Expression of Fas/CD95 and Bcl-2 by primitive hematopoietic progenitors freshly isolated from human fetal liver

The cell-surface expression and the functional status of the CD95/Fas antigen on primitive hematopoietic progenitors PHPs) freshly isolated from human fetal liver (FL) were studied. PHPs were phenotypically defined as CD34(++)CD38(-/+) cells. The most immature subfractions of PHPs, CD34(++)CD38(-) and CD34(++)CD38(+) FL cells, expressed CD95, whereas the more mature CD34(++)CD38(++) and CD34(+)CD38(++) FL cells displayed low CD95 expression. Combinations of cytokines, such as kit ligand (KL) + interleukin-3 or KL + granulocyte-macrophage colony-stimulating factor (GM-CSF) upregulated the expression of CD95 on PHPs upon in vitro culture. Tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) further increased the CD95 expression induced by KL + GM-CSF. The hematopoietic potential of sorted CD34(++)lineage (lin)(-) CD95(+) versus CD34(++)lin(-)CD95(-) FL cells was compared by colony-forming unit-culture (CFU-C) assays performed in serum-deprived medium. Lin(+) cells were composed of erythrocytes, monocytes, T cells, B cells, and natural killer cells. Our results indicated that both CD95(-) and CD95(+) subsets contained pluripotent progenitors, generating myeloid and erythroid progenitors. The functional status of CD95 and the effects of TNF-alpha and IFN-gamma, cytokines known to induce CD95-mediated apoptosis, were analyzed by incubation of PHPs in the presence of anti-CD95 monoclonal antibodies (MoAbs). The effect of anti-CD95 MoAbs was measured by viable cell counting, flow cytometry, and CFU-C assays. A decrease of CFU-C numbers was observed in the presence of anti-CD95 MoAbs and TNF-alpha and/or IFN-gamma. However, whereas growth factor deprivation induced apoptosis of PHPs, cross-linking of CD95 did not lead to apoptosis of PHPs measured by flow cytometry and viable cell counting. The correlation of increased intracytoplasmic levels of bcl-2 with high levels of cell-surface CD34 and the presence of CD95 on fresh FL cells suggests that bcl-2 may be involved in protecting against CD95-mediated apoptosis of FL PHPs. (C) 1996 by The American Society of Hematology.