Lentivector cryptic splicing mediates increase in CD34+ clones expressing truncated HMGA2 in human X-linked severe combined immunodeficiency.

X-linked Severe Combined Immunodeficiency (SCID-X1) due to IL2RG mutations is potentially fatal in infancy where 'emergency' life-saving stem cell transplant may only achieve incomplete immune reconstitution following transplant. Salvage therapy SCID-X1 patients over 2 years old (NCT01306019) is a non-randomized, open-label, phase I/II clinical trial for administration of lentiviral-transduced autologous hematopoietic stem cells following busulfan (6 mg/kg total) conditioning. The primary and secondary objectives assess efficacy in restoring immunity and safety by vector insertion site analysis (VISA). In this ongoing study (19 patients treated), we report VISA in blood lineages from first eight treated patients with longer follow up found a > 60-fold increase in frequency of forward-orientated VIS within intron 3 of the High Mobility Group AT-hook 2 gene. All eight patients demonstrated emergence of dominant HMGA2 VIS clones in progenitor and myeloid lineages, but without disturbance of hematopoiesis. Our molecular analysis demonstrated a cryptic splice site within the chicken ß-globin hypersensitivity 4 insulator element in the vector generating truncated mRNA transcripts from many transcriptionally active gene containing forward-oriented intronic vector insert. A two base-pair change at the splice site within the lentiviral vector eliminated splicing activity while retaining vector functional capability. This highlights the importance of functional analysis of lentivectors for cryptic splicing for preclinical safety assessment and a redesign of clinical vectors to improve safety.

Correction of X-CGD patient HSPCs by targeted CYBB cDNA insertion using CRISPR/Cas9 with 53BP1 inhibition for enhanced homology-directed repair.

X-linked chronic granulomatous disease is an immunodeficiency characterized by defective production of microbicidal reactive oxygen species (ROS) by phagocytes. Causative mutations occur throughout the 13 exons and splice sites of the CYBB gene, resulting in loss of gp91phox protein. Here we report gene correction by homology-directed repair in patient hematopoietic stem/progenitor cells (HSPCs) using CRISPR/Cas9 for targeted insertion of CYBB exon 1-13 or 2-13 cDNAs from adeno-associated virus donors at endogenous CYBB exon 1 or exon 2 sites. Targeted insertion of exon 1-13 cDNA did not restore physiologic gp91phox levels, consistent with a requirement for intron 1 in CYBB expression. However, insertion of exon 2-13 cDNA fully restored gp91phox and ROS production upon phagocyte differentiation. Addition of a woodchuck hepatitis virus post-transcriptional regulatory element did not further enhance gp91phox expression in exon 2-13 corrected cells, indicating that retention of intron 1 was sufficient for optimal CYBB expression. Targeted correction was increased ~1.5-fold using i53 mRNA to transiently inhibit nonhomologous end joining. Following engraftment in NSG mice, corrected HSPCs generated phagocytes with restored gp91phox and ROS production. Our findings demonstrate the utility of tailoring donor design and targeting strategies to retain regulatory elements needed for optimal expression of the target gene.

Preclinical Optimization and Safety Studies of a New Lentiviral Gene Therapy for p47phox-Deficient Chronic Granulomatous Disease.

Chronic granulomatous disease (CGD) is an inherited blood disorder of phagocytic cells that renders patients susceptible to infections and inflammation. A recent clinical trial of lentiviral gene therapy for the most frequent form of CGD, X-linked, has demonstrated stable correction over time, with no adverse events related to the gene therapy procedure. We have recently developed a parallel lentiviral vector for p47phox-deficient CGD (p47phoxCGD), the second most common form of this disease. Using this vector, we have observed biochemical correction of CGD in a mouse model of the disease. In preparation for clinical trial approval, we have performed standardized preclinical studies following Good Laboratory Practice (GLP) principles, to assess the safety of the gene therapy procedure. We report no evidence of adverse events, including mutagenesis and tumorigenesis, in human hematopoietic stem cells transduced with the lentiviral vector. Biodistribution studies of transduced human CD34+ cells indicate that the homing properties or engraftment ability of the stem cells is not negatively affected. CD34+ cells derived from a p47phoxCGD patient were subjected to an optimized transduction protocol and transplanted into immunocompromised mice. After the procedure, patient-derived neutrophils resumed their function, suggesting that gene correction was successful. These studies pave the way to a first-in-man clinical trial of lentiviral gene therapy for the treatment of p47phoxCGD.

Enhanced homology-directed repair for highly efficient gene editing in hematopoietic stem/progenitor cells.

Lentivector gene therapy for X-linked chronic granulomatous disease (X-CGD) has proven to be a viable approach, but random vector integration and subnormal protein production from exogenous promoters in transduced cells remain concerning for long-term safety and efficacy. A previous genome editing-based approach using Streptococcus pyogenes Cas9 mRNA and an oligodeoxynucleotide donor to repair genetic mutations showed the capability to restore physiological protein expression but lacked sufficient efficiency in quiescent CD34+ hematopoietic cells for clinical translation. Here, we report that transient inhibition of p53-binding protein 1 (53BP1) significantly increased (2.3-fold) long-term homology-directed repair to achieve highly efficient (80% gp91phox+ cells compared with healthy donor control subjects) long-term correction of X-CGD CD34+ cells.

MAGT1 messenger RNA-corrected autologous T and natural killer cells for potential cell therapy in X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia disease.

BACKGROUND AIM: X-linked MAGT1 deficiency with increased susceptibility to EBV-infection and N-linked glycosylation defect' (XMEN) disease is caused by mutations in the magnesium transporter 1 (MAGT1) gene. Loss of MAGT1 function results in a glycosylation defect that abrogates expression of key immune proteins such as the NKG2D receptor on CD8+ T and NK cells, which is critical for the recognition and killing of virus-infected and transformed cells, a biomarker for MAGT1 function. Patients with XMEN disease frequently have increased susceptibility to EBV infections and EBV-associated B cell malignancies, for which no specific treatment options are currently available. Experimental transfer of donor EBV-specific cytotoxic T cells may be beneficial but carries the risks of eliciting alloimmune responses. An approach for cell therapy to address viral infections and associated complications that avoids the risks of alloimmunity is needed. METHODS: Here the authors assess the feasibility and efficiency of correcting autologous lymphocytes from XMEN patients by MAGT1 mRNA electroporation (EP) that avoids genomic integration and can be scaled for clinical application. RESULTS AND CONCLUSIONS: Restoration of NKG2D expression was demonstrated in XMEN patient lymphocytes after MAGT1 mRNA electroporation that reach healthy donor levels in CD8+ T and NK cells at 1-2 days after EP. NKG2D expression persisted at ∼50% for 2 weeks after EP. Functionally, mRNA-correction of XMEN NK cells rescued cytotoxic activity also to healthy donor NK cell level. The restored NKG2D receptor expression and function were unaffected by cryopreservation, which will make feasible repeat infusions of MAGT1 mRNA-corrected autologous XMEN CD8+ T and NK cells for potential short term therapy for XMEN patients without the risks of alloimmunization.

NADPH oxidase correction by mRNA transfection of apheresis granulocytes in chronic granulomatous disease.

Granulocytes from patients with chronic granulomatous disease (CGD) have dysfunctional phagocyte reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase that fails to generate sufficient antimicrobial reactive oxidative species. CGD patients with severe persistent fungal or bacterial infection who do not respond to antibiotic therapy may be given apheresis-derived allogeneic granulocyte transfusions from healthy volunteers to improve clearance of intractable infections. Allogeneic granulocyte donors are not HLA matched, so patients who receive the donor granulocyte products may develop anti-HLA alloimmunity. This not only precludes future use of allogeneic granulocytes in an alloimmunized CGD recipient, but increases the risk of graft failure of those recipients who go on to need an allogeneic bone marrow transplant. Here, we provide the first demonstration of efficient functional restoration of CGD patient apheresis granulocytes by messenger RNA (mRNA) electroporation using a scalable, Good Manufacturing Practice-compliant system to restore protein expression and NADPH oxidase function. Dose-escalating clinical-scale in vivo studies in a nonhuman primate model verify the feasibility, safety, and persistence in peripheral blood of infusions of mRNA-transfected autologous granulocyte-enriched apheresis cells, supporting this novel therapeutic approach as a potential nonalloimmunizing adjunct treatment of intractable infections in CGD patients.

Failure to Prevent Severe Graft-Versus-Host Disease in Haploidentical Hematopoietic Cell Transplantation with Post-Transplant Cyclophosphamide in Chronic Granulomatous Disease.

PURPOSE: Haploidentical related donor (HRD) transplantation was performed in 7 recipients with chronic granulomatous disease (CGD) who had no matched-related or unrelated donor. METHODS: Peripheral blood cell (PBC) products were used with a conditioning regimen consisting of low-dose cyclophosphamide, fludarabine, total body irradiation, and busulfan. Graft-versus-host disease (GVHD) prophylaxis consisted of high-dose post-transplant cyclophosphamide and sirolimus. Recipients were ages 14-26 years, and 3 had severe infections active at transplant. RESULTS: All 7 recipients achieved full engraftment with complete donor chimerism early in the post-transplant period. Acute GVHD occurred in all cases and was grade 3 or steroid refractory in 3. Two patients with steroid-refractory GVHD died. Three patients with severe infectious complications active at transplant, 1 Nocardia pneumonia and 2 extensive invasive fungal infections), survived and were cured of their infection at last follow-up. Bacterial disease occurred post-transplant in all recipients, and viral infections/reactivation were common, including 4 cases of BK virus-associated hemorrhagic cystitis. CONCLUSIONS: Seven patients with CGD achieved rapid and full-donor engraftment from HRDs utilizing PBCs and a conditioning regimen with PTCy and sirolimus GVHD prophylaxis. However, the incidence of grade 3 and steroid-refractory GVHD was high and led to 2 deaths. Patients with active infections at transplant had successful transplant courses and were cured of their disease. Although there was an initial success with this regimen, the cumulative experience does not support its use in CGD due to an unacceptable rate of severe GVHD.

Allogeneic Reduced-Intensity Hematopoietic Stem Cell Transplantation for Chronic Granulomatous Disease: a Single-Center Prospective Trial.

PURPOSE: The purpose of this study was to evaluate engraftment and adverse events with a conditioning and prophylactic regimen intended to achieve high rates of engraftment with minimal graft-versus-host disease (GVHD) in allogeneic transplantation for chronic granulomatous disease in a single center. METHODS: Forty patients, 37 male, with chronic granulomatous disease were transplanted. Transplant products were matched sibling peripheral blood stem cells (PBSCs) in four and matched unrelated donor (MUD) bone marrow in three, and one patient received mismatched unrelated PBSCs. Thirty-two patients received MUD PBSCs. All patients received a conditioning regimen of busulfan/alemtuzumab (with low-dose total body irradiation for MUD recipients) with sirolimus graft-versus-host disease prophylaxis. RESULTS: Engraftment occured in 38/40 recipients (95%). Acute or chronic GVHD occurred in 18 (45%) and 5 (12.5%), respectively, with 6 episodes of grades III-IV and/or steroid refractory GVHD. Overall survival was 33/40 (82.5%) and event-free survival was 30/40 (80%). Successful engraftment was associated with myeloid and NK cell, but not CD3+ chimerism. Myeloid engraftment was greater than 70% in 30/32 recipients at mean follow-up of 3.4 years. Evidence of persistent immunodeficiency was not seen in successful transplants. Attempts to rescue failed or poorly functioning grafts were associated with unacceptable morbidity and mortality. CONCLUSIONS: A reduced-intensity allogeneic transplant protocol based on alemtuzumab and busulfan with sirolimus GVHD prophylaxis produced high rates of successful engraftment and minimal regimen-related toxicity. Prolonged clinical follow-up has confirmed its efficacy in ameliorating CGD-related disease. Outcomes were not acceptable with donor cell infusion rescue of cause with poor graft function.

Lentiviral hematopoietic stem cell gene therapy for X-linked severe combined immunodeficiency.

X-linked severe combined immunodeficiency (SCID-X1) is a profound deficiency of T, B, and natural killer (NK) cell immunity caused by mutations inIL2RGencoding the common chain (γc) of several interleukin receptors. Gamma-retroviral (γRV) gene therapy of SCID-X1 infants without conditioning restores T cell immunity without B or NK cell correction, but similar treatment fails in older SCID-X1 children. We used a lentiviral gene therapy approach to treat five SCID-X1 patients with persistent immune dysfunction despite haploidentical hematopoietic stem cell (HSC) transplant in infancy. Follow-up data from two older patients demonstrate that lentiviral vector γc transduced autologous HSC gene therapy after nonmyeloablative busulfan conditioning achieves selective expansion of gene-marked T, NK, and B cells, which is associated with sustained restoration of humoral responses to immunization and clinical improvement at 2 to 3 years after treatment. Similar gene marking levels have been achieved in three younger patients, albeit with only 6 to 9 months of follow-up. Lentiviral gene therapy with reduced-intensity conditioning appears safe and can restore humoral immune function to posthaploidentical transplant older patients with SCID-X1.

Targeted gene addition in human CD34(+) hematopoietic cells for correction of X-linked chronic granulomatous disease.

Gene therapy with genetically modified human CD34(+) hematopoietic stem and progenitor cells (HSPCs) may be safer using targeted integration (TI) of transgenes into a genomic 'safe harbor' site rather than random viral integration. We demonstrate that temporally optimized delivery of zinc finger nuclease mRNA via electroporation and adeno-associated virus (AAV) 6 delivery of donor constructs in human HSPCs approaches clinically relevant levels of TI into the AAVS1 safe harbor locus. Up to 58% Venus(+) HSPCs with 6-16% human cell marking were observed following engraftment into mice. In HSPCs from patients with X-linked chronic granulomatous disease (X-CGD), caused by mutations in the gp91phox subunit of the NADPH oxidase, TI of a gp91phox transgene into AAVS1 resulted in ∼15% gp91phox expression and increased NADPH oxidase activity in ex vivo-derived neutrophils. In mice transplanted with corrected HSPCs, 4-11% of human cells in the bone marrow expressed gp91phox. This method for TI into AAVS1 may be broadly applicable to correction of other monogenic diseases.

Haploidentical Hematopoietic Cell Transplantation with Post-Transplant Cyclophosphamide in a Patient with Chronic Granulomatous Disease and Active Infection: A First Report.

PURPOSE: We describe haploidentical hematopoietic cell transplantation (HCT) with high-dose post-transplant cyclophosphamide (PTCy) in a boy with x-linked chronic granulomatous disease (CGD). METHODS: A persistent and life-threatening fungal infection was the indication for HSCT. Non-myeloablative conditioning with PTCy (50 mg/kg days 3 and 4) was used in the absence of fully matched donors. RESULTS: Engraftment occurred on day 24. The patient experienced Grade 2 graft-versus-host disease of the skin and gastrointestinal tract and CMV infection, both of which were controlled. Chimerism was 100 % at days 30 and 6 months. Cessation of antifungal therapy was consistent with cure of the infection. CONCLUSIONS: Haploidentical HCT with high-dose PTCy for CGD is feasible and succeeded even in the context of active infection.

Chromothriptic cure of WHIM syndrome.

Chromothripsis is a catastrophic cellular event recently described in cancer in which chromosomes undergo massive deletion and rearrangement. Here, we report a case in which chromothripsis spontaneously cured a patient with WHIM syndrome, an autosomal dominant combined immunodeficiency disease caused by gain-of-function mutation of the chemokine receptor CXCR4. In this patient, deletion of the disease allele, CXCR4(R334X), as well as 163 other genes from one copy of chromosome 2 occurred in a hematopoietic stem cell (HSC) that repopulated the myeloid but not the lymphoid lineage. In competitive mouse bone marrow (BM) transplantation experiments, Cxcr4 haploinsufficiency was sufficient to confer a strong long-term engraftment advantage of donor BM over BM from either wild-type or WHIM syndrome model mice, suggesting a potential mechanism for the patient's cure. Our findings suggest that partial inactivation of CXCR4 may have general utility as a strategy to promote HSC engraftment in transplantation.

Enhancers are major targets for murine leukemia virus vector integration.

UNLABELLED: Retroviral vectors have been used in successful gene therapies. However, in some patients, insertional mutagenesis led to leukemia or myelodysplasia. Both the strong promoter/enhancer elements in the long terminal repeats (LTRs) of murine leukemia virus (MLV)-based vectors and the vector-specific integration site preferences played an important role in these adverse clinical events. MLV integration is known to prefer regions in or near transcription start sites (TSS). Recently, BET family proteins were shown to be the major cellular proteins responsible for targeting MLV integration. Although MLV integration sites are significantly enriched at TSS, only a small fraction of the MLV integration sites (<15%) occur in this region. To resolve this apparent discrepancy, we created a high-resolution genome-wide integration map of more than one million integration sites from CD34(+) hematopoietic stem cells transduced with a clinically relevant MLV-based vector. The integration sites form ∼60,000 tight clusters. These clusters comprise ∼1.9% of the genome. The vast majority (87%) of the integration sites are located within histone H3K4me1 islands, a hallmark of enhancers. The majority of these clusters also have H3K27ac histone modifications, which mark active enhancers. The enhancers of some oncogenes, including LMO2, are highly preferred targets for integration without in vivo selection. IMPORTANCE: We show that active enhancer regions are the major targets for MLV integration; this means that MLV preferentially integrates in regions that are favorable for viral gene expression in a variety of cell types. The results provide insights for MLV integration target site selection and also explain the high risk of insertional mutagenesis that is associated with gene therapy trials using MLV vectors.

Transgene-free iPSCs generated from small volume peripheral blood nonmobilized CD34+ cells.

A variety of somatic cells can be reprogrammed to induced pluripotent stem cells (iPSCs), but CD34(+) hematopoietic stem cells (HSCs) present in nonmobilized peripheral blood (PB) would be a convenient target. We report a method for deriving iPSC from PB HSCs using immunobead purification and 2- to 4-day culture to enrich CD34(+) HSCs to 80% ± 9%, followed by reprogramming with loxP-flanked polycistronic (human Oct4, Klf4, Sox2, and c-Myc) STEMCCA-loxP lentivector, or with Sendai vectors. Colonies arising with STEMCCA-loxP were invariably TRA-1-60(+), yielding 5.3 ± 2.8 iPSC colonies per 20 mL PB (n = 17), where most colonies had single-copy STEMCCA-loxP easily excised by transient Cre expression. Colonies arising with Sendai were variably reprogrammed (10%-80% TRA-1-60(+)), with variable yield (6 to >500 TRA-1-60(+) iPSC colonies per 10 mL blood; n = 6). Resultant iPSC clones expressed pluripotent cell markers and generated teratomas. Genomic methylation patterns of STEMCCA-loxP-reprogrammed clones closely matched embryonic stem cells. Furthermore, we showed that iPSCs are derived from the nonmobilized CD34(+) HSCs enriched from PB rather than from any lymphocyte or monocyte contaminants because they lack somatic rearrangements typical of T or B lymphocytes and because purified CD14(+) monocytes do not yield iPSC colonies under these reprogramming conditions.

Hypomorphic Rag mutations can cause destructive midline granulomatous disease.

Destructive midline granulomatous disease characterized by necrotizing granulomas of the head and neck is most commonly caused by Wegener granulomatosis, natural killer/T-cell lymphomas, cocaine abuse, or infections. An adolescent patient with myasthenia gravis treated with thymectomy subsequently developed extensive granulomatous destruction of midface structures, palate, nasal septum, airways, and epiglottis. His lymphocyte numbers, total immunoglobulin G level, and T-cell receptor (TCR) repertoire appeared normal. Sequencing of Recombination activating gene-1 (Rag1) showed compound heterozygous Rag1 mutations; a novel deletion with no recombinase activity and a missense mutation resulting in 50% Rag activity. His thymus was dysplastic and, although not depleted of T cells, showed a notable absence of autoimmune regulator (AIRE) and Foxp3(+) regulatory T cells. This distinct Rag-deficient phenotype characterized by immune dysregulation with granulomatous hyperinflammation and autoimmunity, with relatively normal T and B lymphocyte numbers and a diverse TCR repertoire expands the spectrum of presentation in Rag deficiency. This study was registered at www.clinicaltrials.gov as #NCT00128973.

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.

Diprotin A infusion into nonobese diabetic/severe combined immunodeficiency mice markedly enhances engraftment of human mobilized CD34+ peripheral blood cells.

Hematopoietic stem cell (HSC) graft cell dose impacts significantly on allogeneic transplant. Similarly, HSC gene therapy outcome is affected by loss of repopulating cells during culture required for ex vivo retrovirus transduction. Stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 play a central role in marrow trafficking of HSCs, and maneuvers that enhance CXCR4 activation might positively impact outcome in settings of limiting graft dose. CD26/dipeptidyl peptidase IV (DPP-IV) is an ectoenzyme protease that cleaves SDF-1, thus reducing CXCR4 activation. We show that injection of irradiated nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice with >or=2 micromol Diprotin A (a tripeptide specific inhibitor of CD26 protease activity) at the time of transplant of human granulocyte colony-stimulating factor (G-CSF) mobilized CD34(+) peripheral blood cells (CD34(+) PBCs) results in a >3.4-fold enhancement of engraftment of human cells. We also show that CD26 on residual stromal cells in the irradiated recipient marrow milieu, and not any CD26 activity in the human CD34(+) PBC graft itself, plays the critical role in regulating receptivity of this environment for the incoming graft. Human marrow stromal cells also express CD26, raising the possibility that Diprotin A treatment could significantly enhance engraftment of HSCs in humans in settings of limiting graft dose just as we observed in the NOD/SCID mouse human xenograft model.

The late dividing population of gamma-retroviral vector transduced human mobilized peripheral blood progenitor cells contributes most to gene-marked cell engraftment in nonobese diabetic/severe combined immunodeficient mice.

We used the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model to assess the repopulation potential of subpopulations of mobilized human CD34+ peripheral blood progenitor cells (PBPC). First, PBPC were transduced with gamma-retrovirus vector RD114-MFGS-CFP, which requires cell division for successful transduction, at 24 hours, 48 hours, and 72 hours to achieve 96% cyan fluorescent protein (CFP)-positive cells. Cells were sorted 12 hours after the last transduction into CFP-positive (divided cells) and CFP-negative populations. CFP-positive cells were transplanted postsort, whereas the CFP-negative cells were retransduced and injected at 120 hours. The CFP-negative sorted and retransduced cells contained markedly fewer vector copies and resulted in a 32-fold higher overall engraftment and in a 13-fold higher number of engrafted transgene positive cells. To assess cell proliferation as an underlying cause for the different engraftment levels, carboxyfluorescein succinimidyl ester-labeling of untransduced PBPC was performed to track the number of cell divisions. At 72 hours after initiation of culture, when 95% of all cells have divided, PBPC were sorted into nondivided and divided fractions and transplanted into NOD/SCID mice. Nondivided cells demonstrated 45-fold higher engraftment than divided cells. Late dividing PBPC in ex vivo culture retain high expression of the stem cell marker CD133, whereas rapidly proliferating cells lose CD133 in correlation to the number of cell divisions. Our studies demonstrate that late dividing progenitors transduced with gamma-retroviral vectors contribute most to NOD/SCID engraftment and transgene marking. Confining the gamma-retroviral transduction to CD133-positive cells on days 3 and 4 could greatly reduce the number of transplanted vector copies, limiting the risk of leukemia from insertional mutagenesis. Disclosure of potential conflicts of interest is found at the end of this article.

Gene therapy improves immune function in preadolescents with X-linked severe combined immunodeficiency.

Retroviral gene therapy can restore immunity to infants with X-linked severe combined immunodeficiency (XSCID) caused by mutations in the IL2RG gene encoding the common gamma chain (gammac) of receptors for interleukins 2 (IL-2), -4, -7, -9, -15, and -21. We investigated the safety and efficacy of gene therapy as salvage treatment for older XSCID children with inadequate immune reconstitution despite prior bone marrow transplant from a parent. Subjects received retrovirus-transduced autologous peripherally mobilized CD34(+) hematopoietic cells. T-cell function significantly improved in the youngest subject (age 10 years), and multilineage retroviral marking occurred in all 3 children.

WHIM syndrome myelokathexis reproduced in the NOD/SCID mouse xenotransplant model engrafted with healthy human stem cells transduced with C-terminus-truncated CXCR4.

WHIM(warts, hypogammaglobulinemia, recurrent bacterial infection, and myelokathexis) syndrome is a rare immunodeficiency caused in many cases by autosomal dominant C-terminal truncation mutations in the chemokine receptor CXCR4. A prominent and unexplained feature of WHIM is myelokathexis (hypercellularity with apoptosis of mature myeloid cells in bone marrow and neutropenia). We transduced healthy human CD34(+) peripheral blood-mobilized stem cells (PBSCs) with retrovirus vector encoding wild-type (wt) CXCR4 or WHIM-type mutated CXCR4 and studied these cells ex vivo in culture and after engraftment in a nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mouse xenograft model. Neither wt CXCR4 nor mutated CXCR4 transgene expression itself enhanced apoptosis of neutrophils arising in transduced PBSC cultures even with stimulation by a CXCR4 agonist, stromal cell-derived factor-1 (SDF-1 [CXCL12]). Excess wt CXCR4 expression by transduced human PBSCs enhanced marrow engraftment, but did not affect bone marrow (BM) apoptosis or the release of transduced leukocytes into PB. However, mutated CXCR4 transgene expression further enhanced BM engraftment, but was associated with a significant increase in apoptosis of transduced cells in BM and reduced release of transduced leukocytes into PB. We conclude that increased apoptosis of mature myeloid cells in WHIM is secondary to a failure of marrow release and progression to normal myeloid cell senescence, and not a direct effect of activation of mutated CXCR4.