CNS trafficking of T cells: immune surveillance vs. neuroinflammtion

neuroinflammtion

As a result of the specialized structure of the CNS barriers, immune cell entry into the CNS parenchyma involves two differently regulated steps: migration of the immune cells across the BBB or BSCFB into the CSF drained spaces of the CNS (during immune surveillance) followed by progression across the glia limitans into the CNS parenchyma (during neuroinflammation)⁴³¹. It is generally assumed that the mechanisms of physiological immune surveillance of the CNS are distinct from those operating during disease³⁹⁰. Thus, immune cell trafficking to the CSF across the choroid plexus or into the subarachnoid space via carotid artery is probably of physiological relevance whereas migration via the BBB to the parenchymal perivascular space is speculated to be the route for activated leukocytes in case of inflammation412,432,433.

d Figure and legend adapted from 420. Ransohoff, R.M. & Engelhardt, B. The anatomical and cellular basis of immune surveillance in the central nervous system. Nat. Rev. Immunol. 12, 623-635 (2012).

Unlike the BBB, the fenestrated endothelial cells of the choroid plexus lack tight junctions that would normally limit diapedesis of leukocytes. Thus, although immune cells still have to negotiate the tight junctions of the choroid plexus epithelium, it appears that this site is specialized to allow lymphocytes access to the CSF filled subarachnoid space (see Figure 6)412,434-436. Under normal circumstances, the CSF contains few innate immune cells but a much higher percentage of memory or antigen-experienced CD4⁺ T cells compared to the blood, suggesting that these cells are specifically involved in immune surveillance434,436,437. Multiple integrins, chemokine receptors, and adhesion molecules expressed on circulating and CNS resident cells have been implicated in leukocyte extravasation into the CNS during immune surveillance. The most likely candidates are those which are constitutively expressed in the CNS in the absence of ongoing inflammation. These molecules include the vascular cell adhesion molecule (VCAM) 1, the intercellular adhesion molecule (ICAM) 1, and the chemokines CC ligand (CCL)19 and CCL20.

T cell entry into the choroid plexus parenchyma in the absence of neuroinflammation seems to be specifically dependent on the presence of P-selectin which is constitutively expressed by endothelial cells of the choroid plexus434,436,438-443. In order to reach the CSF-filled ventricles, T cells need to breach the BCSFB. CC receptor (CCR)6⁺ Th17 cells seem to use CCL20 expressed by the choroid plexus epithelium as a guidance and require lymphocyte function-associated antigen (LFA)-1 (aLb2) for their entry into the perivascular and subarachnoid spaces.

The molecular mechanism for the extravasation of the high numbers of central memory T cells found in the CSF, however, is less well understood431,440,444. Studies of EAE (see 2.2.4) suggested that the molecular mechanisms for T cell extravasation might be dependent on the nature of both the lymphocyte and the site of entry⁴³¹. Hence, Th1 cells, in contrast to Th17 cells, seem to preferentially cross the BBB in the spinal cord microvessels in a very late antigen (VLA)4 (α4β1)-dependent manner^444-446. In contrast, the expression of CXC ligand (CXCL)12 seems to mediate retention of CXC receptor (CXCR)4⁺ encephalitogenic T cells in order to block their entry into the CNS parenchyma during immune surveillance^447,448.

Figure 6: Molecular mechanisms involved in T cell migration across the epithelial blood-cerebrospinal fluid barrier (BCSFB)^e. The choroid plexus might be a preferential T cell entry site into the central nervous system (CNS) during immunosurveillance, that is, in the absence of neuroinflammation. Circulating T cells extravasate in a P-selectin-dependent manner across fenestrated capillaries to reach the choroid plexus parenchyma, which is outside the CNS.

To reach the CSF-filled ventricles, T cells need to breach the BCSFB established by choroid plexus epithelial cells. The paracellular pathway is sealed by unique tight junctions between the choroid plexus epithelial cells. CCR6⁺ T helper (Th)17 cells may use chemokine CC ligand (CCL)20 expressed by choroid plexus epithelium as a guidance cue to migrate across the BCSFB into the CSF-filled ventricular space. The high number of central memory T cells found in the CSF of humans suggests that this T cell subset preferentially crosses the BCSFB. The molecular mechanisms used by TCM cells to migrate across the BCSFB are unknown. Functional expression of intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, and under inflammatory conditions, mucosal addressin cell adhesion molecule (MAdCAM)-1 is restricted to the apical surface of choroid plexus epithelial cells, and is thus not available for the basolateral to apical migration of immune cells across the BCSFB. The choroid plexus is in constant movement and these adhesion molecules might instead allow T cells to crawl along the surface of the choroid plexus epithelium or alternatively might mediate the adhesion of antigen presenting cells, so called epiplexus cells, ensuring their strategic localization behind the BCSFB to present CNS antigens to the T cells during immunosurveillance.

Reactivation of patrolling T cells by APCs within the subarachnoid spaces of the CNS triggers the production of soluble mediators including cytokines and chemokines that activate perivascular endothelial cells to express adhesion molecules and chemokines resulting in the recruitment of other inflammatory cells.

Reactivated T cells might enter the brain parenchyma directly to initiate tissue injury. Exposure to inflammatory cytokines leads to the disruption of the parenchymal BBB which is followed by a massive influx of T cells and myeloid cells to the brain parenchyma⁴²⁰. T cell migration across the inflamed BBB is a multistep process mediated by the sequential interaction of different cell adhesion and signaling molecules on the T cell surface with their respective ligands on the highly specialized endothelium of the BBB (see Figure 7)⁴³¹. T cell tethering and rolling are mediated by P-selectin glyocoprotein ligand (PSGL)-1 interacting with its endothelial ligand P-selectin in the inflamed leptomeningeal brain and spinal cord microvessels.

e Figure and legend adapted from 431. Engelhardt, B. & Ransohoff, R.M. Capture, crawl, cross: the T cell code to breach the blood-brain barriers. Trends Immunol. 33, 579-589 (2012).

Although the importance of P-selectin-mediated rolling on T cell entry into the CNS is still controversial, a specific polymorphism in PSGL-1 associated with primary progressive multiple sclerosis supports an impact on neuroinflammation^449-452. P-selectin-mediated rolling of T cells on the luminal surface of the BBB is followed by G-protein coupled receptor (GPCR)-mediated activation of T cell integrins, which allows for integrin-mediated T cell arrest and subsequent crawling to sites permissive for T cell diapedesis across the BBB. Chemokines like CCL2, CCL19, CXCL21, and CCL12 are upregulated in brain endothelial cells and were related to pathology in a mouse model of EAE431,441,442,453-455. Once a chemokine engages its respective chemokine receptor on the T cell, this induces a G-protein-dependent signaling cascade in the T cell affecting cytoskeletal dynamics and integrin conformation. However, both the chemokines and their respective receptors accounting for integrin activation in the process of T cell arrest during CNS entry need to be identified^431,456. T cell arrest is mediated to a large degree on the interaction of the leukocyte integrins LFA-1 and VLA-4 with their respective ligands ICAM-1 and vascular adhesion molecule VCAM-1, which are upregulated on the inflamed brain endothelium. Subsequent T cell polarization and crawling against the direction of flow required ICAM-1 and -2 but no longer VCAM-1.

Similarly to CD4⁺ T cells, the entry of CD8⁺ T cell into the CNS also originates in the subarachnoid spaces⁴⁵⁷. In case of CTL trafficking to the CSF-filled perivascular spaces during acute LCMV-induced meningitis (see 2.2.5), several candidates for chemokine-GPCR interaction-induced activation of integrins have been identified. It has been shown that the chemokine receptors CXCR2, CXCR3, and CCR5 are expressed on CD8⁺ T cells after LCMV infection^458-460. While CCR5 and CXCR3 are dispensable for initial T cell entry into the CSF of LCMV infected mice, subsequent positioning of T cells in the brain parenchyma during LCMV-induced meningitis is controlled in part by interactions between CXCR3 and CXCL10^460-462. In fact, CCR3- or CXCL10-deficient mice display a reduced susceptibility to LCMV-induced meningitis which was attributed to a reduced number of CD8⁺ T cells found in the brain parenchyma^460,462. As seen for T helper cells, extravasation of activated CTLs seems to depend on the expression of PSGL-1, VLA-4, and LFA-1450,463-466. The cellular pathways as well as the molecules mediating T cell diapedesis (especially of CD8⁺ T cells) across the BBB are not well understood and the contribution of chemokine receptors and adhesion molecules to the pathogenesis of neuroinflammatory diseases might vary between different disease models, site of entry, and the characteristics of the CNS infiltrating T cells380,430,431,444,467-470.

However, entry of T cells into the CNS parenchyma is dependent on the sequestration of CXCL12 from the perivascular space by increased endothelial expression of CXCR7. Thereby, T cells are released from this compartment and are able to migrate across the glia limitans (see Figure 5B). T cell penetration of the glia limitans requires GM-CSF to induce focal activity of matrix metalloproteinases (MMPs) namely the gelatinases MMP-2 and -9 in order to cleave β-dystroglycan, a cell surface receptor anchoring astrocyte endfeet to the parenchymal basement membrane431,471-474.

Figure 7: Molecular mechanisms involved in T cell migration across endothelial blood–brain barrier (BBB)^f. As a result of the different neuroanatomy and expression of adhesion and tight junction proteins in parenchymal versus leptomeningeal endothelial cells, the molecular mechanisms of T cell migration across the BBB might be distinct from those controlling T cell migration across the blood–leptomeningeal barrier (BLMB). A predominant role of α4β1-integrins in mediating T cell capture and subsequent G-protein-dependent T cell arrest might be unique to the BBB in the spinal cord. P-selectin can be upregulated during neuroinflammation, providing a cue for P-selectin glycoprotein ligand (PSGL)-1-mediated T cell rolling during ongoing neuroinflammation. G-protein-dependent integrin activation on T cells triggers their arrest on the BBB in a Leukocyte function-associated antigen (LFA)-1−intercellular adhesion molecule (ICAM)-1- and α4β1-integrin−vascular cell adhesion molecule (VCAM)-1-dependent manner. At least in an in vitro BBB model, T cells polarize and crawl against the direction of flow with a mean velocity of 4 μm/min in an ICAM-1- and ICAM-2-dependent manner. Diapedesis across the BBB is observed to occur preferentially through the endothelium, leaving the tight junctions molecularly intact.

f Figure and legend adapted from 431. Engelhardt, B. & Ransohoff, R.M. Capture, crawl, cross: the T cell code to breach the blood-brain barriers. Trends Immunol. 33, 579-589 (2012).

2.2.4 Autoimmune-mediated CNS immunopathology: Multiple sclerosis