Human acute myelogenous leukemia (AML) is thought to arise from a rare population of malignant stem cells. Cells of this nature, herein referred to as leuke-mic stem cells (LSCs), have been documented for nearly all AML subtypes and appear to fulfill the criteria for stem cells in that they are self-renewing and give rise to the cells found in many leukemic populations. Because these cells are likely to be critical for the genesis and perpetu-ation of leukemic disease, the present studies sought to characterize unique molecular properties of the LSC population , with particular emphasis on the transcription factor, nuclear factor-B (NF-B). Previous experiments have shown that unstimulated human CD34 progenitor cells do not express NF-B. In contrast , primary AML CD34 cells display readily detectable NF-B activity as assessed by electrophoretic mobility shift assay and gene expression studies. Furthermore , detailed analyses of enriched AML stem cells (CD34 /CD38 /CD123) indicate that NF-B is also active in the LSC population. Given the expression of NF-B in leukemic, but not normal primitive cells, the hypothesis that inhibition of NF-B might induce leukemia-specific apoptosis was tested by treating primary cells with the proteasome inhibitor MG-132, a well-known inhibitor of NF-B. Leukemic CD34 /CD38 cells displayed a rapid induction of cell death in response to MG-132, whereas normal CD34 /CD38 cells showed little if any effect. Taken together, these data indicate that primitive AML cells aberrantly express NF-B and that the presence of this factor may provide unique opportunities to preferentially ablate LSCs. (Blood. 2001;98: 2301-2307)

Recent studies suggest that the population of malignant cells found in human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs). LSCs have been documented for nearly all AML subtypes and have been phenotypically described as CD34 + /CD38 − or CD34 + /HLA-DR − . Given the potentially critical role of these primitive cells in perpetuating leukemic disease, we sought to further investigate their molecular and cellular characteristics. Flow cytometric studies using primary AML tissue showed that the interleukin-3 receptor alpha chain (IL-3R␣ or CD123) was strongly expressed in CD34 + /CD38 − cells (98 ± 2% positive) from 16 of 18 primary specimens. Conversely, normal bone marrow derived CD34 + /CD38 − cells showed virtually no detectable expression of the CD123 antigen. To assess the functional role of IL-3R␣ positive cells, purified CD34 + /CD123 + leukemia cells were transplanted into immune deficient NOD/SCID mice. These experiments showed that CD123 + cells were competent to establish and maintain leukemic populations in vivo. To begin to elucidate a biological role for CD123 in leukemia, primary AML samples were analyzed with respect to signal transduction activity in the MAPK, Akt, and Stat5 pathways. Phosphorylation was not detected in response to IL-3 stimulation, thereby suggesting CD123 is not active in conventional IL-3-mediated signaling. Collectively, these data indicate that CD123 represents a unique marker for primitive leukemic stem cells. Given the strong expression of this receptor on LSCs, we propose that targeting of CD123 may be a promising strategy for the preferential ablation of AML cells.