142754186 ebola virus pdf

4
Ebola Viruses Anthony Sanchez, Centers for Disease Control and Prevention, Atlanta, Georgia, USA A small group of exotic and mysterious viral agents that cause a severe haemorrhagic disease in human and/or nonhuman primates. Molecular Biology Ebola viruses have evolved to occupy some unknown niche. The ability of these filoviruses to exist in the wild and cause disease in human and nonhuman primates derives from the peculiarities and dynamics of the molecules that are produced by these pathogens. Ebola viruses evolved from a progenitor virus, which also gave rise to a multitude of other human pathogens, such as Measles virus and Rabies virus. The manner in which these viruses replicate their genomes and express proteins from their genes is greatly influenced by the types of host cells they are capable of infecting. The characteristic effects of their molecular interactions within the host dictates the type, severity and duration of infection. An important aspect in the pathogenesis of Ebola viruses is their ability to affect the immune responses of human and/or nonhuman primates and the wide range of tissues that support the growth of these viruses. Genome organization The single-stranded ribonucleic acid (RNA) genome of the Zaire species of Ebola virus is nearly 19 000 bases in length and is very similar in organization to that of Marburg virus. The genome organization and gene structure is consistent with those of paramyxoviruses and rhabdoviruses. Figure 1 illustrates the genome with seven linearly arranged genes (Sanchez et al., 1993). At the extreme 3- and 5-ends are short, extragenic regions (known as leader and trailer sequences) that are important in the initiation and regulation of viral RNA synthesis. Genes are delineated by conserved transcriptional signals for synthesis of positive-sense messenger RNA (mRNA) (a start site at the 3-end and a stop or polyadenylation site at the 5-end), and adjacent genes are either separated by a short intergenic region (also extragenic) or share short sequences that are limited to transcriptional signals (overlap). Intergenic regions and overlaps alternate along the genome of the Zaire species. The Reston species differs from Zaire species in that the overlap between the glycoprotein and VP30 genes is absent and an intergenic region separates them. The sequence 3-UUAAU is common to both start and stop signals and is positioned in the centre of overlaps. As seen in Figure 1, there is a large amount of noncoding sequence in the genome and its function in expression is unclear. Stem–loop structures are also predicted for the sequences at the beginnings of the genes and also the extreme 3- and 5-ends of the genome (also seen in Marburg virus). These structures are present in mRNA and may improve their translation; comparable structures have not been identified in viruses from other families in the order Mononegavirales. Expression of the glycoprotein gene and structures of SGP and GP As with many other viruses, products of the glycoprotein gene of Ebola viruses play an important role in virus entry and pathogenesis. The organization of the glycoprotein genes of all Ebola viruses is unusual and differs from similar genes of other viruses, including Marburg virus in the same Filoviridae family. Figure 2 depicts the organiza- tion and expression strategy of an Ebola virus glycoprotein gene. This gene encodes two prominent proteins, a secreted glycoprotein (SGP) that is released from infected cells in large amounts as a homodimer, and a structural glycopro- tein (GP) that forms trimers of a membrane-anchored heterodimer; GP trimers form the peplomers or ‘spikes’ on the surface of virions. The nonstructural SGP is expressed in preference to the structural GP, with mRNA for GP produced through a transcriptional editing mechanism (Sanchez et al., 1996). The editing event that leads to GP expression is an insertion of a single extra adenosine at a run of seven adenosines in the middle of the coding region. This insertion connects the 0 and 2 1 frames that encode the full-length GP and occurs in about 25% of the transcripts. The coding frame shared by SGP and GP and the frame accessed by editing are seen in Figure 2. As a Article Contents Secondary article . Molecular Biology . Pathology and Pathogenesis . Epidemiology . Patient Management and Control 0 19 2 1 3 4 Trailer 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Leader IR Overlap IR Edit site Overlap IR Overlap NP 3’ VP35 VP40 SGP/GPVP30 VP24 L 5’ Figure 1 Ebola virus (Zaire species) genomic RNA. Coding regions for seven linearly arranged genes are shown as hatched boxes. The scale at the bottom of the figure indicates units of sequence length numbered in kilobases. IR, intergenic region. 1 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

Upload: cheryl-horath

Post on 22-Nov-2015

149 views

Category:

Documents


1 download

TRANSCRIPT

  • Ebola VirusesAnthony Sanchez, Centers for Disease Control and Prevention, Atlanta, Georgia, USA

    A small group of exotic and mysterious viral agents that cause a severe haemorrhagic

    disease in human and/or nonhuman primates.

    Molecular Biology

    Ebola viruses have evolved to occupy some unknownniche. The ability of these loviruses to exist in thewild andcause disease in human and nonhuman primates derivesfrom the peculiarities and dynamics of the molecules thatare produced by these pathogens. Ebola viruses evolvedfrom a progenitor virus, which also gave rise to amultitudeof other human pathogens, such as Measles virus andRabies virus. The manner in which these viruses replicatetheir genomes and express proteins from their genes isgreatly inuenced by the types of host cells they are capableof infecting. The characteristic eects of their molecularinteractions within the host dictates the type, severity andduration of infection. An important aspect in thepathogenesis of Ebola viruses is their ability to aect theimmune responses of human and/or nonhuman primatesand the wide range of tissues that support the growth ofthese viruses.

    Genome organization

    The single-stranded ribonucleic acid (RNA) genome of theZaire species of Ebola virus is nearly 19 000 bases in lengthand is very similar in organization to that ofMarburg virus.The genome organization and gene structure is consistentwith those of paramyxoviruses and rhabdoviruses. Figure 1illustrates the genome with seven linearly arranged genes(Sanchez et al., 1993). At the extreme 3- and 5-ends areshort, extragenic regions (known as leader and trailersequences) that are important in the initiation andregulation of viral RNA synthesis. Genes are delineatedby conserved transcriptional signals for synthesis ofpositive-sense messenger RNA (mRNA) (a start site at

    the 3-end and a stop or polyadenylation site at the 5-end),and adjacent genes are either separated by a shortintergenic region (also extragenic) or share short sequencesthat are limited to transcriptional signals (overlap).Intergenic regions andoverlaps alternate along the genomeof the Zaire species. The Reston species diers from Zairespecies in that the overlap between the glycoprotein andVP30 genes is absent and an intergenic region separatesthem. The sequence 3-UUAAU is common to both startand stop signals and is positioned in the centre of overlaps.As seen in Figure 1, there is a large amount of noncodingsequence in the genome and its function in expression isunclear. Stemloop structures are also predicted for thesequences at the beginnings of the genes and also theextreme 3- and 5-ends of the genome (also seen inMarburg virus). These structures are present inmRNAandmay improve their translation; comparable structures havenot been identied in viruses from other families in theorderMononegavirales.

    Expression of the glycoprotein gene andstructures of SGP and GP

    As with many other viruses, products of the glycoproteingene of Ebola viruses play an important role in virus entryand pathogenesis. The organization of the glycoproteingenes of all Ebola viruses is unusual and diers fromsimilar genes of other viruses, including Marburg virus inthe same Filoviridae family. Figure 2 depicts the organiza-tion and expression strategy of anEbola virus glycoproteingene. This gene encodes two prominent proteins, a secretedglycoprotein (SGP) that is released from infected cells inlarge amounts as a homodimer, and a structural glycopro-tein (GP) that forms trimers of a membrane-anchoredheterodimer; GP trimers form the peplomers or spikes onthe surface of virions. The nonstructural SGP is expressedin preference to the structural GP, with mRNA for GPproduced through a transcriptional editing mechanism(Sanchez et al., 1996). The editing event that leads to GPexpression is an insertion of a single extra adenosine at arun of seven adenosines in the middle of the coding region.This insertion connects the 0 and 2 1 frames that encodethe full-length GP and occurs in about 25% of thetranscripts. The coding frame shared by SGP and GPand the frame accessed by editing are seen in Figure 2. As a

    Article Contents

    Secondary article

    . Molecular Biology

    . Pathology and Pathogenesis

    . Epidemiology

    . Patient Management and Control

    0 1921 3 4

    Trailer

    5 6 7 8 9 10 11 12 13 14 15 16 17 18

    Leader IR

    Overlap

    IR Editsite

    Overlap

    IR

    Overlap

    NP

    3

    VP35VP40 SGP/GPVP30VP24 L

    5

    Figure 1 Ebola virus (Zaire species) genomic RNA. Coding regions forseven linearly arranged genes are shown as hatched boxes. The scale at thebottom of the figure indicates units of sequence length numbered inkilobases. IR, intergenic region.

    1ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

  • consequence of this organization, SGP and GP share thesame N-terminal 300 amino acids with unique C-termini, both in length and sequence.

    Roles of SGP and GP molecules

    The role of the SGP molecule is unclear, but it is knownthat SGP dimers are structurally very distinct frommultimers of GP (Sanchez et al., 1998). The SGP dimeralso has a dierent cell binding pattern, so it appears thattheir roles are separate (Yang et al., 1998). It is likely thatexpression of signicant quantities of SGP is important inthe natural host and could function in establishing apersistent infection through interactions with certain whiteblood cells, or possibly binding soluble mediators pro-duced in response to infections. In acute human cases, SGPis circulating in the blood in fairly large amounts and maycontribute to the pathogenesis of Ebola virus infections ofhuman and nonhuman primates. Eorts are under way toexamine the interaction of SGPwith cells andmolecules inthe human body, and also to determine the three-dimensional structure of SGP to better understand theactions of this molecule.The GP molecule has an important role in initiating

    infections, as the large peplomers covering the surface ofvirions function in the recognition of and attachment tospecic but currently unknown cell receptors (Wool-Lewisand Bates, 1998). The spike structures of Ebola virus havebeen shown to bind to endothelial cells and not tolymphocytes, which are not infected by Ebola virus. It isalso believed to cause fusion of virion and cell membranes,which results in the release of the nucleocapsid into thecytoplasm. Structural studies have shown that GP iscleaved at a single site in the N-terminal third of themolecule by furin, a cellular subtilisin/kexin-like conver-tase, intoGP1 (N-terminal end) andGP2 that are joined by

    a disulde bond (Volchkov et al., 1998). GP1 is highlyglycosylated and contains most of the N- and O-linkedglycans present in the surface spike. GP1 projects awayfrom the surface of the virion and probably functions inreceptor binding. The glycosylation of GP1 aects thefolding of the molecule and may also be important inimmune evasion. Virions of Ebola viruses (and lovirusesin general) are resistant to neutralization by antibodyspecic for the surface glycoprotein, and thismay be due tothe large amount of nonimmunogenic carbohydrate cover-ing GP1, accounting for 60% or more of the molecularweight (GP1=130kDa). GP2 contains an a-helical se-quence important in trimerization through the formationof coiled coils. The GP2 trimer forms a membrane-anchored, rod-like stalk that stabilizes the peplomer andcontains a sequence positioned near its N-terminus thathas been shown in vitro to promote fusion of membranes(fusion peptide) dependent on the presence of phosphati-dylinositol and calcium (Weissenhorn et al., 1998). Itwould thus appear that the role ofGP2 lies in the formationand stabilization of the virion spike and fusion of the virusand host cell membranes during infection. The structuralpicture that is being revealed for theEbola virusGP (aswellas that of Marburg virus) is very similar to the glycopro-teins of retroviruses and inuenza viruses.

    Pathology and Pathogenesis

    Histology

    In severe and fatal cases of Ebola virus infection, as withinfections with other haemorrhagic fever viruses, there isan extensive multiorgan involvement, with focal to wide-spread necrosis. There is no inammation (i.e. inltrationof neutrophils and other phagocytic cells) in tissuesinfected with Ebola virus, which has been suggested to bethe result of an immunosuppression induced by the virus.Immunohistochemical staining of the liver reveals massiveamounts of virus antigen in hepatocytes, Kuper cells, andendothelial cells, and antigen is also deposited in theintercellular connective tissues; the reticuloendothelial cellsystem is heavily involved. The spleen is another targetorgan for the replication of Ebola virus, and in addition tolarge amounts of antigen, there is follicular necrosis and ageneral loss of structural organization. Examination oftissues from infectedhumans,monkeys andguineapigs hasshown that cells of the mononuclear phagocytic system(macrophages and monocytes) are infected by Ebola virus.It is possible that infection and replication in these celltypes early in the course of the disease may contribute tosome form of immunosuppression, especially if these andother cells are induced to secrete substances that disruptsignalling or are rendered ineective in clearing the virusfrom the body.

    0 frame1 frame

    mRNA synthesis

    poly(A)

    poly(A)

    SGP and GP SGPGP

    Unedited (75%)

    Edited (25%)

    SGP homodimer(secreted)

    +A

    Membrane-anchoredtrimer of GP heterodimers

    (virion)

    Translation andprocessing

    Figure 2 Organization and expression of the glycoprotein genes of Ebolaviruses, showing the production of the secreted SGP homodimer as theprimary gene product from an unedited transcript, and the structural GPtrimer through a transcriptional editing event (single adenosine insertion).

    Ebola Viruses

    2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

  • Disruption of immune responses

    It is known that an immunosuppression is established earlyin the course of infections of human and nonhumanprimates, and development of a strong cellular immuneresponse is critical to surviving Ebola virus infections.There is some indication that inactivated virions of Ebolavirus are capable of inhibiting the proliferation of Tlymphocytes, which would contribute to immunosuppres-sion. Humoral (antibody) responses do not appear to be asimportant to virus clearance, as virions of loviruses aredicult to neutralize (even when treated with high-titreanti-GP sera), and antibody levels are usually very low orundetectable at the critical time when cell-mediatedclearance of the virus either begins or fails to develop.Based on results from histological and in vitro studies,

    Ebola viruses are capable of infecting endothelial cells.Infection of primary cell cultures of human umbilical veinendothelial cells with theZaire species ofEbola virus resultsin a disruption of the signalling processes in these cellswithout a shutdown of host cell protein synthesis. Thisdisruption suppresses the induction of immunomodula-tory genes (e.g., major histocompatibility complex (MHC)class I and class II, interleukin 6 (IL-6), intracellularadhesion molecule 1 (ICAM-1) etc.) that are important inhelping to clear the body of infections. The mechanism ofthis suppression has not been characterized, so disruptionof cellular functions by Ebola virus may occur at the cellmembrane, within the cytoplasm, at the nuclear mem-brane, or at any combination of these sites.

    Epidemiology

    Outbreaks of Ebola viruses that cause severe disease inhumans have originated in rural areas (tropical forests or

    savannah) of what was formerly Zaire (now the Demo-cratic Republic of the Congo), Sudan and Cote dIvoire(Table 1). The original 1976 outbreaks in northern Zaireand southern Sudan involved twodierent species ofEbolavirus that emerged simultaneously. The events or condi-tions that triggered these concurrent outbreaks areunexplained, and subsequent reemergences have failed toshed any light on the factors involved in precipitatingEbola outbreaks. The Reston species of Ebola virus, whichhas not been shown to be pathogenic for humans, has beenisolated from monkeys (Macaca fascicularis) native to thePhilippines. However, the Reston species has not beendetected in monkeys directly from the wild and has onlybeen associated with a single Philippine breeding/export-ing facility that has been the source of all outbreaks causedby the Reston species. The possibility that this virus wasintroduced into the facility through tracking of nonhu-man primates (from Africa?) cannot be dismissed.Because the natural reservoir of loviruses remains a

    mystery, one cannot easily predict the occurrence ofoutbreaks. Eorts to detect Ebola virus in animal speci-mens from the wild have not been successful. These eortshave been few and too limited to adequately address theproblem of identifying the natural host from animalsinhabiting the forests. Results of experimental infection ofbats indicate they can support the replication ofEbola virus(1995 Zaire isolate) and shed the virus without showingovert signs of disease. Chimpanzees in the wild are knownto be naturally infected with Ebola virus, and, fromepidemiological investigations in Cote dIvoire and Ga-bon, it would seem likely that other nonhuman primatesbecome infected as well. Ebola virus infections of thesenonhuman primates in Africa probably result from director indirect contact with the natural host, leading to

    Table 1 Outbreaks of Ebola virus infections of humans and/or nonhuman primates

    Ebola virus speciesa Year Locations of outbreaks Human cases (%mortality)

    Zaire 1976 Yambuku, Zaire 318 (88)Sudan 1976 Nzara and Maridi, Sudan 284 (53)Zaire 1977 Tandala, Zaire 1 (100)Sudan 1979 Nzara and Yambio, Sudan 34 (65)Reston 1989 Reston, Virginia, USA 4 (0)

    Calamba, PhilippinesReston 1992 Siena, Italy 0

    Calamba, PhilippinesCote dIvoire 1994 Tai Forest, Cote dIvoire 1 (0)Zaire 1994 Minkouka, Gabon 49 (59)Zaire 1995 Kikwit, Zaire 315 (77)Reston 1996 Alice, Texas, USA 0

    Calamba, PhilippinesZaire 1996 Makokou and Booue, Gabon 91 (73)

    Johannesburg, South Africa

    aOutbreak conrmed by virus isolation.

    Ebola Viruses

    3ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

  • amplication of the virus and introductions into humanpopulations if these animals are encountered.During the 1995 Kikwit outbreak, it was noted that

    secondary transmission occurred only through direct andclose contact with patients, and that aerosol transmissiondid not occur. The spread of disease among humans duringsome of the outbreaks in Africa, including the 1995Kikwitepisode, was accelerated when cases were treated in localhospitals. The nosocomial spread of disease was attributedto the reuse of contaminated needles and the lack of properbarrier nursing procedures to protect patients and stafrom contact with highly infectious body excretions andsecretions. These epidemics stopped or were slowed whenmedical care facilities ceased to function as sources ofinfectious virus, usually as a result of mortality among thesta or when proper barrier nursing techniques anddecontamination practices were established. Becausespread of the virus requires contact with infected personsand/or body uids, the risk of infection is easily decreasedby avoiding behaviours that subject persons to suchcontact.

    Patient Management and Control

    No specic treatment for lovirus haemorrhagic fever isavailable for use in the event of an outbreak. The antiviraldrug ribavirin, which is useful in treating other haemor-rhagic fever diseases such as Lassa fever and haemorrhagicfever with renal syndrome (HFRS) due to Hantaan virus,has no eect on loviruses in vitro and has not beenconsidered for use in infected patients. Experimentaltreatment of infected monkeys with interferon failed toincrease survival of infection. Currently, only supportivecare is available for treating patients; the administration ofintravenous uids and/or transfusionsmay be therapeutic.The development of vaccines against Ebola virus has

    been slow, and the use of killed virus preparations has notstimulated protective immune responses when evaluatedusing guinea-pig and monkey models. However, geneticimmunization, a technique that uses injections of plasmiddeoxyribonucleic acid (DNA) that programmes de novosynthesis of a foreign protein, has proven to be an eectivemethod of inducing protective immune responses inguinea-pigs (Xu et al., 1997). This technique mimics anactual infection by producing newly synthesized proteinwithin antigen-processing cells that stimulate the develop-ment of cytotoxic T cells capable of killing Ebola virus-

    infected cells. Genetic immunizationwithDNAexpressingtheGP, SGP or nucleoprotein of the Zaire species ofEbolavirus has proven eective in inducing immunity. It remainsto be seen if this method of vaccination, as well as otherrecombinant DNA-based strategies, can be successfullytransferred to monkeys and humans. If successful, thedevelopment of a safe and protective vaccinewould be veryimportant in safeguarding the lives of persons workingwith Ebola viruses in research facilities and persons at riskduring future outbreaks.

    References

    Sanchez A, Kiley MP, Holloway BP and Auperin DD (1993) Sequence

    analysis of the Ebola virus genome: organization, genetic elements,

    and comparisonwith the genomeofMarburg virus.VirusResearch 29:

    215240.

    Sanchez A, Trappier S,Mahy BWJ, Peters CJ andNichol ST (1996) The

    virion glycoproteins of Ebola viruses are encoded in two reading

    frames and are expressed through transcriptional editing. Proceedings

    of the National Academy of Sciences of the USA 93: 36023607.

    Sanchez A, Yang Z-Y, Xu L et al. (1998) Biochemical analysis of the

    secreted and virion glycoproteins of Ebola virus. Journal of Virology

    72: 64426447.

    Volchkov VE, Feldmann H, Volchkova VA, and Klenk H-D (1998)

    Processing of the Ebola virus glycoprotein by the proprotein

    convertase furin. Proceedings of the National Academy of Sciences of

    the USA 95: 57625767.

    WeissenhornW,CalderLJ,WhartonSA, Skehel JJ andWileyDC (1998)

    The central structural feature of the membrane fusion protein subunit

    from the Ebola virus glycoprotein is a long triple-stranded coiled coil.

    Proceedings of the National Academy of Sciences of the USA 95: 6032

    6036.

    Wool-LewisRJ andBates P (1998)Characterization ofEbola virus entry

    using pseudotyped viruses: identication of receptor-decient cell

    lines. Journal of Virology 72: 31553160.

    Xu L, SanchezA, Yang Z-Y,Nabel EG,Nichol ST andNabel GJ (1997)

    Genetic immunization for Ebola virus infection. Nature Medicine 4:

    3742.

    Yang Z-Y,DelgadoR,XuL et al. (1998) Distinct cellular interactions of

    secreted and transmembrane Ebola virus glycoproteins. Science 279:

    10341037.

    Further Reading

    FeldmannH, SanchezA andKlenkH-D (1998) Filoviruses. In: Collier L

    et al. (eds) Topley & Wilsons Microbiology and Microbial Infections,

    9th edn, vol. 1, pp. 651664. London: Arnold.

    Peters CJ, Sanchez A, Rollin PE, Ksiazek TG and Murphy FA (1996)

    Filoviridae: Marburg and Ebola viruses. In: Fields BN, Knipe DM,

    Howley PM et al. (eds) Fields Virology, 3rd edn, pp. 11611176.

    Philadelphia: Lippincott-Raven.

    Ebola Viruses

    4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net