Moloney murine leukemia virus-derived retroviral vectors decay intracellularly with a half-life in the range of 5.5 to 7.5 hours

Determinants of Retroviral Integration and Implications for Gene Therapeutic MLV-Based Vectors and for a Cure for HIV-1 Infection

To complete their replication cycle, retroviruses need to integrate a DNA copy of their RNA genome into a host chromosome. Integration site selection is not random and is driven by multiple viral and cellular host factors specific to different classes of retroviruses. Today, overwhelming evidence from cell culture, animal experiments and clinical data suggests that integration sites are important for retroviral replication, oncogenesis and/or latency. In this review, we will summarize the increasing knowledge of the mechanisms underlying the integration site selection of the gammaretrovirus MLV and the lentivirus HIV-1. We will discuss how host factors of the integration site selection of retroviruses may steer the development of safer viral vectors for gene therapy. Next, we will discuss how altering the integration site preference of HIV-1 using small molecules could lead to a cure for HIV-1 infection.

Evaluation of the stability and intratumoral delivery of foreign transgenes encoded by an oncolytic Foamy Virus vector

Foamy Viruses are cell cycle-dependent retroviruses capable of persisting unintegrated in quiescent cells until cell division occurs. This unique ability allows them to target slowly dividing human tumor cells which remains an unmet need in oncolytic virotherapy. We have previously reported the generation of oncolytic Foamy Virus (oFV) vector system and demonstrated its superiority over oncolytic Murine Leukemia Virus vectors in infecting slowly dividing cancer cells. In the present study we evaluated (i) the ability of oFV to carry foreign transgenes and (ii) the genetic stability of these vectors upon serial passage. The thymidine kinase (TK) and inducible caspase 9 (iCasp9) cDNAs could be detected in the oFV backbone for up to 3 in vitro passages. In vivo, GFP-, TK- and iCasp9- carrying oFV vectors propagated efficiently in subcutaneous xenograft glioblastoma tumors and drove transgene expression for up to 66 days. However, in vivo oFV vector spread eventually resulted in complete loss of the iCasp9 cDNA, minor loss of the TK cDNA and negligible loss of the GFP. Our results suggest that oFV is a promising gene delivery platform and that transgenes smaller than 1 kb might be most suitable for oFV arming.

Oncolytic Foamy Virus - generation and properties of a nonpathogenic replicating retroviral vector system that targets chronically proliferating cancer cells

Nonpathogenic retroviruses of the Spumaretrovirinae subfamily can persist long-term in the cytoplasm of infected cells, completing their lifecycle only after the nuclear membrane dissolves at the time of cell division. Since the targeting of slowly dividing cancer cells remains an unmet need in oncolytic virotherapy we constructed a replication competent Foamy Virus vector (oFV) from the genomes of two chimpanzee Simian Foamy Viruses (PAN1 and PAN2) and inserted a GFP transgene in place of the bel-2 open reading frame. oFV-GFP infected and propagated with slow kinetics in multiple human tumor cell lines, inducing a syncytial cytopathic effect. Infection of growth arrested MRC5 cells was not productive, but oFV genomes persisted in the cytoplasm and the productive viral lifecycle resumed when cell division was later restored. In vivo, the virus propagated extensively in intraperitoneal ovarian cancer xenografts, slowing tumor growth, significantly prolonging survival of the treated mice and sustaining GFP transgene expression for at least 45 days. Our data indicate that oFV is a promising new replication-competent viral and gene delivery platform for efficient targeting of the most fundamental trait of cancer cells, their ability to sustain chronic proliferation.Significance:The infectivity of certain retroviruses is limited to dividing cells, which makes them attractive tools for targeting cancer cell proliferation. Previously developed replication-competent gammaretroviral vectors spread efficiently in rapidly dividing cancer cells, but not in cancer cells that divide more slowly. In contrast to rapidly proliferating transplantable mouse tumors, slow proliferation is a hallmark of human cancers and may have contributed to the clinical failure of the preclinically promising Murine Leukemia Virus vector Toca511 which failed to show efficacy in a phase 3 clinical trial in patients with glioblastoma. The studies presented in our manuscript show that oncolytic Foamy Virus (oFV) vectors are capable of persisting unintegrated in quiescent cells and resuming their life cycle once the cells start dividing again. This property of oFVs, together with their lack of pathogenicity and their ability to catalyze the fusion of infected cancer cells, makes them an attractive platform for further investigation.

Memory formation orchestrates the wiring of adult-born hippocampal neurons into brain circuits

During memory formation, structural rearrangements of dendritic spines provide a mean to durably modulate synaptic connectivity within neuronal networks. New neurons generated throughout the adult life in the dentate gyrus of the hippocampus contribute to learning and memory. As these neurons become incorporated into the network, they generate huge numbers of new connections that modify hippocampal circuitry and functioning. However, it is yet unclear as to how the dynamic process of memory formation influences their synaptic integration into neuronal circuits. New memories are established according to a multistep process during which new information is first acquired and then consolidated to form a stable memory trace. Upon recall, memory is transiently destabilized and vulnerable to modification. Using contextual fear conditioning, we found that learning was associated with an acceleration of dendritic spines formation of adult-born neurons, and that spine connectivity becomes strengthened after memory consolidation. Moreover, we observed that afferent connectivity onto adult-born neurons is enhanced after memory retrieval, while extinction training induces a change of spine shapes. Together, these findings reveal that the neuronal activity supporting memory processes strongly influences the structural dendritic integration of adult-born neurons into pre-existing neuronal circuits. Such change of afferent connectivity is likely to impact the overall wiring of hippocampal network, and consequently, to regulate hippocampal function.

Serotonergic Projections Govern Postnatal Neuroblast Migration

In many vertebrates, postnatally generated neurons often migrate long distances to reach their final destination, where they help shape local circuit activity. Concerted action of extrinsic stimuli is required to regulate long-distance migration. Some migratory principles are evolutionarily conserved, whereas others are species and cell type specific. Here we identified a serotonergic mechanism that governs migration of postnatally generated neurons in the mouse brain. Serotonergic axons originating from the raphe nuclei exhibit a conspicuous alignment with subventricular zone-derived neuroblasts. Optogenetic axonal activation provides functional evidence for serotonergic modulation of neuroblast migration. Furthermore, we show that the underlying mechanism involves serotonin receptor 3A (5HT3A)-mediated calcium influx. Thus, 5HT3A receptor deletion in neuroblasts impaired speed and directionality of migration and abolished calcium spikes. We speculate that serotonergic modulation of postnatally generated neuroblast migration is evolutionarily conserved as indicated by the presence of serotonergic axons in migratory paths in other vertebrates.

Blockade of type I interferon (IFN) production by retroviral replicating vectors and reduced tumor cell responses to IFN likely contribute to tumor selectivity

We developed a Moloney mouse leukemia virus (MLV)-based retroviral replicating vector (RRV), Toca 511, which has displayed tumor specificity in resected brain tumor material and blood in clinical trials. Here, we investigated the interaction between Toca 511 and human host cells, and we show that RRVs do not induce type I interferon (IFN) responses in cultured human tumor cells or cultured human primary cells. However, exogenous type I IFN inhibited RRV replication in tumor cells and induced IFN-regulated genes, albeit at a lower level than in primary cells. Unexpectedly, RRVs did not induce IFN-α production upon incubation in vitro with human plasmacytoid dendritic cells (pDCs), whereas lentivirus vector and heat-treated RRVs did. Coincubation of RRVs with heat-treated RRVs or with lentivirus vector suppressed IFN-α production in pDCs, suggesting that native RRV has a dominant inhibitory effect on type I IFN induction. This effect is sensitive to trypsin treatment. In addition, heat treatment inactivated that activity but exposed an immune-stimulatory activity. The immune-stimulating component is sensitive to deglycosidases, trypsin, and phospholipase C treatment. Experiments with retroviral nonreplicating vectors and virus-like particles demonstrated that the immunosuppressive activity is not associated with the amphotropic envelope or the glyco-Gag protein. In summary, our data provide evidence that RRVs do not directly trigger type I IFN responses in IFN-responsive tumor cells. Moreover, RRVs appear to carry a heat-labile component that actively suppresses activation of cellular innate immune responses in pDCs. Inhibition of IFN induction by RRVs and the reduced response to IFN should facilitate tumor-specific infection in vivo.

IMPORTANCE

RRVs have a convincing preference for replicating in tumor cells in animal models, and we observed similar preferences in the initial treatment of human glioblastoma patients. This study investigates the basis for the interaction between RRV and human host cells (tumor versus nontumor) in vitro. We found that RRVs do not trigger an IFN-α/β response in tumor cells, but the cells are capable of responding to type I IFNs and of producing them when stimulated with known agonists. Surprisingly, the data show that RRVs can actively inhibit induction of cellular innate immunity and that this inhibitory activity is heat labile and trypsin sensitive and not attributable to the envelope protein. These data partially explain the observed in vivo tumor specificity.

An efficient large-scale retroviral transduction method involving preloading the vector into a RetroNectin-coated bag with low-temperature shaking

In retroviral vector-mediated gene transfer, transduction efficiency can be hampered by inhibitory molecules derived from the culture fluid of virus producer cell lines. To remove these inhibitory molecules to enable better gene transduction, we had previously developed a transduction method using a fibronectin fragment-coated vessel (i.e., the RetroNectin-bound virus transduction method). In the present study, we developed a method that combined RetroNectin-bound virus transduction with low-temperature shaking and applied this method in manufacturing autologous retroviral-engineered T cells for adoptive transfer gene therapy in a large-scale closed system. Retroviral vector was preloaded into a RetroNectin-coated bag and incubated at 4°C for 16 h on a reciprocating shaker at 50 rounds per minute. After the supernatant was removed, activated T cells were added to the bag. The bag transduction method has the advantage of increasing transduction efficiency, as simply flipping over the bag during gene transduction facilitates more efficient utilization of the retroviral vector adsorbed on the top and bottom surfaces of the bag. Finally, we performed validation runs of endoribonuclease MazF-modified CD4(+) T cell manufacturing for HIV-1 gene therapy and T cell receptor-modified T cell manufacturing for MAGE-A4 antigen-expressing cancer gene therapy and achieved over 200-fold (≥ 10(10)) and 100-fold (≥ 5 × 10(9)) expansion, respectively. In conclusion, we demonstrated that the large-scale closed transduction system is highly efficient for retroviral vector-based T cell manufacturing for adoptive transfer gene therapy, and this technology is expected to be amenable to automation and improve current clinical gene therapy protocols.

Engineering a serum-resistant and thermostable vesicular stomatitis virus G glycoprotein for pseudotyping retroviral and lentiviral vectors

Vesicular stomatitis virus G glycoprotein (VSV-G) is the most widely used envelope protein for retroviral and lentiviral vector pseudotyping; however, serum inactivation of VSV-G pseudotyped vectors is a significant challenge for in vivo gene delivery. To address this problem, we conducted directed evolution of VSV-G to increase its resistance to human serum neutralization. After six selection cycles, numerous common mutations were present. Based on their location within VSV-G, we analyzed whether substitutions in several surface exposed residues could endow viral vectors with higher resistance to serum. S162T, T230N, and T368A mutations enhanced serum resistance, and additionally K66T, T368A, and E380K substitutions increased the thermostability of VSV-G pseudotyped retroviral vectors, an advantageous byproduct of the selection strategy. Analysis of a number of combined mutants revealed that VSV-G harboring T230N + T368A or K66T + S162T + T230N + T368A mutations exhibited both higher in vitro resistance to human serum and higher thermostability, as well as enhanced resistance to rabbit and mouse serum. Finally, lentiviral vectors pseudotyped with these variants were more resistant to human serum in a murine model. These serum-resistant and thermostable VSV-G variants may aid the application of retroviral and lentiviral vectors to gene therapy.

Serum starvation induced cell cycle synchronization facilitates human somatic cells reprogramming

Human induced pluripotent stem cells (iPSCs) provide a valuable model for regenerative medicine and human disease research. To date, however, the reprogramming efficiency of human adult cells is still low. Recent studies have revealed that cell cycle is a key parameter driving epigenetic reprogramming to pluripotency. As is well known, retroviruses such as the Moloney murine leukemia virus (MoMLV) require cell division to integrate into the host genome and replicate, whereas the target primary cells for reprogramming are a mixture of several cell types with different cell cycle rhythms. Whether cell cycle synchronization has potential effect on retrovirus induced reprogramming has not been detailed. In this study, utilizing transient serum starvation induced synchronization, we demonstrated that starvation generated a reversible cell cycle arrest and synchronously progressed through G2/M phase after release, substantially improving retroviral infection efficiency. Interestingly, synchronized human dermal fibroblasts (HDF) and adipose stem cells (ASC) exhibited more homogenous epithelial morphology than normal FBS control after infection, and the expression of epithelial markers such as E-cadherin and Epcam were strongly activated. Futhermore, synchronization treatment ultimately improved Nanog positive clones, achieved a 15–20 fold increase. These results suggested that cell cycle synchronization promotes the mesenchymal to epithelial transition (MET) and facilitates retrovirus mediated reprogramming. Our study, utilization of serum starvation rather than additional chemicals, provide a new insight into cell cycle regulation and induced reprogramming of human cells.

Improved retroviral suicide gene transfer in colon cancer cell lines after cell synchronization with methotrexate

Background

Cancer gene therapy by retroviral vectors is mainly limited by the level of transduction. Retroviral gene transfer requires target cell division. Cell synchronization, obtained by drugs inducing a reversible inhibition of DNA synthesis, could therefore be proposed to precondition target cells to retroviral gene transfer. We tested whether drug-mediated cell synchronization could enhance the transfer efficiency of a retroviral-mediated gene encoding herpes simplex virus thymidine kinase (HSV-tk) in two colon cancer cell lines, DHDK12 and HT29.

Methods

Synchronization was induced by methotrexate (MTX), aracytin (ara-C) or aphidicolin. Gene transfer efficiency was assessed by the level of HSV-TK expression. Transduced cells were driven by ganciclovir (GCV) towards apoptosis that was assessed using annexin V labeling by quantitative flow cytometry.

Results

DHDK12 and HT29 cells were synchronized in S phase with MTX but not ara-C or aphidicolin. In synchronized DHDK12 and HT29 cells, the HSV-TK transduction rates were 2 and 1.5-fold higher than those obtained in control cells, respectively. Furthermore, the rate of apoptosis was increased two-fold in MTX-treated DHDK12 cells after treatment with GCV.

Conclusions

Our findings indicate that MTX-mediated synchronization of target cells allowed a significant improvement of retroviral HSV-tk gene transfer, resulting in an increased cell apoptosis in response to GCV. Pharmacological control of cell cycle may thus be a useful strategy to optimize the efficiency of retroviral-mediated cancer gene therapy.

Effect of serum on transfection by polyethylenimine/virus-like particle hybrid gene delivery vectors

PURPOSE

Murine leukemia virus-like particles (M-VLP) complexed with polymers to promote cellular uptake and endosomal escape represent a new class of effective gene delivery vectors. Building upon recent studies of viral-synthetic hybrid vectors, we report the effects of serum on the formation, activity and stability of PEI/M-VLP complexes.

METHODS

M-VLP were produced by cells grown in serum-supplemented media (M-VLP-S), serum-free media (M-VLP-SF) or serum-free Opti-MEM® I (M-VLP-OM). PEI/M-VLP stoichiometry was varied to investigate complex formation and optimal transfection conditions. The effects of prolonged storage, freeze-thaw cycles, and ultracentrifugation of M-VLP on the stability of vector transduction efficiency were also observed.

RESULTS

M-VLP-S required more PEI to form infective complexes than M-VLP-SF and M-VLP-OM. The stoichiometry of PEI/M-VLP-S was dependent on total PEI concentration (7-8 μg/100 μL M-VLP supernatant), while optimal infectivity of PEI/M-VLP-SF and PEI/M-VLP-OM depended on PEI/M-VLP ratios (12-17 μg and 10-14 μg PEI/10(9) M-VLP, respectively). PEI/M-VLP-SF and PEI/M-VLP-OM complexes were significantly more efficient than PEI/M-VLP-S. Stability of the hybrid vectors was not significantly affected by serum.

CONCLUSIONS

PEI/M-VLP complexes exhibiting increased efficiency were constructed by producing M-VLP in serum-free media. M-VLP could be stored by freezing or refrigeration and concentrated by ultracentrifugation without unacceptable loss of infectivity.

Mifepristone increases gamma-retroviral infection efficiency by enhancing the integration of virus into the genome of infected cells

Gamma-retroviruses are commonly used to deliver genes to cells. Previously, we demonstrated that the synthetic anti-glucocorticoid and anti-progestin agent, mifepristone, increased gamma-retroviral infection efficiency in different target cells, independent of viral titer. In this study, we examine how this occurs. We studied the effect of mifepristone on different steps of viral infection (viral entry, viral survival, viral DNA synthesis and retrovirus integration into the host genome) in three distinct retroviral backbones using different virus recognition receptors. We also tested the potential role of glucocorticoid and progesterone receptors in mediating mifepristone's ability to increase gamma-retroviral infectivity. We show that mifepristone increases gamma-retroviral infection efficiency by facilitating viral integration into the host genome and that this effect seems to be due to mifepristone's anti-glucocorticoid, but not its anti-progestin, activity. These results suggest that inhibition of the glucocorticoid receptor enhances retroviral integration into the host genome and indicates that cells may have a natural protection again retroviral infection that may be reduced by glucocorticoid receptor antagonists.

Fibrin-mediated lentivirus gene transfer: implications for lentivirus microarrays

We employed fibrin hydrogel as a bioactive matrix for lentivirus mediated gene transfer. Fibrin-mediated gene transfer was highly efficient and exhibited strong dependence on fibrinogen concentration. Efficient gene transfer was achieved with fibrinogen concentration between 3.75 and 7.5mg/ml. Lower fibrinogen concentrations resulted in diffusion of virus out of the gel while higher concentrations led to ineffective fibrin degradation by target cells. Addition of fibrinolytic inhibitors decreased gene transfer in a dose-dependent manner suggesting that fibrin degradation by target cells may be necessary for successful gene delivery. Under these conditions transduction may be limited only to cells interacting with the matrix thereby providing a method for spatially-localized gene delivery. Indeed, when lentivirus-containing fibrin microgels were spotted in an array format gene transfer was confined to virus-containing fibrin spots with minimal cross-contamination between neighboring sites. Collectively, our data suggest that fibrin may provide an effective matrix for spatially-localized gene delivery with potential applications in high-throughput lentiviral microarrays and in regenerative medicine.

Lentiviral vectors in gene therapy: their current status and future potential

The concept of gene therapy originated in the mid twentieth century and was perceived as a revolutionary technology with the promise to cure almost any disease of which the molecular basis was understood. Since then, several gene vectors have been developed and the feasibility of gene therapy has been shown in many animal models of human disease. However, clinical efficacy could not be demonstrated until the beginning of the new century in a small-scale clinical trial curing an otherwise fatal immunodeficiency disorder in children. This first success, achieved after retroviral therapy, was later overshadowed by the occurrence of vector-related leukemia in a significant number of the treated children, demonstrating that the future success of gene therapy depends on our understanding of vector biology. This has led to the development of later-generation vectors with improved efficiency, specificity, and safety. Amongst these are HIV-1 lentivirus-based vectors (lentivectors), which are being increasingly used in basic and applied research. Human gene therapy clinical trials are currently underway using lentivectors in a wide range of human diseases. The intention of this review is to describe the main scientific steps leading to the engineering of HIV-1 lentiviral vectors and place them in the context of current human gene therapy.

Viral vectors for gene transfer: current status of gene therapeutics

Gene therapy for the correction of inherited or acquired disease has gained increasing importance in recent years. Successful treatment of children suffering from severe combined immunodeficiency (SCID) was achieved using retrovirus vectors for gene transfer. Encouraging improvements of vision were reported in a genetic eye disorder (LCA) leading to early childhood blindness. Adeno-associated virus (AAV) vectors were used for gene transfer in these trials. This chapter gives an overview of the design and delivery of viral vectors for the transport of a therapeutic gene into a target cell or tissue. The construction and production of retrovirus, lentivirus, and AAV vectors are covered. The focus is on production methods suitable for biopharmaceutical upscaling and for downstream processing. Quality control measures and biological safety considerations for the use of vectors in clinical trials are discussed.

Using lentiviral vectors for efficient pancreatic cancer gene therapy

Pancreatic cancer (PC) remains a life-threatening disease. Efficient therapeutic gene delivery to PC-derived cells continues to present challenges. We used self-inactivated lentiviral vectors to transduce PC-derived cells in vitro and in vivo. We showed that lentiviral vectors transduce PC-derived cell lines with high efficiency (>90%), regardless of the differentiation state of the cell. Next, we transferred human interferon beta (hIFN-beta) gene. Expression of hIFN-beta in PC cells using lentiviral vectors resulted in the inhibition of cell proliferation and the induction of cell death by apoptosis. In vivo, lentiviral administration of hIFN-beta prevented PC tumor progression for up to 15 days following gene therapy, and induced tumor regression/stabilization in 50% of the mice treated. Again, hIFN-beta expression resulted in cancer cell proliferation inhibition and apoptosis induction. We provide evidence that human immunodeficiency virus (HIV)-1-based lentiviral vectors are very efficient for gene transfer in PC-derived cells in vitro and in vivo. As a consequence, delivery of hIFN-beta stopped PC tumor progression. Thus, our approach could be applied to the 85% of PC patients with a locally advanced disease.

Repeated aerosol delivery of carboxyl-terminal modulator protein suppresses tumor in the lungs of K-rasLA1 mice

RATIONALE

Difficulties in achieving long-term survival of patients with lung cancer treated with conventional therapies suggest that novel approaches are required. Recent advances in aerosol-mediated gene delivery have provided the possibility of an alternative for the safe and effective treatment of lung cancer.

OBJECTIVES

To investigate the repeated effect of carboxyl-terminal modulator protein (CTMP) on multistage lung tumorigenesis. In this study, we addressed this question by studying the effects of lentivirus-based CTMP in the lungs of 9- and 13-week-old K-ras(LA1) mice, a model of lung cancer.

METHODS

An aerosol of lentivirus-based CTMP was delivered into 9- and 13-week-old K-ras(LA1) mice, a model of lung cancer, through a nose-only inhalation system twice a week for 4 weeks. The effects of CTMP on lung cancer progression and Akt-related signals were evaluated.

MEASUREMENTS AND MAIN RESULTS

Long-term repeated delivery of CTMP effectively reduced tumor progression in the lungs at different stages of development. Lentiviral-CTMP inhibited protein synthesis and cell cycle and altered Akt signaling pathway in the lungs of 9-week-old K-ras(LA1) mice, and increased apoptosis was observed in the lungs of 13-week-old K-ras(LA1) mice.

CONCLUSIONS

Long-term repeated viral delivery of CTMP may provide a useful tool for designing lung tumor treatment.

IL-13 cytotoxin has potent antitumor activity and synergizes with paclitaxel in a mouse model of oral squamous cell carcinoma

Interleukin-13 receptor-targeted cytotoxin (IL13-PE38) is highly cytotoxic to certain types of human cancers expressing abundant levels of IL-13Ralpha2 chain. Although IL13-PE38 is being tested in a Phase III clinical trial in brain tumors, the activity of IL13-PE38 alone or when combined with taxane, a chemotherapeutic drug for oral squamous cell carcinoma (OSCC), has not been investigated. Here, we show that approximately 40% of OSCCs (n = 50) in a tissue array are strongly positive for IL-13Ralpha2, whereas normal oral mucosa (n = 10) expresses very low or undetectable levels evaluated by immunohistochemistry. IL13-PE38 was highly cytotoxic to OSCC cell lines, but not cytotoxic to normal oral fibroblasts. IL13-PE38 mediated a synergistic antitumor effect with paclitaxel in OSC-19 in vitro and in vivo in the orthotopic OSCC tongue tumor model. Real-time tumor growth was monitored by optical imaging using a Xenogen-IVIS imaging system. Treated animals showed significant (p < 0.05) improvement in survival, which correlated with in vivo imaging of tumor response without evidence of visible toxicity. Gene transfer of IL-13Ralpha2 in oral cancer cells increased sensitivity of OSCC cell line to IL13-PE38 in vitro. Retrovirus-mediated gene-transfer of IL-13Ralpha2 in HSC-3 into tongue tumors in vivo dramatically enhanced the antitumor activity of IL13-PE38, providing complete elimination of established tumors and prolonging survival of these animals. These results indicate that IL13-PE38 in combination with paclitaxel acting via different mechanisms may be a potential treatment option for IL-13Ralpha2 expressing OSCC or for the treatment of non-IL-13Ralpha2 expressing OSCC combined with gene transfer of IL-13Ralpha2.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Overnight transduction with foamyviral vectors restores the long-term repopulating activity of Fancc-/- stem cells

Fanconi anemia (FA) is a complex genetic disorder characterized by congenital abnormalities, bone marrow failure, and myeloid malignancies. Identification of 13 FA genes has been instrumental to explore gene transfer technologies aimed at correction of autologous FA-deficient stem cells. To date, 3 human FA stem cell gene therapy trials with standard 4-day transduction protocols using gammaretroviral vectors failed to provide clinical benefit. In addition, 2- to 4 day ex vivo manipulation of bone marrow from mice containing a disruption of the homologue of human FANCC (Fancc) results in a time-dependent increase in apoptosis and a risk for malignant transformation of hematopoietic cells. Here, we show that a 14-hour transduction period allows a foamyviral vector construct expressing the human FANCC cDNA to efficiently transduce murine FA stem cells with 1 to 2 proviral integrations per genome. Functionally, the repopulating activity of Fancc(-/-) stem cells from reconstituted mice expressing the recombinant FANCC transgene was comparable with wild-type controls. Collectively, these data provide evidence that short-term transduction of c-kit(+) cells with a foamyviral vector is sufficient for functional correction of a stem cell phenotype in a murine FA model. These data could have implications for future gene therapy trials for FA patients.

Rapid production of retroviruses for efficient gene delivery to mammalian cells using 293T cell-based systems

This unit details the applications of one of the more common retroviral packaging systems, based on the highly transfectable 293T cell. The packaging system employs the use of the Phoenix cell lines. Calcium phosphate-mediated transfection is described for efficient introduction of retroviral vector plasmid DNA into the cells to generate high yields of virion-containing supernatant. An alternate protocol describes a method for transfecting retroviruses that contain a vesicular stomatitis virus G (VSV G) protein. Such virions are said to be "pseudotyped" with VSV G glycoprotein. Support protocols provide a simple method for concentrating VSV-G-pseudotyped retroviruses, as well as methods for culturing, cryopreserving, thawing, and drug selecting the Phoenix packaging cell lines. Finally, several methods for transfecting adherent or suspension cells with retroviruses are described.

A microfluidic bioreactor for increased active retrovirus output

Retroviruses are one of the most commonly used vectors in ongoing gene therapy clinical trials. To evaluate and advance virus production on the microscale platform, we have created a novel microfluidic bioreactor for continuous retrovirus production. We investigated the growth kinetics of a retroviral packaging cell line in microfluidic bioreactors for several compartment sizes, and packaging cells perfused in the microdevices showed similar growth kinetics to those cultured in conventional static conditions. To evaluate the efficiency of retrovirus production, virus titers from the microdevices were compared to those obtained from static tissue culture. When retrovirus production and collection were maintained at 37 degrees C, virus production levels were comparable for the microdevices and static tissue culture conditions. However, immediate cold storage downstream of the packaging cells in the microdevices resulted in 1.4- to 3.7-fold greater active virus production levels with the microdevices compared to the conventional static conditions over a 5 day period. Lastly, the use of microfluidics for virus production provides a continuous supply of virus supernatant for immediate infection of target cells or for preservation and storage. Such devices will be valuable for the optimization of production and evaluation of retroviruses and other viral vectors for gene therapy applications.

Distinct mammalian precursors are committed to generate neurons with defined dendritic projection patterns

The mechanisms that regulate how dendrites target different neurons to establish connections with specific cell types remain largely unknown. In particular, the formation of cell-type-specific connectivity during postnatal neurogenesis could be either determined by the local environment of the mature neuronal circuit or by cell-autonomous properties of the immature neurons, already determined by their precursors. Using retroviral fate mapping, we studied the lamina-specific dendritic targeting of one neuronal type as defined by its morphology and intrinsic somatic electrical properties in neonatal and adult neurogenesis. Fate mapping revealed the existence of two separate populations of neuronal precursors that gave rise to the same neuronal type with two distinct patterns of dendritic targeting-innervating either a deep or superficial lamina, where they connect to different types of principal neurons. Furthermore, heterochronic and heterotopic transplantation demonstrated that these precursors were largely restricted to generate neurons with a predetermined pattern of dendritic targeting that was independent of the host environment. Our results demonstrate that, at least in the neonatal and adult mammalian brain, the pattern of dendritic targeting of a given neuron is a cell-autonomous property of their precursors.

Lentivirus-mediated carboxyl-terminal modulator protein gene transfection via aerosol in lungs of K-ras null mice

The low efficiency of conventional therapies in achieving long-term survival of lung cancer patients calls for development of novel options. Aerosol gene delivery may provide the alternative for safe and effective treatment for lung cancer. Therefore, current study was performed to elucidate the potential effects of C-terminal modulator protein (CTMP) via aerosol on lung tumorigenesis. Lentiviral vector-CTMP was delivered into K-ras null lung cancer mice through the nose-only inhalation system for 30 min. After 48 h, the potential effects of CTMP on Akt1-related signals and cell cycle regulation in the lungs were evaluated by western blot, immunohistochemistry and zymography. Lentivirus-based CTMP delivery inhibited the Akt1 activity through selective suppression of Akt1 phosphorylation at Ser473. Aerosol delivery of CTMP inhibited proteins important for Akt1 signals, cell cycle and tumor metastasis in lungs of K-ras null mice. Together, our results suggest that lentivirus-mediated aerosol delivery of CTMP may be compatible with noninvasive in vivo gene therapy. Our results emphasize the importance of noninvasive-targeted delivery of CTMP for lung cancer therapy in the future. While the studies are conducted in mice, it is envisioned that noninvasive targeting the specific genes responsible for cancer progression is an attractive strategy for effective anticancer therapeutics.

Lentiviral vectors for cancer immunotherapy: transforming infectious particles into therapeutics

Lentiviral vectors have emerged as promising tools for both gene therapy and immunotherapy purposes. They exhibit several advantages over other viral systems in that they are less immunogenic and are capable of transducing a wide range of different cell types, including dendritic cells (DC). DC transduced ex vivo with a whole range of different (tumor) antigens were capable of inducing strong antigen-specific T-cell responses, both in vitro and in vivo. Recently, the administration of lentiviral vectors in vivo has gained substantial interest as an alternative method for antigen-specific immunization. This method offers a number of advantages over DC vaccines as the same lentivirus can in principle be used for all patients resulting in a significantly reduced cost and requirement for considerably less expertise for the generation and administration of lentiviral vaccines. By selectively targeting lentiviral vectors to, or restricting transgene expression in certain cell types, selectivity, safety and efficacy can be further improved. This review will focus on the use of direct administration of lentiviral vectors encoding tumor-associated antigens (TAA) for the induction of tumor-specific immune responses in vivo, with a special focus on problems related to the generation of large amounts of highly purified virus and specific targeting of antigen-presenting cells (APC).

EGF Receptor Activation Decreases Retroviral Gene Transfer through Protein Kinase C-δ

Although much progress has been made in the design of retrovirus vectors, the interactions of recombinant retrovirus with host cells remain largely elusive. The inability of recombinant retrovirus to transduce non-dividing cells prompted several studies to determine optimal cocktails of growth factors and/or extracellular matrix molecules to promote gene transfer to slowly diving cells and stem cells. In contrast to previous reports that growth factors increased gene transfer, we found that treatment of human epidermal keratinocytes and several cell lines with epidermal growth factor receptor (EGFR) ligands EGF, transforming growth factor-alpha, or heparin-binding-EGF decreased gene transfer. Conversely, treatment with an EGFR function-blocking antibody or inhibition of EGFR tyrosine phosphorylation enhanced gene transfer in a dose-dependent manner. In addition, blocking protein kinase C (PKC)-delta but not PKC-zeta, with chemical inhibitors or small interfering RNA reversed the effects of EGF and restored gene transfer, indicating that the effect of EGFR activation is mediated through PKC-delta. Lastly, cell cycle analysis showed that the effect of EGFR activation on retroviral gene transfer was independent of the cell cycle status of target cells. Our results implicate EGFR and PKC-delta in retroviral infection and may have implications for retrovirus gene transfer or design of antiretroviral therapies.

Functional analysis of gammaretroviral vector transduction by quantitative PCR

BACKGROUND

In a clinical setting of gene therapy, quantitative methods are required to determine recombinant viral titres and transgene mRNA expression, avoiding the use of reporter genes.

METHODS

We describe procedures based on quantitative polymerase chain reaction (qPCR) designed to assess functional titres of murine leukaemia virus (MLV) vectors, determine proviral copy numbers in transduced cells, and estimate retroviral transgene expression in both target cell lines and mice with transduced chimeric haematopoiesis.

RESULTS

Compared to EGFP titration, proviral DNA detection by qPCR was more accurate in assessing the number of infective particles in supernatants, such that average viral titres in terms of proviral copies per cell were two-fold higher. Transgene mRNA expression was directly determined from the vectors used without the need for reporter assays. A new parameter, defined here as the 'transcription index' (TI), served to establish the association between transcribed transgenic mRNA and each proviral insertion. The TI represents the potential expression of every vector or insertion in each cell type, and is thus useful as a control parameter for monitoring preclinical or clinical protocols.

CONCLUSIONS

The practical use of qPCR is demonstrated as a valuable alternative to reporter genes for the assessment and surveillance of insertion numbers and transgene expression. In combination with protein expression, this approach should be capable of establishing safer therapeutic gene doses, avoiding the potential side effects of high transduction and expression levels.

Gene-modified tissue-engineered skin: the next generation of skin substitutes

Tissue engineering combines the principles of cell biology, engineering and materials science to develop three-dimensional tissues to replace or restore tissue function. Tissue engineered skin is one of most advanced tissue constructs, yet it lacks several important functions including those provided by hair follicles, sebaceous glands, sweat glands and dendritic cells. Although the complexity of skin may be difficult to recapitulate entirely, new or improved functions can be provided by genetic modification of the cells that make up the tissues. Gene therapy can also be used in wound healing to promote tissue regeneration or prevent healing abnormalities such as formation of scars and keloids. Finally, gene-enhanced skin substitutes have great potential as cell-based devices to deliver therapeutics locally or systemically. Although significant progress has been made in the development of gene transfer technologies, several challenges have to be met before clinical application of genetically modified skin tissue. Engineering challenges include methods for improved efficiency and targeted gene delivery; efficient gene transfer to the stem cells that constantly regenerate the dynamic epidermal tissue; and development of novel biomaterials for controlled gene delivery. In addition, advances in regulatable vectors to achieve spatially and temporally controlled gene expression by physiological or exogenous signals may facilitate pharmacological administration of therapeutics through genetically engineered skin. Gene modified skin substitutes are also employed as biological models to understand tissue development or disease progression in a realistic three-dimensional context. In summary, gene therapy has the potential to generate the next generation of skin substitutes with enhanced capacity for treatment of burns, chronic wounds and even systemic diseases.

Transduction of Schistosoma mansoni by vesicular stomatitis virus glycoprotein-pseudotyped Moloney murine leukemia retrovirus

Retroviral transduction of cultured schistosomes offers a potential means to establish transgenic lines of schistosomes and thereby to facilitate the elucidation of schistosome gene function and expression. The Moloney murine leukemia retroviral (MMLV) vector pLNHX was modified to incorporate EGFP or luciferase reporter genes under control of schistosome endogenous gene promoters from the spliced leader RNA and HSP70 genes. These constructs and a plasmid encoding vesicular stomatitis virus glycoprotein (VSVG) were utilized along with GP2-293 cells to produce replication incompetent retrovirus particles pseudotyped with the VSVG envelope. Exposure of several developmental stages, including sporocysts, of Schistosoma mansoni to these virions was facilitated by incubation with polybrene and/or by centrifugation. The early stages of binding and uptake of virus to the parasite tegument were demonstrated by the immunofluorescence colocalization of VSVG envelope and retroviral capsid proteins. Southern hybridization analysis indicated the integration of proviral forms of the MMLV constructs in genomic DNA isolated from the virus exposed schistosomes. Furthermore, analysis of RNA isolated from virus treated parasites demonstrated the presence of transcripts encoding reporter transgenes. Together these results indicated productive transduction by VSVG pseudotyped MMLV of cultured schistosomes, and suggest a tractable route forward towards heritable schistosome transgenesis.

Downstream processing of oncoretroviral and lentiviral gene therapy vectors

Retroviral vectors from both oncoretroviral and lentiviral origins have a great potential as gene delivery vehicles. A number of research groups have devoted considerable effort to the development of large-scale production strategies for retroviral vectors. However, the manufacturing of clinical-grade vectors for gene therapy, especially for in vivo applications, additionally requires scaleable purification strategies to remove the contaminants present in the harvested supernatants while preserving the functionality of the vectors. In this article, we review recent advances made in the field of downstream processing of retroviral vectors. The methods currently described in the literature for clarification, concentration and purification of retroviral vectors will be presented, with special emphasis on novel chromatography methods that open up the possibility to selectively and efficiently purify retroviruses on a large-scale. Problems associated with stability and quantification of retroviral particles will be outlined and future challenges will be discussed.

Inactivation of murine leukaemia virus by exposure to visible light

Prolonged storage of murine leukaemia virus in ambient light leads to a loss of infectivity. Particle integrity and envelope incorporation are unaffected; rather, the defect is functional and intrinsic to the viral core. Light in the violet part of the visible spectrum (wavelength 420-430 nm) is responsible for virus inactivation. Reduced reverse transcriptase-dependent cDNA generation post-entry accounts for the loss in infectivity and is likely due to a polymerase processivity defect. The virion-associated reverse transcription complex is thus photolabile. The phenomenon could be important in certain experimental situations, notably at elevated temperatures or when exposure to light is extensive. Additionally, our study suggests that the reverse transcription complex is a suitable target for an anti-retroviral strategy; identification of the nature of the lesion and the mechanism of its induction may inform the design of novel inhibitors.

Stoichiometric limitations in assembly of active recombinant retrovirus

Although recombinant retroviruses are widely used in gene therapy and as gene transfer vehicles for basic biological studies, their titers are very low as compared to other recombinant viral systems, e.g., adenovirus. We investigated the rate-limiting steps in production of LacZ-encoding ecotropic (CRE BAG 2) and amphotropic (Psi-CRIP) retrovirus. We found that ecotropic retrovirus producer cells produced a large number of inactive viral particles because they were severely limited by the amount of mRNA that was packaged into viral capsids. Introduction of the gene for green fluorescence protein (GFP) increased retroviral titers 40-fold, without affecting the viral matrix protein, p30, or the activity of reverse transcriptase. Surprisingly, while transfer of GFP gene increased retrovirus production, beta-gal activity and X-gal titer decreased significantly. Quantitative real-time polymerase chain reaction (PCR) showed that although producer cells synthesized similar amounts of both mRNAs, retroviral supernatants contained significantly lower amount of LacZ mRNA, possibly due to competition between LacZ and GFP mRNAs for encapsidation into virions. In contrast to ecotropic producers, introduction of GFP gene copies into amphotropic producers resulted in a moderate twofold increase in retrovirus production. However, delivery of genes encoding for the viral proteins gp70 and p30 increased virus production by fivefold, suggesting that amphotropic producers may also be limited by synthesis of structural viral proteins. Our data show that in addition to the amount of viral genome or proteins, assembly of viral components into active viral particles may limit production of high titer retroviral preparations.

Efficient expansion and gene transduction of mouse neural stem/progenitor cells on recombinant fibronectin

Neural stem/progenitor cells (NSCs) are commonly grown as floating neurospheres in medium containing basic fibroblast growth factor and epidermal growth factor. Under these conditions, about 1% of the cells retain multipotentiality. We developed a protocol based on culture of NSCs in adherence on recombinant fibronectin (rFN) to transduce up to 90% NSCs at a multiplicity of infection of 2 with no need for viral concentration or production of serum-free retroviral supernatants. NSCs grew faster on rFN than as neurospheres on tissue culture plastic and did not lose their stem cell nature or multipotentiality. Furthermore, retroviral-mediated transgene expression was sustained with time in culture and upon differentiation into neurons and astrocytes. These experimental conditions may be utilized to study the function of various genes in NSCs, and to manipulate NSCs for gene and cell therapy of several neurological diseases.

Retroviral vectors for human gene delivery

The potential for gene therapy to cure a wide range of diseases has lead to high expectations and a great increase in research efforts in this area. At present, viral vectors are the most efficient means of delivering a corrective gene into human cells. While a number of different viral vectors are under development, retroviral vectors are currently the most common type used in clinical trials today. However, the production of retroviral vectors for gene therapy applications faces a number of challenges. Of primary concern is the low titre of vector stocks produced by packaging cells in culture and the inherent instability of retroviral vector activity. The problems facing large-scale retroviral vector production are outlined in this review and the research efforts by a number of groups who have attempted to optimise production methods are presented.

Retrovirus vector production and transduction: modulation by the cell cycle

In this study, the cell cycle modulation of retrovirus vector production and transduction was analysed. Retrovirus vector expression was found to be similar in all phases of the cell cycle and, in contrast to some other virus promoters shown previously to be upregulated by G(2)/M arrest, Moloney murine leukaemia virus LTR-driven expression was upregulated neither by G(2)/M growth arrest nor by G(1)/S growth arrest. In contrast, cultures enriched for S phase cells produced more infectious virions, apparently by modulation of stages consequent to provirus expression. In terms of retrovirus transduction, limitations appear to be slow progression through the cell cycle and short half-life of the virus. Synchronization of cells prior to mitosis can increase transduction efficiency. Cell cycle modulation can be used to modify retrovirus vector production and transduction and can allow short transduction periods.

Size-exclusion chromatography purification of high-titer vesicular stomatitis virus G glycoprotein-pseudotyped retrovectors for cell and gene therapy applications

Vesicular stomatitis virus G glycoprotein (VSV-G)-pseudotyped replication-defective retroviral particles are pantropic and amenable to concentration to high titer by ultracentrifugation. These features have allowed development of effective retroviral transduction protocols for stem cells in vitro as well as for tissue engineering in vivo. However, retroparticle ultracentrifugation protocols will also copellet cellular and subcellular debris released from retroviral producer cell lines during vector manufacture. We have analyzed concentrated vector preparations by chromatography and have found that a significant amount of genomic DNA released from producer cells coconcentrates with retroviral particles. In an effort to generate high-purity retroparticle preparations, devoid of subcellular contaminants and contaminating genomic DNA, we have developed a process using size-exclusion chromatography combined with host cell nucleic acid digestion and concentration by ultrafiltration. The procedure allowed for a final recovery of 19 +/- 0.4% infectious viral particles from unfractionated starting material, with an average retroparticle concentration of 7.7 x 10(7) +/- 1.5 x 10(6)/ml. The intact virus is of high purity, >90% as determined by anion-exchange high-performance liquid chromatography. Retroparticle structure appeared intact as determined by negative stain electron microscopy and purified virus was functional and allowed for efficient transduction of primary human bone marrow stromal cells in vitro. In conclusion, we have developed a VSV-G retrovector purification process that can be applied to large-scale retroviral production ideal for cell and gene therapy applications.

The temperature stability of mouse retroviruses depends on the cholesterol levels of viral lipid shell and cellular plasma membrane

To delineate parameters contributing to the extracellular lifetime of retroviral vectors, we carried out stability tests of retroviruses derived from cell lines of different origin and kept under different cultivation conditions. Results show that amphotropic mouse retroviruses (MLV-A) derived from human and hamster cells exhibit 2- to 3-fold higher half-lives compared to retroviruses from mouse cells. Cultivation at 32 degrees C has been reported to yield high virus titers. However, the benefit of virus production in mouse cells at 32 degrees C is controversial. In our hands the cultivation temperature affected, hitherto not noticed, the half-life time of MLV-A. The 37/32 degrees C shift resulted in a 3-fold decrease of viral half-lifes compared to MLV-A released from mouse cells at 37 degrees C. Thus, MLV-A released at 37 degrees C is phenotypically different from MLV-A synthesized at 32 degrees C. Increased virus stability was inversely correlated with the level of cholesterol in the viral membrane. Finally, depletion of viral cholesterol in vitro resulted in intact virus with increased thermal stability. Thus, retrovirus lability depends on the host cell and parallels the cholesterol amount in the viral lipid shell.

Efficient transduction of murine primary T cells requires a combination of high viral titer, preferred tropism, and proper timing of transduction

Retroviral vectors have been used exclusively for genetic modification of primary T cells. Most T cell infection protocols have been developed for human T cells, whereas systematic investigations of the optimal conditions for transduction of murine primary T cells are limited. In this study, ecotropic and 10A1-pseudotyped retroviral vectors were compared for their efficiency in infecting murine primary T cell cells, as well as T cell lines. Various factors that affect transduction efficiency were also explored, including virus titer, times of exposure, timing of infection, low-speed centrifugation, and use of fibronectin fragment. Our results showed that up to 80% of murine primary T cells could be infected after a single exposure. Successful infection required a combination of high virus titer (>10(7) CFU/ml), proper timing of infection (within 24 h after mitogen stimulation), and preferred tropism (ecotropic vectors). These optimization results may help to establish a standard protocol for infection of murine primary T cells and provide some insight into the obstacles to retroviral infection of T cells.

Retroviral gene transfer to human epidermal keratinocytes correlates with integrin expression and is significantly enhanced on fibronectin

Human epidermal keratinocytes are an important target for gene therapy because they can be easily expanded in culture and used to generate skin substitutes for the treatment of wounds, genetic diseases of the skin, and for delivery of proteins to the systemic circulation. Although retroviral transduction results in permanent genetic modification, differentiation and loss of transduced cells from the epidermis results in temporary transgene expression. To ensure permanent genetic modification, epidermal stem cells must be transduced with high efficiency. We evaluated gene transfer on two different substrates and found that the efficiency of gene transfer is substantially higher on a substrate of recombinant fibronectin (FN), when compared to tissue culture plastic (TCP). The rate of retroviral transduction on FN is four times faster than transduction on tissue culture plates and is independent of polybrene (PB). The transduction efficiency correlates with the levels of expression of integrin subunits alpha5, alpha2, and beta1, which have been shown to correlate with stem cell phenotype. Notably, cells that adhere rapidly to FN are transduced more efficiently than slowly adherent cells. In addition, integrin-blocking antibodies decrease the efficiency of gene transfer in a dose-dependent manner. Our results suggest that FN may enhance retroviral gene transfer to the least differentiated cells, thereby increasing the potential of genetically modified keratinocytes to treat short- and long-term disease states.

Biodistribution and retention time of retrovirally labeled T lymphocytes in mice is strongly influenced by the culture period before infusion

T lymphocytes used for adoptive immunotherapy are often cultured before transfer to generate sufficient amounts of effector cells with desired specificity. Modification of lymphocytes induced by in vitro activation and expansion may influence their potential effector capacity by altering the survival and trafficking patterns after transfer. In this report, the authors show that the culture period of T cells after ConA/IL-2 stimulation strongly influences the retention and tissue distribution of these cells after infusion into syngeneic C57BL/6 mice. Infused labeled cells that have been cultured for 3 days remained in the peripheral blood and organs in at least a ten-fold higher number than cells cultured for 8 days. In addition, cells cultured for 3 days preferentially migrate to lungs and liver shortly after infusion and subsequently to lymph nodes and spleen. Cells cultured for 8 days preferentially migrate to liver and can be hardly detected in lymph nodes. In contrast, labeled cells cultured for 3 days are predominantly present in lymph nodes starting from day 8 until day 28. We showed that accurate monitoring of transferred cells is feasible, which may contribute to understanding response to adoptive immunotherapy.

Nucleic acid biotechnology

Driven by advances in the acquisition of genetic sequence information and the ability to manipulate small quantities of nucleic acid, a number of technologies are emerging that exploit nucleic acids for research, diagnostic, and therapeutic utility. In this review, we cover three technologies based on nucleic acids--DNA microarrays, antisense technology, and gene therapy--that are especially promising and may make a substantial impact in the laboratory and in the clinic during the coming years. For each of these areas, an overview of the current status and applications is provided, followed by a discussion of critical issues and challenges to be faced for further advancement of the technology; an emphasis is placed on quantitative and engineering aspects.

Relationship between cell proliferation, cell-cycle phase, and retroviral vector production in FLYRD18 human packaging cells

The relatively low concentrations of retroviral vectors produced by most packaging cells requires the optimization and intensification of their production to make a commercially viable product for gene therapy. While a number of reports exist concerning target cell-cycle effects on retroviral vector infection efficiency, no studies have been reported on the effects of packaging cell cycle on vector production. We have studied the effect of proliferation of the human packaging cell line, FLYRD18, on vector production. In addition, the titer levels of vector produced by cells in each phase of the cell cycle were compared. Numerous studies suggested progression of the cells through the cell cycle to be essential for vector production. However, vector release was found not to be predominant in any particular phase of the cell cycle. These findings indicate that packaging cell proliferation is important for optimal virus production and that arrest of the cells in any particular phase of the cell cycle affords no benefits in retroviral vector production. In contrast to previous reports (using other cell lines), we observed no temporary inhibition of cell cycle progression after detachment of cells from their substratum and that virus production occurred immediately after re-plating of the cells. The findings in this report are important for determining the optimal culture conditions for vector production by packaging cells in vitro.

Infectious entry by amphotropic as well as ecotropic murine leukemia viruses occurs through an endocytic pathway

Infectious entry of enveloped viruses is thought to proceed by one of two mechanisms. pH-dependent viruses enter the cells by receptor-mediated endocytosis and are inhibited by transient treatment with agents that prevent acidification of vesicles in the endocytic pathway, while pH-independent viruses are not inhibited by such agents and are thought to enter the cell by direct fusion with the plasma membrane. Nearly all retroviruses, including amphotropic murine leukemia virus (MuLV) and human immunodeficiency virus type 1, are classified as pH independent. However, ecotropic MuLV is considered to be a pH-dependent virus. We have examined the infectious entry of ecotropic and amphotropic MuLVs and found that they were equally inhibited by NH4Cl and bafilomycin A. These agents inhibited both viruses only partially over the course of the experiments. Agents that block the acidification of endocytic vesicles also arrest vesicular trafficking. Thus, partial inhibition of the MuLVs could be the result of virus inactivation during arrest in this pathway. In support of this contention, we found that that the loss of infectivity of the MuLVs during treatment of target cells with the drugs closely corresponded to the loss of activity due to spontaneous inactivation at 37 degrees C in the same period of time. Furthermore, the drugs had no effect on the efficiency of infection under conditions in which the duration of infection was held to a very short period to minimize the effects of spontaneous inactivation. These results indicate that the infectious processes of both ecotropic and amphotropic MuLVs were arrested rather than aborted by transient treatment of the cells with the drugs. We also found that infectious viruses were efficiently internalized during treatment. This indicated that the arrest occurred in an intracellular compartment and that the infectious process of both the amphotropic and ecotropic MuLVs very likely involved endocytosis. An important aspect of this study pertains to the interpretation of experiments in which agents that block endocytic acidification inhibit infectivity. As we have found with the MuLVs, inhibition of infectivity may be secondary to the block of endocytic acidification. While this strongly suggests the involvement of an endocytic pathway, it does not necessarily indicate a requirement for an acidic compartment during the infectious process. Likewise, a lack of inhibition during transient treatment with the drugs would not preclude an endocytic pathway for viruses that are stable during the course of the treatment.

Issues in the manufacture and transplantation of genetically modified hematopoietic stem cells

The advent of safe and practical means to correct, enhance or protect blood cells at the genetic level offers tantalizing therapeutic perspectives. At present, gene delivery using a replication-defective retrovirus is the most efficient method to stably transduce hematopoietic cells. The successful adaptation of retroviral infection to hematopoietic stem cells requires optimized transduction conditions that maximize gene transfer while preserving the cells' potential for engraftment and longterm hematopoiesis. The successful establishment of effective transduction protocols hinges on retrovirus biology as well as stem cell and transplantation biology. Interestingly, the genetic approach could permit novel strategies to promote host repopulation by transplanted stem cells. However, regulated and predictable expression of any transgene integrated at random chromosomal locations cannot be taken for granted. Investigation of the control of transgene expression and prevention of vector silencing will become increasingly important.

Gene transfer by lentiviral vectors is limited by nuclear translocation and rescued by HIV-1 pol sequences

Gene-transfer vectors based on lentiviruses are distinguished by their ability to transduce non-dividing cells. The HIV-1 proteins Matrix, Vpr and Integrase have been implicated in the nuclear import of the viral genome in non-dividing cells. Here we show that a sequence within pol is also required in cis. It contains structural elements previously associated with the progress of reverse transcription in target cells. We restored these elements in cis within late-generation lentiviral vectors. The new vector transduced to a much higher efficiency several types of human primary cells, when both growing and growth-arrested, including haematopoietic stem cells assayed by long-term repopulation of NOD/SCID mice. On in vivo administration into SCID mice, the vector induced higher plasma levels of human clotting factor IX (F.IX) than non-modified vector. Our results indicate that nuclear translocation of the genome is a rate-limiting step in lentiviral infection of both dividing and non-dividing cells, and that it depends on protein and nucleic acid sequence determinants. Full rescue of this step in lentivirus-based vectors improves performance for gene-therapy applications.

Retroviral labeling is an appropriate marker for dividing cells

5-Bromo-2'-deoxyuridne (BrdU) and 3H-thymidine label mitotically active cells, but they do not adequately mark the progeny of dividing cells for long term study. An alternative method is to label cells using the replication-defective CXL retroviral vector, which carries the lacZ gene encoding beta-galactosidase; however, the ability of the CXL retroviral vector to pulse-label mitotically active cells selectively is not known. Cultures of proliferating muscle cells were simultaneously incubated with the CXL retrovirus and BrdU (10 microM) for 2 hr. After removing the retrovirus containing medium, the cells were maintained for an additional 24 hr in vitro before they were stained to detect beta-galactosidase and BrdU simultaneously. More than 95% of beta-galactosidase positive cells were also BrdU positive suggesting that the majority of beta-galactosidase positive cells were in the S-phase of the cell cycle at the time of CXL retroviral administration. Therefore, the CXL retroviral vector is an appropriate pulse marker for dividing cells, and it is useful when it is desirable to know the fate of the progeny of a particular cell following a mitotic event.