RSV neutralization assays - Use in immune response assessment

Shared clinical and immunologic features of mRNA vaccines: preliminary results from a comparative clinical study

Introduction

Clinical trials do not typically assess underlying molecular mechanisms of vaccine immunogenicity or reactogenicity. We evaluated the reactogenicity and immunogenicity of 4 mRNA vaccines and potential contributing mechanisms and identified shared and unique clinical and immunologic features.

Methods

This ongoing, open-label, phase 1 trial randomized healthy adults (18-75 years) to receive a single dose of mRNA-1273.222 (bivalent COVID-19), mRNA-1345 (RSV), mRNA-1010 (influenza), and FLUAD (active influenza comparator) or 2 or 3 doses of mRNA-1647 (CMV). The primary objective was to assess the safety and reactogenicity of each study vaccine, with humoral immunogenicity (neutralizing antibody [nAb] responses) as the secondary objective. This interim analysis reports safety and reactogenicity in all study vaccines and humoral immunogenicity in single-dose vaccines (mRNA-1273.222, mRNA-1345, mRNA-1010, and FLUAD). Exploratory objectives included antigen-specific T-cell responses after single-dose mRNA-1345 or mRNA-1273.222, and soluble mediators of inflammation and innate immunity following vaccination in single-dose vaccine groups and two doses of mRNA-1647.

Results

At the interim analysis data cutoff (February 1, 2023), 302 participants received 1 dose of the study vaccines. Reactogenicity exhibited a consistent trend across vaccine groups; most solicited local and systemic adverse reactions within 7 days were mild or moderate in severity. There were no deaths or serious, severe, or treatment-related adverse events leading to study discontinuation. At Day 29, nAb titers against vaccine-specific antigens increased 2- to 8-fold versus baseline for all single-dose vaccine groups. In an exploratory analysis, mRNA-1273.222 and mRNA-1345 induced antigen-specific Th1-biased CD4+ and CD8+ T-cell responses at Day 29. The cytokine response analysis showed increased levels of IFN-γ, IL-6, IL-2Ra, CXCL9, IP-10, MCP-2, and MIP-1β on Day 2 following vaccination, with generally greater increases observed with mRNA vaccines versus FLUAD. Regardless of age and across mRNA vaccine groups, peak serum levels of IL-1Ra and MCP-1/MCP-2 on Day 2 weakly correlated with systemic reactogenicity scores (correlation coefficient range: 0.15-0.27).

Conclusions

The 4 mRNA vaccines had acceptable reactogenicity, demonstrated changes in serum biomarkers of innate immune activation, and were immunogenic. This suggests that the observed reactogenicity of mRNA vaccines may be related to shared features of the mRNA platform (LNP platform).

Clinical trial registration

ClinicalTrials.gov, identifier NCT05397223.

Coumarins and Hesperetin Inhibit Human Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is one of the most prevalent viruses that causes severe acute lower respiratory tract infections (ALRTIs) in the elderly and young children. There is no specific drug to treat RSV, only a broad-spectrum antiviral, ribavirin, which is only used in critical cases. Our research group is investigating antiviral agents of natural origin, such as coumarins and flavonoids, that may help reduce or prevent RSV infection. The cytotoxic concentrations of coumarins and hesperetin were tested on A549 and HEp-2 cells and used in inhibition tests in which 80% of the cells were viable. The anti-RSV action of the molecules was analyzed in A549 and HEp-2 cells and in HBE cell cultures infected with RSV-luc or rgRSV. We also encapsulated the compounds using β-cyclodextrin to improve the permeability and solubility of the molecules. Esculetin and 4-methyl inhibited rgRSV effectively on A549 and HEp-2 cells after 24 hpi, and when they were encapsulated, coumarin, esculetin, and hesperetin presented inhibition against rgRSV in HBE culture. The coumarins inhibit RSV replication in cell culture and even manage to overcome the mucus barriers of the HBE cultures, and β-cyclodextrin was essential for some of the coumarins to enter the cell and therefore to reach their targets.

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.

Clinical validation of an RSV neutralization assay and analysis of cross-sectional sera associated with 2021-2023 RSV outbreaks to investigate the immunity debt hypothesis

Respiratory syncytial virus (RSV) is a leading cause of acute respiratory infections and hospitalization in infants and the elderly. Newly approved vaccines and the prophylactic antibody nirsevimab have heightened interest in RSV immunologic surveillance, necessitating the development of high-throughput assays assessing anti-RSV neutralizing activity. Quantitative viral neutralization remains the best correlate of protection for RSV infection and the gold standard for RSV immunological testing. Here, we developed a high-throughput RSV strain A2 focus-reduction neutralization test validated to Clinical Laboratory Improvement Amendments (CLIA)/ Good Clinical Laboratory Practices (GCLP) standards using both clinical specimens and commercially available reference sera. The assay is highly accurate, generating reference serum neutralizing titers within twofold of established assays, with an analytical measurement range between 8 and 1,798 international units per mL (IU/mL). Neutralizing activity measured by the assay strongly correlated with antibody titer determined via indirect enzyme-linked immunosorbent assay (ELISA) (ρ = 1.0, P = 0.0014). Individuals recently having tested positive via quantitative reverse transcription polymerase chain reaction (RT-qPCR) for RSV had a 9.1-fold higher geometric mean neutralizing titer relative to RSV PCR negatives (P-value = 0.09). The validated assay was then used to investigate the immunity debt hypothesis for resurgent RSV outbreaks in the 2022-2023 season, using adult clinical remnant sera sent for herpes simplex virus (HSV)-1/2 antibody testing. There was no difference in geometric mean anti-RSV neutralizing titers between sera sampled before and after the 2022-2023 RSV outbreak (P = 0.68). These data are consistent with limited changes in RSV-neutralizing antibody levels in adults across the 2022-23 RSV outbreak.

IMPORTANCE

Population surveillance studies of serum-neutralizing activity against RSV are crucial for evaluating RSV vaccine efficacy and vulnerabilities to new strains. Here, we designed and validated a high-throughput assay for assessing anti-RSV neutralizing activity, standardized its measurements for comparison with other methodologies, and demonstrated its applicability to real-world samples. Our assay is precise, linear, and yields measurements consistent with other standardized assays, offering a methodology useful for large-scale studies of RSV immunity. We also find no significant difference in neutralizing titers among adults between those taken before and after large RSV outbreaks associated with the latter stages of the coronavirus disease of 2019 (COVID-19) public health emergency, underlining the need for a greater understanding of the dynamics of serological responses to RSV infection.

RSV Neutralizing Antibodies in Dried Blood

BACKGROUND

The key correlate of protection of respiratory syncytial virus (RSV) vaccines and monoclonal antibodies (mAbs) is virus neutralization, measured via sera obtained through venipuncture. Dried blood obtained with a finger prick can simplify acquisition, processing, storage, and transport in trials and thereby reduce costs. In this study, we validate an assay to measure RSV neutralization in dried capillary blood.

METHODS

Functional antibodies were compared between matched serum and dried blood samples from a phase 1 trial with RSM01, an investigational anti-RSV prefusion F mAb. Hep-2 cells were infected with a serial dilution of sample-virus mixture by using RSV-A2-mKate to determine the half-maximal inhibitory concentration. Stability of dried blood was evaluated over time and during temperature stress.

RESULTS

Functional antibodies in dried blood were highly correlated with serum (R2 = 0.98, P < .0001). The precision of the assay for dried blood was similar to serum. The function of mAb remained stable for 9 months at room temperature and frozen dried blood samples.

CONCLUSIONS

We demonstrated the feasibility of measuring RSV neutralization using dried blood as a patient-centered solution that may replace serology testing in trials against RSV or other viruses, such as influenza and SARS-CoV-2. Clinical Trials Registration.  NCT05118386 (ClinicalTrials.gov).

Virus neutralization assays for human respiratory syncytial virus using airway organoids

Neutralizing antibodies are considered a correlate of protection against severe human respiratory syncytial virus (HRSV) disease. Currently, HRSV neutralization assays are performed on immortalized cell lines like Vero or A549 cells. It is known that assays on these cell lines exclusively detect neutralizing antibodies (nAbs) directed to the fusion (F) protein. For the detection of nAbs directed to the glycoprotein (G), ciliated epithelial cells expressing the cellular receptor CX3CR1 are required, but generation of primary cell cultures is expensive and labor-intensive. Here, we developed a high-throughput neutralization assay based on the interaction between clinically relevant HRSV grown on primary cells with ciliated epithelial cells, and validated this assay using a panel of infant sera. To develop the high-throughput neutralization assay, we established a culture of differentiated apical-out airway organoids (Ap-O AO). CX3CR1 expression was confirmed, and both F- and G-specific monoclonal antibodies neutralized HRSV in the Ap-O AO. In a side-by-side neutralization assay on Vero cells and Ap-O AO, neutralizing antibody levels in sera from 125 infants correlated well, although titers on Ap-O AO were consistently lower. We speculate that these lower titers might be an actual reflection of the neutralizing antibody capacity in vivo. The organoid-based neutralization assay described here holds promise for further characterization of correlates of protection against HRSV disease.

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

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

Value profile for respiratory syncytial virus vaccines and monoclonal antibodies

Respiratory syncytial virus (RSV) is the predominant cause of acute lower respiratory infection (ALRI) in young children worldwide, yet no licensed RSV vaccine exists to help prevent the millions of illnesses and hospitalizations and tens of thousands of young lives taken each year. Monoclonal antibody (mAb) prophylaxis exists for prevention of RSV in a small subset of very high-risk infants and young children, but the only currently licensed product is impractical, requiring multiple doses and expensive for the low-income settings where the RSV disease burden is greatest. A robust candidate pipeline exists to one day prevent RSV disease in infant and pediatric populations, and it focuses on two promising passive immunization approaches appropriate for low-income contexts: maternal RSV vaccines and long-acting infant mAbs. Licensure of one or more candidates is feasible over the next one to three years and, depending on final product characteristics, current economic models suggest both approaches are likely to be cost-effective. Strong coordination between maternal and child health programs and the Expanded Program on Immunization will be needed for effective, efficient, and equitable delivery of either intervention. This 'Vaccine Value Profile' (VVP) for RSV is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships and multi-lateral organizations, and in collaboration with stakeholders from the WHO headquarters. All contributors have extensive expertise on various elements of the RSV VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

Reduced Respiratory Syncytial Virus Load, Symptoms, and Infections: A Human Challenge Trial of MVA-BN-RSV Vaccine

BACKGROUND

Respiratory syncytial virus (RSV) causes significant disease burden in older adults. MVA-BN-RSV is a novel poxvirus-vectored vaccine encoding internal and external RSV proteins.

METHODS

In a phase 2a randomized double-blind, placebo-controlled trial, healthy participants aged 18 to 50 years received MVA-BN-RSV or placebo, then were challenged 4 weeks later with RSV-A Memphis 37b. Viral load was assessed from nasal washes. RSV symptoms were collected. Antibody titers and cellular markers were assessed before and after vaccination and challenge.

RESULTS

After receiving MVA-BN-RSV or placebo, 31 and 32 participants, respectively, were challenged. Viral load areas under the curve from nasal washes were lower (P = .017) for MVA-BN-RSV (median = 0.00) than placebo (median = 49.05). Total symptom scores also were lower (median = 2.50 and 27.00, respectively; P = .004). Vaccine efficacy against symptomatic, laboratory-confirmed or culture-confirmed infection was 79.3% to 88.5% (P = .022 and .013). Serum immunoglobulin A and G titers increased approximately 4-fold after MVA-BN-RSV vaccination. Interferon-γ-producing cells increased 4- to 6-fold after MVA-BN-RSV in response to stimulation with the encoded RSV internal antigens. Injection site pain occurred more frequently with MVA-BN-RSV. No serious adverse events were attributed to vaccination.

CONCLUSIONS

MVA-BN-RSV vaccination resulted in lower viral load and symptom scores, fewer confirmed infections, and induced humoral and cellular responses.

CLINICAL TRIALS REGISTRATION

NCT04752644.

A Pseudovirus-Based Entry Assay to Evaluate Neutralizing Activity against Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) infection can cause life-threatening pneumonia and bronchiolitis, posing a significant threat to human health worldwide, especially to children and the elderly. Currently, there is no specific treatment for RSV infection. The most effective measures for preventing RSV infection are vaccines and prophylactic medications. However, not all population groups are eligible for the approved vaccines or antibody-based preventive medications. Therefore, there is an urgent need to develop novel vaccines and prophylactic drugs available for people of all ages. High-throughput assays that evaluate the efficacy of viral entry inhibitors or vaccine-induced neutralizing antibodies in blocking RSV entry are crucial for evaluating vaccine and prophylactic drug candidates. We developed an efficient entry assay using a lentiviral pseudovirus carrying the fusion (F) protein of type A or B RSV. In addition, the essential parameters were systematically optimized, including the number of transfected plasmids, storage conditions of the pseudovirus, cell types, cell numbers, virus inoculum, and time point of detection. Furthermore, the convalescent sera exhibited comparable inhibitory activity in this assay as in the authentic RSV virus neutralization assay. We established a robust pseudovirus-based entry assay for RSV, which holds excellent promise for studying entry mechanisms, evaluating viral entry inhibitors, and assessing vaccine-elicited neutralizing antibodies against RSV.

Respiratory syncytial virus infection in the modern era

PURPOSE OF REVIEW

Respiratory syncytial virus (RSV) continues to be a major cause of severe lower respiratory tract infection in infants, young children, and older adults. In this review, changes in the epidemiology of RSV during the coronavirus disease 2019 (COVID-19) pandemic are highlighted together with the role which increased molecular surveillance efforts will have in future in assessing the efficacy of vaccines and therapeutics.

RECENT FINDINGS

The introduction of nonpharmaceutical intervention (NPIs) strategies during the COVID-19 pandemic between 2020 and 2022 resulted in worldwide disruption to the epidemiology of RSV infections, especially with respect to the timing and peak case rate of annual epidemics. Increased use of whole genome sequencing along with efforts to better standardize the nomenclature of RSV strains and discrimination of RSV genotypes will support increased monitoring of relevant antigenic sites in the viral glycoproteins. Several RSV vaccine candidates based on subunit, viral vectors, nucleic acid, or live attenuated virus strategies have shown efficacy in Phase 2 or 3 clinical trials with vaccines using RSVpreF protein currently the closest to approval and use in high-risk populations. Finally, the recent approval and future use of the extended half-life human monoclonal antibody Nirsevimab will also help to alleviate the morbidity and mortality burden caused by annual epidemics of RSV infections.

SUMMARY

The ongoing expansion and wider coordination of RSV molecular surveillance efforts via whole genome sequencing will be crucial for future monitoring of the efficacy of a new generation of vaccines and therapeutics.

The Quest for a Respiratory Syncytial Virus Vaccine for Older Adults: Thinking beyond the F Protein

Respiratory syncytial virus (RSV) is a common cause of paediatric respiratory tract infection and causes a significant health burden in older adults. Natural immunity to RSV is incomplete, permitting recurrent symptomatic infection over an individual's lifespan. When combined with immunosenescence, this increases older adults' susceptibility to more severe disease symptoms. As RSV prophylaxis is currently limited to infants, older adults represent an important target population for RSV vaccine development. The relationship between RSV and our immune systems is complex, and these interactions require deeper understanding to tailor an effective vaccine candidate towards older adults. To date, vaccine candidates targeting RSV antigens, including pre-F, F, G (A), G (B), M2-1, and N, have shown efficacy against RSV infection in older adults in clinical trial settings. Although vaccine candidates have demonstrated robust neutralising IgG and cellular responses, it is important that research continues to investigate the RSV immune response in order to further understand how the choice of antigenic target site may impact vaccine effectiveness. In this article, we discuss the Phase 3 vaccine candidates being tested in older adults and review the hurdles that must be overcome to achieve effective protection against RSV.

Correlates of Protection Against Respiratory Syncytial Virus Infection in Infancy

The highest morbidity and mortality from respiratory syncytial virus (RSV) infection occurs in young infants. Immunization of expectant mothers during pregnancy has the potential to substantially reduce the burden of RSV disease in a majority of infants. Correlates of protection (COP) are important in guiding the development of maternal RSV vaccines and the design of maternal RSV vaccine trials, as immune response to candidate vaccines should mirror protective RSV immunity at birth. Here, we review the literature reporting correlations between RSV immune measures at birth and clinical RSV outcomes during infancy. Less than a dozen studies have investigated immunological COP with RSV disease or related hospitalization, yielding inconsistent findings overall. The differences in findings between studies could be due to differences in inclusion/exclusion criteria (e.g., the inclusion of older infants who may benefit less from maternal antibodies or infants followed during inter-seasonal periods where RSV is absent), differences in semi-quantitative RSV antibody neutralization assays, or differences in RSV outcome measures such as the sensititivity/specificity of diagnostic tests. Future research in this field should seek to standardize RSV immunological measures and outcomes, expand the breadth of functional RSV measures beyond antibody neutralization, and consider infants' age and seasonality of RSV infection.