P-selectin glycoprotein ligand 1 is a ligand for L-selectin on neutrophils, monocytes, and CD34+ hematopoietic progenitor cells

A non-human primate derived anti-P-selectin glycoprotein ligand-1 antibody curtails acute pancreatitis by alleviating the inflammatory responses

Acute pancreatitis (AP) is a devastating disease characterized by an inflammatory disorder of the pancreas. P-selectin glycoprotein ligand-1 (PSGL-1) plays a crucial role in the initial steps of the adhesive at process to inflammatory sites, blockade of PSGL-1 might confer potent anti-inflammatory effects. In this study, we generated two non-human primate derived monoclonal antibodies capable of efficiently targeting human PSGL-1, RH001-6 and RH001-22, which were screened from immunized rhesus macaques. We found that RH001-6, can effectively block the binding of P-selectin to PSGL-1, and abolish the adhesion of leukocytes to endothelial cells in vitro. In vivo, we verified that RH001-6 relieved inflammatory responses and pancreatic injury in both caerulein and l-arginine induced AP models. We also evaluated the safety profile after RH001-6 treatment in mice, and verified that RH001-6 did not cause any significant pathological damages in vivo. Taken together, we developed a novel non-human primate derived PSGL-1 blocking antibody with high-specificity, named RH001-6, which can interrupt the binding of PSGL-1 and P-selectin and attenuate inflammatory responses during AP. Therefore, RH001-6 is highly potential to be further developed into therapeutics against acute inflammatory diseases, such as AP.

HIV Infection: Shaping the Complex, Dynamic, and Interconnected Network of the Cytoskeleton

HIV-1 has evolved a plethora of strategies to overcome the cytoskeletal barrier (i.e., actin and intermediate filaments (AFs and IFs) and microtubules (MTs)) to achieve the viral cycle. HIV-1 modifies cytoskeletal organization and dynamics by acting on associated adaptors and molecular motors to productively fuse, enter, and infect cells and then traffic to the cell surface, where virions assemble and are released to spread infection. The HIV-1 envelope (Env) initiates the cycle by binding to and signaling through its main cell surface receptors (CD4/CCR5/CXCR4) to shape the cytoskeleton for fusion pore formation, which permits viral core entry. Then, the HIV-1 capsid is transported to the nucleus associated with cytoskeleton tracks under the control of specific adaptors/molecular motors, as well as HIV-1 accessory proteins. Furthermore, HIV-1 drives the late stages of the viral cycle by regulating cytoskeleton dynamics to assure viral Pr55Gag expression and transport to the cell surface, where it assembles and buds to mature infectious virions. In this review, we therefore analyze how HIV-1 generates a cell-permissive state to infection by regulating the cytoskeleton and associated factors. Likewise, we discuss the relevance of this knowledge to understand HIV-1 infection and pathogenesis in patients and to develop therapeutic strategies to battle HIV-1.

Single-cell RNA sequencing reveals the suppressive effect of PPP1R15A inhibitor Sephin1 in antitumor immunity

Protein phosphatase 1 regulatory subunit 15A (PPP1R15A) is an important factor in the integrated stress response (ISR) in mammals and may play a crucial role in tumorigenesis. In our studies, we found an inhibitor of PPP1R15A, Sephin1, plays a protumorigenic role in mouse tumor models. By analyzing the single-cell transcriptome data of the mouse tumor models, we found that in C57BL/6 mice, Sephin1 treatment could lead to higher levels of ISR activity and lower levels of antitumor immune activities. Specifically, Sephin1 treatment caused reductions in antitumor immune cell types and lower expression levels of cytotoxicity-related genes. In addition, T cell receptor (TCR) repertoire analysis demonstrated that the clonal expansion of tumor-specific T cells was inhibited by Sephin1. A special TCR + macrophage subtype in tumor was identified to be significantly depleted upon Sephin1 treatment, implying its key antitumor role. These results suggest that PPP1R15A has the potential to be an effective target for tumor therapy.

Immune Responses to IAV Infection and the Roles of L-Selectin and ADAM17 in Lymphocyte Homing

Influenza A virus (IAV) infection is a global public health burden causing up to 650,000 deaths per year. Yearly vaccination programmes and anti-viral drugs currently have limited benefits; therefore, research into IAV is fundamental. Leukocyte trafficking is a crucial process which orchestrates the immune response to infection to protect the host. It involves several homing molecules and receptors on both blood vessels and leukocytes. A key mediator of this process is the transmembrane glycoprotein L-selectin, which binds to vascular addressins on blood vessel endothelial cells. L-selectin classically mediates homing of naïve and central memory lymphocytes to lymph nodes via high endothelial venules (HEVs). Recent studies have found that L-selectin is essential for homing of activated CD8⁺ T cells to influenza-infected lungs and reduction in virus load. A disintegrin and metalloproteinase 17 (ADAM17) is the primary regulator of cell surface levels of L-selectin. Understanding the mechanisms that regulate these two proteins are central to comprehending recruitment of T cells to sites of IAV infection. This review summarises the immune response to IAV infection in humans and mice and discusses the roles of L-selectin and ADAM17 in T lymphocyte homing during IAV infection.

P-Selectin Glycoprotein Ligand 1: A Potential HIV-1 Therapeutic Target

Antiretroviral therapy (ART), which is a life-long therapeutic option, remains the only currently effective clinical method to treat HIV-1 infection. However, ART may be toxic to vital organs including the liver, brain, heart, and kidneys, and may result in systemic complications. In this context, to consider HIV-1 restriction factors from the innate immune system to explore novel HIV therapeutics is likely to be a promising investigative strategy. In light of this, P-selectin glycoprotein ligand 1 (PSGL-1) has recently become the object of close scrutiny as a recognized cell adhesion molecule, and has become a major focus of academic study, as researchers believe that PSGL-1 may represent a novel area of interest in the research inquiry into the field of immune checkpoint inhibition. In this article, we review PSGL-1's structure and functions during infection and/or inflammation. We also outline a comprehensive review of its role and potential therapeutic utility during HIV-1 infection as published in contemporary academic literature.

New Pharmacological Tools to Target Leukocyte Trafficking in Lung Disease

Infection and inflammation of the lung results in the recruitment of non-resident immune cells, including neutrophils, eosinophils and monocytes. This swift response should ensure clearance of the threat and resolution of stimuli which drive inflammation. However, once the threat is subdued this influx of immune cells should be followed by clearance of recruited cells through apoptosis and subsequent efferocytosis, expectoration or retrograde migration back into the circulation. This cycle of cell recruitment, containment of threat and then clearance of immune cells and repair is held in exquisite balance to limit host damage. Advanced age is often associated with detrimental changes to the balance described above. Cellular functions are altered including a reduced ability to traffic accurately towards inflammation, a reduced ability to clear pathogens and sustained inflammation. These changes, seen with age, are heightened in lung disease, and most chronic and acute lung diseases are associated with an exaggerated influx of immune cells, such as neutrophils, to the airways as well as considerable inflammation. Indeed, across many lung diseases, pathogenesis and progression has been associated with the sustained presence of trafficking cells, with examples including chronic diseases such as Chronic Obstructive Pulmonary Disease and Idiopathic Pulmonary Fibrosis and acute infections such as Pneumonia and Pneumonitis. In these instances, there is evidence that dysfunctional and sustained recruitment of cells to the airways not only increases host damage but impairs the hosts ability to effectively respond to microbial invasion. Targeting leukocyte migration in these instances, to normalise cellular responses, has therapeutic promise. In this review we discuss the current evidence to support the trafficking cell as an immunotherapeutic target in lung disease, and which potential mechanisms or pathways have shown promise in early drug trials, with a focus on the neutrophil, as the quintessential trafficking immune cell.

PSGL-1 Immune Checkpoint Inhibition for CD4+ T Cell Cancer Immunotherapy

Immune checkpoint inhibition targeting T cells has shown tremendous promise in the treatment of many cancer types and are now standard therapies for patients. While standard therapies have focused on PD-1 and CTLA-4 blockade, additional immune checkpoints have shown promise in promoting anti-tumor immunity. PSGL-1, primarily known for its role in cellular migration, has also been shown to function as a negative regulator of CD4+ T cells in numerous disease settings including cancer. PSGL-1 is highly expressed on T cells and can engage numerous ligands that impact signaling pathways, which may modulate CD4+ T cell differentiation and function. PSGL-1 engagement in the tumor microenvironment may promote CD4+ T cell exhaustion pathways that favor tumor growth. Here we highlight that blocking the PSGL-1 pathway on CD4+ T cells may represent a new cancer therapy approach to eradicate tumors.

Neuromedin U: potential roles in immunity and inflammation

Since the discovery of neuromedin U (NmU) from porcine spinal cord in 1985, this neuropeptide has been subsequently identified in many other species with multiple physiological and pathophysiological roles detected, ranging from smooth muscle contraction, feeding, energy balance to tumorigenesis. Intriguingly, NmU is also emerging to play pro-inflammatory roles involving immune cell activation and cytokine release in a neuron-dependent or neuron-independent manner. The NmU-mediated inflammatory responses have already been observed in worm infection, sepsis, autoimmune arthritis and allergic animal models. In this review, we focus on the roles of NmU in immunity and inflammation by highlighting the interactions between NmU and immune cells, summarizing the signalling mechanism involved in their reactions and discussing its potential contributions to inflammatory diseases.

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.

Platelets in Skin Autoimmune Diseases

Systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and small vessel vasculitis are three autoimmune diseases frequently manifested in the skin. They share common pathogenic features, including production of autoantibodies, loss of tolerance to self-antigens, tissue necrosis and fibrosis, vasculopathy and activation of the coagulation system. Platelets occupy a central part within the coagulation cascade and are well-recognized for their hemostatic role. However, recent cumulative evidence implicates their additional and multifaceted immunoregulatory functions. Platelets express immune receptors and they store growth factors, cytokines, and chemokines in their granules enabling a significant contribution to inflammation. A plethora of activating triggers such as damage associated molecular patterns (DAMPs) released from damaged endothelial cells, immune complexes, or complement effector molecules can mediate platelet activation. Activated platelets further foster an inflammatory environment and the crosstalk with the endothelium and leukocytes by the release of immunoactive molecules and microparticles. Further insight into the pathogenic implications of platelet activation will pave the way for new therapeutic strategies targeting autoimmune diseases. In this review, we discuss the inflammatory functions of platelets and their mechanistic contribution to the pathophysiology of SSc, ANCA associated small vessel vasculitis and other autoimmune diseases affecting the skin.

Impact of real-time shedding on binding kinetics of membrane-remaining L-selectin to PSGL-1

L-selectin shedding induced by various cytokines is crucial in activating neutrophils (PMNs) in inflammatory cascade. While the real-time shedding in vivo lasts ~10 min after PMN activation, the impact of time-dependent shedding on binding kinetics of membrane-remaining L-selectins to its ligands is poorly understood at transient or steady state. Here, we developed an in vitro L-selectin shedding dynamics approach, together with competitive assays of cell adhesion, and proposed a theoretical model for quantifying the impact of real-time shedding on the binding kinetics of membrane-remaining L-selectins to P-selectin glycoprotein ligand-1 (PSGL-1). Our data indicated that the extent of L-selectin shedding on PMA activation is higher, but the terminating time is longer for Jurkat cells than those for human PMNs. Meanwhile, fMLF or IL-8 stimulation yields the longer terminating time than that on PMA stimulation but results in a similar shedding extent for PMNs. L-selectin shedding reduces L-selectin-PSGL-1-mediated cell adhesion in three ways: decreasing membrane-anchored L-selectins, increasing soluble L-selectins competitively binding to ligands, and presenting conformational alteration of membrane-remaining L-selectins themselves. Compared with those on intact cells, the binding affinities of membrane-remaining L-selectin-PSGL-1 pairs were all enhanced at initial and lowered at the late shedding phase for both PMN and Jurkat cells even with varied transition time points. The rolling velocities of both PMNs and Jurkat cells were increased following mechanically or biochemically induced shedding of L-selectin under shear flow. These findings help to further our understanding of the function of time-dependent L-selectin shedding during the inflammation cascade.

The Many and Varied Roles of Tetraspanins in Immune Cell Recruitment and Migration

Immune cell recruitment and migration is central to the normal functioning of the immune system in health and disease. Numerous adhesion molecules on immune cells and the parenchymal cells they interact with are well recognized for their roles in facilitating the movements of immune cells throughout the body. A growing body of evidence now indicates that tetraspanins, proteins known for their capacity to organize partner molecules within the cell membrane, also have significant impacts on the ability of immune cells to migrate around the body. In this review, we examine the tetraspanins expressed by immune cells and endothelial cells that influence leukocyte recruitment and motility and describe their impacts on the function of adhesion molecules and other partner molecules that modulate the movements of leukocytes. In particular, we examine the functional roles of CD9, CD37, CD63, CD81, CD82, and CD151. This reveals the diversity of the functions of the tetraspanin family in this setting, both in the nature of adhesive and migratory interactions that they regulate, and the positive or inhibitory effects mediated by the individual tetraspanin proteins.

Elevated plasma levels of P-selectin glycoprotein ligand-1-positive microvesicles in patients with unprovoked venous thromboembolism

Essentials PSGL-1+ microvesicles (MVs) may be important in venous thromboembolism (VTE). We measured plasma levels and parental origin of PSGL-1+ MVs in patients with unprovoked VTE. VTE patients had higher plasma levels of PSGL-1+ MVs than healthy controls. The PSGL-1+ MVs originated mainly from monocytes and endothelial cells.

SUMMARY

Background Microvesicles (MVs) express antigens from their parental cells and have a highly procoagulant surface. Animal studies suggest that P-selectin glycoprotein ligand-1-positive (PSGL-1+ ) MVs play a role in the pathogenesis of venous thromboembolism (VTE). Objective The aim of this study was to determine plasma levels, the cellular origin and the morphological characteristics of PSGL-1+ MVs in patients with unprovoked VTE. Methods We conducted a population-based case-control study in 20 patients with a history of unprovoked VTE and 20 age- and sex-matched healthy controls recruited from the general population. Plasma levels, the cellular origin and the morphological characteristics of PSGL-1+ MVs were evaluated using flow cytometry, electron microscopy and confocal microscopy. Results Plasma levels of PSGL-1+ MVs were associated with increased risk of VTE. The odds ratio per one standard deviation increase in PSGL-1+ MVs was 3.11 (95% confidence interval [CI], 1.41-6.88) after adjustment for age and sex, and 2.88 (95% CI, 1.29-6.41) after further adjustment for body mass index. The PSGL-1+ MVs originated mainly from monocytes and endothelial cells determined by double staining with markers of parental cells using flow cytometry and transmission electron microscopy. Scanning electron microscopy of PSGL-1-labeled plasma-derived MVs displayed dominantly spherical vesicles that varied between 50 and 300 nm in diameter. Conclusions Increased plasma levels of PSGL-1+ MVs are associated with the risk of unprovoked VTE. Large population-based prospective studies are required to validate our findings.

Distinguishing features of microglia- and monocyte-derived macrophages after stroke

After stroke, macrophages in the ischemic brain may be derived from either resident microglia or infiltrating monocytes. Using bone marrow (BM)-chimerism and dual-reporter transgenic fate mapping, we here set out to delimit the responses of either cell type to mild brain ischemia in a mouse model of 30 min transient middle cerebral artery occlusion (MCAo). A discriminatory analysis of gene expression at 7 days post-event yielded 472 transcripts predominantly or exclusively expressed in blood-derived macrophages as well as 970 transcripts for microglia. The differentially regulated genes were further collated with oligodendrocyte, astrocyte, and neuron transcriptomes, resulting in a dataset of microglia- and monocyte-specific genes in the ischemic brain. Functional categories significantly enriched in monocytes included migration, proliferation, and calcium signaling, indicative of strong activation. Whole-cell patch-clamp analysis further confirmed this highly activated state by demonstrating delayed outward K⁺ currents selectively in invading cells. Although both cell types displayed a mixture of known phenotypes pointing to the significance of 'intermediate states' in vivo, blood-derived macrophages were generally more skewed toward an M2 neuroprotective phenotype. Finally, we found that decreased engraftment of blood-borne cells in the ischemic brain of chimeras reconstituted with BM from Selplg^-/- mice resulted in increased lesions at 7 days and worse post-stroke sensorimotor performance. In aggregate, our study establishes crucial differences in activation state between resident microglia and invading macrophages after stroke and identifies unique genomic signatures for either cell type.

Not just a marker: CD34 on human hematopoietic stem/progenitor cells dominates vascular selectin binding along with CD44

CD34 is routinely used to identify and isolate human hematopoietic stem/progenitor cells (HSPCs) for use clinically in bone marrow transplantation, but its function on these cells remains elusive. Glycoprotein ligands on HSPCs help guide their migration to specialized microvascular beds in the bone marrow that express vascular selectins (E- and P-selectin). Here, we show that HSPC-enriched fractions from human hematopoietic tissue expressing CD34 (CD34^pos) bound selectins, whereas those lacking CD34 (CD34^neg) did not. An unbiased proteomics screen identified potential glycoprotein ligands on CD34^pos cells revealing CD34 itself as a major vascular selectin ligand. Biochemical and CD34 knockdown analyses highlight a key role for CD34 in the first prerequisite step of cell migration, suggesting that it is not just a marker on these cells. Our results also entice future potential strategies to investigate the glycoforms of CD34 that discriminate normal HSPCs from leukemic cells and to manipulate CD34^neg HSPC-enriched bone marrow or cord blood populations as a source of stem cells for clinical use.

Nanoparticulate Drug Delivery Systems Targeting Inflammation for Treatment of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic, idiopathic inflammatory set of conditions that can affect the entire gastrointestinal (GI) tract and is associated with an increased risk of colorectal cancer. To date there is no curative therapy for IBD; therefore life-long medication can be necessary for IBD management if surgery is to be avoided. Drug delivery systems specific to the colon have improved IBD treatment and several such systems are available to patients. However, current delivery systems for IBD do not target drugs to the site of inflammation, which leads to frequent dosing and potentially severe side effects that can adversely impact patients' adherence to medication. There is a need for novel drug delivery systems that can target drugs to the site of inflammation, prolong local drug availability, improve therapeutic efficacy, and reduce drug side effects. Nanoparticulate (NP) systems are attractive in designing targeted drug delivery systems for the treatment of IBD because of their unique physicochemical properties and capability of targeting the site of disease. This review analyzes the microenvironment at the site of inflammation in IBD, highlighting the pathophysiological features as possible cues for targeted delivery; discusses different strategies and mechanisms of NP targeting IBD, including size-, charge-, ligand-receptor, degradation- and microbiome-mediated approaches; and summarizes recent progress on using NPs towards improved therapies for IBD. Finally, challenges and future directions in this field are presented to advance the development of targeted drug delivery for IBD treatment.

Deep insight into neutrophil trafficking in various organs

Neutrophils are professional phagocytes that constitute the first line of defense in humans. The primary function of neutrophils is to eliminate invading pathogens through oxidative and nonoxidative mechanisms. Because neutrophils rapidly migrate into inflammatory foci via diapedesis and chemotaxis, neutrophil recruitment has long been considered a hallmark of inflammation. Recent advances in intravital microscopic technologies using animal model systems have enabled researchers to directly visualize neutrophil trafficking. Consequently, the specific mechanisms of neutrophil transmigration have been identified, and even the reverse migration of neutrophils can be verified visually. Moreover, the detailed phenomena of neutrophil infiltration into various organs, such as the liver, lymphoid organs, and CNS have been identified. This progress in the study of neutrophil migration from the blood vessels to organs results in a deeper understanding of these immune cells' motility and morphology, which are closely related to the spatiotemporal regulation of the overall immune response. In this review, we discuss our current understanding of neutrophil trafficking in various organs.

PSGL-1: A New Player in the Immune Checkpoint Landscape

P-selectin glycoprotein ligand-1 (PSGL-1) has long been studied as an adhesion molecule involved in immune cell trafficking and is recognized as a regulator of many facets of immune responses by myeloid cells. PSGL-1 also regulates T cell migration during homeostasis and inflammatory settings. However, recent findings indicate that PSGL-1 can also negatively regulate T cell function. Because T cell differentiation is finely tuned by multiple positive and negative regulatory signals that appropriately scale the magnitude of the immune response, PSGL-1 has emerged as an important checkpoint during this process. We summarize what is known regarding PSGL-1 structure and function and highlight how it may act as an immune checkpoint inhibitor in T cells.

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.

The role of platelets in inflammation

There is growing recognition of the critical role of platelets in inflammation and immune responses. Recent studies have indicated that antiplatelet medications may reduce mortality from infections and sepsis, which suggests possible clinical relevance of modifying platelet responses to inflammation. Platelets release numerous inflammatory mediators that have no known role in haemostasis. Many of these mediators modify leukocyte and endothelial responses to a range of different inflammatory stimuli. Additionally, platelets form aggregates with leukocytes and form bridges between leukocytes and endothelium, largely mediated by platelet P-selectin. Through their interactions with monocytes, neutrophils, lymphocytes and the endothelium, platelets are therefore important coordinators of inflammation and both innate and adaptive immune responses.

Polymorphisms of inflammatory markers and risk of essential hypertension in Tatars from Russia

Essential hypertension (EH) is a common disease with a clear genetic component. Inflammation and endothelial dysfunction play a prominent role in the development of persistent blood pressure elevation. The aim of the current study was to detect an association between EH and polymorphic markers in genes encoding for molecules involved in the control of intercellular interactions during the inflammation process. We analysed SNPs in SELE, SELP, SELL, ICAM1, VEGFA, IL1B, IL6, IL10 and IL12B genes in a group of 534 men of Tatar ethnicity (217 patients with EH and 317 controls). Using a Markov chain Monte-Carlo-based approach (APSampler), we found genotype and allelic combinations associated with EH. The most significant associations were observed for SELE rs2076059*C-SELP rs6131*A-VEGFA -2549*I-IL1B rs16944*C (p = 3.42 × 10(-5), FDR q = 0.035) and SELE rs2076059*C-SELP rs6131*A-IL12B rs3212227*C-IL1B rs16944*C (p = 323 × 10(-4), FDR q = 0.035).

Spatiotemporal Expression Dynamics of Selectins Govern the Sequential Extravasation of Neutrophils and Monocytes in the Acute Inflammatory Response

Objective—

Leukocyte recruitment to the site of inflammation is a key event in a variety of cardiovascular pathologies. Infiltrating neutrophils constitute the first line of defense that precedes a second wave of emigrating monocytes reinforcing the inflammatory reaction. The mechanisms initiating this sequential process remained largely obscure.

Approach and Results—

Using advanced in vivo microscopy and in vitro/ex vivo techniques, we identified individual spatiotemporal expression patterns of selectins and their principal interaction partners on neutrophils, resident/inflammatory monocytes, and endothelial cells. Coordinating the intraluminal trafficking of neutrophils and inflammatory monocytes to common sites of extravasation, selectins assign different sites to these immune cells for their initial interactions with the microvascular endothelium. Whereas constitutively expressed leukocyte L-selectin/CD62L and endothelial P-selectin/CD62P together with CD44 and P-selectin glycoprotein ligand-1/CD162 initiate the emigration of neutrophils, de novo synthesis of endothelial E-selectin/CD62E launches the delayed secondary recruitment of inflammatory monocytes. In this context, P-selectin/CD62P and L-selectin/CD62L together with P-selectin glycoprotein ligand-1/CD162 and CD44 were found to regulate the flux of rolling neutrophils and inflammatory monocytes, whereas E-selectin/CD62E selectively adjusts the rolling velocity of inflammatory monocytes. Moreover, selectins and their interaction partners P-selectin glycoprotein ligand-1/CD162 and CD44 differentially control the intraluminal crawling behavior of neutrophils and inflammatory monocytes collectively enabling the sequential extravasation of these immune cells to inflamed tissue.

Conclusions—

Our findings provide novel insights into the mechanisms initiating the sequential infiltration of the perivascular tissue by neutrophils and monocytes in the acute inflammatory response and might thereby contribute to the development of targeted therapeutic strategies for prevention and treatment of cardiovascular diseases.

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.

Functional significance of mononuclear phagocyte populations generated through adult hematopoiesis

Tissue homeostasis requires a complete repertoire of functional macrophages in peripheral tissues. Recent evidence indicates that many resident tissue macrophages are seeded during embryonic development and persist through adulthood as a consequence of localized proliferation. Mononuclear phagocytes are also produced during adult hematopoiesis; these cells are then recruited to sites throughout the body, where they function in tissue repair and remodeling, resolution of inflammation, maintenance of homeostasis, and disease progression. The focus of this review is on mononuclear phagocytes that comprise the nonresident monocyte/macrophage populations in the body. Key features of monocyte differentiation are presented, focusing primarily on the developmental hierarchy that is established through this process, the markers used to identify discrete cell populations, and novel, functional attributes of these cells. These features are then explored in the context of the tumor microenvironment, where mononuclear phagocytes exhibit extensive plasticity in phenotype and function.

Glycopolymer probes of signal transduction

Glycans are key participants in biological processes ranging from reproduction to cellular communication to infection. Revealing glycan roles and the underlying molecular mechanisms by which glycans manifest their function requires access to glycan derivatives that vary systematically. To this end, glycopolymers (polymers bearing pendant carbohydrates) have emerged as valuable glycan analogs. Because glycopolymers can readily be synthesized, their overall shape can be varied, and they can be altered systematically to dissect the structural features that underpin their activities. This review provides examples in which glycopolymers have been used to effect carbohydrate-mediated signal transduction. Our objective is to illustrate how these powerful tools can reveal the molecular mechanisms that underlie carbohydrate-mediated signal transduction.

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.

Lack of functional selectin ligand interactions compromises long term tumor protection by CD8+ T cells

Central memory CD8⁺ T cells expressing the adhesion molecule CD62L (L-selectin) are potent mediators of anti-cancer immunity due to their ability to proliferate extensively upon antigen re-stimulation. The interaction of selectin with its ligands mediates leukocyte rolling along high endothelial venules. Mice deficient in α(1,3) Fucosyltransferase IV and VII (FtDKO) lack functional L, P and E selectin ligands. Thus, we addressed whether the lack of selectin ligand interactions alters tumor protection by CD8⁺ T cells in FtDKO mice. Listeria monocytogenes-OVA (LM-OVA) infection evoked potent OVA-specific CD8⁺ T cells that proliferated and contracted at similar kinetics and phenotype in FtDKO and wild-type mice. Additionally, OVA-specific CD8⁺ T cells in both mouse strains exhibited similar phenotypic differentiation, in vivo cytolytic activity and IFN-γ expression. However, FtDKO mice succumbed to B16-OVA tumors significantly earlier than wild-type mice. In contrast, FtDKO mice evoked strong recall memory CD8⁺ T cell responses and protection to systemic LM-OVA re-challenge. The diminished tumor protection in FtDKO mice was not related to defective antigen presentation by dendritic cells or reduced proliferation of antigen-specific CD8⁺ T cells. However, WT or FtDKO OVA-specific CD8⁺ T cells showed significantly reduced ability to traffic to lymph nodes upon adoptive transfer into naïve FtDKO recipients. Furthermore, FtDKO OVA-specific CD8⁺ T cells displayed poor ability to infiltrate tumors growing in WT mice. These results reveal that selectin ligand expression on host endothelium as well CD8⁺ T cells may be important for their efficient and continued extravasation into peripheral tumors.

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.

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.

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.

Novel molecular imaging approach for subclinical detection of iritis and evaluation of therapeutic success

There is an urgent need for early diagnosis in medicine, whereupon effective treatments could prevent irreversible tissue damage. Acute anterior chamber inflammation is the most common form of uveitis and a major cause of vision loss. The proximity of the iris vasculature to the light-permeable cornea and its involvement in ocular inflammation make it an ideal target for noninvasive molecular imaging. To accomplish this, carboxylated fluorescent microspheres (MSs) were conjugated with recombinant P-selectin glycoprotein ligand-1 and systemically injected in endotoxin-induced uveitic animals. MS adhesion in the microcirculation of the anterior and posterior chamber was visualized by intravital microscopy and scanning laser ophthalmoscopy. In iritic animals, significantly higher numbers of recombinant P-selectin glycoprotein ligand-1-conjugated MSs adhered to the endothelium (P = 0.03) matching the increase in leukocyte adhesion. Conjugated MSs specifically interacted with firmly adhering leukocytes, allowing quantification of the endogenous immune response. Topical eye drop treatment with dexamethasone (P < 0.01) or cyclosporine A (P < 0.01) significantly lowered MS adhesion in iris vessels. Surprisingly, topical dexamethasone significantly reduced MS interaction in the fundus vessels (P < 0.01), while cyclosporine A did not. In vivo MS accumulation preceded clinical signs of anterior uveitis and leukocyte adhesion in iris vasculature. This work introduces noninvasive subclinical detection of endothelial injury in the iris vasculature, providing a unique opportunity for quantifying vascular injury and immune response in vivo.

Human L-selectin preferentially binds synthetic glycosulfopeptides modeled after endoglycan and containing tyrosine sulfate residues and sialyl Lewis x in core 2 O-glycans

Endoglycan is a mucin-like glycoprotein expressed by endothelial cells and some leukocytes and is recognized by L-selectin, a C-type lectin important in leukocyte trafficking and extravasation during inflammation. Here, we show that recombinant L-selectin and human T lymphocytes expressing L-selectin bind to synthetic glycosulfopeptides (GSPs). These synthetic glycosulfopeptides contain 37 amino acid residues modeled after the N-terminus of human endoglycan and contain one or two tyrosine sulfates (TyrSO(3)) along with a nearby core-2-based Thr-linked O-glycan with sialyl Lewis x (C2-SLe(x)). TyrSO(3) at position Y118 was more critical for binding than at Y97. C2-SLe(x) at T124 was required for L-selectin recognition. Interestingly, under similar conditions, neither L-selectin nor T lymphocytes showed appreciable binding to the sulfated carbohydrate epitope 6-sulfo-SLe(x). P-selectin also bound to endoglycan-based GSPs but with lower affinity than toward GSPs modeled after PSGL-1, the physiological ligand for P- and L-selectin that is expressed on leukocytes. These results demonstrate that TyrSO(3) residues in association with a C2-SLe(x) moiety within endoglycan and PSGL-1 are preferentially recognized by L-selectin.

Common polymorphisms in the NOD2 gene region are associated with leprosy and its reactive states

BACKGROUND

Because of its wide spectrum of clinical manifestations and its well-defined immunological complications, leprosy is a useful disease for studying genetic regulation of the host response to infection. We hypothesized that polymorphisms in the nucleotide-binding oligomerization domain containing 2 (NOD2) gene, for a cytosolic receptor known to detect mycobacteria, are associated with susceptibility to leprosy and its clinical outcomes.

METHODS

We used a case-control study design with 933 patients in Nepal. Our study included 240 patients with type 1 (reversal) reactions and 124 patients with type 2 (erythema nodosum leprosum) reactions. We compared the frequencies of 32 common polymorphisms in the NOD2 gene region between patients with the different clinical types of leprosy as well as between the patients and 101 control participants without leprosy.

RESULTS

Four polymorphisms were associated with susceptibility to leprosy when comparing allele frequencies, and 8 were associated when comparing genotype frequencies with a dominant model. Five polymorphisms were associated with protection from reversal reaction in an allelic analysis, and 7 were associated with reversal reaction with a dominant model. Four polymorphisms were associated with increased susceptibility to erythema nodosum leprosum in an allelic analysis, whereas 7 of 32 polymorphisms were associated with a dominant model.

CONCLUSION

These data suggest that NOD2 genetic variants are associated with susceptibility to leprosy and the development of leprosy reactive states.

The transmembrane domains of L-selectin and CD44 regulate receptor cell surface positioning and leukocyte adhesion under flow

During inflammation and immune surveillance, initial contacts (tethering) between free-flowing leukocytes and the endothelium are vitally dependent on the presentation of the adhesion receptor L-selectin on leukocyte microvilli. Determinants that regulate receptor targeting to microvilli are, however, largely elusive. Therefore, we systematically swapped the extracellular (EC), transmembrane (TM), and intracellular (IC) domains of L-selectin and CD44, a hyaluronan receptor expressed on the cell body and excluded from microvilli. Electron microscopy of transfected human myeloid K562 cells showed that the highly conserved TM domains are responsible for surface positioning. The TM segment of L-selectin forced chimeric molecules to microvilli, and the CD44 TM domain evoked expression on the cell body, whereas the IC and EC domains hardly influenced surface localization. Transfectants with microvillus-based chimeras showed a significantly higher adhesion rate under flow but not under static conditions compared with cells with cell body-expressed receptors. Substitution of the IC domain of L-selectin caused diminished tethering but no change in surface distribution, indicating that both microvillus positioning and cytoskeletal anchoring contribute to leukocyte tethering. These findings demonstrate that TM domains of L-selectin and CD44 play a crucial role in cell adhesion under flow by targeting receptors to microvilli or the cell body, respectively.

Superior sensitivity of novel molecular imaging probe: simultaneously targeting two types of endothelial injury markers

The need remains great for early diagnosis of diseases. The special structure of the eye provides a unique opportunity for noninvasive light-based imaging of fundus vasculature. To detect endothelial injury at the early and reversible stage of adhesion molecule up-regulation, we generated novel imaging agents that target two distinct types of endothelial molecules, a mediator of rolling, P-selectin, and one that mediates firm adhesion, ICAM-1. Interactions of these double-conjugated fluorescent microspheres (MSs) in retinal or choroidal microvasculature were visualized in live animals by scanning laser ophthalmoscopy. The new imaging agents showed significantly higher sensitivity for detection of endothelial injury than singly conjugated MSs (rPSGL-1- or alpha-ICAM-1-conjugated), both in terms of rolling (P<0.01) and firm adhesion (P<0.01). The rolling flux of alpha-ICAM-1-conjugated MSs did not differ in EIU animals, whereas double-conjugated MSs showed significantly higher rolling flux (P<0.01), revealing that ICAM-1 in vivo supports rolling, once MS interaction with the endothelium is initiated. Double-conjugated MSs specifically detected firmly adhering leukocytes (P<0.01), allowing in vivo quantification of immune response. Antiinflammatory treatment with dexamethasone led to reduced leukocyte accumulation (P<0.01) as well as MS interaction (P<0.01), which suggests that treatment success and resolution of inflammation is quantitatively reflected with this molecular imaging approach. This work introduces novel imaging agents for noninvasive detection of endothelial injury in vivo. Our approach may be developed further to diagnose human disease at a much earlier stage than currently possible.

Mucin AgC10 from Trypanosoma cruzi Interferes with L-selectin-mediated monocyte adhesion

The protozoan parasite Trypanosoma cruzi has evolved sophisticated systems to evade the immune response. An important requirement for a productive immune response is recruitment of the appropriate immune cells from the bloodstream to the sites of infection. Here, we show that a mucin expressed and secreted by the metacyclic infective form of T. cruzi, AgC10, is able to interfere with L-selectin-mediated monocyte adhesion. Thus, incubation of U937 monocytic cells stably expressing L-selectin (U937LAM) with AgC10 strongly reduced their adhesion on P-selectin under flow, which is dependent on L-selectin. This treatment also results in a significant inhibition by AgC10 of U937LAM and human primary monocyte adhesion to activated vascular endothelium. This effect was specific for L-selectin, because vascular cell adhesion molecule 1 (VCAM-1)-mediated adhesion was not affected by AgC10 pretreatment. This effect of AgC10 is likely due to its ability to induce L-selectin shedding from the monocyte membrane, since pharmacologic blocking of this shedding prevents AgC10 activity. This is the first description of a mechanism that prevents leukocyte adhesion to the endothelium by a parasite and represents a new potential countermeasure to evade the generation of a correct immune response.

L-selectin: role in regulating homeostasis and cutaneous inflammation

The maintenance of immune surveillance and the generation of normal immune responses are dependent on leukocyte migration to appropriate lymphoid and non-lymphoid tissues. The process of leukocyte migration occurs through complex and highly regulated interactions between the circulating leukocytes and the vascular endothelium. Multiple families of adhesion molecules as well as specific chemoattractants and their cognate receptors function to stabilize these interactions and induce migration into the tissue. L-selectin is a key adhesion molecule that regulates both the migration of leukocytes at sites of inflammation and the recirculation of lymphocytes between blood and lymphoid tissues. L-selectin-mediated lymphocyte recirculation is required for maintaining the appropriate tissue distribution of lymphocyte subpopulations including naïve and effector subsets such as regulatory T cells. In addition, L-selectin-mediated entry into peripheral lymph nodes is required for optimal induction of lymphocyte homeostatic proliferation during lymphopenia. Importantly, L-selectin has been shown to have both adhesive and signaling functions during leukocyte migration. Specifically, L-selectin is highly efficient at capturing free-flowing leukocytes from the blood and supporting subsequent fast rolling interactions along the vascular endothelium. During rolling, synergistic interactions between L-selectin and integrin functions slow leukocyte rolling velocities allowing for chemoattractant-induced activation and eventual firm adhesion of the leukocyte to the vascular endothelium. Engagement of L-selectin by ligand generates transmembrane signals leading to activation of intracellular signaling pathways, increased integrin binding affinity, and enhanced chemotaxis. L-selectin has also been shown to mediate leukocyte recruitment during chronic inflammatory and autoimmune diseases and thus is a potential therapeutic target for drug development.

PSGL-1 function in immunity and steady state homeostasis

The substantial importance of P-selectin glycoprotein ligand 1 (PSGL-1) in leukocyte trafficking has continued to emerge beyond its initial identification as a selectin ligand. PSGL-1 seemed to be a relatively simple molecule with an extracellular mucin domain extended as a flexible rod, teleologically consistent with its primary role in tethering leukocytes to endothelial selectins. The rolling interaction between leukocyte and endothelium mediated by this selectin-PSGL-1 interaction requires branched O-glycan extensions on specific PSGL-1 amino acid residues. In some cells, such as neutrophils, the glycosyltransferases involved in formation of the O-glycans are constitutively expressed, while in other cells, such as T cells, they are expressed only after appropriate activation. Thus, PSGL-1 supports leukocyte recruitment in both innate and adaptive arms of the immune response. A complex array of amino acids within the selectins engage multiple sugar residues of the branched O-glycans on PSGL-1 and provide the molecular interactions responsible for the velcro-like catch bonds that support leukocyte rolling. Such binding of PSGL-1 can also induce signaling events that influence cell phenotype and function. Scrutiny of PSGL-1 has revealed a better understanding of how it performs as a selectin ligand and yielded unexpected insights that extend its scope from supporting leukocyte rolling in inflammatory settings to homeostasis including stem cell homing to the thymus and mature T-cell homing to secondary lymphoid organs. PSGL-1 has been found to bind homeostatic chemokines CCL19 and CCL21 and to support the chemotactic response to these chemokines. Surprisingly, the O-glycan modifications of PSGL-1 that support rolling mediated by selectins in inflammatory conditions interfere with PSGL-1 binding to homeostatic chemokines and thereby limit responsiveness to the chemotactic cues used in steady state T-cell traffic. The multi-level influence of PSGL-1 on cell traffic in both inflammatory and steady state settings is therefore substantially determined by the orchestrated addition of O-glycans. However, central as specific O-glycosylation is to PSGL-1 function, in vivo regulation of PSGL-1 glycosylation in T cells remains poorly understood. It is our purpose herein to review what is known, and not known, of PSGL-1 glycosylation and to update understanding of PSGL-1 functional scope.

Surprising up-regulation of P-selectin glycoprotein ligand-1 (PSGL-1) in endotoxin-induced uveitis

P-selectin glycoprotein ligand-1 (PSGL-1) is constitutively expressed on leukocytes and was thought to be down-regulated with cell activation. However, this work shows the surprising finding of functional PSGL-1 up-regulation during acute inflammation. PSGL-1 function was studied in our autoperfusion assay, in which blood from a mouse carotid flows through a microchamber coated with a fixed density of P-selectin. Under the inflammatory conditions--uveitis induced by systemic lipopolysaccharide injection--we recorded significantly reduced leukocyte rolling velocity, which suggests PSGL-1 up-regulation; however, flow cytometry showed reduced PSGL-1. When bound leukocytes were released from the vasculature by PSGL-1 blockade, a large peripheral blood leukocyte (PBL) population showed elevated PSGL-1, which could account for the reduced PSGL-1 in the remaining unbound population. In the eye, systemic blockade of PSGL-1 with a monoclonal antibody or recombinant soluble PSGL-1 drastically reduced the severe manifestations of uveitis. Furthermore, PSGL-1 blockade was significantly more effective in reducing retinal leukostasis than was P-selectin blockade. Our results provide surprising evidence for functional PSGL-1 up-regulation in PBLs during acute inflammation. The temporal overlap between PSGL-1 and P-selectin up-regulation reveals an as yet unrecognized collaboration between this receptor-ligand pair, increasing efficiency of the first steps of the leukocyte recruitment cascade.

Leukocyte adhesion molecules in animal models of inflammatory bowel disease

The dysregulated recruitment of leukocytes into the intestine is required for the initiation and maintenance of inflammatory bowel disease (IBD). Several families of molecules regulate the influx of these cells into sites of inflammation. Interference with some of these molecules has already shown efficacy in the clinics and antibodies that target the molecules involved have been approved by the FDA for use in Crohn's disease (CD), multiple sclerosis (i.e., natalizumab), and psoriasis (i.e., efalizumab). Here, we discuss basic aspects of the different families of relevant molecules and compile a large body of preclinical studies that supported the targeting of specific steps of the leukocyte adhesion cascade for therapeutic purposes in colitis and in novel models of CD-like ileitis.

P-selectin glycoprotein ligand-1 decameric repeats regulate selectin-dependent rolling under flow conditions

P-selectin glycoprotein ligand-1 (PSGL-1) interacts with selectins to support leukocyte rolling along vascular wall. L- and P-selectin bind to N-terminal tyrosine sulfate residues and to core-2 O-glycans attached to Thr-57, whereas tyrosine sulfation is not required for E-selectin binding. PSGL-1 extracellular domain contains decameric repeats, which extend L- and P-selectin binding sites far above the plasma membrane. We hypothesized that decamers may play a role in regulating PSGL-1 interactions with selectins. Chinese hamster ovary cells expressing wild-type PSGL-1 or PSGL-1 molecules exhibiting deletion or substitution of decamers with the tandem repeats of platelet glycoprotein Ibalpha were compared in their ability to roll on selectins and to bind soluble L- or P-selectin. Deletion of decamers abrogated soluble L-selectin binding and cell rolling on L-selectin, whereas their substitution partially reversed these diminutions. P-selectin-dependent interactions with PSGL-1 were less affected by decamer deletion. Videomicroscopy analysis showed that decamers are required to stabilize L-selectin-dependent rolling. Importantly, adhesion assays performed on recombinant decamers demonstrated that they directly bind to E-selectin and promote slow rolling. Our results indicate that the role of decamers is to extend PSGL-1 N terminus far above the cell surface to support and stabilize leukocyte rolling on L- or P-selectin. In addition, they function as a cell adhesion receptor, which supports approximately 80% of E-selectin-dependent rolling.

Systems-level studies of glycosyltransferase gene expression and enzyme activity that are associated with the selectin binding function of human leukocytes

The application of systems biology methods in the emerging field of glycomics requires the collection and integration of glycosyltransferase data at the gene and enzyme level for the purpose of hypothesis generation. We systematically examined the relationship between gene expression, glycosyltransferase activity, glycan expression, and selectin-binding function in different systems, including human neutrophils, undifferentiated HL-60 (human promyelocytic cells), differentiated HL-60, and HL-60 synchronized in specific growth phases. Results demonstrate that 1) the sLe(X) (sialyl-Lewis-X) epitope is expressed in P-selectin glycoprotein ligand-1 (PSGL-1) from neutrophils at higher levels compared with HL-60. This variation may be due to differences in the relative activities of alpha1,3-fucosyltransferases and alpha2,3-sialyltransferases in these two cell types. 2) HL-60 cell differentiation along granulocyte lineage increased the activity of beta1,4GalT and beta1,3GlcNAcT by 1.6- to 3.2-fold. This may contribute to LacNAc chain extension as evidenced by the 1.7-fold increase in DSA-lectin (lectin recognizing LacNAc) binding to cells after differentiation. 3) The activity of enzymes contributing to sLe(X) formation in leukocytes likely varies as ST3[Galbeta1,4GlcNAc] < or = alpha1,3FT[sialyl-LacNAc] < beta1,3GlcNAcT. 4) O-glycan specific glycosyltransferase activity does not undergo periodic variation with cell cycle phases. Overall, gene expression and enzyme activity data combined with knowledge of biochemistry can predict the resulting glycan structures and yield viable experimentally testable hypothesis.

Endoglycan, a member of the CD34 family of sialomucins, is a ligand for the vascular selectins

The interactions of the selectin family of adhesion molecules with their ligands are essential for the initial rolling stage of leukocyte trafficking. Under inflammatory conditions, the vascular selectins, E- and P-selectin, are expressed on activated vessels and interact with carbohydrate-based ligands on the leukocyte surface. While several ligands have been characterized on human T cells, monocytes and neutrophils, there is limited information concerning ligands on B cells. Endoglycan (EG) together with CD34 and podocalyxin comprise the CD34 family of sialomucins. We found that EG, previously implicated as an L-selectin ligand on endothelial cells, was present on human B cells, T cells and peripheral blood monocytes. Upon activation of B cells, EG increased with a concurrent decrease in PSGL-1. Expression of EG on T cells remained constant under the same conditions. We further found that native EG from several sources (a B cell line, a monocyte line and human tonsils) was reactive with HECA-452, a mAb that recognizes sialyl Lewis X and related structures. Moreover, immunopurified EG from these sources was able to bind to P-selectin and where tested E-selectin. This interaction was divalent cation-dependent and required sialylation of EG. Finally, an EG construct supported slow rolling of E- and P-selectin bearing cells in a sialic acid and fucose dependent manner, and the introduction of intact EG into a B cell line facilitated rolling interactions on a P-selectin substratum. These in vitro findings indicate that EG can function as a ligand for the vascular selectins.

The stem cell movement

Stem or progenitor cell-based strategies to combat ischemic heart disease and myocardial infarction, whether autologous transplantation or stimulation of resident populations, not only require detailed insight into transdifferentiation potential and functional coupling, but the efficacy of this approach is underpinned by the need to induce appropriate migration and homing to the site of injury. This review focuses on existing insights into the trafficking of stem cells in the context of cardiac regenerative therapy, with particular focus on the wide variety of potential sources of cells, critical factors that may regulate their migration, and how extrapolating from embryonic stem/progenitor cell behavior during cardiogenesis may reveal pathways implicit in the adult heart postinjury.

No detectable endothelial- or leukocyte-derived L-selectin ligand activity on the endothelium in inflamed cremaster muscle venules

L-selectin is important in mediating leukocyte recruitment in inflammation. The role of L-selectin was for long believed to be influenced by an inducible endothelial ligand; however, L-selectin ligand activity was recently shown to be mediated by leukocytic P-selectin glycoprotein ligand 1 (PSGL-1). Still, it is unknown whether PSGL-1 is deposited on the endothelium or whether leukocyte fragments or leukocytic uropods are presented on the venular surface. Moreover, it is unclear whether ligands for L-selectin other than PSGL-1 are present in inflammation. Overall, this has complicated understanding of the mechanisms that guide recruitment of inflammatory cells. Here, I used intravital microscopy on mouse cremaster muscle venules to show that L-selectin influences leukocyte rolling in inflammation exclusively by mediating L-selectin/PSGL-1-dependent, secondary capture to rolling and adherent leukocytes. I show that leukocyte primary capture in inflammation is mediated almost entirely by P-selectin, whereas the capacity of E-selectin to mediate capture appears to be minimal. In parallel, primary capture remaining after function inhibition of P-selectin is not decreased by blockage or absence of L-selectin. Rolling along the endothelium in venules following a number of inflammatory treatments was abolished by simultaneous blockage of P-selectin, E-selectin, and VCAM-1, indicating that there is no additional adhesive pathway involving L-selectin or any other molecule that can mediate leukocyte rolling in inflamed cremaster muscle venules in response to the used stimuli. Moreover, in vivo staining failed to detect any L-selectin ligand activity on the endothelium. These data demonstrate that expression of L-selectin on leukocytes is insufficient for mediating rolling and efficient recruitment of leukocytes in inflammation.