Recovery of infectious respiratory syncytial virus expressing an additional, foreign gene

A comprehensive review of research advances in the study of lactoferrin to treat viral infections

Lactoferrin (Lf) is a naturally occurring glycoprotein known for its antiviral and antibacterial properties and is present in various physiological fluids. Numerous studies have demonstrated its antiviral effectiveness against multiple viruses, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), influenza virus (IFV), herpes simplex virus (HSV), hepatitis B virus (HBV), and human immunodeficiency virus (HIV). Lf, a vital component of the mucosal defense system, plays a crucial role in inhibiting viral infection by binding to both host cells and viral particles, such as the Hepatitis C virus (HCV). This interaction enables Lf to keep viral particles away from their target cells, emphasizing its significance as a fundamental element of mucosal defense against viral infections. Additionally, Lf has the ability to modulate cytokine expression and enhance cellular immune responses. In the innate immune system, Lf serves as a unique iron transporter and helps suppress various pathogens like bacteria, fungi, and viruses. This article summarises the potential antiviral properties of Lf against various viruses, along with its other mentioned functions. The advancement of Lf-based therapies supports the homology of food and medicine, providing a promising avenue to address viral infections and other public health challenges.

Development of Improved Mumps Vaccine Candidates by Mutating Viral mRNA Cap Methyltransferase Sites in the Large Polymerase Protein

Although a live attenuated vaccine is available for controlling mumps virus (MuV), mumps still outbreaks frequently worldwide. The attenuated MuV vaccine strain S79 is widely used in mumps vaccination in China, but still with many shortcomings, among which the most prominent are the side effects and decreased immunity. Therefore, there is a need to further improve the safety and efficacy of the current MuV vaccine. In the present study, we further attenuated MuV S79 vaccine strain by inhibiting viral mRNA methyltransferase (MTase). We generated a panel of eight recombinant MuVs (rMuVs) carrying mutations in the MTase catalytic site or S-adenosylmethionine (SAM) binding site in the large (L) polymerase protein. These rMuVs are genetically stable and seven rMuVs are more attenuated in replication in cell culture and five rMuVs are more attenuated in replication in lungs of cotton rats compared with the parental vaccine strain S79. Importantly, cotton rats vaccinated with these seven rMuV mutants produced high levels of serum neutralizing antibodies and were completely protected against challenge with a wild-type MuV strain (genotype F). Therefore, our results demonstrate that alteration in the MTase catalytic site or SAM binding site in MuV L protein improves the safety or the immunogenicity of the MuV vaccine and thus mRNA cap MTase may be an effective target for the development of new vaccine candidates for MuV.

In vitro trackable assembly of RNA-specific nucleocapsids of the respiratory syncytial virus

The templates for transcription and replication by respiratory syncytial virus (RSV) polymerase are helical nucleocapsids (NCs), formed by viral RNAs that are encapsidated by the nucleoprotein (N). Proper NC assembly is vital for RSV polymerase to engage the RNA template for RNA synthesis. Previous studies of NCs or nucleocapsid-like particles (NCLPs) from RSV and other nonsegmented negative-sense RNA viruses have provided insights into the overall NC architecture. However, in these studies, the RNAs were either random cellular RNAs or average viral genomic RNAs. An in-depth mechanistic understanding of NCs has been hampered by lack of an in vitro assay that can track NC or NCLP assembly. Here we established a protocol to obtain RNA-free N protein (N⁰) and successfully demonstrated the utility of a new assay for tracking assembly of N with RNA oligonucleotides into NCLPs. We discovered that the efficiency of the NCLP (N–RNA) assembly depends on the length and sequence of the RNA incorporated into NCLPs. This work provides a framework to generate purified N⁰ and incorporate it with RNA into NCLPs in a controllable manner. We anticipate that our assay for in vitro trackable assembly of RSV-specific nucleocapsids may enable in-depth mechanistic analyses of this process.

Establishment of an efficient reverse genetic system of Mumps virus S79 from cloned DNA

BACKGROUND

Mumps is a common type of respiratory infectious disease caused by mumps virus (MuV), and can be effectively prevented by vaccination. In this study, a reverse genetic system of MuV that can facilitate the rational design of safer, more efficient mumps vaccine candidates is established.

METHODS

MuV-S79 cDNA clone was assembled into a full-length plasmid by means of the GeneArt™ High-Order Genetic Assembly System, and was rescued via reverse genetic technology. RT-PCR, sequencing, and immunofluorescence assays were used for rMuV-S79 authentication. Viral replication kinetics and in vivo experimental models were used to evaluate the replication, safety, and immunogenicity of rMuV-S79.

RESULTS

A full-length cDNA clone of MuV-S79 in the assembly process was generated by a novel plasmid assemble strategy, and a robust reverse genetic system of MuV-S79 was successfully established. The established rMuV-S79 strain could reach a high virus titer in vitro. The average viral titer of rMuV-S79 in the lung tissues was 2.68 ± 0.14 log₁₀PFU/g lung tissue, and rMuV-S79 group did not induce inflammation in the lung tissues in cotton rats. Neutralizing antibody titers induced by rMuV-S79 were high, long-lasting and could provide complete protection against MuV wild strain challenge.

CONCLUSION

We have established a robust reverse genetic system of MuV-S79 which can facilitate the optimization of mumps vaccines. rMuV-S79 rescued could reach a high virus titer and the safety was proven in vivo. It could also provide complete protection against MuV wild strain challenge.

Enhancement of safety and immunogenicity of the Chinese Hu191 measles virus vaccine by alteration of the S-adenosylmethionine (SAM) binding site in the large polymerase protein

The live-attenuated measles virus (MV) vaccine based on the Hu191 strain has played a significant role in controlling measles in China. However, it has considerable adverse effects that may cause public health burden. We hypothesize that the safety and efficacy of MV vaccine can be improved by altering the S-adenosylmethionine (SAM) binding site in the conserved region VI of the large polymerase protein. To test this hypothesis, we established an efficient reverse genetics system for the rMV-Hu191 strain and generated two recombinant MV-Hu191 carrying mutations in the SAM binding site. These two mutants grew to high titer in Vero cells, were genetically stable, and were significantly more attenuated in vitro and in vivo compared to the parental rMV-Hu191 vaccine strain. Importantly, both MV-Hu191 mutants triggered a higher neutralizing antibody than rMV-Hu191 vaccine and provided complete protection against MV challenge. These results demonstrate its potential for an improved MV vaccine candidate.

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An efficient reverse genetics system for Chinese MV-Hu191 strain was developed.

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rMV-Hu191 mutants in SAM binding site are attenuated in vitro and in vivo.

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rMV-Hu191 mutants in SAM binding site enhance the safety and immunogenicity of MV vaccine.

Ebola virus disease candidate vaccines under evaluation in clinical trials

Filoviruses are the etiological agents of two human illnesses: Ebola virus disease and Marburg virus disease. Until 2013, medical countermeasure development against these afflictions was limited to only a few research institutes worldwide as both infections were considered exotic due to very low case numbers. Together with the high case-fatality rate of both diseases, evaluation of any candidate countermeasure in properly controlled clinical trials seemed impossible. However, in 2013, Ebola virus was identified as the etiological agent of a large disease outbreak in Western Africa including almost 30,000 infections and more than 11,000 deaths, including case exportations to Europe and North America. These large case numbers resulted in medical countermeasure development against Ebola virus disease becoming a global public-health priority. This review summarizes the status quo of candidate vaccines against Ebola virus disease, with a focus on those that are currently under evaluation in clinical trials.

Applications of Replicating-Competent Reporter-Expressing Viruses in Diagnostic and Molecular Virology

Commonly used tests based on wild-type viruses, such as immunostaining, cannot meet the demands for rapid detection of viral replication, high-throughput screening for antivirals, as well as for tracking viral proteins or virus transport in real time. Notably, the development of replicating-competent reporter-expressing viruses (RCREVs) has provided an excellent option to detect directly viral replication without the use of secondary labeling, which represents a significant advance in virology. This article reviews the applications of RCREVs in diagnostic and molecular virology, including rapid neutralization tests, high-throughput screening systems, identification of viral receptors and virus-host interactions, dynamics of viral infections in vitro and in vivo, vaccination approaches and others. However, there remain various challenges associated with RCREVs, including pathogenicity alterations due to the insertion of a reporter gene, instability or loss of the reporter gene expression, or attenuation of reporter signals in vivo. Despite all these limitations, RCREVs have become powerful tools for both basic and applied virology with the development of new technologies for generating RCREVs, the inventions of novel reporters and the better understanding of regulation of viral replication.

Partial Attenuation of Respiratory Syncytial Virus with a Deletion of a Small Hydrophobic Gene Is Associated with Elevated Interleukin-1β Responses

The small hydrophobic (SH) gene of respiratory syncytial virus (RSV), a major cause of infant hospitalization, encodes a viroporin of unknown function. SH gene knockout virus (RSV ΔSH) is partially attenuated in vivo, but not in vitro, suggesting that the SH protein may have an immunomodulatory role. RSV ΔSH has been tested as a live attenuated vaccine in humans and cattle, and here we demonstrate that it protected against viral rechallenge in mice. We compared the immune response to infection with RSV wild type and RSV ΔSH in vivo using BALB/c mice and in vitro using epithelial cells, neutrophils, and macrophages. Strikingly, the interleukin-1β (IL-1β) response to RSV ΔSH infection was greater than to wild-type RSV, in spite of a decreased viral load, and when IL-1β was blocked in vivo, the viral load returned to wild-type levels. A significantly greater IL-1β response to RSV ΔSH was also detected in vitro, with higher-magnitude responses in neutrophils and macrophages than in epithelial cells. Depleting macrophages (with clodronate liposome) and neutrophils (with anti-Ly6G/1A8) demonstrated the contribution of these cells to the IL-1β response in vivo, the first demonstration of neutrophilic IL-1β production in response to viral lung infection. In this study, we describe an increased IL-1β response to RSV ΔSH, which may explain the attenuation in vivo and supports targeting the SH gene in live attenuated vaccines.

IMPORTANCE

There is a pressing need for a vaccine for respiratory syncytial virus (RSV). A number of live attenuated RSV vaccine strains have been developed in which the small hydrophobic (SH) gene has been deleted, even though the function of the SH protein is unknown. The structure of the SH protein has recently been solved, showing it is a pore-forming protein (viroporin). Here, we demonstrate that the IL-1β response to RSV ΔSH is greater in spite of a lower viral load, which contributes to the attenuation in vivo. This potentially suggests a novel method by which viruses can evade the host response. As all Pneumovirinae and some Paramyxovirinae carry similar SH genes, this new understanding may also enable the development of live attenuated vaccines for both RSV and other members of the Paramyxoviridae.

A recombinant canine distemper virus expressing a modified rabies virus glycoprotein induces immune responses in mice

Canine distemper virus (CDV) and rabies virus (RV) are two important pathogens of the dog. CDV, a member of the morbillivirus genus, has shown promise as an expression vector. The glycoprotein from RV is a main contributor to protective immunity and capable of eliciting the production of virus-neutralizing antibodies. In this study, we recovered an attenuated strain of canine distemper virus and constructed a recombinant virus, rCDV-RV-G, expressing a modified (R333Q) rabies virus glycoprotein (RV-G) of RV Flury strain LEP. RV-G expression by the recombinant viruses was confirmed. Furthermore, G was proved to be incorporated into the surface of CDV particles. While replication of the recombinant virus was slightly reduced compared with the parental CDV, it stably expressed the RV-G over ten serial passages. Inoculation of mice induced specific neutralizing antibodies against both RV-G and CDV. Therefore, the rCDV-RV-G has the potential as a vaccine that may be used to control rabies virus infection in dogs and other animals.

Development and application of reporter-expressing mononegaviruses: current challenges and perspectives

Reverse genetics allows the generation of recombinant viruses entirely from cDNA. One application of this technology is the creation of reporter-expressing viruses, which greatly increase the detail and ease with which these viruses can be studied. However, there are a number of challenges when working with reporter-expressing viruses. Both the reporter protein itself as well as the genetic manipulations within the viral genome required for expression of this reporter can result in altered biological properties of the recombinant virus, and lead to attenuation in vitro and/or in vivo. Further, instability of reporter expression and purging of the genetic information encoding for the reporter from the viral genome can be an issue. Finally, a practical challenge for in vivo studies lies in the attenuation of light signals when traversing tissues. Novel expression strategies and the continued development of brighter, red and far-red shifted reporters and the increased use of bioluminescent reporters for in vivo applications promise to overcome some of these limitations in future. However, a "one size fits all" approach to the design of reporter-expressing viruses has thus far not been possible. Rather, a reporter suited to the intended application must be selected and an appropriate expression strategy and location for the reporter in the viral genome chosen. Still, attenuating effects of the reporter on viral fitness are difficult to predict and have to be carefully assessed with respect to the intended application. Despite these limitations the generation of suitable reporter-expressing viruses will become more common as technology and our understanding of the intricacies of viral gene expression and regulation improves, allowing deeper insight into virus biology both in living cells and in animals.

A stabilized respiratory syncytial virus reverse genetics system amenable to recombination-mediated mutagenesis

We describe the first example of combining bacterial artificial chromosome (BAC) recombination-mediated mutagenesis with reverse genetics for a negative strand RNA virus. A BAC-based respiratory syncytial virus (RSV) rescue system was established. An important advantage of this system is that RSV antigenomic cDNA was stabilized in the BAC vector. The RSV genotype chosen was A2-line19F, a chimeric strain previously shown to recapitulate in mice key features of RSV pathogenesis. We recovered two RSV reporter viruses, one expressing the red fluorescent protein monomeric Katushka 2 (A2-K-line19F) and one expressing Renilla luciferase (A2-RL-line19F). As proof of principle, we efficiently generated a RSV gene deletion mutant (A2-line19FΔNS1/NS2) and a point mutant (A2-K-line19F-I557V) by recombination-mediated BAC mutagenesis. Together with sequence-optimized helper expression plasmids, BAC-RSV is a stable, versatile, and efficient reverse genetics platform for generation of a recombinant Pneumovirus.

Expression and stability of foreign epitopes introduced into 3A nonstructural protein of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) is an aphthovirus that belongs to the Picornaviridae family and causes one of the most important animal diseases worldwide. The capacity of other picornaviruses to express foreign antigens has been extensively reported, however, little is known about FMDV. To explore the potential of FMDV as a viral vector, an 11-amino-acid (aa) HSV epitope and an 8 aa FLAG epitope were introduced into the C-terminal different regions of 3A protein of FMDV full-length infectious cDNA clone. Recombinant viruses expressing the HSV or FLAG epitope were successfully rescued after transfection of both modified constructs. Immunofluorescence assay, Western blot and sequence analysis showed that the recombinant viruses stably maintained the foreign epitopes even after 11 serial passages in BHK-21 cells. The 3A-tagged viruses shared similar plaque phenotypes and replication kinetics to those of the parental virus. In addition, mice experimentally infected with the epitope-tagged viruses could induce tag-specific antibodies. Our results demonstrate that FMDV can be used effectively as a viral vector for the delivery of foreign tags.

A reverse genetics system for the Great Lakes strain of viral hemorrhagic septicemia virus: the NV gene is required for pathogenicity

Viral hemorrhagic septicemia virus (VHSV), belonging to the genus Novirhabdovirus in the family of Rhabdoviridae, causes a highly contagious disease of fresh and saltwater fish worldwide. Recently, a novel genotype of VHSV, designated IVb, has invaded the Great Lakes in North America, causing large-scale epidemics in wild fish. An efficient reverse genetics system was developed to generate a recombinant VHSV of genotype IVb from cloned cDNA. The recombinant VHSV (rVHSV) was comparable to the parental wild-type strain both in vitro and in vivo, causing high mortality in yellow perch (Perca flavescens). A modified recombinant VHSV was generated in which the NV gene was substituted with an enhanced green fluorescent protein gene (rVHSV-ΔNV-EGFP), and another recombinant was made by inserting the EGFP gene into the full-length viral clone between the P and M genes (rVHSV-EGFP). The in vitro replication kinetics of rVHSV-EGFP was similar to rVHSV; however, the rVHSV-ΔNV-EGFP grew 2 logs lower. In yellow perch challenges, wtVHSV and rVHSV induced 82-100% cumulative per cent mortality (CPM), respectively, whereas rVHSV-EGFP produced 62% CPM and rVHSV-ΔNV-EGFP caused only 15% CPM. No reversion of mutation was detected in the recovered viruses and the recombinant viruses stably maintained the foreign gene after several passages. These results indicate that the NV gene of VHSV is not essential for viral replication in vitro and in vivo, but it plays an important role in viral replication efficiency and pathogenicity. This system will facilitate studies of VHSV replication, virulence, and production of viral vectored vaccines.

Interleukin 18 coexpression during respiratory syncytial virus infection results in enhanced disease mediated by natural killer cells

Respiratory syncytial virus (RSV) causes bronchiolitis, the main cause of infantile hospitalization. Immunity against reinfection is poor, and there is great interest in boosting vaccine responses using live vectors expressing host cytokines. We therefore constructed a recombinant RSV expressing murine interleukin 18 (RSV/IL-18), a cytokine capable of inducing strong antiviral immune responses. In vitro RSV/IL-18 replicated at wild-type levels and produced soluble IL-18. In naïve BALB/c mice, RSV/IL-18 infection significantly increased both IL-18 mRNA and protein and attenuated the peak viral load 3-fold. Despite a reduced viral load, RSV/IL-18 infection caused a biphasic weight loss at days 2 and 6 postinfection that was not seen in wild-type infection. Day 2 disease was associated with enhanced pulmonary natural killer (NK) cell numbers and activity and was prevented by NK cell depletion during infection; day 6 disease was correlated with CD8 T-cell recruitment and was enhanced by NK cell depletion. IL-18 expression during priming also enhanced RSV-specific antibody responses and T-cell responses on secondary RSV infection. Therefore, while IL-18 boosted antiviral immunity and reduced the viral load, its coexpression worsened disease. This is the first recombinant RSV with this property, and these are the first studies to demonstrate that NK cells can induce pathology during pulmonary viral infections.

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

Generation of measles virus with a segmented RNA genome

Viruses classified in the order Mononegavirales have a single nonsegmented RNA molecule as the genome and employ similar strategies for genome replication and gene expression. Infectious particles of Measles virus (MeV), a member of the family Paramyxoviridae in the order Mononegavirales, with two or three RNA genome segments (2 seg- or 3 seg-MeV) were generated using a highly efficient reverse genetics system. All RNA segments of the viruses were designed to have authentic 3' and 5' self-complementary termini, similar to those of negative-stranded RNA viruses that intrinsically have multiple RNA genome segments. The 2 seg- and 3 seg-MeV were viable and replicated well in cultured cells. 3 seg-MeV could accommodate up to six additional transcriptional units, five of which were shown to be capable of expressing foreign proteins efficiently. These data indicate that the MeV genome can be segmented, providing an experimental insight into the divergence of the negative-stranded RNA viruses with nonsegmented or segmented RNA genomes. They also illustrate a new strategy to develop mononegavirus-derived vectors harboring multiple additional transcriptional units.

Mutational analysis of the avian pneumovirus conserved transcriptional gene start sequence identifying critical residues

Seven of the eight genes in the avian pneumovirus (APV) genome contain a conserved 9 nt transcriptional start sequence with the virus large (L) polymerase gene differing from the consensus at three positions. The sequence requirements of the APV transcriptional gene start sequence were investigated by generating a series of mutations in which each of the nine conserved bases was mutated to each of the other three possible nucleotides in a minigenome containing two reporter genes. The effect of each mutation was assessed by measuring the relative levels of expression from the altered and unaltered gene start sequences. Mutations at positions 2, 7 and 9 significantly reduced transcription levels while alterations to position 5 had little effect. The L gene start sequence directed transcription at levels approximately 50 % below that of the consensus gene start sequence. These data suggest that there are common features in pneumovirus transcriptional control sequences.

Expression of interleukin-4 by recombinant respiratory syncytial virus is associated with accelerated inflammation and a nonfunctional cytotoxic T-lymphocyte response following primary infection but not following challenge with wild-type virus

The outcome of a viral infection or of immunization with a vaccine can be influenced by the local cytokine environment. In studies of experimental vaccines against respiratory syncytial virus (RSV), an increased stimulation of Th2 (T helper 2) lymphocytes was associated with increased immunopathology upon subsequent RSV infection. For this study, we investigated the effect of increased local expression of the Th2 cytokine interleukin-4 (IL-4) from the genome of a recombinant RSV following primary infection and after a challenge with wild-type (wt) RSV. Mice infected with RSV/IL-4 exhibited an accelerated pulmonary inflammatory response compared to those infected with wt RSV, although the wt RSV group caught up by day 8. In the first few days postinfection, RSV/IL-4 was associated with a small but significant acceleration in the expansion of pulmonary T lymphocytes specific for an RSV CD8(+) cytotoxic T-lymphocyte (CTL) epitope presented as a major histocompatibility complex class I tetramer. However, by day 7 the response of tetramer-positive T lymphocytes in the wt RSV group caught up and exceeded that of the RSV/IL-4 group. At all times, the CTL response of the RSV/IL-4 group was deficient in the production of gamma interferon and was nonfunctional for in vitro cell killing. The accelerated inflammatory response coincided with an accelerated accumulation and activation of pulmonary dendritic cells early in infection, but thereafter the dendritic cells were deficient in the expression of B7-1, which governs the acquisition of cytolytic activity by CTL. Following a challenge with wt RSV, there was an increase in Th2 cytokines in the animals that had previously been infected with RSV/IL-4 compared to those previously infected with wt RSV, but the CD8(+) CTL response and the amount of pulmonary inflammation were not significantly different. Thus, a strong Th2 environment during primary pulmonary immunization with live RSV resulted in early inflammation and a largely nonfunctional primary CTL response but had a minimal effect on the secondary response.

Comparison of the full-length genome sequence of avian metapneumovirus subtype C with other paramyxoviruses

We determined the nucleotide (nt) sequence of the small hydrophobic (SH), attachment glycoprotein (G), and RNA polymerase (L) genes, plus the leader and trailer regions of the Colorado strain of Avian metapneumovirus subtype C (aMPV/C) in order to complete the genome sequencing. The complete genome comprised of 13,134 nucleotides, with a 40 nt leader at its 3' end and a 45 nt trailer at its 5' end. The aMPV/C L gene was the largest with 6173 nt and consisting of a single open reading frame encoding a 2005 amino acids (aa) protein. Comparison of the aMPV/C SH, G, and L nt and predicted aa sequences with those of Human metapneumoviruses (hMPV) revealed higher nt and aa sequence identities than the sequence identities between the aMPV subtypes A, B, C, and D, supporting earlier finding that aMPV/C was closer evolutionary to hMPV than the other aMPV subtypes.

A recombinant Newcastle disease virus (NDV) expressing VP2 protein of infectious bursal disease virus (IBDV) protects against NDV and IBDV

Infectious bursal disease virus (IBDV) causes a highly immunosuppressive disease in chickens. Currently available, live IBDV vaccines can lead to generation of variant viruses. We have developed an alternative vaccine that will not create variant IBDV. By using the reverse genetics approach, we devised a recombinant Newcastle disease virus (NDV) vector from a commonly used vaccine strain LaSota to express the host-protective immunogen VP2 of a variant IBDV strain GLS-5. The gene encoding the VP2 protein of the IBDV was inserted into the most 3'-proximal locus of a full-length NDV cDNA for high-level expression. We successfully recovered the recombinant virus, rLaSota/VP2. The rLaSota/VP2 was genetically stable, at least up to 12 serial passages in chicken embryos, and was shown to express the VP2 protein. The VP2 protein was not incorporated into the virions of recombinant virus. Recombinant rLaSota/VP2 replicated to a titer similar to that of parental NDV strain LaSota in chicken embryos and cell cultures. To assess protective efficacy of the rLaSota/VP2, 2-day-old specific-pathogen-free chickens were vaccinated with the recombinant virus and challenged with a highly virulent NDV strain Texas GB or IBDV variant strain GLS-5 at 3 weeks postvaccination. Vaccination with rLaSota/VP2 generated antibody responses against both NDV and IBDV and provided 90% protection against NDV and IBDV. Booster immunization induced higher levels of antibody responses against both NDV and IBDV and conferred complete protection against both viruses. These results indicate that the recombinant NDV can be used as a vaccine vector for other avian pathogens.

The future of respiratory syncytial virus vaccine development

BACKGROUND

Respiratory syncytial virus (RSV) is the leading cause of viral lower respiratory tract illness in infants and children and is an important cause of lower respiratory tract illness in other populations. Despite decades of research there are currently no licensed vaccines for prevention of RSV disease.

METHODS

A review of the obstacles to RSV vaccine development; current live, attenuated and subunit RSV vaccines in clinical development; and the potential for developing additional vaccine candidates based on recombinant technology.

RESULTS

A number of biologically derived live attenuated RSV vaccines were evaluated in Phase I clinical trials in adults and children, and one vaccine (cpts 248/404) was evaluated in infants as young as 1 month of age. These vaccines displayed a spectrum of attenuation, with cpts 248/955 being the least attenuated and cpts 248/404 being the most attenuated candidate vaccine. None of these was sufficiently attenuated for young infants. The ability to generate recombinant RSV vaccines has led to the development of large numbers of candidate vaccines containing combinations of known attenuating point mutations and deletions of nonessential genes. Clinical evaluation of many of these candidates is in progress. Three types of RSV subunit vaccines have recently been evaluated in clinical trials: purified F glycoprotein vaccines (PFP-1, PFP-2 and PFP-3), BBG2Na and copurified F, G and M proteins. Additional studies of the F/G/M protein vaccine are being conducted.

CONCLUSIONS

During the past 10 years, considerable progress has been made in RSV vaccine development. It is likely that different RSV vaccines will be needed for the various populations at risk.

Transcription and replication of nonsegmented negative-strand RNA viruses

The nonsegmented negative-strand (NNS) RNA viruses of the order Mononegavirales include a wide variety of human, animal, and plant pathogens. The NNS RNA genomes of these viruses are templates for two distinct RNA synthetic processes: transcription to generate mRNAs and replication of the genome via production of a positive-sense antigenome that acts as template to generate progeny negative-strand genomes. The four virus families within the Mononegavirales all express the information encoded in their genomes by transcription of discrete subgenomic mRNAs. The key feature of transcriptional control in the NNS RNA viruses is entry of the virus-encoded RNA-dependent RNA polymerase at a single 3' proximal site followed by obligatory sequential transcription of the linear array of genes. Levels of gene expression are primarily regulated by position of each gene relative to the single promoter and also by cis-acting sequences located at the beginning and end of each gene and at the intergenic junctions. Obligatory sequential transcription dictates that termination of each upstream gene is required for initiation of downstream genes. Therefore, termination is a means to regulate expression of individual genes within the framework of a single transcriptional promoter. By engineering either whole virus genomes or subgenomic replicon derivatives, elements important for signaling transcript initiation, 5' end modification, 3' end polyadenylation, and transcription termination have been identified. Although the diverse families of NNS RNA virus use different sequences to control these processes, transcriptional termination is a common theme in controlling gene expression and overall transcriptional regulation is key in controlling the outcome of viral infection. The latest models for control of replication and transcription are discussed.

Toward novel vaccines and therapies based on negative-strand RNA viruses

The study of negative-strand RNA viruses has suggested new strategies to produce more attenuated viruses. Reverse genetics has allowed the implementation of the strategies, and new or improved monovalent vaccines are being developed. In addition, recombinant viruses expressing foreign proteins or epitopes have been produced with the aim of developing multivalent vaccines capable of stimulating humoral and cellular immune responses against more than one pathogen. Finally, recombinant viruses that selectively enter cells expressing tumor markers or the HIV envelope protein have been engineered and shown to lyse target cells. Preclinical and clinical trials of improved and multivalent vaccines and therapeutic (oncolytic) viruses are ongoing.

Progress in the development of respiratory syncytial virus and parainfluenza virus vaccines

Respiratory syncytial virus (RSV) and human parainfluenza viruses (hPIVs) are leading causes of viral lower respiratory tract illness in children and in high-risk adult populations. Despite decades of research, licensed vaccines for RSV and hPIVs do not exist. Recently, however, genetically engineered live attenuated RSV and hPIV candidate vaccines have been generated, several of which are already being evaluated in clinical trials. Recombinant technology allows candidate vaccines to be "fine-tuned" in response to clinical data, which should hasten the development of vaccines against these important respiratory pathogens.

Recombinant Newcastle disease virus as a vaccine vector

Veterinary vaccines remained conventional for more than fifty years. Recent advances in the recombinant genetic engineering techniques brought forward a leap in designing vaccines for veterinary use. A novel approach of delivering protective immunogens of many different pathogens in a single virus vector was made possible with the introduction of a "reverse genetics" system for nonsegmented negative-sense RNA viruses. Newcastle disease virus (NDV), a nonsegmented negative-sense virus, is one of the major viruses of economic importance in the poultry industry throughout the world. Despite the availability of live virus vaccines of good potency, the intrinsic ability of attenuated strains to revert in virulence makes control of this disease by vaccination difficult. Armed with the knowledge of virulence factors of this virus, it is now possible to produce genetically stable vaccines and to engineer mutations that enhance immunogenicity. The modular nature of the genome of this virus facilitates engineering additional genes from several different pathogens or tumor-specific antigens to design contemporary vaccines for animals and humans. This review will summarize the developments in using NDV as a vaccine vector and the potential of this approach in designing next generation vaccines for veterinary use.

Recombinant Newcastle disease virus as a viral vector: effect of genomic location of foreign gene on gene expression and virus replication

Newcastle disease virus (NDV) was examined for its suitability as a vector for the expression and delivery of foreign genes for vaccination and gene therapy. A reporter gene encoding human secreted alkaline phosphatase (SEAP) was inserted as an additional transcription unit at four different positions in the NDV genome, between the NP and P, M and F, and HN and L genes and behind the L gene. Eight infectious recombinant NDV (rNDV) viruses, four in the non-virulent strain NDFL and four in the virulent derivative NDFLtag, were generated by reverse genetics. SEAP expression levels, replication kinetics and virus yield were examined. Replication kinetics of the rNDV viruses in primary chicken embryo fibroblasts showed that the insertion of an additional gene resulted in a delay in the onset of replication. This effect was most prominent when the gene was inserted between the NP and P genes. With the exception of the strain that carried the SEAP gene behind the L gene, all recombinant strains expressed high levels of SEAP, both in cell culture and in embryonated chicken eggs. In embryonated eggs, the rNDV viruses showed a 2.6- to 5.6-fold (NDFL) or 2.1- to 8.1-fold (NDFLtag) reduction in yield compared with the parent strains. These results show that foreign genes can be inserted at different positions in the NDV genome without severely affecting replication efficiency or virus yield.

Characterization of a recombinant Newcastle disease virus expressing the green fluorescent protein

A recombinant Newcastle disease virus (NDV) expressing the green fluorescent protein (GFP) was generated by applying reverse genetics techniques. The GFP open reading frame flanked by NDV transcription start and stop sequences was inserted between the fusion (F)- and hemagglutinin-neuraminidase genes in a full-length cDNA clone of NDV. This plasmid transcribing antigenome RNA was cotransfected with helper plasmids expressing viral nucleoprotein, phosphoprotein and large protein into cells stably expressing T7 RNA polymerase. The rescued virus was first propagated in embryonated eggs and the allantoic fluid was used to infect cells. Northern blot analysis of RNA isolated from infected cells demonstrated the proper transcription of the introduced GFP-mRNA. The appearance of GFP in live infected cells confirmed further the recovery of a recombinant NDV (rNDVGFP1) expressing the reporter gene. The expression of the heterologous gene was maintained stably for at least five passages in embryonated eggs. The replication kinetics in embryonated eggs and pathogenicity in chickens of rNDVGFP1 did not differ significantly from that of the parent virus. Using GFP autofluorescence, virus infected cells could be tracked easily in native preparations, organ explants and primary tracheal cell cultures. Taken together, these data demonstrate the use of GFP-expressing recombinant NDV for analysis of NDV dissemination and pathogenesis and indicate the potential usefulness of NDV as a vaccine vector.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Transcriptional control of the RNA-dependent RNA polymerase of vesicular stomatitis virus

The nonsegmented negative strand (NNS) RNA viruses include some of the mosr problematic human, animal and plant pathogens extant: for example, rabies virus, Ebola virus, respiratory syncytial virus, the parainfluenza viruses, measles and infectious hemapoietic necrosis virus. The key feature of transcriptional control in the NNS RNA viruses is polymerase entry at a single 3' proximal site followed by obligatory sequential transcription of the linear array of genes. The levels of gene expression are primarily regulated by their position on the genome. The promoter proximal gene is transcribed in greatest abundance and each successive downstream gene is synthesized in progressively lower amounts due to attenuation of transcription at each successive gene junction. In addition, NNS RNA virus gene expression is regulated by cis-acting sequences that reside at the beginning and end of each gene and the intergenic junctions. Using vesicular stomatitis virus (VSV), the prototypic NNS, many of these control elements have been identified.The signals for transcription initiation and 5' end modification and for 3' end polyadenylation and termination have been elucidated. The sequences that determine the ability of the polymerase to slip on the template to generate polyadenylate have been identified and polyadenylation has been shown to be template dependent and integral to the termination process. Transcriptional termination is a key element in control of gene expression of the negative strand RNA viruses and a means by which expression of individual genes may be silenced or regulated within the framework of a single transcriptional promoter. In addition, the fundamental question of the site of entry of the polymerase during transcription has been reexamined and our understanding of the process altered and updated. The ability to engineer changes into infectious viruses has confirmed the action of these elements and as a consequence, it has been shown that transcriptional control is key to controlling the outcome of a viral infection. Finally, the principles of transcriptional regulation have been utilized to develop a new paradigm for systematic attenuation of virulence to develop live attenuated viral vaccines.

Transcriptional termination modulated by nucleotides outside the characterized gene end sequence of respiratory syncytial virus

The genes of respiratory syncytial (RS) virus are transcribed sequentially by the viral RNA polymerase from a single 3'-proximal promoter. Polyadenylation and termination are directed by a sequence at the end of each gene, after which the polymerase crosses an intergenic region and reinitiates at the start sequence of the next gene. The 10 viral genes have different gene end sequences and different termination efficiencies, which allow for regulation of gene expression, since termination of each gene is required for initiation of the downstream gene. RNA sequences within the previously characterized 13 nucleotide gene end, including a conserved sequence 3'-UCAAU-5' and a tract of U residues, are important for termination. In this study, two additional sequence elements outside of the 13 nucleotide gene end were found to modulate termination efficiency: the A residue upstream of the 3'-UCAAU-5' sequence, and the first nucleotide of the intergenic region when it follows a U(4) tract.

Adding genes to the RNA genome of vesicular stomatitis virus: positional effects on stability of expression

Gene expression of the nonsegmented negative strand (NNS) RNA viruses is controlled primarily at the level of transcription by the position of the genes relative to the single transcriptional promoter. We tested this principle by generating engineered variants of vesicular stomatitis virus in which an additional, identical, transcriptional unit was added to the genome at each of the viral gene junctions. Analysis of transcripts confirmed that the level of transcription was determined by the position of the gene relative to the promoter. However, the position at which a gene was inserted affected the replication potential of the viruses. Adding a gene between the first two genes, N and P, reduced replication by over an order of magnitude, whereas addition of a gene at the other gene junctions had no effect on replication levels. All genes downstream of the inserted gene had decreased levels of expression, since transcription of the extra gene introduced an additional transcriptional attenuation event. The added gene was stably maintained in the genome upon repeated passage in all cases. However, expression of the added gene was stable at only three of the four positions. In the case of insertion between the N and P genes, a virus population arose within two passages that had restored replication to wild-type levels. In this population, expression of the additional gene as a monocistronic mRNA was suppressed by mutations at the end of the upstream (N) gene that abolished transcriptional termination. Because transcription is obligatorily sequential, this prevented transcription of the inserted downstream gene as a monocistronic mRNA and resulted instead in polymerase reading through the gene junction to produce a bicistronic mRNA. This eliminated the additional attenuation step and restored expression of all downstream genes and viral replication to wild-type levels. These data show that transcriptional termination is a key element in control of gene expression of the negative strand RNA viruses and a means by which expression of individual genes may be regulated within the framework of a single transcriptional promoter. Further, these results are directly relevant to the use of NNS viruses as vectors and vaccine delivery agents, as they show that the level of expression of an added gene can be controlled by its insertion position but that not all positions of insertion yield stable expression of the added gene.

Evaluation of the replication and immunogenicity of recombinant human parainfluenza virus type 3 vectors expressing up to three foreign glycoproteins

The level of replication and immunogenicity of recombinant parainfluenza virus type 3 (rHPIV3) bearing one, two, or three gene insertions expressing foreign protective antigens was examined. cDNA-derived recombinant HPIV3s bearing genes encoding the open reading frames (ORFs) of the hemagglutinin-neuraminidase (HN) of HPIV1, the HN of HPIV2, or the hemagglutinin (HA) of measles virus replicated efficiently in vitro, including the largest recombinant, which had three gene unit insertions and which was almost 23 kb in length, 50% longer than unmodified HPIV3. Several viruses were recovered from cDNAs whose genome length was not a multiple of six nucleotides and these contained nucleotide insertions that corrected the length to be a multiple of 6, confirming that the "rule of six" applies to HPIV3. Using a hemagglutination inhibition assay, we determined that the HPIV1 HN expressed by recombinant HPIV3 was incorporated into HPIV3 virions, whereas using this assay incorporation of the HPIV2 HN could not be detected. HPIV3 virions bearing HPIV1 HN were not neutralized by HPIV1 antiserum but were readily neutralized by antibodies to the HPIV3 HN or fusion protein (F). Viruses with inserts were restricted for replication in the respiratory tract of hamsters, and the level of restriction was a function of the total number of genes inserted, the nature of the insert, and the position of the inserted gene in the gene order. A single insert of HPIV2 HN or measles virus HA reduced the in vivo replication of rHPIV3 up to 25-fold, whereas the HPIV1 HN insert decreased replication almost 1000-fold. This indicates that the HPIV1 HN insert has an attenuating effect in addition to that of the extra gene insert itself, presumably because it is incorporated into the virus particle. Viruses containing two inserts were generally more attenuated than those with a single insert, and viruses with three inserts were over-attenuated for replication in hamsters. Inserts between the N and P genes were slightly more attenuating than those between the P and the M genes. A recombinant HPIV3 bearing both the HPIV1 and the HPIV2 HN genes (r1HN 2HN) was attenuated, immunogenic, and protected immunized hamsters from challenge with HPIV1, HPIV2, and HPIV3. Thus, it is possible to use a single HPIV vector expressing two foreign gene inserts to protect infants and young children from the severe lower respiratory tract disease caused by the three major human PIV pathogens.

Mucosal immunization of rhesus monkeys against respiratory syncytial virus subgroups A and B and human parainfluenza virus type 3 by using a live cDNA-derived vaccine based on a host range-attenuated bovine parainfluenza virus type 3 vector backbone

Reverse genetics was used to develop a two-component, trivalent live attenuated vaccine against human parainfluenza virus type 3 (HPIV3) and respiratory syncytial virus (RSV) subgroups A and B. The backbone for each of the two components of this vaccine was the attenuated recombinant bovine/human PIV3 (rB/HPIV3), a recombinant BPIV3 in which the bovine HN and F protective antigens are replaced by their HPIV3 counterparts (48). This chimera retains the well-characterized host range attenuation phenotype of BPIV3, which appears to be appropriate for immunization of young infants. The open reading frames (ORFs) for the G and F major protective antigens of RSV subgroup A and B were each placed under the control of PIV3 transcription signals and inserted individually or in homologous pairs as supernumerary genes in the promoter proximal position of rB/HPIV3. The level of replication of rB/HPIV3-RSV chimeric viruses in the respiratory tract of rhesus monkeys was similar to that of their parent virus rB/HPIV3, and each of the chimeras induced a robust immune response to both RSV and HPIV3. RSV-neutralizing antibody titers induced by rB/HPIV3-RSV chimeric viruses were equivalent to those induced by infection with wild-type RSV, and HPIV3-specific antibody responses were similar to, or slightly less than, after infection with the rB/HPIV3 vector itself. This study describes a novel vaccine strategy against RSV in which vaccine viruses with a common attenuated backbone, specifically rB/HPIV3 derivatives expressing the G and/or F major protective antigens of RSV subgroup A and of RSV subgroup B, are used to immunize by the intranasal route against RSV and HPIV3, which are the first and second most important viral agents of pediatric respiratory tract disease worldwide.

Granulocyte-macrophage colony-stimulating factor expressed by recombinant respiratory syncytial virus attenuates viral replication and increases the level of pulmonary antigen-presenting cells

An obstacle to developing a vaccine against human respiratory syncytial virus (RSV) is that natural infection typically does not confer solid immunity to reinfection. To investigate methods to augment the immune response, recombinant RSV (rRSV) was constructed that expresses murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) from a transcription cassette inserted into the G-F intergenic region. Replication of rRSV/mGM-CSF in the upper and lower respiratory tracts of BALB/c mice was reduced 23- to 74- and 5- to 588-fold, respectively, compared to that of the parental rRSV. Despite this strong attenuation of replication, the level of RSV-specific serum antibodies induced by rRSV/mGM-CSF was comparable to, or marginally higher than, that of the parental rRSV. The induction of RSV-specific CD8(+) cytotoxic T cells was moderately reduced during the initial infection, which might be a consequence of reduced antigen expression. Mice infected with rRSV/mGM-CSF had elevated levels of pulmonary mRNA for gamma interferon (IFN-gamma) and interleukin 12 (IL-12) p40 compared to animals infected by wild-type rRSV. Elevated synthesis of IFN-gamma could account for the restriction of RSV replication, as was observed previously with an IFN-gamma-expressing rRSV. The accumulation of total pulmonary mononuclear cells and total CD4(+) T lymphocytes was accelerated in animals infected with rRSV/mGM-CSF compared to that in animals infected with the control virus, and the level of IFN-gamma-positive or IL-4-positive pulmonary CD4(+) cells was elevated approximately twofold. The number of pulmonary lymphoid and myeloid dendritic cells and macrophages was increased up to fourfold in mice infected with rRSV/mGM-CSF compared to those infected with the parental rRSV, and the mean expression of major histocompatibility complex class II molecules, a marker of activation, was significantly increased in the two subsets of dendritic cells. Enhanced antigen presentation likely accounts for the maintenance of a strong antibody response despite reduced viral replication and would be a desirable property for a live attenuated rRSV vaccine.

High-level expression of a foreign gene from the most 3'-proximal locus of a recombinant Newcastle disease virus

A previous report showed that insertion of a foreign gene encoding chloramphenicol acetyltransferase (CAT) between the HN and L genes of the full-length cDNA of a virulent Newcastle disease virus (NDV) yielded virus with growth retardation and attenuation. The NDV vector used in that study was pathogenic to chickens; it is therefore not suitable for use as a vaccine vector. In the present study, an avirulent NDV vector was generated and its potential to express CAT protein was evaluated. The CAT gene was under the control of NDV transcriptional start and stop signals and was inserted immediately before the open reading frame of the viral 3'-proximal nucleocapsid protein gene. A recombinant NDV expressing CAT activity at a high level was recovered. The replication and pathogenesis of the CAT-expressing recombinant NDV were not modified significantly. These results indicate the potential utility of an avirulent NDV as a vaccine vector.

Construction of a live-attenuated bivalent vaccine virus against human parainfluenza virus (PIV) types 1 and 2 using a recombinant PIV3 backbone

PIV1 and PIV2 are important agents of pediatric respiratory tract disease. We are developing live-attenuated vaccines against these viruses. We earlier constructed a PIV3/PIV1 antigenic chimeric virus, designated rPIV3-1, in which the hemagglutinin-neuraminidase (HN) and fusion (F) proteins of wild type rPIV3 were replaced by their PIV1 counterparts. In the present study, rPIV3-1 was used as a vector to express the HN protein of PIV2 to generate a single virus capable of inducing immunity to both PIV1 and PIV2. The PIV2 HN open reading frame was expressed from an extra gene cassette, under the control of PIV3 cis-acting transcription signals, inserted between the F and HN genes of rPIV3-1. The recombinant derivative, designated rPIV3-1.2HN, was readily recovered and exhibited a level of temperature sensitivity and in vitro growth similar to that of its parental virus. The rPIV3-1.2HN virus was restricted in replication in both the upper and lower respiratory tracts of hamsters compared with rPIV3-1, identifying an attenuating effect of the PIV2 HN insert in hamsters. rPIV3-1.2HN elicited serum antibodies to both PIV1 and PIV2 and induced resistance against challenge with wild type PIV1 or PIV2. Thus, rPIV3-1.2HN, a virus attenuated solely by the insertion of the PIV2 HN gene, functioned as a live attenuated bivalent vaccine candidate against both PIV1 and PIV2.

Respiratory syncytial virus vaccines for otitis media

RSV is a high priority for vaccine development because of its propensity to cause pneumonia and bronchiolitis in the infant and young child. Since RSV infection is likely to be a substantial contributor to otitis media, a vaccine could also decrease rates of this disease. No vaccine has yet been developed but it is hoped that the availability of an RSV infectious clone will make it possible to develop a live virus vaccine for the infant and young child. Subunit RSV vaccines are being developed for previously infected persons, i.e. in older children at high risk for RSV disease and the elderly. An effective RSV vaccine for the infant and young child could markedly decrease otitis media disease.

Recovery of a virulent strain of newcastle disease virus from cloned cDNA: expression of a foreign gene results in growth retardation and attenuation

A recombinant mesogenic NDV strain, Beaudette C, and an engineered recombinant NDV expressing an additional gene were generated entirely from cloned cDNAs. For this purpose, a full-length cDNA clone of the virus genome, represented in eight different subgenomic fragments, was assembled in a transcription plasmid between a T7 RNA polymerase promoter and a hepatitis delta virus ribozyme sequence. Infectious NDV could be generated in the cells infected with recombinant vaccinia virus, which expressed T7 RNA polymerase, by simultaneous expression of antigenome-sense NDV RNA from the full-length plasmid and NDV NP, P, and L proteins from cotransfected plasmids. Recombinant virus was then amplified and recovered, either after inoculation of transfection supernatant into the allantoic cavity of embryonated specific-pathogen-free eggs or after further passage in cell culture. Characterization of the recombinant NDV showed similarities in growth and pathogenicity to that of the parental wild-type virus. By using this system, a recombinant NDV containing a foreign gene encoding chloramphenicol acetyltransferase (CAT) was generated. To do this, the CAT transcription cassette containing the CAT open reading frame, flanked by NDV gene start and gene end sequence motifs, was inserted into the region between the HN and L genes of the full-length cDNA. This construct was then used in the generation of a recombinant NDV expressing CAT protein. The CAT gene was maintained stably for at least eight passages without any detectable loss of the gene from the recombinant. Generation of the recombinant virus, however, was associated with reduced plaque size, slower replication kinetics, and more than 100-fold decrease in yield. In addition, the virus showed an increase in mean death time for eggs and a lower intracerebral pathogenicity index in day-old chicks, implicating attenuation of the recombinant virus. Thus, introduction of an additional gene into the NDV genome represents a method to achieve growth retardation and attenuation. These results also indicate that NDV can be engineered to express foreign protein stably and can be manipulated in the future for use as a vaccine vector.

Respiratory syncytial virus can tolerate an intergenic sequence of at least 160 nucleotides with little effect on transcription or replication in vitro and in vivo

The intergenic sequences (IGS) between the first nine genes of human respiratory syncytial virus (RSV) vary in length from 1 to 56 nucleotides and lack apparent conserved sequence motifs. To investigate their influence on sequential transcription and viral growth, recombinant RSV strain A2, from which the SH gene had been deleted to facilitate manipulation, was further modified to contain an M-G IGS of 16, 30, 44, 58, 65, 72, 86, 100, 120, 140, or 160 nucleotides. All of the viruses were viable. For viruses with an M-G IGS of 100 nucleotides or more, plaque size decreased with increasing IGS length. In this same length range, increasing IGS length was associated with modest attenuation during single-step, but not multistep, growth in HEp-2 cells. Surprisingly, Northern blot analysis of the accumulation of six different mRNAs indicated that there was little or no change in transcription with increasing IGS length. Thus, the RSV polymerase apparently can readily cross IGS of various lengths, including unnaturally long ones, with little or no effect on the efficiency of termination and reinitiation. This finding supports the view that the IGS do not have much effect on sequential transcription and provides evidence from infectious virus that IGS length is not an important regulatory feature. To evaluate replication in vivo, BALB/c mice were infected intranasally with RSV containing an M-G IGS of 65, 140, or 160 nucleotides. Replication of the latter two viruses was decreased up to 5- and 25-fold in the upper and lower respiratory tracts, respectively, on day 3 following infection. However, the level of replication at both sites on days 4 and 5 was very similar to that of the virus with an IGS of 65 nucleotides. Thus, the modest attenuation in vivo associated with the longer IGS was additive to that conferred by deletion of the SH gene and might be useful to incrementally increase the level of attenuation of a live-attenuated vaccine virus.

Effect of coexpression of interleukin-2 by recombinant respiratory syncytial virus on virus replication, immunogenicity, and production of other cytokines

We constructed rRSV/mIL-2, a recombinant respiratory syncytial virus (rRSV) containing the coding sequence of murine interleukin-2 (mIL-2) in a transcription cassette inserted into the G-F intergenic region. The recovered virus (rRSV/mIL-2) expressed high levels (up to 2.8 microg/ml) of mIL-2 in cell culture. Replication of rRSV/mIL-2 in vitro was reduced up to 13.6-fold from that of wild-type (wt) rRSV, an effect that was due to the presence of the foreign insert but was not specific to mIL-2. Replication of the rRSV/mIL-2 virus in the upper and lower respiratory tracts of BALB/c mice was reduced up to 6.3-fold, an effect that was specific to mIL-2. The antibody response, including the levels of RSV-specific serum immunoglobulin G1 (IgG1), IgG2a, IgA, and total IgG, and the level of protective efficacy against wt RSV challenge were not significantly different from those of wt rRSV. Analysis of total pulmonary cytokine mRNA isolated 1 and 4 days following infection with rRSV/mIL-2 revealed elevated levels of mRNA for IL-2, gamma interferon (IFN-gamma), IL-4, IL-5, IL-6, IL-10, IL-13, and IL-12 p40 compared to those for wt rRSV. Flow cytometry of total pulmonary mononuclear cells isolated 10 days following infection with rRSV/mIL-2 revealed increased levels of CD4(+) T lymphocytes expressing either IFN-gamma or IL-4 compared to those of wt rRSV. These elevations in cytokine mRNA or cytokine-expressing CD4(+) cells relative to those of wt rRSV-primed animals were not observed following challenge with wt RSV on day 28. Thus, the expression of mIL-2 by rRSV was associated with a modest attenuation of virus growth in vivo, induction of serum antibodies at levels comparable to that of wt rRSV, and transient increases in both the Th1 and Th2 CD4(+) lymphocytes and cytokine mRNAs compared to those of wt rRSV.

A cytoplasmic RNA vector derived from nontransmissible Sendai virus with efficient gene transfer and expression

We have recovered a virion from defective cDNA of Sendai virus (SeV) that is capable of self-replication but incapable of transmissible-virion production. This virion delivers and expresses foreign genes in infected cells, and this is the first report of a gene expression vector derived from a defective viral genome of the Paramyxoviridae. First, functional ribonucleoprotein complexes (RNPs) were recovered from SeV cloned cDNA defective in the F (envelope fusion protein) gene, in the presence of plasmids expressing nucleocapsid protein and viral RNA polymerase. Then the RNPs were transfected to the cells inducibly expressing F protein. Virion-like particles thus obtained had a titer of 0.5 x 10(8) to 1. 0 x 10(8) cell infectious units/ml and contained F-defective RNA genome. This defective vector amplified specifically in an F-expressing packaging cell line in a trypsin-dependent manner but did not spread to F-nonexpressing cells. This vector infected and expressed an enhanced green fluorescent protein reporter gene in various types of animal and human cells, including nondividing cells, with high efficiency. These results suggest that this vector has great potential for use in human gene therapy and vaccine delivery systems.

Long nucleotide insertions between the HN and L protein coding regions of human parainfluenza virus type 3 yield viruses with temperature-sensitive and attenuation phenotypes

Recombinant parainfluenza virus 3 (rPIV3) is being developed as a vector to express foreign genes as a bivalent or multivalent live attenuated virus vaccine. In the present study, we examined the effect of inserted foreign sequence on virus replication in vitro and in vivo, focusing on the parameter of insert length. In one type of construct, foreign sequence of increasing length was flanked by PIV3 transcription signals and inserted as an additional gene unit (GU insert) between the HN and L genes, so that one additional mRNA would be made. In a second type of construct, foreign sequence was inserted into the downstream NCR (NCR insert) of the HN gene, so that the number of encoded mRNAs remained unchanged. In each case, the foreign sequence was designed to lack any significant open reading frame, which permitted an evaluation of the effect of insert length on replication independent of an effect of an expressed protein. The GU or NCR insert sizes ranged from 168 nucleotides (nt) to 3918 nt. rPIV3s containing GU insertions of up to 3918 nt in length, the largest size tested, were viable and replicated efficiently at permissive temperatures in vitro, but a reduction in plaque size was seen at 39 degrees C and 40 degrees C. The rPIV3 with a 3918-nt GU insertion was restricted in replication in the upper (fivefold) and lower (25-fold) respiratory tracts of hamsters. Although a 1908-nt GU insertion did not significantly modify replication of wild-type PIV3 in vitro or in vivo, its introduction significantly augmented the level of temperature sensitivity (ts) and attenuation (att) specified by three mutations in the L protein of a cold-passaged attenuated PIV3 vaccine virus. rPIV3s bearing a 3126- or 3894-nt NCR insertion exhibited in vitro and in vivo phenotypes like those of the rPIV3s bearing similar-sized GU insertions. These findings indicate that rPIV3s whose genome length has been increased by more than 3000 nt by either a GU or an NCR insertion exhibit an unexpected host-range phenotype, that is, efficient replication in vitro but restricted replication in hamsters, especially in the lower respiratory tract. Furthermore, these effects were greatly enhanced when the rPIV3 backbone contained other ts or att mutations. The implications of these findings for the use of single-stranded, negative-sense RNA viruses as vectors for vaccines are discussed.

Rational design of live-attenuated recombinant vaccine virus for human respiratory syncytial virus by reverse genetics

RSV is a major cause of pediatric respiratory tract disease worldwide, but a vaccine is not yet available. It is now possible to prepare live infectious RSV completely from cDNA. This provides a method for introducing defined mutations into infectious virus, making possible the rational design of a live-attenuated vaccine virus for intranasal administration. This is particularly important for RSV, for which achieving the appropriate balance between attenuation and immunogenicity by conventional methods has proven elusive. We took advantage of the existence of a panel of biologically derived vaccine candidate viruses that were incompletely attenuated but well characterized biologically. The mutations in these viruses were identified by sequence analysis and characterized by insertion into recombinant virus, thereby providing a menu of known attenuating mutations. These included a series of amino acid point mutations, mostly in the L polymerase, and a nucleotide substitution in a transcription gene-start signal, a cis-acting RNA element. The second source of mutations was from experimental mutational analysis of recombinant virus and involves deletion of the NS1, NS2, or SH gene. We have reconstructed a previously tested, biologically derived attenuated virus, cpts248/404, in recombinant form and are now proceeding to introduce additional mutations from the menu to achieve stepwise increases in attenuation. The ability to modify the attenuation phenotype incrementally in a directed manner should result in an appropriate vaccine virus.

Sendai virus gene start signals are not equivalent in reinitiation capacity: moderation at the fusion protein gene

In paramyxovirus transcription, viral RNA polymerase synthesizes each monocistronic mRNA by recognizing the gene start (S) and end (E) signals flanking each gene. These signal sequences are well conserved in the virus family; nevertheless, they do exhibit some variations even within a virus species. In Sendai virus (SeV) Z strain, the E signals are identical for all six genes but there are four (N, P/M/HN, F, and L) different S signals with one or two nucleotide variations. The significance of these variations for in vitro and in vivo replication has been unknown. We addressed this issue by SeV reverse genetics. The luciferase gene was placed between the N and P gene so that recombinant SeVs expressed luciferase under the control of each of the four different S signals. The S signal for the F gene was found to drive a lower level of transcription than that of the other three, which exhibited comparable reinitiation capacities. The polar attenuation of SeV transcription thus appeared to be not linear but biphasic. Then, a mutant SeV whose F gene S signal was replaced with that used for the P, M, and HN genes was created, and its replication capability was examined. The mutant produced a larger amount of F protein and downstream gene-encoded proteins and replicated faster than wild-type SeV in cultured cells and in embryonated eggs. Compared with the wild type, the mutant virus also replicated faster in mice and was more virulent, requiring a dose 20 times lower to kill 50% of mice. On the other hand, the unique F start sequence as well as the other start sequences are perfectly conserved in all SeV isolates sequenced to date, including highly virulent fresh isolates as well as egg-adapted strains, with a virulence several magnitudes lower than that of the fresh isolates. This moderation of transcription at the F gene may therefore be relevant to viral fitness in nature.

Observation of measles virus cell-to-cell spread in astrocytoma cells by using a green fluorescent protein-expressing recombinant virus

A recombinant measles virus (MV) which expresses enhanced green fluorescent protein (EGFP) has been rescued. This virus, MVeGFP, expresses the reporter gene from an additional transcription unit which is located prior to the gene encoding the measles virus nucleocapsid protein. The recombinant virus was used to infect human astrocytoma cells (GCCM). Immunocytochemistry (ICC) together with EGFP autofluorescence showed that EGFP is both an early and very sensitive indicator of cell infection. Cells that were EGFP-positive and ICC-negative were frequently observed. Confocal microscopy was used to indirectly visualize MV infection of GCCM cells and to subsequently follow cell-to-cell spread in real time. These astrocytoma cells have extended processes, which in many cases are intimately associated. The processes appear to have an important role in cell-to-cell spread, and MVeGFP was observed to utilize them in the infection of surrounding cells. Heterogeneity was seen in cell-to-cell spread in what was expected to be a homogeneous monolayer. In tissue culture, physical constraints govern the integrity of the syncytia which are formed upon extensive cell fusion. When around 50 cells were fused, the syncytia rapidly disintegrated and many of the infected cells detached. Residual adherent EGFP-positive cells were seen to either continue to be involved in the infection of surrounding cells or to remain EGFP positive but no longer participate in the transmission of MV infection to neighboring cells.

Components of vectors for gene transfer and expression in mammalian cells

Progress in diverse scientific fields has been realized partly by the continued refinement of mammalian gene expression vectors. A growing understanding of biological processes now allows the design of vector components to meet specific objectives. Thus, gene expression in a tissue-selective or ubiquitous manner may be accomplished by selecting appropriate promoter/enhancer elements; stabilization of labile mRNAs may be effected through removal of 3' untranslated regions or fusion to heterologous stabilizing sequences; protein targeting to selected tissues or different organelles is carried out using specific signal sequences; fusion moieties effect the detection, enhanced yield, surface expression, prolongation of half-life, and facile purification of recombinant proteins; and careful tailoring of the codon content of heterologous genes enhances protein production from poorly translated transcripts. The use of viral as well as nonviral genetic elements in vectors allows the stable replication of episomal elements without the need for chromosomal integration. The development of baculovirus vectors for both transient and stable gene expression in mammalian cells has expanded the utility of such vectors for a broad range of cell types. Internal ribosome entry sites are now widely used in many applications that require coexpression of different genes. Progress in gene targeting techniques is likely to transform gene expression and amplification in mammalian cells into a considerably less labor-intensive operation. Future progress in the elucidation of eukaryotic protein degradation pathways holds promise for developing methods to minimize proteolysis of specific recombinant proteins in mammalian cells and tissues.