The Chemokine Receptor CXCR4 Enhances Integrin-mediated in Vitro Adhesion and Facilitates Engraftment of Leukemic Precursor-B Cells in the Bone Marrow

Objective: It has been demonstrated that acute lymphoblastic leukemia (ALL) blasts migrate into layers of bone marrow fibroblasts (BMF) in vitro using the beta1 integrins VLA-4 and VLA-5, and that the chemokine SDF-1 and its receptor CXCR4 influences ALL migration. We investigated whether this effect was due to SDF-1-mediated induction of adhesion through beta1 integrins.

Methods: Adhesion of pre-B ALL cells or the cell line NALM6 to extracellular matrix proteins was examined using short-term in vitro binding assays. The effects of exposure of cells to SDF-1, antibodies to CXCR4, and the G protein inhibitor pertussis toxin (PTX) were assessed. The consequences of down regulation of CXCR4 on the in vivo behavior of pre-B ALL cells after injection into sublethally irradiated NOD/SCID mice was studied.

Results: Treatment with SDF-1 of NALM6 cells or cells from cases of precursor-B ALL resulted in a doubling of adhesion to fibronectin, laminin, and VCAM-1, but had no effect on binding to collagens I or IV. Antibodies to CXCR4 and PTX inhibited SDF-1-induced adhesion on these substrates. NALM6 cells with CXCR4 expression downregulated by SDF-1 exposure demonstrated a reduced capacity to engraft into the bone marrow of NOD/SCID mice, with only 22 +/- 11% of marrow cells being of human origin in mice receiving SDF-1-treated cells compared to 48 +/- 5% in mice receiving untreated cells (p < 0.001). The homing of SDF-1-treated cells to the bone marrow after 24 hours was also reduced by 72 +/- 16% compared to control cells.

Conclusions: These data show that SDF-1 and CXCR4 are involved in regulation of beta1 integrin function, and are important for the localization of pre-B cells to the bone marrow in vivo.