Adaptation of influenza A viruses to cells expressing low levels of sialic acid leads to loss of neuraminidase activity

Influenza Virus Carrying a Codon-Reprogrammed Neuraminidase Gene as a Strategy for Live Attenuated Vaccine

Live attenuated influenza vaccines offer broader and longer-lasting protection in comparison to inactivated influenza vaccines. The neuraminidase (NA) surface glycoprotein of influenza A virus is essential for the release and spread of progeny viral particles from infected cells. In this study, we de novo synthesized the NA gene, in which 62% of codons were synonymously changed based on mammalian codon bias usage. The codon-reprogrammed NA (repNA) gene failed to be packaged into the viral genome, which was achievable with partial restoration of wild-type NA sequence nucleotides at the 3' and 5' termini. Among a series of rescued recombinant viruses, we selected 20/13repNA, which contained 20 and 13 nucleotides of wild-type NA at the 3' and 5' termini of repNA, respectively, and evaluated its potential as a live attenuated influenza vaccine. The 20/13repNA is highly attenuated in mice, and the calculated LD₅₀ was about 10,000-fold higher than that of the wild-type (WT) virus. Intranasal inoculation of the 20/13repNA virus in mice induced viral-specific humoral, cell-mediated, and mucosal immune responses. Mice vaccinated with the 20/13repNA virus were protected from the lethal challenge of both homologous and heterologous viruses. This strategy may provide a new method for the development of live, attenuated influenza vaccines for a better and more rapid response to influenza threats.

Phenotypic effects of mutations observed in the neuraminidase of human origin H5N1 influenza A viruses

Global spread and regional endemicity of H5Nx Goose/Guangdong avian influenza viruses (AIV) pose a continuous threat for poultry production and zoonotic, potentially pre-pandemic, transmission to humans. Little is known about the role of mutations in the viral neuraminidase (NA) that accompanied bird-to-human transmission to support AIV infection of mammals. Here, after detailed analysis of the NA sequence of human H5N1 viruses, we studied the role of A46D, L204M, S319F and S430G mutations in virus fitness in vitro and in vivo. Although H5N1 AIV carrying avian- or human-like NAs had similar replication efficiency in avian cells, human-like NA enhanced virus replication in human airway epithelia. The L204M substitution consistently reduced NA activity of H5N1 and nine other influenza viruses carrying NA of groups 1 and 2, indicating a universal effect. Compared to the avian ancestor, human-like H5N1 virus has less NA incorporated in the virion, reduced levels of viral NA RNA replication and NA expression. We also demonstrate increased accumulation of NA at the plasma membrane, reduced virus release and enhanced cell-to-cell spread. Furthermore, NA mutations increased virus binding to human-type receptors. While not affecting high virulence of H5N1 in chickens, the studied NA mutations modulated virulence and replication of H5N1 AIV in mice and to a lesser extent in ferrets. Together, mutations in the NA of human H5N1 viruses play different roles in infection of mammals without affecting virulence or transmission in chickens. These results are important to understand the genetic determinants for replication of AIV in mammals and should assist in the prediction of AIV with zoonotic potential.

Influenza A (H6N6) viruses isolated from chickens replicate in mice and human lungs without prior adaptation

The H6H6 subtype avian influenza virus (AIV) is currently prevalent in wild birds and poultry. Its host range has gradually expanded to mammals, such as swines. Some strains have even acquired the ability to bind to human-like SAα-2,6 Gal receptors, thus increasing the risk of animal to human transmission. To investigate whether the H6N6 AIV can overcome interspecies barriers from poultry to mammals and even to humans, we have assessed the molecular characteristics, receptor-binding preference, replication in mice and human lungs of three chicken-originated H6N6 strains. Among these, the A/CK/Zhangzhou/346/2014 (ZZ346) virus with the P186T, H156R, and S263G mutations of the hemagglutinin molecule showed the ability to bind to avian-like SAα-2,3 Gal and human-like SAα-2,6 Gal receptors. Moreover, H6N6 viruses, especially the ZZ346 strain, could replicate and infect mice and human lungs. Our study showed the H6N6 virus binding to both avian-like and human-like receptors, confirming its ability to cross the species barrier to infect mice and human lungs without prior adaptation. This study emphasizes the importance of continuous and intense monitoring of the H6N6 evolution in terrestrial birds.

Quantitative RT-PCR Assays for Quantification of Undesirable Mutants in the Novel Type 2 Oral Poliovirus Vaccine

Emergence of mutations is an inherent property of RNA viruses with several implications for their replication, pathogenesis, and evolutionary adaptation. Oral poliovirus vaccine (OPV), developed by Albert Sabin, is composed of live attenuated polioviruses of three serotypes that can revert to neurovirulence during replication in cell culture and in vaccine recipients. Recently, a new modified variant of Sabin 2 virus was developed by introducing changes in its genome, making it more genetically stable to prevent the reversion. The new strain was used to manufacture novel OPV2 (nOPV2), which was approved by the World Health Organization for emergency use to stop outbreaks caused by circulating vaccine-derived poliovirus (cVDPV2). Manufacture of this improved vaccine requires close attention to the genetic heterogenicity to ensure that the levels of the undesirable mutations are limited. Preliminary studies using whole-genome Illumina sequencing (NGS) identified several genomic sites where mutations tend to occur with regularity. They include VP1-I₁₄₃T amino acid change at the secondary attenuation site; VP1-N₁₇₁D, a substitution that modestly increases neurovirulence in mice; and VP1-E₂₉₅K, which may reduce the immunogenicity of the nOPV2. Therefore, to ensure the molecular consistency of vaccine batches, the content of these mutants must be quantified and kept within specifications. To do this, we have developed quantitative, multiplex, one-step reverse-transcriptase polymerase chain reactions (qmosRT-PCRs) as simple methods for quantification of these mutations. Each method uses specific short TaqMan probes with different dyes for the analysis of both mutants and non-mutants in the same sample. The quantification is done using calibration curves developed using validated reference materials. To evaluate the sensitivity and the linearity of the qmosRT-PCR method, the mutant viruses were spiked in non-mutant viruses, and nOPV2 batches were used to validate the method. The spiked samples and the nOPV2 batches were analyzed by qmosRT-PCR and NGS assays. The results showed that qmosRT-PCR is sensitive enough to detect around 1% of mutants. The percentages of mutants determined by qmosRT-PCR correlate well with the results of the NGS. Further, the analysis of the nOPV2 batches showed that the results of qmosRT-PCR correlated well with the results of NGS. In conclusion, the qmosRT-PCR is a specific, sensitive, and linear method. It could be used for quality control of the nOPV2 batches.

Animal models for the risk assessment of viral pandemic potential

Pandemics affect human lives severely and globally. Experience predicts that there will be a pandemic for sure although the time is unknown. When a viral epidemic breaks out, assessing its pandemic risk is an important part of the process that characterizes genomic property, viral pathogenicity, transmission in animal model, and so forth. In this review, we intend to figure out how a pandemic may occur by looking into the past influenza pandemic events. We discuss interpretations of the experimental evidences resulted from animal model studies and extend implications of viral pandemic potentials and ingredients to emerging viral epidemics. Focusing on the pandemic potential of viral infectious diseases, we suggest what should be assessed to prevent global catastrophes from influenza virus, Middle East respiratory syndrome coronavirus, dengue and Zika viruses.

Influenza A Virus Hemagglutinin-Neuraminidase-Receptor Balance: Preserving Virus Motility

Influenza A viruses (IAVs) occasionally cross the species barrier and adapt to novel host species. This requires readjustment of the functional balance of the sialic acid receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA) to the sialoglycan-receptor repertoire of the new host. Novel techniques have revealed mechanistic details of this HA-NA-receptor balance, emphasizing a previously underappreciated crucial role for NA in driving the motility of receptor-associated IAV particles. Motility enables virion penetration of the sialylated mucus layer as well as attachment to, and uptake into, underlying epithelial cells. As IAVs are essentially irreversibly bound in the absence of NA activity, the fine-tuning of the HA-NA-receptor balance rather than the binding avidity of IAV particles per se is an important factor in determining host species tropism.

Neuraminidase Gene Variations in Influenza A(H1N1)pdm09 Virus among Patients Admitted to Refferal Pulmonary Hospital, Tehran, Iran in 2009-2013

BACKGROUND

Neuraminidase (NA) is one of the surface proteins of influenza A virus, which plays an important role in immunization against influenza infection and is recognized as an important therapeutic target. Genetic and antigenic changes and substitutions can influence the efficacy of vaccine and change viral sensitivity to NA inhibitors (NAIs). In this study, we performed phylogenetic and molecular analyses of NA changes in influenza A(H1N1)pdm09 virus, compared them with the corresponding vaccine strain, and examined drug resistance mutations in isolates from patients.

MATERIALS AND METHODS

The complete sequence of NA genes from 34 pandemic H1N1 isolates (identified in 2009-2010, 2010-2011, and 2013) was determined and analyzed both genetically and antigenically. The phylogenetic tree was plotted relative to the corresponding vaccine strain, using MEGA6 software package, based on the maximum likelihood method and JTT matrix (bootstrap value of 1000).

RESULTS

The phylogenetic analysis of pandemic isolates showed 31 amino acid substitutions in NA genes, compared to the vaccine strain. Some of these substitutions (N248D, V241I, N369K, N44S, and N200S) were important in terms of phylogenetic relationship, while the rest (D103N, V106I, R130T, N200S, G201E, and G414R) influenced the antigenic indices of B-cell epitopes. The catalytic sites, framework sites, and N-glycosylation remained unchanged in the studied samples. Meanwhile, H275Y substitution, related to oseltamivir resistance, was detected in 3 isolates. The average nucleotide identity of NAs with the corresponding vaccine strain was 99.415%, 98.607%, and 98.075% in 2009-2010, 2010-2011, and 2012-2013, respectively.

CONCLUSION

In this study, we provided basic information on the genetic and antigenic changes of NA genes in influenza A(H1N1)pdm09 virus from patients in 3 different seasons in Tehran, Iran. Considering the viral NAI resistance and changes in NA gene sequences of the isolates in comparison with the vaccine strain, further studies should be performed to monitor genetic changes in Iran. Moreover, the efficacy of vaccines should be examined.

Reasons for influenza vaccination underutilization: A case-control study

BACKGROUND

Influenza vaccines are underused.

METHODS

Most (131/140) patients from a pediatric practice who were tested for influenza in the 2012-2013 season were enrolled. Medical records plus questionnaires determined vaccine and past disease histories and influenza vaccine attitudes. Influenza-negative tested cases (n = 65) and negative controls (n = 110) closely age-matched to 55 test-positive cases were compared with influenza-positive cases (n = 66) regarding prior influenza, vaccine efficacy, and limited vaccine season conflicting with birth dates and preventative visit timing to determine possible validity of reasons given for underutilization.

RESULTS

The most common parental reason for not vaccinating was lack of perceived need. History of previous influenza was significantly (P < .0001) associated with disease. Live attenuated vaccine rates were greater in controls than in influenza patients for ages 2-18 years (P < .005) and for ages 6-18 years (P < .0001), whereas injectable vaccine rates were not (P = .30 and P = .60, respectively). Most positive cases (59%) and controls (89%) had no prior influenza.

CONCLUSIONS

Prior influenza disease may be a risk factor for infection that could influence vaccination benefit. Live attenuated influenza vaccine outperformed trivalent inactivated influenza vaccine. Limited disease experience in individuals with low influenza vaccination rates, along with vaccine efficacy limitations, lends validity to some underutilization.

Experimental Approaches to Study Genome Packaging of Influenza A Viruses

The genome of influenza A viruses (IAV) consists of eight single-stranded negative sense viral RNAs (vRNAs) encapsidated into viral ribonucleoproteins (vRNPs). It is now well established that genome packaging (i.e., the incorporation of a set of eight distinct vRNPs into budding viral particles), follows a specific pathway guided by segment-specific cis-acting packaging signals on each vRNA. However, the precise nature and function of the packaging signals, and the mechanisms underlying the assembly of vRNPs into sub-bundles in the cytoplasm and their selective packaging at the viral budding site, remain largely unknown. Here, we review the diverse and complementary methods currently being used to elucidate these aspects of the viral cycle. They range from conventional and competitive reverse genetics, single molecule imaging of vRNPs by fluorescence in situ hybridization (FISH) and high-resolution electron microscopy and tomography of budding viral particles, to solely in vitro approaches to investigate vRNA-vRNA interactions at the molecular level.

Deep Sequencing for Evaluation of Genetic Stability of Influenza A/California/07/2009 (H1N1) Vaccine Viruses

Virus growth during influenza vaccine manufacture can lead to mutations that alter antigenic properties of the virus, and thus may affect protective potency of the vaccine. Different reassortants of pandemic "swine" H1N1 influenza A vaccine (121XP, X-179A and X-181) viruses as well as wild type A/California/07/2009(H1N1) and A/PR/8/34 strains were propagated in embryonated eggs and used for DNA/RNA Illumina HiSeq and MiSeq sequencing. The RNA sequences of these viruses published in NCBI were used as references for alignment of the sequencing reads generated in this study. Consensus sequences of these viruses differed from the NCBI-deposited sequences at several nucleotides. 121XP stock derived by reverse genetics was more heterogeneous than X-179A and X-181 stocks prepared by conventional reassortant technology. Passaged 121XP virus contained four non-synonymous mutations in the HA gene. One of these mutations (Lys₂₂₆Glu) was located in the Ca antigenic site of HA (present in 18% of the population). Two non-synonymous mutations were present in HA of viruses derived from X-179A: Pro₃₁₄Gln (18%) and Asn₁₄₆Asp (78%). The latter mutation located in the Sa antigenic site was also detected at a low level (11%) in the wild-type A/California/07/2009(H1N1) virus, and was present as a complete substitution in X-181 viruses derived from X-179A virus. In the passaged X-181 viruses, two mutations emerged in HA: a silent mutation A₁₃₉₈G (31%) in one batch and G₇₅₆T (Glu₂₅₂Asp, 47%) in another batch. The latter mutation was located in the conservative region of the antigenic site Ca. The protocol for RNA sequencing was found to be robust, reproducible, and suitable for monitoring genetic consistency of influenza vaccine seed stocks.

Allele-specific PCR for quantitative analysis of mutants in live viral vaccines

Monitoring consistency of genetic composition of oral polio vaccine (OPV) is a part of its quality control. It is performed by mutant analysis by PCR and restriction enzyme cleavage (MAPREC) used to quantify neurovirulent revertants in the viral genome. Here an alternative method based on quantitative PCR is proposed. Allele-specific quantitative polymerase chain reaction (asqPCR) uses a "tethered" oligonucleotide primer consisting of two specific parts connected by a polyinosine stretch. Homogeneous DNA from plasmids containing wild Leon/37 and attenuated Sabin 3 sequences with 100% 472(C) and 100% 472(T) could only be amplified using homologous primers. Real-time implementation of the allele-specific PCR resulted in sensitive detection of 472(C) revertants with the limit of quantitation of less than 0.05%. Monovalent vaccine batches and international viral references for MAPREC test were used to validate the method. asqPCR performed with the WHO references and monovalent batches of vaccine showed that the new method could measure accurately and reproducibly the content of revertants producing values comparable to MAPREC results. This suggests that asqPCR could be used as an alternative to MAPREC for lot release of OPV. The method could also be used for the quantitation of other mutants in populations of microorganisms.

Deep sequencing approach for genetic stability evaluation of influenza A viruses

Assessment of genetic stability of viruses could be used to monitor manufacturing process of both live and inactivated viral vaccines. Until recently such studies were limited by the difficulty of detecting and quantifying mutations in heterogeneous viral populations. High-throughput sequencing technologies (deep sequencing) can generate massive amounts of genetic information and could be used to reveal and quantify mutations. Comparison of different approaches for deep sequencing of the complete influenza A genome was performed to determine the best way to detect and quantify mutants in attenuated influenza reassortant strain A/Brisbane/59/2007 (H1N1) and its passages in different cell substrates. Full-length amplicons of influenza A virus segments as well as multiple overlapping amplicons covering the entire viral genome were subjected to several ways of DNA library preparation followed by deep sequencing using Solexa (Illumina) and pyrosequencing (454 Life Science) technologies. Sequencing coverage (the number of times each nucleotide was determined) of mutational profiles generated after 454-pyrosequencing of individually synthesized overlapping amplicons were relatively low and insufficiently uniform. Amplification of the entire genome of influenza virus followed by its enzymatic fragmentation, library construction, and Illumina sequencing resulted in high and uniform sequencing coverage enabling sensitive quantitation of mutations. A new bioinformatic procedure was developed to improve the post-alignment quality control for deep-sequencing data analysis.

An NA-deficient 2009 pandemic H1N1 influenza virus mutant can efficiently replicate in cultured cells

We identified a novel neuraminidase (NA)-deficient virus that was a 2009 pandemic influenza H1N1 virus mutant. The mutant virus had a deletion of 1,009 nt in the NA gene and lacked an enzymatic domain. Although the yield of the NA-deficient virus was limited, it formed large plaques when applied to MDCK cell cultures, indicating that the virus was able to spread to adjacent cells. Furthermore, the NA-deficient virus was eluted from chicken erythrocytes at 37 °C, even in the presence of the antiviral drug peramivir. Spread of this NA-deficient virus may pose a potential threat to anti-influenza therapies.

Influenza HA subtypes demonstrate divergent phenotypes for cleavage activation and pH of fusion: implications for host range and adaptation

The influenza A virus (IAV) HA protein must be activated by host cells proteases in order to prime the molecule for fusion. Consequently, the availability of activating proteases and the susceptibility of HA to protease activity represents key factors in facilitating virus infection. As such, understanding the intricacies of HA cleavage by various proteases is necessary to derive insights into the emergence of pandemic viruses. To examine these properties, we generated a panel of HAs that are representative of the 16 HA subtypes that circulate in aquatic birds, as well as HAs representative of the subtypes that have infected the human population over the last century. We examined the susceptibility of the panel of HA proteins to trypsin, as well as human airway trypsin-like protease (HAT) and transmembrane protease, serine 2 (TMPRSS2). Additionally, we examined the pH at which these HAs mediated membrane fusion, as this property is related to the stability of the HA molecule and influences the capacity of influenza viruses to remain infectious in natural environments. Our results show that cleavage efficiency can vary significantly for individual HAs, depending on the protease, and that some HA subtypes display stringent selectivity for specific proteases as activators of fusion function. Additionally, we found that the pH of fusion varies by 0.7 pH units among the subtypes, and notably, we observed that the pH of fusion for most HAs from human isolates was lower than that observed from avian isolates of the same subtype. Overall, these data provide the first broad-spectrum analysis of cleavage-activation and membrane fusion characteristics for all of the IAV HA subtypes, and also show that there are substantial differences between the subtypes that may influence transmission among hosts and establishment in new species.

IAV is associated with significant morbidity and mortality, and represents a challenging public health threat that affects social and economic welfare each year, particularly during IAV pandemics. Although we know that all human strains derive, either directly or via intermediate hosts, from avian viral sources, we know very little about the phenotypic characteristics of the 16 HA subtypes that circulate in aquatic birds and have potential to infect mammals. HA membrane fusion properties, in conjunction with the characteristics for protease activation of HA, a requirement for fusion, are critical factors involved in the ecology and transmission of IAVs, and need to be understood if we are to derive explanations for how pandemic viruses emerge in humans. We examined the cleavage-activation and membrane fusion characteristics for the 16 HA subtypes by transiently expressing HA proteins in cells. Our findings show that the cleavability of the HAs vary considerably between subtypes and depending on the protease. Additionally, analysis of the pH of fusion for each subtype showed that HA stability varied significantly among the subtypes, as well as within subtypes from viruses isolated from different species. Overall, these data have implications for host range, potential for adaptation, and persistence in natural environments.

Attenuation of an influenza A virus due to alteration of its hemagglutinin-neuraminidase functional balance in mice

Influenza A viruses possess two surface glycoproteins, hemagglutinin (HA), which binds to sialic-acid-containing receptors, and neuraminidase (NA), which removes sialic acid from host cells. It is well established that the HA-NA functional balance regulates the efficiency of virus replication. Here, we selected a plaque variant of the WSN (H1N1) strain that grew better than the wild-type virus in NA-expressing MDCK cell culture. A reverse genetics study revealed that the single mutation HA E190K, which occurs infrequently in naturally isolated H1N1 viruses, was responsible for the phenotype of this variant. Receptor assays indicated that this mutation did not affect the receptor specificity of HA but enhanced its receptor-binding affinity, resulting in altered HA-NA functional balance relative to that of the wild-type virus. We also found that this variant replicated in nasal turbinates at an equivalent level but in lungs at a lower level compared with wild-type virus, demonstrating its attenuation in mice. Together, our data demonstrated the importance of the HA-NA functional balance for influenza virus replication in an in vivo biological setting.

Emerged HA and NA mutants of the pandemic influenza H1N1 viruses with increasing epidemiological significance in Taipei and Kaohsiung, Taiwan, 2009-10

The 2009 influenza pandemic provided an opportunity to observe dynamic changes of the hemagglutinin (HA) and neuraminidase (NA) of pH1N1 strains that spread in two metropolitan areas -Taipei and Kaohsiung. We observed cumulative increases of amino acid substitutions of both HA and NA that were higher in the post–peak than in the pre-peak period of the epidemic. About 14.94% and 3.44% of 174 isolates had one and two amino acids changes, respective, in the four antigenic sites. One unique adaptive mutation of HA2 (E374K) was first detected three weeks before the epidemic peak. This mutation evolved through the epidemic, and finally emerged as the major circulated strain, with significantly higher frequency in the post-peak period than in the pre-peak (64.65% vs 9.28%, p<0.0001). E374K persisted until ten months post-nationwide vaccination without further antigenic changes (e.g. prior to the highest selective pressure). In public health measures, the epidemic peaked at seven weeks after oseltamivir treatment was initiated. The emerging E374K mutants spread before the first peak of school class suspension, extended their survival in high-density population areas before vaccination, dominated in the second wave of class suspension, and were fixed as herd immunity developed. The tempo-spatial spreading of E374K mutants was more concentrated during the post–peak (p = 0.000004) in seven districts with higher spatial clusters (p<0.001). This is the first study examining viral changes during the naïve phase of a pandemic of influenza through integrated virological/serological/clinical surveillance, tempo-spatial analysis, and intervention policies. The vaccination increased the percentage of E374K mutants (22.86% vs 72.34%, p<0.001) and significantly elevated the frequency of mutations in Sa antigenic site (2.36% vs 23.40%, p<0.001). Future pre-vaccination public health efforts should monitor amino acids of HA and NA of pandemic influenza viruses isolated at exponential and peak phases in areas with high cluster cases.

Deletions of neuraminidase and resistance to oseltamivir may be a consequence of restricted receptor specificity in recent H3N2 influenza viruses

Background

Influenza viruses attach to cells via sialic acid receptors. The viral neuraminidase (NA) is needed to remove sialic acids so that newly budded virions can disperse. Known mechanisms of resistance to NA inhibitors include mutations in the inhibitor binding site, or mutations in the hemagglutinin that reduce avidity for sialic acid and therefore reduce the requirement for NA activity.

Results

Influenza H3N2 isolates A/Oklahoma/323/03 (Fujian-like), A/Oklahoma/1992/05 (California-like), and A/Oklahoma/309/06 (Wisconsin-like) lost NA activity on passage in MDCK cells due to internal deletions in the NA-coding RNA segment. The viruses grow efficiently in MDCK cells despite diminished NA activity. The full length NA enzyme activity is sensitive to oseltamivir but replication of A/Oklahoma/323/03 and A/Oklahoma/309/06 in MDCK cells was resistant to this inhibitor, indicating that NA is not essential for replication. There was no change in HA activity or sequence after the NA activity was lost but the three viruses show distinct, quite restricted patterns of receptor specificity by Glycan Array analysis. Extensive predicted secondary structure in RNA segment 6 that codes for NA suggests the deletions are generated by polymerase skipping over base-paired stem regions. In general the NA deletions were not carried into subsequent passages, and we were unable to plaque-purify virus with a deleted NA RNA segment.

Conclusion

H3N2 viruses from 2003 to the present have reduced requirement for NA when passaged in MDCK cells and are resistant to NA inhibitors, possibly by a novel mechanism of narrow receptor specificity such that virus particles do not self-aggregate. These viruses delete internal regions of the NA RNA during passage and are resistant to oseltamivir. However, deletions are independently generated at each passage, suggesting that virus with a full length NA RNA segment initiates the first round of infection.

Up to new tricks - a review of cross-species transmission of influenza A viruses

Influenza is a highly contagious disease that has burdened both humans and animals since ancient times. In humans, the most dramatic consequences of influenza are associated with periodically occurring pandemics. Pandemics require the emergence of an antigenically novel virus to which the majority of the population lacks protective immunity. Historically, influenza A viruses from animals have contributed to the generation of human pandemic viruses and they may do so again in the future. It is, therefore, critical to understand the epidemiological and molecular mechanisms that allow influenza A viruses to cross species barriers. This review summarizes the current knowledge of influenza ecology, and the viral factors that are thought to determine influenza A virus species specificity.

Search for additional influenza virus to cell interactions

Sialyl oligosaccharides have long been considered to be the sole receptors for influenza virus. However, according to [1] some viruses are able to grow in sialic-free MDCK cells. Here we attempted to reveal a possible second, non-sialic receptor, hypothesizing the involvement of additional carbohydrate lectin recognition in influenza virus reception process, first of all in situations when a lectin of the host cell could recognize the viral carbohydrate ligand. We tested the presence of galactose- and sialic acid-binding lectins, as well as mannoside- and sulfo-N-acetyllactosamine-recognizing properties of MDCK and Vero cells using polyacrylamide neoglycoconjugates and antibodies. MDCK cells bind galactoside probes stronger than Vero cells, whereas Vero cells bind preferentially sialoside, mannoside and various sulfo-oligosaccharide probes. The probing of viruses with the neoglycoconjugates revealed specific 6'-HSO (3) LacNAc (but not other sulfated oligosaccharides) binding property of A and B human strains. Affinity of 6'-HSO (3) LacNAc probe was comparable with affinity of 6'-SiaLac probe but the binding was not inhibited by the sialooligosaccharide.

Measurement of antibodies to avian influenza virus A(H7N7) in humans by hemagglutination inhibition test

During the epizootic of highly pathogenic avian influenza A(H7N7) in 2003 in The Netherlands, RT-PCR and culture confirmed infection was detected in 89 persons who were ill. A modified hemagglutination inhibition (HI) test using horse erythrocytes and 2 hemagglutinating units of virus was applied to assess retrospectively the extent of human (subclinical) infection. Validation of the HI-test with sera from 34 RT-PCR and culture confirmed A(H7) infected persons and sera from 100 persons from a human influenza vaccine trial in autumn 2002 showed that this HI-test had a sensitivity of 85% and a specificity of 100% when using a cut-off titer of > or =10. Using this cut-off value, A(H7) specific antibodies were detected in 49% of 508 persons exposed to poultry and in 64% of 63 persons exposed to A(H7) infected persons. Correlation of seropositivity with the occurrence of eye symptoms in exposed persons who had not received antiviral prophylaxis and of reduced seropositivity with taking antiviral prophylaxis provided further evidence that the A(H7) HI antibody titers were real. In conclusion, by applying an HI-test using horse erythrocytes human antibodies against the avian A(H7N7) virus were detected with high sensitivity and specificity in an unexpectedly high proportion of exposed persons.

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

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

Characterization of a neuraminidase-deficient influenza a virus as a potential gene delivery vector and a live vaccine

We recently identified a packaging signal in the neuraminidase (NA) viral RNA (vRNA) segment of an influenza A virus, allowing us to produce a mutant virus [GFP(NA)-Flu] that lacks most of the NA open reading frame but contains instead the gene encoding green fluorescent protein (GFP). To exploit the expanding knowledge of vRNA packaging signals to establish influenza virus vectors for the expression of foreign genes, we studied the replicative properties of this virus in cell culture and mice. Compared to wild-type virus, GFP(NA)-Flu was highly attenuated in normal cultured cells but was able to grow to a titer of >10(6) PFU/ml in a mutant cell line expressing reduced levels of sialic acid on the cell surface. GFP expression from this virus was stable even after five passages in the latter cells. In intranasally infected mice, GFP was detected in the epithelial cells of nasal mucosa, bronchioles, and alveoli for up to 4 days postinfection. We attribute the attenuated growth of GFP(NA)-Flu to virion aggregation at the surface of bronchiolar epithelia. In studies to test the potential of this mutant as a live attenuated influenza vaccine, all mice vaccinated with >/==" BORDER="0">10(5) PFU of GFP(NA)-Flu survived when challenged with lethal doses of the parent virus. These results suggest that influenza virus could be a useful vector for expressing foreign genes and that a sialidase-deficient virus may offer an alternative to the live influenza vaccines recently approved for human use.

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.

Polymorphism in the infectious salmon anemia virus hemagglutinin gene: importance and possible implications for evolution and ecology of infectious salmon anemia disease

Infectious salmon anemia (ISA) is an emerging disease in farmed Atlantic salmon with important commercial consequences. The pathogenicity of the ISA virus (ISAV; an orthomyxovirus) varies, observed as differences in disease development and clinical signs. A small polymorphic region (PR) in the ISAV genomic segment encoding the hemagglutinin (HA) has been described. An analysis of 33 HA gene sequences from historical and recent ISA outbreaks was performed, added to a selection of previously published HA sequences. A differential deletion model explaining the generation of HA polymorphism is proposed. The European ISAV sequences could be grouped according to deletion patterns in PR. Cell-culture replication and cytopathic effect varied between viruses from different PR groups. A rather complex epidemiology is suggested, as (a) HA sequences representing several PR variants were detected in three samples; (b) identical mutations occurred in different genetic lineages; and (c) large genetic differences were present in closely related viruses.

A release-competent influenza A virus mutant lacking the coding capacity for the neuraminidase active site

Both influenza A virus surface glycoproteins, the haemagglutinin (HA) and neuraminidase (NA), interact with neuraminic acid-containing receptors. The influenza virus A/Charlottesville/31/95 (H1N1) has shown a substantially reduced sensitivity to NA inhibitor compared with the A/WSN/33 (H1N1) isolate by plaque-reduction assays in Madin-Darby canine kidney (MDCK) cells. However, there was no difference in drug sensitivity in an NA inhibition assay. The replacement of the HA gene of A/WSN/33 with the HA gene of A/Charlottesville/31/95 led to a drastic reduction in sensitivity of A/WSN/33 to NA inhibitor in MDCK cells. Passage of A/Charlottesville/31/95 in cell culture in the presence of an NA inhibitor resulted in the emergence of mutant viruses (delNA) whose genomes lacked the coding capacity for the NA active site. The delNA mutants were plaque-to-plaque purified and further characterized. The delNA-31 mutant produced appreciable yields ( approximately 10(6) p.f.u./ml) in MDCK cell culture supernatants in the absence of viral or bacterial NA activity. Sequence analysis of the delNA mutant genome revealed no compensatory substitutions in the HA or other genes compared with the wild-type. Our data indicate that sialylation of the oligosaccharide chains in the vicinity of the HA receptor-binding site of A/Charlottesville/31/95 virus reduces the HA binding efficiency and thus serves as a compensatory mechanism for the loss of NA activity. Hyperglycosylation of HA is common in influenza A viruses circulating in humans and has the potential to reduce virus sensitivity to NA inhibitors.

The emergence of novel swine influenza viruses in North America

Since 1997, novel viruses of three different subtypes and five different genotypes have emerged as agents of influenza among pigs in North America. The appearance of these viruses is remarkable because there were no substantial changes in the overall epidemiology of swine influenza in the United States and Canada for over 60 years prior to this time. Viruses of the classical H1N1 lineage were virtually the exclusive cause of swine influenza from the time of their initial isolation in 1930 through 1998. Antigenic drift variants of these H1N1 viruses were isolated in 1991-1998, but a much more dramatic antigenic shift occurred with the emergence of H3N2 viruses in 1997-1998. In particular, H3N2 viruses with genes derived from human, swine and avian viruses have become a major cause of swine influenza in North America. In addition, H1N2 viruses that resulted from reassortment between the triple reassortant H3N2 viruses and classical H1N1 swine viruses have been isolated subsequently from pigs in at least six states. Finally, avian H4N6 viruses crossed the species barrier to infect pigs in Canada in 1999. Fortunately, these H4N6 viruses have not been isolated beyond their initial farm of origin. If these viruses spread more widely, they will represent another antigenic shift for our swine population, and could pose a threat to the world's human population. Research on these novel viruses may offer important clues to the genetic basis for interspecies transmission of influenza viruses.