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

VE-cadherin in arachnoid and pia mater cells serves as a suitable landmark for in vivo imaging of CNS immune surveillance and inflammation

Meninges cover the surface of the brain and spinal cord and contribute to protection and immune surveillance of the central nervous system (CNS). How the meningeal layers establish CNS compartments with different accessibility to immune cells and immune mediators is, however, not well understood. Here, using 2-photon imaging in female transgenic reporter mice, we describe VE-cadherin at intercellular junctions of arachnoid and pia mater cells that form the leptomeninges and border the subarachnoid space (SAS) filled with cerebrospinal fluid (CSF). VE-cadherin expression also marked a layer of Prox1+ cells located within the arachnoid beneath and separate from E-cadherin+ arachnoid barrier cells. In vivo imaging of the spinal cord and brain in female VE-cadherin-GFP reporter mice allowed for direct observation of accessibility of CSF derived tracers and T cells into the SAS bordered by the arachnoid and pia mater during health and neuroinflammation, and detection of volume changes of the SAS during CNS pathology. Together, the findings identified VE-cadherin as an informative landmark for in vivo imaging of the leptomeninges that can be used to visualize the borders of the SAS and thus potential barrier properties of the leptomeninges in controlling access of immune mediators and immune cells into the CNS during health and neuroinflammation.

The interplay between T helper cells and brain barriers in the pathogenesis of multiple sclerosis

The blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) represent two complex structures protecting the central nervous system (CNS) against potentially harmful agents and circulating immune cells. The immunosurveillance of the CNS is governed by immune cells that constantly patrol the BCSFB, whereas during neuroinflammatory disorders, both BBB and BCSFB undergo morphological and functional alterations, promoting leukocyte intravascular adhesion and transmigration from the blood circulation into the CNS. Multiple sclerosis (MS) is the prototype of neuroinflammatory disorders in which peripheral T helper (Th) lymphocytes, particularly Th1 and Th17 cells, infiltrate the CNS and contribute to demyelination and neurodegeneration. Th1 and Th17 cells are considered key players in the pathogenesis of MS and its animal model, experimental autoimmune encephalomyelitis. They can actively interact with CNS borders by complex adhesion mechanisms and secretion of a variety of molecules contributing to barrier dysfunction. In this review, we describe the molecular basis involved in the interactions between Th cells and CNS barriers and discuss the emerging roles of dura mater and arachnoid layer as neuroimmune interfaces contributing to the development of CNS inflammatory diseases.

ACKR1 favors transcellular over paracellular T-cell diapedesis across the blood-brain barrier in neuroinflammation in vitro

The migration of CD4+ effector/memory T cells across the blood-brain barrier (BBB) is a critical step in MS or its animal model, EAE. T-cell diapedesis across the BBB can occur paracellular, via the complex BBB tight junctions or transcellular via a pore through the brain endothelial cell body. Making use of primary mouse brain microvascular endothelial cells (pMBMECs) as in vitro model of the BBB, we here directly compared the transcriptome profile of pMBMECs favoring transcellular or paracellular T-cell diapedesis by RNA sequencing (RNA-seq). We identified the atypical chemokine receptor 1 (Ackr1) as one of the main candidate genes upregulated in pMBMECs favoring transcellular T-cell diapedesis. We confirmed upregulation of ACKR1 protein in pMBMECs promoting transcellular T-cell diapedesis and in venular endothelial cells in the CNS during EAE. Lack of endothelial ACKR1 reduced transcellular T-cell diapedesis across pMBMECs under physiological flow in vitro. Combining our previous observation that endothelial ACKR1 contributes to EAE pathogenesis by shuttling chemokines across the BBB, the present data support that ACKR1 mediated chemokine shuttling enhances transcellular T-cell diapedesis across the BBB during autoimmune neuroinflammation.

Intercellular Adhesion Molecule-1 (ICAM-1) and ICAM-2 Differentially Contribute to Peripheral Activation and CNS Entry of Autoaggressive Th1 and Th17 Cells in Experimental Autoimmune Encephalomyelitis

In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), myelin-specific T cells are activated in the periphery and differentiate in T helper (Th) 1 and Th17 effector cells, which cross the blood-brain barrier (BBB) to reach the central nervous system (CNS), where they induce neuroinflammation. Here, we explored the role of intercellular adhesion molecule-1 (ICAM-1) and ICAM-2 in the activation of naïve myelin-specific T cells and in the subsequent migration of differentiated encephalitogenic Th1 and Th17 cells across the BBB in vitro and in vivo. While on antigen-presenting cells ICAM-1, but not ICAM-2 was required for the activation of naïve CD4⁺ T cells, endothelial ICAM-1 and ICAM-2 mediated both Th1 and Th17 cell migration across the BBB. ICAM-1/-2-deficient mice developed ameliorated typical and atypical EAE transferred by encephalitogenic Th1 and Th17 cells, respectively. Our study underscores important yet cell-specific contributions for ICAM-1 and ICAM-2 in EAE pathogenesis.

Claudin-12 is not required for blood-brain barrier tight junction function

BACKGROUND

The blood-brain barrier (BBB) ensures central nervous system (CNS) homeostasis by strictly controlling the passage of molecules and solutes from the bloodstream into the CNS. Complex and continuous tight junctions (TJs) between brain endothelial cells block uncontrolled paracellular diffusion of molecules across the BBB, with claudin-5 being its dominant TJs protein. However, claudin-5 deficient mice still display ultrastructurally normal TJs, suggesting the contribution of other claudins or tight-junction associated proteins in establishing BBB junctional complexes. Expression of claudin-12 at the BBB has been reported, however the exact function and subcellular localization of this atypical claudin remains unknown.

METHODS

We created claudin-12-lacZ-knock-in C57BL/6J mice to explore expression of claudin-12 and its role in establishing BBB TJs function during health and neuroinflammation. We furthermore performed a broad standardized phenotypic check-up of the mouse mutant.

RESULTS

Making use of the lacZ reporter allele, we found claudin-12 to be broadly expressed in numerous organs. In the CNS, expression of claudin-12 was detected in many cell types with very low expression in brain endothelium. Claudin-12^lacZ/lacZ C57BL/6J mice lacking claudin-12 expression displayed an intact BBB and did not show any signs of BBB dysfunction or aggravated neuroinflammation in an animal model for multiple sclerosis. Determining the precise localization of claudin-12 at the BBB was prohibited by the fact that available anti-claudin-12 antibodies showed comparable detection and staining patterns in tissues from wild-type and claudin-12^lacZ/lacZ C57BL/6J mice.

CONCLUSIONS

Our present study thus shows that claudin-12 is not essential in establishing or maintaining BBB TJs integrity. Claudin-12 is rather expressed in cells that typically lack TJs suggesting that claudin-12 plays a role other than forming classical TJs. At the same time, in depth phenotypic screening of clinically relevant organ functions of claudin-12^lacZ/lacZ C57BL/6J mice suggested the involvement of claudin-12 in some neurological but, more prominently, in cardiovascular functions.

Sialomucin CD43 regulates T helper type 17 cell intercellular adhesion molecule 1 dependent adhesion, apical migration and transendothelial migration

T helper type 17 lymphocytes (Th17 cells) infiltrate the central nervous system (CNS), induce inflammation and demyelination and play a pivotal role in the pathogenesis of multiple sclerosis. Sialomucin CD43 is highly expressed in Th17 cells and mediates adhesion to endothelial selectin (E-selectin), an initiating step in Th17 cell recruitment to sites of inflammation. CD43^-/- mice have impaired Th17 cell recruitment to the CNS and are protected from experimental autoimmune encephalomyelitis (EAE), the mouse model of multiple sclerosis. However, E-selectin is dispensable for the development of EAE, in contrast to intercellular and vascular cell adhesion molecules (ICAM-1 and VCAM-1). We report that CD43^-/- mice have decreased demyelination and T-cell infiltration, but similar up-regulation of ICAM-1 and VCAM-1 in the spinal cord, compared with wild-type (WT) mice, at the initiation of EAE. CD43^-/- Th17 cells have impaired adhesion to ICAM-1 under flow conditions in vitro, despite having similar expression of LFA-1, the main T-cell ligand for ICAM-1, as WT Th17 cells. Regardless of the route of integrin activation, CD43^-/- Th17 cell firm arrest on ICAM-1 was comparable to that of WT Th17 cells, but CD43^-/- Th17 cells failed to optimally apically migrate on immobilized ICAM-1-coated coverslips and endothelial cells, and to transmigrate under shear flow conditions in an ICAM-1-dependent manner. Collectively, these findings unveil novel roles for CD43, facilitating adhesion of Th17 cells to ICAM-1 and modulating apical and transendothelial migration, as mechanisms potentially responsible for Th17 cell recruitment to sites of inflammation such as the CNS.

Claudin-3-deficient C57BL/6J mice display intact brain barriers

The tight junction protein claudin-3 has been identified as a transcriptional target of the Wnt/β-catenin signaling pathway regulating blood-brain barrier (BBB) maturation. In neurological disorders loss of claudin-3 immunostaining is observed at the compromised BBB and blood-cerebrospinal fluid barrier (BCSFB). Although these observations support a central role of claudin-3 in regulating brain barriers’ tight junction integrity, expression of claudin-3 at the brain barriers has remained a matter of debate. This prompted us to establish claudin-3^−/− C57BL/6J mice to study the role of claudin-3 in brain barrier integrity in health and neuroinflammation. Bulk and single cell RNA sequencing and direct comparative qRT-PCR analysis of brain microvascular samples from WT and claudin-3^−/− mice show beyond doubt that brain endothelial cells do not express claudin-3 mRNA. Detection of claudin-3 protein at the BBB in vivo and in vitro is rather due to junctional reactivity of anti-claudin-3 antibodies to an unknown antigen still detected in claudin-3^−/− brain endothelium. We confirm expression and junctional localization of claudin-3 at the BCSFB of the choroid plexus. Our study clarifies that claudin-3 is not expressed at the BBB and shows that absence of claudin-3 does not impair brain barrier function during health and neuroinflammation in C57BL/6J mice.

Lack of junctional adhesion molecule (JAM)-B ameliorates experimental autoimmune encephalomyelitis

In multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE) autoaggressive CD4⁺ T cells cross the blood-brain barrier (BBB) and cause neuroinflammation. Therapeutic targeting of CD4⁺ T-cell trafficking into the CNS by blocking α4-integrins has proven beneficial for the treatment of MS but comes with associated risks, probably due to blocking CD8⁺ T cell mediated CNS immune surveillance. Our recent observations show that CD8⁺ T cells also rely on α4β1-integrins to cross the BBB. Besides vascular cell adhesion molecule-1 (VCAM-1), we identified junctional adhesion molecule-B (JAM-B) as a novel vascular α4β1-integrin ligand involved in CD8⁺ T-cell migration across the BBB. This prompted us to investigate, if JAM-B also mediates CD4⁺ T-cell migration across the BBB. We first ensured that encephalitogenic T cells can bind to JAM-B in vitro and next compared EAE pathogenesis in JAM-B^-/- C57BL/6J mice and their wild-type littermates. Following immunization with MOG_aa35-55 peptide, JAM-B^-/- mice developed ameliorated EAE compared to their wild-type littermates. At the same time, we isolated higher numbers of CD45⁺ infiltrating immune cells from the CNS of JAM-B^-/- C57BL/6J mice suffering from EAE. Immunofluorescence staining revealed that the majority of CD45⁺ inflammatory cells accumulated in the leptomeningeal and perivascular spaces of the CNS behind the BBB but do not gain access to the CNS parenchyma. Trapping of CNS inflammatory cells was not due to increased inflammatory cell proliferation. Neither a loss of BBB integrity or BBB polarity potentially affecting local chemokine gradients nor a lack of focal gelatinase activation required for CNS parenchymal immune cell entry across the glia limitans could be detected in JAM-B^-/- mice. Lack of a role for JAM-B in the effector phase of EAE was supported by the observation that we did not detect any role for JAM-B in EAE pathogenesis, when EAE was elicited by in vitro activated MOG _aa35-55-specific CD4⁺ effector T cells. On the other hand, we also failed to demonstrate any role of JAM-B in in vivo priming, proliferation or polarization of MOG_aa35-55-specific CD4⁺ T cells in peripheral immune organs. Finally, our study excludes expression of and thus a role for JAM-B on peripheral and CNS infiltrating myeloid cells. Taken together, although endothelial JAM-B is not required for immune cell trafficking across the BBB in EAE, in its absence accumulation of inflammatory cells mainly in CNS leptomeningeal spaces leads to amelioration of EAE.

P-selectin glycoprotein ligand-1 in T cells

PURPOSE OF REVIEW

We review P-selectin glycoprotein ligand-1 (PSGL-1) as a selectin and chemokine-binding adhesion molecule. PSGL-1 is widely studied in neutrophils. Here, we focus on T cells, because PSGL-1 was recently described as a major immunomodulatory molecule during viral infection. PSGL-1 also plays a crucial role in T-cell homeostasis by binding to lymphoid chemokines, and can induce tolerance by enhancing the functions of regulatory T cells.

RECENT FINDINGS

PSGL-1 was originally described as a leukocyte ligand for P-selectin, but it is actually a ligand for all selectins (P-, L- and E-selectin), binds chemokines, activates integrins and profoundly affects T-cell biology. It has been shown recently that PSGL-1 can modulate T cells during viral infection by acting as a negative regulator for T-cell functions. Absence of PSGL-1 promotes effector CD4 and CD8 T-cell differentiation and prevents T-cell exhaustion. Consistent with this, tumor growth was significantly reduced in PSGL-1-deficient mice because of an enhanced number of effector T cells together with reduced levels of inhibitory receptors that induce T-cell exhaustion.

SUMMARY

PSGL-1 is the best-studied selectin ligand and has become a posterchild of versatility in leukocyte adhesion, inflammation and immunology. The direct involvement of PSGL-1 in T-cell biology suggests that it might be a drug target. Indeed, PSGL-1 has been tested in some clinical trials and recently, PSGL-1 blockers were proposed as a potential cotherapy in cancer immunotherapy.

A Novel Cervical Spinal Cord Window Preparation Allows for Two-Photon Imaging of T-Cell Interactions with the Cervical Spinal Cord Microvasculature during Experimental Autoimmune Encephalomyelitis

T-cell migration across the blood-brain barrier (BBB) is a crucial step in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Two-photon intravital microscopy (2P-IVM) has been established as a powerful tool to study cell-cell interactions in inflammatory EAE lesions in living animals. In EAE, central nervous system inflammation is strongly pronounced in the spinal cord, an organ in which 2P-IVM imaging is technically very challenging and has been limited to the lumbar spinal cord. Here, we describe a novel spinal cord window preparation allowing to use 2P-IVM to image immune cell interactions with the cervical spinal cord microvascular endothelium during EAE. We describe differences in the angioarchitecture of the cervical spinal cord versus the lumbar spinal cord, which will entail different hemodynamic parameters in these different vascular beds. Using T cells as an example, we demonstrate the suitability of this novel methodology in imaging the post-arrest multistep T-cell extravasation across the cervical spinal cord microvessels. The novel methodology includes an outlook to the analysis of the cellular pathway of T-cell diapedesis across the BBB by establishing visualization of endothelial junctions in this vascular bed.

Postarrest stalling rather than crawling favors CD8(+) over CD4(+) T-cell migration across the blood-brain barrier under flow in vitro

Although CD8⁺ T cells have been implied in the pathogenesis of multiple sclerosis (MS), the molecular mechanisms mediating CD8⁺ T‐cell migration across the blood–brain barrier (BBB) into the central nervous system (CNS) are ill defined. Using in vitro live cell imaging, we directly compared the multistep extravasation of activated CD4⁺ and CD8⁺ T cells across primary mouse brain microvascular endothelial cells (pMBMECs) as a model for the BBB under physiological flow. Significantly higher numbers of CD8⁺ than CD4⁺ T cells arrested on pMBMECs under noninflammatory and inflammatory conditions. While CD4⁺ T cells polarized and crawled prior to their diapedesis, the majority of CD8⁺ T cells stalled and readily crossed the pMBMEC monolayer preferentially via a transcellular route. T‐cell arrest and crawling were independent of G‐protein‐coupled receptor signaling. Rather, absence of endothelial ICAM‐1 and ICAM‐2 abolished increased arrest of CD8⁺ over CD4⁺ T cells and abrogated T‐cell crawling, leading to the efficient reduction of CD4⁺, but to a lesser degree of CD8⁺, T‐cell diapedesis across ICAM‐1^null/ICAM‐2^−/− pMBMECs. Thus, cellular and molecular mechanisms mediating the multistep extravasation of activated CD8⁺ T cells across the BBB are distinguishable from those involved for CD4⁺ T cells.

PSGL-1 Is an Immune Checkpoint Regulator that Promotes T Cell Exhaustion

Chronic viruses and cancers thwart immune responses in humans by inducing T cell dysfunction. Using a murine chronic virus that models human infections, we investigated the function of the adhesion molecule, P-selectin glycoprotein ligand-1 (PSGL-1), that is upregulated on responding T cells. PSGL-1-deficient mice cleared the virus due to increased intrinsic survival of multifunctional effector T cells that had downregulated PD-1 as well as other inhibitory receptors. Notably, this response resulted in CD4(+)-T-cell-dependent immunopathology. Mechanistically, PSGL-1 ligation on exhausted CD8(+) T cells inhibited T cell receptor (TCR) and interleukin-2 (IL-2) signaling and upregulated PD-1, leading to diminished survival with TCR stimulation. In models of melanoma cancer in which T cell dysfunction occurs, PSGL-1 deficiency led to PD-1 downregulation, improved T cell responses, and tumor control. Thus, PSGL-1 plays a fundamental role in balancing viral control and immunopathology and also functions to regulate T cell responses in the tumor microenvironment.

Neuropathologic, phenotypic and functional analyses of Mucosal Associated Invariant T cells in Multiple Sclerosis

BACKGROUND

The involvement of Mucosal Associated Invariant T (MAIT) cells, which are anti-microbial semi-invariant T cells, remains elusive in Multiple Sclerosis (MS).

OBJECTIVE

Deciphering the potential involvement of MAIT cells in the MS inflammatory process.

METHODS

By flow cytometry, blood MAIT cells from similar cohorts of MS patients and healthy volunteers (HV) were compared for frequency, phenotype, activation potential after in vitro TCR engagement by bacterial ligands and transmigration abilities through an in vitro model of blood-brain barrier. MS CNS samples were also studied by immunofluorescent staining and quantitative PCR.

RESULTS AND CONCLUSION

Blood MAIT cells from relapsing-remitting MS patients and HV presented similar frequency, ex vivo effector phenotype and activation abilities. MAIT cells represented 0.5% of the total infiltrating T cells on 39 MS CNS lesions. This is low as compared to blood frequency (p<0.001), but consistent with their low transmigration rate. Finally, transcriptional over-expression of MR1 - which presents cognate antigens to MAIT cells - and of the activating cytokines IL-18 and IL-23 was evidenced in MS lesions, suggesting that the CNS microenvironment is suited to activate the few infiltrating MAIT cells. Taken together, these data place MAIT cells from MS patients as minor components of the inflammatory pathological process.

Cell surface levels of endothelial ICAM-1 influence the transcellular or paracellular T-cell diapedesis across the blood-brain barrier

The extravasation of CD4(+) effector/memory T cells (TEM cells) across the blood-brain barrier (BBB) is a crucial step in the pathogenesis of experimental autoimmune encephalomyelitis (EAE) or multiple sclerosis (MS). Endothelial ICAM-1 and ICAM-2 are essential for CD4(+) TEM cell crawling on the BBB prior to diapedesis. Here, we investigated the influence of cell surface levels of endothelial ICAM-1 in determining the cellular route of CD4(+) TEM -cell diapedesis across cytokine treated primary mouse BBB endothelial cells under physiological flow. Inflammatory conditions, inducing high levels of endothelial ICAM-1, promoted rapid initiation of transcellular diapedesis of CD4(+) T cells across the BBB, while intermediate levels of endothelial ICAM-1 favored paracellular CD4(+) T-cell diapedesis. Importantly, the route of T-cell diapedesis across the BBB was independent of loss of BBB barrier properties. Unexpectedly, a low number of CD4(+) TEM cells was found to cross the inflamed BBB in the absence of endothelial ICAM-1 and ICAM-2 via an obviously alternatively regulated transcellular pathway. In vivo, this translated to the development of ameliorated EAE in ICAM-1(null) //ICAM-2(-/-) C57BL/6J mice. Taken together, our study demonstrates that cell surface levels of endothelial ICAM-1 rather than the inflammatory stimulus or BBB integrity influence the pathway of T-cell diapedesis across the BBB.

PSGL-1 and E/P-selectins are essential for T-cell rolling in inflamed CNS microvessels but dispensable for initiation of EAE

T-cell migration across the blood-brain barrier is a crucial step in the pathogenesis of EAE, an animal model for MS. Live cell imaging studies demonstrated that P-selectin glycoprotein ligand-1 (PSGL-1) and its endothelial ligands E- and P-selectin mediate the initial rolling of T cells in brain vessels during EAE. As functional absence of PSGL-1 or E/P-selectins does not result in ameliorated EAE, we speculated that T-cell entry into the spinal cord is independent of PSGL-1 and E/P-selectin. Performing intravital microscopy, we observed the interaction of WT or PSGL-1(-/-) proteolipid protein-specific T cells in inflamed spinal cord microvessels of WT or E/P-selectin(-/-) SJL/J mice during EAE. T-cell rolling but not T-cell capture was completely abrogated in the absence of either PSGL-1 or E- and P-selectin, resulting in a significantly reduced number of T cells able to firmly adhere in the inflamed spinal cord microvessels, but did not lead to reduced T-cell invasion into the CNS parenchyma. Thus, PSGL-1 interaction with E/P-selectin is essential for T-cell rolling in inflamed spinal cord microvessels during EAE. Taken together with previous observations, our findings show that T-cell rolling is not required for successful T-cell entry into the CNS and initiation of EAE.

DARC shuttles inflammatory chemokines across the blood-brain barrier during autoimmune central nervous system inflammation

The Duffy antigen/receptor for chemokines, DARC, belongs to the family of atypical heptahelical chemokine receptors that do not couple to G proteins and therefore fail to transmit conventional intracellular signals. Here we show that during experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, the expression of DARC is upregulated at the blood-brain barrier. These findings are corroborated by the presence of a significantly increased number of subcortical white matter microvessels staining positive for DARC in human multiple sclerosis brains as compared to control tissue. Using an in vitro blood-brain barrier model we demonstrated that endothelial DARC mediates the abluminal to luminal transport of inflammatory chemokines across the blood-brain barrier. An involvement of DARC in experimental autoimmune encephalomyelitis pathogenesis was confirmed by the observed ameliorated experimental autoimmune encephalomyelitis in Darc(-/-) C57BL/6 and SJL mice, as compared to wild-type control littermates. Experimental autoimmune encephalomyelitis studies in bone marrow chimeric Darc(-/-) and wild-type mice revealed that increased plasma levels of inflammatory chemokines in experimental autoimmune encephalomyelitis depended on the presence of erythrocyte DARC. However, fully developed experimental autoimmune encephalomyelitis required the expression of endothelial DARC. Taken together, our data show a role for erythrocyte DARC as a chemokine reservoir and that endothelial DARC contributes to the pathogenesis of experimental autoimmune encephalomyelitis by shuttling chemokines across the blood-brain barrier.

New and emerging immune-targeted drugs for the treatment of multiple sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease with a major inflammatory component that constitutes the most common progressive and disabling neurological condition in young adults. Injectable immunomodulatory medicines such as interferon drugs and glatiramer acetate have dominated the MS market for over the past two decades but this situation is set to change. This is because of: (i) patent expirations, (ii) the introduction of natalizumab, which targets the interaction between leukocytes and the blood-CNS barrier, (iii) the launch of three oral immunomodulatory drugs (fingolimod, dimethyl fumarate and teriflunomide), with another (laquinimod) under regulatory review and (iv) a number of immunomodulatory monoclonal antibodies (alemtuzumab, daclizumab and ocrelizumab) about to enter the market. Current and emerging medicines are reviewed and their impact on people with MS considered.

Selectins and their ligands as potential immunotherapeutic targets in neurological diseases

Selectins are a family of adhesion receptors that bind to highly glycosylated molecules expressed on the surface of leukocytes and endothelial cells. The interactions between selectins and their ligands control tethering and rolling of leukocytes on the vascular wall during the process of leukocyte migration into the tissues under physiological and pathological conditions. In recent years, it has been shown that leukocyte recruitment in the CNS plays a pivotal role in diseases such as multiple sclerosis, ischemic stroke, epilepsy and traumatic brain injury. In this review, we discuss the role of selectins in leukocyte–endothelial interactions in the pathogenesis of neurological diseases, highlighting new findings suggesting that selectins and their ligands may represent novel potential therapeutic targets for the treatment of CNS diseases.

Regulatory T cells suppress the late phase of the immune response in lymph nodes through P-selectin glycoprotein ligand-1

Regulatory T cells (Tregs) maintain tolerance toward self-antigens and suppress autoimmune diseases, although the underlying molecular mechanisms are unclear. In this study, we show that mice deficient for P-selectin glycoprotein ligand-1 (PSGL-1) develop a more severe form of experimental autoimmune encephalomyelitis than wild type animals do, suggesting that PSGL-1 has a role in the negative regulation of autoimmunity. We found that Tregs lacking PSGL-1 were unable to suppress experimental autoimmune encephalomyelitis and failed to inhibit T cell proliferation in vivo in the lymph nodes. Using two-photon laser-scanning microscopy in the lymph node, we found that PSGL-1 expression on Tregs had no role in the suppression of early T cell priming after immunization with Ag. Instead, PSGL-1-deficient Tregs lost the ability to modulate T cell movement and failed to inhibit the T cell-dendritic cell contacts and T cell clustering essential for sustained T cell activation during the late phase of the immune response. Notably, PSGL-1 expression on myelin-specific effector T cells had no role in T cell locomotion in the lymph node. Our data show that PSGL-1 represents a previously unknown, phase-specific mechanism for Treg-mediated suppression of the persistence of immune responses and autoimmunity induction.

Overview of experimental models of the blood-brain barrier in CNS drug discovery

The blood-brain barrier (BBB) is a physical and metabolic entity that isolates the brain from the systemic circulation. The barrier consists of tight junctions between endothelial cells that contain egress transporters and catabolic enzymes. To cross the BBB, a drug must possess the appropriate physicochemical properties to achieve a sufficient time-concentration profile in brain interstitial fluid (ISF). In this overview, we review techniques to measure BBB permeation, which is evidenced by the free concentration of compound in brain ISF over time. We consider a number of measurement techniques, including in vivo microdialysis and brain receptor occupancy following perfusion. Consideration is also given to the endothelial and nonendothelial cell systems used to assess both the BBB permeation of a test compound and its interactions with egress transporters, and computer models employed for predicting passive permeation and the probability of interactions with BBB transporters.

Multiple sclerosis and the blood-central nervous system barrier

The central nervous system (CNS) is isolated from the blood system by a physical barrier that contains efflux transporters and catabolic enzymes. This blood-CNS barrier (BCNSB) plays a pivotal role in the pathophysiology of multiple sclerosis (MS). It binds and anchors activated leukocytes to permit their movement across the BCNSB and into the CNS. Once there, these immune cells target particular self-epitopes and initiate a cascade of neuroinflammation, which leads to the breakdown of the BCNSB and the formation of perivascular plaques, one of the hallmarks of MS. Immunomodulatory drugs for MS are either biologics or small molecules, with only the latter having the capacity to cross the BCNSB and thus have a propensity to cause CNS side effects. However, BCNSB penetration is a desirable feature of MS drugs that have molecular targets within the CNS. These are nabiximols and dalfampridine, which target cannabinoid receptors and potassium channels, respectively. Vascular cell adhesion molecule-1, present on endothelial cells of the BCNSB, also serves as a drug discovery target since it interacts with α4-β1-integrin on leucocytes. The MS drug natalizumab, a humanized monoclonal antibody against α4-β1-integrin, blocks this interaction and thus reduces the movement of immune cells into the CNS. This paper further elaborates on the role of the BCNSB in the pathophysiology and pharmacotherapy of MS.

Genetic mouse models to study blood-brain barrier development and function

The blood–brain barrier (BBB) is a complex physiological structure formed by the blood vessels of the central nervous system (CNS) that tightly regulates the movement of substances between the blood and the neural tissue. Recently, the generation and analysis of different genetic mouse models has allowed for greater understanding of BBB development, how the barrier is regulated during health, and its response to disease. Here we discuss: 1) Genetic mouse models that have been used to study the BBB, 2) Available mouse genetic tools that can aid in the study of the BBB, and 3) Potential tools that if generated could greatly aid in our understanding of the BBB.

Going against the tide--how encephalitogenic T cells breach the blood-brain barrier

During multiple sclerosis or its animal model, experimental autoimmune encephalomyelitis, circulating immune cells enter the central nervous system (CNS) causing neuroinflammation. Extravasation from the blood circulation across the vessel wall occurs through a multistep process regulated by adhesion and signal transducing molecules on the immune cells and on the endothelium. Since the CNS is shielded by the highly specialized blood-brain barrier (BBB), immune cell extravasation into the CNS requires breaching this particularly tight endothelial border. Consequently, travelling into the CNS demands unique adaptations which account for the extreme tightness of the BBB. Modern imaging tools have shown that after arresting on BBB endothelium, in vivo or in vitro encephalitogenic effector/memory T cells crawl for long distances, possibly exceeding 150 µm along the surface of the BBB endothelium before rapidly crossing the BBB. Interestingly, in addition to the distance of crawling, the preferred direction of crawling against the flow is unique for T cell crawling on the luminal surface of CNS microvessels. In this review, we will summarize the cellular and molecular mechanisms involved in the unique T cell behavior that is obviously required for finding a site permissive for diapedesis across the unique vascular bed of the BBB.

Mechanisms of dendritic cell trafficking across the blood-brain barrier

Although the central nervous system (CNS) is considered to be an immunoprivileged site, it is susceptible to a host of autoimmune as well as neuroinflammatory disorders owing to recruitment of immune cells across the blood-brain barrier into perivascular and parenchymal spaces. Dendritic cells (DCs), which are involved in both primary and secondary immune responses, are the most potent immune cells in terms of antigen uptake and processing as well as presentation to T cells. In light of the emerging importance of DC traficking into the CNS, these cells represent good candidates for targeted immunotherapy against various neuroinflammatory diseases. This review focuses on potential physiological events and receptor interactions between DCs and the microvascular endothelial cells of the brain as they transmigrate into the CNS during degeneration and injury. A clear understanding of the underlying mechanisms involved in DC migration may advance the development of new therapies that manipulate these mechanistic properties via pharmacologic intervention. Furthermore, therapeutic validation should be in concurrence with the molecular imaging techniques that can detect migration of these cells in vivo. Since the use of noninvasive methods to image migration of DCs into CNS has barely been explored, we highlighted potential molecular imaging techniques to achieve this goal. Overall, information provided will bring this important leukocyte population to the forefront as key players in the immune cascade in the light of the emerging contribution of DCs to CNS health and disease.

Claudin-1 induced sealing of blood-brain barrier tight junctions ameliorates chronic experimental autoimmune encephalomyelitis

In experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis (MS), loss of the blood-brain barrier (BBB) tight junction (TJ) protein claudin-3 correlates with immune cell infiltration into the CNS and BBB leakiness. Here we show that sealing BBB TJs by ectopic tetracycline-regulated expression of the TJ protein claudin-1 in Tie-2 tTA//TRE-claudin-1 double transgenic C57BL/6 mice had no influence on immune cell trafficking across the BBB during EAE and furthermore did not influence the onset and severity of the first clinical disease episode. However, expression of claudin-1 did significantly reduce BBB leakiness for both blood borne tracers and endogenous plasma proteins specifically around vessels expressing claudin-1. In addition, mice expressing claudin-1 exhibited a reduced disease burden during the chronic phase of EAE as compared to control littermates. Our study identifies BBB TJs as the critical structure regulating BBB permeability but not immune cell trafficking into CNS during EAE, and indicates BBB dysfunction is a potential key event contributing to disease burden in the chronic phase of EAE. Our observations suggest that stabilizing BBB barrier function by therapeutic targeting of TJs may be beneficial in treating MS, especially when anti-inflammatory treatments have failed.

Mechanisms of T-cell migration across the BBB

Under physiological conditions, the highly specialized BBB strictly limits the entrance of immune cells into the CNS. By contrast, in the course of neuroinflammation such as that observed in multiple sclerosis, circulating T cells readily breach the BBB and initiate a cascade of events culminating in disease onset. Lymphocyte extravasation across the BBB occurs through a sequential multistep process, orchestrated by chemokines and cell adhesion molecules that precisely regulate the dynamic interaction of T cells with the endothelial cells forming the BBB. In this article, we will discuss the molecular players triggering the sophisticated process of T-cell migration across the BBB during pathological conditions.

TET inducible expression of the α4β7-integrin ligand MAdCAM-1 on the blood-brain barrier does not influence the immunopathogenesis of experimental autoimmune encephalomyelitis

Inhibiting the α4 subunit of the integrin heterodimers α4β1 and α4β7 with the mab natalizumab is an effective treatment of multiple sclerosis (MS). Which of the two α4 heterodimers is involved in disease pathogenesis has, however, remained controversial. Whereas the development of experimental autoimmune encephalomyelitis (EAE), an animal model of MS, is ameliorated in β7-integrin-deficient C57BL/6 mice, neutralizing antibodies against the β7-integrin subunit or the α4β7-integrin heterodimer fail to interfere with EAE pathogenesis in the SJL mouse. To facilitate α4β7-integrin-mediated immune-cell trafficking across the blood-brain barrier (BBB), we established transgenic C57BL/6 mice with endothelial cell-specific, inducible expression of the α4β7-integrin ligand mucosal addressin cell adhesion molecule (MAdCAM)-1 using the tetracycline (TET)-OFF system. Although TET-regulated MAdCAM-1 induced α4β7-integrin mediated interaction of α4β7(+) /α4β1(-) T cells with the BBB in vitro and in vivo, it failed to influence EAE pathogenesis in C57BL/6 mice. TET-regulated MAdCAM-1 on the BBB neither changed the localization of central nervous system (CNS) perivascular inflammatory cuffs nor did it enhance the percentage of α4β7-integrin(+) inflammatory cells within the CNS during EAE. In conclusion, our study demonstrates that ectopic expression of MAdCAM-1 at the BBB does not increase α4β7-integrin-mediated immune cell trafficking into the CNS during MOG(aa35-55)-induced EAE.

Fluids and barriers of the CNS establish immune privilege by confining immune surveillance to a two-walled castle moat surrounding the CNS castle

Neuronal activity within the central nervous system (CNS) strictly depends on homeostasis and therefore does not tolerate uncontrolled entry of blood components. It has been generally believed that under normal conditions, the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB) prevent immune cell entry into the CNS. This view has recently changed when it was realized that activated T cells are able to breach the BBB and the BCSFB to perform immune surveillance of the CNS. Here we propose that the immune privilege of the CNS is established by the specific morphological architecture of its borders resembling that of a medieval castle. The BBB and the BCSFB serve as the outer walls of the castle, which can be breached by activated immune cells serving as messengers for outside dangers. Having crossed the BBB or the BCSFB they reach the castle moat, namely the cerebrospinal fluid (CSF)-drained leptomeningeal and perivascular spaces of the CNS. Next to the CNS parenchyma, the castle moat is bordered by a second wall, the glia limitans, composed of astrocytic foot processes and a parenchymal basement membrane. Inside the castle, that is the CNS parenchyma proper, the royal family of sensitive neurons resides with their servants, the glial cells. Within the CSF-drained castle moat, macrophages serve as guards collecting all the information from within the castle, which they can present to the immune-surveying T cells. If in their communication with the castle moat macrophages, T cells recognize their specific antigen and see that the royal family is in danger, they will become activated and by opening doors in the outer wall of the castle allow the entry of additional immune cells into the castle moat. From there, immune cells may breach the inner castle wall with the aim to defend the castle inhabitants by eliminating the invading enemy. If the immune response by unknown mechanisms turns against self, that is the castle inhabitants, this may allow for continuous entry of immune cells into the castle and lead to the death of the castle inhabitants, and finally members of the royal family, the neurons. This review will summarize the molecular traffic signals known to allow immune cells to breach the outer and inner walls of the CNS castle moat and will highlight the importance of the CSF-drained castle moat in maintaining immune surveillance and in mounting immune responses in the CNS.

Disruption of central nervous system barriers in multiple sclerosis

The delicate microenvironment of the central nervous system (CNS) is protected by the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCB). These barriers function in distinct CNS compartments and their anatomical basis lay on the junctional proteins present in endothelial cells for the BBB and in the choroidal epithelium for the BCB. During neuroinflammatory conditions like multiple sclerosis (MS) and its murine model experimental autoimmune encephalomyelitis (EAE), activation or damage of the various cellular components of these barriers facilitate leukocyte infiltration leading to oligodendrocyte death, axonal damage, demyelination and lesion development. This manuscript will review in detail the features of these barriers under physiological and pathological conditions, particularly when focal immune activation promotes the loss of the BBB and BCB phenotype, the upregulation of cell adhesion molecules (CAMs) and the recruitment of immune cells.

Alpha4beta1 integrin mediates the recruitment of immature dendritic cells across the blood-brain barrier during experimental autoimmune encephalomyelitis

Dendritic cells (DCs) within the CNS are recognized to play an important role in the effector phase and propagation of the immune response in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis. However, the mechanisms regulating DC trafficking into the CNS still need to be characterized. In this study, we show by performing intravital fluorescence videomicroscopy of the inflamed spinal cord white-matter microvasculature in SJL mice with EAE that immature, and to a lesser extent, LPS-matured, bone marrow-derived DCs efficiently interact with the CNS endothelium by rolling, capturing, and firm adhesion. Immature but not LPS-matured DCs efficiently migrated across the wall of inflamed parenchymal microvessels into the CNS. Blocking alpha4 integrins interfered with the adhesion but not the rolling or capturing of immature and LPS-matured DCs to the CNS microvascular endothelium, inhibiting their migration across the vascular wall. Functional absence of beta1 integrins but not of beta7 integrins or alpha4beta7 integrin similarly reduced the adhesion of immature DCs to the CNS microvascular endothelium, demonstrating that alpha4beta1 but not alpha4beta7 integrin mediates this step of immature DCs interaction with the inflamed blood-brain barrier during EAE. Our study shows that during EAE, especially immature DCs migrate into the CNS, where they may be crucial for the perpetuation of the CNS-targeted autoimmune response. Thus therapeutic targeting of alpha4 integrins affects DC trafficking into the CNS and may therefore lead to the resolution of the CNS autoimmune inflammation by reducing the number of CNS professional APCs.

The blood-brain and the blood-cerebrospinal fluid barriers: function and dysfunction

The central nervous system (CNS) is tightly sealed from the changeable milieu of blood by the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier (BCSFB). While the BBB is considered to be localized at the level of the endothelial cells within CNS microvessels, the BCSFB is established by choroid plexus epithelial cells. The BBB inhibits the free paracellular diffusion of water-soluble molecules by an elaborate network of complex tight junctions (TJs) that interconnects the endothelial cells. Combined with the absence of fenestrae and an extremely low pinocytotic activity, which inhibit transcellular passage of molecules across the barrier, these morphological peculiarities establish the physical permeability barrier of the BBB. In addition, a functional BBB is manifested by a number of permanently active transport mechanisms, specifically expressed by brain capillary endothelial cells that ensure the transport of nutrients into the CNS and exclusion of blood-borne molecules that could be detrimental to the milieu required for neural transmission. Finally, while the endothelial cells constitute the physical and metabolic barrier per se, interactions with adjacent cellular and acellular layers are prerequisites for barrier function. The fully differentiated BBB consists of a complex system comprising the highly specialized endothelial cells and their underlying basement membrane in which a large number of pericytes are embedded, perivascular antigen-presenting cells, and an ensheathment of astrocytic endfeet and associated parenchymal basement membrane. Endothelial cell morphology, biochemistry, and function thus make these brain microvascular endothelial cells unique and distinguishable from all other endothelial cells in the body. Similar to the endothelial barrier, the morphological correlate of the BCSFB is found at the level of unique apical tight junctions between the choroid plexus epithelial cells inhibiting paracellular diffusion of water-soluble molecules across this barrier. Besides its barrier function, choroid plexus epithelial cells have a secretory function and produce the CSF. The barrier and secretory function of the choroid plexus epithelial cells are maintained by the expression of numerous transport systems allowing the directed transport of ions and nutrients into the CSF and the removal of toxic agents out of the CSF. In the event of CNS pathology, barrier characteristics of the blood-CNS barriers are altered, leading to edema formation and recruitment of inflammatory cells into the CNS. In this review we will describe current knowledge on the cellular and molecular basis of the functional and dysfunctional blood-CNS barriers with focus on CNS autoimmune inflammation.

Functions of lipid raft membrane microdomains at the blood-brain barrier

The blood-brain barrier (BBB) is a highly specialized structural and functional component of the central nervous system that separates the circulating blood from the brain and spinal cord parenchyma. Brain endothelial cells (BECs) that primarily constitute the BBB are tightly interconnected by multiprotein complexes, the adherens junctions and the tight junctions, thereby creating a highly restrictive cellular barrier. Lipid-enriched membrane microdomain compartmentalization is an inherent property of BECs and allows for the apicobasal polarity of brain endothelium, temporal and spatial coordination of cell signaling events, and actin remodeling. In this manuscript, we review the role of membrane microdomains, in particular lipid rafts, in the BBB under physiological conditions and during leukocyte transmigration/diapedesis. Furthermore, we propose a classification of endothelial membrane microdomains based on their function, or at least on the function ascribed to the molecules included in such heterogeneous rafts: (1) rafts associated with interendothelial junctions and adhesion of BECs to basal lamina (scaffolding rafts); (2) rafts involved in immune cell adhesion and migration across brain endothelium (adhesion rafts); (3) rafts associated with transendothelial transport of nutrients and ions (transporter rafts).

Flotillins interact with PSGL-1 in neutrophils and, upon stimulation, rapidly organize into membrane domains subsequently accumulating in the uropod

Background

Neutrophils polarize and migrate in response to chemokines. Different types of membrane microdomains (rafts) have been postulated to be present in rear and front of polarized leukocytes and disruption of rafts by cholesterol sequestration prevents leukocyte polarization. Reggie/flotillin-1 and -2 are two highly homologous proteins that are ubiquitously enriched in detergent resistant membranes and are thought to shape membrane microdomains by forming homo- and hetero-oligomers. It was the goal of this study to investigate dynamic membrane microdomain reorganization during neutrophil activation.

Methodology/Principal Findings

We show now, using immunofluorescence staining and co-immunoprecipitation, that endogenous flotillin-1 and -2 colocalize and associate in resting spherical and polarized primary neutrophils. Flotillins redistribute very early after chemoattractant stimulation, and form distinct caps in more than 90% of the neutrophils. At later time points flotillins accumulate in the uropod of polarized cells. Chemotactic peptide-induced redistribution and capping of flotillins requires integrity and dynamics of the actin cytoskeleton, but does not involve Rho-kinase dependent signaling related to formation of the uropod. Both flotillin isoforms are involved in the formation of this membrane domain, as uropod location of exogenously expressed flotillins is dramatically enhanced by co-overexpression of tagged flotillin-1 and -2 in differentiated HL-60 cells as compared to cells expressing only one tagged isoform.

Flotillin-1 and -2 associate with P-selectin glycoprotein ligand 1 (PSGL-1) in resting and in stimulated neutrophils as shown by colocalization and co-immunoprecipitation. Neutrophils isolated from PSGL-1-deficient mice exhibit flotillin caps to the same extent as cells isolated from wild type animals, implying that PSGL-1 is not required for the formation of the flotillin caps. Finally we show that stimulus-dependent redistribution of other uropod-located proteins, CD43 and ezrin/radixin/moesin, occurs much slower than that of flotillins and PSGL-1.

Conclusions/Significance

These results suggest that flotillin-rich actin-dependent membrane microdomains are importantly involved in neutrophil uropod formation and/or stabilization and organize uropod localization of PSGL-1.

P-selectin glycoprotein ligand-1 contributes to wound healing predominantly as a p-selectin ligand and partly as an e-selectin ligand

Cell adhesion molecules are critical to wound healing through leukocyte recruitment. Although P-selectin glycoprotein ligand-1 (PSGL-1) regulates leukocyte rolling by binding P-selectin, but also binding E- and L-selectins with lower affinity, little is known about a role of PSGL-1 in wound healing. To clarify a role of PSGL-1 and its interaction with E- and P-selectins in wound healing, we investigated cutaneous wound healing in PSGL-1-deficient (PSGL-1(-/-)) mice in comparison with E-selectin(-/-), P-selectin(-/-), and P-selectin(-/-) mice treated with an anti-E-selectin antibody. PSGL-1 deficiency inhibited early wound healing, which was accompanied by decreased inflammatory cell infiltration and growth factor expression. By contrast, E-selectin deficiency did not affect wound healing. In general, the inhibitory effect of PSGL-1 deficiency on wound healing was similar to that of P-selectin deficiency either alone or with E-selectin blockade. However, early granulation tissue formation, late angiogenesis, and early infiltration of neutrophils and macrophages in PSGL-1(-/-) mice were inhibited beyond the inhibition in P-selectin(-/-) mice, but to a similar level of inhibition in P-selectin(-/-) mice with E-selectin blockade. These results suggest that PSGL-1 contributes to wound healing predominantly as a P-selectin ligand and partly as an E-selectin ligand by mediating infiltration of inflammatory cells.

Perivascular spaces and the two steps to neuroinflammation

Immune cells enter the central nervous system (CNS) from the circulation under normal conditions for immunosurveillance and in inflammatory neurologic diseases. This review describes the distinct anatomic features of the CNS vasculature that permit it to maintain parenchymal homeostasis and which necessitate specific mechanisms for neuroinflammation to occur. We review the historical evolution of the concept of the blood-brain barrier and discuss distinctions between diffusion/transport of solutes and migration of cells from the blood to CNS parenchyma. The former is regulated at the level of capillaries, whereas the latter takes place in postcapillary venules. We summarize evidence that entry of immune cells into the CNS parenchyma in inflammatory conditions involves 2 differently regulated steps: transmigration of the vascular wall into the perivascular space and progression across the glia limitans into the parenchyma.

VE-PTP maintains the endothelial barrier via plakoglobin and becomes dissociated from VE-cadherin by leukocytes and by VEGF

We have shown recently that vascular endothelial protein tyrosine phosphatase (VE-PTP), an endothelial-specific membrane protein, associates with vascular endothelial (VE)-cadherin and enhances VE-cadherin function in transfected cells (Nawroth, R., G. Poell, A. Ranft, U. Samulowitz, G. Fachinger, M. Golding, D.T. Shima, U. Deutsch, and D. Vestweber. 2002. EMBO J. 21:4885-4895). We show that VE-PTP is indeed required for endothelial cell contact integrity, because down-regulation of its expression enhanced endothelial cell permeability, augmented leukocyte transmigration, and inhibited VE-cadherin-mediated adhesion. Binding of neutrophils as well as lymphocytes to endothelial cells triggered rapid (5 min) dissociation of VE-PTP from VE-cadherin. This dissociation was only seen with tumor necrosis factor alpha-activated, but not resting, endothelial cells. Besides leukocytes, vascular endothelial growth factor also rapidly dissociated VE-PTP from VE-cadherin, indicative of a more general role of VE-PTP in the regulation of endothelial cell contacts. Dissociation of VE-PTP and VE-cadherin in endothelial cells was accompanied by tyrosine phoshorylation of VE-cadherin, beta-catenin, and plakoglobin. Surprisingly, only plakoglobin but not beta-catenin was necessary for VE-PTP to support VE-cadherin adhesion in endothelial cells. In addition, inhibiting the expression of VE-PTP preferentially increased tyrosine phosphorylation of plakoglobin but not beta-catenin. In conclusion, leukocytes interacting with endothelial cells rapidly dissociate VE-PTP from VE-cadherin, weakening endothelial cell contacts via a mechanism that requires plakoglobin but not beta-catenin.

L-Selectin-deficient SJL and C57BL/6 mice are not resistant to experimental autoimmune encephalomyelitis

L-selectin has been suggested to play a role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. Here we demonstrate that L-selectin(-/-) SJL mice are susceptible to proteolipid protein (PLP)-induced EAE because the compromised antigen-specific T cell proliferation in peripheral lymph nodes is fully compensated by the T cell response raised in their spleen. Transfer of PLP-specific T cells into syngeneic recipients induced EAE independent of the presence or absence of L-selectin on PLP-specific T cells or in the recipient. Leukocyte infiltration into the central nervous system parenchyma was detectable independent of the mode of disease induction and the presence or absence of L-selectin. In addition, we found L-selectin(-/-) C57BL/6 mice to be susceptible to myelin oligodendrocyte glycoprotein-induced EAE. Taken together, we demonstrate that in SJL and C57BL/6 mice L-selectin is not required for EAE pathogenesis. The apparent discrepancy of our present observation to previous findings, demonstrating a role of L-selectin in EAE pathogenesis in C57BL/6 mice or myelin-basic protein (MBP)-specific TCR-transgenic B10.PL mice, may be attributed to background genes rather than L-selectin and to a unique role of L-selectin in EAE pathogenesis in MBP-TCR-transgenic mice.

Immune cell entry into the central nervous system: involvement of adhesion molecules and chemokines

In multiple sclerosis and in its animal model experimental autoimmune encephalomyelitis (EAE), inflammatory cells migrate across the highly specialized endothelial blood-brain barrier (BBB) and gain access to the central nervous system (CNS). It is well established that leukocyte recruitment across this vascular bed is unique due to the predominant involvement of alpha4-integrins in mediating the initial contact to as well as firm adhesion with the endothelium. In contrast, the involvement of the selectins, L-selectin, E- and P-selectin and their respective carbohydrate ligands such as P-selectin glycoprotein (PSGL)-1 in this process has been controversially discussed. Intravital microscopic analysis of immune cell interaction with superficial brain vessels demonstrates a role for E- and P-selectin and their common ligand PSGL-1 in lymphocyte rolling. However, E- and P-selectin-deficient SJL- or C57Bl/6 mice or PSGL-1-deficient C57Bl/6 mice develop EAE indistinguishable from wild-type mice. Considering these apparently discrepant observations, it needs to be discussed whether the molecular mechanisms involved in leukocyte trafficking across superficial brain vessels are irrelevant for EAE pathogenesis or whether the therapeutic efficacy of targeting alpha4-integrins in EAE is truly dependent on the inhibition of leukocyte trafficking across the BBB.

E- and P-selectin are not required for the development of experimental autoimmune encephalomyelitis in C57BL/6 and SJL mice

In multiple sclerosis and in its animal model experimental autoimmune encephalomyelitis (EAE), inflammatory cells migrate across the endothelial blood-brain barrier (BBB) and gain access to the CNS. It is well-established that alpha4 integrins are actively involved in leukocyte recruitment across the BBB during EAE. In contrast, the role of endothelial E- and P-selectin in this process has been a controversial issue. In this study, we demonstrate that P-selectin protein can be detected in meningeal blood vessel endothelial cells in healthy SJL and C57BL/6 mice and on rare parenchymal CNS blood vessels in C57BL/6, but not SJL, mice. During EAE, expression of P-selectin but not E-selectin was found up-regulated on inflamed CNS microvessels surrounded by inflammatory infiltrates irrespective of their meningeal or parenchymal localization with a more prominent immunostaining detected in C57BL/6 as compared with SJL mice. P-selectin immunostaining could be localized to CNS endothelial cells and to CD41-positive platelets adhering to the vessel wall. Despite the presence of P-selectin in wild-type mice, E/P-selectin-deficient SJL and C57BL/6 mice developed clinical EAE indistinguishable from wild-type mice. Absence of E- and P-selectin did neither influence the activation of myelin-specific T cells nor the composition of the cellular infiltrates in the CNS during EAE. Finally, endothelial-specific tetracycline-inducible expression of E-selectin at the BBB in transgenic C57BL/6 mice did not alter the development of EAE. Thus, E- and P-selectin are not required for leukocyte recruitment across the BBB and the development of EAE in C57BL/6 and in SJL mice.

Brain endothelial PPARγ controls inflammation-induced CD4+ T cell adhesion and transmigration in vitro

An important step in the pathogenesis of multiple sclerosis is adhesion and transmigration of encephalitogenic T cells across brain endothelial cells (EC) which strongly relies on interaction with EC-expressed adhesion molecules. We provide molecular evidence that the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) is a negative regulator of brain EC inflammation. The PPARgamma agonist pioglitazone reduces transendothelial migration of encephalitogenic T cells across TNFalpha-stimulated brain EC. This effect is clearly PPARgamma mediated, as lentiviral PPARgamma overexpression in brain EC results in selective abrogation of inflammation-induced ICAM-1 and VCAM-1 upregulation and subsequent adhesion and transmigration of T cells. We therefore propose that PPARgamma in brain EC may be exploited to target detrimental EC-T cell interactions under inflammatory conditions.

Preferential expression of very late antigen-4 on type 1 CTL cells plays a critical role in trafficking into central nervous system tumors

We have previously shown preferential tumor-homing and therapeutic efficacy of adoptively transferred type 1 CTL (Tc1) when compared with type 2 CTL (Tc2) in mice bearing intracranial ovalbumin-transfected melanoma (M05). Further characterizing the expression of a panel of homing receptors on Tc1 and Tc2 cells, we found that very late antigen (VLA)-4 (a heterodimer of CD49d and CD29), but none of other receptors evaluated, was expressed at significantly higher levels on Tc1 cells than on Tc2 cells. Although CD49d (alpha(4) integrin) can form heterodimers with both beta(1) (CD29) and beta(7) integrins, alpha(4)beta(7) complexes were not expressed by either Tc1 or Tc2 cells, suggesting that CD49d is solely expressed in VLA-4 complexes. VLA-4 expression on Tc2 cells was down-regulated in an interleukin (IL)-4 dose-dependent manner but not by other type 2 cytokines, such as IL-10 and IL-13, suggesting that IL-4 uniquely down-regulates VLA-4 expression on these cells. In accordance with the differential expression of VLA-4 on Tc1 versus Tc2 cells, Tc1 cells alone were competent to adhere to plate-bound VCAM-1-Ig fusion protein. Finally, the efficient trafficking of Tc1 cells into intracranial M05 lesions in vivo was efficiently blocked by administration of monoclonal antibodies against CD49d or VCAM-1 or small interfering RNA-mediated silencing of CD49d on Tc1 cells. Collectively, these data support the critical role of VLA-4 in the effective intracranial tumor homing of adoptive-transferred, antigen-specific Tc1 cells and suggest that more effective vaccine and/or ex vivo T-cell activation regimens may be developed by promoting the generation of VLA-4(+) antitumor Tc1 cells.

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.

T cell-associated CD18 but not CD62L, ICAM-1, or PSGL-1 is required for the induction of chronic colitis

The induction and perpetuation of chronic colitis are thought to involve a complex set of adhesive interactions between T cells and endothelial cells located on the vasculature within secondary lymphoid tissue and the intestine. The objective of this study was to assess the roles of T cell-associated CD18, CD62L (L-selectin), ICAM-1, and P-selectin glycoprotein ligand-1 (PSGL-1) in the induction of chronic colitis in mice. CD4(+)CD25(-) T cells derived from either wild-type (WT), CD18-deficient [CD18 knockout (KO)], CD62L KO, ICAM-1 KO, or PSGL-1 KO mice were adoptively transferred into recombinase activating gene-1 (RAG-1)-deficient mice (RAG KO mice) to assess the potential of these T cells to induce chronic colitis. At 8-10 wk following T cell transfer, we observed moderate to severe colitis as assessed by increases in colon weight-to-length ratios and by blinded histopathological analysis. In contrast, we found that transfer of CD18 KO T cells into RAG KO recipients resulted in the significant attenuation of colonic inflammation in these mice. Furthermore, we observed fewer infiltrating CD4(+) T cells in the colonic lamina propria in the CD18 KO-->RAG KO group compared with the WT-->RAG KO group. Finally, message levels of colonic TNF-alpha, IL-1beta, and IFN-gamma were significantly reduced in CD18 KO-->RAG KO mice compared with colitic control animals. We conclude that T cell-associated CD18, but not CD62L, ICAM-1, or PSGL-1, is required for the development of chronic colitis.

Disruption of the beta2-integrin CD11d (alphaDbeta2) gene fails to protect against experimental autoimmune encephalomyelitis

The fourth member of the beta(2)-integrin family of adhesion molecules, CD11d (alpha(D)beta(2)), is expressed on a wide variety of immune cells, however its function in autoimmune diseases, including EAE remains unknown. We induced EAE in wild-type and CD11d(-/-) C57BL/6 mice using myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide. The clinical course and histopathology of EAE were identical in both groups of mice throughout the disease course. There were no significant differences in the infiltration of leukocyte subsets into the central nervous system or in the production of cytokines from T cells isolated from the spleen or spinal cord from both groups of mice. Our data demonstrate that CD11d is not required for the development of EAE and, to date, is the only beta(2)-integrin molecule whose deletion does not result in attenuated disease.