Multiparameter flow-cytometric analysis of bcl-2 and Fas expression in normal and neoplastic hematopoiesis

Apoptosis (programmed cell death) is an important regulatory mechanism in hematopoiesis, and is thought to be a principal mechanism of action of cytotoxic chemotherapy. Proteins that modulate apoptosis include bcl-2, which inhibits apoptosis, and Fas (CD95, also known as APO-1), which induces apoptosis in susceptible cells bound by Fas ligand (FasL). To characterize precisely the expression of these apoptosis-regulatory proteins in normal and neoplastic hematopoiesis, the authors have performed multiparameter flow cytometric (FCM) analysis in a series of normal and abnormal marrow specimens. Among normal hematopoietic elements, bcl-2 expression was highest in myeloblasts (29 [± 9] x 10³ molecules of equivalent soluble fluorochrome [MESF]), and lymphocytes (28 [± 7] x 10³ MESF). bcl-2 was essentially undetectable in granulocytes and nucleated red blood cells, whereas monocytes and B-cell precursors expressed intermediate levels of bcl-2 (11 [± 4] x 10³ and 7 [± 1] x 10³ MESF, respectively), Fas expression increased with granulocytic and monocytic differentiation; myeloblasts expressed 8 (± 2) x 10³ MESF, whereas granulocytes (15 [± 2] x 10³ MESF) and monocytes (28 [± 5] x 10³ MESF) displayed relatively greater intensity of staining for Fas. Among lymphoid cells, Fas expression was heterogeneous. B cells expressed lower intensity Fas staining than both CD4+ and CD8+ T cells. Myeloblasts in 30 cases of myeloid leukemia and myelodysplasia studied for bcl-2 and/or Fas expression manifested variable levels of these molecules (range 9-105 x 10³ MESF for bcl-2 and 3-33 x 10³ MESF for Fas). In addition, intraclonal heterogeneity of bcl-2 and Fas expression was seen in certain cases of AML, which correlated with extent of differentiation. Among 28 cases of B- precursor ALL studied for bcl-2 and/or Fas expression, bcl-2 ranged from 22 to 60 x 10³ MESF (P <.001 versus normal marrow B-cell precursors), and Fas varied between essentially undetectable levels and 6 x 10³ MESF. In summary, normal marrow subsets display characteristic levels of the apoptosis- regulatory molecules, bcl-2 and Fas. In hematopoietic neoplasms, expression of bcl-2 and Fas varies among cases, and in some instances, within leukemic blast populations. Further study is required to understand the potential significance of this heterogeneous expression of bcl-2 and Fas in hematologic neoplasia.