Activation of lpxR gene through enterohaemorrhagic Escherichia coli virulence regulators mediates lipid A modification to attenuate innate immune response.

During the course of infection, pathogens must overcome a variety of host defence systems. Modulation of lipid A, which is a strong stimulant for host immune systems, is one of the strategies used by microorganisms to evade the host response. The lpxR gene, which encodes a lipid A 3'-O-deacylase, is commonly found in several pathogens and has been shown to reduce the inflammatory response. Here, we demonstrated that the lpxR gene of enterohaemorrhagic Escherichia coli (EHEC) was positively regulated by two virulence regulators, Pch and Ler, and that this regulation was coordinated with the locus of enterocyte effacement genes, which encode major virulence factors for colonisation. The lpxR promoter was repressed by the binding of H-NS, but the competitive binding of both regulators resulted in transcription activation. Next, we showed that lipid A from the lpxR mutant was more stimulatory of the inflammatory response in macrophage-like cells than lipid A from wild-type EHEC. Furthermore, phagocytic activity and phagosome maturation in host cells infected with the lpxR mutant were increased in a p38 mitogen-activated protein kinase-dependent manner in comparison with wild-type EHEC infection. Finally, we demonstrated that the pch mutant, which is deficient in activation of the locus of enterocyte effacement genes, was phagocytised more efficiently than the wild type. Thus, EHEC modulates lipid A to dampen the host immune response when activating virulence genes for colonisation.