Identification of Therapeutic Targets for Quiescent, Chemotherapy-Resistant Human Leukemia Stem Cells

Abstract

Human acute myeloid leukemia (AML) originates from rare leukemia stem cells (LSCs). Because these chemotherapy-resistant LSCs are thought to underlie disease relapse, effective therapeutic strategies specifically targeting these cells may be beneficial. Here, we report identification of a primary human LSC gene signature and functional characterization of human LSC-specific molecules in vivo in a mouse xenotransplantation model. In 32 of 61 (53%) patients with AML, either CD32 or CD25 or both were highly expressed in LSCs. CD32-or CD25-positive LSCs could initiate AML and were cell cycle-quiescent and chemotherapy-resistant in vivo. Normal human hematopoietic stem cells depleted of CD32-and CD25-positive cells maintained long-term multilineage hematopoietic reconstitution capacity in vivo, indicating the potential safety of treatments targeting these molecules. In addition to CD32 and CD25, quiescent LSCs within the bone marrow niche also expressed the transcription factor WT1 and the kinase HCK. These molecules are also promising targets for LSC-specific therapy.

Leukemia-associated antigens and their relevance to the immunotherapy of acute myeloid leukemia

“…Selective overexpression in LSCs (that is, expression in LSCs with minimal or no expression in normal HSCs) has been demonstrated for Aurora A (AurA) kinase, 32 B-cell lymphoma protein-2 (Bcl-2), 29 cyclin A1, 29,33 mucin 1 (MUC1), 34 PRAME, 30 synovial sarcoma X breakpoint 2-interacting protein (SSX2IP) 29 and WT1 (Table 1). 29 The overexpression of WT1 in LSCs has been corroborated by Saito et al, 28 who demonstrated that WT1 was among the most frequently upregulated genes in AML-LSCs relative to normal HSCs. 28 This is consistent with the observation that WT1-specific CTLs were capable of selectively eliminating leukemic cells while sparing normal marrow progenitors.…”

“…29 The overexpression of WT1 in LSCs has been corroborated by Saito et al, 28 who demonstrated that WT1 was among the most frequently upregulated genes in AML-LSCs relative to normal HSCs. 28 This is consistent with the observation that WT1-specific CTLs were capable of selectively eliminating leukemic cells while sparing normal marrow progenitors. 35 The utility of WT1 for LSC-targeted immunotherapy is further reinforced by its specific overexpression in endosteal LSCs, a subpopulation of LSCs notorious for cell cycle quiescence and resistance to chemotherapeutic agents.…”

“…4 LSCs, which constitute a small subpopulation of leukemic cells and are contained within the CD34 þ CD38 À cell fraction, are responsible for leukemia initiation and relapse because of their potent self-renewal and repopulating capacities. 28 This explains the impetus behind latest efforts to better characterize LSCs and to unravel their gene expression profiles. Majeti et al 29 compared the gene expression signature of LSCs with that of normal HSCs and identified over 3000 genes with dysregulated expression in LSCs.…”

“…35 The utility of WT1 for LSC-targeted immunotherapy is further reinforced by its specific overexpression in endosteal LSCs, a subpopulation of LSCs notorious for cell cycle quiescence and resistance to chemotherapeutic agents. 28 …”

“…64 According to Greiner et al, 25 up to 70% of AML patients in complete remission spontaneously exhibit circulating PRA 300-309 -specific CTLs, further underscoring the significant natural immunogenicity of PRAME. 25 CD8 þ T cells specific for G250/CAIX [24][25][26][27][28][29][30][31][32] , G250/CAIX 254-262 and RHAMM [165][166][167][168][169][170][171][172][173] are also regularly detected in the peripheral blood of AML patients, with frequencies of 60%, 13% and 47%, respectively. 25 In a series of 15 AML patients, Siegel et al 22 observed spontaneous T-cell immune responses to different HLA-A*0201-restricted T-cell epitopes derived from OFA-iLRP, including iLR1 [59][60][61][62][63][64][65][66][67][68] (6/15) …”