Increased readthrough transcription across the simian virus 5 M-F gene junction leads to growth defects and a global inhibition of viral mRNA synthesis

Reverse Genetics Systems for the De Novo Rescue of Diverse Members of Paramyxoviridae

Paramyxoviruses place significant burdens on both human and wildlife health; while some paramyxoviruses are established within human populations, others circulate within diverse animal reservoirs. Concerningly, bat-borne paramyxoviruses have spilled over into humans with increasing frequency in recent years, resulting in severe disease. The risk of future zoonotic outbreaks, as well as the persistence of paramyxoviruses that currently circulate within humans, highlights the need for efficient tools through which to interrogate paramyxovirus biology. Reverse genetics systems provide scientists with the ability to rescue paramyxoviruses de novo, offering versatile tools for implementation in both research and public health settings. Reverse genetics systems have greatly improved over the past 30 years, with several key innovations optimizing the success of paramyxovirus rescue. Here, we describe the significance of such advances and provide a generally applicable guide for the development and use of reverse genetics systems for the rescue of diverse members of Paramyxoviridae.

Single-dose vaccination of a recombinant parainfluenza virus 5 expressing NP from H5N1 virus provides broad immunity against influenza A viruses

Influenza viruses often evade host immunity via antigenic drift and shift despite previous influenza virus infection and/or vaccination. Vaccines that match circulating virus strains are needed for optimal protection. Development of a universal influenza virus vaccine providing broadly cross-protective immunity will be of great importance. The nucleoprotein (NP) of influenza A virus is highly conserved among all strains of influenza A viruses and has been explored as an antigen for developing a universal influenza virus vaccine. In this work, we generated a recombinant parainfluenza virus 5 (PIV5) containing NP from H5N1 (A/Vietnam/1203/2004), a highly pathogenic avian influenza (HPAI) virus, between HN and L (PIV5-NP-HN/L) and tested its efficacy. PIV5-NP-HN/L induced humoral and T cell responses in mice. A single inoculation of PIV5-NP-HN/L provided complete protection against lethal heterosubtypic H1N1 challenge and 50% protection against lethal H5N1 HPAI virus challenge. To improve efficacy, NP was inserted into different locations within the PIV5 genome. Recombinant PIV5 containing NP between F and SH (PIV5-NP-F/SH) or between SH and HN (PIV5-NP-SH/HN) provided better protection against H5N1 HPAI virus challenge than did PIV5-NP-HN/L. These results suggest that PIV5 expressing NP from H5N1 has the potential to be utilized as a universal influenza virus vaccine.

Elements in the canine distemper virus M 3' UTR contribute to control of replication efficiency and virulence

Canine distemper virus (CDV) is a negative-sense, single-stranded RNA virus within the genus Morbillivirus and the family Paramyxoviridae. The Morbillivirus genome is composed of six transcriptional units that are separated by untranslated regions (UTRs), which are relatively uniform in length, with the exception of the UTR between the matrix (M) and fusion (F) genes. This UTR is at least three times longer and in the case of CDV also highly variable. Exchange of the M-F region between different CDV strains did not affect virulence or disease phenotype, demonstrating that this region is functionally interchangeable. Viruses carrying the deletions in the M 3' UTR replicated more efficiently, which correlated with a reduction of virulence, suggesting that overall length as well as specific sequence motifs distributed throughout the region contribute to virulence.

IFNgamma-producing, virus-specific CD8+ effector cells acquire the ability to produce IL-10 as a result of entry into the infected lung environment

It has become clear that T cells with the potential to negatively regulate the immune response are normal constituents of the immune system. These cells often mediate their effects through the production of immunosuppressive factors. At present our understanding of how these cells are generated is limited. Here we report the presence of a population of IL-10-producing, virus-specific CD8+ T cells in the lungs of mice following acute respiratory infection. These cells were only found at minimal levels in the spleen and draining lymph node; instead they were restricted primarily to the infected lung tissue. A major finding from this study is demonstration that the ability to produce IL-10 can be acquired by IFNgamma-producing effector cells following entry into the infected lung. These studies suggest IL-10 production is the result of further differentiation of an antigen-specific CD8+ T cell that is governed by signals present in infected lung tissue.

Region between the canine distemper virus M and F genes modulates virulence by controlling fusion protein expression

Morbilliviruses, including measles and canine distemper virus (CDV), are nonsegmented, negative-stranded RNA viruses that cause severe diseases in humans and animals. The transcriptional units in their genomes are separated by untranslated regions (UTRs), which contain essential transcription and translation signals. Due to its increased length, the region between the matrix (M) protein and fusion (F) protein open reading frames is of particular interest. In measles virus, the entire F 5' region is untranslated, while several start codons are found in most other morbilliviruses, resulting in a long F protein signal peptide (Fsp). To characterize the role of this region in morbillivirus pathogenesis, we constructed recombinant CDVs, in which either the M-F UTR was replaced with that between the nucleocapsid (N) and phosphoprotein (P) genes, or 106 Fsp residues were deleted. The Fsp deletion alone had no effect in vitro and in vivo. In contrast, substitution of the UTR was associated with a slight increase in F gene and protein expression. Animals infected with this virus either recovered completely or experienced prolonged disease and death due to neuroinvasion. The combination of both changes resulted in a virus with strongly increased F gene and protein expression and complete attenuation. Taken together, our results provide evidence that the region between the morbillivirus M and F genes modulates virulence through transcriptional control of the F gene expression.

Distinct pathways for signaling maturation in macrophages and dendritic cells after infection with paramyxovirus simian virus 5

Professional antigen-presenting cells are critical components of both the innate and adaptive immune responses. Although dendritic cells (DCs) are generally thought to be the primary activators of naive T cells, macrophages have also been shown to fulfill this role. As with DCs, the capacity to induce optimal activation of T cells requires that macrophages undergo a process that results in the increased expression of costimulatory molecules, such as CD40, CD80, and CD86, and the production of cytokines. In this study we analyzed the effect of infection of macrophages generated from BALB/c mice with the paramyxovirus simian virus 5 (SV5). Here we have shown that bone marrow-derived macrophages (BMMs) are not productively infected at any multiplicity of infection tested. Analysis of activation markers revealed that SV5-infected BMMs robustly upregulated CD40 and modestly upregulated CD86, but did not upregulate the expression of CD80. Further, SV5-infected BMMs secreted low levels of interferon-beta and interleukin (IL)-12p40, but high levels of tumor necrosis factor-alpha and IL-6. Intriguingly, upregulation of these molecules on BMMs, unlike our previous results using bone marrow-derived dendritic cells, was not dependent on live virus. These findings provide evidence that different professional antigen-presenting cells can detect and respond to virus via distinct mechanisms.

TLR-4 and -6 agonists reverse apoptosis and promote maturation of simian virus 5-infected human dendritic cells through NFkB-dependent pathways

Infection of primary cultures of human immature monocyte-derived dendritic cells (moDC) with the paramyxovirus Simian Virus 5 (SV5) results in extensive cytopathic effect (CPE) and induction of apoptosis, but DC maturation pathways are not activated. In this study, we investigated the relationship between SV5-induced apoptosis and the lack of DC maturation. Reducing CPE and apoptosis in SV5-infected immature DC by the addition of a pancaspase inhibitor resulted in only low level expression of maturation markers CD40, CD80 and CD86, suggesting that SV5 infection either actively blocked maturation pathways or failed to provide sufficient signals to activate maturation. To distinguish between these hypotheses, SV5-infected immature DC were challenged with agonists that stimulate toll-like receptors (TLRs). Treatment with the TLR-4 agonist LPS or TLR-6 agonist FSL1 enhanced cell surface expression of CD40, CD80 and CD86 on SV5-infected cells to levels approaching that of mock-infected TLR-treated moDC, but treatment with agonists for TLR-2, -3, -5 or -8 had little effect. Addition of TLR-4 or -6 agonists to SV5-infected DC also dramatically reduced CPE and apoptosis, but the levels of viral protein and virus yield were not affected. Similarly, SV5-infected immature moDC were matured by treatment with IL-1beta, and these mature infected cells also showed reduced CPE and apoptosis. In the presence of NFkB inhibitors, TLR-4 and -6 agonists did not promote maturation or reduce apoptosis of SV5-infected DC, indicating that maturation and cell survival were both dependent on signaling through NFkB-dependent pathways. Our results suggest a model whereby SV5 replication induces apoptosis in immature DC but fails to provide strong maturation signals, while activation of NFkB-dependent pathways by exogenous ligands can lead to moDC maturation and override SV5-induced cell death.

Antiviral activity and RNA polymerase degradation following Hsp90 inhibition in a range of negative strand viruses

We have analyzed the effectiveness of Hsp90 inhibitors in blocking the replication of negative-strand RNA viruses. In cells infected with the prototype negative strand virus vesicular stomatitis virus (VSV), inhibiting Hsp90 activity reduced viral replication in cells infected at both high and low multiplicities of infection. This inhibition was observed using two Hsp90 inhibitors geldanamycin and radicicol. Silencing of Hsp90 expression using siRNA also reduced viral replication. Hsp90 inhibition changed the half-life of newly synthesized L protein (the large subunit of the VSV polymerase) from >1 h to less than 20 min without affecting the stability of other VSV proteins. Both the inhibition of viral replication and the destabilization of the viral L protein were seen when either geldanamycin or radicicol was added to cells infected with paramyxoviruses SV5, HPIV-2, HPIV-3, or SV41, or to cells infected with the La Crosse bunyavirus. Based on these results, we propose that Hsp90 is a host factor that is important for the replication of many negative strand viruses.

Ligation of CD80 is critical for high-level CD25 expression on CD8+ T lymphocytes

CD80 and CD86 have been shown to play a critical role in the optimal activation of T cells. Although these two molecules bind the same ligand, CD28, the question of whether CD80 and CD86 provide unique signals or serve redundant roles remains controversial. Previous studies have suggested that CD80 binding to CD28 may be superior to CD86 for the activation of naive CD8+ T cells. This study provides a potential mechanism to explain these observations. Our study demonstrates a previously unappreciated role for CD80, its superiority over CD86 in promoting CD25 expression, increasing both the number of cells that express CD25 and the level expressed on a per cell basis. These findings provide new insights into the role of CD80 vs CD86 and have important implications for the design of vaccines and immunotherapeutics aimed at the generation of a robust CD8+ T cell response in vivo.

A simian virus 5 (SV5) P/V mutant is less cytopathic than wild-type SV5 in human dendritic cells and is a more effective activator of dendritic cell maturation and function

Human epithelial cells infected with the parainfluenza virus simian virus 5 (SV5) show minimal activation of host cell interferon (IFN), cytokine, and cell death pathways. In contrast, a recombinant SV5 P/V gene mutant (rSV5-P/V-CPI-) overexpresses viral gene products and is a potent inducer of IFN, proinflammatory cytokines, and apoptosis in these cells. In this study, we have compared the outcomes of wild-type (WT) SV5 and rSV5-P/V-CPI- infections of primary human dendritic cells (DC), important antigen-presenting cells for initiating adaptive immune responses. We have tested the hypothesis that a P/V mutant which activates host antiviral responses will be a more potent inducer of DC maturation and function than WT rSV5, which suppresses host cell responses. Infection of peripheral blood mononuclear cell-derived immature DC with WT rSV5 resulted in high levels of viral protein and progeny virus but very little increase in cell surface costimulatory molecules or secretion of IFN and proinflammatory cytokines. In contrast, immature DC infected with the rSV5-P/V-CPI- mutant produced only low levels of viral protein and progeny virus, but these infected cells were induced to secrete IFN-alpha and other cytokines and showed elevated levels of maturation markers. Unexpectedly, DC infected with WT rSV5 showed extensive cytopathic effects and increased levels of active caspase-3, while infection of DC with the P/V mutant was largely noncytopathic. In mixed-culture assays, WT rSV5-infected DC were impaired in the ability to stimulate proliferation of autologous CD4+ T cells, whereas DC infected with the P/V mutant were very effective at activating T-cell proliferation. The addition of a pancaspase inhibitor to DC infected with WT rSV5 reduced cytopathic effects and resulted in higher surface expression levels of maturation markers. Our finding that the SV5 P/V mutant has both a reduced cytopathic effect in human DC compared to WT SV5 and an enhanced ability to induce DC function has implications for the rational design of novel recombinant paramyxovirus vectors based on engineered mutations in the viral P/V gene.

Variants of the paramyxovirus Simian virus 5 with accelerated or delayed viral gene expression activate proinflammatory cytokine synthesis

Our previous results have shown that the parainfluenza virus SV5 is a poor inducer of proinflammatory cytokines interleukin-8 (IL-8) and macrophage chemoattractant protein 1 (MCP-1). By contrast, an engineered P/V mutant rSV5-P/V-CPI- and a naturally-occurring variant WF-PIV (Wake Forest-Parainfluenza Virus) are both potent activators of IL-8 and MCP-1. In the present study, we addressed the question of why rSV5-WT is such a poor inducer of host cytokine responses relative to the two SV5 variants, and we used the CC chemokine RANTES as a measure of host responses. Time course experiments showed high-level secretion of IL-6 and RANTES following infections of human A549 lung epithelial cells with the P/V-CPI- mutant and WF-PIV. By contrast, SV5-WT induced very low cytokine responses, with the notable exception of moderate induction of RANTES. The mechanism of RANTES induction by the two SV5 variants shared common properties, since RANTES secretion from infected cells had similar kinetics, depended on virus replication, correlated with increased RANTES mRNA levels and promoter activation, and was reduced by inhibitors of the p38 MAPK, ERK, and PI3K pathways. Despite the similar mechanisms of RANTES induction, the two SV5 variants differed dramatically in their growth and gene expression kinetics. By comparison to the P/V mutant rSV5-P/V-CPI- which has accelerated viral gene expression, WF-PIV infection showed a prolonged delay in viral replication, and infected cells did not show high-level viral RNA and protein expression until approximately 12-24 hpi. Sequence analysis revealed that the N, P, V, and M genes from WF-PIV differed by 3, 8, 5, and 10 amino acids compared to rSV5-WT, respectively. Chimeric viruses harboring the WF-PIV P/V or M genes in the context of the other rSV5 genes had growth properties similar to rSV5-WT but had a RANTES-inducing phenotype similar to that of the bone fide WF-PIV virus. Our data indicate a role for both the P/V and the M gene products as determinants of RANTES induction in response to SV5 infection.

Interferon-induced alterations in the pattern of parainfluenza virus 5 transcription and protein synthesis and the induction of virus inclusion bodies

Although parainfluenza virus 5 (simian virus 5 [SV5]) circumvents the interferon (IFN) response by blocking IFN signaling and by reducing the amount of IFN released by infected cells, its ability to circumvent the IFN response is not absolute. The effects of IFN on SV5 infection were examined in Vero cells, which do not produce but can respond to IFN, using a strain of SV5 (CPI-) which does not block IFN signaling. Thus, by infecting Vero cells with CPI- and subsequently treating the cells with exogenous IFN, it was possible to observe the effects that IFN had on SV5 infection in the absence of virus countermeasures. IFN rapidly (within 6 h) induced alterations in the relative levels of virus mRNA and protein synthesis and caused a redistribution of virus proteins within infected cells that led to the enhanced formation of virus cytoplasmic inclusion bodies. IFN induced a steeper gradient of mRNA transcription from the 3' to the 5' end of the genome and the production of virus mRNAs with longer poly(A) tails, suggesting that the processivity of the virus polymerase was altered in cells in an IFN-induced antiviral state. Additional evidence is presented which suggests that these findings also apply to the replication of strains of SV5, parainfluenza virus type 2, and mumps virus that block IFN signaling when they infect cells that are already in an IFN-induced antiviral state.

Replication strategies of rabies virus

Rabies virus (RV) is a prototype neurotropic virus that causes fatal disease in human and animals. RV infects hosts at the periphery, enters motoneurons or sensory nerves and moves to the central nervous system (CNS) via retrograde axonal transport. At later stages, there is also centrifugal spread to major exit portals, such as the salivary glands. Transmission to other hosts is facilitated by behavioral changes related to the CNS infection. Successful accomplishment of the RV infectious cycle depends on multiple functions of the virus, and of individual virus proteins, all together defining the typical pathogenicity and virulence, i.e. the biological fitness of this virus. In particular, it appears important for RV to sneak into the host without causing pronounced host responses and to preserve, at least for some time, the integrity of infected cells and of the neuronal network. The availability of reverse genetics systems that allow generation of engineered recombinant RV has provided tools for a more detailed analysis of viral functions relevant to the typical RV pathogenesis. Novel developments such as tracking of live fluorescent RV are further increasing the opportunities to decipher RV pathogenicity factors. In this review, we describe different aspects of the molecular biology of RV that are relevant to pathogenesis, with a particular emphasis on the accurate control of RV transcription, gene expression, and replication. In addition, the role of individual virus proteins in maintaining host cell integrity and supporting retrograde transport is discussed. The potential of recombinant RVs with single or multiple pathogenicity factors eliminated is being discussed in terms of vaccine and virus vector development.

Abortive versus productive viral infection of dendritic cells with a paramyxovirus results in differential upregulation of select costimulatory molecules

Dendritic cells are the most potent antigen-presenting cell for priming naive T cells. Optimal activation of T cells requires that dendritic cells undergo a process of maturation resulting in the increased expression of costimulatory molecules, such as CD40, CD86, and CD80, and the production of cytokines. In this study we analyzed the effect of infection of dendritic cells obtained from two strains of mice, BALB/c and C57BL/6, with the paramyxovirus simian virus 5 (SV5). Our results show that C57BL/6 bone marrow-derived dendritic cells (BMDC) are much more permissive to infection with SV5 at a multiplicity of infection (MOI) of 10 PFU/cell compared to BALB/c BMDC, as determined by the production of viral proteins and progeny. However, infection of BALB/c BMDC with a higher MOI of 50 PFU/cell resulted in a productive infection with the production of significant amounts of viral proteins and progeny. Regardless of the permissivity to infection, both BALB/c and C57BL/6 BMDC efficiently upregulated CD40 and CD86. However, CD80 upregulation correlated with the level of expression of viral proteins and the production of viral progeny. While secreted alpha/beta interferon was required for increased expression of all three molecules, optimal CD80 expression was dependent on an additional signal provided by a productive viral infection. These findings provide evidence that the signals controlling the expression of costimulatory molecules following viral infection are distinct. Further, they suggest that the amount of virus encountered and/or the permissivity of a dendritic cell to infection can alter the resulting maturation phenotype and functional capacity of the infected dendritic cell.

Growth sensitivity of a recombinant simian virus 5 P/V mutant to type I interferon differs between tumor cell lines and normal primary cells

A paramyxovirus SV5 mutant (rSV5-P/V-CPI-) that encodes 6 naturally-occurring P/V gene substitutions is a potent inducer of type I interferon (IFN) and is restricted for low moi growth, two phenotypes not seen with WT SV5. In this study, we have compared the IFN sensitivity of WT SV5 and the rSV5-P/V-CPI- mutant in tumor cell lines and in cultures of normal primary cells. We have tested the hypothesis that differences in IFN induction elicited by WT rSV5 and rSV5-P/V-CPI- are responsible for differences in low moi growth and spread. In contrast to WT SV5, low moi infection of A549 lung carcinoma cells with rSV5-P/V-CPI- resulted in a plateau of virus production by 24-48 h pi when secreted IFN levels were between approximately 100 and 1000 U/ml. Gene microarray and RT-PCR analyses identified IFN genes and IFN-stimulated genes whose expression were increased by infection of A549 cells with WT and P/V mutant viruses. Restricted low moi growth and spread of rSV5-P/V-CPI- in A549 cells was relieved in the presence of neutralizing antibodies to IFN-beta but not TNF-alpha. When A549 or MDA-MB-435 breast tumor cells were pretreated with IFN, both WT and P/V mutant viruses showed delayed spread and approximately 10-fold reduction in virus yield, but infections were not eliminated. Using normal primary human epithelial cells that have undergone limited passage in culture, WT rSV5 and rSV5-P/V-CPI- displayed high moi growth properties that were similar to that seen in A549 cells. However, IFN pretreatment of these primary cells as well as normal human lung cells eliminated low moi spread of both mutant and WT rSV5 infections. Together, these data demonstrate that SV5 growth in normal primary human cells is highly sensitive to IFN compared to growth in some tumor cell lines, regardless of whether the P/V gene is WT or mutant. These results suggest a model in which spread of WT SV5 in normal human cells is dependent on the ability of the virus to prevent IFN synthesis. The implications of these results for the use of recombinant paramyxoviruses as vectors are discussed.

Variations in intergenic region sequences of Human respiratory syncytial virus clinical isolates: analysis of effects on transcriptional regulation

Sequences at the beginnings and ends of Human respiratory syncytial virus (HRSV) genes are necessary for efficient initiation and termination of transcription. The gene start sequences are well conserved and contain signals required for initiation, while the semi-conserved sequences at the gene ends direct transcriptional termination with varying efficiencies. The intergenic regions, which lie between the gene ends and the downstream gene start sequences, are not conserved in length or sequence, and certain positions have been reported to play a role in transcriptional regulation. We have previously shown that the gene end sequences in HRSV subgroup A clinical isolates are variable and that variations found at certain gene ends decreased transcriptional termination and downstream mRNA expression. Here, we have extended this work to examine variation in the intergenic regions between the genes of clinical isolates. We determined the sequences of the eight intergenic regions and the M2/L overlap from clinical isolates from the US and UK and found that all of these regions contained variations from the prototype A2 strain. The amount of variation observed was disparate among the different intergenic regions and did not correlate with length. The effects of selected variant sequences on transcription were examined in the context of subgenomic replicons. While some changes in the intergenic regions had minor effects, certain sequence variations significantly altered transcription termination or initiation. A single nucleotide deletion in the M/SH intergenic region decreased initiation at the SH gene start seven-fold, while changes in the F/M2 intergenic region were found that in some cases increased and in others decreased termination at the F gene end. The P/M intergenic region was the most variable, but none of the changes examined affected either termination at the P gene end or initiation of the downstream M gene start. These results show that in HRSV clinical isolates the intergenic region sequences are variable and that changes in these regions have the potential to affect transcriptional control at the gene junctions.

Modulation of CD8+ T cell avidity by increasing the turnover of viral antigen during infection

The increased potency of high avidity CD8+ T cells for the clearance of viral infections has been well documented. We have previously reported the novel finding that intranasal infection with the paramyxovirus SV5 induces a CD8+ T cell response to the SV5 P protein that is almost exclusively of high avidity. Based on our results that the level of peptide presentation is a critical factor in the selective expansion of high versus low avidity cells in vitro, we hypothesized that the avidity of the anti-viral response generated in vivo could be altered by increasing the turnover of the P protein during viral infection through linkage to ubiquitin (UbP). Infection with a virus expressing UbP (VV-UbP) elicited a significant increase in low avidity cells in both BALB/c and C3H mice compared to the almost exclusively high avidity response elicited by VV-P. Our results are the first demonstration of the control of avidity during the antiviral response through an engineered change to a viral antigen. The implications of our findings for vaccine development are discussed.

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.

Variations in transcription termination signals of human respiratory syncytial virus clinical isolates affect gene expression

Human respiratory syncytial virus (HRSV) has a single-stranded, negative-sense RNA genome with 10 genes encoding 11 proteins. Sequences at the beginning of the HRSV genes are highly conserved; however, the gene end sequences vary around a semiconserved consensus sequence, and the nontranscribed intergenic regions vary in both length and sequence. The regions at the junctions between HRSV genes (the gene end sequence of an upstream gene, intergenic region, and the gene start sequence of a downstream gene) contain elements required for efficient termination of the upstream gene and transcription of the downstream gene. Previous studies have examined variation in the HRSV coding sequences, but none have systematically analyzed the noncoding transcriptional control regions for variability. We determined the gene start and gene end sequences of each of the 10 HRSV genes from 14 clinical isolates for variations from the sequence of the prototype A2 strain. No changes were found in any of the gene start sequences. Eight of the 10 gene end sequences, however, contained variations. Several of these, a U(4)-tract instead of a U(6)- or U(5)-tract at the M and SH gene ends, respectively, (U(4)A) and an A-to-G change at position four in the G gene end (A4G), were predicted to affect termination and were examined for their effects on transcription. The changes were found to inhibit transcriptional termination, resulting in increased polycistronic readthrough and correspondingly reduced initiation of the downstream monocistronic mRNA. Viruses with the A4G variant G gene end sequence produced less F protein than those with A2-like G gene end sequences. Examination of additional G gene end sequences available in GenBank revealed that the observed A4G variation was restricted to one phylogenetic lineage of HRSV. All viruses examined within this lineage possessed this variant G gene end sequence. The data presented show that the gene end sequences of naturally occurring HRSV clinical isolates vary from those of the prototypic A2 strain and that certain of these changes inhibit efficient transcriptional termination and downstream gene expression.

Simian virus 5 is a poor inducer of chemokine secretion from human lung epithelial cells: identification of viral mutants that activate interleukin-8 secretion by distinct mechanisms

We have compared chemokine secretion from human lung A549 cells infected with simian virus 5 (SV5) with other members of the Rubulavirus genus of paramyxoviruses. High levels of the chemokines interleukin-8 (IL-8) and macrophage chemoattractant protein-1 (MCP-1) were secreted from A549 cells infected with Human parainfluenza virus type 2 (HPIV-2) but not from cells infected with wild-type (WT) SV5. The lack of IL-8 secretion from SV5-infected cells was not due to a global block in all signal transduction pathways leading to IL-8 secretion, since SV5-infected A549 cells secreted IL-8 after stimulation with exogenously added tumor necrosis factor alpha or by coinfection with HPIV-2. A previously described, recombinant SV5 containing substitutions in the shared region of the P/V gene (rSV5-P/V-CPI-) induced IL-8 secretion by a mechanism that was dependent on viral gene expression. By contrast, an SV5 variant isolated from persistently infected cells (Wake Forest strain of Canine parainfluenza virus) induced IL-8 secretion by a mechanism that was largely not affected by inhibitors of viral gene expression. Together, these data demonstrate that SV5 is unusual compared to other closely related paramyxoviruses, since SV5 is a very poor inducer of the cytokines IL-8 and MCP-1. The isolation of two recombinant SV5 mutants that are defective in preventing chemokine induction will allow an identification of mechanisms utilized by WT SV5 to avoid activation of host cell innate immune responses to infection.

Mutations affecting transcriptional termination in the p gene end of subacute sclerosing panencephalitis viruses

Numerous mutations are found in subacute sclerosing panencephalitis (SSPE) viruses, and the M gene is the gene most commonly affected. In some SSPE viruses, such as the MF, Osaka-1, Osaka-2, and Yamagata-1 strains, translation of the M protein is complicated by a transcriptional defect that leads to an almost exclusive synthesis of dicistronic P-M mRNA. To understand the molecular mechanisms of this defect, we sequenced the P gene at the P-M gene junction for several virus strains and probed the involvement of several mutations in the readthrough region via their expression in measles virus minigenomes containing different sequences of the P-M gene junction and flanking reporter genes. The deletion of a single U residue in the U tract of the Osaka-1 strain (3'-UAAUAUUUUU-5') compared with the consensus sequence resulted in a marked reduction of the expression of the downstream reporter gene. In addition, the expression of the downstream gene was markedly decreased by (i) the substitution of a C residue in the U tract of the P gene end of the OSA-2/Fr/B strain of the Osaka-2 virus (3'-UGAUAUUCUU-5' compared with the sequence 3'-UGAUAUUUUU-5' from a sibling virus of the same strain, OSA-2/Fr/V), and (ii) the substitution of a G in the sequence of the P gene end of the Yamagata-1 strain at a variable site immediately upstream from the six-U tract (3'-UGAUGUUUUUU-5' instead of 3'-UGAUUUUUUUU-5'). Mutations at the P gene end can account for the readthrough transcription variation at the P-M gene junction, which directly affects M protein expression.

Naturally occurring substitutions in the P/V gene convert the noncytopathic paramyxovirus simian virus 5 into a virus that induces alpha/beta interferon synthesis and cell death

The V protein of the paramyxovirus simian virus 5 (SV5) is responsible for targeted degradation of STAT1 and the block in alpha/beta interferon (IFN-alpha/beta) signaling that occurs after SV5 infection of human cells. We have analyzed the growth properties of a recombinant SV5 that was engineered to be defective in targeting STAT1 degradation. A recombinant SV5 (rSV5-P/V-CPI-) was engineered to contain six naturally occurring P/V protein mutations, three of which have been shown in previous transfection experiments to disrupt the V-mediated block in IFN-alpha/beta signaling. In contrast to wild-type (WT) SV5, human cells infected with rSV5-P/V-CPI- had STAT1 levels similar to those in mock-infected cells. Cells infected with rSV5-P/V-CPI- were found to express higher-than-WT levels of viral proteins and mRNA, suggesting that the P/V mutations had disrupted the regulation of viral RNA synthesis. Despite the inability to target STAT1 for degradation, single-step growth assays showed that the rSV5-P/V-CPI- mutant virus grew better than WT SV5 in all cell lines tested. Unexpectedly, cells infected with rSV5-P/V-CPI- but not WT SV5 showed an activation of a reporter gene that was under control of the IFN-beta promoter. The secretion of IFN from cells infected with rSV5-P/V-CPI- but not WT SV5 was confirmed by a bioassay for IFN. The rSV5-P/V-CPI- mutant grew to higher titers than did WT rSV5 at early times in multistep growth assays. However, rSV5-P/V-CPI- growth quickly reached a final plateau while WT rSV5 continued to grow and produced a final titer higher than that of rSV5-P/V-CPI- by late times postinfection. In contrast to WT rSV5, infection of a variety of cell lines with rSV5-P/V-CPI- induced cell death pathways with characteristics of apoptosis. Our results confirm a role for the SV5 V protein in blocking IFN signaling but also suggest new roles for the P/V gene products in controlling viral gene expression, the induction of IFN-alpha/beta synthesis, and virus-induced apoptosis.

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.

High avidity cytotoxic T lymphocytes to a foreign antigen are efficiently activated following immunization with a recombinant paramyxovirus, simian virus 5

Our previous work has shown that high avidity cytotoxic T lymphocytes (CTL) are optimal for virus clearance in vivo and thus it is necessary that an effective vaccine is capable of eliciting high avidity CTL. To determine if vaccination with the paramyxovirus simian virus 5 (SV5) elicits a high avidity response to a model foreign antigen, a recombinant virus was engineered to express chicken ovalbumin (rSV5-Ova). To compare the CTL response elicited with rSV5-Ova and a recombinant vaccinia virus expressing ovalbumin (rVV-Ova), mice were vaccinated intranasally with various doses of each vector and the Ova-specific CTL response was determined by ELISPOT analysis. Here, it has been shown that rSV5 can be equally as effective as rVV in eliciting antigen-specific CTL, in terms of both the total number of CTL and the number of high avidity cells. This has implications for both the design of vaccine vectors and the route utilized for vaccine administration for the elicitation of high avidity CTL responses. The advantages and future potential use of rSV5 vaccine vectors are discussed.

Controlled cell killing by a recombinant nonsegmented negative-strand RNA virus

In most tissue culture cell lines tested, infection with the paramyxovirus simian virus 5 (SV5) results in very little cell death. To determine if SV5 could be used as a vector for controlled killing of tumor cells, a recombinant SV5 (rSV5-TK) was constructed to encode the herpes simplex virus thymidine kinase (TK) gene. MDBK cells infected with rSV5-TK showed a time-dependent loss of viability when infected cells were cultured in the presence of the prodrug acyclovir (ACV) or ganciclovir (GCV) while no significant toxicity was observed in the absence of prodrug. Cells infected with a control rSV5 expressing GFP and cultured with prodrug showed only a slight reduction in growth rate and little cell death. Time-lapse video microscopy of rSV5-TK-infected MDBK cells that were cultured in the presence of ACV showed an accumulation of cells with morphological effects characteristic of apoptotic cell death. An MDBK cell line persistently infected with rSV5-TK retained long-term expression of TK and sensitivity to prodrug-mediated cell killing that were comparable to those found in an acute infection. Titration experiments showed that the rSV5-TK plus GCV combination resulted in cell death for all mouse and human cell lines tested, although the kinetics and efficiency of cell death varied between cell types. Our results demonstrating controlled cell killing by a recombinant paramyxovirus support the use of negative-strand RNA viruses as therapeutic vectors for targeted killing of cancer cells.

A novel CD8-independent high-avidity cytotoxic T-lymphocyte response directed against an epitope in the phosphoprotein of the paramyxovirus simian virus 5

Adoptive transfer studies have shown that cytotoxic T lymphocytes (CTL) of high avidity, capable of recognizing low levels of peptide-MHC I molecules, are more efficient at reducing viral titers than are low-avidity CTL, thus establishing CTL avidity as a critical parameter for the ability of a CTL to clear virus in vivo. It has been well documented that CTL of high avidity are relatively CD8 independent, whereas low-avidity CTL require CD8 engagement in order to become activated. In this study we have analyzed the antiviral CTL response elicited following infection with the paramyxovirus simian virus 5 (SV5). We have identified the immunodominant and subdominant CTL responses and subsequently assessed the avidity of these responses by their CD8 dependence. This is the first study in which the relationship between immunodominance and CTL avidity has been investigated. The immunodominant response was directed against an epitope present in the viral M protein, and subdominant responses were directed against epitopes present in the P, F, and HN proteins. Similarly to other CTL responses we have analyzed, the immunodominant response and the subdominant F and HN responses were comprised of both high- and low-avidity CTL. However, the subdominant response directed against the epitope present in the P protein is novel, as it is exclusively high avidity. This high-avidity response is independent of both the route of infection and expression by recombinant SV5. A further understanding of the inherent properties of P that elicit only high-avidity CTL may allow for the design of more efficacious vaccine vectors that preferentially elicit high-avidity CTL in vivo.