Platelet-activating factor mediates procoagulant activity on the surface of endothelial cells by promoting leukocyte adhesion

Efficacy of synthetic glucocorticoids in COVID-19 endothelites

Since March 2020, the world has been fighting a global pandemic caused by a new coronavirus SARS-CoV-2 (COVID-19). SARS-CoV-2 is responsible for severe acute respiratory syndrome, an airway disease that can be severe and fatal in a percentage of cases. Patients with severe COVID-19 can develop extrapulmonary lesions, with renal, hepatic, cardiac, neurological, and tissue involvement that can cause further severe complications. On December 21, 2021, the European Medicines Agency (EMA) authorized the marketing of the first COVID-19 vaccine. However, several randomized trials are ongoing to find effective, safe, and widely available treatments. The most severe stages of COVID-19 infection are characterized by a multi-system inflammatory state induced by a cytokine storm causing multi-organ injury. Epidemiologic evidence has shown that glucocorticoids (GCs), particularly dexamethasone, are used in severe, hospitalized patients with COVID-19 with good therapeutic benefit. COVID-19 can also damage the endothelial system, causing microcirculatory disturbances and consequently leading to functional organ disorders. The combination of endothelial dysfunction with a generalized inflammatory state may contribute to the general pro-coagulative state described in patients with COVID-19 with increased risk of venous and arterial occlusions. The aim of this article is to describe the therapeutic utility of GCs in stabilizing the vascular endothelial barrier in COVID-19 infection. Indeed, we believe that the stabilization of the endothelial barrier and the anti-inflammatory effect of GCs could be the main effect underlying the therapeutic efficacy in COVID-19 patients.

Endothelial cell-glucocorticoid receptor interactions and regulation of Wnt signaling

Vascular inflammation is present in many cardiovascular diseases, and exogenous glucocorticoids have traditionally been used as a therapy to suppress inflammation. However, recent data have shown that endogenous glucocorticoids, acting through the endothelial glucocorticoid receptor, act as negative regulators of inflammation. Here, we performed ChIP for the glucocorticoid receptor, followed by next-generation sequencing in mouse endothelial cells to investigate how the endothelial glucocorticoid receptor regulates vascular inflammation. We identified a role of the Wnt signaling pathway in this setting and show that loss of the endothelial glucocorticoid receptor results in upregulation of Wnt signaling both in vitro and in vivo using our validated mouse model. Furthermore, we demonstrate glucocorticoid receptor regulation of a key gene in the Wnt pathway, Frzb, via a glucocorticoid response element gleaned from our genomic data. These results suggest a role for endothelial Wnt signaling modulation in states of vascular inflammation.

Microparticles and cardiovascular diseases

Microparticles are a distinctive group of small vesicles, without nucleus, which are involved as significant modulators in several physiological and pathophysiological mechanisms. Plasma microparticles from various cellular lines have been subject of research. Data suggest that they are key players in development and manifestation of cardiovascular diseases and their presence, in high levels, is associated with chronic inflammation, endothelial damage and thrombosis. The strong correlation of microparticle levels with several outcomes in cardiovascular diseases has led to their utilization as biomarkers. Despite the limited clinical application at present, their significance emerges, mainly because their detection and enumeration methods are improving. This review article summarizes the evidence derived from research, related with the genesis and the function of microparticles in the presence of various cardiovascular risk factors and conditions. The current data provide a substrate for several theories of how microparticles influence various cellular mechanisms by transferring biological information.

Exploring the diversity, infectivity and metabolomic landscape of Rickettsial infections for developing novel therapeutic intervention strategies

Rickettsioses are zoonotic infections caused by obligate intracellular bacteria of the genera Rickettsia that affect human health; sometimes humans being considered as accidental hosts. At a molecular level, the rickettsiae infection triggers molecular signaling leading to the secretion of proinflammatory cytokines. These cytokines direct the immune response to the host cell damage and pathogen removal. In this review, we present metabolic aspects of the host cell in the presence of rickettsiae and how this presence triggers an inflammatory response to cope with the pathogen. We also reviewed the secretion of cytokines that modulates host cell response at immune and metabolic levels.

Microvesicles as cell-cell messengers in cardiovascular diseases

Cell-cell communication has proven to be even more complex than previously thought since the discovery that extracellular vesicles serve as containers of biological information on various pathophysiological settings. Extracellular vesicles are classified into exosomes, microvesicles/microparticles, or apoptotic bodies, originating from different subcellular compartments. The cellular machinery controlling their formation and composition, as well as the mechanisms regulating their extracellular release, remain unfortunately much unknown. Extracellular vesicles have been found in plasma, urine, saliva, and inflammatory tissues. Their biomarker potential has raised significant interest in the cardiovascular field because the vesicle composition and microRNA content are specific signatures of cellular activation and injury. More than simply cell dust, extracellular vesicles are capable of transferring biological information to neighboring cells and play an active role in inflammatory diseases, including atherosclerosis and angiogenesis. The molecular interactions regulating these effects involve specific receptor activation, proteolytic enzymes, reactive oxygen species, or delivery of genetic information to target cells. Unraveling their mechanisms of action will likely open new therapeutic avenues.

Microparticles: protagonists of a novel communication network for intercellular information exchange

Microparticles represent a heterogeneous population of vesicles with a diameter of 100 to 1000 nm that are released by budding of the plasma membrane and express antigens specific of their parental cells. Although microparticle formation represents a physiological phenomenon, a multitude of pathologies are associated with a considerable increase in circulating microparticles, including inflammatory and autoimmune diseases, atherosclerosis, and malignancies. Microparticles display an broad spectrum of bioactive substances and receptors on their surface and harbor a concentrated set of cytokines, signaling proteins, mRNA, and microRNA. Recent studies provided evidence for the concept of microparticles as veritable vectors for the intercellular exchange of biological signals and information. Indeed, microparticles may transfer part of their components and content to selected target cells, thus mediating cell activation, phenotypic modification, and reprogramming of cell function. Because microparticles readily circulate in the vasculature, they may serve as shuttle modules and signaling transducers not only in their local environment but also at remarkable distance from their site of origin. Altogether, this transcellular delivery system may extend the confines of the limited transcriptome and proteome of recipient cells and establishes a communication network in which specific properties and information among cells can be efficiently shared. At least in same cases, the sequential steps of the transfer process underlie complex regulatory mechanisms, including selective sorting ("packaging") of microparticle components and content, specificity of interactions with target cells determined by surface receptors, and ultimately finely tuned and signal-dependent release and delivery of microparticle content.

Might platelet-leucocyte complexes be playing a role in major vascular involvement of Behçet's disease? A comparative study

We determined platelet-leucocyte complexes, which play roles in the thrombosis-inflammation relationship, in Behçet's disease patients with and without major vascular involvement (MVI) and in healthy controls. We included 36 Behçet's disease patients (22 male, 14 female, mean age: 34.4 +/- 8.3 years) and 20 healthy individuals (14 male, six female, mean age: 31.8 +/- 4.4 years). Whole blood count, CRP and ESR were determined in both groups. Clinical data about the patients were obtained from medical charts. Individuals with hypertension, diabetes, coronary artery disease, and smokers were excluded. Behçet's disease patients with MVI were taken as a separate group (8 male, 5 female, mean age: 37 +/- 8 years). MVI was defined as the presence of pulmonary arterial aneurysm, deep venous thrombosis, vena cava inferior or superior thrombosis, or venous sinus thrombosis. Flow cytometry was used to determine platelet-monocyte complexes (PMC), platelet-neutrophil complexes (PNC), basal and adenosine diphosphate (ADP)-stimulated platelet CD62P expression. Behçet's disease patients with MVI had significantly higher PNC than Behçet's disease patients without MVI and healthy controls (P values = 0.01). PMC levels in Behçet's disease patients with MVI were significantly higher than in healthy controls (P = 0.01). The groups were similar in basal and ADP-stimulated platelet CD62P expression (P values >0.05). Basal and ADP-stimulated CD62P expression, PMC and PNC were not significantly different between active Behçet's disease versus inactive Behçet's disease patients. The evaluated parameters were similar in Behçet's disease patients with and without uveitis, and pathergy-positive and pathergy-negative groups. Our results might suggest that the formation of PMC and PNC might play a role in thrombosis and MVI of Behçet's disease.

JAM-A mediates neutrophil transmigration in a stimulus-specific manner in vivo: evidence for sequential roles for JAM-A and PECAM-1 in neutrophil transmigration

Junctional adhesion molecule-A (JAM-A) is a transmembrane protein expressed at tight junctions of endothelial and epithelial cells and on the surface of platelets and leukocytes. The role of JAM-A in leukocyte transmigration in vivo was directly investigated by intravital microscopy using both a JAM-A-neutralizing monoclonal antibody (mAb) (BV-11) and JAM-A-deficient (knockout [KO]) mice. Leukocyte transmigration (but not adhesion) through mouse cremasteric venules as stimulated by interleukin 1beta (IL-1beta) or ischemia/reperfusion (I/R) injury was significantly reduced in wild-type mice treated with BV-11 and in JAM-A KO animals. In contrast, JAM-A blockade/genetic deletion had no effect on responses elicited by leukotriene B(4) (LTB(4)) or platelet-activating factor (PAF). Furthermore, using a leukocyte transfer method and mice deficient in endothelial-cell JAM-A, evidence was obtained for the involvement of endothelial-cell JAM-A in leukocyte transmigration mediated by IL-1beta. Investigation of the functional relationship between JAM-A and PECAM-1 (CD31) determined that dual blockade/deletion of these proteins does not lead to an inhibitory effect greater than that seen with blockade/deletion of either molecule alone. The latter appeared to be due to the fact that JAM-A and PECAM-1 can act sequentially to mediate leukocyte migration through venular walls in vivo.

Effect of fast-food Mediterranean-type diet on type 2 diabetics and healthy human subjects' platelet aggregation

OBJECTIVE

The aim of our work was to carry out a randomized clinical trial with a fast-food Mediterranean type diet rich in platelet activating factor (PAF) antagonist to investigate the effect on type 2 diabetics and healthy human subject's platelet aggregation.

RESEARCH DESIGN AND METHODS

We extracted lipids from fast-food Mediterranean type foodstuffs, and tested them in vitro for their ability to inhibit or antagonize PAF towards washed rabbit platelets. We chose the foodstuffs that exerted the most potent in vitro anti-PAF activity and fed 22 healthy (group A) and 23 type 2 diabetics (group B) subjects on a diet containing the chosen foodstuffs. The 22 type 2 diabetics (group C) subjects were kept on their regular diet that was being followed before entering the study. Before and after a 4-week diet, all enrolled subjects underwent the following examinations; measurement of total cholesterol, low density lipoprotein (LDL-cholesterol), high density lipoprotein (HDL-cholesterol), triglycerides, glucose, HbA(1c), body mass index (BMI), and platelet aggregation in response to PAF, adenosine 5' diphosphate (ADP) and arachidonic acid (AA).

RESULTS

The chosen diet significantly increased the EC(50) values of PAF and ADP to groups A and B (p<0.05). No statistical difference was observed on the EC(50) value of group C. No statistical differences were detected among Cholesterol, LDL-cholesterol, triglycerides, glucose, HBA(1c), BMI, and EC(50) for AA values, for any of the three groups.

CONCLUSIONS

Consumption of a fast-food Mediterranean type diet rich in PAF antagonist improved platelet response of type 2 diabetics and healthy human subjects against thrombotic, inflammatory and proatherogenic factors.

Gene expression profiling of acute promyelocytic leukaemia identifies two subtypes mainly associated with Flt3 mutational status

Acute promyelocytic leukaemia (APL) is a well-defined disease characterized by a typical morphology of leukaemic cells, the presence of t(15;17) translocation and the unique sensitivity to the differentiating effect of all-trans retinoic acid. Nevertheless, some aspects are variable among APL patients, with differences substantially related to morphological variants, peripheral leukocytes count, the presence of a disseminated intravascular coagulopathy, different PML/RARalpha isoforms (long, variable or short) and Fms-like tyrosine kinase 3 (Flt3) mutations. In order to better define this variability, we investigated the gene expression profiles of 18 APL cases revealing, besides a high uniformity in gene expression pattern, the presence of few robust differences among patients able to identify, by an unsupervised analysis, two major clusters of patients characterized by different phenotypes (hypogranular M3v vs classical M3) and by the presence or absence of Flt3 internal tandem duplications (ITDs). Further supervised analysis confirmed that Flt3 status was the APL parameter best associated with these two subgroups. We identified, between Flt3 wild-type and Flt3-ITDs subsets, 147 differentially expressed genes that were involved in the cytoskeleton organization, in the cell adhesion and migration, in the proliferation and the coagulation/inflammation pathways as well as in differentiation and myeloid granules constitution suggesting a role of Flt3 mutations in the pathogenesis and clinical manifestations of APL.

Recombinant human platelet-activating factor acetylhydrolase for treatment of severe sepsis: results of a phase III, multicenter, randomized, double-blind, placebo-controlled, clinical trial

OBJECTIVE

Platelet-activating factor (PAF) and structurally-related oxidized phospholipids are proinflammatory mediators in systemic inflammatory states such as severe sepsis. The enzyme platelet-activating factor acetylhydrolase (PAF-AH) rapidly degrades PAF and oxidized phospholipids into inactive metabolites. Reduced PAF-AH activity has been observed in patients with severe sepsis and may contribute to their systemic inflammatory response and organ dysfunction. A previous clinical trial with recombinant human PAF-AH (rPAF-AH, Pafase) suggested that this treatment may decrease 28-day all-cause mortality in patients with severe sepsis. The current study was undertaken to confirm this result.

DESIGN

A prospective, randomized, double-blind, placebo-controlled, multicenter, international trial.

SETTING

One hundred forty-six intensive care units from nine countries.

PATIENTS

Approximately 2,522 patients were planned to be enrolled < or =12 hrs after the onset of severe sepsis. Eligible patients were randomized to receive either rPAF-AH 1.0 mg/kg or placebo administered intravenously once daily for five consecutive days.

MEASUREMENTS AND MAIN RESULTS

The study was terminated based on the recommendation of an independent data and safety monitoring committee after the second of three planned interim analyses, and the enrollment of 1,425 patients. rPAF-AH treatment was well tolerated among the 1,261 patients included in the interim analysis (643 rPAF-AH and 618 placebo), but did not decrease 28-day all-cause mortality compared with placebo (25% for rPAF-AH vs. 24% for placebo; relative risk, 1.03; 95% confidence interval, 0.85-1.25; p =.80). There were no statistically significant differences between treatment groups in any of the secondary efficacy end points. The overall incidence of adverse events was similar among rPAF-AH and placebo-treated patients, and no rPAF-AH-treated patients developed antibodies to PAF-AH.

CONCLUSIONS

rPAF-AH was well tolerated and not antigenic, but did not decrease 28-day all-cause mortality in patients with severe sepsis.

Platelet-activating factor and P-selectin activities in thrombotic and nonthrombotic Behçet’s patients

OBJECTIVE

The aim of this study was to compare plasma Platelet-activating factor (PAF) and P-selectin (CD62P) activities in Behçet's disease patients with and without thrombosis.

METHODS

In this cross-sectional and descriptive study, 30 consecutive Behçet's patients were included, 15 of them with venous thrombosis. All patients were also divided into two subgroups according to the presence or absence of clinical activity. Plasma PAF levels, basal and Ca++ ionophore (A23187)-induced leukocyte (cellular) PAF activities, and platelet-rich plasma DeltaCD62P activity (the mean fluorescent density difference between CD62P phycoerythrin-positive and -negative stains) were evaluated.

RESULTS

In the thrombotic group, plasma PAF (P=0.001), basal leukocyte PAF (P=0.017), induced leukocyte PAF (P=0.024), and DeltaCD62P (P=0.023) levels were significantly higher than in the nonthrombotic group. In the whole group of Behçet's patients, there was a positive correlation between plasma PAF and DeltaCD62P levels (r=0.533, P=0.002). When we compared clinically active and inactive patients with respect to the above parameters, there was no significant difference, irrespective of thrombosis. Plasma PAF (P=0.001), basal leukocyte PAF (P=0.004), and DeltaCD62P (P=0.038) levels were significantly higher in the presence of both clinical activity and thrombosis than of clinical activity alone.

CONCLUSION

Platelet-activating factor and CD62P may contribute to endothelial injury and thrombosis development in Behçet's disease. These two parameters seem related to the presence of thrombosis rather than clinical activity.

The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis

OBJECTIVES

To review the platelet-activating factor (PAF) signaling system, its regulation, and its dysregulation in acute inflammation and thrombosis and in syndromes that involve these cascades, including sepsis.

DATA SOURCES

A summary of published literature from MEDLINE search files and published reviews. DATA EXTRACTION, SYNTHESIS, AND SUMMARY: PAF, a phospholipid signaling molecule, transmits outside-in signals to intracellular transduction systems and effector mechanisms in a variety of cell types, including key cells of the innate immune and hemostatic systems: neutrophils, monocytes, and platelets. Thus, the PAF signaling system is a point of convergence at which injurious stimuli can trigger and amplify both acute inflammatory and thrombotic cascades. The biological activities of PAF are regulated by several precise mechanisms that, together, constrain and control its action in physiologic inflammation. Unregulated synthesis of PAF or defects in the mechanisms that limit its biological activities have the potential to cause pathologic inflammation and thrombosis. In addition, nonenzymatic generation of oxidized phospholipids that are recognized by the PAF receptor can trigger inflammatory and thrombotic events. There is evidence that the PAF signaling system is dysregulated in sepsis, shock, and traumatic injury and that interruption or termination of its effector responses leads to beneficial outcomes. Plasma PAF acetylhydrolase, an enzyme that hydrolyzes PAF and structurally related oxidized phospholipids, yielding products that are no longer recognized by the PAF receptor, may be a particularly important signal terminator.

CONCLUSION

The PAF signaling system can trigger inflammatory and thrombotic cascades, amplify these cascades when acting with other mediators, and mediate molecular and cellular interactions (cross talk) between inflammation and thrombosis. Evidence from in vitro experiments, studies of experimental animals, and clinical observations in humans indicates that the PAF signaling system is important in sepsis and other syndromes of inflammatory injury and that therapeutic strategies to interrupt or terminate signaling via the PAF signaling system may be useful in these conditions.

Platelet-Activating Factor Acetylhydrolase Prevents Myocardial Ischemia-Reperfusion Injury

Background —Platelet-activating factor (PAF) is one of the most potent biological mediators of tissue injury. PAF acetylhydrolase (PAF-AH) is a recently isolated naturally occurring enzyme that hydrolyzes PAF and renders it inactive. We hypothesize that inhibition of PAF with PAF-AH will reduce myocardial ischemia-reperfusion (I/R) injury in vivo.

Methods and Results —The coronary ligation model was used in New Zealand white rabbits. The large branch of the marginal coronary artery was occluded for 45 minutes, followed by 2 hours of reperfusion. Fifteen minutes before reperfusion, animals were given either 2 mg/kg of vehicle or of PAF-AH. At the completion of 120 minutes of reperfusion, percentage of necrosis, degree of neutrophil infiltration, and measurements of regional contractility were assessed. Data are expressed as the mean±SEM and compared by Student’s t test or Mann-Whitney ANOVA. Both groups of animals showed an equivalent area at risk; however, 46.7±11% was necrotic in the animal treated with vehicle. In contrast, 20.9±7.0% was necrotic in the animals treated with PAF-AH ( P <0.05). Systolic shortening and wall thickness were significantly greater in those animals treated with PAF-AH at 15, 30, 60, and 120 minutes of reperfusion ( P <0.05). Quantification of neutrophil infiltration showed a 62% reduction in the PAF-AH treated animals compared with those treated with vehicle alone.

Conclusions —PAF-AH is a potent cardioprotective agent in an in vivo model of I/R injury.

Regulation of platelet-activating factor (PAF) activity in human diseases by phospholipase A2 inhibitors, PAF acetylhydrolases, PAF receptor antagonists and free radical scavengers

The aim of this review is to present recent findings indicating the likely involvement of platelet-activating factor (PAF) in human diseases, and possible ways of alleviating its harmful effects. PAF is a potent proinflammatory mediator and promotes adhesive interactions between leukocytes and endothelial cells, leading to transendothelial migration of leukocytes, by a process of juxtacrine intercellular signalling. This process leads to activation of leukocytes and the release of reactive oxygen radicals, lipid mediators, cytokines and enzymes. These reaction products subsequently contribute to the pathological features of various inflammatory diseases. The reactive oxygen radicals cause low density lipoprotein (LDL) oxidation which mediates the development of atherosclerosis. Oxidized LDL may damage cellular and subcellular membranes, leading to tissue injury and cell death. Among the therapeutic approaches considered are agents that inhibit/degrade proinflammatory mediators and thereby have anti-inflammatory and/or anti-atherogenic potential. These include inhibitors of phospholipase A2 activity, PAF-acetylhydrolases, PAF antagonists and free radical scavengers/antioxidants, the latter protecting against oxidized LDL-induced cytotoxicity.

The interface of atherosclerosis and thrombosis: basic mechanisms

Occlusive vascular disease most often results from thrombosis superimposed on atherosclerotic plaque. Disruption of plaque exposes thrombogenic substances within the plaque to blood and may result in thrombotic occlusion of the affected vessel. Mural thrombi may be incorporated into plaque, enhancing the evolution of atherosclerotic lesions. Inflammation plays a key role in the formation and complication of atherosclerosis. Inflammatory mediators regulate processes that determine the composition of the plaque's fibrous cap, a structure that separates blood from the thrombogenic lipid core. Several inflammatory mediators control the release of metalloproteinases (enzymes that break down cap constituents) from smooth muscle cells, macrophages and other cells within plaque. Inflammatory mediators also control the production of connective tissue matrix by cells in the plaque. Factors involved in coagulation, such as thrombin, can regulate non-thrombotic functions of vascular wall cells such as smooth muscle proliferation or cytokine release. The many mechanisms involved in arterial occlusive disease present numerous points at which intervention with pharmacologic agents may prove effective in lowering the risk of acute arterial thrombotic complications.

Human allogeneic lymphocytes trigger endothelial cell tissue factor expression by a tumor necrosis factor-dependent pathway

Lymphocyte adhesion to endothelial cells and the extravascular deposition of fibrin are 2 important processes during pathologic situations such as allograft rejection. Tissue factor (TF) expression was therefore measured on human umbilical vein endothelial cells (HUVECs) after coculture with allogeneic lymphocytes (PBLs) by a factor Xa generation assay. When cocultured with PBLs, HUVECs expressed strong procoagulant activity related to the TF/factor VII-dependent pathway, which was enhanced when endothelial cells were treated with interferon-gamma (IFN-gamma). The highest TF activity was measured when 10(5) lymphocytes were incubated with 10(4) HUVECs (ratio 10: 1) for 4 hours, a time-dependent course similar to that obtained with tumor necrosis factor-alpha (TNF-alpha), and direct contact between the 2 cell types was necessary. PBL-induced TF activity was inhibited by cycloheximide or actinomycin D, indicating active protein synthesis that was confirmed by the increase in TF mRNA detected by reverse transcription-polymerase chain reaction. It was then demonstrated that 1 of the primary signaling pathways leading to endothelial cell TF expression was a rapid initial interaction between membrane TNF expressed on PBLs and the 75-kd TNF receptor, with subsequent involvement of platelet-activating factor and P-selectin. Finally, we showed that the transduction of external signals involving the activation of protein kinase C and protein tyrosine kinases also contributed to the regulation of TF expression.

Cellular activation through the ligation of intercellular adhesion molecule-1

In addition to its role in mediating leukocyte adherence to and migration across the endothelium, intercellular adhesion molecule (ICAM)-1 on the surface of interstitial cells has been implicated as a principal adhesion molecule controlling leukocyte infiltration at inflammatory sites. The present study demonstrates that leukocyte binding to fibroblasts isolated from both the human renal cortex and lung and to endothelial cells induced the de novo synthesis of ICAM-1 mRNA and protein through the ICAM-1-dependent activation of the cultured cells. This was mimicked by specifically cross-linking the ICAM-1 receptor with anti-ICAM-1 antibodies. Following cross-linking there was a two- to threefold increase in the steady state levels of ICAM-1 mRNA isolated from the cells. The expression of this mRNA peaked at 1–3 hours and was dose-dependent on the concentration of secondary cross-linking antibody. Peak protein expression was between 18 and 48 hours after cross-linking. Additional data demonstrated a similar increase in the expression of VCAM-1 following ICAM-1 cross-linking. In contrast, there was no reponse as a result of incubation with an isotype-matched control antibody. Both the binding of leukocytes and the cross-linking of ICAM-1 triggered a rise in cytosolic free calcium as the result, in part, of a calcium influx from the extracellular medium. Using BAPTA-AM to chelate intracellular calcium ions indicated that this increase in cytosolic free calcium was directly involved in the initiation of adhesion molecule upregulation. The present study demonstrates that both endothelial cells and fibroblasts can be stimulated by the direct cell surface interaction of ICAM-1 with its ligands on inflammatory cells. One phenotypic change resulting from this interaction is the upregulated synthesis and expression of more cellular adhesion molecules. This may have profound implications for the control and persistence of a leukocyte infiltrate and the progression of inflammatory disease.

Ca2+ signalling delays in neutrophils: effects of prior exposure to platelet activating factor or formyl-met-leu-phe

Rapid-time confocal scanning of fluo3-loaded neutrophils revealed that in individual cells there were grossly heterogeneous time intervals between stimulation with either f-met-leu-phe or platelet activating factor (PAF) and the initiation of Ca2+ influx, ranging from 75 msec to several seconds. The distribution of lag times after stimulation with f-met-leu-phe (100 nM) was influenced by prior stimulation with either f-met-leu-phe or PAF. However, whereas prior stimulation with f-met-leu-phe (50 nM) caused the subsequent cytosolic free Ca2+ response to second challenge with f-met-leu-phe to be delayed, prior stimulation with PAF (100 nM) caused an increase in the rapidity of the onset of the second response to f-met-leu-phe. With both stimuli, the cytosolic free Ca2+ in some neutrophils (non-Ca2+ responders) in the population did not increase significantly. However, some of these cells responded to the subsequent challenge. However, with both pre-treatment stimuli, those cells in which a significant Ca2+ response was provoked by the first stimulus, responded significantly faster than the initial 'non-Ca2+ responders.' However, the reduced lag time provoked by pre-stimulation was not inhibited in neutrophils in which cytosolic free Ca2+ changes were dampened by intracellular BAPTA. These data point to post-receptor events, other than prior cytosolic free Ca2+ elevation, being important in determining the response Ca2+ delay to subsequent stimulation.

Arterial thrombosis induces early upregulation of intercellular adhesion molecule in the media

PURPOSE

Thrombosed peripheral vessels that are pharmacologically or mechanically recanalized have diminished long-term patency rates compared with vessels that are repaired before occlusion. We hypothesized that thrombosis induces proinflammatory changes in the arterial media that may contribute to postthrombotic vascular remodeling.

METHODS

We studied expression of intercellular adhesion molecule (ICAM), a mediator of leukocyte recruitment, in the arterial wall after thrombosis. Thrombosis was induced in rabbit superficial femoral arteries by embolizing polystyrene beads (Thr-emb) or by ligation (Thr-lig). Control vessels were dissected but not ligated (C-dis) or were subjected to bead embolization and immediate removal (C-emb). Arterial wall ICAM expression was measured by indirect immunohistochemical analysis at 6 hours, 24 hours, and 1 week. Staining intensity was graded from 0 (none) to 4 (intense) by observers who were blinded to the experimental conditions.

RESULTS

No increase in ICAM expression by thrombosed vessels was present at 6 hours. After 24 hours, ICAM expression in the media of thrombosed vessels was increased (Thr-emb, 2.3 +/- 0.5; Thr-lig, 2.0 +/- 0) compared with control vessels (C-dis, 0 +/- 0; C-emb, 0.8 +/- 0.5; p < 0.004). This difference became more marked at 1 week. ICAM staining localized to actin-staining regions of the media.

CONCLUSIONS

Arterial thrombosis, but not surgical injury, induces pronounced early and sustained upregulation of ICAM expression in smooth muscle-containing regions of the arterial media. Upregulation of ICAM is likely to promote recruitment of inflammatory cells or mediate vascular remodeling after luminal thrombosis.

Alpha toxin from Clostridium perfringens induces proinflammatory changes in endothelial cells

Alpha toxin from Clostridium perfringens type A, a phospholipase C, has been implicated in many of the localized and systemic features of gas gangrene. We demonstrated that human endothelial cells synthesize two vasoactive lipids, platelet-activating factor (PAF) and prostacyclin, in response to alpha toxin treatment. The stimulated synthesis of PAF required the enzymatic activity of the toxin and subsequent protein kinase C activation. Alpha toxin-treated endothelial cells accumulated the products of the phospholipase C reaction, diacylglycerol and ceramide, and exhibited a decrease in the enzymatic precursors phosphatidylcholine and sphingomyelin. Furthermore, the temporal accumulation of PAF depended on the concentration of the toxin in the overlying medium and was blocked in the presence of a neutralizing antibody. The cultured endothelial cells also exhibited enhanced neutrophil adhesion in response to alpha toxin which was mediated through the PAF receptor and P-selectin. P-selectin expression by endothelial cells and extravascular neutrophil accumulation were also observed in tissue sections from alpha toxin-injected Sprague-Dawley rats. These endothelial cell-mediated processes are important in maintaining vascular homeostasis and, when activated in a dysregulated manner by C. perfringens alpha toxin, may contribute to localized and systemic manifestations of gas gangrene including enhanced vascular permeability, localized neutrophil accumulation, and myocardial dysfunction.