Ryan Bertoli, Yang Jo Chung, and Peter Aplan

• Submitter: Ryan Bertoli (Student or postbac)
• email: RYAN.BERTOLI@NIH.GOV

Introduction: Acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML) are aggressive forms of blood cancer which are difficult to treat and often fatal. These cancers hinder hematopoiesis, or the ability to produce new blood cells of diverse lineage. Leukemic cell expansion often leads to bone marrow failure, caused by a depletion of hematopoietic stem and progenitor cells (HSPCs) in the marrow.

While bone marrow failure is thought to be mediated by the loss of HSPCs triggered by expansion of leukemic cells in the marrow, the mechanisms underlying this process remain unclear. In this study, we aim to characterize the mechanisms by which HSPCs are depleted in the bone marrow in both AML and ALL, including the specific interactions between leukemic cells and normal HSPCs, the agents mediating these interactions, the functional output of affected HSPC’s, and any differences in the process between ALL and AML in these assays.

Methods: AML and B-cell precursor ALL were generated by transplantation of the 961C and T259 cell lines respectively in B6/Ly5.1 recipient mice. Neither cell line requires irradiation of mice for engraftment. Groups of mice were euthanized at pre-determined conditions, based on time or blood parameters, to examine rates of leukemic engraftment. Complete Blood Counts (CBCs), Multi-Potent Progenitor (MPP) FACS analysis, and morphology were used to characterize the change in hematopoiesis.

Results: Preliminary results show that mice with AML demonstrate severe anemia and thrombocytopenia, along with a depletion of HSPCs. In addition, the loss of wild type hematopoietic cells was much greater than the gain of leukemic cells in the bone marrow. Finally, although there was a clear decrease in WT HSPCs in the bone marrow, a compensatory 10x increase in HSPCs was found in the spleen and peripheral blood of leukemic mice vs. WT mice.

In contrast, in mice engrafted with ALL, anemia was not evident, despite WBC counts >200K/uL. In addition, although the total number of WT hematopoietic cells in the bone marrow was decreased, the proportion of WT HSPCs in the bone marrow was increased with respect to non-leukemic WT mice. In contrast to AML, the loss of WT hematopoietic cells was equivalent to the gain of leukemic cells in the bone marrow. Finally, the spleen in this condition displayed a 100x increase in native HSPCs, matching or even exceeding the HSPC numbers found in a healthy bone marrow.

Conclusions: The results from these experiments demonstrate a clear distinction between the interaction of healthy HSPCs with AML vs ALL, and provide clues to elucidate the mechanisms behind these effects. In the case of AML, the leukemic cells may directly affect the healthy HSC, either through niche competition for resources, or even through secretion or cell-cell interactions. In contrast, in the case of ALL, there may be a physical competition for space within the bone marrow, causing the displaced but still functional HSC’s to engraft in the spleen and provide the needed life-sustaining extra medullary hematopoiesis.