Interaction of Ebola Virus with the Innate Immune System

Abstract

The Ebola viruses (EBOVs) are known as one the most lethal viruses. EBOV systemic infection can cause damage to vital organs and lead to death. The immune responses of the innate immune system and inflammatory cascade are critical elements in the EBOV pathogenesis and mortality. The primary innate immune system response can shape the adaptive immune responses. The innate immune response, due to the pattern-recognition receptors (PRRs), can induce interferons (IFN). IFN is a critical element in the antiviral response. The EBOV can evade the IFN and innate immunity using different mechanisms, whereas a well-controlled and sufficient innate immune response is vital for limiting the EBOV infection. In this regard, a hyperactive inflammation response may lead to cytokine storms and death. In this chapter, we have tried to provide a perspective on the pathogenesis and molecular mechanisms of the innate immune system and its interaction with EBOV infection.

1. Introduction

The Ebola virus (EBOV) genus and Marburg viruses are classified in the Filoviridae family and Mononegavirales order [1]. The EBOV contains a linear single-stranded RNA genome of 18.9-kilobase length and a membrane glycoprotein [2, 3]. Over the past 40 years, there have been 34 episodes of EBOV outbreaks in 11 different Sub-Saharan African countries. The first outbreaks occurred in 1971 in the Democratic Republic of the Congo (DRC) and Sudan [3]. These outbreaks amassed a total of more than 34000 cases and led to 14000 deaths [3]. EBOV Sudan, Bundibugyo, Zaire, and Reston are important species of the EBOV infection in humans [4]. The basic reproductive number (R0) of the EBOV is estimated to be in the range of 1.51–2.53 [4].

Some therapeutic and vaccination strategies have been introduced against the EBOV so far. The remdesivir, monoclonal antibodies [5], and rVSV-ZEBOV (vesicular stomatitis virus-based vaccine which is expressing the glycoprotein of a Zaire Ebola virus) [6] are considered ongoing advances in this area for EBOV treatment and transmission control.

Innate immunity and interferons (IFN) production are one of the most important immune responses to viral infections. Interaction between the Ebola virus and innate immunity with a well-regulated inflammation response can be life-saving in patients with the Ebola virus disease (EBD) [7]. The interaction of the EBOV with the innate immune system is a multifactorial condition, and it is largely associated with cytokines, chemokines, inflammation mediators, the NK cell receptors, and pathogen-associated molecular patterns (PAMPs), such as killer immunoglobulin-like receptor (KIR) and Toll-like receptors (TLRs) [8, 9]. The signaling pathways in the innate immune system are triggered after PAMPs detected by pattern-recognition receptors (PRRs) [10]. Based on protein domain homology, the PRRs are classified into six groups TLRs, a retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), nucleotide oligomerization domain (NOD)-like receptors (NLRs), C-type lectin receptors (CLRs), absent in melanoma-2 (AIM2)-like receptors (ALRs), and cyclic GMP-AMP synthase and stimulator of interferon genes (cGAS-STING pathway) [11, 12]. PRRs are also able to recognize molecules released by damaged cells (The damage-associated molecular pattern-DAMPs) and activate natural immunity [13, 14]. The detection of PAMPs or DAMPs by PRRs increases the transcription of genes encoding cytokine and chemokine, IFN type 1, and antimicrobial proteins (AMPs) [15]. Furthermore, most of the TLRs recognize double-stranded RNA (dsRNA) whereas TLR7 and TLR8 bind single-stranded RNA (ssRNA) in endosomes [16]. After recognition, TLRs recruit several adaptor proteins, including Myeloid Differentiation primary response 88 (MyD88), TIR-domain-containing adaptor protein (TIRAP), TIR-domain-containing adapter-inducing interferon-β (TRIF), and TRIF-related adapter molecule (TRAM) [17]. This initiates downstream signaling cascades that lead to the activation of transcription factors, such as transcription factors NF-κB, interferon regulatory factor 3/7 (IRF3/7), and activator protein-1 (AP-1). These factors stimulate the transcription of genes in the cell nucleus and increase the secretion of pro-inflammatory cytokines and IFN [15, 18]. RIG-1 and melanoma differentiation-associated gene 5 (MDA5), recognize viral ss/dsRNA molecule leads to the translocation of NF-kB, mitogen-activated protein kinases (MAPKs), as well as interferon regulatory factors IRF3, IRF7 [19]. RIG-1 signaling pathway plays a crucial role in the antiviral innate immune response. Activated RIG-1 can interact with signaling adaptor protein mitochondrial antiviral signaling protein (MAVS), also known as IFN-β promoter stimulator 1 (IPS-1), to induction of NF-κB signaling or interferon pathways. It was revealed that upon viral infection, MAVs form high-molecular-weight aggregates downstream of RIG-1 and MDA5 signaling [20]. Activated MAVS–RIG-I signaling ultimately results in the activation of the antiviral IFN-I pathway [21]. The binding of type I and III IFNs to their receptors activates the Jak–STAT pathway, which is responsible for enhancing IFN-stimulated gene expression, which includes antiviral genes, such as Viperin, MxA, MxB, IFITMs, OAS, and PKR [22, 23]. One of the most important complexes, which participate in host defense by sensing viral infection and promoting innate immune system response, is the inflammasome, first described by Martinon in 2002 [24]. The best-studied inflammasome sensor is NOD-like receptor pyrin domain-containing protein 3 (NLRP3), which consists of a sensor molecule (NLRP3), the adaptor protein ASC, also called PYCARD, and an effector pro-caspase-1 [25]. Activated caspase is required for the cleavage of the inactivated interleukin-1 family (IL-1), such as pro-IL-18 and pro-IL-1β to the mature forms to initiate inflammasome [26].

Dendritic cells (DCs) and macrophages are one of the main axes for filoviruses infections due to the activation of the adaptive immune [27, 28]. In addition, the macrophage’s infection and attachment by EBOV result in downstream signaling for alteration of the expression profile in these cells. This alteration leads to the pro-inflammatory cytokine release and virus spreading, which both are important elements in the disease progression and outcome [29, 30]. In this regard, in the current chapter, we tried to provide a perspective on the pathophysiology and molecular mechanisms of the innate immune system and its interaction with EBOV infection.

2. Ebola virus: history and epidemiology

Over the past 40 years, there are 34 episodes of the EBOV outbreak in more than 10 different Sub-Saharan African countries leading to numerous cases and deaths [31]. Different species of the EBOV have been reported thus far. Three important species are known as Sudan, Bundibugyo, and Reston [32]. The responsible species for the 2014 outbreak of EBOV was identified as the Zaire ebolavirus [32]. The EBOV disease is a zoonotic disease in nature and the major route for transmission is contact with the infected animal especially chimpanzees, fruit bats, and antelope [2, 33]. The virus genome could be detectable for 10 weeks postmortem [34]. Any contact with bodily fluids is a major route for transmission of EBOV [35]. Even after the survival from the disease the viral genome can be detected in semen for 179 days and leads to sexual transmission in some cases [36]. Fever, fatigue, diarrhea, and tachycardia are the most common symptoms and based on the infected strain and the patient’s age, 43% of mortality is reported over the 8 days following the infection [37]. In convalesce or a long time after the remission, some rare complications and long-term sequels, such as uveitis or blurred vision, sleeping problems, and meningoencephalitis are reported [38, 39, 40].

3. EBOVE virology and pathogenicity

3.1 Virology and classification

The Ebola virus (EBOV) genus in Filoviridae is a member of the great order of Mononegavirales [1]. There are seven important genes in EBOV. The nucleoprotein (NP) is curial for replication and virus formation. Also, viral protein 35 (VP35) is important in the IFN synthesis blocking and virus replication while VP40 acts as an element in virus formation and intracellular trafficking. The glycoprotein (GP) is a major element for viral entry and induces lymphocyte apoptosis by soluble GPs. EBOV VP30 has been assumed to function as a transcription activation factor, which is essential for viral replication. VP24 is necessary for the formation of nucleocapsids (NC) and nucleocapsid-like structures, and Ebola virus L proteins act as subunits of RNA-dependent RNA polymerase, which along with VP35, is necessary for viral replication and transcription [41, 42, 43]. A summary of the function of genes is provided in Table 1.

Viral gene name	Important functions
VP24	formation of nucleocapsids (NC) and nucleocapsid-like structures, inhibit IFN signaling
L	RNA-dependent RNA polymerase
glycoprotein (GP)	viral entry induces lymphocyte apoptosis and reduces neutralizing antibodies by soluble GPs
NP	Nucleoprotein formation
VP40	Virus assembly
VP35	inhibits the induction of IFN type I
VP30	transcription activation factor

Table 1.

A summary of the important functions of the Ebola virus genes [41, 44].

The VP35 is a key viral protein in the EBOV virulence. This unique protein inhibits the induction of IFN type I in the infected cell. The IFN type I has long been declared a key element in the host’s innate antiviral response [45]. In this regard, the VP35 is considered the most important for virus immune evasion [46]. The IFN blocking strategy in EBOV is not limited to VP35. Other EBOV protein VP24 can actively bind to the karyopherin alpha (KPNA) and interfere with the IFN STAT1 downstream signaling [47]. Furthermore, VP24 seems to be essential for the EBOV replication in macrophages of the guinea pig as an animal model [48].

3.2 Pathogenicity and treatment

The EBOV represents an affinity to a wide range of the cellular receptors for virus attachment to the cells. Following the systemic virus infection, viral cytopathology and immune-mediated cell damage are two essential steps of the EBOV pathogenesis and tissue damage [49]. One of the suggested pathogenic mechanisms of the EBOV is the antibody-dependent augmentation of the infection through the attachment of the antibodies to the virus and the C1q-C1q receptor of the complement in the cell surfaces [50].

One of the most important therapeutic agents for EBOV is the remdesivir. The remdesivir acts as an inhibitor of the virus RNA polymerase and leads to chain termination [51]. Efforts for the treatment of the EBOV also lead to some monoclonal antibodies against this virus; for instance, old and newer versions of these monoclonal antibodies are ZMapp, MAb114, and REGN-EB3 [5].

4. Innate immune system and EBOV

4.1 Primary innate immune response and inflammation

Interaction between the Ebola virus and innate immunity suggested that a fast and well-regulated inflammation response could be life-saving in patients with the Ebola virus disease (EBD). While a massive monocyte/macrophage activation could be lethal [7]. The mononuclear phagocytes are import element in the EBD pathology [52].

Biomarkers are suggestive elements for disease prognosis and pathogenesis. Some markers such as apoptosis antigen-Fas, IFN-β, IL-29, IL-5, TNFR-II, and FAS ligand levels are associated with the moderate disease while the D-dimer, Granzyme B, IL-10, IL-6, IL-8, TNFR-I, vWF (von Willebrand factor), monocyte chemoattractant proteins and thrombomodulin are associate with severe disease [53]. Furthermore, up-regulation of the IL-1β and IL-6 can suggest a non-fatal infection while the increase in TNF-α, IFN-ɤ, IL-10, IL-1 receptor antagonist, neopterin, IL-8, IL-15, and IL-16 is associated with the lethal outcome [7, 54, 55, 56]. By considering all these markers, it has been suggested that in a general view an increase in cytokines and cytokine storm, which it is, represents a hyperactive of the innate immune responses and in the other way, suppression of the adaptive immune responses and lymphocyte apoptosis is the main pathogenesis feature of the EBD and lethal infections [55].

The lymphocyte apoptosis leads to lymphocyte depletion. This apoptotic feature is not due to the replication and infection of the lymphocytes but it mediates through viral and immune system stimulations [57]. sGP, a viral protein produced during EBOV infection and accumulates at high concentrations in the serum, serves as a decoy to prevent the immune system from fighting the infection by binding EBOV-neutralizing antibodies [58]. It is assumed that glycosylation of transmembrane GP may affect neutralizing antibody binding [59]. Virally infected cells, release inflammation mediators that induce Fas and TNF-associated apoptosis-inducing ligands (TRAIL) pathways that can result in lymphocyte apoptosis and lack of an effective adaptive immune response [60, 61, 62]. In EBOV infection, lymphocytes are not directly infected, but apoptosis of lymphocytes is a pathological feature of infection. It is hypothesized that the factors (such as TRAIL, TNF-a, and Fas ligand) secreted by macrophages and dendritic cells infected in EBOV, cause lymphocyte apoptosis [52, 60].

Investigation of the EBOV infection in asymptomatic people suggested strong activation of the innate immune system and inflammation, which leads to adaptive immune activation and cytotoxic T cell responses. The strong activation of the innate immune system, inflammation and adaptive immune activation are considered as the optimum immune response to EBOV [63]. The main concept of the current section is summarized in Figure 1. The figure represents the EBOV infection in lethal and non-lethal scenarios and the role of the innate immune and inflammation in this process. In addition, some important interactions of the EBOV proteins are noted, for instance, the role of the VP35 and VP24 in type I interferon blocking or the role of the soluble glycoproteins in lymphocyte apoptosis.

Figure 1.

A summary of the EBOV infection in lethal and non-lethal scenarios and the role of the innate immune and inflammation. After the EBOV, infection such as any other viral infection innate immunity induces inflammation and IFN. The VP35 and VP24 of the EBOV block the IFN production. A well-controlled and sufficient innate immune response, while it leads to the adaptive immune response, is assumed main cause of asymptomatic or non-lethal infections.

4.2 Innate immunity receptors and EBOV

The interaction of the EBOV and the innate immune system is not only limited to the cytokines, chemokines, or inflammation mediators. In this regard, the NK cell and T cell receptors, which are known as killer immunoglobulin-like receptors (KIR), are critical. The role of the NK cells in response to the EBOV viral-like particles highlighted the importance of these cells in the innate immune response to the EBOV [64]. The KIRs are important elements in the host response to infectious diseases. KIR2DS1 and KIR2DS3 of repertoire genotypes of the KIR represent more susceptibility to fatal EBOV [8].

In the EBOV infection, the interaction of the innate and adaptive immune responses is critical for the disease outcome. The Toll-like receptor 4 (TLR4) is a vital element in response to the EBOV glycoprotein and activation of the antigen-presenting cells and T cells. By considering this, TLR4 responses represent a vital role in the regulation of innate and adaptive immunity [9]. The interaction of the TLR4 and monocytes (as antigen-presenting cells) highlighted the importance of the monocytes in the regulation of the immune response. The evidence supports the alteration of the transcriptional patterns in monocytes in EBD [65]. Furthermore, one of the major elements in inflammation and IFN stimulation is NLRP3 [66]. The EBOV infection could increase the IL-1β and IL-18 by the NLRP3 inflammasome activation [67]. This factor highlights the importance of the NLRP3 in pro-inflammatory cytokine production and innate immune system responses.

5. Macrophages and other important innate immune cells in Ebola virus innate immune responses

The DCs and macrophages infection with the EBOV is critical for adaptive immune activation. The EBOV infection leads to macrophage activation (by increasing the CD163 marker) and decreases T cell activation (by reducing in CD25 marker) in severe cases [68]. The macrophage’s infection leads to reductions in the co-stimulatory molecules in these cells, which are known as the main adaptive immune activation and the axis of the antigen presentation [27, 28]. The macrophage infection also leads to alteration in pro-inflammatory cytokine releases, such as IL-1β, TNF-α, and IL-6, which could dysregulate the inflammation [69]. Furthermore, monocytes infection with EBOV is a toll on the virus spreading all around the body [69]. The EBOV uses a mimicking of the apoptosis process for attachment and entry macrophages due to the TAM receptor tyrosine kinases and integrin αV [70]. However, it seems that other cell surface receptors such as DC-SIGN and DC-SIGNR or macrophage galactose-type calcium-type lectin (MGL) are critical for the EBOV infection in DC and macrophages [71, 72]. The attachment of the virus to macrophages regardless of the virus entry or macrophage infection affects the macrophage’s expression profile. This alteration is led macrophages to produce high levels of pro-inflammatory and pro-apoptotic signals [30].

Macrophages and dendritic cells are important cell targets of EBOV and the main innate immune cells that secrete cytokines, and chemokines following infection [29, 69]. Although macrophages and DCs are still able to initiate coagulation and inflammation, they are not able to stop the spreading of the Ebola virus systemically due to their impaired ability [28, 73]. These dysfunctions have a major impact on the innate and adaptive immune systems [74]. Macrophages and DCs are the essential cells of innate immunity and provide a bridge between innate and adaptive immunity [75]. It will highlight the importance of these cells in the disease outcome (Figure 1). Furthermore, VP24 and VP35 block latent lymphocyte stimulation through the IFN response [73]. All these clues are critical to combine and work as chains for limiting the infection through sufficient and well-controlled innate immune response activation, which leads to adaptive immune responses.

6. Conclusions

In this chapter, we tried to provide a perspective on the EBOV infection and innate immune responses. In a glimpse, it is worth mentioning that innate immune responses are critical in the EBOV infection. Sufficient and well-controlled innate immune responses may lead to optimum cytokine release and adaptive immune activation. In contrast, an overreacted innate immunity could affect and hyper-inflammation response.

The macrophages and DCs are also key elements in EBOV infection due to pro-inflammatory response and virus spreading. The EBOV uses different strategies to dysregulate and evade innate immune responses.

VP35 and VP24 of the virus inhibit the IFN type I stimulation in infected cells. Furthermore, the soluble GPs of the EBOV can induce apoptosis in T cells. The interaction of the EBOV with innate immunity is the most fundamental feature of the infection and determines the disease outcome.

Acknowledgments

We sincerely acknowledge the IntechOpen publication editorial office and Jelena Vrdoljak the Author Service Manager in IntechOpen for her incredible efforts.

Conflict of interest

The authors declare no conflict of interest.