Antigenic Site-Specific Competitive Antibody Responses to the Fusion Protein of Respiratory Syncytial Virus Were Associated With Viral Clearance in Hematopoietic Cell Transplantation Adults

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.

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.

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.

Structural basis for respiratory syncytial virus and human metapneumovirus neutralization

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) continue to be a global burden to infants, the elderly, and immunocompromised individuals. In the past ten years, there has been substantial progress in the development of new vaccine candidates and therapies against these viruses. These advancements were guided by the structural elucidation of the major surface glycoproteins for these viruses, the fusion (F) protein and attachment (G) protein. The identification of immunodominant epitopes on the RSV F and hMPV F proteins has expanded current knowledge on antibody-mediated immune responses, which has led to new approaches for vaccine and therapeutic development through the stabilization of pre-fusion constructs of the F protein and pre-fusion-specific monoclonal antibodies with high potency and efficacy. In this review, we describe structural characteristics of known antigenic sites on the RSV and hMPV proteins, their influence on the immune response, and current progress in vaccine and therapeutic development.

Cationic-nanogel nasal vaccine containing the ectodomain of RSV-small hydrophobic protein induces protective immunity in rodents

Respiratory syncytial virus (RSV) is a leading cause of upper and lower respiratory tract infection, especially in children and the elderly. Various vaccines containing the major transmembrane surface proteins of RSV (proteins F and G) have been tested; however, they have either afforded inadequate protection or are associated with the risk of vaccine-enhanced disease (VED). Recently, F protein-based maternal immunization and vaccines for elderly patients have shown promising results in phase III clinical trials, however, these vaccines have been administered by injection. Here, we examined the potential of using the ectodomain of small hydrophobic protein (SHe), also an RSV transmembrane surface protein, as a nasal vaccine antigen. A vaccine was formulated using our previously developed cationic cholesteryl-group-bearing pullulan nanogel as the delivery system, and SHe was linked in triplicate to pneumococcal surface protein A as a carrier protein. Nasal immunization of mice and cotton rats induced both SHe-specific serum IgG and mucosal IgA antibodies, preventing viral invasion in both the upper and lower respiratory tracts without inducing VED. Moreover, nasal immunization induced greater protective immunity against RSV in the upper respiratory tract than did systemic immunization, suggesting a critical role for mucosal RSV-specific IgA responses in viral elimination at the airway epithelium. Thus, our nasal vaccine induced effective protection against RSV infection in the airway mucosa and is therefore a promising vaccine candidate for further development.

The RSV F p27 peptide: current knowledge, important questions

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

Antibody responses of healthy adults to the p27 peptide of respiratory syncytial virus fusion protein

The respiratory syncytial virus (RSV) fusion (F) protein undergoes two furin-cleavage events to become fusion competent, resulting in the release of a twenty-seven amino acid peptide (p27). Recent studies indicate that the p27 region of the F protein was an immunodominant antigen in young children. In this study, we evaluated the kinetics of the serum antibody response to the p27 peptide following natural RSV reinfection in adults. Nineteen healthy adults under sixty-five years of age were enrolled during the 2018-2019 RSV season in Houston, TX. Blood was collected at three study visits and RSV infection status was defined by changes in neutralizing antibody resulting in three groups: uninfected (n = 12), acutely infected (n = 4), and recently infected (n = 3). Serum IgG and IgA antibodies against RSV/A and RSV/B p27 peptides were measured by enzyme-linked immunosorbent assays, and serum p27-like antibodies were detected by a p27 competitive antibody assay. Anti-p27 antibodies were detected in all subjects at each study visit. The measured IgG and IgA anti-p27 antibody levels followed the same pattern as other RSV site-specific and neutralizing antibody responses described for this cohort previously: the uninfected group had stable responses for the duration of the study period, the acutely infected group had a significant increase following RSV infection, and the recently infected group had a decrease in anti-p27 antibody during the study period. These results indicate that antibodies to the p27 region of the F protein are generated following natural RSV reinfection and suggest that some of the F protein is potentially in a partially cleaved state on the surface of virions, expanding on the previous assumption that all of p27 is post-translationally released and not present on mature F. Additionally, antibody responses were significantly lower (1.4-1.5-fold) toward RSV/B than to RSV/A p27 at each study visit, despite being an RSV/B dominant outbreak. Understanding the mechanism for the differences in the magnitude of the RSV/A and RSV/B p27 antibody response may enhance our understanding of the intracellular processing of the F protein.

Humoral and Mucosal Antibody Response to RSV Structural Proteins in RSV-Infected Adult Hematopoietic Cell Transplant (HCT) Recipients

Respiratory syncytial virus (RSV) is an important cause of lower respiratory tract infection in infants, the elderly, and immunocompromised patients. RSV antibodies play a role in preventing reinfection and in clearance of RSV, but data regarding the levels of viral protein-specific antibodies elicited and their contribution to patient recovery from RSV-induced disease are limited. We prospectively enrolled a cohort of RSV-infected adult hematopoietic cell transplant (HCT) recipients (n = 40). Serum and nasal-wash samples were obtained at enrollment (acute samples) and convalescence (convalescent samples). We measured (1) humoral IgG and mucosal IgA binding antibody levels to multiple RSV proteins (F, G, N, P, and M2-1) by Western blot (WB); (2) neutralizing antibody (Nt Ab) titers by microneutralization assay; and (3) palivizumab-like antibody (PLA) concentrations by an ELISA-based competitive binding assay developed in the lab. Finally, we tested for correlations between protein-specific antibody levels and duration of viral shedding (normal: cleared in <14 days and delayed: cleared ≥14 days), as well as RSV/A and RSV/B subtypes. Convalescent sera from HCT recipients had significantly higher levels of anti-RSV antibodies to all 5 RSV structural proteins assayed (G, F, N, P, M2-1), higher Nt Abs to both RSV subtypes, and higher serum PLAs than at enrollment. Significantly higher levels of mucosal antibodies to 3 RSV structural proteins (G, N, and M2-1) were observed in the convalescent nasal wash versus acute nasal wash. Normal viral clearance group had significantly higher levels of serum IgG antibodies to F, N, and P viral proteins, higher Nt Ab to both RSV subtypes, and higher PLA, as well as higher levels of mucosal IgA antibodies to G and M2-1 viral proteins, and higher Nt Ab to both RSV subtypes compared to delayed viral clearance group. Normal RSV clearance was associated with higher IgG serum antibody levels to F and P viral proteins, and PLAs in convalescent serum (p < 0.05). Finally, overall antibody levels in RSV/A- and/B-infected HCT recipients were not significantly different. In summary, specific humoral and mucosal RSV antibodies are associated with viral clearance in HCT recipients naturally infected with RSV. In contrast to the humoral response, the F surface glycoprotein was not a major target of mucosal immunity. Our findings have implications for antigen selection in the development of RSV vaccines.

Profile of respiratory syncytial virus prefusogenic fusion protein nanoparticle vaccine

INTRODUCTION

Respiratory Syncytial Virus (RSV) is a leading cause of acute lower respiratory infections worldwide. The RSV fusion (F) glycoprotein is a major focus of vaccine development. Despite over 60 years of research, there is no licensed vaccine for RSV.

AREAS COVERED

The primary focus of this review is a novel RSV-F recombinant nanoparticle vaccine from Novavax utilizing the F protein, a conserved and immunodominant surface glycoprotein. This RSV F recombinant nanoparticle vaccine adsorbed to 0.4 mg of aluminum phosphate was ultimately administered by a single intramuscular injection during the third trimester of pregnancy in an effort to induce passive immunity in newborns. Its mechanism, performance in clinical trials, and place in RSV vaccine history are discussed.

EXPERT OPINION

The vaccine was safe and well tolerated in pregnant women and the results suggest potential benefits with respect to other medically relevant end-point events involving RSV-associated respiratory and all-cause disease in infants. However, the RSV-F recombinant nanoparticle vaccine did not meet the pre-specified primary success criteria for efficacy against RSV-associated, medically significant lower respiratory tract infection in infants up to 90 days of life. The potential benefits to infants from maternal immunization and excellent safety profile warrant further confirmatory studies.

A prospective surveillance study on the kinetics of the humoral immune response to the respiratory syncytial virus fusion protein in adults in Houston, Texas

Respiratory syncytial virus (RSV)-specific serum antibody has been correlated to protection of infection and reduction of severe disease, but reinfection is still frequent. In this study, we evaluated RSV-specific serum antibody activity following natural RSV re-infection to examine the longevity of the humoral immune response in adults. Nineteen healthy adult volunteers under sixty-five years of age were enrolled during the 2018-2019 RSV season in Houston, TX. Blood was collected at three study visits. The kinetics of RSV-neutralizing, RSV F site-specific competitive, and RSV-binding antibodies in serum samples were measured by microneutralization and enzyme-linked immunosorbent assays. Three distinct profiles of RSV-specific antibody kinetics were identified that were consistent with RSV infection status: uninfected, acutely infected, and recently infected. The uninfected group had stable antibody titers for the duration of the study period (185 days). The acutely infected group had lower antibody responses at the beginning of the study, supporting a correlate of infection, followed by a significant antibody response after infection that was maintained for at least 125 days. Unlike the acutely infected group, the recently infected group had a significant precipitous decrease in RSV antibody in only 60 days. This study is the first, to our knowledge, to describe this abrupt loss of RSV-specific antibody in detail. This rapid decline of antibody may present an obstacle for the development of vaccines with lasting protection against RSV, and perhaps other respiratory pathogens. Neutralizing antibody responses were greater to prototypic than contemporaneous RSV strains, regardless of infection status, indicating that original antigenic sin may impact the humoral immune response to new or emerging RSV strains.

Antigenic Fingerprinting of Respiratory Syncytial Virus (RSV)-A-Infected Hematopoietic Cell Transplant Recipients Reveals Importance of Mucosal Anti-RSV G Antibodies in Control of RSV Infection in Humans

BACKGROUND

Respiratory syncytial virus (RSV) infection causes significant morbidity in hematopoietic cell transplant (HCT) recipients. However, antibody responses that correlate with recovery from RSV disease are not fully understood.

METHODS

In this study, antibody repertoire in paired serum and nasal wash samples from acutely RSV-A-infected HCT recipients who recovered early (<14 days of RSV shedding) were compared with late-recovered patients (≥14 days of shedding) using gene fragment phage display libraries and surface plasmon resonance.

RESULTS

Anti-F serum responses were similar between these 2 groups for antibody repertoires, neutralization titers, anti-F binding antibodies (prefusion and postfusion proteins), antibody avidity, and binding to specific antigenic sites. In contrast, nasal washes from early-recovered individuals demonstrated higher binding to F peptide containing p27. While the serum RSV G antibody repertoires in the 2 groups were similar, the strongest difference between early-recovered and late-recovered patients was observed in the titers of nasal wash antibodies, especially binding to the central conserved domain. Most importantly, a significantly higher antibody affinity to RSV G was observed in nasal washes from early-recovered individuals compared with late-recovered HCT recipients.

CONCLUSIONS

These findings highlight the importance of mucosal antibodies in resolution of RSV-A infection in the upper respiratory tract.

Antibody Response to the Furin Cleavable Twenty-Seven Amino Acid Peptide (p27) of the Fusion Protein in Respiratory Syncytial Virus (RSV) Infected Adult Hematopoietic Cell Transplant (HCT) Recipients

Background: Cleavage of the inactive precursor fusion protein (F0) of respiratory syncytial virus (RSV) at two furin-recognition sites is required for membrane fusion activity, and the cleavage releases the twenty-seven amino acid peptide (p27). However, a recent study shows that p27 was an immunodominant epitope in RSV infected children, indicating that p27 was recognized as an immunogen. In the present study, we investigated the immunogenicity of p27 in an immunocompromised population of adults by measuring serum and mucosal antibody responses to p27 in samples from adult hematopoietic cell transplant (HCT) recipients. Methods: We prospectively enrolled a cohort of RSV infected HCT recipients. Serum and nasal-wash samples were obtained within the first week of RSV infection (acute) and 3 to 5 weeks post-infection (convalescent). We quantified the serum and mucosal IgG and IgA anti-p27 antibodies by a RSV/A p27 peptide enzyme-linked immunosorbent assay (ELISA) and serum and mucosal p27 like antibodies (P₂₇LA) by a p27 competitive antibody (P₂₇CA) assay. Results: The lower limit of detection for the ELISA and P₂₇CA assays was 0.2 and 50 ng/mL, respectively with no cross-reaction detected with a panel of monoclonal antibodies targeting pre-fusion and post-fusion antigenic sites. P27 antibodies were detected at nanogram concentration in sera and nasal washes in the majority of RSV infected HCT recipients. However, there was no significant difference in the geometric mean antibody concentrations between the acute and convalescent sera (except for serum P₂₇LA), between HCT recipients who shed RSV <14 days and ≥14 days, as well as between RSV/A and RSV/B infected HCT recipients. In addition, approximately 30% of HCT recipients had a 4-fold or greater decrease in mucosal IgG and IgA anti-p27 antibodies during viral clearance. Conclusion: In conclusion, in RSV naturally infected adult HCT recipients, the antibodies against p27 were detectable in both serum and nasal wash samples with higher concentration in serum than that in nasal washes. However, nearly 30% of RSV infected HCT recipients had a significant decrease in their mucosal anti-p27 antibody, suggesting that IgG and IgA anti-p27 antibodies were binding to either free viruses or RSV infected cells containing p27, and that anti-p27 antibodies in the respiratory tract were part of the mucosal antibody response in controlling RSV infection.

Structural Insight into Paramyxovirus and Pneumovirus Entry Inhibition

Paramyxoviruses and pneumoviruses infect cells through fusion (F) protein-mediated merger of the viral envelope with target membranes. Members of these families include a range of major human and animal pathogens, such as respiratory syncytial virus (RSV), measles virus (MeV), human parainfluenza viruses (HPIVs), and highly pathogenic Nipah virus (NiV). High-resolution F protein structures in both the metastable pre- and the postfusion conformation have been solved for several members of the families and a number of F-targeting entry inhibitors have progressed to advanced development or clinical testing. However, small-molecule RSV entry inhibitors have overall disappointed in clinical trials and viral resistance developed rapidly in experimental settings and patients, raising the question of whether the available structural information may provide a path to counteract viral escape through proactive inhibitor engineering. This article will summarize current mechanistic insight into F-mediated membrane fusion and examine the contribution of structural information to the development of small-molecule F inhibitors. Implications are outlined for future drug target selection and rational drug engineering strategies.