Blood, 15 April 2001, Vol. 97, No. 8, pp. 2278-2285
HEMATOPOIESIS
Cyclophosphamide/granulocyte colony-stimulating factor causes
selective mobilization of bone marrow hematopoietic stem cells into the
blood after M phase of the cell cycle
Douglas E. Wright,
Samuel
H. Cheshier,
Amy J. Wagers,
Troy D. Randall,
Julie L. Christensen, and
Irving L. Weissman
From the Departments of Pathology and Developmental
Biology, Stanford University School of Medicine, Stanford, California.
Cytokine-mobilized peripheral blood hematopoietic stem cells (MPB
HSC) are widely used for transplantation in the treatment of
malignancies, but the mechanism of HSC mobilization is unclear. Although many HSC in bone marrow (BM) cycle rapidly and expand their
numbers in response to cytoreductive agents, such as cyclophosphamide (CY), and cytokines, such as granulocyte colony-stimulating factor (G-CSF), MPB HSC are almost all in the G0 or G1
phase of the cell cycle. This has raised the question of whether a
subset of noncycling BM HSC is selectively released, or whether cycling
BM HSC are mobilized after M phase, but before the next S phase of the
cell cycle. To distinguish between these possibilities, mice were
treated with one dose of CY followed by daily doses of G-CSF, and
dividing cells were marked by administration of bromodeoxyuridine
(BrdU) during the interval that BM HSC are expanding. After CY and 4 days of G-CSF, 98.5% of the 2n DNA content long-term repopulating MPB
(LT)-HSC stained positively for BrdU, and therefore derived from cells
that divided during the treatment interval. Next, LT-HSC from mice
previously treated with a single dose of CY, which kills cycling cells,
and 3 daily doses of G-CSF, were nearly all killed by a second dose of
CY, suggesting that CY/G-CSF causes virtually all LT-HSC to cycle.
Analysis of cyclin D2 messenger RNA (mRNA) expression and total RNA
content of MPB HSC suggests that these cells are mostly in
G1 phase. After CY/G-CSF treatment, virtually all BM LT-HSC
enter the cell cycle; some of these HSC then migrate into the blood,
specifically after M phase, and are rapidly recruited to particular
hematopoietic organs.
