Total body irradiation causes profound changes in endothelial traffic molecules for hematopoietic progenitor cell recruitment to bone marrow

Cyclical mobilization and gated importation of thymocyte progenitors in the adult mouse: evidence for a thymus-bone marrow feedback loop

It has recently been observed, as in the fetal thymus, that the importation of hematogenous thymocyte progenitors by the adult thymus is a gated phenomenon, whereby saturating numbers of progenitors periodically enter the thymus and occupy a finite number of intrathymic niches. In addition, the mobilization of thymocyte progenitors from the bone marrow appears to be a cyclical process that coincides temporally with the periods of thymic receptivity (open gate). It is proposed that these events are coordinated by a thymus-bone marrow feedback loop in which a wave of developing triple negative (CD3- CD4- CD8-) thymocytes interacts with stromal cells in the stratified regions of the thymus cortex to sequentially induce the release of diffusible cytokines that regulate the production, mobilization, and recruitment of thymocyte progenitors. The likely components of this feedback loop are described here, as are the properties of the intrathymic vascular gates and niches for thymocyte progenitors. The cyclical production and release of thymocyte progenitors from the bone marrow is placed in the context of a general phenomenon of oscillatory feedback regulation involving all lymphohemopoietic cell lineages. Lastly, the question of whether the gated (as opposed to the continuous) entry of thymocyte progenitors is essential for normal thymocytopoiesis in adult life is discussed.

In vivo imaging of lymphocyte trafficking

Over the past decades, intravital microscopy (IVM), the imaging of cells in living organisms, has become a valuable tool for studying the molecular determinants of lymphocyte trafficking. Recent advances in microscopy now make it possible to image cell migration and cell-cell interactions in vivo deep within intact tissues. Here, we summarize the principal techniques that are currently used in IVM, discuss options and tools for fluorescence-based visualization of lymphocytes in microvessels and tissues, and describe IVM models used to explore lymphoid and non-lymphoid organs. The latter will be introduced according to the physiologic itinerary of developing and differentiating T and B lymphocytes as they traffic through the body, beginning with their development in bone marrow and thymus and continuing with their migration to secondary lymphoid organs and peripheral tissues.

Reduced retention of radioprotective hematopoietic cells within the bone marrow microenvironment in CXCR4-/- chimeric mice

The physiologic role of CXCR4 on hematopoietic stem/progenitor cells (HSPCs) is not fully understood. Here, we show that radioprotection of lethally irradiated mice by embryonic day 14.5 (E14.5) CXCR4-/- fetal liver (FL) cells was markedly impaired when compared with CXCR4+/+ counterparts, but this defect was rescued when hosts were engrafted with high cell numbers. This quantitative defect contrasted with a similar content in hematopoietic colony-forming cells (CFCs), splenic colony-forming units (CFUs-S), and Lin- Sca-1+ c-kit+ cells in E14.5 CXCR4-/- and CXCR4+/+ livers. In addition, the homing of HSPCs in the bone marrow was not altered as detected with a CFSE-staining assay. In contrast, a 30-fold increase in CFCs was seen in the circulation of mice stably reconstituted with CXCR4-/- FL cells and this increment was already observed before hematopoiesis had reached a steady-state level. Together, the data strongly suggest that impaired retention may, at least in short-term hematopoietic reconstitution, lead to a diminution in the number of available progenitors required for radioprotection.

Recruitment of adult thymic progenitors is regulated by P-selectin and its ligand PSGL-1

The molecular mechanisms that direct the migration of early T lymphocyte progenitors to the thymus are unknown. We show here that P-selectin is expressed by thymic endothelium and that lymphoid progenitors in bone marrow and thymus bind P-selectin. Parabiosis, competitive thymus reconstitution and short-term homing assays indicated that P-selectin and its ligand PSGL-1 are functionally important components of the thymic homing process. Accordingly, thymi of mice lacking PSGL-1 contained fewer early thymic progenitors and had increased empty niches for prothymocytes compared with wild-type mice. Furthermore, the number of resident thymic progenitors controls thymic expression of P-selectin, suggesting that regulation of P-selectin expression by a thymic 'niche occupancy sensor' may be used to direct progenitor access.

B-cell reconstitution by transplantation of B220 CD117 B-lymphoid progenitors into irradiated mice

Bone marrow cells of 4-week-old CBA/J mice were searched for progenitors that can reconstitute CBA-type B lineage (B220+) cells after transplantation into irradiated (BALB/c x CBA/J) F(1) mice. In recipients of B lineage-committed B220+ CD117+ cells, which were CD19+ CD127+ CD25- IgM-, numbers of CBA-type B220+ cells had increased greatly by day 8 after transplantation. This increase stopped by day 10 but cell numbers remained at this level for at least 8 weeks. B-cell production in the bone marrow of B220+ CD117+ cell recipients occurred even 8 weeks after transplantation. Probably, the transplanted B-lymphoid progenitors are capable of self-renewing up to 8 weeks after transplantation. B lineage- uncommitted CD117high CD71- cells, which were B220- and included haematopoietic stem cells, could also reconstitute B lineage bone marrow cells after transplantation. Sequential development of CD117+, CD25+ and IgM+ cells in CBA-type B220+ bone marrow cells occurred 2-4 days faster in recipients of B220+ CD117+ progenitors than in recipients of CD117high CD71- progenitors, suggesting that the development of the former progenitors from the latter may take 2-4 days.

Bone marrow is a major reservoir and site of recruitment for central memory CD8+ T cells

Normal bone marrow (BM) contains T cells whose function and origin are poorly understood. We observed that CD8+ T cells in BM consist chiefly of CCR7+ L-selectin+ central memory cells (TCMs). Adoptively transferred TCMs accumulated more efficiently in the BM than naive and effector T cells. Intravital microscopy (IVM) showed that TCMs roll efficiently in BM microvessels via L-, P-, and E-selectin, whereas firm arrest required the VCAM-1/alpha4beta1 pathway. alpha4beta1 integrin activation did not depend on pertussis toxin (PTX)-sensitive Galphai proteins but was reduced by anti-CXCL12. In contrast, TCM diapedesis did not require CXCL12 but was blocked by PTX. After extravasation, TCMs displayed agile movement within BM cavities, remained viable, and mounted potent antigen-specific recall responses for at least two months. Thus, the BM functions as a major reservoir for TCMs by providing specific recruitment signals that act in sequence to mediate the constitutive recruitment of TCMs from the blood.

C1q governs deposition of circulating immune complexes and leukocyte Fcgamma receptors mediate subsequent neutrophil recruitment

Inflammation induced by circulating immunoglobulin G–immune complexes (ICs) characterizes many immune-mediated diseases. In this work, the molecular requirements for the deposition of circulating ICs and subsequent acute leukocyte recruitment in mice were elucidated. We show that after intravenous injection, preformed soluble ICs are rapidly deposited in the postcapillary venules of the cremaster microcirculation, secondary to increased vascular permeability. This deposition is dependent on complement C1q. IC deposition is associated with leukocyte recruitment. Leukocyte rolling, which is mediated by P-selectin in the exteriorized cremaster muscle, is not further increased in response to ICs. In contrast, leukocyte rolling velocity is significantly decreased and leukocyte adhesion is significantly increased in the presence of ICs. The IC-mediated slow leukocyte rolling velocity and subsequent adhesion and emigration are dependent on Fcγ receptors (FcγRs), particularly FcγRIII, with complement C3 and C5 having no detectable role. These studies suggest a regulatory mechanism of IC deposition and leukocyte trafficking in IC-mediated inflammation requiring C1q and FcγRs in sequential, noninteracting roles.

Degeneration of stroma reduces retention of homed cells in bone marrow of lethally irradiated mice

Cytotoxic drugs or irradiation are generally administered before bone marrow (BM) transplantation because of the idea that host bone marrow 'niches' become available to the donor cells for engraftment. How BM stromal cells respond to the radiation, which ultimately modulates grafting of donor cells, is poorly understood. In this study, we examined homing and marrow retention of PKH26+ donor cells in BM of age-matched C57BL/6J mice conditioned at different doses of irradiation. When we injected donor cells into mice that received 900 cGy, the percent homing was highest (15.8 +/- 1.5%) as compared to the lower doses of radiation. Despite the highest levels of homing of donor cells in these mice, about 70% (p < 0.005) homed cells were detached from the marrow within 72 h of transplantation. In contrast, a 2- to 2.5-fold (p < 0.03) multiplication of homed PKH-26+ Sca-1+ cells was observed in sublethally irradiated mice. While determining that CD45- CD106+ cells in BM of the mice received 900 cGy, we found that more than 80% of cells were depleted. It was also revealed from this investigation that grafted cells conferred partial protection to the endogenous myeloid colony-forming cells from radiation injury. Collectively, the present study implicates radiation-induced degeneration of stroma as a cause of poor retention of donor cells in BM of lethally irradiated mice. These results may have important clinical implications in designing conditioning regimens for BM transplantation.

Intravital microscopy: visualizing immunity in context

Recent advances in photonics, particularly multi-photon microscopy (MPM) and new molecular and genetic tools are empowering immunologists to answer longstanding unresolved questions in living animals. Using intravital microscopy (IVM) investigators are dissecting the cellular and molecular underpinnings controlling immune cell motility and interactions in tissues. Recent IVM work showed that T cell responses to antigen in lymph nodes are different from those observed in vitro and appear dictated by factors uniquely relevant to intact organs. Other IVM models, particularly in the bone marrow, reveal how different anatomic contexts regulate leukocyte development, immunity, and inflammation. This article will discuss the current state of the field and outline how IVM can generate new discoveries and serve as a "reality check" for areas of research that were formerly the exclusive domain of in vitro experimentation.

PTX-sensitive signals in bone marrow homing of fetal and adult hematopoietic progenitor cells

Several examples suggest a relationship between in vitro migratory capacity and bone marrow (BM) homing. Pertussis toxin (PTX) is a potent inhibitor of serpentine receptor–associated inhibitory trimeric guanidine nucleotide binding (Gi) protein signals. As such, it blocks hematopoietic progenitor cell migration in vitro, but contrary to expectation, no effects on BM homing were observed in previous studies. We therefore re-examined the effect of PTX on homing of murine BM and fetal liver (FL). We found that BM homing of PTX-incubated progenitor cells (colony-forming cells in culture [CFU-Cs]) from BM or FL in irradiated and nonirradiated recipients was reduced by more than 75%, with a concomitant increase in circulating CFU-Cs in peripheral blood. Additional studies confirmed the functional significance of this reduction in homing: PTX-treated cells did not provide radioprotection, and their short-term engraftment in BM and spleen was drastically reduced. Furthermore, several approaches show that cell-intrinsic rather than host-derived mechanisms are responsible for the PTX-induced homing defect. In summary, we show that Gi protein signals are required for BM homing and, as such, provide a new example of the association between BM homing and in vitro migration. Moreover, our data suggest that the behavior of hematopoietic progenitors in obeying Gi signaling does not diverge from that of mature leukocytes.

Hematopoietic Progenitor Stem Cell Homing in Mice Lethally Irradiated with Ionizing Radiation at Differing Dose Rates

It has recently been shown that specific lineage-depleted murine hematopoietic stem cells that home to the bone marrow 2 days after transplantation of ablated primary recipients are capable of long-term engraftment and repopulation of secondary recipients. We were interested in determining whether the rate at which the ablating radiation dose was delivered to the mice affected the homing of lineage-depleted stem cells to the bone marrow and/or sites of tissue damage. Fractionated, lineage-depleted donor marrow cells were isolated and labeled with the membrane dye PKH26. Recipient mice were lethally irradiated with 11 Gy ionizing radiation using varying dose rates and were immediately injected with PKH26-labeled progenitor stem cells. With the exception of the lowest dose-rate group, all irradiated mice had an approximately fivefold (P = 0.014 to 0.025) reduction in stem cell homing to the bone marrow compared to unirradiated control animals. A fivefold reduction of stem cell homing to the spleen compared to unirradiated animals was also observed, though this was not statistically significant for any dose-rate group (P = 0.072 to 0.233). This difference in homing could not be explained by increased stem cell apoptosis/necrosis or non-marrow tissue homing to the intestine, lung or liver. We show that the dose rate at which a lethal dose of total-body radiation is delivered does not augment hematopoietic progenitor stem cell homing to the bone marrow, spleen or sites of early radiation-mediated tissue damage at either 2 or 5 days postirradiation/transplantation. The observation that greater homing was seen in unirradiated control mice calls into question the concept that adequate bone marrow stem cell homing requires radiation-induced "space" to be made in the marrow, certainly for the enriched early progenitor hematopoietic stem cells used for this set of experiments. Further experiments will be needed to determine whether these homed cells are as capable of giving rise to long-term engraftment/repopulation of the marrow of secondary recipients as they are in irradiated recipients.

Unique SDF-1–induced activation of human precursor-B ALL cells as a result of altered CXCR4 expression and signaling

The mechanisms governing migration and extramedullary dissemination of leukemic cells remain obscure. In this study the migration and in vivo homing to the bone marrow of nonobese diabetic severe combined immunodeficient (NOD/SCID) mice injected with human precursor-B acute lymphoblastic leukemia (ALL) cells in comparison to normal CD34+ progenitors (both cord blood and mobilized peripheral blood) was investigated. Although migration and homing of both cell populations was dependent on stromal cell-derived factor 1 (SDF-1)/CXCR4 interactions, major differences in receptor expression as well as the migratory capacity toward various concentrations of SDF-1 were found. Furthermore, unlike normal CD34+ progenitors, in vivo homing of the leukemic cells was superior when recipient NOD/SCID mice were not irradiated prior to transplantation. In addition, we report differences in the adhesion molecules activated following SDF-1 stimulation, documenting a major role for very late antigen 4 (VLA-4), but not VLA-5 and lymphocyte function-associated antigen-1 (LFA-1), in homing of precursor-B ALL cells. Interestingly, Toxin-B and pertussis toxin inhibited the homing of the leukemic cells but not that of normal CD34+ progenitors or normal CD10+/CD19+ precursor-B cells, revealing differences in CXCR4 signaling pathways that are based on changes that acquired by the leukemic cells. Altogether, our data provide new insights into different SDF-1–induced signaling, activation, and consequent motility between normal CD34+ and precursor-B ALL progenitors, which may lead to improved clinical protocols. (Blood. 2004;103: 2900-2907)

Circulation and chemotaxis of fetal hematopoietic stem cells

The major site of hematopoiesis transitions from the fetal liver to the spleen and bone marrow late in fetal development. To date, experiments have not been performed to evaluate functionally the migration and seeding of hematopoietic stem cells (HSCs) during this period in ontogeny. It has been proposed that developmentally timed waves of HSCs enter the bloodstream only during distinct windows to seed the newly forming hematopoietic organs. Using competitive reconstitution assays to measure HSC activity, we determined the localization of HSCs in the mid-to-late gestation fetus. We found that multilineage reconstituting HSCs are present at low numbers in the blood at all timepoints measured. Seeding of fetal bone marrow and spleen occurred over several days, possibly while stem cell niches formed. In addition, using dual-chamber migration assays, we determined that like bone marrow HSCs, fetal liver HSCs migrate in response to stromal cell-derived factor-1α (SDF-1α); however, unlike bone marrow HSCs, the migratory response of fetal liver HSCs to SDF-1α is greatly increased in the presence of Steel factor (SLF), suggesting an important role for SLF in HSC homing to and seeding of the fetal hematopoietic tissues. Together, these data demonstrate that seeding of fetal organs by fetal liver HSCs does not require large fluxes of HSCs entering the fetal bloodstream, and that HSCs constitutively circulate at low levels during the gestational period from 12 to 17 days postconception. Newly forming hematopoietic tissues are seeded gradually by HSCs, suggesting initial seeding is occurring as hematopoietic niches in the spleen and bone marrow form and become capable of supporting HSC self-renewal. We demonstrate that fetal and adult HSCs exhibit specific differences in chemotactic behavior. While both migrate in response to SDF-1α, fetal HSCs also respond significantly to the cytokine SLF. In addition, the combination of SDF-1α and SLF results in substantially enhanced migration of fetal HSCs, leading to migration of nearly all fetal HSCs in this assay. This finding indicates the importance of the combined effects of SLF and SDF-1α in the migration of fetal HSCs, and is, to our knowledge, the first demonstration of a synergistic effect of two chemoattractive agents on HSCs.

New results on the migratory behavior of blood cell precursors in the early embryo might be relevant to bone marrow transplants and other clinical therapies

Characterization of adhesion and viability of early seeding hematopoietic cells in the host bone marrow in vivo and in situ

OBJECTIVE

Homing and seeding are essential early events of engraftment that depend on the interaction of hematopoietic cells with the host bone marrow (BM) stroma. We used optical techniques to characterize the adhesion patterns and viability of bone marrow cells (BMC) at the level of recipient BM microenvironment.

MATERIALS AND METHODS

Donor cells labeled with PKH dyes were tracked in vivo through an optical window placed over the femoral epiphysis of nonconditioned recipients. Adhesion to BM stroma was assessed with laser tweezers, and viability was assayed by fluorescence resonance energy transfer of the pair PKH67-propidium iodide (PI) in freshly excised femurs.

RESULTS

Three hours after intravenous injection, an estimated 30% of the labeled cells in the femur were immobile. The percent of adherent cells increased to 74+/-3% and 97+/-2% on days +1 and +3, respectively, (p<0.001), and similar fractions of cells were viable at these times (p<0.001). The observation that all adherent cells were viable suggested a correlation between these parameters. The day +3 BM-seeded cells rescued secondary myeloablated allogeneic hosts. Seeding in the host BM was accompanied by (4.5-fold) enrichment of cells expressing SCA-1 and was 22-fold higher for lineage-negative Lin(-) cells compared to lineage-positive cells (p<0.001). Pretreatment of Lin(-) cells with anti-VLA-4 antibodies caused a 2.4-fold decrease in homing and a 4.6-fold decrease in seeding (p<0.001).

CONCLUSIONS

The data indicate that adhesion is rate-limiting determinant of homing and early seeding, and a crucial event that preserves the viability of cells toward successful engraftment. The role of VLA-4 is more important for primary seeding than it is for homing to the BM.

A novel endothelial L-selectin ligand activity in lymph node medulla that is regulated by alpha(1,3)-fucosyltransferase-IV

Lymphocytes home to peripheral lymph nodes (PLNs) via high endothelial venules (HEVs) in the subcortex and incrementally larger collecting venules in the medulla. HEVs express ligands for L-selectin, which mediates lymphocyte rolling. L-selectin counterreceptors in HEVs are recognized by mAb MECA-79, a surrogate marker for molecularly heterogeneous glycans termed peripheral node addressin. By contrast, we find that medullary venules express L-selectin ligands not recognized by MECA-79. Both L-selectin ligands must be fucosylated by α(1,3)-fucosyltransferase (FucT)-IV or FucT-VII as rolling is absent in FucT-IV+VII^−/− mice. Intravital microscopy experiments revealed that MECA-79–reactive ligands depend primarily on FucT-VII, whereas MECA-79–independent medullary L-selectin ligands are regulated by FucT-IV. Expression levels of both enzymes paralleled these anatomical distinctions. The relative mRNA level of FucT-IV was higher in medullary venules than in HEVs, whereas FucT-VII was most prominent in HEVs and weak in medullary venules. Thus, two distinct L-selectin ligands are segmentally confined to contiguous microvascular domains in PLNs. Although MECA-79–reactive species predominate in HEVs, medullary venules express another ligand that is spatially, antigenically, and biosynthetically unique. Physiologic relevance for this novel activity in medullary microvessels is suggested by the finding that L-selectin–dependent T cell homing to PLNs was partly insensitive to MECA-79 inhibition.

PSGL-1 participates in E-selectin-mediated progenitor homing to bone marrow: evidence for cooperation between E-selectin ligands and alpha4 integrin

The nature and exact function of selectin ligands involved in hematopoietic progenitor cell (HPC) homing to the bone marrow (BM) are unclear. Using murine progenitor homing assays in lethally irradiated recipients, we found that the P-selectin glycoprotein ligand-1 (PSGL-1) plays a partial role in HPC homing to the BM (a reduction of about 35% when the P-selectin binding region is blocked). Blockade of both PSGL-1 and alpha4 integrin did not further enhance the effect of anti-alpha4 integrin (a reduction of about 55%). We suspected that E-selectin ligands might contribute to the remaining homing activity. To test this hypothesis, HPC homing assays were carried out in E-selectin-deficient recipients and revealed a profound alteration in HPC homing when E-selectin and alpha4 integrin were inactivated (> 90% reduction). Competitive assays to test homing of long-term repopulating stem cells revealed a drastic reduction (> 99%) of the homed stem cell activity when both alpha4 integrin and E-selectin functions were absent. Further homing studies with PSGL-1-deficient HPCs pretreated with anti-alpha4 integrin antibody revealed that PSGL-1 contributes to approximately 60% of E-selectin ligand-mediated homing activity. Our results thus underscore a major difference between mature myeloid cells and immature stem/progenitor cells in that E-selectin ligands cooperate with alpha4 integrin rather than P-selectin ligands.

Involvement of CXCR4 and IL-2 in the homing and retention of human NK and NK T cells to the bone marrow and spleen of NOD/SCID mice

Human natural killer (NK) and NK T cells play an important role in allogeneic bone marrow (BM) transplantation and graft-versus-leukemia (GVL) effect. The mechanisms by which these cells home to the BM and spleen are not well understood. Here we show that treatment of these cells with pertussis toxin and neutralizing antibodies to the chemokine receptor CXCR4 inhibited homing of the cells to the BM, but not the spleen, of NOD/SCID mice. The retention of NK and NK T cells within the spleen and BM was dependent on Galphai signaling and CXCR4 function. The chemokine receptors CXCR4 and CXCR3 are expressed predominantly on the cell surface of NK T cells. Following activation with interleukin-2 (IL-2), the levels of CXCR4 on NK and NK T cells decreased significantly. Treatment of cells with IL-2 inhibited their migration in response to CXCL12 and their homing and retention in the BM and spleen of NOD/SCID mice. In contrast to CXCR4, the expression levels of the chemokine receptor CXCR3 and the migration of cells in response to CXCL9 and CXCL10 increased after IL-2 treatment. Thus, down-regulation of CXCR4 and up-regulation of CXCR3 may direct the trafficking of cells to the site of inflammation, rather than to hematopoietic organs, and therefore may limit their alloreactive potential.

Biology of hematopoietic stem cells and progenitors: implications for clinical application

Stem cell biology is scientifically, clinically, and politically a current topic. The hematopoietic stem cell, the common ancestor of all types of blood cells, is one of the best-characterized stem cells in the body and the only stem cell that is clinically applied in the treatment of diseases such as breast cancer, leukemias, and congenital immunodeficiencies. Multicolor cell sorting enables the purification not only of hematopoietic stem cells, but also of their downstream progenitors such as common lymphoid progenitors and common myeloid progenitors. Recent genetic approaches including gene chip technology have been used to elucidate the gene expression profile of hematopoietic stem cells and other progenitors. Although the mechanisms that control self-renewal and lineage commitment of hematopoietic stem cells are still ambiguous, recent rapid advances in understanding the biological nature of hematopoietic stem and progenitor cells have broadened the potential application of these cells in the treatment of diseases.

The topologic and chronologic patterns of hematopoietic cell seeding in host femoral bone marrow after transplantation

The early stages of homing, seeding, and engraftment of hematopoietic stem and progenitor cells are poorly characterized. We have developed an optical technique that allows in vivo tracking of transplanted, fluorescent-tagged cells in the host femurs. In this study we used fluorescence microscopy to monitor the topologic and chronologic patterns of hematopoietic cell seeding in the femoral bone marrow (BM) of mice. PKH-labeled cells homed to the femur within minutes after injection into a peripheral vein. Most cells drifted within the marrow space and gradually seeded in clusters close to the endosteal surface of the epiphyseal cortex. Three days after transplantation 85% to 94% (14%) of PKH-labeled cells in the femoral marrow were located within 100 microm of the epiphyseal bone surface (P <.001 versus the more central cells), whereas labeled cells were absent in the femoral diaphysis. Primary seeding of juxtaendosteal, epiphyseal marrow occurred independently of recipient conditioning (myeloablated and nonconditioned hosts), donor-recipient antigen disparity, or the phenotype of the injected cells (whole BM and lineage-negative cells) and was consistently observed in secondary recipients of BM-homed cells. Seeding in regions close to the epiphyseal bone was also observed in freshly excised femurs perfused ex vivo and in femurs assessed without prior placement of optical windows, indicating that the site of primary seeding was not affected by surgical placement of optical windows. Four to 5 days after transplantation, cellular clusters appeared in the more central regions of the epiphyses and in the diaphyses. Centrally located cells showed decreased PKH fluorescence, suggesting that they were progeny of the seeding cells, and brightly fluorescent cells (quiescent first-generation seeding cells) were observed close to the bone surface for as long as 24 days after transplantation. These data indicate that the periphery of the femoral marrow hosts primary seeding and that quiescent cells continue to reside in the periphery for weeks and do not divide. The site of proliferation of transplanted cells is the center of the marrow space.

The role of G-protein signaling in hematopoietic stem/progenitor cell mobilization

The directed migration of mature leukocytes to inflammatory sites and the lymphocyte trafficking in vivo are dependent on G protein-coupled receptors and delivered through pertussis toxin (Ptx)-sensitive Gi-protein signaling. In the present study, we explored the in vivo role of G-protein signaling on the redistribution or mobilization of hematopoietic stem/progenitor cells (HPCs). A single injection of Ptx in mice elicits a long-lasting leukocytosis and a progressive increase in circulating colony-forming unit-culture (CFU-C) and colony-forming unit spleen (CFU-S). We found that the prolonged effect is sustained by a continuous slow release of Ptx bound to red blood cells or other cells and is potentially enhanced by an indirect influence on cell proliferation. Plasma levels of certain cytokines (interleukin 6 [IL-6], granulocyte colony-stimulating factor [G-CSF]) increase days after Ptx treatment, but these are unlikely initiators of mobilization. In addition to normal mice, mice genetically deficient in monocyte chemotactic protein 1 (MCP-1), matrix metalloproteinase 9 (MMP-9), G-CSF receptor, beta2 integrins, or selectins responded to Ptx treatment, suggesting independence of Ptx-response from the expression of these molecules. Combined treatments of Ptx with anti-very late activation antigen (anti-VLA-4), uncovered potentially important insight in the interplay of chemokines/integrins, and the synergy of Ptx with G-CSF appeared to be dependent on MMP-9. As Ptx-mobilized kit+ cells display virtually no response to stromal-derived factor 1 (SDF-1) in vitro, our data suggest that disruption of CXCR4/SDF-1 signaling may be the underlying mechanism of Ptx-induced mobilization and indirectly reinforce the notion that active signaling through this pathway is required for continuous retention of cells within the bone marrow. Collectively, our data unveil a novel example of mobilization through pharmacologic modulation of signaling.

Intravital microscopy comparing T lymphocyte trafficking to the spleen and the mesenteric lymph node

Lymphocyte rolling velocity is determined largely by interactions between leukocyte alpha(4)-integrin (CD49d) and L-selectin and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in mesenteric postcapillary venules and Peyer's patch high endothelial venules (HEVs). The role of these interactions in other tissue sites of lymphocyte emigration is not known. With the use of real-time intravital confocal microscopy, we found that rolling velocities of T lymphocytes in the murine mesenteric lymph node (MLN) HEV also depend on L-selectin and CD49d. However, in the murine spleen, rolling velocities of T lymphocytes are not influenced by the loss of L-selectin and CD49d. With the use of FITC-dextran and TIE2-GFP mice, we further defined the microvascular compartments of the spleen and showed that adherence of T cells is localized to regions in the white pulp that are not lined by endothelial cells and have shear rates similar to bone marrow sinusoids. These results establish that T cell trafficking to the spleen differs from trafficking to other secondary lymphoid organs and suggest that the mechanical properties of the blood-filtering role of the spleen are important in T cell accumulation in the organ.

Bone marrow homing: the players, the playfield, and their evolving roles

Although homing of hematopoietic cells to the bone marrow was described as a functional concept several decades ago, the analysis of its components and the molecular pathways involved remains an ongoing challenge. Because of the biologic and clinical significance of homing, a resurgence of studies delving into the mechanistic and molecular aspects of homing is appearing. These recent studies, highlighted in this brief review, emphasize newly appreciated roles of some known regulators and their interacting partners in homing, although some novel regulators also seem to enter the scene. The homing field still has disparities in experimental design and evaluation of data; however, the ever-expanding search for new information, together with the application of novel technologies, especially when combinations of approaches are used, will certainly bring our understanding of homing to a new level.