IN-VITRO EXPANSION OF HEMATOPOIETIC PROGENITOR CELLS INDUCES FUNCTIONAL EXPRESSION OF FAS ANTIGEN (CD95)

Fas antigen (Fas Ag; CD95) is a cell surface molecule that can mediate apoptosis. Bcl-2 is a cytoplasmic molecule that prolongs cellular sur vival by inhibiting apoptosis. To investigate the role of both molecul es in hematopoiesis, we evaluated the expression of Fas Ag and Bcl-2 o n CD34(+) hematopoietic progenitor cells expanded in vitro. CD34(+) ce lls isolated from bone marrow were cultured in Iscove's modified Dulbe cco's medium supplemented with 10% fetal calf serum, 1% bovine serum a lbumin, 50 ng/mL stem cell factor, 50 ng/mL interleukin-3 (IL-3), 50 n g/mL IL-6, 100 ng/mL granulocyte colony-stimulating factor, and 3 U/mL erythropoietin for 7 days. Colony-forming unit of granulocytes/macrop hages (CFU-GM) and burst-forming unit of erythroids (BFU-E) were expan ded 6.9-fold and 8.8-fold in number at day 5 of culture, respectively. Freshly isolated CD34(+) cells did not express Fas Ag, whereas approx imately half of them expressed Bcl-2. CD34(+) cells cultured with hema topoietic growth factors gradually became positive for Fas Ag and rapi dly lost Bcl-2 expression. Furthermore, apoptosis was induced in the c ultured CD34(+) population when anti-Fas antibody (IgM; 1 mu g/mL) was added, as shown by significant decrease in the number of viable cells , morphologic changes, induction of DNA fragmentation, and significant decrease in the number of clonogenic progenitor cells including CFU-G M and BFU-E. These results indicate that functional expression of Fas Ag is induced on CD34(+) cells expanded in vitro in the presence of he matopoietic growth factors. Induction of Fas Ag and downregulation of Bcl-2 may be expressed as part of the differentiation program of hemat opoietic cells and may be involved in the regulation of hematopoiesis. (C) 1996 by The American Society of Hematology.