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.

Expansion and transduction of nonenriched human cord blood cells using HS-5 conditioned medium and FLT3-L

Cord blood (CB) stem cell transplantations have been associated with delayed hematopoietic engraftment. This has most likely been due to the limited numbers of hematopoietic short-term repopulating cells in CB. Ex vivo expansion of CB has been attempted, and expansion of CD34-enriched CB has been successful; however, CD34 enrichment procedures are in general associated with substantial cell loss. Thus, we have studied culture conditions for expansion of nonenriched CB. Nonenriched CB cells were cultured for 21 days in the presence of conditioned medium from the HS-5 stromal cell line and FLT3-L or alternatively in the presence of FLT3-L, stem cell factor (SCF), megakaryocite growth and development factor (MGDF), and granulocyte colony-stimulating factor (G-CSF) (FSMG), either on fibronectin fragment CH-296-coated dishes or on uncoated dishes. With all four culture conditions, the number of mononuclear cells initially decreased until day 7 and then increased until the end of the expansion cultures. Overall expansion using HS-5 and FLT3-L resulted in superior expansion of MNC and CFU-C (44-/34-fold) for both cultures with and without CH-296 compared to FSMG (18-/17-fold). Expansion on CH-296 was less efficient than expansion on tissue culture-treated wells without CH-296 for both conditions. We then studied the best time for transduction on nonenriched CB. In contrast to enriched CD34 cells, we found for both conditions, HS-5/FLT3-L and growth factor cocktail, higher transduction efficiencies when cells were transduced on day 7 as compared to day 2. Gene transfer rates up to 45% were achieved with both conditions, which corresponded with the increased number of cells in S phase on day 7 compared to day 2. We conclude that HS-5 and FLT-3L allow efficient expansion and transduction of nonenriched CB.

Highly efficient gene transfer into preterm CD34 hematopoietic progenitor cells

OBJECTIVE

Retrovirus-mediated gene transfer has been shown to transduce CD34(+) cells from term gestation umbilical cord blood with relatively high efficiency. The purpose of this study was to compare the efficiencies of retrovirus-mediated gene transfer into early (23-28 weeks' gestation) and term (37-41 weeks' gestation) umbilical cord blood CD34(+) hematopoietic progenitor cells.

STUDY DESIGN

CD34(+) cells were purified from cyropreserved early (23-28 weeks' gestation) and term (37-40 weeks' gestation) umbilical cord blood specimens with fluorescence-activated cell sorting. The CD34(+) cells were then transduced in virus-containing medium (gibbon ape leukemia virus pseudotype vector LAPSN [PG13]) in wells coated with the recombinant human fibronectin fragment CH-296 and in the presence of multiple hematopoietic growth factors (interleukin 6, stem cell factor, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and megakaryocyte growth and development factor) and protamine sulfate. The LAPSN (PG13) virus-containing medium was changed every 12 hours for 48 hours, after which time colony-forming cells were assayed in soft agar. The gibbon ape leukemia virus pseudotype vector LAPSN (PG13) contains the human placental alkaline phosphatase and neomycin phosphotransferase (neo ) genes. The efficiency of gene transfer was assessed by histochemical staining of colony-forming cells in agar for expression of heat-stable alkaline phosphatase.

RESULTS

Gene transfers, as assessed by alkaline phosphatase staining of colony-forming cells (granulocyte-macrophage colony-forming units and erythroid burst-forming units), were similar for CD34(+) hematopoietic progenitor cells from early (58.4% +/- 11.8%) and term (63.2% +/- 12.5%) gestation fetal umbilical cord blood.

CONCLUSION

CD34(+) hematopoietic progenitor cells from midgestation fetal blood can be transduced with high efficiency using techniques optimized for postnatal samples with a gibbon ape leukemia virus pseudotype vector. The early fetus may be a preferable target for gene therapy because of the higher number of circulating CD34(+) and CD38(-) cells relative to term cord blood, their greater proliferative capacity, and the rapid expansion of the fetal hematopoietic system that occurs from the second trimester to delivery. Because in vitro studies of gene transfer into hematopoietic progenitor cells and long-term culture-initiation cells have not been predictive of the efficiency of gene transfer into marrow-repopulating cells in vivo, studies that examine clinically applicable approaches to in utero gene therapy in appropriate animal models are still needed.

Progress towards hematopoietic stem cell gene therapy

The introduction of recombinant genetic material into human cells for therapeutic purposes offers tremendous potential. However, almost from the beginning, the application of gene therapy has been characterized by the striking discrepancy between its promise and realization. Over the past 15 years, much has been learned about the various gene transfer systems and the requirements for efficient hematopoietic stem cell gene transfer. In the current review, we will summarize recent improvements in hematopoietic stem cell gene transfer, describe some of the promising results from recent clinical applications and the impediments that remain.

Expression of herpes simplex virus ICP47 and human cytomegalovirus US11 prevents recognition of transgene products by CD8(+) cytotoxic T lymphocytes

The in vivo persistence of gene-modified cells may be limited by the development of a host immune response to vector-encoded proteins. Herpesviruses evade cytotoxic T-lymphocyte (CTL) recognition by expressing genes which interfere selectively with presentation of viral antigens by class I major histocompatibility complex (MHC) molecules. Here, we studied the use of retroviral vectors encoding herpes simplex virus ICP47, human cytomegalovirus (HCMV) US3, or HCMV US11 to decrease presentation of viral proteins and transgene products to CD8(+) CTL. Human fibroblasts and T cells transduced to express the ICP47, US3, or US11 genes alone exhibited a decrease in cell surface class I MHC expression. The combination of ICP47 and US11 rendered fibroblasts negative for surface class I MHC and allowed a class I MHC-low population of T cells to be sorted by flow cytometry. Fibroblasts and T cells expressing both ICP47 and US11 were protected from CTL-mediated lysis and failed to stimulate specific memory T-cell responses to transgene products in vitro. Our findings suggest that expression of immunoregulatory viral gene products could be a potential strategy to prolong transgene expression in vivo.

Differential engraftment of genetically modified CD34(+) and CD34(-) hematopoietic cell subsets in lethally irradiated baboons

OBJECTIVE

To test gibbon ape leukemia virus (GALV) pseudotype vector transduction of marrow subpopulations that contribute to hematopoietic reconstitution in vivo.

MATERIALS AND METHODS

Autologous CD34(+) Lin(-), CD34(+) Lin(+), and CD34(-) Lin(-) marrow cells, transduced by coculture with PG13/LN, PG13/LNX, and PG13/LNY vector-producing cells, respectively, were transplanted in three female baboons. Two female baboons also were transplanted with fresh allogeneic CD34(-)Lin(-) marrow cells from MHC-matched male siblings and, to ensure survival, with autologous CD34(+)Lin(-) and CD34(+)Lin(+) marrow cells transduced with PG13/LN and PG13/LNX, respectively. The LN, LNX, and LNY vectors are identical except for different length sequences at the 3' end of the bacterial neomycin phosphotransferase (neo) gene.

RESULTS

LN(+) and LNX(+) cells from CD34(+)Lin(-) and CD34(+)Lin(+) cells, respectively, but no LNY(+) from CD34(-)Lin(-) cells were detectable in blood and marrow of all animals after transplant. LN(+), CD34(+)Lin(-) cells contributed to reconstitution of the T, B, and myeloid lineages. LNX(+), CD34(+)Lin(+) cells contributed only to B and myeloid lineages. Male cells, CD34(-)Lin(-), were detected by polymerase chain reaction in blood and marrow of the two allogeneic transplanted animals at estimated frequencies of </=0.001% 1 month after transplant in both animals. Male cells became undetectable in one animal and have remained detectable, with declining frequency, in the other for more than 15 months. In this animal, no male CD34(+) or colony-forming cells have been detected.

CONCLUSIONS

CD34(+)Lin(-) and CD34(+)Lin(+) marrow cells can serve as targets for GALV pseudotype retrovirus-mediated gene transfer. CD34(+)Lin(-) cells contribute to reconstitution of all hematopoietic lineages. Autologous CD34(-)Lin(-) cells were either not transduced by GALV pseudotype retrovirus vectors using current approaches or did not contribute significantly to reconstitution, as suggested by allogeneic transplants.

Stable mixed hematopoietic chimerism in dogs given donor antigen, CTLA4Ig, and 100 cGy total body irradiation before and pharmacologic immunosuppression after marrow transplant

Stable mixed chimerism can be established in dogs given a sublethal dose of 200 cGy total body irradiation (TBI) before and immunosuppression with mycophenolate mofetil (MMF) and cyclosporine (CSP) for 28 and 35 days, respectively, after dog leukocyte antigen-identical marrow transplantation. Most likely, the role of pretransplant TBI was to provide host immunosuppression, since stable mixed chimerism was also achieved in MMF/CSP-treated dogs when 450 cGy irradiation, targeted to cervical, thoracic, and upper abdominal lymph nodes, was substituted for TBI. When TBI was reduced from 200 to 100 cGy, all grafts were rejected within 3 to 12 weeks. Here, we asked whether stable engraftment after 100 cGy TBI could be accomplished by first reducing the intensity of host immune responsiveness with help of the fusion peptide CTLA4Ig, which blocks T-cell costimulation through the B7-CD28 signal pathway. Accordingly, recipient T cells were activated with intravenous (IV) injections of 10(6) donor peripheral blood mononuclear cells (PBMC)/kg per day on days -7 to -1 before 100 cGy TBI, with concurrent administration of CTLA4Ig 4 mg/kg/d IV. All 7 dogs so treated showed initial mixed chimerism. Two rejected their allografts after 8 and 20 weeks, respectively, and survived with autologous marrow recovery; 1 mixed chimera was unevaluable because of death at 3 weeks from intussusception; and 4 showed persisting mixed chimerism, including unirradiated marrow and lymph node spaces, for now more than 46 to 70 weeks after transplant. Data support the hypothesis that stable marrow allografts can be established by combining nonmyeloablative pretransplant host immunosuppression with posttransplant host and donor cell immunosuppression using MMF/CSP.

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.

Stable mixed hematopoietic chimerism in dog leukocyte antigen-identical littermate dogs given lymph node irradiation before and pharmacologic immunosuppression after marrow transplantation

Stable mixed donor/host hematopoietic chimerism can be accomplished in dog leukocyte antigen (DLA)-identical littermate dogs given sublethal (200 cGy) total-body irradiation (TBI) before and immunosuppression with mycophenolate mofetil (MMF) and cyclosporine (CSP) after transplant (Blood 89:3048, 1997). Studies were based on the hypothesis that drugs that prevent graft-versus-host disease (GVHD) after transplant also suppress host-versus-graft (HVG) reactions and thereby enhance engraftment. Here, we asked whether pretransplant TBI provided marrow space for the graft to home or caused host immunosuppression. To address the questions, recipients were given pretransplant irradiation to cervical, thoracic, and abdominal lymph nodes (except pelvis), DLA-identical littermate marrow grafts, and MMF/CSP posttransplant. Six dogs that received 450 cGy irradiation showed initial engraftment. Two rejected their grafts after 8 and 18 weeks, 1 died with GVHD and engraftment, and 3 are alive as mixed chimeras after 57 to 97 weeks. Four dogs given 200 cGy irradiation also showed initial engraftment, but rejected their grafts after 10 to 18 weeks. Mixed chimerism was present in nonirradiated marrow and lymph node spaces and involved granulocytes, T cells, and monocytes. While other explanations are possible, results seem consistent with the hypothesis that pretransplant radiation provides host immunosuppression, and grafts can create their own marrow space. These data set the stage for the development of novel transplant regimens that substitute immunosuppressive for cytotoxic agents.

Improved gene transfer into canine hematopoietic repopulating cells using CD34-enriched marrow cells in combination with a gibbon ape leukemia virus-pseudotype retroviral vector

We have used dogs to study gene transfer into hematopoietic stem cells, because of the applicability of results in dogs to human transplantation and the availability of canine disease models that mimic human diseases. Previously we reported successful gene transfer into canine marrow repopulating cells, however, gene transfer efficiency was low, usually below 0.1% (Kiem et al, Hum Gene Ther 1996; 7: 89). In this study we have used CD34-enriched marrow cells to study different retroviral pseudotypes for their ability to transduce canine hematopoietic repopulating cells. Cells were divided into two equal fractions that were cocultivated for 72 h with irradiated packaging cells producing vector with different retroviral pseudotypes (GALV, amphotropic or 10A1). The vectors used contained small sequence differences to allow differentiation of cells genetically marked by the different vectors. Nonadherent and adherent cells from the cultures were infused into four dogs after a myeloablative dose of 920 cGy total body irradiation. Polymerase chain reaction (PCR) analysis of DNA from peripheral blood and marrow after transplant showed that the highest gene transfer rates (up to 10%) were obtained with the GALV-pseudotype vector. Gene transfer levels have remained stable now for more than 18 months. Southern blot analysis confirmed the high gene transfer rate. Interference studies on canine D17 cells revealed that 10A1 virus behaved like an amphotropic virus and was not able to use the GALV receptor. In summary, our results show improved gene transfer into canine hematopoietic repopulating cells when CD34-enriched cells are transduced by cocultivation on a GALV-pseudotype packaging cell line in combination with a GALV-pseudotype vector. Furthermore, these results demonstrate that the monoclonal antibody to canine CD34 used in this study is able to enrich for hematopoietic repopulating cells.

Gene transfer into fetal baboon hematopoietic progenitor cells

We studied hematopoietic progenitors from fetal baboon blood, marrow, and liver at four time points (125, 140, 160, and 175 days) during the third trimester (gestation approximately 180 days) to determine if fetal baboons might be an appropriate model for in utero gene therapy of hematopoietic stem cells (HSCs). Cells were studied for expression of CD34, CD33, CD38, and HLA-DR, for progenitor content in colony-forming cell assays, and for susceptibility of CD34+ progenitors to retrovirus-mediated gene transfer. Throughout the third trimester, the frequency of CD34+ progenitors in blood and marrow appears to remain unchanged at approximately 0.6 and 5.0%, respectively. In liver, progenitors progressively decrease to undetectable levels by day 175. The proportion of fetal baboon bone marrow and liver CD34+ cells expressing CD38 and HLA-DR appears to increase with increasing fetal age, similar to changes reported for human cord blood CD34+ cells. In fetal baboon blood the proportion of CD34+ cells expressing CD33 appears to decrease with increasing gestational age, also similar to changes reported for human cord blood cells. Progenitors from human cord blood and baboon fetal tissues were similarly susceptible to transduction by the gibbon ape leukemia pseudotyped retroviral vector LAPSN(PG13) containing the genes for human placental alkaline phosphatase (AP) and the bacterial neomycin phosphotransferase (neo). Fetal baboon and human hematopoietic progenitor cells undergo similar phenotypic changes during the third trimester of fetal development and are similarly susceptible to retrovirus-mediated gene transfer. The fetal baboon may be a model in which approaches to mobilization and gene transfer into fetal HSCs can be studied.

Efficient transduction by an amphotropic retrovirus vector is dependent on high-level expression of the cell surface virus receptor

Transduction by murine leukemia virus-based retrovirus vectors is limited in certain cell types, particularly in nondividing cells. But transduction can be inefficient even in cells that divide rapidly. For example, exposure of 208F rat embryo fibroblasts to an excess of an amphotropic retrovirus vector encoding alkaline phosphatase results in a transduction efficiency of only about 10%, even though these cells divide rapidly. Here we show that transduction of 208F cells is limited by cell surface retrovirus receptor levels; overexpression of the amphotropic retrovirus receptor Pit2 markedly improved the transduction efficiency to 50%. To characterize receptor levels and binding affinity, we synthesized a fusion protein that joins the amino terminus of the amphotropic envelope protein to the Fc region of a human immunoglobulin G1 molecule for use in binding assays. In comparison to the parental cell line, the modified cell line showed an order of magnitude increase in binding sites of from 18,000 to 150,000 per cell. Thus, efficient transduction by an amphotropic retrovirus vector requires high-level expression of the retrovirus receptor Pit2. These results provide the rationale for further examination of the role of receptor levels in inefficient transduction, especially with regard to target cells for gene therapy, where a high transduction rate is often crucial.

Improved gene transfer into baboon marrow repopulating cells using recombinant human fibronectin fragment CH-296 in combination with interleukin-6, stem cell factor, FLT-3 ligand, and megakaryocyte growth and development factor

We have used a competitive repopulation assay in baboons to develop improved methods for hematopoietic stem cell transduction and have previously shown increased gene transfer into baboon marrow repopulating cells using a gibbon ape leukemia virus (GALV)-pseudotype retroviral vector (Kiem et al, Blood 90:4638, 1997). In this study using GALV-pseudotype vectors, we examined additional variables that have been reported to increase gene transfer into hematopoietic progenitor cells in culture for their ability to increase gene transfer into baboon hematopoietic repopulating cells. Baboon marrow was harvested after in vivo administration (priming) of stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF). CD34-enriched marrow cells were divided into two equal fractions to directly compare transduction efficiencies under different gene transfer conditions. Transduction by either incubation with retroviral vectors on CH-296-coated flasks or by cocultivation on vector-producing cells was studied in five animals; in one animal, transduction on CH-296 was compared with transduction on bovine serum albumin (BSA)-coated flasks. The highest level of gene transfer was obtained after 24 hours of prestimulation followed by 48 hours of incubation on CH-296 in vector-containing medium in the presence of multiple hematopoietic growth factors (interleukin-6, stem cell factor, FLT-3 ligand, and megakaryocyte growth and development factor). Using these conditions, up to 20% of peripheral blood and marrow cells contained vector sequences for more than 20 weeks, as determined by both polymerase chain reaction and Southern blot analysis. Gene transfer rates were higher for cells transduced on CH-296 as compared with BSA or cocultivation. In one animal, we have used a vector expressing a cell surface protein (human placental alkaline phosphatase) and have detected 10% and 5% of peripheral blood cells expressing the transduced gene 2 and 4 weeks after transplantation as measured by flow cytometry. In conclusion, the conditions described here have resulted in gene transfer rates that will allow detection of transduced cells by flow cytometry to facilitate the evaluation of gene expression. The levels of gene transfer obtained with these conditions suggest the potential for therapeutic efficacy in diseases affecting the hematopoietic system.

Canine T cells transduced with a herpes simplex virus thymidine kinase gene: a model to study effects on engraftment and control of graft-versus-host disease

BACKGROUND

Alloreactive donor T cells in marrow grafts mediate graft-versus-host disease (GVHD), but T-cell depletion has resulted in increased graft failure. Add-back of gene-modified alloreactive donor T cells could prevent graft rejection. After engraftment, in vivo depletion of those modified T cells with ganciclovir may control GVHD.

METHODS

Canine recipient-specific donor cytotoxic T lymphocytes (CTL) were retrovirally transduced with the herpes simplex virus thymidine kinase gene.

RESULTS

Gibbon ape leukemia virus-pseudotyped vector yielded primary CTL transduction efficiency of 22.9+/-9.9%. After selection and expansion, 96.7+/-0.8% of CTL expressed retrovirally transferred genes. Recipient-specific cytotoxic activity was maintained with 84.3% specific lysis. After ganciclovir treatment, herpes simplex virus thymidine kinase-transduced CTL proliferation was reduced 98.7+/-0.2% compared with controls.

CONCLUSIONS

We have demonstrated efficient ex vivo transduction, expansion, maintenance of alloreactivity, and ganciclovir-mediated ablation of canine CTL, which will permit in vivo studies in the dog, a well-established model for GVHD and engraftment.

Current and future preparative regimens for bone marrow transplantation in thalassemia

Preparative regimens for marrow allografts in thalassemia have two objectives. One is eradication of diseased marrow and the other suppression of host-versus-graft (HVG) reactions so that the allograft survives. A common regimen to accomplish these goals has combined high-dose busulfan with cyclophosphamide. Postgrafting immunosuppression with cyclosporine/methotrexate has been used for GVHD prevention. Some patients may die from regimen-related toxicity. Overall event-free survival is 75%. Occasional patients have become mixed donor/host hematopoietic chimeras and, yet, disease symptoms have abated. This has raised the possibility of developing safer and less toxic transplant programs that result in stable mixed hematopoietic chimerism. We have devised such a program in dogs consisting of a nonlethal dose of total body irradiation (200 cGy) before and a novel combination of mycophenolate mofetil and cyclosporine after transplant. Mixed donor/host chimerism (> or = 50% donor cells in all lineages) has persisted for > 80 weeks, even though immunosuppression was discontinued after five weeks.

Efficient serum-free retroviral gene transfer into primitive human hematopoietic progenitor cells by a defined, high-titer, nonconcentrated vector-containing medium

Defined serum-free conditions have great conceptual advantages for the biological safety and standardization of clinical gene transfer into hematopoietic stem cells. In the only study reported to date, Sekhar et al. achieved low serum conditions by a complex concentration procedure of a retroviral supernatant initially containing 10% fetal bovine serum. The high cost, small volume, possible coenrichment of serum-derived pathogens, limited recovery of vector particles, and low titer of the final diluted medium restrict the clinical application of this procedure. Transduction of primitive hematopoietic progenitor cells was not demonstrated. In the present study, a defined serum-free medium containing high titers of the pseudotyped retroviral vector PG13/LN was generated from PG13/LN producer cells without requiring a physical enrichment procedure. The transduction of committed hematopoietic progenitor cells in the serum-free vector-containing medium was efficient, and similar to that occurring under serum-containing control conditions. The number of primitive human hematopoietic long-term culture-initiating cell-derived colonies (LTC-IC-derived colonies) generated from CD34+ and CD34+/HLA-DRlo peripheral blood progenitor "stem" cells (PBSCs) increased during 7 days of treatment in this vector-containing medium in the presence of IL-3, SCF, and flt-3 ligand. The described procedure allowed efficient transduction of LTC-IC-derived colonies generated from CD34+, CD34+/HLA-DRlo, and CD34+/CD38lo PBSCs. This is the first report to demonstrate an increase in primitive peripheral blood LTC-IC-derived colonies in vitro as well as their efficient transduction in a high-titer, serum-free vector-containing medium that can be produced exclusively from defined pharmaceutical-grade components, making it ideally suited for applications in clinical gene therapy.

Gene transfer into marrow repopulating cells: comparison between amphotropic and gibbon ape leukemia virus pseudotyped retroviral vectors in a competitive repopulation assay in baboons

Many diseases might be treated by gene therapy targeted to the hematopoietic system, but low rates of gene transfer achieved in humans and large animals have limited the application of this technique. We have developed a competitive hematopoietic repopulation assay in baboons to evaluate methods for improving gene transfer and have used this method to compare gene transfer rates for retroviral vectors having an envelope protein (pseudotype) from amphotropic murine retrovirus with similar vectors having an envelope protein derived from gibbon ape leukemia virus (GALV). We hypothesized that vectors with a GALV pseudotype might perform better based on our previous work with cultured human hematopoietic cells. CD34(+) marrow cells from each of four untreated baboons were divided into two equal portions that were cocultivated for 48 hours with packaging cells producing equivalent titers of either amphotropic or GALV pseudotyped vectors containing the neo gene. The vectors contained small sequence differences to allow differentiation of cells genetically marked by the different vectors. Nonadherent and adherent cells from the cultures were infused into animals after they received a myeloablative dose of total body irradiation. Polymerase chain reaction (PCR) analysis for neo gene-specific sequences in colony-forming unit-granulocyte-macrophage from cell populations used for transplant showed gene transfer rates of 2.7%, 7.1%, <15%, and 3.9% with the amphotropic vectors and 7.1%, 11.3%, <15%, and 26.4% with the GALV pseudotyped vector. PCR analysis of peripheral blood and marrow cells after engraftment showed the neo gene to be present in all four animals analyzed at levels between 0.1% and 5%. Overall gene transfer efficiency was higher with the GALVpseudotyped vector than with the amphotropic vectors. Southern blot analysis in one animal confirmed a gene transfer efficiency of between 1% and 5%. The higher gene transfer efficiency with the GALV-pseudotyped vector correlated with higher levels of GALV receptor RNA compared with the amphotropic receptor in CD34(+) hematopoietic cells. These results show that GALV-pseudotyped vectors are capable of transducing baboon marrow repopulating cells and may allow more efficient gene transfer rates for human gene therapy directed at hematopoietic cells. In addition, our data show considerable differences in gene transfer efficiency between individual baboons, suggesting that a competitive repopulation assay will be critical for evaluation of methods designed to improve gene transfer into hematopoietic stem cells.

Efficient gene transfer in primitive CD34+/CD38lo human bone marrow cells reselected after long-term exposure to GALV-pseudotyped retroviral vector

Successful retroviral gene transfer into human hematopoietic stem cells was demonstrated in preliminary clinical trials at low efficiency. We have shown previously that gene transfer into committed hematopoietic progenitor cells is more efficient using a gibbon ape leukemia virus (GALV)-pseudotyped retroviral vector instead of an amphotropic retroviral vector. Here, we have conducted a systematic study of human hematopoietic progenitor cells after extended transduction with a GALV-pseudotyped retroviral vector. CD34+/CD38lo Cells were transduced for 5 days and reselected according to phenotype after culture and analyzed for cell cycle status, long-term culture-initiating cell (LTC-IC) activity, and gene transfer. Reselection of rare, very primitive progenitor cells was successful. Equal to fresh CD34+/CD38lo cells, >90% of reselected CD34+/CD38lo cells were in G0/G1. CD34+/CD38lo reselection enriched for LTC-IC (10-fold), as compared to freshly isolated CD34+/CD38lo cells with excellent specificity (82.7% of total LTC-IC were recovered in the reselected CD34+/CD38lo population) and recovery (62% of initial LTC-IC number in CD34+/CD38lo cells were recovered in the reselected fraction after transduction). Gene transfer into primitive progenitor cells was efficient with 50.5% G418-resistant LTC-IC colonies and more than 40 copies of vector provirus detectable per 100 nuclei of CD34+/CD38lo cells. To our knowledge, this is the first systematic analysis of phenotype, function, and cell cycle demonstrating retroviral gene transfer into rare, very primitive human hematopoietic progenitor cells. The chosen strategy should be of considerable value for analyzing and improving gene therapy of the hematopoietic system.

Effect of recombinant canine stem cell factor, a c-kit ligand, on hematopoietic recovery after DLA-identical littermate marrow transplants in dogs

We studied the effect of recombinant canine stem cell factor (rcSCF) on hematopoietic recovery, incidence of graft failure, graft-vs.-host disease (GVHD), and survival after marrow transplantation from dog leukocyte antigen (DLA)-identical canine littermates. Ten animals received 100 microg rcSCF/kg/day b.i.d. by subcutaneous injection on days 1 through 10 after 920 cGy total body irradiation and transplantation of a mean of 3.7x10(8) marrow cells/kg body weight. None of the dogs received GVHD prophylaxis. All animals showed hematopoietic engraftment. The median number of days to achieve 1000 neutrophils/mm3 was 9; 100 monocytes/mm3 were reached after 15 days, 500 lymphocytes/mm3 after 21 days, and 20,000 platelets/mm3 after 16 days. One animal developed GVHD involving skin, gut, and liver and died of bacterial pneumonia 21 days after transplantation. The remaining nine dogs were observed for a median of 37 days (range 29-84 days) posttransplantation until they were killed. Facial edema was seen in three dogs during the first 2-3 days of rcSCF administration. These results show that within the limits of this study it appears to be safe to administer SCF after DLA-identical littermate marrow transplants in dogs. Comparison with previously published data in the same model showed that neutrophil and monocyte recovery was significantly faster in dogs receiving SCF treatment compared with dogs without growth factor treatment (recovery to achieve 1000 neutrophils/mm3: median 9 days vs. 13 days, p = 0.002; recovery to 100 monocytes/mm3: median 15 days vs. 105 days, p = 0.0002). Otherwise, no significant differences were seen. Results obtained with SCF treatment were similar to those previously obtained in the same model with recombinant human granulocyte colony-stimulating factor (rhG-CSF) treatment except that recovery of lymphocytes to 500/mm3 appeared to be more rapid in G-CSF-treated dogs (median 15 days vs. 21 days, p = 0.03).

GaLV pseudotyped vectors and cationic lipids transduce human CD34+ cells

High transduction frequency of hematopoietic stem/progenitor cells is essential to derive clinical benefits for treating certain inherited and acquired diseases. We demonstrate here stable gene transfer into human bone marrow-derived CD34+ progenitors using cationic lipids to facilitate GaLV and amphotroic pseudotyped retroviral-mediated transductions. Furthermore, the transgene was detected only in the progeny of flow cytometer sorted CD34+ population transduced by the LAPSN (PG13) viral vector in the presence of cationic lipids but not when transduction was facilitated with conventional polycations Polybrene or protamine sulfate. Thus, a combination of GaLV pseudotyped vectors and cationic lipids results in increased transduction frequencies of the CD34+ cells without a requirement of extended in vitro culture, or co-cultivation with producer cell lines. These improvements may result in the production of therapeutically significant quantities of genetically modified hematopoietic cells.

Stable mixed hematopoietic chimerism in DLA-identical littermate dogs given sublethal total body irradiation before and pharmacological immunosuppression after marrow transplantation

Postgrafting cyclosporine (CSP) given for 35 days resulted in establishment of stable marrow grafts from DLA-identical canine littermates after otherwise suboptimal, but nevertheless, lethal conditioning with 450 cGy of total body irradiation (TBI). We now asked whether sustained allografts could be achieved after sublethal TBI or without TBI. Five groups of recipients were studied. Dogs in group 1 were given 200 cGy TBI and postgrafting CSP, 15 mg/kg twice daily by mouth on days -1 to 35 posttransplant. Dogs in group 2 were given 200 cGy TBI and methotrexate (MTX), 0.4 mg/kg intravenously (I.V.) on days 1, 3, 6, and 11 along with CSP. Dogs in group 3 were given 200 cGy TBI and CSP along with mycophenolate mofetil (MMF), 10 mg/kg twice daily subcutaneously (S.C.) on days 0 to 27 after transplant, a novel immunosuppressive combination. Dogs in group 4 were given 100 cGy TBI and MMF/CSP. Dogs in group 5 were not given TBI and they received MMF/CSP posttransplant. Allografts were assessed by (Ca)n dinucleotide repeat polymorphism studies in cells from peripheral blood, lymph nodes, and marrow. Dogs in group 1 had transient mixed donor-host hematopoietic chimerism for no more than 4 weeks. Three of six dogs in group 2 had transient mixed chimerism for 3 to 11 weeks, and three have remained stable mixed chimeras for up to 60 weeks now. Four of five dogs in group 3 have remained stable mixed chimeras for 54 to 57 weeks now, while one lost the allograft after 12 weeks. All dogs in groups 4 and 5 rejected their allografts after 2 to 12 weeks. In summary, the establishment of stable mixed hematopoietic chimerism following nonmyelosuppressive and nontoxic conditioning programs has remained a difficult goal. Here we present evidence in a large random-bred animal species that this goal may be achievable with pharmacological immunosuppression postgrafting, capable of inhibiting both host-versus-graft (HVG) and graft-versus-host (GVH) reactions in the setting of DLA-identical grafts.

Human CD34+ cells do not express glutathione S-transferases alpha

The expression of glutathione S-transferases alpha (GST alpha) in human hematopoietic CD34+ cells and bone marrow was studied using RT-PCR and immunoblotting. The GSTA1 protein conjugates glutathione to the stem cell selective alkylator busulfan. This reaction is the major pathway of elimination of the compound from the human body. Human hematopoietic CD34+ cells and bone marrow do not express GSTA1 message, which was present at a high level in liver, an organ relatively resistant to busulfan toxicity in comparison to bone marrow. Similarly, baboon CD34+ cells and dog bone marrow do not express GSTA1. Human GSTA1 may be useful as a chemoprotective selectable marker in human stem cell gene therapy.