Optimal conditions for lentiviral transduction of engrafting human CD34+ cells

Advances in HIV Gene Therapy

Early gene therapy studies held great promise for the cure of heritable diseases, but the occurrence of various genotoxic events led to a pause in clinical trials and a more guarded approach to progress. Recent advances in genetic engineering technologies have reignited interest, leading to the approval of the first gene therapy product targeting genetic mutations in 2017. Gene therapy (GT) can be delivered either in vivo or ex vivo. An ex vivo approach to gene therapy is advantageous, as it allows for the characterization of the gene-modified cells and the selection of desired properties before patient administration. Autologous cells can also be used during this process which eliminates the possibility of immune rejection. This review highlights the various stages of ex vivo gene therapy, current research developments that have increased the efficiency and safety of this process, and a comprehensive summary of Human Immunodeficiency Virus (HIV) gene therapy studies, the majority of which have employed the ex vivo approach.

TGFβ Inhibitor A83-01 Enhances Murine HSPC Expansion for Gene Therapy

Murine hematopoietic stem and progenitor cells (HSPCs) are commonly used as model systems during gene therapeutic retroviral vector development and preclinical biosafety assessment. Here, we developed cell culture conditions to maintain stemness and prevent differentiation during HSPC culture. We used the small compounds A83-01, pomalidomide, and UM171 (APU). Highly purified LSK SLAM cells expanded in medium containing SCF, IL-3, FLT3-L, and IL-11 but rapidly differentiated to myeloid progenitors and mast cells. The supplementation of APU attenuated the differentiation and preserved the stemness of HSPCs. The TGFβ inhibitor A83-01 was identified as the major effector. It significantly inhibited the mast-cell-associated expression of FcεR1α and the transcription of genes regulating the formation of granules and promoted a 3800-fold expansion of LSK cells. As a functional readout, we used expanded HSPCs in state-of-the-art genotoxicity assays. Like fresh cells, APU-expanded HSPCs transduced with a mutagenic retroviral vector developed a myeloid differentiation block with clonal restriction and dysregulated oncogenic transcriptomic signatures due to vector integration near the high-risk locus Mecom. Thus, expanded HSPCs might serve as a novel cell source for retroviral vector testing and genotoxicity studies.

Post-Transplant Administration of G-CSF Impedes Engraftment of Gene Edited Human Hematopoietic Stem Cells by Exacerbating the p53-Mediated DNA Damage Response

Granulocyte colony stimulating factor (G-CSF) is commonly used as adjunct treatment to hasten recovery from neutropenia following chemotherapy and autologous transplantation of hematopoietic stem and progenitor cells (HSPCs) for malignant disorders. However, the utility of G-CSF administration after ex vivo gene therapy procedures targeting human HSPCs has not been thoroughly evaluated. Here, we provide evidence that post-transplant administration of G-CSF impedes engraftment of CRISPR-Cas9 gene edited human HSPCs in xenograft models. G-CSF acts by exacerbating the p53-mediated DNA damage response triggered by Cas9- mediated DNA double-stranded breaks. Transient p53 inhibition in culture attenuates the negative impact of G-CSF on gene edited HSPC function. In contrast, post-transplant administration of G-CSF does not impair the repopulating properties of unmanipulated human HSPCs or HSPCs genetically engineered by transduction with lentiviral vectors. The potential for post-transplant G-CSF administration to aggravate HSPC toxicity associated with CRISPR-Cas9 gene editing should be considered in the design of ex vivo autologous HSPC gene editing clinical trials.

A novel preclinical model of mucopolysaccharidosis type II for developing human hematopoietic stem cell gene therapy

A hematopoietic stem cell (HSC) gene therapy (GT) using lentiviral vectors has attracted interest as a promising treatment approach for neuropathic lysosomal storage diseases. To proceed with the clinical development of HSC-GT, evaluation of the therapeutic potential of gene-transduced human CD34+ (hCD34+) cells in vivo is one of the key issues before human trials. Here, we established an immunodeficient murine model of mucopolysaccharidosis type II (MPS II), which are transplantable human cells, and demonstrated the application of those mice in evaluating the therapeutic efficacy of gene-modified hCD34+ cells. NOG/MPS II mice, which were generated using CRISPR/Cas9, exhibited a reduction of disease-causing enzyme iduronate-2-sulfatatase (IDS) activity and the accumulation of glycosaminoglycans in their tissues. When we transplanted hCD34+ cells transduced with a lentiviral vector carrying the IDS gene into NOG/MPS II mice, a significant amelioration of biochemical pathophenotypes was observed in the visceral and neuronal tissues of those mice. In addition, grafted cells in the NOG/MPS II mice showed the oligoclonal integration pattern of the vector, but no obvious clonal dominance was detected in the mice. Our findings indicate the promising application of NOG/MPS II mice to preclinical study of HSC-GT for MPS II using human cells.

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.

Sustained fetal hemoglobin induction in vivo is achieved by BCL11A interference and coexpressed truncated erythropoietin receptor

Hematopoietic stem cell gene therapy for hemoglobin disorders, including sickle cell disease, requires high-efficiency lentiviral gene transfer and robust therapeutic globin expression in erythroid cells. Erythropoietin is a key cytokine for erythroid proliferation and differentiation (erythropoiesis), and truncated human erythropoietin receptors (thEpoR) have been reported in familial polycythemia. We reasoned that coexpression of thEpoR could enhance the phenotypic effect of a therapeutic vector in erythroid cells in xenograft mouse and autologous nonhuman primate transplantation models. We generated thEpoR by deleting 40 amino acids from the carboxyl terminus, allowing for erythropoietin-dependent enhanced erythropoiesis of gene-modified cells. We then designed lentiviral vectors encoding both thEpoR and B cell lymphoma/leukemia 11A (BCL11A)-targeting microRNA-adapted short hairpin RNA (shmiR BCL11A) driven by an erythroid-specific promoter. thEpoR expression enhanced erythropoiesis among gene-modified cells in vitro. We then transplanted lentiviral vector gene-modified CD34⁺ cells with erythroid-specific expression of both thEpoR and shmiR BCL11A and compared to cells modified with shmiR BCL11A only. We found that thEpoR enhanced shmiR BCL11A-based fetal hemoglobin (HbF) induction in both xenograft mice and rhesus macaques, whereas HbF induction with shmiR BCL11A only was robust, yet transient. thEpoR/shmiR BCL11A coexpression allowed for sustained HbF induction at 20 to 25% in rhesus macaques for 4 to 8 months. In summary, we developed erythroid-specific thEpoR/shmiR BCL11A-expressing vectors, enhancing HbF induction in xenograft mice and rhesus macaques. The sustained HbF induction achieved by addition of thEpoR and shmiR BCL11A may represent a viable gene therapy strategy for hemoglobin disorders.

Nonviral genome engineering of natural killer cells

Natural killer (NK) cells are cytotoxic lymphocytes of the innate immune system capable of immune surveillance. Given their ability to rapidly and effectively recognize and kill aberrant cells, especially transformed cells, NK cells represent a unique cell type to genetically engineer to improve its potential as a cell-based therapy. NK cells do not express a T cell receptor and thus do not contribute to graft-versus-host disease, nor do they induce T cell-driven cytokine storms, making them highly suited as an off-the-shelf cellular therapy. The clinical efficacy of NK cell-based therapies has been hindered by limited in vivo persistence and the immunosuppressive tumor microenvironment characteristic of many cancers. Enhancing NK cell resistance to tumor inhibitory signaling through genome engineering has the potential to improve NK cell persistence in the tumor microenvironment and restore cytotoxic functions. Alongside silencing NK cell inhibitory receptors, NK cell killing can be redirected by the integration of chimeric antigen receptors (CARs). However, NK cells are associated with technical and biological challenges not observed in T cells, typically resulting in low genome editing efficiencies. Viral vectors have achieved the greatest gene transfer efficiencies but carry concerns of random, insertional mutagenesis given the high viral titers necessary. As such, this review focuses on nonviral methods of gene transfer within the context of improving cancer immunotherapy using engineered NK cells.

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.

Preclinical evaluation for engraftment of CD34+ cells gene-edited at the sickle cell disease locus in xenograft mouse and non-human primate models

Sickle cell disease (SCD) is caused by a 20A > T mutation in the β-globin gene. Genome-editing technologies have the potential to correct the SCD mutation in hematopoietic stem cells (HSCs), producing adult hemoglobin while simultaneously eliminating sickle hemoglobin. Here, we developed high-efficiency viral vector-free non-footprint gene correction in SCD CD34+ cells with electroporation to deliver SCD mutation-targeting guide RNA, Cas9 endonuclease, and 100-mer single-strand donor DNA encoding intact β-globin sequence, achieving therapeutic-level gene correction at DNA (∼30%) and protein (∼80%) levels. Gene-edited SCD CD34+ cells contributed corrected cells 6 months post-xenograft mouse transplant without off-target δ-globin editing. We then developed a rhesus β-to-βs-globin gene conversion strategy to model HSC-targeted genome editing for SCD and demonstrate the engraftment of gene-edited CD34+ cells 10-12 months post-transplant in rhesus macaques. In summary, gene-corrected CD34+ HSCs are engraftable in xenograft mice and non-human primates. These findings are helpful in designing HSC-targeted gene correction trials.

Cas9 protein delivery non-integrating lentiviral vectors for gene correction in sickle cell disease

Gene editing with the CRISPR-Cas9 system could revolutionize hematopoietic stem cell (HSC)-targeted gene therapy for hereditary diseases, including sickle cell disease (SCD). Conventional delivery of editing tools by electroporation limits HSC fitness due to its toxicity; therefore, efficient and non-toxic delivery remains crucial. Integrating lentiviral vectors are established for therapeutic gene delivery to engraftable HSCs in gene therapy trials; however, their sustained expression and size limitation preclude their use for CRISPR-Cas9 delivery. Here, we developed a Cas9 protein delivery non-integrating lentiviral system encoding guide RNA and donor DNA, allowing for transient endonuclease function and inclusion of all editing tools in a single vector (all-in-one). We demonstrated efficient one-time correction of the SCD mutation in the endogenous βs-globin gene up to 42% at the protein level (p < 0.01) with the Cas9 protein delivery non-integrating lentiviral all-in-one system without electroporation. Our findings improve prospects for efficient and safe genome editing.

Assessment and streamlined preparation of low-cytotoxicity lentiviral vectors for mobilized human hematopoietic stem cell transduction

As important vectors for ectopic protein expression, gene silencing, and progenitor cell barcoding, lentiviruses continue to emerge as versatile research and clinical tools. For studies employing cell types that are relatively resistant to transduction, high-titer lentivirus preparations with low cytotoxicity are required. During lentivirus production, carryover plasmid DNA endotoxins, transfection reagents, damaged packaging cells, and virus concentration procedures are potential sources of cytotoxicity. As an often unevaluated property of lentivirus preparations, cytotoxicity can unwittingly skew estimates of functional titers and complicate interpretations of transduced cell phenotypes. By employing hematopoietic UT7epo cells cultured in erythropoietin (EPO) below maximal dosing, we first define a sensitive flow cytometric bioassay for critically assessing the cytotoxicity (and titers) of lentivirus preparations. Bioassay of custom preparations of research-grade lentiviruses from six commercial sources unexpectedly revealed substantial cytotoxicity (with certain preparations additionally registering titers several log below designated values). To overcome such limiting properties, we further report on unique, efficient workflows for reproducibly preparing and processing high-titer, low-cytotoxicity (HTLC) lentiviruses at research scale. These HTLC lentiviruses reliably transduce peripheral blood hematopoietic stem/progenitor cells (PB-HSPCs) at frequencies ≥40%, with low cytotoxicity. In addition, by employing cyclosporin H (to inhibit IFITM3), PB-HSPCs can be transduced at heightened efficiency with nominal cytotoxicity. Overall, this work provides straightforward approaches to (1) critical assessment of the cytotoxicity of lentivirus preparations; (2) reproducible generation (and concentration) of high-quality lentiviruses via a streamlined workflow; and (3) transduction of PB-HSPCs at benchmark levels with nominal cytotoxicity.

Successfully transfected primary peripherally mobilized human CD34+ hematopoietic stem and progenitor cells (HSPCs) demonstrate increased susceptibility to retroviral infection

Transfection, the process of introducing purified nucleic acids into cells, and viral transduction, viral-mediated nucleic acid transfer, are two commonly utilized techniques for gene delivery in the research setting. Transfection allows purified nucleic acid to be introduced into target cells through chemical-based techniques, nonchemical methods or particle-based methods, while viral transduction employs genomes or vectors based on adenoviruses, retroviruses (e.g. lentiviruses), adeno-associated viruses, or hybrid viruses. Transfected DNAs are often tested for potential effects on subsequent transduction, but it is not clear whether transfection itself rather than the particular nucleic acid being introduced might impact subsequent viral transfection. We observed a significant association between successfully transfected mobilized peripheral blood CD34+ human stem and progenitor cells (HSPCs) and permissiveness to subsequent lentiviral transduction, which was not evident in other cells such as 293 T cells and Jurkat cells. This association, apparently specific to CD34+ human stem and progenitor cells (HSPCs), is critical to both research and clinical applications as these cells are a frequent target of transfection and viral transduction owing to the durable nature of these cells in living systems. This finding may also present a significant opportunity to enhance the success of viral transduction for clinical applications.

Biallelic correction of sickle cell disease-derived induced pluripotent stem cells (iPSCs) confirmed at the protein level through serum-free iPS-sac/erythroid differentiation

New technologies of induced pluripotent stem cells (iPSCs) and genome editing have emerged, allowing for the development of autologous transfusion therapies. We previously demonstrated definitive β‐globin production from human embryonic stem cell (hESC)‐derived erythroid cell generation via hemangioblast‐like ES‐sacs. In this study, we demonstrated normal β‐globin protein production from biallelic corrected sickle cell disease (SCD) iPSCs. We optimized our ES/iPS‐sac method for feeder cell‐free hESC maintenance followed by serum‐free ES‐sac generation, which is preferred for electroporation‐based genome editing. Surprisingly, the optimized protocol improved yields of ES‐sacs (25.9‐fold), hematopoietic‐like spherical cells (14.8‐fold), and erythroid cells (5.8‐fold), compared with our standard ES‐sac generation. We performed viral vector‐free gene correction in SCD iPSCs, resulting in one clone with monoallelic and one clone with biallelic correction, and using this serum‐free iPS‐sac culture, corrected iPSC‐generated erythroid cells with normal β‐globin, confirmed at DNA and protein levels. Our serum‐free ES/iPS‐sac protocol with gene correction will be useful to develop regenerative transfusion therapies for SCD.

Low-Dose Busulfan Reduces Human CD34+ Cell Doses Required for Engraftment in c-kit Mutant Immunodeficient Mice

Humanized animal models are central to efforts aimed at improving hematopoietic stem cell (HSC) transplantation with or without genetic modification. Human cell engraftment is feasible in immunodeficient mice; however, high HSC doses and conditioning limit broad use of xenograft models. We assessed human CD45⁺ chimerism after transplanting varying doses of human CD34⁺ HSCs (2 × 10⁵ to 2 × 10⁶ cells/mouse) with or without busulfan (BU) pretransplant conditioning in c-kit mutant mice that do not require conditioning (non-obese diabetic [NOD]/B6/severe combined immunodeficiency [SCID]/ interleukin-2 receptor gamma chain null (IL-2rγ^-/-) Kit^W41/W41 [NBSGW]). We then tested a range of BU (5-37.5 mg/kg) using 2 × 10⁵ human CD34⁺ cells. Glycophorin-A erythrocyte chimerism was assessed after murine macrophage depletion using clodronate liposomes. We demonstrated successful long-term engraftment of human CD34⁺ cells at all cell doses in this model, and equivalent engraftment using 10-fold less CD34⁺ cells with the addition of BU conditioning. Low-dose BU (10 mg/kg) was sufficient to allow human engraftment using 2 × 10⁵ CD34⁺ cells, whereas higher doses (≥37.5 mg/kg) were toxic. NBSGW mice support human erythropoiesis in the bone marrow; however, murine macrophage depletion provided only minimal and transient increases in peripheral blood human erythrocytes. Our xenograft model is therefore useful in HSC gene therapy and genome-editing studies, especially for modeling in disorders, such as sickle cell disease, where access to HSCs is limited.

Highly efficient ex vivo lentiviral transduction of primary human pancreatic exocrine cells

The lack of efficient gene transfer methods into primary human pancreatic exocrine cells hampers studies on the plasticity of these cells and their possible role in beta cell regeneration. Therefore, improved gene transfer protocols are needed. Lentiviral vectors are widely used to drive ectopic gene expression in mammalian cells, including primary human islet cells. Here we aimed to optimize gene transfer into primary human exocrine cells using modified lentiviral vectors or transduction conditions. We evaluated different promoters, viral envelopes, medium composition and transduction adjuvants. Transduction efficiency of a reporter vector was evaluated by fluorescence microscopy and flow cytometry. We show that protamine sulfate-assisted transduction of a VSV-G-pseudotyped vector expressing eGFP under the control of a CMV promoter in a serum-free environment resulted in the best transduction efficiency of exocrine cells, reaching up to 90% of GFP-positive cells 5 days after transduction. Our findings will enable further studies on pancreas (patho)physiology that require gene transfer such as gene overexpression, gene knockdown or lineage tracing studies.

Development of a forward-oriented therapeutic lentiviral vector for hemoglobin disorders

Hematopoietic stem cell (HSC) gene therapy is being evaluated for hemoglobin disorders including sickle cell disease (SCD). Therapeutic globin vectors have demanding requirements including high-efficiency transduction at the HSC level and high-level, erythroid-specific expression with long-term persistence. The requirement of intron 2 for high-level β-globin expression dictates a reverse-oriented globin-expression cassette to prevent its loss from RNA splicing. Current reverse-oriented globin vectors can drive phenotypic correction, but they are limited by low vector titers and low transduction efficiencies. Here we report a clinically relevant forward-oriented β-globin-expressing vector, which has sixfold higher vector titers and four to tenfold higher transduction efficiency for long-term hematopoietic repopulating cells in humanized mice and rhesus macaques. Insertion of Rev response element (RRE) allows intron 2 to be retained, and β-globin production is observed in transplanted macaques and human SCD CD34+ cells. These findings bring us closer to a widely applicable gene therapy for hemoglobin disorders.

Busulfan Combined with Immunosuppression Allows Efficient Engraftment of Gene-Modified Cells in a Rhesus Macaque Model

Busulfan conditioning is utilized for hematopoietic stem cell (HSC) depletion in the context of HSC gene-therapy conditioning but may result in insufficient immunosuppression. In this study, we evaluated whether additional immunosuppression is required for efficient engraftment of gene-modified cells using a rhesus HSC lentiviral gene-therapy model. We transduced half of rhesus CD34+ cells with an enhanced green fluorescent protein (GFP)-encoding vector (immunogenic) and the other half with a γ-globin-encoding vector (no predicted immunogenicity). After autologous transplantation of both transduced cell populations following myeloablative busulfan conditioning (5.5 mg/kg/day for 4 days), we observed immunological rejection of GFP-transduced cells up to 3 months post-transplant and stable engraftment of γ-globin-transduced cells in two animals, demonstrating that ablative busulfan conditioning is sufficient for engraftment of gene-modified cells producing non-immunogenic proteins but insufficient to permit engraftment of immunogenic proteins. We then added immunosuppression with abatacept and sirolimus to busulfan conditioning and observed engraftment of both GFP- and γ-globin-transduced cells in two animals, demonstrating that additional immunosuppression allows for engraftment of gene-modified cells expressing immunogenic proteins. In conclusion, myeloablative busulfan conditioning should permit engraftment of gene-modified cells producing non-immunogenic proteins, while additional immunosuppression is required to prevent immunological rejection of a neoantigen.

High-Efficiency Lentiviral Transduction of Human CD34+ Cells in High-Density Culture with Poloxamer and Prostaglandin E2

Hematopoietic stem cell (HSC) gene therapy is curative for various hereditary diseases; however, high-efficiency transduction in HSCs remains crucial to improve the prospects for hemoglobinopathies. We previously optimized lentiviral transduction in human CD34⁺ cells with serum-free medium containing minimal cytokines, allowing efficient transduction (∼50%) and robust xenograft engraftment. In this study, we further improved lentiviral transduction in human CD34⁺ cells. High-density culture conditions (4e6/mL) resulted in ∼5-fold more efficient transduction in CD34⁺ cells (p < 0.01) compared with standard cell density (1e5/mL). After co-culturing vector-exposed CD34⁺ cells with non-transduced CD34⁺ cells, high-density culture conditions enhanced lentiviral gene marking in the non-transduced population (p < 0.01) compared with low-density conditions, suggesting that increasing cell-to-cell contact allows more efficient transduction. Two adjuvants, poloxamer 407 (100 μg/mL) and prostaglandin E2 (10 μM), were added to high-density CD34⁺ cells, resulting in ∼4-fold more efficient transduction (p < 0.01) without significant toxicity compared with no adjuvant control. In summary, we developed a highly efficient lentiviral transduction method in high-density CD34⁺ cell culture with poloxamer 407 and prostaglandin E2, allowing overall ∼10-fold improvement in transduction efficiency and consistently achieving more than 90% transduction and an average vector copy number of ∼10. Our optimized transduction method should improve gene therapy approaches using lentiviral vectors targeting HSCs.

AAV-Vectored Fms-Related Tyrosine Kinase 3 Ligand Inhibits CD34+ Progenitor Cell Engraftment in Humanized Mice

Humanized mice have become useful animal models for HIV/AIDS. Since NOD.Cg-Prkdc scid Il2rgtm1Wjl/SzJ (NSG) mice allow the engraftment of primary human immune cells, we aim to determine the role of human Fms-related tyrosine kinase 3 ligand (hFlt3L), a major growth factor for dendritic cells (DCs), in regulating the differentiation of cord blood-derived CD34⁺ progenitor cells in this murine species. Soluble recombinant hFlt3L protein and AAV-vectored hFlt3L were administrated before or after human CD34⁺ progenitor cell transplantation, respectively. We then measured the peripheral levels of hFlt3L by ELISA. Meantime, reconstituted human immune cells were analyzed by flow cytometry over time. We found that without hFlt3L there were significantly increased types of human immune cells in NSG-huCD34 compared with NSG-huPBL mice but the frequency of human DCs remains low. Transient treatment with recombinant hFlt3L expanded human conventional CD1c⁺ and CD141⁺ DCs as well as plasmacytoid DCs in humanized NSG-huCD34 mice. Surprisingly, however, the prolonged in vivo expression of AAV-vectored hFlt3L resulted in significant suppression of total human CD34⁺ cell engraftment and differentiation. The suppression occurred within 2 weeks when AAV-vectored hFlt3L was administered either before or after the transplantation of CD34⁺ progenitor cells, which was likely associated with the induction of murine myeloid-derived immune suppressive cells and reactive oxygen species in NSG-huCD34 mice. Since chronic  HIV-1 patients displayed significantly high levels of hFlt3L expression, our findings may have implication to explore the role of prolonged hFlt3L in regulating  the differentiation of human CD34⁺ progenitor cells in both NSG-huCD34 mice and infected people. Graphical Abstract ᅟ.

Robust erythroid differentiation system for rhesus hematopoietic progenitor cells allowing preclinical screening of genetic treatment strategies for the hemoglobinopathies

BACKGROUND AIMS

γ-globin expression can be induced by various gene modification strategies, which could be beneficial for hemoglobin (Hb) disorders. To translate promising ideas into clinics, large animal models have proven valuable to evaluate safety and efficacy of the approaches; however, in vitro erythroid differentiation methods have not been established to determine whether they can be modeled in nonhuman primates.

METHODS

We optimized erythroid differentiation culture to produce high-level adult Hb from rhesus hematopoietic progenitor cells by using low (LC) or high cytokine concentration (HC) protocols with or without feeder cells. In addition, we established rhesus globin protein analysis using reverse-phase high performance liquid chromatography and mass spectrometry.

RESULTS

Robust adult Hb production at protein levels was observed in the LC protocol when feeder cells were used, whereas the HC protocol resulted in higher baseline fetal Hb levels (P < 0.01). We then compared lentiviral transduction of rhesus cells between serum-containing LC media and serum-free StemSpan-based differentiation media, revealing 100-fold more efficient transduction in serum-free differentiation media (P < 0.01). Finally, rhesus CD34⁺ cells were transduced with lentiviral vectors encoding artificial zinc finger proteins (ZF-Ldb1), which can reactivate γ-globin expression via tethering the transcriptional co-regulator Ldb1 to γ-globin promoters, and were differentiated in the optimized erythroid differentiation method. This resulted in marked increases of γ-globin levels compared with control groups (P < 0.01).

DISCUSSION

In conclusion, we developed an efficient rhesus erythroid differentiation protocol from hematopoietic progenitor cells with low fetal and high adult Hb production. Further studies are warranted to optimize gene modification and transplantation of rhesus hematopoietic progenitor cells.

Analysis of ROR1 Protein Expression in Mice with Reconstituted Human Immune System Components

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is an oncofetal antigen expressed on multiple tumors and has no significant expression on normal human tissues. ROR1 is highly upregulated in chronic lymphocytic leukemia (CLL) B cells. NOD-scid IL2rg^−/− (NSG) mice engrafted with human CD34⁺ hematopoietic progenitor cells (huNSG) achieved multilineage human immune cell reconstitution including B cells, T cells, NK cells, and DCs. Like the CLL patients, huNSG mice have abnormally high percentage of CD5-expressing B cells in the periphery. In light of this, we aim to determine whether ROR1 is expressed on huNSG B cells. Using flow cytometry analysis, we found that ROR1 was highly expressed in a proportion of bone marrow, spleen, and blood B cells, which were mostly immature B cells. Transplantation of the oncogene TCL-1-transduced CD34⁺ cells in neonatal NSG mice did not increase the frequency of ROR1-expressing B cells, but the mouse with the highest engraftment of transduced cells developed a tumor-like lump consisting of a high percentage of ROR1-expressing B cells. This study highlights the potential use of huNSG mice to study B cell malignant diseases and to evaluate immunotherapeutics targeting ROR1.

High-level embryonic globin production with efficient erythroid differentiation from a K562 erythroleukemia cell line

A reliable cell line capable of robust in vitro erythroid differentiation would be useful to investigate red blood cell (RBC) biology and genetic strategies for RBC diseases. K562 cells are widely utilized for erythroid differentiation; however, current differentiation methods are insufficient to analyze globin proteins. In this study, we sought to improve erythroid differentiation from K562 cells to enable protein-level globin analysis. K562 cells were exposed to a variety of reagents, including hemin, rapamycin, imatinib, and/or decitabine (known erythroid inducers), and cultured in a basic culture medium or erythropoietin-based differentiation medium. All single reagents induced observable erythroid differentiation with higher glycophorin A (GPA) expression but were insufficient to produce detectable globin proteins. We then evaluated various combinations of these reagents and developed a method incorporating imatinib preexposure and an erythropoietin-based differentiation culture containing both rapamycin and decitabine capable of efficient erythroid differentiation, high-level GPA expression (>90%), and high-level globin production at protein levels detectable by hemoglobin electrophoresis and high performance liquid chromatography. In addition, β-globin gene transfer resulted in detectable adult hemoglobin. In summary, we developed an in vitro K562 erythroid differentiation model with high-level globin production. This model provides a practical evaluation tool for hemoglobin production in human erythroid cells.

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.

The use of RetroNectin in studies requiring in vitro HIV-1 infection of human hematopoietic stem/progenitor cells

Human immunodeficiency virus (HIV) causes damage, directly or indirectly, to the whole hematopoietic system, including CD34⁺ hematopoietic stem/progenitor cells (HSPCs). CXCR4-tropic strains of HIV-1 may affect the function of CD34⁺CXCR4⁺ progenitor cells either by infecting the cells or modifying the dynamics of more differentiated hematopoietic cells. However, CD34⁺ cells are known for their resistance to HIV-1 infection in vitro, which restricts any detailed analysis of the impact of HIV on HSPCs. We report the use of RetroNectin, a recombinant fibronectin fragment used for gene transfer with lentiviral vectors, to overcome the limitation associated with CD34⁺ cell resistance to HIV-1 infection. RetroNectin coating of plates improved in vitro HIV-1 infectivity on human CD34⁺ cells by 10 fold. This resulted in stable HIV-1 infection for 5 weeks in an OP9-DL1 coculture. These results suggest that RetroNectin may be a useful tool for long-term monitoring of in vitro HIV-infected CD34⁺ cells.

RNA Trans-Splicing Targeting Endogenous β-Globin Pre-Messenger RNA in Human Erythroid Cells

Sickle cell disease results from a point mutation in exon 1 of the β-globin gene (total 3 exons). Replacing sickle β-globin exon 1 (and exon 2) with a normal sequence by trans-splicing is a potential therapeutic strategy. Therefore, this study sought to develop trans-splicing targeting β-globin pre-messenger RNA among human erythroid cells. Binding domains from random β-globin sequences were comprehensively screened. Six candidates had optimal binding, and all targeted intron 2. Next, lentiviral vectors encoding RNA trans-splicing molecules were constructed incorporating a unique binding domain from these candidates, artificial 5' splice site, and γ-globin cDNA, and trans-splicing was evaluated in CD34⁺ cell-derived erythroid cells from healthy individuals. Lentiviral transduction was efficient, with vector copy numbers of 9.7 to 15.3. The intended trans-spliced RNA product, including exon 3 of endogenous β-globin and γ-globin, was detected at the molecular level. Trans-splicing efficiency was improved to 0.07-0.09% by longer binding domains, including the 5' splice site of intron 2. In summary, screening was performed to select efficient binding domains for trans-splicing. Detectable levels of trans-splicing were obtained for endogenous β-globin RNA in human erythroid cells. These methods provide the basis for future trans-splicing directed gene therapy.

The feasibility of incorporating Vpx into lentiviral gene therapy vectors

While current antiretroviral therapy has significantly improved, challenges still remain in life-long targeting of HIV-1 reservoirs. Lentiviral gene therapy has the potential to deliver protective genes into the HIV-1 reservoir. However, inefficient reverse transcription (RT) occurs in HIV-1 reservoirs during lentiviral gene delivery. The viral protein Vpx is capable of increasing lentiviral RT by antagonizing the restriction factor SAMHD1. Incorporating Vpx into lentiviral vectors could substantially increase gene delivery into the HIV-1 reservoir. The feasibility of this Vpx approach was tested in resting cell models utilizing macrophages and dendritic cells. Our results showed Vpx exposure led to increased permissiveness of cells over a period that exceeded 2 weeks. Consequently, significant lower potency of HIV-1 antiretrovirals inhibiting RT and integration was observed. When Vpx was incorporated with anti-HIV-1 genes inhibiting either pre-RT or post-RT stages of the viral life-cycle, transduction levels significantly increased. However, a stronger antiviral effect was only observed with constructs that inhibit pre-RT stages of the viral life cycle. In conclusion this study demonstrates a way to overcome the major delivery obstacle of gene delivery into HIV-1 reservoir cell types. Importantly, incorporating Vpx with pre-RT anti-HIV-1 genes, demonstrated the greatest protection against HIV-1 infection.

Mice engrafted with human hematopoietic stem cells support a human myeloid cell inflammatory response in vivo

Mice engrafted with human CD34⁺ hematopoietic stem and progenitor cells (CD34⁺ -HSPCs) have been used to study human infection, diabetes, sepsis, and burn, suggesting that they could be highly amenable to characterizing the human inflammatory response to injury. To this end, human leukocytes infiltrating subcutaneous implants of polyvinyl alcohol (PVA) sponges were analyzed in immunodeficient NSG mice reconstituted with CD34⁺ -HSPCs. It was reported that human CD45⁺ (hCD45⁺ ) leukocytes were present in PVA sponges 3 and 7 days postimplantation and could be localized within the sponges by immunohistochemistry. The different CD45⁺ subtypes were characterized by flow cytometry and the profile of human cytokines they secreted into PVA wound fluid was assessed using a human-specific multiplex bead analyses of human IL-12p70, TNFα, IL-10, IL-6, IL1β, and IL-8. This enabled tracking the functional contributions of HLA-DR⁺ , CD33⁺ , CD19⁺ , CD62L⁺ , CD11b⁺ , or CX3CR1⁺ hCD45⁺ infiltrating inflammatory leukocytes. PCR of cDNA prepared from these cells enabled the assessment and differentiation of human, mouse, and uniquely human genes. These findings support the hypothesis that mice engrafted with CD34⁺ -HSPCs can be deployed as precision avatars to study the human inflammatory response to injury.

Mathematical modeling of erythrocyte chimerism informs genetic intervention strategies for sickle cell disease

Recent advances in gene therapy and genome-engineering technologies offer the opportunity to correct sickle cell disease (SCD), a heritable disorder caused by a point mutation in the β-globin gene. The developmental switch from fetal γ-globin to adult β-globin is governed in part by the transcription factor (TF) BCL11A. This TF has been proposed as a therapeutic target for reactivation of γ-globin and concomitant reduction of β-sickle globin. In this and other approaches, genetic alteration of a portion of the hematopoietic stem cell (HSC) compartment leads to a mixture of sickling and corrected red blood cells (RBCs) in periphery. To reverse the sickling phenotype, a certain proportion of corrected RBCs is necessary; the degree of HSC alteration required to achieve a desired fraction of corrected RBCs remains unknown. To address this issue, we developed a mathematical model describing aging and survival of sickle-susceptible and normal RBCs; the former can have a selective survival advantage leading to their overrepresentation. We identified the level of bone marrow chimerism required for successful stem cell-based gene therapies in SCD. Our findings were further informed using an experimental mouse model, where we transplanted mixtures of Berkeley SCD and normal murine bone marrow cells to establish chimeric grafts in murine hosts. Our integrative theoretical and experimental approach identifies the target frequency of HSC alterations required for effective treatment of sickling syndromes in humans. Our work replaces episodic observations of such target frequencies with a mathematical modeling framework that covers a large and continuous spectrum of chimerism conditions. Am. J. Hematol. 91:931-937, 2016. © 2016 Wiley Periodicals, Inc.

Restriction of HIV-1-based lentiviral vectors in adult primary marrow-derived and peripheral mobilized human CD34+ hematopoietic stem and progenitor cells occurs prior to viral DNA integration

Background

Gene therapy is currently being attempted using a number of delivery vehicles including lentiviral-based vectors. The delivery and insertion of a gene using lentiviral-based vectors involves multiple discrete steps, including reverse transcription of viral RNA into DNA, nuclear entry, integration of viral DNA into the host genome and expression of integrated genes. Transduction of murine stem cells by the murine leukemia viruses is inefficient because the expression of the integrated DNA is profoundly blocked. Transduction of human stem cells by lentivirus vectors is also inefficient, but the cause and specific part of the retroviral lifecycle where this block occurs is unknown.

Results

Here we demonstrate that the dominant point of restriction of an HIV-1-based lentiviral vector in adult human hematopoietic stem and progenitor cells (HSPCs) from bone marrow and also those obtained following peripheral mobilization is prior to viral DNA integration. We specifically show that restriction of HSPCs to an HIV-1-based lentiviral vector is prior to formation of nuclear DNA forms.

Conclusions

Murine restriction of MLV and human cellular restriction of HIV-1 are fundamentally different. While murine restriction of MLV occurs post integration, human restriction of HIV-1 occurs before integration.

Kinetics of lentiviral vector transduction in human CD34(+) cells

Unlike cell lines, human hematopoietic stem cells (HSCs) are less efficiently transduced with HIV-1 vectors, potentially limiting this approach. To investigate which step (internalization, reverse transcription, nuclear transport, and integration) limits lentiviral transduction, we evaluated the kinetics of lentiviral transduction in human CD34(+) cells. We transduced HeLa and CD34(+) cells with self-inactivating HIV-1 vector at low and tenfold higher multiplicity of infection (MOI) and evaluated vector amounts at various time points based on the rationale that if a given step was not limiting, tenfold greater vector amounts would be obtained at the tenfold higher MOI. We observed slower internalization (>60 min), a peak in reverse transcription at 24 hours, and completion of integration at 3 days in CD34(+) cells. In HeLa cells, there were approximately tenfold greater amounts at high MOI at all time points. When compared with HeLa cells, CD34(+) cells exhibited larger differences in vector amounts between high and low MOIs at 2-6 hours and a smaller difference at 12 hours to 10 days, revealing a limitation in human CD34(+) cell transduction around 12 hours, which corresponds to reverse transcription. In serial measurements of reverse transcription at 24 hours, vector amounts did not decrease once detected among CD34(+) cells. When using an HSC expansion medium, we observed less limitation for starting reverse transcription and more efficient transduction among CD34(+) cells in vitro and in xenografted mice. These data suggest that it is the initiation of reverse transcription that limits lentiviral transduction of human CD34(+) cells. Our findings provide an avenue for optimizing human CD34(+) cell transduction.

Highly efficient in vitro and in vivo delivery of functional RNAs using new versatile MS2-chimeric retrovirus-like particles

RNA delivery is an attractive strategy to achieve transient gene expression in research projects and in cell- or gene-based therapies. Despite significant efforts investigating vector-directed RNA transfer, there is still a requirement for better efficiency of delivery to primary cells and in vivo. Retroviral platforms drive RNA delivery, yet retrovirus RNA-packaging constraints limit gene transfer to two genome-molecules per viral particle. To improve retroviral transfer, we designed a dimerization-independent MS2-driven RNA packaging system using MS2-Coat-retrovirus chimeras. The engineered chimeric particles promoted effective packaging of several types of RNAs and enabled efficient transfer of biologically active RNAs in various cell types, including human CD34(+) and iPS cells. Systemic injection of high-titer particles led to gene expression in mouse liver and transferring Cre-recombinase mRNA in muscle permitted widespread editing at the ROSA26 locus. We could further show that the VLPs were able to activate an osteoblast differentiation pathway by delivering RUNX2- or DLX5-mRNA into primary human bone-marrow mesenchymal-stem cells. Thus, the novel chimeric MS2-lentiviral particles are a versatile tool for a wide range of applications including cellular-programming or genome-editing.

Decitabine suspends human CD34+ cell differentiation and proliferation during lentiviral transduction

Efficient ex vivo transduction of hematopoietic stem cells (HSCs) is encumbered by differentiation which reduces engraftment. We hypothesized that inhibiting DNA methyltransferase with decitabine would block differentiation of transduced CD34+ cells under cytokine stimulation and thus improve transduction efficiency for engrafting HSCs. Human CD34+ cells in cytokine-containing media were treated with or without decitabine for 24 or 48 hours, and then these cells were transduced with a GFP-expressing lentiviral vector. Utilizing decitabine pre-treatment for 48 hours, we observed an equivalent percentage of successfully transduced cells (GFP-positivity) and a higher percentage of cells that retained CD34 positivity, compared to no decitabine exposure. Cell proliferation was inhibited after decitabine exposure. Similar results were observed among CD34+ cells from six different donors. Repopulating activity was evaluated by transplantation into NOD/SCID/IL2Rγnull mice and demonstrated an equivalent percentage of GFP-positivity in human cells from decitabine-treated samples and a trend for higher human cell engraftment (measured 20-24 weeks after transplantation), compared to no decitabine exposure. In conclusion, ex vivo decitabine exposure inhibits both differentiation and proliferation in transduced human CD34+ cells and modestly increases the engraftment ability in xenograft mice, while the transduction efficiency is equivalent in decitabine exposure, suggesting improvement of lentiviral transduction for HSCs.

Evaluation of engraftment and immunological tolerance after reduced intensity conditioning in a rhesus hematopoietic stem cell gene therapy model

Reduced intensity conditioning (RIC) is desirable for hematopoietic stem cell (HSC) targeted gene therapy; however, RIC may be insufficient for efficient engraftment and inducing immunological tolerance to transgenes. We previously established long-term gene marking in our rhesus macaque autologous HSC transplantation model following 10 Gy total body irradiation (TBI). In this study, we evaluated RIC transplantation with 4 Gy TBI in two rhesus macaques that received equal parts of CD34(+) cells transduced with green fluorescent protein (GFP)-expressing lentiviral vector and empty vector not expressing transgenes. In both animals, equivalently low gene marking between GFP and empty vectors was observed 6 months post-transplantation, even with efficient transduction of CD34(+) cells in vitro. Autologous lymphocyte infusion with GFP marking resulted in an increase of gene marking in lymphocytes in a control animal with GFP tolerance, but not in the two RIC-transplanted animals. In vitro assays revealed strong cellular and humoral immune responses to GFP protein in the two RIC-transplanted animals, but this was not observed in controls. In summary, 4 Gy TBI is insufficient to permit engraftment of genetically modified HSCs and induce immunological tolerance to transgenes. Our findings should help in the design of conditioning regimens in gene therapy trials.

Preclinical demonstration of lentiviral vector-mediated correction of immunological and metabolic abnormalities in models of adenosine deaminase deficiency

Gene transfer into autologous hematopoietic stem cells by γ-retroviral vectors (gRV) is an effective treatment for adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID). However, current gRV have significant potential for insertional mutagenesis as reported in clinical trials for other primary immunodeficiencies. To improve the efficacy and safety of ADA-SCID gene therapy (GT), we generated a self-inactivating lentiviral vector (LV) with a codon-optimized human cADA gene under the control of the short form elongation factor-1α promoter (LV EFS ADA). In ADA(-/-) mice, LV EFS ADA displayed high-efficiency gene transfer and sufficient ADA expression to rescue ADA(-/-) mice from their lethal phenotype with good thymic and peripheral T- and B-cell reconstitution. Human ADA-deficient CD34(+) cells transduced with 1-5 × 10(7) TU/ml had 1-3 vector copies/cell and expressed 1-2x of normal endogenous levels of ADA, as assayed in vitro and by transplantation into immune-deficient mice. Importantly, in vitro immortalization assays demonstrated that LV EFS ADA had significantly less transformation potential compared to gRV vectors, and vector integration-site analysis by nrLAM-PCR of transduced human cells grown in immune-deficient mice showed no evidence of clonal skewing. These data demonstrated that the LV EFS ADA vector can effectively transfer the human ADA cDNA and promote immune and metabolic recovery, while reducing the potential for vector-mediated insertional mutagenesis.

TRIM5α variations influence transduction efficiency with lentiviral vectors in both human and rhesus CD34(+) cells in vitro and in vivo

Human immunodeficiency virus type 1 (HIV-1) vectors can transduce human hematopoietic stem cells (HSC), but transduction efficiency varies among individuals. The innate immune factor tripartite motif-containing protein 5α (TRIM5α) plays an important role for restriction of retroviral infection. In this study, we examined whether TRIM5α could account for variations in transduction efficiency using both an established rhesus gene therapy model and human CD34(+) cell culture. Evaluation of TRIM5α genotypes (Mamu-1, -2, -3, -4, -5, and TrimCyp) in 16 rhesus macaques that were transplanted with transduced CD34(+) cells showed a significant correlation between TRIM5α Mamu-4 and high gene marking in both lymphocytes and granulocytes 6 months after transplantation. Since significant human TRIM5α coding polymorphisms were not known, we evaluated TRIM5α expression levels in human CD34(+) cells from 14 donors. Three days after HIV-1 vector transduction, measured transduction efficiency varied significantly among donors and was negatively correlated with TRIM5α expression levels. In summary, transduction efficiency in both rhesus and human CD34(+) cells was influenced by TRIM5α variations (genotypes and expression levels). Our findings are important for both understanding and mitigating the variability of transduction efficiency for rhesus and human CD34(+) cells.

Integration-specific In Vitro Evaluation of Lentivirally Transduced Rhesus CD34(+) Cells Correlates With In Vivo Vector Copy Number

Hematopoietic stem cell (HSC) gene therapy using integrating vectors has a potential leukemogenic risk due to insertional mutagenesis. To reduce this risk, a limitation of ≤2 average vector copy number (VCN) per cell is generally accepted. We developed an assay for VCN among transduced CD34(+) cells that reliably predicts in vivo VCN in 16 rhesus recipients of CD34(+) cells transduced with a green fluorescent protein (GFP) (or yellow fluorescent protein (YFP))-encoding lentiviral vector. Using GFP (or YFP)-specific probe/primers by real-time PCR, VCN among transduced CD34(+) cells had no correlation with VCN among granulocytes or lymphocytes in vivo assayed 6 months post-transplantation. This was a likely result of residual plasmids present in the vector preparation. We then designed self-inactivating long terminal repeat (SIN-LTR)-specific probe/primers, which detect only integrated provirus. Evaluation with SIN-LTR probe/primers resulted in a positive correlation of VCN among transduced CD34(+) cells with granulocytes and lymphocytes in vivo. The transduced CD34(+) cells had higher VCN (25.1 ± 5.6) as compared with granulocytes (2.8 ± 1) and lymphocytes (2.4 ± 0.7). In summary, an integrated provirus-specific real-time PCR system demonstrated nine- to tenfold higher VCN in transduced CD34(+) cells in vitro, as compared with VCN in vivo. Therefore, the restriction of ≤2 VCN before infusion might unnecessarily limit gene transfer efficacy.Molecular Therapy-Nucleic Acids (2013) 2, e122; doi:10.1038/mtna.2013.49; published online 17 September 2013.

Current progress on gene therapy for primary immunodeficiencies

Primary immunodeficiencies have played a major role in the development of gene therapy for monogenic diseases of the bone marrow. The last decade has seen convincing evidence of long-term disease correction as a result of ex vivo viral vector-mediated gene transfer into autologous haematopoietic stem cells. The success of these early studies has been balanced by the development of vector-related insertional mutagenic events. More recently the use of alternative vector designs with self-inactivating designs, which have an improved safety profile has led to the initiation of a wave of new studies that are showing early signs of efficacy. The ongoing development of safer vector platforms and gene-correction technologies together with improvements in cell-transduction techniques and optimised conditioning regimes is likely to make gene therapy amenable for a greater number of PIDs. If long-term efficacy and safety are shown, gene therapy will become a standard treatment option for specific forms of PID.

Efficient transduction of human hematopoietic repopulating cells with a chimeric HIV1-based vector including SIV capsid

Innate immune factors, such as TRIM5α and cyclophilin A (CypA), act as a major restriction factor of retroviral infection among species. When HIV1 infects human cells, HIV1 capsid binds to human CypA to escape from human TRIM5α restriction. However, in rhesus cells, the mismatch between HIV1 capsid and rhesus CypA is recognized by rhesus TRIM5α to reduce HIV1 infectivity through proteasomal degradation. To circumvent this block, we previously developed a chimeric HIV1 vector (χHIV) that substituted HIV1 capsid with SIV capsid, and it significantly increased transduction efficiency for nonhuman primate cells. In this study, we evaluated whether the χHIV vector efficiently transduces human cells, and the transduction efficiency might increase by a CypA inhibitor (cyclosporine) and a proteasome inhibitor (MG132). The χHIV vector could transduce human CD34⁺ cells, as efficiently as the HIV1 vector, in vitro and in xenograft mice, even in the mismatch between SIV capsid and human CypA. Cyclosporine decreased transduction efficiency with the HIV1 vector, whereas it slightly increased transduction efficiency with the χHIV vector in human CD34⁺ cells. MG132 increased transduction efficiency with both χHIV and HIV1 vectors in the same manner. However, MG132 was toxic to human CD34⁺ cells at high concentrations, and both drugs had a small range of effective dosage. These findings demonstrate that both χHIV and HIV1 vectors have similar transduction efficiency for human hematopoietic repopulating cells, suggesting that the χHIV vector escapes from TRIM5α restriction, which is independent of human CypA.

Humanised mice have functional human neutrophils

The differences between murine and human neutrophils mean that findings in mice may not translate to humans, and therefore an in vivo model with human neutrophils would be an important methodological advance. We generated humanised mice by injecting human cord blood derived CD34+ stem cells into irradiated NOD-scid-γc(-/-) mice. At least 3 months after engraftment, treatment of mice with GCSF mobilised circulating human neutrophils, which comprised 2.6% of human leukocytes, and led to L-selectin shedding and upregulation of CD66b, CD11b and CD63. Subsequent in vivo LPS treatment led to further downregulation of L-selectin with upregulation of CD66b and CD63, and also resulted in human neutrophil sequestration in the lungs. Furthermore, human neutrophils from these mice were capable of robust functional responses. They were shown to undergo a respiratory burst, and to degranulate with upregulation of CD63 and CD66b, in response to fMLP and Escherichia coli. These data show that functional human neutrophils develop from CD34+ cord blood stem cells in NOD-scid-γc(-/-) mice. They suggest that this approach may facilitate the in vivo study of human neutrophils in clinically relevant models of infection and autoimmunity.

Transduction of human CD34+ repopulating cells with a self-inactivating lentiviral vector for SCID-X1 produced at clinical scale by a stable cell line

Self-inactivating (SIN)-lentiviral vectors have safety and efficacy features that are well suited for transduction of hematopoietic stem cells (HSCs), but generation of vector at clinical scale has been challenging. Approximately 280 liters of an X-Linked Severe Combined Immunodeficiency Disorder (SCID-X1) SIN-lentiviral vector in two productions from a stable cell line were concentrated to final titers of 4.5 and 7.2×10(8) tu/ml. These two clinical preparations and three additional development-scale preparations were evaluated in human CD34(+) hematopoietic cells in vitro using colony forming cell (CFU-C) assay and in vivo using the NOD/Lt-scid/IL2Rγ(null) (NSG) mouse xenotransplant model. A 40-hour transduction protocol using a single vector exposure conferred a mean NSG repopulating cell transduction of 0.23 vector genomes/human genome with a mean myeloid vector copy number of 3.2 vector genomes/human genome. No adverse effects on engraftment were observed from vector treatment. Direct comparison between our SIN-lentiviral vector using a 40-hour protocol and an MFGγ(c) γ-retroviral vector using a five-day protocol demonstrated equivalent NSG repopulating cell transduction efficiency. Clonality survey by linear amplification-mediated polymerase chain reaction (LAM-PCR) with Illumina sequencing revealed common clones in sorted myeloid and lymphoid populations from engrafted mice demonstrating multipotent cell transduction. These vector preparations will be used in two clinical trials for SCID-X1.

High-efficiency transduction of rhesus hematopoietic repopulating cells by a modified HIV1-based lentiviral vector

Human immunodeficiency virus type 1 (HIV1) vectors poorly transduce rhesus hematopoietic cells due to species-specific restriction factors, including the tripartite motif-containing 5 isoformα (TRIM5α) which targets the HIV1 capsid. We previously developed a chimeric HIV1 (χHIV) vector system wherein the vector genome is packaged with the simian immunodeficiency virus (SIV) capsid for efficient transduction of both rhesus and human CD34(+) cells. To evaluate whether χHIV vectors could efficiently transduce rhesus hematopoietic repopulating cells, we performed a competitive repopulation assay in rhesus macaques, in which half of the CD34(+) cells were transduced with standard SIV vectors and the other half with χHIV vectors. As compared with SIV vectors, χHIV vectors achieved higher vector integration, and the transgene expression rates were two- to threefold higher in granulocytes and red blood cells and equivalent in lymphocytes and platelets for 2 years. A recipient of χHIV vector-only transduced cells reached up to 40% of transgene expression rates in granulocytes and lymphocytes and 20% in red blood cells. Similar to HIV1 and SIV vectors, χHIV vector frequently integrated into gene regions, especially into introns. In summary, our χHIV vector demonstrated efficient transduction for rhesus long-term repopulating cells, comparable with SIV vectors. This χHIV vector should allow preclinical testing of HIV1-based therapeutic vectors in large animal models.

Ex vivo expansion of haematopoietic stem/progenitor cells from human umbilical cord blood on acellular scaffolds prepared from MS-5 stromal cell line

Lineage-specific expansion of haematopoietic stem/progenitor cells (HSPCs) from human umbilical cord blood (UCB) is desirable because of their several applications in translational medicine, e.g. treatment of cancer, bone marrow failure and immunodeficiencies. The current methods for HSPC expansion use either cellular feeder layers and/or soluble growth factors and selected matrix components coated on different surfaces. The use of cell-free extracellular matrices from bone marrow cells for this purpose has not previously been reported. We have prepared insoluble, cell-free matrices from a murine bone marrow stromal cell line (MS-5) grown under four different conditions, i.e. in presence or absence of osteogenic medium, each incubated under 5% and 20% O₂ tensions. These acellular matrices were used as biological scaffolds for the lineage-specific expansion of magnetically sorted CD34⁺ cells and the results were evaluated by flow cytometry and colony-forming assays. We could get up to 80-fold expansion of some HSPCs on one of the matrices and our results indicated that oxygen tension played a significant role in determining the expansion capacity of the matrices. A comparative proteomic analysis of the matrices indicated differential expression of proteins, such as aldehyde dehydrogenase and gelsolin, which have previously been identified as playing a role in HSPC maintenance and expansion. Our approach may be of value in identifying factors relevant to tissue engineering-based ex vivo HSPC expansion, and it may also provide insights into the constitution of the niche in which these cells reside in the bone marrow.