Background: Fas antigen (Ag) has recently been identified as the putative surface molecule capable of transducing apoptotic signals into cells. Alterations in the expression of proto-oncogene bcl-2 have been implicated in the regulation of apoptosis.
Materials and methods: By employing a monoclonal antibody to bcl-2 protein (124 clone) and to Fas Ag (UB2 clone) the expression of these molecules was analyzed at flow cytometry on bone marrow (BM) and peripheral blood (PB) samples from patients suffering from different lymphoid and myeloid leukemic diseases (27 acute non-lymphocytic leukemia [ANLL]; 14 acute lymphocytic leukemia [ALL]; 19 B-cell chronic lymphocytic leukemia [CLL]; 2 Ph1+ chronic myeloid leukemia [CML]; one CD8+ T-cell chronic lymphoproliferative disorders). Results were compared with those observed on normal PB leukocytes and BM B-cell precursors from patients with non-neoplastic hematological disorders.
Results: Fas Ag was constitutively expressed by both monocytes and neutrophils, while lymphocytes expressed bcl-2 with no difference between B and T cell subsets. Interestingly, bcl-2 expression was always absent on neutrophils. When dealing with ANLL patients, a relatively low bcl-2 and high Fas Ag phenotype characterized subtypes with granulocytic (M2) or promyelocytic (M3) differentiation. This observation was confirmed in a small number of patients for whom bcl-2 levels were quantified as antibody binding capacity (ABC) in molecules/cell. Leukemic cells from patients with ALL constitutively expressed bcl-2, the pattern of this expression being quantitatively lower than that of immature B-cell precursors. Finally, high bcl-2 and low Fas Ag expression represented a crucial part of the B-cell CLL immunophenotype.
Conclusions: Although based on a small number of patient and control samples, our results suggest that bcl-2 and Fas Ag are coordinately expressed on normal PB leukocytes. Fas Ag is expressed at low levels on B-CLL cells, generally considered long-surviving cells. The relatively lower bcl-2-expression detected in both M2 and M3 subtypes may explain, at least in part, the higher remission rates obtained in these forms of ANLL than in other less differentiated morphological variants.