DC-SIGN and DC-SIGNR bind ebola glycoproteins and enhance infection of macrophages and endothelial cells

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Fcγ-receptor IIa-mediated Src Signaling Pathway Is Essential for the Antibody-Dependent Enhancement of Ebola Virus Infection

Antibody-dependent enhancement (ADE) of Ebola virus (EBOV) infection has been demonstrated in vitro, raising concerns about the detrimental potential of some anti-EBOV antibodies. ADE has been described for many viruses and mostly depends on the cross-linking of virus-antibody complexes to cell surface Fc receptors, leading to enhanced infection. However, little is known about the molecular mechanisms underlying this phenomenon. Here we show that Fcγ-receptor IIa (FcγRIIa)-mediated intracellular signaling through Src family protein tyrosine kinases (PTKs) is required for ADE of EBOV infection. We found that deletion of the FcγRIIa cytoplasmic tail abolished EBOV ADE due to decreased virus uptake into cellular endosomes. Furthermore, EBOV ADE, but not non-ADE infection, was significantly reduced by inhibition of the Src family protein PTK pathway, which was also found to be important to promote phagocytosis/macropinocytosis for viral uptake into endosomes. We further confirmed a significant increase of the Src phosphorylation mediated by ADE. These data suggest that antibody-EBOV complexes bound to the cell surface FcγRIIa activate the Src signaling pathway that leads to enhanced viral entry into cells, providing a novel perspective for the general understanding of ADE of virus infection.

Evidence that Processing of the Severe Fever with Thrombocytopenia Syndrome Virus Gn/Gc Polyprotein Is Critical for Viral Infectivity and Requires an Internal Gc Signal Peptide

The severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging, highly pathogenic bunyavirus against which neither antivirals nor vaccines are available. The SFTSV glycoproteins, Gn and Gc, facilitate viral entry into host cells. Gn and Gc are generated from a precursor protein, Gn/Gc, but it is currently unknown how the precursor is converted into the single proteins and whether this process is required for viral infectivity. Employing a rhabdoviral pseudotyping system, we demonstrate that a predicted signal sequence at the N-terminus of Gc is required for Gn/Gc processing and viral infectivity while potential proprotein convertase cleavage sites in Gc are dispensable. Moreover, we show that expression of Gn or Gc alone is not sufficient for host cell entry while particles bearing both proteins are infectious, and we provide evidence that Gn facilitates Golgi transport and virion incorporation of Gc. Collectively, these results suggest that signal peptidase liberates mature Gc from the Gn/Gc precursor and that this process is essential for viral infectivity and thus constitutes a potential target for antiviral intervention.

Prion pathogenesis is unaltered following down-regulation of SIGN-R1

Prion diseases are infectious neurodegenerative disorders characterised by accumulations of abnormal prion glycoprotein in affected tissues. Following peripheral exposure, many prion strains replicate upon follicular dendritic cells (FDC) in lymphoid tissues before infecting the brain. An intact splenic marginal zone is important for the efficient delivery of prions to FDC. The marginal zone contains a ring of specific intercellular adhesion molecule-3-grabbing non-integrin related 1 (SIGN-R1)-expressing macrophages. This lectin binds dextran and capsular pneumococcal polysaccharides, and also enhances the clearance of apoptotic cells via interactions with complement components. Since prions are acquired as complement-opsonized complexes we determined the role of SIGN-R1 in disease pathogenesis. We show that transient down-regulation of SIGN-R1 prior to intravenous prion exposure had no effect on the early accumulation of prions upon splenic FDC or their subsequent spread to the brain. Thus, SIGN-R1 expression by marginal zone macrophages is not rate-limiting for peripheral prion disease pathogenesis.

Filovirus proteins for antiviral drug discovery: A structure/function analysis of surface glycoproteins and virus entry

This review focuses on the recent progress in our understanding of filovirus protein structure/function and its impact on antiviral research. Here we focus on the surface glycoprotein GP_1,2 and its different roles in filovirus entry. We first describe the latest advances on the characterization of GP gene-overlapping proteins sGP, ssGP and Δ-peptide. Then, we compare filovirus surface GP_1,2 proteins in terms of structure, synthesis and function. As they bear potential in drug-design, the discovery of small organic compounds inhibiting filovirus entry is a currently very active field. Although it is at an early stage, the development of antiviral drugs against Ebola and Marburg virus entry might prove essential to reduce outbreak-associated fatality rates through post-exposure treatment of both suspected and confirmed cases.

•

The filovirus surface glycoprotein is the key player protein responsible for viral entry.

•

Secreted forms of the glycoprotein have been suggested to participate to filovirus virus pathogenicity.

•

Recent structural insights of the filovirus surface glycoprotein highlight new antiviral perspectives.

•

Interesting compounds and innovative antiviral strategies emerge from research and development to inhibit filovirus entry.

New Perspectives on Ebola Virus Evolution

Since the recent devastating outbreak of Ebola virus disease in western Africa, there has been significant effort to understand the evolution of the deadly virus that caused the outbreak. There has been a considerable investment in sequencing Ebola virus (EBOV) isolates, and the results paint an important picture of how the virus has spread in western Africa. EBOV evolution cannot be understood outside the context of previous outbreaks, however. We have focused this study on the evolution of the EBOV glycoprotein gene (GP) because one of its products, the spike glycoprotein (GP_1,2), is central to the host immune response and because it contains a large amount of the phylogenetic signal for this virus. We inferred the maximum likelihood phylogeny of 96 nonredundant GP gene sequences representing each of the outbreaks since 1976 up to the end of 2014. We tested for positive selection and considered the placement of adaptive amino acid substitutions along the phylogeny and within the protein structure of GP_1,2. We conclude that: 1) the common practice of rooting the phylogeny of EBOV between the first known outbreak in 1976 and the next outbreak in 1995 provides a misleading view of EBOV evolution that ignores the fact that there is a non-human EBOV host between outbreaks; 2) the N-terminus of GP₁ may be constrained from evolving in response to the host immune system by the highly expressed, secreted glycoprotein, which is encoded by the same region of the GP gene; 3) although the mucin-like domain of GP₁ is essential for EBOV in vivo, it evolves rapidly without losing its twin functions: providing O-linked glycosylation sites and a flexible surface.

Depletion of Alveolar Macrophages Does Not Prevent Hantavirus Disease Pathogenesis in Golden Syrian Hamsters

Andes virus (ANDV) is associated with a lethal vascular leak syndrome in humans termed hantavirus pulmonary syndrome (HPS). The mechanism for the massive vascular leakage associated with HPS is poorly understood; however, dysregulation of components of the immune response is often suggested as a possible cause. Alveolar macrophages are found in the alveoli of the lung and represent the first line of defense to many airborne pathogens. To determine whether alveolar macrophages play a role in HPS pathogenesis, alveolar macrophages were depleted in an adult rodent model of HPS that closely resembles human HPS. Syrian hamsters were treated, intratracheally, with clodronate-encapsulated liposomes or control liposomes and were then challenged with ANDV. Treatment with clodronate-encapsulated liposomes resulted in significant reduction in alveolar macrophages, but depletion did not prevent pathogenesis or prolong disease. Depletion also did not significantly reduce the amount of virus in the lung of ANDV-infected hamsters but altered neutrophil recruitment, MIP-1α and MIP-2 chemokine expression, and vascular endothelial growth factor (VEGF) levels in hamster bronchoalveolar lavage (BAL) fluid early after intranasal challenge. These data demonstrate that alveolar macrophages may play a limited protective role early after exposure to aerosolized ANDV but do not directly contribute to hantavirus disease pathogenesis in the hamster model of HPS.

IMPORTANCE

Hantaviruses continue to cause disease worldwide for which there are no FDA-licensed vaccines, effective postexposure prophylactics, or therapeutics. Much of this can be attributed to a poor understanding of the mechanism of hantavirus disease pathogenesis. Hantavirus disease has long been considered an immune-mediated disease; however, by directly manipulating the Syrian hamster model, we continue to eliminate individual immune cell types. As the most numerous immune cells present in the respiratory tract, alveolar macrophages are poised to defend against hantavirus infection, but those antiviral responses may also contribute to hantavirus disease. Here, we demonstrate that, like in our prior T and B cell studies, alveolar macrophages neither prevent hantavirus infection nor cause hantavirus disease. While these studies reflect pathogenesis in the hamster model, they should help us rule out specific cell types and prompt us to consider other potential mechanisms of disease in an effort to improve the outcome of human HPS.

Development and evaluation of a double antibody sandwich ELISA for the detection of human sDC-SIGN

sDC-SIGN is the soluble form of dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN, CD209), which is a molecule involved with pathogen recognition and immune regulation. However, there is no commercially available ELISA kit for detecting human sDC-SIGN, and the normal range of this molecule is unknown. Here, we describe an ELISA for detecting human sDC-SIGN with high specificity. First, sDC-SIGN protein was expressed and purified. Monoclonal and polyclonal antibodies were then raised against the purified protein and subsequently characterized. A sandwich ELISA was developed using polyclonal antibodies specific for sDC-SIGN for capture and a biotin-labeled monoclonal antibody specific for sDC-SIGN for detection of protein. This method has sensitivity up to 0.2 ng/ml. Using this ELISA, we found that the concentration of sDC-SIGN in sera of healthy volunteers ranges from 0-319 ng/ml with a mean concentration of 27.14 ng/ml. Interestingly, the concentration of sDC-SIGN in sera from patients with cancer or chronic hepatitis B virus (CHB) infection was lower than that of health controls. The mean concentrations of sDC-SIGN in cancer patients and chronic hepatitis B virus infection patients were 3.2 ng/ml and 3.8 ng/ml, respectively. We developed a sandwich ELISA for detecting human sDC-SIGN and demonstrated its use by assessing sera concentrations of sDC-SIGN in patients with cancer and chronic CHB infection compared to that of healthy controls.

Enhancement of Ebola Virus Infection via Ficolin-1 Interaction with the Mucin Domain of GP Glycoprotein

Ebola virus infection requires the surface viral glycoprotein to initiate entry into the target cells. The trimeric glycoprotein is a highly glycosylated viral protein which has been shown to interact with host C-type lectin receptors and the soluble complement recognition protein mannose-binding lectin, thereby enhancing viral infection. Similarly to mannose-binding lectin, ficolins are soluble effectors of the innate immune system that recognize particular glycans at the pathogen surface. In this study, we demonstrate that ficolin-1 interacts with the Zaire Ebola virus (EBOV) glycoprotein, and we characterized this interaction by surface plasmon resonance spectroscopy. Ficolin-1 was shown to bind to the viral glycoprotein with a high affinity. This interaction was mediated by the fibrinogen-like recognition domain of ficolin-1 and the mucin-like domain of the viral glycoprotein. Using a ficolin-1 control mutant devoid of sialic acid-binding capacity, we identified sialylated moieties of the mucin domain to be potential ligands on the glycoprotein. In cell culture, using both pseudotyped viruses and EBOV, ficolin-1 was shown to enhance EBOV infection independently of the serum complement. We also observed that ficolin-1 enhanced EBOV infection on human monocyte-derived macrophages, described to be major viral target cells,. Competition experiments suggested that although ficolin-1 and mannose-binding lectin recognized different carbohydrate moieties on the EBOV glycoprotein, the observed enhancement of the infection likely depended on a common cellular receptor/partner. In conclusion, ficolin-1 could provide an alternative receptor-mediated mechanism for enhancing EBOV infection, thereby contributing to viral subversion of the host innate immune system.

IMPORTANCE

A specific interaction involving ficolin-1 (M-ficolin), a soluble effector of the innate immune response, and the glycoprotein (GP) of EBOV was identified. Ficolin-1 enhanced virus infection instead of tipping the balance toward its elimination. An interaction between the fibrinogen-like recognition domain of ficolin-1 and the mucin-like domain of Ebola virus GP occurred. In this model, the enhancement of infection was shown to be independent of the serum complement. The facilitation of EBOV entry into target host cells by the interaction with ficolin-1 and other host lectins shunts virus elimination, which likely facilitates the survival of the virus in infected host cells and contributes to the virus strategy to subvert the innate immune response.

Ebola Viral Glycoprotein Bound to Its Endosomal Receptor Niemann-Pick C1

Filoviruses, including Ebola and Marburg, cause fatal hemorrhagic fever in humans and primates. Understanding how these viruses enter host cells could help to develop effective therapeutics. An endosomal protein, Niemann-Pick C1 (NPC1), has been identified as a necessary entry receptor for this process, and priming of the viral glycoprotein (GP) to a fusion-competent state is a prerequisite for NPC1 binding. Here, we have determined the crystal structure of the primed GP (GPcl) of Ebola virus bound to domain C of NPC1 (NPC1-C) at a resolution of 2.3 Å. NPC1-C utilizes two protruding loops to engage a hydrophobic cavity on head of GPcl. Upon enzymatic cleavage and NPC1-C binding, conformational change in the GPcl further affects the state of the internal fusion loop, triggering membrane fusion. Our data therefore provide structural insights into filovirus entry in the late endosome and the molecular basis for design of therapeutic inhibitors of viral entry.

•

Structural basis of Ebola virus endosomal-receptor binding

•

NPC1 domain C (NPC1-C) displays a helical core structure with two protruding loops

•

NPC1-C binds to the primed Ebola virus GP (GPcl) protein with a low affinity

•

NPC1-C utilizes two protruding loops to engage a hydrophobic cavity on head of GPcl

The Glycoproteins of All Filovirus Species Use the Same Host Factors for Entry into Bat and Human Cells but Entry Efficiency Is Species Dependent

Ebola and marburgviruses, members of the family Filoviridae, can cause severe hemorrhagic fever in humans. The ongoing Ebola virus (EBOV) disease epidemic in Western Africa claimed more than 11,300 lives and was associated with secondary cases outside Africa, demonstrating that filoviruses pose a global health threat. Bats constitute an important natural reservoir of filoviruses, including viruses of the recently identified Cuevavirus genus within the Filoviridae family. However, the interactions of filoviruses with bat cells are incompletely understood. Here, we investigated whether filoviruses employ different strategies to enter human and bat cells. For this, we examined host cell entry driven by glycoproteins (GP) from all filovirus species into cell lines of human and fruit bat origin. We show that all GPs were able to mediate entry into human and most fruit bat cell lines with roughly comparable efficiency. In contrast, the efficiency of entry into the cell line EidNi/41 derived from a straw-colored fruit bat varied markedly between the GPs of different filovirus species. Furthermore, inhibition studies demonstrated that filoviruses employ the same host cell factors for entry into human, non-human primate and fruit bat cell lines, including cysteine proteases, two pore channels and NPC1 (Niemann-Pick C1 molecule). Finally, processing of GP by furin and the presence of the mucin-like domain in GP were dispensable for entry into both human and bat cell lines. Collectively, these results show that filoviruses rely on the same host cell factors for entry into human and fruit bat cells, although the efficiency of the usage of these factors might differ between filovirus species.

Direct Visualization of Ebola Virus Fusion Triggering in the Endocytic Pathway

Ebola virus (EBOV) makes extensive and intricate use of host factors in the cellular endosomal/lysosomal pathway to release its genome into the cytoplasm and initiate infection. Following viral internalization into endosomes, host cysteine proteases cleave the EBOV fusion glycoprotein (GP) to unmask the binding site for its intracellular receptor, the cholesterol transporter Niemann-Pick C1 (NPC1). GP-NPC1 interaction is required for viral entry. Despite these and other recent discoveries, late events in EBOV entry following GP-NPC1 binding and culminating in GP-catalyzed fusion between viral and cellular lipid bilayers remain enigmatic. A mechanistic understanding of EBOV membrane fusion has been hampered by the failure of previous efforts to reconstitute fusion in vitro or at the cell surface. This report describes an assay to monitor initial steps directly in EBOV membrane fusion-triggering of GP and virus-cell lipid mixing-by single virions in live cells. Fusogenic triggering of GP occurs predominantly in Rab7-positive (Rab7(+)) endosomes, absolutely requires interaction between proteolytically primed GP and NPC1, and is blocked by key GP-specific neutralizing antibodies with therapeutic potential. Unexpectedly, cysteine protease inhibitors do not inhibit lipid mixing by virions bearing precleaved GP, even though they completely block cytoplasmic entry by these viruses, as shown previously. These results point to distinct cellular requirements for different steps in EBOV membrane fusion and suggest a model in which host cysteine proteases are dispensable for GP fusion triggering after NPC1 binding but are required for the formation of fusion pores that permit genome delivery.

IMPORTANCE

Ebola virus (EBOV) causes outbreaks of highly lethal disease for which no approved vaccines or treatments exist. Recent work has elucidated key molecular features of the complex EBOV entry process, including stepwise interactions with multiple host factors. However, there is a critical gap in our understanding of events that surround the final membrane fusion step which persists due to the paucity of direct and extensive investigation of EBOV fusion. Here, we report a real-time assay for EBOV glycoprotein fusion triggering and use it to define its cellular location and requirements. We also uncover an unexpected requirement for host proteases at a step after fusion triggering that may reflect their role in formation of fusion pores for genome delivery.

Characterization of the inhibitory effect of an extract of Prunella vulgaris on Ebola virus glycoprotein (GP)-mediated virus entry and infection

Currently, no approved antiviral therapeutic is available for treatment or prevention of Ebola virus (EBOV) infection. In this study, we characterized an EBOV-glycoprotein (GP) pseudotyped HIV-1-based vector system in different cell cultures, including human umbilical vein endothelial cells (HUVECs) and human macrophages, for the screening of anti-EBOV-GP agent(s). Based on this system, we demonstrated that an aqueous extract (CHPV) from the Chinese herb Prunella vulgaris displayed a potent inhibitory effect on EBOV-GP pseudotyped virus (EBOV-GP-V)-mediated infection in various cell lines, including HUVEC and macrophage. In addition, our results indicated that CHPV was able to block an eGFP-expressing Zaire ebola virus (eGFP-ZEBOV) infection in VeroE6 cells. The anti-EBOV activity of CHPV was exhibited in a dose-dependent manner. At a 12.5 μg/ml concentration, the CHPV showed a greater than 80% inhibition of EBOV-GP-V and eGFP-EBOV infections. Likewise, our studies suggested that the inhibitory effect of CHPV occurred by binding directly to EBOV-GP-Vs and blocking the early viral events. Interestingly, our results have shown that CHPV was able to enhance the anti-EBOV activity of the monoclonal antibody MAb 2G4 against EBOV-GP. Overall, this study provides evidence that CHPV has anti-EBOV activity and may be developed as a novel antiviral approach against EBOV infection.

Ebola vaccine, therapeutics, and diagnostics

Ebolaviruses, members of the family Filoviridae, cause severe hemorrhagic fever in humans and nonhuman primates, with human case fatality rates of up to 90%. No effective prophylaxis or treatment for Ebola virus disease (EVD) is yet commercially available. During the latest outbreak of EVD in West Africa, several unapproved drugs were used for the treatment of patients. This outbreak has indeed accelerated efforts to develop antiviral strategies and some of the vaccine and drug candidates have undergone clinical trials. This article reviews previous researches and recent advances on the development of vaccine, therapeutics, and diagnostics for EVD.

Ebola virus mediated infectivity is restricted in canine and feline cells

Ebolaviruses and marburgviruses belong to the Filoviridae family and often cause severe, fatal hemorrhagic fever in humans and non-human primates. The magnitude of the 2014 outbreak in West Africa and the unprecedented emergence of Ebola virus disease (EVD) in the United States underscore the urgency to better understand the dynamics of Ebola virus infection, transmission and spread. To date, the susceptibility and possible role of domestic animals and pets in the transmission cycle and spread of EVD remains unclear. We utilized infectious VSV recombinants and lentivirus pseudotypes expressing the EBOV surface glycoprotein (GP) to assess the permissiveness of canine and feline cells to EBOV GP-mediated entry. We observed a general restriction in EBOV-mediated infection of primary canine and feline cells. To address the entry mechanism, we used cells deficient in NPC1, a host protein implicated in EBOV entry, and a pharmacological blockade of cholesterol transport, to show that an NPC1-dependent mechanism of EBOV entry is conserved in canine and feline cells. These data demonstrate that cells of canine and feline origin are susceptible to EBOV GP mediated infection; however, infectivity of these cells is reduced significantly compared to controls. Moreover, these data provide new insights into the mechanism of EBOV GP mediated entry into cells of canine and feline origin.

Ebola Virus Disease: A Review of Its Past and Present

Ebola virus, the virus responsible for Ebola virus disease, has spawned several epidemics during the past 38 years. In 2014, an Ebola epidemic spread from Africa to other continents, becoming a pandemic. The virus's relatively unique structure, its infectivity and lethality, the difficulty in stopping its spread, and the lack of an effective treatment captured the world's attention. This article provides a brief review of the known history of Ebola virus disease, its etiology, epidemiology, and pathophysiology and a review of the limited information on managing patients with Ebola virus disease.

Filovirus pathogenesis and immune evasion: insights from Ebola virus and Marburg virus

Ebola viruses and Marburg viruses, members of the filovirus family, are zoonotic pathogens that cause severe disease in people, as highlighted by the latest Ebola virus epidemic in West Africa. Filovirus disease is characterized by uncontrolled virus replication and the activation of host responses that contribute to pathogenesis. Underlying these phenomena is the potent suppression of host innate antiviral responses, particularly the type I interferon response, by viral proteins, which allows high levels of viral replication. In this Review, we describe the mechanisms used by filoviruses to block host innate immunity and discuss the links between immune evasion and filovirus pathogenesis.

Ebola virus entry into host cells: identifying therapeutic strategies

Filoviruses cause severe hemorrhagic fever in humans. The archetypal virus of this group, Ebola virus, is responsible for the current filovirus epidemic in West Africa. Filoviruses infect most mammalian cells, resulting in broad species tropism and likely contributing to rapid spread of virus throughout the body. A thorough understanding of filovirus entry events will facilitate the development of therapeutics against these critical steps in the viral life cycle. This review summarizes the current understanding of filovirus entry and discusses some of the recent advancements in therapeutic strategies that target entry.

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.

Immunological features underlying viral hemorrhagic fevers

Several enveloped RNA viruses of the arenavirus, bunyavirus, filovirus and flavivirus families are associated with a syndrome known as viral hemorrhagic fever (VHF). VHF is characterized by fever, vascular leakage, coagulation defects and multi organ system failure. VHF is currently viewed as a disease precipitated by viral suppression of innate immunity, which promotes systemic virus replication and excessive proinflammatory cytokine responses that trigger the manifestations of severe disease. However, the mechanisms by which immune dysregulation contributes to disease remain poorly understood. Infection of nonhuman primates closely recapitulates human VHF, notably Ebola and yellow fever, thereby providing excellent models to better define the immunological basis for this syndrome. Here we review the current state of our knowledge and suggest future directions that will better define the immunological mechanisms underlying VHF.

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.

Interferon-Induced Transmembrane Protein-Mediated Inhibition of Host Cell Entry of Ebolaviruses

Ebolaviruses are highly pathogenic in humans and nonhuman primates and pose a severe threat to public health. The interferon-induced transmembrane (IFITM) proteins can restrict entry of ebolaviruses, influenza A viruses, and other enveloped viruses. However, the breadth and mechanism of the antiviral activity of IFITM proteins are incompletely understood. Here, we employed ebolavirus glycoprotein–pseudotyped vectors and ebolavirus-like particles to address this question. We show that IFITM proteins inhibit the cellular entry of diverse ebolaviruses and demonstrate that type I interferon induces IFITM protein expression in macrophages, major viral targets. Moreover, we show that IFITM proteins block entry of influenza A viruses and ebolaviruses by different mechanisms and provide evidence that antibodies and IFITM proteins can synergistically inhibit cellular entry of ebolaviruses. These results provide insights into the role of IFITM proteins in infection by ebolaviruses and suggest a mechanism by which antibodies, though poorly neutralizing in vitro, might contribute to viral control in vivo.

Ebola virus outbreak, updates on current therapeutic strategies

Filoviruses are enveloped negative‐sense single‐stranded RNA viruses, which include Ebola and Marburg viruses, known to cause hemorrhagic fever in humans with a case fatality of up to 90%. There have been several Ebola virus outbreaks since the first outbreak in the Democratic Republic of Congo in 1976 of which, the recent 2013–2015 epidemic in Guinea, Liberia, and Sierra Leone is the largest in recorded history. Within a few months of the start of the outbreak in December 2013, thousands of infected cases were reported with a significant number of deaths. As of March 2015, according to the Centers for Disease Control and Prevention, there have been nearly 25 000 suspected cases, with 15 000 confirmed by laboratory testing, and over 10 000 deaths. The large number of cases and the high mortality rate, combined with the lack of effective Food and Drug Administration‐approved treatments, necessitate the development of potent and safe therapeutic measures to combat the current and future outbreaks. Since the beginning of the outbreak, there have been considerable efforts to develop and characterize protective measures including vaccines and antiviral small molecules, and some have proven effective in vitro and in animal models. Most recently, a cocktail of monoclonal antibodies has been shown to be highly effective in protecting non‐human primates from Ebola virus infection. In this review, we will discuss what is known about the nature of the virus, phylogenetic classification, genomic organization and replication, disease transmission, and viral entry and highlight the current approaches and efforts, in the development of therapeutics, to control the outbreak. Copyright © 2015 John Wiley & Sons, Ltd.

Analysis of Ebola Virus Entry Into Macrophages

Ebolaviruses constitute a public health threat, particularly in Central and Western Africa. Host cell factors required for spread of ebolaviruses may serve as targets for antiviral intervention. Lectins, TAM receptor tyrosine kinases (Tyro3, Axl, Mer), T cell immunoglobulin and mucin domain (TIM) proteins, integrins, and Niemann-Pick C1 (NPC1) have been reported to promote entry of ebolaviruses into certain cellular systems. However, the factors used by ebolaviruses to invade macrophages, major viral targets, are poorly defined. Here, we show that mannose-specific lectins, TIM-1 and Axl augment entry into certain cell lines but do not contribute to Ebola virus (EBOV)-glycoprotein (GP)-driven transduction of macrophages. In contrast, expression of Mer, integrin αV, and NPC1 was required for efficient GP-mediated transduction and EBOV infection of macrophages. These results define cellular factors hijacked by EBOV for entry into macrophages and, considering that Mer and integrin αV promote phagocytosis of apoptotic cells, support the concept that EBOV relies on apoptotic mimicry to invade target cells.

Comparative Analysis of Host Cell Entry of Ebola Virus From Sierra Leone, 2014, and Zaire, 1976

The ongoing Ebola virus (EBOV) disease (EVD) epidemic in Western Africa is the largest EVD outbreak recorded to date and requires the rapid development and deployment of antiviral measures. The viral glycoprotein (GP) facilitates host cell entry and, jointly with cellular interaction partners, constitutes a potential target for antiviral intervention. However, it is unknown whether the GPs of the currently and previously circulating EBOVs use the same mechanisms for cellular entry and are thus susceptible to inhibition by the same antivirals and cellular defenses. Here, we show that the GPs of the EBOVs circulating in 1976 and 2014 transduce the same spectrum of target cells, use the same cellular factors for host cell entry, and are comparably susceptible to blockade by antiviral interferon-induced transmembrane proteins and neutralizing antibody KZ52. Thus, the viruses responsible for the ongoing EVD epidemic should be fully susceptible to established antiviral strategies targeting GP and cellular entry factors.

Tissue and cellular tropism, pathology and pathogenesis of Ebola and Marburg viruses

Ebola viruses and Marburg viruses include some of the most virulent and fatal pathogens known to humans. These viruses cause severe haemorrhagic fevers, with case fatality rates in the range 25-90%. The diagnosis of filovirus using formalin-fixed tissues from fatal cases poses a significant challenge. The most characteristic histopathological findings are seen in the liver; however, the findings overlap with many other viral and non-viral haemorrhagic diseases. The need to distinguish filovirus infections from other haemorrhagic fevers, particularly in areas with multiple endemic viral haemorrhagic agents, is of paramount importance. In this review we discuss the current state of knowledge of filovirus infections and their pathogenesis, including histopathological findings, epidemiology, modes of transmission and filovirus entry and spread within host organisms. The pathogenesis of filovirus infections is complex and involves activation of the mononuclear phagocytic system, with release of pro-inflammatory cytokines, chemokines and growth factors, endothelial dysfunction, alterations of the innate and adaptive immune systems, direct organ and endothelial damage from unrestricted viral replication late in infection, and coagulopathy. Although our understanding of the pathogenesis of filovirus infections has rapidly increased in the past few years, many questions remain unanswered.

Camouflage and misdirection: the full-on assault of ebola virus disease

Ebolaviruses cause a severe hemorrhagic fever syndrome that is rapidly fatal to humans and nonhuman primates. Ebola protein interactions with host cellular proteins disrupt type I and type II interferon responses, RNAi antiviral responses, antigen presentation, T-cell-dependent B cell responses, humoral antibodies, and cell-mediated immunity. This multifaceted approach to evasion and suppression of innate and adaptive immune responses in their target hosts leads to the severe immune dysregulation and "cytokine storm" that is characteristic of fatal ebolavirus infection. Here, we highlight some of the processes by which Ebola interacts with its mammalian hosts to evade antiviral defenses.

IFITM proteins inhibit entry driven by the MERS-coronavirus spike protein: evidence for cholesterol-independent mechanisms

The interferon-inducible transmembrane (IFITM) proteins 1, 2 and 3 inhibit the host cell entry of several enveloped viruses, potentially by promoting the accumulation of cholesterol in endosomal compartments. IFITM3 is essential for control of influenza virus infection in mice and humans. In contrast, the role of IFITM proteins in coronavirus infection is less well defined. Employing a retroviral vector system for analysis of coronavirus entry, we investigated the susceptibility of human-adapted and emerging coronaviruses to inhibition by IFITM proteins. We found that entry of the recently emerged Middle East respiratory syndrome coronavirus (MERS-CoV) is sensitive to inhibition by IFITM proteins. In 293T cells, IFITM-mediated inhibition of cellular entry of the emerging MERS- and SARS-CoV was less efficient than blockade of entry of the globally circulating human coronaviruses 229E and NL63. Similar differences were not observed in A549 cells, suggesting that cellular context and/or IFITM expression levels can impact inhibition efficiency. The differential IFITM-sensitivity of coronaviruses observed in 293T cells afforded the opportunity to investigate whether efficiency of entry inhibition by IFITMs and endosomal cholesterol accumulation correlate. No such correlation was observed. Furthermore, entry mediated by the influenza virus hemagglutinin was robustly inhibited by IFITM3 but was insensitive to accumulation of endosomal cholesterol, indicating that modulation of cholesterol synthesis/transport did not account for the antiviral activity of IFITM3. Collectively, these results show that the emerging MERS-CoV is a target of the antiviral activity of IFITM proteins and demonstrate that mechanisms other than accumulation of endosomal cholesterol can contribute to viral entry inhibition by IFITMs.

DESC1 and MSPL activate influenza A viruses and emerging coronaviruses for host cell entry

The type II transmembrane serine protease (TTSP) TMPRSS2 cleaves and activates the influenza virus and coronavirus surface proteins. Expression of TMPRSS2 is essential for the spread and pathogenesis of H1N1 influenza viruses in mice. In contrast, H3N2 viruses are less dependent on TMPRSS2 for viral amplification, suggesting that these viruses might employ other TTSPs for their activation. Here, we analyzed TTSPs, reported to be expressed in the respiratory system, for the ability to activate influenza viruses and coronaviruses. We found that MSPL and, to a lesser degree, DESC1 are expressed in human lung tissue and cleave and activate the spike proteins of the Middle East respiratory syndrome and severe acute respiratory syndrome coronaviruses for cell-cell and virus-cell fusion. In addition, we show that these proteases support the spread of all influenza virus subtypes previously pandemic in humans. In sum, we identified two host cell proteases that could promote the amplification of influenza viruses and emerging coronaviruses in humans and might constitute targets for antiviral intervention. Importance: Activation of influenza viruses by host cell proteases is essential for viral infectivity and the enzymes responsible are potential targets for antiviral intervention. The present study demonstrates that two cellular serine proteases, DESC1 and MSPL, activate influenza viruses and emerging coronaviruses in cell culture and, because of their expression in human lung tissue, might promote viral spread in the infected host. Antiviral strategies aiming to prevent viral activation might thus need to encompass inhibitors targeting MSPL and DESC1.

Inhibition of proprotein convertases abrogates processing of the middle eastern respiratory syndrome coronavirus spike protein in infected cells but does not reduce viral infectivity

Middle East respiratory syndrome coronavirus (MERS-CoV) infection is associated with a high case-fatality rate, and the potential pandemic spread of the virus is a public health concern. The spike protein of MERS-CoV (MERS-S) facilitates viral entry into host cells, which depends on activation of MERS-S by cellular proteases. Proteolytic activation of MERS-S during viral uptake into target cells has been demonstrated. However, it is unclear whether MERS-S is also cleaved during S protein synthesis in infected cells and whether cleavage is required for MERS-CoV infectivity. Here, we show that MERS-S is processed by proprotein convertases in MERS-S–transfected and MERS-CoV–infected cells and that several RXXR motifs located at the border between the surface and transmembrane subunit of MERS-S are required for efficient proteolysis. However, blockade of proprotein convertases did not impact MERS-S–dependent transduction of target cells expressing high amounts of the viral receptor, DPP4, and did not modulate MERS-CoV infectivity. These results show that MERS-S is a substrate for proprotein convertases and demonstrate that processing by these enzymes is dispensable for S protein activation. Efforts to inhibit MERS-CoV infection by targeting host cell proteases should therefore focus on enzymes that process MERS-S during viral uptake into target cells.

Human herpesvirus 8 glycoprotein B binds the entry receptor DC-SIGN

We have previously shown that human herpesvirus 8 (HHV-8) uses DC-SIGN as an entry receptor for dendritic cells, macrophages and B cells. The viral attachment protein for DC-SIGN is unknown. HHV-8 virions contain five conserved herpesvirus glycoproteins, a single unique glycoprotein, and two predicted glycoproteins. Previous studies have shown that DC-SIGN binds highly mannosylated glycoproteins. The HHV-8 glycoprotein B (gB) has been reported to be highly mannosylated, and therefore we hypothesized that gB will bind to DC-SIGN. In this report we confirm that gB has a high mannose carbohydrate structure and demonstrate for the first time that it binds DC-SIGN in a dose-dependent manner. We also identify key amino acids in the DC-SIGN carbohydrate recognition domain that are required for HHV-8 infection and compare these results with published binding regions for ICAM-2/3 and HIV-1 gp120. These results clarify some of the initial events in HHV-8 entry and can be used for the design of targeted preventive therapies.

Computational and experimental prediction of human C-type lectin receptor druggability

Mammalian C-type lectin receptors (CTLRS) are involved in many aspects of immune cell regulation such as pathogen recognition, clearance of apoptotic bodies, and lymphocyte homing. Despite a great interest in modulating CTLR recognition of carbohydrates, the number of specific molecular probes is limited. To this end, we predicted the druggability of a panel of 22 CTLRs using DoGSiteScorer. The computed druggability scores of most structures were low, characterizing this family as either challenging or even undruggable. To further explore these findings, we employed a fluorine-based nuclear magnetic resonance screening of fragment mixtures against DC-SIGN, a receptor of pharmacological interest. To our surprise, we found many fragment hits associated with the carbohydrate recognition site (hit rate = 13.5%). A surface plasmon resonance-based follow-up assay confirmed 18 of these fragments (47%) and equilibrium dissociation constants were determined. Encouraged by these findings we expanded our experimental druggability prediction to Langerin and MCL and found medium to high hit rates as well, being 15.7 and 10.0%, respectively. Our results highlight limitations of current in silico approaches to druggability assessment, in particular, with regard to carbohydrate-binding proteins. In sum, our data indicate that small molecule ligands for a larger panel of CTLRs can be developed.

Influenza A virus encoding secreted Gaussia luciferase as useful tool to analyze viral replication and its inhibition by antiviral compounds and cellular proteins

Reporter genes inserted into viral genomes enable the easy and rapid quantification of virus replication, which is instrumental to efficient in vitro screening of antiviral compounds or in vivo analysis of viral spread and pathogenesis. Based on a published design, we have generated several replication competent influenza A viruses carrying either fluorescent proteins or Gaussia luciferase. Reporter activity could be readily quantified in infected cultures, but the virus encoding Gaussia luciferase was more stable than viruses bearing fluorescent proteins and was therefore analyzed in detail. Quantification of Gaussia luciferase activity in the supernatants of infected culture allowed the convenient and highly sensitive detection of viral spread, and enzymatic activity correlated with the number of infectious particles released from infected cells. Furthermore, the Gaussia luciferase encoding virus allowed the sensitive quantification of the antiviral activity of the neuraminidase inhibitor (NAI) zanamivir and the host cell interferon-inducible transmembrane (IFITM) proteins 1–3, which are known to inhibit influenza virus entry. Finally, the virus was used to demonstrate that influenza A virus infection is sensitive to a modulator of endosomal cholesterol, in keeping with the concept that IFITMs inhibit viral entry by altering cholesterol levels in the endosomal membrane. In sum, we report the characterization of a novel influenza A reporter virus, which allows fast and sensitive detection of viral spread and its inhibition, and we show that influenza A virus entry is sensitive to alterations of endosomal cholesterol levels.

The essential role of clathrin-mediated endocytosis in yellow head virus propagation in the black tiger shrimp Penaeus monodon

Yellow head virus (YHV) is one of the most widespread viruses seriously affecting black tiger shrimp (Penaeus monodon) cultivation. A previous microarray study demonstrated that clathrin coat assembly protein 17 (AP17) was significantly up-regulated after YHV infection (Pongsomboon et al., 2011). Clathrin coat AP17 is a part of the assembly protein σ2 (AP-2) complex which is involved in clathrin-mediated endocytosis. Quantitative RT-PCR (qRT-PCR) revealed that the clathrin coat AP17 gene was up-regulated 3-fold at 12 h post YHV infection. In addition, immunofluorescence microscopy showed that clathrin coat AP17 was highly expressed in the cytoplasm of the YHV-infected hemocytes. Knockdown of the clathrin coat AP17 gene dramatically reduced YHV replicativity by 32-fold. Interestingly, shrimp pre-treated with chlorpromazine, a commercial drug that inhibits clathrin-dependent endocytosis, exhibited significantly low levels of YHV infection. Taken together, these results suggest that clathrin-mediated endocytosis is involved in YHV propagation in P. monodon.

Analysis of determinants in filovirus glycoproteins required for tetherin antagonism

The host cell protein tetherin can restrict the release of enveloped viruses from infected cells. The HIV-1 protein Vpu counteracts tetherin by removing it from the site of viral budding, the plasma membrane, and this process depends on specific interactions between the transmembrane domains of Vpu and tetherin. In contrast, the glycoproteins (GPs) of two filoviruses, Ebola and Marburg virus, antagonize tetherin without reducing surface expression, and the domains in GP required for tetherin counteraction are unknown. Here, we show that filovirus GPs depend on the presence of their authentic transmembrane domains for virus-cell fusion and tetherin antagonism. However, conserved residues within the transmembrane domain were dispensable for membrane fusion and tetherin counteraction. Moreover, the insertion of the transmembrane domain into a heterologous viral GP, Lassa virus GPC, was not sufficient to confer tetherin antagonism to the recipient. Finally, mutation of conserved residues within the fusion peptide of Ebola virus GP inhibited virus-cell fusion but did not ablate tetherin counteraction, indicating that the fusion peptide and the ability of GP to drive host cell entry are not required for tetherin counteraction. These results suggest that the transmembrane domains of filoviral GPs contribute to tetherin antagonism but are not the sole determinants.

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.

Ebola hemorrhagic Fever: novel biomarker correlates of clinical outcome

BACKGROUND

Ebola hemorrhagic fever (EHF) outbreaks occur sporadically in Africa and result in high rates of death. The 2000-2001 outbreak of Sudan virus-associated EHF in the Gulu district of Uganda led to 425 cases, of which 216 were laboratory confirmed, making it the largest EHF outbreak on record. Serum specimens from this outbreak had been preserved in liquid nitrogen from the time of collection and were available for analysis.

METHODS

Available samples were tested using a series of multiplex assays to measure the concentrations of 55 biomarkers. The data were analyzed to identify statistically significant associations between the tested biomarkers and hemorrhagic manifestations, viremia, and/or death.

RESULTS

Death, hemorrhage, and viremia were independently associated with elevated levels of several chemokines and cytokines. Death and hemorrhage were associated with elevated thrombomodulin and ferritin levels. Hemorrhage was also associated with elevated levels of soluble intracellular adhesion molecule. Viremia was independently associated with elevated levels of tissue factor and tissue plasminogen activator. Finally, samples from nonfatal cases had higher levels of sCD40L.

CONCLUSIONS

These novel associations provide a better understanding of EHF pathophysiology and a starting point for researching new potential targets for therapeutic interventions.

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.

TMPRSS2 and ADAM17 cleave ACE2 differentially and only proteolysis by TMPRSS2 augments entry driven by the severe acute respiratory syndrome coronavirus spike protein

The type II transmembrane serine proteases TMPRSS2 and HAT can cleave and activate the spike protein (S) of the severe acute respiratory syndrome coronavirus (SARS-CoV) for membrane fusion. In addition, these proteases cleave the viral receptor, the carboxypeptidase angiotensin-converting enzyme 2 (ACE2), and it was proposed that ACE2 cleavage augments viral infectivity. However, no mechanistic insights into this process were obtained and the relevance of ACE2 cleavage for SARS-CoV S protein (SARS-S) activation has not been determined. Here, we show that arginine and lysine residues within ACE2 amino acids 697 to 716 are essential for cleavage by TMPRSS2 and HAT and that ACE2 processing is required for augmentation of SARS-S-driven entry by these proteases. In contrast, ACE2 cleavage was dispensable for activation of the viral S protein. Expression of TMPRSS2 increased cellular uptake of soluble SARS-S, suggesting that protease-dependent augmentation of viral entry might be due to increased uptake of virions into target cells. Finally, TMPRSS2 was found to compete with the metalloprotease ADAM17 for ACE2 processing, but only cleavage by TMPRSS2 resulted in augmented SARS-S-driven entry. Collectively, our results in conjunction with those of previous studies indicate that TMPRSS2 and potentially related proteases promote SARS-CoV entry by two separate mechanisms: ACE2 cleavage, which might promote viral uptake, and SARS-S cleavage, which activates the S protein for membrane fusion. These observations have interesting implications for the development of novel therapeutics. In addition, they should spur efforts to determine whether receptor cleavage promotes entry of other coronaviruses, which use peptidases as entry receptors.

Role of DC-SIGN in Lassa virus entry into human dendritic cells

The arenavirus Lassa virus (LASV) causes a severe hemorrhagic fever with high mortality in humans. Antigen-presenting cells, in particular dendritic cells (DCs), are early and preferred targets of LASV, and their productive infection contributes to the virus-induced immunosuppression observed in fatal disease. Here, we characterized the role of the C-type lectin DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN) in LASV entry into primary human DCs using a chimera of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) expressing the LASV glycoprotein (rLCMV-LASVGP). We found that differentiation of human primary monocytes into DCs enhanced virus attachment and entry, concomitant with the upregulation of DC-SIGN. LASV and rLCMV-LASVGP bound to DC-SIGN via mannose sugars located on the N-terminal GP1 subunit of LASVGP. We provide evidence that DC-SIGN serves as an attachment factor for rLCMV-LASVGP in monocyte-derived immature dendritic cells (MDDC) and can accelerate the capture of free virus. However, in contrast to the phlebovirus Uukuniemi virus (UUKV), which uses DC-SIGN as an authentic entry receptor, productive infection with rLCMV-LASVGP was less dependent on DC-SIGN. In contrast to the DC-SIGN-mediated cell entry of UUKV, entry of rLCMV-LASVGP in MDDC was remarkably slow and depended on actin, indicating the use of different endocytotic pathways. In sum, our data reveal that DC-SIGN can facilitate cell entry of LASV in human MDDC but that its role seems distinct from the function as an authentic entry receptor reported for phleboviruses.

Utilization of human DC-SIGN and L-SIGN for entry and infection of host cells by the New World arenavirus, Junín virus

•

The C-type lectins hDC-SIGN and hL-SIGN are important for entry of Junín arenavirus.

•

hDC-SIGN and hL-SIGN substantially augment the use of transferrin receptor.

•

hL-SIGN may act as a co-receptor for Junín arenavirus.

The target cell tropism of enveloped viruses is regulated by interactions between viral proteins and cellular receptors determining susceptibility at a host cell, tissue or species level. However, a number of additional cell-surface moieties can also bind viral envelope glycoproteins and could act as capture receptors, serving as attachment factors to concentrate virus particles on the cell surface, or to disseminate the virus infection to target organs or susceptible cells within the host. Here, we used Junín virus (JUNV) or JUNV glycoprotein complex (GPC)-pseudotyped particles to study their ability to be internalized by the human C-type lectins hDC- or hL-SIGN. Our results provide evidence that hDC- and hL-SIGN can mediate the entry of Junín virus into cells, and may play an important role in virus infection and dissemination in the host.

Characterization of the envelope glycoprotein of a novel filovirus, lloviu virus

Lloviu virus (LLOV), a novel filovirus detected in bats, is phylogenetically distinct from viruses in the genera Ebolavirus and Marburgvirus in the family Filoviridae. While filoviruses are known to cause severe hemorrhagic fever in humans and/or nonhuman primates, LLOV is biologically uncharacterized, since infectious LLOV has never been isolated. To examine the properties of LLOV, we characterized its envelope glycoprotein (GP), which likely plays a key role in viral tropism and pathogenicity. We first found that LLOV GP principally has the same primary structure as the other filovirus GPs. Similar to the other filoviruses, virus-like particles (VLPs) produced by transient expression of LLOV GP, matrix protein, and nucleoprotein in 293T cells had densely arrayed GP spikes on a filamentous particle. Mouse antiserum to LLOV VLP was barely cross-reactive to viruses of the other genera, indicating that LLOV is serologically distinct from the other known filoviruses. For functional study of LLOV GP, we utilized a vesicular stomatitis virus (VSV) pseudotype system and found that LLOV GP requires low endosomal pH and cathepsin L, and that human C-type lectins act as attachment factors for LLOV entry into cells. Interestingly, LLOV GP-pseudotyped VSV infected particular bat cell lines more efficiently than viruses bearing other filovirus GPs. These results suggest that LLOV GP mediates cellular entry in a manner similar to that of the other filoviruses while showing preferential tropism for some bat cells.

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.

Role of the phosphatidylserine receptor TIM-1 in enveloped-virus entry

The cell surface receptor T cell immunoglobulin mucin domain 1 (TIM-1) dramatically enhances filovirus infection of epithelial cells. Here, we showed that key phosphatidylserine (PtdSer) binding residues of the TIM-1 IgV domain are critical for Ebola virus (EBOV) entry through direct interaction with PtdSer on the viral envelope. PtdSer liposomes but not phosphatidylcholine liposomes competed with TIM-1 for EBOV pseudovirion binding and transduction. Further, annexin V (AnxV) substituted for the TIM-1 IgV domain, supporting a PtdSer-dependent mechanism. Our findings suggest that TIM-1-dependent uptake of EBOV occurs by apoptotic mimicry. Additionally, TIM-1 enhanced infection of a wide range of enveloped viruses, including alphaviruses and a baculovirus. As further evidence of the critical role of enveloped-virion-associated PtdSer in TIM-1-mediated uptake, TIM-1 enhanced internalization of pseudovirions and virus-like proteins (VLPs) lacking a glycoprotein, providing evidence that TIM-1 and PtdSer-binding receptors can mediate virus uptake independent of a glycoprotein. These results provide evidence for a broad role of TIM-1 as a PtdSer-binding receptor that mediates enveloped-virus uptake. Utilization of PtdSer-binding receptors may explain the wide tropism of many of these viruses and provide new avenues for controlling their virulence.

Development of novel entry inhibitors targeting emerging viruses

Emerging viral diseases pose a unique risk to public health, and thus there is a need to develop therapies. A current focus of funding agencies, and hence research, is the development of broad-spectrum antivirals, and in particular, those targeting common cellular pathways. The scope of this article is to review screening strategies and recent advances in this area, with a particular emphasis on antivirals targeting the step of viral entry for emerging lipid-enveloped viruses such as Ebola virus and SARS-coronavirus.