ENHANCED ENGRAFTMENT OF HEMATOPOIETIC PROGENITOR CELLS IN MICE TREATED WITH GRANULOCYTE-COLONY-STIMULATING FACTOR BEFORE LOW-DOSE IRRADIATION - IMPLICATIONS FOR GENE-THERAPY

Gene therapy for inherited disorders of blood cells will require both efficient methods for stable gene transfer and nonablative bone marrow conditioning regimens to allow engraftment of modified hematopoietic progenitor cells (HPCs). We have used a sensitive murine system for de tecting HPC engraftment using congenic C57BL/6 mice that differ at the Ly5 locus, which encodes the leukocyte common antigen. The system rel ies on the ability of monoclonal antibodies with specificity for Ly5.1 and Ly5.2 (revised nomenclature: CD45.1 and CD45.2, respectively) to distinguish do-nor and recipient peripheral blood leukocytes after tra nsplantation of purified Sca-1(+) bone marrow-derived HPCs. No detecta ble engraftment occurred in nonirradiated recipient mice, even when as many as 2.0 x 10(6) Sca-1(+)HPCs were transplanted. However, in mice receiving total body irradiation (TBI), engraftment increased as a fun ction of pretransplantation radiation dose, number of transplanted cel ls, and time after transplantation. Moreover, mice receiving either gr anulocyte colony-stimulating factor (G-CSF) or G-CSF + stem cell facto r before low-dose TBI (160 cGy) exhibited a marked increase in engraft ment compared with mice receiving a vehicle control before low-dose TB I (18.9% and 20.6% v 5.6% at 1 month, respectively; 29% and 35% v 15.1 % at 4 months, respectively). Use of growth factor pretreatment even a llowed TBI doses as low as 30, 70, or 120 cGy to achieve significant e ngraftment of donor progenitors (0.3%, 1.5%, and 6.8% at 1 month, resp ectively; 1.7%, 5.8%, and 13.9% at 4 months, respectively). All animal s remained healthy during the observation periods. Thus, growth factor preconditioning of the recipient followed by low-dose TBI may provide an optimal balance between safety and efficacy in achieving required levels of engraftment for gene therapy of blood disorders.