Approximately 2-5% of children with newly diagnosed acute lymphoblastic leukemia (ALL) have a Philadelphia (Ph) chromosome detectable on cytogenetic analysis, which is associated with a poor prognosis. In rare ALL cases the Ph chromosome may appear in leukemic cells during the course of the disease. We report here the case of a 5.5-year-old male patient with T-ALL who was found to have the b2a2 BCR-ABL mRNA transcript by reverse transcriptase-polymerase chain reaction (RT-PCR) at first marrow relapse. At the time of initial diagnosis, no BCR-ABL transcripts had been detected by PCR in the patient's blood and marrow samples. Further studies were performed using a competitive PCR titration assay and the fluorescence in situ hybridization (FISH) method to monitor the leukemic clone. Progression of the disease was associated with a higher BCR-ABL transcript level and an increasing proportion of BCR-ABL-positive cells. Metaphase FISH analysis identified the presence of the BCR-ABL fusion gene on a normal chromosome 22. This study shows that a late-appearing Ph translocation in ALL may be cytogenetically invisible. Quantitative RT-PCR and FISH techniques are appropriate and efficient methods for detecting these rare ALL variants expressing the BCR-ABL fusion gene and for estimating the level of residual disease following treatment.

Immunoselected CD34+ peripheral blood progenitor cell (PBPC) transplantation is now frequently used to support autologous hematopoiesis after myeloablative therapy, its feasability having been proved by several groups. However, we and others observed delayed platelet recovery. We hypothesized that immunoselection processing might induce selective loss of megakaryocyte progenitors, or a decrease in their proliferation. We used a colony-forming units megakaryocyte (CFU-Mk) assay to evaluate these consequences and predict platelet recovery in patients. In CD34+ PBPCs from 10 children with solid tumors, we observed no selective loss in CFU-Mk numbers during immunoselection processing and no impairment of clonogenicity. The CFU-Mk yield (59.2 +/- 11.3%) was at least similar to the CD34+ yield (44.2 +/- 3.8%). We assessed the predictive value of CFU-Mk numbers infused for recovery of platelet lineage. We found an inverse correlation between the time taken to reach a platelet count greater than 50 x 10(9)/L and only the CFU-Mk dose (r = -0.71; p = 0.022) among the different type of progenitors, including colony-forming units granulocyte-macrophage (CFU-GM), burst-forming units erythrocyte (BFU-E) and colony-forming units-mixed (CFU-Mix). These findings suggest that CFU-Mk number could be used as sole predictive functional parameter for platelet reconstitution in children after immunoselection of CD34+ cells, in particular for low CD34+ cell dose, and thus as an indicator for initial quality of hematopoietic cells before in vitro expansion.

Autologous peripheral blood progenitor cell (PBPC) transplantation is now commonly used in children. The ontogenic differences in haematopoiesis published in recent years suggest differences in the categories of mobilized PBPCs between children and adults. We investigated the frequency and distribution of mature progenitor cells (colony‐forming cells, CFCs) and primitive progenitor cells [CD34+ CD38− and CD34+ Thy‐1+ cells, long‐term culture‐initiating cells (LTC‐ICs)] in children and adults mobilized using granulocyte colony‐stimulating factor alone. We found similar proportions of granulocyte colony‐forming units (CFU‐G) and/or macrophage CFUs (CFU‐M), mixed lineage CFUs (CFU‐Mix) and megakarocyte CFUs (CFU‐Mk), CD34+ CD38− and CD34+ Thy‐1+ cells, and LTC‐ICs (16·5 ± 3·5 vs. 10·65 ± 5 per 104 CD34+ cells), which produced the same number of CFCs (5 ± 1 vs. 6 ± 1 CFCs/LTC‐ICs) in PB CD34+ cells from children and adults. However, we noted a higher proportion of erythroid blast‐forming units (BFU‐E) in PB CD34+ cells from adults (× 1·5, P = 0·003). Using cord blood as a third ageing point, we observed an inverse age‐related propensity for commitment to the monocyte/macrophage lineage that was still found after normalizing the data per body weight and processed blood mass. This ontogeny‐related programming was detected from the LTC‐IC level, which produced 1·7 times more CFU‐M in children than in adults (P = 0·048). These subtle differences in commitment between children and adults, shown here for the first time, are of interest for the in vitro manipulation of PBPCs and, in particular, for application in adoptive immunotherapy in children.

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