RELEVANCE OF MYELOABLATIVE CONDITIONING IN THE ENGRAFTMENT OF LIMITING NUMBERS OF NORMAL AND GENETICALLY MARKED LYMPHO-HEMATOPOIETIC STEM-CELLS

We have studied the relevance of using myeloablative conditioning in t he engraftment of limiting numbers of normal and retrovirally transduc ed bone marrow (BM) cells, The administration of high doses (10-12 Gy) of split-dose irradiation (4 h or 24 h intervals) to mice transplante d with small grafts (10(4)-10(6) cells) of normal BM not only minimize d the endogenous reconstitution of recipients but also increased, with respect to single irradiation protocols, the survival rate of the ani mals in the long-term, The efficiency of these myeloablative regimens in BM transplantation protocols involving the use of transduced grafts was tested in mice transfused with 5x10(4)-2x10(6) genetically marked BM cells, Ninety percent of recipients survived in the long-term and, in most cases, predominant engraftment of the transduced population w as apparent for up to 11 months post-transplantation. This was confirm ed in hematopoietic samples corresponding to the CFU-S, the preCFU-S a nd the long-term repopulating cells of primary recipients, It was of s ignificance, however, that reductions in the engraftment of the geneti cally marked cells were not associated with the engraftment of exogeno us untransduced cells, but rather with rises in the extent of endogeno us repopulation, revealing the difficulties of preventing the repopula tion of residual endogenous stem cells when limiting numbers of transd uced cells are transplanted, Our results emphasize the relevance of us ing efficient myeloablative conditioning regimens in those cases in wh ich predominant and sustained engraftment of limiting numbers of trans duced repopulating cells is required.