Long-term high-level reconstitution of NADPH oxidase activity in murine X-linked chronic granulomatous disease using a bicistronic vector expressing gp91phox and a Delta LNGFR cell surface marker

CRISPR-Mediated Knockout of Cybb in NSG Mice Establishes a Model of Chronic Granulomatous Disease for Human Stem-Cell Gene Therapy Transplants

Chronic granulomatous disease (CGD) is characterized by defects in the production of microbicidal reactive oxygen species (ROS) by phagocytes. Testing of gene and cell therapies for the treatment of CGD in human hematopoietic cells requires preclinical transplant models. The use of the lymphocyte-deficient NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ (NSG) mouse strain for human hematopoietic cell xenografts to test CGD therapies is complicated by the presence of functional mouse granulocytes capable of producing ROS for subsequent bacterial and fungal killing. To establish a phagocyte-defective mouse model of X-linked CGD (X-CGD) in NSG mice, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 was utilized for targeted knockout of mouse Cybb on the X-chromosome by microinjection of NSG mouse zygotes with Cas9 mRNA and CRISPR single-guide RNA targeting Cybb exon 1 or exon 3. This resulted in a high incidence of indel formation at the CRISPR target site, with all mice exhibiting deletions in at least one Cybb allele based on sequence analysis of tail snip DNA. A female mouse heterozygous for a 235-bp deletion in Cybb exon 1 was bred to an NSG male to establish the X-CGD NSG mouse strain, NSG.Cybb[KO]. Resulting male offspring with the 235 bp deletion were found to be defective for production of ROS by neutrophils and other phagocytes, and demonstrated increased susceptibility to spontaneous bacterial and fungal infections with granulomatous inflammation. The establishment of the phagocyte-defective NSG.Cybb[KO] mouse model enables the in vivo assessment of gene and cell therapy strategies for treating CGD in human hematopoietic cell transplants without obfuscation by functional mouse phagocytes, and may also be useful for modeling other phagocyte disorders in humanized NSG mouse xenografts.

Genetic correction of X-linked chronic granulomatous disease with novel foamy virus vectors

OBJECTIVE

The X-linked form of chronic granulomatous disease (X-CGD) results from mutations in the CYBB gene encoding gp91(phox), the larger subunit of the oxidase flavocytochrome b(558). Affected individuals suffer from recurrent life-threatening infections due to impaired superoxide production by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in phagocytes. Novel foamy virus vectors expressing the human codon-optimized gp91(phox) were evaluated for the genetic correction of the disease in the X-CGD cell line and in X-CGD mouse model.

MATERIALS AND METHODS

The vectors were evaluated in vitro, in the human X-CGD PLB-985 cell line and in the X-CGD bone marrow Lin(-) cells. Transplantation of transduced Lin(-) cells was performed in X-CGD mice after busulfan conditioning. Real-time polymerase chain reaction was used for chimerism and vector copy number determination. Restoration of reduced NADPH oxidase production was assessed by nitrobluetetrazolium and dihydrorhodamine assays.

RESULTS

High and stable gp91(phox) expression, as well as reconstitution of reduced NADPH activity, was achieved in the human X-CGD PLB-985 cell line and in primary murine X-CGD hematopoietic stem cells ex vivo. Transplantation of transduced bone marrow hematopoietic stem cells in the murine model of X-CGD, even with low multiplicities of infection (MOI), reconstituted the levels of oxidase-producing neutrophils and provided enzymatic activity that reached 70% of normal.

CONCLUSIONS

Foamy virus vectors expressing the human gp91(phox) transgene constitute potential candidates for the gene therapy of CGD because they combine lack of pathogenicity with efficacy even at low MOI.

Gene therapy of chronic granulomatous disease: the engraftment dilemma

The potential of gene therapy as a curative treatment for monogenetic disorders has been clearly demonstrated in a series of recent Phase I/II clinical trials. Among primary immunodeficiencies, gene transfer into hematopoietic stem (HSC)/progenitor cells has resulted in the long-term correction of immune and metabolic defects in treated patients. In most cases, successes were augmented by a recognized biological selection for successfully treated cells in vivo, perhaps even to some extent at the HSC level. In contrast, similar achievements have not turned into reality for immunodeficiencies in which gene-transduced cells lack selective advantages in vivo. This is the case for chronic granulomatous disease (CGD), a primary immunodeficiency, characterized by deficient antimicrobial activity in phagocytic cells. Several attempts to correct CGD by gene transfer in combination with bone marrow conditioning have resulted in low-level long-term engraftment and transient clinical benefits despite high levels of gene marking and high numbers of reinfused cells. This review summarizes the data from clinical trials for CGD and provides some insights into treatment options that may lead to a successful application of gene therapy for CGD.

Biochemical correction of X-CGD by a novel chimeric promoter regulating high levels of transgene expression in myeloid cells

X-linked chronic granulomatous disease (X-CGD) is a primary immunodeficiency caused by mutations in the CYBB gene encoding the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase catalytic subunit gp91(phox). A recent clinical trial for X-CGD using a spleen focus-forming virus (SFFV)-based γ-retroviral vector has demonstrated clear therapeutic benefits in several patients although complicated by enhancer-mediated mutagenesis and diminution of effectiveness over time due to silencing of the viral long terminal repeat (LTR). To improve safety and efficacy, we have designed a lentiviral vector that directs transgene expression primarily in myeloid cells. To this end, we created a synthetic chimeric promoter that contains binding sites for myeloid transcription factors CAAT box enhancer-binding family proteins (C/EBPs) and PU.1, which are highly expressed during granulocytic differentiation. As predicted, the chimeric promoter regulated higher reporter gene expression in myeloid than in nonmyeloid cells, and in human hematopoietic progenitors upon granulocytic differentiation. In a murine model of stem cell gene therapy for X-CGD, the chimeric vector resulted in high levels of gp91(phox) expression in committed myeloid cells and granulocytes, and restored normal NADPH-oxidase activity. These findings were recapitulated in human neutrophils derived from transduced X-CGD CD34(+) cells in vivo, and suggest that the chimeric promoter will have utility for gene therapy of myeloid lineage disorders such as CGD.

Retroviral vector integration in post-transplant hematopoiesis in mice conditioned with either submyeloablative or ablative irradiation

X-linked chronic granulomatous disease (X-CGD) is an inherited immunodeficiency with absent phagocyte NADPH oxidase activity caused by defects in the gene encoding gp91^phox. Here we evaluated strategies for less intensive conditioning for gene therapy of genetic blood disorders without selective advantage for gene correction, such as might be used in a human X-CGD protocol. We compared submyeloablative with ablative irradiation as conditioning in murine X-CGD, examining engraftment, oxidase activity and vector integration in mice transplanted with marrow transduced with a gamma-retroviral vector for gp91^phox expression. The frequency of oxidase-positive neutrophils in the donor population was unexpectedly higher in many 300 cGy-conditioned mice compared to lethally irradiated recipients, as was the fraction of vector-marked donor secondary CFU-S12. Vector integration sites in marrow, spleen, and secondary CFU-S12 DNA from primary recipients were enriched for cancer-associated genes, including Evi1, and integrations in or near cancer-associated genes were more frequent in marrow and secondary CFU-S12 from 300 cGy-conditioned mice compared to fully ablated mice. These findings support the concept that vector integration can confer a selection bias, and suggest that intensity of the conditioning regimen may further influence the effects of vector integration on clonal selection in post-transplant engraftment and hematopoiesis.

Retrovirus gene therapy for X-linked chronic granulomatous disease can achieve stable long-term correction of oxidase activity in peripheral blood neutrophils

Chronic granulomatous disease (CGD) is associated with significant morbidity and mortality from infection. The first CGD gene therapy trial resulted in only short-term marking of 0.01% to 0.1% of neutrophils. A recent study, using busulfan conditioning and an SFFV retrovirus vector, achieved more than 20% marking in 2 patients with X-linked CGD. However, oxidase correction per marked neutrophil was less than normal and not sustained. Despite this, patients clearly benefited in that severe infections resolved. As such, we initiated a gene therapy trial for X-CGD to treat severe infections unresponsive to conventional therapy. We treated 3 adult patients using busulfan conditioning and an MFGS retroviral vector encoding gp91(phox), achieving early marking of 26%, 5%, and 4% of neutrophils, respectively, with sustained long-term marking of 1.1% and 0.03% of neutrophils in 2 of the patients. Gene-marked neutrophils have sustained full correction of oxidase activity for 34 and 11 months, respectively, with full or partial resolution of infection in those 2 patients. Gene marking is polyclonal with no clonal dominance. We conclude that busulfan conditioning together with an MFGS vector is capable of achieving long-term correction of neutrophil oxidase function sufficient to provide benefit in management of severe infection. This study was registered at www.clinicaltrials.gov as #NCT00394316.

Enhanced homing and engraftment of fresh but not ex vivo cultured murine marrow cells in submyeloablated hosts following CD26 inhibition by Diprotin A

OBJECTIVE

We recently reported that murine marrow cultured ex vivo for gamma-retrovirus transduction engrafts approximately 10-fold less well than fresh marrow upon transplantation into submyeloablated hosts. Here, we evaluated homing efficiency as a potential mechanism for this engraftment disparity, and whether CD26 inhibition with the tripeptide Diprotin A (DipA) would enhance engraftment of ex vivo cultured cells in submyeloablated hosts.

MATERIALS AND METHODS

Homing and engraftment of fresh and ex vivo cultured lineage-negative (lin(-)) marrow cells in submyeloablated congenic hosts with and without DipA treatment was evaluated. Expression of CXCR4 and CD26 on fresh and cultured lin(-) marrow cells was compared.

RESULTS

Homing of lin(-) cells cultured for gamma-retrovirus transduction was at least threefold less than that of fresh lin(-) cells 20 hours after transplantation into submyeloablated hosts. DipA treatment of fresh lin(-) cells resulted in at least twofold increased homing and engraftment in submyeloablated hosts. DipA treatment, however, did not significantly improve homing or engraftment of cells undergoing a 3-day culture protocol for gamma-retrovirus transduction in submyeloablated hosts. CXCR4 expression on lin(-) cells was significantly decreased following 3 days of culture; CXCR4 expression was not significantly altered following overnight culture.

CONCLUSIONS

Ex vivo culture of lin(-) cells for gamma-retroviral transduction downregulates CXCR4 expression and markedly impairs homing and engraftment of murine lin(-) marrow in submyeloablated hosts. While inhibition of CD26 activity with DipA increases homing and engraftment of fresh lin(-) cells, DipA treatment does not improve homing and engraftment of cultured lin(-) marrow cells in submyeloablated congenic hosts.

A novel competitive repopulation strategy to quantitate engraftment of ex vivo manipulated murine marrow cells in submyeloablated hosts

OBJECTIVE

Standard competitive repopulation assays have proven valuable in evaluating engraftment potential in ablated hosts, permitting comparisons between various test cell populations. However, no similar method exists to compare engraftment of test cells in submyeloablated hosts, which would be helpful given the applications of reduced-intensity conditioning for hematopoietic gene-replacement therapy and other cellular therapies. Here, we developed a novel assay to quantitate engraftment of hematopoietic stem cells in submyeloablated hosts.

MATERIALS AND METHODS

Engraftment of murine marrow cells transduced with retroviral vectors using two separate protocols was compared to engraftment of fresh untreated competitor cells within low-dose radiation-conditioned hosts using a "three-way" marking system, so that test, competitor, and host cell chimerism could be reliably determined posttransplantation.

RESULTS

We demonstrate that the repopulating ability of marrow cells transduced using two distinct protocols was reduced approximately 10-fold compared to fresh competitor cells in submyeloablated hosts utilizing the novel "three-way" transplant assay.

CONCLUSIONS

Murine marrow cells transduced using a clinically applicable protocol acquire an engraftment defect in submyeloablated hosts, similar to cells transduced using a research protocol. We conclude that the submyeloablative competitive repopulation assay described here will be of benefit to comparatively assess the engraftment ability of manipulated hematopoietic stem cells using various culture protocols, such as to test the impact of modifications in transduction protocols needed to attain therapeutic levels of gene-corrected blood cells, or the effect of ex vivo expansion protocols on engraftment potential.

Gamma-glutamylcysteine synthetase-based selection strategy for gene therapy of chronic granulomatous disease and graft-vs.-host disease

Efficient ex vivo/in vivo selection of genetically modified hematopoietic stem/progenitor cells (HPCs) and T lymphocytes could greatly improve several gene therapy strategies. We have previously reported that primary murine HPCs, transduced with a bicistronic retroviral vector, co-expressing the catalytic subunit of gamma-glutamylcysteine synthetase (gamma-GCSh) and eGFP, could be selected by l-buthionine-S,R-sulfoximine (BSO). Upon ex vivo transduction with a low, defined gene dosage and BSO selection, HPCs were able to repopulate the bone marrow of syngeneic myeloablated hosts, showing multi-lineage expression [Hum Gene Ther, 16 (2005), 711]. We now provide 'proof-of-principle' that the same strategy can be applied to the gene therapy of graft-vs.-host disease (GVHD) subsequent to allogeneic bone marrow transplantation (ABMT), and of chromosome X-associated chronic granulomatous disease (CGD). Transfer of the herpes simplex virus-thymidine kinase (HSV-Tk) 'suicide' gene into donor T lymphocytes is a potential method to control GVHD after ABMT. However, an efficient selection system is required to eliminate non-HSV-Tk-expressing T lymphocytes before administration to the patient. We now report that, upon transduction with a retroviral vector, co-expressing gamma-GCSh and eGFP, and subsequent selection by BSO, over 95% human T lymphocytes were found to express eGFP; moreover, upon transduction with a novel retroviral vector co-expressing gamma-GCSh and HSV-Tk, and subsequent BSO treatment, over 95% of T lymphocytes could be eliminated by ganciclovir. The efficacy of the gamma-GCSh-BSO selection strategy was then tested on an in vitro model of CGD. Upon transduction of gp91 (phox)-deficient PLBKO cells with a novel bicistronic retroviral vector co-expressing human gp91 (phox) and gamma-GCSh, exposure to BSO for 48 h eliminated most non-transduced cells, resulting in selection of gp91 (phox)-expressing cells, and reconstitution of NADPH oxidase activity.

Correction of X-linked chronic granulomatous disease by gene therapy, augmented by insertional activation of MDS1-EVI1, PRDM16 or SETBP1

Gene transfer into hematopoietic stem cells has been used successfully for correcting lymphoid but not myeloid immunodeficiencies. Here we report on two adults who received gene therapy after nonmyeloablative bone marrow conditioning for the treatment of X-linked chronic granulomatous disease (X-CGD), a primary immunodeficiency caused by a defect in the oxidative antimicrobial activity of phagocytes resulting from mutations in gp91(phox). We detected substantial gene transfer in both individuals' neutrophils that lead to a large number of functionally corrected phagocytes and notable clinical improvement. Large-scale retroviral integration site-distribution analysis showed activating insertions in MDS1-EVI1, PRDM16 or SETBP1 that had influenced regulation of long-term hematopoiesis by expanding gene-corrected myelopoiesis three- to four-fold in both individuals. Although insertional influences have probably reinforced the therapeutic efficacy in this trial, our results suggest that gene therapy in combination with bone marrow conditioning can be successfully used to treat inherited diseases affecting the myeloid compartment such as CGD.

γ-Glutamylcysteine Synthetase and L-Buthionine-(S,R)-Sulfoximine: A New Selection Strategy for Gene-Transduced Neural and Hematopoietic Stem/Progenitor Cells

In most experimental gene therapy protocols involving stem/progenitor cells, only a small fraction of cells, often therapeutically inadequate, can be transduced and made to express the therapeutic gene. A promising strategy for overcoming this problem is the use of a dominant selection marker, such as a drug resistance gene. In this paper, we explore the potential of the heavy subunit of gamma-glutamylcysteine synthetase (gamma-GCSh) to act as a selection marker. We found that 3T3 fibroblasts transduced with the bicistronic retroviral vector SF91/GCSh-eGFP, encoding gamma-GCSh and the enhanced green fluorescent protein (eGFP), were highly resistant to L-buthionine-(S,R)-sulfoximine (BSO), a gamma-GCS inhibitor with a low clinical toxicity profile. The level of resistance was not proportional to the increase in intracellular glutathione. In fact, cells overexpressing both heavy and light gamma-GCS subunits had higher intracellular GSH levels, and a lower level of resistance to the cytotoxic activity of BSO, compared with cells overexpressing gamma-GCSh alone. 3T3 fibroblasts overexpressing gamma-GCSh could be selected from cultures containing both naive and gene-modified cells by application of exogenous BSO selection pressure for 4 days. Also, primary neural stem/progenitor cells derived from the lateral ventricles of mouse neonatal brains and primary hematopoietic stem/progenitor cells (HSCs/HPCs) from mouse bone marrow, transduced with the gamma-GCSh-eGFP vector, could be selected by BSO treatment in vitro. On ex vivo BSO selection and reimplantation into a syngeneic myeloablated host, donor HSCs/HPCs repopulated the marrow and continued to express the transgene(s). These results provide proof of principle that somatic stem/progenitor cells, transduced simultaneously with a potentially curative gene and gamma-GCSh, can be selected by treatment with BSO before in vivo transplantation.

Stable long-term gene correction with low-dose radiation conditioning in murine X-linked chronic granulomatous disease

We previously demonstrated that low-dose radiation conditioning impairs murine hematopoietic stem cell function, permitting engraftment of syngeneic fresh and transduced marrow cells. In this study, we directly examined the ability of low-dose radiation conditioning to permit engraftment of transduced long-term repopulating cells in murine X-linked chronic granulomatous disease (X-CGD), which closely mimics the human disease. X-CGD mice conditioned with 160 cGy were transplanted with 20 x 10(6) MSCV-m91Neo-transduced syngeneic X-CGD marrow cells. The presence of oxidase-positive neutrophils in two independent cohorts of transplanted 160-cGy-conditioned X-CGD recipients was determined by nitroblue tetrazolium testing. Transplanted X-CGD mice (n = 9 total) displayed 1-17% oxidase-positive neutrophils 6-16 months post-transplant. Retroviral marking and NADPH-oxidase-positive neutrophils persisted through serial transplantation, verifying that stem cells were transduced. These results establish that low-dose radiation conditioning results in durable engraftment of low but potentially clinically relevant numbers of functionally reconstituted blood cells in a murine model of X-CGD.

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.

Rac GTPase isoform-specific regulation of NADPH oxidase and chemotaxis in murine neutrophils in vivo. Role of the C-terminal polybasic domain

The Rho family GTPase Rac acts as a molecular switch for signal transduction to regulate various cellular functions. Mice deficient in the hematopoietic-specific Rac2 isoform exhibit agonist-specific defects in neutrophil chemotaxis and superoxide production, despite expression of the highly homologous Rac1 isoform. To examine whether functional defects in rac2(-/-) neutrophils reflect effects of an overall decrease in total cellular Rac or an isoform-specific role for Rac2, retroviral vectors were used to express exogenous Rac1 or Rac2 at levels similar to endogenous. In rac2(-/-) neutrophils differentiated from transduced myeloid progenitors in vitro, increasing cellular Rac levels by expression of either exogenous Rac1 or Rac2 increased formylmethionylleucylphenylalanine- or phorbol ester-stimulated NADPH oxidase activity. Of note, placement of an epitope tag on the N terminus of Rac1 or Rac2 blunted reconstitution of responses in rac2(-/-) neutrophils. In rac2(-/-) neutrophils isolated from mice transplanted with Rac-transduced bone marrow cells, superoxide production and chemotaxis were fully reconstituted by expression of exogenous Rac2, but not Rac1. A chimeric Rac1 protein in which the Rac1 C-terminal polybasic domain, which contains six lysines or arginines, was replaced with that of the human Rac2 polybasic domain containing only three basic residues, also reconstituted superoxide production and chemotaxis, whereas expression of a Rac2 derivative in which the polybasic domain was replaced with that of Rac1 did not and resulted in disoriented cell motility. Thus, the composition of the polybasic domain is sufficient for determining Rac isoform specificity in the production of superoxide and chemotaxis in murine neutrophils in vivo.