Structure and ion channel activity of the human respiratory syncytial virus (hRSV) small hydrophobic protein transmembrane domain

Advancing the field of viroporins-Structure, function and pharmacology: IUPHAR Review X

Viroporins possess important potential as antiviral targets due to their critical roles during virus life cycles, spanning from virus entry to egress. Although the antiviral amantadine targets the M2 viroporin of influenza A virus, successful progression of other viroporin inhibitors into clinical use remains challenging. These challenges relate in varying proportions to a lack of reliable full-length 3D-structures, difficulties in functionally characterising individual viroporins, and absence of verifiable direct binding between inhibitor and viroporin. This review offers perspectives to help overcome these challenges. We provide a comprehensive overview of the viroporin family, including their structural and functional features, highlighting the moldability of their energy landscapes and actions. To advance the field, we suggest a list of best practices to aspire towards unambiguous viroporin identification and characterisation, along with considerations of potential pitfalls. Finally, we present current and future scenarios of, and prospects for, viroporin targeting drugs.

All Eyes on the Prefusion-Stabilized F Construct, but Are We Missing the Potential of Alternative Targets for Respiratory Syncytial Virus Vaccine Design?

Respiratory Syncytial Virus (RSV) poses a significant global health concern as a major cause of lower respiratory tract infections (LRTIs). Over the last few years, substantial efforts have been directed towards developing vaccines and therapeutics to combat RSV, leading to a diverse landscape of vaccine candidates. Notably, two vaccines targeting the elderly and the first maternal vaccine have recently been approved. The majority of the vaccines and vaccine candidates rely solely on a prefusion-stabilized conformation known for its highly neutralizing epitopes. Although, so far, this antigen design appears to be successful for the elderly, our current understanding remains incomplete, requiring further improvement and refinement in this field. Pediatric vaccines still have a long journey ahead, and we must ensure that vaccines currently entering the market do not lose efficacy due to the emergence of mutations in RSV's circulating strains. This review will provide an overview of the current status of vaccine designs and what to focus on in the future. Further research into antigen design is essential, including the exploration of the potential of alternative RSV proteins to address these challenges and pave the way for the development of novel and effective vaccines, especially in the pediatric population.

Linearly Polarized Infrared Spectroscopy for the Analysis of Biological Materials

The analysis of biological samples with polarized infrared spectroscopy (p-IR) has long been a widely practiced method for the determination of sample orientation and structural properties. In contrast to earlier works, which employed this method to investigate the fundamental chemistry of biological systems, recent interests are moving toward "real-world" applications for the evaluation and diagnosis of pathological states. This focal point review provides an up-to-date synopsis of the knowledge of biological materials garnered through linearly p-IR on biomolecules, cells, and tissues. An overview of the theory with special consideration to biological samples is provided. Different modalities which can be employed along with their capabilities and limitations are outlined. Furthermore, an in-depth discussion of factors regarding sample preparation, sample properties, and instrumentation, which can affect p-IR analysis is provided. Additionally, attention is drawn to the potential impacts of analysis of biological samples with inherently polarized light sources, such as synchrotron light and quantum cascade lasers. The vast applications of p-IR for the determination of the structure and orientation of biological samples are given. In conclusion, with considerations to emerging instrumentation, findings by other techniques, and the shift of focus toward clinical applications, we speculate on the future directions of this methodology.

The respiratory syncytial virus SH protein is incorporated into infectious virus particles that form on virus-infected cells

The association of the SH protein with respiratory syncytial virus (RSV) particles was examined in HEp2 cells and human ciliated nasal epithelial cells. Imaging of infected cells demonstrated the presence of the SH protein in virus filaments, and analysis of purified RSV particles revealed a SH protein species whose size was consistent with the glycosylated SH protein. Although the SH protein was detected in virus filaments it was not required for virus filament formation. Analysis of RSV-infected ciliated cells also revealed that the SH protein was trafficked into the cilia, and this correlated with reduced cilia density on these cells. Reduced cilia loss was not observed on ciliated cells infected with a RSV isolate that failed to express the SH protein. These data provide direct evidence that the SH protein is trafficked into virus particles, and suggests that the SH protein may also promote cilia dysfunction on nasal epithelial cells.

Defining the Assembleome of the Respiratory Syncytial Virus

During respiratory syncytial virus (RSV) particle assembly, the mature RSV particles form as filamentous projections on the surface of RSV-infected cells. The RSV assembly process occurs at the / on the cell surface that is modified by a virus infection, involving a combination of several different host cell factors and cellular processes. This induces changes in the lipid composition and properties of these lipid microdomains, and the virus-induced activation of associated Rho GTPase signaling networks drives the remodeling of the underlying filamentous actin (F-actin) cytoskeleton network. The modified sites that form on the surface of the infected cells form the nexus point for RSV assembly, and in this review chapter, they are referred to as the RSV assembleome. This is to distinguish these unique membrane microdomains that are formed during virus infection from the corresponding membrane microdomains that are present at the cell surface prior to infection. In this article, an overview of the current understanding of the processes that drive the formation of the assembleome during RSV particle assembly is given.

Receptors for Respiratory Syncytial Virus Infection and Host Factors Regulating the Life Cycle of Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract infections and responsible for a large proportion of mortality in children and the elderly. There are no licensed vaccines available to date. Prophylaxis and therapeutic RSV-specific antibodies are limited to populations at high risk owing to high cost and uncertain clinical value. Receptors and host factors are two determinants important for virus entry and establishment of infection in vivo. The identification and understanding of viral receptors and host factors can help us to gain insight into the pathogenesis of RSV infection. Herein, we reviewed receptors and host factors that have been reported thus far. RSV could bind to CX3C chemokine receptor 1 and heparan sulfate proteoglycans via the G protein, and to nucleolin, insulin-like growth factor-1 receptor, epidermal growth factor, and intercellular adhesion molecule-1 via the F protein. Seven host restriction factors and 13 host factors essential for RSV infection were reviewed. We characterized the functions and their roles in the life cycle of RSV, trying to provide an update on the information of RSV-related receptors and host factors.

Respiratory Syncytial Virus

Human respiratory syncytial virus (RSV) is a negative sense single-stranded RNA virus that can result in epidemics of seasonal respiratory infections. Generally, one of the two genotypes (A and B) predominates in a single season and alternate annually with regional variation. RSV is a known cause of disease and death at both extremes of ages in the pediatric and elderly, as well as immunocompromised populations. The clinical impact of RSV on the hospitalized adults has been recently clarified with the expanded use of multiplex molecular assays. Among adults, RSV can produce a wide range of clinical symptoms due to upper respiratory tract infections potentially leading to severe lower respiratory tract infections, as well as exacerbations of underlying cardiac and lung diseases. While supportive care is the mainstay of therapy, there are currently multiple therapeutic and preventative options under development.

Innate Immune Components that Regulate the Pathogenesis and Resolution of hRSV and hMPV Infections

The human respiratory syncytial virus (hRSV) and human Metapneumovirus (hMPV) are two of the leading etiological agents of acute lower respiratory tract infections, which constitute the main cause of mortality in infants. However, there are currently approved vaccines for neither hRSV nor hMPV. Moreover, despite the similarity between the pathology caused by both viruses, the immune response elicited by the host is different in each case. In this review, we discuss how dendritic cells, alveolar macrophages, neutrophils, eosinophils, natural killer cells, innate lymphoid cells, and the complement system regulate both pathogenesis and the resolution of hRSV and hMPV infections. The roles that these cells play during infections by either of these viruses will help us to better understand the illnesses they cause. We also discuss several controversial findings, relative to some of these innate immune components. To better understand the inflammation in the lungs, the role of the respiratory epithelium in the recruitment of innate immune cells is briefly discussed. Finally, we review the main prophylactic strategies and current vaccine candidates against both hRSV and hMPV.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Contribution of Dendritic Cells in Protective Immunity against Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in infants and the elderly. The socioeconomic burden of RSV infection is substantial because it leads to serious respiratory problems, subsequent hospitalization, and mortality. Despite its clinical significance, a safe and effective vaccine is not yet available to prevent RSV infection. Upon RSV infection, lung dendritic cells (DCs) detecting pathogens migrate to the lymph nodes and activate the adaptive immune response. Therefore, RSV has evolved various immunomodulatory strategies to inhibit DC function. Due to the capacity of RSV to modulate defense mechanisms in hosts, RSV infection results in inappropriate activation of immune responses resulting in immunopathology and frequent reinfection throughout life. This review discusses how DCs recognize invading RSV and induce adaptive immune responses, as well as the regulatory mechanisms mediated by RSV to disrupt DC functions and ultimately avoid host defenses.

Respiratory syncytial virus infection in adults

Human respiratory syncytial virus (RSV) belongs to the recently defined Pneumoviridae family, Orthopneumovirus genus. It is a negative sense, single stranded RNA virus that results in epidemics of respiratory infections that typically peak in the winter in temperate climates and during the rainy season in tropical climates. Generally, one of the two genotypes (A and B) predominates in a single season, alternating annually, although regional variation occurs. RSV is a cause of disease and death in children, older people, and immunocompromised patients, and its clinical effect on adults admitted to hospital is clarified with expanded use of multiplex molecular assays. Among adults, RSV produces a wide range of clinical symptoms including upper respiratory tract infections, severe lower respiratory tract infections, and exacerbations of underlying disease. Here we discuss the latest evidence on the burden of RSV related disease in adults, especially in those with immunocompromise or other comorbidities. We review current therapeutic and prevention options, as well as those in development.

Phylogenetic evidence of a novel lineage of canine pneumovirus and a naturally recombinant strain isolated from dogs with respiratory illness in Thailand

BACKGROUND

Canine pneumovirus (CPV) is a pathogen that causes respiratory disease in dogs, and recent outbreaks in shelters in America and Europe have been reported. However, based on published data and documents, the identification of CPV and its variant in clinically symptomatic individual dogs in Thailand through Asia is limited. Therefore, the aims of this study were to determine the emergence of CPV and to consequently establish the genetic characterization and phylogenetic analysis of the CPV strains from 209 dogs showing respiratory distress in Thailand.

RESULTS

This study identified and described the full-length CPV genome from three strains, designated herein as CPV_CP13 TH/2015, CPV_CP82 TH/2016 and CPV_SR1 TH/2016, that were isolated from six dogs out of 209 dogs (2.9%) with respiratory illness in Thailand. Phylogenetic analysis suggested that these three Thai CPV strains (CPV TH strains) belong to the CPV subgroup A and form a novel lineage; proposed as the Asian prototype. Specific mutations in the deduced amino acids of these CPV TH strains were found in the G/glycoprotein sequence, suggesting potential substitution sites for subtype classification. Results of intragenic recombination analysis revealed that CPV_CP82 TH/2016 is a recombinant strain, where the recombination event occurred in the L gene with the Italian prototype CPV Bari/100-12 as the putative major parent. Selective pressure analysis demonstrated that the majority of the nucleotides in the G/glycoprotein were under purifying selection with evidence of positive selection sites.

CONCLUSIONS

This collective information on the CPV TH strains is the first evidence of CPV emergence with genetic characterization in Thailand and as first report in Asia, where homologous recombination acts as a potential force driving the genetic diversity and shaping the evolution of canine pneumovirus.

Potential Viroporin Candidates From Pathogenic Viruses Using Bacteria-Based Bioassays

Viroporins are a family of small hydrophobic proteins found in many enveloped viruses that are capable of ion transport. Building upon the ability to inhibit influenza by blocking its archetypical M2 H⁺ channel, as a family, viroporins may represent a viable target to curb viral infectivity. To this end, using three bacterial assays we analyzed six small hydrophobic proteins from biomedically important viruses as potential viroporin candidates. Our results indicate that Eastern equine encephalitis virus 6k, West Nile virus MgM, Dengue virus 2k, Dengue virus P1, Variola virus gp170, and Variola virus gp151 proteins all exhibit channel activity in the bacterial assays, and as such may be considered viroporin candidates. It is clear that more studies, such as patch clamping, will be needed to characterize the ionic conductivities of these proteins. However, our approach presents a rapid procedure to analyze open reading frames in other viruses, yielding new viroporin candidates for future detailed investigation. Finally, if conductivity is proven vital to their cognate viruses, the bio-assays presented herein afford a simple approach to screen for new channel blockers.

Targeting Intracellular Ion Homeostasis for the Control of Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a leading cause of mortality in infants and young children. Despite the RSV disease burden, no vaccine is available, and treatment remains nonspecific. New drug candidates are needed to combat RSV. Toward this goal, we screened over 2,000 compounds to identify approved drugs with novel anti-RSV activity. Cardiac glycosides, inhibitors of the membrane-bound Na⁺/K⁺-ATPase, were identified to have anti-RSV activity. Cardiac glycosides diminished RSV infection in human epithelial type 2 cells and in primary human airway epithelial cells grown at an air-liquid interface. Digoxin, a U.S. Food and Drug Administration-approved cardiac glycoside, was also able to inhibit infection of primary nasal epithelial cells with community isolates of RSV. Our results suggest that the antiviral effects of cardiac glycosides may be dependent on changes in the intracellular Na⁺ and K⁺ composition. Consistent with this mechanism, we demonstrated that the ionophoric antibiotics salinomycin, valinomycin, and monensin inhibited RSV in human epithelial type 2 cells and primary nasal epithelial cells. Our data indicate that the K⁺/Na⁺-sensitive steps in the RSV life cycle occur within the initial 4 hours of viral infection but do not include virus binding/entry. Rather, our findings demonstrated a negative effect on the RSV transcription and/or replication process. Overall, this work suggests that targeting intracellular ion concentrations offers a novel antiviral strategy.

Immune-Modulation by the Human Respiratory Syncytial Virus: Focus on Dendritic Cells

The human respiratory syncytial virus (hRSV) is the leading cause of pneumonia in infants and produces a significant burden in the elderly. It can also infect and produce disease in otherwise healthy adults and recurrently infect those previously exposed to the virus. Importantly, recurrent infections are not necessarily a consequence of antigenic variability, as described for other respiratory viruses, but most likely due to the capacity of this virus to interfere with the host's immune response and the establishment of a protective and long-lasting immunity. Although some genes encoded by hRSV are known to have a direct participation in immune evasion, it seems that repeated infection is mainly given by its capacity to modulate immune components in such a way to promote non-optimal antiviral responses in the host. Importantly, hRSV is known to interfere with dendritic cell (DC) function, which are key cells involved in establishing and regulating protective virus-specific immunity. Notably, hRSV infects DCs, alters their maturation, migration to lymph nodes and their capacity to activate virus-specific T cells, which likely impacts the host antiviral response against this virus. Here, we review and discuss the most important and recent findings related to DC modulation by hRSV, which might be at the basis of recurrent infections in previously infected individuals and hRSV-induced disease. A focus on the interaction between DCs and hRSV will likely contribute to the development of effective prophylactic and antiviral strategies against this virus.

A Reversible Proton Generator with On/Off Thermoswitch

A reversible polymer photoacid with a thermal on/off switch at physiological temperature able to trigger a large pH modulation of its environment is prepared. Light is used to control the acidity of the solution. Additionally, the temperature could be used to modulate the photoacid efficiency, practically turning on and off the ability of the polymer to produce protons. The behavior of this thermoresponsive photoacid copolymer is the result of the combined action of the temperature-responsive N-isopropylacrylamide and of a reversible photoacid monomer based on a spiropyran derivative. The acidification of the aqueous medium is activated by irradiation at λ = 460 nm. The reverse reaction is achieved by removing the light stimuli or by exposing the solution to UV-light. Increasing the temperature above the lower critical solution temperature of the copolymer deactivates the photoacid and irradiation at λ = 460 nm does not lead to the generation of protons or to any detectable change in the pH value of the solution. Hence, the addition of N-isopropylacrylamide as a comonomer acts as a thermal on/off switch for the photoacid and the coupling of temperature-and light-responsiveness in the polyphotoacids yields a "thermophotoacid".

Beyond Channel Activity: Protein-Protein Interactions Involving Viroporins

Viroporins are short polypeptides encoded by viruses. These small membrane proteins assemble into oligomers that can permeabilize cellular lipid bilayers, disrupting the physiology of the host to the advantage of the virus. Consequently, efforts during the last few decades have been focused towards the discovery of viroporin channel inhibitors, but in general these have not been successful to produce licensed drugs. Viroporins are also involved in viral pathogenesis by engaging in critical interactions with viral proteins, or disrupting normal host cellular pathways through coordinated interactions with host proteins. These protein-protein interactions (PPIs) may become alternative attractive drug targets for the development of antivirals. In this sense, while thus far most antiviral molecules have targeted viral proteins, focus is moving towards targeting host proteins that are essential for virus replication. In principle, this largely would overcome the problem of resistance, with the possibility of using repositioned existing drugs. The precise role of these PPIs, their strain- and host- specificities, and the structural determination of the complexes involved, are areas that will keep the fields of virology and structural biology occupied for years to come. In the present review, we provide an update of the efforts in the characterization of the main PPIs for most viroporins, as well as the role of viroporins in these PPIs interactions.

Modulation of Host Immunity by Human Respiratory Syncytial Virus Virulence Factors: A Synergic Inhibition of Both Innate and Adaptive Immunity

The Human Respiratory Syncytial Virus (hRSV) is a major cause of acute lower respiratory tract infections (ARTIs) and high rates of hospitalizations in children and in the elderly worldwide. Symptoms of hRSV infection include bronchiolitis and pneumonia. The lung pathology observed during hRSV infection is due in part to an exacerbated host immune response, characterized by immune cell infiltration to the lungs. HRSV is an enveloped virus, a member of the Pneumoviridae family, with a non-segmented genome and negative polarity-single RNA that contains 10 genes encoding for 11 proteins. These include the Fusion protein (F), the Glycoprotein (G), and the Small Hydrophobic (SH) protein, which are located on the virus surface. In addition, the Nucleoprotein (N), Phosphoprotein (P) large polymerase protein (L) part of the RNA-dependent RNA polymerase complex, the M2-1 protein as a transcription elongation factor, the M2-2 protein as a regulator of viral transcription and (M) protein all of which locate inside the virion. Apart from the structural proteins, the hRSV genome encodes for the non-structural 1 and 2 proteins (NS1 and NS2). HRSV has developed different strategies to evade the host immunity by means of the function of some of these proteins that work as virulence factors to improve the infection in the lung tissue. Also, hRSV NS-1 and NS-2 proteins have been shown to inhibit the activation of the type I interferon response. Furthermore, the hRSV nucleoprotein has been shown to inhibit the immunological synapsis between the dendritic cells and T cells during infection, resulting in an inefficient T cell activation. Here, we discuss the hRSV virulence factors and the host immunological features raised during infection with this virus.

Structure and functional dynamics characterization of the ion channel of the human respiratory syncytial virus (hRSV) small hydrophobic protein (SH) transmembrane domain by combining molecular dynamics with excited normal modes

The human respiratory syncytial virus (hRSV) is the major cause of lower respiratory tract infection in children and elderly people worldwide. Its genome encodes 11 proteins including SH protein, whose functions are not well known. Studies show that SH protein increases RSV virulence degree and permeability to small compounds, suggesting it is involved in the formation of ion channels. The knowledge of SH structure and function is fundamental for a better understanding of its infection mechanism. The aim of this study was to model, characterize, and analyze the structural behavior of SH protein in the phospholipids bilayer environment. Molecular modeling of SH pentameric structure was performed, followed by traditional molecular dynamics (MD) simulations of the protein immersed in the lipid bilayer. Molecular dynamics with excited normal modes (MDeNM) was applied in the resulting system in order to investigate long time scale pore dynamics. MD simulations support that SH protein is stable in its pentameric form. Simulations also showed the presence of water molecules within the bilayer by density distribution, thus confirming that SH protein is a viroporin. This water transport was also observed in MDeNM studies with histidine residues of five chains (His22 and His51), playing a key role in pore permeability. The combination of traditional MD and MDeNM was a very efficient protocol to investigate functional conformational changes of transmembrane proteins that act as molecular channels. This protocol can support future investigations of drug candidates by acting on SH protein to inhibit viral infection. Graphical Abstract The ion channel of the human respiratory syncytial virus (hRSV) small hydrophobic protein (SH) transmembrane domainᅟ.

Pathophysiological Consequences of Calcium-Conducting Viroporins

Eukaryotic cells have evolved a myriad of ion channels, transporters, and pumps to maintain and regulate transmembrane ion gradients. As intracellular parasites, viruses also have evolved ion channel proteins, called viroporins, which disrupt normal ionic homeostasis to promote viral replication and pathogenesis. The first viral ion channel (influenza M2 protein) was confirmed only 23 years ago, and since then studies on M2 and many other viroporins have shown they serve critical functions in virus entry, replication, morphogenesis, and immune evasion. As new candidate viroporins and viroporin-mediated functions are being discovered, we review the experimental criteria for viroporin identification and characterization to facilitate consistency within this field of research. Then we review recent studies on how the few Ca(2+)-conducting viroporins exploit host signaling pathways, including store-operated Ca(2+) entry, autophagy, and inflammasome activation. These viroporin-induced aberrant Ca(2+) signals cause pathophysiological changes resulting in diarrhea, vomiting, and proinflammatory diseases, making both the viroporin and host Ca(2+) signaling pathways potential therapeutic targets for antiviral drugs.

New antiviral approaches for respiratory syncytial virus and other mononegaviruses: Inhibiting the RNA polymerase

Worldwide, respiratory syncytial virus (RSV) causes severe disease in infants, the elderly, and immunocompromised people. No vaccine or effective antiviral treatment is available. RSV is a member of the non-segmented, negative-strand (NNS) group of RNA viruses and relies on its RNA-dependent RNA polymerase to transcribe and replicate its genome. Because of its essential nature and unique properties, the RSV polymerase has proven to be a good target for antiviral drugs, with one compound, ALS-8176, having already achieved clinical proof-of-concept efficacy in a human challenge study. In this article, we first provide an overview of the role of the RSV polymerase in viral mRNA transcription and genome replication. We then review past and current approaches to inhibiting the RSV polymerase, including use of nucleoside analogs and non-nucleoside inhibitors. Finally, we consider polymerase inhibitors that hold promise for treating infections with other NNS RNA viruses, including measles and Ebola.

Human Respiratory Syncytial Virus: Infection and Pathology

The human respiratory syncytial virus (hRSV) is by far the major cause of acute lower respiratory tract infections (ALRTIs) worldwide in infants and children younger than 2 years. The overwhelming number of hospitalizations due to hRSV-induced ALRTI each year is due, at least in part, to the lack of licensed vaccines against this virus. Thus, hRSV infection is considered a major public health problem and economic burden in most countries. The lung pathology developed in hRSV-infected individuals is characterized by an exacerbated proinflammatory and unbalanced Th2-type immune response. In addition to the adverse effects in airway tissues, hRSV infection can also cause neurologic manifestations in the host, such as seizures and encephalopathy. Although the origins of these extrapulmonary symptoms remain unclear, studies with patients suffering from neurological alterations suggest an involvement of the inflammatory response against hRSV. Furthermore, hRSV has evolved numerous mechanisms to modulate and evade the immune response in the host. Several studies have focused on elucidating the interactions between hRSV virulence factors and the host immune system, to rationally design new vaccines and therapies against this virus. Here, we discuss about the infection, pathology, and immune response triggered by hRSV in the host.

Targeting the Channel Activity of Viroporins

Since the discovery that certain small viral membrane proteins, collectively termed as viroporins, can permeabilize host cellular membranes and also behave as ion channels, attempts have been made to link this feature to specific biological roles. In parallel, most viroporins identified so far are virulence factors, and interest has focused toward the discovery of channel inhibitors that would have a therapeutic effect, or be used as research tools to understand the biological roles of viroporin ion channel activity. However, this paradigm is being shifted by the difficulties inherent to small viral membrane proteins, and by the realization that protein-protein interactions and other diverse roles in the virus life cycle may represent an equal, if not, more important target. Therefore, although targeting the channel activity of viroporins can probably be therapeutically useful in some cases, the focus may shift to their other functions in following years. Small-molecule inhibitors have been mostly developed against the influenza A M2 (IAV M2 or AM2). This is not surprising since AM2 is the best characterized viroporin to date, with a well-established biological role in viral pathogenesis combined the most extensive structural investigations conducted, and has emerged as a validated drug target. For other viroporins, these studies are still mostly in their infancy, and together with those for AM2, are the subject of the present review.

Understanding respiratory syncytial virus (RSV) vaccine development and aspects of disease pathogenesis

Respiratory syncytial virus (RSV) is the most important cause of lower respiratory tract infections causing bronchiolitis and some mortality in young children and the elderly. Despite decades of research there is no licensed RSV vaccine. Although significant advances have been made in understanding the immune factors responsible for inducing vaccine-enhanced disease in animal models, less information is available for humans. In this review, we discuss the different types of RSV vaccines and their target population, the need for establishing immune correlates for vaccine efficacy, and how the use of different animal models can help predict vaccine efficacy and clinical outcomes in humans.

Recent advances in the development of subunit-based RSV vaccines

Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections causing pneumonia and bronchiolitis in infants. RSV also causes serious illness in elderly populations, immunocompromised patients and individuals with pulmonary or cardiac problems. The significant morbidity and mortality associated with RSV infection have prompted interest in RSV vaccine development. In the 1960s, a formalin-inactivated vaccine trial failed to protect children, and indeed enhanced pathology when naturally infected later with RSV. Hence, an alternative approach to traditional killed virus vaccines, which can induce protective immunity without serious adverse events, is desired. Several strategies have been explored in attempts to produce effective vaccine candidates including gene-based and subunit vaccines. Subunit-based vaccine approaches have shown promising efficacy in animal studies and several have reached clinical trials. The current stage of development of subunit-based vaccines against RSV is reviewed in this article.

Modulation of host adaptive immunity by hRSV proteins

Globally, the human respiratory syncytial virus (hRSV) is the major cause of lower respiratory tract infections (LRTIs) in infants and children younger than 2 years old. Furthermore, the number of hospitalizations due to LRTIs has shown a sustained increase every year due to the lack of effective vaccines against hRSV. Thus, this virus remains as a major public health and economic burden worldwide. The lung pathology developed in hRSV-infected humans is characterized by an exacerbated inflammatory and Th2 immune response. In order to rationally design new vaccines and therapies against this virus, several studies have focused in elucidating the interactions between hRSV virulence factors and the host immune system. Here, we discuss the main features of hRSV biology, the processes involved in virus recognition by the immune system and the most relevant mechanisms used by this pathogen to avoid the antiviral host response.

Relevance of Viroporin Ion Channel Activity on Viral Replication and Pathogenesis

Modification of host-cell ionic content is a significant issue for viruses, as several viral proteins displaying ion channel activity, named viroporins, have been identified. Viroporins interact with different cellular membranes and self-assemble forming ion conductive pores. In general, these channels display mild ion selectivity, and, eventually, membrane lipids play key structural and functional roles in the pore. Viroporins stimulate virus production through different mechanisms, and ion channel conductivity has been proved particularly relevant in several cases. Key stages of the viral cycle such as virus uncoating, transport and maturation are ion-influenced processes in many viral species. Besides boosting virus propagation, viroporins have also been associated with pathogenesis. Linking pathogenesis either to the ion conductivity or to other functions of viroporins has been elusive for a long time. This article summarizes novel pathways leading to disease stimulated by viroporin ion conduction, such as inflammasome driven immunopathology.

New insights on the viral and host factors contributing to the airway pathogenesis caused by the respiratory syncytial virus

The respiratory syncytial virus (RSV) is the most prevalent etiological agent of lower respiratory tract infections and the first cause of hospitalization in infants due to respiratory disease worldwide. However, efforts to develop safe and effective vaccines and antivirals have been challenged by an incomplete understanding of the RSV pathogenesis and the host immune response to RSV infection in the airways. Here, we discuss recent advances in understanding the interaction between RSV and the epithelium to induce pathogenesis in the airways, such as the role of the RSV NS2 protein in the airway epithelium, as well as the events involved in the RSV entry process. In addition, we summarize the cellular factors produced by airway epithelial cells (AECs) in response to RSV infection that lead to the activation of innate and adaptive immune responses, inducing lung inflammation and disease. Further, we discuss the possible contribution of a recently identified cytokine, thymic stromal lymphopoitein (TSLP), in the lung immunopathology caused by RSV.

Partial Attenuation of Respiratory Syncytial Virus with a Deletion of a Small Hydrophobic Gene Is Associated with Elevated Interleukin-1β Responses

The small hydrophobic (SH) gene of respiratory syncytial virus (RSV), a major cause of infant hospitalization, encodes a viroporin of unknown function. SH gene knockout virus (RSV ΔSH) is partially attenuated in vivo, but not in vitro, suggesting that the SH protein may have an immunomodulatory role. RSV ΔSH has been tested as a live attenuated vaccine in humans and cattle, and here we demonstrate that it protected against viral rechallenge in mice. We compared the immune response to infection with RSV wild type and RSV ΔSH in vivo using BALB/c mice and in vitro using epithelial cells, neutrophils, and macrophages. Strikingly, the interleukin-1β (IL-1β) response to RSV ΔSH infection was greater than to wild-type RSV, in spite of a decreased viral load, and when IL-1β was blocked in vivo, the viral load returned to wild-type levels. A significantly greater IL-1β response to RSV ΔSH was also detected in vitro, with higher-magnitude responses in neutrophils and macrophages than in epithelial cells. Depleting macrophages (with clodronate liposome) and neutrophils (with anti-Ly6G/1A8) demonstrated the contribution of these cells to the IL-1β response in vivo, the first demonstration of neutrophilic IL-1β production in response to viral lung infection. In this study, we describe an increased IL-1β response to RSV ΔSH, which may explain the attenuation in vivo and supports targeting the SH gene in live attenuated vaccines.

IMPORTANCE

There is a pressing need for a vaccine for respiratory syncytial virus (RSV). A number of live attenuated RSV vaccine strains have been developed in which the small hydrophobic (SH) gene has been deleted, even though the function of the SH protein is unknown. The structure of the SH protein has recently been solved, showing it is a pore-forming protein (viroporin). Here, we demonstrate that the IL-1β response to RSV ΔSH is greater in spite of a lower viral load, which contributes to the attenuation in vivo. This potentially suggests a novel method by which viruses can evade the host response. As all Pneumovirinae and some Paramyxovirinae carry similar SH genes, this new understanding may also enable the development of live attenuated vaccines for both RSV and other members of the Paramyxoviridae.

Novel antigens for RSV vaccines

Respiratory syncytial virus (RSV) remains a leading global cause of infant mortality and adult morbidity. Infection, which recurs throughout life, elicits only short-lived immunity. The development of a safe and efficacious vaccine has, thus far, been elusive. Recent technological advances, however, have yielded promising RSV vaccine candidates that are based on solving atomic-level structures of surface glycoproteins interacting with neutralizing antibodies. The class I fusion glycoprotein, F, serves as the primary antigenic component of most vaccines, and is the target of the only licensed monoclonal antibody product used to reduce the frequency of severe disease in high-risk neonates. However, success of prior F-based vaccines has been limited by the lack of understanding how the conformational rearrangement between a metastable prefusion F (pre-F) and a stable postfusion F (post-F) affected the epitope content. Neutralizing epitopes reside on both conformations, but those specific to pre-F are far more potent than those previously identified and present on post-F. The solution of the pre-F structure and its subsequent characterization and stabilization illustrates the value of a structure-based approach to vaccine development, and provides hope that a safe and effective RSV vaccine is possible.

Protein-Protein Interactions of Viroporins in Coronaviruses and Paramyxoviruses: New Targets for Antivirals?

Viroporins are members of a rapidly growing family of channel-forming small polypeptides found in viruses. The present review will be focused on recent structural and protein-protein interaction information involving two viroporins found in enveloped viruses that target the respiratory tract; (i) the envelope protein in coronaviruses and (ii) the small hydrophobic protein in paramyxoviruses. Deletion of these two viroporins leads to viral attenuation in vivo, whereas data from cell culture shows involvement in the regulation of stress and inflammation. The channel activity and structure of some representative members of these viroporins have been recently characterized in some detail. In addition, searches for protein-protein interactions using yeast-two hybrid techniques have shed light on possible functional roles for their exposed cytoplasmic domains. A deeper analysis of these interactions should not only provide a more complete overview of the multiple functions of these viroporins, but also suggest novel strategies that target protein-protein interactions as much needed antivirals. These should complement current efforts to block viroporin channel activity.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Sedimentation equilibrium of a small oligomer-forming membrane protein: effect of histidine protonation on pentameric stability

Analytical ultracentrifugation (AUC) can be used to study reversible interactions between macromolecules over a wide range of interaction strengths and under physiological conditions. This makes AUC a method of choice to quantitatively assess stoichiometry and thermodynamics of homo- and hetero-association that are transient and reversible in biochemical processes. In the modality of sedimentation equilibrium (SE), a balance between diffusion and sedimentation provides a profile as a function of radial distance that depends on a specific association model. Herein, a detailed SE protocol is described to determine the size and monomer-monomer association energy of a small membrane protein oligomer using an analytical ultracentrifuge. AUC-ES is label-free, only based on physical principles, and can be used on both water soluble and membrane proteins. An example is shown of the latter, the small hydrophobic (SH) protein in the human respiratory syncytial virus (hRSV), a 65-amino acid polypeptide with a single α-helical transmembrane (TM) domain that forms pentameric ion channels. NMR-based structural data shows that SH protein has two protonatable His residues in its transmembrane domain that are oriented facing the lumen of the channel. SE experiments have been designed to determine how pH affects association constant and the oligomeric size of SH protein. While the pentameric form was preserved in all cases, its association constant was reduced at low pH. These data are in agreement with a similar pH dependency observed for SH channel activity, consistent with a lumenal orientation of the two His residues in SH protein. The latter may experience electrostatic repulsion and reduced oligomer stability at low pH. In summary, this method is applicable whenever quantitative information on subtle protein-protein association changes in physiological conditions have to be measured.

Viroporins

Virus encoded ion channels, termed viroporins, are expressed by a diverse set of viruses and have been found to target nearly every host cell membrane and compartment, including endocytic/exocytic vesicles, ER, mitochondria, Golgi, and the plasma membrane. Viroporins are generally very small (<100 amino acids) integral membrane proteins that share common structure motifs (conserved cluster of basic residues adjacent to an amphipathic alpha-helix) but only limited sequence homology between viruses. Ion channel activity of viroporins is either required for replication or greatly enhances replication and pathogenesis. Channel characteristics have been investigated using standard electrophysiological techniques, including planar lipid bilayer, liposome patch clamp or whole-cell voltage clamp. In general, viroporins are voltage-independent non-specific monovalent cation channels, with the exception of the influenza A virus M2 channel that forms a highly specific proton channel due to a conserved HXXXW motif. Viroporin channel currents range between highly variable (‘burst-like’) fluctuations to well resolved unitary (‘square-top’) transitions, and emerging data indicates the quality of channel activity is influenced by many factors, including viroporin synthesis/solubilization, the lipid environment and the ionic composition of the buffers, as well as intrinsic differences between the viroporins themselves. Compounds that block viroporin channel activity are effective antiviral drugs both in vitro and in vivo. Surprisingly distinct viroporins are inhibited by the same compounds (e.g., amantadines and amiloride derivatives), despite wide sequence divergence, raising the possibility of broadly acting antiviral drugs that target viroporins. Electrophysiology of viroporins will continue to play a critical role in elucidating the functional roles viroporins play in pathogenesis and to develop new drugs to combat viroporin-encoding pathogens.

Central nervous system alterations caused by infection with the human respiratory syncytial virus

Worldwide, the human respiratory syncytial virus (hRSV) is the leading cause of infant hospitalization because of acute respiratory tract infections, including severe bronchiolitis and pneumonia. Despite intense research, to date there is neither vaccine nor treatment available to control hRSV disease burden globally. After infection, an incubation period of 3-5 days is usually followed by symptoms, such as cough and low-grade fever. However, hRSV infection can also produce a larger variety of symptoms, some of which relate to the individual's age at infection. Indeed, infants can display severe symptoms, such as dyspnea and chest wall retractions. Upon examination, crackles and wheezes are also common features that suggest infection by hRSV. Additionally, infection in infants younger than 1 year is associated with several non-specific symptoms, such as failure to thrive, periodic breathing or apnea, and feeding difficulties that usually require hospitalization. Recently, neurological symptoms have also been associated with hRSV respiratory infection and include seizures, central apnea, lethargy, feeding or swallowing difficulties, abnormalities in muscle tone, strabismus, abnormalities in the CSF, and encephalopathy. Here, we discuss recent findings linking the neurological, extrapulmonary effects of hRSV with infection and functional impairment of the CNS.

Respiratory Syncytial Virus: pathology, therapeutic drugs and prophylaxis

Human Respiratory Syncytial Virus (hRSV) is the leading cause of lower respiratory tract diseases, affecting particularly newborns and young children. This virus is able to modulate the immune response, generating a pro-inflammatory environment in the airways that causes obstruction and pulmonary alterations in the infected host. To date, no vaccines are available for human use and the first vaccine that reached clinical trials produced an enhanced hRSV-associated pathology 50 years ago, resulting in the death of two children. Currently, only two therapeutic approaches have been used to treat hRSV infection in high risk children: 1. Palivizumab, a humanized antibody against the F glycoprotein that reduces to half the number of hospitalized cases and 2. Ribavirin, which fails to have a significant therapeutic effect. A major caveat for these approaches is their high economical cost, which highlights the need of new and affordable therapeutic or prophylactic tools to treat or prevents hRSV infection. Accordingly, several efforts are in progress to understand the hRSV-associated pathology and to characterize the immune response elicited by this virus. Currently, preclinical and clinical trials are being conducted to evaluate safety and efficacy of several drugs and vaccines, which have shown promising results. In this article, we discuss the most important advances in the development of drugs and vaccines, which could eventually lead to better strategies to treat or prevent the detrimental inflammation triggered by hRSV infection.

A calcium-fortified viral matrix protein

In this issue of Structure, Leyrat and colleagues provide the first structural analysis of the human metapneumovirus (HMPV) matrix protein, a key regulator of viral assembly. Though structurally similar to other matrix proteins, two calcium binding sites suggest intriguing differences in regulation.

Paramyxovirus glycoprotein incorporation, assembly and budding: a three way dance for infectious particle production

Paramyxoviruses are a family of negative sense RNA viruses whose members cause serious diseases in humans, such as measles virus, mumps virus and respiratory syncytial virus; and in animals, such as Newcastle disease virus and rinderpest virus. Paramyxovirus particles form by assembly of the viral matrix protein, the ribonucleoprotein complex and the surface glycoproteins at the plasma membrane of infected cells and subsequent viral budding. Two major glycoproteins expressed on the viral envelope, the attachment protein and the fusion protein, promote attachment of the virus to host cells and subsequent virus-cell membrane fusion. Incorporation of the surface glycoproteins into infectious progeny particles requires coordinated interplay between the three viral structural components, driven primarily by the matrix protein. In this review, we discuss recent progress in understanding the contributions of the matrix protein and glycoproteins in driving paramyxovirus assembly and budding while focusing on the viral protein interactions underlying this process and the intracellular trafficking pathways for targeting viral components to assembly sites. Differences in the mechanisms of particle production among the different family members will be highlighted throughout.

Inhibition of the human respiratory syncytial virus small hydrophobic protein and structural variations in a bicelle environment

The small hydrophobic (SH) protein is a 64-amino-acid polypeptide encoded by the human respiratory syncytial virus (hRSV). SH protein has a single α-helical transmembrane (TM) domain that forms pentameric ion channels. Herein, we report the first inhibitor of the SH protein channel, pyronin B, and we have mapped its binding site to a conserved surface of the RSV SH pentamer, at the C-terminal end of the transmembrane domain. The validity of the SH protein structural model used has been confirmed by using a bicellar membrane-mimicking environment. However, in bicelles the α-helical stretch of the TM domain extends up to His-51, and by comparison with previous models both His-22 and His-51 adopt an interhelical/lumenal orientation relative to the channel pore. Neither His residue was found to be essential for channel activity although His-51 protonation reduced channel activity at low pH, with His-22 adopting a more structural role. The latter results are in contrast with previous patch clamp data showing channel activation at low pH, which could not be reproduced in the present work. Overall, these results establish a solid ground for future drug development targeting this important viroporin. Importance: The human respiratory syncytial virus (hRSV) is responsible for 64 million reported cases of infection and 160,000 deaths each year. Lack of adequate antivirals fuels the search for new targets for treatment. The small hydrophobic (SH) protein is a 64-amino-acid polypeptide encoded by hRSV and other paramyxoviruses, and its absence leads to viral attenuation in vivo and early apoptosis in infected cells. SH protein forms pentameric ion channels that may constitute novel drug targets, but no inhibitor for this channel activity has been reported so far. A small-molecule inhibitor, pyronin B, can reduce SH channel activity, and its likely binding site on the SH protein channel has been identified. Black lipid membrane (BLM) experiments confirm that protonation of both histidine residues reduces stability and channel activity. These results contrast with previous patch clamp data that showed low-pH activation, which we have not been able to reproduce.

The human metapneumovirus small hydrophobic protein has properties consistent with those of a viroporin and can modulate viral fusogenic activity

Human metapneumovirus (HMPV) encodes three glycoproteins: the glycoprotein, which plays a role in glycosaminoglycan binding, the fusion (F) protein, which is necessary and sufficient for both viral binding to the target cell and fusion between the cellular plasma membrane and the viral membrane, and the small hydrophobic (SH) protein, whose function is unclear. The SH protein of the closely related respiratory syncytial virus has been suggested to function as a viroporin, as it forms oligomeric structures consistent with a pore and alters membrane permeability. Our analysis indicates that both the full-length HMPV SH protein and the isolated SH protein transmembrane domain can associate into higher-order oligomers. In addition, HMPV SH expression resulted in increases in permeability to hygromycin B and alteration of subcellular localization of a fluorescent dye, indicating that SH affects membrane permeability. These results suggest that the HMPV SH protein has several characteristics consistent with a putative viroporin. Interestingly, we also report that expression of the HMPV SH protein can significantly decrease HMPV F protein-promoted membrane fusion activity, with the SH extracellular domain and transmembrane domain playing a key role in this inhibition. These results suggest that the HMPV SH protein could regulate both membrane permeability and fusion protein function during viral infection.

IMPORTANCE

Human metapneumovirus (HMPV), first identified in 2001, is a causative agent of severe respiratory tract disease worldwide. The small hydrophobic (SH) protein is one of three glycoproteins encoded by all strains of HMPV, but the function of the HMPV SH protein is unknown. We have determined that the HMPV SH protein can alter the permeability of cellular membranes, suggesting that HMPV SH is a member of a class of proteins termed viroporins, which modulate membrane permeability to facilitate critical steps in a viral life cycle. We also demonstrated that HMPV SH can inhibit the membrane fusion function of the HMPV fusion protein. This work suggests that the HMPV SH protein has several functions, though the steps in the HMPV life cycle impacted by these functions remain to be clarified.

Gene sequence variability of the three surface proteins of human respiratory syncytial virus (HRSV) in Texas

Human respiratory syncytial virus (HRSV) has three surface glycoproteins: small hydrophobic (SH), attachment (G) and fusion (F), encoded by three consecutive genes (SH-G-F). A 270-nt fragment of the G gene is used to genotype HRSV isolates. This study genotyped and investigated the variability of the gene and amino acid sequences of the three surface proteins of HRSV strains collected from 1987 to 2005 from one center. Sixty original clinical isolates and 5 prototype strains were analyzed. Sequences containing SH, F and G genes were generated, and multiple alignments and phylogenetic trees were analyzed. Genetic variability by protein domains comparing virus genotypes was assessed. Complete sequences of the SH-G-F genes were obtained for all 65 samples: HRSV-A = 35; HRSV-B = 30. In group A strains, genotypes GA5 and GA2 were predominant. For HRSV-B strains, the genotype GB4 was predominant from 1992 to 1994 and only genotype BA viruses were detected in 2004-2005. Different genetic variability at nucleotide level was detected between the genes, with G gene being the most variable and the highest variability detected in the 270-nt G fragment that is frequently used to genotype the virus. High variability (>10%) was also detected in the signal peptide and transmembrane domains of the F gene of HRSV A strains. Variability among the HRSV strains resulting in non-synonymous changes was detected in hypervariable domains of G protein, the signal peptide of the F protein, a not previously defined domain in the F protein, and the antigenic site Ø in the pre-fusion F. Divergent trends were observed between HRSV -A and -B groups for some functional domains. A diverse population of HRSV -A and -B genotypes circulated in Houston during an 18 year period. We hypothesize that diverse sequence variation of the surface protein genes provide HRSV strains a survival advantage in a partially immune-protected community.

De novo selection of oncogenes

All cellular proteins are derived from preexisting ones by natural selection. Because of the random nature of this process, many potentially useful protein structures never arose or were discarded during evolution. Here, we used a single round of genetic selection in mouse cells to isolate chemically simple, biologically active transmembrane proteins that do not contain any amino acid sequences from preexisting proteins. We screened a retroviral library expressing hundreds of thousands of proteins consisting of hydrophobic amino acids in random order to isolate four 29-aa proteins that induced focus formation in mouse and human fibroblasts and tumors in mice. These proteins share no amino acid sequences with known cellular or viral proteins, and the simplest of them contains only seven different amino acids. They transformed cells by forming a stable complex with the platelet-derived growth factor β receptor transmembrane domain and causing ligand-independent receptor activation. We term this approach de novo selection and suggest that it can be used to generate structures and activities not observed in nature, create prototypes for novel research reagents and therapeutics, and provide insight into cell biology, transmembrane protein-protein interactions, and possibly virus evolution and the origin of life.

Structure and self-assembly of the calcium binding matrix protein of human metapneumovirus

The matrix protein (M) of paramyxoviruses plays a key role in determining virion morphology by directing viral assembly and budding. Here, we report the crystal structure of the human metapneumovirus M at 2.8 Å resolution in its native dimeric state. The structure reveals the presence of a high-affinity Ca²⁺ binding site. Molecular dynamics simulations (MDS) predict a secondary lower-affinity site that correlates well with data from fluorescence-based thermal shift assays. By combining small-angle X-ray scattering with MDS and ensemble analysis, we captured the structure and dynamics of M in solution. Our analysis reveals a large positively charged patch on the protein surface that is involved in membrane interaction. Structural analysis of DOPC-induced polymerization of M into helical filaments using electron microscopy leads to a model of M self-assembly. The conservation of the Ca²⁺ binding sites suggests a role for calcium in the replication and morphogenesis of pneumoviruses.

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M is a calcium binding protein

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Calcium stabilizes the structure of M

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M forms an obligate dimer in solution

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M self-assembles in the presence of lipids

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The Paramyxoviruses and the Filoviruses have a common ancestor

The role of dendritic cells in innate and adaptive immunity to respiratory syncytial virus, and implications for vaccine development

Respiratory syncytial virus (RSV) is a common human pathogen that causes cold-like symptoms in most healthy adults and children. However, RSV often moves into the lower respiratory tract in infants and young children predisposed to respiratory illness, making it the most common cause of pediatric broncheolitis and pneumonia. The development of an appropriate balanced immune response is critical for recovery from RSV, while an unbalanced and/or excessively vigorous response may lead to immunopathogenesis. Different dendritic cell (DC) subsets influence the magnitude and quality of the host response to RSV infection, with myeloid DCs mediating and plasmacytoid DCs modulating immunopathology. Furthermore, stimulation of DCs through Toll-like receptors is essential for induction of protective immunity to RSV. These characteristics have implications for the rational design of a RSV vaccine.

The comparative genomics of human respiratory syncytial virus subgroups A and B: genetic variability and molecular evolutionary dynamics

Genomic variation and related evolutionary dynamics of human respiratory syncytial virus (RSV), a common causative agent of severe lower respiratory tract infections, may affect its transmission behavior. RSV evolutionary patterns are likely to be influenced by a precarious interplay between selection favoring variants with higher replicative fitness and variants that evade host immune responses. Studying RSV genetic variation can reveal both the genes and the individual codons within these genes that are most crucial for RSV survival. In this study, we conducted genetic diversity and evolutionary rate analyses on 36 RSV subgroup B (RSV-B) whole-genome sequences. The attachment protein, G, was the most variable protein; accordingly, the G gene had a higher substitution rate than other RSV-B genes. Overall, less genetic variability was found among the available RSV-B genome sequences than among RSV-A genome sequences in a comparable sample. The mean substitution rates of the two subgroups were, however, similar (for subgroup A, 6.47 × 10(-4) substitutions/site/year [95% credible interval {CI 95%}, 5.56 × 10(-4) to 7.38 × 10(-4)]; for subgroup B, 7.76 × 10(-4) substitutions/site/year [CI 95%, 6.89 × 10(-4) to 8.58 × 10(-4)]), with the time to their most recent common ancestors (TMRCAs) being much lower for RSV-B (19 years) than for RSV-A (46.8 years). The more recent RSV-B TMRCA is apparently the result of a genetic bottleneck that, over longer time scales, is still compatible with neutral population dynamics. Whereas the immunogenic G protein seems to require high substitution rates to ensure immune evasion, strong purifying selection in conserved proteins such as the fusion protein and nucleocapsid protein is likely essential to preserve RSV viability.

Advances in understanding respiratory syncytial virus infection in airway epithelial cells and consequential effects on the immune response

This article reviews aspects of respiratory syncytial virus (RSV) infection in airway epithelial cells (AECs), including cytopathogenesis, entry, replication and the induction of immune response to the virus, including a new role for thymic stromal lymphopoietin in RSV immunopathology.

Development of asthma is determined by the age-dependent host response to respiratory virus infection: therapeutic implications

Lower respiratory tract virus infections are the major cause of asthma exacerbations. Severity of infection and age at initial encounter with virus appear to be major determinants of the risk for allergic asthma later in life. In animal models, reinfection of mice initially infected as neonates leads to markedly enhanced alterations in airway function and inflammation, unlike reinfection of older mice. Both innate and adaptive immune responses contribute to this susceptibility with lung dendritic cells showing marked differences in phenotype and function in young compared to older mice, and these differences are further enhanced following virus infection. These findings have implications for therapeutic targeting, for example, of RSV G and F surface proteins at different stages of the response to infection.