Convergent structural features of respiratory syncytial virus neutralizing antibodies and plasticity of the site V epitope on prefusion F

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.

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

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

A potent broad-spectrum neutralizing antibody targeting a conserved region of the prefusion RSV F protein

Respiratory syncytial virus (RSV) poses a significant public health challenge, especially among children. Although palivizumab and nirsevimab, neutralizing antibodies (nAbs) targeting the RSV F protein, have been used for prophylaxis, their limitations underscore the need for more effective alternatives. Herein, we present a potent and broad nAb, named 5B11, which exhibits nanogram level of unbiased neutralizing activities against both RSV-A and -B subgroups. Notably, 5B11 shows a ~20-fold increase in neutralizing efficacy compared to 1129 (the murine precursor of palivizumab) and approximately a 3-fold increase in neutralizing efficacy against B18537 in comparison to nirsevimab. Cryo-electron microscopy analysis reveals 5B11's mechanism of action by targeting a highly conserved epitope within site V, offering a promising strategy with potentially lower risk of escape mutants. Antiviral testing in a female cotton rat model demonstrated that low-dose (1.5 mg/kg) administration of 5B11 achieved comparable prophylactic efficacy to that achieved by high-dose (15 mg/kg) of 1129. Furthermore, the humanized 5B11 showed a superior in vivo antiviral activity against B18537 infection compared to nirsevimab and palivizumab. Therefore, 5B11 is a promising RSV prophylactic candidate applicable to broad prevention of RSV infection.

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.

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

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

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

IMPORTANCE

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

The RSVPreF3-AS01 vaccine elicits broad neutralization of contemporary and antigenically distant respiratory syncytial virus strains

The RSVPreF3-AS01 vaccine, containing the respiratory syncytial virus (RSV) prefusion F protein and the AS01 adjuvant, was previously shown to boost neutralization responses against historical RSV strains and to be efficacious in preventing RSV-associated lower respiratory tract diseases in older adults. Although RSV F is highly conserved, variation does exist between strains. Here, we characterized variations in the major viral antigenic sites among contemporary RSV sequences when compared with RSVPreF3 and showed that, in older adults, RSVPreF3-AS01 broadly boosts neutralization responses against currently dominant and antigenically distant RSV strains. RSV-neutralizing responses are thought to play a central role in preventing RSV infection. Therefore, the breadth of RSVPreF3-AS01-elicited neutralization responses may contribute to vaccine efficacy against contemporary RSV strains and those that may emerge in the future.

Discovery and multimerization of cross-reactive single-domain antibodies against SARS-like viruses to enhance potency and address emerging SARS-CoV-2 variants

Coronaviruses have been the causative agent of three epidemics and pandemics in the past two decades, including the ongoing COVID-19 pandemic. A broadly-neutralizing coronavirus therapeutic is desirable not only to prevent and treat COVID-19, but also to provide protection for high-risk populations against future emergent coronaviruses. As all coronaviruses use spike proteins on the viral surface to enter the host cells, and these spike proteins share sequence and structural homology, we set out to discover cross-reactive biologic agents targeting the spike protein to block viral entry. Through llama immunization campaigns, we have identified single domain antibodies (VHHs) that are cross-reactive against multiple emergent coronaviruses (SARS-CoV, SARS-CoV-2, and MERS). Importantly, a number of these antibodies show sub-nanomolar potency towards all SARS-like viruses including emergent CoV-2 variants. We identified nine distinct epitopes on the spike protein targeted by these VHHs. Further, by engineering VHHs targeting distinct, conserved epitopes into multi-valent formats, we significantly enhanced their neutralization potencies compared to the corresponding VHH cocktails. We believe this approach is ideally suited to address both emerging SARS-CoV-2 variants during the current pandemic as well as potential future pandemics caused by SARS-like coronaviruses.

Cross-protective antibodies against common endemic respiratory viruses

Respiratory syncytial virus (RSV), human metapneumovirus (HMPV), and human parainfluenza virus types one (HPIV1) and three (HPIV3) can cause severe disease and death in immunocompromised patients, the elderly, and those with underlying lung disease. A protective monoclonal antibody exists for RSV, but clinical use is limited to high-risk infant populations. Hence, therapeutic options for these viruses in vulnerable patient populations are currently limited. Here, we present the discovery, in vitro characterization, and in vivo efficacy testing of two cross-neutralizing monoclonal antibodies, one targeting both HPIV3 and HPIV1 and the other targeting both RSV and HMPV. The 3 × 1 antibody is capable of targeting multiple parainfluenza viruses; the MxR antibody shares features with other previously reported monoclonal antibodies that are capable of neutralizing both RSV and HMPV. We obtained structures using cryo-electron microscopy of these antibodies in complex with their antigens at 3.62 Å resolution for 3 × 1 bound to HPIV3 and at 2.24 Å for MxR bound to RSV, providing a structural basis for in vitro binding and neutralization. Together, a cocktail of 3 × 1 and MxR could have clinical utility in providing broad protection against four of the respiratory viruses that cause significant morbidity and mortality in at-risk individuals.

Profiling of hMPV F-specific antibodies isolated from human memory B cells

Human metapneumovirus (hMPV) belongs to the Pneumoviridae family and is closely related to respiratory syncytial virus (RSV). The surface fusion (F) glycoprotein mediates viral fusion and is the primary target of neutralizing antibodies against hMPV. Here we report 113 hMPV-F specific monoclonal antibodies (mAbs) isolated from memory B cells of human donors. We characterize the antibodies' germline usage, epitopes, neutralization potencies, and binding specificities. We find that unlike RSV-F specific mAbs, antibody responses to hMPV F are less dominant against the apex of the antigen, and the majority of the potent neutralizing mAbs recognize epitopes on the side of hMPV F. Furthermore, neutralizing epitopes that differ from previously defined antigenic sites on RSV F are identified, and multiple binding modes of site V and II mAbs are discovered. Interestingly, mAbs that bind preferentially to the unprocessed prefusion F show poor neutralization potency. These results elucidate the immune recognition of hMPV infection and provide novel insights for future hMPV antibody and vaccine development.

Improved epitope resolution of the prefusion trimer-specific antibody AM14 bound to the RSV F glycoprotein

Respiratory syncytial virus (RSV) is the most common cause of acute lower respiratory tract infections resulting in medical intervention and hospitalizations during infancy and early childhood, and vaccination against RSV remains a public health priority. The RSV F glycoprotein is a major target of neutralizing antibodies, and the prefusion stabilized form of F (DS-Cav1) is under investigation as a vaccine antigen. AM14 is a human monoclonal antibody with the exclusive capacity of binding an epitope on prefusion F (PreF), which spans two F protomers. The quality of recognizing a trimer-specific epitope makes AM14 valuable for probing PreF-based immunogen conformation and functionality during vaccine production. Currently, only a low-resolution (5.5 Å) X-ray structure is available of the PreF-AM14 complex, revealing few reliable details of the interface. Here, we perform complementary structural studies using X-ray crystallography and cryo-electron microscopy (cryo-EM) to provide improved resolution structures at 3.6 Å and 3.4 Å resolutions, respectively. Both X-ray and cryo-EM structures provide clear side-chain densities, which allow for accurate mapping of the AM14 epitope on DS-Cav1. The structures help rationalize the molecular basis for AM14 loss of binding to RSV F monoclonal antibody-resistant mutants and reveal flexibility for the side chain of a key antigenic residue on PreF. This work provides the basis for a comprehensive understanding of RSV F trimer specificity with implications in vaccine design and quality assessment of PreF-based immunogens.