Mechanotransduction via endothelial adhesion molecule CD31 initiates transmigration and reveals a role for VEGFR2 in diapedesis.

Engagement of platelet endothelial cell adhesion molecule 1 (PECAM, PECAM-1, CD31) on the leukocyte pseudopod with PECAM at the endothelial cell border initiates transendothelial migration (TEM, diapedesis). We show, using fluorescence lifetime imaging microscopy (FLIM), that physical traction on endothelial PECAM during TEM initiated the endothelial signaling pathway. In this role, endothelial PECAM acted as part of a mechanotransduction complex with VE-cadherin and vascular endothelial growth factor receptor 2 (VEGFR2), and this predicted that VEGFR2 was required for efficient TEM. We show that TEM required both VEGFR2 and the ability of its Y1175 to be phosphorylated, but not VEGF or VEGFR2 endogenous kinase activity. Using inducible endothelial-specific VEGFR2-deficient mice, we show in three mouse models of inflammation that the absence of endothelial VEGFR2 significantly (by ≥75%) reduced neutrophil extravasation by selectively blocking diapedesis. These findings provide a more complete understanding of the process of transmigration and identify several potential anti-inflammatory targets.

Autophagy modulates endothelial junctions to restrain neutrophil diapedesis during inflammation.

The migration of neutrophils from the blood circulation to sites of infection or injury is a key immune response and requires the breaching of endothelial cells (ECs) that line the inner aspect of blood vessels. Unregulated neutrophil transendothelial cell migration (TEM) is pathogenic, but the molecular basis of its physiological termination remains unknown. Here, we demonstrated that ECs of venules in inflamed tissues exhibited a robust autophagic response that was aligned temporally with the peak of neutrophil trafficking and was strictly localized to EC contacts. Genetic ablation of EC autophagy led to excessive neutrophil TEM and uncontrolled leukocyte migration in murine inflammatory models, while pharmacological induction of autophagy suppressed neutrophil infiltration into tissues. Mechanistically, autophagy regulated the remodeling of EC junctions and expression of key EC adhesion molecules, facilitating their intracellular trafficking and degradation. Collectively, we have identified autophagy as a modulator of EC leukocyte trafficking machinery aimed at terminating physiological inflammation.

Neutrophil Infiltration and Function in the Pathogenesis of Inflammatory Airspace Disease.

Neutrophils are an important cell type often considered the body's first responders to inflammatory insult or damage. They are recruited to the tissue of the lungs in patients with inflammatory airspace diseases and have unique and complex functions that range from helpful to harmful. The uniqueness of these functions is due to the heterogeneity of the inflammatory cascade and retention in the vasculature. Neutrophils are known to marginate, or remain stagnant, in the lungs even in nondisease conditions. This review discusses the ways in which the recruitment, presence, and function of neutrophils in the airspace of the lungs are unique from those of other tissues, and the complex effects of neutrophils on pathogenesis. Inflammatory mediators produced by neutrophils, such as neutrophil elastase, proresolving mediators, and neutrophil extracellular traps, dramatically affect the outcomes of patients with disease of the lungs.

Human CD99L2 Regulates a Unique Step in Leukocyte Transmigration.

CD99-like 2 (CD99L2 [L2]) is a highly glycosylated 52-kDa type 1 membrane protein that is important for leukocyte transendothelial migration (TEM) in mice. Inhibiting L2 using function-blocking Ab significantly reduces the recruitment of leukocytes to sites of inflammation in vivo. Similarly, L2 knockout mice have an inherent defect in leukocyte transmigration into sites of inflammation. However, the role of L2 in inflammation has only been studied in mice. Furthermore, the mechanism by which it regulates TEM is not known. To study the relevance to human inflammation, we studied the role of L2 on primary human cells in vitro. Our data show that like PECAM and CD99, human L2 is constitutively expressed at the borders of endothelial cells and on the surface of leukocytes. Inhibiting L2 using Ab blockade or genetic knockdown significantly reduces transmigration of human neutrophils and monocytes across endothelial cells. Furthermore, our data also show that L2 regulates a specific, sequential step of TEM between PECAM and CD99, rather than operating in parallel or redundantly with these molecules. Similar to PECAM and CD99, L2 promotes transmigration by recruiting the lateral border recycling compartment to sites of TEM, specifically downstream of PECAM initiation. Collectively, our data identify a novel functional role for human L2 in TEM and elucidate a mechanism that is distinct from PECAM and CD99.

Experimental Colitis Enhances Temporal Variations in CX3CR1 Cell Colonization of the Gut and Brain Following Irradiation.

Peripheral monocyte-derived CX3C chemokine receptor 1 positive (CX3CR1+) cells play important roles in tissue homeostasis and gut repopulation. Increasing evidence also supports their role in immune repopulation of the brain parenchyma in response to systemic inflammation. Adoptive bone marrow transfer from CX3CR1 fluorescence reporter mice and high-resolution confocal microscopy was used to assess the time course of CX3CR1+ cell repopulation of steady-state and dextran sodium sulfate (DSS)-inflamed small intestine/colon and the brain over 4 weeks after irradiation. CX3CR1+ cell colonization and morphologic polarization into fully ramified cells occurred more rapidly in the small intestine than in the colon. For both organs, the crypt/mucosa was more densely populated than the serosa/muscularis layer, indicating preferential temporal and spatial occupancy. Repopulation of the brain was delayed compared with that of gut tissue, consistent with the immune privilege of this organ. However, DSS-induced colon injury accelerated the repopulation. Expression analyses confirmed increased chemokine levels and macrophage colonization within the small intestine/colon and the brain by DSS-induced injury. Early increases of transmembrane protein 119 and ionized calcium binding adaptor molecule 1 expression within the brain after colon injury suggest immune-priming effect of brain resident microglia in response to systemic inflammation. These findings identify temporal differences in immune repopulation of the gut and brain in response to inflammation and show that gut inflammation can impact immune responses within the brain.

Platelet Endothelial Cell Adhesion Molecule (PECAM/CD31) Blockade Modulates Neutrophil Recruitment Patterns and Reduces Infarct Size in Experimental Ischemic Stroke.

The infiltration of polymorphonuclear leukocytes (PMNs) in ischemia-reperfusion injury (I/RI) has been implicated as a critical component of inflammatory damage following ischemic stroke. However, successful blockade of PMN transendothelial migration (TEM) in preclinical studies has not translated to meaningful clinical outcomes. To investigate this further, leukocyte infiltration patterns were quantified, and these patterns were modulated by blocking platelet endothelial cell adhesion molecule-1 (PECAM), a key regulator of TEM. LysM-eGFP mice and microscopy were used to visualize all myeloid leukocyte recruitment following ischemia/reperfusion. Visual examination showed heterogeneous leukocyte distribution across the infarct at both 24 and 72 hours after I/RI. A semiautomated process was designed to precisely map PMN position across brain sections. Treatment with PECAM function-blocking antibodies did not significantly affect total leukocyte recruitment but did alter their distribution, with more observed at the cortex at both early and later time points (24 hours: 89% PECAM blocked vs. 72% control; 72 hours: 69% PECAM blocked vs. 51% control). This correlated with a decrease in infarct volume. These findings suggest that TEM, in the setting of I/RI in the cerebrovasculature, occurs primarily at the cortical surface. The reduction of stroke size with PECAM blockade suggests that infiltrating PMNs may exacerbate I/RI and indicate the potential therapeutic benefit of regulating the timing and pattern of leukocyte infiltration after stroke.

Endothelial Cell Calcium Signaling during Barrier Function and Inflammation.

Calcium is an essential second messenger in endothelial cells and plays a pivotal role in regulating a number of physiologic processes, including cell migration, angiogenesis, barrier function, and inflammation. An increase in intracellular Ca2+ concentration can trigger a number of diverse signaling pathways under both physiologic and pathologic conditions. In this review, we discuss how calcium signaling pathways in endothelial cells play an essential role in affecting barrier function and facilitating inflammation. Inflammatory mediators, such as thrombin and histamine, increase intracellular calcium levels. This calcium influx causes adherens junction disassembly and cytoskeletal rearrangements to facilitate endothelial cell retraction and increased permeability. During inflammation endothelial cell calcium entry and the calcium-related signaling events also help facilitate several leukocyte-endothelial cell interactions, such as leukocyte rolling, adhesion, and ultimately transendothelial migration.

Return of Individual Research Results: A Guide for Biomedical Researchers Utilizing Human Biospecimens.

The recent movement toward returning individual research results to study subjects/participants generates ethical and legal challenges for laboratories performing research on human biospecimens. The concept of an individual's interest in knowing the results of testing on their tissue is pitted against individual and systemic risks and an established legal framework regulating the performance of laboratory testing for medical care purposes. This article discusses the rationale for returning individual research results to subjects, the potential risks associated with returning these results, and the legal framework in the United States that governs testing of identifiable human biospecimens. On the basis of these considerations, this article provides recommendations for investigators to consider when planning and executing human biospecimen research, with the objective of appropriately balancing the interests of research subjects, the need for ensuring integrity of the research process, and compliance with US laws and regulations.

Regulate globally, act locally: adrenergic nerves promote leukocyte recruitment.

In this issue of Immunity, Scheiermann et al. (2012) demonstrate that circadian regulation of the expression of endothelial cell adhesion molecules via adrenergic innervation of local vasculature promotes clinically significant changes in leukocyte homing and bone marrow engraftment.

Transendothelial migration: unifying principles from the endothelial perspective.

Transendothelial migration (TEM) of polymorphonuclear leukocytes (PMN) involves a carefully orchestrated dialog of adhesion and signaling events between leukocyte and endothelial cell. This article focuses on the contribution of endothelial cells to transmigration. The initiation of TEM itself generally requires interaction of PECAM on the leukocyte with PECAM at the endothelial cell border. This is responsible for the transient elevation of cytosolic-free calcium ions in endothelium that is required for TEM and for recruitment of membrane from the lateral border recycling compartment (LBRC). TEM requires LBRC to move to the site at which TEM will take place and for VE-cadherin to move away. Targeting of the LBRC to this site likely precedes movement of VE-cadherin and may play a role in clearing VE-cadherin from the site of TEM. The process of TEM can be dissected into steps mediated by distinct pairs of PMN/endothelial interacting molecules. CD99 regulates a step at or close to the end of TEM. CD99 signals through soluble adenylyl cyclase to activate PKA to trigger ongoing targeted recycling of the LBRC. Paracellular transmigration predominates (≥90% of events) in the cremaster muscle circulation, but transcellular migration may be more important at sites such as the blood-brain barrier. Both processes involve many of the same molecules and recruitment of the LBRC.

MerTK Cleavage on Resident Cardiac Macrophages Compromises Repair After Myocardial Ischemia Reperfusion Injury.

RATIONALE: Clinical benefits of reperfusion after myocardial infarction are offset by maladaptive innate immune cell function, and therapeutic interventions are lacking. OBJECTIVE: We sought to test the significance of phagocytic clearance by resident and recruited phagocytes after myocardial ischemia reperfusion. METHODS AND RESULTS: In humans, we discovered that clinical reperfusion after myocardial infarction led to significant elevation of the soluble form of MerTK (myeloid-epithelial-reproductive tyrosine kinase; ie, soluble MER), a critical biomarker of compromised phagocytosis by innate macrophages. In reperfused mice, macrophage Mertk deficiency led to decreased cardiac wound debridement, increased infarct size, and depressed cardiac function, newly implicating MerTK in cardiac repair after myocardial ischemia reperfusion. More notably, Mertk(CR) mice, which are resistant to cleavage, showed significantly reduced infarct sizes and improved systolic function. In contrast to other cardiac phagocyte subsets, resident cardiac MHCIILOCCR2- (major histocompatibility complex II/C-C motif chemokine receptor type 2) macrophages expressed higher levels of MerTK and, when exposed to apoptotic cells, secreted proreparative cytokines, including transforming growth factor-ß. Mertk deficiency compromised the accumulation of MHCIILO phagocytes, and this was rescued in Mertk(CR) mice. Interestingly, blockade of CCR2-dependent monocyte infiltration into the heart reduced soluble MER levels post-ischemia reperfusion. CONCLUSIONS: Our data implicate monocyte-induced MerTK cleavage on proreparative MHCIILO cardiac macrophages as a novel contributor and therapeutic target of reperfusion injury.

Cutting Edge: CD99 Is a Novel Therapeutic Target for Control of T Cell-Mediated Central Nervous System Autoimmune Disease.

Leukocyte trafficking into the CNS is a prominent feature driving the immunopathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis. Blocking the recruitment of inflammatory leukocytes into the CNS represents an exploitable therapeutic target; however, the adhesion molecules that specifically regulate the step of leukocyte diapedesis into the CNS remain poorly understood. We report that CD99 is critical for lymphocyte transmigration without affecting adhesion in a human blood-brain barrier model. CD99 blockade in vivo ameliorated experimental autoimmune encephalomyelitis and decreased the accumulation of CNS inflammatory infiltrates, including dendritic cells, B cells, and CD4(+) and CD8(+) T cells. Anti-CD99 therapy was effective when administered after the onset of disease symptoms and blocked relapse when administered therapeutically after disease symptoms had recurred. These findings underscore an important role for CD99 in the pathogenesis of CNS autoimmunity and suggest that it may serve as a novel therapeutic target for controlling neuroinflammation.

Segregation of VE-cadherin from the LBRC depends on the ectodomain sequence required for homophilic adhesion.

The lateral border recycling compartment (LBRC) is a reticulum ofperijunctional tubulovesicular membrane that is continuous with the plasmalemma of endothelial cells and is essential for efficient transendothelial migration (TEM) of leukocytes. The LBRC contains molecules involved in TEM, such as PECAM, PVR and CD99, but not VE-cadherin. Despite its importance, how membrane proteins are included in or excluded from the LBRC is not known. Immunoelectronmicroscopy and biochemical approaches demonstrate that inclusion into the LBRC is the default pathway for transmembrane molecules present at endothelial cell borders. A chimeric molecule composed of the extracellular domain of VE-cadherin and cytoplasmic tail of PECAM (VE-CAD/PECAM) did not enter the LBRC, suggesting that VE-cadherin was excluded by a mechanism involving its extracellular domain. Deletion of the homophilic interaction domain EC1 or the homophilic interaction motif RVDAE allowed VE-CAD/PECAM and even native VE-cadherin to enter the LBRC. Similarly, treatment with RVDAE peptide to block homophilic VE-cadherin interactions allowed endogenous VE-cadherin to enter the LBRC. This suggests that homophilic interactions of VE-cadherin stabilize it at cell borders and prevent entry into the LBRC.

A Unique Role for Endothelial Cell Kinesin Light Chain 1, Variant 1 in Leukocyte Transendothelial Migration.

A reservoir of parajunctional membrane in endothelial cells, the lateral border recycling compartment (LBRC), is critical for transendothelial migration (TEM). We have previously shown that targeted recycling of the LBRC to the site of TEM requires microtubules and a kinesin molecular motor. However, the identity of the kinesin and mechanism of cargo binding were not known. We show that microinjection of endothelial cells with a monoclonal antibody specific for kinesin-1 significantly blocked LBRC-targeted recycling and TEM. In complementary experiments, knocking down KIF5B, a ubiquitous kinesin-1 isoform, in endothelial cells significantly decreased targeted recycling of the LBRC and leukocyte TEM. Kinesin heavy chains move cargo along microtubules by one of many kinesin light chains (KLCs), which directly bind the cargo. Knocking down KLC 1 isoform variant 1 (KLC1C) significantly decreased LBRC-targeted recycling and TEM, whereas knocking down other isoforms of KLC1 had no effect. Re-expression of KLC1C resistant to the knockdown shRNA restored targeted recycling and TEM. Thus kinesin-1 and KLC1C are specifically required for targeted recycling and TEM. These data suggest that of the many potential combinations of the 45 kinesin family members and multiple associated light chains, KLC1C links the LBRC to kinesin-1 (KIF5B) during targeted recycling and TEM. Thus, KLC1C can potentially be used as a target for anti-inflammatory therapy.

Targeted Recycling of the Lateral Border Recycling Compartment Precedes Adherens Junction Dissociation during Transendothelial Migration.

Leukocyte transendothelial migration (TEM) requires two major events: local dissociation of adherens junctions manifested as gaps in vascular endothelial (VE)-cadherin staining at the site of TEM and targeted trafficking of the lateral border recycling compartment (LBRC) to the site of TEM. However, the association between LBRC recycling and VE-cadherin gaps remains unknown. We found that when targeting of the LBRC is selectively inhibited using established methods, such as a function blocking anti-platelet endothelial cell adhesion molecule 1 antibody, depolymerizing microtubules, or microinjection of an antibody that inhibits kinesin, VE-cadherin gaps do not form around the blocked leukocyte. This is the first time that the LBRC has been implicated in this process. We obtained similar results for neutrophils and monocytes and in studies using live cell imaging microscopy conducted under fluid shear conditions. Depolymerizing microtubules did not affect the ability of leukocytes to induce tyrosine phosphorylation of VE-cadherin. A VE-cadherin double mutant (Y658F, Y731F) expressed in endothelial cells acted as a dominant negative and inhibited VE-cadherin gap formation and TEM, yet targeting of the LBRC still occurred. These data suggest that targeting of the LBRC to the site of TEM precedes VE-cadherin clearance. Recruitment of the LBRC may play a role in clearing VE-cadherin from the site of TEM.

Isolation of the lateral border recycling compartment using a diaminobenzidine-induced density shift.

The migration of leukocytes across the endothelium and into tissue is critical to mounting an inflammatory response. The lateral border recycling compartment (LBRC), a complex vesicular-tubule invagination of the plasma membrane found at endothelial cell borders, plays an important role in this process. Although a few proteins have been shown to be present in the LBRC, no unique marker is known. Here, we detail methods that can be used to characterize a subcellular compartment that lacks an identifying marker. Initial characterization of the LBRC was performed using standard subcellular fractionation with sucrose gradients and took advantage of the observation that the compartment migrated at a lower density than other membrane compartments. To isolate larger quantities of the compartment, we modified a classic technique known as a diaminobenzidine (DAB)-induced density shift. The DAB-induced density shift allowed for specific isolation of membranes labeled with horseradish peroxidase-conjugated antibody. Because the LBRC could be differentially labeled at 4 °C and 37 °C, we were able to identify proteins that are enriched in the compartment, despite lacking a unique marker. These methods serve as a model to others studying poorly characterized compartments and organelles and are applicable to a wide variety of biological systems.

4D intravital microscopy uncovers critical strain differences for the roles of PECAM and CD99 in leukocyte diapedesis.

Leukocyte transendothelial migration (TEM) is an essential component of the inflammatory response. In vitro studies with human cells have demonstrated that platelet/endothelial cell adhesion molecule (PECAM) functions upstream of CD99 during TEM; however, results in vivo with mice have been apparently contradictory. In this study we use four-dimensional (4D) intravital microscopy to demonstrate that the site and order of function of PECAM and CD99 in vivo are dependent on the strain of mice. In FVB/n mice, PECAM functions upstream of CD99, as in human cells in vitro, and blocking antibodies against either molecule arrest neutrophils before they traverse the endothelium. However, in C57BL/6 mice, PECAM and CD99 appear to function at a different step, as the same antibodies arrest leukocyte migration through the endothelial basement membrane. These results are the first direct comparison of PECAM and CD99 function in different murine strains as well as the first demonstration of the sequential function of PECAM and CD99 in vivo.

Rapid remodeling of tight junctions during paracellular diapedesis in a human model of the blood-brain barrier.

Leukocyte transendothelial migration (TEM; diapedesis) is a critical event in immune surveillance and inflammation. Most TEM occurs at endothelial cell borders (paracellular). However, there is indirect evidence to suggest that, at the tight junctions of the blood-brain barrier (BBB), leukocytes migrate directly through the endothelial cell body (transcellular). Why leukocytes migrate through the endothelial cell body rather than the cell borders is unknown. To test the hypothesis that the tightness of endothelial cell junctions influences the pathway of diapedesis, we developed an in vitro model of the BBB that possessed 10-fold higher electrical resistance than standard culture conditions and strongly expressed the BBB tight junction proteins claudin-5 and claudin-3. We found that paracellular TEM was still the predominant pathway (≥98%) and TEM was dependent on PECAM-1 and CD99. We show that endothelial tight junctions expressing claudin-5 are dynamic and undergo rapid remodeling during TEM. Membrane from the endothelial lateral border recycling compartment is mobilized to the exact site of tight junction remodeling. This preserves the endothelial barrier by sealing the intercellular gaps with membrane and engaging the migrating leukocyte with unligated adhesion molecules (PECAM-1 and CD99) as it crosses the cell border. These findings provide new insights into leukocyte-endothelial interactions at the BBB and suggest that tight junctions are more dynamic than previously appreciated.

How endothelial cells regulate transmigration of leukocytes in the inflammatory response.

Leukocytes attach to vascular endothelial cells at the site of inflammation via a series of intercellular adhesive interactions. In a separate step in leukocyte extravasation, transendothelial migration is regulated by molecules that play no role in the preceding steps of tethering, rolling, adhesion, and locomotion. Transendothelial migration itself can be dissected into a series of distinct interactions regulated sequentially by molecules concentrated at the endothelial cell border; these include platelet/endothelial cell adhesion molecule, poliovirus receptor (CD155), and CD99. These molecules are components of the lateral border recycling compartment (LBRC), a perijunctional network of interconnected tubulovesicular membrane that traffics to surround the leukocyte as it passes across the endothelial cell. This targeted recycling of LBRC requires kinesin to move the membrane along microtubules, and interfering with LBRC trafficking blocks transmigration of neutrophils, monocytes, and lymphocytes. The LBRC is also recruited to mediate transcellular migration when that occurs. Movement of the LBRC is coordinated with events on the luminal surface, such as clustering of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 under the migrating leukocyte, as well as movement of vascular endothelial cadherin and its associated catenins out of the junction at the site of transendothelial migration. How these events are coordinated is not known, but their regulation shares common signaling pathways that may serve to connect these steps.

CD99-like 2 (CD99L2)-deficient mice are defective in the acute inflammatory response.

CD99-Like 2 (CD99L2) is a Type I glycoprotein expressed on leukocytes and endothelial cells as well as other cell types. It is related to CD99, although it shows only 38% sequence identity. CD99L2 has been shown to play a role in leukocyte extravasation in mice under various inflammatory conditions using anti-CD99L2 antibodies and, in one case by targeted deletion of CD99L2. We report here studies on an independently made CD99L2 "knockout mouse" that extend our knowledge of the role of CD99L2 in inflammation. CD99L2 deficiency did not affect the total or relative numbers of circulating leukocyte subsets, red blood cells, or platelets. Neither did CD99L2 deficiency affect the expression of ICAM-1, PECAM, or CD99 on endothelial cells. Mice lacking CD99L2 had a defective inflammatory response in the thioglycollate peritonitis model with a greater than 80% block in neutrophil infiltration and a nearly complete block in monocyte emigration into the peritoneal cavity measured 16h after the inflammatory challenge. The mice will be a useful resource to study the role of CD99L2 in various acute and chronic inflammatory diseases.