A highly stable prefusion RSV F vaccine derived from structural analysis of the fusion mechanism

Immunogenicity, safety, and tolerability of a β-glucan-CpG-adjuvanted respiratory syncytial virus vaccine in Japanese healthy participants aged 60 to 80 years: A phase 2, randomized, double-blind, dose-finding study

VN-0200 is an investigational β-glucan-CpG-adjuvanted respiratory syncytial virus (RSV) vaccine (antigen: VAGA-9001a [RSV F glycoprotein], adjuvant: MABH-9002b). This multicenter, randomized, double-blind, dose-finding phase 2 study explored the optimal VN-0200 dose and confirmed its humoral and cellular immunity and safety. In total, 342 healthy Japanese participants aged 60 to 80 years were randomized to one of 10 vaccination groups, each receiving a different combination of VAGA-9001a and MABH-9002a. VN-0200 was administered intramuscularly on Day 1 and Day 29. Geometric mean titer (GMT) and geometric mean fold rise (GMFR) of neutralization activity for anti-RSV subgroups A (RSV/A) and B (RSV/B), anti-VAGA-9001a antibody titer, and VAGA-9001a-specific interferon (IFN)-γ response were evaluated. Safety was monitored throughout the study. GMTs of serum anti-RSV/A neutralization activity increased from baseline to Day 57 and lower limits of the 95% confidence intervals of the corresponding GMFRs were >1.0 relative to baseline in all treatment groups (primary endpoint). Findings were similar for anti-RSV/A (Day 29) and anti-RSV/B (Day 29 and Day 57) neutralization activity, anti-VAGA-9001a antibody titer (Day 29 and Day 57), and VAGA-9001a-specific IFN-γ response (Day 29 and Day 57) (secondary endpoints). There was no clear influence of adjuvant or dose - response relationship of the antigen or adjuvant for any of the study endpoints. There were no serious vaccine-related treatment-emergent adverse events (TEAEs) or vaccine-related TEAEs leading to death. All antigen/adjuvant dose combinations of VN-0200 were well tolerated and elicited an increase in anti-RSV/A and anti-RSV/B neutralization activity from baseline to Day 29 and Day 57.

Hydrophobic residue substitutions enhance the stability and in vivo immunogenicity of respiratory syncytial virus fusion protein

Respiratory syncytial virus (RSV) entry into host cells is facilitated by viral fusion, wherein the metastable RSV fusion (F) protein undergoes a conformational change from a prefusion state to a highly stable postfusion structure. The prefusion F elicits a more robust human antibody response than its postfusion F and is a primary target for RSV vaccine development. However, the inherent instability of the prefusion F trimer and its low protein expression level in host cells are a significant challenge for developing a high-potency RSV vaccine. Here, we report that the introduction of four hydrophobic residue substitutions in the RSV F protein resulted in a highly stable prefusion F trimer (pre-F-IFLP). This engineered variant exhibits enhanced expression and stability compared to DS-Cav1, with improved thermal stability, increased resistance to acid and base, and extended storage life. Furthermore, pre-F-IFLP induced neutralizing antibody responses 72-fold higher than those elicited by DS-Cav1 following a second booster immunization and fully protected mice against RSV infection.

IMPORTANCE

In this study, we demonstrate that introducing four hydrophobic residue substitutions into the RSV F protein leads to the generation of a highly stable prefusion F trimer (pre-F-IFLP) with improved expression levels in cultured cells and superior stability compared to DS-Cav1, the first-generation prefusion F-stabilized RSV vaccine. Furthermore, pre-F-IFLP induced significantly higher neutralizing antibody responses than DS-Cav1 following both the first and second booster immunizations and conferred complete protection against RSV infection in a mouse model. These findings present an alternative approach for stabilizing the trimeric prefusion F protein, enhancing its expression, and significantly improving its protective efficacy for the prevention of RSV infection in vivo.

Mucosal vaccines with STING-agonist liposomal formulations inhibit RSV (respiratory syncytial virus) replication in cotton rats

Respiratory syncytial virus (RSV) is responsible for severe lower respiratory tract infections (LRTI) in immunocompromised individuals. While recent breakthroughs in vaccine design have led to approved vaccines for the elderly, these vaccines are all administered through the parenteral route. Vaccine administration through the mucosal route could protect the viral route of entry and can be advantageous over injected vaccines. There is however a lack of safe and efficacious mucosal adjuvants that can facilitate both mucosal and systemic immune responses. Here, we present preclinical data based on liposomal nanoparticles, NanoSTING, that encapsulate the endogenous STING-agonist 2'3'-cGAMP (cyclic guanosine adenosine monophosphate) as adjuvant for prefusion protein-based intranasal vaccines against RSV. NanoSTING significantly increased the immunogenicity of well-documented RSV prefusion protein antigens DS-CaV1, sc9-10 DS-CaV1, and SC-TM after a single intranasal dose, when compared to the protein-only and naked-cGAMP adjuvanted groups. Two doses of NanoSTING adjuvanted vaccines yielded robust secretory IgA titers at the mucosal surfaces and induced potent Th1 T-cell responses in the lungs of vaccinated mice. Both NanoSTING-sc9-10 DS-CaV1 and NanoSTING-SCTM vaccines protect against viral replication at the upper (nose) and lower (lung) respiratory tract of RSV-challenged cotton rats. The ability of our mucosal vaccines against RSV to elicit immunity in the respiratory tract can prevent the establishment of infection in individuals and potentially prevent disease transmission.

Precise targeting of HIV broadly neutralizing antibody precursors in humans

A protective HIV vaccine will need to induce broadly neutralizing antibodies (bnAbs) in humans, but priming rare bnAb precursor B cells has been challenging. In a double-blinded, placebo-controlled phase 1 human clinical trial, the recombinant, germline-targeting envelope glycoprotein (Env) trimer BG505 SOSIP.v4.1-GT1.1, adjuvanted with AS01B, induced bnAb precursors of the VRC01-class at a high frequency in the majority of vaccine recipients. These bnAb precursors, that target the CD4 receptor binding site, had undergone somatic hypermutation characteristic of the VRC01-class. A subset of isolated VRC01-class monoclonal antibodies neutralized wild-type pseudoviruses and was structurally extremely similar to bnAb VRC01. These results further support germline-targeting approaches for human HIV vaccine design and demonstrate atomic-level manipulation of B cell responses with rational vaccine design.

Unveiling the Spectrum of Respiratory Syncytial Virus Disease in Adults: From Community to Hospital

BACKGROUND

Respiratory syncytial virus can cause severe disease in the older adult population. Three vaccines for RSV are currently market approved but the risk of RSV-hospitalization in (older) adults from a community level remains elusive. We aimed to estimate the risk of RSV-hospitalization and characterize the patients that end up in hospital.

METHODS

We manually analyzed records of adults aged ≥ 20 with RSV-infection between 2022 and 2024 in three hospitals in the Netherlands. These hospitals implemented routine RSV-testing at emergency departments. Using population-based data in combination with the in-hospital data, we estimated the population risk of RSV-hospitalization. Hospital records were analyzed to characterize the role RSV played in their course of disease.

RESULTS

We analyzed 709 RSV cases of whom 503 (70.9%) were hospitalized. Five hundred twenty-six patients were ≥ 60, and 183 were < 60 years of age. The population RSV-hospitalization rate was 6-20 per 100.000 patients aged 20-59 years and 43-236 per 100.000 for those ≥ 60. The highest risks were observed in those with COPD (1702 per 100.000) and with congestive heart disease (2383 per 100.000). RSV caused clinically relevant infection in 88% of hospitalized cases but was only mentioned using specific ICD-codes in 4.4%. Comorbidity was prevalent (88.5%) and exacerbation of underlying disease caused of 46.3% of RSV-related hospital admissions. ICU admittance was 11.2% and in-hospital mortality was 8.1%.

CONCLUSION

The risk of RSV-hospitalization from the community is low but is increased substantially in those with underlying disease. RSV is often clinically relevant in hospitalized patients by causing exacerbation of underlying disease but is infrequently mentioned in specific ICD-codes.

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.

Intranasal Inoculation of Cationic Crosslinked Carbon Dots-Adjuvanted Respiratory Syncytial Virus F Subunit Vaccine Elicits Mucosal and Systemic Humoral and Cellular Immunity

Respiratory syncytial virus (RSV) causes severe acute lower respiratory tract infections, especially in infants and the elderly. Developing an RSV vaccine that promotes a robust mucosal immune response is necessary to successfully prevent viral transmission and the development of severe disease. We previously reported that crosslinked carbon dots (CCD) may be an excellent adjuvant candidate for intranasal (IN) protein subunit vaccines. Considering the strong immunogenicity of RSV prefused F protein (preF), we prepared an IN RSV vaccine composed of the CCD adjuvant and the preF protein as antigen (CCD/preF) and evaluated the induced antigen-specific humoral and cellular immunity. We found that IN immunization with the CCD/preF vaccine elicited strong serum IgG responses and mucosal immunity, including secreted IgA antibodies, tissue-resident memory T (TRM) cells, and antigen-specific B cells, which lasted for at least 1 year. In addition, a combination of intramuscular and IN immunization with CCD/preF vaccine induced stronger systemic and mucosal immunity. Together, this study proved the high immunogenicity of the CCD/preF vaccines and supported the university of the mucosal CCD adjuvant, supporting further development of the CCD/preF vaccine in larger animal models and clinical studies.

Generation of antigen-specific paired chain antibody sequences using large language models

The traditional process of antibody discovery is limited by inefficiency, high costs, and low success rates. Recent approaches employing artificial intelligence (AI) have been developed to optimize existing antibodies and generate antibody sequences in a target-agnostic manner. In this work, we present MAGE (Monoclonal Antibody GEnerator), a sequence-based Protein Language Model (PLM) fine-tuned for the task of generating paired human variable heavy and light chain antibody sequences against targets of interest. We show that MAGE can generate novel and diverse antibody sequences with experimentally validated binding specificity against SARS-CoV-2, an emerging avian influenza H5N1, and respiratory syncytial virus A (RSV-A). MAGE represents a first-in-class model capable of designing human antibodies against multiple targets with no starting template.

A bivalent mRNA vaccine against RSV infection in rodent models

Because of the higher conservation of RSV Fusion (F) protein than the glycoprotein (G) across RSV strains and serotypes, the majority of vaccine candidates targets to viral fusion protein (F) rather than glycoprotein to elicit a broader range of protective neutralizing antibodies from infection. In this study, we screened two chemically modified mRNA vaccines expressing RSV prefusion stabilized protein (preF) targeting RSV A2 and B subtypes. After immunization, the antigen-specific binding antibody, neutralizing antibody, and T cell-mediated immune response were evaluated. After challenge with live RSV A2 virus in cotton rats, the protection and safety of vaccine was further evaluated. The results showed that the mRNA vaccine candidates elicited robust antigen-specific binding antibody, neutralizing antibody responses and Th1-biased T-cell responses in both mice and cotton rats. Moreover, cotton rats vaccinated with mRNA vaccine, lung pathology and lung infectious viral loads were significantly reduced, and no vaccine enhanced respiratory disease (VERD) happened. These results collectively demonstrated that mRNA-based vaccine induced strong humoral and cellular immunity, provided outstanding protection against both RSV A2 and RSV B subtypes in rodent animals as well. Our data demonstrated that these mRNA vaccines should be further evaluated in clinical trials.

Progress on Respiratory Syncytial Virus Vaccine Development and Evaluation Methods

Respiratory syncytial virus (RSV) remains a significant global health threat, especially to infants, the elderly, and immunocompromised individuals. This review comprehensively explores the progress in RSV vaccine development, the immune evaluation methods, and immunological surrogate. The RSV fusion (F) protein, a primary target for vaccine development, has been engineered in prefusion conformation to elicit potent neutralizing antibodies, while the attachment (G) glycoprotein and other immunogens are also being explored to broaden immune responses. Advances in diverse vaccine platforms, ranging from live attenuated and protein subunit vaccines to cutting-edge mRNA- and nanoparticle-based formulations, highlight the field's progress, yet challenges in balancing safety, immunogenicity, and durability persist. Central to these efforts is the identification and validation of immunological surrogates, which may serve as critical benchmarks for vaccine efficacy. Neutralizing antibody titers, multifunctional T cell responses, and B cell memory have emerged as key correlates of protection. However, the feasibility of these surrogates depends on their ability to predict clinical outcomes across diverse populations and settings. While neutralizing antibodies block the virus directly, T cell responses are essential for clearing infected cells and preventing severe disease, and B cell memory ensures long-term immunity. Integrating these immunological markers into a cohesive framework requires standardized assays, robust clinical validation, and an in-depth understanding of RSV-induced immune response.

RSV F evolution escapes some monoclonal antibodies but does not strongly erode neutralization by human polyclonal sera

Vaccines and monoclonal antibodies targeting the respiratory syncytial virus (RSV) fusion protein (F) have recently begun to be widely used to protect infants and high-risk adults. Some other viral proteins evolve to erode polyclonal antibody neutralization and escape individual monoclonal antibodies. However, little is known about how RSV F evolution affects antibodies. Here we develop an experimental system for measuring neutralization titers against RSV F using pseudotyped lentiviral particles. This system is easily adaptable to evaluate neutralization of relevant clinical strains. We apply this system to demonstrate that natural evolution of RSV F leads to escape from some monoclonal antibodies, but at most modestly affects neutralization by polyclonal serum antibodies. Overall, our work sheds light on RSV antigenic evolution and describes a tool to measure the ability of antibodies and sera to neutralize contemporary RSV strains.

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.

Rational design of next-generation filovirus vaccines with glycoprotein stabilization, nanoparticle display, and glycan modification

Filoviruses pose a significant threat to human health with frequent outbreaks and high mortality. Although two vector-based vaccines are available for Ebola virus, a broadly protective filovirus vaccine remains elusive. In this study, we evaluate a general strategy for stabilizing glycoprotein (GP) structures of Ebola, Sudan, and Bundibugyo ebolaviruses and Ravn marburgvirus. A 3.2 Å-resolution crystal structure provides atomic details for the redesigned Ebola virus GP, and cryo-electron microscopy reveals how a pan-ebolavirus neutralizing antibody targets a conserved site on the Sudan virus GP (3.13 Å-resolution), in addition to a low-resolution model of antibody-bound Ravn virus GP. A self-assembling protein nanoparticle (SApNP), I3-01v9, is redesigned at the N-terminus to allow the optimal surface display of filovirus GP trimers. Following detailed in vitro characterization, the lymph node dynamics of Sudan virus GP and GP-presenting SApNPs are investigated in a mouse model. Compared with soluble GP trimer, SApNPs show ~112 times longer retention in lymph node follicles, up-to-28 times greater presentation on follicular dendritic cell dendrites, and up-to-3 times stronger germinal center reactions. Functional antibody responses induced by filovirus GP trimers and SApNPs bearing wildtype and modified glycans are assessed in mice. Our study provides a foundation for next-generation filovirus vaccine development.

Respiratory syncytial virus: health burden, disease prevention, and treatment-recent progress and lessons learned

Respiratory syncytial virus (RSV), a negative-sense single-stranded RNA virus of the Pneumoviridae family, represents the most important pathogen of lower respiratory tract infections in young infants causing yearly epidemics. RSV is also an important respiratory viral pathogen for older subjects, which is second only to seasonal influenza virus infections. RSV represents a substantial public health burden with respect to morbidity and mortality, particularly in developing countries. Prevention and treatment options would therefore lessen the global disease burden. A formalin-inactivated RSV vaccine in the 1960s induced an enhanced disease upon exposure to natural RSV. After this tragical vaccine failure, it took nearly five decades of intensive research before prevention tools were approved by health authorities. The lead was taken by passive immunity approaches with injected monoclonal antibodies directed against the fusion protein F of RSV. The elucidation of the three-dimensional structure of the F protein revealed pre- and postfusion conformations. Subsequently, structure-based antigen engineering of the F protein paved the way for development of a prophylactic vaccine. In 2023, RSV vaccines were approved for maternal vaccination to protect young infants by placental transfer of antibodies and for vaccination in older subjects. Antiviral drugs that target the RSV fusion process, the RSV replicase, or the cytoplasmic viral factories are in development. Important research papers leading to these developments are reviewed here.

A truncated pre-F protein mRNA vaccine elicits an enhanced immune response and protection against respiratory syncytial virus

The Food and Drug Administration (FDA) has approved vaccines designed by GSK, Pfizer and Moderna to protect high-risk populations against respiratory syncytial virus (RSV). These vaccines employ the pre-fusion F (pre-F) protein as the immunogen. In this study, we explored an mRNA vaccine based on a modified pre-F protein called LC2DM-lipid nanoparticle (LC2DM-LNP). This vaccine features a truncated version of the pre-F protein that is anchored to the cell membrane. Our experiments in young and old female mice revealed that the LC2DM-LNP vaccine elicited robust neutralizing antibody titers. Moreover, LC2DM-LNP prompted a Th1-skewed T-cell immune response in female rodent models. Female cotton rats immunized with LC2DM-LNP demonstrated strong immunity to RSV, without signs of vaccine-enhanced respiratory disease (VERD), even in cases of breakthrough infection. Importantly, when administered to pregnant female cotton rats, LC2DM-LNP ensured the transfer of pre-F-specific antibodies to the offspring and provided protection against RSV without increasing lung inflammation. Our findings suggest that LC2DM-LNP could serve as an alternative RSV vaccine candidate for high-risk groups.

Self-amplifying mRNA vaccines protect elderly BALB/c mice against a lethal respiratory syncytial virus infection

Respiratory syncytial virus (RSV) represents a significant threat, being a primary cause of critical lower respiratory tract infections and fatalities among infants and the elderly worldwide, and poses a challenge to global public health. This urgent public health challenge necessitates the swift development of safe and effective vaccines capable of eliciting robust immune responses at low doses. Addressing this need, our study investigated five self-amplifying mRNA (sa-mRNA) candidate vaccines that encode the various pre-fusion conformations of the RSV fusion protein. When administered via low-dose intramuscular injection to 8-month-old elderly mice, these vaccines triggered potent humoral reactions and T helper type 1-biased cellular immunity. A prime-boost strategy followed by challenge with a lethal, mouse-adapted RSV strain showed that three of these sa-mRNA candidates achieved greater than 80% survival rates. An immune correlates of protection analysis contrasting immunized survivors with non-survivors suggest that the titers of IgG and neutralizing antibody are associated with vaccine-mediated protection from RSV infection. Our results highlight the usefulness of sa-mRNA vaccines to play a crucial role in forging an effective defense against RSV, addressing a critical need in protecting vulnerable populations against this virus.

The Interplay of Furin Cleavage and D614G in Modulating SARS-CoV-2 Spike Protein Dynamics

We report a detailed analysis of the full-length SARS-CoV-2 spike dynamics within a native-like membrane environment and variants inaccessible to studies on soluble constructs by conducting hydrogen-deuterium exchange mass spectrometry (HDX-MS) on enveloped virus-like particles (eVLPs) displaying various spike constructs. We find that the previously identified open-interface trimer conformation is sampled in all eVLP-displayed spike variants studied including sequences from engineered vaccine constructs and native viral sequences. The D614G mutation, which arose early in the pandemic, favors the canonical 'closed-interface' prefusion conformation, potentially mitigating premature S1 shedding in the presence of a cleaved furin site and providing an evolutionary advantage to the virus. Remarkably, furin cleavage at the S1/S2 boundary allosterically increases the flexibility of the S2' site, which may facilitate increased TMPRSS2 processing, enhancing viral infectivity. The use of eVLPs in HDX-MS studies provides a powerful platform for studying viral and membrane proteins in near-native environments.

Robust and Long-Lasting Immunity and Protection in Mice Induced by Lipopolyplex-Delivered mRNA Vaccines Expressing the Prefusion Protein of Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants and children. mRNA vaccines based on the lipopolyplex (LPP) platform have been previously reported, but they remain unapplied in RSV vaccine development. In this study, we developed a novel LPP-delivered mRNA vaccine that expresses the respiratory syncytial virus prefusion protein (RSV pre-F) to evaluate its immunogenicity and protective effect in a mouse model. We synthesized mRNAs with gene modification for RSV pre-F and prepared mRNA vaccines using the LPP delivery platform, referred to as RSV pre-F LPP-mRNA. RSV pre-F protein expression in mRNA vaccines was characterized in vitro. Then, we evaluated the effects of the immune response and protection of this mRNA vaccine in mice up to 24 weeks post-vaccination. Following booster immunization, robust and long-lasting RSV pre-F-specific IgG antibodies were detected in the serum of mice, which exhibited Th1/Th2 balanced IgG response and cross-neutralizing antibodies against different subtypes (RSV A2, B18537, and clinical isolate hRSV/C-Tan/BJ 202301), with a clear dose-response relationship observed. RSV pre-F-specific IgG antibodies were maintained in the mice for an extended period, lasting up to 18 weeks post-immunization. Concurrently, multifunctional RSV F-specific CD8+ T cells (IFN-γ, IL-2, and TNF-α) were detected in the mice. After RSV A2 challenge, the RSV pre-F LPP-mRNA vaccine led to a significant reduction in viral replication, while reduced pathological damage was observed in lung tissue. The LPP-delivered mRNA vaccine expressing RSV pre-F induces a robust and long-lasting immune response and protection, indicating good prospects for further development and application.

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.

DS2 designer pre-fusion F vaccine induces strong and protective antibody response against RSV infection

DS-Cav1, SC-TM, and DS2 are distinct designer pre-fusion F proteins (pre-F) of respiratory syncytial virus (RSV) developed for vaccines. However, their immunogenicity has not been directly compared. In this study, we generated three recombinant vaccines using the chimpanzee adenovirus vector AdC68 to express DS-Cav1, SC-TM, and DS2. All three vaccines elicited robust serum binding and neutralizing antibodies following intramuscular priming and boosting. DS2 induced the strongest antibody responses, followed by SC-TM and DS-Cav1. DS2 also provided strong protection against live RSV challenge. Monoclonal antibodies (mAbs) isolated from long-lived antibody-secreting cells (ASCs) in the bone marrow six months post-immunization with AdC68-DS2 predominantly targeted site Ø as well as site II. One neutralizing antibody against site II, mAb60, conferred strong protection against live RSV infection in mice. These findings highlight the strong ability of the DS2 design in eliciting long-lived antibody responses and guide the development of next-generation RSV vaccines.

Favorable Nonclinical Safety Profile of RSVpreF Bivalent Vaccine in Rats and Rabbits

Background: Respiratory syncytial virus (RSV) infections usually cause mild, cold-like symptoms in most people, but are a leading infectious disease causing infant death and hospitalization and can result in increased morbidity and mortality in older adults and at-risk individuals. Pfizer has developed Abrysvo®, an unadjuvanted bivalent recombinant protein subunit vaccine containing prefusion-stabilized fusion (F) proteins representing RSV A and RSV B subgroups (RSVpreF). It is the only RSV vaccine approved for both maternal immunization to protect infants and active immunization of older adults (≥60 years) and 18-59-year-old individuals with high-risk conditions for prevention of RSV disease. Methods: Nonclinical safety studies, including a repeat-dose toxicity (RDT) study in rats and a combined developmental and reproductive toxicity (DART) study in rabbits, were conducted to support early clinical development. Study designs and parameters evaluated in these studies were consistent with principles and practices as outlined in relevant regulatory guidelines. RSVpreF bivalent vaccine, with or without Al(OH)3, was administered intramuscularly (IM) at 2× the human dose to animals in both studies. Results: Locally tolerated, reversible, inflammatory responses at the injection sites and the draining lymph nodes were observed as typical findings following vaccination. No effect of RSVpreF, with or without Al(OH)3, was observed on female fertility or on embryo-fetal or postnatal survival, growth, or development in the DART study. In both studies, robust immune responses to both RSV A and B antigens were observed, especially with the Al(OH)3 formulation. Conclusions: RSVpreF was well-tolerated both locally and systemically without any adverse effects on reproductive and developmental endpoints.

From setbacks to success: lessons from the journey of RSV vaccine development

Respiratory syncytial virus (RSV) causes high worldwide infant mortality, as well as a high disease burden in the elderly. Efforts in vaccine development over the past 60 years have recently delivered three approved vaccines and two monoclonal antibodies (mAbs). Looking back at the eventful history of RSV vaccine development, several factors can be identified that have hampered the developmental pathway, including the occurrence of enhanced RSV disease (ERD) in the first vaccine attempt and the difficulty in characterizing and stabilizing the pre-fusion F protein as a vaccine target. Moreover, the need for large trials to test vaccine efficacy, usually done late in development, and the lack of a correlate of protection (CoP) result in significant uncertainties in RSV vaccine development. The use of controlled human infection models (CHIMs) may provide a solution for some of these problems: through swift, cost-efficient and closely monitored assessment of vaccine safety and efficacy in early clinical phases, vaccines can either 'fail fast' or show results supporting further investments. Moreover, CHIMs facilitate the assessment of disease and could assist in the identification of a CoP supporting late-stage development. Although some factors may affect translatability to real-world vaccine efficacy, CHIMs can support the clinical development pathway in various ways. We advocate for, and demonstrate, a conceptual and rational design of RSV vaccine development. Assessing protective efficacy early on would result in the most cost-efficient pathway and identification of target populations should be done as early as possible. For RSV, elderly individuals and people in low- and middle-income countries are high-impact populations for RSV prevention. While RSV immunization is now available in certain regions, global access is not accomplished yet, and worldwide prevention does not seem within reach. Quick and cost-effective assessments of candidates currently in the pipeline could contribute to future successes in the battle against RSV.

Vaccine and therapeutic agents against the respiratory syncytial virus: resolved and unresolved issue

Respiratory syncytial virus (RSV) is a predominant pathogen responsible for respiratory tract infections among infants, the elderly, and immunocompromised individuals. In recent years, significant progress has been made in innovative vaccines and therapeutic agents targeting RSV. Nevertheless, numerous challenges and bottlenecks persist in the prevention and treatment of RSV infections. This review will provide an overview of the resolved and unresolved issues surrounding the development of vaccines and therapeutic agents against RSV. As of September 2024, three RSV vaccines against acute lower respiratory infections (ALRI) have been approved globally. Additionally, there have been notable progress in the realm of passive immunoprophylactic antibodies, with the monoclonal antibody nirsevimab receiving regulatory approval for the prevention of RSV infections in infants. Furthermore, a variety of RSV therapeutic agents are currently under clinical investigation, with the potential to yield breakthrough advancements in the foreseeable future. This review delineates the advancements and challenges faced in vaccines and therapeutic agents targeting RSV. It aims to provide insights that will guide the development of effective preventive and control measures for RSV.

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.

Ad26.RSV.preF completely protects calves from severe respiratory disease induced by bovine RSV challenge

Vaccination with Ad26.RSV.preF, an Adenoviral serotype 26 vector encoding RSV F protein stabilized in its prefusion conformation, has previously shown to be immunogenic and protective in RSV seropositive adults and immunogenic in seropositive infants. Human and bovine RSV (bRSV) are genetically highly related and share many aspects of pathogenesis, epidemiology and clinical manifestations at young age. As such, infection of calves with bRSV represents a clinically relevant model with high translational value, enabling preclinical evaluation of Ad26.RSV.preF vaccine efficacy in seronegative young animals. Immunization of young calves with Ad26.RSV.preF induced antibodies neutralizing both human and bovine RSV as well as RSV-specific cellular responses. After bRSV challenge, placebo immunized calves showed viral replication in the respiratory tract, and developed fever and lethargy accompanied with severe respiratory distress, resulting in pre-termination of 7/8 calves. In contrast, all Ad26.RSV.preF immunized calves completed the study with only mild clinical symptoms, strongly and significantly diminished viral loads in nasopharynx and lungs, and only minimal lung pathology. Thus, Ad26.RSV.preF is immunogenic in young calves and efficacious in a stringent heterologous bRSV challenge model, demonstrating induction of broadly protective immunity against severe disease.

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.

A self-amplifying RNA RSV prefusion-F vaccine elicits potent immunity in pre-exposed and naïve non-human primates

Newly approved subunit and mRNA vaccines for respiratory syncytial virus (RSV) demonstrate effectiveness in preventing severe disease, with protection exceeding 80% primarily through the generation of antibodies. An alternative vaccine platform called self-amplifying RNA (saRNA) holds promise in eliciting humoral and cellular immune responses. We evaluate the immunogenicity of a lipid nanoparticle (LNP)-formulated saRNA vaccine called SMARRT.RSV.preF, encoding a stabilized form of the RSV fusion protein, in female mice and in non-human primates (NHPs) that are either RSV-naïve or previously infected. Intramuscular vaccination with SMARRT.RSV.preF vaccine induces RSV neutralizing antibodies and cellular responses in naïve mice and NHPs. Importantly, a single dose of the vaccine in RSV pre-exposed NHPs elicits a dose-dependent anamnestic humoral immune response comparable to a subunit RSV preF vaccine. Notably, SMARRT.RSV.preF immunization significantly increases polyfunctional RSV.F specific memory CD4+ and CD8+ T-cells compared to RSV.preF protein vaccine. Twenty-four hours post immunization with SMARRT.RSV.preF, there is a dose-dependent increase in the systemic levels of inflammatory and chemotactic cytokines associated with the type I interferon response in NHPs, which is not observed with the protein vaccine. We identify a cluster of analytes including IL-15, TNFα, CCL4, and CXCL10, whose levels are significantly correlated with each other after SMARRT.RSV.preF immunization. These findings suggest saRNA vaccines have the potential to be developed as a prophylactic RSV vaccine based on innate, cellular, and humoral immune profiles they elicit.

RSVpreF vaccination in pregnancy: a meta-analysis of maternal-fetal safety and infant efficacy

In May 2023, the United States Food and Drug Administration approved a Pfizer©-sponsored (Pfizer, New York, NY, USA) bivalent respiratory syncytial virus prefusion F protein-based vaccine (RSVpreF) RSV vaccine (AbrysvoTM [Pfizer]) for use during pregnancy to prevent neonatal/infant RSV infection. In February of 2022, trials sponsored by GSK© (Brentford, England, UK) on a similar RSVpreF vaccine were halted because of the identification of a safety signal related to preterm births. As these vaccines use identical pre-fusion F-protein technology, we sought to synthesize the existing data on their effectiveness and safety. We identified all randomized controlled trials and used RevMan 5.4.1 (The Cochrane Collaboration, England, UK) to perform the analysis with 95% confidence intervals and risk ratios (RRs). We found many maternal side effects were more prevalent in the RSVpreF group, with more local reactions, blood disorders, fatigue, joint pain, cardiac disorders, headache, fever, gastrointestinal disorders and pregnancy complications. The vaccinated group demonstrated significant reductions in RSV-lower respiratory tract cases (RR, 0.44 [0.33, 0.57]; P<0.00001), severe respiratory illness (RR, 0.29 [0.19, 0.44]; P<0.00001), and hospitalizations (RR, 0.40 [0.24, 0.67]; P=0.0005). RSVpreF vaccination was associated with a higher incidence of preterm delivery (RR, 1.24 [1.08, 1.44]; P=0.003). No significant difference in neonatal deaths was observed (RR, 1.42 [0.70, 2.89]; P=0.34). In conclusion, RSVpreF vaccination results in systemic adverse events and an increase in preterm delivery. Vaccination appears to have acceptable short-term newborn safety, but is not related to a significant decrease in neonatal death.

A potently neutralizing and protective human antibody targeting antigenic site V on RSV and hMPV fusion glycoprotein

Human respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) are frequent drivers of morbidity and mortality in susceptible populations, most often infantile, older adults, and immunocompromised. The primary target of neutralizing antibodies is the fusion (F) glycoprotein on the surface of the RSV and hMPV virion. As a result of the structural conservation between RSV and hMPV F, three antigenic regions are known to induce cross-neutralizing responses: sites III, IV, and V. Leveraging LIBRA-seq, we identify five RSV/hMPV cross-reactive human antibodies. One antibody, 5-1, potently neutralizes all tested viruses from the major subgroups of RSV and hMPV and provides protection against RSV and hMPV in a mouse challenge model. Structural analysis reveals that 5-1 utilizes an uncommon genetic signature to bind an epitope that spans sites Ø, II and V, defining a new mode of antibody cross-reactivity between RSV and hMPV F. These findings highlight the molecular and structural elements influencing RSV and hMPV cross-reactivity as well as the potential of antibody 5-1 for translational development.

Respiratory Syncytial Virus Vaccination in the Adult Pulmonary Patient

TOPIC IMPORTANCE

Since its discovery in 1957, respiratory syncytial virus (RSV) has been widely recognized as a common and deadly pathogen. Although early studies focused on the impact of RSV on the health of children, more recent data show that RSV imposes a significant burden on individuals aged ≥ 70 years. RSV also substantially harms the health of individuals with cardiopulmonary diseases.

REVIEW FINDINGS

Early efforts to develop an RSV vaccine were hampered by toxicity due to antibody-enhanced viral pneumonia and a lack of efficacy in vaccines that targeted the postfusion configuration of the F fusion protein, which is crucial to the pathogenesis of RSV-mediated injury. A newer wave of vaccines has targeted a stabilized prefusion F protein, generating effective neutralizing antibodies and reducing the burden of mild and severe RSV lower respiratory tract injury. This review focuses on the burden of RSV in patients with pulmonary diseases, highlights the tumultuous path from the early days of RSV vaccine development to the modern era, and offers insights into key gaps in knowledge that must be addressed to adequately protect the vulnerable population of patients with severe pulmonary diseases.

SUMMARY

RSV vaccination with bivalent RSVPreF or RSVPreF3OA, which target the stabilized prefusion F protein, can be broadly recommended to adults aged ≥ 60 years with pulmonary diseases. However, more data are needed to understand how these vaccinations affect key clinical outcomes in individuals with pulmonary disease.

The Cumulative Variations of Respiratory Syncytial Virus Fusion Protein (F) in Ten Consecutive Years in China

BACKGROUND

Variations in the fusion (F) protein of respiratory syncytial virus (RSV) with main antigenic sites I-V and Ø may affect the development of RSV vaccines and therapies.

METHODS

In the study, 30 respiratory specimens positive for RSV were randomly selected from children with acute lower respiratory infections (ALRI) in Beijing every year from 2012 to 2021 for F gene sequencing. Then, 300 F gene sequences and 508 uploaded to GenBank from China were subjected to phylogenetic analysis.

RESULTS

The results indicated the nucleotide identities were 95.4-100% among 446 sequences of RSV A, and 96.3-100% among 362 of RSV B. The most common variant loci were N80K (100.00%) and R213S (97.76%) for site Ø, and V384I/T (98.43%) for site I among sequences of RSV A, and M152I (100.00%), I185V (100.00%), and L172Q/H (94.48%) for site V, and R202Q (99.45%) for site Ø among sequences of RSV B. N276S appears in 95.29% sequences of RSV A, while S276N and N262 I/S appear in 1.38% and 0.55% sequences of RSV B, respectively. No variation was found in all sequences at the binding sites of 14N4 and motavizumab.

CONCLUSIONS

There were cumulative variations of the RSV F gene, especially at some binding sites of antigenic sites.

Induction of neutralizing antibody responses by AAV5-based vaccine for respiratory syncytial virus in mice

Introduction

Respiratory Syncytial Virus (RSV) is a significant cause of respiratory illnesses worldwide, particularly in infants and elderly individuals. Despite the burden RSV imposes, effective preventive measures are limited. The research application of adeno-associated virus (AAV) in vaccine platforms has been expanding, and its potential in prevention and treatment has garnered much attention.

Methods

In this study, we explored the potential application of a recombinant adeno-associated virus 5 (rAAV5) vector-based RSV vaccine, focusing on the expression of the pre-fusion (Pre-F) protein structure. Through intramuscular immunization in mice. The immunogenicity of the vaccine was evaluated in Balb/c mice immunized intramuscularly and intranasal, respectively.

Results

The rAAV5-RSV-Fm vaccine demonstrated positive humoral and induced antibody titers against RSV strains A and B for up to 120 days post-immunization. Notably, intranasal administration also elicited protective antibodies. Characterization studies confirmed the ability of the vac-cine to express the Pre-F protein and its superior immunogenicity compared to that of full-length F protein.

Conclusion

These findings underscore the potential application of rAAV5 vector platforms in RSV vaccine development and further investigation into their protective efficacy is warranted.

Structures of the Foamy virus fusion protein reveal an unexpected link with the F protein of paramyxo- and pneumoviruses

Foamy viruses (FVs) constitute a subfamily of retroviruses. Their envelope (Env) glycoprotein drives the merger of viral and cellular membranes during entry into cells. The only available structures of retroviral Envs are those from human and simian immunodeficiency viruses from the subfamily of orthoretroviruses, which are only distantly related to the FVs. We report the cryo-electron microscopy structures of the FV Env ectodomain in the pre- and post-fusion states, which unexpectedly demonstrate structural similarity with the fusion protein (F) of paramyxo- and pneumoviruses, implying an evolutionary link between the viral fusogens. We describe the structural features that are unique to the FV Env and propose a mechanistic model for its conformational change, highlighting how the interplay of its structural elements could drive membrane fusion and viral entry. The structural knowledge on the FV Env now provides a framework for functional investigations, which can benefit the design of FV Env variants with improved features for use as gene therapy vectors.

Prevention of respiratory syncytial virus from 1991 to 2024: a systematic review and bibliometrics analysis

Background

Respiratory syncytial virus (RSV) puts children and elderly individuals worldwide at risk for severe health issues and financial difficulties. Prevention is the main treatment for RSV infection, as there is currently no particular therapy. By using bibliometrics analysis, this study attempted to map the increasing tendency in the prevention of RSV infection from January 1991 to August 2024 and to examine the frontiers and hotspots of related research.

Methods

We extracted pertinent articles through the Web of Science Core Collection (WoSCC) on August 26, 2024, covering the period between January 1991 and August 2024. Then, an online bibliometrix interface (https://bibliometrics.com), R software (version 4.3.2), CiteSpace V6.1R6 (64-bit) software, and the Online Analysis Platform of Literature Metrology were used to analyze the data.

Results

A total of 709 eligible data points pertaining to the prevention of RSV were included. The United States, England, and the Netherlands were the three major contributors to this field. The most productive journal was Vaccine. Centers for Disease Control and Prevention ranked first, with 22 publications in this field. The fusion (F) protein, nonstructural (NS) protein and glycoprotein (G) protein are the target proteins of RSV prevention drugs.

Conclusions

In the past 30 years, the research on RSV prevention has entered a stage of rapid development, and many vaccines and monoclonal antibodies have entered the clinical research stage, and some have been marketed.

Engineering a cleaved, prefusion-stabilized influenza B virus hemagglutinin by identification and locking of all six pH switches

Vaccine components based on viral fusion proteins require high stability of the native prefusion conformation for optimal potency and manufacturability. In the case of influenza B virus hemagglutinin (HA), the stem's conformation relies on efficient cleavage. In this study, we identified six pH-sensitive regions distributed across the entire ectodomain where protonated histidines assume either a repulsive or an attractive role. Substitutions in these areas enhanced the protein's expression, quality, and stability in its prefusion trimeric state. Importantly, this stabilization enabled the production of a cleavable HA0, which is further processed into HA1 and HA2 by furin during exocytic pathway passage, thereby facilitating correct folding, increased stability, and screening for additional stabilizing substitutions in the core of the metastable fusion domain. Cryo-EM analysis at neutral and low pH revealed a previously unnoticed pH switch involving the C-terminal residues of the natively cleaved HA1. This switch keeps the fusion peptide in a clamped state at neutral pH, averting premature conformational shift. Our findings shed light on new strategies for possible improvements of recombinant or genetic-based influenza B vaccines.

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.

AntigenBoost: enhanced mRNA-based antigen expression through rational amino acid substitution

Messenger RNA (mRNA) vaccines represent a groundbreaking advancement in immunology and public health, particularly highlighted by their role in combating the COVID-19 pandemic. Optimizing mRNA-based antigen expression is a crucial focus in this emerging industry. We have developed a bioinformatics tool named AntigenBoost to address the challenge posed by destabilizing dipeptides that hinder ribosomal translation. AntigenBoost identifies these dipeptides within specific antigens and provides a range of potential amino acid substitution strategies using a two-dimensional scoring system. Through a combination of bioinformatics analysis and experimental validation, we significantly enhanced the in vitro expression of mRNA-derived Respiratory Syncytial Virus fusion glycoprotein and Influenza A Hemagglutinin antigen. Notably, a single amino acid substitution improved the immune response in mice, underscoring the effectiveness of AntigenBoost in mRNA vaccine design.

Structure-based design of a soluble human cytomegalovirus glycoprotein B antigen stabilized in a prefusion-like conformation

Human cytomegalovirus (HCMV) glycoprotein B (gB) is a class III membrane fusion protein required for viral entry. HCMV vaccine candidates containing gB have demonstrated moderate clinical efficacy, but no HCMV vaccine has been approved. Here, we used structure-based design to identify and characterize amino acid substitutions that stabilize gB in its metastable prefusion conformation. One variant containing two engineered interprotomer disulfide bonds and two cavity-filling substitutions (gB-C7), displayed increased expression and thermostability. A 2.8 Å resolution cryoelectron microscopy structure shows that gB-C7 adopts a prefusion-like conformation, revealing additional structural elements at the membrane-distal apex. Unlike previous observations for several class I viral fusion proteins, mice immunized with postfusion or prefusion-stabilized forms of soluble gB protein displayed similar neutralizing antibody titers, here specifically against an HCMV laboratory strain on fibroblasts. Collectively, these results identify initial strategies to stabilize class III viral fusion proteins and provide tools to probe gB-directed antibody responses.

The role of immunoglobulin transport receptor, neonatal Fc receptor in mucosal infection and immunity and therapeutic intervention

The neonatal Fc receptor (FcRn) can transport IgG and antigen-antibody complexes participating in mucosal immune responses that protect the host from most pathogens' invasion via the respiratory, digestive, and urogenital tracts. FcRn expression can be triggered upon stimulation with pathogenic invasion on mucosal surfaces, which may significantly modulate the innate immune response of the host. As an immunoglobulin transport receptor, FcRn is implicated in the pathophysiology of immune-related diseases such as infection and autoimmune disorders. In this review, we thoroughly summarize the recent advancement of FcRn in mucosal immunity and its therapeutic strategy. This includes insights into its regulation mechanisms of FcRn expression influenced by pathogens, its emerging role in mucosal immunity and its potential probability as a therapeutic target in infection and autoimmune diseases.

A Phase 1/2a Study Evaluating Safety and Immunogenicity of Ad26.RSV.preF in RSV-seronegative Toddlers Aged 12-24 Months

Background

Respiratory syncytial virus (RSV) causes serious illness in children. The Ad26.RSV.preF vaccine candidate was immunogenic with acceptable safety in a phase 1/2a study of RSV-seropositive children. Here, we assessed its safety and immunogenicity in RSV-seronegative children.

Methods

In this randomized, observer-blinded, placebo-controlled, phase 1/2a study (NCT03606512; https://www.clinicaltrials.gov/ct2/show/NCT03606512), RSV-seronegative toddlers aged 12-24 months received Ad26.RSV.preF (2.5 × 1010 viral particles) or placebo on days 1, 29, and 57 (a meningococcal vaccine [Nimenrix] could substitute for day 57 placebo). Primary endpoints were solicited local and systemic adverse events (AEs; 7 days after each vaccination), unsolicited AEs (28 days postvaccination), and serious AEs (first vaccination until study end). Participants were monitored for RSV-respiratory tract infection to assess infection rates and for severe RSV-lower respiratory tract infection as an indication of enhanced disease. RSV-A2 neutralizing, RSV (A and B) preF binding, and RSV postF immunoglobulin G-binding antibodies were evaluated on days 1 (predose), 8, and 85, and after RSV season 1.

Results

Thirty-eight participants were enrolled and vaccinated (Ad26.RSV.preF, n = 20; placebo, placebo/Nimenrix, n = 18). Solicited AEs were more common following Ad26.RSV.preF than placebo; most were mild/moderate. No vaccine-related serious AEs were reported. Five of 19 participants receiving Ad26.RSV.preF and 2/18 receiving placebo or placebo/Nimenrix had confirmed RSV-respiratory tract infection or RSV-associated otitis media; none were considered severe. At the final season 1 study visit, most Ad26.RSV.preF recipients had ≥2-fold increases from baseline in RSV-A2 neutralizing, RSV A and B preF binding, and RSV postF antibodies.

Conclusions

Ad26.RSV.preF was well tolerated and immunogenic in RSV-seronegative toddlers.

Generation of novel respiratory syncytial virus vaccine candidate antigens that can induce high levels of prefusion-specific antibodies

Respiratory syncytial virus (RSV) infection is an acute respiratory infection caused by RSV. It occurs worldwide, and for over 50 years, several attempts have been made to research and develop vaccines to prevent RSV infection; effective preventive vaccines are eagerly awaited. The RSV fusion (F) protein, which has gained attention as a vaccine antigen, causes a dynamic structural change from the preF to postF state. Therefore, the structural changes in proteins must be regulated to produce a vaccine antigen that can efficiently induce antibodies with high virus-neutralizing activity. We successfully discovered several mutations that stabilized the antigen site Ø in the preF state, trimerized it, and improved the level of protein expression through observation and computational analysis of the RSV-F protein structure and amino acid mutation analysis of RSV strains. The four RSV-F protein mutants that resulted from the combination of these effective mutations stably conserved a wide range of preF- and trimeric preF-specific epitopes with high virus-neutralizing activity. Absorption assay using human serum revealed that mutants constructed bound to antibodies with virus-neutralizing activity that were induced by natural RSV infection, whereas they hardly bound to anti-postF antibodies without virus-neutralizing activity. Furthermore, mouse immunization demonstrated that our constructed mutants induced a high percentage of antibodies that bind to the preF-specific antigen site. These characteristics suggest that the mutants constructed can be superior vaccine antigens from the viewpoint of RSV infection prevention effect and safety.

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.

The Central Conserved Peptides of Respiratory Syncytial Virus G Protein Enhance the Immune Response to the RSV F Protein in an Adenovirus Vector Vaccine Candidate

Respiratory syncytial virus (RSV) is a serious human respiratory pathogen that commonly affects children, older adults, and immunocompromised individuals. At present, the design of licensed vaccines focuses on the incorporation of the pre-fusion protein (PreF protein) of RSV, as this protein has the ability to induce antibodies that offer a high level of protection. Moreover, the G protein contains the CX3C motif that binds the chemokine receptor CX3CR1 in respiratory epithelial cells, which plays an essential role in viral infection. Therefore, incorporating the G antigen into vaccine design may prove more advantageous for RSV prevention. In this study, we developed a human adenoviral vector-based RSV vaccine containing highly neutralizing immunogens, a modified full-length PreF protein fused with the central conserved peptides of the G protein (Gcc) from both RSV subgroups trimerized via a C-terminal foldon, and evaluated its immune response in mice through intranasal (i.n.) immunization. Our results showed that immunization with Ad5-PreF-Qa-Gcc elicited a balanced Th1/Th2 immune response and robust mucosal immunity with higher neutralizing antibody titers against RSV Long and RSV B1. Importantly, immunization with Ad5-PreF-Qa-Gcc enhanced CD4+ CD25+ FoxP3+ Treg cell response and protected the mice against RSV infection. Our data demonstrate that the combination of Gcc and the PreF antigen is a viable strategy for developing effective RSV 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.

Proscan: a structure-based proline design web server

The ability to control protein conformations and dynamics through structure-based design has been useful in various scenarios, including engineering of viral antigens for vaccines. One effective design strategy is the substitution of residues to proline amino acids, which due to its unique cyclic side chain can favor and rigidify key backbone conformations. To provide the community with a means to readily identify and explore proline designs for target proteins of interest, we developed the Proscan web server. Proscan provides assessment of backbone angles, energetic and deep learning-based favorability scores, and other parameters for proline substitutions at each position of an input structure, along with interactive visualization of backbone angles and candidate substitution sites on structures. It identifies known favorable proline substitutions for viral antigens, and was benchmarked against datasets of proline substitution stability effects from deep mutational scanning and thermodynamic measurements. This tool can enable researchers to identify and prioritize designs for prospective vaccine antigen targets, or other designs to favor stability of key protein conformations. Proscan is available at: https://proscan.ibbr.umd.edu.

Structural basis for potent neutralization of human respirovirus type 3 by protective single-domain camelid antibodies

Respirovirus 3 is a leading cause of severe acute respiratory infections in vulnerable human populations. Entry into host cells is facilitated by the attachment glycoprotein and the fusion glycoprotein (F). Because of its crucial role, F represents an attractive therapeutic target. Here, we identify 13 F-directed heavy-chain-only antibody fragments that neutralize recombinant respirovirus 3. High-resolution cryo-EM structures of antibody fragments bound to the prefusion conformation of F reveal three distinct, previously uncharacterized epitopes. All three antibody fragments bind quaternary epitopes on F, suggesting mechanisms for neutralization that may include stabilization of the prefusion conformation. Studies in cotton rats demonstrate the prophylactic efficacy of these antibody fragments in reducing viral load in the lungs and nasal passages. These data highlight the potential of heavy-chain-only antibody fragments as effective interventions against respirovirus 3 infection and identify neutralizing epitopes that can be targeted for therapeutic development.

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.