Effects of the chemokine stromal cell-derived factor-1 on the migration and localization of precursor-B acute lymphoblastic leukemia cells within bone marrow stromal layers

Acute lymphoblastic leukemia (ALL) blasts undergo migration into layers of bone marrow fibroblasts (BMF) in vitro, utilizing the beta 1 integrins VLA- 4 and VL-5 as adhesion molecules. However, it has been unclear as to whethe r this is a selective process mediated by specific chemoattractant molecule s, or simply a reflection of the highly motile nature of early B cell precu rsors. We further characterized this process using a transwell culture syst em, in which the two chambers were separated by an 8 mu m diameter micropor ous membrane, through which leukemic cells could move. When a BMF layer was grown on the upper surface of the membrane there was an 84.1% reduction in transmigration of the human pre-B ALL cell line NALM-6 into the lower cham ber, compared to control membrane with no BMF layer. Localization of leukem ic cells under the BMF layer was confirmed ultrastructurally, suggesting th e possibility that the migration of leukemic cells was directed by a chemot actic agent secreted by BMF. The involvement of the chemokine stromal cell- derived factor-1 (SDF-1) in this process was next investigated. BMF were sh own to express m-RNA for SDF-1. Addition of SDF-1 at 100 ng/ml into the low er chamber increased transmigration of NALM-6 across the membrane by 2.2-fo ld, and also induced a 1.4- to 6.1-fold increase in movement of NALM-6 thro ugh a BMF layer into the lower chamber. The receptor for SDF-1, CXCR4, was demonstrated by flow cytometry on all 10 cases of precursor-B ALL analyzed, as well as on NALM-6, KM-3 and REH lines. An inhibitory antibody to CXCR4 was able to block the migration of NALM-6 cells into BMF monolayers grown o n plastic by 51%, and in nine cases of ALL by 8-40%, as well as partially i nhibit transmigration of leukemic cells through BMF layers along an SDF-1 c oncentration gradient. These results confirm that precursor-B ALL cells sel ectively localize within bone marrow stroma in vitro, and that this process is partially due to the stromal chemokine SDF-1 binding to its receptor CX CR4 on leukemic cells. SDF-1 may be important in influencing the localizati on of precursor-B ALL cells in marrow microenvironmental inches which regul ate their survival and proliferation.