Stromal cells retard the differentiation of CD34(+)CD38(low/neg) human primitive progenitors exposed to cytokines independent of their mitotic history

Stem cell proliferation induced by potent cytokines usually leads to a loss of primitive potential through differentiation. In this study, the ability of cytokines and murine MS5 stromal cells to independently regulate the pr oliferation and longterm culture-initiating cell (LTC-IC) activity of primi tive CD34(+)CD38(low/neg) human bone marrow cells was evaluated. To compare populations with identical proliferation histories, cells were labeled wit h carboxy fluorescein diacetate succinimidyl ester, and LTC-IC activity was assessed 4 days later in cells that had accomplished the same number of di visions with or without MS5 cells. MS5 cells counteracted dramatically the loss of LTC-IC activity observed in the presence of cytokines alone. Thus, in the presence of MS5 cells, means of 1233 (n = 5) and 355 (n = 9) LTC-IC- derived colony-forming cells (CFCs) were generated by 1000 cells that perfo rmed 3 and 4 divisions respectively, whereas 311 (n = 5) and 64 (n = 5) CFC s were generated by 1000 cells cultured without MS5 cells. Interestingly, M S5 cells had no detectable effect on the LTC-IC activity of cells that divi ded only twice in 4 days-1606 CFCs (n = 6) and 1993 (n = 6) CFCs, respectiv ely, without and with MS5 cells-and a 48 additional hours of coculture were necessary to unmask changes in the LTC-IC activity mediated by stromal cel ls, These results indicate that cytokines and stroma-derived signals can re gulate independently the proliferation and differentiation of primitive cel ls and that these stroma-derived extracellular factors act directly on thei r target cells, (C) 2001 by The American Society of Hematology.