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. Pathol 2007;17:243-250. The central nervous system (CNS) parenchyma is an immune-privileged organ, largely because it lacks resident dendritic cells, which are the cellular elements required for afferent communication to the immune system (34). Intraparenchymal immune-mediated inflammation reliably follows peripheral sensitization against a pathogen or autoantigen harbored within the CNS, illustrating efficient immune surveillance of the CNS and competent effector mechanisms. Once begun, immune-mediated CNS inflammation differs from that in peripheral tissues, in part because of the special character of the blood-brain barrier (BBB), which, along with factors produced by resident neuroepithelial cells, governs trafficking, survival and effector functions of hematogenous leukocytes during inflammatory states (1,28,69,79). Understanding how leukocytes migrate across the BBB has important implications. Recent clinical trials in relapsing-remitting multiple sclerosis (MS) using natalizumab, humanized antibodies directed against α 4 integrins, demonstrated efficacy (6). 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 (28). BrainLeukocyte entry into tissues is governed by the presence of chemokines and adhesion molecules at post-capillary venules (14,85,90,108). Compared with peripheral microvasculature, the molecular mechan...

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How circulating T cells infiltrate into the brain in Alzheimer disease (AD) remains unclear. We previously reported that amyloid β (Aβ)-dependent CCR5 expression in brain endothelial cells is involved in T cell transendothelial migration. In this study, we explored the signaling pathway of CCR5 up-regulation by Aβ. We showed that inhibitors of JNK, ERK, and PI3K significantly decreased Aβ-induced CCR5 expression in human brain microvascular endothelial cells (HBMECs). Chromatin immunoprecipitation assay revealed that Aβ-activated JNK, ERK, and PI3K promoted brain endothelial CCR5 expression via transcription factor Egr-1. Furthermore, neutralization Ab of receptor for advanced glycation end products (RAGE; an Aβ receptor) effectively blocked Aβ-induced JNK, ERK, and PI3K activation, contributing to CCR5 expression in HBMECs. Aβ fails to induce CCR5 expression when truncated RAGE was overexpressed in HBMECs. Transendothelial migration assay showed that the migration of MIP-1α (a CCR5 ligand)-expressing AD patients’ T cells through in vitro blood-brain barrier model was effectively blocked by anti-RAGE Ab, overexpression of truncated RAGE, and dominant-negative PI3K, JNK/ERK, or Egr-1 RNA interference in HBMECs, respectively. Importantly, blockage of intracerebral RAGE abolished the up-regulation of CCR5 on brain endothelial cells and the increased T cell infiltration in the brain induced by Aβ injection in rat hippocampus. Our results suggest that intracerebral Aβ interaction with RAGE at BBB up-regulates endothelial CCR5 expression and causes circulating T cell infiltration in the brain in AD. This study may provide a new insight into the understanding of inflammation in the progress of AD.

Endothelial cells that functionally express blood brain barrier (BBB) properties are useful surrogates for studying leukocyte-endothelial cell interactions at the BBB. In this study, we compared two different endothelial cellular models: transfected human brain microvascular endothelial cells (THBMECs) and human umbilical vein endothelial cells (HUVECs). With each grow under optimal conditions, confluent THBMEC cultures showed continuous occludin and ZO-1 immunoreactivity, while HUVEC cultures exhibited punctate ZO-1 expression at sites of cell-cell contact only. Confluent THBMEC cultures on 24-well collagen-coated transwell inserts had significantly higher transendothelial electrical resistance (TEER) and lower solute permeability than HUVECs. Confluent THBMECs were more restrictive for mononuclear cell migration than HUVECs. Only THBMECs utilized abluminal CCL5 to facilitate T-lymphocyte migration in vitro although both THBMECs and HUVECs employed CCL3 to facilitate T cell migration. These data establish baseline conditions for using THBMECs to develop in vitro BBB models for studying leukocyte-endothelial interactions during neuroinflammation.

S100B is a reporter of blood-brain barrier (BBB) integrity which appears in blood when the BBB is breached. Circulating S100B derives from either extracranial sources or release into circulation by normal fluctuations in BBB integrity or pathologic BBB disruption (BBBD). Elevated S100B matches the clinical presence of indices of BBBD (gadolinium enhancement or albumin coefficient). After repeated sub-concussive episodes, serum S100B triggers an antigen-driven production of anti-S100B autoantibodies. We tested the hypothesis that the presence of S100B in extracranial tissue is due to peripheral cellular uptake of serum S100B by antigen presenting cells, which may induce the production of auto antibodies against S100B. To test this hypothesis, we used animal models of seizures, enrolled patients undergoing repeated BBBD, and collected serum samples from epileptic patients. We employed a broad array of techniques, including immunohistochemistry, RNA analysis, tracer injection and serum analysis. mRNA for S100B was segregated to barrier organs (testis, kidney and brain) but S100B protein was detected in immunocompetent cells in spleen, thymus and lymph nodes, in resident immune cells (Langerhans, satellite cells in heart muscle, etc.) and BBB endothelium. Uptake of labeled S100B by rat spleen CD4+ or CD8+ and CD86+ dendritic cells was exacerbated by pilocarpine-induced status epilepticus which is accompanied by BBBD. Clinical seizures were preceded by a surge of serum S100B. In patients undergoing repeated therapeutic BBBD, an autoimmune response against S100B was measured. In addition to its role in the central nervous system and its diagnostic value as a BBBD reporter, S100B may integrate blood-brain barrier disruption to the control of systemic immunity by a mechanism involving the activation of immune cells. We propose a scenario where extravasated S100B may trigger a pathologic autoimmune reaction linking systemic and CNS immune responses.