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Acute pulmonary embolism generally resolves within 6 months. However, if the thrombus is infected, venous thrombi transform into fibrotic vascular obstructions leading to chronic deep vein thrombosis and/or chronic thromboembolic pulmonary hypertension (CTEPH), but precise mechanisms remain unclear. Neutrophils are crucial in sequestering pathogens; therefore, we investigated the role of neutrophil extracellular traps (NETs) in chronic thrombosis. Because chronic pulmonary thrombotic obstructions are biologically identical to chronic deep venous thrombi, the murine inferior vena cava ligation model was used to study the transformation of acute to chronic thrombus. Mice with staphylococcal infection presented with larger thrombi containing more neutrophils and NETs but less resolution. Targeting NETs with DNase1 diminished fibrosis and promoted thrombus resolution. For translational studies in humans, we focused on patients with CTEPH, a severe type of deep venous and pulmonary artery fibrotic obstruction after thrombosis. Neutrophils, markers of neutrophil activation, and NET formation were increased in CTEPH patients. NETs promoted the differentiation of monocytes to activated fibroblasts with the same cellular phenotype as fibroblasts from CTEPH vascular occlusions. RNA sequencing of fibroblasts isolated from thrombo-endarterectomy specimens and pulmonary artery biopsies revealed transforming growth factor-β (TGF-β) as the central regulator, a phenotype which was replicated in mice with fibroblast-specific TGF-β overactivity. Our findings uncover a role of neutrophil-mediated inflammation to enhance TGF-β signaling, which leads to fibrotic thrombus remodeling. Targeting thrombus NETs with DNases may serve as a new therapeutic concept to treat thrombosis and prevent its sequelae., (© 2021 by The American Society of Hematology.)

Fluid shear stress in the vasculature is the driving force for natural bypass growth, a fundamental endogenous mechanism to counteract the detrimental consequences of vascular occlusive disease, such as stroke or myocardial infarction. This process, referred to as "arteriogenesis," relies on local recruitment of leukocytes, which supply growth factors to preexisting collateral arterioles enabling them to grow. Although several mechanosensing proteins have been identified, the series of mechanotransduction events resulting in local leukocyte recruitment is not understood. In a mouse model of arteriogenesis (femoral artery ligation), we found that endothelial cells release RNA in response to increased fluid shear stress and that administration of RNase inhibitor blocking plasma RNases improved perfusion recovery. In contrast, treatment with bovine pancreatic RNase A or human recombinant RNase1 interfered with leukocyte recruitment and collateral artery growth. Our results indicated that extracellular RNA (eRNA) regulated leukocyte recruitment by engaging vascular endothelial growth factor receptor 2 (VEGFR2), which was confirmed by intravital microscopic studies in a murine cremaster model of inflammation. Moreover, we found that release of von Willebrand factor (VWF) as a result of shear stress is dependent on VEGFR2. Blocking VEGFR2, RNase application, or VWF deficiency interfered with platelet-neutrophil aggregate formation, which is essential for initiating the inflammatory process in arteriogenesis. Taken together, the results show that eRNA is released from endothelial cells in response to shear stress. We demonstrate this extracellular nucleic acid as a critical mediator of mechanotransduction by inducing the liberation of VWF, thereby initiating the multistep inflammatory process responsible for arteriogenesis., (© 2019 by The American Society of Hematology.)

The tight electrostatic binding of the chemokine platelet factor 4 (PF4) to polyanions induces heparin-induced thrombocytopenia, a prothrombotic adverse drug reaction caused by immunoglobulin G directed against PF4/polyanion complexes. This study demonstrates that nucleic acids, including aptamers, also bind to PF4 and enhance PF4 binding to platelets. Systematic assessment of RNA and DNA constructs, as well as 4 aptamers of different lengths and secondary structures, revealed that increasing length and double-stranded segments of nucleic acids augment complex formation with PF4, while single nucleotides or single-stranded polyA or polyC constructs do not. Aptamers were shown by circular dichroism spectroscopy to induce structural changes in PF4 that resemble those induced by heparin. Moreover, heparin-induced anti-human-PF4/heparin antibodies cross-reacted with human PF4/nucleic acid and PF4/aptamer complexes, as shown by an enzyme immunoassay and a functional platelet activation assay. Finally, administration of PF4/44mer-DNA protein C aptamer complexes in mice induced anti-PF4/aptamer antibodies, which cross-reacted with murine PF4/heparin complexes. These data indicate that the formation of anti-PF4/heparin antibodies in postoperative patients may be augmented by PF4/nucleic acid complexes. Moreover, administration of therapeutic aptamers has the potential to induce anti-PF4/polyanion antibodies and a prothrombotic diathesis.

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a regulator of energy balance at the cellular and whole-body levels, but little is known about the role of AMPK in platelet activation. We report that both the α1 and α2 AMPK isoforms are expressed by human and murine platelets and that thrombin elicits the phosphorylation of AMPKα as well as the upstream kinase, liver kinase B1 (LKB1). In human platelets, the kinase inhibitors iodotubercidin and compound C significantly inhibited thrombin-induced platelet aggregation and clot retraction without affecting the initial increase in [Ca(2+)](i). Clot retraction was also impaired in platelets from AMPKα2(-/-) mice but not from wild-type littermates or AMPKα1(-/-) mice. Moreover, rebleeding was more frequent in AMPKα2(-/-) mice, and the FeCl(3)-induced thrombi formed in AMPKα2(-/-) mice were unstable. Mechanistically, AMPKα2 was found to phosphorylate in vitro the Src-family kinase, Fyn, and isoform deletion resulted in the attenuated threonine phosphorylation of Fyn as well as the subsequent tyrosine phosphorylation of its substrate, β3 integrin. These data indicate that AMPKα2-by affecting Fyn phosphorylation and activity-plays a key role in platelet αIIbβ3 integrin signaling, leading to clot retraction and thrombus stability.

Cell surface-associated proteolysis plays a crucial role in the migration of mononuclear phagocytes to sites of inflammation. The glycolytic enzyme enolase-1 (ENO-1) binds plasminogen at the cell surface, enhancing local plasmin production. This study addressed the role played by ENO-1 in lipopolysaccharide (LPS)-driven chemokine-directed monocyte migration and matrix invasion in vitro, as well as recruitment of monocytes to the alveolar compartment in vivo. LPS rapidly up-regulated ENO-1 cell-surface expression on human blood monocytes and U937 cells due to protein translocation from cytosolic pools, which increased plasmin generation, enhanced monocyte migration through epithelial monolayers, and promoted matrix degradation. These effects were abrogated by antibodies directed against the plasminogen binding site of ENO-1. Overexpression of ENO-1 in U937 cells increased their migratory and matrix-penetrating capacity, which was suppressed by overexpression of a truncated ENO-1 variant lacking the plasminogen binding site (ENO-1DeltaPLG). In vivo, intratracheal LPS application in mice promoted alveolar recruitment of monocytic cells that overexpressed ENO-1, but not of cells overexpressing ENO-1DeltaPLG. Consistent with these data, pneumonia-patients exhibited increased ENO-1 cell-surface expression on blood monocytes and intense ENO-1 staining of mononuclear cells in the alveolar space. These data suggest an important mechanism of inflammatory cell invasion mediated by increased cell-surface expression of ENO-1.

Various virus infections cause dysfunctional hemostasis and in some instances lead to the development of viral hemorrhagic fever syndrome. How do diverse viruses induce the expression of tissue factor on vascular cells? We hypothesize that a direct stimulation of pattern recognition receptors (PRR) by viral nucleic acids may be the key. Double-stranded RNA (dsRNA) is produced by many viruses and is recognized by various PRR, including Toll-like receptor-3 (TLR3). We have investigated whether poly I:C, a model for viral dsRNA, can influence cellular hemostasis. Poly I:C could up-regulate tissue factor and down-regulate thrombomodulin expression on endothelial cells but not on monocytes. The response to poly I:C was diminished upon small interfering RNA (siRNA)-mediated inhibition of TLR3, but not other PRR. In vivo, application of poly I:C induced similar changes in the aortic endothelium of mice as determined by enface microscopy. D-dimer, a circulating marker for enhanced coagulation and fibrinolysis, and tissue fibrin deposition was elevated. All the hemostasis-related responses to poly I:C, but not cytokine secretion, were blunted in TLR3(-/-) mice. Hence, the activation of TLR3 can induce the procoagulant state in the endothelium, and this could be relevant for understanding the mechanisms of viral stimulation of hemostasis.

Cell injury leads to exposure of intracellular material and is associated with increased permeability of vessels in the vicinity of the damage. Here, we demonstrate that natural extracellular RNA as well as artificial RNA (poly-I:C), or single-stranded RNA but not DNA, significantly increased the permeability across brain microvascular endothelial cells in vitro and in vivo. RNA-induced hyperpermeability of tight monolayers of endothelial cells correlated with disintegration of tight junctions and was mediated through vascular endothelial growth factor (VEGF), reminiscent of heparin's activities. Antisense oligonucleotides against VEGF-receptor 2 (VEGF-R2) prevented the permeability-inducing activity of extracellular RNA and heparin completely. Hence, these polyanionic substances can lead to mobilization/stabilization of VEGF with the subsequent activation of VEGF-R2. In accordance with these functional data, strong binding of VEGF as well as other growth factors to RNA was demonstrable. In in vivo rat models of FeCl(3)-induced sinus sagittal is superior thrombosis and stroke/brain edema, pretreatment of animals with RNase (but not DNase) resulted in a significant reduction of vessel occlusion, infarct volume, and prevention of brain edema formation. Together, these results identify extracellular RNA as a novel natural permeability factor, upstream of VEGF, whereas counteracting RNase treatment may serve as new vessel-protective modality.

Staphylococcus aureus is a major human pathogen interfering with host-cell functions. Impaired wound healing is often observed in S aureus-infected wounds, yet, the underlying mechanisms are poorly defined. Here, we identify the extracellular adherence protein (Eap) of S aureus to be responsible for impaired wound healing. In a mouse wound-healing model wound closure was inhibited in the presence of wild-type S aureus and this effect was reversible when the wounds were incubated with an isogenic Eap-deficient strain. Isolated Eap also delayed wound closure. In the presence of Eap, recruitment of inflammatory cells to the wound site as well as neovascularization of the wound were prevented. In vitro, Eap significantly reduced intercellular adhesion molecule 1 (ICAM-1)-dependent leukocyte-endothelial interactions and diminished the consequent activation of the proinflammatory transcription factor nuclear factor kappaB (NFkappaB) in leukocytes associated with a decrease in expression of tissue factor. Moreover, Eap blocked alphav-integrin-mediated endothelial-cell migration and capillary tube formation, and neovascularization in matrigels in vivo. Collectively, the potent anti-inflammatory and antiangiogenic properties of Eap provide an underlying mechanism that may explain the impaired wound healing in S aureus-infected wounds. Eap may also serve as a lead compound for new anti-inflammatory and antiangiogenic therapies in several pathologies.

Proliferative retinopathies, such as those complicating prematurity and diabetes, are major causes of blindness. A prominent feature of these retinopathies is excessive neovascularization, which is orchestrated by the hypoxia-induced vascular endothelial growth factor (VEGF) stimulating endothelial cells and the integrin-mediated adhesive interactions of endothelial cells with extracellular matrix components such as fibronectin (FN). Recently, we demonstrated that alpha-defensins interfere with alpha5beta1-FN interactions and dependent endothelial cell functions. Here, alpha-defensins were studied in hypoxia-induced proliferative retinopathy. In vitro, alpha-defensins specifically inhibited alpha5beta1-integrin-dependent migration of bovine retinal endothelial cells (BRECs) to FN, attenuated the VEGF-stimulated increase in endothelial permeability, and blocked BREC proliferation and capillary sprout formation in 3-dimensional fibrin-matrices. An up-regulation of beta1-integrin and FN was observed in the retinal vessels in the mouse model of hypoxia-induced retinal angiogenesis. Systemic and local administration of alpha-defensins reduced retinal neovascularization by 45% and 60%, respectively, and this effect was comparable to the inhibitory effect of alpha5beta1-blocking antibody. alpha-Defensins were detected in human diabetic retinas associated with normal retinal vessels but were absent from proliferative lesions. Together, these data show that alpha-defensins inhibit pathologic retinal neovascularization in vivo and may provide a clinically efficient strategy against proliferative retinopathies.

Angiogenesis and inflammation are closely related biologic processes in wound healing and the responses to vascular injury as well as in cardiovascular diseases; however, the molecular connections are poorly defined. In particular, it is yet unclear whether endogenous factors can regulate both angiogenesis and inflammation. Here, we show that the endogenous angiogenesis inhibitor, angiostatin (containing kringle domains 1-4 of plasminogen), serves an anti-inflammatory role, since the kringles 1-3 and its kringle 4 directly interact with leukocyte beta1- and beta2-integrins, respectively. In particular, a specific interaction between kringle 4 and alphaMbeta2-integrin (Mac-1) but not leukocyte function antigen 1 (LFA-1) was identified. Angiostatin thereby inhibited beta1- and beta2-integrin-mediated adhesion of leukocytes to extracellular matrix proteins and the endothelium as well as their transmigration through the endothelium in vitro. Moreover, angiostatin blocked the peritonitis-induced neutrophil emigration in vivo. In addition, through its interaction with Mac-1, angiostatin reduced activation of the proinflammatory transcription factor nuclear factor kappaB (NFkappaB), as well as the NFkappaB-related expression of tissue factor, a potent initiator of hemostasis following vascular injury. Finally, angiostatin forms were generated in vivo following skin injury/inflammation and were detectable during the following entire period of wound healing peaking at the terminal phase of the healing process. Taken together, over and above inhibition of neovascularization, angiostatin was identified as an antiadhesive/anti-inflammatory substance. These observations could provide the basis for new therapeutic applications of angiostatin to target chronic inflammatory processes in different pathologic situations.

The urokinase receptor (urokinase plasminogen activator receptor; uPAR) regulates monocyte adhesion by direct binding to vitronectin and by forming complexes with integrins. Therefore, possible up-regulation of uPAR in acute myocardial infarction (AMI) may affect monocyte adhesion. In 20 patients with AMI, uPAR surface expression (measured by flow cytometry) was increased compared with that in patients with chronic stable angina (mean +/- SD fluorescence, 179 +/- 96 vs 80 +/- 53; P =.002). Expression of uPAR correlated with activation of beta(2)-integrins lymphocyte function-associated antigen 1 (LFA-1) and macrophage antigen 1 (Mac-1), measured by using monoclonal antibodies (mAbs) 24 and CBRM1/5. Isolated mononuclear cells (MNCs) from patients with AMI showed enhanced adhesiveness to human umbilical vein endothelial cells (HUVECs), to fibrinogen (Mac-1 ligand), and to vitronectin (uPAR ligand). Excessive adhesion of MNCs to HUVECs was inhibited by mAbs anti-CD18 (84%), anti-CD11a (51%), and anti-CD11b (57%), indicating a major contribution of LFA-1 and Mac-1. The mAb anti-uPAR R3 blocked adhesion of cells from patients with AMI to vitronectin (95%) but also beta(2)-integrin-mediated adhesion to fibrinogen (79%) and HUVECs (66%). Incubation of monocytic MonoMac6 cells with plasma from patients with AMI enhanced uPAR messenger RNA expression and cell adhesion to HUVECs. Thus, released soluble factors may contribute to enhanced monocyte adhesion in AMI. Mouse pre-B lymphocytes (BAF3 cells) transfected with various amounts of uPAR complementary DNA showed a strong correlation of uPAR expression with beta(2)-integrin-dependent adhesion to intercellular adhesion molecule 1, thus providing evidence for the functional relevance of uPAR up-regulation in an isolated in vitro system. In conclusion, we found that uPAR expression is elevated on monocytes in AMI and contributes to enhanced cell adhesion. Thus, uPAR may be a novel target for prevention of unwanted monocyte recruitment as part of inflammatory cardiovascular processes.

During acute inflammatory processes, beta(2) and beta(1) integrins sequentially mediate leukocyte recruitment into extravascular tissues. We studied the influence of VLA-4 (very late antigen-4) (alpha(4)beta(1)) engagement on beta(2) integrin activation-dependent cell-to-cell adhesion. Ligation of VLA-4 by the soluble chimera fusion product vascular cell adhesion molecule-1 (VCAM-1)-Fc or by 2 anti-CD29 (beta(1) chain) monoclonal antibodies (mAb) rapidly induced adhesion of myelomonocytic cells (HL60, U937) to human umbilical vein endothelial cells (HUVECs). Cell adhesion was mediated via beta(2) integrin (LFA-1 and Mac-1) activation: induced adhesion to HUVECs was inhibited by blocking mAbs anti-CD18 (70%-90%), anti-CD11a (50%-60%), or anti-CD11b (60%-70%). Adhesion to immobilized ligands of beta(2) integrins (intercellular adhesion molecule-1 [ICAM-1], fibrinogen, keyhole limpet hemocyanin) as well as to ICAM-1-transfected Chinese hamster ovary cells, but not to ligands of beta(1) integrins (VCAM-1, fibronectin, laminin, and collagen), was augmented. VCAM-1-Fc binding provoked the expression of the activation-dependent epitope CBRM1/5 of Mac-1 on leukocytes. Clustering of VLA-4 through dimeric VCAM-1-Fc was required for beta(2) integrin activation and induction of cell adhesion, whereas monovalent VCAM-1 or Fab fragments of anti-beta(1) integrin mAb were ineffective. Activation of beta(2) integrins by alpha(4)beta(1) integrin ligation (VCAM-1-Fc or anti-beta(1) mAb) required the presence of urokinase receptor (uPAR) on leukocytic cells, because the removal of uPAR from the cell surface by phosphatidylinositol-specific phospholipase C reduced cell adhesion to less than 40%. Adhesion was reconstituted when soluble recombinant uPAR was allowed to reassociate with the cells. Finally, VLA-4 engagement by VCAM-1-Fc or anti-beta(1) integrin mAb induced uPAR-dependent adhesion to immobilized vitronectin as well. These results elucidate a novel activation pathway of beta(2) integrin-dependent cell-to-cell adhesion that requires alpha(4)beta(1) integrin ligation for initiation and uPAR as activation transducer. (Blood. 2000;96:506-513)

Proteolytic cleavage of single-chain high molecular weight kininogen (HK) by kallikrein releases the short-lived vasodilator bradykinin and leaves behind 2-chain high molecular weight kininogen (HKa) that has been previously reported to exert antiadhesive properties as well as to bind to the urokinase receptor (uPAR) on endothelial cells. In this study we defined the molecular mechanisms for the antiadhesive effects of HKa related to disruption of integrin- and uPAR-mediated cellular interactions. Vitronectin (VN) but not fibrinogen or fibronectin-dependent alphavbeta(3) integrin-mediated adhesion of endothelial cells was blocked by HKa or its isolated domain 5. In a purified system, HKa but not HK competed for the interaction of VN with alphavbeta(3) integrin, because HKa and the isolated domain 5 but not HK bound to both multimeric and native VN in a Zn(2+)-dependent manner. The interaction between HKa or domain 5 with VN was prevented by heparin, plasminogen activator inhibitor-1, and a recombinant glutathione-S-transferase (GST)-fusion peptide GST-VN (1-77) consisting of the amino terminal portion of VN (amino acids 1-77), but not by a cyclic arginyl-glycyl-aspartyl peptide, indicating that HKa interacts with the amino terminal portion of VN ("somatomedin B region"). Furthermore, we have confirmed that HKa but not HK bound to uPAR and to the truncated 2-domain form of uPAR lacking domain 1 in a Zn(2+)-dependent manner. Through these interactions, HKa or its recombinant His-Gly-Lys-rich domain 5 completely inhibited the uPAR-dependent adhesion of myelomonocytic U937 cells and uPAR-transfected BAF-3 cells to VN and thereby promoted cell detachment. By immunogold electron microscopy, both VN and HK/HKa were found to be colocalized in sections from human atherosclerotic coronary artery, indicating that the described interactions are likely to take place in vivo. Taken together, HK and HKa inhibit different VN-responsive adhesion receptor systems and may thereby influence endothelial cell- or leukocyte-related interactions in the vasculature, particularly under inflammatory conditions. (Blood. 2000;96:514-522)

The trace element Zinc (Zn2+) has been implicated as a mediator in host defense, yet the molecular basis for its extracellular functions remains obscure. Here, we demonstrate that Zn2+ can induce the adhesion of myelomonocytic cells to the endothelium, as well as to the provisional matrix proteins vitronectin (VN) and fibrinogen (FBG), which are pivotal steps for the recruitment of leukocytes into inflamed/injured tissue. Physiologic concentrations of Zn2+ increased the urokinase receptor (uPAR)-mediated adhesion of myelomonocytic cells to VN, whereas other divalent cations had smaller effects. Zn2+-induced cell adhesion to VN was abolished by cation chelators such as 1-10-phenanthroline, as well as by plasminogen activator inhibitor-1 (PAI-1) and a monoclonal antibody (MoAb) against uPAR. These characteristics could be recapitulated with a uPAR-transfected cell line emphasizing the specificity of this receptor system for Zn2+-dependent cell adhesion. Like urokinase (uPA), Zn2+ increased the binding of radiolabeled VN to uPAR-expressing cells, as well as the interaction of VN with immobilized uPAR in an isolated system. Moreover, Zn2+ enhanced leukocytic cell adhesion to FBG and endothelial cell monolayers by activating beta2-integrins. Instead of the direct beta2-integrin activation through the divalent cation binding site, Zn2+-induced integrin activation was mediated via uPAR, a crucial regulator of this system. The present study uncovers for the first time Zn2+-mediated cell adhesion mechanisms that may play a crucial role in modulating leukocyte adhesion to vessel wall components.

Urokinase-type-plasminogen activator (uPA) and its receptor are localized in the vessel wall where they are involved in cellular activation and remodelling processes. Besides the cell surface glycolipid (GPI)-anchored urokinase receptor (uPAR), which binds uPA with high affinity, recent evidence points to the existence of soluble uPAR (suPAR), as well. In the present study, the origin, binding mechanism, and cellular effects of suPAR were examined. Under basal conditions human vascular smooth muscle cells (HVSMC), human umbilical vein endothelial cells (HUVEC), and monocytic cells released 0.1 to 2 ng/mL suPAR, which was increased twofold to fivefold after phorbol ester (PMA) stimulation, as measured by a function-dependent enzyme-linked immunosorbent assay (ELISA). suPAR alone did not bind to HVSMC or HUVEC, but reduced cellular uPA binding by 50% to 70%. However, after removal of GPI-uPAR with phosphatidylinositol-specific phospholipase C, suPAR dose-dependently increased uPA binding by fourfold to fivefold. This increase in binding was completely inhibited by vitronectin (VN) and by a monoclonal antibody against VN, but not by other matrix proteins or antibodies. Thus, VN-mediated uPA binding to cells was regulated by the ratio of soluble to surface-associated uPAR. In a uPAR-deficient cell line (LM-TK-), suPAR increased uPA binding up to 10-fold, whereas the truncated receptor lacking the amino-terminal uPA-binding domain was ineffective. The formation of a ternary uPA/suPAR/VN-complex on the cell surface and the free extracellular matrix could be inhibited by a monoclonal antibody against VN, as well as by plasminogen activator inhibitor-1 (PAI-1). Moreover, VN-mediated binding of the uPA/suPAR-complex led to a fivefold increase in plasminogen activator activity. Through this novel pathway, VN concentrates the uPA/suPAR-complex to cell surfaces and extracellular matrix sites, leading to the accumulation of plasminogen activator activity required for cell migration and tissue remodelling processes.