Update on the use of nonhuman primate models for preclinical testing of gene therapy approaches targeting hematopoietic cells

Lentiviral Transduction of Nonhuman Primate Hematopoietic Stem and Progenitor Cells

The nonhuman primate (NHP) animal model is an important predictive preclinical model for developing gene and cell therapies. It is also an experimental animal model used to study hematopoietic stem and progenitor cell (HSPC) biology, with the capability of serving as a step for the translation of the basic research concepts from small animals to humans. Lentiviral vectors are currently the standard gene delivery vehicles for transduction of HSPCs in the clinical setting. They have proven to be less genotoxic and more efficient than the previously used murine γ-retroviruses. Transplantation of lentiviral vector-transduced HSPCs into autologous macaques has been well developed over the past two decades. In this chapter, we provide detailed methodologies for lentiviral vector transduction of rhesus macaque HSPCs, including production and titration of lentiviral vector, purification of CD34⁺ HSPCs, and lentiviral vector transduction and assessment.

Comparison of busulfan and total body irradiation conditioning on hematopoietic clonal dynamics following lentiviral gene transfer in rhesus macaques

The clonal dynamics following hematopoietic stem progenitor cell (HSPC) transplantation with busulfan conditioning are of great interest to the development of HSPC gene therapies. Compared with total body irradiation (TBI), busulfan is less toxic and more clinically relevant. We used a genetic barcoded HSPC autologous transplantation model to investigate the impact of busulfan conditioning on hematopoietic reconstitution in rhesus macaques. Two animals received lower busulfan dose and demonstrated lower vector marking levels compared with the third animal given a higher busulfan dose, despite similar busulfan pharmacokinetic analysis. We observed uni-lineage clonal engraftment at 1 month post-transplant, replaced by multilineage clones by 2 to 3 months in all animals. The initial multilineage clones in the first two animals were replaced by a second multilineage wave at 9 months; this clonal pattern disappeared at 13 months in the first animal, though was maintained in the second animal. The third animal maintained stable multilineage clones from 3 months to the most recent time point. In addition, busulfan animals exhibit more rapid HSPC clonal mixing across bone marrow sites and less CD16⁺ NK-biased clonal expansion compared with TBI animals. Therefore, busulfan conditioning regimens can variably impact the marrow niche, resulting in differences in clonal patterns with implications for HSPC gene therapies.

Comparative engraftment and clonality of macaque HSPCs expanded on human umbilical vein endothelial cells versus non-expanded cells

Ex vivo hematopoietic stem and progenitor cell (HSPC) expansion platforms are under active development, designed to increase HSPC numbers and thus engraftment ability of allogeneic cord blood grafts or autologous HSPCs for gene therapies. Murine and in vitro models have not correlated well with clinical outcomes of HSPC expansion, emphasizing the need for relevant pre-clinical models. Our rhesus macaque HSPC competitive autologous transplantation model utilizing genetically barcoded HSPC allows direct analysis of the relative short and long-term engraftment ability of lentivirally transduced HSPCs, along with additional critical characteristics such as HSPC clonal diversity and lineage bias. We investigated the impact of ex vivo expansion of macaque HSPCs on the engineered endothelial cell line (E-HUVECs) platform regarding safety, engraftment of transduced and E-HUVEC-expanded HSPC over time compared to non-expanded HSPC for up to 51 months post-transplantation, and both clonal diversity and lineage distribution of output from each engrafted cell source. Short and long-term engraftment were comparable for E-HUVEC expanded and the non-expanded HSPCs in both animals, despite extensive proliferation of CD34⁺ cells during 8 days of ex vivo culture for the E-HUVEC HSPCs, and optimization of harvesting and infusion of HSPCs co-cultured on E-HUVEC in the second animal. Long-term hematopoietic output from both E-HUVEC expanded and unexpanded HSPCs was highly polyclonal and multilineage. Overall, the comparable HSPC kinetics of macaques to humans, the ability to study post-transplant clonal patterns, and simultaneous multi-arm comparisons of grafts without the complication of interpreting allogeneic effects makes our model ideal to test ex vivo HSPC expansion platforms, particularly for gene therapy applications.

BCL11A enhancer-edited hematopoietic stem cells persist in rhesus monkeys without toxicity

Gene editing of the erythroid-specific BCL11A enhancer in hematopoietic stem and progenitor cells (HSPCs) from patients with sickle cell disease (SCD) induces fetal hemoglobin (HbF) without detectable toxicity, as assessed by mouse xenotransplant. Here, we evaluated autologous engraftment and HbF induction potential of erythroid-specific BCL11A enhancer-edited HSPCs in 4 nonhuman primates. We used a single guide RNA (sgRNA) with identical human and rhesus target sequences to disrupt a GATA1 binding site at the BCL11A +58 erythroid enhancer. Cas9 protein and sgRNA ribonucleoprotein complex (RNP) was electroporated into rhesus HSPCs, followed by autologous infusion after myeloablation. We found that gene edits persisted in peripheral blood (PB) and bone marrow (BM) for up to 101 weeks similarly for BCL11A enhancer- or control locus-targeted (AAVS1-targeted) cells. Biallelic BCL11A enhancer editing resulted in robust γ-globin induction, with the highest levels observed during stress erythropoiesis. Indels were evenly distributed across PB and BM lineages. Off-target edits were not observed. Nonhomologous end-joining repair alleles were enriched in engrafting HSCs. In summary, we found that edited HSCs can persist for at least 101 weeks after transplant and biallelic-edited HSCs provide substantial HbF levels in PB red blood cells, together supporting further clinical translation of this approach.

Clonal tracking of erythropoiesis in rhesus macaques

The classical model of hematopoietic hierarchies is being reconsidered on the basis of data from in vitro assays and single cell expression profiling. Recent experiments suggested that the erythroid lineage might differentiate directly from multipotent hematopoietic stem cells / progenitors or from a highly biased subpopulation of stem cells, rather than transiting through common myeloid progenitors or megakaryocyte-erythrocyte progenitors. We genetically barcoded autologous rhesus macaque stem and progenitor cells, allowing quantitative tracking of the in vivo clonal output of thousands of individual cells over time following transplantation. CD34⁺ cells were lentiviral-transduced with a high diversity barcode library, with the barcode in an expressed region of the provirus, allowing barcode retrieval from DNA or RNA, with each barcode representing an individual stem or progenitor cell clone. Barcode profiles from bone marrow CD45^-CD71⁺ maturing nucleated red blood cells were compared with other lineages purified from the same bone marrow sample. There was very high correlation of barcode contributions between marrow nucleated red blood cells and other lineages, with the highest correlation between nucleated red blood cells and myeloid lineages, whether at earlier or later time points post transplantation, without obvious clonal contributions from highly erythroid-biased or restricted clones. A similar profile occurred even under stressors such as aging or erythropoietin stimulation. RNA barcode analysis on circulating mature red blood cells followed over long time periods demonstrated stable erythroid clonal contributions. Overall, in this nonhuman primate model with great relevance to human hematopoiesis, we documented continuous production of erythroid cells from multipotent, non-biased hematopoietic stem cell clones at steady-state or under stress.

MHC-identical and transgenic cynomolgus macaques for preclinical studies

Cynomolgus macaques are useful experimental animals that are physiologically and genetically close to humans. We have developed two kinds of experimental usage of cynomolgus macaque: transplantation and disease models. First, we identified certain major histocompatibility complex (MHC) haplotypes including homozygotes and heterozygotes in cynomolgus macaques native to the Philippines, because they have less polymorphism in the MHC than that in other origins such as Vietnam and Indonesia. As a preclinical model of the induced pluripotent stem cell (iPSC) stock project, we established iPSCs from various types of MHC homozygous macaques, which were transplanted into compatible MHC heterozygous macaques, the iPSC stock project was experimentally shown to be effective. Second, to obtain disease models of cynomolgus macaques for studies on regenerative medicine including cell therapies, we established two kinds of genetic technology to modify cynomolgus macaques: transgenic technology and gene editing technology using CRISPR-Cas9. We will establish disease models, such as Alzheimer's disease and progeria (Werner syndrome). In future, we will distribute the MHC-identical cynomolgus monkeys and genetically modified macaques to researchers, especially those engaging in regenerative medicine.

Improving the Transduction of Bone Marrow-Derived Cells with an Integrase-Defective Lentiviral Vector

In lentiviral vector (LV) applications where transient transgene expression is sufficient, integrase-defective lentiviral vectors (IDLVs) are beneficial for reducing the potential for off-target effects associated with insertional mutagenesis. It was previously demonstrated that human RPE65 mRNA expression from an integrating lentiviral vector (ILV) induces endogenous Rpe65 and Cralbp mRNA expression in murine bone marrow–derived cells (BMDCs), initiating programming of the cells to retinal pigment epithelium (RPE)-like cells. These cells regenerate RPE in retinal degeneration models when injected systemically. As transient expression of RPE65 is sufficient to activate endogenous RPE-associated genes for programming BMDCs, use of an ILV is an unnecessary risk. In this study, an IDLV expressing RPE65 (IDLV3-RPE65) was generated. Transduction with IDLV3-RPE65 is less efficient than the integrating vector (ILV3-RPE65). Therefore, IDLV3-RPE65 transduction was enhanced with a combination of preloading 20 × -concentrated viral supernatant on RetroNectin at a multiplicity of infection of 50 and transduction of BMDCs by low-speed centrifugation. RPE65 mRNA levels increased from ∼12-fold to ∼25-fold (p < 0.05) after modification of the IDLV3-RPE65 transduction protocol, achieving expression similar to the ∼27-fold (p < 0.05) increase observed with ILV3-RPE65. Additionally, the study shows that the same preparation of RetroNectin can be used to coat up to three wells with no reduction in transduction. Critically, IDLV3-RPE65 transduction initiates endogenous Rpe65 mRNA expression in murine BMDCs and Cralbp/CRALBP mRNA in both murine and human BMDCs, similar to expression observed in ILV3-RPE65-transduced cells. Systemic administration of ILV3-RPE65 or IDLV3-RPE65 programmed BMDCs in a mouse model of retinal degeneration is sufficient to retain visual function and reduce retinal degeneration compared to mice receiving no treatment or naïve BMDC. It is concluded that IDLV3-RPE65 is appropriate for programming BMDCs to RPE-like cells.

Geographic clonal tracking in macaques provides insights into HSPC migration and differentiation

Wu et al. use barcode tracking to uncover prolonged geographic bone marrow segregation of regenerating hematopoietic stem and progenitor cell clones after transplantation and provide evidence for local bone marrow production of T cells.

The geographic distribution of hematopoiesis at a clonal level is of interest in understanding how hematopoietic stem and progenitor cells (HSPCs) and their progeny interact with bone marrow (BM) niches during regeneration. We tagged rhesus macaque autologous HSPCs with genetic barcodes, allowing clonal tracking over time and space after transplantation. We found marked geographic segregation of CD34⁺ HSPCs for at least 6 mo posttransplantation, followed by very gradual clonal mixing at different BM sites over subsequent months to years. Clonal mapping was used to document local production of granulocytes, monocytes, B cells, and CD56⁺ natural killer (NK) cells. In contrast, CD16⁺CD56⁻ NK cells were not produced in the BM, and in fact were clonally distinct from multipotent progenitors producing all other lineages. Most surprisingly, we documented local BM production of CD3⁺ T cells early after transplantation, using both clonal mapping and intravascular versus tissue-resident T cell staining, suggesting a thymus-independent T cell developmental pathway operating during BM regeneration, perhaps before thymic recovery.

Transplantation of iPS-Derived Tumor Cells with a Homozygous MHC Haplotype Induces GRP94 Antibody Production in MHC-Matched Macaques

Immune surveillance is a critical component of the antitumor response in vivo, yet the specific components of the immune system involved in this regulatory response remain unclear. In this study, we demonstrate that autoantibodies can mitigate tumor growth in vitro and in vivo We generated two cancer cell lines, embryonal carcinoma and glioblastoma cell lines, from monkey-induced pluripotent stem cells (iPSC) carrying a homozygous haplotype of major histocompatibility complex (MHC, Mafa in Macaca fascicularis). To establish a monkey cancer model, we transplanted these cells into monkeys carrying the matched Mafa haplotype in one of the chromosomes. Neither Mafa-homozygous cancer cell line grew in monkeys carrying the matched Mafa haplotype heterozygously. We detected in the plasma of these monkeys an IgG autoantibody against GRP94, a heat shock protein. Injection of the plasma prevented growth of the tumor cells in immunodeficient mice, whereas plasma IgG depleted of GRP94 IgG exhibited reduced killing activity against cancer cells in vitro These results indicate that humoral immunity, including autoantibodies against GRP94, plays a role in cancer immune surveillance. Cancer Res; 77(21); 6001-10. ©2017 AACR.

Quantitative stability of hematopoietic stem and progenitor cell clonal output in rhesus macaques receiving transplants

Autologous transplantation of hematopoietic stem and progenitor cells lentivirally labeled with unique oligonucleotide barcodes flanked by sequencing primer targets enables quantitative assessment of the self-renewal and differentiation patterns of these cells in a myeloablative rhesus macaque model. Compared with other approaches to clonal tracking, this approach is highly quantitative and reproducible. We documented stable multipotent long-term hematopoietic clonal output of monocytes, granulocytes, B cells, and T cells from a polyclonal pool of hematopoietic stem and progenitor cells in 4 macaques observed for up to 49 months posttransplantation. A broad range of clonal behaviors characterized by contribution level and biases toward certain cell types were extremely stable over time. Correlations between granulocyte and monocyte clonalities were greatest, followed by correlations between these cell types and B cells. We also detected quantitative expansion of T cell-biased clones consistent with an adaptive immune response. In contrast to recent data from a nonquantitative murine model, there was little evidence for clonal succession after initial hematopoietic reconstitution. These findings have important implications for human hematopoiesis, given the similarities between macaque and human physiologies.

Advantages of nonhuman primates as preclinical models for evaluating stem cell-based therapies for Parkinson's disease

The derivation of dopaminergic neurons from induced pluripotent stem cells brings new hope for a patient-specific, stem cell-based replacement therapy to treat Parkinson's disease (PD) and related neurodegenerative diseases; and this novel cell-based approach has already proven effective in animal models. However, there are several aspects of this procedure that have yet to be optimized to the extent required for translation to an optimal cell-based transplantation protocol in humans. These challenges include pinpointing the optimal graft location, appropriately scaling up the graft volume, and minimizing the risk of chronic immune rejection, among others. To advance this procedure to the clinic, it is imperative that a model that accurately and fully recapitulates characteristics most pertinent to a cell-based transplantation to the human brain is used to optimize key technical aspects of the procedure. Nonhuman primates mimic humans in multiple ways including similarities in genomics, neuroanatomy, neurophysiology, immunogenetics, and age-related changes in immune function. These characteristics are critical to the establishment of a relevant model in which to conduct preclinical studies to optimize the efficacy and safety of cell-based therapeutic approaches to the treatment of PD. Here we review previous studies in rodent models, and emphasize additional advantages afforded by nonhuman primate models in general, and the baboon model in particular, for preclinical optimization of cell-based therapeutic approaches to the treatment of PD and other neurodegenerative diseases. We outline current unresolved challenges to the successful application of stem cell therapies in humans and propose that the baboon model in particular affords a number of traits that render it most useful for preclinical studies designed to overcome these challenges.

Pigtailed macaques as a model to study long-term safety of lentivirus vector-mediated gene therapy for hemoglobinopathies

Safely achieving long-term engraftment of genetically modified hematopoietic stem cells (HSCs) that maintain therapeutic transgene expression is the benchmark for successful application of gene therapy for hemoglobinopathies. We used the pigtailed macaque HSC transplantation model to ascertain the long-term safety and stability of a γ-globin lentivirus vector. We observed stable gene-modified cells and fetal hemoglobin expression for 3 years. Retrovirus integration site (RIS) analysis spanning 6 months to 3.1 years revealed vastly disparate integration profiles, and dynamic fluctuation of hematopoietic contribution from different gene-modified HSC clones without evidence for clonal dominance. There were no perturbations of the global gene-expression profile or expression of genes within a 300 kb region of RIS, including genes surrounding the most abundantly marked clones. Overall, a 3-year long follow-up revealed no evidence of genotoxicity of the γ-globin lentivirus vector with multilineage polyclonal hematopoiesis, and HSC clonal fluctuations that were not associated with transcriptome dysregulation.

Clonal tracking of rhesus macaque hematopoiesis highlights a distinct lineage origin for natural killer cells

Analysis of hematopoietic stem cell function in nonhuman primates provides insights that are relevant for human biology and therapeutic strategies. In this study, we applied quantitative genetic barcoding to track the clonal output of transplanted autologous rhesus macaque hematopoietic stem and progenitor cells over a time period of up to 9.5 months. We found that unilineage short-term progenitors reconstituted myeloid and lymphoid lineages at 1 month but were supplanted over time by multilineage clones, initially myeloid restricted, then myeloid-B clones, and then stable myeloid-B-T multilineage, long-term repopulating clones. Surprisingly, reconstitution of the natural killer (NK) cell lineage, and particularly the major CD16(+)/CD56(-) peripheral blood NK compartment, showed limited clonal overlap with T, B, or myeloid lineages, and therefore appears to be ontologically distinct. Thus, in addition to providing insights into clonal behavior over time, our analysis suggests an unexpected paradigm for the relationship between NK cells and other hematopoietic lineages in primates.

Myeloid-lymphoid ontogeny in the rhesus monkey (Macaca mulatta)

Establishment of a functional immune system has important implications for health and disease, yet questions remain regarding the mechanism, location, and timing of development of myeloid and lymphoid cell compartments. The goal of this study was to characterize the ontogeny of the myeloid-lymphoid system in rhesus monkeys to enhance current knowledge of the developmental sequence of B-cell (CD20, CD79), T-cell (CD3, CD4, CD8, FoxP3), dendritic cell (CD205), and macrophage (CD68) lineages in the fetus and infant. Immunohistochemical assessments addressed the temporal and spatial expression of select phenotypic markers in the developing liver, thymus, spleen, lymph nodes, gut-associated lymphoid tissue (GALT), and bone marrow with antibodies known to cross-react with rhesus cells. CD3 was the earliest lymphoid marker identified in the first trimester thymus and, to a lesser extent, in the spleen. T-cell markers were also expressed midgestation on cells of the liver, spleen, thymus, and in Peyer's patches of the small and large intestine, and where CCR5 expression was noted. A myeloid marker, CD68, was found on hepatic cells near blood islands in the late first trimester. B-cell markers were observed mid-second trimester in the liver, spleen, thymus, lymph nodes, bone marrow spaces, and occasionally in GALT. By the late third trimester and postnatally, secondary follicles with germinal centers were present in the thymus, spleen, and lymph nodes. These results suggest that immune ontogeny in monkeys is similar in temporal and anatomical sequence when compared to humans, providing important insights for translational studies.

Path to the clinic: assessment of iPSC-based cell therapies in vivo in a nonhuman primate model

Induced pluripotent stem cell (iPSC)-based cell therapies have great potential for regenerative medicine but are also potentially associated with tumorigenic risks. Current rodent models are not optimal predictors of efficiency and safety for clinical application. Therefore, we developed a clinically relevant nonhuman primate model to assess the tumorigenic potential and in vivo efficacy of both undifferentiated and differentiated iPSCs in autologous settings without immunosuppression. Undifferentiated autologous iPSCs indeed form mature teratomas in a dose-dependent manner. However, tumor formation is accompanied by an inflammatory reaction. On the other hand, iPSC-derived mesodermal stromal-like cells form new bone in vivo without any evidence of teratoma formation. We therefore show in a large animal model that closely resembles human physiology that undifferentiated autologous iPSCs form teratomas, and that iPSC-derived progenitor cells can give rise to a functional tissue in vivo.

Antigen-specific activation and cytokine-facilitated expansion of naive, human CD8+ T cells

Antigen-specific priming of human, naive T cells has been difficult to assess. Owing to the low initial frequency in the naive cell pool of specific T cell precursors, such an analysis has been obscured by the requirements for repeated stimulations and prolonged culture time. In this protocol, we describe how to evaluate antigen-specific priming of CD8(+) cells 10 d after a single specific stimulation. The assay provides reference conditions, which result in the expansion of a substantial population of antigen-specific T cells from the naive repertoire. Various conditions and modifications during the priming process (e.g., testing new cytokines, co-stimulators and so on) can now be directly compared with the reference conditions. Factors relevant to achieving effective priming include the dendritic cell preparation, the T cell preparation, the cell ratio at the time of priming, the serum source used for the experiment and the timing of addition and concentration of the cytokines used for expansion. This protocol is relevant for human immunology, vaccine biology and drug development.

Better leukemia-free and overall survival in AML in first remission following cyclophosphamide in combination with busulfan compared with TBI

Cyclophosphamide combined with total body irradiation (Cy/TBI) or busulfan (BuCy) are the most widely used myeloablative conditioning regimens for allotransplants. Recent data regarding their comparative effectiveness are lacking. We analyzed data from the Center for International Blood and Marrow Transplant Research for 1230 subjects receiving a first hematopoietic cell transplant from a human leukocyte antigen-matched sibling or from an unrelated donor during the years 2000 to 2006 for acute myeloid leukemia (AML) in first complete remission (CR) after conditioning with Cy/TBI or oral or intravenous (IV) BuCy. Multivariate analysis showed significantly less nonrelapse mortality (relative risk [RR] = 0.58; 95% confidence interval [CI]: 0.39-0.86; P = .007), and relapse after, but not before, 1 year posttransplant (RR = 0.23; 95% CI: 0.08-0.65; P = .006), and better leukemia-free survival (RR = 0.70; 95% CI: 0.55-0.88; P = .003) and survival (RR = 0.68; 95% CI: 0.52-0.88; P = .003) in persons receiving IV, but not oral, Bu compared with TBI. In combination with Cy, IV Bu is associated with superior outcomes compared with TBI in patients with AML in first CR.

No impact of lentiviral transduction on hematopoietic stem/progenitor cell telomere length or gene expression in the rhesus macaque model

The occurrence of clonal perturbations and leukemia in patients transplanted with gamma-retroviral (RV) vector-transduced autologous hematopoietic stem and progenitor cells (HSPCs) has stimulated extensive investigation, demonstrating that proviral insertions may perturb adjacent proto-oncogene expression. Although enhancer-deleted lentiviruses are less likely to result in insertional oncogenesis, there is evidence that they may perturb transcript splicing, and one patient with a benign clonal expansion of lentivirally transduced HPSC has been reported. The rhesus macaque model provides an opportunity for informative long-term analysis to ask whether transduction impacts on long-term HSPC properties. We used two techniques to examine whether lentivirally transduced HSPCs from eight rhesus macaques transplanted 1-13.5 years previously are perturbed at a population level, comparing telomere length as a measure of replicative history and gene expression profile of vector positive versus vector negative cells. There were no differences in telomere lengths between sorted GFP+ and GFP- blood cells, suggesting that lentiviral (LV) transduction did not globally disrupt replicative patterns. Bone marrow GFP+ and GF- CD34+ cells showed no differences in gene expression using unsupervised and principal component analysis. These studies did not uncover any global long-term perturbation of proliferation, differentiation, or other important functional parameters of transduced HSPCs in the rhesus macaque model.

Assessing the risks of genotoxicity in the therapeutic development of induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) have great potential for regenerative medicine as well as for basic and translational research. However, following the initial excitement over the enormous prospects of this technology, several reports uncovered serious concerns regarding its safety for clinical applications and reproducibility for laboratory applications such as disease modeling or drug screening. In particular, the genomic integrity of iPSCs is the focus of extensive research. Epigenetic remodeling, aberrant expression of reprogramming factors, clonal selection, and prolonged in vitro culture are potential pathways for acquiring genomic alterations. In this review, we will critically discuss current reprogramming technologies particularly in the context of genotoxicity, and the consequences of these alternations for the potential applications of reprogrammed cells. In addition, current strategies of genetic modification of iPSCs, as well as applicable suicide strategies to control the risk of iPSC-based therapies will be introduced.

Contributions of gene marking to cell and gene therapies

The first human genetic modification studies used replication-incompetent integrating vector vectors to introduce marker genes into T lymphocytes and subsequently into hematopoietic stem cells. Such studies have provided numerous insights into the biology of hematopoiesis and immune reconstitution and contributed to clinical development of gene and cell therapies. Tracking of hematopoietic reconstitution and analysis of the origin of residual malignant disease after hematopoietic transplantation has been possible via gene marking. Introduction of selectable marker genes has enabled preselection of specific T-cell populations for tumor and viral immunotherapy and reduced the threat of graft-versus-host disease, improving the survival of patients after allogeneic marrow transplantation. Marking studies in humans, murine xenografts, and large animals have helped optimize conditions for gene transfer into CD34(+) hematopoietic progenitors, contributing to the achievement of gene transfer efficiencies sufficient for clinical benefit in several serious genetic diseases such as X-linked severe combined immunodeficiency and adrenoleukodystrophy. When adverse events linked to insertional mutagenesis arose in clinical gene therapy trials for inherited immunodeficiencies, additional animal studies using gene-marking vectors have greatly increased our understanding of genotoxicity. The knowledge gained from these studies is being translated into new vector designs and clinical protocols, which we hope will continue to improve the efficiency, effectiveness and safety of these promising therapeutic approaches.

Repetitive busulfan administration after hematopoietic stem cell gene therapy associated with a dominant HDAC7 clone in a nonhuman primate

The risk of genotoxicity of retroviral vector-delivered gene therapy targeting hematopoietic stem cells (HSCs) has been highlighted by the development of clonal dominance and malignancies in human and animal gene therapy trials. Large-animal models have proven invaluable to test the safety of retroviral vectors, but the detection of clonal dominance may require years of follow-up. We hypothesized that hematopoietic stress may accelerate the proliferation and therefore the detection of abnormal clones in these models. We administered four monthly busulfan (Bu) infusions to induce hematopoietic stress in a healthy rhesus macaque previously transplanted with CD34+ cells transduced with retroviral vectors carrying a simple marker gene. Busulfan administration resulted in significant cytopenias with each cycle, and prolonged pancytopenia after the final cycle with eventual recovery. Before busulfan treatment there was highly polyclonal marking in all lineages. After Bu administration clonal diversity was markedly decreased in all lineages. Unexpectedly, we found no evidence of selection of the MDS1/EVI1 clones present before Bu administration, but a clone with a vector integration in intron 1 of the histone deacetylase-7 (HDAC7) gene became dominant in granulocytes over time after Bu administration. The overall marking level in the animal was increased significantly after Bu treatment and coincident with expansion of the HDAC7 clone, suggesting an in vivo advantage for this clone under stress. HDAC7 expression was upregulated in marrow progenitors containing the vector. Almost 5 years after Bu administration, the animal developed progressive cytopenias, and at autopsy the marrow showed complete lack of neutrophil or platelet maturation, with a new population of approximately 20% undifferentiated blasts. These data suggest that chemotherapeutic stress may accelerate vector-related clonal dominance, even in the absence of drug resistance genes expressed by the vector. This model may both accelerate the detection of abnormal clones to facilitate analysis of genotoxicity for human gene therapy, and help assess the safety of administering myelotoxic chemotherapeutic agents in patients previously engrafted with vector-containing cells.

Sustained high-level polyclonal hematopoietic marking and transgene expression 4 years after autologous transplantation of rhesus macaques with SIV lentiviral vector-transduced CD34+ cells

We previously reported that lentiviral vectors derived from the simian immunodeficiency virus (SIV) were efficient at transducing rhesus hematopoietic repopulating cells. To evaluate the persistence of vector-containing and -expressing cells long term, and the safety implications of SIV lentiviral vector-mediated gene transfer, we followed 3 rhesus macaques for more than 4 years after transplantation with transduced CD34+ cells. All 3 animals demonstrated significant vector marking and expression of the GFP transgene in T cells, B cells, and granulocytes, with mean GFP+ levels of 6.7% (range, 3.3%-13.0%), 7.4% (4.2%-13.4%), and 5.6% (3.1%-10.5%), respectively. There was no vector silencing in hematopoietic cells over time. Vector insertion site analysis of granulocytes demonstrated sustained highly polyclonal reconstitution, with no evidence for progression to oligoclonality. A significant number of clones were found to contribute at both 1-year and 3- or 4-year time points. No vector integrations were detected in the MDS1/EVI1 region, in contrast to our previous findings with a gamma-retroviral vector. These data show that lentiviral vectors can mediate stable and efficient long-term expression in the progeny of transduced hematopoietic stem cells, with an integration profile that may be safer than that of standard Moloney murine leukemia virus (MLV)-derived retroviral vectors.

Reconstitution of the myeloid and lymphoid compartments after the transplantation of autologous and genetically modified CD34+ bone marrow cells, following gamma irradiation in cynomolgus macaques

Background

Prolonged, altered hematopoietic reconstitution is commonly observed in patients undergoing myeloablative conditioning and bone marrow and/or mobilized peripheral blood-derived stem cell transplantation. We studied the reconstitution of myeloid and lymphoid compartments after the transplantation of autologous CD34⁺ bone marrow cells following gamma irradiation in cynomolgus macaques.

Results

The bone marrow cells were first transduced ex vivo with a lentiviral vector encoding eGFP, with a mean efficiency of 72% ± 4%. The vector used was derived from the simian immunodeficiency lentivirus SIVmac251, VSV-g pseudotyped and encoded eGFP under the control of the phosphoglycerate kinase promoter. After myeloid differentiation, GFP was detected in colony-forming cells (37% ± 10%). A previous study showed that transduction rates did not differ significantly between colony-forming cells and immature cells capable of initiating long-term cultures, indicating that progenitor cells and highly immature hematopoietic cells were transduced with similar efficiency. Blood cells producingeGFP were detected as early as three days after transplantation, and eGFP-producing granulocyte and mononuclear cells persisted for more than one year in the periphery.

Conclusion

The transplantation of CD34⁺ bone marrow cells had beneficial effects for the ex vivo proliferation and differentiation of hematopoietic progenitors, favoring reconstitution of the T- and B-lymphocyte, thrombocyte and red blood cell compartments.

Potent inhibition of simian immunodeficiency virus (SIV) replication by an SIV-based lentiviral vector expressing antisense Env

In light of findings demonstrating that the macaque TRIM5alpha protein inhibits infection of cells by human immunodeficiency virus (HIV)-1, simian immunodeficiency virus (SIV)-based lentiviral vectors may have distinct advantages over HIV-1 vectors for the transduction of macaque hematopoietic stem cells. We evaluated the ability of an SIV vector (VRX859) encoding an antisense SIV envelope sequence and enhanced green fluorescent protein (GFP) to inhibit viral replication and to transduce rhesus CD34(+) lymphoid progenitor cells. After infection with homologous SIV strains, CD4(+) cell lines transduced with VRX859 exhibited more than 600-fold inhibition of viral replication compared with control cells. Less inhibition was observed with the divergent SIV strain SIVsmE660. Partial inhibition of a chimeric simian-human immunodeficiency virus, which contains an HIV-1 envelope in an SIV backbone, was observed, suggesting that the SIV vector also contributes to viral inhibition independent of the antisense envelope inhibitor. Transduction of rhesus CD34(+) cells with VRX859 at various multiplicities of infection resulted in transduction efficiencies comparable to those obtained with the HIV vector VRX494. However, when we evaluated transduction of rhesus T lymphocyte progenitors by examining GFP expression in CD4(+) T cells derived from transduced CD34(+) cells, we observed more efficient transduction with the SIV-based vector. GFP(+)CD4(+) T cells derived from VRX859-transduced CD34(+) cells strongly inhibited SIVmac239 replication as compared with control CD4(+) T cells. The ability of this SIV-based vector to mediate potent inhibition of SIV replication, coupled with its efficient transduction of rhesus hematopoietic progenitor cells, make it an important candidate for proof-of-principle experiments of stem cell gene therapy in the SIV-macaque model.

Hematopoietic stem-cell behavior in nonhuman primates

Little is known about the behavior of hematopoietic stem cells (HSCs) in primates because direct observations and competitive-repopulation assays are not feasible. Therefore, we used 2 different and independent experimental strategies, the tracking of transgene expression after retroviral-mediated gene transfer (N = 11 baboons; N = 7 rhesus macaques) and quantitation of the average telomere length of granulocytes (N = 132 baboons; N = 14 macaques), together with stochastic methods, to study HSC kinetics in vivo. The average replication rate for baboon HSCs is once per 36 weeks according to gene-marking analyses and once per 23 weeks according to telomere-shortening analyses. Comparable results were derived from the macaque data. These rates are substantially slower than the average replication rates previously reported for HSCs in mice (once per 2.5 weeks) and cats (once per 8.3 weeks). Because baboons and macaques live for 25 to 45 years, much longer than mice ( approximately 2 years) and cats (12-18 years), we can compute that HSCs undergo a relatively constant number ( approximately 80-200) of lifetime replications. Thus, our data suggest that the self-renewal capacity of mammalian stem cells in vivo is defined and evolutionarily conserved.

Stem cell gene transfer: insights into integration and hematopoiesis from primate genetic marking studies

Genetic marking strategies in the nonhuman primate model have elucidated a number of principles with relevance to implementation of clinical stem cell therapies, including the lineage potential, number, and life span of hematopoietic stem and progenitor cells. The recent occurrence of leukemias likely related to insertional activation of a proto-oncogene in two patients with X-severe combined immunodeficiency (SCID) syndromes treated with CD34(+) cells transduced with retroviral vectors expressing the corrective common gamma cytokine receptor gene has refocused attention on the issue of retroviral integration. We have analyzed >1500 independent insertions from rhesus macaques transplanted with CD34(+) cells transduced with either MLV or SIV vectors. Of these, 46 rhesus macaques followed long term have not had progression to leukemia, abnormal hematopoiesis, or clonal hematopoiesis. However, the pattern of both MLV and SIV integrants in cells of these animals was found to be highly nonrandom, with a propensity for insertions of both vectors within genes: for MLV particularly near the transcription start site, and for SIV particularly in gene-dense regions.

Gene therapy to inhibit xenoantibody production using lentiviral vectors in non-human primates

Xenoantibodies to the gal alpha1,3 gal (gal) epitope impede the use of pig tissues for xenotransplantation, a procedure that may help overcome the shortage of human organ donors. Stable gal chimerism and tolerance to gal(+) hearts could be achieved in alpha1,3-galactosyltransferase (alpha1,3GT)(-/-) mice using lentiviral vectors expressing porcine alpha1,3GT, the enzyme that synthesizes the gal carbohydrate. In this study, we evaluated whether chimerism sufficient to inhibit anti-gal xenoantibody responses can be achieved using lentivectors in non-human primates. Rhesus macaques were transplanted with autologous, alpha1,3GT-transduced bone marrow (BM) following sublethal irradation. Simian immunodeficiency virus (SIV)- and human immunodeficiency virus (HIV)-1-derived lentiviral constructs were compared. Chimerism was observed in several hematopoietic lineages in all monkeys. Engraftment in animals receiving SIV-based alpha1,3GT constructs was similar to that achieved using the HIV-1-derived lentivector for the first 2 months post-transplantation, but increased thereafter to reach higher levels by 5 months. Upon immunization with porcine hepatocytes, the production of anti-gal immunoglobulin M xenoantibody was substantially reduced in the gal(+) BM recipients compared to controls. This study is the first to report the application of gene therapy to achieve low-level, long-term gal chimerism sufficient to inhibit production of anti-gal antibodies after immunization with porcine cells in rhesus macaques.

AMD3100 mobilizes hematopoietic stem cells with long-term repopulating capacity in nonhuman primates

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4, has been shown to induce rapid mobilization of CD34(+) hematopoietic cells in mice, dogs, and humans, offering an alternative to G-CSF mobilization of peripheral-blood hematopoietic stem cells. In this study, AMD3100-mobilized CD34(+) cells were phenotypically analyzed, marked with Neo(R)-containing retroviral vectors, and subsequently transplanted into myeloablated rhesus macaques. We show engraftment of transduced AMD3100-mobilized CD34(+) cells with Neo(R) gene marked myeloid and lymphoid cells up to 32 months after transplantation, demonstrating the ability of AMD3100 to mobilize true long-term repopulating hematopoietic stem cells. More AMD3100-mobilized CD34(+) cells are in the G(1) phase of the cell cycle and more cells express CXCR4 and VLA-4 compared with G-CSF-mobilized CD34(+) cells. In vivo gene marking levels obtained with AMD3100-mobilized CD34(+) cells were better than those obtained using CD34(+) cells mobilized with G-CSF alone. Overall, these results indicate that AMD3100 mobilizes a population of hematopoietic stem cells with intrinsic characteristics different from those of hematopoietic stem cells mobilized with G-CSF, suggesting fundamental differences in the mechanism of AMD3100-mediated and G-CSF-mediated hematopoietic stem cell mobilization. Thus, AMD3100-mobilized CD34(+) cells represent an alternative source of hematopoietic stem cells for clinical stem cell transplantation and genetic manipulation with integrating retroviral vectors.

Large animal models and gene therapy

Over the last two decades, gene transfer experiments for the treatment of inherited or acquired diseases have mainly been performed in mice. While mice provide proof of principle and allow testing of a variety of therapeutic modalities, mouse models have some limitations, as only short-term experiments can be performed, their homogenous genetic background is unlike humans, and the knockout models do not always faithfully represent the human disease. Naturally occurring large animal models of human genetic diseases have become increasingly important despite the costs and the extensive clinical attention they require because of their similarities to human patients. Large animals are reasonably outbred, long lived allowing for longitudinal studies, are more similar in size to a neonate or small child providing an opportunity to address issues related to scaling up therapy, and many physiological parameters including the immune system are more similar to those in humans versus those in mice.

Nonhuman primate embryonic stem cells as a preclinical model for hematopoietic and vascular repair

Stem cell-based regenerative medicine therapies have been touted recently as a novel therapeutic approach to treat and cure a wide range of diseases. Both adult and embryonic stem (ES) cells can serve as important sources of precursor cells to derive more mature cells potentially utilized for clinical applications. Nonhuman primates have proven useful as a preclinical model, as demonstrated in studies of hematopoietic cell transplantation, gene therapy, and other areas. The derivation of nonhuman primate ES cells now provides an optimal resource to characterize and test ES cell-based therapies prior to trials with human ES cells. This review describes work to define strategies and mechanisms to derive blood and endothelial cells from nonhuman primate ES cells isolated from various species. Preclinical testing that solely relies on studies of putative therapeutic cells derived from mouse ES cells transplanted into other mice, or analyses of human ES cell-derived cells transplanted into immunodeficient or immunosuppressed rodents may not be predictive of efficacy in subsequent human trials. However, future testing using nonhuman primate ES cell-derived therapeutic cells done as an allogeneic transplant may best predict success for subsequent studies using human ES cells. Therefore, additional research on nonhuman primate ES cells, in addition to work on mouse and human ES cells, is greatly needed to facilitate clinical translation of new stem cell treatments.

Large animal models for stem and progenitor cell analysis

Extrapolation of an understanding regarding hematopoiesis and, in particular, hematopoietic stem cells (HSCs) from rodent models or in vitro human cell models to applications in humans has proven very difficult. This is not surprising, given the differences between rodent and human hematopoietic physiology and the lack of true in vitro assays for HSCs. Therefore, translational preclinical development of genetic and cellular therapies is dependent on the utilization of practical and well-defined large animal models. This chapter will introduce the most commonly used model species, including macaques, baboons, dogs, cats, and sheep, and explain the particular advantages and limitations of each. Specific protocols for the support of macaques through ablative cell and gene therapy procedures will be included to introduce investigators to the types of resources and support required to maintain a large animal facility dedicated to high-intensity experimentation, and also to introduce investigators to the types of procedures that are possible.

Hematopoietic stem cell gene therapy: dead or alive?

Despite some reports of toxicity in recent clinical trials, many scientists believe that the use of gene therapy in the treatment of congenital genetic defects and acquired disorders has too much potential to abandon. Hematopoietic stem cells (HSCs) have been primary targets for gene therapy owing to their capacity for differentiation and self-renewal, whereby multiple cell lineages can potentially be corrected for the lifetime of an individual. These efforts represent a long-term investment towards broadening physicians' treatment options for patients whose diseases, in particular certain immunodeficiencies, are fatal and where no other therapy is available. We review the recent progress and clinical triumphs as well as the reported toxicity related to insertional mutagenesis. We also discuss the current risk-to-benefit estimates and future strategies to reduce the risks and allow full realization of clinical potential. Scientists are continually revising protocols: going both from "bench to bedside" and, as strikingly demonstrated by HSC gene therapy, from "bedside to bench."

Current status of gene therapy strategies to treat HIV/AIDS

Progress in developing effective gene transfer approaches to treat HIV-1 infection has been steady. Many different transgenes have been reported to inhibit HIV-1 in vitro. However, effective translation of such results to clinical practice, or even to animal models of AIDS, has been challenging. Among the reasons for this failure are uncertainty as to the most effective cell population(s) to target, the diffuseness of these target cells in the body, and ineffective or insufficiently durable gene delivery. Better understanding of the HIV-1 replicative cycle, host factors involved in HIV-1 infection, vector biology and application, transgene technology, animal models, and clinical study design have all contributed vastly to planning current and future strategies for application of gene therapeutic approaches to the treatment of AIDS. This review focuses on the newest developments in these areas and provides a strong basis for renewed optimism that gene therapy will have an important role to play in treating people infected with HIV-1.

Stem and progenitor cell therapies: insights from non-human primate models

Genetic marking strategies in the non-human primate model have elucidated a number of principles relevant to implementation of clinical stem cell therapies, including the lineage potential, number and lifespan of hematopoietic stem and progenitor cells, and differences in the functional properties of marrow cells mobilized into the peripheral blood utilizing different regimens.

Recurrent retroviral vector integration at the Mds1/Evi1 locus in nonhuman primate hematopoietic cells

Recent reports linking insertional activation of LMO2 following gene therapy for X-linked severe combined immunodeficiency (X-SCID) have led to a re-evaluation of risks following gene therapy with retroviral vectors. In our analysis of 702 integration sites in rhesus macaques that underwent transplantation up to 7 years earlier with autologous CD34+ cells transduced with amphotropic murine leukemia virus (MLV)-derived retroviral vectors containing marker genes, we detected insertion into one locus, the Mds1/Evi1 region, a total of 14 times in 9 animals. Mds1/Evi1 integrations were observed stably long term, primarily in myeloid cells. We hypothesize that this over-representation likely results from an impact on the self-renewal and engraftment potential of CD34+ progenitor cells via insertional mutagenesis at this specific locus. There is no evidence of ongoing in vivo clonal expansion of the Mds1/Evi1 populations, and all animals are hematologically normal without evidence for leukemia. Characterization of integration sites in this relevant preclinical model provides critical information for gene therapy risk assessment as well as identification of genes controlling hematopoiesis.

Expression of human IL-1alpha after intramarrow gene transfer into healthy non-human primate by adenoviral vector

Interleukin-1alpha (IL-1alpha) is a multifunctional cytokine that stimulates myelopoiesis in macaque. However, daily systemic injections of IL-1alpha are associated with severe side effects. We therefore investigated the feasibility of a gene therapy strategy aimed at increasing the IL-1alpha local production in bone marrow with limited release of the vector into the blood circulation. Intra-medullar administration of adenoviral vector containing human IL-1alpha (huIL-1alpha) gene resulted in enhanced neutrophil, monocyte and platelet counts during the two first weeks after injection. The DNA vector, the transgene expression and the huIL-1alpha production was detected in treated bone marrow without significant detection of huIL-1alpha in the peripheral blood. Associated with huIL-1alpha production, we observed concomitant plasma C reactive protein and IL-1Ra peaks in the acellular fraction of treated bone marrow at days 3 and 7. No abnormal clinical side effects were observed in any of the animals following the adenoviral vector injection.

Gene therapy for chronic granulomatous disease

Chronic granulomatous disease (CGD) is a congenital immune deficiency that is a promising therapeutic target for gene replacement into haematopoietic stem cells (HSCs). CGD results from mutations in any one of four genes encoding subunits of the superoxide-generating NADPH oxidase of phagocytes. Life-threatening, recurrent bacterial and fungal infections, as well as inflammatory granulomas, are the hallmarks of the disease. NADPH oxidase activity can be reconstituted by retroviral- or lentiviral-mediated gene transfer to human CGD marrow in vitro and in xenograft transplant models. Gene transfer studies in knockout mouse models that resemble the human disease suggest that correction of oxidase activity in a minority of phagocytes will be of clinical benefit. Phase I clinical studies in unconditioned CGD patients showed transient expression of small numbers of gene-corrected neutrophils. Areas of research at present include efforts to enhance gene transfer rates into repopulating HSCs using vectors that transduce quiescent cells, and to increase the engraftment of genetically corrected HSCs using non-myeloablative conditioning and drug resistance genes for selection.

Distinct genomic integration of MLV and SIV vectors in primate hematopoietic stem and progenitor cells

Murine leukemia virus (MLV)-derived vectors are widely used for hematopoietic stem cell (HSC) gene transfer, but lentiviral vectors such as the simian immunodeficiency virus (SIV) may allow higher efficiency transfer and better expression. Recent studies in cell lines have challenged the notion that retroviruses and retroviral vectors integrate randomly into their host genome. Medical applications using these vectors are aimed at HSCs, and thus large-scale comprehensive analysis of MLV and SIV integration in long-term repopulating HSCs is crucial to help develop improved integrating vectors. We studied integration sites in HSCs of rhesus monkeys that had been transplanted 6 mo to 6 y prior with MLV- or SIV-transduced CD34⁺ cells. Unique MLV (491) and SIV (501) insertions were compared to a set of in silico-generated random integration sites. While MLV integrants were located predominantly around transcription start sites, SIV integrants strongly favored transcription units and gene-dense regions of the genome. These integration patterns suggest different mechanisms for integration as well as distinct safety implications for MLV versus SIV vectors.

A primate model of gene transfer into hematopoietic stem cells demonstrated MLV integration around transcription start sites whereas SIV integrated into gene-dense regions, indicating distinct safety implications for each

Genetic manipulation of hematopoietic stem cells

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

Low-dose total body irradiation causes clonal fluctuation of primate hematopoietic stem and progenitor cells

Due to high frequency of side effects caused by high-dose total body irradiation (TBI) the nonmyeloablative regimen together with cytotoxic agents is currently used especially for elderly patients. However, immediate and long-term effects of low-dose irradiation used in allogeneic transplantation on stem cells is less well known. We have studied the effect of low-dose 3 Gy TBI on the number of hematopoietic stem cell (HSC) clones contributing simultaneously to granulocyte production in rhesus macaque. The number of clones after 3 Gy TBI decreased markedly by 2 to 3 weeks after 3 Gy TBI, followed by a period of clonal instability, and recovery to almost pre-3 Gy TBI clonal diversity. The clones accounting for this recovery contributed before 3 Gy TBI, suggesting the profound initial impact of TBI was on a pool of progenitor cells, whereas most of the more primitive HSCs remained unaffected and were able to again contribute to hematopoiesis after recovery. Clonal fluctuation may indirectly suggest the presence of short-term/long-term HSC populations in rhesus macaque bone marrow as reported in a mouse model. The results indicate that even low-dose irradiation affects hematopoietic clonal dynamics and have implications for design of conditioning regimens for transplantation purposes.

Induction of transgene-specific immunological tolerance in myeloablated nonhuman primates using lentivirally transduced CD34+ progenitor cells

Modeling human hematopoietic progenitor cell gene therapy in nonhuman primates allows long-term evaluation of safety, maintenance of gene expression, and potential immune response against transgene products. We transplanted autologous G-CSF/SCF-mobilized CD34+ cells transduced with lentiviral vectors expressing EGFP into myeloablated rhesus macaques. To date, more than 4 years posttransplantation, 0.5-8% EGFP expression is maintained in multiple cell lineages. The animals remain healthy with no evidence of hematopoietic abnormalities or malignancies. To assess immune functions, we actively immunized two of our transplanted animals with purified rEGFP proteins and CpG adjuvant and demonstrated stable levels of EGFP+ cell populations maintained for over 29 months despite four active immunizations. We did not detect a persistent anti-EGFP antibody response or anti-EGFP T cell response in these immunized animals. Immune response to an irrelevant antigen was normal. Taken together, our data provide formal support that transplantation of lentivirally transduced CD34+ progenitor cells in myeloablated rhesus macaques induces specific immunological tolerance toward a foreign transgene.

The presence of the carboxy-terminal fragment of fibronectin allows maintenance of non-human primate long-term hematopoietic repopulating cells during extended ex vivo culture and transduction

OBJECTIVE

Ex vivo expansion of primitive hematopoietic cells remains of interest for gene therapy and transplantation. Previous studies reported loss of repopulating activity following culture of cells for more than 4-7 days in the presence of cytokines or stromal cells. In the current study, we investigated whether prolonged culture and transduction in the presence of the carboxy-terminal portion of fibronectin (FN) could maintain or expand retrovirally transduced repopulating hematopoietic stem cells (HSCs).

METHODS

The impact of culture and transduction on rhesus macaque CD34+ peripheral blood stem cells (PBSCs) was assessed in the presence of FN and stimulatory cytokines. A competitive repopulation design using up to three retroviral vectors allowed direct comparison of repopulating activity between cells transduced and cultured for 4 days vs 10 days.

RESULTS

In the first animal, all cells were cultured and transduced for 10 days, with one vector used on days 0-4 and a second on days 4-10. There was stable long-term marking from both vectors, indicating that cells cycling both early and late could engraft. In three animals, we compared cells that were cryopreserved following a 4-day transduction to cells that were continued in culture for an additional 6 days. Total marking derived from the 10-day expanded cells was significantly higher than marking from the 4-day cultured cells.

CONCLUSIONS

These results suggest that culture on FN support allows prolonged ex vivo maintenance and even expansion of transduced repopulating stem cells.

The impact of low-dose busulfan on clonal dynamics in nonhuman primates

An understanding of the number and contribution of individual pluripotent hematopoietic stem cells (HSCs) to the formation of blood lineages has important clinical implications for gene therapy and stem cell transplantation. We have been able to efficiently mark rhesus macaque long-term repopulating stem and progenitor cells with retroviral vectors, and track their in vivo contributions to hematopoiesis using the linear amplification mediated-polymerase chain reaction (LAM-PCR) technique of insertion site analysis. We assessed the impact of busulfan on contributions of individual retrovirally marked clones to hematopoiesis. There were 2 macaques that received transplants of retrovirally transduced CD34(+) cells 2 years previously that were then treated with 4 mg/kg busulfan. Despite only transient and mild suppression of peripheral blood counts, the numbers of individual stem/progenitor clones contributing to granulocyte production decreased dramatically, by 80% in the first monkey and by 60% in the second monkey. A similar impact was seen on clones contributing to T cells. The clone numbers recovered gradually back toward baseline by 5 months following busulfan in the first monkey and by 3 months in the second monkey, and have remained stable for more than one year in both animals. Tracking of individual clones with insertion-site-specific primers suggested that clones contributing to hematopoiesis prior to busulfan accounted for the majority of this recovery, but that some previously undetected clones began to contribute during this recovery phase. These results indicate that even low-dose busulfan significantly affects stem and progenitor cell dynamics. The clonal diversity of hematopoiesis was significantly decreased after even a single, clinically well-tolerated dose of busulfan, with slow but almost complete recovery over the next several months, suggesting that true long-term repopulating stem cells were not permanently deleted. However, the prolonged period of suppression of many clones suggests that transplanted HSCs may have a marked competitive advantage if they can engraft and proliferate during this time period, and supports the use of this agent in nonmyeloablative regimens

Long-Term Clinical and Molecular Follow-up of Large Animals Receiving Retrovirally Transduced Stem and Progenitor Cells: No Progression to Clonal Hematopoiesis or Leukemia

There has been significant progress toward clinically relevant levels of retroviral gene transfer into hematopoietic stem cells (HSC), and the therapeutic potential of HSC-based gene transfer has been convincingly demonstrated in children with severe combined immunodeficiency syndrome (SCID). However, the subsequent development of leukemia in two children with X-linked SCID who were apparently cured after transplantation of retrovirally corrected CD34+ cells has raised concerns regarding the safety of gene therapy approaches utilizing integrating vectors. Nonhuman primates and dogs represent the best available models for gene transfer safety and efficacy and are particularly valuable for evaluation of long-term effects. We have followed 42 rhesus macaques, 23 baboons, and 17 dogs with significant levels of gene transfer for a median of 3.5 years (range 1-7) after infusion of CD34+ cells transduced with retroviral vectors expressing marker or drug-resistance genes. None developed abnormal hematopoiesis or leukemia. Integration site analysis confirmed stable, polyclonal retrovirally marked hematopoiesis, without progression toward mono- or oligoclonality over time. These results suggest that retroviral integrations using replication-incompetent vectors, at copy numbers achieved using standard protocols, are unlikely to result in leukemogenesis and that patient- or transgene-specific factors most likely contributed to the occurrence of leukemia in the X-SCID gene therapy trial.

Functional endothelial cells derived from rhesus monkey embryonic stem cells

We have used rhesus monkey embryonic stem (ES) cells to study endothelial cell development. Rhesus ES cells (R366.4 cell line) exposed to medium containing vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF), and epidermal growth factor (EGF) assumed a relatively uniform endothelial cell morphology and could be propagated and expanded with a consistent phenotype and normal karyotype. When placed in Matrigel, these rhesus ES cell–derived endothelial cells (RESDECs) formed capillary-like structures characteristic of endothelial cells. Immunohistochemical and flow cytometric analysis of RESDECs showed that they take up acetylated low-density lipoprotein (LDL), express CD146, von Willebrand factor, and the integrin αvβ3, and bind the lectin ulex europaeus agglutinin-1. These cells also express the VEGF receptor Flk-1 and secrete VEGF. When introduced in a Matrigel plug implanted subcutaneously in mice, RESDECs formed intact vessels and recruited new endothelial cell growth. In vivo function was demonstrated by coinjection of RESDECs with murine tumor cells subcutaneously into immunocompromised adult mice. RESDECs injected alone did not form measurable tumors. Tumor cells grew more rapidly and had increased vascularization when coinjected with the RESDECs. Immunohistochemical staining demonstrated that the RESDECs participated in forming the tumor neovasculature. RESDECs provide a novel means to examine the mechanisms of endothelial cell development, and may open up new therapeutic strategies.

Predictable and efficient retroviral gene transfer into murine bone marrow repopulating cells using a defined vector dose

OBJECTIVE

Current protocols of retroviral gene transfer into murine hematopoietic stem cells (HSC) result in variable gene transfer efficiency and involve various procedures that are not clinically applicable. We developed and evaluated a reliable transduction protocol that is more related to clinical methods.

MATERIALS AND METHODS

HSC were enriched from steady-state bone marrow by magnetic cell sorting (lineage depletion) and cultured in defined serum-free medium containing an improved growth factor cocktail (Flt3-ligand, stem cell factor, interleukin-3, interleukin-11). Cell-free ecotropic retroviral vector particles, generated by transient transfection of human 293T-based packaging cells, were preloaded at defined titers on CH296-coated tissue culture plates, thus largely avoiding serum contamination. These conditions were evaluated in 17 experiments involving 29 transduction cultures and 185 recipient mice.

RESULTS

After two rounds of infection, the gene marking rates in cultured mononuclear cells and stem/progenitor cells (Lin(-)c-Kit(+)) were 15 to 85% (53.7%+/-21.7%, n=23) and 30 to 95% (69.8%+/-20.4%, n=17), respectively. Even after one round of infection, gene transfer was efficient (31.2%+/-15.1%, n=12). Using identical conditions, gene transfer rates were highly reproducible. Average transgene expression in reconstituted animals correlated well with pretransplant data. Using a moderate multiplicity of infection, the majority of transduced cells carried less than three transgene copies. In addition, coinfection was possible to establish two different vectors in single cells.

CONCLUSION

The protocol described here achieves efficient retroviral transduction of murine bone marrow repopulating cells with a defined gene dosage, largely avoiding procedures that decrease stem cell output and repopulating capacity. This protocol may help to improve the predictive value of preclinical efficiency/toxicity studies for gene therapeutic interventions and basic research.