Differential Expression of SLAM Family Receptor Markers in Normal Human Hematopoietic Stem Cells and Their Malignant Counterpart in MDS and AML.

Members of the signaling lymphocytic activation molecule (SLAM) family, including CD150, CD48 and CD244, were shown to precisely distinguish more committed lineage restricted progenitor cells from pluripotent and multipotent murine hematopoietic stem cells (HSC; Kiel et al; 2005 Cell). Similar SLAM profiles may also be present on HSC subsets in humans. We hypothesized that these SLAM markers may be indicators not only of stem cell potential in normal hematopoiesis but also distinguish a subset of the most immature malignant precursors of leukemia. In agreement with the concept of a “cancer stem cell,” the presence of leukemic stem cell population may be an indicator of important clinical and biological properties. We first tested the distribution of CD150, CD48 and CD244 antigens on human CD34⁺ cells derived from 7 control individuals using 4-color flow cytometry. CD34⁺ cells were measured in the blast gate based on side scatter and CD45 expression. Within CD34⁺ blasts, expression of CD48, CD150, and CD244 was detected on 16.71%±9.69, 6.53%±2.93, and 26.92%±6.95 of cells respectively. Subsequently, we investigated SLAM expression in 9 immature leukemic cell lines, including KG-1, K562, U937, HEL, HL60, MKN-95, NB-4, Kasumi and UT7, and found increased expression of SLAM markers in KG-1 (CD48⁺, CD150⁺, CD244⁺) and Kasumi (CD48⁻, CD150⁻, CD244⁺). Consequently, none of the leukemic cells showed pluripotent/multipotent SLAM profiles. We then compared the SLAM marker expression on blasts from patients with AML and MDS with that of CD34⁺ cells from normal controls. We studied a total of 28 patients: 11 MDS (2 low grade, 5 advanced MDS, 3 MDS/MPD overlap) and 10 AML (FAB: 3 M1, 2 M2, 1 M3, 2 M4/M4E0 and 2 M6). In our cohort, 8/10 AML patients expressed one of the three SLAM markers; 6/10 were CD150⁻CD48⁻CD244⁺ (63.57%±6.96) and 2/10 were CD150⁺CD48⁻CD244⁻(46%±10.96) suggestive of the presence of either pluripotent or multipotent leukemic stem cell phenotype. In the MDS cohort, 8/11 patients expressed one of three SLAM markers, 7/11 were CD150⁻CD48⁻CD244⁺ (41.21% ± 8.9) and 1/11 were CD150⁺CD48⁻CD244⁻ (1.26%±0.59) again consistent with a profile derived from either pluripotent or multipotent stem cells. None of the MDS and AML patients had either co-expression of CD244 and CD48 or increased expression of CD48 alone. Two of the M1 type AML patients with CD150⁻CD48⁻CD244⁺ phenotype received prior chemotherapy and achieved complete remission on bone marrow biopsy and flow cytometry using traditional blast markers. In some, we conclude that the SLAM receptor markers may be associated with certain types of leukemic blasts and may be useful in the identification of leukemic stem cell population in both MDS and AML.