A Cysteine Zipper Stabilizes a Pre-Fusion F Glycoprotein Vaccine for Respiratory Syncytial Virus

Identifying Key Drivers of Efficient B Cell Responses: On the Role of T Help, Antigen-Organization, and Toll-like Receptor Stimulation for Generating a Neutralizing Anti-Dengue Virus Response

T help (Th), stimulation of toll-like receptors (pathogen-associated molecular patterns, PAMPs), and antigen organization and repetitiveness (pathogen-associated structural patterns, PASPs) were shown numerous times to be important in driving B-cell and antibody responses. In this study, we dissected the individual contributions of these parameters using newly developed "Immune-tag" technology. As model antigens, we used eGFP and the third domain of the dengue virus 1 envelope protein (DV1 EDIII), the major target of virus-neutralizing antibodies. The respective proteins were expressed alone or genetically fused to the N-terminal fragment of the cucumber mosaic virus (CMV) capsid protein-nCMV, rendering the antigens oligomeric. In a step-by-step manner, RNA was attached as a PAMP, and/or a universal Th-cell epitope was genetically added for additional Th. Finally, a PASP was added to the constructs by displaying the antigens highly organized and repetitively on the surface of CMV-derived virus-like particles (CuMV VLPs). Sera from immunized mice demonstrated that each component contributed stepwise to the immunogenicity of both proteins. All components combined in the CuMV VLP platform induced by far the highest antibody responses. In addition, the DV1 EDIII induced high levels of DENV-1-neutralizing antibodies only if displayed on VLPs. Thus, combining multiple cues typically associated with viruses results in optimal antibody responses.

Influence of glycosylation on the immunogenicity and antigenicity of viral immunogens

A key aspect of successful viral vaccine design is the elicitation of neutralizing antibodies targeting viral attachment and fusion glycoproteins that embellish viral particles. This observation has catalyzed the development of numerous viral glycoprotein mimetics as vaccines. Glycans can dominate the surface of viral glycoproteins and as such, the viral glycome can influence the antigenicity and immunogenicity of a candidate vaccine. In one extreme, glycans can form an integral part of epitopes targeted by neutralizing antibodies and are therefore considered to be an important feature of key immunogens within an immunization regimen. In the other extreme, the existence of peptide and bacterially expressed protein vaccines shows that viral glycosylation can be dispensable in some cases. However, native-like glycosylation can indicate native-like protein folding and the presence of conformational epitopes. Furthermore, going beyond native glycan mimicry, in either occupancy of glycosylation sites or the glycan processing state, may offer opportunities for enhancing the immunogenicity and associated protection elicited by an immunogen. Here, we review key determinants of viral glycosylation and how recombinant immunogens can recapitulate these signatures across a range of enveloped viruses, including HIV-1, Ebola virus, SARS-CoV-2, Influenza and Lassa virus. The emerging understanding of immunogen glycosylation and its control will help guide the development of future vaccines in both recombinant protein- and nucleic acid-based vaccine technologies.

Structure of a SARS-CoV-2 spike S2 subunit in a pre-fusion, open conformation

The 800 million human infections with SARS-CoV-2 and the likely emergence of new variants and additional coronaviruses necessitate a better understanding of the essential spike glycoprotein and the development of immunogens that foster broader and more durable immunity. The S2 fusion subunit is more conserved in sequence, is essential to function, and would be a desirable immunogen to boost broadly reactive antibodies. It is, however, unstable in structure and in its wild-type form, cannot be expressed alone without irreversible collapse into a six-helix bundle. In addition to the irreversible conformational changes of fusion, biophysical measurements indicate that spike also undergoes a reversible breathing action. However, spike in an open, "breathing" conformation has not yet been visualized at high resolution. Here we describe an S2-only antigen, engineered to remain in its relevant, pre-fusion viral surface conformation in the absence of S1. We also describe a panel of natural human antibodies specific for S2 from vaccinated and convalescent individuals. One of these mAbs, from a convalescent individual, afforded a high-resolution cryo-EM structure of the prefusion S2. The structure reveals a complex captured in an "open" conformation with greater stabilizing intermolecular interactions at the base and a repositioned fusion peptide. Together, this work provides an antigen for advancement of next-generation "booster" immunogens and illuminates the likely breathing adjustments of the coronavirus spike.

Respiratory Syncytial Virus Vaccines: A Review of the Candidates and the Approved Vaccines

Respiratory syncytial virus (RSV) is responsible for a significant proportion of global morbidity and mortality affecting young children and older adults. In the aftermath of formalin-inactivated RSV vaccine development, the effort to develop an immunizing agent was carefully guided by epidemiologic and pathophysiological evidence of the virus, including various vaccine technologies. The pipeline of RSV vaccine development includes messenger ribonucleic acid (mRNA), live-attenuated (LAV), subunit, and recombinant vector-based vaccine candidates targeting different virus proteins. The availability of vaccine candidates of various technologies enables adjustment to the individualized needs of each vulnerable age group. Arexvy® (GSK), followed by Abrysvo® (Pfizer), is the first vaccine available for market use as an immunizing agent to prevent lower respiratory tract disease in older adults. Abrysvo is additionally indicated for the passive immunization of infants by maternal administration during pregnancy. This review presents the RSV vaccine pipeline, analyzing the results of clinical trials. The key features of each vaccine technology are also mentioned. Currently, 24 vaccines are in the clinical stage of development, including the 2 licensed vaccines. Research in the field of RSV vaccination, including the pharmacovigilance methods of already approved vaccines, promotes the achievement of successful prevention.

Leveraging deep learning to improve vaccine design

Deep learning has led to incredible breakthroughs in areas of research, from self-driving vehicles to solutions, to formal mathematical proofs. In the biomedical sciences, however, the revolutionary results seen in other fields are only now beginning to be realized. Vaccine research and development efforts represent an application with high public health significance. Protein structure prediction, immune repertoire analysis, and phylogenetics are three principal areas in which deep learning is poised to provide key advances. Here, we opine on some of the current challenges with deep learning and how they are being addressed. Despite the nascent stage of deep learning applications in immunological studies, there is ample opportunity to utilize this new technology to address the most challenging and burdensome infectious diseases confronting global populations.

A prefusion-stabilized RSV F subunit vaccine elicits B cell responses with greater breadth and potency than a postfusion F vaccine

There is currently no licensed vaccine for respiratory syncytial virus (RSV). Here, we assess the effect of RSV fusion protein (F) conformation on B cell responses in a post hoc comparison of samples from the DS-Cav1 [prefusion (pre-F)] and MEDI7510 [postfusion (post-F)] vaccine clinical trials. We compared the magnitude and quality of the serological and B cell responses across time points and vaccines. We measured RSV A and B neutralization, F-binding immunoglobulin G titers, and competition assays at week 0 (before vaccination) and week 4 (after vaccination) to evaluate antibody specificity and potency. To compare B cell specificity and activation, we used pre-F and post-F probes in tandem with a 17-color immunophenotyping flow cytometry panel at week 0 (before vaccination) and week 1 (after vaccination). Our data demonstrate that both DS-Cav1 and MEDI7510 vaccination robustly elicit F-specific antibodies and B cells, but DS-Cav1 elicited antibodies that more potently neutralized both RSV A and B. The superior potency was mediated by antibodies that bind antigenic sites on the apex of pre-F that are not present on post-F. In the memory (CD27⁺) B cell compartment, vaccination with DS-Cav1 or MEDI7510 elicited B cells with different epitope specificities. B cells preferentially binding the pre-F probe were activated in DS-Cav1-vaccinated participants but not in MEDI7510-vaccinated participants. Our findings emphasize the importance of using pre-F as an immunogen in humans because of its deterministic role in eliciting highly potent neutralizing antibodies and memory B cells.

Stabilisation of Viral Membrane Fusion Proteins in Prefusion Conformation by Structure-Based Design for Structure Determination and Vaccine Development

The membrane surface of enveloped viruses contains dedicated proteins enabling the fusion of the viral with the host cell membrane. Working with these proteins is almost always challenging because they are membrane-embedded and naturally metastable. Fortunately, based on a range of different examples, researchers now have several possibilities to tame membrane fusion proteins, making them amenable for structure determination and immunogen generation. This review describes the structural and functional similarities of the different membrane fusion proteins and ways to exploit these features to stabilise them by targeted mutational approaches. The recent determination of two herpesvirus membrane fusion proteins in prefusion conformation holds the potential to apply similar methods to this group of viral fusogens. In addition to a better understanding of the herpesviral fusion mechanism, the structural insights gained will help to find ways to further stabilise these proteins using the methods described to obtain stable immunogens that will form the basis for the development of the next generation of vaccines and antiviral drugs.

A single-shot adenoviral vaccine provides hemagglutinin stalk-mediated protection against heterosubtypic influenza challenge in mice

Conventional influenza vaccines fail to confer broad protection against diverse influenza A viruses with pandemic potential. Efforts to develop a universal influenza virus vaccine include refocusing immunity towards the highly conserved stalk domain of the influenza virus surface glycoprotein, hemagglutinin (HA). We constructed a non-replicating adenoviral (Ad) vector, encoding a secreted form of H1 HA, to evaluate HA stalk-focused immunity. The Ad5_H1 vaccine was tested in mice for its ability to elicit broad, cross-reactive protection against homologous, heterologous, and heterosubtypic lethal challenge in a single-shot immunization regimen. Ad5_H1 elicited hemagglutination inhibition (HI+) active antibodies (Abs), which conferred 100% sterilizing protection from homologous H1N1 challenge. Furthermore, Ad5_H1 rapidly induced H1-stalk-specific Abs with Fc-mediated effector function activity, in addition to stimulating both CD4+ and CD8+ stalk-specific T cell responses. This phenotype of immunity provided 100% protection from lethal challenge with a head-mismatched, reassortant influenza virus bearing a chimeric HA, cH6/1, in a stalk-mediated manner. Most importantly, 100% protection from mortality following lethal challenge with a heterosubtypic avian influenza virus, H5N1, was observed following a single immunization with Ad5_H1. In conclusion, Ad-based influenza vaccines can elicit significant breadth of protection in naive animals and could be considered for pandemic preparedness and stockpiling.

HIV and SARS-CoV-2: Tracing a Path of Vaccine Research and Development

PURPOSE OF REVIEW

This review examines the major advances and obstacles in the field of HIV vaccine research as they pertain to informing the development of vaccines against SARS-CoV-2.

RECENT FINDINGS

Although the field of HIV research has yet to deliver a licensed vaccine, the technologies developed and knowledge gained in basic scientific disciplines, translational research, and community engagement have positively impacted the development of vaccines for other viruses, most notably and recently for SARS-CoV-2. These advances include the advent of viral vectors and mRNA as vaccine delivery platforms; the use of structural biology for immunogen design; an emergence of novel adjuvant formulations; a more sophisticated understanding of viral phylogenetics; improvements in the development and harmonization of accurate assays for vaccine immunogenicity; and maturation of the fields of bioethics and community engagement for clinical trials conducted in diverse populations. Decades of foundational research and investments into HIV biology, though yet to yield an authorized or approved vaccine for HIV/AIDS, have now paid dividends in the rapid development of safe and effective SARS-CoV-2 vaccines. This latter success presents an opportunity for feedback on improved pathways for development of safe and efficacious vaccines against HIV and other pathogens.

Interprotomer disulfide-stabilized variants of the human metapneumovirus fusion glycoprotein induce high titer-neutralizing responses

Human metapneumovirus (HMPV) is a major cause of respiratory disease worldwide, particularly among children and the elderly. Although there is no licensed HMPV vaccine, promising candidates have been identified for related pneumoviruses based on the structure-based stabilization of the fusion (F) glycoprotein trimer, with prefusion-stabilized F glycoprotein trimers eliciting significantly higher neutralizing responses than their postfusion F counterparts. However, immunization with HMPV F trimers in either prefusion or postfusion conformations has been reported to elicit equivalent neutralization responses. Here we investigate the impact of stabilizing disulfides, especially interprotomer disulfides (IP-DSs) linking protomers of the F trimer, on the elicitation of HMPV-neutralizing responses. We designed F trimer disulfides, screened for their expression, and used electron microscopy (EM) to confirm their formation, including that of an unexpected postfusion variant. In mice, IP-DS-stabilized prefusion and postfusion HMPV F elicited significantly higher neutralizing responses than non-IP-DS-stabilized HMPV Fs. In macaques, the impact of IP-DS stabilization was more measured, although IP-DS-stabilized variants of either prefusion or postfusion HMPV F induced neutralizing responses many times the average titers observed in a healthy human cohort. Serological and absorption-based analyses of macaque responses revealed elicited HMPV-neutralizing responses to be absorbed differently by IP-DS-containing and by non-IP-DS-containing postfusion Fs, suggesting IP-DS stabilization to alter not only the immunogenicity of select epitopes but their antigenicity as well. We speculate the observed increase in immunogenicity by IP-DS trimers to be related to reduced interprotomer flexibility within the HMPV F trimer.

The impact of high-resolution structural data on stemming the COVID-19 pandemic

The coronavirus disease 2019 (COVID-19) pandemic has had a catastrophic impact on human health and the world economy. The response of the scientific community was unparalleled, and a combined global effort has resulted in the creation of vaccines in a shorter time frame than previously unimaginable. Reflecting this concerted effort, the structural analysis of the etiological agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has progressed with an unprecedented pace. Since the onset of the pandemic, over 1000 high-resolution structures of a broad range of SARS-CoV-2 proteins have been solved and made publicly available. These structures have aided in the identification of numerous potential druggable targets and have contributed to the design of different vaccine candidates. This opinion article will discuss the impact of high-resolution structures in understanding SARS-CoV-2 biology and explore their role in the development of vaccines and antivirals.

Targeting Viral Surface Proteins through Structure-Based Design

The emergence of novel viral infections of zoonotic origin and mutations of existing human pathogenic viruses represent a serious concern for public health. It warrants the establishment of better interventions and protective therapies to combat the virus and prevent its spread. Surface glycoproteins catalyzing the fusion of viral particles and host cells have proven to be an excellent target for antivirals as well as vaccines. This review focuses on recent advances for computational structure-based design of antivirals and vaccines targeting viral fusion machinery to control seasonal and emerging respiratory viruses.

The Versatile Manipulations of Self-Assembled Proteins in Vaccine Design

Protein assemblies provide unique structural features which make them useful as carrier molecules in biomedical and chemical science. Protein assemblies can accommodate a variety of organic, inorganic and biological molecules such as small proteins and peptides and have been used in development of subunit vaccines via display parts of viral pathogens or antigens. Such subunit vaccines are much safer than traditional vaccines based on inactivated pathogens which are more likely to produce side-effects. Therefore, to tackle a pandemic and rapidly produce safer and more effective subunit vaccines based on protein assemblies, it is necessary to understand the basic structural features which drive protein self-assembly and functionalization of portions of pathogens. This review highlights recent developments and future perspectives in production of non-viral protein assemblies with essential structural features of subunit vaccines.

Vaccines for COVID-19

Since the emergence of COVID-19, caused by the SARS-CoV-2 virus at the end of 2019, there has been an explosion of vaccine development. By 24 September 2020, a staggering number of vaccines (more than 200) had started preclinical development, of which 43 had entered clinical trials, including some approaches that have not previously been licensed for human vaccines. Vaccines have been widely considered as part of the exit strategy to enable the return to previous patterns of working, schooling and socializing. Importantly, to effectively control the COVID-19 pandemic, production needs to be scaled-up from a small number of preclinical doses to enough filled vials to immunize the world's population, which requires close engagement with manufacturers and regulators. It will require a global effort to control the virus, necessitating equitable access for all countries to effective vaccines. This review explores the immune responses required to protect against SARS-CoV-2 and the potential for vaccine-induced immunopathology. We describe the profile of the different platforms and the advantages and disadvantages of each approach. The review also addresses the critical steps between promising preclinical leads and manufacturing at scale. The issues faced during this pandemic and the platforms being developed to address it will be invaluable for future outbreak control. Nine months after the outbreak began we are at a point where preclinical and early clinical data are being generated for the vaccines; an overview of this important area will help our understanding of the next phases.

Cryo-EM structure of the prefusion state of canine distemper virus fusion protein ectodomain

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Expression and purification of prefusion CDV solF in presence of a fusion inhibitor.

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Elucidation of the CDV fusion protein ectodomain by cryo-EM.

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High structural similarity between MeV and CDV solF suggests common fusion mechanisms.

Measles virus (MeV) and canine distemper virus (CDV), two members of the Morbillivirus genus, are still causing important global diseases of humans and animals, respectively. To enter target cells, morbilliviruses rely on an envelope-anchored machinery, which is composed of two interacting glycoproteins: a tetrameric receptor binding (H) protein and a trimeric fusion (F) protein. To execute membrane fusion, the F protein initially adopts a metastable, prefusion state that refolds into a highly stable postfusion conformation as the result of a finely coordinated activation process mediated by the H protein. Here, we employed cryo-electron microscopy (cryo-EM) and single particle reconstruction to elucidate the structure of the prefusion state of the CDV F protein ectodomain (solF) at 4.3 Å resolution. Stabilization of the prefusion solF trimer was achieved by fusing the GCNt trimerization sequence at the C-terminal protein region, and expressing and purifying the recombinant protein in the presence of a morbilliviral fusion inhibitor class compound. The three-dimensional cryo-EM map of prefusion CDV solF in complex with the inhibitor clearly shows density for the ligand at the protein binding site suggesting common mechanisms of membrane fusion activation and inhibition employed by different morbillivirus members.

Structure-Based Vaccine Antigen Design

Enabled by new approaches for rapid identification and selection of human monoclonal antibodies, atomic-level structural information for viral surface proteins, and capacity for precision engineering of protein immunogens and self-assembling nanoparticles, a new era of antigen design and display options has evolved. While HIV-1 vaccine development has been a driving force behind these technologies and concepts, clinical proof-of-concept for structure-based vaccine design may first be achieved for respiratory syncytial virus (RSV), where conformation-dependent access to neutralization-sensitive epitopes on the fusion glycoprotein determines the capacity to induce potent neutralizing activity. Success with RSV has motivated structure-based stabilization of other class I viral fusion proteins for use as immunogens and demonstrated the importance of structural information for developing vaccines against other viral pathogens, particularly difficult targets that have resisted prior vaccine development efforts. Solving viral surface protein structures also supports rapid vaccine antigen design and application of platform manufacturing approaches for emerging pathogens.

The Optimal Concentration of Formaldehyde is Key to Stabilizing the Pre-Fusion Conformation of Respiratory Syncytial Virus Fusion Protein

Background: To date, there is no licensed vaccine available to prevent respiratory syncytial virus (RSV) infection. The valuable pre-fusion conformation of the fusion protein (pre-F) is prone to lose high neutralizing antigenic sites. The goals of this study were to stabilize pre-F protein by fixatives and try to find the possibility of developing an inactivated RSV vaccine. Methods: The screen of the optimal fixative condition was performed with flow cytometry. BALB/c mice were immunized intramuscularly with different immunogens. The serum neutralizing antibody titers of immunized mice were determined by neutralization assay. The protection and safety of these immunogens were assessed. Results: Fixation in an optimal concentration of formaldehyde (0.0244%–0.0977%) or paraformaldehyde (0.0625%–1%) was able to stabilize pre-F. Additionally, BALB/c mice inoculated with optimally stabilized pre-F protein (opti-fixed) induced a higher anti-RSV neutralization (9.7 log₂, mean value of dilution rate) than those inoculated with unstable (unfixed, 8.91 log₂, p < 0.01) or excessively fixed (exce-fixed, 7.28 log₂, p < 0.01) pre-F protein. Furthermore, the opti-fixed immunogen did not induce enhanced RSV disease. Conclusions: Only the proper concentration of fixatives could stabilize pre-F and the optimal formaldehyde condition provides a potential reference for development of an inactivated RSV vaccine.

Transient opening of trimeric prefusion RSV F proteins

The respiratory syncytial virus (RSV) F glycoprotein is a class I fusion protein that mediates viral entry and is a major target of neutralizing antibodies. Structures of prefusion forms of RSV F, as well as other class I fusion proteins, have revealed compact trimeric arrangements, yet whether these trimeric forms can transiently open remains unknown. Here, we perform structural and biochemical studies on a recently isolated antibody, CR9501, and demonstrate that it enhances the opening of prefusion-stabilized RSV F trimers. The 3.3 Å crystal structure of monomeric RSV F bound to CR9501, combined with analysis of over 25 previously determined RSV F structures, reveals a breathing motion of the prefusion conformation. We also demonstrate that full-length RSV F trimers transiently open and dissociate on the cell surface. Collectively, these findings have implications for the function of class I fusion proteins, as well as antibody prophylaxis and vaccine development for RSV.

Common Features of Enveloped Viruses and Implications for Immunogen Design for Next-Generation Vaccines

Enveloped viruses enter cells by inducing fusion of viral and cellular membranes, a process catalyzed by a specialized membrane-fusion protein expressed on their surface. This review focuses on recent structural studies of viral fusion proteins with an emphasis on their metastable prefusion form and on interactions with neutralizing antibodies. The fusion glycoproteins have been difficult to study because they are present in a labile, metastable form at the surface of infectious virions. Such metastability is a functional requirement, allowing these proteins to refold into a lower energy conformation while transferring the difference in energy to catalyze the membrane fusion reaction. Structural studies have shown that stable immunogens presenting the same antigenic sites as the labile wild-type proteins efficiently elicit potently neutralizing antibodies, providing a framework with which to engineer the antigens for stability, as well as identifying key vulnerability sites that can be used in next-generation subunit vaccine design.

Structure-based design of a quadrivalent fusion glycoprotein vaccine for human parainfluenza virus types 1-4

Parainfluenza virus types 1-4 (PIV1-4) are highly infectious human pathogens, of which PIV3 is most commonly responsible for severe respiratory illness in newborns, elderly, and immunocompromised individuals. To obtain a vaccine effective against all four PIV types, we engineered mutations in each of the four PIV fusion (F) glycoproteins to stabilize their metastable prefusion states, as such stabilization had previously enabled the elicitation of high-titer neutralizing antibodies against the related respiratory syncytial virus. A cryoelectron microscopy structure of an engineered PIV3 F prefusion-stabilized trimer, bound to the prefusion-specific antibody PIA174, revealed atomic-level details for how introduced mutations improved stability as well as how a single PIA174 antibody recognized the trimeric apex of prefusion PIV3 F. Nine combinations of six newly identified disulfides and two cavity-filling mutations stabilized the prefusion PIV3 F immunogens and induced 200- to 500-fold higher neutralizing titers in mice than were elicited by PIV3 F in the postfusion conformation. For PIV1, PIV2, and PIV4, we also obtained stabilized prefusion Fs, for which prefusion versus postfusion titers were 2- to 20-fold higher. Elicited murine responses were PIV type-specific, with little cross-neutralization of other PIVs. In nonhuman primates (NHPs), quadrivalent immunization with prefusion-stabilized Fs from PIV1-4 consistently induced potent neutralizing responses against all four PIVs. For PIV3, the average elicited NHP titer from the quadrivalent immunization was more than fivefold higher than any titer observed in a cohort of over 100 human adults, highlighting the ability of a prefusion-stabilized immunogen to elicit especially potent neutralization.

Sustainable vaccine development: a vaccine manufacturer's perspective

Vaccination remains the most cost-effective public health intervention after clean water, and the benefits impressively outweigh the costs. The efforts needed to fulfill the steadily growing demands for next-generation and novel vaccines designed for emerging pathogens and new indications are only realizable in a sustainable business model. Vaccine development can be fast-tracked through strengthening international collaborations, and the continuous innovation of technologies to accelerate their design, development, and manufacturing. However, these processes should be supported by a balanced project portfolio, and by managing sustainable vaccine procurement strategies for different types of markets. Collectively this will allow a gradual shift to a more streamlined and profitable vaccine production, which can significantly contribute to the worldwide effort to shape global health.

Structures of the prefusion form of measles virus fusion protein in complex with inhibitors

Measles virus (MeV), a major cause of childhood morbidity and mortality, is highly immunotropic and one of the most contagious pathogens. MeV may establish, albeit rarely, persistent infection in the central nervous system, causing fatal and intractable neurodegenerative diseases such as subacute sclerosing panencephalitis and measles inclusion body encephalitis. Recent studies have suggested that particular substitutions in the MeV fusion (F) protein are involved in the pathogenesis by destabilizing the F protein and endowing it with hyperfusogenicity. Here we show the crystal structures of the prefusion MeV-F alone and in complex with the small compound AS-48 or a fusion inhibitor peptide. Notably, these independently developed inhibitors bind the same hydrophobic pocket located at the region connecting the head and stalk of MeV-F, where a number of substitutions in MeV isolates from neurodegenerative diseases are also localized. Since these inhibitors could suppress membrane fusion mediated by most of the hyperfusogenic MeV-F mutants, the development of more effective inhibitors based on the structures may be warranted to treat MeV-induced neurodegenerative diseases.

Efficacy of mucosal polyanhydride nanovaccine against respiratory syncytial virus infection in the neonatal calf

Human respiratory syncytial virus (HRSV) is a leading cause of severe acute lower respiratory tract infection in infants and children worldwide. Bovine RSV (BRSV) is closely related to HRSV and a significant cause of morbidity in young cattle. BRSV infection in calves displays many similarities to RSV infection in humans, including similar age dependency and disease pathogenesis. Polyanhydride nanoparticle-based vaccines (i.e., nanovaccines) have shown promise as adjuvants and vaccine delivery vehicles due to their ability to promote enhanced immunogenicity through the route of administration, provide sustained antigen exposure, and induce both antibody- and cell-mediated immunity. Here, we developed a novel, mucosal nanovaccine that encapsulates the post-fusion F and G glycoproteins from BRSV into polyanhydride nanoparticles and determined the efficacy of the vaccine against RSV infection using a neonatal calf model. Calves receiving the BRSV-F/G nanovaccine exhibited reduced pathology in the lungs, reduced viral burden, and decreased virus shedding compared to unvaccinated control calves, which correlated with BRSV-specific immune responses in the respiratory tract and peripheral blood. Our results indicate that the BRSV-F/G nanovaccine is highly immunogenic and, with optimization, has the potential to significantly reduce the disease burden associated with RSV infection in both humans and animals.

Adjuvants and the vaccine response to the DS-Cav1-stabilized fusion glycoprotein of respiratory syncytial virus

Appropriate adjuvant selection may be essential to optimize the potency and to tailor the immune response of subunit vaccines. To induce protective responses against respiratory syncytial virus (RSV)-a highly prevalent childhood pathogen without a licensed vaccine-we previously engineered a pre-fusion-stabilized trimeric RSV F (pre-F) "DS-Cav1" immunogen, which induced high titer RSV-neutralizing antibodies, in mice and non-human primates, when formulated with adjuvants Poly (I:C) and Poly (IC:LC), respectively. To assess the impact of different adjuvants, here we formulated RSV F DS-Cav1 with multiple adjuvants and assessed immune responses. Very high RSV-neutralizing antibody responses (19,006 EC50) were observed in naïve mice immunized with 2 doses of DS-Cav1 adjuvanted with Sigma adjuvant system (SAS), an oil-in-water adjuvant, plus Carbopol; high responses (3658-7108) were observed with DS-Cav1 adjuvanted with Alum, SAS alone, Adjuplex, Poly (I:C) and Poly (IC:LC); and moderate responses (1251-2129) were observed with DS-Cav1 adjuvanted with the TLR4 agonist MPLA, Alum plus MPLA or AddaVax. In contrast, DS-Cav1 without adjuvant induced low-level responses (6). A balanced IgG1 and IgG2a (Th2/Th1) immune response was elicited in most of the high to very high response groups (all but Alum and Adjuplex). We also tested the immune response induced by DS-Cav1 in elderly mice with pre-existing DS-Cav1 immunity; we observed that DS-Cav1 adjuvanted with SAS plus Carbopol boosted the response 2-3-fold, whereas DS-Cav1 adjuvanted with alum boosted the response 5-fold. Finally, we tested whether a mixture of ISA 71 VG and Carbopol would enhanced the antibody response in DS-Cav1 immunized calves. While pre-F-stabilized bovine RSV F induced very high titers in mice when adjuvanted with SAS plus Carbopol, the addition of Carbopol to ISA 71 VG did not enhance immune responses in calves. The vaccine response to pre-F-stabilized RSV F is augmented by adjuvant, but the degree of adjuvant-induced enhancement appears to be both context-dependent and species-specific.

Characterization of Pre-F-GCN4t, a Modified Human Respiratory Syncytial Virus Fusion Protein Stabilized in a Noncleaved Prefusion Conformation

The human respiratory syncytial virus (hRSV) fusion (F) protein is considered a major target of the neutralizing antibody response to hRSV. This glycoprotein undergoes a major structural shift from the prefusion (pre-F) to the postfusion (post-F) state at the time of virus-host cell membrane fusion. Recent evidences suggest that the pre-F state is a superior target for neutralizing antibodies compared to the post-F state. Therefore, for vaccine purposes, we have designed and characterized a recombinant hRSV F protein, called Pre-F-GCN4t, stabilized in a pre-F conformation. To show that Pre-F-GCN4t does not switch to a post-F conformation, it was compared with a recombinant post-F molecule, called Post-F-XC. Pre-F-GCN4t was glycosylated and trimeric and displayed a conformational stability different from that of Post-F-XC, as shown by chemical denaturation. Electron microscopy analysis suggested that Pre-F-GCN4t adopts a lollipop-like structure. In contrast, Post-F-XC had a typical elongated conical shape. Hydrogen/deuterium exchange mass spectrometry demonstrated that the two molecules had common rigid folding core and dynamic regions and provided structural insight for their biophysical and biochemical properties and reactivity. Pre-F-GCN4t was shown to deplete hRSV-neutralizing antibodies from human serum more efficiently than Post-F-XC. Importantly, Pre-F-GCN4t was also shown to bind D25, a highly potent monoclonal antibody specific for the pre-F conformation. In conclusion, this construct presents several pre-F characteristics, does not switch to the post-F conformation, and presents antigenic features required for a protective neutralizing antibody response. Therefore, Pre-F-GCN4t can be considered a promising candidate vaccine antigen.IMPORTANCE Human respiratory syncytial virus (RSV) is a global leading cause of infant mortality and adult morbidity. The development of a safe and efficacious RSV vaccine remains an important goal. The RSV class I fusion (F) glycoprotein is considered one of the most promising vaccine candidates, and recent evidences suggest that the prefusion (pre-F) state is a superior target for neutralizing antibodies. Our study presents the physicochemical characterization of Pre-F-GCN4t, a molecule designed to be stabilized in the pre-F conformation. To confirm its pre-F conformation, Pre-F-GCN4t was analyzed in parallel with Post-F-XC, a molecule in the post-F conformation. Our results show that Pre-F-GCN4t presents characteristics of a stabilized pre-F conformation and support its use as an RSV vaccine antigen. Such an antigen may represent a significant advance in the development of an RSV vaccine.

Respiratory syncytial virus: prospects for new and emerging therapeutics

INTRODUCTION

Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract infections (LRTI) in infants, the elderly, and the immunocompromised. Although the development of a RSV vaccine has been a priority for >50 years, there is still no vaccine available. Treatment of RSV LRTI has remained mostly supportive, i.e. hydration and oxygenation. Palivizumab and ribavirin are the only options currently available for prevention and treatment of RSV infection, but evidence suggests that they are not fully effective. This creates a significant unmet medical need for new therapeutics for prevention and treatment of RSV worldwide. Areas covered: This article reviews the antiviral drugs and monoclonal antibodies (mAb) for RSV that are in different stages of clinical development. Expert commentary: Over the last 10 years, new antiviral drugs and mAb have shown clinical promise against RSV, and may become available in the coming years. Although the RSV fusion protein has been the most popular target for inhibitors and mAbs, new approaches targeting other viral proteins have shown promising results. To overcome the emergence of RSV escape mutants, combination antiviral therapy may be explored in the future.

Passive and active immunization against respiratory syncytial virus for the young and old

INTRODUCTION

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants worldwide and also causes significant disease in the elderly. Despite 60 years of RSV research and vaccine development, there is only one approved medicine to prevent RSV infections. Palivizumab, a monoclonal antibody (mAb) against the RSV fusion (F) protein, is indicated for preterm infants and children at high-risk for RSV infections. It is an active time in RSV vaccine and mAb development with 14 vaccines and 2 mAbs currently being tested in clinical trials as of 13 February 2017. Active vaccination of women in the third trimester or passive immunization of infants with a mAb are particularly attractive approaches as the most severe disease occurs within the first 6 months of life. Areas covered: Here, we review current approaches for preventing RSV in the young and old, describe proposed clinical endpoints for studies in pediatric and adult clinical trials and highlight results from recent and ongoing clinical studies. Expert commentary: With 16 candidates in clinical development, approval of the first RSV vaccine or mAb for the prevention of RSV in all infants or the elderly is likely to occur in the next five years.

One-step design of a stable variant of the malaria invasion protein RH5 for use as a vaccine immunogen

Many promising vaccine candidates from pathogenic viruses, bacteria, and parasites are unstable and cannot be produced cheaply for clinical use. For instance, Plasmodium falciparum reticulocyte-binding protein homolog 5 (PfRH5) is essential for erythrocyte invasion, is highly conserved among field isolates, and elicits antibodies that neutralize in vitro and protect in an animal model, making it a leading malaria vaccine candidate. However, functional RH5 is only expressible in eukaryotic systems and exhibits moderate temperature tolerance, limiting its usefulness in hot and low-income countries where malaria prevails. Current approaches to immunogen stabilization involve iterative application of rational or semirational design, random mutagenesis, and biochemical characterization. Typically, each round of optimization yields minor improvement in stability, and multiple rounds are required. In contrast, we developed a one-step design strategy using phylogenetic analysis and Rosetta atomistic calculations to design PfRH5 variants with improved packing and surface polarity. To demonstrate the robustness of this approach, we tested three PfRH5 designs, all of which showed improved stability relative to wild type. The best, bearing 18 mutations relative to PfRH5, expressed in a folded form in bacteria at >1 mg of protein per L of culture, and had 10-15 °C higher thermal tolerance than wild type, while also retaining ligand binding and immunogenic properties indistinguishable from wild type, proving its value as an immunogen for a future generation of vaccines against the malaria blood stage. We envision that this efficient computational stability design methodology will also be used to enhance the biophysical properties of other recalcitrant vaccine candidates from emerging pathogens.

The Structure-Forming Juncture in Oxidative Protein Folding: What Happens in the ER?

The folding of disulfide bond containing proteins proceeds in a biphasic manner. Initially, cysteines are oxidized to form disulfide bonds. Structure is largely absent during this phase. Next, when a minimally correct number of native linkages of disulfide bonds have been acquired, the biopolymer conformationally folds into the native, or a native-like, state. Thus, at the end of this "oxidative folding" process, a stable and biologically active protein is formed. This review focuses on dissecting the "structure-forming step" in oxidative protein folding. The ability to follow this pivotal step in protein maturation in somewhat detail is uniquely facilitated in "oxidative" folding scenarios. We review this step using bovine pancreatic Ribonuclease A as a model while recognizing the impact that this step has in subcellular trafficking and protein aggregation.

Iterative structure-based improvement of a fusion-glycoprotein vaccine against RSV

Structure-based design of vaccines, particularly the iterative optimization used so successfully in the structure-based design of drugs, has been a long-sought goal. We previously developed a first-generation vaccine antigen called DS-Cav1, comprising a prefusion-stabilized form of the fusion (F) glycoprotein, which elicits high-titer protective responses against respiratory syncytial virus (RSV) in mice and macaques. Here we report the improvement of DS-Cav1 through iterative cycles of structure-based design that significantly increased the titer of RSV-protective responses. The resultant second-generation 'DS2'-stabilized immunogens have their F subunits genetically linked, their fusion peptides deleted and their interprotomer movements stabilized by an additional disulfide bond. These DS2 immunogens are promising vaccine candidates with superior attributes, such as their lack of a requirement for furin cleavage and their increased antigenic stability against heat inactivation. The iterative structure-based improvement described here may have utility in the optimization of other vaccine antigens.

Structure-Based Design of Head-Only Fusion Glycoprotein Immunogens for Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a significant cause of severe respiratory illness worldwide, particularly in infants, young children, and the elderly. Although no licensed vaccine is currently available, an engineered version of the metastable RSV fusion (F) surface glycoprotein-stabilized in the pre-fusion (pre-F) conformation by "DS-Cav1" mutations-elicits high titer RSV-neutralizing responses. Moreover, pre-F-specific antibodies, often against the neutralization-sensitive antigenic site Ø in the membrane-distal head region of trimeric F glycoprotein, comprise a substantial portion of the human response to natural RSV infection. To focus the vaccine-elicited response to antigenic site Ø, we designed a series of RSV F immunogens that comprised the membrane-distal head of the F glycoprotein in its pre-F conformation. These "head-only" immunogens formed monomers, dimers, and trimers. Antigenic analysis revealed that a majority of the 70 engineered head-only immunogens displayed reactivity to site Ø-targeting antibodies, which was similar to that of the parent RSV F DS-Cav1 trimers, often with increased thermostability. We evaluated four of these head-only immunogens in detail, probing their recognition by antibodies, their physical stability, structure, and immunogenicity. When tested in naïve mice, a head-only trimer, half the size of the parent RSV F trimer, induced RSV titers, which were statistically comparable to those induced by DS-Cav1. When used to boost DS-Cav1-primed mice, two head-only RSV F immunogens, a dimer and a trimer, boosted RSV-neutralizing titers to levels that were comparable to those boosted by DS-Cav1, although with higher site Ø-directed responses. Our results provide proof-of-concept for the ability of the smaller head-only RSV F immunogens to focus the vaccine-elicited response to antigenic site Ø. Decent primary immunogenicity, enhanced physical stability, potential ease of manufacture, and potent immunogenicity upon boosting suggest these head-only RSV F immunogens, engineered to retain the pre-fusion conformation, may have advantages as candidate RSV vaccines.

Immobilization of the N-terminal helix stabilizes prefusion paramyxovirus fusion proteins

Parainfluenza virus 5 (PIV5) is an enveloped, single-stranded, negative-sense RNA virus of the Paramyxoviridae family. PIV5 fusion and entry are mediated by the coordinated action of the receptor-binding protein, hemagglutinin-neuraminidase (HN), and the fusion protein (F). Upon triggering by HN, F undergoes an irreversible ATP- and pH-independent conformational change, going down an energy gradient from a metastable prefusion state to a highly stable postfusion state. Previous studies have highlighted key conformational changes in the F-protein refolding pathway, but a detailed understanding of prefusion F-protein metastability remains elusive. Here, using two previously described F-protein mutations (S443D or P22L), we examine the capacity to modulate PIV5 F stability and the mechanisms by which these point mutants act. The S443D mutation destabilizes prefusion F proteins by disrupting a hydrogen bond network at the base of the F-protein globular head. The introduction of a P22L mutation robustly rescues destabilized F proteins through a local hydrophobic interaction between the N-terminal helix and a hydrophobic pocket. Prefusion stabilization conferred by a P22L-homologous mutation is demonstrated in the F protein of Newcastle disease virus, a paramyxovirus of a different genus, suggesting a conserved stabilizing structural element within the paramyxovirus family. Taken together, the available data suggest that movement of the N-terminal helix is a necessary early step for paramyxovirus F-protein refolding and presents a novel target for structure-based drug design.

Antibody-guided vaccine design: identification of protective epitopes

In the last decade, progress in the analysis of the human immune response and in the isolation of human monoclonal antibodies have provided an innovative approach to the identification of protective antigens which are the basis for the design of vaccines capable of eliciting effective B-cell immunity. In this review we illustrate, with relevant examples, the power of this approach that can rapidly lead to the identification of protective antigens in complex pathogens, such as human cytomegalovirus and Plasmodium falciparum, and of conserved sites in highly variable antigens, such as influenza hemagglutinin and HIV-1 Env. We will also discuss how the genealogical analysis of antigen-stimulated B cell clones provides the basis to delineate the best suitable prime-boost vaccination strategy for the induction of broadly neutralizing antibodies.

A Chimeric Pneumovirus Fusion Protein Carrying Neutralizing Epitopes of Both MPV and RSV

Respiratory syncytial virus (RSV) and human metapneumovirus (HMPV) are paramyxoviruses that are responsible for substantial human health burden, particularly in children and the elderly. The fusion (F) glycoproteins are major targets of the neutralizing antibody response and studies have mapped dominant antigenic sites in F. Here we grafted a major neutralizing site of RSV F, recognized by the prophylactic monoclonal antibody palivizumab, onto HMPV F, generating a chimeric protein displaying epitopes of both viruses. We demonstrate that the resulting chimeric protein (RPM-1) is recognized by both anti-RSV and anti-HMPV F neutralizing antibodies indicating that it can be used to map the epitope specificity of antibodies raised against both viruses. Mice immunized with the RPM-1 chimeric antigen generate robust neutralizing antibody responses to MPV but weak or no cross-reactive recognition of RSV F, suggesting that grafting of the single palivizumab epitope stimulates a comparatively limited antibody response. The RPM-1 protein provides a new tool for characterizing the immune responses resulting from RSV and HMPV infections and provides insights into the requirements for developing a chimeric subunit vaccine that could induce robust and balanced immunity to both virus infections.

Influence of Respiratory Syncytial Virus F Glycoprotein Conformation on Induction of Protective Immune Responses

Human respiratory syncytial virus (hRSV) vaccine development has received new impetus from structure-based studies of its main protective antigen, the fusion (F) glycoprotein. Three soluble forms of F have been described: monomeric, trimeric prefusion, and trimeric postfusion. Most human neutralizing antibodies recognize epitopes found exclusively in prefusion F. Although prefusion F induces higher levels of neutralizing antibodies than does postfusion F, postfusion F can also induce protection against virus challenge in animals. However, the immunogenicity and protective efficacy of the three forms of F have not hitherto been directly compared. Hence, BALB/c mice were immunized with a single dose of the three proteins adjuvanted with CpG and challenged 4 weeks later with virus. Serum antibodies, lung virus titers, weight loss, and pulmonary pathology were evaluated after challenge. Whereas small amounts of postfusion F were sufficient to protect mice, larger amounts of monomeric and prefusion F proteins were required for protection. However, postfusion and monomeric F proteins were associated with more pathology after challenge than was prefusion F. Antibodies induced by all doses of prefusion F, in contrast to other F protein forms, reacted predominantly with the prefusion F conformation. At high doses, prefusion F also induced the highest titers of neutralizing antibodies, and all mice were protected, yet at low doses of the immunogen, these antibodies neutralized virus poorly, and mice were not protected. These findings should be considered when developing new hRSV vaccine candidates.

IMPORTANCE

Protection against hRSV infection is afforded mainly by neutralizing antibodies, which recognize mostly epitopes found exclusively in the viral fusion (F) glycoprotein trimer, folded in its prefusion conformation, i.e., before activation for membrane fusion. Although prefusion F is able to induce high levels of neutralizing antibodies, highly stable postfusion F (found after membrane fusion) is also able to induce neutralizing antibodies and protect against infection. In addition, a monomeric form of hRSV F that shares epitopes with prefusion F was recently reported. Since each of the indicated forms of hRSV F may have advantages and disadvantages for the development of safe and efficacious subunit vaccines, a direct comparison of the immunogenic properties and protective efficacies of the different forms of hRSV F was made in a mouse model. The results obtained show important differences between the noted immunogens that should be borne in mind when considering the development of hRSV vaccines.