Cell-extrinsic effects in T cell acute lymphoblastic pre-leukemia stem cells mediated by EphA3

Our recent study of a novel model of T-ALL pre-leukemic stem cells, the NUP98-HOXD13 (NHD13) mouse, showed that the abnormal self-renewal of these stem cells was dependent on Lyl1 yet, when Lyl1 was deleted, the T-ALL still developed. In the present study, we observe that the thymocytes in these mice also overexpress EphA3, and we characterise the thymocytes in NHD13-EphA3-/- mice. NHD13-EphA3-/- thymocytes retain their abnormal self-renewal activity demonstrated by their capacity to engraft following primary and secondary transplants. Strikingly, NHD13-EphA3-/- thymocytes fail to engraft upon the third serial transplant, whereas the NHD13 thymocytes engraft indefinitely. Seeking to explain this, we find that NHD13 DN2 thymocytes are capable of halting the normal differentiation process of incoming WT progenitor cells, and remarkably, this capacity is severely impaired in the absence of EphA3. Therefore EphA3 is not critical for engraftment, but is essential for enabling the halt in differentiation of neighbouring WT cells, which in turn allows the incumbent progenitors to remain longer in the thymus due to an absence of normal cell competition, a property that in itself has been demonstrated to be oncogenic. We suggest that pre-leukemic self-renewal in this model is a complex interplay of cell intrinsic and extrinsic factors, and that multiple redundant pathways to leukemogenesis are active in this model.