Open access peer-reviewed chapter

Interaction of Ebola Virus with the Innate Immune System

Written By

Felix B. He, Krister Melén, Laura Kakkola and Ilkka Julkunen

Submitted: February 26th, 2019 Reviewed: May 8th, 2019 Published: June 19th, 2019

DOI: 10.5772/intechopen.86749

Chapter metrics overview

1,046 Chapter Downloads

View Full Metrics


Ebola viruses (EBOV) are zoonotic pathogens that cause severe diseases in humans and have been responsible for several disease outbreaks over the past 40 years. Ebola virus disease (EVD) leads to death on an average of 45–50% of cases, but in some outbreaks, the figures have been higher. The largest EVD outbreak in West Africa in 2014–2015 lead to more than 28,000 cases and 11,300 fatalities. Host innate immune responses are vital in restricting the spread of viral infections including that of Ebola virus. EBOV and some other filoviruses are known to trigger uncontrolled virus replication by suppressing host innate immune responses, mainly by targeting the antiviral response through virus proteins. At least EBOV VP24 and VP35 proteins have been shown to inhibit the expression of type I and III interferon (IFN) genes as well as to inhibit IFN signaling leading to downregulated IFN-induced antiviral responses. In this review we concentrate on describing the mechanisms by which EBOV contributes to the pathogenesis of severe disease and on how the virus interacts with the host innate immune system.


  • Ebola virus
  • filovirus
  • innate immunity
  • RIG-I pathway
  • MDA5 pathway
  • VP24

1. Introduction

Ebola virus (EBOV) belongs to the family of filoviruses which include seven viral species. Currently, eight virus types have been identified within this virus family [1]. The virus particles have a uniform diameter of 80 nm but can extend even up to 10,000 nm [2]. So far the largest outbreak of Ebola virus disease (EVD) has taken place in West Africa, in Guinea, Sierra Leone, and Liberia in 2014–2015 [3, 4]. In humans EVD is characterized by a severe disease with high fever, diarrhea and vomiting, occasionally hemorrhagic manifestations, and suppressed immune and inflammatory responses which often lead to sepsis-like symptoms and hypovolemic shock [5]. Because of its high case-fatality rate and limited treatment and vaccination options, EBOV is classified as a biothreat pathogen of category A [6] and should be handled at biosafety level 4 (BSL-4) laboratories. EBOV is also considered as one of the deadliest human pathogens and a potential bioterrorism agent [7].

EBOV infection targets many tissues and cell types leading to dysregulation of inflammatory mediators, disrupted homeostasis, and impaired host immune responses. Together with abnormalities in the coagulation and vascular system, the infection often leads to a fatal outcome in humans due to a multiorgan failure [8, 9, 10].

Invading and replicating viruses are recognized by the host via cellular pattern recognition receptors (PRRs). PRRs recognize pathogens via pathogen-associated molecular patterns (PAMPs), such as viral structural components and nucleic acids, which then activate host innate immune responses. RNA virus infection activates different PRRs like Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), and nucleotide-binding oligomerization domain-containing (NOD)-like receptors (NLRPs). Cell membrane-associated TLR3 and intracellular vacuole-located TLR7 and TLR8 are activated by viral dsRNA and ssRNA molecules, respectively, leading to the activation and nuclear translocation of transcription factors NF-κB, interferon regulatory factor 3 (IRF3) and IRF7 as well as MAP kinases activated transcription factors (MAPK TFs). Cytosolic RLRs, RIG-I, and melanoma differentiation-associated antigen 5 (MDA5) are activated by viral ss/dsRNA molecules leading to activation and nuclear translocation of IRF3 (and IRF7), NF-κB, and MAPK TFs [11, 12, 13]. NLRP activation, especially NLRP3, leads to the activation of the inflammasome and the production of inflammatory cytokines IL-1β and IL-18 [14].

It has been shown that the RIG-I pathway has a significant role in host innate immune responses when the pathogen is an RNA virus. RIG-I recognizes 5′-triphosphate and short ss/dsRNA structures present in genomic and replicated viral RNAs. RIG-I activates mitochondrial antiviral signaling protein (MAVS) which is located in mitochondrial membranes. MAVS triggers the activation of inhibitor kappaB kinases (IKKα/β/γ/ε) and TANK binding kinase 1 (TBK1) through tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins. Activated TBK1 and IKKε then phosphorylate IRF3 [15], which forms dimers and translocates into the nucleus. At the same time, the canonical IKKα/β/γ complex activates NF-κB by phosphorylating the inhibitor of NF-κB (IκB) leading to degradation of IκB and the release and nuclear translocation of active p50-p65 NF-κB complex. NF-κB and dimerized IRF3 bind to the promoter elements of type I and type III IFN genes. This then leads to RNA polymerase II complex-initiated expression of IFN genes and secretion of type I IFN-α/β and type III IFN-λs [16]. The produced interferons are important in activating the second phase of innate immune responses in epithelial cells, fibroblasts, leukocytes, or basically any cell that has functional IFN receptors. IFN α/β and IFN-λs bind to their specific type I and type III cell surface IFN receptors (IFNAR and IFNLR, respectively) leading to activation of janus kinases (JAK) and phosphorylation and activation of signal transducers and activators of transcriptions 1 and 2 (STAT1 and STAT2). Activated STAT1-STAT2 complexes translocate into nucleus and together with IRF9 induce the expression of hundreds of host genes, which include antiviral genes like Viperin, IFITMs, PKR, OAS, and Mx genes [17].


2. Ebola virus, virus proteins, and virus replication

Single-stranded viruses with negative-sense RNA genomes can be assigned to three different subgroups whether they are multisegmented, circular, or unsegmented [18]. Unsegmented viruses belong to the order of Mononegavirales, and the filovirus group is one of the eight mononegaviral families [19]. Filoviruses are enveloped, non-segmented, negative-stranded RNA viruses of varying morphology. They are called filoviruses because of their filamentous particle structure [20]. Filoviruses are assigned to seven species in three genera Cuevavirus, Ebolavirus(EBOV), and Marburgvirus(MARV) [21]. Most of the filoviruses are human pathogens, and the diseases caused by two of these viruses, EBOV and MARV, are well-known because of their high case-fatality rate [3].

Ebola virus group includes five virus species, Zaire ebolavirus(ZEBOV), Bundibugyo ebolavirus(BEBOV), Ivory Coast ebolavirus(ICEBOV), Sudan ebolavirus(SEBOV), and Reston ebolavirus(REBOV). Filoviruses consist of ssRNA genomes of 19 kilobases. EBOV genome encodes eight different proteins that all have specific functions [22]. Table 1 summarizes the major characteristics of EBOV proteins. The gene order of EBOV genome is NP, VP35, VP40, GP/sGP, VP30, VP24, and L (Figure 1). Nucleocapsid-associated proteins include the major nucleoprotein NP and the minor nucleoprotein VP30. Both of these proteins interact with the RNA genome and protect the viral RNA. Nucleocapsid structures also include VP35 and RNA-dependent RNA polymerase (RDRP) protein L [23]. Ribonucleoprotein complex regulates viral replication and transcription of the viral genome. The RDRP complex consists of L polymerase and VP35, the latter of which acts as a polymerase cofactor [24, 25, 26]. NP with RDRP complex catalyzes the viral genome with VP30 to initiate transcription and replication. VP40 is required for viral particle formation, and it is the major matrix protein [27]. Viral envelope glycoprotein (GP) is the only viral envelope protein, and its function is to attach the host cell surface and mediate the entry of viral nucleocapsids [28]. EBOV GP is heavily N- and O-glycosylated. On the surface of virus particles, GP is cleaved into two subunits (GP1 and GP2), and it exists as a trimeric protein complex (peplomers). In addition to full-length GP, there are several other forms of proteins encoded by the GP gene: nonstructural soluble glycoprotein (sGP) and a small soluble GP (ssGP) [29]. The functions of sGP and ssGP are presently not known, but they have been suggested to neutralize EBOV GP-specific antibodies. The viral genome encodes also VP24 which is a minor matrix protein, and its functions are dealing with virion assembly and downregulation of host innate immune responses (see below).

  • Minor matrix protein, virion assembly

  • Inhibits type I and type III interferon (IFN) gene expression

  • Inhibits type I and III IFN signaling reducing the expression of IFN-induced genes (blocks STAT1/2 nuclear import)

  • Minor nucleoprotein

  • Transcription activator

  • Polymerase cofactor

  • Binds dsRNA and inhibits type I IFN production

  • Inhibits dendritic cell maturation

  • Blocks IKKƐ/TBK1 activation and IRF3 phosphorylation

  • Viral matrix protein

  • Required in virion assembly and budding

  • Structural protein of nucleocapsid complex

  • Catalyzes viral replication and transcription of the RNA genome

  • Viral envelope glycoprotein

  • Attachment to host cell surface

  • Mediates virus entry

  • Target of anti-GP neutralizing antibodies

sGP (ssGP)
  • Soluble glycoprotein (small soluble GP)

  • Possible decoy of anti-GP antibodies

L polymerase
  • Viral RNA-dependent RNA polymerase

Table 1.

Ebola virus proteins and their functions in virus replication cycle and in host cell functions.

Figure 1.

Ebola virus genome structure and the expression of individual EBOV proteins in transfected cells. Panel A. EBOV genome encodes for eight different proteins, nucleoprotein (NP), viral protein 35 (VP35), VP40, secreted glycoprotein (sGP), GP, VP30, VP24, and RNA polymerase L are schematically shown. Panel B. Human hepatoma HuH7 cells were transfected with pcDNA3-His or HA-EBB expression constructs for different EBOV genes, and 24 h after transfections, the cells were stained with monoclonal anti-His (NP, VP35, VP40, GP, sGP, and VP30) or anti-HA (VP24 and L) antibodies and secondary rabbit anti-mouse immunoglobulin. Note that VP40 and especially VP24 are localized into the cell nucleus, and GP-expressing cells show significant cytotoxicity.

EBOV can infect a wide variety of cells, which may explain the ability of the virus to spread to many tissues and different types of cells. At present there is no direct evidence of one specific EBOV receptor; rather many types of molecules such as integrins, C-type lectins, and TIM-1 have been suggested to function as a cellular receptor. After attachment EBOV is endocytosed followed by a fusion of viral and endosomal membranes and release of viral nucleocapsid into the cell cytoplasm. In the cytoplasm virus-specific mRNAs are synthesized from the genomic RNA template. Viral RNA polymerase complex is responsible for the synthesis of individual mRNA molecules for each EBOV proteins. Both the transcription and translation of EBOV proteins takes place in the cell cytoplasm. Virus replication is regulated by the L polymerase, VP30, VP35, and NP followed by the assembly of viral nucleocapsid (NC) structures. GP synthesis and glycosylation occurs in the Golgi complex, and the assembly of newly produced virus particles takes place at the plasma membrane where NCs, VP40, VP24, and GP are assembled followed by virus budding from the plasma membrane [22, 23, 24, 25, 26, 27].


3. Ebola virus disease (EVD)

Ebola virus disease was first recognized in 1976 simultaneously in two different geographic locations, in Sudan and in the Democratic Republic of Congo [9, 30]. The newly identified viral agent was named Ebola virus, and the symptoms resembled those of Marburg virus disease (MVD). Most human cases have been caused by the ZEBOV species, and most of the outbreaks have occurred in Central and West Africa [5]. Like in many other zoonotic diseases, EBOV is considered to have a natural reservoir in animals, but humans may also transmit the disease via blood, serum, and bodily secretions (Figure 2). Patients that recovered from the primary infection were found to excrete the virus for several weeks or months also via the genital tract, especially in semen [31]. Humans and possibly some other mammalian species like primates are considered as the dead-end hosts [32]. Even though there are no firm links to natural reservoirs of EBOV, many studies suggest that rodents and bats likely play a role in virus transmission [33, 34, 35, 36]. There is strong evidence that fruit bats, in case they are in close contact with humans or when they are used as food, transmit the disease to humans. EBOV may exist silently in reservoir species and be activated through certain stimuli such as stress, coinfection, pregnancy of the carrier animals, ecological changes, and change in food habits [37, 38].

Figure 2.

Target cells and tissues infected by EBOV. Zoonotic and human-to-human transmission of EBOV through initial infection sites lead to viremia which targets the lymph nodes, liver, spleen, adrenal cortex, and vascular system. The widespread viral dissemination leads to tissue and vascular damage in these organs possibly resulting in sepsis-like state and multiorgan failure.

Nevertheless, the route of primary transmission from possible reservoirs to humans needs to be studied in more detail in order to prevent direct infection routes from animals to humans. During outbreaks the dominant mode of transmission is human-to-human either through mucosa or lacerations [39]. An average incubation time in EBOV epidemics with human-to-human spread has been around 9–10 days [40]. Analysis of EBOV transmission between the patient and the secondary case(s) indicates an association with an exposure to infectious bodily fluids [41]. A large meta-analysis conducted on the secondary transmissions in the same household showed that the risk of transmission was less than 1% when the person was not in direct contact with an EVD patient [42].

Once the transmission has occurred, symptoms normally arise after 4–10 days of exposure, though there is a wide variation in the incubation time ranging from 2 to 21 days [43, 44]. The typical symptoms of EVD are flu-like symptoms with fever, myalgia, and chills. Also, gastrointestinal symptoms occur as vomiting and diarrhea. After these common symptoms, the disease may rapidly evolve as hemorrhagic complications, anuria, dysthesia, and sepsis-like symptoms resulting in multiorgan failure [44, 45]. Other reported symptoms include headache, profound weakness, coughing, and rhinorrhea. Also, when systemic symptoms related to cardiovascular system occur, they often result in septic shock and edema [5, 44, 45]. Hematological changes in laboratory parameters include leukopenia, decreased neutrophil counts, and increase in liver enzymes. When the infection proceeds, patients develop thrombocytopenia, prolonged prothrombin time, and activated partial thromboplastin time. This may result in disseminated intravascular coagulation, which finally leads to a multiorgan failure and death [5]. Patients who have survived EVD were found to develop long-term symptoms and disorders such as recurrent hepatitis, myalgia, arthralgia, prolonged hair loss, psychosis, and uveitis [5, 43, 45], which in rural areas often do not receive adequate therapy.

Rapid EVD diagnosis is done by antigen detection methods (e.g., ELISA) or by the detection of viral RNA using RT-PCR techniques. High levels of viruses/viral RNA are generally seen after 48 h of clinical infection. ELISA-based EBOV-specific IgG and IgM antibody detection methods have also been developed [2]. Due to rural conditions and the fatal nature of the disease, EVD is often diagnosed based on anamnestic information and patient’s symptoms [46].

Fortunately, there are promising novel therapeutic alternatives of antiviral compounds identified in in vitroand in animal studies [46]. Humanized monoclonal neutralizing antibody cocktails have also been used to treat EVD patients [47]. Due to the very high case-fatality rate of EVD, WHO has declared that it is ethical to use experimental drugs to treat and prevent EVD. However, to date, there are no EBOV-specific therapies that have proven their efficiency in controlled studies in humans, and thus, supportive care remains the main treatment modality for EVD patients [5, 48]. Possible future therapies would include slowing down virus replication and disease progression allowing host innate and adaptive immune responses to overcome the infection [49, 50].

Another way to approach EBOV epidemics is to use vaccines in high-risk areas. Vaccine candidates must show good efficacy in experimental EVD models [51]. Recent reviews summarize the progress made in the field of EBOV vaccines [52, 53]. Currently there are two promising vaccine candidates that have entered clinical studies: monovalent and bivalent recombinant adenovirus and VSV-based vaccines [52], the latter of which has been used in the most recent epidemic in the Democratic Republic of Congo.


4. The effect of Ebola virus infection and EBOV proteins on cytokine gene expression

Filoviruses can infect many different cell types, for example, macrophages, monocytes, dendritic cells, Kupffer cells in the liver, fibroblasts, hepatocytes, cells of adrenal gland tissue, endothelial cells, and epithelial cells (Figure 2) [54, 55]. In nonhuman primates it has been shown that the virus first replicates in macrophages and dendritic cells. These cells are considered to be responsible for an unbalanced immune response [55]. Studies have shown that EBOV efficiently infect these cells after they differentiate from monocytes [56, 57, 58]. Histopathological studies in human tissues have proven that macrophages are readily infected [59]. The data on cytokines and inflammatory responses show that there is a correlation between poor prognosis and intense inflammatory response characterized by excessive cytokine and chemokine production [60]. After the initial infection phase in monocyte/macrophages and dendritic cells, the virus is spreading to lymph nodes and other organs such as the liver and the spleen which takes place via the lymphatic system [54, 55]. EBOV infection in these target organs leads to strong inflammatory responses and the release of pro-inflammatory cytokines and chemokines, such as interleukin-1β (IL-1β), IL-6, IL-8, IL-10, monocyte chemoattractant protein 1 (MCP1), macrophage inflammatory protein 1α (MIP1α), MIP1β, and tumor necrosis factor (TNF) as well as to reactive oxygen species and nitric oxide [8, 61, 62].

MIP1α and MCP1 create a positive feedback loop where secreted cytokines recruit more monocyte/macrophages to the site of infection enabling EBOV to infect more target cells [55]. The infection caused by EBOV inhibits the maturation of dendritic cells and prevents antigen presentation to T cells. This event is due to EBOV infection to inhibit upregulation of CD40, CD80, CD86, and major histocompatibility complex (MHC) class II molecules [63, 64]. A commonly seen characteristic of EBOV infection is lymphopenia which occurs among CD4+ and CD8+ T cells and natural killer (NK) cells [65, 66]. The same effect was detected in vitro with EBOV-infected human CD4+ and CD8+ T cells [67]. The differences in lymphopenia profiles between the survivors and deceased patients have been linked to uncommon innate immune response and suppression of adaptive immunity [68, 69]. However, the connection between pathogenesis and the consequences of lymphopenia is presently not known. Loss of CD4+ T cells may also lead to reduced production of EBOV-specific immunoglobulin M (IgM) and IgG antibodies stating that early events that occur in the immune system in EBOV infection determine the outcome of EVD [70].

The morbidity and mortality of EVD are considered to be due to a burst of immunological mediators better known as a “cytokine storm” [60, 68]. The cytokine storm is a response caused by a wide variety of infectious and noninfectious agents where they induce the production of pro- and anti-inflammatory factors usually consisting of IFNs, TNFs, interleukins, and chemokines [60, 71]. Unfortunately, the precise mechanisms triggering the cytokine storm is not known. Yet there are some studies showing that certain viruses and bacteria trigger cytokine storm through T-cell receptors and CD28 and/or by activating PAMP recognition pathways [72, 73]. Since EBOV infection in macrophages and dendritic cells suppresses their cytokine and chemokine production, including that of antiviral IFNs, it is likely that the excessive production of pro- and anti-inflammatory mediators occurs in other cell types apart from macrophages and DCs [74].

The immune evasion mediated by individual EBOV proteins has also been studied. So far two of the eight or nine EBOV proteins, namely, VP24 and VP35, have been shown to interfere with the activation of innate immune responses (Figure 3). VP35 has been shown to inhibit the maturation of dendritic cells. It interferes with the RIG-I signaling pathway to prevent enhanced expression of MHC class I and class II and the costimulatory molecules CD40, CD80, and CD86. This leads to impaired antigen presentation to CD8+ and CD4+ T cells and to impaired T-cell activation which disrupts the linkage between innate and adaptive immune responses [75, 76]. VP35 also inhibits RIG-I signaling by preventing IFN-α/β gene expression. VP35 binds to dsRNA which inhibits the interaction of RIG-I with viral RNAs. Also, the interaction between PKR activator PACT and RIG-I is disrupted which does not allow the normal RIG-I ATPase activation to take place [77]. VP35 has also been reported to increase the SUMOylation of IRF7 by SUMO-conjugating enzyme UBC9 and SUMO E3 protein ligase PIAS1 leading to reduced transcriptional activity of IRF7 [78]. IRF7 is one of the key transcription factors regulating IFN-α/β and IFN-λ gene expression [79]. EBOV VP35 also inhibits IKKε/TBK1 kinase complex functions [80]. In addition to all the abovementioned functions, VP35 has an inhibitory effect on PKR activation that contributes to inhibition of dendritic cell maturation [61, 81].

Figure 3.

Panel A. EBOV prevents type I and III IFN production and expression. Two of the eight proteins encoded by EBOV have shown inhibitory effect in previous in vitro studies: VP35 and VP24. VP35 blocks RIG-I-like signaling by binding to dsRNA or PACT and prevents IFN α/β production. It also promotes degradation of IRF3 and IRF7 by interacting with host SUMOylation process. VP35 also prevents phosphorylation of IKKε. VP24 instead inhibits IFN-λ1 gene expression downstream of IRF3. The exact mechanism is still unknown. Panel B. VP24 blocks the nuclear import of phosphorylated STAT1-STAT2 dimers by binding to importin α isoforms which limits the nuclear accumulation of activated STATs and reduces IFN-induced gene expression.

EBOV VP24, in addition of having a role in virion assembly, is downregulating the activation of innate immune responses. In virus-infected and in VP24 gene-transfected cells, VP24 protein is expressed in the cell cytoplasm and especially in the nucleus (Figure 1) [82]. The expression of VP24 genes from different EBOV viruses has shown that they all inhibit RIG-I-induced IFN gene expression [83]. The analyses have been done by cotransfecting cultured cells, often human embryonal kidney cells (HEK293 cells), with the expression constructs for VP24 and activators of the RIG-I pathway (deltaRIG-I, MAVS, IKKε, or TBK1) together with IFN promoter-reporter constructs (e.g., luciferase). These analyses have revealed that VP24 is efficiently inhibiting IFN gene expression on all components of the RIG-I pathway. Interestingly, the IFN expression-inducing capacity of constitutively active form of IRF3, dimerized IRF35D construct was also inhibited by VP24. However, a mutant VP24 protein, which lacked a nuclear localization signal and was thus mostly cytoplasmic, could not interfere with RIG-I-induced IFN gene expression (Figure 4) [82]. This indicates that VP24 likely interferes IFN gene expression by presently unidentified mechanism in the cell nucleus.

Figure 4.

Panel A. Interaction of EBOV VP24 with human importin α isoforms and intracellular location of wild-type (wt) VP24 and NLS mutant (mut) VP24. Baculovirus orE. coli-expressed GST-importin α isoforms were allowed to bind to glutathione Sepharose. In vitro-translated 35S-methionine-labeled wt and mut VP24 proteins were allowed to bind to immobilized GST-importin α isoforms. NLS mutant VP24 shows clearly reduced binding to importin α molecules. Panel B shows the amount of Sepharose-bound GST-importin α molecules. Panel C shows the intracellular location of wt VP24 and reduced nuclear translocation of NLS mutant of VP24.


5. Downregulation of IFN-induced antiviral activities by EBOV proteins

One of the factors dictating EBOV lethality is its ability to replicate in many cell types and evade host immune responses. There are multiple mechanisms that allow filoviruses to surpass the host innate antiviral responses, for instance, interferon-induced antiviral responses [84]. Type I and III IFNs (IFN α/β/λ) have a major role in antiviral response in viral infections [82, 85]. The activation of RLRs and TLRs and their downstream signaling cascades lead to the expression of type I and type III IFNs [11]. Type I IFNs (mainly IFN-α/β) bind to their specific cell surface receptors IFNAR1 and IFNAR2, while type III IFNs (IFN-λ1-4) have their own cell-specific receptor composed of IFNLR and IL-10R2 receptor chains (Figure 3). Activation of type I or type III IFN receptors leads to activation of JAK–STAT signaling pathway which ultimately leads to phosphorylation and dimerization of STAT1 and STAT2 and the expression of IFN-stimulated genes [86]. Several studies have shown that especially EBOV VP24 protein interacts with this antiviral defense system by interfering with nuclear translocation of activated STAT1-STAT2 dimers (Figure 3B). VP24 has a nuclear localization signal (NLS), which mediates a tight interaction with importin α molecules that mediate the nuclear translocation of nuclear-targeted proteins together with importin β. Humans have six different importin α isoforms, and VP24 is capable of binding to all importin α molecules, especially to importin α5, α6, and α7 [82]. Importin α-bound VP24 is able to prevent the interaction of STAT1-SAT2 complexes with the NLS-binding armadillo domains of importin α isoforms and thus prevent the nuclear import and subsequent STAT-induced activation of IFN-stimulated genes (ISGs). However, if the NLS of VP24 is mutated, VP24 is incapable of inhibiting importin α-STAT interaction, and IFN-induced genes are expressed normally [82]. As mentioned above, EBOV VP35 is able to inhibit dsRNA-induced PKR activation. PKR is one of the ISGs that has antiviral activity against many different viruses. EBOV GP is also able to induce cytotoxic activities in cells (Figure 1) even though the precise mechanisms behind this activity are present unknown.


6. Concluding remarks

Filoviruses target many cell types and tissues that regulate the activation of host immune responses and blood coagulation and hemostatic systems. Even if many of the processes in EBOV-host cell interactions have recently been revealed, there are still many open questions, e.g., by which molecular mechanisms are involved in EVD. In addition, more detailed information is needed to determine the activity of individual EBOV proteins, in addition to VP24 and VP35, on host innate and adaptive responses. Collectively, with this information it would be possible to design novel drugs or new modalities of treatment of Ebola and other filovirus infections.


7. Conclusions

Ebola virus infection is characterized by a severe infection with distorted regulation of blood coagulation and hemodynamic system and enhanced expression of inflammatory cytokines. In human infections Ebola virus targets macrophages and dendritic cells followed by systemic spread to the liver, spleen, and adrenal tissues. Individual EBOV proteins, such as VP24 and VP35, can interfere with the activation of host interferon gene expression and downregulate host antiviral responses.



The authors thank Ali Mirazimi and Helen Karlsson at Karolinska Institutet, Stockholm, Sweden, and Miao Jiang and Pamela Österlund at National Institute for Health and Welfare, Helsinki, Finland, for collaboration in the original publications. The technical assistance of Sari Maljanen is greatly acknowledged. The original research was funded by the Innovative Medicines Initiative 2 Joint Undertaking under grant agreement No. 115843. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation program and EFPIA. The study was also supported by the Medical Research Council of the Academy of Finland (grants 252252, 256159, and 297329), the Sigrid Juselius Foundation, and the Jane and Aatos Erkko Foundation.


Conflict of interest

The authors declare no conflict of interest.


  1. 1. Burk R, Bollinger L, Johnson JC, et al. Neglected filoviruses. FEMS Microbiology Reviews. 2016;40(4):494-519. DOI: 10.1093/femsre/fuw010
  2. 2. Rougeron V, Feldmann H, Grard G, et al. Ebola and Marburg haemorrhagic fever. Journal of Clinical Virology. 2015;64:111-119. DOI: 10.1016/j.jcv.2015.01.014
  3. 3. World Health Organization. Ebola Virus Disease. 2019. Available from:[Accessed: April 25, 2019]
  4. 4. Baize S, Pannetier D, Oestereich L, et al. Emergence of Zaire Ebola virus disease in Guinea. The New England Journal of Medicine. 2014;371(15):1418-1425. DOI: 10.1056/NEJMoa1404505
  5. 5. Feldmann H, Geisbert TW. Ebola haemorrhagic fever. Lancet. 2011;377(9768):849-862. DOI: 10.1016/S0140-6736(10)60667-8
  6. 6. Centers for Disease Control and Prevention. Bioterrorism Agents/Diseases. 2019. Available from:[Accessed: April 25, 2019]
  7. 7. Borio L, Inglesby T, Peters CJ, et al. Hemorrhagic fever viruses as biological weapons: Medical and public health management. Journal of the American Medical Association. 2002;287(18):2391-2405
  8. 8. Baize S, Leroy EM, Georges AJ, et al. Inflammatory responses in Ebola virus-infected patients. Clinical and Experimental Immunology. 2002;128(1):163-168
  9. 9. WHO. Ebola haemorrhagic fever in Zaire, 1976. Bulletin of the World Health Organization. 1978;56(2):271-293
  10. 10. Yang ZY, Duckers HJ, Sullivan NJ, et al. Identification of the Ebola virus glycoprotein as the main viral determinant of vascular cell cytotoxicity and injury. Nature Medicine. 2000;6(8):886-889
  11. 11. Jensen S, Thomsen AR. Sensing of RNA viruses: A review of innate immune receptors involved in recognizing RNA virus invasion. Journal of Virology. 2012;86(6):2900-2910. DOI: 10.1128/JVI.05738-11
  12. 12. Jiang M, Österlund P, Sarin LP, et al. Innate immune responses in human monocyte-derived dendritic cells are highly dependent on the size and the 5′ phosphorylation of RNA molecules. Journal of Immunology. 2011;187(4):1713-1721. DOI: 10.4049/jimmunol.1100361
  13. 13. Jiang M, Österlund P, Fagerlund R, et al. MAP kinase p38alpha regulates type III interferon (IFN- λ1) gene expression in human monocyte-derived dendritic cells in response to RNA stimulation. Journal of Leukocyte Biology. 2015;97(2):307-320. DOI: 10.1189/jlb.2A0114-059RR
  14. 14. Pedraza-Alva G, Pérez-Martínez L, Valdez-Hernández L, et al. Negative regulation of the inflammasome: Keeping inflammation under control. Immunological Reviews. 2015;265(1):231-257. DOI: 10.1111/imr.12294
  15. 15. Liu S, Cai X, Wu J, et al. Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation. Science. 2015;347(6227):aaa2630. DOI: 10.1126/science.aaa2630
  16. 16. Gack MU. Mechanisms of RIG-I-like receptor activation and manipulation by viral pathogens. Journal of Virology. 2014;88(10):5213-5216. DOI: 10.1128/JVI.03370-13
  17. 17. Lazear HM, Nice TJ, Diamond MS. Interferon-λ: Immune functions at barrier surfaces and beyond. Immunity. 2015;43(1):15-28. DOI: 10.1016/j.immuni.2015.07.001
  18. 18. Li CX, Shi M, Tian JH, et al. Unprecedented genomic diversity of RNA viruses in arthropods reveals the ancestry of negative-sense RNA viruses. eLife. 2015;4:1-26. DOI: 10.7554/eLife.05378
  19. 19. Afonso CL, Amarasinghe GK, Bányai K, et al. Taxonomy of the orderMononegavirales: Update 2016. Archives of Virology. 2016;161(8):2351-2360. DOI: 10.1007/s00705-016-2880-1
  20. 20. Kiley MP, Bowen ET, Eddy GA, et al. Filoviridae: A taxonomic home for Marburg and Ebola viruses? Intervirology. 1982;18(1-2):24-32
  21. 21. Kuhn JH, Becker S, Ebihara H, et al. Proposal for a revised taxonomy of the family Filoviridae: Classification, names of taxa and viruses, and virus abbreviations. Archives of Virology. 2010;155(12):2083-2103. DOI: 10.1007/s00705-010-0814-x
  22. 22. Elliott LH, Kiley MP, McCormick JB. Descriptive analysis of Ebola virus proteins. Virology. 1985;147(1):169-176
  23. 23. Volchkov VE, Volchkova VA, Chepurnov AA, et al. Characterization of the L gene and 5′ trailer region of Ebola virus. The Journal of General Virology. 1999;80(Pt 2):355-362
  24. 24. Muhlberger E, Lotfering B, Klenk HD, et al. Three of the four nucleocapsid proteins of Marburg virus, NP, VP35, and L, are sufficient to mediate replication and transcription of Marburg virus-specific monocistronic minigenomes. Journal of Virology. 1998;72(11):8756-8764
  25. 25. Mühlberger E. Filovirus replication and transcription. Future Virology. 2007;2(2):205-215. DOI: 10.2217/17460794.2.2.205
  26. 26. Mühlberger E, Weik M, Volchkov VE, et al. Comparison of the transcription and replication strategies of Marburg virus and Ebola virus by using artificial replication systems. Journal of Virology. 1999;73(3):2333-2342
  27. 27. Noda T, Sagara H, Suzuki E, et al. Ebola virus VP40 drives the formation of virus-like filamentous particles along with GP. Journal of Virology. 2002;76(10):4855-4865
  28. 28. Carette JE, Raaben M, Wong AC, et al. Ebola virus entry requires the cholesterol transporter niemann-pick C1. Nature. 2011;477(7364):340-343. DOI: 10.1038/nature10348
  29. 29. Mehedi M, Falzarano D, Seebach J, et al. A new Ebola virus nonstructural glycoprotein expressed through RNA editing. Journal of Virology. 2011;85(11):5406-5414. DOI: 10.1128/JVI.02190-10
  30. 30. WHO. Ebola haemorrhagic fever in Sudan, 1976. Bulletin of the World Health Organization. 1978;56(2):247-270
  31. 31. Deen GF, Knust B, Broutet N, et al. Ebola RNA persistence in semen of Ebola virus disease survivors—Preliminary report. The New England Journal of Medicine. 2017;377(15):1428-1437. DOI: 10.1056/NEJMoa1511410
  32. 32. Groseth A, Feldmann H, Strong JE. The ecology of Ebola virus. Trends in Microbiology. 2007;15(9):408-416
  33. 33. Morvan JM, Deubel V, Gounon P, et al. Identification of Ebola virus sequences present as RNA or DNA in organs of terrestrial small mammals of the Central African Republic. Microbes and Infection. 1999;1(14):1193-1201
  34. 34. Leroy EM, Kumulungui B, Pourrut X, et al. Fruit bats as reservoirs of Ebola virus. Nature. 2005;438(7068):575-576. DOI: 10.1038/438575a
  35. 35. Berge T, Bowong S, Lubuma J, et al. Modeling Ebola virus disease transmissions with reservoir in a complex virus life ecology. Mathematical Biosciences and Engineering. 2018;15(1):21-56. DOI: 10.3934/mbe.2018002
  36. 36. Swanepoel R, Leman PA, Burt FJ, et al. Experimental inoculation of plants and animals with Ebola virus. Emerging Infectious Diseases. 1996;2(4):321-325
  37. 37. Gupta M, Mahanty S, Greer P, et al. Persistent infection with Ebola virus under conditions of partial immunity. Journal of Virology. 2004;78(2):958-967
  38. 38. Strong JE, Wong G, Jones SE, et al. Stimulation of Ebola virus production from persistent infection through activation of the Ras/MAPK pathway. Proceedings of the National Academy of Sciences of the United States of America. 2008;105(46):17982-17987. DOI: 10.1073/pnas.0809698105
  39. 39. Fischer R, Judson S, Miazgowicz K, et al. Ebola virus stability on surfaces and in fluids in simulated outbreak environments. Emerging Infectious Diseases. 2015;21(7):1243-1246. DOI: 10.3201/eid2107.150253
  40. 40. Ajelli M, Parlamento S, Bome D, et al. The 2014 Ebola virus disease outbreak in Pujehun, Sierra Leone: Epidemiology and impact of interventions. BMC Medicine. 2015;13:281. DOI: 10.1186/s12916-015-0524-z
  41. 41. Dowell SF, Mukunu R, Ksiazek TG, et al. Transmission of Ebola hemorrhagic fever: A study of risk factors in family members, Kikwit, Democratic Republic of the Congo, 1995. Commission de Lutte contre les Épidémies à Kikwit. Journal of Infectious Diseases. 1999;179(Suppl 1):S87-S91
  42. 42. Dean NE, Halloran ME, Yang Y, et al. Transmissibility and pathogenicity of Ebola virus: A systematic review and meta-analysis of household secondary attack rate and asymptomatic infection. Clinical Infectious Diseases. 2016;62(10):1277-1286. DOI: 10.1093/cid/ciw114
  43. 43. Kortepeter MG, Bausch DG, Bray M. Basic clinical and laboratory features of filoviral hemorrhagic fever. The Journal of Infectious Diseases. 2011;204(Suppl 3):S810-S816. DOI: 10.1093/infdis/jir299
  44. 44. Jeffs B. A clinical guide to viral haemorrhagic fevers: Ebola, Marburg and Lassa. Tropical Doctor. 2006;36(1):1-4
  45. 45. Hartman AL, Towner JS, Nichol ST. Ebola and Marburg hemorrhagic fever. Clinics in Laboratory Medicine. 2010;30(1):161-177. DOI: 10.1016/j.cll.2009.12.001
  46. 46. Goeijenbier M, van Kampen JJ, Reusken CB, et al. Ebola virus disease: A review on epidemiology, symptoms, treatment and pathogenesis. The Netherlands Journal of Medicine. 2014;72(9):442-448
  47. 47. World Health Organization. Ebola Virus Disease. 2019. Available from:[Accessed: April 25, 2019]
  48. 48. Malvy D, McElroy AK, de Clerck H, et al. Ebola virus disease. Lancet. 2019;393(10174):936-948. DOI: 10.1016/S0140-6736(18)33132-5
  49. 49. Feldmann H, Jones SM, Schnittler HJ, et al. Therapy and prophylaxis of Ebola virus infections. Current Opinion in Investigational Drugs. 2005;6(8):823-830
  50. 50. Fischer WA 2nd, Vetter P, Bausch DG, et al. Ebola virus disease: An update on post-exposure prophylaxis. The Lancet Infectious Diseases. 2018;18(6):e183-e192. DOI: 10.1016/S1473-3099(17)30677-1
  51. 51. Geisbert TW, Pushko P, Anderson K, et al. Evaluation in nonhuman primates of vaccines against Ebola virus. Emerging Infectious Diseases. 2002;8(5):503-507
  52. 52. Venkatraman N, Silman D, Folegatti PM, et al. Vaccines against Ebola virus. Vaccine. 2018;36(36):5454-5459. DOI: 10.1016/j.vaccine.2017.07.054
  53. 53. Wang Y, Li J, Hu Y, et al. Ebola vaccines in clinical trial: The promising candidates. Human Vaccines and Immunotherapeutics. 2017;13(1):153-168. DOI: 10.1080/21645515.2016.1225637
  54. 54. Geisbert TW, Young HA, Jahrling PB, et al. Pathogenesis of Ebola hemorrhagic fever in primate models: Evidence that hemorrhage is not a direct effect of virus-induced cytolysis of endothelial cells. American Journal of Pathology. 2003;163(6):2371-2382. DOI: 10.1016/S0002-9440(10)63592-4
  55. 55. Geisbert TW, Hensley LE, Larsen T, et al. Pathogenesis of Ebola hemorrhagic fever in cynomolgus macaques: Evidence that dendritic cells are early and sustained targets of infection. American Journal of Pathology. 2003;163(6):2347-2370. DOI: 10.1016/S0002-9440(10)63591-2
  56. 56. Yonezawa A, Cavrois M, Greene WC. Studies of Ebola virus glycoprotein-mediated entry and fusion by using pseudotyped human immunodeficiency virus type 1 virions: Involvement of cytoskeletal proteins and enhancement by tumor necrosis factor alpha. Journal of Virology. 2005;79(2):918-926. DOI: 10.1128/JVI.79.2.918-926.2005
  57. 57. Dube D, Schornberg KL, Stantchev TS, et al. Cell adhesion promotes Ebola virus envelope glycoprotein-mediated binding and infection. Journal of Virology. 2008;82(14):7238-7242. DOI: 10.1128/JVI.00425-08
  58. 58. Martinez O, Johnson JC, Honko A, et al. Ebola virus exploits a monocyte differentiation program to promote its entry. Journal of Virology. 2013;87(7):3801-3814. DOI: 10.1128/JVI.02695-12
  59. 59. Zaki SR, Shieh WJ, Greer PW, et al. A novel immunohistochemical assay for the detection of Ebola virus in skin: Implications for diagnosis, spread, and surveillance of Ebola hemorrhagic fever. The Journal of Infectious Diseases. 1999;179(Suppl 1):S36-S47. DOI: 10.1086/514319
  60. 60. Reynard S, Journeaux A, Gloaguen E, et al. Immune parameters and outcomes during Ebola virus disease. JCI Insight. 2019;4(1):1-16. pii: 125106. DOI: 10.1172/jci.insight.125106
  61. 61. Messaoudi I, Amarasinghe GK, Basler CF. Filovirus pathogenesis and immune evasion: Insights from Ebola virus and marburg virus. Nature Reviews. Microbiology. 2015;13(11):663-676. DOI: 10.1038/nrmicro3524
  62. 62. Villinger F, Rollin PE, Brar SS, et al. Markedly elevated levels of interferon (IFN)-γ, IFN-α, interleukin (IL)-2, IL-10, and tumor necrosis factor-α associated with fatal Ebola virus infection. The Journal of Infectious Diseases. 1999;179(Suppl 1):S188-S191. DOI: 10.1086/514283
  63. 63. Lubaki NM, Ilinykh P, Pietzsch C, et al. The lack of maturation of Ebola virus-infected dendritic cells results from the cooperative effect of at least two viral domains. Journal of Virology. 2013;87(13):7471-7485. DOI: 10.1128/JVI.03316-12
  64. 64. Mahanty S, Hutchinson K, Agarwal S, et al. Cutting edge: impairment of dendritic cells and adaptive immunity by Ebola and Lassa viruses. Journal of Immunology. 2003;170(6):2797-2801
  65. 65. Bradfute SB, Braun DR, Shamblin JD, et al. Lymphocyte death in a mouse model of Ebola virus infection. The Journal of Infectious Diseases. 2007;196(Suppl 2):S296-S304. DOI: 10.1086/520602
  66. 66. Reed DS, Hensley LE, Geisbert JB, et al. Depletion of peripheral blood T lymphocytes and NK cells during the course of Ebola hemorrhagic fever in cynomolgus macaques. Viral Immunology. 2004;17(3):390-400. DOI: 10.1089/vim.2004.17.390
  67. 67. Gupta M, Spiropoulou C, Rollin PE. Ebola virus infection of human PBMCs causes massive death of macrophages, CD4 and CD8 T cell sub-populations in vitro. Virology. 2007;364(1):45-54. DOI: 10.1016/j.virol.2007.02.017
  68. 68. Wauquier N, Becquart P, Padilla C, et al. Human fatal zaire Ebola virus infection is associated with an aberrant innate immunity and with massive lymphocyte apoptosis. PLoS Neglected Tropical Diseases. 2010;4(10). pii: e837. DOI: 10.1371/journal.pntd.0000837
  69. 69. Sanchez A, Lukwiya M, Bausch D, et al. Analysis of human peripheral blood samples from fatal and nonfatal cases of Ebola (Sudan) hemorrhagic fever: Cellular responses, virus load, and nitric oxide levels. Journal of Virology. 2004;78(19):10370-10377. DOI: 10.1128/JVI.78.19.10370-10377.2004
  70. 70. Baize S, Leroy EM, Georges-Courbot MC, et al. Defective humoral responses and extensive intravascular apoptosis are associated with fatal outcome in Ebola virus-infected patients. Nature Medicine. 1999;5(4):423-426
  71. 71. Tisoncik JR, Korth MJ, Simmons CP, et al. Into the eye of the cytokine storm. Microbiology and Molecular Biology Reviews. 2012;76(1):16-32. DOI: 10.1128/MMBR.05015-11
  72. 72. Arad G, Levy R, Nasie I, et al. Binding of superantigen toxins into the CD28 homodimer interface is essential for induction of cytokine genes that mediate lethal shock. PLoS Biology. 2011;9(9):e1001149. DOI: 10.1371/journal.pbio.1001149
  73. 73. Scherer MT, Ignatowicz L, Winslow GM, et al. Superantigens: Bacterial and viral proteins that manipulate the immune system. Annual Review of Cell Biology. 1993;9:101-128. DOI: 10.1146/annurev.cb.09.110193.000533
  74. 74. Younan P, Iampietro M, Nishida A, et al. Ebola virus binding to Tim-1 on T lymphocytes induces a cytokine storm. MBio. 2017;8(5):1-20. pii: e00845-17. DOI: 10.1128/mBio.00845-17
  75. 75. Yen B, Mulder LC, Martinez O, et al. Molecular basis for ebolavirus VP35 suppression of human dendritic cell maturation. Journal of Virology. 2014;88(21):12500-12510. DOI: 10.1128/JVI.02163-14
  76. 76. Jin H, Yan Z, Prabhakar BS, et al. The VP35 protein of Ebola virus impairs dendritic cell maturation induced by virus and lipopolysaccharide. The Journal of General Virology. 2010;91(Pt 2):352-361. DOI: 10.1099/vir.0.017343-0
  77. 77. Luthra P, Ramanan P, Mire CE, et al. Mutual antagonism between the Ebola virus VP35 protein and the RIG-I activator PACT determines infection outcome. Cell Host And Microbe. 2013;14(1):74-84. DOI: 10.1016/j.chom.2013.06.010
  78. 78. Chang TH, Kubota T, Matsuoka M, et al. Ebola zaire virus blocks type I interferon production by exploiting the host SUMO modification machinery. PLoS Pathogens. 2009;5(6):e1000493. DOI: 10.1371/journal.ppat.1000493
  79. 79. Österlund PI, Pietilä TE, Veckman V, et al. IFN regulatory factor family members differentially regulate the expression of type III IFN (IFN-lambda) genes. Journal of Immunology. 2007;179(6):3434-3442
  80. 80. Prins KC, Cardenas WB, Basler CF. Ebola virus protein VP35 impairs the function of interferon regulatory factor-activating kinases IKKepsilon and TBK-1. Journal of Virology. 2009;83(7):3069-3077. DOI: 10.1128/JVI.01875-08
  81. 81. Feng Z, Cerveny M, Yan Z, et al. The VP35 protein of Ebola virus inhibits the antiviral effect mediated by double-stranded RNA-dependent protein kinase PKR. Journal of Virology. 2007;81(1):182-192. DOI: 10.1128/JVI.01006-06
  82. 82. He F, Melén K, Maljanen S, et al. Ebolavirus protein VP24 interferes with innate immune responses by inhibiting interferon-λ1 gene expression. Virology. 2017;509:23-34. DOI: 10.1016/j.virol.2017.06.002
  83. 83. Guito JC, Albarino CG, Chakrabarti AK, et al. Novel activities by ebolavirus and marburgvirus interferon antagonists revealed using a standardized in vitro reporter system. Virology. 2017;501:147-165. DOI: 10.1016/j.virol.2016.11.015
  84. 84. Basler CF, Amarasinghe GK. Evasion of interferon responses by Ebola and Marburg viruses. Journal of Interferon and Cytokine Research. 2009;29(9):511-520. DOI: 10.1089/jir.2009.0076
  85. 85. Schneider WM, Chevillotte MD, Rice CM. Interferon-stimulated genes: A complex web of host defenses. Annual Review of Immunology. 2014;32:513-545. DOI: 10.1146/annurev-immunol-032713-120231
  86. 86. Schoggins JW. Interferon-stimulated genes: Roles in viral pathogenesis. Current Opinion in Virology. 2014;6:40-46. DOI: 10.1016/j.coviro.2014.03.006

Written By

Felix B. He, Krister Melén, Laura Kakkola and Ilkka Julkunen

Submitted: February 26th, 2019 Reviewed: May 8th, 2019 Published: June 19th, 2019