PHENOTYPIC AND FUNCTIONAL-CHANGES INDUCED AT THE CLONAL LEVEL IN HEMATOPOIETIC STEM-CELLS AFTER 5-FLUOROURACIL TREATMENT

A significant fraction of hematopoietic stem cells (HSCs) have been sh own to be resistant to the effects of cytotoxic agents such as 5-fluor ouracil (5-FU), which is thought to eliminate many of the rapidly divi ding, more committed progenitors in the bone marrow and to provide a r elatively enriched population of the most primitive hematopoietic prog enitor cells. Although differences between 5-FU-enriched progenitor po pulations and those from normal bone marrow have been described, it re mained unclear if these differences reflected characteristics of the m ost primitive stem cells that were revealed by 5-FU, or if there were changes in the stem-cell population itself. Here, we have examined som e of the properties of the stem cells in the bone. marrow before and a fter 5-FU treatment and have defined several activation-related change s in the stem-cell population. We found that long-term reconstituting stem cells decrease their expression of the growth factor receptor c-k it by 10-fold and increase their expression of the integrin Mac-1 (CD1 1b). These changes begin as early as 24 hours after 5-FU treatment and are most pronounced within 2 to 3 days, This activated phenotype of H SCs isolated from 5-FU-treated mice is similar to the phenotype of ste m cells found in the fetal liver and to the phenotype of transiently r epopulating progenitors in normal bone marrow, We found that cell cycl e is induced concomitantly with these physical changes, and within 2 d ays as many as 29% of the stem-cell population is in the S/G(2)/M phas es of the cell cycle, Furthermore, when examined at a clonal level, we found that 5-FU did not appear to eliminate many of the transient, mu ltipotent progenitors from the bone marrow that were found to be copur ified with long-term repopulating, activated stem cells, These results demonstrate the sensitivity of the hematopoietic system to changes in its homeostasis and correlate the expression of several important sur face molecules with the activation state of HSCs. (C) 1997 by The Amer ican Society of Hematology.