Folate receptor-alpha is a cofactor for cellular entry by Marburg and Ebola viruses

Metabolic Enzymes in Viral Infection and Host Innate Immunity

Metabolic enzymes are central players for cell metabolism and cell proliferation. These enzymes perform distinct functions in various cellular processes, such as cell metabolism and immune defense. Because viral infections inevitably trigger host immune activation, viruses have evolved diverse strategies to blunt or exploit the host immune response to enable viral replication. Meanwhile, viruses hijack key cellular metabolic enzymes to reprogram metabolism, which generates the necessary biomolecules for viral replication. An emerging theme arising from the metabolic studies of viral infection is that metabolic enzymes are key players of immune response and, conversely, immune components regulate cellular metabolism, revealing unexpected communication between these two fundamental processes that are otherwise disjointed. This review aims to summarize our present comprehension of the involvement of metabolic enzymes in viral infections and host immunity and to provide insights for potential antiviral therapy targeting metabolic enzymes.

Identification of CCZ1 as an essential lysosomal trafficking regulator in Marburg and Ebola virus infections

Marburg and Ebola filoviruses are two of the deadliest infectious agents and several outbreaks have occurred in the last decades. Although several receptors and co-receptors have been reported for Ebola virus, key host factors remain to be elucidated. In this study, using a haploid cell screening platform, we identify the guanine nucleotide exchange factor CCZ1 as a key host factor in the early stage of filovirus replication. The critical role of CCZ1 for filovirus infections is validated in 3D primary human hepatocyte cultures and human blood-vessel organoids, both critical target sites for Ebola and Marburg virus tropism. Mechanistically, CCZ1 controls early to late endosomal trafficking of these viruses. In addition, we report that CCZ1 has a role in the endosomal trafficking of endocytosis-dependent SARS-CoV-2 infections, but not in infections by Lassa virus, which enters endo-lysosomal trafficking at the late endosome stage. Thus, we have identified an essential host pathway for filovirus infections in cell lines and engineered human target tissues. Inhibition of CCZ1 nearly completely abolishes Marburg and Ebola infections. Thus, targeting CCZ1 could potentially serve as a promising drug target for controlling infections caused by various viruses, such as SARS-CoV-2, Marburg, and Ebola.

The Role of Folic Acid in SARS-CoV-2 Infection: An Intriguing Linkage under Investigation

BACKGROUND

SARS-CoV-2 is a life-threatening RNA virus that may cause an acute respiratory syndrome associated with extremely high morbidity and mortality rates. Folic acid (FA), also known as folate, is an essential vitamin vital for human homeostasis, participating in many biochemical pathways, and its deficiency has been associated with viral infection vulnerability. In this review, we investigated the association between FA intake and SARS-CoV-2 infection, along with the existence of any potential impact of FA on the health outcome of patients suffering from this new viral infection.

METHODS

Studies included were patients' and in silico and molecular docking studies.

RESULTS

Data from in silico studies and molecular docking support that FA inhibits SARS-CoV-2 entry into the host and viral replication, binding at essential residues. Accordingly, in patients' studies, a protective role of FA supplementation against SARS-CoV-2 infection is indicated. However, contradictory data from observational studies indicate that FA supplementation, often linked to deficits during systemic inflammation due to SARS-CoV-2, increases the risk of post-infection mortality.

CONCLUSIONS

Future randomized controlled trial studies, including the FA pharmacological group, are needed to better understand the role of FA as a potential protective or mortality risk indicator in COVID-19 patients.

Pseudotyped Viruses for Marburgvirus and Ebolavirus

Marburg virus (MARV) and Ebola virus (EBOV) of the Filoviridae family are the most lethal viruses in terms of mortality rate. However, the development of antiviral treatment is hampered by the requirement for biosafety level-4 (BSL-4) containment. The establishment of BSL-2 pseudotyped viruses can provide important tools for the study of filoviruses. This chapter summarizes general information on the filoviruses and then focuses on the construction of replication-deficient pseudotyped MARV and EBOV (e.g., lentivirus system and vesicular stomatitis virus system). It also details the potential applications of the pseudotyped viruses, including neutralization antibody detection, the study of infection mechanisms, the evaluation of antibody-dependent enhancement, virus entry inhibitor screening, and glycoprotein mutation analysis.

Application of Pseudotyped Viruses

Highly pathogenic emerging and reemerging viruses have serious public health and socioeconomic implications. Although conventional live virus research methods can more reliably investigate disease pathogenicity and evaluate antiviral products, they usually depend on high-level biosafety laboratories and skilled researchers; these requirements hinder in vitro assessments of efficacy, as well as efforts to test vaccines and antibody drugs. In contrast, pseudotyped viruses (i.e., single-round infectious viruses that mimic the membrane structures of various live viruses) are widely used in studies of highly pathogenic viruses because they can be handled in biosafety level 2 facilities. This chapter provides a concise overview of various aspects of pseudotyped virus technologies, including (1) exploration of the mechanisms of viral infection; (2) evaluation of the efficacies of vaccines and monoclonal antibodies based on pseudovirion-based neutralization assay; (3) assessment of antiviral agents (i.e., antibody-based drugs and inhibitors); (4) establishment of animal models of pseudotyped virus infection in vivo; (5) investigation of the evolution, infectivity, and antigenicity of viral variants and viral glycosylation; and (6) prediction of antibody-dependent cell-mediated cytotoxic activity.

Filoviruses: Innate Immunity, Inflammatory Cell Death, and Cytokines

Filoviruses are a group of single-stranded negative sense RNA viruses. The most well-known filoviruses that affect humans are ebolaviruses and marburgviruses. During infection, they can cause life-threatening symptoms such as inflammation, tissue damage, and hemorrhagic fever, with case fatality rates as high as 90%. The innate immune system is the first line of defense against pathogenic insults such as filoviruses. Pattern recognition receptors (PRRs), including toll-like receptors, retinoic acid-inducible gene-I-like receptors, C-type lectin receptors, AIM2-like receptors, and NOD-like receptors, detect pathogens and activate downstream signaling to induce the production of proinflammatory cytokines and interferons, alert the surrounding cells to the threat, and clear infected and damaged cells through innate immune cell death. However, filoviruses can modulate the host inflammatory response and innate immune cell death, causing an aberrant immune reaction. Here, we discuss how the innate immune system senses invading filoviruses and how these deadly pathogens interfere with the immune response. Furthermore, we highlight the experimental difficulties of studying filoviruses as well as the current state of filovirus-targeting therapeutics.

Structural and Functional Aspects of Ebola Virus Proteins

Ebola virus (EBOV), member of genus Ebolavirus, family Filoviridae, have a non-segmented, single-stranded RNA that contains seven genes: (a) nucleoprotein (NP), (b) viral protein 35 (VP35), (c) VP40, (d) glycoprotein (GP), (e) VP30, (f) VP24, and (g) RNA polymerase (L). All genes encode for one protein each except GP, producing three pre-proteins due to the transcriptional editing. These pre-proteins are translated into four products, namely: (a) soluble secreted glycoprotein (sGP), (b) Δ-peptide, (c) full-length transmembrane spike glycoprotein (GP), and (d) soluble small secreted glycoprotein (ssGP). Further, shed GP is released from infected cells due to cleavage of GP by tumor necrosis factor α-converting enzyme (TACE). This review presents a detailed discussion on various functional aspects of all EBOV proteins and their residues. An introduction to ebolaviruses and their life cycle is also provided for clarity of the available analysis. We believe that this review will help understand the roles played by different EBOV proteins in the pathogenesis of the disease. It will help in targeting significant protein residues for therapeutic and multi-protein/peptide vaccine development.

The Methanolic Extract of Perilla frutescens Robustly Restricts Ebola Virus Glycoprotein-Mediated Entry

Ebola virus (EBOV), one of the most infectious human viruses and a leading cause of viral hemorrhagic fever, imposes a potential public health threat with several recent outbreaks. Despite the difficulties associated with working with this pathogen in biosafety level-4 containment, a protective vaccine and antiviral therapeutic were recently approved. However, the high mortality rate of EBOV infection underscores the necessity to continuously identify novel antiviral strategies to help expand the scope of prophylaxis/therapeutic management against future outbreaks. This includes identifying antiviral agents that target EBOV entry, which could improve the management of EBOV infection. Herein, using EBOV glycoprotein (GP)-pseudotyped particles, we screened a panel of natural medicinal extracts, and identified the methanolic extract of Perilla frutescens (PFME) as a robust inhibitor of EBOV entry. We show that PFME dose-dependently impeded EBOV GP-mediated infection at non-cytotoxic concentrations, and exerted the most significant antiviral activity when both the extract and the pseudoparticles are concurrently present on the host cells. Specifically, we demonstrate that PFME could block viral attachment and neutralize the cell-free viral particles. Our results, therefore, identified PFME as a potent inhibitor of EBOV entry, which merits further evaluation for development as a therapeutic strategy against EBOV infection.

Repurposing Methotrexate in Dampening SARS-CoV2-S1-Mediated IL6 Expression: Lessons Learnt from Lung Cancer

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (COVID-19) is associated with uncontrolled inflammatory responses. Loss of pulmonary angiotensin-converting enzyme 2 (ACE2) function has been associated with SARS-CoV-2 infection. The aberrant signalling and dysregulated inflammation characteristic of lung cancer have marked similarities with SARS-CoV-2 infection. Spearman's correlation analysis of The Cancer Genome Atlas (TCGA) datasets indicated an inverse correlation between ACE2 and IL6 in lung adenocarcinoma. qRT-PCR analysis revealed CoV-2-SRBD-mediated diminished ACE2 expression in lung cancer cells that was concomitant with increased IL6 expression. Western blot and qRT-PCR analysis suggested that treatment with methotrexate (MTx) dampened CoV-2-SRBD-mediated increase in JAK1/STAT3 phosphorylation, gp130, IL6, and folate-binding protein (FBP) expressions. MTx also rescued the diminished expression of ACE2 in CoV-2-SRBD transfected cells. As lung tissue injury in severely affected COVID-19 patients is characterised by aberrant inflammatory response, repurposing MTx as an effective therapy against critical regulators of inflammation in SARS-CoV-2 infection warrants investigation.

Cross-Neutralisation of Novel Bombali Virus by Ebola Virus Antibodies and Convalescent Plasma Using an Optimised Pseudotype-Based Neutralisation Assay

Ebolaviruses continue to pose a significant outbreak threat, and while Ebola virus (EBOV)-specific vaccines and antivirals have been licensed, efforts to develop candidates offering broad species cross-protection are continuing. The use of pseudotyped virus in place of live virus is recognised as an alternative, safer, high-throughput platform to evaluate anti-ebolavirus antibodies towards their development, yet it requires optimisation. Here, we have shown that the target cell line impacts neutralisation assay results and cannot be selected purely based on permissiveness. In expanding the platform to incorporate each of the ebolavirus species envelope glycoprotein, allowing a comprehensive assessment of cross-neutralisation, we found that the recently discovered Bombali virus has a point mutation in the receptor-binding domain which prevents entry into a hamster cell line and, importantly, shows that this virus can be cross-neutralised by EBOV antibodies and convalescent plasma.

Modulation of immune response in Ebola virus disease

Ebola virus disease targets and destroys immune cells, including macrophages and dendritic cells, leading to impairment of host response. After infection, a combination of strategies including alteration and evasion of immune response culminating in a strong inflammatory response can lead to multi-organ failure and death in most infected patients. This review discusses immune response dynamics, mainly focusing on how Ebola manipulates innate and adaptive immune responses and strategizes to thwart host immune responses. We also discuss the challenges and prospects of developing therapeutics and vaccines against Ebola.

Exosomes and GPI-anchored proteins: Judicious pairs for investigating biomarkers from body fluids

Exosomes are 50-100 nm membranous vesicles actively released by cells which can be indicative of a diseased cell status. They contain various kinds of molecule - proteins, mRNA, miRNA, lipids - that are actively being studied as potential biomarkers. Hereafter I put forward several arguments in favor of the potential use of glycosylphosphatidylinositol-anchored proteins (GPI-APs) as biomarkers especially of cancerous diseases. I will briefly update readers on the exosome field and review various features of GPI-APs, before further discussing the advantages of this class of proteins as potential exosomal biomarkers. I will finish with a few examples of exosomal GPI-APs that have already been demonstrated to be good prognostic markers, as well as innovative approaches developed to quantify these exosomal biomarkers.

Ebola virus disease: An emerging and re-emerging viral threat

The genus Ebolavirus from the family Filoviridae is composed of five species including Sudan ebolavirus, Reston ebolavirus, Bundibugyo ebolavirus, Taï Forest ebolavirus, and Ebola virus (previously known as Zaire ebolavirus). These viruses have a large non-segmented, negative-strand RNA of approximately 19 kb that encodes for glycoproteins (i.e., GP, sGP, ssGP), nucleoproteins, virion proteins (i.e., VP 24, 30,40) and an RNA dependent RNA polymerase. These viruses have become a global health concern because of mortality, their rapid dissemination, new outbreaks in West-Africa, and the emergence of a new condition known as "Post-Ebola virus disease syndrome" that resembles inflammatory and autoimmune conditions such as rheumatoid arthritis, systemic lupus erythematosus and spondyloarthritis with uveitis. However, there are many gaps in the understanding of the mechanisms that may induce the development of such autoimmune-like syndromes. Some of these mechanisms may include a high formation of neutrophil extracellular traps, an uncontrolled "cytokine storm", and the possible formation of auto-antibodies. The likely appearance of autoimmune phenomena in Ebola survivors suppose a new challenge in the management and control of this disease and opens a new field of research in a special subgroup of patients. Herein, the molecular biology, pathogenesis, clinical manifestations, and treatment of Ebola virus disease are reviewed and some strategies for control of disease are discussed.

Comparative analysis of shrimp (Penaeus vannamei) miRNAs expression profiles during WSSV infection under experimental conditions and in pond culture

In recent years, the importance of viral and host microRNAs (miRNAs) in mediating viral replication and control of host cellular machinery, has been realised and increasing efforts have been taken in order to understand the interactions of miRNAs from host and pathogen during infection. However, all existing studies has thus far been conducted in controlled experimental conditions and the veracity of these data for field conditions are yet to be established. In this framework, small RNA sequencing was performed to identify the miRNAs involved in shrimp (Penaeus vannamei) immune responses under two different WSSV infection conditions of natural infection and experimentally challenged conditions. The expression profiles of miRNAs of shrimp infected with WSSV under two contrasting conditions were compared and as a result, 23365 known miRNAs and 481 novel miRNAs were identified. Amongst the most abundantly expressed miRNAs, the hypoxia related miR-210 and immune pathway related miR-29b were expressed only in infected shrimps of both conditions. miR-8-5p, having a functional role in modulation of chitin biosynthesis was exclusively represented in higher numbers in the WSSV -infected shrimps under natural conditions whilst four of the miRNAs (mja-miR-6493-5p, mja-miR-6492, mmu-miR-3968, tcf-miR-9b-5p) identified from shrimps collected from pond culture targeted chitinase, an important enzyme involved in growth and moulting in shrimps, indicating an interaction between WSSV infection and moult cycle under culture conditions. Some of the miRNAs (tca-miR-87b-3p, cte-miR-277a) and miRNAs belonging to class miR-9, miR-981 that were identified only in WSSV infected shrimps under experimental conditions, are known to respond against WSSV infection in shrimps. Moreover, the miRNA target prediction revealed several immune-related gene targets such as cathepsin, c-type lectin, haemocyanin and ubiquitin protein ligase were commonly identified under both the conditions. However, the miRNAs identified from challenge experiment had wide number of gene targets as compared to the miRNAs of natural infection. The shrimp miRNA mja-miR-6489-3p, was also found to target early virus gene wsv001 of WSSV. Our study, therefore, provides the comparative analysis of miRNA expression from shrimp during WSSV infection in two different conditions.

A Critical Domain of Ebolavirus Envelope Glycoprotein Determines Glycoform and Infectivity

Ebolaviruses comprises 5 species that exert varying degrees of mortality/infectivity in humans with Reston ebolaviruses (REBOV) showing the lowest and Zaire ebolaviruses (ZEBOV) showing the highest. However, the molecular basis of this differential mortality/infectivity remains unclear. Here, we report that the structural features of ebolavirus envelope glycoproteins (GPs) and one of their counter receptors, macrophage galactose-type calcium-type lectin (MGL/CD301), play crucial roles in determining viral infectivity. The low infectivity of REBOV mediated by the interaction between GPs and MGL/CD301 dramatically increased when the N-terminal 18 amino acids (33rd through 50th) of GPs were replaced with that of ZEBOV. Furthermore, structural analysis of glycans of GPs revealed that N-glycans were more extended in REBOV than in ZEBOV. N-glycan extension was reversed by the replacement of aforementioned N-terminal 18 amino acid residues. Therefore, these data strongly suggest that extended N-glycans on GPs reduce MGL/CD301-mediated viral infectivity by hindering the interaction between GPs and MGL/CD301 preferentially binds O-glycans.

How have retrovirus pseudotypes contributed to our understanding of viral entry?

Study of virus entry into host cells is important for understanding viral tropism and pathogenesis. Studying the entry of in vitro cultured viruses is not always practicable. Study of highly pathogenic viruses, viruses that do not grow in culture, and viruses that rapidly change phenotype in vitro can all benefit from alternative models of entry. Retrovirus particles can be engineered to display the envelope proteins of heterologous enveloped viruses. This approach, broadly termed ‘pseudotyping’, is an important technique for interrogating virus entry. In this perspective we consider how retrovirus pseudotypes have addressed these challenges and improved our understanding of the entry pathways of diverse virus species, including Ebolavirus, human immunodeficiency virus and hepatitis C virus.

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

The lifecycle of the Ebola virus in host cells

Ebola haemorrhagic fever causes deadly disease in humans and non-human primates resulting from infection with the Ebola virus (EBOV) genus of the family Filoviridae. However, the mechanisms of EBOV lifecycle in host cells, including viral entry, membrane fusion, RNP formation, GP-tetherin interaction, and VP40-inner leaflet association remain poorly understood. This review describes the biological functions of EBOV proteins and their roles in the lifecycle, summarizes the factors related to EBOV proteins or RNA expression throughout the different phases, and reviews advances with regards to the molecular events and mechanisms of the EBOV lifecycle. Furthermore, the review outlines the aspects remain unclear that urgently need to be solved in future research.

Ebola virus glycoprotein directly triggers T lymphocyte death despite of the lack of infection

Fatal outcomes of Ebola virus (EBOV) infections are typically preceded by a 'sepsis-like' syndrome and lymphopenia despite T cells being resistant to Ebola infection. The mechanisms that lead to T lymphocytes death remain largely unknown; however, the degree of lymphopenia is highly correlative with fatalities. Here we investigated whether the addition of EBOV or its envelope glycoprotein (GP) to isolated primary human CD4+ T cells induced cell death. We observed a significant decrease in cell viability in a GP-dependent manner, which is suggestive of a direct role of GP in T cell death. Using immunoprecipitation assays and flow cytometry, we demonstrate that EBOV directly binds to CD4+ T cells through interaction of GP with TLR4. Transcriptome analysis revealed that the addition of EBOV to CD4+ T cells results in the significant upregulation of pathways associated with interferon signaling, pattern recognition receptors and intracellular activation of NFκB signaling pathway. Both transcriptome analysis and specific inhibitors allowed identification of apoptosis and necrosis as mechanisms associated with the observed T cell death following exposure to EBOV. The addition of the TLR4 inhibitor CLI-095 significantly reduced CD4+ T cell death induced by GP. EBOV stimulation of primary CD4+ T cells resulted in a significant increase in secreted TNFα; inhibition of TNFα-mediated signaling events significantly reduced T cell death while inhibitors of both necrosis and apoptosis similarly reduced EBOV-induced T cell death. Lastly, we show that stimulation with EBOV or GP augments monocyte maturation as determined by an overall increase in expression levels of markers of differentiation. Subsequently, the increased rates of cellular differentiation resulted in higher rates of infection further contributing to T cell death. These results demonstrate that GP directly subverts the host's immune response by increasing the susceptibility of monocytes to EBOV infection and triggering lymphopenia through direct and indirect mechanisms.

Ebola Vaccine: How Far are we?

Ebola viruses have been identified as an emerging threat as it causes severe haemorrhagic fever in human with mortality rates ranging from 50 to 90%. In addition to being a global health concern, the virus also is considered a potential biological threat agent. As for now, no licensed vaccine is available for pre or post exposure treatment. Recent epidemic of this disease in South Africa has led to concern towards development of an effective vaccine on a priority basis. This review is an attempt to look upon current progress in the development of Ebola virus vaccines and highlights strategies that have the greatest potential for commercial development.

Overexpression of Ebola virus envelope GP1 protein

Ebola virus uses its envelope GP1 and GP2 for viral attachment and entry into host cells. Due to technical difficulty expressing full-length envelope, many structural and functional studies of Ebola envelope protein have been carried out primarily using GP1 lacking its mucin-like domain. As a result, the viral invasion mechanisms involving the mucin-like domain are not fully understood. To elucidate the role of the mucin-like domain of GP1 in Ebola-host attachment and infection and to facilitate vaccine development, we constructed a GP1 expression vector containing the entire attachment region (1-496). Cysteine 53 of GP1, which forms a disulfide bond with GP2, was mutated to serine to avoid potential disulfide bond mispairing. Stable expression clones using codon optimized open reading frame were developed in human 293-H cells with yields reaching ∼25 mg of GP1 protein per liter of spent medium. Purified GP1 was functional and bound to Ebola attachment receptors, DC-SIGN and DC-SIGNR. The over-expression and easy purification characteristic of this system has implications in Ebola research and vaccine development. To further understand the differential expression yields between the codon optimized and native GP1, we analyzed the presence of RNA structural motifs in the first 100 nucleotides of translational initiation AUG site. RNA structural prediction showed the codon optimization removed two potential RNA pseudoknot structures. This methodology is also applicable to the expression of other difficult virus envelope proteins.

Unraveling the role of membrane microdomains during microbial infections

Infectious diseases pose major socioeconomic and health-related threats to millions of people across the globe. Strategies to combat infectious diseases derive from our understanding of the complex interactions between the host and specific bacterial, viral, and fungal pathogens. Lipid rafts are membrane microdomains that play important role in life cycle of microbes. Interaction of microbial pathogens with host membrane rafts influences not only their initial colonization but also their spread and the induction of inflammation. Therefore, intervention strategies aimed at modulating the assembly of membrane rafts and/or regulating raft-directed signaling pathways are attractive approaches for the. management of infectious diseases. The current review discusses the latest advances in terms of techniques used to study the role of membrane microdomains in various pathological conditions and provides updated information regarding the role of membrane rafts during bacterial, viral and fungal infections.

Anti-Ebola therapies based on monoclonal antibodies: current state and challenges ahead

The 2014 Ebola outbreak, the largest recorded, took us largely unprepared, with no available vaccine or specific treatment. In this context, the World Health Organization declared that the humanitarian use of experimental therapies against Ebola Virus (EBOV) is ethical. In particular, an experimental treatment consisting of a cocktail of three monoclonal antibodies (mAbs) produced in tobacco plants and specifically directed to the EBOV glycoprotein (GP) was tested in humans, apparently with good results. Several mAbs with high affinity to the GP have been described. This review discusses our current knowledge on this topic. Particular emphasis is devoted to those mAbs that have been assayed in animal models or humans as possible therapies against Ebola. Engineering aspects and challenges for the production of anti-Ebola mAbs are also briefly discussed; current platforms for the design and production of full-length mAbs are cumbersome and costly.

Surface-engineered mesoporous silica particles with luminescent, cytocompatible and targeting properties for cancer cell imaging

Mechanochemically-treated europium(iii)-doped mesoporous silica particles were prepared, and a targeting ligand for cancer cells was immobilized. The surface-engineered particles exhibited the clear imaging along with all the cellular shapes.

Forty Years of Ebolavirus Molecular Biology: Understanding a Novel Disease Agent Through the Development and Application of New Technologies

Molecular biology is a broad discipline that seeks to understand biological phenomena at a molecular level, and achieves this through the study of DNA, RNA, proteins, and/or other macromolecules (e.g., those involved in the modification of these substrates). Consequently, it relies on the availability of a wide variety of methods that deal with the collection, preservation, inactivation, separation, manipulation, imaging, and analysis of these molecules. As such the state of the art in the field of ebolavirus molecular biology research (and that of all other viruses) is largely intertwined with, if not driven by, advancements in the technical methodologies available for these kinds of studies. Here we review of the current state of our knowledge regarding ebolavirus biology and emphasize the associated methods that made these discoveries possible.

Molecular mechanisms of Ebola pathogenesis

Ebola viruses (EBOVs) and Marburg viruses (MARVs) are among the deadliest human viruses, as highlighted by the recent and widespread Ebola virus outbreak in West Africa, which was the largest and longest epidemic of Ebola virus disease (EVD) in history, resulting in significant loss of life and disruptions across multiple continents. Although the number of cases has nearly reached its nadir, a recent cluster of 5 cases in Guinea on March 17, 2016, has extended the enhanced surveillance period to June 15, 2016. New, enhanced 90-d surveillance windows replaced the 42-d surveillance window to ensure the rapid detection of new cases that may arise from a missed transmission chain, reintroduction from an animal reservoir, or more important, reemergence of the virus that has persisted in an EVD survivor. In this review, we summarize our current understanding of EBOV pathogenesis, describe vaccine and therapeutic candidates in clinical trials, and discuss mechanisms of viral persistence and long-term health sequelae for EVD survivors.

Glycopeptide Antibiotics Potently Inhibit Cathepsin L in the Late Endosome/Lysosome and Block the Entry of Ebola Virus, Middle East Respiratory Syndrome Coronavirus (MERS-CoV), and Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)

Ebola virus infection can cause severe hemorrhagic fever with a high mortality in humans. The outbreaks of Ebola viruses in 2014 represented the most serious Ebola epidemics in history and greatly threatened public health worldwide. The development of additional effective anti-Ebola therapeutic agents is therefore quite urgent. In this study, via high throughput screening of Food and Drug Administration-approved drugs, we identified that teicoplanin, a glycopeptide antibiotic, potently prevents the entry of Ebola envelope pseudotyped viruses into the cytoplasm. Furthermore, teicoplanin also has an inhibitory effect on transcription- and replication-competent virus-like particles, with an IC₅₀ as low as 330 nm. Comparative analysis further demonstrated that teicoplanin is able to block the entry of Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS) envelope pseudotyped viruses as well. Teicoplanin derivatives such as dalbavancin, oritavancin, and telavancin can also inhibit the entry of Ebola, MERS, and SARS viruses. Mechanistic studies showed that teicoplanin blocks Ebola virus entry by specifically inhibiting the activity of cathepsin L, opening a novel avenue for the development of additional glycopeptides as potential inhibitors of cathepsin L-dependent viruses. Notably, given that teicoplanin has routinely been used in the clinic with low toxicity, our work provides a promising prospect for the prophylaxis and treatment of Ebola, MERS, and SARS virus infection.

A parallel genome-wide RNAi screening strategy to identify host proteins important for entry of Marburg virus and H5N1 influenza virus

BACKGROUND

Genome-wide RNAi screening has been widely used to identify host proteins involved in replication and infection of different viruses, and numerous host factors are implicated in the replication cycles of these viruses, demonstrating the power of this approach. However, discrepancies on target identification of the same viruses by different groups suggest that high throughput RNAi screening strategies need to be carefully designed, developed and optimized prior to the large scale screening.

METHODS

Two genome-wide RNAi screens were performed in parallel against the entry of pseudotyped Marburg viruses and avian influenza virus H5N1 utilizing an HIV-1 based surrogate system, to identify host factors which are important for virus entry. A comparative analysis approach was employed in data analysis, which alleviated systematic positional effects and reduced the false positive number of virus-specific hits.

RESULTS

The parallel nature of the strategy allows us to easily identify the host factors for a specific virus with a greatly reduced number of false positives in the initial screen, which is one of the major problems with high throughput screening. The power of this strategy is illustrated by a genome-wide RNAi screen for identifying the host factors important for Marburg virus and/or avian influenza virus H5N1 as described in this study.

CONCLUSIONS

This strategy is particularly useful for highly pathogenic viruses since pseudotyping allows us to perform high throughput screens in the biosafety level 2 (BSL-2) containment instead of the BSL-3 or BSL-4 for the infectious viruses, with alleviated safety concerns. The screening strategy together with the unique comparative analysis approach makes the data more suitable for hit selection and enables us to identify virus-specific hits with a much lower false positive rate.

Predictive and comparative analysis of Ebolavirus proteins

Ebolavirus is the pathogen for Ebola Hemorrhagic Fever (EHF). This disease exhibits a high fatality rate and has recently reached a historically epidemic proportion in West Africa. Out of the 5 known Ebolavirus species, only Reston ebolavirus has lost human pathogenicity, while retaining the ability to cause EHF in long-tailed macaque. Significant efforts have been spent to determine the three-dimensional (3D) structures of Ebolavirus proteins, to study their interaction with host proteins, and to identify the functional motifs in these viral proteins. Here, in light of these experimental results, we apply computational analysis to predict the 3D structures and functional sites for Ebolavirus protein domains with unknown structure, including a zinc-finger domain of VP30, the RNA-dependent RNA polymerase catalytic domain and a methyltransferase domain of protein L. In addition, we compare sequences of proteins that interact with Ebolavirus proteins from RESTV-resistant primates with those from RESTV-susceptible monkeys. The host proteins that interact with GP and VP35 show an elevated level of sequence divergence between the RESTV-resistant and RESTV-susceptible species, suggesting that they may be responsible for host specificity. Meanwhile, we detect variable positions in protein sequences that are likely associated with the loss of human pathogenicity in RESTV, map them onto the 3D structures and compare their positions to known functional sites. VP35 and VP30 are significantly enriched in these potential pathogenicity determinants and the clustering of such positions on the surfaces of VP35 and GP suggests possible uncharacterized interaction sites with host proteins that contribute to the virulence of Ebolavirus.

Emerging intracellular receptors for hemorrhagic fever viruses

Ebola virus and Lassa virus belong to different virus families that can cause viral hemorrhagic fever, a life-threatening disease in humans with limited treatment options. To infect a target cell, Ebola and Lassa viruses engage receptors at the cell surface and are subsequently shuttled into the endosomal compartment. Upon arrival in late endosomes/lysosomes, the viruses trigger membrane fusion to release their genome into the cytoplasm. Although contact sites at the cell surface were recognized for Ebola virus and Lassa virus, it was postulated that Ebola virus requires a critical receptor inside the cell. Recent screens for host factors identified such internal receptors for both viruses: Niemann-Pick disease type C1 protein (NPC1) for Ebola virus and lysosome-associated membrane protein 1 (LAMP1) for Lassa virus. A cellular trigger is needed to permit binding of the viral envelope protein to these intracellular receptors. This 'receptor switch' represents a previously unnoticed step in virus entry with implications for host-pathogen interactions and viral tropism.

Role of EXT1 and Glycosaminoglycans in the Early Stage of Filovirus Entry

Filoviruses, including both Ebola virus (EBOV) and Marburg virus (MARV), can infect humans and other animals, causing hemorrhagic fever with a high mortality rate. Entry of these viruses into the host is mediated by a single filoviral glycoprotein (GP). GP is composed of two subunits: GP1, which is responsible for attachment and binding to receptor(s) on susceptible cells, and GP2, which mediates viral and cell membrane fusion. Although numerous host factors have been implicated in the entry process, the initial attachment receptor(s) has not been well defined. In this report, we demonstrate that exostosin 1 (EXT1), which is involved in biosynthesis of heparan sulfate (HS), plays a role in filovirus entry. Expression knockdown of EXT1 by small interfering RNAs (siRNAs) impairs GP-mediated pseudoviral entry and that of infectious EBOV and MARV in tissue cultured cells. Furthermore, HS, heparin, and other related glycosaminoglycans (GAGs), to different extents, can bind to and block GP-mediated viral entry and that of infectious filoviruses. These results strongly suggest that HS and other related GAGs are attachment receptors that are utilized by filoviruses for entry and infection. These GAGs may have therapeutic potential in treating EBOV- and MARV-infected patients.

IMPORTANCE

Infection by Ebola virus and Marburg virus can cause severe illness in humans, with a high mortality rate, and currently there is no FDA-approved vaccine or therapeutic treatment available. The ongoing 2014 outbreak in West Africa underscores a lack of our understanding in the infection and pathogenesis of these viruses and the urgency of drug discovery and development. In this study, we provide several pieces of evidence that demonstrate that heparan sulfate and other closely related glycosaminoglycans are the molecules that are used by filoviruses for initial attachment. Furthermore, we demonstrate that these glycosaminoglycans can block entry of and infection by filoviruses. Thus, this work provides mechanistic insights on the early step of filoviral infection and suggests a possible therapeutic option for diseases caused by filovirus infection.

Hsa-miR-1246, hsa-miR-320a and hsa-miR-196b-5p inhibitors can reduce the cytotoxicity of Ebola virus glycoprotein in vitro

Ebola virus (EBOV) causes a highly lethal hemorrhagic fever syndrome in humans and has been associated with mortality rates of up to 91% in Zaire, the most lethal strain. Though the viral envelope glycoprotein (GP) mediates widespread inflammation and cellular damage, these changes have mainly focused on alterations at the protein level, the role of microRNAs (miRNAs) in the molecular pathogenesis underlying this lethal disease is not fully understood. Here, we report that the mi-RNAs hsa-miR-1246, hsa-miR-320a and hsa-miR-196b-5p were induced in human umbilical vein endothelial cells (HUVECs) following expression of EBOV GP. Among the proteins encoded by predicted targets of these miRNAs, the adhesion-related molecules tissue factor pathway inhibitor (TFPI), dystroglycan1 (DAG1) and the caspase 8 and FADD-like apoptosis regulator (CFLAR) were significantly downregulated in EBOV GP-expressing HUVECs. Moreover, inhibition of hsa-miR-1246, hsa-miR-320a and hsa-miR-196b-5p, or overexpression of TFPI, DAG1 and CFLAR rescued the cell viability that was induced by EBOV GP. Our results provide a novel molecular basis for EBOV pathogenesis and may contribute to the development of strategies to protect against future EBOV pandemics.

Package of NDV-pseudotyped HIV-Luc virus and its application in the neutralization assay for NDV infection

Newcastle disease virus (NDV) is a member of the Paramyxovirinae subfamily and can infect most species of birds. It has been a great threat for the poultry industry all around the world. In this report, we successfully produced infectious pseudotyped pNL4-3-Luc-R⁻E⁻ (HIV-Luc) viruses with the HN and F envelope proteins of NDV. Further investigation revealed the cytoplasmic domains of HN and F, especially HN, plays a significant role in the infection efficiency of these pseudotyped HIV-Luc viruses. Replacement of, or direct fusion to the cytoplasmic domain of the HN protein by that of vesicular stomatitis virus G (VSV-G) could greatly enhance or destroy the infective potential of HN and F-pseudotyped (NDV-pseudotyped) HIV-Luc virus. We further established a novel neutralization assay to evaluate neutralizing antibodies against NDV with the NDV-pseudotyped HIV-Luc viruses. Comparative neutralization data indicate that the results determined by using the NDV-pseudotyped HIV-Luc viruses are as reliable as those by the conventional virus-neutralization assay (VN test) with native NDV. Moreover, the results show that the novel neutralization assay is more sensitive than the VN test.

The clinically approved drugs amiodarone, dronedarone and verapamil inhibit filovirus cell entry

Objectives

Filoviruses such as Ebola virus and Marburg virus cause a severe haemorrhagic fever syndrome in humans for which there is no specific treatment. Since filoviruses use a complex route of cell entry that depends on numerous cellular factors, we hypothesized that there may be drugs already approved for human use for other indications that interfere with signal transduction or other cellular processes required for their entry and hence have anti-filoviral properties.

Methods

We used authentic filoviruses and lentiviral particles pseudotyped with filoviral glycoproteins to identify and characterize such compounds.

Results

We discovered that amiodarone, a multi-ion channel inhibitor and adrenoceptor antagonist, is a potent inhibitor of filovirus cell entry at concentrations that are routinely reached in human serum during anti-arrhythmic therapy. A similar effect was observed with the amiodarone-related agent dronedarone and the L-type calcium channel blocker verapamil. Inhibition by amiodarone was concentration dependent and similarly affected pseudoviruses as well as authentic filoviruses. Inhibition of filovirus entry was observed with most but not all cell types tested and was accentuated by the pre-treatment of cells, indicating a host cell-directed mechanism of action. The New World arenavirus Guanarito was also inhibited by amiodarone while the Old World arenavirus Lassa and members of the Rhabdoviridae (vesicular stomatitis virus) and Bunyaviridae (Hantaan) families were largely resistant.

Conclusions

The ion channel blockers amiodarone, dronedarone and verapamil inhibit filoviral cell entry.

Filovirus-like particles as vaccines and discovery tools

Ebola and Marburg viruses are members of the family Filoviridae, which cause severe hemorrhagic fevers in humans. Filovirus outbreaks have been sporadic, with mortality rates currently ranging from 30 to 90%. Unfortunately, there is no efficacious human therapy or vaccine available to treat disease caused by either Ebola or Marburg virus infection. Expression of the filovirus matrix protein, VP40, is sufficient to drive spontaneous production and release of virus-like particles (VLPs) that resemble the distinctively filamentous infectious virions. The addition of other filovirus proteins, including virion proteins (VP)24, 30 and 35 and glycoprotein, increases the efficiency of VLP production and results in particles containing multiple filovirus antigens. Vaccination with Ebola or Marburg VLPs containing glycoprotein and VP40 completely protects rodents from lethal challenge with the homologous virus. These candidate vaccines are currently being tested for immunogenicity and efficacy in nonhuman primates. Furthermore, the Ebola and Marburg VLPs are being used as a surrogate model to further understand the filovirus life cycle, with the goal of developing rationally designed vaccines and therapeutics. Thus, in addition to their use as a vaccine, VLPs are currently being used as tools to learn lessons about filovirus pathogenesis, immunology, replication and assembly requirements.

Functionalized mesoporous carbon nanoparticles for targeted chemo-photothermal therapy of cancer cells under near-infrared irradiation

Chemo-photothermal therapy with the combination of chemotherapy and photothermal therapy using mesoporous carbon nanoparticles has emerged as a promising anticancer treatment for its synergistic effects.

Emerging targets and novel approaches to Ebola virus prophylaxis and treatment

Ebola is a highly virulent pathogen causing severe hemorrhagic fever with a high case fatality rate in humans and non-human primates (NHPs). Although safe and effective vaccines or other medicinal agents to block Ebola infection are currently unavailable, a significant effort has been put forth to identify several promising candidates for the treatment and prevention of Ebola hemorrhagic fever. Among these, recombinant adenovirus-based vectors have been identified as potent vaccine candidates, with some affording both pre- and post-exposure protection from the virus. Recently, Investigational New Drug (IND) applications have been approved by the US Food and Drug Administration (FDA) and phase I clinical trials have been initiated for two small-molecule therapeutics: anti-sense phosphorodiamidate morpholino oligomers (PMOs: AVI-6002, AVI-6003) and lipid nanoparticle/small interfering RNA (LNP/siRNA: TKM-Ebola). These potential alternatives to vector-based vaccines require multiple doses to achieve therapeutic efficacy, which is not ideal with regard to patient compliance and outbreak scenarios. These concerns have fueled a quest for even better vaccination and treatment strategies. Here, we summarize recent advances in vaccines or post-exposure therapeutics for prevention of Ebola hemorrhagic fever. The utility of novel pharmaceutical approaches to refine and overcome barriers associated with the most promising therapeutic platforms are also discussed.

The cytoprotective enzyme heme oxygenase-1 suppresses Ebola virus replication

Ebola virus (EBOV) is the causative agent of a severe hemorrhagic fever in humans with reported case fatality rates as high as 90%. There are currently no licensed vaccines or antiviral therapeutics to combat EBOV infections. Heme oxygenase-1 (HO-1), an enzyme that catalyzes the rate-limiting step in heme degradation, has antioxidative properties and protects cells from various stresses. Activated HO-1 was recently shown to have antiviral activity, potently inhibiting the replication of viruses such as hepatitis C virus and human immunodeficiency virus. However, the effect of HO-1 activation on EBOV replication remains unknown. To determine whether the upregulation of HO-1 attenuates EBOV replication, we treated cells with cobalt protoporphyrin (CoPP), a selective HO-1 inducer, and assessed its effects on EBOV replication. We found that CoPP treatment, pre- and postinfection, significantly suppressed EBOV replication in a manner dependent upon HO-1 upregulation and activity. In addition, stable overexpression of HO-1 significantly attenuated EBOV growth. Although the exact mechanism behind the antiviral properties of HO-1 remains to be elucidated, our data show that HO-1 upregulation does not attenuate EBOV entry or budding but specifically targets EBOV transcription/replication. Therefore, modulation of the cellular enzyme HO-1 may represent a novel therapeutic strategy against EBOV infection.

Filovirus entry

A number of advances in recent years have significantly furthered our understanding of filovirus attachment and cellular tropism. For example, several cell-surface molecules have been identified as attachment factors with the potential to facilitate the in vivo targeting of particular cell types such as macrophages and hepatic cells. Furthermore, our knowledge of internalization and subsequent events during filovirus entry has also been widened, adding new variations to the paradigms for viral entry established for HIV and influenza. In particular, host cell factors such as endosomal proteases and the intracellular receptor Niemann-Pick C1 are now known to play a vital role in activating the membrane fusion potential of filovirus glycoproteins.

TIM-family proteins promote infection of multiple enveloped viruses through virion-associated phosphatidylserine

Human T-cell Immunoglobulin and Mucin-domain containing proteins (TIM1, 3, and 4) specifically bind phosphatidylserine (PS). TIM1 has been proposed to serve as a cellular receptor for hepatitis A virus and Ebola virus and as an entry factor for dengue virus. Here we show that TIM1 promotes infection of retroviruses and virus-like particles (VLPs) pseudotyped with a range of viral entry proteins, in particular those from the filovirus, flavivirus, New World arenavirus and alphavirus families. TIM1 also robustly enhanced the infection of replication-competent viruses from the same families, including dengue, Tacaribe, Sindbis and Ross River viruses. All interactions between TIM1 and pseudoviruses or VLPs were PS-mediated, as demonstrated with liposome blocking and TIM1 mutagenesis experiments. In addition, other PS-binding proteins, such as Axl and TIM4, promoted infection similarly to TIM1. Finally, the blocking of PS receptors on macrophages inhibited the entry of Ebola VLPs, suggesting that PS receptors can contribute to infection in physiologically relevant cells. Notably, infection mediated by the entry proteins of Lassa fever virus, influenza A virus and SARS coronavirus was largely unaffected by TIM1 expression. Taken together our data show that TIM1 and related PS-binding proteins promote infection of diverse families of enveloped viruses, and may therefore be useful targets for broad-spectrum antiviral therapies.

To infect cells, enveloped viruses typically utilize cellular receptors, which mediate specific, high-affinity interactions with the viral entry protein and prime the entry protein for subsequent steps in the viral entry process. Viral entry is also enhanced by attachment factors. Although less specific than receptors, attachment factors can alter the course of infection and thus severity of viral disease by increasing the infection efficiency of specific target cells. Here we observed that TIM proteins, a group of proteins that promote phagocytosis of apoptotic cells, can dramatically enhance the entry of a number of viruses, including Ebola, West Nile and dengue viruses, whereas they have little effect on the entry of other viruses. The inability of a virus to use TIM proteins may be due to the presence of an abundant, high-affinity receptor (Lassa fever virus), or because the TIM proteins direct virions to a non-productive internalization pathway (SARS coronavirus, influenza A virus). Mechanistically, TIM proteins appear to interact with enveloped viruses and apoptotic cells similarly by binding phosphatidylserine residues exposed on the viral and cellular membranes. Collectively our studies show that TIM proteins are attachment factors that can substantially improve the infection efficiency of a number of pathogenic viruses.

Cellular factors implicated in filovirus entry

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