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Proliferation of purified murine hemopoietic stem cells in serum-free cultures stimulated with purified stem-cell-activating factor.

Under conditions of steady-state hemopoiesis in normal mice, the majority of hemopoietic stem cells in the bone marrow are in the quiescent state of the cell cycle. These cells can be stimulated to proliferate in vitro by the addition of a factor termed the "stem-cell-activating factor" (SAF), which is present in medium conditioned by various cell types. This factor is indistinguishable in antigenic and molecular properties from the lymphokine interleukin 3 (IL-3). The action of SAF on the stem cell cycle was studied by examination of the survival of the spleen colony-forming unit(s) (CFU-S) after four days of serum-free culture in the presence of purified SAF. CFU-S subtypes were distinguished on the basis of the day of colony counting (i.e., days 7, 9, and 12 after transplantation). The results indicate that SAF selectively induces an increase of the day-7 CFU-S: the CFU-S number increased 2.7-fold on day 7 and 1.2-fold on day 9, and decreased fivefold for day-12 CFU-S. Similar results were obtained in SAF culture of partially and highly purified stem cells. The proliferation of day-9 CFU-S in cultures of low-density bone marrow cells was found to be similar to that of unfractionated bone marrow cells until day 4 of culture. However, in the culture of partially purified stem cells, this proliferation stopped between days 4 and 10, whereas it continued with unfractionated cells. This indicates that cocultured bone marrow cells affect the proliferation of stem cells upon induction by SAF; day-4 cultures of highly purified resting stem cells with purified SAF resulted in a similar decrease in day-12 CFU-S and an increase in day-7 CFU-S, as was observed with unfractionated bone marrow cells. The DNA histogram of the stimulated sorted cells clearly revealed an actively DNA-synthesizing population. The results are in agreement with those of a selective induction of proliferation by SAF of resting pluripotent hemopoietic stem cells.