Differential Expression of Homing-Associated Cell Adhesion Molecule, Very Late Antigen-4 and L-Selectin in Hematopoietic Progenitor Cell Trafficking Between the Marrow and Blood

This study addresses the hypothesis that the following cell adhesion molecules (CAMs): homing-associated cell adhesion molecule (HCAM), very late antigen-4 (VLA-4) and L-selectin play a role in the trafficking of hematopoietic progenitor cells (HPCs) between the bone marrow microenvironment and the peripheral circulation. In order to ascertain differences in CAM expression based on physiologic compartment, the expression of HCAM, VLA-4 or L-selectin per CD34+ myeloid progenitor cell was assessed between paired samples of blood and marrow. CAM expression was flow cytometrically quantitated in paired samples obtained from patients treated with mobilizing doses of granulocyte-colony stimulating factor (G-CSF) or from normal donors donating for allogeneic transplant. In G-CSF mobilized patients, marrow derived CD34+ myeloid progenitor cells expressed more VLA-4 per cell than those in circulation. In normal donors, marrow derived myeloid progenitor cells expressed more CD34 per CAM expressing (HCAM+, VLA-4+ or L-selectin+) CD34+ myeloid progenitors, colony forming unit (CFU) and long term culture initiating cell (LTCIC) assays of flow cytometrically sorted normal marrow and blood CAM+/-CD34+ myeloid progenitors were performed. L-selectin+CD34+ myeloid progenitors formed a greater percentage of BFU-E colonies and a lower percentage of CFU-GM colonies than all other CAM+/-CD34+ myeloid progenitors sorted from normal blood. In normal donors, CAM+/-CD34+ myeloid progenitors sorted from blood formed significantly more colonies per 106 plated cells than those derived from marrow. L-selectin+CD34+ myeloid progenitors derived from marrow contained significantly more LTCIC (per 106 sorted CAM+CD34+ myeloid progenitors) than those expressing HCAM or VLA-4. In order to determine whether CD34+ myeloid progenitors utilize VLA-4 to bind to fibronectin (FN), in vitro binding assays were performed. Adhesion of normal blood derived VLA-4+CD34+ myeloid progenitors to FN was blocked by the addition of monoclonal antibodies against the α4 subunit of VLA-4. These data suggest a model of HPC trafficking, in which HPCs utilize VLA-4 to adhere to components of the bone marrow microenvironment, while HPC modulation of L-selectin affinity plays an important role in HPC homing and a less direct role in hematopoietic reconstitution