Hematopoietic stem cell regulatory volumes as revealed in studies of the bgj/bgj:W/WV chimera

High c-Kit expression identifies hematopoietic stem cells with impaired self-renewal and megakaryocytic bias

c-Kit^lo HSCs exhibit enhanced self-renewal and long-term reconstitution potential and give rise to c-Kit^hi HSCs that have a megakaryocytic bias.

Hematopoietic stem cells (HSCs) are heterogeneous with respect to their self-renewal, lineage, and reconstitution potentials. Although c-Kit is required for HSC function, gain and loss-of-function c-Kit mutants suggest that even small changes in c-Kit signaling profoundly affect HSC function. Herein, we demonstrate that even the most rigorously defined HSCs can be separated into functionally distinct subsets based on c-Kit activity. Functional and transcriptome studies show HSCs with low levels of surface c-Kit expression (c-Kit^lo) and signaling exhibit enhanced self-renewal and long-term reconstitution potential compared with c-Kit^hi HSCs. Furthermore, c-Kit^lo and c-Kit^hi HSCs are hierarchically organized, with c-Kit^hi HSCs arising from c-Kit^lo HSCs. In addition, whereas c-Kit^hi HSCs give rise to long-term lymphomyeloid grafts, they exhibit an intrinsic megakaryocytic lineage bias. These functional differences between c-Kit^lo and c-Kit^hi HSCs persist even under conditions of stress hematopoiesis induced by 5-fluorouracil. Finally, our studies show that the transition from c-Kit^lo to c-Kit^hi HSC is negatively regulated by c-Cbl. Overall, these studies demonstrate that HSCs exhibiting enhanced self-renewal potential can be isolated based on c-Kit expression during both steady state and stress hematopoiesis. Moreover, they provide further evidence that the intrinsic functional heterogeneity previously described for HSCs extends to the megakaryocytic lineage.

Effects of heavy ions on cycling stem cells

Murine marrow stem cells assayed with the spleen colony assay have been shown to be largely in a noncycling state, Go. In the unirradiated animal where these spleen-colony forming units (CFUs) transit normally between a non-proliferative state and active proliferation, exposure to a sufficient dose of ionizing radiation increases the frequency (probability) of this transition. For low-LET irradiation, marrow stem cells are not induced into cycle until a threshold dose is achieved. This dose appears to be in the range 50 to 100 cGy, inducing proliferation in an all-or-nothing manner. For irradiation with heavy charged-particles, however, the threshold dose is dependent on mass and energy. Irradiation with particles of sufficient mass and energy stimulates active proliferation even at the smallest doses tested, 5 cGy. Further, this response does not appear to result from an all-or-nothing effect. Rather, individual animals with intermediate levels of stem cell cycling have been observed. These data support the notion that locally controlled hemopoiesis can be affected by local deposition of radiation damage.

The stem cell concept revisited: self-renewal capacity is a dynamic property of hemopoietic cells

A rigid developmental program of stem cell division and progressive maturation into blood cells is challenged. It is proposed that the capacity for self-renewal is not limited to pluripotent stem cells but is shared by committed progenitors and even cells of later compartments. The relative probability of self-replication vs maturation in mitotic cells is controlled by extracellular influences. At the cell population level, the balance between proliferation and maturation and between compartments is regulated by feedback interactions. Inducibility of maturation in response to regulatory signals is smaller at earlier stages; consequently, at steady state primitive cells self-renew while their more differentiated progeny are forced to be transitory. The proposed dynamic linkage between compartments can be destabilized in a number of ways, resulting in defective hemopoiesis or leukemia. At all stages hemopoietic cells are able to change their patterns of gene expression, in an inheritable manner, in response to changes in their microenvironment. In particular, the capacity for self-renewal itself can vary even within a conventionally-defined compartment. On this basis of adaptive differentiation and self-renewal it is possible to account for the progression of chronic myelocytic leukemia and its "blastic conversion"; to analyse the hemopoietic system's response to various physiological and experimental perturbations; and to re-interpret the excessive phenotypic plasticity and apparent "lineage infidelity" manifested by leukemic cells and cell lines.

Special proliferative sites are not needed for seeding and proliferation of transfused bone marrow cells in normal syngeneic mice

The widely held view that transfused bone marrow cells will not proliferate in normal mice, not exposed to irradiation or other forms of bone marrow ablation, was reinvestigated. Forty million bone marrow cells from male donors were given to female recipients on each of 5 consecutive days, 5 to 10 times the number customarily used in the past. When the recipients were examined 2-13 weeks after the last transfusion, donor cells were found to average 16-25% of total marrow cells. Similar percentages of donor cells were found when variants of the enzyme phosphoglycerate kinase determined electrophoretically were used for identification of donor and recipient cells. Evidence is presented that the proportion of donor cells is compatible with a linear dependence on the number of cells transfused over the range tested--i.e., 20-200 million bone marrow cells injected intravenously. Special proliferative sites thus do not appear to be required.

Partitioning of bone marrow into stem cell regulatory domains

To examine the hypothesis that bone marrow consists of discrete stem cell regulatory volumes or domains, we studied spleen colony-forming unit (CFU-S) population growth kinetics in unirradiated WBB6F1-W/Wv mice receiving various doses of +/+ bone marrow cells. Assay of femoral marrow CFU-S content in the eight recipient dose groups revealed a family of growth curves having an initial dose-independent exponential phase and a subsequent dose-dependent deceleration phase. CFU-S content at the growth transition (inflection point) was not a simple linear function of inoculum dose but was shown rather to reflect a random distribution of initially seeded donor CFU-S in discrete volumes of recipient bone marrow. The inoculum dose resulting in a mean of 1 CFU-S per bone marrow sampling unit was estimated to be 17 x 10(6) bone marrow cells, corresponding to a total marrow uptake of approximately 5100 CFU-S (based on a seeding efficiency factor of 10%). If we assume single-hit kinetics, it follows that the recipient W/Wv bone marrow may contain approximately 5100 domains in which stem cell proliferation is geared to the density of the stem cell population. When the various inocula were corrected for multiple seeding in a given domain, the mean inflection point per domain was similar and indicative of five or so divisions before departure from exponential growth at approximately 20% of final CFU-S content 8 days after bone marrow injection. The partitioning of bone marrow into highly localized functional units is consistent with the putative regulatory role of short-range interactions between stem cells and essential stromal elements.

Different marrow cell number requirements for the haemopoietic colony formation and the curve of the W/Wv anemia

The lowest cell number in the normal marrow transplant, which allows the cure of W/Wv anemia was found to be between 10(4) and 10(5). This exceeds by several times the lowest cell number necessary for the haemopoietic colony formation. Therefore, either the colony forming cell is not the haemopoietic stem cell but rather its progeny, or this cell requires an aid from some other cells to exert is activity.