Vitronectin stabilizes intravascular adhesion of neutrophils by coordinating ß2 integrin clustering.

The recruitment of neutrophils from the microvasculature to the site of injury or infection represents a key event in the inflammatory response. Vitronectin (VN) is a multifunctional macromolecule abundantly present in blood and extracellular matrix. The role of this glycoprotein in the extravasation process of circulating neutrophils remains elusive. Employing advanced in vivo/ex vivo imaging techniques in different mouse models as well as in vitro methods, we uncovered a previously unrecognized function of VN in the transition of dynamic to static intravascular interactions of neutrophils with microvascular endothelial cells. These distinct properties of VN require the heteromerization of this glycoprotein with plasminogen activator inhibitor-1 (PAI- 1) on the activated venular endothelium and subsequent interactions of this protein complex with the scavenger receptor low-density lipoprotein receptor-related protein-1 on intravascularly adhering neutrophils. This induces p38 mitogen-activated protein kinases-dependent intracellular signaling events which, in turn, regulates the proper clustering of the b2 integrin lymphocyte function associated antigen-1 on the surface of these immune cells. As a consequence of this molecular interplay, neutrophils become able to stabilize their adhesion to the microvascular endothelium and, subsequently, to extravasate to the perivascular tissue. Hence, endothelial-bound VN-PAI-1 heteromers stabilize intravascular adhesion of neutrophils by coordinating b2 integrin clustering on the surface of these immune cells, thereby effectively controlling neutrophil trafficking to inflamed tissue. Targeting this protein complex might be beneficial for the prevention and treatment of inflammatory pathologies.

Platelets Guide Leukocytes to Their Sites of Extravasation.

Effective immune responses require the directed migration of leukocytes from the vasculature to the site of injury or infection. How immune cells "find" their site of extravasation remains largely obscure. Here, we identified a previously unrecognized role of platelets as pathfinders guiding leukocytes to their exit points in the microvasculature: upon onset of inflammation, circulating platelets were found to immediately adhere at distinct sites in venular microvessels enabling these cellular blood components to capture neutrophils and, in turn, inflammatory monocytes via CD40-CD40L-dependent interactions. In this cellular crosstalk, ligation of PSGL-1 by P-selectin leads to ERK1/2 MAPK-dependent conformational changes of leukocyte integrins, which promote the successive extravasation of neutrophils and monocytes to the perivascular tissue. Conversely, blockade of this cellular partnership resulted in misguided, inefficient leukocyte responses. Our experimental data uncover a platelet-directed, spatiotemporally organized, multicellular crosstalk that is essential for effective trafficking of leukocytes to the site of inflammation.

Plasminogen Activator Inhibitor-1 Promotes Neutrophil Infiltration and Tissue Injury on Ischemia-Reperfusion.

OBJECTIVE: Ischemia-reperfusion (I/R) injury significantly contributes to organ dysfunction and failure after myocardial infarction, stroke, and transplantation. In addition to its established role in the fibrinolytic system, plasminogen activator inhibitor-1 has recently been implicated in the pathogenesis of I/R injury. The underlying mechanisms remain largely obscure. APPROACH AND RESULTS: Using different in vivo microscopy techniques as well as ex vivo analyses and in vitro assays, we identified that plasminogen activator inhibitor-1 rapidly accumulates on microvascular endothelial cells on I/R enabling this protease inhibitor to exhibit previously unrecognized functional properties by inducing an increase in the affinity of ß2 integrins in intravascularly rolling neutrophils. These events are mediated through low-density lipoprotein receptor-related protein-1 and mitogen-activated protein kinase-dependent signaling pathways that initiate intravascular adherence of these immune cells to the microvascular endothelium. Subsequent to this process, extravasating neutrophils disrupt endothelial junctions and promote the postischemic microvascular leakage. Conversely, deficiency of plasminogen activator inhibitor-1 effectively reversed leukocyte infiltration, microvascular dysfunction, and tissue injury on experimental I/R without exhibiting side effects on microvascular hemostasis. CONCLUSIONS: Our experimental data provide novel insights into the nonfibrinolytic properties of the fibrinolytic system and emphasize plasminogen activator inhibitor-1 as a promising target for the prevention and treatment of I/R injury.

Aged neutrophils contribute to the first line of defense in the acute inflammatory response.

Under steady-state conditions, aged neutrophils are removed from the circulation in bone marrow, liver, and spleen, thereby maintaining myeloid cell homeostasis. The fate of these aged immune cells under inflammatory conditions, however, remains largely obscure. Here, we demonstrate that in the acute inflammatory response during endotoxemia, aged neutrophils cease returning to the bone marrow and instead rapidly migrate to the site of inflammation. Having arrived in inflamed tissue, aged neutrophils were found to exhibit a higher phagocytic activity as compared with the subsequently recruited nonaged neutrophils. This distinct behavior of aged neutrophils under inflammatory conditions is dependent on specific age-related changes in their molecular repertoire that enable these "experienced" immune cells to instantly translate inflammatory signals into immune responses. In particular, aged neutrophils engage Toll-like receptor-4- and p38 MAPK-dependent pathways to induce conformational changes in ß2 integrins that allow these phagocytes to effectively accomplish their mission in the front line of the inflammatory response. Hence, ageing in the circulation might represent a critical process for neutrophils that enables these immune cells to properly unfold their functional properties for host defense.

Early pulmonary response is critical for extra-pulmonary carbon nanoparticle mediated effects: comparison of inhalation versus intra-arterial infusion exposures in mice.

BACKGROUND: The death toll associated with inhaled ambient particulate matter (PM) is attributed mainly to cardio-vascular rather than pulmonary effects. However, it is unclear whether the key event for cardiovascular impairment is particle translocation from lung to circulation (direct effect) or indirect effects due to pulmonary particle-cell interactions. In this work, we addressed this issue by exposing healthy mice via inhalation and intra-arterial infusion (IAI) to carbon nanoparticles (CNP) as surrogate for soot, a major constituent of (ultrafine) urban PM. METHODS: Equivalent surface area CNP doses in the blood (30mm2 per animal) were applied by IAI or inhalation (lung-deposited dose 10,000mm2; accounting for 0.3% of lung-to-blood CNP translocation). Mice were analyzed for changes in hematology and molecular markers of endothelial/epithelial dysfunction, pro-inflammatory reactions, oxidative stress, and coagulation in lungs and extra-pulmonary organs after CNP inhalation (4 h and 24 h) and CNP infusion (4 h). For methodological reasons, we used two different CNP types (spark-discharge and Printex90), with very similar physicochemical properties [≥98 and ≥95% elemental carbon; 10 and 14 nm primary particle diameter; and 800 and 300 m2/g specific surface area] for inhalation and IAI respectively. RESULTS: Mild pulmonary inflammatory responses and significant systemic effects were observed following 4 h and 24 h CNP inhalation. Increased retention of activated leukocytes, secondary thrombocytosis, and pro-inflammatory responses in secondary organs were detected following 4 h and 24 h of CNP inhalation only. Interestingly, among the investigated extra-pulmonary tissues (i.e. aorta, heart, and liver); aorta revealed as the most susceptible extra-pulmonary target following inhalation exposure. Bypassing the lungs by IAI however did not induce any extra-pulmonary effects at 4 h as compared to inhalation. CONCLUSIONS: Our findings indicate that extra-pulmonary effects due to CNP inhalation are dominated by indirect effects (particle-cell interactions in the lung) rather than direct effects (translocated CNPs) within the first hours after exposure. Hence, CNP translocation may not be the key event inducing early cardiovascular impairment following air pollution episodes. The considerable response detected in the aorta after CNP inhalation warrants more emphasis on this tissue in future studies.

Synchronized renal tubular cell death involves ferroptosis.

Receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis is thought to be the pathophysiologically predominant pathway that leads to regulated necrosis of parenchymal cells in ischemia-reperfusion injury (IRI), and loss of either Fas-associated protein with death domain (FADD) or caspase-8 is known to sensitize tissues to undergo spontaneous necroptosis. Here, we demonstrate that renal tubules do not undergo sensitization to necroptosis upon genetic ablation of either FADD or caspase-8 and that the RIPK1 inhibitor necrostatin-1 (Nec-1) does not protect freshly isolated tubules from hypoxic injury. In contrast, iron-dependent ferroptosis directly causes synchronized necrosis of renal tubules, as demonstrated by intravital microscopy in models of IRI and oxalate crystal-induced acute kidney injury. To suppress ferroptosis in vivo, we generated a novel third-generation ferrostatin (termed 16-86), which we demonstrate to be more stable, to metabolism and plasma, and more potent, compared with the first-in-class compound ferrostatin-1 (Fer-1). Even in conditions with extraordinarily severe IRI, 16-86 exerts strong protection to an extent which has not previously allowed survival in any murine setting. In addition, 16-86 further potentiates the strong protective effect on IRI mediated by combination therapy with necrostatins and compounds that inhibit mitochondrial permeability transition. Renal tubules thus represent a tissue that is not sensitized to necroptosis by loss of FADD or caspase-8. Finally, ferroptosis mediates postischemic and toxic renal necrosis, which may be therapeutically targeted by ferrostatins and by combination therapy.

Intercellular Transport of Nanomaterials is Mediated by Membrane Nanotubes In Vivo.

So-called membrane nanotubes are cellular protrusions between cells whose functions include cell communication, environmental sampling, and protein transfer. It has been previously reported that systemically administered carboxyl-modified quantum dots (cQDs) are rapidly taken up by perivascular macrophages in skeletal muscle of healthy mice. Expanding these studies, it is found, by means of in vivo fluorescence microscopy on the mouse cremaster muscle, rapid uptake of cQDs not only by perivascular macrophages but also by tissue-resident cells, which are localized more than 100 µm distant from the closest vessel. Confocal microscopy on muscle tissue, immunostained for the membrane dye DiI, reveals the presence of continuous membranous structures between MHC-II-positive, F4/80-positive cells. These structures contain microtubules, components of the cytoskeleton, which clearly colocalize with cQDs. The cQDs are exclusively found inside endosomal vesicles. Most importantly, by using in vivo fluorescence microscopy, this study detected fast (0.8 µm s(-1) , mean velocity), bidirectional movement of cQDs in such structures, indicating transport of cQD-containing vesicles along microtubule tracks by the action of molecular motors. The findings are the first to demonstrate membrane nanotube function in vivo and they suggest a previously unknown route for the distribution of nanomaterials in tissue.

Influence of Surface Modifications on the Spatiotemporal Microdistribution of Quantum Dots In Vivo.

For biomedical applications of nanoconstructs, it is a general prerequisite to efficiently reach the desired target site. In this regard, it is crucial to determine the spatiotemporal distribution of nanomaterials at the microscopic tissue level. Therefore, the effect of different surface modifications on the distribution of microinjected quantum dots (QDs) in mouse skeletal muscle tissue has been investigated. In vivo real-time fluorescence microscopy and particle tracking reveal that carboxyl QDs preferentially attach to components of the extracellular matrix (ECM), whereas QDs coated with polyethylene glycol (PEG) show little interaction with tissue constituents. Transmission electron microscopy elucidates that carboxyl QDs adhere to collagen fibers as well as basement membranes, a type of ECM located on the basolateral side of blood vessel walls. Moreover, carboxyl QDs have been found in endothelial junctions as well as in caveolae of endothelial cells, enabling them to translocate into the vessel lumen. The in vivo QD distribution is confirmed by in vitro experiments. The data suggest that ECM components act as a selective barrier depending on QD surface modification. For future biomedical applications, such as targeting of blood vessel walls, the findings of this study offer design criteria for nanoconstructs that meet the requirements of the respective application.

Multiphoton Microscopy of Nonfluorescent Nanoparticles In Vitro and In Vivo.

Nanotechnology holds great promise for a plethora of potential applications. The interaction of engineered nanomaterials with living cells, tissues, and organisms is, however, only partly understood. Microscopic investigations of nano-bio interactions are mostly performed with a few model nanoparticles (NPs) which are easy to visualize, such as fluorescent quantum dots. Here the possibility to visualize nonfluorescent NPs with multiphoton excitation is investigated. Signals from silver (Ag), titanium dioxide (TiO2 ), and silica (SiO2 ) NPs in nonbiological environments are characterized to determine signal dependency on excitation wavelength and intensity as well as their signal stability over time. Ag NPs generate plasmon-induced luminescence decaying over time. TiO2 NPs induce photoluminescent signals of variable intensities and in addition strong third harmonic generation (THG). Optimal settings for microscopic detection are determined and then applied for visualization of these two particle types in living cells, in murine muscle tissue, and in the murine blood stream. Silica NPs produce a THG signal, but in living cells it cannot be discriminated sufficiently from endogenous cellular structures. It is concluded that multiphoton excitation is a viable option for studies of nano-bio interactions not only for fluorescent but also for some types of nonfluorescent NPs.

Spatiotemporal expression dynamics of selectins govern the sequential extravasation of neutrophils and monocytes in the acute inflammatory response.

OBJECTIVE: Leukocyte recruitment to the site of inflammation is a key event in a variety of cardiovascular pathologies. Infiltrating neutrophils constitute the first line of defense that precedes a second wave of emigrating monocytes reinforcing the inflammatory reaction. The mechanisms initiating this sequential process remained largely obscure. APPROACH AND RESULTS: Using advanced in vivo microscopy and in vitro/ex vivo techniques, we identified individual spatiotemporal expression patterns of selectins and their principal interaction partners on neutrophils, resident/inflammatory monocytes, and endothelial cells. Coordinating the intraluminal trafficking of neutrophils and inflammatory monocytes to common sites of extravasation, selectins assign different sites to these immune cells for their initial interactions with the microvascular endothelium. Whereas constitutively expressed leukocyte L-selectin/CD62L and endothelial P-selectin/CD62P together with CD44 and P-selectin glycoprotein ligand-1/CD162 initiate the emigration of neutrophils, de novo synthesis of endothelial E-selectin/CD62E launches the delayed secondary recruitment of inflammatory monocytes. In this context, P-selectin/CD62P and L-selectin/CD62L together with P-selectin glycoprotein ligand-1/CD162 and CD44 were found to regulate the flux of rolling neutrophils and inflammatory monocytes, whereas E-selectin/CD62E selectively adjusts the rolling velocity of inflammatory monocytes. Moreover, selectins and their interaction partners P-selectin glycoprotein ligand-1/CD162 and CD44 differentially control the intraluminal crawling behavior of neutrophils and inflammatory monocytes collectively enabling the sequential extravasation of these immune cells to inflamed tissue. CONCLUSIONS: Our findings provide novel insights into the mechanisms initiating the sequential infiltration of the perivascular tissue by neutrophils and monocytes in the acute inflammatory response and might thereby contribute to the development of targeted therapeutic strategies for prevention and treatment of cardiovascular diseases.

Matrix metalloproteinases modulate ameboid-like migration of neutrophils through inflamed interstitial tissue.

In vitro studies suggest that leukocytes locomote in an ameboid fashion independently of pericellular proteolysis. Whether this motility pattern applies for leukocyte migration in inflamed tissue is still unknown. In vivo microscopy on the inflamed mouse cremaster muscle revealed that blockade of serine proteases or of matrix metalloproteinases (MMPs) significantly reduces intravascular accumulation and transmigration of neutrophils. Using a novel in vivo chemotaxis assay, perivenular microinjection of inflammatory mediators induced directional interstitial migration of neutrophils. Blockade of actin polymerization, but not of actomyosin contraction abolished neutrophil interstitial locomotion. Multiphoton laser scanning in vivo microscopy showed that the density of the interstitial collagen network increases in inflamed tissue, thereby providing physical guidance to infiltrating neutrophils. Although neutrophils locomote through the interstitium without pericellular collagen degradation, inhibition of MMPs, but not of serine proteases, diminished their polarization and interstitial locomotion. In this context, blockade of MMPs was found to modulate expression of adhesion/signaling molecules on neutrophils. Collectively, our data indicate that serine proteases are critical for neutrophil extravasation, whereas these enzymes are dispensable for neutrophil extravascular locomotion. By contrast, neutrophil interstitial migration strictly relies on actin polymerization and does not require the pericellular degradation of collagen fibers but is modulated by MMPs.

Tissue plasminogen activator promotes postischemic neutrophil recruitment via its proteolytic and nonproteolytic properties.

OBJECTIVE: Neutrophil infiltration of the postischemic tissue considerably contributes to organ dysfunction on ischemia/reperfusion injury. Beyond its established role in fibrinolysis, tissue-type plasminogen activator (tPA) has recently been implicated in nonfibrinolytic processes. The role of this serine protease in the recruitment process of neutrophils remains largely obscure. APPROACH AND RESULTS: Using in vivo microscopy on the postischemic cremaster muscle, neutrophil recruitment and microvascular leakage, but not fibrinogen deposition at the vessel wall, were significantly diminished in tPA(-/-) mice. Using cell transfer techniques, leukocyte and nonleukocyte tPA were found to mediate ischemia/reperfusion-elicited neutrophil responses. Intrascrotal but not intra-arterial application of recombinant tPA induced a dose-dependent increase in the recruitment of neutrophils, which was significantly higher compared with stimulation with a tPA mutant lacking catalytic activity. Whereas tPA-dependent transmigration of neutrophils was selectively reduced on the inhibition of plasmin or gelatinases, neutrophil intravascular adherence was significantly diminished on the blockade of mast cell activation or lipid mediator synthesis. Moreover, stimulation with tPA caused a significant elevation in the leakage of fluorescein isothiocyanate dextran to the perivascular tissue, which was completely abolished on neutrophil depletion. In vitro, tPA-elicited macromolecular leakage of endothelial cell layers was abrogated on the inhibition of its proteolytic activity. CONCLUSIONS: Endogenously released tPA promotes neutrophil transmigration to reperfused tissue via proteolytic activation of plasmin and gelatinases. As a consequence, tPA on transmigrating neutrophils disrupts endothelial junctions allowing circulating tPA to extravasate to the perivascular tissue, which, in turn, amplifies neutrophil recruitment through the activation of mast cells and release of lipid mediators.

C-C motif chemokine CCL3 and canonical neutrophil attractants promote neutrophil extravasation through common and distinct mechanisms.

Initial observations suggested that C-C motif chemokines exclusively mediate chemotaxis of mononuclear cells. In addition, recent studies also implicated these chemotactic cytokines in the recruitment of neutrophils. The underlying mechanisms remained largely unknown. Using in vivo microscopy on the mouse cremaster muscle, intravascular adherence and subsequent paracellular transmigration of neutrophils elicited by the chemokine (C-C motif) ligand 3 (CCL3, synonym MIP-1α) were significantly diminished in mice with a deficiency of the chemokine (C-C motif) receptor 1 (Ccr1(-/-)) or 5 (Ccr5(-/-)). Using cell-transfer techniques, neutrophil responses required leukocyte CCR1 and nonleukocyte CCR5. Furthermore, neutrophil extravasation elicited by CCL3 was almost completely abolished on inhibition of G protein-receptor coupling and PI3Kγ-dependent signaling, while neutrophil recruitment induced by the canonical neutrophil attractants chemokine (C-X-C motif) ligand 1 (CXCL1, synonym KC) or the lipid mediator platetelet-activating factor (PAF) was only partially reduced. Moreover, Ab blockade of ß(2) integrins, of α(4) integrins, or of their putative counter receptors ICAM-1 and VCAM-1 significantly attenuated CCL3-, CXCL1-, or PAF-elicited intravascular adherence and paracellular transmigration of neutrophils. These data indicate that the C-C motif chemokine CCL3 and canonical neutrophil attractants exhibit both common and distinct mechanisms for the regulation of intravascular adherence and transmigration of neutrophils.

Carbon-based nanomaterials accelerate arteriolar thrombus formation in the murine microcirculation independently of their shape.

Although carbon-based nanomaterials (CBNs) have been shown to exert prothrombotic effects in microvessels, it is poorly understood whether CBNs also have the potential to interfere with the process of leukocyte-endothelial cell interactions and whether the shape of CBNs plays a role in these processes. Thus, the aim of this study was to compare the acute effects of two differently shaped CBNs, fiber-shaped single-walled carbon nanotubes (SWCNT) and spherical ultrafine carbon black (CB), on thrombus formation as well as on leukocyte-endothelial cell interactions and leukocyte transmigration in the murine microcirculation upon systemic administration in vivo. Systemic administration of both SWCNT and CB accelerated arteriolar thrombus formation at a dose of 1 mg kg(-1) body weight, whereas SWCNT exerted a prothrombotic effect also at a lower dose (0.1 mg kg(-1) body weight). In vitro, both CBNs induced P-selectin expression on human platelets and formation of platelet-granulocyte complexes. In contrast, injection of fiber-shaped SWCNT or of spherical CB did not induce leukocyte-endothelial cell interactions or leukocyte transmigration. In vitro, both CBNs slightly increased the expression of activation markers on human monocytes and granulocytes. These findings suggest that systemic administration of CBNs accelerates arteriolar thrombus formation independently of the CBNs' shape, but does not induce leukocyte-endothelial cell interactions or leukocyte transmigration.

Transient targeting of phosphoinositide 3-kinase acts as a roadblock in mast cells' route to allergy.

BACKGROUND: Tissue mast cell numbers are dynamically regulated by recruitment of progenitors from the vasculature. It is unclear whether progenitors are recruited during allergic sensitization and whether recruitment promotes allergic responses. OBJECTIVE: We sought to (1) determine the effect of mast cell recruitment on acute allergic responses and (2) to define the role of phosphoinositide 3-kinase (PI3K) isoforms in sequential steps to allergic responses. METHODS: Gene-targeted mice for PI3Kγ or PI3Kδ or mice treated with isoform-specific PI3K inhibitors (a novel PI3Kγ-specific inhibitor [NVS-PI3-4] and the PI3Kδ inhibitor IC87114) were used to monitor IgE-mediated mast cell recruitment, migration, adhesion by means of intravital microscopy, degranulation, TNF-α release, and subsequent endothelial cell activation in vivo or in bone marrow-derived mast cells. RESULTS: Functional PI3Kγ, but not PI3Kδ, was crucial for mast cell accumulation in IgE-challenged skin, TNF-α release from IgE/antigen-stimulated mast cells, and mast cell/endothelial interactions and chemotaxis. PI3Kγ-deficient bone marrow-derived mast cells did not adhere to the endothelium in TNF-α-treated cremaster muscle, whereas PI3Kδ was not required. Depletion of TNF-α blocked IgE-induced mast cell recruitment, which links tissue mast cell-derived cytokine release to endothelial activation and mast cell recruitment. Interference with mast cell recruitment protected against anaphylaxis and was superior to blockage of tissue mast cell degranulation. CONCLUSIONS: Interference with mast cell recruitment to exacerbated tissues provides a novel strategy to alleviate allergic reactions and surpassed attenuation of tissue mast cell degranulation. This results in prolonged drug action and allows for reduction of drug doses required to block anaphylaxis, an important feature for drugs targeting inflammatory disease in general.

Endogenous and exogenous pentraxin-3 limits postischemic acute and chronic kidney injury.

Ischemia-reperfusion activates innate immunity and sterile inflammation, resulting in acute kidney injury. Since pentraxin 3 (PTX3) regulates multiple aspects of innate immunity and tissue inflammation, we tested whether PTX3 would be involved in renal ischemia-reperfusion injury. Renal pedicle clamping increased PTX3 serum levels, as well as PTX3 expression, inside the kidney but predominantly in CD45/CD11c(+) cells, a subpopulation of intrarenal mononuclear phagocytes. Lack of PTX3 aggravated postischemic acute kidney injury as evidenced by massive tubular necrosis, and TNF and IL-6 release, as well as massively increased neutrophil and macrophage infiltrates at 24 h. This was followed by tubular atrophy, interstitial fibrosis, and kidney shrinking 10 weeks later. In vivo microscopy uncovered increased leukocyte adhesion and transmigration in postischemic microvessels of Ptx3-deficient mice. Furthermore, injection of recombinant PTX3 up to 6 h after reperfusion prevented renal leukocyte recruitment and postischemic kidney injury. Thus, local PTX3 release from a subpopulation of intrarenal mononuclear phagocytes or delayed PTX3 treatment limits postischemic renal inflammation. Conversely, Ptx3 loss-of-function mutations predispose to postischemic acute kidney injury and subsequent chronic kidney disease.

Histones from dying renal cells aggravate kidney injury via TLR2 and TLR4.

In AKI, dying renal cells release intracellular molecules that stimulate immune cells to secrete proinflammatory cytokines, which trigger leukocyte recruitment and renal inflammation. Whether the release of histones, specifically, from dying cells contributes to the inflammation of AKI is unknown. In this study, we found that dying tubular epithelial cells released histones into the extracellular space, which directly interacted with Toll-like receptor (TLR)-2 (TLR2) and TLR4 to induce MyD88, NF-κB, and mitogen activated protein kinase signaling. Extracellular histones also had directly toxic effects on renal endothelial cells and tubular epithelial cells in vitro. In addition, direct injection of histones into the renal arteries of mice demonstrated that histones induce leukocyte recruitment, microvascular vascular leakage, renal inflammation, and structural features of AKI in a TLR2/TLR4-dependent manner. Antihistone IgG, which neutralizes the immunostimulatory effects of histones, suppressed intrarenal inflammation, neutrophil infiltration, and tubular cell necrosis and improved excretory renal function. In summary, the release of histones from dying cells aggravates AKI via both its direct toxicity to renal cells and its proinflammatory effects. Because the induction of proinflammatory cytokines in dendritic cells requires TLR2 and TLR4, these results support the concept that renal damage triggers an innate immune response, which contributes to the pathogenesis of AKI.

A novel approach to prevent endothelial hyperpermeability: the Crataegus extract WS® 1442 targets the cAMP/Rap1 pathway.

Endothelial hyperpermeability followed by edema formation is a hallmark of many severe disorders. Effective drugs directly targeting endothelial barrier function are widely lacking. We hypothesized that the hawthorn (Crataegus spp.) extract WS® 1442, a proven multi-component drug against moderate forms of heart failure, would prevent vascular leakage by affecting endothelial barrier-regulating systems. In vivo, WS® 1442 inhibited the histamine-evoked extravasation of FITC-dextran from mouse cremaster muscle venules. In cultured human endothelial cells, WS® 1442 blocked the thrombin-induced FITC-dextran permeability. By applying biochemical and microscopic techniques, we revealed that WS® 1442 abrogates detrimental effects of thrombin on adherens junctions (vascular endothelial-cadherin), the F-actin cytoskeleton, and the contractile apparatus (myosin light chain). Mechanistically, WS® 1442 inhibited the thrombin-induced rise of intracellular calcium (ratiometric measurement), followed by an inactivation of PKC and RhoA (pulldown assay). Moreover, WS® 1442 increased endothelial cAMP levels (ELISA), which consequently activated PKA and Rap1 (pulldown assay). Utilizing pharmacological inhibitors or siRNA, we found that PKA is not involved in barrier protection, whereas Epac1, Rap1, and Rac1 play a crucial role in the WS® 1442-induced activation of cortactin, which triggers a strong cortical actin rearrangement. In summary, WS® 1442 effectively protects against endothelial barrier dysfunction in vitro and in vivo. It specifically interacts with endothelial permeability-regulating systems by blocking the Ca(2+)/PKC/RhoA and activating the cAMP/Epac1/Rap1 pathway. As a proven safe herbal drug, WS® 1442 opens a novel pharmacological approach to treat hyperpermeability-associated diseases. This in-depth mechanistic work contributes to a better acceptance of this herbal remedy.

Urokinase-type plasminogen activator promotes paracellular transmigration of neutrophils via Mac-1, but independently of urokinase-type plasminogen activator receptor.

BACKGROUND: Urokinase-type plasminogen activator (uPA) has recently been implicated in the pathogenesis of ischemia-reperfusion (I/R) injury. The underlying mechanisms remain largely unclear. METHODS AND RESULTS: Using in vivo microscopy on the mouse cremaster muscle, I/R-elicited firm adherence and transmigration of neutrophils were found to be significantly diminished in uPA-deficient mice and in mice treated with the uPA inhibitor WX-340, but not in uPA receptor (uPAR)-deficient mice. Interestingly, postischemic leukocyte responses were significantly reduced on blockade of the integrin CD11b/Mac-1, which also serves as uPAR receptor. Using a cell transfer technique, postischemic adherence and transmigration of wild-type leukocytes were significantly decreased in uPA-deficient animals, whereas uPA-deficient leukocytes exhibited a selectively reduced transmigration in wild-type animals. On I/R or stimulation with recombinant uPA, >90% of firmly adherent leukocytes colocalized with CD31-immunoreactive endothelial junctions as detected by in vivo fluorescence microscopy. In a model of hepatic I/R, treatment with WX-340 significantly attenuated postischemic neutrophil infiltration and tissue injury. CONCLUSIONS: Our data suggest that endothelial uPA promotes intravascular adherence, whereas leukocyte uPA facilitates the subsequent paracellular transmigration of neutrophils during I/R. This process is regulated via CD11b/Mac-1, and does not require uPAR. Pharmacological blockade of uPA interferes with these events and effectively attenuates postischemic tissue injury.

ESAM supports neutrophil extravasation, activation of Rho, and VEGF-induced vascular permeability.

Endothelial cell-selective adhesion molecule (ESAM) is specifically expressed at endothelial tight junctions and on platelets. To test whether ESAM is involved in leukocyte extravasation, we have generated mice carrying a disrupted ESAM gene and analyzed them in three different inflammation models. We found that recruitment of lymphocytes into inflamed skin was unaffected by the gene disruption. However, the migration of neutrophils into chemically inflamed peritoneum was inhibited by 70% at 2 h after stimulation, recovering at later time points. Analyzing neutrophil extravasation directly by intravital microscopy in the cremaster muscle revealed that leukocyte extravasation was reduced (50%) in ESAM(-/-) mice without affecting leukocyte rolling and adhesion. Depletion of >98% of circulating platelets did not abolish the ESAM deficiency-related inhibitory effect on neutrophil extravasation, indicating that it is only ESAM at endothelial tight junctions that is relevant for the extravasation process. Knocking down ESAM expression in endothelial cells resulted in reduced levels of activated Rho, a GTPase implicated in the destabilization of tight junctions. Indeed, vascular permeability stimulated by vascular endothelial growth factor was reduced in ESAM(-/-) mice. Collectively, ESAM at endothelial tight junctions participates in the migration of neutrophils through the vessel wall, possibly by influencing endothelial cell contacts.