Interaction of P-selectin and PSGL-1 generates microparticles that correct hemostasis in a mouse model of hemophilia A

High plasma levels of soluble P-selectin are associated with thrombotic disorders and may predict future cardiovascular events. Mice with high levels of soluble P-selectin have more microparticles in their plasma than do normal mice. Here we show that chimeras of P-selectin and immunoglobulin (P-sel-Ig) induced formation of procoagulant microparticles in human blood through P-selectin glycoprotein ligand-1 (PSGL-1; encoded by the Psgl1 gene, officially known as Selpl). In addition, Psgl1-/- mice produced fewer microparticles after P-sel-Ig infusion and did not spontaneously increase their microparticle count in old age as do wild-type mice. Injected microparticles specifically bound to thrombi and thus could be involved in thrombin generation at sites of injury. Infusion of P-sel-Ig into hemophilia A mice produced a 20-fold increase over control immunoglobulin in microparticles containing tissue factor. This significantly improved the kinetics of fibrin formation in the hemophilia A mice and normalized their tail-bleeding time. P-sel-Ig treatment could become a new approach to sustained control of bleeding in hemophilia.

Homing of antibody secreting cells

As activated B cells differentiate into plasma cells they complete a final series of migration steps that take them to locations where they can efficiently carry out their effector function, secreting immunoglobulin (Ig) M or IgG into circulation or releasing dimeric IgA adjacent to the epithelium for transcytosis. Recent experiments have established a key role for chemokines in directing antibody secreting cell (ASC) movement within the secondary lymphoid organs where they are generated, as well as in guiding the cells to the bone marrow or mucosal surfaces. This review discusses the chemokines involved in directing ASC movements, particularly focusing on the role of CXCR4 and CXCL12/SDF1. The function of CCR9 and CCR10 in IgA ASC homing and contributions made by integrins and lectins are also discussed.

Endoglycan, a member of the CD34 family, functions as an L-selectin ligand through modification with tyrosine sulfation and sialyl Lewis x

During lymphocyte homing to secondary lymphoid organs and instances of inflammatory trafficking, the rolling of leukocytes on vascular endothelium is mediated by transient interactions between L-selectin on leukocytes and several carbohydrate-modified ligands on the endothelium. Most L-selectin ligands such as CD34 and podocalyxin present sulfated carbohydrate structures (6-sulfated sialyl Lewis x or 6-sulfo-sLex) as a recognition determinant within their heavily glycosylated mucin domains. We recently identified endoglycan as a new member of the CD34 family. We report here that endoglycan, like the two other members of this family (CD34 and podocalyxin) can function as a L-selectin ligand. However, endoglycan employs a different binding mechanism, interacting with L-selectin through sulfation on two tyrosine residues and O-linked sLex structures that are presented within its highly acidic amino-terminal region. Our analysis establishes striking parallels with PSGL-1, a leukocyte ligand that interacts with all three selectins, mediating leukocyte-endothelial, leukocyte-leukocyte, and platelet-leukocyte interactions. Since the distribution of endoglycan includes hematopoietic precursors and leukocyte subpopulations, in addition to endothelial cells, our findings suggest several potential settings for endoglycan-mediated adhesion events.

Accumulation of tissue factor into developing thrombi in vivo is dependent upon microparticle P-selectin glycoprotein ligand 1 and platelet P-selectin

Using a laser-induced endothelial injury model, we examined thrombus formation in the microcirculation of wild-type and genetically altered mice by real-time in vivo microscopy to analyze this complex physiologic process in a system that includes the vessel wall, the presence of flowing blood, and the absence of anticoagulants. We observe P-selectin expression, tissue factor accumulation, and fibrin generation after platelet localization in the developing thrombus in arterioles of wild-type mice. However, mice lacking P-selectin glycoprotein ligand 1 (PSGL-1) or P-selectin, or wild-type mice infused with blocking P-selectin antibodies, developed platelet thrombi containing minimal tissue factor and fibrin. To explore the delivery of tissue factor into a developing thrombus, we identified monocyte-derived microparticles in human platelet-poor plasma that express tissue factor, PSGL-1, and CD14. Fluorescently labeled mouse microparticles infused into a recipient mouse localized within the developing thrombus, indicating that one pathway for the initiation of blood coagulation in vivo involves the accumulation of tissue factor- and PSGL-1-containing microparticles in the platelet thrombus expressing P-selectin. These monocyte-derived microparticles bind to activated platelets in an interaction mediated by platelet P-selectin and microparticle PSGL-1. We propose that PSGL-1 plays a role in blood coagulation in addition to its known role in leukocyte trafficking.

The role of selectins in inflammation and disease

Selectins are carbohydrate-binding molecules that bind to fucosylated and sialylated glycoprotein ligands, and are found on endothelial cells, leukocytes and platelets. They are involved in trafficking of cells of the innate immune system, T lymphocytes and platelets. An absence of selectins or selectin ligands has serious consequences in mice or humans, leading to recurrent bacterial infections and persistent disease. Selectins are involved in constitutive lymphocyte homing, and in chronic and acute inflammation processes, including post-ischemic inflammation in muscle, kidney and heart, skin inflammation, atherosclerosis, glomerulonephritis and lupus erythematosus. Selectin-neutralizing monoclonal antibodies, recombinant soluble P-selectin glycoprotein ligand 1 and small-molecule inhibitors of selectins have been tested in clinical trials on patients with multiple trauma, cardiac indications and pediatric asthma, respectively. Anti-selectin antibodies have also been successfully used in preclinical models to deliver imaging contrast agents and therapeutics to sites of inflammation. Further improvements in the efficiency, availability, specificity and pharmacokinetics of selectin inhibitors, and specialized application routes and schedules, hold promise for therapeutic indications.

P-selectin glycoprotein ligand-1 mediates L-selectin-dependent leukocyte rolling in venules

Leukocyte rolling in postcapillary venules of inflamed tissues is reduced in L-selectin-deficient mice and mice treated with L-selectin blocking antibodies, but the glycoprotein ligand for L-selectin in inflamed venules is unknown. Here, we show that L-selectin-dependent rolling after P-selectin blockade is completely absent in P-selectin glycoprotein ligand-1 (PSGL-1)-/- mice or wild-type mice treated with a PSGL-1 blocking monoclonal antibody. Immunohistochemistry and flow cytometry failed to show PSGL-1 expression on resting or inflamed endothelium or on platelets. To investigate whether leukocyte-expressed PSGL-1 is mediating L-selectin-dependent rolling, we reconstituted lethally irradiated wild-type mice with PSGL-1-/- bone marrow cells. These chimeric mice showed no L-selectin-dependent rolling, suggesting that leukocyte-expressed PSGL-1 mediates L-selectin-dependent rolling. Frame-to-frame video analysis of L-selectin-dependent rolling in wild-type mice showed that the majority of observed L-selectin-dependent leukocyte rolling was between free flowing leukocytes and already adherent leukocytes or possibly leukocyte fragments, followed by E-selectin-dependent leukocyte rolling along the endothelium. Leukocyte rolling was significantly slower for leukocyte-endothelial than leukocyte-leukocyte interactions. We conclude that leukocyte-expressed PSGL-1 serves as the main L-selectin ligand in inflamed postcapillary venules. L-selectin binding to PSGL-1 initiates tethering events that enable L-selectin-independent leukocyte-endothelial interactions. These findings provide a molecular mechanism for the inflammatory defects seen in L-selectin-deficient mice.

Recombinant P-selectin glycoprotein ligand-1 directly inhibits leukocyte rolling by all 3 selectins in vivo: complete inhibition of rolling is not required for anti-inflammatory effect

Selectin-dependent leukocyte rolling is one of the earliest steps of an acute inflammatory response and, as such, contributes to many inflammatory diseases. Although inhibiting leukocyte rolling with selectin antagonists is a strategy that promises far-reaching clinical benefit, the perceived value of this strategy has been limited by studies using inactive, weak, or poorly characterized antagonists. Recombinant P-selectin glycoprotein ligand-1-immunoglobulin (rPSGL-Ig) is a recombinant form of the best-characterized selectin ligand (PSGL-1) fused to IgG, and is one of the best prospects in the search for effective selectin antagonists. We have used intravital microscopy to investigate the ability of rPSGL-Ig to influence leukocyte rolling in living blood vessels and find that it can reduce rolling dependent on each of the selectins in vivo. Interestingly, doses of rPSGL-Ig required to reverse pre-existing leukocyte rolling are 30-fold higher than those required to limit inflammation, suggesting additional properties of this molecule. In support of this, we find that rPSGL-Ig can bind the murine chemokine KC and inhibit neutrophil migration toward this chemoattractant in vitro.

Cell-cell interactions: leukocyte-endothelial interactions

Interactions of leukocytes with endothelial cells are early events in acute and chronic inflammation, immune surveillance of tissues, and wound defense and repair. In contrast with their requisite roles in host defense, dysregulated leukocyte-endothelial interactions mediate inflammatory tissue injury, thrombosis, and other pathologic sequelae. Recent observations also identify dysregulated leukocyte-endothelial interactions in neoplasia and sickle cell vasculopathy. Leukocyte interactions with inflamed endothelial cells are mediated by selectins, signaling molecules that include lipids and chemokines, integrins and their ligands, and junctional molecules. They provide multiple checkpoints for regulation in physiologic inflammation and hemostasis and for dysregulation in pathologic syndromes. Neutrophil-endothelial encounters illustrate a multistep paradigm for inflammatory cell-cell interactions and provide the basis for multiple variations on the central themes.

N-terminal residues in murine P-selectin glycoprotein ligand-1 required for binding to murine P-selectin

P-selectin binds to the N-terminal region of human P-selectin glycoprotein ligand-1 (PSGL-1). For optimal binding, this region requires sulfation on 3 tyrosines and specific core-2 O-glycosylation on a threonine. P-selectin is also thought to bind to the N terminus of murine PSGL-1, although it has a very different amino acid sequence than human PSGL-1. Murine PSGL-1 has potential sites for sulfation at Tyr13 and Tyr15 and for O-glycosylation at Thr14 and Thr17. We expressed murine PSGL-1 or constructs with substitutions of these residues in transfected Chinese hamster ovary cells that coexpressed the glycosyltransferases required for binding to P-selectin. The cells were assayed for binding to fluid-phase P-selectin and for tethering and rolling on P-selectin under flow. In both assays, substitution of Tyr13 or Thr17 markedly diminished, but did not eliminate, binding to P-selectin. In contrast, substitution of Tyr15 or Thr14 did not affect binding. Substitution of all 4 residues eliminated binding. Treatment of cells with chlorate, an inhibitor of sulfation, markedly reduced binding of wild-type PSGL-1 to P-selectin but did not further decrease binding of PSGL-1 with substitutions of both tyrosines. These data suggest that sulfation of Tyr13 and O-glycosylation of Thr17 are necessary for murine PSGL-1 to bind optimally to P-selectin. Because it uses only one tyrosine, murine PSGL-1 may rely more on other peptide components and O-glycosylation to bind to P-selectin than does human PSGL-1.

Discordant expression of selectin ligands and sialyl Lewis x-related epitopes on murine myeloid cells

Murine leukocytes are thought to express alpha2-3-sialylated and alpha1-3-fucosylated selectin ligands such as sialyl Lewis x (sLe(x)), although monoclonal antibodies (mAbs) to sLe(x) or Le(x) reportedly do not bind to murine leukocytes. We observed that P- and E-selectin bound to pronase-sensitive ligands on murine monocytic WEHI-3 cells and murine neutrophils, indicating that the ligands for both selectins are glycoproteins. CSLEX-1, HECA-452, and other widely used mAbs to sLe(x) and Le(x) did not bind to WEHI-3 cells and bound at very low levels to murine neutrophils. Only the anti-sLe(x) mAbs 2H5 and KM93, which also recognize nonfucosylated glycans, bound to WEHI-3 cells. 2H5 and KM93 bound to pronase-resistant structures, indicating that the mAbs did not identify selectin ligands. Treatment of WEHI-3 cells with glycosidases or chlorate demonstrated that sialic acid modifications, alpha1-3-galactosylation, or sulfation did not mask epitopes for mAbs to sLe(x) or Le(x). Compared to human promyelocytic HL-60 cells, WEHI-3 cells and murine neutrophils expressed low alpha1-3-fucosyltransferase activities. Consistent with very low endogenous fucosylation, forced fucosylation of intact WEHI-3 cells or murine neutrophils by exogenous alpha1-3-fucosyltransferase FTVI and GDP-fucose created many new epitopes for anti-sLe(x) mAbs such as HECA-452 and CSLEX-1. Nevertheless, forced fucosylation of intact cells did not significantly augment their ability to bind to fluid-phase P- or E-selectin or to roll on immobilized P- or E-selectin under flow. These data suggest that murine myeloid leukocytes fucosylate only a few specific glycans, which interact preferentially with P- and E-selectin.

Glycosulfopeptides with O-glycans containing sialylated and polyfucosylated polylactosamine bind with low affinity to P-selectin

P-selectin glycoprotein ligand-1 (PSGL-1), a dimeric mucin on leukocytes, is the best characterized ligand for selectins. P-selectin binds stereospecifically to the extreme N terminus of PSGL-1, which contains three clustered tyrosine sulfates (TyrSO3-) adjacent to a Thr residue with a core 2-based O-glycan expressing sialyl Lewis x (C2-O-sLe(x)). GSP-6, a synthetic glycosulfopeptide modeled after the N terminus of PSGL-1, containing three TyrSO3- residues and a short, monofucosylated C2-O-sLe(x) bound to P-selectin with high affinity (K(d) approximately 650 nm). However, PSGL-1 from human HL-60 cells contains higher levels of O-glycans that are sialylated and polyfucosylated polylactosamines (PFPL). Furthermore, studies with fucosyltransferase-deficient mice suggest that sialylated PFPL structures contribute to binding to P-selectin. To resolve whether sialylated PFPL O-glycans participate in binding of PSGL-1 to human P-selectin, we synthesized glycosulfopeptides, designated GSP-6' and GSP-6", with three TyrSO3- residues and either difucosylated polylactosamine (C2-O-Le(x)-sLe(x)) or trifucosylated polylactosamine (C2-O-Le(x)-Le(x)-sLe(x)). Binding of the GSPs to P-selectin was measured by affinity chromatography, fluorescence solid-phase assays, and equilibrium gel filtration. Unexpectedly, both GSP-6' and GSP-6" bound to P-selectin with low affinity (K(d) approximately 37 microm for GSP-6' and K(d) approximately 50 microm for GSP-6"). Binding of GSP-6' and GSP-6" to P-selectin required fucosylation and, to a lesser extent, sialylation as well as the sulfated peptide backbone of GSP-6' and GSP-6". These results demonstrate that contrary to expectations, a core 2 O-glycan containing sialylated PFPL does not promote high affinity binding of PSGL-1 to P-selectin.

ITAM-based interaction of ERM proteins with Syk mediates signaling by the leukocyte adhesion receptor PSGL-1

P-selectin glycoprotein ligand 1 (PSGL-1) is a leukocyte adhesion molecule involved in cell tether and rolling on activated endothelium. Our work shows that PSGL-1 associates with Syk. This association is mediated by the actin-linking proteins moesin and ezrin, which directly interact with Syk in an ITAM-dependent manner. PSGL-1 engagement induces tyrosine phosphorylation of Syk and SRE-dependent transcriptional activity. Treatment of cells with the Syk inhibitor piceatannol and overexpression of either a Syk dead kinase mutant or an ITAM-mutated moesin abrogated PSGL-1-induced transcriptional activation. These data unveil a new functional role for the ERMs (ezrin/radixin/moesin) as adaptor molecules in the interactions of adhesion receptors and intracellular tyrosine kinases and show that PSGL-1 is a signaling molecule in leukocytes.

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.

Regulatory roles of carbohydrate ligands for selectins in the homing of lymphocytes

Cell surface carbohydrate determinants figure heavily in the regulation of lymphocyte homing and the inflammatory recruitment of leukocytes. The recently described sulfated carbohydrate ligand for selectins, sialyl 6-sulfo Lewis x, is mainly involved in the routine homing of various subsets of lymphocytes, such as nai;ve helper T cells, and skin- and gut-homing helper memory T cells. The homing of lymphocytes is regulated by a unique post-translational modification of sialic acid moieties that occurs specifically in sulfated selectin ligands, whereby the sialic acid loses its N-acetyl group and is converted into a 1-5 cyclic derivative. By contrast, nonsulfated carbohydrate ligands of the selectins, such as sialyl Lewis x, are mainly involved in the recruitment of leukocytes during inflammation. The increment of sialyl Lewis x expression upon inflammatory stimuli is mediated by transcriptional induction of fucosyltransferase genes.

Glycosylation in the control of selectin counter-receptor structure and function

Leukocyte trafficking is characterized by sequential cell adhesion and activation events that deliver specific leukocyte subsets to distinct extravascular locations under different pathophysiological circumstances. E-, P- and/or L-selectin-dependent leukocyte-endothelial cell adhesive interactions contribute essentially to this process. Selectin counter-receptor activity on leukocyte and high endothelial venules is borne by specific glycoproteins whose ability to support adhesion requires specific post-translational modifications. These modifications are typified by serine/threonine-linked oligosaccharides capped with the sialyl Lewis x moiety, an alpha2-3sialylated, alpha1-3ucosylated tetrasaccharide synthesized by specific glycosyltransferases. Recent advances in glycan structure analysis and in characterizing mice with targeted deletions of glycosyltransferase and sulfotransferase genes discloses an essential role for 6-O GlcNAc sulfate modification of the sialyl Lewis x tetrasaccharide in L-selectin counter-receptor activity. Related studies identify novel extended Core 1 type O-glycans bearing the 6-sulfosialyl Lewis x moiety, define the molecular nature of the MECA-79 epitope, and disclose a requirement for the alpha1-3fucosyltransferases FucT-IV and FucT-VII in the elaboration of L-selectin counter-receptor activities. Parallel studies also demonstrate that these 2 fucosyltransferases, a core 2 GlcNAc transferase, and core 2-type sialyl Lewis x determinants make essential contributions to leukocyte P-selectin counter-receptor activity, and figure prominently in the control of leukocyte E-selectin counter-receptor activity.

Integration of inflammatory signals by rolling neutrophils

In inflammation, neutrophils roll along the endothelial wall of postcapillary venules and sample inflammatory signals. Neutrophil activation is required to generate beta(2) integrin bonds with the endothelium that are strong enough to withstand the flow forces and thus achieve arrest from the rolling state. Unlike naïve T cells, neutrophils are not only activated by ligation of G-protein coupled receptors with chemokines and other chemoattractants but also receive signals from engagement of adhesion molecules including the selectins and beta(2) integrins. Rolling neutrophils integrate the sum total of inputs received while scanning the inflamed endothelium. In this process, the velocity of rolling neutrophils systematically decreases as a function of their contact time with the inflamed endothelium. If an activation threshold is reached, beta(2) integrins switch to the high-affinity conformation, redistribute on the cell surface, and trigger arrest and adhesion. Rolling cells that do not reach the activation threshold detach from the endothelium and are released back into the circulation. The role of chemokines, adhesion molecules, and other activating inputs involved in this response as well as signaling pathways are the subjects of ongoing investigations. This review provides a conceptual framework for neutrophil recruitment from the flowing blood.