FLOW CYTOMETRIC IDENTIFICATION OF CANDIDATE NORMAL STEM-CELL POPULATIONS IN CD45-NEGATIVE B-CELL PRECURSOR ACUTE LYMPHOBLASTIC-LEUKEMIA

CD45-negative B-cell precursor acute lymphoblastic leukaemia (ALL) pro vides a unique model to study the stem cell compartment in ALL as leuk aemic CD34-positive cells, unlike their normal counterparts, do not ex press CD45. By increasing the number of events analysed to 10(6), stor ing only the events in the region of interest (storage gate), using ap propriate isotype controls and stringent washing procedures, a now cyt ometric protocol was established to characterize rare CD34(+)CD19(-) e vents. In eight of 12 patients (67%) with CD45-negative B-cell precurs or ALL, a distinct CD34(+)CD19(-)CD45(+) candidate normal stem cell po pulation could be detected. In one patient analysed by four-colour sta ining, the CD34(+)CD19(-)CD45(+) cells, unlike the CD45-negative leuka emic cells, expressed CD117 (c-kit), providing further evidence that t hese cells represent residual nonleukaemic normal cells. By multiparam eter analysis, this population of candidate normal stem cells could be separated from contaminating leukaemic CD34(+)CD19(-)CD45(-) cells, w hich were detected in 11 of the 12 patients within the CD34(+)CD19 - c ompartment.