Use of imaging to study leukocyte trafficking in the central nervous system

Expression of the membrane tetraspanin claudin 18 on cancer cells promotes T lymphocyte infiltration and antitumor immunity in pancreatic cancer

Tumors weakly infiltrated by T lymphocytes poorly respond to immunotherapy. We aimed to unveil malignancy-associated programs regulating T cell entrance, arrest, and activation in the tumor environment. Differential expression of cell adhesion and tissue architecture programs, particularly the presence of the membrane tetraspanin claudin (CLDN)18 as a signature gene, demarcated immune-infiltrated from immune-depleted mouse pancreatic tumors. In human pancreatic ductal adenocarcinoma (PDAC) and non-small cell lung cancer, CLDN18 expression positively correlated with more differentiated histology and favorable prognosis. CLDN18 on the cell surface promoted accrual of cytotoxic T lymphocytes (CTLs), facilitating direct CTL contacts with tumor cells by driving the mobilization of the adhesion protein ALCAM to the lipid rafts of the tumor cell membrane through actin. This process favored the formation of robust immunological synapses (ISs) between CTLs and CLDN18-positive cancer cells, resulting in increased T cell activation. Our data reveal an immune role for CLDN18 in orchestrating T cell infiltration and shaping the tumor immune contexture.

An isoform of the giant protein titin is a master regulator of human T lymphocyte trafficking

Response to multiple microenvironmental cues and resilience to mechanical stress are essential features of trafficking leukocytes. Here, we describe unexpected role of titin (TTN), the largest protein encoded by the human genome, in the regulation of mechanisms of lymphocyte trafficking. Human T and B lymphocytes express five TTN isoforms, exhibiting cell-specific expression, distinct localization to plasma membrane microdomains, and different distribution to cytosolic versus nuclear compartments. In T lymphocytes, the LTTN1 isoform governs the morphogenesis of plasma membrane microvilli independently of ERM protein phosphorylation status, thus allowing selectin-mediated capturing and rolling adhesions. Likewise, LTTN1 controls chemokine-triggered integrin activation. Accordingly, LTTN1 mediates rho and rap small GTPases activation, but not actin polymerization. In contrast, chemotaxis is facilitated by LTTN1 degradation. Finally, LTTN1 controls resilience to passive cell deformation and ensures T lymphocyte survival in the blood stream. LTTN1 is, thus, a critical and versatile housekeeping regulator of T lymphocyte trafficking.

Alpha4 beta7 integrin controls Th17 cell trafficking in the spinal cord leptomeninges during experimental autoimmune encephalomyelitis

Th1 and Th17 cell migration into the central nervous system (CNS) is a fundamental process in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis (MS). Particularly, leptomeningeal vessels of the subarachnoid space (SAS) constitute a central route for T cell entry into the CNS during EAE. Once migrated into the SAS, T cells show an active motility behavior, which is a prerequisite for cell-cell communication, in situ reactivation and neuroinflammation. However, the molecular mechanisms selectively controlling Th1 and Th17 cell trafficking in the inflamed leptomeninges are not well understood. By using epifluorescence intravital microscopy, we obtained results showing that myelin-specific Th1 and Th17 cells have different intravascular adhesion capacity depending on the disease phase, with Th17 cells being more adhesive at disease peak. Inhibition of αLβ2 integrin selectively blocked Th1 cell adhesion, but had no effect on Th17 rolling and arrest capacity during all disease phases, suggesting that distinct adhesion mechanisms control the migration of key T cell populations involved in EAE induction. Blockade of α4 integrins affected myelin-specific Th1 cell rolling and arrest, but only selectively altered intravascular arrest of Th17 cells. Notably, selective α4β7 integrin blockade inhibited Th17 cell arrest without interfering with intravascular Th1 cell adhesion, suggesting that α4β7 integrin is predominantly involved in Th17 cell migration into the inflamed leptomeninges in EAE mice. Two-photon microscopy experiments showed that blockade of α4 integrin chain or α4β7 integrin selectively inhibited the locomotion of extravasated antigen-specific Th17 cells in the SAS, but had no effect on Th1 cell intratissue dynamics, further pointing to α4β7 integrin as key molecule in Th17 cell trafficking during EAE development. Finally, therapeutic inhibition of α4β7 integrin at disease onset by intrathecal injection of a blocking antibody attenuated clinical severity and reduced neuroinflammation, further demonstrating a crucial role for α4β7 integrin in driving Th17 cell-mediated disease pathogenesis. Altogether, our data suggest that a better knowledge of the molecular mechanisms controlling myelin-specific Th1 and Th17 cell trafficking during EAE delevopment may help to identify new therapeutic strategies for CNS inflammatory and demyelinating diseases.

The interplay between T helper cells and brain barriers in the pathogenesis of multiple sclerosis

The blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) represent two complex structures protecting the central nervous system (CNS) against potentially harmful agents and circulating immune cells. The immunosurveillance of the CNS is governed by immune cells that constantly patrol the BCSFB, whereas during neuroinflammatory disorders, both BBB and BCSFB undergo morphological and functional alterations, promoting leukocyte intravascular adhesion and transmigration from the blood circulation into the CNS. Multiple sclerosis (MS) is the prototype of neuroinflammatory disorders in which peripheral T helper (Th) lymphocytes, particularly Th1 and Th17 cells, infiltrate the CNS and contribute to demyelination and neurodegeneration. Th1 and Th17 cells are considered key players in the pathogenesis of MS and its animal model, experimental autoimmune encephalomyelitis. They can actively interact with CNS borders by complex adhesion mechanisms and secretion of a variety of molecules contributing to barrier dysfunction. In this review, we describe the molecular basis involved in the interactions between Th cells and CNS barriers and discuss the emerging roles of dura mater and arachnoid layer as neuroimmune interfaces contributing to the development of CNS inflammatory diseases.

Effects of acute and chronic oxidative stress on the blood-brain barrier in 2D and 3D in vitro models

Oxidative stress is a shared pathology of neurodegenerative disease and brain injuries, and is derived from perturbations to normal cell processes by aging or environmental factors such as UV exposure and air pollution. As oxidative cues are often present in systemic circulation, the blood-brain barrier (BBB) plays a key role in mediating the effect of these cues on brain dysfunction. Therefore, oxidative damage and disruption of the BBB is an emergent focus of neurodegenerative disease etiology and progression. We assessed barrier dysfunction in response to chronic and acute oxidative stress in 2D and 3D in vitro models of the BBB with human iPSC-derived brain microvascular endothelial-like cells (iBMECs). We first established doses of hydrogen peroxide to induce chronic damage (modeling aging and neurodegenerative disease) and acute damage (modeling the response to traumatic brain injury) by assessing barrier function via transendothelial electrical resistance in 2D iBMEC monolayers and permeability and monolayer integrity in 3D tissue-engineered iBMEC microvessels. Following application of these chronic and acute doses in our in vitro models, we found local, discrete structural changes were the most prevalent responses (rather than global barrier loss). Additionally, we validated unique functional changes in response to oxidative stress, including dysfunctional cell turnover dynamics and immune cell adhesion that were consistent with changes in gene expression.

Biology of the human blood-nerve barrier in health and disease

A highly regulated endoneurial microenvironment is required for normal axonal function in peripheral nerves and nerve roots, which structurally consist of an outer collagenous epineurium, inner perineurium consisting of multiple concentric layers of specialized epithelioid myofibroblasts that surround the innermost endoneurium, which consists of myelinated and unmyelinated axons embedded in a looser mesh of collagen fibers. Endoneurial homeostasis is achieved by tight junction-forming endoneurial microvessels that control ion, solute, water, nutrient, macromolecule and leukocyte influx and efflux between the bloodstream and endoneurium, and the innermost layers of the perineurium that control interstitial fluid component flux between the freely permeable epineurium and endoneurium. Strictly speaking, endoneurial microvascular endothelium should be considered the blood-nerve barrier (BNB) due to direct communication with circulating blood. The mammalian BNB is considered the second most restrictive vascular system after the blood-brain barrier (BBB) based on classic in situ permeability studies. Structural alterations in endoneurial microvessels or interactions with hematogenous leukocytes have been described in several human peripheral neuropathies; however major advances in BNB biology in health and disease have been limited over the past 50 years. Guided by transcriptome and proteome studies of normal and pathologic human peripheral nerves, purified primary and immortalized human endoneurial endothelial cells that form the BNB and leukocytes from patients with well-characterized peripheral neuropathies, validated by in situ or ex vivo protein expression studies, data are emerging on the molecular and functional characteristics of the human BNB in health and in specific peripheral neuropathies, as well as chronic neuropathic pain. These early advancements have the potential to not only increase our understanding of how the BNB works and adapts or fails to adapt to varying insult, but provide insights relevant to pathogenic leukocyte trafficking, with translational potential and specific therapeutic application for chronic peripheral neuropathies and neuropathic pain.

LFA-1 Controls Th1 and Th17 Motility Behavior in the Inflamed Central Nervous System

Leukocyte trafficking is a key event during autoimmune and inflammatory responses. The subarachnoid space (SAS) and cerebrospinal fluid are major routes for the migration of encephalitogenic T cells into the central nervous system (CNS) during experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis, and are sites of T cell activation before the invasion of CNS parenchyma. In particular, autoreactive Th1 and Th17 cell trafficking and reactivation in the CNS are required for the pathogenesis of EAE. However, the molecular mechanisms controlling T cell dynamics during EAE are unclear. We used two-photon laser microscopy to show that autoreactive Th1 and Th17 cells display distinct motility behavior within the SAS in the spinal cords of mice immunized with the myelin oligodendrocyte glycoprotein peptide MOG₃₅₋₅₅. Th1 cells showed a strong directional bias at the disease peak, moving in a straight line and covering long distances, whereas Th17 cells exhibited more constrained motility. The dynamics of both Th1 and Th17 cells were strongly affected by blocking the integrin LFA-1, which interfered with the deformability and biomechanics of Th1 but not Th17 cells. The intrathecal injection of a blocking anti-LFA-1 antibody at the onset of disease significantly inhibited EAE progression and also strongly reduced neuro-inflammation in the immunized mice. Our results show that LFA-1 plays a pivotal role in T cell motility during EAE and suggest that interfering with the molecular mechanisms controlling T cell motility can help to reduce the pathogenic potential of autoreactive lymphocytes.

Imaging the dynamic interactions between immune cells and the neurovascular interface in the spinal cord

Imaging the dynamic interactions between immune cells, glia, neurons and the vasculature in living rodents has revolutionized our understanding of physiological and pathological mechanisms of the CNS. Emerging microscopy and imaging technologies have enabled longitudinal tracking of structural and functional changes in a plethora of different cell types in the brain. The development of novel methods also allowed stable and longitudinal optical access to the spinal cord with minimum tissue perturbation. These important advances facilitated the application of in vivo imaging using two-photon microscopy for studies of the healthy, diseased, or injured spinal cord. Indeed, decoding the interactions between peripheral and resident cells with the spinal cord vasculature has shed new light on neuroimmune and vascular mechanisms regulating the onset and progression of neurological diseases. This review focuses on imaging studies of the interactions between the vasculature and peripheral immune cells or microglia, with emphasis on their contribution to neuroinflammation. We also discuss in vivo imaging studies highlighting the importance of neurovascular changes following spinal cord injury. Real-time imaging of blood-brain barrier (BBB) permeability and other vascular changes, perivascular glial responses, and immune cell entry has revealed unanticipated cellular mechanisms and novel molecular pathways that can be targeted to protect the injured or diseased CNS. Imaging the cell-cell interactions between the vasculature, immune cells, and neurons as they occur in real time, is a powerful tool both for testing the efficacy of existing therapeutic approaches, and for identifying new targets for limiting damage or enhancing the potential for repair of the affected spinal cord tissue.

Blockade of α4 integrins reduces leukocyte-endothelial interactions in cerebral vessels and improves memory in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive decline associated with the deposition of amyloid-β (Aβ) plaques, hyperphosphorylation of tau protein, and neuronal loss. Vascular inflammation and leukocyte trafficking may contribute to AD pathogenesis, and a better understanding of these inflammation mechanisms could therefore facilitate the development of new AD therapies. Here we show that T cells extravasate in the proximity of cerebral VCAM-1+ vessels in 3xTg-AD transgenic mice, which develop both Aβ and tau pathologies. The counter-ligand of VCAM-1 - α4β1 integrin, also known as very late antigen-4 (VLA-4) - was more abundant on circulating CD4+ T cells and was also expressed by a significant proportion of blood CD8+ T cells and neutrophils in AD mice. Intravital microscopy of the brain microcirculation revealed that α4 integrins control leukocyte-endothelial interactions in AD mice. Therapeutic targeting of VLA-4 using antibodies that specifically block α4 integrins improved the memory of 3xTg-AD mice compared to an isotype control. These antibodies also reduced neuropathological hallmarks of AD, including microgliosis, Aβ load and tau hyperphosphorylation. Our results demonstrate that α4 integrin-dependent leukocyte trafficking promotes cognitive impairment and AD neuropathology, suggesting that the blockade of α4 integrins may offer a new therapeutic strategy in AD.

The cerebral endothelial cell as a key regulator of inflammatory processes in sterile inflammation

Cerebral endothelial cells accomplish numerous tasks connected to the maintenance of homeostasis of the central nervous system. They create a barrier between the central nervous system and peripheral blood and regulate mechanotransduction, vascular permeability, rheology, thrombogenesis, and leukocyte adhesion. In pathophysiological conditions (e.g., stroke or ischemia-reperfusion injury) the endothelial functions are impaired, leading to increased vascular permeability, vascular inflammation, leukocyte-endothelium interactions, and transendothelial migration, driving CNS inflammation and neuronal destruction. This review describes the current knowledge on the regulatory roles of endothelial cells in neuroinflammatory processes.

Nanovesicles from adipose-derived mesenchymal stem cells inhibit T lymphocyte trafficking and ameliorate chronic experimental autoimmune encephalomyelitis

Cell based-therapies represent promising strategies for the treatment of neurological diseases. We have previously shown that adipose stem cells (ASC) ameliorate chronic experimental autoimmune encephalomyelitis (EAE). Recent evidence indicates that most ASC paracrine effects are mediated by extracellular vesicles, i.e. micro- and nanovesicles (MVs and NVs). We show that preventive intravenous administration of NVs isolated from ASC (ASC-NVs) before disease onset significantly reduces the severity of EAE and decreases spinal cord inflammation and demyelination, whereas therapeutic treatment with ASC-NVs does not ameliorate established EAE. This treatment marginally inhibits antigen-specific T cell activation, while reducing microglial activation and demyelination in the spinal cord. Importantly, ASC-NVs inhibited integrin-dependent adhesion of encephalitogenic T cells in vitro, with no effect on adhesion molecule expression. In addition, intravital microscopy showed that encephalitogenic T cells treated with ASC NVs display a significantly reduced rolling and firm adhesion in inflamed spinal cord vessels compared to untreated cells. Our results show that ASC-NVs ameliorate EAE pathogenesis mainly by inhibiting T cell extravasation in the inflamed CNS, suggesting that NVs may represent a novel therapeutic approach in neuro-inflammatory diseases, enabling the safe administration of ASC effector factors.

Molecular mobility and activity in an intravital imaging setting - implications for cancer progression and targeting

Molecular mobility, localisation and spatiotemporal activity are at the core of cell biological processes and deregulation of these dynamic events can underpin disease development and progression. Recent advances in intravital imaging techniques in mice are providing new avenues to study real-time molecular behaviour in intact tissues within a live organism and to gain exciting insights into the intricate regulation of live cell biology at the microscale level. The monitoring of fluorescently labelled proteins and agents can be combined with autofluorescent properties of the microenvironment to provide a comprehensive snapshot of in vivo cell biology. In this Review, we summarise recent intravital microscopy approaches in mice, in processes ranging from normal development and homeostasis to disease progression and treatment in cancer, where we emphasise the utility of intravital imaging to observe dynamic and transient events in vivo We also highlight the recent integration of advanced subcellular imaging techniques into the intravital imaging pipeline, which can provide in-depth biological information beyond the single-cell level. We conclude with an outlook of ongoing developments in intravital microscopy towards imaging in humans, as well as provide an overview of the challenges the intravital imaging community currently faces and outline potential ways for overcoming these hurdles.

A Novel Cervical Spinal Cord Window Preparation Allows for Two-Photon Imaging of T-Cell Interactions with the Cervical Spinal Cord Microvasculature during Experimental Autoimmune Encephalomyelitis

T-cell migration across the blood-brain barrier (BBB) is a crucial step in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Two-photon intravital microscopy (2P-IVM) has been established as a powerful tool to study cell-cell interactions in inflammatory EAE lesions in living animals. In EAE, central nervous system inflammation is strongly pronounced in the spinal cord, an organ in which 2P-IVM imaging is technically very challenging and has been limited to the lumbar spinal cord. Here, we describe a novel spinal cord window preparation allowing to use 2P-IVM to image immune cell interactions with the cervical spinal cord microvascular endothelium during EAE. We describe differences in the angioarchitecture of the cervical spinal cord versus the lumbar spinal cord, which will entail different hemodynamic parameters in these different vascular beds. Using T cells as an example, we demonstrate the suitability of this novel methodology in imaging the post-arrest multistep T-cell extravasation across the cervical spinal cord microvessels. The novel methodology includes an outlook to the analysis of the cellular pathway of T-cell diapedesis across the BBB by establishing visualization of endothelial junctions in this vascular bed.

Live Imaging of Immune Responses in Experimental Models of Multiple Sclerosis

Experimental autoimmune encephalomyelitis (EAE) is the most common animal model of multiple sclerosis (MS), a chronic inflammatory autoimmune disease of the central nervous system (CNS) characterized by multifocal perivascular infiltrates that predominantly comprise lymphocytes and macrophages. During EAE, autoreactive T cells first become active in the secondary lymphoid organs upon contact with antigen-presenting cells (APCs), and then gain access to CNS parenchyma, through a compromised blood–brain barrier, subsequently inducing inflammation and demyelination. Two-photon laser scanning microscopy (TPLSM) is an ideal tool for intravital imaging because of its low phototoxicity, deep tissue penetration, and high resolution. In the last decade, TPLSM has been used to visualize the behavior of T cells and their contact with APCs in the lymph nodes (LNs) and target tissues in several models of autoimmune diseases. The leptomeninges and cerebrospinal fluid represent particularly important points for T cell entry into the CNS and reactivation following contact with local APCs during the preclinical phase of EAE. In this review, we highlight recent findings concerning the pathogenesis of EAE and MS, emphasizing the use of TPLSM to characterize T cell activation in the LNs and CNS, as well as the mechanisms of tolerance induction. Furthermore, we discuss how advanced imaging unveils disease mechanisms and helps to identify novel therapeutic strategies to treat CNS autoimmunity and inflammation.

Imaging of Leukocyte Trafficking in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is characterized by a progressive decline of cognitive functions. The neuropathological features of AD include amyloid beta (Aβ) deposition, intracellular neurofibrillary tangles derived from the cytoskeletal hyperphosphorylated tau protein, amyloid angiopathy, the loss of synapses, and neuronal degeneration. In the last decade, inflammation has emerged as a key feature of AD, but most studies have focused on the role of microglia-driven neuroinflammation mechanisms. A dysfunctional blood-brain barrier has also been implicated in the pathogenesis of AD, and several studies have demonstrated that the vascular deposition of Aβ induces the expression of adhesion molecules and alters the expression of tight junction proteins, potentially facilitating the transmigration of circulating leukocytes. Two-photon laser scanning microscopy (TPLSM) has become an indispensable tool to dissect the molecular mechanisms controlling leukocyte trafficking in the central nervous system (CNS). Recent TPLSM studies have shown that vascular deposition of Aβ in the CNS promotes intraluminal neutrophil adhesion and crawling on the brain endothelium and also that neutrophils extravasate in the parenchyma preferentially in areas with Aβ deposits. These studies have also highlighted a role for LFA-1 integrin in neutrophil accumulation in the CNS of AD-like disease models, revealing that LFA-1 inhibition reduces the corresponding cognitive deficit and AD neuropathology. In this article, we consider how current imaging techniques can help to unravel new inflammation mechanisms in the pathogenesis of AD and identify novel therapeutic strategies to treat the disease by interfering with leukocyte trafficking mechanisms.

Modeling leukocyte trafficking at the human blood-nerve barrier in vitro and in vivo geared towards targeted molecular therapies for peripheral neuroinflammation

Peripheral neuroinflammation is characterized by hematogenous mononuclear leukocyte infiltration into peripheral nerves. Despite significant clinical knowledge, advancements in molecular biology and progress in developing specific drugs for inflammatory disorders such as rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis, there are currently no specific therapies that modulate pathogenic peripheral nerve inflammation. Modeling leukocyte trafficking at the blood-nerve barrier using a reliable human in vitro model and potential intravital microscopy techniques in representative animal models guided by human observational data should facilitate the targeted modulation of the complex inflammatory cascade needed to develop safe and efficacious therapeutics for immune-mediated neuropathies and chronic neuropathic pain.