Proliferative status of primitive hematopoietic progenitors from patients with acute myelogenous leukemia (AML)

One possible explanation for the competitive advantage that malignant cells in patients with acute myelogenous leukemia (AML) appear to have over norm al hematopoietic elements is that leukemic progenitors proliferate more rap idly than their normal progenitor cell counterparts. To test this hypothesi s, an overnight H-3-thymidine (H-3-Tdr) suicide assay was used to analyze t he proliferative status of malignant progenitors detected in both colony-fo rming cell (CFC) and long-term culture initiating cell (LTC-IC) assays from the peripheral blood of nine patients with newly diagnosed AML. Culture of AML cells in serum-free medium with 100 ng/ml Steel factor (SF), 20 ng/ml interleukin 3 (IL-3) and 20 ng/ml granulocyte colony-stimulating factor (G- CSF) for 16-24 h maintained the number of AML-CFC and LTC-IC at near input values (mean % input +/- s.d. for CFC and LTC-IC were 78+/-33 and 126+53, r espectively). The addition of 20 mu Ci/ml high specific activity 3H-Tdr to these cultures reduced the numbers of both progenitor cell types from most of the patient samples substantially: mean % kill+/-s.d. for AML-CFC and LT C-IC were 64 +/- 27 and 82 +/- 16, respectively, indicating that a large pr oportion of both progenitor populations were actively cycling. FISH analysi s of colonies from CFC and LTC-IC assays confirmed that most cytogeneticall y abnormal CFC and LTC-IC were actively cycling (mean % kill +/- s.d.: 68 /- 26 and 85 +/- 13, respectively). Interestingly, in six patient samples w here a significant number of cytogenetically normal LTC-ICs were detected, the % kill of these cells (74 +/- 20) was similar to that of the abnormal p rogenitors. These data contrast with the predominantly quiescent cell cycle status of CFC and LTC-IC previously observed in steady-state peripheral bl ood from normal individuals but also provide evidence that a significant pr oportion of primitive malignant progenitors from AML patients are quiescent and therefore may be resistant to standard chemotherapeutic regimens.