The clinical and functional relevance of microparticles induced by activated protein C treatment in sepsis

Therapeutic Potential of Extracellular Vesicles for Sepsis Treatment

Sepsis is a deadly condition lacking a specific treatment despite decades of research. This has prompted the exploration of new approaches, with extracellular vesicles (EVs) emerging as a focal area. EVs are nanosized, cell-derived particles that transport bioactive components (i.e., proteins, DNA, and RNA) between cells, enabling both normal physiological functions and disease progression depending on context. In particular, EVs have been identified as critical mediators of sepsis pathophysiology. However, EVs are also thought to constitute the biologically active component of cell-based therapies and have demonstrated anti-inflammatory, anti-apoptotic, and immunomodulatory effects in sepsis models. The dual nature of EVs in sepsis is explored here, discussing their endogenous roles and highlighting their therapeutic properties and potential. Related to the latter component, prior studies involving EVs from mesenchymal stem/stromal cells (MSCs) and other sources are discussed and emerging producer cells that could play important roles in future EV-based sepsis therapies are identified. Further, how methodologies could impact therapeutic development toward sepsis treatment to enhance and control EV potency is described.

Preliminary study of microparticle coagulation properties in septic patients with disseminated intravascular coagulation

Background

Sepsis typically results in enhanced coagulation system activation and microthrombus formation. Microparticle (MP) production promotes coagulation and enhances pro-coagulation. This study investigated how circulating MP levels and tissue factor-bearing MP (TF+-MP) activity caused coagulation in patients with septic disseminated intravascular coagulation (DIC).

Methods

Thirty patients with septic DIC and 30 healthy controls were studied from December 2017 to March 2019. Patient blood samples were collected at enrolment (day 1) and on days 3 and 5; DIC scores and Sequential Organ Failure Assessment (SOFA) scores were recorded. TF+-MP activity was measured using TF-dependent factor Xa generation experiments. Circulating MP concentrations were determined by MP capture assay. Clotting factor activity, antithrombin level, soluble thrombomodulin, and serum tissue factor pathway inhibitor (TFPI) concentrations were measured.

Results

Patients with septic DIC had lower circulating MP levels than healthy control patients. Circulating MP levels in patients with septic DIC were positively correlated with DIC scores and negatively correlated with coagulation factors, but TF+-MP activity did not correlate with clotting factor levels and TFPI.

Conclusions

In patients with septic DIC, circulating MP levels are important in promoting coagulation activation and increasing clotting factor consumption. TF+-MP activity may not be the main form of active TF.

Extracellular Vesicles as Markers and Mediators in Sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. It remains a highly lethal condition in which current tools for early diagnosis and therapeutic decision-making are far from ideal. Extracellular vesicles (EVs), 30 nm to several micrometers in size, are released from cells upon activation and apoptosis and express membrane epitopes specific for their parental cells. Since their discovery two decades ago, their role as biomarkers and mediators in various diseases has been intensively studied. However, their potential importance in the sepsis syndrome has gained attention only recently. Sepsis and EVs are both complex fields in which standardization has long been overdue. In this review, several topics are discussed. First, we review current studies on EVs in septic patients with emphasis on their variable quality and clinical utility. Second, we discuss the diagnostic and therapeutic potential of EVs as well as their role as facilitators of cell communication via micro RNA and the relevance of micro-organism-derived EVs. Third, we give an overview over the potential beneficial but also detrimental roles of EVs in sepsis. Finally, we focus on the role of EVs in selected intensive care scenarios such as coagulopathy, mechanical ventilation and blood transfusion. Overall, the prospect for EV use in septic patients is bright, ranging from rapid and precise (point-of-care) diagnostics, prevention of harmful iatrogenic interventions, to using EVs as guides of individualized therapy. Before the above is achieved, however, the EV research field requires reliable standardization of the current methods and development of new analytical procedures that can close the existing technological gaps.

Immunohaemostasis: a new view on haemostasis during sepsis

Host infection by a micro-organism triggers systemic inflammation, innate immunity and complement pathways, but also haemostasis activation. The role of thrombin and fibrin generation in host defence is now recognised, and thrombin has become a partner for survival, while it was seen only as one of the "principal suspects" of multiple organ failure and death during septic shock. This review is first focused on pathophysiology. The role of contact activation system, polyphosphates and neutrophil extracellular traps has emerged, offering new potential therapeutic targets. Interestingly, newly recognised host defence peptides (HDPs), derived from thrombin and other "coagulation" factors, are potent inhibitors of bacterial growth. Inhibition of thrombin generation could promote bacterial growth, while HDPs could become novel therapeutic agents against pathogens when resistance to conventional therapies grows. In a second part, we focused on sepsis-induced coagulopathy diagnostic challenge and stratification from "adaptive" haemostasis to "noxious" disseminated intravascular coagulation (DIC) either thrombotic or haemorrhagic. Besides usual coagulation tests, we discussed cellular haemostasis assessment including neutrophil, platelet and endothelial cell activation. Then, we examined therapeutic opportunities to prevent or to reduce "excess" thrombin generation, while preserving "adaptive" haemostasis. The fail of international randomised trials involving anticoagulants during septic shock may modify the hypothesis considering the end of haemostasis as a target to improve survival. On the one hand, patients at low risk of mortality may not be treated to preserve "immunothrombosis" as a defence when, on the other hand, patients at high risk with patent excess thrombin and fibrin generation could benefit from available (antithrombin, soluble thrombomodulin) or ongoing (FXI and FXII inhibitors) therapies. We propose to better assess coagulation response during infection by an improved knowledge of pathophysiology and systematic testing including determination of DIC scores. This is one of the clues to allocate the right treatment for the right patient at the right moment.

Circulating microparticles: challenges and perspectives of flow cytometric assessment

Circulating blood microparticles are likely to play a significant role as messengers of biological information. Their accurate quantification and characterisation is challenging and needs to be carefully designed with preferable usage of fresh minimally-processed blood samples. Utilisation of flow cytometers specifically designed for analysis of small-size particles is likely to provide considerable methodological advantages and should be the preferable option. This viewpoint manuscript provides a critical summary of the key methodological aspects of microparticle analysis.

Note: The review process for this viewpoint article was fully handled by Christian Weber, Editor in Chief.

Pharmacological modulation of procoagulant microparticles improves haemodynamic dysfunction during septic shock in rats

Circulating microparticles play a pro-inflammatory and procoagulant detrimental role in the vascular dysfunction of septic shock. It was the objective of this study to investigate mechanisms by which a pharmacological modulation of microparticles could affect vascular dysfunction in a rat model of septic shock. Septic or sham rats were treated by activated protein C (aPC) and resuscitated during 4 hours. Their microparticles were harvested and inoculated to another set of healthy recipient rats. Haemodynamic parameters were monitored, circulating total procoagulant microparticles assessed by prothrombinase assay, and their cell origin characterised. Mesenteric resistance arteries, aorta and heart were harvested for western blotting analysis. We found that a) the amount and phenotype of circulating microparticles were altered in septic rats with an enhanced endothelial, leucocyte and platelet contribution; b) aPC treatment significantly reduced the generation of leucocyte microparticles and norepinephrine requirements to reach the mean arterial pressure target in septic rats; c) Microparticles from untreated septic rats, but not from aPC-treated ones, significantly reduced the healthy recipients’ mean arterial pressure; d) Microparticle thromboxane content and aPC activity were significantly increased in aPC-treated septic rats. In inoculated naïve recipients, microparticles from aPC-treated septic rats prompted reduced NF-κB and cyclooxygenase-2 arterial activation, blunted the generation of pro-inflammatory iNOS and secondarily increased platelet and endothelial microparticles. In conclusion, in this septic shock model, increased circulating levels of procoagulant microparticles led to negative haemodynamic outcomes. Pharmacological treatment by aPC modified the cell origin and levels of circulating microparticles, thereby limiting vascular inflammation and favouring haemodynamic improvement.

Endothelial microparticles released by activated protein C protect beta cells through EPCR/PAR1 and annexin A1/FPR2 pathways in islets

Islet transplantation is associated with early ischaemia/reperfusion, localized coagulation and redox-sensitive endothelial dysfunction. In animal models, islet cytoprotection by activated protein C (aPC) restores islet vascularization and protects graft function, suggesting that aPC triggers various lineages. aPC also prompts the release of endothelial MP that bear EPCR, its specific receptor. Microparticles (MP) are plasma membrane procoagulant vesicles, surrogate markers of stress and cellular effectors. We measured the cytoprotective effects of aPC on endothelial and insulin-secreting Rin-m5f β-cells and its role in autocrine and paracrine MP-mediated cell crosstalk under conditions of oxidative stress. MP from aPC-treated primary endothelial (EC) or β-cells were applied to H2 O2 -treated Rin-m5f. aPC activity was measured by enzymatic assay and ROS species by dihydroethidium. The capture of PKH26-stained MP and the expression of EPCR were probed by fluorescence microscopy and apoptosis by flow cytometry. aPC treatment enhanced both annexin A1 (ANXA1) and PAR-1 expression in EC and to a lesser extent in β-cells. MP from aPC-treated EC (eMaPC ) exhibited high EPCR and annexin A1 content, protected β-cells, restored insulin secretion and were captured by 80% of β cells in a phosphatidylserine and ANXA1-dependent mechanism. eMP activated EPCR/PAR-1 and ANXA1/FPR2-dependent pathways and up-regulated the expression of EPCR, and of FPR2/ALX, the ANXA1 receptor. Cytoprotection was confirmed in H2 O2 -treated rat islets with increased viability (62% versus 48% H2 O2 ), reduced apoptosis and preserved insulin secretion in response to glucose elevation (16 versus 5 ng/ml insulin per 10 islets). MP may prove a promising therapeutic tool in the protection of transplanted islets.

Applicability of extracellular vesicles in clinical studies

BACKGROUND

Extracellular vesicles (EVs) are submicron cellular fragments that mediate intercellular communication. EVs have in the last decade attracted major interest as biomarkers or platforms for biomarkers of health and disease. To better understand the reasons why despite great expectations and considerable effort, EV-based methods have not yet been introduced into clinical practice, we present a systematic analysis of published results of clinical studies.

MATERIALS AND METHODS

Clinical studies on populations of body fluid samples, published from 2010 to including 2015, applying centrifugation of fluid human samples with centrifuge accelerations up to about 25 000 g and flow cytometry for detection of EVs were analysed with respect to statistical significance (p), statistical power (P), clinical significance (CS), defined as the difference between the means divided by the sum of standard deviations, and size of the populations (N_min ), defined as the number of samples in the smaller group.

RESULTS

Final analysis included 65 publications with 716 comparisons reporting 308 (43%) statistically significant differences (P < 0·05), 242 (34%) had statistical power P > 0·8 and 88 (12%) had clinical importance CS > 1·96. None of comparison with CS > 1·96 included populations in which the smaller group consisted of 50 or more samples.

CONCLUSIONS

To fulfil claimed expectations for EV-based methods as promising diagnostic tools, more evidence on EV-based mechanisms of diseases should be gathered. Also, the methods of EV harvesting and assessment should be improved to yield better repeatability and thus allow clinical studies with larger number of samples.

Nano-zymography Using Laser-Scanning Confocal Microscopy Unmasks Proteolytic Activity of Cell-Derived Microparticles

Cell-derived microparticles (MPs) are nano-sized vesicles released by activated cells in the extracellular milieu. They act as vectors of biological activity by carrying membrane-anchored and cytoplasmic constituents of the parental cells. Although detection and characterization of cell-derived MPs may be of high diagnostic and prognostic values in a number of human diseases, reliable measurement of their size, number and biological activity still remains challenging using currently available methods. In the present study, we developed a protocol to directly image and functionally characterize MPs using high-resolution laser-scanning confocal microscopy. Once trapped on annexin-V coated micro-wells, we developed several assays using fluorescent reporters to measure their size, detect membrane antigens and evaluate proteolytic activity (nano-zymography). In particular, we demonstrated the applicability and specificity of this method to detect antigens and proteolytic activities of tissue-type plasminogen activator (tPA), urokinase and plasmin at the surface of engineered MPs from transfected cell-lines. Furthermore, we were able to identify a subset of tPA-bearing fibrinolytic MPs using plasma samples from a cohort of ischemic stroke patients who received thrombolytic therapy and in an experimental model of thrombin-induced ischemic stroke in mice. Overall, this method is promising for functional characterization of cell-derived MPs.

Enhanced expression of cell-specific surface antigens on endothelial microparticles in sepsis-induced disseminated intravascular coagulation

Sepsis-induced disseminated intravascular coagulation (DIC) is a major cause of death in patients admitted to intensive care units. Endothelial injury with microparticle production is reported in the pathogenesis of sepsis. Endothelial microparticles (EMPs) present several cell-specific surface antigens with different bioactivities, for example, tissue factor (TF), thrombomodulin (TM), and endothelial protein C receptor (EPCR). We investigated associations between these three different surface antigen-positive EMPs and sepsis-induced DIC. This cross-sectional study composed of 24 patients with sepsis and 23 healthy controls was conducted from November 2012 to September 2013. Blood samples were collected from patients within 24 h of diagnosis of severe sepsis and from healthy controls. Numbers of TF-positive EMPs (TF EMPs), TM-positive EMPs (TM EMPs), and EPCR-positive EMPs (EPCR EMPs) were measured by flow cytometry immediately thereafter. Acute Physiology and Chronic Health Evaluation II and Sequential Organ Failure Assessment scores were assessed in the severe sepsis patients at enrollment. We assessed DIC with the International Society of Thrombosis and Haemostasis (ISTH) overt DIC diagnostic criteria algorithm. Numbers of antigen-positive EMPs were increased significantly in both severe sepsis patients and controls and with the increase in ISTH DIC score. Numbers of TF EMPs and EPCR EMPs correlated significantly with Sequential Organ Failure Assessment score, and numbers of EPCR EMPs correlated significantly with Acute Physiology and Chronic Health Evaluation II score. Numbers of the three antigen-positive EMPs were increased significantly in severe sepsis patients versus those in healthy controls and with the increase of ISTH DIC score, suggesting that the specific bioactivity of each antigen-positive EMP may play a role in the progression of sepsis-induced DIC.

Association of the Plasma Platelet-Derived Microparticles to Platelet Count Ratio with Hospital Mortality and Disseminated Intravascular Coagulopathy in Critically Ill Patients

AIM

The role of platelet-derived microparticles (PDMPs) in the crosstalk between coagulopathy and inflammation in critically ill patients remains unclear. The aim of this cohort observational study was to investigate the associations between the PDMP levels and hospital mortality or disseminated intravascular coagulopathy (DIC).

METHODS

This study included 119 patients who were admitted to the ICU. The PDMP levels were measured using an enzyme-linked immunosorbent assay three times a week, for a total of 372 samples. We calculated the maximum (max) PDMP value, max PDMP/platelet (PDMP/Plts) ratio (converted to the PDMP levels per 10(4) platelets) and nadir platelet count during the ICU stay. Baseline patient data and scores, including the Japanese Association for Acute Medicine (JAAM) DIC score, were collected, and potential predictors were analyzed for possible associations with hospital mortality.

RESULTS

The max PDMP/Plts ratio was significantly different comparing the survivors (n=98: median, 2.54) and non-survivors (n=21: median 17.59; p＜0.001). There was a weak but statistically significant negative correlation between the max PDMP level and nadir platelet count (r=-0.332, p＜0.001). The max PDMP level and max PDMP/Plts ratio were higher in the DIC group (81.48 and 9.27, respectively) than in the non-DIC group (34.88 and 2.35, p=0.001 and p＜0.001, respectively). The max PDMP/Plts ratio was the only variable found to be independently associated with hospital mortality according to a multivariate logistic regression analysis.

CONCLUSIONS

PDMPs are involved in the development of DIC but are not related to hospital mortality. There is a good association between the PDMP/Plts ratio and hospital mortality and/or DIC in critically ill patients.

Endothelial barrier dysfunction in septic shock

The endothelium provides an essential and selective membrane barrier that regulates the movement of water, solutes, gases, macromolecules and the cellular elements of the blood from the tissue compartment in health and disease. Its structure and continuous function is essential for life for all vertebrate organisms. Recent evidence indicates that the endothelial surface does not have a passive role in systemic inflammatory states such as septic shock. In fact, endothelial cells are in dynamic equilibrium with a myriad of inflammatory mediators and elements of the innate immune and coagulation systems to orchestrate the host response in sepsis. The barrier function of the endothelial surface is almost uniformly impaired in septic shock, and it is likely that this contributes to adverse outcomes. In this review, we will highlight recent advances in the understanding of the signalling events that regulate endothelial function and molecular events that induce endothelial dysfunction in sepsis. Endothelial barrier repair strategies as a treatment for sepsis include modulation of C5a, high-mobility group box 1 and VEGF receptor 2; stimulation of angiopoietin-1, sphingosine 1 phosphate receptor 1 and Slit; and a number of other innovative approaches.

Comparative response of platelet fV and plasma fV to activated protein C and relevance to a model of acute traumatic coagulopathy

Background

Acute traumatic coagulopathy (ATC) has been linked to an increase in activated protein C (aPC) from 40 pM in healthy individuals to 175 pM. aPC exerts its activity primarily through cleavage of active coagulation factor Va (fVa). Platelets reportedly possess fVa which is more resistant to aPC cleavage than plasma fVa; this work examines the hypothesis that normal platelets are sufficient to maintain coagulation in the presence of elevated aPC.

Methods

Coagulation responses of normal plasma, fV deficient plasma (fVdp), and isolated normal platelets in fVdp were conducted: prothrombin (PT) tests, turbidimetry, and thromboelastography (TEG), including the dose response of aPC on the samples.

Results

PT and turbidimetric assays demonstrate that normal plasma is resistant to aPC at doses much higher than those found in ATC. Additionally, an average physiological number of washed normal platelets (200,000 platelets/mm³) was sufficient to eliminate the anti-coagulant effects of aPC up to 10 nM, nearly two orders of magnitude above the ATC concentration and even the steady-state pharmacological concentration of human recombinant aPC, as measured by TEG. aPC also demonstrated no significant effect on clot lysis in normal plasma samples with or without platelets.

Conclusions

Although platelet fVa shows slightly superior resistance to aPC's effects compared to plasma fVa in static models, neither fVa is sufficiently cleaved in simulations of ATC or pharmacologically-delivered aPC to diminish coagulation parameters. aPC is likely a correlative indicator of ATC or may play a cooperative role with other activity altering products generated in ATC.

The coagulation system and its function in early immune defense

Blood coagulation has a Janus-faced role in infectious diseases. When systemically activated, it can cause serious complications associated with high morbidity and mortality. However, coagulation is also part of the innate immune system and its local activation has been found to play an important role in the early host response to infection. Though the latter aspect has been less investigated, phylogenetic studies have shown that many factors involved in coagulation have ancestral origins which are often combined with anti-microbial features. This review gives a general overview about the most recent advances in this area of research also referred to as immunothrombosis.

The endothelium, a protagonist in the pathophysiology of critical illness: focus on cellular markers

The endotheliumis key in the pathophysiology of numerous diseases as a result of its precarious function in the regulation of tissue homeostasis. Therefore, its clinical evaluation providing diagnostic and prognostic markers, as well as its role as a therapeutic target, is the focus of intense research in patientswith severe illnesses. In the critically ill with sepsis and acute brain injury, the endothelium has a cardinal function in the development of organ failure and secondary ischemia, respectively. Cellular markers of endothelial function such as endothelial progenitor cells (EPC) and endothelialmicroparticles (EMP) are gaining interest as biomarkers due to their accessibility, although the lack of standardization of EPC and EMP detection remains a drawback for their routine clinical use. In this paper we will review data available on EPC, as a general marker of endothelial repair, and EMP as an equivalent of damage in critical illnesses, in particular sepsis and acute brain injury. Their determination has resulted in new insights into endothelial dysfunction in the critically ill. It remains speculative whether their determination might guide therapy in these devastating acute disorders in the near future.

Microparticles are new biomarkers of septic shock-induced disseminated intravascular coagulopathy

PURPOSE

Septic shock-induced disseminated intravascular coagulopathy (DIC) contributes to multiple organ failure. Mechanisms governing vascular responses to open occurrence of DIC have not yet been established. Circulating plasma microparticles (MPs), released upon cell stress, constitute a catalytic procoagulant surface and are surrogates of vascular cell activation/injury. Herein, MPs were assessed as possible markers of haemostatic and vascular dysfunction in the DIC time course.

METHODS

One hundred patients with septic shock from three ICUs were enrolled and their haemostatic status evaluated at admission (D1), D2, D3 and D7. Circulating procoagulant MPs were isolated, quantified by prothrombinase assay and their cellular origin determined. DIC diagnosis was made according to the JAAM 2006 score.

RESULTS

Ninety-two patients were analysed and 40 had DIC during the first 24 h. Routine clotting times and factor/inhibitor activity did not allow assessing vascular cell involvement. At admission, thrombin generation and fibrinolysis were observed in both groups while impaired fibrin polymerisation was evidenced only in DIC patients. Sustained thrombin generation persisted over time in both groups at D7. While total microparticle concentrations were in the same range regardless of DIC diagnosis, specific phenotypes were already detected at admission in DIC patients. Endothelial- and leucocyte-derived MPs were higher in DIC while an increased soluble glycoprotein V/platelet ratio was delayed, underscoring the first involvement of endothelial cells and leucocytes whereas platelet activation was delayed. Endothelium-derived CD105-MPs (OR 6.55) and CD31-MPs (OR 0.49) were strongly associated with early DIC in multivariate analysis.

CONCLUSION

Endothelial-derived microparticles are relevant biomarkers of septic shock-induced DIC and could be used to evaluate early vascular injury.

Circulating microparticles: square the circle

Background

The present review summarizes current knowledge about microparticles (MPs) and provides a systematic overview of last 20 years of research on circulating MPs, with particular focus on their clinical relevance.

Results

MPs are a heterogeneous population of cell-derived vesicles, with sizes ranging between 50 and 1000 nm. MPs are capable of transferring peptides, proteins, lipid components, microRNA, mRNA, and DNA from one cell to another without direct cell-to-cell contact. Growing evidence suggests that MPs present in peripheral blood and body fluids contribute to the development and progression of cancer, and are of pathophysiological relevance for autoimmune, inflammatory, infectious, cardiovascular, hematological, and other diseases. MPs have large diagnostic potential as biomarkers; however, due to current technological limitations in purification of MPs and an absence of standardized methods of MP detection, challenges remain in validating the potential of MPs as a non-invasive and early diagnostic platform.

Conclusions

Improvements in the effective deciphering of MP molecular signatures will be critical not only for diagnostics, but also for the evaluation of treatment regimens and predicting disease outcomes.

Microparticles from mycobacteria-infected macrophages promote inflammation and cellular migration

Mycobacterium tuberculosis infection is characterized by a strong inflammatory response whereby a few infected macrophages within the granuloma induce sustained cellular accumulation. The mechanisms coordinating this response are poorly characterized. We hypothesized that microparticles (MPs), which are submicron, plasma membrane-derived vesicles released by cells under both physiological and pathological conditions, are involved in this process. Aerosol infection of mice with M. tuberculosis increased CD45(+) MPs in the blood after 4 wk of infection, and in vitro infection of human and murine macrophages with mycobacteria enhanced MP release. MPs derived from mycobacteria-infected macrophages were proinflammatory, and when injected into uninfected mice they induced significant neutrophil, macrophage, and dendritic cell recruitment to the injection site. When incubated with naive macrophages, these MPs enhanced proinflammatory cytokine and chemokine release, and they aided in the disruption of the integrity of a respiratory epithelial cell monolayer, providing a mechanism for the egress of cells to the site of M. tuberculosis infection in the lung. In addition, MPs colocalized with the endocytic recycling marker Rab11a within macrophages, and this association increased when the MPs were isolated from mycobacteria-infected cells. M. tuberculosis-derived MPs also carried mycobacterial Ag and were able to activate M. tuberculosis-specific CD4(+) T cells in vivo and in vitro in a dendritic cell-dependent manner. Collectively, these data identify an unrecognized role for MPs in host response against M. tuberculosis by promoting inflammation, intercellular communication, and cell migration.

Microparticles and acute lung injury

The pathophysiology of acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), is characterized by increased vascular and epithelial permeability, hypercoagulation and hypofibrinolysis, inflammation, and immune modulation. These detrimental changes are orchestrated by cross talk between a complex network of cells, mediators, and signaling pathways. A rapidly growing number of studies have reported the appearance of distinct populations of microparticles (MPs) in both the vascular and alveolar compartments in animal models of ALI/ARDS or respective patient populations, where they may serve as diagnostic and prognostic biomarkers. MPs are small cytosolic vesicles with an intact lipid bilayer that can be released by a variety of vascular, parenchymal, or blood cells and that contain membrane and cytosolic proteins, organelles, lipids, and RNA supplied from and characteristic for their respective parental cells. Owing to this endowment, MPs can effectively interact with other cell types via fusion, receptor-mediated interaction, uptake, or mediator release, thereby acting as intrinsic stimulators, modulators, or even attenuators in a variety of disease processes. This review summarizes current knowledge on the formation and potential functional role of different MPs in inflammatory diseases with a specific focus on ALI/ARDS. ALI has been associated with the formation of MPs from such diverse cellular origins as platelets, neutrophils, monocytes, lymphocytes, red blood cells, and endothelial and epithelial cells. Because of their considerable heterogeneity in terms of origin and functional properties, MPs may contribute via both harmful and beneficial effects to the characteristic pathological features of ALI/ARDS. A better understanding of the formation, function, and relevance of MPs may give rise to new promising therapeutic strategies to modulate coagulation, inflammation, endothelial function, and permeability either through removal or inhibition of "detrimental" MPs or through administration or stimulation of "favorable" MPs.

Leukocyte-derived microparticles in vascular homeostasis

Leukocyte-derived microparticles (LMPs) may originate from neutrophils, monocytes/macrophages, and lymphocytes. They express markers from their parental cells and harbor membrane and cytoplasmic proteins as well as bioactive lipids implicated in a variety of mechanisms, maintaining or disrupting vascular homeostasis. When they carry tissue factor or coagulation inhibitors, they participate in hemostasis and pathological thrombosis. Both proinflammatory and anti-inflammatory processes can be affected by LMPs, thus ensuring an appropriate inflammatory response. LMPs also play a dual role in the endothelium by either improving the endothelial function or inducing an endothelial dysfunction. LMPs are implicated in all stages of atherosclerosis. They circulate at a high level in the bloodstream of patients with high atherothrombotic risk, such as smokers, diabetics, and subjects with obstructive sleep apnea, where their prolonged contact with the vessel wall may contribute to its overall deterioration. Numbering microparticles, including LMPs, might be useful in predicting cardiovascular events. LMPs modify the endothelial function and promote the recruitment of inflammatory cells in the vascular wall, necessary processes for the progression of the atherosclerotic lesion. In addition, LMPs favor the neovascularization within the vulnerable plaque and, in the ruptured plaque, they take part in coagulation and platelet activation. Finally, LMPs participate in angiogenesis. They might represent a novel therapeutic tool to reset the angiogenic switch in pathologies with altered angiogenesis. Additional studies are needed to further investigate the role of LMPs in cardiovascular diseases. However, large-scale studies are currently difficult to set up because microparticle measurement still requires elaborate techniques which lack standardization.

Microparticles as biological vectors of activated protein C treatment in sepsis

Activated protein C (APC), a physiological coagulation inhibitor, has been shown to reduce mortality in patients with severe sepsis. APC exerts pleiotropic cytoprotection by a mechanism that requires its interaction with endothelial cell protein C receptor and protease-activated receptor-1 on target cells. In the previous issue, Pérez-Casal and colleagues elegantly demonstrate that APC, using its recombinant form (rhAPC), can communicate to target cells through release of microparticles (MPs), small membrane vesicles released from activated cells, to induce anti-apoptotic and anti-inflammatory properties that might participate in the improvement of patient outcome. Of interest is the fact that APC itself promotes the release of MPs from target cells including endothelial cells and monocytes. These MPs bear the endothelial cell protein C receptor/APC molecules and can transfer the message to target cells including those of origin to induce cytoprotection. The long-range APC signal can thus be mediated by MPs in vivo upon pharmacological treatment using rhAPC in severe septic patients. A novel pharmacological approach targeting MP production and properties could therefore be used to treat severe sepsis in addition to other well-known actions of APC via direct interaction with the cells of interest.