Retroviral Marking of Canine Bone Marrow: Long-Term, High-Level Expression of Human Interleukin-2 Receptor Common Gamma Chain in Canine Lymphocytes

Immune Reconstitution After Gene Therapy Approaches in Patients With X-Linked Severe Combined Immunodeficiency Disease

X-linked severe immunodeficiency disease (SCID-X1) is an inherited, rare, and life-threating disease. The genetic origin is a defect in the interleukin 2 receptor γ chain (IL2RG) gene and patients are classically characterized by absence of T and NK cells, as well as presence of partially-functional B cells. Without any treatment the disease is usually lethal during the first year of life. The treatment of choice for these patients is hematopoietic stem cell transplantation, with an excellent survival rate (>90%) if an HLA-matched sibling donor is available. However, when alternative donors are used, the success and survival rates are often lower. Gene therapy has been developed as an alternative treatment initially using γ-retroviral vectors to correct the defective γ chain in the absence of pre-conditioning treatment. The results were highly promising in SCID-X1 infants, showing long-term T-cell recovery and clinical benefit, although NK and B cell recovery was less robust. However, some infants developed T-cell acute lymphoblastic leukemia after the gene therapy, due to vector-mediated insertional mutagenesis. Consequently, considerable efforts have been made to develop safer vectors. The most recent clinical trials using lentiviral vectors together with a low-dose pre-conditioning regimen have demonstrated excellent sustained T cell recovery, but also B and NK cells, in both children and adults. This review provides an overview about the different gene therapy approaches used over the last 20 years to treat SCID-X1 patients, particularly focusing on lymphoid immune reconstitution, as well as the developments that have improved the process and outcomes.

Gene therapy: therapeutic gene causing lymphoma

The development of T-cell leukaemia following the otherwise successful treatment of three patients with X-linked severe combined immune deficiency (X-SCID) in gene-therapy trials using haematopoietic stem cells has led to a re-evaluation of this approach. Using a mouse model for gene therapy of X-SCID, we find that the corrective therapeutic gene IL2RG itself can act as a contributor to the genesis of T-cell lymphomas, with one-third of animals being affected. Gene-therapy trials for X-SCID, which have been based on the assumption that IL2RG is minimally oncogenic, may therefore pose some risk to patients.

Gene therapy of X-linked severe combined immunodeficiency by use of a pseudotyped gammaretroviral vector

BACKGROUND

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the common cytokine-receptor gamma chain (gamma(c)), resulting in disruption of development of T lymphocytes and natural-killer cells. B-lymphocyte function is also intrinsically compromised. Allogeneic bone-marrow transplantation is successful if HLA-matched family donors are available, but HLA-mismatched procedures are associated with substantial morbidity and mortality. We investigated the application of somatic gene therapy by use of a gibbon-ape-leukaemia-virus pseudotyped gammaretroviral vector.

METHODS

Four children with SCID-X1 were enrolled. Autologous CD34-positive haemopoietic bone-marrow stem cells were transduced ex vivo and returned to the patients without preceding cytoreductive chemotherapy. The patients were monitored for integration and expression of the gamma(c) vector and for functional immunological recovery.

FINDINGS

All patients have shown substantial improvements in clinical and immunological features, and prophylactic medication could be withdrawn in two. No serious adverse events have been recorded. T cells responded normally to mitogenic and antigenic stimuli, and the T-cell-receptor (TCR) repertoire was highly diverse. Where assessable, humoral immunity, in terms of antibody production, was also restored and associated with increasing rates of somatic mutation in immunoglobulin genes.

INTERPRETATION

Gene therapy for SCID-X1 is a highly effective strategy for restoration of functional cellular and humoral immunity.

Genetic manipulation of hematopoietic stem cells

Over the past two decades, the ability to transfer genes into hematopoietic stem cells (HSCs) has provided new insights into the behavior of individual stem cells and offered a novel approach for the treatment of various inherited or acquired disorders. At present, gene transfer into HSCs has been achieved mainly using modified retroviruses. While retrovirus-based vectors could efficiently transduce murine HSCs, extrapolation of these methods to large mammals and human clinical trials resulted in very low numbers of gene-marked engrafted cells. In addition, in vitro progenitor assays used to optimize gene transfer procedures were found to poorly predict the outcome of stem cell gene transfer. The focus rapidly turned to the development of superior and more relevant preclinical assays in human stem cell gene transfer research. Xenogeneic transplant models and large animal transplantation system have been invaluable. The development of better assays for evaluating human gene therapy protocols and a better understanding of stem cell and vector biology has culminated over the past decade in multiple strategies to improve gene transfer efficiency into HSCs. Improved gene transfer vectors, optimization of cytokine combination, and incorporation of a recombinant fragment of fibronectin during transduction are examples of novel successful additions to the early gene transfer protocols that have contributed to the first unequivocal clinical benefits resulting from genetic manipulation of HSC.

Retroviral transduction of IL2RG into CD34(+) cells from X-linked severe combined immunodeficiency patients permits human T- and B-cell development in sheep chimeras

X-linked severe combined immunodeficiency (XSCID) is caused by mutations of the common gamma chain of cytokine receptors, gamma(c). Because bone marrow transplantation (BMT) for XSCID does not provide complete immune reconstitution for many patients and because of the natural selective advantage conferred on lymphoid progenitors by the expression of normal gamma(c), XSCID is a good candidate disease for therapeutic retroviral gene transfer to hematopoietic stem cells. We studied XSCID patients who have persistent defects in B-cell and/or combined B- and T-cell function despite having received T cell-depleted haploidentical BMT. We compared transduction of autologous B-cell lines and granulocyte colony-stimulating factor-mobilized peripheral CD34(+) cells from these patients using an MFGS retrovirus vector containing the gamma(c) gene IL2RG pseudotyped with amphotropic, gibbon ape leukemia virus, or RD114 envelopes. Transduced B-cell lines and peripheral CD34(+) cells demonstrated provirus integration and new cell-surface gamma(c) expression. The chimeric sheep model was exploited to test development of XSCID CD34(+) cells into mature myeloid and lymphoid lineages. Transduced and untransduced XSCID CD34(+) cells injected into developing sheep fetuses gave rise to myeloid cells. However, only transduced gamma progenitors from XSCID patients developed into T and B cells. These results suggest that gene transfer to autologous peripheral CD34(+) cells using MFGS-gc retrovirus may benefit XSCID patients with persistent T- and B-cell deficits despite prior BMT.

Correction of genetic blood defects by gene transfer

Recent clinical trials in patients with a severe combined immunodeficiency disease demonstrate that gene therapy is a powerful tool in the treatment of genetic blood defects. Recent identification of the genes involved in the pathogenesis of inherited lymphohemopoietic disorders led to animal models of gene transfer. Extensive preclinical studies have overcome some of the obstacles involved in the transduction of hemopoietic cells. These promising results led to the approval of several clinical trials that are currently underway. This review focuses on the clinical outcome in patients with genetic blood defects treated by gene transfer and examines the progress achieved to date and the problems that have been encountered. Despite the obstacles, improved clinical results for several of these diseases are expected within the next 5 years.

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.

Gene therapy of severe combined immunodeficiencies

Recent advances in gene transfer in human hematopoietic cells, combined with a better understanding of the genetic aspects of several immunodeficiencies, has offered new opportunities in the domain of gene therapy. Severe combined immunodeficiency (SCID) appear to represent a good model for the application of gene therapy, combining an expected selective advantage for transduced cells, an absence of immunological response to the vector and/or the therapeutic transgene, together with accessibility to hematopoietic stem cells (HSC). Ex vivo retroviral transduction of a therapeutic transgene in HSC prior to transplantation appears to be a particularly effective and long-lasting means of restoring the expression of a mutated gene in the lymphoid lineage. Furthermore, encouraging therapeutic benefits as a result of a gene therapy protocol for the treatment of X-linked severe combined immunodeficiencies (SCID-X1) invites many questions as to the reasons for this therapeutic benefit. This review outlines the results that have been achieved in gene therapy for SCID-X1, ADA-SCID as well as other types of SCID, and discusses the possible relationship between the physiopathology of each disease and the success of relevant trials.

Comparison of five retrovirus vectors containing the human IL-2 receptor gamma chain gene for their ability to restore T and B lymphocytes in the X-linked severe combined immunodeficiency mouse model

X-linked severe combined immunodeficiency (XSCID) is caused by mutations in the IL-2 receptor gamma chain (IL2RG) gene, resulting in absent T lymphocytes and nonfunctional B lymphocytes. Recently T lymphocyte production and B lymphocyte function were restored in XSCID patients infused with autologous stem cells transduced with a retrovirus containing the human IL2RG cDNA. To optimize the expression of human IL2RG for future clinical trials, we compared five retroviral vectors expressing human IL2RG from different LTR enhancer-promoter elements in a mouse model. Northern and Southern blot analysis of hematopoietic tissues from repopulated mice revealed that the retroviral vector with the highest expression per copy number was MFG-S-hIL2RG, followed by MND-hIL2RG. All five vectors were capable of restoring lymphopoiesis in irradiated XSCID mice transplanted with transduced IL2RG-deficient hematopoietic stem cells. Transduction of IL2RG-deficient hematopoietic stem cells with all five vectors restored T lymphopoiesis in transplanted stem cell-deficient W/W(v) mouse recipients. However, only XSCID stem cells transduced with the MFG-S-hIL2RG vector generated B lymphocytes in W/W(v) mice. We conclude that the MFG-S-hIL2RG vector provides the best opportunity for in vivo selection and development of B and T lymphocytes for human XSCID gene therapy.

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.

Gene therapy of lymphoid primary immunodeficiencies

Gene therapy offers an attractive option to the most severe forms of primary immunodeficiency diseases. Identification of disease-associated genes as well as advances in the technology of gene transfer into hematopoietic progenitor cells have set the basis for the first clinical trials. Settings characterized by the potential for a selective advantage provided to transduced cells are the first diseases to target. The recent example of successful treatment of severe combined immunodeficiency-X1 (gammac deficiency) illustrates this potential.

Lentivirus-based vectors transduce mouse hematopoietic stem cells with similar efficiency to Moloney murine leukemia virus–based vectors

The low levels of transduction of human hematopoietic stem cells (HSCs) with Moloney murine leukemia virus (MLV) vectors have been an obstacle to gene therapy for hematopoietic diseases. It has been demonstrated that lentivirus vectors are more efficient than MLV vectors at transducing nondividing cell lines as well as human CD34+ cells and severe combined immunodeficiency disease repopulating cells. We compared transduction of cell lines and Lin− bone marrow cells, using a vesicular stomatitis virus G (VSV-G)-pseudotyped lentivirus or MLV vectors carrying a green fluorescent protein marker gene. As predicted, the lentivirus vector was more efficient at transducing mouse and human growth-inhibited cell lines. The transduction of mouse HSC by lentivirus vectors was compared directly to MLV vectors in a co-transduction assay. In this assay, transduction by ecotropic MLV is a positive internal control for downstream steps in retrovirus transduction, including cell division. Both the VSV-G lentivirus and MLV vectors transduced mouse HSCs maintained in cytokine-free medium at very low frequency, as did the ecotropic control. The lentivirus vector and the MLV vector were equally efficient at transducing bone marrow HSCs cultured in interleukin 3 (IL-3), IL-6, and stem cell factor for 96 hours. In conclusion, although lentivirus vectors are able to transduce growth-inhibited cell lines, the cell cycle status of HSCs render them resistant to lentivirus-mediated transduction, and it is hypothesized that entry into cycle, not necessarily division, may be a requirement for efficient lentivirus-mediated transduction.

Lentivirus-based vectors transduce mouse hematopoietic stem cells with similar efficiency to Moloney murine leukemia virus–based vectors

The low levels of transduction of human hematopoietic stem cells (HSCs) with Moloney murine leukemia virus (MLV) vectors have been an obstacle to gene therapy for hematopoietic diseases. It has been demonstrated that lentivirus vectors are more efficient than MLV vectors at transducing nondividing cell lines as well as human CD34+ cells and severe combined immunodeficiency disease repopulating cells. We compared transduction of cell lines and Lin− bone marrow cells, using a vesicular stomatitis virus G (VSV-G)-pseudotyped lentivirus or MLV vectors carrying a green fluorescent protein marker gene. As predicted, the lentivirus vector was more efficient at transducing mouse and human growth-inhibited cell lines. The transduction of mouse HSC by lentivirus vectors was compared directly to MLV vectors in a co-transduction assay. In this assay, transduction by ecotropic MLV is a positive internal control for downstream steps in retrovirus transduction, including cell division. Both the VSV-G lentivirus and MLV vectors transduced mouse HSCs maintained in cytokine-free medium at very low frequency, as did the ecotropic control. The lentivirus vector and the MLV vector were equally efficient at transducing bone marrow HSCs cultured in interleukin 3 (IL-3), IL-6, and stem cell factor for 96 hours. In conclusion, although lentivirus vectors are able to transduce growth-inhibited cell lines, the cell cycle status of HSCs render them resistant to lentivirus-mediated transduction, and it is hypothesized that entry into cycle, not necessarily division, may be a requirement for efficient lentivirus-mediated transduction.

Highly efficient gene transfer into cord blood nonobese diabetic/severe combined immunodeficiency repopulating cells by oncoretroviral vector particles pseudotyped with the feline endogenous retrovirus (RD114) envelope protein

Limited expression of the amphotropic envelope receptor is a recognized barrier to efficient oncoretroviral vector–mediated gene transfer. Human hematopoietic cell lines and cord blood–derived CD34+ and CD34+, CD38− cell populations and the progenitors contained therein were transduced far more efficiently with oncoretroviral particles pseudotyped with the envelope protein of feline endogenous virus (RD114) than with conventional amphotropic vector particles. Similarly, human repopulating cells from umbilical cord blood capable of establishing hematopoiesis in immunodeficient mice were efficiently transduced with RD114-pseudotyped particles, whereas amphotropic particles were ineffective at introducing the proviral genome. After only a single exposure of CD34+ cord blood cells to RD114-pseudotyped particles, all engrafted nonobese diabetic/severe combined immunodeficiency mice (15 of 15) contained genetically modified human bone marrow cells. Human cells that were positive for enhanced green fluorescent protein represented as much as 90% of the graft. The use of RD114-pseudotyped vectors may be advantageous for therapeutic gene transfer into hematopoietic stem cells.

Superior transduction of mouse hematopoietic stem cells with 10A1 and VSV-G pseudotyped retrovirus vectors

The inefficient transduction of human hematopoietic stem cells (HSC) with amphotropic retroviral vectors has been an obstacle to gene therapy for hematopoietic diseases. We have previously reported low levels of amphotropic retrovirus receptor (Pit-2) mRNA and higher levels of gibbon ape leukemia virus (GALV) or 10A1 retrovirus receptor (Pit-1) mRNA in mouse and human HSC. The vesicular stomatitis virus (VSV-G) uses an abundant membrane phospholipid as a receptor. We hypothesized that transduction of HSC requires relatively high levels of retrovirus receptor molecules. Because mouse HSC can be efficiently transduced by ecotropic virus through the abundant ecotropic receptor, the mouse is an ideal model to compare receptor levels and transduction. We have developed a cotransduction assay where ecotropic retrovirus transduction is a positive internal control for downstream steps in retrovirus transduction. A comparison of mouse HSC transduction with amphotropic, 10A1, and VSV-G envelopes showed that the level of amphotropic and 10A1 receptor mRNA in HSC correlated with the frequency of transduction. Transduction with VSV-G vectors was similar to that with 10A1 vectors. We conclude that the level of retrovirus receptor on HSC is critical for HSC transduction and that GALV or VSV-G vectors would be better for human HSC transduction.

Restoration of Lymphoid Populations in a Murine Model of X-Linked Severe Combined Immunodeficiency by a Gene-Therapy Approach

X-linked severe combined immunodeficiency (XSCID) is a life-threatening syndrome in which both cellular and humoral immunity are profoundly compromised. This disease results from mutations in theIL2RG gene, which encodes the common cytokine receptor γ chain, γc. Previously, we generated γc-deficient mice as a murine model of XSCID. We have now used lethally irradiated γc-deficient mice to evaluate a gene therapeutic approach for treatment of this disease. Transfer of the human γc gene to repopulating hematopoietic stem cells using an ecotropic retrovirus resulted in an increase in T cells, B cells, natural killer (NK) cells, and intestinal intraepithelial lymphocytes, as well as normalization of the CD4:CD8 T-cell ratio and of serum Ig levels. In addition, the restored cells could proliferate in response to interleukin-2 (IL-2). Thus, our results provide added support that gene therapy is a feasible therapeutic strategy for XSCID. Moreover, because we used a vector directing expression of human γc to correct a defect in γc-deficient mice, these data also indicate that human γc can cooperate with the distinctive cytokine receptor chains such as IL-2Rβ and IL-7R to mediate responses to murine cytokines in vivo.

Restoration of Lymphoid Populations in a Murine Model of X-Linked Severe Combined Immunodeficiency by a Gene-Therapy Approach

X-linked severe combined immunodeficiency (XSCID) is a life-threatening syndrome in which both cellular and humoral immunity are profoundly compromised. This disease results from mutations in theIL2RG gene, which encodes the common cytokine receptor γ chain, γc. Previously, we generated γc-deficient mice as a murine model of XSCID. We have now used lethally irradiated γc-deficient mice to evaluate a gene therapeutic approach for treatment of this disease. Transfer of the human γc gene to repopulating hematopoietic stem cells using an ecotropic retrovirus resulted in an increase in T cells, B cells, natural killer (NK) cells, and intestinal intraepithelial lymphocytes, as well as normalization of the CD4:CD8 T-cell ratio and of serum Ig levels. In addition, the restored cells could proliferate in response to interleukin-2 (IL-2). Thus, our results provide added support that gene therapy is a feasible therapeutic strategy for XSCID. Moreover, because we used a vector directing expression of human γc to correct a defect in γc-deficient mice, these data also indicate that human γc can cooperate with the distinctive cytokine receptor chains such as IL-2Rβ and IL-7R to mediate responses to murine cytokines in vivo.

The Use of Granulocyte Colony-Stimulating Factor During Retroviral Transduction on Fibronectin Fragment CH-296 Enhances Gene Transfer Into Hematopoietic Repopulating Cells in Dogs

A competitive repopulation assay in the dog was used to develop improved gene transfer protocols for hematopoietic stem cell gene therapy. Using this assay, we previously showed improved gene transfer into canine hematopoietic repopulating cells when CD34-enriched marrow cells were cocultivated on gibbon ape leukemia virus (GALV)–based retrovirus vector-producing cells. In the present study, we have investigated the use of fibronectin fragment CH-296 and 2 growth factor combinations to further improve gene transfer efficiency. CD34-enriched marrow cells from each dog were prestimulated for 24 hours and then divided into 3 equal fractions. Two fractions were placed into flasks coated with either CH-296 or bovine serum albumin (BSA) and virus-containing medium supplemented with growth factors, and protamine sulfate was replaced 4 times over a 48-hour period. One fraction was cocultivated on irradiated PG13 (GALV-pseudotype) packaging cells for 48 hours. In 2 animals, cells of the different fractions were transduced in the presence of human FLT-3 ligand (FLT3L), canine stem cell factor (cSCF), and human megakaryocyte growth and development factor (MGDF), and in 2 other dogs, transduction was performed in the presence of FLT3L, cSCF, and canine granulocyte-colony stimulating factor (cG-CSF). The vectors used contained small sequence differences, allowing differentiation of cells genetically marked by the different vectors. After transduction, nonadherent and adherent cells from all 3 fractions were pooled and infused into lethally irradiated dogs. Polymerase chain reaction and Southern blot analysis were used to determine the persistence of the transferred vectors in the peripheral blood and marrow cells after transplantation. The highest levels of gene transfer were obtained when cells were transduced in the presence of FLT3L, cSCF, and cG-CSF (gene transfer levels of more than 10% for more than 8 months so far). Compared with the 2 animals that received cells transduced with FLT3L, cSCF, and MGDF, gene transfer levels were significantly higher when dogs received cells that were transduced in the presence of cG-CSF. Transduction on CH-296 resulted in gene transfer levels that were at least as high as transduction by cocultivation. In summary, the overall levels of gene transfer obtained with these conditions should be sufficiently high to allow stem cell gene therapy studies aimed at correcting genetic diseases in dogs as a model for human gene therapy.