Retroviral transduction and engraftment ability of primate hematopoietic progenitor and stem cells transduced under serum-free versus serum-containing conditions

Hematopoietic Stem Cell Gene-Addition/Editing Therapy in Sickle Cell Disease

Autologous hematopoietic stem cell (HSC)-targeted gene therapy provides a one-time cure for various genetic diseases including sickle cell disease (SCD) and β-thalassemia. SCD is caused by a point mutation (20A > T) in the β-globin gene. Since SCD is the most common single-gene disorder, curing SCD is a primary goal in HSC gene therapy. β-thalassemia results from either the absence or the reduction of β-globin expression, and it can be cured using similar strategies. In HSC gene-addition therapy, patient CD34+ HSCs are genetically modified by adding a therapeutic β-globin gene with lentiviral transduction, followed by autologous transplantation. Alternatively, novel gene-editing therapies allow for the correction of the mutated β-globin gene, instead of addition. Furthermore, these diseases can be cured by γ-globin induction based on gene addition/editing in HSCs. In this review, we discuss HSC-targeted gene therapy in SCD with gene addition as well as gene editing.

Hematopoietic-Stem-Cell-Targeted Gene-Addition and Gene-Editing Strategies for β-hemoglobinopathies

Sickle cell disease (SCD) is caused by a well-defined point mutation in the β-globin gene and therefore is an optimal target for hematopoietic stem cell (HSC) gene-addition/editing therapy. In HSC gene-addition therapy, a therapeutic β-globin gene is integrated into patient HSCs via lentiviral transduction, resulting in long-term phenotypic correction. State-of-the-art gene-editing technology has made it possible to repair the β-globin mutation in patient HSCs or target genetic loci associated with reactivation of endogenous γ-globin expression. With both approaches showing signs of therapeutic efficacy in patients, we discuss current genetic treatments, challenges, and technical advances in this field.

Serum-free Erythroid Differentiation for Efficient Genetic Modification and High-Level Adult Hemoglobin Production

In vitro erythroid differentiation from primary human cells is valuable to develop genetic strategies for hemoglobin disorders. However, current erythroid differentiation methods are encumbered by modest transduction rates and high baseline fetal hemoglobin production. In this study, we sought to improve both genetic modification and hemoglobin production among human erythroid cells in vitro. To model therapeutic strategies, we transduced human CD34⁺ cells and peripheral blood mononuclear cells (PBMCs) with lentiviral vectors and compared erythropoietin-based erythroid differentiation using fetal-bovine-serum-containing media and serum-free media. We observed more efficient transduction (85%-93%) in serum-free media than serum-containing media (20%-69%), whereas the addition of knockout serum replacement (KSR) was required for serum-free media to promote efficient erythroid differentiation (96%). High-level adult hemoglobin production detectable by electrophoresis was achieved using serum-free media similar to serum-containing media. Importantly, low fetal hemoglobin production was observed in the optimized serum-free media. Using KSR-containing, serum-free erythroid differentiation media, therapeutic adult hemoglobin production was detected at protein levels with β-globin lentiviral transduction in both CD34⁺ cells and PBMCs from sickle cell disease subjects. Our in vitro erythroid differentiation system provides a practical evaluation platform for adult hemoglobin production among human erythroid cells following genetic manipulation.

Optimal conditions for lentiviral transduction of engrafting human CD34+ cells

Cytokines are required for γ-retroviral transduction of human CD34+ cells. However, cytokines may reduce engraftment of CD34+ cells and may not be necessary for their lentiviral transduction. We sought to optimize transduction and engraftment of human CD34+ cells using lentiviral vectors. Single 24 h transduction of human CD34+ cells with human immunodeficiency virus type 1 (HIV1)-based lentiviral vectors in media containing stem cell factor (SCF), FMS-like tyrosine kinase 3 (FLT3) ligand, thrombopoietin (each 100 ng ml⁻¹) and 10% fetal bovine serum was compared with various cytokine conditions during ex vivo culture and assayed using humanized xenograft mice for 6 months after transplantation. Serum-free media improved transduction efficiency of human CD34+ cells. Interleukin-3 (20 ng ml⁻¹) had little effect on transduction efficiency or engraftment. Threefold higher cytokine mixture (each 300 ng ml⁻¹) reduced engraftment of CD34+ cells. SCF alone (100 ng ml⁻¹) proved insufficient for maintaining engraftment ability and reduced transduction efficiency. Short-term prestimulation had little effect on transduction efficiency or engraftment, yet 24 h prestimulation showed higher transduction efficiency, higher gene expression levels and lower engraftment. In summary, 24 h prestimulation followed by single 24-h lentiviral transduction in serum-free media with SCF, FLT3 ligand and thrombopoietin yields high transduction efficiency to engrafting human CD34+ cells, and is applicable in human clinical gene therapy trials.

Myeloablative irradiation in non-human primates

PURPOSE

We used total body irradiation (TBI) as conditioning for cord blood transplantation studies in pigtailed macaques. In these studies, different doses of TBI were explored to obtain optimal myelosuppression with acceptable radiation-related side effects.

METHODS

Four macaques received TBI ranging from 800 to 1320 cGy, followed by standard post-transplant care. Hematopoietic recovery was monitored by CBC and donor contribution by variable number of tandem repeats analysis.

RESULTS

Animals receiving 800 or 1020 cGy TBI tolerated the irradiation well with autologous recovery of neutrophils within 24 days. At a dose of 1200 cGy, neither autologous recovery nor extramedullary toxicity was observed. A fourth animal received 1320 cGy of TBI and suffered significant toxicity necessitating termination of the study.

CONCLUSIONS

We conclude that previously considered myeloablative doses of TBI allowed for autologous recovery in the pigtailed macaque, and that a dose of 1200 cGy may be most appropriate, providing both myeloablation and acceptable non-hematopoietic toxicities.

Genetic modification of human hematopoietic cells: preclinical optimization of oncoretroviral-mediated gene transfer for clinical trials

This chapter provides information about the oncoretroviral transduction of human hematopoietic stem/ progenitor cells under clinically applicable conditions. We describe in detail a short -60 h transduction protocol which consistently yields transduction efficiencies in the range of 30-50% with five different oncoretroviral vectors. We discuss a number of parameters that affect transduction efficiency, including the oncoretroviral vector characteristics, the vector stock collection, the source of CD34+ cells and transduction conditions.

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.

Functional assessment of the engraftment potential of gammaretrovirus-modified CD34+ cells, using a short serum-free transduction protocol

The successful transduction and engraftment of human mobilized peripheral blood (MBP) CD34(+) cells are determined to a large extent by the ex vivo cell-processing conditions. In preparation for upcoming clinical trials, we investigated essential culture parameters and devised a short and efficient gammaretroviral transduction protocol entailing minimal manipulation of MBP CD34(+) cells. The engraftment potential and in vivo transgene expression in the progeny of repopulating CD34(+) cells were measured to assess the functionality of CD34(+) cells transduced under these conditions. Using a competitive in vivo repopulation assay in nonobese diabetic/severe combined immunodeficient mice, we demonstrate equivalent engraftment of CD34(+) cells transduced under serum-free conditions as compared with CD34(+) cells cultured with serum. We also took advantage of this in vivo model to demonstrate that ex vivo manipulation of CD34(+) cells can be shortened to 60 hr, using 36 hr of prestimulation and two cycles of transduction 12 hr apart. These minimally manipulated CD34(+) cells engraft in a manner similar to cells transduced under longer protocols and the vector-encoded transgene is expressed at the same frequency in cells derived from repopulating CD34(+) cells in vivo. We have thus developed a short and efficient human MBP CD34(+) transduction protocol under serum-free conditions that is suitable and broadly applicable for phase I clinical trials.

Isolated hepatocyte transplantation for Crigler-Najjar syndrome type 1

Crigler-Najjar syndrome type 1 (CN1) is an inherited disorder characterized by the absence of hepatic uridine diphosphoglucuronate glucuronosyltransferase (UDPGT), the enzyme responsible for the conjugation and excretion of bilirubin. We performed allogenic hepatocyte transplantation (AHT) in a child with CN1, aiming to improve bilirubin glucuronidation in this condition. A 9-year-old boy with CN1 was prepared with plasmapheresis and immunosuppression with prednisolone and tacrolimus. When a graft was made available, 7.5 x 10(9) hepatocytes were isolated and infused into the portal vein percutaneously. After 2 weeks phenobarbitone was added to promote the enzymatic activity of UDPGT of the transplanted hepatocytes. Nocturnal phototherapy was continued throughout the studied period. Total bilirubin was considered a reliable marker of allogenic cell function. There was no significant variation of vital signs nor complications during the infusion. Mean +/- SD bilirubin level was 530 +/- 38 micromol/L before and 359 +/- 46 micromol/L after AHT (t-test, p < 0.001). However, the introduction of phenobarbitone was followed by a drop of tacrolimus level with increase of alanine aminotransferase (ALT) and increase of bilirubin. After standard treatment of cellular rejection bilirubin fell again but from then on it was maintained at a greater level. After discharge the patient experienced a further increase of bilirubin that returned to predischarge levels after readmission to the hospital. This was interpreted as poor compliance with phototherapy. Only partial correction of clinical jaundice and the poor tolerability to nocturnal phototherapy led the parents to refuse further hepatocyte infusions and request an orthotopic liver transplant. After 24 months the child is well, with good liver function on tacrolimus and prednisolone-based immunosuppression. Isolated AHT, though effective and safe, is not sufficient to correct CN1. Maintenance of adequate immunosuppression and family compliance are the main factors hampering the success of this procedure.

Efficient in vitro transduction of naive murine B cells with lentiviral vectors

The aim of this study was to determine the impact of lentiviral transduction on primary murine B cells. Studying B cell activities in vivo or using them for tolerance induction requires that the cells remain unaltered in their biological behavior except for expression of the transgene. As we show here, murine B cells can efficiently be transduced by lentiviral, VSV-G-pseudotyped vectors without the necessity of prior activation. Culture with LPS gave enhanced transduction efficiencies but led to the upregulation of CD86 and proliferation of the cells. Transduction of naive B cells by lentiviral vectors was dependent on multiplicity of infection and did not lead to a concomitant activation. Furthermore, the transduced cells could be used for studies in the NOD mouse system without altering the onset of diabetes. We conclude that lentiviral gene transfer into naive B cells is a powerful tool for manipulation of B cells for therapeutic applications.

Hoxb4-deficient mice undergo normal hematopoietic development but exhibit a mild proliferation defect in hematopoietic stem cells

Enforced expression of Hoxb4 dramatically increases the regeneration of murine hematopoietic stem cells (HSCs) after transplantation and enhances the repopulation ability of human severe combined immunodeficiency (SCID) repopulating cells. Therefore, we asked what physiologic role Hoxb4 has in hematopoiesis. A novel mouse model lacking the entire Hoxb4 gene exhibits significantly reduced cellularity in spleen and bone marrow (BM) and a subtle reduction in red blood cell counts and hemoglobin values. A mild reduction was observed in the numbers of primitive progenitors and stem cells in adult BM and fetal liver, whereas lineage distribution was normal. Although the cell cycle kinetics of primitive progenitors was normal during endogenous hematopoiesis, defects in proliferative responses of BM Lin- Sca1+ c-kit+ stem and progenitor cells were observed in culture and in vivo after the transplantation of BM and fetal liver HSCs. Quantitative analysis of mRNA from fetal liver revealed that a deficiency of Hoxb4 alone changed the expression levels of several other Hox genes and of genes involved in cell cycle regulation. In summary, the deficiency of Hoxb4 leads to hypocellularity in hematopoietic organs and impaired proliferative capacity. However, Hoxb4 is not required for the generation of HSCs or the maintenance of steady state hematopoiesis.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Current developments in the design of onco-retrovirus and lentivirus vector systems for hematopoietic cell gene therapy

Over the past dozen years, the majority of clinical gene therapy trials for inherited genetic diseases and cancer therapy have been performed using murine onco-retrovirus as the gene delivery vector. The earliest systems used were relatively inefficient in both the rates of transduction and expression of the transgene. Formidable obstacles inherent in the cell biology and/or the immunology of the target cell systems limited the efficacy of gene therapy for many target diseases. Development of novel retrovirus gene transfer systems that are in progress have begun to overcome these obstacles. Evidence of this progress is the recent successful functional correction of the immune T and B lymphocyte deficiency in patients with X-linked severe combined immunodeficiency (X-SCID) and adenosine deaminase (ADA)-deficient SCID following onco-retrovirus vector ex vivo transduction of autologous marrow stem cells [Science 296 (2002) 2410; Science 288 (2000) 669; N. Engl. J. Med. 346 (2002) 1185]. These achievements of prolonged clinical benefit from gene therapy were tempered by the finding of insertional mutageneses in two of the treated X-SCID patients [N. Engl. J. Med. 348 (2003) 255].

Retroviral transduction efficiency of G-CSF+SCF-mobilized peripheral blood CD34+ cells is superior to G-CSF or G-CSF+Flt3-L-mobilized cells in nonhuman primates

Gene transfer experiments in nonhuman primates have been shown to be predictive of success in human clinical gene therapy trials. In most nonhuman primate studies, hematopoietic stem cells (HSCs) collected from the peripheral blood or bone marrow after administration of granulocyte colony-stimulating factor (G-CSF) + stem cell factor (SCF) have been used as targets, but this cytokine combination is not generally available for clinical use, and the optimum target cell population has not been systematically studied. In our current study we tested the retroviral transduction efficiency of rhesus macaque peripheral blood CD34(+) cells collected after administration of different cytokine mobilization regimens, directly comparing G-CSF+SCF versus G-CSF alone or G-CSF+Flt3-L in competitive repopulation assays. Vector supernatant was added daily for 96 hours in the presence of stimulatory cytokines. The transduction efficiency of HSCs as assessed by in vitro colony-forming assays was equivalent in all 5 animals tested, but the in vivo levels of mononuclear cell and granulocyte marking was higher at all time points derived from target CD34(+) cells collected after G-CSF+SCF mobilization compared with target cells collected after G-CSF (n = 3) or G-CSF+Flt3-L (n = 2) mobilization. In 3 of the animals long-term marking levels of 5% to 25% were achieved, but originating only from the G-CSF+SCF-mobilized target cells. Transduction efficiency of HSCs collected by different mobilization regimens can vary significantly and is superior with G-CSF+SCF administration. The difference in transduction efficiency of HSCs collected from different sources should be considered whenever planning clinical gene therapy trials and should preferably be tested directly in comparative studies.