Filoviridae: a taxonomic home for Marburg and Ebola viruses?

Characterizing alpha helical properties of Ebola viral proteins as potential targets for inhibition of alpha-helix mediated protein-protein interactions

Ebola, considered till recently as a rare and endemic disease, has dramatically transformed into a potentially global humanitarian crisis. The genome of Ebola, a member of the Filoviridae family, encodes seven proteins. Based on the recently implemented software (PAGAL) for analyzing the hydrophobicity and amphipathicity properties of alpha helices (AH) in proteins, we characterize the helices in the Ebola proteome. We demonstrate that AHs with characteristically unique features are involved in critical interactions with the host proteins. For example, the Ebola virus membrane fusion subunit, GP2, from the envelope glycoprotein ectodomain has an AH with a large hydrophobic moment. The neutralizing antibody (KZ52) derived from a human survivor of the 1995 Kikwit outbreak recognizes a protein epitope on this AH, emphasizing the critical nature of this secondary structure in the virulence of the Ebola virus. Our method ensures a comprehensive list of such `hotspots'. These helices probably are or can be the target of molecules designed to inhibit AH mediated protein-protein interactions. Further, by comparing the AHs in proteins of the related Marburg viruses, we are able to elicit subtle changes in the proteins that might render them ineffective to previously successful drugs. Such differences are difficult to identify by a simple sequence or structural alignment. Thus, analyzing AHs in the small Ebola proteome can aid rational design aimed at countering the `largest Ebola epidemic, affecting multiple countries in West Africa' ( http://www.cdc.gov/vhf/ebola/outbreaks/2014-west-africa/index.html).

Characterizing alpha helical properties of Ebola viral proteins as potential targets for inhibition of alpha-helix mediated protein-protein interactions

Ebola, considered till recently as a rare and endemic disease, has dramatically transformed into a potentially global humanitarian crisis. The genome of Ebola, a member of the Filoviridae family, encodes seven proteins. Based on the recently implemented software (PAGAL) for analyzing the hydrophobicity and amphipathicity properties of alpha helices (AH) in proteins, we characterize the helices in the Ebola proteome. We demonstrate that AHs with characteristically unique features are involved in critical interactions with the host proteins. For example, the Ebola virus membrane fusion subunit, GP2, from the envelope glycoprotein ectodomain has an AH with a large hydrophobic moment. The neutralizing antibody (KZ52) derived from a human survivor of the 1995 Kikwit outbreak recognizes a protein epitope on this AH, emphasizing the critical nature of this secondary structure in the virulence of the Ebola virus. Our method ensures a comprehensive list of such `hotspots'. These helices probably are or can be the target of molecules designed to inhibit AH mediated protein-protein interactions. Further, by comparing the AHs in proteins of the related Marburg viruses, we are able to elicit subtle changes in the proteins that might render them ineffective to previously successful drugs. Such differences are difficult to identify by a simple sequence or structural alignment. Thus, analyzing AHs in the small Ebola proteome can aid rational design aimed at countering the `largest Ebola epidemic, affecting multiple countries in West Africa' ( http://www.cdc.gov/vhf/ebola/outbreaks/2014-west-africa/index.html).

Filoviruses in bats: current knowledge and future directions

Filoviruses, including Ebolavirus and Marburgvirus, pose significant threats to public health and species conservation by causing hemorrhagic fever outbreaks with high mortality rates. Since the first outbreak in 1967, their origins, natural history, and ecology remained elusive until recent studies linked them through molecular, serological, and virological studies to bats. We review the ecology, epidemiology, and natural history of these systems, drawing on examples from other bat-borne zoonoses, and highlight key areas for future research. We compare and contrast results from ecological and virological studies of bats and filoviruses with those of other systems. We also highlight how advanced methods, such as more recent serological assays, can be interlinked with flexible statistical methods and experimental studies to inform the field studies necessary to understand filovirus persistence in wildlife populations and cross-species transmission leading to outbreaks. We highlight the need for a more unified, global surveillance strategy for filoviruses in wildlife, and advocate for more integrated, multi-disciplinary approaches to understand dynamics in bat populations to ultimately mitigate or prevent potentially devastating disease outbreaks.

Cellular factors implicated in filovirus entry

Although filoviral infections are still occurring in different parts of the world, there are no effective preventive or treatment strategies currently available against them. Not only do filoviruses cause a deadly infection, but they also have the potential of being used as biological weapons. This makes it imperative to comprehensively study these viruses in order to devise effective strategies to prevent the occurrence of these infections. Entry is the foremost step in the filoviral replication cycle and different studies have reported the involvement of a myriad of cellular factors including plasma membrane components, cytoskeletal proteins, endosomal components, and cytosolic factors in this process. Signaling molecules such as the TAM family of receptor tyrosine kinases comprising of Tyro3, Axl, and Mer have also been implicated as putative entry factors. Additionally, filoviruses are suggested to bind to a common receptor and recent studies have proposed T-cell immunoglobulin and mucin domain 1 (TIM-1) and Niemann-Pick C1 (NPC1) as potential receptor candidates. This paper summarizes the existing literature on filoviral entry with a special focus on cellular factors involved in this process and also highlights some fundamental questions. Future research aimed at answering these questions could be very useful in designing novel antiviral therapeutics.

Potential vaccines and post-exposure treatments for filovirus infections

Viruses of the family Filoviridae represent significant health risks as emerging infectious diseases as well as potentially engineered biothreats. While many research efforts have been published offering possibilities toward the mitigation of filoviral infection, there remain no sanctioned therapeutic or vaccine strategies. Current progress in the development of filovirus therapeutics and vaccines is outlined herein with respect to their current level of testing, evaluation, and proximity toward human implementation, specifically with regard to human clinical trials, nonhuman primate studies, small animal studies, and in vitro development. Contemporary methods of supportive care and previous treatment approaches for human patients are also discussed.

Structural dissection of Ebola virus and its assembly determinants using cryo-electron tomography

Ebola virus is a highly pathogenic filovirus causing severe hemorrhagic fever with high mortality rates. It assembles heterogenous, filamentous, enveloped virus particles containing a negative-sense, single-stranded RNA genome packaged within a helical nucleocapsid (NC). We have used cryo-electron microscopy and tomography to visualize Ebola virus particles, as well as Ebola virus-like particles, in three dimensions in a near-native state. The NC within the virion forms a left-handed helix with an inner nucleoprotein layer decorated with protruding arms composed of VP24 and VP35. A comparison with the closely related Marburg virus shows that the N-terminal region of nucleoprotein defines the inner diameter of the Ebola virus NC, whereas the RNA genome defines its length. Binding of the nucleoprotein to RNA can assemble a loosely coiled NC-like structure; the loose coil can be condensed by binding of the viral matrix protein VP40 to the C terminus of the nucleoprotein, and rigidified by binding of VP24 and VP35 to alternate copies of the nucleoprotein. Four proteins (NP, VP24, VP35, and VP40) are necessary and sufficient to mediate assembly of an NC with structure, symmetry, variability, and flexibility indistinguishable from that in Ebola virus particles released from infected cells. Together these data provide a structural and architectural description of Ebola virus and define the roles of viral proteins in its structure and assembly.

Ebola and Marburg haemorrhagic fever viruses: major scientific advances, but a relatively minor public health threat for Africa

Ebola and Marburg viruses are the only members of the Filoviridae family (order Mononegavirales), a group of viruses characterized by a linear, non-segmented, single-strand negative RNA genome. They are among the most virulent pathogens for humans and great apes, causing acute haemorrhagic fever and death within a matter of days. Since their discovery 50 years ago, filoviruses have caused only a few outbreaks, with 2317 clinical cases and 1671 confirmed deaths, which is negligible compared with the devastation caused by malnutrition and other infectious diseases prevalent in Africa (malaria, cholera, AIDS, dengue, tuberculosis …). Yet considerable human and financial resourses have been devoted to research on these viruses during the past two decades, partly because of their potential use as bioweapons. As a result, our understanding of the ecology, host interactions, and control of these viruses has improved considerably.

Ebola haemorrhagic fever

Ebola viruses are the causative agents of a severe form of viral haemorrhagic fever in man, designated Ebola haemorrhagic fever, and are endemic in regions of central Africa. The exception is the species Reston Ebola virus, which has not been associated with human disease and is found in the Philippines. Ebola virus constitutes an important local public health threat in Africa, with a worldwide effect through imported infections and through the fear of misuse for biological terrorism. Ebola virus is thought to also have a detrimental effect on the great ape population in Africa. Case-fatality rates of the African species in man are as high as 90%, with no prophylaxis or treatment available. Ebola virus infections are characterised by immune suppression and a systemic inflammatory response that causes impairment of the vascular, coagulation, and immune systems, leading to multiorgan failure and shock, and thus, in some ways, resembling septic shock.

Proposal for a revised taxonomy of the family Filoviridae: classification, names of taxa and viruses, and virus abbreviations

The taxonomy of the family Filoviridae (marburgviruses and ebolaviruses) has changed several times since the discovery of its members, resulting in a plethora of species and virus names and abbreviations. The current taxonomy has only been partially accepted by most laboratory virologists. Confusion likely arose for several reasons: species names that consist of several words or which (should) contain diacritical marks, the current orthographic identity of species and virus names, and the similar pronunciation of several virus abbreviations in the absence of guidance for the correct use of vernacular names. To rectify this problem, we suggest (1) to retain the current species names Reston ebolavirus, Sudan ebolavirus, and Zaire ebolavirus, but to replace the name Cote d'Ivoire ebolavirus [sic] with Taï Forest ebolavirus and Lake Victoria marburgvirus with Marburg marburgvirus; (2) to revert the virus names of the type marburgviruses and ebolaviruses to those used for decades in the field (Marburg virus instead of Lake Victoria marburgvirus and Ebola virus instead of Zaire ebolavirus); (3) to introduce names for the remaining viruses reminiscent of jargon used by laboratory virologists but nevertheless different from species names (Reston virus, Sudan virus, Taï Forest virus), and (4) to introduce distinct abbreviations for the individual viruses (RESTV for Reston virus, SUDV for Sudan virus, and TAFV for Taï Forest virus), while retaining that for Marburg virus (MARV) and reintroducing that used over decades for Ebola virus (EBOV). Paying tribute to developments in the field, we propose (a) to create a new ebolavirus species (Bundibugyo ebolavirus) for one member virus (Bundibugyo virus, BDBV); (b) to assign a second virus to the species Marburg marburgvirus (Ravn virus, RAVV) for better reflection of now available high-resolution phylogeny; and (c) to create a new tentative genus (Cuevavirus) with one tentative species (Lloviu cuevavirus) for the recently discovered Lloviu virus (LLOV). Furthermore, we explain the etymological derivation of individual names, their pronunciation, and their correct use, and we elaborate on demarcation criteria for each taxon and virus.

Ebola virus uses clathrin-mediated endocytosis as an entry pathway

Ebola virus (EBOV) infects several cell types and while viral entry is known to be pH-dependent, the exact entry pathway(s) remains unknown. To gain insights into EBOV entry, the role of several inhibitors of clathrin-mediated endocytosis in blocking infection mediated by HIV pseudotyped with the EBOV envelope glycoprotein (EbGP) was examined. Wild type HIV and envelope-minus HIV pseudotyped with Vesicular Stomatitis Virus glycoprotein (VSVg) were used as controls to assess cell viability after inhibiting clathrin pathway. Inhibition of clathrin pathway using dominant-negative Eps15, siRNA-mediated knockdown of clathrin heavy chain, chlorpromazine and sucrose blocked EbGP pseudotyped HIV infection. Also, both chlorpromazine and Bafilomycin A1 inhibited entry of infectious EBOV. Sensitivity of EbGP pseudotyped HIV as well as infectious EBOV to inhibitors of clathrin suggests that EBOV uses clathrin-mediated endocytosis as an entry pathway. Furthermore, since chlorpromazine inhibits EBOV infection, novel therapeutic modalities could be designed based on this lead compound.

Ebolavirus VP24 binding to karyopherins is required for inhibition of interferon signaling

The Ebolavirus VP24 protein counteracts alpha/beta interferon (IFN-alpha/beta) and IFN-gamma signaling by blocking the nuclear accumulation of tyrosine-phosphorylated STAT1 (PY-STAT1). According to the proposed model, VP24 binding to members of the NPI-1 subfamily of karyopherin alpha (KPNalpha) nuclear localization signal receptors prevents their binding to PY-STAT1, thereby preventing PY-STAT1 nuclear accumulation. This study now identifies two domains of VP24 required for inhibition of IFN-beta-induced gene expression and PY-STAT1 nuclear accumulation. We demonstrate that loss of function correlates with loss of binding to KPNalpha proteins. Thus, the VP24 IFN antagonist function requires the ability of VP24 to interact with KPNalpha.

Ebola virus VP30 is an RNA binding protein

The Ebola virus (EBOV) genome encodes for several proteins that are necessary and sufficient for replication and transcription of the viral RNAs in vitro; NP, VP30, VP35, and L. VP30 acts in trans with an RNA secondary structure upstream of the first transcriptional start site to modulate transcription. Using a bioinformatics approach, we identified a region within the N terminus of VP30 with sequence features that typify intrinsically disordered regions and a putative RNA binding site. To experimentally assess the ability of VP30 to directly interact with the viral RNA, we purified recombinant EBOV VP30 to >90% homogeneity and assessed RNA binding by UV cross-linking and filter-binding assays. VP30 is a strongly acidophilic protein; RNA binding became stronger as pH was decreased. Zn(2+), but not Mg(2+), enhanced activity. Enhancement of transcription by VP30 requires a RNA stem-loop located within nucleotides 54 to 80 of the leader region. VP30 showed low binding affinity to the predicted stem-loop alone or to double-stranded RNA but showed a good binding affinity for the stem-loop when placed in the context of upstream and downstream sequences. To map the region responsible for interacting with RNA, we constructed, purified, and assayed a series of N-terminal deletion mutations of VP30 for RNA binding. The key amino acids supporting RNA binding activity map to residues 26 to 40, a region rich in arginine. Thus, we show for the first time the direct interaction of EBOV VP30 with RNA and the importance of the N-terminal region for binding RNA.

Ebolavirus and other filoviruses

Since Ebola fever emerged in Central Africa in 1976, a number of studies have been undertaken to investigate its natural history and to characterize its transmission from a hypothetical reservoir host(s) to humans. This research has comprised investigations on a variety of animals and their characterization as intermediate, incidental, amplifying, reservoir, or vector hosts. A viral transmission chain was recently unveiled after a long absence of epidemic Ebola fever. Animal trapping missions were carried out in the Central African rain forest in an area where several epidemics and epizootics had occurred between 2001 and 2005. Among the various animals captured and analyzed, three species of fruit bats (suborder Megachiroptera) were found asymptomatically and naturally infected with Ebola virus: Hypsignathus monstrosus (hammer-headed fruit beats), Epomops franqueti (singing fruit bats), and Myonycteris torquata (little collared fruit bats). From experimental data, serological studies and virus genetic analysis, these findings confirm the importance of these bat species as potential reservoir species of Ebola virus in Central Africa. While feeding bats drop partially eaten fruit and masticated fruit pulp (spats) to the ground, possibly promoting indirect transmission of Ebola virus to certain ground dwelling mammals, if virus is being shed in saliva by chronically and asymptomatically infected bats. Great apes and forest duikers are particularly sensitive to lethal Ebola virus infection. These terrestrial mammals feed on fallen fruits and possibly spats, suggesting a chain of events leading to Ebola virus spillover to these incidental hosts. This chain of events may occur sporadically at different sites and times depending on a combination of the phenology of fruit production by different trees, animal behavior, and various, but as yet still unknown environmental factors, which could include drought. During the reproductive period, infected body fluid can also be shed in the environment and present a potential risk for indirect transmission to other vertebrates.

The ERK mitogen-activated protein kinase pathway contributes to Ebola virus glycoprotein-induced cytotoxicity

Ebola virus is a highly lethal pathogen that causes hemorrhagic fever in humans and nonhuman primates. Among the seven known viral gene products, the envelope glycoprotein (GP) alone induces cell rounding and detachment that ultimately leads to cell death. Cellular cytoxicity is not seen with comparable levels of expression of a mutant form of GP lacking a mucin-like domain (GPDeltamuc). GP-induced cell death is nonapoptotic and is preceded by downmodulation of cell surface molecules involved in signaling pathways, including certain integrins and epidermal growth factor receptor. To investigate the mechanism of GP-induced cellular toxicity, we analyzed the activation of several signal transduction pathways involved in cell growth and survival. The active form of extracellular signal-regulated kinases types 1 and 2 (ERK1/2), phospho-ERK1/2, was reduced in cells expressing GP compared to those expressing GPDeltamuc as determined by flow cytometry, in contrast to the case for several other signaling proteins. Subsequent analysis of the activation states and kinase activities of related kinases revealed a more pronounced effect on the ERK2 kinase isoform. Disruption of ERK2 activity by a dominant negative ERK or by small interfering RNA-mediated ERK2 knockdown potentiated the decrease in alphaV integrin expression associated with toxicity. Conversely, activation of the pathway through the expression of a constitutively active form of ERK2 significantly protected against this effect. These results indicate that the ERK signaling cascade mediates GP-mediated cytotoxicity and plays a role in pathogenicity induced by this gene product.

Production of monoclonal antibodies and development of an antigen capture ELISA directed against the envelope glycoprotein GP of Ebola virus

Ebola virus (EBOV) causes severe outbreaks of Ebola hemorrhagic fever in endemic regions of Africa and is considered to be of impact for other parts of the world as an imported viral disease. To develop a new diagnostic test, monoclonal antibodies to EBOV were produced from mice immunized with inactivated EBOV species Zaire. Antibodies directed against the viral glycoprotein GP were characterized by ELISA, Western blot and immunofluorescence analyses. An antigen capture ELISA was established, which is specific for EBOV-Zaire and shows a sensitivity of approximately 10(3) plaque-forming units/ml. Since the ELISA is able to detect even SDS-inactivated EBOV in spiked human sera, it could complement the existing diagnostic tools in the field and in routine laboratories where high containment facilities are not available.

Polyploid measles virus with hexameric genome length

Particles of most virus species accurately package a single genome, but there are indications that the pleomorphic particles of parainfluenza viruses incorporate multiple genomes. We characterized a stable measles virus mutant that efficiently packages at least two genomes. The first genome is recombinant and codes for a defective attachment protein with an appended domain interfering with fusion-support function. The second has one adenosine insertion in a purine run that interrupts translation of the appended domain and restores function. In that genome, a one base deletion in a different purine run abolishes polymerase synthesis, but restores hexameric genome length, thus ensuring accurate RNA encapsidation, which is necessary for efficient replication. Thus, the two genomes are complementary. The infection kinetics of this mutant indicate that packaging of multiple genomes does not negatively affect growth. We also show that polyploid particles are produced in standard infections at no expense to infectivity. Our results illustrate how the particles of parainfluenza viruses efficiently accommodate cargoes of different volume, and suggest a mechanism by which segmented genomes may have evolved.

Viral hemorrhagic fever hazards for travelers in Africa

This short review covers 6 viral hemorrhagic fevers (VHFs) that are known to occur in Africa: yellow fever, Rift Valley fever, Crimean-Congo hemorrhagic fever, Lassa fever, Marburg virus disease, and Ebola hemorrhagic fever. All of these have at one time or another affected travelers, often the adventurous kind who are "roughing it" in rural areas, who should therefore be made aware by their physicians or travel health clinics about their potential risk of exposure to any VHF along their travel route and how to minimize the risk. A significant proportion of VHF cases involving travelers have affected expatriate health care workers who were nosocomially exposed in African hospitals or clinics. The VHFs are associated with a high case-fatality rate but are readily prevented by well-known basic precautions.

Suppressive effect of Ebola virus on T cell proliferation in vitro is provided by a 125-kDa GP viral protein

Ebola virus (EV), an extremely infectious pathogen, causes severe hemorrhagic fever in humans and nonhuman primates. The disease pattern includes damage of parenchymal cells of vital organs in association with hemostatic and immune disorders. Vaccination with the inactivated virions does not provide an effective immune protection against the disease. The inadequate immune response may be directly caused by the virus, and, hence, it may presumably be crucial in the pathogenic process and prophylactic treatment of Ebola infection. The suggested immunosuppressive properties of EV were examined in this study. We have demonstrated that the whole heat-inactivated virions can dose-dependently suppress human lymphocyte mitogen-stimulated proliferation in vitro. In further analyses, we identified the viral protein responsible for the suppressive effect, and we showed that it was provided by a protein corresponding to a 125-kDa envelope glycoprotein (GP-125). The protein alone inhibited lymphocyte proliferation, whereas the other viral proteins were without significant effect on blastogenesis. To determine the immunosuppressive properties of different portions of GP-125, deletion mutants of GP were designed based on predicted localisation of antigen sites. They were expressed as recombinant proteins and studied in proliferation assays. We identified a 40-amino acid sequence at the N-terminus of GP-125 that exerted a suppressive effect on blastogenesis.

The nucleoprotein of Marburg virus is target for multiple cellular kinases

The nucleoprotein (NP) of Marburg virus is phosphorylated at serine and threonine residues in a ratio of 85:15, regardless of whether the protein is isolated from virions or from eukaryotic expression systems. Phosphotyrosine is absent. Although many potential phosphorylation sites are located in the N-terminal half of NP, this part of the protein is not phosphorylated. Analyses of phosphorylation state and phosphoamino acid content of truncated NPs expressed in HeLa cells using the vaccinia virus T7 expression system led to the identification of seven phosphorylated regions (region I*, amino acids 404-432; II*, amino acids 446-472; III*, amino acids 484-511; IV*, amino acids 534-543; V*, amino acid 549; VI*, amino acids 599-604; and VII*, amino acid 619) with a minimum of seven phosphorylated amino acid residues located in the C-terminal half of NP. All phosphothreonine residues and consensus recognition sequences for protein kinase CKII are located in regions I*-V*. Regions VI* and VII* contain only phosphoserine with three of four serine residues in consensus recognition motifs for proline-directed protein kinases. Mutagenesis of proline-adjacent serine residues to alanine or aspartic acid did not influence the function of NP in a reconstituted transcription/replication system; thus it is concluded that serine phosphorylation in the most C-terminal part of NP is not a regulatory factor in viral RNA synthesis.

Emerging Viruses

Diseases caused by microbial infections are present throughout human evolution. Large proportions are the result of virus infections. A commonly cited example of resurgent or recurrent disease is the yearly appearance of new antigenically different influenza viruses. These new variants are able to evoke disease in their host while causing the centuries-old symptoms of influenza. However, conventional virological techniques have failed to identify the agent, even though evidence suggested non-A, non-B hepatitis (NANBH) is caused by a blood-borne, small enveloped virus, readily transmissible to chimpanzees. Modern molecular biology techniques are used to identify completely new viruses. These viruses can be associated with a new disease or associated with a well-characterized disease present in humans for many years. In identifying an emerging virus, one is often presented with epidemiological data and clinical specimens that have no reactivity with diagnostic reagents available for known pathogens. The primary aim therefore is to identify any new infectious agent and build a body of data to support the existence of a causal link between organism and disease.

Marburg virus vaccines based upon alphavirus replicons protect guinea pigs and nonhuman primates

Marburg virus (MBGV), for which no vaccines or treatments currently exist, causes an acute hemorrhagic fever with a high mortality rate in humans. We previously showed that immunization with either killed MBGV or a glycoprotein (GP) subunit prevented lethal infection in guinea pigs. In the studies reported here, an RNA replicon, based upon Venezuelan equine encephalitis (VEE) virus, was used as a vaccine vector; the VEE structural genes were replaced by genes for MBGV GP, nucleoprotein (NP), VP40, VP35, VP30, or VP24. Guinea pigs were vaccinated with recombinant VEE replicons (packaged into VEE-like particles), inoculated with MBGV, and evaluated for viremia and survival. Results indicated that either GP or NP were protective antigens while VP35 afforded incomplete protection. As a more definitive test of vaccine efficacy, nonhuman primates (cynomolgus macaques) were inoculated with VEE replicons expressing MBGV GP and/or NP. Three monkeys received packaged control replicons (influenza HA); these died 9 or 10 days after challenge, with typical MBGV disease. MBGV NP afforded incomplete protection, sufficient to prevent death but not disease in two of three macaques. Three monkeys vaccinated with replicons which expressed MBGV GP, and three others vaccinated with both replicons that expressed GP or NP, remained aviremic and were completely protected from disease.

Characterization of Ebola virus entry by using pseudotyped viruses: identification of receptor-deficient cell lines

Studies analyzing Ebola virus replication have been severely hampered by the extreme pathogenicity of this virus. To permit analysis of the host range and function of the Ebola virus glycoprotein (Ebo-GP), we have developed a system for pseudotyping these glycoproteins into murine leukemia virus (MLV). This pseudotyped virus, MLV(Ebola), can be readily concentrated to titers which exceed 5 x 10(6) infectious units/ml and is effectively neutralized by antibodies specific for Ebo-GP. Analysis of MLV(Ebola) infection revealed that the host range conferred by Ebo-GP is very broad, extending to cells of a variety of species. Notably, all lymphoid cell lines tested were completely resistant to infection; we speculate that this is due to the absence of a cellular receptor for Ebo-GP on B and T cells. The generation of high-titer MLV(Ebola) pseudotypes will be useful for the analysis of immune responses to Ebola virus infection, development of neutralizing antibodies, analysis of glycoprotein function, and isolation of the cellular receptor(s) for the Ebola virus.

Threat to Humans from Virus Infections of Non-human Primates

Several hundred distinct non human primate species are recognised, and they are likely to harbour a similar range of viruses to humans. Simians such as cynomolgus and rhesus macaques, African green monkeys, and marmosets are widely used for biomedical research, but despite this extensive close contact very few simian viruses have been shown to pose a threat of infection or illness to humans. Herpesvirus Simiae is the best recognised zoonotic hazard of simians. It is an alphaherpes virus of Asiatic macaques, which causes a mild or subclinical primary infection followed by latency in its natural host. It can be acquired by humans following a bite and causes an ascending meningoencephalitis. Less than 40 human cases have been described and the mortality rate in untreated human infections is 70%. The infection is treatable with acyclovir and extensive guidelines for managing simians and potential exposures have been developed. Ebola virus and Marburg virus have caused epizootics in cynomolgus macaques and vervet monkeys respectively, which have resulted in human infection and fatalities. However, non human primates are unlikely to be their natural host. More recently simian immunodeficiency virus and simian foamy virus have infected researchers, but infection has not been linked to illness. Simian viruses also pose a direct threat to humans through the use of primary monkey tissue cultures in laboratory work and vaccine manufacture, indeed a significant exposure of the human population occurred when cells contaminated with SV40 a polyomavirus of rhesus monkeys were used for polio vaccine production. New medical interventions such as xenotransplantation using primate organs pose a potential risk which requires careful assessment. Copyright 1997 by John Wiley & Sons Ltd.

Filovirus-induced endothelial leakage triggered by infected monocytes/macrophages

The pathogenetic mechanisms underlying hemorrhagic fevers are not fully understood, but hemorrhage, activation of coagulation, and shock suggest vascular instability. Here, we demonstrate that Marburg virus (MBG), a filovirus causing a severe form of hemorrhagic fever in humans, replicates in human monocytes/macrophages, resulting in cytolytic infection and release of infectious virus particles. Replication also led to intracellular budding and accumulation of viral particles in vacuoles, thus providing a mechanism by which the virus may escape immune surveillance. Monocytes/macrophages were activated by MBG infection as indicated by tumor necrosis factor alpha (TNF-alpha) release. Supernatants of monocyte/macrophage cultures infected with MBG increased the permeability of cultured human endothelial cell monolayers. The increase in endothelial permeability correlated with the time course of TNF-alpha release and was inhibited by a TNF-alpha specific monoclonal antibody. Furthermore, recombinant TNF-alpha added at concentrations present in supernatants of virus-infected macrophage cultures increased endothelial permeability in the presence of 10 micron H2O2. These results indicate that TNF-alpha plays a critical role in mediating increased permeability, which was identified as a paraendothelial route shown by formation of interendothelial gaps. The combination of viral replication in endothelial cells (H.-J. Schnittler, F. Mahner, D. Drenckhahn, H.-D. Klenk, and H. Feldmann, J. Clin. Invest. 19:1301-1309, 1993) and monocytes/macrophages and the permeability-increasing effect of virus-induced cytokine release provide the first experimental data for a novel concept in the pathogenesis of viral hemorrhagic fever.

Emerging and reemerging of filoviruses

Filoviruses are causative agents of a hemorrhagic fever in man with mortalities ranging from 22 to 88%. They are enveloped, nonsegmented negative-stranded RNA viruses and are separated into two types, Marburg and Ebola, which can be serologically, biochemically and genetically distinguished. In general, there is little genetic variability among viruses belonging to the Marburg type. The Ebola type, however, is subdivided into at least three distinct subtypes. Marburg virus was first isolated during an outbreak in Europe in 1967. Ebola virus emerged in 1976 as the causative agent of two simultaneous outbreaks in southern Sudan and northern Zaire. The reemergence of Ebola, subtype Zaire, in Kikwit 1995 caused a worldwide sensation, since it struck after a sensibilization on the danger of Ebola virus disease. Person-to-person transmission by intimate contact is the main route of infection, but transmission by droplets and small aerosols among infected individuals is discussed. The natural reservoir for filoviruses remains a mystery. Filoviruses are prime examples for emerging pathogens. Factors that may be involved in emergence are international commerce and travel, limited experience in diagnosis and case management, import of nonhuman primates, and the potential of filoviruses for rapid evolution.

Experimental infection of cynomolgus macaques with Ebola-Reston filoviruses from the 1989-1990 U.S. epizootic

This study describes the pathogenesis of the Ebola-Reston (EBO-R) subtype of Ebola virus for experimentally infected cynomolgus monkeys. The disease course of EBO-R in macaques was very similar to human disease and to experimental diseases in macaques following EBO-Zaire and EBO-Sudan infections. Cynomolgus monkeys infected with EBO-R in this experiment developed anorexia, occasional nasal discharge, and splenomegaly, petechial facial hemorrhages and severe subcutaneous hemorrhages in venipuncture sites, similar to human Ebola fever. Five of the six EBO-R infected monkeys died, 8 to 14 days after inoculation. One survived and developed high titered neutralizing antibodies specific for EBO-R. The five acutely ill monkeys shed infectious virus in various bodily secretions. Further, abundant virus was visualized in alveolar interstitial cells and free in the alveoli suggesting the potential for generating infectious aerosols. Thus, taking precautions against aerosol exposures to filovirus infected primates, including humans, seems prudent. This experiment demonstrated that EBO-R was lethal for macaques and was capable of initiating and sustaining the monkey epizootic. Further investigation of this animal model should facilitate development of effective immunization, treatment, and control strategies for Ebola hemorrhagic fever.

Differentiation of filoviruses by electron microscopy

Cultured monolayers of MA-104, Vero 76, SW-13, and DBS-FRhL-2 cells were infected with Marburg (MBG), Ebola-Sudan (EBO-S), Ebola-Zaire (EBO-Z), and Ebola-Reston (EBO-R) viruses (Filoviridae, Filovirus) and examined by electron microscopy to provide ultrastructural details of morphology and morphogenesis of these potential human pathogens. Replication of each filovirus was seen in all cell systems employed. Filoviral particles appeared to enter host cells by endocytosis. Filoviruses showed a similar progression of morphogenic events, from the appearance of nascent intracytoplasmic viral inclusions to formation of mature virions budded through plasma membranes, regardless of serotype or host cell. However, ultrastructural differences were demonstrated between MBG and other filoviruses. MBG virions recovered from culture fluids were uniformly shorter in mean unit length than EBO-S, EBO-Z, or EBO-R particles. Examination of filovirus-infected cells revealed that intermediate MBG inclusions were morphologically distinct from EBO-S, EBO-Z, and EBO-R inclusions. No structural difference of viral inclusion material was observed among EBO-S, EBO-Z, and EBO-R. Immunoelectron microscopy showed that the filoviral matrix protein (VP40) and nucleoprotein (NP) accumulated in EBO-Z inclusions, and were closely associated during viral morphogenesis. These details facilitate the efficient and definitive diagnosis of filoviral infections by electron microscopy.

Tissue factor initiation of disseminated intravascular coagulation in filovirus infection

Filovirus infections in humans and primates cause intrinsic activation of the clotting cascade. Tissue factor, the normal activator of the clotting cascade, is released into the bloodstream from activated leukocytes and viral budding from infected cells. This release of tissue factor, a trans-membrane protein found in large amounts in cells preferred by filoviruses for replication, initiates the hemorrhagic complications characteristic of filovirus infection. These complications contribute to the high mortality rates of filovirus infections. Directing chemotheraputic measures at the release of tissue factor, which causes the hemorrhagic complications, will result in significant reductions of mortality rates in man and primates.

The complete nucleotide sequence of the Popp (1967) strain of Marburg virus: a comparison with the Musoke (1980) strain

The nucleotide sequence of genomic RNA of Marburg virus strain Popp was determined. Strain Popp was isolated in 1967 during the first filoviral outbreak. The virus was purified from blood of infected guinea pigs in which it had been maintained. The length of the determined sequence was 19112 nucleotides. Amino acid sequences of seven known virion proteins were deduced. Nucleotide and amino acid sequences were compared with those of strain Musoke of Marburg virus isolated in 1980 in Kenya and purified from Vero cells. Homology between nucleotide sequences of two strains was 93.9%. Comparisons revealed conserved and variable regions of the nucleotide and amino acid sequences. The GP, the envelope protein of the virion, was found to be the most variable protein. The greatest differences in the protein were located in the supposedly external part of the molecule. Amino acid substitutions in the L protein, the main component of viral RNA-dependent RNA polymerase, were also distributed extremely non-randomly. It was shown that the non-coding regions of the genome were more variable than the coding ones; 37.6% of nucleotide differences corresponded to the former. 72.6% of nucleotide substitutions located in the coding regions were found to be at the third codon position.

Ebola protein analyses for the determination of genetic organization

Amino-acid sequencing of the purified major nucleoprotein (NP), VP35 and VP40 from purified Ebola virus proved that they are the protein products of the first three genes, and that the open reading frame (ORF) of the NP begins at nucleotide 470. Because of the many unusual features of the ORFs of Ebola virus, we thought that our conclusions should be substantiated. Comparisons of in vitro-translation products to purified viral proteins were used to demonstrate conclusively that the NP, VP35 and VP40 were the protein products of genes one, two, and three, respectively. Studies using antibodies to synthetic peptides matching the N- and C-termini of the deduced sequences from these genes confirmed these conclusions and that the ORF for the NP begins at nucleotide 470. Subsequent studies confirmed that VP30 is encoded by the fifth gene.

Replication of Marburg virus in human endothelial cells. A possible mechanism for the development of viral hemorrhagic disease

Marburg and Ebola virus, members of the family Filoviridae, cause a severe hemorrhagic disease in humans and primates. The disease is characterized as a pantropic virus infection often resulting in a fulminating shock associated with hemorrhage, and death. All known histological and pathophysiological parameters of the disease are not sufficient to explain the devastating symptoms. Previous studies suggested a nonspecific destruction of the endothelium as a possible mechanism. Concerning the important regulatory functions of the endothelium (blood pressure, anti-thrombogenicity, homeostasis), we examined Marburg virus replication in primary cultures of human endothelial cells and organ cultures of human umbilical cord veins. We show here that Marburg virus replicates in endothelial cells almost as well as in monkey kidney cells commonly used for virus propagation. Our data support the concept that the destruction of endothelial cells resulting from Marburg virus replication is a possible mechanism responsible for the hemorrhagic disease and the shock syndrome typical of this infection.

Marburg virus gene 4 encodes the virion membrane protein, a type I transmembrane glycoprotein

Gene 4 of Marburg virus, strain Musoke, was subjected to nucleotide sequence analysis. It is 2,844 nucleotides long and extends from genome position 5821 to position 8665 (EMBL Data Library, emnew: MVREPCYC [accession no. Z12132]). The gene is flanked by transcriptional signal sequences (start signal, 3'-UACUUCUUGUAAUU-5'; termination signal, 3'-UAAUUCUUUUU-5') which are conserved in all Marburg virus genes. The major open reading frame encodes a polypeptide of 681 amino acids (M(r), 74,797). After in vitro transcription and translation, as well as expression in Escherichia coli, this protein was identified by its immunoreactivity with specific antisera as the unglycosylated form of the viral membrane glycoprotein (GP). The GP is characterized by the following four different domains: (i) a hydrophobic signal peptide at the amino terminus (1 to 18), (ii) a predominantly hydrophilic external domain (19 to 643), (iii) a hydrophobic transmembrane anchor (644 to 673), and (iv) a small hydrophilic cytoplasmic tail at the carboxy terminus (674 to 681). Amino acid analysis indicated that the signal peptide is removed from the mature GP. The GP therefore has the structural features of a type I transmembrane glycoprotein. The external domain of the protein has 19 N-glycosylation sites and several clusters of hydroxyamino acids and proline residues that are likely to be the attachment sites for about 30 O-glycosidic carbohydrate chains. The region extending from positions 585 to 610 shows significant homology to a domain observed in the envelope proteins of several retroviruses and Ebola virus that has been suspected to be responsible for immunosuppressive properties of these viruses. A second open reading frame of gene 4 has the coding capacity for an unidentified polypeptide 112 amino acids long.

Molecular biology and evolution of filoviruses

The family Filoviridae contains extremely pathogenic human viruses causing a fulminating, febrile hemorrhagic disease. Filoviruses are enveloped, filamentous particles with a nonsegmented negative-strand RNA genome showing the gene arrangement 3'-NP-VP35-VP40-GP-VP30-VP24-L-5'. Genes are flanked by highly conserved transcriptional signals and are generally separated by variable intergenic regions. They are transcribed into monocistronic polyadenylated messenger RNAs which contain relatively long 5' and 3' untranslated regions. Seven structural proteins are encoded by the genome of which four form the helical nucleocapsid (NP-VP35-VP30-L), two are membrane-associated (VP40-VP24), and one is a transmembrane glycoprotein (GP). Comparison of filovirus genomes with those of other nonsegmented negative-strand RNA viruses suggest comparable mechanisms of transcription and replication and a common evolutionary lineage for all these viruses. Sequence analyses of single genes, however, showed that filoviruses are more closely related to paramyxoviruses, particularly human respiratory syncytial virus. These data support the concept of the taxonomic order Mononegavirales for all nonsegmented negative-strand RNA viruses and the classification of Marburg virus, Ebola virus, and Reston virus in the family Filoviridae, separate from the families Paramyxoviridae and Rhabdoviridae.

The envelope glycoprotein of Ebola virus contains an immunosuppressive-like domain similar to oncogenic retroviruses

Genomic RNA of a Zaire strain of Ebola virus was cloned, and cDNA inserts specific for the glycoprotein gene were isolated and sequenced. The determined sequence has only one open reading frame encoding 318 amino acids and is part of ORF-4 on the plus RNA strand. The putative transcriptional stop site (3' AAUUCUUUUU 5') and the transcriptional start site (3' AACUACUUCUAAUU..5') were identified. Computer-assisted comparison of the amino acid sequence of the C-terminal part of protein encoded by ORF-4 of Ebola virus with sequences of the proteins present in the SWISSPROT and EMBL banks revealed significant homology with the 'immunosuppressive domain' of the p15E envelope proteins of various oncogenic retroviruses. The possible role of such a homology is discussed.

Marburg virus, a filovirus: messenger RNAs, gene order, and regulatory elements of the replication cycle

The genome of Marburg virus (MBG), a filovirus, is 19.1 kb in length and thus the largest one found with negative-strand RNA viruses. The gene order - 3' untranslated region-NP-VP35-VP40-GP-VP30-VP24-L-5' untranslated region-resembles that of other non-segmented negative-strand (NNS) RNA viruses. Six species of polyadenylated subgenomic RNAs, isolated from MBG-infected cells, are complementary to the negative-strand RNA genome. They can be translated in vitro into the known structural proteins NP, GP (non-glycosylated form), VP40, VP35, VP30 and VP24. At the gene boundaries conserved transcriptional start (3'-NNCUNCNUNUAAUU-5') and stop signals (3'-UAAUUCUUUUU-5') are located containing the highly conserved pentamer 3'-UAAUU-5'. Comparison with other NNS RNA viruses shows conservation primarily in the termination signals, whereas the start signals are more variable. The intergenic regions vary in length and nucleotide composition. All genes have relatively long 3' and 5' end non-coding regions. The putative 3' and 5' leader RNA sequences of the MBG genome resemble those of other NNS RNA viruses in length, conservation at the 3' and 5' ends, and in being complementary at their extremities. The data support the concept of a common taxonomic order Mononegavirales comprising the Filoviridae, Paramyxoviridae, and Rhabdoviridae families.

The nucleotide sequence of the L gene of Marburg virus, a filovirus: homologies with paramyxoviruses and rhabdoviruses

The nucleotide sequence of the L gene of Marburg virus, strain Musoke, has been determined. The L gene has a single long open reading frame encoding a polypeptide of 2330 amino acids (MW 267,175) that represents the viral RNA-dependent RNA polymerase. The putative transcription start signal (3'CUACCUAUAAUU 5') and the termination signal (3' UAAUUCUUUUU 5') of the gene could be identified. Computer-assisted comparison of the L protein with L proteins of other nonsegmented negative-stranded RNA viruses (Paramyxoviridae: Sendai virus, Newcastle disease virus, human parainfluenza 3 virus, measles virus, human respiratory syncytial virus; Rhabdoviridae: vesicular stomatitis virus, rabies virus) revealed significant homologies primarily in the N-terminal half of the proteins. We have identified three common conserved boxes (A, B, and C) among filo-, paramyxo-, and rhabdovirus L proteins, which are probably involved in the polymerase function. The L proteins can be divided into an N-terminal half, which seems to accommodate the common enzymatic sites, and a C-terminal half carrying virus specific peculiarities. The data presented here suggest a common evolutionary history for all nonsegmented negative-stranded RNA viruses and show that filoviruses are more closely related to paramyxo- than to rhabdoviruses.

Nucleotide sequences of the 3' leader and 5' trailer regions of human respiratory syncytial virus genomic RNA

The nucleotide sequences of the 3' extracistronic (leader) and 5' extracistronic (trailer) regions were determined for genomic RNA (vRNA) of human respiratory syncytial virus (RSV) strain A2. To sequence the 3' leader region, vRNA was extracted from purified virions, size-selected, polyadenylated, copied into cDNA, amplified by the polymerase chain reaction, cloned, and sequenced. The 3' leader sequence is 44 nt, which is somewhat shorter than its counterparts (50 to 70 nt) in other nonsegmented negative-strand viruses sequenced to date. The 5' trailer region was mapped and sequenced in part directly by dideoxynucleotide sequencing of vRNA. The sequence was confirmed and completed by analysis of cDNA clones derived from vRNA. The 5' trailer sequence is 155 nt in length, which is substantially longer than its counterparts (40 to 70 nt) in other nonsegmented negative-strand viruses. Ten of the 11 terminal nt of the 3' leader and 5' trailer regions were complementary. Among the other paramyxoviruses, the terminal 5 to 16 nt of the leader and trailer regions are highly conserved, but the corresponding RSV sequences were identical to the others only for the terminal 2 nt of each end. Surprisingly, the termini of the RSV leader and trailer regions were in somewhat better agreement with those of the rhabdoviruses vesicular stomatitis virus and rabies virus, sharing identity for the first 3 or 4 nt.

Glycosylation and oligomerization of the spike protein of Marburg virus

The oligosaccharide side chains of the glycoprotein of Marburg virus (MW 170,000) have been analyzed by determining their sensitivity to enzymatic degradation and their reactivity with lectins. It was found that they consist of N- and O-glycans. Studies employing chemical cross-linking showed that the glycoprotein is present as a homotrimer in the viral envelope.

Electron microscopy of the novel barley yellow streak mosaic virus

Unique particles of barley yellow streak mosaic virus (BYSMV) were detected in diseased barley, wheat, and several species of grass. They appeared to be about 64 nm in width and from 127 nm to an astonishing 4000 nm in length. Individual particles were circular in transverse section. The outermost layer of each particle seemed to be a membrane-like envelope. The internal structure of many particles was bead-like. Some particles had centers that were translucent. The BYSMV particles were distributed throughout the leaf, sheath, root, and awn organs of barley. Virus particles were present in all cell types of the epidermis, mesophyll, phloem, and xylem. However, mesophyll cells contained the greatest number of particles. Most BYSMV particles occurred in large clusters of quasi-parallel arrays. Both individual and groups of particles were located within the cavities of ER elements. Ribosomes were attached to some outer surfaces of the ER bounding membrane. BYSMV particles are unique because they do not resemble any in presently classified groups or families of plant viruses: they are, however, similar to those of some unclassified viruses that infect insects.

Virus-Like Particles and a Spider Mite Intimately Associated with a New Disease of Barley

The malting barley-producing regions in Montana and Canada are threatened with a new virus-like barley disease that appears to be etiologically novel. Ultrathin sections of diseased tissue contained enveloped, filamentous virus-like particles that measured 64 nanometers by 126 to 4000 nanometers. These lengths are unique for plant viruses. Unexpectedly, the spider mite,Petrobia latens, which has never been reported to be a vector of a pathogen, was found to transmit the causal agent from diseased plants to healthy barley, while noninfective mites failed to do so unless they were allowed prior access to diseased tissue.

Conservation of the 3' terminal nucleotide sequences of Ebola and Marburg virus

The 3' RNA base sequences of several Marburg (MBG) and Ebola (EBO) virus isolates have been determined. A comparison of these 3' terminal noncoding sequences with those of other negative strand RNA viruses suggests a unique phylogenic niche for Marburg and Ebola viruses. The translation initiation site and 35 N-terminal amino acids of the 3' proximal coding gene of a Zaire strain of Ebola virus was predicted. In addition, putative leader RNA sequences preceding the first gene are discussed in terms of possible regulatory functions.

Descriptive analysis of Ebola virus proteins

The virion proteins of two strains of Ebola virus were compared by SDS-polyacrylamide gel electrophoresis (PAGE) and radioimmunoprecipitation (RIP). Seven virion proteins were described; an L (180K), GP (125K), NP (104K), VP40 (40K), VP35 (35K), VP30 (30K), and VP24 (24K). The RNP complex of the virus contained the L, the NP, and VP30, with VP35 in loose association with them. The GP was the major spike protein, with VP40 and VP24 making up the remaining protein content of the multilayered envelope.

Physicochemical inactivation of Lassa, Ebola, and Marburg viruses and effect on clinical laboratory analyses

Clinical specimens from patients infected with Lassa, Ebola, or Marburg virus may present a serious biohazard to laboratory workers. We have examined the effects of heat, alteration of pH, and gamma radiation on these viruses in human blood and on the electrolytes, enzymes, and coagulation factors measured in laboratory tests that are important in the care of an infected patient. Heating serum at 60 degrees C for 1 h reduced high titers of these viruses to noninfectious levels without altering the serum levels of glucose, blood urea nitrogen, and electrolytes. Dilution of blood in 3% acetic acid, diluent for a leukocyte count, inactivated all of these viruses. All of the methods tested for viral inactivation markedly altered certain serum proteins, making these methods unsuitable for samples that are to be tested for certain enzyme levels and coagulation factors.