The inflammatory response in the CNS begins with the movement of leukocytes across the blood-brain barrier in a multistep process that requires cells to pass through a perivascular space before entering the parenchyma. The molecular mechanisms that orchestrate this movement are not known. The chemokine CXCL12 is highly expressed throughout the CNS by microendothelial cells under normal conditions, suggesting it might play a role maintaining the blood-brain barrier. We tested this hypothesis in the setting of experimental autoimmune encephalomyelitis (EAE) by using AMD3100, a specific antagonist of the CXCL12 receptor CXCR4. We demonstrate that the loss of CXCR4 activation enhances the migration of infiltrating leukocytes into the CNS parenchyma. CXCL12 is expressed at the basolateral surface of CNS endothelial cells in normal spinal cord and at the onset of EAE. This polarity is lost in vessels associated with an extensive parenchymal invasion of mononuclear cells during the peak of disease. Inhibition of CXCR4 activation during the induction of EAE leads to loss of the typical intense perivascular cuffs, which are replaced with widespread white matter infiltration of mononuclear cells, worsening the clinical severity of the disease and increasing inflammation. Taken together, these data suggest a novel anti-inflammatory role for CXCL12 during EAE in that it functions to localize CXCR4-expressing mononuclear cells to the perivascular space, thereby limiting the parenchymal infiltration of autoreactive effector cells.

During CNS autoimmunity, brain endothelial cell CXCR7 internalizes CXCL12 from the perivascular space, thereby permitting leukocyte migration into the CNS parenchyma.

Multiple sclerosis is a neurodegenerative disease characterized by episodes of autoimmune attack of oligodendrocytes leading to demyelination and progressive functional deficits. Because many patients exhibit functional recovery in between demyelinating episodes, understanding mechanisms responsible for repair of damaged myelin is critical for developing therapies that promote remyelination and prevent disease progression. The chemokine CXCL12 is a developmental molecule known to orchestrate the migration, proliferation, and differentiation of neuronal precursor cells within the developing CNS. Although studies suggest a role for CXCL12 in oligodendroglia ontogeny in vitro, no studies have investigated the role of CXCL12 in remyelination in vivo in the adult CNS. Using an experimental murine model of demyelination mediated by the copper chelator cuprizone, we evaluated the expression of CXCL12 and its receptor, CXCR4, within the demyelinating and remyelinating corpus callosum (CC). CXCL12 was significantly up-regulated within activated astrocytes and microglia in the CC during demyelination, as were numbers of CXCR4+ NG2+ oligodendrocyte precursor cells (OPCs). Loss of CXCR4 signaling via either pharmacological blockade or in vivo RNA silencing led to decreased OPCs maturation and failure to remyelinate. These data indicate that CXCR4 activation, by promoting the differentiation of OPCs into oligodendrocytes, is critical for remyelination of the injured adult CNS.M ultiple sclerosis (MS), a progressive, neurodegenerative disease of the CNS, occurs most often in a relapsing/remitting form, in which a period of demyelination is followed by a period of functional recovery (1). The recovery stage involves remyelination via the migration and maturation of oligodendrocyte precursor cells (OPCs) (2). However, as the disease progresses, remyelination fails with continuous loss of function (3). Possible explanations for remyelination failure of intact axons include defects in OPC recruitment to the site of demyelination or in OPC differentiation into myelinating oligodendrocytes. Although studies indicate that both aspects of OPC biology are altered in MS (4, 5), the molecular mechanisms that orchestrate these processes within the adult CNS are incompletely understood.Studies in mice indicate that neural precursors that give rise to cells of oligodendrocytes lineage can be identified within the ventral half of the ventricular zones of all CNS regions by embryonic days 12-14 (E12-E14) via their expression of NG2 chondroitin sulfate proteoglycan (6). In the final stage of oligodendrocyte differentiation, which occurs primarily during the postnatal period (P4-P12), OPCs begin to express mature markers of oligodendrocytes including 2′3′-cyclic nucleotide phosphohydrolase (CNPase), myelin basic protein (MBP), proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG). Similar events occur during remyelination; NG2+ OPCs proliferate within subventricular zones, migrate to areas of demyelination, and differentiate ...

Dysregulation of blood-brain barrier (BBB) function and transendothelial migration of leukocytes are essential components of the development and propagation of active lesions in multiple sclerosis (MS). Animal studies indicate that polarized expression of the chemokine CXCL12 at the BBB prevents leukocyte extravasation into the central nervous system (CNS) and that disruption of CXCL12 polarity promotes entry of autoreactive leukocytes and inflammation. In the present study, we examined expression of CXCL12 and its receptor, CXCR4, within CNS tissues from MS and non-MS patients. Immunohistochemical analysis of CXCL12 expression at the BBB revealed basolateral localization in tissues derived from non-MS patients and at uninvolved sites in tissues from MS patients. In contrast, within active MS lesions, CXCL12 expression was redistributed toward vessel lumena and was associated with CXCR4 activation in infiltrating leukocytes, as revealed by phospho-CXCR4-specific antibodies. Quantitative assessment of CXCL12 expression by the CNS microvasculature established a positive correlation between CXCL12 redistribution, leukocyte infiltration, and severity of histological disease. These results suggest that CXCL12 normally functions to localize infiltrating leukocytes to perivascular spaces, preventing CNS parenchymal infiltration. In the patient cohort studied, altered patterns of CXCL12 expression at the BBB were specifically associated with MS, possibly facilitating trafficking of CXCR4-expressing mononuclear cells into and out of the perivascular space and leading to progression of disease.

The migration of lymphocytes into the CNS during viral encephalitis is hindered by the blood-brain barrier (BBB) such that most infiltrating cells remain localized to perivascular spaces. This sequestration of leukocytes away from the parenchyma is believed to protect the CNS from immunopathologic injury. Infections of the CNS with highly cytopathic neurotropic viruses, such as West Nile virus (WNV), however, require the parenchymal penetration of T lymphocytes for virus clearance and survival, suggesting that perivascular localization might hinder antiviral immune responses during WNV encephalitis. Using human and murine brain specimens from individuals with WNV encephalitis, we evaluated the expression of CXCL12 and its receptor, CXCR4, at the BBB and tested the hypothesis that inhibition of CXCR4 would promote T lymphocyte entry into the CNS parenchyma and increase viral clearance. Antagonism of CXCR4 significantly improved survival from lethal infection through enhanced intraparenchymal migration of WNV-specific CD8 ؉ T cells within the brain, leading to reduced viral loads and, surprisingly, decreased immunopathology at this site. The benefits of enhanced CD8 ؉ T cell infiltration suggest that pharmacologic targeting of CXCR4 may have therapeutic utility for the treatment of acute viral infections of the CNS.CD8 T cell ͉ CNS ͉ CXCL12 ͉ chemokine ͉ neuropathology U nder normal, uninflamed conditions, the CNS is an immuneprivileged site with minimal infiltration by inflammatory cells (1). The limited trafficking of leukocytes is the result of restriction at the blood-brain barrier (BBB); the specialized microvasculature of the CNS that prevents the entry of cells through mechanisms that are incompletely understood. Although immune cell restriction may protect the brain from inappropriate immune activation and neuropathology (2), it may also act as a barrier to successful pathogen clearance during acute infections of the CNS (3). Indeed, one of the hallmarks of viral encephalitis is the development of perivascular infiltrates comprised of virus-specific T cells with minimal invasion of the CNS parenchyma (4). Thus, BBB restriction may limit CNS antiviral immune responses at the expense of delayed clearance of microbial agents.Studies in mice suggest that polarized expression of the chemokine CXCL12 at the BBB localizes infiltrating mononuclear cells to the perivascular spaces of the CNS microvasculature, therefore limiting their entry into the CNS parenchyma (5). CXCL12 expression is present along the basolateral surfaces of CNS endothelial cells where leukocytes, which ubiquitously express its receptor CXCR4, engage CXCL12 when attempting to enter the CNS. This subcompartment retention, which is analogous to the role of CXCL12 in lymphoid compartments (6), could be an integral component of CNS protection from the pathologic consequences of immune cell activation (7). In CNS autoimmune diseases, such as multiple sclerosis (MS) and its murine model experimental autoimmune encephalomyelitis (EAE), alterations in...