Phenotyping of acute myelocytic leukemia (AML) progenitors: an approach for tracing minimal numbers of AML cells among normal bone marrow.

Previous studies have shown that the phenotypes of progenitors of human AML (AML-CFU) are variable, reflecting arrests at different stages of maturation. We were interested to seek discrepancies between the surface properties of AML precursors and normal bone marrow colony formers in order to detect minimal numbers of AML cells among normal bone marrow cells in remission bone marrow. Therefore, we selected two surface markers, the MoAb CD34, reactive with blast cells, and Vim-2, a surface marker reactive with mature myeloid cells, and determined the antigen density of these markers (relative fluorescence intensity using fluorescence-activated cell sorting) for normal marrow and AML progenitors. While these markers defined an identical phenotype (CD34++/Vim-2-/+) for a broad spectrum of normal progenitors, i.e., CFU-GEMM, BFU-e, day 15 CFU-GM, and day 7 CFU-GM, referred to as the "normal" progenitor phenotype, AML progenitors frequently exhibited different phenotypes. In 12 of 20 cases the phenotypes of the majority of AML progenitors were discrepant from the normal surface profile, i.e., according to one marker in 8 cases (CD34-/+/Vim-2-/+ or CD34++/Vim-2++) and two markers in 4 cases (CD34-/+/Vim-2++). Since these data indicate that AML and normal progenitors were frequently distinguishable, we then determined the potential utility of these phenotypic dissimilarities for detection of minimal disease. Artificial mixtures of normal bone marrow and minimal numbers (0.1-1%) of AML cells were prepared. Based upon the phenotypic discrepancies, AML metaphases were successfully demonstrated in these mixtures following cell sorting and culture. Thus, it appears that minimal numbers of AML mitoses can be identified with an approximate 10(-2) to 10(-3) sensitivity by taking advantage of differential coexpression of surface antigens.