Vascular Niche E-Selectin Protects Acute Myeloid Leukaemia Stem Cells from Chemotherapy

We recently reported that vascular adhesion molecule E-selectin is a key component of the bone marrow vascular niche, ‘awakening’ otherwise dormant Haematopoietic Stem Cells (HSC) (Winkler et al., Nat Med 2012). Following cytotoxic chemotherapy or radiation injury, E-selectin expression in the bone marrow increases ~10 to 20-fold during the recovery phase, at a time when HSC must cycle to replenish the blood and immune systems. When E-selectin is absent (in gene deleted mice) or E-selectin is therapeutically blocked using the small molecule glycomimetic antagonist GMI-1271, a greater proportion of HSC return to quiescence following radiation or chemotherapy.

We now report cell surface E-selectin to be also upregulated 5 to 10-fold on the BM vasculature in mice with acute myeloid leukaemia (AML). This raises the interesting question: how do AML leukaemia stem cells (LSC) respond to E-selectin at the vascular niche? Using models of murine AML generated by retroviral transduction of the MLL-AF9 fusion oncogene into HSC, we found leukemic blasts rapidly upregulate E-selectin binding potential upon oncogenic transformation. In fact targeted disruption of these E-selectin-mediated interactions by administration of GMI-1271 injection distrupts adhesion and localization of AML cells and was sufficient to continually mobilise leukaemic blasts into the blood for at least 24 hours after a single injection at 40 mg/kg, suggesting that E-selectin-mediated interactions play a role in retaining LSC within BM niches.

We next queried whether E-selectin-mediated signalling may help promote LSC survival following therapy. To test this, cohorts of 20 wildtype or 20 E-selectin knock-out mice were transplanted with the same AML cells, then 4 weeks later, half were treated with high dose cytarabine (2 x 900mg/kg at 12hour interval) while the other half remained untreated. At 24 hours after the first cytarabine injection, BM cells were harvested to measure numbers of surviving functional LSC by limiting-dilution transplantation assays in irradiated wild-type syngenic recipients and the proportion of these recipients that developed leukemia was used to calculate the original number of surviving LSC by Poisson’s distribution. We found that although the absence of E-selectin had no effect on total LSC numbers per femur, the absence of E-selectin dramatically increased sensitivity of LSC to cytarabine treatment (20-fold). These results indeed suggest that E-selectin is a key vascular niche component mediating LSC chemoresistance.

Our data are also consistent with previous xenograft models in immune-deficient mice showing that the few human CD34+ AML LSC that survived chemotherapy, were observed clustered around endosteal vascular endothelium in recipient mice (Ishikawa et al., Nat BioTechnol 2007; Ninomiya et al., Leukemia 2007) where E-selectin is expressed. In summary our data confirm that niche factors alone can strongly influence LSC sensitivity to chemotherapy, and suggest a chemoprotective role for the vascular adhesion molecule E-selectin which is upregulated in the bone marrow of leukaemic mice, Taken together, these data identify E-selectin as a novel therapeutic target for the treatment of AML leukemic stem cells in that in vivo inhibition by the small molecule glycomimetic E-selectin antagonist GMI-1271 may improve chemosensitivity.