Distinct mechanisms of entry by envelope glycoproteins of Marburg and Ebola (Zaire) viruses

Ebolavirus delta-peptide immunoadhesins inhibit marburgvirus and ebolavirus cell entry

With the exception of Reston and Lloviu viruses, filoviruses (marburgviruses, ebolaviruses, and "cuevaviruses") cause severe viral hemorrhagic fevers in humans. Filoviruses use a class I fusion protein, GP(1,2), to bind to an unknown, but shared, cell surface receptor to initiate virus-cell fusion. In addition to GP(1,2), ebolaviruses and cuevaviruses, but not marburgviruses, express two secreted glycoproteins, soluble GP (sGP) and small soluble GP (ssGP). All three glycoproteins have identical N termini that include the receptor-binding region (RBR) but differ in their C termini. We evaluated the effect of the secreted ebolavirus glycoproteins on marburgvirus and ebolavirus cell entry, using Fc-tagged recombinant proteins. Neither sGP-Fc nor ssGP-Fc bound to filovirus-permissive cells or inhibited GP(1,2)-mediated cell entry of pseudotyped retroviruses. Surprisingly, several Fc-tagged Δ-peptides, which are small C-terminal cleavage products of sGP secreted by ebolavirus-infected cells, inhibited entry of retroviruses pseudotyped with Marburg virus GP(1,2), as well as Marburg virus and Ebola virus infection in a dose-dependent manner and at low molarity despite absence of sequence similarity to filovirus RBRs. Fc-tagged Δ-peptides from three ebolaviruses (Ebola virus, Sudan virus, and Taï Forest virus) inhibited GP(1,2)-mediated entry and infection of viruses comparably to or better than the Fc-tagged RBRs, whereas the Δ-peptide-Fc of an ebolavirus nonpathogenic for humans (Reston virus) and that of an ebolavirus with lower lethality for humans (Bundibugyo virus) had little effect. These data indicate that Δ-peptides are functional components of ebolavirus proteomes. They join cathepsins and integrins as novel modulators of filovirus cell entry, might play important roles in pathogenesis, and could be exploited for the synthesis of powerful new antivirals.

Vaccine and adjuvant design for emerging viruses: mutations, deletions, segments and signaling

Vaccination is currently the most effective strategy to medically control viral diseases. However, developing vaccines is a long and expensive process, and traditional methods, such as attenuating wild-type viruses by serial passage, may not be suitable for all viruses and may lead to vaccine safety considerations, particularly in the case of the vaccination of particular patient groups, such as the immunocompromised and the elderly. In particular, developing vaccines against emerging viral pathogens adds a further level of complexity, as they may only be administered to small groups of people or only in response to a specific event or threat, limiting our ability to study and evaluate responses. In this commentary, we discuss how novel techniques may be used to engineer a new generation of vaccine candidates as we move toward a more targeted vaccine design strategy, driven by our understanding of the mechanisms of viral pathogenesis, attenuation and the signaling events which are required to develop a lasting, protective immunity. We will also briefly discuss the potential future role of vaccine adjuvants, which could be used to bridge the gap between vaccine safety, and lasting immunity from a single vaccination. 

Production of influenza pseudotyped lentiviral particles and their use in influenza research and diagnosis: an update

Pseudotyped viral particles are being used as safe surrogates to mimic the structure and surface of many viruses, including highly pathogenic viruses such as avian influenza H5N1, to investigate biological functions mediated by the envelope proteins derived from these viruses. The first part of this article evaluates and discusses the differences in the production and characterization of influenza pseudoparticles. The second part focuses on the applications that such a flexible tool can provide in modern influenza research, in particular in the fields of drug discovery, molecular biology and diagnosis.

Discovery, synthesis, and biological evaluation of a novel group of selective inhibitors of filoviral entry

Herein, we report the development of an antifiloviral screening system, based on a pseudotyping strategy, and its application in the discovery of a novel group of small molecules that selectively inhibit the Ebola and Marburg glycoprotein (GP)-mediated infection of human cells. Using Ebola Zaire GP-pseudotyped HIV particles bearing a luciferase reporter gene and 293T cells, a library of 237 small molecules was screened for inhibition of GP-mediated viral entry. From this assay, lead compound 8a was identified as a selective inhibitor of filoviral entry with an IC(50) of 30 μM. To analyze functional group requirements for efficacy, a structure-activity relationship analysis of this 3,5-disubstituted isoxazole was then conducted with 56 isoxazole and triazole derivatives prepared using "click" chemistry. This study revealed that while the isoxazole ring can be replaced by a triazole system, the 5-(diethylamino)acetamido substituent found in 8a is required for inhibition of viral-cell entry. Variation of the 3-aryl substituent provided a number of more potent antiviral agents with IC(50) values ranging to 2.5 μM. Lead compound 8a and three of its derivatives were also found to block the Marburg glycoprotein (GP)-mediated infection of human cells.

A functional henipavirus envelope glycoprotein pseudotyped lentivirus assay system

Background

Hendra virus (HeV) and Nipah virus (NiV) are newly emerged zoonotic paramyxoviruses discovered during outbreaks in Queensland, Australia in 1994 and peninsular Malaysia in 1998/9 respectively and classified within the new Henipavirus genus. Both viruses can infect a broad range of mammalian species causing severe and often-lethal disease in humans and animals, and repeated outbreaks continue to occur. Extensive laboratory studies on the host cell infection stage of HeV and NiV and the roles of their envelope glycoproteins have been hampered by their highly pathogenic nature and restriction to biosafety level-4 (BSL-4) containment. To circumvent this problem, we have developed a henipavirus envelope glycoprotein pseudotyped lentivirus assay system using either a luciferase gene or green fluorescent protein (GFP) gene encoding human immunodeficiency virus type-1 (HIV-1) genome in conjunction with the HeV and NiV fusion (F) and attachment (G) glycoproteins.

Results

Functional retrovirus particles pseudotyped with henipavirus F and G glycoproteins displayed proper target cell tropism and entry and infection was dependent on the presence of the HeV and NiV receptors ephrinB2 or B3 on target cells. The functional specificity of the assay was confirmed by the lack of reporter-gene signals when particles bearing either only the F or only G glycoprotein were prepared and assayed. Virus entry could be specifically blocked when infection was carried out in the presence of a fusion inhibiting C-terminal heptad (HR-2) peptide, a well-characterized, cross-reactive, neutralizing human mAb specific for the henipavirus G glycoprotein, and soluble ephrinB2 and B3 receptors. In addition, the utility of the assay was also demonstrated by an examination of the influence of the cytoplasmic tail of F in its fusion activity and incorporation into pseudotyped virus particles by generating and testing a panel of truncation mutants of NiV and HeV F.

Conclusions

Together, these results demonstrate that a specific henipavirus entry assay has been developed using NiV or HeV F and G glycoprotein pseudotyped reporter-gene encoding retrovirus particles. This assay can be conducted safely under BSL-2 conditions and will be a useful tool for measuring henipavirus entry and studying F and G glycoprotein function in the context of virus entry, as well as in assaying and characterizing neutralizing antibodies and virus entry inhibitors.

Endogenous CD317/Tetherin limits replication of HIV-1 and murine leukemia virus in rodent cells and is resistant to antagonists from primate viruses

Human CD317 (BST-2/tetherin) is an intrinsic immunity factor that blocks the release of retroviruses, filoviruses, herpesviruses, and arenaviruses. It is unclear whether CD317 expressed endogenously in rodent cells has the capacity to interfere with the replication of the retroviral rodent pathogen murine leukemia virus (MLV) or, in the context of small-animal model development, contributes to the well-established late-phase restriction of human immunodeficiency virus type 1 (HIV-1). Here, we show that small interfering RNA (siRNA)-mediated knockdown of CD317 relieved a virion release restriction and markedly enhanced the egress of HIV-1, HIV-2, and simian immunodeficiency virus (SIV) in rat cells, including primary macrophages. Moreover, rodent CD317 potently inhibited MLV release, and siRNA-mediated depletion of CD317 in a mouse T-cell line resulted in the accelerated spread of MLV. Several virus-encoded antagonists have recently been reported to overcome the restriction imposed by human or monkey CD317, including HIV-1 Vpu, envelope glycoproteins of HIV-2 and Ebola virus, Kaposi's sarcoma-associated herpesvirus K5, and SIV Nef. In contrast, both rat and mouse CD317 showed a high degree of resistance to these viral antagonists. These data suggest that CD317 is a broadly acting and conserved mediator of innate control of retroviral infection and pathogenesis that restricts the release of retroviruses and lentiviruses in rodents. The high degree of resistance of the rodent CD317 restriction factors to antagonists from primate viruses has implications for HIV-1 small-animal model development and may guide the design of novel antiviral interventions.

Antibody-mediated neutralization of Ebola virus can occur by two distinct mechanisms

Human Ebola virus causes severe hemorrhagic fever disease with high mortality and there is no vaccine or treatment. Antibodies in survivors occur early, are sustained, and can delay infection when transferred into nonhuman primates. Monoclonal antibodies (mAbs) from survivors exhibit potent neutralizing activity in vitro and are protective in rodents. To better understand targets and mechanisms of neutralization, we investigated a panel of mAbs shown previously to react with the envelope glycoprotein (GP). While one non-neutralizing mAb recognized a GP epitope in the nonessential mucin-like domain, the rest were specific for GP1, were neutralizing, and could be further distinguished by reactivity with secreted GP. We show that survivor antibodies, human KZ52 and monkey JP3K11, were specific for conformation-dependent epitopes comprising residues in GP1 and GP2 and that neutralization occurred by two distinct mechanisms; KZ52 inhibited cathepsin cleavage of GP whereas JP3K11 recognized the cleaved, fusion-active form of GP.

Vpu-dependent block to incorporation of GaLV Env into lentiviral vectors

Background

The gibbon ape leukemia virus (GaLV) Env protein mediates entry into a wide range of human cells and is frequently used to pseudotype retroviral vectors. However, an incompatibility exists between GaLV Env and lentiviral vectors that results in decreased steady-state levels of the mature GaLV Env in cells and prevents its incorporation into lentiviral vector particles.

Results

We identified the HIV-1 Vpu protein as the major cause of the depletion in GaLV Env levels that occurs when lentiviral vector components are present. This activity of Vpu targeted the mature (cleaved) form of the GaLV Env that exists within or beyond the trans-Golgi. The activity required two conserved phospho-serines in the cytoplasmic tail of Vpu that are known to recruit β TrCP, a substrate adaptor for an SCF E3 ubiquitin ligase complex, and could be blocked by mutation of lysine 618 in the GaLV Env tail. Moreover, the Vpu-mediated decrease of GaLV Env levels was inhibited by the lysosomal inhibitor, bafilomycin A1. Interestingly, this activity of Vpu was only observed in the presence of other lentiviral vector components.

Conclusions

Similar to the mechanism whereby Vpu targets BST-2/tetherin for degradation, these findings implicate β-TrCP-mediated ubiquitination and the endo-lysosomal pathway in the degradation of the GaLV Env by lentiviral vector components. Possibly, the cytoplasmic tail of the GaLV Env contains features that mimic bona fide targets of Vpu, important to HIV-1 replication. Furthermore, the lack of effect of Vpu on GaLV Env in the absence of other HIV-1 proteins, suggests that a more complex interaction may exist between Vpu and its target proteins, with the additional involvement of one or more component(s) of the HIV-1 replication machinery.

Ebolavirus glycoprotein structure and mechanism of entry

Ebolavirus (EBOV) is a highly virulent pathogen capable of causing a severe hemorrhagic fever with 50-90% lethality. The EBOV glycoprotein (GP) is the only virally expressed protein on the virion surface and is critical for attachment to host cells and catalysis of membrane fusion. Hence, the EBOV GP is a critical component of vaccines as well as a target of neutralizing antibodies and inhibitors of attachment and fusion. The crystal structure of the Zaire ebolavirus GP in its trimeric, prefusion conformation (3 GP(1) plus 3 GP(2)) in complex with a neutralizing antibody fragment, derived from a human survivor of the 1995 Kikwit outbreak, was recently determined. This is the first near-complete structure of any filovirus glycoprotein. The overall molecular architecture of the Zaire ebolavirus GP and its role in viral entry and membrane fusion are discussed in this article.

Zaire Ebola virus entry into human dendritic cells is insensitive to cathepsin L inhibition

Cathepsins B and L contribute to Ebola virus (EBOV) entry into Vero cells and mouse embryonic fibroblasts. However, the role of cathepsins in EBOV-infection of human dendritic cells (DCs), important targets of infection in vivo, remains undefined. Here, EBOV-like particles containing a beta-lactamase-VP40 fusion reporter and Ebola virus were used to demonstrate the cathepsin dependence of EBOV entry into human monocyte-derived DCs. However, while DC infection is blocked by cathepsin B inhibitor, it is insensitive to cathepsin L inhibitor. Furthermore, DCs pre-treated for 48 h with TNFalpha were generally less susceptible to entry and infection by EBOV. This decrease in infection was associated with a decrease in cathepsin B activity. Thus, cathepsin L plays a minimal, if any, role in EBOV infection in human DCs. The inflammatory cytokine TNFalpha modulates cathepsin B activity and affects EBOV entry into and infection of human DCs.

Rho GTPases modulate entry of Ebola virus and vesicular stomatitis virus pseudotyped vectors

To explore mechanisms of entry for Ebola virus (EBOV) glycoprotein (GP) pseudotyped virions, we used comparative gene analysis to identify genes whose expression correlated with viral transduction. Candidate genes were identified by using EBOV GP pseudotyped virions to transduce human tumor cell lines that had previously been characterized by cDNA microarray. Transduction profiles for each of these cell lines were generated, and a significant positive correlation was observed between RhoC expression and permissivity for EBOV vector transduction. This correlation was not specific for EBOV vector alone as RhoC also correlated highly with transduction of vesicular stomatitis virus GP (VSVG) pseudotyped vector. Levels of RhoC protein in EBOV and VSV permissive and nonpermissive cells were consistent with the cDNA gene array findings. Additionally, vector transduction was elevated in cells that expressed high levels of endogenous RhoC but not RhoA. RhoB and RhoC overexpression significantly increased EBOV GP and VSVG pseudotyped vector transduction but had minimal effect on human immunodeficiency virus (HIV) GP pseudotyped HIV or adeno-associated virus 2 vector entry, indicating that not all virus uptake was enhanced by expression of these molecules. RhoB and RhoC overexpression also significantly enhanced VSV infection. Similarly, overexpression of RhoC led to a significant increase in fusion of EBOV virus-like particles. Finally, ectopic expression of RhoC resulted in increased nonspecific endocytosis of fluorescent dextran and in formation of increased actin stress fibers compared to RhoA-transfected cells, suggesting that RhoC is enhancing macropinocytosis. In total, our studies implicate RhoB and RhoC in enhanced productive entry of some pseudovirions and suggest the involvement of actin-mediated macropinocytosis as a mechanism of uptake of EBOV GP and VSVG pseudotyped viral particles.

Analysis of hemagglutinin-mediated entry tropism of H5N1 avian influenza

BACKGROUND

Avian influenza virus H5N1 is a major concern as a potential global pandemic. It is thought that multiple key events must take place before efficient human-to-human transmission of the virus occurs. The first step in overcoming host restriction is viral entry which is mediated by HA, responsible for both viral attachment and viral/host membrane fusion. HA binds to glycans-containing receptors with terminal sialic acid (SA). It has been shown that avian influenza viruses preferentially bind to alpha2,3-linked SAs, while human influenza A viruses exhibit a preference for alpha2,6-linked SAs. Thus it is believed the precise linkage of SAs on the target cells dictate host tropism of the viruses.

RESULTS

We demonstrate that H5N1 HA/HIV pseudovirus can efficiently transduce several human cell lines including human lung cells. Interestingly, using a lectin binding assay we show that the presence of both alpha2,6-linked and alpha2,3-linked SAs on the target cells does not always correlate with efficient transduction. Further, HA substitutions of the residues implicated in switching SA-binding between avian and human species did not drastically affect HA-mediated transduction of the target cells or target cell binding.

CONCLUSION

Our results suggest that a host factor(s), which is yet to be identified, is required for H5N1 entry in the host cells.

Comparison of vesicular stomatitis virus pseudotyped with the S proteins from a porcine and a human coronavirus

The surface proteins S of severe acute respiratory syndrome coronavirus (SARS-CoV) and transmissible gastroenteritis virus (TGEV) were compared for their ability to mediate infection of viral pseudotypes based on vesicular stomatitis virus (VSV). The cell tropism of the respective pseudotypes corresponded to the tropism of the viruses from which the S protein was derived. Higher infectivity values were obtained with the SARS-CoV S protein than with the TGEV S protein. Differences were observed with respect to the importance of the cytoplasmic tail and the membrane anchor of the S proteins. In the case of the SARS-CoV S protein, truncation of the cytoplasmic tail resulted in increased infectivity. For the TGEV S protein, the inactivation of an intracellular retention signal in the cytoplasmic tail was required. Exchange of the membrane anchor of the S proteins led to a low infection efficiency. Our results indicate that related glycoproteins may show substantial differences in their ability to mediate pseudotype infection.

Protection against filovirus infection: virus-like particle vaccines

Significant progress has been made in vaccine development against infection by Ebola and Marburg viruses, members of the Filoviridae, which cause severe hemorrhagic fevers in humans with no effective treatment and a mortality rate of up to 90%. Several vaccine strategies have been shown to effectively protect immunized animals against filovirus infection. Among these candidate vaccine strategies, virus-like particles represent a promising approach and have been shown to protect small laboratory animals as well as nonhuman primates against lethal challenge by Ebola and/or Marburg viruses. This review briefly summarizes filovirus epidemiology and pathogenesis, and focuses on the discussion of recent advances in filovirus vaccine development and the current understanding of protective immune responses against filovirus infection with an emphasis on the progress and challenge of filovirus virus-like particle vaccine development.

Anti-HIV activity mediated by natural killer and CD8+ cells after toll-like receptor 7/8 triggering

We previously found that triggering TLR7/8 either by single stranded HIV RNA or synthetic compounds induced changes in the lymphoid microenvironment unfavorable to HIV. In this study, we used selective TLR7 and 8 agonists to dissect their contribution to the anti-HIV effects. While triggering TLR7 inhibited efficiently HIV replication in lymphoid suspension cells from tonsillar origin, its effect was inconsistent in peripheral blood mononuclear cells (PBMC). In contrast, triggering TLR8 showed a very prominent and overall very consistent effect in PBMC and tonsillar lymphoid suspension cells. Depletion of dendritic cells (DC), Natural killer cells (NK) and CD8+ T-cells from PBMC resulted in the reversal of TLR8 induced anti-HIV effects. Especially noteworthy, depletion of either NK or CD8+ T-cells alone was only partially effective. We interpret these findings that DC are the initiator of complex changes in the microenvironment that culminates in the anti-HIV active NK and CD8+ effector cells. The near lack of NK and the low number of CD8+ T-cells in tonsillar lymphoid suspension cells may explain the lower TLR8 agonist's anti-HIV effects in that tissue. However, additional cell-type specific differences must exist since the TLR7 agonists had a very strong inhibitory effect in tonsillar lymphoid suspension cells. Separation of effector from the CD4+ target cells did not abolish the anti-HIV effects pointing to the critical role of soluble factors. Triggering TLR7 or 8 were accompanied by major changes in the cytokine milieu; however, it appeared that not a single soluble factor could be assigned for the potent effects.

These results delineate the complex effects of triggering TLR7/8 for an efficient antiviral defense. While the ultimate mechanism(s) remains unknown, the potent effects described may have therapeutic value for treating chronic viral diseases. Notably, HIV replication is blocked by TLR triggering before HIV integrates into the host chromosome which would prevent the establishment or maintenance of the latent reservoir.

Analysis of filovirus entry into vero e6 cells, using inhibitors of endocytosis, endosomal acidification, structural integrity, and cathepsin (B and L) activity

Ebola and Marburg viruses are believed to enter host cells by receptor-mediated endocytosis. The process has been studied through the use of inhibitors that affect host cell properties and recombinant pseudotyping systems in which filovirus structural glycoproteins mediate entry of foreign virus particles. The aim of the present study was to determine the effects of such treatments on the entry of wild-type filoviruses. Vero E6 cells were exposed to various inhibitors before, during, and after infection with filoviruses. Infected cultures were harvested early (18-24 h) and late (72 h) after infection, and effects of treatment on entry were measured by fluorescent antibody staining of cells or by antigen capture immunoassays, respectively. These prelimary results suggest that filoviruses enter host cells through receptor-mediated endocytosis via clathrin-coated pits and caveolae, that actin filaments and microtubules are important in the entry process, and that proteolytic digestion of glycoprotein 1 by endosomal proteases facilitates entry. These observations obtained using wild-type viruses confirm the results of studies utilizing recombinant systems and offer additional insights into filovirus entry.

Ebolavirus and Marburgvirus: insight the Filoviridae family

Ebolavirus and Marburgvirus (belonging to the Filoviridae family) emerged four decades ago and cause epidemics of haemorrhagic fever with high case-fatality rates. The genome of filoviruses encodes seven proteins. No significant homology is observed between filovirus proteins and any known macromolecule. Moreover, Marburgvirus and Ebolavirus show significant differences in protein homology. The natural maintenance cycle of filoviruses is unknown, the natural reservoir, the mode of transmission, the epidemic disease generation, and temporal dynamics are unclear. Lastly, Ebolavirus and Marburgvirus are considered as potential biological weapons. Vaccine appears the unique therapeutic frontier. Here, molecular and clinical aspects of filoviral haemorrhagic fevers are summarized.

Proteolysis of the Ebola virus glycoproteins enhances virus binding and infectivity

Cellular cathepsins are required for Ebola virus infection and are believed to proteolytically process the Ebola virus glycoprotein (GP) during entry. However, the significance of cathepsin cleavage during infection remains unclear. Here we demonstrate a role for cathepsin L (CatL) cleavage of Ebola virus GP in the generation of a stable 18-kDa GP1 viral intermediate that exhibits increased binding to and infectivity for susceptible cell targets. Cell binding to a lymphocyte line was increased when CatL-proteolysed pseudovirions were used, but lymphocytes remained resistant to Ebola virus GP-mediated infection. Genetic removal of the highly glycosylated mucin domain in Ebola virus GP resulted in cell binding similar to that observed with CatL-treated full-length GP, and no overall enhancement of binding or infectivity was observed when mucin-deleted virions were treated with CatL. These results suggest that cathepsin cleavage of Ebola virus GP facilitates an interaction with a cellular receptor(s) and that removal of the mucin domain may facilitate receptor binding. The influence of CatL in Ebola virus GP receptor binding should be useful in future studies characterizing the mechanism of Ebola virus entry.

Micro-scale signature of purifying selection in Marburg virus genomes

In the seven protein-coding genes in the Marburg virus (MARV) genome, the synonymous nucleotide diversity substantially exceeded the nonsynonymous nucleotide diversity, indicating strong purifying selection. Likewise, there was evidence of purifying selection on 5'UTR and 3'UTR, where nucleotide diversity (pi) was significantly less than piS in the coding regions. Nonsynonymous polymorphic sites showed significantly reduced mean gene diversity in comparison to other polymorphic sites, indicating that purifying selection at certain slightly deleterious nonsynonymous polymorphisms is ongoing. Moreover, nonsynonymous polymorphic sites showed significantly reduced gene diversity in comparison to adjacent synonymous sites, even though the vast majority of such adjacent synonymous sites were in the same codon or an adjacent codon. Thus purifying selection, in conjunction with recombination and/or backward mutation, can act to break up linkage relationships at a micro-scale in the MARV genome. The ability of purifying selection to break up linkage between synonymous and nonsynonymous polymorphisms on such a fine scale has not been reported in any other genome.

Development of vaccines for Marburg hemorrhagic fever

Marburg (MARV) and Ebola viruses (EBOV) emerged from the rainforests of Central Africa more than 30 years ago causing outbreaks of severe and, usually, fatal hemorrhagic fever. EBOV has garnered the lion's share of the attention, fueled by the higher frequency of EBOV outbreaks, high mortality rates and importation into the USA, documented in such popular works as the best-selling novel 'The Hot Zone'. However, recent large outbreaks of hundreds of cases of MARV infection in the Democratic Republic of the Congo and Angola with case fatalities approaching 90% dramatically highlight its lethal potential. Although no vaccines or antiviral drugs for MARV are currently available, remarkable progress has been made over the last few years in developing potential countermeasures against MARV in nonhuman primate models. In particular, a vaccine based on attenuated recombinant vesicular stomatitis virus was recently shown to have both preventive and postexposure efficacy.

Gene transfer in human skin with different pseudotyped HIV-based vectors

Pseudotyping lentiviral vector with other viral surface proteins could be applied for treating genetic anomalies in human skin. In this study, the modification of HIV vector tropism by pseudotyping with the envelope glycoprotein from vesicular stomatitis virus (VSV), the Zaire Ebola (EboZ) virus, murine leukemia virus (MuLV), lymphocytic choriomeningitis virus (LCMV), Rabies or the rabies-related Mokola virus encoding LacZ as a reporter gene was evaluated qualitatively and quantitatively in human skin xenografts. High transgene expression was detected in dermal fibroblasts transduced with VSV-G-, EboZ- or MuLV-pseudotyped HIV vector with tissue irregularities in the dermal compartments following repeated injections of EboZ- or LCMV-pseudotyped vectors. Four weeks after transduction, double-labeling immunofluorescence of beta-galactosidase and involucrin or integrin beta1 demonstrated that VSV-G-, EboZ- or MuLV-pseudotyped HIV vector effectively targeted quiescent epidermal stem cells which underwent terminal differentiation resulting in transgene expression in their progenies. Among the six different pseudotyped HIV-based vectors evaluated, VSV-G-pseudotyped vector was found to be the most efficient viral glycoprotein for cutaneous transduction as demonstrated by the highest level of beta-galactosidase expression and genome copy number evaluated by TaqMan PCR.

The ERK mitogen-activated protein kinase pathway contributes to Ebola virus glycoprotein-induced cytotoxicity

Ebola virus is a highly lethal pathogen that causes hemorrhagic fever in humans and nonhuman primates. Among the seven known viral gene products, the envelope glycoprotein (GP) alone induces cell rounding and detachment that ultimately leads to cell death. Cellular cytoxicity is not seen with comparable levels of expression of a mutant form of GP lacking a mucin-like domain (GPDeltamuc). GP-induced cell death is nonapoptotic and is preceded by downmodulation of cell surface molecules involved in signaling pathways, including certain integrins and epidermal growth factor receptor. To investigate the mechanism of GP-induced cellular toxicity, we analyzed the activation of several signal transduction pathways involved in cell growth and survival. The active form of extracellular signal-regulated kinases types 1 and 2 (ERK1/2), phospho-ERK1/2, was reduced in cells expressing GP compared to those expressing GPDeltamuc as determined by flow cytometry, in contrast to the case for several other signaling proteins. Subsequent analysis of the activation states and kinase activities of related kinases revealed a more pronounced effect on the ERK2 kinase isoform. Disruption of ERK2 activity by a dominant negative ERK or by small interfering RNA-mediated ERK2 knockdown potentiated the decrease in alphaV integrin expression associated with toxicity. Conversely, activation of the pathway through the expression of a constitutively active form of ERK2 significantly protected against this effect. These results indicate that the ERK signaling cascade mediates GP-mediated cytotoxicity and plays a role in pathogenicity induced by this gene product.

Tyro3 family-mediated cell entry of Ebola and Marburg viruses

Filoviruses, represented by the genera Ebolavirus and Marburgvirus, cause a lethal hemorrhagic fever in humans and in nonhuman primates. Although filovirus can replicate in various tissues or cell types in these animals, the molecular mechanisms of its broad tropism remain poorly understood. Here we show the involvement of members of the Tyro3 receptor tyrosine kinase family-Axl, Dtk, and Mer-in cell entry of filoviruses. Ectopic expression of these family members in lymphoid cells, which otherwise are highly resistant to filovirus infection, enhanced infection by pseudotype viruses carrying filovirus glycoproteins on their envelopes. This enhancement was reduced by antibodies to Tyro3 family members, Gas6 ligand, or soluble ectodomains of the members. Live Ebola viruses infected both Axl- and Dtk-expressing cells more efficiently than control cells. Antibody to Axl inhibited infection of pseudotype viruses in a number of Axl-positive cell lines. These results implicate each Tyro3 family member as a cell entry factor in filovirus infection.

[Properties of the Ebola virus glycoprotein]

In central and west Africa, Ebola virus, a member of the filovirus group, has produced sporadic outbreaks of lethal disease. This virus causes hemorrhagic fever in humans and nonhuman primates, resulting in mortality rates of up to 90%. Although there are no satisfactory biologic explanations for this extreme virulence, it has been suggested that functions of the envelope glycoprotein are likely to play important roles in the pathogenicity of Ebola virus.

Ebola virus glycoprotein GP is not cytotoxic when expressed constitutively at a moderate level

Transient expression of Ebola virus (EBOV) glycoprotein GP causes downregulation of surface proteins, cell rounding and detachment, a phenomenon believed to play a central role in the pathogenicity of the virus. In this study, evidence that moderate expression of GP does not result in such morphological changes was provided. It was shown that GP continuously produced in 293T cells from the Kunjin virus replicon was correctly processed and transported to the plasma membrane without affecting the surface expression of β1 and α5 integrins and major histocompatibility complex I molecules. The level of GP expression in Kunjin replicon GP-expressing cells was similar to that observed in cells infected with EBOV early in infection and lower than that produced in cells transfected with plasmid DNA, phCMV-GP, expressing GP from a strong promoter. Importantly, transient transfection of Kunjin replicon GP-expressing cells with GP-coding plasmid DNA resulted in overexpression of GP, which lead to the downregulation of surface molecules and massive rounding and detachment of transfected cells. Here, it was also demonstrated that cell rounding and downregulation of the surface markers are the late events in EBOV infection, whereas synthesis and massive release of virus particles occur at early steps and do not cause significant cytotoxic effects. These findings indicate that the synthesis of EBOV GP in virus-infected cells is controlled well by several mechanisms that do not allow GP overexpression and hence the early appearance of its cytotoxic properties.

Conserved receptor-binding domains of Lake Victoria marburgvirus and Zaire ebolavirus bind a common receptor

The GP(1,2) envelope glycoproteins (GP) of filoviruses (marburg- and ebolaviruses) mediate cell-surface attachment, membrane fusion, and entry into permissive cells. Here we show that a 151-amino acid fragment of the Lake Victoria marburgvirus GP1 subunit bound filovirus-permissive cell lines more efficiently than full-length GP1. An homologous 148-amino acid fragment of the Zaire ebolavirus GP1 subunit similarly bound the same cell lines more efficiently than a series of longer GP1 truncation variants. Neither the marburgvirus GP1 fragment nor that of ebolavirus bound a nonpermissive lymphocyte cell line. Both fragments specifically inhibited replication of infectious Zaire ebolavirus, as well as entry of retroviruses pseudotyped with either Lake Victoria marburgvirus or Zaire ebolavirus GP(1,2). These studies identify the receptor-binding domains of both viruses, indicate that these viruses utilize a common receptor, and suggest that a single small molecule or vaccine can be developed to inhibit infection of all filoviruses.

TLR7/8 Triggering Exerts Opposing Effects in Acute versus Latent HIV Infection

TLRs trigger innate immunity by recognizing conserved motifs of microorganisms. Recently, ssRNAs from HIV and influenza virus were shown to trigger TLR7 and 8. Thus, we hypothesized that HIV ssRNA, by triggering TLR7/8, affects HIV pathogenesis. Indeed, HIV ssRNA rendered human lymphoid tissue of tonsillar origin or PBMC barely permissive to HIV replication. The synthetic compound R-848, which also triggers TLR7/8, showed similar anti-HIV activity. Loss of R-848's activity in lymphoid tissue depleted of B cells suggested a role for B cells in innate immunity. TLR7/8 triggering appears to exert antiviral effects through soluble factors: conditioned medium reduced HIV replication in indicator cells. Although a number of cytokines and chemokines were increased upon adding R-848 to lymphoid tissue, blocking those cytokines/chemokines (i.e., IFN-alpha receptor, IFN-gamma, MIP-1alpha, -1beta, RANTES, and stromal cell-derived factor-1) did not result in the reversal of R-848's anti-HIV activity. Thus, the nature of this soluble factor(s) remains unknown. Unlike lymphoid tissue acutely infected with HIV, triggering latently infected promonocytic cells induced the release of HIV virions. The anti-HIV effects of triggering TLR7/8 may inhibit rapid killing, while pro-HIV effects may guarantee a certain replication level. Compounds triggering TLR7/8 may be attractive drug candidates to purge latent HIV while preventing new infections.

Lentiviral vectors

We review the use of lentiviral vectors in current human gene therapy applications that involve genetic modification of nondividing tissues with integrated transgenes. Safety issues, including insertional mutagenesis and replication-competent retroviruses, are discussed. Innate cellular defenses against retroviruses and their implications for human gene therapy with different lentiviral vectors are also addressed.

Mannose-binding lectin binds to Ebola and Marburg envelope glycoproteins, resulting in blocking of virus interaction with DC-SIGN and complement-mediated virus neutralization

Mannose-binding lectin (MBL), a serum lectin that mediates innate immune functions including activation of the lectin complement pathway, binds to carbohydrates expressed on some viral glycoproteins. In this study, the ability of MBL to bind to virus particles pseudotyped with Ebola and Marburg envelope glycoproteins was evaluated. Virus particles bearing either Ebola (Zaire strain) or Marburg (Musoke strain) envelope glycoproteins bound at significantly higher levels to immobilized MBL compared with virus particles pseudotyped with vesicular stomatitis virus glycoprotein or with no virus glycoprotein. As observed in previous studies, Ebola-pseudotyped virus bound to cells expressing the lectin DC-SIGN (dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin). However, pre-incubation of virus with MBL blocked DC-SIGN-mediated binding to cells, suggesting that the two lectins bind at the same or overlapping sites on the Ebola glycoprotein. Neutralization experiments showed that virus pseudotyped with Ebola or Marburg (Musoke) glycoprotein was neutralized by complement, while the Marburg (Ravn strain) glycoprotein-pseudotyped virus was less sensitive to neutralization. Neutralization was partially mediated through the lectin complement pathway, since a complement source deficient in MBL was significantly less effective at neutralizing viruses pseudotyped with filovirus glycoproteins and addition of purified MBL to the MBL-deficient complement increased neutralization. These experiments demonstrated that MBL binds to filovirus envelope glycoproteins resulting in important biological effects and suggest that MBL can interact with filoviruses during infection in humans.

Altering the tropism of lentiviral vectors through pseudotyping

The host range of retroviral vectors including lentiviral vectors can be expanded or altered by a process known as pseudotyping. Pseudotyped lentiviral vectors consist of vector particles bearing glycoproteins (GPs) derived from other enveloped viruses. Such particles possess the tropism of the virus from which the GP was derived. For example, to exploit the natural neural tropism of rabies virus, vectors designed to target the central nervous system have been pseudotyped using rabies virus-derived GPs. Among the first and still most widely used GPs for pseudotyping lentiviral vectors is the vesicular stomatitis virus GP (VSV-G), due to the very broad tropism and stability of the resulting pseudotypes. Pseudotypes involving VSV-G have become effectively the standard for evaluating the efficiency of other pseudotypes. This review samples a few of the more prominent examples from the ever-expanding list of published lentiviral pseudotypes, noting comparisons made with pseudotypes involving VSV-G in terms of titer, viral particle stability, toxicity, and host-cell specificity. Particular attention is paid to publications of successfully targeting a specific organ or cell types.

Comprehensive analysis of ebola virus GP1 in viral entry

Ebola virus infection is initiated by interactions between the viral glycoprotein GP1 and its cognate receptor(s), but little is known about the structure and function of GP1 in viral entry, partly due to the concern about safety when working with the live Ebola virus and the difficulty of manipulating the RNA genome of Ebola virus. In this study, we have used a human immunodeficiency virus-based pseudotyped virus as a surrogate system to dissect the role of Ebola virus GP1 in viral entry. Analysis of more than 100 deletion and amino acid substitution mutants of GP1 with respect to protein expression, processing, viral incorporation, and viral entry has allowed us to map the region of GP1 responsible for viral entry to the N-terminal 150 residues. Furthermore, six amino acids in this region have been identified as critical residues for early events in Ebola virus entry, and among these, three are clustered and are implicated as part of a potential receptor-binding pocket. In addition, substitutions of some 30 residues in GP1 are shown to adversely affect GP1 expression, processing, and viral incorporation, suggesting that these residues are involved in the proper folding and/or overall conformation of GP. Sequence comparison of the GP1 proteins suggests that the majority of the critical residues for GP folding and viral entry identified in Ebola virus GP1 are conserved in Marburg virus. These results provide information for elucidating the structural and functional roles of the filoviral glycoproteins and for developing potential therapeutics to block viral entry.

Viral delivery of recombinant short hairpin RNAs

Recent work demonstrates that RNA interference (RNAi) can coordinate protein expression. Inhibitory RNAs are expressed naturally in cells as microRNAs (miRNAs) or introduced into cells as small interfering RNAs (siRNAs). Both types of small RNAs can be used at the bench to silence mRNA expression. For many researchers, transfection of siRNAs synthesized in vitro or purchased from commercial sources is impractical for the cellular system under study. As an alternative to transfection-based methods, we provide a practical approach to accomplish siRNA-mediated gene silencing through the generation and introduction of recombinant viral vectors expressing short hairpin RNAs (shRNAs). shRNAs are subsequently processed to siRNAs in vivo, leading to efficient, and, in some cases, long-term silencing.

Studies of ebola virus glycoprotein-mediated entry and fusion by using pseudotyped human immunodeficiency virus type 1 virions: involvement of cytoskeletal proteins and enhancement by tumor necrosis factor alpha

The Ebola filoviruses are aggressive pathogens that cause severe and often lethal hemorrhagic fever syndromes in humans and nonhuman primates. To date, no effective therapies have been identified. To analyze the entry and fusion properties of Ebola virus, we adapted a human immunodeficiency virus type 1 (HIV-1) virion-based fusion assay by substituting Ebola virus glycoprotein (GP) for the HIV-1 envelope. Fusion was detected by cleavage of the fluorogenic substrate CCF2 by beta-lactamase-Vpr incorporated into virions and released as a result of virion fusion. Entry and fusion induced by the Ebola virus GP occurred with much slower kinetics than with vesicular stomatitis virus G protein (VSV-G) and were blocked by depletion of membrane cholesterol and by inhibition of vesicular acidification with bafilomycin A1. These properties confirmed earlier studies and validated the assay for exploring other properties of Ebola virus GP-mediated entry and fusion. Entry and fusion of Ebola virus GP pseudotypes, but not VSV-G or HIV-1 Env pseudotypes, were impaired in the presence of the microtubule-disrupting agent nocodazole but were enhanced in the presence of the microtubule-stabilizing agent paclitaxel (Taxol). Agents that impaired microfilament function, including cytochalasin B, cytochalasin D, latrunculin A, and jasplakinolide, also inhibited Ebola virus GP-mediated entry and fusion. Together, these findings suggest that both microtubules and microfilaments may play a role in the effective trafficking of vesicles containing Ebola virions from the cell surface to the appropriate acidified vesicular compartment where fusion occurs. In terms of Ebola virus GP-mediated entry and fusion to various target cells, primary macrophages proved highly sensitive, while monocytes from the same donors displayed greatly reduced levels of entry and fusion. We further observed that tumor necrosis factor alpha, which is released by Ebola virus-infected monocytes/macrophages, enhanced Ebola virus GP-mediated entry and fusion to human umbilical vein endothelial cells. Thus, Ebola virus infection of one target cell may induce biological changes that facilitate infection of secondary target cells that play a key role in filovirus pathogenesis. Finally, these studies indicate that pseudotyping in the HIV-1 virion-based fusion assay may be a valuable approach to the study of entry and fusion properties mediated through the envelopes of other viral pathogens.

Genetic manipulation of hematopoietic stem cells

Over the past two decades, the ability to transfer genes into hematopoietic stem cells (HSCs) has provided new insights into the behavior of individual stem cells and offered a novel approach for the treatment of various inherited or acquired disorders. At present, gene transfer into HSCs has been achieved mainly using modified retroviruses. While retrovirus-based vectors could efficiently transduce murine HSCs, extrapolation of these methods to large mammals and human clinical trials resulted in very low numbers of gene-marked engrafted cells. In addition, in vitro progenitor assays used to optimize gene transfer procedures were found to poorly predict the outcome of stem cell gene transfer. The focus rapidly turned to the development of superior and more relevant preclinical assays in human stem cell gene transfer research. Xenogeneic transplant models and large animal transplantation system have been invaluable. The development of better assays for evaluating human gene therapy protocols and a better understanding of stem cell and vector biology has culminated over the past decade in multiple strategies to improve gene transfer efficiency into HSCs. Improved gene transfer vectors, optimization of cytokine combination, and incorporation of a recombinant fragment of fibronectin during transduction are examples of novel successful additions to the early gene transfer protocols that have contributed to the first unequivocal clinical benefits resulting from genetic manipulation of HSC.

Lentivirus-mediated gene transfer to the respiratory epithelium: a promising approach to gene therapy of cystic fibrosis

Gene therapy of cystic fibrosis (CF) lung disease needs highly efficient delivery and long-lasting complementation of the CFTR (cystic fibrosis transmembrane conductance regulator) gene into the respiratory epithelium. The development of lentiviral vectors has been a recent advance in the field of gene transfer and therapy. These integrating vectors appear to be promising vehicles for gene delivery into respiratory epithelial cells by virtue of their ability to infect nondividing cells and mediate long-term persistence of transgene expression. Studies in human airway tissues and animal models have highlighted the possibility of achieving gene expression by lentiviral vectors, which outlasted the normal lifespan of the respiratory epithelium, indicating targeting of a 'stem cell' compartment. Modification of the paracellular permeability and pseudotyping with heterologous envelopes are the strategies currently used to overcome the paucity of specific viral receptors on the apical surface of airway epithelial cells and to reach the basolateral surface receptors. Preclinical studies on CF mice, demonstrating complementation of the CF defect, offer hope that lentivirus gene therapy can be translated into an effective treatment of CF lung disease. Besides a direct targeting of the stem/progenitor niche(s) in the CF airways, an alternative approach may envision homing of hematopoietic stem cells engineered to express the CFTR gene by lentiviral vectors. In the context of lentivirus-mediated CFTR gene transfer to the CF airways, biosafety aspects should be of primary concern.

Ebola virus glycoproteins: guidance devices for targeting gene therapy vectors

Replacing the native viral envelope protein on the surface of a retrovirus or lentivirus with the glycoprotein of a foreign enveloped virus, a process called pseudotyping, can expand the set of potential target cells for a viral vector or can restrict entry to specific cells. The Ebola virus glycoprotein, because of its evolutionary origins and the route of viral entry promoted by it, possesses distinct advantages in forming the outer shell of such pseudotyped retroviruses for gene therapy applications. Studies of the transduction of human airway epithelia by lentivirus pseudotyped with a modified Ebola virus glycoprotein from which the region of O- glycosylation has been removed have demonstrated that such recombinant viruses possess particular promise for the treatment of cystic fibrosis. This result highlights the synergism between basic studies of virus entry and gene therapy advances.

Ebola virus glycoprotein-mediated anoikis of primary human cardiac microvascular endothelial cells

Ebola virus glycoprotein (EGP) has been implicated for the induction of cytotoxicity and injury in vascular cells. On the other hand, EGP has also been suggested to induce massive cell rounding and detachment from the plastic surface by downregulating cell adhesion molecules without causing cytotoxicity. In this study, we have examined the cytotoxic role of EGP in primary endothelial cells by transduction with a replication-deficient recombinant adenovirus expressing EGP (Ad-EGP). Primary human cardiac microvascular endothelial cells (HCMECs) transduced with Ad-EGP displayed loss of cell adhesion from the plastic surface followed by cell death. Transfer of conditioned medium from EGP-transduced HCMEC into naive cells did not induce loss of adhesion or cell death, suggesting that EGP needs to be expressed intracellularly to exert its cytotoxic effect. Subsequent studies suggested that HCMEC death occurred through apoptosis. Results from this study shed light on the EGP-induced anoikis in primary human cardiac endothelial cells, which may have significant pathological consequences.

Transduction of bone-marrow-derived mesenchymal stem cells by using lentivirus vectors pseudotyped with modified RD114 envelope glycoproteins

Bone-marrow-derived mesenchymal stem cells (MSCs) have attracted considerable attention as tools for the systemic delivery of therapeutic proteins in vivo, and the ability to efficiently transfer genes of interest into such cells would create a number of therapeutic opportunities. We have designed and tested a series of human immunodeficiency virus type 1 (HIV-1)-based vectors and vectors based on the oncogenic murine stem cell virus to deliver and express transgenes in human MSCs. These vectors were pseudotyped with either the vesicular stomatitis virus G (VSV-G) glycoprotein (GP) or the feline endogenous virus RD114 envelope GP. Transduction efficiencies and transgene expression levels in MSCs were analyzed by quantitative flow cytometry and quantitative real-time PCR. While transduction efficiencies with virus particles pseudotyped with the VSV-G GP were found to be high, RD114 pseudotypes revealed transduction efficiencies that were 1 to 2 orders of magnitude below those observed with VSV-G pseudotypes. However, chimeric RD114 GPs, with the transmembrane and extracellular domains fused to the cytoplasmic domain derived from the amphotropic Moloney murine leukemia virus 4070A GP, revealed about 15-fold higher titers relative to the unmodified RD114 GP. The transduction efficiencies in human MSCs of HIV-1-based vectors pseudotyped with the chimeric RD114 GP were similar to those obtained with HIV-1 vectors pseudotyped with the VSV-G GP. Our results also indicate that RD114 pseudotypes were less toxic than VSV-G pseudotypes in human MSC progenitor assays. Taken together, these results suggest that lentivirus pseudotypes bearing alternative Env GPs provide efficient tools for ex vivo modification of human MSCs.

Production of novel ebola virus-like particles from cDNAs: an alternative to ebola virus generation by reverse genetics

We established a plasmid-based system for generating infectious Ebola virus-like particles (VLPs), which contain an Ebola virus-like minigenome consisting of a negative-sense copy of the green fluorescent protein gene. This system produced nearly 10(3) infectious particles per ml of supernatant, equivalent to the titer of Ebola virus generated by a reverse genetics system. Interestingly, infectious Ebola VLPs were generated, even without expression of VP24. Transmission and scanning electron microscopic analyses showed that the morphology of the Ebola VLPs was indistinguishable from that of authentic Ebola virus. Thus, this system allows us to study Ebola virus entry, replication, and assembly without biosafety level 4 containment. Furthermore, it may be useful in vaccine production against this highly pathogenic agent.

Mannosyl glycodendritic structure inhibits DC-SIGN-mediated Ebola virus infection in cis and in trans

We have designed a glycodendritic structure, BH30sucMan, that blocks the interaction between dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and Ebola virus (EBOV) envelope. BH30sucMan inhibits DC-SIGN-mediated EBOV infection at nanomolar concentrations. BH30sucMan may counteract important steps of the infective process of EBOV and, potentially, of microorganisms shown to exploit DC-SIGN for cell entry and infection.

Folate receptor alpha and caveolae are not required for Ebola virus glycoprotein-mediated viral infection

Folate receptor alpha (FRalpha) has been described as a factor involved in mediating Ebola virus entry into cells (6). Furthermore, it was suggested that interaction with FRalpha results in internalization and subsequent viral ingress into the cytoplasm via caveolae (9). Descriptions of cellular receptors for Ebola virus and its entry mechanisms are of fundamental importance, particularly with the advent of vectors bearing Ebola virus glycoprotein (GP) being utilized for gene transfer into cell types such as airway epithelial cells. Thus, the ability of FRalpha to mediate efficient entry of viral pseudotypes carrying GP was investigated. We identified cell lines and primary cell types such as macrophages that were readily infected by GP pseudotypes despite lacking detectable surface FRalpha, indicating that this receptor is not essential for Ebola virus infection. Furthermore, we find that T-cell lines stably expressing FRalpha are not infectible, suggesting that FRalpha is also not sufficient to mediate entry. T-cell lines lack caveolae, the predominant route of FRalpha-mediated folate metabolism. However, the coexpression of FRalpha with caveolin-1, the major structural protein of caveolae, was not able to rescue infectivity in a T-cell line. In addition, other cell types lacking caveolae are fully infectible by GP pseudotypes. Finally, a panel of ligands to and soluble analogues of FRalpha were unable to inhibit infection on a range of cell lines, questioning the role of FRalpha as an important factor for Ebola virus entry.

Transduction Patterns of Pseudotyped Lentiviral Vectors in the Nervous System

We have developed a non-primate-based lentiviral vector based on the equine infectious anemia virus (EIAV) for efficient gene transfer to the central and peripheral nervous systems. Previously we have demonstrated that pseudotyping lentiviral vectors with the rabies virus glycoprotein confers retrograde axonal transport to these vectors. In the present study we have successfully produced high-titer EIAV vectors pseudotyped with envelope glycoproteins from Rhabdovirus vesicular stomatitis virus (VSV) serotypes (Indiana and Chandipura strains); rabies virus [various Evelyn-Rokitnicki-Abelseth ERA strains and challenge virus standard (CVS)]; Lyssavirus Mokola virus, a rabies-related virus; and Arenavirus lymphocytic choriomeningitis virus (LCMV). These vectors were delivered to the striatum or spinal cord of adult rats or muscle of neonatal mice by direct injection. We report that the lentiviral vectors pseudotyped with envelopes from the VSV Indiana strain, wild-type ERA, and CVS strains resulted in strong transduction in the striatum, while Mokola- and LCMV-pseudotyped vectors exhibited moderate and weak transduction, respectively. Furthermore ERA- and CVS-pseudotyped lentiviral vectors demonstrated retrograde transport and expression in distal neurons after injection in brain, spinal cord, and muscle. The differences in transduction efficiencies and retrograde transport conferred by these envelope glycoproteins present novel opportunities in designing therapeutic strategies for different neurological diseases.

Molecular mechanisms of filovirus cellular trafficking

The filoviruses, Ebola and Marburg, are two of the most pathogenic viruses, causing lethal hemorrhagic fever in humans. Recent discoveries suggest that filoviruses, along with other phylogenetically or functionally related viruses, utilize a complex mechanism of replication exploiting multiple cellular components including lipid rafts, endocytic compartments, and vacuolar protein sorting machinery. In this review, we summarize these recent findings and discuss the implications for vaccine and therapeutics development.

Vaccine research efforts for filoviruses

Ebola and Marburg viruses belong to the family Filoviridae, and cause acute, frequently fatal, haemorrhagic fever in humans and non-human primates. No vaccines are available for human use. This review describes the status of research efforts to develop vaccines for these viruses and to identify the immune mechanisms of protection. The vaccine approaches discussed include DNA-based vaccines, and subunit vaccines vectored by adenovirus, alphavirus replicons, and vaccinia virus.

Biochemical and functional characterization of the Ebola virus VP24 protein: implications for a role in virus assembly and budding

The VP24 protein of Ebola virus is believed to be a secondary matrix protein and minor component of virions. In contrast, the VP40 protein of Ebola virus is the primary matrix protein and the most abundant virion component. The structure and function of VP40 have been well characterized; however, virtually nothing is known regarding the structure and function of VP24. Wild-type and mutant forms of VP24 were expressed in mammalian cells to gain a better understanding of the biochemical and functional nature of this viral protein. Results from these experiments demonstrated that (i) VP24 localizes to the plasma membrane and perinuclear region in both transfected and Ebola virus-infected cells, (ii) VP24 associates strongly with lipid membranes, (iii) VP24 does not contain N-linked sugars when expressed alone in mammalian cells, (iv) VP24 can oligomerize when expressed alone in mammalian cells, (v) progressive deletions at the N terminus of VP24 resulted in a decrease in oligomer formation and a concomitant increase in the formation of high-molecular-weight aggregates, and (vi) VP24 was present in trypsin-resistant virus like particles released into the media covering VP24-transfected cells. These data indicate that VP24 possesses structural features commonly associated with viral matrix proteins and that VP24 may have a role in virus assembly and budding.

Covalent modifications of the ebola virus glycoprotein

The role of covalent modifications of the Ebola virus glycoprotein (GP) and the significance of the sequence identity between filovirus and avian retrovirus GPs were investigated through biochemical and functional analyses of mutant GPs. The expression and processing of mutant GPs with altered N-linked glycosylation, substitutions for conserved cysteine residues, or a deletion in the region of O-linked glycosylation were analyzed, and virus entry capacities were assayed through the use of pseudotyped retroviruses. Cys-53 was the only GP(1) ( approximately 130 kDa) cysteine residue whose replacement resulted in the efficient secretion of GP(1), and it is therefore proposed that it participates in the formation of the only disulfide bond linking GP(1) to GP(2) ( approximately 24 kDa). We propose a complete cystine bridge map for the filovirus GPs based upon our analysis of mutant Ebola virus GPs. The effect of replacement of the conserved cysteines in the membrane-spanning region of GP(2) was found to depend on the nature of the substitution. Mutations in conserved N-linked glycosylation sites proved generally, with a few exceptions, innocuous. Deletion of the O-linked glycosylation region increased GP processing, incorporation into retrovirus particles, and viral transduction. Our data support a common evolutionary origin for the GPs of Ebola virus and avian retroviruses and have implications for gene transfer mediated by Ebola virus GP-pseudotyped retroviruses.

Lentiviral vectors for gene delivery into cells

Human immunodeficiency virus type I (HIV) is the etiologic agent of acquired immunodeficiency syndrome or AIDS. Vectors based upon HIV have been in use for over a decade. Beginning in 1996, with the demonstration of improved pseudotyping using vesicular stomatitis virus (VSV) G protein along with transduction of resting mammalian cells, a series of improvements have been made in these vectors, making them both safer and more efficacious. Taking a cue from vector development of murine leukemia virus (MLV), split coding and self-inactivating HIV vectors now appear quite suitable for phase I clinical trials. In parallel, a number of pre-clinical efficacy studies in animals have demonstrated the utility of these vectors for various diseases processes, especially neurodegenerative and hematopoietic illnesses. These vectors are also appropriate for the study of other viruses (specifically of viral entry) and investigation of the HIV replicative cycle, along with straightforward transgene delivery to target cells of interest. Vectors based upon other lentiviruses have shown similar abilities and promise. Although concerns remain, particularly with regards to detection and propagation of replication-competent lentivirus, it is almost certain that these vectors will be introduced into the clinic within the next 3-5 years.

Evidence against Ebola virus sGP binding to human neutrophils by a specific receptor

The issue of whether Ebola secretory glycoprotein (sGP) binds to human neutrophils via the IgG Fc receptor IIIb (FcgammaRIIIb, CD16b) or other receptors has been controversial. To clarify this, FACS analysis, an sGP absorption assay, and direct binding of (125)I-sGP to neutrophils were performed. Results from FACS analysis demonstrated that limited washing conditions leads to the nonspecific formation of immune complexes on the neutrophil surface and this, but not a specific interaction between sGP and CD16b, is responsible for the previous observations. An sGP absorption assay also demonstrated that sGP is not specifically bound but is nonspecifically proteolysed by proteases released from neutrophils. Finally, there was no difference in (125)I-sGP binding to neutrophils compared to other control cell types. Taken together, these results demonstrate that neutrophils do not express a specific receptor for Ebola virus sGP. It is unlikely that sGP plays a role in the Ebola virus pathogenesis through interfering with the innate immunity by targeting neutrophils.

Efficient transduction of liver and muscle after in utero injection of lentiviral vectors with different pseudotypes

In this study we investigate the efficacy of lentiviral vectors of different pseudotypes for gene transfer to tissues of the preimmune fetus. BALB/c fetuses at 14-15 days' gestation received lentiviral vectors carrying the transgene lacZ under the control of the human cytomegalovirus (CMV) promoter by intramuscular (i.m.) or intrahepatic (i.h.) injection. We pseudotyped the lentiviral vectors with vesicular stomatitis virus (VSV-G), with Mokola virus, or with Ebola virus envelope glycoproteins. We harvested the pups at time points between 5 days and 9 months following injection and performed a detailed histologic assessment. The efficiency and distribution of transduction after in utero administration was highly dependent upon the route of administration and the pseudotype of vector used. Biodistribution studies showed widespread distribution of vector sequences in multiple tissues, albeit at very low levels, and transduced cells were found in significant numbers only in liver, heart, and muscle. Overall, VSV-G was the most efficient in transducing hepatocytes, whereas Mokola and Ebola were more efficient in transducing myocytes. Transduction of cardiomyocytes was observed after both i.m. and i.h. injection of all three vectors. Our findings of long-term transduction of skeletal myocytes and cardiomyocytes after in utero administration suggest a novel strategy for the treatment of congenital muscular dystrophies.