**2.6 CD25 and CD32**

Saito et al. conducted microarray analysis and found that CD25 and CD35 were expressed on quiescent LSCs, but not on HSCs [28]. The activation of CD25, namely IL2Rα, can control cell proliferation, survival, and differentiation. CD32 is a member of the Fc-gamma receptor family and is mainly found on immune cells. Transplantation of CD34<sup>+</sup> CD38<sup>−</sup>CD25+ cells and CD34+ CD38<sup>−</sup>CD32+ cells into NO/ SCID mice can trigger leukemia and resistance to cytarabine. It has been reported that overexpression of CD25 in AML cells may be caused by the activation of STAT5

**15**

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

and MYC [29, 30]. Gönen et al. analyzed the correlation between the expression of CD25 (IL-2 receptor alpha) and prognosis in 657 patients with primary AML (≤60 years old); they concluded that CD25 can be used as a biomarker for poor prognosis of AML [31]. Cerny et al. also indicated that CD25 expression can be used

The most fundamental reason for the relapse of AML is the existence of LSCs. It is necessary to investigate the key mechanisms of resistance of LSCs to the current

Dean et al. showed that 96% of LSCs are in the G0 phase of the cell cycle [33]. Chemotherapeutic drugs acting on the cell cycle or on rapidly differentiating cells can inhibit only differentiated mature leukemia cells, while LSCs in the G0 phase cannot be completely inhibited because they do not divide. Once they are properly stimulated to re-enter the cell cycle, they will continue to proliferate and differentiate into daughter leukemia cells, thus causing recurrence. According to some studies, LSCs are much less sensitive to daunorubicin and cytarabine than differentiated leukemia cells.

The expression of multidrug resistance genes on the surface of LSCs can induce

after BCRP inhibition is increased, but it is much lower than that of cells expressing only BCRP. Therefore, it is indicated that the drug resistance of LSC is related to the interaction of multiple drug resistance proteins. Some other reports have revealed that LSC has higher MDR1, MRP, BCRP, and lung resistance related protein (LRP) expression relative to HSC, thus giving LSC a stronger drug resistance advantage. The high expression of multidrug resistance gene in LSCs is the main mechanism by

Hope et al. proved that LSCs have self-renewal ability, which is one of the most prominent features of CSCs [37]. The self-renewal ability of LSCs may be one of the key factors that promote the development and metastasis of leukemia, and the molecular regulation mechanism is very complicated. Bmi-1 is a member of the PcG (polycomb group) transcriptional repressor family and is an essential factor in maintaining HSC self-renewal. Raffel et al. showed that miR-126 overexpression renders AML cells more resistant to standard chemotherapy and that treatment of primary AML cells results in the enrichment of LSC-like cells with increased

which LSCs exhibit primary resistance to chemotherapeutic drugs [35, 36].

CD38<sup>−</sup> LSCs. The intracellular drug concentration

the production of various membrane transporters that can pump a variety of chemotherapeutic drugs out of the cell, which results in lowering the concentration of the drug in the cancer cells. The ABC membrane transporter plays a pivotal role in this drug efflux process. The ABC transporter, namely the ATP-binding cassette transporter, has an ATP-dependent drug-release function [34]. The most representative multidrug resistance genes are ABCB1, ABCC1, and ABCG2, which encode P-glycoproteins (P-gp, P-170, and MDR1), multidrug resistance protein (MRP), and breast cancer resistance protein (BCRP), respectively. BCRP is pref-

**3.2 LSCs highly express multidrug resistance genes and proteins**

as an indicator to predict early treatment failure in AML [32].

**3. LSCs are the source of treatment resistance**

treatment strategy for effective clearance of LSC.

**3.1 LSCs are mostly in the G0 quiescent phase**

erentially expressed in CD34+

**3.3 LSC display higher self-renewal ability**

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

*Advances in Hematologic Malignancies*

identical marker of LSCs in CD34+

LSCs in a primitive state.

**2.4 CD123 (IL3Rα)**

**2.5 CD47**

**2.6 CD25 and CD32**

Transplantation of CD34<sup>+</sup>

**2.2 CD96**

**2.3 CD44**

DNA-binding payload in CLL1-ADC is critical because such a payload affords the ADC the ability to kill both proliferative and quiescent cells, thus making CLL1- ADC a very compelling candidate for the treatment of patients with AML.

CD96 is a member of the immunoglobulin superfamily, a transmembrane glycoprotein, and a T-cell surface-specific receptor 6. By using blood samples from 55 patients with AML, Du et al. found that CD96 (>10%)-enriched patients showed a poor response to chemotherapy [20]. Of note, CD96 was proved to be an efficient

CD38<sup>−</sup> groups.

CD44 is a surface glycoprotein and a receptor for hyaluronic acid, which is mainly involved in cell-cell interaction, adhesion, and migration [16]. CD44<sup>+</sup> cancer cells show higher sphere-forming ability and treatment resistance. CD44 is not only a special marker of LSC, but it is also a key regulator of LSC function that is essential for homing of LSCs to microenvironmental niches and for maintaining

Approximately 45% of AML cells that overexpress CD123 have higher proliferative activity and are more tolerant to apoptotic stimulation. Clinical studies have also demonstrated patients overexpressing CD123 usually have a poor prognosis. Williams et al. found that NK-92 preferentially inhibits leukemic stem cells compared with bulk leukemia cells [21]. NK-92 combined with the anti-CD123 antibody, 7G3, enhanced survival in a primary AML xenograft model when compared with control arms. Some other IL3R antibodies (DT388IL3, CSL362, and MGD006)

can significantly prolong the survival rate of patients with AML [22, 23].

not susceptible to Ara-C treatment but highly expressed CD47 [27].

CD38<sup>−</sup>CD25+

CD47 is a transmembrane glycoprotein that is widely expressed in human tissues. CD47 also functions as a marker of "self" on host cells within an organism. When expressed, CD47 binds to SIRPα on the surface of circulating immune cells to deliver an inhibitory "don't eat me" signal [24]. Higher expression of CD47 has been demonstrated in LSCs [25, 26]. Anti-CD47 antibody treatment has also been shown to act synergistically with cytarabine (Ara-C) chemotherapy in a model of AML. While Ara-C effectively eliminated TSP-1 cancer cells in the proliferative phase, anti-CD47 antibodies were putatively able to target quiescent LSCs that were

Saito et al. conducted microarray analysis and found that CD25 and CD35 were expressed on quiescent LSCs, but not on HSCs [28]. The activation of CD25, namely IL2Rα, can control cell proliferation, survival, and differentiation. CD32 is a member of the Fc-gamma receptor family and is mainly found on immune cells.

SCID mice can trigger leukemia and resistance to cytarabine. It has been reported that overexpression of CD25 in AML cells may be caused by the activation of STAT5

cells and CD34+

CD38<sup>−</sup>CD32+

cells into NO/

**14**

and MYC [29, 30]. Gönen et al. analyzed the correlation between the expression of CD25 (IL-2 receptor alpha) and prognosis in 657 patients with primary AML (≤60 years old); they concluded that CD25 can be used as a biomarker for poor prognosis of AML [31]. Cerny et al. also indicated that CD25 expression can be used as an indicator to predict early treatment failure in AML [32].
