Cathelicidins prime platelets to mediate arterial thrombosis and tissue inflammation

Abstract

Leukocyte-released antimicrobial peptides contribute to pathogen elimination and activation of the immune system. Their role in thrombosis is incompletely understood. Here we show that the cathelicidin LL-37 is abundant in thrombi from patients with acute myocardial infarction. Its mouse homologue, CRAMP, is present in mouse arterial thrombi following vascular injury, and derives mainly from circulating neutrophils. Absence of hematopoietic CRAMP in bone marrow chimeric mice reduces platelet recruitment and thrombus formation. Both LL-37 and CRAMP induce platelet activation in vitro by involving glycoprotein VI receptor with downstream signaling through protein tyrosine kinases Src/Syk and phospholipase C. In addition to acute thrombosis, LL-37/CRAMP-dependent platelet activation fosters platelet–neutrophil interactions in other inflammatory conditions by modulating the recruitment and extravasation of neutrophils into tissues. Absence of CRAMP abrogates acid-induced lung injury, a mouse pneumonia model that is dependent on platelet–neutrophil interactions. We suggest that LL-37/CRAMP represents an important mediator of platelet activation and thrombo-inflammation.

The endogenous antimicrobial cathelicidin LL37 induces platelet activation and augments thrombus formation

“…LL37 directly induced platelet aggregation in a concentrationdependent manner, notably, 20 mM LL37 induced 100% aggregation ( Figure 1F). In line with a recent study, 21 in our experiments, LL37 failed to induce aggregation in human PRP (supplemental Figure 1). Similarly, to determine whether LL37 influences inside-out signaling to integrin aIIbb3 and a-granule secretion, the level of fibrinogen binding and P-selectin exposure was measured respectively using human isolated platelets and PRP by flow cytometry.…”

“…Immediately prior to the submission of this manuscript, a recent study demonstrated the functional impact of LL37 in priming platelets and inducing thromboinflammatory conditions, although the molecular mechanisms that regulate such effects were not fully established. 21 However, in this study, we have investigated the impact of LL37 in the modulation of platelet reactivity, thrombosis, and hemostasis under physiological conditions and uncovered the functional dependence of LL37 on FPR2/ALX in platelets.…”

“…Because the level of LL37 released during inflammation is significantly higher than normal, 19,20 understanding its critical functions in the modulation of platelet reactivity will pave the way for determining fundamental mechanisms underlying platelet-related complications in various inflammatory diseases. Immediately prior to the submission of this manuscript, Pircher et al 21 demonstrated the ability of LL37 to prime circulating platelets and induce thromboinflammation. Although they have established a significant functional impact of LL37 in platelets, its actions via FPR2/ALX were not determined.…”

Histamine deficiency facilitates coronary microthrombosis after myocardial infarction by increasing neutrophil‐platelet interactions

“…Neutrophil extracellular traps, containing decondensed chromatin and antimicrobial proteins, stimulate the intrinsic pathway of coagulation, increase platelet-dependent thrombin generation, activate platelets and increase thrombi size. 11,19,31,32 Thus, activated neutrophils play a procoagulant role in arterial thrombosis. Activated platelets excite neutrophils, which, in return, reinforces platelet activation.…”

Role of RXRβ in platelet function and arterial thrombosis

“…Arterial thrombosis in vivo was analyzed following a FeCl3‐induced vessel injury in the mouse common carotid artery as described before . Briefly, mice were anesthetized using 2% isoflurane and intraperitoneal injection of fentanyl (0.05 mg/kg), midazolam (5 mg/kg), and medetomidine (0.5 mg/kg).…”