven potential glycosylation sites. According to the molecular weight as determined by electrophoretic migration in polyacrylamide gel all of these sites might carry a carbohydrate side chain. When the parrot Ulster NA was compared with two other N1 neuraminidases, those of the human PR8 and WSN strains, deletions in the stalk region of 15 amino acids for PR8 NA and of 16 amino acids for WSN NA were apparent. No further rearrangements were found within N1 neuraminidases. Although the parrot Ulster strain was isolated 40 years after the two human strains, the base sequence homology of their NA genes is still 83 or 82%, respectively.