Zaire Ebolavirus Glycoprotein Research Articles

Are we any closer to combating Ebola infections?

Are we any closer to combating Ebola infections?

Ebolavirus Glycoprotein Structure and Mechanism of Entry

Ebolavirus (EBOV) is a highly virulent pathogen capable of causing a severe hemorrhagic fever with 50-90% lethality. The EBOV glycoprotein (GP) is the only virally expressed protein on the virion surface and is critical for attachment to host cells and catalysis of membrane fusion. Hence, the EBOV GP is a critical component of vaccines as well as a target of neutralizing antibodies and inhibitors of attachment and fusion. The crystal structure of the Zaire ebolavirus GP in its trimeric, prefusion conformation (3 GP(1) plus 3 GP(2)) in complex with a neutralizing antibody fragment, derived from a human survivor of the 1995 Kikwit outbreak, was recently determined. This is the first near-complete structure of any filovirus glycoprotein. The overall molecular architecture of the Zaire ebolavirus GP and its role in viral entry and membrane fusion are discussed in this article.

Vesicular Stomatitis Virus-Based Ebola Vaccine Is Well-Tolerated and Protects Immunocompromised Nonhuman Primates

Ebola virus (EBOV) is a significant human pathogen that presents a public health concern as an emerging/re-emerging virus and as a potential biological weapon. Substantial progress has been made over the last decade in developing candidate preventive vaccines that can protect nonhuman primates against EBOV. Among these prospects, a vaccine based on recombinant vesicular stomatitis virus (VSV) is particularly robust, as it can also confer protection when administered as a postexposure treatment. A concern that has been raised regarding the replication-competent VSV vectors that express EBOV glycoproteins is how these vectors would be tolerated by individuals with altered or compromised immune systems such as patients infected with HIV. This is especially important as all EBOV outbreaks to date have occurred in areas of Central and Western Africa with high HIV incidence rates in the population. In order to address this concern, we evaluated the safety of the recombinant VSV vector expressing the Zaire ebolavirus glycoprotein (VSVΔG/ZEBOVGP) in six rhesus macaques infected with simian-human immunodeficiency virus (SHIV). All six animals showed no evidence of illness associated with the VSVΔG/ZEBOVGP vaccine, suggesting that this vaccine may be safe in immunocompromised populations. While one goal of the study was to evaluate the safety of the candidate vaccine platform, it was also of interest to determine if altered immune status would affect vaccine efficacy. The vaccine protected 4 of 6 SHIV-infected macaques from death following ZEBOV challenge. Evaluation of CD4+ T cells in all animals showed that the animals that succumbed to lethal ZEBOV challenge had the lowest CD4+ counts, suggesting that CD4+ T cells may play a role in mediating protection against ZEBOV.

Vesicular stomatitis virus-based vaccines protect nonhuman primates against aerosol challenge with Ebola and Marburg viruses

Considerable progress has been made over the last decade in developing candidate preventive vaccines that can protect nonhuman primates against Ebola and Marburg viruses. A vaccine based on recombinant vesicular stomatitis virus (VSV) seems to be particularly robust as it can also confer protection when administered as a postexposure treatment. While filoviruses are not thought to be transmitted by aerosol in nature the inhalation route is among the most likely portals of entry in the setting of a bioterrorist event. At present, all candidate filoviral vaccines have been evaluated against parenteral challenges but none have been tested against an aerosol exposure. Here, we evaluated our recombinant VSV-based Zaire ebolavirus (ZEBOV) and Marburg virus (MARV) vaccines against aerosol challenge in cynomolgus macaques. All monkeys vaccinated with a VSV vector expressing the glycoprotein of ZEBOV were completely protected against an aerosol exposure of ZEBOV. Likewise, all monkeys vaccinated with a VSV vector expressing the glycoprotein of MARV were completely protected against an aerosol exposure of MARV. All control animals challenged by the aerosol route with either ZEBOV or MARV succumbed. Interestingly, disease in control animals appeared to progress slower than previously seen in macaques exposed to comparable doses by intramuscular injection.

Epitopes Required for Antibody‐Dependent Enhancement of Ebola Virus Infection

We have shown that antibody-dependent enhancement (ADE) of infection with Zaire Ebola virus (ZEBOV) is mediated by interaction of virus-specific antibodies with Fc receptors or complement component C1q and its receptors in vitro. ADE activities of the antisera to the viral glycoprotein (GP) were virus species specific and were primarily correlated with immunoglobulin (Ig) G2a and IgM levels but not with IgG1 levels. Interestingly, compared with ZEBOV, Reston Ebola virus (REBOV) had substantially weaker potential to induce ADE antibodies. Using monoclonal antibodies, we identified ZEBOV-specific ADE epitopes. To confirm epitope specificity, we constructed a chimeric ZEBOV GP, the ADE epitopes of which were replaced with the corresponding regions of REBOV GP. We found that mouse antisera to the chimeric ZEBOV GP showed less potential to induce ADE activity than did mouse antisera to wild-type ZEBOV GP, although they retained neutralizing activity. These data suggest that GP lacking the ADE-inducing epitopes may increase the potential of GP as a vaccine antigen.

Mucosal Delivery of Adenovirus‐Based Vaccine Protects against Ebola Virus Infection in Mice

Mucosal vaccination can offer several advantages over conventional intramuscular immunization to protect against Ebola virus (EBOV) infection, such as immune protection at sites of viral entry into susceptible individuals, and can be administered using needle-free devices. The present study evaluated oral and nasal vaccination of mice with human adenovirus serotype 5 (Ad) expressing the Zaire ebolavirus glycoprotein (Ad-ZGP) in terms of their protection against and underlying immune responses to EBOV. Similar to intramuscular administration, oral or nasal vaccination of mice with Ad-ZGP fully protected the mice against a lethal challenge with mouse-adapted EBOV. Both T and B cell responses developed in mice receiving oral or nasal vaccination in different body compartments, indicating qualitative improvement of the immune response after mucosal immunization, compared with intramuscular vaccination. Overall, the breadth of the immune response noted after nasal or oral immunization, including stimulation of CD8+ T cells or effector memory T cells from the gastrointestinal tract or the lungs, was superior to that noted after intramuscular administration of the vaccine. The present study showed that adenovirus-based vaccine is effective against EBOV infection in mice after oral and nasal immunization.

Identification of two amino acid residues on Ebola virus glycoprotein 1 critical for cell entry

Using site-directed mutagenesis and retroviral vector pseudotyping of the wild type or mutated glycoprotein of Zaire ebolavirus (ZEBOV), we analyzed 15 conserved residues in the N-terminus of the filovirus glycoprotein 1 (GP1) in order to identify residues critical for cell entry. Results from infectivity assays and Western blot analyses identified two phenylalanine residues at positions 88 and 159 that appear to be critical for ZEBOV entry in vitro. We extended this observation by introduction of alanines at either position 88 or 159 of Ivory Coast Ebolavirus (CIEBOV) and observed the same phenotype. Further, we showed that introduction of each of the two mutations in a recombinant full-length clone of ZEBOV (Mayinga strain) that also carried the coding sequence for GFP could not be rescued, suggesting the mutants rendered the virus non-infectious. The two phenylalanines that are critical for both ZEBOV and CIEBOV entry are found in two linear domains of GP1 that are highly conserved among filoviruses, and thus could provide a target for rational development of broadly cross-protective vaccines or antiviral therapies.

Chimpanzee adenovirus vaccine protects against Zaire Ebola virus

This study evaluated the use of a chimpanzee-based adenovirus vaccine in mouse and Guinea pigs models of Zaire Ebola virus (ZEBOV) infection. Vaccine vector expressing the envelope glycoprotein of ZEBOV was created from the molecular clone of chimpanzee adenovirus pan7 (AdC7). AdC7 vaccine stimulated robust T and B cell responses to ZEBOV in naïve mice inducing complete protection to an otherwise lethal challenge of ZEBOV. Complete protection to Zaire Ebola virus was also observed in Guinea pigs vaccinated with a relatively low dose of AdC7 (5 × 109/kg). Pre-existing immunity to AdHu5 was generated in mice following pre-exposure to AdHu5 or administration of pooled human immune globulin. Pre-existing immunity to human adenoviruses severely compromised the efficacy of the human AdHu5 vaccine but not the chimpanzee AdC7 vaccine. These results validate further development of Chimpanzee-based vaccine and highlight the impact of pre-existing immunity to the vaccine carrier.

141. Adenovirus Vaccine to Ebola Virus: Mechanism of Protection and Impact of Pre-Existing Immunity to the Vaccine Carrier

A number of viral pathogens could be used as agents for biological warfare or bioterrorism. The Zaire strain of Ebola virus, with mortality rate of up to 90%, often with death occurring in less than 20 days after infection, represents one of the most dangerous viral pathogen. The profile of Ebola virus infections that makes it such a dangerous pathogen also contributes to its utility as a potential agent of biological warfare. Development of an efficacious vaccine is thus considered a high priority in spite of the limited impact of Ebola virus on human morbidity and mortality world-wide. This study evaluated the use of human and simian-based adenovirus vaccines in mouse models of Zaire Ebola virus (ZEBOV) infection. E1-deleted vaccine vectors expressing the envelope glycoprotein of ZEBOV were created from molecular clones of human adenovirus serotype 5 (AdHu5) and chimpanzee adenovirus pan7 (AdC7). Both vaccines stimulated robust T and B cell responses to ZEBOV in na|[iuml]|ve mice after a single intramuscular administration, inducing complete protection to an otherwise lethal challenge of mouse-adapted ZEBOV. The use of different knockout mice revealed that protection against challenge at one month post-vaccination is mainly mediated by antibodies with likely involvement of NK and CD4+ T cells. Preliminary data suggest that the T-cell response plays an important role for protection of vaccinated mice early after immunization (e.g. day 10). Pre-existing immunity to AdHu5, which is found in a high percentage of humans, was generated in mice following pre-exposure to AdHu5 or administration of pooled human immune globulin. Pre-existing immunity to human adenoviruses severely compromised the efficacy of the human AdHu5 vaccine but not the simian AdC7 vaccine. These results highlight the impact of pre-existing immunity to the vaccine carrier and define protection mechanism of adenovirus vaccine vector to Ebola virus in mice.