Flow cytometric characterization of differentiation events and primitive stent cell populations in childhood B-cell precursor ALL

Bone marrow and peripheral blood from children with acute lymphoblastic leukemia was analyzed bq flow cytometry to assess leukemic cell differentiation and to characterize the profile of cell surface marker expression on rare CD34(+) cell populations. The goal of this study was to determine if patterns of cell surface antigens could be identified on CD34(+) subpopulations which may allow distinction bt tween normal and 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 increasing 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 patients which did not express CD19 and comprised less than 0.1% of all bone marrow or peripheral blood cells. Within this population, there was differentiation from primitive CD34(+) CD38(+) to more mature CD34(+) CD38(+) cells. In 4 of these patients, an additional CD34(+) population with low expression of CD19 (CD34(+) CD19(lo)) was detected. The lack of CD45 expression on the leukemic cells of 2 patients was used as a marker for the leukemic cell clone. In both patients, the CD34(+) CD19(-) cells did express CD15 while CD34(+) CCD19(lo/+)cells were CD45 negative. This suggests that the CD34(+)CD19(lo) cells were part of the leukemic clone and that the CD34(+)CD38(-)CD19(-) cells may represent residual normal primitive hematopoietic cells. In conclusion. flow cytometry allowed identification of primitive CD34(+) cell populations in children with ALL, which can now be functionally characterized by transplantation onto immune-deficient mice.