Retroviral gene transfer in autologous bone marrow transplantation for adult acute leukemia

Cancer gene therapy update

Objective. To provide an update about gene marking and gene therapy trials in cancer patients.

Data Sources. A MEDLINE search using the term “gene therapy” was conducted for the period 1985 to 1998. The reference lists from retrieved articles were reviewed. Meeting abstracts from the American Society of Clinical Oncology annual meeting (published in their proceedings) and the Annual Cancer Gene Therapy Symposium (published in Cancer Gene Therapy) that concerned gene therapy in cancer patients were also included.

Data Extraction. Both authors reviewed the retrieved material and included preclinical data, case reports, and clinical trials related to gene transfer or gene therapy in cancer patients.

Data Synthesis. There are several possible approaches to using gene therapy for the diagnosis and treatment of cancer and for the monitoring of cancer therapy. Exogenous genes may be used to mark cells to help better understand cancer biology or may be used directly for cancer treatment. Gene-marking trials have already provided new information about cancer biology and have demonstrated that reinfused progenitor cells may be a source of relapse in patients with acute or chronic myelogenous leukemia and neuroblastoma.

Approaches using gene therapy for cancer treatment include: using lymphocytes as gene carriers, using foreign genes to increase tumor immunogenicity, introducing tumor regression antigen genes into viruses, introducing “sensitivity” genes to produce new cytotoxic agent(s) within tumors, producing new protein product(s) to protect normal cells, replacing missing or mutant tumor suppressor genes, and inactivating oncogenes. Clinical trials using these strategies have demonstrated that gene transfer is feasible (albeit with low transduction efficiency) and that gene expression occurs; in addition, clinical responses have been noted.

The evolution of gene therapy in X-linked severe combined immunodeficiency

OBJECTIVES

To review the evolution of gene therapy in infants with X-linked severe combined immunodeficiency (XL-SCID) and to evaluate the current challenges facing this evolving field.

DATA SOURCES

The MEDLINE, OVID, CINAHL, and HealthSTAR databases were searched to identify pertinent articles using the following keywords: gene therapy, XL-SCID, bone marrow transplant, and viral vectors.

STUDY SELECTION

Journal articles were selected for their relevance to human gene therapy in patients with XL-SCID.

RESULTS

Gene therapy with a retrovirus-derived vector has been used to treat 20 patients with XL-SCID internationally. Although most patients derived improvements in T- and B-cell immune numbers and function, severe adverse effects have occurred. After gene therapy, 5 of the 20 patients developed leukemia. This outcome has been associated with insertion of the corrected gene near the T-cell proto-oncogene LMO2. One of the 5 patients subsequently died.

CONCLUSIONS

Within the past decade, effective improvements in vectorology and cell culture conditions have resulted in clinical success in some infants with SCID and have revived interest after many years of setbacks. However, clinical success and significant adverse events have been reported in patients with XL-SCID who have undergone gene therapy using a retroviral vector. As extensive research into improving safety through vector development and monitoring of gene therapy continues, further progress in gene therapy development can be anticipated.

The continuing contribution of gene marking to cell and gene therapy

Gene-marking studies were the first gene-transfer protocols approved for human use. Their intent was not directly therapeutic but rather to track the behavior and fate of cells in vivo, and to use this information to improve treatment protocols. For more than fifteen years, gene-marking studies using retroviral vectors have provided invaluable information about the biology of human hematopoietic cells and T lymphocytes, and have helped guide cell therapies intended to treat malignant disease. Although the safety record of marking studies has been impeccable, the development of leukemia by immunodeficient children treated with retroviral vectors cast a pall over the entire field and essentially brought the era of pure gene-marking studies to an abrupt end. Paradoxically, the impetus these events gave to studying retroviral integration sites in host cell DNA emphasized the additional information that marker studies could provide about the behavior of cells at the clonal level. As confidence has slowly returned, marker studies have reappeared, usually as components of gene therapy protocols in which a marker gene or sequence is incorporated to allow the modified cells to be tracked or imaged in vivo. Hence, gene marking continues to have much to offer in terms of our understanding of the behavior, fate, and safety of gene-modified cells in vivo.

A review of gene therapy for haematological disorders

Gene therapy aims to correct the disease process by restoring, modifying or enhancing cellular functions through the introduction of a functional gene into a target cell. Whilst the concept of gene therapy is simple, the practical reality of translating this new technology to the clinic has proven to be more difficult than first imagined. Recent progress in gene transfer technology has shown impressive clinical success in infants with immunodeficiency. However, two of these children have subsequently developed leukaemia as a result of insertional mutagenesis, thus, raising important questions about the safety of genetic therapeutics. This article reviews the current status of gene therapy and outlines the challenges faced by this emerging technology that holds so much promise for many suffering from catastrophic disorders.

Genetic modification of hematopoietic stem cells: recent advances in the gene therapy of inherited diseases

Hematopoietic stem cells constitute a rare population of precursor cells with remarkable properties for being used as targets in gene therapy protocols. The last years have been particularly productive both in the fields of gene therapy and stem cell biology. Results from ongoing clinical trials have shown the first unquestionable clinical benefits of immunodeficient patients transplanted with genetically modified autologous stem cells. On the other hand, severe side effects in a few patients treated with gene therapy have also been reported, indicating the usefulness of further improving the vectors currently used in gene therapy clinical trials. In the field of stem cell biology, evidence showing the plastic potential of adult hematopoietic stem cells and data indicating the multipotency of adult mesenchymal precursor cells have been presented. Also, the generation of embryonic stem cells by means of nuclear transfer techniques has appeared as a new methodology with direct implications in gene therapy.

Tumor cell contamination in re-infused stem cell autografts: does it have clinical significance?

Tumor cells frequently contaminate autologous stem cell products in patients having a variety of malignancies. Mobilized peripheral blood stem cells may be less contaminated with tumor cells than bone marrow harvests are, but they are still frequently infiltrated. Gene-marking studies using retroviral vectors provide evidence that tumor cells contained in autografts contribute to relapse in myeloid leukemia and neuroblastoma patients. Also clinical studies have shown that tumor cell contamination of autografts is associated with shortened disease-free survival; on the other hand, successful ex vivo purging of tumor cells is associated with superior clinical outcome. However, the presence of tumor cells in autografts or insufficient purging may correlate with the extent of systemic residual disease and/or tumor chemosensitivity; therefore, there is no direct evidence that reinfused tumor cells alone cause relapse. Particularly in patients having highly chemosensitive disease and no detected systemic residual disease following high-dose transplant chemotherapy, the relative number of tumor cells contained in autografts and eventually reinfused, may become a determining factor for clinical outcome. There are no randomized trials showing improved (disease-free) survival with purging. In the absence of such trials, the contribution of tumor cells in the stem cell autografts to subsequent relapse remains controversial.

Gene transfer and the treatment of haematological malignancy

Gene therapy offers an additional therapeutic modality for treating haematological malignancy. Because gene therapies could be truly specific for the malignancy, they should ultimately prove both safe and effective. We have far to go before this full potential is realized, but gene transfer strategies are already showing therapeutic promise. Gene transfer may be used to correct the genetic defect in the tumour, to render it more susceptible to conventional therapies, or the normal host cells more resistant, to induce or amplify an antitumour immune response, or simply as a means of tracking the tumour or cells used for treatment. This article describes examples of each approach and discusses future prospects for the field.

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.

Gene-Marking studies of hematopoietic cells

Gene-marking studies were the first approved clinical protocols introducing exogenous genetic material into human cells. Such studies were never intended to provide direct therapeutic benefit. Instead, they were expected to provide information about normal cell biology and disease pathogenesis that could not be obtained in any other way. However, the information gained from such studies has had a significant impact on disease management. Gene-marking studies have provided valuable insights into the biology of the human stem cell, factors that influence the efficiency of gene transfer, mechanisms of relapse after stem cell transplantation, and the pharmacodynamics of adoptive cellular immunotherapy. With continuing advances in gene-marking technology, the value of the information provided by these studies increases, thereby ensuring their continued relevance to the field of gene transfer.

Gene therapy for pediatric AIDS

Gene therapy is an experimental treatment modality under investigation for applications to HIV-1 infection. We have developed retroviral vectors carrying anti-HIV-1 genes, demonstrated that these genes cause significant suppression of HIV-1 replication in cultures of primary hematopoietic cells, and performed a clinical trial in pediatric AIDS patients. Four HIV-1-infected children and adolescents underwent bone marrow harvest from which CD34+ cells were isolated and transduced by a retroviral vector carrying an RRE decoy gene. The cells were re-infused into the subjects, without complications, showing that gene transfer in pediatric AIDS patients in safe and feasible. However, gene-containing leukocytes in the peripheral blood were seen only at a low level and only in the first months following cell infusion. To attain some degree of efficacy, it will be necessary to achieve a higher level of gene transfer and to obtain sustained gene expression. We are currently developing new gene transfer methods and vectors designed to improve the results in future trials. If it becomes possible to reach the ideal goal of producing high percentages of T lymphocytes and monocytic cells that are resistant to HIV-1 infection, gene therapy could serve as a complement to antiretroviral drug therapy and help to sustain immunologic function.

Assessment of transduction rates of porcine bone marrow

Although drug resistance is commonly used as an indicator of gene transfer in various cellular contexts, the assessment of drug resistance is often imprecise and over-estimated. To measure accurately transduction efficiencies of the retroviral-mediated transfer of genes encoding the neomycine phosphotransferase (Neo(r)) and porcine major histocompatibility (MHC) class II in pig bone marrow cells (BMC), the fraction of targeted progenitors was evaluated by both colony-forming unit granulocytes/macrophages assays (G418r CFU-GM) and by PCR analysis of the transgenes (Tg). Transduced and untransduced BMC were selected at different concentrations of G418 and revealed high individual variability of drug sensitivity. Comparison of the results obtained by estimating the CFU frequency and the PCR assays on drug-resistant colonies demonstrated a marked overestimation of BM transduction rates when determined by G418 resistance alone, because only approximately one-third of individual colonies were positive for both the Neo(r) and the class II Tg. Because this discrepancy is likely to affect the overall assessment of transduction rates using drug resistance markers, our data attest for the need of a combination of molecular assays to determine transduction efficiencies accurately.

Ex vivo expansion of hematopoietic stem and progenitor cells: are we there yet?

Ex vivo expansion of hematopoietic stem and progenitor cells is a very ambitious idea that would have major implications in the areas of stem cell transplantation and somatic gene therapy. However, successful ex vivo expansion has evaded and frustrated scientists for a number of years. The goal of ex vivo expansion is to induce cell division and proliferation of stem cells while maintaining their primary functional characteristic, namely, their ability to engraft and sustain long-term hematopoiesis. Only when a balance between these two requirements is reached can ex vivo expansion of stem cells be considered successful. Establishing such a balance has not been easy. However, many lessons have been learned along the way, and today we have a more profound understanding of the potential obstacles facing ex vivo expansion than we did only a few years ago. In this review, we discuss these obstacles and evaluate the current status of ex vivo expansion of stem and progenitor cells both from the perspective of basic stem cell biology and from the viewpoint of clinical utility of these cells in transplantation.

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

Reports of 1- to 2-log higher gene transfer levels in purified CD34+ cells or marrow CFU compared with levels in mature circulating blood cells after transplantation of retrovirally transduced primitive human hematopoietic cells have resulted in concern that transduced progenitors do not contribute proportionally to ongoing hematopoiesis (Kohn et al., 1995; Brenner, 1996). To study the issue in a relevant large animal, we analyzed samples of mature blood cells, marrow CD34-enriched cells and marrow CD34-depleted cells, and marrow CFU from a cohort of 11 rhesus transplanted with retrovirally transduced cells and followed for up to 5.5 years. They were transplanted with CD34-enriched bone marrow (BM) or G-CSF/SCF-mobilized peripheral blood (PB) cells transduced with vectors containing either neo, human glucocerebrosidase, or murine adenosine deaminase genes. There were no significant differences between the levels of vector sequences found in BM CD34+ cells, BM CD34- cells, PB granulocytes, or PB mononuclear cells (MNCs) in any animal. In four animals transplanted with SCF/G-CSF-primed BM cells and analyzed 3-6 months posttransplantation, the percentage of CFU containing the neo vector appeared to be 1 log higher than the representation of marked cells in the PB of these animals, but this discrepancy did not persist at time points greater than 6 months posttransplantation. The level of CFU marking was no higher than PB granulocyte or MNC marking at any time points in the other animals. Low levels of mature gene-modified cells probably reflect poor transduction of repopulating stem cells, not a block in differentiation or specific immune rejection of mature cells. This study represents the longest follow-up of primates transplanted with transduced hematopoietic cells, and it is encouraging that the levels of vector-containing cells appear stable for up to 5 years.

Increased transduction efficiency of primary hematopoietic cells by physical colocalization of retrovirus and target cells

Efficient gene transfer into hematopoietic stem cells offers a number of potential therapeutic applications. However, the relatively low titer of retroviral supernatants and the requirement for cell division to ensure integration have meant that transduction efficiency has been low. We have modified a flowthrough approach to cell transduction and have been able consistently to increase gene transfer efficiency into human hematopoietic progenitor cells. We transduced CD34 cells with retroviral vectors encoding a truncated nerve growth factor receptor (NGFR) or neo. Retroviral supernatant was pulled through 0.2-micron polycarbonated membranes, followed by placement of cells on the filter. In the absence of cytokines, the transduction efficiency of CD34 cells with a NGFR vector was increased 3-11-fold over that obtained at an identical MOI in liquid culture to produce 11%-44% transduction. Furthermore, both Thy1+ and Thy1- subsets in a total CD34 population were transduced with similar efficiency, and transduction with a neo vector, as measured by G418 resistance in clonogenic assays, increased 1.5-5-fold. The mechanism by which gene transfer is improved may reflect colocalization of cells and retrovirus. Costaining of cells transduced on the filter with an NGFR retrovirus with both an NGFR antibody and a gp70 antibody that recognizes viral coat protein revealed high-level coexpression. The levels of in vitro gene transfer we obtain are equivalent to those observed when CD34 cells are cocultured in liquid culture with cytokines. However, culture with cytokines may commit CD34 cells to differentiation and has produced disappointingly low levels of subsequent in vivo gene transfer. Gene marking studies using distinguishable retroviral vectors will provide a means of learning whether the effects of flowthrough transduction genuinely enhance the efficiency of gene transfer to human marrow-repopulating cells.

Gene therapy for leukemia and lymphoma

Gene therapy of malignant diseases can be divided into four basic approaches: gene interference, gene insertion, immunopotentiation, and suicide gene approaches. This article reviews the application of these approaches in the therapy of leukemias and lymphomas.

Murine bone marrow expressing the neomycin resistance gene has no competitive disadvantage assessed in vivo

The neomycin phosphotransferase (neo) gene is one of the most common marker genes used in gene transfer experimentation, but potential effects of neo gene expression in vivo have not been systematically investigated. Several early clinical retroviral gene transfer studies have suggested that neo gene expression could have deleterious effects on hematopoiesis, owing to a discrepancy between the level of neo-marked transduced marrow progenitor cells compared with mature circulating progeny cells posttransplantation (Brenner et al., 1993; Kohn et al., 1995; Brenner, 1996b). We examined the long-term in vivo repopulating ability of bone marrow from transgenic mice expressing neo from a strong constitutive promoter using a competitive repopulation assay. Different ratios of neo transgenic and wild-type congenic marrow cells were cotransplanted into W/Wv recipient mice. The percentages of blood cells containing the neo transgene in each group of recipient mice monitored for 4 months posttransplantation closely matched the input ratios of neo transgenic to congenic control marrow cells. Similar concordances of engraftment with input ratios of neo transgenic cells were also found in spleen, thymus, and whole marrow of recipient mice at 4 months posttransplantation. Analysis of the beta-hemoglobin phenotype (beta(single) for the neo transgenic and C57 control cells and beta(diffuse) for the congenic competitor HW80 cells) in recipients confirmed erythroid repopulation from neo transgenic marrow cells at levels matching the input ratios. We conclude that hematopoietic cells expressing neo had no engraftment or maturation defects detectable in vivo. These results suggest that the low-level contribution of vector-marked cells to circulating populations in clinical trials is not due to direct deleterious effects of neo gene expression on hematopoiesis.

Delayed Targeting of Cytokine-Nonresponsive Human Bone Marrow CD34+ Cells With Retrovirus-Mediated Gene Transfer Enhances Transduction Efficiency and Long-Term Expression of Transduced Genes

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 usingneoR-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 ofneoR 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 transducedneoR 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.

Delayed Targeting of Cytokine-Nonresponsive Human Bone Marrow CD34+ Cells With Retrovirus-Mediated Gene Transfer Enhances Transduction Efficiency and Long-Term Expression of Transduced Genes

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 usingneoR-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 ofneoR 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 transducedneoR 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.

Human gene therapy

Human gene therapy and its application for the treatment of human genetic disorders, such as cystic fibrosis, cancer, and other diseases, are discussed. Gene therapy is a technique in which a functioning gene is inserted into a human cell to correct a genetic error or to introduce a new function to the cell. Many methods, including retroviral vectors and non-viral vectors, have been developed for both ex vivo and in vivo gene transfer into cells. Vectors need to be developed that efficiently transfer genes to target cells, and promoter systems are required that regulate gene expression according to physiologic needs of the host cell. There are several safety and ethical issues related to manipulating the human genome that need to be resolved. Current gene therapy efforts focus on gene insertion into somatic cells only. Gene therapy has potential for the effective treatment of genetic disorders, and gene transfer techniques are being used for basic research, for example, in cancer, to examine the underlying mechanism of disease. There are still many technical obstacles to be overcome before human gene therapy can become a routine procedure. The current human genome project provides the sequences of a vast number of human genes, leading to the identification, characterization, and understanding of genes that are responsible for many human diseases.

Gene transfer into hematopoietic cells

Transfer of a gene into stem cells with subsequent lineage-specific gene expression is a desired goal with many potential therapeutic applications. Retroviral vectors developed from murine leukemia viruses reproducibly transfer genes into murine stem cells, but are inefficient at gene insertion into stem cells of larger animals or man. A growing knowledge of stem cell biology suggests that this inefficiency reflects the quiescent state of stem cells, even when incubated in the presence of multiple cytokines and low expression of the receptor for amphotropic retroviral vectors. Alternative vector systems are being explored in an effort to overcome these barriers to stem cell-targeted gene transfer. Our work has shown that recombinant adeno-associated virus vectors, which have the potential for transducing quiescent cells, transfer, express and integrate a globin gene linked to its normal regulatory elements in human erythroid cells, but only at very high multiplicities of infection. The integrated genome was stable and the encoded globin gene was expressed at levels equivalent to a normal globin gene.

Retroviral Gene Transduction of Adult Peripheral Blood or Marrow-Derived CD34+ Cells for Six Hours Without Growth Factors or on Autologous Stroma Does Not Improve Marking Efficiency Assessed In Vivo

Our previous work in patients undergoing autologous transplant for multiple myeloma (MM) or breast cancer (BC) has shown that retroviral transduction of adult CD34+ cells for 72 hours in the presence of interleukin-3 (IL-3), IL-6, and stem cell factor (SCF ) resulted in .01% to 1% long-term marking of peripheral blood and marrow cells (Blood 85:3948, 1995). In this study we compare these previous studies to transduction with no added growth factors, previously shown to result in higher levels of marking in children (Lancet 342:1134, 1993) or transduction in the presence of an autologous stromal layer. Peripheral blood (PB) mononuclear cells were collected via apheresis after high-dose cyclophosphamide and granulocyte colony-stimulating factor. Bone marrow (BM) was also harvested in all patients. One third of both BM and PB collections were enriched for CD34+ cells and transduced with one of two marking vectors containing the neomycin-resistance gene to distinguish cells originating from BM and PB posttransplantation. Cells from 3 MM and 2 BC patients were transduced without growth factors for 6 hours and cells from 2 MM and 2 BC patients were transduced in the presence of autologous marrow stroma. Immediately posttransduction, the percentage of Neo-resistant PB and BM progenitors (colony-forming units) were: 0% to 19% in the 6-hour no growth factor group and 0% to 36% in the autologous stroma group. After conditioning therapy, both transduced and untransduced PB and BM fractions were infused into the patients. Semi-quantitative nested DNA polymerase chain reaction was performed on total, mononuclear, and granulocyte fractions of PB and BM at 1, 3, 6, 9, 12, and 18 months. Poor marking has been observed in both groups, with no consistently positive patients. These results compare unfavorably with our prior experience using growth factors during transduction. Further optimization of transduction conditions and vectors needs to be developed to improve transduction efficiency of adult human repopulating hematopoietic cells.

Gene marking

Gene marking studies were the first gene transfer protocols to enter clinical practice. To date, clinical marking studies have been limited to the hematopoietic stem cell and its progeny. In this setting, they have provided valuable information about stem cell biology, the factors that influence gene transfer efficiency, and the mechanism of relapse in patients receiving stem cell rescue as therapy for malignant disease. Second-generation studies are beginning to provide even more information about a wider variety of clinical and biological issues. Although marker studies have been useful, it is becoming apparent that the indicator genes used up to now have a number of undesirable characteristics. Future applications of marking, in the hematopoietic system and elsewhere, will require the use of marker elements that will not produce any modification of the cells' behavior. Finally, marker studies have proved safe so far, but follow-up of the treated patients continues.