Selective lodgement or homing of transplanted hemopoietic stem cells in the recipient's bone marrow (BM) is a critical step in the establishment of long-term hemopoiesis after BM transplantation. However, despite its biologic and clinical significance, little is understood about the process of homing. In the present study, we have concentrated on the initial stages of homing and explored the functional role in vivo of some of the adhesion pathways previously found to mediate in vitro adhesion of hemopoietic cells to cultured BM stroma. We have found that homing of murine hemopoietic progenitors of the BM of lethally irradiated recipients at 3 h after transplant was significantly reduced after pretreatment of the donor cells with an antibody to the integrin very late antigen 4 (VLA4). This inhibition of marrow homing was accompanied by an increase in hemopoietic progenitors circulating in the blood and an increased uptake of these progenitors by the spleen. Similar results were obtained by treatment of the recipients with an antibody to vascular cell adhesion molecule 1 (VCAM-1), a ligand for VLA4. Furthermore, we showed that administration of the same antibodies (anti-VLA4 or anti-VCAM-1) to normal animals causes mobilization of hemopoietic progenitors into blood. These data suggest that hemopoietic cell lodgement in the BM is a regulatable process and can be influenced by VLA4/VCAM-1 adhesion pathway. Although additional molecular pathways are not excluded and may be likely, our data establish VCAM-1 as a BM endothelial addressin, analogous to the role that mucosal addressin cell adhesion molecule (MAdCAM) plays in lymphocyte homing. Whether splenic uptake of hemopoietic progenitors is passive or controlled through different mechanisms remains to be clarified. In addition, we provide experimental evidence that homing and mobilization are related phenomena involving, at least partly, similar molecular pathways.

We have explored the functional implications of inducible ␣4 integrin deletion during adult hematopoiesis by generating a conditional-knockout mouse model, and we show that ␣4 integrin-deficient hematopoietic progenitor cells accumulate in the peripheral blood soon after interferon-induced gene deletion. Although their numbers gradually stabilize at a lower level, progenitor cell influx into the circulation continues at abovenormal levels for more than 50 weeks. Concomitantly, a progressive accumulation of progenitors occurs within the spleen. In addition, the regeneration of erythroid and myeloid progenitor cells is delayed during stress hematopoiesis induced by phenylhydrazine or by 5-fluorouracil, suggesting impairment in early progenitor expansion in the absence of ␣4 integrin. Moreover, in transplantation studies, homing of ␣4 ؊/؊ cells to the bone marrow, but not to the spleen, is selectively impaired, and short-term engraftment is critically delayed in the early weeks after transplantation. Thus, conditional deletion of ␣4 integrin in adult mice is accompanied by a novel hematopoietic phenotype during both homeostasis and recovery from stress, a phenotype that is distinct from the ones previously described in ␣4 integrin-null chimeras and ␤1 integrin-conditional knockouts.Development of hematopoietic stem cells and their differentiated descendants in selective anatomic sites during fetal or adult life relies on favorable interactions with a specialized microenvironment that provides mechanical support and facilitates the enhancement of hematopoietic stem cells' proliferation and differentiation. The relationship between hematopoietic cells and cells within the microenvironment is a highly dynamic one, such that in response to stimuli, their coordinated responses can accommodate acute demands in cell expansion and migration in or out of the hematopoietic compartment to meet physiologic needs. As the molecular demands of stem cells residing in each anatomical site are likely to be different from those of differentiated cells, specialized niches are envisioned to accommodate these requirements (61). Members of the integrin family of cytoadhesive molecules are widely expressed in the hematopoietic system (56, 63) and exercise decisive roles in the interactions between hematopoietic cells and their microenvironment. This specialized function is dependent on the ability of integrins to serve not only as adhesion receptors but also as bona fide bidirectional signaling molecules that transduce signals to downstream effectors (20, 55). Integration of signaling networks is of particular importance in hematopoiesis, permitting cross talk with additional integrin molecules or with growth factor receptors, metalloproteinases, and chemokines in order to influence motility and other cellular functions (22,26,47). Moreover, the specificity in the actions of integrins in different anatomic locations may rely on the fact that signaling after ligation to immobilized ligands is both topographically constrained and cell type ...

It was previously reported that treatment with the sulfated polysaccharide fucoidan or the structurally similar dextran sulfate increased circulating mature white blood cells and hematopoietic progenitor/stem cells (HPCs) in mice and nonhuman primates; however, the mechanism mediating these effects was unclear. It is reported here that plasma concentrations of the highly potent chemoattractant stromal-derived factor 1 (SDF-1) increase rapidly and dramatically after treatment with fucoidan in monkeys and in mice, coinciding with decreased levels in bone marrow. In vitro and in vivo data suggest that the SDF-1 increase is due to its competitive displacement from heparan sulfate proteoglycans that sequester the chemokine on endothelial cell surfaces or extracellular matrix in bone marrow and other tissues. Although moderately increased levels of interleukin-8, MCP1, or MMP9 were also present after fucoidan treatment, studies in gene-ablated mice (GCSFR ؊/؊ , MCP1 ؊/؊ , or MMP9 ؊/؊ ) and the use of metalloprotease inhibitors do not support their involvement in the concurrent mobilization. Instead, SDF-1 increases, uniquely associated with sulfated glycan-mobilizing treatments and not with several other mobilizing agents tested, are likely responsible. To the authors' knowledge, this is the first published report of disrupting the SDF-1 gradient between bone marrow and peripheral blood through a physiologically relevant mechanism, resulting in mobilization with kinetics similar to other mobilizing CXC chemokines. The study further underscores the importance of the biological roles of carbohydrates. IntroductionStromal-derived factor 1 (SDF-1) is a highly potent chemoattractant both in vitro and in vivo for mature leukocytes and hematopoietic progenitor/stem cells (HPCs), which carry its receptor CXCR4. [1][2][3][4][5][6][7] This highly conserved chemokine is constitutively expressed by virtually all tissues, 8 including bone marrow (BM). 3 It is expressed as 2 alternatively spliced isoforms, the predominant ␣ form and the ␤ form containing 4 additional amino acids at the C terminus, each possessing a heparin-binding domain. 9,10 The SDF-1-CXCR4 interaction plays a dominant role in hematopoiesis, and mice deficient in either gene die in utero exhibiting defects in B-cell lymphopoiesis and BM myelopoiesis. 4,5 Additionally, a critical role for CXCR4 on human cells in engraftment to the BM of nonobese diabetic/severe combined immunodeficiency mice 6,7 has been shown. Although involvement of SDF-1 in mobilization-the egress of HPCs from the BM to the peripheral blood (PB)-has also been speculated, direct evidence has only recently been obtained in mice using a synthetic SDF-1 analog 11 or following injection with an adenovirus expressing human SDF-1. 12 Previously, we reported that the sulfated polysaccharide fucoidan (FucS) and the structurally similar dextran sulfate (DexS) can elevate circulating white blood cells (WBCs) and mobilize HPCs within hours in a selectin-independent manner in mice and nonhuman primates. 13...

The specific retention of intravenously administered hemopoietic cells within bone marrow is a complex multistep process. Despite recent insights, the molecular mechanics governing this process remain largely undefined. This study explored the influence of ␤ 2 -integrins on the homing to bone marrow and repopulation kinetics of progenitor cells. Both antifunctional antibodies and genetically deficient cells were used. In addition, triple selectin-deficient mice were used as recipients of either deficient (selectin or ␤ 2 ) or normal cells in homing experiments. The homing patterns of either ␤ 2 null or selectin null cells into normal or selectin-deficient recipients were similar to those of normal cells given to normal recipients. Furthermore, spleen colonyforming units and the early bone marrow repopulating activity for the first 2 weeks after transplantation were not significantly different from those of control cells. These data are in contrast to the importance of ␤ 2 -integrin and selectins in the adhesion/migration cascade of mature leukocytes. The special bone marrow flow hemodynamics may account for these differences. Although early deaths after transplantation can be seen in recipients deficient in CD18 and selectin, these are attributed to septic complications rather than homing defects. However, when ␤ 2 -or selectin-null donor cells are treated with anti-␣ 4 antibodies before their transplantation to normal or selectin-deficient recipients, a dramatic inhibition of homing (>90%) was found. The data suggest that the ␣ 4 ␤ 1 /vascular cell adhesion molecule-1 pathway alone is capable of providing effective capture of cells within the bone marrow, but if its function is compromised, the synergistic contribution of other pathways, that is, ␤ 2 -integrins or selectins, is uncovered. IntroductionRestriction of the developing hemopoietic cells to certain anatomic sites within the body, that is, the extravascular spaces of the bone marrow (BM), signifies a unique microenvironment capable of providing not only anchorage to hemopoietic cells, but of transmitting signals enabling their proliferation and differentiation. The relationship of hemopoietic cells with their microenvironment is a highly dynamic one allowing further expansion on demand and, depending on the stimuli, the movement of cells in and out of their microenvironment. The ability of intravenously administered cells to re-establish connections within the BM environment is a clinically exploited example of that flexibility. As the sinusoidal endothelial cells separate the extravascular space from the circulation, intravenously administered cells need first to tether to the sinusoidal endothelium, then to firmly anchor themselves within the extravascular space through reversible adhesion and transmigration steps. Although several insights regarding the molecular pathways that guide these processes have been obtained, our knowledge of this complex process remains incomplete. Early reports suggest that galactose-and mannose-specific lectins (present...

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