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

Designing a T cell multi-epitope vaccine against hRSV with reverse vaccinology: An immunoinformatics approach

As an infectious viral pathogen, human respiratory syncytial virus (hRSV) can cause severe respiratory infections and is recognized as one of the highest priority pathogens by the World Health Organization (WHO). Although vaccines play an important role in disease prevention and transmission, the wild-type virus is usually prone to immune escape due to the relatively high mutation rate of biological proteins. Therefore, designing a broad-spectrum hRSV vaccine is essential to provide extensive protection against multiple viral variants. Using a consensus sequence approach, we designed a broad-spectrum T-cell epitope vaccine composed of 385 amino acids, consisting of 12 CTLs and 5 HTLs from the fusion protein and glycoprotein. The designed multi-epitope vaccine was expected to have non-allergenicity, high population coverage, strong antigenicity and immunogenicity, appropriate physical and chemical properties, and high solubility. Meanwhile, the structure of the vaccine had a high similarity to that of the natural virus. In addition, through structural biology analysis, the constructed vaccine achieved robust structural compactness and binding stability. Computer-generated immunological simulations indicated that the vaccine could elicit realistic immune responses in humans. The designed vaccine showed good binding affinity and molecular and immune simulation. In conclusion, the broad-spectrum hRSV vaccine could be an excellent candidate for preventing hRSV infection. The employed prediction pipeline was proved to be an efficient method for screening immunogenic epitopes of additional pathogens.

A single amino acid mutation alters multiple neutralization epitopes in the respiratory syncytial virus fusion glycoprotein

Respiratory syncytial virus (RSV) is the leading cause of infant hospitalization. All current RSV therapeutics, including antibody prophylaxis and adult vaccination, target the RSV fusion glycoprotein (RSV-F). The seven neutralization sites on RSV-F are highly conserved and infrequently mutate. Here, we show that a single amino acid mutation at position 305 in RSV-F significantly alters antigenic recognition of RSV-F binding sites and reduces the susceptibility of RSV to neutralizing antibodies. In an in vitro evolution assay, we show that RSV-F L305I occurs in a majority of RSV quasi-species. Computational modeling predicted that the L305I mutation altered the epitope landscape of RSV-F, resulting in changes to neutralizing antibody sensitivity and affinity towards the RSV-F glycoprotein. Screening of published RSV-F sequences revealed that position 305 in RSV-F was conserved with a leucine and isoleucine in RSV-A and RSV-B subtypes respectively. Our study suggests that select amino acids in RSV-F may act as 'conformational switches' for RSV to evade host serum antibodies. This work has important implications in understanding RSV evolution and resistance as it suggests that mutational resistance to neutralizing antibodies can occur at sites distal to antigenic epitopes, significantly altering antibody sensitivity to viral infection. These unique antigenic landscape changes should be considered in the context of vaccine and therapeutic development in order to better understand viral mechanisms of evasion and resistance.

Group IIC self-splicing intron-derived novel circular RNA vaccine elicits superior immune response against RSV

Introduction

The remarkable commercial success of mRNA vaccines against COVID-19 and tumors, along with their potential as therapeutic drugs, has significantly boosted enthusiasm for circular RNAs (circRNA) as a promising next-generation therapeutic platform. The development of novel circRNA cyclization technologies represents a significant leap forward in RNA engineering and therapeutic applications. Recent advancements in group I and IIB self-splicing intron-based ribozymes have enabled precise cyclization of RNA molecules. However, this approach faces significant limitations, including low cyclization efficiency and the requirement for additional additives, which restrict its broader application. Group IIC self-splicing introns represent the shortest known selfsplicing ribozymes and employ a splicing mechanism that is fundamentally distinct from that of group IIB self-splicing introns. However, the potential of group IIC self-splicing introns to carry exogenous sequences for the development of circular RNA-based platforms remains an open question and warrants further investigation.

Methods

Here, we demonstrate that group IIC self-splicing introns can efficiently circularize and express exogenous proteins of varying lengths, as evidenced by luciferase and GFP reporter systems. Leveraging structural biology-based design, we engineered the RSV pre-F protein and validated the potential of IIC self-splicing introns as a vaccine platform for preventing infectious diseases.

Results

In mouse models, the novel nucleic acid vaccine developed using IIC self-splicing introns elicited superior immunogenicity and in vivo protective efficacy compared to protein-adjuvant vaccines.

Discussion

The development of the novel circular RNA vaccine platform holds significant promise for advancing next-generation therapeutics for disease treatment and prevention.

Enhanced construction of a bivalent adenovirus-based vaccine for preventing childhood pathogens: Human respiratory syncytial virus and norovirus

To impede the spread of respiratory syncytial virus (RSV) and norovirus (NoV) in children, we developed novel recombinant adenoviruses expressing RSV fusion (F) and NoV VP1 proteins driven by different promoter elements (EF1α, SV40, and CMV), resulting in Ad-F/E/NoV, Ad-F/S/NoV, and Ad-F/C/NoV, respectively. The expressed F can be recognized by postfusion-specific antibody targeting to site II and prefusion-specific antibodies targeting to sites V and Ø in the Ad-F-infected cells. However, NoV VP1 is only expressed in Ad-F/E/NoV and Ad-F/C/NoV-infected cells. Intraperitoneal two-dose with monovalent Ad-F and all three bicistronic Ads individually in rodents induced anti-F neutralizing antibodies similarly. Ad-F/C/NoV and Ad-F/E/NoV but not Ad-F/S/NoV significantly induced NoV VP1-specific IgG. The serum from Ad-F/C/NoV-immunized subjects elicited superior anti-NoV activity, as evidenced by reduced binding of NoV VLPs to histo-blood group antigens. Ad-F/C/NoV and Ad-F/E/NoV-immunized mice exhibited elevated cellular immune-mediated splenic IFN-γ and IL-4 secretions. In the RSV challenge study, pulmonary virions were significantly decreased during the vaccination with each of the three bicistronic Ads, confirming their efficacy in preventing RSV infection. Finally, the mucosal (intranasal) immunization in mice with Ad-F/C/NoV induced superior anti-RSV and NoV IgA in both serum and vaginal washes specific to RSV and NoV were highly induced. These findings highlight the optimization of an Ad vaccine targeting two pathogens prevalent in childhood. Additionally, the results underscore the utility of mucosal delivery of Ad vaccines as a valuable strategy for public vaccination efforts.

Development of a genetically modified full-length human respiratory syncytial virus preF protein vaccine

Human Respiratory Syncytial Virus (hRSV) is a major cause of acute lower respiratory tract infections (ALRTI) in infants, the elderly, and immunocompromised individuals. The recent approval of recombinant protein-based hRSV vaccines represents significant progress in combating hRSV. However, these vaccines utilized optimized preF ectodomain attached with an exogenous trimeric motif, which may induce immunological complications. Our research addresses these concerns by employing modified "full-length" preF proteins, preF-TMCT, designed to mimic the natural F protein structure and avoid potential immunological complications. We characterized a group of preF constructs and identified two candidates that exhibited desirable expression levels, high antigenicity and good stability. Immunization of Balb/c mice confirmed the robust immunogenicity and effective in induction of cross-reactive neutralizing antibodies of these antigens, particularly the lead-construct BR40. This investigation aims to contribute new insights to hRSV vaccine development. The near-native structure of the "full-length" preF-TMCT antigen also makes it valuable for producing therapeutic monoclonal antibodies (mAbs) and other biopharmaceuticals against hRSV infection.

Respiratory syncytial virus fuses with plasma membrane to infect primary cultures of bronchial epithelial cells

Background

Respiratory syncytial virus (RSV) is a common cause of bronchiolitis in children under the age of five. RSV infection proceeds by fusion of the viral envelope with the target cell membrane, but it is unclear whether fusion occurs with plasma or endosomal membranes.

Methods

Entry and/or infection was studied in undifferentiated primary cultures of human bronchial epithelial cells. Synchronization of viral entry or infection was achieved by attaching the virus to the plasma membrane at temperatures of 4°C or 22°C. Cells in which entry events had occurred were identified by the enzymatic action of beta-lactamase M (BlaM) fused to the RSV P protein (BlaM-P) carried by rgRSV virions. BlaM cleaves the beta-lactam ring of CCF2 loaded into the cells, disrupting FRET and allowing blue light to be emitted. Green fluorescent protein (GFP) expression, encoded by the rgRSV genome, was used to identify infected cells.

Results

We found that adsorption of RSV at 4°C favors entry via endocytosis, whereas binding of the virus to the membrane at 22°C favors RSV entry via the plasma membrane. The induction of endocytosis by synchronization at 4°C is, therefore, an artifact. In addition, we found that all drugs that interfered with RSV infection reduced cell membrane deformations such as filopodia and lamellipodia, suggesting a mechanism by which they may interfere with RSV fusion with the cell membrane.

Discussion

In conclusion, RSV enters the cell by direct fusion of its envelope with the plasma membrane.

Epitope-focused vaccine immunogens design using tailored horseshoe-shaped scaffold

The continuous emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) variants highlights the need to update coronavirus 2019 disease (COVID-19) vaccine components. Epitope-based vaccine designs targeting conserved and immunorecessive regions of SARS-CoV-2 are critically needed. Here, we report an engineered epitope-focused immunogen design based on a novel horseshoe-shaped natural protein scaffold, named ribonuclease inhibitor 1 (RNH1), that can multiply display of conserved neutralizing epitopes from SARS-CoV-2 S2 stem helix. The designed immunogen RNH1-S1139 demonstrates high binding affinity to S2-specific neutralizing antibodies and elicits robust epitope-targeted antibody responses either through homologous or heterologous vaccination regimens. RNH1-S1139 immune serum has been proven to have similar binding ability against SARS-CoV, SARS-CoV-2 and its variants, providing broad-spectrum protection as a membrane fusion inhibitor. Further studies showed that RNH1 has the potential to serve as a versatile scaffold that displays other helical epitopes from various antigens, including respiratory syncytial virus (RSV) F glycoprotein. Our proposed immunogen engineering strategy via tailored horseshoe-shape nano-scaffold supports the continued development of epitope-focused vaccines as part of a next-generation vaccine design.

State of the Art and Emerging Technologies in Vaccine Design for Respiratory Pathogens

In this review, we present the efforts made so far in developing effective solutions to prevent infections caused by seven major respiratory pathogens: influenza virus, respiratory syncytial virus (RSV), the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Bordetella pertussis, Streptococcus pneumoniae (pneumococcus), Mycobacterium tuberculosis, and Pseudomonas aeruginosa. Advancements driven by the recent coronavirus disease 2019 (COVID-19) crisis have largely focused on viruses, but effective prophylactic solutions for bacterial pathogens are also needed, especially in light of the antimicrobial resistance (AMR) phenomenon. Here, we discuss various innovative key technologies that can help address this critical need, such as (a) the development of Lung-on-Chip ex vivo models to gain a better understanding of the pathogenesis process and the host-microbe interactions; (b) a more thorough investigation of the mechanisms behind mucosal immunity as the first line of defense against pathogens; (c) the identification of correlates of protection (CoPs) which, in conjunction with the Reverse Vaccinology 2.0 approach, can push a more rational and targeted design of vaccines. By focusing on these critical areas, we expect substantial progress in the development of new vaccines against respiratory bacterial pathogens, thereby enhancing global health protection in the framework of the increasingly concerning AMR emergence.

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.

An mRNA-Based Respiratory Syncytial Virus Vaccine Elicits Strong Neutralizing Antibody Responses and Protects Rodents Without Vaccine-Associated Enhanced Respiratory Disease

BACKGROUND

Respiratory syncytial virus (RSV) causes the most common type of severe lower respiratory tract infection worldwide, and the fusion (F) protein is a target for neutralizing antibodies and vaccine development. This study aimed to investigate the immunogenicity and efficacy of an mRNA-based RSV vaccine with an F protein sequence.

METHODS

We designed an mRNA construct encoding a modified RSV F protein, which was further developed into an LNP-encapsulated mRNA vaccine (LVRNA007). LVRNA007 was administered to mice and cotton rats, followed by immunogenicity analysis and viral challenge studies. Protection of rodents from the viral infection was evaluated based on the presence of the virus in the lung and pathological examination of respiratory tissues.

RESULTS

LVRNA007 induced robust humoral and cellular immune responses in both mice and cotton rats, with neutralization antibody levels in the immunized animals maintained at high levels for over one year. Vaccination of LVRNA007 also protected the rodents from RSV challenge, judged by the much decreased virus titer and the pathological score in the lung tissue. In addition, no vaccine-enhanced disease (VED) phenomenon was observed with LVRNA007 vaccination.

CONCLUSIONS

Based on the preclinical immunogenicity and efficacy data, LVRNA007 could be a potential promising vaccine for prophylaxis of RSV infection.

Anti-Idiotypic Antibody as a Booster Vaccine Against Respiratory Syncytial Virus

Background/Objectives: The respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in children and adults. With nearly everyone infected by the age of five, there is an opportunity to develop booster vaccines that enhance B-cell immunity, promoting potent and broadly neutralizing antibodies. One potential approach involves using anti-idiotypic antibodies (anti-IDs) to mimic specific antigenic sites and enhance preexisting immunity in an epitope-specific manner. RB1, a monoclonal antibody (mAb) that binds to site IV of the RSV fusion (RSV F) protein, is a potent and broadly neutralizing against RSV A and B viruses. It is the precursor for MK1654 (clesrovimab), which successfully completed a Phase III clinical trial. Methods: In this study, we isolated two anti-IDs, 1A6 and 1D4, targeting RB1 CDR regions, demonstrating that 1A6 competes fully with RSV F in binding to RB1. Results: We resolved the RB1-1A6 and RB1-1D4 Fab-Fab complex structures and proved that 1A6 mimics the RSV F site IV better than 1D4. In an immunogenicity study, mice primed with RSV F and boosted with 1A6 Fab showed a site IV-specific antibody response with a concurrent increase in RSV virus neutralization. Conclusions: These results suggest that anti-IDs could be potentially used as booster vaccines for specific epitopes.

Neutralizing activity of anti-respiratory syncytial virus monoclonal antibody produced in Nicotiana benthamiana

Respiratory syncytial virus (RSV) is a highly contagious virus that affects the lungs and respiratory passages of many vulnerable people. It is a leading cause of lower respiratory tract infections and clinical complications, particularly among infants and elderly. It can develop into serious complications such as pneumonia and bronchiolitis. The development of RSV vaccine or immunoprophylaxis remains highly active and a global health priority. Currently, GSK's Arexvy™ vaccine is approved for the prevention of lower respiratory tract disease in older adults (>60 years). Palivizumab and currently nirsevimab are the approved monoclonal antibodies (mAbs) for RSV prevention in high-risk patients. Many studies are ongoing to develop additional therapeutic antibodies for preventing RSV infections among newborns and other susceptible groups. Recently, additional antibodies have been discovered and shown greater potential for development as therapeutic alternatives to palivizumab and nirsevimab. Plant expression platforms have proven successful in producing recombinant proteins, including antibodies, offering a potential cost-effective alternative to mammalian expression platforms. Hence in this study, an attempt was made to use a plant expression platform to produce two anti-RSV fusion (F) mAbs 5C4 and CR9501. The heavy-chain and light-chain sequences of both these antibodies were transiently expressed in Nicotiana benthamiana plants using a geminiviral vector and then purified using single-step protein A affinity column chromatography. Both these plant-produced mAbs showed specific binding to the RSV fusion protein and demonstrate effective viral neutralization activity in vitro. These preliminary findings suggest that plant-produced anti-RSV mAbs are able to neutralize RSV in vitro.

The respiratory syncytial virus vaccine and monoclonal antibody landscape: the road to global access

Respiratory syncytial virus (RSV) is the second most common pathogen causing infant mortality. Additionally, RSV is a major cause of morbidity and mortality in older adults (age ≥60 years) similar to influenza. A protein-based maternal vaccine and monoclonal antibody (mAb) are now market-approved to protect infants, while an mRNA and two protein-based vaccines are approved for older adults. First-year experience protecting infants with nirsevimab in high-income countries shows a major public health benefit. It is expected that the RSV vaccine landscape will continue to develop in the coming years to protect all people globally. The vaccine and mAb landscape remain active with 30 candidates in clinical development using four approaches: protein-based, live-attenuated and chimeric vector, mRNA, and mAbs. Candidates in late-phase trials aim to protect young infants using mAbs, older infants and toddlers with live-attenuated vaccines, and children and adults using protein-based and mRNA vaccines. This Review provides an overview of RSV vaccines highlighting different target populations, antigens, and trial results. As RSV vaccines have not yet reached low-income and middle-income countries, we outline urgent next steps to minimise the vaccine delay.

A foldon-free prefusion F trimer vaccine for respiratory syncytial virus to reduce off-target immune responses

Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in infants and older people. Current RSV subunit vaccines are based on a fusion protein that is stabilized in the prefusion conformation and linked to a heterologous foldon trimerization domain to obtain a prefusion F (preF) trimer. Here we show that current RSV vaccines induce undesirable anti-foldon antibodies in non-human primates, mice and humans. To overcome this, we designed a foldon-free RSV preF trimer by elucidating the structural basis of trimerization-induced preF destabilization through molecular dynamics simulations and by introducing amino acid substitutions that negate hotspots of charge repulsion. The highly stable prefusion conformation was validated using antigenic and cryo-electron microscopy analysis. The preF is immunogenic and protective in naive mouse models and boosts neutralizing antibody titres in RSV-pre-exposed mice and non-human primates, while achieving similar titres to approved RSV vaccines in mice. This stable preF design is a promising option as a foldon-independent candidate for a next-generation RSV vaccine immunogen.

Biology of human respiratory syncytial virus: Current perspectives in immune response and mechanisms against the virus

Human respiratory syncytial virus (hRSV) remains a leading cause of morbidity and mortality in infants, young children, and older adults. hRSV infection's limited treatment and vaccine options significantly increase bronchiolitis' morbidity rates. The severity and outcome of viral infection hinge on the innate immune response. Developing vaccines and identifying therapeutic interventions suitable for young children, older adults, and pregnant women relies on comprehending the molecular mechanisms of viral PAMP recognition, genetic factors of the inflammatory response, and antiviral defense. This review covers fundamental elements of hRSV biology, diagnosis, pathogenesis, and the immune response, highlighting prospective options for vaccine development.

Rational design of uncleaved prefusion-closed trimer vaccines for human respiratory syncytial virus and metapneumovirus

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) cause human respiratory diseases and are major targets for vaccine development. In this study, we design uncleaved prefusion-closed (UFC) trimers for the fusion protein (F) of both viruses by examining mutations critical to F metastability. For RSV, we assess four previous prefusion F designs, including the first and second generations of DS-Cav1, SC-TM, and 847A. We then identify key mutations that can maintain prefusion F in a native-like, closed trimeric form (up to 76%) without introducing any interprotomer disulfide bond. For hMPV, we develop a stable UFC trimer with a truncated F2-F1 linkage and an interprotomer disulfide bond. Dozens of UFC constructs are characterized by negative-stain electron microscopy (nsEM), x-ray crystallography (11 RSV-F structures and one hMPV-F structure), and antigenic profiling. Using an optimized RSV-F UFC trimer as bait, we identify three potent RSV neutralizing antibodies (NAbs) from a phage-displayed human antibody library, with a public NAb lineage targeting sites Ø and V and two cross-pneumovirus NAbs recognizing site III. In mouse immunization, rationally designed RSV-F and hMPV-F UFC trimers induce robust antibody responses with high neutralizing titers. Our study provides a foundation for future prefusion F-based RSV and hMPV vaccine development.

Targeting RSV-neutralizing B cell receptors with anti-idiotypic antibodies

Respiratory syncytial virus (RSV) causes lower respiratory tract infections with significant morbidity and mortality at the extremes of age. Vaccines based on the viral fusion protein are approved for adults over 60, but infant protection relies on passive immunity via antibody transfer or maternal vaccination. An infant vaccine that rapidly elicits protective antibodies would fulfill a critical unmet need. Antibodies arising from the VH3-21/VL1-40 gene pairing can neutralize RSV without the need for affinity maturation, making them attractive to target through vaccination. Here, we develop an anti-idiotypic monoclonal antibody (ai-mAb) immunogen that is specific for unmutated VH3-21/VL1-40 B cell receptors (BCRs). The ai-mAb efficiently engages B cells with bona fide target BCRs and does not activate off-target non-neutralizing B cells, unlike recombinant pre-fusion (preF) protein used in current RSV vaccines. These results establish proof of concept for using an ai-mAb-derived vaccine to target B cells hardwired to produce RSV-neutralizing antibodies.

Bovine Respiratory Syncytial Virus Nanovaccine Induces Long-Lasting Humoral Immunity in Mice

With limited therapies and vaccines available, human respiratory syncytial virus (HRSV) has a significant negative health impact on all age groups but particularly on infants, young children, and older adults. Bovine respiratory syncytial virus (BRSV) is pathogenically and antigenically similar to HRSV. Building upon previous studies using a BRSV nanovaccine coencapsulating multiple proteins, this work demonstrates the development and comparative evaluation of a coencapsulated nanovaccine to a cocktail nanovaccine formulation composed of polyanhydride nanoparticles encapsulating BRSV postfusion (F) glycoprotein and CpG ODN 1668 coadjuvant delivered simultaneously with nanoparticles encapsulating BRSV attachment glycoprotein (G) and CpG ODN 1668. These nanovaccine formulations were administered to C57BL/6 mice by one of two prime-boost regimens (i.e., intranasal/intranasal or intranasal/subcutaneous) followed by assessment of humoral immunity. The cocktail nanovaccine induced sustained anti-F and anti-G serum IgG antibody responses for 12 weeks postprimary immunization. Using polyanhydride particles to deliver G protein in a prime-boost regime also significantly induced serum anti-G antibodies compared to protein and coadjuvant alone. Serum IgG induced by the nanovaccine demonstrated virus-neutralizing capability from 42 to 119 days postprimary immunization. Further, anti-F IgG antibodies were detected in the bronchoalveolar lavage fluid of vaccinated animals. Finally, the nanovaccine induced long-lived anti-F antibody secreting plasma cells that were detectable in the bone marrow 205 days postprimary immunization. Overall, the BRSV nanovaccine(s) successfully induced long-lived humoral immune responses capable of virus neutralization, making this a promising vaccine candidate for further evaluation in other relevant animal models.

Mutating a flexible region of the RSV F protein can stabilize the prefusion conformation

The respiratory syncytial virus (RSV) fusion (F) glycoprotein is highly immunogenic in its prefusion (pre-F) conformation. However, the protein is unstable, and its conformation must be stabilized for it to function effectively as an immunogen in vaccines. We present a mutagenesis strategy to arrest the RSV F protein in its pre-F state by blocking localized changes in protein structure that accompany large-scale conformational rearrangements. We generated a series of mutants and screened them in vitro to assess their potential for forming a stable pre-F. In animals, the immunogenicity of a representative mutant F protein, with a conformation confirmed by cryo-electron microscopy, elicited levels of neutralizing antibodies and protection against RSV-induced lung damage that were comparable to those of DS-Cav1, a pre-F used in a licensed vaccine.

Efficacious human metapneumovirus vaccine based on AI-guided engineering of a closed prefusion trimer

The prefusion conformation of human metapneumovirus fusion protein (hMPV Pre-F) is critical for eliciting the most potent neutralizing antibodies and is the preferred immunogen for an efficacious vaccine against hMPV respiratory infections. Here we show that an additional cleavage event in the F protein allows closure and correct folding of the trimer. We therefore engineered the F protein to undergo double cleavage, which enabled screening for Pre-F stabilizing substitutions at the natively folded protomer interfaces. To identify these substitutions, we developed an AI convolutional classifier that successfully predicts complex polar interactions often overlooked by physics-based methods and visual inspection. The combination of additional processing, stabilization of interface regions and stabilization of the membrane-proximal stem, resulted in a Pre-F protein vaccine candidate without the need for a heterologous trimerization domain that exhibited high expression yields and thermostability. Cryo-EM analysis shows the complete ectodomain structure, including the stem, and a specific interaction of the newly identified cleaved C-terminus with the adjacent protomer. Importantly, the protein induces high and cross-neutralizing antibody responses resulting in near complete protection against hMPV challenge in cotton rats, making the highly stable, double-cleaved hMPV Pre-F trimer an attractive vaccine candidate.

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.

Systematic computer-aided disulfide design as a general strategy to stabilize prefusion class I fusion proteins

Numerous enveloped viruses, such as coronaviruses, influenza, and respiratory syncytial virus (RSV), utilize class I fusion proteins for cell entry. During this process, the proteins transition from a prefusion to a postfusion state, undergoing substantial and irreversible conformational changes. The prefusion conformation has repeatedly shown significant potential in vaccine development. However, the instability of this state poses challenges for its practical application in vaccines. While non-native disulfides have been effective in maintaining the prefusion structure, identifying stabilizing disulfide bonds remains an intricate task. Here, we present a general computational approach to systematically identify prefusion-stabilizing disulfides. Our method assesses the geometric constraints of disulfide bonds and introduces a ranking system to estimate their potential in stabilizing the prefusion conformation. We hypothesized that disulfides restricting the initial stages of the conformational switch could offer higher stability to the prefusion state than those preventing unfolding at a later stage. The implementation of our algorithm on the RSV F protein led to the discovery of prefusion-stabilizing disulfides that supported our hypothesis. Furthermore, the evaluation of our top design as a vaccine candidate in a cotton rat model demonstrated robust protection against RSV infection, highlighting the potential of our approach for vaccine development.

Assembly of respiratory syncytial virus matrix protein lattice and its coordination with fusion glycoprotein trimers

Respiratory syncytial virus (RSV) is an enveloped, filamentous, negative-strand RNA virus that causes significant respiratory illness worldwide. RSV vaccines are available, however there is still significant need for research to support the development of vaccines and therapeutics against RSV and related Mononegavirales viruses. Individual virions vary in size, with an average diameter of ~130 nm and ranging from ~500 nm to over 10 µm in length. Though the general arrangement of structural proteins in virions is known, we use cryo-electron tomography and sub-tomogram averaging to determine the molecular organization of RSV structural proteins. We show that the peripheral membrane-associated RSV matrix (M) protein is arranged in a packed helical-like lattice of M-dimers. We report that RSV F glycoprotein is frequently observed as pairs of trimers oriented in an anti-parallel conformation to support potential interactions between trimers. Our sub-tomogram averages indicate the positioning of F-trimer pairs is correlated with the underlying M lattice. These results provide insight into RSV virion organization and may aid in the development of RSV vaccines and anti-viral targets.

RSV Prevention Within Reach for Older Infants and Toddlers: The Role of Active Immunization

This review article will summarize the vaccines and monoclonal antibodies currently under evaluation for the prevention of RSV disease in older infants, toddlers and young children. We will review the rationale for passive protection during the first months of life, and the role of active immunization afterwards, either with live attenuated, protein-based or mRNA vaccines.

KD-409, a Respiratory Syncytial Virus FG Chimeric Protein without the CX3C Chemokine Motif, Is an Efficient Respiratory Syncytial Virus Vaccine Preparation for Passive and Active Immunization in Mice

Although respiratory syncytial virus (RSV) vaccine development initiatives have existed for half a century, no candidate has been approved for application at all ages from neonates to children. Developing an effective and safe RSV vaccine for pediatric use is challenging owing to RSV-associated disease and vaccine-enhanced disease (VED). We aimed to design an RSV vaccine, KD-409, by structurally incorporating the F ectodomain and G protein central conserved domain without the CX3C chemokine motif and test its efficacy and safety. KD-409 formed rosette particles or trimmers. KD-409 immunization of mice mainly induced anti-RSV F protein IgG. The induced anti-F antibodies had a higher IgG2a/IgG1 ratio than pre-fusion F, suggesting that they induced Th1-dominant immunity. Active and passive immunities were assessed by analyzing the viral titers in BALB/c mice intranasally challenged with RSV after intramuscular KD-409 immunization and pups derived from mothers who were intramuscularly vaccinated with KD-409 twice, respectively. KD-409 was more effective than post-fusion F and had a lower minimum effective dose than pre-fusion F. Thus, KD-409 demonstrated great potential as a novel RSV vaccine candidate, outperforming existing RSV F-based candidates. Our findings provide a promising strategy to overcome RSV-associated acute lower respiratory infections without the risk of VED associated with traditional approaches.

Systemic prime mucosal boost significantly increases protective efficacy of bivalent RSV influenza viral vectored vaccine

Although licensed vaccines against influenza virus have been successful in reducing pathogen-mediated disease, they have been less effective at preventing viral infection of the airways and current seasonal updates to influenza vaccines do not always successfully accommodate viral drift. Most licensed influenza and recently licensed RSV vaccines are administered via the intramuscular route. Alternative immunisation strategies, such as intranasal vaccinations, and "prime-pull" regimens, may deliver a more sterilising form of protection against respiratory viruses. A bivalent ChAdOx1-based vaccine (ChAdOx1-NP + M1-RSVF) encoding conserved nucleoprotein and matrix 1 proteins from influenza A virus and a modified pre-fusion stabilised RSV A F protein, was designed, developed and tested in preclinical animal models. The aim was to induce broad, cross-protective tissue-resident T cells against heterotypic influenza viruses and neutralising antibodies against RSV in the respiratory mucosa and systemically. When administered via an intramuscular prime-intranasal boost (IM-IN) regimen in mice, superior protection was generated against challenge with either RSV A, Influenza A H3N2 or H1N1. These results support further clinical development of a pan influenza & RSV vaccine administered in a prime-pull regimen.

Respiratory Syncytial Virus in Adult Patients at a Tertiary Care Hospital in Germany: Clinical Features and Molecular Epidemiology of the Fusion Protein in the Severe Respiratory Season of 2022/2023

Following an interseasonal rise in mainly pediatric respiratory syncytial virus (RSV) cases in Germany in 2021, an exceptionally high number of adult cases was observed in the subsequent respiratory season of 2022/2023. The aim of this study was to compare the clinical presentation of RSV infections in the pre- and post-SARS-CoV-2 pandemic periods. Additionally, the local epidemiology of the RSV fusion protein was analyzed at a molecular genetic and amino acid level. RSV detections in adults peaked in calendar week 1 of 2023, 8 weeks earlier than the earliest peak observed in the three pre-pandemic seasons. Although the median age of the adult patients was not different (66.5 vs. 65 years), subtle differences between both periods regarding comorbidities and the clinical presentation of RSV cases were noted. High rates of comorbidities prevailed; however, significantly lower numbers of patients with a history of lung transplantation (p = 0.009), chronic kidney disease (p = 0.013), and immunosuppression (p = 0.038) were observed in the 2022/2023 season. In contrast, significantly more lower respiratory tract infections (p < 0.001), in particular in the form of pneumonia (p = 0.015) and exacerbations of obstructive lung diseases (p = 0.008), were detected. An ICU admission was noted for 23.7% of all patients throughout the study period. Sequence analysis of the fusion protein gene revealed a close phylogenetic relatedness, regardless of the season of origin. However, especially for RSV-B, an accumulation of amino acid point substitutions was noted, including in antigenic site Ø. The SARS-CoV-2 pandemic had a tremendous impact on the seasonality of RSV, and the introduction of new vaccination and immunization strategies against RSV warrants further epidemiologic studies of this important pathogen.

Farnesyltransferase inhibitor lonafarnib suppresses respiratory syncytial virus infection by blocking conformational change of fusion glycoprotein

Respiratory syncytial virus (RSV) is the major cause of bronchiolitis and pneumonia in young children and the elderly. There are currently no approved RSV-specific therapeutic small molecules available. Using high-throughput antiviral screening, we identified an oral drug, the prenylation inhibitor lonafarnib, which showed potent inhibition of the RSV fusion process. Lonafarnib exhibited antiviral activity against both the RSV A and B genotypes and showed low cytotoxicity in HEp-2 and human primary bronchial epithelial cells (HBEC). Time-of-addition and pseudovirus assays demonstrated that lonafarnib inhibits RSV entry, but has farnesyltransferase-independent antiviral efficacy. Cryo-electron microscopy revealed that lonafarnib binds to a triple-symmetric pocket within the central cavity of the RSV F metastable pre-fusion conformation. Mutants at the RSV F sites interacting with lonafarnib showed resistance to lonafarnib but remained fully sensitive to the neutralizing monoclonal antibody palivizumab. Furthermore, lonafarnib dose-dependently reduced the replication of RSV in BALB/c mice. Collectively, lonafarnib could be a potential fusion inhibitor for RSV infection.

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.

Systematic identification of structure-specific protein-protein interactions

The physical interactome of a protein can be altered upon perturbation, modulating cell physiology and contributing to disease. Identifying interactome differences of normal and disease states of proteins could help understand disease mechanisms, but current methods do not pinpoint structure-specific PPIs and interaction interfaces proteome-wide. We used limited proteolysis-mass spectrometry (LiP-MS) to screen for structure-specific PPIs by probing for protease susceptibility changes of proteins in cellular extracts upon treatment with specific structural states of a protein. We first demonstrated that LiP-MS detects well-characterized PPIs, including antibody-target protein interactions and interactions with membrane proteins, and that it pinpoints interfaces, including epitopes. We then applied the approach to study conformation-specific interactors of the Parkinson's disease hallmark protein alpha-synuclein (aSyn). We identified known interactors of aSyn monomer and amyloid fibrils and provide a resource of novel putative conformation-specific aSyn interactors for validation in further studies. We also used our approach on GDP- and GTP-bound forms of two Rab GTPases, showing detection of differential candidate interactors of conformationally similar proteins. This approach is applicable to screen for structure-specific interactomes of any protein, including posttranslationally modified and unmodified, or metabolite-bound and unbound protein states.

VLPs generated by the fusion of RSV-F or hMPV-F glycoprotein to HIV-Gag show improved immunogenicity and neutralizing response in mice

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) vaccines have been long overdue. Structure-based vaccine design created a new momentum in the last decade, and the first RSV vaccines have finally been approved in older adults and pregnant individuals. These vaccines are based on recombinant stabilized pre-fusion F glycoproteins administered as soluble proteins. Multimeric antigenic display could markedly improve immunogenicity and should be evaluated in the next generations of vaccines. Here we tested a new virus like particles-based vaccine platform which utilizes the direct fusion of an immunogen of interest to the structural human immunodeficient virus (HIV) protein Gag to increase its surface density and immunogenicity. We compared, in mice, the immunogenicity of RSV-F or hMPV-F based immunogens delivered either as soluble proteins or displayed on the surface of our VLPs. VLP associated F-proteins showed better immunogenicity and induced superior neutralizing responses. Moreover, when combining both VLP associated and soluble immunogens in a heterologous regimen, VLP-associated immunogens provided added benefits when administered as the prime immunization.

Structure and design of Langya virus glycoprotein antigens

Langya virus (LayV) is a recently discovered henipavirus (HNV), isolated from febrile patients in China. HNV entry into host cells is mediated by the attachment (G) and fusion (F) glycoproteins which are the main targets of neutralizing antibodies. We show here that the LayV F and G glycoproteins promote membrane fusion with human, mouse, and hamster target cells using a different, yet unknown, receptor than Nipah virus (NiV) and Hendra virus (HeV) and that NiV- and HeV-elicited monoclonal and polyclonal antibodies do not cross-react with LayV F and G. We determined cryoelectron microscopy structures of LayV F, in the prefusion and postfusion states, and of LayV G, revealing their conformational landscape and distinct antigenicity relative to NiV and HeV. We computationally designed stabilized LayV G constructs and demonstrate the generalizability of an HNV F prefusion-stabilization strategy. Our data will support the development of vaccines and therapeutics against LayV and closely related HNVs.

Potential Effects on Elderly People From Nirsevimab Use in Infants

Nirsevimab therapy has the potential to revolutionize infant respiratory syncytial virus (RSV) prophylaxis. But other populations suffering RSV, such the elderly or those over 60, may also be protected by using this novel antibody in the infant group. It is true that some studies link the use of nirsevimab to a reduction in the virus's ability to spread by lowering the viral load in infants as a result of the drug's long half-life. However, this protective effect may not be very significant because RSV transmission in the elderly typically comes from other elderly people or from school-aged children. Furthermore, RSV may be transmitted at any time of the year and not just during the period of nirsevimab protection due to its existence in human reservoirs. The reasons made here show that, even though nirsevimab treatment in infants may protect the elderly, this benefit would be limited and testimonial. Therefore, immunizing the elderly with currently licensed and developing vaccines should be a priority.

Outpatient respiratory syncytial virus infections and novel preventive interventions

PURPOSE OF REVIEW

With interventions to prevent respiratory syncytial virus (RSV) infection within reach, this review aims to provide healthcare professionals with the latest information necessary to inform parents and assess the potential impact of RSV prevention on everyday practice. We address frequently asked questions for parental counseling.

RECENT FINDINGS

Numerous studies emphasize the major burden of RSV on young children, parents, healthcare and society. In the first year of life, about 14% of healthy term infants visit a doctor and 2% require hospitalization due to RSV. In older children (1--5 years), RSV infections and associated morbidity (wheeze, acute otitis media) are major drivers of outpatient visits. A novel maternal RSV vaccine and long-acting mAb can provide protection during infants' first months of life. This maternal vaccine showed 70.9% efficacy against severe RSV infection within 150 days after birth; the mAb nirsevimab reduces medically attended RSV infections by 79.5% within 150 days after administration. Both gained regulatory approval in the USA (FDA) and Europe (EMA).

SUMMARY

Novel RSV immunizations hold promise to reduce the RSV burden in infants, with substantial impact on everyday practice. Tailored parental guidance will be instrumental for successful implementation. Awaiting pediatric vaccines, RSV infections beyond infancy will still pose a significant outpatient burden.

Design of hepadnavirus core protein-based chimeric virus-like particles carrying epitopes from respiratory syncytial virus

Respiratory syncytial virus (RSV) is one of the most important pathogens causing respiratory tract infection in humans, especially in infants and the elderly. The identification and structural resolution of the potent neutralizing epitopes on RSV fusion (F) protein enable an "epitope-focused" vaccine design. However, the display of RSV F epitope II on the surface of the widely-used human hepatitis B virus core antigen (HBcAg) has failed to induce neutralizing antibody response in mice. Here, we used the hepadnavirus core protein (HcAg) from different mammalian hosts as scaffolds to construct chimeric virus-like particles (VLPs) presenting the RSV F epitope II. Mouse immunization showed that different HcAg-based chimeric VLPs elicited significantly different neutralizing antibody responses, among which the HcAg derived from roundleaf bat (RBHcAg) is the most immunogenic. Furthermore, RBHcAg was used as the scaffold platform to present multiple RSV F epitopes, and the immunogenicity was further improved in comparison to that displaying a single epitope II. The designed RBHcAg-based multiple-epitope-presenting VLP formulated with MF59-like adjuvant elicited a potent and balanced Th1/Th2 immune response, and offered substantial protection in mice against the challenge of live RSV A2 virus. The designed chimeric VLPs may serve as the potential starting point for developing epitope-focused vaccines against RSV. Our study also demonstrated that RBHcAg is an effective VLP carrier for presenting foreign epitopes, providing a promising platform for epitope-focused vaccine design.

A tale of two fusion proteins: understanding the metastability of human respiratory syncytial virus and metapneumovirus and implications for rational design of uncleaved prefusion-closed trimers

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) cause human respiratory diseases and are major targets for vaccine development. In this study, we designed uncleaved prefusion-closed (UFC) trimers for the fusion (F) proteins of both viruses by examining mutations critical to F metastability. For RSV, we assessed four previous prefusion F designs, including the first and second generations of DS-Cav1, SC-TM, and 847A. We then identified key mutations that can maintain prefusion F in a native-like, closed trimeric form (up to 76%) without introducing any interprotomer disulfide bond. For hMPV, we developed a stable UFC trimer with a truncated F2-F1 linkage and an interprotomer disulfide bond. Tens of UFC constructs were characterized by negative-stain electron microscopy (nsEM), x-ray crystallography (11 RSV-F and one hMPV-F structures), and antigenic profiling. Using an optimized RSV-F UFC trimer as bait, we identified three potent RSV neutralizing antibodies (NAbs) from a phage-displayed human antibody library, with a public NAb lineage targeting sites Ø and V and two cross-pneumovirus NAbs recognizing site III. In mouse immunization, rationally designed RSV-F and hMPV-F UFC trimers induced robust antibody responses with high neutralizing titers. Our study provides a foundation for future prefusion F-based RSV and hMPV vaccine development.

Evidence that an interaction between the respiratory syncytial virus F and G proteins at the distal ends of virus filaments mediates efficient multiple cycle infection

Evidence for a stable interaction between the respiratory syncytial virus (RSV) F and G proteins on the surface of virus filaments was provided using antibody immunoprecipitation studies on purified RSV particles, and by the in situ analysis on the surface of RSV-infected cells using the proximity ligation assay. Imaging of the F and G protein distribution on virus filaments suggested that this protein complex was localised at the distal ends of the virus filaments, and suggested that this protein complex played a direct role in mediating efficient localised cell-to-cell virus transmission. G protein expression was required for efficient localised cell-to-cell transmission of RSV in cell monolayers which provided evidence that this protein complex mediates efficient multiple cycle infection. Collectively, these data provide evidence that F and G proteins form a complex on the surface of RSV particles, and that a role for this protein complex in promoting virus transmission is suggested.

A general computational design strategy for stabilizing viral class I fusion proteins

Many pathogenic viruses rely on class I fusion proteins to fuse their viral membrane with the host cell membrane. To drive the fusion process, class I fusion proteins undergo an irreversible conformational change from a metastable prefusion state to an energetically more stable postfusion state. Mounting evidence underscores that antibodies targeting the prefusion conformation are the most potent, making it a compelling vaccine candidate. Here, we establish a computational design protocol that stabilizes the prefusion state while destabilizing the postfusion conformation. With this protocol, we stabilize the fusion proteins of the RSV, hMPV, and SARS-CoV-2 viruses, testing fewer than a handful of designs. The solved structures of these designed proteins from all three viruses evidence the atomic accuracy of our approach. Furthermore, the humoral response of the redesigned RSV F protein compares to that of the recently approved vaccine in a mouse model. While the parallel design of two conformations allows the identification of energetically sub-optimal positions for one conformation, our protocol also reveals diverse molecular strategies for stabilization. Given the clinical significance of viruses using class I fusion proteins, our algorithm can substantially contribute to vaccine development by reducing the time and resources needed to optimize these immunogens.

A molecular perspective for the development of antibodies against the human respiratory syncytial virus

The human respiratory syncytial virus (hRSV) is the leading etiologic agent causing respiratory infections in infants, children, older adults, and patients with comorbidities. Sixty-seven years have passed since the discovery of hRSV, and only a few successful mitigation or treatment tools have been developed against this virus. One of these is immunotherapy with monoclonal antibodies against structural proteins of the virus, such as Palivizumab, the first prophylactic approach approved by the Food and Drug Administration (FDA) of the USA. In this article, we discuss different strategies for the prevention and treatment of hRSV infection, focusing on the molecular mechanisms against each target that underly the rational design of antibodies against hRSV. At the same time, we describe the latest results regarding currently approved therapies against hRSV and the challenges associated with developing new candidates.

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.

Entry of Enveloped Viruses into Host Cells: Membrane Fusion

Viruses are intracellular parasites that hijack the cellular machinery for their own replication. Therefore, an obligatory step in the virus life cycle is the delivery of the viral genome inside the cell. Enveloped viruses (i.e., viruses with a lipid envelope) use a two-step procedure to release their genetic material into the cell: (1) they first bind to specific surface receptors of the target cell membrane and then (2) they fuse the viral and cell membranes. This last step may occur at the cell surface or after internalization of the virus particle by endocytosis or by some other route (e.g., macropinocytosis). Remarkably, the virus-cell membrane fusion process goes essentially along the same intermediate steps than other membrane fusions that occur, for instance, in vesicular fusion at the nerve synapsis or cell-cell fusion in yeast mating. Specialized viral proteins, fusogens, promote virus-cell membrane fusion. The viral fusogens experience drastic structural rearrangements during fusion, releasing the energy required to overcome the repulsive forces that prevent spontaneous fusion of the two membranes. This chapter provides an overview of the different types of viral fusogens and their mode of action, as they are currently known. Furthermore, it outlines novel strategies for vaccine development related to stabilized viral fusogens.

Structural characterization of M8C10, a neutralizing antibody targeting a highly conserved prefusion-specific epitope on the metapneumovirus fusion trimerization interface

IMPORTANCE

Human metapneumovirus (hMPV) is a common pathogen causing lower respiratory tract infections worldwide and can develop severe symptoms in high-risk populations such as infants, the elderly, and immunocompromised patients. There are no approved hMPV vaccines or neutralizing antibodies available for therapeutic or prophylactic use. The trimeric hMPV fusion F protein is the major target of neutralizing antibodies in human sera. Understanding the immune recognition of antibodies to hMPV-F antigen will provide critical insights into developing efficacious hMPV monoclonal antibodies and vaccines.

Non-glycosylated G protein with CpG ODN provides robust protection against respiratory syncytial virus without inducing eosinophilia

Introduction

Respiratory syncytial virus (RSV) vaccines targeting the fusion glycoprotein (F protein) are highly effective clinically in preventing RSV challenges. The attachment glycoprotein (G protein) is a potentially effective vaccine antigen candidate, as it is important for cell adhesion during infection. However, vaccine-associated enhanced diseases in mice, such as eosinophilic lung inflammation following RSV challenge, are a concern with G protein vaccines. This study aimed to design an effective G protein vaccine with enhanced safety and efficacy by evaluating the efficacy and adverse reactions of vaccines composed of different recombinant G proteins and adjuvants in mice.

Methods

Mice were subcutaneously immunized with glycosylated G protein expressed in mammalian cells (mG), non-glycosylated G protein expressed in Escherichia coli (eG), or F protein with or without aluminum salts (alum), CpG oligodeoxynucleotide (CpG ODN), or AddaVax. After vaccination, the levels of G-specific antibody and T-cell responses were measured. The immunized mice were challenged with RSV and examined for the viral load in the lungs and nasal turbinates, lung-infiltrating cells, and lung pathology.

Results

mG with any adjuvant was ineffective at inducing G-specific antibodies and had difficulty achieving both protection against RSV challenge and eosinophilia suppression. In particular, mG+CpG ODN induced G-specific T helper 1 (Th1) cells but only a few G-specific antibodies and did not protect against RSV challenge. However, eG+CpG ODN induced high levels of G-specific antibodies and Th1 cells and protected against RSV challenge without inducing pulmonary inflammation. Moreover, the combination vaccine of eG+F+CpG ODN showed greater protection against upper respiratory tract RSV challenge than using each single antigen vaccine alone.

Discussion

These results indicate that the efficacy of recombinant G protein vaccines can be enhanced without inducing adverse reactions by using appropriate antigens and adjuvants, and their efficacy is further enhanced in the combination vaccine with F protein. These data provide valuable information for the clinical application of G protein vaccines.

CpG ODN enhances the efficacy of F protein vaccine against respiratory syncytial virus infection in the upper respiratory tract via CD4+ T cells

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory illness worldwide, particularly in infants and older adults. Vaccines targeting the fusion glycoprotein (F protein) -one of the surface antigens of RSV- are highly effective in preventing RSV-associated severe lower respiratory tract disease. However, the efficacy of these vaccines against upper respiratory tract challenge needs improvement. Here, we aimed to examine the efficacy of F protein vaccines with or without CpG oligodeoxynucleotide (CpG ODN) as an adjuvant in the upper and lower respiratory tracts in mice. F + CpG ODN induced higher levels of F-specific IgG than that induced by F alone; however, levels of neutralizing antibodies did not increase compared to those induced by F alone. F + CpG ODN induced T helper 1 (Th1) cells while F alone induced T helper 2 (Th2) cells. Moreover, F + CpG ODN improved the protection against RSV challenge in the upper respiratory tract compared to F alone, which was largely dependent on CD4+ T cells. Meanwhile, both F + CpG ODN and F alone protected the lower respiratory tract. In conclusion, we demonstrated that induction of F-specific Th1 cells is an effective strategy to prevent RSV challenge in the upper respiratory tract in F protein vaccines. These data support the development of novel F protein vaccines.

New Insights on Respiratory Syncytial Virus Prevention

Respiratory syncytial virus (RSV) is a well-known infant pathogen transmitted mainly by droplets. It is a leading cause of upper respiratory tract infections in children, usually with a mild course of illness. RSV has also been a threat to older people, especially those with underlying medical conditions. For a long time, prevention was limited to passive immunoprophylaxis with palivizumab for high-risk infants. There was a strong need to find other treatment or prevention methods against RSV infections. In addition, after the coronavirus disease 2019 (COVID-19) pandemic, some significant changes in RSV epidemiology have been observed. Researchers noticed the shift in RSV seasonality and age distribution and the increased number of cases in older infants and adults. All of these made the need to find other medical options even stronger. Fortunately, two protein-based vaccines against RSV have successfully passed all phases of clinical trials and have been approved for use by adults and older people. One of them is also approved for infants from birth to 6 months of age (after maternal immunisation during pregnancy) and for pregnant women between 24 and 36 weeks of pregnancy. Also, a new passive immunisation option named nirsevimab (a highly potent monoclonal antibody with a long half-life) is now available for the paediatric group. In this review, we will discuss the previous and current RSV prevention methods in the light of structural discoveries of RSV antigens.

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.

Evaluation of a stabilized RSV pre-fusion F mRNA vaccine: Preclinical studies and Phase 1 clinical testing in healthy adults

Human respiratory syncytial virus (RSV) causes a substantial proportion of respiratory tract infections worldwide. Although RSV reinfections occur throughout life, older adults, particularly those with underlying comorbidities, are at risk for severe complications from RSV. There is no RSV vaccine available to date, and treatment of RSV in adults is largely supportive. A correlate of protection for RSV has not yet been established, but antibodies targeting the pre-fusion conformation of the RSV F glycoprotein play an important role in RSV neutralization. We previously reported a Phase 1 study of an mRNA-based vaccine (V171) expressing a pre-fusion-stabilized RSV F protein (mDS-Cav1) in healthy adults. Here, we evaluated an mRNA-based vaccine (V172) expressing a further stabilized RSV pre-fusion F protein (mVRC1). mVRC1 is a single chain version of RSV F with interprotomer disulfides in addition to the stabilizing mutations present in the mDS-Cav1 antigen. The immunogenicity of the two mRNA-based vaccines encoding mVRC1 (V172) or a sequence-optimized version of mDS-Cav1 to improve transcriptional fidelity (V171.2) were compared in RSV-naïve and RSV-experienced African green monkeys (AGMs). V172 induced higher neutralizing antibody titers than V171.2 and demonstrated protection in the AGM challenge model. We conducted a Phase 1, randomized, placebo-controlled, clinical trial of 25 μg, 100 μg, 200 μg, or 300 μg of V172 in healthy older adults (60-79 years old; N = 112) and 100 μg, 200 μg, or 300 μg of V172 in healthy younger adults (18-49 years old; N = 48). The primary clinical objectives were to evaluate the safety and tolerability of V172, and the secondary objective was to evaluate RSV serum neutralization titers. The most commonly reported solicited adverse events were injection-site pain, injection-site swelling, headache, and tiredness. V172 was generally well tolerated in older and younger adults and increased serum neutralizing antibody titers, pre-fusion F-specific competing antibody titers, and RSV F-specific T-cell responses.

Modeling Human Respiratory Syncytial Virus (RSV) Infection: Recent Contributions and Future Directions Using the Calf Model of Bovine RSV Disease

The human orthopneumovirus (human respiratory syncytial virus [RSV]) is a leading cause of respiratory disease in children worldwide and a significant cause of infant mortality in low- and middle-income countries. The natural immune response to the virus has a preponderant role in disease progression, with a rapid neutrophil infiltration and dysbalanced T cell response in the lungs associated with severe disease in infants. The development of preventive interventions against human RSV has been difficult partly due to the need to use animal models that only partially recapitulate the immune response as well as the disease progression seen in human infants. In this brief review, we discuss the contributions of the calf model of RSV infection to understanding immunity to RSV and in developing vaccine and drug candidates, focusing on recent research areas. We propose that the bovine model of RSV infection is a valuable alternative for assessing the translational potential of interventions aimed at the human population.

Multivalent antigen display on nanoparticle immunogens increases B cell clonotype diversity and neutralization breadth to pneumoviruses

Nanoparticles for multivalent display and delivery of vaccine antigens have emerged as a promising avenue for enhancing B cell responses to protein subunit vaccines. Here, we evaluated B cell responses in rhesus macaques immunized with prefusion-stabilized respiratory syncytial virus (RSV) F glycoprotein trimer compared with nanoparticles displaying 10 or 20 copies of the same antigen. We show that multivalent display skews antibody specificities and drives epitope-focusing of responding B cells. Antibody cloning and repertoire sequencing revealed that focusing was driven by the expansion of clonally distinct B cells through recruitment of diverse precursors. We identified two antibody lineages that developed either ultrapotent neutralization or pneumovirus cross-neutralization from precursor B cells with low initial affinity for the RSV-F immunogen. This suggests that increased avidity by multivalent display facilitates the activation and recruitment of these cells. Diversification of the B cell response by multivalent nanoparticle immunogens has broad implications for vaccine design.

Subtractive Immunization as a Method to Develop Respiratory Syncytial Virus (RSV)-Specific Monoclonal Antibodies

Respiratory Syncytial Virus (RSV) is a significant cause of lower respiratory tract infections in the young, the elderly, and in immunodeficient patients. As such, the virus represents an important cause of morbidity and mortality worldwide. Development of monoclonal antibodies against RSV has resulted in a commercial prophylaxis, palivizumab (Synagis®), and different antibodies that have improved our understanding of the structure of the viral proteins. In this study, a different immunization technique, subtractive immunization, was evaluated for its applicability to develop RSV-specific antibodies. One hybridoma which produced antibodies with the strongest staining of RSV infected cells, ATAC-0025, was selected for further characterization. This antibody belongs to the IgG1 class, has neutralizing capacity and recognizes the envelope F-protein. The antibody has a broad reactivity against a range of RSV reference strains and clinical isolates.