Platelets regulate ischemia-induced revascularization and angiogenesis by secretion of growth factor-modulating factors.

In ischemic tissue, platelets can modulate angiogenesis. The specific factors influencing this function, however, are poorly understood. Here, we characterized the complement anaphylatoxin C5a-mediated activation of C5a receptor 1 (C5aR1) expressed on platelets as a potent regulator of ischemia-driven revascularization. We assessed the relevance of the anaphylatoxin receptor C5aR1 on platelets in patients with coronary artery disease as well as those with peripheral artery disease and used genetic mouse models to characterize its significance for ischemia and growth factor-driven revascularization. The presence of C5aR1-expressing platelets was increased in the hindlimb ischemia model. Ischemia-driven angiogenesis was significantly improved in C5aR1-/- mice but not in C5-/- mice, suggesting a specific role of C5aR1. Experiments using the supernatant of C5a-stimulated platelets suggested a paracrine mechanism of angiogenesis inhibition by platelets by means of antiangiogenic CXC chemokine ligand 4 (CXCL4, PF4). Lineage-specific C5aR1 deletion verified that the secretion of CXCL4 depends on C5aR1 ligation on platelets. Using C5aR1-/-CXCL4-/- mice, we observed no additional effect in the revascularization response, underscoring a strong dependence of CXCL4 secretion on the C5a-C5aR1-axis. We identified a novel mechanism for inhibition of neovascularization via platelet C5aR1, which was mediated by the release of antiangiogenic CXCL4.

Protocol to isolate and analyze mouse bone marrow derived dendritic cells (BMDC).

Different types of immune cells are involved in atherogenesis and may act atheroprotective or atheroprogressive. Here, we describe an in vitro approach to analyze CD11c+ cells and CD11c+-derived ApoE in atherosclerosis. The major steps include harvesting mouse bone marrow, plating cells in culture dishes, treating them with differentiation factors, and collecting cells after removal of undesirable populations. This protocol can be adapted for CD11c+ cells in different contexts, thus, serving as models for different diseases and to analyze cell-specific molecules. For complete details on the use and execution of this protocol, please refer to Sauter et al. (2021).

Platelet depletion does not alter long-term functional outcome after cerebral ischaemia in mice.

Platelets are key mediators of thrombus formation and inflammation during the acute phase of ischaemic stroke. Particularly, the platelet glycoprotein (GP) receptors GPIbα and GPVI have been shown to mediate platelet adhesion and activation in the ischaemic brain. GPIbα and GPVI blockade could reduce infarct volumes and improve functional outcome in mouse models of acute ischaemic stroke, without concomitantly increasing intracerebral haemorrhage. However, the functional role of platelets during long-term stroke recovery has not been elucidated so far. Thus, we here examined the impact of platelet depletion on post-stroke recovery after transient middle cerebral artery occlusion (tMCAO) in adult male mice. Platelet depleting antibodies or isotype control were applied from day 3-28 after tMCAO in mice matched for infarct size. Long-term functional recovery was assessed over the course of 28 days by behavioural testing encompassing motor and sensorimotorical functions, as well as anxiety-like or spontaneous behaviour. Whole brain flow cytometry and light sheet fluorescent microscopy were used to identify resident and infiltrated immune cell types, and to determine the effects of platelet depletion on the cerebral vascular architecture, respectively. We found that delayed platelet depletion does not improve long-term functional outcome in the tMCAO stroke model. Immune cell abundance, the extent of thrombosis and the organisation of the cerebral vasculature were also comparable between platelet-depleted and control mice. Our study demonstrates that, despite their critical role in the acute stroke setting, platelets appear to contribute only marginally to tissue reorganisation and functional recovery at later stroke stages.

Protocol to visualize CD11c+ cells in atherosclerosis using LacZ reporter mice.

Here, we describe an in vivo approach to visualize CD11c+ cells in atherosclerosis. In particular, we use a protocol for X-Gal staining of immune cells within atherosclerotic plaques, which can be used as an alternative to analyze plaque composition and cell-specific molecules in atherogenesis. LacZ knockin mice have to be bred to mice carrying the CD11ccre recombinase-both brought onto an ApoE-/- background-to be able to visualize this cell type of interest in the plaques by X-Gal staining. With this approach, different immune cells in atherogenesis can be examined. For complete details on the use and execution of this protocol, please refer to Sauter et al. (2021).

Activated Platelets Upregulate ß2 Integrin Mac-1 (CD11b/CD18) on Dendritic Cells, Which Mediates Heterotypic Cell-Cell Interaction.

Recent evidence suggests interaction of platelets with dendritic cells (DCs), while the molecular mechanisms mediating this heterotypic cell cross-talk are largely unknown. We evaluated the role of integrin Mac-1 (αMß2, CD11b/CD18) on DCs as a counterreceptor for platelet glycoprotein (GP) Ibα. In a dynamic coincubation model, we observed interaction of human platelets with monocyte-derived DCs, but also that platelet activation induced a sharp increase in heterotypic cell binding. Inhibition of CD11b or GPIbα led to significant reduction of DC adhesion to platelets in vitro independent of GPIIbIIIa, which we confirmed using platelets from Glanzmann thrombasthenia patients and transgenic mouse lines on C57BL/6 background (GPIbα-/-, IL4R-GPIbα-tg, and muMac1 mice). In vivo, inhibition or genetic deletion of CD11b and GPIbα induced a significant reduction of platelet-mediated DC adhesion to the injured arterial wall. Interestingly, only intravascular antiCD11b inhibited DC recruitment, suggesting a dynamic DC-platelet interaction. Indeed, we could show that activated platelets induced CD11b upregulation on Mg2+-preactivated DCs, which was related to protein kinase B (Akt) and dependent on P-selectin and P-selectin glycoprotein ligand 1. Importantly, specific pharmacological targeting of the GPIbα-Mac-1 interaction site blocked DC-platelet interaction in vitro and in vivo. These results demonstrate that cross-talk of platelets with DCs is mediated by GPIbα and Mac-1, which is upregulated on DCs by activated platelets in a P-selectin glycoprotein ligand 1-dependent manner.

Apolipoprotein E derived from CD11c+ cells ameliorates atherosclerosis.

Atherosclerosis is studied in models with dysfunctional lipid homeostasis-predominantly the ApoE-/- mouse. The role of antigen-presenting cells (APCs) for lipid homeostasis is not clear. Using a LacZ reporter mouse, we showed that CD11c+ cells were enriched in aortae of ApoE-/- mice. Systemic long-term depletion of CD11c+ cells in ApoE-/- mice resulted in significantly increased plaque formation associated with reduced serum ApoE levels. In CD11ccre+ApoEfl/fl and Albumincre+ApoEfl/fl mice, we could show that ≈70% of ApoE is liver-derived and ≈25% originates from CD11c+ cells associated with significantly increased atherosclerotic plaque burden in both strains. Exposure to acLDL promoted cholesterol efflux from CD11c+ cells and cell-specific deletion of ApoE resulted in increased inflammation reflected by increased IL-1ß serum levels. Our results determined for the first time the level of ApoE originating from CD11c+ cells and demonstrated that CD11c+ cells ameliorate atherosclerosis by the secretion of ApoE.

The C5a/C5a receptor 1 axis controls tissue neovascularization through CXCL4 release from platelets.

Platelets contribute to the regulation of tissue neovascularization, although the specific factors underlying this function are unknown. Here, we identified the complement anaphylatoxin C5a-mediated activation of C5a receptor 1 (C5aR1) on platelets as a negative regulatory mechanism of vessel formation. We showed that platelets expressing C5aR1 exert an inhibitory effect on endothelial cell functions such as migration and 2D and 3D tube formation. Growth factor- and hypoxia-driven vascularization was markedly increased in C5ar1-/- mice. Platelet-specific deletion of C5aR1 resulted in a proangiogenic phenotype with increased collateralization, capillarization and improved pericyte coverage. Mechanistically, we found that C5a induced preferential release of CXC chemokine ligand 4 (CXCL4, PF4) from platelets as an important antiangiogenic paracrine effector molecule. Interfering with the C5aR1-CXCL4 axis reversed the antiangiogenic effect of platelets both in vitro and in vivo.In conclusion, we identified a mechanism for the control of tissue neovascularization through C5a/C5aR1 axis activation in platelets and subsequent induction of the antiangiogenic factor CXCL4.

Complement, inflammation and thrombosis.

A mutual relationship exists between immune activation and mechanisms of thrombus formation. In particular, elements of the innate immune response such as the complement system can modulate platelet activation and subsequently thrombus formation. Several components of the complement system including C3 or the membrane attack complex have been reported to be associated with platelets and become functionally active in the micromilieu of platelet activation. The exact mechanisms how this interplay is regulated and its consequences for tissue inflammation, damage or recovery remain to be defined. This review addresses the current state of knowledge on this topic and puts it into context with diseases featuring both thrombosis and complement activation. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Myocardial expression of the anaphylatoxin receptor C3aR is associated with cardiac inflammation and prognosis in patients with non-ischaemic heart failure.

AIM: The aim of this study is to analyse the prognostic value of complement anaphylatoxin receptors in patients with non-ischaemic cardiomyopathy undergoing endomyocardial biopsy. METHODS AND RESULTS: In 102 patients (72.5% male patients, median age 54 years) with non-ischaemic cardiomyopathy, myocardial expression of C3aR was assessed among other parameters. The primary study endpoint was a composite of death, heart transplantation, heart failure-related re-hospitalization, and deterioration of left ventricular ejection fraction within a mean follow-up of 11.9 months. The number of cells, which stained positive for C3aR, was significantly increased in patients with inflammatory compared with non-inflammatory cardiomyopathy (1.75 ± 0.31 cells in inflammatory cardiomyopathy vs. 0.94 ± 0.26 in non-inflammatory cardiomyopathy, P = 0.049). Subsequently, positive expression for C3aR was judged based on a semi-quantitative scoring system. Significantly, more patients with positive MHCII and CD68 expression showed an increased number of C3aR-positive cells. C3aR expression based on this score was more pronounced in patients with human herpesvirus 6 viral genome detection. Kaplan-Meier curves illustrate that the C3aR-negative group reached the primary endpoint significantly more often (mean follow-up 11.9 months, log rank 5.963, P = 0.015). Lack of C3aR expression was a strong independent predictor for the primary endpoint in Cox regression analysis [hazard ratio 0.46 (0.26-0.82, P = 0.009)]. CONCLUSIONS: C3aR-positive cells are found more often in patients with inflammatory cardiomyopathy. The relevance of C3aR-positive cells in patients with non-ischaemic cardiomyopathy should be further evaluated as potential predictors or modulators of adverse cardiac remodelling, the substrate of progressive heart failure.

Functional Relevance of the Anaphylatoxin Receptor C3aR for Platelet Function and Arterial Thrombus Formation Marks an Intersection Point Between Innate Immunity and Thrombosis.

BACKGROUND: Platelets have distinct roles in the vascular system in that they are the major mediator of thrombosis, critical for restoration of tissue integrity, and players in vascular inflammatory conditions. In close spatiotemporal proximity, the complement system acts as the first line of defense against invading microorganisms and is a key mediator of inflammation. Whereas the fluid phase cross-talk between the complement and coagulation systems is well appreciated, the understanding of the pathophysiological implications of such interactions is still scant. METHODS: We analyzed coexpression of the anaphylatoxin receptor C3aR with activated glycoprotein IIb/IIIa on platelets of 501 patients with coronary artery disease using flow cytometry; detected C3aR expression in human or murine specimen by polymerase chain reaction, immunofluorescence, Western blotting, or flow cytometry; and examined the importance of platelet C3aR by various in vitro platelet function tests, in vivo bleeding time, and intravital microscopy. The pathophysiological relevance of C3aR was scrutinized with the use of disease models of myocardial infarction and stroke. To approach underlying molecular mechanisms, we identified the platelet small GTPase Rap1b using nanoscale liquid chromatography coupled to tandem mass spectrometry. RESULTS: We found a strong positive correlation of platelet complement C3aR expression with activated glycoprotein IIb/IIIa in patients with coronary artery disease and coexpression of C3aR with glycoprotein IIb/IIIa in thrombi obtained from patients with myocardial infarction. Our results demonstrate that the C3a/C3aR axis on platelets regulates distinct steps of thrombus formation such as platelet adhesion, spreading, and Ca2+ influx. Using C3aR-/- mice or C3-/- mice with reinjection of C3a, we uncovered that the complement activation fragment C3a regulates bleeding time after tail injury and thrombosis. Notably, C3aR-/- mice were less prone to experimental stroke and myocardial infarction. Furthermore, reconstitution of C3aR-/- mice with C3aR+/+ platelets and platelet depletion experiments demonstrated that the observed effects on thrombosis, myocardial infarction, and stroke were specifically caused by platelet C3aR. Mechanistically, C3aR-mediated signaling regulates the activation of Rap1b and thereby bleeding arrest after injury and in vivo thrombus formation. CONCLUSIONS: Overall, our findings uncover a novel function of the anaphylatoxin C3a for platelet function and thrombus formation, highlighting a detrimental role of imbalanced complement activation in cardiovascular diseases.