Nonhemopoietic Cell TLR4 Signaling Is Critical in Causing Early Lipopolysaccharide-Induced Ileus

Abstract

Endotoxin-mediated ileus is poorly understood. Our objective was to mechanistically investigate the role of cell-specific TLR4 expression/signaling in causing gastrointestinal dysmotility. TLR4 chimeras and CSF-1-dependent macrophage-deficient mice were subjected to i.p. ultrapure (UP)-LPS (5 mg/kg). At 6 h, gastric emptying and gastrointestinal transit assessed in vivo motility, and jejunal circular muscle contractility was measured in vitro. Muscularis infiltration of neutrophils and monocytes were counted, and intestinal muscularis inflammatory mediators were quantified by quantitative PCR. Demonstrating TLR4 dependency, UP-LPS-induced gastric stasis and ileus of TLR4(WT) mice were absent in mutant TLR4(LPS-d) mice. Unexpectedly, engraftment of TLR4-mutant bone marrow into TLR4-competent mice (bmTLR4(LPS-d)/TLR4(WT)) exhibited a significant transit delay to UP-LPS similar to bmTLR4(WT)/TLR4(WT) mice. CSF-1(-/-) mice were not protected from ileus. Contrary, UP-LPS-treated bmTLR4(WT)/TLR4(LPS-d) and bmTLR4(LPS-d)/TLR4(LPS-d) mice had normal transit. No leukocytic infiltration was detected at 6 h. Spontaneous jejunal contractions were markedly suppressed in UP-LPS-treated TLR4-competent mice, but bethanechol-stimulated contractions were not altered by UP-LPS in any group. UP-LPS-induced inflammatory mRNAs in a TLR4-dependent manner, but TLR4 mRNA itself was not significantly altered. In chimera mice, UP-LPS induction of IL-1beta and IL-10 were hemopoietic dependent, and GM-CSF was nonhemopoietic dependent, whereas IL-6 and inducible NO synthase were derived from both cell types. Hemopoietic and nonhemopoietic cells contribute to TLR4-sensitive muscularis inflammatory signaling, but nonhemopoietic TLR4 signaling plays an exclusive primary role in causing functional UP-LPS-induced gastric stasis and ileus. Direct LPS suppression of spontaneous contractility participates in mediating early TLR4-transduced dysmotility.