The hematopoietic stem cell (HCS) compartment does not represent a homogenous group of cells but instead consists of 3 subsets termed myeloid-biased (My-bi), lymphoid-biased (Ly-bi), and balanced (Bala) HSCs. Each of these possess a distinct pre-programmed proliferation and differentiation behavior, which helps us to determine if only 1 of these subsets contains cells that induce acute myeloid leukemia (AML). Method: Highly purified HSCs were separated into 2 different subsets Ly-bi (CD150-)

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... My-bi (CD150+) using fluorescence-activated cell sorting. The sorted cells were transduced with a retroviral vector expressing HoxA9, a gene known to induce AML, and injected into mice (primary hosts). Blood samples were taken regularly and analyzed in terms of leukemic behavior. After euthanizing the mice, spleen and bone marrow were analyzed, and whole bone marrow cells from these mice were serially transplanted into naive mice (secondary hosts). In line with the previous experiments, the leukemic behavior was again examined using blood analysis. Finally, whole bone marrow and spleen from these secondary hosts was transplanted into new tertiary hosts, and the time of survival of these mice was determined. Results: Mice injected with Ly-bi HSCs expressing HoxA9 displayed a shift of differentiation towards the myeloid lineage and a high percentage of Mac-1 positive cells, which is typical for AML. Additionally, typical signs of leukemia were observed in 70% of the primary hosts, which was characterized by an enlarged spleen. After serial transplantation, the development of leukemia became more aggressive as indicated by increased spleen weight and the loss of blood cell markers. The tertiary host mice died within 4 weeks of transplantation. In contrast, mice injected with My-bi HSCs expressing HoxA9 showed no signs of leukemia in primary host mice and lower levels of HoxA9 expression in bone marrow and spleen. Transduced cells did not differentiate towards the myeloid lineage and did not behave like those of [...]