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 po tential occurring in a stepwise irreversible fashion linked with diffe rentiation. Recent work shows that in vitro cytokine stimulation of mu rine marrow cells induces cell cycle transit of primitive stem cells, taking 40 h for progression from Go to mitosis and 12 h for subsequent doublings. At 48 h of culture, progenitors are expanded, but stem cel l engraftment is markedly diminished. We have investigated whether thi s effect on engraftment was an irreversible step or a reversible plast ic feature correlated with cell cycle progression. Long-term engraftme nt (2 and 6 mo) of male BALB/c marrow cells exposed in vitro to interl eukin (IL)-3, IL-6, IL-11, and steel factor was assessed at 2-4-h inte rvals of culture over 24-48 h using irradiated female hosts; the engra ftment phenotype showed marked fluctuations over 2-4-h intervals, with engraftment nadirs occurring in late S and early G(2). These data sho w that early stem cell regulation is cell cycle based, and have critic al 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.