Transforming growth factor beta 1 inhibits cytokine-induced CNS endothelial cell activation

The Blood-Brain Barrier: Implications for Vasculitis

There has been extraordinary research in the blood-brain barrier. Once considered a static anatomic barrier to the traffic of molecules in and out of the central nervous system when fully developed in adults, the blood-brain barrier is now known to be not only fully functional in development but also vital in cerebrovascular angiogenesis. Blood-brain barrier breakdown has been recognized as an important factor in a variety of primary neurologic diseases; however, such disturbances have yet to be critically analyzed. This article reviews the history, neurodevelopment, ultrastructure, function, and clinicopathologic correlation and relevance to central nervous system vasculitis.

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.

Pericyte coculture models to study astrocyte, pericyte, and endothelial cell interactions

The microvascular pericyte is an integral component of the blood-brain barrier and the neurovascular unit. Most model systems that have been developed to study the functional parameters of these systems have not incorporated the pericyte. In this chapter, we consider pericyte coculture and triple culture systems and detail the methodology, suggestions, and problems with isolation of these unique cells. We also present data to show that triple cultures are ideal to study the role of the CNS pericyte in CNS angiogenesis.

Decreased production of TNF-alpha by lymph node cells indicates experimental autoimmune encephalomyelitis remission in Lewis rats

Experimental autoimmune encephalomyelitis (EAE) is mediated by CD4+ Th1 cells that mainly secrete IFN-gamma and TNF-alpha, important cytokines in the pathophysiology of the disease. Spontaneous remission is, in part, attributed to the down regulation of IFN-gamma and TNF-alpha by TGF-beta. In the current paper, we compared weight, histopathology and immunological parameters during the acute and recovery phases of EAE to establish the best biomarker for clinical remission. Female Lewis rats were immunised with myelin basic protein (MBP) emulsified with complete Freund's adjuvant. Animals were evaluated daily for clinical score and weight prior to euthanisation. All immunised animals developed the expected characteristics of EAE during the acute phase, including significant weight loss and high clinical scores. Disease remission was associated with a significant reduction in clinical scores, although immunised rats did not regain their initial weight values. Brain inflammatory infiltrates were higher during the acute phase. During the remission phase, anti-myelin antibody levels increased, whereas TNF-alpha and IFN-gamma production by lymph node cells cultured with MBP or concanavalin A, respectively, decreased. The most significant difference observed between the acute and recovery phases was in the induction of TNF-alpha levels in MBP-stimulated cultures. Therefore, the in vitro production of this cytokine could be used as a biomarker for EAE remission.

Neutrophils and the blood–brain barrier dysfunction after trauma

Despite the fact that neutrophils are essential for the protection from invading pathogens, hyperactive neutrophils may elicit detrimental cerebral damage after severe trauma. The neutrophil interactions with the neurovascular unit entail endothelial dysfunction involving endothelial leakage, formation of edema, coagulation abnormalities, disturbed hemodynamics, tissue infiltration etc. These elements of the "whole body inflammation," designated systemic inflammatory response syndrome (SIRS) in conjunction with intracerebral proinflammatory activities, are important triggers of post-traumatic cerebral damage and mortality according to the "second hit" concept. From the immunologic point of view, the brain is an immune privileged site, known to resist autodestructive inflammatory activity much more efficiently than other organs because of the highly efficient diverse functions of the blood-brain barrier (BBB). However, both the underlying strategy of the BBB to maintain cerebral protecting functions against the post-traumatic neutrophil-mediated "second hit" and how activated neutrophils may overcome the BBB are currently unknown. Therefore, this review summarizes the current understanding of the "second hit," the BBB physiology, and its role in the maintenance of cerebral immune privilege, and discusses recent findings that may explain the pathophysiologic neutrophil-BBB interactions occurring after severe trauma, thus offering novel therapeutic options to protect from post-traumatic brain damage.

Traumatic brain injury increases TGFβRII expression on endothelial cells

Transforming growth factor beta (TGFbeta) modulates a variety of growth related functions following traumatic injury. The cellular response to TGFbeta is predominantly mediated through TGFbeta receptor I (TGFbetaRI) and receptor II (TGFbetaRII) on the cell surface and SMAD proteins intracellularly. We investigated the expression of TGFbeta receptors in the acute and chronic phases of a traumatic cerebral injury (TCI) by immunohistochemistry and in cultures of murine brain microvascular endothelial (EN) cells using cytofluorimetry. Here, we report that TGFbetaRII expression significantly increases on brain endothelial cells in the chronic phase of TCI. SMAD3 and SMAD4 protein expression were also upregulated suggesting the activation of TGFbeta receptor intracellular signaling. When TGFbetaRI and TGFbetaRII expression was studied in in vitro cultures of murine brain microvessel EN cells, TGFbetaRII showed increased expression on proliferating cells that are incorporating BrdU. These data show a differential expression of TGFbetaRI and TGFbetaRII on brain microvessel EN cells in the acute and chronic phases of TCI that might be associated with EN proliferation following injury.

Angiogenic factors in the central nervous system

The past decade has seen considerable advances in the understanding of angiogenesis. Blood vessel development and growth in the central nervous system are tightly controlled processes that are regulated by angiogenic factors. Angiogenic factors have been implicated in the pathogenesis of a wide variety of disorders, including primary and metastatic brain tumors, aneurysms, arteriovenous malformations, and cavernous malformations. The potential clinical applications of angiogenesis research include inhibition of angiogenesis to control brain tumors and therapeutic angiogenesis to promote collateral blood vessel formation among patients at risk of ischemia. This article summarizes the processes of blood vessel formation in the brain, examines the angiogenic factors that are prominent in the central nervous system, reviews the clinical use of angiogenesis inhibitors, and identifies areas for future investigation.

Cellular mechanisms of CNS pericytes

Three major functional roles have been ascribed to pericytes associated with central nervous system microvasculature-contractility, regulation o f endothelial cell activity, and macrophage activity. A host of different cell factors and signalling agents appear to be involved with these cellular functions, some effecting the pericyte and others produced by this cell. These include neuromodulators, vasoactive peptides, metabolic factors, growth factors and cytokines. The specific compounds and their actions are collectively viewed in an effort to provide an overall picture of the regulation of pericyte functional activity. This small vascular cell is emerging as a significant entity in several physiological processes through the functions of above; these processes include control of blood flow, regulation of vascular development and immune responses. Defining the regulatory agents and their mechanisms is key to understanding the role that pericytes play in these processes. Because these cells have begun to receive increasing attention in neurobiological studies, an overview of signalling properties should be timely and beneficial.

Murine endothelia do not express MHC class II I-Ealpha subunit and differentially regulate I-Aalpha expression along the vascular tree

Cellular elements of the vascular wall, such as endothelium (En) and smooth muscle cells/pericytes (SM/P) possess important immunologic properties. We have previously reported that murine brain microvessel En cells and SM/P express Major Histocompatibility (MHC) class II molecules and activate syngeneic CD4+ T cells in a class II dependent way. Herein we compare MHC class II expression on brain microvessel En to aorta large vessel En cells in order to explore the mechanisms of immune responses in brain tissue versus other peripheral tissues. Interestingly, we demonstrate that En cells from brain microvessel and large aortic vessel express the I-A but not the I-E subunit of MHC class II molecules. The expression of I-A class II molecules can be upregulated on brain microvessel and aortic En cells by interferon-gamma (IFN-gamma). Similarly, the expression of I-A, but not I-E, MHC class II molecules on brain microvessel endothelial cells was upregulated in the presence of activated T cells. Interleukin-10 (IL-10) was found to inhibit IFN-gamma-mediated upregulation of I-A class II molecule expression on aortic but not on microvessel En cells. Our data may indicate that some differences in organ-specific immune responses, are defined by local parameters, such as MHC distribution and regulation.

Role of the CNS microvascular pericyte in the blood-brain barrier

Pericytes are a very important cellular constituent of the blood-brain barrier. They play a regulatory role in brain angiogenesis, endothelial cell tight junction formation, blood-brain barrier differentiation, as well as contribute to the microvascular vasodynamic capacity and structural stability. Central nervous system pericytes express macrophage functions and are actively involved in the neuroimmune network operating at the blood-brain barrier. They exhibit unique functional characteristics critical for the pathogenesis of a number of cerebrovascular, neurodegenerative, and neuroimmune diseases.

Concordance and contradiction concerning cytokines and chemokines in experimental demyelinating disease

Experimental autoimmune encephalomyelitis (EAE) has served as a model for human demyelinating diseases, including multiple sclerosis. EAE is mediated by CD4+ T lymphocytes of the TH1 subset. These T cells produce inflammatory cytokines and chemokines that are associated with pathogenicity. The disease is downregulated by other T cells, presumably of the TH2 subset that secrete a different pattern of cytokines which modulate the activity of the pathogenic TH1 cells. Ongoing studies should provide insight into how the interactions of T-cell subsets impact on the pathogenesis of autoimmune demyelinating diseases.

Altered peptide ligand modulation of experimental allergic encephalomyelitis: immune responses within the CNS

An altered peptide ligand (analog) of the encephalitogenic epitope of proteolipid protein residues 139-151 (p139-151) in which residues 144 and 147 are substituted with leucine and arginine, respectively (LR), protects from clinical but not histological experimental allergic encephalomyelitis (EAE). To understand in situ events associated with this protection, T cells from brains of mice immunized with either native p139-151, the analog LR or a combination of the two were isolated and characterized. High proportions of cells from co-immunized mice (38%) and LR-immunized mice (58%) reacted to both p139-151 and LR, whereas fewer cells from p139-151 immunized mice (7%) were cross-reactive. T cell clones derived from brains of LR- and co-immunized mice were also cross-reactive in vitro. By reverse transcriptase-based polymerase chain reaction, higher levels of TGF-beta mRNA, and lower levels of TNF-alpha and IFN-gamma mRNA were found in the central nervous system (CNS) tissue of LR and co-immunized mice. Immunohistochemistry demonstrated greater TGF-beta immunoreactivity in CNS inflammatory foci in co-immunized and LR-immunized mice. There were no significant differences in CD4+ or CD8+ cell infiltrates among the groups and differences in other cytokines were not identified by immunocytochemistry. Protection from clinical EAE in LR and co-immunized mice was partially abolished by anti-TGF-beta antibody treatment. Thus, protection from clinical disease following immunization with the analog LR is associated with infiltration into the CNS of a T cell population that could potentially recognize the native PLP peptide and with enhanced TGF-beta production by cells within CNS inflammatory foci.

The effect of interferon beta-1b on lymphocyte-endothelial cell adhesion

Interferon beta-1b (IFN beta-1b) (Betaseron) has been recently approved for treatment of multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS). The mechanism of action of IFN beta-1b is not understood, but its effect in reducing gadolinium enhanced MRI lesions suggest an effect at the blood brain barrier (BBB). Thus the objective of this study is to examine the effect of IFN beta-1b treatment of endothelial cells (EC) on lymphocyte-EC adhesion, and on the expression of the adhesion molecules (AM) ICAM-1, VCAM and E-selectin induced by IFN-gamma, TNF-alpha, or IL-1 beta. Primary cultures of human umbilical vein EC (HUVEC) were used which under basal conditions expressed low levels of ICAM-1 but not VCAM or E-selectin. IFN beta-1b (1-1000 IU/ml) had minimal effect on basal expression of AM on HUVEC, but AM could be substantially upregulated by IFN-gamma, IL-1 beta or TNF-alpha which was associated with a parallel increase in lymphocyte-EC adhesion. The effect of IFN beta-1b on AM expression induced by IFN-gamma, IL-1 beta or TNF-alpha was slightly additive, and was associated with a modest increase in lymphocyte-EC adhesion. In contrast TGF-beta, shown previously to downregulate lymphocyte-EC adhesion, inhibited this adhesion in our experiments. It is concluded that IFN-beta does not downregulate the inducible expression of ICAM-1, VCAM or E-selectin on HUVEC and does not inhibit the adhesion of lymphocytes to HUVEC. These findings have implications on the mechanism of action of IFN beta-1b in MS.

Glial transforming growth factor (TGF)-beta isotypes in multiple sclerosis: differential glial expression of TGF-beta 1, 2 and 3 isotypes in multiple sclerosis

We studied glial transforming growth factor (TGF)-beta isotype expression in 14 cases of multiple sclerosis. Acute active lesions exhibited selective TGF-beta 2 immunoreactivity of lesion encircling ramified microglia. In contrast, astrocytes within chronic active white matter lesions expressed all three isotypes. Chronic active lesions which extended into cortex exhibited selective cortical astrocyte TGF-beta 2 expression. This isotype was also selectively expressed by astrocytes in apparently normal white matter. A similar pattern of glial TGF-beta expression was seen in the pathological control, progressive multifocal leukoencephalopathy. The results suggest that TGF-beta cytokines are locally expressed in demyelination and that the beta 2 isotype may be uniquely regulated.

Regulation of cytokine gene expression in experimental autoimmune encephalomyelitis

We previously reported that recovery of Lewis rats from experimental autoimmune encephalomyelitis (EAE) is associated with the appearance of suppressor T cells (Ts). These Ts secrete TGF-beta which down-regulates the production of inflammatory cytokines by the effector T cells that mediate this disease. In the present study, we immunized Lewis rats with myelin basic protein (MBP)+CFA, and evaluated purified T cells and MBP-activated spleen cells (SpC) during the paralytic phase (day 12) and after recovery (days 30-33) for TGF-beta and interferon (IFN)-gamma mRNA. We used reverse transcriptase-polymerase chain reaction (RT-PCR), quantitated on the basis of beta-actin mRNA. Abundant IFN-gamma mRNA was present in MBP-activated SpC obtained on day 12. In contrast, only trace IFN-gamma mRNA was detected in day 30 activated SpC, and no IFN-gamma mRNA was present in purified, nonactivated T cells obtained at either time. The level of IFN-gamma mRNA correlated with secretion of IFN-gamma as determined by ELISA on SpC culture supernatants, and with severity of adoptively transferred EAE by the activated SpC. Thus, it appears that IFN-gamma mRNA is both transcribed and translated in response to antigen activation, resulting in secretion of IFN-gamma by the disease-inducing Te. In contrast, when we used RT-PCR to investigate the expression of TGF-beta mRNA, we found the transcript present in isolated T cells and MBP-activated SpC obtained from rats at both days 12 and 30. The presence of TGF-beta mRNA at time points corresponding to both clinical EAE and recovery suggests post-transcriptional regulation of the production of this immunoregulatory cytokine.

In vitro studies of glial cells: what can we learn about demyelinating diseases?

Acquired demyelinating diseases of the central and peripheral nervous systems comprise an important group of neurologic diseases of unknown etiology and incompletely understood pathogenesis. Cultures of glial cells are proving highly useful in investigating the role of both antibodies and cytokines in the pathogenesis of these disorders. While there clearly is need for comparative studies employing more complex systems and using patient derived tissues, glial cell cultures provide important advantages by allowing researchers to characterize the effect of cytokines and growth factors on specific cell types in controlled conditions.

Recovery phase of acute experimental autoimmune encephalomyelitis in rats corresponds to development of endothelial cell unresponsiveness to interferon gamma activation

Activation of the vascular endothelium is important in the development of inflammation. Activated endothelial cells (EC) express surface markers not expressed by quiescent EC. These surface markers augment adhesion reactions and leukocyte migration. We examined microvessel EC activation longitudinally in experimental autoimmune encephalomyelitis (EAE) in Lewis rats. CNS microvessels were isolated at 0, 3, 7, 12, 20, and 30 days post-inoculation (PI). Normal and CFA-injected rat microvessels do not express activation antigens (Ag). Increased expression of major histocompatibility complex (MHC) class II molecule and intercellular adhesion molecule-1 (ICAM-1) were detected on CNS microvessels from immunized rats at 7 days PI, prior to development of clinical signs, and at 12 days PI. Enhanced MHC class I molecule was seen only at 12 days. MHC class II molecule expression was focally expressed along microvessel fragments. By 20 days PI, EC did not exhibit increased levels of any of the markers tested. Perivascular cells (possibly pericytes), however, were found to express MHC class II molecule and ICAM-1 up to 30 days PI. During the recovery phase isolated CNS microvessels from MBP-immunized rats were unresponsive to IFN gamma-mediated endothelial activation. Unresponsiveness was independent of IFN gamma concentration. These results suggest that the endothelium is restored to functional quiescence during the recovery phase of acute EAE.

Methylprednisolone prevents rejection of intrastriatal grafts of xenogeneic embryonic neural tissue in adult rats

We studied the effects of high-dose methylprednisolone on the survival of intrastriatal neural xenografts and the host responses against them. Dissociated mesencephalic tissue from inbred mouse (CBA-strain) embryos was transplanted to the intact striatum of adult Sprague-Dawley rats. The rats received either daily injections of methylprednisolone (30 mg/kg), or cyclosporin A (10 mg/kg), or no immunosuppressive treatment. Two or six weeks after transplantation, there was good survival of xenografts in both the methylprednisolone- and cyclosporin A-treated rats. In contrast, the xenografts in untreated control rats were all rejected by six weeks. There was no marked difference in the degree of expression of MHC class I and II antigens and the accumulation of activated astrocytes and microglial cells/macrophages between the three groups. However, both methylprednisolone and cyclosporin A reduced infiltration of T lymphocytes to the transplantation sites. The expression of pro-inflammatory cytokines (interferon-gamma, tumour necrosis factor-alpha, interleukin-6) in and around the grafts was lower in the methylprednisolone- and cyclosporin A-treated groups than in untreated control rats. Although high-dose methylprednisolone caused significant body weight loss, we conclude that this treatment can prevent rejection of intrastriatal grafts of xenogeneic embryonic neural tissue in the adult.

Focal ischemia due to traumatic contusions documented by stable xenon-CT and ultrastructural studies

A traumatic cerebral contusion causes a zone of perifocal neuronal necrosis, the cause of which is not known; the surgical management of these lesions remains controversial. To determine the pathophysiological mechanisms responsible for brain damage after contusions, the authors performed cerebral blood flow (CBF) mapping studies and related these to change in local cerebral blood volume (CBV) and ultrastructure. In 11 severely head injured patients with contusion, CBF and CBV were measured in pericontusional areas using stable xenon-computerized tomography (CT). These studies demonstrated a profound reduction in perilesional CBF (mean 17.5 +/- 4 ml/100 g/min), which was always accompanied by a zone of edema defined by CT density measurements. Mean CBV in these regions was 2.3 +/- 0.4 ml/100 g, a reduction to approximately one-half the value of 4.8 ml/100 g found in the nonedematous regions, and to approximately 35% of the value of 6.0 ml/100 g found in normal volunteers. Ultrastructural analysis of the pericontusional tissue, taken at surgery in four patients with high intracranial pressure showed glial swelling with narrowing of the microvascular lumina due to massive podocytic process swelling. Additionally, some suggestion of vascular occlusion due to erythrocyte and leukocyte stasis was seen. These data support the conclusion that microvascular compromise by compression and/or occlusion is a major event associated with profound perilesional hypoperfusion, which is a uniform finding within edematous pericontusional tissue.