title: Toll-like receptor 7 stimulation by imiquimod induces macrophage autophagy and inflammation in atherosclerotic plaques.
abstract: Atherosclerotic plaques tend to rupture as a consequence of a weakened fibrous cap, particularly in the shoulder regions where most macrophages reside. Macrophages express Toll-like receptors to recognize pathogens and eliminate intracellular pathogens by inducing autophagy. Because Toll-like receptor 7 (TLR7) is thought to be expressed in macrophages but not in smooth muscle cells (SMCs), we investigated whether induction of macrophage autophagic death by TLR7 ligand imiquimod can affect the composition of atherosclerotic plaques in favor of their stability. Immunohistochemical staining of human carotid plaques as well as Western blotting of cultured macrophages and SMCs confirmed that TLR7 was expressed in macrophages, but not in SMCs. In vitro experiments showed that only TLR7 expressing cells underwent imiquimod-induced cell death, which was characterized by autophagosome formation. Imiquimod-treated macrophages activated nuclear factor-κB (NF-κB) and released pro-inflammatory cytokines and chemokines. This effect was inhibited by the glucocorticoid dexamethasone. Imiquimod-induced cytokine release was significantly decreased in autophagy-deficient macrophages because these cells died by necrosis at an accelerated pace. Local in vivo administration of imiquimod to established atherosclerotic lesions in rabbit carotid arteries induced macrophage autophagy without induction of cell death, and triggered cytokine production, upregulation of vascular adhesion molecule-1, infiltration of T-lymphocytes, accumulation of macrophages and enlargement of plaque area. Treatment with dexamethasone suppressed these pro-inflammatory effects in vivo. SMCs and endothelial cells in imiquimod-treated plaques were not affected. In conclusion, imiquimod induces macrophage autophagy in atherosclerotic plaques, but stimulates plaque progression through cytokine release and enhanced infiltration of inflammatory cells.
authors.affiliation: Laboratory of Physiopharmacology, University of Antwerp, Antwerp, Belgium.
authors.forename: Inge
authors.initials: I
authors.lastname: De Meyer
authors.forename: Wim
authors.initials: W
authors.lastname: Martinet
authors.forename: Dorien M
authors.initials: DM
authors.lastname: Schrijvers
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Timmermans
authors.forename: Hidde
authors.initials: H
authors.lastname: Bult
authors.forename: Guido R Y
authors.initials: GR
authors.lastname: De Meyer
mesh_terms: D000634:Aminoquinolines / Q000494:pharmacology* / Q000633:toxicity; D000818:Animals; D000893:Anti-Inflammatory Agents / Q000494:pharmacology; D001343:Autophagy / Q000187:drug effects*; D015153:Blotting, Western; D016207:Cytokines / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D006801:Humans; D000077271:Imiquimod; D007150:Immunohistochemistry; D007249:Inflammation / Q000139:chemically induced* / Q000276:immunology / Q000473:pathology; D018836:Inflammation Mediators / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects* / Q000276:immunology / Q000473:pathology; D008297:Male; D008562:Membrane Glycoproteins / Q000819:agonists* / Q000172:deficiency / Q000235:genetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D016328:NF-kappa B / Q000378:metabolism; D009336:Necrosis; D058226:Plaque, Atherosclerotic / Q000188:drug therapy* / Q000276:immunology / Q000473:pathology; D011817:Rabbits; D013997:Time Factors; D051199:Toll-Like Receptor 7 / Q000819:agonists* / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D020298:U937 Cells
chemical_list: D000634:Aminoquinolines; D000893:Anti-Inflammatory Agents; D016207:Cytokines; D018836:Inflammation Mediators; D008562:Membrane Glycoproteins; D016328:NF-kappa B; C495348:TLR7 protein, human; C477038:Tlr7 protein, mouse; D051199:Toll-Like Receptor 7; D003907:Dexamethasone; D000077271:Imiquimod