Trafficking of human CD34 ؉ stem/progenitor cells (HSCs/HPCs) is regulated by chemokines, cytokines, proteolytic enzymes, and adhesion molecules. We report that the adhesion receptor CD44 and its major ligand, hyaluronic acid (HA), are essential for homing into the bone marrow (BM) and spleen of nonobese diabetic/ severe combined immunodeficient (NOD/ SCID) mice and engraftment by human HSCs. Homing was blocked by anti-CD44 monoclonal antibodies (mAbs) or by soluble HA, and it was significantly impaired after intravenous injection of hyaluronidase. Furthermore, stromal cellderived factor-1 (SDF-1) was found to be a rapid and potent stimulator of progenitor adhesion to immobilized HA, leading to formation of actin-containing protrusions with CD44 located at their tips. HPCs migrating on HA toward a gradient of SDF-1 acquired spread and polarized morphology with CD44 concentrating at the pseudopodia at the leading edge. These morphologic alterations were not observed when the progenitors were first exposed to anti-CD44 mAbs, demonstrating a crosstalk between CD44 and CXCR4 signaling. Unexpectedly, we found that HA is expressed on human BM sinusoidal endothelium and endosteum, the regions where SDF-1 is also abundant. Taken IntroductionThe outcome of hematopoietic stem cell transplantation is influenced by the ability of the cells to home and repopulate their specialized bone marrow (BM) niches. The crosstalk between the hematopoietic stem/progenitor cells (HSCs/HPCs) and the microenvironment, which regulates homing to the BM, is not fully elucidated. Data indicate that transplanted HSCs/HPCs lodge into their BM niches by a sequence of highly regulated events that mimic the migration of leukocytes to inflammatory sites. This process includes tethering and rolling on E-and P-selectins, firm adhesion to the vessel wall, transendothelial extravasation, and migration through the extracellular matrix (ECM). [1][2][3] This multistep process is mediated by an interplay between chemokines, growth factors, proteolytic enzymes, and adhesion molecules. 4,5 The chemokine stromal cell-derived factor-1 (SDF-1), also named CXCL-12, and its receptor, CXCR4, play key roles in human HSC trafficking and repopulation. 6 This chemokine, expressed by both human and murine BM endothelium and stroma, 7,8 is the most powerful chemoattractant of HSCs/HPCs 9,10 that also regulates their survival. 11,12 It induces the integrin-mediated firm arrest of human HPCs under physiologic shear flow, facilitates their transendothelial migration, 3,8 and regulates homing 13 and BM engraftment. 14 Furthermore, SDF-1 is also required for the retention of murine stem and progenitor cells within the BM. 15,16 HSCs/HPCs express several types of adhesion molecules that are responsible for cell-cell and cell-ECM interactions 17 ; among them CD44 is of particular interest.The importance of CD44 in cell migration is reported for a variety of normal and malignant cells. 18 CD44 is a multifunctional and multistructural receptor that has a large array of isoforms....

Stem cell homing into the bone microenvironment is the first step in the initiation of marrow-derived blood cells. It is reported that human severe combined immunodeficient (SCID) repopulating cells home and accumulate rapidly, within a few hours, in the bone marrow and spleen of immunodeficient mice previously conditioned with total body irradiation. Primitive CD34 ؉ CD38 ؊/low CXCR4 ؉ cells capable of engrafting primary and secondary recipient mice selectively homed to the bone marrow and spleen, whereas CD34 ؊ CD38 ؊/low Lin ؊ cells were not detected. Moreover, whereas freshly isolated CD34 ؉ CD38 ؉/high cells did not home, in vivo stimulation with granulocyte colony-stimulating factor as part of the mobilization process, or in vitro stem cell factor stimulation for 2 to 4 days, potentiated the homing capabilities of cytokinestimulated CD34 ؉ CD38 ؉ cells. Homing of enriched human CD34 ؉ cells was inhibited by pretreatment with anti-CXCR4 antibodies. Moreover, primitive CD34 ؉ -CD38 ؊/low CXCR4 ؉ cells also homed in response to a gradient of human stromal cell-derived factor 1 (SDF-1), directly injected into the bone marrow or spleen of nonirradiated NOD/SCID mice. IntroductionDuring development hematopoietic stem cells migrate from the fetal liver into the bone marrow (BM) and continuously produce maturing hematopoietic cells that are released into the blood circulation. Hematopoietic stem cells are functionally defined, based on their ability to home to the BM microenvironment and to durably repopulate transplanted recipients with both myeloid and lymphoid cells. [1][2][3] In vivo repopulating assays for human stem cells have been developed by several groups, including ours. [4][5][6][7] In these assays human severe combined immunodeficient (SCID) repopulating cells (SRCs), characterized as CD34 ϩ CD38 Ϫ or as CD34 ϩ CD38 Ϫ/low cells, engraft the BM of sublethally irradiated immune-deficient NOD/ SCID and NOD/SCID/B2m null mice with high levels of myeloid and lymphoid cells. 7,8 Other primitive cells such as CD34 ϩ CD38 ϩ or CD34 Ϫ CD38 Ϫ cells were also found to have limited repopulation potential. 9,10 The chemokine stromal cell-derived factor 1 (SDF-1), which is also secreted by stromal cells in the BM microenvironment, was shown to attract human CD34 ϩ cells in vitro. [11][12][13] Recently we reported that engraftment of human SRC is dependent on the expression of SDF-1 and its receptor CXCR4, recharacterizing SRCs as CD34 ϩ CD38 Ϫ/low CXCR4 ϩ cells with major stem cell properties. 14 To home from the blood circulation into the BM microenvironment, hematopoietic stem and progenitor cells must first roll on E and P selectins, which are expressed on the BM vascular endothelial cells. [15][16][17] Following firm arrest and adhesion to the vessel wall under shear flow, a process mediated by the major integrins (VLA-4, VLA-5, and LFA-1) and their vascular ligands (VCAM-1 and ICAM-1), the cells extravasate through the endothelium into the hematopoietic compartment. 18-22 SDF-1 is constitutively expressed by ...

Mast cells, essential effector cells in allergic inflammation, have been found to be activated in T cell-mediated inflammatory processes in accordance with their residence in close physical proximity to T cells. We have recently reported that mast cells release granule-associated mediators and TNF-α upon direct contact with activated T cells. This data suggested an unrecognized activation pathway, where mast cells may be activated during T cell-mediated inflammation. Herein, we show that this cell-cell contact results in the release of matrix metalloproteinase (MMP)-9 and the MMP inhibitor tissue inhibitor of metalloproteinase 1 from HMC-1 human mast cells or from mature peripheral blood-derived human mast cells. The expression and release of these mediators, as well as of β-hexosaminidase and several cytokines, were also induced when mast cells were incubated with cell membranes isolated from activated, but not resting, T cells. Subcellular fractionation revealed that the mature form of MMP-9 cofractionated with histamine and tryptase, indicating its localization within the secretory granules. MMP-9 release was first detected at 6 h and peaked at 22 h of incubation with activated T cell membranes, while TNF-α release peaked after only 6 h. Anti-TNF-α mAb inhibited the T cell membrane-induced MMP-9 release, indicating a possible autocrine regulation of MMP release by mast cell TNF-α. This cascade of events, whereby mast cells are activated by T cells to release cytokines and MMP-9, which are known to be essential for leukocyte extravasation and recruitment to affected sites, points to an important immunoregulatory function of mast cells within the context of T cell-mediated inflammatory processes.