Clinical studies of bone marrow transplantation (BMT) suggest that the immune system contributes to the eradication of acute myeloid leukemia (AML). A recent study also showed that the Fas (CD95/AP01) mediates apoptotic signal from cytotoxic T lymphocytes. Sixty-four patients with AML were studied for the expression of Fas in the context of CD34 and CD38 coexpression. The clinical relevance of Fas expression and function on AML was also investigated. Fas was expressed on 2% to 98% of AML cells (2% to 20% in 11 patients, 20% to 50% in 20 patients, 50% to 80% in 24 patients, and 80% to 98% in nine patients). Only 44.4% of patients with AML M1 (French-American-British [FAB] classification) were Fas(+) (greater than or equal to 20% of leukemia cells expressed Fas), whereas 89.1% of patients with AML M2, M3, M4, M5 were Fas(+) (P < .01). Among 43 CD34(+) patients (greater than or equal to 20% leukemia cells were CD34(+)), 34 were Fas(+), and 19 of 21 CD34(-) patients were Fas(+) (P = NS). Thirteen cases were studied for their expression of Fas in the context of CD34 and CD38 using three-color analysis. Fas is expressed at a high level in the gated CD34(+)CD38(+/-) and CD34(+)CD38(+) population. In 10 AML samples, Fas was expressed at a higher level in CD34(+)/CD38(+) population than in CD34(+)/CD38(+/-) or CD34(-) cell populations. Fas-induced apoptosis by anti-fas monoclonal antibody (MoAb) was determined by morphologic features and colorimetric DNA fragmentation assay. Induction of apoptosis was found in 14 of 24 cases. However, no statistically significant correlation was observed between Fas expression and induction of apoptosis. Leukemia colony-forming unit assays suggested that in some cases, Fas-induced apoptosis occurred in the clonogenic cell populations. Parameters such as laboratory and clinical data at initial diagnosis were correlated with Fas expression and only response to initial induction chemotherapy showed significant correlation with Fas expression (P < .05). We conclude that the majority of AML cells exhibit variable expression of Fas, and apoptosis could be induced by anti-fas MoAb in some cases. Our results suggest the Fas-mediated apoptosis may be clinically relevant, whereas the issue of clonogenic leukemia cells and Fas expression needs further studies. (C) 1997 by The American Society of Hematology.
