Flow Cytometric Analysis of Immature Cell Populations in B-Cell Precursor ALL

A flow cytometric protocol was developed that allowed identification and analysis of rare CD34+CD19- cells in the bone marrow of patients with B-cell precursor acute lymphoblastic leukemia (ALL). Within this CD34+CD19- cell population an expression of CD38 was observed that was compatible with differentiation from primitive CD34+CD38- to more mature CD34+ CD38+ cells. Lack or very low levels of CD45 expression on the leukemic cells of 8 patients was used as a marker for the leukemic cell clone. In 3 of these 8 patients, all CD34+CD19- cells did express CD45; in the other 5 patients, some or most of the CD34+CD19- population were CD45-. In normal bone marrow, CD34+CD19- cells were always CD45+ (n= 6). This suggests heterogeneity of stem cell involvement in childhood B-cell precursor ALL. In some patients the leukemic transformation may arise from a lymphoid-committed cell already expressing CD19, while the CD34+CD19- cells may represent residual normal cells. To identify early differentiation events within the leukemic cell clone CD24 and CD31 expression was analyzed. Our preliminary data are compatible with the most immature leukemic blasts being CD34+ CD31+ CD24-. These data need now to be confirmed by molecular methods and transplantion of these populations onto immune-deficient mice.