Biochemical analysis of the secreted and virion glycoproteins of Ebola virus

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.

Anti-Ebola virus mAb 3A6 with unprecedented potency protects highly viremic animals from fatal outcome and physically lifts its glycoprotein target from the virion membrane

Monoclonal antibodies (mAbs) against Ebola virus (EBOV) glycoprotein (GP1,2) are the standard of care for Ebola virus disease (EVD). Anti-GP1,2 mAbs targeting the stalk and membrane proximal external region (MPER) potently neutralize EBOV in vitro. However, their neutralization mechanism is poorly understood because they target a GP1,2 epitope that has evaded structural characterization. Moreover, their in vivo efficacy has only been evaluated in the mouse model of EVD. Using x-ray crystallography and cryo-electron tomography of 3A6 complexed with its stalk- GP1,2 MPER epitope we reveal a novel mechanism in which 3A6 elevates the stalk or stabilizes a conformation of GP1,2 that is lifted from the virion membrane. In domestic guinea pig and rhesus monkey EVD models, 3A6 provides therapeutic benefit at high viremia levels, advanced disease stages, and at the lowest dose yet demonstrated for any anti-EBOV mAb-based monotherapy. These findings can guide design of next-generation, highly potent anti-EBOV mAbs.

Antibodies against the Ebola virus soluble glycoprotein are associated with long-term vaccine-mediated protection of non-human primates

The 2013 Ebola epidemic in Central and West Africa heralded the emergence of wide-spread, highly pathogenic viruses. The successful recombinant vector vaccine against Ebola (rVSVΔG-ZEBOV-GP) will limit future outbreaks, but identifying mechanisms of protection is essential to protect the most vulnerable. Vaccine-induced antibodies are key determinants of vaccine efficacy, yet the mechanism by which vaccine-induced antibodies prevent Ebola infection remains elusive. Here, we exploit a break in long-term vaccine efficacy in non-human primates to identify predictors of protection. Using unbiased humoral profiling that captures neutralization and Fc-mediated functions, we find that antibodies specific for soluble glycoprotein (sGP) drive neutrophil-mediated phagocytosis and predict vaccine-mediated protection. Similarly, we show that protective sGP-specific monoclonal antibodies have elevated neutrophil-mediated phagocytic activity compared with non-protective antibodies, highlighting the importance of sGP in vaccine protection and monoclonal antibody therapeutics against Ebola virus.

Structure of the Inmazeb cocktail and resistance to Ebola virus escape

Monoclonal antibodies can provide important pre- or post-exposure protection against infectious disease for those not yet vaccinated or in individuals that fail to mount a protective immune response after vaccination. Inmazeb (REGN-EB3), a three-antibody cocktail against Ebola virus, lessened disease and improved survival in a controlled trial. Here, we present the cryo-EM structure at 3.1 Å of the Ebola virus glycoprotein, determined without symmetry averaging, in a simultaneous complex with the antibodies in the Inmazeb cocktail. This structure allows the modeling of previously disordered portions of the glycoprotein glycan cap, maps the non-overlapping epitopes of Inmazeb, and illuminates the basis for complementary activities and residues critical for resistance to escape by these and other clinically relevant antibodies. We further provide direct evidence that Inmazeb protects against the rapid emergence of escape mutants, whereas monotherapies even against conserved epitopes do not, supporting the benefit of a cocktail versus a monotherapy approach.

Ligand-based design of peptide entry inhibitors targeting the endosomal receptor binding site of filoviruses

Filoviruses enter cells through micropinocytosis and trafficking into the endosomes in which they bind to the receptor Niemann-Pick C1 protein (NPC1) for membrane fusion and entry into the cytoplasm. The endosomal receptor-binding is critical step for filovirus entry. Designing inhibitors to block receptor binding will prevent viral entry. Using available binding structural information from the co-crystal structures of the viral GP with the receptor NPC1 or with monoclonal antibodies, we have conducted structure-based design of peptide inhibitors to target the receptor binding site (RBS). The designed peptides were tested for their inhibition activity against pseudo-typed or replication-competent viruses in a cell-based assay. The results indicate that these peptides exhibited strong activities against both Ebola and Marburg virus infection. It is expected that these peptides can be further developed for therapeutic use to treat filovirus infection and combat the outbreaks.

Glycan shield of the ebolavirus envelope glycoprotein GP

The envelope glycoprotein GP of the ebolaviruses is essential for host cell entry and the primary target of the host antibody response. GP is heavily glycosylated with up to 17 N-linked sites, numerous O-linked glycans in its disordered mucin-like domain (MLD), and three predicted C-linked mannosylation sites. Glycosylation is important for host cell attachment, GP stability and fusion activity, and shielding from neutralization by serum antibodies. Here, we use glycoproteomics to profile the site-specific glycosylation patterns of ebolavirus GP. We detect up to 16 unique O-linked glycosylation sites in the MLD, and two O-linked sites in the receptor-binding GP1 subunit. Multiple O-linked glycans are observed within N-linked glycosylation sequons, suggesting crosstalk between the two types of modifications. We confirmed C-mannosylation of W288 in full-length trimeric GP. We find complex glycosylation at the majority of N-linked sites, while the conserved sites N257 and especially N563 are enriched in unprocessed glycans, suggesting a role in host-cell attachment via DC-SIGN/L-SIGN. Our findings illustrate how N-, O-, and C-linked glycans together build the heterogeneous glycan shield of GP, guiding future immunological studies and functional interpretation of ebolavirus GP-antibody interactions.

Site-specific N-, O-, and C-linked glycans are characterized in the ebolavirus envelope glycoprotein GP using mass spectrometry-based glycoproteomics.

Rapid detection of an Ebola biomarker with optical microring resonators

Ebola virus (EBOV) is a highly infectious pathogen, with a case mortality rate as high as 89%. Rapid therapeutic treatments and supportive measures can drastically improve patient outcome; however, the symptoms of EBOV disease (EVD) lack specificity from other endemic diseases. Given the high mortality and significant symptom overlap, there is a critical need for sensitive, rapid diagnostics for EVD. Facile diagnosis of EVD remains a challenge. Here, we describe a rapid and sensitive diagnostic for EVD through microring resonator sensors in conjunction with a unique biomarker of EBOV infection, soluble glycoprotein (sGP). Microring resonator sensors detected sGP in under 40 min with a limit of detection (LOD) as low as 1.00 ng/mL in serum. Furthermore, we validated our assay with the detection of sGP in serum from EBOV-infected non-human primates. Our results demonstrate the utility of a high-sensitivity diagnostic platform for detection of sGP for diagnosis of EVD.

Protective CD8+ T Cell Response Induced by Modified Vaccinia Virus Ankara Delivering Ebola Virus Nucleoprotein

The urgent need for vaccines against Ebola virus (EBOV) was underscored by the large outbreak in West Africa (2014–2016). Since then, several promising vaccine candidates have been tested in pre-clinical and clinical studies. As a result, two vaccines were approved for human use in 2019/2020, of which one includes a heterologous adenovirus/Modified Vaccinia virus Ankara (MVA) prime-boost regimen. Here, we tested new vaccine candidates based on the recombinant MVA vector, encoding the EBOV nucleoprotein (MVA-EBOV-NP) or glycoprotein (MVA-EBOV-GP) for their efficacy after homologous prime-boost immunization in mice. Our aim was to investigate the role of each antigen in terms of efficacy and correlates of protection. Sera of mice vaccinated with MVA-EBOV-GP were virus-neutralizing and MVA-EBOV-NP immunization readily elicited interferon-γ-producing NP-specific CD8+ T cells. While mock-vaccinated mice succumbed to EBOV infection, all vaccinated mice survived and showed drastically decreased viral loads in sera and organs. In addition, MVA-EBOV-NP vaccinated mice became susceptible to lethal EBOV infection after depletion of CD8+ T cells prior to challenge. This study highlights the potential of MVA-based vaccines to elicit humoral immune responses as well as a strong and protective CD8+ T cell response and contributes to understanding the possible underlying mechanisms.

Development and Structural Analysis of Antibody Therapeutics for Filoviruses

The filoviruses, including ebolaviruses and marburgviruses, are among the world’s deadliest pathogens. As the only surface-exposed protein on mature virions, their glycoprotein GP is the focus of current therapeutic monoclonal antibody discovery efforts. With recent technological developments, potent antibodies have been identified from immunized animals and human survivors of virus infections and have been characterized functionally and structurally. Structural insight into how the most successful antibodies target GP further guides vaccine development. Here we review the recent developments in the identification and characterization of neutralizing antibodies and cocktail immunotherapies.

Enterotoxigenic Escherichia coli Heat-Stable Toxin and Ebola Virus Delta Peptide: Similarities and Differences

Enterotoxigenic Escherichia coli (ETEC) STb toxin exhibits striking structural similarity to Ebola virus (EBOV) delta peptide. Both ETEC and EBOV delta peptide are enterotoxins. Comparison of the structural and functional similarities and differences of these two toxins illuminates features that are important in induction of pathogenesis by a bacterial and viral pathogen.

Structural and Functional Aspects of Ebola Virus Proteins

Ebola virus (EBOV), member of genus Ebolavirus, family Filoviridae, have a non-segmented, single-stranded RNA that contains seven genes: (a) nucleoprotein (NP), (b) viral protein 35 (VP35), (c) VP40, (d) glycoprotein (GP), (e) VP30, (f) VP24, and (g) RNA polymerase (L). All genes encode for one protein each except GP, producing three pre-proteins due to the transcriptional editing. These pre-proteins are translated into four products, namely: (a) soluble secreted glycoprotein (sGP), (b) Δ-peptide, (c) full-length transmembrane spike glycoprotein (GP), and (d) soluble small secreted glycoprotein (ssGP). Further, shed GP is released from infected cells due to cleavage of GP by tumor necrosis factor α-converting enzyme (TACE). This review presents a detailed discussion on various functional aspects of all EBOV proteins and their residues. An introduction to ebolaviruses and their life cycle is also provided for clarity of the available analysis. We believe that this review will help understand the roles played by different EBOV proteins in the pathogenesis of the disease. It will help in targeting significant protein residues for therapeutic and multi-protein/peptide vaccine development.

The Ebola virus soluble glycoprotein contributes to viral pathogenesis by activating the MAP kinase signaling pathway

Ebola virus (EBOV) expresses three different glycoproteins (GPs) from its GP gene. The primary product, soluble GP (sGP), is secreted in abundance during infection. EBOV sGP has been discussed as a potential pathogenicity factor, however, little is known regarding its functional role. Here, we analyzed the role of sGP in vitro and in vivo. We show that EBOV sGP has two different functions that contribute to infectivity in tissue culture. EBOV sGP increases the uptake of virus particles into late endosomes in HEK293 cells, and it activates the mitogen-activated protein kinase (MAPK) signaling pathway leading to increased viral replication in Huh7 cells. Furthermore, we analyzed the role of EBOV sGP on pathogenicity using a well-established mouse model. We found an sGP-dependent significant titer increase of EBOV in the liver of infected animals. These results provide new mechanistic insights into EBOV pathogenicity and highlight EBOV sGP as a possible therapeutic target.

Since its discovery in 1976, Ebola virus (EBOV) has caused infrequent outbreaks of hemorrhagic disease in Africa. The virus’ replication cycle has been well-characterized in cell culture using natural isolates and reverse genetics systems. For many EBOV proteins the key functions have been defined, however, the role the primary products of the glycoprotein (GP) gene, soluble GP (sGP), is not well understood. Our studies focused on sGP’s impact on different stages in the viral life cycle. While sGP increased the uptake of EBOV particles into late endosomes in a human kidney-derived cell line, it activated the mitogen-activated protein kinase signaling pathway leading to increased viral replication in a human liver-derived cell line. Analysis of sGP treatment in the well-established mouse model for EBOV infection demonstrated that sGP treatment indeed increases virus replication in key target tissues like liver and spleen. This data suggests a contribution of sGP to EBOV pathogenicity and identifies it as a new target for therapeutic approaches.

Activating Natural Killer Cell Receptors, Selectins, and Inhibitory Siglecs Recognize Ebolavirus Glycoprotein

Expression of the extensively glycosylated Ebolavirus glycoprotein (EBOV-GP) induces physical alterations of surface molecules and plays a crucial role in viral pathogenicity. Here we investigate the interactions of EBOV-GP with host surface molecules using purified EBOV-GP, EBOV-GP-transfected cell lines, and EBOV-GP-pseudotyped lentiviral particles. Subsequently, we wanted to examine which receptors are involved in this recognition by binding studies to cells transfected with the EBOV-GP as well as to recombinant soluble EBOV-GP. As the viral components can also bind to inhibitory receptors of immune cells (e.g., Siglecs, TIM-1), they can even suppress the activity of immune effector cells. Our data show that natural killer (NK) cell receptors NKp44 and NKp46, selectins (CD62E/P/L), the host factors DC-SIGNR/DC-SIGN, and inhibitory Siglecs function as receptors for EBOV-GP. Our results show also moderate to strong avidity of homing receptors (P-, L-, and E-selectin) and DC-SIGNR/DC-SIGN to purified EBOV-GP, to cells transfected with EBOV-GP, as well as to the envelope of a pseudotyped lentiviral vector carrying the EBOV-GP. The concomitant activation and inhibition of the immune system exemplifies the evolutionary antagonism between the immune system and pathogens. Altogether these interactions with activating and inhibitory receptors result in a reduced NK cell-mediated lysis of EBOV-GP-expressing cells. Modulation of these interactions may provide new strategies for treating infections caused by this virus.

Ultrasensitive point-of-care immunoassay for secreted glycoprotein detects Ebola infection earlier than PCR

Ebola virus (EBOV) hemorrhagic fever outbreaks have been challenging to deter due to the lack of health care infrastructure in disease-endemic countries and a corresponding inability to diagnose and contain the disease at an early stage. EBOV vaccines and therapies have improved disease outcomes, but the advent of an affordable, easily accessed, mass-produced rapid diagnostic test (RDT) that matches the performance of more resource-intensive polymerase chain reaction (PCR) assays would be invaluable in containing future outbreaks. Here, we developed and demonstrated the performance of a new ultrasensitive point-of-care immunoassay, the EBOV D4 assay, which targets the secreted glycoprotein of EBOV. The EBOV D4 assay is 1000-fold more sensitive than the U.S. Food and Drug Administration-approved RDTs and detected EBOV infection earlier than PCR in a standard nonhuman primate model. The EBOV D4 assay is suitable for low-resource settings and may facilitate earlier detection, containment, and treatment during outbreaks of the disease.

Unconventional viral gene expression mechanisms as therapeutic targets

Unlike the human genome that comprises mostly noncoding and regulatory sequences, viruses have evolved under the constraints of maintaining a small genome size while expanding the efficiency of their coding and regulatory sequences. As a result, viruses use strategies of transcription and translation in which one or more of the steps in the conventional gene-protein production line are altered. These alternative strategies of viral gene expression (also known as gene recoding) can be uniquely brought about by dedicated viral enzymes or by co-opting host factors (known as host dependencies). Targeting these unique enzymatic activities and host factors exposes vulnerabilities of a virus and provides a paradigm for the design of novel antiviral therapies. In this Review, we describe the types and mechanisms of unconventional gene and protein expression in viruses, and provide a perspective on how future basic mechanistic work could inform translational efforts that are aimed at viral eradication.

The sphingosine kinase 1 activator, K6PC-5, attenuates Ebola virus infection

Ebola virus (EBOV) is responsible for outbreaks with case fatality rates of up to 90% and for an epidemic in West Africa with more than ten thousand deaths. EBOV glycoprotein (EBOV-GP) is the only viral surface protein and is responsible for viral entry into cells. Here, by employing pseudotyped EBOV-GP viral particles, we uncover a critical role for sphingolipids in inhibiting viral entry. Sphingosine kinase 1 (SphK1) catalyzes the phosphorylation of sphingosine to sphingosine 1-phosphate (S1P). The administration of the SphK1 activator, K6PC-5, or S1P, or the overexpression of SphK1 consistently exhibited striking inhibitory effects in EBOV-GP-driven entry in diverse cell lines. Finally, K6PC-5 markedly reduced the EBOV titer in infected cells and the de novo production of viral proteins. These data present K6PC-5 as an efficient tool to inhibit EBOV infection in endothelial cells and suggest further studies to evaluate its systemic effects.

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K6PC-5, a sphingosine kinase 1 activator, inhibits Ebola virus infection

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Sphingosine 1-phosphate, the product of SphK1, attenuates the viral entry

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Inhibiton/activation of S1P receptors has no influence on Ebola virus entry

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These data support the endogen effect of S1P in Ebola virus infection

Immunotherapeutic strategies to target vulnerabilities in the Ebolavirus glycoprotein

The 2014 Ebola virus disease (EVD) outbreak in West Africa and the subsequent outbreaks of 2018-2020 in Equator and North Kivu provinces of the Democratic Republic of the Congo illustrate the public health challenges of emerging and reemerging viruses. EVD has a high case fatality rate with a rapidly progressing syndrome of fever, rash, vomiting, diarrhea, and bleeding diathesis. Recently, two monoclonal-antibody-based therapies received United States Food and Drug Administration (FDA) approval, and there are several other passive immunotherapies that hold promise as therapeutics against other species of Ebolavirus. Here, we review concepts needed to understand mechanisms of action, present an expanded schema to define additional sites of vulnerability on the viral glycoprotein, and review current antibody-based therapeutics. The concepts described are used to gain insights into the key characteristics that represent functional targets for immunotherapies against Zaire Ebolavirus and other emerging viruses within the Ebolavirus genus.

Development of a Multivalent Kunjin Virus Reporter Virus-Like Particle System Inducing Seroconversion for Ebola and West Nile Virus Proteins in Mice

Kunjin virus (KUNV) is an attenuated strain of the severe neurotropic West Nile virus (WNV). The virus has a single-strand positive-sense RNA genome that encodes a polyprotein. Following gene expression, the polyprotein is cleaved into structural proteins for viral packaging and nonstructural proteins for viral replication and expression. Removal of the structural genes generate subgenomic replicons that maintain replication capacity. Co-expression of these replicons with the viral structural genes produces reporter virus-like particles (RVPs) which infect cells in a single round. In this study, we aimed to develop a system to generate multivalent RVPs based on KUNV to elicit an immune response against different viruses. We selected the Ebola virus (EBOV) glycoprotein (GP) and the matrix protein (VP40) genes, as candidates to be delivered by KUNV RVPs. Initially, we enhanced the production of KUNV RVPs by generating a stable cell line expressing the KUNV packaging system comprising capsid, precursor membrane, and envelope. Transfection of the DNA-based KUNV replicon into this cell line resulted in an enhanced RVP production. The replicon was expressed in the stable cell line to produce the RVPs that allowed the delivery of EBOV GP and VP40 genes into other cells. Finally, we immunized BALB/cN mice with RVPs, resulting in seroconversion for EBOV GP, EBOV VP40, WNV nonstructural protein 1, and WNV E protein. Thus, our study shows that KUNV RVPs may function as a WNV vaccine candidate and RVPs can be used as a gene delivery system in the development of future EBOV vaccines.

Development of an antigen detection assay for early point-of-care diagnosis of Zaire ebolavirus

The 2013–2016 Ebola virus (EBOV) outbreak in West Africa and the ongoing cases in the Democratic Republic of the Congo have spurred development of a number of medical countermeasures, including rapid Ebola diagnostic tests. The likelihood of transmission increases as the disease progresses due to increasing viral load and potential for contact with others. Early diagnosis of EBOV is essential for halting spread of the disease. Polymerase chain reaction assays are the gold standard for diagnosing Ebola virus disease (EVD), however, they rely on infrastructure and trained personnel that are not available in most resource-limited settings. Rapid diagnostic tests that are capable of detecting virus with reliable sensitivity need to be made available for use in austere environments where laboratory testing is not feasible. The goal of this study was to produce candidate lateral flow immunoassay (LFI) prototypes specific to the EBOV glycoprotein and viral matrix protein, both targets known to be present during EVD. The LFI platform utilizes antibody-based technology to capture and detect targets and is well suited to the needs of EVD diagnosis as it can be performed at the point-of-care, requires no cold chain, provides results in less than twenty minutes and is low cost. Monoclonal antibodies were isolated, characterized and evaluated in the LFI platform. Top performing LFI prototypes were selected, further optimized and confirmed for sensitivity with cultured live EBOV and clinical samples from infected non-human primates. Comparison with a commercially available EBOV rapid diagnostic test that received emergency use approval demonstrates that the glycoprotein-specific LFI developed as a part of this study has improved sensitivity. The outcome of this work presents a diagnostic prototype with the potential to enable earlier diagnosis of EVD in clinical settings and provide healthcare workers with a vital tool for reducing the spread of disease during an outbreak.

Ebola virus (EBOV) causes a severe hemorrhagic fever and has an extremely high fatality rate that ranges from 60%-90%. There is no approved treatment or vaccine for this infectious disease and halting spread of the virus relies on identifying and isolating infected patients quickly. The current gold standard, polymerase chain reaction assay, requires patient samples be transported to regional reference laboratories where it often takes days to get results. A handful of Ebola rapid diagnostic tests have been developed, but lack the sensitivity required to detect the virus in earlier stages of the disease. There is great need for more sensitive rapid diagnostic tests that can identify the EBOV infected patients when they first become symptomatic. This study focused on production of high affinity mAbs to two target EBOV proteins for development of a more sensitivity rapid diagnostic test. Efforts have resulted in production of prototype detecting the EBOV glycoprotein that shows a notable improvement in sensitivity and offers the potential for earlier diagnosis of infection.

Pseudotyping of VSV with Ebola virus glycoprotein is superior to HIV-1 for the assessment of neutralising antibodies

Ebola virus (EBOV) is an enveloped, single-stranded RNA virus that can cause Ebola virus disease (EVD). It is thought that EVD survivors are protected against subsequent infection with EBOV and that neutralising antibodies to the viral surface glycoprotein (GP) are potential correlates of protection. Serological studies are vital to assess neutralising antibodies targeted to EBOV GP; however, handling of EBOV is limited to containment level 4 laboratories. Pseudotyped viruses can be used as alternatives to live viruses, which require high levels of bio-containment, in serological and viral entry assays. However, neutralisation capacity can differ among pseudotyped virus platforms. We evaluated the suitability of EBOV GP pseudotyped human immunodeficiency virus type 1 (HIV-1) and vesicular stomatitis virus (VSV) to measure the neutralising ability of plasma from EVD survivors, when compared to results from a live EBOV neutralisation assay. The sensitivity, specificity and correlation with live EBOV neutralisation were greater for the VSV-based pseudotyped virus system, which is particularly important when evaluating EBOV vaccine responses and immuno-therapeutics. Therefore, the EBOV GP pseudotyped VSV neutralisation assay reported here could be used to provide a better understanding of the putative correlates of protection against EBOV.

Epitopes of Naturally Acquired and Vaccine-Induced Anti-Ebola Virus Glycoprotein Antibodies in Single Amino Acid Resolution

The Ebola virus (EBOV) can cause severe infections in humans, leading to a fatal outcome in a high percentage of cases. Neutralizing antibodies against the EBOV surface glycoprotein (GP) can prevent infections, demonstrating a straightforward way for an efficient vaccination strategy. Meanwhile, many different anti-EBOV antibodies have been identified, whereas the exact binding epitopes are often unknown. Here, the analysis of serum samples from an EBOV vaccine trial with the recombinant vesicular stomatitis virus-Zaire ebolavirus (rVSV-ZEBOV) and an Ebola virus disease survivor, using high-density peptide arrays, is presented. In this proof-of-principle study, distinct IgG and IgM antibodies binding to different epitopes of EBOV GP is detected: By mapping the whole GP as overlapping peptide fragments, new epitopes and confirmed epitopes from the literature are found. Furthermore, the highly selective binding epitope of a neutralizing monoclonal anti-EBOV GP antibody could be validated. This shows that peptide arrays can be a valuable tool to study the humoral immune response to vaccines in patients and to support Ebola vaccine development.

The Proteolytic Regulation of Virus Cell Entry by Furin and Other Proprotein Convertases

A wide variety of viruses exploit furin and other proprotein convertases (PCs) of the constitutive protein secretion pathway in order to regulate their cell entry mechanism and infectivity. Surface proteins of enveloped, as well as non-enveloped, viruses become processed by these proteases intracellularly during morphogenesis or extracellularly after egress and during entry in order to produce mature virions activated for infection. Although viruses also take advantage of other proteases, it is when some viruses become reactive with PCs that they may develop high pathogenicity. Besides reacting with furin, some viruses may also react with the PCs of the other specificity group constituted by PC4/PC5/PACE4/PC7. The targeting of PCs for inhibition may result in a useful strategy to treat infections with some highly pathogenic viruses. A wide variety of PC inhibitors have been developed and tested for their antiviral activity in cell-based assays.

Recombinant subunit vaccines protect guinea pigs from lethal Ebola virus challenge

Ebola virus (EBOV) is among the deadliest pathogens known to man causing infrequent outbreaks of hemorrhagic disease. In humans, the case fatality rates in the outbreaks can reach 90%. During the West African epidemic almost 30,000 people were infected and of these over 11,000 fatalities were reported. Currently, we are facing an uncontained larger outbreak in the Democratic Republic of the Congo. Even though EBOV was discovered in 1976, extensive efforts to develop countermeasures, particularly therapeutics and vaccines, started late and there is still no FDA-approved product available. Nevertheless, one candidate vaccine, the rVSV-ZEBOV, is being used in clinical trials during the current outbreak with the hope of ending the human transmission chains. However, adverse reactions to administration of some EBOV vaccines have been reported; therefore, we have developed a safe and efficacious formulation of insect-cell derived adjuvanted protein vaccines. Vaccine candidates containing the EBOV glycoprotein with or without matrix proteins VP24 and VP40 formulated with one of three different adjuvants were tested in guinea pigs for immunogenicity and efficacy against lethal EBOV challenge. The results demonstrated that these vaccine candidates engendered high titers of antigen-specific antibodies in immunized animals and two of these vaccine candidates afforded complete or nearly complete protection against lethal challenge. Interestingly, we found a sex bias in partially protected immunized groups with male guinea pigs succumbing to disease and females surviving. In summary, we developed a safe and immunogenic adjuvanted subunit vaccine uniformly protective against EBOV disease in guinea pigs.

Conserved B and T cell epitopes prediction of ebola virus glycoprotein for vaccine development: An immuno-informatics approach

Ebola virus (EBOV), a non-segmented single-stranded RNA virus, is often-most transmitted through body fluids like sweat, tears, saliva, and nasal secretions. Till date, there is no licensed vaccine of EBOV is available in the market; however, the world is increasingly vulnerable to this emerging threat. Hence, it is the need of time to develop a vaccine for EBOV to hinder its dissemination. The current study has been designed for identification and characterization of the potential B and T-cell epitopes using the Immuno-informatics tools, and it helped in finding the potent vaccine candidates against EBOV. Prediction, antigenicity and allergenicity testing of predicted B and T cells' epitopes was done as well to identify their potential as a vaccine candidate and to measure their safety level respectively. Among B-cell epitopes "WIPAGIGVTGVIIA" showed a high antigenicity score and it would play an important role in evoking the immune response. In T-cell epitopes, peptides "AIGLAWIPY" and "IRGFPRCRY" presented high antigenicity score, which binds to MHC class-I and MHC class-II alleles respectively. All predicted epitopes were analyzed and compared with already reported peptides carefully. Comparatively, Peptides predicted in the present study showed more immunogenicity score than already reported peptides, used as positive control, and are more immunogenic as compared to them. Peptides reported in the present study do not target only Zaire EBOV (ZEBOV), as in previous studies, but also other species, i.e. Tai Forest EBOV (TAFV), Sudan EBOV (SUDV), Bundibugyo EBOV (BDBV), and Reston EBOV (RESTV) and would bring the promising results as potent vaccine candidates.

Cross-reactive neutralizing human survivor monoclonal antibody BDBV223 targets the ebolavirus stalk

Three Ebolavirus genus viruses cause lethal disease and lack targeted therapeutics: Ebola virus, Sudan virus and Bundibugyo virus. Monoclonal antibody (mAb) cocktails against the surface glycoprotein (GP) present a potential therapeutic strategy. Here we report two crystal structures of the antibody BDBV223, alone and complexed with its GP2 stalk epitope, an interesting site for therapeutic/vaccine design due to its high sequence conservation among ebolaviruses. BDBV223, identified in a human survivor of Bundibugyo virus disease, neutralizes both Bundibugyo virus and Ebola virus, but not Sudan virus. Importantly, the structure suggests that BDBV223 binding interferes with both the trimeric bundle assembly of GP and the viral membrane by stabilizing a conformation in which the monomers are separated by GP lifting or bending. Targeted mutagenesis of BDBV223 to enhance SUDV GP recognition indicates that additional determinants of antibody binding likely lie outside the visualized interactions, and perhaps involve quaternary assembly or membrane-interacting regions.

Human antibodies cross-reactive for several viruses within the Ebolavirus genus have been identified. Here the authors present the crystal structure of such a neutralizing monoclonal antibody (mAb) targeting the stalk of Bundibugyo virus glycoprotein and show that mAb binding may interfere with trimeric bundle assembly and/or the viral membrane.

Glycoprotein GP as a basis for the universal vaccine against Ebola virus disease

Ebola virus disease (EVD) is one of the deadliest viral infections affecting humans and nonhuman primates. Of 6 known representatives of the Ebolavirus genus responsible for the disease, 3 can infect humans, causing acute highly contagious fever characterized by up to 90% fatality. These include Bundibugyo ebolavirus (BDBV), Zaire ebolavirus (ZEBOV) and Sudan ebolavirus (SUDV). The majority of the reported EVD cases are caused by ZEBOV. Vaccine development against the virus started in 1976, immediately after the causative agent of the infection was identified. So far, 4 vaccines have been approved. All of them are based on the protective epitope of the ZEBOV glycoprotein GP. Because SUDV and BDBV can also cause outbreaks and epidemics, it is vital to design a vaccine capable of conferring protection against all known ebolaviruses posing a threat to the human population. This article presents systematized data on the structure, immunogenicity and protective properties of ebolavirus glycoprotein GP, looks closely at the immunodominant epitopes of ZEBOV, SUDV and BDBV glycoprotein GP required to elicit a protective immune response, and offers a rational perspective on the development of a universal vaccine against EVD that relies on the use of vectors expressing two variants of GP represented by ZEBOV and SUDV.

Achieving cross-reactivity with pan-ebolavirus antibodies

Filoviruses are the causative agents of highly lethal outbreaks in sub-Saharan Africa. Although an experimental vaccine and several therapeutics are being deployed in the Democratic Republic of Congo to combat the ongoing Ebola virus outbreak, these therapies are specific for only one filovirus species. There is currently significant interest in developing broadly reactive monoclonal antibodies (mAbs) with utility against the variety of ebolaviruses that may emerge. Thus far, the primary target of these mAbs has been the viral spike glycoprotein (GP). Here we present an overview of GP-targeted antibodies that exhibit broad reactivity and the structural characteristics that could confer this cross-reactivity. We also discuss how these structural features could be leveraged to design vaccine antigens that elicit cross-reactive antibodies.

Thermostable Ebola virus vaccine formulations lyophilized in the presence of aluminum hydroxide

No United States Food and Drug Administration-licensed vaccines protective against Ebola virus (EBOV) infections are currently available. EBOV vaccine candidates currently in development, as well as most currently licensed vaccines in general, require transport and storage under a continuous cold chain in order to prevent potential decreases in product efficacy. Cold chain requirements are particularly difficult to maintain in developing countries. To improve thermostability and reduce costly cold chain requirements, a subunit protein vaccine against EBOV was formulated as a glassy solid using lyophilization. Formulations of the key antigen, Ebola glycoprotein (EBOV-GP), adjuvanted with microparticulate aluminum hydroxide were prepared in liquid and lyophilized forms, and the vaccines were incubated at 40 °C for 12 weeks. Aggregation and degradation of EBOV-GP were observed in liquid formulations during the 12-week incubation period, whereas changes were minimal in lyophilized formulations. Antibody responses against EBOV-GP following three intramuscular immunizations in BALB/c mice were used to determine vaccine immunogenicity. EBOV-GP formulations were equally immunogenic in liquid and lyophilized forms. After lyophilization and reconstitution, adjuvanted vaccine formulations produced anti-EBOV-GP IgG antibody responses in mice similar to those generated against corresponding adjuvanted liquid vaccine formulations. More importantly, antibody responses in mice injected with reconstituted lyophilized vaccine formulations that had been incubated at 40 °C for 12 weeks prior to injection indicated that vaccine immunogenicity was fully retained after high-temperature storage, showing promise for future vaccine development efforts.

Review of computational methods for virus-host protein interaction prediction: a case study on novel Ebola-human interactions

Identification of potential virus-host interactions is useful and vital to control the highly infectious virus-caused diseases. This may contribute toward development of new drugs to treat the viral infections. Recently, database records of clinically and experimentally validated interactions between a small set of human proteins and Ebola virus (EBOV) have been published. Using the information of the known human interaction partners of EBOV, our main objective is to identify a set of proteins that may interact with EBOV proteins. Here, we first review the state-of-the-art, computational methods used for prediction of novel virus-host interactions for infectious diseases followed by a case study on EBOV-human interactions. The assessment result shows that the predicted human host proteins are highly similar with known human interaction partners of EBOV in the context of structure and semantics and are responsible for similar biochemical activities, pathways and host-pathogen relationships.

Immunogenicity of a Candidate Ebola Hemorrhagic Fever Vaccine in Mice Based on Controlled In Vitro Expression of Ebolavirus Glycoprotein

Ebolavirus (EBOV) is the etiology of Ebola hemorrhagic fever (EHF). A major EHF outbreak in 2014-2015 in West Africa claimed >11,000 lives. A licensed vaccine is not available for EHF, although several vaccines have undergone clinical trials. We developed a human adenovirus (Ad) serotype 5-based candidate EHF vaccine based on controlled expression of the EBOV (Makona strain) glycoprotein (GP) as the immunogen. Two clones, AdGP72 and AdGP75, and a control Ad515 vector, were generated and tested for protein expression in vitro and immunogenicity in mice. Eight groups of mice were immunized with three doses of buffer, Ad515, AdGP72, and AdGP75, by two different dose regimens. Three different antigens (AdGP75-infected Vero E6 cell extract and two baculovirus expressed EBOV GP antigens, namely, GP alone or GP with EBOV VP40) were used to evaluate the immune response. Expression studies indicated that full-length GP was cleaved into its component subunits when expressed in mammalian cells through the Ad vectors. Moreover, in coimmunoprecipitation studies, EBOV GP was found to be associated with VP40 when expressed in baculoviruses. The candidate vaccines were immunogenic in mice, as evaluated by enzyme-linked immunosorbent assay using mammalian- or baculovirus-derived antigens. Further characterization and development of the candidate vaccines are warranted.

Systematic Analysis of Monoclonal Antibodies against Ebola Virus GP Defines Features that Contribute to Protection

Antibodies are promising post-exposure therapies against emerging viruses, but which antibody features and in vitro assays best forecast protection are unclear. Our international consortium systematically evaluated antibodies against Ebola virus (EBOV) using multidisciplinary assays. For each antibody, we evaluated epitopes recognized on the viral surface glycoprotein (GP) and secreted glycoprotein (sGP), readouts of multiple neutralization assays, fraction of virions left un-neutralized, glycan structures, phagocytic and natural killer cell functions elicited, and in vivo protection in a mouse challenge model. Neutralization and induction of multiple immune effector functions (IEFs) correlated most strongly with protection. Neutralization predominantly occurred via epitopes maintained on endosomally cleaved GP, whereas maximal IEF mapped to epitopes farthest from the viral membrane. Unexpectedly, sGP cross-reactivity did not significantly influence in vivo protection. This comprehensive dataset provides a rubric to evaluate novel antibodies and vaccine responses and a roadmap for therapeutic development for EBOV and related viruses.

The structural basis for filovirus neutralization by monoclonal antibodies

Filoviruses, including ebolaviruses and marburgviruses, are the causative agents of highly lethal disease outbreaks. The 2013-2016 Ebola virus outbreak was responsible for >28000 infections and >11000 deaths. Although there are currently no licensed vaccines or therapeutics for any filovirus-induced disease, monoclonal antibodies (mAbs) are among the most promising options for therapeutic development. Hundreds of mAbs have been isolated from human survivors of filovirus infections that target the viral spike glycoprotein (GP). The binding, neutralization, and cross-reactivity of many of these mAbs has been determined. Several mAbs have been characterized structurally, and this information has been crucial for strategizing therapeutic and vaccine design. Here we present an overview of the structural features of the neutralizing/protective epitopes on filovirus glycoproteins.

Statins Suppress Ebola Virus Infectivity by Interfering with Glycoprotein Processing

Ebola virus (EBOV) infection is a major public health concern due to high fatality rates and limited effective treatments. Statins, widely used cholesterol-lowering drugs, have pleiotropic mechanisms of action and were suggested as potential adjunct therapy for Ebola virus disease (EVD) during the 2013-2016 outbreak in West Africa. Here, we evaluated the antiviral effects of statin (lovastatin) on EBOV infection in vitro Statin treatment decreased infectious EBOV production in primary human monocyte-derived macrophages and in the hepatic cell line Huh7. Statin treatment did not interfere with viral entry, but the viral particles released from treated cells showed reduced infectivity due to inhibition of viral glycoprotein processing, as evidenced by decreased ratios of the mature glycoprotein form to precursor form. Statin-induced inhibition of infectious virus production and glycoprotein processing was reversed by exogenous mevalonate, the rate-limiting product of the cholesterol biosynthesis pathway, but not by low-density lipoprotein. Finally, statin-treated cells produced EBOV particles devoid of the surface glycoproteins required for virus infectivity. Our findings demonstrate that statin treatment inhibits EBOV infection and suggest that the efficacy of statin treatment should be evaluated in appropriate animal models of EVD.IMPORTANCE Treatments targeting Ebola virus disease (EVD) are experimental, expensive, and scarce. Statins are inexpensive generic drugs that have been used for many years for the treatment of hypercholesterolemia and have a favorable safety profile. Here, we show the antiviral effects of statins on infectious Ebola virus (EBOV) production. Our study reveals a novel molecular mechanism in which statin regulates EBOV particle infectivity by preventing glycoprotein processing and incorporation into virus particles. Additionally, statins have anti-inflammatory and immunomodulatory effects. Since inflammation and dysregulation of the immune system are characteristic features of EVD, statins could be explored as part of EVD therapeutics.

Ebola virus requires a host scramblase for externalization of phosphatidylserine on the surface of viral particles

Cell surface receptors for phosphatidylserine contribute to the entry of Ebola virus (EBOV) particles, indicating that the presence of phosphatidylserine in the envelope of EBOV is important for the internalization of EBOV particles. Phosphatidylserine is typically distributed in the inner layer of the plasma membrane in normal cells. Progeny virions bud from the plasma membrane of infected cells, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the plasma membrane in infected cells for EBOV virions to acquire it. Currently, the intracellular dynamics of phosphatidylserine during EBOV infection are poorly understood. Here, we explored the role of XK-related protein (Xkr) 8, which is a scramblase responsible for exposure of phosphatidylserine in the plasma membrane of apoptotic cells, to understand its significance in phosphatidylserine-dependent entry of EBOV. We found that Xkr8 and transiently expressed EBOV glycoprotein GP often co-localized in intracellular vesicles and the plasma membrane. We also found that co-expression of GP and viral major matrix protein VP40 promoted incorporation of Xkr8 into ebolavirus-like particles (VLPs) and exposure of phosphatidylserine on their surface, although only a limited amount of phosphatidylserine was exposed on the surface of the cells expressing GP and/or VP40. Downregulating Xkr8 or blocking caspase-mediated Xkr8 activation did not affect VLP production, but they reduced the amount of phosphatidylserine on the VLPs and their uptake in recipient cells. Taken together, our findings indicate that Xkr8 is trafficked to budding sites via GP-containing vesicles, is incorporated into VLPs, and then promote the entry of the released EBOV to cells in a phosphatidylserine-dependent manner.

Although Ebola virus causes severe hemorrhagic fever with a high mortality rate, there are no approved therapeutics. The viral entry process is one of the targets for antiviral development. Previous studies suggest that binding of phosphatidylserine, a component of the viral envelop, to the receptors promotes the entry of Ebola virus. Ebola virus is released from the surface membrane of infected cells. However, phosphatidylserine normally distributes in the inner layer of the cell surface membrane, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the membrane in infected cells for Ebola virus to acquire it. Because the mechanism by which phosphatidylserine changes its orientation in Ebola virus-infected cells is poorly understood, we studied and identified a cellular enzyme, XK-related protein 8 (Xkr8), as a responsible factor involved in this process. We demonstrated that the Ebola virus glycoprotein promoted the incorporation of Xkr8 in viral particles, which flips phosphatidylserine on their surface, enhancing their entry to cells. Our findings provide new insights into the mechanism of Ebola virus infection, which may be exploited for the development of therapeutics against Ebola virus infection.

Proteolytic Processing of Filovirus Glycoproteins

Filoviruses (Marburg virus and Ebola virus) have a single envelope glycoprotein (GP) that initiates infection. GP is a class I fusion protein that forms trimeric spikes composed of heterodimers of the subunits GP1 and GP2. GP1 and GP2 are derived from the precursor pre-GP by furin cleavage during exocytosis. GP1 contains a receptor-binding core topped by a glycan cap and a heavily glycosylated mucin-like domain, while GP2 contains a fusion loop and a membrane anchor. After entering cells by macropinocytosis, the glycan cap and the mucin-like domain are removed from GP1 by endosomal cathepsins B and L exposing the binding site for the Niemann-Pick C1 receptor. It appears that there is no strict requirement for specific proteases involved in GP processing. Thus, furin is not indispensible for GP1-2 cleavage, and GP1 may be trimmed not only by cathepsins B and L but also by other endosomal proteases.

Two soluble glycoproteins of Ebola virus are also processed by host proteases. A significant amount of GP1,2 is cleaved by the metalloprotease TACE and shed from the surface of infected cells (GP1,2 delta). The secreted protein sGP is derived from the precursor pre-sGP by furin cleavage.

Ebola virus VP24 interacts with NP to facilitate nucleocapsid assembly and genome packaging

Ebola virus causes devastating hemorrhagic fever outbreaks for which no approved therapeutic exists. The viral nucleocapsid, which is minimally composed of the proteins NP, VP35, and VP24, represents an attractive target for drug development; however, the molecular determinants that govern the interactions and functions of these three proteins are still unknown. Through a series of mutational analyses, in combination with biochemical and bioinformatics approaches, we identified a region on VP24 that was critical for its interaction with NP. Importantly, we demonstrated that the interaction between VP24 and NP was required for both nucleocapsid assembly and genome packaging. Not only does this study underscore the critical role that these proteins play in the viral replication cycle, but it also identifies a key interaction interface on VP24 that may serve as a novel target for antiviral therapeutic intervention.

Overexpression of Ebola virus envelope GP1 protein

Ebola virus uses its envelope GP1 and GP2 for viral attachment and entry into host cells. Due to technical difficulty expressing full-length envelope, many structural and functional studies of Ebola envelope protein have been carried out primarily using GP1 lacking its mucin-like domain. As a result, the viral invasion mechanisms involving the mucin-like domain are not fully understood. To elucidate the role of the mucin-like domain of GP1 in Ebola-host attachment and infection and to facilitate vaccine development, we constructed a GP1 expression vector containing the entire attachment region (1-496). Cysteine 53 of GP1, which forms a disulfide bond with GP2, was mutated to serine to avoid potential disulfide bond mispairing. Stable expression clones using codon optimized open reading frame were developed in human 293-H cells with yields reaching ∼25 mg of GP1 protein per liter of spent medium. Purified GP1 was functional and bound to Ebola attachment receptors, DC-SIGN and DC-SIGNR. The over-expression and easy purification characteristic of this system has implications in Ebola research and vaccine development. To further understand the differential expression yields between the codon optimized and native GP1, we analyzed the presence of RNA structural motifs in the first 100 nucleotides of translational initiation AUG site. RNA structural prediction showed the codon optimization removed two potential RNA pseudoknot structures. This methodology is also applicable to the expression of other difficult virus envelope proteins.

Mechanistic understanding of N-glycosylation in Ebola virus glycoprotein maturation and function

The Ebola virus (EBOV) trimeric envelope glycoprotein (GP) precursors are cleaved into the receptor-binding GP₁ and the fusion-mediating GP₂ subunits and incorporated into virions to initiate infection. GP₁ and GP₂ form heterodimers that have 15 or two N-glycosylation sites (NGSs), respectively. Here we investigated the mechanism of how N-glycosylation contributes to GP expression, maturation, and function. As reported before, we found that, although GP₁ NGSs are not critical, the two GP₂ NGSs, Asn⁵⁶³ and Asn⁶¹⁸, are essential for GP function. Further analysis uncovered that Asn⁵⁶³ and Asn⁶¹⁸ regulate GP processing, demannosylation, oligomerization, and conformation. Consequently, these two NGSs are required for GP incorporation into EBOV-like particles and HIV type 1 (HIV-1) pseudovirions and determine viral transduction efficiency. Using CRISPR/Cas9 technology, we knocked out the two classical endoplasmic reticulum chaperones calnexin (CNX) and/or calreticulin (CRT) and found that both CNX and CRT increase GP expression. Nevertheless, NGSs are not required for the GP interaction with CNX or CRT. Together, we conclude that, although Asn⁵⁶³ and Asn⁶¹⁸ are not required for EBOV GP expression, they synergistically regulate its maturation, which determines its functionality.

Forty Years of Ebolavirus Molecular Biology: Understanding a Novel Disease Agent Through the Development and Application of New Technologies

Molecular biology is a broad discipline that seeks to understand biological phenomena at a molecular level, and achieves this through the study of DNA, RNA, proteins, and/or other macromolecules (e.g., those involved in the modification of these substrates). Consequently, it relies on the availability of a wide variety of methods that deal with the collection, preservation, inactivation, separation, manipulation, imaging, and analysis of these molecules. As such the state of the art in the field of ebolavirus molecular biology research (and that of all other viruses) is largely intertwined with, if not driven by, advancements in the technical methodologies available for these kinds of studies. Here we review of the current state of our knowledge regarding ebolavirus biology and emphasize the associated methods that made these discoveries possible.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Filovirus Structural Biology: The Molecules in the Machine

In this chapter, we describe what is known thus far about the structures and functions of the handful of proteins encoded by filovirus genomes. Amongst the fascinating findings of the last decade is the plurality of functions and structures that these polypeptides can adopt. Many of the encoded proteins can play multiple, distinct roles in the virus life cycle, although the mechanisms by which these functions are determined and controlled remain mostly veiled. Further, some filovirus proteins are multistructural: adopting different oligomeric assemblies and sometimes, different tertiary structures to achieve their separate, and equally essential functions. Structures, and the functions they dictate, are described for components of the nucleocapsid, the matrix, and the surface and secreted glycoproteins.

Molecular modeling, simulation and docking study of ebola virus glycoprotein

Ebola virus (EBOV) is a filamentous, enveloped, non-segmented, negative-strand ribonucleic acid (RNA) virus which belongs to family Filoviridae. Ebola virus includes different glycoproteins each of which plays their roles in different aspects of viral life cycle. In this study secreted glycoprotein (Q7T9E0) of Ebola virus was acquired from Uniprot. The formation of alpha helix and beta sheets of secondary structures were predicted through online servers. Higher flexibility and disordered regions of proteins were determined through RONN, GLOBPLOT and DISSEMBLE. Three dimensional (3D) structure of the protein was built through homology modeling techniques and MOE software. The validation and evaluation of the refined models were determined with two stereochemical tests i-e RAMPAGE and ERRAT servers. Further docking studies of given protein was performed with different derivatives of two antiviral drugs dronedarone and amiodarone through MOE. Docking score and binding affinity of respective derivatives demonstrate that these might be used as protein receptors.

The roles of ebolavirus glycoproteins in viral pathogenesis

Ebolaviruses are highly dangerous pathogens exhibiting extreme virulence in humans and nonhuman primates. The majority of ebolavirus species, most notably Zaire ebolavirus, can cause Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, in humans. EVD is associated with case-fatality rates as high as 90%, and there is currently no specific treatment or licensed vaccine available against EVD. Understanding the molecular biology and pathogenesis of ebolaviruses is important for the development of antiviral therapeutics. Ebolavirus encodes several forms of glycoproteins (GPs), which have some interesting characteristics, including the transcriptional editing coding strategy and extensive O-glycosylation modification, clustered in the mucin-like domain of GP1, full-length GP (GP_1,2), and shed GP. In addition to the canonical role of the spike protein, GP_1,2, in viral entry, ebolavirus GPs appear to have multiple additional functions, likely contributing to the complex pathogenesis of the virus. Here, we review the roles of ebolavirus GPs in viral pathogenesis.

Rodent-Adapted Filoviruses and the Molecular Basis of Pathogenesis

Ebola, Marburg, and Ravn viruses, all filoviruses, are the causative agents of severe hemorrhagic fever. Much of what we understand about the pathogenesis of filovirus disease is derived from work with animal models, including nonhuman primates, which are considered the "gold standard" filovirus model since they faithfully recapitulate the clinical hallmarks of filovirus disease. However, rodent models, including the mouse, guinea pig, and hamster, also exist for Ebola, Marburg, and Ravn viruses, and although they may not reproduce all the clinical signs of filovirus disease, thanks to their relative ease of use and low cost, they are often the first choice for initial descriptions of virus pathogenesis and evaluation of antiviral prophylactics and therapeutics. Since filoviruses do not cause significant disease in adult, immunocompetent rodents, these models rely on "rodent-adapted" viruses that have been passaged several times through their host until virulence and lethality are achieved. In the process of adaptation, the viruses acquire numerous nucleotide/amino acid mutations that contribute to virulence in their rodent host. Interestingly, virus protein 24 (VP24) and nucleoprotein (NP) appear to be major virulence factors for ebolaviruses in rodents, whereas VP40 appears to be the major virulence factor for marburgviruses. By characterizing these mutations and understanding the molecular mechanisms that lead to the acquisition of virulence, we can gain better insight into the pathogenic processes that underlie filovirus disease in humans. These processes, and the viral and/or cellular proteins that contribute to them, will make attractive targets for the development of novel therapeutics and counter-measures.

Structures of Ebola virus GP and sGP in complex with therapeutic antibodies

The Ebola virus (EBOV) GP gene encodes two glycoproteins. The major product is a soluble, dimeric glycoprotein termed sGP that is secreted abundantly. Despite the abundance of sGP during infection, little is known regarding its structure or functional role. A minor product, resulting from transcriptional editing, is the transmembrane-anchored, trimeric viral surface glycoprotein termed GP. GP mediates attachment to and entry into host cells, and is the intended target of antibody therapeutics. Because large portions of sequence are shared between GP and sGP, it has been hypothesized that sGP may potentially subvert the immune response or may contribute to pathogenicity. In this study, we present cryo-EM structures of GP and sGP in complex with GP-specific and GP/sGP cross-reactive antibodies undergoing human clinical trials. The structure of the sGP dimer presented here, in complex with both an sGP-specific antibody and a GP/sGP cross-reactive antibody, permits us to unambiguously assign the oligomeric arrangement of sGP and compare its structure and epitope presentation to those of GP. Further, we provide biophysical evaluation of naturally occurring GP/sGP mutations that fall within the footprints identified by our high-resolution structures. Taken together, our data provide a detailed and more complete picture of the accessible Ebolavirus glycoprotein landscape and a structural basis to evaluate patient and vaccine antibody responses toward differently structured products of the GP gene.

Human Survivors of Disease Outbreaks Caused by Ebola or Marburg Virus Exhibit Cross-Reactive and Long-Lived Antibody Responses

A detailed understanding of serological immune responses to Ebola and Marburg virus infections will facilitate the development of effective diagnostic methods, therapeutics, and vaccines. We examined antibodies from Ebola or Marburg survivors 1 to 14 years after recovery from disease, by using a microarray that displayed recombinant nucleoprotein (NP), viral protein 40 (VP40), envelope glycoprotein (GP), and inactivated whole virions from six species of filoviruses. All three outbreak cohorts exhibited significant antibody responses to antigens from the original infecting species and a pattern of additional filoviruses that varied by outbreak. NP was the most cross-reactive antigen, while GP was the most specific. Antibodies from survivors of infections by Marburg marburgvirus (MARV) species were least cross-reactive, while those from survivors of infections by Sudan virus (SUDV) species exhibited the highest cross-reactivity. Based on results revealed by the protein microarray, persistent levels of antibodies to GP, NP, and VP40 were maintained for up to 14 years after infection, and survival of infection caused by one species imparted cross-reactive antibody responses to other filoviruses.

Identification of the Fusion Peptide-Containing Region in Betacoronavirus Spike Glycoproteins

The fusion peptides (FP) play an essential role in fusion of viral envelope with cellular membranes. The location and properties of the FPs in the spike (S) glycoproteins of different coronaviruses (CoV) have not yet been determined. Through amino acid sequence analysis of S proteins of representative CoVs, we identified a common region as a possible FP (pFP) that shares the characteristics of FPs of class I viral fusion proteins, including high Ala/Gly content, intermediate hydrophobicity, and few charged residues. To test the hypothesis that this region contains the CoV FP, we systemically mutated every residue in the pFP of Middle East respiratory syndrome betacoronavirus (MERS-CoV) and found that 11 of the 22 residues in the pFP (from G953 to L964, except for A956) were essential for S protein-mediated cell-cell fusion and virus entry. The synthetic MERS-CoV pFP core peptide (955IAGVGWTAGL964) induced extensive fusion of liposome membranes, while mutant peptide failed to induce any lipid mixing. We also selectively mutated residues in pFPs of two other β-CoVs, severe acute respiratory syndrome coronavirus (SARS-CoV) and mouse hepatitis virus (MHV). Although the amino acid sequences of these two pFPs differed significantly from that of MERS-CoV and each other, most of the pFP mutants of SARS-CoV and MHV also failed to mediate membrane fusion, suggesting that these pFPs are also the functional FPs. Thus, the FPs of 3 different lineages of β-CoVs are conserved in location within the S glycoproteins and in their functions, although their amino acid sequences have diverged significantly during CoV evolution.

IMPORTANCE

Within the class I viral fusion proteins of many enveloped viruses, the FP is the critical mediator of fusion of the viral envelope with host cell membranes leading to virus infection. FPs from within a virus family, like influenza viruses or human immunodeficiency viruses (HIV), tend to share high amino acid sequence identity. In this study, we determined the location and amino acid sequences of the FPs of S glycoproteins of 3 β-CoVs, MERS-CoV, SARS-CoV, and MHV, and demonstrated that they were essential for mediating cell-cell fusion and virus entry. Interestingly, in marked contrast to the FPs of influenza and HIV, the primary amino acid sequences of the FPs of β-CoVs in 3 different lineages differed significantly. Thus, during evolution the FPs of β-CoVs have diverged significantly in their primary sequences while maintaining the same essential biological functions. Our findings identify a potential new target for development of drugs against CoVs.

Direct Visualization of Ebola Virus Fusion Triggering in the Endocytic Pathway

Ebola virus (EBOV) makes extensive and intricate use of host factors in the cellular endosomal/lysosomal pathway to release its genome into the cytoplasm and initiate infection. Following viral internalization into endosomes, host cysteine proteases cleave the EBOV fusion glycoprotein (GP) to unmask the binding site for its intracellular receptor, the cholesterol transporter Niemann-Pick C1 (NPC1). GP-NPC1 interaction is required for viral entry. Despite these and other recent discoveries, late events in EBOV entry following GP-NPC1 binding and culminating in GP-catalyzed fusion between viral and cellular lipid bilayers remain enigmatic. A mechanistic understanding of EBOV membrane fusion has been hampered by the failure of previous efforts to reconstitute fusion in vitro or at the cell surface. This report describes an assay to monitor initial steps directly in EBOV membrane fusion-triggering of GP and virus-cell lipid mixing-by single virions in live cells. Fusogenic triggering of GP occurs predominantly in Rab7-positive (Rab7(+)) endosomes, absolutely requires interaction between proteolytically primed GP and NPC1, and is blocked by key GP-specific neutralizing antibodies with therapeutic potential. Unexpectedly, cysteine protease inhibitors do not inhibit lipid mixing by virions bearing precleaved GP, even though they completely block cytoplasmic entry by these viruses, as shown previously. These results point to distinct cellular requirements for different steps in EBOV membrane fusion and suggest a model in which host cysteine proteases are dispensable for GP fusion triggering after NPC1 binding but are required for the formation of fusion pores that permit genome delivery.

IMPORTANCE

Ebola virus (EBOV) causes outbreaks of highly lethal disease for which no approved vaccines or treatments exist. Recent work has elucidated key molecular features of the complex EBOV entry process, including stepwise interactions with multiple host factors. However, there is a critical gap in our understanding of events that surround the final membrane fusion step which persists due to the paucity of direct and extensive investigation of EBOV fusion. Here, we report a real-time assay for EBOV glycoprotein fusion triggering and use it to define its cellular location and requirements. We also uncover an unexpected requirement for host proteases at a step after fusion triggering that may reflect their role in formation of fusion pores for genome delivery.

Induction of Cell-Cell Fusion by Ebola Virus Glycoprotein: Low pH Is Not a Trigger

Ebola virus (EBOV) is a highly pathogenic filovirus that causes hemorrhagic fever in humans and animals. Currently, how EBOV fuses its envelope membrane within an endosomal membrane to cause infection is poorly understood. We successfully measure cell-cell fusion mediated by the EBOV fusion protein, GP, assayed by the transfer of both cytoplasmic and membrane dyes. A small molecule fusion inhibitor, a neutralizing antibody, as well as mutations in EBOV GP known to reduce viral infection, all greatly reduce fusion. By monitoring redistribution of small aqueous dyes between cells and by electrical capacitance measurements, we discovered that EBOV GP-mediated fusion pores do not readily enlarge—a marked difference from the behavior of other viral fusion proteins. EBOV GP must be cleaved by late endosome-resident cathepsins B or L in order to become fusion-competent. Cleavage of cell surface-expressed GP appears to occur in endosomes, as evidenced by the fusion block imposed by cathepsin inhibitors, agents that raise endosomal pH, or an inhibitor of anterograde trafficking. Treating effector cells with a recombinant soluble cathepsin B or thermolysin, which cleaves GP into an active form, increases the extent of fusion, suggesting that a fraction of surface-expressed GP is not cleaved. Whereas the rate of fusion is increased by a brief exposure to acidic pH, fusion does occur at neutral pH. Importantly, the extent of fusion is independent of external pH in experiments in which cathepsin activity is blocked and EBOV GP is cleaved by thermolysin. These results imply that low pH promotes fusion through the well-known pH-dependent activity of cathepsins; fusion induced by cleaved EBOV GP is a process that is fundamentally independent of pH. The cell-cell fusion system has revealed some previously unappreciated features of EBOV entry, which could not be readily elucidated in the context of endosomal entry.

The devastation and transmissibility of Ebola virus (EBOV) are well known. However, the manner in which EBOV enters host cells through endosomal membrane remains elusive. Here, we have developed a convenient experimental system to mimic EBOV fusion in endosomes: cells expressing the fusion protein of EBOV, GP, on their surface are fused to target cells. This system exhibits the known key properties of EBOV fusion. We show that the pH-dependence of EBOV fusion is caused by the pH-dependence of cathepsins, proteases known to cleave EBOV GP into a fusion-competent form. We demonstrate that the fusion activity of this cleaved form is independent of pH. We further show that the enlargement of the fusion pore created by EBOV GP is unusually slow in reaching sizes necessary to pass EBOV’s genome—this is atypical of virally created fusion pores. This cell-cell fusion system should provide a useful platform for developing drugs against EBOV infection.