The role of antigen-presenting cells in filoviral hemorrhagic fever: gaps in current knowledge

Development and characterization of influenza M2 ectodomain and/or hemagglutinin stalk-based dendritic cell-targeting vaccines

A universal influenza vaccine is required for broad protection against influenza infection. Here, we revealed the efficacy of novel influenza vaccine candidates based on Ebola glycoprotein dendritic cell (DC)-targeting domain (EΔM) fusion protein technology. The four copies of ectodomain matrix protein of influenza (tM2e) or M2e hemagglutinin stalk (HA stalk) peptides (HM2e) were fused with EΔM to generate EΔM-tM2e or EΔM-HM2e, respectively. We demonstrated that EΔM-HM2e- or EΔM-tM2e-pseudotyped viral particles can efficiently target DC/macrophages in vitro and induced significantly high titers of anti-HA and/or anti-M2e antibodies in mice. Significantly, the recombinant vesicular stomatitis virus (rVSV)-EΔM-tM2e and rVSV-EΔM-HM2e vaccines mediated rapid and potent induction of M2 or/and HA antibodies in mice sera and mucosa. Importantly, vaccination of rVSV-EΔM-tM2e or rVSV-EΔM-HM2e protected mice from influenza H1N1 and H3N2 challenges. Taken together, our study suggests that rVSV-EΔM-tM2e and rVSV-EΔM-HM2e are promising candidates that may lead to the development of a universal vaccine against different influenza strains.

HIV-1 trans-Infection Mediated by DCs: The Tip of the Iceberg of Cell-to-Cell Viral Transmission

HIV-1 cell-to-cell transmission is key for an effective viral replication that evades immunity. This highly infectious mechanism is orchestrated by different cellular targets that utilize a wide variety of processes to efficiently transfer HIV-1 particles. Dendritic cells (DCs) are the most potent antigen presenting cells that initiate antiviral immune responses, but are also the cells with highest capacity to transfer HIV-1. This mechanism, known as trans-infection, relies on the capacity of DCs to capture HIV-1 particles via lectin receptors such as the sialic acid-binding I-type lectin Siglec-1/CD169. The discovery of the molecular interaction of Siglec-1 with sialylated lipids exposed on HIV-1 membranes has enlightened how this receptor can bind to several enveloped viruses. The outcome of these interactions can either mount effective immune responses, boost the productive infection of DCs and favour innate sensing, or fuel viral transmission via trans-infection. Here we review these scenarios focusing on HIV-1 and other enveloped viruses such as Ebola virus or SARS-CoV-2.

Development and Evaluation of an Ebola Virus Glycoprotein Mucin-Like Domain Replacement System as a New Dendritic Cell-Targeting Vaccine Approach against HIV-1

The development of efficient vaccine approaches against HIV infection remains challenging in the vaccine field. Here, we developed an Ebola virus envelope glycoprotein (EboGP)-based chimeric fusion protein system and demonstrated that replacement of the mucin-like domain (MLD) of EboGP with HIV C2-V3-C3 (134 amino acids [aa]) or C2-V3-C3-V4-C4-V5-C5 (243 aa) polypeptides (EbGPΔM-V3 and EbGPΔM-V3-V5, respectively) still maintained the efficiency of EboGP-mediated viral entry into human macrophages and dendritic cells (DCs). Animal studies using mice revealed that immunization with virus-like particles (VLPs) containing the above chimeric proteins, especially EbGPΔM-V3, induced significantly more potent anti-HIV antibodies than HIV gp120 alone in mouse serum and vaginal fluid. Moreover, the splenocytes isolated from mice immunized with VLPs containing EbGPΔM-V3 produced significantly higher levels of gamma interferon (IFN-γ), interleukin 2 (IL-2), IL-4, IL-5, and macrophage inflammatory protein 1α (MIP-1α). Additionally, we demonstrated that coexpression of EbGPΔM-V3 and the HIV Env glycoprotein in a recombinant vesicular stomatitis virus (rVSV) vector elicited robust anti-HIV antibodies that may have specifically recognized epitopes outside or inside the C2-V3-C3 region of HIV-1 gp120 and cross-reacted with the gp120 from different HIV strains. Thus, this study has demonstrated the great potential of this DC-targeting vaccine platform as a new vaccine approach for improving immunogen delivery and increasing vaccine efficacy.

IMPORTANCE Currently, there are more than 38.5 million reported cases of HIV globally. To date, there is no approved vaccine for HIV-1 infection. Thus, the development of an effective vaccine against HIV infection remains a global priority. This study revealed the efficacy of a novel dendritic cell (DC)-targeting vaccination approach against HIV-1. The results clearly show that the immunization of mice with virus-like particles (VLPs) and VSVs containing HIV Env and a fusion protein composed of a DC-targeting domain of Ebola virus GP with HIV C2-V3-C3 polypeptides (EbGPΔM-V3) could induce robust immune responses against HIV-1 Env and/or Gag in serum and vaginal mucosa. These findings provide a proof of concept of this novel and efficient DC-targeting vaccine approach in delivering various antigenic polypeptides of HIV-1 and/or other emergent infections to the host antigen-presenting cells to prevent HIV and other viral infections.

IL-23/Th17 Axis: A Potential Therapeutic Target of Psoriasis

Psoriasis is an immune-mediated skin disease that leads to the initiation of abnormal production of inflammatory mediators and keratinocytes hyper-proliferation. Th-1 cell expressing cytokines such as IL-1β and TNF-α have been the important hallmarks in the management of psoriasis. However, investigations carried out in the previous few years underline the involvement of another subset of T helper cells, i.e. Th-17 in psoriasis exacerbation, and hence become the point of focus now. The immunopathogenesis of Th-17 is the result of the IL-23/Th-17 axis. It involves the release of IL-17 and IL-22 in response to the activated NF-kβ dependent activation of IL-23. The function of human Th-17 cells as well as the crucial role of IL-23/Th-17 axis in the exacerbation of psoriasis and treatment have been well explored. Therefore, considering IL-23/Th17 axis as a pertinent therapeutic target in immune driven disorders, extensive investigations are now highlighting the utility of biopharmaceuticals and/or biological agents acting on these targets. Here, we review the IL-23/Th-17 axis based therapeutic targets, different types of active moieties based on their source of availability and most useful USFDA approved Mabs targeting the IL-23/Th17 axis in psoriasis for a better understanding of the future possibilities in this area.

DC/L-SIGN recognition of spike glycoprotein promotes SARS-CoV-2 trans-infection and can be inhibited by a glycomimetic antagonist

The efficient spread of SARS-CoV-2 resulted in a unique pandemic in modern history. Despite early identification of ACE2 as the receptor for viral spike protein, much remains to be understood about the molecular events behind viral dissemination. We evaluated the contribution of C-type lectin receptors (CLRS) of antigen-presenting cells, widely present in respiratory mucosa and lung tissue. DC-SIGN, L-SIGN, Langerin and MGL bind to diverse glycans of the spike using multiple interaction areas. Using pseudovirus and cells derived from monocytes or T-lymphocytes, we demonstrate that while virus capture by the CLRs examined does not allow direct cell infection, DC/L-SIGN, among these receptors, promote virus transfer to permissive ACE2+ Vero E6 cells. A glycomimetic compound designed against DC-SIGN, enable inhibition of this process. These data have been then confirmed using authentic SARS-CoV-2 virus and human respiratory cell lines. Thus, we described a mechanism potentiating viral spreading of infection.

When Dendritic Cells Go Viral: The Role of Siglec-1 in Host Defense and Dissemination of Enveloped Viruses

Dendritic cells (DCs) are among the first cells that recognize incoming viruses at the mucosal portals of entry. Initial interaction between DCs and viruses facilitates cell activation and migration to secondary lymphoid tissues, where these antigen presenting cells (APCs) prime specific adaptive immune responses. Some viruses, however, have evolved strategies to subvert the migratory capacity of DCs as a way to disseminate infection systemically. Here we focus on the role of Siglec-1, a sialic acid-binding type I lectin receptor potently upregulated by type I interferons on DCs, that acts as a double edge sword, containing viral replication through the induction of antiviral immunity, but also favoring viral spread within tissues. Such is the case for distant enveloped viruses like human immunodeficiency virus (HIV)-1 or Ebola virus (EBOV), which incorporate sialic acid-containing gangliosides on their viral membrane and are effectively recognized by Siglec-1. Here we review how Siglec-1 is highly induced on the surface of human DCs upon viral infection, the way this impacts different antigen presentation pathways, and how enveloped viruses have evolved to exploit these APC functions as a potent dissemination strategy in different anatomical compartments.

Dendritic Cells/Macrophages-Targeting Feature of Ebola Glycoprotein and its Potential as Immunological Facilitator for Antiviral Vaccine Approach

In the prevention of epidemic and pandemic viral infection, the use of the antiviral vaccine has been the most successful biotechnological and biomedical approach. In recent times, vaccine development studies have focused on recruiting and targeting immunogens to dendritic cells (DCs) and macrophages to induce innate and adaptive immune responses. Interestingly, Ebola virus (EBOV) glycoprotein (GP) has a strong binding affinity with DCs and macrophages. Shreds of evidence have also shown that the interaction between EBOV GP with DCs and macrophages leads to massive recruitment of DCs and macrophages capable of regulating innate and adaptive immune responses. Therefore, studies for the development of vaccine can utilize the affinity between EBOV GP and DCs/macrophages as a novel immunological approach to induce both innate and acquired immune responses. In this review, we will discuss the unique features of EBOV GP to target the DC, and its potential to elicit strong immune responses while targeting DCs/macrophages. This review hopes to suggest and stimulate thoughts of developing a stronger and effective DC-targeting vaccine for diverse virus infection using EBOV GP.

Cytokine Effects on the Entry of Filovirus Envelope Pseudotyped Virus-Like Particles into Primary Human Macrophages

Macrophages are one of the first and also a major site of filovirus replication and, in addition, are a source of multiple cytokines, presumed to play a critical role in the pathogenesis of the viral infection. Some of these cytokines are known to induce macrophage phenotypic changes in vitro, but how macrophage polarization may affect the cell susceptibility to filovirus entry remains largely unstudied. We generated different macrophage subsets using cytokine pre-treatment and subsequently tested their ability to fuse with beta-lactamase containing virus-like particles (VLP), pseudotyped with the surface glycoprotein of Ebola virus (EBOV) or the glycoproteins of other clinically relevant filovirus species. We found that pre-incubation of primary human monocyte-derived macrophages (MDM) with interleukin-10 (IL-10) significantly enhanced filovirus entry into cells obtained from multiple healthy donors, and the IL-10 effect was preserved in the presence of pro-inflammatory cytokines found to be elevated during EBOV disease. In contrast, fusion of IL-10-treated macrophages with influenza hemagglutinin/neuraminidase pseudotyped VLPs was unchanged or slightly reduced. Importantly, our in vitro data showing enhanced virus entry are consistent with the correlation established between elevated serum IL-10 and increased mortality in filovirus infected patients and also reveal a novel mechanism that may account for the IL-10-mediated increase in filovirus pathogenicity.

Incorporation of Ebola glycoprotein into HIV particles facilitates dendritic cell and macrophage targeting and enhances HIV-specific immune responses

The development of an effective vaccine against HIV infection remains a global priority. Dendritic cell (DC)-based HIV immunotherapeutic vaccine is a promising approach which aims at optimizing the HIV-specific immune response using primed DCs to promote and enhance both the cellular and humoral arms of immunity. Since the Ebola virus envelope glycoprotein (EboGP) has strong DC-targeting ability, we investigated whether EboGP is able to direct HIV particles towards DCs efficiently and promote potent HIV-specific immune responses. Our results indicate that the incorporation of EboGP into non-replicating virus-like particles (VLPs) enhances their ability to target human monocyte-derived dendritic cells (MDDCs) and monocyte-derived macrophages (MDMs). Also, a mucin-like domain deleted EboGP (EboGPΔM) can further enhanced the MDDCs and MDMs-targeting ability. Furthermore, we investigated the effect of EboGP on HIV immunogenicity in mice, and the results revealed a significantly stronger HIV-specific humoral immune response when immunized with EboGP-pseudotyped HIV VLPs compared with those immunized with HIV VLPs. Splenocytes harvested from mice immunized with EboGP-pseudotyped HIV VLPs secreted increased levels of macrophage inflammatory proteins-1α (MIP-1α) and IL-4 upon stimulation with HIV Env and/or Gag peptides compared with those harvested from mice immunized with HIV VLPs. Collectively, this study provides evidence for the first time that the incorporation of EboGP in HIV VLPs can facilitate DC and macrophage targeting and induce more potent immune responses against HIV.

The role of mononuclear phagocytes in Ebola virus infection

The filovirus, Zaire Ebolavirus (EBOV), infects tissue macrophages (Mϕs) and dendritic cells (DCs) early during infection. Viral infection of both cells types is highly productive, leading to increased viral load. However, virus infection of these two cell types results in different consequences for cellular function. Infection of Mϕs stimulates the production of proinflammatory and immunomodulatory cytokines and chemokines, leading to the production of a cytokine storm, while simultaneously increasing tissue factor production and thus facilitating disseminated intravascular coagulation. In contrast, EBOV infection of DCs blocks DC maturation and antigen presentation rendering these cells unable to communicate with adaptive immune response elements. Details of the known interactions of these cells with EBOV are reviewed here. We also identify a number of unanswered questions that remain about interactions of filoviruses with these cells.

Ebola virus infection kinetics in chimeric mice reveal a key role of T cells as barriers for virus dissemination

Ebola virus (EBOV) causes severe systemic disease in humans and non-human primates characterized by high levels of viremia and virus titers in peripheral organs. The natural portals of virus entry are the mucosal surfaces and the skin where macrophages and dendritic cells (DCs) are primary EBOV targets. Due to the migratory properties of DCs, EBOV infection of these cells has been proposed as a necessary step for virus dissemination via draining lymph nodes and blood. Here we utilize chimeric mice with competent hematopoietic-driven immunity, to show that EBOV primarily infects CD11b⁺ DCs in non-lymphoid and lymphoid tissues, but spares the main cross-presenting CD103⁺ DC subset. Furthermore, depletion of CD8 and CD4 T cells resulted in loss of early control of virus replication, viremia and fatal Ebola virus disease (EVD). Thus, our findings point out at T cell function as a key determinant of EVD progress and outcome.

Filovirus Strategies to Escape Antiviral Responses

This chapter describes the various strategies filoviruses use to escape host immune responses with a focus on innate immune and cell death pathways. Since filovirus replication can be efficiently blocked by interferon (IFN), filoviruses have evolved mechanisms to counteract both type I IFN induction and IFN response signaling pathways. Intriguingly, marburg- and ebolaviruses use different strategies to inhibit IFN signaling. This chapter also summarizes what is known about the role of IFN-stimulated genes (ISGs) in filovirus infection. These fall into three categories: those that restrict filovirus replication, those whose activation is inhibited by filoviruses, and those that have no measurable effect on viral replication. In addition to innate immunity, mammalian cells have evolved strategies to counter viral infections, including the induction of cell death and stress response pathways, and we summarize our current knowledge of how filoviruses interact with these pathways. Finally, this chapter delves into the interaction of EBOV with myeloid dendritic cells and macrophages and the associated inflammatory response, which differs dramatically between these cell types when they are infected with EBOV. In summary, we highlight the multifaceted nature of the host-viral interactions during filoviral infections.

The Tetherin Antagonism of the Ebola Virus Glycoprotein Requires an Intact Receptor-Binding Domain and Can Be Blocked by GP1-Specific Antibodies

The glycoprotein of Ebola virus (EBOV GP), a member of the family Filoviridae, facilitates viral entry into target cells. In addition, EBOV GP antagonizes the antiviral activity of the host cell protein tetherin, which may otherwise restrict EBOV release from infected cells. However, it is unclear how EBOV GP antagonizes tetherin, and it is unknown whether the GP of Lloviu virus (LLOV), a filovirus found in dead bats in Northern Spain, also counteracts tetherin. Here, we show that LLOV GP antagonizes tetherin, indicating that tetherin may not impede LLOV spread in human cells. Moreover, we demonstrate that appropriate processing of N-glycans in tetherin/GP-coexpressing cells is required for tetherin counteraction by EBOV GP. Furthermore, we show that an intact receptor-binding domain (RBD) in the GP1 subunit of EBOV GP is a prerequisite for tetherin counteraction. In contrast, blockade of Niemann-Pick disease type C1 (NPC1), a cellular binding partner of the RBD, did not interfere with tetherin antagonism. Finally, we provide evidence that an antibody directed against GP1, which protects mice from a lethal EBOV challenge, may block GP-dependent tetherin antagonism. Our data, in conjunction with previous reports, indicate that tetherin antagonism is conserved among the GPs of all known filoviruses and demonstrate that the GP1 subunit of EBOV GP plays a central role in tetherin antagonism.

IMPORTANCE

Filoviruses are reemerging pathogens that constitute a public health threat. Understanding how Ebola virus (EBOV), a highly pathogenic filovirus responsible for the 2013-2016 Ebola virus disease epidemic in western Africa, counteracts antiviral effectors of the innate immune system might help to define novel targets for antiviral intervention. Similarly, determining whether Lloviu virus (LLOV), a filovirus detected in bats in northern Spain, is inhibited by innate antiviral effectors in human cells might help to determine whether the virus constitutes a threat to humans. The present study shows that LLOV, like EBOV, counteracts the antiviral effector protein tetherin via its glycoprotein (GP), suggesting that tetherin does not pose a defense against LLOV spread in humans. Moreover, our work identifies the GP1 subunit of EBOV GP, in particular an intact receptor-binding domain, as critical for tetherin counteraction and provides evidence that antibodies directed against GP1 can interfere with tetherin counteraction.

Ebola Virus Disease Is Characterized by Poor Activation and Reduced Levels of Circulating CD16+ Monocytes

A number of previous studies have identified antigen-presenting cells (APCs) as key targets of Ebola virus (EBOV), but the role of APCs in human Ebola virus disease (EVD) is not known. We have evaluated the phenotype and kinetics of monocytes, neutrophils, and dendritic cells (DCs) in peripheral blood of patients for whom EVD was diagnosed by the European Mobile Laboratory in Guinea. Acute EVD was characterized by reduced levels of circulating nonclassical CD16⁺ monocytes with a poor activation profile. In survivors, CD16⁺ monocytes were activated during recovery, coincident with viral clearance, suggesting an important role of this cell subset in EVD pathophysiology.

Ebola Virus Infections in Nonhuman Primates Are Temporally Influenced by Glycoprotein Poly-U Editing Site Populations in the Exposure Material

Recent experimentation with the variants of the Ebola virus that differ in the glycoprotein's poly-uridine site, which dictates the form of glycoprotein produced through a transcriptional stutter, has resulted in questions regarding the pathogenicity and lethality of the stocks used to develop products currently undergoing human clinical trials to combat the disease. In order to address these concerns and prevent the delay of these critical research programs, we designed an experiment that permitted us to intramuscularly challenge statistically significant numbers of naïve and vaccinated cynomolgus macaques with either a 7U or 8U variant of the Ebola virus, Kikwit isolate. In naïve animals, no difference in survivorship was observed; however, there was a significant delay in the disease course between the two groups. Significant differences were also observed in time-of-fever, serum chemistry, and hematology. In vaccinated animals, there was no statistical difference in survivorship between either challenge groups, with two succumbing in the 7U group compared to 1 in the 8U challenge group. In summary, survivorship was not affected, but the Ebola virus disease course in nonhuman primates is temporally influenced by glycoprotein poly-U editing site populations.

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.

Amino Acid Residue at Position 79 of Marburg Virus VP40 Confers Interferon Antagonism in Mouse Cells

Marburg viruses (MARVs) cause highly lethal infections in humans and nonhuman primates. Mice are not generally susceptible to MARV infection; however, if the strain is first adapted to mice through serial passaging, it becomes able to cause disease in this animal. A previous study correlated changes accrued during mouse adaptation in the VP40 gene of a MARV strain known as Ravn virus (RAVV) with an increased capacity to inhibit interferon (IFN) signaling in mouse cell lines. The MARV strain Ci67, which belongs to a different phylogenetic clade than RAVV, has also been adapted to mice and in the process the Ci67 VP40 acquired a different collection of genetic changes than did RAVV VP40. Here, we demonstrate that the mouse-adapted Ci67 VP40 more potently antagonizes IFN-α/β-induced STAT1 and STAT2 tyrosine phosphorylation, gene expression, and antiviral activity in both mouse and human cell lines, compared with the parental Ci67 VP40. Ci67 VP40 is also demonstrated to target the activation of kinase Jak1. A single change at VP40 residue 79 was found to be sufficient for the increased VP40 IFN antagonism. These data argue that VP40 IFN-antagonist activity plays a key role in MARV pathogenesis in mice.

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.

Effective binding of a phosphatidylserine-targeting antibody to Ebola virus infected cells and purified virions

Ebola virus is responsible for causing severe hemorrhagic fevers, with case fatality rates of up to 90%. Currently, no antiviral or vaccine is licensed against Ebola virus. A phosphatidylserine-targeting antibody (PGN401, bavituximab) has previously been shown to have broad-spectrum antiviral activity. Here, we demonstrate that PGN401 specifically binds to Ebola virus and recognizes infected cells. Our study provides the first evidence of phosphatidylserine-targeting antibody reactivity against Ebola virus.

Reservoir host immune responses to emerging zoonotic viruses

Zoonotic viruses, such as HIV, Ebola virus, coronaviruses, influenza A viruses, hantaviruses, or henipaviruses, can result in profound pathology in humans. In contrast, populations of the reservoir hosts of zoonotic pathogens often appear to tolerate these infections with little evidence of disease. Why are viruses more dangerous in one species than another? Immunological studies investigating quantitative and qualitative differences in the host-virus equilibrium in animal reservoirs will be key to answering this question, informing new approaches for treating and preventing zoonotic diseases. Integrating an understanding of host immune responses with epidemiological, ecological, and evolutionary insights into viral emergence will shed light on mechanisms that minimize fitness costs associated with viral infection, facilitate transmission to other hosts, and underlie the association of specific reservoir hosts with multiple emerging viruses. Reservoir host studies provide a rich opportunity for elucidating fundamental immunological processes and their underlying genetic basis, in the context of distinct physiological and metabolic constraints that contribute to host resistance and disease tolerance.

Human Ebola virus infection in West Africa: a review of available therapeutic agents that target different steps of the life cycle of Ebola virus

The recent outbreak of the human Zaire ebolavirus (EBOV) epidemic is spiraling out of control in West Africa. Human EBOV hemorrhagic fever has a case fatality rate of up to 90%. The EBOV is classified as a biosafety level 4 pathogen and is considered a category A agent of bioterrorism by Centers for Disease Control and Prevention, with no approved therapies and vaccines available for its treatment apart from supportive care. Although several promising therapeutic agents and vaccines against EBOV are undergoing the Phase I human trial, the current epidemic might be outpacing the speed at which drugs and vaccines can be produced. Like all viruses, the EBOV largely relies on host cell factors and physiological processes for its entry, replication, and egress. We have reviewed currently available therapeutic agents that have been shown to be effective in suppressing the proliferation of the EBOV in cell cultures or animal studies. Most of the therapeutic agents in this review are directed against non-mutable targets of the host, which is independent of viral mutation. These medications are approved by the Food and Drug Administration (FDA) for the treatment of other diseases. They are available and stockpileable for immediate use. They may also have a complementary role to those therapeutic agents under development that are directed against the mutable targets of the EBOV.

Virus particle release from glycosphingolipid-enriched microdomains is essential for dendritic cell-mediated capture and transfer of HIV-1 and henipavirus

Human immunodeficiency virus type 1 (HIV-1) exploits dendritic cells (DCs) to promote its transmission to T cells. We recently reported that the capture of HIV-1 by mature dendritic cells (MDCs) is mediated by an interaction between the glycosphingolipid (GSL) GM3 on virus particles and CD169/Siglec-1 on MDCs. Since HIV-1 preferentially buds from GSL-enriched lipid microdomains on the plasma membrane, we hypothesized that the virus assembly and budding site determines the ability of HIV-1 to interact with MDCs. In support of this hypothesis, mutations in the N-terminal basic domain (29/31KE) or deletion of the membrane-targeting domain of the HIV-1 matrix (MA) protein that altered the virus assembly and budding site to CD63(+)/Lamp-1-positive intracellular compartments resulted in lower levels of virion incorporation of GM3 and attenuation of virus capture by MDCs. Furthermore, MDC-mediated capture and transmission of MA mutant viruses to T cells were decreased, suggesting that HIV-1 acquires GSLs via budding from the plasma membrane to access the MDC-dependent trans infection pathway. Interestingly, MDC-mediated capture of Nipah and Hendra virus (recently emerged zoonotic paramyxoviruses) M (matrix) protein-derived virus-like particles that bud from GSL-enriched plasma membrane microdomains was also dependent on interactions between virion-incorporated GSLs and CD169. Moreover, capture and transfer of Nipah virus envelope glycoprotein-pseudotyped lentivirus particles by MDCs were severely attenuated upon depletion of GSLs from virus particles. These results suggest that GSL incorporation into virions is critical for the interaction of diverse enveloped RNA viruses with DCs and that the GSL-CD169 recognition nexus might be a conserved viral mechanism of parasitization of DC functions for systemic virus dissemination.

IMPORTANCE

Dendritic cells (DCs) can capture HIV-1 particles and transfer captured virus particles to T cells without establishing productive infection in DCs, a mechanism of HIV-1 trans infection. We have recently identified CD169-mediated recognition of GM3, a host-derived glycosphingolipid (GSL) incorporated into the virus particle membrane, as the receptor and ligand for the DC-HIV trans infection pathway. In this study, we have identified the matrix (MA) domain of Gag to be the viral determinant that governs incorporation of GM3 into HIV-1 particles, a previously unappreciated function of the HIV-1 MA. In addition, we demonstrate that the GSL-CD169-dependent trans infection pathway is also utilized as a dissemination mechanism by henipaviruses. GSL incorporation in henipaviruses was also dependent on the viral capsid (M) protein-directed assembly and budding from GSL-enriched lipid microdomains. These findings provide evidence of a conserved mechanism of retrovirus and henipavirus parasitization of cell-to-cell recognition pathways for systemic virus dissemination.

Pyridinyl imidazole inhibitors of p38 MAP kinase impair viral entry and reduce cytokine induction by Zaire ebolavirus in human dendritic cells

Antigen presenting cells (APCs), including macrophages and dendritic cells, are early and sustained targets of Ebola virus (EBOV) infection in vivo. Because EBOV activates mitogen-activated protein kinase (MAPK) signaling upon infection of APCs, we evaluated the effect of pyridinyl imidazole inhibitors of p38 MAPK on EBOV infection of human APCs and EBOV mediated cytokine production from human DCs. The p38 MAPK inhibitors reduced viral replication in PMA-differentiated macrophage-like human THP-1 cells with an IC50 of 4.73μM (SB202190), 8.26μM (p38kinhIII) and 8.21μM (SB203580) and primary human monocyte-derived dendritic cells (MDDCs) with an IC50 of 2.67μM (SB202190). Furthermore, cytokine production from EBOV-treated MDDCs was inhibited in a dose-dependent manner. A control pyridinyl imidazole compound failed to inhibit either EBOV infection or cytokine induction. Using an established EBOV virus-like particle (VLP) entry assay, we demonstrate that inhibitor pretreatment blocked VLP entry suggesting that the inhibitors blocked infection and replication at least in part by blocking EBOV entry. Taken together, our results indicate that pyridinyl imidazole p38 MAPK inhibitors may serve as leads for the development of therapeutics to treat EBOV infection.

Differential induction of apoptosis, interferon signaling, and phagocytosis in macrophages infected with a panel of attenuated and nonattenuated poxviruses

Due to the essential role macrophages play in antiviral immunity, it is important to understand the intracellular and molecular processes that occur in macrophages following infection with various strains of vaccinia virus, particularly those used as vaccine vectors. Similarities as well as differences were found in macrophages infected with different poxvirus strains, particularly at the level of virus-induced apoptosis and the expression of immunomodulatory genes, as determined by microarray analyses. Interestingly, the attenuated modified vaccinia Ankara virus (MVA) was particularly efficient in triggering apoptosis and beta interferon (IFN-β) secretion and in inducing changes in the expression of genes associated with increased activation of innate immunity, setting it apart from the other five vaccinia virus strains tested. Taken together, these results increase our understanding of how these viruses interact with human macrophages, at the cellular and molecular levels, and suggest mechanisms that may underlie their utility as recombinant vaccine vectors.

IMPORTANCE

Our studies clearly demonstrate that there are substantial biological differences in the patterns of cellular gene expression between macrophages infected with different poxvirus strains and that these changes are due specifically to infection with the distinct viruses. For example, a clear induction in IFN-β mRNA was observed after infection with MVA but not with other poxviruses. Importantly, antiviral bioassays confirmed that MVA-infected macrophages secreted a high level of biologically active type I IFN. Similarly, the phagocytic capacity of macrophages was also specifically increased after infection with MVA. Although the main scope of this study was not to test the vaccine potential of MVA as there are several groups in the field working extensively on this aspect, the characteristics/phenotypes we observed at the in vitro level clearly highlight the inherent advantages that MVA possesses in comparison to other poxvirus strains.

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 lack of maturation of Ebola virus-infected dendritic cells results from the cooperative effect of at least two viral domains

Ebola virus (EBOV) infections are characterized by deficient T lymphocyte responses, T lymphocyte apoptosis, and lymphopenia in the absence of direct infection of T lymphocytes. In contrast, dendritic cells (DC) are infected but fail to mature appropriately, thereby impairing the T cell response. We investigated the contributions of EBOV proteins in modulating DC maturation by generating recombinant viruses expressing enhanced green fluorescent protein and carrying mutations affecting several potentially immunomodulating domains. They included envelope glycoprotein (GP) domains, as well as innate response antagonist domains (IRADs) previously identified in the VP24 and VP35 proteins. GP expressed by an unrelated vector, but not the wild-type EBOV, was found to strongly induce DC maturation, and infections with recombinant EBOV carrying mutations disabling GP functional domains did not restore DC maturation. In contrast, each of the viruses carrying mutations disabling any IRAD in VP35 induced a dramatic upregulation of DC maturation markers. This was dependent on infection, but not interaction with GP. Disabling of IRADs also resulted in up to a several hundredfold increase in secretion of cytokines and chemokines. Furthermore, these mutations induced formation of homotypic DC clusters, which represent close correlates of their maturation and presumably facilitate transfer of antigen from migratory DC to lymph node DC. Thus, an individual IRAD is insufficient to suppress DC maturation; rather, the suppression of DC maturation and the "immune paralysis" observed during EBOV infections results from a cooperative effect of two or more individual IRADs.

Ebola virus exploits a monocyte differentiation program to promote its entry

Antigen-presenting cells (APCs) are critical targets of Ebola virus (EBOV) infection in vivo. However, the susceptibility of monocytes to infection is controversial. Studies indicate productive monocyte infection, and yet monocytes are also reported to be resistant to EBOV GP-mediated entry. In contrast, monocyte-derived macrophages and dendritic cells are permissive for both EBOV entry and replication. Here, freshly isolated monocytes are demonstrated to indeed be refractory to EBOV entry. However, EBOV binds monocytes, and delayed entry occurs during monocyte differentiation. Cultured monocytes spontaneously downregulate the expression of viral entry restriction factors such as interferon-inducible transmembrane proteins, while upregulating the expression of critical EBOV entry factors cathepsin B and NPC1. Moreover, these processes are accelerated by EBOV infection. Finally, ectopic expression of NPC1 is sufficient to rescue entry into an undifferentiated, normally nonpermissive monocytic cell line. These results define the molecular basis for infection of APCs and suggest means to limit APC infection.

A mutation in the Ebola virus envelope glycoprotein restricts viral entry in a host species- and cell-type-specific manner

Zaire Ebola virus (EBOV) is a zoonotic pathogen that causes severe hemorrhagic fever in humans. A single viral glycoprotein (GP) mediates viral attachment and entry. Here, virus-like particle (VLP)-based entry assays demonstrate that a GP mutant, GP-F88A, which is defective for entry into a variety of human cell types, including antigen-presenting cells (APCs), such as macrophages and dendritic cells, can mediate viral entry into mouse CD11b(+) APCs. Like that of wild-type GP (GP-wt), GP-F88A-mediated entry occurs via a macropinocytosis-related pathway and requires endosomal cysteine proteases and an intact fusion peptide. Several additional hydrophobic residues lie in close proximity to GP-F88, including L111, I113, L122, and F225. GP mutants in which these residues are mutated to alanine displayed preferential and often impaired entry into several cell types, although not in a species-specific manner. Niemann-Pick C1 (NPC1) protein is an essential filovirus receptor that binds directly to GP. Overexpression of NPC1 was recently demonstrated to rescue GP-F88A-mediated entry. A quantitative enzyme-linked immunosorbent assay (ELISA) demonstrated that while the F88A mutation impairs GP binding to human NPC1 by 10-fold, it has little impact on GP binding to mouse NPC1. Interestingly, not all mouse macrophage cell lines permit GP-F88A entry. The IC-21 cell line was permissive, whereas RAW 264.7 cells were not. Quantitative reverse transcription (RT)-PCR assays demonstrate higher NPC1 levels in GP-F88A permissive IC-21 cells and mouse peritoneal macrophages than in RAW 264.7 cells. Cumulatively, these studies suggest an important role for NPC1 in the differential entry of GP-F88A into mouse versus human APCs.

Mouse models for filovirus infections

The filoviruses marburg- and ebolaviruses can cause severe hemorrhagic fever (HF) in humans and nonhuman primates. Because many cases have occurred in geographical areas lacking a medical research infrastructure, most studies of the pathogenesis of filoviral HF, and all efforts to develop drugs and vaccines, have been carried out in biocontainment laboratories in non-endemic countries, using nonhuman primates (NHPs), guinea pigs and mice as animal models. NHPs appear to closely mirror filoviral HF in humans (based on limited clinical data), but only small numbers may be used in carefully regulated experiments; much research is therefore done in rodents. Because of their availability in large numbers and the existence of a wealth of reagents for biochemical and immunological testing, mice have become the preferred small animal model for filovirus research. Since the first experiments following the initial 1967 marburgvirus outbreak, wild-type or mouse-adapted viruses have been tested in immunocompetent or immunodeficient mice. In this paper, we review how these types of studies have been used to investigate the pathogenesis of filoviral disease, identify immune responses to infection and evaluate antiviral drugs and vaccines. We also discuss the strengths and weaknesses of murine models for filovirus research, and identify important questions for further study.