Structure and function of L-selectin

Group XIV C-type lectins: emerging targets in tumor angiogenesis

C-type lectins, distinguished by a C-type lectin binding domain (CTLD), are an evolutionarily conserved superfamily of glycoproteins that are implicated in a broad range of physiologic processes. The group XIV subfamily of CTLDs are comprised of CD93, CD248/endosialin, CLEC14a, and thrombomodulin/CD141, and have important roles in creating and maintaining blood vessels, organizing extracellular matrix, and balancing pro- and anti-coagulative processes. As such, dysregulation in the expression and downstream signaling pathways of these proteins often lead to clinically relevant pathology. Recently, group XIV CTLDs have been shown to play significant roles in cancer progression, namely tumor angiogenesis and metastatic dissemination. Interest in therapeutically targeting tumor vasculature is increasing and the search for novel angiogenic targets is ongoing. Group XIV CTLDs have emerged as key moderators of tumor angiogenesis and metastasis, thus offering substantial therapeutic promise for the clinic. Herein, we review our current knowledge of group XIV CTLDs, discuss each's role in malignancy and associated potential therapeutic avenues, briefly discuss group XIV CTLDs in the context of two other relevant lectin families, and offer future direction in further elucidating mechanisms by which these proteins function and facilitate tumor growth.

Molecular mechanisms of catch bonds and their implications for platelet hemostasis

Adhesive molecular bonds between blood cells are essential for thrombosis and hemostasis as they provide means for platelet adhesion, aggregation, and signaling in flowing blood. According to the nowadays conventional definition, a "catch" bond is a type of non-covalent bio-molecular bridge, whose dissociation lifetime counter-intuitively increases with applied tensile force. Following recent experimental findings, such receptor-ligand protein bonds are vital to the blood cells involved in the prevention of bleeding (hemostatic response) and infection (immunity). In this review, we examine the up-to-date experimental discoveries and theoretical insights about catch bonds between the blood cells, their biomechanical principles at the molecular level, and their role in platelet thrombosis and hemostasis.

From the Cradle to the Grave of an Infection: Host-Pathogen Interaction Visualized by Intravital Microscopy

During infections, interactions between host immune cells and the pathogen occur in distinct anatomical locations and along defined time scales. This can best be assessed in the physiological context of an infection in the living tissue. Consequently, intravital imaging has enabled us to dissect the critical phases and events throughout an infection in real time in living tissues. Specifically, advances in visualizing specific cell types and individual pathogens permitted tracking the early events of tissue invasion of the pathogen, cellular interactions involved in the induction of the immune response as well the events implicated in clearance of the infection. In this respect, two vantage points have evolved since the initial employment of this technique in the field of infection biology. On the one hand, strategies acquired by the pathogen to establish within the host and circumvent or evade the immune defenses have been elucidated. On the other hand, analyzing infections from the immune system's perspective has led to insights into the dynamic cellular interactions that are involved in the initial recognition of the pathogen, immune induction as well as effector function delivery and immunopathology. Furthermore, an increasing interest in probing functional parameters in vivo has emerged, such as the analysis of pathogen reactivity to stress conditions imposed by the host organism in order to mediate clearance upon pathogen encounter. Here, we give an overview on recent intravital microscopy findings of host-pathogen interactions along the course of an infection, from both the immune system's and pathogen's perspectives. We also discuss recent developments and future perspectives in extracting intravital information beyond the localization of pathogens and their interaction with immune cells. Such reporter systems on the pathogen's physiological state and immune cell functions may prove useful in dissecting the functional dynamics of host-pathogen interactions. © 2019 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

A CD40 targeting peptide prevents severe symptoms in experimental autoimmune encephalomyelitis

CD40/CD154-interaction is critical in the development of Experimental Autoimmune Encephalomyelitis (EAE; mouse model of Multiple Sclerosis). Culprit CD4⁺CD40⁺ T cells drive a more severe form of EAE than conventional CD4 T cells. Blocking CD40/CD154-interaction with CD154-antibody prevents or ameliorates disease but had thrombotic complications in clinical trials. We targeted CD40 using a CD154-sequence based peptide. Peptides in human therapeutics demonstrate good safety. A small peptide, KGYY₆, ameliorates EAE when given as pretreatment or at first symptoms. KGYY₆ binds Th40 and memory T cells, affecting expression of CD69 and IL-10 in the CD4 T cell compartment, ultimately hampering disease development.

Immunomodulatory Function of Myeloid-Derived Suppressor Cells during B Cell-Mediated Immune Responses

Myeloid-derived suppressor cells (MDSCs) play roles in immune regulation during neoplastic and non-neoplastic inflammatory responses. This immune regulatory function is directed mainly toward T cells. However, MDSCs also regulate other cell populations, including B cells, during inflammatory responses. Indeed, B cells are essential for antibody-mediated immune responses. MDSCs regulate B cell immune responses directly via expression of effector molecules and indirectly by controlling other immune regulatory cells. B cell-mediated immune responses are a major component of the overall immune response; thus, MDSCs play a prominent role in their regulation. Here, we review the current knowledge about MDSC-mediated regulation of B cell responses.

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 PSGL-1-L-selectin signaling complex regulates neutrophil adhesion under flow

Neutrophils are recruited from the blood to sites of inflammation, where they contribute to immune defense but may also cause tissue damage. During inflammation, neutrophils roll along the microvascular endothelium before arresting and transmigrating. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen 1 (LFA-1), which can be induced by selectin engagement. Here, we demonstrate that a subset of P-selectin glycoprotein ligand-1 (PSGL-1) molecules is constitutively associated with L-selectin. Although this association does not require the known lectin-like interaction between L-selectin and PSGL-1, the signaling output is dependent on this interaction and the cytoplasmic tail of L-selectin. The PSGL-1-L-selectin complex signals through Src family kinases, ITAM domain-containing adaptor proteins, and other kinases to ultimately result in LFA-1 activation. The PSGL-1-L-selectin complex-induced signaling effects on neutrophil slow rolling and recruitment in vivo demonstrate the functional importance of this pathway. We conclude that this is a signaling complex specialized for sensing adhesion under flow.

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.

Inhibition of Na+/H+ exchanger enhances low pH-induced L-selectin shedding and beta2-integrin surface expression in human neutrophils

Ischemia-reperfusion injury is a common pathological occurrence causing tissue damage in heart attack and stroke. Entrapment of neutrophils in the vasculature during ischemic events has been implicated in this process. In this study, we examine the effects that lactacidosis and consequent reductions in intracellular pH (pH(i)) have on surface expression of adhesion molecules on neutrophils. When human neutrophils were exposed to pH 6 lactate, there was a marked decrease in surface L-selectin (CD62L) levels, and the decrease was significantly enhanced by inclusion of Na(+)/H(+) exchanger (NHE) inhibitor 5-(N,N-hexamethylene)amiloride (HMA). Similar effects were observed when pH(i) was reduced while maintaining normal extracellular pH, by using an NH(4)Cl prepulse followed by washes and incubation in pH 7.4 buffer containing NHE inhibitors [HMA, cariporide, or 5-(N,N-dimethyl)amiloride (DMA)]. The amount of L-selectin shedding induced by different concentrations of NH(4)Cl in the prepulse correlated with the level of intracellular acidification with an apparent pK of 6.3. In contrast, beta(2)-integrin (CD11b and CD18) was only slightly upregulated in the low-pH(i) condition and was enhanced by NHE inhibition to a much lesser extent. L-selectin shedding was prevented by treating human neutrophils with inhibitors of extracellular metalloproteases (RO-31-9790 and KD-IX-73-4) or with inhibitors of intracellular signaling via p38 MAP kinase (SB-203580 and SB-239063), implying a transmembrane effect of pH(i). Taken together, these data suggest that the ability of NHE inhibitors such as HMA to reduce ischemia-reperfusion injury may be related to the nearly complete removal of L-selectin from the neutrophil surface.

LeuCAM and Reactive Oxygen Species during Long-Term Exercise Combined with Sleep and Energy Deficiency

INTRODUCTION

During the Norwegian military ranger-training course, cadets are exposed to prolonged physical exercise combined with sleep-, energy-, and food deficiency. The open-window postexercise hypothesis indicates that after hard physical activity, there is an increased risk of contracting infectious diseases.

PURPOSE

The purpose of the present study was to determine leukocyte reactive oxygen species (ROS) levels, total antioxidant status (TAS), leukocyte expression of the cell adhesion molecules CD62L and CD11b, and plasma levels of soluble adhesion molecule L-selectin before, during, and in the recovery phase of a military ranger-training course.

METHODS

Ten cadets from the Norwegian Military Academy were recruited to the study. Flow cytometry was used to study the intracellular levels of ROS in leukocytes (basally, as well as after in vitro stimulation with phorbol myristate acetate (PMA)), applying the probes dihydroethidium (DHE) and dihydrorhodamine 123 (DHR) and the leukocyte expression of adhesion molecules CD62L and CD11b. ELISA was used to assess the plasma levels of soluble L-selectin, and TAS in plasma was measured using the ABTS+ reduction assay kit.

RESULTS

The basal levels of ROS as well as PMA-stimulated ROS in leukocytes declined gradually during the ranger-training course, being lowest on the last day (P < 0.05). The level of TAS increased (P < 0.01) during the course. A striking decrease (P < 0.001) was observed in leukocyte CD62L expression and was sustained even after 3 d of recovery. The leukocyte expression of CD11b remained unchanged.

CONCLUSION

The ranger-training course leads to a partial exhaustion of the leukocyte ROS-generating machinery and to a nearly total extinguishing of leukocyte CD62L expression. These changes may support the open-window hypothesis indicating reduced ability to combat microbial invasions before total restitution.

Long-distance running modulates the expression of leucocyte and endothelial adhesion molecules

There still exist many unanswered questions whether physical exercise is beneficial or harmful to the immune system. The 'open-window' post-exercise hypothesis states that athletes are more susceptible to infections after exercise, but there is a need for further elucidation. The aim of the present study was to investigate the effect of long-distance running on leucocyte expression of selected adhesion molecules as well as the plasma levels of soluble leucocyte- and endothelium-derived adhesion molecules. Twenty-seven men participating in Oslo marathon together with 16 entrants (eight men and eight women) in the Oslo half-marathon were recruited to this study. Venous blood was collected before and immediately after the races for analysing the leucocyte expression of CD62L, CD11b and CD14 with the help of flow cytometry, and plasma concentrations of soluble (s) sE-selectin, sL-selectin, sP-selectin, sVCAM-1, sICAM-1 and sCD14 were assessed by means of enzyme-linked immunosorbent assays. A significant increase of leucocyte CD11b expression was observed following both races, compared to the pre-race situation. Monocyte CD14 expression increased only after the marathon race. After both races, CD62L expression was significantly lowered on all leucocyte subsets, whereas the plasma levels of sE-selectin, sP-selectin, sL-selectin, sVCAM-1, sICAM-1 and sCD14 were all increased. Altogether, these changes negatively influence the ability of leucocytes to adhere to and actively transmigrate the endothelium to reach the tissues. Our study thus supports the 'open-window' hypothesis, indicating a reduced capacity to combat infectious agents during the immediate post-exercise period.

The interaction of protein kinase C isozymes alpha, iota, and theta with the cytoplasmic domain of L-selectin is modulated by phosphorylation of the receptor

The leukocyte adhesion molecule L-selectin has an important role in the initial steps of leukocyte extravasation during inflammation and lymphocyte homing. Its cytoplasmic domain is involved in signal transduction after L-selectin cross-linking and in the regulation of receptor binding activity in response to intracellular signals. However, the signaling events occurring at the level of the receptor are largely unknown. This study therefore addressed the question of whether protein kinases associate with the cytoplasmic domain of the receptor and mediate its phosphorylation. Using a glutathione S-transferase fusion protein of the L-selectin cytoplasmic domain, we isolated a kinase activity from cellular extracts of the human leukemic Jurkat T-cell line that phosphorylated L-selectin on serine residues. This kinase showed characteristics of the protein kinase C (PKC) family. Moreover, the Ca(2+)-independent PKC isozymes theta and iota were found associated with the cytoplasmic domain of L-selectin. Pseudosubstrate inhibitors of these isozymes abolished phosphorylation of the cytoplasmic domain, demonstrating that these kinases are responsible for the phosphorylation. Analysis of proteins specifically bound to the phosphorylated cytoplasmic tail of L-selectin revealed that PKCalpha and -theta are strongly associated with the phosphorylated cytoplasmic domain of L-selectin. Binding of these isozymes to L-selectin was also found in intact cells after phorbol ester treatment inducing serine phosphorylation of the receptor. Furthermore, stimulation of Jurkat T-cells by CD3 cross-linking induced association of PKCalpha and -theta with L-selectin, indicating a role of these kinases in the regulation of L-selectin through the T-cell receptor complex. The phosphorylation-regulated association of PKC isozymes with the cytoplasmic domain of L-selectin indicates an important role of this kinase family in L-selectin signal transduction.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

CDNA microarray gene expression analysis of B-cell chronic lymphocytic leukemia proposes potential new prognostic markers involved in lymphocyte trafficking

Human cancer is characterized by complex molecular perturbations leading to variable clinical behavior, often even in single-disease entities. We performed a feasibility study systematically comparing large-scale gene expression profiles with clinical features in human B-cell chronic lymphocytic leukemia (B-CLL). cDNA microarrays were employed to determine the expression levels of 1,024 selected genes in 54 peripheral blood lymphocyte samples obtained from patients with B-CLL. Statistical analyses were applied to correlate the expression profiles with a number of clinical parameters including patient survival and disease staging. We were able to identify genes whose expression levels significantly correlated with patient survival and/or with clinical staging. Most of these genes code either for cell adhesion molecules (L-selectin, integrin-beta2) or for factors inducing cell adhesion molecules (IL-1beta, IL-8, EGR1), suggesting that prognosis of this disease may be related to a defect in lymphocyte trafficking. This report demonstrates the feasibility of a systematic integration of large-scale gene expression profiles with clinical data as a general approach for dissecting human diseases.

Intracranial paracrine interleukin-2 therapy stimulates prolonged antitumor immunity that extends outside the central nervous system

To explore the potential efficacy of local cytokine delivery against tumors in the central nervous system (CNS), C57BL6 mice were simultaneously given intracranial injections of tumor challenge and of irradiated B16F10 melanoma cells transduced to secrete interleukin-2 (IL-2). Intracranial IL-2 therapy generated antitumor responses capable of extending the survival of animals that received simultaneous intracranial tumor challenge either locally or at distant sites in the brain. Nontransduced melanoma cells had little effect. Animals that survived intracranial IL-2 therapy and tumor challenge showed prolonged survival compared with controls when challenged with a second tumor dose 70 days after initial treatment. In addition, animals that rejected intracranial tumors were also protected from tumor growth upon rechallenge at sites outside the CNS (i.e., subcutaneous tumor challenge). Conversely, identical or 10-fold larger doses of IL-2-transduced cells administered by subcutaneous injection failed to generate protection against intracranial tumor challenges. Elimination of T-cell and natural killer (NK) subsets using gene knockout mice and antibody-depletion techniques demonstrated that NK cells were most important for the initial antitumor response, whereas CD4+ T-cells were not necessary. These studies demonstrate that local IL-2 therapy in the brain not only generates an immediate local antitumor immune response, but also establishes long-term immunologic memory capable of eliminating subsequent tumor challenges within and outside of the CNS. Furthermore, the antitumor response to paracrine IL-2 in the brain differed significantly from that in the flank, suggesting that the intrinsic CNS cells involved in initiating immunity within the brain have different cytokine requirements from their peripheral counterparts.

Inverse association between endogenous glucocorticoid secretion and L-selectin (CD62L) expression in trauma patients

Exogenously administered glucocorticoids downregulate inflammatory host response, i.e. by inhibition of adhesion molecule expression on leukocyte surfaces. Here, possible associations between the trauma-induced endogenous secretion of cortisol and the expression of neutrophil adhesion molecules (L-selectin/CD62L, CD 11b, CD54) were studied in humans. Standardized elective hip arthroplasty was investigated as an exemplary condition of acute inflammation. In 20 patients, blood for quantification of cortisol and adrenocorticotropic hormone was obtained at minutes 10, 20, 30, 60, hours 1, 2, 4 and 10 and days 1,3 and 7. Expression of L-selectin/CD62L, CD11b and CD54 on neutrophil surfaces was determined preoperatively, and postoperatively at hours 1, 2, 3, 4, and 10 and at days 1 and 3. Secretion of both, adrenocorticotropic hormone and cortisol was significantly increased between 1-10 hours after onset of tissue injury. Compared to baseline values, CD11b expression was increased at hour 1 and normalized after day 1, whereas L-selectin/CD62L expression, mirroring this pattern was decreased until day 1. Patients with high endogenous glucocorticoid secretion exhibited significantly decreased expression selectively of L-selectin/CD62L. However, we also observed that glucocorticoids do not directly induce L-selectin shedding from neutrophil surfaces in vitro, arguing for more indirect glucocorticoid action on adhesion molecule expression. Together, this study showed that increased endogenous cortisol secretion is associated with lower expression of L-selectin on neutrophil surfaces in humans that is consistent with a downmodulating role of this neuroendocrine stress response in inflammatory leukocyte recruitment.

Differential Homing Commitments of Antigen-Specific T Cells After Oral or Parenteral Immunization in Humans

Animal experiments show that lymphocytes activated in the intestine circulate through mesenteric lymph nodes, lymphatics, and blood, returning to the gut. Homing into intestinal lamina propria is mediated by lymphocyte surface homing receptors, mainly the α4β7-integrin. We studied in humans whether intestinal T cells entering the blood upon antigenic activation would exhibit homing commitments to the gut. Volunteers were immunized with keyhole limpet hemocyanin (KLH) first orally and then parenterally or only parenterally, and the expression of α4β7 on T cells specific for KLH or tetanus toxoid was studied. Circulating T cells were depleted of α4β7+ cells by immunomagnetic selection. This depletion removed a significant proportion of the KLH-specific cells (mean decrease in proliferative response of 71%) in the orally immunized volunteers. No difference in the KLH-induced proliferation was found between the total and the α4β7-depleted populations in volunteers parenterally immunized with KLH, regardless of whether a preceding mucosal priming had taken place or not. In both immunization groups, the depletion of α4β7+ cells had no influence on the proliferative response to tetanus toxoid. We conclude that, in contrast to T cells activated by parenteral immunization, gut-derived T cells have preferential homing commitments to the gut. This commitment was no longer observed after a subsequent parenteral Ag administration. Besides showing that the site of Ag encounter determines the expression of homing receptors, the present study is the first to provide evidence for a circulation of newly activated Ag-specific intestinal T cells back to the gut in humans.

Homing potentials of circulating antibody-secreting cells after administration of oral or parenteral protein or polysaccharide vaccine in humans

The site of antigen encounter influences the Ig-distribution and homing potentials of circulating antibody-secreting cells (ASC) induced. After oral antigen administration, the majority ASC secrete the mucosal Ig-isotype, IgA, and all of them express the gut homing receptor (HR), alpha 4 beta 7, thus implying mucosal homing of these cells. Parenteral protein vaccine induces an IgG-dominated response with a low proportion of alpha 4 beta 7 expressing cells. However, a polysaccharide vaccine, even if administered parenterally, elicits an IgA-dominated response, hence suggesting homing to the mucosa. In order to study the influence of the nature of the antigen on the targeting of the ASC response, the present work compares the homing potentials of circulating ASC in humans after administration of an oral Salmonella Typhi Ty21a vaccine (antigen studied: O-9,12 polysaccharide), an oral recombinant cholera vaccine (antigen studied: cholera toxin B-subunit, CTB protein), a parenteral pneumococcal vaccine (antigen studied: Pnc capsular polysaccharide 19F) or a parenteral tetanus toxoid vaccine (antigen studied: TT protein). alpha 4 beta 7 was expressed on a higher proportion of ASC induced by oral O-9,12 (99%) and CTB (99%) than by parenteral Pnc (70%) or TT (63%). L-selectin, the peripheral lymph node HR, was expressed on a smaller proportion of ASC induced by O-9,12 (37%) or CTB (43%) than of those induced by Pnc (78%) or TT (81%). The results imply that even if the nature of the antigen has a profound effect on the Ig-distribution of the ASC response, it does not seem to influence the targeting of the response.

L-selectin-mediated lymphocyte aggregation: role of carbohydrates, activation and effects on cellular interactions

L-selectin on lymphocytes reacts with glycosylated ligands on high endothelial venule walls in lymphoid organs. Through this carbohydrate-dependent interaction, rolling and initial attachment of lymphocytes to endothelium is mediated. Here we have studied an earlier described L-selectin-induced homotypic aggregation, to further elucidate the events that occur after engagement of L-selectin. It was found that the interaction of L-selectin with fucoidan, but not with other carbohydrates, or with monoclonal antibodies directed against the carbohydrate recognition domain of L-selectin, resulted in homotypic aggregation among both B- or T lymphocytes. Importantly, this aggregation was shown to be both lymphocyte function-associated antigen-1 (LFA-1) and calcium-independent. Furthermore, for aggregation metabolic energy was required, and signalling via protein tyrosine kinase appeared to be involved. Neither de novo protein synthesis, protein kinase C mediated signalling, Gi-protein mediated signal transduction, nor calcium mobilization were required for aggregation. During aggregation, L-selectin was not shed from the lymphocyte's cell surface. Finally, it was found that the lymphocyte binding capacity to high endothelial venules on cryostat sections was not altered upon triggering these lymphocytes via L-selectin. Interestingly, L-selectin-triggered cells showed increased binding to paracortical areas in peripheral lymph nodes. Our data suggest that signals via L-selectin, might lead to altered expression of cell surface molecules, important in interactions other than the first stage of lymphocyte rolling.

Differences in immune responses induced by oral and rectal immunizations with Salmonella typhi Ty21a: evidence for compartmentalization within the common mucosal immune system in humans

Based on the concept of the common mucosal immune system, immunization at various inductive sites can induce an immune response at other, remote mucosal surfaces. The immune responses elicited through rectal and oral routes of antigen delivery were compared with respect to (i) measurement of antibody responses in serum and various external secretions of the vaccinees and (ii) characterization of the nature and homing potentials of circulating antibody-secreting cells (ASC). Specific ASC appeared in the circulation in 4 of 5 volunteers after oral and 9 of 11 volunteers after rectal immunization with Salmonella typhi Ty21a. The kinetics, magnitude, and immunoglobulin isotype distribution of the ASC responses were similar in the two groups. In both groups, almost all ASC (99 or 95% after oral or rectal immunization, respectively) expressed alpha4 beta7, the gut homing receptor (HR), whereas L-selectin, the peripheral lymph node HR, was expressed only on 22 or 38% of ASC, respectively. Oral immunization elicited a more pronounced immune response in saliva and vaginal secretion, while rectal immunization was more potent in inducing a response in nasal secretion, rectum, and tears. No major differences were found in the abilities of the two immunization routes to induce a response in serum or intestinal secretion. Thus, the rectal antigen delivery should be considered as an alternative to the oral immunization route. The different immune response profiles found in various secretions after oral versus rectal antigen administration provide evidence for a compartmentalization within the common mucosal immune system in humans.

L-selectin stimulates the neutral sphingomyelinase and induces release of ceramide

Selectins have been shown to be crucial in the rolling process of leukocytes during lymphocyte homing and in the early phase of inflammatory processes. Recently, we and others have shown that binding of L-selectin to its ligands correlates with a rapid induction of several intracellular signaling molecules, in particular, Src-like tyrosine kinases, MAP-kinases, Jun NH2-terminal kinase, the small G-proteins Ras and Rac, and a release of Ca2+ in leukocytes. Here, we demonstrate the activation of a novel signaling pathway by L-selectin. Stimulation of Jurkat T-lymphocytes via L-selectin results in an increase of neutral sphingomyelinase activity. This activity correlates with a consumption of cellular sphingomyelin and a release of ceramide. The activation of the neutral sphingomyelinase by L-selectin does not depend on tyrosine kinase activity and, therefore, represents an alternative and novel pathway to stimulate lymphocytes via L-selectin.

Flow-cytometric analysis of leukocyte activation induced by polyethylene-terephthalate with and without pyrolytic carbon coating

Leukocyte activation is one test for the evaluation of blood-materials interaction. The expression of adhesion molecules analyzed by flow cytometry provides a simple method to evaluate leukocyte activation by biomaterials: any change in these molecules can be predictive of the inflammatory activity of the materials. In this study the contact between leukocytes and uncoated polyethylene terephthalate or pyrolytic carbon-coated polyethylene terephthalate (PET and PET-PC, respectively) was inspected by analyzing whether the expression of some adhesion molecules involved in leukocyte activation, namely LFA-1 (CD11a/ CD18), Mac-1/CR3 (CD11b/CD18), and LECAM-1 (CD62L) can be modified. By flow cytometry expression of the adhesion molecules can be studied separately on lymphocytes and myeloid cells. The materials tested reduced the total numbers of both leukocytes and neutrophils, although not significantly. Neither PET nor PET-PC changed the expression of the adhesion molecules in lymphocytes: this suggests that no specific immune response is stimulated. On the contrary, statistically significant changes were observed for monocytes and granulocytes: the percentage of cells expressing Mac-1 and the density of such antigens on cell membranes increased while the percentage of LECAM-1 positive cells decreased. Similar changes were observed when the cells underwent the inflammatory stimulus provided by an in vitro challenge with bacterial endotoxin. Our results demonstrated that polyethylene terephthalate activates leukocytes by modifying the expression in neutrophils of the molecules involved in the early phase of the inflammatory response. Even after coating PET with pyrolytic carbon, the ability of this material to activate circulating leukocytes was maintained.

L-selectin regulates actin polymerisation via activation of the small G-protein Rac2

L-selectin mediated adhesion to endothelial cells is a crucial step in the immune response to pathogens (1, 2) and in lymphocyte homing (3, 4). Selectin molecules mediate leukocyte rolling on endothelial cells, the initial step of adhesion (5, 6). We have previously shown that stimulation of Jurkat T-lymphocytes via L-selectin results in activation of the p21Ras pathway and synthesis of reactive oxygen intermediates (7). Here, we show that cellular stimulation via L-selectin induces a change of cytoskeleton organisation demonstrated by a tenfold increase of actin filament polymerisation. This actin polymerisation is mediated by a Ras and Rac2 regulated pathway, since inhibition of Ras by transient transfection of transdominant inhibitory N17Ras or suppression of Rac2 protein expression by antisense oligonucleotides prevents L-selectin triggered actin polymerisation. Our results point to a signaling cascade from L-selectin via Ras and Rac2 to actin filaments, which might be important for leukocyte adhesion.

The selectins: insights into selectin-induced intracellular signaling in leukocytes

Characteristic features of the inflammatory and immune responses involve the recruitment of leukocytes to sites of tissue injury and the recirculation of lymphocytes through hematopoietic and lymphoid tissues. Recent studies indicate that the regulated cell surface expression of a family of protein adhesion molecules known as selectins and their counterreceptors on both leukocytes and endothelium play critical roles in both biologic processes. Initially, the function of these molecules was thought to be restricted to regulating cell-cell adhesive interactions. Selectin-dependent cell-cell binding has been shown to be essential in localizing leukocytes within tissues by promoting cell rolling along endothelium prior to the development of tight adhesion and subsequent cell migration. However, recent studies suggest that these molecules also play an active role in regulating additional leukocyte functions. This article will review the emerging evidence that indicates a broader and significant role of selectin molecules and their counterreceptors in the initiation of intracellular signaling pathways and regulation of other leukocyte functional responses including degranulation, cytokine expression, activation of the respiratory burst, and T lymphocyte activation.

Evidence for a lymphotropic nature of circulating plasmacytoid monocytes: findings from a case of CD56+ chronic myelomonocytic leukemia

Because the cells previously designated plasmacytoid T cells share major immunophenotypic features with cells of the mononuclear-phagocyte system, they have been re-named and are now known as plasmacytoid monocytes (PM). We describe a unique case of chronic myelomonocytic leukemia with circulating PM. The patient, a 48-year-old man, presented initially with refractory anemia. Four years later his general condition deteriorated, accompanied by an increase in leukocytes to 200,000/microliters blood. The bone marrow histology was interpreted as compatible with a diagnosis of chronic myelomonocytic leukemia. Two months before he died, the patient developed generalized lymphadenopathy clinically simulating malignant lymphoma. Histologic examination of an axillary lymph node revealed diffuse infiltration by PM. The PM in the lymph node and some circulating cells closely resembling PM expressed L-selectin, a finding that could be interpreted as a morphologic correlate of their marked lymphotropism. The detection of large numbers of CD56/CD33 double-positive circulating blast cells by FACS analysis strongly supported the diagnosis of a leukemia of myelogenous origin. The patient died of tumor cachexia. Autopsy revealed widespread leukemic infiltrates (always containing clusters of PM) in bone marrow, spleen, liver, lymph nodes, and mucosa-associated lymphoid tissue of the oropharynx. The final diagnosis was one of chronic myelomonocytic leukemia with marked lymphotropism and partial differentiation towards PM. We consider that the rare instances of a hematologic tumor with differentiation towards PM should be classified amongst the myelogenous leukemias.

Correlation between serum level of soluble L-selectin and leukocyte count in chronic myeloid and lymphocytic leukemia and during bone marrow transplantation

L-selectin is a glycoprotein which is one of three members in a family of cell adhesion molecules called selectins. L-selectin is present in distinct forms on both neutrophil granulocytes and lymphocytes, and it appears to play an important role in the early stages of leukocyte-endothelial cell interaction. Activation of leukocytes leads to shedding of the extracellular part of L-selectin which thus forms a soluble adhesion molecule, sL-selectin, which retains functional capacity and can be detected in serum. In the present study we have developed a specific, sensitive sandwich ELISA to measure the serum level of sL-selectin in patients with hematological and infectious disorders. Three patients with acute myeloid leukemia in remission and 1 patient with chronic myeloid leukemia in chronic phase were followed during bone marrow transplantation and the level of sL-selectin was found to correlate closely to the leukocyte counts with no detectable sL-selectin during periods of severe leukopenia. In 11 patients with chronic phase chronic myeloid leukemia and 13 patients with chronic lymphocytic leukemia the sL-selectin level was also found to correlate closely to the leukocyte count (R = 0.98; p = 0.001 and R = 0.83; p = 0.004 respectively). One CML patient with a leukocytosis of 385 x 10(9)/l was found to have an sL-selectin concentration 625 times above normal. Ten patients with acute pneumonia were evaluated at diagnosis and at the time of follow-up 4-8 weeks later. In all patients the initial sL-selectin level was higher than at follow-up. However, no close correlation between sL-selectin and leukocyte count or CRP (C-reactive protein) at the time of diagnosis was found. In summary, we have found that the sL-selectin level in human serum closely correlates to the leukocyte count in both CML and CLL and during bone marrow transplantation.