A DNA transfection system for generation of influenza A virus from eight plasmids

Protective efficacy in chickens, geese and ducks of an H5N1-inactivated vaccine developed by reverse genetics

We generated a high-growth H5N1/PR8 virus by plasmid-based reverse genetics. The virulence associated multiple basic amino acids of the HA gene were removed, and the resulting virus is attenuated for chickens and chicken eggs. A formalin-inactivated oil-emulsion vaccine was prepared from this virus. When SPF chickens were inoculated with 0.3 ml of the vaccine, the hemagglutinin-inhibition (HI) antibody became detectable at 1 week post-vaccination (p.v.) and reached a peak of 10log2 at 6 weeks p.v. then slowly declined to 4log2 at 43 weeks p.v. Challenge studies performed at 2, 3 and 43 weeks p.v. indicated that all of the chickens were completely protected from disease signs and death. Ducks and geese were completely protected from highly pathogenic H5N1 virus challenge 3 weeks p.v. The duration of protective immunity in ducks and geese was investigated by detecting the HI antibody of the field vaccinated birds, and the results indicated that 3 doses of the vaccine inoculation in geese could induce a 34 weeks protection, while 2 doses induced more than 52 weeks protection in ducks. We first reported that an oil-emulsion inactivated vaccine derived from a high-growth H5N1 vaccine induced approximately 10 months of protective immunity in chickens and demonstrated that the oil-emulsion inactivated avian influenza vaccine is immunogenic for geese and ducks. These results provide useful information for the application of vaccines to the control of H5N1 avian influenza in poultry, including chickens and domestic waterfowl.

World Health Organization Global Influenza Program Surveillance Network. Evolution of H5N1 avian influenza viruses in Asia

An outbreak of highly pathogenic avian influenza A (H5N1) has recently spread to poultry in 9 Asian countries. H5N1 infections have caused > or =52 human deaths in Vietnam, Thailand, and Cambodia from January 2004 to April 2005. Genomic analyses of H5N1 isolates from birds and humans showed 2 distinct clades with a nonoverlapping geographic distribution. All the viral genes were of avian influenza origin, which indicates absence of reassortment with human influenza viruses. All human H5N1 isolates tested belonged to a single clade and were resistant to the adamantane drugs but sensitive to neuraminidase inhibitors. Most H5N1 isolates from humans were antigenically homogeneous and distinct from avian viruses circulating before the end of 2003. Some 2005 isolates showed evidence of antigenic drift. An updated nonpathogenic H5N1 reference virus, lacking the polybasic cleavage site in the hemagglutinin gene, was produced by reverse genetics in anticipation of the possible need to vaccinate humans.

Multiple gene segments control the temperature sensitivity and attenuation phenotypes of ca B/Ann Arbor/1/66

Cold-adapted (ca) B/Ann Arbor/1/66 is the influenza B virus strain master donor virus for FluMist, a live, attenuated, influenza virus vaccine licensed in 2003 in the United States. Each FluMist vaccine strain contains six gene segments of the master donor virus; these master donor gene segments control the vaccine's replication and attenuation. These gene segments also express characteristic biological traits in model systems. Unlike most virulent wild-type (wt) influenza B viruses, ca B/Ann Arbor/1/66 is temperature sensitive (ts) at 37 degrees C and attenuated (att) in the ferret model. In order to define the minimal genetic components of these phenotypes, the amino acid sequences of the internal genes of ca B/Ann Arbor/1/66 were aligned to those of other influenza B viruses. These analyses revealed eight unique amino acids in three proteins: two in the polymerase subunit PA, two in the M1 matrix protein, and four in the nucleoprotein (NP). Using reverse genetics, these eight wt amino acids were engineered into a plasmid-derived recombinant of ca B/Ann Arbor/1/66, and these changes reverted both the ts and the att phenotypes. A detailed mutational analysis revealed that a combination of two sites in NP (A114 and H410) and one in PA (M431) controlled expression of ts, whereas these same changes plus two additional residues in M1 (Q159 and V183) controlled the att phenotype. Transferring this genetic signature to the divergent wt B/Yamanashi/166/98 strain conferred both the ts and the att phenotypes on the recombinant, demonstrating that this small, complex, genetic signature encoded the essential elements for these traits.

Functional constraints of influenza A virus epitopes limit escape from cytotoxic T lymphocytes

Viruses can exploit a variety of strategies to evade immune surveillance by cytotoxic T lymphocytes (CTL), including the acquisition of mutations in CTL epitopes. Also for influenza A viruses a number of amino acid substitutions in the nucleoprotein (NP) have been associated with escape from CTL. However, other previously identified influenza A virus CTL epitopes are highly conserved, including the immunodominant HLA-A*0201-restricted epitope from the matrix protein, M1(58-66). We hypothesized that functional constraints were responsible for the conserved nature of influenza A virus CTL epitopes, limiting escape from CTL. To assess the impact of amino acid substitutions in conserved epitopes on viral fitness and recognition by specific CTL, we performed a mutational analysis of CTL epitopes. Both alanine replacements and more conservative substitutions were introduced at various positions of different influenza A virus CTL epitopes. Alanine replacements for each of the nine amino acids of the M1(58-66) epitope were tolerated to various extents, except for the anchor residue at the second position. Substitution of anchor residues in other influenza A virus CTL epitopes also affected viral fitness. Viable mutant viruses were used in CTL recognition experiments. The results are discussed in the light of the possibility of influenza viruses to escape from specific CTL. It was speculated that functional constraints limit variation in certain epitopes, especially at anchor residues, explaining the conserved nature of these epitopes.

cis-Acting packaging signals in the influenza virus PB1, PB2, and PA genomic RNA segments

The influenza A virus genome consists of eight negative-sense RNA segments. The cis-acting signals that allow these viral RNA segments (vRNAs) to be packaged into influenza virus particles have not been fully elucidated, although the 5' and 3' untranslated regions (UTRs) of each vRNA are known to be required. Efficient packaging of the NA, HA, and NS segments also requires coding sequences immediately adjacent to the UTRs, but it is not yet known whether the same is true of other vRNAs. By assaying packaging of genetically tagged vRNA reporters during plasmid-directed influenza virus assembly in cells, we have now mapped cis-acting sequences that are sufficient for packaging of the PA, PB1, and PB2 segments. We find that each involves portions of the distal coding regions. Efficient packaging of the PA or PB1 vRNAs requires at least 40 bases of 5' and 66 bases of 3' coding sequences, whereas packaging of the PB2 segment requires at least 80 bases of 5' coding region but is independent of coding sequences at the 3' end. Interestingly, artificial reporter vRNAs carrying mismatched ends (i.e., whose 5' and 3' ends are derived from different vRNA segments) were poorly packaged, implying that the two ends of any given vRNA may collaborate in forming specific structures to be recognized by the viral packaging machinery.

Molecular basis of replication of duck H5N1 influenza viruses in a mammalian mouse model

We recently analyzed a series of H5N1 viruses isolated from healthy ducks in southern China since 1999 and found that these viruses had progressively acquired the ability to replicate and cause disease in mice. In the present study, we explored the genetic basis of this change in host range by comparing two of the viruses that are genetically similar but differ in their ability to infect mice and have different pathogenicity in mice. A/duck/Guangxi/22/2001 (DKGX/22) is nonpathogenic in mice, whereas A/duck/Guangxi/35/2001 (DKGX/35) is highly pathogenic. We used reverse genetics to create a series of single-gene recombinants that contained one gene from DKGX/22 and the remaining seven gene segments from DKGX/35. We find that the PA, NA, and NS genes of DKGX/22 could attenuate DKGX/35 virus to some extent, but PB2 of DKGX/22 virus attenuated the DKGX/35 virus dramatically, and an Asn-to-Asp substitution at position 701 of PB2 plays a key role in this function. Conversely, of the recombinant viruses in the DKGX/22 background, only the one that contains the PB2 gene of DKGX/35 was able to replicate in mice. A single amino acid substitution (Asp to Asn) at position 701 of PB2 enabled DKGX/22 to infect and become lethal for mice. These results demonstrate that amino acid Asn 701 of PB2 is one of the important determinants for this avian influenza virus to cross the host species barrier and infect mice, though the replication and lethality of H5N1 influenza viruses involve multiple genes and may result from a constellation of genes. Our findings may help to explain the expansion of the host range and lethality of the H5N1 influenza viruses to humans.

Attenuation of equine influenza viruses through truncations of the NS1 protein

Equine influenza is a common disease of the horse, causing significant morbidity worldwide. Here we describe the establishment of a plasmid-based reverse genetics system for equine influenza virus. Utilizing this system, we generated three mutant viruses encoding carboxy-terminally truncated NS1 proteins. We have previously shown that a recombinant human influenza virus lacking the NS1 gene (delNS1) could only replicate in interferon (IFN)-incompetent systems, suggesting that the NS1 protein is responsible for IFN antagonist activity. Contrary to previous findings with human influenza virus, we found that in the case of equine influenza virus, the length of the NS1 protein did not correlate with the level of attenuation of that virus. With equine influenza virus, the mutant virus with the shortest NS1 protein turned out to be the least attenuated. We speculate that the basis for attenuation of the equine NS1 mutant viruses generated is related to their level of NS1 protein expression. Our findings show that the recombinant mutant viruses are impaired in their ability to inhibit IFN production in vitro and they do not replicate as efficiently as the parental recombinant strain in embryonated hen eggs, in MDCK cells, or in vivo in a mouse model. Therefore, these attenuated mutant NS1 viruses may have potential as candidates for a live equine influenza vaccine.

Perforin and Fas cytolytic pathways coordinately shape the selection and diversity of CD8+-T-cell escape variants of influenza virus

Antigenic variation is a viral strategy exploited to promote survival in the face of the host immune response and represents a major challenge for efficient vaccine development. Influenza viruses are pathogens with high transmissibility and mutation rates, enabling viral escape from immunity induced by prior infection or vaccination. Intense selection from neutralizing antibody drives antigenic changes in the surface glycoproteins, resulting in emergence of new strains able to reinfect hosts immune to previously circulating viruses. CD8+ cytotoxic T cells (CTLs) also provide protective immunity from influenza virus infection and may contribute to the antigenic evolution of influenza viruses. Utilizing mice transgenic for an influenza virus NP366-374 peptide-specific T-cell receptor, we demonstrated that the respiratory tract is a suitable site for generation of escape variants of influenza virus selected by CTL in vivo. In this report the contributions of the perforin and Fas pathways utilized by influenza virus-specific CTLs in viral clearance and selection of CTL escape variants have been evaluated. While transgenic CTLs deficient in either perforin- or Fas-mediated pathways are efficient in initial pulmonary viral control, variant virus emergence was observed in all the mice studied, although the spectrum of viral CTL escape variants selected varied profoundly. Thus, a less-restricted repertoire of escape variants was observed in mice with an intact perforin cytotoxic pathway compared with a limited variant diversity in perforin pathway-deficient mice, although maximal variant diversity was observed in mice having both Fas and perforin pathways intact. We conclude that selection of viral CTL escape variants reflects coordinate action between the tightly controlled perforin/granzyme pathway and the more promiscuous Fas/FasL pathway.

Influenza: lessons from past pandemics, warnings from current incidents

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

A single amino acid change in the C-terminal domain of the matrix protein M1 of influenza B virus confers mouse adaptation and virulence

Serial passage of an initially avirulent influenza B virus, B/Memphis/12/97, resulted in the selection of a variant which was lethal in mice. Virulence correlated with improved growth in vivo and prolonged replication. Sequencing of the complete coding regions of the parent and mouse-adapted viruses revealed 8 amino acid differences. Sequencing and characterization of intermediate passages suggested that one change in the C-terminal domain of the M1 protein, an asparagine to a serine at position 221, was responsible for acquisition of virulence and lethality. Site-directed mutagenesis of the M segment of a different virus, B/Yamanashi/166/98, to change this amino acid residue confirmed its importance by conferring improved growth and virulence in mice. This observation suggests a role for the C domain of the M1 protein in growth and virulence in a mammalian host.

Full restoration of viral fitness by multiple compensatory co-mutations in the nucleoprotein of influenza A virus cytotoxic T-lymphocyte escape mutants

Amino acid substitutions have been identified in the influenza A virus nucleoprotein that are associated with escape from recognition by virus-specific cytotoxic T lymphocytes (CTLs). One of these is the arginine-to-glycine substitution at position 384 (R384G). This substitution alone, however, is detrimental to viral fitness, which is overcome in part by the functionally compensating co-mutation E375G. Here, the effect on viral fitness of four other co-mutations associated with R384G was investigated by using plasmid-driven rescue of mutant viruses. Whilst none of these alternative co-mutations alone compensated functionally for the detrimental effect of the R384G substitution, the M239V substitution improved viral fitness of viruses containing 375G and 384R. The nucleoprotein displays unexpected flexibility to overcome functional constraints imposed by CTL epitope sequences, allowing influenza viruses to escape from specific CTLs.

Two residues in the hemagglutinin of A/Fujian/411/02-like influenza viruses are responsible for antigenic drift from A/Panama/2007/99

The H3N2 vaccine strain (A/Panama/2007/99) for the 2003-2004 influenza season did not antigenically match the circulating A/Fujian/411/02-like H3N2 viruses and had reduced effectiveness against influenza outbreaks. A/Wyoming/03/2003, an A/Fujian-like virus, was recommended as the vaccine strain for the 2004-2005 season. A/Wyoming differed from A/Panama by 16 amino acids in the HA1 molecule. Reverse genetics was used to determine the minimal amino acid changes that were responsible for the antigenic drift from A/Panama to A/Wyoming. After substitutions of 2 of the 16 amino acids in the HA (H155T, Q156H), the A/Panama HA variant was antigenically equivalent to A/Wyoming as determined by hemagglutination inhibition and microneutralization assays using ferret postinfection antisera. Conversely, A/Wyoming containing the His-155 and Gln-156 residues from A/Panama was antigenically equivalent to A/Panama. These results indicated that only these two HA residues specified the antigenic drift from A/Panama to A/Wyoming; other amino acid differences between these two H3N2 viruses had minimal impact on virus antigenicity but impacted virus replication efficiency in eggs.

Mutations in the NS1 protein of swine influenza virus impair anti-interferon activity and confer attenuation in pigs

It has been shown previously that the nonstructural protein NS1 of influenza virus is an alpha/beta interferon (IFN-alpha/beta) antagonist, both in vitro and in experimental animal model systems. However, evidence of this function in a natural host has not yet been obtained. Here we investigated the role of the NS1 protein in the virulence of a swine influenza virus (SIV) isolate in pigs by using reverse genetics. The virulent wild-type A/Swine/Texas/4199-2/98 (TX/98) virus and various mutants encoding carboxy-truncated NS1 proteins were rescued. Growth properties of TX/98 viruses with mutated NS1, induction of IFN in tissue culture, and virulence-attenuation in pigs were analyzed and compared to those of the recombinant wild-type TX/98 virus. Our results indicate that deletions in the NS1 protein decrease the ability of the TX/98 virus to prevent IFN-alpha/beta synthesis in pig cells. Moreover, all NS1 mutant viruses were attenuated in pigs, and this correlated with the amount of IFN-alpha/beta induced in vitro. These data suggest that the NS1 protein of SIV is a virulence factor. Due to their attenuation, NS1-mutated swine influenza viruses might have a great potential as live attenuated vaccine candidates against SIV infections of pigs.

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

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

Influenza virus neuraminidase contributes to secondary bacterial pneumonia

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

Generation of recombinant influenza A virus without M2 ion-channel protein by introduction of a point mutation at the 5' end of the viral intron

The aim of this study was to inhibit influenza virus M2 protein expression by mutating the splicing signal of the M gene. Mutations were introduced into the GU dinucleotide sequence at the 5'-proximal splicing site of the M gene (corresponding to nt 52-53 of M cRNA). Transfected cells expressing mutated M viral ribonucleoproteins failed to generate M2 mRNA. Interestingly, recombinant viruses with mutations at the dinucleotide sequence were viable, albeit attenuated, in cell culture. These recombinants failed to express M2 mRNA and M2 protein. These observations demonstrated that the GU invariant dinucleotide sequence at the 5'-proximal splicing site of M gene is essential for M2 mRNA synthesis. These results also indicated that the M2 ion-channel protein is critical, but not essential, for virus replication in cell culture. This approach may provide a new way of producing attenuated influenza A virus.

A new approach to an influenza live vaccine: modification of the cleavage site of hemagglutinin

A promising approach to reduce the impact of influenza is the use of an attenuated, live virus as a vaccine. Using reverse genetics, we generated a mutant of strain A/WSN/33 with a modified cleavage site within its hemagglutinin, which depends on proteolytic activation by elastase. Unlike the wild-type, which requires trypsin, this mutant is strictly dependent on elastase. Both viruses grow equally well in cell culture. In contrast to the lethal wild-type virus, the mutant is entirely attenuated in mice. At a dose of 10(5) plaque-forming units, it induced complete protection against lethal challenge. This approach allows the conversion of any epidemic strain into a genetically homologous attenuated virus.

Generation of influenza vaccine viruses on Vero cells by reverse genetics: an H5N1 candidate vaccine strain produced under a quality system

Human influenza vaccine reference strains are prepared as required when an antigenically new strain is recommended by WHO for inclusion in the vaccine. Currently, for influenza A, these strains are produced by a double infection of embryonated hens' eggs using the recommended strain and the laboratory strain PR8 which grows to high titre in eggs, in order to produce a high growth reassortant (HGR). HGRs are provided by WHO reference laboratories to the vaccine manufacturing industry which use them to prepare seed virus for vaccine production. The use of reverse genetics in preparing vaccine reference strains offers several advantages over the traditional method: (i) the reverse genetics approach is a direct rational approach compared with the potentially hit-or-miss traditional approach; (ii) reverse genetics will decontaminate a wild type virus that may have been derived in a non-validated system, e.g. a cell line not validated for vaccine purposes, or that may contain additional pathogens; (iii) at the plasmid stage, the HA can be engineered to remove pathogenic traits. The use of reverse genetics in deriving HGRs has been demonstrated by several laboratories, including its use in deriving a non-pathogenic reassortant strain from a highly pathogenic virus. In this report, we have advanced the use of reverse genetics by making use of a cell line acceptable for human vaccine production, by demonstrating directly the short time frame in which a reassortant virus can be derived, and by deriving a non-pathogenic pandemic vaccine reference virus in cells validated for vaccine production and under quality controlled conditions.

Consequences of immunodominant epitope deletion for minor influenza virus-specific CD8+-T-cell responses

The extent to which CD8+ T cells specific for other antigens expand to compensate for the mutational loss of the prominent DbNP366 and DbPA224 epitopes has been investigated using H1N1 and H3N2 influenza A viruses modified by reverse genetics. Significantly increased numbers of CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ T cells were found in the spleen and in the inflammatory population recovered by bronchoalveolar lavage from mice that were first given the -NP-PA H1N1 virus intraperitoneally and then challenged intranasally with the homologous H3N2 virus. The effect was less consistent when this prime-boost protocol was reversed. Also, though the quality of the response measured by cytokine staining showed some evidence of modification when these minor CD8+-T-cell populations were forced to play a more prominent part, the effects were relatively small and no consistent pattern emerged. The magnitude of the enhanced clonal expansion following secondary challenge suggested that the prime-boost with the -NP-PA viruses gave a response overall that was little different in magnitude from that following comparable exposure to the unmanipulated viruses. This was indeed shown to be the case when the total response was measured by ELISPOT analysis with virus-infected cells as stimulators. More surprisingly, the same effect was seen following primary challenge, though individual analysis of the CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ sets gave no indication of compensatory expansion. A possible explanation is that novel, as yet undetected epitopes emerge following primary exposure to the -NP-PA deletion viruses. These findings have implications for both natural infections and vaccines.

Reverse genetics of negative-stranded RNA viruses: a global perspective

The advent of reverse genetics technology has revolutionized the field of RNA viruses. It is now possible to manipulate even negative-stranded RNA viruses at will, and evaluate the effects of these changes on the biology and pathogenesis of these viruses. The fundamental insights gleaned from the reverse genetics-based studies over the last several years have provided a new momentum for the development of designed therapies for the control and prevention of these viral pathogens. The recombinant viruses have been exploited also as vectors for devising targeted therapies for non-viral diseases such as malignancies, and in gene therapy for inherited disorders. This review provides a brief summary of the stumbling blocks and the successes in the development of the technology for the negative-stranded RNA viruses. The many and varied applications of the recombinant vectors are also outlined.

Protection afforded by intranasal immunization with the neuraminidase-lacking mutant of influenza A virus in a ferret model

Protective efficacy of the intranasal immunization with the neuraminidase (NA)-deficient mutant of the influenza A virus was investigated in ferrets. Despite the highly attenuated replication in vivo, the mutant completely protected the animals against the wild type virus challenge. When challenge was done with antigenic drift variants, significant reductions in the viral titers, inflammatory cell counts, and protein concentrations were observed in the nasal washes of the immunized animals. The genetically engineered NA-deficient mutant also protected animals against the challenge and induced humoral immune response against the foreign protein that replaced the NA. We conclude that the NA as antigen is dispensable in the live attenuated influenza virus vaccine and that the NA-lacking mutant can be used as a virus vector.

Pathogenesis of Hong Kong H5N1 influenza virus NS gene reassortants in mice: the role of cytokines and B- and T-cell responses

The severity of disease caused in humans by H5N1 influenza viruses remains unexplained. The NS gene of Hong Kong H5N1/97 viruses was shown to contribute to high pathogenicity of reassortants in a pig model. However, the molecular pathogenesis and host immune response underlying this phenomenon remain unclear. Here, in a mouse model, H1N1 A/Puerto Rico/8/34 (PR/8) reassortants that contained the H5N1/97 NS gene, the H5N1/01 NS gene, or an altered H5N1/97 NS gene encoding a Glu92→Asp substitution in NS1 was studied. The pathogenicity of reassortant viruses, the induction of cytokines and chemokine CXCL1 (KC) in the lungs and specific B- and T-cell responses was characterized. In mice infected with reassortant virus containing the H5N1/97 NS gene, the mouse lethal dose (50 %) and lung virus titres were similar to those of PR/8, which is highly pathogenic to mice. This reassortant virus required two more days than PR/8 to be cleared from the lungs of infected mice. Reassortants containing the altered H5N1/97 NS gene or the H5N1/01 NS gene demonstrated attenuated pathogenicity and lower lung titres in mice. Specific B- and T-cell responses were consistent with viral pathogenicity and did not explain the delayed clearance of the H5N1/97 NS reassortant. The reassortant induced elevated pulmonary concentrations of the inflammatory cytokines IL1α, IL1β, IL6, IFN-γand chemokine KC, and decreased concentrations of the anti-inflammatory cytokine IL10. This cytokine imbalance is reminiscent of the clinical findings in two humans who died of H5N1/97 infection and may explain the unusual severity of the disease.

Lack of prominent peptide-major histocompatibility complex features limits repertoire diversity in virus-specific CD8+ T cell populations

Using both 'reverse genetics' and structural analysis, we have examined the in vivo relationship between antigenicity and T cell receptor (TCR) repertoire diversity. Influenza A virus infection of C57BL/6 mice induces profoundly different TCR repertoires specific for the nucleoprotein peptide of amino acids 366-374 (NP366) and the acid polymerase peptide of amino acids 224-233 (PA224) presented by H-2D(b). Here we show the H-2D(b)-NP366 complex with a 'featureless' structure selected a limited TCR repertoire characterized by 'public' TCR usage. In contrast, the prominent H-2D(b)-PA224 complex selected diverse, individually 'private' TCR repertoires. Substitution of the arginine at position 7 of PA224 with an alanine reduced the accessible side chains of the epitope. Infection with an engineered virus containing a mutation at the site encoding the exposed arginine at position 7 of PA224 selected a restricted TCR repertoire similar in diversity to that of the H-2D(b)-NP366-specific response. Thus, the lack of prominent features in an antigenic complex of peptide and major histocompatibility complex class I is associated with a diminished spectrum of TCR usage.

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

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

Biology of plant rhabdoviruses

The Rhabdoviridae, whose members collectively infect invertebrates, animals, and plants, form a large family that has important consequences for human health, agriculture, and wildlife ecology. Plant rhabdoviruses can be separated into the genera Cytorhabdovirus and Nucleorhabdovirus, based on their sites of replication and morphogenesis. This review presents a general overview of classical and contemporary findings about rhabdovirus ecology, pathology, vector relations, and taxonomy. The genome organization and structure of several recently sequenced nucleorhabdoviruses and cytorhabdoviruses is integrated with new cell biology findings to provide a model for the replication of the two genera. A prospectus outlines the exciting opportunities for future research that will contribute to a more detailed understanding of the biology, biochemistry, replication and host interactions of the plant rhabdoviruses.

Infectious cDNA clones of porcine reproductive and respiratory syndrome virus and their potential as vaccine vectors

Full-length infectious cDNA clones have recently become available for both European and North American genotypes of porcine reproductive and respiratory syndrome virus (PRRSV), and it is now possible to alter the PRRSV genome and create genetically defined mutant viruses. Among many possible applications of the PRRSV infectious cDNA clones, development of genetically modified vaccines is of particular interest. Using infectious clones, the PRRSV genome has been manipulated by changing individual amino acids, deleting coding regions, inserting foreign sequences, and generating arterivirus chimeras. The limited available data suggest that all structural proteins of PRRSV are essential for replication of the virus, and that PRRSV infectivity is relatively intolerant of subtle changes within the structural proteins. The major tasks in PRRSV research are to identify virulence factors and pathogenic mechanisms, and to understand the structure–function relationships of individual viral proteins. Utilizing these infectious clones as tools, a new generation of safe and efficacious PRRS vaccines may be constructed.

Predicting antigenic variants of influenza A/H3N2 viruses

Models based on amino acid changes in influenza hemagglutinin protein were compared to predict antigenic variants of influenza A/H3N2 viruses.

Current inactivated influenza vaccines provide protection when vaccine antigens and circulating viruses share a high degree of similarity in hemagglutinin protein. Five antigenic sites in the hemagglutinin protein have been proposed, and 131 amino acid positions have been identified in the five antigenic sites. In addition, 20, 18, and 32 amino acid positions in the hemagglutinin protein have been identified as mouse monoclonal antibody–binding sites, positively selected codons, and substantially diverse codons, respectively. We investigated these amino acid positions for predicting antigenic variants of influenza A/H3N2 viruses in ferrets. Results indicate that the model based on the number of amino acid changes in the five antigenic sites is best for predicting antigenic variants (agreement = 83%). The methods described in this study could be applied to predict vaccine-induced cross-reactive antibody responses in humans, which may further improve the selection of vaccine strains.

Characterization of CD8+ T cell repertoire diversity and persistence in the influenza A virus model of localized, transient infection

Influenza virus infection of C57BL/6 mice provides a well-characterized model for the study of acute CD8(+) T cell responses and for the analysis of memory in the absence of antigen persistence. The advent of tetramer reagents and intracellular cytokine staining, coupled with techniques such as single cell RT-PCR and influenza reverse genetics, has enabled the detailed molecular dissection of different epitope-specific primary, memory and secondary immune CD8(+) T cell responses. The approach offers novel insights into the factors determining the selection of immune repertoires, and their functional consequences for CD8(+) T cell-mediated immunity.

Efficient bunyavirus rescue from cloned cDNA

Bunyaviruses are trisegmented, negative-sense RNA viruses. Previously, we described a rescue system to recover infectious Bunyamwera virus (genus Orthobunyavirus) entirely from cloned cDNA (Bridgen, A. and Elliott, R.M. (1996) Proc. Nat. Acad. Sci. USA 93, 15400-15404) utilizing a recombinant vaccinia virus expressing bacteriophage T7 RNA polymerase to drive intracellular transcription of transfected T7 promoter-containing plasmids. Here we report efforts to improve the efficiency of the system by comparing different methods of providing T7 polymerase. We found that a BHK-derived cell line BSR-T7/5 that constitutively expresses T7 RNA polymerase supported efficient and reproducible recovery of Bunyamwera virus, routinely generating >10(7) pfu per rescue experiment. Furthermore, we show that the virus can be recovered from transfecting cells with just three plasmids that express full-length antigenome viral RNAs, greatly simplifying the procedure. We suggest that this procedure should be applicable to viruses in other genera of the family Bunyaviridae and perhaps also to arenaviruses.

Influenza B virus BM2 protein is a crucial component for incorporation of viral ribonucleoprotein complex into virions during virus assembly

Influenza B virus contains four integral membrane proteins in its envelope. Of these, BM2 has recently been found to have ion channel activity and is considered to be a functional counterpart to influenza A virus M2, but the role of BM2 in the life cycle of influenza B virus remains unclear. In an effort to explore its function, a number of BM2 mutant viruses were generated by using a reverse genetics technique. The BM2DeltaATG mutant virus synthesized BM2 at markedly lower levels but exhibited similar growth to wild-type (wt) virus. In contrast, the BM2 knockout virus, which did not produce BM2, did not grow substantially but was able to grow normally when BM2 was supplemented in trans by host cells expressing BM2. These results indicate that BM2 is a required component for the production of infectious viruses. In the one-step growth cycle, the BM2 knockout virus produced progeny viruses lacking viral ribonucleoprotein complex (vRNP). The inhibited incorporation of vRNP was regained by trans-supplementation of BM2. An immunofluorescence study of virus-infected cells revealed that distribution of hemagglutinin, nucleoprotein, and matrix (M1) protein of the BM2 knockout virus at the apical membrane did not differ from that of wt virus, whereas the sucrose gradient flotation assay revealed that the membrane association of M1 was greatly affected in the absence of BM2, resulting in a decrease of vRNP in membrane fractions. These results strongly suggest that BM2 functions to capture the M1-vRNP complex at the virion budding site during virus assembly.

From lethal virus to life-saving vaccine: developing inactivated vaccines for pandemic influenza

Over the past eight years, cases of human infection with highly pathogenic avian influenza viruses have raised international concern that we could be on the brink of a global influenza pandemic. Many of these human infections have proved fatal and if the viruses had been able to transmit efficiently from person to person, the effects would have been devastating. How can we arm ourselves against this pandemic threat when these viruses are too dangerous to use in conventional vaccine production? Recent technological developments (reverse genetics) have allowed us to manipulate the influenza virus genome so that we can construct safe, high-yielding vaccine strains. However, the transition of reverse-genetic technologies from the research laboratory to the manufacturing environment has presented new challenges for vaccine manufacturers as well as veterinary and public health authorities.

Introduction of a temperature-sensitive phenotype into influenza A/WSN/33 virus by altering the basic amino acid domain of influenza virus matrix protein

Our previous studies with influenza A viruses indicated that the association of M1 with viral RNA and nucleoprotein (NP) is required for the efficient formation of helical ribonucleoprotein (RNP) and for the nuclear export of RNPs. RNA-binding domains of M1 map to the following two independent regions: a zinc finger motif at amino acid positions 148 to 162 and a series of basic amino acids (RKLKR) at amino acid positions 101 to 105. Altering the zinc finger motif of M1 reduces viral growth slightly. A substitution of Ser for Arg at either position 101 or position 105 of the RKLKR domain partially reduces the nuclear export of RNP and viral replication. To further understand the role of the zinc finger motif and the RKLKR domain in viral assembly and replication, we introduced multiple mutations by using reverse genetics to modify these regions of the M gene of influenza virus A/WSN/33. Of multiple mutants analyzed, a double mutant, R101S-R105S, of RKLKR resulted in a temperature-sensitive phenotype. The R101S-R105S double mutant had a greatly reduced ratio of M1 to NP in viral particles and a weaker binding of M1 to RNPs. These results suggest that mutations can be introduced into the RKLKR domain to control viral replication.

Reverse genetics systems for the generation of segmented negative-sense RNA viruses entirely from cloned cDNA

Reverse genetics is defined as the generation of virus entirely from cloned cDNA. For negative-sense RNA viruses, whose genomes are complementary to mRNA in their orientation, the viral RNA(s) and the viral proteins required for replication and translation must be provided to initiate the viral replication cycle. Segmented negative-sense RNA viruses were refractory to genetic manipulation until 1989. In this chapter, we review developments in the reverse genetics of segmented negative-sense RNA viruses, beginning with the in vitro reconstitution of viral polymerase complexes in the late 1980s and culminating in the generation of Bunyamwera and influenza virus entirely from plasmid DNA almost a decade later.

Influenza vaccines generated by reverse genetics

Influenza viruses cause annual epidemics and occasional pandemics of acute respiratory disease. Vaccination is the primary means to prevent and control the disease. However, influenza viruses undergo continual antigenic variation, which requires the annual reformulation of trivalent influenza vaccines, making influenza unique among pathogens for which vaccines have been developed. The segmented nature of the influenza virus genome allows for the traditional reassortment between two viruses in a coinfected cell. This technique has long been used to generate strains for the preparation of either inactivated or live attenuated influenza vaccines. Recent advancements in reverse genetics techniques now make it possible to generate influenza viruses entirely from cloned plasmid DNA by cotransfection of appropriate cells with 8 or 12 plasmids encoding the influenza virion sense RNA and/or mRNA. Once regulatory issues have been addressed, this technology will enable the routine and rapid generation of strains for either inactivated or live attenuated influenza vaccine. In addition, the technology offers the potential for new vaccine strategies based on the generation of genetically engineered donors attenuated through directed mutation of one or more internal genes. Reverse genetics techniques are also proving to be important for the development of pandemic influenza vaccines, because the technology provides a means to modify genes to remove virulence determinants found in highly pathogenic avian strains. The future of influenza prevention and control lies in the application of this powerful technology for the generation of safe and more effective influenza vaccines.

Functional compensation of a detrimental amino acid substitution in a cytotoxic-T-lymphocyte epitope of influenza a viruses by comutations

Influenza A viruses accumulate amino acid substitutions in cytotoxic-T-lymphocyte (CTL) epitopes, allowing these viruses to escape from CTL immunity. The arginine-to-glycine substitution at position 384 of the viral nucleoprotein is associated with escape from CTLs. Introduction of the R384G substitution in the nucleoprotein gene segment of influenza virus A/Hong Kong/2/68 by site-directed mutagenesis was detrimental to viral fitness. Introduction of one of the comutations associated with R384G, E375G, partially restored viral fitness and nucleoprotein functionality. We hypothesized that influenza A viruses need to overcome functional constraints to accumulate mutations in CTL epitopes and escape from CTLs.

Inhibition of influenza virus matrix (M1) protein expression and virus replication by U6 promoter-driven and lentivirus-mediated delivery of siRNA

Small interfering RNA (siRNA)-induced RNA degradation has been used recently as an antivirus agent to inhibit specific virus replication. This report shows that 21 nt duplexes of siRNA of the influenza virus M gene can cause specific inhibition of influenza virus matrix (M1) protein expression in transfected 293T cells. Furthermore, it is shown that a lentivirus vector can be used to effectively deliver M gene siRNAs into Madin-Darby canine kidney cells and can cause specific inhibition of M1 protein expression and influenza virus replication. Therefore, lentivirus-mediated delivery of siRNA and gene silencing can be used in studying the specific functions of virus genes in replication and may have a potential therapeutic application.

Efficient generation and growth of influenza virus A/PR/8/34 from eight cDNA fragments

A reverse genetics system for the generation of influenza virus A/PR/8/34 (NIBSC vaccine strain) from plasmid DNA was developed. Upon transfection of eight bidirectional transcription plasmids encoding the gene segments of A/PR/8/34 into 293T cells, virus titers in the supernatant were about 10(4) TCID50/ml. The production of A/PR/8/34 in 293T cells was compared to that of A/WSN/33, for which virus titers in the supernatant were 10(7)-10(8) TCID50/ml. Time-course analysis of virus production indicated that the differences in virus titers were due to reinfection of 293T cells by A/WSN/33 but not A/PR/8/34. Indeed, virus titers of A/PR/8/34 comparable to those of A/WSN/33 were achieved upon addition of trypsin to the culture medium of transfected cells. The production of chimeric viruses revealed that the difference in virus titers between A/PR/8/34 and A/WSN/33 are determined primarily by differences in the surface glycoproteins hemagglutinin and neuraminidase and the polymerase protein PB1. In conclusion, high-titer virus stocks of recombinant influenza A/PR/8/34 virus can be produced as well as virus stocks with much lower titers, but without the requirement of virus amplification through replication.

Identification of an amino acid residue on influenza C virus M1 protein responsible for formation of the cord-like structures of the virus

Influenza C virus-like particles (VLPs) have been generated from cloned cDNAs. A cDNA of the green fluorescent protein (GFP) gene in antisense orientation was flanked by the 5' and 3' non-coding regions of RNA segment 5 of the influenza C virus. The cDNA cassette was inserted between an RNA polymerase I promoter and terminator of the Pol I vector. This plasmid DNA was transfected into 293T cells together with plasmids encoding virus proteins of C/Ann Arbor/1/50 or C/Yamagata/1/88. Transfer of the supernatants of the transfected 293T cells to HMV-II cells resulted in GFP expression in the HMV-II cells. The quantification of the GFP-positive HMV-II cells indicated the presence of approximately 10(6) VLPs (ml supernatant)(-1). Cords 50-300 microm in length were observed on transfected 293T cells, although the cords were not observed when the plasmid for M1 protein of C/Ann Arbor/1/50 was replaced with that of C/Taylor/1233/47. A series of transfection experiments with plasmids encoding M1 mutants of C/Ann Arbor/1/50 or C/Taylor/1233/47 showed that an amino acid at residue 24 of the M1 protein is responsible for cord formation. This finding provides direct evidence for a previous hypothesis that M1 protein is involved in the formation of cord-like structures protruding from the C/Yamagata/1/88-infected cells. Evidence was obtained by electron microscopy that transfected cells bearing cords produced filamentous VLPs, suggesting the potential role of the M1 protein in determining the filamentous/spherical morphology of influenza C virus.

Role of transmembrane domain and cytoplasmic tail amino acid sequences of influenza a virus neuraminidase in raft association and virus budding

Influenza virus neuraminidase (NA), a type II transmembrane glycoprotein, possesses receptor-destroying activity and thereby facilitates virus release from the cell surface. Among the influenza A viruses, both the cytoplasmic tail (CT) and transmembrane domain (TMD) amino acid sequences of NA are highly conserved, yet their function(s) in virus biology remains unknown. To investigate the role of amino acid sequences of the CT and TMD on the virus life cycle, we systematically mutagenized the entire CT and TMD of NA by converting two to five contiguous amino acids to alanine. In addition, we also made two chimeric NA by replacing the CT proximal one-third amino acids of the NA TMD [NA(1T2N)NA] and the entire NA TMD (NATRNA) with that of human transferrin receptor (TR) (a type II transmembrane glycoprotein). We rescued transfectant mutant viruses by reverse genetics and examined their phenotypes. Our results show that all mutated and chimeric NAs could be rescued into transfectant viruses. Different mutants showed pleiotropic effects on virus growth and replication. Some mutants (NA2A5, NA3A7, and NA4A10) had little effect on virus growth while others (NA3A2, NA5A27, and NA5A31) produced about 50- to 100-fold-less infectious virus and still some others (NA5A14, NA4A19, and NA4A23) exhibited an intermediate phenotype. In general, mutations towards the ectodomain-proximal sequences of TMD progressively caused reduction in NA enzyme activity, affected lipid raft association, and attenuated virus growth. Electron microscopic analysis showed that these mutant viruses remained aggregated and bound to infected cell surfaces and could be released from the infected cells by bacterial NA treatment. Moreover, viruses containing mutations in the extreme N terminus of the CT (NA3A2) as well as chimeric NA containing the TMD replaced partially [NA(1T2N)NA] or fully (NATRNA) with TR TMD caused reduction in virus growth and exhibited the morphological phenotype of elongated particles. These results show that although the sequences of NA CT and TMD per se are not absolutely essential for the virus life cycle, specific amino acid sequences play a critical role in providing structural stability, enzyme activity, and lipid raft association of NA. In addition, aberrant morphogenesis including elongated particle formation of some mutant viruses indicates the involvement of NA in virus morphogenesis and budding.

Responsiveness to a pandemic alert: use of reverse genetics for rapid development of influenza vaccines

BACKGROUND

In response to the emergence of severe infection capable of rapid global spread, WHO will issue a pandemic alert. Such alerts are rare; however, on Feb 19, 2003, a pandemic alert was issued in response to human infections caused by an avian H5N1 influenza virus, A/Hong Kong/213/03. H5N1 had been noted once before in human beings in 1997 and killed a third (6/18) of infected people. The 2003 variant seemed to have been transmitted directly from birds to human beings and caused fatal pneumonia in one of two infected individuals. Candidate vaccines were sought, but no avirulent viruses antigenically similar to the pathogen were available, and the isolate killed embryonated chicken eggs. Since traditional strategies of vaccine production were not viable, we sought to produce a candidate reference virus using reverse genetics.

METHODS

We removed the polybasic aminoacids that are associated with high virulence from the haemagglutinin cleavage site of A/Hong Kong/213/03 using influenza reverse genetics techniques. A reference vaccine virus was then produced on an A/Puerto Rico/8/34 (PR8) backbone on WHO-approved Vero cells. We assessed this reference virus for pathogenicity in in-vivo and in-vitro assays.

FINDINGS

A reference vaccine virus was produced in Good Manufacturing Practice (GMP)-grade facilities in less than 4 weeks from the time of virus isolation. This virus proved to be non-pathogenic in chickens and ferrets and was shown to be stable after multiple passages in embryonated chicken eggs.

INTERPRETATION

The ability to produce a candidate reference virus in such a short period of time sets a new standard for rapid response to emerging infectious disease threats and clearly shows the usefulness of reverse genetics for influenza vaccine development. The same technologies and procedures are currently being used to create reference vaccine viruses against the 2004 H5N1 viruses circulating in Asia.

Generation of high-yielding influenza A viruses in African green monkey kidney (Vero) cells by reverse genetics

Influenza A viruses are the cause of annual epidemics of human disease with occasional outbreaks of pandemic proportions. The zoonotic nature of the disease and the vast viral reservoirs in the aquatic birds of the world mean that influenza will not easily be eradicated and that vaccines will continue to be needed. Recent technological advances in reverse genetics methods and limitations of the conventional production of vaccines by using eggs have led to a push to develop cell-based strategies to produce influenza vaccine. Although cell-based systems are being developed, barriers remain that need to be overcome if the potential of these systems is to be fully realized. These barriers include, but are not limited to, potentially poor reproducibility of viral rescue with reverse genetics systems and poor growth kinetics and yields. In this study we present a modified A/Puerto Rico/8/34 (PR8) influenza virus master strain that has improved viral rescue and growth properties in the African green monkey kidney cell line, Vero. The improved properties were mediated by the substitution of the PR8 NS gene for that of a Vero-adapted reassortant virus. The Vero growth kinetics of viruses with H1N1, H3N2, H6N1, and H9N2 hemagglutinin and neuraminidase combinations rescued on the new master strain were significantly enhanced in comparison to those of viruses with the same combinations rescued on the standard PR8 master strain. These improvements pave the way for the reproducible generation of high-yielding human and animal influenza vaccines by reverse genetics methods. Such a means of production has particular relevance to epidemic and pandemic use.

The influenza B virus nonstructural NS1 protein is essential for efficient viral growth and antagonizes beta interferon induction

We analyzed the functions of the influenza B virus nonstructural NS1-B protein, both by utilizing a constructed mutant virus (Delta NS1-B) lacking the NS1 gene and by testing the activities of the protein when expressed in cells. The mutant virus replicated to intermediate levels in 6-day-old embryonated chicken eggs that contain an immature interferon (IFN) system, whereas older eggs did not support viral propagation to a significant extent. The Delta NS1-B virus was a substantially stronger inducer of beta IFN (IFN-beta) transcripts in human lung epithelial cells than the wild type, and furthermore, transiently expressed NS1-B protein efficiently inhibited virus-dependent activation of the IFN-beta promoter. Interestingly, replication of the Delta NS1-B knockout virus was attenuated by more than 4 orders of magnitude in tissue culture cells containing or lacking functional IFN-alpha/beta genes. These findings show that the NS1-B protein functions as a viral IFN antagonist and indicate a further requirement of this protein for efficient viral replication that is unrelated to blocking IFN effects.

Imparting temperature sensitivity and attenuation in ferrets to A/Puerto Rico/8/34 influenza virus by transferring the genetic signature for temperature sensitivity from cold-adapted A/Ann Arbor/6/60

The four temperature-sensitive (ts) loci identified in the PB1 and PB2 gene segments of cold-adapted A/Ann Arbor/6/60 influenza virus, the master donor virus for influenza A virus (MDV-A) FluMist vaccines, were introduced into a divergent A/Puerto Rico/8/34 influenza virus strain. Recombinant A/Puerto Rico/8/34 virus with these four introduced ts loci exhibited both ts and att phenotypes similar to those of MDV-A, which could be used as a donor virus for manufacturing large quantities of inactivated influenza virus vaccine against potential pandemic strains.

Reverse genetics of mononegavirales

"Reverse genetics" or de novo synthesis of nonsegmented negative-sense RNA viruses (Mononegavirales) from cloned cDNA has become a reliable technique to study this group of medically important viruses. Since the first generation of a negative-sense RNA virus entirely from cDNA in 1994, reverse genetics systems have been established for members of most genera of the Rhabdo-, Paramyxo-, and Filoviridae families. These systems are based on intracellular transcription of viral full-length RNAs and simultaneous expression of viral proteins required to form the typical viral ribonucleoprotein complex (RNP). These systems are powerful tools to study all aspects of the virus life cycle as well as the roles of virus proteins in virus-host interplay and pathogenicity. In addition, recombinant viruses can be designed to have specific properties that make them attractive as biotechnological tools and live vaccines.

Implementation of Biosurety Systems in a Department of Defense Medical Research Laboratory

New biosurety regulations and guidelines were implemented in 2003 because of increased concern for the safety and security of biological select agents and toxins (BSAT) that may be used as weapons of mass destruction. Biosurety is defined as the combination of security, biosafety, agent accountability, and personnel reliability needed to prevent unauthorized access to select agents of bioterrorism. These new regulations will lead to increased scrutiny of the use of select biological agents in registered research laboratories, but the regulations may have unintended effects on cost, progress, and perceptions in programs previously considered part of the academic research community. We review the history of biosurety, evolving guidelines, implementation of the regulations, and impacts at the lead research laboratory for medical biological defense for the Department of Defense.

Respiratory viruses predisposing to bacterial infections: role of neuraminidase

BACKGROUND AND METHODS

Viral-bacterial coinfections in humans are well-documented. Viral infections often lead to bacterial superinfections. In vitro and animal models for influenza, as well as molecular microbiology study of viruses and bacteria, provide an understanding of the mechanisms that explain how respiratory viruses and bacteria combine to cause disease. This article focuses on viral and bacterial combinations, particularly synergism between influenza and Streptococcus pneumoniae.

RESULTS

Potential mechanisms for synergism between viruses and bacteria include: virus destruction of respiratory epithelium may increase bacterial adhesion; virus-induced immunosuppression may cause bacterial superinfections; and inflammatory response to viral infection may up-regulate expression of molecules that bacteria utilize as receptors. Influenza and parainfluenza viruses possess neuraminidase (NA) activity, which appears to increase bacterial adherence after viral preincubation. Experimental studies demonstrate that viral NA exposes pneumococcal receptors on host cells by removing terminal sialic acids. Other studies show that inhibition of viral NA activity reduces adherence and invasion of S. pneumoniae, independently of effects on viral replication. Clinical studies reveal that influenza vaccination reduces the incidence of secondary bacterial respiratory tract infections.

CONCLUSIONS

Detection of viral factors (e.g. high NA activity) that increase the likely potential of epidemic/pandemic influenza strains for causing morbidity and mortality from secondary bacterial infections provides new possibilities for intervention. Additional study is needed to identify the mechanisms for the development of bacterial complications after infections with respiratory syncytial virus and other important respiratory viruses that lack NA activity. Prevention of bacterial superinfection is likely to depend on effective antiviral measures.