LINEAGE COMMITMENT IN HUMAN HEMATOPOIESIS INVOLVES ASYMMETRIC CELL-DIVISION OF MULTIPOTENT PROGENITORS AND DOES NOT APPEAR TO BE INFLUENCEDBY CYTOKINES

Different models have been proposed to explain lineage commitment in h emopoiesis. Some suggest that lineage commitment occurs in a stochasti c manner without the direct influence of extracellular factors; others postulate that cytokines determine whether multipotent cells will bec ome erythroid or granulocyte/macrophage progenitors. In the present st udy, the patterns of proliferation and differentiation of individually sorted human cord blood-derived primitive hemopoietic cells (highly e nriched for multipotent progenitors) were analyzed in a serum-free cul ture system supplemented with different cytokine combinations. In a fi rst set of experiments, the response of individual cells to different cytokine combinations was compared, whereas in a second set of experim ents, single cells were allowed to undergo one division after which th e two daughter cells were physically separated and cultured in either the same or different cytokine combinations. Proliferation of progenit or cells was absolutely dependent on cytokines, and the combination of mast cell growth factor plus interleukin 6 was sufficient to induce m itosis. When cytokine combinations favoring erythropoiesis and/or myel opoiesis were added to the cultures, a more vigorous proliferative res ponse of the sorted primitive progenitors was observed. Interestingly, the relative proportions of granulocyte/macrophage, erythroid, and mu ltipotent progenitors remained more or less the same regardless of the cytokine combination used, indicating a permissive rather than an ins tructive role for cytokines in hemopoietic differentiation. Asymmetric cell divisions, defined as a division that yields two daughter cells with distinct functional properties, were observed in 3-17% of the pro genitor cells capable of forming colonies under our experimental condi tions. In the rest, symmetric divisions involving multipotent and line age-committed progenitors were observed. The results of this study dem onstrate that the asymmetric cell divisions that occur in the early st ages of hemopoiesis at the level of multipotent progenitors cannot be skewed by the addition of specific cytokine combinations. These findin gs support the hypothesis that lineage commitment in hemopoiesis occur s in a stochastic manner by mechanisms that remain to be elucidated. ( C) 1993 Wiley-Liss, Inc.