Choroid Plexus Stroma Research Articles

The choroid plexus synergizes with immune cells during neuroinflammation

The choroid plexus (ChP) is a vital brain barrier and source of cerebrospinal fluid (CSF). Here, we use longitudinal two-photon imaging in awake mice and single-cell transcriptomics to elucidate the mechanisms of ChP regulation of brain inflammation. We used intracerebroventricular injections of lipopolysaccharides (LPS) to model meningitis in mice and observed that neutrophils and monocytes accumulated in the ChP stroma and surged across the epithelial barrier into the CSF. Bi-directional recruitment of monocytes from the periphery and, unexpectedly, macrophages from the CSF to the ChP helped eliminate neutrophils and repair the barrier. Transcriptomic analyses detailed the molecular steps accompanying this process and revealed that ChP epithelial cells transiently specialize to nurture immune cells, coordinating their recruitment, survival, and differentiation as well as regulation of the tight junctions that control the permeability of the ChP brain barrier. Collectively, we provide a mechanistic understanding and a comprehensive roadmap of neuroinflammation at the ChP brain barrier.

Transporters, Ion Channels, and Junctional Proteins in Choroid Plexus Epithelial Cells.

The choroid plexus (CP) plays significant roles in secreting cerebrospinal fluid (CSF) and forming circadian rhythms. A monolayer of epithelial cells with tight and adherens junctions of CP forms the blood-CSF barrier to control the movement of substances between the blood and ventricles, as microvessels in the stroma of CP have fenestrations in endothelial cells. CP epithelial cells are equipped with several kinds of transporters and ion channels to transport nutrient substances and secrete CSF. In addition, junctional components also contribute to CSF production as well as blood-CSF barrier formation. However, it remains unclear how junctional components as well as transporters and ion channels contribute to the pathogenesis of neurodegenerative disorders. In this manuscript, recent findings regarding the distribution and significance of transporters, ion channels, and junctional proteins in CP epithelial cells are introduced, and how changes in expression of their epithelial proteins contribute to the pathophysiology of brain disorders are reviewed.

Deep analysis of CD4 T cells in the rhesus CNS during SIV infection.

Virologic suppression with antiretroviral therapy (ART) has significantly improved health outcomes for people living with HIV, yet challenges related to chronic inflammation in the central nervous system (CNS)-known as Neuro-HIV- persist. As primary targets for HIV-1 with the ability to survey and populate the CNS and interact with myeloid cells to co-ordinate neuroinflammation, CD4 T cells are pivotal in Neuro-HIV. Despite their importance, our understanding of CD4 T cell distribution in virus-targeted CNS tissues, their response to infection, and potential recovery following initiation of ART remain limited. To address these gaps, we studied ten SIVmac251-infected rhesus macaques using an ART regimen simulating suboptimal adherence. We evaluated four macaques during the acute phase pre-ART and six during the chronic phase. Our data revealed that HIV target CCR5+ CD4 T cells inhabit both the brain parenchyma and adjacent CNS tissues, encompassing choroid plexus stroma, dura mater, and the skull bone marrow. Aligning with the known susceptibility of CCR5+ CD4 T cells to viral infection and their presence within the CNS, high levels of viral RNA were detected in the brain parenchyma and its border tissues during acute SIV infection. Single-cell RNA sequencing of CD45+ cells from the brain revealed colocalization of viral transcripts within CD4 clusters and significant activation of antiviral molecules and specific effector programs within T cells, indicating CNS CD4 T cell engagement during infection. Acute infection led to marked imbalance in the CNS CD4/CD8 ratio which persisted into the chronic phase. These observations underscore the functional involvement of CD4 T cells within the CNS during SIV infection, enhancing our understanding of their role in establishing CNS viral presence. Our findings offer insights for potential T cell-focused interventions while underscoring the challenges in eradicating HIV from the CNS, particularly in the context of sub-optimal ART.

Shifting from ependyma to choroid plexus epithelium and the changing expressions of aquaporin-1 and aquaporin-4.

The cells of the choroid plexus (CP) epithelium are specialized ependymal cells (ECs) but have distinct properties. The CP cells and ECs form single-cell sheets contiguous to each other at a transitional zone. The CP is underlined by a basal lamina and has barrier properties, whereas the ECs do not. The basal lamina of the CP is continuous with the glia limitans superficialis and, consequently, the CP stroma is continuous with the meninges along entering blood vessels. The CP has previously been reported to express aquaporin-1 (AQP1) mostly apically, and ECs show mostly basolateral aquaporin-4 (AQP4) expression. Recent evidence in various systems has shown that in changing conditions the expression and distribution of AQP4 can be modified, involving phosphorylation and calmodulin-triggered translocation. Studies on the human CP revealed that AQP4 is also expressed in some CP cells, which is likely to be increased during ageing based on mouse data. Moreover, subependymal astrocytic processes in the ependyma-CP transition, forming a glial plate around blood vessels and facing the CP stroma, were strongly positive for AQP4. We propose that the increased AQP4 expression might be a compensatory mechanism for the observed reduction in CSF production in the ageing human brain. The high AQP4 density in the transition zone might facilitate the transport of water into and out of the CP stroma and serve as a drainage and clearing pathway for metabolites in the CNS.

Aquaporin-1 and Aquaporin-4 Expression in Ependyma, Choroid Plexus and Surrounding Transition Zones in the Human Brain.

The choroid plexus (CP) is a structure in the brain ventricles that produces the main part of the cerebrospinal fluid (CSF). It is covered with specialized cells which show epithelial characteristics and are the site of the blood-CSF barrier. These cells form a contiguous cell sheet with ventricle-lining ependymal cells which are known to express aquaporin-4 (AQP4). In contrast, CP epithelial cells express aquaporin-1 (AQP1) apically. We investigated the expression patterns of aquaporins in the CP-ependyma transition from human body donors using immunofluorescence and electron microscopy. Ependymal cells and subependymal astrocytes at the base of the CP showed a particularly high AQP4 immunoreactivity. Astrocytic processes formed a dense meshwork or glial plate around the blood vessels entering the CP. Interestingly, some of these astrocytic processes were in direct contact with the CP stroma, which contains fenestrated blood vessels, separated only by a basal lamina. Electron microscopy confirmed the continuity of the subastrocytic basal lamina with the CP epithelium. We also probed for components of the AQP4 anchoring dystrophin-dystroglycan complex. Immunolabeling for dystrophin and AQP4 showed an overlapping staining pattern in the glial plate but not in previously reported AQP4-positive CP epithelial cells. In contrast, dystroglycan expression was associated with laminin staining in the glial plate and the CP epithelium. This suggests different mechanisms for AQP4 anchoring in the cell membrane. The high AQP4 density in the connecting glial plate might facilitate the transport of water in and out of the CP stroma and could possibly serve as a drainage and clearing pathway for metabolites.

Increased cytokine expression in the choroid plexus stroma and epithelium in response to endotoxin-induced systemic inflammation in mice.

Sepsis-associated encephalopathy (SAE) is characterized as diffuse brain dysfunction in patients with excessive systemic inflammatory reaction to an infection. In our previous studies using a mouse model of SAE with intraperitoneal injection of lipopolysaccharide (LPS), tissue concentrations of various cytokines were elevated in the entire brain parenchyma 4 and 24 h following LPS administration. Cytokines elevated at 4 h were produced by the choroid plexus, leptomeninges and vascular endothelium, while those at 24 h were produced by astrocytes. Interleukin (IL)-1β did not increase in the concentration in the brain parenchyma during the period from 1 to 24 h following LPS. In the present study, we hypothesized that the intracranial cells that initially respond to systemic inflammation are situated in the choroid plexus and produce IL-1β to initiate cytokine-mediated reactions. We quantified the transcript levels of related cytokines within the choroid plexus and specified the choroid plexus cells that are involved in the immediate cytokine-mediated responses. Mice received LPS or saline by intraperitoneal injection. Four hours after treatments, the choroid plexuses were isolated and subjected to cytokine gene expression analyses using real-time reverse transcription-polymerase chain reaction. Another group of mice was fixed at 1, 4 and 24 h after treatments and the expression of cytokines and receptors was studied with double immunohistofluorescence staining. The transcript levels of IL-1β, CC-motif ligand (CCL)2, CXC-motif ligand (CXCL)1, CXCL2 and IL-6 in the choroid plexus were significantly increased in mice treated with LPS compared to saline control. The IL-1β expression was remarkable in choroid plexus macrophages at 1 and 4 h but not in the brain parenchyma. Choroid plexus stromal cells expressed IL-1 receptor type 1 (IL-1R1). The IL-1R1-bearing stromal cells produced CCL2, CXCL1, CXCL2 and IL-6 at 4 h. Choroid plexus epithelial cells expressed CXCR2, a common receptor for CXCL1 and CXCL2. Choroid plexus epithelial cells also expressed CCL2, CXCL1 and CXCL2 at 4 h, and IL-1R1-bearing stromal cells expressed CXCR2. Therefore, in response to systemic LPS injection, one of the intracranial reactions was initiated within the choroid plexus using IL-1β derived from macrophages. The choroid plexus stromal cells subsequently had elevated expression of CCL2, CXCL1, CXCL2 and IL-6. The choroid plexus epithelial cells also had elevated expression of CCL2, CXCL1 and CXCL2. The presence of receptors for these cytokines on both epithelial and stromal cells raised the possibility of reciprocal interactions between these cells. The results suggested that the immediate early responses exerted by the choroid plexus are relevant to understanding how SAE is initiated in clinical settings.

The choroid plexus stroma constitutes a sanctuary for paediatric B-cell precursor acute lymphoblastic leukaemia in the central nervous system.

Despite current central nervous system‐directed therapies for childhood B‐cell precursor acute lymphoblastic leukaemia, relapse at this anatomical site still remains a challenging issue. Few reports have addressed the study of the specific cellular microenvironments which can promote the survival, quiescence, and therefore chemoresistance of B‐cell precursor acute lymphoblastic leukaemia cells in the central nervous system. Herein, we showed by immunofluorescence and electron microscopy that in xenotransplanted mice, leukaemic cells infiltrate the connective tissue stroma of the choroid plexus, the brain structure responsible for the production of cerebrospinal fluid. The ultrastructural study also showed that leukaemia cells are able to migrate through blood vessels located in the choroid plexus stroma. In short‐term co‐cultures, leukaemic cells established strong interactions with human choroid plexus fibroblasts, mediated by an increased expression of ITGA4 (VLA‐4)/ITGAL (LFA‐1) and their ligands VCAM1/ICAM1. Upon contact with leukaemia cells, human choroid plexus fibroblasts acquired a cancer‐associated fibroblast phenotype, with an increased expression of α‐SMA and vimentin as well as pro‐inflammatory factors. Human choroid plexus fibroblasts also have the capacity to reduce the proliferative index of leukaemic blasts and promote their survival and chemoresistance to methotrexate and cytarabine. The inhibition of VLA‐4/VCAM‐1 interactions using anti‐VLA‐4 antibodies, and the blockade of Notch signalling pathway by using a γ‐secretase inhibitor partially restored chemotherapy sensitivity of leukaemia cells. We propose that the choroid plexus stroma constitutes a sanctuary for B‐cell precursor acute lymphoblastic leukaemia cells in the central nervous system. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Principles of neuroanatomical architecture supporting brain–immune cell–cell interactions

AbstractThe brain interacts with the immune system under inflammatory and non‐inflammatory conditions. Brain cells have the capacity to produce a variety of cytokines in response to systemic immune activation. Therefore, we hypothesized that brain cells and immune cells move close to each other, so that they can carry out intercellular interactions. In the present article, I review our studies to determine the site for the brain–immune cell–cell interactions. The leptomeninges, choroid plexus stroma and epithelium, attachments of the choroid plexuses, vascular endothelial cells, cells of the perivascular space, and astrocytic end‐feet compose the histological architecture that supports interactions between bone marrow‐derived myeloid cells and brain parenchymal cells under non‐inflammatory conditions. This architecture also functions as the interface at which systemic inflammation‐induced responses can be conveyed to the brain parenchyma under inflammatory conditions. It should be noted that such brain–immune interactions occur in the absence of florid inflammatory cell infiltrates in the brain parenchyma, and with relative preservation of the blood‐brain barrier. Rather, cells comprising the blood‐brain barrier act as the interface linking peripheral immune activation and brain responses. These responses, known as “neuroinflammation”, are unique to the brain. Neuroinflammation differs from the histological inflammation that is accompanied by a florid infiltration of the leptomeninges (meningitis) and/or brain parenchyma (encephalitis) by inflammatory cells. In this review, the relevance of histological studies to unravel the mechanisms linking systemic inflammation, brain–immune interface cells and brain parenchymal cells, with a special emphasis on sepsis‐associated encephalopathy, is discussed.

PS913 ROLE OF CHOROID PLEXUS STROMA IN TUMOUR CHEMORESISTANCE: A SANCTUARY FOR LEUKAEMIA CELLS IN THE CENTRAL NERVOUS SYSTEM

Background:Central Nervous System (CNS) relapses are still one of the main causes of morbimortality in childhood acute lymphoblastic leukaemia (ALL) patients despite the reduction achieved by CNS‐directed prophylactic chemotherapy. Although CNS colonization by ALL cells has been already associated to both increased survival and enhanced chemoresistance of blasts to intrathecal therapy, the mechanisms of protection and tumour cell maintenance in this compartment, as well as the interactions established between ALL and CNS cells, still need to be better understood.Aims:We propose in vivo and in vitro studies aimed at determining the location of leukaemia cells in the choroid plexus (CP) and the relevance of their interaction with stromal fibroblastic cells to create a tumour supportive microenvironment. Also, we explore the contribution of stromal cells to the ALL chemoresistance.Methods:Brain histopathology evaluation of immunodeficient NSG mice transplanted with human primary ALL cells or the Nalm‐6 cell line was performed by using electron and fluorescence microscopy. Leukemic co‐cultures with human CP cells in the presence or absence of different concentrations of chemotherapeutic drugs were used for viability assays. Changes in CP microenvironment were further tested by RT‐qPCR, flow cytometry, ELISA and CBA assays.Results:The histological analysis of a xenograft model of ALL revealed the presence of leukaemic cells not only in the cerebral ventricles floating in the cerebrospinal fluid, but also interacting with the CP epithelium and within the CP stroma of those animals with CNS infiltration. The proximity of ALL cells to CP capillaries suggests they may easily enter through this fenestrated endothelium. Evaluation of interactions between leukaemic cells and stromal cells showed an upregulated expression of very late antigen‐4 (VLA‐4) in ALL cells and vascular cell adhesion molecule (VCAM1) in CP fibroblasts. Furthermore, in these co‐cultures, ALL cells exhibited a reduced cell proliferation and an enhanced cell viability following treatment with chemotherapeutic agents. However, the blockade of VCAM1‐VLA‐4 interaction was found to impair those effects suggesting that VCAM1‐VLA‐4 interactions are, at least in part, responsible of protecting leukemic cells from cell death.Summary/Conclusion:Choroid plexus is proposed as a possible sanctuary for leukemic cells. There, ALL cells may remain attached to fibroblasts in a dormant state and protected from intrathecal chemotherapy treatment, thus favouring later relapses.

PS914 CONDITIONAL MEF2C EXPRESSION IN THE MOUSE THYMUS DRIVES DEVELOPMENT OF T/B-BIPHENOTYPIC LYMPHOID LEUKEMIA/LYMPHOMA

Background: Central Nervous System (CNS) relapses are still one of the main causes of morbimortality in childhood acute lymphoblastic leukaemia (ALL) patients despite the reduction achieved by CNS-directed prophylactic chemotherapy. Although CNS colonization by ALL cells has been already associated to both increased survival and enhanced chemoresistance of blasts to intrathecal therapy, the mechanisms of protection and tumour cell maintenance in this compartment, as well as the interactions established between ALL and CNS cells, still need to be better understood. Aims: We propose in vivo and in vitro studies aimed at determining the location of leukaemia cells in the choroid plexus (CP) and the relevance of their interaction with stromal fibroblastic cells to create a tumour supportive microenvironment. Also, we explore the contribution of stromal cells to the ALL chemoresistance. Methods: Brain histopathology evaluation of immunodeficient NSG mice transplanted with human primary ALL cells or the Nalm-6 cell line was performed by using electron and fluorescence microscopy. Leukemic co-cultures with human CP cells in the presence or absence of different concentrations of chemotherapeutic drugs were used for viability assays. Changes in CP microenvironment were further tested by RT-qPCR, flow cytometry, ELISA and CBA assays. Results: The histological analysis of a xenograft model of ALL revealed the presence of leukaemic cells not only in the cerebral ventricles floating in the cerebrospinal fluid, but also interacting with the CP epithelium and within the CP stroma of those animals with CNS infiltration. The proximity of ALL cells to CP capillaries suggests they may easily enter through this fenestrated endothelium. Evaluation of interactions between leukaemic cells and stromal cells showed an upregulated expression of very late antigen-4 (VLA-4) in ALL cells and vascular cell adhesion molecule (VCAM1) in CP fibroblasts. Furthermore, in these co-cultures, ALL cells exhibited a reduced cell proliferation and an enhanced cell viability following treatment with chemotherapeutic agents. However, the blockade of VCAM1-VLA-4 interaction was found to impair those effects suggesting that VCAM1-VLA-4 interactions are, at least in part, responsible of protecting leukemic cells from cell death. Summary/Conclusion: Choroid plexus is proposed as a possible sanctuary for leukemic cells. There, ALL cells may remain attached to fibroblasts in a dormant state and protected from intrathecal chemotherapy treatment, thus favouring later relapses.

Abstract # 3098 Cytokine-mediated cell–cell interaction between choroid plexus epithelium and stroma in response to systemic inflammation

We reported that the choroid plexus stromal and epithelial cells produce cytokines 4 h after i.p. administration of lipopolysaccharide (LPS). These choroid plexus-derived as well as leptomeningeal-derived cytokines act on receptors expressed by astrocytic processes to stimulate astrocytes to produce other cytokines 24 h after LPS injection, resulting in brain microenvironmental alteration. We hypothesized the presence of choroid plexus cells that produce cytokines to exert the immediate response earlier than 4 h after LPS injection. Mice received LPS or saline i.p. and were fixed with 4% paraformaldehyde 1 and 4 h later. Using frozen sections of the brain containing the choroid plexus, the expression of cytokines, receptors and cell markers were investigated with double immunofluorescence. The results indicated that choroid plexus stromal macrophages expressed IL-1 β 1 h after LPS and some stromal cells expressed IL-1R1 (IL-1 receptor). These stromal cells expressed IL-6, CXCL2 and CCL2 4 h after LPS and the same cells bore CXCR2 (CXCL1/CXCL2 receptor). Furthermore, the choroid plexus epithelial cells expressed CXCL1 and CXCL2 4 h after LPS and bore CXCR2 and IL-6 receptor. Therefore, the signaling using IL-1 β from choroid plexus macrophages and IL-1R1 on choroid plexus stromal cells is the immediate reaction and triggers the cytokine-mediated cell–cell interactions between choroid plexus epithelial and stromal cells. These interactions within the choroid plexus ignite the cytokine responses exerted by brain parenchymal cells.

Histological Architecture Underlying Brain-Immune Cell-Cell Interactions and the Cerebral Response to Systemic Inflammation.

Although the brain is now known to actively interact with the immune system under non-inflammatory conditions, the site of cell–cell interactions between brain parenchymal cells and immune cells has been an open question until recently. Studies by our and other groups have indicated that brain structures such as the leptomeninges, choroid plexus stroma and epithelium, attachments of choroid plexus, vascular endothelial cells, cells of the perivascular space, circumventricular organs, and astrocytic endfeet construct the histological architecture that provides a location for intercellular interactions between bone marrow-derived myeloid lineage cells and brain parenchymal cells under non-inflammatory conditions. This architecture also functions as the interface between the brain and the immune system, through which systemic inflammation-induced molecular events can be relayed to the brain parenchyma at early stages of systemic inflammation during which the blood–brain barrier is relatively preserved. Although brain microglia are well known to be activated by systemic inflammation, the mechanism by which systemic inflammatory challenge and microglial activation are connected has not been well documented. Perturbed brain–immune interaction underlies a wide variety of neurological and psychiatric disorders including ischemic brain injury, status epilepticus, repeated social defeat, and neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Proinflammatory status associated with cytokine imbalance is involved in autism spectrum disorders, schizophrenia, and depression. In this article, we propose a mechanism connecting systemic inflammation, brain–immune interface cells, and brain parenchymal cells and discuss the relevance of basic studies of the mechanism to neurological disorders with a special emphasis on sepsis-associated encephalopathy and preterm brain injury.

Psammoma bodies as signs of choroid plexus ageing: A morphometric analysis

Background/Aim. Psammoma bodies (PB) are regarded as benign consequences of ageing in choroid plexus stroma. The aim of this study was to assess the morphometric characteristics of psammoma bodies of all four choroid plexuses during the ageing process. Our intention was to find the possible relations between psammoma bodies and choroid plexus and blood vessels parameters. Methods. This study was conducted on the material taken from 15 cadavers during routine autopsies. Tissue samples were collected from both lateral, third and forth ventricles? choroid plexus. Slices were stained with Mallory trichrome stains. In each image, we analyzed morphometrically the epithelium, blood vessels present and all the psammoma bodies. Results. With age, right choroid plexus surface density decreases (p < 0.05), while the psammoma bodies volume density increases (p < 0.05). A decrease in the blood vessels volume density was observed in the third ventricle?s choroid plexus (p < 0.05), as well as an age-related decrease in the psammoma bodies perimeter (p < 0.01). Not associated with ageing, the increase in psammoma bodies perimeter and volume density predicts a decrease in choroid plexus surface density (p < 0.05 and p < 0.001, respectively). There was a decrease in the volume density of blood vessels with age and with the increase in Feret?s diameter of psammoma bodies, (p < 0.001 and p < 0.05, respectively). Conclusion. We want to point out that there is an association between ageing and increased size and volume density of psammoma bodies. More important is the fact that psammoma bodies? presence and their morphometric characteristics are good predictors of changes occurring on the level of choroid plexus structure and vascularization, which may have crucial effects on the choroid plexus physiology.

Endotoxemia-induced cytokine reactions by hippocampal astrocytes mediated by cells in the brain-immune interface

Systemic immune activation is known to change brain cytokine microenvironment. Here we identified cells and cytokines involved in the brain reaction to endotoxin-induced systemic inflammation. Adult male C57BL/6 mice were intraperitoneally injected with lipopolysaccharide (LPS) or saline. One, 4 and 24 h after LPS and 1 h after saline injection, spleens and hippocampi were homogenized and tissue concentrations of multiple cytokines were determined by the Multiplex assays. Another group of mice were fixed under the same schedule and studied immunohistologically. Fifteen out of 33 cytokines examined increased in concentration in the spleen 1 and/or 4 h after LPS injection, compared to saline control. Ten cytokines increased in the hippocampus: TNF-a only at 1 h, CCL2, CXCL1, CXCL2, CXCL9, IL-6 and LIF only at 4h, CCL11 and CXCL10 at 4 and 24 h and G-CSF at only 24h after LPS injection. Cytokines elevated at 4 h were expressed by hippocampal vascular endothelial cells, CXCL12-immunopositive stromal cells of the leptomeninges and choroid plexus stroma, and choroid plexus epithelial cells, all of which were located at the brain-immune interface as defined in our previous bone marrow chimera study. The receptors of these cytokines were expressed in astrocytic endfeet. Cytokines elevated at 24 h were expressed widespreadly by astrocytes. Thus, astrocytes are the major hippocampal responder cells to endotoxemia, whose reactions are mediated by cells in the brain-immune interface.

Alterations in tight junction protein and IgG permeability accompany leukocyte extravasation across the choroid plexus during neuroinflammation.

The choroid plexus (CP) is considered to be a point of leukocyte entry into the CNS during normal immune surveillance and in neuroinflammatory diseases. The structural and functional alterations within the CP that support this migration are not understood. We used quantitative, high-resolution, 3-dimensional (3-D) fluorescence imaging to analyze CP alterations associated with inflammatory responses in C57/Bl6 mice after the induction of experimental autoimmune encephalomyelitis by immunization with myelin oligodendrocyte glycoprotein (MOG) and complete Freund adjuvant/pertussis toxin (MOG-CFA/PTX) or adjuvants alone (CFA-PTX). The MOG-CFA/PTX and CFA/PTX produced similar effects, although those caused by the former were consistently more marked. Both treatments resulted in the accumulation of serum immunoglobulin G and leukocytes in the CP stroma, consistent with elevated stromal capillary permeability. They also provoked distortions and diminished immunostaining patterns of the tight junction adaptor protein ZO-1 in the choroidal epithelium but no obvious change in the patterns of the tight junction associated protein claudin-2. Only MOG-CFA/PTX triggered visible extravasation of immunoglobulin G and leukocytes across the choroidal epithelium. Our results suggest that CFA/PTX primes the CP for neuroinflammation by inducing structural changes that are exacerbated when there is an immune response to MOG and reinforce the CP as a gateway for leukocytes to enter the CNS by accessing the CSF and leptomeninges.

IFN-γ-dependent activation of the brain’s choroid plexus for CNS immune surveillance and repair

Infiltrating T cells and monocyte-derived macrophages support central nervous system repair. Although infiltration of leucocytes to the injured central nervous system has recently been shown to be orchestrated by the brain's choroid plexus, the immunological mechanism that maintains this barrier and regulates its activity as a selective gate is poorly understood. Here, we hypothesized that CD4(+) effector memory T cells, recently shown to reside at the choroid plexus stroma, regulate leucocyte trafficking through this portal through their interactions with the choroid plexus epithelium. We found that the naïve choroid plexus is populated by T helper 1, T helper 2 and regulatory T cells, but not by encephalitogenic T cells. In vitro findings revealed that the expression of immune cell trafficking determinants by the choroid plexus epithelium is specifically induced by interferon-γ. Tumour necrosis factor-α and interferon-γ reciprocally controlled the expression of their receptors by the choroid plexus epithelium, and had a synergistic effect in inducing the epithelial expression of trafficking molecules. In vivo, interferon-γ-dependent signalling controlled trafficking through the choroid plexus; interferon-γ receptor knockout mice exhibited reduced levels of T cells and monocyte entry to the cerebrospinal fluid and impaired recovery following spinal cord injury. Moreover, reduced expression of trafficking molecules by the choroid plexus was correlated with reduced CD4(+) T cells in the choroid plexus and cerebrospinal fluid of interferon-γ receptor knockout mice. Similar effect on the expression of trafficking molecules by the choroid plexus was found in bone-marrow chimeric mice lacking interferon-γ receptor in the central nervous system, or reciprocally, lacking interferon-γ in the circulation. Collectively, our findings attribute a novel immunological plasticity to the choroid plexus epithelium, allowing it to serve, through interferon-γ signalling, as a tightly regulated entry gate into the central nervous system for circulating leucocytes immune surveillance under physiological conditions, and for repair following acute injury.

17. Selective localization of bone marrow-derived ramified cells in the brain adjacent to the attachments of choroid plexus

Although the immune system modulates higher functions of the brain under non-inflammatory conditions, how immune cells interact with brain parenchymal cells remains to be determined. Using bone marrow chimeric mice produced by syngeneic intra-bone marrow-bone marrow transplantation (IBM-BMT) with GFP-transgenic mice being as a donor, we examined the distribution, density and differentiation of donor-derived marrow cells in the brain parenchyma 2 weeks and 1, 4 and 8 months after BMT. Marrow-derived cells started to populate discrete brain regions from 1 to 4 months after BMT, exhibited ramified morphology and expressed Iba-1. Most of these discrete regions were adjacent to the attachments of choroid plexus that comprised thinned brain parenchyma consisting of astroglial processes in the narrow channel between the ependyma and pia. These specific portions of astroglial processes expressed fractalkine. In the choroid plexus stroma, not only Iba-1 + myeloid cells but also non-myeloid CXCL12-expressing cells were of bone marrow-origin. Transcripts of fractalkine, CXCL12 and their related molecules such as CX3CR1, ADAM10 and CXCR4 were detected in the tissue consisting of the choroid plexus, the attachments and adjacent brain parenchyma. Thus, bone marrow cells selectively enter the discrete brain regions adjacent to the attachments of choroid plexus and differentiate into ramified myeloid cells. Fractalkine in the attachments of choroid plexus and CXCL12 in the choroid plexus stroma may be involved in these brain–immune interactions.

Selective localization of bone marrow-derived ramified cells in the brain adjacent to the attachments of choroid plexus

Although the immune system modulates higher functions of the brain under non-inflammatory conditions, how immune cells interact with brain parenchymal cells remains to be determined. Using bone marrow chimeric mice in which the recipients' immune system was reconstituted by marrow cells derived from GFP-transgenic mice by syngeneic intra-bone marrow-bone marrow transplantation (IBM-BMT) and by intravenous (IV)-BMT, we examined the distribution, density and differentiation of donor-derived marrow cells in the brain parenchyma 2 weeks and 1, 4 and 8 months after BMT. Marrow-derived cells started to populate discrete brain regions from 1 to 4 months after BMT, exhibited ramified morphology and expressed Iba-1. The ramified marrow-derived cells were distributed in more brain regions and for a longer time after IBM-BMT than IV-BMT. Most of these discrete regions were adjacent to the attachments of choroid plexus that comprised thinned brain parenchyma consisting of astroglial processes in the narrow channel between the ependyma and pia. These specific portions of astroglial processes expressed fractalkine. In the choroid plexus stroma, not only Iba-1+ myeloid cells but also non-myeloid CXCL12-expressing cells were of bone marrow-origin. Transcripts of fractalkine, CXCL12 and their related molecules such as CX3CR1, ADAM10 and CXCR4 were detected in the tissue consisting of the choroid plexus, the attachments and adjacent brain parenchyma. Thus, bone marrow cells selectively enter the discrete brain regions adjacent to the attachments of choroid plexus and differentiate into ramified myeloid cells. Fractalkine in the attachments of choroid plexus and CXCL12 in the choroid plexus stroma may be involved in these brain-immune interactions.

5. Intra bone marrow procedure facilitates entry of transplanted bone marrow cells through the tenia of choroid plexus into brain parenchyma

Although the immune system modulates the brain function under steady-state conditions, it has not been clear where and how the peripheral immune cells interact with brain parenchymal cells. We created radiation chimera mice in which recipients’ immune system was reconstituted by bone marrow cells derived from donor GFP transgenic mice by a novel intra-bone marrow–bone marrow transplantation (IBM–BMT) method and by conventional intravenous (IV)–BMT. We examined the distribution, differentiation and density of donor-derived bone marrow (BM) cells in the brain parenchyma 2 weeks and 1, 4 and 8 months after the BMT. We found BM-derived cells in several discrete brain regions mostly adjacent to the tenia of the choroid plexus from 1 to 4 months after BMT, but not in the remaining major parts until 8 months after BMT. BM-derived cells exhibited ramified morphology and expressed Iba-1 but not GFAP, CNPase or NeuN, indicative of the myeloid lineage. The densities of BM-derived cells in the brain parenchyma increased in a time dependent manner after IBM–BMT but not after IV–BMT, therefore the BM-derived cell densities 8 months after IBM–BMT were significantly higher than those after IV–BMT. The choroid plexus stroma contained more BM-derived cells after IBM–BMT than after IV–BMT. These results suggested that the tenia of the choroid plexus represents a novel route of entry for peripheral immune cells into the brain parenchyma under steady-state conditions.

Psammoma bodies – Friends or foes of the aging choroid plexus

Psammoma bodies are structures classified in the group of dystrophic calcifications, which occur in some kind of tumors and in choroid plexus during the aging process. Despite early discovery of their presence in choroid plexus stroma, mechanisms responsible for their formation remained unclear. Their presence in some kind of tumors was even more extensively studied, but significant breakthrough in the field of their etiology was not attained, too. However, till today correlation between their presence in tumors and aging is not established. Also, there are not any data about structural differences between ones found in tumors and ones found in choroid plexus. This might points to the assumption that besides the aging, some other causes might be involved in their formation in choroid plexus. Furthermore, it is contradictory that forms, like psammoma bodies, present in such malignant formations as tumors, represent quite benign phenomenon in choroid plexus. Literature data and the results of our previous researches revealed that there might be connections between, these, on the first sight quite different processes. Firstly, psammoma bodies are present in stroma of tumors with predominantly papillomatous morphology, which is present in choroid plexus, too. Initial forms of psammoma bodies might be formed in fibrovascular core of choroid plexus villi, similarly like in tumors papillae of papillary thyroid cancer. Their further growth leads to the progressive destruction of both tumors papillae and choroidal villi. Choroid plexus stroma is characterized by the fenestrated blood vessels presence, which are similar to newly formed vessels in tumors. This makes it vulnerable to the noxious agents from circulation. It can contain lymphocytes, macrophages, dendritic cells and myofibroblasts in cases with psammoma bodies, similarly to tumors stroma which is in activated, proinflammatory state. So, all these facts can suggest that similar processes can lead to psammoma bodies formation in both tumors and choroid plexus and, that they might have harmful effect on choroid plexus structure and function during the aging process. Significantly higher degree of choroidal epithelial cells atrophy, in cases with present psammoma bodies proves that partially. Further researches should be focused on detection of osteopontin and nanobacteria, already detected in tumors psammoma bodies, in choroid plexus ones. Discovery of choroidal psammoma bodies mechanisms formation can be important for elucidation of some aspects in pathogenesis of some tumors, too. Application of choroid plexus epithelial cells functional markers in cases with psammoma bodies should show their functional status.