Hematopoietc stem cells: Biology and transplantation

Hematopoietic stem cells represent up to 0.05% of bone marrow cells in mouse and in man. Multipotent hematopoietic progenitors can be purified based on their expression of cell surface markers: Thy-1.1 ^l0 Lin ^l0/10 Sca-1 ⁺ c-kit ⁺ in mouse and Thy-1 ^lo Lin ^-/l0CD34 ⁺ in humans. In both cases clonogenic cells within these populations give rise to all lineages of blood cells, including lymphoid, myeloid, and erythroid cells; however, only a subset of stem cells in each species is capable of long term self-renewal and sustained production of all blood cell types. In mouse, three subpopulations of multipotent progenitors have been identified based on differences in low level lineage marker expression. Cells expressing low levels of CD4 and/or Mac-1 give transient multilineage reconstitution, while the lineage negative subset gives long term reconstitution. The fact that we can predict a progenitor's self-renewal potential based on its expression of cell-surface markers indicates that the self-renewal potential of stem cells is a determined rather than 3 stochastic trait. Only around 4% of long term self-renewing stem cells (LTHSC) from adult mouse bone marrow are in the S/G2/M phases of cell cycle at any single time. In contrast, most stem cells found in the fetal liver have long term self-renewal potential, and are in cycle. Fetal liver stem cell clones yield more progeny in irradiated adult recipients than do adull) stem cells. These traits suggest that fetal liver stem cells may be capable of more self-renewing divisions than adult stem cells. It has been proposed that the survival and self-renewal of dividing stem cells could be limited by telomere length and the presence of anti-apoptosis genes. We have shown that most self-renewing multipotent progenitors express relatively high levels of telomerase, while non-self renewing multipotent progenitors express little detectable telomerase activity. When mice are fed BrDU for extended intervals all stem cell subsets go into and out of eel) cycle regularly. In fact, -9-10% of the long term HSC subset enter cycle each day, apparently randomly. The daughter cells of such divisions almost certainly include LT and ST HSC. We attempted hydroxyurea treatment {HU) of mice to test whether cycling HSCs can be accumulated, with the intent that they will thus be more likely to be targets for retroviral-mediated gene therapy. We show that HU treatment of mice can lead to ∼20-30% of LT HSCs to be in cell cycle after 3 days treatment.