Acute myeloid leukemia (AML) originates from self-renewing leukemic stem cells (LSCs), an ultimate therapeutic target for AML. We have reported that the T-cell immunoglobulin mucin-3 (TIM-3) is expressed on LSCs in most types of AML but not on normal hematopoietic stem cells (HSCs). TIM-3+ AML cells reconstituted human AML in immunodeficient mice, whereas TIM3− AML cells did not, suggesting that the TIM-3+ population contains all functional LSCs. We established an anti-human TIM-3 mouse IgG2a antibody having complement-dependent and antibody-dependent cellular cytotoxic activities. This antibody did not harm reconstitution of normal human HSCs, but blocked engraftment of AML after xenotransplantation. Furthermore, when it is administered into mice grafted with human AML, this treatment dramatically diminished their leukemic burden, and eliminated LSCs capable of reconstituting human AML in secondary recipients (Kikushige et al, Cell Stem Cell, 2010).We extended the analysis of TIM-3 expression into various types of human hematological malignancies, and found that human TIM-3 is expressed in the vast majority of CD34+CD38− LSCs of human myeloid malignancies including chronic myeloid leukemia, chronic myelomonocytic leukemia and myelodysplastic syndromes (MDS). Although TIM-3 was not expressed in CD34+CD38− stem cell fraction in normal bone marrow cells, TIM-3 was progressively up-regulated in this population of MDS, along with disease progression into leukemia: The average percentages of TIM-3+ cells in the CD34+CD38− population was 7.8% in RCMD (n=10), 19.2% in RAEB-1 (n=10), 84.0% in RAEB-2 (n=10) and 92.2% in overt AML (n=10). Thus, TIM-3 might be useful to isolate malignant stem cells responsible for progression into AML in MDS patients. The close association of TIM-3 expression with transformation into AML led us to hypothesize that TIM-3 itself has a function in AML stem cell development. TIM-3 is type 1 cell-surface glycoprotein and has a structure that includes an N-terminal immunoglobulin variable domain followed by a mucin domain, a transmembrane domain and a cytoplasmic tail. Tyrosine residues are clustered in the cytoplasmic tail, suggesting that TIM-3 can induce signal transduction in TIM-3+ AML cells. Previous reports have shown that galectin-9 and HMGB-1 are the ligand of TIM-3 in lymphocytes and dendritic cells. TIM-3 is reported to signal differently in lymphocytes and myeloid cells, because TIM-3 ligation results in different patterns of tyrosine phosphorylation in these cell types, suggesting that TIM-3 has lineage- or cellular context-dependent signal transduction pathways or functions. Therefore, we considered that it should be critical to identify the function of TIM-3 in primary AML cells. We cultured TIM-3+ AML cells in the presence or absence of galectin-9 or HMGB-1, and performed cDNA microarray analysis to find genes activated in response to TIM-3 ligation. Interestingly, pro-apoptotic genes such as BAX and SIVA were significantly down-regulated in the presence of galectin-9 or HMGB-1, suggesting that TIM-3 signaling could promote survival of TIM-3-expressing LSCs. These data suggest that TIM-3 is a surface marker useful to track malignant LSCs in progression from MDS to AML, and TIM-3 may function for maintenance of LSC through inducing survival-promoting signaling.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.