Colony-forming cells expressing high levels of CD34 are the main targets for granulocyte colony-stimulating factor and macrophage colony-stimulating factor in the human fetal liver

The effects of the granulocyte (G) and macrophage (M) colony-stimulating factors (CSFs) on the growth of purified subpopulations of human fetal liver progenitors were investigated. In contradiction to the characterization of these cytokines as CSFs acting late in the course of hematopoiesis, both G-CSF and M-CSF were most potent in promoting the growth of fetal liver colony-forming cells (CFCs) that express high levels of CD34 and CD38 (CD34(++)CD38(+)) and are depleted of cells expressing a panel of lineage markers (Lin(-)). Cultures of these cells in serum-deprived conditions generated a mean of 11.2 and 39.1 low-proliferative potential (LPP)-CFCs per 1.0 x 10(3) CD34(++)CD38(+)Lin(-) cells grown in G-CSF and M-CSF, respectively. Cultures of more mature progenitors, isolated based on a lower level of CD34 expression (CD34(+) Lin(-)), generated few LPP-CFCs and 6.3 and 4.7 clusters per 1.0 x 10(3) CD34(+)Lin(-) cells in response to G-CSFs and M-CSF, respectively. G-CSF was also found to synergistically enhance colony growth by either kit-ligand (KL) or flt-3/flk-2 ligand (FL) in cultures of CD34(++)CD38(+)Lin(-) cells as well as the more primitive compartment of CD34(++)CD38(-)Lin(-) cells. Synergism between G-CSF and KL or FL was also observed in liquid cultures of CD34(++)CD38(-)Lin(-) cells. The effects of G-CSF on CD34(++)CD38(-)Lin(-) cells were further demonstrated by the ability of G-CSF to support the short-term survival of these cells in clonal cultures. In contrast, M-CSF did not affect the growth or survival of CD34(++)CD38(-)Lin(-) cells, a finding that was also supported by the observation that the receptor for M-CSF (CD115 or fms) was only expressed on CD34(++)CD38(+)Lin(-) cells. G-CSF receptor expression and flt-3/flk-2 expression were detected by flow cytometry on both the CD38(-) and CD38(+) subpopulations of CD34(++)Lin(-) cells, but these receptors were not detected on CD34(+) cells. Receptors for KL (CD117) and interleukin-3 (CD123), for which the ligands are active on a broad range of fetal liver progenitors, were detected on cells expressing both high and low levels of CD34. These data help to define the potential roles of cytokines in human fetal hematopoiesis.