Clinical virology of Ebola hemorrhagic fever (EHF): virus, virus antigen, and IgG and IgM antibody findings among EHF patients in Kikwit, Democratic Republic of the Congo, 1995

Ebola hemorrhagic fever (EHF) patients treated at Kikwit General Hospital during the 1995 outbreak were tested for viral antigen, IgG and IgM antibody, and infectious virus. Viral antigen could be detected in virtually all patients during the acute phase of illness, while antibody was not always detectable before death. Virus was also isolated from patients during the course of their febrile illness, but attempts to quantify virus in Vero E6 cells by standard plaque assay were often unsuccessful. IgG and IgM antibody appeared at approximately the same time after disease onset (8-10 days), but IgM persisted for a much shorter period among the surviving convalescent patients. IgG antibody was detectable in surviving patients through about 2 years after onset, the latest time that samples were obtained. Detection of Ebola virus antigens or virus isolation appears to be the most reliable means of diagnosis for patients with suspected acute EHF, since patients with this often-fatal disease (80% mortality) may not develop detectable antibodies before death.

Experimental inoculation of plants and animals with Ebola virus

Thirty-three varieties of 24 species of plants and 19 species of vertebrates and invertebrates were experimentally inoculated with Ebola Zaire virus. Fruit and insectivorous bats supported replication and circulation of high titers of virus without necessarily becoming ill; deaths occurred only among bats that had not adapted to the diet fed in the laboratory.

The assembly of Ebola virus nucleocapsid requires virion-associated proteins 35 and 24 and posttranslational modification of nucleoprotein

Ebola virus encodes seven viral structural and regulatory proteins that support its high rates of replication, but little is known about nucleocapsid assembly of this virus in infected cells. We report here that three viral proteins are necessary and sufficient for formation of Ebola virus particles and that intracellular posttranslational modification regulates this process. Expression of the nucleoprotein (NP) and virion-associated proteins VP35 and VP24 led to spontaneous assembly of nucleocapsids in transfected 293T cells by transmission electron microscopy. A specific biochemical interaction of these three proteins was demonstrated, and, interestingly, O-glycosylation and sialation of NP were demonstrated and necessary for their association. This distinct mechanism of regulation for filovirus assembly suggests new approaches for viral therapies and vaccines for Ebola and related viruses.

Molecular determinants of Ebola virus virulence in mice

Zaire ebolavirus (ZEBOV) causes severe hemorrhagic fever in humans and nonhuman primates, with fatality rates in humans of up to 90%. The molecular basis for the extreme virulence of ZEBOV remains elusive. While adult mice resist ZEBOV infection, the Mayinga strain of the virus has been adapted to cause lethal infection in these animals. To understand the pathogenesis underlying the extreme virulence of Ebola virus (EBOV), here we identified the mutations responsible for the acquisition of the high virulence of the adapted Mayinga strain in mice, by using reverse genetics. We found that mutations in viral protein 24 and in the nucleoprotein were primarily responsible for the acquisition of high virulence. Moreover, the role of these proteins in virulence correlated with their ability to evade type I interferon-stimulated antiviral responses. These findings suggest a critical role for overcoming the interferon-induced antiviral state in the pathogenicity of EBOV and offer new insights into the pathogenesis of EBOV infection.

Zaire ebolavirus causes severe hemorrhagic fever in humans with up to 90% case-fatality rates. Currently, there are no vaccines or specific therapeutic interventions available for this devastating viral disease due, at least in part, to a lack of knowledge regarding the molecular basis of virulence for this extremely pathogenic agent. While adult mice resist wild-type Zaire ebolavirus infection, the virus has recently been adapted to cause lethal infection in mice. In order to understand the pathogenesis underlying Zaire ebolavirus infection, the authors identified the mutations responsible for the acquisition of virulence in mice, using reverse genetics technology, which allows the generation of genetically altered mutant viruses from cloned cDNA. By testing the virulence of mutant viruses, two viral proteins, viral protein 24 and the nucleoprotein, were found to be primarily responsible for the acquisition of virulence in mice. Moreover, the role of these proteins in virulence correlated with their ability to confer resistance to interferon-stimulated antiviral responses in mouse cells. These findings suggest a critical role of these proteins in overcoming the interferon-induced antiviral state in the pathogenicity of Zaire ebolavirus and offer new insights into the pathogenesis of Zaire ebolavirus infection.

Vesicular release of ebola virus matrix protein VP40

We have analysed the expression and cellular localisation of the matrix protein VP40 from Ebola virus. Full-length VP40 and an N-terminal truncated construct missing the first 31 residues [VP40(31-326)] both locate to the plasma membrane of 293T cells when expressed transiently, while a C-terminal truncation of residues 213 to 326 [VP40(31-212)] shows only expression in the cytoplasm, when analysed by indirect immunofluorescence and plasma membrane preparations. In addition, we find that full-length VP40 [VP40(1-326)] and VP40(31-326) are both released into the cell culture supernatant and float up in sucrose gradients. The efficiency of their release, however, is dependent on the presence of the N-terminal 31 residues. VP40 that is released into the supernatant is resistant to trypsin digestion, a finding that is consistent with the formation of viruslike particles detected by electron microscopy. Together, these results provide strong evidence that Ebola virus VP40 is sufficient for virus assembly and budding from the plasma membrane.

Containing a haemorrhagic fever epidemic: the Ebola experience in Uganda (October 2000-January 2001)

INTRODUCTION

The Ebola virus, belonging to the family of filoviruses, was first recognized in 1976 when it caused concurrent outbreaks in Yambuku in the Democratic Republic of Congo (DRC), and in the town of Nzara in Sudan. Both countries share borders with Uganda. A total of 425 cases and 224 deaths attributed to Ebola haemorrhagic fever (EHF) were recorded in Uganda in 2000/01. Although there was delayed detection at the community level, prompt and efficient outbreak investigation led to the confirmation of the causative agent on 14 October 2000 by the National Institute of Virology in South Africa, and the subsequent institution of control interventions.

CONTROL INTERVENTIONS

Public health interventions to contain the epidemic aimed at minimizing transmission in the health care setting and in the community, reducing the case fatality rate due to the epidemic, strengthening co-ordination for the response and building capacity for on-going surveillance and control. Co-ordination of the control interventions was organized through the Interministerial Committee, National Ebola Task Force, District Ebola Task Forces, and the Technical Committees at national and district levels. The World Health Organization (WHO) under the Global Outbreak Alert and Response Network co-ordinated the international response. The post-outbreak control interventions addressed weaknesses prior to outbreak detection and aimed at improving preparations for future outbreak detection and response. Challenges to control efforts included inadequate and poor quality protective materials, deaths of health workers, numerous rumors and the rejection of convalescent cases by members of the community.

CONCLUSIONS

This was recognized as the largest reported outbreak of EHF in the world. Control interventions were very successful in containing the epidemic. The community structures used to contain the epidemic have continued to perform well after containment of the outbreak, and have proved useful in the identification of other outbreaks. This was also the first outbreak response co-ordinated by the WHO under the Global Outbreak Alert and Response Network, a voluntary organization recently created to co-ordinate technical and financial resources to developing countries during outbreaks.

Molecular characterization of guinea pig-adapted variants of Ebola virus

Serial passage of initially nonlethal Ebola virus (EBOV) in outbred guinea pigs resulted in the selection of variants with high pathogenicity. Nucleotide sequence analysis of the complete genome of the guinea pig-adapted variant 8mc revealed that it differed from wild-type virus by eight mutations. No mutations were identified in nontranscribed regions, including leader, trailer, and intragenic sequences. Among noncoding regions the only base change was found in the VP30 gene. Two silent base changes were found in the open reading frame (ORF) encoding NP protein. Nucleotide changes resulting in single-amino-acid exchanges were identified in both NP and L genes. Three other mutations found in VP24 caused amino acid substitutions, which are responsible for larger structural changes of this protein, as indicated by an alteration in electrophoretic mobility. A highly pathogenic EBOV variant K5 from another passaging series showed an amino acid substitution at nearly the same location in the VP24 gene, suggesting the importance of this protein in the adaptation process. In addition, sequence variability of the GP gene was found when plaque-purified clones of EBOV-8mc were analyzed. Three of five viral clones showed insertion of one uridine residue at the GP gene-editing site, which led to a significant change in the expression of virus glycoproteins. This observation suggests that the editing site is a hot spot for insertion and deletion of nucleotides, not only at the level of transcription but also of genome replication. Irrespective of the number of uridine residues at the editing site, all plaque-purified clones of EBOV variant 8mc resembled each other in their pathogenicity for guinea pigs, indicating either the absence or only supportive role of mutations in the GP gene on the adaptation process.

Pathophysiology of shock and hemorrhage in a fulminating viral infection (Ebola)

Eleven rhesus monkeys were monitored intensively during experimental infection with Ebola virus. Prominent neutrophilia with left shift and lymphopenia were the earliest abnormalities and were statistically significant by day 4 (P less than .02 and P less than .01, respectively). By day 4 falls in platelet counts were not statistically significant, whereas in vitro platelet aggregation was markedly depressed, progressing rapidly to complete failure by the time of maximum illness. Intraplatelet protein studies suggested this event was the result of in vivo activation and degranulation. Coagulation cascade defects were mainly in the intrinsic system and were surprisingly mild, with no evidence of selective consumption or production deficit of factor VII or VIII. When the possibility of indirectly mediated damage to endothelium possibly by a nonspecific immune response was examined, weight loss was less severe in drug-treated monkeys, and all had detectable plasma prostacyclin metabolites, but there was no improvement in survival.

Mannosyl glycodendritic structure inhibits DC-SIGN-mediated Ebola virus infection in cis and in trans

We have designed a glycodendritic structure, BH30sucMan, that blocks the interaction between dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and Ebola virus (EBOV) envelope. BH30sucMan inhibits DC-SIGN-mediated EBOV infection at nanomolar concentrations. BH30sucMan may counteract important steps of the infective process of EBOV and, potentially, of microorganisms shown to exploit DC-SIGN for cell entry and infection.

Budding of PPxY-containing rhabdoviruses is not dependent on host proteins TGS101 and VPS4A

Viral matrix proteins of several enveloped RNA viruses play important roles in virus assembly and budding and are by themselves able to bud from the cell surface in the form of lipid-enveloped, virus-like particles (VLPs). Three motifs (PT/SAP, PPxY, and YxxL) have been identified as late budding domains (L-domains) responsible for efficient budding. L-domains can functionally interact with cellular proteins involved in vacuolar sorting (VPS4A and TSG101) and endocytic pathways (Nedd4), suggesting involvement of these pathways in virus budding. Ebola virus VP40 has overlapping PTAP and PPEY motifs, which can functionally interact with TSG101 and Nedd4, respectively. As for vesicular stomatitis virus (VSV), a PPPY motif within M protein can interact with Nedd4. In addition, M protein has a PSAP sequence downstream of the PPPY motif, but the function of PSAP in budding is not clear. In this study, we compared L-domain functions between Ebola virus and VSV by constructing a chimeric M protein (M40), in which the PPPY motif of VSV M is replaced by the L domains of VP40. The budding efficiency of M40 was 10-fold higher than that of wild-type (wt) M protein. Overexpression of a dominant negative mutant of VPS4A or depletion of cellular TSG101 reduced the budding of only M40-containing VLPs but not that of wt M VLPs or live VSV. These findings suggest that the PSAP motif of M protein is not critical for budding and that there are fundamental differences between PTAP-containing viruses (Ebola virus and human immunodeficiency virus type 1) and PPPY-containing viruses (VSV and rabies virus) regarding their dependence on specific host factors for efficient budding.

Ebola virus defective interfering particles and persistent infection

Ebola virus (Zaire subtype) is associated with high mortality disease outbreaks that commonly involve human to human transmission. Surviving patients can show evidence of prolonged virus persistence. The potential for Ebola virus to generate defective interfering (DI) particles and establish persistent infections in tissue culture was investigated. It was found that serial undiluted virus passages quickly resulted in production of an evolving population of virus minireplicons possessing both deletion and copyback type DI genome rearrangements. The tenth undiluted virus passage resulted in the establishment of virus persistently infected cell lines. Following one or two crises, these cells were stably maintained for several months with continuous shedding of infectious virus. An analysis of the estimated genome lengths of a selected set of the Ebola virus minireplicons and standard filoviruses revealed no obvious genome length rule, such as "the rule of six" found for the phylogenetically related Paramyxovirinae subfamily viruses. Minimal promoters for Ebola virus replication were found to be contained within 156 and 177 nucleotide regions of the genomic and antigenomic RNA 3' termini, respectively, based on the length of authentic termini retained in the naturally occurring minireplicons analyzed. In addition, using UV-irradiated preparations of virus released from persistently infected cells, it was demonstrated that Ebola virus DI particles could potentially be used as natural minireplicons to assay standard virus support functions.

Rescue of recombinant Marburg virus from cDNA is dependent on nucleocapsid protein VP30

Here we report recovery of infectious Marburg virus (MARV) from a full-length cDNA clone. Compared to the wild-type virus, recombinant MARV showed no difference in terms of morphology of virus particles, intracellular distribution in infected cells, and growth kinetics. The nucleocapsid protein VP30 of MARV and Ebola virus (EBOV) contains a Zn-binding motif which is important for the function of VP30 as a transcriptional activator in EBOV, whereas its role for MARV is unclear. It has been reported previously that MARV VP30 is able to support transcription in an EBOV-specific minigenome system. When the Zn-binding motif was destroyed, MARV VP30 was shown to be inactive in the EBOV system. While it was not possible to rescue recombinant MARV when the VP30 plasmid was omitted from transfection, MARV VP30 with a destroyed Zn-binding motif and EBOV VP30 were able to mediate virus recovery. In contrast, rescue of recombinant EBOV was not supported by EBOV VP30 containing a mutated Zn-binding domain.

Protection from lethal infection is determined by innate immune responses in a mouse model of Ebola virus infection

A mouse-adapted strain of Ebola Zaire virus produces a fatal infection when BALB/cj mice are infected intraperitoneally (ip) but subcutaneous (sc) infection with the same virus fails to produce illness and confers long-term protection from lethal ip rechallenge. To identify immune correlates of protection in this model, we compared viral replication and cytokine/chemokine responses to Ebola virus in mice infected ip (10 PFU/mouse), or sc (100 PFU/mouse) and sc "immune" mice rechallenged ip (10(6) PFU/mouse) at several time points postinfection (pi). Ebola viral antigens were detected in the serum, liver, spleen, and kidneys of ip-infected mice by day 2 pi, increasing up to day 6. Sc-infected mice and immune mice rechallenged ip had no detectable viral antigens until day 6 pi, when low levels of viral antigens were detected in the livers of sc-infected mice only. TNF-alpha and MCP-1 were detected earlier and at significantly higher levels in the serum and tissues of ip-infected mice than in sc-infected or immune mice challenged ip. In contrast, high levels of IFN-alpha and IFN-gamma were found in tissues within 2 days after challenge in sc-infected and immune mice but not in ip-infected mice. Mice became resistant to ip challenge within 48 h of sc infection, coinciding with the rise in tissue IFN-alpha levels. In this model of Ebola virus infection, the nonlethal sc route of infection is associated with an attenuated inflammatory response and early production of antiviral cytokines, particularly IFN-alpha, as compared with lethal ip infection.

Oligomerization of Ebola virus VP30 is essential for viral transcription and can be inhibited by a synthetic peptide

Transcription of Ebola virus (EBOV)-specific mRNA is driven by the nucleocapsid proteins NP, VP35, and L. This process is further dependent on VP30, an essential EBOV-specific transcription factor. The present study addresses the self-assembly of VP30 and the functional significance of this process for viral transcription and propagation. Essential for oligomerization of VP30 is a region spanning amino acids 94-112. Within this region a cluster of four leucine residues is of critical importance. Mutation of only one of these leucine residues resulted in oligomerization-deficient VP30 molecules that were no longer able to support EBOV-specific transcription. The essential role of homo-oligomerization for the function of VP30 was further corroborated by the finding that mixed VP30 oligomers consisting of VP30 and transcriptionally inactive VP30 mutants were impaired in their ability to support EBOV transcription. The dominant negative effect of these VP30 mutants was dependent on their ability to bind to VP30. The oligomerization of VP30 could be dose dependently inhibited by a 25-mer peptide (E30pep-wt) derived from the presumed oligomerization domain (IC50,1 mum). A control peptide (E30pep-3LA), in which three leucines were changed to alanine, had no inhibitory effect. Thus, E30pep-wt seemed to bind efficiently to VP30 and consequently blocked the oligomerization of the protein. When E30pep-wt was transfected into EBOV-infected cells, the peptide inhibited viral replication suggesting that inhibition of VP30 oligomerization represents a target for EBOV antiviral drugs.

Particle-to-PFU ratio of Ebola virus influences disease course and survival in cynomolgus macaques

This study addresses the role of Ebola virus (EBOV) specific infectivity in virulence. Filoviruses are highly lethal, enveloped, single-stranded negative-sense RNA viruses that can cause hemorrhagic fever. No approved vaccines or therapies exist for filovirus infections, and infectious virus must be handled in maximum containment. Efficacy testing of countermeasures, in addition to investigations of pathogenicity and immune response, often requires a well-characterized animal model. For EBOV, an obstacle in performing accurate disease modeling is a poor understanding of what constitutes an infectious dose in animal models. One well-recognized consequence of viral passage in cell culture is a change in specific infectivity, often measured as a particle-to-PFU ratio. Here, we report that serial passages of EBOV in cell culture resulted in a decrease in particle-to-PFU ratio. Notably, this correlated with decreased potency in a lethal cynomolgus macaque (Macaca fascicularis) model of infection; animals were infected with the same viral dose as determined by plaque assay, but animals that received more virus particles exhibited increased disease. This suggests that some particles are unable to form a plaque in a cell culture assay but are able to result in lethal disease in vivo. These results have a significant impact on how future studies are designed to model EBOV disease and test countermeasures.

IMPORTANCE

Ebola virus (EBOV) can cause severe hemorrhagic disease with a high case-fatality rate, and there are no approved vaccines or therapies. Specific infectivity can be considered the total number of viral particles per PFU, and its impact on disease is poorly understood. In stocks of most mammalian viruses, there are particles that are unable to complete an infectious cycle or unable to cause cell pathology in cultured cells. We asked if these particles cause disease in nonhuman primates by infecting monkeys with equal infectious doses of genetically identical stocks possessing either high or low specific infectivities. Interestingly, some particles that did not yield plaques in cell culture assays were able to result in lethal disease in vivo. Furthermore, the number of PFU needed to induce lethal disease in animals was very low. Our results have a significant impact on how future studies are designed to model EBOV disease and test countermeasures.

Inactivation of Ebola virus with a surfactant nanoemulsion

Hemorrhagic fever caused by Ebola virus (EBO) is a highly contagious infection. This necessitates that the contaminated instruments, clothes, and hospital premises must be completely disinfected. Nanoemulsions are a new form of disinfectant composed of detergents and vegetable oil suspended in water. The antiviral activity of nanoemulsion ATB has been investigated against EBO. The nanoemulsion was tested against two preparations of EBO (strain Zaire) obtained from Vero cell culture fluid (EBO-zc) and from blood of infected monkeys (EBO-zb). The nanoemulsion ATB was virucidal against both preparations of EBO, inactivating the purified virus within 20 min even when diluted 1:100 with the growth medium. Inactivation of the virus in tissue preparations was also complete, but required 1:10 dilutions with media or higher. After treatment with ATB (10 and 1% concentrations), no EBO was apparent even after two passages in Vero cell culture. These data indicate that the nanoemulsion is an effective disinfectant for EBO. Because of the excellent biocompatibility of nanoemulsions, studies are planned to determine whether the nanoemulsion-killed virus is suitable for developing a vaccine against EBO.

Establishment of fruit bat cells (Rousettus aegyptiacus) as a model system for the investigation of filoviral infection

Background

The fruit bat species Rousettus aegyptiacus was identified as a potential reservoir for the highly pathogenic filovirus Marburg virus. To establish a basis for a molecular understanding of the biology of filoviruses in the reservoir host, we have adapted a set of molecular tools for investigation of filovirus replication in a recently developed cell line, R06E, derived from the species Rousettus aegyptiacus.

Methodology/Principal Findings

Upon infection with Ebola or Marburg viruses, R06E cells produced viral titers comparable to VeroE6 cells, as shown by TCID₅₀ analysis. Electron microscopic analysis of infected cells revealed morphological signs of filovirus infection as described for human- and monkey-derived cell lines. Using R06E cells, we detected an unusually high amount of intracellular viral proteins, which correlated with the accumulation of high numbers of filoviral nucleocapsids in the cytoplasm. We established protocols to produce Marburg infectious virus-like particles from R06E cells, which were then used to infect naïve target cells to investigate primary transcription. This was not possible with other cell lines previously tested. Moreover, we established protocols to reliably rescue recombinant Marburg viruses from R06E cells.

Conclusion/Significance

These data indicated that R06E cells are highly suitable to investigate the biology of filoviruses in cells derived from their presumed reservoir.

Marburg virus and several species of Ebola virus are endemic in central Africa and cause sporadic outbreaks in this region with mortality rates of up to 90%. So far, there is no vaccination or therapy available to protect people at risk in these regions. Recently, different fruit bats have been identified as potential reservoirs. One of them is Rousettus aegyptiacus. It seems that within huge bat populations only relatively small numbers are positive for filovirus-specific antibodies or filoviral RNA, a phenomenon that is currently not understood. As a first step towards understanding the biology of filoviruses in bats, we sought to establish a model system to investigate filovirus replication in cells derived from their natural reservoir. Here, we provide the first insights into this topic by monitoring filovirus infection of a Rousettus aegyptiacus derived cell line, R06E. We were able to show that filoviruses propagate well in R06E cells, which can, therefore, be used to investigate replication and transcription of filovirus RNA and to very efficiently perform rescue of recombinant Marburg virus using reverse genetics. These results emphasize the suitability of the newly established bat cell line for filovirus research.

Spontaneous Mutation at Amino Acid 544 of the Ebola Virus Glycoprotein Potentiates Virus Entry and Selection in Tissue Culture

Ebolaviruses have a surface glycoprotein (GP1,2) that is required for virus attachment and entry into cells. Mutations affecting GP1,2 functions can alter virus growth properties. We generated a recombinant vesicular stomatitis virus encoding Ebola virus Makona variant GP1,2 (rVSV-MAK-GP) and observed emergence of a T544I mutation in the Makona GP1,2 gene during tissue culture passage in certain cell lines. The T544I mutation emerged within two passages when VSV-MAK-GP was grown on Vero E6, Vero, and BS-C-1 cells but not when it was passaged on Huh7 and HepG2 cells. The mutation led to a marked increase in virus growth kinetics and conferred a robust growth advantage over wild-type rVSV-MAK-GP on Vero E6 cells. Analysis of complete viral genomes collected from patients in western Africa indicated that this mutation was not found in Ebola virus clinical samples. However, we observed the emergence of T544I during serial passage of various Ebola Makona isolates on Vero E6 cells. Three independent isolates showed emergence of T544I from undetectable levels in nonpassaged virus or virus passaged once to frequencies of greater than 60% within a single passage, consistent with it being a tissue culture adaptation. Intriguingly, T544I is not found in any Sudan, Bundibugyo, or Tai Forest ebolavirus sequences. Furthermore, T544I did not emerge when we serially passaged recombinant VSV encoding GP1,2 from these ebolaviruses. This report provides experimental evidence that the spontaneous mutation T544I is a tissue culture adaptation in certain cell lines and that it may be unique for the species Zaire ebolavirusIMPORTANCE The Ebola virus (Zaire) species is the most lethal species of all ebolaviruses in terms of mortality rate and number of deaths. Understanding how the Ebola virus surface glycoprotein functions to facilitate entry in cells is an area of intense research. Recently, three groups independently identified a polymorphism in the Ebola glycoprotein (I544) that enhanced virus entry, but they did not agree in their conclusions regarding its impact on pathogenesis. Our findings here address the origins of this polymorphism and provide experimental evidence showing that it is the result of a spontaneous mutation (T544I) specific to tissue culture conditions, suggesting that it has no role in pathogenesis. We further show that this mutation may be unique to the species Zaire ebolavirus, as it does not occur in Sudan, Bundibugyo, and Tai Forest ebolaviruses. Understanding the mechanism behind this mutation can provide insight into functional differences that exist in culture conditions and among ebolavirus glycoproteins.

Filovirus Strategies to Escape Antiviral Responses

This chapter describes the various strategies filoviruses use to escape host immune responses with a focus on innate immune and cell death pathways. Since filovirus replication can be efficiently blocked by interferon (IFN), filoviruses have evolved mechanisms to counteract both type I IFN induction and IFN response signaling pathways. Intriguingly, marburg- and ebolaviruses use different strategies to inhibit IFN signaling. This chapter also summarizes what is known about the role of IFN-stimulated genes (ISGs) in filovirus infection. These fall into three categories: those that restrict filovirus replication, those whose activation is inhibited by filoviruses, and those that have no measurable effect on viral replication. In addition to innate immunity, mammalian cells have evolved strategies to counter viral infections, including the induction of cell death and stress response pathways, and we summarize our current knowledge of how filoviruses interact with these pathways. Finally, this chapter delves into the interaction of EBOV with myeloid dendritic cells and macrophages and the associated inflammatory response, which differs dramatically between these cell types when they are infected with EBOV. In summary, we highlight the multifaceted nature of the host-viral interactions during filoviral infections.

Molecular mechanism of de novo replication by the Ebola virus polymerase

Non-segmented negative-strand RNA viruses, including Ebola virus (EBOV), rabies virus, human respiratory syncytial virus and pneumoviruses, can cause respiratory infections, haemorrhagic fever and encephalitis in humans and animals, and are considered a substantial health and economic burden worldwide1. Replication and transcription of the viral genome are executed by the large (L) polymerase, which is a promising target for the development of antiviral drugs. Here, using the L polymerase of EBOV as a representative, we show that de novo replication of L polymerase is controlled by the specific 3' leader sequence of the EBOV genome in an enzymatic assay, and that formation of at least three base pairs can effectively drive the elongation process of RNA synthesis independent of the specific RNA sequence. We present the high-resolution structures of the EBOV L-VP35-RNA complex and show that the 3' leader RNA binds in the template entry channel with a distinctive stable bend conformation. Using mutagenesis assays, we confirm that the bend conformation of the RNA is required for the de novo replication activity and reveal the key residues of the L protein that stabilize the RNA conformation. These findings provide a new mechanistic understanding of RNA synthesis for polymerases of non-segmented negative-strand RNA viruses, and reveal important targets for the development of antiviral drugs.

Ebola and Marburg viruses: II. Thier development within Vero cells and the extra-cellular formation of branched and torus forms

The development of Marburg virus and the Sudanese and Zaire strains of Ebola virus in Vero cells as visualized by electron microscopy is described. Despite differences in timing, all three strains appear to pass through identical stages of development. Initially there is a large increase in nucleolus material, and viral precursor material arranges itself in spirals and then into tubes. The cells fill with core material, which passes to the plasmalemma, which often proliferates. Each virion passes through the plasmalemma, acquiring a coat of host material. The formation of torus forms is discussed; the branched appearance that is often seen is believed to be an aberrant form. The reasons for this view are put forward.

A luciferase-based budding assay for Ebola virus

The VP40 matrix protein of Ebola virus (EBOV) is capable of budding from mammalian cells as a virus-like particle (VLP) and is the major protein involved in virus egress. A functional budding assay has been developed based upon this characteristic of VP40 to assess the contributions of VP40 sequences as well as host proteins to the budding process. This well-defined assay has been modified for potential use in a high-throughput format in which the detection and quantification of firefly luciferase protein in VLPs represents a direct measure of VP40 budding efficiency. Luciferase was found to be incorporated into budding VP40 VLPs. Furthermore, co-expression of EBOV glycoprotein (GP) enhances release of VLPs containing VP40 and luciferase. In contrast, when luciferase is co-expressed with a budding deficient mutant of VP40, luciferase levels in the VLP fraction decrease significantly. This assay represents a promising high-throughput approach to identify inhibitors of EBOV budding.

[Ebola virus host cell entry]

Ebola virus is an enveloped virus with filamentous structure and causes a severe hemorrhagic fever in human and nonhuman primates. Host cell entry is the first essential step in the viral life cycle, which has been extensively studied as one of the therapeutic targets. A virus factor of cell entry is a surface glycoprotein (GP), which is an only essential viral protein in the step, as well as the unique particle structure. The virus also interacts with a lot of host factors to successfully enter host cells. Ebola virus at first binds to cell surface proteins and internalizes into cells, followed by trafficking through endosomal vesicles to intracellular acidic compartments. There, host proteases process GPs, which can interact with an intracellular receptor. Then, under an appropriate circumstance, viral and endosomal membranes are fused, which is enhanced by major structural changes of GPs, to complete host cell entry. Recently the basic research of Ebola virus infection mechanism has markedly progressed, largely contributed by identification of host factors and detailed structural analyses of GPs. This article highlights the mechanism of Ebola virus host cell entry, including recent findings.