Seminars in Immunopathology

, 33:469 | Cite as

Microparticles: a critical component in the nexus between inflammation, immunity, and thrombosis

  • Olivier Morel
  • Nicolas Morel
  • Laurence Jesel
  • Jean-Marie Freyssinet
  • Florence Toti
Review

Abstract

Plasma membrane remodeling characterized by phosphatidylserine exposure and consecutive microparticle (MP) shedding is an ubiquitous process enabling the clearance of senescent cells and the maintenance of tissue homeostasis. MPs are released as fragments from the budding plasma membrane of virtually all eukaryotic cell types undergoing stimulation or apoptosis and may be considered a broad primitive response to stress. MP release is dependent on cytoskeleton degradation pathways involving caspases, requires a sustained increase in intracellular calcium triggering K+ and Cl efflux and is possibly tuned by mitochondria permeability changes. Because they convey a broad spectrum of bioactive molecules, circulating MPs may serve as shuttles promoting cellular cross talk in various pathological settings such as inflammation or immunity-induced thrombotic disorders. If the drastic shedding of procoagulant MPs appears clearly noxious in thrombotic disorders or in some models of inflammation-induced coagulopathy, this does not necessarily endorse their invariably harmful nature. In the vessel, endothelial cytoprotection reported in the early regulation of inflammation-induced coagulopathy is emblematic of the beneficial effects provided by MPs. In addition, MPs would prove beneficial in the prevention of blood leakage. Because of their multiple properties that are characteristic of a private response of the parental cell, MPs could act as cytoprotective and anti-inflammatory agents through the delivery of activated protein C or annexin 1 and could contribute to the limitation of vascular hyporeactivity. Owing to their ability to cargo bioactive signals, MPs could be viewed as an integrated communication network enabling the coordination of complex cellular responses in biological fluids and the maintenance of the homeostasis equation. A better understanding of the molecular mechanisms involved in MP shedding would pave the way of a new pharmacological approach aiming at the control of MP-driven cellular responses.

Keywords

Phosphatidylserine Ca++ Thrombosis Apoptosis Platelet Scott syndrome Immunity inflammation Tissue factor TMEM16F 

Introduction

Microparticles (MPs) are released from the budding plasma membrane of virtually every eukaryotic cell types after stimulation or apoptosis. They can also result from the resealing of membrane fragments disrupted during cell necrosis. In multicellular organisms, mostly functioning in a minimal energy consumption mode with permanent recycling, MPs are thought to reflect a dynamic balance between cell proliferation, stimulation, and death [1]. At the surface of senescent or apoptotic cells, the exposure of phosphatidylserine (PS) constitutes a recognition signal that enables phagocytosis clearance in multicellular organisms [2]. Because the randomization of membrane lipid asymmetry is an important physiological process, it is likely that the machinery for accomplishing it is common to all eukaryotic cells [3]. Owing to their smaller size and therefore greater ability to diffuse more rapidly and transiently escape phagocytosis, MPs survive longer than the activated cells they stem from. Detectable in small amounts in the peripheral blood of healthy individuals, elevated concentrations of MPs originating from platelet, granulocyte, erythrocyte, and endothelial lineages are associated with a variety of pathophysiological issues including thrombosis, inflammation, sepsis, malaria, metabolic disorder, auto-immune disease, trauma, cancer, or sickle-cell disease. MPs convey a broad spectrum of bioactive molecules thought to contribute to cell cross talk. Because they display autoantigens, such as RNA and DNA, in a highly immunostimulatory manner, they may also act as potent autoadjuvants. Released in the vasculature or in other biological fluids, MPs may tune cellular interactions and behave as key regulators in the nexus between inflammation, immunity and thrombosis. The aim of this review is to (1) describe the molecular mechanisms possibly governing MP generation (2) discuss the role of MPs in the nexus between inflammation–immunity and coagulopathies.

Generation of microparticles by stimulated cells

In eukaryotic cells, each of the two leaflets of the plasma membrane bilayer has a specific lipid composition. Aminophospholipids [PS and phosphatidyl-ethanolamine (PE)] are more specifically segregated in the inner leaflet, whereas phosphatidylcholine (PC) and sphingomyelin are enriched in the external one. The maintenance of this asymmetric distribution or its loss is the consequence of opposite transports, their balance being under control of specific transporters governing inward (flip) or outward (flop) translocation [4]. Many years after this first observation, the identity and actual biological function of proteins involved in lipid translocation remains yet incomplete. The maintenance of lipid asymmetry relies on an adenosine triphosphate (ATP)-dependent transporter, the aminophospholipid translocase, that specifically and rapidly conveys PS and PE from the outer to the inner leaflet and that is inhibited by calcium ions [5]. Because transbilayer phospholipid transport is slow in the absence of a translocator, lipid asymmetry is stable in quiescent cells [3].

Membrane remodeling is a crucial event in cell response and is involved in apoptosis, hemostasis, and cell clearance that is tightly regulated. To provide an adequate procoagulant surface upon stimulation, at least one, and probably more than one, transporters facilitate the rapid egress of PS at the outer leaflet. Possible translocators are members of the floppase family (outward-directed transporters including the ATP-binding cassette transporter A1, ABCA1), and the so-called calcium-dependent scramblase that facilitate bidirectional movement between membrane leaflets [6, 7, 8]. Cell stimulation, on the broad acceptance of the term, triggers increased cytosolic [Ca2+] concentration and the outward transport of PS that is enabled by the concerted activity of lipid transporters. Aminophospholipids are rapidly translocated to the outer leaflet, flippase activity is reduced while an overwhelming floppase activity and the activation of the scramblase that favors phospholipid exchanges between the two leaflets, regardless of the chemical nature of the polar head group, account for transient phospholipids mass imbalance between the two leaflets. Surface tension and membrane budding are ultimately resolved into MP shedding.

The procoagulant nature of micropartcicles

PS exerts a key catalytic role by promoting the assembly of the vitamin K-dependent enzyme complexes of the coagulation cascade. Its exposure by the surface of stimulated/adherent platelets is part of the physiological hemostatic response at site of the wound, also triggered by MPs and other activated blood and vascular cells. Vitamin K-dependent factors (FVII, FIX, FX, and FII) bind to accessible PS through their γ-carboxyglutamyl residues. Local concentrations of focused enzymes, cofactors (FVIIIa and FVa) and zymogens enable the fulfillment of the kinetic constraints for optimal thrombin generation that will in turn stimulate platelets in an amplification loop at the basis of the rapidity of the hemostatic response [4]. Platelets, with a high rate of floppase and scramblase activity behave as true sensors that trigger blood coagulation at site of vascular injury [3]. In the vasculature, MPs shed from platelets provide an additional phopholipidic surface with approximately 50–100-fold higher procoagulant activity [9]. In other cell types, such as T cells or EBV transformed cells, the scrambling rate is intermediate, the lowest range being measured in erythrocytes [3]. The relevance of “flip-flop” to hemostasis, is suggested from the phenotype of Scott syndrome, a rare bleeding disorder in which both PS exposure and MP shedding are deficient [10, 11]. Severe to moderate bleeding episodes, generally provoked, constitute the clinical phenotype, with no other apparent disorders in homozygous-like patients. Very recently, a hereditary bleeding disorder with a deficient platelet procoagulant activity was identified in German Shepherd dogs with features of Scott syndrome [12]. As yet, the paucity of human cases has hindered genetic identification of the trait. Phospholipid scramblase 1 and the transporter ABCA1 are putative aminophospholipid floppases that were not confirmed the defective element responsible for Scott syndrome.

Factors governing plasma membrane remodeling and microparticle shedding

Calcium homeostasis and PS exposure

Calcium concentration in the cytosol is tightly regulated and an essential factor in the regulation of aminophospholipid transbilayer migration (Fig. 1). The Ca2+-dependent mechanisms associated with rapid phospholipid redistribution include a variety of processes, such as the activation of outward PS transporters, inhibition of the inward aminophospholipid translocase, and cytoskeleton degradation. Notably, cell shrinkage itself that accompanies stimulation-driven ions and water fluxes across the membrane modifies lipid packing in both leaflets and PS redistribution between leaflets. Intracellular Ca2+ rises as a consequence of its release from internal stores and of its entry across the plasma membrane. In platelets, agonist-induced stimulation of different receptors leads to the activation of phospholipase C isoforms, which hydrolyze phosphoinositide-4,5-bisphosphate to inositol-1,4,5-trisphosphate (IP3) and 1,2-diacyl-glycerol (DAG). Ca2+ IP3 releases Ca2+ from the intracellular stores through IP3 receptor channels, while DAG is involved in Ca2+ entry from the extracellular compartment. The agonist-induced Ca2+release from stores is followed by a Ca2+ influx through the plasma membrane, a particular mode of refilling referred to as store-operated calcium entry (SOCE). Prurigenric P2X1 channel are another source of calcium entry in platelets [13]. Stromal interacting protein 1 (STIM1), a transmembrane protein located in the tubular dense system and the platelet equivalent to endoplasmic reticulum (ER), was recently assessed as a Ca2+ sensor that detects release from the ER. When translocated to the platelet plasma membrane, it is in turn able to trigger a sustained influx of extracellular calcium. Orail1 was very recently identified as the principal actor of SOCE sensored by STIMI-1 (Fig. 1). In mice expressing a mutated form of Orai1 (R93W), a marked inhibition of platelet SOCE together with impaired agonist-induced cytosolic [Ca2+] could be evidenced (Fig. 1). The relevance of the STIM-1/Orai1 SOCE pathway in thrombosis was underlined by the fact that platelet deficiency in either Orai1 or STIM1 protects against collagen-dependent arterial thrombus formation and brain infarction in vivo [14, 15]. Conversely, one surprising finding is that platelets from Stim1−/− and Orai1−/− mice, despite a complete lack of SOCE are still able to fulfill most of their physiological functions. Whilst the Ca2+ release from internal stores appears to be sufficient to trigger shape change, integrin activation and granule release, the Orai1 pathway would be of particular relevance in coagulant activity, MP shedding, and the stabilization of the thrombus. These observations fit well with the phylogenic perspective that platelets have to trigger a rapid response to stimulation. It appears reasonable that basic platelet functions would be regulated without the delay of an indirect SOCE process, which involves a molecular coupling that necessitates up to 15 s to be accomplished [13, 16].
Fig. 1

Role of calcium in the formation of microparticles. In platelets, activation can occur through different pathways that culminate in the activation of phospholipase C isoforms and the production of diacyl-glycerol (DAG) and inositol triphosphate (IP3). The binding of IP3 to its receptors located in the dense tubular system mediates Ca2+ release from cytosolic stores. In turn, the resulting decline in Ca2+ store content, sensored by the stromal interacting molecules 1 (STIM-1) triggers a sustained influx of extracellular calcium by a mechanism known as store-operated calcium entry. It was recently demonstrated that Orai1 is the principal Ca2+ entry channel involved in store-operated calcium entry. The role of direct-operated calcium channel such as the P2X1 prurinoreceptor needs further investigation. Interestingly in transgenic mouse that overexpresses P2X1 receptor increased PS exposure was observed. The raise of calcium is supposed to promote PS exposure through several mechanisms including flippase inhibition and the activation of floppase and scamblase. In addition, the elevation of Ca2+ cytosolic concentration could activate the K+ Gardos channel and the TMEM16F Cl channel. Water follows K+ and Cl efflux inducing cell dehydration and shrinkage. In two patients with Scott syndrome 3 identified mutations in TMEM16F predicted the truncation of the protein and premature termination of the protein devoid of its calcium sensing domain

Another important regulatory protein triggered by calcium is calmodulin (CaM). Upon binding of Ca2+, CaM affinity for downstream target proteins like kinases or phosphatases increases. This wide array of kinases includes myosin light chain kinase, CaM-dependent protein kinases I, II, and IV, and calcineurin [7]. Best known as an immune-suppressor in vivo, CsA mechanistically inhibits calcineurin by forming a complex with its effector proteins cyclophilins (see below). Interestingly, earlier studies have reported that cyclosporin A (CsA) abrogates ABCA1-dependent cholesterol efflux [17, 18]. Very recently, Karwatsky et al. have demonstrated that cholesterol efflux to apoA-I in ABCA1-expressing cells is regulated by Ca2+-dependent calcineurin signaling [19].

Cytoskeleton, membrane lateral organization and PS exposure

Cytoskeleton integrity is believed to participate in the maintenance of membrane asymmetry and cell shape, its cleavage being therefore a facilitator of membrane budding in stimulated cells. Cytoplasmic caspases and calpains are Ca2+-dependent thiol proteases that contribute to cytoskeleton reorganization by favoring talin, filamin, and gelsolin cleavage. Whereas baseline calpain activity appears insufficient, agonist-induced Ca2+ influx was found necessary for maximal protease activity and cytoskeleton degradation [20]. Caspase-3 was also demonstrated a major factor in cytoskeleton reorganization. It mediates the cleavage of ROCK I, a rho-kinase acting by myosine light chain phosphorylation that induces cell membrane contraction, shrinkage, and MP release [21]. Independently of cell death, ROCK-II, another Rho-kinase isoform, was demonstrated to be activated by caspase-2 following endothelial cell stimulation by thrombin and to be involved in MP shedding [22]. Interestingly, prolonged exposure of endothelial cells to thrombin triggers the expression of a cell-associated and of a soluble form of TRAIL that participates in the release of MPs [23].

Rafts that are organized as functional transbilayer domains through the specific recruitment of particular proteins and lipids may also contribute to the membrane response [24]. PS exposure is dramatically reduced after raft disruption by methyl-cyclodextrin and proteins with strong cytoskeleton interactions are represented to a lesser extent in MPs [25]. In this view, a given stimulus could thus be expected to elicit a “private” membrane response after raft reorganization leading to inclusive or exclusive protein sorting. Such lateral remodeling would explain how MPs of the same cellular origin may have different protein and lipid compositions [4]. As an illustration, CD63 and P-selectin (CD62) were found increased in PS+-MPs whereas GPIb (CD42b) was less represented [26]. The evidence of an increased expression of platelet activation markers (CD62P and CD 63) on PS+-MPs could suggest that those particular MPs are preferential markers of platelet stimulation [26]. Indeed, data from Perez-Pujol and coworkers suggest that the exposure of PS is accompanied by the selective shedding of MPs and that their phenotype is dependent on the stress that elicited their release [27, 28].

Physical mechanisms involved in PS exposure and MP shedding

Challenging the current paradigm of a membrane remodeling driven by specific lipid transporters, other authors have suggested that the rapid diffusion of lipids across the plasma membrane could be the consequence of physical constraints resulting from the formation of blebs or transient transmembrane pores triggered by ion fluxes. Indeed, following stimulation by a Ca2+ ionophore, K+ and Cl ions leave the cell, water follows, and the cell shrinks. According to Eliott and coworkers, the plasma membrane buckles prior the shedding of MPs, as a consequence of cell shrinkage. At the apex of the blebs, the packing of PC in the outward leaflet is relatively loose, reducing the energetic barrier of the outward movement of PS and PE (“flop”). Conversely, at the base of the blebs, PS and PE are tightly packed in the inner leaflet, increasing the energetic probability of the inverse transport of PC (“flip”). The swift randomization of the membrane phospholipids independently of specific transporters would be ultimately resolved into the shedding of MPs. This hypothesis gives to ion channels a central role in membrane remodeling and raises the question of their ability to sensor different types of stress [29].

By triggering endogenous sphingomyelinase in the inner monolayer of the membrane, the raise of cytosolic [Ca2+] may also lead to the formation of temporary pores. As a result of swift sphingomyelin hydrolysis by sphingomyelinase, ceramide concentrations reach a threshold enabling a mismatch between the two membrane leaflets and the creation of an invagination. The unstability between leaflets is eventually solved through the formation of transient pores. During the lifetime of pores, lipids have free access to both sides of the membrane [30, 31]. Ceramide-induced pore formation which fosters cell membrane scrambling and PS exposure was found determinant in erythrocyte programmed cell death (eryptosis), in sepsis, haemolytic uremic syndrome or malaria [32].

Transmembrane ion transport

Cell shrinkage results from cell dehydration involving a Ca2+-activated efflux of K+ and Cl ions [29, 33]. While numerous experimental data point at cytosolic [Ca2+] as the trigger of PS exposure, investigations of other ion fluxes provide further insights. For instance, the α-hemolysin from Escherichia coli triggers erythrocyte shrinkage and PS exposure through the dual Ca2+activation of the K+ Gardos channel and of the TMEM16A Cl channel [34]. The relevance of K+ Gardos channels as other possible contributors to PS exposure was also illustrated in Scott syndrome [35]. The lack of procoagulant platelet response observed in one patient, was partially restored by pre-treatment with valinomycin, a selective K+ ionophore that favors K+ efflux. The prime importance of Ca2+-activated efflux of Cl in the regulation of PS exposure was very recently illustrated in a subline of a mouse B cell line, Ba/F3,that strongly exposed PS. A cDNA that caused PS spontaneous exposure and encoded a constitutively active mutant of TMEM16F was identified by expression cloning (Fig. 1). Wild-type TMEM16F was localized on the plasma membrane and conferred Ca2+-dependent scrambling of phospholipids. In two patients with Scott syndrome 3 identified mutations in TMEM16F predicted the truncation of the protein and premature termination of the protein devoid of its calcium sensing domain [36, 37]. Interestingly, other authors showed that in platelets stimulated by thrombin/collagen, Na+ influx is able to trigger phospholipid scrambling via a Na+/H+ exchanger [38]. Hence, intracellular Ca2+ may well not be the sole sensor of the collapse of membrane asymmetry.

Mitochondria and tubular dense system in the regulation of membrane remodeling

Initially known as the power factory of the cells, mitochondria are now considered a check point in the apoptotic machinery. Recently, their incidence in the procoagulant functions of vascular cells and platelets has been investigated. Mitochondrial permeability appears an important event for MP shedding. Various agonists promote the opening of the mitochondrial permeability transition pores (mPTP) leading to the release in cytosol of cytochrome c and apoptosis-inducing factor that enable the activation of effector caspases [39]. Consitutive proteins of the mPTP include the adenine nucleotide translocator, the voltage-dependent anion channel, and cyclophylin D. mPTP opening is blocked by CsA that binds to cyclophylin D and dissociates it from the translocator [40]. Mitochondrial membrane depolarization appears an integral event leading to the outward transport of PS in response to physiological agonist such as thrombin and collagen [41, 42]. The contribution of mPTP to plasma membrane remodeling is suggested by the observation that CsA prevents the loss of the mitochondrial transmembrane potential together with MP release and PS exposure in thrombin/convulxin stimulated platelets. This link was confirmed in platelets from mice in which cyclophilin D was invalidated. They were characterized by a defective PS externalization and prothrombinase activity in response to thrombin and convulxin [43]. Interestingly, in the canine Scott syndrome [44], the trait locus includes several genes coding for mitochondria-associated proteins and apoptosis regulators. It is thus likely, that multiple actors govern PS exposure and MP release at different steps of the cell response machinery among which (1) specific caspase-dependent cytoskeleton degradation (2) enhancement of mitochondria permeability (3) sustained increase in intracellular calcium.

Role of MPS in transcellular communication

Whatever the type of cell stimulation responsible for membrane shedding, MPs do not behave as inert cellular debris but should be considered as potent mediators of a new cellular communication network enabling information exchanges in body fluids. It should however be emphasized that the mode of cellular activation, the type of stress (i.e., stimulation vs. apoptosis...) together with the micro-environnement where MPs were generated are probably determinant in the nature and composition of MPs. For instance, past studies have suggested that different MP protein expression patterns are generated under conditions of rapid stimulation or of apoptosis [45]. Even the type of stimulus is able to elicit a private response characterized by a different MP composition. By comparison to MPs shed by LPS-stimulated leukocytes those shed after P-selectin stimulation are enriched in lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18). Importantly, the presence of LFA-1 at MP surface suggests a possible role in immune responses. In addition, LFA-1 may promote MP interaction with the endothelium, causing endothelial activation or dysfunction [46].

Because they harbor or contain ligands, receptors, counter-receptors, active lipids, mRNA, MPs are able to modulate biological properties of neighboring cells through several mechanisms including (1) transfer of membrane-associated receptors (2) release of proteins or active biolipids (3) exchange of genetic information by transfer of RNA (4) induction of adaptative immune response. This MP-mediated intercellular delivery system allows the establishment of an integrated communication network in which specific properties and information among cells can be swiftly shared enabling the coordination of complex responses such as immunoregulation and the maintenance of the homeostasis equation [47]. However, under pathophysiological conditions, the spreading of MPs can amplify noxious responses such as inflammation, cell invasion, thrombosis and vascular damage.

MPs as a source prothrombotic vascular effectors

A dual procoagulant character: blood-borne tissue factor

In the vessel, rapid phospholipid membrane remodeling and PS exposure characterize a physiologically relevant procoagulant response. Owing to their high membrane plasticity ensuring a rapid procoagulant response at site of vascular injury, platelets are the main provider of circulating MPs in healthy individuals. In other vascular cell types, scrambling is less effective, with the lowest ability being found in red blood cells. Beside platelets, leukocytes and endothelial cells, MPs are key determinants of the hemostasis equation. Furthermore, MPs constitute the main reservoir of blood-borne tissue factor (TF) released from [48] cells with inducible (monocytes, macrophages, endothelial cells, etc.) or constitutive TF expression (vascular smooth cells, fibroblasts, and tumor cells). TF is the major cellular initiator of the coagulation cascade that acts synergistically with PS when expressed by the membrane of stimulated cells (Fig. 2). In this context, shed MPs could be viewed as protective in case of bleeding, whereas their dramatic release could favor thrombotic complications in the arterial or venous bed. In the vasculature, various agonists such as cytokines, microorganisms, thrombin, low shear stress, oxidative stress, hyperglycemia, smoking, dylipidemia, complement, immune reaction induce procoagulant phospholipid exposure, TF induction and its release in an active MP-borne form [49, 50]. For instance, in heart transplant recipients, acute rejection triggers the release of a specific MP pattern. Significant elevation of MPs bearing TF, Fas, or E-selectin was associated with acute allograft rejection and suggested endothelial cell activation and Fas-mediated apoptosis [51]. Of note, human leukocyte antigen class I antibodies was demonstrated a potent inducer of TF expression by endothelial cells.
Fig. 2

Role of microparticles (MPs) in the vasculature. Microparticles escape phagocytosis and can promote amplification and propagation of cellular activation. Owing to their ability to cargo bioactive signals, MPs could be viewed as an integrated communication network enabling the coordination of complex cellular responses in biological fluids. Under pathophysiological conditions, the spreading of MPs can amplify noxious responses such as inflammation, cell invasion, thrombosis and vascular damage. Owing to their high membrane plasticity ensuring a rapid procoagulant response at site of vascular injury, platelets are the main provider of circulating MPs in healthy individuals. In other vascular cell types, scrambling is less effective, with the lowest ability being found in red blood cells. Beside platelets, leukocytes and endothelial cells MPs are key determinants of the hemostasis equation. Furthermore, MPs constitute the main reservoir of blood-borne tissue factor (TF) released from cells with inducible (monocytes, macrophages, endothelial cells, etc.) or constitutive TF expression (vascular smooth cells, fibroblasts, and tumor cells). TF is the major cellular initiator of the coagulation cascade that acts synergistically with PS when expressed by the membrane of stimulated cells

At the onset of thrombosis, it is likely that the accumulation of leukocyte-derived MPs harboring TF activities is mandatory whereas the level of platelet-derived MPs mostly testify to the extent of platelet stimulation. In an experimental mouse model of venous thrombosis, circulating leukocyte-derived MP levels were negatively correlated with the thrombus weight, suggesting their early incorporation within the growing thrombus [52]. Conversely, platelet-derived MPs were positively correlated with the thrombus growth [52]. In acute myocardial infarction, sequestered inflammatory cells together with the shedding of leukocyte MPs harboring TF activity are crucial to thrombus formation. Independent of the eventual role played by platelet-derived MPs, we have shown that leukocyte-derived MPs are the main contributor of TF activity at the vicinity of the arterial thrombus (see below). Altogether, these data underline the prime importance of leukocyte-derived MPs in the concentration of TF activity at site of endothelium injury [53]. However, besides the central role of the leukocyte-platelet-endothelium triad in thrombus generation, it is worth mentioning that other cellular lineages may contribute to MP-driven blood coagulation. In traumatic brain injury, we have demonstrated an increased generation of procoagulant MPs in the cerebrospinal fluid that may reflect neuronal apoptosis. MPs were also detected in the peripheral blood, a mechanism possibly involved in trauma-induced coagulopathy [54]. Accordingly, recent studies raised the possibility that central nervous system-derived MPs may enter the bloodstream as the result of blood brain-barrier disruption and interact with endothelial cells [55]. In others clinical settings (malaria, sickle-cell disease, etc.), erythrocyte-derived MPs appeared important contributors to the circulating storage pool of MPs, levels being correlated with the extent of blood coagulation activation [56, 57].

Intertwined proinflammatory and prothrombotic MP-mediated responses

The first hint suggesting a prothrombotic pathway mediated by MPs in inflammatory conditions was brought by Mesri and Altieri that evidenced the up-regulation of circulating leukocyte MPs in vivo and a stress signaling pathway in endothelial cells, leading to increased TF activity [58]. Similarly, monocyte-derived-MPs up-regulated the expression of active TF by endothelial cells and a rapid expression of von Willebrand factor at the cell surface favoring a transient attachment of non-activated platelets to the endothelium. In vitro experiments demonstrated that these two major cell responses are under the control of reactive oxygen species (ROS) delivered by MPs [59]. Under conditions of oxidative stress, endothelial-derived MPs contain oxidized phospholipids that promote monocyte–endothelial interactions. Importantly, platelet-derived MPs (PMPs) are able to up-regulate cytoadhesin expression in monocytes and endothelial cells through the delivery of arachidonic acid, thus reinforcing cell adhesiveness. At high shear stress, platelet-derived MPs (PMPs) GPIb and P-selectin-dependent rolling would enable the delivery of RANTES, a CC chemokine, to the inflamed endothelium and favor monocyte adhesion [60]. MPs also contribute to endothelial inflammation as providers of proinflammatory interleukin (IL)-1β. The relevance of these findings is also suggested in human pathological settings. For instance, in patients with pulmonary arterial hypertension, we have previously shown that RANTES was associated with high levels of procoagulant MPs of leukocyte, endothelial, and platelet origin in the pulmonary vascular bed, possibly reflecting multiple pathways in which MPs contribute to inflammation and thrombus formation [61]. In the same clinical setting, increased platelet-, leukocyte- and endothelium-derived MPs were recently found to predict enhanced coagulation and vascular inflammation [62].

P-selectin: soluble and membranous

During the past decade, selectins, leukocytes-derived MPs, and TF have merged into a determinant triad in thrombosis [63]. The first demonstration of the importance of leukocyte shedding in the hemostatic equation came from the group of Denisa Wagner. Using a mice model of hemophilia A, Hrachovinova et al. demonstrated that the infusion of soluble P-selectin prompts the generation of leukocyte-derived MPs harboring TF and able to correct a bleeding phenotype [64]. Other mechanisms triggered by P-selectin could contribute to increased thrombotic propensity. P-selectin was shown to favor the transfer of TF sorted from raft into monocyte-derived MPs and delivered as a functional entity to platelets [65]. P-selectin also promotes PS exposure by monocytes and TF expression [66, 67]. Finally, P-selectin stimulation of monocytes generated MPs enriched in lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) that enabled interactions between leukocytes and endothelial cells [46]. The prime importance of this pathway was later emphasized in experimental models of thrombosis. Using intravital real time microscopy several groups have established that the swift recruitment at the edge of the thrombus of leukocytes and leukocyte-derived MPs through P-selection/P-selectin glycoprotein ligand-1 interactions promotes thrombus growth [68, 69, 70].

Atherothrombosis and MP storages pools: inflammation, thrombosis, and plaque vulnerability

Beside the circulating pool of MPs, another reservoir worth considering in the setting of atherothrombosis is the pool of MPs sequestrated in the plaque. Exposed upon plaque disruption, the acellular lipid-rich core of an atherosclerotic plaque represents its most thrombogenic part, with enhanced TF activity being directly supported by TF+-MPs exposing PS [71]. Apoptotic macrophages together with smooth muscle cells (SMCs) may also contribute to TF+-MP accumulation in the lipid core. In vitro, under settings of minimal apoptosis, human SMC release TF+-MPs whereas TF expression is up-regulated by both native or aggregated low-density lipoprotein (LDL), the engagement of the latter with LDL receptor-related protein leading to the expression of cellular TF activity and TF+-MP release [72]. A recent report has demonstrated that plaque MPs induce T cell proliferation, and that immunoglobulins are trapped within macrophage-derived MPs. Plaque sequestered MPs express major histocompatibility complexes on their surface and activate CD4+ T lymphocytes, possibly cooperating locally with the scarce B lymphocytes to produce immunoglobulins directed against plaque antigens. Besides being an important determinant of plaque thrombogenicity, MPs might thus play a previously unrecognized role in modulating tissue inflammation as supported by their proliferative effect on CD4+ T lymphocytes [73].

In the kinetics of arterial thrombus formation, the respective contributions of blood-borne TF spread by MPs and the trapped pool of TF+-MPs are still on debate [74]. To decipher the role of circulating or sequestered TF+-MPs, reciprocal bone marrow transplants were performed between wild-type and engineered mice expressing minimal TF [75, 76]. Results suggest that whereas arterial vessel-wall TF is involved in the initiation of platelet activation, blood-borne TF spread by MPs mediates thrombus propagation. Once vessel-wall TF is covered by a layer of fibrin and platelets, the plaque might become impermeable to circulating clotting factors. Thus, blood-borne TF would prevail in puncture wounds or venous thrombosis, its contribution being less effective in the presence of high amounts of plaque TF.

In addition to their prothrombotic and proinflammatory potential within the atheromatous plaque, MPs are also likely to contribute to plaque neoangiogenesis and proteolysis, 2 major determinants of plaque vulnerability. MPs isolated from plaques enhance VEGF production endothelial proliferation and neovessel formation in a CD40L dependent pathway [77]. Increased vasa vasorum neovascularization can be associated with a higher risk of intraplaque hemorrhage and subsequent rupture or thrombosis [78]. The recent recognition of erythrocyte-derived MPs as another important contributor to the pool of trapped MPs is in line with the latter paradigm [73]. However, although deleterious within the plaque, MPs contribute to vascular repair. For instance, platelet-derived MPs (PMPs) augment the adhesion of early angiogenic cell outgrowth to extracellular matrix components resulting in accelerated re-endothelialization after arterial denudation injury [79]. Because MPs harbored functional metalloproteinase, one could not exclude that they promote matrix proteolysis, a main factor in plaque destabilization [80]. Recently, monocyte- and endothelial-derived MPs were shown to harbor functional urokinase able to promote the transformation of plasminogen into plasmin. In situ, this uPA-driven cross talk generates plasmin with a high efficiency underlining its potential physiological relevance in fibronolysis and matrix proteolysis induced by inflammatory cells or derived-MPs. Indeed, plasmin formed on these cellular membranes is implicated in the proteolytic processing of extracellular matrix components, cell migration, angiogenesis through endothelial progenitor cell homing, and in cell blebbing and shedding [81, 82, 83]. TACE/ADAM-17 is another metalloprotease evidenced in human atherosclerotic plaques that may contribute to plaque vulnerability and cleaves tumor necrosis factor (TNF) and its receptors, TNFR-1 and TNFR-2. TACE+-MPs from the plaque were shown to promote the shedding of the endothelial protein C receptor (EPCR) from endothelial cells, providing another mechanism by which they contribute to enhanced in situ thrombogenicity [84, 85]. Indeed, the loss of EPCR from the endothelial cell surface would impede the binding of anticoagulant activated protein C and the limitation of thrombin auto-amplification. Finally, endothelial MPs themselves are part of the amplification loop leading to the [85] enhanced in situ thrombogenicity. Released upon various types of aggression, they induce monocyte TF expression and procoagulant activity [86].

Microparticles in venous thrombosis

By contrast to the mechanisms underlying arterial thrombosis, several lines of evidence have suggest that blood-borne TF could be a determinant factor in venous thrombosis, especially in the setting of neoplasia, the contribution of tissular MPs trapped within the vascular wall being probably marginal [87, 88, 89]. Another crucial difference between arterial and venous thrombotic process rely on the differential impact of blood flow, venous stasis being a crucial determinant of venous thrombosis whereas arterial thrombosis likely occurs at site of high shear stress. It should also be emphasized that venous stasis promotes cell-cell and MP-cell interactions reinforcing thrombotic and proinflammatory loops. The careful dissection of the sequence linking endothelial damage and thrombus growth in animal models has allow a better understanding of the role of MP and TF in venous thrombosis. In a rat model of inferior vena cava ligation, TF staining was observed in both leukocytes and endothelial cells associated with the clot [90]. Likewise, one study showed that inhibition of the TF factor VIIa complex reduced thrombosis in an inferior vena cava ligation model in primates [91]. As depicted in animal models using intravital real time microscopy to monitor the thrombus growth, it is likely that under stasis, leukocyte- and tumor-derived MPs expressing P-selectin glycoprotein-1 bind to endothelial and platelet P-selectin. In this setting, it is worth mentioning that high plasma levels of soluble P-selectin were found predictive of venous thromboembolism in cancer patients [92]. Several studies demonstrated that levels of procoagulant MPs, eventually harboring enhanced circulating TF activity are significantly elevated in cancer patients with respect to healthy individuals or non cancer patients [89]. A recent retrospective analysis using impedance-based flow cytometry to detect TF-positive MPs reported that tumor-derived TF-positive MPs are elevated in cancer patients with venous thromboembolism [93, 94]. The putative role of TF+-MPs as a marker of venous thrombotic risk was recently assessed by measurement of MP TF activity. Among 11 pancreatic cancer patients, MP TF activity increased over time (30 weeks) in the two patients who developed VTE whereas such activity remained at baseline in patients with no sign of thrombosis [88, 89]. Although the limited size of the cohort did not allow to raise definite conclusion, this result suggested that MP TF activity may be predictive of cancer-associated VTE and that TF+-MPs may be the vehicle that transports the procoagulant activity from the tumor site to the deep veins to trigger thrombosis [73].

The deleterious role of MPs in the regulation of vascular tone

Apart their role in the direct modulation of prothrombotic, proadhesive and proinflammatory properties of vascular cells, circulating MPs could also be an important regulator of the vascular tone, a key component of the Virchow’s triad. Indeed, circulating endothelial-derived MPs (EMPs) correlate with endothelial dysfunction in patients with ischemic left ventricular dysfunction [95] or within the coronary circulation [96]. Platelet-derived MPs are a source of thromboxane A2, a potent regulator of vascular tone, as shown in rabbit aorta [97]. In addition, MPs from apoptotic T lymphocytes impair endothelium-dependent relaxation through eNOS down-regulation and caveolin-1 overexpression [98]. Endothelial dysfunction was also targeted by MPs derived from apoptotic SMCs that diminished NO production in mouse aorta [99].

Microparticles in immune-mediated acquired thrombophilic disorders

In “immune-mediated acquired thrombophilic disorders” such as the antiphospholipid syndrome and heparin-induced thrombocytopenia, auto-antibodies could contribute to the release of procoagulant MPs. Elevated levels of EMPs are a common characteristic of antiphospholipid syndrome, solely detected in patients with systemic lupus erythematosus presenting antiphospholipid antibodies and correlated with the lupus anticoagulant moiety that is strongly associated with thrombotic propensity. Plasma samples from antiphospholipid syndrome patients promote EMP release, pointing at a possible antiphospholipid antibody-driven effect. Heparin-induced thrombocytopenia is a common cause of drug-related immune-mediated thrombocytopenia known to favor thrombotic diathesis and the production of antibodies against circulating heparin platelet factor-4 complexes, involved in platelet crosslinking and activation. In patients, circulating MPs expose GPIb, GPIIbIIIa, P-selectin, and thrombospondin and were found highly thrombogenic with the potency to trigger the activation. Likewise, in other drug-induced thrombocytopenia, elevated levels of procoagulant MPs could be detectable as the consequence of drastic platelet stimulation. In the general context of cytopenia, because shed MPs provide an additional phospholipid procoagulant catalytic surface one should keep in mind that a careful evaluation of the thrombotic and bleeding risk should take into account (1) the extent of thrombocytopenia and (2) the extent of procoagulant MP shedding. For instance, in patients with paroxysmal nocturnal hemoglobinura (PNH) patients with pancytopenia and high levels of circulating MPs should be at low risk of bleeding, whereas PNH patients with pancytopenia but low levels of circulating MPs might probably be [100]. Likewise, elevated levels of circulating platelet MPs were precisely found protective against bleeding in patients with auto-immune thrombocytopenia but were associated with the occurrence of small cerebral vessel infarcts when very high [101].

Microparticles in the restoration of hemostasis: role in transfusion and in hemophilia

If the drastic shedding of procoagulant MPs appears clearly noxious in thrombotic disorder, this does not necessarily endorse their invariably harmful nature, the activation of blood coagulation being considered beneficial in bleeding disorders (see above) [64]. In humans, part of the beneficial effect of rVIIa in hemophilic patients could rely on the shedding of procoagulant MPs, some of them expressing TF [102]. Because MPs are also present, and accumulate, in blood products such as erythrocyte and platelet concentrates during storage, one could not exclude that they act as submicron clotting bumbs. Recently, the use of warm fresh whole blood (WFWB) transfusion has gained interest in patients with severe life-threatening hemorrhagic traumatic injuries, retrospective analysis of American casualties in Iraq and Afghanistan showing lower mortality rates associated with this approach [103]. One of the singularities of the WFWB is the transfusion of leukocytes and derived MPs. Because cell stimulation leads to functional TF exposure on both moieties, it is likely that they might act as potent effectors of the coagulation process in transfused patients, explaining the benefit of WFWB in the restoration of efficient vascular homeostasis [104].

MPs as a source of proinflammatory effectors

Proinflammatory effects of MPs in the vessel

In the settings of inflammatory disorders, various agonists such as cytokines, LPS, microorganisms, toxins, auto-antibodies, heatstroke [105] were described as potent inducers of membrane remodeling, PS exposure and MP release. In the early 1990s, we have demonstrated that following LPS stimulation, monocyte disseminate procoagulant and proadhesives activities in the form of circulating MPs enriched in TF, PS, and cytoadhesins [106]. The importance of this pathway in inflammatory induced coagulopathy was underlined by several experimental models. In endotoxemic mice, levels of TF+-MPs activity correlate with coagulation [107]. Likewise, in enterohemorrhagic E. coli infection associated with the release of Shiga toxin, the formation of aggregates between platelets and leukocytes led to the generation of TF+-MPs that could contribute to microthrombi, tissue damage, and the onset of hemolytic and uremic syndrome [108]. Similarly, in Ebola hemorrhagic fever, overexpression of TF in monocyte/macrophages and derived-MPs is a key event triggering the onset of intravascular disseminated intravascular coagulopathy [109]. The link between sepsis and increased coagulopathy was nicely illustrated by the work of Aras and coworkers in human endotoxemia [110]. Volunteers receiving an infusion of endotoxin exhibited an early increase in TF+-MPs and to a lesser extent of EMPs [110].

Numerous studies have pointed at the proinflammatory effects of MPs on the endothelium and its phenotype alteration. It is likely that MPs, by releasing a broad panel of inflammatory mediators are key features in the regulation of inflammatory process and cellular infiltration within tissues. The first demonstration that MPs may affect the endothelial phenotype came from studies depicting a transcellular PMP-mediated delivery of arachidonic acid to endothelial cells and the concomitant expression of cyclooxygenase type 2 [111]. In addition, MPs are able to stimulate the release of proinflammatory endothelial cytokines, including IL-6 or MCP-1 and to induce the expression of ICAM-1, VCAM-1 and E-selectin cytoadhesins [58, 111, 112, 113]. Amplification loops are probably maintained through the MP-driven expression of appropriate counter-receptors at leukocyte membrane surface. More recently, the transcellular delivery of the chemokines RANTES by platelet-derived MPs was demonstrated to promote monocyte arrest at the inflamed endothelial surface and diapedesis [60].

Beyond their effect on vascular cells, circulating MPs also contribute to an enhanced inflammatory vascular response. MPs are lipid substrate for secretory phospholipase A2 enabling the production of lysophosphatidic acid, a potent proinflammatory mediator and platelet agonist. In addition, the release by activated platelets of inorganic polyphosphates, a polymer that directly binds to and activates the plasma protease factor XII, triggers the secretion of the inflammatory mediator bradykinin and induces fluid extravasation in microvessels [114]. Amplification loops may also result from MP-driven cell crosstalk in aggregates, as suggested in vitro by PMP-driven neutrophil aggregation and activation [115]. Monocyte-derived MPs constitute a secretion pathway for IL-1β and have chemotactism abilities [116]. Another reservoir of proinflammatory MPs is TACE+-MPs from the atherosclerotic plaque that are able to stimulate the release of TNF-α by the endothelial cell (see above).

Proinflammatory effects of MPs in other biological fluids: insights from clinical issues

Inflammatory diseases

Apart from their putative role in the regulation of inflammatory status within the vasculature, a growing literature reporting the proinflammatory character of MPs prompted the clinical investigation in other storage compartments and fluids. In immunoinflammatory diseases and in the absence of an infectious agent (i.e., in non-septic conditions), the nature of the factors triggering the production of proinflammatory TNF and IL-1β cytokines, mainly produced upon activation of monocytes, is still elusive. Imbalance in cytokine homeostasis plays an important role in the pathogenesis of chronic inflammatory diseases such as multiple sclerosis and rheumatoid arthritis (RA). In such settings, mechanisms ruling the balance between TNF and IL-1β and their inhibitors (soluble receptors and sIL-1Ra secreted IL-1 receptor antagonist), escape normal controls. It was previously demonstrated that direct cellular contact with stimulated T cells induces the massive up-regulation of IL-1β and TNF-α in monocytes. Interestingly, MPs shed by stimulated T cells mimic the cellular contact achieved by their parental cell [117]. At the opposite, HDL were demonstrated to interfere in the binding of T lymphoyte-derived MPs to monocytes, thereby reducing the production of proinflammatory cytokines [118].

In the synovial fluid of patients with RA, MPs from monocytes and granulocytes have been identified as procoagulant entities bearing TF and as modulators of chemokine and cytokine production by Fibroblast like synoviocytes (FLS) [119, 120]. Using genetically deficient mice and pharmacological blockade, PMPs could recently be identified in joint synovial fluids as a possible marker of platelet adhesion on the collagen of the FLS extracellular matrix via platelet glycoprotein GPVI. In this particular setting, it is likely that the contact between the extracellular matrix enriched in collagen IV and the platelets contributed to MP release. Moreover, released MPs were identified as potent effectors contributing to joint inflammation through the up-regulation of IL-6 and IL-8 in FLS [121].

Cerebral malaria

Cerebral malaria is another example of the MP proinflammatory potential. During the final stages, alteration of the endothelial blood–brain barrier and the sequestration of Plasmodium falciparum-parasitized red blood cells (PRBCs), platelets, and leukocytes within the brain microvasculature occur. MP generation appears a critical step in this noxious loop. Indeed, circulating EMPs in Malawian children were found significantly elevated during crisis and were correlated with plasma TNF concentration. At follow-up, both EMP and TNF levels returned to control values [122]. Interestingly in deficient mice for the ABCA-1 gene, an occasional floppase, protection against neuropaludism could be evidenced, possibly related to the reduced shedding of vascular cells, including platelets and macrophages. Importantly, MPs from infected mice displayed enhanced proinflammatory and procoagulant properties with respect to normal mice [123]. Furthermore, they induced macrophage activation as mirrored by CD40 up-regulation and TNF production.

Experimental data have established erythrocytes-derived MPs as the central inducer of systemic inflammation during malaria infection. Evidences were provided that the binding of PMPs to PRBCs enables the transfer of cytoadhesins to PRBCs leading to a major enhancement in PRBC adherence to the endothelium. It is thus likely that PMPs participate in the pathogenesis of cerebral malaria while interacting with both PRBCs and endothelium [124].

Acute lung injury

Elevated levels of TF+-MPs could be evidenced in the broncho-alveolar fluid of patients with adult respiratory distress syndrome. In murine models, EMPs contribute by themselves to acute lung injury. This conclusion was initially based on multiple measures of histological parameters, lung permeability indexes and dysregulation of endothelial nitric oxide formation and release [125]. Moreover, recent data point at EMPs as inducers of inflammation. They significantly increase pulmonary and systemic IL-1β and TNF-α levels which correlate with increased neutrophil recruitment to the lung. Within the lung, the release of myeloperoxidase, a large source of oxidative injury, is thought to reinforce the noxious action of MPs release [126].

Sepsis

In sepsis, NO and bacterial elements are responsible for the generation of platelet-derived exosome that have an active role in vascular signaling. They behave as redox-active particles that can activate endothelial cell caspase-3 and induce apoptosis through the generation of superoxide, NO and peroxynitrite [127]. Inoculation of MPs isolated from septic rats in healthy counterparts reproduced the hemodynamic septic inflammatory patterns associated with oxidative and nitrosative stresses. The increase in superoxide ion production and NF-KB activity, the overexpression of inducible NO synthase and the overproduction of NO together with decreased endothelial NO synthase activation were evidenced in MP-treated rats [128].

Protective effects of MPs

Altogether, it is likely that the respective importance of each cell lineage and stress in the development of a MP-driven response varies with respect to the location of the thrombotic process and according to the size and nature of the vessel (macroangiopathy/microangiopathy and arterial/venous). MPs could substantially interfere in the three aspects of the Virchow’s triad namely vascular inflammation and thrombogenicity, blood thrombogenicity and alteration of vascular reactivity. At the opposite, the shedding of endothelial-derived MPs could be viewed as beneficial since they contribute to the “sorting” of caspase-3, thereby preventing endothelial cell apoptosis and detachment [129].

Anti-inflammatory effects of MPs

Another complexity was brought to our current understanding of MP-mediated inflammatory responses by Gasser and Schifferli [130] who suggested that MPs could promote anti-inflammatory properties. It is likely that neutrophils are able to release potent anti-inflammatory effectors, in the form of membrane vesicles, at the earliest stage of inflammation, already providing a drive to its resolution [130]. Indeed, neutrophil MPs have no proinflammatory activity on human macrophages assessed by the release of IL-8 and TNF-α but increase the secretion of TGF-β, a potent inhibitor of macrophage activation. As a proof of evidence, such vesicles are able to inhibit the macrophage anti-inflammatory response to LPS.

Similarly, erythrocytes-derived ectosomes, another name for MPs, may react with and down-regulate cells of the immune system. MPs are taken up by macrophages, and they significantly reduce their activation by zymosan A and LPS, with a drastic drop in TNF-α and IL-8 release, an effect that lasted for at least 24 h [131]. Other mechanisms were recently proposed to depict the link between MPs and the resolution of the inflammatory response. When shed from adherent neutrophils to endothelial cells, MPs bear annexin A1, an endogenous anti-inflammatory protein able to inhibit the adhesion of stimulated neutrophils adhesion to the endothelium [132]. AnxA1 acts in a paracrine/autocrine fashion and triggers signaling pathways that down-regulate PMN activation while it up-regulates the anti-inflammatory IL-10, leading to the inhibition of inducible NO synthase mRNA expression. The relevance of this anti-inflammatory pathway was nicely illustrated in cystic fibrosis (CF), a disease caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. Typical chronic bacterial infections and inflammation in the lung in CF was associated with lower expression of AnxA1. In vitro assays with human PMN demonstrated that a CFTR inhibitor prompted the sorting of AnxA1 in MPs. A defective counter-regulatory property of AnxA1 in CF cells, owing to its sorting in MPs, might thus contribute to the inflammatory background of the disease, at least in situ [133].

Anticoagulant potential of MPs: a possible role in cytoprotection and in the limitation of the inflammatory response

Anionic phospholipids exposed by activated vascular cells or derived MPs promote the assembly of both procoagulant and protein C anticoagulant enzyme complexes, the latter probably requiring ~10-fold higher PS concentrations. Depending on their cell origin, the presence of anticoagulant thrombomodulin, TFPI, EPCR or protein S at MP surface is another indication of an eventual MP-driven anticoagulant regulatory pathway [85, 134]. Strikingly, although MPs from stimulated endothelial cells are appropriately recognized as potent procoagulant mediators, especially when they harbor active TF, EMPs induced by APC were recently demonstrated to support anticoagulant activities enabling efficient FVa and FVIIIa inactivation at membrane surface. Of note, the MP anticoagulant effect was borne by MPs that exposed EPCR and could be also observed on monocyte-derived MPs. Both APC and PAR-1 active sites were mandatory in anticoagulant MP release [135]. Treatment by anticoagulant rhAPC known to reduce disseminated coagulation and mortality in human severe sepsis is also neuroprotective in ischemic stroke. In a recent report, Pérez-Casal and coworkers provide evidences that part of the cytoprotective effects of APC is mediated by MPs and that APC+-MPs levels are raised in patients with sepsis after treatment by rhAPC [136]. A variety of APC cytoprotective effects like the modulation of cytokine or cytoadhesin transcription levels, antiapoptotic activity and endothelial barrier stabilization, could be induced in vitro by APC+-MPs [137]. In a baboon, heatstroke model and despite a lack of demonstrable antithrombotic effect, rhAPC cytoprotective and anti-inflammatory properties were confirmed by a decrease in thrombomodulin, IL-6, and procoagulant MP levels presumably reflecting diminished cell apoptosis. [138]. This observation also suggests that MP-conveyed APC displays cytoprotective effects at lower doses than those necessary to blunt the activation of coagulation. In humans, heatstroke is characterized by substantial leukocyte activation and MP release that could mediate intravascular coagulopathy in severe clinical settings [105]. Altogether, it is likely that during sepsis or heatstroke, the infusion of APC induces the release of anticoagulant MPs harboring EPCR and APC whilst it diminishes the generation of procoagulant MPs shed upon vascular cell apoptosis.

Of particular importance for pharmacological development, the report by Griffin demonstrating that the APC cytoprotective effects could at least in part be dissociated from its anticoagulant activity, supports the concept that EMPs do not convey an univoque prothrombotic potential, and that anticoagulant and cytoprotective properties may override thrombotic ones upon APC stimulation [137]. Altogether, these observations shed light on the physiopathological relevance of MPs as modulators of cell death and survival eventually associated to that of the inflammatory balance.

Protective effect of MPs on vascular hyporeactivity

In critical disease conditions such as septic shock or trauma, MPs were first considered as deleterious conveyers prompting endothelial dysfunction, blood coagulation, intravascular disseminated coagulopathy and death. Elevated MP levels of platelet, granulocyte and endothelial origin were reported in patients with septic shock, meningococcemia or traumatic brain injury [54, 139, 140]. A beneficial effect conveyed by MPs in patients with septic shock was more recently suggested by the observation that higher mortality rates and organ dysfunction were associated with lower levels of endothelial, platelet, and leukocyte-derived MPs [141]. Because MPs from patients with septic shock enhance contraction of aorta in lipopolyssacharide-treated mice and possibly through the delivery of thromboxane A2, a challenging hypothesis would be that MPs protect septic patients from vascular hyporeactivity by maintaining a tonic pressure response [139]. In line with the observation, ex vivo application of rhAPC, that increases MP generation, also improves arterial contractility and endothelial dysfunction in a murine endotoxemia model [136, 142].

Microparticles and immunity

If the immune response is able to trigger the release of procoagulant MPs as demonstrated in acute allograft rejection following heart transplantation [51], shed MPs are also known to harbor major histocompatibility complex molecules, making them responsible for the stimulation of T cells in an antigen-specific manner, eventually promoting an immune response in vivo. Exosomes, that are other small vesicles, released as a consequence of the fusion of intraluminal membrane-bound multivesicular bodies with the plasma membrane are important mediators in immunity. When released by antigen-presenting cells, one of the major properties of exosomes is to induce antigen-specific T cell activation [143]. Because MPs display autoantigens, such as RNA and DNA, in a highly immunostimulatory manner, they may also act as potent autoadjuvants. In the bone marrow, nucleic-acid-containing MP autoadjuvants might induce B cell tolerance, whereas in the periphery, they might stimulate mature B cells that have escaped central tolerance. Indeed, because MP autoadjuvants can trigger several receptors, they could effectively provide apoptotic or activating signals to B cells [144]. Conversely, MPs could also support a sort of physiological immune escape as that described in pregnancy, a situation in which the invading trophoblast represents a semi-allograft. In this particular setting, the shedding of Fas ligand (CD95 ligand)-exposing MPs may constitute a mechanism by which trophoblast cells promote a state of immune privilege and, therefore protect themselves from maternal immune recognition. Indeed, Abrahams et al. have demonstrated that first-trimester trophoblast cells lack membrane-associated FasL but constitutively secrete active FasL through the release of MPs, and that such MPs are able to induce Fas-presenting T cell death by apoptosis [145]. Cancer represents another situation in which MPs constitute a way to escape from the immune system. Epithelial ovarian cancer cells were shown to secrete functional FasL via the release of MPs [146]. In contrast, normal ovarian epithelial cells express but do not secrete FasL. Tumor-derived MPs may serve as shuttle for FasL, which is known to induce apoptosis of sensitive human lymphoid target via interaction with the Fas receptor. Together, these two studies suggest a mechanism by which tumors might neutralize Fas-bearing immune cells, thus facilitating escape and promoting survival [146]. Of note, tumor-derived vesicles were proposed as actors in the establishment of tumoral niche through the degradation of the extracellular matrix, the activation of fibroblasts, of their resistance to apoptosis and the promotion of an increased motility [147].

Conclusions

MPs serve as biological shuttles enabling cell interactions in various pathological settings and may represent one of the elements coupling thrombosis and inflammation. If the drastic shedding of procoagulant MPs appears clearly noxious in thrombotic disorders or in some models of inflammation-induced coagulopathy, this does not necessarily endorse their invariably harmful nature. In the vessel, endothelial cytoprotection reported in the early regulation of inflammation-induced coagulopathy is emblematic of the beneficial effects other than the prevention of blood leakage. Because of their multiple properties that are characteristic of a private response of the parental cell, MPs prompt target cells through different ways: they are able to cargo active enzymes or anti-inflammatory proteins and to deliver effectors of cell signaling. MP favor the establishment of an integrated communication network in biological fluids. Further investigations of the mechanisms tuning MP generation and of their mode of action should help our current understanding of the significance of MPs in physiopathological issues and appears a prerequisite in the development of a pharmacological regulation.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Olivier Morel
    • 1
    • 2
    • 3
  • Nicolas Morel
    • 4
  • Laurence Jesel
    • 1
    • 2
    • 3
  • Jean-Marie Freyssinet
    • 1
    • 3
    • 5
  • Florence Toti
    • 1
    • 3
    • 5
  1. 1.Faculté de Médecine, Institut d’Hématologie et d’ImmunologieUniversité de StrasbourgStrasbourgFrance
  2. 2.Pôle de CardiologieHôpitaux Universitaires de StrasbourgStrasbourgFrance
  3. 3.INSERM, U.770Le Kremlin-BicêtreFrance
  4. 4.Département de Réanimation des UrgencesHôpital PellegrinBordeauxFrance
  5. 5.Faculté de MédecineUniversité Paris-Sud 11Le Kremlin-BicêtreFrance

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