MYELOSUPPRESSIVE CONDITIONING IMPROVES AUTOLOGOUS ENGRAFTMENT OF GENETICALLY MARKED HEMATOPOIETIC REPOPULATING CELLS IN DOGS

We have studied the role of different conditioning regimens for engraf tment of genetically marked hematopoietic repopulating cells in dogs. Peripheral blood (PB) and/or marrow cells collected after treatment wi th recombinant canine stem cell factor (rcSCF) or cyclophosphamide wer e transduced in a vector-containing long-term culture system. Three di fferent vector-producing cell lines with similar viral titers were use d. In two of them, the neo-containing LN vector was packaged either in the PA317 cell line with an amphotropic murine retrovirus envelope or the PG13 cell line with the gibbon ape leukemia virus (GALV) envelope . The MFG/GC vector produced in PA317 cells contained the human glucoc erebrosidase gene. Nineteen dogs received either no conditioning (grou p A, n = 5), irradiation to both humeri with 1,000 cGy (group B, n = 5 ), a sublethal dose of cyclophosphamide 40 mg/kg (group C, n = 4), a s ublethal dose of 200 or 300 cGy total body irradiation (TBI) (group D, n = 3), or an otherwise lethal dose of 920 cGy TBI (group E, n = 3) b efore intravenous (groups A, C, D, E) or intramedullary (group B) infu sion of the transduced autologous hematopoietic cells. Transduction ef ficiency of hematopoietic cells at the time of infusion into the anima ls was similar among the different conditioning groups. Dogs were obse rved for at least 6 months. PB granulocytes were obtained at least eve ry 3 weeks after transplant and analyzed by polymerase chain reaction for the presence of the transduced genes. The percentages of positive results in dogs more than 4 weeks after transplantation were 0% withou t conditioning, 5% with local irradiation, 18% with sublethal cyclopho sphamide, 33% with sublethal TBI, and 17% with otherwise lethal TBI. A nalyzing the influence of conditioning regimens by a generalized estim ating equation (GEE) technique, which considered the use of different retrovirus vectors and the number of mononuclear cells infused as pote ntial confounding variables, we found that engraftment of genetically marked repopulating cells was significantly improved (P < .001) in dog s receiving systemic conditioning with either otherwise lethal TBI, su blethal TBI, or sublethal cyclophosphamide compared to dogs with local irradiation only or no conditioning. Within the limitation of the exp erimental design, these data suggest that myeloablative or myelosuppre ssive conditioning improves engraftment of genetically marked hematopo ietic repopulating cells. (C) 1995 by The American Society of Hematolo gy.