Hematopoietic stem cells (HSCs) are present in the marrow of adult mice at a frequency of 1/104, as measured by limiting dilution transplantation assays for individual cells that produce lymphoid (B and T) as well as myeloid (GM) cells for at least 4 months in irradiated recipients. HSCs thus defined can be reproducibly isolated in the CD45midlinRhoSP fraction of adult mouse bone marrow at a purity of >30%. In mice, mutations in c-kit, the receptor for Steel factor (SF) lead to substantial reductions in the adult HSC population. In vitro, SF has been identified as a potent regulator of HSC self-renewal divisions. High concentrations of SF in combination with IL-11 allow adult HSCs to divide with a net 2–4 fold expansion in HSC numbers after 10 days and low concentrations of SF result in loss of HSC activity. To investigate the cellular mechanisms underlying these different outcomes, we cultured 114 CD45midlinRhoSP adult mouse bone marrow cells in single cell cultures containing serum-free medium + 20 ng/ml IL-11 and either 300 or 10 ng/ml of SF. Each culture was then examined every 4–6 hr. The kinetics of division of these cells under both conditions was identical with completion of the 1st division occurring between 22–68 hr. During that time none of the input cells died (<1%). After 10 days of culture, during which time all input cells divided at least 5 times (>50 cells), the HSC content of pooled clones (as measured by in vivo transplantation assays) was found to be >10-fold higher in the clones generated under high vs. low SF conditions (p<0.05). To characterize the types of self-renewal divisions undertaken, 9 doublets generated under the high SF condition were harvested between 4 and 8 hr after they underwent their 1st division and then each of the daughters was injected into a separate irradiated mouse. Analysis of the 18 mice showed that for one of the input cells both daughters were HSCs (evidence of a symmetric self-renewal division) and for 3 more, only one of the 2 daughters was an HSC (evidence of an asymmetric self-renewal division). In contrast no daughter HSCs were identified when 6 doublets produced under the low SF condition were assayed. To determine whether the loss of HSC activity under low SF conditions was a pre- or post-mitotic event, additional in vivo HSC assays were performed on cells harvested from individual wells after 8, 16 and 96 hours of incubation. The results revealed no change in the proportion of wells with either low or high concentrations of SF that contained HSCs after 8 hr of incubation (10/36 positive mice injected with starting single cells and 5/17 (low SF) vs. 6/17 (high SF) positive mice injected with 8-hr single cells, respectively). However, a significant difference (p<0.01) was seen after 96 hr (5/35 vs. 2/43 positive mice, respectively) and, after only 16 hr, before a first mitosis was seen under either condition, a decline in HSCs was apparent under the low SF condition (4/15 vs. 1/15 positive mice injected with cells from the high vs. low SF condition). Together, these studies indicate that HSC exposure to different SF concentrations can rapidly and irreversibly alter the ability of HSCs to execute symmetric as well asymmetric self-renewal divisions in vitro.

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