Platelets: Orchestrators of immunity in host defense and beyond.

Platelets prevent blood loss during vascular injury and contribute to thrombus formation in cardiovascular disease. Beyond these classical roles, platelets are critical for the host immune response. They guard the vasculature against pathogens via specialized receptors, intracellular signaling cascades, and effector functions. Platelets also skew inflammatory responses by instructing innate immune cells, support adaptive immunosurveillance, and influence antibody production and T cell polarization. Concomitantly, platelets contribute to tissue reconstitution and maintain vascular function after inflammatory challenges. However, dysregulated activation of these multitalented cells exacerbates immunopathology with ensuing microvascular clotting, excessive inflammation, and elevated risk of macrovascular thrombosis. This dichotomy underscores the critical importance of precisely defining and potentially modulating platelet function in immunity.

Mural cell-derived chemokines provide a protective niche to safeguard vascular macrophages and limit chronic inflammation.

Maladaptive, non-resolving inflammation contributes to chronic inflammatory diseases such as atherosclerosis. Because macrophages remove necrotic cells, defective macrophage programs can promote chronic inflammation with persistent tissue injury. Here, we investigated the mechanisms sustaining vascular macrophages. Intravital imaging revealed a spatiotemporal macrophage niche across vascular beds alongside mural cells (MCs)-pericytes and smooth muscle cells. Single-cell transcriptomics, co-culture, and genetic deletion experiments revealed MC-derived expression of the chemokines CCL2 and MIF, which actively preserved macrophage survival and their homeostatic functions. In atherosclerosis, this positioned macrophages in viable plaque areas, away from the necrotic core, and maintained a homeostatic macrophage phenotype. Disruption of this MC-macrophage unit via MC-specific deletion of these chemokines triggered detrimental macrophage relocalizing, exacerbated plaque necrosis, inflammation, and atheroprogression. In line, CCL2 inhibition at advanced stages of atherosclerosis showed detrimental effects. This work presents a MC-driven safeguard toward maintaining the homeostatic vascular macrophage niche.

Migrating Platelets Are Mechano-scavengers that Collect and Bundle Bacteria.

Blood platelets are critical for hemostasis and thrombosis and play diverse roles during immune responses. Despite these versatile tasks in mammalian biology, their skills on a cellular level are deemed limited, mainly consisting in rolling, adhesion, and aggregate formation. Here, we identify an unappreciated asset of platelets and show that adherent platelets use adhesion receptors to mechanically probe the adhesive substrate in their local microenvironment. When actomyosin-dependent traction forces overcome substrate resistance, platelets migrate and pile up the adhesive substrate together with any bound particulate material. They use this ability to act as cellular scavengers, scanning the vascular surface for potential invaders and collecting deposited bacteria. Microbe collection by migrating platelets boosts the activity of professional phagocytes, exacerbating inflammatory tissue injury in sepsis. This assigns platelets a central role in innate immune responses and identifies them as potential targets to dampen inflammatory tissue damage in clinical scenarios of severe systemic infection.

Neutrophil "plucking" on megakaryocytes drives platelet production and boosts cardiovascular disease.

Intravascular neutrophils and platelets collaborate in maintaining host integrity, but their interaction can also trigger thrombotic complications. We report here that cooperation between neutrophil and platelet lineages extends to the earliest stages of platelet formation by megakaryocytes in the bone marrow. Using intravital microscopy, we show that neutrophils "plucked" intravascular megakaryocyte extensions, termed proplatelets, to control platelet production. Following CXCR4-CXCL12-dependent migration towards perisinusoidal megakaryocytes, plucking neutrophils actively pulled on proplatelets and triggered myosin light chain and extracellular-signal-regulated kinase activation through reactive oxygen species. By these mechanisms, neutrophils accelerate proplatelet growth and facilitate continuous release of platelets in steady state. Following myocardial infarction, plucking neutrophils drove excessive release of young, reticulated platelets and boosted the risk of recurrent ischemia. Ablation of neutrophil plucking normalized thrombopoiesis and reduced recurrent thrombosis after myocardial infarction and thrombus burden in venous thrombosis. We establish neutrophil plucking as a target to reduce thromboischemic events.

Plasmacytoid dendritic cells control homeostasis of megakaryopoiesis.

Platelet homeostasis is essential for vascular integrity and immune defence1,2. Although the process of platelet formation by fragmenting megakaryocytes (MKs; thrombopoiesis) has been extensively studied, the cellular and molecular mechanisms required to constantly replenish the pool of MKs by their progenitor cells (megakaryopoiesis) remains unclear3,4. Here we use intravital imaging to track the cellular dynamics of megakaryopoiesis over days. We identify plasmacytoid dendritic cells (pDCs) as homeostatic sensors that monitor the bone marrow for apoptotic MKs and deliver IFNα to the MK niche triggering local on-demand proliferation and maturation of MK progenitors. This pDC-dependent feedback loop is crucial for MK and platelet homeostasis at steady state and under stress. pDCs are best known for their ability to function as vigilant detectors of viral infection5. We show that virus-induced activation of pDCs interferes with their function as homeostatic sensors of megakaryopoiesis. Consequently, activation of pDCs by SARS-CoV-2 leads to excessive megakaryopoiesis. Together, we identify a pDC-dependent homeostatic circuit that involves innate immune sensing and demand-adapted release of inflammatory mediators to maintain homeostasis of the megakaryocytic lineage.

Thrombin: A Gas Pedal Driving Innate Immunity.

Hemostasis and immunity were long considered entirely separate entities. In this issue of Immunity, Burzynski et al. (2019) find that thrombin, the key enzyme within the coagulation cascade, activates IL-1α, a central pleiotropic pro-inflammatory cytokine, to promote wound healing and platelet production following ectoderm injury.

Transcatheter or Surgical Treatment of Aortic-Valve Stenosis.

BACKGROUND: Among low-risk patients with severe, symptomatic aortic stenosis who are eligible for both transcatheter aortic-valve implantation (TAVI) and surgical aortic-valve replacement (SAVR), data are lacking on the appropriate treatment strategy in routine clinical practice. METHODS: In this randomized noninferiority trial conducted at 38 sites in Germany, we assigned patients with severe aortic stenosis who were at low or intermediate surgical risk to undergo either TAVI or SAVR. Percutaneous- and surgical-valve prostheses were selected according to operator discretion. The primary outcome was a composite of death from any cause or fatal or nonfatal stroke at 1 year. RESULTS: A total of 1414 patients underwent randomization (701 to the TAVI group and 713 to the SAVR group). The mean (±SD) age of the patients was 74±4 years; 57% were men, and the median Society of Thoracic Surgeons risk score was 1.8% (low surgical risk). The Kaplan-Meier estimate of the primary outcome at 1 year was 5.4% in the TAVI group and 10.0% in the SAVR group (hazard ratio for death or stroke, 0.53; 95% confidence interval [CI], 0.35 to 0.79; P<0.001 for noninferiority). The incidence of death from any cause was 2.6% in the TAVI group and 6.2% in the SAVR group (hazard ratio, 0.43; 95% CI, 0.24 to 0.73); the incidence of stroke was 2.9% and 4.7%, respectively (hazard ratio, 0.61; 95% CI, 0.35 to 1.06). Procedural complications occurred in 1.5% and 1.0% of patients in the TAVI and SAVR groups, respectively. CONCLUSIONS: Among patients with severe aortic stenosis at low or intermediate surgical risk, TAVI was noninferior to SAVR with respect to death from any cause or stroke at 1 year. (Funded by the German Center for Cardiovascular Research and the German Heart Foundation; DEDICATE-DZHK6 ClinicalTrials.gov number, NCT03112980.).

The AIM2 inflammasome exacerbates atherosclerosis in clonal haematopoiesis.

Clonal haematopoiesis, which is highly prevalent in older individuals, arises from somatic mutations that endow a proliferative advantage to haematopoietic cells. Clonal haematopoiesis increases the risk of myocardial infarction and stroke independently of traditional risk factors1. Among the common genetic variants that give rise to clonal haematopoiesis, the JAK2V617F (JAK2VF) mutation, which increases JAK-STAT signalling, occurs at a younger age and imparts the strongest risk of premature coronary heart disease1,2. Here we show increased proliferation of macrophages and prominent formation of necrotic cores in atherosclerotic lesions in mice that express Jak2VF selectively in macrophages, and in chimeric mice that model clonal haematopoiesis. Deletion of the essential inflammasome components caspase 1 and 11, or of the pyroptosis executioner gasdermin D, reversed these adverse changes. Jak2VF lesions showed increased expression of AIM2, oxidative DNA damage and DNA replication stress, and Aim2 deficiency reduced atherosclerosis. Single-cell RNA sequencing analysis of Jak2VF lesions revealed a landscape that was enriched for inflammatory myeloid cells, which were suppressed by deletion of Gsdmd. Inhibition of the inflammasome product interleukin-1ß reduced macrophage proliferation and necrotic formation while increasing the thickness of fibrous caps, indicating that it stabilized plaques. Our findings suggest that increased proliferation and glycolytic metabolism in Jak2VF macrophages lead to DNA replication stress and activation of the AIM2 inflammasome, thereby aggravating atherosclerosis. Precise application of therapies that target interleukin-1ß or specific inflammasomes according to clonal haematopoiesis status could substantially reduce cardiovascular risk.

Jak2 V617F clonal hematopoiesis promotes arterial thrombosis via platelet activation and cross talk.

ABSTRACT: JAK2 V617F (JAK2VF) clonal hematopoiesis (CH) has been associated with atherothrombotic cardiovascular disease (CVD). We assessed the impact of Jak2VF CH on arterial thrombosis and explored the underlying mechanisms. A meta-analysis of 3 large cohort studies confirmed the association of JAK2VF with CVD and with platelet counts and adjusted mean platelet volume (MPV). In mice, 20% or 1.5% Jak2VF CH accelerated arterial thrombosis and increased platelet activation. Megakaryocytes in Jak2VF CH showed elevated proplatelet formation and release, increasing prothrombogenic reticulated platelet counts. Gp1ba-Cre-mediated expression of Jak2VF in platelets (VFGp1ba) increased platelet counts to a similar level as in 20% Jak2VF CH mice while having no effect on leukocyte counts. Like Jak2VF CH mice, VFGp1ba mice showed enhanced platelet activation and accelerated arterial thrombosis. In Jak2VF CH, both Jak2VF and wild-type (WT) platelets showed increased activation, suggesting cross talk between mutant and WT platelets. Jak2VF platelets showed twofold to threefold upregulation of COX-1 and COX-2, particularly in young platelets, with elevated cPLA2 activation and thromboxane A2 production. Compared with controls, conditioned media from activated Jak2VF platelets induced greater activation of WT platelets that was reversed by a thromboxane receptor antagonist. Low-dose aspirin ameliorated carotid artery thrombosis in VFGp1ba and Jak2VF CH mice but not in WT control mice. This study shows accelerated arterial thrombosis and platelet activation in Jak2VF CH with a major role of increased reticulated Jak2VF platelets, which mediate thromboxane cross talk with WT platelets and suggests a potential beneficial effect of aspirin in JAK2VF CH.

Scrutinizing the Role of Venoarterial Extracorporeal Membrane Oxygenation: Has Clinical Practice Outpaced the Evidence?

The use of venoarterial extracorporeal membrane oxygenation (VA-ECMO) for temporary mechanical circulatory support in various clinical scenarios has been increasing consistently, despite the lack of sufficient evidence regarding its benefit and safety from adequately powered randomized controlled trials. Although the ARREST trial (Advanced Reperfusion Strategies for Patients with Out-of-Hospital Cardiac Arrest and Refractory Ventricular Fibrillation) and a secondary analysis of the PRAGUE OHCA trial (Prague Out-of-Hospital Cardiac Arrest) provided some evidence in favor of VA-ECMO in the setting of out-of-hospital cardiac arrest, the INCEPTION trial (Early Initiation of Extracorporeal Life Support in Refractory Out-of-Hospital Cardiac Arrest) has not found a relevant improvement of short-term mortality with extracorporeal cardiopulmonary resuscitation. In addition, the results of the recently published ECLS-SHOCK trial (Extracorporeal Life Support in Cardiogenic Shock) and ECMO-CS trial (Extracorporeal Membrane Oxygenation in the Therapy of Cardiogenic Shock) discourage the routine use of VA-ECMO in patients with infarct-related cardiogenic shock. Ongoing clinical trials (ANCHOR [Assessment of ECMO in Acute Myocardial Infarction Cardiogenic Shock, NCT04184635], REVERSE [Impella CP With VA ECMO for Cardiogenic Shock, NCT03431467], UNLOAD ECMO [Left Ventricular Unloading to Improve Outcome in Cardiogenic Shock Patients on VA-ECMO, NCT05577195], PIONEER [Hemodynamic Support With ECMO and IABP in Elective Complex High-risk PCI, NCT04045873]) may clarify the usefulness of VA-ECMO in specific patient subpopulations and the efficacy of combined mechanical circulatory support strategies. Pending further data to refine patient selection and management recommendations for VA-ECMO, it remains uncertain whether the present usage of this device improves outcomes.

Cerebral Embolic Protection during Transcatheter Aortic-Valve Replacement.

BACKGROUND: Transcatheter aortic-valve replacement (TAVR) for the treatment of aortic stenosis can lead to embolization of debris. Capture of debris by devices that provide cerebral embolic protection (CEP) may reduce the risk of stroke. METHODS: We randomly assigned patients with aortic stenosis in a 1:1 ratio to undergo transfemoral TAVR with CEP (CEP group) or without CEP (control group). The primary end point was stroke within 72 hours after TAVR or before discharge (whichever came first) in the intention-to-treat population. Disabling stroke, death, transient ischemic attack, delirium, major or minor vascular complications at the CEP access site, and acute kidney injury were also assessed. A neurology professional examined all the patients at baseline and after TAVR. RESULTS: A total of 3000 patients across North America, Europe, and Australia underwent randomization; 1501 were assigned to the CEP group and 1499 to the control group. A CEP device was successfully deployed in 1406 of the 1489 patients (94.4%) in whom an attempt was made. The incidence of stroke within 72 hours after TAVR or before discharge did not differ significantly between the CEP group and the control group (2.3% vs. 2.9%; difference, -0.6 percentage points; 95% confidence interval, -1.7 to 0.5; P = 0.30). Disabling stroke occurred in 0.5% of the patients in the CEP group and in 1.3% of those in the control group. There were no substantial differences between the CEP group and the control group in the percentage of patients who died (0.5% vs. 0.3%); had a stroke, a transient ischemic attack, or delirium (3.1% vs. 3.7%); or had acute kidney injury (0.5% vs. 0.5%). One patient (0.1%) had a vascular complication at the CEP access site. CONCLUSIONS: Among patients with aortic stenosis undergoing transfemoral TAVR, the use of CEP did not have a significant effect on the incidence of periprocedural stroke, but on the basis of the 95% confidence interval around this outcome, the results may not rule out a benefit of CEP during TAVR. (Funded by Boston Scientific; PROTECTED TAVR ClinicalTrials.gov number, NCT04149535.).

Capillary and arteriolar pericytes attract innate leukocytes exiting through venules and 'instruct' them with pattern-recognition and motility programs.

Coordinated navigation within tissues is essential for cells of the innate immune system to reach the sites of inflammatory processes, but the signals involved are incompletely understood. Here we demonstrate that NG2(+) pericytes controlled the pattern and efficacy of the interstitial migration of leukocytes in vivo. In response to inflammatory mediators, pericytes upregulated expression of the adhesion molecule ICAM-1 and released the chemoattractant MIF. Arteriolar and capillary pericytes attracted and interacted with myeloid leukocytes after extravasating from postcapillary venules, 'instructing' them with pattern-recognition and motility programs. Inhibition of MIF neutralized the migratory cues provided to myeloid leukocytes by NG2(+) pericytes. Hence, our results identify a previously unknown role for NG2(+) pericytes as an active component of innate immune responses, which supports the immunosurveillance and effector function of extravasated neutrophils and macrophages.

Mechanosensing via a GpIIb/Src/14-3-3ζ axis critically regulates platelet migration in vascular inflammation.

Platelets are not only the first responders in thrombosis and hemostasis but also central players in inflammation. Compared with platelets recruited to thrombi, immune-responsive platelets use distinct effector functions including actin-related protein complex 2/3-dependent migration along adhesive substrate gradients (haptotaxis), which prevents inflammatory bleeding and contributes to host defense. How platelet migration in this context is regulated on a cellular level is incompletely understood. Here, we use time-resolved morphodynamic profiling of individual platelets to show that migration, in contrast to clot retraction, requires anisotropic myosin IIa-activity at the platelet rear which is preceded by polarized actin polymerization at the front to initiate and maintain migration. Integrin GPIIb-dependent outside-in signaling via Gα13 coordinates polarization of migrating platelets to trigger tyrosine kinase c-Src/14-3-3ζ-dependent lamellipodium formation and functions independent of soluble agonists or chemotactic signals. Inhibitors of this signaling cascade, including the clinically used ABL/c-Src inhibitor dasatinib, interfere predominantly with the migratory capacity of platelets, without major impairment of classical platelet functions. In murine inflammation models, this translates to reduced migration of platelets visualized by 4D intravital microscopy, resulting in increased inflammation-associated hemorrhage in acute lung injury. Finally, platelets isolated from patients with leukemia treated with dasatinib who are prone to clinically relevant hemorrhage exhibit prominent migration defects, whereas other platelet functions are only partially affected. In summary, we define a distinct signaling pathway essential for migration and provide novel mechanistic insights explaining dasatinib-related platelet dysfunction and bleeding.

Venoarterial extracorporeal membrane oxygenation in patients with infarct-related cardiogenic shock: an individual patient data meta-analysis of randomised trials.

BACKGROUND: Venoarterial extracorporeal membrane oxygenation (VA-ECMO) is increasingly used in patients with cardiogenic shock despite the lack of evidence from adequately powered randomised clinical trials. Three trials reported so far were underpowered to detect a survival benefit; we therefore conducted an individual patient-based meta-analysis to assess the effect of VA-ECMO on 30-day death rate. METHODS: Randomised clinical trials comparing early routine use of VA-ECMO versus optimal medical therapy alone in patients presenting with infarct-related cardiogenic shock were identified by searching MEDLINE, Cochrane Central Register of Controlled Trials, Embase, and trial registries until June 12, 2023. Trials were included if at least all-cause death rate 30 days after in-hospital randomisation was reported and trial investigators agreed to collaborate (ie, providing individual patient data). Odds ratios (ORs) as primary outcome measure were pooled using logistic regression models. This study is registered with PROSPERO (CRD42023431258). FINDINGS: Four trials (n=567 patients; 284 VA-ECMO, 283 control) were identified and included. Overall, there was no significant reduction of 30-day death rate with the early use of VA-ECMO (OR 0·93; 95% CI 0·66-1·29). Complication rates were higher with VA-ECMO for major bleeding (OR 2·44; 95% CI 1·55-3·84) and peripheral ischaemic vascular complications (OR 3·53; 95% CI 1·70-7·34). Prespecified subgroup analyses were consistent and did not show any benefit for VA-ECMO (pinteraction ≥0·079). INTERPRETATION: VA-ECMO did not reduce 30-day death rate compared with medical therapy alone in patients with infarct-related cardiogenic shock, and an increase in major bleeding and vascular complications was observed. A careful review of the indication for VA-ECMO in this setting is warranted. FUNDING: Foundation Institut für Herzinfarktforschung.

A randomized comparison of the treatment sequence of percutaneous coronary intervention and transcatheter aortic valve implantation: Rationale and design of the TAVI PCI trial.

BACKGROUND: About half of patients with severe aortic stenosis present with concomitant coronary artery disease. The optimal timing of percutaneous coronary intervention (PCI) and transcatheter aortic valve implantation (TAVI) in patients with severe aortic stenosis and concomitant coronary artery disease remains unknown. STUDY DESIGN: The TAVI PCI trial is a prospective, international, multicenter, randomized, 2-arm, open-label study planning to enroll a total of 986 patients. It is designed to investigate whether the strategy "angiography-guided complete revascularization after (within 1-45 days) TAVI" is noninferior to the strategy "angiography-guided complete revascularization before (within 1-45 days) TAVI" using the Edwards SAPIEN 3 or 3 Ultra Transcatheter Heart Valve in patients with severe aortic stenosis and concomitant coronary artery disease. Patients are randomized in a 1:1 ratio to one of the 2 treatment strategies. The primary end point is a composite of all-cause death, nonfatal myocardial infarction, ischemia-driven revascularization, rehospitalization (valve- or procedure-related including heart failure), or life-threatening/disabling or major bleeding at 1 year. CONCLUSIONS: The TAVI PCI trial tests the hypothesis that the strategy "PCI after TAVI" is noninferior to the strategy "PCI before TAVI" in patients with severe aortic stenosis and concomitant coronary artery disease.

Atrial Fibrillation Risk Assessment after Embolic Stroke of Undetermined Source.

OBJECTIVE: Approximately 20% of strokes are embolic strokes of undetermined source (ESUS). Undetected atrial fibrillation (AF) remains an important cause. Yet, oral anticoagulation in unselected ESUS patients failed in secondary stroke prevention. Guidance on effective AF detection is lacking. Here, we introduce a novel, non-invasive AF risk assessment after ESUS. METHODS: Catch-Up ESUS is an investigator-initiated, observational cohort study conducted between 2018 and 2019 at the Munich University Hospital. Besides clinical characteristics, patients received ≥72 h digital electrocardiogram recordings to generate the rhythm irregularity burden. Uni- and multivariable regression models predicted the primary endpoint of incident AF, ascertained by standardized follow-up including implantable cardiac monitors. Predictors included the novel rhythm irregularity burden constructed from digital electrocardiogram recordings. We independently validated our model in ESUS patients from the University Hospital Tübingen, Germany. RESULTS: A total of 297 ESUS patients were followed for 15.6 ± 7.6 months. Incident AF (46 patients, 15.4%) occurred after a median of 105 days (25th to 75th percentile 31-33 days). Secondary outcomes were recurrent stroke in 7.7% and death in 6.1%. Multivariable-adjusted analyses identified the rhythm irregularity burden as the strongest AF-predictor (hazard ratio 3.12, 95% confidence interval 1.62-5.80, p < 0001) while accounting for the known risk factors age, CHA2 DS2 -VASc-Score, and NT-proBNP. Independent validation confirmed the rhythm irregularity burden as the most significant AF-predictor (hazard ratio 2.20, 95% confidence interval 1.45-3.33, p < 0001). INTERPRETATION: The novel, non-invasive, electrocardiogram-based rhythm irregularity burden may help adjudicating AF risk after ESUS, and subsequently guide AF-detection after ESUS. Clinical trials need to clarify if high-AF risk patients benefit from tailored secondary stroke prevention. ANN NEUROL 2023;93:479-488.

Thrombocytopenia and splenic platelet-directed immune responses after IV ChAdOx1 nCov-19 administration.

Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are based on a range of novel platforms, with adenovirus-based approaches (like ChAdOx1 nCov-19) being one of them. Recently, a novel complication of SARS-CoV-2-targeted adenovirus vaccines has emerged: immune thrombocytopenia, either isolated, or accompanied by thrombosis (then termed VITT). This complication is characterized by low platelet counts, and in the case of VITT, also by platelet-activating platelet factor 4 antibodies reminiscent of heparin-induced thrombocytopenia, leading to a prothrombotic state with clot formation at unusual anatomic sites. Here, we detected antiplatelet antibodies targeting platelet glycoprotein receptors in 30% of patients with proven VITT (n = 27) and 42% of patients with isolated thrombocytopenia after ChAdOx1 nCov-19 vaccination (n = 26), indicating broad antiplatelet autoimmunity in these clinical entities. We use in vitro and in vivo models to characterize possible mechanisms of these platelet-targeted autoimmune responses leading to thrombocytopenia. We show that IV but not intramuscular injection of ChAdOx1 nCov-19 triggers platelet-adenovirus aggregate formation and platelet activation in mice. After IV injection, these aggregates are phagocytosed by macrophages in the spleen, and platelet remnants are found in the marginal zone and follicles. This is followed by a pronounced B-cell response with the emergence of circulating antibodies binding to platelets. Our work contributes to the understanding of platelet-associated complications after ChAdOx1 nCov-19 administration and highlights accidental IV injection as a potential mechanism of platelet-targeted autoimmunity. Hence, preventing IV injection when administering adenovirus-based vaccines could be a potential measure against platelet-associated pathologies after vaccination.

Procoagulant platelet sentinels prevent inflammatory bleeding through GPIIBIIIA and GPVI.

Impairment of vascular integrity is a hallmark of inflammatory diseases. We recently reported that single immune-responsive platelets migrate and reposition themselves to sites of vascular injury to prevent bleeding. However, it remains unclear how single platelets preserve vascular integrity once encountering endothelial breaches. Here we demonstrate by intravital microscopy combined with genetic mouse models that procoagulant activation (PA) of single platelets and subsequent recruitment of the coagulation cascade are crucial for the prevention of inflammatory bleeding. Using a novel lactadherin-based compound, we detect phosphatidylserine (PS)-positive procoagulant platelets in the inflamed vasculature. We identify exposed collagen as the central trigger arresting platelets and initiating subsequent PA in a CypD- and TMEM16F-dependent manner both in vivo and in vitro. Platelet PA promotes binding of the prothrombinase complex to the platelet membrane, greatly enhancing thrombin activity and resulting in fibrin formation. PA of migrating platelets is initiated by costimulation via integrin αIIbß3 (GPIIBIIIA)/Gα13-mediated outside-in signaling and glycoprotein VI signaling, leading to an above-threshold intracellular calcium release. This effectively targets the coagulation cascade to breaches of vascular integrity identified by patrolling platelets. Platelet-specific genetic loss of either CypD or TMEM16F as well as combined blockade of platelet GPIIBIIIA and glycoprotein VI reduce platelet PA in vivo and aggravate pulmonary inflammatory hemorrhage. Our findings illustrate a novel role of procoagulant platelets in the prevention of inflammatory bleeding and provide evidence that PA of patrolling platelet sentinels effectively targets and confines activation of coagulation to breaches of vascular integrity.

Transcatheter aortic valve implantation and its impact on endothelial function in patients with aortic stenosis.

Vascular function is impaired in patients with aortic valve stenosis (AS). The impact of transcatheter aortic valve implantation (TAVI) on endothelial function is inconclusive so far. Therefore, we sought to assess the short-term influence of TAVI on endothelial dysfunction in patients with AS. We recruited 47 patients (76.6 % male, 80.04 years old) with AS scheduled for TAVI. Endothelial function was assessed by fingertip reactive hyperemia peripheral arterial tonometry (RH-PAT). Measurements were conducted one day before and three days after TAVI. Patients were grouped according to RH-PAT change after TAVI. Overall, RH-PAT measurements did not significantly improve after TAVI (Reactive Hyperemia Index: 1.5 vs 1.6, p = 0.883; logarithm of the Reactive Hyperemia Index: 0.44 vs. 0.49, p = 0.523). Interestingly, patients with no RH-PAT improvement after TAVI displayed a more severe AS and had lower blood pressure after TAVI. This might be due to a more disturbed blood flow in patients with a smaller aortic valve area and higher peak aortic valve velocity. The relationship between AS severity, endothelial dysfunction and TAVI has to be investigated in future research that apply longitudinal study designs.