Acute leukemias are often associated with the suppression of hematopoiesis. This condition is generally believed to reflect the “crowding-out” effect by the rapidly proliferating leukemia cells. Emerging data, however, has indicated that leukemia cells can instructively modulate the marrow microenvironment that is essential for the maintenance of hematopoietic homeostasis. One of the underlying mechanisms likely implicates leukemia-associated disruption of the marrow stem cell niche; however, the mechanisms by which acute leukemia employs to modulate niche architectural alterations are not well understood. A better understanding of the leukemia niche elements and the implicated molecular pathways involved in the niche regulation will facilitate the development of therapies to accelerate hematopoietic recovery and reconstitution in leukemia patients.
We used AML human NSG xenografts and the murine T-ALL model induced by the retroviral transfection of marrow progenitors with activated form of Notch1 (ICN1). In the murine T-ALL model, the transformed progenitors were co-transplanted with the wild type (WT) marrow cells into irradiated immune-sufficient C57BL/6 mice. The activated Notch1 drives leukemia transformation characterized by the expansion of an immature CD4+/CD8+ population of T progenitors. We tracked the niche location of leukemia using multi-photon intravital microscopy. The niche cell compartment and the reconstitution of co-transplanted marrow cells were characterized by flow cytometry, RT-PCR and in vitro differentiation assays.
In NSG mice receiving primary human AML cells, we observed that leukemia development was accompanied by a suppression of the host hematopoietic stem cell (HSC) compartment. A similar hematopoietic suppression was observed in the C57BL/6 mice receiving ICN1-expressing progenitors but not in the mice receiving control cells in a disease stage-dependent manner, characterized by a loss of the HSC population at the later disease stage and a prominent suppression of B lymphopoiesis in the companion non-transformed marrow compartment. Co-culturing with leukemia cells only modestly affected HSPC differentiation, indicating elements of leukemia microenvironment are also important to mediate the hematopoietic suppression.
To understand how leukemia cells modulate niche, we longitudinally tracked the niche localization and the proliferation of pre-leukemia cells in lethally irradiated host mice. We found that ICN1-expressing individual pre-leukemia cells can be identified on day1 at the peri-vascular niche after adoptive transfer, and appeared as clusters around day8 followed by a rapid expansion phase 2-3 weeks after transplantation. We then demonstrated that the CD8+ leukemia cells, which are enriched for leukemia initiating cells, but not the CD8- cells, homed to the peri-vascular niche in the secondary leukemia transplanted hosts. Further, co-transplantation of leukemia cells with WT HSCs decreased HSC homing to the marrow and displaced them from the peri-vascular niche. In addition, leukemia mice displayed a marked loss of endosteal-lining osteoblasts, accompanied with a down-regulation of the transcription factors essential for the osteoblast differentiation in both osteoblastic progenitors and the multi-potent stromal cells (MSCs), and leukemia MSCs displayed a suppressed osteoblast formation when compared to control MSCs. In contrast, expressions of Notch signaling targets as well as CXCL12 were up-regulated in the leukemia stromal cells. The requirement of Notch in the alteration of leukemia niche function is supported by the observation that gamma-secretase inhibitor (GSI) treatment of mice receiving GSI-insensitive ICN1 cells attenuated the HSC suppression.
In summary, we found a potentially common mechanism employed by different types of acute leukemia to instructively modulate niche components essential for normal hematopoiesis. Specifically, we identified that T-ALL leukemia cells driven by ICN1 induces a hematopoietic suppression through competitive displacement of the normal HSCs from the proliferating peri-vascular niche and an ablation of the endosteal osteoblasts. Our future studies will target to elucidate the mechanistic co-regulation of normal and malignant hematopoiesis by leukemia microenvironment.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.