Rescue from lethal irradiation correlates with transplantation of 10-20 CFU-S-day 12

Repopulation by donor cells of a bone marrow ablated by irradiation is now recognized to proceed in two phases: initial repopulation that may be temporary followed by permanent engraftment of longterm repopulating cells (LTRC). While a single LTRC has been shown to be capable of restoring the entire lymph-hemopoietic system of an irradiated animal, the identity of the temporary repopulating cells has not been established unequivocally. We used the results of transplantation of subpopulations successively enriched for LTRC and containing varying numbers of CFU-S-12 (colony-forming units in the spleen at day 12 post transplantation) and progenitors to determine the likely cell type and number of cells needed for initial survival after radiation. Subpopulations from untreated and 5-fluorouracil-treated mice were discriminated on the basis of antibody reactivity, Hoechst 33342 and rhodamine 123 fluorescence intensity and light scattering properties. The minimum rescue inocula varied greatly in CFU-GEMM, BFU-E and CFU-GM content. One to two CFU-S-12 were uniformly present in all isolated suspensions that rescued 50% of lethally irradiated animals. In view of the known average seeding efficiency of CFU-S, our studies suggest that transfusion of 10-20 CFU-S day 12/13 is responsible for radioprotection. Evidence that multiple CFU-S day 12/13 are needed for initial repopulation is also supported by quantitative estimates of the number of mature cells that can be produced by CFU-S. Transfusion of a single CFU-S day 12/13 can be shown to be grossly inadequate to provide the number of peripheral blood cells needed to ameliorate the severe pancytopenia following lethal irradiation by day 12-14. Our data also indicate that 5-fluorouracil-treated marrow subpopulations appear inferior to untreated subpopulations in their ability to contribute to initial repopulation when transfused at low cell doses into lethally irradiated recipients.