Complete sequence of the RNA genome of pneumonia virus of mice (PVM)

Current Animal Models for Understanding the Pathology Caused by the Respiratory Syncytial Virus

The human respiratory syncytial virus (hRSV) is the main etiologic agent of severe lower respiratory tract infections that affect young children throughout the world, associated with significant morbidity and mortality, becoming a serious public health problem globally. Up to date, no licensed vaccines are available to prevent severe hRSV-induced disease, and the generation of safe-effective vaccines has been a challenging task, requiring constant biomedical research aimed to overcome this ailment. Among the difficulties presented by the study of this pathogen, it arises the fact that there is no single animal model that resembles all aspects of the human pathology, which is due to the specificity that this pathogen has for the human host. Thus, for the study of hRSV, different animal models might be employed, depending on the goal of the study. Of all the existing models, the murine model has been the most frequent model of choice for biomedical studies worldwide and has been of great importance at contributing to the development and understanding of vaccines and therapies against hRSV. The most notable use of the murine model is that it is very useful as a first approach in the development of vaccines or therapies such as monoclonal antibodies, suggesting in this way the direction that research could have in other preclinical models that have higher maintenance costs and more complex requirements in its management. However, several additional different models for studying hRSV, such as other rodents, mustelids, ruminants, and non-human primates, have been explored, offering advantages over the murine model. In this review, we discuss the various applications of animal models to the study of hRSV-induced disease and the advantages and disadvantages of each model, highlighting the potential of each model to elucidate different features of the pathology caused by the hRSV infection.

Murine Pneumonia Virus Expressing the Fusion Glycoprotein of Human Respiratory Syncytial Virus from an Added Gene Is Highly Attenuated and Immunogenic in Rhesus Macaques

Human respiratory syncytial virus (RSV) continues to be the leading viral cause of severe acute lower respiratory tract disease in infants and children worldwide. A licensed vaccine or antiviral drug suitable for routine use remains unavailable. Like RSV, Murine pneumonia virus (MPV) is a member of the genus Orthopneumovirus, family Pneumoviridae Humans are not normally exposed to MPV, and MPV is not cross-protective with RSV. We evaluated MPV as an RSV vaccine vector expressing the RSV fusion (F) glycoprotein. The RSV F open reading frame (ORF) was codon optimized, and the encoded RSV F protein was made identical to an early passage of RSV strain A2. The RSV F ORF was placed under the control of MPV transcription signals and inserted at the first (rMPV-F1), third (rMPV-F3), or fourth (rMPV-F4) gene position of a version of the MPV genome that contained a codon-pair-optimized polymerase (L) gene. The recovered viruses replicated in vitro as efficiently as the empty vector, with stable expression of RSV F protein. Replication and immunogenicity of rMPV-F1 and rMPV-F3 were evaluated in rhesus macaques following intranasal and intratracheal administration. Both viruses replicated at low levels in the upper and lower respiratory tracts, maintained stable RSV F expression, and induced RSV-neutralizing serum antibodies at high levels similar to those induced by wild-type RSV replicating to a 5- to 25-fold-higher titer. In conclusion, this study demonstrated that rMPV provides a highly attenuated yet immunogenic vector for the expression of RSV F protein, with potential application in RSV-naive and RSV-experienced populations.IMPORTANCE Human respiratory syncytial virus (RSV) is an important human pathogen that lacks a licensed vaccine or antiviral drug suitable for routine use. We describe here the evaluation of recombinant murine pneumonia virus (rMPV) as a live-attenuated vector that expresses the RSV F protein, the major RSV neutralization antigen, as an experimental RSV vaccine. The rMPV-RSV-F vectors expressing RSV F from the first, third, or fourth gene position were genetically stable and were not restricted for replication in vitro In contrast, the vectors exhibited highly attenuated replication in the respiratory tract of rhesus macaques, maintained stable RSV F expression, and induced RSV-neutralizing serum antibodies at high titers similar to those conferred by wild-type RSV. Given the lack of preexisting immunity to MPV in humans and the lack of cross-neutralization and cross-protection between MPV and RSV, an rMPV-vectored RSV vaccine should be immunogenic in both RSV-naive children and RSV-experienced adults.

The nonstructural proteins of Pneumoviruses are remarkably distinct in substrate diversity and specificity

Background

Interferon (IFN) inhibits viruses by inducing several hundred cellular genes, aptly named ‘interferon (IFN)-stimulated genes’ (ISGs). The only two RNA viruses of the Pneumovirus genus of the Paramyxoviridae family, namely Respiratory Syncytial Virus (RSV) and Pneumonia Virus of Mice (PVM), each encode two nonstructural (NS) proteins that share no sequence similarity but yet suppress IFN. Since suppression of IFN underlies the ability of these viruses to replicate in the host cells, the mechanism of such suppression has become an important area of research. This Short Report is an important extension of our previous efforts in defining this mechanism.

Results

We show that, like their PVM counterparts, the RSV NS proteins also target multiple members of the ISG family. While significantly extending the substrate repertoire of the RSV NS proteins, these results, unexpectedly, also reveal that the target preferences of the NS proteins of the two viruses are entirely different. This is surprising since the two Pneumoviruses are phylogenetically close with similar genome organization and gene function, and the NS proteins of both also serve as suppressors of host IFN response.

Conclusion

The finding that the NS proteins of the two highly similar viruses suppress entirely different members of the ISG family raises intriguing questions of pneumoviral NS evolution and mechanism of action.

Epitope mapping and kinetics of CD4 T cell immunity to pneumonia virus of mice in the C57BL/6 strain

Pneumonia virus of mice (PVM) infection has been widely used as a rodent model to study the closely related human respiratory syncytial virus (hRSV). While T cells are indispensable for viral clearance, they also contribute to immunopathology. To gain more insight into mechanistic details, novel tools are needed that allow to study virus-specific T cells in C57BL/6 mice as the majority of transgenic mice are only available on this background. While PVM-specific CD8 T cell epitopes were recently described, so far no PVM-specific CD4 T cell epitopes have been identified within the C57BL/6 strain. Therefore, we set out to map H2-IA^b-restricted epitopes along the PVM proteome. By means of in silico prediction and subsequent functional validation, we were able to identify a MHCII-restricted CD4 T cell epitope, corresponding to amino acids 37–47 in the PVM matrix protein (M_37–47). Using this newly identified MHCII-restricted M_37–47 epitope and a previously described MHCI-restricted N_339–347 epitope, we generated peptide-loaded MHCII and MHCI tetramers and characterized the dynamics of virus-specific CD4 and CD8 T cell responses in vivo. The findings of this study can provide a basis for detailed investigation of T cell-mediated immune responses to PVM in a variety of genetically modified C57BL/6 mice.

Unique nonstructural proteins of Pneumonia Virus of Mice (PVM) promote degradation of interferon (IFN) pathway components and IFN-stimulated gene proteins

Pneumonia Virus of Mice (PVM) is the only virus that shares the Pneumovirus genus of the Paramyxoviridae family with Respiratory Syncytial Virus (RSV). A deadly mouse pathogen, PVM has the potential to serve as a robust animal model of RSV infection, since human RSV does not fully replicate the human pathology in mice. Like RSV, PVM also encodes two nonstructural proteins that have been implicated to suppress the IFN pathway, but surprisingly, they exhibit no sequence similarity with their RSV equivalents. The molecular mechanism of PVM NS function, therefore, remains unknown. Here, we show that recombinant PVM NS proteins degrade the mouse counterparts of the IFN pathway components. Proteasomal degradation appears to be mediated by ubiquitination promoted by PVM NS proteins. Interestingly, NS proteins of PVM lowered the levels of several ISG (IFN-stimulated gene) proteins as well. These results provide a molecular foundation for the mechanisms by which PVM efficiently subverts the IFN response of the murine cell. They also reveal that in spite of their high sequence dissimilarity, the two pneumoviral NS proteins are functionally and mechanistically similar.

Feral swine virome is dominated by single-stranded DNA viruses and contains a novel Orthopneumovirus which circulates both in feral and domestic swine

Feral swine are known reservoirs for various pathogens that can adversely affect domestic animals. To assess the viral ecology of feral swine in the USA, metagenomic sequencing was performed on 100 pooled nasal swabs. The virome was dominated by small, ssDNA viruses belonging to the families Circoviridae, Anelloviridae and Parvovirinae. Only four RNA viruses were identified: porcine kobuvirus, porcine sapelovirus, atypical porcine pestivirus and a novel Orthopneumovirus, provisionally named swine orthopneumovirus (SOV). SOV shared ~90 % nucleotide identity to murine pneumonia virus (MPV) and canine pneumovirus. A modified, commercially available ELISA for MPV found that approximately 30 % of both feral and domestic swine sera were positive for antibodies cross-reactive with MPV. Quantitative reverse transcription-PCR identified two (2 %) and four (5.0 %) positive nasal swab pools from feral and domestic swine, respectively, confirming that SOV circulates in both herds.

Extraribosomal l13a is a specific innate immune factor for antiviral defense

We report a novel extraribosomal innate immune function of mammalian ribosomal protein L13a, whereby it acts as an antiviral agent. We found that L13a is released from the 60S ribosomal subunit in response to infection by respiratory syncytial virus (RSV), an RNA virus of the Pneumovirus genus and a serious lung pathogen. Unexpectedly, the growth of RSV was highly enhanced in L13a-knocked-down cells of various lineages as well as in L13a knockout macrophages from mice. In all L13a-deficient cells tested, translation of RSV matrix (M) protein was specifically stimulated, as judged by a greater abundance of M protein and greater association of the M mRNA with polyribosomes, while general translation was unaffected. In silico RNA folding analysis and translational reporter assays revealed a putative hairpin in the 3'untranslated region (UTR) of M mRNA with significant structural similarity to the cellular GAIT (gamma-activated inhibitor of translation) RNA hairpin, previously shown to be responsible for assembling a large, L13a-containing ribonucleoprotein complex that promoted translational silencing in gamma interferon (IFN-γ)-activated myeloid cells. However, RNA-protein interaction studies revealed that this complex, which we named VAIT (respiratory syncytial virus-activated inhibitor of translation) is functionally different from the GAIT complex. VAIT is the first report of an extraribosomal L13a-mediated, IFN-γ-independent innate antiviral complex triggered in response to virus infection. We provide a model in which the VAIT complex strongly hinders RSV replication by inhibiting the translation of the rate-limiting viral M protein, which is a new paradigm in antiviral defense.

IMPORTANCE

The innate immune mechanisms of host cells are diverse in nature and act as a broad-spectrum cellular defense against viruses. Here, we report a novel innate immune mechanism functioning against respiratory syncytial virus (RSV), in which the cellular ribosomal protein L13a is released from the large ribosomal subunit soon after infection and inhibits the translation of a specific viral mRNA, namely, that of the matrix protein M. Regarding its mechanism, we show that the recognition of a specific secondary structure in the 3' untranslated region of the M mRNA leads to translational arrest of the mRNA. We also show that the level of M protein in the infected cell is rate limiting for viral morphogenesis, providing a rationale for L13a to target the M mRNA for suppression of RSV growth. Translational silencing of a viral mRNA by a deployed ribosomal protein is a new paradigm in innate immunity.

Animal model of respiratory syncytial virus: CD8+ T cells cause a cytokine storm that is chemically tractable by sphingosine-1-phosphate 1 receptor agonist therapy

The cytokine storm is an intensified, dysregulated, tissue-injurious inflammatory response driven by cytokine and immune cell components. The cytokine storm during influenza virus infection, whereby the amplified innate immune response is primarily responsible for pulmonary damage, has been well characterized. Now we describe a novel event where virus-specific T cells induce a cytokine storm. The paramyxovirus pneumonia virus of mice (PVM) is a model of human respiratory syncytial virus (hRSV). Unexpectedly, when C57BL/6 mice were infected with PVM, the innate inflammatory response was undetectable until day 5 postinfection, at which time CD8(+) T cells infiltrated into the lung, initiating a cytokine storm by their production of gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α). Administration of an immunomodulatory sphingosine-1-phosphate (S1P) receptor 1 (S1P1R) agonist significantly inhibited PVM-elicited cytokine storm by blunting the PVM-specific CD8(+) T cell response, resulting in diminished pulmonary disease and enhanced survival.

IMPORTANCE

A dysregulated overly exuberant immune response, termed a "cytokine storm," accompanies virus-induced acute respiratory diseases (VARV), is primarily responsible for the accompanying high morbidity and mortality, and can be controlled therapeutically in influenza virus infection of mice and ferrets by administration of sphingosine-1-phosphate 1 receptor (S1P1R) agonists. Here, two novel findings are recorded. First, in contrast to influenza infection, where the cytokine storm is initiated early by the innate immune system, for pneumonia virus of mice (PVM), a model of RSV, the cytokine storm is initiated late in infection by the adaptive immune response: specifically, by virus-specific CD8 T cells via their release of IFN-γ and TNF-α. Blockading these cytokines with neutralizing antibodies blunts the cytokine storm and protects the host. Second, PVM infection is controlled by administration of an S1P1R agonist.

Full-genome analysis of a canine pneumovirus causing acute respiratory disease in dogs, Italy

An outbreak of canine infectious respiratory disease (CIRD) associated to canine pneumovirus (CnPnV) infection is reported. The outbreak occurred in a shelter of the Apulia region and involved 37 out of 350 dogs that displayed cough and/or nasal discharge with no evidence of fever. The full-genomic characterisation showed that the causative agent (strain Bari/100-12) was closely related to CnPnVs that have been recently isolated in the USA, as well as to murine pneumovirus, which is responsible for respiratory disease in mice. The present study represents a useful contribution to the knowledge of the pathogenic potential of CnPnV and its association with CIRD in dogs. Further studies will elucidate the pathogenicity and epidemiology of this novel pneumovirus, thus addressing the eventual need for specific vaccines.

In situ evolution of virus-specific cytotoxic T cell responses in the lung

Cytotoxic T cells (CTL) play a critical role in the clearance of respiratory viral infections, but they also contribute to disease manifestations. In this study, we infected mice with a genetically modified pneumonia virus of mice (PVM) that allowed visualization of virus-specific CTL and infected cells in situ. The first virus-specific T cells entered the lung via blood vessels in the scattered foci of PVM-infected cells, which densely clustered around the bronchi at day 7 after infection. At this time, overall pulmonary virus load was maximal, but the mice showed no overt signs of disease. On days 8 to 9, T cells gained access to the infected bronchial epithelium and to the lung interstitium, which was associated with a reduction in the number of virus-infected cells within the initial clusters but could not prevent further virus spread throughout the lung tissue. Interestingly, recruitment of virus-specific CTL throughout the parenchyma was still ongoing on day 10, when the virus infection was already largely controlled. This also represented the peak of clinical disease. Thus, disease was associated with an exuberant T cell infiltration late in the course of the infection, which may be required to completely eliminate virus at residual foci of infection. PVM-induced immunopathology may thus result from the need to generate widespread T cell infiltrates to complete the elimination of virus-infected cells in a large organ like the lung. This experimental model provides the first insights into the spatiotemporal evolution of pulmonary antiviral T cell immunity in vivo.

CD8+ T-cell epitope mapping for pneumonia virus of mice in H-2b mice

The paramyxovirus pneumonia virus of mice (PVM) is a rodent model of human respiratory syncytial virus (hRSV) pathogenesis. Here we characterized the PVM-specific CD8(+) T-cell repertoire in susceptible C57BL/6 mice. In total, 15 PVM-specific CD8(+) T-cell epitopes restricted by H-2D(b) and/or H-2K(b) were identified. These data open the door for using widely profiled, genetically manipulated C57BL/6 mice to study the contribution of epitope-specific CD8(+) T cells to PVM pathogenesis.

Novel pneumoviruses (PnVs): Evolution and inflammatory pathology

A previous report of a novel pneumovirus (PnV) isolated from the respiratory tract of a dog described its significant homology to the rodent pathogen, pneumonia virus of mice (PVM). The original PnV-Ane4 pathogen replicated in and could be re-isolated in infectious state from mouse lung but elicited minimal mortality compared to PVM strain J3666. Here we assess phylogeny and physiologic responses to 10 new PnV isolates. The G/glycoprotein sequences of all PnVs include elongated amino-termini when compared to the characterized PVMs, and suggest division into groups A and B. While we observed significant differences in cytokine production and neutrophil recruitment to the lungs of BALB/c mice in response to survival doses (50 TCID50 units) of representative group A (114378-10-29-KY-F) and group B (7968-11-OK) PnVs, we observed no evidence for positive selection (dN > dS) among the PnV/PnV, PVM/PnV or PVM/PVM G/glycoprotein or F/fusion protein sequence pairs.

Innate and adaptive immune response to pneumonia virus of mice in a resistant and a susceptible mouse strain

Respiratory syncytial virus (RSV) is the leading cause of infant bronchiolitis. The closely related pneumonia virus of mice (PVM) causes a similar immune-mediated disease in mice, which allows an analysis of host factors that lead to severe illness. This project was designed to compare the immune responses to lethal and sublethal doses of PVM strain 15 in Balb/c and C57Bl/6 mice. Balb/c mice responded to PVM infection with an earlier and stronger innate response that failed to control viral replication. Production of inflammatory cyto- and chemokines, as well as infiltration of neutrophils and IFN-γ secreting natural killer cells into the lungs, was more predominant in Balb/c mice. In contrast, C57Bl/6 mice were capable of suppressing both viral replication and innate inflammatory responses. After a sublethal infection, PVM-induced IFN-γ production by splenocytes was stronger early during infection and weaker at late time points in C57Bl/6 mice when compared to Balb/c mice. Furthermore, although the IgG levels were similar and the mucosal IgA titres lower, the virus neutralizing antibody titres were higher in C57Bl/6 mice than in Balb/c mice. Overall, the difference in susceptibility of these two strains appeared to be related not to an inherent T helper bias, but to the capacity of the C57Bl/6 mice to control both viral replication and the immune response elicited by PVM.

Induction of IL-6 and CCL5 (RANTES) in human respiratory epithelial (A549) cells by clinical isolates of respiratory syncytial virus is strain specific

BACKGROUND

Respiratory syncytial virus (RSV) is the major respiratory pathogen of infants and young children. During each seasonal epidemic, multiple strains of both subgroup A and B viruses circulate in the community. Like other RNA viruses, RSV genome replication is prone to errors that results in a heterogeneous population of viral strains some of which may possess differences in virulence. We sought to determine whether clinical isolates of RSV differ in their capacity to induce inflammatory cytokines IL-6 and CCL5 (previously known as RANTES [regulated upon activation, normal T-cell expressed and secreted protein]), which are known to be induced in vitro and in vivo in response to RSV, during infection of A549 cells.

RESULTS

Screening of subgroup A and B isolates revealed heterogeneity among strains to induce IL-6 and CCL5. We chose two subgroup B strains, New Haven (NH)1067 and NH1125, for further analysis because of their marked differences in cytokine inducing properties and because subgroup B strains, in general, are less genetically heterogeneous as compared to subgroup A strains. At 12 and 24 hours post infection RSV strains, NH1067 and NH1125 differed in their capacity to induce IL-6 by an order of magnitude or more. The concentrations of IL-6 and CCL5 were dependent on the dose of infectious virus and the concentration of these cytokines induced by NH1125 was greater than that of those induced by NH1067 when the multiplicity of infection of NH1067 used was as much as 10-fold higher than that of NH1125. The induction of IL-6 was dependent on viable virus as infection with UV-inactivated virus did not induce IL-6. The difference in IL-6 induction most likely could not be explained by differences in viral replication kinetics. The intracellular level of RSV RNA, as determined by quantitative RT-PCR, was indistinguishable between the 2 strains though the titer of progeny virus produced by NH1125 was greater than that produced by NH1067 at 16, 24 and 36 hours but essentially equal at 48 and 72 hours. Full genome sequencing of the 2 strains revealed 193 polymorphisms and 4 insertions in NH1067 when compared to NH1125 (2 single base insertions in non-coding regions and 2 duplications of 3 and 60 bases in the RSV G gene). Of the polymorphisms, 147 occurred in coding regions and only 30 resulted in amino acid changes in 7 of the RSV genes.

CONCLUSIONS

These data suggest that RSV strains may not be homogeneous with regard to pathogenesis or virulence. Identification of the genetic polymorphisms associated with variations in cytokine induction may lead to insights into RSV disease and to the development of effective antiviral agents and vaccines.

Evaluation of pneumonia virus of mice as a possible human pathogen

Pneumonia virus of mice (PVM), a relative of human respiratory syncytial virus (RSV), causes respiratory disease in mice. There is serologic evidence suggesting widespread exposure of humans to PVM. To investigate replication in primates, African green monkeys (AGM) and rhesus macaques (n = 4) were inoculated with PVM by the respiratory route. Virus was shed intermittently at low levels by a subset of animals, suggesting poor permissiveness. PVM efficiently replicated in cultured human cells and inhibited the type I interferon (IFN) response in these cells. This suggests that poor replication in nonhuman primates was not due to a general nonpermissiveness of primate cells or poor control of the IFN response. Seroprevalence in humans was examined by screening sera from 30 adults and 17 young children for PVM-neutralizing activity. Sera from a single child (6%) and 40% of adults had low neutralizing activity against PVM, which could be consistent with increasing incidence of exposure following early childhood. There was no cross-reaction of human or AGM sera between RSV and PVM and no cross-protection in the mouse model. In native Western blots, human sera reacted with RSV but not PVM proteins under conditions in which AGM immune sera reacted strongly. Serum reactivity was further evaluated by flow cytometry using unfixed Vero cells infected with PVM or RSV expressing green fluorescent protein (GFP) as a measure of viral gene expression. The reactivity of human sera against RSV-infected cells correlated with GFP expression, whereas reactivity against PVM-infected cells was low and uncorrelated with GFP expression. Thus, PVM specificity was not evident. Our results indicate that the PVM-neutralizing activity of human sera is not due to RSV- or PVM-specific antibodies but may be due to low-affinity, polyreactive natural antibodies of the IgG subclass. The absence of PVM-specific antibodies and restriction in nonhuman primates makes PVM unlikely to be a human pathogen.

Canine pneumovirus replicates in mouse lung tissue and elicits inflammatory pathology

Canine pneumovirus (CnPnV) was recently isolated from the respiratory tracts of shelter dogs and shares sequence similarity with the rodent pathogen, pneumonia virus of mice (PVM). We show here that CnPnV replicates in and can elicit local proinflammatory cytokine production and neutrophil recruitment to lung tissue and the airways. In contrast to PVM J3666 infection, fatal CnPnV infections are observed only in response to high titer intranasal inocula (>67 TCID(50) units). Sera from mice that recover from CnPnV infection contain antibodies that cross-react with PVM antigens; these mice are protected against lethal PVM infection. Given these findings, it will be intriguing to determine the relative role(s) of CnPnV and PVM in eliciting respiratory symptoms in susceptible canine species.

Both nonstructural proteins NS1 and NS2 of pneumonia virus of mice are inhibitors of the interferon type I and type III responses in vivo

Infection of mice with pneumonia virus of mice (PVM) provides a convenient experimental pathogenesis model in a natural host for a human respiratory syncytial virus-related virus. Extending our previous work showing that the PVM nonstructural (NS) proteins were pathogenicity factors in mice, we identify both the NS1 and NS2 proteins as antagonists of alpha/beta interferon (IFN-α/β) and IFN-λ by use of recombinant PVM (rPVM) with single and combined deletions of the NS proteins (ΔNS1, ΔNS2, and ΔNS1 ΔNS2). Wild-type and NS deletion PVMs were evaluated for growth and pathogenesis by infecting knockout mice that lack functional receptors to IFN-α/β, IFN-λ, or both. The absence of the receptor to IFN-α/β (IFNAR) or IFN-λ (interleukin-28 receptor α chain [IL-28Rα]) individually did not reverse the attenuated virulence of the NS deletion viruses although loss of IFNAR partially restored replication efficiency. When both receptors were deleted, replication and virulence were largely rescued for rPVM ΔNS1 and were significantly but not completely rescued for rPVM ΔNS2. As for rPVM ΔNS1 ΔNS2, the effect was mostly limited to partial enhancement of replication. This indicates that both IFN-α/β and IFN-λ contributed to restricting the NS deletion viruses, with the former playing the greater role. Interestingly, the replication and virulence of wild-type PVM were completely unaffected by the presence or absence of functional receptors to IFN-α/β and IFN-λ, indicating that both systems are strongly suppressed during infection. However, pretreatment of mice with IFN-α/β was protective against lethal rPVM challenge, whereas pretreatment with IFN-λ delayed but did not prevent disease and, in some cases, reduced mortality. The fact that virulence of rPVM lacking NS2 was not recovered completely when both interferon receptors were deleted suggests that NS2 may have further functions outside the IFN system.

Genomic analysis of a pneumovirus isolated from dogs with acute respiratory disease

A previously unrecognized virus belonging to the subfamily Pneumovirinae and most closely related to murine pneumovirus (MPV) was identified in domestic dogs in 2 related animal shelters. Additional diagnostic testing yielded 3 new viral isolates and identified 6 additional PCR positive dogs from other USA locations indicating that its distribution is not geographically limited. Nucleotide sequences encompassing 9 of the 10 genes were compared to the only 2 available MPV strains, 15 and J3666. Several features distinguished the canine pneumovirus (CnPnV) from the murine strains. Two regions of diversity were identified in the amino-proximal region of P and the overlapping P2 ORF was only 54 amino acids (aa) compared to 137aa in MPV. The G protein had an amino-terminal cytoplasmic tail 18aa longer than in the MPV strains. The CnPnV SH protein showed the highest divergence with only 90.2% aa identity when compared to MPV strain 15. Like strain 15, the CnPnV SH ORF coded for a protein of 92aa while J3666 has a 114aa variant. Comparison of CnPnV isolates at culture passages 4 and 17 revealed 7nt differences within the 8598nt sequenced. Of note was a substitution at nt 364 in G resulting in a termination codon that would produce a truncated G protein of 122aa. Analysis of early passage and ex vivo samples showed the termination codon in G to be predominant after 6 days in culture indicating rapid selection of the mutation in A72 cells.

Pneumonia Virus of Mice (PVM): Exploring Novel Therapeutic Options In a Severe Respiratory Disease Model

Respiratory syncytial virus (RSV) is the most important respiratory pathogen among infants and toddlers, with infections prevalent and nearly universal in this age group. Severe infections are more common among premature infants, those with cardiac and pulmonary anomalies, and the immunosupressed. Effective prophylactic monoclonal antibody treatment is available for high-risk infants, but there is no effective vaccine. Mouse challenge models have been used for the study of the human RSV pathogen, but the most severe forms of RSV disease are not replicated by this approach. Pneumonia virus of mice (PVM; family Paramyxoviridae, genus Pneumovirus) is a mouse pathogen of the same family as human respiratory syncytial virus. PVM replicates efficiently in mouse-lung epithelial cells in vivo in response to a minimal virus inoculum, and replication is accompanied by local production of proinflammatory cytokines (MIP-1α, MIP-2, MCP-1, and IFN-γ) and granulocyte recruitment to the lung. PVM infection and the ensuing inflammatory response can lead to pulmonary edema and respiratory compromise. Our laboratories have pioneered the use of the PVM model for the study of human clinical disease, which has provided important insights into the role of the inflammatory response in the pathogenesis of severe respiratory virus infection. As part of this work, we have presented several immunomodulatory strategies that clearly reduce morbidity and mortality when administered to PVM infected, symptomatic mice, and thus hold promise as realistic therapeutic strategies for severe RSV infection in human subjects.

The role of viruses in the etiology and pathogenesis of common cold

Numerous viruses are able to cause respiratory tract infections. With the availability of new molecular techniques, the number of pathogens detected in specimens from the human respiratory tract has increased. Some of these viral infections have the potential to lead to severe systemic disease. Other viruses are limited to playing a role in the pathogenesis of the common cold syndrome. This chapter focuses on the viral pathogens that are linked to common cold. It is not the intention to comprehensively review all the viruses that are able to cause respiratory tract infections—this would go beyond the scope of this book. The list of viruses that are briefly reviewed here includes rhinoviruses, respiratory syncytial virus, parainfluenza virus, adenovirus, metapneumovirus and coronavirus. Bocavirus is discussed as one example of a newly identified pathogen with a less established role in the etiology and pathogenesis of common cold. Influenza virus does not cause what is defined as common cold. However, influenza viruses are associated with respiratory disease and the clinical picture of mild influenza and common cold frequently overlaps. Therefore, influenza virus has been included in this chapter. It is important to note that a number of viruses are frequently co-detected with other viruses in humans with respiratory diseases. Therefore, the viral etiology and the role of viruses in the pathogenesis of common cold is complex, and numberous questions remain to be answered.

Deletion of nonstructural proteins NS1 and NS2 from pneumonia virus of mice attenuates viral replication and reduces pulmonary cytokine expression and disease

Pneumonia virus of mice (PVM) strain 15 causes fatal pneumonia in mice and provides a convenient model for human respiratory syncytial virus pathogenesis and immunobiology. We prepared PVM mutants lacking the genes for nonstructural proteins NS1 and/or NS2. In Vero cells, which lack type I interferon (IFN), deletion of these proteins had no effect on the efficiency of virus growth. In IFN-competent mouse embryo fibroblasts, wild-type (wt) PVM and the DeltaNS1 virus grew efficiently and strongly inhibited the IFN response, whereas virus lacking NS2 was highly attenuated and induced high levels of IFN and IFN-inducible genes. In BALB/c mice, intranasal infection with wt PVM caused overt disease that began on day 6 and was lethal by day 9 postinoculation. In comparison, DeltaNS1 induced transient, reduced disease, and DeltaNS2 and DeltaNS12 caused no disease. Thus, NS1 and NS2 are virulence factors, with NS2 being a major antagonist of the type I IFN system. The pulmonary titers of wt PVM and DeltaNS1 were high on day 3 and increased further by day 6; in addition, expression of IFN and representative proinflammatory cytokines/chemokines and T lymphocyte-related cytokines was undetectable on day 3 but increased dramatically by day 6 coincident with the onset of disease. The titers of DeltaNS2 and DeltaNS12 were somewhat lower on day 3 and decreased further by day 6; in addition, these viruses induced a more circumscribed set of cytokines/chemokines (IFN, interleukin-6 [IL-6], and CXCL10) that were detected on day 3 and had largely subsided by day 6. Lung immunohistology revealed abundant PVM-positive pneumocytes and bronchial and bronchiolar epithelial cells in wt PVM- and DeltaNS1-infected mice on day 6 compared to few PVM-positive foci with DeltaNS2 and DeltaNS12. These results indicate that severe PVM disease is associated with high, poorly controlled virus replication driving the expression of high levels of pulmonary IFN and a broad array of cytokines/chemokines. In contrast, in the absence of NS2, there was an early, transient innate response involving moderate levels of IFN, IL-6, and CXCL10 that restricted virus replication and prevented disease.

Role of T cells in virus control and disease after infection with pneumonia virus of mice

Infection of mice with pneumonia virus of mice (PVM) is used as a natural host experimental model for studying the pathogenesis of infection with the closely related human respiratory syncytial virus. We analyzed the contribution of T cells to virus control and pathology after PVM infection. Control of a sublethal infection with PVM strain 15 in C57BL/6 mice was accompanied by a 100-fold increase in pulmonary cytotoxic T lymphocytes, 20% of which were specific for PVM. T-cell-deficient mice failed to eliminate PVM and became virus carriers in the absence of the clinical or histopathological signs of pneumonia that occurred after infection of control mice. Mice with limited T-cell numbers did not achieve virus control without weight loss, indicating that T-cell-mediated virus control was closely linked to immunopathology. Both CD4 and CD8 T cells independently contributed to virus elimination and disease. Virus control and disease were similar in the absence of perforin, gamma interferon, or tumor necrosis factor alpha. Interestingly, disease and mortality after lethal high-dose PVM infection were independent of T cells. These data illustrate a key role for T cells in control of PVM infection and demonstrate that both T-cell-dependent and -independent pathways contribute to disease in a viral dose-dependent fashion.

Pneumonia virus of mice: severe respiratory infection in a natural host

Pneumonia virus of mice (PVM; family Paramyxoviridae, genus Pneumovirus) is a natural mouse pathogen that is closely related to human and bovine respiratory syncytial viruses. Among the prominent features of this infection, robust replication of PVM takes place in bronchial epithelial cells in response to a minimal virus inoculum. Virus replication in situ results in local production of proinflammatory cytokines (MIP-1alpha, MIP-2, MCP-1 and IFNgamma) and granulocyte recruitment to the lung. If left unchecked, PVM infection and the ensuing inflammatory response ultimately lead to pulmonary edema, respiratory compromise and death. In this review, we consider the recent studies using the PVM model that have provided important insights into the role of the inflammatory response in the pathogenesis of severe respiratory virus infection. We also highlight several works that have elucidated acquired immune responses to this pathogen, including T cell responses and the development of humoral immunity. Finally, we consider several immunomodulatory strategies that have been used successfully to reduce morbidity and mortality when administered to PVM-infected, symptomatic mice, and thus hold promise as realistic therapeutic strategies for severe respiratory virus infections in human subjects.

Identification of a novel virulence factor in recombinant pneumonia virus of mice

Pneumonia virus of mice (PVM) is a murine relative of human respiratory syncytial virus (HRSV). Here we developed a reverse genetics system for PVM based on a consensus sequence for virulent strain 15. Recombinant PVM and a version engineered to express green fluorescent protein replicated as efficiently as the biological parent in vitro but were 4- and 12.5-fold attenuated in vivo, respectively. The G proteins of HRSV and PVM have been suggested to contribute to viral pathogenesis, but this had not been possible to study in a defined manner in a fully permissive host. As a first step, we evaluated recombinant mutants bearing a deletion of the entire G gene (Delta G) or expressing a G protein lacking its cytoplasmic tail (Gt). Both G mutants replicated as efficiently in vitro as their recombinant parent, but both were nonpathogenic in mice at doses that would otherwise be lethal. We could not detect replication of the Delta G mutant in mice, indicating that its attenuation is based on a severe reduction in the virus load. In contrast, the Gt mutant appeared to replicate as efficiently in mice as its recombinant parent. Thus, the reduction in virulence associated with the Gt mutant could not be accounted for by a reduction in viral replication. These results identified the cytoplasmic tail of G as a virulence factor whose effect is not mediated solely by the viral load. In addition to its intrinsic interest, a recombinant virus that replicates with wild-type-like efficiency but does not cause disease defines optimal properties for vaccine development.

Immunization strategies for the prevention of pneumovirus infections

Pneumoviruses, which are viruses of the family Paramyxoviridae, subfamily Pneumovirinae, are pathogens that infect the respiratory tract of their host species. The human pneumovirus pathogen, human respiratory syncytial virus (RSV), has counterparts that infect cows (bovine RSV), sheep (ovine RSV), goats (caprine RSV) and rodents (pneumonia virus of mice). Each pneumovirus is host specific and results in a spectrum of disease, ranging from mild upper-respiratory illness to severe bronchiolitis and pneumonia with significant morbidity and mortality. Given the public health burden caused by human RSV and the concomitant agricultural impact of bovine RSV, these two viruses are considered as prime targets for the development of safe and effective vaccines. In this review, we describe the strategies used to develop vaccines against human and bovine RSV and introduce the pneumonia virus mouse model as a novel and invaluable tool for preclinical studies and new vaccine strategies.

Mucosal inoculation with an attenuated mouse pneumovirus strain protects against virulent challenge in wild type and interferon-gamma receptor deficient mice

Protective mechanisms underlying the responses to mucosal vaccination are not yet clearly defined. Using the natural mouse pneumovirus pathogen, pneumonia virus of mice (PVM), we explore responses of wild type and interferon-gamma (IFNgamma) receptor gene-deleted mice to virulent challenge after mucosal vaccination with an attenuated virus strain. Serum neutralizing antibodies develop after intranasal inoculation with 30 pfu of attenuated, replication-competent PVM strain 15, which correlate with diminished gross and microscopic pulmonary pathology and protection from weight loss in response to subsequent challenge with the virulent parent PVM strain J3666. Virus replication in response to challenge was blunted in PVM strain 15 vaccinated mice, as was local production of secretory mediators IFNgamma, TNF-alpha, MIP-1 alpha, and MIP-2. Interestingly, responses of vaccinated IFNgamma receptor gene-deleted mice were indistinguishable from those of the wild type, suggesting that IFNgamma signaling may not be crucial for the generation of adaptive responses to pneumovirus infection in vivo.

Respiratory syncytial virus disease mechanisms implicated by human, animal model, and in vitro data facilitate vaccine strategies and new therapeutics

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis, pneumonia, mechanical ventilation, and respiratory failure in infants in the US. No effective post-infection treatments are widely available, and currently there is no vaccine. RSV disease is the result of virus-induced airway damage and complex inflammatory processes. The outcome of infection depends on host and viral genetics. Here, we review disease mechanisms in primary RSV infection that are implicated by clinical studies, in vitro systems, and animal models. Defining RSV disease mechanisms is difficult because there is a wide range of RSV disease phenotypes in humans, and there are disparities in RSV disease phenotypes among the animal models of RSV infection. However, host factors identified by multiple lines of investigation as playing important roles in RSV pathogenesis are providing key insights. A better understanding of RSV molecular biology and RSV pathogenesis is facilitating rational vaccine design strategies and molecular targets for new therapeutics.

Activation and inactivation of antiviral CD8 T cell responses during murine pneumovirus infection

Pneumonia virus of mice (PVM) is a natural pathogen of mice and has been proposed as a tractable model for the replication of a pneumovirus in its natural host, which mimics human infection with human respiratory syncytial virus (RSV). PVM infection in mice is highly productive in terms of virus production compared with the situation seen with RSV in mice. Because RSV suppresses CD8 T cell effector function in the lungs of infected mice, we have investigated the nature of PVM-induced CD8 T cell responses to study pneumovirus-induced T cell responses in a natural virus-host setting. PVM infection was associated with a massive influx of activated CD8 T cells into the lungs. After identification of three PVM-specific CD8 T cell epitopes, pulmonary CD8 T cell responses were enumerated. The combined frequency of cytokine-secreting CD8 T cells specific for the three epitopes was much smaller than the total number of activated CD8 T cells. Furthermore, quantitation of the CD8 T cell response against one of these epitopes (residues 261-270 from the phosphoprotein) by MHC class I pentamer staining and by in vitro stimulation followed by intracellular IFN-gamma and TNF-alpha staining indicated that the majority of pulmonary CD8 specific for the P261 epitope were deficient in cytokine production. This deficient phenotype was retained up to 96 days postinfection, similar to the situation in the lungs of human RSV-infected mice. The data suggest that PVM suppresses T cell effector functions in the lungs.