Multiple sclerosis (MS) is a neurodegenerative disease characterized by myelin degradation, diminished electrical conduction along axons, and axon degeneration. Demyelination occurs in localized lesions throughout the brain and spinal cord following immune cell infiltration into the central nervous system (CNS).1 Although the initiating factor(s) is unknown, CD4+ T helper 17 (Th17) cells have been detected in human MS lesions, and Th17 cell-mediated pathogenicity has been validated in experimental autoimmune encephalomyelitis (EAE), a mouse model of MS that is induced following myelin oligodendrocyte glycoprotein immunization.2,3 While CD4+ T cell infiltration into the CNS is a critical step in MS progression, the molecular details of immune cell homing and initial entry into the CNS are not well understood.
T cell recruitment and diapedesis in lymphoid tissues are well described and are influenced by chemokines such as CCL20 and its receptor CCR6.4 Consistent with the involvement of the chemoattractant CCL20 in lymphocyte CNS infiltration, brain samples from unaffected or MS patients constitutively express CCL20 in choroid plexus epithelial cells.5 In addition, activated astrocytes in MS patients and in mice with EAE overexpress CCL20, which may drive subsequent waves of pathogenic T cell infiltration into the inflamed CNS.5,6,7,8 Previous studies conducted thirteen days post-EAE induction suggested that CCR6+ Th17 cells crossed the blood-brain barrier via the choroid plexus.6 However, the mechanisms that promote initial immune cell entry and accumulation in the CNS to induce MS lesions in humans remain to be clearly demonstrated.
To determine the initial site of immune cell infiltration into the CNS under pathological conditions, Arima et al. transferred pathogenic CD4+ T cells into wild-type mice to induce EAE.9 Just prior to disease onset, no T cells were detected in the choroid plexus at five days post-EAE induction, suggesting that another gateway exists for initial T cell entry into the CNS. Importantly, histological examination of other CNS regions revealed that pathogenic CD4+ T cells significantly accumulated in the dorsal blood vessels of the fifth lumbar spinal cord region. These results are consistent with previous observations that T cells accumulated in the spinal cord of animals displaying EAE characteristics.10 Moreover, these findings suggest that, in this model, the blood vessels within the spinal cord rather than the blood-brain barrier serve as a gateway for T cell infiltration into the CNS.
To investigate the factors that regulate T cell infiltration into the spinal cord, the authors performed T cell transfers to induce EAE in knockout mice. CD4+ T cell accumulation in the spinal cord was suppressed when pathogenic cells were transferred into host mice deficient for IL-6, gp130, or STAT3, which is consistent with previous results in an arthritis model showing that Th17 cells accumulated in response to IL-6-dependent signaling.9,11 Given that CCL20 is induced in response to IL-6-dependent signaling, the authors analyzed chemokine expression in the fifth lumbar region of wild-type mice and found that CCL20 was expressed in dorsal blood vessel endothelial cells.6,9,11 Moreover, anti-CCL20 antibody injection into wild-type mice significantly blocked T cell accumulation in the CNS and reduced EAE pathology.
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FIGURE 1. Neural Activation Stimulates Local CCL20 Expression and Th17 Cell Infiltration in the Spinal Cord. In this model, sensory nerve activation is translated into an inflammatory response by a localized IL-6 signaling feedback loop within dorsal blood vessel endothelial cells (1). IL-6 receptor signaling induces STAT3 binding at the CCL20 promoter, transcriptional activation, and CCL20 secretion (2). CCL20 functions as a chemoattractant to recruit circulating CCR6+ Th17 cells to sites of neural activation where they infiltrate the central nervous system and cause localized MS lesions (3). Moreover, secretion of IL-17 from circulating Th17 cells amplifies the proinflammatory response by enhancing CCL20 transcription via NFkB signaling.
To explore how CCL20 production is activated locally, Arima and colleagues tested the hypothesis that regional neural activation near specific blood vessels induces IL-6-dependent signaling.9 Specifically, the authors focused on the soleus muscle sensory nerves that have dorsal root ganglia positioned at the fifth lumbar region. Following pathogenic T cell transfer into wild-type mice, inhibition of soleus muscle contraction reduced CCL20 levels in dorsal blood vessels, prevented CD4+ T cell accumulation in the fifth lumbar region of the spinal cord, and attenuated the pathological characteristics of EAE. These protective effects were reversed following direct electrical stimulation of the soleus muscle, suggesting that local neural activation is a critical event during autoreactive T cell infiltration and the development of EAE.
Collectively, these findings demonstrate that sensory nerve activation, in the presence of circulating pathogenic T cells, can amplify IL-6-dependent signaling in endothelial cells and promote T cell infiltration at specific sites within the CNS. These data suggest that the locations of lesions in MS patients may be explained, in part, by regional physiological neural activity that induces CCL20 expression and drives autoreactive T cell accumulation. Further understanding of the molecular signals that transduce neural activity to immune cell extravasation during EAE progression may identify novel therapeutic strategies for MS.
In addition to endothelial cell-expressed CCL20, studies have shown that astrocytes also express CCL20 within human MS lesions and in mice with relapsing EAE.7,8 Consistent with these observations, a recent report by Meares et al. showed that IL-17 enhanced the IL-6 signaling pathway and increased CCL20 expression in astrocytes via Nuclear Factor kappa B (NFkB) and STAT3 activation.6 These results suggest that CCL20 may be a common factor that promotes T cell extravasation in multiple regions of the CNS. Moreover, in response to IL-1beta astrocytes were shown to express VEGF-A, which facilitated blood-brain barrier (BBB) disruption by downregulating Claudin and Occludin expression in endothelial cells. These results suggest a model whereby astrocytes secrete factors such as VEGF, which increases BBB permeability, and CCL20, which recruits circulating T cells through the leaky BBB to induce MS lesion formation.12 Collectively, these studies indicate that cytokine-dependent chemokine production in choroid plexus epithelial cells, endothelial cells within specific lumbar spinal cord regions, and astrocytes may promote inflammatory cell recruitment into specific regions of the CNS.
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FIGURE 2. Astrocyte-secreted VEGF and CCL20 may Cooperate during Th17 Cell Brain Infiltration. In response to IL-beta-induced inflammation in the brain (1), astrocytes express and secrete VEGF (2) into the extravascular space. VEGF signals through VEGF R2 on endothelial cell membranes and induces blood-brain barrier (BBB) breakdown by downregulating the expression of Claudin and Occludin and removing endothelial cell to cell adhesions (3).12 In this model, astrocytes also produce the chemokine CCL20 (4), which promotes Th17 cell diapedesis through the compromised BBB (5) and drives localized immune cell infiltration into the central nervous system.
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