Contribution of cysteine residues in the extracellular domain of the F protein of human respiratory syncytial virus to its function

Host immune response to respiratory syncytial virus infection and its contribution to protection and susceptibility in adults: a systematic literature review

INTRODUCTION

Respiratory syncytial virus (RSV) is an important pathogen in infants, children, older adults, and those with comorbidities. Mechanisms involving viral proteins appear to underlie the ability of RSV to evade and modulate host immunity. We aimed to understand virus- and host-dependent factors regulating the development and severity of RSV infection, as related to the prevention and treatment of RSV-associated disease in adults, through a systematic literature review (SLR).

METHODS

An SLR was conducted to identify immune mechanisms involved in the protective response to RSV infection in adults, and responses that may contribute to the development of severe disease. Concurrent searches (MEDLINE/Embase) using embase.com identified relevant papers published between 1990 and 19 April 2023.

RESULTS

Of 1813 records identified, 113 were selected for review. Inclusion criteria were based on relevant patient populations, outcomes, and study methodologies. RSV is common, recurrent, and associated with high morbidity and mortality in older adults and people with underlying chronic diseases. Immune responses differ between younger and older adults. The approval of effective vaccines may protect older individuals from symptomatic RSV infection.

CONCLUSIONS

We established the complexities of RSV immune response, but further research is required to fully understand anti-RSV immunology.

Preventing RSV Infection in Children: Current Passive Immunizations and Vaccine Development

Human respiratory syncytial virus (RSV) is a leading cause of acute respiratory tract infection and lower respiratory tract infection, associated with high morbidity and mortality in young children, the elderly, and immunocompromised individuals. Initial attempts to develop an RSV vaccine in the 1960s were faced with a setback due to the enhanced RSV disease developed by vaccinated children. More recent advancements have led to the generation of RSV vaccines for older adults and pregnant women. However, there are still no commercially available RSV vaccines for infants. This work summarizes the current passive immunizations and the ongoing efforts to develop an RSV vaccine for infants.

Recent Progress toward the Discovery of Small Molecules as Novel Anti-Respiratory Syncytial Virus Agents

Respiratory syncytial virus (RSV) stands as the foremost cause of infant hospitalization globally, ranking second only to malaria in terms of infant mortality. Although three vaccines have recently been approved for the prophylaxis of adults aged 60 and above, and pregnant women, there is currently no effective antiviral drug for treating RSV infections. The only preventive measure for infants at high risk of severe RSV disease is passive immunization through monoclonal antibodies. This Perspective offers an overview of the latest advancements in RSV drug discovery of small molecule antivirals, with particular focus on the promising findings from agents targeting the fusion and polymerase proteins. A comprehensive reflection on the current state of RSV research is also given, drawing inspiration from the lessons gleaned from HCV and HIV, while also considering the impact of the recent approval of the three vaccines.

Microbiological and epidemiological features of respiratory syncytial virus

The properties of the main surface proteins and the viral cycle of the respiratory syncytial virus (RSV) make it an attractive pathogen from the perspective of microbiology. The virus gets its name from the manner it infects cells, which enables it to produce syncytia, which allow the virus' genetic material to move across cells without having to release viral offspring to the cellular exterior, reducing immune system identification. This causes a disease with a high impact in both children and adults over 60, which has sparked the development of several preventive interventions based on vaccines and monoclonal antibodies for both age groups. The epidemiological characteristics of this virus, which circulates in epidemics throughout the coldest months of the year and exhibits a marked genetic and antigenic drift due to its high mutation capability, must be taken into consideration while using these preventive methods. The most important microbiological and epidemiological elements of RSV are covered in this study, along with how they have affected the creation of preventive medications and their use in the future.

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.

Current state and challenges in respiratory syncytial virus drug discovery and development

Human respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections (LRTI) in young children and elderly people worldwide. Recent significant progress in our understanding of the structure and function of RSV proteins has led to the discovery of several clinical candidates targeting RSV fusion and replication. These include both the development of novel small molecule interventions and the isolation of potent monoclonal antibodies. In this review, we summarize the state-of-the-art of RSV drug discovery, with a focus on the characteristics of the candidates that reached the clinical stage of development. We also discuss the lessons learned from failed and discontinued clinical developments and highlight the challenges that remain for development of RSV therapies.

Vaccination with prefusion-stabilized respiratory syncytial virus fusion protein elicits antibodies targeting a membrane-proximal epitope

IMPORTANCE

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and pneumonia in infants, infecting all children by age 5. RSV also causes substantial morbidity and mortality in older adults, and a vaccine for older adults based on a prefusion-stabilized form of the viral F glycoprotein was recently approved by the FDA. Here, we investigate a set of antibodies that belong to the same public clonotype and were isolated from individuals vaccinated with a prefusion-stabilized RSV F protein. Our results reveal that these antibodies are highly potent and recognize a previously uncharacterized antigenic site on the prefusion F protein. Vaccination with prefusion RSV F proteins appears to boost the elicitation of these neutralizing antibodies, which are not commonly elicited by natural infection.

The RSV F p27 peptide: current knowledge, important questions

Respiratory syncytial virus (RSV) remains a leading cause of hospitalizations and death for young children and adults over 65. The worldwide impact of RSV has prioritized the search for an RSV vaccine, with most targeting the critical fusion (F) protein. However, questions remain about the mechanism of RSV entry and RSV F triggering and fusion promotion. This review highlights these questions, specifically those surrounding a cleaved 27 amino acids long peptide within F, p27.

Respiratory syncytial virus: from pathogenesis to potential therapeutic strategies

Respiratory syncytial virus (RSV) is one of the most important viral pathogens causing respiratory tract infection in infants, the elderly and people with poor immune function, which causes a huge disease burden worldwide every year. It has been more than 60 years since RSV was discovered, and the palivizumab monoclonal antibody, the only approved specific treatment, is limited to use for passive immunoprophylaxis in high-risk infants; no other intervention has been approved to date. However, in the past decade, substantial progress has been made in characterizing the structure and function of RSV components, their interactions with host surface molecules, and the host innate and adaptive immune response to infection. In addition, basic and important findings have also piqued widespread interest among researchers and pharmaceutical companies searching for effective interventions for RSV infection. A large number of promising monoclonal antibodies and inhibitors have been screened, and new vaccine candidates have been designed for clinical evaluation. In this review, we first briefly introduce the structural composition, host cell surface receptors and life cycle of RSV virions. Then, we discuss the latest findings related to the pathogenesis of RSV. We also focus on the latest clinical progress in the prevention and treatment of RSV infection through the development of monoclonal antibodies, vaccines and small-molecule inhibitors. Finally, we look forward to the prospects and challenges of future RSV research and clinical intervention.

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.

Respiratory syncytial virus entry and how to block it

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease in young children and elderly people. Although the virus was isolated in 1955, an effective RSV vaccine has not been developed, and the only licensed intervention is passive immunoprophylaxis of high-risk infants with a humanized monoclonal antibody. During the past 5 years, however, there has been substantial progress in our understanding of the structure and function of the RSV glycoproteins and their interactions with host cell factors that mediate entry. This period has coincided with renewed interest in developing effective interventions, including the isolation of potent monoclonal antibodies and small molecules and the design of novel vaccine candidates. In this Review, we summarize the recent findings that have begun to elucidate RSV entry mechanisms, describe progress on the development of new interventions and conclude with a perspective on gaps in our knowledge that require further investigation.

Viral Entry Properties Required for Fitness in Humans Are Lost through Rapid Genomic Change during Viral Isolation

Human parainfluenza viruses cause a large burden of human respiratory illness. While much research relies upon viruses grown in cultured immortalized cells, human parainfluenza virus 3 (HPIV-3) evolves in culture. Cultured viruses differ in their properties compared to clinical strains. We present a genome-wide survey of HPIV-3 adaptations to culture using metagenomic next-generation sequencing of matched pairs of clinical samples and primary culture isolates (zero passage virus). Nonsynonymous changes arose during primary viral isolation, almost entirely in the genes encoding the two surface glycoproteins-the receptor binding protein hemagglutinin-neuraminidase (HN) or the fusion protein (F). We recovered genomes from 95 HPIV-3 primary culture isolates and 23 HPIV-3 strains directly from clinical samples. HN mutations arising during primary viral isolation resulted in substitutions at HN's dimerization/F-interaction site, a site critical for activation of viral fusion. Alterations in HN dimer interface residues known to favor infection in culture occurred within 4 days (H552 and N556). A novel cluster of residues at a different face of the HN dimer interface emerged (P241 and R242) and imply a role in HPIV-3-mediated fusion. Functional characterization of these culture-associated HN mutations in a clinical isolate background revealed acquisition of the fusogenic phenotype associated with cultured HPIV-3; the HN-F complex showed enhanced fusion and decreased receptor-cleaving activity. These results utilize a method for identifying genome-wide changes associated with brief adaptation to culture to highlight the notion that even brief exposure to immortalized cells may affect key viral properties and underscore the balance of features of the HN-F complex required for fitness by circulating viruses.IMPORTANCE Human parainfluenza virus 3 is an important cause of morbidity and mortality among infants, the immunocompromised, and the elderly. Using deep genomic sequencing of HPIV-3-positive clinical material and its subsequent viral isolate, we discover a number of known and novel coding mutations in the main HPIV-3 attachment protein HN during brief exposure to immortalized cells. These mutations significantly alter function of the fusion complex, increasing fusion promotion by HN as well as generally decreasing neuraminidase activity and increasing HN-receptor engagement. These results show that viruses may evolve rapidly in culture even during primary isolation of the virus and before the first passage and reveal features of fitness for humans that are obscured by rapid adaptation to laboratory conditions.

GENETIC AND ANTIGENIC CHARACTERISTICS OF RESPIRATORY SYNCYTIAL VIRUS STRAINS ISOLATED IN ST. PETERSBURG IN 2013-2016

Antigenic and genetic characteristics of Russian RSV isolates are presented for the first time. Of the 69 strains isolated in St. Petersburg, 93% belonged to the RSV-A antigenic group. The antigenic variations in the F-protein RSV were analyzed using a panel from 6 monoclonal antibodies by the method of micro-cultural ELISA. Depending on the decrease in the effectiveness of interaction with monoclonal antibodies (relative to the reference strain Long), RSV-A isolates were divided into 4 antigenic subgroups. The results of 24 isolates sequencing showed that more than 60% of them had substitutions in significant F-protein sites compared to the ON67-1210A reference strain of the current RSV genotype ON1/GA2. The most variable were the signal peptide and antigenic site II. When comparing the results of ELISA and sequencing, it was not possible to identify any specific key substitutions in the amino acid sequence of the F-protein that affect the interaction of the virus with antibodies. The nucleotide sequence of the F-gene from 19 of the 24 characterized isolates was close to that of ON67-1210A reference virus and was significantly different from RSV-A Long and A2 viruses. A separate group consisted of 5 strains, in which the F-protein structure was approximated to RSV Long.

Neutralizing epitopes of RSV and palivizumab resistance in Japan

Respiratory Syncytial Virus (RSV) is one of the most important viral pathogen related to acute lower respiratory infection in young children. The virus surface envelope contains the G, F, and SH proteins as spike proteins. The F protein is considered to be a major antigenic target for the neutralizing (NT) epitope as only the F protein is essential for cell infection among the three viral envelope proteins, and it is more highly conserved than the G protein. Recently, four antigenic targets related to NT activity have been reported;site I, site II, site IV, and site zero (0). Site II is the target for palivizumab used throughout the world to suppress severe RSV infection as passive immunity in high-risk children since 1998. Under the recent conditions in which indications for palivizumab administered subjects are being expanded, palivizumab-resistant mutations have been confirmed overseas in children with RSV infection, although they remain infrequent. Therefore, continuous genetic analysis of the palivizumab-binding region of the F protein is necessary. In addition, as vaccine development progresses, RSV infection control is expected to improve greatly over the next decade.

Modulation of the host immune response by respiratory syncytial virus proteins

Respiratory syncytial virus (RSV) causes severe respiratory disease in both the very young and the elderly. Nearly all individuals become infected in early childhood, and reinfections with the virus are common throughout life. Despite its clinical impact, there remains no licensed RSV vaccine. RSV infection in the respiratory tract induces an inflammatory response by the host to facilitate efficient clearance of the virus. However, the host immune response also contributes to the respiratory disease observed following an RSV infection. RSV has evolved several mechanisms to evade the host immune response and promote virus replication through interactions between RSV proteins and immune components. In contrast, some RSV proteins also play critical roles in activating, rather than suppressing, host immunity. In this review, we discuss the interactions between individual RSV proteins and host factors that modulate the immune response and the implications of these interactions for the course of an RSV infection.

Identification and functional analysis of inter-subunit disulfide bonds of the F protein of Helicoverpa armigera nucleopolyhedrovirus

The major envelope fusion protein F of the budded virus of baculoviruses consists of two disulfide-linked subunits: an N-terminal F2 subunit and a C-terminal, membrane-anchored F1 subunit. There is one cysteine in F2 and there are 15 cysteines in F1, but their role in disulfide linking is largely unknown. In this study, the inter- and intra-subunit disulfide bonds of the Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) F protein were analysed by site-directed mutagenesis. Results indicated that in a functional F protein, an inter-subunit disulfide bond exists between amino acids C108 (F2) and C241 (F1). When C241 was mutated, an alternative disulfide bond was formed between C108 and C232, rendering F non-functional. No inter-subunit bridge was observed in a double C232/C241 mutant of F1. C403 was not involved in the formation of inter-subunit disulfide bonding, but mutation of this amino acid decreased viral infectivity significantly, suggesting that it might be involved in intra-subunit disulfide bonds. The influence of reductant [tris(2-carboxyethyl) phosphine (TCEP)] and free-thiol inhibitors [4-acetamido-4'-maleimidylstilbene 2,2'-disulfonic acid (AMS) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB)] on the infectivity of HearNPV was tested. The results indicated that TCEP greatly decreased the infection of HzAm1 cells by HearNPV. In contrast, AMS and DTNB had no inhibitory effect on viral infectivity. The data suggested that free thiol/disulfide isomerization was not likely to play a role in viral entry and infectivity.

Detection of respiratory syncytial virus fusion protein variants between 2009 and 2012 in China

Respiratory syncytial virus (RSV) causes respiratory tract infection, particularly acute lower respiratory tract infection (ALRTI), in early childhood. The RSV fusion protein (F protein) is an important surface protein, and it is the target of both cytotoxic T lymphocytes (CTL) and neutralizing antibodies; thus, it may be useful as a candidate for vaccine research. This study investigated the genetic diversity of the RSV F protein. To this end, a total of 1800 nasopharyngeal aspirates from hospitalized children with ALRTI were collected for virus isolation between June 2009 and March 2012. There were 333 RSV-positive cases (277 cases of RSV A, 55 of RSV B, and 1 with both RSV A and RSV B), accounting for 18.5 % of the total cases. Next, 130 clinical strains (107 of RSV A, 23 of RSV B) were selected for F gene sequencing. Phylogenetic analysis revealed that the F gene sequence is highly conserved, with significant amino acid changes at residues 16, 25, 45, 102, 122, 124, 209, and 447. Mutations in human histocompatibility leukocyte antigen (HLA)-restricted CTL epitopes were also observed. Variations in RSV A F protein at the palivizumab binding site 276 (N→S) increased between 2009 and 2012 and became predominant. Western blot analysis and microneutralization data showed a substitution at residue 276 (N→S) in RSV A that did not cause resistance to palivizumab. In conclusion, the RSV F gene is geographically and temporally conserved, but limited genetic variations were still observed. These data could be helpful for the development of vaccines against RSV infection.

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.

A single amino acid in the F2 subunit of respiratory syncytial virus fusion protein alters growth and fusogenicity

Respiratory syncytial virus (RSV) causes severe lower respiratory tract infection in children, especially in infants less than 1 year of age. There are currently no licensed vaccines against RSV. rA2ΔM2-2 is a promising live-attenuated vaccine candidate that is currently being evaluated in the clinic. Attenuation of rA2ΔM2-2 is achieved by a single deletion of the M2-2 gene, which disrupts the balance between viral transcription and replication. Whilst performing a manufacturing feasibility study in a serum-free adapted Vero cell line, differences in growth kinetics and cytopathic effect (CPE) were identified between two rA2ΔM2-2 vaccine candidates. Comparative sequence analysis identified four amino acid differences between the two vaccine viruses. Recombinant rA2ΔM2-2 viruses carrying each of the four amino acid differences identified a K66E mutation in the F₂ fragment of the fusion (F) protein as the cause of the growth and CPE differences. Syncytium-formation experiments with RSV F protein carrying mutations at aa 66 suggested that a change in charge at this residue within the F₂ fragment can have a significant impact on fusion.

Structure and function of respiratory syncytial virus surface glycoproteins

The two major glycoproteins on the surface of the respiratory syncytial virus (RSV) virion, the attachment glycoprotein (G) and the fusion glycoprotein (F), control the initial phases of infection. G targets the ciliated cells of the airways, and F causes the virion membrane to fuse with the target cell membrane. The F protein is the major target for antiviral drug development, and both G and F glycoproteins are the antigens targeted by neutralizing antibodies induced by infection. In this chapter, we review the structure and function of the RSV surface glycoproteins, including recent X-ray crystallographic data of the F glycoprotein in its pre- and postfusion conformations, and discuss how this information informs antigen selection and vaccine development.

Characterization of neutralizing affinity-matured human respiratory syncytial virus F binding antibodies in the sub-picomolar affinity range

In the human adaptation and optimization of a mouse anti-human respiratory syncytial virus neutralizing antibody, affinity assessment was crucial to distinguish among potential candidates and to evaluate whether this correlated with function in vitro and in vivo. This affinity assessment was complicated by the trimeric nature of the antigen target, respiratory syncytial virus F (RSV-F) glycoprotein. In the initial affinity screen, surface plasmon resonance was used to determine the intrinsic binding affinities of anti-RSV-F Fab and immunoglobulin G (IgG) to the extracellular domain of RSV-F. This assessment required minimal biotinylation of the RSV-F protein and design of a capture strategy to minimize avidity effects. Approximately 30 Fabs were selected from three optimization phage display libraries on the basis of an initial ELISA screen. Surface plasmon resonance analysis demonstrated the success of optimization with some candidates from the screened libraries having low picomolar dissociation constants, more than 700-fold tighter than the parental monoclonal antibody (B21M). The affinities of these antibodies were further evaluated by a kinetic exclusion assay, a solution binding technology. One IgG (monoclonal antibody 029) displayed a low picomolar K(D) comparable with that of motavizumab, an RSV antibody in clinical study. Kinetic exclusion assay showed that two other of the matured IgGs (011 and 019) had sub-picomolar dissociation constants that could not be resolved further. We discuss the relevance of these interaction analysis results in the light of recently published data on the mechanism of F-driven viral fusion during paramyxoviral infection and 101F epitope conservation revealed from the recent crystal structure of RSV-F in the post-fusion state.

Cholesterol-rich microdomains as docking platforms for respiratory syncytial virus in normal human bronchial epithelial cells

Respiratory syncytial virus (RSV) is one of the major causes of respiratory infections in children, and it is the main pathogen causing bronchiolitis in infants. The binding and entry mechanism by which RSV infects respiratory epithelial cells has not yet been determined. In this study, the earliest stages of RSV infection in normal human bronchial epithelial cells were probed by tracking virions with fluorescent lipophilic dyes in their membranes. Virions colocalized with cholesterol-containing plasma membrane microdomains, identified by their ability to bind cholera toxin subunit B. Consistent with an important role for cholesterol in RSV infection, cholesterol depletion profoundly inhibited RSV infection, while cholesterol repletion reversed this inhibition. Merger of the outer leaflets of the viral envelope and the cell membrane appeared to be triggered at these sites. Using small-molecule inhibitors, RSV infection was found to be sensitive to Pak1 inhibition, suggesting the requirement of a subsequent step of cytoskeletal reorganization that could involve plasma membrane rearrangements or endocytosis. It appears that RSV entry depends on its ability to dock to cholesterol-rich microdomains (lipid rafts) in the plasma membrane where hemifusion events begin, assisted by a Pak1-dependent process.

Structure of respiratory syncytial virus fusion glycoprotein in the postfusion conformation reveals preservation of neutralizing epitopes

Respiratory syncytial virus (RSV) invades host cells via a type I fusion (F) glycoprotein that undergoes dramatic structural rearrangements during the fusion process. Neutralizing monoclonal antibodies, such as 101F, palivizumab, and motavizumab, target two major antigenic sites on the RSV F glycoprotein. The structures of these sites as peptide complexes with motavizumab and 101F have been previously determined, but a structure for the trimeric RSV F glycoprotein ectodomain has remained elusive. To address this issue, we undertook structural and biophysical studies on stable ectodomain constructs. Here, we present the 2.8-Å crystal structure of the trimeric RSV F ectodomain in its postfusion conformation. The structure revealed that the 101F and motavizumab epitopes are present in the postfusion state and that their conformations are similar to those observed in the antibody-bound peptide structures. Both antibodies bound the postfusion F glycoprotein with high affinity in surface plasmon resonance experiments. Modeling of the antibodies bound to the F glycoprotein predicts that the 101F epitope is larger than the linear peptide and restricted to a single protomer in the trimer, whereas motavizumab likely contacts residues on two protomers, indicating a quaternary epitope. Mechanistically, these results suggest that 101F and motavizumab can bind to multiple conformations of the fusion glycoprotein and can neutralize late in the entry process. The structural preservation of neutralizing epitopes in the postfusion state suggests that this conformation can elicit neutralizing antibodies and serve as a useful vaccine antigen.

Soluble respiratory syncytial virus fusion protein in the fully cleaved, pretriggered state is triggered by exposure to low-molarity buffer

The paramyxovirus fusion (F) glycoprotein is anchored in the virion membrane in a metastable, pretriggered form. Once triggered, the F protein undergoes a dramatic conformational extension that inserts its hydrophobic fusion peptide into the target cell membrane, then folds back on itself to bring the membranes together and initiate fusion. Unlike most other paramyxoviruses, the respiratory syncytial virus (RSV) F protein alone is sufficient to mediate membrane fusion and virus infection. To study the triggering mechanism of the RSV F protein, we have generated a soluble F (sF) protein by replacing the transmembrane and cytoplasmic tail domains with a 6His tag. The sF protein is secreted efficiently from 293T cells in a fully cleaved form. It is recognized by neutralizing monoclonal antibodies, appears spherical by electron microscopic analysis, and is not aggregated, all consistent with a native, pretriggered trimer. The sF protein was purified on a Ni(+2) column and eluted with 50 mM phosphate buffer containing 500 mM NaCl and 250 mM imidazole. Dialysis against 10 mM buffer caused the sF protein to trigger, forming "hat pin"-shaped molecules that aggregated as rosettes, characteristic of the posttriggered form. Further dialysis experiments indicated that the efficiency of triggering correlated well with the reduction of buffer molarity. Reduction of buffer molarity by dilution also resulted in exposure of the fusion peptide, as detected by liposome association, confirming sF protein triggering. Mutation of the furin cleavage site adjacent to the fusion peptide prevented liposome association, further confirming that association is via the fusion peptide.

Characterization of the epitope for anti-human respiratory syncytial virus F protein monoclonal antibody 101F using synthetic peptides and genetic approaches

Chimeric 101F (ch101F) is a mouse–human chimeric anti-human respiratory syncytial virus (HRSV) neutralizing antibody that recognizes residues within antigenic site IV, V, VI of the fusion (F) glycoprotein. The binding of ch101F to a series of peptides overlapping aa 422–438 spanning antigenic site IV, V, VI was analysed. Residues 423–436 comprise the minimal peptide sequence for ch101F binding. Substitution analysis revealed that R429 and K433 are critical for ch101F binding, whilst K427 makes a minor contribution. Binding of ch101F to a series of single mutations at positions 427, 429 and 433 in the F protein expressed recombinantly on the cell surface confirmed the peptide results. Sequence analysis of viruses selected for resistance to neutralization by ch101F indicated that a single change (K433T) in the F protein allowed ch101F escape. The results confirm that ch101F and palivizumab have different epitope specificity and define key residues for ch101F recognition.

Relationship between the loss of neutralizing antibody binding and fusion activity of the F protein of human respiratory syncytial virus

To elucidate the relationship between resistance to HRSV neutralizing antibodies directed against the F protein and the fusion activity of the F protein, a recombinant approach was used to generate a panel of mutations in the major antigenic sites of the F protein. These mutant proteins were assayed for neutralizing mAb binding (ch101F, palivizumab, and MAb19), level of expression, post-translational processing, cell surface expression, and fusion activity. Functional analysis of the fusion activity of the panel of mutations revealed that the fusion activity of the F protein is tolerant to multiple changes in the site II and IV/V/VI region in contrast with the somewhat limited spectrum of changes in the F protein identified from the isolation of HRSV neutralizing antibody virus escape mutants. This finding suggests that aspects other than fusion activity may limit the spectrum of changes tolerated within the F protein that are selected for by neutralizing antibodies.

A comparative immunogenicity study of HIV-1 virus-like particles bearing various forms of envelope proteins, particles bearing no envelope and soluble monomeric gp120

To assess the potential of native Envelope glycoprotein (Env) trimers as neutralizing antibody vaccines, we immunized guinea pigs with three types of VLPs and soluble gp120. Particles included "SOS-VLPs" (bearing disulfide-shackled functional trimers), "UNC-VLPs" (bearing uncleaved nonfunctional Env) and "naked VLPs" (bearing no Env). The SOS-VLPs were found to have a density of about 27 native trimers per particle, approximately twice that of live inactivated HIV-1 preparations. As immunogens, UNC- and SOS-VLP rapidly elicited anti-gp120 antibodies focused on the V3 loop and the gp120 coreceptor binding site. Reactivity to the gp41 immunodominant domain was absent in SOS-VLP sera, presumably because gp120-gp41 association is stabilized, effectively covering this epitope. Gp120-immune sera reacted with the receptor binding sites of gp120 and were less focused on the V3 loop. Some Env-VLP sera neutralized primary isolates at modest titers. The measurement of neutralization was found to be affected by the cell lines used. Depending on the assay particulars, non-Env specific antibodies in VLP sera could enhance infection, or nonspecifically neutralize. However, a neutralization assay using TZM-BL cells was essentially clear of these effects. We also describe a native trimer binding assay to confirm neutralization activity in a manner that completely eliminates nonspecific effects. Overall, our data suggests that Env-VLP sera were primarily focused on nonfunctional forms of Env on VLP surfaces, possibly gp120/gp41 monomers and not the trimers. Therefore, to make progress toward a more effective VLP-based vaccine, we will need to find ways to refocus the attention of B cells on native trimers.

Intranasal vaccination with peptides and cholera toxin subunit B as adjuvant to enhance mucosal and systemic immunity to respiratory syncytial virus

Amino acid sequences 200-225 and 255-278 of the F protein of human respiratory syncytial virus (HRSV) are T cell epitopes (Bourgeois et al., 1991; Corvaisier et al, 1993). Peptides corresponding to these two regions were synthesized and coupled with keyhole limpet haemocyanin (KLH). The two conjugated proteins were administered intranasally to BALB/c mice alone or together with cholera toxin B (CTB). ELISAs revealed that the mixture of the conjugates with CTB increased not only the systemic response but also the mucosal immune response of the saliva. The systemic response was lower and the mucosal immune response was undetectable in mice immunized with the conjugates on their own. These results suggest that these two peptide sequences are effective epitopes for inducing systemic and mucosal immune responses in conjunction with CTB, and may provide the basis for a nasal peptide vaccine against RSV for human use.

A conserved region between the heptad repeats of paramyxovirus fusion proteins is critical for proper F protein folding

Paramyxoviruses are a diverse family that utilizes a fusion (F) protein to enter cells via fusion of the viral lipid bilayer with a target cell membrane. Although certain regions of the F protein are known to play critical roles in membrane fusion, the function of much of the protein remains unclear. Sequence alignment of a set of paramyxovirus F proteins and analysis utilizing Block Maker identified a region of conserved amino acid sequence in a large domain between the heptad repeats of F1, designated CBF1. We employed site-directed mutagenesis to analyze the function of completely conserved residues of CBF1 in both the simian virus 5 (SV5) and Hendra virus F proteins. The majority of CBF1 point mutants were deficient in homotrimer formation, proteolytic processing, and transport to the cell surface. For some SV5 F mutants, proteolytic cleavage and surface expression could be restored by expression at 30 degrees C, and varying levels of fusion promotion were observed at this temperature. In addition, the mutant SV5 F V402A displayed a hyperfusogenic phenotype at both 30 and 37 degrees C, indicating that this mutation allows for efficient fusion with only an extremely small amount of cleaved, active protein. The recently published prefusogenic structure of PIV5/SV5 F (Yin, H. S., et al. (2006) Nature 439, 38-44) indicates that residues within and flanking CBF1 interact with the fusion peptide domain. Together, these data suggest that CBF1-fusion peptide interactions are critical for the initial folding of paramyxovirus F proteins from this important viral family and can also modulate subsequent membrane fusion promotion.