Donor source and post-transplantation cyclophosphamide influence outcome in allogeneic stem cell transplantation for GATA2 deficiency.

GATA2 deficiency was described in 2011, and shortly thereafter allogeneic hematopoietic stem cell transplantation (HSCT) was shown to reverse the hematologic disease phenotype. However, there remain major unanswered questions regarding the type of conditioning regimen, type of donors, and graft-versus-host disease (GVHD) prophylaxis. We report 59 patients with GATA2 mutations undergoing HSCT at National Institutes of Health between 2013 and 2020. Primary endpoints were engraftment, reverse of the clinical phenotype, secondary endpoints were overall survival (OS), event-free survival (EFS), and the incidence of acute and chronic GVHD. The OS and EFS at 4 years were 85·1% and 82·1% respectively. Ninety-six percent of surviving patients had reversal of the hematologic disease phenotype by one-year post-transplant. Incidence of grade III-IV aGVHD in matched related donor (MRD) and matched unrelated donor recipients (URD) patients receiving Tacrolimus/Methotrexate for GVHD prophylaxis was 32%. In contrast, in the MRD and URD who received post-transplant cyclophosphamide (PT/Cy), no patient developed grade III-IV aGVHD. Six percent of haploidentical related donor (HRD) recipients developed grade III-IV aGVHD. In summary, a busulfan-based HSCT regimen in GATA2 deficiency reverses the hematologic disease phenotype, and the use of PT/Cy reduced the risk of both aGVHD and cGVHD.

Long-term follow-up of foamy viral vector-mediated gene therapy for canine leukocyte adhesion deficiency.

The development of leukemia following gammaretroviral vector-mediated gene therapy for X-linked severe combined immunodeficiency disease and chronic granulomatous disease (CGD) has emphasized the need for long-term follow-up in animals treated with hematopoietic stem cell gene therapy. In this study, we report the long-term follow-up (4-7 years) of four dogs with canine leukocyte adhesion deficiency (CLAD) treated with foamy viral (FV) vector-mediated gene therapy. All four CLAD dogs previously received nonmyeloablative conditioning with 200 cGy total body irradiation followed by infusion of autologous, CD34(+) hematopoietic stem cells transduced by a FV vector expressing canine CD18 from an internal Murine Stem Cell Virus (MSCV) promoter. CD18(+) leukocyte levels were >2% following infusion of vector-transduced cells leading to ongoing reversal of the CLAD phenotype for >4 years. There was no clinical development of lymphoid or myeloid leukemia in any of the four dogs and integration site analysis did not reveal insertional oncogenesis. These results showing disease correction/amelioration of disease in CLAD without significant adverse events provide support for the use of a FV vector to treat children with leukocyte adhesion deficiency type 1 (LAD-1) in a human gene therapy clinical trial.

Gains of glycosylation comprise an unexpectedly large group of pathogenic mutations.

Mutations involving gains of glycosylation have been considered rare, and the pathogenic role of the new carbohydrate chains has never been formally established. We identified three children with mendelian susceptibility to mycobacterial disease who were homozygous with respect to a missense mutation in IFNGR2 creating a new N-glycosylation site in the IFNgammaR2 chain. The resulting additional carbohydrate moiety was both necessary and sufficient to abolish the cellular response to IFNgamma. We then searched the Human Gene Mutation Database for potential gain-of-N-glycosylation missense mutations; of 10,047 mutations in 577 genes encoding proteins trafficked through the secretory pathway, we identified 142 candidate mutations ( approximately 1.4%) in 77 genes ( approximately 13.3%). Six mutant proteins bore new N-linked carbohydrate moieties. Thus, an unexpectedly high proportion of mutations that cause human genetic disease might lead to the creation of new N-glycosylation sites. Their pathogenic effects may be a direct consequence of the addition of N-linked carbohydrate.

Leukocyte integrin activation mediates transient neutropenia after G-CSF administration.

After administration of granulocyte colony-stimulating factor (G-CSF), there is a marked, albeit transient, drop in circulating neutrophils. To determine the role of leukocyte integrins in this disappearance, a dog having canine leukocyte adhesion deficiency (CLAD) or CLAD dogs who had undergone gene correction either by matched littermate allogeneic transplant or autologous gene therapy were evaluated. Shortly after G-CSF administration, a dramatic, yet transient, neutropenia was observed in the control littermates. This neutropenia was not as marked in the CLAD dogs. In all instances, it was CD18(+) neutrophils that preferentially egressed from the circulation. The association of CD18 with this rapid loss suggested leukocyte integrin activation after G-CSF administration. To determine the activation status of the integrin, a monoclonal antibody recognizing the activated α-subunit cation binding domain (mAb24) was used to evaluate human leukocytes after G-CSF administration. Mirroring the dramatic decrease in circulating neutrophil numbers, there was a dramatic and specific increase in the activation of the α-subunit after G-CSF expression on polymorphonuclear leukocytes. This activation, like the drop in neutrophil count, was transient. These results demonstrate that the leukocyte integrin on circulating neutrophils is transiently activated after G-CSF administration and mediates the transient neutropenia observed after G-CSF administration.

A novel GATA2 distal enhancer mutation results in MonoMAC syndrome in 2 second cousins.

Mutations in the transcription factor GATA2 can cause MonoMAC syndrome, a GATA2 deficiency disease characterized by several findings, including disseminated nontuberculous mycobacterial infections, severe deficiencies of monocytes, natural killer cells, and B lymphocytes, and myelodysplastic syndrome. GATA2 mutations are found in âˆ¼90% of patients with a GATA2 deficiency phenotype and are largely missense mutations in the conserved second zinc-finger domain. Mutations in an intron 5 regulatory enhancer element are also well described in GATA2 deficiency. Here, we present a multigeneration kindred with the clinical features of GATA2 deficiency but lacking an apparent GATA2 mutation. Whole genome sequencing revealed a unique adenine-to-thymine variant in the GATA2 -110 enhancer 116,855 bp upstream of the GATA2 ATG start site. The mutation creates a new E-box consensus in position with an existing GATA-box to generate a new hematopoietic regulatory composite element. The mutation segregates with the disease in several generations of the family. Cell type-specific allelic imbalance of GATA2 expression was observed in the bone marrow of a patient with higher expression from the mutant-linked allele. Allele-specific overexpression of GATA2 was observed in CRISPR/Cas9-modified HL-60 cells and in luciferase assays with the enhancer mutation. This study demonstrates overexpression of GATA2 resulting from a single nucleotide change in an upstream enhancer element in patients with MonoMAC syndrome. Patients in this study were enrolled in the National Institute of Allergy and Infectious Diseases clinical trial and the National Cancer Institute clinical trial (both trials were registered at www.clinicaltrials.gov as #NCT01905826 and #NCT01861106, respectively).

Gene therapy of canine leukocyte adhesion deficiency using lentiviral vectors with human CD11b and CD18 promoters driving canine CD18 expression.

To identify cellular promoters in a self-inactivating (SIN) lentiviral vector that might be beneficial in treating children with leukocyte adhesion deficiency type 1 (LAD-1), we tested lentiviral vectors with human CD11 and CD18 leukocyte integrin proximal promoter elements directing expression of canine CD18 in animals with canine LAD (CLAD). Lentiviral vectors with either the human CD11b (637 bp) proximal promoter or the human CD18 (1,060 bp) proximal promoter resulted in the highest percentages of CD18(+) CLAD CD34(+) cells in vitro. Subsequently, two CLAD dogs were infused with autologous CD34(+) cells transduced with the hCD11b (637 bp)-cCD18 vector, and two CLAD dogs were infused with autologous CD34(+) cells transduced with the hCD18 (1,060 bp)-cCD18 vector. Each dog received a nonmyeloablative dose of 200 cGy total body irradiation (TBI) before the infusion of transduced cells. The two CLAD dogs treated with the hCD18 (1,060 bp)-cCD18 vector, and one of the two dogs treated with the hCD11b (637 bp)-cCD18 vector, had reversal of the CLAD phenotype. These studies using endogenous leukocyte integrin proximal promoters represent an important step in the development of gene therapy for children with LAD-1.

Multiorgan neutrophilic inflammation in a Border Collie with "trapped" neutrophil syndrome.

Trapped neutrophil syndrome is a rare congenital disease recognized in Border Collies and is characterized by persistent neutropenia with myeloid hyperplasia. The mechanism of neutropenia has not been described. We document the case of a young Border Collie diagnosed with trapped neutrophil syndrome based on clinical features, blood and bone marrow evaluation, and presence of the associated homozygous mutation. Results from flow cytometric and storage studies suggested lower neutrophil survival time. The dog had substantial neutrophilic inflammation in multiple organs, indicating that neutrophils could leave the marrow and enter tissues, making the term "trapped" neutrophil syndrome a misnomer.

ASXL1 and STAG2 are common mutations in GATA2 deficiency patients with bone marrow disease and myelodysplastic syndrome.

Patients with GATA2 deficiencyharbor de novo or inherited germline mutations in the GATA2 transcription factor gene, predisposing them to myeloid malignancies. There is considerable variation in disease progression, even among family members with the same mutation in GATA2. We investigated somatic mutations in 106 patients with GATA2 deficiency to identify acquired mutations that are associated with myeloid malignancies. Myelodysplastic syndrome (MDS) was the most common diagnosis (∼44%), followed by GATA2 bone marrow immunodeficiency disorder (G2BMID; ∼37%). Thirteen percent of the cohort had GATA2 mutations but displayed no disease manifestations. There were no correlations between age or sex with disease progression or survival. Cytogenetic analyses showed a high incidence of abnormalities (∼43%), notably trisomy 8 (∼23%) and monosomy 7 (∼12%), but the changes did not correlate with lower survival. Somatic mutations in ASXL1 and STAG2 were detected in ∼25% of patients, although the mutations were rarely concomitant. Mutations in DNMT3A were found in ∼10% of patients. These somatic mutations were found similarly in G2BMID and MDS, suggesting clonal hematopoiesis in early stages of disease, before the onset of MDS. ASXL1 mutations conferred a lower survival probability and were more prevalent in female patients. STAG2 mutations also conferred a lower survival probability, but did not show a statistically significant sex bias. There was a conspicuous absence of many commonly mutated genes associated with myeloid malignancies, including TET2, IDH1/2, and the splicing factor genes. Notably, somatic mutations in chromatin-related genes and cohesin genes characterized disease progression in GATA2 deficiency.

Preclinical Evaluation of Foamy Virus Vector-Mediated Gene Addition in Human Hematopoietic Stem/Progenitor Cells for Correction of Leukocyte Adhesion Deficiency Type 1.

Ex vivo gene therapy procedures targeting hematopoietic stem and progenitor cells (HSPCs) predominantly utilize lentivirus-based vectors for gene transfer. We provide the first pre-clinical evidence of the therapeutic utility of a foamy virus vector (FVV) for the genetic correction of human leukocyte adhesion deficiency type 1 (LAD-1), an inherited primary immunodeficiency resulting from mutation of the ß2 integrin common chain, CD18. CD34+ HSPCs isolated from a severely affected LAD-1 patient were transduced under a current good manufacturing practice-compatible protocol with FVV harboring a therapeutic CD18 transgene. LAD-1-associated cellular chemotactic defects were ameliorated in transgene-positive, myeloid-differentiated LAD-1 cells assayed in response to a strong neutrophil chemoattractant in vitro. Xenotransplantation of vector-transduced LAD-1 HSPCs in immunodeficient (NSG) mice resulted in long-term (∼5 months) human cell engraftment within murine bone marrow. Moreover, engrafted LAD-1 myeloid cells displayed in vivo levels of transgene marking previously reported to ameliorate the LAD-1 phenotype in a large animal model of the disease. Vector insertion site analysis revealed a favorable vector integration profile with no overt evidence of genotoxicity. These results coupled with the unique biological features of wild-type foamy virus support the development of FVVs for ex vivo gene therapy of LAD-1.

Hematopoietic Cell Transplantation and Outcomes Related to Human Papillomavirus Disease in GATA2 Deficiency.

GATA2 deficiency is a bone marrow failure syndrome effectively treated with hematopoietic cell transplantation (HCT), which also addresses the predisposition to many infections (prominently mycobacterial). However, many GATA2-deficient persons who come to HCT also have prevalent and refractory human papilloma virus disease (HPVD), which can be a precursor to cancer. We analyzed 75 HCT recipients for the presence of HPVD to identify patient characteristics and transplantation results that influence HPVD outcomes. We assessed the impact of cellular recovery and iatrogenic post-transplantation immunosuppression, as per protocol (PP) or intensified/prolonged (IP) graft-versus-host disease (GVHD) prophylaxis or treatment, on the persistence or resolution of HPVD. Our experience with 75 HCT recipients showed a prevalence of 49% with anogenital HPVD, which was either a contributing or primary factor in the decision to proceed to HCT. Of 24 recipients with sufficient follow-up, 13 had resolution of HPVD, including 8 with IP and 5 with PP. Eleven recipients had persistent HPVD, including 5 with IP and 6 with PP immunosuppression. No plausible cellular recovery group (natural killer cells or T cells) showed a significant difference in HPV outcomes. One recipient died of metastatic squamous cell carcinoma, presumably of anogenital origin, at 33 months post-transplantation after prolonged immunosuppression for chronic GVHD. Individual cases demonstrate the need for continued aggressive monitoring, especially in the context of disease prevalent at transplantation or prior malignancy. HCT proved curative in many cases in which HPVD was refractory and recurrent prior to transplantation, supporting a recommendation that HPVD should be considered an indication rather than contraindication to HCT, but post-transplantation monitoring should be prolonged with a high level of vigilance for new or recurrent HPVD.

Haploidentical in utero hematopoietic cell transplantation improves phenotype and can induce tolerance for postnatal same-donor transplants in the canine leukocyte adhesion deficiency model.

In the murine model, in utero hematopoietic cell transplantation (IUHCT) has been shown to achieve low levels of allogeneic chimerism and associated donor-specific tolerance permitting minimal conditioning postnatal hematopoietic stem cell transplantation (HSCT). In this pilot study, we investigated IUHCT in the canine leukocyte adhesion deficiency (CLAD) model. Haploidentical IUHCT resulted in stable low-level donor cell chimerism in all dogs that could be analyzed by sensitive detection methodology (4 of 10) through 18 months of follow-up. In the 2 CLAD recipients, low-level chimerism resulted in amelioration and complete reversal of the CLAD phenotype, respectively. Six recipients of IUHCT (5 carriers and 1 CLAD) subsequently received postnatal HSCT from the same haploidentical prenatal donor after minimal conditioning with busulfan 10 mg/kg. Chimerism in 2 of 5 CLAD carriers that underwent HSCT increased from < 1% pre-HSCT to sustained levels of 35% to 45%. Control animals undergoing postnatal haploidentical HSCT without IUHCT had no detectable donor chimerism. These results demonstrate that haploidentical IUHCT in the CLAD model can result in low-level donor chimerism that can prevent the lethal phenotype in CLAD dogs, and can result in donor-specific tolerance that can facilitate postnatal minimal conditioning HSCT.

Feline leukocyte adhesion (CD18) deficiency caused by a deletion in the integrin ß2 (ITGB2) gene.

BACKGROUND: Leukocyte adhesion deficiency (LAD) or CD18 deficiency is an autosomal recessive immunodeficiency which has been described in people, cattle, dogs, and knockout mice. OBJECTIVES: The study goals were to characterize the clinicopathologic, immunologic, and molecular genetic features of feline LAD (FLAD) in a neutered male adult Domestic Longhair cat with severe leukocytosis and recurrent infections. METHODS: Flow cytometry evaluated surface expression of CD18 on neutrophils. In vitro functional assays assessed CD18-dependent neutrophil adhesion and T-cell proliferation. Genomic DNA and cDNA were used to identify a causative mutation in the coding sequence of the integrin ß2 subunit (ITGB2) gene. RESULTS: The affected cat developed periodontitis during the first months of life followed by recurrent infections poorly responsive to antibiotic therapy, accompanied by extreme neutrophilia. Neutrophils from the proband, compared to feline controls, did not express any CD18 on the cell surface. Adhesion of affected neutrophils was severely impaired with and without phorbol-myristate-acetate activation. The proband's T-cells proliferated weakly to 1 pg but normally to 100 pg staphylococcal enterotoxin A, suggesting a CD18-independent T-cell response at higher doses. Molecular genetic analysis of the ITGB2 gene revealed a 24 bp deletion at the exon 2 to intron 2 boundary (c.46_58 + 11del), predicting premature translational termination due to abnormal splicing of exon 1 to exon 3 or 4. CONCLUSIONS: Feline LAD exhibits features similar to LAD in other species. However, clinical episodes in FLAD appeared milder allowing for an extended life expectancy under long-term antimicrobial therapy, possibly due to an alternative, CD18-independent T-cell proliferation pathway.

Nonmyeloablative hematopoietic stem cell transplantation corrects the disease phenotype in the canine model of leukocyte adhesion deficiency.

OBJECTIVE: The aim of this study was to test a nonmyeloablative hematopoietic stem cell transplant regimen applicable to children with leukocyte adhesion deficiency (LAD) who have a histocompatible sibling donor by using the canine model of LAD, namely canine leukocyte adhesion deficiency or CLAD. METHODS: Thirteen CLAD pups received a hematopoietic stem cell transplant from a dog leukocyte antigen (DLA)-matched littermate donor after pretransplant nonmyeloablative conditioning with 200 cGy total-body irradiation and posttransplant immunosuppression with cyclosporine and mycophenolate mofetil. Donor chimerism following transplant was assessed by flow cytometry for the presence of donor CD18 peripheral blood leukocytes and leukocyte subsets. RESULTS: Eleven of the 13 transplanted animals achieved stable mixed donor chimerism and reversal of the severe CLAD phenotype without graft-vs-host disease. The level of donor chimerism ranged from 3.9 to 95.5% at 1 year following transplant. There was one early death 3 weeks after transplant from thrombocytopenia and hemorrhage, and one dog with donor microchimerism (0.5% CD18+ donor leukocytes) who had attenuation of the CLAD phenotype. CONCLUSION: These results demonstrate that a nonmyeloablative transplant regimen from a DLA-matched littermate donor leads to mixed chimerism and reversal of the severe disease phenotype in dogs with CLAD, and provides support for the use of this approach in children with LAD who possess a histocompatible sibling donor.

Canine leukocyte adhesion deficiency colony for investigation of novel hematopoietic therapies.

The genetic immunodeficiency disease canine leukocyte adhesion deficiency (CLAD) was originally described in juvenile Irish Setters with severe, recurrent bacterial infections. CLAD was subsequently shown to result from a mutation in the leukocyte integrin CD18 subunit which prevents leukocyte surface expression of the CD11/CD18 complex. We describe the development of a mixed-breed CLAD colony with clinical features that closely parallel those described in Irish Setters. We demonstrate that the early identification of CLAD heterozygotes and CLAD-affected dogs by a combination of flow cytometry and DNA sequencing allows the CLAD-affected animals to receive life-saving antibiotic therapy. The distinct clinical phenotype in CLAD, the ability to detect CD18 on the leukocyte surface by flow cytometry, and the history of the canine model in marrow transplantation, enable CLAD to serve as an attractive large-animal model for the investigation of novel hematopoietic stem cell and gene therapy strategies.

Mixed chimeric hematopoietic stem cell transplant reverses the disease phenotype in canine leukocyte adhesion deficiency.

The genetic disease canine leukocyte adhesion deficiency (CLAD) is characterized by recurrent, severe bacterial infections, typically culminating in death by 6 months of age. CLAD is due to a mutation in the leukocyte integrin CD18 subunit, which prevents surface expression of the CD11/CD18 leukocyte integrin complex. We demonstrate that stable mixed donor:host hematopoietic chimerism, achieved by a non-myeloablative bone marrow transplant from a histocompatible littermate, reverses the disease phenotype in CLAD. Donor chimerism following the transplant was demonstrated both by flow cytometric detection of donor-derived CD18-positive leukocytes in the peripheral blood of the recipient, and by the demonstration of donor-derived DNA microsatellite repeats in the peripheral blood leukocytes of the recipient. These results indicate that mixed hematopoietic chimerism reverses the clinical phenotype in CLAD and represents a potential therapeutic approach for the human disease leukocyte adhesion deficiency.

The genetic immunodeficiency disease, leukocyte adhesion deficiency, in humans, dogs, cattle, and mice.

This review highlights the genotype-phenotype relationship of the genetic immunodeficiency disease leukocyte adhesion deficiency (LAD) in humans, dogs, cattle, and mice, and provides assessment of the opportunities that each animal species provides in the understanding of leukocyte biology and in developing new therapeutic approaches to LAD in humans. This comparison is important since animal models of genetic diseases in humans provide the opportunity to test new therapeutic approaches in an appropriate, disease-specific model. The success of this approach is dependent on the relationship of the phenotype in the animal to the phenotype of the disease in humans.

Tracking of specific integrant clones in dogs treated with foamy virus vectors.

Vector integration can lead to proto-oncogene activation and malignancies during hematopoietic stem cell gene therapy. We previously used foamy virus vectors to deliver the CD18 gene under the control of an internal murine stem cell virus promoter and successfully treated dogs with canine leukocyte adhesion deficiency. Here we have tracked the copy numbers of 11 specific proviruses found in these animals for 36-42 months after transplantation, including examples within or near proto-oncogenes, tumor suppressor genes, and genes unrelated to cancer. We found no evidence for clonal expansion of any of the clones, including those with proviruses in the MECOM gene (MDS1-EVI1 complex). These results suggest that although foamy virus vectors may integrate near proto-oncogenes, this does not necessarily lead to clonal expansion and malignancies. Additionally, we show that copy number estimates of these specific proviruses based on linker-mediated PCR results are different from those obtained by quantitative PCR, but can provide a qualitative assessment of provirus levels.

Gene therapy for canine leukocyte adhesion deficiency with lentiviral vectors using the murine stem cell virus and human phosphoglycerate kinase promoters.

Children with leukocyte adhesion deficiency type 1 (LAD-1) and dogs with canine LAD (CLAD) develop life-threatening bacterial infections due to mutations in the leukocyte integrin CD18. Here, we compared the human phosphoglycerate kinase (hPGK) promoter to the murine stem cell virus (MSCV) promoter/enhancer in a self-inactivating HIV-1-derived lentiviral vector to treat animals with CLAD. Four CLAD dogs were infused with CD34(+) cells transduced with the hPGK vector, and two CLAD dogs received MSCV vector-transduced CD34(+) cells. Infusions were preceded by a nonmyeloablative dose of 200 cGy total body irradiation. Comparable numbers of transduced cells were infused in each group of animals. Only one of four CLAD animals treated with the hPGK-cCD18 vector had reversal of CLAD, whereas both MSCV-cCD18 vector-treated dogs had reversal of the phenotype. Correction of CLAD depends both upon the percentage of CD18(+) myeloid cells and the level of expression of CD18 on individual myeloid cells. In this regard, the hPGK promoter directed low levels of expression of CD18 on neutrophils compared to the MSCV promoter, likely contributing to the suboptimal clinical outcome with the hPGK vector.

Potential large animal models for gene therapy of human genetic diseases of immune and blood cell systems.

Genetic mutations involving the cellular components of the hematopoietic system--red blood cells, white blood cells, and platelets--manifest clinically as anemia, infection, and bleeding. Although gene targeting has recapitulated many of these diseases in mice, these murine homologues are limited as translational models by their small size and brief life span as well as the fact that mutations induced by gene targeting do not always faithfully reflect the clinical manifestations of such mutations in humans. Many of these limitations can be overcome by identifying large animals with genetic diseases of the hematopoietic system corresponding to their human disease counterparts. In this article, we describe human diseases of the cellular components of the hematopoietic system that have counterparts in large animal species, in most cases carrying mutations in the same gene (CD18 in leukocyte adhesion deficiency) or genes in interacting proteins (DNA cross-link repair 1C protein and protein kinase, DNA-activated catalytic polypeptide in radiation-sensitive severe combined immunodeficiency). Furthermore, we describe the potential of these animal models to serve as disease-specific preclinical models for testing the efficacy and safety of clinical interventions such as hematopoietic stem cell transplantation or gene therapy before their use in humans with the corresponding disease.

Successful treatment of canine leukocyte adhesion deficiency by foamy virus vectors.

Recent successes in treating genetic immunodeficiencies have demonstrated the therapeutic potential of stem cell gene therapy. However, the use of gammaretroviral vectors in these trials led to insertional activation of nearby oncogenes and leukemias in some study subjects, prompting studies of modified or alternative vector systems. Here we describe the use of foamy virus vectors to treat canine leukocyte adhesion deficiency (CLAD). Four of five dogs with CLAD that received nonmyeloablative conditioning and infusion of autologous, CD34+ hematopoietic stem cells transduced by a foamy virus vector expressing canine CD18 had complete reversal of the CLAD phenotype, which was sustained more than 2 years after infusion. In vitro assays showed correction of the lymphocyte proliferation and neutrophil adhesion defects that characterize CLAD. There were no genotoxic complications, and integration site analysis showed polyclonality of transduced cells and a decreased risk of integration near oncogenes as compared to gammaretroviral vectors. These results represent the first successful use of a foamy virus vector to treat a genetic disease, to our knowledge, and suggest that foamy virus vectors will be effective in treating human hematopoietic diseases.