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 ma y be temporary followed by permanent engraftment of long-term repopula ting 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 establ ished unequivocally. We used the results of transplantation of subpopu lations successively enriched for LTRC and containing varying numbers of CFU-S-12 (colony-forming units in the spleen at day 12 post transpl antation) 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 t he basis of antibody reactivity, Hoechst 33342 and rhodamine 123 fluor escence intensity and light scattering properties. The minimum rescue inocula varied greatly in CFU-GEMM, BFU-E and CFU-GM content. One to t wo CFU-S-12 were uniformly present in all isolated suspensions that re scued 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 tha t multiple CFU-S day 12/13 are needed for initial repopulation is also supported by quantitative estimates of the number of mature cells tha t can be produced by CFU-S, Transfusion of a single CFU-S day 12/13 ca n be shown to be grossly inadequate to provide the number of periphera l blood cells needed to ameliorate the severe pancytopenia following l ethal irradiation by day 12-14. Our data also indicate that 5-fluorour acil-treated marrow subpopulations appear inferior to untreated subpop ulations in their ability to contribute to initial repopulation when t ransfused at low cell doses into lethally irradiated recipients.