Human CD34⁺ hematopoietic stem/progenitor cells express high levels of FLIP and are resistant to fas-mediated apoptosis

We sought to determine whether lympho-hematopoietic stem-progenitor cells (HSC) from human placental/umbilical cord blood (CB) or adult mobilized blood (PBSC) are sensitive to Fas-induced apoptosis. Human CD34⁺ cells from CB or PBSC were cultured in serum-free medium, with or without hematopoietic growth factors (FKT: FLT-3 ligand [FL], KIT ligand [KL], and thrombopoietin [TPO]), and with or without soluble Fas ligand (sFasL) or agonistic anti-Fas antibody. After 5-48 hours of culture, cells were assessed for viability and stained with Annexin V and 7-Aminoactinomycin D for apoptosis analysis by fluorescence-activated cell sorting. Cultured cells were also assessed by in vitro hematopoietic colony-forming cell (CFC) and in vivo nonobese diabetic/severe combined immunodeficient mouse engraftment potential (SEP) assays. Levels of Fas, FLICE inhibitory protein (FLIP), and Caspase 8 mRNA in CD34⁺ cells were determined by real-time quantitative polymerase chain reaction. Expression of FLIP was confirmed by Western blotting. No decrease in viability, CFC, or SEP was observed in CB or PBSC CD34⁺ cells cultured in the presence of sFasL or agonistic anti-Fas antibody. Human CB and mobilized PBSC CD34⁺ cells expressed high levels of FLIP, low ratios of Caspase 8:FLIP, and low levels of Fas. Thus, human CB and PBSC CD34⁺ HSC were resistant to Fas pathway agonists. High-level expression of FLIP likely provides one level of protection of CD34⁺ cells from Fas-mediated apoptosis.