Ornithine Lipid Activates Both TLR4 and the non-canonical NLRP3 Inflammasome.
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-1{beta} 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-1{beta} 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.