Adhesion of acute myeloid leukemia to bone marrow endothelial cells provides protection from chemotherap

Causes of refractory acute myeloid leukemia (AML) are unknown, but may be related to the bone marrow (BM) vascular network given the close relationship between hematopoiesis and the vasculature. We hypothesized that bone marrow endothelial cells (BMECs) provide a protective advantage to AML cells. To test this hypothesis, co-cultures of AML cells and BMECs were exposed to increasing doses of cytarabine chemotherapy. There was a 2-fold decrease in leukemia cell death of AML cells when adhered to BMECs compared to non-adhered. Even irradiated BMECs protected AML cells from cytarabine chemotherapy. A protein array was used to show an upregulation of adhesion molecules on BMEC membranes during chemotherapy treatment. One novel adhesion molecule, BCAM, is an important player in cell adhesion mediated drug resistance of AML cells. When we co-cultured AML cells with BMECs in the presence of a BCAM blocking peptide, the protective effect of the AML-BMEC interaction was reversed. Cell cycle analysis shows an increase of S phase AML cells in presence of BCAM blocking peptide. Together, these in vitro results support the concept that ECs may be a protective reservoir for AML cells, at a minimum by means of adhesion molecules.;Prior intravital efforts have focused on calvaria bone, which may over-represent the endosteal niche and under-represent the vascular niche due to the very close approximation of bone surfaces. Therefore, we created an intravital animal model of human AML to track single AML cells in the bone marrow of mouse long bones. In brief, we irradiated NOD/scid/IL2Rgammanull (NSG) mice, drilled a window on the tibia surface, xenotransplanted fluorescently tagged human AML cells via IV injection, and then analyzed the tibias by fluorescent microscopy for the presence of AML cells at various time points after transplant. This technique was also used to follow the resurgence of AML cells after chemotherapy treatment to predict the areas of relapse.;Transwell assays of AML cells in culture with BMECs also provided protection from chemotherapy. AML cell lines were shown to express receptors also found on ECs such as VEGFR, PDGFR, and cKit. Pazopanib is a receptor tyrosine kinase inhibitor of these receptors and was able to remove the protection provided by ECs during chemotherapy treatment. Xenograft models of AML showed regression of AML and reduction of BM vascularity when treated with pazopanib.;Angiogenic cytokines such as IL-8 present in the bone marrow of AML patients promote the proliferation and survival of leukemic blasts during chemotherapy treatment. The addition of an IL-8 inhibitor significantly reduced AML cell proliferation and viability, and enhanced the cytotoxic ability of cytarabine treatment.;These studies suggest an adhesion and soluble mediated protective environment present in the bone marrow that can be abrogated when blocked or interfered with appropriate strategies.