Shear flow-dependent integration of apical and subendothelial chemokines in T-cell transmigration: implications for locomotion and the multistep paradigm

Mechanical regulation of lymphocyte activation and function

Mechanosensing, or how cells sense and respond to the physical environment, is crucial for many aspects of biological function, ranging from cell movement during development to cancer metastasis, the immune response and gene expression driving cell fate determination. Relevant physical stimuli include the stiffness of the extracellular matrix, contractile forces, shear flows in blood vessels, complex topography of the cellular microenvironment and membrane protein mobility. Although mechanosensing has been more widely studied in non-immune cells, it has become increasingly clear that physical cues profoundly affect the signaling function of cells of the immune system. In this Review, we summarize recent studies on mechanical regulation of immune cells, specifically lymphocytes, and explore how the force-generating cytoskeletal machinery might mediate mechanosensing. We discuss general principles governing mechanical regulation of lymphocyte function, spanning from the molecular scale of receptor activation to cellular responses to mechanical stimuli.

Microengineered In Vitro Assays for Screening and Sorting Manufactured Therapeutic T Cells

Adoptively transferred T cells constitute a major class of current and emergent cellular immunotherapies for the treatment of disease, including but not limited to cancer. Although key advancements in molecular recognition, genetic engineering, and manufacturing have dramatically enhanced their translational potential, therapeutic potency remains limited by poor homing and infiltration of transferred cells within target host tissues. In vitro microengineered homing assays with precise control over micromechanical and biological cues can address these shortcomings by enabling interrogation, screening, sorting, and optimization of therapeutic T cells based on their homing capacity. In this article, the working principles, application, and integration of microengineered homing assays for the mechanistic study of biophysical and biomolecular cues relevant to homing of therapeutic T cells are reviewed. The potential for these platforms to enable scalable enrichment and screening of next-generation manufactured T cell therapies for cancer is also discussed.

Single-cell characterization of deformation and dynamics of mesenchymal stem cells in microfluidic systems: A computational study

Understanding the homing dynamics of individual mesenchymal stem cells (MSCs) in physiologically relevant microenvironments is crucial for improving the efficacy of MSC-based therapies for therapeutic and targeting purposes. This study investigates the passive homing behavior of individual MSCs in micropores that mimic interendothelial clefts through predictive computational simulations informed by previous microfluidic experiments. Initially, we quantified the size-dependent behavior of MSCs in micropores and elucidated the underlying mechanisms. Subsequently, we analyzed the shape deformation and traversal dynamics of each MSC. In addition, we conducted a systematic investigation to understand how the mechanical properties of MSCs impact their traversal process. We considered geometric and mechanical parameters, such as reduced cell volume, cell-to-nucleus diameter ratio, and cytoskeletal prestress states. Furthermore, we quantified the changes in the MSC traversal process and identified the quantitative limits in their response to variations in micropore length. Taken together, the computational results indicate the complex dynamic behavior of individual MSCs in the confined microflow. This finding offers an objective way to evaluate the homing ability of MSCs in an interendothelial-slit-like microenvironment.

Human BBB-on-a-chip reveals barrier disruption, endothelial inflammation, and T cell migration under neuroinflammatory conditions

The blood-brain barrier (BBB) is a highly selective barrier that ensures a homeostatic environment for the central nervous system (CNS). BBB dysfunction, inflammation, and immune cell infiltration are hallmarks of many CNS disorders, including multiple sclerosis and stroke. Physiologically relevant human in vitro models of the BBB are essential to improve our understanding of its function in health and disease, identify novel drug targets, and assess potential new therapies. We present a BBB-on-a-chip model comprising human brain microvascular endothelial cells (HBMECs) cultured in a microfluidic platform that allows parallel culture of 40 chips. In each chip, a perfused HBMEC vessel was grown against an extracellular matrix gel in a membrane-free manner. BBBs-on-chips were exposed to varying concentrations of pro-inflammatory cytokines tumor necrosis factor alpha (TNFα) and interleukin-1 beta (IL-1β) to mimic inflammation. The effect of the inflammatory conditions was studied by assessing the BBBs-on-chips' barrier function, cell morphology, and expression of cell adhesion molecules. Primary human T cells were perfused through the lumen of the BBBs-on-chips to study T cell adhesion, extravasation, and migration. Under inflammatory conditions, the BBBs-on-chips showed decreased trans-endothelial electrical resistance (TEER), increased permeability to sodium fluorescein, and aberrant cell morphology in a concentration-dependent manner. Moreover, we observed increased expression of cell adhesion molecules and concomitant monocyte adhesion. T cells extravasated from the inflamed blood vessels and migrated towards a C-X-C Motif Chemokine Ligand 12 (CXCL12) gradient. T cell adhesion was significantly reduced and a trend towards decreased migration was observed in presence of Natalizumab, an antibody drug that blocks very late antigen-4 (VLA-4) and is used in the treatment of multiple sclerosis. In conclusion, we demonstrate a high-throughput microfluidic model of the human BBB that can be used to model neuroinflammation and assess anti-inflammatory and barrier-restoring interventions to fight neurological disorders.

The T Cell Journey: A Tour de Force

T cells act as the puppeteers in the adaptive immune response, and their dysfunction leads to the initiation and progression of pathological conditions. During their lifetime, T cells experience myriad forces that modulate their effector functions. These forces are imposed by interacting cells, surrounding tissues, and shear forces from fluid movement. In this review, a journey with T cells is made, from their development to their unique characteristics, including the early studies that uncovered their mechanosensitivity. Then the studies pertaining to the responses of T cell activation to changes in antigen-presenting cells' physical properties, to their immediate surrounding extracellular matrix microenvironment, and flow conditions are highlighted. In addition, it is explored how pathological conditions like the tumor microenvironment can hinder T cells and allow cancer cells to escape elimination.

Immune cell behaviour and dynamics in the kidney - insights from in vivo imaging

The actions of immune cells within the kidney are of fundamental importance in kidney homeostasis and disease. In disease settings such as acute kidney injury, anti-neutrophil cytoplasmic antibody-associated vasculitis, lupus nephritis and renal transplant rejection, immune cells resident within the kidney and those recruited from the circulation propagate inflammatory responses with deleterious effects on the kidney. As in most forms of inflammation, intravital imaging - particularly two-photon microscopy - has been critical to our understanding of immune cell responses in the renal microvasculature and interstitium, enabling visualization of immune cell dynamics over time rather than statically. These studies have demonstrated differences in the recruitment and function of these cells from those in more conventional vascular beds, and provided a wealth of information on the actions of blood-borne immune cells such as neutrophils, monocytes and T cells, as well as kidney-resident mononuclear phagocytes, in a range of diseases affecting different kidney compartments. In particular, in vivo imaging has furthered our understanding of leukocyte function within the glomerulus in acute glomerulonephritis, and in the tubulointerstitium and interstitial microvasculature during acute kidney injury and following transplantation, revealing mechanisms of immune surveillance, antigen presentation and inflammation in the kidney.

Mechanosensing by Piezo1 and its implications for physiology and various pathologies

Piezo1 is a mechanosensitive ion channel with essential roles in cardiovascular, lung, urinary, and immune functions. Piezo1 is widely distributed in different tissues in the human body and its specific roles have been identified following a decade of research; however, not all are well understood. Many structural and functional characteristics of Piezo1 have been discovered and are known to differ greatly from the characteristics of other mechanosensitive ion channels. Understanding the mechanisms by which this ion channel functions may be useful in determining its physiological roles in various organ systems. This review provides insight into the signalling pathways activated by mechanical stimulation of Piezo1 in various organ systems and cell types. We discuss downstream targets of Piezo1 and the overall effects resulting from Piezo1 activation, which may provide insights into potential treatment targets for diseases involving this ion channel.

Differential immunological signature at the culprit site distinguishes acute coronary syndrome with intact from acute coronary syndrome with ruptured fibrous cap: results from the prospective translational OPTICO-ACS study

AIMS

Acute coronary syndromes with intact fibrous cap (IFC-ACS), i.e. caused by coronary plaque erosion, account for approximately one-third of ACS. However, the underlying pathophysiological mechanisms as compared with ACS caused by plaque rupture (RFC-ACS) remain largely undefined. The prospective translational OPTICO-ACS study programme investigates for the first time the microenvironment of ACS-causing culprit lesions (CL) with intact fibrous cap by molecular high-resolution intracoronary imaging and simultaneous local immunological phenotyping.

METHODS AND RESULTS

The CL of 170 consecutive ACS patients were investigated by optical coherence tomography (OCT) and simultaneous immunophenotyping by flow cytometric analysis as well as by effector molecule concentration measurements across the culprit lesion gradient (ratio local/systemic levels). Within the study cohort, IFC caused 24.6% of ACS while RFC-ACS caused 75.4% as determined and validated by two independent OCT core laboratories. The IFC-CL were characterized by lower lipid content, less calcification, a thicker overlying fibrous cap, and largely localized near a coronary bifurcation as compared with RFC-CL. The microenvironment of IFC-ACS lesions demonstrated selective enrichment in both CD4+ and CD8+ T-lymphocytes (+8.1% and +11.2%, respectively, both P < 0.05) as compared with RFC-ACS lesions. T-cell-associated extracellular circulating microvesicles (MV) were more pronounced in IFC-ACS lesions and a significantly higher amount of CD8+ T-lymphocytes was detectable in thrombi aspirated from IFC-culprit sites. Furthermore, IFC-ACS lesions showed increased levels of the T-cell effector molecules granzyme A (+22.4%), perforin (+58.8%), and granulysin (+75.4%) as compared with RFC plaques (P < 0.005). Endothelial cells subjected to culture in disturbed laminar flow conditions, i.e. to simulate coronary flow near a bifurcation, demonstrated an enhanced adhesion of CD8+T cells. Finally, both CD8+T cells and their cytotoxic effector molecules caused endothelial cell death, a key potential pathophysiological mechanism in IFC-ACS.

CONCLUSIONS

The OPTICO-ACS study emphasizes a novel mechanism in the pathogenesis of IFC-ACS, favouring participation of the adaptive immune system, particularly CD4+ and CD8+ T-cells and their effector molecules. The different immune signatures identified in this study advance the understanding of coronary plaque progression and may provide a basis for future development of personalized therapeutic approaches to ACS with IFC.

TRIAL REGISTRATION

The study was registered at clinicalTrials.gov (NCT03129503).

Regulatory T Cell Transmigration and Intravascular Migration Undergo Mechanistically Distinct Regulation at Different Phases of the Inflammatory Response

Regulatory T cells (Tregs) play important roles in limiting inflammatory responses in the periphery. During these responses, Treg abundance in affected organs increases and interfering with their recruitment results in exacerbation of inflammation. However, the mechanisms whereby Tregs enter the skin remain poorly understood. The aim of this study was to use intravital microscopy to investigate adhesion and transmigration of Tregs in the dermal microvasculature in a two-challenge model of contact sensitivity. Using intravital confocal microscopy of Foxp3-GFP mice, we visualized endogenous Tregs and assessed their interactions in the dermal microvasculature. Four hours after hapten challenge, Tregs underwent adhesion with ∼25% of these cells proceeding to transmigration, a process dependent on CCR4. At 24 h, Tregs adhered but no longer underwent transmigration, instead remaining in prolonged contact with the endothelium, migrating over the endothelial surface. Four hours after a second challenge, Treg transmigration was restored, although in this case transmigration was CCR4 independent, instead involving the CCR6/CCL20 pathway. Notably, at 24 h but not 4 h after challenge, endothelial cells expressed MHC class II (MHC II). Moreover, at this time of peak MHC II expression, inhibition of MHC II reduced Treg adhesion, demonstrating an unexpected role for MHC II in Treg attachment to the endothelium. Together these data show that Treg adhesion and transmigration can be driven by different molecular mechanisms at different stages of an Ag-driven inflammatory response. In addition, Tregs can undergo prolonged migration on the inflamed endothelium.

T-Cell Mechanobiology: Force Sensation, Potentiation, and Translation

A T cell is a sensitive self-referential mechanical sensor. Mechanical forces influence the recognition, activation, differentiation, and function throughout the lifetime of a T cell. T cells constantly perceive and respond to physical stimuli through their surface receptors, cytoskeleton, and subcellular structures. Surface receptors receive physical cues in the form of forces generated through receptor-ligand binding events, which are dynamically regulated by contact tension, shear stress, and substrate rigidity. The resulting mechanotransduction not only influences T-cell recognition and signaling but also possibly modulates cell metabolism and gene expression. Moreover, forces also dynamically regulate the deformation, organization, and translocation of cytoskeleton and subcellular structures, leading to changes in T-cell mobility, migration, and infiltration. However, the roles and mechanisms of how mechanical forces modulate T-cell recognition, signaling, metabolism, and gene expression, are largely unknown and underappreciated. Here, we review recent technological and scientific advances in T-cell mechanobiology, discuss possible roles and mechanisms of T-cell mechanotransduction, and propose new research directions of this emerging field in health and disease.

A silicon nanomembrane platform for the visualization of immune cell trafficking across the human blood-brain barrier under flow

Here we report on the development of a breakthrough microfluidic human in vitro cerebrovascular barrier (CVB) model featuring stem cell-derived brain-like endothelial cells (BLECs) and nanoporous silicon nitride (NPN) membranes (µSiM-CVB). The nanoscale thinness of NPN membranes combined with their high permeability and optical transparency makes them an ideal scaffold for the assembly of an in vitro microfluidic model of the blood-brain barrier (BBB) featuring cellular elements of the neurovascular unit (NVU). Dual-chamber devices divided by NPN membranes yield tight barrier properties in BLECs and allow an abluminal pericyte-co-culture to be replaced with pericyte-conditioned media. With the benefit of physiological flow and superior imaging quality, the µSiM-CVB platform captures each phase of the multi-step T-cell migration across the BBB in live cell imaging. The small volume of <100 µL of the µSiM-CVB will enable in vitro investigations of rare patient-derived immune cells with the human BBB. The µSiM-CVB is a breakthrough in vitro human BBB model to enable live and high-quality imaging of human immune cell interactions with the BBB under physiological flow. We expect it to become a valuable new tool for the study of cerebrovascular pathologies ranging from neuroinflammation to metastatic cancer.

CD11c+ Cells Are Gatekeepers for Lymphocyte Trafficking to Infiltrated Islets During Type 1 Diabetes

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease that affects more than 19 million people with incidence increasing rapidly worldwide. For T cells to effectively drive T1D, they must first traffic to the islets and extravasate through the islet vasculature. Understanding the cues that lead to T cell entry into inflamed islets is important because diagnosed T1D patients already have established immune infiltration of their islets. Here we show that CD11c⁺ cells are a key mediator of T cell trafficking to infiltrated islets in non-obese diabetic (NOD) mice. Using intravital 2-photon islet imaging we show that T cell extravasation into the islets is an extended process, with T cells arresting in the islet vasculature in close proximity to perivascular CD11c⁺ cells. Antigen is not required for T cell trafficking to infiltrated islets, but T cell chemokine receptor signaling is necessary. Using RNAseq, we show that islet CD11c⁺ cells express over 20 different chemokines that bind chemokine receptors expressed on islet T cells. One highly expressed chemokine-receptor pair is CXCL16-CXCR6. However, NOD. CXCR6^-/- mice progressed normally to T1D and CXCR6 deficient T cells trafficked normally to the islets. Even with CXCR3 and CXCR6 dual deficiency, T cells trafficked to infiltrated islets. These data reinforce that chemokine receptor signaling is highly redundant for T cell trafficking to inflamed islets. Importantly, depletion of CD11c⁺ cells strongly inhibited T cell trafficking to infiltrated islets of NOD mice. We suggest that targeted depletion of CD11c⁺ cells associated with the islet vasculature may yield a therapeutic target to inhibit T cell trafficking to inflamed islets to prevent progression of T1D.

Production via good manufacturing practice of exofucosylated human mesenchymal stromal cells for clinical applications

BACKGROUND

The regenerative and immunomodulatory properties of human mesenchymal stromal cells (hMSCs) have raised great hope for their use in cell therapy. However, when intravenously infused, hMSCs fail to reach sites of tissue injury. Fucose addition in α(1,3)-linkage to terminal sialyllactosamines on CD44 creates the molecule known as hematopoietic cell E-/L-selectin ligand (HCELL), programming hMSC binding to E-selectin that is expressed on microvascular endothelial cells of bone marrow (BM), skin and at all sites of inflammation. Here we describe how this modification on BM-derived hMSCs (BM-hMSCs) can be adapted to good manufacturing practice (GMP) standards.

METHODS

BM-hMSCs were expanded using xenogenic-free media and exofucosylated using α(1,3)-fucosyltransferases VI (FTVI) or VII (FTVII). Enforced fucosylation converted CD44 into HCELL, and HCELL formation was assessed using Western blot, flow cytometry and cell-binding assays. Untreated (unfucosylated), buffer-treated and exofucosylated BM-hMSCs were each analyzed for cell viability, immunophenotype and differentiation potential, and E-selectin binding stability was assessed at room temperature, at 4°C, and after cryopreservation. Cell product safety was evaluated using microbiological testing, karyotype analysis, and c-Myc messenger RNA (mRNA) expression, and potential effects on genetic reprogramming and in cell signaling were analyzed using gene expression microarrays and receptor tyrosine kinase (RTK) phosphorylation arrays.

RESULTS

Our protocol efficiently generates HCELL on clinical-scale batches of BM-hMSCs. Exofucosylation yields stable HCELL expression for 48 h at 4°C, with retained expression after cell cryopreservation. Cell viability and identity are unaffected by exofucosylation, without changes in gene expression or RTK phosphorylation.

DISCUSSION

The described exofucosylation protocol using xenogenic-free reagents enforces HCELL expression on hMSCs endowing potent E-selectin binding without affecting cell viability or native phenotype. This described protocol is readily scalable for GMP-compliant clinical production.

Biomimetic post-capillary venule expansions for leukocyte adhesion studies

Leukocyte adhesion and extravasation are maximal near the transition from capillary to post-capillary venule, and are strongly influenced by a confluence of scale-dependent physical effects. Mimicking the scale of physiological vessels using in vitro microfluidic systems allows the capture of these effects on leukocyte adhesion assays, but imposes practical limits on reproducibility and reliable quantification. Here we present a microfluidic platform that provides multiple (54-512) technical replicates within a 15-minute sample collection time, coupled with an automated computer vision analysis pipeline that captures leukocyte adhesion probabilities as a function of shear and extensional stresses. We report that in post-capillary channels of physiological scale, efficient leukocyte adhesion requires erythrocytes forcing leukocytes against the wall, a phenomenon that is promoted by the transitional flow in post-capillary venule expansions and dependent on the adhesion molecule ICAM-1.

Microfluidic lung airway-on-a-chip with arrayable suspended gels for studying epithelial and smooth muscle cell interactions

Chronic lung diseases (CLDs) are regulated by complex interactions between many different cell types residing in lung airway tissues. Specifically, interactions between airway epithelial cells (ECs) and airway smooth muscle cells (SMCs) have been shown in part to play major roles in the pathogenesis of CLDs, but the underlying molecular mechanisms are not well understood. To advance our understanding of lung pathophysiology and accelerate drug development processes, new innovative in vitro tissue models are needed that can reconstitute the complex in vivo microenvironment of human lung tissues. Organ-on-a-chip technologies have recently made significant strides in recapitulating physiological properties of in vivo lung tissue microenvironments. However, novel advancements are still needed to enable the study of airway SMC-EC communication with matrix interactions, and to provide higher throughput capabilities and manufacturability. We have developed a thermoplastic-based microfluidic lung airway-on-a-chip model that mimics the lung airway tissue microenvironment, and in particular, the interactions between SMCs, ECs, and supporting extracellular matrix (ECM). The microdevice is fabricated from acrylic using micromilling and solvent bonding techniques, and consists of three vertically stacked microfluidic compartments with a bottom media reservoir for SMC culture, a middle thin hydrogel layer, and an upper microchamber for achieving air-liquid interface (ALI) culture of the epithelium. A unique aspect of the design lies in the suspended hydrogel with upper and lower interfaces for EC and SMC culture, respectively. A mixture of type I collagen and Matrigel was found to promote EC adhesion and monolayer formation, and SMC adhesion and alignment. Optimal culturing protocols were established that enabled EC-SMC coculture for more than 31 days. Epithelial monolayers displayed common morphological markers including ZO-1 tight junctions and F-actin cell cortices, while SMCs exhibited enhanced cell alignment and expression of α-SMA. The thermoplastic device construction facilitates mass manufacturing, allows EC-SMC coculture systems to be arrayed for increased throughput, and can be disassembled to allow extraction of the suspended gel for downstream analyses. This airway-on-a-chip device has potential to significantly advance our understanding of SMC-EC-matrix interactions, and their roles in the development of CLDs.

Endothelial Cell Focal Adhesion Regulates Transendothelial Migration and Subendothelial Crawling of T Cells

Leukocytes circulating in the blood stream leave out of blood vessels and infiltrate into inflamed tissues to perform immune responses. Endothelial cells (ECs) lining interior of the post-capillary venules regulate various steps of leukocyte extravasation. In response to inflammatory signals, ECs upregulate adhesion molecules and produce/present chemokines to support firm adhesion and intraluminal crawling of leukocytes. They also remodel junctions to facilitate leukocyte transendothelial migration (TEM). While roles of apical/lateral components of EC layers in regulating leukocyte extravasation have been extensively investigated, relatively little attention has been paid to the basal part of EC layers comprising subendothelial spaces. In this study, we employed interference reflection microscopy (IRM), a microscopy technique specialized for label-free visualization of cell–substrate contact, to study detailed dynamic interactions between basal part of ECs and T cells underneath EC monolayer. For TEM, T cells on EC monolayer extended protrusions through junctions to explore subendothelial spaces, and EC focal adhesions (EC-FAs) acted as physical barrier for the protrusion. Therefore, preferential TEM occurred through junctions where near-junction focal adhesion (NJ-FA) density of ECs was low. After TEM, T cells performed subendothelial crawling (SEC) with flattened morphology and reduced migration velocity due to tight confinement. T cell SEC mostly occurred through gaps formed in between EC-FAs with minimally breaking EC-FAs. Tumor necrosis factor-α (TNF-α) treatment significantly loosened confinement in subendothelial spaces and reduced NJ-FA density of ECs, thus remodeled basal part of EC layer to facilitate leukocyte extravasation.

Regulation of genome organization and gene expression by nuclear mechanotransduction

It is well established that cells sense chemical signals from their local microenvironment and transduce them to the nucleus to regulate gene expression programmes. Although a number of experiments have shown that mechanical cues can also modulate gene expression, the underlying mechanisms are far from clear. Nevertheless, we are now beginning to understand how mechanical cues are transduced to the nucleus and how they influence nuclear mechanics, genome organization and transcription. In particular, recent progress in super-resolution imaging, in genome-wide application of RNA sequencing, chromatin immunoprecipitation and chromosome conformation capture and in theoretical modelling of 3D genome organization enables the exploration of the relationship between cell mechanics, 3D chromatin configurations and transcription, thereby shedding new light on how mechanical forces regulate gene expression.

T-cell regulation through a basic suppressive mechanism targeting low-density lipoprotein receptor-related protein 1

Cell adhesion is generally considered to depend on positive regulation through ligation of integrins and cytokine receptors. However, here we show that T-cell adhesion, and notably also T-cell receptor (TCR) -induced activation, are subject to constant suppression through shedding of low-density lipoprotein receptor-related protein 1 (LRP1). The broad-spectrum metalloprotease inhibitor GM6001 abrogated shedding, so inducing prominent cell surface expression of LRP1 while enhancing TCR-induced activation and adhesion to β₁ and β₂ integrin ligands, hence arresting the cells. Integrin ligands also inhibited shedding but the effect was less potent than that of GM6001. Unlike GM6001, integrin ligands also induced cell surface expression of full-length thrombospondin-1 (TSP170) and TSP130, which associated with LRP1, and TSP110, which did not associate with LRP1. Cell surface expression of LRP1 and TSP130 were induced exclusively in adhering cells, expression of TSP110 preferentially in non-adhering cells and expression of TSP170 correlated with T-cell motility. The pro-adhesive chemokine CXCL12 also inhibited LRP1 shedding and induced surface expression of TSP170 and TSP130 while inhibiting TSP110. Exogenous TSP-1 and ligation of CD28 inhibited shedding although less effectively than GM6001, and the inhibition through CD28 was independent of TSP-1. Small interfering RNA silencing experiments confirmed involvement of LRP1 and TSP-1 in integrin-dependent adhesion and TCR-induced activation. Hence, the poor LRP1 expression in T cells depends on shedding. Integrin ligands and CXCL12 antagonize shedding through a TSP-1-dependent pathway and ligation of CD28 antagonizes shedding independent of TSP-1. The disappearance of LRP1 from the cell surface may provide basic immunosuppression at the T-cell level.

Migration of bone marrow progenitor cells in the adult brain of rats and rabbits

Neurogenesis takes place in the adult mammalian brain in three areas: Subgranular zone of the dentate gyrus (DG); subventricular zone of the lateral ventricle; olfactory bulb. Different molecular markers can be used to characterize the cells involved in adult neurogenesis. It has been recently suggested that a population of bone marrow (BM) progenitor cells may migrate to the brain and differentiate into neuronal lineage. To explore this hypothesis, we injected recombinant SV40-derived vectors into the BM and followed the potential migration of the transduced cells. Long-term BM-directed gene transfer using recombinant SV40-derived vectors leads to expression of the genes delivered to the BM firstly in circulating cells, then after several months in mature neurons and microglial cells, and thus without central nervous system (CNS) lesion. Most of transgene-expressing cells expressed NeuN, a marker of mature neurons. Thus, BM-derived cells may function as progenitors of CNS cells in adult animals. The mechanism by which the cells from the BM come to be neurons remains to be determined. Although the observed gradual increase in transgene-expressing neurons over 16 mo suggests that the pathway involved differentiation of BM-resident cells into neurons, cell fusion as the principal route cannot be totally ruled out. Additional studies using similar viral vectors showed that BM-derived progenitor cells migrating in the CNS express markers of neuronal precursors or immature neurons. Transgene-positive cells were found in the subgranular zone of the DG of the hippocampus 16 mo after intramarrow injection of the vector. In addition to cells expressing markers of mature neurons, transgene-positive cells were also positive for nestin and doublecortin, molecules expressed by developing neuronal cells. These cells were actively proliferating, as shown by short term BrdU incorporation studies. Inducing seizures by using kainic acid increased the number of BM progenitor cells transduced by SV40 vectors migrating to the hippocampus, and these cells were seen at earlier time points in the DG. We show that the cell membrane chemokine receptor, CCR5, and its ligands, enhance CNS inflammation and seizure activity in a model of neuronal excitotoxicity. SV40-based gene delivery of RNAi targeting CCR5 to the BM results in downregulating CCR5 in circulating cells, suggesting that CCR5 plays an important role in regulating traffic of BM-derived cells into the CNS, both in the basal state and in response to injury. Furthermore, reduction in CCR5 expression in circulating cells provides profound neuroprotection from excitotoxic neuronal injury, reduces neuroinflammation, and increases neuronal regeneration following this type of insult. These results suggest that BM-derived, transgene-expressing, cells can migrate to the brain and that they become neurons, at least in part, by differentiating into neuron precursors and subsequently developing into mature neurons.

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.

Characterization of T cell repertoire of blood, tumor, and ascites in ovarian cancer patients using next generation sequencing

Tumor-infiltrating lymphocytes (TILs) play an important role in regulating the host immune response and are one of key factors in defining tumor microenvironment. Some studies have indicated that T cell infiltration in malignant ascites is associated with clinical outcome, but few studies have performed detailed characterization of T cell diversity or clonality in malignant effusions. We have applied a next generation sequencing method to characterize T cell repertoire of a set of primary cancers, ascites, and blood from 12 ovarian cancer patients and also analyzed the T cell subtype populations in malignant fluids from 3 ovarian cancer patients. We observed enrichment of certain T cells in tumors and ascites, but most of the enriched T cell receptor (TCR) sequences in tumors and ascites were not common. Moreover, we analyzed TCR sequences of T cell subtypes (CD4⁺, CD8⁺, and regulatory T cells) isolated from malignant effusions and also found clonal expansion of certain T cell populations, but the TCR sequences were almost mutually exclusive among the three subgroups. Although functional studies of clonally expanded T cell populations are definitely required, our approach offers a detailed characterization of T cell immune microenvironment in tumors and ascites that might differently affect antitumor immune response.

A novel device to concurrently assess leukocyte extravasation and interstitial migration within a defined 3D environment

Leukocyte extravasation and interstitial migration are key events during inflammation. Traditional in vitro techniques address only specific steps of cell recruitment to tissues and fail to recapitulate the whole process in an appropriate three-dimensional (3D) microenvironment. Herein, we describe a device that enables us to qualitatively and quantitatively assess in 4D the interdependent steps underlying leukocyte trafficking in a close-to-physiology in vitro context. Real-time tracking of cells, from initial adhesion to the endothelium and subsequent diapedesis to interstitial migration towards the source of the chemoattractant within the 3D collagen matrix, is enabled by the use of optically transparent porous membranes laid over the matrix. Unique features of the device, such as the use of non-planar surfaces and the contribution of physiological flow to the establishment of a persistent chemoattractant gradient, were assessed by numerical simulations and validated by proof-of-concept, simultaneous testing of differentially treated primary mouse neutrophils. This microfluidic platform offers new and versatile tools to thoroughly investigate the stepwise process of circulating cell recruitment to target tissues in vitro and to test novel therapeutics targeting various steps of the process.

Regulation of T-lymphocyte motility, adhesion and de-adhesion by a cell surface mechanism directed by low density lipoprotein receptor-related protein 1 and endogenous thrombospondin-1

T lymphocytes are highly motile and constantly reposition themselves between a free-floating vascular state, transient adhesion and migration in tissues. The regulation behind this unique dynamic behaviour remains unclear. Here we show that T cells have a cell surface mechanism for integrated regulation of motility and adhesion and that integrin ligands and CXCL12/SDF-1 influence motility and adhesion through this mechanism. Targeting cell surface-expressed low-density lipoprotein receptor-related protein 1 (LRP1) with an antibody, or blocking transport of LRP1 to the cell surface, perturbed the cell surface distribution of endogenous thrombospondin-1 (TSP-1) while inhibiting motility and potentiating cytoplasmic spreading on intercellular adhesion molecule 1 (ICAM-1) and fibronectin. Integrin ligands and CXCL12 stimulated motility and enhanced cell surface expression of LRP1, intact TSP-1 and a 130,000 MW TSP-1 fragment while preventing formation of a de-adhesion-coupled 110 000 MW TSP-1 fragment. The appearance of the 130 000 MW TSP-1 fragment was inhibited by the antibody that targeted LRP1 expression, inhibited motility and enhanced spreading. The TSP-1 binding site in the LRP1-associated protein, calreticulin, stimulated adhesion to ICAM-1 through intact TSP-1 and CD47. Shear flow enhanced cell surface expression of intact TSP-1. Hence, chemokines and integrin ligands up-regulate a dominant motogenic pathway through LRP1 and TSP-1 cleavage and activate an associated adhesion pathway through the LRP1-calreticulin complex, intact TSP-1 and CD47. This regulation of T-cell motility and adhesion makes pro-adhesive stimuli favour motile responses, which may explain why T cells prioritize movement before permanent adhesion.

Blood vessels pattern heparan sulfate gradients between their apical and basolateral aspects

A hallmark of immune cell trafficking is directional guidance via gradients of soluble or surface bound chemokines. Vascular endothelial cells produce, transport and deposit either their own chemokines or chemokines produced by the underlying stroma. Endothelial heparan sulfate (HS) was suggested to be a critical scaffold for these chemokine pools, but it is unclear how steep chemokine gradients are sustained between the lumenal and ablumenal aspects of blood vessels. Addressing this question by semi-quantitative immunostaining of HS moieties around blood vessels with a pan anti-HS IgM mAb, we found a striking HS enrichment in the basal lamina of resting and inflamed post capillary skin venules, as well as in high endothelial venules (HEVs) of lymph nodes. Staining of skin vessels with a glycocalyx probe further suggested that their lumenal glycocalyx contains much lower HS density than their basolateral extracellular matrix (ECM). This polarized HS pattern was observed also in isolated resting and inflamed microvascular dermal cells. Notably, progressive skin inflammation resulted in massive ECM deposition and in further HS enrichment around skin post capillary venules and their associated pericytes. Inflammation-dependent HS enrichment was not compromised in mice deficient in the main HS degrading enzyme, heparanase. Our results suggest that the blood vasculature patterns steep gradients of HS scaffolds between their lumenal and basolateral endothelial aspects, and that inflammatory processes can further enrich the HS content nearby inflamed vessels. We propose that chemokine gradients between the lumenal and ablumenal sides of vessels could be favored by these sharp HS scaffold gradients.

Detection of bidirectional signaling during integrin activation and neutrophil adhesion

Neutrophil arrest and migration on inflamed endothelium is dependent upon a conformational shift in CD11a/CD18 (LFA-1) from a low to high affinity and clustered state which determines the strength and lifetime of bond formation with intracellular adhesion molecule 1 (ICAM-1). Cytoskeletal adaptor proteins kindlin-3 and talin-1 anchor clustered LFA-1 to the cytoskeleton and support the transition from neutrophil rolling to arrest. We employ microfluidic flow channels and total internal reflection fluorescence microscopy to evaluate the spatiotemporal regulation of LFA-1 affinity and bond formation that facilitate the transition from neutrophil rolling to arrest. Methodology is presented to correlate the relationship between integrin conformation, bond formation with ICAM-1, and cytoskeletal engagement and adhesion strengthening necessary to achieve a migratory phenotype.

T lymphocytes orient against the direction of fluid flow during LFA-1-mediated migration

As they leave the blood stream and travel to lymph nodes or sites of inflammation, T lymphocytes are captured by the endothelium and migrate along the vascular wall to permissive sites of transmigration. These processes take place under the influence of hemodynamic shear stress; therefore, we investigated how migrational speed and directionality are influenced by variations in shear stress. We examined human effector T lymphocytes on intercellular adhesion molecule 1 (ICAM-1)-coated surfaces under the influence of shear stresses from 2 to 60 dyn.cm(-2). T lymphocytes were shown to respond to shear stress application by a rapid (30 s) and fully reversible orientation of their migration against the fluid flow without a change in migration speed. Primary T lymphocytes migrating on ICAM-1 in the presence of uniformly applied SDF-1α were also found to migrate against the direction of shear flow. In sharp contrast, neutrophils migrating in the presence of uniformly applied fMLP and leukemic HSB2 T lymphocytes migrating on ICAM-1 alone oriented their migration downstream, with the direction of fluid flow. Our findings suggest that, in addition to biochemical cues, shear stress is a contributing factor to leukocyte migration directionality.

Microfluidics for in vitro biomimetic shear stress-dependent leukocyte adhesion assays

Recruitment of leukocytes from blood to tissues is a multi-step process playing a major role in the activation of inflammatory responses. Tethering and rolling of leukocytes along the vessel wall, followed by arrest and transmigration through the endothelium result from chemoattractant-dependent signals, inducing adhesive and migratory events. Shear forces exerted by the blood flow on leukocytes induce rolling via selectin-mediated interactions with endothelial cells and increase the probability of leukocytes to engage their chemokine receptors, facilitating integrin activation and consequent arrest. Flow-derived shear forces generate mechanical stimuli concurring with biochemical signals in the modulation of leukocyte-endothelial cell interactions. In the last few years, a host of in vitro studies have clarified the biochemical adhesion cascade and the role of shear stress in leukocyte extravasation. The limitation of the static environment in Boyden devices has been overcome both by the use of parallel-plate flow chambers and by custom models mimicking the in vivo conditions, along with widespread microfluidic approaches to in vitro modeling. These devices create an in vitro biomimetic environment where the multi-step transmigration process can be imaged and quantified under mechanical and biochemical controlled conditions, including fluid dynamic settings, channel design, materials and surface coatings. This paper reviews the technological solutions recently proposed to model, observe and quantify leukocyte adhesion behavior under shear flow, with a final survey of high-throughput solutions featuring multiple parallel assays as well as thorough and time-saving statistical interpretation of the experimental results.

α(M)β(2)-integrin-intercellular adhesion molecule-1 interactions drive the flow-dependent trafficking of Guillain-Barré syndrome patient derived mononuclear leukocytes at the blood-nerve barrier in vitro

The mechanisms of hematogenous leukocyte trafficking at the human blood-nerve barrier (BNB) are largely unknown. Intercellular adhesion molecule-1 (ICAM-1) has been implicated in the pathogenesis of Guillain-Barré syndrome (GBS). We developed a cytokine-activated human in vitro BNB model using primary endoneurial endothelial cells. Endothelial treatment with 10 U/ml tissue necrosis factor-α and 20 U/ml interferon-γ resulted in de novo expression of pro-inflammatory chemokines CCL2, CXCL9, CXCL11, and CCL20, with increased expression of CXCL2-3, CXCL8, and CXCL10 relative to basal levels. Cytokine treatment induced/enhanced ICAM-1, E- and P-selectin, vascular cell adhesion molecule-1 and the alternatively spliced pro-adhesive fibronectin variant, fibronectin connecting segment-1 expression in a time-dependent manner, without alterations in junctional adhesion molecule-A expression. Lymphocytes and monocytes from untreated GBS patients express ICAM-1 counterligands, α(M)- and α(L)-integrin, with differential regulation of α(M) -integrin expression compared to healthy controls. Under flow conditions that mimic capillary hemodynamics in vivo, there was a >3-fold increase in total GBS patient and healthy control mononuclear leukocyte adhesion/migration at the BNB following cytokine treatment relative to the untreated state. Function neutralizing monoclonal antibodies against human α(M)-integrin (CD11b) and ICAM-1 reduced untreated GBS patient mononuclear leukocyte trafficking at the BNB by 59% and 64.2%, respectively. Monoclonal antibodies against α(L)-integrin (CD11a) and human intravenous immunoglobulin reduced total leukocyte adhesion/migration by 22.8% and 17.6%, respectively. This study demonstrates differential regulation of α(M)-integrin on circulating mononuclear cells in GBS, as well as an important role for α(M)-integrin-ICAM-1 interactions in pathogenic GBS patient leukocyte trafficking at the human BNB in vitro.

Inflammatory cell trafficking across the blood-brain barrier: chemokine regulation and in vitro models

The blood-brain barrier (BBB) is the brain-specific capillary barrier that is critical for preventing toxic substances from entering the central nervous system (CNS). In contrast to vessels of peripheral organs, the BBB limits the exchange of inflammatory cells and mediators under physiological and pathological conditions. Clarifying these limitations and the role of chemokines in regulating the BBB would provide new insights into neuroprotective strategies in neuroinflammatory diseases. Because there is a paucity of in vitro BBB models, however, some mechanistic aspects of transmigration across the BBB still remain largely unknown. In this review, we summarize current knowledge of BBB cellular components, the multistep process of inflammatory cells crossing the BBB, functions of CNS-derived chemokines, and in vitro BBB models for transmigration, with a particular focus on new and recent findings.

CXCL12-induced monocyte-endothelial interactions promote lymphocyte transmigration across an in vitro blood-brain barrier

The accumulation of inflammatory cells in the brain parenchyma is a critical step in the pathogenesis of neuroinflammatory diseases such as multiple sclerosis (MS). Chemokines and adhesion molecules orchestrate leukocyte transmigration across the blood-brain barrier (BBB), but the dynamics of chemokine receptor expression during leukocyte transmigration are unclear. We describe an in vitro BBB model system using human brain microvascular endothelial cells that incorporates shear forces mimicking blood flow to elucidate how chemokine receptor expression is modulated during leukocyte transmigration. In the presence of the chemokine CXCL12, we examined modulation of its receptor CXCR4 on human T cells, B cells, and monocytes transmigrating across the BBB under flow conditions. CXCL12 stimulated transmigration of CD4(+) and CD8(+) T cells, CD19(+) B cells, and CD14(+) monocytes. Transmigration was blocked by CXCR4-neutralizing antibodies. Unexpectedly, CXCL12 selectively down-regulated CXCR4 on transmigrating monocytes, but not T cells. Monocytes underwent preferential CXCL12-mediated adhesion to the BBB in vitro compared with lymphocytes. These findings provide new insights into leukocyte-endothelial interactions at the BBB under conditions mimicking blood flow and suggest that in vitro BBB models may be useful for identifying chemokine receptors that could be modulated therapeutically to reduce neuroinflammation in diseases such as MS.

Transendothelial migration of lymphocytes mediated by intraendothelial vesicle stores rather than by extracellular chemokine depots

Chemokines presented by the endothelium are critical for integrin-dependent adhesion and transendothelial migration of naive and memory lymphocytes. Here we found that effector lymphocytes of the type 1 helper T cell (T(H)1 cell) and type 1 cytotoxic T cell (T(C)1 cell) subtypes expressed adhesive integrins that bypassed chemokine signals and established firm arrests on variably inflamed endothelial barriers. Nevertheless, the transendothelial migration of these lymphocytes strictly depended on signals from guanine nucleotide-binding proteins of the G(i) type and was promoted by multiple endothelium-derived inflammatory chemokines, even without outer endothelial surface exposure. Instead, transendothelial migration-promoting endothelial chemokines were stored in vesicles docked on actin fibers beneath the plasma membranes and were locally released within tight lymphocyte-endothelial synapses. Thus, effector T lymphocytes can cross inflamed barriers through contact-guided consumption of intraendothelial chemokines without surface-deposited chemokines or extraendothelial chemokine gradients.

CD44 and HCELL: preventing hematogenous metastasis at step 1

Despite great strides in our knowledge of the genetic and epigenetic changes underlying malignancy, we have limited information on the molecular basis of metastasis. Over 90% of cancer deaths are caused by spread of tumor cells from a primary site to distant organs and tissues, highlighting the pressing need to define the molecular effectors of cancer metastasis. Mounting evidence suggests that circulating tumor cells (CTCs) home to specific tissues by hijacking the normal leukocyte trafficking mechanisms. Cancer cells characteristically express CD44, and there is increasing evidence that hematopoietic cell E-/L-selectin ligand (HCELL), a sialofucosylated glycoform of CD44, serves as the major selectin ligand on cancer cells, allowing interaction of tumor cells with endothelium, leukocytes, and platelets. Here, we review the structural biology of CD44 and of HCELL, and present current data on the function of these molecules in mediating organ-specific homing/metastasis of CTCs.

Migrational guidance of neutrophils is mechanotransduced via high-affinity LFA-1 and calcium flux

Acute inflammation triggers the innate immune response of neutrophils that efficiently traffic from the bloodstream to concentrate at high numbers at the site of tissue infection or wounding. A gatekeeper in this process is activation of β(2) integrins, which form bond clusters with ICAM-1 on the endothelial surface. These bond clusters serve dual functions of providing adhesive strength to anchor neutrophils under the shear forces of blood flow and directional guidance for cell polarization and subsequent transmigration on inflamed endothelium. We hypothesized that shear forces transmitted through high-affinity LFA-1 facilitates the cooperation with the calcium release-activated channel Orai1 in directing localized cytoskeletal activation and directed migration. By using vascular mimetic microfluidic channels, we observed neutrophil arrest on a substrate of either ICAM-1 or allosteric Abs that stabilize a high- or low-affinity conformation of LFA-1. Neutrophils captured via low-affinity LFA-1 did not exhibit intracellular calcium flux, F-actin polymerization, cell polarization, or directional migration under shear flow. In contrast, high-affinity LFA-1 provided orientation along a uropod-pseudopod axis that required calcium flux through Orai1. We demonstrate how the shear stress of blood flow can transduce distinct outside-in signals at focal sites of high-affinity LFA-1 that provide contact-mediated guidance for neutrophil emigration.

Chemokine triggered integrin activation and actin remodeling events guiding lymphocyte migration across vascular barriers

Chemokine signals activate leukocyte integrins and actin remodeling machineries critical for leukocyte adhesion and motility across vascular barriers. The arrest of leukocytes at target blood vessel sites depends on rapid conformational activation of their α4 and β2 integrins by the binding of endothelial-displayed chemokines to leukocyte Gi-protein coupled receptors (GPCRs). A universal regulator of this event is the integrin-actin adaptor, talin1. Chemokine-stimulated GPCRs can transmit within fractions of seconds signals via multiple Rho GTPases, which locally raise plasma membrane levels of the talin activating phosphatidyl inositol, PtdIns(4,5)P2 (PIP2). Additional pools of GPCR stimulated Rac-1 and Rap-1 GTPases together with GPCR stimulated PLC and PI3K family members regulate the turnover of focal contacts of leukocyte integrins, induce the collapse of leukocyte microvilli, and promote polarized leukocyte crawling in search of exit cues. Concomitantly, other leukocyte GTPases trigger invasive protrusions into and between endothelial cells in search of basolateral chemokine exit cues. We will review here major findings and open questions related to these sequential guiding activities of endothelial presented chemokines, focusing mainly on lymphocyte-endothelial interactions as a paradigm for other leukocytes.

CXCR4 and CCR5 ligands cooperate in monocyte and lymphocyte migration and in inhibition of dual-tropic (R5/X4) HIV-1 infection

One of the most important functions of chemokines and their receptors is the regulation of directional migration of leukocytes within tissues. In specific tissue compartments, cells are exposed to multiple chemokines presented in complex dimensional and temporal patterns. Therefore, a leukocyte requires the mechanisms to integrate the various directional signals it receives from different chemoattractants. In this study, we report that CCL3, CCL5, and CCL8, three potent mononuclear cell chemoattractants, are able to synergize with the homeostatic chemokine CXCL12 in the migration of CD14(+) monocytes, CD3(+) T-lymphocytes, or PHA-activated lymphoblasts. In addition, CCL5 augmented the CXCR4 ligand-driven ERK phosphorylation in mononuclear cells. Furthermore, the synergistic effect between CCL5 and CXCL12 in monocyte chemotaxis is inhibited in the presence of specific CCR1 antibody and AMD3100, but not by maraviroc. In HIV-1 infection assays, a combination of CXCL12 and CCL5 cooperated to inhibit the replication of the dual-tropic (R5/X4) HIV-1 HE strain. Finally, although the dual-tropic HIV-1 strain was barely suppressed by AMD3100 or maraviroc alone, HIV-1 infection was completely blocked by the combination of these two receptor antagonists. Our data demonstrate the cooperation between CCL5 and CXCL12, which has implications in migration of monocytes/lymphocytes during inflammation and in HIV-1 infection.

Enforced hematopoietic cell E- and L-selectin ligand (HCELL) expression primes transendothelial migration of human mesenchymal stem cells

According to the multistep model of cell migration, chemokine receptor engagement (step 2) triggers conversion of rolling interactions (step 1) into firm adhesion (step 3), yielding transendothelial migration. We recently reported that glycosyltransferase-programmed stereosubstitution (GPS) of CD44 on human mesenchymal stem cells (hMSCs) creates the E-selectin ligand HCELL (hematopoietic cell E-selectin/L-selectin ligand) and, despite absence of CXCR4, systemically administered HCELL(+)hMSCs display robust osteotropism visualized by intravital microscopy. Here we performed studies to define the molecular effectors of this process. We observed that engagement of hMSC HCELL with E-selectin triggers VLA-4 adhesiveness, resulting in shear-resistant adhesion to ligand VCAM-1. This VLA-4 activation is mediated via a Rac1/Rap1 GTPase signaling pathway, resulting in transendothelial migration on stimulated human umbilical vein endothelial cells without chemokine input. These findings indicate that hMSCs coordinately integrate CD44 ligation and integrin activation, circumventing chemokine-mediated signaling, yielding a step 2-bypass pathway of the canonical multistep paradigm of cell migration.

Pathways implicated in stem cell migration: the SDF-1/CXCR4 axis

The hallmark of hematopoietic stem and progenitor cells (HSPCs) is their motility, which is essential for their function, such as recruitment upon demand. Stromal Derived Factor-1 (SDF-1, CXCL12) and its major receptor CXCR4 play major roles in stem cell motility and development. In vitro migration assays, implicating either gradients or cell surface-bound forms of SDF-1, are easy to perform and provide vital information regarding directional and random stem cell motility, which correlate with their repopulation potential in clinical and experimental transplantations. In vivo stem cell homing to the bone marrow, their retention, engraftment, and egress to the circulation, all involve SDF-1/CXCR4 interactions. Finally, other stem cell features such as stem cell survival and proliferation, are also dependent on the SDF-1/CXCR4 axis.

Comparison of immortalized bEnd5 and primary mouse brain microvascular endothelial cells as in vitro blood-brain barrier models for the study of T cell extravasation

Important insights into the molecular mechanism of T cell extravasation across the blood-brain barrier (BBB) have already been obtained using immortalized mouse brain endothelioma cell lines (bEnd). However, compared with bEnd, primary brain endothelial cells have been shown to establish better barrier characteristics, including complex tight junctions and low permeability. In this study, we asked whether bEnd5 and primary mouse brain microvascular endothelial cells (pMBMECs) were equally suited as in vitro models with which to study the cellular and molecular mechanisms of T cell extravasation across the BBB. We found that both in vitro BBB models equally supported both T cell adhesion under static and physiologic flow conditions, and T cell crawling on the endothelial surface against the direction of flow. In contrast, distances of T cell crawling on pMBMECs were strikingly longer than on bEnd5, whereas diapedesis of T cells across pMBMECs was dramatically reduced compared with bEnd5. Thus, both in vitro BBB models are suited to study T cell adhesion. However, because pMBMECs better reflect endothelial BBB specialization in vivo, we propose that more reliable information about the cellular and molecular mechanisms of T cell diapedesis across the BBB can be attained using pMBMECs.

Role of CCR5 and its ligands in the control of vascular inflammation and leukocyte recruitment required for acute excitotoxic seizure induction and neural damage

Chemokines may play a role in leukocyte migration across the blood-brain barrier (BBB) during neuroinflammation and other neuropathological processes, such as epilepsy. We investigated the role of the chemokine receptor CCR5 in seizures. We used a rat model based on intraperitoneal kainic acid (KA) administration. Four months before KA injection, adult rats were given femoral intramarrow inoculations of SV (RNAiR5-RevM10.AU1), which carries an interfering RNA (RNAi) against CCR5, plus a marker epitope (AU1), or its monofunctional RNAi-carrying homologue, SV(RNAiR5). This treatment lowered expression of CCR5 in circulating cells. In control rats, seizures induced elevated expression of CCR5 ligands MIP-1α and RANTES in the microvasculature, increased BBB leakage and CCR5(+) cells, as well as neuronal loss, inflammation, and gliosis in the hippocampi. Animals given either the bifunctional or the monofunctional vector were largely protected from KA-induced seizures, neuroinflammation, BBB damage, and neuron loss. Brain CCR5 mRNA was reduced. Rats receiving RNAiR5-bearing vectors showed far greater repair responses: increased neuronal proliferation, and decreased production of MIP-1α and RANTES. Controls received unrelated SV(BUGT) vectors. Decrease in CCR5 in circulating cells strongly protected from excitotoxin-induced seizures, BBB leakage, CNS injury, and inflammation, and facilitated neurogenic repair.

The blood-brain barrier, chemokines and multiple sclerosis

The infiltration of leukocytes into the central nervous system (CNS) is an essential step in the neuropathogenesis of multiple sclerosis (MS). Leukocyte extravasation from the bloodstream is a multistep process that depends on several factors including fluid dynamics within the vasculature and molecular interactions between circulating leukocytes and the vascular endothelium. An important step in this cascade is the presence of chemokines on the vascular endothelial cell surface. Chemokines displayed along the endothelial lumen bind chemokine receptors on circulating leukocytes, initiating intracellular signaling that culminates in integrin activation, leukocyte arrest, and extravasation. The presence of chemokines at the endothelial lumen can help guide the movement of leukocytes through peripheral tissues during normal immune surveillance, host defense or inflammation. The expression and display of homeostatic or inflammatory chemokines therefore critically determine which leukocyte subsets extravasate and enter the peripheral tissues. Within the CNS, however, infiltrating leukocytes that cross the endothelium face additional boundaries to parenchymal entry, including the abluminal presence of localizing cues that prevent egress from perivascular spaces. This review focuses on the differential display of chemokines along endothelial surfaces and how they impact leukocyte extravasation into parenchymal tissues, especially within the CNS. In particular, the display of chemokines by endothelial cells of the blood brain barrier may be altered during CNS autoimmune disease, promoting leukocyte entry into this immunologically distinct site. Recent advances in microscopic techniques, including two-photon and intravital imaging have provided new insights into the mechanisms of chemokine-mediated capture of leukocytes within the CNS.

The role of chemokines in the recruitment of lymphocytes to the liver

Chemokines direct leukocyte trafficking and positioning within tissues, thus playing critical roles in regulating immune responses and inflammation. The chemokine system is complex, involving interactions between multiple chemokines and their receptors that operate in combinatorial cascades with adhesion molecules. The involvement of multiple chemokines and chemokine receptors in these processes brings flexibility and specificity to recruitment. The hepatic vascular bed is a unique low-flow environment through which leukocytes are recruited to the liver during homeostatic immune surveillance and in response to infection or injury. The rate of leukocyte recruitment and the nature of cells recruited through the sinusoids in response to inflammatory signals will shape the severity of disease. At one end of the spectrum, fulminant liver failure results from a rapid recruitment of leukocytes that leads to hepatocyte destruction and liver failure; at the other end, diseases such as chronic hepatitis C infection may progress over many years from hepatitis to fibrosis and cirrhosis. Chronic hepatitis is characterized by a T lymphocyte-rich infiltrate and the nature and outcome of hepatitis will depend on the T cell subsets recruited, their activation and function within the liver. Different subsets of effector T cells have been described based on their secretion of cytokines and specific functions. These include Th1 and Th2 cells, and more recently Th17 and Th9 cells, which are associated with different types of immune response and which express distinct patterns of chemokine receptors that promote their recruitment under particular conditions. The effector function of these cells is balanced by the recruitment of regulatory T cells that are able to suppress antigen-specific effectors to allow resolution of immune responses and restoration of immune homeostasis. Understanding the signals that are responsible for recruiting different lymphocyte subsets to the liver will elucidate disease pathogenesis and open up new therapeutic approaches to modulate recruitment in favor of resolution rather than injury.

Dynamic alterations in chemokine gradients induce transendothelial shuttling of human T cells under physiologic shear conditions

The active movement of cells from subendothelial compartments into the bloodstream (intravasation) has been recognized for several decades by histologic and physiologic studies, yet the molecular effectors of this process are relatively uncharacterized. For extravasation, studies based predominantly on static transwell assays support a general model, whereby transendothelial migration (TEM) occurs via chemoattraction toward increasing chemokine concentrations. However, this model of chemotaxis cannot readily reconcile how chemokines influence intravasation, as shear forces of blood flow would likely abrogate luminal chemokine gradient(s). Thus, to analyze how T cells integrate perivascular chemokine signals under physiologic flow, we developed a novel transwell-based flow chamber allowing for real-time modulation of chemokine levels above (luminal/apical compartment) and below (abluminal/subendothelial compartment) HUVEC monolayers. We routinely observed human T cell TEM across HUVEC monolayers with the combination of luminal CXCL12 and abluminal CCL5. With increasing concentrations of CXCL12 in the luminal compartment, transmigrated T cells did not undergo retrograde transendothelial migration (retro-TEM). However, when exposedto abluminal CXCL12, transmigrated T cells underwent striking retro-TEM and re-entered the flow stream [corrected]. This CXCL12 fugetactic (chemorepellant) effect was concentration-dependent, augmented by apical flow, blocked by antibodies to integrins, and reduced by AMD3100 in a dose-dependent manner. Moreover, CXCL12-induced retro-TEM was inhibited by PI3K antagonism and cAMP agonism. These findings broaden our understanding of chemokine biology and support a novel paradigm by which temporospatial modulations in subendothelial chemokine display drive cell migration from interstitial compartments into the bloodstream.

Chemokines and chemokine receptors: standing at the crossroads of immunobiology and neurobiology

There are several molecular entities common to the immune and nervous systems. Salient among them are the chemokines and their receptors, which play remarkably varied and potent roles in immunobiology and neurobiology. This review limns several illustrative examples and presents general principles of chemokine action that are manifest in both systems. These include the following: (1) chemokines tend equally to arrest cells and to make them move, in the process of positioning target cells with spatiotemporal precision; (2) signaling and nonsignaling receptors collaborate to adjust the chemokine environment for maximal efficacy; and (3) expression of a single chemokine receptor on circulating blood cells and parenchymal cells is often used to coordinate complex tissue responses. The challenge is to integrate knowledge of the roles of key receptors (as well as their ligands) into a coherent account of events during pathologic processes, in order to guide therapeutic development.

Glycosyltransferase-programmed stereosubstitution (GPS) to create HCELL: engineering a roadmap for cell migration

During evolution of the vertebrate cardiovascular system, the vast endothelial surface area associated with branching vascular networks mandated the development of molecular processes to efficiently and specifically recruit circulating sentinel host defense cells and tissue repair cells at localized sites of inflammation/tissue injury. The forces engendered by high-velocity blood flow commensurately required the evolution of specialized cell surface molecules capable of mediating shear-resistant endothelial adhesive interactions, thus literally capturing relevant cells from the blood stream onto the target endothelial surface and permitting subsequent extravasation. The principal effectors of these shear-resistant binding interactions comprise a family of C-type lectins known as 'selectins' that bind discrete sialofucosylated glycans on their respective ligands. This review explains the 'intelligent design' of requisite reagents to convert native CD44 into the sialofucosylated glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), the most potent E-selectin counter-receptor expressed on human cells, and will describe how ex vivo glycan engineering of HCELL expression may open the 'avenues' for the efficient vascular delivery of cells for a variety of cell therapies.