FLOW CYTOMETRIC CHARACTERIZATION OF DIFFE RENTIATION EVENTS AND PRIMITIVE STENT CELL-POPULATIONS IN CHILDHOOD B-CELL PRECURSOR ALL

Bone marrow and peripheral blood from children with acute lymphoblasti c leukemia was analyzed bq flow cytometry to assess leukemic cell diff erentiation and to characterize the profile of cell surface marker exp ression on rare CD34(+) cell populations. The goal of this study was t o determine if patterns of cell surface antigens could be identified o n CD34(+) subpopulations which may allow distinction bt tween normal a nd leukemic stem cells. Expression of the progenitor cell antigen CD34 on leukemic blasts was very heterogeneous and variied between 0.5 and 100% in 20 patients analyzed in this study. In cALL and pre-B-ALL, a variable percentage of the leukemic cells coexpressed CD20 in addition to CD10. Only in one case, differentiation characteristic for normal B cell development with coordinated downregulation of CD10 with increa sing expression of CD20 was observed. By analysing 5 x 10(5)-1 x 10(6) cells, a CD34(+) cell population could de identified in 8 out of 8 pa tients which did not express CD19 and comprised less than 0.1% of all bone marrow or peripheral blood cells. Within this population, there w as differentiation from primitive CD34(+) CD38(+) to more mature CD34( +) CD38(+) cells. In 4 of these patients, an additional CD34(+) popula tion with low expression of CD19 (CD34(+) CD19(lo)) was detected. The lack of CD45 expression on the leukemic cells of 2 patients was used a s a marker for the leukemic cell clone. In both patients, the CD34(+) CD19(-) cells did express CD15 while CD34(+) CCD19(lo/+)cells were CD4 5 negative. This suggests that the CD34(+)CD19(lo) cells were part of the leukemic clone and that the CD34(+)CD38(-)CD19(-) cells may repres ent residual normal primitive hematopoietic cells. In conclusion. flow cytometry allowed identification of primitive CD34(+) cell population s in children with ALL, which can now be functionally characterized by transplantation onto immune-deficient mice.