This project uses the genetic immunodeficiency disease leukocyte adhesion deficiency or LAD as a model to which to apply advances in understanding of stem cell biology developed in the context of this SCOR to enhance gene transfer into the hematopoietic stem cell. In LAD molecular defects in the leukocyte integrin CD18 molecule result in the failure of leukocytes to adhere to the vessel wall and migrate to the site of infection culminating in recurrent episodes of life-threatening bacterial. LAD is an attractive model of the proposed studies in that: 1) the defect in LAD involves a membrane receptor, therefore efficacy of gene transfer can be assessed by flow cytometric analysis of peripheral blood leukocytes; 2) the skin chamber assay allows CD18 gene corrected cells to be selectively detected in vivo; 3) the presence of severe and moderate deficiency phenotypes of LAD facilitates correlation between the phenotype and the persistence of CD18+ cells following the infusion of gene-corrected cells; and 4) a canine form of LAD (CLAD) enables the efficacy and safety of novel therapeutic approaches to be tested in a appropriate, large-animal model prior to their application in humans with the disease. The specific aims of this project are: 1) to expand our current clinical trial of ex vivo retroviral-mediated gene transfer of CD18 into CD34+ cells from patients with LAD using the PG13/LgCD18 retroviral vector to include moderate deficiency as well as severe deficiency patients; 2) to utilize the CLAD model and retroviral-mediated gene transfer of CD18 to determine whether a conditioning regimen will enable the engraftment of sufficient autologous, CD18 gene corrected hematopoietic stem cells to reverse the clinical phenotype; and 3) to design future clinical gene therapy trials in LAD based on the results from this project and the other projects in this SCOR. In both LAD and CLAD the efficacy of therapy will be assessed by monitoring the persistence of CD18+ SCOR. In both LAD and CLAD the efficacy of therapy will be assessed by monitoring the persistence of CD18+ cells in the peripheral blood flow by flow cytometry, the migration of CD18+ neutrophils into skin chambers, and the reversal of the clinical phenotype. Including the advances in understanding of the hematopoietic stem cell from other projects in this SCOR, as well as the results from this Project, will expedite the translation of basic science and clinical observations into novel approaches to hematopoietic stem cell gene therapy in humans.