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

Therapeutic utility of engineered myeloid cells in the tumor microenvironment

Despite promising results shown in hematologic tumors, immunotherapies for the treatment of solid tumors have mostly failed so far. The immunosuppressive tumor microenvironment and phenotype of tumor infiltrating macrophages are among the more prevalent reasons for this failure. Tumor associated macrophages (TAMs, M2-macrophages) are circulating myeloid cells recruited to the local tumor microenvironment, and together with regulatory T cells (T-regs), are reprogrammed to become immune suppressive. This results in the inactivation or hampered recruitment of cytotoxic CD8 + T and Natural Killer (NK) cells. Recently, attempts have been made to try to leverage specific myeloid functions and properties, including their ability to reach the TME and to mediate the phagocytosis of cancer cells. Additionally, myeloid cells have been used for drug delivery and reprogramming the tumor microenvironment in cancer patients. This approach, together with the advancements in genome editing, paved the way for the development of novel cell-mediated immunotherapies. This article focuses on the latest studies that detail the therapeutic properties of genetically engineered or pharmacologically modulated myeloid cells in cancer preclinical models, limitations, pitfalls, and evaluations of these approaches in patients with cancer.

Current status on the development of pseudoviruses for enveloped viruses

Emerging and reemerging infectious diseases have a strong negative impact on public health. However, because many of these pathogens must be handled in biosafety level, 3 or 4 containment laboratories, research and development of antivirals or vaccines against these diseases are often impeded. Alternative approaches to address this issue have been vigorously pursued, particularly the use of pseudoviruses in place of wild‐type viruses. As pseudoviruses have been deprived of certain gene sequences of the virulent virus, they can be handled in biosafety level 2 laboratories. Importantly, the envelopes of these viral particles may have similar conformational structures to those of the wild‐type viruses, making it feasible to conduct mechanistic investigation on viral entry and to evaluate potential neutralizing antibodies. However, a variety of challenging issues remain, including the production of a sufficient pseudovirus yield and the inability to produce an appropriate pseudotype of certain viruses. This review discusses current progress in the development of pseudoviruses and dissects the factors that contribute to low viral yields.

Codon Optimization Leads to Functional Impairment of RD114-TR Envelope Glycoprotein

Lentiviral vectors (LVs) are a highly valuable tool for gene transfer currently exploited in basic, applied, and clinical studies. Their optimization is therefore very important for the field of vectorology and gene therapy. A key molecule for LV function is the envelope because it guides cell entry. The most commonly used in transiently produced LVs is the vesicular stomatitis virus glycoprotein (VSV-G) envelope, whose continuous expression is, however, toxic for stable LV producer cells. In contrast, the feline endogenous retroviral RD114-TR envelope is suitable for stable LV manufacturing, being well tolerated by producer cells under constitutive expression. We have previously reported successful, transient and stable production of LVs pseudotyped with RD114-TR for good transduction of T lymphocytes and CD34⁺ cells. To further improve RD114-TR-pseudotyped LV cell entry by increasing envelope expression, we codon-optimized the RD114-TR open reading frame (ORF). Here we show that, despite the RD114-TRco precursor being produced at a higher level than the wild-type counterpart, it is unexpectedly not duly glycosylated, exported to the cytosol, and processed. Correct cleavage of the precursor in the functional surface and transmembrane subunits is prevented in vivo, and, consequently, the unprocessed precursor is incorporated into LVs, making them inactive.

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.

Production of lentiviral vectors

Lentiviral vectors (LV) have seen considerably increase in use as gene therapy vectors for the treatment of acquired and inherited diseases. This review presents the state of the art of the production of these vectors with particular emphasis on their large-scale production for clinical purposes. In contrast to oncoretroviral vectors, which are produced using stable producer cell lines, clinical-grade LV are in most of the cases produced by transient transfection of 293 or 293T cells grown in cell factories. However, more recent developments, also, tend to use hollow fiber reactor, suspension culture processes, and the implementation of stable producer cell lines. As is customary for the biotech industry, rather sophisticated downstream processing protocols have been established to remove any undesirable process-derived contaminant, such as plasmid or host cell DNA or host cell proteins. This review compares published large-scale production and purification processes of LV and presents their process performances. Furthermore, developments in the domain of stable cell lines and their way to the use of production vehicles of clinical material will be presented.

Baboon envelope pseudotyped LVs outperform VSV-G-LVs for gene transfer into early-cytokine-stimulated and resting HSCs

Hematopoietic stem cell (HSC)-based gene therapy holds promise for the cure of many diseases. The field is now moving toward the use of lentiviral vectors (LVs) as evidenced by 4 successful clinical trials. These trials used vesicular-stomatitis-virus-G protein (VSV-G)-LVs at high doses combined with strong cytokine-cocktail stimulation to obtain therapeutically relevant transduction levels; however, they might compromise the HSC character. Summarizing all these disadvantages, alternatives to VSV-G-LVs are urgently needed. We generated here high-titer LVs pseudotyped with a baboon retroviral envelope glycoprotein (BaEV-LVs), resistant to human complement. Under mild cytokine prestimulation to preserve the HSC characteristics, a single BaEV-LV application at a low dose, resulted in up to 90% of hCD34(+) cell transduction. Even more striking was that these new BaEV-LVs allowed, at low doses, efficient transduction of up to 30% of quiescent hCD34(+) cells, whereas high-dose VSV-G-LVs were insufficient. Importantly, reconstitution of NOD/Lt-SCID/γc(-/-) (NSG) mice with BaEV-LV-transduced hCD34(+) cells maintained these high transduction levels in all myeloid and lymphoid lineages, including early progenitors. This transduction pattern was confirmed or even increased in secondary NSG recipient mice. This suggests that BaEV-LVs efficiently transduce true HSCs and could improve HSC-based gene therapy, for which high-level HSC correction is needed for life-long cure.

Non-integrating gamma-retroviral vectors as a versatile tool for transient zinc-finger nuclease delivery

Designer nucleases, like zinc-finger nucleases (ZFNs), represent valuable tools for targeted genome editing. Here, we took advantage of the gamma-retroviral life cycle and produced vectors to transfer ZFNs in the form of protein, mRNA and episomal DNA. Transfer efficacy and ZFN activity were assessed in quantitative proof-of-concept experiments in a human cell line and in mouse embryonic stem cells. We demonstrate that retrovirus-mediated protein transfer (RPT), retrovirus-mediated mRNA transfer (RMT), and retrovirus-mediated episome transfer (RET) represent powerful methodologies for transient protein delivery or protein expression. Furthermore, we describe complementary strategies to augment ZFN activity after gamma-retroviral transduction, including serial transduction, proteasome inhibition, and hypothermia. Depending on vector dose and target cell type, gene disruption frequencies of up to 15% were achieved with RPT and RMT, and >50% gene knockout after RET. In summary, non-integrating gamma-retroviral vectors represent a versatile tool to transiently deliver ZFNs to human and mouse cells.

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.

New developments in lentiviral vector design, production and purification

INTRODUCTION

Lentiviruses are a very potent class of viral vectors for which there is presently a rapidly growing interest for a number of gene therapy. However, their construction, production and purification need to be performed according to state-of-the-art techniques in order to obtain sufficient quantities of high purity material of any usefulness and safety.

AREAS COVERED

The recent advances in the field of recombinant lentivirus vector design, production and purification will be reviewed with an eye toward its utilization for gene therapy. Such a review should be helpful for the potential user of this technology.

EXPERT OPINION

The principal hurdles toward the use of recombinant lentivirus as a gene therapy vector are the low titer at which it is produced as well as the difficulty to purify it at an acceptable level without degrading it. The recent advances in the bioproduction of this vector suggest these issues are about to be resolved, making the retrovirus gene therapy a mature technology.

A novel lentiviral vector targets gene transfer into human hematopoietic stem cells in marrow from patients with bone marrow failure syndrome and in vivo in humanized mice

In vivo lentiviral vector (LV)-mediated gene delivery would represent a great step forward in the field of gene therapy. Therefore, we have engineered a novel LV displaying SCF and a mutant cat endogenous retroviral glycoprotein, RDTR. These RDTR/SCF-LVs outperformed RDTR-LVs for transduction of human CD34(+) cells (hCD34(+)). For in vivo gene therapy, these novel RDTR/SCF-displaying LVs can distinguish between the target hCD34(+) cells of interest and nontarget cells. Indeed, they selectively targeted transduction to 30%-40% of the hCD34(+) cells in cord blood mononuclear cells and in the unfractionated BM of healthy and Fanconi anemia donors, resulting in the correction of CD34(+) cells in the patients. Moreover, RDTR/SCF-LVs targeted transduction to CD34(+) cells with 95-fold selectivity compared with T cells in total cord blood. Remarkably, in vivo injection of the RDTR/SCF-LVs into the BM cavity of humanized mice resulted in the highly selective transduction of candidate hCD34(+)Lin(-) HSCs. In conclusion, this new LV will facilitate HSC-based gene therapy by directly targeting these primitive cells in BM aspirates or total cord blood. Most importantly, in the future, RDTR/SCF-LVs might completely obviate ex vivo handling and simplify gene therapy for many hematopoietic defects because of their applicability to direct in vivo inoculation.

Stem cell gene therapy for fanconi anemia: report from the 1st international Fanconi anemia gene therapy working group meeting

Survival rates after allogeneic hematopoietic cell transplantation (HCT) for Fanconi anemia (FA) have increased dramatically since 2000. However, the use of autologous stem cell gene therapy, whereby the patient's own blood stem cells are modified to express the wild-type gene product, could potentially avoid the early and late complications of allogeneic HCT. Over the last decades, gene therapy has experienced a high degree of optimism interrupted by periods of diminished expectation. Optimism stems from recent examples of successful gene correction in several congenital immunodeficiencies, whereas diminished expectations come from the realization that gene therapy will not be free of side effects. The goal of the 1st International Fanconi Anemia Gene Therapy Working Group Meeting was to determine the optimal strategy for moving stem cell gene therapy into clinical trials for individuals with FA. To this end, key investigators examined vector design, transduction method, criteria for large-scale clinical-grade vector manufacture, hematopoietic cell preparation, and eligibility criteria for FA patients most likely to benefit. The report summarizes the roadmap for the development of gene therapy for FA.

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.

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.

Transduction of human primitive repopulating hematopoietic cells with lentiviral vectors pseudotyped with various envelope proteins

Lentiviral vectors are useful for transducing primitive hematopoietic cells. We examined four envelope proteins for their ability to mediate lentiviral transduction of mobilized human CD34(+) peripheral blood cells. Lentiviral particles encoding green fluorescent protein (GFP) were pseudotyped with the vesicular stomatitis virus envelope glycoprotein (VSV-G), the amphotropic (AMPHO) murine leukemia virus envelope protein, the endogenous feline leukemia viral envelope protein or the feline leukemia virus type C envelope protein. Because the relative amount of genome RNA per ml was similar for each pseudotype, we transduced CD34(+) cells with a fixed volume of each vector preparation. Following an overnight transduction, CD34(+) cells were transplanted into immunodeficient mice which were sacrificed 12 weeks later. The average percentages of engrafted human CD45(+) cells in total bone marrow were comparable to that of the control, mock-transduced group (37-45%). Lenti-particles pseudotyped with the VSV-G envelope protein transduced engrafting cells two- to tenfold better than particles pseudotyped with any of the gamma-retroviral envelope proteins. There was no correlation between receptor mRNA levels for the gamma-retroviral vectors and transduction efficiency of primitive hematopoietic cells. These results support the use of the VSV-G envelope protein for the development of lentiviral producer cell lines for manufacture of clinical-grade vector.

Cocal-pseudotyped lentiviral vectors resist inactivation by human serum and efficiently transduce primate hematopoietic repopulating cells

Lentiviral vectors are established as efficient and convenient vehicles for gene transfer. They are almost always pseudotyped with the envelope glycoprotein of vesicular stomatitis virus (VSV-G) due to the high titers that can be achieved, their stability, and broad tropism. We generated a novel cocal vesiculovirus envelope glycoprotein plasmid and compared the properties of lentiviral vectors pseudotyped with cocal, VSV-G, and a modified feline endogenous retrovirus envelope glycoprotein (RD114/TR). Cocal-pseudotyped lentiviral vectors can be produced at titers as high as with VSV-G, have a broad tropism, and are stable, allowing for efficient concentration by centrifugation. Additionally, cocal vectors are more resistant to inactivation by human serum than VSV-G-pseudotyped vectors, and efficiently transduce human CD34(+) nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse-repopulating cells (SRCs), and long-term primate hematopoietic repopulating cells. These studies establish the potential of cocal-pseudotyped lentiviral vectors for a variety of scientific and therapeutic gene transfer applications, including in vivo gene delivery and hematopoietic stem cell (HSC) gene therapy.

Towards a clinically relevant lentiviral transduction protocol for primary human CD34 hematopoietic stem/progenitor cells

Background

Hematopoietic stem cells (HSC), in particular mobilized peripheral blood stem cells, represent an attractive target for cell and gene therapy. Efficient gene delivery into these target cells without compromising self-renewal and multi-potency is crucial for the success of gene therapy. We investigated factors involved in the ex vivo transduction of CD34⁺ HSCs in order to develop a clinically relevant transduction protocol for gene delivery. Specifically sought was a protocol that allows for efficient transduction with minimal ex vivo manipulation without serum or other reagents of animal origin.

Methodology/Principal Findings

Using commercially available G-CSF mobilized peripheral blood (PB) CD34⁺ cells as the most clinically relevant target, we systematically examined factors including the use of serum, cytokine combinations, pre-stimulation time, multiplicity of infection (MOI), transduction duration and the use of spinoculation and/or retronectin. A self-inactivating lentiviral vector (SIN-LV) carrying enhanced green fluorescent protein (GFP) was used as the gene delivery vehicle. HSCs were monitored for transduction efficiency, surface marker expression and cellular function. We were able to demonstrate that efficient gene transduction can be achieved with minimal ex vivo manipulation while maintaining the cellular function of transduced HSCs without serum or other reagents of animal origin.

Conclusions/Significance

This study helps to better define factors relevant towards developing a standard clinical protocol for the delivery of SIN-LV into CD34⁺ cells.

Efficient siRNA delivery by the cationic liposome DOTAP in human hematopoietic stem cells differentiating into dendritic cells

RNA interference technology is an ideal strategy to elucidate the mechanisms associated with human CD34(+) hematopoietic stem cell differentiation into dendritic cells. Simple manipulations in vitro can unequivocally yield alloreactive or tolerogenic populations, suggesting key implications of biochemical players that might emerge as therapeutic targets for cancer or graft-versus-host disease. To knockdown proteins typically involved in the biology of dendritic cells, we employed an siRNA delivery system based on the cationic liposome DOTAP as the carrier. Freshly-isolated CD34(+) cells were transfected with siRNA for cathepsin S with negligible cytotoxicity and transfection rates (>60%) comparable to the efficiency shown by lentiviral vectors. Further, cathepsin S knockdown was performed during both cell commitment and through the entire 14-day differentiation process with repeated transfection rounds that had no effect per se on cell development. Tested in parallel, other commercially-available chemical reagents failed to meet acceptable standards. In addition to safe and practical handling, a direct advantage of DOTAP over viral-mediated techniques is that transient silencing effects can be dynamically appraised through the recovery of targeted proteins. Thus, our findings identify DOTAP as an excellent reagent for gene silencing in resting and differentiating CD34(+) cells, suggesting a potential for applications in related preclinical models.

Identification of parameters required for efficient lentiviral vector transduction and engraftment of human cord blood CD34(+) NOD/SCID-repopulating cells

OBJECTIVE

Human cord blood (CB) is a potential source of hematopoietic stem cells (HSC) for gene therapy to treat patients with hematopoietic disorders. However, limited numbers of CB CD34(+) cells, low transduction efficiency with lentiviral vectors (LVs), and low engraftment efficiency of nonobese diabetic/severe combined immunodeficient (NOD/SCID) repopulating cells (SRC), a measure of HSC, are blocks to this procedure. To optimize culture and transduction conditions, we compared various lengths of time for prestimulation before transduction, transduction duration, and posttransduction cell culture.

MATERIALS AND METHODS

We used a LV to transduce human CB CD34(+) cells followed by engraftment into NOD/SCID mice. We evaluated the effects of prestimulation and transduction time and optimized ex vivo cell culture duration before transplantation.

RESULTS

We were able to achieve up to 40% transduction efficiency and up to 50% engraftment efficiency of SRC in CB CD34(+) cells when CB CD34(+) cells were either not prestimulated or prestimulated in 1% fetal bovine serum medium for 1 hour, followed by 5 hours transduction and 3 days culture in a cocktail of growth factors after transduction. No apparent functional changes of CB CD34(+) cells were noted under these conditions.

CONCLUSION

This gene-transduction/cell-expansion protocol is the first systematic study to optimize prestimulation time, transduction time, and, very importantly, ex vivo culture time after transduction, and may be of use for LV gene transduction in a gene therapy setting.

Towards a rAAV-based gene therapy for ADA-SCID: from ADA deficiency to current and future treatment strategies

Adenosine deaminase deficiency fosters a rare, devastating pediatric immune deficiency with concomitant opportunistic infections, metabolic anomalies and multiple organ system pathology. The standard of care for adenosine deaminase deficient severe combined immune deficiency (ADA-SCID) includes enzyme replacement therapy or bone marrow transplantation. Gene therapies for ADA-SCID over nearly two decades have exclusively involved retroviral vectors targeted to lymphocytes and hematopoetic progenitors. These groundbreaking gene therapies represent a revolution in clinical medicine, but come with several challenges, including the risk of insertional mutagenesis. An alternative gene therapy for ADA-SCID may utilize recombinant adeno-associated virus vectors in vivo, with numerous target tissues, to foster ectopic expression and secretion of adenosine deaminase. This review endeavors to describe ADA-SCID, the traditional treatments, previous retroviral gene therapies, and primarily, alternative recombinant adeno-associated virus-based strategies to remedy this potentially fatal genetic disease.

Lentiviral transduction of apoAI into hematopoietic progenitor cells and macrophages: applications to cell therapy of atherosclerosis

OBJECTIVE

We used genetically engineered mouse hematopoietic progenitor cells (HPCs) to investigate the therapeutic effects of human apoAI on atherosclerosis in apoE(-/-) mice.

METHODS AND RESULTS

Lentiviral constructs expressing either human apoAI (LV-apoAI) or green fluorescent protein (LV-GFP) cDNA under a macrophage specific promoter (CD68) were generated and used for ex vivo transduction of mouse HPCs and macrophages. The transduction efficiency was >25% for HPCs and >70% for macrophages. ApoAI was found in the macrophage culture media, mostly associated with the HDL fraction. Interestingly, a significant increase in mRNA and protein levels for ATP binding cassette A1 (ABCA1) and ABCG1 were found in apoAI-expressing macrophages after acLDL loading. Expression of apoAI significantly increased cholesterol efflux in wild-type and apoE(-/-) macrophages. HPCs transduced with LV-apoAI ex vivo and then transplanted into apoE(-/-) mice caused a 50% reduction in atherosclerotic lesion area compared to GFP controls, without influencing plasma HDL-C levels.

CONCLUSIONS

Lentiviral transduction of apoAI into HPCs reduces atherosclerosis in apoE(-/-) mice. Expression of apoAI in macrophages improves cholesterol trafficking in wild-type apoE-producing macrophages and causes upregulation of ABCA1 and ABCG1. These novel observations set the stage for a cell therapy approach to atherosclerosis regression, exploiting the cooperation between apoE and apoAI to maximize cholesterol exit from the plaque.