HEMATOPOIETIC STEM-CELLS FOUND IN LINEAGE-POSITIVE SUBSETS IN THE BONE-MARROW OF 5-FLUOROURACIL-TREATED MICE

It is known that treatment of mice with 5-fluorouracil (5-FU, 150 mg/kg) confers radioprotection. To investigate this effect, we performed bone marrow transplantation (BMT) using C57BL/6-Ly5 congenic mice treated with 5-FU five days prior to experiments. The mononuclear cells (MNC) in 5-FU-treated bone marrow (BM) were 10 times more radioprotective than those in untreated BM. Moreover, the number of BM MNC expressing c-kit on their surface from 5-FU-treated mice was markedly decreased relative to those from untreated controls. These results showed that the surface characteristics of cells that contributed to this radioprotective effect differ from those of stem cells as reported recently. BM MNC of mice treated with 5-FU were separated on the basis of expression of the lineage-specific antigens (Lin), c-kit, and Ly6A/E. When injected into lethally irradiated mice, 1,000 Lin(+) and Lin(-)c-kit(+)Ly6A/E(+) cells showed radioprotective effects such that 100% and 60% survived, respectively, Flow cytometric analysis 165 days after BMT showed that 88.8% and 65.1% of peripheral blood (PB) in mice transplanted with Lin(+) and Lin(-)c-kit(+)Ly6A/E(+) was derived from donor mice, respectively. After six months, donor-derived Lin(-)c-kit(+)Ly6A/E(+) cells which showed radioprotective effects on a secondary irradiated host were detected from mice transplanted with Lin(+) cells from 5-FU-treated mice. Taken together, these findings demonstrated that stem cells expressing Lin(+) present in the BM of mice treated with 5-FU other than Lin(-)c-kit(+)Ly6A/E(+) cells and these Lin(+) cells play an important role in the recovery of myeloablative mice.