SEARCHING FOR HEMATOPOIETIC STEM-CELLS .2. THE HETEROGENEITY OF THY-1.1(LO)LIN(- LO)SCA-1(+) MOUSE HEMATOPOIETIC STEM-CELLS SEPARATED BY COUNTERFLOW CENTRIFUGAL ELUTRIATION/

Mouse hematopoietic stem cells (HSCs) are highly enriched in the rare (similar to 0.05%) Thy-1.1(lo)Lin(-/lo)Sca-1(+) fraction of hematopoie tic tissues. It has been demonstrated that Thy-1.1(lo)Lin(-/lo)Sca-1() cells are the only HSCs in C57BL/Ka-Thy-1.1 bone marrow. In this stu dy, we separated C57/Ka-Thy-1.1 bone marrow cells by counterflow centr ifugal elutriation (CCE) into four fractions and characterized Thy-1.1 (lo)Lin(-/lo)Sca-1(+) cells in each eluted fraction. The peak number o f Thy-1.1(lo)Lin(-/lo)Sca-1(+) cells was highly enriched in one eluted fraction, which was also highly enriched for day-12 to -13 CFU-S. Act ivities for day-13 CFU-S, radioprotection, and long-term multilineage reconstitution correlated with, and could be generally predicted by de termining, the frequency of Thy-1.1(lo)Lin(-/lo)Sca-1(+) cells in a gi ven eluted fraction. However, the fraction that was highly enriched fo r blast cells and contained a low frequency of Thy-1.1(lo)Lin(-/lo)Sca -1(+) cells, only provided short-term but not long-term radioprotectio n, with a predicted cell number (100 cells) that should have protected greater than or equal to 95% (PD95) of hosts. Still, when 300 Thy-1.1 (lo)Lin(-/lo)Sca-1(+) cells from this fraction (3 x PD95 for unfractio nated HSCs) were injected, mice were radioprotected and donor cells pr ovided long-term multilineage reconstitution. We propose that such bla st cells may contain two Thy-1.1(lo)Lin(-/lo)Sca-1(+) subsets, one pro viding short-term and the other long-term multilineage sustained hemat opoiesis, the latter presumably due to HSC self-renewal.