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J. Exp. Med.,
Volume 188, Number 2, July 20, 1998 393-398
By


§
From the * Cancer Center, the The most primitive engrafting hematopoietic stem cell has been assumed to have a fixed phenotype, with changes in engraftment and renewal potential occurring in a stepwise irreversible
fashion linked with differentiation. Recent work shows that in vitro cytokine stimulation of
murine marrow cells induces cell cycle transit of primitive stem cells, taking 40 h for progression from G0 to mitosis and 12 h for subsequent doublings. At 48 h of culture, progenitors are
expanded, but stem cell engraftment is markedly diminished. We have investigated whether
this effect on engraftment was an irreversible step or a reversible plastic feature correlated with
cell cycle progression. Long-term engraftment (2 and 6 mo) of male BALB/c marrow cells exposed in vitro to interleukin (IL)-3, IL-6, IL-11, and steel factor was assessed at 2-4-h intervals of culture over 24-48 h using irradiated female hosts; the engraftment phenotype showed
marked fluctuations over 2-4-h intervals, with engraftment nadirs occurring in late S and early
G2. These data show that early stem cell regulation is cell cycle based, and have critical implications for strategies for stem cell expansion and engraftment or gene therapy, since position in
cell cycle will determine whether effective engraftment occurs in either setting.
Department of Medicine, and the § Department of Cell Biology,
University of Massachusetts Medical Center, Worcester, Massachusetts 01605
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