Potentiation of NETs release is novel characteristic of TREM-1 activation and the pharmacological inhibition of TREM-1 could prevent from the deleterious consequences of NETs release in sepsis.

During sepsis, neutrophil activation induces endothelial cell (EC) dysfunction partly through neutrophil extracellular trap (NET) release. The triggering receptor expressed on myeloid cell-1 (TREM-1) is an orphan immune receptor that amplifies the inflammatory response mediated by Toll-like receptor-4 (TLR4) engagement. Although the key role of TLR4 signaling in NETosis is known, the role of TREM-1 in this process has not yet been investigated. Here, we report that TREM-1 potentiates NET release by human and murine neutrophils and is a component of the NET structure. In contrast, pharmacologic inhibition or genetic ablation of TREM-1 decreased NETosis in vitro and during experimental septic shock in vivo. Moreover, isolated NETs were able to activate ECs and impair vascular reactivity, and these deleterious effects were dampened by TREM-1 inhibition. TREM-1 may, therefore, constitute a new therapeutic target to prevent NETosis and associated endothelial dysfunction.

Inhibition of triggering receptor expressed on myeloid cells-1 impairs thrombin generation.

BACKGROUND: New evidence has shown the link between inflammation and thrombosis. Triggering receptor expressed on myeloid cells-1 (TREM-1) is an immunoreceptor expressed mostly on neutrophils and monocytes/macrophages. TREM-1 acts as an amplifier of the inflammatory response, and its pharmacological inhibition displays protective effects in various models of inflammatory disorders, in particular by dampening coagulation abnormalities and thrombocytopenia observed during acute inflammation. OBJECTIVES: We aimed to decipher the role of TREM-1 in fostering thrombin generation. METHODS: We measured thrombin generation (TG) by the use of calibrated automated thrombography with whole blood, and isolated primary human neutrophils and monocytes upon stimulation with lipopolysaccharide (LPS). Tissue factor (TF) expression was measured by flow cytometry and its activity by ELISA. Phosphatidylserine (PtdSer) exposure was determined by flow cytometry. A dodecapeptide (LR12) was used as a specific inhibitor of TREM-1. RESULTS: LPS increased TG, TF expression, and activity, as well as the exposure of PtdSer on the surface of monocytes. LR12 dampened TF activity through the decrease of PtdSer exposure, leading to a reduction of thrombin generation. CONCLUSIONS: TREM-1 inhibition decreases thrombin generation and could be an interesting target for the development of new inhibitors of leukocyte-associated thrombotic activity.

Triggering Receptor Expressed on Myeloid Cells-1 Inhibitor Targeted to Endothelium Decreases Cell Activation.

TREM-1 (Triggering Receptor Expressed on Myeloid cells-1) is an immunoreceptor expressed on neutrophils, monocytes/macrophages, and endothelial cells. It amplifies the inflammatory response driven by Toll-Like Receptors (TLR) engagement. The pharmacological inhibition of TREM-1 confers protection in several pre-clinical models of acute inflammation. In this study, we aimed to investigate the role of TREM-1 in endothelial cells using a sneaking ligand construct (SLC) inhibiting TREM-1 in the endothelium. The SLC was made of 3 modules: an E-selectin targeting domain, a Pseudomonas aeruginosa exotoxin a translocation domain, and a 7 aa peptide (LSKSLVF) that contains the interaction site between TREM-1 and its adaptor protein DAP-12. SLC peptide was effectively picked up by endothelial cells following LPS stimulation. It decreased LPS induced TREM-1 up-regulation and cell activation, neutrophils extravasation, and improved median survival time during experimental peritonitis in mice. We reported that a targeted endothelial TREM-1 inhibition is able to dampen cell activation and to confer protection during septic shock in mice. The use of such cell-specific, ligand- independent TREM-1 inhibitors deserve further investigations during acute or chronic inflammatory disorders.

TREM-1 multimerization is essential for its activation on monocytes and neutrophils.

The triggering receptor expressed on myeloid cells-1 (TREM-1) is a receptor expressed on innate immune cells. By promoting the amplification of inflammatory signals that are initially triggered by Toll-like receptors (TLRs), TREM-1 has been characterized as a major player in the pathophysiology of acute and chronic inflammatory diseases, such as septic shock, myocardial infarction, atherosclerosis, and inflammatory bowel diseases. However, the molecular events leading to the activation of TREM-1 in innate immune cells remain unknown. Here, we show that TREM-1 is activated by multimerization and that the levels of intracellular Ca2+ release, reactive oxygen species, and cytokine production correlate with the degree of TREM-1 aggregation. TREM-1 activation on primary human monocytes by LPS required a two-step process consisting of upregulation followed by clustering of TREM-1 at the cell surface, in contrast to primary human neutrophils, where LPS induced a rapid cell membrane reorganization of TREM-1, which confirmed that TREM-1 is regulated differently in primary human neutrophils and monocytes. In addition, we show that the ectodomain of TREM-1 is able to homooligomerize in a concentration-dependent manner, which suggests that the clustering of TREM-1 on the membrane promotes its oligomerization. We further show that the adapter protein DAP12 stabilizes TREM-1 surface expression and multimerization. TREM-1 multimerization at the cell surface is also mediated by its endogenous ligand, a conclusion supported by the ability of the TREM-1 inhibitor LR12 to limit TREM-1 multimerization. These results provide evidence for ligand-induced, receptor-mediated dimerization of TREM-1. Collectively, our findings uncover the mechanisms necessary for TREM-1 activation in monocytes and neutrophils.

Effects of Porphyromonas gingivalis LPS and LR12 peptide on TREM-1 expression by monocytes.

Periodontal disease involves the activation of host immune response, acting not only as defender of periodontal tissues against bacterial aggression but also as mediator of tissue destruction. Triggering receptor expressed on myeloid cells 1 (TREM-1) is an immune receptor that synergizes with Toll-like receptors in amplifying the inflammatory response mediated by microbial molecules. AIM: To investigate the role of P. gingivalis lipopolysaccharide (LPS) and the effect of LR12, a TREM-1 inhibitory peptide, on the expression of membrane-bound and soluble form of TREM-1 on human primary monocytes, as well as the production of proinflammatory cytokines. MATERIAL AND METHODS: Cells were stimulated with 1 µg/ml of LPS with or without LR12. PCR, flow cytometry and ELISA were used to determine TREM-1 expressions and cytokines release by monocytes. RESULTS: P. gingivalis LPS can induce a significant increase in TREM-1 expression (mRNA, membrane-bound and soluble form, p < 0.001) as well as cytokines (IL-1ß, TNFα) and chemokines (IL-8) production by monocytes. This monocytes' activation was partly prevented by LR12. CONCLUSIONS: TREM-1 inhibitors such as LR12 could be interesting for the modulation of the excessive inflammatory response that occurs during periodontal disease.

Targeted endothelial gene deletion of triggering receptor expressed on myeloid cells-1 protects mice during septic shock.

Aims: TREM-1 (Triggering Receptor Expressed on Myeloid cells-1) is an immunoreceptor expressed on neutrophils and monocytes/macrophages whose role is to amplify the inflammatory response driven by Toll-Like Receptors engagement. The pharmacological inhibition of TREM-1 confers protection in several pre-clinical models of acute inflammation. In this study, we aimed to decipher the role of TREM-1 on the endothelium. Methods and results: We first showed by qRT-PCR, flow cytometry and confocal microscopy that TREM-1 was expressed in human pulmonary microvascular endothelial cells as well as in mouse vasculature (aorta, mesenteric artery, and pulmonary vessels). TREM-1 expression was upregulated following septic insult. We next observed that TREM-1 engagement impaired mouse vascular reactivity and promoted vascular inflammation. The pharmacological inhibition of TREM-1 (using the synthetic inhibitory peptide LR12) prevented these disorders both in vitro and in vivo. We generated endothelium-conditional Trem-1 ko mice (EndoTREM-1-/-) and submitted them to a caecal ligation and puncture-induced septic shock. As compared with wild-type littermates, targeted endothelial Trem-1 deletion conferred protection during septic shock in modulating inflammatory cells mobilization and activation, in restoring vasoreactivity, and in improving the survival. Conclusion: We reported that TREM-1 is expressed and inducible in endothelial cells and plays a direct role in vascular inflammation and dysfunction. The targeted deletion of endothelial Trem-1 conferred protection during septic shock in modulating inflammatory cells mobilization and activation, restoring vasoreactivity, and improving survival. The effect of TREM-1 on vascular tone, while impressive, deserves further investigations including the design of endothelium-specific TREM-1 inhibitors.

Triggering Receptor Expressed on Myeloid cells-1: a new player in platelet aggregation.

Triggering Receptor Expressed on Myeloid cells-1 (TREM-1) is an immunoreceptor initially known to be expressed on neutrophils and monocytes/macrophages. TREM-1 acts as an amplifier of the inflammatory response during both infectious and aseptic inflammatory diseases. Another member of the TREM family, The Triggering receptor expressed on myeloid cells Like Transcript-1 (TLT-1) is exclusively expressed in platelets and promotes platelet aggregation. As the gene that encodes for TLT-1 is located in the TREM-1 gene cluster, this prompted us to investigate the expression of TREM-1 on platelets. Here we show that TREM-1 is constitutively expressed in α-granules and mobilised at the membrane upon platelet activation. Pharmacologic inhibition of TREM-1 reduces platelet activation as well as platelet aggregation induced by collagen, ADP, and thrombin in human platelets. Aggregation is similarly impaired in platelets from Trem-1-/- mice. In vivo, TREM-1 inhibition decreases thrombus formation in a carotid artery model of thrombosis and protects mice during pulmonary embolism without excessive bleeding. These findings suggest that TREM-1 inhibition could be useful adducts in antiplatelet therapies.