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In our previous phase 1/2 study aimed at controlling graft-versus-host disease, 12 patients received Herpes simplex virus thymidine kinase (HSV-tk(+))/neomycin phosphotransferase (NeoR(+))-expressing donor gene-modified T cells (GMCs) and underwent an HLA-identical sibling T-cell-depleted bone marrow transplantation (BMT). This study's objective was to follow up, to quantify, and to characterize persistently circulating GMCs more than 10 years after BMT. Circulating GMCs remain detectable in all 4 evaluable patients. However, NeoR- and HSV-tk-polymerase chain reaction (PCR) differently quantified in vivo counts, suggesting deletions within the HSV-tk gene. Further experiments, including a novel "transgene walking" PCR method, confirmed the presence of deletions. The deletions were unique, patient-specific, present in most circulating GMCs expressing NeoR, and shown to occur at time of GMC production. Unique patient-specific retroviral insertion sites (ISs) were found in all GMCs capable of in vitro expansion/cloning as well. These findings suggest a rare initial gene deletion event and an in vivo survival advantage of rare GMC clones resulting from an anti-HSV-tk immune response and/or ganciclovir treatment. In conclusion, we show that donor mature T cells infused with a T-cell-depleted graft persist in vivo for more than a decade. These cells, containing transgene deletions and subjected to significant in vivo selection, represent a small fraction of T cells infused at transplantation.

Genetically modified donor T cells with an inducible "suicide" gene have the potential to improve the safety and availability of allogeneic hematopoietic stem cell transplantation by enhancing engraftment and permitting control of graft-versus-host disease (GVHD). However, several clinical studies of gene-modified T cells have shown limited to no in vivo function of the ex vivo expanded T cells. Using the well-established dog model of allogeneic marrow transplantation, the question was asked if retrovirally transduced, donor derived, ex vivo expanded cytotoxic T lymphocytes (CTLs) that are recipient specific could enhance engraftment of dog leukocyte antigen (DLA)-haploidentical marrow following a single dose of 9.2 Gy total body irradiation and no postgrafting immunosuppression. In this setting, only 4 of 11 control recipients of DLA-haploidentical marrow without added CTLs engrafted. CTLs did not enhance engraftment of CD34(+) selected peripheral blood stem cells. However, recipient-specific CTLs enhanced engraftment of DLA-haploidentical marrow in 9 of 11 evaluable recipients (P =.049). All dogs that engrafted developed multiorgan GVHD. To facilitate in vivo tracking, 8 dogs received CTLs transduced with a retroviral vector encoding green fluorescent protein (GFP) and neomycin phosphotransferase (neo). Recipients that engrafted had sharp increases in the numbers of circulating GFP(+) CTLs on days +5 to +6 after transplantation. GFP(+) CTLs isolated from blood were capable of recipient-specific lysis. At necropsy, up to 7.1% of CD3(+) cells in tissues were GFP(+) and polymerase chain reaction in situ hybridization for neo showed infiltration of transduced CTLs in GVHD-affected organs. These results show that ex vivo expanded, transduced T cells maintained in vivo function and enhanced marrow engraftment.

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.

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin(-) CD34(+)CD38(-) cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin(-)CD34(+) CD38(-) cells and SRC, CD34(+)-enriched lineage-depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34(+)CD38(-) cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP(+) human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34(+)CD38(-) phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34(+)CD38(-) phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34(+)CD38(+) cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)

The possibility of primitive hematopoietic cell ex vivo expansion is of interest for both gene therapy and transplantation applications. The engraftment of autologous rhesus peripheral blood (PB) progenitors expanded 10 to 14 days were tracked in vivo using genetic marking. Stem cell factor (SCF)/granulocyte colony-stimulating factor (G-CSF)-mobilized and CD34-enriched PB cells were divided into two equal aliquots and transduced with one of two retroviral vectors carrying the neomycin-resistance gene (neo) for 4 days in the presence of interleukin-3 (IL-3), IL-6, and SCF in the first 5 animals, IL-3/IL-6/SCF/Flt-3 ligand (FLT) in 2 subsequent animals, or IL-3/IL-6/SCF/FLT plus an autologous stromal monolayer (STR) in the final 2. At the end of transduction period, one aliquot (nonexpanded) from each animal was frozen, whereas the other was expanded under the same conditions but without vector for a total of 14 days before freezing. After total body irradiation, both the nonexpanded and expanded transduced cells were reinfused. Despite 5- to 13-fold higher cell and colony-forming unit (CFU) doses from the expanded fraction of marked cells, there was greater short- and long-term marking from the nonexpanded cells in all animals. In animals receiving cells transduced and expanded in the presence of IL-3/IL-6/SCF/FLT, engraftment by the marked expanded cells was further diminished. This discrepancy was even more pronounced in the animals who received cells transduced and expanded in the presence of FLT and autologous stroma, with no marking detectable from the expanded cells. Despite lack of evidence for expansion of engrafting cells, we found that the addition of FLT and especially STR during the initial brief transduction period increased engraftment with marked cells into a clinically relevant range. Levels of marked progeny cells originating from the nonexpanded aliqouts were significantly higher than that seen in previous 4 animals receiving cells transduced in the presence of IL-3/IL-6/SCF, with levels of 10% to 20% confirmed by Southern blotting from the nonexpanded IL-3/IL-6/SCF/FLT/STR graft compared with 0.01% in the original IL-3/IL-6/SCF cohort. These results suggest that, although expansion of PB progenitors is feasible ex vivo, their contribution towards both short- and long-term engraftment is markedly impaired. However, a brief transduction in the presence of specific cytokines and stromal support allows engraftment with an encouraging number of retrovirally modified cells.

Primitive hematopoietic progenitor cells (HPCs) are potential targets for treatment of numerous hematopoietic diseases using retroviral-mediated gene transfer (RMGT). To achieve high efficiency of gene transfer into primitive HPCs, a delicate balance between cellular activation and proliferation and maintenance of hematopoietic potential must be established. We have demonstrated that a subpopulation of human bone marrow (BM) CD34(+) cells, highly enriched for primitive HPCs, persists in culture in a mitotically quiescent state due to their cytokine-nonresponsive (CNR) nature, a characteristic that may prevent efficient RMGT of these cells. To evaluate and possibly circumvent this, we designed a two-step transduction protocol using neoR-containing vectors coupled with flow cytometric cell sorting to isolate and examine transduction efficiency in different fractions of cultured CD34(+) cells. BM CD34(+) cells stained on day 0 (d0) with the membrane dye PKH2 were prestimulated for 24 hours with stem cell factor (SCF), interleukin-3 (IL-3), and IL-6, and then transduced on fibronectin with the retroviral vector LNL6 on d1. On d5, half of the cultured cells were transduced with the retroviral vector G1Na and sorted on d6 into cytokine-responsive (d6 CR) cells (detected via their loss of PKH2 fluorescence relative to d0 sample) and d6 CNR cells that had not divided since d0. The other half of the cultured cells were first sorted on d5 into d5 CR and d5 CNR cells and then infected separately with G1Na. Both sets of d5 and d6 CR and CNR cells were cultured in secondary long-term cultures (LTCs) and assayed weekly for transduced progenitor cells. Significantly higher numbers of G418-resistant colonies were produced in cultures initiated with d5 and d6 CNR cells compared with respective CR fractions (P < .05). At week 2, transduction efficiency was comparable between d5 and d6 transduced CR and CNR cells (P > .05). However, at weeks 3 and 4, d5 and d6 CNR fractions generated significantly higher numbers of neoR progenitor cells relative to the respective CR fractions (P < .05), while no difference in transduction efficiency between d5 and d6 CNR cells could be demonstrated. Polymerase chain reaction (PCR) analysis of the origin of transduced neoR gene in clonogenic cells demonstrated that mature progenitors (CR fractions) contained predominantly LNL6 sequences, while more primitive progenitor cells (CNR fractions) were transduced with G1Na. These results demonstrate that prolonged stimulation of primitive HPCs is essential for achieving efficient RMGT into cells capable of sustaining long-term in vitro hematopoiesis. These findings may have significant implications for the development of clinical gene therapy protocols.