Cell cycle distribution of primitive haematopoietic cells stimulated in vitro and in vivo

A novel approach is used to study the proliferating behaviour of primitive haematopoietic cell populations in response to different stimuli. A mathema tical model based on the average proportion of apoptotic, dividing and quie scent cells in primitive haematopoietic cell populations is developed to de scribe the mitotic history of 5- (and 6-) carboxyfluorescein diacetate succ inimidyl ester-labelled cells. The cell cycle distributions in different cy tokine-supplemented cultures of primitive human and mouse bone marrow cells are determined and compared with those found in vivo. The results indicate that a combination of flt-3 ligand, Steel factor and interleukin-11 or hyp er-interleukin-6 provide a level of mitogenic stimulation similar to that e xisting in vivo after a myeloablative radiation dose. The comparison of the cell cycle distribution obtained for different cultures of human bone marr ow CD34(+)(45RA/71)(-) cells demonstrates that the addition of flt-3 ligand in these cultures decreases apoptosis significantly but does not reduce qu iescence. In addition, in vivo and in vitro, it was found that more than 3 days of stimulation are required to recruit a maximum number of quiescent c ells into active cell cycle. These kinetics of cell cycle activation are fo und to be similar to those identified for the haematopoietic stem cells com partment in the same cultures. This mathematical analysis provides a useful tool for the development of haematopoietic stem cell culture processes for clinical applications.