CD9 Defines Acute Myeloid Leukemia Stem Cells and is Regulated by Alpha-2-Macroglobulin

Abstract Background: Leukemia stem cells (LSCs) are responsible for the initiation, progress and relapse of acute myeloid leukemia (AML). Therefore, the therapy strategy of targeting LSCs is hopeful to eradicate AML. In this study, we aim to identify LSC-specific surface markers and uncover the underlying mechanism of AML LSCs.Methods: Microarray gene expression data were used to investigate the candidate AML-LSC specific markers. CD9 expression was evaluated by flow cytometry in AML cell lines, patients with AML and normal donors. The biological characteristics of CD9-positive (CD9+) cells were analyzed by in vitro proliferation, chemotherapeutic drug resistance, migration and in vivo xenotransplantation assays. The molecular mechanism involved in CD9+ cell function was investigated by gene expression profiling. Effect of alpha-2-macroglobulin (A2M) on CD9+ cells was analyzed by proliferation, drug resistance and migration assays.Results: CD9 as a cell surface protein was specifically expressed on AML LSCs, but almost not expressed on normal hematopoietic stem cells (HSCs). CD9+ cells exhibited more resistance to chemotherapy drugs and higher migration potential than CD9-negative (CD9-) cells. More importantly, CD9+ cells possess the ability to reconstitute human AML in immunocompromised mice and promote tumor growth, suggesting CD9+ cells define the LSC population. Furthermore, we identified A2M plays a crucial role in CD9+ LSCs stemness maintenance. Down-regulation of A2M impairs drug-resistance and migration of CD9+ cells.Conclusion: Our findings suggest that CD9 is a new biomarker of AML LSCs and may serve as a promising therapeutic target.