High-resolution tracking of cell division suggests similar cell cycle kinetics of hematopoietic stem cells stimulated in vitro and in vivo

The kinetics of proliferation of primitive murine bone marrow (BM) cells st imulated either in vitro with growth factors (fetal liver tyrosine kinase l igand 3 [FL], Steel factor [SF], and interleukin-11 [IL-11], or hyper-IL-6) or in vivo by factors active in myeloablated recipients were examined. Cel ls were first labeled with 5- and B-carboxyfluorescein diacetate succinimid yl ester (CFSE) and then incubated overnight prior to isolating CFSE+ cells , After 2 more days in culture, more than 90% of the in vivo lymphomyeloid repopulating activity was associated with the most fluorescent CFSE+ cells (ie, cells that had not yet divided), although this accounted for only 25% of the repopulating stem cells measured in the CFSE+ "start" population. Af ter a total of 4 days in culture (1 day rater), 15-fold more stem cells wer e detected (ie, 4-fold more than the day 1 input number), and these had bec ome (and thereafter remained) exclusively associated with cells that had di vided at least once in vitro. Flow cytometric analysis of CFSE+ cells recov ered from the BM of transplanted mice indicated that these cells proliferat ed slightly faster (up to 5 divisions completed within 2 days and up to 8 d ivisions completed within 3 days in vivo versus 5 and 7 divisions, respecti vely, in vitro). FL, SF, and ligands which activate gp130 are thus efficien t stimulators of transplantable stem cell self-renewal divisions in vitro. The accompanying failure of these cells to accumulate rapidly indicates imp ortant changes in their engraftment potential independent of accompanying c hanges in their differentiation status. (C) 2000 by The American Society of Hematology.