Platelets Independently Recruit into Asthmatic Lungs and Models of Allergic Inflammation via CCR3.

Platelet activation and pulmonary recruitment occur in patients with asthma and in animal models of allergic asthma, in which leukocyte infiltration, airway remodeling, and hyperresponsiveness are suppressed by experimental platelet depletion. These observations suggest the importance of platelets to various characteristics of allergic disease, but the mechanisms of platelet migration and location are not understood. The aim of this study was to assess the mechanism of platelet recruitment to extravascular compartments of lungs from patients with asthma and after allergen challenge in mice sensitized to house dust mite (HDM) extract (contains the DerP1 [Dermatophagoides pteronyssinus extract peptidase 1] allergen); in addition, we assessed the role of chemokines in this process. Lung sections were immunohistochemically stained for CD42b+ platelets. Intravital microscopy in allergic mice was used to visualize platelets tagged with an anti-mouse CD49b-PE (phycoerythrin) antibody. Platelet-endothelial interactions were measured in response to HDM (DerP1) exposure in the presence of antagonists to CCR3, CCR4, and CXCR4. Extravascular CD42b+ platelets were detected in the epithelium and submucosa in bronchial biopsy specimens taken from subjects with steroid-naive mild asthma. Platelets were significantly raised in the lung parenchyma from patients with fatal asthma compared with postmortem control-lung tissue. Furthermore, in DerP1-sensitized mice, subsequent HDM exposure induced endothelial rolling, endothelial adhesion, and recruitment of platelets into airway walls, compared with sham-sensitized mice, via a CCR3-dependent mechanism in the absence of aggregation or interactions with leukocytes. Localization of singular, nonaggregated platelets occurs in lungs of patients with asthma. In allergic mice, platelet recruitment occurs via recognized vascular adhesive and migratory events, independently of leukocytes via a CCR3-dependent mechanism.

Red Blood Cells Elicit Platelet-Dependent Neutrophil Recruitment Into Lung Airspaces.

ABSTRACT: Hemolysis that occurs in intravascular hemolytic disorders, such as sickle cell disease and malaria, is associated with inflammation and platelet activation. Alveolar hemorrhage, for example following primary blast lung injury or acute respiratory distress syndrome, results in the escape of erythrocytes (RBCs) into alveolar spaces, where they subsequently lyse and release their intracellular contents. However, the inflammatory effects of RBCs in the airways are not fully understood. We hypothesized that RBCs in the airway induce an inflammatory response, associated with platelet activation. By instilling whole RBCs or lysed RBCs into the airways of mice, we have demonstrated that whole RBCs elicit macrophage accumulation in the lung. On the other hand, lysed RBCs induce significant inflammatory cell recruitment, particularly neutrophils and this was associated with a 50% increase in circulating platelet neutrophil complexes. Platelet depletion prior to lysed RBC exposure in the lung resulted in reduced neutrophil recruitment, suggesting that the presence of intracellular RBC components in the airways can elicit inflammation that is platelet dependent. To identify specific platelet-dependent signaling pathways involved in neutrophil recruitment, anti-P-selectin ligand and anti-PSGL1 blocking antibodies were tested; however, neither affected neutrophil recruitment. These findings implicate an involvement for other, as yet unidentified platelet-dependent signaling and adhesion mechanisms. Further understanding of how platelets contribute to lung inflammation induced by the presence of RBCs could offer novel therapeutic approaches to attenuate inflammation that occurs in conditions associated with alveolar hemorrhage.

A dichotomy in platelet activation: Evidence of different functional platelet responses to inflammatory versus haemostatic stimuli.

INTRODUCTION: Platelets participate in inflammatory disorders through a variety of different functional responses, including chemotaxis, platelet-leukocyte complex formation and facilitation of leukocyte recruitment that are thought to be distinct from platelet aggregation. This may account for why classical anti-platelet drugs have failed to ameliorate inflammatory disorders where platelets are known to participate, suggesting that distinct pathways may control inflammatory and haemostatic functions of platelets. In the present study, we have therefore investigated the effect of different stimuli on several different functions of platelets preferentially involved either in haemostasis or in inflammation. MATERIALS AND METHODS: Human platelets were stimulated with either inflammatory (fMLP, histamine, IL-1ß, LPS, MDC/CCL22, SDF-1α/CXCL12 and 5-HT) or haemostatic (ADP, collagen, convulxin, epinephrine, TRAP-6 and U46619) stimuli. Aggregation, platelet-leukocyte complex formation, platelet migration and platelet protein phosphorylation were assessed. RESULTS: Haemostatic stimuli induced platelet aggregation, whilst inflammatory agonists induced platelet migration. The haemostatic stimuli, with the exception of epinephrine, and some of the inflammatory stimuli induced platelet-leukocyte complex formation, even if to a different extent. Furthermore, inflammatory stimuli induced a shorter lasting profile of platelet protein phosphorylation compared with haemostatic stimuli. CONCLUSIONS: Stimulation of platelets with inflammatory stimuli triggers the activation of non haemostatic functions different from those induced by haemostatic stimuli, supporting the existence of alternative platelet responses depending on the stimulus (haemostatic or inflammatory). A deeper understanding of the biochemical pathways behind these functional differences may lead to the development of novel therapeutic options targeting the inflammatory actions of platelets, without affecting their critical role in haemostasis.

Platelet-Eosinophil Interactions As a Potential Therapeutic Target in Allergic Inflammation and Asthma.

The importance of platelet activation during hemostasis is well understood. An understanding of these mechanisms has led to the use of several classes of anti-platelet drugs to inhibit aggregation for the prevention of thrombi during cardiovascular disease. It is now also recognized that platelets can function very differently during inflammation, as part of their role in the innate immune response against pathogens. This dichotomy in platelet function occurs through distinct physiological processes and alternative signaling pathways compared to that of hemostasis (leading to platelet aggregation) and is manifested as increased rheological interactions with leukocytes, the ability to undergo chemotaxis, communication with antigen-presenting cells, and direct anti-pathogen responses. Mounting evidence suggests platelets are also critical in the pathogenesis of allergic diseases such as asthma, where they have been associated with antigen presentation, bronchoconstriction, bronchial hyperresponsiveness, airway inflammation, and airway remodeling in both clinical and experimental studies. In particular, platelets have been reported bound to eosinophils in the blood of patients with asthma and the incidence of these events increases after both spontaneous asthma attacks in a biphasic manner, or after allergen challenge in the clinic. Platelet depletion in animal models of allergic airway inflammation causes a profound reduction in eosinophil recruitment to the lung, suggesting that the association of platelets with eosinophils is indeed an important event during eosinophil activation. Furthermore, in cases of severe asthma, and in animal models of allergic airways inflammation, platelet-eosinophil complexes move into the lung through a platelet P-selectin-mediated, eosinophil ß1-integrin activation-dependent process, while platelets increase adherence of eosinophils to the vascular endothelium in vitro, demonstrating a clear interaction between these cell types in allergic inflammatory diseases. This review will explore non-thrombotic platelet activation in the context of allergy and the association of platelets with eosinophils, to reveal how these phenomena may lead to the discovery of novel therapeutic targets.