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.

The Pneumonia Virus of Mice (PVM) model of acute respiratory infection

Pneumonia Virus of Mice (PVM) is related to the human and bovine respiratory syncytial virus (RSV) pathogens, and has been used to study respiratory virus replication and the ensuing inflammatory response as a component of a natural host—pathogen relationship. As such, PVM infection in mice reproduces many of the clinical and pathologic features of the more severe forms of RSV infection in human infants. Here we review some of the most recent findings on the basic biology of PVM infection and its use as a model of disease, most notably for explorations of virus infection and allergic airways disease, for vaccine evaluation, and for the development of immunomodulatory strategies for acute respiratory virus infection.

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.

ChemR23 dampens lung inflammation and enhances anti-viral immunity in a mouse model of acute viral pneumonia

Viral diseases of the respiratory tract, which include influenza pandemic, children acute bronchiolitis, and viral pneumonia of the elderly, represent major health problems. Plasmacytoid dendritic cells play an important role in anti-viral immunity, and these cells were recently shown to express ChemR23, the receptor for the chemoattractant protein chemerin, which is expressed by epithelial cells in the lung. Our aim was to determine the role played by the chemerin/ChemR23 system in the physiopathology of viral pneumonia, using the pneumonia virus of mice (PVM) as a model. Wild-type and ChemR23 knock-out mice were infected by PVM and followed for functional and inflammatory parameters. ChemR23^−/− mice displayed higher mortality/morbidity, alteration of lung function, delayed viral clearance and increased neutrophilic infiltration. We demonstrated in these mice a lower recruitment of plasmacytoid dendritic cells and a reduction in type I interferon production. The role of plasmacytoid dendritic cells was further addressed by performing depletion and adoptive transfer experiments as well as by the generation of chimeric mice, demonstrating two opposite effects of the chemerin/ChemR23 system. First, the ChemR23-dependent recruitment of plasmacytoid dendritic cells contributes to adaptive immune responses and viral clearance, but also enhances the inflammatory response. Second, increased morbidity/mortality in ChemR23^−/− mice is not due to defective plasmacytoid dendritic cells recruitment, but rather to the loss of an anti-inflammatory pathway involving ChemR23 expressed by non-leukocytic cells. The chemerin/ChemR23 system plays important roles in the physiopathology of viral pneumonia, and might therefore be considered as a therapeutic target for anti-viral and anti-inflammatory therapies.

Infections of the lower respiratory tract by single-stranded RNA viruses represent a major health problem worldwide. Animal models indicate that the severity of infections caused by these viruses is due essentially to an excessive primary immune response of the host, rather than the direct cytopathogenicity of the viruses. Plasmacytoid dendritic cells have been reported to play an important role in anti-viral immunity, but the factors responsible for the recruitment of these cells to the infected lung were unknown. This study depicts the roles of the G protein-coupled receptor ChemR23 in the recruitment of plasmacytoid dendritic cells and anti-viral immunity in a mouse model of acute viral pneumonia. The data also highlight the role of ChemR23 in dampening the lung inflammatory response. This latter effect is independent of pDC recruitment but involves non-leukocytic cells. This observation is of particular interest considering the established role of airway endothelial and epithelial cells in the immune responses following bacterial, viral and fungal infections. Our results suggest therefore that the chemerin/ChemR23 system might be considered as a target for anti-viral and anti-inflammatory therapies.

Interferon-gamma coordinates CCL3-mediated neutrophil recruitment in vivo

BACKGROUND

We have shown previously that acute infection with the respiratory pathogen, pneumonia virus of mice (PVM), results in local production of the proinflammatory chemokine, CCL3, and that neutrophil recruitment in response to PVM infection is reduced dramatically in CCL3 -/- mice.

RESULTS

In this work, we demonstrate that CCL3-mediated neutrophil recruitment is coordinated by interferon-gamma (IFNgamma). Neutrophil recruitment in response to PVM infection was diminished five-fold in IFNgamma receptor gene-deleted mice, although neutrophils from IFNgammaR -/- mice expressed transcripts for the CCL3 receptor, CCR1 and responded functionally to CCL3 ex vivo. Similarly, in the absence of PVM infection, CCL3 overexpression alone could not elicit neutrophil recruitment in the absence of IFNgamma. Interestingly, although supplemental IFNgamma restored neutrophil recruitment and resulted in a sustained weight loss among CCL3-overexpressing IFNgamma -/- mice, CCL3-mediated neutrophil recruitment alone did not result in the pulmonary edema or respiratory failure characteristic of severe viral infection, suggesting that CCL3 and IFN-gamma together are sufficient to promote neutrophil recruitment but not pathologic activation.

CONCLUSION

Our findings reveal a heretofore unrecognized hierarchical interaction between the IFNgamma and CCL3, which demonstrate that IFNgamma is crucial for CCL3-mediated neutrophil recruitment in vivo.

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.

Ribavirin and cysteinyl leukotriene-1 receptor blockade as treatment for severe bronchiolitis

In this work we have evaluated the clinical responses of pneumovirus-infected mice to combination therapy with the antiviral agent, ribavirin, and the CysLT1 cysteinyl leukotriene receptor antagonist, montelukast. We observed substantial virus replication in our mouse model of pneumovirus infection and significant accumulation of cysteinyl leukotrienes in lung tissue, the latter detected at levels that correlate directly with granulocyte recruitment to the airways. While administration of the nucleoside analog, ribavirin, reduced virus replication approximately 2,000-fold, the clinical outcomes as measured by morbidity and mortality, in response to ribavirin monotherapy were indistinguishable from those of the no-treatment controls. Similarly, montelukast therapy alone did not reduce granulocyte recruitment nor did it improve the clinical outcome. However, combined therapy with ribavirin and montelukast resulted in a significant reduction in morbidity and a substantial reduction in mortality (50% survival at t = 14 days and onward, compared to 10-20% survival in response to montelukast alone or to ribavirin alone, respectively, p < 0.01). These findings define further the independent contributions made by virus replication and by the ensuing inflammatory response to the detrimental sequelae of pneumovirus infection in vivo.

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

Pneumonia virus of mice (PVM) is an enveloped RNA-containing virus of Family Paramyxoviridae. Sequences had been determined previously for a number of PVM genes, although these represented cloned cDNAs rather than consensus sequences. Sequences were not available for the 3' -leader and 5' -trailer regions that constitute the genome termini or for the large polymerase L gene that accounts for 43% of the genome. Also, the available sequences were from an attenuated variant of strain 15, whereas the present study analyzed the version of strain 15 that is available from the American Type Culture Collection (ATCC) and is highly virulent in mice. Analysis of unclosed RT-PCR products yielded a complete consensus sequence of 14,886 nt (GenBank accession number AY729016). Of the regions for which sequences had been previously reported for the non-pathogenic strain, there were 13 nucleotide differences and 10 amino acid differences compared to the present consensus sequence for the virulent isolate. The various genes of PVM shared 29-62% nucleotide sequence identity and 10-60% amino acid sequence identity with human or bovine respiratory syncytial virus (HRSV and BRSV), its closest relatives.

Reevaluation of the virulence of prototypic strain 15 of pneumonia virus of mice

Prototypic strain 15 of pneumonia virus of mice (PVM) has been described as being nonpathogenic in mice, in contrast to the mouse-passaged, highly virulent strain J3666. Previous sequence analysis also indicated that strain 15 encodes an attachment G protein that is truncated at the amino terminus, which for the amino terminally anchored protein deletes the cytoplasmic tail. However, we found that PVM strain 15 obtained from the American Type Culture Collection was highly virulent in mice and was essentially indistinguishable on that basis from strain J3666. Sequence analysis showed that this preparation of virus encodes a G protein with an intact cytoplasmic tail: the truncated predicted protein in the previous sequence appeared to be due to a single nucleotide insertion that disrupted the upstream end of the open reading frame and shifted the translational start site to the next downstream AUG. Taken together, the two studies indicate that strain 15 is an inherently virulent strain but that a nonpathogenic variant that was generated during passage in vitro and encodes a truncated G protein exists. Interestingly, the majority sequence of strain J3666 was found to encode a G protein with an extended cytoplasmic tail, suggesting that there is the potential for considerable plasticity in the cytoplasmic tail of the G protein of PVM.

Diminished expression of an antiviral ribonuclease in response to pneumovirus infection in vivo

The mouse eosinophil-associated ribonucleases (mEars) are species specific, divergent orthologs of the human antiviral RNase A ribonucleases, eosinophil-derived neurotoxin (RNase 2) and eosinophil cationic protein (RNase 3). We show here that mEar 2 is also an antiviral ribonuclease, as micromolar concentrations promote a approximately sixfold reduction in the infectivity of pneumonia virus of mice (PVM) for target respiratory epithelial cells in vitro. Although initially identified as a component of eosinophilic leukocytes, mEar 2 mRNA and protein were also detected in lung tissue accompanied by enzymatically active mEar 2 in bronchoalveolar lavage fluid (BALF). At t=3 days post-inoculation with PVM (strain J3666), we observed the characteristic inflammatory response accompanied by diminished expression of total mEar mRNA and protein in lung tissue and a corresponding fivefold drop in ribonuclease activity in BALF. No change in mEar expression was observed in response to infection with PVM strain 15, a replication-competent strain of PVM that does not elicit a cellular inflammatory response. However, mEar expression is not directly dependent on inflammation per se, as diminished expression of mEar mRNA and BAL ribonuclease activity were also observed in PVM-infected, inflammation-deficient, MIP-1alpha -/- mice. We propose that this mechanism may represent a novel virus-mediated evasion strategy, with a mechanism that is linked in some fashion to virus-specific pathogenicity.

Detection of sendai virus and pneumonia virus of mice by use of fluorogenic nuclease reverse transcriptase polymerase chain reaction analysis

Sendai virus may induce acute respiratory tract disease in laboratory mice and is a common contaminant of biological materials. Pneumonia virus of mice (PVM) also infects the respiratory tract and, like Sendai virus, may induce a persistent wasting disease syndrome in immunodeficient mice. Reverse transcriptase-polymerase chain reaction (RT-PCR) assays have proven useful for detection of Sendai virus and PVM immunodeficient animals and contaminated biomaterials. Fluorogenic nuclease RT-PCR assays (fnRT-PCR) combine RT-PCR with an internal fluorogenic hybridization probe, thereby potentially enhancing specificity and eliminating post-PCR processing. Therefore, fnRT-PCR assays specific for Sendai virus and PVM were developed by targeting primer andprobe sequences to unique regions of the Sendai virus nucleocapsid (NP) gene and the PVM attachment (G) gene, respectively. The Sendai virus and PVM fnRT-PCR assays detected only Sendai virusand PVM , respectively. Neither assay detected other viruses of the family Paramyxoviridae or other RNA viruses that naturally infect rodents. The fnRT-PCR assays detected as little as 10 fg of Sendai virus RNA and one picogram of PVM RNA, respectively, andthe Sendai virus fnRT-PCR assay had comparable sensitivity when directly compared with the mouse antibody production test. The fnRT-PCR assays were also able to detect viral RNA in respiratory tract tissues and cage swipe specimens collected from experimentally inoculated C.B-17 severe combined immunodeficient mice, but did not detect viral RNA in age- and strain-matched mock-infected mice. In conclusion, these fnRT-PCR assays offer potentially high-throughput diagnostic assays to detect Sendai virus and PVM in immunodeficient mice, and to detect Sendai virus in contaminated biological materials.

Altered pathogenesis of severe pneumovirus infection in response to combined antiviral and specific immunomodulatory agents

We report here the responses of mice with symptomatic pneumovirus infection to combined antiviral and specific immunomodulatory agents. Mice infected with pneumonia virus of mice, a natural mouse pathogen that replicates the signs and symptoms of severe infection with respiratory syncytial virus (RSV), responded to the antiviral agent ribavirin when it was administered in the setting of endogenous (gene deletion) or exogenous (antibody-mediated) blockade of the MIP-1alpha proinflammatory signaling cascade. Although neither treatment is effective alone, together they offer a dramatic reduction in symptoms and pathology, the most impressive of which is a significant reduction in morbidity and mortality. The findings presented are consistent with the notion of unique and independent contributions of virus replication and ongoing inflammation to the pathogenesis of severe respiratory virus infection, and they provide the impetus for the study of this treatment regimen in RSV-infected humans.

IgG isolated from LP-BM5 infected mouse brain activates ionotropic glutamate receptors

Biochemical and immunological studies have shown that mice infected with LP-BM5 virus develop antibodies to ionotropic glutamate receptors. Here, IgG isolated from brain of infected mice has been tested electrophysiologically on cultured rat cortical and hippocampal neurons. The IgG elicited glycine-independent currents that reversed at approximately 0 mV. Equivalent concentrations of IgG from uninfected mice were inactive. The glycine-independent currents were less influenced by DNQX and GYKI-52466 than currents elicited by AMPA and KA. The IgG also elicited glycine-dependent currents that reversed at -10 mV and were blocked by dl-AP5, 5,7-DCKA, and polyamine amides. Glycine-dependent and -independent currents were unaffected by tetrodotoxin, strychnine, the transmembrane Cl- gradient or d-tubocurare. Although part of the glycine-independent current remains uncharacterized, these results confirm that a virus-induced immunopathology produces IgG clones that activate ionotropic glutamate receptors and that could, thereby, contribute to the excitotoxic neurological syndrome observed in LP-BM5-infected mice.