Stem cell homing and repopulation are not well understood. The chemokine stromal cell–derived factor-1 (SDF-1) and its receptor CXCR4 were found to be critical for murine bone marrow engraftment by human severe combined immunodeficient (SCID) repopulating stem cells. Treatment of human cells with antibodies to CXCR4 prevented engraftment. In vitro CXCR4-dependent migration to SDF-1 of CD34 ⁺ CD38 ^−/low cells correlated with in vivo engraftment and stem cell function. Stem cell factor and interleukin-6 induced CXCR4 expression on CD34 ⁺ cells, which potentiated migration to SDF-1 and engraftment in primary and secondary transplanted mice. Thus, up-regulation of CXCR4 expression may be useful for improving engraftment of repopulating stem cells in clinical transplantation.

In vivo, leukocyte transendothelial migration (TEM) occurs at endothelial cell junctions (paracellular) and nonjunctional (transcellular) locations, whereas in vitro models report that TEM is mostly paracellular. The mechanisms that control the route of leukocyte TEM remain unknown. Here we tested the hypothesis that elevated intercellular adhesion molecule-1 (ICAM-1) expression regulates the location of polymorphonuclear leukocyte (PMN) TEM. We used an in vitro flow model of tumor necrosis factor-α (TNF-α)-activated human umbilical vein endothelium cells (HUVECs) or an HUVEC cell line transfected with ICAM-1GFP (green fluorescent protein) and live-cell fluorescence microscopy to quantify the location of PMN adhesion and TEM. We observed robust transcellular TEM with TNF-α-activated HUVECs and ICAM-1GFP immortalized HUVECS (iHUVECs). In contrast, primary CD3⁺ T lymphocytes exclusively used a paracellular route. Endothelial ICAM-1 was identified as essential for both paracellular and transcellular PMN transmigration, and interfering with ICAM-1 cytoplasmic tail function preferentially reduced transcellular TEM. We also found that ICAM-1 surface density and distribution as well as endothelial cell shape contributed to transcellular TEM. In summary, ICAM-1 promotes junctional and nonjunctional TEM across inflamed vascular endothelium via distinct cytoplasmic tail associations. (Blood. 2005;106:584-592)

Selectins have previously been shown to tether a flowing leukocyte to a vessel wall and mediate rolling. Here, we report that an intergrin, VLA-4, can also support tethering and rolling. Blood T lymphocytes and alpha 4 integrin-transfected cells can tether in shear flow, and then roll, through binding of the intergrin VLA-4 to purified VCAM-1 on the wall of a flow chamber. VLA-4 transfectants showed similar tethering and rolling on TNF-stimulated endothelium. Tethering efficiency, rolling velocity, and resistance to detachment are related to VCAM-1 density. Tethering and rolling did not occur on ICAM-1, fibronectin, or fibronectin fragments, and tethering did not require integrin activation or the presence of an alpha 4 cytoplasmic domain. Arrest of rolling cells on VCAM-1 occurred spontaneously, and/or was triggered by integrin activating agents Mn2+, phorbol ester, and mAb TS2/16. These agents, and the alpha 4 cytoplasmic domain, promoted increased resistance to detachment. Together the results show that VLA-4 is a versatile integrin that can mediate tethering, rolling, and firm arrest on VCAM-1.

The chemokine SDF-1 plays a central role in the repopulation of the bone marrow (BM) by circulating CD34⁺ progenitors, but the mechanisms of its action remain obscure. To extravasate to target tissue, a blood-borne cell must arrest firmly on vascular endothelium. Murine hematopoietic progenitors were recently shown in vivo to roll along BM microvessels that display selectins and integrins. We now show that SDF-1 is constitutively expressed by human BM endothelium. In vitro, human CD34⁺ cells establish efficient rolling on P-selectin, E-selectin, and the CD44 ligand hyaluronic acid under physiological shear flow. ICAM-1 alone did not tether CD34⁺ cells under flow, but, in the presence of surface-bound SDF-1, CD34⁺ progenitors rolling on endothelial selectin rapidly developed firm adhesion to the endothelial surface, mediated by an interaction between ICAM-1 and its integrin ligand, which coimmobilized with SDF-1. Human CD34⁺ cells accumulated efficiently on TNF-activated human umbilical cord endothelial cells in the absence of SDF-1, but they required immobilized SDF-1 to develop firm integrin-mediated adhesion and spreading. In the absence of selectins, SDF-1 also promoted VLA-4–mediated, Gi protein–dependent tethering and firm adhesion to VCAM-1 under shear flow. To our knowledge, this is the first demonstration that SDF-1 expressed on vascular endothelium is crucial for translating rolling adhesion of CD34⁺ progenitors into firm adhesion by increasing the adhesiveness of the integrins VLA-4 and LFA-1 to their respective endothelial ligands, VCAM-1 and ICAM-1.