Lysophospholipid sensing triggers secretion of flagellin from pathogenic salmonella.

Flagellin induces inflammatory and innate immune responses through activation of Toll-like receptor 5. Here we show that proinflammatory monomeric flagellin produced by salmonella during infection of intestinal epithelial cells was not derived from polymeric bacterial cell wall-associated flagellum but instead was synthesized and secreted de novo by the bacterium after direct sensing of host-produced lysophospholipids. Inhibition of lysophospholipid biosynthesis in intestinal epithelial cells reduced flagellin production and release from salmonella. Lysophospholipids induced a cAMP-dependent signaling pathway in salmonella that resulted in production and secretion of active flagellin. The induction of Toll-like receptor ligand synthesis and secretion by a host signal represents a previously unknown regulatory mechanism for inflammation and innate immunity during infection with a bacterial pathogen.