Kinetics of the respiratory syncytial virus growth cycle in HeLa cells

Clinical and biological consequences of respiratory syncytial virus genetic diversity

Respiratory syncytial virus (RSV) is one of the most common etiological agents of global acute respiratory tract infections with a disproportionate burden among infants, individuals over the age of 65, and immunocompromised populations. The two major subtypes of RSV (A and B) co-circulate with a predominance of either group during different epidemic seasons, with frequently emerging genotypes due to RSV's high genetic variability. Global surveillance systems have improved our understanding of seasonality, disease burden, and genomic evolution of RSV through genotyping by sequencing of attachment (G) glycoprotein. However, the integration of these systems into international infrastructures is in its infancy, resulting in a relatively low number (~2200) of publicly available RSV genomes. These limitations in surveillance hinder our ability to contextualize RSV evolution past current canonical attachment glycoprotein (G)-oriented understanding, thus resulting in gaps in understanding of how genetic diversity can play a role in clinical outcome, therapeutic efficacy, and the host immune response. Furthermore, utilizing emerging RSV genotype information from surveillance and testing the impact of viral evolution using molecular techniques allows us to establish causation between the clinical and biological consequences of arising genotypes, which subsequently aids in informed vaccine design and future vaccination strategy. In this review, we aim to discuss the findings from current molecular surveillance efforts and the gaps in knowledge surrounding the consequence of RSV genetic diversity on disease severity, therapeutic efficacy, and RSV-host interactions.

Contribution of Nanotechnologies to Vaccine Development and Drug Delivery against Respiratory Viruses

According to the Center for Disease Control and Prevention (CDC), the coronavirus disease 2019, a respiratory viral illness linked to significant morbidity, mortality, production loss, and severe economic depression, was the third-largest cause of death in 2020. Respiratory viruses such as influenza, respiratory syncytial virus, SARS-CoV-2, and adenovirus, are among the most common causes of respiratory illness in humans, spreading as pandemics or epidemics throughout all continents. Nanotechnologies are particles in the nanometer range made from various compositions. They can be lipid-based, polymer-based, protein-based, or inorganic in nature, but they are all bioinspired and virus-like. In this review, we aimed to present a short review of the different nanoparticles currently studied, in particular those which led to publications in the field of respiratory viruses. We evaluated those which could be beneficial for respiratory disease-based viruses; those which already have contributed, such as lipid nanoparticles in the context of COVID-19; and those which will contribute in the future either as vaccines or antiviral drug delivery systems. We present a short assessment based on a critical selection of evidence indicating nanotechnology's promise in the prevention and treatment of respiratory infections.

Patterns of virus growth across the diversity of life

Although viruses in their natural habitats add up to less than 10% of the biomass, they contribute more than 90% of the genome sequences [1]. These viral sequences or 'viromes' encode viruses that populate the Earth's oceans [2, 3] and terrestrial environments [4, 5], where their infections impact life across diverse ecological niches and scales [6, 7], including humans [8-10]. Most viruses have yet to be isolated and cultured [11-13], and surprisingly few efforts have explored what analysis of available data might reveal about their nature. Here, we compiled and analyzed seven decades of one-step growth and other data for viruses from six major families, including their infections of archaeal, bacterial and eukaryotic hosts [14-191]. We found that the use of host cell biomass for virus production was highest for archaea at 10%, followed by bacteria at 1% and eukarya at 0.01%, highlighting the degree to which viruses of archaea and bacteria exploit their host cells. For individual host cells, the yield of virus progeny spanned a relatively narrow range (10-1000 infectious particles per cell) compared with the million-fold difference in size between the smallest and largest cells. Furthermore, healthy and infected host cells were remarkably similar in the time they needed to multiply themselves or their virus progeny. Specifically, the doubling time of healthy cells and the delay time for virus release from infected cells were not only correlated (r = 0.71, p < 10-10, n = 101); they also spanned the same range from tens of minutes to about a week. These results have implications for better understanding the growth, spread and persistence of viruses in complex natural habitats that abound with diverse hosts, including humans and their associated microbes.

Evidence for a biphasic mode of respiratory syncytial virus transmission in permissive HEp2 cell monolayers

BACKGROUND

During respiratory syncytial virus (RSV) infection filamentous virus particles are formed on the cell surface. Although the virus infectivity remains cell-associated, low levels of cell-free virus is detected during advanced infection. It is currently unclear if this cell-free virus infectivity is due to a low-efficiency specific cell-release mechanism, or if it arises due to mechanical breakage following virus-induced cell damage at the advanced stage of infection. Understanding the origin of this cell-free virus is a prerequisite for understanding the mechanism of RSV transmission in permissive cells. In this study we describe a detailed examination of RSV transmission in permissive HEp2 cell monolayers.

METHODS

HEp2 cell monolayers were infected with RSV using a multiplicity of infection of 0.0002, and the course of infection monitored over 5 days. The progression of the virus infection within the cell monolayers was performed using bright-field microscopy to visualise the cell monolayer and immunofluorescence microscopy to detect virus-infected cells. The cell-associated and cell-free virus infectivity were determined by virus plaque assay, and the virus-induced cell cytotoxicity determined by measuring cell membrane permeability and cellular DNA fragmentation.

RESULTS

At 2 days-post infection (dpi), large clusters of virus-infected cells could be detected indicating localised transmission in the cell monolayer, and during this stage we failed to detect either cell-free virus or cell cytotoxicity. At 3 dpi the presence of much larger infected cell clusters correlated with the begining of virus-induced changes in cell permeability. The presence of cell-free virus correlated with continued increase in cell permeability and cytotoxicity at 4 and 5 dpi. At 5 dpi extensive cell damage, syncytial formation, and increased cellular DNA fragmentation was noted. However, even at 5 dpi the cell-free virus constituted less than 1 % of the total virus infectivity.

CONCLUSIONS

Our data supports a model of RSV transmission that initially involves the localised cell-to-cell spread of virus particles within the HEp2 cell monolayer. However, low levels of cell free-virus infectivity was observed at the advanced stages of infection, which correlated with a general loss in cell monolayer integrity due to virus-induced cytotoxicity.

Human Respiratory Syncytial Virus: An Introduction

Human respiratory syncytial virus (RSV) is understood to be a significant human pathogen in infants, young children, and the elderly and the immunocompromised. Over the last decade many important mechanisms contributing to RSV infection, replication, and disease pathogenesis have been revealed; however, there is still insufficient knowledge which has in part hampered vaccine development. Considerable information is accumulating regarding how RSV proteins modulate molecular signaling and immune responses to infection. Understanding how RSV interacts with its host is crucial to facilitate the development of safe and effective vaccines and therapeutic treatments.In this chapter, we provide a brief introduction into RSV replication, pathogenesis, and host immune response, and summarize the state of RSV vaccine and antiviral compounds in clinical stages of development. This chapter frames features of this book and the molecular methods used for understanding RSV interaction with the host.

Role of peroxiredoxin 1 and peroxiredoxin 4 in protection of respiratory syncytial virus-induced cysteinyl oxidation of nuclear cytoskeletal proteins

The respiratory epithelium plays a central role in innate immunity by secreting networks of inflammatory mediators in response to respiratory syncytial virus (RSV) infection. Previous proteomic studies focusing on the host cellular response to RSV indicated the existence of a nuclear heat shock response and cytoplasmic depletion of antioxidant proteins in model type II-like airway epithelial cells. Here, we increased the depth of nuclear proteomic interrogation by using fluorescence difference labeling followed by liquid isoelectric focusing prefractionation/two-dimensional gel electrophoresis (2-DE) to identify an additional 41 proteins affected by RSV infection. Surprisingly, we found inducible oligomers and shifts in isoelectric points for peroxiredoxin 1 (Prdx-1), Prdx-3, and Prdx-4 isoforms without changes in their total abundance, indicating that Prdxs were being oxidized in response to RSV. To address the role of Prdx-1 and Prdx-4 in RSV infection, isoforms were selectively knocked down by small interfering RNA (siRNA) transfection. Cells lacking Prdx-1, Prdx-4, or both showed increased levels of reactive oxygen species formation and a higher level of protein carbonylation in response to RSV infection. Using a novel saturation fluorescence labeling 2-DE analysis, we showed that 15 unique proteins had enhanced oxidative modifications of at least >1.2-fold in the Prdx knockdowns in response to RSV, including annexin A2 and desmoplakin. Our results suggest that Prdx-1 and Prdx-4 are essential for preventing RSV-induced oxidative damage in a subset of nuclear intermediate filament and actin binding proteins in epithelial cells.

The host response and molecular pathogenesis associated with respiratory syncytial virus infection

Since the isolation of respiratory syncytial virus (RSV) in 1956, its significance as an important human pathogen in infants, the elderly and the immunocompromised has been established. Many important mechanisms contributing to RSV infection, replication and disease pathogenesis have been uncovered; however, there is still insufficient knowledge in these and related areas, which must be addressed to facilitate the development of safe and effective vaccines and therapeutic treatments. A better understanding of the molecular pathogenesis of RSV infection, particularly the host-cell response and transcription profiles to RSV infection, is required to advance disease intervention strategies. Substantial information is accumulating regarding how RSV proteins modulate molecular signaling and regulation of cytokine and chemokine responses to infection, molecular signals regulating programmed cell death, and innate and adaptive immune responses to infection. This review discusses RSV manipulation of the host response to infection and related disease pathogenesis.

The baculovirus GP64 protein mediates highly stable infectivity of a human respiratory syncytial virus lacking its homologous transmembrane glycoproteins

Baculovirus GP64 is a low-pH-dependent membrane fusion protein required for virus entry and cell-to-cell transmission. Recently, GP64 has generated interest for practical applications in mammalian systems. Here we examined the membrane fusion function of GP64 from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) expressed in mammalian cells, as well as its capacity to functionally complement a mammalian virus, human respiratory syncytial virus (HRSV). Both authentic GP64 and GP(64/F), a chimeric protein in which the GP64 cytoplasmic tail domain was replaced with the 12 C-terminal amino acids of the HRSV fusion (F) protein, induced low-pH-dependent cell-cell fusion when expressed transiently in HEp-2 (human) cells. Levels of surface expression and syncytium formation were substantially higher at 33 degrees C than at 37 degrees C. The open reading frames (ORFs) encoding GP64 or GP(64/F), along with two marker ORFs encoding green fluorescent protein (GFP) and beta-glucuronidase (GUS), were used to replace all three homologous transmembrane glycoprotein ORFs (small hydrophobic SH, attachment G, and F) in a cDNA of HRSV. Infectious viruses were recovered that lacked the HRSV SH, G, and F proteins and expressed instead the GP64 or GP(64/F) protein and the two marker proteins GFP and GUS. The properties of these viruses, designated RSDeltaSH,G,F/GP64 or RSDeltaSH,G,F/GP(64/F), respectively, were compared to a previously described HRSV expressing GFP in place of SH but still containing the wild-type HRSV G and F proteins (RSDeltaSH [A. G. Oomens, A. G. Megaw, and G. W. Wertz, J. Virol., 77:3785-3798, 2003]). By immunoelectron microscopy, the GP64 and GP(64/F) proteins were shown to incorporate into HRSV-induced filaments at the cell surface. Antibody neutralization, ammonium chloride inhibition, and replication levels in cell culture showed that both GP64 proteins efficiently mediated infectivity of the respective viruses in a temperature-sensitive, low-pH-dependent manner. Furthermore, RSDeltaSH,G,F/GP64 and RSDeltaSH,G,F/GP(64/F) replicated to higher levels and had significantly higher stability of infectivity than HRSVs containing the homologous HRSV G and F proteins. Thus, GP64 and a GP64/HRSV F chimeric protein were functional and efficiently complemented an unrelated human virus in mammalian cells, producing stable, infectious virus stocks. These results demonstrate the potential of GP64 for both practical applications requiring stable pseudotypes in mammalian systems and for studies of viral glycoprotein requirements in assembly and pathogenesis.

Infectivity of a human respiratory syncytial virus lacking the SH, G, and F proteins is efficiently mediated by the vesicular stomatitis virus G protein

To examine the requirements of the human respiratory syncytial virus (HRSV) SH (small hydrophobic), G (attachment), and F (fusion) proteins for virus infectivity and morphology, we used the prototype A2 strain of HRSV to generate a series of cDNAs from which (i) the SH open reading frame (ORF), (ii) the SH and G ORFs, or (iii) the SH, G, and F ORFs were deleted. Each deleted ORF was replaced as follows: the SH ORF was replaced with that of green fluorescent protein; the G ORF was replaced with that of G(vsv), a chimeric glycoprotein consisting of the vesicular stomatitis Indiana virus (VSIV) G protein ecto- and transmembrane domains coupled to the HRSV F cytoplasmic tail; and the F ORF was replaced with that of marker protein beta-glucuronidase. The number of genes and the intergenic junctions in the constructs were kept as found in A2 virus in order to maintain authentic levels of transcription. Infectious viruses were recovered from all three engineered cDNAs and designated RSdeltash, RSdeltash,g/G(vsv), and RSdeltash,g,f/G(vsv), respectively. Low-pH-induced syncytium formation was observed in cells infected with viruses RSdeltaSH,G/G(vsv) and RSdeltaSH,G,F/G(vsv), indicating that G(vsv) was expressed and functional. Neutralization of infectivity by anti-VSIV G antibodies and inhibition of entry by ammonium chloride showed that RSdeltaSH,G,F/G(vsv) infectivity was mediated by G(vsv) and that an acidification step was required for entry into the host cell, similar to VSIV virions. All three engineered viruses displayed growth kinetics and virus yields similar to a wild-type A2 virus, both in Vero and HEp-2 cells. Abundant virus-induced filaments were observed at the surface of cells infected with each of the three engineered viruses or with virus A2, indicating that neither the SH and G proteins nor the F protein ecto- and transmembrane domains were required for the formation of these structures. This is the first report of the recovery of an infectious HRSV lacking a fusion protein of the Paramyxoviridae family and of manipulation of the HRSV entry pathway via incorporation of a nonparamyxoviral transmembrane glycoprotein.

MAPK activation is involved in posttranscriptional regulation of RSV-induced RANTES gene expression

Airway epithelial cells represent the primary cell target of respiratory syncytial virus (RSV) infection. They actively participate in the lung immune/inflammatory response that follows RSV infection by expressing chemokines, small chemotactic cytokines that recruit and activate leukocytes. Regulated on activation, normal T cell expressed, and presumably secreted (RANTES) is a member of the CC chemokine subfamily and is strongly chemotactic for T lymphocytes, monocytes, basophils, and eosinophils, cell types that are present or activated in the inflammatory infiltrate that follows RSV infection of the lung. RSV infection of airway epithelial cells induces RANTES expression by increasing gene transcription and stabilizing RNA transcripts. The signaling pathway regulating RANTES gene expression after RSV infection has not been determined. In this study, we examined the role of extracellular signal-regulated kinase (ERK) and p38, members of the mitogen-activated protein (MAP) kinase (MAPK) family, in RSV-induced RANTES production. RSV infection of alveolar epithelial cells induced increased phosphorylation and catalytic activity of ERK and the upstream kinases Raf-1 and MAP ERK kinase. Induction of the MAP signaling cascade required a replication-competent virus. RSV infection of alveolar epithelial cells also induced activation of p38 MAPK. Inhibition of ERK and p38 activation significantly reduced RSV-induced RANTES mRNA and protein secretion without affecting RANTES gene transcription or transcription factor activation. These results indicate that the MAPK signaling cascade regulates RANTES production in alveolar epithelial cells through a posttranscriptional mechanism.

Identification of NF-kappaB-dependent gene networks in respiratory syncytial virus-infected cells

Respiratory syncytial virus (RSV) is a mucosa-restricted virus that is a leading cause of epidemic respiratory tract infections in children. In epithelial cells, RSV replication activates nuclear translocation of the inducible transcription factor nuclear factor kappaB (NF-kappaB) through proteolysis of its cytoplasmic inhibitor, IkappaB. In spite of a putative role in mediating virus-inducible gene expression, the spectrum of NF-kappaB-dependent genes induced by RSV infection has not yet been determined. To address this, we developed a tightly regulated cell system expressing a nondegradable, epitope-tagged IkappaBalpha isoform (Flag-IkappaBalpha Mut) whose expression could be controlled by exogenous addition of nontoxic concentrations of doxycycline. Flag-IkappaBalpha Mut expression potently inhibited IkappaBalpha proteolysis, NF-kappaB binding, and NF-kappaB-dependent gene transcription in cells stimulated with the prototypical NF-kappaB-activating cytokine tumor necrosis factor alpha (TNF-alpha) and in response to RSV infection. High-density oligonucleotide microarrays were then used to profile constitutive and RSV-induced gene expression in the absence or presence of Flag-IkappaBalpha Mut. Comparison of these profiles revealed 380 genes whose expression was significantly changed by the dominant-negative NF-kappaB. Of these, 236 genes were constitutive (not RSV regulated), and surprisingly, only 144 genes were RSV regulated, representing numerically approximately 10% of the total population of RSV-inducible genes at this time point. Hierarchical clustering of the 144 RSV- and Flag-IkappaBalpha Mut-regulated genes identified two discrete gene clusters. The first group had high constitutive expression, and its expression levels fell in response to RSV infection. In this group, constitutive mRNA expression was increased by Flag-IkappaBalpha Mut expression, and the RSV-induced decrease in expression was partly inhibited. In the second group, constitutive expression was very low (or undetectable) and, after RSV infection, expression levels strongly increased. In this group, NF-kappaB was required for RSV-inducible expression because Flag-IkappaBalpha Mut expression blocked their induction by RSV. This latter cluster includes chemokines, transcriptional regulators, intracellular proteins regulating translation and proteolysis, and secreted proteins (complement components and growth factor regulators). These data suggest that NF-kappaB action induces global cellular responses after viral infection.

Respiratory syncytial virus infection of human airway epithelial cells is polarized, specific to ciliated cells, and without obvious cytopathology

Gene therapy for cystic fibrosis (CF) lung disease requires efficient gene transfer to airway epithelial cells after intralumenal delivery. Most gene transfer vectors so far tested have not provided the efficiency required. Although human respiratory syncytial virus (RSV), a common respiratory virus, is known to infect the respiratory epithelium, the mechanism of infection and the epithelial cell type targeted by RSV have not been determined. We have utilized human primary airway epithelial cell cultures that generate a well-differentiated pseudostratified mucociliary epithelium to investigate whether RSV infects airway epithelium via the lumenal (apical) surface. A recombinant RSV expressing green fluorescent protein (rgRSV) infected epithelial cell cultures with high gene transfer efficiency when applied to the apical surface but not after basolateral inoculation. Analyses of the cell types infected by RSV revealed that lumenal columnar cells, specifically ciliated epithelial cells, were targeted by RSV and that cultures became susceptible to infection as they differentiated into a ciliated phenotype. In addition to infection of ciliated cells via the apical membrane, RSV was shed exclusively from the apical surface and spread to neighboring ciliated cells by the motion of the cilial beat. Gross histological examination of cultures infected with RSV revealed no evidence of obvious cytopathology, suggesting that RSV infection in the absence of an immune response can be tolerated for >3 months. Therefore, rgRSV efficiently transduced the airway epithelium via the lumenal surface and specifically targeted ciliated airway epithelial cells. Since rgRSV appears to breach the lumenal barriers encountered by other gene transfer vectors in the airway, this virus may be a good candidate for the development of a gene transfer vector for CF lung disease.

Perflubron reduces lung inflammation in respiratory syncytial virus infection by inhibiting chemokine expression and nuclear factor-kappa B activation

Airway mucosa inflammation plays a critical role in the pathogenesis of lower respiratory tract infections caused by respiratory syncytial virus (RSV), the major etiologic agent of bronchiolitis in infancy. Type and intensity of cellular infiltration are dictated by inflammatory chemokines, which are rapidly and abundantly induced in lung tissue by RSV. This process is, to a large extent, transcriptionally regulated by RSV-mediated activation of the nuclear factor-kappa B. The administration of a perfluorocarbon (PFC) liquid, such as perflubron, during partial liquid ventilation improves lung function and also reduces inflammation. In this study we demonstrate that treatment of BALB/c mice with perflubron intranasally 6 hours after RSV infection significantly inhibited lung cellular inflammation as well as the expression of the chemokines RANTES, MIP-1 alpha, MIP-1 beta, and MIP-2, compared with phosphate-buffered saline-treated control mice. However, perflubron treatment did not affect RSV replication. Strikingly, treatment with perflubron abrogated nuclear factor-kappa B activation in lung of RSV-infected mice. These results demonstrate a novel mechanism by which PFC may exert antiinflammatory activity and suggest that partial liquid ventilation with PFC may be considered in future clinical trials for infants with severe RSV infections requiring mechanical ventilation.

Functional analysis of recombinant respiratory syncytial virus deletion mutants lacking the small hydrophobic and/or attachment glycoprotein gene

Respiratory syncytial virus (RSV) produces three envelope glycoproteins, the attachment glycoprotein (G), the fusion (F) protein, and the small hydrophobic (SH) protein. It had been assumed, by analogy with other paramyxoviruses, that the G and F proteins would be required for the first two steps of viral entry, attachment and fusion. However, following repeated passage in cell culture, a viable mutant RSV that lacked both the G and SH genes was isolated (R. A. Karron, D. A. Buonagurio, A. F. Georgiu, S. S. Whitehead, J. E. Adamus, M. L. Clements-Mann, D. O. Harris, V. B. Randolph, S. A. Udem, B. R. Murphy, and M. S. Sidhu, Proc. Natl. Acad. Sci. USA 94:13,961--13,966, 1997). To explore the roles of the G, F, and SH proteins in virion assembly, function, and cytopathology, we have modified the full-length RSV cDNA and used it to rescue infectious RSV lacking the G and/or SH genes. The three resulting viruses and the parental virus all contain the green fluorescent protein (GFP) gene that serves to identify infected cells. We have used purified, radiolabeled virions to examine virus production and function, in conjunction with GFP to quantify infected cells. We found that the G protein enhances virion binding to target cells but plays no role in penetration after attachment. The G protein also enhances cell-to-cell fusion, presumably via cell-to-cell binding, and enhances virion assembly or release. The presence or absence of the G protein in virions has no obvious effect on the content of F protein or host cell proteins in the virion. In growth curve experiments, the viruses lacking the G protein produced viral titers that were at least 10-fold lower than titers of viruses containing the G protein. This reduction is due in large part to the less efficient release of virions and the lower infectivity of the released virions. In the absence of the G protein, virus expressing both the F and SH proteins displayed somewhat smaller plaques, lower fusion activity, and slower viral entry than the virus expressing the F protein alone, suggesting that the SH protein has a negative effect on virus fusion in cell culture.

Inducible translational regulation of the NF-IL6 transcription factor by respiratory syncytial virus infection in pulmonary epithelial cells

Respiratory syncytial virus (RSV), the most common etiologic agent of epidemic pediatric respiratory disease, infects and replicates in the human airway epithelium, resulting in the induction of cellular gene products essential for immune and inflammatory responses. We describe the effect of RSV infection on nuclear factor-IL6 (NF-IL6) expression, a human basic domain-leucine zipper-containing transcription factor that alone in combination with other inducible transcription factors regulates the expression of cytokine and adhesion molecule genes. RSV-infected human type II pulmonary alveolar epithelial cells (A549) synthesize a single 45.7-kDa isoform of NF-IL6 rapidly and in a time-dependent manner. NF-IL6 is first detectable after 3 h of infection and continues to accumulate until 48 h (until the cells lose viability). NF-IL6 production could not be induced by UV-inactivated virus, demonstrating the requirement of viral replication for NF-IL6 synthesis. Immunoprecipitation after [35S]methionine metabolic labeling was done to investigate the mechanism for NF-IL6 production. There was robust NF-IL6 protein synthesis within RSV-infected (24 h) cells. Protein synthesis occurred without detectable changes in the abundance or size of the single 1.8-kb NF-IL6 mRNA. RNase protection assay of transfected chloramphenicol acetyltransferase reporter genes driven by either wild-type or mutated NF-IL6 binding sites show a virus-induced increase in NF-IL6-dependent transcription. These studies have demonstrated a novel inducible mechanism for translational control of NF-IL6 synthesis and identify this transcription factor as a potential effector of the host response to RSV infection.

Neutralization of respiratory syncytial virus after cell attachment

Little is known about the mechanisms of antibody-mediated neutralization of respiratory syncytial virus (RSV) which causes recurrent infections in human despite the virtually universal presence of neutralizing serum antibodies. Human serum neutralization titers showed strong correlation with post-cell-attachment neutralizing titers for both RSV-convalescent sera and control sera but showed less strong correlation with cell-attachment blocking titers. Neutralization was effective for the first 60 min of infection, indicating that immune serum-mediated neutralization of RSV infection largely involves inhibition of early events following cell attachment.

Cycles, chaos, and evolution in virus cultures: a model of defective interfering particles

Defective interfering particles (DIP) are spontaneous deletion mutants of viruses that replicate at the expense of the parent virus. DIP have complex effects on the growth of viruses in vitro, including the establishment of persistent infection, cyclical variation in virus titer, eradication of replicating virus, and rapid evolution of the virus. We show here that a simple mathematical model, based only on experimental observations, can explain all of the major effects of DIP on the population dynamics of virus growth. The variation in virus titer caused by DIP has many features that are characteristic of deterministic chaos: it follows that the quantitative effects of DIP are intrinsically unpredictable beyond a short time. We conclude (i) that other factors, such as temperature-sensitive virus mutants or interferons, need not be invoked to explain the complex effects of DIP; and (ii) that dominantly interfering viruses should only be used with great caution for therapeutic purposes, since their effects are, in principle, unpredictable.

A simple method for obtaining highly viable virus from culture supernatant

Traditionally density-gradient methods are used to purify viruses. However, these procedures are not only time consuming and cumbersome, recovery of viable viruses are often quite low. In this report, a single-step concentration technique was used to concentrate a mutant of Moloney murine leukemia virus (ts1) virus from culture supernatants by ultrafiltration. A special ultrafiltration unit with a 100,000 mol wt cut-off was able to concentrate viruses about 30-fold without losing any infectivity. In comparison, traditional sucrose density gradient purified viruses lost a significant portion of their infectivity. This technique could be used for concentrating other viruses for many useful purposes where more viable viruses are needed, e.g., study of virus-cell binding.

Active respiratory syncytial virus purified by ion-exchange chromatography: characterization of binding and elution requirements

Two viruses, respiratory syncytial virus (RSV) and vesicular stomatitis virus (VSV) were used to evaluate viral purification by an affinity resin column (Matrex Cellufine Sulfate (MCS); Amicon Division, WR Grace & Co.). Viable RSV was purified significantly from crude cell lysate by a single pass through a column containing the anionic MCS resin. Most cell protein and albumin eluted from the MCS resin with phosphate buffered saline (PBS) but RSV eluted at high ionic strength, i.e., greater than or equal to 0.6 M NaCl. Further purification was possible by sucrose step gradient centrifugation. The RSV prepared by column purification or by column plus sucrose gradient separation was both intact and infective. RSV and pure samples of VSV were used to optimize ionic strength and salts for elution from the MCS column: 0.8 M NaCl removed most of the viral protein. The capacity of the MCS gel for RSV or VSV was found to be about 0.6-0.8 mg viral protein per ml of hydrated resin. Detergent-solubilized viral membrane proteins bound to the MCS resin in 0.145 M NaCl and eluted with higher salt concentrations. Thus, this resin also may be a useful aid for relatively gentle purification of these proteins.

Comparison of caprine, human and bovine strains of respiratory syncytial virus

A new continuous ovine kidney cell line allowing the growth of caprine, human and bovine respiratory syncytial virus was used to minimize host cell related variations for the direct comparison of the viral ultrastructures, serological relationships and structural protein profiles. Results show that all three strains are closely related although a closer relationship was found between bovine and caprine RS.

Direct observation of the budding and fusion of an enveloped virus by video microscopy of viable cells

Video-enhanced microscopy and digital image processing were used to observe the assembly, budding, and fusion of Respiratory Syncytial virus. Viral filaments were seen to bud from the plasma membrane of viable infected cells to a final length of 5-10 micron with an average speed of elongation of 110-250 nm/s. The rapidity of viral assembly and its synchronous occurrence (leading to the production of several viral particles per minute from the same surface domain) suggests a directed process of recruitment of viral components to an area selected for virus maturation. Virions were also seen to adsorb to the cell surface, and to fuse with the plasma membrane. These are the first real time observations of viral morphogenesis and penetration which are crucial events in the infectious cycle of enveloped viruses.

A microplate method for isolation of viruses from infants and children with acute respiratory infections

Between December 1984 and December 1986, a microplate technique was adopted for isolation of viruses from infants and children with acute respiratory infections. By using two kinds of tissue culture microplates, i.e., the HHVM plate, containing human embryonic fibroblast (HEF), HEp-2, Vero and MDCK cells, and the MK plate which contains secondary monkey kidney cells, 1,080 field viruses were isolated from 1,061 (24.9%) out of 4,254 throat swabs. Of these 1,080 isolates, 1,003 (92.9%) were recovered in the HHVM plates and the remaining 77 (7.1%) in the MK plates. With the HHVM plate, influenza A and B viruses were cultivated in MDCK, RS virus in HEp-2, parainfluenza and mumps viruses in Vero, adenoviruses in both HEF and HEp-2, polioviruses in HEF, HEp-2 and Vero, coxsackie B viruses in both HEp-2 and Vero, rhino and echo viruses in HEF, herpes simplex virus in both HEF and HEp-2, and cytomegalovirus in HEF, although MK were more sensitive than Vero to parainfluenza and coxsackie B viruses. There was no difference in the rate of isolation of viruses between the microplate and ordinary tube methods. Cross contamination in the microplates was negligible for routine work.

Immunoprophylaxis and immunotherapy of respiratory syncytial virus infection in the cotton rat

Human convalescent antiserum to respiratory syncytial virus (RSV) administered intraperitoneally to cotton rats prior to RSV challenge provided near-complete protection from pulmonary infection. Antiserum given subsequent to viral challenge reduced pulmonary viral titers 100-fold or greater within 24 h. Sandoglobulin, a preparation of purified human IgG with high titer of anti-RSV neutralizing activity, produced the same effects as convalescent antiserum. Sandoglobulin was absorbed rapidly and produced a significant therapeutic reduction in virus titer within 3 h. The level of virus reduction in pulmonary and nasal tissues was directly proportional to the neutralizing antibody titer in the cotton rat serum, and was always greater in the lungs than the nose. Animals treated therapeutically with Sandoglobulin had a depressed primary antibody response to infection, but were completely resistant to reinfection with RSV. Histologic examination of pulmonary tissues from Sandoglobulin-treated animals showed no pathologic changes.

Enzyme-linked immunosorbent assay for detection of respiratory syncytial virus infection: development and description

An indirect enzyme-linked immunosorbent assay for detection of respiratory syncytial virus (RSV) antigens was developed, using commercially available antisera. Horse anti-RSV and calf antiserum to bovine RSV were used as capture and detector antibodies, respectively. The assay could detect as few as 50 PFU of unpurified RSV per ml in infected cell culture supernatant fluids and as little as 10 ng of affinity-purified RSV antigen per ml. No cross-reactions were observed with heterologous virus types. Freeze-thaw treatment had no effect on RSV enzyme-linked immunosorbent assay titers, but viral transport medium inhibited RSV enzyme-linked immunosorbent assay titers from 10- to 100-fold. The assay can be easily performed in 24 h and is a sensitive and specific method for the detection of RSV antigens.

Development of antibody-dependent cell-mediated cytotoxicity in the respiratory tract after natural infection with respiratory syncytial virus

Antibody-dependent cell-mediated cytotoxicity (ADCC) was measured in nasopharyngeal secretions collected from 42 infants and young children at various intervals after primary or secondary infection with respiratory syncytial virus. ADCC was determined by specific immune release of 51Cr from respiratory syncytial virus-infected HEp-2 cell culture monolayers, with lymphocytes from adult volunteers as effector cells. Specific ADCC responses in nasopharyngeal secretions after primary infection were observed as early as 3 days after the onset of clinical symptoms, and peak activity was observed 14 to 29 days after the onset of illness. ADCC responses after reinfection were significantly greater in both the acute and convalescent phases (P less than 0.05) than were ADCC responses after primary infection. ADCC in secretions was mediated primarily by the immunoglobulin G isotype of respiratory syncytial virus antibody.

Respiratory syncytial virus proteins: identification by immunoprecipitation

The proteins of respiratory syncytial virus have not been clearly identified due to the lability of the virus and difficulties in its purification. We have pulse-labeled respiratory syncytial virus with [35S]methionine and [35S]cysteine and analyzed cell lysates by polyacrylamide gel electrophoresis. Five 35S-labeled viral proteins ranging in molecular weight from 21,000 to 73,000 (VP73, VP44, VP35, VP28, and VP21) were easily discernable above background cellular proteins. Treatment of the infected cells with 0.15 M NaCl before labeling suppressed host cell protein synthesis and allowed clearer visualization of the five viral proteins by polyacrylamide gel electrophoresis. Three glycoproteins (VGP 92, VGP 50, and VGP 17) were also identified after labeling with [3H]glucosamine. Five of these polypeptides (VP51, VP44, VP35, VP28, and VGP92) were shown to be antigenically active because they could be immunoprecipitated with anti-respiratory syncytial virus antibody produced in New Zealand white rabbits, cotton rats, and humans before analysis by polyacrylamide gel electrophoresis.

Determination of human immunoglobulin M rheumatoid factor by a solid-phase radioimmunoassay which uses human immunoglobulin G in antigen-antibody complexes

A solid-phase radioimmunoassay for the rapid determination of human immunoglobulin M (IgM) rheumatoid factor (RF) has been developed. Preparation of the solid phase for the assay involved the formation of complexes between respiratory syncytial virus-specific human IgG antibodies and virus antigen on the surface of polystyrene balls. Binding of serum RF to IgG in the immune complex was subsequently detected by 125I-labeled mu-chain-specific antibodies to human IgM. The amount of radioactive indicator antibody bound was converted to units of RF by comparison to the standard curve for an RF reference-serum pool. This assay should prove useful in studies of the physiological role of RF, since it can effectively measure low levels of circulating RF.

Polypeptides of respiratory syncytial virus

Radiolabeled respiratory syncytial virus was purified from medium that had been harvested from infected HeLa cell monolayers before it contained much cellular debris. After isopycnic centrifugation in linear gradients prepared with sucrose dissolved in Hanks balanced salt solution, almost all the infectivity and most of the radioactivity were recovered in a single band with density from 1.16 to 1.23 g/cm3 and a peak at 1.2 g/cm3. Analysis by polyacrylamide gel electrophoresis resolved the purified virus into seven polypeptides of approximate molecular weights 20,000 to 80,000, of which the two largest and the smallest proved to by glycoproteins.

Morphogenesis and ultrastructure of respiratory syncytial virus

Respiratory syncytial (RS) virus was grown in Vero cells and fixed for electron microscopy at various stages of maturation. Both filamentous and round or kidney-shaped particles, either with (complete) or without (incomplete) internal structure, were observed. All four morphological forms were identical with respect to their reactivity with ferritin-labeled antibody to RS virus. Freezeetching revealed a structural feature apparently unique for RS virus, namely helical striations around the core on the internal aspect of the envelope. This specific configuration was already detectable during the early stages of viral differentiation of the host cell membrane. Concentration of free virus by zonal ultracentrifugation of culture fluids onto sucrose cushions yielded predominantly filamentous forms up to 10 mum in length.

Growth of respiratory syncytial virus in suspension cell culture

Respiratory syncytial virus, Burnett strain, adsorbed efficiently and grew to high titers in suspension cultures of HEp-2 and MA-160 cells. Our results compared favorably with previous experience with the growth of respiratory syncytial virus in monolayer cell cultures. The use of suspension cell cultures provides a convenient and simple procedure for producing high-titering respiratory syncytial virus pools.