E- and P-Selectin Are Not Involved in the Recruitment of Inflammatory Cells Across the Blood-Brain Barrier in Experimental Autoimmune Encephalomyelitis

Molecular targets for disrupting leukocyte trafficking during multiple sclerosis

Autoimmune diseases of the central nervous system (CNS) involve the migration of abnormal numbers of self-directed leukocytes across the blood–brain barrier that normally separates the CNS from the immune system. The cardinal lesion associated with neuroinflammatory diseases is the perivascular infiltrate, which comprises leukocytes that have traversed the endothelium and have congregated in a subendothelial space between the endothelial-cell basement membrane and the glial limitans. The exit of mononuclear cells from this space can be beneficial, as when virus-specific lymphocytes enter the CNS for pathogen clearance, or might induce CNS damage, such as in the autoimmune disease multiple sclerosis when myelin-specific lymphocytes invade and induce demyelinating lesions. The molecular mechanisms involved in the movement of lymphocytes through these compartments involve multiple signalling pathways between these cells and the microvasculature. In this review, we discuss adhesion, costimulatory, cytokine, chemokine and signalling molecules involved in the dialogue between lymphocytes and endothelial cells that leads to inflammatory infiltrates within the CNS, and the targeting of these molecules as therapies for the treatment of multiple sclerosis.

Inflammatory cell migration into the central nervous system: a few new twists on an old tale

Understanding the mechanisms of leukocyte trafficking into the brain might provide insights into how to modulate pathologic immune responses or enhance host protective mechanisms in neuroinflammatory diseases such as multiple sclerosis. This review summarized our knowledge about the sites for leukocyte entry into the central nervous system, highlighting the routes from blood into the perivascular space and brain parenchyma through the blood-brain barrier. We further discussed the multistep paradigm of leukocyte-endothelial interactions at the blood-brain barrier, focusing on the adhesion molecules and chemokines involved in leukocyte transmigration. Luminal chemokines, which are immobilized on endothelial surfaces, initiate leukocyte integrin clustering and conformational change, leading to leukocyte arrest. Some leukocytes undergo post-arrest locomotion across the endothelial surface until interendothelial junctions are identified. Leukocytes then extend protrusions through the interendothelial junctions, in search of abluminal chemokines, which will serve as guidance cues for transmigration. Extravasating cells first accumulate in the perivascular space between the endothelial basement membrane and the basement membrane of the glia limitans. Matrix metalloproteases may be involved in leukocyte transverse across glia limitans into the brain parenchyma. The adhesion molecules and chemokine receptors provide attractive targets for neuroinflammatory diseases because of their important role in mediating central nervous system inflammation.

A CD99-related antigen on endothelial cells mediates neutrophil but not lymphocyte extravasation in vivo

CD99 is a long-known leukocyte antigen that does not belong to any of the known protein families. It was recently found on endothelial cells, where it mediates transendothelial migration of human monocytes and lymphocyte recruitment into inflamed skin in the mouse. Here, we show that CD99L2, a recently cloned, widely expressed antigen of unknown function with moderate sequence homology to CD99, is expressed on mouse leukocytes and endothelial cells. Using antibodies, we found that CD99L2 and CD99 are involved in transendothelial migration of neutrophils in vitro and in the recruitment of neutrophils into inflamed peritoneum. Intravital and electron microscopy of cremaster venules revealed that blocking CD99L2 inhibited leukocyte transmigration through the vessel wall (diapedesis) at the level of the perivascular basement membrane. We were surprised to find that, in contrast to CD99, CD99L2 was not relevant for the extravasation of lymphocytes into inflamed tissue. Although each protein promoted cell aggregation of transfected cells, endothelial CD99 and CD99L2 participated in neutrophil extravasation independent of these proteins on neutrophils. Our results establish CD99L2 as a new endothelial surface protein involved in neutrophil extravasation. In addition, this is the first evidence for a role of CD99 and CD99L2 in the process of leukocyte diapedesis in vivo.

New insights into adaptive immunity in chronic neuroinflammation

Understanding the immune response in the central nervous system (CNS) is crucial for the development of new therapeutic concepts in chronic neuroinflammation, which differs considerably from other autoimmune diseases. Special immunologic properties of inflammatory processes in the CNS, which is often referred to as an immune privileged site, imply distinct features of CNS autoimmune disease in terms of disease initiation, perpetuation, and therapeutic accessibility. Furthermore, the CNS is a stress-sensitive organ with a low capacity for self-renewal and is highly prone to bystander damage caused by CNS inflammation. This leads to neuronal degeneration that contributes considerably to the phenotype of the disease. In this chapter, we discuss recent findings emphasizing the predominant role of the adaptive immune system in the pathogenesis of chronic neuroinflammation, that is, multiple sclerosis (MS) in patients and experimental autoimmune encephalomyelitis (EAE) in rodents. In addition, we report on efforts to translate these findings into clinical practice with the aim of developing selective treatment regimens.

Immune cell migration as a means to control immune privilege: lessons from the CNS and tumors

Certain organs, such as the brain, eye, and gonads, are particularly sensitive to damage by inflammation. Therefore, these tissues have developed unique immunological properties that curtail inflammatory responses, a phenomenon termed immune privilege. In addition, by co-opting some of the regulatory cues operant in immune privilege in normal organs, tumors can evade immunosurveillance. While many different mechanisms contribute to immune privilege, there is evidence that leukocyte migration is an important checkpoint in its control. This hypothesis is based on the fact that leukocyte entry into these organs is restricted by physical barriers and that the collapse of these obstacles marks a critical step in the development of inflammatory/autoimmune disease at these sites. Numerous studies in a variety of experimental systems have characterized the molecular and cellular mechanisms involved in leukocyte homing to immune-privileged organs. Recently, two-photon microscopy has revealed critical insights into the events occurring in the extravascular space of immune-privileged organs, including locomotion patterns and interactive behavior of leukocytes in the interstitial space. Here, we review our current understanding of immune cell migration to and within immune-privileged organs and highlight how this knowledge may be exploited for immunotherapeutic purposes.

Dysferlin is a new marker for leaky brain blood vessels in multiple sclerosis

Dysferlin is a muscle protein involved in cell membrane repair and its deficiency is associated with muscular dystrophy. We describe that dysferlin is also expressed in leaky endothelial cells. In the normal central nervous system (CNS), dysferlin is only present in endothelial cells of circumventricular organs. In the inflamed CNS of patients with multiple sclerosis (MS) or in animals with experimental autoimmune encephalomyelitis, dysferlin reactivity is induced in endothelial cells and the expression is associated with vascular leakage of serum proteins. In MS, dysferlin expression in endothelial cells is not restricted to vessels with inflammatory cuffs but is also present in noninflamed vessels. In addition, many blood vessels with perivascular inflammatory infiltrates lack dysferlin expression in inactive lesions or in the normal-appearing white matter. In vitro, dysferlin can be induced in endothelial cells by stimulation with tumor necrosis factor-alpha. Hence, dysferlin is not only a marker for leaky brain vessels, but also reveals dissociation of perivascular inflammatory infiltrates and blood-brain barrier disturbance in multiple sclerosis.

CD62L is required for the priming of encephalitogenic T cells but does not play a major role in the effector phase of experimental autoimmune encephalomyelitis

CD62L (l-selectin, mel 14) regulates naïve T cell homing into lymph nodes and the migration of leucocytes to sites of inflammation. The requirement of CD62L in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, has been demonstrated previously. However, it remains controversial as to whether CD62L is required for the induction or the effector phase of EAE. It is also unclear whether other non-T effector cells need CD62L to enter the central nervous system (CNS) parenchyma and exert their damaging effects on myelin. We report that mice with a targeted mutation of CD62L are resistant to Myelin oligodendrocyte glycoprotein peptide-induced EAE. CD62L-deficient mice had no peptide-specific T cell responses in the draining lymph nodes and had lower levels of peptide-specific T cell responses in spleens at a later time point. Adoptive transfer studies showed that CD62L-deficient mice were fully susceptible to adoptive transfer EAE induced by either wildtype or CD62L-deficient T cells. Moreover, CD62L-deficient, F4/80(+) macrophages can be efficiently recruited into the CNS parenchyma. These data suggest that CD62L is required for the induction of encephalitogenic T cells during EAE development, but is not required by T and non-T effector cells to attack the CNS parenchyma.

Determinants of CCL5-driven mononuclear cell migration across the blood-brain barrier. Implications for therapeutically modulating neuroinflammation

Chemokine receptors and adhesion molecules are used selectively for the transmigration of leukocytes across the blood-brain barrier (BBB) during neuroinflammation. We established an activated in vitro BBB (aIVBBB) using physiological concentrations of cytokines. We studied CCL5-driven migration as a model to determine how chemokine receptors and adhesion molecules regulate T-cell and monocyte migration across the aIVBBB. Increased expression of CCL5 and its receptors, CCR1 and CCR5 have been described in the perivascular space of multiple sclerosis (MS) lesions. Elucidating the determinants of CCL5-mediated mononuclear cell migration may clarify appropriate targets for therapeutic modulation in neuroinflammatory conditions. In response to CCL5, there was a significant increase in total mononuclear cell migration across the aIVBBB. Neutralizing monoclonal antibodies to CCR1 and CCR5 abrogated CCL5-driven transmigration, suggestive of non-redundant receptor usage in mononuclear cell migration to this chemokine in vitro. CCL5-driven transmigration was also dependent on alpha(4)beta(1) integrin/fibronectin connecting segment-1 (FN CS-1) and alpha(L)beta(2) integrin/intercellular adhesion molecule (ICAM-1) interactions. Monocyte migration to CCL5 was solely dependent on alpha(4)beta(1) integrin/FN CS-1 while T-cell migration required both alpha(L)beta(2) integrin/ICAM-1 and alpha(4)beta(1) integrin/FN CS-1 interactions. These findings provide plausible molecular targets for the selective inhibition of mononuclear cell trafficking during the acute immune effector phases of MS and other neuroinflammatory diseases.

Cerebral leucocyte infiltration in lupus-prone MRL/MpJ-fas lpr mice--roles of intercellular adhesion molecule-1 and P-selectin

The autoimmune disease which affects MRL/MpJ-fas(lpr) mice results in cerebral leucocyte recruitment and cognitive dysfunction. We have previously observed increased leucocyte trafficking in the cerebral microcirculation of these mice; however, the types of leucocytes recruited have not been analysed thoroughly, and the roles of key endothelial adhesion molecules in recruitment of these leucocytes have not been investigated. Therefore the aim of this study was to classify the phenotypes of leucocytes present in inflamed brains of MRL/MpJ-fas(lpr) mice, and dissect the roles of endothelial adhesion molecules in their accumulation in the brain. Immunohistochemical analysis revealed significant leucocyte infiltration in the brains of 16- and 20-week-old MRL/MpJ-fas(lpr) mice, affecting predominantly the choroid plexus. Isolation of brain-infiltrating leucocytes revealed that lymphocytes and neutrophils were the main populations present. The CD3(+) lymphocytes in the brain consisted of similar proportions of CD4(+), CD8(+) and CD4(-)/CD8(-)[double negative (DN)] populations. Assessment of MRL/MpJ-fas(lpr) mice deficient in endothelial adhesion molecules intercellular adhesion molecule-1 (ICAM-1) or P-selectin indicated that cerebral leucocyte recruitment persisted in the absence of these molecules, with only minor changes in the phenotypes of infiltrating cells. Together these data indicate that the brains of MRL/MpJ-fas(lpr) mice are affected by a mixed leucocyte infiltrate, of which the unusual DN lymphocyte phenotype contributes a substantial proportion. In addition, endothelial adhesion molecules ICAM-1 and P-selectin, which modulate survival of MRL/MpJ-fas(lpr) mice, do not markedly inhibit leucocyte entry into the central nervous system.

Reevaluation of P-selectin and alpha 4 integrin as targets for the treatment of experimental autoimmune encephalomyelitis

There has been a great deal of interest in adhesion molecules as targets for the treatment of multiple sclerosis and other inflammatory diseases. In this study, we systematically evaluate alpha(4) integrin and P-selectin as targets for therapy in murine models of multiple sclerosis-for the first time directly measuring the ability of their blockade to inhibit recruitment and relate this to clinical efficacy. Experimental autoimmune encephalomyelitis was induced in C57BL/6 or SJL/J mice and intravital microscopy was used to quantify leukocyte interactions within the CNS microvasculature. In both strains, pretreatment with blocking Abs to either alpha(4) integrin or P-selectin reduced firm adhesion to a similar extent, but did not block it completely. The combination of the Abs was more effective than either Ab alone, although the degree of improvement was more evident in SJL/J mice. Similarly, dual blockade was much more effective at preventing the subsequent accumulation of fluorescently labeled leukocytes in the tissue in both strains. Despite evidence of blockade of leukocyte recruitment mechanisms, no clinical benefit was observed with anti-adhesion molecule treatments or genetic deletion of P-selectin in the C57BL/6 model, or in a pertussis toxin-modified model in SJL/J mice. In contrast, Abs to alpha(4) integrin resulted in a significant delay in the onset of clinical signs of disease in the standard SJL/J model. Despite evidence of a similar ability to block firm adhesion, Abs to P-selectin had no effect. Importantly, combined blockade of both adhesion molecules resulted in significantly better clinical outcome than anti-alpha(4) integrin alone.

Abrogation of Functional Selectin-Ligand Expression Reduces Migration of Pathogenic CD8+T Cells into Heart

CD8+ T cells are involved in autoimmune and infectious myocarditis and cardiac allograft rejection. The role of selectins in cardiac recruitment of CD8+ T cells is not understood. In this study, the contribution of T cell selectin ligands to effector CD8+ T cell recruitment into the heart was examined using a model of myocarditis, which depends on transfer of OVA peptide-specific CD8+ T cells (OT-I) into mice (CMy-mOva) that express OVA in the heart. alpha-(1,3)-Fucosyltransferase (FucT)-VII-deficient OT-I cells displayed over a 95% reduction in their ability to interact with P-selectin under flow conditions in vitro, compared with wild-type OT-I cells. Interaction of FucT-VII-deficient OT-I cells with E-selectin was reduced approximately 50%. FucT-VII-deficient OT-I cells were also less efficiently recruited into a dermal site of Ag and adjuvant injection. Significantly, FucT-VII-deficient OT-I cells were also impaired in their ability to migrate into CMy-mOva hearts, compared with wild-type OT-I cells. Transfer of FucT-VII-deficient T cells caused less severe early myocarditis and myocyte damage than transfer of wild-type T cells. Combined FucT-IV/VII-deficient OT-I cells displayed a more profound reduction in E-selectin interactions in vitro compared with FucT-VII-deficient T cells, and the FucT-IV/VII-deficient T cells also showed less early recruitment and pathogenicity in the CMy-mOva myocarditis model. These results identify a prominent role for selectin ligands in contributing to effector CD8+ T cell recruitment into the myocardium and indicate that selectin-dependent T cell recruitment is relevant to other tissues besides the skin.

Leukocyte recruitment and the acute inflammatory response

Leukocyte recruitment is a hallmark feature of the inflammatory response. This review summarizes the generally accepted paradigm of leukocyte recruitment based on studies using intravital microscopy to visualize the microcirculation. The role of selectins and alpha4-integrin in rolling as well as integrin-mediated adhesion is discussed. However, it is becoming increasingly clear that the recruitment cascade within organs differs and therefore the review also attempts to highlight what is and is not known regarding leukocyte recruitment into the brain microvasculature. In the second part of this review, we provide some discussion of mechanisms by which the inflammatory response may be terminated. Particular emphasis on nuclear factor Nf kappaB and how IL10, IL13 and secreted leukocyte protease inhibitor (SLPI) may impact upon the Nf kappaB-dependent inflammatory response is presented.

Molecular mechanisms involved in T cell migration across the blood–brain barrier

In the healthy individuum lymphocyte traffic into the central nervous system (CNS) is very low and tightly controlled by the highly specialized blood-brain barrier (BBB). In contrast, under inflammatory conditions of the CNS such as in multiple sclerosis or in its animal model experimental autoimmune encephalomyelitis (EAE) circulating lymphocytes and monocytes/macrophages readily cross the BBB and gain access to the CNS leading to edema, inflammation and demyelination. Interaction of circulating leukocytes with the endothelium of the blood-spinal cord and blood-brain barrier therefore is a critical step in the pathogenesis of inflammatory diseases of the CNS. Leukocyte/endothelial interactions are mediated by adhesion molecules and chemokines and their respective chemokine receptors. We have developed a novel spinal cord window preparation, which enables us to directly visualize CNS white matter microcirculation by intravital fluorescence videomicroscopy. Applying this technique of intravital fluorescence videomicroscopy we could provide direct in vivo evidence that encephalitogenic T cell blasts interact with the spinal cord white matter microvasculature without rolling and that alpha4-integrin mediates the G-protein independent capture and subsequently the G-protein dependent adhesion strengthening of T cell blasts to microvascular VCAM-1. LFA-1 was found to neither mediate the G-protein independent capture nor the G- protein dependent initial adhesion strengthening of encephalitogenic T cell blasts within spinal cord microvessel, but was rather involved in T cell extravasation across the vascular wall into the spinal cord parenchyme. Our observation that G-protein mediated signalling is required to promote adhesion strengthening of encephalitogenic T cells on BBB endothelium in vivo suggested the involvement of chemokines in this process. We found functional expression of the lymphoid chemokines CCL19/ELC and CCL21/SLC in CNS venules surrounded by inflammatory cells in brain and spinal cord sections of mice afflicted with EAE suggesting that the lymphoid chemokines CCL19 and CCL21 besides regulating lymphocyte homing to secondary lymphoid tissue might be involved in T lymphocyte migration into the immuneprivileged CNS during immunosurveillance and chronic inflammation. Here, I summarize our current knowledge on the sequence of traffic signals involved in T lymphocyte recruitment across the healthy and inflamed blood-brain and blood-spinal cord barrier based on our in vitro and in vivo investigations.

Regulation of immune cell entry into the central nervous system

The central nervous system (CNS) has long been regarded as an immune privileged organ implying that the immune system avoids the CNS to not disturb its homeostasis, which is critical for proper function of neurons. Meanwhile, it is accepted that immune cells do in fact gain access to the CNS and that immune responses can be mounted within this tissue. However, the unique CNS microenvironment strictly controls these immune reactions starting with tightly controlling immune cell entry into the tissue. The endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid (CSF) barrier, which protect the CNS from the constantly changing milieu within the bloodstream, also strictly control immune cell entry into the CNS. Under physiological conditions, immune cell migration into the CNS is kept at a very low level. In contrast, during a variety of pathological conditions of the CNS such as viral or bacterial infections, or during inflammatory diseases such as multiple sclerosis, immunocompetent cells readily traverse the BBB and likely also the choroid plexus and subsequently enter the CNS parenchyma or CSF spaces. This chapter summarizes our current knowledge of immune cell entry across the blood CNS barriers. A large body of the currently available information on immune cell entry into the CNS has been derived from studying experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. Therefore, most of this chapter discussing immune cell entry during CNS pathogenesis refers to observations in the EAE model, allowing for the possibility that other mechanisms of immune cell entry into the CNS might apply under different pathological conditions such as bacterial meningitis or stroke.

The LPS receptor, CD14, in experimental autoimmune encephalomyelitis and multiple sclerosis

Innate immune receptors are crucial for defense against microorganisms. Recently, a cross-talk between innate and adaptive immunity has been considered. Here, we provide first evidence for a role of the key innate immune receptor, LPS receptor (CD14) in pathophysiology of experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis. Indicating a functional importance in vivo, we show that CD14 deficiency increased clinical symptoms in active experimental autoimmune encephalomyelitis. Consistent with these observations, CD14 deficient mice exhibited a markedly enhanced infiltration of monocytes and neutrophils in brain and spinal cord. Moreover, we observed an increased immunoreactivity of CD14 in biopsy and post mortem brain tissues of multiple sclerosis patients compared to age-matched controls. Thus, the key innate immune receptor, CD14, may be of pathophysiological relevance in experimental autoimmune encephalomyelitis and multiple sclerosis.

The role of alpha-4 integrin in the aetiology of multiple sclerosis: current knowledge and therapeutic implications

Multiple sclerosis (MS) has been recognised as a disease since the mid-19th century. The delineation of its CNS pathology, revealing the presence of inflammatory demyelination and relative sparing of axons, was originally interpreted as evidence of infection. Despite many studies, a primary infectious aetiology of MS has not been found. However, the occurrence of acute demyelinating disease following a variety of infections and vaccinations, leading to MS in about a third of cases, provides evidence for the existence of an auto-allergic pathogenesis for the disease. Improved understanding of the role of the blood-brain barrier in protecting the CNS, and the mechanisms by which cells gain entry into the brain and spinal cord has advanced the understanding of MS. Evidence of the central role of the adhesion molecule alpha4beta1-integrin (very late activation antigen-4 [VLA-4]) for lymphocytes in endothelial transmigration into the CNS specifically, has provided a major insight into the pathogenesis of human demyelinating disease and its experimental model, experimental autoimmune encephalomyelitis (EAE). This finding has led to a new window of therapeutic opportunity in MS. Monoclonal antibodies to VLA-4 abrogate the development of EAE in sensitised animals and may actually reverse its clinical and pathological findings in manifest disease. Natalizumab, one such monoclonal antibody, which is administered intravenously, has been found to be a promising agent in the treatment of MS. Although single doses produced no improvement in the speed or quality of recovery from acute exacerbations of MS in a phase II trial, long-term administration (in phase II and phase III trials) have produced significant benefits with results showing both a marked reduction in the risk of new magnetic resonance imaging lesions and a significant reduction in the risk of exacerbations within 2 months of the initiation of therapy. Phase III double-blinded controlled trials have provided additional evidence of safety and a favourable impact on exacerbation rates over the 1 year of administration. Unfortunately, the success of natalizumab has been curtailed by three cases of progressive multifocal leukoencephalopathy, which have prompted the manufacturer to voluntary withdraw the drug from the market. An independent review board is currently investigating the safety of the drug to determine whether it should return to the market. The demonstration that selective modulation (blocking) of the adhesion molecule VLA-4 by natalizumab in MS, resembling that observed in experimental disease, represents a major advance in rational therapy.

IL-12 driven upregulation of P-selectin ligand on myelin-specific T cells is a critical step in an animal model of autoimmune demyelination

Experimental autoimmune encephalomyelitis (EAE) is an inflammatory demyelinating disease of the central nervous system. IL-12p40 monokines play a critical role in the generation of EAE-inducing CD4+T cells. Here we show that IL-12 directly upregulates the expression of the adhesion molecule, P-selectin glycoprotein ligand (PSGL-1), on B10.PL MBP-TCR transgenic T cells during their initial encounter with antigen. Pre-incubation of IL-12-stimulated myelin-reactive CD4+T cells with a blocking antibody against PSGL-1 reduced the incidence and severity of EAE. We conclude that IL-12-driven PSGL-1 expression can facilitate the development of autoimmune demyelination.

The ins and outs of T-lymphocyte trafficking to the CNS: anatomical sites and molecular mechanisms

This review addresses current knowledge of the molecular trafficking signals involved in the migration of circulating leukocytes across the highly specialized blood-central nervous system (CNS) barriers during immunosurveillance and inflammation. In this regard, adhesion molecules and activating and chemotactic factors are also discussed and the regional variability in the brain and spinal cord parenchyma are also considered. Furthermore, direct passage into cerebrospinal fluid (CSF) is discussed, in the context of CNS immunosurveillance. The potential differences that characterize leukocyte entry into these varied anatomical sites are highlighted, with special emphasis on studies of the pathogenesis of multiple sclerosis and its animal models. An update on findings from clinical trials of natalizumab is also provided.

Selectin Ligand-Independent Priming and Maintenance of T Cell Immunity during Airborne Tuberculosis

Immunity to Mycobacterium tuberculosis infection is critically dependent on the timely priming of T effector lymphocytes and their efficient recruitment to the site of mycobacterial implantation in the lung. E-, P-, and L-selectin counterreceptors control lymphocyte homing to lymph nodes and leukocyte trafficking to peripheral sites of acute inflammation, their adhesive function depending on fucosylation by fucosyltransferases (FucT) IV and VII. To address the relative importance of differentially glycosylated selectin counterreceptors for priming of T cell effector functions in a model of mycobacteria-induced granulomatous pulmonary inflammation, we used aerosol-borne M. tuberculosis to infect FucT-IV-/-, FucT-VII-/-, FucT-IV-/-/FucT-VII-/-, or wild-type control mice. In lymph nodes, infected FucT-IV-/-/FucT-VII-/- and, to a lesser extent, FucT-VII-/- mice had severely reduced numbers of T cells and reduced Ag-specific effector responses. By contrast, recruitment of activated T cells into the lungs was similar in all four groups of mice during infection and expression of T cell, and macrophage effector functions were only delayed in lungs of FucT-IV-/-/FucT-VII-/- mice. Importantly, lungs from all groups expressed CXCL13, CCL21, and CCL19 and displayed organized follicular neolymphoid structures after infection with M. tuberculosis, which suggests that the lung served as a selectin ligand-independent priming site for immune responses to mycobacterial infection. All FucT-deficient strains were fully capable of restricting M. tuberculosis growth in infected organs until at least 150 days postinfection. Our observations indicate that leukocyte recruitment functions dictated by FucT-IV and FucT-VII-dependent selectin ligand activities are not critical for inducing or maintaining T cell effector responses at levels necessary to control pulmonary tuberculosis.

Peripheral injection of lipopolysaccharide prevents brain recruitment of leukocytes induced by central injection of interleukin-1

I.c.v. injection of interleukin-1beta induces infiltration of leukocytes into the brain. I.p. injection of bacterial endotoxin lipopolysaccharide induces the expression of interleukin-1 in the CNS without causing the entry of leukocytes into the brain. This suggests that during systemic inflammation trafficking of potentially damaging leukocytes into the CNS is inhibited. In this study, we investigated the effects of peripheral injection of lipopolysaccharide on brain leukocyte recruitment induced by i.c.v.-interleukin-1 in mice. I.c.v.-interleukin-1 induced widespread infiltration of leukocytes into the brain 16 h after the injection. Pretreatment with i.p.-lipopolysaccharide 2 h before the i.c.v. interleukin-1 injection completely blocked interleukin-1-induced leukocyte infiltration, whereas i.p.-LPS only attenuated the effect of interleukin-1 if it was given 12 h before i.c.v. interleukin-1 injection. I.p.-lipopolysaccharide given 24 h before i.c.v. interleukin-1 injection did not alter interleukin-1 induced leukocyte infiltration. I.c.v.-interleukin-1 induced expression of p- and e-selectins in brain vasculatures prior to the appearance of leukocytes in the brain parenchyma. Induction of p- and e-selectin was inhibited by the pretreatment of i.p.-lipopolysaccharide 2 h, but not 24 h, before i.c.v.-interleukin-1 injection. I.c.v.-interleukin-1-induced leukocyte infiltration was diminished in both e- and p- selectin knockout animals. These results suggest that systemic inflammation actively inhibits recruitment of leukocytes by CNS. Inhibition of the expression of p- and e-selectins is a mechanism by which peripheral inflammation regulate CNS leukocyte recruitment.

Activation of cerebral endothelium is required for mononuclear cell recruitment in a novel in vitro model of brain inflammation

Brain inflammation is a common event in the pathogenesis of several neurological diseases. It is unknown whether leukocyte/endothelium interactions are sufficient to promote homing of blood-borne cells into the brain compartment. The role of mononuclear cells and endothelium was analyzed in a new experimental model, the isolated guinea-pig brain maintained in vitro by arterial perfusion. This preparation allows one to investigate early steps of brain inflammation that are impracticable in vivo. We demonstrate by confocal microscopy analysis that in vitro co-perfusion of pro-inflammatory agents and pre-activated fluorescent mononuclear cells induced endothelial expression of selectins and intracellular adhesion molecule-1 in correspondence of arrested mononuclear cells, and correlates with a moderate increase in blood-brain barrier permeability. Separate perfusion of pro-inflammatory agents and mononuclear cells induced neither mononuclear cell adhesion nor adhesion molecule expression. We demonstrate that co-activation of mononuclear cells and cerebral endothelium is an essential requirement for cell arrest and adhesion in the early stages of experimental cerebral inflammation.

Immune cell migration across the blood–brain barrier: molecular mechanisms and therapeutic targeting

The endothelial blood–brain barrier (BBB) and the epithelial blood–cerebrospinal fluid barrier protect the CNS from the constantly changing milieu within the bloodstream. The BBB strictly controls immune cell entry into the CNS, which is rare under physiological conditions. During a variety of pathological conditions of the CNS, such as viral or bacterial infections, or during inflammatory diseases, such as multiple sclerosis, immunocompetent cells readily traverse the BBB and subsequently enter the CNS parenchyma. Most of the available information on immune cell entry into the CNS is derived from studying experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. Consequently, our current knowledge on traffic signals mediating immune cell entry across the BBB during immunosurveillance and disease results mainly from experimental data in the EAE model. Therefore, a large part of this review summarizes these findings. Similarly, the potential benefits and risks associated with therapeutic targeting of immune cell trafficking across the BBB will be discussed in the context of multiple sclerosis, since elucidation of the molecular mechanisms relevant to this disease have largely relied on the use of its in vivo model, EAE.

Mechanisms of lymphocyte migration in autoimmune disease

The recruitment of leukocytes to inflamed tissues plays an essential role in combating infection and promoting wound healing. However, in autoimmune diseases such as multiple sclerosis and diabetes, leukocytes enter tissues and contribute to inappropriate inflammatory responses, which cause tissue injury and dysfunction. In diseases of this type, lymphocytes play critical roles in initiating and maintaining these aberrant inflammatory responses. The aim of this review is to examine the mechanisms whereby T-lymphocytes enter tissues in autoimmune diseases and to compare these mechanisms between various organs and diseases. An overview of the mechanisms of leukocyte recruitment and the techniques used to study leukocyte trafficking is provided, focusing on the use of intravital microscopy as a tool to assess the functional microvasculature in vivo. We also discuss the series of tissue homing events which allow naïve lymphocytes to first enter lymph nodes and undergo activation, then subsequently to home to the peripheral organ where their cognate antigen is present. Finally, we examine mechanisms of leukocyte recruitment in diseases such as multiple sclerosis, autoimmune diabetes, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease and asthma.

[Possible target specific molecular imaging with ultrasound contrast agents]

Non-invasive molecular imaging technologies provide researchers with the opportunity to study cellular and molecular processes. Among different imaging technologies, ultrasound based molecular imaging methods are also of interest, since the use of ultrasound contrast agents allows specific and sensitive depiction of molecular targets. Recent studies are encouraging and have demonstrated the feasibility of ultrasound based molecular imaging. This review summarizes current experiences and recent preclinical studies with target-specific ultrasound contrast agents.

Differential expression of selectins by mouse brain capillary endothelial cells in vitro in response to distinct inflammatory stimuli

Increased lymphocyte trafficking across blood-brain barrier (BBB) is a prominent and early event in inflammatory and immune-mediated CNS diseases. The adhesion molecules that control the entry of leukocytes into the brain have not been fully elucidated. Although the role of ICAM-1 and VCAM-1 has been well documented, the expression and role of selectins is still a matter of controversy. In a mouse syngenic in vitro BBB model, highly relevant for examining immunological events, mouse brain capillary endothelial cells (MBCECs) do not express selectins. Treatment of MBCECs with LPS, induced E- and P-selectin expression, whereas TNF-alpha or IFN-gamma treatments did not. Finally, P-selectin but not E-selectin expression was induced in IL-1beta treated MBCECs. Thus, our study suggests that diverse inflammatory stimuli could differentially regulate selectin expression at the BBB.

The circumventricular organs participate in the immunopathogenesis of experimental autoimmune encephalomyelitis

Background

During inflammatory conditions of the central nervous system (CNS), such as in multiple sclerosis or in its animal model, experimental autoimmune encephalomyelitis (EAE), immune cells migrate from the blood stream into the CNS parenchyma and into the cerebrospinal fluid (CSF) spaces. The endothelial blood-brain barrier (BBB) has been considered the most obvious entry site for circulating immune cells. Recently, the choroid plexus has been considered as an alternative entry site for circulating lymphocytes into the CSF. The choroid plexus, belongs to the circumventricular organs (CVOs) localized in the walls of the ventricles. Other CVOs, which similar to the choroid plexus lack an endothelial BBB, have not been considered as possible entry sites for immune cells into the CNS parenchyma or the CSF. Here we asked, whether CVOs are involved in the recruitment of inflammatory cells into the brain during EAE.

Methods

We performed an extensive immunohistological study on the area postrema (AP), the subfornical organ (SFO), the organum vasculosum of the lamina terminalis (OVLT) and the median eminence (ME) in frozen brain sections from healthy SJL mice and mice suffering from EAE. Expression of cell adhesion molecules, the presence of leukocyte subpopulations and the detection of major histocompatibility complex antigen expression was compared.

Results

Similar changes were observed for all four CVOs included in this study. During EAE significantly increased numbers of CD45⁺ leukocytes were detected within the four CVOs investigated, the majority of which stained positive for the macrophage markers F4/80 and Mac-1. The adhesion molecules ICAM-1 and VCAM-1 were upregulated on the fenestrated capillaries within the CVOs. A considerable upregulation of MHC class I throughout the CVOs and positive immunostaining for MHC class II on perivascular cells additionally documented the immune activation of the CVOs during EAE. A significant enrichment of inflammatory infiltrates was observed in close vicinity to the CVOs.

Conclusion

Our data indicate that the CVOs are a site for the entry of immune cells into the CNS and CSF and consequently are involved in the inflammatory process in the CNS during EAE.

PSGL-1 is not required for development of experimental autoimmune encephalomyelitis

Adhesion molecules are essential mediators for lymphocyte trafficking through the blood-brain barrier into the CNS in multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). However, the role of the selectin molecules and their ligand, P-selectin glycoprotein-1 (PSGL-1) which mediates tethering and rolling of the leukocytes in demyelinating disease remains controversial. This study demonstrates that mice deficient in PSGL-1 are not significantly different in the development and progression of EAE compared to wild type controls. Our observations suggest that PSGL-1-selectin interactions are redundant and not required for the development of EAE. Our data also indicate that other adhesion molecules are necessary for the initial rolling events leading to leukocyte infiltration into the CNS during EAE.

P-Selectin Glycoprotein Ligand 1 Is Not Required for the Development of Experimental Autoimmune Encephalomyelitis in SJL and C57BL/6 Mice

In multiple sclerosis and in its animal model experimental autoimmune encephalomyelitis (EAE), inflammatory cells migrate across the endothelial blood-brain barrier and gain access to the CNS. The involvement of P-selectin glycoprotein ligand 1 (PSGL-1) and of its major endothelial ligand P-selectin in this process have been controversial. In this study we demonstrate that although encephalitogenic T cells express functional PSGL-1, which can bind to soluble and immobilize P-selectin if presented in high concentrations, PSGL-1 is not involved T cell interaction with P-selectin expressing brain endothelial cells in vitro. Furthermore, neither anti-PSGL-1 Abs nor the lack of PSGL-1 in PSGL-1-deficient mice inhibits the recruitment of inflammatory cells across the blood-brain barrier or the development of clinical EAE. Taken together, our findings demonstrate that PSGL-1 is not required for the pathogenesis of EAE.

Efficient Recruitment of Lymphocytes in Inflamed Brain Venules Requires Expression of Cutaneous Lymphocyte Antigen and Fucosyltransferase-VII

Lymphocyte migration into the brain represents a critical event in the pathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). However, the mechanisms controlling the recruitment of lymphocytes to the CNS via inflamed brain venules are poorly understood, and therapeutic approaches to inhibit this process are consequently few. In this study, we demonstrate for the first time that human and murine Th1 lymphocytes preferentially adhere to murine inflamed brain venules in an experimental model that mimics early inflammation during EAE. A virtually complete inhibition of rolling and arrest of Th1 cells in inflamed brain venules was observed with a blocking anti-P-selectin glycoprotein ligand 1 Ab and anti-E- and P-selectin Abs. Th1 lymphocytes produced from fucosyltransferase (FucT)-IV(-/-) mice efficiently tethered and rolled, whereas in contrast, primary adhesion of Th1 lymphocytes obtained from FucT-VII(-/-) or Fuc-VII(-/-)FucT-IV(-/-) mice was drastically reduced, indicating that FucT-VII is critical for the recruitment of Th1 cells in inflamed brain microcirculation. Importantly, we show that Abs directed against cutaneous lymphocyte Ag (CLA), a FucT-VII-dependent carbohydrate modification of P-selectin glycoprotein ligand 1, blocked rolling of Th1 cells. By exploiting a system that allowed us to obtain Th1 and Th2 cells with skin- vs gut-homing (CLA(+) vs integrin beta(7)(+)) phenotypes, we observed that induced expression of CLA on Th cells determined a striking increase of rolling efficiency in inflamed brain venules. These observations allow us to conclude that efficient recruitment of activated lymphocytes to the brain in the contexts mimicking EAE is controlled by FucT-VII and its cognate cell surface Ag CLA.

Diapedesis of mononuclear cells across cerebral venules during experimental autoimmune encephalomyelitis leaves tight junctions intact

Diapedesis of leukocytes across endothelial barriers is generally believed to require the opening of endothelial tight junctions. At the blood-brain barrier (BBB), endothelial cells are interconnected by complex tight junctions. Here, we show by serial section conventional electron microscopy that during experimental autoimmune encephalomyelitis mononuclear cells traverse cerebral microvessels by a transcellular pathway, leaving the endothelial tight junctions intact. Cerebral endothelial cells were found to form filopodia-like membrane protrusions on their luminal aspect, thus embracing the mononuclear cells and forming cup-like structures, and eventually pores, through which the traversing cell could reach the abluminal side. At the abluminal side endothelial cell protrusions surrounding a migrating inflammatory cell were found to be progressively lined with basal lamina, suggesting a change from luminal to abluminal membrane characteristics of endothelial cell membranes during inflammatory cell diapedesis. Morphological evidence for the involvement of tight junctions in the diapedesis of mononuclear cells across the inflamed BBB could not be obtained in any case. Taken together, the presence of morphologically intact tight junctions and our novel finding of the presence of a basal lamina on both sides of abluminal endothelial cell protrusions surrounding migrating inflammatory cells suggests that during experimental autoimmune encephalomyelitis diapedesis of mononuclear cells occurs via a transendothelial process.

Gene and protein expression profiling of the microvascular compartment in experimental autoimmune encephalomyelitis in C57Bl/6 and SJL mice

Dysfunction of the blood-brain barrier (BBB) is a hallmark of inflammatory diseases of the central nervous system (CNS) such as multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). The molecular mechanisms leading to BBB breakdown are not well understood. In order to find molecules involved in this process, we used oligonucleotide microarrays and proteomics to analyze gene and protein expression of the microvascular compartment isolated from brains of C57Bl/6 and SJL/N mice afflicted with EAE and the microvascular compartment isolated from healthy controls. Out of the 6500 known genes and expressed sequence tags (ESTs) studied, expression of 288 genes was found to be changed. Of these genes 128 were altered in the microvascular compartment in both EAE models. Six proteins were identified to be present at altered levels. In addition to the expected increased expression of genes coding for molecules involved in leukocyte recruitment, genes not yet ascribed to EAE pathogenesis were identified. Thus, proteomics and gene array screens of the microvascular compartment are valid approaches, that can be used to define novel candidate molecules involved in EAE pathogenesis at the level of the BBB.

Generation and characterization of telomerase-transfected human lymphatic endothelial cells with an extended life span

The study of lymphatic endothelial cells and lymphangiogenesis has, in the past, been hampered by the lack of lymphatic endothelial-specific markers. The recent discovery of several such markers has permitted the isolation of lymphatic endothelial cells (LECs) from human skin. However, cell numbers are limited and purity is variable with the different isolation procedures. To overcome these problems, we have transfected human dermal microvascular endothelial cells (HDMVECs) with a retrovirus containing the coding region of human telomerase reverse transcriptase (hTERT), and have produced a cell line, hTERT-HDLEC, with an extended lifespan. hTERT-HDLEC exhibit a typical cobblestone morphology when grown in culture, are contact-inhibited, and express endothelial cell-specific markers. hTERT-HDLEC also express the recognized lymphatic markers, Prox-1, LYVE-1 and podoplanin, as well as integrin alpha9, but do not express CD34. They also form tube-like structures in three-dimensional collagen gels when stimulated with vascular endothelial growth factors -A and -C. Based on these currently recognized criteria, these cells are LEC. Surprisingly, we also found that the widely studied HMEC-1 cell line expresses recognized lymphatic markers; however, these cells are also CD34-positive. In summary, the ectopic expression of hTERT increases the life span of LECs and does not affect their capacity to form tube-like structures in a collagen matrix. The production and characterization of hTERT-HDLEC will facilitate the study of the properties of lymphatic endothelium in vitro.

Mouse CD99 participates in T-cell recruitment into inflamed skin

Human CD99 is a small highly O-glycosylated cell-surface protein expressed on most leukocytes. It was recently found to be expressed at endothelial cell contacts and to participate in the transendothelial migration (TEM) of monocytes in vitro. In order to analyze the physiologic relevance of CD99 in vivo we searched for the mouse homolog. We cloned a mouse cDNA coding for a protein 45% identical in its sequence with human CD99. Based on the cDNA, we generated antibodies against this mouse homolog of CD99, which detected the antigen on most leukocytes, on endothelia of various tissues, and at cell contacts of cultured endothelial cells. Cell aggregation of CD99-transfected Chinese hamster ovary (CHO) cells was completely blocked by anti-CD99 antibodies. The same antibodies inhibited TEM of lymphocytes in vitro, independent of whether T cells or endothelial cells were preincubated with antibodies. In a cutaneous delayed-type hypersensitivity (DTH) reaction, anti-CD99 antibodies inhibited the recruitment of in vivo-activated T cells into inflamed skin as well as edema formation. We conclude that mouse CD99 participates in the TEM of lymphocytes and in their recruitment to inflamed skin in vivo. This establishes CD99 as a valid target for interference with cutaneous inflammatory processes.

T-cell interaction with ICAM-1/ICAM-2 double-deficient brain endothelium in vitro: the cytoplasmic tail of endothelial ICAM-1 is necessary for transendothelial migration of T cells

Endothelial intercellular adhesion molecule 1 (ICAM-1) and ICAM-2 are both involved in lymphocyte extravasation during immunosurveillance and inflammation. To define their exact role during T-cell extravasation, we used mouse T cells and ICAM-1-/-ICAM-2-/- brain endothelioma cells. ICAM-1-/-ICAM-2-/- brain endothelioma cells did not support transendothelial migration (TEM) of T cells in vitro. Re-expression of different ICAM-1 mutants in the ICAM-1-/-ICAM-2-/- endothelioma line bEndI1/2.1 or in the ICAM-1-/- endothelioma line bEndI1.1 demonstrated that the extracellular domain of ICAM-1 suffices to support T-cell adhesion while the presence of the cytoplasmic tail was strictly required for TEM. Surprisingly, tyrosine phosphorylation of endothelial ICAM-1 was not necessary for TEM of T cells or for Rho guanosine triphosphatase (RhoGTPase) activation. Furthermore, cytoplasmic deletion mutants of ICAM-1 were unable to mediate RhoGTPase activation. Thus, our data demonstrate that the cytoplasmic tail of endothelial ICAM-1-independently from tyrosine phosphorylation-is essential for supporting TEM of T lymphocytes, while Rho signaling is involved in endothelial cells.

Role of ICAM-1 and P-selectin expression in the development and effector function of CD4+CD25+regulatory T cells

Dynamic regulatory mechanisms prevent autoreactive T cell activation. Upon T cell receptor crosslinking, CD4+CD25+ T regulatory (T(R)) cells block both the proliferation and cytokine production of CD4+CD25- effector cells in an apparent antigen non-specific manner. Within the T(R)population, L-selectin (CD62L)(hi)T(R)cells have been described as more efficient suppressors of T cell proliferation than CD62L(low)T(R)cells. We have previously reported that CD4+CD25+CD62L(hi)T(R)cells express elevated levels of two additional adhesion molecules, ICAM-1 (CD54) and P-selectin (CD62P) in comparison to non-T(R)cells. In the current study, we investigated the functional contribution of CD54 and CD62P expression to the suppressive phenotype of T(R)cells both in vitro and in vivo. While the CD4+CD25+ T(R)cell population was demonstrated to be significantly larger in CD62P-/- mice than in wild-type C57BL/6 mice, CD62P-/- T(R)cell function was deficient in vitro, but not in vivo. Interestingly, we detected no deficiencies in T(R)cell numbers or effector function in CD54-/- mice suggesting that T(R)cells may differ from effector CD4+ T cells in the requirement for CD54 expression within the immunological synapse. Collectively, these findings indicate that CD62P may influence T(R)cell differentiation/development and that T(R)cell activation occurs independently of CD54 expression.

Inhibition of monocyte/endothelial cell interactions and monocyte adhesion molecule expression by the immunosuppressant mycophenolate mofetil

STUDY DESIGN

In vitro study on the effects of mycophenolate mofetil (MMF) on isolated human monocytes and endothelial cells.

OBJECTIVES

Haematogenous macrophages play an essential role in the development of secondary damage following spinal cord injury (SCI), and there is evidence that the use of immunosuppressants such as MMF can reduce monocyte invasion and neuronal damage.

SETTING

University Hospital for Orthopaedic Surgery, Frankfurt am Main, Germany.

METHODS

The effects of MMF on the adhesion of human monocytes to human umbilical vein endothelial cells (HUVEC), monocyte binding to immobilised E-selectin, and monocyte expression of intercellular adhesion molecule (ICAM)-1, sialyl Lewis X (sLeX) and major histocompatibility complex (MHC)-II were studied. The binding of monocytes to E-selectin was examined by using purified and immobilised E-selectin fusion protein. Adhesion molecule expression was investigated by flow cytometry.

RESULTS

The binding of monocytes to HUVEC was significantly reduced by 30.1% after treatment of monocytes with MMF (10 microg/ml), whereas the pretreatment of HUVEC with MMF did not result in significant changes in monocyte adhesion. MMF forcefully inhibited monocyte binding to immobilised E-selectin by 55.7%. Furthermore, MMF significantly inhibited the upregulation of ICAM-1- and MHC-II-expression on monocytes stimulated with either lipopolysaccharide or interferon-gamma, whereas the expression of sLeX was not impaired. Toxic effects were excluded by propidium-iodide staining and measurement of fluorescein-diacetate metabolism.

CONCLUSION

MMF can downregulate important monocytic adhesion molecules and inhibits monocyte adhesion to endothelial cells, thus indicating that treatment with MMF could be beneficial after SCI.

SPONSORSHIP

This study was supported by the DFG (Ha 2721/1-3), the Paul und Ursula Klein-Stiftung and the Stiftung Friedrichsheim.

Human cerebrospinal fluid central memory CD4+ T cells: evidence for trafficking through choroid plexus and meninges via P-selectin

Cerebrospinal fluid (CSF) from healthy individuals contains between 1,000 and 3,000 leukocytes per ml. Little is known about trafficking patterns of leukocytes between the systemic circulation and the noninflamed CNS. In the current study, we characterized the surface phenotype of CSF cells and defined the expression of selected adhesion molecules on vasculature in the choroid plexus, the subarachnoid space surrounding the cerebral cortex, and the cerebral parenchyma. Using multicolor flow cytometry, we found that CSF cells predominantly consisted of CD4+/CD45RA-/CD27+/CD69+-activated central memory T cells expressing high levels of CCR7 and L-selectin. CD3+ T cells were present in the choroid plexus stroma in autopsy CNS tissue sections from individuals who died without known neurological disorders. P- and E-selectin immunoreactivity was detected in large venules in the choroid plexus and subarachnoid space, but not in parenchymal microvessels. CD4+ T cells in the CSF expressed high levels of P-selectin glycoprotein ligand 1, and a subpopulation of circulating CD4+ T cells displayed P-selectin binding activity. Intercellular adhesion molecule 1, but not vascular cell adhesion molecule 1 or mucosal addressin cell adhesion molecule 1, was expressed in choroid plexus and subarachnoid space vessels. Based on these findings, we propose that T cells are recruited to the CSF through interactions between P-selectin/P-selectin ligands and intercellular adhesion molecule 1/lymphocyte function-associated antigen 1 in choroid plexus and subarachnoid space venules. These results support the overall hypothesis that activated memory T cells enter CSF directly from the systemic circulation and monitor the subarachnoid space, retaining the capacity to either initiate local immune reactions or return to secondary lymphoid organs.

Three or more routes for leukocyte migration into the central nervous system

Leukocyte migration into and through tissues is fundamental to normal physiology, immunopathology and host defence. Leukocyte entry into the central nervous system (CNS) is restricted, in part, because of the blood-brain barrier (BBB). During the past decade, crucial components that are involved in the process of leukocyte migration have been identified and progress has been made in understanding the mechanisms of neuroinflammatory reactions. In this review, present knowledge of the trafficking determinants that guide the migration of leukocytes is superimposed onto the vascular and compartmental anatomy of the CNS. We discuss three distinct routes for leukocytes to enter the CNS and consider how different populations of leukocytes use trafficking signals to gain entry.

Localization of claudin-3 in tight junctions of the blood-brain barrier is selectively lost during experimental autoimmune encephalomyelitis and human glioblastoma multiforme

In the central nervous system (CNS) complex endothelial tight junctions (TJs) form a restrictive paracellular diffusion barrier, the blood-brain barrier (BBB). During inflammation, BBB properties are frequently lost, resulting in brain edema. To investigate whether BBB leakiness correlates with molecular changes at BBB TJs, we performed immunofluorescence stainings for TJ molecules in a mouse model of experimental autoimmune encephalomyelitis (EAE) and in human tissue with glioblastoma multiforme (GBM). In TJs of healthy CNS vessels in both mouse and man we detected occludin, ZO-1, claudin-5 and claudin-3. In EAE brain and spinal cord sections we observed the selective loss of claudin-3 immunostaining from TJs of venules surrounded by inflammatory cuffs, whereas the localization of the other TJ proteins remained unchanged. In addition, selective loss of claudin-3 immunostaining was also observed in altered cerebral microvessels of human GBM. Our data demonstrate the selective loss of claudin-3 from BBB TJs under pathological conditions such as EAE or GBM when the integrity of the BBB is compromised, and therefore suggest that claudin-3 is a central component determining the integrity of BBB TJs in vivo.

Time- and cell type-specific induction of platelet-derived growth factor receptor-beta during cerebral ischemia

During cerebral ischemia, angiogenesis occurs inside and around the infarcted area. The growth of new blood vessels may contribute to a better outcome after stroke due to accelerated and increased delivery of nutrients and oxygen to the ischemic tissue. The platelet-derived growth factor (PDGF)-B/PDGF receptor (PDGFR)-beta system, hitherto thought to contribute mainly to neuroprotection, may also support angiogenesis and vascular remodeling by mediating interactions of endothelial cells with pericytes after cerebral ischemia. While platelet-derived growth factor (PDGF)-B and its receptor PDGFR-beta are essential factors for the recruitment of pericytes to brain capillaries during embryonic development, their role in blood vessel maturation during cerebral ischemia is not clear. The aim of the present study was to investigate the time course and location of PDGF-B and PDGFR-beta expression in a mouse model of focal cerebral ischemia. In contrast to the early and continuous induction of PDGF-B, PDGFR-beta mRNA was specifically upregulated in vascular structures in the infarcted area 48 h after occlusion of the middle cerebral artery. Immunohistology and confocal microscopy analysis revealed the specific upregulation of PDGFR-beta on blood vessels and suggested expression mainly on pericytes. Our results imply PDGFR-beta as a key factor in vascular remodeling during stroke and suggest that the pleiotropic functions of PDGF-B may be regulated via the expression of its receptor. Influencing the PDGF system therapeutically might improve angiogenesis, cellular protection, and edema inhibition.

Adhesion molecules as therapeutic targets for autoimmune diseases and transplant rejection

Inflammatory disorders such as autoimmune diseases and graft rejection are mediated by activated leukocytes, particularly T lymphocytes, which penetrate the inflamed tissue and perpetuate or amplify the immune reaction. In an unstimulated state, leukocytes do not readily adhere to the vascular endothelium. However, inflammatory signals induce the expression of proteins on the endothelial cell surface that promote the adhesion and extravasation of activated immune cells from the circulation into the underlying tissues. Key among these molecules are P- and E-selectin, intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) on the endothelial cells, and their respective counter receptors, P-selectin glycoprotein ligand-1 (PSGL-1), leukocyte function-associated antigen-1 (LFA-1) and very late antigen-4 (VLA-4), on the leukocytes. In vitro blockade of these molecules inhibits the adhesion of leukocytes. In many cases there is attenuation of leukocyte activation as well. Adhesion blockade in animal models prevents or ameliorates graft rejection and disease severity in autoimmune models. Clinical studies with humanised monoclonal antibodies which interfere with LFA-1/ICAM-1 or VLA-4/VCAM-1 interactions have shown significant efficacy and good safety profiles in autoimmune disease, including psoriasis, multiple sclerosis and inflammatory bowel disease. Thus, adhesion blockade is emerging as a useful therapeutic strategy in several inflammatory settings.

Encephalitogenic T cells use LFA-1 for transendothelial migration but not during capture and initial adhesion strengthening in healthy spinal cord microvessels in vivo

LFA-1 on the surface of encephalitogenic T cells has been suggested to be involved in the pathogenesis of experimental autoimmune encephalomyelitis. By applying a novel technique of intravital fluorescence microscopy that enables us to visualize the interaction of circulating encephalitogenic T lymphoblasts within the healthy spinal cord white matter microvasculature in vivo, we investigated the possible involvement of LFA-1 on circulating encephalitogenic T cells in their multi-step interaction with the blood-brain barrier endothelium in vivo. LFA-1 was found to mediate neither the G-protein-independent capture nor the G-protein-dependent initial adhesion strengthening of encephalitogenic T cell blasts within spinal cord microvessels. In contrast, blocking of LFA-1 on encephalitogenic T lymphoblasts resulted in a significantly reduced number of T cells firmly adhering within spinal cord microvessels 2 h after injection and in a significantly reduced number of T cells subsequently migrating across the vascular wall into the spinal cord parenchyme. Our study provides the first direct evidence that encephalitogenic T cells use LFA-1 for transendothelial migration but not for capture and initial adhesion in spinal cord microvessels in vivo.

Glycosylation processing inhibition by castanospermine prevents experimental autoimmune encephalomyelitis by interference with IL-2 receptor signal transduction

In this study, we explored the therapeutic targets of the glycosylation processing inhibitor, castanospermine (CAST), in murine passive transfer experimental autoimmune encephalomyelitis (EAE), a model disease of multiple sclerosis. By using lymphocytic-endothelial adhesion and transmigration assays, FACScan and Western blotting, we defined the effects of CAST on expression, function and signal transduction of glycoproteins crucial in the pathophysiology of this disease. CAST prevented clinical signs of EAE and completely inhibited inflammatory CNS infiltrates associated with this disease. Here, we showed that CAST blocks antigen-induced lymphocytic activation and clonal expansion in a dose-dependent manner. Importantly, we observed that CAST strongly impairs IL-2-induced signal transduction of the IL-2 receptor. In contrast, neither expression nor binding ability of the IL-2 receptor was affected by this drug. In addition, we were able to exclude major effects of CAST on expression and function of different glycoproteins important in antigen presentation as well as lymphocytic-endothelial adhesion and transmigration. In conclusion, CAST strongly interferes in the signal transduction of the IL-2 receptor. This could explain both inhibitory effects of CAST in clonal T cell expansion and development of transfer EAE. This relatively selective pharmacological effect of CAST highlights its potential as a novel immunomodulatory approach in multiple sclerosis.

Subset-specific reductions in lung lymphocyte accumulation following intratracheal antigen challenge in endothelial selectin-deficient mice

We previously demonstrated induction and expression of CD62E and CD62P in the lungs of mice primed and then challenged with intratracheal (i.t.) SRBC. The current study examined accumulation of endogenous lymphocytes in the lungs of endothelial E- and P-selectin-deficient (E(-)P(-)) mice after i.t. SRBC challenge. Compared with syngeneic wild-type (wt) mice, E(-)P(-) mice showed an 85-95% decrease in CD8(+) T cells and B cells in the lungs at both early and late time points. In contrast, CD4(+) T cell accumulation was reduced by approximately 60% early, but equivalent to wt levels later. Surprisingly, many gammadelta T cells were found in lungs and blood of E(-)P(-) mice but were undetectable in the lungs and blood of wt mice. Absolute numbers of peripheral blood CD4, CD8, and B lymphocytes in E(-)P(-) mice equaled or exceeded the levels in wt mice, particularly after challenge. Trafficking studies using alphabeta T lymphoblasts confirmed that the recruitment of circulating cells after challenge was markedly reduced in E(-)P(-) mice. Furthermore, Ag priming occurred normally in both the selectin-deficient and wt mice, because primed lymphocytes from both groups transferred Ag sensitivity into naive wt mice. Lung production of mRNA for six CC and two CXC chemokines after challenge was equivalent by RT-PCR analysis in wt and E(-)P(-) mice. Therefore, reduced lung accumulation of alphabeta T cells and B cells in E(-)P(-) mice did not result from reduced delivery of circulating lymphocytes to the lungs, unsuccessful Ag priming, or defective pulmonary chemokine production. Selectin-dependent lymphocyte recruitment into the lungs following i.t.-SRBC challenge is subset specific and time dependent.

Role of genetic background in P selectin-dependent immune surveillance of the central nervous system

Although the blood-brain barrier and blood cerebrospinal fluid barrier maintain the central nervous system (CNS) as an immunologically privileged site, T lymphocytes can migrate through unstimulated brain endothelium and epithelium to perform immune surveillance or initiate inflammation. Our prior results suggested that early CNS migration of a CD4 Th1 cell line was facilitated by P selectin (CD62P) in (PL/JxSJL/J)F1 mice. Here, quantitative analysis of migration 2 h following adoptive transfer of fluorescently labeled cells revealed a 53-72% decrease in activated splenocyte, CD4 Th1 and CD8 migration, but not CD4 Th2, in CD62P-deficient C57BL6/J mice. Immunohistochemistry revealed constitutive expression of CD62P within the meninges and choroid plexus epithelia in C57BL6/J and SJL/J, but not BALB/cJ, mice. Activated splenocyte migration was approximately three- to four-fold greater in SJL/J as compared to BALB/cJ mice. Anti-CD62P treatment normalized this difference. Based on these results, we hypothesize that genetically determined kinetics of immune surveillance may regulate the phenotype of subsequent CNS inflammation.

Overlapping roles of P-selectin and alpha 4 integrin to recruit leukocytes to the central nervous system in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is mediated by inflammatory cells recruited from the circulation to the CNS. We used intravital microscopy to investigate the mechanisms of this recruitment. No leukocyte rolling and very little adhesion was observed in healthy control mice. In contrast, both rolling and adhesion was observed in brain postcapillary venules before onset of physical symptoms of EAE. Rolling and adhesion remained elevated for 2 wk and returned to near normal levels by 5 wk postsymptom onset. Consistent with a role for P-selectin in recruitment to the CNS, P-selectin protein was detected in the brains and spinal cords of EAE mice. Expression was highest before symptom onset and decreased over the next 2 wk. The importance of alpha(4) integrin increased with time as anti-alpha(4) integrin blocked approximately 20, 50, and 60% of leukocyte rolling 2 days before disease onset, 5 days and 2 wk postonset of symptoms, respectively, and 85% of rolling 5 wk postsymptoms. Addition of anti-P-selectin to alpha(4) integrin Ab-treated mice blocked all remaining rolling at each time point. Interestingly, however, alpha(4) integrin-mediated rolling appeared to be entirely dependent on P-selectin as anti-P-selectin alone was able to completely block all leukocyte rolling. In the absence of rolling (with P-selectin Ab), a 70% reduction in adhesion was noted. A very similar reduction was seen when mice were treated with alpha(4) integrin-blocking Ab. In conclusion, we describe increased leukocyte trafficking in the brains of EAE mice with important overlapping roles for both P-selectin and alpha(4) integrin in mediating leukocyte-endothelial cell interactions.