An improved plaque reduction virus neutralization assay for human metapneumovirus

The role of M2 proteins of pneumoviruses in transcription regulation, prevention of hypermutation, and activation of the type I interferon pathway

Human metapneumovirus (HMPV) is an important causative agent of respiratory tract disease. Fundamental knowledge of the interaction between HMPV and the innate immune system could lead to the design of novel antiviral therapies. Previously, we demonstrated that HMPV M2-2 deletion mutants had hypermutated genomes and contained defective interfering particles (DIs), which are potent inducers of the IFN response. Here, we investigated the role of the HMPV M2-2 protein as IFN antagonist using chimeric HMPV expressing M2 proteins of other pneumoviruses: respiratory syncytial virus (RSV) and avian metapneumovirus type C (AMPV/C). Chimeric HMPVs expressing the M2 proteins of RSV or AMPV/C were attenuated in HEp-2 cells but did not activate the IFN response, and their genomes were not hypermutated. In contrast, chimeric HMPVs expressing the M2-2 proteins of RSV and AMPV/C, in combination with HMPV M2-1, did activate the IFN response, and their genomes were hypermutated. Investigation of the role of the pneumovirus M2 proteins in transcription regulation demonstrated that the M2-2 protein, only in concerted action with autologous M2-1 protein, acted as a transcription elongation factor. As a second approach, chimeric RSV in which the IFN antagonists NS1 and NS2 were replaced by the HMPV M2-2 gene failed to suppress an IFN response, indicating that the HMPV M2-2 protein is not a potent IFN antagonist. These data indicate that expression of autologous M2-1 and M2-2 proteins is important for the fidelity of the RNA-dependent RNA polymerase, necessary to prevent the accumulation of mutations, and possibly DIs, thereby preventing activation of the IFN responses.IMPORTANCEThe M2-2 protein of human metapneumovirus is suggested to function as a type I interferon antagonist, a function so far not assigned to the M2 proteins of other pneumoviruses. Although M2-2 deletion mutants of HMPV activate the type I interferon pathway, these mutants have hypermutated genomes and contain defective interfering RNAs, known to activate the interferon pathway. Here, we show that the M2-2 protein, in concerted action with autologous M2-1 protein, acts as a transcription elongation factor, which could explain the accumulation of DIs in M2-2 deletion mutants. Additionally, chimeric RSV in which the IFN antagonists NS1 and NS2 were replaced by the HMPV M2-2 gene failed to suppress an IFN response. These data indicate that expression of autologous M2-1 and M2-2 proteins is required for the fidelity of the RNA-dependent RNA polymerase to prevent genome hypermutation and activation of the type I IFN pathway.

A neutralizing single-domain antibody that targets the trimer interface of the human metapneumovirus fusion protein

IMPORTANCE

Human metapneumovirus (hMPV) is an important respiratory pathogen for which no licensed antivirals or vaccines exist. Single-domain antibodies represent promising antiviral biologics that can be easily produced and formatted. We describe the isolation and detailed characterization of two hMPV-neutralizing single-domain antibodies that are directed against the fusion protein F. One of these single-domain antibodies broadly neutralizes hMPV A and B strains, can prevent proteolytic maturation of F, and binds to an epitope in the F trimer interface. This suggests that hMPV pre-F undergoes trimer opening or "breathing" on infectious virions, exposing a vulnerable site for neutralizing antibodies. Finally, we show that this single-domain antibody, fused to a human IgG1 Fc, can protect cotton rats against hMPV replication, an important finding for potential future clinical applications.

Continued adaptation of A/H2N2 viruses during pandemic circulation in humans

Influenza A viruses of the H2N2 subtype sparked a pandemic in 1957 and circulated in humans until 1968. Because A/H2N2 viruses still circulate in wild birds worldwide and human population immunity is low, the transmissibility of six avian A/H2N2 viruses was investigated in the ferret model. None of the avian A/H2N2 viruses was transmitted between ferrets, suggesting that their pandemic risk may be low. The transmissibility, receptor binding preference and haemagglutinin (HA) stability of human A/H2N2 viruses were also investigated. Human A/H2N2 viruses from 1957 and 1958 bound to human-type α2,6-linked sialic acid receptors, but the 1958 virus had a more stable HA, indicating adaptation to replication and spread in the new host. This increased stability was caused by a previously unknown stability substitution G205S in the 1958 H2N2 HA, which became fixed in A/H2N2 viruses after 1958. Although individual substitutions were identified that affected the HA receptor binding and stability properties, they were not found to have a substantial effect on transmissibility of A/H2N2 viruses via the air in the ferret model. Our data demonstrate that A/H2N2 viruses continued to adapt during the first year of pandemic circulation in humans, similar to what was previously shown for the A/H1N1pdm09 virus.

Fetal Transfer of Human Metapneumovirus-Neutralizing Antibodies Is Reduced From Mothers Living With HIV-1

Transplacental hMPV-neutralizing antibody transfer was reduced from mothers living with HIV-1. However, a comparison of antibody titers at birth between hMPV hospitalization cases at <6 months and matched controls suggested that reduced maternal antibody might not be the primary cause of the previously reported elevated hMPV risk in HIV-1-exposed infants.

Comparison of three air samplers for the collection of four nebulized respiratory viruses - Collection of respiratory viruses from air

Viral respiratory tract infections are a leading cause of morbidity and mortality worldwide. Unfortunately, the transmission routes and shedding kinetics of respiratory viruses remain poorly understood. Air sampling techniques to quantify infectious viruses in the air are indispensable to improve intervention strategies to control and prevent spreading of respiratory viruses. Here, the collection of infectious virus with the six-stage Andersen cascade impactor was optimized with semi-solid gelatin as collection surface. Subsequently, the collection efficiency of the cascade impactor, the SKC BioSampler, and an in-house developed electrostatic precipitator was compared. In an in vitro set-up, influenza A virus, human metapneumovirus, parainfluenza virus type 3, and respiratory syncytial virus were nebulized and the amount of collected infectious virus and viral RNA was quantified with each air sampler. Whereas only low amounts of virus were collected using the electrostatic precipitator, high amounts were collected with the BioSampler and cascade impactor. The BioSampler allowed straight-forward sampling in liquid medium, whereas the more laborious cascade impactor allowed size fractionation of virus-containing particles. Depending on the research question, either the BioSampler or the cascade impactor can be applied in laboratory and field settings, such as hospitals to gain more insight into the transmission routes of respiratory viruses.

Safe-in-Man Broad Spectrum Antiviral Agents

Emerging and re-emerging viral diseases occur with regularity within the human population. The conventional 'one drug, one virus' paradigm for antivirals does not adequately allow for proper preparedness in the face of unknown future epidemics. In addition, drug developers lack the financial incentives to work on antiviral drug discovery, with most pharmaceutical companies choosing to focus on more profitable disease areas. Safe-in-man broad spectrum antiviral agents (BSAAs) can help meet the need for antiviral development by already having passed phase I clinical trials, requiring less time and money to develop, and having the capacity to work against many viruses, allowing for a speedy response when unforeseen epidemics arise. In this chapter, we discuss the benefits of repurposing existing drugs as BSAAs, describe the major steps in safe-in-man BSAA drug development from discovery through clinical trials, and list several database resources that are useful tools for antiviral drug repositioning.

Human Paramyxovirus Infections Induce T Cells That Cross-React with Zoonotic Henipaviruses

Humans are infected with paramyxoviruses of different genera early in life, which induce cytotoxic T cells that may recognize conserved epitopes. This raises the question of whether cross-reactive T cells induced by antecedent paramyxovirus infections provide partial protection against highly lethal zoonotic Nipah virus infections. By characterizing a measles virus-specific but paramyxovirus cross-reactive human T cell clone, we discovered a highly conserved HLA-B*1501-restricted T cell epitope in the fusion protein. Using peptides, tetramers, and single cell sorting, we isolated a parainfluenza virus-specific T cell clone from a healthy adult and showed that both clones cleared Nipah virus-infected cells. We identified multiple conserved hot spots in paramyxovirus proteomes that contain other potentially cross-reactive epitopes. Our data suggest that, depending on HLA haplotype and history of paramyxovirus exposures, humans may have cross-reactive T cells that provide protection against Nipah virus. The effect of preferential boosting of these cross-reactive epitopes needs to be further studied in light of paramyxovirus vaccination studies.IMPORTANCE Humans encounter multiple paramyxoviruses early in life. This study shows that infection with common paramyxoviruses can induce T cells cross-reactive with the highly pathogenic Nipah virus. This demonstrates that the combination of paramyxovirus infection history and HLA haplotype affects immunity to phylogenetically related zoonotic paramyxoviruses.

Advances in laboratory assays for detecting human metapneumovirus

Human metapneumovirus (HMPV) is one of the major causes of acute respiratory tract infection (ARI) and shows high morbidity and mortality, particularly in children and immunocompromised patients. Various methods for detecting HMPV have been developed and applied in clinical laboratories. When reviewing the literature, we found that polymerase chain reaction (PCR)-based assays have been most frequently and consistently used to detect HMPV. The most commonly used method was multiplex reverse transcriptase-PCR (RT-PCR; 57.4%), followed by real-time RT-PCR (38.3%). Multiplex RT-PCR became the more popular method in 2011-2019 (69.7%), in contrast to 2001-2009 (28.6%). The advent of multiplex PCR in detecting broader viral pathogens in one run and coinfected viruses influenced the change in user preference. Further, newly developed microarray technologies and ionization mass spectrometry were introduced in 2011-2019. Viral culture (including shell vial assays) and fluorescent immunoassays (with or without culture) were once the mainstays. However, the percentage of studies employing culture and fluorescent immunoassays decreased from 21.4% in 2001-2010 to 15.2% in 2011-2019. Meanwhile, the use of PCR-based methods of HMPV detection increased from 78.6% in 2001-2010 to 84.8% in 2011-2019. The increase in PCR-based methods might have occurred because PCR methods demonstrated better diagnostic performance, shorter hands-on and run times, less hazards to laboratory personnel, and more reliable results than traditional methods. When using these assays, it is important to acquire a comprehensive understanding of the principles, advantages, disadvantages, and precautions for data interpretation. In the future, the combination of nanotechnology and advanced genetic platforms such as next-generation sequencing will benefit patients with HMPV infection by facilitating efficient therapeutic intervention. Analytical and clinical validation are required before using new techniques in clinical laboratories.

Discovery and development of safe-in-man broad-spectrum antiviral agents

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We reviewed the discovery and development process of broad-spectrum antiviral agents.

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We summarized the information on 120 safe-in-man agents in a freely accessible database.

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Further studies will increase the number of broad-spectrum antivirals, expand the spectrum of their indications, and identify drug combinations for treatment of emerging and re-emerging viral infections.

Viral diseases are one of the leading causes of morbidity and mortality in the world. Virus-specific vaccines and antiviral drugs are the most powerful tools to combat viral diseases. However, broad-spectrum antiviral agents (BSAAs, i.e. compounds targeting viruses belonging to two or more viral families) could provide additional protection of the general population from emerging and re-emerging viral diseases, reinforcing the arsenal of available antiviral options. Here, we review discovery and development of BSAAs and summarize the information on 120 safe-in-man agents in a freely accessible database (https://drugvirus.info/). Future and ongoing pre-clinical and clinical studies will increase the number of BSAAs, expand the spectrum of their indications, and identify drug combinations for treatment of emerging and re-emerging viral infections as well as co-infections.

Novel Antiviral Activities of Obatoclax, Emetine, Niclosamide, Brequinar, and Homoharringtonine

Viruses are the major causes of acute and chronic infectious diseases in the world. According to the World Health Organization, there is an urgent need for better control of viral diseases. Repurposing existing antiviral agents from one viral disease to another could play a pivotal role in this process. Here, we identified novel activities of obatoclax and emetine against herpes simplex virus type 2 (HSV-2), echovirus 1 (EV1), human metapneumovirus (HMPV) and Rift Valley fever virus (RVFV) in cell cultures. Moreover, we demonstrated novel activities of emetine against influenza A virus (FLUAV), niclosamide against HSV-2, brequinar against human immunodeficiency virus 1 (HIV-1), and homoharringtonine against EV1. Our findings may expand the spectrum of indications of these safe-in-man agents and reinforce the arsenal of available antiviral therapeutics pending the results of further in vitro and in vivo tests.

Common Nodes of Virus-Host Interaction Revealed Through an Integrated Network Analysis

Viruses are one of the major causes of acute and chronic infectious diseases and thus a major contributor to the global burden of disease. Several studies have shown how viruses have evolved to hijack basic cellular pathways and evade innate immune response by modulating key host factors and signaling pathways. A collective view of these multiple studies could advance our understanding of virus-host interactions and provide new therapeutic perspectives for the treatment of viral diseases. Here, we performed an integrative meta-analysis to elucidate the 17 different host-virus interactomes. Network and bioinformatics analyses showed how viruses with small genomes efficiently achieve the maximal effect by targeting multifunctional and highly connected host proteins with a high occurrence of disordered regions. We also identified the core cellular process subnetworks that are targeted by all the viruses. Integration with functional RNA interference (RNAi) datasets showed that a large proportion of the targets are required for viral replication. Furthermore, we performed an interactome-informed drug re-purposing screen and identified novel activities for broad-spectrum antiviral agents against hepatitis C virus and human metapneumovirus. Altogether, these orthogonal datasets could serve as a platform for hypothesis generation and follow-up studies to broaden our understanding of the viral evasion landscape.

Consensus and variations in cell line specificity among human metapneumovirus strains

Human metapneumovirus (HMPV) has been a notable etiological agent of acute respiratory infection in humans, but it was not discovered until 2001, because HMPV replicates only in a limited number of cell lines and the cytopathic effect (CPE) is often mild. To promote the study of HMPV, several groups have generated green fluorescent protein (GFP)-expressing recombinant HMPV strains (HMPV^GFP). However, the growing evidence has complicated the understanding of cell line specificity of HMPV, because it seems to vary notably among HMPV strains. In addition, unique A2b clade HMPV strains with a 180-nucleotide duplication in the G gene (HMPV A2b_180nt-dup strains) have recently been detected. In this study, we re-evaluated and compared the cell line specificity of clinical isolates of HMPV strains, including the novel HMPV A2b_180nt-dup strains, and six recombinant HMPV^GFP strains, including the newly generated recombinant HMPV A2b_180nt-dup strain, MG0256-EGFP. Our data demonstrate that VeroE6 and LLC-MK2 cells generally showed the highest infectivity with any clinical isolates and recombinant HMPV^GFP strains. Other human-derived cell lines (BEAS-2B, A549, HEK293, MNT-1, and HeLa cells) showed certain levels of infectivity with HMPV, but these were significantly lower than those of VeroE6 and LLC-MK2 cells. Also, the infectivity in these suboptimal cell lines varied greatly among HMPV strains. The variations were not directly related to HMPV genotypes, cell lines used for isolation and propagation, specific genome mutations, or nucleotide duplications in the G gene. Thus, these variations in suboptimal cell lines are likely intrinsic to particular HMPV strains.

Interferon-Induced Transmembrane Protein 1 Restricts Replication of Viruses That Enter Cells via the Plasma Membrane

The acute antiviral response is mediated by a family of interferon-stimulated genes (ISGs), providing cell-intrinsic immunity. Mutations in genes encoding these proteins are often associated with increased susceptibility to viral infections. One family of ISGs with antiviral function is the interferon-inducible transmembrane proteins (IFITMs), of which IFITM3 has been studied extensively. In contrast, IFITM1 has not been studied in detail. Since IFITM1 can localize to the plasma membrane, we investigated its function with a range of enveloped viruses thought to infect cells by fusion with the plasma membrane. Overexpression of IFITM1 prevented infection by a number of Paramyxoviridae and Pneumoviridae, including respiratory syncytial virus (RSV), mumps virus, and human metapneumovirus (HMPV). IFITM1 also restricted infection with an enveloped DNA virus that can enter via the plasma membrane, herpes simplex virus 1 (HSV-1). To test the importance of plasma membrane localization for IFITM1 function, we identified blocks of amino acids in the conserved intracellular loop (CIL) domain that altered the subcellular localization of the protein and reduced antiviral activity. By screening reported data sets, 12 rare nonsynonymous single nucleotide polymorphisms (SNPs) were identified in human IFITM1, some of which are in the CIL domain. Using an Ifitm1-/- mouse, we show that RSV infection was more severe, thereby extending the range of viruses restricted in vivo by IFITM proteins and suggesting overall that IFITM1 is broadly antiviral and that this antiviral function is associated with cell surface localization.IMPORTANCE Host susceptibility to viral infection is multifactorial, but early control of viruses not previously encountered is predominantly mediated by the interferon-stimulated gene (ISG) family. There are upwards of 300 of these genes, the majority of which do not have a clearly defined function or mechanism of action. The cellular location of these proteins may have an important effect on their function. One ISG located at the plasma membrane is interferon-inducible transmembrane protein 1 (IFITM1). Here we demonstrate that IFITM1 can inhibit infection with a range of viruses that enter via the plasma membrane. Mutant IFITM1 proteins that were unable to localize to the plasma membrane did not restrict viral infection. We also observed for the first time that IFITM1 plays a role in vivo, and Ifitm1-/- mice were more susceptible to viral lung infection. These data contribute to our understanding of how ISGs prevent viral infections.

Development and optimization of a direct plaque assay for trypsin-dependent human metapneumovirus strains

Human metapneumovirus (HMPV) is a non-segmented, negative strand RNA virus belonging to the family Pneumoviridae, previously a subfamily of Paramyxoviridae. It is a leading cause of lower respiratory tract infection in infants, children, and adults with underlying medical conditions. HMPV grows poorly in cell culture and requires trypsin to cleave and mature the virus particles, which adds to the challenge of HMPV research. Currently, an indirect immuno-staining assay is commonly used to titrate HMPV, which is time-consuming and costly. In order to simplify virus quantification for HMPV, a direct plaque assay was developed. By optimizing trypsin concentration and other supplements in the agarose overlay, it was found that HMPV strains from all four subgroups formed clear and countable plaques 5-7 days post-infection. Animal tissue homogenate can also be directly titrated with this assay. Compared with the traditional assay, the direct plaque assay yields similar titer result, but saves time and eliminates the use of antibodies. Potentially, it can also be applied to plaque purification for HMPV clinical isolates. The direct plaque assay will be a valuable tool in HMPV research.

Paramyxovirus Infections in Ex Vivo Lung Slice Cultures of Different Host Species

In vivo experiments in animal models of disease are of crucial importance for viral tropism and pathogenesis studies. However, these experiments must be complemented with in vitro and ex vivo experiments. Here, we describe a protocol for the preparation and ex vivo infection of lung slices from different mammalian host species with various respiratory paramyxoviruses expressing fluorescent reporter proteins, and suggest follow-up experiments including immunohistochemistry, flow cytometry and confocal microscopy.

Fluorescent and Bioluminescent Reporter Myxoviruses

The advent of virus reverse genetics has enabled the incorporation of genetically encoded reporter proteins into replication-competent viruses. These reporters include fluorescent proteins which have intrinsic chromophores that absorb light and re-emit it at lower wavelengths, and bioluminescent proteins which are luciferase enzymes that react with substrates to produce visible light. The incorporation of these reporters into replication-competent viruses has revolutionized our understanding of molecular virology and aspects of viral tropism and transmission. Reporter viruses have also enabled the development of high-throughput assays to screen antiviral compounds and antibodies and to perform neutralization assays. However, there remain technical challenges with the design of replication-competent reporter viruses, and each reporter has unique advantages and disadvantages for specific applications. This review describes currently available reporters, design strategies for incorporating reporters into replication-competent paramyxoviruses and orthomyxoviruses, and the variety of applications for which these tools can be utilized both in vitro and in vivo.

Antiviral Activity of Favipiravir (T-705) against a Broad Range of Paramyxoviruses In Vitro and against Human Metapneumovirus in Hamsters

The clinical impact of infections with respiratory viruses belonging to the family Paramyxoviridae argues for the development of antiviral therapies with broad-spectrum activity. Favipiravir (T-705) has demonstrated potent antiviral activity against multiple RNA virus families and is presently in clinical evaluation for the treatment of influenza. Here we demonstrate in vitro activity of T-705 against the paramyxoviruses human metapneumovirus (HMPV), respiratory syncytial virus, human parainfluenza virus, measles virus, Newcastle disease virus, and avian metapneumovirus. In addition, we demonstrate activity against HMPV in hamsters. T-705 treatment inhibited replication of all paramyxoviruses tested in vitro, with 90% effective concentration (EC90) values of 8 to 40 μM. Treatment of HMPV-challenged hamsters with T-705 at 200 mg/kg of body weight/day resulted in 100% protection from infection of the lungs. In all treated and challenged animals, viral RNA remained detectable in the respiratory tract. The observation that T-705 treatment had a significant effect on infectious viral titers, with a limited effect on viral genome titers, is in agreement with its proposed mode of action of viral mutagenesis. However, next-generation sequencing of viral genomes isolated from treated and challenged hamsters did not reveal (hyper)mutation. Polymerase activity assays revealed a specific effect of T-705 on the activity of the HMPV polymerase. With the reported antiviral activity of T-705 against a broad range of RNA virus families, this small molecule is a promising broad-range antiviral drug candidate for limiting the viral burden of paramyxoviruses and for evaluation for treatment of infections with (re)emerging viruses, such as the henipaviruses.

Effect of in vitro syncytium formation on the severity of human metapneumovirus disease in a murine model

Human metapneumovirus (HMPV) is an important cause of acute respiratory tract infections (ARTI) in children, elderly individuals and immunocompromised patients. In vitro, different HMPV strains can induce variable cytopathic effects ranging from large multinucleated syncytia to focal cell rounding. In this study, we investigated the impact of different in vitro phenotypes of two HMPV strains on viral replication and disease severity in a BALB/c mouse model. We first generated two recombinant GFP-expressing HMPV viruses: C-85473, a syncytium-inducing strain (rC-85473) belonging to the A1 subtype and CAN98-75, a focal cell rounding-inducing strain (rCAN98-75) of the B2 subtype. We subsequently exchanged the F genes of both strains to create the chimeric viruses rC-85473_F and rCAN98-75_F. We demonstrated that the F protein was the sole protein responsible for the syncytium phenotype and that viruses carrying a syncytium-inducing F protein replicated to significantly higher titers in vitro. In vivo, however, the virulence and replicative capacity of the different HMPV strains did not appear to be solely dependent on the F gene but also on the viral background, with the strains containing the C-85473 background inducing more weight loss as well as increased lung viral titers, pro-inflammatory cytokines and inflammation than strains containing the CAN98-75 background. In conclusion, the F protein is the main determinant of syncytium formation and replication kinetics in vitro, although it is not the only factor implicated in HMPV disease severity in mice.

Roles of the putative integrin-binding motif of the human metapneumovirus fusion (f) protein in cell-cell fusion, viral infectivity, and pathogenesis

Human metapneumovirus (hMPV) is a relatively recently identified paramyxovirus that causes acute upper and lower respiratory tract infection. Entry of hMPV is unusual among the paramyxoviruses, in that fusion is accomplished by the fusion (F) protein without the attachment glycoprotein (G protein). It has been suggested that hMPV F protein utilizes integrin αvβ1 as a cellular receptor. Consistent with this, the F proteins of all known hMPV strains possess an integrin-binding motif ((329)RGD(331)). The role of this motif in viral entry, infectivity, and pathogenesis is poorly understood. Here, we show that α5β1 and αv integrins are essential for cell-cell fusion and hMPV infection. Mutational analysis found that residues R329 and G330 in the (329)RGD(331) motif are essential for cell-cell fusion, whereas mutations at D331 did not significantly impact fusion activity. Furthermore, fusion-defective RGD mutations were either lethal to the virus or resulted in recombinant hMPVs that had defects in viral replication in cell culture. In cotton rats, recombinant hMPV with the R329K mutation in the F protein (rhMPV-R329K) and rhMPV-D331A exhibited significant defects in viral replication in nasal turbinates and lungs. Importantly, inoculation of cotton rats with these mutants triggered a high level of neutralizing antibodies and protected against hMPV challenge. Taken together, our data indicate that (i) α5β1 and αv integrins are essential for cell-cell fusion and viral replication, (ii) the first two residues in the RGD motif are essential for fusion activity, and (iii) inhibition of the interaction of the integrin-RGD motif may serve as a new target to rationally attenuate hMPV for the development of live attenuated vaccines.

IMPORTANCE

Human metapneumovirus (hMPV) is one of the major causative agents of acute respiratory disease in humans. Currently, there is no vaccine or antiviral drug for hMPV. hMPV enters host cells via a unique mechanism, in that viral fusion (F) protein mediates both attachment and fusion activity. Recently, it was suggested that hMPV F protein utilizes integrins as receptors for entry via a poorly understood mechanism. Here, we show that α5β1 and αv integrins are essential for hMPV infectivity and F protein-mediated cell-cell fusion and that the integrin-binding motif in the F protein plays a crucial role in these functions. Our results also identify the integrin-binding motif to be a new, attenuating target for the development of a live vaccine for hMPV. These findings not only will facilitate the development of antiviral drugs targeting viral entry steps but also will lead to the development new live attenuated vaccine candidates for hMPV.

Human metapneumovirus keeps dendritic cells from priming antigen-specific naive T cells

Human metapneumovirus (hMPV) is the second most common cause of acute lower respiratory tract infections in children, causing a significant public health burden worldwide. Given that hMPV can repeatedly infect the host without major antigenic changes, it has been suggested that hMPV may have evolved molecular mechanisms to impair host adaptive immunity and, more specifically, T-cell memory. Recent studies have shown that hMPV can interfere with superantigen-induced T-cell activation by infecting conventional dendritic cells (DCs). Here, we show that hMPV infects mouse DCs in a restricted manner and induces moderate maturation. Nonetheless, hMPV-infected DCs are rendered inefficient at activating naive antigen-specific CD4(+) T cells (OT-II), which not only display reduced proliferation, but also show a marked reduction in surface activation markers and interleukin-2 secretion. Decreased T-cell activation was not mediated by interference with DC-T-cell immunological synapse formation as recently described for the human respiratory syncytial virus (hRSV), but rather by soluble factors secreted by hMPV-infected DCs. These data suggest that although hMPV infection is restricted within DCs, it is sufficient to interfere with their capacity to activate naive T cells. Altogether, by interfering with DC function and productive priming of antigen-inexperienced T cells, hMPV could impair the generation of long-term immunity.

Small hydrophobic protein of human metapneumovirus does not affect virus replication and host gene expression in vitro

Human metapneumovirus (HMPV) encodes a small hydrophobic (SH) protein of unknown function. HMPV from which the SH open reading frame was deleted (HMPVΔSH) was viable and displayed similar replication kinetics, cytopathic effect and plaque size compared with wild type HMPV in several cell-lines. In addition, no differences were observed in infection efficiency or cell-to-cell spreading in human primary bronchial epithelial cells (HPBEC) cultured at an air-liquid interphase. Host gene expression was analyzed in A549 cells infected with HMPV or HMPVΔSH using microarrays and mass spectrometry (MS) based techniques at multiple time points post infection. Only minor differences were observed in mRNA or protein expression levels. A possible function of HMPV SH as apoptosis blocker, as proposed for several members of the family Paramyxoviridae, was rejected based on this analysis. So far, a clear phenotype of HMPV SH deletion mutants in vitro at the virus and host levels is absent.

An improved respiratory syncytial virus neutralization assay based on the detection of green fluorescent protein expression and automated plaque counting

Background

Virus neutralizing antibodies against respiratory syncytial virus (RSV) are considered important correlates of protection for vaccine evaluation. The established plaque reduction assay is time consuming, labor intensive and highly variable.

Methods

Here, a neutralization assay based on a modified RSV strain expressing the green fluorescent protein in combination with automated detection and quantification of plaques is described.

Results

The fluorescence plaque reduction assay in microplate format requires only two days to complete and is simple and reproducible. A good correlation between visual and automated counting methods to determine RSV neutralizing serum antibody titers was observed.

Conclusions

The developed virus neutralization assay is suitable for high-throughput testing and can be used for both animal studies and (large scale) vaccine clinical trials.

Expeditious neutralization assay for human metapneumovirus based on a recombinant virus expressing Renilla luciferase

BACKGROUND

Human metapneumovirus (HMPV) is a common cause of respiratory diseases in persons of all ages. Because of its slow replication and weak cytopathic effect in cultured cells, conventional neutralization assays for HMPV require around one week for completion.

OBJECTIVES

The purpose of this study is to establish a rapid neutralization assay based on a recombinant virus expressing Renilla luciferase (Rluc).

STUDY DESIGN

A recombinant HMPV expressing both Rluc and green fluorescent protein (GFP) was created by reverse genetics method. Two-fold serial dilutions of human 23 sera were made in a 96-well plate and incubated with 50 pfu/well of the recombinant virus at 4°C for 1 h. The mixtures were then transferred to LLC-MK2 cells in a 96-well plate, incubated for 2 h, and replaced with trypsin-free fresh media. After incubation at 32°C for 24 h, the cells were lysed and measured for Rluc activity. The neutralization titer was defined as the reciprocal of the highest serum dilution that resulted in 50% reduction of Rluc activity.

RESULTS

The novel assay could be completed within 24 h and eliminated the requirement of trypsin supporting multistep replication in cultured cells, as well as laborious processes including the plaque assay with immunostaining. Neutralization titers correlated well with those determined by a GFP-based assay previously developed.

CONCLUSIONS

The neutralization assay based on Rluc activity is the fastest and the most straightforward of all previous assays, and may be available for high throughput screening of neutralizing antibodies.

Human Metapneumovirus: lessons learned over the first decade

It has been 10 years since human metapneumovirus (HMPV) was identified as a causative agent of respiratory illness in humans. Since then, numerous studies have contributed to a substantial body of knowledge on many aspects of HMPV. This review summarizes our current knowledge on HMPV, HMPV disease pathogenesis, and disease intervention strategies and identifies a number of areas with key questions to be addressed in the future.

Physical analysis of virus particles using electrospray differential mobility analysis

This review critically examines an emerging tool to measure viral clearance from biomanufacturing streams, monitor assembly of viruses and virus-like particles, rapidly identify viruses from biological milieu, assay virus neutralization, and prepare bionanoconjugates for bacterial detection. Electrospray differential mobility analysis (ES-DMA) is a tool of choice to simultaneously determine viral size and concentration because it provides full multimodal size distributions with subnanometer precision from individual capsid proteins to intact viral particles. The review contrasts ES-DMA to similar tools and highlights expected growth areas including at-line process sensing as a process analytical technology (PAT), bioseparating as a distinct unit operation, monitoring viral reactions, and interrogating virus-host protein interactions.

Residues of the human metapneumovirus fusion (F) protein critical for its strain-related fusion phenotype: implications for the virus replication cycle

The paramyxovirus F protein promotes fusion of the viral and cell membranes for virus entry, as well as cell-cell fusion for syncytium formation. Most paramyxovirus F proteins are triggered at neutral pH to initiate membrane fusion. Previous studies, however, demonstrated that human metapneumovirus (hMPV) F proteins are triggered at neutral or acidic pH in transfected cells, depending on the strain origin of the F sequences (S. Herfst et al., J. Virol. 82:8891-8895, 2008). We now report an extensive mutational analysis which identifies four variable residues (294, 296, 396, and 404) as the main determinants of the different syncytial phenotypes found among hMPV F proteins. These residues lie near two conserved histidines (H368 and H435) in a three-dimensional (3D) model of the pretriggered hMPV F trimer. Mutagenesis of H368 and H435 indicates that protonation of these histidines (particularly His435) is a key event to destabilize the hMPV F proteins that require low pH for cell-cell fusion. The syncytial phenotypes were reproduced in cells infected with the corresponding hMPV strains. However, the low-pH dependency for syncytium formation could not be related with a virus entry pathway dependent on an acidic environment. It is postulated that low pH may be acting for some hMPV strains as certain destabilizing mutations found in unusual strains of other paramyxoviruses. In any case, the results presented here and those reported by Schowalter et al. (J. Virol. 83:1511-1522, 2009) highlight the relevance of certain residues in the linker region and domain II of the pretriggered hMPV F protein for the process of membrane fusion.

Streptococcus pneumoniae exposure is associated with human metapneumovirus seroconversion and increased susceptibility to in vitro HMPV infection

It remains largely unknown which factors determine the clinical outcome of human metapneumovirus (HMPV) infections. The aim of the present study was to analyse whether exposure to bacterial pathogens can influence HMPV infections. From 57 children, serum samples and colonization data for Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus and Streptococcus pneumoniae were collected at 1.5, 6, 14 and 24 months of age. Seroconversion rates to HMPV were determined and related to bacterial carriage. Frequent nasopharyngeal carriage (≥2 times in the first 2 years of life) of S. pneumoniae, but not of the other three pathogens, was associated with increased seroconversion rates of infants to HMPV at the age of 2 years (frequently vs. less exposed, 93% vs. 59%; p <0.05). Subsequently, the susceptibility of well-differentiated normal human bronchial epithelial cells (wd-NHBE) pre-incubated with bacterial pathogens to in vitro HMPV infection was evaluated. Pre-incubation of wd-NHBE with S. pneumoniae resulted in increased susceptibility to infection with HMPV-enhanced green fluorescent protein (EGFP), as determined by enumeration of EGFP-positive cells. This was not the case for cells pre-incubated with H. influenzae, M. catarrhalis on S. aureus. We conclude that exposure to S. pneumoniae can modulate HMPV infection.

Effects of N-linked glycosylation of the fusion protein on replication of human metapneumovirus in vitro and in mouse lungs

The fusion (F) protein is an important membrane glycoprotein necessary for cellular entry and replication of human metapneumovirus (hMPV). Selective prevention of N-linked glycosylation may compromise the catalytic and fusion functions of the F protein. By using site-directed mutagenesis and reverse genetics, recombinant mutant viruses lacking one or two N-linked glycosylation sites in the F protein were constructed. M1, which lacked glycosylation at position 57 of the F protein, had slightly compromised replication, whereas M2 and M4, which lacked glycosylation at position(s) 172 or 57 and 172, respectively, showed profound impairment of replication when compared with wild-type (WT) NL/1/00-GFP virus in both Vero E6 cells and mouse lungs. M2 was less fit than WT virus in vitro and in immunocompromised mouse lungs. The F proteins of WT and mutant viruses were similarly expressed on the infected cell membrane, while the activated fusion protein subunits, F1 of M2 and M4, were produced in lower quantities compared with those of WT and M1 virus. The mutated viruses lacking N-linked glycosylation at position 353, either individually or together with other sites, could not be recovered. Thus, N-linked glycosylation may be involved in the catalysis of the fusion protein from F0 to F1 and F2, which is critical for fusion function. Strategies targeting N-linked glycosylation may be helpful for developing attenuated live vaccines or antiviral drugs for hMPV.

The synthetic bacterial lipopeptide Pam3CSK4 modulates respiratory syncytial virus infection independent of TLR activation

Respiratory syncytial virus (RSV) is an important cause of acute respiratory disease in infants, immunocompromised subjects and the elderly. However, it is unclear why most primary RSV infections are associated with relatively mild symptoms, whereas some result in severe lower respiratory tract infections and bronchiolitis. Since RSV hospitalization has been associated with respiratory bacterial co-infections, we have tested if bacterial Toll-like receptor (TLR) agonists influence RSV-A2-GFP infection in human primary cells or cell lines. The synthetic bacterial lipopeptide Pam3-Cys-Ser-Lys4 (Pam3CSK4), the prototype ligand for the heterodimeric TLR1/TLR2 complex, enhanced RSV infection in primary epithelial, myeloid and lymphoid cells. Surprisingly, enhancement was optimal when lipopeptides and virus were added simultaneously, whereas addition of Pam3CSK4 immediately after infection had no effect. We have identified two structurally related lipopeptides without TLR-signaling capacity that also modulate RSV infection, whereas Pam3CSK4-reminiscent TLR1/2 agonists did not, and conclude that modulation of infection is independent of TLR activation. A similar TLR-independent enhancement of infection could also be demonstrated for wild-type RSV strains, and for HIV-1, measles virus and human metapneumovirus. We show that the effect of Pam3CSK4 is primarily mediated by enhanced binding of RSV to its target cells. The N-palmitoylated cysteine and the cationic lysines were identified as pivotal for enhanced virus binding. Surprisingly, we observed inhibition of RSV infection in immortalized epithelial cell lines, which was shown to be related to interactions between Pam3CSK4 and negatively charged glycosaminoglycans on these cells, which are known targets for binding of laboratory-adapted but not wild-type RSV. These data suggest a potential role for bacterial lipopeptides in enhanced binding of RSV and other viruses to their target cells, thus affecting viral entry or spread independent of TLR signaling. Moreover, our results also suggest a potential application for these synthetic lipopeptides as adjuvants for live-attenuated viral vaccines.

Respiratory syncytial virus (RSV) infections are an important cause of hospitalization of infants during the winter season. However, RSV is often not the only detectable pathogen, but co-infections with respiratory bacteria are common. It has been hypothesized that this results from epithelial damage caused by the virus, facilitating colonization by pathogenic bacteria such as Streptococcus pneumoniae. However, an inverse order of events is not impossible: bacterial infections may activate respiratory epithelial cells through TLR signaling, resulting in increased susceptibility to virus infections. We tested this hypothesis by screening bacterial TLR agonists for their capacity to modulate RSV infection in different cell types, and identified the lipopeptide and prototype TLR1/2 agonist Pam3CSK4 as an enhancer of RSV infections. However, to our surprise this proved independent of TLR activation, but was mediated by enhancement of binding between virus and target cell. Two structurally related lipopeptides unable to stimulate TLR responses were identified that enhanced infections with RSV, but also with other enveloped viruses including HIV-1, human metapneumovirus, and measles virus. We speculate that bacterial infections may influence the pathogenesis of virus infections by facilitating binding to target cells.

Microneutralization assay for the measurement of neutralizing antibodies to human metapneumovirus

BACKGROUND

Human metapneumovirus (hMPV) is a newly discovered virus which causes respiratory illness in persons of all ages.

OBJECTIVE

A simple and rapid method to determine neutralizing antibody titers against hMPV is needed to facilitate the development of vaccines and therapeutics for hMPV. Therefore, we sought to adapt the methodology used for RSV microneutralization assay (MNA) to measure neutralizing antibody titers against hMPV.

STUDY DESIGN

Serial 2-fold dilutions of serum were made in 96 well microtiter plates and incubated with approximately 50pfu of hMPV A or B strain for 60min at room temperature. LLC-MK2 cells were added to the serum-virus mixtures and plates incubated at 35 degrees C in CO(2) for 5 days. Plates were fixed with acetone; air dried, blocked and then developed with monoclonal antibody to the hMPV N protein followed by horse radish peroxidase labeled antibody and substrate. Neutralization titer was defined as the titer of serum that reduced color development by 50% compared to the positive control wells.

RESULTS

Titers measured by MNA correlated well with those determined by standard plaque reduction assay (R=0.77). Neutralization titers determined by MNA demonstrated excellent inter-assay variability (coefficient of variance=7%). In addition, there was good correlation of antibody titers from 10 hMPV infected adults measured by MNA using either group A or group B hMPV (R=0.87).

CONCLUSION

MNA is a simple and reproducible method for the measurement of serum neutralizing antibody against hMPV.

Generation of temperature-sensitive human metapneumovirus strains that provide protective immunity in hamsters

Human metapneumovirus (HMPV) causes acute respiratory tract illness primarily in young children, immunocompromised individuals and the elderly. Vaccines would be desirable to prevent severe illnesses in these risk groups. Here, we describe the generation and evaluation of cold-passage (cp) temperature-sensitive (ts) HMPV strains as vaccine candidates. Repeated passage of HMPV at low temperatures in Vero cells resulted in the accumulation of mutations in the viral genome. Introduction of these mutations in a recombinant HMPV by reverse genetics resulted in a ts-phenotype, judged on the decreased shut-off temperature for virus replication in vitro. As an alternative approach, three previously described cp-respiratory syncytial virus (cp-HRSV) mutations were introduced in a recombinant HMPV, which also resulted in a low shut-off temperature in vitro. Replication of these ts-viruses containing either the cp-HMPV or cp-HRSV mutations was reduced in the upper respiratory tract (URT) and undetectable in the lower respiratory tract (LRT) of hamsters. Nevertheless, high titres of HMPV-specific antibodies were induced by both ts-viruses. Upon immunization with the ts-viruses, the LRT of hamsters were completely protected against challenge infection with a heterologous HMPV strain, and URT viral titres were reduced by 10 000-fold. In conclusion, we provide proof-of-principle for two candidate live-attenuated HMPV vaccines that induce cross-protective immunity to prevent infection of the LRT in Syrian golden hamsters. Further mapping of the molecular determinants of attenuation of HMPV should be the subject of future studies.

Efficient multiplication of human metapneumovirus in Vero cells expressing the transmembrane serine protease TMPRSS2

Human metapneumovirus (HMPV) is a major causative agent of severe bronchiolitis and pneumonia. Its fusion (F) protein must be cleaved by host proteases to cause membrane fusion, a critical step for virus infection. By generating Vero cells constitutively expressing the transmembrane serine protease TMPRSS2 and green fluorescent protein-expressing recombinant HMPV, we show that TMPRSS2, which is expressed in the human lung epithelium, cleaves the HMPV F protein efficiently and supports HMPV multiplication. The results indicate that TMPRSS2 is a possible candidate protease involved in the development of lower respiratory tract illness in HMPV-infected patients.

Immunogenicity and efficacy of two candidate human metapneumovirus vaccines in cynomolgus macaques

Human metapneumovirus (HMPV) is an important cause of acute respiratory tract disease for which the development of vaccine candidates is warranted. We have previously described the generation of an iscom matrix-adjuvanted HMPV fusion protein subunit vaccine (Fsol) and a live-attenuated vaccine (HMPVM11). Here, we evaluate the immunogenicity and efficacy of these vaccines in cynomolgus macaques. Immunization with Fsol induced HMPV F-specific antibody responses, virus neutralizing antibody titers, and cellular immune responses, but the induced humoral immune response waned rapidly over time. HMPVM11 was strongly attenuated and displayed limited immunogenicity, although immunization with this virus primed for a good secondary HMPV-specific lymphoproliferative response after challenge infection. The duration of virus shedding in HMPVM11-immunized animals was reduced compared to sham-immunized animals. Both vaccines induced HMPV-specific immune responses, but the rapid waning of immunity is a challenging obstacle for vaccine development.