CD44 is a major E-selectin ligand on human hematopoietic progenitor cells

CD43 is a ligand for E-selectin on CLA+ human T cells

The recruitment of memory T cells from blood into tissues is a central element of immune surveillance and adaptive immune responses and a key feature of chronic cutaneous inflammatory diseases such as psoriasis and atopic dermatitis. Human memory T cells that infiltrate skin express the carbohydrate epitope cutaneous lymphocyte-associated antigen (CLA). Expression of the CLA epitope on T cells has been described on P-selectin glycoprotein ligand-1 (PSGL-1) and associated with the acquisition of both E-selectin and P-selectin ligand functions. In this report, we show that CD43, a sialomucin expressed constitutively on T cells, can also be decorated with the CLA epitope and serve as an E-selectin ligand. CLA expressed on CD43 was found exclusively on the high-molecular-weight (125 kDa) glycoform bearing core-2-branched O-linked glycans. CLA+ CD43 purified from human T cells supported tethering and rolling in shear flow via E-selectin but did not support binding of P-selectin. The identification and characterization of CD43 as a T-cell E-selectin ligand distinct from PSGL-1 expands the role of CD43 in the regulation of T-cell trafficking and provides new targets for the modulation of immune functions in skin.

CD44 on LS174T colon carcinoma cells possesses E-selectin ligand activity

Metastasis of circulating tumor cells requires a multistep cascade of events initiated by adhesion of tumor cells to the vascular endothelium of involved tissues. This process occurs under the forces of blood flow and is promoted by adhesion molecules specialized to interact under shear conditions. The endothelial molecule E-selectin is a major mediator of these adhesive events, and there is strong evidence that E-selectin receptor-ligand interactions contribute to the formation of metastasis. However, little is known about the identity of E-selectin ligand(s) expressed on cancer cells. To address this issue, we did SDS-PAGE analysis of membrane proteins, metabolic inhibition studies, and blot rolling assays of LS174T, a colon carcinoma cell line known to interact with E-selectin under physiologic flow conditions. Our studies show that LS174T cells express the hematopoietic cell E/L-selectin (HCELL) glycoform of CD44, which functions as a high-affinity E-selectin glycoprotein ligand on these cells. However, in contrast to the HCELL glycoform on human hematopoietic progenitor cells, which expresses carbohydrate-binding determinant(s) for E-selectin primarily on N-glycans of standard CD44, the relevant determinant(s) on LS174T cells is expressed on O-glycans and is predominantly found on variant isoforms of CD44 (CD44v). Our finding that tumor-associated CD44 splice variant(s) express E-selectin ligand activity provides novel perspectives on the biology of CD44 in cancer metastasis.

Identification of leukocyte E-selectin ligands, P-selectin glycoprotein ligand-1 and E-selectin ligand-1, on human metastatic prostate tumor cells

Prostate tumor cells, which characteristically metastasize to bone, initiate binding interactions with bone marrow endothelium under blood flow conditions through binding interactions with E-selectin. We hypothesized that E-selectin ligands on prostate tumor cells are directly associated with bone-metastatic potential. In this report, we elucidate the identity of E-selectin ligands on human metastatic prostate tumor cells and examine their association with prostate tumor progression and metastasis in vivo. To our surprise, we found that the E-selectin-binding form of P-selectin glycoprotein ligand-1 (PSGL-1) is expressed on the human bone-metastatic prostate tumor MDA PCa 2b cell line. Interestingly, we also found that human prostate tumor cells derived from bone, lymph node, and brain metastases expressed another leukocyte E-selectin ligand, E-selectin ligand-1 (ESL-1). Immunohistochemical analysis of PSGL-1 and ESL-1 in normal prostate tissue and in localized and metastatic prostate tumors revealed that ESL-1 was principally localized to intracellular cell membrane and expressed on all normal and malignant prostate tissue, whereas PSGL-1 was notably detected on the surfaces of bone-metastatic prostate tumor cells. These findings implicate a functional role of PSGL-1 in the bone tropism of prostate tumor cells and establish a new perspective into the molecular mechanism of human prostate tumor metastasis.

Hierarchy of molecular-pathway usage in bone marrow homing and its shift by cytokines

Efficient bone marrow (BM) homing is a prerequisite for successful engraftment of transplanted hematopoietic cells (HPCs). Contradictory conclusions about the contribution of SDF-1/CXCR4 have clouded our understanding of its role within the molecular pathway cooperation needed for BM homing, particularly with the well-defined hierarchic network of adhesion molecules. In the present study we sought to unravel cooperative and compensatory molecular pathways guiding BM homing. Fresh BM-HPCs, rendered either SDF-1 unresponsive or Gi-signaling refractory, homed quite efficiently, because of compensation by alpha4-integrin interacting with VCAM-1. The contribution of SDF-1/CXCR4- or Gi-protein-mediated signals to BM homing became apparent after their blockade was combined with deletion of alpha4-integrin, leading to dramatic reduction in BM homing. Similar conclusions were revealed when VCAM-1-deficient hosts were used. Cytokine incubation changed the functional properties of BM-HPCs and hierarchy of molecular pathway usage in homing, by shifting the dominance among the homing mediators: loss of CXCR4 or Gi-signaling now significantly reduced BM homing, with only partial compensation through alpha4/VCAM-1 and endothelial selectins. These studies depict a flexible hierarchy of cooperating homing pathways, in which dominant players are repositioned with changing cytokine milieu, and possibly source of HPCs.

Interleukin-4 induced down-regulation of skin homing receptor expression by human viral-specific CD8 T cells may contribute to atopic risk of cutaneous infection

Factors controlling the expression of cutaneous lymphocyte-associated antigen (CLA) by T cells are poorly understood, but data from murine and human CD4(+) T cell systems have suggested that cytokines play an important role. However, there are no data examining the influence of cytokines on the expression of CLA by human antigen-specific CD8(+) T cells. Peripheral blood mononuclear cells (PBMC) were isolated from 10 HLA-A*0201-positive healthy individuals. Using HLA-peptide tetrameric complexes refolded with immunodominant peptides from Epstein-Barr virus (EBV), cytomegalovirus (CMV) and influenza A virus, we investigated the temporal associations of CLA expression by viral-specific CD8(+) T cells following stimulation with antigen. Ex vivo influenza matrix-specific CD8(+) T cells expressed significantly (P < 0.05) greater levels of CLA than EBV BMLF1 and CMV pp65-specific CD8(+) T cells (mean 9.7% influenza matrix versus 1.4% BMLF1 versus 1.1% pp65) and these differences were sustained on culture. However, regardless of viral specificity, interleukin (IL)-12 and IL-4 induced significant (P < 0.05) dose-dependent up-regulation and down-regulation of CLA expression, respectively, with IL-4 showing a dominant negative effect. In many cases, IL-4 resulted in complete abrogation of detectable CLA expression by the viral-specific CD8(+) T cells. Overall these data demonstrate that CLA expression by human viral-specific CD8(+) T cells is highly dynamic and that IL-4 causes significant down-regulation. Disorders associated with a type 2 cytokine shift may reduce the efficiency of skin homing by viral-specific CD8(+) T cells. Furthermore, the ability to modify the local and systemic microenvironment may offer novel therapeutic strategies that influence tissue-specific T cell homing.

Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size

Stem cells reside in a specialized niche that regulates their abundance and fate. Components of the niche have generally been defined in terms of cells and signaling pathways. We define a role for a matrix glycoprotein, osteopontin (OPN), as a constraining factor on hematopoietic stem cells within the bone marrow microenvironment. Osteoblasts that participate in the niche produce varying amounts of OPN in response to stimulation. Using studies that combine OPN-deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell number and function in a stem cell–nonautonomous manner. The OPN-null microenvironment was sufficient to increase the number of stem cells associated with increased stromal Jagged1 and Angiopoietin-1 expression and reduced primitive hematopoietic cell apoptosis. The activation of the stem cell microenvironment with parathyroid hormone induced a superphysiologic increase in stem cells in the absence of OPN. Therefore, OPN is a negative regulatory element of the stem cell niche that limits the size of the stem cell pool and may provide a mechanism for restricting excess stem cell expansion under conditions of niche stimulation.

CD44 is a physiological E-selectin ligand on neutrophils

The selectin family of adhesion molecules and their glycoconjugated ligands are essential for blood polymorphonuclear neutrophil (PMN) extravasation into inflammatory and infectious sites. However, E-selectin ligands on PMNs are not well characterized. We show here that CD44 immunopurified from G-CSF–differentiated 32D cells or from peripheral blood PMNs binds specifically to E-selectin. In contrast, CD44 extracted from bone marrow stromal or brain endothelial cell lines does not interact with E-selectin, suggesting cell-specific posttranslational modifications of CD44. PMN-derived CD44 binding activity is mediated by sialylated, α(1,3) fucosylated, N-linked glycans. CD44 enables slow leukocyte rolling on E-selectin expressed on inflamed endothelium in vivo and cooperates with P-selectin glycoprotein ligand–1 to recruit neutrophils into thioglycollate-induced peritonitis and staphylococcal enterotoxin A–injected skin pouch. CD44 extracted from human PMNs also binds to E-selectin. Moreover, we demonstrate that CD44 is hypofucosylated in PMNs from a patient with leukocyte adhesion deficiency type II, suggesting that it contributes to the syndrome. These findings thus suggest broader roles for CD44 in the innate immune response and uncover a potential new target for diseases in which selectins play a prominent role.

PSGL-1 derived from human neutrophils is a high-efficiency ligand for endothelium-expressed E-selectin under flow

P-selectin glycoprotein ligand-1 (PSGL-1) has been proposed as an important tethering ligand for E-selectin and is expressed at a modest level on human leukocytes. Sialyl Lewis x (sLe(x))-like glycans bind to E-selectin and are expressed at a relatively high level on circulating leukocytes. It is unclear whether PSGL-1 has unique biochemical attributes that contribute to its role as an E-selectin ligand. To probe this issue, we conjugated microspheres with either sLe(x) or PSGL-1 purified from myeloid cells (neutrophils and HL-60) and compared their adhesion to endothelial expressed E-selectin under defined shear conditions. We found that both sLe(x) and PSGL-1 microspheres adhere to 4 h of IL-1beta-activated human umbilical vein endothelial cells predominantly through E-selectin. Analysis of the adhesion revealed that the rate of initial tethering of the PSGL-1 microspheres to E-selectin was significantly greater than the rate of initial tethering of the sLe(x) microspheres despite the fact that the sLe(x) microspheres tested had higher ligand densities than the PSGL-1 microspheres. We also found that pretreatment of the PSGL-1 or sLe(x) microspheres with HECA-452 had no significant effect on initial tethering to E-selectin. These results support the hypotheses that 1) PSGL-1 is a high-efficiency tethering ligand for E-selectin, 2) ligand biochemistry can significantly influence initial tethering to E-selectin, and 3) PSGL-1 tethering to E-selectin can occur via non-HECA-452 reactive epitopes.

Enforced fucosylation of neonatal CD34+ cells generates selectin ligands that enhance the initial interactions with microvessels but not homing to bone marrow

Hematopoietic progenitor/stem cell homing to the bone marrow requires the concerted action of several adhesion molecules. Endothelial P- and E-selectins play an important role in this process, but their ligands on a large subset of neonate-derived human CD34+ cells are absent, leading to a reduced ability to interact with the bone marrow (BM) microvasculature. We report here that this deficiency results from reduced α1,3-fucosyltransferase (FucT) expression and activity in these CD34+ cells. Incubation of CD34+ cells with recombinant human FucTVI rapidly corrected the deficiency in nonbinding CD34+ cells and further increased the density of ligands for both P- and E-selectins on all cord blood–derived CD34+ cells. Intravital microscopy studies revealed that these FucTVI-treated CD34+ cells displayed a marked enhancement in their initial interactions with the BM microvasculature, but unexpectedly, homing into the BM was not improved by FucTVI treatment. These data indicate that, although exogenous FucT enzyme activity can rapidly modulate selectin binding avidity of cord blood CD34+ cells, further studies are needed to understand how to translate a positive effect on progenitor cell adhesion in bone marrow microvessels into one that significantly influences migration and lodgement into the parenchyma.

Recombinant CD44-HABD is a novel and potent direct angiogenesis inhibitor enforcing endothelial cell-specific growth inhibition independently of hyaluronic acid binding

CD44 is the main cellular receptor for hyaluronic acid (HA). We previously found that overexpression of CD44 inhibited tumor growth of mouse fibrosarcoma cells in mice. Here, we show that soluble recombinant CD44 HA-binding domain (CD44-HABD) acts directly onto endothelial cells by inhibiting endothelial cell proliferation in a cell-specific manner. Consequently, soluble recombinant CD44-HABD also blocked angiogenesis in vivo in chick and mouse, and thereby inhibited tumor growth of various origins at very low doses (0.25 mg/kg x day). The antiangiogenic effect of CD44 is independent of its HA-binding capacity, since mutants deficient in HA binding still maintain their antiangiogenic and antiproliferative properties. Recombinant CD44-HABD represents a novel class of angiogenesis inhibitors based on a cell-surface receptor.

Rolling of Human Bone-Metastatic Prostate Tumor Cells on Human Bone Marrow Endothelium under Shear Flow Is Mediated by E-Selectin

Prostate tumor cells preferentially adhere to bone marrow endothelial cells (BMECs) compared with endothelial linings from other tissue microvessels, implicating the importance of BMEC adhesion in the predilection of prostate tumor metastasis to bone. E (endothelial)-selectin, which functions as an initiator of leukocyte adhesion to target tissue endothelium, is constitutively expressed on BMECs, suggesting that prostate tumor cells could use this adhesive mechanism to initiate their migration into bone. In this report, we demonstrate for the first time that human bone-metastatic prostate tumor cells roll on human BMECs under physiological flow conditions. We show that these dynamic adhesive interactions are dependent on the expression of BMEC E-selectin and sialylated glycoconjugates on bone-metastatic prostate tumor cells. We also establish the importance of both glycoprotein(s) and glycosphingolipid structures displaying sialyl Lewis X epitopes as potential E-selectin ligands on bone-metastatic prostate tumor cells. Coexpression of sialylated glycoproteins and glycolipids on bone-metastatic prostate tumor cells triggers robust E-selectin binding activity, which is identical to that observed on human hematopoietic progenitor cells. By Western blot analysis, we identify candidate E-selectin glycoprotein ligand(s); distinct sialyl Lewis X (or HECA-452 antigen)-bearing membrane proteins were resolved at M(r) 130,000 and M(r) 220,000 as well as others ranging from M(r) 100,000 to M(r) 220,000. Immunohistochemical analysis of HECA-452 antigen expression on normal prostate tissue and on low- and high-grade prostate adenocarcinoma shows that HECA-452 antigen expression is directly associated with prostate tumor progression and may indicate acquisition of E-selectin ligand expression. These findings provide novel insight into potential adhesive mechanisms promoting hematogenous dissemination of prostate tumor cells into bone.

Comparative proteomics of glycoproteins based on lectin selection and isotope coding

Lectins have been widely used in glycan structure analysis. The studies described here exploit this fact to select glycopeptides carrying disease-associated modifications in their oligosaccharides. Coupling lectin affinity selection with recent advances in stable isotope coding for quantitative proteomics allowed a comparative proteomics method to be developed for examining aberrant glycosylation in cancer. Control and experimental samples were individually tryptic digested and differentially coded with stable isotope coding agents before they were mixed and affinity selected with a lectin affinity chromatography column. Glycopeptides carrying an alpha-L-fucose residue were selected with Lotus tetragonolobus agglutinin (LTA) immobilized on a chromatography matrix. Because the oligosaccharides of glycoproteins are generally heterogeneous and often of unknown structure, it was necessary to deglycosylate the selected peptides with PNGase F before they could be compared to sequences in DNA and protein databases. After deglycosylated peptides were transferred to a reversed phase chromatography (RPC) column and fractionated by gradient elution with increasing amounts of acetonitrile. The RPC fractions were then analyzed by both matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS). When this method was applied to a study of lymphosarcoma in canines, it was found that during chemotherapy, a series of fucosylated proteins in the blood of patients decreased in concentration more than 2-fold. Two of the proteins identified, CD44 and E-selectin, are known to be involved in cell adhesion and cancer cell migration. The observed aberrant fucosylation of these proteins is consistent with the hypothesis that CD44 and E-selectin play a key role in metastasis and the spread of cancer cells to remote sites.

The bone marrow is akin to skin: HCELL and the biology of hematopoietic stem cell homing

The recent findings that adult stem cells are capable of generating new blood vessels and parenchymal cells within tissues they have colonized has raised immense optimism that these cells may provide functional recovery of damaged organs. The use of adult stem cells for regenerative therapy poses the challenging task of getting these cells into the requisite sites with minimum morbidity and maximum efficiency. Ideally, tissue-specific colonization could be achieved by introducing the stem cells intravascularly and exploiting the native physiologic processes governing cell trafficking. Critical to the success of this approach is the use of stem cells bearing appropriate membrane molecules that mediate homing from vascular to tissue compartments. Hematopoietic stem cells (HSC) express a novel glycoform of CD44 known as hematopoietic cell E-/L-selectin ligand (HCELL). This molecule is the most potent E-selectin ligand natively expressed on any human cell. This article reviews our current understanding of the molecular basis of HSC homing and will describe the fundamental "roll" of HCELL in opening the avenues for efficient HSC trafficking to the bone marrow, the skin and other extramedullary sites.

A mouse model to study organ homing behaviour of haemopoietic progenitor cells reveals high selectivity but low efficiency of multipotent progenitors to home into haemopoietic organs

To study the homing behaviour of an enriched multipotent primitive haemopoietic progenitor cell (HPC) population in mice, undifferentiated murine factor-dependent multipotent HPCs (FDCP-mix), stably transfected with the green fluorescence protein gene, were intravenously injected into congenic mice. After 2 or 24 h, cell suspensions were prepared from bone marrow, spleen, lung, liver, muscle, colon, kidney, brain or blood of the mice and analysed by flow cytometry. Using direct quantifiable determination of total HPC numbers homed per organ and a method to estimate the degree of organ contamination by HPC that were present in blood vessels within the organs before preparation, the highest absolute numbers of HPC were detected in the liver and lungs at 2 h but this was sharply decreased at 24 h, whereas HPC selectively accumulated in the bone marrow and spleen at 24 h after transplantation. Only a few HPC were detected in other organs. The seeding efficiency of homed FDCP-mix HPC to the bone marrow and spleen was approximately 1.5% and ranged between that of primary whole bone marrow cells and lineage-depleted freshly isolated bone marrow cells. Pretreatment of HPC with inhibitors of signal transduction indicated that short-term homing of multipotent HPC into haemopoietic organs is an active process requiring co-ordinated intracellular signalling through Rho family small GTPases and protein kinases. Thus, short-term homing of FDCP-mix HPC into haemopoietic organs is of low efficiency but high selectivity, and provides a system to analyse the mechanisms and manipulation of primitive HPC which saves large numbers of donor animals.

Genetic determinants of arterial calcification associated with atherosclerosis

Increasing research interest has focused on arterial calcification in the setting of atherosclerosis. Many features of atherosclerosis-related calcification provide useful clinical information. For example, calcium mineral deposits frequently form in atherosclerotic plaque, and intimal arterial calcification can be used as a surrogate marker for atherosclerosis; also, calcium deposits are readily and noninvasively quantified, which is useful because greater amounts of coronary calcification predict a higher risk of myocardial infarction and death. Several mechanisms leading to calcification associated with atherosclerosis have been proposed; however, no direct testing of proposed mechanisms has yet been reported. Studies in genetically altered animals and in humans have shed light on potential genetic determinants, which in turn could form the basis for a more comprehensive understanding of the factors affecting calcification within plaque and the associated pathobiologic implications. We review proposed molecular and cellular mechanisms of atherosclerosis-associated arterial calcification, summarize genetic influences, and suggest areas in which further investigation is needed. Understanding the molecular and genetic determinants of specific structural plaque components such as calcification can provide a solid foundation for the development of novel therapeutic approaches to favorably alter plaque structure and minimize vulnerability to arterial rupture.

Adhesion to E-selectin promotes growth inhibition and apoptosis of human and murine hematopoietic progenitor cells independent of PSGL-1

Although both P- and E-selectin are constitutively expressed on bone marrow endothelial cells, their role in the regulation of hematopoiesis has only recently been investigated. We have previously shown that P-selectin glycoprotein ligand-l (PSGL-1/CD162) is expressed by primitive human bone marrow CD34+ cells, mediates their adhesion to P-selectin, and, more importantly, inhibits their proliferation. We now demonstrate that adhesion to E-selectin inhibits the proliferation of human CD34+ cells isolated either from human umbilical cord blood, adult mobilized blood, or steady-state bone marrow. Furthermore, a subpopulation, which does not contain the most primitive hematopoietic progenitor cells, undergoes apoptosis following E-selectin-mediated adhesion. The same phenomenon was observed in cells isolated from mouse bone marrow. Using lineage-negative Sca-1+ c-KIT+ bone marrow cells from PSGL-1(-/-) and wild-type mice, we establish that PSGL-1 is not the ligand involved in E-selectin-mediated growth inhibition and apoptosis. Moreover, stable transfection of the human myeloid cell line K562 (which does not express PSGL-1) with alpha(1,3) fucosyltransferase VII alone was sufficient to recapitulate the E-selectin-mediated growth inhibition and apoptosis observed in hematopoietic progenitor cells. These data demonstrate that an E-selectin ligand(s) other than PSGL-1 transduces growth inhibitory and proapoptotic signals and requires posttranslational fucosylation to be functional.

In vitro model for hematopoietic progenitor cell homing reveals endothelial heparan sulfate proteoglycans as direct adhesive ligands

Proteoglycans (PGs) play a dominant role within the bone marrow (BM), but their role in homing of transplanted hematopoietic progenitor cells (HPC) is unknown. In this study, the role of heparan sulfate (HS) PGs on BM endothelium as adhesive structures was investigated. HPC (primary CD34+ cells and cell line KG-1a) were able to bind fractionated heparin, which could be competed by highly sulfated heparin/HS-glycosaminoglycans (GAGs). Under flow conditions, HPC adhered to immobilized heparin after rolling over E-selectin. Rolling of KG-1a on BM endothelial cell (EC) line 4LHBMEC was completely E selectin-dependent. Addition of heparin/HS-GAGs, endothelial treatment with chlorate, or anti-HS all partially inhibited firm adhesion. Moreover, enzymatic removal of endothelial HS-GAGs reduced initial adhesion. Finally, HPC-bound PGs isolated from 4LHBMEC, which was largely inhibited by enzymatic HS-degradation. In summary, we identified sulfated structures on BM endothelium, most likely HSPGs, as a novel class of glycoconjugates involved in the multistep homing cascade of HPC.

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.

A novel CD44 antibody identifies an epitope that is aberrantly expressed on acute lymphoblastic leukaemia cells

Previous studies have shown that the antibody 7H9D6 identifies CD44, a glycoprotein receptor for hyaluronic acid. 7H9D6 recognizes an epitope of CD44 that is not always present on CD44 molecules. The 7H9D6 antibody bound to the hyaluronic acid binding domain of CD44 and inhibited cell adhesion to immobilized hyaluronic acid. However, the expression of the 7H9D6 epitope was not sufficient for hyaluronic acid binding. Immunofluorescent staining with 7H9D6 revealed a punctate surface staining pattern, suggesting that CD44 molecules recognized by 7H9D6 are located in clusters on the cell surface. In contrast, other CD44 antibodies produced a uniform staining pattern. Early bone marrow B cells were negative for 7H9D6 but reactive with other CD44 monoclonal antibodies. In contrast, leukaemic cells from 65% of patients (28 of 43) with B lineage acute lymphoblastic leukaemia bound 7H9D6. Patients expressing the 7H9D6 epitope on their leukaemic cells had an increased risk of death (HR 3.5 95% CI 1.1-10.9, P = 0.029) and of disease relapse (HR 3.2 95% CI 1.2-8.5, P = 0.017) when corrected for white cell count. This antibody may be useful for the detection of residual disease in B lineage acute lymphoblastic leukaemia and as a prognostic indicator and for the study of CD44 function.

Murine Lewis lung carcinoma-derived endothelium expresses markers of endothelial activation and requires tumor-specific extracellular matrix in vitro

The purpose of the study was to identify characteristics specific to tumor-derived endothelium that may be important in tumor biology, or for the development of targeted therapeutics or imaging agents. Normal C57Bl/6 murine heart or lung endothelium, or C57Bl/6 murine Lewis lung carcinoma tumor-derived endothelium was isolated from excised tissues using specific antibodies. The endothelium was cultured using either native fibronectin, or the oncofetal form of fibronectin. Cell surface adhesion molecule expression was analyzed by flow cytometry, and the cellular distribution of specific molecules was examined using indirect immunofluorescence staining. Oncofetal fibronectin was critical for maintaining the phenotype of tumor-derived endothelium, which demonstrated an elongated morphology in vitro, with few cell-cell contacts. They expressed high levels of CD31, CD102, and vascular endothelial cadherin, and constitutively expressed CD62E, CD54, and CD106, indicating an "activated" phenotype. Moreover, they expressed significantly greater levels of Sca-1 and Flk-1 than normal murine endothelium. Cellular distribution of CD31, beta-catenin, and CD106 in tumor-derived endothelium was not continuous at cell borders, as observed in cultures of murine heart endothelium. In conclusion, Lewis lung carcinoma-derived tumor endothelium exhibits a specific phenotype in vitro, distinct from normal endothelium, and could be used as an in vitro tool for developing targeted agents.

Glycosylation-dependent inhibition of cutaneous lymphocyte-associated antigen expression: implications in modulating lymphocyte migration to skin

Constitutive E-selectin expression on dermal microvascular endothelial cells plays a critical role in mediating rolling adhesive interactions of human skin-homing T cells and in pathologic accumulation of lymphocytes in skin. The major E-selectin ligand on human skin-homing T cells is cutaneous lymphocyte-associated antigen (CLA), a specialized glycoform of P-selectin glycoprotein ligand-1 (PSGL-1) defined by monoclonal antibody HECA-452. Since HECA-452 reactivity, and not PSGL-1 polypeptide itself, confers the specificity of human T cells to enter dermal tissue, inhibition of HECA-452 expression is a potential strategy for modulating lymphocyte migration to skin. In this study, we examined the efficacy of several well-characterized metabolic inhibitors of glycosylation and of a novel fluorinated analog of N-acetylglucosamine (2-acetamido-1,3,6-tri-O-acetyl-4-deoxy-4-fluoro-D-glucopyranose [4-F-GlcNAc]) to alter HECA-452 expression on human CLA(+) T cells and prevent cell tethering and rolling on selectins under shear stress. At concentrations that did not affect PSGL-1 expression, we found that swainsonine (inhibitor of complex-type N-glycan synthesis) had no effect on HECA-452 expression or selectin ligand activity, whereas benzyl-O-N-acetylgalactosamide (BAG; inhibitor of O-glycan biosynthesis) ablated HECA-452 expression on PSGL-1 and significantly lowered selectin ligand activity. We found that 4-F-GlcNAc (putative inhibitor of poly-N-acetyllactosamine biosynthesis) was more potent than BAG at lowering HECA-452 expression and selectin binding. In addition, we show that 4-F-GlcNAc was directly incorporated into native CLA expressed on T cells, indicating direct inhibition on poly-N-acetyllactosamine elongation and selectin-binding determinants on PSGL-1 O-glycans. These observations establish a potential treatment approach for targeting pathologic lymphocyte trafficking to skin and indicate that 4-F-GlcNAc may be a promising agent for treatment of dermal tropism associated with malignancies and inflammatory disorders.

In vitro adherence of lymphocytes to dermal endothelium under shear stress: implications in pathobiology and steroid therapy of acute cutaneous GVHD

The extravasation of leukocytes at sites of inflammation critically depends on initial shear-resistant adhesive interactions between leukocytes in blood flow and target tissue endothelium. Dermal lymphocytic infiltrates are a hallmark feature of acute cutaneous graft-versus-host disease (acGVHD) following allogeneic hematopoietic stem cell (allo-HSC) transplantation. These infiltrates occur commonly during periods of profound lymphopenia, suggesting that the dermal endothelial adhesive mechanism(s) promoting lymphocyte emigration in acGVHD are highly efficient. To examine this issue, we performed Stamper-Woodruff assays on frozen sections of biopsy specimens of cutaneous lesions occurring within 100 days of HSC transplantation in 22 autologous hematopoietic stem cell transplant (auto-HSCT) and 25 allo-HSCT recipients. By using this shear-based assay, we observed lymphocyte adherence to papillary dermal vascular structures in all punch biopsy specimens of allo-HSCT recipients who had clinicohistologic evidence of acGVHD and who were not receiving steroids, whereas no lymphocyte adherence was observed within skin specimens from allo-HSCT recipients who did not develop acGVHD. Within the group of auto-HSCT recipients, 2 of 22 skin biopsies demonstrated lymphocyte binding to dermal vessels. Among allo-HSCT patients receiving steroid therapy for acGVHD, lymphocyte binding to dermal endothelium was abrogated prior to resolution of rash in those who responded, yet binding was persistent in skin from one patient whose rash did not respond to steroid therapy. Collectively, these data indicate that the papillary endothelium of skin in acGVHD displays heightened capacity to support lymphocyte adhesion under shear stress conditions and suggest that down-modulation of this endothelial adhesive capability may be one mechanism by which steroids abrogate acGVHD reactions.

CD44 in cancer

CD44 is a multistructural and multifunctional cell surface molecule involved in cell proliferation, cell differentiation, cell migration, angiogenesis, presentation of cytokines, chemokines, and growth factors to the corresponding receptors, and docking of proteases at the cell membrane, as well as in signaling for cell survival. All these biological properties are essential to the physiological activities of normal cells, but they are also associated with the pathologic activities of cancer cells. Experiments in animals have shown that targeting of CD44 by antibodies, antisense,and CD44-soluble proteins markedly reduces the malignant activities of various neoplasms, stressing the therapeutic potential of anti-CD44 agents. Furthermore, because alternative splicing and posttranslational modifications generate many different CD44 sequences, including, perhaps, tumor-specific sequences, the production of anti-CD44 tumor-specific agents may be a realistic therapeutic approach. However, in many cancers (renal cancer and non-Hodgkin's lymphomas are exceptions), a high level of CD44 expression is not always associated with an unfavorable outcome. On the contrary, in some neoplams CD44 upregulation is associated with a favorable outcome. Even worse, in many cases different research grows analyzing the same neoplastic disease reached contradictory conclusions regarding the correlation between CD44 expression and disease prognosis, possibly due to differences in methodology. These problems must be resolved before applying anti-CD44 therapy to human cancers.

Selectins: lectins that initiate cell adhesion under flow

Interactions of selectins with cell-surface glycoconjugates mediate tethering and rolling adhesion of leukocytes and platelets on vascular surfaces. Recent studies have helped elucidate the molecular details of selectin-ligand interactions, the biosynthetic pathways for constructing selectin ligands, and the biophysical and cell biological features that modulate selectin-dependent rolling under flow.

The hyaluronan receptor, CD44

CD44 is a widely expressed cell surface hyaluronan receptor which plays a key role in mediating cell migration. A number of recent papers demonstrating an interplay between CD44 and matrix metalloproteinases (MMPs) have shed important insights into the molecular mechanisms underlying these events. This has important implication for understanding how mis-regulation of CD44 can contribute to disease pathologies.

Homing and clonogenic outgrowth of CD34(+) peripheral blood stem cells: a role for L-selectin?

OBJECTIVE

After transplantation of hematopoietic stem cells, adhesion molecules play a major role in the multistep process of engraftment in which L-selectin is suggested to be of relevance. A positive correlation previously was found between the number of reinfused L-selectin(+) stem cells and platelet recovery. In the present study, we determined the role of L-selectin in different engraftment steps, i.e., adhesion to endothelial cells, migration, and clonogenic outgrowth by in vitro assays that closely mimic the in vivo situation.

MATERIALS AND METHODS

Flow adhesion and migration experiments were performed using the human bone marrow endothelial cell line 4LHBMEC and isolated peripheral CD34(+) cells with or without blocking of L-selectin-ligand interaction. Various clonogenic assays, including serum-free colony-forming unit-megakaryocytes (CFU-MK) and burst-forming unit-megakaryocytes (BFU-MK), were performed with sorted L-selectin(+)L-selectin(-) cells or in the presence of antibodies.

RESULTS

Blocking of L-selectin on CD34(+) cells did not significantly affect rolling over and firm adhesion to 4LHBMEC. In addition, no role for L-selectin was found in transendothelial migration experiments. Finally, in clonogenic outgrowth of sorted or anti-L-selectin monoclonal antibody-incubated CD34(+) cells, no key role for L-selectin expression could be defined in BFU-MK and CFU-MK assays.

CONCLUSION

Using in vitro assays for CD34(+) stem cell adhesion, migration, and clonogenic capacity, we were not able to define a major role for L-selectin.

Direct real-time observation of E- and P-selectin-mediated rolling on cutaneous lymphocyte-associated antigen immobilized on Western blots

Human memory T cells associated with cutaneous inflammatory responses are characterized by their expression of cutaneous lymphocyte-associated Ag (CLA), a carbohydrate determinant differentially expressed on P-selectin glycoprotein ligand-1 (PSGL-1). Although expression of the CLA epitope on PSGL-1 (CLA(+) PSGL-1) by memory T cells is associated with acquisition of E-selectin ligand activity, it is not known whether CLA(+) PSGL-1, itself, is a ligand for E-selectin on human T cells or whether other glycoproteins, with or without CLA modification, support E-selectin-dependent rolling in shear flow. To address this issue, we developed a method for real-time analysis of functional adhesive interactions between selectin-bearing cells in shear flow with leukocyte ligands resolved by SDS-PAGE and immobilized on standard Western blots. The results of these studies provide direct evidence that CLA(+) PSGL-1 is a functional ligand for both E- and P-selectin, confirm that the P-selectin ligand activity of PSGL-1 is independent of CLA modification, and identify a distinct, non-PSGL-1 E-selectin ligand on CLA-positive human memory T cells.

P-selectin glycoprotein ligand-1-deficient mice have impaired leukocyte tethering to E-selectin under flow

P-selectin glycoprotein ligand-1 (PSGL-1) mediates rolling of leukocytes on P-selectin under flow. The glycoproteins that enable leukocyte tethering to or rolling on E-selectin are not known. We used gene targeting to prepare PSGL-1-deficient (PSGL-1-/-) mice, which were healthy but had moderately elevated total blood leukocytes. Fluid-phase E-selectin bound to approximately 70% fewer sites on PSGL-1-/- than PSGL-1+/+ neutrophils. Compared with PSGL-1+/+ leukocytes, significantly fewer PSGL-1-/- leukocytes rolled on E-selectin in vitro, because their initial tethering to E-selectin was impaired. The residual cells that tethered rolled with the same shear resistance and velocities as PSGL-1+/+ leukocytes. Compared with PSGL-1+/+ mice, significantly fewer PSGL-1-/- leukocytes rolled on E-selectin in TNF-alpha-treated venules of cremaster muscle in which P-selectin function was blocked by an mAb. The residual PSGL-1-/- leukocytes that tethered rolled with slow velocities equivalent to those of PSGL-1+/+ leukocytes. These results reveal a novel function for PSGL-1 in tethering leukocytes to E-selectin under flow.

differential L-selectin binding activities of human hematopoietic cell L-selectin ligands, HCELL and PSGL-1

Expression of L-selectin on human hematopoietic cells (HC) is associated with a higher proliferative activity and a more rapid engraftment after hematopoietic stem cell transplantation. Two L-selectin ligands are expressed on human HCs, P-selectin glycoprotein ligand-1 (PSGL-1) and a specialized glycoform of CD44 (hematopoietic cell E- and L-selectin ligand, HCELL). Although the structural biochemistry of HCELL and PSGL-1 is well characterized, the relative capacity of these molecules to mediate L-selectin-dependent adhesion has not been explored. In this study, we examined under shear stress conditions L-selectin-dependent leukocyte adhesive interactions mediated by HCELL and PSGL-1, both as naturally expressed on human HC membranes and as purified molecules. By utilizing both Stamper-Woodruff and parallel-plate flow chamber assays, we found that HCELL displayed a 5-fold greater capacity to support L-selectin-dependent leukocyte adherence across a broad range of shear stresses compared with that of PSGL-1. Moreover, L-selectin-mediated leukocyte binding to immunopurified HCELL was consistently >5-fold higher than leukocyte binding to equivalent amounts of PSGL-1. Taken together, these data indicate that HCELL is a more avid L-selectin ligand than PSGL-1 and may be the preferential mediator of L-selectin-dependent adhesive interactions among human HCs in the bone marrow.

Homing of human hematopoietic stem and progenitor cells: new insights, new challenges?

In healthy adults, hematopoiesis takes place in the bone marrow, where the majority of hematopoietic progenitor cells (HPC) reside. In patients undergoing chemo- and/or radiotherapy, hematopoiesis is seriously disturbed. Reconstitution of bone-marrow function can be achieved by bone marrow transplantation or peripheral blood stem cell transplantation. The success of stem cell transplantation depends on the ability of intravenously infused stem cells to lodge in the bone marrow, a process referred to as homing. However, the molecular mechanisms that govern this process are poorly understood. It is hypothesized that homing is a multistep process, consisting of adhesion of the HPC to endothelial cells of the marrow sinusoids, followed by transendothelial migration directed by chemoattractants, and finally anchoring within the extravascular bone marrow spaces where proliferation and differentiation will occur. In this review, we discuss the factors that determine the engraftment potential of stem cells, and focus on various aspects of migration and homing of HPC, i.e., the role of the chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR-4, the involvement of adhesion molecules, and the induction of actin polymerization in the HPC. Defining the role of chemokines and adhesion molecules in human stem cell migration and engraftment will help us uncover the underlying mechanisms that regulate stem cell homing and will eventually advance clinical stem cell transplantation.