Capture of platelets to the endothelium of the femoral vein is mediated by CD62P and CD162

CXCR4 and CXCR7 Inhibition Ameliorates the Formation of Platelet-Neutrophil Complexes and Neutrophil Extracellular Traps through Adora2b Signaling

Peritonitis and peritonitis-associated sepsis are characterized by an increased formation of platelet-neutrophil complexes (PNCs), which contribute to an excessive migration of polymorphonuclear neutrophils (PMN) into the inflamed tissue. An important neutrophilic mechanism to capture and kill invading pathogens is the formation of neutrophil extracellular traps (NETs). Formation of PNCs and NETs are essential to eliminate pathogens, but also lead to aggravated tissue damage. The chemokine receptors CXCR4 and CXCR7 on platelets and PMNs have been shown to play a pivotal role in inflammation. Thereby, CXCR4 and CXCR7 were linked with functional adenosine A2B receptor (Adora2b) signaling. We evaluated the effects of selective CXCR4 and CXCR7 inhibition on PNCs and NETs in zymosan- and fecal-induced sepsis. We determined the formation of PNCs in the blood and, in addition, their infiltration into various organs in wild-type and Adora2b-/- mice by flow cytometry and histological methods. Further, we evaluated NET formation in both mouse lines and the impact of Adora2b signaling on it. We hypothesized that the protective effects of CXCR4 and CXCR7 antagonism on PNC and NET formation are linked with Adora2b signaling. We observed an elevated CXCR4 and CXCR7 expression in circulating platelets and PMNs during acute inflammation. Specific CXCR4 and CXCR7 inhibition reduced PNC formation in the blood, respectively, in the peritoneal, lung, and liver tissue in wild-type mice, while no protective anti-inflammatory effects were observed in Adora2b-/- animals. In vitro, CXCR4 and CXCR7 antagonism dampened PNC and NET formation with human platelets and PMNs, confirming our in vivo data. In conclusion, our study reveals new protective aspects of the pharmacological modulation of CXCR4 and CXCR7 on PNC and NET formation during acute inflammation.

Gut Microbiota Restricts NETosis in Acute Mesenteric Ischemia-Reperfusion Injury

OBJECTIVE

Recruitment of neutrophils and formation of neutrophil extracellular traps (NETs) contribute to lethality in acute mesenteric infarction. To study the impact of the gut microbiota in acute mesenteric infarction, we used gnotobiotic mouse models to investigate whether gut commensals prime the reactivity of neutrophils towards formation of neutrophil extracellular traps (NETosis). Approach and Results: We applied a mesenteric ischemia-reperfusion (I/R) injury model to germ-free (GF) and colonized C57BL/6J mice. By intravital imaging, we quantified leukocyte adherence and NET formation in I/R-injured mesenteric venules. Colonization with gut microbiota or monocolonization with Escherichia coli augmented the adhesion of leukocytes, which was dependent on the TLR4 (Toll-like receptor-4)/TRIF (TIR-domain-containing adapter-inducing interferon-β) pathway. Although neutrophil accumulation was decreased in I/R-injured venules of GF mice, NETosis following I/R injury was significantly enhanced compared with conventionally raised mice or mice colonized with the minimal microbial consortium altered Schaedler flora. Also ex vivo, neutrophils from GF and antibiotic-treated mice showed increased LPS (lipopolysaccharide)-induced NETosis. Enhanced TLR4 signaling in GF neutrophils was due to elevated TLR4 expression and augmented IRF3 (interferon regulatory factor-3) phosphorylation. Likewise, neutrophils from antibiotic-treated conventionally raised mice had increased NET formation before and after ischemia. Increased NETosis in I/R injury was abolished in conventionally raised mice deficient in the TLR adaptor TRIF. In support of the desensitizing influence of enteric LPS, treatment of GF mice with LPS via drinking water diminished LPS-induced NETosis in vitro and in the mesenteric I/R injury model.

CONCLUSIONS

Collectively, our results identified that the gut microbiota suppresses NETing neutrophil hyperreactivity in mesenteric I/R injury, while ensuring immunovigilance by enhancing neutrophil recruitment.

N-cadherin knockdown leads to disruption of trophoblastic and endothelial cell interaction in a 3D cell culture model - New insights in trophoblast invasion failure

INTRODUCTION

Trophoblast homing to maternal spiral arteries is mandatory for successful placentation. Cell-cell adhesion molecules regulate this process and adhesion molecule expression is altered in impaired placentation. We hypothesize that, similar to immune cell recruitment, trophoblast cell adherence and rolling are primarily mediated by adhesion molecules like, cadherins, immunoglobulins, selectins and their partnering ligands. Here, the interdependence of adhesion molecule expression in trophoblastic cell lines of diverse origin was investigated in relation to their interaction with endothelial cell networks on Matrigel® co-cultures and the effect of specific adhesion molecule knockdown analyzed.

METHODS

Trophoblastic cells were labeled in red and co-cultured with green HUVEC networks on Matrigel®. Association was quantified after collection of fluorescence microscopy pictures using Wimasis® internet platform and software. Expression of adhesion molecules was analyzed by PCR and Western blot, immuno-fluorescence and flow cytometry. The impact of adhesion molecules on trophoblast-endothelial-cell interaction was investigated using siRNA technique.

RESULTS

N-cadherin and CD162 were specifically expressed in the trophoblast cell line HTR-8/SVneo, which closely adhere to and actively migrate toward HUVEC networks on Matrigel®. Suppression of N-cadherin led to a significant alteration in trophoblast-endothelial cell interaction. Expression of VE-cadherin in closely interacting trophoblast cells was not confirmed in vitro.

DISCUSSION

We identified N-cadherin to mediate specific interaction between HUVEC and the migrating trophoblast cells HTR-8/SVneo in a Matrigel® co-culture model. VE-cadherin contribution could not be confirmed in vitro. Our results support the hypothesis that impaired N-cadherin but not VE-cadherin expression is involved in trophoblast recruitment to the maternal endothelium.

The Interaction of Selectins and PSGL-1 as a Key Component in Thrombus Formation and Cancer Progression

Cellular interaction is inevitable in the pathomechanism of human disease. Formation of heterotypic cellular aggregates, between distinct cells of hematopoietic and nonhematopoietic origin, may be involved in events leading to inflammation and the complex process of cancer progression. Among adhesion receptors, the family of selectins with their ligands have been considered as one of the major contributors to cell-cell interactions. Consequently, the inhibition of the interplay between selectins and their ligands may have potential therapeutic benefits. In this review, we focus on the current evidence on the selectins as crucial modulators of inflammatory, thrombotic, and malignant disorders. Knowing that there is promiscuity in selectin binding, we outline the importance of a key protein that serves as a ligand for all selectins. This dimeric mucin, the P-selectin glycoprotein ligand 1 (PSGL-1), has emerged as a major player in inflammation, thrombus, and cancer development. We discuss the interaction of PSGL-1 with various selectins in physiological and pathological processes with particular emphasis on mechanisms that lead to severe disease.

Endotoxaemia-augmented murine venous thrombosis is dependent on TLR-4 and ICAM-1, and potentiated by neutropenia

Venous thromboembolism is a major cause of death during and immediately post-sepsis. Venous thrombosis (VT) is mediated by cell adhesion molecules and leukocytes, including neutrophil extracellular traps (NETs). Sepsis, or experimentally, endotoxaemia, shares similar characteristics and is modulated via toll like receptor 4 (TLR4). This study was undertaken to determine if endotoxaemia potentiates early stasis thrombogenesis, and secondarily to determine the role of VT TLR4, ICAM-1 and neutrophils (PMNs). Wild-type (WT), ICAM-1^-/- and TLR4^-/- mice underwent treatment with saline or LPS (10 mg/kg i. p.) alone, or followed by inferior vena cava (IVC) ligation to generate stasis VT. In vivo microscopy of leukocyte trafficking was performed in non-thrombosed mice, and tissue and plasma were harvested during early VT formation. Pre-thrombosis, circulating ICAM-1 was elevated and increased leukocyte adhesion and rolling occurred on the IVC of LPS-treated mice. Post-thrombosis, endotoxaemic mice formed larger, platelet-poor thrombi. Endotoxaemic TLR4^-/- mice did not have an augmented thrombotic response and exhibited significantly decreased circulating ICAM-1 compared to endotoxaemic WT controls. Endotoxaemic ICAM-1^-/- mice had significantly smaller thrombi compared to controls. Hypothesising that PMNs localised to the inflamed endothelium were promoting thrombosis, PMN depletion using anti-Ly6G antibody was performed. Paradoxically, VT formed without PMNs was amplified, potentially related to endotoxaemia induced elevation of PAI-1 and circulating FXIII, and decreased uPA. Endotoxaemia enhanced early VT occurs in a TLR-4 and ICAM-1 dependent fashion, and is potentiated by neutropenia. ICAM-1 and/or TLR-4 inhibition may be a unique strategy to prevent sepsis-associated VT.

Inflammation in venous thromboembolism: Cause or consequence?

Venous thromboembolism (VTE) which includes deep vein thrombosis (DVT) and pulmonary thromboembolism (PTE) is a moderately common disease especially in elderly population with high rate of recurrence and complications. Evidence is accumulating that VTE is not restricted to coagulation system and immune system appears to be involved in formation and resolution of thrombus. The present study was aimed at reviewing current evidences on immune system abnormalities such as alterations in cytokines, chemokines and immune cells. Also, current evidences suggest that; a, inflammation in general functions as a double-edged sword, b, inflammation can be both a cause and a consequence of VTE, and c, current anti-coagulation therapies are not well-equipped with the capacity to selectively inhibit inflammatory cells and pathways. Applying such inferences for selective pharmacological targeting of immune mediators in VTE and thereby for adoption of higher effective anti-thromboinflammatory strategies, either therapeutic or prophylactic, is henceforth to be considered as the line of research for future.

Equid herpesvirus type 1 activates platelets

Equid herpesvirus type 1 (EHV-1) causes outbreaks of abortion and neurological disease in horses. One of the main causes of these clinical syndromes is thrombosis in placental and spinal cord vessels, however the mechanism for thrombus formation is unknown. Platelets form part of the thrombus and amplify and propagate thrombin generation. Here, we tested the hypothesis that EHV-1 activates platelets. We found that two EHV-1 strains, RacL11 and Ab4 at 0.5 or higher plaque forming unit/cell, activate platelets within 10 minutes, causing α-granule secretion (surface P-selectin expression) and platelet microvesiculation (increased small events double positive for CD41 and Annexin V). Microvesiculation was more pronounced with the RacL11 strain. Virus-induced P-selectin expression required plasma and 1.0 mM exogenous calcium. P-selectin expression was abolished and microvesiculation was significantly reduced in factor VII- or X-deficient human plasma. Both P-selectin expression and microvesiculation were re-established in factor VII-deficient human plasma with added purified human factor VIIa (1 nM). A glycoprotein C-deficient mutant of the Ab4 strain activated platelets as effectively as non-mutated Ab4. P-selectin expression was abolished and microvesiculation was significantly reduced by preincubation of virus with a goat polyclonal anti-rabbit tissue factor antibody. Infectious virus could be retrieved from washed EHV-1-exposed platelets, suggesting a direct platelet-virus interaction. Our results indicate that EHV-1 activates equine platelets and that α-granule secretion is a consequence of virus-associated tissue factor triggering factor X activation and thrombin generation. Microvesiculation was only partly tissue factor and thrombin-dependent, suggesting the virus causes microvesiculation through other mechanisms, potentially through direct binding. These findings suggest that EHV-1-induced platelet activation could contribute to the thrombosis that occurs in clinically infected horses and provides a new mechanism by which viruses activate hemostasis.

Platelets orchestrate remote tissue damage after mesenteric ischemia-reperfusion

Ischemia-reperfusion (I/R) injury is a leading cause of morbidity and mortality. A functional role for platelets in tissue damage after mesenteric I/R is largely unknown. The hypothesis that mesenteric I/R local and remote injury are platelet dependent was tested. Using a murine mesenteric I/R model, we demonstrate that platelets orchestrate remote lung tissue damage that follows mesenteric I/R injury and also contribute, albeit to a lesser degree, to local villi damage. While lung damage is delayed compared with villi damage, it increased over time and was characterized by accumulation of platelets in the pulmonary vasculature early, followed by alveolar capillaries and extravasation into the pulmonary space. Both villi and lung tissues displayed complement deposition. We demonstrate that villi and lung damage are reduced in mice made platelet deficient before I/R injury and that platelet transfusion into previously platelet-depleted mice before I/R increased both villi and lung tissue damage. Increased C3 deposition accompanied platelet sequestration in the lung, which was mostly absent in platelet-depleted mice. In contrast, C3 deposition was only minimally reduced on villi of platelet-depleted mice. Our findings position platelets alongside complement as a significant early upstream component that orchestrates remote lung tissue damage after mesenteric I/R and strongly suggest that reperfusion injury mitigating modalities should consider the contribution of platelets.

Platelet-associated CD40/CD154 mediates remote tissue damage after mesenteric ischemia/reperfusion injury

Several innate and adaptive immune cell types participate in ischemia/reperfusion induced tissue injury. Amongst them, platelets have received little attention as contributors in the process of tissue damage after ischemia reperfusion (I/R) injury. It is currently unknown whether platelets participate through the immunologically important molecules including, CD40 and when activated, CD154 (CD40L), in the pathogenesis of I/R injury. We hypothesized that constitutive expression of CD40 and activation-induced expression of CD154 on platelets mediate local mesenteric and remote lung tissue damage after I/R injury. Wild type (WT; C57BL/6J), CD40 and CD154 deficient mice underwent mesenteric ischemia for 30 minutes followed by reperfusion for 3 hours. WT mice subjected to mesenteric I/R injury displayed both local intestinal and remote lung damage. In contrast, there was significantly less intestinal damage and no remote lung injury in CD40 and CD154 deficient mice when compared to WT mice. Platelet-depleted WT mice transfused with platelets from CD40 or CD154 deficient mice failed to reconstitute remote lung damage. In contrast, when CD40 or CD154 deficient mice were transfused with WT platelets lung tissue damage was re-established. Together, these findings suggest that multiple mechanisms are involved in local and remote tissue injury and also identify platelet-expressed CD40 and/or CD154 as mediators of remote tissue damage.

Signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation

Platelet activation is strongly affected by nitric oxide/cyclic GMP (NO/cGMP) signaling involving cGMP-dependent protein kinase I (cGKI). Previously it was shown that interaction of the cGKI substrate IRAG with InsP(3)RI is essential for NO/cguanosine monophosphate (GMP)-dependent inhibition of platelet aggregation in vitro and in vivo. However, the role of Inositol-trisphosphate receptor associated cGMP kinase substrate (IRAG) for platelet adhesion or granule secretion was unknown. Here, we analysed the functional role of IRAG for platelet activation. Murine IRAG-deficient platelets displayed enhanced aggregability towards several agonists (collagen, thrombin and TxA2). NO- or cGMP-dependent inhibition of agonist induced ATP- or 5-HT secretion from dense granules, and P-selectin secretion from alpha granules was severely affected in IRAG-deficient platelets. Concomitantly, the effect of NO/cGMP on platelet aggregation was strongly reduced in IRAG-deficient platelets. Furthermore, GPIIb/IIIa-mediated adhesion of platelets to fibrinogen could only weakly be inhibited in IRAG-deficient mice contrary to wild-type (WT) mice. Our results suggest that signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation regarding granule secretion, aggregation and adhesion. This platelet disorder might cause that the bleeding time of IRAG-deficient mice was reduced.

Lack of P-selectin glycoprotein ligand-1 protects mice from thrombosis after collagen/epinephrine challenge

INTRODUCTION

In thrombotic processes, during the association of leukocytes with platelets and endothelial cells, P-selectin glycoprotein ligand-1 (PSGL-1) binds to P-selectin, expressed on activated platelets and endothelial cells. Our aim was to establish the role of PSGL-1 in thrombus formation by evaluating the response to thrombotic stimuli in wild type and PSGL-1 knockout mice.

MATERIALS AND METHODS

Mice were challenged by tail vein injection of (i) 15 μg collagen plus 3 μg epinephrine (coll/epi) (ii) 7.5 μg collagen plus 1.5 μg epinephrine or (iii) saline. Retro-orbital blood samples were collected in ACD anticoagulaed tubes and platelet and leukocyte counts were measured. In addition, kidneys, liver, spleen and lungs were investigated for fibrin deposition by immunohistochemistry and Western-blotting. Frozen sections were analysed for double labeling for platelet and leukocyte presence.

RESULTS

After coll/epi challenge, the number of platelets and leukocytes decreased significantly in both genotypes. Lower agonist concentration resulted in an attenuated platelet decrease in PSGL-1 knockout mice compared to the controls, however changes in leukocyte and neutrophil counts were not significantly different in the two strains. In knockout mice considerably less fibrin deposition has been observed in the lungs by Western-blotting and immunohistochemistry. After coll/epi challenge the lungs of the PSGL-1 knockout animals contained both platelets and leukocytes but less thrombi has been detected than in wild-type mice.

CONCLUSIONS

Our results indicate that the deficiency of PSGL-1 results in milder thrombocytopenia, less fibrin deposition and lower number of thrombosed blood vessels, suggesting that this molecule is essential for multicellular interactions during thrombus formation.

Differentiation of enzymatic from platelet hypercoagulability using the novel thrombelastography parameter delta (delta)

BACKGROUND

Thrombelastography (TEG) allows for rapid global assessment of coagulation function. Our previous work demonstrated that a hypercoagulable state identified by TEG's G value was associated with thromboembolic events in a cohort of critically ill surgical patients despite routine chemoprophylaxis. We hypothesized that a hypercoagulable state could be differentiated into enzymatic or platelet etiology through the use of thrombus velocity curves; specifically the time to maximum rate of thrombus generation (TMRTG) and the novel TEG parameter, delta. (Delta)

METHODS

We retrospectively studied 10 critically ill surgical patients receiving thromboprophylaxis for at least 72 h by TEG, using kaolin activated citrated samples. Thrombus velocity curves were plotted for each patient, and delta was calculated as the difference between the TEG parameters R and SP, corresponding to the time to maximum rate of thrombus generation (TMRTG), which reflects the enzymatic contribution to clot formation. The TEG parameter G, (G = 5000 x A/100-A) also was determined for each patient. As G is derived from amplitude (A), it reflects overall net clot strength. A hypercoagulable state was defined as delta < 0.6 min and/or G > 11 dynes/cm(2).

RESULTS

A hypercoagulable state was identified via delta in 6 patients (60%); all of whom remained hypercoagulable following heparinase addition, suggesting chemoprophylaxis was ineffective. Of six patients with a hypercoagulable G value, 50% had a normal delta suggesting the presence of platelet hypercoagulability. Delta closely correlated with TMRTG (r = 0.94). However, the varying contribution of platelets to hypercoagulability, was shown by a nonlinear, weak correlation of delta and TMRTG with G (r = 0.11 and r = 0.14, respectively).

CONCLUSION

Delta reflects changes in thrombin generation as measured by TMRTG, allowing for differentiation of enzymatic from platelet hypercoagulability. Future studies will be required to validate these findings.