CELL-EXTRINSIC EFFECTS IN T CELL PRE-LEUKEMIA STEM CELLS

Current therapies for T cell acute lymphoblastic leukaemia (T-ALL) have achieved relatively successful results. However, relapse cases are often fatal. Relapse is caused by the presence of leukemic stem cells that are resistant to treatment. Using the recently described model of T-ALL pre-leukemic stem cells, the NUP98-HOXD13 (NHD13) mouse, we investigated novel mechanisms of self-renewal in these cells. We found that the thymocytes in these mice overexpressed EphA3, a gene associated with abnormal self-renewal in other cancers. We generated NHD13/EphA3-knockout (NHD13-EA3KO) mice and performed serial thymocyte transplantation. NHD13-EA3KO thymocytes have abnormal self-renewal activity similar to NHD13, demonstrated by their capacity to engraft following primary and secondary transplants. Strikingly, the NHD13-EA3KO thymocytes failed to engraft upon the third serial transplant, whereas the NHD13 thymocytes engraft indefinitely, suggesting that EphA3 is essential for long term self-renewal. Additionally, we found that EphA3 is required for the typical NHD13 thymocyte “import block”, where (in a recipient animal that has received NHD13 thymocytes) incoming wild type thymic progenitors from the bone marrow cannot differentiate beyond DN1. When EphA3 is deleted from NHD13 thymocytes, they can still engraft but the “import block” is lifted. NHD13 thymocytes are quiescent and express less Ki67+ compared to WT cells. We analysed thymocytes from primary transplant recipients and found that, independently of EphA3, DN2 NHD13 cells can modify the cell cycle status of the surrounding WT cells, reported as a decreased proportion of Ki67+ WT cells. We have identified two ways in which NHD13 thymocytes can influence the behaviour of the wild type thymocytes around them. We suggest self-renewal in this model is a complex interplay of cell intrinsic and extrinsic factors.