Identification of a Small Subpopulation of Candidate Leukemia Initiating Cells within the Side Population (SP) of Patients with Acute Myeloid Leukemia.

Acute myeloid leukemia (AML) likely originates from the CD34+CD38- hematopoietic stem cell (HSC). The so-called side population (SP), defined by Hoechst 33342 dye, might offer an alternative/supplementary stem cell compartment. The relationship between both compartments is largely unknown. We found that the CD34+CD38- compartment can be subdivided in an AML and normal compartment, based on expression of AML stem cell specific antigen CLL-1: CLL-1 positive CD34+CD38- cells carry AML specific cytogenetic aberrations and initiate leukemia in NOD/SCID mice (van Rhenen, Blood 2007, in press). Lineage aberrancies including CD7, CD19, CD56 and aberrant myeloid aberrancies, enabled to further define AML and normal CD34+CD38- sub-compartments (

1. 3 with lymphoid characteristics, ie CD7+ (median 7% of total SP), CD19+ (2%) and CD56+ (4%), all 3 CD45^high and CD48+;

2. a myeloid population (median 54% of SP population; range 4–91%), CD45^low and CD48-, with relatively high forward and sideward scatter (FSC^high/SSC^high) and high CD38 expression (median 82%) and usually with aberrant marker expression;

3. a low-frequency FSC^low/SSC^low myeloid fraction, CD45^low and CD48-, with lower CD38 expression (median 48%), and negative for aberrant markers and

4. a similar population but with aberrant markers present. The latter, presumably primitive, malignant population had median frequency (of WBC) of 0.0018% (range 0.00016–0.0056%).

The CD34+CD38- content herein was 20% at maximum. NBM too had the 3 lymphoid populations (median 8%, 2% and 5%, resp), the FSC^high/SSC^high myeloid population (in 9/12 cases) and the FSC^low/SSC^low myeloid population (12/12 cases). The putative primitive character of the AML FSC^low/SSC^low SP subpopulation was substantiated by suspension culture for 5 weeks with subsequent CFU assay (14 days): FSC^low/SSC^low cells had >200 fold clonogenic ability compared to FSC^high/SSC^high cells. In conclusion, the AML SP compartment is highly heterogeneous and contains a low-frequency (median 1:50,000) subpopulation, defined by aberrant markers and with primitive characteristics, as a likely candidate stem cell population. In a stem cell concept integrating the CD34+CD38- and SP compartment, the presumed AML stem cell frequency would be in the order of 1:250,000. which probably is close to the presumed frequency of leukemia initiating cells in diagnosis AML.