AC133 expression on acute myeloid leukemia blasts: correlation to FAB and to CD34 expression and possible implications for peripheral blood progenitor cell purging in AML

AC133 is an antigen expressed on CD34+ hematopoietic progenitor cells. In acute myeloid leukemia (AML) it is expressed on leukemic blasts of most FAB subtypes. However, few data are available regarding coexpression of other surface antigens. We measured AC133 expression on AML blasts from 28 consecutive patients at initial diagnosis (n=26) or at diagnosis of first relapse (n=2) and on 26 leukapheresis products from 14 patients. In AML AC133 correlated with CD34 expression (Spearman r=0.4711, P=0.0114) and even stronger with combined CD34/CD33 expression (Spearman r=0.5083, P=0.0068). In leukapheresis products AC133 expression correlated with CD34 expression (Spearman r=0.7495, P=0.002) and the yield of the obtained amount of CD34+ cells (Spearman r=0.6484, P=0.0121). In conclusion AC133 expression is closely related to CD34 expression in AML. In leukapheresis products AC133 provides an additional marker for selection of PBPC autografts in AC133- AML.     

Immunologic purging of autologous peripheral blood stem cell products based on CD34 and CD133 expression can be effectively and safely applied in half of the acute myeloid leukemia patients.

PURPOSE: Several studies have shown survival benefit by autologous stem cell transplantation in acute myeloid leukemia (AML) after purging of grafts. This has, however, not been confirmed in randomized studies due to high toxicity of purging modalities for normal progenitor/stem cells. In this study, we investigated whether positive selection for CD34+ and/or CD133+ cells, which results in high recovery of normal progenitor/stem cells, is applicable for purging AML grafts. EXPERIMENTAL DESIGN: Positive selections of normal stem cells using CD34 and/or CD133 can be done if one or both markers are absent or have dim expression and remain so during the course of the disease. Marker expressions in newly diagnosed AML were measured with flow cytometry using a cutoff value for positivity of 1%. Stability of marker expression was studied by pairwise comparison of material at diagnosis and relapse. Leukemia associated phenotype expression was used to measure the efficacy of tumor cell reduction. RESULTS: In newly diagnosed AML (n = 165), we found no CD34 and/or CD133 expression in 32% of the cases and dim expression in 20% of the cases. No increase in the percentage of CD34+ cells (n = 44) and CD133+ cells (n = 29) was found in corresponding relapses. Positive selection using grafts contaminated with AML blasts, showing either no or dim expression of CD34 or CD133, resulted in a 3 to 4 log tumor cell reduction (n = 11) with median 50% recovery of normal stem cells.CONCLUSIONS: Purging by positive selection of CD34+ and/or CD133+ cells can safely, effectively, and reproducibly be applied in about 50% of AML cases.     

Detection of cytogenetic aberrations both in CD90 (Thy-1)-positive and (Thy-1)-negative stem cell (CD34) subfractions of patients with acute and chronic myeloid leukemias.

Acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) are thought to arise from malignant hematopoietic progenitor cells representing early and undifferentiated stem cell clones. In CML there is evidence for a progenitor cell subset free of leukemic clones, depending on the course of the disease. Additionally, it has been suggested that in AML, the early stem cell compartment (CD34+/90+) does not harbor the malignant clone. We analyzed white blood cells from leukemia patients for the presence of aberrant cells in stem cell subfractions. Sixteen patients with CML, six patients with AML, two patients with acute lymphatic leukemia (ALL) and one with chronic myelomonocytic leukemia (CMMOL), all with known cytogenetic abnormalities, were evaluated according to their CD90 (Thy-1)-positive or -negative phenotype. Subsets were sorted on to slides and further characterized by FISH and/or standard cytogenetic testing. The bcr-abl translocation or gross chromosomal abnormalities could be detected in equally high amounts of 92.2% and 89.2% in both stem cell subsets. We conclude, that in progressed AML and CML cells characterized by specific genetic aberrations implicated in the malignant state can be found in the CD34+/CD90+ and CD90- population, thus making CD90 an inappropriate marker to distinguish benign from malignant cells in these leukemias.     

The interleukin-3 receptor alpha chain is a unique marker for human acute myelogenous leukemia stem cells.

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-3Ralpha 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-3Ralpha 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.     

Expression of c-kit Receptor (CD 117) and CD34 in Leukemic Cells

The expression of c-kit receptor (c-kit R; CD117) and CD34 was examined in acute myeloid leukemia (AML), acute lymphoid leukemia (ALL), chronic myeloid leukemia (CML) in blastic transformation (BT), and myelofibrosis (MF) in myeloid BT. In myeloid leukemia including AML, CML-myeloid BT and MF-myeloid BT, both c-kit R and CD34 were expressed synchronously, while in lymphoid leukemia including ALL and CML-lymphoid BT, only CD34 was highly expressed. A close correlation between c-kit R and CD33 expression and an inverse correlation between c-kit R and CD 19 expression were observed when all of the myeloid plus lymphoid leukemia cells were analysed. There was a close correlation between c-kit R and CD34 expression in the myeloid leukemia cells, c-kit R expression may be associated with myeloid phenotypes of leukemic cells and may be useful for the diagnosis of myeloid leukemia. The literature of c-kit R expression in leukemic cells is reviewed here and the comparison of c-kit R and CD34 expression in normal hematopoietic progenitor cells with those on the leukemic counterparts was discussed.     

CD34+/CD38- Stem Cell Burden Could Predict Chronic Myeloid Leukemia Patients’ Outcome

Background:The current predictor of the Chronic myeloid leukemia (CML) patients’ outcome is the degree of response to targeted therapy; here we search for a biomarker predicting CML outcome before start of therapy. This study aimed to assess the impact of the CD34+/CD38- stem cells (SCs) burden in chronic myeloid leukemia (CML) on treatment response and patients’ outcomes. Methods:Our study included 65 CML patients in the chronic phase. The patients’ CD34+/CD38- stem cells were quantified using flowcytometry before and after treatment by frontline imatinib (IM) therapy. The median follow-up for all patients was 18 months. Results:CD34+/CD38- stem cells frequency at diagnosis and after therapies are correlated to known prognostic markers (blast cells count, spleen size, total White cell count, and clinical scores). After therapy, the leukemic stem cells count dropped rapidly. The pretreatment CD34+/CD38- stem cells burden predicts response to frontline therapy. In addition, high SCs frequency at diagnosis predicts poor molecular response, transformation to AML, and poor patients’ outcomes. Conclusion:The percentage of CD34+/CD38- SCs burden at diagnosis reflects the CML disease behavior and is considered a biomarker for predicting CML patients’ response to first-line Tyrosine kinase inhibitors (TKI) therapy.Key Words: CML, CD34+/CD38, stem cells, outcome     

Small-molecule inhibition of BRD4 as a new potent approach to eliminate leukemic stem- and progenitor cells in acute myeloid leukemia (AML)

Acute myeloid leukemia (AML) is a life-threatening stem cell disease characterized by uncontrolled proliferation and accumulation of myeloblasts. Using an advanced RNAi screen-approach in an AML mouse model we have recently identified the epigenetic ‘reader’ BRD4 as a promising target in AML. In the current study, we asked whether inhibition of BRD4 by a small-molecule inhibitor, JQ1, leads to growth-inhibition and apoptosis in primary human AML stem- and progenitor cells. Primary cell samples were obtained from 37 patients with freshly diagnosed AML (n=23) or refractory AML (n=14). BRD4 was found to be expressed at the mRNA and protein level in unfractionated AML cells as well as in highly enriched CD34⁺/CD38⁻ and CD34⁺/CD38⁺ stem- and progenitor cells in all patients examined. In unfractionated leukemic cells, submicromolar concentrations of JQ1 induced major growth-inhibitory effects (IC₅₀ 0.05-0.5 μM) in most samples, including cells derived from relapsed or refractory patients. In addition, JQ1 was found to induce apoptosis in CD34⁺/CD38⁻ and CD34⁺/CD38⁺ stem- and progenitor cells in all donors examined as evidenced by combined surface/Annexin-V staining. Moreover, we were able to show that JQ1 synergizes with ARA-C in inducing growth inhibition in AML cells. Together, the BRD4-targeting drug JQ1 exerts major anti-leukemic effects in a broad range of human AML subtypes, including relapsed and refractory patients and all relevant stem- and progenitor cell compartments, including CD34⁺/CD38⁻ and CD34⁺/CD38⁺ AML cells. These results characterize BRD4-inhibition as a promising new therapeutic approach in AML which should be further investigated in clinical trials.     

CD133在急性白血病中的表达

目的研究CD133与急性白血病FAB分型、免疫分型及细胞遗传学改变间的关系.方法对60例急性白血病患者骨髓标本进行细胞形态学及细胞化学检查,确定其FAB亚型;运用直接免疫荧光标记法检测CD133及系列相关免疫标记进行免疫分型;采用RHG法进行核型分析.结果CD133在急性髓性白血病(AML)及急性淋巴细胞性白血病(ALL)细胞膜上均可表达,且往往与CD34或CD34、CD33共表达,CD133急性白血病患者细胞遗传学改变无特异性.结论CD133与CD34一样为早期造血干和(或)祖细胞膜上标记.     

Retinoic acid induction of CD38 antigen expression on normal and leukemic human myeloid cells: relationship with cell differentiation

Differentiation in the hematopoietic system involves, among other changes, altered expression of antigens, including the CD34 and CD38 surface antigens. In normal hematopoiesis, the most immature stem cells have the CD34+CD34 -phenotype. In acute myeloid leukemia (AML), although blasts from most patients are CD38+, some are CD38 -. AML blasts are blocked at early stages of differentiation; in some leukemic cells this block can be overcome by a variety of agents, including retinoids, that induce maturation into macrophages and granulocytes both in vitro and in vivo . Retinoids can also induce CD38 expression. In the present study, we investigated the relationship between induction of CD38 expression and induction of myeloid differentiation by retinoic acid (RA) in normal and leukemic human hematopoietic cells. In the promyelocytic (PML) CD34 -cell lines, HL60 and CB-1, as well as in normal CD34+CD34 -hematopietic progenitor cells RA induced both CD38 expression as well as morphological and functional myeloid differentiation that resulted in loss of self-renewal. In contrast, in the myeloblastic CD34+ leukemic cell lines, ML-1 and KG-1a, as well as in primary cultures of cells derived from CD34+-AML (M 0 and M 1 ) patients, RA caused an increase in CD38+ that was not associated with significant differentiation. Yet, long exposure of ML-1, but not KG-1, cells to RA resulted in loss of self-renewal. The results suggest that while in normal hematopoietic cells and in PML CD34 -cells induction of CD38 antigen expression by RA results in terminal differentiation along the myeloid lineage, in early myeloblastic leukemic CD34+ cells, induction of CD38 and differentiation are not functionally related. Since, several lines of evidence suggest that the CD38 -cells are the targets of leukemic transformation, transition of these cells into CD38+ phenotype by RA or other drugs may have therapeutic effect, either alone or in conjunction with cytotoxic drugs, regardless the ability of the cells to undergo differentiation.     

Aberrant marker expression patterns on the CD34+CD38- stem cell compartment in acute myeloid leukemia allows to distinguish the malignant from the normal stem cell compartment both at diagnosis and in remission.

Acute myeloid leukemia (AML) is generally regarded as a stem cell disease. In CD34-positive AML, the leukemic stem cell has been recognized as CD38 negative. This CD34+CD38- population survives chemotherapy and is most probable the cause of minimal residual disease (MRD). The outgrowth of MRD causes relapse and MRD can therefore serve as a prognostic marker. The key role of leukemogenic CD34+CD38- cells led us to investigate whether they can be detected under MRD conditions. Various markers were identified to be aberrantly expressed on the CD34+CD38- population in AML and high-risk MDS samples at diagnosis, including C-type lectin-like molecule-1 and several lineage markers/marker-combinations. Fluorescent in situ hybridization analysis revealed that marker-positive cells were indeed of malignant origin. The markers were neither expressed on normal CD34+CD38- cells in steady-state bone marrow (BM) nor in BM after chemotherapy. We found that these markers were indeed expressed in part of the patients on malignant CD34+CD38- cells in complete remission, indicating the presence of malignant CD34+CD38- cells. Thus, by identifying residual malignant CD34+CD38- cells after chemotherapy, MRD detection at the stem cell level turned out to be possible. This might facilitate characterization of these chemotherapy-resistant leukemogenic cells, thereby being of help to identify new targets for therapy.     

CD34+/CD38- Stem Cell Burden Could Predict Chronic Myeloid Leukemia Patients’ Outcome

Background: The current predictor of the Chronic myeloid leukemia (CML)  patients’ outcome is the degree of response to targeted therapy; here we search for a biomarker predicting CML outcome before start of therapy. This study aimed to assess the impact of the  CD34+/CD38- stem cells (SCs) burden in chronic myeloid leukemia (CML) on  treatment response and patients’ outcomes. Methods: Our study included 65 CML patients in the chronic phase. The patients’  CD34+/CD38- stem cells were quantified  using flowcytometry before and after treatment by frontline imatinib (IM) therapy. The median follow-up for all patients was 18 months. Results: CD34+/CD38- stem cells frequency at diagnosis and after therapies are correlated to known prognostic markers (blast cells count, spleen size, total White cell count, and clinical scores). After therapy, the leukemic stem cells count dropped rapidly. The pretreatment CD34+/CD38- stem cells burden predicts response to frontline therapy. In addition, high SCs frequency at diagnosis predicts poor molecular response, transformation to AML, and poor patients’ outcomes. Conclusion: The percentage of CD34+/CD38- SCs burden at diagnosis reflects the CML disease behavior and is considered a biomarker for predicting CML patients’ response to first-line Tyrosine kinase inhibitors (TKI) therapy.     

Clonal chromosomal abnormalities in the stem cell compartment of patients with acute myeloid leukemia in morphological complete remission.

Acute myeloid leukemia arises from the clonal expansion of a malignant transformed progenitor cell. Despite intensive chemotherapy, final disease eradication is achieved by a small proportion of cases only and 50-70% of adults with AML will ultimately relapse and die from their disease. Hence residual disease below the level of morphological detectability must be assumed in clinical and morphological complete remission. CD34+/CD38- and CD34+/CD38+ subpopulations of seven patients in morphological complete remission were isolated by FACS (purity >98%) and were analyzed by conventional cytogenetics or FISH for chromosomal aberrations. In five of seven patients, clonal chromosomal abnormalities were detected in the CD34+/CD38+ subpopulation and in one patient with AML M2 (add (2)(q37)) in the most immature CD34+/CD38- stem cell compartment. One patient with AML M4Eo (inv(16),+8), showed a normal karyotype by conventional cytogenetic analysis, whereas four of 15 metaphases of the sorted CD34+/CD38+ subpopulation revealed the inversion 16. These observations underline that leukemic cells can survive intensive chemotherapy in the niche of the stem cell compartment. In some patients the sensitivity for the detection of persistent leukemic cells seems to be higher in FACS-sorted subpopulations than conventional cytogenetic analysis of the unseparated bone marrow. Immunophenotyping revealed minimal residual disease in four of the patients. Functional analysis has to be performed to investigate the leukemogenic potential of these residual cells.     

139例急性髓系白血病免疫分型特点分析

目的探讨急性髓性白血病（AML）的免疫分型特点及意义。方法采用单克隆抗体和流式细胞仪检测AML的免疫表型。结果（1）139例AML病例中各种抗原的阳性表达率依次为为MPO（92.1%）,CD33（92.1%）,CD13(89.2%),其中53例AML伴淋巴系抗原表达,分别为CD19(20.9%),CD7(16.2%),CD2(7.2%),CD10(0.72%)。(2)CD14在M4、M5型AML中高表达。(3)干祖细胞分化抗原表达率依次为CD117（83.8%）〉HLA DR(80.3%)>CD34(67.6%),CD34阳性的完全缓解率（CR）分别明显低于CD34阴性组（P=0.034）。（4）CD7阳性患者CR明显低于其抗原表达阴性者（P=0.041）。结论白血病免疫分型能确诊某些特殊类型的白血病,对免疫分型的研究将有助于指导临床诊断、治疗及判断预后。     

Enrichment of N-Cadherin and Tie2-bearing CD34/CD38/CD123 leukemic stem cells by chemotherapy-resistance

Acute myeloid leukemia (AML) arises from genetic changes at the level of stem cell, various mutations have been elucidated, including AML1–ETO fusion gene has been shown as the representative target of cellular transformation for LSCs originating from hematopoietic stem cells (HSCs) compartment. LSCs resemble HSCs with respect to self-renewal capacity and chemotherapy-resistance. However, LSCs possess specific cell-surface markers, they are proposed to reside within the CD34⁺/CD38⁻/CD123⁺ compartment. And the interaction mediated by adhesion molecules between LSCs and niche played a role in chemoresistance of LSCs. Therefore, study on the LSCs surface makers related to niche is helpful for the potential target therapy in the future. In this study, the proportions of CD34⁺/CD38⁻/CD123⁺ LSCs compartment co-expressing the three adhesion molecules, N-Cadherin, Tie2 and CD44, respectively, from AML patients before and after chemotherapy were analyzed. We demonstrated N-Cadherin and Tie2 positive CD34⁺/CD38⁻/CD123⁺ LSCs populations could be enriched by chemotherapy. Furthermore, AML1/ETO fusion signals and MDR1 expression were detected on the CD34⁺/CD38⁻/CD123⁺ LSCs populations expressing N-Cadherin and Tie2. Therefore, N-Cadherin and Tie2 are probably the potential markers for identification of LSCs.     

CD47在初诊正常核型急性髓系白血病患者中的表达及其临床意义

目的 探讨CD47在初诊正常核型急性髓系白血病(AML)患者中的表达水平及临床意义.方法 选取137例初诊正常核型AML患者及3名健康志愿者.采用流式细胞术及实时荧光定量聚合酶链反应(qPCR)对健康志愿者骨髓造血干细胞(HSC)和造血祖细胞(MPP)及AML患者骨髓单个核细胞(MNC)和白血病干细胞(LSC,Lin-CD34+CD38-CD90-)的CD47表达水平进行检测;采用基因组分析平台检测患者FMS样酪氨酸激酶3内部串联重复(FLT3-ITD).采用超高速流式分选系统分选CD34+CD38-CD47lo和CD34+CD38-CD47hi细胞.将两组细胞均用MethoCult H4445培养液培养接种于含有琼脂糖的甲基纤维素板,12 d后计数MPP集落形成单位(CFU),同时将1×105个CD34+CD38-CD47lo和CD34+CD38-CD47hi细胞分别移植到经280 cGy照射的NSG(NOD-SCID IL-2Rγnull)小鼠,8周后处死小鼠,流式细胞术检测人CD45+细胞比例.结果 CD47的表达在初诊正常核型AML患者中高于健康志愿者,CD47在AML各法、美、英协作组(FAB)亚型中均有表达,相对表达量比较差异无统计学意义(F=0.545,P>0.05).137例患者中CD34+CD38-CD47hi 37例,其中17例(46%)FLT3-ITD阴性,20例(54%)为FLT3-ITD阳性;CD34+CD38-CD47lo的患者100例,其中63例(63%)FLT3-ITD阴性,37例(37%)FLT3-ITD阳性,CD34+CD38-CD47hi与CD34+CD38-CD47lo患者相比较,FLT3-ITD阳性率差异无统计学意义(χ2=3.79,P>0.05).将FACS分选的CD34+CD38-CD47lo和CD34+CD38-CD47hi接种于含有琼脂糖的甲基纤维素板12 d后,仅CD34+CD38-CD47lo细胞可以形成CFU.NSG小鼠移植实验显示CD34+CD38-CD47lo细胞可重建造血,CD34+CD38-CD47hi植入失败.结论CD34+CD38-CD47hi富集LSC,可作为AML患者化疗后微小残留病的监测指标.     

Flow cytometric characterization of acute myeloid leukemia. Part II. Phenotypic heterogeneity at diagnosis.

The frequency and distribution of aberrant antigen expression are analyzed on bone marrow aspirates from 80 patients with newly diagnosed acute myeloid leukemia (AML) by multidimensional flow cytometry. Parameters examined are the light scatter profile of the leukemic cells and the correlative expression of different combinations of the CD2, 4, 5, 7, 11b, 11c, 13, 14, 15, 16, 33, 34, 38, and HLA-DR antigens. Antigen expression on leukemic cells in bone marrow is described by characteristic antigen expression patterns describing: (i) the percentage of cells expressing the antigen; (ii) the antigen density; and (iii) the distribution of the antigen on the leukemic cells. Typically the non-myeloid antigens are homogeneously expressed by the leukemic cells, whereas the myeloid associated antigen CD11b, CD11c, CD14, and CD15 are heterogeneously expressed. Comparison of the antigenic profiles of 80 bone marrow aspirates revealed an extreme interclonal heterogeneity. Comparison of the antigen expression patterns found in AML patients with the antigen expression in normal bone marrow revealed four patterns of aberrant antigen expression in AML: (i) expression of nonmyeloid antigens (i.e. CD2, CD5, and CD7 were present in 57, 60, and 37% of the patients, respectively); (ii) asynchronous expression of myeloid associated antigens (i.e. co-expression of CD34 and CD15 in 25% of the patients and expression of CD16 on immature myeloid cells in 15% of the cases); (iii) over-expression of myeloid associated antigens (e.g. CD34 in 16% of the cases and CD14 on neutrophilic cells in 19% of all patients); and (iv) absence of expression of myeloid associated antigens (e.g. lack of CD33 in 21% of the cases and lack of both CD11b and CD15 in 6% of all patients. Multidimensional flow cytometric analysis of bone marrow aspirates of AML patients disclosed that the leukemic cells of each AML patient had a unique antigenic profile and could be discriminated from their normal counterparts based on aberrant antigen expression and typical light scatter profiles. The ability to distinguish leukemic cells from normal cells allows the detection of residual leukemic cells during and after chemotherapy.     

C-type lectin-like molecule-1: a novel myeloid cell surface marker associated with acute myeloid leukemia

Acute myeloid leukemia (AML) has a poor prognosis due to treatment-resistant relapses. A humanized anti-CD33 antibody (Mylotarg) showed a limited response rate in relapsed AML. To discover novel AML antibody targets, we selected a panel of single chain Fv fragments using phage display technology combined with flow cytometry on AML tumor samples. One selected single chain Fv fragment broadly reacted with AML samples and with myeloid cell lineages within peripheral blood. Expression cloning identified the antigen recognized as C-type lectin-like molecule-1 (CLL-1), a previously undescribed transmembrane glycoprotein. CLL-1 expression was analyzed with a human anti-CLL-1 antibody that was generated from the single chain Fv fragment. CLL-1 is restricted to the hematopoietic lineage, in particular to myeloid cells present in peripheral blood and bone marrow. CLL-1 is absent on uncommitted CD34(+)/CD38(-) or CD34(+)/CD33(-) stem cells and present on subsets of CD34(+)/CD38(+) or CD34(+)/CD33(+) progenitor cells. CLL-1 is not expressed in any other tissue. In contrast, analysis of primary AMLs demonstrated CLL-1 expression in 92% (68 of 74) of the samples. As an AML marker, CLL-1 was able to complement CD33, because 67% (8 of 12) of the CD33(-) AMLs expressed CLL-1. CLL-1 showed variable expression (10-60%) in CD34(+) cells in chronic myelogenous leukemia and myelodysplastic syndrome but was absent in 12 of 13 cases of acute lymphoblastic leukemia. The AML reactivity combined with the restricted expression on normal cells identifies CLL-1 as a novel potential target for AML treatment.