Ornithine Lipid is a Partial TLR4 Agonist and NLRP3 Activator

Myeloid cells recognise Gram-negative bacterial lipopolysaccharides (LPS). LPS recognition triggers inflammatory reactions through Toll-like receptor 4 (TLR4) and primes the cells for inflammasome activation. In phosphate-depleted environments, bacteria cannot produce LPS. Instead, they increase their synthesis of ornithine lipid (OL), which is constitutively present in some pathogenic Gram-negative and -positive bacteria but absent in commensals. OL is implicated in bacterial pathogenicity, but the mechanism is unclear. Using primary murine macrophages and human peripheral blood mononuclear cells, we identify OL as a partial TLR4 agonist and an NLRP3 inflammasome activator. For this, OL directly activates TLR4 and indirectly activates NLRP3 in a potassium-efflux-dependent manner. OL also upregulates the expression of NLRP3 and pro-IL-1beta and induces cytokine secretion in primed and unprimed cells. By contrast, in the presence of LPS, OL functions as a partial TLR4 antagonist; LPS-induced TLR4 activation and inflammasome priming are inhibited by OL, leading to reduced TNF and IL-1beta secretion. We thus suggest that in phosphate-depleted environments OL replaces LPS bacterial immunogenicity, while constitutively present OL may allow bacteria to escape immune surveillance. ### Competing Interest Statement As cofounders of Viva in vitro diagnostics LHN and PP declare that the research presented herein was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. PP is scientific advisor of Viva in vitro diagnostics. KS is a co-inventor on patent applications for NLRP3 inhibitors licensed to Inflazome Ltd., a company headquartered in Dublin, Ireland. Inflazome is developing drugs that target the NLRP3 inflammasome to address unmet clinical needs in inflammatory disease. K Schroder served on the Scientific Advisory Board of Inflazome from 2016 to 2017 and served as a consultant to Quench Bio, USA and Novartis, Switzerland. The authors have no additional financial interests.