Vascular inflammation in central nervous system diseases: adhesion receptors controlling leukocyte-endothelial interactions

Leukocyte trafficking from the blood into the tissues represents a key process during inflammation and requires multiple steps mediated by adhesion molecules and chemoattractants. Inflammation has a detrimental role in several diseases, and in such cases, the molecular mechanisms controlling leukocyte migration are potential therapeutic targets. Over the past 20 years, leukocyte migration in the CNS has been investigated almost exclusively in the context of stroke and MS. Experimental models of ischemic stroke have led to the characterization of adhesion molecules controlling leukocyte migration during acute inflammation, whereas EAE, the animal model of MS, has provided similar data for chronic inflammation. Such experiments have led to clinical trials of antileukocyte adhesion therapy, with consistently positive outcomes in human subjects with MS, showing that interference with leukocyte adhesion can ameliorate chronic inflammatory CNS diseases. This review summarizes our current understanding of the roles of adhesion molecules controlling leukocyte-endothelial interactions in stroke and MS, focusing on recently discovered, novel migration mechanisms. We also discuss the growing evidence suggesting a role for vascular inflammation and leukocyte trafficking in neurodegenerative diseases such as AD. Moreover, we highlight recent findings suggesting a role for leukocyte-endothelial interactions in the pathogenesis of seizures and epilepsy, thus linking endothelial activation and leukocyte trafficking to neuronal electrical hyperactivity. These emerging roles for leukocytes and leukocyte adhesion mechanisms in CNS diseases provide insight into the mechanisms of brain damage and may contribute to the development of novel therapeutic strategies.     

CC chemokines mediate leukocyte trafficking into the central nervous system during murine neurocysticercosis: role of gamma delta T cells in amplification of the host immune response

According to a previous report, the degree of the host immune response highly correlates with severity of the disease in the murine model for neurocysticercosis. In wild-type mice, Mesocestoides corti infection induced a rapid and extensive accumulation of gamma delta T cells and macrophages in the brain. NK cells, dendritic cells, alpha beta T cells, and B cells were also recruited to the brain but at lower levels. In contrast, gamma delta T-cell-deficient mice exhibited decreased cellular infiltration and reduced central nervous system (CNS) pathology. To understand the mechanisms of leukocyte recruitment into the CNS, chemokine expression was analyzed in infected brains in the present study. MCP-1 (CCL2), MIP-1 alpha (CCL3), and MIP-1 beta (CCL4) were up-regulated within 2 days after M. corti infection. Protein expression of RANTES (CCL5), eotaxin (CCL11), and MIP-2 was detected later, at 1 week postinfection. Correlating with the decreased cellular infiltration, delta chain T-cell receptor-deficient (TCR delta(-/-)) mice exhibited substantially reduced levels of most of the chemokines analyzed (with the exception of eotaxin). The results suggest that gamma delta T cells play an important role in the CNS immune response by producing chemokines such as MCP-1 and MIP-1 alpha, enhancing leukocyte trafficking into the brain during murine neurocysticercosis.     

Anti-viral effector T cell responses and trafficking are not dependent upon DRAK2 signaling following viral infection of the central nervous system

The signaling events involved in T cell trafficking into the central nervous system (CNS) following viral infection are not fully understood. Intracerebral infection of mice with mouse hepatitis virus (MHV) results in an acute encephalomyelitis followed by an immune-mediated demyelinating disease. Although chemokine signaling is critical in promoting T cell infiltration into the CNS and control of viral replication, additional signaling pathways have not been completely explored. DRAK2, a lymphoid-restricted serine/threonine kinase, prevents spurious T cell activation. Yet Drak2- / - mice are resistant to MOG-induced experimental autoimmune encephalomyelitis (EAE), suggesting that DRAK2 may influence T cell trafficking into the CNS. In order to further characterize the molecular mechanisms governing T cell activation and accumulation within the CNS in response to viral infection, MHV was instilled into the CNS of Drak2- / - mice. Drak2-deficient T cells possessed no obvious defects in trafficking into the CNS following MHV infection. Moreover, Drak2-deficient T cell activation, expansion and cytokine production were unimpaired in response to acute MHV infection. These results demonstrate that DRAK2 signaling is dispensable for T cell recruitment into the CNS following viral infection, suggesting that the resistance of Drak2- / - mice to EAE is not due to overt T cell trafficking defects.     

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.     

Lipid-cytokine-chemokine cascades orchestrate leukocyte recruitment in inflammation

Chemoattractants are pivotal mediators of host defense, orchestrating the recruitment of immune cells into sites of infection and inflammation. Chemoattractants display vast chemical diversity and include bioactive lipids, proteolytic fragments of serum proteins, and chemokines (chemotactic cytokines). All chemoattractants induce chemotaxis by activating seven-transmembrane-spanning GPCRs expressed on immune cells, establishing the concept that all chemoattractants are related in function. However, although chemoattractants have overlapping functions in vitro, recent in vivo data have revealed that they function, in many cases, nonredundantly in vivo. The chemically diverse nature of chemoattractants contributes to the fine control of leukocyte trafficking in vivo, with sequential chemoattractant use guiding immune cell recruitment into inflammatory sites. Lipid mediators frequently function as initiators of leukocyte recruitment, attracting the first immune cells into tissues. These initial responding immune cells produce cytokines locally, which in turn, induce the local release of chemokines. Local chemokine production then markedly amplifies subsequent waves of leukocyte recruitment. These new discoveries establish a paradigm for leukocyte recruitment in inflammation--described as lipid-cytokine-chemokine cascades--as a driving force in the effector phase of immune responses.     

The acute inflammatory response to lipopolysaccharide in CNS parenchyma differs from that in other body tissues.

Acute inflammation is important for defence against infection, wound repair and the mediation of auto-immune tissue destruction. Myelomonocytic recruitment in acute inflammation is a stereotyped and non-specific response to tissue insult which begins within 2 h. In this study, lipopolysaccharide was injected into the murine CNS and other body sites of mice to compare the inflammatory responses. Doses of lipopolysaccharide which induced typical myelomonocytic recruitment in skin and the choroid plexus had no effect in CNS parenchyma, apart from the morphological activation of local resident microglia. The CNS parenchymal response proceeded independently of that in the choroid plexus-cerebral ventricles and had three distinct and unique phases. Initially there was minimal neutrophil exudation and a two-day delay before any increase in macrophage-microglial cell number. Next, there was a rapid increase in macrophage-microglial cell numbers during the third day, mainly due to recruitment of blood monocytes. During this phase, leukocyte recruitment was restricted to monocytes which rapidly adopted the arborized microglial phenotype. Monocytes migrated through an intact blood-brain barrier independent of changes in solute permeability. Finally, there was a florid myelomonocytic reaction predominantly in the white matter, one week after intracerebral injection of 2 micrograms lipopolysaccharide. At this time, the leukocyte reaction disrupted the blood-brain barrier, mononuclear phagocytes expressed macrophage morphology and abundant major histocompatibility complex Class II antigen, and T lymphocytes were present. Myelomonocytic entry into the CNS was partially inhibited by prior blockade of the type 3 complement receptor, known to mediate leukocyte adhesion to endothelium elsewhere. The processes which lead to rapid myelomonocytic recruitment in other tissues are absent in CNS parenchyma. Understanding the molecular mechanisms responsible could have considerable significance both for CNS pathophysiology as well as possible anti-inflammatory therapeutic application elsewhere in the body.     

Mouse hepatitis virus infection of the central nervous system: chemokine-mediated regulation of host defense and disease

Infection of the central nervous system (CNS) of susceptible mice with mouse hepatitis virus (MHV), a positive-strand RNA virus that is a member of the Coronaviridae family, reproducibly results in an acute encephalomyelitis followed by a demyelinating disease similar to the human demyelinating disease multiple sclerosis (MS). MHV infection triggers a robust cell-mediated response in which both CD4+ and CD8+ T cells are essential in controlling viral replication and spread. However, viral clearance is incomplete and viral RNA and protein can persist within white matter tracts, areas of viral persistence are often associated with demyelinating lesions, and recent studies have indicated an important role for both T cells and macrophages in contributing to myelin destruction. The molecular mechanisms governing leukocyte trafficking and accumulation within the CNS of MHV-infected mice are just now being understood and recent studies indicate that chemokines and chemokine receptors have an important role in this process. This article will provide an overview on how these molecules regulate T cell and macrophage trafficking into the CNS of MHV-infected mice and illustrate the delicate balance that exists with regards to expression of chemokines and their receptors as it relates to both host defense and disease development.     

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.     

Involvement of CD44 in leukocyte trafficking at the blood-retinal barrier

In the present study, we investigated the involvement of CD44 in leukocyte trafficking in vivo at the blood-retinal barrier using experimental autoimmune uveoretinitis (EAU) as a model system. Leukocyte trafficking was evaluated using adoptive transfer of calcein-AM (C-AM)-labeled spleen cells harvested from syngeneic mice at prepeak severity of EAU to mice at a similar stage of disease. CD44 and its ligand hyaluronan were up-regulated in the eye during EAU. CD44-positive leukocytes were found sticking in the retinal venules and postcapillary venules but not in the retinal arterioles nor in mesenteric vessels. Preincubation of in vitro C-AM-labeled leukocytes with anti-CD44 monoclonal antibodies (mAb; IM7) or high molecular weight hyaluronic acid (HA) before transfer significantly suppressed leukocyte rolling but not sticking in retinal venules and also reduced cell infiltration in the retinal parenchyma. Administration of the HA-specific enzyme hyaluronidase to mice before cell transfer also reduced leukocyte infiltration, suggesting that CD44-HA interactions are involved in leukocyte recruitment in EAU. This was further supported by the observation that disease severity was reduced by administration of anti-CD44 mAb (IM7) at the early leukocyte-infiltration stage. Further studies also indicated that CD44 activation was associated with increased levels of apoptosis, and this may also be in part responsible for the reduction in disease severity. These findings demonstrate that CD44 is directly involved in leukocyte-endothelial interaction in vivo and influence the trafficking of primed leukocytes to the retina and their overall survival.     

Two-photon microscopy analysis of leukocyte trafficking and motility

During the last several years, live tissue imaging, in particular using two-photon laser microscopy, has advanced our understanding of leukocyte trafficking mechanisms. Studies using this technique are revealing distinct molecular requirements for leukocyte migration in different tissue environments. Also emerging from the studies are the ingenious infrastructures for leukocyte trafficking, which are produced by stromal cells. This review summarizes the recent imaging studies that provided novel mechanistic insights into in vivo leukocyte migration essential for immunosurveillance.     

Phospholipase D1 mediates lymphocyte adhesion and migration in experimental autoimmune encephalomyelitis

Lymphocyte adhesion and subsequent trafficking across endothelial barriers are essential steps in various immune‐mediated disorders of the CNS, including MS. The molecular mechanisms underlying these processes, however, are still unknown. Phospholipase D1 (PLD1), an enzyme that generates phosphatidic acid through hydrolysis of phosphatidylcholine and additionally yields choline as a product, has been described as regulator of the cell mobility. By using PLD1‐deficient mice, we investigated the functional significance of PLD1 for lymphocyte adhesion and migration in vitro and after myelin oligodendrocyte glycoprotein (MOG)_35–55‐induced EAE, a model of human MS. The lack of PLD1 reduced chemokine‐mediated static adhesion of lymphocytes to the endothelial adhesion molecules vascular cell adhesion molecule 1 (VCAM‐1) and intercellular adhesion molecule 1 (ICAM‐1) in vitro, and was accompanied by a decreased migratory capacity in both blood brain barrier and cell migration models. Importantly, PLD1 is also relevant for the recruitment of immune cells into the CNS in vivo since disease severity after EAE was significantly attenuated in PLD1‐deficient mice. Furthermore, PLD1 expression could be detected on lymphocytes in MS patients. Our findings suggest a critical function of PLD1‐dependent intracellular signaling cascades in regulating lymphocyte trafficking during autoimmune CNS inflammation.     

The alpha(1,3)fucosyltransferases FucT-IV and FucT-VII exert collaborative control over selectin-dependent leukocyte recruitment and lymphocyte homing

E-, P-, and L-selectin counterreceptor activities, leukocyte trafficking, and lymphocyte homing are controlled prominently but incompletely by alpha(1,3)fucosyltransferase FucT-VII-dependent fucosylation. Molecular determinants for FucT-VII-independent leukocyte trafficking are not defined, and evidence for contributions by or requirements for other FucTs in leukocyte recruitment is contradictory and incomplete. We show here that inflammation-dependent leukocyte recruitment retained in FucT-VII deficiency is extinguished in FucT-IV(-/-)/FucT-VII(-/-) mice. Double deficiency yields an extreme leukocytosis characterized by decreased neutrophil turnover and increased neutrophil production. FucT-IV also contributes to HEV-born L-selectin ligands, since lymphocyte homing retained in FucT-VII(-/-) mice is revoked in FucT-IV(-/-)/FucT-VII(-/-) mice. These observations reveal essential FucT-IV-dependent contributions to E-, P-, and L-selectin ligand synthesis and to the control of leukocyte recruitment and lymphocyte homing.     

CXCR3 blockade inhibits T-cell migration into the CNS during EAE and prevents development of adoptively transferred, but not actively induced, disease

Autoreactive T-cell infiltration into the CNS is critical in MS and EAE. The chemokine receptor CXCR3 and its ligands are implicated in MS and mouse EAE, but the contribution of CXCR3 to T-cell migration into the inflamed CNS remains controversial. During active disease in a rat EAE model, blood T-cell, spleen T-cell and T lymphoblast migration into the CNS was inhibited by a CXCR3 blocking mAb by, 30-70%, ～75% and 50-80%, respectively. However, CXCR3 blockade after active immunization did not inhibit EAE, did not alter total T-cell accumulation in the CNS and did not affect Treg accumulation or the presence of cells producing IFN-γ or IL-17. Conversely, CXCR3 blockade during EAE induced by adoptive transfer of myelin basic protein-activated T cells delayed disease onset, shortened its duration and reduced disease severity. Moreover, CXCR3 blockade inhibited leukocyte infiltration of the CNS>95%, virtually abolishing infiltration of transferred T cells. Thus, CXCR3 plays a major role in T-cell migration to the CNS and can be critical for encephalitogenic T-cell migration into the CNS to induce disease, but CXCR3-independent recruitment can also produce EAE.     

JAM-related proteins in mucosal homeostasis and inflammation

Mucosal surfaces are lined by epithelial cells that form a physical barrier protecting the body against external noxious substances and pathogens. At a molecular level, the mucosal barrier is regulated by tight junctions (TJs) that seal the paracellular space between adjacent epithelial cells. Transmembrane proteins within TJs include junctional adhesion molecules (JAMs) that belong to the cortical thymocyte marker for Xenopus family of proteins. JAM family encompasses three classical members (JAM-A, JAM-B, and JAM-C) and related molecules including JAM4, JAM-like protein, Coxsackie and adenovirus receptor (CAR), CAR-like membrane protein and endothelial cell-selective adhesion molecule. JAMs have multiple functions that include regulation of endothelial and epithelial paracellular permeability, leukocyte recruitment during inflammation, angiogenesis, cell migration, and proliferation. In this review, we summarize the current knowledge regarding the roles of the JAM family members in the regulation of mucosal homeostasis and leukocyte trafficking with a particular emphasis on barrier function and its perturbation during pathological inflammation.     

The role of chemokines in the recruitment of lymphocytes to the liver

Chemokines direct leukocyte trafficking and positioning within tissues. They thus play critical roles in regulating immune responses and inflammation. The chemokine system is complex involving interactions between multiple chemokines and their receptors that operate in combinatorial cascades with adhesion molecules. The involvement of multiple chemokines and chemokine receptors in these processes brings flexibility and specificity to recruitment. The hepatic vascular bed is a unique low flow environment through which leukocyte are recruited to the liver during homeostatic immune surveillance and in response to infection or injury. The rate of leukocyte recruitment and the nature of cells recruited through the sinusoids in response to inflammatory signals will shape the severity of disease. At one end of the spectrum fulminant liver failure results from a rapid recruitment of leukocytes that leads to hepatocyte destruction and liver failure at the other diseases such as chronic hepatitis C infection may progress over many years from hepatitis to fibrosis and cirrhosis. Chronic hepatitis is charactezised by a T lymphocyte rich infiltrate and the nature and outcome of hepatitis will depend on the T cell subsets recruited, their activation and function within the liver. Different subsets of effector T cells have been described based on their secretion of cytokines and specific functions. These include Th1 and Th2 cells and more recently Th17 and Th9 cells which are associated with different types of immune response and which express distinct patterns of chemokine receptors that promote their recruitment under particular conditions. The effector function of these cells is balanced by the recruitment of regulatory T cells that are able to suppress antigen-specific effectors to allow resolution of immune responses and restoration of immune homeostasis. Understanding the signals that are responsible for recruiting different lymphocyte subsets to the liver will elucidate disease pathogenesis and open up new therapeutic approaches to modulate recruitment in favour of resolution rather than injury.     

Chemokine expression during the development and resolution of a pulmonary leukocyte response to influenza A virus infection in mice

Influenza A virus replicates in the respiratory epithelium and induces an inflammatory infiltrate comprised of mononuclear cells and neutrophils. To understand the development of the cell-mediated immune response to influenza and how leukocyte trafficking to sites of inflammation is regulated, we examined the chemokine expression pattern in lung tissue from A/PR/8/34-infected C57BL/6 mice using an RNase protection assay. Monocyte chemoattractant protein 1, macrophage inflammatory protein 1alpha (MIP-1alpha), MIP-1beta, MIP-3alpha, regulated on activation, normal T expressed and secreted (RANTES), MIP-2, and interferon-inducible protein 10 (IP-10) mRNA expression was up-regulated between days 5 and 15 after infection, consistent with a role for these chemokines in leukocyte recruitment to the lung. Low levels of expression were detected for the CC chemokine receptors (CCR)2 and CCR5, whereas CXC chemokine receptor (CXCR)3 was significantly up-regulated by day 10 after infection, coinciding with peak inflammatory cell infiltration in the airways. As RANTES, IP-10, and their receptors were up-regulated during influenza virus infection, we investigated leukocyte recruitment and viral clearance in mice deficient in RANTES or CXCR3, the receptor for IP-10. Leukocyte recruitment and viral replication in influenza-infected RANTES knockout(-/-) mice were similar to that in control mice, showing that RANTES is not essential for the immune response to influenza infection. Similarly, leukocyte recruitment and viral replication in CXCR3-/- mice were identical to control mice, except at day 8 postinfection, where fewer lymphocytes, neutrophils, and eosinophils were detected in the bronchoalveolar lavage of CXCR3-/- mice. These studies suggest that although the chemokines detected may play a role in regulating leukocyte trafficking to the lung during influenza infection, some may be functionally redundant.     

The blood-brain-barrier in multiple sclerosis: functional roles and therapeutic targeting

In most regions of the central nervous system (CNS), the composition of the neuronal microenvironment is maintained by virtue of particular blood-brain-barrier (BBB) characteristics, to which vascular endothelial cells (ECs) contribute an important role. Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS, characterized at tissue level by multifocal perivascular infiltrates, predominantly of lymphocytes and macrophages. Thus, lymphocyte recruitment into the brain across ECs of the BBB represents a critical event in disease pathogenesis, which is highly restricted and carefully regulated. In recent years, different investigations have identified the crucial components involved in leukocyte migration, providing new insights into mechanisms modulating neuroinflammatory reactions. In this review, several topics relating to these events are discussed, namely: (1) cellular and molecular characteristics of the BBB regulating permeability, as well as signals inducing EC differentiation in the brain and specific cell properties; (2) pathogenic mechanisms guiding the migration of different leukocyte populations through the BBB in MS; and (3) current knowledge on how different MS therapies targeting leukocytes migration across the BBB function. Furthermore, because the BBB has proven to be an important retaining wall preventing drug passage into the CNS, novel strategies directed at successful delivery of large molecules for effective treatment of various inflammatory conditions of the brain, both currently available or still under development, are discussed.     

The blood–brain-barrier in multiple sclerosis: Functional roles and therapeutic targeting

In most regions of the central nervous system (CNS), the composition of the neuronal microenvironment is maintained by virtue of particular blood–brain-barrier (BBB) characteristics, to which vascular endothelial cells (ECs) contribute an important role. Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS, characterized at tissue level by multifocal perivascular infiltrates, predominantly of lymphocytes and macrophages. Thus, lymphocyte recruitment into the brain across ECs of the BBB represents a critical event in disease pathogenesis, which is highly restricted and carefully regulated. In recent years, different investigations have identified the crucial components involved in leukocyte migration, providing new insights into mechanisms modulating neuroinflammatory reactions. In this review, several topics relating to these events are discussed, namely: (1) cellular and molecular characteristics of the BBB regulating permeability, as well as signals inducing EC differentiation in the brain and specific cell properties; (2) pathogenic mechanisms guiding the migration of different leukocyte populations through the BBB in MS; and (3) current knowledge on how different MS therapies targeting leukocytes migration across the BBB function. Furthermore, because the BBB has proven to be an important retaining wall preventing drug passage into the CNS, novel strategies directed at successful delivery of large molecules for effective treatment of various inflammatory conditions of the brain, both currently available or still under development, are discussed.     

Chemokines in and out of the central nervous system: much more than chemotaxis and inflammation

Actions of chemokines and the interaction with specific receptors go beyond their original, defined role of recruiting leukocytes to inflamed tissues. Chemokine receptor expression in peripheral elements and resident cells of the central nervous system (CNS) represents a relevant communication system during neuroinflammatory conditions. The following examples are described in this review: Chemokine receptors play important homeostatic properties by regulating levels of specific ligands in blood and tissues during healthy and pathological conditions; chemokines and their receptors are clearly involved in leukocyte extravasation and recruitment to the CNS, and current studies are directed toward understanding the interaction between chemokine receptors and matrix metalloproteinases in the process of blood brain barrier breakdown. We also propose novel functions of chemokine receptors during demyelination/remyelination, and developmental processes.     

Activated leukocyte cell adhesion molecule promotes leukocyte trafficking into the central nervous system

Adhesion molecules of the immunoglobulin superfamily are crucial effectors of leukocyte trafficking into the central nervous system. Using a lipid raft-based proteomic approach, we identified ALCAM as an adhesion molecule involved in leukocyte migration across the blood-brain barrier (BBB). ALCAM expressed on BBB endothelium localized together with CD6 on leukocytes and with BBB endothelium transmigratory cups. ALCAM expression on BBB cells was upregulated in active multiple sclerosis and experimental autoimmune encephalomyelitis lesions. Moreover, ALCAM blockade restricted the transmigration of CD4+ lymphocytes and monocytes across BBB endothelium in vitro and in vivo and reduced the severity and delayed the time of onset of experimental autoimmune encephalomyelitis. Our findings indicate an important function for ALCAM in the recruitment of leukocytes into the brain and identify ALCAM as a potential target for the therapeutic dampening of neuroinflammation.