Lentiviral gene transfer into primary and secondary NOD/SCID repopulating cells

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

In vivo selection of hematopoietic stem cells transduced at a low multiplicity-of-infection with a foamy viral MGMT(P140K) vector

OBJECTIVE

Using a clinically relevant transduction strategy, we investigated to what extent hematopoietic stem cells in lineage-negative bone marrow (Lin(neg) BM) could be genetically modified with an foamy virus (FV) vector that expresses the DNA repair protein, O(6)-methylguanine DNA methyltransferase (MGMT(P140K)) and selected in vivo with submyeloablative or myeloablative alkylator therapy.

MATERIALS AND METHODS

Lin(neg) BM was transduced at a low multiplicity-of-infection with the FV vector, MD9-P140K, which coexpresses MGMT(P140K) and the enhanced green fluorescent protein, transplanted into C57BL/6 mice, and mice treated with submyeloablative or myeloablative alkylator therapy. The BM was analyzed for the presence of in vivo selected, MD9-P140K-transduced cells at 6 months post-transplantation and subsequently transplanted into secondary recipient animals.

RESULTS

Following submyeloablative therapy, 55% of the mice expressed MGMT(P140K) in the BM. Proviral integration was observed in approximately 50% of committed BM-derived progenitors and analysis of proviral insertion sites indicated up to two integrations per transduced progenitor colony. Transduced BM cells selected with submyeloablative therapy reconstituted secondary recipient mice for up to 6 months post-transplantation. In contrast, after delivery of myeloablative therapy to primary recipient mice, only 25% survived. Hematopoietic stem cells were transduced because BM cells from the surviving animals reconstituted secondary recipients with MGMT(P140K)-positive cells for 5 to 6 months.

CONCLUSIONS

In vivo selection of MD9-P140K-transduced BM cells was more efficient following submyeloablative than myeloablative therapy. These data indicate that a critical number of transduced stem cells must be present to produce sufficient numbers of genetically modified progeny to protect against acute toxicity associated with myeloablative therapy.

Sustained long-term engraftment and transgene expression of peripheral blood CD34+ cells transduced with third-generation lentiviral vectors

As mobilized peripheral blood (MPB) represents an attractive cell source for gene therapy, we investigated the ability of third-generation lentiviral vectors (LVs) to transfer the enhanced green fluorescent protein gene into MPB CD34(+) cells in culture conditions allowing expansion of transplantable human hematopoietic stem cells. To date, few studies have reported transduction of MPB cells with vesicular stomatitis virus G pseudotyped LVs. The critical issue remains whether primitive, hematopoietic repopulating cells have, indeed, been transduced. In vitro (5 weeks' culture in FLT3 ligand + thrombopoietin + stem cell factor + interleukin 6) and in vivo (serial transplantation in NOD/SCID mice) experiments show that MPB CD34(+) cells can be effectively long-term transduced by LV and maintain their proliferation, self-renewal, and multilineage differentiation potentials. We show that expansion following transduction improves the engraftment of transduced MPB CD34(+) (4.6-fold expansion of SCID repopulating cells by limiting dilution studies). We propose ex vivo expansion after transduction as an effective tool to improve gene therapy protocols with MPB. Disclosure of potential conflicts of interest is found at the end of this article.

Real-time quantitative PCR for the design of lentiviral vector analytical assays

From the recent and emerging concerns for approving lentiviral vector-mediated gene transfer in human clinical applications, several analytical methods have been applied in preclinical models to address the lentiviral vector load in batches, cells or tissues. This review points out the oldest generation methods (blots, RT activity, standard PCR) as well as a full description of the newest real-time quantitative PCR (qPCR) applications. Combinations of primer and probe sequences, which have worked in the lentiviral amplification context, have been included in the effort to dress an exhaustive list. Also, great variations have been observed from interlaboratory results, we have tempted to compare between them the different analytical methods that have been used to consider (i) the titration of lentiviral vector batches, (ii) the absence of the susceptible emerging replicative lentiviruses or (iii) the lentiviral vector biodistribution in the organism.

Long-term persistence of a nonintegrated lentiviral vector in mouse hematopoietic stem cells

OBJECTIVE

Lentiviral transduction is an established method for efficiently modifying the gene expression program of primary cells, but the ability of the introduced construct to persist as an episome has not been well studied.

MATERIAL AND METHODS

Here we investigated this issue in lethally irradiated female mice injected with 300 or 3000 doubly sorted male lin(neg), Sca-1(high), c-kit(high), Thy-1.1(low) mouse bone marrow cells that had been exposed in vitro to self-inactivating lentivirus vector encoding a green fluorescence protein (GFP) cDNA. Seven to sixteen months later, bone marrow cells from primary mice were injected into secondary female recipients and another 8 months later into tertiary female recipients. Integration study was performed on individual spleen colonies by Southern blot analysis. Inverse polymerase chain reaction (PCR) and sequence of amplified vector-derived DNA was used to verify Southern blot results.

RESULTS

Spleen colony-forming cell study revealed that a small fraction of the spleen colonies contained integrated provirus as shown by Southern blot analysis. Unexpectedly, many spleen colonies were found to contain a nonintegrated episomal form of the provirus, which was confirmed by an inverse PCR analysis. In some of the spleen colonies containing only the episomal form, GFP-expressing cells were also detected. Lentiviral sequences were present in hematopoietic tissues of primary mice but not in other tissues.

CONCLUSIONS

These results demonstrate that lentiviral vectors produce episomal circles in hematopoietic stem cells that can be transferred through many cell generations and expressed in their progeny.

Expression of human coagulation factor VIII in adipocytes transduced with the simian immunodeficiency virus agmTYO1-based vector for hemophilia A gene therapy

We demonstrate that transduction of adipocytes with a simian immunodeficiency virus agm TYO1 (SIVagm)-based lentiviral vector carrying the human coagulation factor VIII gene (SIVhFVIII) resulted in expression of the human FVIII transgene in vitro and in db/db mice in vivo. Cultured human adipocytes were transduced with the SIVagm vector carrying the GFP gene in a dose-dependent manner and transduction of adipocytes with SIVhFVIII resulted in efficient expression of human coagulation factor VIII (hFVIII; 320 +/- 39.8 ng/10(6) adipocytes/24 h) in vitro. Based upon successful transduction of adipocytes by SIV vectors carrying the lacZ gene in vivo in mice, the adipose tissue of db/db mice was transduced with SIVhFVIII. There was a transient appearance of human FVIII in mouse plasma (maximum 1.8 ng/ml) on day 11 after the injection. Transcripts of human FVIII transgene and human FVIII antigen also were detected in the adipose tissue by RT-PCR and immunofluorescence, respectively, on day 14. Emergence of anti-human FVIII antibodies 14 days after the injection of SIVhFVIII may explain the disappearance of human FVIII from the circulation. These results suggest that transduction of the adipocytes with vectors carrying the human FVIII gene may be potentially applicable for gene therapy of hemophilia A.

Hematopoietic stem cell gene therapy: progress toward therapeutic targets

The concept of hematopoietic stem cell gene therapy is as exciting as that of stem cell transplantation itself. The past 20 years of research have led to improved techniques for transferring and expressing genes in hematopoietic stem cells and preclinical models now routinely indicate the ease with which new genes can be expressed in repopulating stem cells of multiple species. Both modified murine oncoretroviruses and lentiviruses transmit genes into the genome of hematopoietic stem cells and allow expression in the host following transplantation. Using oncoretroviruses, therapeutic genes for severe combined immunodeficiency, common variable gamma chain immunodeficiency, chronic granulomatous disease, Hurler's and Gaucher's Disease have all been used clinically with only modest success except for the patients with immunodeficiency in whom a partial T-cell chimerism has been dramatic. Since stem cell selection in vivo appears important to the therapeutic success of gene transfer, drug resistance selection, most recently using the MGMT gene, has been developed and appears to be safe. Future trials combining a drug resistance and therapeutic gene are planned, as are trials using safety-modified lentiviruses. The therapeutic potential of hematopoietic stem cell gene therapy, particularly given recent advances in stem cell plasticity, remains an exceptionally exciting area of clinical research.

Vectors for the treatment of autoimmune disease

Gene therapy has been applied in a variety of experimental models of autoimmunity with some success. In this article, we outline recent developments in gene therapy vectors, discuss advantages and disadvantages of each, and highlight their recent applications in autoimmune models. We also consider progress in vector targeting and components for regulating transgene expression, which will both improve gene therapy safety and empower gene therapy to fullfil its potential as a therapeutic modality. In conclusion, we consider candidate vectors that satisfy requirements for application in the principal therapeutic strategies in which gene therapy will be applied to autoimmune conditions.

Efficient gene transfer into the CNS by lentiviral vectors purified by anion exchange chromatography

Lentiviral vectors have been shown to stably transduce dividing and non-dividing target cells in vitro and in vivo. However, in vivo gene transfer applications with viral vectors in the central nervous system require highly efficient vector preparations, because only very small volumes can be injected stereotactically without damage to the brain tissue. Since lentiviral vectors are generated in transient co-transfection systems, viral preparations need to be purified and efficiently concentrated before injection into the brain. We describe an alternative procedure to concentrate lentiviral preparations by binding viral particles to an anion exchange column. Viral particles are eluted with sodium chloride, desalted and further concentrated by ultrafiltration. These vector preparations allowed high levels of gene transfer into terminally differentiated neuronal and glial cells and long-term transgene expression without any signs of acute and long-term toxicity or inflammation. The purification of lentiviral vectors from large-scale preparations by anion exchange chromatography allowed us to concentrate the virus to small volumes and to use these preparations to genetically modified target cells in vivo without signs of acute inflammatory responses.

Development of gene therapy for blood disorders by gene transfer into haematopoietic stem cells

Haematopoietic stem cells (HSCs) are important target cells for gene therapy of blood disorders due to their pluripotency and ability to reconstitute haematopoiesis following myeloablation and transplantation. HSCs can 'self-renew' and generate new stem cells. Genetically modified stem cells are therefore expected to last a lifetime in the recipient following blood and marrow transplantation, and can potentially cure haematological disorders. Oncoretroviral vectors have been the main vectors used for HSCs because of their ability to integrate into the chromosomes of their target cells. Because oncoretroviral vectors require dividing target cells for successful localization of the preintegration complex and subsequent chromosomal integration of the provirus, only the dividing fraction of the target cells can be transduced. As only a small fraction of haematopoietic stem cells is dividing at any one time, oncoretroviral vector transduction of human HSCs has been low in clinical trials. However, patients with severe combined immune deficiency-X1 (SCID-X1) have recently been treated successfully by gene therapy of autologous bone marrow cells using oncoretroviral vectors containing the common gamma chain gene. While several additional disorders may potentially be treated successfully using oncoretroviral gene transfer to HSCs, many disorders may require much higher gene transfer efficiency than was achieved in the SCID-X1 study. Therefore, lentiviral vectors have recently emerged as promising vectors for human HSCs because they can transduce dividing and nondividing HSCs efficiently, and may become the vectors of choice in the future for treatment of blood disorders where a large fraction of HSCs has to be corrected.

Transcriptional targeting of lentiviral vectors by long terminal repeat enhancer replacement

Gene therapy of many genetic diseases requires permanent gene transfer into self-renewing stem cells and restriction of transgene expression to specific progenies. Human immunodeficiency virus (HIV)-derived lentiviral vectors are very effective in transducing rare, nondividing stem cell populations (e.g., hematopoietic stem cells) without altering their long-term repopulation and differentiation capacities. We developed a strategy for transcriptional targeting of lentiviral vectors based on replacing the viral long terminal repeat (LTR) enhancer with cell lineage-specific, genomic control elements. An upstream enhancer (HS2) of the erythroid-specific GATA-1 gene was used to replace most of the U3 region of the LTR, immediately upstream of the HIV type 1 (HIV-1) promoter. The modified LTR was used to drive the expression of a reporter gene (the green fluorescent protein [GFP] gene), while a second gene (a truncated form of the p75 nerve growth factor receptor [DeltaLNGFR]) was placed under the control of an internal constitutive promoter to monitor cell transduction, or to immunoselect transduced cells, independently from the expression of the targeted promoter. The transcriptionally targeted vectors were used to transduce cell lines, human CD34+ hematopoietic stem-progenitor cells, and murine bone marrow (BM)-repopulating stem cells. Gene expression was analyzed in the stem cell progeny in vitro and in vivo after xenotransplantation into nonobese diabetic-SCID mice or BM transplantation in coisogenic mice. The modified LTR directed high levels of transgene expression specifically in mature erythroblasts, in a TAT-independent fashion and with no alteration in titer, infectivity, and genomic stability of the lentiviral vector. Expression from the modified LTR was higher, better restricted, and showed less position-effect variegation than that obtained by the same combination of enhancer-promoter elements placed in a conventional, internal position. Cloning of the woodchuck hepatitis virus posttranscriptional regulatory element at a defined position in the targeted vector allowed selective accumulation of the genomic transcripts with respect to the internal RNA transcript, with no loss of cell-type restriction. A critical advantage of this targeting strategy is the use of a spliced, major viral transcript to express a therapeutic gene and that of an internal, independently regulated promoter to express an additional gene for either cell marking or in vivo selection purposes.

Development of gene therapy for hematopoietic stem cells using lentiviral vectors

Lentiviral vectors are promising tools for the development of gene therapy since they can transduce both quiescent and dividing target cells. Lentiviral vectors may be particularly promising gene delivery tools for hematopoietic stem cells since these target cells tend to be quiescent and are therefore difficult target cells for vectors that require dividing targets. Human hematopoietic stem cells that can repopulate NOD/SCID mice have been efficiently transduced using HIV-1-based lentiviral vectors and similar vectors can also transduce murine hematopoietic stem cells. HIV-1 vectors that contain strong general promoters can generate high levels of transgene expression and very high expression levels can be generated in erythroid cells in vivo using beta-globin regulatory sequences to control the expression of the transgene. Current lentiviral vectors have a similar level of biosafety as oncoretroviral vectors and can therefore theoretically be used in clinical gene therapy protocols. Future challenges include the generation of lentiviral vectors that can express more than one transgene at high levels and the generation of safe permanent packaging cells for practical use in clinical gene therapy trials.

Lentiviral vector-mediated transduction of murine CD34(-) hematopoietic stem cells

OBJECTIVE

Efficient gene transfer into murine hematopoietic stem cells (HSCs) provides a powerful tool for exploring hematopoietic stem cell biology. In this study, we evaluated the efficiency of lentiviral vector-mediated gene transfer into murine CD34(-/low)c-Kit(+)Sca-1(+)Lin(-) (CD34(-) KSL) cells that are highly enriched for HSCs.

MATERIALS AND METHODS

FACS-sorted CD34(-) KSL cells were transduced with the vesicular stomatitis virus G glycoprotein-pseudotyped HIV-1-based lentiviral vector containing the green fluorescent protein (GFP) gene under the control of the cytomegalovirus promoter, and then 50 transduced cells were transplanted into lethally irradiated mice. Transduction efficiency was assessed by FACS analysis for GFP expression in peripheral blood (PB) cells. FACS-sorted GFP(+) KSL bone marrow (BM) cells from primary recipients were used for secondary transplantation, and GFP expression in PB cells of reconstituted mice was analyzed by FACS.

RESULTS

GFP expression was detected in PB cells of all primary recipients (n = 10) at an average of 40% (range 26-58%) when the lentiviral vector containing the woodchuck hepatitis virus posttranscriptional regulatory element was used. GFP(+) cells were found in multilineage cells in PB, BM, spleen, and thymus for at least 8 months posttransplantation. In secondary recipients, donor-derived GFP(+) KSL BM cells could reconstitute and GFP expression was detected in both myeloid and lymphoid cells in PB.

CONCLUSION

Our results indicate that lentiviral vectors can efficiently transduce highly enriched murine HSCs and sustain long-term expression of the transgene in the multilineage differentiated progeny in reconstituted mice.

Ex vivo purging by adenoviral p53 gene therapy does not affect NOD-SCID repopulating activity of human CD34+ cells

Co-incubation of a replication-deficient, recombinant adenovirus carrying the wild-type p53 gene (rAd-p53) and hematopoietic stem cell (HSC) products from patients with breast cancer can significantly reduce tumor cell contamination. Whereas this approach provides a powerful tumor cell purging strategy, potential detrimental effects on the HSC population have not been investigated. The ability of human HSC to reconstitute hematopoiesis in severe combined immunodeficient (SCID) mice and to undergo secondary transplantation provides the only nonclinical measure of self-renewing, stem cell function. The objective of this study was to investigate whether co-incubation with rAd-p53 compromised the SCID repopulating activity (SRA) of HSC. Granulocyte colony-stimulating factor-mobilized human CD34+ cells were co-cultured with rAd-p53 at our targeted clinical dose, and the ability of these cells to establish multilineage hematopoiesis in sublethally irradiated, nonobese diabetic (NOD)-SCID mice was investigated. The persistence of human cells in the mice was investigated by flow cytometry, granulocyte-macrophage colony-forming unit assay, and polymerase chain reaction of human Alu sequences. Further, limiting dilution analysis provided a quantitative comparison between the SRA of CD34+ cells co-incubated with rAd-p53 and control CD34+ cells (no rAd-p53 co-incubation). We conclude that co-incubation with rAd-p53 has little effect on the SRA of HSC.

High Levels of Transgene Expression Following Transduction of Long-Term NOD/SCID-Repopulating Human Cells with a Modified Lentiviral Vector

Both oncoretroviral and lentiviral vectors have been shown to transduce CD34(+) human hematopoietic stem cells (HSC) capable of establishing human hematopoiesis in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice that support partially human hematopoiesis. We and others have reported that murine stem cell virus (MSCV)-based oncoretroviral vectors efficiently transduced HSC that had been cultured ex vivo for 4-7 days with cytokines, resulting in transgene expression in lymphoid and myeloid progenies of SCID-engrafting cells 4-8 weeks post-transplantation. Although lentiviral vectors have been demonstrated to transduce HSC under minimal ex vivo culture conditions, concerns exist regarding the level of transgene expression mediated by these vectors. We therefore evaluated a novel hybrid lentiviral vector (GIN-MU3), in which the U3 region of the HIV-1 long terminal repeat was replaced by the MSCV U3 region (MU3). Human cord blood CD34(+) cells were transduced with vesicular stomatitis virus G envelope protein-pseudotyped lentiviruses during a 48-hour culture period. After a total of 4 days in culture, transduced cells were transplanted into NOD/SCID mice to examine gene transfer and expression in engrafting human cells. Fifteen weeks post-transplantation, 37% +/- 12% of engrafted human cells expressed the green fluorescence protein (GFP) gene introduced by the lentiviral vector. High levels of GFP expression were observed in lymphoid, myeloid and erythroid progenies, and in engrafted human cells that retained the CD34(+) phenotype 15 weeks post-transplantation. This study provides evidence that lentiviral vectors transduced both short-term and long-term engrafting human cells, and mediated persistent transgene expression at high levels in multiple lineages of hematopoietic cells.