Therapeutic Levels of Gene Corrected Neutrophils Following Foamy Viral Vector Mediated Gene Transfer in a Canine Model of Leukocyte Adhesion Deficiency.
Authors: Bauer Thomas R. , Allen James M. , Tuschong Laura M. , Olson Erik M. , Burkholder Tanya , Hickstein Dennis D. , Russell David W. 
Published: October 2019

Abstract
Children with the genetic immunodeficiency disease leukocyte adhesion deficiency (LAD) experience life-threatening bacterial infections due to the inability of their leukocytes to adhere and migrate to sites of infection. Heterogenous molecular defects in the leukocyte integrin CD18 molecule are responsible for LAD. The canine form of LAD, known as CLAD, represents a disease-specific, large animal model in which affected animals typically die within the first few months of life. We evaluated a gene therapy approach in treating CLAD using foamy viral vectors. Foamy viral vectors were selected because of their efficacy of marking in several animal models, including human CD34+ cells in NOD-SCID mice, and because foamy viral vectors may be less likely to cause insertional mutagenesis than conventional Moloney murine leukemia virus (MLV) based vectors. In contrast to MLV, no case of leukemia or disease has been reported in animals or humans as a consequence of infection by foamy viruses. Each of three CLAD dogs received a single infusion of autologous, foamy viral vector-transduced CD34+ cells following non-myeloablative conditioning with 200-cGy total body irradiation (TBI). CLAD CD34+ cells were transduced via a short-duration (14–20 hr) exposure to foamy viral vector ΔΦ Mscv-CD18 (2-3 MOI) and 50 ng/mL growth factors cG-CSF, cSCF, and hFlt3-L, on 10 μg/cm2 Retronectin™. Estimated transduction of the CLAD CD34+ cells, using a sample of transduced cells incubated in vitro for 3 additional days, ranged from 13.7 to 24.6% by flow cytometric detection of CD18 expression. The total infused cell doses ranged from 1.7 to 2.3 x 106 CD34+ cells / kg. Two of the three CLAD dogs are alive and well 2 and 6 months post-infusion. By 3 weeks post infusion, both dogs displayed approximately 1.2–1.4% CD18+ neutrophils in the peripheral blood. This level has increased progressively in each animal, to 2.1% CD18+ neutrophils by 6 months in the first dog and to 2.3% by 2 months in the second dog following infusion. Both animals have had resolution or pre-emption of severe CLAD disease when compared to untreated controls, who were euthanized by 6 months due to refractory bacterial infections. The third dog died 6 days after infusion from intussusception arising as a complication from the TBI. These results represent the first report of successful gene therapy in a disease-specific, large-animal model using foamy viral vectors to reverse the disease phenotype. Foamy viral vectors offer potential advantages over MLV-based vectors, including the ability to transduce non-dividing cells and the safety of a non-pathogenic virus backbone. The simplified, short-duration transduction regimen used with foamy viral vectors, coupled with a clinically applicable, non-myeloablative regimen of 200 cGy TBI, produced levels of CD18-gene corrected leukocytes in the peripheral blood that reversed or prevented the disease phenotype in CLAD. These studies indicate that foamy viral vectors may represent a new therapeutic gene therapy approach for the treatment of children with LAD. Long-term follow-up of these animals will be required to establish the efficacy and safety of this vector.