TREM ‐1 Blockade Inhibits Inflammasome Activation and Pyroptosis: Novel Insights on the Role of TREM ‐1 and Syk in Monosodium Urate Crystal‐Induced Inflammation

Gout is associated with the upregulation of triggering receptor expressed on myeloid cells-1 (TREM-1) and soluble TREM-1 (sTREM-1). Cell activation signalling induced by monosodium urate (MSU) crystals and TREM-1 signalling both converge on spleen tyrosine kinase (Syk). The aim of this study was to decipher the role of the TREM-1 peptidomimetic inhibitor LP17, as well as Syk inhibitor, and their interaction during MSU-induced cell inflammation, focusing on NLRP3 inflammasome activation, IL-1β production and pyroptosis. In MSU-activated cells, both LP17 and Syk inhibitor (iSyk) significantly reduced the secretion of IL-1β and the release and activity of caspase-1. LP17 changed the phosphorylation of Syk, indicating that inhibition of TREM-1 modulates the activation state of Syk. Both LP17 and iSyk reduced the level of NLRP3-induced apoptosis-associated speck-like protein (ASC) transcripts and MSU-induced immunolabelling of ASC. Under confocal immunomicroscopy, TREM-1 in MSU-crystal-activated cells was localised to the same perimembranal compartment with NLRP3 inflammasome; inhibition of TREM-1 hindered the colocalisation of Syk with ASC. These results were corroborated by use of the in vitro ASC oligomerszation assay that showed that blocking TREM-1 induced Syk redistribution from ASC complexes, indicating that LP17 can repress the ability of Syk to incorporate into the forming ASC speckle. Additionally, unlike iSyk, LP17 hampered MSU-induced cleavage of gasdermin D, the hallmark of pyroptosis. Together, our findings suggest that blocking TREM-1 may prove beneficial as a novel strategy in the treatment of gout as well as other inflammasome-mediated diseases.