The resolution of neuroinflammation in neurodegeneration: leukocyte recruitment via the choroid plexus

Review20 December 2013free access The resolution of neuroinflammation in neurodegeneration: leukocyte recruitment via the choroid plexus Michal Schwartz Corresponding Author Michal Schwartz Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel Search for more papers by this author Kuti Baruch Kuti Baruch Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel Search for more papers by this author Michal Schwartz Corresponding Author Michal Schwartz Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel Search for more papers by this author Kuti Baruch Kuti Baruch Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel Search for more papers by this author Author Information Michal Schwartz 1 and Kuti Baruch1 1Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel *Corresponding author. Tel: +972 8 934 2467; Fax: +972 8 934 6018; E-mail: [email protected] The EMBO Journal (2014)33:7-22https://doi.org/10.1002/embj.201386609 See the Glossary for abbreviations used in this article. PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract Inflammation is an integral part of the body's physiological repair mechanism, unless it remains unresolved and becomes pathological, as evident in the progressive nature of neurodegeneration. Based on studies from outside the central nervous system (CNS), it is now understood that the resolution of inflammation is an active process, which is dependent on well-orchestrated innate and adaptive immune responses. Due to the immunologically privileged status of the CNS, such resolution mechanism has been mostly ignored. Here, we discuss resolution of neuroinflammation as a process that depends on a network of immune cells operating in a tightly regulated sequence, involving the brain's choroid plexus (CP), a unique neuro-immunological interface, positioned to integrate signals it receives from the CNS parenchyma with signals coming from circulating immune cells, and to function as an on-alert gate for selective recruitment of inflammation-resolving leukocytes to the inflamed CNS parenchyma. Finally, we propose that functional dysregulation of the CP reflects a common underlying mechanism in the pathophysiology of neurodegenerative diseases, and can thus serve as a potential novel target for therapy. Glossary AD Alzheimer's disease ALS Amyotrophic Lateral Sclerosis Aβ Amyloid beta BBB Blood-Brain Barrier BCSFB Blood-Cerebrospinal fluid Barrier CCL Chemokine (C-C motif) ligand CCR Chemokine (C-C motif) receptor CNS Central Nervous System CP Choroid plexus CSF Cerebrospinal fluid EAE Experimental Autoimmune Encephalomyelitis GM-CSF Granulocyte-macrophage colony-stimulating factor ICAM-1 Intercellular Adhesion Molecule 1 IFN Interferon IL Interleukin PD Parkinson's disease PPMS Primary-Progressive Multiple Sclerosis RRMS Relapsing-Remitting Multiple Sclerosis SCI Spinal Cord Injury SPMS Secondary-Progressive Multiple Sclerosis Tregs Regulatory T cells VCAM-1 Vascular Cell Adhesion Molecule Introduction The definition of ‘inflammation’ has changed dramatically since first described by the Roman scholar, Aulus Cornelius Celsus, 2000 years ago. This phenomenon, defined by Celsus's four cardinal signs of ‘rubor et tumor cum calore et dolore’ (redness and swelling with heat and pain), and later recognized by Matchnikoff in the 19th century as a productive phagocytic process (Scott et al, 2004), is known today to reflect complex physiological interactions between resident and recruited immune cells, soluble factors and tissue-specific elements. This process, when properly orchestrated, results in protection from spread of infection or damage, followed by a resolution phase in which the affected tissues are restored to their original structural and functional state. However, as much as inflammation is a pivotal process in fighting off many threatening conditions, when it is unresolved, it forms the basis of a wide range of persistent/chronic diseases; while often not the primary cause of destruction in these diseases, secondary damage mediated by the inflammatory response constantly disrupts the return to homeostasis. Traditionally, resolution of inflammation was considered to be a passive process, through which inflammation spontaneously subsides. However, there is a growing appreciation that similarly to the initiation of inflammation, resolution of inflammation is an active process in which inflammation-resolving cells and their cytokines are pivotal for the termination of the inflammatory response (Nathan & Ding, 2010; Buckley et al, 2013). Inflammation in the central nervous system (CNS), neuroinflammation, is common to all neurodegenerative conditions, and is frequently viewed as detrimental to neurological function. In many of these diseases, such as Amyotrophic Lateral Sclerosis (ALS), Parkinson's disease (PD), Alzheimer's disease (AD) and Primary Progressive Multiple Sclerosis (PPMS), the etiology of the diseases is primarily sporadic, and no specific cellular or soluble components can account for the inflammatory response (Frank-Cannon et al, 2009). Importantly, while the mechanisms that ultimately lead to neurodegeneration are different in each disease, chronic neuroinflammation is typically a prominent feature in the progressive nature of neurodegeneration. Up until recently, it was believed that such local neuroinflammatory response reflects systemic inflammation. This contention, together with the immunologically privileged nature of the CNS, and the fact that neuroinflammation is often destructive to the neural parenchyma, led to the common view that entry of circulating immune cells to the CNS could only escalate the parenchymal damage, and therefore to the attempts to use systemic anti-inflammatory drugs to mitigate neuroinflammation in neurodegenerative diseases (Schwartz & Shechter, 2010b). Studies initiated by our group more than a decade ago, demonstrated that the recovery of the CNS from acute damage is non-tissue autonomous and requires the involvement of circulating leukocytes (Rapalino et al, 1998; Moalem et al, 1999; Yoles et al, 2001). These findings have led to subsequent studies which highlighted the possibility that infiltrating monocyte-derived macrophages are needed for fighting off neurodegenerative conditions (Butovsky et al, 2006, 2007) and brought to appreciation the pivotal role of CNS-specific T cells in CNS maintenance and repair (Kipnis et al, 2004c; Ziv et al, 2006), which led to the formulation of our theory of ‘protective autoimmunity’. According to this theory, a T-cell response, involving CD4+ T cells that recognize CNS self-antigens, provides the body's mechanism of protection, maintenance and repair in health and disease (Box 1). How the CNS parenchyma can benefit from circulating leukocytes under the constraints of being an immune privileged site, and how the cross-talk between the CNS and circulating immune cells can take place, despite the complex barrier systems that separate the CNS from the circulation, are now becoming more fully understood. Box 1: Overview of “protective autoimmunity” ‘Protective autoimmunity’ (Moalem et al, 1999) has been proposed by our team as an essential physiological mechanism for CNS protection, repair and maintenance in both health and pathological disease. Accordingly, autoimmune T cells do not necessarily reflect immune system malfunction, as originally believed; a well-controlled generation and activation of CNS-specific T cells is a purposeful process, and only when it is dysregulated these cells become destructive. This model challenges the dogma that an organism needs to completely delete self-reacting cells, and suggests that the anti-self response is pivotal for CNS tissue maintenance, though it requires more rigorous control than the response to non-self. It remains to be established whether protective autoimmunity is a more general phenomenon which occurs in tissues other than the CNS. Here, we will discuss the complexity of the immunological processes involved in chronic neuroinflammation and neurodegeneration, and emphasize that they are mediated by interactions of a physiological immune cell network, encompassing effector and regulatory, resident and infiltrating immune cells, which ultimately culminate in the recruitment of inflammation-resolving cells to the CNS via a designated ‘gate’ within the CNS territory but outside the parenchyma. Under physiological conditions, this immunological network maintains immune surveillance of the CNS from outside the parenchyma, and under pathological conditions, it participates in the resolution of neuroinflammation. We will suggest that this response is mediated by a unique neuro-immunological interface, the brain's choroid plexus (CP), which serves as a selective gateway for leukocyte entry. Accordingly, the fate of this interface under disease conditions can be viewed as a limiting factor in controlling the levels of systemic immune support provided to the CNS. Circulating immune cells fight off neuroinflammation in neurodegenerative diseases The vicious cycle of non-resolved neuroinflammation As briefly described above, for decades, neuroinflammation was viewed as a unified pathological phenomenon that should be completely eliminated regardless of its primary etiology. As a result of progress in understanding the pathophysiology of many neurodegenerative diseases, it has become slowly understood that the etiology of each disease has great impact on the nature of the local inflammatory response, and the role that is played by innate and adaptive immunity. Thus, the inflammatory component of the autoimmune disease Relapsing-Remitting MS (RRMS) differs from that of neurodegenerative diseases such as AD, ALS, and even other forms of MS [Secondary-Progressive (SPMS) and Primary-Progressive (PPMS)], with respect to both the local and the systemic immune response (Sospedra & Martin, 2005; Venken et al, 2008; He & Balling, 2013). Most CNS pathologies are characterized by an early acute reparative phase of microglial activation, which is needed for the effective removal of threatening compounds, toxic agents, and misfolded proteins (Block et al, 2007). This response often fails to lead to complete removal of the threats, or even results in an escalating effect in the form of a vicious cycle of unresolved local cytotoxic inflammation. Such a phenomenon led our group to suggest the possibility that CNS pathologies emerge after a prolonged struggle between a pathological process unique to a given disease, and an attempt for local tissue restoration by the immune system, and that such a process is reminiscent of the ‘competition’ between tumors and immune cells (Schwartz & Ziv, 2008). The triumph of cancer and its rapid propagation occurs when the tumor ultimately escapes from immune control. Such a competition between the microglia and the local source of CNS pathology is seen for example in AD, in which amyloid beta (Aβ) deposits (amyloid plaques) locally activate the resident, ‘resting’, microglia. In their activated state, microglia can potentially restrict plaque formation by secreting proteolytic enzymes, or clear Aβ by receptor-mediated phagocytosis (Lai & McLaurin, 2012; Sierra et al, 2013). However, chronic microglial activation in neurodegenerative diseases is accompanied by the production of pro-inflammatory cytokines which might override the beneficial effect of these cells (Block et al, 2007; Hanisch & Kettenmann, 2007). Arresting such a vicious cycle requires an active resolving immune response and the recruitment of systemic monocyte-derived macrophages (Simard et al, 2006; Butovsky et al, 2007), for which the unique structure of the CNS as an immune privileged site may pose an obstacle. As will be discussed below, we suggest that under acute injurious conditions microglial activation is among the first immune-related events at the lesion site (Fig 1), yet it seems that these cells either cannot acquire a resolving-phenotype, or fail to be skewed to this phenotype in a timely manner (Shechter et al, 2009; Shechter & Schwartz, 2013). Figure 1. Local and systemic immune cell response to acute or chronic CNS damageUnder neuroinflammatory situations, either acute (upper part) or chronic (bottom part), CNS parenchymal damage (black line) leads to glial cell activation and to local inflammatory response (red line). In response to acute CNS damage, circulating leukocytes are recruited to the CNS (blue line) and participate in the resolution of the innate inflammatory response. When such a response is not resolved it may lead to chronic neuroinflammation, associated with escalating toxicity and neuronal death, which is the case in chronic neurodegenerative diseases; the lack of resolution reflects insufficient recruitment of systemic inflammation-resolving immune cells to the CNS. Download figure Download PowerPoint CNS-specific T cells: from offenders to protectors The notion of the CNS as an immune privileged site (Ehrlich, 1885), from which immune cells should be excluded, was initially supported by the seminal observations of Shirai (Shirai, 1921) and Medawar (Medawar, 1948) who demonstrated that tissue grafts in the eye or brain survive longer than those implanted in other areas of the body. In the following decades, this notion was further substantiated by mounting experimental data demonstrating the spatial and immunological separation of the CNS from circulating immunity. Spatially, the CNS was found to be an enclosed compartment, secluded from the circulation by physical barriers, and the fact that the CNS has its own population of phagocytic cells, the resident microglia, was used to support the view that it is an immunologically autonomous unit, which ideally functions in the absence of immune surveillance (Barker & Billingham, 1977). In apparent support of this view, whenever immune cells were found in the perivascular spaces of the CNS, in the cerebrospinal fluid (CSF), or in the parenchyma itself, it was almost always considered a sign of autoimmune disease or at least of the beginning of such a disease. Accordingly, adaptive immune cells inside the CNS were repeatedly described as the prime culprits in experimental autoimmune encephalomyelitis (EAE) (Swanborg, 2001), a murine model of MS, and were shown to directly attack CNS myelin, leading to neurodegeneration (Sospedra & Martin, 2005). Several populations of immune cells were shown to be involved in autoimmune pathologies, among which are CNS-specific T cells (both CD4+ and CD8+) and monocyte-derived macrophages. Passive transfer of T cells specific for components of CNS myelin was shown to suffice for evoking EAE, and macrophages were shown to accumulate in the inflamed parenchyma (Martin et al, 1992; Steinman, 1996; Owens et al, 1998); such macrophages appeared morphologically identical to the activated microglia. It is thus perhaps not surprising that interactions between adaptive immune cells and the CNS have received a negative reputation, leading to widespread clinical attempts to use anti-inflammatory drugs to treat CNS pathologies, without necessarily differentiating between local neuroinflammation occurring under non-inflammatory neurodegenerative diseases, and inflammatory CNS diseases such as RRMS; in most of these cases, anti-inflammatory treatments failed (Cudkowicz et al, 2006; Gordon et al, 2007; Wolinsky et al, 2007; Fondell et al, 2012; Wyss-Coray & Rogers, 2012). Over the past decades, the widely held view of autoimmune cells as an indiscriminately negative feature of the immune response has fundamentally changed. We now know that the CNS is constantly surveyed by circulating immune cells within the CSF (but not within the parenchyma), and that under physiological conditions, activated T cells patrol the CNS, without the appearance of autoimmune disease (Hickey, 1999; Engelhardt & Ransohoff, 2005; Kunis et al, 2013). In addition, CNS-specific T cells were shown to support brain plasticity, both in health and in response to CNS trauma [thoroughly reviewed in (Kipnis et al, 2012; Rook et al, 2011; Schwartz & Shechter, 2010a)]. Nevertheless, the fact that under homeostatic conditions, circulating immune cells are rarely found in the brain parenchyma raises several key questions regarding the locations from which CNS-specific T cells affect the healthy brain. The theory of ‘protective autoimmunity’, which attributes a beneficial role to autoimmune CNS-specific CD4+ T cells in healthy CNS maintenance and repair, has provided insights to this enigma. The neuroprotective capacity of autoimmune cells was demonstrated across different models of CNS pathologies, including mechanical injuries (Moalem et al, 1999; Hauben et al, 2001; Kipnis et al, 2002b; Hofstetter et al, 2003; Ling et al, 2006), chronic neurodegenerative diseases (Benner et al, 2004; Butovsky et al, 2006; Laurie et al, 2007; Mosley et al, 2007), and imbalances in neurotransmitter levels (Schori et al, 2001). Moreover, in the healthy brain, autoimmune CD4+ T cells were found to play a role in maintenance of neuronal plasticity, including neurogenesis and spatial learning/memory (Kipnis et al, 2004c; Ziv et al, 2006; Radjavi et al, 2014). Examining the potential mechanism by which these cells exert a neuroprotective role in models of CNS trauma and neurodegeneration has highlighted their ability to control CNS inflammation as part of a wider cellular network. Circulating myeloid cells are recruited to the injured CNS by CNS-specific T cells While the unexpected experimental findings that recovery from CNS injuries is impaired in immune compromised mice (Bakalash et al, 2002; Kipnis et al, 2001), and is boosted in animals vaccinated with CNS-specific antigens (Hauben et al, 2000, 2001), were well-documented, the underlying mechanism remained puzzling. A few years ago, an insight was obtained when it was shown that CNS-specific T cells have the ability to augment the recruitment of anti-inflammatory monocyte-derived macrophages to the injured spinal cord (Shechter et al, 2009). Through these studies, it became evident that infiltrating myeloid cells, which are virtually identical in morphology to the resident microglia and were often viewed as infiltrating ‘microglia’, have distinct and non-redundant activities from those of the resident cells (Shechter et al, 2009; Jung & Schwartz, 2012). Independent studies have revealed that each population of myeloid cells has a distinct origin (Ginhoux et al, 2010) (Box 2). Box 2: Microglia and CNS-infiltrating monocyte-derived macrophages Distributed throughout the various regions of the brain, the spinal cord and the retina, the microglia are the resident myeloid cells of the CNS (Río-Hortega, 1937); though long suspected (Alliot et al, 1991), these cells were only recently shown to have a distinct origin than monocyte-derived macrophages (Ginhoux et al, 2010), which infiltrate to the brain under pathological conditions. The microglia, which originate from the yolk sac (Ginhoux et al, 2010), populate the CNS during early development, and serve a sentinel role in maintaining adult brain homeostasis, with limited self-renewal capacity (Hanisch & Kettenmann, 2007; Saijo & Glass, 2011). Under both acute and chronic conditions of neuroinflammation, blood-borne myeloid cells which home to the damaged CNS share many morphological and phenotypical features with the activated microglia, a fact which indistinctively associated them with pathological CNS inflammation. Over the past two decades, intensive research has revealed that infiltrating blood-derived macrophages are not part of the microglia turnover, and that they can exhibit enhanced phagocytic capacity, neurotropic support, and anti-inflammatory characteristics, compared to the microglia (Shechter et al, 2009; Jung & Schwartz, 2012; London et al, 2013). Thus, the potential beneficial role of blood-derived macrophages was demonstrated in various CNS pathologies, ranging from acute insults to neurodegenerative diseases, and neurodevelopmental mental disorders [thoroughly reviewed in (Shechter & Schwartz, 2013)]. Evidence for a role of monocyte-derived macrophages in response to neuroinflammation was first obtained in experimental murine models of spinal cord injury, where it was demonstrated that ‘alternatively activated’ blood macrophages, when locally transplanted at the margin of a spinal cord lesion, resulted in improved recovery (Rapalino et al, 1998). The success of such macrophage transplantation was found to be dependent on the site of their injection (for example, no effect was found when cells were administered at the center of the lesion or far from its margins), the number of injected cells, and the time elapsed between the injury and the injection (Schwartz & Yoles, 2006). Using animal models of bone marrow chimeric mice, which allowed distinction between activated microglia and CNS-infiltrating monocyte-derived macrophages, as well as their selective ablation, revealed that the infiltrating cells, display a local anti-inflammatory phenotype, which was critically dependent upon their expression of interleukin-10 (Shechter et al, 2009), a key factor in suppressing microglial activity (Taylor et al, 2006). This inflammation-resolving role of the recruited macrophages was further demonstrated and characterized in a model of retinal insult, in which monocyte-derived macrophages were shown to infiltrate the injured retina and support cell renewal and survival by skewing the local pro-inflammatory cytokine milieu (London et al, 2011). Circulating myeloid cells were also shown to support the CNS under conditions of chronic inflammation and neurodegeneration. In ALS, a fatal neurodegenerative disease affecting motor neurons, a gradual increase in microglial activation at the spinal cord during the course of the disease leads to accumulation of toxic inflammatory compounds in a vicious cycle of microglial toxicity (Turner et al, 2004; Barbeito et al, 2010; Liao et al, 2012). In vitro studies have shown that microglia isolated from mice over-expressing human mutant superoxide dismutase (mSOD1), an ALS mouse model, produce higher levels of TNF-α when stimulated with LPS compared with wild-type microglia (Weydt et al, 2004), and microglia that express less mSOD1 can attenuate the local inflammatory response (Beers et al, 2006; Xiao et al, 2007). In line with the in vitro findings, animals whose bone marrow is replaced with bone marrow cells deficient in expression of myeloid differentiation primary response protein (MyD88) have an earlier disease onset and a shorter lifespan than mSOD1 mice receiving normal bone marrow (Kang & Rivest, 2007). In murine models of AD, conditional ablation and reconstitution strategies demonstrated that amyloid beta (Aβ) plaque formation in the diseased brain can be attenuated by blood-borne macrophages (Simard et al, 2006; Butovsky et al, 2007; Town et al, 2008). This reduction in amyloid plaque load is correlated with arrest of the local neuroinflammatory response, reduction of pro-inflammatory cytokine levels, and local elevation of neurotrophic factors. The neuroprotective effect of circulating myeloid-derived cells was also demonstrated using various ‘microglial replacement’ strategies in MECP2 mutant mice, a model for Rett syndrome (Derecki et al, 2012), and these cells were shown to correct abnormal behavior in Hoxb8 mutant mice, a model for obsessive-compulsive disorder (Chen et al, 2010). The brain's choroid plexus: a selectively activated gate for leukocytes From barrier to gate Though the immune cell populations described above were repeatedly suggested to exert their beneficial effect on the diseased CNS by controlling neuroinflammation, their limited numbers within the healthy CNS parenchyma and the fact that their infiltration to the CNS upon injury is limited and carefully regulated, raised several key questions with regard to their trafficking routes and sites of interaction with the CNS. The CNS barrier system includes the blood–brain barrier (BBB), which is formed by tightly connected endothelium that surrounds parenchymal microvessels, and the blood–CSF barrier (BCSFB), which is formed by the CP, an epithelial monolayer that surrounds an inner stroma, and is vascularized by blood vessels (the structural and functional differences between the BBB and the BCSFB as neuro-immunological interfaces are summarized in Table 1). The classic role attributed to the CP is the production of the CSF, providing the brain with a nutritive metabolic milieu, and forming a protective mechanical cushion. Over the last decade, however, this compartment was reported to participate in various aspects of brain homeostasis, suggesting that it plays a much greater role than previously believed (Emerich et al, 2005; Johanson et al, 2011; Falcao et al, 2012; Baruch & Schwartz, 2013). Table 1. Structural and functional differences between the BBB and the BCSFB as neuro-immunological interfaces. BBB BCSFB References Location Brain parenchymal capillaries Brain ventricles Emerich et al (2005), Weiss et al (2009), Abbott et al (2010), Johanson et al (2011), Redzic (2011), Ransohoff & Engelhardt (2012) Structure Created by the endothelial cells that form the walls of the capillaries A villous layer of modified cuboidal epithelium which surrounds an inner stroma, and is vascularized by capillaries Capillary type Continuous Fenestrated Barrier properties Maintained at the level of specialized (extremely tight) endothelial tight junctions, and by the glia limitans Maintained at the level of specialized (leaky) epithelial tight-junctions of the choroid plexus Main functions Classically recognized for its barrier role and its disruption in CNS pathologies Classically recognized for its secretory role as the main producer of the CSF Main roles in maintaining CNS biochemical homeostasis Buffering passive diffusion and active transport of blood-borne solutes and nutrients to the CNS Actively modulating the chemical exchange between the CSF and the brain parenchyma, including surveying the chemical and immunological status of the brain, detoxifying the CSF, and secreting a nutritive ‘cocktail’ of neurotrophic polypeptides Immune cell localization Virchow–Robin perivascular spaces At the choroid plexus stroma, and on the epithelial apical side (epiplexus/Kolmer cells) Expression of immune cell trafficking determinants Induced under inflammatory conditions Constitutively expressed and further induced in response to CNS “danger” signals Steffen et al (1996), Carrithers et al (2002), Kivisakk et al (2003), Szmydynger-Chodobska et al (2009, 2012), Kunis et al (2013), Shechter et al (2013b) Immune cell trafficking across the barrier Mainly documented under inflammatory pathological conditions of the CNS Immune surveillance of the CSF in the steady-state, and mediating trafficking of leukocytes to the CNS following parenchymal damage Structurally, in contrast to the BBB, the BCSFB lacks endothelial tight junctions or astrocytic glia limitans, and its barrier properties are mostly restricted to the tight junctions of the epithelial layer of the CP (Redzic, 2011). This relative structural permissiveness for immune cell trafficking, and the fact that the cellular composition of the ventricular and lumbar CSF differs from that of the blood, and is dominated by CD4+ memory T cells (Kivisakk et al, 2003; Provencio et al, 2005), led to the suggestion that T cells enter the CSF in a regulated manner via the choroid plexus. Unlike the BBB, the CP constitutively expresses adhesion molecules and chemokines, which support transepithelial leukocyte trafficking (Steffen et al, 1996; Kunis et al, 2013; Shechter et al, 2013b); the selective expression of integrin receptors, such as intercellular adhesion molecule (ICAM)-1, on the apical side of the CP was recently suggested to serve as a foothold for ‘basal to apical’ transepithelial migration of leukocytes across the CP (Kunis et al, 2013). Experimentally, leukocyte trafficking through the CP-CSF route is supported by findings that adoptively transferred T cell blasts are found in the CP (Carrithers et al, 2000, 2002), and that neutrophils (Szmydynger-Chodobska et al, 2009), monocytes (Szmydynger-Chodobska et al, 2012; Kunis et al, 2013; Shechter et al, 2013b) and T cells (Kunis et al, 2013) enter the injured CNS through this site in response to parenchymal damage. Interestingly, this route was also suggested to serve as a gateway for encephalitogenic cells entering the CNS via CCL20-CCR6 interactions between Th17 cells and CP-derived CCL20 (Ransohoff, 2009; Reboldi et al, 2009). Yet, as CCR6 is also expressed by other cell populations, including T regulatory cells (Tregs), CCL20 may also take part in facilitating CNS immune surveillance under non-pathological conditions. Importantly, encephalitogenic IL-17- or GM-CSF-producing T cells are scarcely found in the stroma of the CP in healthy mice (Kunis et al, 2013), and when CP epithelial cells are exposed to these cytokines, they do not upregulate the expression trafficking determinants (Kunis et al, 2013). IFN-γ-dependent activation of the choroid plexus Using high-throughput analysis of the T-cell receptor (TCR) repertoire, we recently demonstrated that the CP stroma is enriched with CD4+ T cells specific for CNS antigens (Baruch et al, 2013). These cells were found to express cellular markers of ef