Chemokine stromal cell-derived factor-1alpha modulates VLA-4 integrin-dependent adhesion to fibronectin and VCAM-1 on bone marrow hematopoietic progenitor cells

Role and Therapeutic Targeting of SDF-1α/CXCR4 Axis in Multiple Myeloma

Simple Summary

The SDF-1α/CXCR4 axis plays crucial roles in proliferation, survival, invasion, dissemination, and drug resistance in multiple myeloma. This review summarizes the pleiotropic role of the SDF-1α/CXCR4 axis in multiple myeloma and introduces the SDF-1α/CXCR4 axis-targeted therapies in multiple myeloma.

Abstract

The C-X-C chemokine receptor type 4 (CXCR4) is a pleiotropic chemokine receptor that is expressed in not only normal hematopoietic cells but also multiple myeloma cells. Its ligand, stromal cell-derived factor 1α (SDF-1α) is produced in the bone marrow microenvironment. The SDF-1α/CXCR4 axis plays a pivotal role in the major physiological processes associated with tumor proliferation, survival, invasion, dissemination, and drug resistance in myeloma cells. This review summarizes the pleiotropic role of the SDF-1α/CXCR4 axis in multiple myeloma and discusses the future perspective in the SDF-1α/CXCR4 axis-targeted therapies in multiple myeloma.

Combined Single-Cell Manipulation and Chemomechanical Modeling to Probe Cell Migration Mechanism During Cell-to-Cell Interaction

Spatial presentations of chemical and mechanical information are key parameters for cell migration. However, previous theoretical and experimental studies focus on probing the mechanisms caused by a single type of stimulus, while ignoring the synergetic effects, especially for single cell migration during cell-to-cell interaction. Here we develop a chemomechanical model to assess the biochemical and biophysical modulators of single cell migration during cell-to-cell interaction. This model considers the stimulation of chemoattractant concentration gradient, influence of dynamic adhesion strength and relative motion between cells. The model is validated with single cell manipulation of leukemia cancer cell on stromal cell layer using optical tweezers. Both the modeling and experimental results demonstrate that cell migration velocity caused by chemotaxis can be biased by dynamic adhesion force, which is related to the retrograde flow of stromal cell layer. Besides, the biophysical modulators can influence the effect of drug treatment for specific signaling pathway. Our work provides a quantitative description of single cell migration in a complex environment that is close to realistic in vivo situation and is useful for further exploration of cell signaling pathway during cell-to-cell interactions for investigation of potential therapeutic strategy.

Variety matters: Diverse functions of monocyte subtypes in vascular inflammation and atherogenesis

Monocytes are important mediators of the innate immunity by recognizing and attacking especially bacterial pathogens but also play crucial roles in various inflammatory diseases, including vascular inflammation and atherosclerosis. Maturation, differentiation and function of monocytes have been intensively explored for a long time in innumerable experimental and clinical studies. Monocytes do not represent a uniform cell type but could be further subdivided into subpopulations with distinct features and functions. Those subpopulations have been identified in experimental mouse models as well as in humans, albeit distinguished by different cell surface markers. While Ly6C is used for subpopulation differentiation in mice, corresponding human subsets are differentiated by CD14 and CD16. In this review, we specifically focused on new experimental insights from recent years mainly in regard to murine monocyte subpopulations and their roles in vascular inflammation und atherogenesis.

The good and bad faces of the CXCR4 chemokine receptor

Chemokines are chemotactic cytokines that promote cell migration and activation under homeostatic and inflammatory conditions. Chemokines bind to seven transmembrane-spanning receptors that are coupled to heterotrimeric guanine nucleotide-binding (G) proteins, which are the responsible for intracellularly transmitting the activating signals for cell migration. Hematopoiesis, vascular development, lymphoid organ morphogenesis, cardiogenesis and neural differentiation are amongst the processes involving chemokine function. In addition, immune cell trafficking from bone marrow to blood circulation, and from blood and lymph to lymphoid and inflamed tissues, is tightly regulated by chemokines both under physiological conditions and also in autoimmune diseases. Furthermore, chemokine binding to their receptors stimulate trafficking to and positioning of cancer cells into target tissues and organs during tumour dissemination. The CXCL12 chemokine (also known as stromal-cell derived factor-1α, SDF-1α) plays key roles in hematopoiesis and lymphoid tissue architecture, in cardiogenesis, vascular formation and neurogenesis, as well as in the trafficking of solid and hematological cancer cell types. CXCL12 binds to the CXCR4 receptor, a multi-facetted molecule which tightly mirrors CXCL12 functions in homeostasis and disease. This review addresses the important roles of the CXCR4-CXCL12 axis in homeostasis, specially focusing in hematopoiesis, as well as it provides a picture of CXCR4 as mediator of cancer cell spreading, and a view of the available CXCR4 antagonists in different cancer types.

Cutting Edge: CCR2 Is Not Required for Ly6Chi Monocyte Egress from the Bone Marrow but Is Necessary for Migration within the Brain in La Crosse Virus Encephalitis

Inflammatory monocyte (iMO) recruitment to the brain is a hallmark of many neurologic diseases. Prior to entering the brain, iMOs must egress into the blood from the bone marrow through a mechanism, which for known encephalitic viruses, is CCR2 dependent. In this article, we show that during La Crosse Virus-induced encephalitis, egress of iMOs was surprisingly independent of CCR2, with similar percentages of iMOs in the blood and brain of heterozygous and CCR2^-/- mice following infection. Interestingly, CCR2 was required for iMO trafficking from perivascular areas to sites of virus infection within the brain. Thus, CCR2 was not essential for iMO trafficking to the blood or the brain but was essential for trafficking within the brain parenchyma. Analysis of other orthobunyaviruses showed that Jamestown Canyon virus also induced CCR2-independent iMO egress to the blood. These studies demonstrate that the CCR2 requirement for iMO egress to the blood is not universal for all viruses.

Increased Mobilization of CD45+CD34+VLA-4+ Cells in Acute Viral Myocarditis Induced by Coxsackievirus B3

OBJECTIVES

Bone marrow-derived cells (BMCs) have recently been identified to play a vital role in repairing damaged myocardium; however, it is not known whether or not mobilization of BMCs is involved in the pathogenesis of acute viral myocarditis (VMC). Thus, we analyzed the expression of CD45+CD34+VLA-4+ cells and vascular cell adhesion protein-1 (VCAM-1) in a murine model of acute VMC.

METHODS

Male BALB/c mice were intraperitoneally infected with coxsackievirus B3 to establish acute VMC. The frequency of CD45+CD34+VLA-4+ cells in the heart, peripheral blood, and bone marrow was examined by flow cytometry 3, 7, 14, and 28 days after injection. Cardiac VCAM-1 and pathology scores were determined by immunohistochemistry, and myocardial VCAM-1, IL-1β, and TNF-α were analyzed by RT-PCR and Western blot.

RESULTS

In mice with acute VMC, the CD45+CD34+VLA-4+ cell population in the heart was significantly increased by day 7 and then decreased; in contrast, the CD45+CD34+VLA-4+ cell population decreased in the bone marrow and peripheral blood by day 3 and then increased. High expression of VCAM-1 was detected in the heart in parallel with CD45+CD34+VLA-4+ cell expression.

CONCLUSIONS

In mice with acute VMC, VCAM-1-induced CD45+CD34+VLA-4+ cell mobilization into the injured heart is involved in the repair of injured myocardium.

The role of endoglin in post-ischemic revascularization

Following arterial occlusion, blood vessels respond by forming a new network of functional capillaries (angiogenesis), by reorganizing preexisting capillaries through the recruitment of smooth muscle cells to generate new arteries (arteriogenesis) and by growing and remodeling preexisting collateral arterioles into physiologically relevant arteries (collateral development). All these processes result in the recovery of organ perfusion. The importance of endoglin in post-occlusion reperfusion is sustained by several observations: (1) endoglin expression is increased in vessels showing active angiogenesis/remodeling; (2) genetic endoglin haploinsufficiency in humans causes deficient angiogenesis; and (3) the reduction of endoglin expression by gene disruption or the administration of endoglin-neutralizing antibodies reduces angiogenesis and revascularization. However, the precise role of endoglin in the several processes associated with revascularization has not been completely elucidated and, in some cases, the function ascribed to endoglin by different authors is controversial. The purpose of this review is to organize in a critical way the information available for the role of endoglin in several phenomena (angiogenesis, arteriogenesis and collateral development) associated with post-ischemic revascularization.

The mechanistic impact of CD22 engagement with epratuzumab on B cell function: Implications for the treatment of systemic lupus erythematosus

Epratuzumab is a B-cell-directed non-depleting monoclonal antibody that targets CD22. It is currently being evaluated in two phase 3 clinical trials in patients with systemic lupus erythematosus (SLE), a disease associated with abnormalities in B-cell function and activation. The mechanism of action of epratuzumab involves perturbation of the B-cell receptor (BCR) signalling complex and intensification of the normal inhibitory role of CD22 on the BCR, leading to reduced signalling and diminished activation of B cells. Such effects may result from down-modulation of CD22 upon binding by epratuzumab, as well as decreased expression of other proteins involved in amplifying BCR signalling capability, notably CD19. The net result is blunting the capacity of antigen engagement to induce B-cell activation. The functional consequences of epratuzumab binding to CD22 include diminished B-cell proliferation, effects on adhesion molecule expression, and B-cell migration, as well as reduced production of pro-inflammatory cytokines, such as IL-6 and TNF. Studies in patients treated with epratuzumab have revealed a number of pharmacodynamic effects that are linked to the mechanism of action (i.e., a loss of the target molecule CD22 from the B-cell surface followed by a modest reduction in peripheral B-cell numbers after prolonged therapy). Together, these data indicate that epratuzumab therapy affords a unique means to modulate BCR complex expression and signalling.

20-HETE regulates the angiogenic functions of human endothelial progenitor cells and contributes to angiogenesis in vivo

Circulating endothelial progenitor cells (EPC) contribute to postnatal neovascularization. We identified the cytochrome P450 4A/F-20-hydroxyeicosatetraenoic acid (CYP4A/F-20-HETE) system as a novel regulator of EPC functions associated with angiogenesis in vitro. Here, we explored cellular mechanisms by which 20-HETE regulates EPC angiogenic functions and assessed its contribution to EPC-mediated angiogenesis in vivo. Results showed that both hypoxia and vascular endothelial growth factor (VEGF) induce CYP4A11 gene and protein expression (the predominant 20-HETE synthases in human EPC), and this is accompanied by an increase in 20-HETE production by ~1.4- and 1.8-fold, respectively, compared with the control levels. Additional studies demonstrated that 20-HETE and VEGF have a synergistic effect on EPC proliferation, whereas 20-HETE antagonist 20-HEDGE or VEGF-neutralizing antibody negated 20-HETE- or VEGF-induced proliferation, respectively. These findings are consistent with the presence of a positive feedback regulation on EPC proliferation between the 20-HETE and the VEGF pathways. Furthermore, we found that 20-HETE induced EPC adhesion to fibronectin and endothelial cell monolayer by 40 ± 5.6 and 67 ± 10%, respectively, which was accompanied by a rapid induction of very late antigen-4 and chemokine receptor type 4 mRNA and protein expression. Basal and 20-HETE-stimulated increases in adhesion were negated by the inhibition of the CYP4A-20-HETE system. Lastly, EPC increased angiogenesis in vivo by 3.6 ± 0.2-fold using the Matrigel plug angiogenesis assay, and these increases were markedly reduced by the local inhibition of 20-HETE system. These results strengthened the notion that 20-HETE regulates the angiogenic functions of EPC in vitro and EPC-mediated angiogenesis in vivo.

Covalent immobilization of stem cell factor and stromal derived factor 1α for in vitro culture of hematopoietic progenitor cells

Hematopoietic stem cells (HSCs) are currently utilized in the treatment of blood diseases, but widespread application of HSC therapeutics has been hindered by the limited availability of HSCs. With a better understanding of the HSC microenvironment and the ability to precisely recapitulate its components, we may be able to gain control of HSC behavior. In this work we developed a novel, biomimetic PEG hydrogel material as a substrate for this purpose and tested its potential with an anchorage-independent hematopoietic cell line, 32D clone 3 cells. We immobilized a fibronectin-derived adhesive peptide sequence, RGDS; a cytokine critical in HSC self-renewal, stem cell factor (SCF); and a chemokine important in HSC homing and lodging, stromal derived factor 1α (SDF1α), onto the surfaces of poly(ethylene glycol) (PEG) hydrogels. To evaluate the system's capabilities, we observed the effects of the biomolecules on 32D cell adhesion and morphology. We demonstrated that the incorporation of RGDS onto the surfaces promotes 32D cell adhesion in a dose-dependent fashion. We also observed an additive response in adhesion on surfaces with RGDS in combination with either SCF or SDF1α. In addition, the average cell area increased and circularity decreased on gel surfaces containing immobilized SCF or SDF1α, indicating enhanced cell spreading. By recapitulating aspects of the HSC microenvironment using a PEG hydrogel scaffold, we have shown the ability to control the adhesion and spreading of the 32D cells and demonstrated the potential of the system for the culture of primary hematopoietic cell populations.

Stem cell recruitment after injury: lessons for regenerative medicine

Tissue repair and regeneration are thought to involve resident cell proliferation as well as the selective recruitment of circulating stem and progenitor cell populations through complex signaling cascades. Many of these recruited cells originate from the bone marrow, and specific subpopulations of bone marrow cells have been isolated and used to augment adult tissue regeneration in preclinical models. Clinical studies of cell-based therapies have reported mixed results, however, and a variety of approaches to enhance the regenerative capacity of stem cell therapies are being developed based on emerging insights into the mechanisms of progenitor cell biology and recruitment following injury. This article discusses the function and mechanisms of recruitment of important bone marrow-derived stem and progenitor cell populations following injury, as well as the emerging therapeutic applications targeting these cells.

Overcoming drug resistance and treating advanced prostate cancer

Most of the prostate cancers (PCa) in advanced stage will progress to castration-resistant prostate cancer (CRPC). Within CRPC group, 50-70% of the patients will develop bone metastasis in axial and other regions of the skeleton. Once PCa cells spread to the bone, currently, no treatment regimens are available to eradicate the metastasis, and cancer-related death becomes inevitable. In 2012, it is estimated that there will be 28,170 PCa deaths in the United States. Thus, PCa bone metastasis-associated clinical complications and treatment resistance pose major clinical challenges. In this review, we will present recent findings on the molecular and cellular pathways that are responsible for bone metastasis of PCa. We will address several novel mechanisms with a focus on the role of bone and bone marrow microenvironment in promoting PCa metastasis, and will further discuss why prostate cancer cells preferentially metastasize to the bone. Additionally, we will discuss novel roles of several key pathways, including angiogenesis and extracellular matrix remodeling in bone marrow and stem cell niches with their relationship to PCa bone metastasis and poor treatment response. We will evaluate how various chemotherapeutic drugs and radiation therapies may allow aggressive PCa cells to gain advantageous mutations leading to increased survival and rendering the cancer cells to become resistant to treatment. The novel concept relating several key survival and invasion signaling pathways to stem cell niches and treatment resistance will be reviewed. Lastly, we will provide an update of several recently developed novel drug candidates that target metastatic cancer microenvironments or niches, and discuss the advantages and significance provided by such therapeutic approaches in pursuit of overcoming drug resistance and treating advanced PCa.

Inflammatory monocytes and the pathogenesis of viral encephalitis

Monocytes are a heterogeneous population of bone marrow-derived cells that are recruited to sites of infection and inflammation in many models of human diseases, including those of the central nervous system (CNS). Ly6C^hi/CCR2^hi inflammatory monocytes have been identified as the circulating precursors of brain macrophages, dendritic cells and arguably microglia in experimental autoimmune encephalomyelitis; Alzheimer’s disease; stroke; and more recently in CNS infection caused by Herpes simplex virus, murine hepatitis virus, Theiler’s murine encephalomyelitis virus, Japanese encephalitis virus and West Nile virus. The precise differentiation pathways and functions of inflammatory monocyte-derived populations in the inflamed CNS remains a contentious issue, especially in regard to the existence of monocyte-derived microglia. Furthermore, the contributions of monocyte-derived subsets to viral clearance and immunopathology are not well-defined. Thus, understanding the pathways through which inflammatory monocytes migrate to the brain and their functional capacity within the CNS is critical to inform future therapeutic strategies. This review discusses some of the key aspects of inflammatory monocyte trafficking to the brain and addresses the role of these cells in viral encephalitis.

Endothelial endoglin is involved in inflammation: role in leukocyte adhesion and transmigration

Human endoglin is an RGD-containing transmembrane glycoprotein identified in vascular endothelial cells. Although endoglin is essential for angiogenesis and its expression is up-regulated in inflammation and at sites of leukocyte extravasation, its role in leukocyte trafficking is unknown. This function was tested in endoglin heterozygous mice (Eng(+/-)) and their wild-type siblings Eng(+/+) treated with carrageenan or LPS as inflammatory agents. Both stimuli showed that inflammation-induced leukocyte transendothelial migration to peritoneum or lungs was significantly lower in Eng(+/-) than in Eng(+/+) mice. Leukocyte transmigration through cell monolayers of endoglin transfectants was clearly enhanced in the presence of endoglin. Coating transwells with the RGD-containing extracellular domain of endoglin, enhanced leukocyte transmigration, and this increased motility was inhibited by soluble endoglin. Leukocytes stimulated with CXCL12, a chemokine involved in inflammation, strongly adhered to endoglin-coated plates and to endoglin-expressing endothelial cells. This endoglin-dependent adhesion was abolished by soluble endoglin, RGD peptides, the anti-integrin α5β1 inhibitory antibody LIA1/2 and the chemokine receptor inhibitor AMD3100. These results demonstrate for the first time that endothelial endoglin interacts with leukocyte integrin α5β1 via its RGD motif, and this adhesion process is stimulated by the inflammatory chemokine CXCL12, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking.

Molecular mechanisms underlying adhesion and migration of hematopoietic stem cells

Hematopoietic stem cell transplantation is the most powerful treatment modality for a large number of hematopoietic malignancies, including leukemia. Successful hematopoietic recovery after transplantation depends on homing of hematopoietic stem cells to the bone marrow and subsequent lodging of those cells in specific niches in the bone marrow. Migration of hematopoietic stem cells to the bone marrow is a highly regulated process that requires correct regulation of the expression and activity of various molecules including chemoattractants, selectins and integrins. This review will discuss recent studies that have extended our understanding of the molecular mechanisms underlying adhesion, migration and bone marrow homing of hematopoietic stem cells.

FTY720 markedly increases alloengraftment but does not eliminate host anti-donor T cells that cause graft rejection on its withdrawal

The immunomodulator FTY720 (FTY) is beneficial in models of graft-versus-host disease, solid organ transplantation, and autoimmunity and has been approved for use in patients with multiple sclerosis. FTY modifies the homing and migration of many cell types. We report that FTY has profound positive and negative effects on allogeneic bone marrow (BM) engraftment in sublethally irradiated recipients. FTY increased donor hematopoietic progenitors in the BM, resulting in high donor engraftment in the B cell, myeloid cell, and natural killer cell, but not T cell, lineages. Donor T cell progenitors within the thymus of FTY-treated recipients were dramatically reduced, resulting in a lack of donor T cell reconstitution. In addition to preventing the ingress of donor (and host) T cell progenitors, FTY prevented the egress of fully functional host CD4+CD8- and CD4-CD8+ thymocytes that on cessation of FTY administration were able to exit from the thymus and contribute to a rapid and complete rejection of a well-established donor BM graft. When used in combination with anti-CD40L mAbs to block the CD40L:CD40 costimulatory pathway, FTY markedly enhanced anti-CD40L mAb-mediated alloengraftment promotion. In contrast to FTY alone, the combination of anti-CD40L mAb and FTY resulted in a surprisingly stable, multilineage, long-term donor chimerism. These data illustrate FTY's profound migration modulating effects and suggest a use in combinatorial therapy in achieving stable alloengraftment under nonmyeloablative conditions.

Stromal-derived factor-1 and its receptor, CXCR4, are constitutively expressed by mouse liver sinusoidal endothelial cells: implications for the regulation of hematopoietic cell migration to the liver during extramedullary hematopoiesis

Stromal-derived factor (SDF)-1 is the main regulating factor for trafficking/homing of hematopoietic stem cells (HSC) to the bone marrow (BM). It is possible that this chemokine may also play a fundamental role in regulating the migration of HSC to several organs during extramedullary hematopoiesis. Because liver sinusoidal endothelial cells (LSEC) constitute an extramedullary niche for HSC, it is possible that these cells represent one of the main cellular sources of SDF-1 at the liver. Here, we show that LSEC express SDF-1 at the mRNA and protein level. Biological assays showed that conditioned medium from LSEC (LSEC-CM) stimulated the migration of BM progenitor lineage-negative (BM/Lin⁻) cells. This effect was significantly reduced by AMD3100, indicating that the SDF-1/CXCR4 axis is involved in the stimulatory migrating effect induced by LSEC-CM. Early localization of HSC in SDF-1-expressing LSEC microenvironment together with increased levels of this chemokine in hepatic homogenates was found in an experimental model of liver extramedullary hematopoiesis. Flow cytometry studies showed that LSEC express the CXCR4 receptor. Functional assays showed that activation of this receptor by SDF-1 stimulated the migration of LSEC and increased the expression of PECAM-1. Our findings suggest that LSEC through the production of SDF-1 may constitute a fundamental niche for regulation of HSC migration to the liver. To our knowledge, this is the first report showing that LSEC not only express and secrete SDF-1, but also its receptor CXCR4.

Mobilization of hematopoietic stem and progenitor cells using inhibitors of CXCR4 and VLA-4

Successful hematopoietic stem cell transplant requires the infusion of a sufficient number of hematopoietic stem/progenitor cells (HSPCs) that are capable of homing to the bone marrow cavity and regenerating durable trilineage hematopoiesis in a timely manner. Stem cells harvested from peripheral blood are the most commonly used graft source in HSCT. Although granulocyte colony-stimulating factor (G-CSF) is the most frequently used agent for stem cell mobilization, the use of G-CSF alone results in suboptimal stem cell yields in a significant proportion of patients. Both the chemokine receptor CXCR4 and the integrin α(4)β(1) (very late antigen 4 (VLA-4)) have important roles in the homing and retention of HSPCs within the bone marrow microenvironment. Preclinical and/or clinical studies have shown that targeted disruption of the interaction of CXCR4 or VLA-4 with their ligands results in the rapid and reversible mobilization of hematopoietic stem cells into the peripheral circulation and is synergistic when combined with G-CSF. In this review, we discuss the development of small-molecule CXCR4 and VLA-4 inhibitors and how they may improve the utility and convenience of peripheral blood stem cell transplantation.

Guanine nucleotide exchange factor Vav1 regulates perivascular homing and bone marrow retention of hematopoietic stem and progenitor cells

Engraftment and maintenance of hematopoietic stem and progenitor cells (HSPC) depend on their ability to respond to extracellular signals from the bone marrow microenvironment, but the critical intracellular pathways integrating these signals remain poorly understood. Furthermore, recent studies provide contradictory evidence of the roles of vascular versus osteoblastic niche components in HSPC function. To address these questions and to dissect the complex upstream regulation of Rac GTPase activity in HSPC, we investigated the role of the hematopoietic-specific guanine nucleotide exchange factor Vav1 in HSPC localization and engraftment. Using intravital microscopy assays, we demonstrated that transplanted Vav1(-/-) HSPC showed impaired early localization near nestin(+) perivascular mesenchymal stem cells; only 6.25% of Vav1(-/-) HSPC versus 45.8% of wild-type HSPC were located less than 30 μm from a nestin(+) cell. Abnormal perivascular localization correlated with decreased retention of Vav1(-/-) HSPC in the bone marrow (44-60% reduction at 48 h posttransplant, compared with wild-type) and a very significant defect in short- and long-term engraftment in competitive and noncompetitive repopulation assays (<1.5% chimerism of Vav1(-/-) cells vs. 53-63% for wild-type cells). The engraftment defect of Vav1(-/-) HSPC was not related to alterations in proliferation, survival, or integrin-mediated adhesion. However, Vav1(-/-) HSPC showed impaired responses to SDF1α, including reduced in vitro migration in time-lapse microscopy assays, decreased circadian and pharmacologically induced mobilization in vivo, and dysregulated Rac/Cdc42 activation. These data suggest that Vav1 activity is required specifically for SDF1α-dependent perivascular homing of HSPC and suggest a critical role for this localization in retention and subsequent engraftment.

Cholesterol regulates VEGFR-1 (FLT-1) expression and signaling in acute leukemia cells

VEGF receptors 1 (FLT-1) and 2 (KDR) are expressed on subsets of acute myeloid leukemia (AML) and acute lymphoid leukemia cells, in which they induce cell survival, proliferation, and migration. However, little is known about possible cofactors that regulate VEGF receptor expression and activation on leukemia cells. Here we show that cholesterol accumulates in leukemia-rich sites within bone marrow of xenotransplanted severe combined immunodeficient (SCID) mice. Therefore, we hypothesized that cholesterol-rich domains might regulate FLT-1 signaling and chemotaxis of acute leukemias. We then showed that FLT-1 accumulates in discrete cholesterol-rich membrane domains where it associates with caveolin-1 and that placenta growth factor (PlGF)/VEGF stimulation promotes FLT-1 localization in such cholesterol-rich domains. Accordingly, FLT-1 localization and its phosphorylation are abrogated by methyl-β-cyclodextrin (MβCD), which removes cellular cholesterol, and by nystatin, an inhibitor of lipid-raft endocytosis. Mechanistically, cholesterol increases FLT-1 expression and promotes PlGF/VEGF-induced leukemia cells viability and also induces VEGF production by the leukemia cells in vitro. Taken together, we conclude that cholesterol regulates VEGF:VEGFR-1 signaling on subsets of acute leukemias, modulating cell migration, and viability, which may be crucial for disease progression. Finally, we provide evidence obtained from human AML samples that primary leukemia cells accumulate significantly more cholesterol than do normal cells and that cholesterol accumulation correlates with disease aggressiveness.

Altered SDF-1-mediated differentiation of bone marrow-derived endothelial progenitor cells in diabetes mellitus

In diabetic patients and animal models of diabetes mellitus (DM), circulating endothelial progenitor cell (EPC) number is lower than in normoglycaemic conditions and EPC angiogenic properties are inhibited. Stromal cell derived factor-1 (SDF-1) plays a key role in bone marrow (BM) c-kit⁺ stem cell mobilization into peripheral blood (PB), recruitment from PB into ischemic tissues and differentiation into endothelial cells. The aim of the present study was to examine the effect of DM in vivo and in vitro, on murine BM-derived c-kit⁺ cells and on their response to SDF-1. Acute hindlimb ischemia was induced in streptozotocin-treated DM and control mice; circulating c-kit⁺ cells exhibited a rapid increase followed by a return to control levels which was significantly faster in DM than in control mice. CXCR4 expression by BM c-kit⁺ cells as well as SDF-1 protein levels in the plasma and in the skeletal muscle, both before and after the induction of ischemia, were similar between normoglycaemic and DM mice. However, BM-derived c-kit⁺ cells from DM mice exhibited an impaired differentiation towards the endothelial phenotype in response to SDF-1; this effect was associated with diminished protein kinase phosphorylation. Interestingly, SDF-1 ability to induce differentiation of c-kit⁺ cells from DM mice was restored when cells were cultured under normoglycaemic conditions whereas c-kit⁺ cells from normoglycaemic mice failed to differentiate in response to SDF-1 when they were cultured in hyperglycaemic conditions. These results show that DM diminishes circulating c-kit⁺ cell number following hindlimb ischemia and inhibits SDF-1-mediated AKT phosphorylation and differentiation towards the endothelial phenotype of BM-derived c-kit⁺ cells.

Regulation of Lymphoid and Myeloid Leukemic Cell Survival: Role of Stromal Cell Adhesion Molecules

Several laboratories have documented the necessity for direct contact of lymphoid and myeloid leukemic cells with bone marrow stromal cells for optimal survival. Subsequent studies have identified various stromal cell adhesion molecules and soluble factors that facilitate survival through leukemic cell anti-apoptotic signal transduction pathways. This report provides an overview of enhanced leukemic cell survival through adhesive interactions with bone marrow expressed molecules. In addition, we describe the establishment of cloned murine stromal cell lines engineered to constitutively express human VCAM-1 protein on their surface. These stromal cell lines will be useful in studies aimed at better understanding the specific contribution of VCAM-1: VLA-4 signaling in maintenance of residual leukemic disease.

SDF-1 and PDGF enhance alphavbeta5-mediated ERK activation and adhesion-independent growth of human pre-B cell lines

CD23 acts through the alphavbeta5 integrin to promote growth of human pre-B cell lines in an adhesion-independent manner. alphavbeta5 is expressed on normal B-cell precursors in the bone marrow. Soluble CD23 (sCD23), short CD23-derived peptides containing the arg-lys-cys (RKC) motif recognized by alphavbeta5 and anti-alphavbeta5 monoclonal antibodies (MAbs) all sustain growth of pre-B cell lines. The chemokine stromal cell-derived factor-1 (SDF-1) regulates key processes during B-cell development. SDF-1 enhanced the growth-sustaining effect driven by ligation of alphavbeta5 with anti-alphavbeta5 MAb 15F-11, sCD23 or CD23-derived RKC-containing peptides. This effect was restricted to B-cell precursors and was specific to SDF-1. The enhancement in growth was associated with the activation of extracellular signal-regulated kinase (ERK) and both these responses were attenuated by the MEK inhibitor U0126. Finally, platelet-derived growth factor also enhanced both alphavbeta5-mediated cell growth and ERK activation. The data suggest that adhesion-independent growth-promoting signals delivered to B-cell precursors through the alphavbeta5 integrin can be modulated by cross-talk with receptors linked to both G-protein and tyrosine kinase-coupled signalling pathways.

The CysLT1 ligand leukotriene D4 supports alpha4beta1- and alpha5beta1-mediated adhesion and proliferation of CD34+ hematopoietic progenitor cells

Cytokines and chemokines control hematopoietic stem and progenitor cell (HPC) proliferation and trafficking. However, the role of nonpeptide mediators in the bone marrow microenvironment has remained elusive. Particularly CysLT(1), a G protein-coupled receptor recognizing inflammatory mediators of the cysteinyl leukotriene family, is highly expressed in HPCs. We therefore analyzed the effects of its ligands on human CD34(+) HPCs. The most potent CysLT(1) ligand, LTD(4), rapidly and significantly up-regulated alpha(4)beta(1) and alpha(5)beta(1) integrin-dependent adhesion of both primitive and committed HPC. LTD(4)-triggered adhesion was inhibited by specific CysLT(1) antagonists. The effects of other CysLT(1) ligands were weak (LTC(4)) or absent (LTE(4)). In serum-free liquid cultures supplemented with various hematopoietic cytokines including IL-3, only LTD(4) significantly augmented the expansion of HPCs in a dose-dependent manner comparable to that of peptide growth factors. LTC(4) and LTE(4) were less effective. In CD34(+) cell lines and primary HPCs, LTD(4) induced phosphorylation of p44/42 ERK/MAPK and focal adhesion kinase-related tyrosine kinase Pyk2, which is linked to integrin activation. Bone marrow stromal cells produced biologically significant amounts of cysteinyl leukotrienes only when hematopoietic cells were absent, suggesting a regulatory feedback mechanism in the hematopoietic microenvironment. In contrast to antagonists of the homing-related G protein-coupled receptor CXCR4, administration of a CysLT(1) antagonist failed to induce human CD34(+) HPC mobilization in vivo. Our results suggest that cysteinyl leukotriene may contribute to HPC retention and proliferation only when cysteinyl leukotriene levels are increased either systemically during inflammation or locally during marrow aplasia.

Erythrocytic precursor cells show potent shear stress resistant adhesion and home to hematopoietic tissue in vivo

BACKGROUND

Transfusion of erythropoietic precursor cells has been suggested as an alternative to conventional red blood cells. However, little is known about the fate of transfused erythrocytic precursors after they enter the bloodstream.

STUDY DESIGN AND METHODS

Erythrocytic precursors were classified by flow cytometry into different maturation stages. Precursors were enriched using cell surface expression of CD71 and Ter119 antigens and analyzed under shear stress in a parallel plate flow chamber and after fluorescence tagging with PKH and transfusion into anemic mice.

RESULTS

We found that at all maturation stages, erythrocytic precursors expressed the adhesion receptor very late antigen (VLA)-4 with a frequency decreasing from 90% to approximately 60% during maturation. In contrast, expression of the beta(2)-integrins LFA-1 and Mac-1 and the rolling receptor P-selectin glycoprotein ligand-1 increased from 10% to 20% to approximately 50% during erythrocytic maturation. The chemokine receptor CXCR4 was expressed at low levels during differentiation stages. In vitro shear stress adhesion analysis showed that erythrocytic precursors can efficiently activate VLA-4 such that it binds its cognate ligand, vascular cell adhesion molecule (VCAM)-1. The coimmobilization of stromal cell-derived factor-1 alpha with VCAM-1 strengthened this adhesion. Transfusion of primitive (CD71+) or more mature (Ter119+) erythrocytic precursors into mice showed that both populations selectively and efficiently home to hematopoietic tissues.

CONCLUSION

Our results demonstrate that erythrocytic precursor cells of different maturation stages are capable of homing to hematopoietic organs. This work has implications for the development of transfusion protocols that use ex vivo expanded, but not fully matured, erythrocytic precursors from cultured stem cell populations.

Crosstalk between CXCR4/stromal derived factor-1 and VLA-4/VCAM-1 pathways regulates neutrophil retention in the bone marrow

Neutrophil retention in and release from the bone marrow is a critical process that remains incompletely understood. Previous work has implicated the CXCR4/stromal derived factor-1 (SDF-1) chemokine axis in the marrow retention of neutrophils, yet the adhesion pathways responsible for this retention are unknown. Because alpha(4)beta(1) integrin (VLA-4) and its ligand VCAM-1 play a central role in the interactions of hematopoietic stem cells, lymphocytes, and developing neutrophils in the marrow, we investigated whether this integrin might be involved in marrow neutrophil retention and release. In this study, we show that VLA-4 is expressed on murine marrow neutrophils and decreases with maturation, whereas blockade of this integrin leads to the release of marrow neutrophils. Marrow neutrophils adhere via VLA-4 to VCAM-1, which is expressed on marrow endothelium and stroma, and inhibition of VCAM-1 causes release of marrow neutrophils. Furthermore, SDF-1 (CXCL12) signaling through neutrophil CXCR4 augments VLA-4 adhesion to VCAM-1 in vitro, an effect that is blocked by preincubation with pertussis toxin. In vivo blockade of both CXCR4 and alpha(4) causes synergistic release of marrow neutrophils, showing that cross-talk between CXCR4 and VLA-4 modulates marrow retention of these cells. Taken together, these results indicate that the VLA-4/VCAM adhesion pathway is critical in the retention and maturation-controlled release of neutrophils from the marrow, while providing an important link between the CXCR4/SDF-1 signaling axis and the adhesion events that govern this process.

Suppression of CXCL12 production by bone marrow osteoblasts is a common and critical pathway for cytokine-induced mobilization

Current evidence suggests that hematopoietic stem/progenitor cell (HSPC) mobilization by granulocyte colony-stimulating factor (G-CSF) is mediated by induction of bone marrow proteases, attenuation of adhesion molecule function, and disruption of CXCL12/CXCR4 signaling in the bone marrow. The relative importance and extent to which these pathways overlap or function independently are uncertain. Despite evidence of protease activation in the bone marrow, HSPC mobilization by G-CSF or the chemokine Grobeta was abrogated in CXCR4(-/-) bone marrow chimeras. In contrast, HSPC mobilization by a VLA-4 antagonist was intact. To determine whether other mobilizing cytokines disrupt CXCR4 signaling, we characterized CXCR4 and CXCL12 expression after HSPC mobilization with Flt3 ligand (Flt3L) and stem cell factor (SCF). Indeed, treatment with Flt3L or SCF resulted in a marked decrease in CXCL12 expression in the bone marrow and a loss of surface expression of CXCR4 on HSPCs. RNA in situ and sorting experiments suggested that the decreased CXCL12 expression is secondary to a loss of osteoblast lineage cells. Collectively, these data suggest that disruption of CXCR4 signaling and attenuation of VLA-4 function are independent mechanisms of mobilization by G-CSF. Loss of CXCL12 expression by osteoblast appears to be a common and key step in cytokine-induced mobilization.

Identification and Characterization of Vitamin A-Storing Cells in Fetal Liver: Implications for Functional Importance of Hepatic Stellate Cells in Liver Development and Hematopoiesis

Hepatic stellate cells (HpSTCs) are major regulators of hepatic fibrogenesis in adults. However, their early development in fetal liver is largely unknown. To characterize fetal HpSTCs in the liver, in which hepatic development and hematopoiesis occur in parallel, we determined the phenotypic characteristics of HpSTCs from rat fetal livers, using a strategy focused on vitamin A. Storage of vitamin A in the cytoplasm is a unique characteristic of HpSTCs, permitting identification of them by vitamin A-specific autofluorescence (vA+) when excited with UV light using flow cytometry. A characteristic vA+ cell population was identified in liver as early as 13 days post coitum; it had a surface phenotype of RT1A- intercellular adhesion molecule (ICAM)-1+ vascular cell adhesion molecule (VCAM)-1+ beta3-integrin+. Although nonspecific autofluorescent cells were found with the antigenic profile of RT1A- ICAM-1+ VCAM-1+, they were beta3-integrin- and proved to be hepatoblasts, bipotent hepatic parenchymal progenitors. In addition to expression of classic HpSTC markers, the vA+ cells were able to proliferate continuously in a serum-free hormonally defined medium containing leukemia inhibitory factor, which was found to be a key factor for their replication. These results demonstrated that the vA+ cells are fetal HpSTCs with extensive proliferative activity. Furthermore, the vA+ cells strongly express hepatocyte growth factor, stromal-derived factor-1alpha, and Hlx (homeobox transcription factor), indicating that they play important roles for hepatic development and hematopoiesis. The abilities to isolate and expand fetal HpSTCs enable further investigation into their roles in early liver development and facilitate identification of possibly novel signals of potential relevance for liver diseases.

Modulation of beta1-integrins on hemopoietic progenitor cells after allergen challenge in asthmatic subjects

BACKGROUND

Mobilization of hemopoietic progenitor cells from the bone marrow (BM) is a feature of inflammatory asthmatic responses. Understanding the mechanisms regulating progenitor cell mobilization and trafficking to the peripheral circulation might be important for the development of effective asthma therapies.

OBJECTIVE

We investigated the role of adhesion molecules in the mobilization of hemopoietic progenitor cells from the BM during an allergen-induced asthmatic response.

METHODS

BM and peripheral blood samples were obtained from dual-responders with mild asthma before and at several time points after allergen challenge. Fluctuations in expression and adhesive properties of beta1- and beta2-integrins on CD34(+)CD45(+) progenitor cells were assessed by using flow cytometry and adhesion to protein-coated wells, respectively.

RESULTS

On BM-derived CD34(+)CD45(+) cells, expression of very late antigen (VLA) 4, but not VLA-5 or Mac-1, decreased significantly 24 hours after allergen challenge and had begun to recover by 48 hours after challenge. In peripheral blood allergen challenge induced a significant decrease in VLA-4 levels after 6 hours, which had not recovered by 96 hours after challenge. Similarly, VLA-5 expression decreased, most prominently at 72 to 96 hours after allergen challenge. In contrast, Mac-1 levels did not change. Chemokine-stimulated adhesion of BM-derived CD34(+)CD45(+) cells to fibronectin was significantly attenuated 24 hours after challenge. Furthermore, adhesion to fibronectin and vascular cell adhesion molecule 1 was greatly reduced by anti-VLA-4 or anti-VLA-5 antibodies.

CONCLUSIONS

Preferential downregulation of beta1-integrin expression on progenitor cells can reduce the tethering forces to BM components, thus facilitating their egress to the peripheral circulation during an allergic inflammatory response.

The monoclonal anti-VLA-4 antibody natalizumab mobilizes CD34+ hematopoietic progenitor cells in humans

We investigated the role of adhesion molecule VLA-4 in CD34+ blood stem-cell mobilization. Therefore, we examined 20 patients with multiple sclerosis (MS) who were treated with the anti-VLA-4 antibody natalizumab. Treated patients had received a median number of 4 natalizumab infusions (range: 2-9 infusions). Blood samples were taken 4 weeks following the last infusion. With a median proportion of 7.6 CD34+ cells/microL (range: 2.2-30.4 cells/microL), these patients had a significantly higher (P=.003) amount of circulating CD34+ cells compared with 5 healthy volunteers (median: 1.4/microL; range: 0.6-2.4/microL) and 5 untreated MS patients (median: 1.0/microL; range: 0.5-1.7/microL) (P=.001). Serial measurements in 4 patients receiving their first natalizumab infusion showed a maximal significant increase in circulating CD34+ cells from 3.3/microL (range: 1.6-4.8/microL) to 10.4/microL (range: 7.5-12.04/microL) 72 hours following natalizumab infusion (P=.001), including pluripotent cells in colony-forming assays. This mobilizing ability of natalizumab might be useful for patients with poor response to granulocyte colony-stimulating factor (G-CSF)-based protocols.

Mobilization of human CD34+ CD133+ and CD34+ CD133(-) stem cells in vivo by consumption of an extract from Aphanizomenon flos-aquae--related to modulation of CXCR4 expression by an L-selectin ligand?

OBJECTIVE

The goal of this study was to evaluate effects on human stem cells in vitro and in vivo of an extract from the edible cyanobacterium Aphanizomenon flos-aquae (AFA) enriched for a novel ligand for human CD62L (L-selectin).

EXPERIMENTAL APPROACH

Ligands for CD62L provide a mechanism for stem cell mobilization in conjunction with down-regulation of the CXCR4 chemokine receptor for stromal derived factor 1. Affinity immunoprecipitation was used to identify a novel ligand for CD62L from a water extract from AFA. The effects of AFA water extract on CD62L binding and CXCR4 expression was tested in vitro using human bone marrow CD34+ cells and the two progenitor cell lines, KG1a and K562. A double-blind randomized crossover study involving 12 healthy subjects evaluated the effects of consumption on stem cell mobilization in vivo.

RESULTS

An AFA extract rich in the CD62L ligand reduced the fucoidan-mediated externalization of the CXCR4 chemokine receptor on bone marrow CD34+ cells by 30% and the CD62L+ CD34+ cell line KG1A by 50% but did not alter the CXCR4 expression levels on the CD34(-) cell line K562. A transient, 18% increase in numbers of circulating CD34+ stem cells maximized 1 hour after consumption (P<.0003). When 3 noncompliant volunteers were removed from analysis, the increase in CD34+ cells was 25% (P<.0001).

CONCLUSION

AFA water extract contains a novel ligand for CD62L. It modulates CXCR4 expression on CD34+ bone marrow cells in vitro and triggers the mobilization of CD34+ CD133+ and CD34+ CD133(-) cells in vivo.

Stepping out of the flow: capillary extravasation in cancer metastasis

In order for cancer cells to successfully colonize a metastatic site, they must detach from the primary tumor using extracellular matrix-degrading proteases, intravasate and survive in the circulation, evade the immune response, and extravasate the vasculature to invade the target tissue parenchyma, where metastatic foci are established. Though many of the steps of metastasis are widely studied, the precise cellular interactions and molecular alterations associated with extravasation are unknown, and further study is needed to elucidate the mechanisms inherent to this process. Studies of leukocytes localized to inflamed tissue during the immune response may be used to elucidate the process of cancer extravasation, since leukocyte diapedesis through the vasculature involves critical adhesive interactions with endothelial cells, and both leukocytes and cancer cells express similar surface receptors capable of binding endothelial adhesion molecules. Thus, leukocyte extravasation during the inflammatory response has provided a model for transendothelial migration (TEM) of cancer cells. Leukocyte extravasation is characterized by a process whereby rolling mediated by cytokine-activated endothelial selectins is followed by firmer adhesions with beta1 and beta2 integrin subunits to an activated endothelium and subsequent diapedesis, which most likely involves activation of Rho GTPases, regulators of cytoskeletal rearrangements and motility. It is controversial whether such selectin-mediated rolling is necessary for TEM of cancer cells. However, it has been established that similar stable adhesions between tumor and endothelial cells precede cancer cell transmigration through the endothelium. Additionally, there is support for the preferential attachment of tumor cells to the endothelium and, accordingly, site-specific metastasis of cancer cells. Rho GTPases are critical to TEM of cancer cells as well, and some progress has been made in understanding the specific roles of the Rho GTPase family, though much is still unknown. As the mechanisms of cancer TEM are elucidated, new approaches to study and target metastasis may be utilized and developed.

"Thinking without thinking" about natalizumab and PML

The novel multiple sclerosis (MS) therapeutic natalizumab has taken neurologists and their MS patients on a roller-coaster ride: initial encouraging efficacy data led to expedited release in the United States, followed by suspension of dosing with the unexpected occurrence of progressive multifocal leukoencephalopathy (PML) in three clinical trial participants. The drug was re-released in 2006, in a restricted distribution format. Aside from PML, natalizumab treatment was not associated with opportunistic infections, suggesting the possibility that PML in these individuals was mechanism-based, and was not a consequence of generalized immunosuppression. This commentary proposes a hypothesis to account for PML in natalizumab-treated patients.

Focal adhesion kinase is required for CXCL12-induced chemotactic and pro-adhesive responses in hematopoietic precursor cells

Hematopoietic stem/progenitor cells (HSC/P) reside in the bone marrow in distinct anatomic locations (niches) to receive growth, survival and differentiation signals. HSC/P localization and migration between niches depend on cell-cell and cell-matrix interactions, which result from the cooperation of cytokines, chemokines and adhesion molecules. The CXCL12-CXCR4 pathway, in particular, is essential for myelopoiesis and B lymphopoiesis but the molecular mechanisms of CXCL12 action remain unclear. We previously noted a strong correlation between prolonged CXCL12-mediated focal adhesion kinase (FAK) phosphorylation and sustained pro-adhesive responses in progenitor B cells, but not in mature B cells. Although FAK has been well studied in adherent fibroblasts, its function in hematopoietic cells is not defined. We used two independent approaches to reduce FAK expression in (human and mouse) progenitor cells. RNA interference (RNAi)-mediated FAK silencing abolished CXCL12-induced responses in human pro-B leukemia, REH cells. FAK-deficient REH cells also demonstrated reduced CXCL12-induced activation of the GTPase Rap1, suggesting the importance of FAK in CXCL12-mediated integrin activation. Moreover, in FAK(flox/flox) hematopoietic precursor cells, Cre-mediated FAK deletion resulted in impaired CXCL12-induced chemotaxis. These studies suggest that FAK may function as a key intermediary in signaling pathways controlling hematopoietic cell lodgment and lineage development.

Effects of AMD3100 on transmigration and survival of acute myelogenous leukemia cells

Acute myelogenous leukaemia (AML) blasts transmigrate in response to SDF-1alpha. AMD3100, a novel bicyclam molecule which inhibits stromal-derived factor (SDF)-1alpha/CXCR4 interactions, inhibited the transmigration of AML blasts and inhibited outgrowth of leukemia colony forming units. AMD3100 did not abrogate stroma-mediated protection from cytarabine-mediated apoptosis, except in the case of one promyelocytic leukemic sample tested, and it did not influence adhesion of blasts to endothelial monolayers. When AML blasts were pretreated with AMD3100, the positive effects of SDF-1alpha on NOD/SCID engraftment were diminished. This work confirms that AML is influenced by the SDF-1alpha/CXCR4 axis and demonstrates that disruption of this axis by the bicyclam AMD3100 can influence AML microenvironmental interactions.

Differential role for very late antigen-5 in mobilization and homing of hematopoietic stem cells

The role of very late antigen-5 (VLA-5) in homing and mobilization of hematopoietic stem cells from normal bone marrow (NBM) and bone marrow (MBM) and peripheral blood (MPB) from mobilized mice was investigated. We found a decreased number of VLA-5-expressing cells in the lineage-negative fraction of MPB. However, virtually all stem/progenitor cells were present in the VLA-5(+) fraction and hence mobilization of hematopoietic stem cell subsets does not coincide with a downregulation of VLA-5. Stem/progenitor cells from MPB and MBM demonstrated enhanced stromal-derived factor-alpha-induced migration. This enhanced migration correlates with an improved hematopoietic reconstitution potential, with the migrated MPB cells showing the fastest reconstitution. Interestingly, homing of MPB, MBM and NBM stem/progenitor cells in bone marrow and spleen did not differ and is therefore not responsible for the differences in hematopoietic reconstitution. The observed increase in VLA-5(+) cells in the recipients after transplantation can most probably be attributed to selective homing of VLA-5(+) cells instead of an upregulation of VLA-5. Treatment with an antibody to VLA-5 partially inhibited bone marrow homing of progenitor cells, whereas homing in the spleen was hardly affected. These data indicate a differential role for VLA-5 in the movement of stem cells from and toward bone marrow.

SDF-1 expression is elevated in chronic human renal allograft rejection

The exact mechanism of acute and chronic allograft rejection still remains unclear. The chemokine SDF-1 as mediator of allograft rejection has been under intensive investigation in liver, cardiac and bone marrow transplantation, whereas in renal transplantation, there are no reports about SDF-1 to date. This study was performed to evaluate if SDF-1 might also play an important role in human renal graft biopsies. One hundred and ninety formalin-fixed, paraffin-embedded renal allograft biopsies were included in the analysis from patients with normal renal graft morphology (according to Banff 97 classification grade 1, n = 84), with acute interstitial rejection (Banff grade 4 type I, n = 10), with acute vascular rejection (Banff grade 4 type II, n = 21), with chronic allograft nephropathy (CAN, Banff grade 5, n = 23), and without rejection but with various other lesions (Banff grade 6, n = 42). SDF-1 was localized by immunohistochemistry. In biopsies with CAN, SDF-1 expression was significantly elevated in interstitial infiltrates and infiltrating neointimal cells of arteries compared with biopsies with normal renal graft morphology. This is the first study describing a role of SDF-1 in human renal allograft rejection. We were able to demonstrate in a large number of biopsies an upregulation of SDF-1 in patients with CAN. Whether SDF-1 has pro-inflammatory or protective properties in this setting has to be evaluated in further trials.

Endolyn (CD164) modulates the CXCL12-mediated migration of umbilical cord blood CD133+ cells

Hematopoietic stem cell/hematopoietic progenitor cell (HSC/HPC) homing to specific microenvironmental niches involves interactions between multiple receptor ligand pairs. Although CXCL12/CXCR4 plays a central role in these events, CXCR4 regulators that provide the specificity for such cells to lodge and be retained in particular niches are poorly defined. Here, we provide evidence that the sialomucin endolyn (CD164), an adhesion receptor that regulates the adhesion of CD34+ cells to bone marrow stroma and the recruitment of CD34+CD38(lo/-) cells into cycle, associates with CXCR4. The class II 103B2 monoclonal antibody, which binds the CD164 N-linked glycan-dependent epitope or CD164 knockdown by RNA interference, significantly inhibits the migration of CD133+ HPCs toward CXCL12 in vitro. On presentation of CXCL12 on fibronectin, CD164 associates with CXCR4, an interaction that temporally follows the association of CXCR4 with the integrins VLA-4 and VLA-5. This coincides with PKC-zeta and Akt signaling through the CXCR4 receptor, which was disrupted on the loss of CD164 though MAPK signaling was unaffected. We therefore demonstrate a novel association among 3 distinct families of cell-surface receptors that regulate cell migratory responses and identify a new role for CD164. We propose that this lends specificity to the homing and lodgment of these cells within the bone marrow niche.

Prenatal transplantation of cytokine-stimulated marrow improves early chimerism in a resistant strain combination but results in poor long-term engraftment

OBJECTIVE

In the absence of immunodeficiency, only microchimerism (<0.1%) has been achieved in human fetal recipients or nonhuman primates following in utero hematopoietic cell transplantation (IUHCT). We hypothesized that enhanced long-term engraftment might be more reliably achieved in microchimeric systems if higher levels of chimerism existed during development of adaptive immunity. To evaluate this hypothesis, we stimulated the donor cells with vascular endothelial growth factor (VEGF) and stem cell factor (SCF) prior to IUHCT in a chimerism-resistant murine strain combination.

METHODS

Donor Balb/c marrow was cultured in media with or without VEGF and SCF supplementation for 12 hours prior to IUHCT into B6 fetuses at 14 days postcoitum (dpc). Donor cell phenotype, homing, and chimerism were assessed at short and long-term time points and transplanted animals received skin allografts at 8 weeks.

RESULTS

In pretreated allogeneic recipients, early chimerism rates were more than double that of controls (71% vs 33%, p = 0.01). These differences were associated with higher numbers of pretransplant donor cell colony-forming cells without change in donor cell homing. Despite prolonged skin allograft survival for pretreated recipients compared with controls (mean survival = 20.8 vs 8.2 days, p < 0.001), long-term engraftment was unchanged.

CONCLUSIONS

These findings demonstrate that higher levels of early chimerism in recipients of cytokine-stimulated marrow result in improved short-term chimerism and tolerance. Future studies are needed to confirm the existence of a "threshold" level of chimerism necessary to sustain long-term engraftment.

Mechanisms of hematopoietic stem cell mobilization: When innate immunity assails the cells that make blood and bone

Mobilization is now used worldwide to collect large numbers of hematopoietic stem and progenitor cells (HSPCs) for transplantation. Although the first mobilizing agents were discovered largely by accident, discovery of more efficient mobilizing agents will require a better understanding of the molecular mechanisms responsible. During the past 5 years, a number of mechanisms have been identified, shedding new light on the dynamics of the hematopoietic system in vivo and on the intricate relationship between hematopoiesis, innate immunity, and bone. After briefly reviewing the mechanisms by which circulating HSPCs home into the bone marrow and what keeps them there, the current knowledge of mechanisms responsible for HSPC mobilization in response to hematopoietic growth factors such as granulocyte colony-stimulating factor, chemotherapy, chemokines, and polyanions will be discussed together with current strategies developed to further increase HSPC mobilization.

Monocyte emigration from bone marrow during bacterial infection requires signals mediated by chemokine receptor CCR2

Monocytes recruited to tissues mediate defense against microbes or contribute to inflammatory diseases. Regulation of the number of circulating monocytes thus has implications for disease pathogenesis. However, the mechanisms controlling monocyte emigration from the bone marrow niche where they are generated remain undefined. We demonstrate here that the chemokine receptor CCR2 was required for emigration of Ly6C(hi) monocytes from bone marrow. Ccr2(-/-) mice had fewer circulating Ly6C(hi) monocytes and, after infection with Listeria monocytogenes, accumulated activated monocytes in bone marrow. In blood, Ccr2(-/-) monocytes could traffic to sites of infection, demonstrating that CCR2 is not required for migration from the circulation into tissues. Thus, CCR2-mediated signals in bone marrow determine the frequency of Ly6C(hi) monocytes in the circulation.