Identification of direct connections between the dura and the brain.

The arachnoid barrier delineates the border between the central nervous system and dura mater. Although the arachnoid barrier creates a partition, communication between the central nervous system and the dura mater is crucial for waste clearance and immune surveillance1,2. How the arachnoid barrier balances separation and communication is poorly understood. Here, using transcriptomic data, we developed transgenic mice to examine specific anatomical structures that function as routes across the arachnoid barrier. Bridging veins create discontinuities where they cross the arachnoid barrier, forming structures that we termed arachnoid cuff exit (ACE) points. The openings that ACE points create allow the exchange of fluids and molecules between the subarachnoid space and the dura, enabling the drainage of cerebrospinal fluid and limited entry of molecules from the dura to the subarachnoid space. In healthy human volunteers, magnetic resonance imaging tracers transit along bridging veins in a similar manner to access the subarachnoid space. Notably, in neuroinflammatory conditions such as experimental autoimmune encephalomyelitis, ACE points also enable cellular trafficking, representing a route for immune cells to directly enter the subarachnoid space from the dura mater. Collectively, our results indicate that ACE points are a critical part of the anatomy of neuroimmune communication in both mice and humans that link the central nervous system with the dura and its immunological diversity and waste clearance systems.

Inflammation-inducible type 2 deiodinase expression in the leptomeninges, choroid plexus, and at brain blood vessels in male rodents.

Thyroid hormone regulates immune functions and has antiinflammatory effects. In promoter assays, the thyroid hormone-activating enzyme, type 2 deiodinase (D2), is highly inducible by the inflammatory transcription factor nuclear factor-κ B (NF-κB), but it is unknown whether D2 is induced in a similar fashion in vivo during inflammation. We first reexamined the effect of bacterial lipopolysaccharide (LPS) on D2 expression and NF-κB activation in the rat and mouse brain using in situ hybridization. In rats, LPS induced very robust D2 expression in normally non-D2-expressing cells in the leptomeninges, adjacent brain blood vessels, and the choroid plexus. These cells were vimentin-positive fibroblasts and expressed the NF-κB activation marker, inhibitor κ B-α mRNA, at 2 hours after injection, before the increase in D2 mRNA. In mice, LPS induced intense D2 expression in the choroid plexus but not in leptomeninges, with an early expression peak at 2 hours. Moderate D2 expression along numerous brain blood vessels appeared later. D2 and NF-κB activation was induced in tanycytes in both species but with a different time course. Enzymatic assays from leptomeningeal and choroid plexus samples revealed exceptionally high D2 activity in LPS-treated rats and Syrian hamsters and moderate but significant increases in mice. These data demonstrate the cell type-specific, highly inducible nature of D2 expression by inflammation, and NF-κB as a possible initiating factor, but also warrant attention for species differences. The results suggest that D2-mediated T3 production by fibroblasts regulate local inflammatory actions in the leptomeninges, choroid plexus and brain blood vessels, and perhaps also in other organs.

The relationship between localized subarachnoid inflammation and parenchymal pathophysiology after spinal cord injury.

Subarachnoid inflammation following spinal cord injury (SCI) can lead to the formation of localized subarachnoid scarring and the development of post-traumatic syringomyelia (PTS). While PTS is a devastating complication of SCI, its relative rarity (occurring symptomatically in about 5% of clinical cases), and lack of fundamental physiological insights, have led us to examine an animal model of traumatic SCI with induced arachnoiditis. We hypothesized that arachnoiditis associated with SCI would potentiate early parenchymal pathophysiology. To test this theory, we examined early spatial pathophysiology in four groups: (1) sham (non-injured controls), (2) arachnoiditis (intrathecal injection of kaolin), (3) SCI (35-g clip contusion/compression injury), and (4) PTS (intrathecal kaolin+SCI). Overall, there was greater parenchymal inflammation and scarring in the PTS group relative to the SCI group. This was demonstrated by significant increases in cytokine (IL-1α and IL-1ß) and chemokine (MCP-1, GRO/KC, and MIP-1α) production, MPO activity, blood-spinal cord barrier (BSCB) permeability, and MMP-9 activity. However, parenchymal inflammatory mediator production (acute IL-1α and IL-1ß, subacute chemokines), BSCB permeability, and fibrous scarring in the PTS group were larger than the sum of the SCI group and arachnoiditis group combined, suggesting that arachnoiditis does indeed potentiate parenchymal pathophysiology. Accordingly, these findings suggest that the development of arachnoiditis associated with SCI can lead to an exacerbation of the parenchymal injury, potentially impacting the outcome of this devastating condition.

Human cerebrospinal fluid central memory CD4+ T cells: evidence for trafficking through choroid plexus and meninges via P-selectin.

Cerebrospinal fluid (CSF) from healthy individuals contains between 1,000 and 3,000 leukocytes per ml. Little is known about trafficking patterns of leukocytes between the systemic circulation and the noninflamed CNS. In the current study, we characterized the surface phenotype of CSF cells and defined the expression of selected adhesion molecules on vasculature in the choroid plexus, the subarachnoid space surrounding the cerebral cortex, and the cerebral parenchyma. Using multicolor flow cytometry, we found that CSF cells predominantly consisted of CD4+/CD45RA-/CD27+/CD69+-activated central memory T cells expressing high levels of CCR7 and L-selectin. CD3+ T cells were present in the choroid plexus stroma in autopsy CNS tissue sections from individuals who died without known neurological disorders. P- and E-selectin immunoreactivity was detected in large venules in the choroid plexus and subarachnoid space, but not in parenchymal microvessels. CD4+ T cells in the CSF expressed high levels of P-selectin glycoprotein ligand 1, and a subpopulation of circulating CD4+ T cells displayed P-selectin binding activity. Intercellular adhesion molecule 1, but not vascular cell adhesion molecule 1 or mucosal addressin cell adhesion molecule 1, was expressed in choroid plexus and subarachnoid space vessels. Based on these findings, we propose that T cells are recruited to the CSF through interactions between P-selectin/P-selectin ligands and intercellular adhesion molecule 1/lymphocyte function-associated antigen 1 in choroid plexus and subarachnoid space venules. These results support the overall hypothesis that activated memory T cells enter CSF directly from the systemic circulation and monitor the subarachnoid space, retaining the capacity to either initiate local immune reactions or return to secondary lymphoid organs.

HLA-DR expression on arachnoid cells. A role in the fibrotic inflammation surrounding nerve roots in spondylotic cervical myelopathy.

STUDY DESIGN: Human arachnoid cells were examined immunohistochemically for expression of HLA-DR or Fc receptors. OBJECTIVE: To assess the ability of arachnoid cells to express HLA-DR and act as antigen presenting cells. SUMMARY OF BACKGROUND DATA: An intradural inflammatory reaction surrounds the cervical nerve roots and radicular vessels in patients with spondylotic cervical myeloradiculopathy. Active arachnoid proliferation and fibrosis in this reaction suggest these cells are actively involved in this inflammatory process. An inflammatory response is initiated by an antigen presenting cell. Evidence suggests that arachnoid cells may be antigen presenting cells, like macrophages, because they can phagocytize foreign tissue and have immunoglobulin G and complement receptors. METHODS: Spinal arachnoid obtained from patients undergoing spinal surgery for intramedullary processes was grown in culture. The arachnoid cells were characterized and examined immunohistochemically for the expression of HLA-DR and Fc receptors. RESULTS: HLA-DR was expressed in vivo and in vitro by arachnoid cells. The macrophage Fc receptor was not expressed in vivo nor in vitro by arachnoid cells. CONCLUSION: Finding in vivo and in vitro HLA-DR expression on arachnoid cells adds further support to the concept that arachnoid cells have the potential to serve as antigen presenting cells. The absence of Fc receptors, although necessary for macrophages, does not negate antigen presenting function because other types of cells-e.g., endothelium-can present antigen without displaying Fc receptors. These experiments provide new evidence that arachnoid cells may initiate or sustain the intradural inflammatory reaction found in cervical myeloradiculopathy.

Monoclonal antibodies identify three IgG Fc receptors in normal human central nervous system.

Functional Fc receptors have been described in the central nervous system (CNS) in the subependymal periventricular regions, leptomeninges, including brain perivascular tissues, and choroid plexus. The distribution of this receptor activity suggests a role in protection of adjacent nervous tissue from IgG-opsonized antigens, including microorganisms. In this report, we have utilized monoclonal antibodies to human Fc gamma RI, II, and III; 32, IV.3, and 3G8, respectively, to immunohistochemically examine the distribution of these receptors in the CNS. Fc gamma RI was only occasionally present in the CNS where it was identified most often in the choroid plexus. Fc gamma RII was the predominant receptor in brain. It as consistently present in leptomeninges, including brain perivascular regions, arachnoid granulations, and choroid plexus stroma. Some samples of subependymal periventricular tissue also displayed Fc gamma RII. Fc gamma RIII was only identified in subependymal periventricular tissue but not in choroid plexus and arachnoid. These results demonstrate that regions of normal adult brain which produce cerebral spinal fluid (CSF) and border on CSF and vascular compartments display Fc gamma R heterogeneity consistent with that of blood monocytes and systemic macrophages.