Variegation of retroviral vector gene expression in myeloid cells

Stable integrant-specific differences in bimodal HIV-1 expression patterns revealed by high-throughput analysis

HIV-1 gene expression is regulated by host and viral factors that interact with viral motifs and is influenced by proviral integration sites. Here, expression variation among integrants was followed for hundreds of individual proviral clones within polyclonal populations throughout successive rounds of virus and cultured cell replication, with limited findings using CD4+ cells from donor blood consistent with observations in immortalized cells. Tracking clonal behavior by proviral “zip codes” indicated that mutational inactivation during reverse transcription was rare, while clonal expansion and proviral expression states varied widely. By sorting for provirus expression using a GFP reporter in the nef open reading frame, distinct clone-specific variation in on/off proportions were observed that spanned three orders of magnitude. Tracking GFP phenotypes over time revealed that as cells divided, their progeny alternated between HIV transcriptional activity and non-activity. Despite these phenotypic oscillations, the overall GFP+ population within each clone was remarkably stable, with clones maintaining clone-specific equilibrium mixtures of GFP+ and GFP- cells. Integration sites were analyzed for correlations between genomic features and the epigenetic phenomena described here. Integrants inserted in the sense orientation of genes were more frequently found to be GFP negative than those in the antisense orientation, and clones with high GFP+ proportions were more distal to repressive H3K9me3 peaks than low GFP+ clones. Clones with low frequencies of GFP positivity appeared to expand more rapidly than clones for which most cells were GFP+, even though the tested proviruses were Vpr-. Thus, much of the increase in the GFP- population in these polyclonal pools over time reflected differential clonal expansion. Together, these results underscore the temporal and quantitative variability in HIV-1 gene expression among proviral clones that are conferred in the absence of metabolic or cell-type dependent variability, and shed light on cell-intrinsic layers of regulation that affect HIV-1 population dynamics.

Very few HIV-1 infected cells persist in patients for more than a couple days, but those that do pose life-long health risks. Strategies designed to eliminate these cells have been based on assumptions about what viral properties allow infected cell survival. However, such approaches for HIV-1 eradication have not yet shown therapeutic promise, possibly because many assumptions about virus persistence are based on studies involving a limited number of infected cell types, the averaged behavior of cells in diverse populations, or snapshot views. Here, we developed a high-throughput approach to study hundreds of distinct HIV-1 infected cells and their progeny over time in an unbiased way. This revealed that each virus established its own pattern of gene expression that, upon infected cell division, was stably transmitted to all progeny cells. Expression patterns consisted of alternating waves of activity and inactivity, with the extent of activity differing among infected cell families over a 1000-fold range. The dynamics and variability among infected cells and within complex populations that the work here revealed has not previously been evident, and may help establish more accurate correlates of persistent HIV-1 infection.

Alpharetroviral vector-mediated gene therapy for X-CGD: functional correction and lack of aberrant splicing

Comparative integrome analysis has revealed that the most neutral integration pattern among retroviruses is attributed to alpharetroviruses. We chose X-linked chronic granulomatous disease (X-CGD) as model to evaluate the potential of self-inactivating (SIN) alpharetroviral vectors for gene therapy of monogenic diseases. Therefore, we combined the alpharetroviral vector backbone with the elongation factor-1α short promoter, both considered to possess a low genotoxic profile, to drive transgene (gp91(phox)) expression. Following efficient transduction transgene expression was sustained and provided functional correction of the CGD phenotype in a cell line model at low vector copy number. Further analysis in a murine X-CGD transplantation model revealed gene-marking of bone marrow cells and oxidase positive granulocytes in peripheral blood. Transduction of human X-CGD CD34+ cells provided functional correction up to wild-type levels and long-term expression upon transplantation into a humanized mouse model. In contrast to lentiviral vectors, no aberrantly spliced transcripts containing cellular exons fused to alpharetroviral sequences were found in transduced cells, implying that the safety profile of alpharetroviral vectors may extend beyond their neutral integration profile. Taken together, this highlights the potential of this SIN alpharetroviral system as a platform for new candidate vectors for future gene therapy of hematopoietic disorders.

Alpharetroviral self-inactivating vectors: long-term transgene expression in murine hematopoietic cells and low genotoxicity

Comparative integrome analyses have highlighted alpharetroviral vectors with a relatively neutral, and thus favorable, integration spectrum. However, previous studies used alpharetroviral vectors harboring viral coding sequences and intact long-terminal repeats (LTRs). We recently developed self-inactivating (SIN) alpharetroviral vectors with an advanced split-packaging design. In a murine bone marrow (BM) transplantation model we now compared alpharetroviral, gammaretroviral, and lentiviral SIN vectors and showed that all vectors transduced hematopoietic stem cells (HSCs), leading to comparable, sustained multilineage transgene expression in primary and secondary transplanted mice. Alpharetroviral integrations were decreased near transcription start sites, CpG islands, and potential cancer genes compared with gammaretroviral, and decreased in genes compared with lentiviral integrations. Analyzing the transcriptome and intragenic integrations in engrafting cells, we observed stronger correlations between in-gene integration targeting and transcriptional activity for gammaretroviral and lentiviral vectors than for alpharetroviral vectors. Importantly, the relatively "extragenic" alpharetroviral integration pattern still supported long-term transgene expression upon serial transplantation. Furthermore, sensitive genotoxicity studies revealed a decreased immortalization incidence compared with gammaretroviral and lentiviral SIN vectors. We conclude that alpharetroviral SIN vectors have a favorable integration pattern which lowers the risk of insertional mutagenesis while supporting long-term transgene expression in the progeny of transplanted HSCs.

Genetic modification of mesenchymal stem cells

Mesenchymal stem cells (MSC) are currently considered the most promising type of adult stem cells for therapeutic applications, because they can be easily isolated from the bone marrow and other tissues, and manipulated for different applications. The genetic transformation of MSC using genes that enhance their homing ability, as well as their proliferation and survival capacities when transplanted to sites of injury, is an important alternative to improve MSC function, especially for tissue regeneration. This chapter describes protocols for the transformation of MSC using plasmid vectors by lipofection and electroporation, as well as retroviral vectors representing viral transformations.

Physiological regulation of transgene expression by a lentiviral vector containing the A2UCOE linked to a myeloid promoter

Protection against epigenetic silencing is a desirable feature of future gene therapy vectors, in particular for those applications in which transgene expression will not confer growth advantage to gene-transduced cells. The ubiquitous chromatin opening element (UCOE) consisting of the methylation-free CpG island encompassing the dual divergently transcribed promoters of the human HNRPA2B1-CBX3 housekeeping genes (A2UCOE) has been shown to shield constitutive active heterologous promoters from epigenetic modifications and chromosomal position effects. However, it is unclear if this element can be used to improve expression from tissue-specific enhancer/promoters, while maintaining tissue specificity in hematopoietic cells. Here, we evaluated the potential of the A2UCOE in combination with the myeloid-specific myeloid related protein 8 (MRP8) promoter to target transgene expression specifically to myeloid cells in vitro and in vivo from a self-inactivating lentiviral vector. The inclusion of the A2UCOE did not interfere with specific upregulation of MRP8 promoter activity during myeloid differentiation and mediated sustained and vector copy-dependent expression in myeloid cells. Notably, the A2UCOE did not protect the MRP8 promoter from methylation in the P19 embryonal carcinoma cell line, suggesting that this element maintains the inherent epigenetic state and transcriptional activity of cellular promoters in their native configuration. Thus, the A2UCOE could represent a useful protective genetic element in gene therapy vectors, ensuring physiological transcriptional regulation of tissue-specific promoters independent of the chromosomal integration site.

Lentiviral vector design and imaging approaches to visualize the early stages of cellular reprogramming

Induced pluripotent stem cells (iPSCs) can be derived from somatic cells by gene transfer of reprogramming transcription factors. Expression levels of these factors strongly influence the overall efficacy to form iPSC colonies, but additional contribution of stochastic cell-intrinsic factors has been proposed. Here, we present engineered color-coded lentiviral vectors in which codon-optimized reprogramming factors are co-expressed by a strong retroviral promoter that is rapidly silenced in iPSC, and imaged the conversion of fibroblasts to iPSC. We combined fluorescence microscopy with long-term single cell tracking, and used live-cell imaging to analyze the emergence and composition of early iPSC clusters. Applying our engineered lentiviral vectors, we demonstrate that vector silencing typically occurs prior to or simultaneously with the induction of an Oct4-EGFP pluripotency marker. Around 7 days post-transduction (pt), a subfraction of cells in clonal colonies expressed Oct4-EGFP and rapidly expanded. Cell tracking of single cell-derived iPSC colonies supported the concept that stochastic epigenetic changes are necessary for reprogramming. We also found that iPSC colonies may emerge as a genetic mosaic originating from different clusters. Improved vector design with continuous cell tracking thus creates a powerful system to explore the subtle dynamics of biological processes such as early reprogramming events.

Methods for Gene Delivery

The success of any gene transfer procedure, either through in vivo inoculation of the genetic material or after gene transfer into the patient’s cells ex vivo, strictly depends upon the efficiency of nucleic acid internalization by the target cells. As a matter of fact, making gene transfer more efficient continues to represent the most relevant challenge to the clinical success of gene therapy.

Chromatin, gene silencing and HIV latency

Cellular defence mechanisms against HIV contribute to its persistence.

One of the cellular defenses against virus infection is the silencing of viral gene expression. There is evidence that at least two gene-silencing mechanisms are used against the human immuno-deficiency virus (HIV). Paradoxically, this cellular defense mechanism contributes to viral latency and persistence, and we review here the relationship of viral latency to gene-silencing mechanisms.

Efficient in vivo targeting of epidermal stem cells by early gestational intraamniotic injection of lentiviral vector driven by the keratin 5 promoter

At the present time, no efficient in vivo method for gene transfer to skin stem cells exists. In this study, we hypothesized that early in gestation, specific epidermal stem cell populations may be accessible for gene transfer. To test this hypothesis, we injected lentiviral vectors encoding the green fluorescence protein marker gene driven by either the cytomegalovirus promoter or the keratin 5 (K5) promoter into the murine amniotic space at early developmental stages between embryonic days 8 and 12. This resulted in sustained green fluorescent protein (GFP) expression in both basal epidermal stem cells and bulge cells in the hair follicles of the skin. Transduction of stem cell populations was dependent on the developmental stage, and confirmed by the prolonged duration of GFP expression in all skin elements into adulthood. In addition, transduced stem cell populations responded to regenerative signals after wounding and actively participated in wound healing. Finally, we quantified the fraction of epidermal stem cells transduced, and the distribution of transduction related to the promoters utilized, confirming improved efficiency with the K5 promoter. This simple approach has possible biological applications in our study of gene functions in skin, and perhaps future clinical applications for treatment of skin based disorders.

Stable gene expression occurs from a minority of integrated HIV-1-based vectors: transcriptional silencing is present in the majority

Human immunodeficiency virus (HIV)-based vectors are being increasingly used in vitro for gene transfer and in vivo for gene therapy. The proportion of integrated retroviral vectors that are silenced or remain transcriptionally active, and the stability of gene expression in the latter remains poorly explored. To study this, T cells were infected with an HIV-1-based vector construct containing a long terminal repeat-driven reporter gene. Only a small percentage of detectable integrated vector expressed gene product. In clones derived from cells with transcriptionally active vector, gene expression was remarkably stable with more than 80% continuing to express for greater than 18 months. Failure to continue expressing the vector was associated with epigenetic changes. Our data suggest that there are two forms of vector silencing: one occurring immediately after integration affecting the majority of the vectors, and one occurring in the much longer term affecting a small minority of vectors which had previously established expression.

Hematopoietic stem cell gene transfer in a tumor-prone mouse model uncovers low genotoxicity of lentiviral vector integration

Insertional mutagenesis represents a major hurdle to gene therapy and necessitates sensitive preclinical genotoxicity assays. Cdkn2a-/- mice are susceptible to a broad range of cancer-triggering genetic lesions. We exploited hematopoietic stem cells from these tumor-prone mice to assess the oncogenicity of prototypical retroviral and lentiviral vectors. We transduced hematopoietic stem cells in matched clinically relevant conditions, and compared integration site selection and tumor development in transplanted mice. Retroviral vectors triggered dose-dependent acceleration of tumor onset contingent on long terminal repeat activity. Insertions at oncogenes and cell-cycle genes were enriched in early-onset tumors, indicating cooperation in tumorigenesis. In contrast, tumorigenesis was unaffected by lentiviral vectors and did not enrich for specific integrants, despite the higher integration load and robust expression of lentiviral vectors in all hematopoietic lineages. Our results validate a much-needed platform to assess vector safety and provide direct evidence that prototypical lentiviral vectors have low oncogenic potential, highlighting a major rationale for application to gene therapy.

Silencing and variegation of gammaretrovirus and lentivirus vectors

Retrovirus vectors integrate into the genome, providing stable gene transfer, but integration contributes in part to transcriptional silencing that compromises long-term expression. In the case of gammaretrovirus vectors based on murine leukemia virus, many integration events are completely silenced in undifferentiated stem cells and in transgenic mice. Gammaretrovirus vectors are also subject to variegation in which sister cells bearing the same provirus differentially express, and cell differentiation can lead to extinction of vector expression. In contrast, lentivirus vectors based on human immunodeficiency virus type 1 appear to express more efficiently, although other reports indicate that lentivirus vectors can be silenced. This review summarizes the key features of gammaretrovirus vector silencing. The evidence for and against gene silencing of lentivirus vectors is described with special emphasis on the potential effects of vector design, provirus copy number, and integration site preferences on silencing. This analysis suggests that the difference between selfinactivating (SIN) lentivirus vectors and their modified SIN gammaretrovirus counterparts may be less dramatic than previously thought. It will therefore be important to further characterize the mechanisms of silencing, in order to create better gammaretrovirus and lentivirus vectors that consistently express at single copy for gene therapy.

Retrovirus silencing, variegation, extinction, and memory are controlled by a dynamic interplay of multiple epigenetic modifications

Retrovirus silencing in stem cells produces silent or variegated provirus. Additional memory and extinction mechanisms act during differentiation. Here we show that retrovirus is silent or variegated in mouse embryonic stem (ES) cells that are de novo methyltransferase (dnmt3a and dnmt3b) null. Memory is maintained during differentiation, and extinction occurs on variegated retrovirus, indicating that DNA methylation is dispensable for all forms of retrovirus silencing. Silent and variegated provirus are marked by hypoacetylated histone H3 and bound H1. In wild-type ES cells, silent and variegated proviruses are methylated and bound by hypoacetylated H3, MeCP2, and less H1. Silencing, variegation, and extinction are partially reactivated by 5-AzaC in this context. Lentivirus vectors are also silent or variegated, marked by silent chromatin, and exhibit memory and extinction. We conclude that the universal epigenetic mark of retrovirus silencing is silent chromatin established via the dynamic interplay of multiple epigenetic modifications that include but do not require DNA methylation. A molecular mechanism of competitive H1 and MeCP2 binding may account for this epigenetic interplay, and a model for variegation is discussed.

A one-step approach to obtain cell clones expressing tetracycline-responsive transactivators

Despite the wide application of the tetracycline-regulated gene expression system, several drawbacks in establishing the system in in vitro-cultured cells have been described. Most of the problems are related to obtaining a reliable tetracycline-regulated cell clone, which often results in arduous labor. We describe here a new approach to facilitate the screening and selection of such cell clones. We have constructed a tetracycline-responsive plasmid that harbors an antibiotic resistance gene fused to the enhanced green fluorescent protein (EGFP) gene and the luciferase gene, both under the control of a bidirectional promoter. We demonstrate that the selection of tetracycline-regulated clones is highly simplified by using this plasmid. Only clones expressing the system in a functional manner are able to survive under antibiotic selection. In addition, a quick characterization of the responsiveness of the clones is possible by monitoring GFP expression in vivo.

Gene therapy of X-linked severe combined immunodeficiency

This review addresses several questions in the light of the results recently obtained by a gene therapy trial for the treatment of X-linked severe combined immunodeficiency. This primary immunodeficiency, characterized by a complete absence of T and natural killer lymphocytes, appeared as a good model for the application of gene therapy, combining an expected selective advantage for transduced cells, an absence of immunological response to the vector and/or the therapeutic transgene together with accessibility to hematopoietic stem cells. After a brief description of the disease and its physiopathology we summarize the clinical results of the gene therapy trial putting them in perspective with those obtained following allogeneic hematopoietic stem cell transplantation. Definitive conclusions cannot be thrown due to the limited number of gene therapy-treated patients and their relatively short follow-up.

Long-term high-level reconstitution of NADPH oxidase activity in murine X-linked chronic granulomatous disease using a bicistronic vector expressing gp91phox and a Delta LNGFR cell surface marker

A murine model of X-linked chronic granulomatous disease (X-CGD), an inherited immune deficiency with absent phagocyte NADPH oxidase activity caused by defects in the gp91(phox) gene, was used to evaluate a bicistronic retroviral vector in which expression of human gp91(phox) and a linked gene for Delta LNGFR, a truncated form of human low-affinity nerve growth factor receptor, are under the control of a spleen focus-forming virus long-terminal repeat (LTR). Four independent cohorts of 11-Gy irradiated X-CGD mice (total, 22 mice) were transplanted with or without preselection of transduced X-CGD bone marrow (BM). Transplanted mice had high-level correction of neutrophil gp91(phox) expression and reconstitution of NADPH oxidase activity. Expression lasted for at least 14 months in primary transplants, and persisted in secondary and tertiary transplants. Both gp91(phox) and Delta LNGFR were detected on circulating granulocytes, lymphocytes, lymphoid, and (for Delta LNGFR) red blood cells. Mice receiving transduced bone marrow [BM] preselected ex vivo for Delta LNGFR expression had high-level (= 80%) reconstitution with transduced cells, with an improved fraction of oxidase-corrected neutrophils posttransplant. Analysis of secondary and tertiary CFU-S showed that silencing of individual provirus integrants can occur even after preselection for Delta LNGFR prior to transplantation, and that persistent provirus expression was associated with multiple integration sites in most cases. No obvious adverse consequences of transgenic protein expression were observed.

Elevation of transgene expression level by flanking matrix attachment regions (MAR) is promoter dependent: a study of the interactions of six promoters with the RB7 3' MAR

We have analyzed effects of a matrix attachment region (MAR) from the tobacco RB7 gene on transgene expression from six different promoters in stably transformed tobacco cell cultures. The presence of MARs flanking the transgene increased expression of constructs based on the constitutive CaMV 35S, NOS, and OCS promoters. Expression from an induced heat shock promoter was also increased and MARs did not cause expression in the absence of heat shock. There was also no effect of MARs on the pea ferredoxin promoter, which is not normally expressed in this cell line. Importantly, most transgenes flanked by RB7 MAR elements showed a large reduction in the number of low expressing GUS transformants relative to control constructs without MARs.

Kinetics of fluorescence expression in nonhuman primates transplanted with GFP retrovirus-modified CD34 cells

Downregulation and loss of proviral expression have been demonstrated to occur in a variety of in vitro studies and in mouse models. Here we evaluated the kinetics of proviral expression after transplantation in a competitive repopulating model in the baboon. Transgene persistence and green fluorescent protein (GFP) expression in peripheral blood leukocytes (PBL) were analyzed in four animals by semiquantitative PCR and flow cytometry for up to 80 weeks (range 17-80). All animals were transplanted with cells transduced with EGFP or EYFP reporters driven by Moloney murine leukemia virus (MoMuLV) or a modified promoter/enhancer, (MND) respectively. Simultaneous dual-color analysis of fluorescence levels in granulocyte and lymphocyte subsets following hematopoietic reconstitution demonstrated progressive loss of fluorescence intensity occurring predominantly early after transplant in cells transduced with both retrovirus backbones and at serial time points. In addition, we carried out PCR analysis of DNA extracted from sorted EGFP(-)/EYFP(-) cells and confirmed the presence of cells genetically marked by either vector in this population, indicating the persistence of cells that have downregulated or lost retroviral gene expression. In comparison to mouse studies, however, we did not detect substantial differences between MND and MoMuLV backbones.

Robust and efficient regulation of transgene expression in vivo by improved tetracycline-dependent lentiviral vectors

We developed a panel of lentiviral vectors that displayed tetracycline-regulated transgene expression over two orders of magnitude in bulk, non-selected populations of transduced cells in vitro and in vivo. The robust expression and homogeneous response indicated that most transduced vector genomes were transcription competent and responsive to regulation, providing the lentiviral vector with a novel competitive advantage for gene transfer. After ex vivo transduction and transplantation of cord blood CD34+ cells into NOD/SCID mice, reporter gene expression could be switched "on" and "off" in human hematopoietic cells in vivo for prolonged times, proving integration of the regulated expression system into long-term repopulating cells. By vector injection into established tumor grafts, we achieved efficient delivery and quantitative regulation of transgene expression in vivo. By these approaches, gene function studies can now be performed in in vivo models of human hematopoiesis and cancer. In the future, regulated lentiviral vectors will improve the safety and efficacy of gene therapy.

Down-regulation of retroviral transgene expression during differentiation of progenitor-derived dendritic cells

OBJECTIVE

Hematopoietic progenitor cells are a promising source for generation of genetically modified dendritic cells. A prerequisite for using these cells in therapeutic approaches is stable vector-mediated transgene expression during and after cell maturation. We investigated the expression of enhanced green fluorescence protein (EGFP) mediated by retroviral vectors in dendritic cells and other hematopoietic cells differentiated in vitro.

MATERIAL AND METHODS

CD34(+) cells were efficiently transduced with retroviral vector constructs known to mediate different expression levels due to distinct cis-acting elements. EGFP(+) cells were purified by cell sorting and differentiated to monocytes, granulocytes, dendritic cells, and erythrocytes. Coexpression of EGFP and cell type-specific markers was analyzed by flow cytometry.

RESULTS

Transgene expression from various retroviral vectors was silenced exclusively in dendritic cells, but not in other mature myeloid cells. Loss of EGFP was most pronounced in cells initially displaying low expression levels. This was confirmed by using a retroviral vector coding for a variant of EGFP with significantly reduced half-life. In contrast, a majority of dendritic cells showed stable expression when a self-inactivating retroviral construct using an internal cytomegalovirus promotor was used.

CONCLUSIONS

We suggest that expression from the retroviral long terminal repeat is silenced during dendritic cell differentiation in vitro. High levels of stable transgene product in progenitor cells may mask a loss of expression. An improvement of retroviral vectors mediating stable transgenic expression is necessary for therapeutic approaches using gene-modified dendritic cells.

Gene therapy of HIV-1 infection using lentiviral vectors expressing anti-HIV-1 genes

The use of vectors based on primate lentiviruses for gene therapy of human immunodeficiency virus type 1 (HIV-1) infection has many potential advantages over the previous murine retroviral vectors used for delivery of genes that inhibit replication of HIV-1. First, lentiviral vectors have the ability to transduce dividing and nondividing cells that constitute the targets of HIV-1 infection such as resting T cells, dendritic cells, and macrophages. Lentiviral vectors can also transfer genes to hematopoietic stem cells with a superior gene transfer efficiency and without affecting the repopulating capacity of these cells. Second, these vectors could be potentially mobilized in vivo by the wild-type virus to secondary target cells, thus expanding the protection to previously untransduced cells. And finally, lentiviral vector backbones have the ability to block HIV-1 replication by several mechanisms that include sequestration of the regulatory proteins Tat and Rev, competition for packaging into virions, and by inhibition of reverse transcription in heterodimeric virions with possible generation of nonfunctional recombinants between the vector and viral genomes. The inhibitory ability of lentiviral vectors can be further increased by expression of anti-HIV-1 genes. In this case, the lentiviral vector packaging system has to be modified to become resistant to the anti-HIV-1 genes expressed by the vector in order to avoid self-inhibition of the vector packaging system during vector production. This review focuses on the use of lentiviral vectors as the main agents to mediate inhibition of HIV-1 replication and discusses the different genetic intervention strategies for gene therapy of HIV-1 infection.

Purging of chronic myelogenous leukemia cells by retrovirally expressed anti-bcr-abl ribozymes with specific cellular compartmentalization

In patients with chronic myelogenous leukemia (CML), abnormal expansion of myeloid cells is maintained by expression of the p210(bcr-abl) fusion protein. Thus, this protein and its mRNA represent primary targets to inhibit proliferation of these cells. Here we describe the properties of a ribozyme against the bcr-abl mRNA, expressed as a fusion transcript with the human U1 small nuclear RNA or the adenovirus VA1 RNA and delivered to the cells through retroviral vectors. These fusion ribozymes are specifically localized in the nucleus or in the cytoplasm, respectively. Transduction of 32D-LG7 myeloid cells, whose growth is IL-3 independent thanks to deregulated bcr-abl expression, imposed strong negative selective pressure on cell growth and induced restoration of an IL-3-dependent phenotype. Although expressed at a level similar to that of the U1-fusion ribozyme, the cytoplasmic VA1 ribozyme was a more powerful inhibitor of p210(bcr-abl) gene expression. In cells transduced with the vector expressing this ribozyme, the levels of the bcr-abl transcript were reduced up to 10(4)-fold, the p210(bcr-abl) protein became undetectable, and the cells underwent massive apoptosis when cultured in the absence of IL-3. Transduction of primary hematopoietic cells obtained from bone marrow of patients with CML resulted in remarkable reduction of bcr-abl mRNA levels, starting a few days after transduction. These results show the feasibility and efficacy of vector-expressed anti-bcr-abl ribozymes for purging of CML cells.

Ectopic expression of CCAAT/enhancer binding protein beta (C/EBPbeta) in long-term bone marrow cultures induces granulopoiesis and alters stromal cell function

CCAAT/enhancer binding proteins (C/EBP) have been demonstrated to impact directly the development of multiple hematopoietic lineages. However, the role of C/EBPbeta in the differentiation of various hematopoietic lineages has not been thoroughly examined. We used primary bone marrow cultures to assess directly the ability of C/EBPbeta to influence myelopoiesis. Retroviral expression vectors were used to express C/EBPbeta ectopically in murine primary long-term bone marrow cultures. The differentiation potential of these cells was determined using hematopoietic colony assays and differential staining of cells within the cultures. Bone marrow cultures that overexpressed C/EBPbeta had fewer myeloid progenitors and a significant increase in the number of granulocytes. The ability of C/EBPbeta to alter hematopoiesis in vitro was dependent on the presence of the transcriptional activation domain because LIP, which lacks this functional domain, did not decrease the ability of bone marrow cultures to support myeloid progenitors. These data also show that C/EBPbeta influences hematopoiesis by altering stromal cell function rather than the intrinsic developmental potential of myeloid progenitor cells.

The site of HIV-1 integration in the human genome determines basal transcriptional activity and response to Tat transactivation

Because of the heterogeneity of chromatin, the site of integration of human immunodeficiency virus (HIV) in the genome could have dramatic effects on its transcriptional activity. We have used an HIV-1-derived retroviral vector, in which the green fluorescent protein is under the control of the HIV promoter, to generate by infection 34 Jurkat clonal cell lines each containing a single integration of the HIV-1 vector. In the absence of Tat, a 75-fold difference in expression level between the highest and lowest expressing clones was observed. Basal promoter activity was low in 80% of the clones and moderate to high in the remaining 20% of clones. We found that differences in expression levels are due to the integration site and are not controlled by DNA methylation or histone acetylation. Tat activated transcription in each clone, and an inverse correlation was observed between basal transcriptional activity and inducibility by Tat. These observations demonstrate that the chromatin environment influences basal HIV gene expression and that the HIV Tat protein activates transcription independently of the chromatin environment.

Enhanced transgene expression in cord blood CD34(+)-derived hematopoietic cells, including developing T cells and NOD/SCID mouse repopulating cells, following transduction with modified trip lentiviral vectors

The recent development of lentivirus-derived vectors is an important breakthrough in gene transfer technology because these vectors allow transduction of nondividing cells such as hematopoietic stem cells (HSC), due to an active nuclear import of reverse-transcribed vector DNA. We recently demonstrated that addition of the central DNA flap of HIV-1 to an HIV-derived lentiviral vector strikingly increases transduction of CD34(+) cells. We now describe improvements of the transduction protocol designed to preserve HSC properties and two modifications of the previously described TRIP-CMV vector. First, deletion of the enhancer/promoter of the 3' LTR in the TRIP-CMV vector resulted in a safer vector (TRIPDeltaU3-CMV) with conserved transduction efficiency and increased EGFP transgene expression. Second, the original internal CMV promoter was replaced with the promoter for the ubiquitously expressed elongation factor 1alpha (EF1alpha). This promoter substitution resulted in a significantly more homogeneous expression of the EGFP transgene in all hematopoietic cell types, including CD34(+)-derived T lymphocytes, in which the CMV promoter was inactive, and NOD/SCID mouse repopulating cells. We thus present here an HIV-derived lentiviral vector, TRIPDeltaU3-EF1alpha, which can very efficiently transduce human cord blood HSC and results in high long-term transgene expression in CD34(+)-derived T, B, NK, and myeloid hematopoietic cells.

Efficient in vitro and in vivo gene regulation of a retrovirally delivered pro-apoptotic factor under the control of the Drosophila HSP70 promoter

We have developed a self-inactivating retroviral vector system with an internal, inducible Drosophila HSP70 promoter. This vector system delivers the desired transgene into cells rapidly and efficiently. It generates mixed populations of transduced cells where the transgene is inducible, and does not require the isolation of specific clones. Since the transgene is not expressed (or poorly expressed) at the restrictive condition (34 degrees C), mixed populations can be selected in which tumor suppressors or other inhibitory genes can be strongly induced upon changing the conditions (39 degrees C or the plant amino acid L-canavanine). This retroviral vector should be very useful for the expression of sequences that are poorly tolerated by cells, and is also active in animals.

MDR1 gene expression in NOD/SCID repopulating cells after retroviral gene transfer under clinically relevant conditions

We have adapted a recently published protocol for retroviral gene transfer into hematopoietic cells [A. J. Schilz et al. (1998) Blood 92: 3163-3171] with respect to clinical requirements such as large-volume vector stock generation, adequate cell source, high cell numbers, and serum-free conditions. We present data on transduction efficacy and expression of the multidrug resistance 1 (MDR1) gene in human CD34(+) cells from mobilized peripheral blood (PB) mediated by a gibbon ape leukemia virus (GALV)-pseudotyped retroviral vector. Using a 1-day cytokine-mediated prestimulation, consisting of human interleukin (IL)-3, IL-6, stem cell factor (SCF), Flt-3 ligand (FL), and thrombopoietin (TPO), followed by a 3-day transduction procedure, we were able to detect up to 51% CD34(+) cells expressing MDR1. Xenotransplantation of transduced cells into NOD/LtSz-scid/scid (NOD/SCID) mice resulted in a mean engraftment level of 23% (0.1 to 87%). As shown by quantitative PCR analysis, a mean of 12.7% (range 0.3 to 55%) of the engrafted human cells in the bone marrow of chimeric mice contained the MDR1 cDNA. Furthermore, enhanced expression of MDR1 above control levels was detected in up to 15% of the engrafted human cell population. Our data suggest that NOD/SCID repopulating cells derived from mobilized PB can be transduced efficiently with existing retroviral vector systems under clinically applicable conditions.

Lentivirus-mediated gene transfer of gp91phox corrects chronic granulomatous disease (CGD) phenotype in human X-CGD cells

BACKGROUND

Chronic granulomatous diseases (CGD) are caused by impaired antimicrobial activity in phagocytes, due to the absence or malfunction of the respiratory burst NADPH oxidase. Two-thirds of the patients have mutations in their X-linked CGD gene encoding gp91phox, the largest subunit of the NADPH oxidase.

METHODS

Aimed at gene therapy of X-CGD already at the level of resting pluripotent hematopoietic stem cells, we generated an advanced HIV-1-based vector with self-inactivating (SIN2) features containing the therapeutic gp91phox gene. In this vector an internal cytomegalovirus (CMV) promoter exclusively drives transgene expression. The green fluorescent protein (GFP) served as reporter for evaluation of gene transfer and expression in the human myeloid PLB985 X-CGD cell line.

RESULTS

The X-CGD cells were efficiently transduced by the VSV-G pseudotyped lentivirus constructs (up to 74% GFP+ cells at 3 days post-transduction). CMV-driven GFP-expression was stable for at least 3 weeks after transduction and persisted after granulocytic differentiation of the target cells. Using the lentivector with the gp91phox transgene, 26% and 48% of the X-CGD cells expressed gp91phox at Days 2 and 20 after co-culture with 293T producer cells, respectively. Upon granulocytic differentiation of the transduced X-CGD cells with dimethylformamide (DMF), up to 63% (mean 49%, n = 7) of the cells were found to be functionally reconstituted with mean levels of superoxide production of 31% (n = 7) compared to wild-type PLB985 cells.

CONCLUSION

Lentivirus vectors expressing gp91phox are able to at least partially correct human myeloid X-CGD cells.

Variability of transcriptional regulation after gene transfer with the retroviral tetracycline system

Inducible transcription and position-independent expression are critical issues after gene transfer. To gain insight into the amount of variability of transcriptional regulation due to random proviral integration, we analyzed a total of 200 C6 glioma and rat-1 fibroblast clones retrovirally infected with the conventional and reverse tet systems where a luciferase reporter gene was placed under control of a tetracycline-responsive promoter. Repressed luciferase activities differed by up to 81000-fold among individual clones. Repressed activities close to baseline levels were observed in eight clones, all of them transduced with the conventional tet system. Regulation factors ranged from less than two-fold (indicating absence of regulation), observed in 17 clones to 90-fold. Regulation was higher with the conventional tet system as compared with the reverse tet system. Our data show that even under these standardized conditions there was a very high variability in absolute expression levels and regulability between individual clones, and they suggest that homogeneous transcriptional regulation in a cellular population remains a challenge for research in biotechnology.