Evidence for nonspecific adsorption of targeted retrovirus vector particles to cells

Enterprises and challenges in diagnostics for precision medicine: an interview with Eddie Blair

Interview with Dr Eddie Blair, PhD, by Claire Raison (Commissioning Editor) Dr Eddie Blair is Managing Director of Integrated Medicines Ltd (Cambridge, UK), a company he formed in 2003 to enable precision medicine by combining diagnostic testing with new and existing medicines. Dr Blair has raised angel and private equity investments in excess of £12 million, has published over 40 primary peer-reviewed papers, including a series on companion diagnostic valuation, and is named inventor on at least 12 patents. Dr Blair is a member of the Editorial Advisory Board of Expert Review of Molecular Diagnostics and speaks to the Commissioning Editor here about entrepreneurship, obstacles and potential of introducing diagnostics innovations into routine clinical practice.

Paclitaxel-loaded phosphonated calixarene nanovesicles as a modular drug delivery platform

A modular p-phosphonated calix[4]arene vesicle (PCV) loaded with paclitaxel (PTX) and conjugated with folic acid as a cancer targeting ligand has been prepared using a thin film-sonication method. It has a pH-responsive capacity to trigger the release of the encapsulated PTX payload under mildly acidic conditions. PTX-loaded PCV conjugated with alkyne-modified PEG-folic acid ligands prepared via click ligation (fP-PCVPTX) has enhanced potency against folate receptor (FR)-positive SKOV-3 ovarian tumour cells over FR-negative A549 lung tumour cells. Moreover, fP-PCVPTX is also four times more potent than the non-targeting PCVPTX platform towards SKOV-3 cells. Overall, as a delivery platform the PCVs have the potential to enhance efficacy of anticancer drugs by targeting a chemotherapeutic payload specifically to tumours and triggering the release of the encapsulated drug in the vicinity of cancer cells.

A biomimetic approach to hormone resistant prostate cancer cell isolation using inactivated Sendai virus (HVJ-E)

A versatile immobilization method of inactivated Sendai virus (HVJ-E) for metastatic prostate cancer cell isolation.

Shedding of clinical-grade lentiviral vectors is not detected in a gene therapy setting

Gene therapy using viral vectors that stably integrate into ex vivo cultured cells holds great promises for the treatment of monogenic diseases as well as cancer. However, carry-over of infectious vector particles has been described to occur upon ex vivo transduction of target cells. This, in turn, may lead to inadvertent spreading of viral particles to off-target cells in vivo, raising concerns for potential adverse effects, such as toxicity of ectopic transgene expression, immunogenicity from in vivo transduced antigen-presenting cells and, possibly, gene transfer to germline cells. Here, we have investigated factors influencing the extent of lentiviral vector (LV) shedding upon ex vivo transduction of human hematopoietic stem and progenitor cells. Our results indicate that, although vector carry-over is detectable when using laboratory-grade vector stocks, the use of clinical-grade vector stocks strongly decreases the extent of inadvertent transduction of secondary targets, likely because of the higher degree of purification. These data provide supportive evidence for the safe use of the LV platform in clinical settings.

Cell-cell transmission of VSV-G pseudotyped lentivector particles

Many replicating viruses, including HIV-1 and HTLV-1, are efficiently transmitted from the cell surface of actively infected cells upon contact with bystander cells. In a previous study, we reported the prolonged cell surface retention of VSV-G replication-deficient pseudotyped lentivector prior to endocytic entry. However, the competing kinetics of cell surface versus dissociation, neutralization or direct transfer to other cells have received comparatively little attention. Here we demonstrate that the relative efficiency of cell-cell surface transmission can outpace "cell-free" transduction at limiting vector input. This coincides with the prolonged half-life of cell bound vector but occurs, unlike HTLV-1, without evidence for particle aggregation. These studies suggest that cell-surface attachment stabilizes particles and alters neutralization kinetics. Our experiments provide novel insight into the underexplored cell-cell transmission of pseudotyped particles.

A mutant retroviral receptor restricts virus superinfection interference and productive infection

Background

Both cell-free and cell-associated infection routes are important for retroviral dissemination. Regardless of the mechanism, the driving force of retroviral entry is the interaction between the viral envelope and its receptor. To date it remains unclear how decreased affinity of viruses for their receptors affects viral cell-free infection, cell-cell transmission, and spreading kinetics. We have previously characterized a mutant form of the amphotropic murine retrovirus receptor human phosphate transporter 2 (PiT2) wherein the single substitution of a glutamic acid for the lysine residue at position 522 of this receptor is sufficient to render it to function as a gibbon ape leukemia virus (GALV) receptor.

Results

In this study we analyzed the binding affinity of the mutant receptor PiT2K522E and determined that it has a 1000 fold decreased GALV envelope binding affinity compared to the GALV wild type receptor. The decreased affinity does not restrict the initiation of cell-free GALV infection. The diminished binding affinity does, however, correlate with a decrease in the ability of GALV to spread in cells expressing this mutant receptor.

Conclusions

The reduced ability of GALV to subsequently spread among cells expressing PiT2K522E is likely resulted from reduced cell-cell transmission, the decreased ability of PiT2K522E-expressing cells to establish superinfection interference, and attendant cytopathic affects.

Pseudotype-independent nonspecific uptake of gammaretroviral and lentiviral particles in human cells

The effective entry of retroviruses into target cells depends on the presence of viral envelope (Env) proteins and cognate cellular receptors, such as the murine cationic amino acid transporter-1 (mCAT-1) for the ecotropic murine leukemia virus (MLV-E). Here, we examined whether human cells internalize MLV-E or other retroviral pseudotypes irrespective of the presence of a specific receptor. Using fluorescently tagged Gag to monitor viral internalization, and treating cells with chloroquine or bafilomycin A1, we show that endocytosis is the main pathway for productive transduction with ecotropic particles, but endocytosis of retroviral particles itself does not depend on a suitable receptor or Env. Nonspecific endosomal uptake and lysosomal degradation occurred with all "illegitimate" envelope-receptor combinations tested: MLV particles pseudotyped with the ecotropic envelope or measles virus H and F proteins as well as "ecotropic" or "bald" HIV-1 particles. Kinetic studies in cell lines and primary human T lymphocytes showed the persistence of Gag-GFP signals for more than 10 days after exposure to retroviral vector particles, even in the absence of a suitable receptor. Further studies testing the Gag-mediated transfer of protein or retroviral mRNA revealed that nonspecific endocytosis prevented the release of functional particle-associated proteins and nucleic acids into the cytosol. We conclude that receptor-targeted retroviral particles are unlikely to escape nonspecific cellular uptake unless appropriate protective principles are discovered. Conversely, as lysosomal degradation was found to inactivate mRNA and proteins embedded into retroviral particles, receptor targeting is a useful strategy for both transient and permanent cell modification by retrovirus-like particles.

Retargeting of viruses to generate oncolytic agents

Oncolytic virus therapy is based on the ability of viruses to effectively infect and kill tumor cells without destroying the normal tissues. While some viruses seem to have a natural preference for tumor cells, most viruses require the modification of their tropism to specifically enter and replicate in such cells. This review aims to describe the transductional targeting strategies currently employed to specifically redirect viruses towards surface receptors on tumor cells. Three major strategies can be distinguished; they involve (i) the incorporation of new targeting specificity into a viral surface protein, (ii) the incorporation of a scaffold into a viral surface protein to allow the attachment of targeting moieties, and (iii) the use of bispecific adapters to mediate targeting of a virus to a specified moiety on a tumor cell. Of each strategy key features, advantages and limitations are discussed and examples are given. Because of their potential to cause sustained, multiround infection—a desirable characteristic for eradicating tumors—particular attention is given to viruses engineered to become self-targeted by the genomic expression of a bispecific adapter protein.

The receptors for gibbon ape leukemia virus and amphotropic murine leukemia virus are not downregulated in productively infected cells

Background

Over the last several decades it has been noted, using a variety of different methods, that cells infected by a specific gammaretrovirus are resistant to infection by other retroviruses that employ the same receptor; a phenomenon termed receptor interference. Receptor masking is thought to provide an earlier means of blocking superinfection, whereas receptor down regulation is generally considered to occur in chronically infected cells.

Results

We used replication-competent GFP-expressing viruses containing either an amphotropic murine leukemia virus (A-MLV) or the gibbon ape leukemia virus (GALV) envelope. We also constructed similar viruses containing fluorescence-labeled Gag proteins for the detection of viral particles. Using this repertoire of reagents together with a wide range of antibodies, we were able to determine the presence and availability of viral receptors, and detect viral envelope proteins and particles presence on the cell surface of chronically infected cells.

Conclusions

A-MLV or GALV receptors remain on the surface of chronically infected cells and are detectable by respective antibodies, indicating that these receptors are not downregulated in these infected cells as previously proposed. We were also able to detect viral envelope proteins on the infected cell surface and infected cells are unable to bind soluble A-MLV or GALV envelopes indicating that receptor binding sites are masked by endogenously expressed A-MLV or GALV viral envelope. However, receptor masking does not completely prevent A-MLV or GALV superinfection.

Entry kinetics and cell-cell transmission of surface-bound retroviral vector particles

BACKGROUND

Transduction with recombinant HIV-1 derived lentivirus vectors is a multi-step process initiated by surface attachment and subsequent receptor-directed uptake into the target cell. We previously reported the retention of vesicular stomatitis virus G protein pseudotyped particles on murine progenitor cells and their delayed cell-cell transfer.

METHODS

To examine the underlying mechanism in more detail, we used a combination of approaches focused on investigating the role of receptor-independent factors in modulating attachment.

RESULTS

The investigation of synchronized transduction reveals cell-type specific rates of vector particle clearance with substantial delays during particle entry into murine hematopoietic progenitor cells. The observed uptake kinetics from the surface of the 1 degrees cell correlate inversely with the magnitude of transfer to 2 degrees targets, corresponding with our initial observation of preferential cell-cell transfer in the context of brief vector exposures. We further demonstrate that vector particle entry into cells is associated with the cell-type specific abundance of extracellular matrix fibronectin. Residual particle-extracellular fibronectin matrix binding and 2 degrees transfer can be competitively disrupted by heparin exposure without affecting murine progenitor homing and repopulation.

CONCLUSIONS

Although cellular attachment factors, including fibronectin, aid gene transfer by colocalizing particles to cells and disfavoring early dissociation from targets, they also appear to stabilize particles on the cell surface. The present study highlights the inadvertent consequences for cell entry and cell-cell transfer.

Cell entry of enveloped viruses

Enveloped viruses penetrate their cell targets following the merging of their membrane with that of the cell. This fusion process is catalyzed by one or several viral glycoproteins incorporated on the membrane of the virus. These envelope glycoproteins (EnvGP) evolved in order to combine two features. First, they acquired a domain to bind to a specific cellular protein, named "receptor." Second, they developed, with the help of cellular proteins, a function of finely controlled fusion to optimize the replication and preserve the integrity of the cell, specific to the genus of the virus. Following the activation of the EnvGP either by binding to their receptors and/or sometimes the acid pH of the endosomes, many changes of conformation permit ultimately the action of a specific hydrophobic domain, the fusion peptide, which destabilizes the cell membrane and leads to the opening of the lipidic membrane. The comprehension of these mechanisms is essential to develop medicines of the therapeutic class of entry inhibitor like enfuvirtide (Fuzeon) against human immunodeficiency virus (HIV). In this chapter, we will summarize the different envelope glycoprotein structures that viruses develop to achieve membrane fusion and the entry of the virus. We will describe the different entry pathways and cellular proteins that viruses have subverted to allow infection of the cell and the receptors that are used. Finally, we will illustrate more precisely the recent discoveries that have been made within the field of the entry process, with a focus on the use of pseudoparticles. These pseudoparticles are suitable for high-throughput screenings that help in the development of natural or artificial inhibitors as new therapeutics of the class of entry inhibitors.

Therapeutic sentinel lymph node imaging

Improving existing means of sentinel lymph node identification in non-small cell lung cancer will allow for molecular detection of occult micrometastases that may cause recurrence in early stage non-small cell lung cancer. Furthermore, targeted application of chemical and biological cytotoxic agents can potentially improve outcomes in patients with lymph node (LN) metastases. "Therapeutic Sentinel Lymph Node Imaging" incorporates these modalities into a single agent thereby identifying which LNs harbor tumor cells and simultaneously eradicating metastatic disease. In this review, we summarize the novel preclinical agents for identification and treatment of tumor bearing LNs and discuss their potential for clinical translation.

Directional spread of surface-associated retroviruses regulated by differential virus-cell interactions

The spread of viral infections involves the directional progression of virus particles from infected cells to uninfected target cells. Prior to entry, the binding of virus particles to specific cell surface receptors can trigger virus surfing, an actin-dependent lateral transport of viruses toward the cell body (M. J. Lehmann et al., J. Cell Biol. 170:317-325, 2005; M. Schelhaas, et al., PLoS Pathog. 4:e1000148, 2008; J. L. Smith, D. S. Lidke, and M. A. Ozbun, Virology 381:16-21, 2008). Here, we have used live-cell imaging to demonstrate that for cells chronically infected with the gammaretrovirus murine leukemia virus in which receptor has been downregulated, a significant portion of completely assembled virus particles are not immediately released into the supernatant but retain long-term association with the cell surface. Retention can be attributed, at least in part, to nonspecific particle attachment to cell surface glycosylaminoglycans. In contrast to virus surfing, viruses retained at the surface of infected cells undergo a lateral motility that is random and actin independent. This diffusive motility can be abruptly halted and converted into inward surfing after treatment with Polybrene, a soluble cation that increases virus-cell adsorption. In the absence of Polybrene, particle diffusion allows for an outward flow of viruses to the infected cell periphery. Peripheral particles are readily captured by and transmitted to neighboring uninfected target cells in a directional fashion. These data demonstrate a surface-based mechanism for the directional spread of viruses regulated by differential virus-cell interactions.

Cell carriers for oncolytic viruses: Fed Ex for cancer therapy

Oncolytic viruses delivered directly into the circulation face many hazards that impede their localization to, and infection of, metastatic tumors. Such barriers to systemic delivery could be overcome if couriers, which confer both protection, and tumor localization, to their viral cargoes, could be found. Several preclincal studies have shown that viruses can be loaded into, or onto, different types of cells without losing the biological activity of either virus or cell carrier. Importantly, such loading can significantly protect the viruses from immune-mediated virus-neutralizing activities, including antiviral antibody. Moreover, an impressive portfolio of cellular vehicles, which have some degree of tropism for tumor cells themselves, or for the biological properties associated with the tumor stroma, is already available. Therefore, it will soon be possible to initiate clinical protocols to test the hypopthesis that cell-mediated delivery can permit efficient shipping of oncolytic viruses from the loading bay (the production laboratory) directly to the tumor in immune-competent patients with metastatic disease.

A one-step SYBR Green I-based product-enhanced reverse transcriptase assay for the quantitation of retroviruses in cell culture supernatants

PCR-enhanced reverse transcriptase assays (PERT) are sensitive tools for the detection of retroviruses in biological samples. The adaptation of real-time PCR techniques based on fluorescent probes (F-PERT) has added a reliable quantitative capacity to the assay. In the interest of economy and time, the SYBR Green I-based real-time detection system was used to establish a convenient one-step PERT assay (SG-PERT). This assay can be completed in 2h, is linear over six orders of magnitude and can be used to quantify retroviruses belonging to divergent species, such as the human immunodeficiency virus type 1 (HIV-1), murine leukemia virus (MLV) and prototypic foamy virus (PFV).

Doxorubicin-loaded polymeric micelle overcomes multidrug resistance of cancer by double-targeting folate receptor and early endosomal pH

An optimized, pH-sensitive mixed-micelle system conjugated with folic acid is prepared in order to challenge multidrug resistance (MDR) in cancers. The micelles are composed of poly(histidine (His)-co-phenylalanine (Phe))-b-poly(ethylene glycol) (PEG) and poly(L-lactic acid) (PLLA)-b-PEG-folate. Core-forming, pH-sensitive hydrophobic blocks of poly(His-co-Phe) of varying composition are synthesized. The pH sensitivity of the micelles is controlled by the copolymer composition and is fine tuned to early endosomal pH by blending PLLA(3K)-b-PEG(2K)-folate in the presence of a basic anticancer drug, doxorubicin (DOX). In vitro tests are conducted against both wild-type (A2780) and DOX-resistant ovarian carcinoma cell lines. A mixed-micelle system composed of poly(His-co-Phe (16 mole%))-b-PEG (80 wt%) and PLLA-b-PEG-folate (20 wt%) is selected to target early endosomal pH. DOX-loaded micelles effectively kill both wild-type sensitive (A2780) and DOX-resistant ovarian MDR cancer-cell lines (A2780/DOX(R)) through an instantaneous high dose of DOX in the cytosol, which results from active internalization, accelerated DOX release triggered by endosomal pH, and an endosomal membrance disruption.

Cyclophosphamide facilitates antitumor efficacy against subcutaneous tumors following intravenous delivery of reovirus

PURPOSE

The purpose of the present study was to investigate whether it is possible to achieve truly systemic delivery of oncolytic reovirus, in immunocompetent hosts, using cyclophosphamide to overcome some of the barriers to effective intratumoral delivery and replication of i.v. injected virus.

EXPERIMENTAL DESIGN

I.v. delivery of reovirus was combined with different regimens of i.p. administered cyclophosphamide in C57Bl/6 mice bearing established s.c. B16 tumors. Intratumoral viral replication, tumor size, and survival were measured along with levels of neutralizing antibody (NAb) in the blood. Finally, differential toxicities of the virus/cyclophosphamide regimens were monitored through viral replication in systemic organs, survival, and cardiac damage.

RESULTS

Repeated i.v. injection of reovirus was poorly effective at seeding intratumoral viral replication/oncolysis. However, by combining i.v. virus with cyclophosphamide, viral titers of between 10(7) and 10(8) plaque-forming units per milligram were recovered from regressing tumors. Doses of cyclophosphamide that ablated NAb were associated with severe toxicities, characterized by viral replication in systemic organs--toxicities that are mirrored by repeated reovirus injections into B-cell knockout mice. Next, we restructured the dosing of cyclophosphamide and i.v. virus such that a dose of 3 mg cyclophosphamide was administered 24 h before reovirus injection, and this schedule was repeated every 6 days. Using this protocol, high levels of intratumoral viral access and replication ( approximately 10(7) plaque-forming units per milligram tumor) were maintained along with systemically protective levels of NAb and only very mild, non-life-threatening toxicity.

CONCLUSION

NAb to oncolytic viruses play a dual role in the context of systemic viral delivery; on one hand, they hinder repeated administration of virus but on the other, they provide an important safety mechanism by which virus released from vigorous intratumoral replication is neutralized before it can disseminate and cause toxicity. These data support the use of cyclophosphamide to modulate, but not ablate, patient NAb, in development of carefully controlled clinical trials of the systemic administration of oncolytic viruses.

Loading of oncolytic vesicular stomatitis virus onto antigen-specific T cells enhances the efficacy of adoptive T-cell therapy of tumors

Although adoptive T-cell therapy has shown clinical success, efficacy is limited by low levels of T-cell trafficking to, and survival in, the immunosuppressive environment of an established tumor. Oncolytic virotherapy has recently emerged as a promising approach to induce both direct tumor cell killing and local proinflammatory environments within tumors. However, inefficient systemic delivery of oncolytic viruses remains a barrier to use of these agents against metastatic disease that is not directly accessible to the end of a needle. Here we show that the ability of antigen-specific T cells to circulate freely, and to localize to tumors, can be exploited to achieve the systemic delivery of replication-competent, oncolytic vesicular stomatitis virus (VSV). Thus, VSV loaded onto OT-I T cells, specific for the SIINFEKL epitope of the ovalbumin antigen, was efficiently delivered to established B16ova tumors in the lungs of fully immune-competent C57Bl/6 mice leading to significant increases in therapy compared to the use of virus, or T cells, alone. Although OT-I T-cell-mediated delivery of VSV led to viral replication within tumors and direct viral oncolysis, therapy was also dependent upon an intact host immune system. Moreover, VSV loading onto the T cells increased both T-cell activation in vitro and T-cell trafficking in vivo. The combination of adoptive T-cell transfer of antigen-specific T cells, along with oncolytic virotherapy, can, therefore, increase the therapeutic utility of both approaches through multiple mechanisms and should be of direct translational value.

Identification of host proteins associated with retroviral vector particles by proteomic analysis of highly purified vector preparations

The Moloney murine leukemia virus (MMLV) belongs to the Retroviridae family of enveloped viruses, which is known to acquire minute amounts of host cellular proteins both on the surface and inside the virion. Despite the extensive use of retroviral vectors in experimental and clinical applications, the repertoire of host proteins incorporated into MMLV vector particles remains unexplored. We report here the identification of host proteins from highly purified retroviral vector preparations obtained by rate-zonal ultracentrifugation. Viral proteins were fractionated by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in-gel tryptic digested, and subjected to liquid chromatography/tandem mass spectrometry analysis. Immunogold electron microscopy studies confirmed the presence of several host membrane proteins exposed at the vector surface. These studies led to the identification of 27 host proteins on MMLV vector particles derived from 293 HEK cells, including 5 proteins previously described as part of wild-type MMLV. Nineteen host proteins identified corresponded to intracellular proteins. A total of eight host membrane proteins were identified, including cell adhesion proteins integrin beta1 (fibronectin receptor subunit beta) and HMFG-E8, tetraspanins CD81 and CD9, and late endosomal markers CD63 and Lamp-2. Identification of membrane proteins on the retroviral surface is particularly attractive, since they can serve as anchoring sites for the insertion of tags for targeting or purification purposes. The implications of our findings for retrovirus-mediated gene therapy are discussed.

Primary attachment of murine leukaemia virus vector mediated by particle-associated heparan sulfate proteoglycan

Target cell entry of murine leukaemia virus vectors proceeds via primary attachment, independent of the viral envelope protein and subsequent envelope-receptor interaction. Although much attention has been paid to modifying the latter for target cell specificity, the initial binding interaction has been overlooked, despite its opposing involvement both in providing the virus available for receptor binding and in depleting free virus. As a first step towards modifying primary attachment, both to provide specificity and to enhance vector availability, we sought to determine the nature of this interaction. Following an initial screen of GAGs (glycosaminoglycans) for their ability to inhibit virus binding and transduction, we have shown that production of virus from cells in which GAG sulfation is inhibited, or treatment of virus with heparinase III, reduces both particle attachment and infection. Detection in purified virus preparations of a neo-epitope generated by heparinase III confirmed the presence of virus-associated HSPG [HS (heparan sulfate) proteoglycan], acquired from the producer cell. We propose that host-acquired cell-surface HSPG (potentially including syndecan-2) provides a means of virus attachment to target cells that precedes specific receptor interaction and membrane fusion. Inhibition of HS biosynthesis may provide a sufficiently reduced background of primary binding such that novel mechanisms of attachment, ideally with appropriate target cell specificity, can be introduced.

The use of a tropism-modified measles virus in folate receptor-targeted virotherapy of ovarian cancer

PURPOSE

Attenuated measles viruses are promising experimental anticancer agents currently being evaluated in a phase I dose escalation trial for ovarian cancer patients. Virus attachment, entry, and subsequent intercellular fusion between infected and uninfected neighboring cells are mediated via the two measles receptors (CD46 and SLAM). To minimize potential toxicity due to measles virus-associated immunosuppression and infection of nontarget tissues, we sought to develop an ovarian cancer exclusive fully retargeted measles virus.

EXPERIMENTAL DESIGN AND RESULTS

Interactions of measles virus with its natural receptors were ablated, and a single-chain antibody (scFv) specific for alpha-folate receptor (FRalpha), a target overexpressed on 90% of nonmucinous ovarian cancer, was genetically engineered on the viral attachment protein (MV-alphaFR). Specificity of virus tropism was tested on tumor and normal cells. Biodistribution of measles virus infection was evaluated in measles-susceptible CD46 transgenic mice, whereas antitumor activity was monitored noninvasively by bioluminescence imaging in xenograft models. Tropism and fusogenic activity of MV-alphaFR was redirected exclusively to FRalpha without compromise to virus infectivity. In contrast to the parental virus, MV-alphaFR has no background infectivity on normal human cells. The antitumor activity of MV-alphaFR, as assessed by tumor volume reduction and overall survival increase, was equal to the parental virus in two models of human ovarian cancer (s.c. and i.p.).

CONCLUSIONS

A FR-exclusive ovarian cancer targeted oncolytic virus was generated and shown to be therapeutically effective, thus introducing a new modality for FR targeting and a candidate measles virus for clinical testing.

Prolonged adherence of human immunodeficiency virus-derived vector particles to hematopoietic target cells leads to secondary transduction in vitro and in vivo

Human immunodeficiency virus type 1-derived lentivirus vectors bearing the vesicular stomatitis virus G (VSV-G) envelope glycoprotein demonstrate a wide host range and can stably transduce quiescent hematopoietic stem cells. In light of concerns about biosafety and potential germ line transmission, they have been used predominantly for ex vivo strategies, thought to ensure the removal of excess surface-bound particles and prevent in vivo dissemination. Studies presented here instead reveal prolonged particle adherence after ex vivo exposure, despite serial wash procedures, with subsequent transduction of secondary target cells in direct and transwell cocultures. We explored the critical parameters affecting particle retention and transfer and show that attachment to the cell surface selectively protects virus particles from serum complement-mediated inactivation. Moreover, studies with nonmyeloablated murine recipients show that transplantation of vector-exposed, washed hematopoietic cells results in systemic dissemination of functional VSV-G/lentivector particles. We demonstrate genetic marking by inadvertent transfer of vector particles and prolonged expression of transgene product in recipient tissues. Our findings have implications for biosafety, vector design, and cell biology research.

Targeted retroviral vectors displaying a cleavage site-engineered hemagglutinin (HA) through HA-protease interactions

We report here a targeting method that exploits the expression pattern of cell surface proteases to induce gene delivery to specific tissues. We describe retroviral vectors harboring modified surface glycoproteins derived from an avian influenza virus hemagglutinin (HA) for which the cell entry properties, dependent on HA cleavage by producer cells, were conditionally blocked at a postbinding step by insertion of matrix metalloproteinase (MMP) substrates. We demonstrate that such vectors induce gene transfer, both in vitro and in mice harboring human tumor xenografts, only through contact with target cells expressing MMPs that cleave the substrate introduced into the recombinant HA. This selective gene transfer in MMP-rich cells was specifically inhibited by 1,10-phenanthroline, a broad-range MMP inhibitor. Importantly, such MMP-activatable vectors selectively transduced MMP-rich cells in heterogeneous populations containing MMP-rich and MMP-poor cells. These vectors will allow useful gene transfer applications into target cells exhibiting specific protease activities.

Murine leukemia virus particles activate Rac1 in HeLa cells

A number of viruses, when they bind to cells, activate intracellular signals that facilitate post-binding steps of infection. To determine if retroviruses activate intracellular signaling, we transduced HeLa cells with amphotropic retroviruses produced by TelCeB6 cells and examined cell lysates for activated Rac1. We found that retroviruses activate Rac1. Rac1 activation was blocked when cells were depleted of cholesterol, cultured in suspension, or incubated with an anti-beta(1) integrin antibody, and when viruses were treated with heparinase III. Retrovirus activation of Rac1 did not require the amphotropic envelope protein. Gene transfer was reduced 2.4-fold when viruses were treated with heparinase III, but did not change when cells were transduced in the presence of function-blocking anti-beta(1) integrin antibodies. The implications of these findings with respect to retrovirus-cell interactions are discussed.

The perforin-dependent immunological synapse allows T-cell activation-dependent tumor targeting by MLV vector particles

We have reported that retroviral particles adhered to the surface of antigen-specific T cells can be carried to metastases following adoptive transfer in vivo, a process we have called viral hitch hiking. Following antigen-driven T-cell accumulation at tumors, viral particles productively infect tumor cells via envelope/receptor dependent interactions ('hand on' of virus from the T cell to the tumor cell). We describe here a second envelope/receptor independent pathway of viral hand on from T cells, dependent on T-cell activation. We show that the endosomolytic property of perforin promotes release of viral particles from endosomes into which they are co-delivered along with cytotoxic granules from the activated T cell. Therefore, hand on of MLV particles lacking any envelope can be used for in vivo delivery of vectors, where targeting is at the extremely specific level of recognition of antigen by the T-cell receptor, thereby dispensing with the need to engineer viral envelopes. These data reveal a novel pathway by which MLV viral particles exploit a functional immunological synapse and present new opportunities both to improve the efficacy of adoptive T-cell transfer and to target vectors for systemic gene delivery.

Visualization of infectious pancreatic necrosis virus (IPNV) particles labeled with fluorescent probes

Infectious pancreatic necrosis virus (IPNV) particles were labeled with SYBR Green I or a monoclonal antibody and FITC-conjugated secondary antibody and examined in a fluorescence microscope. Labeled viral particles were visualized in a narrow range of pixels. Comparing IPNV particles with fluorescent phage T4 virions, the former, as expected, were seen smaller in size. The method allows the rapid and accurate counting of viral particles both on filters and bound to the cell surface. In addition, IPNV particles can be specifically enumerated in the presence of other virions and the ratio between physical particles and virus infectivity can be easily calculated as well.

Tumor-targeted, systemic delivery of therapeutic viral vectors using hitchhiking on antigen-specific T cells

Antigen-specific T cells circulate freely and accumulate specifically at sites of antigen expression. To enhance the survival and targeting of systemically delivered viral vectors, we exploited the observation that retroviral particles adhere nonspecifically, or 'hitchhike,' to the surface of T cells. Adoptive transfer of antigen-specific T cells, loaded with viruses encoding interleukin (IL)-12 or Herpes Simplex Virus thymidine kinase (HSVtk), cured established metastatic disease where adoptive T-cell transfer alone was not effective. Productive hand off correlated with local heparanase expression either from malignant tumor cells and/or as a result of T-cell activation by antigen, providing high levels of selectivity for viral transfer to metastatic tumors in vivo. Protection, concentration and targeting of viruses by adsorption to cell carriers represent a new technique for systemic delivery of vectors, in fully immunocompetent hosts, for a variety of diseases in which delivery of genes may be therapeutically beneficial.

Increase in infectivity of targeted Moloney murine leukemia virus-based gene-delivery vectors through lowering the threshold for fusion

Many research groups have developed targeted vectors for gene therapy based on Moloney murine leukemia virus (MoMLV). Despite proper binding of the targeted vector to the target molecule, little or no infectivity of human cells expressing the target molecule has been achieved in most studies. One of the reasons for this lack of infectivity may be steric hindrance within the targeted envelope glycoprotein (Env), impeding the conformational changes required for fusion and infection. Here, attempts were made to solve this problem by mutating key residues within Env of two targeted MoMLV-based vectors, MoMLV-E-Sel and MoMLV-FBP. Selection of key residues was based on an Env with reduced threshold for fusion, that of the CD4-independent human immunodeficiency virus type 2 isolate ROD/B. It was shown here that vectors bearing MoMLV-FBP Env with a V512M substitution had higher titres and faster kinetics of entry than vectors bearing parental targeted Env proteins. This could be due to the partial release of steric constraints that result in an Env with a reduced threshold for fusion.

Insertion of targeting domains into the envelope glycoprotein of Moloney murine leukemia virus (MoMLV)-based vectors modulates the route of mCAT-1-mediated viral entry

Several groups have inserted targeting domains into the envelope glycoprotein (Env) of Moloney murine leukemia virus (MoMLV) in an attempt to produce targeted retroviral vectors for human gene therapy. While binding of these modified Envs to the target molecule expressed on the surface of human cells was observed, specific high-titer infection of human cells expressing the target molecule was not achieved. Here we investigate the initial steps in the entry process of targeted MoMLV vectors both in murine and human cells expressing the MoMLV receptor, the mouse cationic amino acid transporter-1 (mCAT-1). We show that insertion of a small ligand targeted to E-selectin and of a single chain antibody (scFv) targeted to folate-binding protein (FBP) into the N-terminus of MoMLV Env results in the reduction of the infectivity and the kinetics of entry of the MoMLV vectors. The use of soluble receptor-binding domain (sRBD), bafilomycin A1 (BafA1) and methyl-beta-cyclodextrin (MbetaC) increase the infectivity of the MoMLV vectors targeted to FBP (MoMLV-FBP) suggesting that the scFv targeted to FBP increases the threshold for fusion and might re-route entry of the targeted MoMLV-FBP vector towards an endocytic, non-productive pathway.

Porcine endogenous retroviruses infect cells lacking cognate receptors by an alternative pathway: implications for retrovirus evolution and xenotransplantation

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors. The H residue of this motif is most critical, with all substitutions we tested being inactive. Interestingly, porcine endogenous retroviruses (PERVs) of all three host-range groups, A, B, and C, lack full PHQ motifs, but most members have an H residue at position 10. H10A PERV mutants are noninfectious but were efficiently transactivated by adding to the assays a PHQ-containing SU or receptor-binding subdomain (RBD) derived from a gibbon ape leukemia virus (GALV). A requirement of this transactivation was a functional GALV receptor on the cells. In contrast to this heterologous transactivation, PERV RBDs and SUs were inactive in all tested cells, including porcine ST-IOWA cells. Surprisingly, transactivation by GALV RBD enabled wild-type or H10A mutant PERVs of all three host-range groups to efficiently infect cells from humans and rodents that lack functional PERV receptors and it substantially enhanced infectivities of wild-type PERVs, even for cells with PERV receptors. Thus, PERVs can suboptimally infect cells that contain cognate receptors or they can employ a transactivation pathway to more efficiently infect all cells. This ability to infect cells lacking cognate receptors was previously demonstrated only for nontransmissible variant gammaretroviruses with recombinant and mutant envelope glycoproteins. We conclude that some endogenously inherited mammalian retroviruses also have a receptor-independent means for overcoming host-range and interference barriers, implying a need for caution in xenotransplantation, especially of porcine tissues.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Efficient Retroviral Vector Targeting of Carcinoembryonic Antigen-Positive Tumors

Many gene therapy approaches require specific, efficient gene delivery to cells in vivo. To target colorectal tumors we fused a single-chain variable fragment (scFv) directed against carcinoembryonic antigen (CEA) to the amphotropic murine leukemia virus envelope. A proline-rich hinge and matrix metalloprotease (MMP) cleavage site linked the two proteins. Following attachment to CEA, MMP cleavage of the envelope at the cell surface removed the scFv and proline-rich hinge, allowing transduction. This allowed selective targeting of CEA-positive cells in vivo after injection of producer cells at the site of the tumor, with up to 10% of cells within a CEA-positive tumor xenograft becoming transduced. Intraperitoneal injection of amphotropic producer cells resulted in transduction of cells in spleen, liver, and kidney, which was not detected when CEA-targeted producer cells were used. These results demonstrate the feasibility of using targeted retroviral vectors for in vivo gene delivery to tumors. Furthermore, the lack of transduction of host cells eliminates the risk of insertional mutagenesis leading to transformation of host hematopoietic cells.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Targeting retroviral and lentiviral vectors

Retroviral vectors capable of efficient in vivo gene delivery to specific target cell types or to specific locations of disease pathology would greatly facilitate many gene therapy applications. The surface glycoproteins of membrane-enveloped viruses stand among the choice candidates to control the target cell receptor recognition and host range of retroviral vectors onto which they are incorporated. This can be achieved in many ways, such as the exchange of glycoprotein by pseudotyping, their biochemical modifications, their conjugation with virus-cell bridging agents or their structural modifications. Understanding the fundamental properties of the viral glycoproteins and the molecular mechanism of virus entry into cells has been instrumental in the functional alteration of their tropism. Here we briefly review the current state of our understanding of the structure and function of viral envelope glycoproteins and we discuss the emerging targeting strategies based on retroviral and lentiviral vector systems.

The Trojan exosome hypothesis

We propose that retroviruses exploit a cell-encoded pathway of intercellular vesicle traffic, exosome exchange, for both the biogenesis of retroviral particles and a low-efficiency but mechanistically important mode of infection. This Trojan exosome hypothesis reconciles current paradigms of retrovirus-directed transmission with the unique lipid composition of retroviral particles, the host cell proteins present in retroviral particles, the complex cell biology of retroviral release, and the ability of retroviruses to infect cells independently of Envelope protein-receptor interactions. An exosomal origin also predicts that retroviruses pose an unsolvable paradox for adaptive immune responses, that retroviral antigen vaccines are unlikely to provide prophylactic protection, and that alloimmunity is a central component of antiretroviral immunity. Finally, the Trojan exosome hypothesis has important implications for the fight against HIV and AIDS, including how to develop new antiretroviral therapies, assess the risk of retroviral infection, and generate effective antiretroviral vaccines.

Gene therapy for ovarian cancer: progress and potential

Gene therapy remains a promising therapeutic modality for ovarian cancer. Yet much work remains to be done to see gene therapy realize its full potential in elucidating the complex genetic interactions of delivered genes within target cancer cells and in the development of improved vector systems. Because most neoplasms involve multiple mutations, the targeting of a single mutation is unlikely to achieve total tumor control: gene therapy strategies that target multiple cellular processes or invoke various antitumor approaches need to be investigated. Additionally, current vector systems do not transduce ovarian cancer cells efficiently and are hampered by immune responses that further limit their efficacy. Additionally, limitations in vector specificity lead to transduction of normal cells and subsequent toxicity. Investigators are developing refinements to current gene therapy approaches that would address these limitations and that are soon to be incorporated into clinical trials. It is hoped that these advances will lead to improvements in the therapeutic index for ovarian cancer gene therapy and provide another effective therapeutic tool for this deadly disease.

Green fluorescent protein-tagged retroviral envelope protein for analysis of virus-cell interactions

Fluorescent retroviral envelope (Env) proteins were developed for direct visualization of viral particles. By fusing the enhanced green fluorescent protein (eGFP) to the N terminus of the amphotropic 4070A envelope protein, extracellular presentation of eGFP was achieved. Viruses incorporated the modified Env protein and efficiently infected cells. We used the GFP-tagged viruses for staining retrovirus receptor-positive cells, thereby circumventing indirect labeling techniques. By generating cells which conditionally expressed the GFP-tagged Env protein, we could confirm an inverse correlation between retroviral Env expression and infectivity (superinfection). eGFP-tagged virus particles are suitable for monitoring the dynamics of virus-cell interactions.

Rescue of retroviral envelope fusion deficiencies by cationic liposomes

BACKGROUND

Retroviral particles that are inappropriately enveloped can transduce target cells if pre-associated with cationic liposomes. This study optimises and addresses the mechanism of liposome-enhanced gene delivery, and explores the potential for such agents to compensate for fusion deficiency associated with chimaeric envelope proteins.

METHODS

Particles bearing wild-type, chimaeric or no envelope proteins were complexed with DOTAP or DC-Chol/DOPE cationic liposomes and added to target cells for various times. Particle binding was determined by detection of cell-associated capsid protein and infectivity was measured histochemically.

RESULTS

Stable association of cationic liposomes with retrovirus particles significantly enhanced their binding rate to target cells in proportion to the increase of transduction kinetics for infectious virus. Binding of virus was equivalent with or without envelope protein and/or virus receptor, indicating that a non-specific interaction precedes receptor recognition. Non-infectious combinations were rescued by the intrinsic fusogenicity of the cationic liposomes, which enabled entry of the viral core, but left subsequent events unaltered. The optimised transduction rate with non-enveloped particles and DOTAP approached that of amphotropic-enveloped virus in some cases, although the effect was target-cell-dependent. DC-Chol/DOPE was less potent at direct fusion but was able to enhance 600-fold the receptor-dependent action of chimaeric envelopes that were deficient in fusion by virtue of the addition of targeting domains.

CONCLUSIONS

These data have implications for the development of retroviral vector targeting strategies from the perspectives of the specificity of target cell interaction and compensating for chimaeric envelope fusion deficiency.

Relationship between SU subdomains that regulate the receptor-mediated transition from the native (fusion-inhibited) to the fusion-active conformation of the murine leukemia virus glycoprotein

Envelope glycoproteins (Env) of retroviruses are trimers of SU (surface) and TM (transmembrane) heterodimers and are expressed on virions in fusion-competent forms that are likely to be metastable. Activation of the viral receptor-binding domain (RBD) via its interaction with a cell surface receptor is thought to initiate a cascade of events that lead to refolding of the Env glycoprotein into its stable fusion-active conformation. While the fusion-active conformation of the TM subunit has been described in detail for several retroviruses, little is known about the fusion-competent structure of the retroviral glycoproteins or the molecular events that mediate the transition between the two conformations. By characterizing Env chimeras between the ecotropic and amphotropic murine leukemia virus (MLV) SUs as well as a set of point mutants, we show that alterations of the conformation of the SU glycoprotein strongly elevate Env fusogenicity by disrupting the stability of the Env complex. Compensatory mutations that restored both Env stability and fusion control were also identified, allowing definition of interactions within the Env complex that maintain the stability of the native Env complex. We show that, in the receptor-unbound form, structural interactions between the N terminus of the viral RBD (NTR domain), the proline-rich region (PRR), and the distal part of the C-terminal domain of the SU subunit maintain a conformation of the glycoprotein that is fusion inhibitory. Additionally, we identified mutations that disrupt this fusion-inhibitory conformation and allow fusion activation in the absence of viral receptors, provided that receptor-activated RBD fragments are added in trans during infection. Other mutations were identified that allow fusion activation in the absence of receptors for both the viral glycoprotein and the trans-acting RBD. Finally, we found mutations of the SU that bypass in cis the requirement for the NTR domain in fusion activation. All these different mutations call for a critical role of the PRR in mediating conformational changes of the Env glycoprotein during fusion activation. Our results suggest a model of MLV Env fusion activation in which unlocking of the fusion-inhibitory conformation is initiated by receptor binding of the viral RBD, which, upon disruption of the PRR, allows the NTR domain to promote further events in Env fusion activation. This involves a second type of interaction, in cis or in trans, between the receptor-activated RBD and a median segment of the freed C-terminal domain.

Folate-mediated delivery of macromolecular anticancer therapeutic agents

The receptor for folic acid constitutes a useful target for tumor-specific drug delivery, primarily because: (1) it is upregulated in many human cancers, including malignancies of the ovary, brain, kidney, breast, myeloid cells and lung, (2) access to the folate receptor in those normal tissues that express it can be severely limited due to its location on the apical (externally-facing) membrane of polarized epithelia, and (3) folate receptor density appears to increase as the stage/grade of the cancer worsens. Thus, cancers that are most difficult to treat by classical methods may be most easily targeted with folate-linked therapeutics. To exploit these peculiarities of folate receptor expression, folic acid has been linked to both low molecular weight drugs and macromolecular complexes as a means of targeting the attached molecules to malignant cells. Conjugation of folic acid to macromolecules has been shown to enhance their delivery to folate receptor-expressing cancer cells in vitro in almost all situations tested. Folate-mediated macromolecular targeting in vivo has, however, yielded only mixed results, largely because of problems with macromolecule penetration of solid tumors. Nevertheless, prominent examples do exist where folate targeting has significantly improved the outcome of a macromolecule-based therapy, leading to complete cures of established tumors in many cases. This review presents a brief mechanistic background of folate-targeted macromolecular therapeutics and then summarizes the successes and failures observed with each major application of the technology.

Gene therapy in transplantation

Gene transfer and gene therapy represent a relatively new field that has grown and expanded enormously in the last 5-10 years. The application of gene transfer and gene medicines to transplantation is currently in its infancy. Consideration for gene medicines in transplantation requires delivery of vectors, either to the graft or to the immune system. Delivery of vectors to the graft provides a choice of potential immunologic targets including: costimulatory signals; inhibitory cytokines; adhesion molecules; and molecules relating to apoptosis. In addition, non-immunologic targets, that increase graft protective mechanisms by reducing ischemic and immunologic damage, represent significant targets for gene transfer. Delivery of vectors to the immune system includes potential targets to modify the immune system, and results in tolerance. Other considerations for gene therapy include the development of additional technologies, such as gene conversion or transgenesis coupled with xenotransplantation, which may provide genetically modified organs. Another important aspect of gene transfer relates to regulation of the transgene expression. A variety of issues concerning innate immunity, adaptive immunity, response to vector components, response to transgene products, and entry of vectors into the antigen presentation and processing pathway require further investigation and refinement of approaches. Lastly, regulatable promoters and the understanding of their interaction with individual cells, tissues and organs, and their interaction with innate and adaptive immunity, are of paramount importance to improving the efficacy and utility of gene transfer. There is no doubt that there is much exciting basic and translational science to be accomplished in the next decade in order to solve these potential barriers and advance gene medicines into the clinical realm in transplantation.

Cell surface heparan sulfate is a receptor for attachment of envelope protein-free retrovirus-like particles and VSV-G pseudotyped MLV-derived retrovirus vectors to target cells

Non-infectious, envelope protein-free, retrovirus-like particles (VLP) derived from either Moloney murine leukemia virus (MLV) or human HIV are able to bind efficiently to, but not infect, target cells. Upon subsequent addition to the bound particles of the G protein of vesicular stomatitis virus (VSV-G), an efficient surrogate retrovirus envelope protein, the VLP are efficiently taken up by the cells to produce infection. Cell attachment of the VLP is efficiently inhibited by soluble heparin and dextran sulfate and less efficiently abrogated by several other glycosaminoglycans (GAGs) including chondroitin sulfate A and chondroitin sulfate B (dermatan sulfate), as determined by deconvolution microscopic immunodetection of the viral gag protein and by quantitative binding studies of metabolically labeled (35)S-VLP. Enzymatic digestion of heparan sulfate (HS) from the cell surface with heparinase I also reduces VLP binding. Furthermore, VLP adsorption onto several CHO cell lines variably deficient in cell surface GAG is significantly but incompletely abrogated. De-sulfated heparins are less efficient than native heparin in inhibiting the Polybrene-mediated binding of VLP, whereas growth of human cells in the presence of sodium chlorate leads to significant reduction of Polybrene-mediated VLP binding. In addition, specific inhibition of VLP binding and infectivity of mature infectious VSV-G-pseudotyped virus is observed in the presence of heparin and HS under Polybrene-free conditions. We conclude from these studies that the presence of Polybrene, the degree of sulfation of cell surface GAG, and possibly the presence of charged cell surface macromolecules create an electrostatic environment that promotes optimum binding of VLP to cells. Additionally, our results demonstrate that, in the absence of Polybrene, initial attachments of non-infectious, envelope protein-free VLP and probably mature infectious virus particles are mediated by interactions of the virus particles with cell surface heparan sulfate, and possibly with other GAG molecules.

Retargeting gene delivery using surface-engineered retroviral vector particles

Retroviral vectors with the capacity to deliver transgenes to specific tissues are expected to be of great value for various gene transfer applications in vivo. Initial attempts to modify vector host-range by the insertion of ligands on their surface glycoproteins have frequently failed, essentially owing to the impairment of the fusogenicity of the vector particles bound to the targeted cell-surface molecules. Several strategies aimed to recover the fusogenic activity of surface-engineered vector particles have recently been explored and have given rise to novel concepts in the field.