Monoclonal antibody to murine PECAM-1 (CD31) blocks acute inflammation in vivo

[Protective effect of FAK inhibitor PF-562271 against human umbilical vein endothelial cell injury induced by aging platelets]

OBJECTIVE

To investigate the protective effect of PF-562271, a FAK inhibitor, against aging platelet-induced injury in human umbilical vein endothelial cells (HUVECs).

METHODS

Cultured HUVECs were treated with vehicle, lipopolysaccharide (LPS), LPS+aging platelets, or LPS+aging platelets+PF-562271. The changes in protein expressions of FAK, pFAK and PECAM-1 in the treated cells were detected using Western blotting and immunofluorescence assay, and the level of reactive oxygen species (ROS) was detected with flow cytometry. The changes of barrier function of the cells were assessed with cell permeability test and transendothelial cell resistance test. RT-qPCR was used to analyze mRNA expressions of inflammatory factors, and pro-inflammatory cytokine levels in the culture supernatants was determined with enzyme-linked immunosorbent assay. Immunofluorescence assay was used to examine the effect of the ROS inhibitor vitamin C on PECAM-1 expression in the cells with different treatments.

RESULTS

Treatment of HUVECs with LPS and aging platelets significantly increased cellular protein expressions of FAK, pFAK and PECAM-1, which were effectively lowered by addition of PF-562271 (P < 0.05). LPS and aged platelets obviously enhanced ROS production in the cells, which was inhibited by the addition of PF-562271 (P < 0.001). PF-562271 significantly alleviated the damage of endothelial cell barrier function of the cells caused by LPS and aging platelets (P < 0.01). The expressions of TNF-α, IL-6 and IL-8 in HUVECs increased significantly after exposure to LPS and aging platelets, and were obviously lowered after treatment with PF-562271 (P < 0.05). Treatment with vitamin C significantly decreased the expression of PECAM-1 protein in the cells (P < 0.01).

CONCLUSION

The FAK inhibitor PF-562271 alleviates endothelial cell damage induced by LPS and aging platelets by lowering cellular oxidative stress levels and reducing inflammatory responses.

Endothelial Jagged1 levels and distribution are post-transcriptionally controlled by ZFP36 decay proteins

Vascular morphogenesis requires a delicate gradient of Notch signaling controlled, in part, by the distribution of ligands (Dll4 and Jagged1). How Jagged1 (JAG1) expression is compartmentalized in the vascular plexus remains unclear. Here, we show that Jag1 mRNA is a direct target of zinc-finger protein 36 (ZFP36), an RNA-binding protein involved in mRNA decay that we find robustly induced by vascular endothelial growth factor (VEGF). Endothelial cells lacking ZFP36 display high levels of JAG1 and increase angiogenic sprouting in vitro. Furthermore, mice lacking Zfp36 in endothelial cells display mispatterned and increased levels of JAG1 in the developing retinal vascular plexus. Abnormal levels of JAG1 at the sprouting front alters NOTCH1 signaling, increasing the number of tip cells, a phenotype that is rescued by imposing haploinsufficiency of Jag1. Our findings reveal an important feedforward loop whereby VEGF stimulates ZFP36, consequently suppressing Jag1 to enable adequate levels of Notch signaling during sprouting angiogenesis.

Effects of Humanin G (HNG) on angiogenesis and neurodegeneration markers in Age-related Macular Degeneration (AMD)

Advanced stages of Age-related Macular Degeneration (AMD) are characterized by retinal neurodegeneration and aberrant angiogenesis, and mitochondrial dysfunction contributes to the pathogenesis of AMD. In this study, we tested the hypothesis that Humanin G (HNG), a cytoprotective mitochondrial-derived peptide, positively regulates cell proliferation, cell death, and the protein levels of angiogenesis and neurodegeneration markers, in normal (control) and AMD RPE transmitochondrial cybrid cell lines. These normal and AMD RPE transmitochondrial cybrid cell lines had identical nuclei derived from mitochondria-deficient ARPE-19 cell line, but differed in mitochondrial DNA (mtDNA) content that was derived from clinically characterized AMD patients and normal (control) subjects. Cell lysates were extracted from untreated and HNG-treated AMD and normal (control) cybrid cell lines, and the Luminex XMAP multiplex assay was used to examine the protein levels of angiogenesis and neurodegeneration markers. Humanin G reduced Caspase-3/7-mediated apoptosis, improved cell proliferation, and normalized the protein levels of angiogenesis and neurodegeneration markers in AMD RPE cybrid cell lines, thereby suggesting Humanin G's positive regulatory role in AMD.

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.

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.

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.

Targeting vascular inflammation through emerging methods and drug carriers

Acute inflammation is a common dangerous component of pathogenesis of many prevalent conditions with high morbidity and mortality including sepsis, thrombosis, acute respiratory distress syndrome (ARDS), COVID-19, myocardial and cerebral ischemia-reperfusion, infection, and trauma. Inflammatory changes of the vasculature and blood mediate the course and outcome of the pathology in the tissue site of insult, remote organs and systemically. Endothelial cells lining the luminal surface of the vasculature play the key regulatory functions in the body, distinct under normal vs. pathological conditions. In theory, pharmacological interventions in the endothelial cells might enable therapeutic correction of the overzealous damaging pro-inflammatory and pro-thrombotic changes in the vasculature. However, current agents and drug delivery systems (DDS) have inadequate pharmacokinetics and lack the spatiotemporal precision of vascular delivery in the context of acute inflammation. To attain this level of precision, many groups design DDS targeted to specific endothelial surface determinants. These DDS are able to provide specificity for desired tissues, organs, cells, and sub-cellular compartments needed for a particular intervention. We provide a brief overview of endothelial determinants, design of DDS targeted to these molecules, their performance in experimental models with focus on animal studies and appraisal of emerging new approaches. Particular attention is paid to challenges and perspectives of targeted therapeutics and nanomedicine for advanced management of acute inflammation.

PECAM-1 supports leukocyte diapedesis by tension-dependent dephosphorylation of VE-cadherin

Leukocyte extravasation is an essential step during the immune response and requires the destabilization of endothelial junctions. We have shown previously that this process depends in vivo on the dephosphorylation of VE-cadherin-Y731. Here, we reveal the underlying mechanism. Leukocyte-induced stimulation of PECAM-1 triggers dissociation of the phosphatase SHP2 which then directly targets VE-cadherin-Y731. The binding site of PECAM-1 for SHP2 is needed for VE-cadherin dephosphorylation and subsequent endocytosis. Importantly, the contribution of PECAM-1 to leukocyte diapedesis in vitro and in vivo was strictly dependent on the presence of Y731 of VE-cadherin. In addition to SHP2, dephosphorylation of Y731 required Ca2+ -signaling, non-muscle myosin II activation, and endothelial cell tension. Since we found that β-catenin/plakoglobin mask VE-cadherin-Y731 and leukocyte docking to endothelial cells exert force on the VE-cadherin-catenin complex, we propose that leukocytes destabilize junctions by PECAM-1-SHP2-triggered dephosphorylation of VE-cadherin-Y731 which becomes accessible by actomyosin-mediated mechanical force exerted on the VE-cadherin-catenin complex.

[Effect of different groups of first line DMT on endothelial damage in multiple sclerosis]

INTRODUCTION

Vascular changes, including destabilization of the blood-brain barrier, are common pathological signs in multiple sclerosis (MS). There are prerequisites, which indicate the direct effects of disease modifying therapy (DMT) on the state of the vascular wall and reduce the damage to the endothelium in MS.

AIM OF THIS STUDY

Was to identify and evaluate the relationship of endothelial dysfunction in patients with multiple sclerosis with used DMT.

MATERIALS AND METHODS

The study included 85 patients with a reliable diagnosis of MS according to the McDonald criteria of 2010 (56 women, 29 men) aged from 17 to 62 years (average age 36.3±1.2 years). All patients underwent a comprehensive clinical and neurological examination, laboratory tests (blood serum analysis for the content of adhesion molecules sICAM-1, sPECAM-1, sE-selectin, sP-selectin, for the content of homocysteine and matrix metalloproteinase 9 (MMR-9) by ELISA; blood plasma analysis for Von Willebrand factor antigen (vWf) by ELISA).

The results of the study indicate a decrease of endothelial damage in MS during interferon therapy. Its also allow the use of indicators such as von Willebrand factor antigen, sPECAM-1, sE-selectin levels as potential markers of the effectiveness of DMT.

Advanced Characterization of Imiquimod-Induced Psoriasis-Like Mouse Model

Many autoimmune disorders such as psoriasis lead to the alteration of skin components which generally manifests as unwanted topical symptoms. One of the most widely approved psoriasis-like animal models is the imiquimod (IMQ)-induced mouse model. This representation mimics various aspects of the complex cutaneous pathology and could be appropriate for testing topical treatment options. We perform a thorough characterization of this model by assessing some parameters that are not fully described in the literature, namely a precise description of skin disruption. It was evaluated by transepidermal water loss measurements and analyses of epidermis swelling as a consequence of keratinocyte hyperproliferation. The extent of neo-angiogenesis and hypervascularity in dermis were highlighted by immunostaining. Moreover, we investigated systemic inflammation through cytokines levels, spleen swelling and germinal centers appearance in draining lymph nodes. The severity of all parameters was correlated to IMQ concentration in skin samples. This study outlines new parameters of interest useful to assess this model. We highlight the skin barrier disruption and report a systemic inflammatory reaction occurring at distance both in spleen and lymph nodes. These newly identified biological endpoints could be exploited to investigate the efficacy of therapeutic candidates for psoriasis and more extensively for several other skin inflammatory diseases.

Semaphorin 3F Promotes Transendothelial Migration of Leukocytes in the Inflammatory Response After Survived Cardiac Arrest

Leukocyte transmigration through the blood vessel wall is a fundamental step of the inflammatory response and requires expression of adhesion molecule PECAM-1. Accumulating evidence implicates that semaphorin (Sema) 3F and its receptor neuropilin (NRP) 2 are central regulators in vascular biology. Herein, we assess the role of Sema3F in leukocyte migration in vitro and in vivo. To determine the impact of Sema3F on leukocyte recruitment in vivo, we used the thioglycollate-induced peritonitis model. After the induction of peritonitis, C57BL/6 mice were intraperitoneally (i.p.) injected daily with recombinant Sema3F or solvent for 3 days. Compared with solvent-treated controls, leukocyte count was increased in the peritoneal lavage of Sema3F-treated mice indicating that Sema3F promotes leukocyte extravasation into the peritoneal cavity. In line with this observation, stimulation of human endothelial cells with Sema3F enhanced the passage of peripheral blood mononuclear cells (PBMCs) through the endothelial monolayer in the transwell migration assays. Conversely, silencing of endothelial Sema3F by siRNA transfection dampened diapedesis of PBMCs through the endothelium in vitro. xMechanistically, Sema3F induced upregulation of adhesion molecule PECAM-1 in endothelial cells and in murine heart tissue shown by immunofluorescence and western blotting. The inhibition of PECAM-1 by blocking antibody HEC7 blunted Sema3F-induced leukocyte migration in transwell assays. SiRNA-based NRP2 knockdown reduced PECAM-1 expression and migration of PBMCs in Sema3F-treated endothelial cells, indicating that PECAM-1 expression and leukocyte migration in response to Sema3F depend on endothelial NRP2. To assess the regulation of Sema3F in human inflammatory disease, we collected serum samples of patients from day 0 to day 7 after survived out-of-hospital cardiac arrest (OHCA, n = 41). First, we demonstrated enhanced migration of PBMCs through endothelial cells exposed to the serum of patients after OHCA in comparison to the serum of patients with stable coronary artery disease or healthy volunteers. Remarkably, serum samples of OHCA patients contained significantly higher Sema3F protein levels compared with CAD patients (CAD, n = 37) and healthy volunteers (n = 11), suggesting a role of Sema3F in the pathophysiology of the inflammatory response after OHCA. Subgroup analysis revealed that elevated serum Sema3F levels after ROSC are associated with decreased survival, myocardial dysfunction, and prolonged vasopressor therapy, clinical findings that determine the outcome of post-resuscitation period after OHCA. The present study provides novel evidence that endothelial Sema3F controls leukocyte recruitment through a NRP2/PECAM-1-dependent mechanism. Sema3F serum concentrations are elevated following successful resuscitation suggesting that Sema3F might be involved in the inflammatory response after survived OHCA. Targeting the Sema3F/NRP2/PECAM-1 pathway could provide a novel approach to abolish overwhelming inflammation after resuscitation.

Endothelial IQGAP1 regulates leukocyte transmigration by directing the LBRC to the site of diapedesis

The function of endothelial cell IQGAP1 during diapedesis requires its actin-binding domain and IQ motifs to recruit the lateral border recycling compartment. Genetic ablation of endothelial cell IQGAP1 expression in vivo causes significant disruption of diapedesis in two models of inflammation.

Transendothelial migration (TEM) of leukocytes across the endothelium is critical for inflammation. In the endothelium, TEM requires the coordination of membrane movements and cytoskeletal interactions, including, prominently, recruitment of the lateral border recycling compartment (LBRC). The scaffold protein IQGAP1 was recently identified in a screen for LBRC-interacting proteins. Knockdown of endothelial IQGAP1 disrupted the directed movement of the LBRC and substantially reduced leukocyte TEM. Expression of truncated IQGAP1 constructs demonstrated that the calponin homology domain is required for IQGAP1 localization to endothelial borders and that the IQ domain, on the same IQGAP1 polypeptide, is required for its function in TEM. This is the first reported function of IQGAP1 requiring two domains to be present on the same polypeptide. Additionally, we show for the first time that IQGAP1 in the endothelium is required for efficient TEM in vivo. These findings reveal a novel function for IQGAP1 and demonstrate that IQGAP1 in endothelial cells facilitates TEM by directing the LBRC to the site of TEM.

PECAM-1 Stabilizes Blood-Brain Barrier Integrity and Favors Paracellular T-Cell Diapedesis Across the Blood-Brain Barrier During Neuroinflammation

Breakdown of the blood-brain barrier (BBB) and increased immune cell trafficking into the central nervous system (CNS) are hallmarks of the pathogenesis of multiple sclerosis (MS). Platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) is expressed on cells of the vascular compartment and regulates vascular integrity and immune cell trafficking. Involvement of PECAM-1 in MS pathogenesis has been suggested by the detection of increased levels of soluble PECAM-1 (sPECAM-1) in the serum and CSF of MS patients. Here, we report profound upregulation of cell-bound PECAM-1 in initial (pre-phagocytic) white matter as well as active cortical gray matter MS lesions. Using a human in vitro BBB model we observed that PECAM-1 is not essential for the transmigration of human CD4⁺ T-cell subsets (Th1, Th1^*, Th2, and Th17) across the BBB. Employing an additional in vitro BBB model based on primary mouse brain microvascular endothelial cells (pMBMECs) we show that the lack of endothelial PECAM-1 impairs BBB properties as shown by reduced transendothelial electrical resistance (TEER) and increases permeability for small molecular tracers. Investigating T-cell migration across the BBB under physiological flow by in vitro live cell imaging revealed that absence of PECAM-1 in pMBMECs did not influence arrest, polarization, and crawling of effector/memory CD4⁺ T cells on the pMBMECs. Absence of endothelial PECAM-1 also did not affect the number of T cells able to cross the pMBMEC monolayer under flow, but surprisingly favored transcellular over paracellular T-cell diapedesis. Taken together, our data demonstrate that PECAM-1 is critically involved in regulating BBB permeability and although not required for T-cell diapedesis itself, its presence or absence influences the cellular route of T-cell diapedesis across the BBB. Upregulated expression of cell-bound PECAM-1 in human MS lesions may thus reflect vascular repair mechanisms aiming to restore BBB integrity and paracellular T-cell migration across the BBB as it occurs during CNS immune surveillance.

Endothelial BMP4 Regulates Leukocyte Diapedesis and Promotes Inflammation

Leukocyte recruitment is a fundamental event in the response of the innate immune system to injury. This process is promoted in part by the opening of endothelial cell adherens junctions that allows leukocyte extravasation through gaps between adjacent endothelial cells. VE-cadherin is a key component of endothelial cell adherens junctions and a negative regulator of leukocyte emigration. Accumulating evidence implicates bone morphogenetic protein (BMP) 4 as a critical regulator in vascular biology, but its role in leukocyte extravasation in vitro and in vivo has not been investigated so far. To assess the impact of BMP4 on leukocyte emigration in vivo, we used the thioglycollate-induced peritonitis model. C57BL/6 mice were intraperitoneally (i.p.) injected with recombinant BMP4 in addition to thioglycollate. Compared to solvent-treated controls, we observed higher accumulation of leukocytes in the peritoneal lavage of BMP4-treated mice indicating that BMP4 promotes leukocyte diapedesis into the inflamed peritoneal cavity. Endothelial cell-specific deletion of BMP4 in mice markedly diminished leukocyte diapedesis following thioglycollate administration suggesting that endothelial BMP4 is required for leukocyte recruitment. Consistent with these in vivo results, transwell migration assays with human umbilical vein endothelial cells (HUVECs) in vitro revealed that recombinant BMP4 enhanced leukocyte transmigration through the endothelial monolayer. Conversely, silencing of endothelial BMP4 by siRNA dampened leukocyte diapedesis in vitro. Mechanistic studies showed that loss of BMP4 improved endothelial junction stability by upregulation of VE-cadherin expression in vitro and in vivo. Vice versa, treatment of HUVECs with recombinant BMP4 decreased expression of VE-cadherin and impaired endothelial junction stability shown by Western blotting and immunocytochemistry. Finally, severe endothelial damage in HUVECs in response to serum of patients collected 24 h after survived cardiac arrest was accompanied by increase in leukocyte migration in transwell assays and activation of the BMP pathway most probably by upregulation of endothelial BMP4 RNA and protein expression. Collectively, the present study provides novel evidence that endothelial BMP4 controls leukocyte recruitment through a VE-cadherin-dependent mechanism and that BMP4-induced inflammation might be involved in the pathogenesis of endothelial cell damage following successful resuscitation after cardiac arrest.

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.

Differential DARC/ACKR1 expression distinguishes venular from non-venular endothelial cells in murine tissues

BACKGROUND

Intravascular leukocyte recruitment in most vertebrate tissues is restricted to postcapillary and collecting venules, whereas capillaries and arterioles usually support little or no leukocyte adhesion. This segmental restriction is thought to be mediated by endothelial, rather than hemodynamic, differences. The underlying mechanisms are largely unknown, in part because effective tools to distinguish, isolate, and analyze venular endothelial cells (V-ECs) and non-venular endothelial cells (NV-ECs) have been unavailable. We hypothesized that the atypical chemokine receptor DARC (Duffy Antigen Receptor for Chemokines, a.k.a. ACKR1 or CD234) may distinguish V-ECs versus NV-ECs in mice.

METHODS

We generated a rat-anti-mouse monoclonal antibody (MAb) that specifically recognizes the erythroid and endothelial forms of native, surface-expressed DARC. Using this reagent, we characterized DARC expression and distribution in the microvasculature of murine tissues.

RESULTS

DARC was exquisitely restricted to post-capillary and small collecting venules and completely absent from arteries, arterioles, capillaries, veins, and most lymphatics in every tissue analyzed. Accordingly, intravital microscopy showed that adhesive leukocyte-endothelial interactions were restricted to DARC+ venules. DARC was detectable over the entire circumference of V-ECs, but was more concentrated at cell-cell junctions. Analysis of single-cell suspensions suggested that the frequency of V-ECs among the total microvascular EC pool varies considerably between different tissues.

CONCLUSIONS

Immunostaining of endothelial DARC allows the identification and isolation of intact V-ECs from multiple murine tissues. This strategy may be useful to dissect the mechanisms underlying segmental microvascular specialization in healthy and diseased tissues and to characterize the role of EC subsets in tissue-homeostasis, immune surveillance, infection, inflammation, and malignancies.

The in Vitro and in Vivo Antitumor Activities of Tetracyclic Triterpenoids Compounds Actein and 26-Deoxyactein Isolated from Rhizome of Cimicifuga foetida L

AIMS

This work aims to study the in vitro and in vivo antitumor activities of tetracyclic triterpenoids compounds actein and 26-deoxyactein. Further, the mechanism is investigated.

METHODS

In vitro, a modified MTT method was used to assay the cytotoxicities of actein and 26-deoxyactein in 12 human tumor cell lines. In vivo, mouse sarcoma S180 and human lung cancer A549 cells were respectively implanted subcutaneously in ICR mice and nude mice to establish implanted tumor models. Flow cytometry (FCM) was used to assay the cycle distribution of the tumor cells. Immunohistochemistry was used to measure CD31-positive expression in the xenogrft tumor by analyzing microvessel density (MVD). In addition, acute toxicities of actein and 26-deoxyactein were also evaluated.

RESULTS

Actein and 26-deoxyactein inhibited the proliferation of the 12 human cancer cell lines tested with the values of 50% inhibitory concentrations (IC50) between 12.29 and 88.39 μg/mL. In vivo, both actein (3-27 mg/kg) and 26-deoxyactein (3-27 mg/kg) significantly inhibited the growth of the implanted sarcoma S180 in a dose-dependent manner. Actein (10, 30 mg/kg) and 26-deoxyactein (10, 30 mg/kg) markedly inhibited the xenograft growth with T/C (%) values of 38%, 55% for actein, and 35%, 49% for 26-deoxyactein. Compared with the vehicle control, actein (10, 30 mg/kg) and 26-deoxyactein (10, 30 mg/kg) significantly reduced the MVD in the xenograft tumor. The FCM result showed that human leukemia HL-60 cells were arrested at G₁ phase after treated with either actein (6.25-25 μg/mL) or 26-deoxyactein (6.25-25 μg/mL) for 48 h. A limited trial in mice showed that both of the minimal lethal doses (MLDs) of actein and 26-deoxyactein were over 5 g/kg.

CONCLUSIONS

Both actein and 26-deoxyactein have low toxicities. Importantly, both these two tetracyclic triterpenoids compounds isolated from rhizome of Cimicifuga foetida L. have significant antitumor activities in vitro and in vivo, which is associated with cell cycle arrest and angiogenesis inhibition.

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.

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.

TRPC6 is the endothelial calcium channel that regulates leukocyte transendothelial migration during the inflammatory response

Weber et al. identify TRPC6 as the calcium channel mediating the transient increase in endothelial cytosolic free calcium concentration required for transendothelial migration of leukocytes during the inflammatory response.

Leukocyte transendothelial migration (TEM) is a tightly regulated, multistep process that is critical to the inflammatory response. A transient increase in endothelial cytosolic free calcium ion concentration (↑[Ca²⁺]_i) is required for TEM. However, the mechanism by which endothelial ↑[Ca²⁺]_i regulates TEM and the channels mediating this ↑[Ca²⁺]_i are unknown. Buffering ↑[Ca²⁺]_i in endothelial cells does not affect leukocyte adhesion or locomotion but selectively blocks TEM, suggesting a role for ↑[Ca²⁺]_i specifically for this step. Transient receptor potential canonical 6 (TRPC6), a Ca²⁺ channel expressed in endothelial cells, colocalizes with platelet/endothelial cell adhesion molecule-1 (PECAM) to surround leukocytes during TEM and clusters when endothelial PECAM is engaged. Expression of dominant-negative TRPC6 or shRNA knockdown in endothelial cells arrests neutrophils apically over the junction, similar to when PECAM is blocked. Selectively activating endothelial TRPC6 rescues TEM during an ongoing PECAM blockade, indicating that TRPC6 functions downstream of PECAM. Furthermore, endothelial TRPC6 is required for trafficking of lateral border recycling compartment membrane, which facilitates TEM. Finally, mice lacking TRPC6 in the nonmyeloid compartment (i.e., endothelium) exhibit a profound defect in neutrophil TEM with no effect on leukocyte trafficking. Our findings identify endothelial TRPC6 as the calcium channel mediating the ↑[Ca²⁺]_i required for TEM at a step downstream of PECAM homophilic interactions.

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.

The regulation of transendothelial migration: new knowledge and new questions

Leucocyte transendothelial migration (TEM) involves a co-operative series of interactions between surface molecules on the leucocyte and cognate counter-ligands on the endothelial cell. These interactions set up a cascade of signalling events inside the endothelial cell that both allow for the junctions to loosen and for membrane to be recruited from the lateral border recycling compartment (LBRC). The LBRC is thought to provide an increased surface area and unligated receptors to the leucocyte to continue the process. The relative importance of the individual adhesion/signalling molecules that promote transmigration may vary depending on the type of leucocyte, the vascular bed, the inflammatory stimulus, and the stage of the inflammatory response. However, the molecular interactions between leucocyte and endothelial cell activate signalling pathways that disengage the adherens and tight junctions and recruit the LBRC to the site of transmigration. With the exception of disengaging the junctions, similar molecules and mechanisms promote transcellular migration as paracellular migration of leucocytes. This review will discuss the molecular interactions and signalling pathways that regulate transmigration, and the common themes that emerge from studying TEM of different leucocyte subsets under different inflammatory conditions. We will also raise some unanswered questions in need of future research.

Monocyte trafficking across the vessel wall

Monocytes fundamentally contribute to immune surveillance and the inflammatory response in immunoinflammatory diseases like atherosclerosis. Recruitment of these cells to the site of injury requires their trafficking across the blood vessel wall. A series of events, including capture, rolling, slow rolling, arrest, adhesion strengthening, and lateral locomotion, precede monocyte transmigration. Recent investigations have revealed new aspects of this cascade. This article revisits some conventional paradigms and selectively highlights new findings, including novel insights into monocyte differentiation and recently identified functional mediators, signalling pathways, and new structural aspects of monocyte extravasation. The emerging roles of endothelial junctional molecules like vascular endothelial-cadherin and the junctional adhesion molecule family, adhesion molecules such as intercellular adhesion molecule-1, molecules localized to the lateral border recycling compartment like cluster of differentiation 99, platelet/endothelial cell adhesion molecule-1, and poliovirus receptor (CD155), as well as other cell surface molecules such as cluster of differentiation 146 and ephrins in transendothelial migration are discussed.

PECAM-1 isoforms, eNOS and endoglin axis in regulation of angiogenesis

Vascular development and maintenance of proper vascular function through various regulatory mechanisms are critical to our wellbeing. Delineation of the regulatory processes involved in development of the vascular system and its function is one of the most important topics in human physiology and pathophysiology. Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31), a cell adhesion molecule with proangiogenic and proinflammatory activity, has been the subject of numerous studies. In the present review, we look at the important roles that PECAM-1 and its isoforms play during angiogenesis, and its molecular mechanisms of action in the endothelium. In the endothelium, PECAM-1 not only plays a role as an adhesion molecule but also participates in intracellular signalling pathways which have an impact on various cell adhesive mechanisms and endothelial nitric oxide synthase (eNOS) expression and activity. In addition, recent studies from our laboratory have revealed an important relationship between PECAM-1 and endoglin expression. Endoglin is an essential molecule during angiogenesis, vascular development and integrity, and its expression and activity are compromised in the absence of PECAM-1. In the present review we discuss the roles that PECAM-1 isoforms may play in modulation of endothelial cell adhesive mechanisms, eNOS and endoglin expression and activity, and angiogenesis.

ROBO4-mediated vascular integrity regulates the directionality of hematopoietic stem cell trafficking

Despite the use of hematopoietic stem cells (HSCs) in clinical therapy for over half a century, the mechanisms that regulate HSC trafficking, engraftment, and life-long persistence after transplantation are unclear. Here, we show that the vascular endothelium regulates HSC trafficking into and out of bone marrow (BM) niches. Surprisingly, we found that instead of acting as barriers to cellular entry, vascular endothelial cells, via the guidance molecule ROBO4, actively promote HSC translocation across vessel walls into the BM space. In contrast, we found that the vasculature inhibits the reverse process, as induced vascular permeability led to a rapid increase in HSCs in the blood stream. Thus, the vascular endothelium reinforces HSC localization to BM niches both by promoting HSC extravasation from blood-to-BM and by forming vascular barriers that prevent BM-to-blood escape. Our results uncouple the mechanisms that regulate the directionality of HSC trafficking and show that the vasculature can be targeted to improve hematopoietic transplantation therapies.

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Endothelial ROBO4 promotes unidirectional HSC trafficking across vessel walls

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Sinusoidal endothelial cells mediate HSC extravasation from blood to bone marrow

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Vascular integrity prevents HSC escape from bone marrow to blood

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Induced vascular permeability rapidly mobilizes HSCs to the blood stream

Getting leukocytes to the site of inflammation

There is no "response" in either the innate or adaptive immune response unless leukocytes cross blood vessels. They do this through the process of diapedesis, in which the leukocyte moves in ameboid fashion through tightly apposed endothelial borders (paracellular transmigration) and in some cases through the endothelial cell itself (transcellular migration). This review summarizes the steps leading up to diapedesis, then focuses on the molecules and mechanisms responsible for transendothelial migration. Surprisingly, many of the same molecules and mechanisms that regulate paracellular migration also control transcellular migration, including a major role for membrane from the recently described lateral border recycling compartment. A hypothesis that integrates the various known mechanisms of transmigration is proposed.

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.

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.

Microhemorrhage is an early event in the pulmonary fibrotic disease of PECAM-1 deficient FVB/n mice

Platelet Endothelial Cell Adhesion Molecule 1 (PECAM-1) deficient mice in the FVB/n strain exhibit fatal chronic pulmonary fibrotic disease. The illness occurs in the absence of a detectable pro-inflammatory event. PECAM-1 is vital to the stability of vascular permeability, leukocyte extravasation, clotting of platelets, and clearance of apoptotic cells. We show here that the spontaneous development of fibrotic disease in PECAM-1 deficient FVB/n mice is characterized by early loss of vascular integrity in pulmonary capillaries, resulting in spontaneous microbleeds. Hemosiderin-positive macrophages were found in interstitial spaces and bronchoalveolar lavage (BAL) fluid in relatively healthy animals. We also observed a gradually increasing presence of hemosiderin-positive macrophages and fibrin deposition in the advanced stages of disease, corresponding to the accumulation of collagen, IL-10 expression, and myofibroblasts expressing alpha smooth muscle actin (SMA). Together with the growing evidence that pulmonary microbleeds and coagulation play an active part in human pulmonary fibrosis, this data further supports our hypothesis that PECAM-1 expression is necessary for vascular barrier function control and regulation of homeostasis specifically, in the pulmonary environment.

Similarities and differences in the regulation of leukocyte extravasation and vascular permeability

Leukocyte extravasation is regulated and mediated by a multitude of adhesion and signaling molecules. Many of them enable the capturing and docking of leukocytes to the vessel wall. Others allow leukocytes to crawl on the apical surface of endothelial cells to appropriate sites of exit. While these steps are well understood and the adhesion molecules mediating these interactions are largely identified, a still growing number of adhesion receptors mediate the diapedesis process, the actual migration of leukocytes through the endothelial cell layer, and the underlying basement membrane. In most cases, it is not known which molecular processes they actually mediate, whether they enable the migration of leukocytes through the endothelial cell layer or whether they are involved in the destabilization of endothelial junctions. In addition, leukocytes are able to circumvent junctions and transcytose directly through the body of endothelial cells. While this latter route indeed exists, recent work has highlighted in vivo the junctional pathway as the prevalent way of leukocyte exit in various inflamed tissues. Recent work elucidating molecular mechanisms that regulate endothelial junctions and thereby leukocyte extravasation and vascular permeability will be discussed.

PECAM-1 and caveolae form the mechanosensing complex necessary for NOX2 activation and angiogenic signaling with stopped flow in pulmonary endothelium

We showed that stop of flow triggers a mechanosignaling cascade that leads to the generation of reactive oxygen species (ROS); however, a mechanosensor coupled to the cytoskeleton that could potentially transduce flow stimulus has not been identified. We showed a role for KATP channel, caveolae (caveolin-1), and NADPH oxidase 2 (NOX2) in ROS production with stop of flow. Based on reports of a mechanosensory complex that includes platelet endothelial cell adhesion molecule-1 (PECAM-1) and initiates signaling with mechanical force, we hypothesized that PECAM-1 could serve as a mechanosensor in sensing disruption of flow. Using lungs in situ, we observed that ROS production with stop of flow was significantly reduced in PECAM-1(-/-) lungs compared with lungs from wild-type (WT) mice. Lack of PECAM-1 did not affect NOX2 activation machinery or the caveolin-1 expression or caveolae number in the pulmonary endothelium. Stop of flow in vitro triggered an increase in angiogenic potential of WT pulmonary microvascular endothelial cells (PMVEC) but not of PECAM-1(-/-) PMVEC. Obstruction of flow in lungs in vivo showed that the neutrophil infiltration as observed in WT mice was significantly lowered in PECAM-1(-/-) mice. With stop of flow, WT lungs showed higher expression of the angiogenic marker VEGF compared with untreated (sham) and PECAM-1(-/-) lungs. Thus PECAM-1 (and caveolae) are parts of the mechanosensing machinery that generates superoxide with loss of shear; the resultant ROS potentially drives neutrophil influx and acts as an angiogenic signal.

An immunologist's guide to CD31 function in T-cells

Although it is expressed by all leukocytes, including T-, B-lymphocytes and dendritic cells, the immunoglobulin-like receptor CD31 is generally regarded by immunologists as a marker of endothelial cell lineage that lacks an established functional role in adaptive immunity. This perception has recently been challenged by studies that reveal a key role for this molecule in the regulation of T-cell homeostasis, effector function and trafficking. The complexity of the biological functions of CD31 results from the integration of its adhesive and signaling functions in both the immune and vascular systems. Signaling by means of CD31 is induced by homophilic engagement during the interactions of immune cells and is mediated by phosphatase recruitment or activation through immunoreceptor tyrosine inhibitory motifs (ITIMs) that are located in its cytoplasmic tail. Loss of CD31 function is associated with excessive immunoreactivity and susceptibility to cytotoxic killing. Here, we discuss recent findings that have brought to light a non-redundant, complex role for this molecule in the regulation of T-cell-mediated immune responses, with large impact on our understanding of immunity in health and disease.

Nectin-3 (CD113) interacts with Nectin-2 (CD112) to promote lymphocyte transendothelial migration

Lymphocyte trafficking and migration through vascular endothelial cells (ECs) in secondary lymphoid tissues is critical for immune protection. In the present study, we investigate the role of nectin cell adhesion molecules for the migration of lymphocytes through ECs. Nectins are key players for the establishment of homotypic and heterotypic cell to cell contacts; they are required for cell to cell adherens junction formation and take part in the transendothelial migration of monocytes during the step of diapedesis, when monocytes migrate through EC junctions. We first show that Nectin-3 (CD113) is the only nectin expressed by T lymphocytes and since nectins are expressed on ECs we explored Nectin-3 potential functions in lymphocyte: EC interactions. We demonstrate that Nectin-2, expressed on ECs, is the major counter-receptor of Nectin-3. A soluble form of Nectin-3 binds to Nectin-2 localized at EC junctions and blocking Nectin-2 trans-interactions with monoclonal antibodies abolishes the binding of soluble Nectin-3 to ECs. Nectin-2 is expressed on High Endothelial venules (HEVs), where lymphocyte homing occurs in vivo. Finally, we show that Nectin-3 trans-interaction with Nectin-2 is essential for the process of lymphocyte transendothelial migration in vitro as targeting with blocking monoclonal antibodies either Nectin-3, expressed on lymphocytes, or Nectin-2, expressed on ECs, inhibits lymphocyte extravasation. The nectin family of CAMs is important for the regulation of endothelial barrier functions and transendothelial migration of immune cells. Our results demonstrate for the first time that Nectin-3 trans-interacts with Nectin-2 to promote lymphocyte and monocyte extravasation.

Suppressing iron oxide nanoparticle toxicity by vascular targeted antioxidant polymer nanoparticles

The biomedical use of superparamagnetic iron oxide nanoparticles has been of continued interest in the literature and clinic. Their ability to be used as contrast agents for imaging and/or responsive agents for remote actuation makes them exciting materials for a wide range of clinical applications. Recently, however, concern has arisen regarding the potential health effects of these particles. Iron oxide toxicity has been demonstrated in in vivo and in vitro models, with oxidative stress being implicated as playing a key role in this pathology. One of the key cell types implicated in this injury is the vascular endothelial cells. Here, we report on the development of a targeted polymeric antioxidant, poly(trolox ester), nanoparticle that can suppress oxidative damage. As the polymer undergoes enzymatic hydrolysis, active trolox is locally released, providing a long term protection against pro-oxidant agents. In this work, poly(trolox) nanoparticles are targeted to platelet endothelial cell adhesion molecules (PECAM-1), which are able to bind to and internalize in endothelial cells and provide localized protection against the cytotoxicity caused by iron oxide nanoparticles. These results indicate the potential of using poly(trolox ester) as a means of mitigating iron oxide toxicity, potentially expanding the clinical use and relevance of these exciting systems.

Progenitor cell homing in the postischemic myocardium: just an unmotivated pitstop in the microcirculation?

OBJECTIVE

We developed a model for direct assessment of BMC sequestration in the postischemic murine myocardium after direct antegrade intracoronary injection.

METHODS

Modified syngeneic heterotopic heart transplantation was used as a basic model for global myocardial I/R injury in a total of n = 29 animals. IVM was employed to analyze the right ventricular subepicardial coronary microcirculation and for tracking fluorescently labeled BMCs.

RESULTS

IVM allowed monitoring all segments of the coronary microcirculation including feeding arterioles, nutritive capillaries, and postcapillary venules. WI and generalized atherosclerosis induced profound reperfusion failure, particularly in nutritive myocardial capillaries. BMCs were found to exclusively sequester in myocardial capillaries, but not in coronary arterioles or postcapillary venules. The sequestration of BMCs in coronary capillaries occurred independent of WI, generalized atherosclerosis, or adhesion molecule function.

CONCLUSIONS

This is the first study allowing direct assessment of BMC homing to the postischemic myocardium. Heterotopic heart transplantation and IVM are proper means to study the myocardial sequestration of BMCs after direct antegrade intracoronary injection in vivo. We show for the first time that intracoronarily injected BMCs sequester exclusively in nutritive myocardial capillaries.

Absence of platelet endothelial cell adhesion molecule 1, PECAM-1/CD31, in vivo increases resistance to Salmonella enterica serovar Typhimurium in mice

PECAM-1/CD31 is known to regulate inflammatory responses and exhibit pro- and anti-inflammatory functions. This study was designed to determine the functional role of PECAM-1 in susceptibility to murine primary in vivo infection with Salmonella enterica serovar Typhimurium and in in vitro inflammatory responses of peritoneal macrophages. Lectin profiling showed that cellular PECAM-1 and recombinant human PECAM-1-Ig chimera contain high levels of mannose sugars and N-acetylglucosamine. Consistent with this carbohydrate pattern, both recombinant human and murine PECAM-1-Ig chimeras were shown to bind S. Typhimurium in a dose-dependent manner in vitro. Using oral and fecal-oral transmission models of S. Typhimurium SL1344 infection, PECAM-1(-/-) mice were found to be more resistant to S. Typhimurium infection than wild-type (WT) C57BL/6 mice. While fecal shedding of S. Typhimurium was comparable in wild-type and PECAM-1(-/-) mice, the PECAM-1-deficient mice had lower bacterial loads in systemic organs such as liver, spleen, and mesenteric lymph nodes than WT mice, suggesting that extraintestinal dissemination was reduced in the absence of PECAM-1. This reduced bacterial load correlated with reduced tumor necrosis factor (TNF), interleukin-6 (IL-6), and monocyte chemoattractant protein (MCP) levels in sera of PECAM-1(-/-) mice. Following in vitro stimulation of macrophages with either whole S. Typhimurium, lipopolysaccharide (LPS) (Toll-like receptor 4 [TLR4] ligand), or poly(I·C) (TLR3 ligand), production of TNF and IL-6 by PECAM-1(-/-) macrophages was reduced. Together, these results suggest that PECAM-1 may have multiple functions in resistance to infection with S. Typhimurium, including binding to host cells, extraintestinal spread to deeper tissues, and regulation of inflammatory cytokine production by infected macrophages.

Monocyte ADAM17 promotes diapedesis during transendothelial migration: identification of steps and substrates targeted by metalloproteinases

Despite expanded definition of the leukocyte adhesion cascade and mechanisms underlying individual steps, very little is known about regulatory mechanisms controlling sequential shifts between steps. We tested the hypothesis that metalloproteinases provide a mechanism to rapidly transition monocytes between different steps. Our study identifies diapedesis as a step targeted by metalloproteinase activity. Time-lapse video microscopy shows that the presence of a metalloproteinase inhibitor results in a doubling of the time required for human monocytes to complete diapedesis on unactivated or inflamed human endothelium, under both static and physiological-flow conditions. Thus, diapedesis is promoted by metalloproteinase activity. In contrast, neither adhesion of monocytes nor their locomotion over the endothelium is altered by metalloproteinase inhibition. We further demonstrate that metalloproteinase inhibition significantly elevates monocyte cell surface levels of integrins CD11b/CD18 (Mac-1), specifically during transendothelial migration. Interestingly, such alterations are not detected for other endothelial- and monocyte-adhesion molecules that are presumed metalloproteinase substrates. Two major transmembrane metalloproteinases, a disintegrin and metalloproteinase (ADAM)17 and ADAM10, are identified as enzymes that control constitutive cleavage of Mac-1. We further establish that knockdown of monocyte ADAM17, but not endothelial ADAM10 or ADAM17 or monocyte ADAM10, reproduces the diapedesis delay observed with metalloproteinase inhibition. Therefore, we conclude that monocyte ADAM17 facilitates the completion of transendothelial migration by accelerating the rate of diapedesis. We propose that the progression of diapedesis may be regulated by spatial and temporal cleavage of Mac-1, which is triggered upon interaction with endothelium.

Endothelial nitric oxide synthase in dorsal root ganglia during chronic inflammatory nociception

Nitric oxide (NO) is a gaseous molecule implicated both in vascular tone and nociceptive transmission. The capillary blood supply to the dorsal root ganglia (DRG) is unique because it is highly permeable to several low and high molecular-weight compounds. This anatomical situation leads to a potential role of endothelial nitric oxide synthase (eNOS) in inflammatory nociception, which is not well established. Therefore, we examined the role of eNOS in DRG in a murine chronic inflammatory pain model induced by complete Freund's adjuvant using L-N(5)-(1-iminoethyl)ornithine (L-NIO), a potent inhibitor of eNOS activity. Pain state was examined using a behavioral test. The expression of eNOS, platelet endothelial cell adhesion molecule-1 (CD31) and vascular endothelial growth factor (VEGF) was examined by immunofluorescence. In control animals, CD31 was detected in vessels; VEGF was localized both in vessels and neurons while a weak eNOS immunopositivity was detected in both vessels and in neurons. Under inflammatory pain conditions, eNOS, CD31 and VEGF immunopositivity increased. Administration of L-NIO significantly attenuated thermal hyperalgesia by 24 h and decreased eNOS activity and CD31 immunopositivity by 7 days. VEGF was unaffected. Our results show that eNOS plays a nociceptive role in the early phases of inflammation while in the later phases it may be involved in neurotrophic support.

CD31 exhibits multiple roles in regulating T lymphocyte trafficking in vivo

The role of CD31, an Ig-like molecule expressed by leukocytes and endothelial cells (ECs), in the regulation of T lymphocyte trafficking remains contentious. Using CD31-deficient mice, we show that CD31 regulates both constitutive and inflammation-induced T cell migration in vivo. Specifically, T cell:EC interactions mediated by CD31 molecules are required for efficient localization of naive T lymphocytes to secondary lymphoid tissue and constitutive recirculation of primed T cells to nonlymphoid tissues. In inflammatory conditions, T cell:EC CD31-mediated interactions facilitate T cell recruitment to Ag-rich sites. However, endothelial CD31 also provides a gate-keeping mechanism to limit the rate of Ag-driven T cell extravasation. This event contributes to the formation of Ag-specific effector T cell infiltrates and is induced by recognition of Ag on the endothelium. In this context, CD31 engagement is required for restoring endothelial continuity, which is temporarily lost upon MHC molecule ligation by migrating cognate T cells. We propose that integrated adhesive and signaling functions of CD31 molecules exert a complex regulation of T cell trafficking, a process that is differentially adapted depending on cell-specific expression, the presence of inflammatory conditions and the molecular mechanism facilitating T cell extravasation.

Migration of leukocytes across the vascular intima molecules and mechanisms

The process of leukocyte emigration from the blood into a site of inflammation can be dissected into four distinct but continuous stages involving sequential interactions of different adhesion molecules on the leukocyte and endothelial surfaces. Although the molecules mediating adhesion to the luminal surface have been well studied, much less is known about the final stage of this process, transendothelial migration. This is the stage in which a leukocyte bound to the vascular wall passes between adjacent endothelial cells; it is a complex process in its own right, involving continuous structural, mechanical, and biochemical changes in both interacting cell types. Distinct cell adhesion molecule and signal transduction mechanisms that appear to be involved in this phase are discussed here.

Role of high endothelial postcapillary venules and selected adhesion molecules in periodontal diseases: a review

Periodontitis is accompanied by the proliferation of small blood vessels in the gingival lamina propria. Specialized postcapillary venules, termed periodontal high endothelial-like venules, are also present, and demonstrate morphological and functional traits similar to those of high endothelial venules (HEVs) in lymphatic organs. The suggested role of HEVs in the pathogenesis of chronic periodontitis involves participation in leukocyte transendothelial migration and therefore proinflammatory effects appear. Recent observations suggest that chronic periodontitis is an independent risk factor for systemic vascular disease and may result in stimulation of the synthesis of acute phase protein by cytokines released by periodontal high endothelial cells (HECs). However, tissue expression of HEV-linked adhesion molecules has not been evaluated in the gingiva of patients with chronic periodontitis. This is significant in relation to potential therapy targeting expression of the adhesion molecules. In this review, current knowledge of HEV structure and the related expression of four surface adhesion molecules of HECs [CD34, platelet endothelial cell adhesion molecule 1, endoglin and intercellular adhesion molecule 1 (ICAM-1)], involved in the key steps of the adhesion cascade in periodontal diseases, are discussed. Most studies on the expression of adhesion molecules in the development and progression of periodontal diseases pertain to ICAM-1 (CD54). Studies by the authors demonstrated quantitatively similar expression of three of four selected surface markers in gingival HEVs of patients with chronic periodontitis and in HEVs of reactive lymph nodes, confirming morphological and functional similarity of HEVs in pathologically altered tissues with those in lymphoid tissues.

Soluble platelet/endothelial cell adhesion molecule (sPECAM)-1 is increased in polycystic ovary syndrome and related to endothelial dysfunction

Striking evidence indicates endothelial impairment in polycystic ovary syndrome (PCOS) but the mechanisms linking PCOS status to cardiovascular risk remain elusive. Platelet/endothelial cell adhesion molecule (PECAM)-1 is a soluble (s) signaling molecule involved in inflammation and angiogenesis with predictive value for endothelial dysfunction in patients at risk. In a prospective, controlled study, sPECAM-1 levels and the relationships to metabolic, inflammatory and vascular PCOS traits were evaluated in 26 patients and 29-age- and body mass index-matched controls. To assess endothelial injury, carotid artery intimae-media thickness (CIMT) and brachial artery flow-mediated vasodilatation (FMD) were employed. Of the 26 women with PCOS, 25 completed a six-month metformin combined with ethinylestradiol 0.3 mg/drospirenone 3 mg therapy. Soluble PECAM-1 levels were increased in PCOS (p = 0.018 vs. Controls) and significantly decreased at follow-up (p = 0.0002). Smoking and weight had no effect on sPECAM-1 dynamics. In both univariate and multivariate analysis, basal sPECAM-1 was inversely related to FMD (r = -0.311, p = 0.021) but not CIMT. To conclude, sPECAM-1 is increased in PCOS, an effect reversed by combined metformin and anti-androgenic contraceptive therapy. Elevated sPECAM-1 contributes to endothelial dysfunction however further studies are inquired to assess its relevance as biomarker and potential therapeutic target in PCOS.

The anti-TNF-α antibody infliximab indirectly regulates PECAM-1 gene expression in two models of in vitro blood cell activation

Chronic inflammatory bowel diseases can be successfully treated with antibodies against the acute phase mediator TNF-α. The process of activation and of extravasation of inflammatory cells from the blood into the 'stressed' tissue site is controlled by cytokines and chemokines, which attract leukocytes and by adhesion molecules, which mediate their attachment and transmigration toward the affected cell(s). The changes in the gene expression of adhesion molecules taking place in those cells before attachment have been less investigated. Changes of PECAM-1, ICAM-1 and vascular cell adhesion molecule-1 (VCAM-1) gene expression were studied in phytohaemagglutinin (PHA)- and lipolysaccharide (LPS)-treated human peripheral blood leukocytes (PBLs), granulocytes and the human monocyte cell line U-937. Cells were treated either with PHA or with LPS in the presence or absence of infliximab and incubated with TNF-α, IFN-γ and/or transforming growth factor beta (TGF-β) and treated as above. Activation of PBLs by PHA or LPS treatment triggered a sharp upregulation of ICAM-1, VCAM-1 gene expression and a time-dependent downregulation of PECAM-1 gene expression reaching a minimum 4 h from start of the experiment. The anti-TNF-α antibody infliximab, by neutralizing TNF-α and IFN-γ production, completely reversed PECAM-1 mRNA downregulation and ICAM-1 and VCAM-1 upregulation. Immunostaining of PBLs cytospins with antibodies against PECAM-1 and ICAM-1 confirmed RT-PCR and western blot results. PBLs IFN-γ or TNF-α treatment downregulated PECAM-1 in parallel with the upregulation of ICAM-1 and VCAM-1 gene expression, whereas TGF-β upregulated PECAM-1- and downregulated ICAM-1 and VCAM-1 gene expression counteracting the effect of TNF-α or IFN-γ. Similar results were obtained in human U937 cells and in granulocyte cultures by TNF-α or IFN-γ treatment. Taken together, these results suggest that infliximab, blocking TNF-α and IFN-γ production, exerts its anti-inflammatory effect through inhibiting downregulation of PECAM-1 gene expression and upregulation of ICAM-1 and VCAM-1 expression in leukocytes of the peripheral blood. These results also suggest that TGF-β may thus be of therapeutic importance as an anti-inflammatory agent.

Mechanisms of leukocyte transendothelial migration

Neither the innate nor adaptive immune system "responds" unless leukocytes cross blood vessels. This process occurs through diapedesis, in which the leukocyte moves in an ameboid fashion through tightly apposed endothelial borders and, in some cases, through the endothelial cell itself. This review focuses on the active role of the endothelial cell in diapedesis. Several mechanisms play a critical role in transendothelial migration, including signals derived from clustering of apically disposed intercellular adhesion molecule 1 and vascular cell adhesion molecule 1, disruption or loosening of adherens junctions, and targeted recycling of platelet/endothelial cell adhesion molecule and other molecules from the recently described lateral border recycling compartment. Surprisingly, many of the same molecules and mechanisms that regulate paracellular migration also control transcellular migration. A hypothesis that integrates the various known mechanisms of transmigration is proposed.

Cellular and molecular events in the localization of diabetogenic T cells to islets of Langerhans

Understanding the entry of autoreactive T cells to their target organ is important in autoimmunity because this entry initiates the inflammatory process. Here, the events that lead to specific localization of diabetogenic CD4 T cells into islets of Langerhans resulting in diabetes were examined. This was evaluated in two models, one in which T cells specific for a hen-egg white lysozyme (HEL) peptide were injected into mice expressing HEL on β cells and the other using T cells in the nonobese diabetic mouse strain, which develops spontaneous diabetes. Only T cells specific for β-cell antigens localized in islets within the first hours after their injection and were found adherent to intraislet dendritic cells (DCs). DCs surrounded blood vessels with dendrites reaching into the vessels. Localization of antigen-specific T cells did not require chemokine receptor signaling but involved class II histocompatibility and intercellular adhesion molecule 1 molecules. We found no evidence for nonspecific localization of CD4 T cells into normal noninflamed islets. Thus, the anatomy of the islet of Langerhans permits the specific localization of diabetogenic T cells at a time when there is no inflammation in the islets.

CD99 and CD99L2 act at the same site as, but independently of, PECAM-1 during leukocyte diapedesis

Leukocyte extravasation depends on various adhesion receptors at endothelial cell contacts. Here we have analyzed how mouse CD99 and CD99L2 cooperate with PECAM-1. We found that antibodies against mouse CD99 and PECAM-1 trap neutrophils between endothelial cells in in vitro transmigration assays. A sequential function, as has been suggested for human PECAM-1 and CD99, could not be demonstrated. In contrast to these in vitro results, blocking CD99 or CD99L2 or gene disruption of PECAM-1 trapped neutrophils in vivo between endothelial cells and the underlying basement membrane as revealed by electron microscopy and by 3-dimensional confocal fluorescence microscopy in the inflamed cremaster tissue. Leukocyte extravasation was inhibited in interleukin-1β-inflamed peritoneum and in the cremaster by PECAM-1 gene disruption and was further attenuated by blocking antibodies against CD99 and CD99L2. In addition, CD99 and CD99L2 were required for leukocyte extravasation in the cremaster after stimulation with tumor necrosis factor-α, where the need for PECAM-1 is known to be bypassed. We conclude that CD99 and CD99L2 act independently of PECAM-1 in leukocyte extravasation and cooperate in an independent way to help neutrophils overcome the endothelial basement membrane.

Intravital imaging of embryonic and tumor neovasculature using viral nanoparticles

Viral nanoparticles are a novel class of biomolecular agents that take advantage of the natural circulatory and targeting properties of viruses to allow the development of therapeutics, vaccines and imaging tools. We have developed a multivalent nanoparticle platform based on the cowpea mosaic virus (CPMV) that facilitates particle labeling at high density with fluorescent dyes and other functional groups. Compared with other technologies, CPMV-based viral nanoparticles are particularly suited for long-term intravital vascular imaging because of their biocompatibility and retention in the endothelium with minimal side effects. The stable, long-term labeling of the endothelium allows the identification of vasculature undergoing active remodeling in real time. In this study, we describe the synthesis, purification and fluorescent labeling of CPMV nanoparticles, along with their use for imaging of vascular structure and for intravital vascular mapping in developmental and tumor angiogenesis models. Dye-labeled viral nanoparticles can be synthesized and purified in a single day, and imaging studies can be conducted over hours, days or weeks, depending on the application.

PECAM-1: conflicts of interest in inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a cell adhesion and signaling receptor that is expressed on hematopoietic and endothelial cells. PECAM-1 is vital to the regulation of inflammatory responses, as it has been shown to serve a variety of pro-inflammatory and anti-inflammatory functions. Pro-inflammatory functions of PECAM-1 include the facilitation of leukocyte transendothelial migration and the transduction of mechanical signals in endothelial cells emanating from fluid shear stress. Anti-inflammatory functions include the dampening of leukocyte activation, suppression of pro-inflammatory cytokine production, and the maintenance of vascular barrier integrity. Although PECAM-1 has been well-characterized and studied, the mechanisms through which PECAM-1 regulates these seemingly opposing functions, and how they influence each other, are still not completely understood. The purpose of this review, therefore, is to provide an overview of the pro- and anti-inflammatory functions of PECAM-1 with special attention paid to mechanistic insights that have thus far been revealed in the literature in hopes of gaining a clearer picture of how these opposing functions might be integrated in a temporal and spatial manner on the whole organism level. A better understanding of how inflammatory responses are regulated should enable the development of new therapeutics that can be used in the treatment of acute and chronic inflammatory disorders.