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 Ca 2+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.

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.