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.

Toll-like Receptor 7 Rapidly Relaxes Human Airways

“…Furthermore, despite TLR7's profound bronchodilatory effect in the airways, TLR7 stimulation had no effect on blood pressure (mean arterial pressure) or heart rate. TLR7 agonists are unlikely to have cardiovascular effects in humans, either, given that human systemic and pulmonary vascular endothelium does not express TLR7 (47,48). A lack of systemic effects is advantageous for designing TLR7-targeted therapies for the airways.…”

WITHDRAWN: Cyclophilin a regulates autophagy of macrophage through PI3K/Akt/mTOR signaling

“…During AS evolution, changes in macrophages are closely related with autophagy, which is involved in occurrence and development of the disease [3]. Correspondingly, recent investigations demonstrated that macrophage autophagy is involve in a novel pathway through which these cells contribute to vascular diseases [4][5][6][7].…”

Autophagy in Vascular Disease

“…Accordingly, local administration of imiquimod in cholesterol-fed rabbits does not result in stabilization of atherosclerotic plaques, as observed for rapalogs, but triggers inflammation and plaque progression. 126 Moreover, some well-known autophagy inducers (eg, lithium chloride) do not induce autophagy in the vessel wall, but stimulate apoptosis, 127 which is detrimental for the stability of advanced atherosclerotic plaques.…”