MPs are produced following cellular activation or apoptosis. The increase in intracellular calcium activates various cytosolic enzymes, including calpains, that cleave the cytoskeleton and facilitate the role of procaspase-3 in apoptosis 11. As a response to stimulus, the cytoskeleton is reorganized and the asymmetric distribution of the phospholipid membrane modified with exposure of phosphatidylserine at the cell surface. Cellular blebbing then occurs, ultimately leading to the release of MPs. In addition to phosphatidylserine exposure, protein-lipid raft domains are formed and furnish the MP with its specificities and biological roles 12 (Figure 1).

The cellular origin of MPs can be determined by assessment of the antigens that they expose at their surface. However, the complete protein content of MPs remains difficult to establish. More than 300 proteins have been reported by proteomics, some of which are cytosolic and some membranous 13. The MP phenotype is, however, known to vary according to cellular origin and parental cell response to stimulus 714.

Although bearing phosphatidylserine, which is a signal for phagocytosis, MPs seem to survive longer than their parental apoptotic cell, probably because of their size, which does not allow optimal exposure of a cluster of senescence signals. Dasgupta and colleagues 15 recently described the major role of lactadherin in the removal of phosphatidylserine-expressing platelet MPs from human plasma. Lactadherin is a macrophage opsonin that mediates the clearance of apoptotic lymphocytes and knockout lactadherin (-/-) mice have increased levels of circulating platelet MPs and a hypercoagulable state; lactadherin supplementation restores the normal clearance of MPs. To date, there are no data on the effect of MP clearance on the haemostatic balance under physiological or pathological settings.

As mediators of cellular communication, MPs are actors and possible mediators in the interplay between throm-bosis and inflammation, a process previously described for vascular injury in inflammatory diseases 5. They can transfer receptors, organelles, mRNA and other proteins to target cells 16 and also comprise a secretion pathway for several cytokines, such as mature IL-1β 17. The multiple properties of MPs and the variety of their possible cellular targets support them having a key role in cell reprogramming and tissue remodeling with physiological or pathological consequences 4. Thus, MPs could play a major role in propagating proinflammatory and procoagulant states in sepsis. In the vascular compartment, including the arterial wall, the particular settings of sepsis and the tuning abilities of MPs point to the endothelium as a pivotal target 1819.

Thrombin generation requires activation of coagulation factors, which is made possible after their assembly on a catalytic surface constituted of anionic phospholipids. Cell activation constitutes the first step by furnishing exposed phosphatidylserine with a negative charge. The required remodelling of plasma membrane, resulting in phosphatidylserine translocation to the outer leaflet of the plasma membrane, occurs in platelets, endothelial cells and monocytes at sites of vascular damage or injury. Calcium ion-mediated interactions between gamma-carboxyl groups of vitamin-K-dependent factors and phosphatidylserine comprise the key step in this assembly, explaining the efficacy of anti-vitamin K treatments in hypercoagulable states 22.

At the monocyte surface a possible encrypted preformed TF would be de-encrypted by plasma membrane remodelling, thereby allowing the (auto-)activation of factor VII. Indeed, TF expression at the surface of monocyte-derived MPs has been demonstrated during in vitro endotoxin stimulation 23. Although TF is the primary initiator of blood coagulation, whether there is a blood-borne TF (activity) is still debated, but there is growing evidence that this activity is directly tied to MPs 524. TF-bearing MPs can interact with neutrophil granulocytes by 'paracrine transfer', as demonstrated in vitro 252627. Circulating MPs bearing active TF have been associated with a thrombotic status in human meningococcal sepsis 28 and a primate Ebola fever model 29, pointing to their possible role in the dissemination of a procoagulant potential.

TF-driven coagulation is under the control of Tissue factor pathway inhibitor (TFPI), an inhibitory complex, with factor Xa and protein S as cofactors. Although this inhibits TF-induced thrombin generation, thrombin is still generated during the propagation phase via the Josso loop: platelet-exposed factor XI (of megakaryocytic origin) is activated by the GP_Ibα-thrombin complex present on the surface of activated platelets. Activated platelets, and released GP_Ibα-FXIa bearing MPs, may, in turn, be responsible for increased thrombin generation 303132. In addition to blood-borne TF conveyed by MPs, polymorphonuclear (PMN)-derived MPs likely contribute to an additional amplification loop in the generation of thrombin mediated by MPs (Figure 2).

MPs could contribute to such amplification loops in sepsis. Indeed, ex vivo activation of human neutrophils by endotoxin, platelet activating factor or phorbol myristate acetate can generate MPs bearing active integrin α_Mβ₂ (CD11b/CD18), which is able to activate GP_Ibα 3334.

Interestingly, several cellular models showed that α_Mβ₂ exposed at the MP surface can interact with other ligands, such as urokinase plasminogen activator, plasminogen and metalloproteases MMP-2 and -5, suggesting a role in fibrinolysis and in local tissue remodelling 303435. MPs may also display antithrombotic activities, which would be overwhelmed by procoagulant activities when MPs are released under highly thrombotic conditions, as observed during sepsis or myocardial infarction. Indeed, in purified monocyte suspensions, thrombomodulin anticoagulant activity and TF coexist at the MP surface, but when released by lipopolysaccharide treatment, the TF activity is predominant on MPs 31. The presence of the anticoagulant endothelial protein C receptor (EPCR) at the surface of endothelial-derived MPs (mpEPCR) is another example of a cytoprotective element attached to MPs 32; EPCR is involved in the activation of anticoagulant protein C by the thrombin-thrombomodulin complex. mpEPCR, released in response to activated protein C (APC), may switch the procoagulant properties of endothelial MPs to anticoagulant and anti-apoptotic properties. On the surface of MPs bearing mpEPCR, APC inactivates procoagulant cofactors factor Va and factor VIIIa, thereby down-regulating thrombin generation. Because a circulating soluble form of EPCR (sEPCR) has been described in sepsis, and its concentration possibly correlates with the severity of the illness, the respective contributions of mpEPCR and sEPCR is a matter of clinical relevance. sEPCR binds protein C and APC, thereby blunting their actions. The efficacy of therapeutic activated protein C (rhAPC; drotrecogin alfa (activated)) may depend on the balance between circulating sEPCR and mpEPCR 32. Recent investigations in human endothelial cells reported that free rhAPC and rhAPC bound to mpEPCR have similar effects. rhAPC cleaves protease activated receptor-1 and induces significant changes in the activation/inhibition of genes with direct anti-apoptotic effects or indirect cell barrier protective effects, the latter requiring transactivation of KDR (vascular endothelial growth factor receptor 2/kinase insert domain receptor) via the phosphoinositide 3-kinase-Akt pathway and S1P₁ (sphingosine 1-phosphate receptor) 36.

In sepsis, procoagulant MPs of endothelial, platelet, erythroid, and leukocyte origins have been reported 2837.

During sepsis, the endothelial function is altered and the endothelial surface becomes proadhesive, procoagulant and antifibrinolytic 42. The endothelium is one of the primary targets of circulating MPs, as demonstrated by Barry and colleagues 43in vitro. Indeed, they showed that arachidonic acid exposed by platelet MPs promotes the up-regulation of cyclooxygenase-2 (COX-2) and intercellular adhesion molecules in endothelial cells. Platelet-derived MPs have been shown to modulate interactions between monocytes and endothelial cells. Released proinflammatory endothelial cytokines may themselves also contribute to the production of MPs 44, thereby amplifying the inflammatory response and the consecutive alteration of the vascular function 45. Platelet activating factor present in endothelial cells and leukocytes is also involved in the proinflammatory effect of MPs 46.

Circulating MPs of endothelial origin may thus vary with respect to quantity and phenotype according to the endothelial response and have been reported in inflammatory diseases and disorders 47; the endothelial response to inflammation stimuli may be immediate, delayed or reflect a chronic endothelial activation. They were reported to participate in the regulation of arterial tone in several diseases in which oxidative stress is involved, such as human acute coronary syndromes 48 or preeclampsia 49 associated with altered NO bioavailability 50.

Sepsis induces a phenotypic change of the endothelium and the endothelial surface becomes proinflammatory, expresses cell adhesion molecules (intercellular cell adhesion molecule 1, vascular cell adhesion molecule 1) 51 and becomes prothrombotic through the increased expression of membrane TF and the inhibition of thrombomodulin and EPCR synthesis. In parallel, endothelial cells become capable of recruiting and activating platelets 52.

MPs may be considered as both the cause and the consequence of inflammatory diseases through multiple amplification and regulatory loops affecting vascular cell functions. In vitro incubation of leukocyte-derived MPs with endothelial cells allowed Mesri and Altieri 53 to demonstrate their role in the secretion of inflammatory IL-6 and in the production of TF 54. Furthermore, platelet MPs are able to deliver RANTES at the inflamed endothelium, thus promoting leukocyte recruitment and diapedesis 55. MPs may affect the smooth muscle tissue through the activation of the transcription factor NF-κB and favour the expression of inducible NO-synthase and COX-2, resulting in an increase in NO and vasodilator prostanoids, leading to arterial hyporeactivity 45.

The interactions between platelets, leukocytes and endothelium clearly contribute to the vascular dysfunction observed in sepsis and various MPs were reported to alter the arterial wall directly or indirectly 5657.

Endothelial MPs may play a role in the spread of sepsis inflammatory responses leading to multiple organ dys-function 1858. They may participate in the potentialisation of the procoagulant state associated with sepsis by providing renewed lipid surfaces of human endothelial cells for the generation of thrombin and by up-regulating monocyte TF expression, as demonstrated in vitro 59. In sepsis, blockade of the human TF pathway by TFPI is very quickly overridden, clearing the way for a detrimental procoagulant state 28. Indeed, in humans, a single endotoxin administration provokes a significant increase in endothelial-cell- or monocyte-derived MPs displaying potentiated TF 60. This state is worsened by the exhaustion and/or faulty activation of the two other regulatory molecules, antithrombin and APC.

Several reports illustrate well the cascade of interwoven events that link cellular activation, TF up-regulation, the release of MPs presenting active TF and the triggering of disseminated intravascular coagulopathy and shock 2829. With regard to vascular tone, MPs could promote the significant vasoplegia observed in septic shock 8. Arachidonic acid transfer may up-regulate COX-2 expression and the production of prostacyclin, which is implicated in vasodilation and the inhibition of platelet activation 32.

During sepsis, generated MPs are involved in the modification of the oxidative status; they form microaggregates with circulating neutrophil granulocytes and markedly increase oxidative activity 8. Subunits of NADPH oxidase have been identified in endothelial- and platelet-derived MPs associated with increased production of reactive oxygen species 1861 (Figure 3).