Live attenuated recombinant vaccine protects nonhuman primates against Ebola and Marburg viruses

Vesicular stomatitis virus genomic RNA persists in vivo in the absence of viral replication

Our previous studies using intranasal inoculation of mice with vesicular stomatitis virus (VSV) vaccine vectors showed persistence of vector genomic RNA (gRNA) for at least 60 days in lymph nodes in the absence of detectable infectious virus. Here we show high-level concentration of virus and gRNA in lymph nodes after intramuscular inoculation of mice with attenuated or single-cycle VSV vectors as well as long-term persistence of gRNA in the lymph nodes. To determine if the persistence of gRNA was due to ongoing viral replication, we developed a tagged-primer approach that was critical for detection of VSV mRNA specifically. Our results show that VSV gRNA persists long-term in the lymph nodes while VSV mRNA is present only transiently. Because VSV transcription is required for replication, our results indicate that persistence of gRNA does not result from continuing viral replication. We also performed macrophage depletion studies that are consistent with initial trapping of VSV gRNA largely in lymph node macrophages and subsequent persistence elsewhere in the lymph node.

Refined methods for propagating vesicular stomatitis virus vectors that are defective for G protein expression

Propagation-defective vesicular stomatitis virus (VSV) vectors that encode a truncated G protein (VSV-Gstem) or lack the G gene entirely (VSV-DeltaG) are attractive vaccine vectors because they are immunogenic, cannot replicate and spread after vaccination, and do not express many of the epitopes that elicit neutralizing anti-VSV immunity. To consider advancing non-propagating VSV vectors towards clinical assessment, scalable technology that is compliant with human vaccine manufacturing must be developed to produce clinical trial material. Accordingly, two propagation methods were developed for VSV-Gstem and VSV-DeltaG vectors encoding HIV gag that have the potential to support large-scale production. One method is based on transient expression of G protein after electroporating plasmid DNA into Vero cells and the second is based on a stable Vero cell line that contains a G gene controlled by a heat shock-inducible transcription unit. Both methods reproducibly supported production of 1 x 10(7) to 1 x 10(8) infectious units (I.U.s) of vaccine vector per milliliter. Results from these studies also showed that optimization of the G gene is necessary for abundant G protein expression from electroporated plasmid DNA or from DNA integrated in the genome of a stable cell line, and that the titers of VSV-Gstem vectors generally exceeded VSV-DeltaG.

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.

Peripheral immunization blocks lethal actions of vesicular stomatitis virus within the brain

Vesicular stomatitis virus (VSV) is the prototype virus for 75 or more negative-strand RNA viruses in the rhabdovirus family. Some of these viruses, including VSV, can cause neurological impairment or death upon brain infection. VSV has shown promise in the prevention and treatment of disease as a vaccine vector and an oncolytic virus, but infection of the brain remains a concern. Three VSV variants, the wild-type-related VSV-G/GFP and two attenuated viruses, VSV-CT1 and VSV-CT9-M51, were compared for neuroinvasiveness and neuromorbidity. In nonimmunized mice, direct VSV-G/GFP injection into the brain invariably resulted in lethal encephalitis; in contrast, partial survival was seen after direct injection of the attenuated VSV strains. In addition, both attenuated VSV strains showed significantly reduced neuroinvasiveness after intranasal inoculation of young postnatal day 16 mice. Of the three tested variants, VSV-CT9-M51 generated the lowest degree of neuropathology. Despite its attenuated state, peripheral inoculations of VSV-CT9-M51 targeted and killed human glioblastoma implanted into the mouse brain. Importantly, we show here that intranasal or intramuscular immunization prevents the lethal effects of subsequent VSV-G/GFP, VSV-CT1, and VSV-CT9-M51 injections into the brain. These results indicate that attenuated recombinant viruses show reduced neurovirulence and that peripheral immunization blocks the lethal actions of all VSVs tested.

A catalytically and genetically optimized beta-lactamase-matrix based assay for sensitive, specific, and higher throughput analysis of native henipavirus entry characteristics

Nipah virus (NiV) and Hendra virus (HeV) are the only paramyxoviruses requiring Biosafety Level 4 (BSL-4) containment. Thus, study of henipavirus entry at less than BSL-4 conditions necessitates the use of cell-cell fusion or pseudotyped reporter virus assays. Yet, these surrogate assays may not fully emulate the biological properties unique to the virus being studied. Thus, we developed a henipaviral entry assay based on a β-lactamase-Nipah Matrix (βla-M) fusion protein. We first codon-optimized the bacterial βla and the NiV-M genes to ensure efficient expression in mammalian cells. The βla-M construct was able to bud and form virus-like particles (VLPs) that morphologically resembled paramyxoviruses. βla-M efficiently incorporated both NiV and HeV fusion and attachment glycoproteins. Entry of these VLPs was detected by cytosolic delivery of βla-M, resulting in enzymatic and fluorescent conversion of the pre-loaded CCF2-AM substrate. Soluble henipavirus receptors (ephrinB2) or antibodies against the F and/or G proteins blocked VLP entry. Additionally, a Y105W mutation engineered into the catalytic site of βla increased the sensitivity of our βla-M based infection assays by 2-fold. In toto, these methods will provide a more biologically relevant assay for studying henipavirus entry at less than BSL-4 conditions.

Prime-boost vaccination with recombinant mumps virus and recombinant vesicular stomatitis virus vectors elicits an enhanced human immunodeficiency virus type 1 Gag-specific cellular immune response in rhesus macaques

Intramuscular inoculation of rhesus macaques with one or more doses of recombinant vesicular stomatitis virus (rVSV) expressing human immunodeficiency virus type 1 (HIV-1) Gag (rVSVgag) typically elicits peak cellular immune responses of 500 to 1,000 gamma interferon (IFN-gamma) enzyme-linked immunospots (ELISPOTS)/10(6) peripheral blood lymphocytes (PBL). Here, we describe the generation of a novel recombinant mumps virus (rMuV) expressing HIV-1 Gag (rMuVgag) and measure the Gag-specific cellular immune responses detected in rhesus macaques following vaccination with a highly attenuated form of rVSV expressing HIV-1 Gag (rVSVN4CT1gag1) and rMuVgag in various prime-boost combinations. Notably, peak Gag-specific cellular immune responses of 3,000 to 3,500 ELISPOTS/10(6) PBL were detected in macaques that were primed with rMuVgag and boosted with rVSVN4CT1gag1. Lower peak cellular immune responses were detected in macaques that were primed with rVSVN4CT1gag1 and boosted with rMuVgag, although longer-term gag-specific responses appeared to remain higher in this group of macaques. These findings indicate that rMuVgag may significantly enhance Gag-specific cellular immune responses when administered with rVSVN4CT1gag1 in heterologous prime-boost regimens.

Fibroblastic reticular cells and their role in viral hemorrhagic fevers

Viral hemorrhagic fevers (VHFs) caused by Ebola, Marburg and Lassa viruses often manifest as multiple organ dysfunction and hemorrhagic shock with high mortality. These viruses target numerous cell types, including monocytes and dendritic cells, which are primary early targets that mediate critical pathogenetic processes. This review focuses on fibroblastic reticular cells (FRCs), another prevalent infected cell type that is known as a key regulator of circulatory and immune functions. Viral infection of FRCs could have debilitating effects in secondary lymphoid organs and various other tissues. FRCs may also contribute to the spread of these deadly viruses throughout the body. Here, we review the salient features of these VHFs and the biology of FRCs, emphasizing the potential role of these cells in VHFs and the rapid deterioration of immune and hemovascular sytems that are characteristic of such acute infections.

Mucosal immunization of cynomolgus macaques with the VSVDeltaG/ZEBOVGP vaccine stimulates strong ebola GP-specific immune responses

BACKGROUND

Zaire ebolavirus (ZEBOV) produces a lethal viral hemorrhagic fever in humans and non-human primates.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate that the VSVDeltaG/ZEBOVGP vaccine given 28 days pre-challenge either intranasally (IN), orally (OR), or intramuscularly (IM) protects non-human primates against a lethal systemic challenge of ZEBOV, and induces cellular and humoral immune responses. We demonstrated that ZEBOVGP-specific T-cell and humoral responses induced in the IN and OR groups, following an immunization and challenge, produced the most IFN-gamma and IL-2 secreting cells, and long term memory responses.

CONCLUSIONS/SIGNIFICANCE

We have shown conclusively that mucosal immunization can protect from systemic ZEBOV challenge and that mucosal delivery, particularly IN immunization, seems to be more potent than IM injection in the immune parameters we have tested. Mucosal immunization would be a huge benefit in any emergency mass vaccination campaign during a natural outbreak, or following intentional release, or for mucosal immunization of great apes in the wild.

Single-injection vaccine protects nonhuman primates against infection with marburg virus and three species of ebola virus

The filoviruses Marburg virus and Ebola virus cause severe hemorrhagic fever with high mortality in humans and nonhuman primates. Among the most promising filovirus vaccines under development is a system based on recombinant vesicular stomatitis virus (VSV) that expresses a single filovirus glycoprotein (GP) in place of the VSV glycoprotein (G). Here, we performed a proof-of-concept study in order to determine the potential of having one single-injection vaccine capable of protecting nonhuman primates against Sudan ebolavirus (SEBOV), Zaire ebolavirus (ZEBOV), Cote d'Ivoire ebolavirus (CIEBOV), and Marburgvirus (MARV). In this study, 11 cynomolgus monkeys were vaccinated with a blended vaccine consisting of equal parts of the vaccine vectors VSVDeltaG/SEBOVGP, VSVDeltaG/ZEBOVGP, and VSVDeltaG/MARVGP. Four weeks later, three of these animals were challenged with MARV, three with CIEBOV, three with ZEBOV, and two with SEBOV. Three control animals were vaccinated with VSV vectors encoding a nonfilovirus GP and challenged with SEBOV, ZEBOV, and MARV, respectively, and five unvaccinated control animals were challenged with CIEBOV. Importantly, none of the macaques vaccinated with the blended vaccine succumbed to a filovirus challenge. As expected, an experimental control animal vaccinated with VSVDeltaG/ZEBOVGP and challenged with SEBOV succumbed, as did the positive controls challenged with SEBOV, ZEBOV, and MARV, respectively. All five control animals challenged with CIEBOV became severely ill, and three of the animals succumbed on days 12, 12, and 14, respectively. The two animals that survived CIEBOV infection were protected from subsequent challenge with either SEBOV or ZEBOV, suggesting that immunity to CIEBOV may be protective against other species of Ebola virus. In conclusion, we developed an immunization scheme based on a single-injection vaccine that protects nonhuman primates against lethal challenge with representative strains of all human pathogenic filovirus species.

Correlates of protective immunity for Ebola vaccines: implications for regulatory approval by the animal rule

Ebola is a rare but deadly disease caused by a filovirus for which no treatment or vaccine is available. Vaccine development for such diseases is hampered by the inability to test vaccines in volunteers or a target population.

The Food and Drug Administration (FDA) has instituted the 'animal rule' to allow approval of vaccines and drugs that cannot be tested in humans based on immune correlates in animal models.

In the case of Ebola, the relevant animal models are non-human primates and mice.

For Ebola, immune correlates consist of immunoglobulin G responses, although other factors, such as T cells, are also likely to be important in a successful immune response.

Vaccine candidates for Ebola vaccine include the glycoprotein and nucleocapsid proteins.

Initial testing of Ebola vaccines has shown a protective effect in non-human primates and positive antibody titres in humans.

For diseases that are both rare and lethal, it is difficult to test vaccines. Sullivan and colleagues describe the use of immune correlates and the animal rule for the licensing of Ebola vaccines.

Ebola virus infection is a highly lethal disease for which there are no effective therapeutic or preventive treatments. Several vaccines have provided immune protection in laboratory animals, but because outbreaks occur unpredictably and sporadically, vaccine efficacy cannot be proven in human trials, which is required for traditional regulatory approval. The Food and Drug Administration has introduced the 'animal rule', to allow laboratory animal data to be used to show efficacy when human trials are not logistically feasible. In this Review, we describe immune correlates of vaccine protection against Ebola virus in animals. This research provides a basis for bridging the gap from basic research to human vaccine responses in support of the licensing of vaccines through the animal rule.

Replication-deficient ebolavirus as a vaccine candidate

Ebolavirus causes severe hemorrhagic fever, with case fatality rates as high as 90%. Currently, no licensed vaccine is available against Ebolavirus. We previously generated a replication-deficient, biologically contained Ebolavirus, EbolaDeltaVP30, which lacks the essential VP30 gene, grows only in cells stably expressing this gene product, and is genetically stable. Here, we evaluated the vaccine potential of EbolaDeltaVP30. First, we demonstrated its safety in STAT-1-knockout mice, a susceptible animal model for Ebolavirus infection. We then tested its protective efficacy in two animal models, mice and guinea pigs. Mice immunized twice with EbolaDeltaVP30 were protected from a lethal infection of mouse-adapted Ebolavirus. Virus titers in the serum of vaccinated mice were significantly lower than those in nonvaccinated mice. Protection of mice immunized with EbolaDeltaVP30 was associated with a high antibody response to the Ebolavirus glycoprotein and the generation of an Ebolavirus NP-specific CD8(+) T-cell response. Guinea pigs immunized twice with EbolaDeltaVP30 were also protected from a lethal infection of guinea pig-adapted Ebolavirus. Our study demonstrates the potential of the EbolaDeltaVP30 virus as a new vaccine platform.

Chimeric human parainfluenza virus bearing the Ebola virus glycoprotein as the sole surface protein is immunogenic and highly protective against Ebola virus challenge

We generated a new live-attenuated vaccine against Ebola virus (EBOV) based on a chimeric virus HPIV3/DeltaF-HN/EboGP that contains the EBOV glycoprotein (GP) as the sole transmembrane envelope protein combined with the internal proteins of human parainfluenza virus type 3 (HPIV3). Electron microscopy analysis of the virus particles showed that they have an envelope and surface spikes resembling those of EBOV and a particle size and shape resembling those of HPIV3. When HPIV3/DeltaF-HN/EboGP was inoculated via apical surface of an in vitro model of human ciliated airway epithelium, the virus was released from the apical surface; when applied to basolateral surface, the virus infected basolateral cells but did not spread through the tissue. Following intranasal (IN) inoculation of guinea pigs, scattered infected cells were detected in the lungs by immunohistochemistry, but infectious HPIV3/DeltaF-HN/EboGP could not be recovered from the lungs, blood, or other tissues. Despite the attenuation, the virus was highly immunogenic, and a single IN dose completely protected the animals against a highly lethal intraperitoneal challenge of guinea pig-adapted EBOV.

Recombinant vesicular stomatitis virus-based west Nile vaccine elicits strong humoral and cellular immune responses and protects mice against lethal challenge with the virulent west Nile virus strain LSU-AR01

Vesicular stomatitis virus (VSV) has been extensively utilized as a viral vector system for the induction of protective immune responses against a variety of pathogens. We constructed recombinant VSVs specifying either the Indiana or Chandipura virus G glycoprotein and expressing the West Nile virus (WNV) envelope (E) glycoprotein. Mice were intranasally vaccinated using a prime (Indiana)-boost (Chandipura) immunization approach and challenged with the virulent WNV-LSU-AR01. Ninety-percent (9 of 10) of the vaccinated mice survived as compared to 10% of the mock-vaccinated mice after WNV lethal challenge. Histopathological examination of brain tissues revealed neuronal necrosis in mock-vaccinated mice but not in vaccinated mice, and vaccinated, but not mock-vaccinated mice developed a strong neutralizing antibody response against WNV. Extensive immunological analysis using polychromatic flow cytometry staining revealed that vaccinated, but not mock-vaccinated mice developed robust cellular immune responses as evidenced by up-regulation of CD4⁺ CD154⁺ IFNγ⁺ T cells in vaccinated, but not mock-vaccinated mice. Similarly, vaccinated mice developed robust E-glycoprotein-specific CD8⁺ T cell immune responses as evidenced by the presence of a high percentage of CD8⁺ CD62L^low IFNγ⁺ cells. In addition, a sizeable population of CD8⁺ CD69⁺ cells was detected indicating E-specific activation of mature T cells and CD4⁺ CD25⁺ CD127^low T regulatory (T reg) cells were down-regulated. These results suggest that VSV-vectored vaccines administered intranasally can efficiently induce protective humoral and cellular immune responses against WNV infections.

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.

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.

Ebola virus is among the most lethal microbes known to man, with case fatality rates often exceeding 80%. Since its discovery in 1976, outbreaks have been sporadic and geographically restricted, primarily to areas of Central Africa. However, concern about the natural or unnatural introduction of Ebola outside of the endemic areas has dramatically increased both research interest and public awareness. A number of candidate vaccines have been developed to combat Ebola virus, and these vaccines have shown varying degrees of success in nonhuman primate models. Safety is a significant concern for any vaccine and in particular for vaccines that replicate in the host. Here, we evaluated the safety of our replication-competent vesicular stomatitus virus (VSV)-based Ebola vaccine in SHIV-infected rhesus monkeys. We found that the vaccine caused no evidence of overt illness in any of these immunocompromised animals. We also demonstrated that this vaccine partially protected the SHIV-infected monkeys against a lethal Ebola challenge and that there appears to be an association with levels of CD4+ lymphocytes and survival. Our study suggests that the VSV-based Ebola vaccine will be safe in immunocompromised populations and supports further study and development of this promising vaccine platform for its use in humans.

Newly discovered ebola virus associated with hemorrhagic fever outbreak in Uganda

Over the past 30 years, Zaire and Sudan ebolaviruses have been responsible for large hemorrhagic fever (HF) outbreaks with case fatalities ranging from 53% to 90%, while a third species, Côte d'Ivoire ebolavirus, caused a single non-fatal HF case. In November 2007, HF cases were reported in Bundibugyo District, Western Uganda. Laboratory investigation of the initial 29 suspect-case blood specimens by classic methods (antigen capture, IgM and IgG ELISA) and a recently developed random-primed pyrosequencing approach quickly identified this to be an Ebola HF outbreak associated with a newly discovered ebolavirus species (Bundibugyo ebolavirus) distantly related to the Côte d'Ivoire ebolavirus found in western Africa. Due to the sequence divergence of this new virus relative to all previously recognized ebolaviruses, these findings have important implications for design of future diagnostic assays to monitor Ebola HF disease in humans and animals, and ongoing efforts to develop effective antivirals and vaccines.

In this report we describe a newly discovered ebolavirus species which caused a large hemorrhagic fever outbreak in western Uganda. The virus is genetically distinct, differing by more than 30% at the genome level from all other known ebolavirus species. The unique nature of this virus created challenges for traditional filovirus molecular based diagnostic assays and genome sequencing approaches. Instead, we quickly determined over 70% of the virus genome using a recently developed random-primed pyrosequencing approach that allowed the rapid development of a molecular detection assay that was deployed in the disease outbreak response. This draft sequence allowed easy completion of the whole genome sequence using a traditional primer walking approach and prompt confirmation that this virus represented a new ebolavirus species. Current efforts to design effective diagnostics, antivirals and vaccines will need to take into account the distinct nature of this important new member of the filovirus family.

Protection against lethal challenge by Ebola virus-like particles produced in insect cells

Ebola virus-like particles (VLPs) were produced in insect cells using a recombinant baculovirus expression system and their efficacy for protection against Ebola virus infection was investigated. Two immunizations with 50 microg Ebola VLPs (high dose) induced a high level of antibodies against Ebola GP that exhibited strong neutralizing activity against GP-mediated virus infection and conferred complete protection of vaccinated mice against lethal challenge by a high dose of mouse-adapted Ebola virus. In contrast, two immunizations with 10 microg Ebola VLPs (low dose) induced 5-fold lower levels of antibodies against GP and these mice were not protected against lethal Ebola virus challenge, similar to control mice that were immunized with 50 microg SIV Gag VLPs. However, the antibody responses against GP were boosted significantly after a third immunization with 10 microg Ebola VLPs to similar levels as those induced by two immunizations with 50 microg Ebola VLPs, and vaccinated mice were also effectively protected against lethal Ebola virus challenge. Furthermore, serum viremia levels in protected mice were either below the level of detection or significantly lower compared to the viremia levels in control mice. These results show that effective protection can be achieved by immunization with Ebola VLPs produced in insect cells, which give high production yields, and lend further support to their development as an effective vaccine strategy against Ebola virus.

Single-dose, virus-vectored vaccine protection against Yersinia pestis challenge: CD4+ cells are required at the time of challenge for optimal protection

We have developed an experimental recombinant vesicular stomatitis virus (VSV) vectored plague vaccine expressing a secreted form of Yersinia pestis low calcium response protein V (LcrV) from the first position of the VSV genome. This vector, given intramuscularly in a single dose, induced high-level antibody titers to LcrV and gave 90-100% protection against pneumonic plague challenge in mice. This single-dose protection was significantly better than that generated by VSV expressing the non-secreted LcrV protein. Increased protection correlated with increased anti-LcrV antibody and a bias toward IgG2a and away from IgG1 isotypes. We also found that the depletion of CD4+ cells, but not CD8+ cells, at the time of challenge resulted in reduced vaccine protection, indicating a role for cellular immunity in protection.

A paramyxovirus-vectored intranasal vaccine against Ebola virus is immunogenic in vector-immune animals

Ebola virus (EBOV) causes outbreaks of a highly lethal hemorrhagic fever in humans. The virus can be transmitted by direct contact as well as by aerosol and is considered a potential bioweapon. Because direct immunization of the respiratory tract should be particularly effective against infection of mucosal surfaces, we previously developed an intranasal vaccine based on replication-competent human parainfluenza virus type 3 (HPIV3) expressing EBOV glycoprotein GP (HPIV3/EboGP) and showed that it is immunogenic and protective against a high dose parenteral EBOV challenge. However, because the adult human population has considerable immunity to HPIV3, which is a common human pathogen, replication and immunogenicity of the vaccine in this population might be greatly restricted. Indeed, in the present study, replication of the vaccine in the respiratory tract of HPIV3-immune guinea pigs was found to be restricted to undetectable levels. This restriction appeared to be based on both neutralizing antibodies and cellular or other components of the immunity to HPIV3. Surprisingly, even though replication of HPIV3/EboGP was highly restricted in HPIV3-immune animals, it induced a high level of EBOV-specific antibodies that nearly equaled that obtained in HPIV3-naive animals. We also show that the previously demonstrated presence of functional GP in the vector particle was not associated with increased replication in the respiratory tract nor with spread beyond the respiratory tract of HPIV3-naive guinea pigs, indicating that expression and functional incorporation of the attachment/penetration glycoprotein of this systemic virus did not mediate a change in tissue tropism.

Structure of the Ebola virus glycoprotein bound to an antibody from a human survivor

Ebola virus (EBOV) entry requires the surface glycoprotein (GP) to initiate attachment and fusion of viral and host membranes. Here we report the crystal structure of EBOV GP in its trimeric, pre-fusion conformation (GP1+GP2) bound to a neutralizing antibody, KZ52, derived from a human survivor of the 1995 Kikwit outbreak. Three GP1 viral attachment subunits assemble to form a chalice, cradled by the GP2 fusion subunits, while a novel glycan cap and projected mucin-like domain restrict access to the conserved receptor-binding site sequestered in the chalice bowl. The glycocalyx surrounding GP is likely central to immune evasion and may explain why survivors have insignificant neutralizing antibody titres. KZ52 recognizes a protein epitope at the chalice base where it clamps several regions of the pre-fusion GP2 to the amino terminus of GP1. This structure provides a template for unravelling the mechanism of EBOV GP-mediated fusion and for future immunotherapeutic development.

Protection against filovirus infection: virus-like particle vaccines

Significant progress has been made in vaccine development against infection by Ebola and Marburg viruses, members of the Filoviridae, which cause severe hemorrhagic fevers in humans with no effective treatment and a mortality rate of up to 90%. Several vaccine strategies have been shown to effectively protect immunized animals against filovirus infection. Among these candidate vaccine strategies, virus-like particles represent a promising approach and have been shown to protect small laboratory animals as well as nonhuman primates against lethal challenge by Ebola and/or Marburg viruses. This review briefly summarizes filovirus epidemiology and pathogenesis, and focuses on the discussion of recent advances in filovirus vaccine development and the current understanding of protective immune responses against filovirus infection with an emphasis on the progress and challenge of filovirus virus-like particle vaccine development.

Monovalent virus-like particle vaccine protects guinea pigs and nonhuman primates against infection with multiple Marburg viruses

BACKGROUND

Virus-like particle (VLP)-based vaccines have the advantage of being morphologically and antigenically similar to the live virus from which they are derived. Expression of the glycoprotein and VP40 matrix protein from Lake Victoria marburgvirus (MARV) results in spontaneous production of VLPs in mammalian cells. Guinea pigs vaccinated with Marburg virus VLPs (mVLPs) or inactivated MARV (iMARV) develop homologous humoral and T-cell responses and are completely protected from a lethal homologous MARV challenge.

AIMS & METHODS

To determine whether mVLPs based on the Musoke (aka Lake Victoria) isolate of MARV could broadly protect against diverse isolates of MARV, guinea pigs were vaccinated with mVLPs or iMARV-Musoke and challenged with MARV-Musoke, -Ravn or -Ci67.

RESULTS

Prior to challenge, the mVLP- and iMARV-vaccinated guinea pigs had high levels of homologous MARV-Musoke and heterologous MARV-Ravn and -Ci67 antibodies. The Musoke-based mVLPs and iMARV vaccines provided complete protection in guinea pigs against viremia, viral replication and pathological changes in tissues, and lethal disease following challenge with MARV-Musoke, -Ravn or -Ci67. Guinea pigs vaccinated with RIBI adjuvant alone and infected with guinea pig-adapted MARV-Musoke, -Ravn or -Ci67 had histopathologic findings similar to those seen in the nonhuman primate model for MARV infection. Based on the strong protection observed in guinea pigs, we next vaccinated cynomolgus macaques with Musoke-based mVLPs and showed the VLP-vaccinated monkeys were broadly protected against three isolates of MARV (Musoke, Ravn and Ci67).

CONCLUSION

Musoke mVLPs are effective at inducing broad heterologous immunity and protection against multiple MARV isolates.

SARS vaccine based on a replication-defective recombinant vesicular stomatitis virus is more potent than one based on a replication-competent vector

A SARS vaccine based on a live-attenuated vesicular stomatitis virus (VSV) recombinant expressing the SARS-CoV S protein provides long-term protection of immunized mice from SARS-CoV infection (Kapadia, S.U., Rose, J. K., Lamirande, E., Vogel, L., Subbarao, K., Roberts, A., 2005. Long-term protection from SARS coronavirus infection conferred by a single immunization with an attenuated VSV-based vaccine. Virology 340(2), 174-82.). Because it is difficult to obtain regulatory approval of vaccine based on live viruses, we constructed a replication-defective single-cycle VSV vector in which we replaced the VSV glycoprotein (G) gene with the SARS-CoV S gene. The virus was only able to infect cells when pseudotyped with the VSV G protein. We measured the effectiveness of immunization with the single-cycle vaccine in mice. We found that the vaccine given intramuscularly induced a neutralizing antibody response to SARS-CoV that was approximately ten-fold greater than that required for the protection from SARS-CoV infection, and significantly greater than that generated by the replication-competent vector expressing SARS-CoV S protein given by the same route. Our results, along with earlier studies showing potent induction of T-cell responses by single-cycle vectors, indicate that these vectors are excellent alternatives to live-attenuated VSV.

Recombinant vesicular stomatitis virus vector mediates postexposure protection against Sudan Ebola hemorrhagic fever in nonhuman primates

Recombinant vesicular stomatitis virus (VSV) vectors expressing homologous filoviral glycoproteins can completely protect rhesus monkeys against Marburg virus when administered after exposure and can partially protect macaques after challenge with Zaire ebolavirus. Here, we administered a VSV vector expressing the Sudan ebolavirus (SEBOV) glycoprotein to four rhesus macaques shortly after exposure to SEBOV. All four animals survived SEBOV challenge, while a control animal that received a nonspecific vector developed fulminant SEBOV hemorrhagic fever and succumbed. This is the first demonstration of complete postexposure protection against an Ebola virus in nonhuman primates and provides further evidence that postexposure vaccination may have utility in treating exposures to filoviruses.

Treatment of Marburg and Ebola hemorrhagic fevers: a strategy for testing new drugs and vaccines under outbreak conditions

The filoviruses, Marburg and Ebola, have the dubious distinction of being associated with some of the highest case-fatality rates of any known infectious disease--approaching 90% in many outbreaks. In recent years, laboratory research on the filoviruses has produced treatments and vaccines that are effective in laboratory animals and that could potentially drastically reduce case-fatality rates and curtail outbreaks in humans. However, there are significant challenges in clinical testing of these products and eventual delivery to populations in need. Most cases of filovirus infection are recognized only in the setting of large outbreaks, often in the most remote and resource-poor areas of sub-Saharan Africa, with little infrastructure and few personnel experienced in clinical research. Significant political, legal, and socio-cultural barriers also exist. Here, we review the present research priorities and environment for field study of the filovirus hemorrhagic fevers and outline a strategy for future prospective clinical research on treatment and vaccine prevention.

Ebola virus-like particle-based vaccine protects nonhuman primates against lethal Ebola virus challenge

BACKGROUND

Currently, there are no licensed vaccines or therapeutics for the prevention or treatment of infection by the highly lethal filoviruses, Ebola virus (EBOV) and Marburg virus (MARV), in humans. We previously had demonstrated the protective efficacy of virus-like particle (VLP)-based vaccines against EBOV and MARV infection in rodents.

METHODS

To determine the efficacy of vaccination with Ebola VLPs (eVLPs) in nonhuman primates, we vaccinated cynomolgus macaques with eVLPs containing EBOV glycoprotein (GP), nucleoprotein (NP), and VP40 matrix protein and challenged the macaques with 1000 pfu of EBOV.

RESULTS

Serum samples from the eVLP-vaccinated nonhuman primates demonstrated EBOV-specific antibody titers, as measured by enzyme-linked immunosorbent assay, complement-mediated lysis assay, and antibody-dependent cell-mediated cytotoxicity assay. CD44+ T cells from eVLP-vaccinated macaques but not from a naive macaque responded with vigorous production of tumor necrosis factor- alpha after EBOV-peptide stimulation. All 5 eVLP-vaccinated monkeys survived challenge without clinical or laboratory signs of EBOV infection, whereas the control animal died of infection.

CONCLUSION

On the basis of safety and efficacy, eVLPs represent a promising filovirus vaccine for use in humans.

Forty years of marburg virus

Forty years ago, in early August 1967, the first filovirus ever detected, Marburg virus, made its appearance in Europe, causing severe and often fatal hemorrhagic fever in laboratory workers in Marburg and Frankfurt and, about 4 weeks later, in Belgrade. The etiological agent was isolated and identified by the combined efforts of virologists in Marburg and Hamburg within the very short time of 3 months. Marburg was not the only town where the virus was isolated and identified for the first time, but most cases of infection occurred in Marburg.

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.

Marburg virus Angola infection of rhesus macaques: pathogenesis and treatment with recombinant nematode anticoagulant protein c2

BACKGROUND

The procoagulant tissue factor (TF) is thought to play a role in the coagulation disorders that characterize filoviral infections. In this study, we evaluated the pathogenesis of lethal infection with the Angola strain of Marburg virus (MARV-Ang) in rhesus macaques and tested the efficacy of recombinant nematode anticoagulant protein c2 (rNAPc2), an inhibitor of TF/factor VIIa, as a potential treatment.

METHODS

Twelve rhesus macaques were challenged with a high dose (1000 pfu) of MARV-Ang. Six macaques were treated with rNAPc2, and 6 macaques served as control animals.

RESULTS

All 6 control animals succumbed to MARV-Ang challenge by day 8 (mean, 7.3 days), whereas 5 of 6 rNAPc2-treated animals died on day 9 and 1 rNAPc2-treated animal survived. The disease course for MARV-Ang infection appeared to progress more rapidly in rhesus macaques than has been previously reported for other strains of MARV. In contrast to Ebola virus (EBOV) infection in macaques, up-regulation of TF was not as striking, and deposition of fibrin was a less prominent pathologic feature of disease in these animals.

CONCLUSIONS

These data show that the pathogenicity of MARV-Ang infection appears to be consistent with the apparent increased human virulence attributed to this strain. The apparent reduced efficacy of rNAPc2 against MARV-Ang infection, compared with its efficacy against EBOV infection, appears to be associated with differences in TF induction and fibrin deposition.

In vitro and in vivo characterization of recombinant Ebola viruses expressing enhanced green fluorescent protein

To facilitate an understanding of the molecular aspects of the pathogenesis of Zaire ebolavirus (ZEBOV) infection, we generated 2 different recombinant viruses expressing enhanced green fluorescent protein (eGFP) from additional transcription units inserted at different positions in the virus genome. These viruses showed in vitro phenotypes similar to that of wild-type ZEBOV (wt-ZEBOV) and were stable over multiple passages. Infection with one of the viruses expressing eGFP produced only mild disease in rhesus macaques, demonstrating a marked attenuation in this animal model. However, in mice lacking signal transducer and activator of transcription 1, both viruses expressing eGFP caused lethal cases of disease that were moderately attenuated, compared with that caused by wt-ZEBOV. In mice, viral replication could be easily tracked by the detection of eGFP-positive cells in tissues, by use of flow cytometry. These findings demonstrate that the incorporation of a foreign gene will attenuate ZEBOV in vivo but that these viruses still have potential for in vitro and in vivo research applications.

High-throughput molecular detection of hemorrhagic fever virus threats with applications for outbreak settings

Within the past dozen years, outbreaks of filoviral hemorrhagic fever within the human population have been occurring with increasing frequency, with an average of 1 epidemic now occurring every 1-2 years. Many of the outbreaks have been large (involving >150 cases), necessitating rapid responses from the international community to help implement infection control and surveillance. This increased activity, combined with today's climate of bioterrorism threats, has heightened the need for high-throughput methodologies for specific detection of these high-hazard viruses in sophisticated laboratory setups and mobile field laboratory situations. Using Zaire Ebola virus as an example, we describe here the development of a high-throughput protocol for RNA extraction and quantitative reverse-transcription polymerase chain reaction analysis that is safe, fast, and reliable. Furthermore, the applicability of this method to an outbreak setting was demonstrated by correct analysis of >500 specimens at a field laboratory established during a recent outbreak of Marburg hemorrhagic fever in Angola.

Vaccine to confer to nonhuman primates complete protection against multistrain Ebola and Marburg virus infections

Filoviruses (Ebola and Marburg viruses) are among the deadliest viruses known to mankind, with mortality rates nearing 90%. These pathogens are highly infectious through contact with infected body fluids and can be easily aerosolized. Additionally, there are currently no licensed vaccines available to prevent filovirus outbreaks. Their high mortality rates and infectious capabilities when aerosolized and the lack of licensed vaccines available to prevent such infectious make Ebola and Marburg viruses serious bioterrorism threats, placing them both on the category A list of bioterrorism agents. Here we describe a panfilovirus vaccine based on a complex adenovirus (CAdVax) technology that expresses multiple antigens from five different filoviruses de novo. Vaccination of nonhuman primates demonstrated 100% protection against infection by two species of Ebola virus and three Marburg virus subtypes, each administered at 1,000 times the lethal dose. This study indicates the feasibility of vaccination against all current filovirus threats in the event of natural hemorrhagic fever outbreak or biological attack.

Replicating and non-replicating viral vectors for vaccine development

Viral vectors provide a convenient means to deliver vaccine antigens to select target cells or tissues. A broad spectrum of replicating and non-replicating vectors is available. An appropriate choice for select applications will depend on the biology of the infectious agent targeted, as well as factors such as whether the vaccine is intended to prevent infection or boost immunity in already infected individuals, prior exposure of the target population to the vector, safety, and the number and size of gene inserts needed. Here several viral vectors under development as HIV/AIDS vaccines are reviewed. A vaccine strategy based on initial priming with a replicating vector to enlist the innate immune system, target mucosal inductive sites, and prime both cellular and humoral systemic and mucosal immune responses is proposed. Subsequently, boosting with a replicating or non-replicating vector and/or protein subunits could lead to induction of necessary levels of protective immunity.

Animal models of highly pathogenic RNA viral infections: hemorrhagic fever viruses

A diverse group of highly pathogenic RNA viruses cause a severe multisystemic illness in humans commonly referred to as viral hemorrhagic fever (VHF). Although they can vary widely in clinical presentation, all VHFs share certain features that include intense fever, malaise, bleeding and shock. Effective antiviral therapies for most of the VHFs are lacking. Complicating development of intervention strategies is the relative infrequency and unpredictability of VHF outbreaks making human clinical trials extremely challenging or unfeasible. Therefore, animal models that can recapitulate human disease are essential to the development of effective antivirals and vaccines. In general, a good animal model of VHF will demonstrate systemic dispersion of the virus through infection of mononuclear phagocytes and dendritic cells, which induces the release of inflammatory mediators that increase vascular permeability and facilitate coagulation. The culmination of this process leads to significant loss of plasma volume and terminal hypovolemic shock. Although it is clear that nonhuman primate models are the most faithful to human disease, the more accessible and less costly rodent models, including those based on infection with related surrogate viruses, can reproduce certain components of VHF and can serve as suitable preclinical models for initial development of effective countermeasures. Such models are sufficient for testing of drugs that directly block viral replication, but may be inadequate for evaluating therapies that depend for their success on the activation or inhibition of host responses.

Antibody therapy as a future treatment option for Ebola virus infection

Ebola virus causes lethal hemorrhagic fever in human and nonhuman primates. Effective prophylaxis and treatment for this disease are not yet available. Antisera and monoclonal antibodies specific to Ebola virus proteins have been tested for passive immunization in experimental animal models and clinical cases, and shown to be effective in mice and guinea pigs, whereas the evidence of protective efficacy in primates, including humans, remains elusive. In this review, we focus on research relevant to prophylaxis and treatment by passive immunization, and discuss the potential use of antibody therapy for Ebola virus infection. Nevertheless, there is no doubt that a comprehensive understanding of Ebola virus pathogenesis will aid in the development of therapeutic strategies against Ebola hemorrhagic fever.

Nucleocapsid formation and RNA synthesis of Marburg virus is dependent on two coiled coil motifs in the nucleoprotein

The nucleoprotein (NP) of Marburg virus (MARV) is responsible for the encapsidation of viral genomic RNA and the formation of the helical nucleocapsid precursors that accumulate in intracellular inclusions in infected cells. To form the large helical MARV nucleocapsid, NP needs to interact with itself and the viral proteins VP30, VP35 and L, which are also part of the MARV nucleocapsid. In the present study, a conserved coiled coil motif in the central part of MARV NP was shown to be an important element for the interactions of NP with itself and VP35, the viral polymerase cofactor. Additionally, the coiled coil motif was essential for the formation of NP-induced intracellular inclusions and for the function of NP in the process of transcription and replication of viral RNA in a minigenome system. Transfer of the coiled coil motif to a reporter protein was sufficient to mediate interaction of the constructed fusion protein with the N-terminus of NP. The coiled coil motif is bipartite, constituted by two coiled coils which are separated by a flexible linker.

Isolates of Zaire ebolavirus from wild apes reveal genetic lineage and recombinants

Over the last 30 years, Zaire ebolavirus (ZEBOV), a virus highly pathogenic for humans and wild apes, has emerged repeatedly in Central Africa. Thus far, only a few virus isolates have been characterized genetically, all belonging to a single genetic lineage and originating exclusively from infected human patients. Here, we describe the first ZEBOV sequences isolated from great ape carcasses in the Gabon/Congo region that belong to a previously unrecognized genetic lineage. According to our estimates, this lineage, which we also encountered in the two most recent human outbreaks in the Republic of the Congo in 2003 and 2005, diverged from the previously known viruses around the time of the first documented human outbreak in 1976. These results suggest that virus spillover from the reservoir has occurred more than once, as predicted by the multiple emergence hypothesis. However, the young age of both ZEBOV lineages and the spatial and temporal sequence of outbreaks remain at odds with the idea that the virus simply emerged from a long-established and widespread reservoir population. Based on data from two ZEBOV genes, we also demonstrate, within the family Filoviridae, recombination between the two lineages. According to our estimates, this event took place between 1996 and 2001 and gave rise to a group of recombinant viruses that were responsible for a series of outbreaks in 2001-2003. The potential for recombination adds an additional level of complexity to unraveling and potentially controlling the emergence of ZEBOV in humans and wildlife species.

Ebolavirus and Marburgvirus: insight the Filoviridae family

Ebolavirus and Marburgvirus (belonging to the Filoviridae family) emerged four decades ago and cause epidemics of haemorrhagic fever with high case-fatality rates. The genome of filoviruses encodes seven proteins. No significant homology is observed between filovirus proteins and any known macromolecule. Moreover, Marburgvirus and Ebolavirus show significant differences in protein homology. The natural maintenance cycle of filoviruses is unknown, the natural reservoir, the mode of transmission, the epidemic disease generation, and temporal dynamics are unclear. Lastly, Ebolavirus and Marburgvirus are considered as potential biological weapons. Vaccine appears the unique therapeutic frontier. Here, molecular and clinical aspects of filoviral haemorrhagic fevers are summarized.

Attenuation of recombinant vesicular stomatitis virus-human immunodeficiency virus type 1 vaccine vectors by gene translocations and g gene truncation reduces neurovirulence and enhances immunogenicity in mice

Recombinant vesicular stomatitis virus (rVSV) has shown great potential as a new viral vector for vaccination. However, the prototypic rVSV vector described previously was found to be insufficiently attenuated for clinical evaluation when assessed for neurovirulence in nonhuman primates. Here, we describe the attenuation, neurovirulence, and immunogenicity of rVSV vectors expressing human immunodeficiency virus type 1 Gag. These rVSV vectors were attenuated by combinations of the following manipulations: N gene translocations (N4), G gene truncations (CT1 or CT9), noncytopathic M gene mutations (Mncp), and positioning of the gag gene into the first position of the viral genome (gag1). The resulting N4CT1-gag1, N4CT9-gag1, and MncpCT1-gag1 vectors demonstrated dramatically reduced neurovirulence in mice following direct intracranial inoculation. Surprisingly, in spite of a very high level of attenuation, the N4CT1-gag1 and N4CT9-gag1 vectors generated robust Gag-specific immune responses following intramuscular immunization that were equivalent to or greater than immune responses generated by the more virulent prototypic vectors. MncpCT1-gag1 also induced Gag-specific immune responses following intramuscular immunization that were equivalent to immune responses generated by the prototypic rVSV vector. Placement of the gag gene in the first position of the VSV genome was associated with increased in vitro expression of Gag protein, in vivo expression of Gag mRNA, and enhanced immunogenicity of the vector. These findings demonstrate that through directed manipulation of the rVSV genome, vectors that have reduced neurovirulence and enhanced immunogenicity can be made.

Ebola virus VP24 proteins inhibit the interaction of NPI-1 subfamily karyopherin alpha proteins with activated STAT1

The Zaire ebolavirus protein VP24 was previously demonstrated to inhibit alpha/beta interferon (IFN-alpha/beta)- and IFN-gamma-induced nuclear accumulation of tyrosine-phosphorylated STAT1 (PY-STAT1) and to inhibit IFN-alpha/beta- and IFN-gamma-induced gene expression. These properties correlated with the ability of VP24 to interact with the nuclear localization signal receptor for PY-STAT1, karyopherin alpha1. Here, VP24 is demonstrated to interact not only with overexpressed but also with endogenous karyopherin alpha1. Mutational analysis demonstrated that VP24 binds within the PY-STAT1 binding region located in the C terminus of karyopherin alpha1. In addition, VP24 was found to inhibit PY-STAT1 binding to both overexpressed and endogenous karyopherin alpha1. We assessed the binding of both PY-STAT1 and the VP24 proteins from Zaire, mouse-adapted Zaire, and Reston Ebola viruses for interaction with all six members of the human karyopherin alpha family. We found, in contrast to previous studies, that PY-STAT1 can interact not only with karyopherin alpha1 but also with karyopherins alpha5 and alpha6, which together comprise the NPI-1 subfamily of karyopherin alphaS. Similarly, all three VP24s bound and inhibited PY-STAT1 interaction with karyopherins alpha1, alpha5, and alpha6. Consistent with their ability to inhibit the karyopherin-PY-STAT1 interaction, Zaire, mouse-adapted Zaire, and Reston Ebola virus VP24s displayed similar capacities to inhibit IFN-beta-induced gene expression in human and mouse cells. These findings suggest that VP24 inhibits interaction of PY-STAT1 with karyopherins alpha1, alpha5, or alpha6 by binding within the PY-STAT1 binding region of the karyopherins and that this function is conserved among the VP24 proteins of different Ebola virus species.