title: Conditional signaling by Toll-like receptor 4.
abstract: Signaling through Toll-like receptor 4 (TLR4) is thought to initiate innate and adaptive immune responses. Signaling of TLR4 is usually studied using isolated cells, which are activated by sub-nanomolar concentrations of lipopolysaccharide (LPS). However, in normal tissues, cells bearing TLR4 reside in microenvironments containing large amounts of endogenous substances that can stimulate the receptor. We developed an in vitro model system using the human cell line HEK 293 and an in vivo model using mice that have normal or that lack TLR4 receptors to study how TLR4 functions in such microenvironments. Here we report that signaling through TLR4 is strongly inhibited by intact extracellular matrix and that inhibition is abrogated and endogenous agonist(s) are liberated when the matrix is degraded. Thus, release from inhibition rather than direct stimulation by agonists such as LPS is the critical first event by which TLR4 initiates immune responses.
authors.affiliation: Transplantation Biology and Department of Pharmacology and Experimental Therapeutics, Mayo Clinic, Rochester, Minnesota 55905, USA.
authors.forename: Gregory J
authors.initials: GJ
authors.lastname: Brunn
authors.forename: Marlo K
authors.initials: MK
authors.lastname: Bungum
authors.forename: Geoffrey B
authors.initials: GB
authors.lastname: Johnson
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Platt
mesh_terms: D000818:Animals; D002460:Cell Line; D002478:Cells, Cultured; D005109:Extracellular Matrix / Q000502:physiology; D006497:Heparan Sulfate / Q000494:pharmacology; D006801:Humans; D007109:Immunity / Q000502:physiology*; D007375:Interleukin-1 / Q000494:pharmacology; D018950:Lipopolysaccharide Receptors / Q000235:genetics; D008070:Lipopolysaccharides / Q000494:pharmacology; D051218:Lymphocyte Antigen 96 / Q000235:genetics; D008264:Macrophages; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D016328:NF-kappa B / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects; D051197:Toll-Like Receptor 4 / Q000172:deficiency / Q000502:physiology*; D014162:Transfection; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
chemical_list: D006497:Heparan Sulfate; D007375:Interleukin-1; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; D016328:NF-kappa B; D051197:Toll-Like Receptor 4; D014409:Tumor Necrosis Factor-alpha; C411728:Ly96 protein, mouse