PADGEM protein: a receptor that mediates the interaction of activated platelets with neutrophils and monocytes

Myosin-1C augments endothelial secretion of von Willebrand factor by linking contractile actomyosin machinery to the plasma membrane

ABSTRACT

Blood endothelial cells control the hemostatic and inflammatory response by secreting von Willebrand factor (VWF) and P-selectin from storage organelles called Weibel-Palade bodies (WPBs). Actin-associated motor proteins regulate this secretory pathway at multiple points. Before fusion, myosin Va forms a complex that anchors WPBs to peripheral actin structures, allowing for the maturation of content. After fusion, an actomyosin ring/coat is recruited and compresses the WPB to forcibly expel the largest VWF multimers. Here, we provide, to our knowledge, the first evidence for the involvement of class I myosins during regulated VWF secretion. We show that the unconventional myosin-1C (Myo1c) is recruited after fusion via its pleckstrin homology domain in an actin-independent process. This provides a link between the actin ring and phosphatidylinositol 4,5-bisphosphate (PIP2) at the membrane of the fused organelle and is necessary to ensure maximal VWF secretion. This is an active process requiring Myo1c ATPase activity because inhibition of class I myosins using the inhibitor pentachloropseudilin or expression of an ATPase-deficient Myo1c rigor mutant perturbs the expulsion of VWF and alters the kinetics of the exocytic actin ring. These data offer a novel insight into the control of an essential physiological process and provide a new way in which it can be regulated.

Flow Cytometry and Platelets

Clinical assessment of platelet activation by flow cytometry is useful in the characterization and diagnosis of platelet-specific disorders and as a measure of risk for thrombosis or bleeding. Platelets circulate in a resting, "unactivated" state, but when activated they undergo alterations in surface glycoprotein function and/or expression level, exposure of granule membrane proteins, and exposure of procoagulant phospholipids. Flow cytometry provides the means to detect these changes and, unlike other platelet tests, is appropriate for measuring platelet function in samples from patients with low platelet counts. The present review will focus on flow cytometric tests for platelet activation markers.

Macrophages and platelets in liver fibrosis and hepatocellular carcinoma

During fibrosis, (myo)fibroblasts deposit large amounts of extracellular matrix proteins, thereby replacing healthy functional tissue. In liver fibrosis, this leads to the loss of hepatocyte function, portal hypertension, variceal bleeding, and increased susceptibility to infection. At an early stage, liver fibrosis is a dynamic and reversible process, however, from the cirrhotic stage, there is significant progression to hepatocellular carcinoma. Both liver-resident macrophages (Kupffer cells) and monocyte-derived macrophages are important drivers of fibrosis progression, but can also induce its regression once triggers of chronic inflammation are eliminated. In liver cancer, they are attracted to the tumor site to become tumor-associated macrophages (TAMs) polarized towards a M2- anti-inflammatory/tumor-promoting phenotype. Besides their role in thrombosis and hemostasis, platelets can also stimulate fibrosis and tumor development by secreting profibrogenic factors and regulating the innate immune response, e.g., by interacting with monocytes and macrophages. Here, we review recent literature on the role of macrophages and platelets and their interplay in liver fibrosis and hepatocellular carcinoma.

Resveratrol reduces platelet storage lesion by preventing free mitochondrial DNA (mtDNA) accumulation in platelet concentrates during storage

Platelet activation and mitochondrial damage are among the crucial events leading to the quality reduction of platelet concentrates (PCs) during preparation and storage, called platelet storage lesion. Platelet activation results in the clearance of transfused platelets. Oxidative stress and platelet activation trigger mitochondrial DNA (mtDNA) release into the extracellular milieu which is associated with adverse transfusion reactions. Therefore, we aimed to investigate the effects of resveratrol, an antioxidant polyphenol, on platelet activation markers and mtDNA release. Ten PCs were divided equally into two bags each, one of them was allocated to the control group (n = 10) and another to the case group (resveratrol-treated, n = 10). Free mtDNA level and CD62P (P-selectin) expression level were measured by absolute quantification Real-Time PCR, and flow cytometry on days 0 (the receiving day), 3, 5, and 7 of storage respectively. Moreover, Lactate dehydrogenase (LDH) enzyme activity, pH, platelet count, mean platelet volume (MPV), and platelet distribution width (PDW) were assessed as well. Treatment of PCs with resveratrol can significantly decrease mtDNA release during storage compared to the control. In addition, platelet activation was significantly mitigated. We also observed significantly lower MPV, PDW, and LDH activity in resveratrol-treated PCs compared to the control group on days 3, 5, and 7. Furthermore, resveratrol maintained the pH of PCs on day 7. Resveratrol diminished free mtDNA and maintained biochemical parameters in PCs, possibly by reducing platelet activation. Therefore, resveratrol might be a possible additive solution for improving the quality of stored PCs.

Interactions between Platelets and Tumor Microenvironment Components in Ovarian Cancer and Their Implications for Treatment and Clinical Outcomes

Platelets, the primary operatives of hemostasis that contribute to blood coagulation and wound healing after blood vessel injury, are also involved in pathological conditions, including cancer. Malignancy-associated thrombosis is common in ovarian cancer patients and is associated with poor clinical outcomes. Platelets extravasate into the tumor microenvironment in ovarian cancer and interact with cancer cells and non-cancerous elements. Ovarian cancer cells also activate platelets. The communication between activated platelets, cancer cells, and the tumor microenvironment is via various platelet membrane proteins or mediators released through degranulation or the secretion of microvesicles from platelets. These interactions trigger signaling cascades in tumors that promote ovarian cancer progression, metastasis, and neoangiogenesis. This review discusses how interactions between platelets, cancer cells, cancer stem cells, stromal cells, and the extracellular matrix in the tumor microenvironment influence ovarian cancer progression. It also presents novel potential therapeutic approaches toward this gynecological cancer.

Early SARS-CoV-2 infection: Platelet-neutrophil complexes and platelet function

Background

Conflicting results have been reported on platelet activity ex vivo and responsiveness in vitro among patients with COVID-19 with or without thromboembolic complications.

Objectives

To assess platelet reactivity in patients with moderate disease at early stages of COVID-19.

Methods

We performed a prospective, descriptive analysis of 100 consecutive patients presenting with suspected SARS-CoV-2 infection at University Medical Center Freiburg during the first or second wave of the pandemic. Following polymerase chain reaction testing and compliance with study inclusion criteria, 20 SARS-CoV-2-positive and 55 SARS-CoV-2-negative patients (serving as patient controls) were enrolled. In addition, 15 healthy subjects were included. Platelet reactivity was assessed using whole-blood impedance aggregometry and flow cytometry in response to various agonists.

Results

Platelet aggregation was significantly impaired in the patients with COVID-19 compared with that in the patient controls or healthy subjects. The reduced platelet responsiveness in the patients with COVID-19 was associated with impaired activation of GPIIb/IIIa (α_IIbβ₃). In contrast, low expression of P-selectin at baseline and intact secretion upon stimulation in vitro suggest that no preactivation in vivo, leading to "exhausted" platelets, had occurred. The proportion of circulating platelet-neutrophil complexes was significantly higher in the patients with COVID-19 (mean ± SD, 41% ± 13%) than in the patient controls (18% ± 7%; 95% CI, 11.1-34.1; P = .0002) or healthy subjects (17% ± 4%; 95% CI, 13.8-33.8; P < .0001). An analysis of neutrophil adhesion receptors revealed upregulation of CD11b (α-subunit of α_Mβ₂) and CD66b (CEACAM8) but not of CD162 (PSGL-1) in the patients with COVID-19.

Conclusion

Despite reduced platelet responsiveness, platelet-neutrophil complexes are increased at early stages of moderate disease. Thus, this cellular interaction may occur during COVID-19 without preceding platelet activation.

Genes involved in platelet aggregation and activation are downregulated during acute anaphylaxis in humans

Objective

Mechanisms underlying the anaphylactic reaction in humans are not fully understood. Here, we aimed at improving our understanding of anaphylaxis by investigating gene expression changes.

Methods

Microarray data set GSE69063 was analysed, describing emergency department (ED) patients with severe anaphylaxis (n = 12), moderate anaphylaxis (n = 6), sepsis (n = 20) and trauma (n = 11). Samples were taken at ED presentation (T0) and 1 h later (T1). Healthy controls were age and sex matched to ED patient groups. Gene expression changes were determined using limma, and pathway analysis applied. Differentially expressed genes were validated in an independent cohort of anaphylaxis patients (n = 31) and matched healthy controls (n = 10), using quantitative reverse transcription-polymerase chain reaction.

Results

Platelet aggregation was dysregulated in severe anaphylaxis at T0, but not in moderate anaphylaxis, sepsis or trauma. Dysregulation was not observed in patients who received adrenaline before T0. Seven genes (GATA1 (adjusted P-value = 5.57 × 10^-4), TLN1 (adjusted P-value = 9.40 × 10^-4), GP1BA (adjusted P-value = 2.15 × 10^-2), SELP (adjusted P-value = 2.29 × 10^-2), MPL (adjusted P-value = 1.20 × 10^-2), F13A1 (adjusted P-value = 1.39 × 10^-2) and SPARC (adjusted P-value = 4.06 × 10^-2)) were significantly downregulated in severe anaphylaxis patients who did not receive adrenaline before ED arrival, compared with healthy controls. One gene (TLN1 (adjusted P-value = 1.29 × 10^-2)) was significantly downregulated in moderate anaphylaxis patients who did not receive adrenaline before ED arrival, compared with healthy controls.

Conclusion

Downregulation of genes involved in platelet aggregation and activation is a unique feature of the early anaphylactic reaction not previously reported and may be associated with reaction severity.

Platelet signaling at the nexus of innate immunity and rheumatoid arthritis

Rheumatoid arthritis (RA) is a debilitating autoimmune disorder characterized by chronic inflammation of the synovial tissues and progressive destruction of bone and cartilage. The inflammatory response and subsequent tissue degradation are orchestrated by complex signaling networks between immune cells and their products in the blood, vascular endothelia and the connective tissue cells residing in the joints. Platelets are recognized as immune-competent cells with an important role in chronic inflammatory diseases such as RA. Here we review the specific aspects of platelet function relevant to arthritic disease, including current knowledge of the molecular crosstalk between platelets and other innate immune cells that modulate RA pathogenesis.

Platelet-leukocyte aggregate formation and inflammation in patients with pulmonary arterial hypertension and CTEPH

Pulmonary hypertension (PH) is defined by increased mean pulmonary artery pressure, and the clinical classification includes five etiologies, of which we investigated subgroup 1, pulmonary arterial hypertension (PAH) and subgroup 4, chronic thrombotic and/or embolic disease (CTEPH). Platelets participate in both innate and adaptive immune responses and could possibly contribute to the suggested systemic inflammation associated with PAH. In this study, we utilized flow cytometry to analyze platelet activation and platelet-monocyte (PMA) and granulocyte (PGA) aggregates in PAH and CTEPH patients and healthy control subjects. The plasma concentration of proinflammatory cytokines was measured by multiplex electrochemiluminescence. Our main finding is that circulating platelets are activated in the circulation and form aggregates with both monocytes and granulocytes in patients with idiopathic PAH (IPAH), associated PAH (APAH) and pulmonary hypertension due to CTEPH. There was a strong correlation between the platelet activation, assessed as P-selectin, and the number of aggregates formed. IL-6, IL-8, IL-10 and TNF-α were increased in all PH subgroups as compared to healthy controls, and PMAs were associated with circulating IL-6, IL-8 and IL-10, whereas PGAs were associated with IL-6. The increased concentrations of platelet-leukocyte aggregates found in PAH/CTEPH patients might thus contribute to the inflammatory state in PH.

Increased platelet activation and platelet-inflammasome engagement during chikungunya infection

Chikungunya fever is a viral disease transmitted by mosquitoes of the genus Aedes. The infection is usually symptomatic and most common symptoms are fever accompanied by joint pain and swelling. In most cases symptoms subside within a week. However, severe prolonged and disabling joint pain, that may persist for several months, even years, are reported. Although the pathogenesis of Chikungunya infection is not fully understood, the evolution to severe disease seems to be associated with the activation of immune mechanisms and the action of inflammatory mediators. Platelets are recognized as inflammatory cells with fundamental activities in the immune response, maintenance of vascular stability and pathogenicity of several inflammatory and infectious diseases. Although the involvement of platelets in the pathogenesis of viral diseases has gained attention in recent years, their activation in Chikungunya has not been explored. The aim of this study was to analyze platelet activation and the possible role of platelets in the amplification of the inflammatory response during Chikungunya infection. We prospectively included 132 patients attended at the Quinta D’Or hospital and 25 healthy volunteers during the 2016 epidemic in Rio de Janeiro, Brazil. We observed increased expression of CD62P on the surface of platelets, as well as increased plasma levels of CD62P and platelet-derived inflammatory mediators indicating that the Chikungunya infection leads to platelet activation. In addition, platelets from chikungunya patients exhibit increased expression of NLRP3, caspase 4, and cleaved IL-1β, suggestive of platelet-inflammasome engagement during chikungunya infection. In vitro experiments confirmed that the Chikungunya virus directly activates platelets. Moreover, we observed that platelet activation and soluble p-selectin at the onset of symptoms were associated with development of chronic forms of the disease. Collectively, our data suggest platelet involvement in the immune processes and inflammatory amplification triggered by the infection.

Platelet Derivatives and the Immunomodulation of Wound Healing

Besides their primary role in hemostasis, platelets contain a plethora of immunomodulatory molecules that profoundly affect the entire process of wound repair. Therefore, platelet derivatives, such as platelet-rich plasma or platelet lysate, have been widely employed with promising results in the treatment of chronic wounds. Platelet derivatives provide growth factors, cytokines, and chemokines targeting resident and immigrated cells belonging to the innate and adaptive immune system. The recruitment and activation of neutrophils and macrophages is critical for pathogen clearance in the early phase of wound repair. The inflammatory response begins with the release of cytokines, such as TGF-β, aimed at damping excessive inflammation and promoting the regenerative phase of wound healing. Dysregulation of the immune system during the wound healing process leads to persistent inflammation and delayed healing, which ultimately result in chronic wound. In this review, we summarize the role of the different immune cells involved in wound healing, particularly emphasizing the function of platelet and platelet derivatives in orchestrating the immunological response.

Increase of Circulating Monocyte-Platelet Conjugates in Rheumatoid Arthritis Responders to IL-6 Blockage

Platelets (PLT) bind to a significant percentage of circulating monocytes and this immunomodulatory interaction is increased in several inflammatory and autoimmune conditions. The therapeutic blockage of IL-6 with Tocilizumab (TCZ) alters PLT and the phenotype and function of monocytes in rheumatoid arthritis (RA). However, the relationship between monocyte−PLT conjugates (CD14+PLT+) and clinical and immunological variables and the regulation of this interaction by IL-6 blockage are still unknown. Here, we compared the presence of monocyte−PLT conjugates (CD14+PLT+) and membrane CD162 expression using flow cytometry, and, by ELISA, the markers of PLT activation (sCD62P and sCD40L) in healthy donors (HD) and patients with long-standing RA before TCZ (baseline). We found higher percentages and absolute counts of CD14+PLT+, and higher plasmatic levels of sCD62P and sCD40L but lower CD162 expression on monocytes from RA patients than those from HD. Additionally, the levels of CD14+PLT+ inversely correlated with inflammatory parameters. Interestingly, 95% of patients with lower percentages of CD14+PLT+ and only 63% of patients with higher percentages of CD14+PLT+ achieved a EULAR-defined response at four weeks (p = 0.036). After TCZ, the percentage of CD14+PLT+ increased in 92% of RA patients who achieved 12 w-remission (p < 0.001). Our results suggest that the binding of PLTs has a modulatory effect, accentuated by the increased binding of PLTs to monocytes in response to the therapeutic blockage of IL-6.

Dual-target synergistic antithrombotic mechanism of a Dabigatran etexilate analogue (HY023016)

Thrombin has long been considered a desirable antithrombotic target, but anti-thrombin therapy without anti-platelet therapy has never achieved the ideal effect. HY023016, derived from dabigatran etexilate, exhibited a potent antithrombotic efficacy. In the present study, mechanisms underlying this effect were explored. HY023016 strongly decreased the binding of thrombin to recombinant GPIbα N-terminal sequence which was confirmed by surface plasmon resonance. Flow cytometry revealed that HY023016 selectively decreased the binding of antibody to GPIbα and inhibited the washed human platelet aggregation induced by thrombin. Fluorescence experiment showed that HY023016 remarkably inhibited exosite II by a loss of affinity for the γ'-peptide of fibrinogen. Using intravital microscopy, we observed and recorded the dynamic process of thrombus formation and found that HY023016 effectively prevented thrombus formation in rat arteriovenous shunt thrombosis model. On the basis of these findings, we propose that HY023016 provides a novel insight into the antithrombotic mechanism, which exerts synergistic anticoagulant and antiplatelet effects through thrombin and GPIbα.

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.

V-ATPase V0a1 promotes Weibel-Palade body biogenesis through the regulation of membrane fission

Membrane fission, the division of a membrane-bound structure into two discrete compartments, is essential for diverse cellular events, such as endocytosis and vesicle/granule biogenesis; however, the process remains unclear. The hemostatic protein von Willebrand factor is produced in vascular endothelial cells and packaged into specialized secretory granules, Weibel–Palade bodies (WPBs) at the trans-Golgi network (TGN). Here, we reported that V0a1, a V-ATPase component, is required for the membrane fission of WPBs. We identified two V0a isoforms in distinct populations of WPBs in cultured endothelial cells, V0a1 and V0a2, on mature and nascent WPBs, respectively. Although WPB buds were formed, WPBs could not separate from the TGN in the absence of V0a1. Screening using dominant–negative forms of known membrane fission regulators revealed protein kinase D (PKD) as an essential factor in biogenesis of WPBs. Further, we showed that the induction of wild-type PKDs in V0a1-depleted cells does not support the segregation of WPBs from the TGN; suggesting a primary role of V0a1 in the membrane fission of WPBs. The identification of V0a1 as a new membrane fission regulator should facilitate the understanding of molecular events that enable membrane fission.

A PSGL-1 glycomimetic reduces thrombus burden without affecting hemostasis

Events mediated by the P-selectin/PSGL-1 pathway play a critical role in the initiation and propagation of venous thrombosis by facilitating the accumulation of leukocytes and platelets within the growing thrombus. Activated platelets and endothelium express P-selectin, which binds P-selectin glycoprotein ligand-1 (PSGL-1) that is expressed on the surface of all leukocytes. We developed a pegylated glycomimetic of the N terminus of PSGL-1, PEG40-GSnP-6 (P-G6), which proved to be a highly potent P-selectin inhibitor with a favorable pharmacokinetic profile for clinical translation. P-G6 inhibits human and mouse platelet-monocyte and platelet-neutrophil aggregation in vitro and blocks microcirculatory platelet-leukocyte interactions in vivo. Administration of P-G6 reduces thrombus formation in a nonocclusive model of deep vein thrombosis with a commensurate reduction in leukocyte accumulation, but without disruption of hemostasis. P-G6 potently inhibits the P-selectin/PSGL-1 pathway and represents a promising drug candidate for the prevention of venous thrombosis without increased bleeding risk.

Thrombosis in COVID-19 infection: Role of platelet activation-mediated immunity

Background

Thrombosis plays an important role in the Coronavrus Disease 2019 (COVID-19) infection-related complications such as acute respiratory distress syndrome and myocardial infarction. Multiple factors such as oxygen demand injuries, endothelial cells injury related to infection, and plaque formation.

Main body

Platelets obtained from the patients may have severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA, showing that the increased activation potential recommends platelet can be hyper-activated in severely ill SARS-CoV-2 cases. Platelets contain multiple receptors that interact with specific ligands. Pathogen’s receptors such as Toll-like receptors (TLRs), NOD-like receptor, C-type lectin receptor family, glycoprotein (GP) such as GPαIIbβ3 and GPIbα which allow pathogens to interact with platelets. Platelet TLRs and NOD2 are involved in platelet activation and thrombosis. Accordingly, TLRs are critical receptors that could recognize various endogenous damage-associated molecular patterns and exogenous pathogen-associated molecular patterns (PAMPs). TLRs are considered as important components in the activation of innate immunity response against pathogenic and non-pathogenic components like damaged tissues. TLRs-1,-2,-4,-6,-7 expression on or within platelets has been reported previously. Various PAMPs were indicated to be capable of binding to platelet-TLRs and inducing both the activation and promotion of downstream proinflammatory signaling cascade.

Conclusion

It is possible that the increased TLRs expression and TLR-mediated platelets activation during COVID-19 may enhance vascular and coronary thrombosis. It may be hypothesized using TLRs antagonist and monoclonal antibody against P-selectin, as the marker of leukocyte recruitment and platelet activation, besides viral therapy provide therapeutic advances in fighting against the thrombosis related complications in COVID-19.

Chitinase 3-like-1 contributes to acetaminophen-induced liver injury by promoting hepatic platelet recruitment

Background

Hepatic platelet accumulation contributes to acetaminophen (APAP)-induced liver injury (AILI). However, little is known about the molecular pathways involved in platelet recruitment to the liver and whether targeting such pathways could attenuate AILI.

Methods

Mice were fasted overnight before intraperitoneally (i.p.) injected with APAP at a dose of 210 mg/kg for male mice and 325 mg/kg for female mice. Platelets adherent to Kupffer cells were determined in both mice and patients overdosed with APAP. The impact of α-chitinase 3-like-1 (α-Chi3l1) on alleviation of AILI was determined in a therapeutic setting, and liver injury was analyzed.

Results

The present study unveiled a critical role of Chi3l1 in hepatic platelet recruitment during AILI. Increased Chi3l1 and platelets in the liver were observed in patients and mice overdosed with APAP. Compared to wild-type (WT) mice, Chil1-/- mice developed attenuated AILI with markedly reduced hepatic platelet accumulation. Mechanistic studies revealed that Chi3l1 signaled through CD44 on macrophages to induce podoplanin expression, which mediated platelet recruitment through C-type lectin-like receptor 2. Moreover, APAP treatment of Cd44-/- mice resulted in much lower numbers of hepatic platelets and liver injury than WT mice, a phenotype similar to that in Chil1-/- mice. Recombinant Chi3l1 could restore hepatic platelet accumulation and AILI in Chil1-/- mice, but not in Cd44-/- mice. Importantly, we generated anti-Chi3l1 monoclonal antibodies and demonstrated that they could effectively inhibit hepatic platelet accumulation and AILI.

Conclusions

We uncovered the Chi3l1/CD44 axis as a critical pathway mediating APAP-induced hepatic platelet recruitment and tissue injury. We demonstrated the feasibility and potential of targeting Chi3l1 to treat AILI.

Funding

ZS received funding from NSFC (32071129). FWL received funding from NIH (GM123261). ALFSG received funding from NIDDK (DK 058369). ZA received funding from CPRIT (RP150551 and RP190561) and the Welch Foundation (AU-0042-20030616). CJ received funding from NIH (DK122708, DK109574, DK121330, and DK122796) and support from a University of Texas System Translational STARs award. Portions of this work were supported with resources and the use of facilities of the Michael E. DeBakey VA Medical Center and funding from Department of Veterans Affairs I01 BX002551 (Equipment, Personnel, Supplies). The contents do not represent the views of the US Department of Veterans Affairs or the US Government.

Phenotypic and Functional Consequences of PLT Binding to Monocytes and Its Association with Clinical Features in SLE

Platelets (PLTs) can modulate the immune system through the release of soluble mediators or through interaction with immune cells. Monocytes are the main immune cells that bind with PLTs, and this interaction is increased in several inflammatory and autoimmune conditions, including systemic lupus erythematosus (SLE). Our aim was to characterize the phenotypic and functional consequences of PLT binding to monocytes in healthy donors (HD) and in SLE and to relate it to the pathogenesis of SLE. We analyzed the phenotypic and functional features of monocytes with non-activated and activated bound PLTs by flow cytometry. We observed that monocytes with bound PLTs and especially those with activated PLTs have an up-regulated HLA-DR, CD86, CD54, CD16 and CD64 expression. Monocytes with bound PLTs also have an increased capacity for phagocytosis, though not for efferocytosis. In addition, monocytes with bound PLTs have increased IL-10, but not TNF-α, secretion. The altered phenotypic and functional features are comparable in SLE and HD monocytes and in bound PLTs. However, the percentages of monocytes with bound PLTs are significantly higher in SLE patients and are associated with undetectable levels of anti-dsDNA antibodies and hematuria, and with normal C3 and albumin/creatinine levels. Our results suggest that PLTs have a modulatory influence on monocytes and that this effect may be highlighted by an increased binding of PLTs to monocytes in autoimmune conditions.

The Maternal Leucocytes in Thrombophilia and Hypothyroidism and their Influence on Fetal Cells

The literature data show that thrombophilia and maternal dysfunction of thyroid gland during pregnancy are associated with an increased risk of miscarriage, placental abruption, hypertensive disorders and fetal growth retardation. It was shown that thyroid hormones and hypercoagulable states influence onto a leucocyte activity. The aim of this study has been to investigate maternal leucocytes changes and their correlation with frequency of fetal cells micronuclei in pregnant women with thrombophilia and hypothyroidism. The samples of blood and amniotic fluid were collected from healthy pregnant women and pregnant women with inherited thrombophilia and hypothyroidism (16 - 18 weeks of gestation). Hematological characteristics were determined by using standard hematological methods. The frequency of micronuclei was determined in fetal cells after amniocentesis by using standard cytogenetic methods. The results of this study showed significant higher levels of β-hCG, number of monocytes and eosinophils in blood of pregnant women with thrombophilia. A large number of eosinophils was documented in blood of pregnant women with hypothyroidism. Increased percentage distribution of eosinophils and basophils is shown in both investigated groups of pregnant women. The increased fetal cells micronuclei frequency and their correlation with percentage distribution of eosinophils and basophils were indicated in pregnant women with hypothyroidism. The obtained results suggest that an increased percentage of eosinophils and basophils in pregnant women with hypothyroidism contribute to a formation of micronuclei in fetal cells.

Identification of immune-related genes with prognostic significance in the microenvironment of cutaneous melanoma

Cutaneous melanoma is one of the most aggressive cancers characterized by increasing incidence and mortality. In recent years, the emergence of immunotherapy has greatly raised the survival rate of patients suffering from cutaneous melanoma, yet some sufferers remain to have poor outcomes after treatment mainly due to the tumor microenvironment (TME). In this study, cutaneous melanoma-associated TME was systematically analyzed using the ESTIMATE algorithm based on the gene transcriptome data obtained from the TCGA database. Totally, 471 patients were included and 553 TME-related genes were screened. Afterwards, a 3-gene signature-based model (CLEC4A, GBP4, KIR2DL4) was constructed via univariate Cox, LASSO, and multivariate Cox regression analyses. To validate the validity of this model, ROC analysis was conducted, and the model was further validated to be an independent prognostic biomarker through univariate and multivariate regression analyses. Finally, the three genes in the model were studied by GSEA and GSVA for their biological significance. We found that the three genes could promote cancer immune response predominantly through affecting immune-related pathways such as antigen processing and presentation, and they may help tumor cells in escaping from surveillance of the immune system when their expression levels were decreased. Additionally, we as well discovered that the expression of the three genes was significantly and positively correlated with the infiltration of related immune cells, but negatively associated with tumor purity. Overall, this study comprehensively analyzed the TME of cutaneous melanoma, identified related biomarkers, and discovered their association with immune system.

Platelet activation and platelet-monocyte aggregate formation trigger tissue factor expression in patients with severe COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emergent pathogen responsible for the coronavirus disease 2019 (COVID-19). Since its emergence, the novel coronavirus has rapidly achieved pandemic proportions causing remarkably increased morbidity and mortality around the world. A hypercoagulability state has been reported as a major pathologic event in COVID-19, and thromboembolic complications listed among life-threatening complications of the disease. Platelets are chief effector cells of hemostasis and pathological thrombosis. However, the participation of platelets in the pathogenesis of COVID-19 remains elusive. This report demonstrates that increased platelet activation and platelet-monocyte aggregate formation are observed in severe COVID-19 patients, but not in patients presenting mild COVID-19 syndrome. In addition, exposure to plasma from severe COVID-19 patients increased the activation of control platelets ex vivo. In our cohort of COVID-19 patients admitted to the intensive care unit, platelet-monocyte interaction was strongly associated with tissue factor (TF) expression by the monocytes. Platelet activation and monocyte TF expression were associated with markers of coagulation exacerbation as fibrinogen and D-dimers, and were increased in patients requiring invasive mechanical ventilation or patients who evolved with in-hospital mortality. Finally, platelets from severe COVID-19 patients were able to induce TF expression ex vivo in monocytes from healthy volunteers, a phenomenon that was inhibited by platelet P-selectin neutralization or integrin α_IIb/β₃ blocking with the aggregation inhibitor abciximab. Altogether, these data shed light on new pathological mechanisms involving platelet activation and platelet-dependent monocyte TF expression, which were associated with COVID-19 severity and mortality.

Immunothrombosis and thromboinflammation in host defense and disease

Platelets are increasingly being recognized for playing roles beyond thrombosis and hemostasis. Today we know that they mediate inflammation by direct interactions with innate immune cells or secretion of cytokines/chemokines. Here we review their interactions with neutrophils and monocytes/macrophages in infection and sepsis, stroke, myocardial infarction and venous thromboembolism. We discuss new roles for platelet surface receptors like GPVI or GPIb and also look at platelet contributions to the formation of neutrophil extracellular traps (NETs) as well as to deep vein thrombosis during infection, e.g. in COVID-19 patients.

Revisiting Platelets and Toll-Like Receptors (TLRs): At the Interface of Vascular Immunity and Thrombosis

While platelet function has traditionally been described in the context of maintaining vascular integrity, recent evidence suggests that platelets can modulate inflammation in a much more sophisticated and nuanced manner than previously thought. Some aspects of this expanded repertoire of platelet function are mediated via expression of Toll-like receptors (TLRs). TLRs are a family of pattern recognition receptors that recognize pathogen-associated and damage-associated molecular patterns. Activation of these receptors is crucial for orchestrating and sustaining the inflammatory response to both types of danger signals. The TLR family consists of 10 known receptors, and there is at least some evidence that each of these are expressed on or within human platelets. This review presents the literature on TLR-mediated platelet activation for each of these receptors, and the existing understanding of platelet-TLR immune modulation. This review also highlights unresolved methodological issues that potentially contribute to some of the discrepancies within the literature, and we also suggest several recommendations to overcome these issues. Current understanding of TLR-mediated platelet responses in influenza, sepsis, transfusion-related injury and cardiovascular disease are discussed, and key outstanding research questions are highlighted. In summary, we provide a resource—a “researcher’s toolkit”—for undertaking further research in the field of platelet-TLR biology.

Chronic intermittent hypoxia vs chronic continuous hypoxia: Effects on vascular endothelial function and myocardial contractility

BACKGROUND AND AIM

Both chronic intermittent hypoxia (CIH) and chronic continuous hypoxia (CCH) are risk factors for cardiovascular disease, which are associated with cardiac systolic function and associated with dysfunction of endothelia and coagulation-fibrinolysis system in the vasculature. However, the different effects of these two hypoxic models are not fully understood. In our study, we systemically compared the effects of CIH and CCH on cardiac function and related factor levels in serum using rat model.

METHODS

Forty-five male Sprague-Dawley rats were randomly divided into the normoxia control (NC), CIH and CCH groups. The rat CIH and CCH models were established, then the blood and tissue samples were collected to analyze the function of endothelium and the coagulation-fibrinolysis system. Also, the ultrasound cardiogram was performed to directly assess myocardial contractility.

RESULTS

Both CIH and CCH significantly decreased the NO, eNOS, P-eNOS and AT-III levels in the rat serum but significantly increased the levels of ET-1, vWF, COX-2, NF-κB, FIB, FVIII and PAI-1 in the rat serum (P < 0.05). The expression of ET-1, VWF and ICAM-1 in CIH group were higher than CCH group (P < 0.05), however, the expression of CD62p was increased in CCH group but not in CIH group. The expression of t-PA in CIH group were lower than CCH group (P < 0.05), but there were no significant differences in CCH group and NC group (P > 0.05). Using transmission electron microscope, we found that the mitochondrial ultrastructure of thoracic aorta endothelial cells in CIH and CCH group were damaged. Moreover, the myocardial contractility in CIH and CCH group were significantly decreased compared with NC group.

CONCLUSION

Our results suggested that CIH and CCH could cause endothelial dysfunction, dysfunction of the coagulation-fibrinolysis system and decreasing of myocardial contractility. Compared with CCH, CIH has greater effect on vasoconstriction and adhesion of vascular endothelial cells, and stronger procoagulant effect.

Amyloid precursor protein 770 is specifically expressed and released from platelets

Platelets not only play an essential role in hemostasis after vascular injury but are also involved in the development of coronary artery disease (CAD) and cerebrovascular lesions. Patients with CAD and cerebral ischemia are recommended to undergo antiplatelet therapy, but they have an increased incidence of major bleeding complications. Both assessment of the platelet activation status and response to antiplatelet therapy in each patient are highly desired. β-Amyloid precursor protein (APP) 770 is expressed in vascular endothelial cells, and its extracellular region, a soluble form of APP770 (sAPP770, also called nexin-2), is proteolytically cleaved for shedding. Abundant sAPP770 is also released from activated platelets. In this study, we used peripheral blood samples from patients with CAD and control subjects and evaluated sAPP770 as a specific biomarker for platelet activation. First, the plasma levels of sAPP770 correlated well with those of the soluble form CD40 ligand (CD40L), an established biomarker for platelet activation. Additionally, flow cytometry analysis using peripheral blood cells showed that CD40L expression is up-regulated in activated T cells, whereas APP770 expression is negligible in all blood cell types except platelets. Following stimulation with collagen or ADP, aggregating platelets immediately released sAPP770. Finally, patients with dual antiplatelet therapy showed significantly lower levels of plasma sAPP770 than those with no therapy. Taken together, our data show that plasma sAPP770 could be a promising biomarker for platelet activation.

Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which a variety of circulating pro-inflammatory cells and dysregulated molecules are involved in disease aetiology and progression. Platelets are an important cellular element in the circulation that can bind several dysregulated molecules (such as collagen, thrombin and fibrinogen) that are present both in the synovium and the circulation of patients with RA. Platelets not only respond to dysregulated molecules in their environment but also transport and express their own inflammatory mediators, and serve as regulators at the boundary between haemostasis and immunity. Activated platelets also produce microparticles, which further convey signalling molecules and receptors to the synovium and circulation, thereby positioning these platelet-derived particles as strategic regulators of inflammation. These diverse functions come together to make platelets facilitators of cellular crosstalk in RA. Thus, the receptor functions, ligand binding potential and dysregulated signalling pathways in platelets are becoming increasingly important for treatment in RA. This Review aims to highlight the role of platelets in RA and the need to closely examine platelets as health indicators when designing effective pharmaceutical targets in this disease.

Effects of Yak skin gelatin on platelet activation

Studies have shown that gelatin is not only a good hemostatic material, but also a food additive with potentially broad use. Yak skin gelatin is a new gelatin resource, but its oral coagulant effects have not been studied. Given the central role of platelets in hemostasis, in this study we examined the pharmacodynamical differences between different molecular Yak skin gelatins on platelet activation. The hemostatic effects of Yak skin gelatins with different molecular weight distributions were evaluated for bleeding time (BT), clotting time (CT), and platelet activity by measuring the contents of P-selectin, platelet membrane glycoprotein Ia/IIa (GP Ia/IIa), platelet membrane glycoprotein IIb/IIIa (GP IIb/IIIa), and platelet membrane glycoprotein IV (GP IV). Intragastric administration of Yak skin gelatin resulted in a significant reduction in CT and BT, and an increase in the contents of P-selectin, GP Ia/IIa, GP IIb/IIIa, and GP IV in all groups in comparison with the control group. The strongest activation of platelets by Yak skin gelatin was observed with size between 0.1 μm and 0.22 μm, and activation may have been in response to improving GP IIb/IIIa and GP IV levels. When measuring the levels of an established indicator of platelet activation, platelet activation-dependent granule membrane protein (CD62P), its promotion was observed for all molecular weight ranges of Yak skin gelatins. In brief, Yak skin gelatin has hemostatic effects, and Yak skin gelatin fractions between 0.1 μm and 0.22 μm are the primary effectors of hemostasis via promoting platelet membrane glycoprotein activities and strengthening platelet function.

Profiling Gene Programs in the Blood During Liver Regeneration in Living Liver Donors

The human liver's capacity to rapidly regenerate to a full-sized functional organ after resection has allowed successful outcomes for living donor liver transplantation (LDLT) procedures. However, the ability to detect and track physiological changes occurring during liver regeneration after resection and throughout the restoration process is still lacking. We performed a comprehensive whole-transcriptome RNA sequencing analysis of liver and circulating blood tissue from 12 healthy LDLT donors to define biomarker signatures for monitoring physiological activities during liver regeneration at 14 time points for up to a 1-year procedural follow-up. LDLT donor liver tissue differentially expressed 1238 coding and noncoding genes after resection, and an additional 1260 genes were selectively regulated after LDLT. A total of 15,011 RNA transcript species were identified in the blood in response to liver resection. The transcripts most highly regulated were sequentially expressed within 3 distinct peaks that correlated with sets of functional genes involved in the induction of liver resection-specific innate immune response (peak 1), activation of the complement system (peak 2), and platelet activation and erythropoiesis (peak 3). Each peak corresponded with progressive phases of extracellular matrix degradation, remodeling, and organization during liver restoration. These processes could be tracked by distinct molecular signatures of up-regulated and down-regulated gene profiles in the blood during phases of liver repair and regeneration. In conclusion, the results establish temporal and dynamic transcriptional patterns of gene expression following surgical liver resection that can be detected in the blood and potentially used as biomarker signatures for monitoring phases of liver regeneration.

Platelets and Complement Cross-Talk in Early Atherogenesis

Atherosclerosis remains a ubiquitous and serious threat to human health. The initial formation of the atherosclerotic lesion (atheroma) is driven by pro-inflammatory signaling involving monocytes and vascular endothelial cells; later stages of the disease involve rupture of well-established atherosclerotic plaques, thrombosis, and blood vessel occlusion. While the central role of platelets in thrombosis is undisputed, platelets exhibit pro-inflammatory activities, and contribute to early-stage atheroma formation. Platelets also engage components of the complement system, an essential element of innate immunity that contributes to vascular inflammation. Here we provide an overview of the complex interplay between platelets and the complement system, with a focus on how the crosstalk between them may impact on the initiation of atheroma formation.

Peptide Antagonists for P-selectin Discriminate between Sulfatide-Dependent Platelet Aggregation and PSGL-1-Mediated Cell Adhesion

BACKGROUND

Membrane-exposed sulfatides are proposed to contribute to P-selectin-dependent platelet aggregation. Here, we demonstrated that P-selectin-mediated platelet aggregation on a collagen-coated surface under flow indeed depended on sulfatides and that this interaction differed considerably from the interaction of P-selectin with P-selectin Glycoprotein Ligand-1 (PSGL-1), which underlies leukocyte-endothelium adhesion.

METHODS AND RESULTS

Upon platelet activation, sulfatides were translocated to the platelet surface to form focal hot-spots. Interestingly, P-selectin was observed to exclusively interact with liposomes with a sulfatide density higher than 21% (w/w), indicating that the binding profile of P-selectin for sulfatide-rich liposomes was dependent on sulfatide density. Sulfatide-liposome binding to P-selectin and sulfatide/P-selectin-dependent platelet aggregation was blunted by peptide antagonists, carrying the EWVDV motif within N-terminal extensions, such as CDVEWVDVSC (half maximal inhibitory concentration IC₅₀ = 0.2 μM), but not by the EWVDV core motif itself (IC₅₀ > 1000 μM), albeit both being equally potent inhibitors of PSGL-1/P-selectin interaction (IC₅₀= 7-12 μM).

CONCLUSIONS

Our data suggest that the sulfatide/P-selectin interaction implicates multiple binding pockets, which only partly overlap with that of PSGL-1. These observations open ways to selectively interfere with sulfatide/P-selectin-dependent platelet aggregation without affecting PSGL-1-dependent cell adhesion.

Platelet-Neutrophil Interplay: Insights Into Neutrophil Extracellular Trap (NET)-Driven Coagulation in Infection

Well established for their central role in hemostasis, platelets have increasingly been appreciated as immune cells in recent years. This emerging role should not come as a surprise as the central immune cells of invertebrates, hemocytes, are able to phagocytose, secrete soluble mediators and promote coagulation of hemolymph, blurring the line between immunity and hemostasis. The undeniable evolutionary link between coagulation and immunity becomes even clearer as the role of platelets in inflammation is better understood. Platelets exert a range of immune-related functions, many of which involve an intimate interplay with leukocytes. Platelets promote leukocyte recruitment via endothelial activation and can serve as “landing pads” for leukocytes, facilitating cellular adhesion in vascular beds devoid of classic adhesion molecules. Moreover, platelets enhance leukocyte function both through direct interactions and through release of soluble mediators. Among neutrophil-platelets interactions, the modulation of neutrophil extracellular traps (NETs) is of great interest. Platelets have been shown to induce NET formation; and, in turn, NET components further regulate platelet and neutrophil function. While NETs have been shown to ensnare and kill pathogens, they also initiate coagulation via thrombin activation. In fact, increased NET formation has been associated with hypercoagulability in septic patients as well as in chronic vascular disorders. This review will delve into current knowledge of platelet-neutrophil interactions, with a focus on NET-driven coagulation, in the context of infectious diseases. A better understanding of these mechanisms will shed a light on the therapeutic potential of uncoupling immunity and coagulation through targeting of NETs.

Impaired frequencies and function of platelets and tissue remodeling in chronic Chagas disease

Chronic inflammation, as a consequence of the persistent infection with Trypanosoma cruzi, leads to continuous activation of the immune system in patients with chronic Chagas disease. We have previously shown that increased sera levels of soluble P-selectin are associated with the severity of the cardiomyopathy distinctive of chronic Chagas disease. In this study, we explored the expression of biomarkers of platelet and endothelial activation, tissue remodeling, and mediators of the coagulation cascade in patients at different clinical stages of chronic Chagas heart disease. The frequencies of activated platelets, measured by the expression of CD41a and CD62P were decreased in patients with chronic Chagas disease compared with those in uninfected subjects, with an inverse association with disease severity. Platelet activation in response to adenosine diphosphate was also decreased in T. cruzi-infected subjects. A major proportion of T. cruzi infected subjects showed increased serum levels of fibrinogen. Patients with severe cardiac dysfunction showed increased levels of endothelin-1 and normal values of procollagen I. In conclusion, chronic infection with T. cruzi induced hemostatic alterations, even in those patients who do not yet present cardiac symptoms.

Platelets: Peripheral Biomarkers of Dementia?

Dementia continues to be the most burdening neurocognitive disorder, having a negative impact on the lives of millions. The search for biomarkers to improve the clinical diagnosis of dementia is ongoing, with the focus on effective use of readily accessible peripheral markers. In this review, we concentrate on platelets as biomarkers of dementia and analyze their potential as easily-accessible clinical biomarkers for various subtypes of dementia. Current platelet protein biomarkers that have been investigated for their clinical utility in the diagnosis of dementia, in particular Alzheimer's disease, include amyloid-β protein precursor (AβPP), the AβPP secretases (BACE1 and ADAM10), α-synuclein, tau protein, serotonin, cholesterol, phospholipases, clusterin, IgG, surface receptors, MAO-B, and coated platelets. Few of them, i.e., platelet tau, AβPP (particularly with regards to coated platelets) and secreted ADAM10 and BACE1 show the most promise to be taken forward into clinical setting to diagnose dementia. Aside from protein biomarkers, changes in factors such as mean platelet volume have the potential to play a very specific role in both the dementia diagnosis and prognosis. This review raises a number of research questions for consideration before application of the above biomarkers to routine clinical setting. It is without doubt that there is a need for more clarification on the effects of dementia on platelet morphology and protein content before these changes can be clinically applied as dementia biomarkers and explored further in differentiating distinct dementia subtypes.

Effect of tumor microenvironment on pathogenesis of the head and neck squamous cell carcinoma: a systematic review

The tumor microenvironment (TME) is comprised of many different cell populations, such as cancer-associated fibroblasts and various infiltrating immune cells, and non-cell components of extracellular matrix. These crucial parts of the surrounding stroma can function as both positive and negative regulators of all hallmarks of cancer development, including evasion of apoptosis, induction of angiogenesis, deregulation of the energy metabolism, resistance to the immune detection and destruction, and activation of invasion and metastasis. This review represents a summary of recent studies focusing on describing these effects of microenvironment on initiation and progression of the head and neck squamous cell carcinoma, focusing on oral squamous cell carcinoma, since it is becoming clear that an investigation of differences in stromal composition of the head and neck squamous cell carcinoma microenvironment and their impact on cancer development and progression may help better understand the mechanisms behind different responses to therapy and help define possible targets for clinical intervention.

Aspirin eugenol ester inhibits agonist-induced platelet aggregation in vitro by regulating PI3K/Akt, MAPK and Sirt 1/CD40L pathways

Aspirin eugenol ester (AEE) was a promising drug candidate for treating inflammation, pain and fever and preventing cardiovascular diseases with fewer side effects than its precursors. Previous researches indicated that AEE could markedly inhibit agonist-induced platelet aggregation in vitro and ex vivo, however, the anti-platelet aggregation mechanisms of AEE remain to be defined. Here, AEE in vitro effects on agonist-induced granule-secretion, intercellular Ca²⁺ mobilization and thromboxane A₂ (TXA₂) generation were examined. Vasodilator-stimulated phosphoprotein (VASP), mitogen-activated protein kinase (MAPK), Akt, Sirt 1 and CD40L expressions were also studied. In agonist-activated platelets in vitro, AEE markedly attenuated granule secretion markers (P-selectin expression and ATP release), intercellular Ca²⁺ mobilization and thromboxane B₂ (TXB₂) formation. AEE also attenuated CD40L activation, suppressed extracellular-signal-regulated protein kinase 2 (ERK2), c-Jun N-terminal kinase 1 (JNK1) and Akt phosphorylation, and recovered Sirt1 expression, but the activation of p38, VASP^Ser157 and VASP^Ser239, and the levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were not affected by AEE. Overall, this study demonstrates that AEE inhibits agonist-induced platelet aggregation in vitro by regulating PI3K/Akt, MAPK and Sirt 1/CD40L pathways.

Peripheral serotonin causes dengue virus-induced thrombocytopenia through 5HT2 receptors

Dengue virus (DENV) is the most prevalent vector-borne viral pathogen, infecting millions of patients annually. Thrombocytopenia, a reduction in circulating platelet counts, is the most consistent sign of DENV-induced disease, independent of disease severity. However, the mechanisms leading to DENV-induced thrombocytopenia are unknown. Here, we show that thrombocytopenia is caused by serotonin derived from mast cells (MCs), which are immune cells that are present in the perivascular space and are a major peripheral source of serotonin. We show that during DENV infection, MCs release serotonin, which prompts platelet activation, aggregation, and enhanced phagocytosis, dependent on 5HT_2A receptors. MC deficiency in mice or pharmacologic inhibition of MCs reversed thrombocytopenia. Furthermore, reconstitution of MC-deficient mice with wild-type MCs, but not MCs lacking serotonin synthesis resulting from deficiency in the enzyme tryptophan hydroxylase-1, restored the thrombocytopenic phenotype. Exogenous serotonin was also sufficient to overcome the effects of drugs that inhibit platelet activation in vitro and to restore thrombocytopenia in DENV-infected MC-deficient mice. Therapeutic targeting of 5HT_2A receptors during DENV infection effectively prevented thrombocytopenia in mice. Similarly, serotonin derived from DENV-activated human MCs led to increased human platelet activation. Thus, MC-derived serotonin is a previously unidentified mechanism of DENV-induced thrombocytopenia and a potential therapeutic target.

The Platelet Response to Tissue Injury

In recent years, various studies have increasingly explained platelet functions not only in their central role as a regulator in cellular hemostasis and coagulation. In fact, there is growing evidence that under specific conditions, platelets act as a mediator between the vascular system, hemostasis, and the immune system. Therefore, they are essential in many processes involved in tissue remodeling and tissue reorganization after injury or inflammatory responses. These processes include the promotion of inflammatory processes, the contribution to innate and adaptive immune responses during bacterial and viral infections, the modulation of angiogenesis, and the regulation of cell apoptosis in steady-state tissue homeostasis or after tissue breakdown. All in all platelets may contribute to the control of tissue homeostasis much more than generally assumed. This review summarizes the current knowledge of platelets as part of the tissue remodeling network and seeks to provide possible translational implications for clinical therapy.

Recent advances in understanding the roles of blood platelets in the pathogenesis of allergic inflammation and bronchial asthma

Platelets play an essential role in hemostasis to minimize blood loss due to traumatic injury. In addition, they contain various immune-associated molecules and contribute to immunological barrier formation at sites of vascular injury, thereby protecting against invading pathogens. Platelets are also crucially involved in development of allergic diseases, including bronchial asthma. Platelets in asthmatics are more activated than those in healthy individuals. By using a murine asthma model, platelets were shown to be actively involved in progression of the disease, including in airway eosinophilia and airway remodeling. In the asthmatic airway, pathological microvascular angiogenesis, a component of airway remodeling, is commonly observed, and the degree of abnormality is significantly associated with disease severity. Therefore, in order to repair the newly formed and structurally fragile blood vessels under inflammatory conditions, platelets may be continuously activated in asthmatics. Importantly, platelets constitutively express IL-33 protein, an alarmin cytokine that is essential for development of bronchial asthma. Meanwhile, the concept of development of allergic diseases has recently changed dramatically, and allergy researchers now share a belief in the centrality of epithelial barrier functions. In particular, IL-33 released from epithelial barrier tissue at sites of eczema can activate the antigen-non-specific innate immune system as an alarmin that is believed to be necessary for subsequent antigen-specific acquired immunological responses. From this perspective, we propose in this review a possible mechanism for how activated platelets act as an alarmin in development of bronchial asthma.

Platelet pathophysiology, pharmacology, and function in coronary artery disease

Platelets play a key role in the pathophysiology of coronary artery disease and acute coronary syndromes. Our understanding of platelet function in thrombus formation has increased considerably, resulting in the development of clinically effective treatment strategies and identification of new targets. An underappreciated platelet function is their contribution toward acute and chronic inflammatory processes including atherogenesis. In this review, we discuss the role of platelets in atherosclerosis and thrombosis, platelet function testing, and the pharmacology of currently available antiplatelet drugs.

Platelet-leucocyte aggregation is augmented in cirrhosis and further increased by platelet transfusion

BACKGROUND

Thrombocytopenia and circulating dysfunctional immune cells are commonly observed in patients with cirrhosis. Platelets may form complexes with neutrophils, monocytes and T cells modulating their function. We recently reported increased frequencies of platelet-complexed neutrophils in cirrhosis with evidence of neutrophil activation upon contact with healthy platelets in vitro. Whether this occurs in vivo following platelet transfusion and contributes to systemic inflammation and endothelial activation is unknown.

AIMS

To characterise platelet-leucocyte aggregation in cirrhosis and to determine whether elective platelet transfusion results in perturbations associated with changes in markers of haemostasis, inflammation or endothelial activation.

METHODS

We collected blood from cirrhotics (n = 19) before and following elective platelet transfusion. We measured platelet-leucocyte aggregation, activation and function, and markers of platelet activation, systemic inflammation and endothelial activation by flow cytometry. Haemostasis was assessed by thromboelastometry and plasma haemostatic proteins.

RESULTS

We observed a 2.5-fold increase in platelet-complexed neutrophils in patients with cirrhosis compared with healthy subjects and twofold more platelets attached per monocyte and T cell. All platelet-complexed leucocytes expressed higher levels of activation markers and platelet-complexed neutrophils had higher resting oxidative burst and phagocytic capacity than their nonplatelet-complexed counterparts (P < 0.001); most pronounced in patients with cirrhosis. Paradoxically, platelet-complexed leucocyte frequency decreased with increasing MELD score. Platelet transfusion increased soluble CD40 ligand (platelet activation marker), the frequency of platelet-complexed monocytes (P < 0.05) and improved haemostatic status.

CONCLUSION

Cirrhotic patients have activated circulating platelet-complexed leucocytes with increased platelet-monocyte aggregation following elective platelet transfusion. Elective platelet transfusion might therefore exacerbate immune dysfunction in cirrhosis.

Platelets in Immune Response to Virus and Immunopathology of Viral Infections

Platelets are essential effector cells in hemostasis. Aside from their role in coagulation, platelets are now recognized as major inflammatory cells with key roles in the innate and adaptive arms of the immune system. Activated platelets have key thromboinflammatory functions linking coagulation to immune responses in various infections, including in response to virus. Recent studies have revealed that platelets exhibit several pattern recognition receptors (PRR) including those from the toll-like receptor, NOD-like receptor, and C-type lectin receptor family and are first-line sentinels in detecting and responding to pathogens in the vasculature. Here, we review the main mechanisms of platelets interaction with viruses, including their ability to sustain viral infection and replication, their expression of specialized PRR, and activation of thromboinflammatory responses against viruses. Finally, we discuss the role of platelet-derived mediators and platelet interaction with vascular and immune cells in protective and pathophysiologic responses to dengue, influenza, and human immunodeficiency virus 1 infections.

Leukocyte Adhesion Molecules and Leukocyte-Platelet Interactions during Hemodialysis: Effects of Different Synthetic Membranes

Membranes made from synthetic polymers, in general, are considered as being biocompatible membranes and tend to be treated as a homogeneous group. However, all of these membranes have multiple and different characteristics that may contribute to interactions with blood components. As a consequence, the biocompatibility profile of synthetic membranes may vary. In the present cross-over study, we examined by flow cytometry the effects (expressed as % change from predialysis values) of three different synthetic polymers (polysulfone, PSF; polyacrylonitrile-co-sodium methallyl sulfonate, AN69; ethylenevinylalcohol, EVAL) on the expression of leukocyte adhesion molecules (CD11b/CD18, CD15s) and the interactions between leukocytes and platelets under conditions of routine clinical use.

For neutrophils, a statistically significant difference was found in CD15s expression for EVAL as compared to AN69 (p<0.05) and in CD11b/CD18 expression for PSF as compared to both EVAL (p<0.01) and AN69 (p<0.05). No difference between membranes was found on the expression of such adhesive molecules on monocytes. Significant differences in platelet-neutrophil (but not in platelet-monocyte) coaggregate formation were observed between PSF and both EVAL (p<0.001) and AN69 (p<0.01). Reactive oxygen species production by neutrophil population during hemodialysis was significantly different between each pair of synthetic polymers (PSF vs EVAL, p<0.001; PSF vs AN69, p<0.001; AN69 vs EVAL, p<0.05).

Our data demonstrate that in terms of leukocyte adhesion receptors and platelet-leukocyte interactions, the biocompatibility profile of the synthetic membranes polysulphone, AN69 and EVAL shows many similarities but also several significant differences. Our results support the concept that biocompatibility evaluation of each membrane should be based exclusively on data generated by that membrane in order to avoid errors based on assumptions about group characteristics.

Extracellular Matrix Metalloproteinase Inducer EMMPRIN (CD147) in Cardiovascular Disease

The receptor EMMPRIN is involved in the development and progression of cardiovascular diseases and in the pathogenesis of myocardial infarction. There are several binding partners of EMMPRIN mediating the effects of EMMPRIN in cardiovascular diseases. EMMPRIN interaction with most binding partners leads to disease progression by mediating cytokine or chemokine release, the activation of platelets and monocytes, as well as the formation of monocyte-platelet aggregates (MPAs). EMMPRIN is also involved in atherosclerosis by mediating the infiltration of pro-inflammatory cells. There is also evidence that EMMPRIN controls energy metabolism of cells and that EMMPRIN binding partners modulate intracellular glycosylation and trafficking of EMMPRIN towards the cell membrane. In this review, we systematically discuss these multifaceted roles of EMMPRIN and its interaction partners, such as Cyclophilins, in cardiovascular disease.