Gene therapy in peripheral blood lymphocytes and bone marrow for ADA- immunodeficient patients

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.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.

Generation of high-titer retroviral vector-producing macrophages as vehicles for in vivo gene transfer

The goal of this project was to develop a novel gene transfer system based on macrophages (Mphi) as shuttles of recombinant retroviral vectors carrying therapeutic or marker genes. The murine Mphi cell line WGL5 was used as a source of Mphi for this study. We generated retrovirus-producing Mphi by transducing the WGL5 cells with a replication-defective retroviral vector carrying the enhanced green fluorescent protein (EGFP) reporter gene and the Moloney murine leukemia virus (MoMLV) as helper virus. We demonstrated stable integration of the recombinant retrovirus in the Mphi genome, efficient recombinant retrovirus production, and EGFP gene delivery to different cell lines in vitro. To evaluate Mphi-mediated EGFP gene transfer in vivo, allogeneic mice were injected s.c. with the retrovirus-producing WGL5 Mphi, that gave rise to solid tumor masses at the injection site, highly infiltrated with host leukocytes. We observed EGFP fluorescence in tumor-infiltrating CD4(+) and CD8(+) host T lymphocytes, providing direct evidence of the ability of engineered Mphi to mediate EGFP gene delivery to host cells in vivo. Moreover, we showed that retrovirus-producing Mphi could home to different organs in vivo following i.v. injection into mice. These data demonstrate that Mphi can be engineered as cellular vehicles for recombinant retroviruses carrying heterologous genes and suggest potential applications of this novel vector system for gene therapy.

Clinical Gene Therapy in Hematology: Past and Future

Gene transfer into hematopoietic cells using viral vectors has focused mostly on lymphocytes and hematopoietic stem cells (HSCs). HSCs have been considered particularly important as target cells because of their pluripotency and ability to reconstitute hematopoiesis after myeloablation and transplantation. HSCs are believed to have the ability to live a long time, perhaps a lifetime, in the recipient following bone marrow transplantation. Genetic correction of HSCs can therefore potentially last a lifetime and permanently cure hematologic disorders in which genetic deficiencies cause the pathology. Oncoretroviral vectors have been the main vectors used for HSCs because of their ability to integrate into the chromosomes of their target cells. Gene-transfer efficiency of murine HSCs is high using oncoretroviral vectors. In contrast, gene-transfer efficiency using the same viral vectors to transduce human HSCs or HSCs from large animals has been much lower. Although these difficulties may have several causes, the main reason for the low efficiency of human HSC transduction with oncoretroviral vectors is probably because of the nondividing nature of HSCs. Murine HSCs can be easily stimulated to divide in culture, whereas it is more problematic to stimulate human HSCs to divide rapidly in vitro. Because oncoretroviral vectors require dividing target cells for successful nuclear import of the preintegration complex and subsequent integration of the provirus, only the dividing fraction of the target cells can be transduced. This review focuses on gene transfer into human hematopoietic cells, particularly human HSCs. We review the clinical studies that have been reported, including the recent successful gene therapy for X-linked severe combined immunodeficiency. We discuss how the gene-transfer efficiency of human HSCs can be improved using oncoretroviral and lentiviral vectors.

Analysis of gene transfer efficiency of retrovirus producer cell transplantation for in situ gene transfer to hematopoietic cells

OBJECTIVE

The aim of this study was to assess the gene transfer efficiency of an in situ administration protocol for hematopoietic stem/progenitor cells in the rhesus macaque (Macaca mulatta) animal model.

MATERIALS AND METHODS

Moloney murine leukemia virus amphotropic vector producer cells (1--2 x 10(8) cells/animal) were transplanted into the femoral bone marrow cavities of six macaques. To determine if the levels of gene transfer could be increased, a second injection at the same dose of producer cells was performed into the iliac crest in three of the six macaques.

RESULTS

We demonstrated that 0.02-0.1% of peripheral blood mononuclear cells contained the vector transgene for up to 12 months following the initial administration of producer cells. Hematopoietic progenitor cell assays indicated that the neomycin phosphotransferase gene was detected in 10--30% of progenitor cell colonies. A humoral immune response directed toward viral particles was demonstrated in all animals. Additionally, we demonstrated that an increase in the levels of transduced cells, up to 1% of circulating peripheral blood mononuclear cells and granulocytes, contain the transgene following producer cell readministration.

CONCLUSIONS

These data demonstrate the successful in situ gene transfer to hematopoietic stem/progenitor cells and circulating peripheral blood mononuclear cells that persists as long as 12 months postinjection, in the absence of any preconditioning.

Bone marrow stem cell protection from chemotherapy by low--molecular-weight compounds

The stem cells of the bone marrow have the capacity for both self-renewal and derivation of all the blood cell lineages. Consequently, toxicity to these cells can result in neutropenia, agranulocytosis, thrombocytopenia, pancytopenia, or aplastic anemia. Many anticancer drugs adversely affect the bone marrow, and neutropenia is a common limiting factor in dose escalation. In this review, we discuss agents that appear to have potential as bone marrow sparing agents. Computerized catalogs of the National Library of Medicine and Medline were searched for reports on low-molecular-weight compounds that detailed effects on the hematopoietic progenitor cells. The most promising agents are the endogenous peptides p-glutamic acid-glutamic acid-aspartic acid-cysteine-lysine and acetyl-serine-aspartic acid-lysine-proline, and the exogenous compounds amifostine and ammonium trichloro[dioxoethylene-O,O']tellurate, but several others are also discussed. These compounds preserve stem cell function in the presence of antineoplastic drugs of diverse pharmacological classes, and they do so by various mechanisms of action. Their present status in clinical practice is also detailed. More needs to be learned about their mechanisms of action and therapeutic potential, but the results are encouraging for some of these compounds and more clinical trials should be expected.

[Gene therapy of X-linked severe combined immunologic deficiency (SCID-X1)]

X-linked severe combined immunodeficiency (SCID-X1) is a recessive hereditary disorder in which early T and Natural Killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common gamma c chain that participates in several cytokine receptors including the interleukin-2 (Il-2), Il-4, Il-7, Il-9, Il-15 receptors. SCID-X1 offers a reliable model for gene therapy as it is a lethal condition that is, in many cases, curable by allogeneic bone marrow transplantation. We have shown that retrovirus-mediated transfer of the gamma c cDNA induced gamma c chain expression and restored the function of the high-affinity IL-2 receptor on SCI-X1 EBV-transformed B-cell lines. We have the designed culture conditions to study NK-cell and T-cell development of CD34+ hematopoietic progenitor cells. In the culture systems, gamma c transduced CD34+ marrow cells from two SCID-X1 patients were able to mature into CD56+ and/or CD16+ NK cells and into CD4+ TCR alpha beta+ T cells. These preclinical results set the basis for a clinical study of ex-vivo gamma c gene transfer into CD34+ cells from SCID-X1 patients.

Lentivirus-mediated gene transfer in primary T cells is enhanced by a central DNA flap

Retroviral vectors have become the primary tool for gene delivery into hematopoietic cells, including T lymphocytes. Lentiviral vectors offer an advantage over Moloney murine leukemia virus (MuLV) vectors because of their ability to translocate across an intact nuclear membrane and integrate into the genome of nonproliferating cells. We have recently demonstrated that a central strand displacement event, controlled by the central polypurine tract (cPPT) and the central termination sequence (CTS), results in the formation of a central DNA flap which acts as a cis-determinant of HIV-1 genome nuclear import. Here, we show that insertion of this DNA determinant in a classical lentiviral vector resulted in a significantly higher level of transduction in activated T cells (51 +/- 12.7% versus 15 +/- 1.4%). CD4(+) and CD8(+) T cells were transduced at equivalent levels. Importantly, freshly isolated T cells stimulated only during the 12-h transduction period could be efficiently transduced with this new flap-containing lentiviral vector, but not with the parental lentiviral vector nor an MuLV vector. Transgene expression in the flap-containing lentiviral vector, under the control of either an internal cytomegalovirus or the elongation factor-1 alpha (EF1 alpha) promoter, was significant and expression remained elevated in resting T cells. Thus, this system allows stable expression of transgenes in T lymphocytes following a short ex vivo transduction protocol.

Nonmyeloablative immunosuppressive regimen prolongs In vivo persistence of gene-modified autologous T cells in a nonhuman primate model

The in vivo persistence of gene-modified cells can be limited by host immune responses to transgene-encoded proteins. In this study we evaluated in a nonhuman primate model whether the administration of a nonmyeloablative regimen consisting of low-dose total-body irradiation with 200 cGy followed by immunosuppression with mycophenolate mofetil and cyclosporin A for 28 and 35 days, respectively, could be used to facilitate persistence of autologous gene-modified T cells when a transgene-specific immune response had already been established or to induce long-lasting tolerance in unprimed recipients. Two macaques (Macaca nemestrina) received infusions of T cells transduced to express either the enhanced green fluorescent protein and neomycin phosphotransferase genes or the hygromycin phosphotransferase and herpes simplex virus thymidine kinase genes. In the absence of immunosuppression, both macaques developed potent class I major histocompatibility complex-restricted CD8(+) cytotoxic T-lymphocyte (CTL) responses that rapidly eliminated the gene-modified T cells and that persisted long term as memory CTL. Treatment with the nonmyeloablative regimen failed to abrogate preexisting memory CTL responses but interfered with the induction of transgene-specific CTL and facilitated in vivo persistence of gene-modified cells in an unprimed host. However, sustained tolerance to gene-modified T cells was not achieved with this regimen, indicating that further modifications will be required to permit sustained persistence of gene-modified T cells.

Gene-transferred oligoclonal T cells predominantly persist in peripheral blood from an adenosine deaminase-deficient patient during gene therapy

Adenosine deaminase (ADA) deficiency is the primary cause of severe combined immunodeficiency disease and has become a focus for developing innovative approaches to gene therapy. We previously described successful treatment of a Japanese ADA-deficient patient by periodic infusions of genetically modified autologous T lymphocytes transduced with a retroviral vector containing human ADA cDNA. In order to investigate whether polyclonality was restored by the gene therapy and whether the gene-transduced T lymphocytes persisted in the peripheral blood of the patient, we analyzed the T cell clonotype using a T cell receptor-specific RT-PCR/SSCP method. Oligoclonal T cell expansion was observed in every Vbeta family, and the expanded T cell clones were stable throughout the periodic gene therapy. Some of these T cell clones are likely carrying the vector, since they were identical to the clones selected by G418 resistance. Therefore, although it is uncertain when oligoclonal T cells started to expand and what percentage of the oligoclones carries the vector, the peripheral blood of the patient administered the gene therapy included oligoclonal T cells, some of which were identical to the ADA-gene-transduced clones.

Systematic analysis of clinically applicable conditions leading to a high efficiency of transduction and transgene expression in human T cells

BACKGROUND

The transduction of human peripheral blood T cells with retroviral vectors constitutes an attractive approach for the correction of a number of genetic diseases. In this study we have conducted a systematic analysis of the relevance of a large number of parameters currently considered to affect the transduction of, and transgene expression in, human T cells.

METHODS

Retroviral vectors encoding the human nerve growth factor receptor (NGFR) were used for transducing human T cells from normal volunteers. The proportion of T cells that expressed the marker transgene was determined by flow cytometry using anti-NGFR antibodies.

RESULTS

Spinoculation and static fibronectin (FN)-assisted infections improved to a similar extent the transduction efficiency of PHA/IL-2 stimulated T cells, when compared with samples subjected to standard static infections. When immobilized anti-CD3 (anti-CD3i) or anti-CD3i/28i-stimulated T cells were considered, static infections in FN-coated plates were reproducibly more efficient than spinoculation infections performed on FN-uncoated plates. Under optimized manipulation conditions (three infection cycles of anti-CD3i/28i-stimulated T cells in FN-coated plates) the total number of NGFR+ T cells harvested after 7 days of incubation represented, on average, twice the total number of T cells seeded at Day 0, and up to 95% of the human T cells efficiently expressed the marker transgene. Similar results were obtained regardless of whether samples were manipulated in medium supplemented with fetal bovine serum or with heat-inactivated autologous serum.

CONCLUSIONS

Our study offers new experimental conditions for the transduction of human T cells, with obvious implications for the development of gene therapy protocols.

Gene therapy for Fabry disease

Fabry disease is an X-linked metabolic disorder caused by a deficiency of alpha-galactosidase A (alpha-Gal A). Lack of this lysosomal hydrolase results in the accumulation of galactose-terminal glycosphingolipids in a number of tissues, including vascular endothelial cells. Premature death is predominantly associated with vascular conditions of the heart, kidneys and brain. Historically, treatment has largely been palliative. Alternative treatments for many lysosomal storage diseases have been developed, including allogeneic organ and bone marrow transplantation, enzyme replacement therapy, and gene therapy. Significant clinical risks still exist with allogeneic transplantations. Alpha-Gal A enzyme replacement therapy has been implemented in clinical trials. This approach has been effective but may have limitations for long-term systemic or cost-effective correction. As an alternative, gene therapy approaches, involving a variety of gene delivery systems, have been pursued for the amelioration of Fabry disease. Fabry disease is a compelling disorder for gene therapy, as target cells are readily accessible and relatively low levels of enzyme correction may suffice to reduce storage. Importantly, metabolic cooperativity effects are also manifested in Fabry disease, wherein corrected cells secrete alpha-Gal A that can correct bystander cells. In addition, a broad therapeutic window probably exists, and mouse models of Fabry disease have been generated to assist studies. As an example, in vitro and in vivo studies using alpha-Gal A-transduced haematopoietic cells from Fabry mice have demonstrated enzymatic correction of recipient cells and dissemination of alpha-Gal A upon transplantation, leading to reduced lipid storage in a number of clinically relevant organs. This corrective enzymatic effect has recently been shown to be even further enhanced upon pre-selection of therapeutically transduced cells prior to transplantation. This review will briefly detail current gene delivery methods and summarize results to date in the context of gene therapy for Fabry disease.

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.

The potential for therapy of immune disorders with gene therapy

Several hurdles remain before gene therapy will be a part of mainstream medical therapy; however, the preliminary report of success in HSC correction in patients with XSCID provides hope that gene therapy will become a reality.

MLV-10A1 retrovirus pseudotype efficiently transduces primary human CD4+ T lymphocytes

BACKGROUND

Previously, we showed that retroviral vectors pseudotyped with the envelope of the amphotropic murine leukemia virus 10A1 (MLV-10A1) more efficiently transduce primary human CD8+ T lymphocytes when compared with other A-MLV, gibbon ape leukemia virus (GaLV) and feline endogenous retrovirus (RD114) vector pseudotypes. For the success of several gene therapeutic approaches (ADA, HIV) it is important to effectively transduce primary human CD4+ T lymphocytes.

METHODS

We have used retroviral vectors encoding the enhanced green fluorescent protein (EGFP) as a marker gene and carrying envelopes of MLV-10A1, A-MLV and GaLV and have analyzed the transduction efficiency of both human CD4+ T cell lines (CEM, H9, HUT78, J16) and primary human CD4+ T lymphocytes using a RetroNectin-assisted transduction protocol and virus-containing supernatant.

RESULTS

In CD4+ T cell lines the MLV-10A1 vector pseudotype was most effective and infected up to 85% of cells which then stably expressed GFP over time. MLV-10A1 was also superior and infected approximately 32% of primary human CD4+ T lymphocytes in comparison to GaLV (18%) and A-MLV (12%). The superior efficiency of MLV-10A1 for the transduction of CD4+ T cells correlates with the longer half-life of this pseudotype in comparison to A-MLV and, as previously shown by interference analysis, with the usage of both the A-MLV (Pit2) and the GaLV receptor (Pitl) for cell entry.

CONCLUSIONS

MLV-10A1 is a suitable vector for transferring genes with high efficacy into primary human CD4+ T lymphocytes. The use of MLV-10A1 pseudotyped vectors should make it easier to obtain a sufficient number of gene-modified T lymphocytes for an adoptive transfer.

T cell-specific expression from Mo-MLV retroviral vectors containing a CD4 mini-promoter/enhancer

BACKGROUND

Gene therapy of various immunological disorders will greatly benefit from improved retroviral vectors (RVs) with T cell specificity. Such vectors can be designed by placing a gene of therapeutic interest under the control of tissue-specific transcriptional elements. However, low titers and loss of specificity are frequently encountered with tissue-specific vectors. The aim of the present study was to develop a T cell-specific RV.

METHODS

We constructed a series of Moloney murine leukemia virus (Mo-MLV)-based RVs expressing enhanced green fluorescent protein (EGFP) under the control of a mini-promoter/enhancer cassette derived from the CD4 gene (CD4pmE) and tested them in cell lines and peripheral blood lymphocytes. Expression of EGFP was monitored by fluorescence microscopy and analyzed by flow cytometry.

RESULTS

The CD4pmE cassette was inserted between the viral long terminal repeats (LTRs) in self-inactivating vectors (SIN vectors) or was substituted to the 3' U3 viral promoter/enhancer (hybrid vectors). High vector titers but poor specific expression of EGFP were achieved when CD4pmE was inserted in sense orientation in SIN vectors. Low titers but high specificity were observed when the CD4pmE cassette was in anti-sense orientation. In contrast, high titers and good T cell specificity were obtained with hybrid vectors.

CONCLUSION

An efficient T cell-specific retroviral vector was obtained.

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

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

Gene therapy: trials and tribulations

The art and science of gene therapy has received much attention of late. The tragic death of 18-year-old Jesse Gelsinger, a volunteer in a Phase I clinical trial, has overshadowed the successful treatment of three children suffering from a rare but fatal immunological disease. In the light of the success and tragedy, it is timely to consider the challenges faced by gene therapy--a novel form of molecular medicine that may be poised to have an important impact on human health in the new millennium.

The use of enzyme therapy to regulate the metabolic and phenotypic consequences of adenosine deaminase deficiency in mice. Differential impact on pulmonary and immunologic abnormalities

Adenosine deaminase (ADA) deficiency results in a combined immunodeficiency brought about by the immunotoxic properties of elevated ADA substrates. Additional non-lymphoid abnormalities are associated with ADA deficiency, however, little is known about how these relate to the metabolic consequences of ADA deficiency. ADA-deficient mice develop a combined immunodeficiency as well as severe pulmonary insufficiency. ADA enzyme therapy was used to examine the relative impact of ADA substrate elevations on these phenotypes. A "low-dose" enzyme therapy protocol prevented the pulmonary phenotype seen in ADA-deficient mice, but did little to improve their immune status. This treatment protocol reduced metabolic disturbances in the circulation and lung, but not in the thymus and spleen. A "high-dose" enzyme therapy protocol resulted in decreased metabolic disturbances in the thymus and spleen and was associated with improvement in immune status. These findings suggest that the pulmonary and immune phenotypes are separable and are related to the severity of metabolic disturbances in these tissues. This model will be useful in examining the efficacy of ADA enzyme therapy and studying the mechanisms underlying the immunodeficiency and pulmonary phenotypes associated with ADA deficiency.

Progress in the use of gene transfer methods to treat genetic blood diseases

A report by French physician-scientists suggests a successful application of gene transfer methods in the treatment of two children with severe combined immunodeficiency (SCID) due to defective interleukin 2 receptor common gamma chain. The protocol used in this clinical trial was derived from a number of preclinical and basic studies leading to improved transduction of hematopoietic stem and primitive progenitor cells using retrovirus vectors. These improvements have also been shown to impact transduction of a long-lived progenitor cell in a chemotherapy protocol in cancer patients. The improved results of these human trials come during a period of increased scrutiny and criticism of human gene therapy trials, due, in part, to significant toxicities in some trials using adenovirus-based vectors. The potential efficacy versus toxicity of phase I trials of human gene therapy is also under question. After many years of research, however, there appears to be real evidence that genetic diseases may be successfully treated by gene transfer techniques. Future clinical studies should be based on continued progress in the understanding of the toxicology of gene delivery systems, vector technology, and target cell manipulation.

FLT3 ligand preserves the uncommitted CD34+CD38- progenitor cells during cytokine prestimulation for retroviral transduction

Before stem cell gene therapy can be considered for clinical applications, problems regarding cytokine prestimulation remain to be solved. In this study, a retroviral vector carrying the genes for the enhanced version of green fluorescent protein (EGFP) and neomycin resistance (neo(r)) was used for transduction of CD34+ cells. The effect of cytokine prestimulation on transduction efficiency and the population of uncommitted CD34+CD38- cells was determined. CD34+ cells harvested from umbilical cord blood were kept in suspension cultures and stimulated with combinations of the cytokines stem cell factor (SCF), FLT3 ligand, interleukin-3 (IL-3), IL-6, and IL-7 prior to transduction. Expression of the two genes was assessed by flow cytometry and determination of neomycin-resistant colonies in a selective colony-forming unit (CFU) assay, respectively. The neomycin resistance gene was expressed in a higher percentage of cells than the EGFP gene, but there seemed to be a positive correlation between expression of the two genes. The effect of cytokine prestimulation was therefore monitored using EGFP as marker for transduction. When SCF was compared to SCF in combination with more potent cytokines, highest transduction efficiency was found with SCF and IL-3 and IL-6 (5.05% +/- 0.80 versus 2.66% +/- 0.53 with SCF alone, p = 0.04). However, prestimulation with SCF in combination with IL-3 and IL-6 also reduced the percentage of CD34+ cells (p = 0.02). Then, prestimulation with SCF and FLT3 ligand was compared. Significant difference in transduction efficiency was not found. Interestingly, FLT3 ligand seemed to preserve the population of CD34+CD38- cells compared to SCF (16.56% +/- 2.02 versus 9.39% +/- 2.35, p = 0.03). In conclusion, prestimulation with potent cytokine combinations increased the transduction efficiency, but reduced the fraction of CD34+ cells. Importantly, the use of FLT3 ligand seemed to preserve the population of uncommitted cells.

Primary T lymphocytes as targets for gene therapy

Peripheral blood T lymphocytes have been considered an attractive target for gene therapy applications. They can be easily harvested and readily expanded ex vivo. The transduction efficiency of primary human lymphocytes with standard retroviral vectors approaches 50% or more using optimized methods of gene transfer. Other methods of gene transfer, including adenoviral, adeno-associated viral, and lentiviral vectors, or nonviral techniques, have also been used for gene transfer into primary lymphocytes. Despite encouraging results in vitro, human clinical trials using retroviral vectors to transduce primary lymphocytes have been hindered by low expression levels of transgenes and immune responses against transgene products. Strategies to overcome these problems need to be developed.

In vivo correction of genetic defects of monocyte/macrophages using attenuated Salmonella as oral vectors for targeted gene delivery

Macrophages are normal targets for Salmonella during natural infections, and it has been demonstrated that attenuated bacteria can deliver nucleic acid vaccine constructs. Therefore, we assessed if attenuated Salmonella can be used for the in vivo delivery of transgenes to their natural cellular target, in an attempt to correct genetic defects associated with monocytes/macrophages. This system would offer the distinct advantage of achieving a specific targeting of defective cells in a non-invasive form. Using a reporter gene, we demonstrated that attenuated Salmonella could be used as an effective in vitro delivery system to transfer genetic material into nondividing cells like murine macrophages. In vivo, the oral administration of attenuated Salmonella allows targeted delivery of transgenes to macrophages and subsequently expression of transgenes at a systemic level. IFNgamma-deficient mice (GKO) were thus selected as a model for the in vivo validation of the Salmonella-based delivery approach. Attenuated Salmonella, used as the carrier for a eukaryotic expression vector encoding the murine IFNgamma gene, was able to restore the production of this cytokine in GKO macrophages. Their oral administration to IFNgamma-deficient mice also re-established, in these immunocompromised animals, the natural resistance to bacterial infections. These results demonstrate, for the first time, that attenuated Salmonella can be successfully used in vivo as a DNA delivery system for the correction of a genetic defect associated with monocyte/macrophages.

Cellular transplants as sources for therapeutic agents

Soluble factors normally produced by cells of the human body are of increasing importance as potential therapeutic agents. Although considerable progress has been made in understanding the etiology and pathogenesis of disease, in developing animal models and newer experimental therapeutics, few discoveries have been translated into clinically effective ways of delivering the multiple therapeutic agents obtained from living mammalian cells. This review examines the use of transplanted cells as alternatives to conventional delivery systems to deliver a variety of protein based therapeutic agents. The chapter begins with a set of questions to establish the complexity and challenges of this form of drug delivery. The following section focuses the discussion on our understanding of genetic engineering, tissue engineering, and some areas of developmental biology as they relate to the development of this nascent field. Much of the discussion has a neuro/endocrine emphasis. The chapter ends by listing the basic ingredients needed to push the use of transplanted cells toward medical practice and some general comments about future developments.

Mild preconditioning and low-level engraftment confer methotrexate resistance in mice transplanted with marrow expressing drug-resistant dihydrofolate reductase activity

Effective engraftment of hematopoietic cells targeted for gene transfer is facilitated by cytoreductive preconditioning such as high-dose total body irradiation (TBI). To minimize the adverse side effects associated with TBI, experiments were conducted to determine whether sublethal doses of TBI would allow sufficient engraftment of MTX-resistant hematopoietic cells to confer survival on recipient mice administered MTX. FVB/N animals were administered 1, 2, or 4 Gy TBI (lethal dose, 8.5 Gy), transplanted with 107 FVB/N transgenic marrow cells expressing an MTX-resistant dihydrofolate reductase (DHFR) transgene, and then administered MTX daily for 60 days. Control mice administered 1 Gy with or without subsequent transplantation of normal marrow cells succumbed to MTX toxicity by day 45. In contrast, nearly all animals transplanted with transgenic marrow survived MTX administration, regardless of the TBI dose used for preconditioning. The donor DHFR transgenic marrow engraftment level was proportional to the preconditioning dose of TBI but was surprisingly reduced in animals given 2 or 4 Gy TBI and subsequently administered MTX when compared with control animals administered phosphate-buffered saline. Animals preconditioned with 1 Gy were also protected from MTX toxicity when transplanted with reduced amounts (5 × 106 and 1 × 106 cells) of DHFR transgenic donor marrow, resulting in low-level (approximately 1%) engraftment. In conclusion, very mild preconditioning allows sufficient low-level engraftment of genetically modified stem cells for in vivo manifestation of the modified phenotype, suggesting the usefulness of mild preconditioning regimens in human gene therapy trials targeting hematopoietic stem cells.

Mild preconditioning and low-level engraftment confer methotrexate resistance in mice transplanted with marrow expressing drug-resistant dihydrofolate reductase activity

Effective engraftment of hematopoietic cells targeted for gene transfer is facilitated by cytoreductive preconditioning such as high-dose total body irradiation (TBI). To minimize the adverse side effects associated with TBI, experiments were conducted to determine whether sublethal doses of TBI would allow sufficient engraftment of MTX-resistant hematopoietic cells to confer survival on recipient mice administered MTX. FVB/N animals were administered 1, 2, or 4 Gy TBI (lethal dose, 8.5 Gy), transplanted with 107 FVB/N transgenic marrow cells expressing an MTX-resistant dihydrofolate reductase (DHFR) transgene, and then administered MTX daily for 60 days. Control mice administered 1 Gy with or without subsequent transplantation of normal marrow cells succumbed to MTX toxicity by day 45. In contrast, nearly all animals transplanted with transgenic marrow survived MTX administration, regardless of the TBI dose used for preconditioning. The donor DHFR transgenic marrow engraftment level was proportional to the preconditioning dose of TBI but was surprisingly reduced in animals given 2 or 4 Gy TBI and subsequently administered MTX when compared with control animals administered phosphate-buffered saline. Animals preconditioned with 1 Gy were also protected from MTX toxicity when transplanted with reduced amounts (5 × 106 and 1 × 106 cells) of DHFR transgenic donor marrow, resulting in low-level (approximately 1%) engraftment. In conclusion, very mild preconditioning allows sufficient low-level engraftment of genetically modified stem cells for in vivo manifestation of the modified phenotype, suggesting the usefulness of mild preconditioning regimens in human gene therapy trials targeting hematopoietic stem cells.

Lentiviral-mediated gene transfer into human lymphocytes: role of HIV-1 accessory proteins

Resting lymphocytes are refractory to gene transfer using Moloney murine leukemia virus (MMLV)-based retroviral vectors because of their quiescent status. Recently, it has been shown that lentiviral vectors are capable of transferring genes into nondividing and terminally differentiated cells. We used human immunodeficiency virus type-1 (HIV-1)–based vectors expressing enhanced green fluorescent protein (EGFP) driven by different promoters (CMV, MPSV, or PGK) and investigated their ability to transduce human T- and B-cell lines, as well as resting or activated primary peripheral and umbilical cord blood lymphocytes. The effects of the presence or the absence of HIV-1 accessory proteins (Vif, Vpr, Vpu, and Nef) in the vector system were also assessed. Flow cytometry analysis showed no differences in the ability of these vectors of transferring the reporter gene into lymphocytic lines and mitogen-stimulated primary lymphocytes in the presence or the absence of HIV-1 accessory proteins (APs). Similarly, viral supernatants generated in the presence of accessory genes could efficiently transduce various subsets of resting lymphocytes and provide long-term expression of the transgene. No significant transduction-induced changes in cell activation or cycling status were observed and Alu-HIV-1 long terminal repeat polymerase chain reaction (LTR PCR) analysis demonstrated integration of the vector sequences at the molecular level. In contrast, in the absence of HIV-1 APs, lentiviral vectors failed to integrate and express the transgene in resting lymphocytes. These results show that transduction of primary resting lymphocytes with HIV-1–based vectors requires the presence of viral accessory proteins.

Lentiviral-mediated gene transfer into human lymphocytes: role of HIV-1 accessory proteins

Resting lymphocytes are refractory to gene transfer using Moloney murine leukemia virus (MMLV)-based retroviral vectors because of their quiescent status. Recently, it has been shown that lentiviral vectors are capable of transferring genes into nondividing and terminally differentiated cells. We used human immunodeficiency virus type-1 (HIV-1)–based vectors expressing enhanced green fluorescent protein (EGFP) driven by different promoters (CMV, MPSV, or PGK) and investigated their ability to transduce human T- and B-cell lines, as well as resting or activated primary peripheral and umbilical cord blood lymphocytes. The effects of the presence or the absence of HIV-1 accessory proteins (Vif, Vpr, Vpu, and Nef) in the vector system were also assessed. Flow cytometry analysis showed no differences in the ability of these vectors of transferring the reporter gene into lymphocytic lines and mitogen-stimulated primary lymphocytes in the presence or the absence of HIV-1 accessory proteins (APs). Similarly, viral supernatants generated in the presence of accessory genes could efficiently transduce various subsets of resting lymphocytes and provide long-term expression of the transgene. No significant transduction-induced changes in cell activation or cycling status were observed and Alu-HIV-1 long terminal repeat polymerase chain reaction (LTR PCR) analysis demonstrated integration of the vector sequences at the molecular level. In contrast, in the absence of HIV-1 APs, lentiviral vectors failed to integrate and express the transgene in resting lymphocytes. These results show that transduction of primary resting lymphocytes with HIV-1–based vectors requires the presence of viral accessory proteins.

Highly efficient gene transfer in naive human T cells with a murine leukemia virus-based vector

Retroviral vectors based on the Moloney murine leukemia virus (MuLV) have become the primary tool for gene delivery into hematopoietic cells, but clinical trials have been hampered by low transduction efficiencies. Recently, we and others have shown that gene transfer of MuLV-based vectors into T cells can be significantly augmented using a fibronectin-facilitated protocol. Nevertheless, the relative abilities of naive (CD45RA+) and memory (CD45RO+) lymphocyte subsets to be transduced has not been assessed. Although naive T cells demonstrate a restricted cytokine profile following antigen stimulation and a decreased susceptibility to infection with human immunodeficiency virus, it was not clear whether they could be efficiently infected with a MuLV vector. This study describes conditions that permitted gene transfer of an enhanced green fluorescent protein-expressing retroviral vector in more than 50% of naive umbilical cord (UC) blood and peripheral blood (PB) T cells following CD3/CD28 ligation. Moreover, treatment of naive T cells with interleukin-7 resulted in the maintenance of a CD45RA phenotype and gene transfer levels approached 20%. Finally, it was determined that parameters for optimal transduction of CD45RA+ T cells isolated from PB and UC blood differed: transduction of the UC cells was significantly increased by the presence of autologous mononuclear cells (24.5% versus 56.5%). Because naive T cells harbor a receptor repertoire that allows them to respond to novel antigens, the development of protocols targeting their transduction is crucial for gene therapy applications. This approach will also allow the functions of exogenous genes to be evaluated in primary nontransformed naive T cells.

Highly efficient gene transfer in naive human T cells with a murine leukemia virus-based vector

Retroviral vectors based on the Moloney murine leukemia virus (MuLV) have become the primary tool for gene delivery into hematopoietic cells, but clinical trials have been hampered by low transduction efficiencies. Recently, we and others have shown that gene transfer of MuLV-based vectors into T cells can be significantly augmented using a fibronectin-facilitated protocol. Nevertheless, the relative abilities of naive (CD45RA+) and memory (CD45RO+) lymphocyte subsets to be transduced has not been assessed. Although naive T cells demonstrate a restricted cytokine profile following antigen stimulation and a decreased susceptibility to infection with human immunodeficiency virus, it was not clear whether they could be efficiently infected with a MuLV vector. This study describes conditions that permitted gene transfer of an enhanced green fluorescent protein-expressing retroviral vector in more than 50% of naive umbilical cord (UC) blood and peripheral blood (PB) T cells following CD3/CD28 ligation. Moreover, treatment of naive T cells with interleukin-7 resulted in the maintenance of a CD45RA phenotype and gene transfer levels approached 20%. Finally, it was determined that parameters for optimal transduction of CD45RA+ T cells isolated from PB and UC blood differed: transduction of the UC cells was significantly increased by the presence of autologous mononuclear cells (24.5% versus 56.5%). Because naive T cells harbor a receptor repertoire that allows them to respond to novel antigens, the development of protocols targeting their transduction is crucial for gene therapy applications. This approach will also allow the functions of exogenous genes to be evaluated in primary nontransformed naive T cells.

Retrovirus-mediated transduction of primary ZAP-70-deficient human T cells results in the selective growth advantage of gene-corrected cells: implications for gene therapy

Humans lacking the ZAP-70 protein tyrosine kinase present with an absence of CD8+ T cells and defective CD4+ T cells in the periphery. This severe combined immunodeficiency is fatal unless treated by allogeneic bone marrow transplantation. However, in the absence of suitable marrow donors, the development of alternative forms of therapy is desirable. Because lymphocytes are long-lived, it is possible that introduction of the wild-type ZAP-70 gene into CD4+ ZAP-70-deficient T cells will restore their immune function in vivo. Initial investigations evaluating the feasibility of gene therapy for ZAP-70 deficiency were performed using HTL V-I-transformed lymphocytes. Although transformation was useful in circumventing problems associated with the maintenance of ZAP-70-deficient T cells and low gene transfer levels, the presence of HTL V-I precluded any biological studies. Here, we investigated a retrovirus-mediated approach for the correction of primary T cells derived from two ZAP-70-deficient patients. Upon introduction of the wild-type ZAP-70 gene, TCR-induced MAPK activation, IL-2 secretion and proliferation were restored to approximately normal levels. Importantly, this gain-of-function was associated with a selective growth advantage of gene-corrected cells, thereby indicating the feasibility of a gene therapy-based strategy.

Biodegradable polyester, poly[alpha-(4-aminobutyl)-L-glycolic acid], as a non-toxic gene carrier

PURPOSE

The aim of this study was to develop a non-toxic polymeric gene carrier. For this purpose, biodegradable cationic polymer, poly[alpha-(4-aminobutyl)-L-glycolic acid] (PAGA) was synthesized. PAGA was designed to have ester linkage because polyesters usually show biodegradability.

METHODS

Degradation of PAGA in an aqueous solution was followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). PAGA/DNA complexes were characterized by gel electrophoresis, atomic force microscopy (AFM), dynamic light scattering (DLS). The transfection was measured by using the beta-galactosidase reporter gene.

RESULTS

PAGA was degraded in aqueous solution very quickly and the final degradation product was a monomer (L-oxylysine). Formation of self-assembling biodegradable complexes between PAGA and DNA at a charge ratio 1:1 (+/-) was confirmed by gel band shift assay and AFM. In these studies, controlled release of DNA from the complexes could be seen. The complexes showed about 2-fold higher transfection efficiency than DNA complexes of poly-L-lysine (PLL), a structural analogue of PAGA, which is the most commonly used poly-cation for gene delivery. The polymer did not show cytotoxicity, possibly because of its degradability and the biocompatibility of the monomer.

CONCLUSIONS

The use of the biodegradable poly-cation, PAGA, as a DNA condensing agent will be useful in safe gene delivery.

Gene therapy for adenosine deaminase deficiency

The clinical gene therapy trials for adenosine deaminase (ADA) deficiency have defined both the potential benefits and the present limitations of gene therapy with hematopoietic stem cells (HSC). Current clinical results indicate that (a) both umbilical cord blood and neonatal bone marrow HSC can be transduced with murine retroviral-based vectors, (b) the transduced HSC can engraft in nonmyeloablated patients, (c) the frequency of HSC transduction/engraftment is low (1/10,000), (d) an in vivo selective advantage can exist for transduced T lymphoid progeny, and (e) the transduced ADA gene is not expressed in nondividing T lymphocytes. Improving the clinical results of gene therapy for ADA deficiency and other genetic diseases involving HSC will require (a) developing new vectors that express the transduced gene in nondividing cells and (b) increasing the frequency of stable HSC transduction.

The importance of adenosine deaminase for lymphocyte development and function

Deficiency in the enzyme adenosine deaminase (ADA) in humans manifests primarily as severe lymphopenia and immunodeficiency, resulting in death by 6 months of age, if untreated. In this review, we discuss phenotypical, biochemical, and metabolic hallmarks of the disease, and describe a mouse model in which levels of ADA can be biochemically and genetically manipulated. This model provides exciting possibilities for uncovering the mechanisms by which this purine catabolic enzyme affects lymphopoiesis.

Efficient gene transfer to hematopoietic progenitor cells using SV40-derived vectors

We used recombinant SV40 (rSV40)-derived vectors to deliver transgenes to human and simian hematopoietic progenitor cells in culture, and in vivo after transduction ex vivo. rSV40 are highly efficient vectors that are made in very high titers. They infect almost all cells, whether resting or dividing. Two rSV40s were used: SV(HBS), carrying hepatitis B surface antigen as a marker; and SV(Aw) carrying IN#33, a single chain Fv antibody against HIV-1 integrase. CD34+ cells derived from human fetal bone marrow (HFBM) and rhesus macaque bone marrow were transduced once with SV(HBS) without selection. On average 60% of colonies derived from transduced CD34+ cells carried and expressed HBsAg, as assessed by PCR and immunochemistry. Transgene carriage persisted following differentiation of transduced rhesus CD34+ cells into T lymphocytes. In an effort to increase the percentage of gene-marked cells, three sequential treatments of CD34+ cells were done using SV(Aw), without selection. Two weeks later, >95% of colonies expressed IN#33. Unselected SV(Aw)-transduced CD34+ cells from HFBM were transplanted into sublethally irradiated SCID mice. Bone marrow harvested 3 months later showed that >50% of bone marrow cells expressed IN#33. This is comparable with the percentage of human cells in these animals' bone marrow as judged by immunostaining for human CD45. The stability and longevity of transduction in this setting suggests that rSV40 vectors integrate into the cellular genome. This possibility was supported by finding that PCR of genomic DNA using primer pairs with one cellular and one viral primer yielded PCR products only in transduced, but not control, cells. These PCR products hybridized with an SV40 DNA fragment. Thus, rSV40 vectors transduce normal human and primate bone marrow progenitor cells effectively without selection, and maintain transgene expression in vivo following reimplantation. Such high efficiency transduction may be useful in treating diseases of CD34+ cells and their derivatives.

Genetics of systemic lupus erythematosus. Clinical implications

Rheumatic diseases have long been recognized as having complex inheritance patterns. It has recently been estimated that over 100 genes may be implicated in the SLE disease process. Identification of these genes has led to a greater understanding of the etiopathogenesis of SLE and is beginning to lead to new types of interventions directed at correcting aberrant biological processes.

Efficient gene transfer into primary human CD8+ T lymphocytes by MuLV-10A1 retrovirus pseudotype

Efficient and stable gene transfer into primary human T lymphocytes would greatly improve their use for adoptive transfer to treat acquired disorders, viral diseases, and cancer. We have constructed retroviral vector pseudotypes of amphotropic murine leukemia viruses (A-MuLV, MuLV-10A1), gibbon ape leukemia virus (GaLV), and feline endogenous virus (RD114) containing the enhanced green fluorescent protein (GFP) as a marker gene. Transduction of primary human CD8+ T lymphocytes by the different GFP-retrovirus pseudotypes revealed the superiority of MuLV-10A1 in comparison with A-MuLV, GaLV, and RD114, respectively. The superior transduction efficacy of CD8+ T cells by MuLV-10A1 correlates with a longer half-life of this pseudotype in comparison with A-MuLV and, as shown by interference analysis with the human T cell line HUT78, by the utilization of both the A-MuLV receptor (Pit2) and the GaLV receptor (Pit1) for cell entry.

Efficiency of transgenic T cell generation from gene-marked cultured human CD34+ cord blood cells is determined by their maturity and the cytokines present in the culture medium

Success of gene therapy for diseases affecting the T cell lineage depends on the thymic repopulation by genetically engineered hematopoietic progenitor cells (HPC). Although it has been shown that retrovirally transduced HPC can repopulate the thymus, little information is available on the effect of the culture protocol. Moreover, for expansion of the number of HPC, cytokine supplemented culture is needed. Here, we transduced purified human umbilical cord blood (CB) CD34+ cells in cultures supplemented with various combinations of the cytokines thrombopoietin (TPO), stem cell factor (SCF), flt3/flk-2 ligand (FL), interleukin-3 (IL-3) and IL-6, and investigated thymus-repopulating ability of gene-marked HPC in vitro. Irrespective of the cytokine cocktail used, transduced CD34+CD38- CB cells, expressing the marker green fluorescent protein (GFP) encoded by the MFG-GFP retrovirus, have both superior proliferative and thymus-repopulating potential compared with transduced CD34+CD38+ CB cells. Effectively transduced GFP+CD34+CD38- HPC, cultured for 3 or 17 days, more readily generated T cells than GFP- HPC from the same culture. The reverse was true in the case of CD34+CD38+ HPC cultures. Finally, our results indicate that the number of GFP+ T cell progenitors actually increased during culture of CD34+CD38- HPC, in a magnitude that is determined by the cytokine cocktail used during culture.

Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease

Severe combined immunodeficiency-X1 (SCID-X1) is an X-linked inherited disorder characterized by an early block in T and natural killer (NK) lymphocyte differentiation. This block is caused by mutations of the gene encoding the gammac cytokine receptor subunit of interleukin-2, -4, -7, -9, and -15 receptors, which participates in the delivery of growth, survival, and differentiation signals to early lymphoid progenitors. After preclinical studies, a gene therapy trial for SCID-X1 was initiated, based on the use of complementary DNA containing a defective gammac Moloney retrovirus-derived vector and ex vivo infection of CD34+ cells. After a 10-month follow-up period, gammac transgene-expressing T and NK cells were detected in two patients. T, B, and NK cell counts and function, including antigen-specific responses, were comparable to those of age-matched controls. Thus, gene therapy was able to provide full correction of disease phenotype and, hence, clinical benefit.

HIV protease as a target for retrovirus vector-mediated gene therapy

The dimeric aspartyl protease of HIV has been the subject of intense research for almost a decade. Knowledge of the substrate specificity and catalytic mechanism of this enzyme initially guided the development of several potent peptidomimetic small molecule inhibitors. More recently, the solution of the HIV protease structure led to the structure-based design of improved peptidomimetic and non-peptidomimetic antiviral compounds. Despite the qualified success of these inhibitors, the high mutation rate associated with RNA viruses continues to hamper the long-term clinical efficacy of HIV protease inhibitors. The dimeric nature of the viral protease has been conducive to the investigation of dominant-negative inhibitors of the enzyme. Some of these inhibitors are defective protease monomers that interact with functional monomers to form inactive protease heterodimers. An advantage of macromolecular inhibitors as compared to small-molecule inhibitors is the increased surface area of interaction between the inhibitor and the target gene product. Point mutations that preserve enzyme activity but confer resistance to small-molecule inhibitors are less likely to have an adverse effect on macromolecular interactions. The use of efficient retrovirus vectors has facilitated the delivery of these macromolecular inhibitors to primary human lymphocytes. The vector-transduced cells were less susceptible to HIV infection in vitro, and showed similar levels of protection compared to other macromolecular inhibitors of HIV replication, such as RevM10. These preliminary results encourage the further development of dominant-negative HIV protease inhibitors as a gene therapy-based antiviral strategy.

Adenosine deaminase deficiency as the first target disorder in gene therapy

In the past decade, the advent of gene therapy has been acclaimed as a revolutionary medical intervention, embraced with great enthusiasm. However, recent disappointing results of the considerable clinical trials have also clearly demonstrated that such an initial expectation was an overestimation of gene therapy. There are only a few successful cases despite the 3000 patients who have been treated with various forms of gene therapy. Gene therapy for severe combined immunodeficiency (SCID) caused by adenosine deaminase (ADA) deficiency is one of the few such cases where results have been promising. In particular, peripheral T-lymphocytes-directed gene therapy provides further immunological improvements for patients with ADA-SCID receiving the PEG-ADA treatment whereas gene therapy targeting haematopoietic stem cell has so far proved insufficient for clinical benefits. This report will review crucial problems elucidated in the past five clinical trials for ADA-SCID and gives an outline of the next generation of stem cell gene therapy in Japan.

Genetic modification of human T cells with CD20: a strategy to purify and lyse transduced cells with anti-CD20 antibodies

A retroviral vector has been constructed that contains the human CD20 cDNA under the control of the Moloney murine leukemia virus (Mo-MuLV) LTR. Freshly isolated mononuclear cells are infected for three consecutive days in the presence of PHA and hrlL-2, and a mean 15.9% of the cells (range, 6.5 to 31.7%) acquire a CD3+CD20+ phenotype. Transduced T lymphocytes grow and expand in vitro for up to 3 weeks like mock-infected cells and, as observed for the T lymphoblastoid CEM cell line, CD20 expression is maintained for several months with no change in the growth curve of the cells. CD20-expressing CEM and fresh T lymphocytes can be positively immunoselected on columns using different anti-CD20 antibodies. Exposure to monoclonal chimeric anti-CD20 IgG1(kappa) Rituximab antibody (Roche), in the presence of complement, results in effective and rapid killing of the transduced CD3+CD20+ human T cells in vitro. This approach represents a new and alternative method to gene manipulation with "suicide" genes for the production of drug-responsive T cell populations, a crucial step for the future management of graft-versus-host disease in bone marrow transplant patients.