An investigation into the effects of bacterial lipopolysaccharide on human platelets

Assessment of platelet biology in equine patients with systemic inflammatory response syndrome

In addition to maintaining hemostasis, platelets have an important role in modulating innate and adaptive immune responses. A low platelet count has been found to be a negative prognostic factor for survival in humans and horses with critical illnesses, such as sepsis or systemic inflammatory response syndrome (SIRS). Decreased platelet aggregation, caused by in vivo activation, has been found in human patients with severe sepsis. In our prospective controlled study, we assessed platelet biology in blood samples from 20 equine SIRS cases and 120 healthy control horses. Platelet variables such as platelet count, large platelet count, clumps, plateletcrit, mean platelet volume, and mean platelet component concentration were analyzed by laser flow cytometry (Advia 2120) from K₃EDTA blood and from citrate blood. Hirudin blood samples were analyzed by impedance aggregometry (Multiplate analyzer; Roche) for platelet aggregation, including spontaneous aggregation and aggregation by 4 different agonists: adenosine diphosphate (ADPtest), ADP + prostaglandin E1 (ADPtestHS), arachidonic acid (ASPItest), and collagen (COLtest). SIRS cases had significantly lower platelet counts in K₃EDTA blood (p < 0.0001) compared to control horses. There were no significant differences in aggregation values between SIRS cases and controls. Non-surviving SIRS horses did not have statistically significant lower platelet counts or lower aggregation values for COLtest, ADPtest, or ADPtestHS compared to surviving SIRS horses, although 5 non-survivors were thrombocytopenic.

Attenuated platelet aggregation in patients with septic shock is independent from the activity state of myosin light chain phosphorylation or a reduction in Rho kinase-dependent inhibition of myosin light chain phosphatase

Background

Impaired coagulation contributes to the morbidity and mortality associated with septic shock. Whether abnormal platelet contraction adds to the bleeding tendency is unknown. Platelets contract when Ca²⁺-dependent myosin light chain kinase (MLCK) phosphorylates Ser19 of myosin light chain (MLC₂₀), promoting actin-myosin cross-bridge cycling. Contraction is opposed when myosin light chain phosphatase (MLCP) dephosphorylates MLC₂₀. It is thought that Rho kinase (ROK) inhibits MLCP by phosphorylating Thr855 of the regulatory subunit MYPT, favouring platelet contraction. This study tested the hypotheses that in septic shock, (i) platelet function is inversely correlated with illness severity and (ii) ROK-dependent MLCP inhibition and myosin light chain phosphorylation are reduced.

Methods

Blood was sampled from non-septic shock patients and patients in the first 24 h of septic shock. Platelet function was assessed using whole blood impedance aggregation induced by 1) ADP (1.6 and 6.5 μM), 2) thrombin receptor-activating protein (TRAP; 32 μM), 3) arachidonic acid (500 μM) and 4) collagen (3.2 μg/ml). Arachidonic acid-induced aggregation was measured in the presence of the ROK inhibitor Y27632. Illness severity was evaluated using sequential organ failure assessment (SOFA) and acute physiology and chronic health evaluation (APACHE) II scores. Western blot analysis of [Ser19]MLC₂₀ and [Thr855]MYPT phosphorylation quantified activation and inhibition of platelet MLC₂₀ and MLCP, respectively. Data were analysed using Spearman's rank correlation coefficient, Student's t-test and Mann-Whitney test; p < 0.05 was considered significant.

Results

Agonist-induced aggregation was attenuated in septic shock patients (n = 22 to 34; p < 0.05). Aggregation correlated inversely with SOFA and APACHE II scores (n = 34; p < 0.05). Thr855 phosphorylation of MYPT from unstimulated platelets was not decreased in patients with septic shock (n = 22 to 24). Both septic shock and ROK inhibition attenuated arachidonic acid-induced platelet aggregation independent of changes in [Ser19]MLC₂₀ and [Thr855]MYPT phosphorylation (n = 14).

Conclusions

Impairment of whole blood aggregation in patients within the first 24 h of septic shock was correlated with SOFA and APACHE II scores. Attenuated aggregation was independent of molecular evidence of diminished platelet contraction or reduced ROK inhibition of MLCP. Efforts to restore platelet function in septic shock should therefore focus on platelet adhesion and degranulation.

Patients with liver cirrhosis suffer from primary haemostatic defects? Fact or fiction?

Patients with cirrhosis can have abnormalities in laboratory tests reflecting changes in primary haemostasis, including bleeding time, platelet function tests, markers of platelet activation, and platelet count. Such changes have been considered particularly relevant in the bleeding complications that occur in cirrhosis. However, several studies have shown that routine diagnostic tests, such as platelet count, bleeding time, PFA-100, thromboelastography are not clinically useful to stratify bleeding risk in patients with cirrhosis. Moreover, treatments used to increase platelet count or to modulate platelet function could potentially do harm. Consequently the optimal management of bleeding complications is still a matter of discussion. Moreover, in the last two decades there has been an increased recognition that not only bleeding but also thrombosis complicates the clinical course of cirrhosis. Thus, we performed a literature search looking at publications studying both qualitative and quantitative aspects of platelet function to verify which primary haemostasis defects occur in cirrhosis. In addition, we evaluated the contribution of qualitative and quantitative aspects of platelet function to the clinical outcome in cirrhosis and their therapeutic management according to the data available in the literature. From the detailed analysis of the literature, it appears clear that primary haemostasis may not be defective in cirrhosis, and a low platelet count should not necessarily be considered as an automatic index of an increased risk of bleeding. Conversely, caution should be observed in patients with severe thrombocytopenia where its correction is advised if bleeding occurs and before invasive diagnostic and therapeutic procedures.

Beyond thrombosis: the versatile platelet in critical illness

Sepsis, acute lung injury, and ARDS contribute substantially to the expanding burden of critical illness within our ICUs. Each of these processes is characterized by a myriad of injurious events, including apoptosis, microvascular dysfunction, abnormal coagulation, and dysregulated host immunity. Only recently have platelets--long considered merely effectors of thrombosis--been implicated in inflammatory conditions and the pathobiology of these disease processes. A growing body of evidence suggests a prominent role for maladaptive platelet activation and aggregation during sepsis and ARDS and has begun to underscore the pluripotential influence of platelets on outcomes in critical illness. Not only do platelets enhance vascular injury through thrombotic mechanisms but also appear to help orchestrate pathologic immune responses and are pivotal players in facilitating leukocyte recruitment to vulnerable tissue. These events contribute to the organ damage and poor patient outcomes that still plague the care of these high-risk individuals. An understanding of the role of platelets in critical illness also highlights the potential for both the development of risk stratification schema and the use of novel, targeted therapies that might alter the natural history of sepsis, acute lung injury, and ARDS. Future studies of adenosine, platelet polyphosphates, and the platelet transcriptome/proteome also should add considerably to our ability to unravel the mysteries of the versatile platelet.

Mechanisms underlying the inhibitory effects of lipopolysaccharide on human platelet adhesion

Alterations in platelet aggregation in septic conditions are well established. However, little is known about the effects of lipopolysaccharide (LPS) on platelet adhesion. We have therefore investigated the effects of LPS in human platelet adhesion, using an in vitro model of platelet adhesion to fibrinogen-coated wells. Microtiter plates were coated with human fibrinogen, after which washed platelets (6 x 10(8) platelets/ml) were allowed to adhere. Adherent platelets were quantified through measurement of acid phosphatase activity. Calcium mobilization in Fura2-AM-loaded platelets was monitored with a spectrofluorimeter. Platelet flow cytometry in thrombin-stimulated platelets was performed using monoclonal mouse anti-platelet GPIIb/IIIa antibody (PAC-1). Prior incubation of washed platelets with LPS (0.01-300 microg/ml) for 5 to 60 min concentration- and time-dependently inhibited non-activated platelet adhesion. In thrombin-activated (50 mU/ml) platelets, LPS inhibited the adhesion to a significantly lesser extent than non-activated platelets. Cyclohexamide, superoxide dismutase polyethylene glycol (PEG-SOD) or catalase polyethylene glycol did not affect the LPS responses. No alterations in cyclic GMP levels were seen after platelet incubation with LPS, except with the highest concentration employed (300 microg/ml) where an increase of 36% (P < 0.05) was observed. Thrombin increased by 7.5-fold the internal Ca(2+) platelet levels, an effect markedly inhibited by LPS. Thrombin induced concentration-dependent platelet GPIIb/IIIa activation, but LPS failed to affect the activation state of this membrane glycoprotein. In conclusion, LPS inhibits human platelet adhesion to fibrinogen by mechanisms involving blockade of external Ca(2+), independently of cGMP generation and activation of GPIIb/IIIa complex.

The effect of clopidogrel, aspirin and both antiplatelet drugs on platelet function in patients with peripheral arterial disease

Peripheral arterial disease (PAD) is associated with platelet hyperactivity. Aspirin and clopidogrel, two platelet inhibitors, act by different mechanisms. Aspirin inhibits thromboxane A2 synthesis and clopidogrel acts on the P2Y12 platelet ADP receptor. We evaluated the effect of clopidogrel (75 mg/day), aspirin (75 mg/day) and then both drugs on several platelet function indices in patients with PAD (n = 20). There was a significant (P = 0.0001) decrease in ADP-induced aggregation, after clopidogrel but not after taking aspirin. Clopidogrel plus aspirin significantly decreased spontaneous platelet aggregation (SPA) (P = 0.01 to P = 0.002) but SPA was not significantly altered by either aspirin or clopidogrel monotherapy. Similarly, monotherapy did not inhibit serotonin (5HT)-induced aggregation but there was a sigificant inhibition (P = 0.03 to P < 0.02) after combination therapy. ADP (0.8 microM)-induced platelet shape change (PSC) was significantly inhibited by clopidogrel (P = 0.004) or aspirin (P = 0.01). This was also true for 5HT-induced PSC (clopidogrel, P = 0.01; aspirin, P = 0.03). Soluble P-selectin decreased significantly (from 32 +/- 24 to 25 +/- 17 ng/ml, P = 0.04) with combination therapy. Plasma platelet-derived growth factor and intraplatelet 5HT levels were not altered by combination therapy. In PAD, clopidogrel is a more potent inhibitor of ADP-induced platelet activation than aspirin; combination therapy is more effective than clopidogrel or aspirin monotherapy. These potentially clinically relevant findings should be evaluated in appropriately designed trials.

The effect of a loading dose (300 mg) of clopidogrel on platelet function in patients with peripheral arterial disease

Clopidogrel acts on the P2Y12 adenosine diphosphate (ADP) purinergic receptors on human platelets. The aim of this study was to establish if a loading dose of clopidogrel inhibits platelet activation in patients with peripheral arterial disease (PAD). Two indices of platelet activation were considered: platelet shape change (PSC) and aggregation. Citrated blood was collected from ten PAD patients who were not on aspirin, at baseline (0 hours) and 2 and 4 hours after these patients ingested a loading dose (300 mg) of clopidogrel. ADP (5 micromo/L)-induced platelet aggregation in whole blood was inhibited after 2 hours (free platelet count, 47% +/- 19% vs. 68% +/- 15%; p < or = 0.001) and 4 hours (47% +/- 19% vs. 66% +/- 16%; p < or = 0.001). There was also a significant inhibition of 5- hydroxytryptamine (SHT, 5.0 micromol/L)-induced platelet aggregation at 2 hours. This trend was also observed for 10-micomol/L ADP-induced aggregation. ADP (0.3-0.4 micromol/L)-induced PSC was significantly inhibited at 4 hours (increase in median platelet volume, 6.3%, 1.8-10.7 vs. 1.2%, 0-5.3; p = 0.01). 5HT (0.5 micromol/L)-induced PSC at 4 hours was also significantly inhibited (8.1, 5.3-10.6 vs. 3.0, 0-8.2; p = 0.03). A loading dose of clopidogrel (300 mg) inhibits platelet activation in PAD patients, as early as 2 hours. To the authors' knowledge, no other study considered the effect of a loading dose of clopidogrel in PAD.

Platelet function in sepsis

OBJECTIVE

To evaluate platelet function in sepsis.

DATA SOURCES

The MEDLINE database and bibliographies of selected articles.

DATA SYNTHESIS

The common occurrence of thrombocytopenia in critically ill patients has been recognized for many years and is known to be associated with an increased mortality rate. Platelet function can be divided into four areas: activation, adhesion, aggregation, and secretion. Studies have found that activated platelets secrete key components of the coagulation and inflammatory cascades and are involved in the regulation of vascular tone. However, studies on platelet function in sepsis have been scarce, and their data are often conflicting. In sepsis, aggregation of circulating platelets seems to be reduced, yet platelet receptors are present in normal amounts.

CONCLUSIONS

Platelets play a complex role in sepsis; they are able to modulate not only their own function but also that of cells around them. Further study is needed to better define the precise mechanisms and effects of platelet activation in sepsis and to determine the benefits and risks of inhibiting platelet function.

Platelet accumulation induced by bacterial endotoxin in rats

We studied the effects of i.v. administration of endotoxin (Escherichia coli, Serotype 0127:B8) on the kinetics of 111In-labelled platelets within the pulmonary, abdominal and splenic vascular beds of the rat, and on the radioactivity present in dissected samples of splenic and hepatic tissues. Bolus i.v. injection of endotoxin to anaesthetised rats caused a dose-dependent, transient accumulation of 111In-labelled platelets in the pulmonary vasculature. Increased radioactivity, suggestive of platelet sequestration, was detected in tissue samples from both the spleen and the liver at 4.5 h compared to the radioactivity detected in those organs in vehicle treated rats. The modulation of endotoxin-induced platelet accumulation within the lungs, spleen and liver by pharmacological agents was investigated. The pulmonary, hepatic and splenic platelet accumulation induced by endotoxin was unaffected by pre-treatment of the animals with indomethacin, Hirulog or L-NAME. Pre-treatment with dexamethasone significantly reduced the platelet accumulation within the liver and spleen, but not the lungs.

Platelet-activating factor (PAF) and its relation to prostaglandins, leukotrienes and other aspects of arachidonate metabolism

This article summarizes some of the previously reported findings regarding a lipid mediator known as platelet-activating factor (PAF), and briefly describes its effects on cells and tissues. The effects of PAF have also been considered in relation to certain products of arachidonate metabolism released in response to PAF.

The effect of ethanol on platelet function and vascular prostanoids

The present review will discuss the effects of ethanol on platelet function and vascular prostanoids. Whenever possible we have considered human studies because there are marked species differences in platelet function and vascular prostanoid release even in the absence of ethanol. Because of the specialised nature of some parts of the text, we have included brief introductions to help readers who are not familiar with this field.