High molecular weight kininogen regulates platelet-leukocyte interactions by bridging Mac-1 and glycoprotein Ib

Serum bridging molecules drive candidal invasion of human but not mouse endothelial cells

During hematogenously disseminated candidiasis, blood borne fungi must invade the endothelial cells that line the blood vessels to infect the deep tissues. Although Candida albicans, which forms hyphae, readily invades endothelial cells, other medically important species of Candida are poorly invasive in standard in vitro assays and have low virulence in immunocompetent mouse models of disseminated infection. Here, we show that Candida glabrata, Candida tropicalis, Candida parapsilosis, and Candida krusei can bind to vitronectin and high molecular weight kininogen present in human serum. Acting as bridging molecules, vitronectin and kininogen bind to αv integrins and the globular C1q receptor (gC1qR), inducing human endothelial cells to endocytose the fungus. This mechanism of endothelial cell invasion is poorly supported by mouse endothelial cells but can be restored when mouse endothelial cells are engineered to express human gC1qR or αv integrin. Overall, these data indicate that bridging molecule-mediated endocytosis is a common pathogenic strategy used by many medically important Candida spp. to invade human vascular endothelial cells.

Complement, inflammation and thrombosis

A mutual relationship exists between immune activation and mechanisms of thrombus formation. In particular, elements of the innate immune response such as the complement system can modulate platelet activation and subsequently thrombus formation. Several components of the complement system including C3 or the membrane attack complex have been reported to be associated with platelets and become functionally active in the micromilieu of platelet activation. The exact mechanisms how this interplay is regulated and its consequences for tissue inflammation, damage or recovery remain to be defined. This review addresses the current state of knowledge on this topic and puts it into context with diseases featuring both thrombosis and complement activation. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

CD147 is a Novel Interaction Partner of Integrin αMβ2 Mediating Leukocyte and Platelet Adhesion

Surface receptor-mediated adhesion is a fundamental step in the recruitment of leukocytes and platelets, as well as platelet-leukocyte interactions. The surface receptor CD147 is crucially involved in host defense against self-derived and invading targets, as well as in thrombosis. In the current study, we describe the previously unknown interaction of CD147 with integrin αMβ2 (Mac-1) in this context. Using binding assays, we were able to show a stable interaction of CD147 with Mac-1 in vitro. Leukocytes from Mac-1^-/- and CD147^+/- mice showed a markedly reduced static adhesion to CD147- and Mac-1-coated surfaces, respectively, compared to wild-type mice. Similarly, we observed reduced rolling and adhesion of monocytes under flow conditions when cells were pre-treated with antibodies against Mac-1 or CD147. Additionally, as assessed by antibody inhibition experiments, CD147 mediated the dynamic adhesion of platelets to Mac-1-coated surfaces. The interaction of CD147 with Mac-1 is a previously undescribed mechanism facilitating the adhesion of leukocytes and platelets.

Inhibition of IP6K1 suppresses neutrophil-mediated pulmonary damage in bacterial pneumonia

The significance of developing host-modulating personalized therapies to counteract the growing threat of antimicrobial resistance is well-recognized because such resistance cannot be overcome using microbe-centered strategies alone. Immune host defenses must be finely controlled during infection to balance pathogen clearance with unwanted inflammation-induced tissue damage. Thus, an ideal antimicrobial treatment would enhance bactericidal activity while preventing neutrophilic inflammation, which can induce tissue damage. We report that disrupting the inositol hexakisphosphate kinase 1 (Ip6k1) gene or pharmacologically inhibiting IP6K1 activity using the specific inhibitor TNP [N2-(m-(trifluoromethyl)benzyl) N6-(p-nitrobenzyl)purine] efficiently and effectively enhanced host bacterial killing but reduced pulmonary neutrophil accumulation, minimizing the lung damage caused by both Gram-positive and Gram-negative bacterial pneumonia. IP6K1-mediated inorganic polyphosphate (polyP) production by platelets was essential for infection-induced neutrophil-platelet aggregate (NPA) formation and facilitated neutrophil accumulation in alveolar spaces during bacterial pneumonia. IP6K1 inhibition reduced serum polyP levels, which regulated NPAs by triggering the bradykinin pathway and bradykinin-mediated neutrophil activation. Thus, we identified a mechanism that enhances host defenses while simultaneously suppressing neutrophil-mediated pulmonary damage in bacterial pneumonia. IP6K1 is, therefore, a legitimate therapeutic target for such disease.

Cell Receptor and Cofactor Interactions of the Contact Activation System and Factor XI

The contact activation system (CAS) or contact pathway is central to the crosstalk between coagulation and inflammation and contributes to diverse disorders affecting the cardiovascular system. CAS initiation contributes to thrombosis but is not required for hemostasis and can trigger plasma coagulation via the intrinsic pathway [through factor XI (FXI)] and inflammation via bradykinin release. Activation of factor XII (FXII) is the principal starting point for the cascade of proteolytic cleavages involving FXI, prekallikrein (PK), and cofactor high molecular weight kininogen (HK) but the precise location and cell receptor interactions controlling these reactions remains unclear. FXII, PK, FXI, and HK utilize key protein domains to mediate binding interactions to cognate cell receptors and diverse ligands, which regulates protease activation. The assembly of contact factors has been demonstrated on the cell membranes of a variety of cell types and microorganisms. The cooperation between the contact factors and endothelial cells, platelets, and leukocytes contributes to pathways driving thrombosis yet the basis of these interactions and the relationship with activation of the contact factors remains undefined. This review focuses on cell receptor interactions of contact proteins and FXI to develop a cell-based model for the regulation of contact activation.

Crosstalk between platelets and the complement system in immune protection and disease

Platelets have a central function in repairing vascular damage and stopping acute blood loss. They are equally central to thrombus formation in cardiovascular diseases such as myocardial infarction and ischaemic stroke. Beyond these classical prothrombotic diseases, immune mediated pathologies such as haemolytic uraemic syndrome (HUS) or paroxysmal nocturnal haemoglobinuria (PNH) also feature an increased tendency to form thrombi in various tissues. It has become increasingly clear that the complement system, part of the innate immune system, has an important role in the pathophysiology of these diseases. Not only does complement influence prothrombotic disease, it is equally involved in idiopathic thrombocytopenic purpura (ITP), an autoimmune disease characterised by thrombocytopenia. Thus, there are complex interrelationships between the haemostatic and immune systems, and platelets and complement in particular. Not only does complement influence platelet diseases such as ITP, HUS and PNH, it also mediates interaction between microbes and platelets during systemic infection, influencing the course of infection and development of protective immunity. This review aims to provide an integrative overview of the mechanisms underlying the interactions between complement and platelets in health and disease.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

Platelets and platelet adhesion molecules: novel mechanisms of thrombosis and anti-thrombotic therapies

Platelets are central mediators of thrombosis and hemostasis. At the site of vascular injury, platelet accumulation (i.e. adhesion and aggregation) constitutes the first wave of hemostasis. Blood coagulation, initiated by the coagulation cascades, is the second wave of thrombin generation and enhance phosphatidylserine exposure, can markedly potentiate cell-based thrombin generation and enhance blood coagulation. Recently, deposition of plasma fibronectin and other proteins onto the injured vessel wall has been identified as a new "protein wave of hemostasis" that occurs prior to platelet accumulation (i.e. the classical first wave of hemostasis). These three waves of hemostasis, in the event of atherosclerotic plaque rupture, may turn pathogenic, and cause uncontrolled vessel occlusion and thrombotic disorders (e.g. heart attack and stroke). Current anti-platelet therapies have significantly reduced cardiovascular mortality, however, on-treatment thrombotic events, thrombocytopenia, and bleeding complications are still major concerns that continue to motivate innovation and drive therapeutic advances. Emerging evidence has brought platelet adhesion molecules back into the spotlight as targets for the development of novel anti-thrombotic agents. These potential antiplatelet targets mainly include the platelet receptors glycoprotein (GP) Ib-IX-V complex, β3 integrins (αIIb subunit and PSI domain of β3 subunit) and GPVI. Numerous efforts have been made aiming to balance the efficacy of inhibiting thrombosis without compromising hemostasis. This mini-review will update the mechanisms of thrombosis and the current state of antiplatelet therapies, and will focus on platelet adhesion molecules and the novel anti-thrombotic therapies that target them.

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.

This study identifies a previously unanticipated role for platelets as pathfinders, guiding leukocytes to the sites at which they can exit the microvasculature; this process appears to be critical for an effective immune response.

White blood cells (leukocytes) are the effector cells of the immune system. The movement (extravasation) of leukocytes from the bloodstream to the surrounding tissue is a prerequisite for proper host defense. Platelets are anucleate cell particles that circulate in the blood and play a fundamental role in hemostasis. Here, we report a previously unrecognized function of platelets as "pathfinders" guiding leukocytes to their site of extravasation. Upon onset of the inflammatory response, platelets were found to immediately adhere to specific sites in the smallest venular microvessels. At these "hot spots", platelets capture intravascularly crawling neutrophils and, in turn, inflammatory monocytes. The cellular crosstalk arising from these interactions leads to conformational changes of distinct adhesion molecules on the surface of leukocytes, subsequently promoting the extravasation of these immune cells to the inflamed tissue. Conversely, blockade of this cellular partnership leads to misguided and inefficient leukocyte responses. Thus, platelet-directed guidance of leukocytes to confined sites of extravasation appears to be a critical step in the recruitment process of immune cells, which might emerge as a promising therapeutic target for the prevention and treatment of inflammatory pathologies.

Developmental endothelial locus-1 modulates platelet-monocyte interactions and instant blood-mediated inflammatory reaction in islet transplantation

Platelet-monocyte interactions are strongly implicated in thrombo-inflammatory injury by actively contributing to intravascular inflammation, leukocyte recruitment to inflamed sites, and the amplification of the procoagulant response. Instant blood-mediated inflammatory reaction (IBMIR) represents thrombo-inflammatory injury elicited upon pancreatic islet transplantation (islet-Tx), thereby dramatically affecting transplant survival and function. Developmental endothelial locus-1 (Del-1) is a functionally versatile endothelial cell-derived homeostatic factor with anti-inflammatory properties, but its potential role in IBMIR has not been previously addressed. Here, we establish Del-1 as a novel inhibitor of IBMIR using a whole blood-islet model and a syngeneic murine transplantation model. Indeed, Del-1 pre-treatment of blood before addition of islets diminished coagulation activation and islet damage as assessed by C-peptide release. Consistently, intraportal islet-Tx in transgenic mice with endothelial cell-specific overexpression of Del-1 resulted in a marked decrease of monocytes and platelet-monocyte aggregates in the transplanted tissues, relative to those in wild-type recipients. Mechanistically, Del-1 decreased platelet-monocyte aggregate formation, by specifically blocking the interaction between monocyte Mac-1-integrin and platelet GPIb. Our findings reveal a hitherto unknown role of Del-1 in the regulation of platelet-monocyte interplay and the subsequent heterotypic aggregate formation in the context of IBMIR. Therefore, Del-1 may represent a novel approach to prevent or mitigate the adverse reactions mediated through thrombo-inflammatory pathways in islet-Tx and perhaps other inflammatory disorders involving platelet-leukocyte aggregate formation.

Contact activation of C3 enables tethering between activated platelets and polymorphonuclear leukocytes via CD11b/CD18

Complement component C3 has a potential role in thrombotic pathologies. It is transformed, without proteolytic cleavage, into C3(H2O) upon binding to the surface of activated platelets. We hypothesise that C3(H2O) bound to activated platelets and to platelet-derived microparticles (PMPs) contributes to platelet-PMN complex (PPC) formation and to the binding of PMPs to PMNs. PAR-1 activation of platelets in human whole blood from normal individuals induced the formation of CD16+/CD42a+ PPC. The complement inhibitor compstatin and a C5a receptor antagonist inhibited PPC formation by 50 %, while monoclonal antibodies to C3(H2O) or anti-CD11b inhibited PPC formation by 75-100 %. Using plasma protein-depleted blood and blood from a C3-deficient patient, we corroborated the dependence on C3, obtaining similar results after reconstitution with purified C3. By analogy with platelets, PMPs isolated from human serum were found to expose C3(H2O) and bind to PMNs. This interaction was also blocked by the anti-C3(H2O) and anti-CD11b monoclonal antibodies, indicating that C3(H2O) and CD11b are involved in tethering PMPs to PMNs. We confirmed the direct interaction between C3(H2O) and CD11b by quartz crystal microbalance analysis using purified native C3 and recombinant CD11b/CD18 and by flow cytometry using PMP and recombinant CD11b. Transfectants expressing CD11b/CD18 were also shown to specifically adhere to surface-bound C3(H2O). We have identified contact-activated C3(H2O) as a novel ligand for CD11b/CD18 that mediates PPC formation and the binding of PMPs to PMNs. Given the various roles of C3 in thrombotic reactions, this finding is likely to have important pathophysiological implications.

Leukocyte integrins: role in leukocyte recruitment and as therapeutic targets in inflammatory disease

Infection or sterile inflammation triggers site-specific attraction of leukocytes. Leukocyte recruitment is a process comprising several steps orchestrated by adhesion molecules, chemokines, cytokines and endogenous regulatory molecules. Distinct adhesive interactions between endothelial cells and leukocytes and signaling mechanisms contribute to the temporal and spatial fine-tuning of the leukocyte adhesion cascade. Central players in the leukocyte adhesion cascade include the leukocyte adhesion receptors of the β2-integrin family, such as the αLβ2 and αMβ2 integrins, or of the β1-integrin family, such as the α4β1-integrin. Given the central involvement of leukocyte recruitment in different inflammatory and autoimmune diseases, the leukocyte adhesion cascade in general, and leukocyte integrins in particular, represent key therapeutic targets. In this context, the present review focuses on the role of leukocyte integrins in the leukocyte adhesion cascade. Experimental evidence that has implicated leukocyte integrins as targets in animal models of inflammatory disorders, such as experimental autoimmune encephalomyelitis, psoriasis, inflammatory bone loss and inflammatory bowel disease as well as preclinical and clinical therapeutic applications of antibodies that target leukocyte integrins in various inflammatory disorders are presented. Finally, we review recent findings on endogenous inhibitors that modify leukocyte integrin function, which could emerge as promising therapeutic targets.

Sex differences of leukocyte-platelet interactions and on-treatment platelet reactivity in patients with atherosclerosis

OBJECTIVE

To investigate differences of platelet activation and on-treatment residual platelet reactivity between female and male patients with atherosclerotic cardiovascular disease.

METHODS

We compared P-selectin expression, activated glycoprotein (GP) IIb/IIIa and leukocyte-platelet aggregates (LPA) by flow cytometry between 110 female and 206 male patients undergoing angioplasty and stenting. On-treatment residual platelet reactivity was determined by two test systems.

RESULTS

The expression of P-selectin and GPIIb/IIIa did not differ significantly between female and male patients. In contrast, females showed a significantly more pronounced formation of LPA in vivo, in response to thrombin receptor-activating peptide-6 and in response to adenosine diphosphate. Further, high LPA were seen more frequently in female patients. Finally, protease-activated receptor (PAR)-1 mediated platelet reactivity by both assays was significantly higher in females.

CONCLUSION

Female sex is associated with a more pronounced formation of LPA and increased PAR-1 mediated platelet reactivity in atherosclerotic cardiovascular disease.

A novel pathway of rapid TLR-triggered activation of integrin-dependent leukocyte adhesion that requires Rap1 GTPase

TLR2 and TLR5 ligation directly induces β2-integrin activation, promoting cell adhesion to ICAM-1. Systemic in vivo administration of the TLR2 ligand Pam3CSK4 increases integrin-dependent adhesion to endothelium within minutes. The signaling pathway linking TLR ligation with β2-integin activation involves Rac-1, NADPH oxidase 2, and Rap1-GTPase.

Rapid β2-integrin activation is indispensable for leukocyte adhesion and recruitment to sites of infection and is mediated by chemokine- or P-selectin glycoprotein ligand-1–induced inside-out signaling. Here we uncovered a novel pathway for rapid activation of integrin-dependent leukocyte adhesion, triggered by toll-like receptor (TLR)–mediated signaling. TLR2 or TLR5 ligation rapidly activated integrin-dependent leukocyte adhesion to immobilized ICAM-1 and fibronectin. Consistently, in vivo administration of the TLR2-ligand Pam3CSK4 increased integrin-dependent slow rolling and adhesion to endothelium within minutes, as identified by intravital microscopy in the cremaster model. TLR2 and TLR5 ligation increased β2-integrin affinity, as assessed by the detection of activation-dependent neoepitopes. TLR2- and TLR5-triggered integrin activation in leukocytes required enhanced Rap1 GTPase activity, which was mediated by Rac1 activation and NADPH oxidase-2–dependent reactive oxygen species production. This novel direct pathway linking initial pathogen recognition by TLRs to rapid β2-integrin activation may critically regulate acute leukocyte infiltration to sites of pathogen invasion.

Structure and function of platelet receptors initiating blood clotting

At the clinical level, recent studies reveal the link between coagulation and other pathophysiological processes, including platelet activation, inflammation, cancer, the immune response, and/or infectious diseases. These links are likely to underpin the coagulopathy associated with risk factors for venous thromboembolic (VTE) and deep vein thrombosis (DVT). At the molecular level, the interactions between platelet-specific receptors and coagulation factors could help explain coagulopathy associated with aberrant platelet function, as well as revealing new approaches targeting platelet receptors in diagnosis or treatment of VTE or DVT. Glycoprotein (GP)Ibα, the major ligand-binding subunit of the platelet GPIb-IX-V complex, that binds the adhesive ligand, von Willebrand factor (VWF), is co-associated with the platelet-specific collagen receptor, GPVI. The GPIb-IX-V/GPVI adheso-signaling complex not only initiates platelet activation and aggregation (thrombus formation) in response to vascular injury or disease but GPIbα also regulates coagulation through a specific interaction with thrombin and other coagulation factors. Here, we discuss the structure and function of key platelet receptors involved in thrombus formation and coagulation in health and disease, with a particular focus on platelet GPIbα.

Platelets and neurovascular inflammation

Platelets participate in haemostasis and in thrombus formation in health and disease. Moreover, they contribute to inflammation and cooperate with immune cells in a magnitude of inflammatory/immune responses. Although the inflammatory response has been recognised to be critical in neuronal diseases such as Alzheimer's disease or multiple sclerosis and its mouse counterpart, experimental autoimmune encephalomyelitis, the participation of platelets in these diseases is poorly investigated so far. Emerging studies, however, point to an interesting crosstalk between platelets and neuroinflammation. For instance, when the integrity of the blood brain barrier is compromised, platelets may be relevant for endothelial inflammation, as well as recruitment and activation of inflammatory cells, thereby potentially contributing to central nervous tissue pathogenesis. This review summarises recent insights in the role of platelets for neurovascular inflammation and addresses potential underlying mechanisms, by which platelets may affect the pathophysiology of neurovascular diseases.

Current viewpoints on platelet contribution to inflammation

Inflammation is an underlying feature of a variety of human diseases. Because inflammatory diseases are a major cause of morbidity and mortality in developed countries, understanding the interaction of the most important factors involved is an important challenge. Although platelets are widely recognized as having a critical role in primary hemostasis and thrombosis, basic and clinical evidence increasingly identifies these enucleated cells as relevant modulators, as both effector and target cells, of the inflammatory response. The cross-talk between platelets, endothelial cells and leukocytes in the inflammatory milieu mat be seen as a double-edged sword which functions not only as an effective first-line defense mechanism but may also lead to organ failure and death in the absence of counter-regulation systems. The molecular mechanisms involved in the reciprocal activation of platelets, endothelial cells and leukocytes are beginning to be elucidated. In the light of the existing data from experimental and clinical studies it is conceivable that platelet adhesion molecules and platelet mediators provide promising targets for novel therapeutic strategies in inflammatory diseases. The potentially adverse effects of these approaches need to be carefully addressed and monitored, including alterations in hemostasis and coagulation and particularly the impairment of host defense mechanisms, given the recently identified pivotal role of platelets in pathogen recognition and bacterial trapping. In this review we discuss the most important recent advances in research into the cross-talk between platelets and vascular cells during inflammation and the clinical consequences of these interactions.

Platelets: key players in vascular inflammation

Review on platelet function in inflammation and atherosclerosis.

Platelets play a crucial role in the physiology of the primary hemostasis and in the pathophysiological activity of arterial thrombosis, provide rapid protection against bleeding, and catalyze the formation of stable blood clots via the coagulation cascade. Over the past years, it has become clear that platelets are important, not only in hemostasis and thrombosis but also in inflammation and in distinct aspects of atherosclerosis. Nowadays, platelets are known to have a large variety of functions. Platelets are able to interact with a large variety of cell types, such as leukocytes, endothelial cells, and SMCs, and these interactions have been implicated in the pathophysiology of vascular inflammation. In addition, platelets carry a highly inflammatory payload and are able to transport, synthesize, and deposit cytokines, chemokines, and lipid mediators, thereby initiating and propagating atherosclerotic disease. In this review, the current state of the art of the proinflammatory functions in the context of atherosclerotic cardiovascular disease will be outlined.

Endogenous modulators of inflammatory cell recruitment

Leukocyte recruitment is a central immune process. Multiple factors have been described to promote leukocyte infiltration into inflamed tissues, but only recently has evidence for endogenous negative modulators of this inflammatory process emerged. The discovery of several locally produced modulators has emerged into a new field of endogenous inhibitors of leukocyte extravasation. Recent findings from several inflammatory disease models show that tissues can self-regulate the recruitment of inflammatory cells, suggesting that local tissues may have a greater 'regulatory say' over the immune response than previously appreciated. Here, we propose that locally produced modulators of leukocyte recruitment may represent local homeostatic mechanisms that tissues and organs may have evolved for protection against the destructive potential of the immune system.

Platelets contribute to the pathogenesis of experimental autoimmune encephalomyelitis

RATIONALE

Multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE), are inflammatory disorders of the central nervous system (CNS). The function of platelets in inflammatory and autoimmune pathologies is thus far poorly defined.

OBJECTIVE

We addressed the role of platelets in mediating CNS inflammation in EAE.

METHODS AND RESULTS

We found that platelets were present in human MS lesions as well as in the CNS of mice subjected to EAE but not in the CNS from control nondiseased mice. Platelet depletion at the effector-inflammatory phase of EAE in mice resulted in significantly ameliorated disease development and progression. EAE suppression on platelet depletion was associated with reduced recruitment of leukocytes to the inflamed CNS, as assessed by intravital microscopy, and with a blunted inflammatory response. The platelet-specific receptor glycoprotein Ibα (GPIbα) promotes both platelet adhesion and inflammatory actions of platelets and targeting of GPIbα attenuated EAE in mice. Moreover, targeting another platelet adhesion receptor, glycoprotein IIb/IIIa (GPIIb/IIIa), also reduced EAE severity in mice.

CONCLUSIONS

Platelets contribute to the pathogenesis of EAE by promoting CNS inflammation. Targeting platelets may therefore represent an important new therapeutic approach for MS treatment.

Kinin generation from exogenous kininogens at the surface of retinoic acid-differentiated human neuroblastoma IMR-32 cells after stimulation with interferon-γ

Bradykinin-related peptides, kinins, ubiquitously occur in the nervous system and together with other pro-inflammatory mediators contribute to pathological states of that tissue such as edema and chronic pain. In the current work we characterized the kinin-forming system of neuronal cells obtained by differentiation of human neuroblastoma cell line IMR-32 with retinoic acid. These cells were shown to concentrate exogenous kinin precursors, kininogens, on the surface, release kinins from kininogens and subsequently convert kinins to their des-Arg metabolites. Significantly higher amounts of kinins and des-Arg-kinins were produced after cell stimulation with interferon-γ, a potent pro-inflammatory mediator involved in many neurological disorders. The expression of the major tissue kininogenase (the human kallikrein 1) and the major cell membrane-bound kininase (the carboxypeptidase M) also increased after cell stimulation with interferon-γ, suggesting the involvement of these enzymes in the kinin production and degradation, respectively. Interferon-γ was also able to up-regulate the expression of two known subtypes of kinin receptors. On the protein level, the changes were only observed in the expression of the des-Arg-kinin-specific type 1 receptor which functions in the propagation of the inflammatory state. Taken together, these results suggest a novel way for local kinin and des-Arg-kinin generation in the nervous tissue during pathological states accompanied by interferon-γ release.

Platelet interactions as therapeutic targets for prevention of atherothrombosis

Physiologically, platelets perform important tasks to maintain the homeostasis of the vascular wall and the surrounding environment. In pathologic conditions, however, platelets contribute to the formation of atherosclerotic plaques as well as to atherothrombotic events (i.e., acute myocardial infarction). This review aims to elucidate the role of platelets in atherogenesis and atherothrombosis and to provide an insight into current and future strategies for platelet inhibition.

Role of platelets in neuroinflammation: a wide-angle perspective

OBJECTIVES

This review summarizes recent developments in platelet biology relevant to neuroinflammatory disorders. Multiple sclerosis (MS) is taken as the "Poster Child" of these disorders but the implications are wide. The role of platelets in inflammation is well appreciated in the cardiovascular and cancer research communities but appears to be relatively neglected in neurological research.

ORGANIZATION

After a brief introduction to platelets, topics covered include the matrix metalloproteinases, platelet chemokines, cytokines and growth factors, the recent finding of platelet PPAR receptors and Toll-like receptors, complement, bioactive lipids, and other agents/functions likely to be relevant in neuroinflammatory diseases. Each section cites literature linking the topic to areas of active research in MS or other disorders, including especially Alzheimer's disease.

CONCLUSION

The final section summarizes evidence of platelet involvement in MS. The general conclusion is that platelets may be key players in MS and related disorders, and warrant more attention in neurological research.

The formation of monocyte-platelet aggregates is independent of on-treatment residual agonists'-inducible platelet reactivity

BACKGROUND

Circulating monocyte-platelet aggregates (MPA) are a sensitive marker of in vivo platelet activation and patients with atherosclerotic vascular disease exhibit higher levels of MPA. Clopidogrel has been shown to reduce MPA formation in these patients to a greater extent than aspirin. However, response to clopidogrel and aspirin shows a wide variability, and patients with high on-treatment residual platelet reactivity are at an increased risk for adverse events after coronary stenting. We therefore investigated the association of MPA with on-treatment residual agonists'-inducible platelet aggregation in 125 patients on dual antiplatelet therapy after peripheral, coronary or carotid artery stenting.

METHODS

MPA were characterized by co-expression of monocyte marker CD14 and platelet-specific markers (CD42b and CD62P) by whole blood flow cytometry. Platelet reactivity was determined by light transmission aggregometry, the VerifyNow P2Y12 and aspirin assays, and the vasodilator-stimulated phosphoprotein phosphorylation assay. Cut-off values for residual platelet reactivity were defined according to quartiles of each assay.

RESULTS

The extent of MPA formation showed no significant differences between patients without and with residual ADP-inducible platelet reactivity, and between individuals without and with residual arachidonic acid (AA)-inducible platelet reactivity. Even patients with combined on-treatment residual ADP- and AA-inducible platelet reactivity did not exhibit significantly higher levels of MPA than patients without any on-treatment residual platelet reactivity.

CONCLUSION

High on-treatment residual agonists'-inducible platelet reactivity results in less than a 25% increase in circulating MPA, suggesting that MPA formation is largely dependent on other factors.

Leukocyte-endothelial interactions in inflammation

At sites of inflammation, infection or vascular injury local proinflammatory or pathogen-derived stimuli render the luminal vascular endothelial surface attractive for leukocytes. This innate immunity response consists of a well-defined and regulated multi-step cascade involving consecutive steps of adhesive interactions between the leukocytes and the endothelium. During the initial contact with the activated endothelium leukocytes roll along the endothelium via a loose bond which is mediated by selectins. Subsequently, leukocytes are activated by chemokines presented on the luminal endothelial surface, which results in the activation of leukocyte integrins and the firm leukocyte arrest on the endothelium. After their firm adhesion, leukocytes make use of two transmigration processes to pass the endothelial barrier, the transcellular route through the endothelial cell body or the paracellular route through the endothelial junctions. In addition, further circulating cells, such as platelets arrive early at sites of inflammation contributing to both coagulation and to the immune response in parts by facilitating leukocyte-endothelial interactions. Platelets have thereby been implicated in several inflammatory pathologies. This review summarizes the major mechanisms and molecules involved in leukocyte-endothelial and leukocyte-platelet interactions in inflammation.

Deletion of murine kininogen gene 1 (mKng1) causes loss of plasma kininogen and delays thrombosis

High-molecular-weight kininogen (HK) plays an important role in the assembly of the plasma kallikrein-kinin system. While the human genome contains a single copy of the kininogen gene, 3 copies exist in the rat (1 encoding K-kininogen and 2 encoding T-kininogen). Here, we confirm that the mouse genome contains 2 homologous kininogen genes, mKng1 and mKng2, and demonstrate that these genes are expressed in a tissue-specific manner. To determine the roles of these genes in murine development and physiology, we disrupted mKng1, which is expressed primarily in the liver. mKng1(-/-) mice were viable, but lacked plasma HK and low-molecular-weight kininogen (LK), as well as DeltamHK-D5, a novel kininogen isoform that lacks kininogen domain 5. Moreover, despite normal tail vein bleeding times, mKng1(-/-) mice displayed a significantly prolonged time to carotid artery occlusion following Rose Bengal administration and laser-induced arterial injury. These results suggest that a single gene, mKng1, is responsible for production of plasma kininogen, and that plasma HK contributes to induced arterial thrombosis in mice.

Cleavage of high-molecular-weight kininogen by elastase and tryptase is inhibited by ferritin

Ferritin is a protein principally known for its role in iron storage. We have previously shown that ferritin can bind high-molecular-weight kininogen (HK). Upon proteolytic cleavage by the protease kallikrein, HK releases the proinflammatory peptide bradykinin (BK) and other biologically active products, such as two-chain high-molecular-weight kininogen, HKa. At inflammatory sites, HK is oxidized, which renders it a poor substrate for kallikrein. However, oxidized HK remains a good substrate for elastase and tryptase, thereby providing an alternative cleavage mechanism for HK during inflammation. Here we report that ferritin can retard the cleavage of both native HK and oxidized HK by elastase and tryptase. Initial rates of cleavage were reduced 45-75% in the presence of ferritin. Ferritin is not a substrate for elastase or tryptase and does not interfere with the ability of either protease to digest a synthetic substrate, suggesting that ferritin may impede HK cleavage through direct interaction with HK. Immunoprecipitation and solid phase binding studies reveal that ferritin and HK bind directly with a Kd of 134 nM. To test whether ferritin regulates HK cleavage in vivo, we used THP-1 cells, a human monocyte/macrophage cell line that has been used to model pulmonary inflammatory cells. We observed that ferritin impedes the cleavage of HK by secretory proteases in stimulated macrophages. Furthermore, ferritin, HK, and elastase are all present in or on alveolar macrophages in a mouse model of pulmonary inflammation. Collectively, these results implicate ferritin in the modulation of HK cleavage at sites of inflammation.

Kininogen binding to the surfaces of macrophages

Kinin generation may be initiated on the cell surfaces via a primary kininogen docking which has been characterized for endothelial cells, platelets, neutrophils, astrocytes and smooth muscle cells. In this work we describe the adsorption of biotin-labeled human kininogens by murine RAW 264.7 macrophages and human U-937 monocytes/macrophages. Both cell types strongly bound high molecular mass kininogen (HK) in a zinc-ion dependent manner with the dissociation constants of 9.1 nM and 3.3 nM, respectively, and the binding capacities of 46 fmol and 71 fmol per million of respective cells. The HK binding was quenched by 50% by antibodies against Mac-1, gC1qR and uPAR proteins indicating that these macrophage surface receptors are involved in the HK adsorption. A significant increase of HK binding was observed after cell activation with phorbol myristate acetate. Our results suggest that macrophages, similarly to neutrophils, may supply kininogens to the inflammatory foci to support the local kinin production at these sites.

Platelets, inflammation and atherosclerosis

An expanding body of evidence continues to build on the role of platelets as initial actors in the development of atherosclerotic lesions. Platelets bind to leukocytes and endothelial cells, and initiate monocyte transformation into macrophages. Platelets internalize oxidized phospholipids and promote foam cell formation. Platelets also recruit progenitor cells to the scene that are able to differentiate into foam cells or endothelial cells depending on conditions. Platelets tip the scales in the initiation, development and total extent of atherosclerotic lesions.

Platelets Recruit Human Dendritic Cells Via Mac-1/JAM-C Interaction and Modulate Dendritic Cell Function In Vitro

OBJECTIVE

Thrombotic events and immunoinflammatory processes take place next to each other during vascular remodeling in atherosclerotic lesions. In this study we investigated the interaction of platelets with dendritic cells (DCs).

METHODS AND RESULTS

The rolling of DCs on platelets was mediated by PSGL-1. Firm adhesion of DCs was mediated through integrin alphaMbeta2 (Mac-1). In vivo, adhesion of DCs to injured carotid arteries in mice was mediated by platelets. Pretreatment with soluble GPVI, which inhibits platelet adhesion to collagen, substantially reduced recruitment of DCs to the injured vessel wall. In addition, preincubation of DCs with sJAM-C significantly reduced their adhesion to platelets. Coincubation of DCs with platelets induced maturation of DCs, as shown by enhanced expression of CD83. In the presence of platelets, DC-induced lymphocyte proliferation was significantly enhanced. Moreover, coincubation of DCs with platelets resulted in platelet phagocytosis by DCs, as verified by different cell phagocytosis assays. Finally, platelet/DC interaction resulted in apoptosis of DCs mediated by a JAM-C-dependent mechanism.

CONCLUSIONS

Recruitment of DCs by platelets, which is mediated via CD11b/CD18 (Mac-1) and platelet JAM-C, leads to DC activation and platelet phagocytosis. This process may be of importance for progression of atherosclerotic lesions.

TREM-1 ligand expression on platelets enhances neutrophil activation

The triggering receptor expressed on myeloid cells 1 (TREM-1) plays an important role in the innate immune response related to severe infections and sepsis. Modulation of TREM-1-associated activation improves the outcome in rodent models for pneumonia and sepsis. However, the identity and occurrence of the natural TREM-1 ligands are so far unknown, impairing the further understanding of the biology of this receptor. Here, we report the presence of a ligand for TREM-1 on human platelets. Using a recombinant TREM-1 fusion protein, we demonstrate specific binding of TREM-1 to platelets. TREM-1-specific signals are required for the platelet-induced augmentation of polymorphonuclear leukocyte (PMN) effector functions (provoked by LPS). However, TREM-1 interaction with its ligand is not required for platelet/PMN complex formation, which is dependent on integrins and selectins. Taken together, the results indicate that the TREM-1 ligand is expressed by platelets, and the TREM-1/ligand interaction contributes to the amplification of LPS-induced PMN activation. Our results shed new light on our understanding of TREM-1 and its role in the innate inflammatory response in infections and might contribute to the development of future concepts to treat sepsis.

Antithrombotic activity of kininogen is mediated by inhibitory effects of domain 3 during arterial injury in vivo

High-molecular-weight kininogen (HK) and its domain 3 (D3) exhibit anticoagulant properties and inhibit platelet activation at low thrombin concentration in vitro. We hypothesized that the rapid occlusive thrombosis in HK-deficient (HKd) rats following endothelial injury of the aorta results from enhanced platelet aggregation by thrombin. The effects of D3 (G235-M357) or D3-derived peptides on thrombosis in vivo were tested. D3 and its exon 7C terminal peptide (E7CP, K270-Q292), expressed as glutathione S-transferase (GST) fusion proteins (GST-D3, GST-E7CP), or GST alone, as well as cleaved HK (HKa) or synthetic peptide E7CP, were infused intravenously 10 min before endothelial injury. Blood flow was reduced down to 10% of baseline flow within 28 +/- 5.2 min by a platelet-fibrin thrombus in GST-treated HKd rats compared with >240 min in GST-treated normal HK rats (wild type). GST-D3, GST-E7CP, HKa, or E7CP infusion prolonged the flow time to 233, >240, 223, and >240 min, respectively, in HKd rats. When GST-E7CP was infused 10 min after the injury, blood flow was maintained for >240 min. Thrombin-antithrombin concentrations were elevated by injury in HKd rats receiving GST from 35 to 55 microg/l and decreased with GST-E7CP, HKa, or E7CP reconstitution to 40, 15, and 9 microg/l, respectively. We conclude that HKd rats are prothrombotic and that HKa, kininogen D3, and its fragment E7CP modulate arterial thrombosis after endothelial injury.

Platelets in inflammation and atherogenesis

Platelets represent an important linkage between inflammation, thrombosis, and atherogenesis. Inflammation is characterized by interactions among platelets, leukocytes, and ECs. These interactions trigger autocrine and paracrine activation processes that lead to leukocyte recruitment into the vascular wall. Platelet-induced chronic inflammatory processes at the vascular wall result in development of atherosclerotic lesions and atherothrombosis. This Review highlights the molecular machinery and inflammatory pathways used by platelets to initiate and accelerate atherothrombosis.

A monoclonal antibody against kininogen reduces inflammation in the HLA-B27 transgenic rat

The human leukocyte antigen B27 (HLA-B27) transgenic rat is a model of human inflammatory bowel disease, rheumatoid arthritis and psoriasis. Studies of chronic inflammation in other rat models have demonstrated activation of the kallikrein–kinin system as well as modulation by a plasma kallikrein inhibitor initiated before the onset of clinicopathologic changes or a deficiency in high-molecular-mass kininogen. Here we study the effects of monoclonal antibody C11C1, an antibody against high-molecular-mass kininogen that inhibits the binding of high-molecular-mass kininogen to leukocytes and endothelial cells in the HLA-B27 rat, which was administered after the onset of the inflammatory changes. Thrice-weekly intraperitoneal injections of monoclonal antibody C11C1 or isotype IgG₁ were given to male 23-week-old rats for 16 days. Stool character as a measure of intestinal inflammation, and the rear limbs for clinical signs of arthritis (tarsal joint swelling and erythema) were scored daily. The animals were killed and the histology sections were assigned a numerical score for colonic inflammation, synovitis, and cartilage damage. Administration of monoclonal C11C1 rapidly decreased the clinical scores of pre-existing inflammatory bowel disease (P < 0.005) and arthritis (P < 0.001). Histological analyses confirmed significant reductions in colonic lesions (P = 0.004) and synovitis (P = 0.009). Decreased concentrations of plasma prekallikrein and high-molecular-mass kininogen were found, providing evidence of activation of the kallikrein–kinin system. The levels of these biomarkers were reversed by monoclonal antibody C11C1, which may have therapeutic potential in human inflammatory bowel disease and arthritis.

Structure and expression of two kininogen genes in mice

Kininogens serve dual functions by forming a scaffold for the assembly of the protein complex initiating the surface-activated blood coagulation cascade and as precursors for the kinin hormones. While rats have three kininogen genes, for mice, cattle, and humans only one gene has been described. Here, we present sequence and expression data of a second mouse kininogen gene. The two genes, kininogen-I and kininogen-II, are located in close proximity on chromosome 16 in a headtohead arrangement. In liver and kidney, both genes are expressed and for each gene three alternative splice variants are synthesized. Two of them are the expected high and low molecular weight isoforms known from all mammalian kininogens. However, for both genes also a third, hitherto unknown splice variant was detected which lacks part of the high molecular weight mRNA due to splicing from the low molecular weight donor site to alternative splice acceptor sites in exon 10. The physiological functions of the six kininogen isoforms predicted by these findings need to be determined.

Mapping the Interaction between High Molecular Mass Kininogen and the Urokinase Plasminogen Activator Receptor

The urokinase plasminogen activator receptor (uPAR) is a multifunctional, GPI-linked receptor that modulates cell adhesion/migration and fibrinolysis. We mapped the interaction sites between soluble uPAR (suPAR) and high molecular mass kininogen (HK). Binding of biotin-HK to suPAR was inhibited by HK, 56HKa, and 46HKa with an IC50 of 60, 110, and 8 nm, respectively. We identified two suPAR-binding sites, a higher affinity site in the light chain of HK and 46HKa (His477-Gly496) and a lower affinity site within the heavy chain (Cys333-Lys345). HK predominantly bound to suPAR fragments containing domains 2 and 3 (S-D2D3). Binding of HK to domain 1 (S-D1) was also detected, and the addition of S-D1 to S-D2D3 completely inhibited biotin-HK or -46HKa binding to suPAR. Using sequential and overlapping 20-amino acid peptides prepared from suPAR, two regions for HK binding were identified. One on the carboxyl-terminal end of D2 (Leu166-Thr195) blocked HK binding to suPAR and to human umbilical vein endothelial cells (HUVEC). This site overlapped with the urokinase-binding region, and urokinase inhibited the binding of HK to suPAR. A second region on the amino-terminal portion of D3 (Gln215-Asn255) also blocked HK binding to HUVEC. Peptides that blocked HK binding to uPAR also inhibited prekallikrein activation on HUVEC. Therefore, HK interacts with suPAR at several sites. HK binds to uPAR as part of its interaction with its multiprotein receptor complex on HUVEC, and the biological functions that depend upon this binding are modulated by urokinase.