Pia Kivisäkk

Three or more routes for leukocyte migration into the central nervous system.

Leukocyte migration into and through tissues is fundamental to normal physiology, immunopathology and host defence. Leukocyte entry into the central nervous system (CNS) is restricted, in part, because of the blood–brain barrier (BBB). During the past decade, crucial components that are involved in the process of leukocyte migration have been identified and progress has been made in understanding the mechanisms of neuroinflammatory reactions. In this review, present knowledge of the trafficking determinants that guide the migration of leukocytes is superimposed onto the vascular and compartmental anatomy of the CNS. We discuss three distinct routes for leukocytes to enter the CNS and consider how different populations of leukocytes use trafficking signals to gain entry.

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.

Expression of CCR7 in multiple sclerosis: Implications for CNS immunity

It is unclear how immune cells traffic between the lymphoid compartment and the central nervous system (CNS), which lacks lymphatic vessels and is shielded by the blood–brain barrier. We studied the expression of CCR7, a chemokine receptor required for migration of T cells and dendritic cells (DCs) to lymphoid organs, in the CNS of patients with multiple sclerosis (MS) to gain insight into pathways for CNS immune cell trafficking. Inflamed MS lesions contained numerous CCR7+ myeloid cells expressing major histocompatibility complex class II, CD68 and CD86, consistent with maturing DCs. CCR7+ DCs also were identified in cerebrospinal fluid (CSF). These observations suggested that the afferent limb of CNS immunity is comprised, in part, of DCs, which are generated within the CNS and migrate to deep cervical lymph nodes through the CSF after antigen capture. Ninety percent of CSF T cells expressed CCR7 and CSF from patients with MS was relatively depleted of CCR7‐negative effector‐memory T cells. In contrast, all T cells in parenchymal MS lesions lacked CCR7, indicating local retention and differentiation of central‐memory T cells upon restimulation by antigen within the CNS. These data suggested that the efferent limb of CNS immunity is executed by central‐memory T cells, which enter CSF directly from the circulation. Ann Neurol 2004

CCR1+/CCR5+ Mononuclear Phagocytes Accumulate in the Central Nervous System of Patients with Multiple Sclerosis

Mononuclear phagocytes (monocytes, macrophages, and microglia) are considered central to multiple sclerosis (MS) pathogenesis. Molecular cues that mediate mononuclear phagocyte accumulation and activation in the central nervous system (CNS) of MS patients may include chemokines RANTES/CCL5 and macrophage inflammatory protein-1alpha/CCL3. We analyzed expression of CCR1 and CCR5, the monocyte receptors for these chemokines, on circulating and cerebrospinal fluid CD14+ cells, and in MS brain lesions. Approximately 70% of cerebrospinal fluid monocytes were CCR1+/CCR5+, regardless of the presence of CNS pathology, compared to less than 20% of circulating monocytes. In active MS lesions CCR1+/CCR5+ monocytes were found in perivascular cell cuffs and at the demyelinating edges of evolving lesions. Mononuclear phagocytes in early demyelinating stages comprised CCR1+/CCR5+ hematogenous monocytes and CCR1-/CCR5- resident microglial cells. In later stages, phagocytic macrophages were uniformly CCR1-/CCR5+. Cultured in vitro, adherent monocytes/macrophages up-regulated CCR5 and down-regulated CCR1 expression, compared to freshly-isolated monocytes. Taken together, these findings suggest that monocytes competent to enter the CNS compartment derive from a minority CCR1+/CCR5+ population in the circulating pool. In the presence of ligand, these cells will be retained in the CNS. During further activation in lesions, infiltrating monocytes down-regulate CCR1 but not CCR5, whereas microglia up-regulate CCR5.

Multiple sclerosis: a study of CXCL10 and CXCR3 co-localization in the inflamed central nervous system

T-cell accumulation in the central nervous system (CNS) is considered crucial to the pathogenesis of multiple sclerosis (MS). We found that the majority of T cells within the cerebrospinal fluid (CSF) compartment expressed the CXC chemokine receptor 3 (CXCR), independent of CNS inflammation. Quantitative immunohistochemistry revealed continuous accumulation of CXCR3+ T cells during MS lesion formation. The expression of one CXCR3 ligand, interferon (IFN)-gamma-inducible protein of 10 kDa (IP-10)/CXC chemokine ligand (CXCL) 10 was elevated in MS CSF, spatially associated with demyelination in CNS tissue sections and correlated tightly with CXCR3 expression. These data suggest a critical role for CXCL10 and CXCR3 in the accumulation of T cells in the CNS of MS patients.

Localizing central nervous system immune surveillance: Meningeal antigen-presenting cells activate T cells during experimental autoimmune encephalomyelitis

The onset of neurological signs in experimental autoimmune encephalomyelitis is tightly associated with infiltration and reactivation of T cells in the central nervous system. The anatomic localization of the initial T cell‐antigen‐presenting cell (APC) interactions leading to reactivation of T cells in the central nervous system is, however, still unclear. We hypothesized that activated CD4+ T cells gain direct access to the subarachnoid space and become reactivated on encounter with cognate antigen in this compartment.

Alterations of the human gut microbiome in multiple sclerosis.

The gut microbiome plays an important role in immune function and has been implicated in several autoimmune disorders. Here we use 16S rRNA sequencing to investigate the gut microbiome in subjects with multiple sclerosis (MS, n=60) and healthy controls (n=43). Microbiome alterations in MS include increases in Methanobrevibacter and Akkermansia and decreases in Butyricimonas, and correlate with variations in the expression of genes involved in dendritic cell maturation, interferon signalling and NF-kB signalling pathways in circulating T cells and monocytes. Patients on disease-modifying treatment show increased abundances of Prevotella and Sutterella, and decreased Sarcina, compared with untreated patients. MS patients of a second cohort show elevated breath methane compared with controls, consistent with our observation of increased gut Methanobrevibacter in MS in the first cohort. Further study is required to assess whether the observed alterations in the gut microbiome play a role in, or are a consequence of, MS pathogenesis.

Circulating MicroRNAs as biomarkers for disease staging in multiple sclerosis

MicroRNAs (miRNAs) are single‐stranded, small noncoding RNAs that regulate gene expression. Because they are stable in serum, they are being developed as biomarkers for cancer and other diseases. In multiple sclerosis (MS), miRNAs have been studied in cell populations but not in the circulation. In MS, a major challenge is to develop immune biomarkers to monitor disease. We asked whether circulating miRNAs could be identified in MS and whether they are linked to disease stage and/or disability.

Regulation of astrocyte activation by glycolipids drives chronic CNS inflammation

Astrocytes have complex roles in health and disease, thus it is important to study the pathways that regulate their function. Here we report that lactosylceramide (LacCer) synthesized by β-1,4-galactosyltransferase 6 (B4GALT6) is upregulated in the central nervous system (CNS) of mice during chronic experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis (MS). LacCer acts in an autocrine manner to control astrocyte transcriptional programs that promote neurodegeneration. In addition, LacCer in astrocytes controls the recruitment and activation of microglia and CNS-infiltrating monocytes in a non–cell autonomous manner by regulating production of the chemokine CCL2 and granulocyte-macrophage colony–stimulating factor (GM-CSF), respectively. We also detected high B4GALT6 gene expression and LacCer concentrations in CNS MS lesions. Inhibition of LacCer synthesis in mice suppressed local CNS innate immunity and neurodegeneration in EAE and interfered with the activation of human astrocytes in vitro. Thus, B4GALT6 regulates astrocyte activation and is a potential therapeutic target for MS and other neuroinflammatory disorders.

T-cells in the cerebrospinal fluid express a similar repertoire of inflammatory chemokine receptors in the absence or presence of CNS inflammation: Implications for CNS trafficking

It is believed that chemokines and their receptors are involved in trafficking of T‐cells to the central nervous system (CNS). The aim of the current study was to define the expression on cerebrospinal fluid (CSF) T‐cells of six chemokine receptors associated with trafficking to sites of inflammation. Flow cytometry was used to detect chemokine receptor expression. We observed that CD3+T‐cells in the CSF express a restricted array of inflammatory chemokine receptors, specifically CXCR3, CCR5 and CCR6, but little CCR1‐3. This repertoire was independent of the presence of CNS inflammation, since comparable findings were obtained in patients with multiple sclerosis (MS) and individuals with non‐inflammatory neurological diseases. The enrichment of CCR5+T‐cells in the CSF could largely be explained by higher frequency of CD4+/CD45RO+T‐cells in this compartment. In contrast, CD4+/CD45RO+T‐cells expressing CXCR3 were significantly enriched in CSF as compared with blood. Similar levels of CCR6+/CD3+T‐cells were observed in blood and CSF, while levels of CCR2+/CD3+T‐cells were lower in CSF than in blood. The CSF was virtually devoid of CCR5+/CXCR3‐ T‐cells, suggesting that the expression of CCR5 alone is not sufficient for the trafficking of CD3+T‐cells to the CSF. We hypothesize that CXCR3 is the principal inflammatory chemokine receptor involved in intrathecal accumulation of T‐cells in MS. Through interactions with its ligands, CXCR3 is proposed to mediate retention of T‐cells in the inflamed CNS.