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Toll-like receptor 7 stimulation by imiquimod induces macrophage autophagy and inflammation in atherosclerotic plaques

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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.

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Acknowledgments

The authors are indebted to Dr. Chantal de Groote, Rita Van Den Bossche, Anne-Elise Van Hoydonck, Hermine Fret, Cédéric Michiels, Lieve Svensson, Francis Terloo and Isabel Pintelon for excellent technical assistance. We thank Prof. P. Van Schil and Dr. E. Van Vré (University Hospital Antwerp) for collecting and processing the human carotid endarterectomy specimens. This work was supported by the Agency for Innovation by Science and Technology in Flanders (IWT), the Fund for Scientific Research (FWO)-Flanders and Bijzonder Onderzoeksfonds of the University of Antwerp (TOP-BOF). The Tecnai G2 Spirit BioTWIN TEM was purchased with support of the Hercules Foundation (Hercules Type 2) [AUHA004].

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The authors declare that they have no conflict of interest.

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Authors and Affiliations

  1. Laboratory of Physiopharmacology, University of Antwerp, Antwerp, Belgium

    Inge De Meyer, Wim Martinet, Dorien M. Schrijvers, Hidde Bult & Guido R. Y. De Meyer

  2. Laboratory of Cell Biology and Histology, University of Antwerp, Antwerp, Belgium

    Jean-Pierre Timmermans

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  1. Inge De Meyer
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  2. Wim Martinet
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  3. Dorien M. Schrijvers
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  6. Guido R. Y. De Meyer
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Correspondence to Guido R. Y. De Meyer.

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De Meyer, I., Martinet, W., Schrijvers, D.M. et al. Toll-like receptor 7 stimulation by imiquimod induces macrophage autophagy and inflammation in atherosclerotic plaques. Basic Res Cardiol 107, 269 (2012). https://doi.org/10.1007/s00395-012-0269-1

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