Selectins and Their Ligands Are Required for Homing and Engraftment of BCR-ABL+ Leukemia-Initiating Cells.

Autologous hematopoietic stem cell (HSC) transplantation is a feasible form of treatment for many types of leukemias and lymphomas, including chronic myeloid leukemia (CML). Malignant cells contaminating the graft, however, can engraft and lead to relapse of the original disease. Previous studies have demonstrated that BCR-ABL+ leukemic progenitors have defects in the adhesive function of beta-1 integrins and in their response to the chemokine SDF-1alpha, pathways that are critical for homing and engraftment of normal HSC. We hypothesized that BCR-ABL-expressing leukemic stem cells differ from normal HSC in their homing and engraftment properties. Using a retroviral transduction/transplantation model of CML and donor/recipient mice with mutations in adhesion molecules, we investigated the role of specific adhesion pathways in the engraftment of CML-like leukemia. We found no difference in the expression levels of integrins, LFA-1, and CXCR4 between normal and BCR-ABL+ c-Kit+ Lin- cells, but lower expression levels of P-selectin glycoprotein ligand-1 (PSGL-1) and of L-selectin. In transplantation experiments, VCAM-1, the principal bone marrow ligand for beta-1 integrin, was not required in the bone marrow endothelium of the recipient for efficient engraftment of CML-like disease, confirming that progenitors capable of initiating CML-like leukemia upon transplantation are independent of the beta-1 integrin pathway for engraftment. Likewise, recipient P-selectin was also not required for the engraftment of CML-like leukemia. By contrast, deficiency of PSGL-1 in the leukemic cells or of E-selectin in the recipient significantly reduced engraftment by BCR-ABL-expressing stem cells, as assessed by Southern blot quantitation of proviral clone frequency. The requirement for recipient E-selectin could be bypassed by direct intrafemoral injection of BCR-ABL-expressing cells, leading to polyclonal leukemia. BCR-ABL-expressing cells that were deficient for the selectin ligand-synthesizing enzymes Core-2 or Fucosyltransferases IV and VII also exhibited decreased engraftment and increased disease latency. Treatment of BCR-ABL-transduced cells with neuraminidase, which destroys selectin binding sites, completely blocked leukemic engraftment. Whereas L-selectin has no role in homing and engraftment of normal HSC, BCR-ABL-expressing L-selectin-deficient progenitors were profoundly defective for engraftment, with decreased disease clonality, increased disease latency, and frequent death of recipients from graft failure. Importantly, efficient engraftment and leukemogenesis of BCR-ABL-expressing L-selectin-deficient cells was restored by co-expression of a chimeric E/L-selectin molecule that is resistant to cell surface shedding. These results establish that BCR-ABL-expressing leukemic stem cells rely to a greater extent on selectins and their ligands for homing and engraftment than normal HSC. Specific blocking of selectin-ligand interactions is a novel clinical strategy to exploit the differences in normal and Ph+ stem cells that may be beneficial in an autologous transplantation setting.