The localization, origin, and impact of platelets in the tumor microenvironment are tumor type-dependent

BACKGROUND

How platelets interact with and influence the tumor microenvironment (TME) remains poorly characterized.

METHODS

We compared the presence and participation of platelets in the TME of two tumors characterized by highly different TME, PyMT AT-3 mammary tumors and B16F1 melanoma.

RESULTS

We show that whereas firmly adherent platelets continuously line tumor vessels of both AT-3 and B16F1 tumors, abundant extravascular stromal clusters of platelets from thrombopoietin-independent origin were present only in AT-3 mammary tumors. We further show that platelets influence the angiogenic and inflammatory profiles of AT-3 and B16F1 tumors, though with very different outcomes according to tumor type. Whereas thrombocytopenia increased bleeding in both tumor types, it further caused severe endothelial degeneration associated with massive vascular leakage, tumor swelling, and increased infiltration of cytotoxic cells, only in AT-3 tumors.

CONCLUSIONS

These results indicate that while platelets are integral components of solid tumors, their localization and origin in the TME, as well as their impact on its shaping, are tumor type-dependent.

Periodontitis in ischemic stroke: impact of Porphyromonas gingivalis on thrombus composition and ischemic stroke outcomes

Background

Periodontitis is associated with an increased risk of ischemic stroke, but the mechanisms underlying this association remain unclear.

Objectives

Our objective was to determine whether Porphyromonas gingivalis (Pg), a periodontal bacterium, could be detected within thrombus aspirates, modify thrombus composition, and endovascular therapy responses.

Methods

The presence of Pg gingipain in 175 consecutive thrombi from patients with large vessel occlusion stroke enrolled in the multicenter research cohort compoCLOT was investigated by immunostaining. Thrombus blood cell composition according to gingipain status was analyzed in a subset of 63 patients.

Results

Pg gingipain immunostaining was positive in 33.7% of thrombi (95% CI, 26.7%-40.8%). The percentage of near to complete reperfusion (modified Thrombolysis in Cerebral Infarction Score 2c/3) at the end of the procedure was lower in the Pgpos group than the Pgneg group (39.0% vs 57.8% respectively; adjusted odds ratio, 0.38; 95% CI, 0.19-0.77). At 3 months, 35.7% of patients in the Pgpos group had a favorable neurological outcome vs 49.5% in the Pgneg group (odds ratio, 0.65; 95% CI, 0.30-1.40). Quantitative analysis of a subset of 63 thrombi showed that neutrophil elastase content was significantly (P < .05) higher in Pgpos thrombi than in Pgneg thrombi.

Conclusion

Our results indicate that intrathrombus Pg gingipain is associated with increased neutrophil content and resistance to endovascular therapy.

Thrombus composition and thrombolysis resistance in stroke

A State of the Art lecture titled "Thrombus Composition and Thrombolysis Resistance in Stroke" was presented at the ISTH Congress in 2022. Intravenous thrombolysis (IVT) remains the only pharmacologic option to re-establish cerebral perfusion at the acute phase of ischemic stroke. IVT is based on the administration of recombinant tissue plasminogen activator with the objective of dissolving fibrin, the major fibrillar protein component of thrombi. Almost 30 years on from its introduction, although the clinical benefits of IVT have been clearly demonstrated, IVT still suffers from a relatively low efficacy, with a rate of successful early recanalization below 50% overall. Analyses of thrombectomy-recovered acute ischemic stroke (AIS) thrombi have shown that apart from occlusion site, thrombus length, and collateral status, AIS thrombus structure and composition are also important modulators of IVT efficacy. In this article, after a brief presentation of IVT principle and current knowledge on IVT resistance, we review recent findings on how compaction and structural alterations of fibrin together with nonfibrin thrombus components such as neutrophil extracellular traps and von Willebrand factor interfere with IVT in AIS. We further discuss how these new insights could soon result in the development of original adjuvant therapies for improved IVT in AIS. Finally, we summarize relevant new data presented during the 2022 ISTH Congress.

High thrombus platelet content is associated with a lower rate of first pass effect in stroke treated by endovascular therapy

Background and purpose:

First pass effect (FPE), the occurrence of complete reperfusion after one pass with no rescue attempt during endovascular therapy (EVT), is associated with the best clinical outcome after an acute ischemic stroke (AIS). Previous studies evaluating FPE occurrence according to EVT technical strategies, occlusion locations, or thrombus composition have provided controversial results. Here, we performed a correlation analysis between FPE occurrence and AIS thrombus cellular composition, as assessed using quantitative biochemical assays.

Patients and methods:

Homogenates of AIS thrombi from 250 patients were prepared by mechanical grinding. Platelet, red blood cell (RBC), and leukocyte contents of AIS thrombi were respectively estimated by quantification of GP (glycoprotein) VI, heme, and DNA in thrombus homogenates. FPE was defined as a modified Thrombolysis in Cerebral Infraction (mTICI) score of 2C or 3 after a single EVT device pass.

Results:

AIS thrombi successfully removed after a single pass were poorer in GPVI (0.098 ± 0.023 vs 0.111 ± 0.024 ng/mg, p < 0.001) compared to those whose removal had required several passes. GPVI content was also significantly associated with a higher number of device passes and a longer procedure time. No such significant correlation was found with DNA and heme content.

Discussion and conclusion:

Thrombus platelet content may hamper thrombus removal by EVT. This result suggests that adjunctive therapies or functionalization of retrieval devices targeting platelets may improve EVT efficacy.

Modeling Large Vessel Occlusion Stroke for the Evaluation of Endovascular Therapy According to Thrombus Composition

More than 40% of endovascular therapy (EVT) fail to achieve complete reperfusion of the territory of the occluded artery in patients with acute ischemic stroke (AIS). Understanding factors influencing EVT could help overcome its limitations. Our objective was to study the impact of thrombus cell composition on EVT procedures, using a simulation system for modeling thrombus-induced large vessel occlusion (LVO) in flow conditions. In an open comparative trial, we analyzed the behavior of size-standardized platelet-rich and red blood cells (RBC)-rich thrombi during simulated stent retriever-mediated EVT procedures. Sixteen simulated EVT procedures were performed (8 RBC- vs. 8 platelet-rich thrombi). Platelet-rich thrombi were associated with a higher number of stent retriever passes (p = 0.03) and a longer procedure duration (p = 0.02) compared to RBC-rich thrombi. Conversely, RBC-rich thrombi released more embolic fragments than platelet-rich thrombi (p = 0.004). Both RBC-rich and platelet-rich thrombi underwent drastic compaction after being injected into the in vitro circulation model, and histologic analyses showed that these EVT-retrieved thrombi displayed features comparable to those previously observed in thrombi from patients with AIS patients having LVO, including a marked structural dichotomy between RBC- and platelet-rich areas. Our results show that the injection of in vitro-produced thrombi in artificial cerebrovascular arterial networks is suitable for testing recanalization efficacy and the risk of embolization of EVT devices and strategies in association with thrombus cell composition.

Intravenous abciximab as a rescue therapy for immediate reocclusion after successful mechanical thrombectomy in acute ischemic stroke patients

Background - Immediate reocclusion after mechanical thrombectomy (MT) for acute ischemic stroke (AIS) is a rare but devastating condition associated with poor functional outcome.Objective - The aim of this study was to gain insights into the mechanisms underlying immediate reocclusion, and to evaluate the efficacy and safety of the glycoprotein IIb/IIIa antagonist abciximab, for its treatment.Methods - Clinical data were collected from April 2015 to April 2019 in a monocentric prospective registry of AIS patients treated by MT. All patients with immediate reocclusion were retrospectively selected and subdivided into 2 groups according to abciximab treatment status. In vitro, the separate and combined effects of abciximab and alteplase on clot formation in whole blood under flow conditions were further investigated in microfluidic chambers.Results - From 929 MT-treated patients, 21 had post-MT immediate reocclusion. Abciximab treatment in reocclusion patients (n = 10) led to higher rate of final recanalization (p < .001) while it did not increase bleeding complications. Flow chamber experiments revealed that, in contrast to alteplase, abciximab efficiently limits thrombus accretion from flowing blood by blocking platelet aggregation.Conclusions - Our results underscore a key role for platelet aggregation and the potential of Glycoprotein IIb/IIIa antagonists as a rescue therapy in post-MT immediate reocclusion.

Pathogenic variants in THSD4, encoding the ADAMTS-like 6 protein, predispose to inherited thoracic aortic aneurysm

Purpose:

Thoracic aortic aneurysm and dissection (TAAD) is a life-threatening disease with often unrecognized inherited forms. We sought to identify novel pathogenic variants associated with autosomal dominant inheritance of TAAD.

Methods:

We analyzed exome sequencing data from 35 French TAAD families and performed next-generation sequencing capture panel of genes in 1114 unrelated TAAD patients. Functional effects of pathogenic variants identified were validated in cell, tissue, and mouse models.

Results:

We identified five functional variants in THSD4 of which two heterozygous variants lead to a premature termination codon. THSD4 encodes ADAMTSL6 (member of the ADAMTS/L superfamily), a microfibril-associated protein that promotes fibrillin-1 matrix assembly. The THSD4 variants studied lead to haploinsufficiency or impaired assembly of fibrillin-1 microfibrils. Thsd4^+/− mice showed progressive dilation of the thoracic aorta. Histologic examination of aortic samples from a patient carrying a THSD4 variant and from Thsd4^+/− mice, revealed typical medial degeneration and diffuse disruption of extracellular matrix.

Conclusion:

These findings highlight the role of ADAMTSL6 in aortic physiology and TAAD pathogenesis. They will improve TAAD management and help develop new targeted therapies.

Protective Effect of ApoA1 (Apolipoprotein A1)-Milano in a Rat Model of Large Vessel Occlusion Stroke

Background and Purpose- Previous experimental studies found that the infusion of human purified nascent HDL (high-density lipoprotein) significantly reduced infarct volume and hemorrhagic transformation rate by decreasing neutrophil recruitment. ApoA1-M (apolipoprotein A1-Milano) is a natural variant of human ApoA1 that confers protection against atherosclerosis. Recombinant ApoA1-M has been formulated as a complex with phospholipids to mimic the properties of nascent HDL. The aim of this study was to assess the impact of intravenous ApoA1-M in a transient middle cerebral artery occlusion stroke model in rats. Methods- In a first experiment, rats were subjected to 120-minute transient middle cerebral artery occlusion and intravenous ApoA1-M was infused immediately or 4 hours after occlusion. In a second experiment, rats were subjected to 240-minute transient middle cerebral artery occlusion and intravenous ApoA1-M was infused with or without recombinant tPA (tissue-type plasminogen activator) immediately after recanalization. Primary outcome criteria were the infarct volume and hemorrhagic transformation rate measured at 24 hours. Platelets, coagulation, and neutrophil activation biomarkers were measured in brain homogenates and plasma. Additional in vitro experiments studied the effects of ApoA1-M on platelet aggregation and platelet-neutrophil interactions. Results- The infusion of ApoA1-M immediately or 4 hours after 120-minute transient middle cerebral artery occlusion significantly reduced the infarct volume compared with saline (P=0.034 and P=0.036, respectively). Compared with tPA alone, co-administration of ApoA1-M and tPA showed similar rates of hemorrhagic transformation. ApoA1-M had no significant inhibition effect on neutrophil activation biomarkers. Platelet activation was slightly decreased in rats treated with ApoA1-M compared with saline. In vitro, the incubation of human and rat platelet-rich plasma with ApoA1-M significantly reduced ADP-induced platelet aggregation (P=0.001 and P=0.02, respectively). Conclusions- ApoA1-Milano significantly decreased the infarct volume through an inhibition of platelet aggregation but did not reduce hemorrhagic transformation and neutrophils activation as expected after previous experimental studies with nascent HDL. Visual Overview- An online visual overview is available for this article.

Two-layered susceptibility vessel sign is associated with biochemically quantified thrombus red blood cell content

BACKGROUND AND PURPOSE

Better characterization of the thrombus could be useful to determine acute ischaemic stroke (AIS) aetiology and predict response to thrombolysis and endovascular therapy (EVT). To test the hypothesis that susceptibility vessel sign (SVS) on baseline magnetic resonance imaging (MRI) is related to red blood cell (RBC) content of AIS thrombi, the total haemoglobin contents (HbCs) of AIS thrombi retrieved by EVT from patients with or without SVS or two-layered SVS (TLSVS) were compared.

METHODS

Baseline MRI of 84 anterior AIS patients was reviewed by neuro-radiologists blinded to clinical and biochemical data. Thrombi from these patients were retrieved by EVT and analysed for HbC by quantitative enzyme-linked immunosorbent assay and measurement of haem concentration.

RESULTS

Susceptibility vessel sign and TLSVS were respectively observed in 85.7% and 50.0% of cases. The median HbC content was 253 µg/mg thrombus (interquartile range 177-333) and the median haem content was 219 µg/mg thrombus (131-264). Thrombus HbC and haem content were highly correlated with thrombus RBC content determined by flow cytometry (r = 0.94). Thrombi from patients with TLSVS weighed more [31.1 (16.5-68.3) mg vs. 17.7 (11.7-33.3) mg; P = 0.005] and had a higher HbC content [278 (221-331) µg/mg vs. 196 (139-301) µg/mg; P = 0.010] compared to thrombi from patients without TLSVS. There was no difference in thrombus weight or HbC content according to SVS status.

CONCLUSIONS

Our study shows that TLSVS is significantly associated with a higher thrombus weight and RBC content, as determined by quantitative assays.

The reversed passive Arthus reaction as a model for investigating the mechanisms of inflammation-associated hemostasis

In recent years, accumulating evidence has indicated that platelets continuously repair vascular damage at sites of inflammation and/or infection. Studies in mouse models of inflammation have highlighted the fact that the mechanisms underlying bleeding prevention by platelets in inflamed organs can substantially differ from those supporting primary hemostasis following tail tip transection or thrombus formation in models of thrombosis. As a consequence, exploration of the hemostatic function of platelets in inflammation, as well as assessment of the risk of inflammation-induced bleeding associated with a platelet deficit and/or the use of anti-thrombotic drugs, require the use of dedicated experimental models. In the present review, we present the pros and cons of the cutaneous reversed passive Arthus reaction, a model of inflammation which has been instrumental in studying how inflammation causes vascular injury and how platelets continuously intervene to repair it. The limitations and common issues encountered when working with mouse models of inflammation for investigating platelet functions in inflammation are also discussed.

Microfluidic Modeling of Thrombolysis

Objective- Despite the high clinical relevance of thrombolysis, models for its study in human flowing blood are lacking. Our objective was to develop a microfluidic model for comparative evaluation of thrombolytic therapeutic strategies. Approach and Results- Citrated human blood was supplemented with 3,3'-dihexyloxacarbocyanine iodide and Alexa Fluor 647 fibrinogen conjugate, recalcified, and perfused for 3 to 4 minutes at venous or arterial wall shear rate in microfluidic flow chambers coated with collagen and tissue factor to generate nonocclusive fluorescent thrombi. A second perfusion was performed for 10 minutes with rhodamine-6G-labeled citrated whole blood, supplemented or not with r-tPA (recombinant tissue-type plasminogen activator), fluorescein isothiocyanate-conjugated r-tPA, and Alexa Fluor 568 plasminogen conjugate. Plasminogen and r-tPA bound to preformed thrombi and r-tPA caused a concentration-dependent decrease in thrombus fibrin content (up to 50% reduction at 15 µg/mL r-tPA) as assessed by fluorescence microscopy. Fibrinolysis was confirmed by measurement of D-dimers in the output flow. Remarkably, despite ongoing fibrinolysis, new platelets continued to be recruited to the thrombus under lysis. Under the arterial condition, combining r-tPA with hirudin enhanced fibrinolysis but did not prevent the recruitment of new platelets, which was, however, prevented by antiplatelet agents (ticagrelor or the GPVI [glycoprotein VI]-blocking antigen-binding fragment 9O12). Conclusions- Our microfluidic thrombolysis model is suitable for studying thrombolysis and testing the efficacy of drugs used in combination with r-tPA. Real-time analysis of fibrin and platelets during r-tPA-mediated fibrinolysis at arterial or venous flow conditions showed that platelets continue to accumulate during fibrinolysis. Such platelet accumulation may impair r-tPA-mediated recanalization.

Spontaneous bleeding in thrombocytopenia: Is it really spontaneous?

Spontaneous bleeding is a clinical hallmark of thrombocytopenia and can take multiple forms including petechiae, epistaxis, gum bleeding, or, in worst cases, intracranial hemorrhage. Those bleeding events are called " spontaneous " because they occur in the absence of overt trauma. Spontaneous bleeding manifestations have long been considered to be a direct consequence of low platelet counts. Nevertheless, although low platelet counts may lead to ultrastructural endothelial alterations, those alterations and the associated state of vascular fragility are unlikely sufficient to cause spontaneous rupture of the microvessel wall. Indeed, in addition to endothelial injury, factors capable of damaging the basement membrane are required to allow escape of red blood cells in the extravascular space. Therefore, despite their misleading name, spontaneous bleeding events in thrombocytopenia are most likely provoked and involve subclinical biological processes in which platelets normally intervene to ensure hemostasis. In this review, we discuss past and more recent studies on the possible triggers of spontaneous bleeding events in thrombocytopenia, with a particular focus on the role of inflammatory reactions.

Erythrocyte Efferocytosis by the Arterial Wall Promotes Oxidation in Early-Stage Atheroma in Humans

Background

Since red blood cells (RBCs) are the predominant cellular blood component interacting with the arterial wall, we explored the role of RBCs efferocytosis by vascular smooth muscle cells (vSMCs) in the initiation of human atheroma.

Methods and results

The comparison of human healthy aortas with aortic fatty streaks or fibroatheromas revealed that RBC angiophagy is implicated from the earliest stages of atherogenesis, as documented by the concomitant detection of redox-active iron, hemoglobin, glycophorin A, and ceroids. RBCs infiltration in the arterial wall was associated with local lipid and protein oxidation, as well as vascular response (expression of heme oxygenase-1 and of genes related to iron metabolism as well as those encoding for phagocytosis). These effects were recapitulated in vitro when vSMCs were co-cultured with phosphatidyl-exposing senescent (s) RBCs but not with fresh RBCs. VSMCs engulfing sRBC increased their intracellular iron content, accumulated hemoglobin, lipids, and activated their phagolysosomes. Strikingly, injections of sRBCs into rats promoted iron accumulation in the aortic wall. In rabbits, hypercholesterolemia increased circulating senescent RBCs and induced the subendothelial accumulation of iron-rich phagocytic foam cells. RBCs bring cholesterol and iron/heme into the vascular wall and interact with vSMCs that phagocytize them.

Conclusion

This study presents a previously unforeseen mechanism of plaque formation that implicates intimal RBC infiltration as one of the initial triggers for foam cell formation and intimal oxidation. Pathogenic effects exerted by several metabolic and hemodynamic factors may rely on their effect on RBC biology, thereby impacting how RBCs interact with the vascular wall.

Exacerbation of Thromboinflammation by Hyperglycemia Precipitates Cerebral Infarct Growth and Hemorrhagic Transformation

BACKGROUND AND PURPOSE

Admission hyperglycemia is associated with a poor outcome in acute ischemic stroke. How hyperglycemia impacts the pathophysiology of acute ischemic stroke remains largely unknown. We investigated how preexisting hyperglycemia increases ischemia/reperfusion cerebral injury.

METHODS

Normoglycemic and streptozotocin-treated hyperglycemic rats were subjected to transient middle cerebral artery occlusion. Infarct growth and brain perfusion were assessed by magnetic resonance imaging. Markers of platelet, coagulation, and neutrophil activation were measured in brain homogenates and plasma. Downstream microvascular thromboinflammation (DMT) was investigated by intravital microscopy.

RESULTS

Hyperglycemic rats had an increased infarct volume with an increased blood-brain barrier disruption and hemorrhagic transformation rate compared with normoglycemic rats. Magnetic resonance imaging scans revealed that hyperglycemia enhanced and accelerated lesion growth and was associated with hemorrhagic transformation originating from territories that were still not completely reperfused at 1 hour after middle cerebral artery recanalization. Intravital microscopy and analysis of brain homogenates showed that DMT began immediately after middle cerebral artery occlusion and was exacerbated by hyperglycemia. Measurement of plasma serotonin and matrix metalloproteinase-9 indicated that platelets and neutrophils were preactivated in hyperglycemic rats. Neutrophils from hyperglycemic diabetic patients showed increased adhesion to endothelial cells as compared with neutrophils from normoglycemic donors in flow chamber experiments.

CONCLUSIONS

We show that hyperglycemia primes the thromboinflammatory cascade, thus, amplifying middle cerebral artery occlusion-induced DMT. DMT exacerbation in hyperglycemic rats impaired reperfusion and precipitated neurovascular damage, blood-brain barrier disruption, and hemorrhagic transformation. Our results designate DMT as a possible target for reduction of the deleterious impact of hyperglycemia in acute ischemic stroke.

Platelet and Erythrocyte Sources of S1P Are Redundant for Vascular Development and Homeostasis, but Both Rendered Essential After Plasma S1P Depletion in Anaphylactic Shock

RATIONALE

Sphingosine-1-phosphate (S1P) signaling is essential for vascular development and postnatal vascular homeostasis. The relative importance of S1P sources sustaining these processes remains unclear.

OBJECTIVE

To address the level of redundancy in bioactive S1P provision to the developing and mature vasculature.

METHODS AND RESULTS

S1P production was selectively impaired in mouse platelets, erythrocytes, endothelium, or smooth muscle cells by targeted deletion of genes encoding sphingosine kinases -1 and -2. S1P deficiency impaired aggregation and spreading of washed platelets and profoundly reduced their capacity to promote endothelial barrier function ex vivo. However, and in contrast to recent reports, neither platelets nor any other source of S1P was essential for vascular development, vascular integrity, or hemostasis/thrombosis. Yet rapid and profound depletion of plasma S1P during systemic anaphylaxis rendered both platelet- and erythrocyte-derived S1P essential for survival, with a contribution from blood endothelium observed only in the absence of circulating sources. Recovery was sensitive to aspirin in mice with but not without platelet S1P, suggesting that platelet activation and stimulus-response coupling is needed. S1P deficiency aggravated vasoplegia in this model, arguing a vital role for S1P in maintaining vascular resistance during recovery from circulatory shock. Accordingly, the S1P2 receptor mediated most of the survival benefit of S1P, whereas the endothelial S1P1 receptor was dispensable for survival despite its importance for maintaining vascular integrity.

CONCLUSIONS

Although source redundancy normally secures essential S1P signaling in developing and mature blood vessels, profound depletion of plasma S1P renders both erythrocyte and platelet S1P pools necessary for recovery and high basal plasma S1P levels protective during anaphylactic shock.

Uptake of Plasmin-PN-1 Complexes in Early Human Atheroma

Zymogens are delivered to the arterial wall by radial transmural convection. Plasminogen can be activated within the arterial wall to produce plasmin, which is involved in evolution of the atherosclerotic plaque. Vascular smooth muscle cells (vSMCs) protect the vessels from proteolytic injury due to atherosclerosis development by highly expressing endocytic LDL receptor-related protein-1 (LRP-1), and by producing anti-proteases, such as Protease Nexin-1 (PN-1). PN-1 is able to form covalent complexes with plasmin. We hypothesized that plasmin-PN-1 complexes could be internalized via LRP-1 by vSMCs during the early stages of human atheroma. LRP-1 is also responsible for the capture of aggregated LDL in human atheroma. Plasmin activity and immunohistochemical analyses of early human atheroma showed that the plasminergic system is activated within the arterial wall, where intimal foam cells, including vSMCs and platelets, are the major sites of PN-1 accumulation. Both PN-1 and LRP-1 are overexpressed in early atheroma at both messenger and protein levels. Cell biology studies demonstrated an increased expression of PN-1 and tissue plasminogen activator by vSMCs in response to LDL. Plasmin-PN-1 complexes are internalized via LRP-1 in vSMCs, whereas plasmin alone is not. Tissue PN-1 interacts with plasmin in early human atheroma via two complementary mechanisms: plasmin inhibition and tissue uptake of plasmin-PN-1 complexes via LRP-1 in vSMCs. Despite this potential protective effect, plasminogen activation by vSMCs remains abnormally elevated in the intima in early stages of human atheroma.

High-density lipoproteins potentiate α1-antitrypsin therapy in elastase-induced pulmonary emphysema

Several studies report that high-density lipoproteins (HDLs) can carry α1-antitrypsin (AAT; an elastase inhibitor). We aimed to determine whether injection of exogenous HDL, enriched or not in AAT, may have protective effects against pulmonary emphysema. After tracheal instillation of saline or elastase, mice were randomly treated intravenously with saline, human plasma HDL (75 mg apolipoprotein A1/kg), HDL-AAT (75 mg apolipoprotein A1-3.75 mg AAT/kg), or AAT alone (3.75 mg/kg) at 2, 24, 48, and 72 hours. We have shown that HDL-AAT reached the lung and prevented the development of pulmonary emphysema by 59.3% at 3 weeks (alveoli mean chord length, 22.9 ± 2.8 μm versus 30.7 ± 4.5 μm; P < 0.001), whereas injection of HDL or AAT alone only showed a moderate, nonsignificant protective effect (28.2 ± 4.2 μm versus 30.7 ± 5 μm [P = 0.23] and 27.3 ± 5.66 μm versus 30.71 ± 4.96 μm [P = 0.18], respectively). Indeed, protection by HDL-AAT was significantly higher than that observed with HDL or AAT (P = 0.006 and P = 0.048, respectively). This protective effect was associated (at 6, 24, and 72 h) with: (1) a reduction in neutrophil and macrophage number in the bronchoalveolar lavage fluid; (2) decreased concentrations of IL-6, monocyte chemoattractant protein-1, and TNF-α in both bronchoalveolar lavage fluid and plasma; (3) a reduction in matrix metalloproteinase-2 and matrix metalloproteinase-9 activities; and (4) a reduction in the degradation of fibronectin, a marker of tissue damage. In addition, HDL-AAT reduced acute cigarette smoke-induced inflammatory response. Intravenous HDL-AAT treatment afforded a better protection against elastase-induced pulmonary emphysema than AAT alone, and may represent a significant development for the management of emphysema associated with AAT deficiency.

Pathology of human plaque vulnerability: mechanisms and consequences of intraplaque haemorrhages

Atherothrombotic diseases are still major causes of inability and mortality and fighting atherothrombosis remains a public health priority. The involvement of repeated intraplaque haemorrhages (IPH) in the evolution of atherothrombotic lesions towards complications was proposed as early as 1936. This important topic has been recently revisited and reviewed. Histological observations have been corroborated by magnetic resonance imaging (MRI) of human carotid atheroma, identifying IPH as the main determinant of plaque evolution towards rupture. Beside the intimal integration of asymptomatic luminal coagulum, inward sprouting of neovessels from the adventitia towards the plaque, is one source of IPH in human atheroma. We recently described that directed neo-angiogenesis from the adventitia towards the plaque, across the media, is initiated by lipid mediators generated by the plaque on the luminal side, outwardly convected to the medial VSMCs. Subsequent stimulation of VSMC PPAR-γ receptors induces VEGF expression which causes centripetal sprouting of adventitial vessels. However, this neovascularization is considered to be immature and highly susceptible to leakage. The main cellular components of IPH are Red Blood Cells (RBCs), which with their haemoglobin content and their cell membrane components, particularly enriched in unesterified cholesterol, participate in both the oxidative process and cholesterol accumulation. The presence of iron, glycophorin A and ceroids provides evidence of RBCs. IPH also convey blood leukocytes and platelets and are sites prone to weak pathogen contamination. Therefore prevention and treatment of the biological consequences of IPH pave the way to innovative preventive strategies and improved therapeutic options in human atherothrombotic diseases.

ADAMTS13 exerts a thrombolytic effect in microcirculation

Recombinant tissue plasminogen activator (r-tPA) is the drug of choice for thrombolysis, but it is associated with a significant risk of bleeding and is not always successful. By cleaving von Willebrand factor (VWF), the metalloprotease ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type I repeats-13) down-regulates thrombus formation in injured vessels. We investigated whether recombinant ADAMTS13 (r-ADAMTS13) induces thrombolysis in vivo in mice. Thrombosis was produced by ferric chloride-induced (FeCl(3)) injury in the venules of a dorsal skinfold chamber. Phosphate-buffered saline (PBS, vehicle), r-tPA or r-ADAMTS13, supplemented with hirudin (to stop on-going thrombin generation), was directly applied onto the occluded vessel, and thrombus dissolution was evaluated by intravital microscopy. The incidence of blood flow restoration significantly increased 30 minutes (min) after r-ADAMTS13 vs. PBS treatment (60% vs. 0%, p<0.05) and 60 min after r-tPA treatment (75% vs. 17%, p<0.05). Both r-tPA and r-ADAMTS13 significantly reduced thrombus size 60 min after their superfusion (53.2% and 62.3% of the initial thrombus size, p<0.05 and p<0.01, respectively). Bleeding occurred in all r-tPA-treated chambers, while it was absent in mice treated with r-ADAMTS13 or PBS. We observed that, similar to r-tPA, r-ADAMTS13 can dissolve occlusive thrombi induced by FeCl(3) injury in venules. In contrast to r-tPA, the in vivo thrombolytic effect of ADAMTS13 was not associated with any signs of haemorrhage. ADAMTS13 could represent a new therapeutic option for thrombolysis.

How platelets safeguard vascular integrity

The haemostatic role of platelets was established in the 1880s by Bizzozero who observed their ability to adhere and aggregate at sites of vascular injury. It was only some 80 years later that the function of platelets in maintaining the structural integrity of intact blood vessels was reported by Danielli. Danielli noted that platelets help preserve the barrier function of endothelium during organ perfusion. Subsequent studies have demonstrated further that platelets are continuously needed to support intact mature blood vessels. More recently, platelets were shown to safeguard developing vessels, lymphatics, as well as the microvasculature at sites of leukocyte infiltration, including inflamed organs and tumours. Interestingly, from a mechanistic point of view, the supporting role of platelets in these various vessels does not necessarily involve the well-understood process of platelet plug formation but, rather, may rely on secretion of the various platelet granules and their many active components. The present review focuses on these nonconventional aspects of platelet biology and function by presenting situations in which platelets intervene to maintain vascular integrity and discusses possible mechanisms of their actions. We propose that modulating these newly described platelet functions may help treat haemorrhage as well as treat cancer by increasing the efficacy of drug delivery to tumours.

Increased efficacy of breast cancer chemotherapy in thrombocytopenic mice

Platelets contribute to homeostasis of the tumor vasculature by helping prevent hemorrhage. Thus, we hypothesized that inducing thrombocytopenia would increase tumor vascular leakiness and facilitate the effective delivery of chemotherapeutic agents to tumors. In a mammary carcinoma murine model, platelet depletion induced bleeding specifically at the tumor site, favoring the accumulation of fluorescently labeled microspheres only in the tumor. Moreover, induction of thrombocytopenia in tumor-bearing mice before injection of paclitaxel increased its intratumoral accumulation and reduced growth of both slow- and fast-growing tumors, compared with mice with normal platelet counts that were treated only with paclitaxel. Histologic analysis confirmed the expectation of an increase in tumor apoptosis and a reduction in tumor proliferation in thrombocytopenic mice receiving chemotherapy. No increased toxicity was seen in other organs or blood cells. Taken together, our results indicate that low platelet count selectively induces leakiness of tumor vessels and favors the delivery of chemotherapy to tumor sites, enhancing its tumoricidal effects.

Innate immune cells induce hemorrhage in tumors during thrombocytopenia

Platelets are crucial regulators of tumor vascular homeostasis and continuously prevent tumor hemorrhage through secretion of their granules. However, the reason for tumor bleeding in the absence of platelets remains unknown. Tumors are associated with inflammation, a cause of hemorrhage in thrombocytopenia. Here, we investigated the role of the inflamed tumor microenvironment in the induction of tumor vessel injury in thrombocytopenic mice. Using s.c. injections of vascular endothelial growth factor or tumor necrosis factor-alpha combined with depletion of neutrophils, we demonstrate that enhancing the opening of endothelial cell junctions was not sufficient to cause bleeding in the absence of platelets; instead, induction of tissue hemorrhage in thrombocytopenia required recruitment of leukocytes. Immunohistology revealed that thrombocytopenia-induced tumor hemorrhage occurs at sites of macrophage and neutrophil accumulation. Mice deficient in beta2 or beta3 integrins, which have decreased neutrophil and/or macrophage infiltration in their tumor stroma, were protected from thrombocytopenia-induced tumor hemorrhage, indicating that, in the absence of platelets, stroma-infiltrating leukocytes induced tumor vessel injury. This injury was independent of reactive oxygen species generation and of complement activation, as suggested by the persistence of tumor hemorrhage in C3- and nicotinamide adenine dinucleotide phosphate oxidase-deficient thrombocytopenic mice. Our results show that platelets counteract tumor-associated inflammation and that the absence of this platelet function elicits vascular injuries by tumor-infiltrating innate immune cells.

Platelets: guardians of tumor vasculature

Solid tumors generate a prothrombotic environment capable of platelet activation. Recent findings indicate that the activated platelets are crucial regulators of tumor vascular homeostasis in that they prevent tumor hemorrhage. Surprisingly, this effect is independent of platelets' capacity to form thrombi and instead relies on the secretion of their granule content. Thus, targeting platelet secretory activities may represent a new approach to specifically destabilize tumor vasculature.

Platelet granule secretion continuously prevents intratumor hemorrhage

Cancer is associated with a prothrombogenic state capable of platelet activation. Platelets, on the other hand, can support angiogenesis, a process involved in the progression of tumor growth and metastasis. However, it is unclear whether platelet/tumor interactions substantially contribute to tumor physiology. We investigated whether platelets stabilize tumor vessels and studied the underlying mechanisms. We induced severe acute thrombocytopenia in mice bearing s.c. Lewis lung carcinoma or B16F10 melanoma. Intravital microscopy revealed that platelet depletion led to a rapid destabilization of tumor vessels with intratumor hemorrhage starting as soon as 30 min after induction of thrombocytopenia. Using an inhibitor of glycoprotein Ibalpha (GPIbalpha) and genetically engineered mice with platelet adhesion defects, we investigated the role of platelet adhesion receptors in stabilizing tumor vessels. We found that a single defect in either GPIbalpha, von Willebrand factor, P-selectin, or platelet integrin activation did not lead to intratumor hemorrhage. We then compared the ability of transfused resting and degranulated platelets to prevent intratumor hemorrhage. Whereas resting platelets prevented thrombocytopenia-induced tumor bleeding, circulating degranulated platelets did not. This suggests that the prevention of intratumor hemorrhage by platelets relies on the secretion of the content of platelet granules. Supporting this hypothesis, we further found that thrombocytopenia dramatically impairs the balance between propermeability and antipermeability factors in tumor-bearing animals, in particular depleting blood of angiopoietin-1 and serotonin. Our results show a crucial contribution of platelets to tumor homeostasis through continuous prevention of severe intratumor hemorrhage and consequent cell death. The study also suggests platelet function as a reasonable target for specific destabilization of tumor vessels.