The roles of ebolavirus glycoproteins in viral pathogenesis

Ebola Virus GP Activates Endothelial Cells via Host Cytoskeletal Signaling Factors

Ebola virus disease (EVD) is a lethal disease caused by the highly pathogenic Ebola virus (EBOV), and its major symptoms in severe cases include vascular leakage and hemorrhage. These symptoms are caused by abnormal activation and disruption of endothelial cells (ECs) whose mediators include EBOV glycoprotein (GP) without the need for viral replication. However, the detailed molecular mechanisms underlying virus-host interactions remain largely unknown. Here, we show that EBOV-like particles (VLPs) formed by GP, VP40, and NP activate ECs in a GP-dependent manner, as demonstrated by the upregulation of intercellular adhesion molecules-1 (ICAM-1) expression. VLPs-mediated ECs activation showed a different kinetic pattern from that of TNF-α-mediated activation and was associated with apoptotic ECs disruption. In contrast to TNF-α, VLPs induced ICAM-1 overexpression at late time points. Furthermore, screening of host cytoskeletal signaling inhibitors revealed that focal adhesion kinase inhibitors were found to be potent inhibitors of ICAM-1 expression mediated by both TNF-α and VLPs. Our results suggest that EBOV GP stimulates ECs to induce endothelial activation and dysfunction with the involvement of host cytoskeletal signaling factors, which represent potential therapeutic targets for EVD.

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.

Construction and evaluation of DNA vaccine encoding Ebola virus glycoprotein fused with lysosome-associated membrane protein

Ebola virus (EBOV) of the genus Ebolavirus belongs to the family Filoviridae, which cause disease in both humans and non-human primates. Zaire Ebola virus accounts for the highest fatality rate, reaching 90%. Considering that EBOV has a high infection and fatality rate, the development of a highly effective vaccine has become a top public health priority. Glycoprotein (GP) plays a critical role during infection and protective immune responses. Herein, we developed an EBOV GP recombinant DNA vaccine that targets the major histocompatibility complex (MHC) class II compartment by fusing with lysosomal-associated membrane protein 1 (LAMP1). Through lysosome trafficking and antigen presentation transferring, the LAMP1 targeting strategy successfully improved both humoral and cellular EBOV-GP-specific immune responses. After three consecutive immunizations, the serum antibody titers, especially the neutralizing activity of mice immunized with the pVAX-LAMP/GPEBO vaccine were significantly higher than those of the other groups. Antigen-specific T cells showed positive activity against three dominant peptides, EAAVSHLTTLATIST, IGEWAFWETKKNLTR, and ELRTFSILNRKAIDF, with high affinity for MHC class II molecules predicted by IEDB-recommended. Preliminary safety observation denied histological alterations. DNA vaccine candidate pVAX-LAMP/GPEBO shows promise against Ebola epidemic and further evaluation is guaranteed.

Investigating the Interaction between Negative Strand RNA Viruses and Their Hosts for Enhanced Vaccine Development and Production

The current pandemic has highlighted the ever-increasing risk of human to human spread of zoonotic pathogens. A number of medically-relevant zoonotic pathogens are negative-strand RNA viruses (NSVs). NSVs are derived from different virus families. Examples like Ebola are known for causing severe symptoms and high mortality rates. Some, like influenza, are known for their ease of person-to-person transmission and lack of pre-existing immunity, enabling rapid spread across many countries around the globe. Containment of outbreaks of NSVs can be difficult owing to their unpredictability and the absence of effective control measures, such as vaccines and antiviral therapeutics. In addition, there remains a lack of essential knowledge of the host–pathogen response that are induced by NSVs, particularly of the immune responses that provide protection. Vaccines are the most effective method for preventing infectious diseases. In fact, in the event of a pandemic, appropriate vaccine design and speed of vaccine supply is the most critical factor in protecting the population, as vaccination is the only sustainable defense. Vaccines need to be safe, efficient, and cost-effective, which is influenced by our understanding of the host–pathogen interface. Additionally, some of the major challenges of vaccines are the establishment of a long-lasting immunity offering cross protection to emerging strains. Although many NSVs are controlled through immunisations, for some, vaccine design has failed or efficacy has proven unreliable. The key behind designing a successful vaccine is understanding the host–pathogen interaction and the host immune response towards NSVs. In this paper, we review the recent research in vaccine design against NSVs and explore the immune responses induced by these viruses. The generation of a robust and integrated approach to development capability and vaccine manufacture can collaboratively support the management of outbreaking NSV disease health risks.

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.

Increased Pathogenicity and Virulence of Middle East Respiratory Syndrome Coronavirus Clade B In Vitro and In Vivo

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe acute respiratory disease in humans. MERS-CoV strains from early epidemic clade A and contemporary epidemic clade B have not been phenotypically characterized to compare their abilities to infect cells and mice. We isolated the clade B MERS-CoV ChinaGD01 strain from a patient infected during the South Korean MERS outbreak in 2015 and compared the phylogenetics and pathogenicity of MERS-CoV EMC/2012 (clade A) and ChinaGD01 (clade B) in vitro and in vivo Genome alignment analysis showed that most clade-specific mutations occurred in the orf1ab gene, including mutations that were predicted to be potential glycosylation sites. Minor differences in viral growth but no significant differences in plaque size or sensitivity to beta interferon (IFN-β) were detected between these two viruses in vitro ChinaGD01 virus infection induced more weight loss and inflammatory cytokine production in human DPP4-transduced mice. Viral titers were higher in the lungs of ChinaGD01-infected mice than with EMC/2012 infection. Decreased virus-specific CD4⁺ and CD8⁺ T cell numbers were detected in the lungs of ChinaGD01-infected mice. In conclusion, MERS-CoV evolution induced changes to reshape its pathogenicity and virulence in vitro and in vivo and to evade adaptive immune response to hinder viral clearance.IMPORTANCE MERS-CoV is an important emerging pathogen and causes severe respiratory infection in humans. MERS-CoV strains from early epidemic clade A and contemporary epidemic clade B have not been phenotypically characterized to compare their abilities to infect cells and mice. In this study, we showed that a clade B virus ChinaGD01 strain caused more severe disease in mice, with delayed viral clearance, increased inflammatory cytokines, and decreased antiviral T cell responses, than the early clade A virus EMC/2012. Given the differences in pathogenicity of different clades of MERS-CoV, periodic assessment of currently circulating MERS-CoV is needed to monitor potential severity of zoonotic disease.

Release of Immunomodulatory Ebola Virus Glycoprotein-Containing Microvesicles Is Suppressed by Tetherin in a Species-Specific Manner

The Ebola virus glycoprotein (EBOV-GP) forms GP-containing microvesicles, so-called virosomes, which are secreted from GP-expressing cells. However, determinants of GP-virosome release and their functionality are poorly understood. We characterized GP-mediated virosome formation and delineated the role of the antiviral factor tetherin (BST2, CD317) in this process. Residues in the EBOV-GP receptor-binding domain (RBD) promote GP-virosome secretion, while tetherin suppresses GP-virosomes by interactions involving the GP-transmembrane domain. Tetherin from multiple species interfered with GP-virosome release, and tetherin from the natural fruit bat reservoir showed the highest inhibitory activity. Moreover, analyses of GP from various ebolavirus strains, including the EBOV responsible for the West African epidemic, revealed the most efficient GP-virosome formation by highly pathogenic ebolaviruses. Finally, EBOV-GP-virosomes were immunomodulatory and acted as decoys for EBOV-neutralizing antibodies. Our results indicate that GP-virosome formation might be a determinant of EBOV immune evasion and pathogenicity, which is suppressed by tetherin.

Ebola virus VP35 has novel NTPase and helicase-like activities

Ebola virus (EBOV) is a non-segmented, negative-sense RNA virus (NNSV) in the family Filoviridae, and is recognized as one of the most lethal pathogens in the planet. For RNA viruses, cellular or virus-encoded RNA helicases play pivotal roles in viral life cycles by remodelling viral RNA structures and/or unwinding viral dsRNA produced during replication. However, no helicase or helicase-like activity has ever been found to associate with any NNSV-encoded proteins, and it is unknown whether the replication of NNSVs requires the participation of any viral or cellular helicase. Here, we show that despite of containing no conserved NTPase/helicase motifs, EBOV VP35 possesses the NTPase and helicase-like activities that can hydrolyse all types of NTPs and unwind RNA helices in an NTP-dependent manner, respectively. Moreover, guanidine hydrochloride, an FDA-approved compound and inhibitor of certain viral helicases, inhibited the NTPase and helicase-like activities of VP35 as well as the replication/transcription of an EBOV minigenome replicon in cells, highlighting the importance of VP35 helicase-like activity during EBOV life cycle. Together, our findings provide the first demonstration of the NTPase/helicase-like activity encoded by EBOV, and would foster our understanding of EBOV and NNSVs.

Dendritic Cells/Macrophages-Targeting Feature of Ebola Glycoprotein and its Potential as Immunological Facilitator for Antiviral Vaccine Approach

In the prevention of epidemic and pandemic viral infection, the use of the antiviral vaccine has been the most successful biotechnological and biomedical approach. In recent times, vaccine development studies have focused on recruiting and targeting immunogens to dendritic cells (DCs) and macrophages to induce innate and adaptive immune responses. Interestingly, Ebola virus (EBOV) glycoprotein (GP) has a strong binding affinity with DCs and macrophages. Shreds of evidence have also shown that the interaction between EBOV GP with DCs and macrophages leads to massive recruitment of DCs and macrophages capable of regulating innate and adaptive immune responses. Therefore, studies for the development of vaccine can utilize the affinity between EBOV GP and DCs/macrophages as a novel immunological approach to induce both innate and acquired immune responses. In this review, we will discuss the unique features of EBOV GP to target the DC, and its potential to elicit strong immune responses while targeting DCs/macrophages. This review hopes to suggest and stimulate thoughts of developing a stronger and effective DC-targeting vaccine for diverse virus infection using EBOV GP.

A versatile platform technology for recombinant vaccines using non-propagative human parainfluenza virus type 2 vector

Ectopic protein with proper steric structure was efficiently loaded onto the envelope of the F gene-defective BC-PIV vector derived from human parainfluenza virus type 2 (hPIV2) by a reverse genetics method of recombinant virus production. Further, ectopic antigenic peptide was successfully loaded either outside, inside, or at both sides of the envelope of the vector. The BC-PIV vector harboring the Ebola virus GP gene was able to elicit neutralizing antibodies in mice. In addition, BC-PIV with antigenic epitopes of both melanoma gp100 and WT1 tumor antigen induced a CD8+ T-cell-mediated response in tumor-transplanted syngeneic mice. Considering the low pathogenicity and recurrent infections of parental hPIV2, BC-PIV can be used as a versatile vector with high safety for recombinant vaccine development, addressing unmet medical needs.

Investigation of Outbreak-Specific Nonsynonymous Mutations on Ebolavirus GP in the Context of Known Immune Reactivity

The global response to the most recent EBOV outbreak has led to increased generation and availability of data, which can be globally analyzed to increase our understanding of immune responses to EBOV. We analyzed the published antibody epitope data to identify regions immunogenic for humans on the main GP antigenic target and determine sequence variance/nonsynonymous mutations between historical isolates and variants from the 2013-2016 outbreak. Approximately half of the GP sequence has been reported as targeted by antibody responses. Our results show an enrichment of nonsynonymous mutations (NSMs) within epitopic regions on GP (70%, p = 0.0133). Mapping NSMs to human epitope reactivity may be useful for future therapeutic and prophylaxis development as well as for our general understanding of immunity against EBOV.

A Critical Domain of Ebolavirus Envelope Glycoprotein Determines Glycoform and Infectivity

Ebolaviruses comprises 5 species that exert varying degrees of mortality/infectivity in humans with Reston ebolaviruses (REBOV) showing the lowest and Zaire ebolaviruses (ZEBOV) showing the highest. However, the molecular basis of this differential mortality/infectivity remains unclear. Here, we report that the structural features of ebolavirus envelope glycoproteins (GPs) and one of their counter receptors, macrophage galactose-type calcium-type lectin (MGL/CD301), play crucial roles in determining viral infectivity. The low infectivity of REBOV mediated by the interaction between GPs and MGL/CD301 dramatically increased when the N-terminal 18 amino acids (33rd through 50th) of GPs were replaced with that of ZEBOV. Furthermore, structural analysis of glycans of GPs revealed that N-glycans were more extended in REBOV than in ZEBOV. N-glycan extension was reversed by the replacement of aforementioned N-terminal 18 amino acid residues. Therefore, these data strongly suggest that extended N-glycans on GPs reduce MGL/CD301-mediated viral infectivity by hindering the interaction between GPs and MGL/CD301 preferentially binds O-glycans.

West Africa 2013 Ebola: From Virus Outbreak to Humanitarian Crisis

The 2013 outbreak of Ebola virus disease (EVD) in West Africa constituted a major humanitarian crisis. The outbreak numbered over 28,500 cases, more than 10 times the number cumulatively registered from all previous EVD outbreaks combined, with at least 11,000 deaths, and resulted in billions of dollars of lost economic growth to an already impoverished region. The unprecedented scale of West Africa 2013 took the world by surprise and laid bare deficiencies in our response capacity to complex humanitarian disasters of highly infectious and lethal pathogens. However, the magnitude of West Africa 2013 also provided a unique opportunity and obligation to better understand not only the biology and epidemiology of EVD, but also the many scientific, economic, social, political, ethical, and logistical challenges in confronting emerging infectious diseases in the modern era.