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-stimulati ng factors (CSFs) on the growth of purified subpopulations of human fe tal liver progenitors were investigated. In contradiction to the chara cterization of these cytokines as CSFs acting late in the course of he matopoiesis, both G-CSF and M-CSF were most potent in promoting the gr owth of fetal liver colony-forming cells (CFCs) that express high leve ls of CD34 and CD38 (CD34(++)CD38(+)) and are depleted of cells expres sing a panel of lineage markers (Lin(-)). Cultures of these cells in s erum-deprived conditions generated a mean of 11.2 and 39.1 low-prolife rative potential (LPP)-CFCs per 1.0 x 10(3) CD34(++)CD38(+)Lin(-) cell s grown in G-CSF and M-CSF, respectively. Cultures of more mature prog enitors, isolated based on a lower level of CD34 expression (CD34(+) L in(-)), 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(++)CD3 8(+)Lin(-) cells as well as the more primitive compartment of CD34(++) CD38(-)Lin(-) cells. Synergism between G-CSF and KL or FL was also obs erved in liquid cultures of CD34(++)CD38(-)Lin(-) cells. The effects o f G-CSF on CD34(++)CD38(-)Lin(-) cells were further demonstrated by th e ability of G-CSF to support the short-term survival of these cells i n clonal cultures. In contrast, M-CSF did not affect the growth or sur vival of CD34(++)CD38(-)Lin(-) cells, a finding that was also supporte d by the observation that the receptor for M-CSF (CD115 or fms) was on ly expressed on CD34(++)CD38(+)Lin(-) cells. G-CSF receptor expression and flt-3/flk-2 expression were detected by flow cytometry on both th e CD38(-) and CD38(+) subpopulations of CD34(++)Lin(-) cells, but thes e receptors were not detected on CD34(+) cells. Receptors for KL (CD11 7) and interleukin-3 (CD123), for which the ligands are active on a br oad range of fetal liver progenitors, were detected on cells expressin g both high and low levels of CD34. These data help to define the pote ntial roles of cytokines in human fetal hematopoiesis.