No discrepancy between in vivo gene marking efficiency assessed in peripheral blood populations compared with bone marrow progenitors or CD34+ cells

Efficient lentiviral gene transfer to canine repopulating cells using an overnight transduction protocol

The use of lentiviral vectors for the transduction of hematopoietic stem cells has evoked much interest owing to their ability to stably integrate into the genome of nondividing cells. However, published large animal studies have reported highly variable gene transfer rates of typically less than 1%. Here we report the use of lentiviral vectors for the transduction of canine CD34+ hematopoietic repopulating cells using a very short, 18-hour transduction protocol. We compared lentiviral transduction of hematopoietic repopulating cells from either stem cell factor (SCF)– and granulocyte-colony stimulating factor (G-CSF)–primed marrow or mobilized peripheral blood in a competitive repopulation assay in 3 dogs. All dogs engrafted rapidly within 9 days. Transgene expression was detected in all lineages (B cells, T cells, granulocytes, and red blood cells as well as platelets) indicating multilineage engraftment of transduced cells, with overall long-term marking levels of up to 12%. Gene transfer levels in mobilized peripheral blood cells were slightly higher than in primed marrow cells. In conclusion, we show efficient lentiviral transduction of canine repopulating cells using an overnight transduction protocol. These results have important implications for the design of stem cell gene therapy protocols, especially for those diseases in which the maintenance of stem cells in culture is a major limitation.

Highly efficient gene transfer into baboon marrow repopulating cells using GALV-pseudotype oncoretroviral vectors produced by human packaging cells

Vector-containing medium harvested from murine packaging cell lines has been shown to contain factors that can negatively influence the transduction and maintenance of hematopoietic stem cells. Thus, we generated a human packaging cell line with a gibbon ape leukemia virus pseudotype (Phoenix-GALV), and we evaluated vectors produced by Phoenix-GALV for their ability to transduce hematopoietic progenitor/stem cells. In 3 baboons, we used a competitive repopulation assay to directly compare GALV-pseudotype retrovirus vectors produced by either Phoenix-GALV or by the NIH 3T3-derived packaging cell line, PG13. In 3 additional baboons we compared Phoenix-GALV-derived vectors to more recently developed lentiviral vectors. Gene transfer efficiency into hematopoietic repopulating cells was assessed by evaluating the number of genetically modified peripheral blood and marrow cells using flow cytometry and real-time polymerase chain reaction. Transduction efficiency of hematopoietic repopulating cells was significantly higher using the Phoenix-GALV-derived vector as compared with the PG13-derived vectors or lentiviral vectors, with stable transduction levels up to 25%. We followed 2 animals for more than one year. Flow cytometric analysis of hematopoietic subpopulations in these animals revealed transgene expression in CD13(+) granulocytes, CD20(+) B lymphocytes, CD3(+) T lymphocytes, CD61(+) platelets, as well as red blood cells, indicating multilineage engraftment of cells transduced by Phoenix-GALV-pseudotype vectors. In addition, transduction of human CD34(+) cells was significantly more efficient than transduction of baboon CD34(+) cells, suggesting that Phoenix-GALV-derived oncoretroviral vectors may be even more efficient in human stem cell gene therapy applications.

Introduction of the green fluorescent protein gene into hematopoietic stem cells results in prolonged discrepancy of in vivo transduction levels between bone marrow progenitors and peripheral blood cells in nonhuman primates

BACKGROUND

The green fluorescent protein (GFP) has proven a useful marker in retroviral gene transfer studies targeting hematopoietic stem cells (HSCs) in mice. However, several investigators have reported very low in vivo peripheral blood marking levels in nonhuman primates after transplantation of HSCs transduced with the GFP gene. We retrovirally marked cynomolgus monkey HSCs with the GFP gene, and tracked in vivo marking levels within both bone marrow progenitor cells and mature peripheral blood cells following autologous transplantation after myeloablative conditioning.

METHODS

Bone marrow cells were harvested from three cynomolgus macaques and enriched for the primitive fraction by CD34 selection. CD34(+) cells were transduced with one of three retroviral vectors all expressing the GFP gene and were infused after myeloablative total body irradiation (500 cGy x 2). Following transplantation, proviral levels and fluorescence were monitored among clonogenic bone marrow progenitors and mature peripheral blood cells.

RESULTS

Although 13-37% of transduced cells contained the GFP provirus and 11-13% fluoresced ex vivo, both provirus and fluorescence became almost undetectable in the peripheral blood within several months after transplantation regardless of the vectors used. However, on sampling of bone marrow at multiple time points, significant fractions (5-10%) of clonogenic progenitors contained the provirus and fluoresced ex vivo reflecting a significant discrepancy between GFP gene marking levels within bone marrow cells and their mature peripheral blood progeny. The discrepancy (at least one log) persisted for more than 1 year after transplantation. Since no cytotoxic T lymphocytes against GFP were detected in the animals, an immune response against GFP is an unlikely explanation for the low levels of transduced peripheral blood cells. Administration of granulocyte colony stimulating factor and stem cell factor resulted in mobilization of transduced bone marrow cells detectable as mature granulocyte progeny which expressed the GFP gene, suggesting that transduced progenitor cells in bone marrow could be mobilized into the peripheral blood and differentiated into granulocytes.

CONCLUSIONS

Low levels of GFP-transduced mature cells in the peripheral blood of nonhuman primates may reflect a block to differentiation associated with GFP; this block can be overcome in part by nonphysiological cytokine treatment ex vivo and in vivo.

Umbilical cord blood cells capable of engrafting in primary, secondary, and tertiary xenogeneic hosts are preserved after ex vivo culture in a noncontact system

This report describes stroma-based and stroma-free cultures that maintain long-term engrafting hematopoietic cells for at least 14 days ex vivo. Umbilical cord blood (UCB) CD34(+) cells were cultured in transwells above AFT024 feeders with fetal-liver-tyrosine-kinase (FL) + stem cell factor (SCF) + interleukin 7 (IL-7), or FL + thrombopoietin (Tpo). CD34(+) progeny were transplanted into nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice or preimmune fetal sheep. SCID repopulating cells (SRC) with multilineage differentiation potential were maintained in FL-SCF-IL-7 or FL-Tpo containing cultures for up to 28 days. Marrow from mice highly engrafted with uncultured or expanded cells induced multilineage human hematopoiesis in 50% of secondary but not tertiary recipients. Day 7 expanded cells engrafted primary, secondary, and tertiary fetal sheep. Day 14 expanded cells, although engrafting primary and to a lesser degree secondary fetal sheep, failed to engraft tertiary recipients. SRC that can be transferred to secondary recipients were maintained for at least 14 days in medium containing glycosaminoglycans and cytokines found in stromal supernatants. This is the first demonstration that ex vivo culture in stroma-noncontact and stroma-free cultures maintains "long-term" engrafting cells, defined by their capacity to engraft secondary or tertiary hosts. (Blood. 2001;97:3441-3449)

Lentivirus vector-mediated hematopoietic stem cell gene transfer of common gamma-chain cytokine receptor in rhesus macaques

Nonhuman primate model systems of autologous CD34+ cell transplant are the most effective means to assess the safety and capabilities of lentivirus vectors. Toward this end, we tested the efficiency of marking, gene expression, and transplant of bone marrow and peripheral blood CD34+ cells using a self-inactivating lentivirus vector (CS-Rh-MLV-E) bearing an internal murine leukemia virus long terminal repeat derived from a murine retrovirus adapted to replicate in rhesus macaques. In vitro cytokine stimulation was not required to achieve efficient transduction of CD34+ cells resulting in marking and gene expression of the reporter gene encoding enhanced green fluorescent protein (EGFP) following transplant of the CD34+ cells. Monkeys transplanted with mobilized peripheral blood CD34+ cells resulted in EGFP expression in 1 to 10% of multilineage peripheral blood cells, including red blood cells and platelets, stable for 15 months to date. The relative level of gene expression utilizing this vector is 2- to 10-fold greater than that utilizing a non-self-inactivating lentivirus vector bearing the cytomegalovirus immediate-early promoter. In contrast, in animals transplanted with autologous bone marrow CD34+ cells, multilineage EGFP expression was evident initially but diminished over time. We further tested our lentivirus vector system by demonstrating gene transfer of the human common gamma-chain cytokine receptor gene (gamma(c)), deficient in X-linked SCID patients and recently successfully used to treat disease. Marking was 0.42 and.001 HIV-1 vector DNA copy per 100 cells in two animals. To date, all EGFP- and gamma(c)-transplanted animals are healthy. This system may prove useful for expression of therapeutic genes in human hematopoietic cells.

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.

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.

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.

Many multipotential gene-marked progenitor or stem cell clones contribute to hematopoiesis in nonhuman primates

Retroviral insertion site analysis was used to track the contribution of retrovirally transduced primitive progenitors to hematopoiesis after autologous transplantation in the rhesus macaque model. CD34-enriched mobilized peripheral blood cells were transduced with retroviral marking vectors containing the neo gene and were reinfused after total body irradiation. High-level gene transfer efficiency allowed insertion site analysis of individual myeloid and erythroid colony-forming units (CFU) and of highly purified B- and T-lymphoid populations in 2 animals. At multiple time points up to 1 year after transplantation, retroviral insertion sites were identified by performing inverse polymerase chain reaction and sequencing vector-containing CFU or more than 99% pure T- and B-cell populations. Forty-eight unique insertion sequences were detected in the first animal and also in the second animal, and multiple clones contributed to hematopoiesis at 2 or more time points. Multipotential clones contributing to myeloid and lymphoid lineages were identified. These results support the concept that hematopoiesis in large animals is polyclonal and that individual multipotential stem or progenitor cells can contribute to hematopoiesis for prolonged periods. Gene transfer to long-lived, multipotent clones is shown and is encouraging for human gene therapy applications.

Many multipotential gene-marked progenitor or stem cell clones contribute to hematopoiesis in nonhuman primates

Retroviral insertion site analysis was used to track the contribution of retrovirally transduced primitive progenitors to hematopoiesis after autologous transplantation in the rhesus macaque model. CD34-enriched mobilized peripheral blood cells were transduced with retroviral marking vectors containing the neo gene and were reinfused after total body irradiation. High-level gene transfer efficiency allowed insertion site analysis of individual myeloid and erythroid colony-forming units (CFU) and of highly purified B- and T-lymphoid populations in 2 animals. At multiple time points up to 1 year after transplantation, retroviral insertion sites were identified by performing inverse polymerase chain reaction and sequencing vector-containing CFU or more than 99% pure T- and B-cell populations. Forty-eight unique insertion sequences were detected in the first animal and also in the second animal, and multiple clones contributed to hematopoiesis at 2 or more time points. Multipotential clones contributing to myeloid and lymphoid lineages were identified. These results support the concept that hematopoiesis in large animals is polyclonal and that individual multipotential stem or progenitor cells can contribute to hematopoiesis for prolonged periods. Gene transfer to long-lived, multipotent clones is shown and is encouraging for human gene therapy applications.

Long-term tracking of murine hematopoietic cells transduced with a bicistronic retrovirus containing CD24 and EGFP genes

Hematopoietic stem cells (HSCs) are attractive targets for gene therapy, but current gene transfer methodologies are inadequate for efficient HSC transduction and perpetual transgene expression. To improve gene transfer vectors and transduction protocols, it is vital to establish a system to evaluate transgene expression and the long-term behavior of transduced cells in vivo. For this purpose, we constructed a bicistronic retrovirus encoding the human CD24 (as the first cistron) and the enhanced green fluorescent protein (EGFP; as the second cistron). Murine bone marrow cells were transduced with this vector and the transgene expression was monitored along with hematopoietic reconstitution. Stable expression of CD24 and EGFP was demonstrated in the long-term repopulating cells for at least 6 months, and multi-parameter flow cytometry illustrated expression of both markers in all the lymphohematopoietic lineages examined (B and T lymphoid, erythroid and myeloid). Sustained expression was also shown in the secondary transplants for 6 months, suggesting that self-renewing HSCs were transduced by this vector. Overall, EGFP-tagged bicistronic retroviruses would provide powerful tools for detailed in vivo analysis of transduced hematopoietic cells, such as transgene expression in conjunction with lineage differentiation. Gene Therapy (2000) 7, 1193-1199.

Introduction of a xenogeneic gene via hematopoietic stem cells leads to specific tolerance in a rhesus monkey model

Host immune responses against foreign transgenes may be a major obstacle to successful gene therapy. To clarify the impact of an immune response to foreign transgene products on the survival of genetically modified cells, we studied the in vivo persistence of cells transduced with a vector expressing a foreign transgene compared to cells transduced with a nonexpressing vector in the clinically predictive rhesus macaque model. We constructed retroviral vectors containing the neomycin phosphotransferase gene (neo) sequences modified to prevent protein expression (nonexpressing vectors). Rhesus monkey lymphocytes or hematopoietic stem cells (HSCs) were transduced with nonexpressing and neo-expressing vectors followed by reinfusion, and their in vivo persistence was studied. While lymphocytes transduced with a nonexpressing vector could be detected for more than 1 year, lymphocytes transduced with a neo-expressing vector were no longer detectable within several weeks of infusion. However, five of six animals transplanted with HSCs transduced with nonexpression or neo-expression vectors, and progeny lymphocytes marked with either vector persisted for more than 2 years. Furthermore, in recipients of transduced HSCs, infusion of mature lymphocytes transduced with a second neo-expressing vector did not result in elimination of the transduced lymphocytes. Our data show that introduction of a xenogeneic gene via HSCs induces tolerance to the foreign gene products. HSC gene therapy is therefore suitable for clinical applications where long-term expression of a therapeutic or foreign gene is required.

Efficient transduction of human hematopoietic repopulating cells generating stable engraftment of transgene-expressing cells in NOD/SCID mice

In an attempt to develop efficient procedures of human hematopoietic gene therapy, retrovirally transduced CD34+ cord blood cells were transplanted into NOD/SCID mice to evaluate the repopulating potential of transduced grafts. Samples were prestimulated on Retronectin-coated dishes and infected with gibbon ape leukemia virus (GALV)-pseudotyped FMEV vectors encoding the enhanced green fluorescent protein (EGFP). Periodic analyses of bone marrow (BM) from transplanted recipients revealed a sustained engraftment of human hematopoietic cells expressing the EGFP transgene. On average, 33.6% of human CD45+ cells expressed the transgene 90 to120 days after transplantation. Moreover, 11.9% of total NOD/SCID BM consisted of human CD45+ cells expressing the EGFP transgene at this time. The transplantation of purified EGFP+ cells increased the proportion of CD45+ cells positive for EGFP expression to 57.7% at 90 to 120 days after transplantation. At this time, 18.9% and 4.3% of NOD/SCID BM consisted of CD45+/EGFP+ and CD34+/EGFP+ cells, respectively. Interestingly, the transplantation of EGFP− cells purified at 24 hours after infection also generated a significant engraftment of CD45+/EGFP+ and CD34+/EGFP+ cells, suggesting that a number of transduced repopulating cells did not express the transgene at that time. Molecular analysis of NOD/SCID BM confirmed the high levels of engraftment of human transduced cells deduced from FACS analysis. Finally, the analysis of the provirus insertion sites by conventional Southern blotting indicated that the human hematopoiesis in the NOD/SCID BM was predominantly oligoclonal.

Efficient transduction of human hematopoietic repopulating cells generating stable engraftment of transgene-expressing cells in NOD/SCID mice

In an attempt to develop efficient procedures of human hematopoietic gene therapy, retrovirally transduced CD34+ cord blood cells were transplanted into NOD/SCID mice to evaluate the repopulating potential of transduced grafts. Samples were prestimulated on Retronectin-coated dishes and infected with gibbon ape leukemia virus (GALV)-pseudotyped FMEV vectors encoding the enhanced green fluorescent protein (EGFP). Periodic analyses of bone marrow (BM) from transplanted recipients revealed a sustained engraftment of human hematopoietic cells expressing the EGFP transgene. On average, 33.6% of human CD45+ cells expressed the transgene 90 to120 days after transplantation. Moreover, 11.9% of total NOD/SCID BM consisted of human CD45+ cells expressing the EGFP transgene at this time. The transplantation of purified EGFP+ cells increased the proportion of CD45+ cells positive for EGFP expression to 57.7% at 90 to 120 days after transplantation. At this time, 18.9% and 4.3% of NOD/SCID BM consisted of CD45+/EGFP+ and CD34+/EGFP+ cells, respectively. Interestingly, the transplantation of EGFP− cells purified at 24 hours after infection also generated a significant engraftment of CD45+/EGFP+ and CD34+/EGFP+ cells, suggesting that a number of transduced repopulating cells did not express the transgene at that time. Molecular analysis of NOD/SCID BM confirmed the high levels of engraftment of human transduced cells deduced from FACS analysis. Finally, the analysis of the provirus insertion sites by conventional Southern blotting indicated that the human hematopoiesis in the NOD/SCID BM was predominantly oligoclonal.