Retrovirus-mediated gene transfer into rhesus monkey hematopoietic stem cells: the effect of viral titers on transduction efficiency

Retroviral transduction and engraftment ability of primate hematopoietic progenitor and stem cells transduced under serum-free versus serum-containing conditions

The ability to efficiently transduce hematopoietic stem and progenitor cells under serum-free conditions would be desirable for safety and standardization of clinical gene therapy protocols. Using rhesus macaques, we studied the transduction efficiency and engraftment ability of CD34-enriched SCF/G-CSF mobilized progenitor cells (PBSC) transduced with standard amphotropic marking vectors under serum-free and serum-containing conditions. Supernatants were collected from producer cells 16 hours after serum-free medium or medium containing 10% fetal calf serum was added. Vector titers were approximately two- to threefold higher when producer cells were cultured in serum-containing medium. However, retroviral transduction of rhesus CFU-GM was improved using serum-free vector-containing medium. For analysis of engraftment with transduced cells, three macaques had CD34+ peripheral blood stem cells split into two fractions for transduction. One fraction was transduced using serum-free vector-containing medium, and the other fraction was transduced using standard serum-containing medium. The two fractions were re-infused simultaneously following total body irradiation. In all three animals, there was equivalent marking from both vectors for 7-9 months post-transplantation. These data are encouraging regarding the removal of serum-containing medium from clinical hematopoietic cell transduction protocols, given the lack of a detrimental effect on transduction and engraftment with transduced cells.

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs has not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs have not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.

Neonatal gene therapy. transfer and expression of exogenous genes in neonatal sheep following direct injection of retroviral vectors into the bone marrow space

We investigated whether gene transfer into hematopoietic cells could be achieved by direct injection of retroviral vector supernatant into the bone marrow space of newborn sheep. Six sheep (5 weeks old) were injected bilaterally with either 1 mL of G1nBgSvNa8.1 vector supernatant (titer: 1 x 10(7)) in each hip (n = 5) or with 3 mL of the same vector preparation/hip (n = 1). In addition, one 3-month-old sheep was injected unilaterally with 1 mL of the same vector preparation. Blood and marrow of these animals were analyzed for the transgene before injection and at intervals thereafter. At 1 week postinjection, an average of 11.6% of the lymphocytes and 25.5% of the granulocytes/monocytes in the marrow, and an average of 0.9% of the lymphocytes and 1.8% of the granulocytes/monocytes in the blood contained and expressed the LacZ gene. The presence/expression of the transgene has persisted for at least 13 months within the blood and bone marrow of these animals. These findings demonstrate that the direct injection of small volumes of high-titer retroviral supernatant into the bone marrow of newborn sheep results in transduction of hematopoietic cells that persists for at least 13 months postinjection.

Intrinsic potential of phenotypically defined human hemopoietic stem cells to self-renew in short-term in vitro cultures

In search for culture conditions that will facilitate hemopoietic stem cell (HSC) replication while preserving their primitive properties, we have made use of a multi-parameter FACS assay to define HSCs on basis of their phenotypic characteristics, i.e., CD34++CD33,38,71(-). Bone marrow and umbilical cord blood samples of CD34(+) cells from 31 donors were loaded with the membrane dye PKH26 and each exposed to various culture conditions for 6 days. The cells that retained the primitive CD34(++)CD33,38,71(-) phenotype were analysed for the number of cell replications they underwent, by measuring loss of PKH26 fluorescence after 6 days. A most striking observation was the large inter-sample variation in the proliferative response of cells that retained the CD34(++)CD33,38,71(-) phenotype. In general, samples could be characterised as either good- or poorly-replicating, according to the proliferation property of their CD34(++)CD33,38,71(-) subset. In comparison to this 'intrinsic' potential, the effects of the applied growth stimuli on CD34(++)CD33,38,71(-) cell replication were negligible. In contrast, the overall recovery of the CD34(++)CD33,38,71(-) cells was clearly dependent on the culture stimuli. Of the various conditions tested, serum-free cultures with pre-established stroma maintained the cells with this primitive phenotype most effectively. In cultures supplemented with various combinations of recombinant HGFs, HSC differentiation prevailed. These findings with phenotypically defined HSCs should assist in the design of systems for expansion and ex vivo gene therapy of early hemopoietic cells.

Isolation and transduction of CD34+ cells from small quantities of peripheral blood from HIV-1-infected patients not treated with hemopoietic growth factors

A proposed hemopoietic stem cell gene therapy for treatment for HIV infection would involve transduction of CD34+ hemopoietic stem cells with vectors encoding anti-HIV constructs. Peripheral blood has proved to be a useful source of these hemopoietic stem cells and this study exploits this finding. Small quantities of peripheral blood were obtained from HIV-negative patients and HIV-positive patients who were and were not receiving hemopoietic growth factors (HGFs). CD34+ cells were obtained from these samples using a simple technique and scored for frequency of colony type. This demonstrated that HIV-negative patients had the highest frequency of colony-forming units (CFUs). HIV-positive patients not treated with HGFs had a lower frequency of CFUs, but the same colony type distribution as HIV-negative patients. HIV-positive patients treated with HGFs had the lowest frequency of CFUs, but their colony type distribution demonstrated that they had responded to treatment. CD34+ cells selected in this way were also transduced with the murine retroviral MFG vector using a technique that demonstrated transduction efficiencies ranging from 2% to 16% (median, 11.5%). This study simplifies the experimental requirements for development of a hemopoietic stem cell gene therapy for HIV infection and offers the possibility that longitudinal studies could be performed on peripheral blood CD34+ cells from HIV-positive or HIV-negative patients without the need for granulocyte colony-stimulating factor mobilization.

In utero gene therapy: transfer and long-term expression of the bacterial neo(r) gene in sheep after direct injection of retroviral vectors into preimmune fetuses

We investigated whether directly injecting retroviral vectors into preimmune fetuses could result in the transfer and long-term expression of exogenous genes. Twenty-nine preimmune sheep fetuses were injected with helper-free retroviral vector preparations. Twenty-two fetuses survived to term, 4 of which were sacrificed at birth. Of the remaining 18 animals, 3 were controls and 15 had received vector preparations. Twelve of these 15 animals demonstrated transduction of hematopoietic cells when blood and marrow were analyzed by neo(r)-specific PCR. Eight experimental sheep have been followed for 5 years, during which time we have consistently observed proviral DNA and G418-resistant hematopoetic progenitors. The G418-resistant colonies were positive when analyzed by neo(r)-specific PCR. neo(r) gene expression was also demonstrated using several immunological and biochemical methods. The transduction of hematopoietic stem cells was confirmed when lambs transplanted with bone marrow from in utero-transduced sheep exhibited neo(r) activity in marrow and blood. Vector distribution was widespread in primary animals without pathology. PCR analysis indicates that the germ line was not altered. These studies demonstrate that direct injection of an engineered retrovirus is a feasible means of safely delivering a foreign gene to a developing fetus and achieving long-term expression without modifying the germ line of the recipient.

Retroviral gene transfer into cord blood stem/progenitor cells using purified vector stocks

Cord blood (CB) progenitor/stem cells (P/SC) are ideal targets for early gene therapy in individuals prenatally diagnosed with genetic disorders. Most retroviral transduction protocols were developed using adult peripheral blood stem cells (PBSC) and bone marrow (BM). Less is known about retroviral transduction of CB P/SC. We examined how timing, multiplicity of infection (MOI), and polycations in the transduction media affect transduction efficiency. Rates of transduction were determined in recently isolated CD34+ enriched CB cells and in colonies derived after various times in liquid cultures (LC). CB mononuclear cells (MNC) were separated by ficoll-hypaque centrifugation and enriched for CD34+ cells. Purity was assessed by flow cytometry. Transduction were performed with clinical-grade retroviral stocks at MOIs of 1-20. Transduction was performed with fetal bovine serum (FBS) or autologous plasma, IL-3, GM-CSF, IL-6, and SCF. The retroviral vector contained LacZ and neomycin resistance (neo) reporter genes. Transduction was determined by X-gal stain and by PCR amplification of the reporter genes. No drug selection was used. Twenty-five experiments were done. CB volumes ranged from 35-150 ml. MNC and CD34+ cell counts ranges were: 0.14-840 x 10(6) and 0.1-4.2 x 10(6), respectively. Transduction efficiency in liquid cultures ranged from 4-63%. Higher rates were seen using MOI > or = 10, 2 microg/ml polybrene, and 10% autologous CB plasma. In colonies, transduction rates were 63 to 72% by PCR and 32% by X-gal staining. In LTC-IC derived colonies, transduction was 7% by PCR. Short incubations of CD34+ CB cells with purified retroviral stocks, polybrene, and autologous sera result in high transduction rates of committed progenitors and moderately low efficiencies of transduction of LTC-IC in the absence of drug selection.

Protection of hematopoietic stem cells from chemotherapy-induced toxicity by multidrug-resistance 1 gene transfer

An increased chemotherapeutic dose intensity is believed to translate into higher survival rates among cancer patients. Pancytopenia is the dose-limiting toxic result of most anticancer agents. Overexpression of the human multidrug resistance 1 (MDR1) gene in transgenic animals resulted in complete myeloprotection against high doses of cytostatic drugs. Stem cell research, vector development, and experimental pharmacology are uniting their efforts in an attempt to achieve a similar effect in human hematopoietic stem cells. This article gives an overview of the crucial steps involved, from retroviral vector design and optimization of viral titers to vector uptake, gene integration, and expression. The authors' own results are presented with special regard in vitro and in vivo assays for the detection of hematopoietic stem cell transduction.

Long-term in vivo expression of the MFG-ADA retroviral vector in rhesus monkeys transplanted with transduced bone marrow cells

We have tested the recombinant human adenosine deaminase (hADA) retroviral vector MFG-ADA for its efficacy in transducing hemopoietic stem cells of nonhuman primates and its expression level in the hematopoietic system. The percentage of provirus-positive granulocytes 1 year after transplantation of bone marrow transduced with MFG-ADA was 0.1%, which was equivalent to previously obtained results with the hADA virus-producing cell line POC-1. However, in MFG-ADA monkeys, significantly more peripheral blood mononuclear cells carried the hADA gene (1% versus 0.1%). Human ADA expression levels in peripheral blood mononuclear cells were different between POC-1 and MFG-ADA monkeys using samples with equal numbers of provirus copies per cell. In contrast, in total red blood cell lysates of MFG-ADA monkeys, the hADA expression was higher (approximately 10-fold) and could be detected longer (20 weeks and up to more than 1 year after bone marrow transplantation in 2 monkeys) than in POC-1 monkeys that were only positive for up to 12 weeks at the most. At 3 years after bone marrow transplantation, the MFG-ADA provirus could still be detected in 0.1% of bone marrow cells and peripheral blood cells and in 1% of cultured T cells. These results show that MFG-ADA virus can give rise to long-term in vivo expression of hADA in the primate hematopoietic system. However, transduction efficiencies remain low.

Use of amphotropic retroviral vectors for gene transfer in human colon carcinoma cells

Previous studies in rodent models have demonstrated the feasibility of gene transfer to the stem cells of the intestinal epithelium using ecotropic retroviral vectors delivered luminally. This report represents a next step toward targeting the human intestine as a site for somatic gene therapy. The first experiment assessed the viability of amphotropic retroviral vectors in the luminal environment. It was found that after 4 hr at 37 degrees C in luminal effluent, the loss of titer was no greater than when incubated in control media. Likewise, neither the vector nor the target cells were adversely affected by N-acetylcysteine, which is likely to be used as a preparatory agent for mucus removal. To determine whether human intestinal cells are transducible by these vectors, three colon carcinoma cell lines were studied: HT-29, T84, and Caco-2. All were transduced; however, the expression of the reporter gene was highest in the HT-29 cells. Subsequent studies using these cells showed that with regular stocks of vector, gene transfer peaked at a stock dilution of 1/10 and declined at full strength. This problem could be partially overcome by centrifugal concentration of the retroviral stocks. With this approach, gene transfer increased with increasing particles up to 10x regular stock titers but was inefficient at 100x. Overall, these findings provide encouraging evidence that amphotropic retroviral vectors may eventually be used for in vivo gene transfer into human intestinal epithelium. However, they also point to the need for improved methods of concentrating retroviral vectors.

Expression of the gibbon ape leukemia virus receptor-1 in rhesus macaque tissues

A cDNA fragment specific for the rhesus macaque gibbon ape leukemia virus (GaLV) receptor (Glvr-1) was isolated by polymerase chain reaction (PCR) amplification and oligonucleotide primers specific for human Glvr-1 and a cDNA library derived from rhesus macaque brain. Sequence analysis of the fragment revealed the polypeptide domain necessary for infection by GaLV. This fragment was utilized to elucidate expression levels of Glvr-1 RNA in rhesus macaque tissues. By Northern blot analysis, Glvr-1 RNA is most abundantly expressed in the thymus and bone marrow, with detectable levels also in the brain and testes of juvenile male macaques. In the developing 70-days gestation fetus, Glvr-1 expression was observed predominately in the liver and spleen. Although additional studies are required, these studies support the notion that cell types involved in hematopoiesis express Glvr-1.

Retroviral stem cell gene therapy

Long-term in vivo gene transfer studies in mice have shown that recombinant murine retroviruses are able to infect murine hemopoietic stem cells with high efficiency. Taken together the results indicated that the proviral structure was present at high frequency in circulating hemopoietic cells resulting in significant expression levels. Because of the success of these murine studies, it was believed that gene therapy would soon be applicable to treat a wide variety of congenital or acquired human diseases associated with the hemopoietic system. However, results from gene transfer studies in nonhuman primates and first human clinical trails have indicated that murine retrovirus infection of primate hemopoietic stem cells is inefficient. Although there are essential differences between the murine and primate gene therapy studies with respect to the recombinant viruses and transduction protocols used, these differences cannot solely account for the differences observed in infection efficiency. Therefore, in recent years effort has been spent on the identification of factors limiting retroviral transduction of primate hemopoietic stem cells. Increasing knowledge concerning hemopoiesis and retroviral infection has helped in identifying a number of limiting factors. Novel transduction strategies and tools have been generated which attempt to circumvent these limiting factors. These factors as well as the strategies that showed increased retroviral infection of primate hemopoietic stem cells will be discussed.

The contribution of animal models to the development of treatments for hematologic recovery following myeloablative therapy: a review

This review describes the role that animal models have played in the development of clinical procedures for growth factor and hematopoietic cell therapies following high-dose cancer chemotherapy, radiotherapy or both. Data are discussed describing animal models that add to the understanding of human hematopoiesis, including myeloid and lymphoid lineage localization and in vivo maturation. Finally, current animal models of cytokine and cell therapies are presented in the context of their contributions to early clinical trials and future therapies. These studies underscore the past and current contributions animal investigations have made to improving clinical therapies.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

Gene transfer into hematopoietic stem cells of nonhuman primates

Nonhuman primates provide an appropriate preclinical large-animal model to test the efficacy of bone marrow gene therapy procedures. Successful retroviral vector-mediated gene transfer into monkey pluripotent hematopoietic stem cells (PHSC) has closed the gap between gene transfer experiments in mouse models and clinical application of bone marrow gene therapy. After initial bone marrow transplant failures, ex vivo bone marrow culture conditions were found that sufficiently supported maintenance of the long-term repopulating ability of genetically modified autologous monkey grafts. The efficiency of gene transfer into primate PHSC has, however, remained at least one order of magnitude lower than has been achieved in mice. Similar gene transfer efficiencies have been obtained with total bone marrow grafts, CD34+ bone marrow grafts, and mobilized peripheral blood progenitor cell grafts; however, various attempts to increase the transduction efficiency have been without significant success. Primate PHSC seem to require quite different culture conditions for their maintenance and transduction than mouse PHSC, in particular regarding hematopoietic growth factor addition. In contrast to observations in other species, some form of conditioning appeared essential for engraftment of transduced PHSC in monkeys. Although it has been shown that mouse retroviruses can replicate in monkeys and are capable of inducing neoplasms, experiments in monkeys have sufficiently confirmed the safety of current gene transfer procedures to allow their clinical application.

Developmental-stage-specific expression and regulation of an amphotropic retroviral receptor in hematopoietic cells

Expression of the transmembrane receptor protein Ram-1 may be critical to optimizing retroviral gene transfer. Ram-1 acts as both a sodium-dependent phosphate transporter and a receptor for amphotropic retroviruses. We previously reported detectable Ram-1 in murine hematopoietic fetal liver cells (FLC) despite resistance of these cells to amphotropic retroviral transduction (infection). We document here that Ram-1 expression is completely absent in murine yolk sac cells from days 9.5 through 13.5 of ontogeny and first appears at low levels in midgestational FLC between days 13.5 and 14.5. In addition, Ram-1 expression is detected only in more differentiated populations within FLC, day 14.5, and not in those highly enriched for stem cells, indicating developmental regulation of Ram-1 during murine hematopoiesis. Others have reported the in vitro use of phosphate-free medium as a stimulus to increase levels of Ram-1 mRNA in nonhematopoietic cells. We now demonstrate that Ram-1 poly(A)+ mRNA increases significantly following culture of FLC in phosphate-free medium. Further, transduction of FLC in phosphate-free medium with an amphotropic retrovirus containing the multiple drug resistance gene leads to gene transfer not observed previously. These data demonstrate that (i) the normal resistance of FLC to amphotropic transduction is most likely due to an insufficient number of Ram-1 molecules for efficient retroviral recognition and binding, and (ii) Ram-1 can be upregulated by increasing the need for phosphate transport across the cell membrane.

Adenovirus-mediated transfer of the amphotropic retrovirus receptor cDNA increases retroviral transduction in cultured cells

The presence of functional amphotropic receptors on the cell surface is necessary for amphotropic retrovirus-mediated gene transfer. A recombinant adenoviral vector that expresses the receptor for amphotropic retrovirus (RAM) was constructed and used to express the receptor cDNA in different cell types in culture. Transfer of the RAM cDNA increased amphotropic retroviral-mediated transfer from 0 to 60% in Chinese hamster ovary cells. RAM expression increased retroviral transduction four- to eight-fold from 2-4% to 18%-35% in HeLa, Namalva, and X63 cells, but had no effect on 208F and HepG2 cells which have high baseline retroviral transduction rates of about 50%. For the purpose of application to ex vivo gene therapy, primary mouse hepatocytes were studied in a similar manner. Hepatocytes had a baseline transduction efficiency of about 40% and did not have increased rates of retroviral-mediated gene transfer with expression of recombinant RAM. This recombinant adenoviral vector conferred infection of amphotropic retrovirus into cells that were relatively resistant to infection, thus offering a rapid and easy method to stably introduce genes into these cell lines.

Retroviral transfer of the nlsLacZ gene into human CD34+ cell populations and into TF-1 cells: future prospects in gene therapy

Few data are available concerning behavior of reimplanted human hematopoietic cells after autologous stem cell transplantation. This paper reports the possibility to transfer gene markers coding for beta-galactosidase (beta-Gal) activity by retroviral vectors into a human leukemic growth factor-dependent cell line, TF-1, and into human hematopoietic progenitors isolated from peripheral blood or bone marrow. Using various combinations of retroviral vectors and packaging cell lines, we demonstrated high expression of a bacterial beta-Gal activity induced by the LacZ gene, the nlsLacZ gene, or the Sh-ble/LacZ gene, in human hematopoietic cells. The expression of the nlsLacZ construct was stable until the end of the culture in infected CD34+ cell-enriched cell populations, and a slow decrease of transgene expression was observed in a transduced TF-1 cell population during a 1-year long-term culture. Data obtained with the nlsLacZ gene demonstrate that both retroviral transfer and corresponding gene expression were not found to modify the pattern of cell proliferation and differentiation. These results open interesting prospectives for the use of the nlsLacZ gene to mark and follow the fate of progenitor cells isolated from patients with cancers prior to reimplantation.

Cell-surface receptors for gibbon ape leukemia virus and amphotropic murine retrovirus are inducible sodium-dependent phosphate symporters

Cell surface receptors for gibbon ape leukemia virus (Glvr-1) and murine amphotropic retrovirus (Ram-1) are distinct but related proteins having multiple membrane-spanning regions. Distant homology with a putative phosphate permease of Neurospora crassa suggested that these receptors might serve transport functions. By expression in Xenopus laevis oocytes and in mammalian cells, we have identified Glvr-1 and Ram-1 as sodium-dependent phosphate symporters. Two-electrode voltage-clamp analysis indicates net cation influx, suggesting that phosphate is transported with excess sodium ions. Phosphate uptake was reduced by > 50% in mouse fibroblasts expressing amphotropic envelope glycoprotein, which binds to Ram-1, indicating that Ram-1 is a major phosphate transporter in these cells. RNA analysis shows wide but distinct tissue distributions, with Glvr-1 expression being highest in bone marrow and Ram-1 in heart. Overexpression of Ram-1 severely repressed Glvr-1 synthesis in fibroblasts, suggesting that transporter expression may be controlled by net phosphate accumulation. Accordingly, depletion of extracellular phosphate increased Ram-1 and Glvr-1 expression 3- to 5-fold. These results suggest simple methods to modulate retroviral receptor expression, with possible applications to human gene therapy.

Circumvention of chemotherapy-induced myelosuppression by transfer of the mdr1 gene

Drug-induced myelosuppression is a frequent reason for curtailing chemotherapy in cancer patients. 'Rescue' of myelosuppressed patients with autologous marrow transplants is reasonably advanced and permits an increase in the dose of anticancer drugs. Despite this improvement, patients often relapse with drug resistance disease. The human multidrug resistance (mdr1) gene might make it possible to render hemopoietic stem cells resistant to anticancer drugs after transfer of this gene. By introducing resistant stem cells into patients it might be possible to treat these patients repeatedly with otherwise ablative therapy. This review explores the feasibility of mdr1 gene therapy.