Lentiviral vectors pseudotyped with envelope glycoproteins derived from gibbon ape leukemia virus and murine leukemia virus 10A1

Development of HIV-1 vectors pseudotyped with envelope proteins of other retroviruses

In the past three decades, human immunodeficiency virus type 1 (HIV-1)-derived vectors were evolved and became indispensable to transduce therapeutic genes into a range of different target cell types to facilitate a variety of gene therapeutic strategies. To achieve this, i) the biosafety profile of the vectors was incrementally enhanced and ii) the CD4-restricted tropism mediated by the envelope proteins (Env) of the parental virus needed to be directed towards recruitment of other receptors expressed on the desired target cells. Here, a closer look is first taken at the development of vector components and the mechanisms of Env incorporation into particles. While envelope proteins originating from a broad range of very diverse virus species were successfully utilized, members of the Retroviridae family most frequently provided Env or further engineered variants thereof to form transduction-competent HIV-1 pseudotype vector particles. The development of these vectors is reviewed and anticipated to further contribute to the future progression of somatic gene therapy.

Molecular Engineering of Virus Tropism

Engineered viral vectors designed to deliver genetic material to specific targets offer significant potential for disease treatment, safer vaccine development, and the creation of novel biochemical research tools. Viral tropism, the specificity of a virus for infecting a particular host, is often modified in recombinant viruses to achieve precise delivery, minimize off-target effects, enhance transduction efficiency, and improve safety. Key factors influencing tropism include surface protein interactions between the virus and host-cell, the availability of host-cell machinery for viral replication, and the host immune response. This review explores current strategies for modifying the tropism of recombinant viruses by altering their surface proteins. We provide an overview of recent advancements in targeting non-enveloped viruses (adenovirus and adeno-associated virus) and enveloped viruses (retro/lentivirus, Rabies, Vesicular Stomatitis Virus, and Herpesvirus) to specific cell types. Additionally, we discuss approaches, such as rational design, directed evolution, and in silico and machine learning-based methods, for generating novel AAV variants with the desired tropism and the use of chimeric envelope proteins for pseudotyping enveloped viruses. Finally, we highlight the applications of these advancements and discuss the challenges and future directions in engineering viral tropism.

Methods for Engineering Binders to Multi-Pass Membrane Proteins

Numerous potential drug targets, including G-protein-coupled receptors and ion channel proteins, reside on the cell surface as multi-pass membrane proteins. Unfortunately, despite advances in engineering technologies, engineering biologics against multi-pass membrane proteins remains a formidable task. In this review, we focus on the different methods used to prepare/present multi-pass transmembrane proteins for engineering target-specific biologics such as antibodies, nanobodies and synthetic scaffold proteins. The engineered biologics exhibit high specificity and affinity, and have broad applications as therapeutics, probes for cell staining and chaperones for promoting protein crystallization. We primarily cover publications on this topic from the past 10 years, with a focus on the different formats of multi-pass transmembrane proteins. Finally, the remaining challenges facing this field and new technologies developed to overcome a number of obstacles are discussed.

Genetic Engineering and Enrichment of Human NK Cells for CAR-Enhanced Immunotherapy of Hematological Malignancies

The great clinical success of chimeric antigen receptor (CAR) T cells has unlocked new levels of immunotherapy for hematological malignancies. Genetically modifying natural killer (NK) cells as alternative CAR immune effector cells is also highly promising, as NK cells can be transplanted across HLA barriers without causing graft-versus-host disease. Therefore, off-the-shelf usage of CAR NK cell products might allow to widely expand the clinical indications and to limit the costs of treatment per patient. However, in contrast to T cells, manufacturing suitable CAR NK cell products is challenging, as standard techniques for genetically engineering NK cells are still being defined. In this study, we have established optimal lentiviral transduction of primary human NK cells by systematically testing different internal promoters for lentiviral CAR vectors and comparing lentiviral pseudotypes and viral entry enhancers. We have additionally modified CAR constructs recognizing standard target antigens for acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) therapy—CD19, CD33, and CD123—to harbor a CD34-derived hinge region that allows efficient detection of transduced NK cells in vitro and in vivo and also facilitates CD34 microbead-assisted selection of CAR NK cell products to >95% purity for potential clinical usage. Importantly, as most leukemic blasts are a priori immunogenic for activated primary human NK cells, we developed an in vitro system that blocks the activating receptors NKG2D, DNAM-1, NKp30, NKp44, NKp46, and NKp80 on these cells and therefore allows systematic testing of the specific killing of CAR NK cells against ALL and AML cell lines and primary AML blasts. Finally, we evaluated in an ALL xenotransplantation model in NOD/SCID-gamma (NSG) mice whether human CD19 CAR NK cells directed against the CD19+ blasts are relying on soluble or membrane-bound IL15 production for NK cell persistence and also in vivo leukemia control. Hence, our study provides important insights into the generation of pure and highly active allogeneic CAR NK cells, thereby advancing adoptive cellular immunotherapy with CAR NK cells for human malignancies further.

Challenges on the development of a pseudotyping assay for Zika glycoproteins

Introduction. Zika virus (ZIKV) emerged as a public health concern on the American continent during late 2015. As the number of infected grew so did the concerns about its capability to cause long-term damage especially with the appearance of the congenital Zika syndrome (CZS). Proteins from the TAM family of receptor tyrosine kinases (RTKs) were proposed as the cellular receptors, however, due to the ability of the virus to infect a variety of cell lines different strategies to elucidate the tropism of the virus should be investigated.Hypothesis. Pseudotyping is a powerful tool to interrogate the ability of the glycoprotein (GP) to permit entry of viruses.Aim. We aimed to establish a highly tractable pseudotype model using lenti- and retro-viral backbones to investigate the entry pathway of ZIKV.Methodology. We used different glycoprotein constructs and different lenti- or retro-viral backbones, in a matrix of ratios to investigate production of proteins and functional pseudotypes.Results. Varying the ratio of backbone and glycoprotein plasmids did not yield infectious pseudotypes. Moreover, the supplementation of the ZIKV protease or the substitution of the backbone had no positive impact on the infectivity. We showed production of the proteins in producer cells implying the lack of infectious pseudotypes is due to a lack of successful glycoprotein incorporation, rather than lack of protein production.Conclusion. In line with other reports, we were unable to successfully produce infectious pseudotypes using the variety of methods described. Other strategies may be more suitable in the development of an efficient pseudotype model for ZIKV and other flaviviruses.

Efficient Pseudotyping of Different Retroviral Vectors Using a Novel, Codon-Optimized Gene for Chimeric GALV Envelope

The Gibbon Ape Leukemia Virus envelope protein (GALV-Env) mediates efficient transduction of human cells, particularly primary B and T lymphocytes, and is therefore of great interest in gene therapy. Using internal domains from murine leukemia viruses (MLV), chimeric GALV-Env proteins such as GALV-C_4070A were derived, which allow pseudotyping of lentiviral vectors. In order to improve expression efficiency and vector titers, we developed a codon-optimized (co) variant of GALV-C_4070A (coGALV-Env). We found that coGALV-Env mediated efficient pseudotyping not only of γ-retroviral and lentiviral vectors, but also α-retroviral vectors. The obtained titers on HEK293T cells were equal to those with the classical GALV-Env, whereas the required plasmid amounts for transient vector production were significantly lower, namely, 20 ng coGALV-Env plasmid per 10⁶ 293T producer cells. Importantly, coGALV-Env-pseudotyped γ- and α-retroviral, as well as lentiviral vectors, mediated efficient transduction of primary human T cells. We propose that the novel chimeric coGALV-Env gene will be very useful for the efficient production of high-titer vector preparations, e.g., to equip human T cells with novel specificities using transgenic TCRs or CARs. The considerably lower amount of plasmid needed might also result in a significant cost advantage for good manufacturing practice (GMP) vector production based on transient transfection.

HLA Class I Knockout Converts Allogeneic Primary NK Cells Into Suitable Effectors for "Off-the-Shelf" Immunotherapy

Cellular immunotherapy using chimeric antigen receptors (CARs) so far has almost exclusively used autologous peripheral blood-derived T cells as immune effector cells. However, harvesting sufficient numbers of T cells is often challenging in heavily pre-treated patients with malignancies and perturbed hematopoiesis and perturbed hematopoiesis. Also, such a CAR product will always be specific for the individual patient. In contrast, NK cell infusions can be performed in non-HLA-matched settings due to the absence of alloreactivity of these innate immune cells. Still, the infused NK cells are subject to recognition and rejection by the patient's immune system, thereby limiting their life-span in vivo and undermining the possibility for multiple infusions. Here, we designed genome editing and advanced lentiviral transduction protocols to render primary human NK cells unsusceptible/resistant to an allogeneic response by the recipient's CD8+ T cells. After knocking-out surface expression of HLA class I molecules by targeting the B2M gene via CRISPR/Cas9, we also co-expressed a single-chain HLA-E molecule, thereby preventing NK cell fratricide of B2M-knockout (KO) cells via "missing self"-induced lysis. Importantly, these genetically engineered NK cells were functionally indistinguishable from their unmodified counterparts with regard to their phenotype and their natural cytotoxicity towards different AML cell lines. In co-culture assays, B2M-KO NK cells neither induced immune responses of allogeneic T cells nor re-activated allogeneic T cells which had been expanded/primed using irradiated PBMNCs of the respective NK cell donor. Our study demonstrates the feasibility of genome editing in primary allogeneic NK cells to diminish their recognition and killing by mismatched T cells and is an important prerequisite for using non-HLA-matched primary human NK cells as readily available, "off-the-shelf" immune effectors for a variety of immunotherapy indications in human cancer.

Gene Therapy Applications of Non-Human Lentiviral Vectors

Recent commercialization of lentiviral vector (LV)-based cell therapies and successful reports of clinical studies have demonstrated the untapped potential of LVs to treat diseases and benefit patients. LVs hold notable and inherent advantages over other gene transfer agents based on their ability to transduce non-dividing cells, permanently transform target cell genome, and allow stable, long-term transgene expression. LV systems based on non-human lentiviruses are attractive alternatives to conventional HIV-1-based LVs due to their lack of pathogenicity in humans. This article reviews non-human lentiviruses and highlights their unique characteristics regarding virology and molecular biology. The LV systems developed based on these lentiviruses, as well as their successes and shortcomings, are also discussed. As the field of gene therapy is advancing rapidly, the use of LVs uncovers further challenges and possibilities. Advances in virology and an improved understanding of lentiviral biology will aid in the creation of recombinant viral vector variants suitable for translational applications from a variety of lentiviruses.

The stability of envelope-pseudotyped lentiviral vectors

Lentiviral vectors have become popular tools for stable genetic modification of mammalian cells. In some applications of lentiviral vector-transduced cells, infectious-lentiviral particles should be absent. Quantification of the free-vector particles that remain from the inoculum can be difficult. Therefore a formula was established that yields an estimation of the ‘Reduction Ratio.’ This ratio represents the loss of titer based on a number of vector-inactivating effects. In this study, we evaluated several parameters and assumptions that were used in the current formula. We generated new data on the stability and trypsin sensitivity of lentiviral vectors pseudotyped with eight heterologous envelope proteins and the loss of vectors by washing or passaging the cell cultures. Our data demonstrate that the loss of virus titer under the influence of trypsin as well as the half-life of the particles in tissue culture medium is dependent on the vector’s envelope protein. While VSV-G-envelope-pseudotyped particles were unsensitive to trypsin, the titer of vectors pseudotyped with other envelope proteins decreased 2–110-fold. The half-life in culture medium ranged from 8 to 40 h for the different envelope-pseudotyped vectors, with 35 h for VSV-G-envelope-pseudotyped vector particles. Additionally, we found that removal of the culture medium from Ø35 mm to Ø10 cm dishes reduces the amount of vector particles in the culture by 50-fold and 20-fold, respectively. Together these data can be used to more precisely estimate the maximum number of free lentiviral vector particles in cell cultures.

Toward a Scalable Purification Protocol of GaLV-TR-Pseudotyped Lentiviral Vectors

Lentiviral vectors (LV) that are used in research and development as well as in clinical trials are in majority vesicular stomatitis virus G glycoprotein (VSVg) pseudotyped. The predominance of this pseudotype choice for clinical gene therapy studies is largely due to a lack of purification schemes for pseudotypes other than VSVg. In this study, we report for the first time the development of a new downstream process protocol allowing high-yield production of stable and infectious gibbon ape leukemia virus (GaLV)-TR-LV particles. We identified critical conditions in tangential flow filtration (TFF) and chromatographic steps for preserving the infectivity/functionality of LV during purification. This was carried out by identifying for each step, the critical parameters affecting LV infectivity, including pH, salinity, presence of stabilizers, temperature, and by defining the optimal order of these steps. A three-step process was developed for GaLV-TR-LV purification consisting of one TFF and two chromatographic steps (ion-exchange chromatography and size exclusion chromatography) permitting recoveries of >27% of infectious particles. With this process, purified GaLV-pseudotyped LV enabled the transduction of 70% human CD34⁺ cells in the presence of the Vectofusin-1 peptide, whereas in the same conditions nonpurified vector transduced only 9% of the cells (multiplicity of infection 20). Our protocol will allow for the first time the purification of GaLV-TR-LV that are biologically active, stable, and with sufficient recovery in the perspective of preclinical studies and clinical applications. Obviously, further optimizations are required to improve final vector yields.

Improved GaLV-TR Glycoproteins to Pseudotype Lentiviral Vectors: Impact of Viral Protease Activity in the Production of LV Pseudotypes

Lentiviral vectors (LVs) are excellent tools for gene transfer into mammalian cells. It is noteworthy that the first gene therapy treatment using LVs was approved for commercialization in 2017. The G glycoprotein from rhabdovirus vesicular stomatitis virus (VSV-G) is the glycoprotein most used to pseudotype LVs, due to its high efficiency in transducing several cell types and its resistance to viral vector purification and storage conditions. However, VSV-G expression induces cytotoxicity, which limits LV production to short periods. As alternative to VSV-G, γ-retrovirus glycoproteins (4070A derived, GaLV derived, and RD114 derived) have been used to pseudotype both γ-retroviral vectors (RVs) and LVs. These glycoproteins do not induce cytotoxicity, allowing the development of stable LV producer cells. Additionally, these LV pseudotypes present higher transduction efficiencies of hematopoietic stem cells when compared to VSV-G. Here, new 4070A-, RD114-TR-, and GaLV-TR-derived glycoproteins were developed with the aim of improving its cytoplasmic tail R-peptide cleavage and thus increase LV infectious titers. The new glycoproteins were tested in transient LV production using the wild-type or the less active T26S HIV-1 protease. The GaLV-TR-derived glycoproteins were able to overcome titer differences observed between LV production using wild-type and T26S protease. Additionally, these glycoproteins were even able to increase LV titers, evidencing its potential as an alternative glycoprotein to pseudotype LVs.

Sequence Determinants in Gammaretroviral Env Cytoplasmic Tails Dictate Virus-Specific Pseudotyping Compatibility

Viruses can incorporate foreign glycoproteins to form infectious particles through a process known as pseudotyping. However, not all glycoproteins are compatible with all viruses. Despite the fact that viral pseudotyping is widely used, what makes a virus/glycoprotein pair compatible is poorly understood. To study this, we chose to analyze a gammaretroviral glycoprotein (Env) whose compatibility with different viruses could be modulated through small changes in its cytoplasmic tail (CT). One form of this glycoprotein is compatible with murine leukemia virus (MLV) particles but incompatible with human immunodeficiency virus type 1 (HIV-1) particles, while the second is compatible with HIV-1 particles but not with MLV particles. To decipher the factors affecting virus-specific Env incompatibility, we characterized Env incorporation, maturation, cell-to-cell fusogenicity, and virus-to-cell fusogenicity of each Env. The HIV-1 particle incompatibility correlated with less efficient cleavage of the R peptide by HIV-1 protease. However, the MLV particle incompatibility was more nuanced. MLV incompatibility appeared to be caused by lack of incorporation into particles, yet incorporation could be restored by further truncating the CT or by using a chimeric MLV Gag protein containing the HIV-1 MA without fully restoring infectivity. The MLV particle incompatibility appeared to be caused in part by fusogenic repression in MLV particles through an unknown mechanism. This study demonstrates that the Env CT can dictate functionality of Env within particles in a virus-specific manner.IMPORTANCE Viruses utilize viral glycoproteins to efficiently enter target cells during infection. How viruses acquire viral glycoproteins has been studied to understand the pathogenesis of viruses and develop safer and more efficient viral vectors for gene therapies. The CTs of viral glycoproteins have been shown to regulate various stages of glycoprotein biogenesis, but a gap still remains in understanding the molecular mechanism of glycoprotein acquisition and functionality regarding the CT. Here, we studied the mechanism of how specific mutations in the CT of a gammaretroviral envelope glycoprotein distinctly affect infectivity of two different viruses. Different mutations caused failure of glycoproteins to function in a virus-specific manner due to distinct fusion defects, suggesting that there are virus-specific characteristics affecting glycoprotein functionality.

Progresses towards safe and efficient gene therapy vectors

The emergence of genetic engineering at the beginning of the 1970′s opened the era of biomedical technologies, which aims to improve human health using genetic manipulation techniques in a clinical context. Gene therapy represents an innovating and appealing strategy for treatment of human diseases, which utilizes vehicles or vectors for delivering therapeutic genes into the patients' body. However, a few past unsuccessful events that negatively marked the beginning of gene therapy resulted in the need for further studies regarding the design and biology of gene therapy vectors, so that this innovating treatment approach can successfully move from bench to bedside. In this paper, we review the major gene delivery vectors and recent improvements made in their design meant to overcome the issues that commonly arise with the use of gene therapy vectors. At the end of the manuscript, we summarized the main advantages and disadvantages of common gene therapy vectors and we discuss possible future directions for potential therapeutic vectors.

Technical considerations for the generation of novel pseudotyped viruses

A pseudotyped virus (PV) is a virus particle with an envelope protein originating from a different virus. The ability to dictate which envelope proteins are expressed on the surface has made pseudotyping an important tool for basic virological studies such as determining the cellular targets of the envelope protein of the virus as well as identification of potential antiviral compounds and measuring specific antibody responses. In this review, we describe the common methodologies employed to generate PVs, with a focus on approaches to improve the efficacy of PV generation.

Pseudotyping of lentiviral vector with novel vesiculovirus envelope glycoproteins derived from Chandipura and Piry viruses

While the envelope glycoprotein of vesicular stomatitis virus (VSV-G) is widely used for pseudotyping of lentiviral vectors, sub-optimal gene transfer into certain cell types and its sensitivity to inactivation by human complement hinders its broader applications. To find alternative candidates, here we evaluated two serologically distinct novel viral envelopes derived from Chandipura (CNV-G) and Piry (PRV-G) vesiculoviruses. Both permitted generation of high titer psuedotyped lentiviral vectors with a capacity for high efficiency gene transfer into various cell types from different species. In human lymphoid and hematopoietic stem cells, their transduction efficiency was significantly lower than that of VSV-G. However, both novel envelopes were found to be more resistant to inactivation by human serum complement compared to VSV-G. Thus CNV-G and PRV-G envelopes can be harnessed for multiple uses in the future based on the cell type that needs to be gene transduced and possibly for in vivo gene transfer.

A neutralization assay for chikungunya virus infections in a multiplex format

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We established a CHIKV neutralization assay in a 384-well format.

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We used CHIKV pseudotyped lentiviral vectors encoding luciferase.

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We showed specific neutralization activity of patient sera.

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We developed a new multiplex neutralization assay for CHIKV.

Chikungunya virus (CHIKV) is a mosquito-transmitted Alphavirus that causes chikungunya fever and has infected millions of people mainly in developing countries. The associated disease is characterized by rash, high fever and severe arthritis that can persist for years. Since the epidemic on La Réunion in 2006, CHIKV has adapted to Aedes albopictus, which also inhabits temperate regions of the eastern and western hemispheres, including Europe and the United States. A. albopictus might continue migrating north with continuing climate change and CHIKV would then no longer be confined to the developing nations. No treatment or licensed CHIKV vaccine exists. A CHIKV neutralization assay in a 384-well format by using CHIKV-pseudotyped lentiviral vectors was established. This assay system can be used for entry inhibitor screening under a reduced safety level (S2). Production of CHIKV-pseudotyped lentiviral vectors and the reaction volume are optimized. A dose dependent, specific neutralization of CHIKV-pseudotyped vectors with sera of CHIKV-infected individuals could be measured in a 384-well format. A safe and simple multiplex assay for the analysis of CHIKV neutralizing activities was developed and will be able to improve drug and vaccine development as well as it would improve the understanding of CHIKV epidemics regarding antibody responses.

A bispecific transmembrane antibody simultaneously targeting intra- and extracellular epitopes of the epidermal growth factor receptor inhibits receptor activation and tumor cell growth

Epidermal growth factor receptor (EGFR) plays an important role in essential cellular processes such as proliferation, survival and migration. Aberrant activation of EGFR is frequently found in human cancers of various origins and has been implicated in cancer pathogenesis. The therapeutic antibody cetuximab (Erbitux) inhibits tumor growth by binding to the extracellular domain of EGFR, thereby preventing ligand binding and receptor activation. This activity is shared by the single chain antibody fragment scFv(225) that contains the same antigen binding domain. The unrelated EGFR-specific antibody fragment scFv(30) binds to the intracellular domain of the receptor and retains antigen binding upon expression as an intrabody in the reducing environment of the cytosol. Here, we used scFv(225) and scFv(30) domains to generate a novel type of bispecific transmembrane antibody termed 225.TM.30, that simultaneously targets intra- and extracellular EGFR epitopes. Bispecific 225.TM.30 and related membrane-anchored monospecific 225.TM and TM.30 proteins carrying extracellular scFv(225) or intracellular scFv(30) antibody fragments linked to a transmembrane domain were expressed in EGFR-overexpressing tumor cells using a doxycycline-inducible retroviral system. Induced expression of 225.TM.30 and 225.TM, but not TM.30 reduced EGFR surface levels and ligand-induced EGFR activation, while all three molecules markedly inhibited tumor cell growth. Co-localization of 225.TM with EGFR was predominantly found on the cell surface, while interaction with 225.TM.30 and TM.30 proteins resulted in the redistribution of EGFR to perinuclear compartments. Our data demonstrate functionality of this novel type of membrane-anchored intrabodies in tumor cells and suggest distinct modes of action of mono- and bispecific variants.

Efficient lentiviral transduction and transgene expression in primary human B cells

Primary human B cells are an attractive target for gene-therapeutic applications, but have been found to be relatively resistant toward transduction with lentiviral vectors (LVVs), even though a number of different envelope pseudotypes were tested. Moreover, low transgene expression in primary human B cells has impeded the use of LVVs for this target cell. We investigated the transduction potential of gibbon-ape leukemia virus (GALV) Env-pseudotyped LVVs for primary human B cells. By establishing optimized transduction kinetics and multiplicities of infection, we were able to regularly obtain transduction efficiencies of more than 50% in CD40L-activated B cells. Noteworthy, with the use of GALV-pseudotyped LVVs we could achieve a more than 10-fold higher yield of transduced activated B cells in direct comparison with LVVs pseudotyped with measles virus glycoproteins. Phenotyping of transduced primary B cells revealed a majority of memory B cells, a long-lived phenotype, presumed to be well suited for enduring therapeutic interventions. Finally, by combining the enhancer (Eμ) and the matrix/scaffold-attachment regions (MARs) of the human immunoglobulin heavy chain with the promoter of spleen focus-forming virus (SFFV) we aimed at generating a novel LVV particularly suitable for B cell transgenesis. We show that the optimized vector facilitated significantly higher transgene expression in various B cell lines and, more importantly, primary human B cells (mean factor of three). In summary, we have established a novel protocol for the efficient lentiviral transduction of primary human B cells and have improved transgene expression in B cells by a specific vector modification.

Library screening and receptor-directed targeting of gammaretroviral vectors

Gene- and cell-based therapies hold great potential for the advancement of the personalized medicine movement. Gene therapy vectors have made dramatic leaps forward since their inception. Retroviral-based vectors were the first to gain clinical attention and still offer the best hope for the long-term correction of many disorders. The fear of nonspecific transduction makes targeting a necessary feature for most clinical applications. However, this remains a difficult feature to optimize, with specificity often coming at the expense of efficiency. The aim of this article is to discuss the various methods employed to retarget retroviral entry. Our focus will lie on the modification of gammaretroviral envelope proteins with an in-depth discussion of the creation and screening of envelope libraries.

Developing novel lentiviral vectors into clinical products

Gene therapy vectors based on murine retroviruses have now been in clinical trials for over 20 years. During that time, a variety of novel vector pseudotypes were developed in an effort to improve gene transfer. Lentiviral vectors are now in clinical trials and a similar evolution of vector technology is anticipated. These modifications present challenges for those producing large-scale clinical materials. This chapter discusses approaches to process development for novel lentiviral vectors, highlight considerations, and methods to be incorporated into the development schema.

Lentiviral vectors: basic to translational

More than two decades have passed since genetically modified HIV was used for gene delivery. Through continuous improvements these early marker gene-carrying HIVs have evolved into safer and more effective lentiviral vectors. Lentiviral vectors offer several attractive properties as gene-delivery vehicles, including: (i) sustained gene delivery through stable vector integration into host genome; (ii) the capability of infecting both dividing and non-dividing cells; (iii) broad tissue tropisms, including important gene- and cell-therapy-target cell types; (iv) no expression of viral proteins after vector transduction; (v) the ability to deliver complex genetic elements, such as polycistronic or intron-containing sequences; (vi) potentially safer integration site profile; and (vii) a relatively easy system for vector manipulation and production. Accordingly, lentivector technologies now have widespread use in basic biology and translational studies for stable transgene overexpression, persistent gene silencing, immunization, in vivo imaging, generating transgenic animals, induction of pluripotent cells, stem cell modification and lineage tracking, or site-directed gene editing. Moreover, in the present high-throughput '-omics' era, the commercial availability of premade lentiviral vectors, which are engineered to express or silence genome-wide genes, accelerates the rapid expansion of this vector technology. In the present review, we assess the advances in lentiviral vector technology, including basic lentivirology, vector designs for improved efficiency and biosafety, protocols for vector production and infection, targeted gene delivery, advanced lentiviral applications and issues associated with the vector system.

Improved lentiviral gene transfer into human embryonic stem cells grown in co-culture with murine feeder and stroma cells

Genetic modification of human embryonic stem cells (hESCs) using biophysical DNA transfection methods are hampered by the very low single cell survival rate and cloning efficiency of hESCs. Lentiviral gene transfer strategies are widely used to genetically modify hESCs but limited transduction efficiencies in the presence of feeder or stroma cells present problems, particularly if vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped viral particles are applied. Here, we investigated whether the recently described semen derived enhancer of virus infection (SEVI) and alternative viral envelope proteins derived from either Gibbon ape leukaemia virus (GALV) or feline leukaemia virus (RD114) are applicable for transducing hESCs during co-culture with feeder or stroma cells. Our first set of experiments demonstrates that SEVI has no toxic effect on murine or hESCs and that exposure to SEVI does not interfere with the pluripotency-associated phenotype. Focusing on hESCs, we were able to further demonstrate that SEVI increases the transduction efficiencies of GALV and RD114 pseudotyped lentiviral vectors. More importantly, aiming at targeted differentiation of hESCs into functional somatic cell types, GALV pseudotyped lentiviral particles could efficiently and exclusively transduce hESCs grown in co-culture with OP9-GFP stroma cells (which were often used to induce differentiation into haematopoietic derivatives).

Sequences in gibbon ape leukemia virus envelope that confer sensitivity to HIV-1 accessory protein Vpu

HIV-1 efficiently forms pseudotyped particles with many gammaretrovirus glycoproteins, such as Friend murine leukemia virus (F-MLV) Env, but not with the related gibbon ape leukemia virus (GaLV) Env or with a chimeric F-MLV Env with a GaLV cytoplasmic tail domain (CTD). This incompatibility is modulated by the HIV-1 accessory protein Vpu. Because the GaLV Env CTD does not resemble tetherin or CD4, the well-studied targets of Vpu, we sought to characterize the modular sequence in the GaLV Env CTD required for this restriction in the presence of Vpu. Using a systematic mutagenesis scan, we determined that the motif that makes GaLV Env sensitive to Vpu is INxxIxxVKxxVxRxK. This region in the CTD of GaLV Env is predicted to form a helix. Mutations in the CTD that would break this helix abolish sensitivity to Vpu. Although many of these positions can be replaced with amino acids with similar biophysical properties without disrupting the Vpu sensitivity, the final lysine residue is required. This Vpu sensitivity sequence appears to be modular, as the unrelated Rous sarcoma virus (RSV) Env can be made Vpu sensitive by replacing its CTD with the GaLV Env CTD. In addition, F-MLV Env can be made Vpu sensitive by mutating two amino acids in its cytoplasmic tail to make it resemble more closely the Vpu sensitivity motif. Surprisingly, the core components of this Vpu sensitivity sequence are also present in the host surface protein CD4, which is also targeted by Vpu through its CTD.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Adeno-associated viral vectors for mapping, monitoring, and manipulating neural circuits

Understanding the structure and function of neural circuits is central is neuroscience research. To address the associated questions, new genetically encoded tools have been developed for mapping, monitoring, and manipulating neurons. Essential to implementation of these tools is their selective delivery to defined neuronal populations in the brain. This has been facilitated by recent improvements in cell type-specific transgene expression using recombinant adeno-associated viral vectors. Here, we highlight these developments and discuss areas for improvement that could further expand capabilities for neural circuit analysis.

Mechanisms for Env glycoprotein acquisition by retroviruses

A mandatory step in the formation of an infectious retroviral particle is the acquisition of its envelope glycoprotein (Env). This step invariably occurs by Env positioning itself in the host membrane at the location of viral budding and being incorporated along with the host membrane into the viral particle. In some ways, this step of the viral life cycle would appear to be imprecise. There is no specific sequence in Env or in the retroviral structural protein, Gag, that is inherently required for the production of an infectious Env-containing particle. Additionally, Env-defective proviruses can efficiently produce infectious particles with any of a number of foreign retroviral Env glycoproteins or even glycoproteins from unrelated viral families, a process termed pseudotyping. However, mounting evidence suggests that Env incorporation is neither passive nor random. Rather, several redundant mechanisms appear to contribute to the carefully controlled process of Env acquisition, many of which are apparently used by a wide variety of enveloped viruses. This review presents and discusses the evidence for these different mechanisms contributing to incorporation.

Retroviral vectors for gene therapy

Since their first clinical trial 20 years ago, retroviral (gretroviral and lentiviral) vectors have now been used in more than 350 gene-therapy studies. Retroviral vectors are particularly suited for gene-correction of cells due to long-term and stable expression of the transferred transgene(s), and also because little effort is required for their cloning and production. Several monogenic inherited diseases, mostly immunodeficiencies, can now be successfully treated. The occurrence of insertional mutagenesis in some studies allowed extensive analysis of integration profiles of retroviral vectors, as well as the design of lentiviral vectors with increased safety properties. These new-generation vectors will enable us to continue the successful story of gene therapy, and treat more patients and even more complex diseases.

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.

Lentiviral vectors

Lentiviral vectors have evolved over the last decade as powerful, reliable, and safe tools for stable gene transfer in a wide variety of mammalian cells. Contrary to other vectors derived from oncoretroviruses, they allow for stable gene delivery into most nondividing primary cells. In particular, lentivectors (LVs) derived from HIV-1 have gradually evolved to display many desirable features aimed at increasing both their safety and their versatility. This is why lentiviral vectors are becoming the most useful and promising tools for genetic engineering, to generate cells that can be used for research, diagnosis, and therapy. This chapter describes protocols and guidelines, for production and titration of LVs, which can be implemented in a research laboratory setting, with an emphasis on standardization in order to improve transposability of results between laboratories. We also discuss latest designs in LV technology.

RD114 envelope proteins provide an effective and versatile approach to pseudotype lentiviral vectors

Lentiviral vectors derived from the HIV-1 genome offer great promise for gene therapy due to their ability to transduce non-dividing cells and sustain long-term expression of transgenes. The majority of current lentiviral vectors are pseudotyped with the vesicular stomatitis viral envelope (VSV-G). VSV-G equips lentiviral vectors with a broad host cell tropism and increased stability. Increased particle stability enables viral supernatants to be concentrated by high-speed centrifugation to enhance their infectivity. Despite its efficacy, VSV-G is cytotoxic - a feature that prohibits the development of stable cell lines that constitutively express this envelope. Therefore, non-toxic envelope proteins are being investigated. RD114 is an attractive alternative because it also provides increased particle stability and its receptor is widely expressed on hematopoietic stem cells (HSCs). In this study, the packaging efficiency of three envelope proteins, RD114, RDpro and VSV-G, were evaluated with two lentiviral vectors (TRIP GFP and HPV-402). RDpro is an RD114-HIV chimera designed to pseudotype lentiviral vectors more efficiently. In transient systems, VSV-G generated titers of 10(8) and 10(7) viral particles/mL for TRIP GFP and HPV-402. RDpro possessed titers of 10(7) and 10(6), while RD114 titers were one log lower for each vector. Despite having relatively lower titers, RD114 proteins are less toxic; this was demonstrated in the extension of transient transfection reactions from 48 to 96 h. VSV-G transfections are generally limited to 48 h. In regard to gene therapy applications, we show that RDpro supernatants efficiently transduce peripheral blood HSCs. The versatility of RD114 envelopes was again demonstrated by using a 'mixed' expression system; composed of stably expressed RD114 envelope proteins to pseudotype lentiviral vectors generated in trans (titer range 10(3)-10(5)). Our data show that RD114 envelope proteins are effective and versatile constructs that could prove to be essential components of therapeutic lentiviral gene transfer systems.

Characterization of retroviral and lentiviral vectors pseudotyped with xenotropic murine leukemia virus-related virus envelope glycoprotein

Retroviral and lentiviral vectors are effective gene delivery vehicles that are being evaluated in clinical trials. Variations in the viral envelope (Env) glycoproteins, which are used to pseudotype retroviral or lentiviral vectors, can alter vector performance, including stability, titers, host range, and tissue tropism. Xenotropic murine leukemia virus (MLV)-related virus (XMRV) is a novel human retrovirus identified in patients with prostate cancer. XMRV targets XPR1 cell surface receptor, which is expressed in a broad range of human tissues including hematopoietic stem cells. Pseudotyping with XMRV Env would allow targeting of XPR1-expressing tissues. Here, we characterized XMRV Env-pseudotyped retroviral and lentiviral vectors. Although HIV and MLV vectors were poorly pseudotyped with wild-type XMRV Env, replacement of the C-terminal 11 amino acid residues in the transmembrane domain of XMRV Env with the corresponding 6 amino acid residues of amphotropic MLV Env (XMRV/R(ampho)) significantly increased XMRV Env-pseudotyped HIV and MLV vector titers. The transduction efficiency in human CD34(+) cells when using the XMRV/R(ampho)-pseudotyped HIV vector (10-20%) was comparable to that achieved when using the same infectious units of vesicular stomatitis virus G glycoprotein-pseudotyped vector (25%); thus the modified XMRV Env offers an alternative pseudotyping strategy for XPR1-mediated gene delivery.

Two distinct mechanisms regulate recruitment of murine leukemia virus envelope protein to retroviral assembly sites

The cytoplasmic tail domain (CTD) of retroviral envelope (Env) proteins has been implicated in modulating Env incorporation into viral particles. We generated a panel of murine leukemia virus (MLV) Env mutants and analyzed their ability to be recruited to human immunodeficiency virus-1 (HIV-1) assembly sites. Surprisingly, the entire CTD was dispensable for recruitment to assembly sites, but a mutation that disrupted the furin cleavage site in Env abolished recruitment. To determine if MLV Env can show selectivity for homologous assembly sites, cells were co-transfected with both HIV-1 and MLV assembly components along with each MLV Env construct and assayed for infectious particle production. MLV Env selectively formed infectious particles with the MLV components at the expense of infectious HIV-1 infectious particle production, but truncation of the CTD progressively reduced this selectivity. Collectively these data suggest that there are two separable mechanisms that govern MLV Env recruitment to viral assembly sites.

Flexibility in cell targeting by pseudotyping lentiviral vectors

Lentiviral vectors have become an important research tool and have just entered into clinical trials. As wild-type lentiviruses engage specific receptors that have limited tropism, most investigators have replaced the endogenous envelope glycoprotein with an alternative envelope. Such pseudotyped vectors have the potential to infect a wide variety of cell types and species. Alternatively, selection of certain viral envelope glycoproteins may also facilitate cell targeting to enhance directed gene transfer. We describe the method for generating pseudotyped vector and provide information regarding available pseudotypes and their respective target tissues.

The influence of semen-derived enhancer of virus infection on the efficiency of retroviral gene transfer

BACKGROUND

An improvement of retroviral infection has been postulated using a naturally occurring fragment of the abundant semen marker prostatic acidic phosphatase. This peptide, termed semen-derived enhancer of virus infection (SEVI), promotes HIV attachment to the target cells.

METHODS

In the present study, we examined whether SEVI would also enhance the infectivity of other viruses with different envelope proteins. We focused on retroviruses pseudotyped with envelopes that are commonly used for the genetic modification of cells, in particular, T cells and hematopoietic progenitor cells. Because the effect of SEVI is considered to be a result of its cationic properties, we compared SEVI with other cationic agents such as protamine sulfate and Polybrene.

RESULTS

We found that SEVI increases the efficiency of gene transfer for lentiviral and gammaretroviral vector constructs pseudotyped with VSV-G, GALV, RD114 or foamy viral envelopes on hematopoietic and nonhematopoietic cell lines. On T cells, the transduction efficiency of GALV and RD114 pseudotyped vectors was significantly increased by SEVI. A significant increase of the gene transfer rate was also detected for foamy virally pseudotyped lentivirus on murine hematopoietic progenitor cells. No toxic effect of SEVI treatment was detected on any cell type tested, including human and murine hematopoietic stem/progenitor cells. When directly comparing the effect of SEVI with Polybrene or protamine sulfate, we show that the semen-derived protein is more efficient in increasing the gene transfer rate.

CONCLUSIONS

SEVI is a promising agent for promoting and improving gene transfer and may also be useful for clinical gene therapy studies.

Minimal amino acid exchange in human TCR constant regions fosters improved function of TCR gene-modified T cells

TCR gene therapy using adoptive transfer of TCR gene-modified T cells is a new strategy for treatment of cancer. One critical prerequisite for TCR gene therapy is sufficient expression of transferred TCRs. Several strategies to achieve optimal expression were developed, including "murinization," which replaces the human TCRalpha and TCRbeta constant regions by their murine counterparts. Using a series of mouse-human hybrid constructs, we have identified nine amino acids responsible for the improved expression of murinized TCRs. Five essential amino acid exchanges were identified in the TCRbeta C region, with exchange of a glutamic acid (human) for a basic lysine (mouse) at position 18 of the C region, being most important. For the TCRalpha C region, an area of four amino acids was sufficient for improved expression. The minimally murinized TCR variants (harboring only nine residues of the mouse sequence) enhanced expression of human TCRs by supporting preferential pairing of transferred TCR chains and a more stable association with the CD3 proteins. Most important, usage of minimally murinized TCR chains improved the function of transduced primary human T cells in comparison with cells transduced with wild-type TCRs. For TCR gene therapy, the utilization of minimally instead of completely murinized constant regions dramatically reduces the number of foreign residues and thereby the risk for immunogenicity of therapeutic TCRs.

Thermosensitivity of the reverse transcription process as an inactivation mechanism of lentiviral vectors

Lentiviral vectors are an important tool for gene transfer research and gene therapy purposes. However, the low stability of these vectors affects their production, storage, and efficacy in preclinical and clinical settings. In the present work the mechanism underlying the thermosensitivity of lentiviral vectors was evaluated. For lentiviral vectors pseudotyped with amphotropic and RDpro envelopes, the capacity to perform reverse transcription was lost rapidly at 37 degrees C, in high correlation with the loss of infectivity. The vector with RDpro envelope presented a higher level of stability than that with amphotropic envelope for both the reverse transcription process and viral infectivity. Reverse transcriptase enzyme inactivation and viral template RNA degradation were not implicated in the loss of the viral capacity to perform reverse transcription. Furthermore, early entry steps in the infection process do not determine the rate of viral inactivation, as the amount of viral RNA and p24 protein entering the cells decreased slowly for both vectors. Taken together, it can be concluded that the reverse transcription process is thermolabile and thus determines the rate of lentiviral inactivation. Strategies to stabilize the reverse transcription process should be pursued to improve the applicability of lentiviral vectors in gene therapy.

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.

Pseudotyping incompatibility between HIV-1 and gibbon ape leukemia virus Env is modulated by Vpu

The Env protein from gibbon ape leukemia virus (GaLV) has been shown to be incompatible with human immunodeficiency virus type 1 (HIV-1) in the production of infectious pseudotyped particles. This incompatibility has been mapped to the C-terminal cytoplasmic tail of GaLV Env. Surprisingly, we found that the HIV-1 accessory protein Vpu modulates this incompatibility. The infectivity of HIV-1 pseudotyped with murine leukemia virus (MLV) Env was not affected by Vpu. However, the infectivity of HIV-1 pseudotyped with an MLV Env with the cytoplasmic tail from GaLV Env (MLV/GaLV Env) was restricted 50- to 100-fold by Vpu. A Vpu mutant containing a scrambled membrane-spanning domain, Vpu(RD), was still able to restrict MLV/GaLV Env, but mutation of the serine residues at positions 52 and 56 completely alleviated the restriction. Loss of infectivity appeared to be caused by reduced MLV/GaLV Env incorporation into viral particles. The mechanism of this downmodulation appears to be distinct from Vpu-mediated CD4 downmodulation because Vpu-expressing cells that failed to produce infectious HIV-1 particles nonetheless continued to display robust surface MLV/GaLV Env expression. In addition, if MLV and HIV-1 were simultaneously introduced into the same cells, only the HIV-1 particle infectivity was restricted by Vpu. Collectively, these data suggest that Vpu modulates the cellular distribution of MLV/GaLV Env, preventing its recruitment to HIV-1 budding sites.

Activation of transgene-specific T cells following lentivirus-mediated gene delivery to mouse lung

Integrating lentiviral vectors based on the human immunodeficiency virus type-1 (HIV-1) can transduce quiescent cells, which in lung account for almost 95% of the epithelial cell population. Pseudotyping lentiviral vectors with the envelope glycoprotein from the Ebola Zaire virus, the lymphocytic choriomeningitis virus (LCMV), the Mokola virus, and the vesicular stomatitis virus (VSV-G) resulted in transduction of mouse alveolar epithelium, but gene expression in the lung of C57BL/6 and BALB/c mice waned within 90 days of vector injection. Intratracheal delivery of the four pseudotyped lentiviral vectors resulted in transgene-specific T-cell activation in both mouse strains, albeit lower than that achieved by intramuscular injection of the vectors. We performed an adoptive transfer of luciferase-specific T cells, isolated from spleen or lung of donor mice injected with VSV-G-pseudotyped lentivirus vector expressing luciferase into the muscle or lung, respectively, into recipient recombination-activating gene (RAG)–deficient mice transduced in lung with adenovirus expressing firefly luciferase (ffluc2). Gene expression declined within 7 days of adoptive transfer approaching background levels by day 36. Taken together, our results suggest that the loss of transduced cells in lung is due to VSV-G.HIV vector–mediated activation of transgene-specific T cells rather than as result of normal turnover of airway cells.

Lentiviral vectors with measles virus glycoproteins - dream team for gene transfer?

Lentiviral vectors are potent gene transfer vehicles frequently applied in research and lately also in clinical applications. Recent improvements have come from combining lentiviral vectors with engineered envelope proteins, which now allow targeting of cell entry to any cell population of interest, as well as the transduction of quiescent cells of the haematopoietic system. We propose that measles virus envelope glycoproteins are especially well suited for this purpose because they can mediate pH-independent cell entry at the cell surface membrane and can induce cytoskeleton rearrangements that facilitate the transport of lentiviral core particles to the cell nucleus. Lentiviral vectors pseudotyped with measles virus glycoproteins are expected to improve the safety and efficacy of gene transfer to human cells.

Foreign glycoproteins can be actively recruited to virus assembly sites during pseudotyping

Retroviruses like human immunodeficiency virus type 1 (HIV-1), as well as many other enveloped viruses, can efficiently produce infectious virus in the absence of their own surface glycoprotein if a suitable glycoprotein from a foreign virus is expressed in the same cell. This process of complementation, known as pseudotyping, often can occur even when the glycoprotein is from an unrelated virus. Although pseudotyping is widely used for engineering chimeric viruses, it has remained unknown whether a virus can actively recruit foreign glycoproteins to budding sites or, alternatively, if a virus obtains the glycoproteins through a passive mechanism. We have studied the specificity of glycoprotein recruitment by immunogold labeling viral glycoproteins and imaging their distribution on the host plasma membrane using scanning electron microscopy. Expressed alone, all tested viral glycoproteins were relatively randomly distributed on the plasma membrane. However, in the presence of budding HIV-1 or Rous sarcoma virus (RSV) particles, some glycoproteins, such as those encoded by murine leukemia virus and vesicular stomatitis virus, were dramatically redistributed to viral budding sites. In contrast, the RSV Env glycoprotein was robustly recruited only to the homologous RSV budding sites. These data demonstrate that viral glycoproteins are not in preformed membrane patches prior to viral assembly but rather that glycoproteins are actively recruited to certain viral assembly sites.

Highly efficient transduction of human plasmacytoid dendritic cells without phenotypic and functional maturation

BACKGROUND

Gene modified dendritic cells (DC) are able to modulate DC functions and induce therapeutic immunity or tolerance in an antigen-specific manner. Among the different DC subsets, plasmacytoid DC (pDC) are well known for their ability to recognize and respond to a variety of viruses by secreting high levels of type I interferon.

METHODS

We analyzed here, the transduction efficiency of a pDC cell line, GEN2.2, and of pDC derived from CD34+ progenitors, using lentiviral vectors (LV) pseudotyped with different envelope glycoproteins such as the vesicular stomatitis virus envelope (VSVG), the gibbon ape leukaemia virus envelope (GaLV) or the feline endogenous virus envelope (RD114). At the same time, we evaluated transgene expression (E-GFP reporter gene) under the control of different promoters.

RESULTS

We found that efficient gene transfer into pDC can be achieved with VSVG-pseudotyped lentiviral vectors (LV) under the control of phoshoglycerate kinase (PGK) and elongation factor-1 (EF1alpha) promoters (28% to 90% of E-GFP+ cells, respectively) in the absence of phenotypic and functional maturation. Surprisingly, promoters (desmin or synthetic C5-12) described as muscle-specific and which drive gene expression in single strand AAV vectors in gene therapy protocols were very highly active in pDC using VSVG-LV.

CONCLUSION

Taken together, our results indicate that LV vectors can serve to design pDC-based vaccines in humans, and they are also useful in vitro to evaluate the immunogenicity of the vector preparations, and the specificity and safety of given promoters used in gene therapy protocols.

Stable transduction of quiescent T cells without induction of cycle progression by a novel lentiviral vector pseudotyped with measles virus glycoproteins

A major limitation of current lentiviral vectors (LVs) is their inability to govern efficient gene transfer into quiescent cells such as primary T cells, which hampers their application for gene therapy. Here we generated high-titer LVs incorporating Edmonston measles virus (MV) glycoproteins H and F on their surface. They allowed efficient transduction through the MV receptors, SLAM and CD46, both present on blood T cells. Indeed, these H/F-displaying vectors outperformed by far VSV-G-LVs for the transduction of IL-7-prestimulated T cells. More importantly, a single exposure to these H/F-LVs allowed efficient gene transfer in quiescent T cells, which are not permissive for VSV-G-LVs that need cell-cycle entry into the G1b phase for efficient transduction. High-level transduction of resting memory (50%) and naive (11%) T cells with H/F-LVs, which seemed to occur mainly through SLAM, was not at cost of cell-cycle entry or of target T-cell activation. Finally, the naive or memory phenotypes of transduced resting T cells were maintained and no changes in cytokine profiles were detected, suggesting that T-cell populations were not skewed. Thus, H/F-LV transduction of resting T cells overcomes the limitation of current lentiviral vectors and may improve the efficacy of T cell-based gene therapy.

Polymers for viral gene delivery

BACKGROUND

The development of viral vectors capable of providing efficient gene transfer in diseased tissues without causing any pathogenic effects is pivotal for overcoming the many challenges facing gene therapy.

OBJECTIVE

Immune responses against viral vectors, inadequate gene expression and inefficient targeting to specific cells in vivo are some of the major problems limiting the clinical utility of viral gene therapy.

METHODS

This review will focus on recent progress in strategic polymer-based modifications to improve the performance and biocompatibility of a variety of viral vectors. We will discuss the preclinical development of four approaches involving injectable polymers, polyelectrolytes, polymer microspheres and polymer-virus conjugates.

RESULTS/CONCLUSION

Much progress has been made in creating 'hybrid' gene delivery vectors that combine the strengths of polymers and viruses. With further optimization, these hybrid vectors, which may be safer and more effective, are likely to succeed in clinical applications.