Neutrophil-neutrophil interactions under hydrodynamic shear stress involve L-selectin and PSGL-1. A mechanism that amplifies initial leukocyte accumulation of P-selectin in vitro

Experimental and clinical perspectives on glycocalyx integrity and its relation to acute respiratory distress syndrome

The development of microcirculation imaging devices has significantly advanced our comprehension of the capillary environment's dynamics. Early research suggested that erythrocytes did not contact the vessel's inner surface due to the Fåhraeus effect, implying the presence of a covering on the endothelial cell surface. Subsequent electron microscopy studies revealed this layer to be a complex part of the vessel wall, now known as the endothelial glycocalyx (EG). The EG is a network of proteoglycans and glycoproteins bound to the endothelial membrane, incorporating soluble molecules from the endothelium and plasma. Over time, studies have elucidated the structure, function, and therapeutic targets of the glycocalyx, underscoring its pivotal role in vascular biology. The presence of cellular extensions of lung tissue cells in both vascular and nonvascular areas demonstrates the pivotal role of the glycocalyx in pulmonary vascular leak, surfactant dysfunction, impaired lung compliance and gas exchange abnormalities, which are hallmarks of acute respiratory distress syndrome (ARDS). It is of the utmost importance to elucidate the mechanisms underlying alveolocapillary glycocalyx degradation to develop efficacious treatments for ARDS, which has a mortality rate of 35 %. An understanding of the glycocalyx's role in vascular integrity provides a foundation for exploring new therapeutic avenues to mitigate lung injury and improve clinical outcomes in ARDS patients.

Antibody blockade of the PSGL-1 immune checkpoint enhances T-cell responses to B-cell lymphoma

Despite advancements in cancer immunotherapy, most lymphomas remain unresponsive to checkpoint inhibitors. P-selectin glycoprotein ligand-1 (PSGL-1), recently identified as a promoter of T-cell exhaustion in murine melanoma models, has emerged as a novel immune checkpoint protein and promising immunotherapeutic target. In this study, we investigated the potential of PSGL-1 antibody targeting in B-cell lymphoma. Using allogeneic co-culture systems, we demonstrated that targeted antibody interventions against human PSGL-1 enhanced T-cell activation and effector cytokine production in response to lymphoma cells. Moreover, in vitro treatment of primary lymphoma cell suspensions with PSGL-1 antibody resulted in increased activation of autologous lymphoma-infiltrating T cells. Using the A20 syngeneic B-cell lymphoma mouse model, we found that PSGL-1 antibody treatment significantly slowed tumor development and reduced the endpoint tumor burden. This antitumoral effect was accompanied by augmented tumor infiltration of CD4+ and CD8+ T cells and reduced infiltration of regulatory T cells. Finally, anti-PSGL-1 administration enhanced the expansion of CAR T cells previously transferred to mice bearing the aggressive Eμ-Myc lymphoma cells and improved disease control. These results demonstrate that PSGL-1 antibody blockade bolsters T-cell activity against B-cell lymphoma, suggesting a potential novel immunotherapeutic approach for treating these malignancies.

Selectins in Biology and Human Disease: Opportunity in E-selectin Antagonism

Selectins are cell adhesion proteins discovered in the 1980s. As C-type lectins, selectins contain an essential calcium ion in the ligand-binding pocket and recognize the isomeric tetrasaccharides sialyl Lewisx (sLex) and sialyl Lewisa (sLea). Three selectins, E-selectin, P-selectin, and L-selectin, play distinct, complementary roles in inflammation, hematopoiesis, and tumor biology. They have been implicated in the pathology of diverse inflammatory disorders, and several selectin antagonists have been tested clinically. E-selectin plays a unique role in leukocyte activation, making it an attractive target for intervention, for example, in sickle cell disease (SCD). This review summarizes selectin biology and pathology, structure and ligand binding, and selectin antagonists that have reached clinical testing with an emphasis on E-selectin.

Antibody targeting of surface P-selectin glycoprotein ligand 1 leads to lymphoma apoptosis and tumorigenesis inhibition

Lymphomas are a heterogeneous group of diseases that originate from T, B or natural killer cells. Lymphoma treatment is based on chemotherapy, radiotherapy, and monoclonal antibody (mAb) or other immunotherapies. The P-selectin glycoprotein ligand 1 (PSGL-1) is expressed at the surface of hematological malignant cells and has been shown to have a pro-oncogenic role in multiple myeloma and lymphoma. Here, we investigated the expression and therapeutic potential of PSGL-1 in T and B cell lymphomas. By flow cytometry analysis, we found that PSGL-1 was expressed in both T and B cell-derived lymphoma cell lines but generally at higher levels in T cell lymphoma cell lines. For most T and B cell-derived lymphoma cell lines, in vitro targeting with the PL1 mAb, which recognizes the PSGL-1 N-terminal extracellular region and blocks functional interactions with selectins, resulted in reduced cell viability. The PL1 mAb pro-apoptotic activity was shown to be dose-dependent, to be linked to increased ERK kinase phosphorylation, and to be dependent on the MAP kinase signaling pathway. Importantly, anti-PSGL-1 treatment of mice xenografted with the HUT-78 cutaneous T-cell lymphoma cell line resulted in decreased tumor growth, had no effect on in vivo proliferation, but increased the levels of apoptosis in tumors. Anti-PSGL-1 treatment of mice xenografted with a Burkitt lymphoma cell line that was resistant to anti-PSGL-1 treatment in vitro, had no impact on tumorigenesis. These findings show that PSGL-1 antibody targeting triggers lymphoma cell apoptosis and substantiates PSGL-1 as a potential target for lymphoma therapy.

Acute Pulmonary Embolism and Immunity in Animal Models

Venous thromboembolism, encompassing acute pulmonary embolism (APE) and deep vein thrombosis (DVT), is a potentially fatal disease with complex pathophysiology. Traditionally, the Virchow triad provided a framework for understanding the pathogenic contributors to thrombus formation, which include endothelial dysfunction, alterations in blood flow and blood hypercoagulability. In the last years, it has become apparent that immunity plays a central role in thrombosis, interacting with classical prothrombotic mechanisms, oxidative stress and vascular factors. Thrombosis amplifies inflammation, and exaggerated inflammatory processes can trigger thrombosis mainly due to the activation of leukocytes, platelets, and endothelial cells. APE-related endothelium injury is a major trigger for immune system activation. Endothelium is also a key component mediating inflammatory reaction and it is relevant to maintain vascular permeability. Exaggerated right ventricular wall stress and overload, with coexisting systemic hypotension and hypoxemia, result in myocardial injury and necrosis. Hypoxia, tissue factor activation and cytokine storm are engaged in the thrombo-inflammatory processes. Thrombus development is characterized by inflammatory state vascular wall caused mainly by an early extravasation of leukocytes and intense selectins and cytokines production. Nevertheless, immunity of DVT is well described, little is known about potential chemokine and cellular differences between thrombus that develops in the vein and thrombus that detaches and lodges in the pulmonary circulation being a cause of APE. There is a paucity of data considering inflammatory state in the pulmonary artery wall during an acute episode of pulmonary embolism. The main aim of this review is to summarize the knowledge of immunity in acute phase of pulmonary embolism in experimental models.

Fast and furious: The neutrophil and its armamentarium in health and disease

Neutrophils are powerful effector cells leading the first wave of acute host-protective responses. These innate leukocytes are endowed with oxidative and nonoxidative defence mechanisms, and play well-established roles in fighting invading pathogens. With microbicidal weaponry largely devoid of specificity and an all-too-well recognized toxicity potential, collateral damage may occur in neutrophil-rich diseases. However, emerging evidence suggests that neutrophils are more versatile, heterogeneous, and sophisticated cells than initially thought. At the crossroads of innate and adaptive immunity, neutrophils demonstrate their multifaceted functions in infectious and noninfectious pathologies including cancer, autoinflammation, and autoimmune diseases. Here, we discuss the kinetics of neutrophils and their products of activation from bench to bedside during health and disease, and provide an overview of the versatile functions of neutrophils as key modulators of immune responses and physiological processes. We focus specifically on those activities and concepts that have been validated with primary human cells.

Monocyte-endothelial cell interactions in vascular and tissue remodeling

Monocytes are circulating leukocytes of innate immunity derived from the bone marrow that interact with endothelial cells under physiological or pathophysiological conditions to orchestrate inflammation, angiogenesis, or tissue remodeling. Monocytes are attracted by chemokines and specific receptors to precise areas in vessels or tissues and transdifferentiate into macrophages with tissue damage or infection. Adherent monocytes and infiltrated monocyte-derived macrophages locally release a myriad of cytokines, vasoactive agents, matrix metalloproteinases, and growth factors to induce vascular and tissue remodeling or for propagation of inflammatory responses. Infiltrated macrophages cooperate with tissue-resident macrophages during all the phases of tissue injury, repair, and regeneration. Substances released by infiltrated and resident macrophages serve not only to coordinate vessel and tissue growth but cellular interactions as well by attracting more circulating monocytes (e.g. MCP-1) and stimulating nearby endothelial cells (e.g. TNF-α) to expose monocyte adhesion molecules. Prolonged tissue accumulation and activation of infiltrated monocytes may result in alterations in extracellular matrix turnover, tissue functions, and vascular leakage. In this review, we highlight the link between interactions of infiltrating monocytes and endothelial cells to regulate vascular and tissue remodeling with a special focus on how these interactions contribute to pathophysiological conditions such as cardiovascular and chronic liver diseases.

The serum proteome of VA-ECMO patients changes over time and allows differentiation of survivors and non-survivors: an observational study

BACKGROUND

Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) is applied in patients with refractory hemodynamic failure. Exposure of blood components to high shear stress and the large extracorporeal surfaces in the ECMO circuit trigger a complex inflammatory response syndrome and coagulopathy which are believed to worsen the already poor prognosis of these patients. Mass spectrometry-based proteomics allow a detailed characterization of the serum proteome as it provides the identity and concentration of large numbers of individual proteins at the same time. In this study, we aimed to characterize the serum proteome of patients receiving VA-ECMO.

METHODS

Serum samples were collected on day 1 and day 3 after initiation of VA-ECMO. Samples underwent immunoaffinity based depletion for the 14 most abundant serum proteins, in-solution digestion and PreOmics clean-up. A spectral library was built with multiple measurements of a master-mix sample using variable mass windows. Individual samples were measured in data independent acquisition (DIA) mode. Raw files were analyzed by DIA-neural network. Unique proteins were log transformed and quantile normalized. Differential expression analysis was conducted with the LIMMA-R package. ROAST was applied to generate gene ontology enrichment analyses.

RESULTS

Fourteen VA-ECMO patients and six healthy controls were recruited. Seven patients survived. Three hundred and fifty-one unique proteins were identified. One hundred and thirty-seven proteins were differentially expressed between VA-ECMO patients and controls. One hundred and forty-five proteins were differentially expressed on day 3 compared to day 1. Many of the differentially expressed proteins were involved in coagulation and the inflammatory response. The serum proteomes of survivors and non-survivors on day 3 differed from each other according to partial least-squares discriminant analysis (PLS-DA) and 48 proteins were differentially expressed. Many of these proteins have also been ascribed to processes in coagulation and inflammation (e.g., Factor IX, Protein-C, Kallikrein, SERPINA10, SEMA4B, Complement C3, Complement Factor D and MASP-1).

CONCLUSION

The serum proteome of VA-ECMO patients displays major changes compared to controls and changes from day 1 until day 3. Many changes in the serum proteome are related to inflammation and coagulation. Survivors and non-survivors can be differentiated according to their serum proteomes using PLS-DA analysis on day 3. Our results build the basis for future studies using mass-spectrometry based serum proteomics as a tool to identify novel prognostic biomarkers.

TRIAL REGISTRATION

DRKS00011106.

P-selectin glycoprotein ligand 1 promotes T cell lymphoma development and dissemination

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PSGL-1 protein is frequently expressed at the surface of malignant T cells.

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Enforced expression of PSGL-1 promotes T cell tumorigenesis in mice.

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PSGL-1 expression accelerates malignant T cell dissemination from tumors to several organs.

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PSGL-1 expression promotes malignant T cell expansion in kidneys and lungs.

P-selectin glycoprotein ligand-1 (PSGL-1) is a membrane-bound glycoprotein expressed in lymphoid and myeloid cells. It is a ligand of P-, E- and L-selectin and is involved in T cell trafficking and homing to lymphoid tissues, among other functions. PSGL-1 expression has been implicated in different lymphoid malignancies, so here we aimed to evaluate the involvement of PSGL-1 in T cell lymphomagenesis and dissemination. PSGL-1 was highly expressed at the surface of human and mouse T cell leukemia and lymphoma cell lines. To assess its impact on T cell malignancies, we stably expressed human PSGL-1 (hPSGL-1) in a mouse thymic lymphoma cell line, which expresses low levels of endogenous PSGL-1 at the cell surface. hPSGL-1-expressing lymphoma cells developed subcutaneous tumors in athymic nude mice recipients faster than control empty vector or parental cells. Moreover, the kidneys, lungs and liver of tumor-bearing mice were infiltrated by hPSGL-1-expressing malignant T cells. To evaluate the role of PSGL-1 in lymphoma cell dissemination, we injected intravenously control and hPSGL-1-expressing lymphoma cells in athymic mice. Strikingly, PSGL-1 expression facilitated disease infiltration of the kidneys, as determined by histological analysis and anti-CD3 immunohistochemistry. Together, these results indicate that PSGL-1 expression promotes T cell lymphoma development and dissemination to different organs.

Biomechanics of Neutrophil Tethers

Leukocytes, including neutrophils, propelled by blood flow, can roll on inflamed endothelium using transient bonds between selectins and their ligands, and integrins and their ligands. When such receptor–ligand bonds last long enough, the leukocyte microvilli become extended and eventually form thin, 20 µm long tethers. Tether formation can be observed in blood vessels in vivo and in microfluidic flow chambers. Tethers can also be extracted using micropipette aspiration, biomembrane force probe, optical trap, or atomic force microscopy approaches. Here, we review the biomechanical properties of leukocyte tethers as gleaned from such measurements and discuss the advantages and disadvantages of each approach. We also review and discuss viscoelastic models that describe the dependence of tether formation on time, force, rate of loading, and cell activation. We close by emphasizing the need to combine experimental observations with quantitative models and computer simulations to understand how tether formation is affected by membrane tension, membrane reservoir, and interactions of the membrane with the cytoskeleton.

Sialic Acid Conjugate-Modified Liposomal Dexamethasone Palmitate Targeting Neutrophils for Rheumatoid Arthritis Therapy: Influence of Particle Size

Many anti-inflammatory therapies targeting neutrophils have been developed so far. A sialic acid (SA)-modified liposomal (SAL) formulation, based on the high expression of L-selectin in peripheral blood neutrophils (PBNs) and SA as its targeting ligand, has proved to be an effective neutrophil-mediated drug delivery system targeting rheumatoid arthritis (RA). The objective of this study was to investigate the influence of particle size of drug-carrying SALs transported and delivered by neutrophils on their anti-RA effect. Dexamethasone palmitate-loaded SALs (DP-SALs) of different particle sizes (300.2 ± 5.5 nm, 150.3 ± 4.3 nm, and 75.0 ± 3.9 nm) were prepared with DP as a model drug. Our study indicated that DP-SALs could efficiently target PBNs, with larger liposomes leading to higher drug accumulation in cells. However, a high intake of large DP-SALs by PBNs inhibited their migration ability and capacity to release the payload at the target site. In contrast, small DP-SALs (75.0 ± 3.9 nm) could maintain the drug delivery potential of PBNs, leading to their efficient accumulation at the inflammatory site, where PBNs would be excessively activated to form neutrophil extracellular traps along with efficient payload release (small DP-SALs) and finally to induce excellent anti-RA effect.

Tumor-derived HMGB1 induces CD62Ldim neutrophil polarization and promotes lung metastasis in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is highly aggressive, difficult to treat and commonly develops visceral metastasis, including lung metastasis. We observed that High mobility group box 1 protein (HMGB1) was highly expressed in human TNBC and positively correlated with cancer metastasis. The hypoxic tumor environment is known to regulate HMGB1 secretion, but an understanding of the underlying mechanism by which tumor-derived HMGB1 regulates interstitial components and promotes breast cancer lung metastasis has remained elusive. The results of the present study showed that the number of CD62L^dim neutrophils, which have a strong ability to produce neutrophil extracellular traps (NETs), increased significantly in both peripheral blood and lung tissues in a mouse TNBC model and were regulated by tumor-derived HMGB1 through the TLR2 pathway. Furthermore, serum HMGB1 levels were positively correlated with CD62L^dim neutrophils in 86 breast cancer patients. We demonstrated that CD62L^dim neutrophils accelerated lung metastasis and that interventions targeting the “HMGB1-CD62L^dim neutrophil-NETs” axis could inhibit lung metastasis. Our results suggest that the combination of HMGB1 and CD62L^dim neutrophils is a potential marker for breast cancer lung metastasis and is novel target for future prevention and therapy.

Maternal, Fetal, and Placental Selectins in Women With Pre-eclampsia; Association With the Renin-Angiotensin-System

Selectins [endothelial (E), platelet (P), and leucocytes (L)] are a class of cell adhesion molecules, stimulated in response to inflammation. Pre-eclampsia is characterized by inflammation, and angiotensin II is pro-inflammatory. We hypothesized that circulating maternal and fetal concentrations and placental expression of selectins would be increased in women with pre-eclampsia and would be associated with the angiotensin receptors (AT1R and AT2R). Maternal and fetal blood and placental tissue was collected at delivery from White European normotensive controls (n = 17) and women with pre-eclampsia (n = 17). Soluble (s) E-, P- and L-selectin protein concentrations were measured by ELISA and placental protein expression was examined by immunohistochemistry. Maternal sE-selectin concentrations were increased in pre-eclampsia (P < 0.001); conversely fetal sE- and sP-selectin levels were lower in pre-eclampsia (P < 0.05 for both). Staining was mainly localized to the syncytiotrophoblast for all selectins. E-selectin expression was increased, while P-selectin was decreased in placental from pre-eclampsia (P < 0.05 for both); no differences were observed for L-selectin expression. Both E- and L-selectin were positively correlated (P < 0.008; P < 0.02) with AT2R placental expression, whilst P-selectin was negatively associated with AT1R (P < 0.005), all only in the pre-eclampsia group. This novel study reports maternal, fetal and placental expression of selectins in pre-eclampsia. The increased E-selectins reflect the endothelial dysfunction, characteristic of pre-eclampsia. In contrast, the reduced P-selectins and the positive association of placental AT2Rs with both E-and L-selectin in pre-eclampsia could be a protective mechanism to limit the endothelial dysfunction.

Acute Myeloid and Lymphoblastic Leukemia Cell Interactions with Endothelial Selectins: Critical Role of PSGL-1, CD44 and CD43

Acute myeloid and lymphoblastic leukemia are poor prognosis hematologic malignancies, which disseminate from the bone marrow into the blood. Blast interactions with selectins expressed by vascular endothelium promote the development of drug resistance and leukostasis. While the role of selectins in initiating leukemia blast adhesion is established, our knowledge of the involved selectin ligands is incomplete. Using various primary acute leukemia cells and U937 monoblasts, we identified here functional selectin ligands expressed by myeloblasts and lymphoblasts by performing biochemical studies, expression inhibition by RNA interference and flow adhesion assays on recombinant selectins or selectin ligands immunoadsorbed from primary blast cells. Results demonstrate that P-selectin glycoprotein ligand-1 (PSGL-1) is the major P-selectin ligand on myeloblasts, while it is much less frequently expressed and used by lymphoblasts to interact with endothelial selectins. To roll on E-selectin, myeloblasts use PSGL-1, CD44, and CD43 to various extents and the contribution of these ligands varies strongly among patients. In contrast, the interactions of PSGL-1-deficient lymphoblasts with E-selectin are mainly supported by CD43 and/or CD44. By identifying key selectin ligands expressed by acute leukemia blasts, this study offers novel insight into their involvement in mediating acute leukemia cell adhesion with vascular endothelium and may identify novel therapeutic targets.

The function of P-selectin glycoprotein ligand-1 is conserved from ancestral fishes to mammals

PSGL-1 is a mucin-like glycoprotein that supports, in mammals, leukocyte rolling on selectins. However, we have limited knowledge whether its function is conserved in non-mammals and how its structure adapted during evolution. To identify conserved amino acid sequences required for selectin binding, we performed multiple alignments of PSGL-1 sequences from 18 mammals, 4 birds, 3 reptiles, 1 amphibian, and 15 fishes. The amino-terminal T[D/E]PP[D/E] motif, which identifies in mammals a core-2 O-glycosylated threonine required for selectin-binding, is partially conserved in some fishes (e.g., T. rubripes) and birds (e.g., G. gallus), however, most non-mammals do not display it. The sulfated tyrosine residues of human PSGL-1, which bind L- and P-selectin, are not observed in non-mammals, suggesting that they are dispensable for selectin-binding or that other amino acids play their role. A mucin-like domain is present in all species. Interestingly, the alignment of cytoplasmic sequences of non-mammals reveals the conservation of ezrin/radixin/moesin binding site and two new motifs (M1 and M2). To examine the conservation of PSGL-1 function, we cloned PSGL-1 cDNA sequences of zebrafish and fugu, and established their cross-reactivity with human selectins under flow conditions. Importantly, deleting the well-conserved M1 motif strongly decreased PSGL-1 expression at leukocyte surface and induced retention of the precursor molecule in the endoplasmic reticulum, indicating that M1 motif provides a signal required to export PSGL-1 precursors to the Golgi complex. These data show for the first time the conservation of PSGL-1 function from fishes to mammals and reveal the function of a new motif.

Myths, reality and future of mesenchymal stem cell therapy

Mesenchymal stem cell (MSC) therapy represents an alternative approach for tissue regeneration and inflammation control. In spite of a huge amount of preclinical data that has been accumulated on the therapeutic properties of MSCs, there are many conflicting results, possibly due to differences in the properties of MSCs obtained from different sources or underestimated mechanisms of MSC in vivo behavior. This review consolidates the in vivo effects of MSC therapy, discusses the fate of MSCs after intravascular and local delivery and proposes possible trends in MSC therapy.

Early Targeting of L-Selectin on Leukocytes Promotes Recovery after Spinal Cord Injury, Implicating Novel Mechanisms of Pathogenesis

L-selectin, a lectin-like receptor on all leukocyte classes, functions in adhesive and signaling roles in the recruitment of myeloid cells from the blood to sites of inflammation. Here, we consider L-selectin as a determinant of neurological recovery in a murine model of spinal cord injury (SCI). Spinal cord-injured, L-selectin knock-out (KO) mice (male) showed improved long-term recovery with greater white matter sparing relative to wild-type (WT) mice and reduced oxidative stress in the injured cord at 72 h post-SCI. There was a partial and transient reduction in accumulation of neutrophils in the injured spinal cords of KOs at 24 h post-injury. To complement these findings with KO mice, we sought a pharmacologic means for lowering L-selectin levels. We found that diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), induced the shedding of L-selectin from the cell surface of myeloid subsets, specifically neutrophils and non-classical monocytes, in the blood and the injured spinal cord. Diclofenac administration to injured WT mice enhanced neurological recovery to a level comparable to that of KOs but did not improve recovery in KOs. While diclofenac treatment had no effect on myeloid cell accumulation, there was a reduction in oxidative stress at 72 h post-SCI. These findings implicate L-selectin in secondary pathogenesis beyond a role in leukocyte recruitment and raise the possibility of repurposing diclofenac for the treatment of SCI.

Biomimetic post-capillary venule expansions for leukocyte adhesion studies

Leukocyte adhesion and extravasation are maximal near the transition from capillary to post-capillary venule, and are strongly influenced by a confluence of scale-dependent physical effects. Mimicking the scale of physiological vessels using in vitro microfluidic systems allows the capture of these effects on leukocyte adhesion assays, but imposes practical limits on reproducibility and reliable quantification. Here we present a microfluidic platform that provides multiple (54-512) technical replicates within a 15-minute sample collection time, coupled with an automated computer vision analysis pipeline that captures leukocyte adhesion probabilities as a function of shear and extensional stresses. We report that in post-capillary channels of physiological scale, efficient leukocyte adhesion requires erythrocytes forcing leukocytes against the wall, a phenomenon that is promoted by the transitional flow in post-capillary venule expansions and dependent on the adhesion molecule ICAM-1.

Neutrophils in traumatic brain injury (TBI): friend or foe?

Our knowledge of the pathophysiology about traumatic brain injury (TBI) is still limited. Neutrophils, as the most abundant leukocytes in circulation and the first-line transmigrated immune cells at the sites of injury, are highly involved in the initiation, development, and recovery of TBI. Nonetheless, our understanding about neutrophils in TBI is obsolete, and mounting evidences from recent studies have challenged the conventional views. This review summarizes what is known about the relationships between neutrophils and pathophysiology of TBI. In addition, discussions are made on the complex roles as well as the controversial views of neutrophils in TBI.

Presence of Rigid Red Blood Cells in Blood Flow Interferes with the Vascular Wall Adhesion of Leukocytes

The symptoms of many blood diseases can often be attributed to irregularities in cellular dynamics produced by abnormalities in blood cells, particularly red blood cells (RBCs). Contingent on the disease and its severity, RBCs can be afflicted with increased membrane rigidity as seen in malaria and sickle cell disease. Despite this understanding, little experimental work has been conducted toward understanding the effect of RBC rigidity on cellular dynamics in physiologic blood flow. Though many have computationally modeled complex blood flow to postulate how RBC rigidity may disrupt normal hemodynamics, to date, there lacks a clear understanding of how rigid RBCs affect the blood cell segregation behavior in blood flow, known as margination, and the resulting change in the adhesion of white blood cells (WBCs). In this work, we utilized an in vitro blood flow model to examine how different RBC rigidities and volume fractions of rigid RBCs impact cell margination and the downstream effect on white blood cell (WBC) adhesion in blood flow. Healthy RBC membranes were rigidified and reconstituted into whole blood and then perfused over activated endothelial cells under physiologically relevant shear conditions. Rigid RBCs were shown to reduce WBC adhesion by up to 80%, contingent on the RBC rigidity and the fraction of treated RBCs present in blood flow. Furthermore, the RBC core was found to be slightly expanded with the presence of rigid RBCs, by up to ∼30% in size fully composed of rigid RBCs. Overall, the obtained results demonstrate an impact of RBC rigidity on cellular dynamics and WBC adhesion, which possibly contributes to the pathological understanding of diseases characterized by significant RBC rigidity.

cfDNA correlates with endothelial damage after cardiac surgery with prolonged cardiopulmonary bypass and amplifies NETosis in an intracellular TLR9-independent manner

Cardiopulmonary bypass (CPB) provokes inflammation culminating in organ dysfunction and increased mortality. Recently, neutrophil extracellular traps (NETs) have been found to be involved in a variety of cardiovascular diseases promoting tissue and organ injury. Here, we aimed to elaborate the proinflammatory potential of circulating cell-free (cf)DNA in patients undergoing cardiac surgery with CPB. Plasma was collected pre- and postoperatively as well as at d1, d3, d5 and d8 after surgery. At d1, we found circulating cfDNA levels to be significantly increased in patients with prolonged CPB duration (>100 min) when compared to those with shorter CPB times (CPB < 100 min). Increased CPB duration yielded in higher levels of circulating mitochondrial (mt)DNA, soluble thrombomodulin (sCD141) and ICAM-1, reflecting endothelial damage. Positive correlation between cfDNA and sCD141 was demonstrated at all time points. Plasma and cfDNA from patients with CPB > 100 min induced NETs release by neutrophils from healthy donors which was not suppressed by inhibitors of intracellular toll-like receptor (TLR)9. DNA binding to neutrophils’ surface (s)TLR9 has been evidenced. Altogether, we demonstrate that elevated plasma cfDNA might be useful to assess CPB-mediated detrimental effects, including endothelial damage, in cardiac surgical patients with prolonged CPB duration. cfDNA-triggered NETosis is independent of classical TLR9 signaling.

Ectodomain Shedding by ADAM17: Its Role in Neutrophil Recruitment and the Impairment of This Process during Sepsis

Neutrophils are specialized at killing bacteria and are recruited from the blood in a rapid and robust manner during infection. A cascade of adhesion events direct their attachment to the vascular endothelium and migration into the underlying tissue. A disintegrin and metalloproteinase 17 (ADAM17) functions in the cell membrane of neutrophils and endothelial cells by cleaving its substrates, typically in a cis manner, at an extracellular site proximal to the cell membrane. This process is referred to as ectodomain shedding and it results in the downregulation of various adhesion molecules and receptors, and the release of immune regulating factors. ADAM17 sheddase activity is induced upon cell activation and rapidly modulates intravascular adhesion events in response to diverse environmental stimuli. During sepsis, an excessive systemic inflammatory response against infection, neutrophil migration becomes severely impaired. This involves ADAM17 as indicated by increased levels of its cleaved substrates in the blood of septic patients, and that ADAM17 inactivation improves neutrophil recruitment and bacterial clearance in animal models of sepsis. Excessive ADAM17 sheddase activity during sepsis thus appears to undermine in a direct and indirect manner the necessary balance between intravascular adhesion and de-adhesion events that regulate neutrophil migration into sites of infection. This review provides an overview of ADAM17 function and regulation and its potential contribution to neutrophil dysfunction during sepsis.

Using CRISPR-Cas9 to quantify the contributions of O-glycans, N-glycans and Glycosphingolipids to human leukocyte-endothelium adhesion

There is often interest in dissecting the relative contributions of the N-glycans, O-glycans and glycosphingolipids (GSLs) in regulating complex biological traits like cell signaling, adhesion, development and metastasis. To address this, we developed a CRISPR-Cas9 toolkit to selectively truncate each of these commonly expressed glycan-types. Here, O-glycan biosynthesis was truncated by knocking-out Core 1 β3Gal-T Specific Molecular Chaperone (COSMC), N-glycans by targeting the β1,2 GlcNAc-transferase (MGAT1) and GSLs by deleting UDP-glucose ceramide glucosyltransferase (UGCG). These reagents were applied to reveal the glycoconjugates regulating human myeloid cell adhesion to selectins under physiological shear-flow observed during inflammation. These functional studies show that leukocyte rolling on P- and L-selectin is ablated in cells lacking O-glycans, with N-glycan truncation also increasing cell rolling velocity on L-selectin. All three glycan families contributed to E-selectin dependent cell adhesion with N-glycans contributing to all aspects of the leukocyte adhesion cascade, O-glycans only being important during initial recruitment, and GSLs stabilizing slow cell rolling and the transition to firm arrest. Overall, the genome editing tools developed here may be broadly applied in studies of cellular glycosylation.

Leukocytes Crossing the Endothelium: A Matter of Communication

Leukocytes cross the endothelial vessel wall in a process called transendothelial migration (TEM). The purpose of leukocyte TEM is to clear the causing agents of inflammation in underlying tissues, for example, bacteria and viruses. During TEM, endothelial cells initiate signals that attract and guide leukocytes to sites of tissue damage. Leukocytes react by attaching to these sites and signal their readiness to move back to endothelial cells. Endothelial cells in turn respond by facilitating the passage of leukocytes while retaining overall integrity. In this review, we present recent findings in the field and we have endeavored to synthesize a coherent picture of the intricate interplay between endothelial cells and leukocytes during TEM.

Regulation of L-selectin-dependent hydrodynamic shear thresholding by leukocyte deformability and shear dependent bond number

BACKGROUND

During inflammation leukocyte attachment to the blood vessel wall is augmented by capture of near-wall flowing leukocytes by previously adherent leukocytes. Adhesive interactions between flowing and adherent leukocytes are mediated by L-selectin and P-selectin Glycoprotein Ligand-1 (PSGL-1) co-expressed on the leukocyte surface and ultimately regulated by hydrodynamic shear thresholding.

OBJECTIVE

We hypothesized that leukocyte deformability is a significant contributory factor in shear thresholding and secondary capture.

METHODS

Cytochalasin D (CD) was used to increase neutrophil deformability and fixation was used to reduce deformability. Neutrophil rolling on PSGL-1 coated planar surfaces and collisions with PSGL-1 coated microbeads were analyzed using high-speed videomicroscopy (250 fps).

RESULTS

Increased deformability led to an increase in neutrophil rolling flux on PSGL-1 surfaces while fixation led to a decrease in rolling flux. Abrupt drops in flow below the shear threshold resulted in extended release times from the substrate for CD-treated neutrophils, suggesting increased bond number. In a cell-microbead collision assay lower flow rates were correlated with briefer adhesion lifetimes and smaller adhesive contact patches.

CONCLUSIONS

Leukocyte deformation may control selectin bond number at the flow rates associated with hydrodynamic shear thresholding. Model analysis supported a requirement for both L-selectin catch-slip bond properties and multiple bond formation for shear thresholding.

The influence of blood on targeted microbubbles

The ability to successfully target the delivery of drugs and other therapeutic molecules has been a key goal of biomedical research for many decades. Despite highly promising in vitro results, however, successful translation of targeted drug delivery into clinical use has been extremely limited. This study investigates the significance of the characteristics of whole blood, which are rarely accounted for in vitro assays, as a possible explanation for the poor correlation between in vitro and in vivo experiments. It is shown using two separate model systems employing either biochemical or magnetic targeting that blood causes a substantial reduction in targeting efficiency relative to saline under the same flow conditions. This finding has important implications for the design of targeted drug delivery systems and the assays used in their development.

Development of an autoimmune syndrome affecting the skin and internal organs in P-selectin glycoprotein ligand 1 leukocyte receptor-deficient mice

OBJECTIVE

To define and characterize the progression of the spontaneous autoimmune disease that develops in mice in the absence of the leukocyte adhesion receptor P-selectin glycoprotein ligand 1 (PSGL-1).

METHODS

Skin-resident immune cells from PSGL-1-deficient mice and C57BL/6 control mice of different ages were isolated and analyzed by flow cytometry. Biochemical parameters were analyzed in mouse serum and urine, and the presence of serum autoantibodies was investigated. Skin and internal organs were extracted, and their structure was analyzed histologically.

RESULTS

Skin-resident innate and adaptive immune cells from PSGL-1(-/-) mice had a proinflammatory phenotype with an imbalanced T effector cell:Treg cell ratio. Sera from PSGL-1(-/-) mice had circulating autoantibodies commonly detected in connective tissue-related human autoimmune diseases. Biochemical and histologic analysis of skin and internal organs revealed skin fibrosis and structural and functional abnormalities in the lungs and kidneys. Furthermore, PSGL-1(-/-) mice exhibited vascular alterations, showing loss of dermal vessels, small vessel medial layer remodeling in the lungs and kidneys, and ischemic processes in the kidney that promote renal infarcts.

CONCLUSION

Our study demonstrates that immune system overactivation due to PSGL-1 deficiency triggers an autoimmune syndrome with characteristics similar to systemic sclerosis, including skin fibrosis, vascular alterations, and systemic organ involvement. These results suggest that PSGL-1 expression contributes to the maintenance of the homeostasis of the immune system and could act as a barrier for autoimmunity in mice.

Leukocyte migration into inflamed tissues

Leukocyte migration through activated venular walls is a fundamental immune response that is prerequisite to the entry of effector cells such as neutrophils, monocytes, and effector T cells to sites of infection, injury, and stress within the interstitium. Stimulation of leukocytes is instrumental in this process with enhanced temporally controlled leukocyte adhesiveness and shape-changes promoting leukocyte attachment to the inner wall of blood vessels under hydrodynamic forces. This initiates polarized motility of leukocytes within and through venular walls and transient barrier disruption facilitated sequentially by stimulated vascular cells, i.e., endothelial cells and their associated pericytes. Perivascular cells such as macrophages and mast cells that act as tissue inflammatory sentinels can also directly and indirectly regulate the exit of leukocytes from the vascular lumen. In this review, we discuss current knowledge and open questions regarding the mechanisms involved in the interactions of different effector leukocytes with peripheral vessels in extralymphoid organs.

L-selectin shedding by NSAIDs: old friends in new dresses

The recruitment of leukocytes to sites of inflammation requires the highly organized interplay of cell adhesion molecules on both leukocytes and inflamed endothelial cells, and disrupting the interaction of these molecules may compromise efficient recruitment of immune cells. Non-steroidal anti-inflammatory drugs inhibit inflammatory responses by several mechanisms including inhibition of prostaglandin synthesis and decreasing the expression of cell surface adhesion molecules. A report by Herrera-Garcia et al. [Eur. J. Immunol. 2013. 43: 55-64] in this issue of the European Journal of Immunology shows that the non-steroidal anti-inflammatory drug N-phenylanthranilic acid (N-Ph) causes L-selectin to be shed from the leukocyte plasma membrane and that this process in turn causes a decrease in leukocyte recruitment during inflammation in vivo. This finding may lead to novel approaches using N-Ph in the control of inflammatory processes as discussed in this Commentary.

Silencing α1,3-fucosyltransferases in human leukocytes reveals a role for FUT9 enzyme during E-selectin-mediated cell adhesion

Leukocyte adhesion during inflammation is initiated by the binding of sialofucosylated carbohydrates expressed on leukocytes to endothelial E/P-selectin. Although the glycosyltransferases (glycoTs) constructing selectin-ligands have largely been identified using knock-out mice, important differences may exist between humans and mice. To address this, we developed a systematic lentivirus-based shRNA delivery workflow to create human leukocytic HL-60 cell lines that lack up to three glycoTs. Using this, the contributions of all three myeloid α1,3-fucosyltransferases (FUT4, FUT7, and FUT9) to selectin-ligand biosynthesis were evaluated. The cell adhesion properties of these modified cells to L-, E-, and P-selectin under hydrodynamic shear were compared with bone marrow-derived neutrophils from Fut4(-/-)Fut7(-/-) dual knock-out mice. Results demonstrate that predominantly FUT7, and to a lesser extent FUT4, forms the selectin-ligand at the N terminus of leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) in humans and mice. Here, 85% reduction in leukocyte interaction was observed in human FUT4(-)7(-) dual knockdowns on P/L-selectin substrates. Unlike Fut4(-/-)Fut7(-/-) mouse neutrophils, however, human knockdowns lacking FUT4 and FUT7 only exhibited partial reduction in rolling interaction on E-selectin. In this case, the third α1,3-fucosyltransferase FUT9 played an important role because leukocyte adhesion was reduced by 50-60% in FUT9-HL-60, 70-80% in dual knockdown FUT7(-)9(-) cells, and ∼85% in FUT4(-)7(-)9(-) triple knockdowns. Gene silencing results are in agreement with gain-of-function experiments where all three fucosyltransferases conferred E-selectin-mediated rolling in HEK293T cells. This study advances new tools to study human glycoT function. It suggests a species-specific role for FUT9 during the biosynthesis of human E-selectin ligands.

Neutrophil rolling at high shear: flattening, catch bond behavior, tethers and slings

Neutrophil recruitment to sites of inflammation involves neutrophil rolling along the inflamed endothelium in the presence of shear stress imposed by blood flow. Neutrophil rolling in post-capillary venules in vivo is primarily mediated by P-selectin on the endothelium binding to P-selectin glycoprotein ligand-1 (PSGL-1) constitutively expressed on neutrophils. Blood flow exerts a hydrodynamic drag on the rolling neutrophil which is partially or fully balanced by the adhesive forces generated in the P-selectin-PSGL-1 bonds. Rolling is the result of rapid formation and dissociation of P-selectin-PSGL-1 bonds at the center and rear of the rolling cell, respectively. Neutrophils roll stably on P-selectin in post-capillary venules in vivo and flow chambers in vitro at wall shear stresses greater than 6 dyn cm(-2). However, the mechanisms that enable neutrophils to roll at such high shear stress are not completely understood. In vitro and in vivo studies have led to the discovery of four potential mechanisms, viz. cell flattening, catch bond behavior, membrane tethers, and slings. Rolling neutrophils undergo flattening at high shear stress, which not only increases the size of the cell footprint but also reduces the hydrodynamic drag experienced by the rolling cell. P-selectin-PSGL-1 bonds behave as catch bonds at small detachment forces and thus become stronger with increasing force. Neutrophils rolling at high shear stress form membrane tethers which can be longer than the cell diameter and promote the survival of P-selectin-PSGL-1 bonds. Finally, neutrophils rolling at high shear stress form 'slings', which act as cell autonomous adhesive substrates and support step-wise peeling. Tethers and slings act together and contribute to the forces balancing the hydrodynamic drag. How the synergy between the four mechanisms leads to stable rolling at high shear stress is an area that needs further investigation.

Particle-cell dynamics in human blood flow: implications for vascular-targeted drug delivery

The outcome of vascular-targeted therapies is generally determined by how efficiently vascular-targeted carriers localize and adhere to the endothelial wall at the targeted site. This study investigates the impact of leukocytes, platelets and red blood cells on the margination of vascular-targeted polymeric nanospheres and microspheres under various physiological blood flow conditions. We report that red blood cells either promote or hinder particle adhesion to an endothelial wall in a parallel plate flow chamber depending on the blood flow pattern, hematocrit, and particle size. Leukocytes prevent microspheres - but not nanospheres - from adhering in laminar and pulsatile flows via (1) competition for the available binding space and (2) physical removal of previously bound spheres. In recirculating blood flow, the negative effect of leukocytes on particle adhesion is minimal for large microspheres in the disturbed flow region beyond the flow reattachment. Resting platelets were found to have no effect on particle binding likely due to their dimensions and minimal interaction with the endothelial wall. Overall, the findings of the present work would be critical for designing effective vascular-targeted carriers for imaging and drug delivery applications in several human diseases.

ADAM17 activation in circulating neutrophils following bacterial challenge impairs their recruitment

Neutrophil infiltration and bacterial clearance occur earlier in conditional knockout mice with leukocytes lacking the metalloprotease ADAM17 than in control mice. We investigated cell-intrinsic changes in neutrophils lacking ADAM17 and alterations in the inflammatory environment in conditional ADAM17 knockout mice to determine how the sheddase exerts its effects on neutrophil recruitment. In vivo analyses comparing control and ADAM17-deficient neutrophils revealed that the latter cells accumulated at increased levels in the inflamed mesenteric microvasculature and in the peritoneal cavity following bacterial challenge, indicating changes in their adhesive properties. Consistent with this, bacterial infection caused a marked down-regulation of L-selectin, an adhesion protein and substrate of ADAM17, from the surface of circulating neutrophils in control mice but not in conditional ADAM17 knockout mice. Neutrophils from gene-targeted mice with leukocytes expressing a noncleavable form of L-selectin also displayed a competitive advantage in the presence of control neutrophils when infiltrating a site of infection. Taken together, our findings reveal that impaired L-selectin shedding is a key mechanism underlying early neutrophil recruitment in conditional ADAM17 knockout mice during bacterial infection. Disrupting only the shedding of L-selectin, however, did not increase bacterial clearance, indicating that additional substrates also contribute to the detrimental role of ADAM17 during severe infection.

Molecular cloning and expression analysis of P-selectin glycoprotein ligand-1 from zebrafish (Danio rerio)

To date, the best characterized glycoprotein ligand for P-selectin is P-selectin glycoprotein ligand-1 (PSGL-1). In this study, we cloned the full-length cDNA of PSGL-1 from zebrafish (Danio rerio). Zebrafish PSGl-1 cDNA is 1,594 bp and encodes a putative 284 amino acid protein with a theoretical molecular weight of 30.33 kDa and isoelectric point of 7.96. A signal peptide of 27 amino acids is predicted. The putative protein contains an extracellular mucin-like domain, a transmembrane domain and a cytoplasmic domain, with homology to mammalian PSGL-1. In the putative P-selectin binding region, there are 1 potential tyrosine sulfation site and 12 potential threonine O-glycosylation sites. A single extracellular cysteine, at the junction of the extracellular and transmembrane domains, suggests a disulfide-bonding pattern. The amino acid sequence of zebrafish PSGL-1 is 19-22% identical to that of mammalian PSGL-1. RT-PCR and whole-mount in situ hybridization analysis revealed that zebrafish PSGL-1 was expressed in early embryonic development, and the expression has an increased trend from 0.2 (1-cell stage) to 72 hpf. The results indicate that the general domain structure of PSGL-1 protein is conserved among species, and zebrafish PSGL-1 plays important roles in embryonic development and probably has similar biological function to that of mammalian PSGL-1.

Ezrin-radixin-moesin-binding sequence of PSGL-1 glycoprotein regulates leukocyte rolling on selectins and activation of extracellular signal-regulated kinases

P-selectin glycoprotein ligand-1 (PSGL-1) mediates the capture (tethering) of free-flowing leukocytes and subsequent rolling on selectins. PSGL-1 interactions with endothelial selectins activate Src kinases and spleen tyrosine kinase (Syk), leading to α(L)β(2) integrin-dependent leukocyte slow rolling, which promotes leukocyte recruitment into tissues. In addition, but through a distinct pathway, PSGL-1 engagement activates ERK. Because ezrin, radixin and moesin proteins (ERMs) link PSGL-1 to actin cytoskeleton and because they serve as adaptor molecules between PSGL-1 and Syk, we examined the role of PSGL-1 ERM-binding sequence (EBS) on cell capture, rolling, and signaling through Syk and MAPK pathways. We carried out mutational analysis and observed that deletion of EBS severely reduced 32D leukocyte tethering and rolling on L-, P-, and E-selectin and slightly increased rolling velocity. Alanine substitution of Arg-337 and Lys-338 showed that these residues play a key role in supporting leukocyte tethering and rolling on selectins. Importantly, EBS deletion or Arg-337 and Lys-338 mutations abrogated PSGL-1-induced ERK activation, whereas they did not prevent Syk phosphorylation or E-selectin-induced leukocyte slow rolling. These studies demonstrate that PSGL-1 EBS plays a critical role in recruiting leukocytes on selectins and in activating the MAPK pathway, whereas it is dispensable to phosphorylate Syk and to lead to α(L)β(2)-dependent leukocyte slow rolling.

Glycomechanics of the metastatic cascade: tumor cell-endothelial cell interactions in the circulation

Hydrodynamic shear force plays an important role in the leukocyte adhesion cascade that involves the tethering and rolling of cells along the endothelial layer, their firm adhesion or arrest, and their extravasation or escape from the circulatory system by inducing passive deformation, or cell flattening, and microvilli stretching, as well as regulating the expression, distribution, and conformation of adhesion molecules on leukocytes and the endothelial layer. Similarly, the dissemination of circulating tumor cells (CTCs) from the primary tumor sites is believed to involve tethering, rolling, and firm adhesion steps before their eventual extravasation which leads to secondary tumor sites (metastasis). Of particular importance to both the leukocyte adhesion cascade and the extravasation of CTCs, glycoproteins are involved in all three steps (capture, rolling, and firm adhesion) and consist of a variety of important selectin ligands. This review article provides an overview of glycoprotein glycosylation associated with the abnormal glycan expression on cancer cell surfaces, where well-established and novel selectin ligands that are cancer related are discussed. An overview of computational approaches on the effects of fluid mechanical force on glycoprotein mediated cancer cell rolling and adhesion is presented with a highlight of recent flow-based and selectin-mediated cell capturing/enriching devices. Finally, as an important branch of the glycoprotein family, mucins, specifically MUC1, are discussed in the context of their aberrant expression on cancer cells and their role as cancer cell adhesion molecules. Since metastasis relies heavily on glycoprotein interactions in the bloodstream where the fluid shear stress highly regulates cell adhesion forces, it is important to study and understand the glycomechanics of all relevant glycoproteins (well-established and novel) as they relate to the metastatic cascade.

Leukocyte ligands for endothelial selectins: specialized glycoconjugates that mediate rolling and signaling under flow

Reversible interactions of glycoconjugates on leukocytes with P- and E-selectin on endothelial cells mediate tethering and rolling of leukocytes in inflamed vascular beds, the first step in their recruitment to sites of injury. Although selectin ligands on hematopoietic precursors have been identified, here we review evidence that PSGL-1, CD44, and ESL-1 on mature leukocytes are physiologic glycoprotein ligands for endothelial selectins. Each ligand has specialized adhesive functions during tethering and rolling. Furthermore, PSGL-1 and CD44 induce signals that activate the β2 integrin LFA-1 and promote slow rolling, whereas ESL-1 induces signals that activate the β2 integrin Mac-1 in adherent neutrophils. We also review evidence for glycolipids, CD43, L-selectin, and other glycoconjugates as potential physiologic ligands for endothelial selectins on neutrophils or lymphocytes. Although the physiologic characterization of these ligands has been obtained in mice, we also note reported similarities and differences with human selectin ligands.

Sialyl Lewisx-dependent binding of human monocyte-derived dendritic cells to selectins

The limited efficacy of monocyte-derived dendritic cell (mo-DC)-based vaccines is primarily attributed to the reduced mo-DC migratory capacity. One undefined aspect is the initial binding of mo-DCs to endothelial cells and vascular selectins. In this study, we investigated the role and modulation of the selectin binding determinant sialyl Lewis(x) (sLe(x)) in selectin-dependent mo-DC binding. Our data reveal that sLe(x) is required for maximal binding of mo-DCs to tumor necrosis factor (TNF)-α-activated endothelial cells under static conditions, as evidenced by the use of sialidase. Sialidase treatment also abrogated mo-DC cell tethering to immobilized, purified P-, L-, or E-selectin under flow. The requirement of sLe(x)-dependent binding of mo-DC to selectins was further substantiated by using sLe(x) free sugar and anti-sLe(x) antibody, which significantly suppressed mo-DC-selectin binding. P-selectin glycoprotein ligand-1 is required for mo-DC binding to both P- and L-selectin, but it is dispensable for E-selectin recognition. Interestingly, the extent of mo-DC tethering was maximal on P-selectin, followed by E- and L- selectin. Accordingly, L-selectin mediated faster mo-DC rolling than E- or P-selectin. Interferon (IFN)-γ induces a significant increase in mo-DC surface sLe(x) expression, which is probably due to the enhanced synthesis of C2GnT-I. These findings may contribute to improving mo-DC-based vaccination protocols.

Mimicking the inflammatory cell adhesion cascade by nucleic acid aptamer programmed cell-cell interactions

Nature has evolved effective cell adhesion mechanisms to deliver inflammatory cells to inflamed tissue; however, many culture-expanded therapeutic cells are incapable of targeting diseased tissues following systemic infusion, which represents a great challenge in cell therapy. Our aim was to develop simple approaches to program cell-cell interactions that would otherwise not exist toward cell targeting and understanding the complex biology of cell-cell interactions. We employed a chemistry approach to engineer P- or L-selectin binding nucleic acid aptamers onto mesenchymal stem cells (MSCs) to enable them to engage inflamed endothelial cells and leukocytes, respectively. We show for the first time that engineered cells with a single artificial adhesion ligand can recapitulate 3 critical cell interactions in the inflammatory cell adhesion cascade under dynamic flow conditions. Aptamer-engineered MSCs adhered on respective selectin surfaces under static conditions >10 times more efficiently than controls including scrambled-DNA modified MSCs. Significantly, engineered MSCs can be directly captured from the flow stream by selectin surfaces or selectin-expressing cells under flow conditions (≤2dyn/cm²). The simple chemistry approach and the versatility of aptamers permit the concept of engineered cell-cell interactions to be generically applicable for targeting cells to diseased tissues and elucidating the biology of cell-cell interactions.

L-selectin--a dynamic regulator of leukocyte migration

The leukocytic cell adhesion receptor L-selectin mediates the initial step of the adhesion cascade, the capture and rolling of leukocytes on endothelial cells. This event enables leukocytes to migrate out of the vasculature into surrounding tissues during inflammation and immune surveillance. Distinct domains of L-selectin contribute to proper leukocyte migration. In this review, we discuss the contributions of these domains with respect to L-selectin function: the regulation by serine phosphorylation of the cytoplasmic tail, the role of the transmembrane domain in receptor positioning on the cell surface as well as the N-glycosylation of the extracellular part and the identification of novel binding partners.

Targeting therapeutics to the vascular wall in atherosclerosis--carrier size matters

OBJECTIVE

Vascular-targeted imaging and drug delivery systems are promising for the treatment of atherosclerosis due to the vast involvement of endothelium in the initiation and growth of plaque. Herein, we investigated the role of particle size in dictating the ability of vascular-targeted spherical particles to interact with the vascular wall (VW) from pulsatile and recirculating human blood flow relevant in atherosclerosis.

METHODS

In vitro parallel plate flow chambers (PPFC) with straight or vertical step channel were used to examine the localization and binding efficiency of inflammation-targeted polymeric spheres sized from 0.2 to 5 μm to inflamed endothelium from disturbed reconstituted and whole blood flow. Apolipoprotein deficient mice were used to study particle localization and binding to plaque in vivo.

RESULTS

The efficiency of particle binding in disturbed reconstituted blood flow increases as spherical diameter increases from 500 nm to 5 μm. No significant difference was observed between adhesion of 200 nm and 500 nm spheres. Binding efficiency for all particle size was enhanced in disturbed whole blood flow except adhesion of 5 μm in pulsatile whole blood. The adhesion trend in the in vivo model confirmed the binding pattern observed in in vitro assays.

CONCLUSIONS

The presented data shows that the binding efficiency of vascular-targeted drug carriers in blood flow is a function of particle size, wall shear rate, flow type, blood composition and ligand characteristics. Overall, the presented results suggest that micron-sized spherical particles (2 μm), not nanospheres, are optimal for vascular-targeted drug delivery applications in medium to large vessel relevant in atherosclerosis.

Cytoplasmic domain of P-selectin glycoprotein ligand-1 facilitates dimerization and export from the endoplasmic reticulum

P-selectin glycoprotein ligand-1 (PSGL-1) is a homodimeric transmembrane mucin on leukocytes. During inflammation, reversible interactions of PSGL-1 with selectins mediate leukocyte rolling on vascular surfaces. The transmembrane domain of PSGL-1 is required for dimerization, and the cytoplasmic domain propagates signals that activate β(2) integrins to slow rolling on integrin ligands. Leukocytes from knock-in "ΔCD" mice express a truncated PSGL-1 that lacks the cytoplasmic domain. Unexpectedly, they have 10-fold less PSGL-1 on their surfaces than WT leukocytes. Using glycosidases, proteases, Western blotting, confocal microscopy, cell-surface cross-linking, FRET, and pulse-chase metabolic labeling, we demonstrate that deleting the cytoplasmic domain impaired dimerization and delayed export of PSGL-1 from the endoplasmic reticulum (ER), markedly increasing a monomeric precursor in the ER and decreasing mature PSGL-1 on the cell surface. A monomeric full-length PSGL-1 made by substituting the transmembrane domain with that of CD43 exited the ER normally, revealing that dimerization was not required for ER export. Thus, the transmembrane and cytoplasmic domains cooperate to promote dimerization of PSGL-1. Furthermore, the cytoplasmic domain provides a key signal to export precursors of PSGL-1 from the ER to the Golgi apparatus en route to the cell surface.

Biophysics of selectin-ligand interactions in inflammation and cancer

Selectins (L-, E- and P-selectin) are calcium-dependent transmembrane glycoproteins that are expressed on the surface of circulating leukocytes, activated platelets, and inflamed endothelial cells. Selectins bind predominantly to sialofucosylated glycoproteins and glycolipids (E-selectin only) present on the surface of apposing cells, and mediate transient adhesive interactions pertinent to inflammation and cancer metastasis. The rapid turnover of selectin-ligand bonds, due to their fast on- and off-rates along with their remarkably high tensile strengths, enables them to mediate cell tethering and rolling in shear flow. This paper presents the current body of knowledge regarding the role of selectins in inflammation and cancer metastasis, and discusses experimental methodologies and mathematical models used to resolve the biophysics of selectin-mediated cell adhesion. Understanding the biochemistry and biomechanics of selectin-ligand interactions pertinent to inflammatory disorders and cancer metastasis may provide insights for developing promising therapies and/or diagnostic tools to combat these disorders.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

CD40/CD40L interaction induces E-selectin dependent leukocyte adhesion to human endothelial cells and inhibits endothelial cell migration

BACKGROUND

CD40 is a receptor expressed on a wide range of cells such as leukocytes and endothelial cells (EC). As a member of the tumor necrosis factor (TNF) superfamily the activation of CD40 by CD40-ligand (CD40L) plays a crucial role for the development and progression of a variety of inflammatory processes including atherosclerosis. The aim of the present study was to investigate the effect of CD40/CD40L interaction on leukocyte adhesion to the endothelium and on endothelial cell migration.

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVEC) were stimulated with either stable transfectants of mouse myeloma cells expressing the CD40L or wild type cells (4 h). Subsequently adhesion of leukocytes expressing Sialyl Lewis X, the counterpart for E-selectin (HL60 cells), was measured under shear stress (2-2.6 dyne/cm(2)) using a flow chamber adhesion assay. Stimulation of CD40 led to a significant increase of E-selectin dependent adhesion of leukocytes to the endothelium. Incubation of cells with either the CD40L blocking antibody TRAP-1 or the E-selectin blocking antibody BBA2 during CD40 stimulation completely abolished adhesion of leukocytes to HUVEC. Similar results were found in human cardiac microvasculature endothelial cells (HCMEC). In contrast stimulation of CD40 had no effect on adhesion of L-selectin expressing NALM6-L cells. Furthermore, CD40/CD40L interaction abrogated VEGF-induced migration of HUVEC compared to non-stimulated controls. In comparison experiments, stimulation of endothelial cells with VEGF led to a significant phosphorylation of ERK1/2, Akt, and eNOS. Stimulation of endothelial CD40 had no effect on VEGF-induced phosphorylation of ERK1/2. However, VEGF-induced activation of Akt and eNOS was reduced to baseline levels when endothelial CD40 was stimulated.

CONCLUSION

CD40/CD40L interaction induces E-selectin dependent adhesion of leukocytes to human endothelial cells and reduces endothelial cell migration by inhibiting the Akt/eNOS signaling pathway.

Vascular Recruitment of Human Retinoblastoma Cells by Multi-Cellular Adhesive Interactions with Circulating Leukocytes

Retinoblastoma (RB) is a retinal cancer of childhood. RB survivors tend to develop additional tumors later in life, although the physical mechanisms of RB metastatic spread are largely unknown. One step in metastasis through the blood stream is tumor cell adherence to the blood vessel wall through specific receptor:ligand interactions. Yet, human RB cell lines RB143 and WERI-Rb27 do not express selectin ligands or beta-2 integrins and cannot directly interact with inflamed endothelium. In this study, we show that RB cells express ICAM-1, a beta-2 integrin ligand that correlates with metastasis and is preferentially co-expressed on RB cells that also express ABCG2, a stem cell marker associated with chemoresistance and metastasis. Based on the presence of ICAM-1+ RB cells, we tested the hypothesis that RB cells could be recruited to an E-selectin surface via attachment to activated polymorphonuclear cells (PMNs). We characterized the dynamic adhesion between RB cells and PMNs within E-selectin coated microtubes under a physiological range of wall shear stress values (0.2-5 dyn/cm²). We show that activated PMNs are necessary for the recruitment of RB cells through ICAM-1:LFA-1 binding. Results from this work may lead to new strategies that target the metastatic spread of tumor cells.