The Heptad Repeat C Domain of the Respiratory Syncytial Virus Fusion Protein Plays a Key Role in Membrane Fusion

Farnesyltransferase inhibitor lonafarnib suppresses respiratory syncytial virus infection by blocking conformational change of fusion glycoprotein

Respiratory syncytial virus (RSV) is the major cause of bronchiolitis and pneumonia in young children and the elderly. There are currently no approved RSV-specific therapeutic small molecules available. Using high-throughput antiviral screening, we identified an oral drug, the prenylation inhibitor lonafarnib, which showed potent inhibition of the RSV fusion process. Lonafarnib exhibited antiviral activity against both the RSV A and B genotypes and showed low cytotoxicity in HEp-2 and human primary bronchial epithelial cells (HBEC). Time-of-addition and pseudovirus assays demonstrated that lonafarnib inhibits RSV entry, but has farnesyltransferase-independent antiviral efficacy. Cryo-electron microscopy revealed that lonafarnib binds to a triple-symmetric pocket within the central cavity of the RSV F metastable pre-fusion conformation. Mutants at the RSV F sites interacting with lonafarnib showed resistance to lonafarnib but remained fully sensitive to the neutralizing monoclonal antibody palivizumab. Furthermore, lonafarnib dose-dependently reduced the replication of RSV in BALB/c mice. Collectively, lonafarnib could be a potential fusion inhibitor for RSV infection.

Intranasal vaccination with a recombinant protein CTA1-DD-RBF protects mice against hRSV infection

Human respiratory syncytial virus (hRSV) infection is a major pediatric health concern worldwide. Despite more than half a century of efforts, there is still no commercially available vaccine. In this study, we constructed and purified the recombinant protein CTA1-DD-RBF composed of a CTA1-DD mucosal adjuvant and prefusion F protein (RBF) using Escherichia coli BL21 cells. We studied the immunogenicity of CTA1-DD-RBF in mice. Intranasal immunization with CTA1-DD-RBF stimulated hRSV F-specific IgG1, IgG2a, sIgA, and neutralizing antibodies as well as T cell immunity without inducing lung immunopathology upon hRSV challenge. Moreover, the protective immunity of CTA1-DD-RBF was superior to that of the RBF protein, as confirmed by the assessment of serum-neutralizing activity and viral clearance after challenge. Compared to formalin-inactivated hRSV (FI-RSV), intranasal immunization with CTA1-DD-RBF induced a Th1 immune response. In summary, intranasal immunization with CTA1-DD-RBF is safe and effective in mice. Therefore, CTA1-DD-RBF represents a potential mucosal vaccine candidate for the prevention of human infection with hRSV.

Disulfide-stapled design of α-helical bundles to target the trimer-of-hairpins motif of human respiratory syncytial virus fusion protein

Human respiratory syncytial virus (RSV) is the major cause of acute lower respiratory tract infections worldwide in infants and young children. The RSV F glycoprotein is a class I fusion protein that mediates viral entry into host cells and is a major target of neutralizing antibodies. Targeting F glycoprotein has been recognized as a promising antiviral therapeutic strategy against RSV infection. Here, we reported the disulfide-stapled design of α-helical bundle to target the trimer-of-hairpins (TOH) motif of RSV F glycoprotein, which is the central regulatory module that triggers viral membrane fusion event. In TOH motif, three N-terminal heptad repeat (NtHR) helices form a trimeric coiled-coil core and other three C-terminal heptad repeat (CtHR) helices add to the core in an antiparallel manner. Interaction analysis between NtHR and CtHR revealed that the C-terminal tail of CtHR packs tightly against NtHR as compared to the N-terminal and middle regions of CtHR. A core binding site in CtHR C-terminus was identified, which represents a 13-mer chp peptide and can effectively interact with NtHR helix in native ordered conformation but would become largely disordered when splitting from the protein context of CtHR helix. Two chp helices were stapled together in a parallel manner with single, double or triple disulfide bridges, thus systematically resulting in seven disulfide-stapled α-helical bundles. Molecular simulations revealed that the double and triple stapling can effectively stabilize the structured conformation of α-helical bundles, whereas the free conformation of single-stapled bundles still remain intrinsically disordered in solvent. The double-stapled bundle chp-ds[508,516] and the triple-stapled bundle chp-ts[508,512,516] were rationally designed to have high potency; they can form a tight three-helix bundle with NtHR helix, thus potently targeting NtHR-CtHR interactions involved in RSV-F TOH motif through a competitive disruption mechanism.

Dissociation of the respiratory syncytial virus F protein-specific human IgG, IgA and IgM response

Human respiratory syncytial virus (RSV) is one of the most important causes of severe respiratory tract infections in early childhood. The only prophylactic protection is the neutralizing antibody, palivizumab, which targets a conformational epitope of the RSV fusion (F) protein. The F protein is generated as a F0 precursor containing two furin cleavage sites allowing excision of the P27 fragment and then gives rise to a fusion-competent version consisting of the N-terminal F2 subunit and the a C-terminal F1 subunits linked by two disulphide bonds. To investigate natural human F-specific antibody responses, F2 conferring the species-specificity of RSV, was expressed in Escherichia coli. Furthermore, the F0 protein, comprising both subunits F2 and F1, was expressed as palivizumab-reactive glycoprotein in baculovirus-infected insect cells. Six overlapping F2-derived peptides lacking secondary structure were synthesized. The analysis of IgG, IgA and IgM responses of adult subjects to native versions and denatured forms of F2 and F0 and to unfolded F2-derived peptides revealed that mainly non-conformational F epitopes, some of which represented cryptic epitopes which are not exposed on the proteins were recognized. Furthermore, we found a dissociation of IgG, IgA and IgM antibody responses to F epitopes with F2 being a major target for the F-specific IgM response. The scattered and dissociated immune response to F may explain why the natural RSV-specific antibody response is only partially protective underlining the need for vaccines focusing human antibody responses towards neutralizing RSV epitopes.

Triterpenoid-Mediated Inhibition of Virus-Host Interaction: Is Now the Time for Discovering Viral Entry/Release Inhibitors from Nature?

Fatal infectious diseases caused by HIV-1, influenza A virus, Ebola virus, and currently pandemic coronavirus highlight the great need for the discovery of antiviral agents in mechanisms different from current viral replication-targeted approaches. Given the critical role of virus-host interactions in the viral life cycle, the development of entry or shedding inhibitors may expand the current repertoire of antiviral agents; the combination of antireplication inhibitors and entry or shedding inhibitors would create a multifaceted drug cocktail with a tandem antiviral mechanism. Therefore, we provide critical information about triterpenoids as potential antiviral agents targeting entry and release, focusing specifically on the emerging aspect of triterpenoid-mediated inhibition of a variety of virus-host membrane fusion mechanisms via a trimer-of-hairpin motif. These properties of triterpenoids supply their host an evolutionary advantage for chemical defense and may protect against an increasingly diverse array of viruses infecting mammals, providing a direction for antiviral drug discovery.

Flexible RSV Prefusogenic Fusion Glycoprotein Exposes Multiple Neutralizing Epitopes that May Collectively Contribute to Protective Immunity

Human respiratory syncytial virus (RSV) is a cause of lower respiratory tract infection in infants, young children, and older adults. There is no licensed vaccine and prophylactic treatment options are limited. The RSV fusion (F) glycoprotein is a target of host immunity and thus a focus for vaccine development. F-trimers are metastable and undergo significant rearrangements from the prefusion to a stable postfusion structure with neutralizing epitopes on intermediate structures. We hypothesize that vaccine strategies that recapitulate the breathable F quaternary structure, and provide accessibility of B-cells to epitopes on intermediate conformations, may collectively contribute to protective immunity, while rigid prefusion F structures restrict access to key protective epitopes. To test this hypothesis, we used the near full-length prefusogenic F as a backbone to construct three prefusion F variants with substitutions in the hydrophobic head cavity: (1) disulfide bond mutant (DS), (2) space filling hydrophobic amino acid substitutions (Cav1), and (3) DS, Cav1 double mutant (DS-Cav1). In this study, we compared the immunogenicity of prefusogenic F to prefusion F variants in two animal models. Native prefusogenic F was significantly more immunogenic, producing high titer antibodies to prefusogenic, prefusion, and postfusion F structures, while animals immunized with DS or DS-Cav1 produced antibodies to prefusion F. Importantly, prefusogenic F elicited antibodies that target neutralizing epitopes including prefusion-specific site zero (Ø) and V and conformation-independent neutralizing sites II and IV. Immunization with DS or DS-Cav1 elicited antibodies primarily to prefusion-specific sites Ø and V with little or no antibodies to other key neutralizing sites. Animals immunized with prefusogenic F also had significantly higher levels of antibodies that cross-neutralized RSV A and B subtypes, while immunization with DS or DS-Cav1 produced antibodies primarily to the A subtype. We conclude that breathable trimeric vaccines that closely mimic the native F-structure, and incorporate strategies for B-cell accessibility to protective epitopes, are important considerations for vaccine design. F structures locked in a single conformation restrict access to neutralizing epitopes that may collectively contribute to destabilizing F-trimers important for broad protection. These results also have implications for vaccine strategies targeting other type 1 integral membrane proteins.

Third Helical Domain of the Nipah Virus Fusion Glycoprotein Modulates both Early and Late Steps in the Membrane Fusion Cascade

Medically important paramyxoviruses, such as measles, mumps, parainfluenza, Nipah, and Hendra viruses, infect host cells by directing fusion of the viral and cellular plasma membranes. Upon infection, paramyxoviruses cause a second type of membrane fusion, cell-cell fusion (syncytium formation), which is linked to pathogenicity. Host cell receptor binding causes conformational changes in the attachment glycoprotein (HN, H, or G) that trigger a conformational cascade in the fusion (F) glycoprotein that mediates membrane fusion. F, a class I fusion protein, contains the archetypal heptad repeat regions 1 (HR1) and 2 (HR2). It is well established that binding of HR1 and HR2 is key to fusing viral and cellular membranes. In this study, we uncovered a novel fusion-modulatory role of a third structurally conserved helical region (HR3) in F. Based on its location within the F structure, and structural differences between its prefusion and postfusion conformations, we hypothesized that the HR3 modulates triggering of the F conformational cascade (still requiring G). We used the deadly Nipah virus (NiV) as an important paramyxoviral model to perform alanine scan mutagenesis and a series of multidisciplinary structural/functional analyses that dissect the various states of the membrane fusion cascade. Remarkably, we found that specific residues within the HR3 modulate not only early F-triggering but also late extensive fusion pore expansion steps in the membrane fusion cascade. Our results characterize these novel fusion-modulatory roles of the F HR3, improving our understanding of the membrane fusion process for NiV and likely for the related Henipavirus genus and possibly Paramyxoviridae family members.IMPORTANCE The Paramyxoviridae family includes important human and animal pathogens, such as measles, mumps, and parainfluenza viruses and the deadly henipaviruses Nipah (NiV) and Hendra (HeV) viruses. Paramyxoviruses infect the respiratory tract and the central nervous system (CNS) and can be highly infectious. Most paramyxoviruses have a limited host range. However, the biosafety level 4 NiV and HeV are highly pathogenic and have a wide mammalian host range. Nipah viral infections result in acute respiratory syndrome and severe encephalitis in humans, leading to 40 to 100% mortality rates. The lack of licensed vaccines or therapeutic approaches against NiV and other important paramyxoviruses underscores the need to understand viral entry mechanisms. In this study, we uncovered a novel role of a third helical region (HR3) of the NiV fusion glycoprotein in the membrane fusion process that leads to viral entry. This discovery sets HR3 as a new candidate target for antiviral strategies for NiV and likely for related viruses.

Human respiratory syncytial virus F protein expressed in Pichia pastoris or Escherichia coli induces protective immunity without inducing enhanced respiratory disease in mice

Human respiratory syncytial virus (hRSV) is the primary cause of severe respiratory tract disease in children and infants as well as in elderly and immunocompromised adults. The fusion protein (F) of hRSV is the major antigen eliciting a neutralizing antibody response and protective immunity in the host, especially those recognizing the prefusion F protein (pre-F). In this study, we made genetic constructs for expression of a recombinant prefusion F protein in Pichia pastoris GS115, called RGF. Using Escherichia coli BL21, we expressed the pre-F and postfusion F protein (Post-F), called RBF and Post-RBF, respectively. RGF and RBF showed high affinity for 5C4, a highly potent monoclonal antibody specific for pre-F. We studied the immunogenicity of RGF and RBF in mice. Compared to mice immunized with formalin-inactivated RSV (FI-RSV), mice immunized with RGF or RBF exhibited superior protective immunity, which was confirmed by serum neutralizing activity and viral clearance after challenge. As judged from the IgG1/IgG2a ratios and numbers of IFN-γ- and IL-4-secreting cells, RGF or RBF with alum adjuvant induced a balanced Th1-biased immune response and produced no signs of enhanced respiratory disease (ERD) upon hRSV challenge. In addition, the immunogenicity and protective efficacy of RGF were superior to those of RBF in mice. Therefore, RGF represents a potential vaccine candidate for the prevention of human infection with hRSV.

Genotyping of Type A Human Respiratory Syncytial Virus Based on Direct F Gene Sequencing

Background and objectives: The human respiratory syncytial virus (hRSV) is among the important respiratory pathogens affecting children. Genotype-specific attachment (G) gene sequencing is usually used to determine the virus genotype. The reliability of the fusion (F) gene vs. G gene genotype-specific sequencing was screened. Materials and Methods: Archival RNA from Saudi children who tested positive for hRSV-A were used. Samples were subjected to a conventional one-step RT-PCR for both F and G genes and direct gene sequencing of the amplicons using the same primer sets. Phylogeny and mutational analysis of the obtained sequences were conducted. Results: The generic primer set succeeded to amplify target gene sequences. The phylogenetic tree based on partial F gene sequencing resulted in an efficient genotyping of hRSV-A strains equivalent to the partial G gene genotyping method. NA1, ON1, and GA5 genotypes were detected in the clinical samples. The latter was detected for the first time in Saudi Arabia. Different mutations in both conserved and escape-mutant domains were detected in both F and G. Conclusion: It was concluded that a partial F gene sequence can be used efficiently for hRSV-A genotyping.

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.