THE KINETICS OF MURINE HEMATOPOIETIC STEM-CELLS IN-VIVO IN RESPONSE TO PROLONGED INCREASED MATURE BLOOD-CELL PRODUCTION INDUCED BY GRANULOCYTE-COLONY-STIMULATING FACTOR

Because of the complexity of appropriate stem cell assays, little info rmation on the in vivo regulation of murine stem cell biology or stemm atopoiesis is available. It is unknown whether and how in vivo the pri mitive hematopoietic stem cell compartment is affected during a contin ued increased production of mature blood cells. In this study, we pres ent data showing that prolonged (3 weeks) administration of granulocyt e colony-stimulating factor (G-CSF), which is a major regulator of mat ure granulocyte production, has a substantial impact on both the size and the location of various stem cell subset pools in mice. We have us ed the novel cobblestone area forming cell (CAFC) assay to assess the effects of G-CSF on the stem cell compartment (CAFC days 7, 14, 21, an d 28). In marrow, in which normally 99% of the total number of stem ce lls can be found, G-CSF induced a severe depletion of particularly the most primitive stem cells to 5% to 10% of normal values. The response after 7 days of G-CSF treatment was an increased amplification betwee n CAFC day 14 and 7. However, this response occurred at the expense of the number of CAFC day 14. It is likely that the resulting gap of CAF C day 14 cell numbers was subsequently replenished from the more primi tive CAFC day 21 and 28 compartments, because these cell numbers remai ned low during the entire treatment period. In the spleen, the number of stem cells increased, likely caused by a migration from the marrow via the blood, leading to an accumulation in the spleen. The increased number of stem cells in the spleen overcompensated for the loss in th e marrow. When total body (marrow and spleen) stem cell numbers were c alculated, it appeared that a continued increased production of mature granulocytes resulted in the establishment of a higher, new steady st ate of the stem cell compartment; most committed stem cells (CAFC day 7) were increased threefold, CAFC day 14 were increased 2.3-fold, CAFC -day 21 were increased 1.8-fold, and the most primitive stem cells eva luated, CAFC day 28, were not different from normal, although now 95% of these cells were located in the spleen. Four weeks after discontinu ation of the G-CSF treatment, the stem cell reserve in the spleen had returned to a normal level, whereas stem cell numbers in marrow had re covered to values above normal. This study shows that the primitive st em cell compartment is seriously perturbed during an increased stimula tion of the production of mature blood cells. Furthermore, it shows th at intricate regulatory feedback loops exist within the stem cell comp artment that will enable proper adaptations to stress situations. (C) 1995 by The American Society of Hematology.