Leukemic Hierarchy and Systemic Factors Dictating Disease Evolution In Chronic Myelogenous Leukemia

Abstract 886

Chronic myelogenous leukemia (CML) is a clonal myeloproliferative neoplasm (MPN) characterized by the t(9;22)(q34;q11) reciprocal translocation, which leads to the expression of the BCR/ABL fusion protein. CML is clinically characterized by the abnormal expansion of the myeloid lineage leading to progressive peripheral neutrophilia. This indolent disease has the propensity to evolve into an acute leukemia-like blastic phase involving either myeloid or B lymphoid cells. As such, CML constitutes a paradigm for understanding cellular and molecular events controlling chronic diseases and their evolution toward frank leukemia. CML arise from a small pool of leukemic stem cells (LSC) that can be operationally defined by their ability to sustain tumor growth over time and to transplant leukemia to recipient mice. Multiple lines of evidence indicate that LSCs originate from abnormally behaving hematopoietic stem cells (HSCs), although it is still largely unknown how BCR/ABL affects the biology of HSCs and the fate of downstream progenitor cells. To study the impact of BCR/ABL expression on these compartments, we used a transgenic mouse model that allows for inducible expression of BCR/ABL in HSCs and downstream progeny (Blood 105: 324, 2005). Induction of BCR/ABL expression in this model recapitulated many features of human CML such as myeloid bone marrow (BM) hyperplasia, myelofibrosis, splenomegaly and myeloid cell infiltration in non-hematopoietic organs. Disease development also correlated with a dramatic reorganization of the stem and progenitor compartments in the BM and their relocation/expansion in the spleen. In the BM, we observed a marked reduction in the number of Lin^– Sca1⁺ c-Kit⁺ Flk2^– CD48^– CD150⁺ long-term (LT) and Lin^– Sca1⁺ c-Kit⁺ Flk2^– CD48^– CD150^– short-term (ST) HSCs associated with an expansion of several types of non-self-renewing multipotent progenitors (MPP) and myeloid committed progenitors. We confirmed by limited dilution transplantation experiments that BCR/ABL⁺ LT-HSCs were true LSCs as they were the only cells capable of transferring sustained CML disease in recipient mice with as few as 50 cells injected. We also found that BCR/ABL expression impacts on the biology of the ST-HSC and Lin^– Sca1⁺ c-Kit⁺ Flk2⁺ MPP compartments. Transplantation of both populations induced dramatic but transient hyperplasia, which could eventually mimic a leukemic phenotype with high doses of cell injected (4,000 cells per mouse). However, while transplantation of BCR/ABL⁺ ST-HSCs led to the expected myeloid hyperplasia, transplantation of BCR/ABL⁺ MPPs led to a massive accumulation of B-cell progenitors in the BM that resembles lymphoid blast crisis. Strikingly, co-transplantation of 4,000 BCR/ABL⁺ LT- or ST-HSCs with 4,000 BCR/ABL⁺ MPPs almost always resulted in myeloid hyperplasia suggesting an active inhibition of MPP-derived lymphoid progeny by the leukemic myeloid compartment. We reasoned that the molecular effectors for this lymphoid inhibitory effect could be extracellular signaling molecules that will be detectable in the serum of CML-developing BCR/ABL mice. Using antibody arrays and enzyme-linked immunosorbent assays (ELISA), we found that the serum concentration of the proinflammatory cytokine interleukine-6 (IL-6) correlates with CML progression both in primary and transplanted mice. Moreover, we showed in vitro that IL-6 controls lineage fate decision of leukemic progenitors by promoting myeloid differentiation from MPPs at the expense of the B lymphoid lineage differentiation. As such, IL-6 targets both normal and malignant MPPs thereby providing a positive feedback loops that promote CML development. In summary, our results identify and functionally characterize a pathological hierarchy in CML that includes LSC and immature leukemic progenitors. They uncover a novel fate-regulatory mechanism at the systemic level that controls the differentiation outcome of the leukemic progenitors and can have key implication for disease progression. Taken together, they demonstrate that CML evolution is the result of a balance between BCR/ABL cell intrinsic effects and environmental cues and provide a rational for the paradoxical myeloid-lymphoid conversion that can be observed during lymphoid blast crisis.