ROLE OF BETA-1-INTEGRINS AND BETA-2-INTEGRINS IN THE ADHESION OF HUMAN CD34HI STEM-CELLS TO BONE-MARROW STROMA

Hematopoietic stem cell interaction with elements of the underlying stroma is essential for sustained normal hematopoiesis. Here we have determined that adhesion receptors in the integrin family play a role in promoting adhesion of human hematopoietic stem cells to cultured human marrow stromal cells. Enriched CD34hi progenitor cells expressed VLA-4, VLA-5, and at least one or more beta-2 integrins. Homogeneous marrow stromal cell monolayers capable of supporting proliferation of cocultivated CD34hi cells expressed VCAM-1 and fibronectin (ligands for VLA-4 and VLA-5) as well as ICAM-1 (ligand for LFA-1 and Mac-1). Adhesion-blocking experiments indicated that VLA-4/VCAM-1, VLA-5/fibronectin, and beta-2-integrin/ICAM-1 pathways all are important for CD34hi cell attachment to stromal cells. Consistent with this suggestion, IL-1 stimulation of stromal cells caused both increased VCAM-1 and ICAM-1 expression and increased attachment by CD34hi bone marrow cells. In addition, CD34hi cells utilized VLA-4 to adhere to purified VCAM-1 and employed VLA-5 (and to a lesser extent VLA-4) to adhere to purified fibronectin. Together these results suggest that CD34hi stem cells may utilize multiple integrin-mediated adhesion pathways to localize within specialized microenvironmental niches created by marrow stromal cells.