Cytokine-Dependent Imatinib Resistance in Mouse Bcr-Abl(+), Arf-Null Lymphoblastic Leukemia.

Retroviral vector-mediated transduction of the BCR-ABL tyrosine kinase into Arf-deficient mouse bone marrow progenitors allows rapid ex vivo outgrowth of pure pre-B cell populations that induce a highly aggressive form of ALL when inoculated intravenously into healthy syngeneic mice. These leukemias resist therapy with the BCR-ABL kinase inhibitor imatinib, and surprisingly, drug resistance is non-tumor-cell autonomous. We now demonstrate that expression of the p185 BCR-ABL isoform (hereafter p185) in Arf-null pre-B cells is sufficient to generate polyclonal populations of leukemia-initiating cells (LICs), only 20 of which induce fatal, imatinib-resistant disease within one month after infusion. These p185+ Arf-null LICs are pre-B cells by immunophenotypic criteria, lack evidence of myeloid and stem cells markers, exhibit immunoglobulin heavy chain gene rearrangements and random proviral insertions, and are almost universally capable of initiating lethal, transplantable leukemias in healthy syngeneic recipient mice. Therefore, these LICs do not represent rare ‘cancer stem cells’. Resistance to targeted therapy with imatinib in this model system is not dependent upon mutations within the BCR-ABL kinase domain, but, rather, is mediated through interactions between the LICs and the host environment. While cytokines like IL-7 are dispensable for the efficient generation of p185+, Arf-null cells, these LICs remain IL-7-responsive and are significantly protected from imatinib-induced cytostasis by IL-7 in vitro. Introduction of p185 into Arf-null hematopoietic progenitors that also lack the common gamma chain (γ_c) for cytokine receptors bypasses their cytokine requirements and permits pre-B cell development. These p185+, Arf-null, γ_c-null LICs also initiate ALL but exhibit much greater sensitivity to imatinib in vivo, allowing a significant fraction of treated mice to sustain long term remissions even when therapy is discontinued. Therefore, salutary cytokines expressed in the hematopoietic microenvironment facilitate leukemic proliferation and confer resistance to targeted therapy. Not surprisingly, single agent therapy with dasatinib, a second generation BCR-ABL kinase inhibitor, is dramatically more efficacious in this pre-clinical Ph+ ALL model, although some recipient mice still succumb to disease despite continuous drug treatment. Using vectors co-expressing p185 and luciferase, we are now initiating dasatinib therapy when recipient animals have detectable disease burdens, thereby permitting a more dynamic assessment of therapeutic response and subsequent failure. We speculate that in human Ph+ ALL, the frequent deletion of the INK4A-ARF locus (∼50% of cases at diagnosis) might help to maintain LIC survival in the face of targeted therapy and so facilitate the emergence of drug-resistant BCR-ABL variants. As such, INK4A-ARF deletion might prove to be a poor prognostic indicator.