Qualitative and quantitative characterization of Fas (CD95) expression andits role in primary human acute leukemia cells

Fas antigen, a cell surface molecule, directly mediates apoptosis, and is e xpressed on a limited number of human tissues. Blood or bone marrow samples from patients with acute myelogenous leukemia (AML), acute lymphoblastic l eukemia (ALL) and mixed leukemia were examined qualitatively and quantitati vely for the expression of Fas as well as its function using flow cytometry and the annexin V staining method. Fas expression was flow cytometrically unimodal with heterogeneous density, and showed quantitatively characterist ic features in different diseases: undetectable in mixed leukemia, faint to weak in ALL, low in MO and M1, and variable (low to strong) in M2, M3, M4, and M5. Both the full-length and the alternatively spliced truncated mRNAs were detected constitutively even in acute leukemia cells with qualitative ly negative and quantitatively faint Fas, and the band density of the forme r transcripts detected by RT-PCR was correlated with the level of expressio n of the Fas protein. Short-term culturing of freshly isolated leukemia cel ls gave rise to an increase of Fas density. In acute leukemia cells, the ap optosis induced by anti-Fas MoAb was compared with that induced by etoposid e (a topoisomerase II inhibitor). We found that fresh ALL and AML cells wer e resistant to the anti-Fas IgM antibody, while etoposide could trigger apo ptosis in all types of leukemia tested. The combined effects of the anti-Fa s MoAb and etoposide were not always synergistic. These results suggest tha t Fas is a biological marker for characterizing ALL and AML cells, and prov ide insight into creating a new therapeutic modality using cytotoxic drugs and cytokines together with modulation of Fas. (C) 2000 Elsevier Science Lt d. All rights reserved.