Elevated platelet-leukocyte complexes are associated with, but dispensable for myocardial ischemia-reperfusion injury.

AIMS: P-selectin is an activatable adhesion molecule on platelets promoting platelet aggregation, and platelet-leukocyte complex (PLC) formation. Increased numbers of PLC are circulating in the blood of patients shortly after acute myocardial infarction and predict adverse outcomes. These correlations led to speculations about whether PLC may represent novel therapeutic targets. We therefore set out to elucidate the pathomechanistic relevance of PLC in myocardial ischemia and reperfusion injury. METHODS AND RESULTS: By generating P-selectin deficient bone marrow chimeric mice, the post-myocardial infarction surge in PLC numbers in blood was prevented. Yet, intravital microscopy, flow cytometry and immunohistochemical staining, echocardiography, and gene expression profiling showed unequivocally that leukocyte adhesion to the vessel wall, leukocyte infiltration, and myocardial damage post-infarction were not altered in response to the lack in PLC. CONCLUSION: We conclude that myocardial infarction associated sterile inflammation triggers PLC formation, reminiscent of conserved immunothrombotic responses, but without PLC influencing myocardial ischemia and reperfusion injury in return. Our experimental data do not support a therapeutic concept of selectively targeting PLC formation in myocardial infarction.

Peripheral serotonin lacks effects on endothelial adhesion molecule expression in acute inflammation.

BACKGROUND: Peripheral, non-neuronal serotonin promotes the recruitment of neutrophils to sites of acute inflammation and tissue damage. Direct effects of serotonin on neutrophil function were shown to be involved. However, the influence of serotonin on the endothelial counterpart is unknown. OBJECTIVES: To investigate whether serotonin alters the function of endothelial cells in leukocyte recruitment during acute inflammation. METHODS: We used two murine models of acute inflammation: intraperitoneal lipopolysaccharide (LPS) injection and mesenteric ischemia/reperfusion injury (I/R). To study effects of peripheral serotonin, leukocyte recruitment and endothelial adhesion molecule expression were compared in wild type (WT) and tryptophan hydroxylase 1 deficient (Tph1-/- ) mice, which are unable to synthesize peripheral serotonin. RESULTS: As expected, neutrophil transmigration into the peritoneal cavity following LPS injection was impaired in Tph1-/- mice. Abdominal blood vessels, however, showed no difference in adhesion molecule expression. In the early reperfusion phase after mesenteric I/R, the number of rolling leukocytes was significantly lower in Tph1-/- compared to WT. In line with the LPS model, endothelial adhesion molecule expression was independent of serotonin. In vitro experiments using human umbilical vein endothelial cells (HUVECs) confirmed that serotonin does not affect endothelial adhesion molecules. CONCLUSIONS: The inflammatory release of peripheral serotonin is dispensable for the regulation of endothelial adhesion molecules.

A DARPin targeting activated Mac-1 is a novel diagnostic tool and potential anti-inflammatory agent in myocarditis, sepsis and myocardial infarction.

The monocyte ß2-integrin Mac-1 is crucial for leukocyte-endothelium interaction, rendering it an attractive therapeutic target for acute and chronic inflammation. Using phage display, a Designed-Ankyrin-Repeat-Protein (DARPin) was selected as a novel binding protein targeting and blocking the αM I-domain, an activation-specific epitope of Mac-1. This DARPin, named F7, specifically binds to activated Mac-1 on mouse and human monocytes as determined by flow cytometry. Homology modelling and docking studies defined distinct interaction sites which were verified by mutagenesis. Intravital microscopy showed reduced leukocyte-endothelium adhesion in mice treated with this DARPin. Using mouse models of sepsis, myocarditis and ischaemia/reperfusion injury, we demonstrate therapeutic anti-inflammatory effects. Finally, the activated Mac-1-specific DARPin is established as a tool to detect monocyte activation in patients receiving extra-corporeal membrane oxygenation, as well as suffering from sepsis and ST-elevation myocardial infarction. The activated Mac-1-specific DARPin F7 binds preferentially to activated monocytes, detects inflammation in critically ill patients, and inhibits monocyte and neutrophil function as an efficient new anti-inflammatory agent.

Expression of the oxygen-sensitive transcription factor subunit HIF-1α in patients suffering from secondary Raynaud syndrome.

Anti-ischemic therapy remains a challenge due to the complexity of hypoxia response pathways. Hypoxia-inducible factor (HIF)-1 is a heterodimer transcription factor consisting of 2 subunits, HIF-1α and HIF-1ß. Hypoxia-dependent activation of HIF-1α regulates cellular O2 homeostasis. Raynaud syndrome (RS), as a comorbidity of the autoimmune disease systemic sclerosis (SS), is characterized by vasospasms that limit blood flow to the limbs, resulting in hypoxia. A single-center randomized study was conducted to compare prostaglandin E1 (PgE1) therapy with a treatment combining PgE1 and an endothelin-1 blocker, bosentan. A total of 30 patients suffering from SS with RS were enrolled. We examined the regulation of HIF-1α, its target heme oxygenase-1 (HMOX-1), and the serum levels of the HIF-1α protein in a subset of patients as well as in ten healthy individuals. The expression of HIF-1α and HMOX-1 in monocytes was measured using absolute plasmid-based quantitative real-time PCR, whereas serum HIF-1α levels were measured with ELISA. Samples were taken at the time of randomization and after 24 weeks. We found that HIF-1α and HMOX-1 mRNA expression in monocytes and serum HIF-1α protein levels were significantly higher in the SS/RS patients compared to the healthy control group. Single-drug therapy significantly increased HIF-1α and HMOX-1 mRNA expression in monocytes and serum HIF-1α protein levels in the SS/RS patients compared to those at the time of randomization, whereas combining PgE1 with an endothelin-1 blocker prevented the further increases in HIF-1α and HMOX-1 expression. We propose HIF-1α and HMOX-1 as novel markers for anti-ischemic therapy in RS.

Platelet Serotonin Aggravates Myocardial Ischemia/Reperfusion Injury via Neutrophil Degranulation.

BACKGROUND: Platelets store large amounts of serotonin that they release during thrombus formation or acute inflammation. This facilitates hemostasis and modulates the inflammatory response. METHODS: Infarct size, heart function, and inflammatory cell composition were analyzed in mouse models of myocardial reperfusion injury with genetic and pharmacological depletion of platelet serotonin. These studies were complemented by in vitro serotonin stimulation assays of platelets and leukocytes in mice and men, and by measuring plasma serotonin levels and leukocyte activation in patients with acute coronary syndrome. RESULTS: Platelet-derived serotonin induced neutrophil degranulation with release of myeloperoxidase and hydrogen peroxide (H2O2) and increased expression of membrane-bound leukocyte adhesion molecule CD11b, leading to enhanced inflammation in the infarct area and reduced myocardial salvage. In patients hospitalized with acute coronary syndrome, plasmatic serotonin levels correlated with CD11b expression on neutrophils and myeloperoxidase plasma levels. Long-term serotonin reuptake inhibition-reported to protect patients with depression from cardiovascular events-resulted in the depletion of platelet serotonin stores in mice. These mice displayed a reduction in neutrophil degranulation and preserved cardiac function. In line, patients with depression using serotonin reuptake inhibition, presented with suppressed levels of CD11b surface expression on neutrophils and lower myeloperoxidase levels in blood. CONCLUSIONS: Taken together, we identify serotonin as a potent therapeutic target in neutrophil-dependent thromboinflammation during myocardial reperfusion injury.

Autocrine and paracrine regulatory functions of platelet serotonin.

Platelets serotonin (5-hydroxytrytamine, 5-HT) uptake and storage in dense granules is tightly regulated by the serotonergic transport system in the blood. Several 5-HT transporters (5-HTTs) have been identified in the vasculature and blood cells, beyond them 5-HTT is the major 5-HT transporter in platelets. Abnormal 5-HT concentrations in the blood plasma or increased platelet 5-HT uptake or abnormal release contribute to the development of various diseases in the vasculature. Consequently, several clinical trials suggested the positive therapeutic effects of 5-HTT blockade in the circulation. Inhibition of 5-HT strongly attenuates autocrine and paracrine functions of platelets, influencing platelet aggregation, vascular contraction, permeability, tissue repair, wound healing, immunity and cancer. Here, we highlight the current state of basic biological research regarding the hemostatic and non-hemostatic functions of platelet-derived 5-HT in normal and disease conditions. We also describe the physiological consequences of targeting platelet 5-HT functions in thrombosis, stroke, inflammation and cancer to overcome common health problems.

Functional Relevance of the Anaphylatoxin Receptor C3aR for Platelet Function and Arterial Thrombus Formation Marks an Intersection Point Between Innate Immunity and Thrombosis.

BACKGROUND: Platelets have distinct roles in the vascular system in that they are the major mediator of thrombosis, critical for restoration of tissue integrity, and players in vascular inflammatory conditions. In close spatiotemporal proximity, the complement system acts as the first line of defense against invading microorganisms and is a key mediator of inflammation. Whereas the fluid phase cross-talk between the complement and coagulation systems is well appreciated, the understanding of the pathophysiological implications of such interactions is still scant. METHODS: We analyzed coexpression of the anaphylatoxin receptor C3aR with activated glycoprotein IIb/IIIa on platelets of 501 patients with coronary artery disease using flow cytometry; detected C3aR expression in human or murine specimen by polymerase chain reaction, immunofluorescence, Western blotting, or flow cytometry; and examined the importance of platelet C3aR by various in vitro platelet function tests, in vivo bleeding time, and intravital microscopy. The pathophysiological relevance of C3aR was scrutinized with the use of disease models of myocardial infarction and stroke. To approach underlying molecular mechanisms, we identified the platelet small GTPase Rap1b using nanoscale liquid chromatography coupled to tandem mass spectrometry. RESULTS: We found a strong positive correlation of platelet complement C3aR expression with activated glycoprotein IIb/IIIa in patients with coronary artery disease and coexpression of C3aR with glycoprotein IIb/IIIa in thrombi obtained from patients with myocardial infarction. Our results demonstrate that the C3a/C3aR axis on platelets regulates distinct steps of thrombus formation such as platelet adhesion, spreading, and Ca2+ influx. Using C3aR-/- mice or C3-/- mice with reinjection of C3a, we uncovered that the complement activation fragment C3a regulates bleeding time after tail injury and thrombosis. Notably, C3aR-/- mice were less prone to experimental stroke and myocardial infarction. Furthermore, reconstitution of C3aR-/- mice with C3aR+/+ platelets and platelet depletion experiments demonstrated that the observed effects on thrombosis, myocardial infarction, and stroke were specifically caused by platelet C3aR. Mechanistically, C3aR-mediated signaling regulates the activation of Rap1b and thereby bleeding arrest after injury and in vivo thrombus formation. CONCLUSIONS: Overall, our findings uncover a novel function of the anaphylatoxin C3a for platelet function and thrombus formation, highlighting a detrimental role of imbalanced complement activation in cardiovascular diseases.

A ligand-specific blockade of the integrin Mac-1 selectively targets pathologic inflammation while maintaining protective host-defense.

Integrin-based therapeutics have garnered considerable interest in the medical treatment of inflammation. Integrins mediate the fast recruitment of monocytes and neutrophils to the site of inflammation, but are also required for host defense, limiting their therapeutic use. Here, we report a novel monoclonal antibody, anti-M7, that specifically blocks the interaction of the integrin Mac-1 with its pro-inflammatory ligand CD40L, while not interfering with alternative ligands. Anti-M7 selectively reduces leukocyte recruitment in vitro and in vivo. In contrast, conventional anti-Mac-1 therapy is not specific and blocks a broad repertoire of integrin functionality, inhibits phagocytosis, promotes apoptosis, and fuels a cytokine storm in vivo. Whereas conventional anti-integrin therapy potentiates bacterial sepsis, bacteremia, and mortality, a ligand-specific intervention with anti-M7 is protective. These findings deepen our understanding of ligand-specific integrin functions and open a path for a new field of ligand-targeted anti-integrin therapy to prevent inflammatory conditions.

Chip-based digital PCR as a novel detection method for quantifying microRNAs in acute myocardial infarction patients.

miRNAs have shown promise as potential biomarkers for acute myocardial infarction (AMI). However, the current used quantitative real-time PCR (qRT-PCR) allows solely for relative expression of nucleic acids and it is susceptible to day-to-day variability, which has limited the validity of using the miRNAs as biomarkers. In this study we explored the technical qualities and diagnostic potential of a new technique, chip-based digital PCR, in quantifying the miRNAs in patients with AMI and ischaemia-reperfusion injury (I/R). In a dilution series of synthetic C.elegans-miR-39, chip-based digital PCR displayed a lower coefficient of variation (8.9% vs 46.3%) and a lower limit of detection (0.2 copies/µL vs 1.1 copies/µL) compared with qRT-PCR. In the serum collected from 24 patients with ST-elevation myocardial infarction (STEMI) and 20 patients with stable coronary artery disease (CAD) patients after percutaneous coronary intervention (PCI), we used qRT-PCR and multiplexed chip-based digital PCR to quantify the serum levels of miRNA-21 and miRNA-499 as they have been validated in AMI in prior studies. In STEMI, I/R injury was assessed via measurement of ST-segment resolution (ST-R). Chip-based digital PCR revealed a statistical significance in the difference of miR-21 levels between stable CAD and STEMI groups (118.8 copies/µL vs 59 copies/µL; P=0.0300), whereas qRT-PCR was unable to reach significance (136.4 copies/µL vs 122.8 copies/µL; P=0.2273). For miR-499 levels, both chip-based digital PCR and qRT-PCR revealed statistically significant differences between stable CAD and STEMI groups (2 copies/µL vs 8.5 copies/µL, P=0.0011; 0 copies/µL vs 19.4 copies/µL; P<0.0001). There was no association between miR-21/499 levels and ST-R post-PCI. Our results show that the chip-based digital PCR exhibits superior technical qualities and promises to be a superior method for quantifying miRNA levels in the circulation, which may become a more accurate and reproducible method for directly quantifying miRNAs, particularly for use in large multi-centre clinical trials.

Peptidylarginine deiminase 4 promotes age-related organ fibrosis.

Aging promotes inflammation, a process contributing to fibrosis and decline in organ function. The release of neutrophil extracellular traps (NETs [NETosis]), orchestrated by peptidylarginine deiminase 4 (PAD4), damages organs in acute inflammatory models. We determined that NETosis is more prevalent in aged mice and investigated the role of PAD4/NETs in age-related organ fibrosis. Reduction in fibrosis was seen in the hearts and lungs of aged PAD4-/- mice compared with wild-type (WT) mice. An increase in left ventricular interstitial collagen deposition and a decline in systolic and diastolic function were present only in WT mice, and not in PAD4-/- mice. In an experimental model of cardiac fibrosis, cardiac pressure overload induced NETosis and significant platelet recruitment in WT but not PAD4-/- myocardium. DNase 1 was given to assess the effects of extracellular chromatin. PAD4 deficiency or DNase 1 similarly protected hearts from fibrosis. We propose a role for NETs in cardiac fibrosis and conclude that PAD4 regulates age-related organ fibrosis and dysfunction.

Acute exposure to air pollution particulate matter aggravates experimental myocardial infarction in mice by potentiating cytokine secretion from lung macrophages.

Clinical, but not experimental evidence has suggested that air pollution particulate matter (PM) aggravates myocardial infarction (MI). Here, we aimed to describe mechanisms and consequences of PM exposure in an experimental model of MI. C57BL/6J mice were challenged with a PM surrogate (Residual Oil Fly Ash, ROFA) by intranasal installation before MI was induced by permanent ligation of the left anterior descending coronary artery. Histological analysis of the myocardium 7 days after MI demonstrated an increase in infarct area and enhanced inflammatory cell recruitment in ROFA-exposed mice. Mechanistically, ROFA exposure increased the levels of the circulating pro-inflammatory cytokines TNF-α, IL-6, and MCP-1, activated myeloid and endothelial cells, and enhanced leukocyte recruitment to the peritoneal cavity and the vascular endothelium. Notably, these effects on endothelial cells and circulating leukocytes could be reversed by neutralizing anti-TNF-α treatment. We identified alveolar macrophages as the primary source of elevated cytokine production after PM exposure. Accordingly, in vivo depletion of alveolar macrophages by intranasal clodronate attenuated inflammation and cell recruitment to infarcted tissue of ROFA-exposed mice. Taken together, our data demonstrate that exposure to environmental PM induces the release of inflammatory cytokines from alveolar macrophages which directly worsens the course of MI in mice. These findings uncover a novel link between air pollution PM exposure and inflammatory pathways, highlighting the importance of environmental factors in cardiovascular disease.

Platelet-neutrophil complex formation-a detailed in vitro analysis of murine and human blood samples.

Platelets form complexes with neutrophils during inflammatory processes. These aggregates migrate into affected tissues and also circulate within the organism. Several studies have evaluated platelet-neutrophil complexes as a marker of cardiovascular diseases in human and mouse. Although multiple publications have reported platelet-neutrophil complex counts, we noticed that different methods were used to analyze platelet-neutrophil complex formation, resulting in significant differences, even in baseline values. We established a protocol for platelet-neutrophil complex measurement with flow cytometry in murine and human whole blood samples. In vitro platelet-neutrophil complex formation was stimulated with ADP or PMA. We tested the effect of different sample preparation steps and cytometer settings on platelet-neutrophil complex detection and noticed false-positive counts with increasing acquisition speed. Platelet-neutrophil complex formation depends on platelet P-selectin expression, and antibody blocking of P-selectin consequently prevented ADP-induced platelet-neutrophil complex formation. These findings may help generating more comparable data among different research groups that examine platelet-neutrophil complexes as a marker for cardiovascular disease and novel therapeutic interventions.

Intravital Microscopy of Leukocyte-endothelial and Platelet-leukocyte Interactions in Mesenterial Veins in Mice.

Intravital microscopy is a method that can be used to investigate different processes in different regions and vessels in living animals. In this protocol, we describe intravital microscopy of mesentery veins. This can be performed in a short period of time with reproducible results showing leukocyte-endothelial interactions in vivo. We describe an inflammatory setting after LPS challenge of the endothelium. But in this model one can apply many different types of inflammatory conditions, like bacterial, chemical or biological and investigate the administration of drugs and their direct effects on the living animal and its impact on leukocyte recruitment. This protocol has been applied successfully to a number of different treatments of mice and their effects on inflammatory response in vessels. Herein, we describe the visualization of leukocytes and platelets by fluorescently labeling these with rhodamine 6G. Additionally, any specific imaging can be performed using targeted fluorescently labeled molecules.

Dual-contrast molecular imaging allows noninvasive characterization of myocardial ischemia/reperfusion injury after coronary vessel occlusion in mice by magnetic resonance imaging.

BACKGROUND: Inflammation and myocardial necrosis play important roles in ischemia/reperfusion injury after coronary artery occlusion and recanalization. The detection of inflammatory activity and the extent of myocardial necrosis itself are of great clinical and prognostic interest. We developed a dual, noninvasive imaging approach using molecular magnetic resonance imaging in an in vivo mouse model of myocardial ischemia and reperfusion. METHODS AND RESULTS: Ischemia/reperfusion injury was induced in 10-week-old C57BL/6N mice by temporary ligation of the left anterior descending coronary artery. Activated platelets were targeted with a contrast agent consisting of microparticles of iron oxide (MPIOs) conjugated to a single-chain antibody directed against a ligand-induced binding site (LIBS) on activated glycoprotein IIb/IIIa (LIBS-MPIOs). After injection and imaging of LIBS-MPIOs, late gadolinium enhancement was used to depict myocardial necrosis; these imaging experiments were also performed in P2Y12 (-/-) mice. All imaging results were correlated to immunohistochemistry findings. Activated platelets were detectable by magnetic resonance imaging via a significant signal effect caused by LIBS-MPIOs in the area of left anterior descending coronary artery occlusion 2 hours after reperfusion. In parallel, late gadolinium enhancement identified the extent of myocardial necrosis. Immunohistochemistry confirmed that LIBS-MPIOs bound significantly to microthrombi in reperfused myocardium. Only background binding was found in P2Y12 (-/-) mice. CONCLUSIONS: Dual molecular imaging of myocardial ischemia/reperfusion injury allows characterization of platelet-driven inflammation by LIBS-MPIOs and myocardial necrosis by late gadolinium enhancement. This noninvasive imaging strategy is of clinical interest for both diagnostic and prognostic purposes and highlights the potential of molecular magnetic resonance imaging for characterizing ischemia/reperfusion injury.

Acute fluoxetine treatment induces slow rolling of leukocytes on endothelium in mice.

OBJECTIVE: Activated platelets release serotonin at sites of inflammation where it acts as inflammatory mediator and enhances recruitment of neutrophils. Chronic treatment with selective serotonin reuptake inhibitors (SSRI) depletes the serotonin storage pool in platelets, leading to reduced leukocyte recruitment in murine experiments. Here, we examined the direct and acute effects of SSRI on leukocyte recruitment in murine peritonitis. METHODS: C57Bl/6 and Tph1-/- (Tryptophan hydroxylase1) mice underwent acute treatment with the SSRI fluoxetine or vehicle. Serotonin concentrations were measured by ELISA. Leukocyte rolling and adhesion on endothelium was analyzed by intravital microscopy in mesentery venules with and without lipopolysaccharide challenge. Leukocyte extravasation in sterile peritonitis was measured by flow cytometry of abdominal lavage fluid. RESULTS: Plasma serotonin levels were elevated 2 hours after fluoxetine treatment (0.70 ± 0.1 µg/ml versus 0.27 ± 0.1, p = 0.03, n = 14), while serum serotonin did not change. Without further stimulation, acute fluoxetine treatment increased the number of rolling leukocytes (63 ± 8 versus 165 ± 17/0.04 mm(2) min(-1)) and decreased their velocity (61 ± 6 versus 28 ± 1 µm/s, both p<0.0001, n = 10). In Tph1-/- mice leukocyte rolling was not significantly influenced by acute fluoxetine treatment. Stimulation with lipopolysaccharide decreased rolling velocity and induced leukocyte adhesion, which was enhanced after fluoxetine pretreatment (27 ± 3 versus 36 ± 2/0.04 mm(2), p = 0.008, n = 10). Leukocyte extravasation in sterile peritonitis, however, was not affected by acute fluoxetine treatment. CONCLUSIONS: Acute fluoxetine treatment increased plasma serotonin concentrations and promoted leukocyte-endothelial interactions in-vivo, suggesting that serotonin is a promoter of acute inflammation. E-selectin was upregulated on endothelial cells in the presence of serotonin, possibly explaining the observed increase in leukocyte-endothelial interactions. However transmigration of neutrophils in sterile peritonitis was not affected by higher serotonin concentrations, indicating that the effect of fluoxetine was restricted to early steps in the leukocyte recruitment. Whether SSRI use in humans alters leukocyte recruitment remains to be investigated.

A novel hollow and perforated flexible wire allows the safe and effective local application of thrombolytic therapy in a mouse model of deep vein thrombosis.

We tested the feasibility of local thrombolytic therapy via a novel hollow flexible and perforated wire in a mouse model of deep vein thrombosis. Inferior vena cava (IVC) thrombosis was induced by vessel wall exposure to ferric chloride after laparotomy in anesthetized C57Bl/6 mice. Thrombus formation was visualized by intravital microscopy of rhodamine-labeled platelets and leukocytes. A nitinol hypotube coronary wire with perforated tip was inserted via a 0.8 × 40 mm canula into the IVC lumen distal to the site of ferric chloride exposure. Either tissue plasminogen activator (tPA, alteplase) or saline (control) was administered via the platinum wire distal to the thrombus, avoiding mechanical fragmentation. Thrombus size was assessed by immunohistochemistry (platelet CD41 staining). Intravital microscopy of the IVC demonstrated platelet-containing thrombus growth starting 1 min after ferric chloride exposure. Alteplase administration resulted in significant thrombus size reduction within 10-20 min observed by intravital microscopy and confirmed by histological assessment of IVC cross-sections. Saline-treated mice (n = 4) demonstrated near total IVC occlusion with thrombotic material (84 ± 8% of cross-sectional area in serial sections), whereas alteplase-treated mice showed a dose-dependent decrease of thrombotic area [56 ± 5% with 1.5, 39 ± 4 % with 15 and 21 ± 6% with 150 mg/kg, respectively (n = 4)]. We demonstrate that a flexible hollow and perforated wire enables the successful application of thrombolytic therapy to IVC thrombi in mice without vessel wall perforation. Flexible wire-based thrombolytic therapy appears to be a safe and reliable method for thrombus dissolution even in fragile small veins and may become a promising strategy for targeted therapy of small vessel thrombosis.

Platelet serotonin promotes the recruitment of neutrophils to sites of acute inflammation in mice.

The majority of peripheral serotonin is stored in platelets, which secrete it on activation. Serotonin releases Weibel-Palade bodies (WPBs) and we asked whether absence of platelet serotonin affects neutrophil recruitment in inflammatory responses. Tryptophan hydroxylase (Tph)1­deficient mice, lacking non-neuronal serotonin, showed mild leukocytosis compared with wild-type (WT), primarily driven by an elevated neutrophil count. Despite this, 50% fewer leukocytes rolled on unstimulated mesenteric venous endothelium of Tph1(-/-) mice. The velocity of rolling leukocytes was higher in Tph1(-/-) mice, indicating fewer selectin-mediated interactions with endothelium. Stimulation of endothelium with histamine, a secretagogue of WPBs, or injection of serotonin normalized the rolling in Tph1(-/-) mice. Diminished rolling in Tph1(-/-) mice resulted in reduced firm adhesion of leukocytes after lipopolysaccharide treatment. Blocking platelet serotonin uptake with fluoxetine in WT mice reduced serum serotonin by > 80% and similarly reduced leukocyte rolling and adhesion. Four hours after inflammatory stimulation, neutrophil extravasation into lung, peritoneum, and skin wounds was reduced in Tph1(-/-) mice, whereas in vitro neutrophil chemotaxis was independent of serotonin. Survival of lipopolysaccharide-induced endotoxic shock was improved in Tph1(-/-) mice. In conclusion, platelet serotonin promotes the recruitment of neutrophils in acute inflammation, supporting an important role for platelet serotonin in innate immunity.

Serotonin antagonism improves platelet inhibition in clopidogrel low-responders after coronary stent placement: an in vitro pilot study.

Increased residual platelet reactivity remains a burden for coronary artery disease (CAD) patients who received a coronary stent and do not respond sufficiently to treatment with acetylsalicylic acid and clopidogrel. We hypothesized that serotonin antagonism reduces high on-treatment platelet reactivity. Whole blood impedance aggregometry was performed with arachidonic acid (AA, 0.5 mM) and adenosine diphosphate (ADP, 6.5 µM) in addition to different concentrations of serotonin (1-100 µM) in whole blood from 42 CAD patients after coronary stent placement and 10 healthy subjects. Serotonin increased aggregation dose-dependently in CAD patients who responded to clopidogrel treatment: After activation with ADP, aggregation increased from 33.7 ± 1.3% to 40.9 ± 2.0% in the presence of 50 µM serotonin (p<0.05) and to 48.2 ± 2.0% with 100 µM serotonin (p<0.001). The platelet serotonin receptor antagonist ketanserin decreased ADP-induced aggregation significantly in clopidogrel low-responders (from 59.9 ± 3.1% to 37.4 ± 3.5, p<0.01), but not in clopidogrel responders. These results were confirmed with light transmission aggregometry in platelet-rich plasma in a subset of patients. Serotonin hence increased residual platelet reactivity in patients who respond to clopidogrel after coronary stent placement. In clopidogrel low-responders, serotonin receptor antagonism improved platelet inhibition, almost reaching responder levels. This may justify further investigation of triple antiplatelet therapy with anti-serotonergic agents.