Recreating the Bone Marrow Microenvironment to Model Leukemic Stem Cell Quiescence.

The main challenge in the treatment of acute myeloid leukaemia (AML) is relapse as it has no good treatment options and 90% of relapsed patients die as a result. It is now well accepted that relapse is due to a persisting subset of AML cells known as leukaemia-initiating cells or leukemic stem cells (LSCs). Haematopoietic stem cells (HSCs) reside in the bone marrow microenvironment (BMM), a specialised niche that co-ordinates HSC self-renewal, proliferation and differentiation. HSCs are divided into two types, long-term HSCs (LT-HSCs) and short-term HSCs (ST-HSCs) where LT-HSCs are typically quiescent and act as a reserve of HSCs. Like LT-HSCs, a quiescent population of LSCs also exist. Like LT-HSCs, quiescent LSCs have low metabolic activity, low RNA synthesis and receive pro-survival signals from the BMM, making them resistant to drugs and upon discontinuation of therapy, they can become activated and re-establish the disease. Several studies have shown that activation of quiescent LSCs may sensitise them to cytotoxic drugs. However, it is very difficult to experimentally model the quiescence-inducing bone marrow microenvironment (BMM). Here we report that culturing AML cells with bone marrow stromal cells, transforming growth factor beta-1 and hypoxia in a three-dimensional system can replicate the quiescence-driving BMM. A quiescent-like state of the AML cells was confirmed by reduced cell proliferation, increased percentage of cells in the G0 cell cycle phase and a decrease in absolute cell numbers, expression of markers of quiescence and reduced metabolic activity. Furthermore, the culture can be established as co-axial microbeads enabling high-throughput screening to which has been used to identify combination drug treatments that can break BMM-mediated LSC quiescence enabling the eradication of quiescent LSCs.