Comparative quantitative expression of bcl-2 by normal and leukaemic myeloid cells

Summary. Expression of the bcl-2 oncoprotein by AML blasts has previously been demonstrated to be heterogenous with high levels of bcl-2 expression being associated with a low complete remission rate and poor survival. We have quantified bcl-2 expression in AML blasts in relation to expression of the CD34 antigen and in comparison to CD34-positive cells from normal bone marrow. When expressed as molecules of equivalent soluble fluorochrome (MESF) per cell, AML blast cell bcl-2 expression varied from 11.1 to 99.9 × lO³ (median 39.4 × 10³, n=56) with 28.5% of patients expressing high MESF values (>50 × 10³) and 16% of patients expressing low MESF values (<20 × 10³), the remainder expressing intermediate values. There was no significant difference between intensity of bcl-2 expression and FAB classification in the de novo AML cases, and there was no significant differences between de novo and secondary AML cases. Blasts from CD34⁺ AML patients expressed significantly higher levels of bcl-2 (mean MESF 43.6 × 10³, n =36) than CD34^? AML patients (mean MESF 31.7 × 10³, n=19). In five cases of CD34⁺ AML, bcl-2 expression was determined on purified CD34⁺ and CD34^? blast cell populations. In all cases CD34⁺ blasts were found to express significantly higher bcl-2 MESF values compared to the CD34^? fraction. Purified CD34⁺ cells from normal bone marrow consistently expressed high levels of bcl-2 (MESF >75 × 10³ n = 4), which was comparable to that found on CD34⁺ AML cells. Our results suggest that the poor prognosis previously associated with AML blasts expressing the CD34 antigen may in part be related to high expression of bcl-2. Also the ability to measure bcl-2 in AML blasts quantitatively by flow cytometry and to categorize patients into discrete groups may be of value as a prognostic indicator in AML.