Chronic myeloid leukemia (CML) is effectively treated with imatinib mesylate (IM), a small molecule inhibitor of the BCR-ABL tyrosine kinase that is expressed in the entire hematopoietic compartment including stem cells (HSC) and progenitors in CML patients. While IM induces disease remission, it does not appear to eradicate BCR-ABL-positive stem cells. We analyzed the HSC/Progenitors profiles using fluorescence-activated cell sorting (FACS) and investigated the minimal residual disease (MRD) in HSC and myeloid progenitors from patients with CML chronic phase (CP) after IM-therapy. HSC identified as CD34+CD38–Lin–, were separated to Thy-1+ (HSC/Thy-1+) and Thy-1– (HSC/Thy-1–) cells. HSC/Thy-1+ (CD34+CD38–Thy-1+Lin–), HSC/Thy-1– (CD34+CD38–Thy-1–Lin–), common myeloid progenitors (CMP: CD34+CD38+IL-3Rα+CD45RA–Lin–), granulocyte–macrophage progenitors (GMP: CD34+CD38+IL-3Rα+CD45RA+Lin–), and megakaryocyte–erythroid progenitors (MEP: CD34+CD38+IL-3Rα–CD45RA– Lin–) were isolated by cell sorting and MRD was quantified with real-time polymerase chain reaction detecting BCR-ABL transcripts. FACS analysis revealed higher levels of the HSC/Thy-1– cells and progenitors (CD34+CD38+Lin– cells) in bone marrow from patients with CML CP than in normal bone marrow although the level of long-term HSC in CML CP was similar with normal bone marrow. After the IM-therapy the proportion of progenitor pools (CD34+Lin– cells) within Lin– were remarkably reduced, especially that of HSC/Thy-1– cells and progenitor cells. The proportion of MEP was increased and that of GMP was decreased in bone marrow from patients with CML CP as compared with their normal counterparts. BCR-ABL positive progenitors in bone marrow were eradicated within 12 month in 5 out of 9 patients. BCR-ABL positive cells, however, remained in the stem cell population. They were positive even after achieving undetectable levels of BCR-ABL transcript in total RNA isolated from the bone marrow. The ratio of BCR-ABL to BCR was significantly decreased with the continuation of imatinib, however the retention of BCR-ABL-positive cells was observed in HSC/Thy-1– or HSC/Thy-1+ populations except one out of 9 cases. These results indicated retention but significant reduction of BCR-ABL positive stem cells in CML during IM-therapy. They also implicate that the sorted and purified stem cells are useful for more sensitive quantification of BCR-ABLpositive MRD.

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