Competition Between Leukemic Cells And Normal Hematopoietic Cells In Non-irradiated Recipient Mice

Background:The biological mechanisms on the development of leukemia still remain nebulous. Our recent study demonstrated that different effects of T-ALL leukemic environments on normal hematopoietic stem (HSCs) or progenitor cells (HPCs) in the irradiated model (Hu X, et al. Blood2009). Moreover, a previous publication showed that the leukemic cells created bone marrow niches and disrupted the behavior of normal HSCs/HPCs (Sipkins DA et al, Science2008). To further understand the actual effects of the leukemic environment on normal HSCs and HPCs in a more clinically-relevant model, we have introduced a non-irradiated mouse model to better mimic the physiological condition. We investigated the dynamic and function of HSC and HPC in the leukemic environment using this non-irradiated mouse model.Methods:The Notch1-over expression murine T-cell leukemic cells were established as previously reported (Hu X et al, Blood2009). In our work, leukemic cells were injected into healthy C57BL/6J (CD45.2+) mice to establish the non-irradiated leukemic model. We identified the Leukemia type though FACS in the peripheral blood and bone marrow. Then we took this mouse model to investigate kinetics of the HPC and HSC in the leukemic environment. Flow cytometry was used to study the HSCs/HPCs at different time points(day7,10,14).The kinetic of sorted HSC were analysed by gene array.Then we used colony-forming cell (CFC) assay and competitive bone marrow transplantation (cBMT) to assess the impact of leukemic environment on HSCs/HPCs. In addition, two-photon microscopy imaging approaches were used to show the competition between leukemic cells and hematopoietic cells.According to the results of two-photon, we study the homing and location of leukemia cells by splenectomy.Results:T-ALL was induced with100%penetrance in non-irradiated recipients. After10days of transplantation, hematopoietic suppression occurred to HSCs but not HPCs in the leukemia-bearing mice. In accordance with the increased frequency of primitive cells, the capacity of colony-forming [CFU-GM (13±1.22vs3±0.35CFCs/104BMNCs), CFU-mix (8±1.13vs3±1.41CFCs/104BMNCs)] and repopulation [(46±4.38)%vs (20±2.26)%, respectively] of residual normal hematopoietic cells in leukemic condition were significantly higher than that in control mice. The two-photon microscopy showed that the normal hematopoietic cells preferred to localize in the endosteum niches, while the leukemic stem cells (LSCs) firstly localized to spleen, and then migrated to bone marrow vascular niches where the LSCs created their own niche to increase their size of cell population. At last, the leukemic cells invaded and occupied the endosteum niches.The process of leukemia could be delayed after removing spleen.This suggested the spleen increased the developmet of the leukemia.Conclusions:Our current findings demonstrate that normal HSCs/HPCs are reversibly suppressed in leukemic environment of the non-irradiated model, which is similar to the results of irradiated model (Hu X, et al. Blood2009). And the data from microscopy imaging approaches suggests there is a paradigm that the leukemic cells cause the suppression of normal hematopoietic cells by hijacking their normal niches.The homing and location of leukemia cells suggest that the spleen could facilitate the development of leukemia.