Ebola Virus Shed Glycoprotein Triggers Differentiation, Infection, and Death of Monocytes Through Toll-Like Receptor 4 Activation

Extracellular vesicle storm during the course of Ebola virus infection in primates

Introduction

Ebola virus (EBOV) is an RNA virus of the Filoviridae family that is responsible for outbreaks of hemorrhagic fevers in primates with a lethality rate as high as 90%. EBOV primarily targets host macrophages leading to cell activation and systemic cytokine storm, and fatal infection is associated with an inhibited interferon response, and lymphopenia. The EBOV surface glycoprotein (GP) has been shown to directly induce T cell depletion and can be secreted outside the virion via extracellular vesicles (EVs), though most studies are limited to epithelial cells and underlying mechanisms remain poorly elucidated.

Methods

To assess the role of GP on EBOV-induced dysregulation of host immunity, we first utilized EBOV virus-like particles (VLPs) expressing VP40 and NP either alone (Bald-VLP) or in conjunction with GP (VLP-GP) to investigate early inflammatory responses in THP-1 macrophages and in a murine model. We then sought to decipher the role of non-classical inflammatory mediators such as EVs over the course of EBOV infection in two EBOV-infected rhesus macaques by isolating and characterizing circulatory EVs throughout disease progression using size exclusion chromatography, nanoparticle tracking-analysis, and LC-MS/MS.

Results

While all VLPs could induce inflammatory mediators and recruit small peritoneal macrophages, pro-inflammatory cytokine and chemokine gene expression was exacerbated by the presence of GP. Further, quantification of EVs isolated from infected rhesus macaques revealed that the concentration of vesicles peaked in circulation at the terminal stage, at which time EBOV GP could be detected in host-derived exosomes. Moreover, comparative proteomics conducted across EV populations isolated from serum at various time points before and after infection revealed differences in host-derived protein content that were most significantly pronounced at the endpoint of infection, including significant expression of mediators of TLR4 signaling.

Discussion

These results suggest a dynamic role for EVs in the modification of disease states in the context of EBOV. Overall, our work highlights the importance of viral factors, such as the GP, and host derived EVs in the inflammatory cascade and pathogenesis of EBOV, which can be collectively further exploited for novel antiviral development.

Interplay of Ebola Virus With Immune Cells Leading to Their Death by Diverse Mechanisms

Inflammation and cytopenia are commonly observed during Ebola virus (EBOV) infection; however, mechanisms responsible for EBOV-induced cell death remain obscure. While apoptosis and necrosis are already identified as mechanisms of cell death induced by the virus, our study demonstrates that THP-1 monocytes and SupT1 T cells exposed to EBOV undergo pyroptosis and necroptosis, respectively, through a direct contact with EBOV, and also mediate pyroptosis or necroptosis of uninfected bystander cells via indirect effects associated with secreted soluble factors. These results emphasize novel aspects of interactions between EBOV and immune cell populations and provide a better understanding of the immunopathogenesis of EBOV disease.

Ebola virus shed glycoprotein is toxic to human T, B, and natural killer lymphocytes

Lymphocyte depletion is a distinctive feature of Ebola virus (EBOV) disease. The ectodomain of EBOV glycoprotein (GP) is cleaved off the surface of infected cells into circulation as shed GP. To test the hypothesis that shed GP induces lymphocyte death, we cultured primary human B, NK, or T cells with shed GP in vitro. We found that shed GP dependably decreased B, NK, and T cell viability across donors. B and NK cells exhibited higher susceptibility than T cells. Continuous monitoring revealed shed GP began to kill B and NK cells by 4 h and T cells by 5 h. We also demonstrated that shed GP-induced lymphocyte death can be both caspase dependent and caspase independent. Our data are evidence that the cytotoxic effect of shed GP on lymphocytes may contribute to EBOV disease and highlight the need for further research to clarify mechanisms of shed GP-induced death.

When the allergy alarm bells toll: The role of Toll-like receptors in allergic diseases and treatment

Toll-like receptors of the human immune system are specialized pathogen detectors able to link innate and adaptive immune responses. TLR ligands include among others bacteria-, mycoplasma- or virus-derived compounds such as lipids, lipo- and glycoproteins and nucleic acids. Not only are genetic variations in TLR-related genes associated with the pathogenesis of allergic diseases, including asthma and allergic rhinitis, their expression also differs between allergic and non-allergic individuals. Due to a complex interplay of genes, environmental factors, and allergen sources the interpretation of TLRs involved in immunoglobulin E-mediated diseases remains challenging. Therefore, it is imperative to dissect the role of TLRs in allergies. In this review, we discuss i) the expression of TLRs in organs and cell types involved in the allergic immune response, ii) their involvement in modulating allergy-associated or -protective immune responses, and iii) how differential activation of TLRs by environmental factors, such as microbial, viral or air pollutant exposure, results in allergy development. However, we focus on iv) allergen sources interacting with TLRs, and v) how targeting TLRs could be employed in novel therapeutic strategies. Understanding the contributions of TLRs to allergy development allow the identification of knowledge gaps, provide guidance for ongoing research efforts, and built the foundation for future exploitation of TLRs in vaccine design.

Scratching the Surface Takes a Toll: Immune Recognition of Viral Proteins by Surface Toll-like Receptors

Early innate viral recognition by the host is critical for the rapid response and subsequent clearance of an infection. Innate immune cells patrol sites of infection to detect and respond to invading microorganisms including viruses. Surface Toll-like receptors (TLRs) are a group of pattern recognition receptors (PRRs) that can be activated by viruses even before the host cell becomes infected. However, the early activation of surface TLRs by viruses can lead to viral clearance by the host or promote pathogenesis. Thus, a plethora of research has attempted to identify specific viral ligands that bind to surface TLRs and mediate progression of viral infection. Herein, we will discuss the past two decades of research that have identified specific viral proteins recognized by cell surface-associated TLRs, how these viral proteins and host surface TLR interactions affect the host inflammatory response and outcome of infection, and address why controversy remains regarding host surface TLR recognition of viral proteins.

Activation of TLR4 by viral glycoproteins: A double-edged sword?

Recognition of viral infection by pattern recognition receptors is paramount for a successful immune response to viral infection. However, an unbalanced proinflammatory response can be detrimental to the host. Recently, multiple studies have identified that the SARS-CoV-2 spike protein activates Toll-like receptor 4 (TLR4), resulting in the induction of proinflammatory cytokine expression. Activation of TLR4 by viral glycoproteins has also been observed in the context of other viral infection models, including respiratory syncytial virus (RSV), dengue virus (DENV) and Ebola virus (EBOV). However, the mechanisms involved in virus-TLR4 interactions have remained unclear. Here, we review viral glycoproteins that act as pathogen-associated molecular patterns to induce an immune response via TLR4. We explore the current understanding of the mechanisms underlying how viral glycoproteins are recognized by TLR4 and discuss the contribution of TLR4 activation to viral pathogenesis. We identify contentious findings and research gaps that highlight the importance of understanding viral glycoprotein-mediated TLR4 activation for potential therapeutic approaches.

Pathogenicity and Virulence of Ebolaviruses with Species- and Variant-specificity

Ebola virus (EBOV), belonging to the species Zaire ebolavirus in the genus Ebolavirus, causes a severe febrile illness in humans with case fatality rates (CFRs) up to 90%. While there have been six virus species classified, which each have a single type virus in the genus Ebolavirus, CFRs of ebolavirus infections vary among viruses belonging to each distinct species. In this review, we aim to define the ebolavirus species-specific virulence on the basis of currently available laboratory and experimental findings. In addition, this review will also cover the variant-specific virulence of EBOV by referring to the unique biological and pathogenic characteristics of EBOV variant Makona, a new EBOV variant isolated from the 2013-2016 EBOV disease outbreak in West Africa. A better definition of species-specific and variant-specific virulence of ebolaviruses will facilitate our comprehensive knowledge on genus Ebolavirus biology, leading to the development of therapeutics against well-focused pathogenic mechanisms of each Ebola disease.

Therapeutic vaccination strategies against EBOV by rVSV-EBOV-GP: the role of innate immunity

Zaire Ebola virus (EBOV) is a member of the Filoviridae family. Infection with EBOV causes Ebola virus disease (EVD) characterized by excessive inflammation, lymphocyte death, coagulopathy, and multi-organ failure. In 2019, the FDA-approved the first anti-EBOV vaccine, rVSV-EBOV-GP (Ervebo® by Merck). This live-recombinant vaccine confers both prophylactic and therapeutic protection to nonhuman primates and humans. While mechanisms conferring prophylactic protection are well-investigated, those underlying protection conferred shortly before and after exposure to EBOV remain poorly understood. In this review, we review data from in vitro and in vivo studies analyzing early immune responses to rVSV-EBOV-GP and discuss the role of innate immune activation in therapeutic protection.

[The latest research findings on Ebola virus]

013-2016 Ebola virus disease (EVD) outbreak was the largest EVD outbreak ever documented that started earlier in Guinea and later widely spread throughout West Africa, ending up a total of > 28,000 human infections. In this review, we outline research findings on Ebola virus (EBOV) variant Makona, a new EBOV variant isolated from the 2013-2016 EVD outbreak, and introduce the unique biological and pathogenic characteristics of Makona variant. We also discuss about the relevance of persistent infection of EBOV in EVD survivors with resurgence of EVD outbreak in Guinea in 2021. Moreover, this review covers a recent case report of EVD relapse and deliberates new interpretations of EBOV biology and EVD outbreak.

To B or Not to B: Mechanisms of Protection Conferred by rVSV-EBOV-GP and the Roles of Innate and Adaptive Immunity

Zaire Ebola virus (EBOV) is a member of the Filoviridae family of negative sense, single-stranded RNA viruses. EBOV infection causes Ebola virus disease (EVD), characterized by coagulopathy, lymphopenia, and multi-organ failure, which can culminate in death. In 2019, the FDA approved the first vaccine against EBOV, a recombinant live-attenuated viral vector wherein the G protein of vesicular stomatitis virus is replaced with the glycoprotein (GP) of EBOV (rVSV-EBOV-GP, Ervebo^® by Merck). This vaccine demonstrates high efficacy in nonhuman primates by providing prophylactic, rapid, and post-exposure protection. In humans, rVSV-EBOV-GP demonstrated 100% protection in several phase III clinical trials in over 10,000 individuals during the 2013–2016 West Africa epidemic. As of 2020, over 218,000 doses of rVSV-EBOV-GP have been administered to individuals with high risk of EBOV exposure. Despite licensure and robust preclinical studies, the mechanisms of rVSV-EBOV-GP-mediated protection are not fully understood. Such knowledge is crucial for understanding vaccine-mediated correlates of protection from EVD and to aid the further design and development of therapeutics against filoviruses. Here, we summarize the current literature regarding the host response to vaccination and EBOV exposure, and evidence regarding innate and adaptive immune mechanisms involved in rVSV-EBOV-GP-mediated protection, with a focus on the host transcriptional response. Current data strongly suggest a protective synergy between rapid innate and humoral immunity.

Extracellular Vesicles and Ebola Virus: A New Mechanism of Immune Evasion

Ebola virus (EBOV) disease can result in a range of symptoms anywhere from virtually asymptomatic to severe hemorrhagic fever during acute infection. Additionally, spans of asymptomatic persistence in recovering survivors is possible, during which transmission of the virus may occur. In acute infection, substantial cytokine storm and bystander lymphocyte apoptosis take place, resulting in uncontrolled, systemic inflammation in affected individuals. Recently, studies have demonstrated the presence of EBOV proteins VP40, glycoprotein (GP), and nucleoprotein (NP) packaged into extracellular vesicles (EVs) during infection. EVs containing EBOV proteins have been shown to induce apoptosis in recipient immune cells, as well as contain pro-inflammatory cytokines. In this manuscript, we review the current field of knowledge on EBOV EVs including the mechanisms of their biogenesis, their cargo and their effects in recipient cells. Furthermore, we discuss some of the effects that may be induced by EBOV EVs that have not yet been characterized and highlight the remaining questions and future directions.

Immune parameters and outcomes during Ebola virus disease

BACKGROUND

The West African Ebola virus epidemic from 2014-2016 highlighted the lack of knowledge about the pathogenicity of the virus and the factors responsible for outcome. A performant and rapid diagnosis is of crucial importance, as is overcoming the difficulty of providing high-quality patient management during such an extensive outbreak. Here, we propose to study the role of the immune mediators during Ebola virus disease and to define some molecules of importance in the outcome.

METHODS

Plasma from Guinean patients sampled during the outbreak were analyzed using RT-qPCR, magnetic bead assay, ELISA, and high-quality statistical analyses. We also performed a transcriptomic analysis in leukocytes samples. Therefore, we deeply characterized the immune responses involved in Ebola virus disease.

RESULTS

We evaluated the immune patterns depending on the outcome of the disease. Survivors presented an efficient and well-balanced immune response, whereas fatalities were characterized by an intense inflammatory response, overexpression of multiple cytokines, and a "chemokine storm." The plasma concentration of most of the parameters tested increased until death. Statistical analyses also allowed us to define a panel of markers highly predictive of outcome.

CONCLUSION

The immune response observed in fatalities was highly similar to that characterizing septic shock syndrome. Our results suggest that immune responses can play a major pathogenic role during severe Ebola virus infection and argue in favor of therapeutic approaches that act on both viral replication and the induction of shock syndrome.

FUNDING

French Ministry of Foreign Affairs, the Agence Française de Développement, and the Institut Pasteur.