Interactions with the bone marrow microenvironment as determinant of biological phenotype and myeloid differentiation in imatinib-resistant chronic myeloid leukemia

Mutations within BCR-ABL1, the oncogene giving rise to chronic myeloid leukaemia (CML), can lead to resistance to tyrosine kinase inhibitors and some are associated with clinically more aggressive disease and worse outcome (Branford et al., 2003). This phenomenon was faithfully recapitulated in our murine transduction/transplantation model of BCR-ABL1+ CML, in which recipients of bone marrow transduced with the imatinib-resistant BCR-ABL1 point mutant T315I (or Y253F) had shortened survival, compared to native BCR-ABL1. To test how BCR-ABL1-positive leukaemia-initiating cells may interact with the bone marrow microenvironment (BMM), we used a combination of confocal and 2-photon intravital microscopy of murine calvarium. We showed that BCR-ABL1T315I + Lin– c-Kit+ Sca-1+ (LKS) cells, homed closer to osteoblastic cells than LKS cells expressing native BCR-ABL1 which suggests a differential homing location of BCR-ABL1T315I leukemic stem cells (LSC) and may be due to differences in adhesion or migration. Indeed, immunofluorescence revealed an altered actin cytoskeleton and striking differences in focal adhesion kinase in BCR-ABL1T315I+ compared to native BCR-ABL1 cells. Additionally, integrin β3, was upregulated in BCR-ABL1T315I cells. Consistent with the situation in humans we found an increase in the number of immature blast-like cells and alterations of the immunophenotypic composition of cells in the bone marrow of murine recipients of BCR-ABL1T315I+. Furthermore, the deposition of fibronectin by BCR-ABL1T315I + cells was decreased compared to native BCR-ABL1 cells. Intrafemoral or intravenous administration of fibronectin, however, led to a significant prolongation of survival in 70% of mice with BCR-ABL1T315I + CML. In summary, these data suggest that interactions with BMM via the integrin β3-mediated signaling pathway may regulate myeloid differentiation and clinical outcome in BCR-ABL1T315I+ imatinib-resistant CML. Targeting the interaction of BCR-ABL1T315I+ leukemia cells with the BMM in the form of fibronectin may offer a beneficial, innovative, adjuvant treatment strategy in patients with BCR-ABL1T315I+ CML.