The fluctuating phenotype of the lymphohematopoietic stem cell with cell cycle transit.

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.

Effects of cytokines on stem cell engraftment depends on time of evaluation post-marrow-infusion.

In vitro treatment of bone marrow cells to expand stem cells may lead to impaired hematopoietic long-term reconstitution. Here we report on studies that show that cytokine-treated cells maintain short-term reconstitution, but lose that potential with time posttransplantation. Hematopoietic progenitors assayed in vitro as high- and low-proliferative potential colony-forming cells, when exposed to four cytokines (interleukin (IL)-3, IL-6, IL-11, and stem cell factor) were significantly expanded and induced to enter the cell cycle. A competitive transplant model--which uses BALB/c mice of opposite genders: cytokine-treated male BALB/c marrow cells competed with fresh, noncultured female cells--gave mean engraftment levels of 47 +/- 3% at 1 week posttransplantation, 49 +/- 6% at 3 weeks, 30 +/- 10% at 6 weeks, 26 +/- 9% at 12 weeks, and 15 +/- 3% at 24 weeks. These data were confirmed using a congenic Ly 5.1/5.2 transplant model and suggest either that cytokines act differentially on separate sets of short-term and long-term repopulating cells or act on one population of stem cells to limit long-term repopulation.