Characterization of HIV-1 vectors with gammaretrovirus envelope glycoproteins produced from stable packaging cells

Efficient clinical-grade γ-retroviral vector purification by high-speed centrifugation for CAR T cell manufacturing

γ-Retroviral vectors (γ-RV) are powerful tools for gene therapy applications. Current clinical vectors are produced from stable producer cell lines which require minimal further downstream processing, while purification schemes for γ-RV produced by transient transfection have not been thoroughly investigated. We aimed to develop a method to purify transiently produced γ-RV for early clinical studies. Here, we report a simple one-step purification method by high-speed centrifugation for γ-RV produced by transient transfection for clinical application. High-speed centrifugation enabled the concentration of viral titers in the range of 10⁷-10⁸ TU/mL with >80% overall recovery. Analysis of research-grade concentrated vector revealed sufficient reduction in product- and process-related impurities. Furthermore, product characterization of clinical-grade γ-RV by BioReliance demonstrated two-logs lower impurities per transducing unit compared with regulatory authority-approved stable producer cell line vector for clinical application. In terms of CAR T cell manufacturing, clinical-grade γ-RV produced by transient transfection and purified by high-speed centrifugation was similar to γ-RV produced from a clinical-grade stable producer cell line. This method will be of value for studies using γ-RV to bridge vector supply between early- and late-stage clinical trials.

Lentiviral Vector Bioprocessing

Lentiviral vectors (LVs) are potent tools for the delivery of genes of interest into mammalian cells and are now commonly utilised within the growing field of cell and gene therapy for the treatment of monogenic diseases and adoptive therapies such as chimeric antigen T-cell (CAR-T) therapy. This is a comprehensive review of the individual bioprocess operations employed in LV production. We highlight the role of envelope proteins in vector design as well as their impact on the bioprocessing of lentiviral vectors. An overview of the current state of these operations provides opportunities for bioprocess discovery and improvement with emphasis on the considerations for optimal and scalable processing of LV during development and clinical production. Upstream culture for LV generation is described with comparisons on the different transfection methods and various bioreactors for suspension and adherent producer cell cultivation. The purification of LV is examined, evaluating different sequences of downstream process operations for both small- and large-scale production requirements. For scalable operations, a key focus is the development in chromatographic purification in addition to an in-depth examination of the application of tangential flow filtration. A summary of vector quantification and characterisation assays is also presented. Finally, the assessment of the whole bioprocess for LV production is discussed to benefit from the broader understanding of potential interactions of the different process options. This review is aimed to assist in the achievement of high quality, high concentration lentiviral vectors from robust and scalable processes.

Progress and Perspectives in the Development of Lentiviral Vector Producer Cells

After two decades of clinical trials, gene therapy demonstrated effectiveness in the treatment of a series of diseases. Currently, several gene therapy products are approved and used in the clinic. Lentiviral vectors (LVs) are one of the most used transfer vehicles to deliver genetic material and the vector of choice to modify hematopoietic cells to correct primary immunodeficiencies, hemoglobinopathies, and leukodystrophies. LVs are also widely used to modify T cells to treat cancers in immunotherapies (e.g., chimeric antigen receptors T cell therapies, CAR-T). In genome editing, LVs are used to deliver sequence-specific designer nucleases and DNA templates. The approval LV gene therapy products (e.g., Kymriah, for B-cell Acute lymphoblastic leukemia treatment; LentiGlobin, for β-thalassemia treatment) reinforced the need to improve their bioprocess manufacturing. The production has been mostly dependent on transient transfection. Production from stable cell lines facilitate GMP compliant processes, providing an easier scale-up, reproducibility and cost-effectiveness. The establishment of stable LV producer cell lines presents, however, several difficulties, with the cytotoxicity of some of the vector proteins being a major challenge. Genome editing technologies pose additional challenges to LV producer cells. Herein the major bottlenecks, recent achievements, and perspectives in the development of LV stable cell lines are revised.

Lentiviral Vector Purification Using Nanofiber Ion-Exchange Chromatography

Lentiviral vectors (LVs) are used in cell and gene therapies due to their ability to transduce both dividing and non-dividing cells while carrying a relatively large genetic payload and providing long-term gene expression via gene integration. Current cultivation methods produce titers of 10⁵–10⁷ transduction unit (TU)/mL; thus, it is necessary to concentrate LVs as well as remove process- and product-related impurities. In this work, we used a packaging cell line WinPac-RD-HV for LV production to simplify upstream processing. A direct capture method based on ion-exchange chromatography and cellulose nanofibers for LV concentration and purification was developed. This novel scalable stationary phase provides a high surface area that is accessible to LV and, therefore, has potential for high-capacity operation compared to traditional bead-based supports. We were able to concentrate LVs 100-fold while achieving a two-log removal of host cell protein and maintaining up to a 90% yield of functional vector.

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.

A simple and highly efficient method for transduction of human adipose-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating into a wide range of cell types and provide a potential to transfer therapeutic protein in vivo, making them valuable candidates for gene therapy and cell therapy. However, using MSCs in in vivo is limited due to the low rate of transfection and transduction efficacy. Therefore, developing methods to efficiently transfer genes into MSCs would provide a number of opportunities for using them in the clinic. Here, we introduce a simple and robust method for efficient transduction of human adipose-derived MSCs by modification under the culture condition of human embryonic kidney cells 293 (HEK293T) and MSCs. Moreover, as a transduction enhancer, polybrene was replaced with Lipofectamine, a cationic lipid. Therefore, we showed that transduction of primary cells can be increased efficiently by modifying the culture condition.

Lentivector Producer Cell Lines with Stably Expressed Vesiculovirus Envelopes

Retroviral and lentiviral vectors often use the envelope G protein from the vesicular stomatitis virus Indiana strain (VSVind.G). However, lentivector producer cell lines that stably express VSVind.G have not been reported, presumably because of its cytotoxicity, preventing simple scale-up of vector production. Interestingly, we showed that VSVind.G and other vesiculovirus G from the VSV New Jersey strain (VSVnj), Cocal virus (COCV), and Piry virus (PIRYV) could be constitutively expressed and supported lentivector production for up to 10 weeks. All G-enveloped particles were robust, allowing concentration and freeze-thawing. COCV.G and PIRYV.G were resistant to complement inactivation, and, using chimeras between VSVind.G and COCV.G, the determinant for complement inactivation of VSVind.G was mapped to amino acid residues 136-370. Clonal packaging cell lines using COCV.G could be generated; however, during attempts to establish LV producer cells, vector superinfection was observed following the introduction of a lentivector genome. This could be prevented by culturing the cells with the antiviral drug nevirapine. As an alternative countermeasure, we demonstrated that functional lentivectors could be reconstituted by admixing supernatant from stable cells producing unenveloped virus with supernatant containing envelopes harvested from cells stably expressing VSVind.G, COCV.G, or PIRYV.G.

LentiPro26: novel stable cell lines for constitutive lentiviral vector production

Lentiviral vectors (LVs) are excellent tools to promote gene transfer and stable gene expression. Their potential has been already demonstrated in gene therapy clinical trials for the treatment of diverse disorders. For large scale LV production, a stable producer system is desirable since it allows scalable and cost-effective viral productions, with increased reproducibility and safety. However, the development of stable systems has been challenging and time-consuming, being the selection of cells presenting high expression levels of Gag-Pro-Pol polyprotein and the cytotoxicity associated with some viral components, the main limitations. Hereby is described the establishment of a new LV producer cell line using a mutated less active viral protease to overcome potential cytotoxic limitations. The stable transfection of bicistronic expression cassettes with re-initiation of the translation mechanism enabled the generation of LentiPro26 packaging populations supporting high titers. Additionally, by skipping intermediate clone screening steps and performing only one final clone screening, it was possible to save time and generate LentiPro26-A59 cell line, that constitutively produces titers above 106 TU.mL-1.day-1, in less than six months. This work constitutes a step forward towards the development of improved LV producer cell lines, aiming to efficiently supply the clinical expanding gene therapy applications.

Integrated Method for Purification and Single-Particle Characterization of Lentiviral Vector Systems by Size Exclusion Chromatography and Tunable Resistive Pulse Sensing

Elements derived from lentiviral particles such as viral vectors or virus-like particles are commonly used for biotechnological and biomedical applications, for example in mammalian protein expression, gene delivery or therapy, and vaccine development. Preparations of high purity are necessary in most cases, especially for clinical applications. For purification, a wide range of methods are available, from density gradient centrifugation to affinity chromatography. In this study we have employed size exclusion columns specifically designed for the easy purification of extracellular vesicles including exosomes. In addition to viral marker protein and total protein analysis, a well-established single-particle characterization technology, termed tunable resistive pulse sensing, was employed to analyze fractions of highest particle load and purity and characterize the preparations by size and surface charge/electrophoretic mobility. With this study, we propose an integrated platform combining size exclusion chromatography and tunable resistive pulse sensing for monitoring production and purification of viral particles.

Mass spectrometry analysis reveals differences in the host cell protein species found in pseudotyped lentiviral vectors

Lentiviral vectors (LVs) have been successfully used in clinical trials showing long term therapeutic benefits. Studying the role of cellular proteins in lentivirus HIV-1 life cycle can help understand virus assembly and budding, leading to improvement of LV production for gene therapy. Lentiviral vectors were purified using size exclusion chromatography (SEC). The cellular protein composition of LVs produced by two different methods was compared: the transient transfection system pseudotyped with the VSV-G envelope, currently used in clinical trials, and a stable producer cell system using a non-toxic envelope derived from cat endogenous retrovirus RD114, RDpro. Proteins of LVs purified by size exclusion chromatography were identified by tandem mass spectrometry (MS/MS). A smaller number of cellular protein species were detected in stably produced vectors compared to transiently produced vector samples. This may be due to the presence of co-purified VSV-G vesicles in transiently produced vectors. AHNAK (Desmoyokin) was unique to RDpro-Env vectors. The potential role in LV particle production of selected proteins identified by MS analysis including AHNAK was assessed using shRNA gene knockdown technique. Down-regulation of the selected host proteins AHNAK, ALIX, and TSG101 in vector producer cells did not result in a significant difference in vector production.

Interaction between von Hippel-Lindau Protein and Fatty Acid Synthase Modulates Hypoxia Target Gene Expression

Hypoxia-inducible factors (HIFs) play a central role in the transcriptional response to changes in oxygen availability. Stability of HIFs is regulated by multi-step reactions including recognition by the von Hippel-Lindau tumour suppressor protein (pVHL) in association with an E3 ligase complex. Here we show that pVHL physically interacts with fatty acid synthase (FASN), displacing the E3 ubiquitin ligase complex. This results in HIF-α protein stabilization and activation of HIF target genes even in normoxia such as during adipocyte differentiation. 25-hydroxycholesterol (25-OH), an inhibitor of FASN expression, also inhibited HIF target gene expression in cultured cells and in mouse liver. Clinically, FASN is frequently upregulated in a broad variety of cancers and has been reported to have an oncogenic function. We found that upregulation of FASN correlated with induction of many HIF target genes, notably in a malignant subtype of prostate tumours. Therefore, pVHL-FASN interaction plays a regulatory role for HIFs and their target gene expression.

Vectofusin-1 Promotes RD114-TR-Pseudotyped Lentiviral Vector Transduction of Human HSPCs and T Lymphocytes

Ex vivo transduction of human CD34⁺ hematopoietic stem/progenitor cells (hCD34⁺ HSPCs) and T lymphocytes is a key process that requires high efficiency and low toxicity to achieve effective clinical results. So far, several enhancers have been used to improve this process. Among them, Retronectin highly meliorates VSV-G and RD114-TR pseudotyped lentiviral vector delivery in hCD34⁺ HSPCs and T lymphocytes. However, Retronectin is expensive and requires pre-coating of culture dishes or bags before cell seeding, resulting in a cumbersome procedure. Recently, an alternative transduction adjuvant has been developed, named Vectofusin-1, whose effect has been demonstrated on gene delivery to cell lines and primary hCD34⁺ HSPCs by lentiviral vectors pseudotyped with different envelope glycoproteins. In this study, we have focused our analysis on the effect of Vectofusin-1 on the transduction of hCD34⁺ HSPCs and T lymphocytes by using mostly RD114-TR pseudotyped lentivectors and clinical transduction protocols. Here, we have proved that Vectofusin-1 reproducibly enhances gene delivery to hCD34⁺ HSPCs and activated T cells without cell toxicity and with efficacy comparable to that of Retronectin. The use of Vectofusin-1 will therefore help to shorten and simplify clinical cell manipulation, especially if automated systems are planned for transducing large-scale clinical lots.

RD-MolPack technology for the constitutive production of self-inactivating lentiviral vectors pseudotyped with the nontoxic RD114-TR envelope

To date, gene therapy with transiently derived lentivectors has been very successful to cure rare infant genetic diseases. However, transient manufacturing is unfeasible to treat adult malignancies because large vector lots are required. By contrast, stable manufacturing is the best option for high-incidence diseases since it reduces the production cost, which is the major current limitation to scale up the transient methods. We have previously developed the proprietary RD2-MolPack technology for the stable production of second-generation lentivectors, based on the RD114-TR envelope. Of note, opposite to vesicular stomatitis virus glycoprotein (VSV-G) envelope, RD114-TR does not need inducible expression thanks to lack of toxicity. Here, we present the construction of RD2- and RD3-MolPack cells for the production of self-inactivating lentivectors expressing green fluorescent protein (GFP) as a proof-of-concept of the feasibility and safety of this technology before its later therapeutic exploitation. We report that human T lymphocytes transduced with self-inactivating lentivectors derived from RD3-MolPack cells or with self-inactivating VSV-G pseudotyped lentivectors derived from transient transfection show identical T-cell memory differentiation phenotype and comparable transduction efficiency in all T-cell subsets. RD-MolPack technology represents, therefore, a straightforward tool to simplify and standardize lentivector manufacturing to engineer T-cells for frontline immunotherapy applications.

A novel intranuclear RNA vector system for long-term stem cell modification

Genetically modified stem and progenitor cells have emerged as a promising regenerative platform in the treatment of genetic and degenerative disorders, highlighted by their successful therapeutic use in inherent immunodeficiencies. However, biosafety concerns over insertional mutagenesis resulting from integrating recombinant viral vectors have overshadowed the widespread clinical applications of genetically modified stem cells. Here, we report an RNA-based episomal vector system, amenable for long-term transgene expression in stem cells. Specifically, we used a unique intranuclear RNA virus, Borna disease virus (BDV), as the gene transfer vehicle, capable of persistent infections in various cell types. BDV-based vectors allowed for long-term transgene expression in mesenchymal stem cells (MSCs) without affecting cellular morphology, cell surface CD105 expression, or the adipogenicity of MSCs. Similarly, replication-defective BDV vectors achieved long-term transduction of human induced pluripotent stem cells (iPSCs), while maintaining the ability to differentiate into three embryonic germ layers. Thus, the BDV-based vectors offer a genomic modification-free, episomal RNA delivery system for sustained stem cell transduction.

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.

Construction of stable packaging cell lines for clinical lentiviral vector production

Lentiviral vectors are useful experimental tools for stable gene delivery and have been used to treat human inherited genetic disorders and hematologic malignancies with promising results. Because some of the lentiviral vector components are cytotoxic, transient plasmid transfection has been used to produce the large batches needed for clinical trials. However, this method is costly, poorly reproducible and hard to scale up. Here we describe a general method for construction of stable packaging cell lines that continuously produce lentiviral vectors. This uses Cre recombinase-mediated cassette exchange to insert a codon-optimised HIV-1 Gag-Pol expression construct in a continuously expressed locus in 293FT cells. Subsequently Rev, envelope and vector genome expression cassettes are serially transfected. Vector titers in excess of 10⁶ transducing units/ml can be harvested from the final producer clones, which can be increased to 10⁸ TU/ml by concentration. This method will be of use to all basic and clinical investigators who wish to produce large batches of lentiviral vectors.

RD2-MolPack-Chim3, a packaging cell line for stable production of lentiviral vectors for anti-HIV gene therapy

Over the last two decades, several attempts to generate packaging cells for lentiviral vectors (LV) have been made. Despite different technologies, no packaging clone is currently employed in clinical trials. We developed a new strategy for LV stable production based on the HEK-293T progenitor cells; the sequential insertion of the viral genes by integrating vectors; the constitutive expression of the viral components; and the RD114-TR envelope pseudotyping. We generated the intermediate clone PK-7 expressing constitutively gag/pol and rev genes and, by adding tat and rd114-tr genes, the stable packaging cell line RD2-MolPack, which can produce LV carrying any transfer vector (TV). Finally, we obtained the RD2-MolPack-Chim3 producer clone by transducing RD2-MolPack cells with the TV expressing the anti-HIV transgene Chim3. Remarkably, RD114-TR pseudovirions have much higher potency when produced by stable compared with transient technology. Most importantly, comparable transduction efficiency in hematopoietic stem cells (HSC) is obtained with 2-logs less physical particles respect to VSV-G pseudovirions produced by transient transfection. Altogether, RD2-MolPack technology should be considered a valid option for large-scale production of LV to be used in gene therapy protocols employing HSC, resulting in the possibility of downsizing the manufacturing scale by about 10-fold in respect to transient technology.

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.

Retroviral and lentiviral vectors for the induction of immunological tolerance

Retroviral and lentiviral vectors have proven to be particularly efficient systems to deliver genes of interest into target cells, either in vivo or in cell cultures. They have been used for some time for gene therapy and the development of gene vaccines. Recently retroviral and lentiviral vectors have been used to generate tolerogenic dendritic cells, key professional antigen presenting cells that regulate immune responses. Thus, three main approaches have been undertaken to induce immunological tolerance; delivery of potent immunosuppressive cytokines and other molecules, modification of intracellular signalling pathways in dendritic cells, and de-targeting transgene expression from dendritic cells using microRNA technology. In this review we briefly describe retroviral and lentiviral vector biology, and their application to induce immunological tolerance.

Effects of over-expression of ANXA10 gene on proliferation and apoptosis of hepatocellular carcinoma cell line HepG2

The effects of over-expression of ANXA10 gene on proliferation and apoptosis of hepato-cellular carcinoma cell line HepG2 were elucidated. The human ANXA10 gene was subcloned into the lentiviral vector, PGC-FU, to generate the lentiviral expression vector, PGC-FU-ANXA10. The corrected ANXA10 was confirmed by endoenzyme digestion, and sequencing. Recombinant lentiviruses were produced by 293T cells following the co-transfection of PGC-FU-ANXA10 with the packaging plasmids pHelper1.0 and pHelper2.0. The resulting recombinant lentiviruses carrying ANXA10 were then used to infect human embryonic kidney epithelial cells, and lentiviral particles were produced. The ANXA10 expression in 293T cells was detected by using fluorescent microscope and Western blotting. HepG2 cells were infected, and divided into PGC-Fu-ANXA10 group, PGC-Fu group and HepG2 cell group. The changes of ANXA10 mRNA and protein expression were detected by using RT-PCR and Western blotting respectively. Flow cytometry and MTT assay were performed to examine the changes in cell apoptosis and proliferation respectively. The recombinant PGC-FU-ANXA10 vector was successfully constructed, the ANXA10 protein was detected by using Western blotting, and virus titer was 2×10(8) TU/mL. The recombinant lentiviruses were effectively infected into HepG2 cells in vitro and the infection efficiency was 70%. At 72 h after infection, the ANXA10 mRNA and protein expression levels in PGC-Fu-ANXA10 group were significantly higher than in PGC-Fu group and HepG2 cell group (P<0.05); the in vitro growth inhibition rate of HepG2 cells in PGC-Fu-ANXA10 group was 24.65%, significantly higher than that in PGC-Fu group and HepG2 cell group (P<0.05), but there was no significant difference between PGC-Fu group and HepG2 cell group; the apoptosis rate in PGC-Fu-ANXA10 group, PGC-Fu group and HepG2 cell group was (51.92±1.41)%, (19.00±1.12)% and (3.59±0.89)% respectively. The apoptosis rate in PGC-Fu-ANXA10 group was significantly higher than in PGC-Fu group and HepG2 cell group (P<0.05). The recombinant lentiviruses PGC-FU-ANXA10 were constructed successfully and infected into HepG2 cells. The overexpression of ANXA10 gene can significantly inhibit proliferation and promote apoptosis of HepG2 cells in vitro.

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.

RGB marking with lentiviral vectors for multicolor clonal cell tracking

Cells transduced with lentiviral vectors are individually marked by a highly characteristic pattern of insertion sites inherited by all their progeny. We have recently extended this principle of clonal cell marking by introducing the method of RGB marking, which makes use of the simultaneous transduction of target cells with three lentiviral gene ontology (LeGO) vectors encoding red, green or blue fluorescent proteins. In accordance with the additive color model, individual RGB-marked cells display a large variety of unique and highly specific colors. Color codes remain stable after cell division and can thus be used for clonal tracking in vivo and in vitro. Our protocol for efficient RGB marking is based on established methods of lentiviral vector production (3-4 d) and titration (3 d). The final RGB-marking step requires concurrent transduction with the three RGB vectors at equalized multiplicities of infection (1-12 h). The initial efficiency of RGB marking can be assessed after 2-4 d by flow cytometry and/or fluorescence microscopy.

A small-molecule-controlled system for efficient pseudotyping of prototype foamy virus vectors

Foamy virus (FV) vector systems have recently demonstrated their power as efficient gene transfer tools for different target tissues. Unfortunately, FVs cannot be naturally pseudotyped by heterologous viral glycoproteins due to an unusual particle morphogenesis involving a FV Env-dependent particle release process. Therefore, current FV vector systems are constrained to the broad host cell range provided by the cognate viral glycoprotein. We evaluated different approaches for pseudotyping of FV vectors, in which the specific FV Gag-Env interaction, essential for particle egress, is substituted by a small-molecule controlled heterodimerization (HD) system. In one system developed, one HD-domain (HDD) is fused to a membrane-targeting domain (MTD), such as the human immunodeficiency virus (HIV) Gag matrix (MA) subunit, with a second fused to the FV capsid protein. Coexpression of both components with different heterologous viral glycoproteins allowed an efficient, dimerizer-dependent pseudotyping of FV capsids. With this system FV vesicular stomatitis virus glycoprotein (VSV-G) pseudotype titers greater than 1 × 10(6) IU/ml were obtained, at levels comparable to authentic FV vector particles. As a proof-of-principle we demonstrate that Pac2 cells, naturally resistant to FV vectors, become permissive to FV VSV-G pseudotypes. Similar to other retroviral vectors, this FV pseudotyping system now enables adaptation of cell-specific targeting approaches for FVs.

Construction of stable producer cells to make high-titer lentiviral vectors for dendritic cell-based vaccination

Lentiviral vectors (LVs) enveloped with an engineered Sindbis virus glycoprotein can specifically bind to dendritic cells (DCs) through the surface receptor DC-SIGN and induce antigen expression, thus providing an efficient method for delivering DC-directed vaccines. In this study, we constructed a stable producer line (LV-MGFP) for synthesizing DC-SIGN-targeted HIV-1-based LVs (DC-LVs) encoding green fluorescent protein (GFP) by a concatemeric array transfection technique. We demonstrated that the established stable clones could routinely produce vector supernatants with titers above 10(7) transduction units per milliliter (TU/mL) during a continuous 3-month cell passage. The producer cells were also capable of generating similar titers of DC-LVs in serum-free medium. Moreover, the addition of 1-deoxymannojirimycin (DMJ) enabled the producer cells to manufacture DC-LVs with both improved titers and enhanced potency to evoke antigen-specific CD8(+) T cell responses in mice. The stable lines could accommodate the replacement of the internal murine stem cell virus (MSCV) promoter with the human ubiquitin-C (Ubi) promoter in the lentiviral backbone. The resulting DC-LVs bearing Ubi exhibited the enhanced potency to elicit vaccine-specific immunity. Based on accumulated evidence, our studies support the application of this production method in manufacturing DC-LVs for preclinical and clinical testing of novel DC-based immunization.

Cell and Tissue Gene Targeting with Lentiviral Vectors

One of the main advantages of using lentivectors is their capacity to transduce a wide range of cell types, independently from the cell cycle stage. However, transgene expression in certain cell types is sometimes not desirable, either because of toxicity, cell transformation, or induction of transgene-specific immune responses. In other cases, specific targeting of only cancerous cells within a tumor is sought after for the delivery of suicide genes. Consequently, great effort has been invested in developing strategies to control transgene delivery/expression in a cell/tissue-specific manner. These strategies can broadly be divided in three; particle pseudotyping (surface targeting), which entails modification of the envelope glycoprotein (ENV); transcriptional targeting, which utilizes cell-specific promoters and/or inducible promoters; and posttranscriptional targeting, recently applied in lentivectors by introducing sequence targets for cell-specific microRNAs. In this chapter we describe each of these strategies providing some illustrative examples.

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.

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.

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.

Efficient human hematopoietic cell transduction using RD114- and GALV-pseudotyped retroviral vectors produced in suspension and serum-free media

Retroviral vectors derived from the Moloney murine leukemia virus have been used in successful and promising gene therapy clinical trials. However, platforms for their large-scale production must be further developed. As a proof of principle, we reported the generation of a packaging cell line that produces amphotropic retroviral vectors in suspension and serum-free medium (SFM). In the present study, we have constructed and characterized two retroviral packaging cell lines designed for gene transfer in hematopoietic cells. These cell lines grow in suspension and SFM, and produce high-titer RD114- and gibbon ape leukemia virus (GALV)-pseudotyped vectors for a 3-month culture period. Viral particles released are as robust during repeated freeze-thaw cycles and on thermal inactivation at 37 degrees C as their counterparts produced in cells cultured adherently with serum. We also show that RD114- and GALV-pseudotyped vectors produced in suspension and SFM efficiently transduce human lymphocytes and hematopoietic stem cells. As these retroviral packaging cell lines distinctively maintain high vector titers while growing in suspension and SFM, we conclude that these cell lines are uniquely suitable for large-scale clinical-grade vector production for late-phase clinical trials involving gene transfer into hematopoietic cells.

Single-chain antibodies that target lentiviral vectors to MHC class II on antigen-presenting cells

Lentiviral vectors are promising vaccines because they can transduce and express antigens in dendritic cells in vivo, leading to potent immunization. To improve the safety and efficacy of lentivector vaccination, we sought to target vector transduction to antigen-presenting cells by modifying the viral envelope. To do this we screened a nonimmunized human single-chain antibody phage display library for phage that bound mouse bone marrow-derived dendritic cells (BMDCs) and isolated three single-chain antibodies (scFvs) that bound to more than 20% of cells in the BMDC culture. The three scFvs also bound to dendritic cells, macrophages, monocytes, and B cells from mouse spleen, but not to neutrophils, eosinophils, or T cells. Immunoblotting demonstrated that two unique scFvs, C2 and C7, recognized MHC class II. We constructed chimeric envelope proteins, by fusing these two scFvs to the amino terminus of the amphotropic murine leukemia virus envelope (MLV-A). These chimeric envelopes were expressed on the surface of lentiviral vector particles and enhanced infection (5- to 10-fold) of BMDC cultures, compared with lentiviral vectors with unmodified MLV-A envelope. Similarly, the chimeric envelopes enhanced (10- to 20-fold) the infection of primary lymph node class II-positive cells. One of the envelopes, C2, gave increased interferon-gamma production from splenocytes of vaccinated mice compared with MLV-A, achieving a level similar to that obtained with vesicular stomatitis virus glycoprotein G, when used to deliver an ovalbumin model antigen gene. These results demonstrate that surface-targeting lentiviral vector transduction of antigen-presenting cells gives efficient and potentially safer immunization.

Improved coinfection with amphotropic pseudotyped retroviral vectors

Amphotropic pseudotyped retroviral vectors have typically been used to infect target cells without prior concentration. Although this can yield high rates of infection, higher rates may be needed where highly efficient coinfection of two or more vectors is needed. In this investigation we used amphotropic retroviral vectors produced by the Plat-A cell line and studied coinfection rates using green and red fluorescent proteins (EGFP and dsRed2). Target cells were primary human fibroblasts (PHF) and 3T3 cells. Unconcentrated vector preparations produced a coinfection rate of approximately 4% (defined as cells that are both red and green as a percentage of all cells infected). Optimized spinoculation, comprising centrifugation at 1200 g for 2 hours at 15 degrees C, increased the coinfection rate to approximately 10%. Concentration by centrifugation at 10,000 g or by flocculation using Polybrene increased the coinfection rate to approximately 25%. Combining the two processes, concentration by Polybrene flocculation and optimized spinoculation, increased the coinfection rate to 35% (3T3) or >50% (PHF). Improved coinfection should be valuable in protocols that require high transduction by combinations of two or more retroviral vectors.

Analysis of human immunodeficiency virus type 1 vector cis- and trans-acting elements production by means of Semliki Forest virus

Recombinant Semliki Forest virus (SFV) is an attractive viral vector system owing to its ability to allow high efficiency of viral protein expression. To produce recombinant pseudotyped human immunodeficiency virus type 1 (HIV-1) virions, we designed a chimeric SFV/HIV vector system that contains both the HIV-1 cis- and trans-acting elements under the transcriptional control of the SFV replicase and investigated the ability of the hybrid SFV/HIV system to produce lentiviral particles capable of transducing target cells. Co-transfection of target cells with the two helper SFV packaging system RNAs along with each SFV/Gag-Pol, SFV/VSV(G) as well as SFV/HIV-1 vector unit replicon led to the generation of efficient transducing competent recombinant SFV/HIV particles. In contrast, co-transduction of target cells with the SFV/HIV chimeric virions produced recombinant particles with low transducing ability. Our data suggest that both the genomic and the subgenomic RNAs containing the HIV-1 vector unit were negatively selected for incorporation into recombinant particles, despite the fact that the SFV-driven HIV-1 vector replicon was the only one containing a lentiviral packaging sequence. The results of this study provide insights relevant to the design of chimeric lentiviral vectors.

Gene delivery by lentivirus vectors

The capacity to efficiently transduce nondividing cells, shuttle large genetic payloads, and maintain stable long-term transgene expression are attributes that have brought lentiviral vectors to the forefront of gene delivery vehicles for research and therapeutic applications in a clinical setting. Our discussion initiates with advances in lentiviral vector development and how these sophisticated lentiviral vectors reflect improvements in safety, regarding the prevention of replication competent lentiviruses (RCLs), vector mobilization, and insertional mutagenesis. Additionally, we describe conventional molecular regulatory systems to manage gene expression levels in a spatial and temporal fashion in the context of a lentiviral vector. State of the art technology for lentiviral vector production by transient transfection and packaging cell lines are explicitly presented with current practices used for concentration, purification, titering, and determining the safety of a vector stock. We summarize lentiviral vector applications that have received a great deal of attention in recent years including the generation of transgenic animals and the stable delivery of RNA interference molecules. Concluding remarks address some of the successes in preclinical animals, and the recent transition of lentiviral vectors to human clinical trials as therapy for a variety of infectious and genetic diseases.

Transduction of Human Hematopoietic Stem Cells by Lentiviral Vectors Pseudotyped with the RD114-TR Chimeric Envelope Glycoprotein

Lentiviral vectors are efficiently pseudotyped with RD114-TR, a chimeric envelope glycoprotein made of the extracellular and transmembrane domains of the feline leukemia virus RD114 and the cytoplasmic tail of the murine leukemia virus amphotropic envelope. RD114-TR-pseudotyped vectors may be concentrated by centrifugation, are resistant to complement inactivation, and are suitable for both ex vivo and in vivo gene therapy applications. We analyzed RD114-TR-pseudotyped, HIV-1-derived lentiviral vectors for their ability to transduce human cord blood, bone marrow, and peripheral blood mobilized CD34(+) hematopoietic stem/progenitor cells. Transduction efficiency was analyzed in CD34(+) cells in liquid culture, in CD34(+) clonogenic progenitors in semisolid culture, and in CD34(+) repopulating stem cells after xenotransplantation in NOD-SCID mice. Compared with a standard VSV-G-based packaging system, RD114-TR-pseudotyped particles transduced hematopoietic stem/progenitor cells at lower multiplicity of infection, with lower toxicity and less pseudo-transduction at comparable vector copy number per genome. Potential changes in the CD34(+) cell transcription profile and phenotype on transduction with RD114-TR-pseudotyped vectors was comparatively investigated by microarray analysis. Our study shows that the biology of repopulating hematopoietic stem cells and their progeny is not affected by transduction with RD114-TR-pseudotyped lentiviral vectors. RD114-TR is compatible with the development of lentiviral stable packaging cell lines, and may become the envelope of choice for clinical studies aiming at safe and efficient genetic modification of human hematopoietic stem cells.

Simian immunodeficiency virus lentivector corrects human X-linked chronic granulomatous disease in the NOD/SCID mouse xenograft

X-linked chronic granulomatous disease (X-CGD) is a primary immunodeficiency caused by mutations in the phagocyte nicotinamide dinucleotide phosphate oxidase catalytic subunit gp91(phox). Gene therapy targeting hematopoietic stem cells (HSCs) can correct CGD, but permanent correction remains a challenge. Lentiviral vectors have become attractive tools for gene transfer, and they may have the potential to transduce very primitive HSCs. We used a self-inactivating RD114/TR-pseudotyped simian immunodeficiency virus (SIVmac)-based vector encoding human gp91(phox) for ex vivo transduction of peripheral blood-mobilized stem cells (PBSCs) from patients with X-CGD. In PBSCs from two patients, ex vivo transduction efficiencies of 40.5 and 46% were achieved, and correction of oxidase activity was observed in myeloid cells differentiating in culture. When transduced PBSCs from these patients were transplanted into nonobese diabetic/severe combined immunodeficient mice and compared to normal control, 10.5 and 7.3% of the human myeloid cells in bone marrow developing at 6 weeks from the human xenografts expressed the gp91(phox) transgene. Sustained functional correction of oxidase activity was documented in myeloid cells differentiated from engrafted transduced PBSCs. Transgene marking was polyclonal as assessed by vector integration site analysis. These data suggest that RD114/TR SIVmac-based vectors might be suitable for gene therapy of CGD and other hereditary hematologic diseases.

Lentiviral vectors pseudotyped with murine ecotropic envelope: increased biosafety and convenience in preclinical research

OBJECTIVE

Lentiviral vectors are increasingly used for preclinical models of gene therapy and other forms of experimental transgenesis. Due to the broad tropism and the ability for concentration by ultracentrifugation, most lentiviral vector preparations are produced using the vesicular stomatitis virus glycoprotein (VSV-g) protein as envelope. Recently, Hanawa and colleagues have demonstrated that the ecotropic envelope protein of murine leukemia viruses allows efficient pseudotyping of HIV-1-derived vector particles. However, this method has found little acceptance, despite potential advantages.

MATERIALS AND METHODS

We produced lentiviral vectors pseudotyped with murine ecotropic envelope using a four-plasmid transient transfection system and evaluated their performance in murine fibroblasts and hematopoietic cells.

RESULTS

Titers of lentiviral "ecotropic" supernatants were only slightly lower than those produced with VSV-g, could be concentrated by overnight centrifugation (13,000g), and efficiently transduced murine fibroblasts and hematopoietic cells but not human cells. Our Institutional Biosafety Committee agreed on the production and use of replication-defective lentiviral vectors pseudotyped with murine ecotropic envelope under biosafety level 1 (BL1) conditions with additional BL2 practices. We also obtained useful guidelines for the work with human infectious lentiviral vectors.

CONCLUSIONS

For the researcher, "ecotropic" lentiviral vectors significantly improve the convenience of daily work, compared to the conditions required for lentiviral pseudotypes that are capable of infecting human cells. High efficiency and superior biosafety in combination with convenient handling will certainly boost the potential applicability of this important vector system.

Retrovirus Vectors: Toward the Plentivirus?

Recombinant retroviral vectors based upon simple gammaretroviruses, complex lentiviruses, or potentially nonpathogenic spumaviruses represent relatively well characterized tools that are widely used for stable gene transfer. Different members of the Retroviridae family have developed distinct and potentially useful features related to their life cycle. These natural differences can be exploited for specialized applications in gene therapy and could conceivably be combined to create future retroviral hybrid vectors, ideally incorporating the following features: an efficient, noncytopathic packaging system with low likelihood of recombination; serum resistance; an ability to pseudotype with cell-specific envelopes; high-fidelity reverse transcription before cell entry; unrestricted cytoplasmic transport and nuclear import; an insulated expression cassette; specific chromosomal targeting; and physiologic or regulated levels of transgene expression. We envisage that, compared to contemporary vectors, a hybrid vector combining these properties would have increased therapeutic efficacy and an enhanced biosafety profile. Many of the above goals will require the inclusion of nonretroviral components into vector particles or transgenes.

Optimisation of a multipartite human immunodeficiency virus based vector system; control of virus infectivity and large-scale production

BACKGROUND

We have previously described a five-plasmid HIV-1 vector system that utilises a codon-optimised gagpol gene. While this system was shown to be safer than systems using proviral type helpers, the titre of virus produced was relatively low. Therefore, a process of optimising all aspects of virus production was initiated.

METHODS

A systematic approach was taken to the optimisation of virus production by transient expression using a five-plasmid packaging system. Codon-manipulation was used to reduce homology between helper and vector constructs. Ultrafiltration and ultracentrifugation were used for large-scale virus production.

RESULTS

We describe codon-optimised reading frames for Tat and Rev and the optimisation of virus production. The optimisation process resulted in an increase in virus titre of 7- to 8-fold. Several other approaches to increasing viral titre described by others proved ineffective in our system after it had been optimised. In addition, we show that by varying the ratio of the GagPol helper construct to vector, the infectivity of the virus could be controlled. The use of a novel codon-optimised HIV-1 GagPol expression construct with reduced homology to vector sequences significantly reduced transfer of gagpol sequences to transduced cells. Virus could be collected in serum-free medium without a significant loss of titre, which facilitated subsequent processing. Processing using a combination of ultrafiltration and ultracentrifugation allowed efficient and rapid processing of litre volumes of virus supernatant.

CONCLUSIONS

By taking a systematic approach to optimising all aspects of our five-plasmid lentiviral vector system we improved titre, safety, large-scale production, and demonstrated that infectivity could be specifically controlled.

Gene therapy progress and prospects: development of improved lentiviral and retroviral vectors--design, biosafety, and production

Replication defective vectors derived from simple retroviruses or the more complex genomes of lentiviruses continue to offer the advantages of long-term expression, cell and tissue specific tropism, and large packaging capacity for the delivery of therapeutic genes. The occurrence of adverse events caused by insertional mutagenesis in three patients in a gene therapy trial for X-linked SCID emphasizes the potential for problems in translating this approach to the clinic. Several genome-wide studies of retroviral integration are now providing novel insights into the integration site preferences of different vector classes. We review recent developments in vector design, integration, biosafety, and production.

Conjugation of lentivirus to paramagnetic particles via nonviral proteins allows efficient concentration and infection of primary acute myeloid leukemia cells

Nonviral producer cell proteins incorporated into retroviral vector surfaces profoundly influence infectivity and in vivo half-life. We report the purification and concentration of lentiviral vectors using these surface proteins as an efficient gene transduction strategy. Biotinylation of these proteins and streptavidin paramagnetic particle concentration enhances titer 400- to 2,500-fold (to 10(9) CFU/ml for vesicular stomatitis virus G protein and 5 x 10(8) for amphotropic murine leukemia virus envelope). This method also uses newly introduced membrane proteins (B7.1 and DeltaLNGFR) directed to lentiviral surfaces, allowing up to 17,000-fold concentrations. Particle conjugation of lentivirus allows facile manipulation in vitro, resulting in the transduction of 48 to 94% of human acute myeloid leukemia blasts.

Altering the tropism of lentiviral vectors through pseudotyping

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

Gene transfer to repopulating human CD34+ cells using amphotropic-, GALV-, or RD114-pseudotyped HIV-1-based vectors from stable producer cells

A novel, stable human immunodeficiency virus type 1 vector packaging system, STAR, was tested for its ability to transduce human cord blood CD34+ progenitor cells assayed both in vitro and after transplantation into NOD/SCID mice. Vectors pseudotyped with three different gammaretrovirus envelopes were used: the amphotropic MLV envelope (MLV-A), a modified gibbon ape leukemia virus envelope (GALV+), and a modified feline endogenous virus RD114 envelope (RDpro). Gene transfer to freshly thawed CD34+ cells in the absence of cytokines was very low. Addition of cytokines increased gene transfer efficiency significantly and this was further augmented if the cells were prestimulated for 24 h. Concentration of the vectors (15-fold) by low-speed centrifugation increased gene transfer to CD34+ cells in vitro even further. More than 90% of cells were transduced with a single exposure to the RDpro vector as determined by GFP expression using flow cytometry. The two other pseudotypes transduced approximately 65-70% of the cells under the same conditions. Transplantation of CD34+ cells prestimulated for 24 h and then transduced with a single exposure to concentrated vector revealed that the RDpro vector transduced 55.1% of NOD/SCID repopulating human cells, which was significantly higher than the MLV-A (12.6%)- or GALV+ (25.1%)-pseudotyped vectors.

Factors influencing the titer and infectivity of lentiviral vectors

Lentiviral vectors have undergone several generations of design improvement to enhance their biosafety and expression characteristics, and have been approved for use in human clinical studies. Most preclinical studies with these vectors have employed easily assayed marker genes for the purpose of determining vector titers and transduction efficiencies. Naturally, the adaptation of these vector systems to clinical use will increasingly involve the transfer of genes whose products may not be easily measured, meaning that the determination of vector titer will be more complicated. One method for determining vector titer that can be universally employed on all human immunodeficiency virus type 1-based lentiviral vector supernatants involves the measurement of Gag (p24) protein concentration in vector supernatants by immunoassay. We have studied the effects that manipulation of several variables involved in vector design and production by transient transfection have on vector titer and infectivity. We have determined that manipulation of the amount of transfer vector, packaging, and envelope plasmids used to transfect the packaging cells does not alter vector infectivity, but does influence vector titer. We also found that modifications to the transfer vector construct, such as replacing the internal promoter or transgene, do not generally alter vector infectivity, whereas inclusion of the central polypurine tract in the transfer vector increases vector infectivity on HEK293 cells and human umbilical cord blood CD34+ hematopoietic progenitor cells (HPCs). The infectivities of vector supernatants can also be increased by harvesting at early time points after the initiation of vector production, collection in serum-free medium, and concentration by ultracentrifugation. For the transduction of CD34+ HPCs, we found that the simplest method of increasing vector infectivity is to pseudotype vector particles with the RD114 envelope instead of vesicular stomatitis virus G glycoprotein (VSV-G).

Human immunodeficiency virus type 1 vectors with alphavirus envelope glycoproteins produced from stable packaging cells

Alphavirus glycoproteins have broad host ranges. Human immunodeficiency virus type 1 (HIV-1) vectors pseudotyped with their glycoproteins could extend the range of tissues that can be transduced in both humans and animal models. Here, we established stable producer cell lines for HIV vectors pseudotyped with alphavirus Ross River virus (RRV) and Semliki Forest virus (SFV) glycoproteins E2E1. RRV E2E1-stable clones could routinely produce high-titer pseudotyped vectors for at least 5 months. SFV E2E1-stable clones, however, produced relatively low titers. We examined the properties of RRV E2E1-pseudotyped vectors [HIV-1(RRV)] and compared them with amphotropic murine leukemia virus Env- and vesicular stomatitis virus glycoprotein G-pseudotyped vectors. HIV-1(RRV) displayed a number of characteristics which would be advantageous in ex vivo and in vivo experiments, including resistance to inactivation by heat-labile components in fresh human sera and thermostability at 37 degrees C. Upon single-step concentration by ultracentrifugation of HIV-1(RRV), we could achieve vector stocks with titers up to 6 x 10(7) IU/ml. HIV-1(RRV) efficiently transduced cells from several different species, including murine primary dendritic cells, but failed to transduce human and murine T cells as well as human hematopoietic stem cells (HSC). These results indicate that HIV-1(RRV) could be used in a number of applications including animal model experiments and suggest that expression of RRV cellular receptors is limited or absent in certain cell types such as T cells and human HSC.