Toll-Like Receptor 7 Stimulation Promotes the Development of Atherosclerosis in Apolipoprotein E-Deficient Mice.

Atherosclerosis is a chronic inflammatory disease with multiple characteristic facets, including vascular inflammation, endothelial dysfunction, plaque development, impaired blood flow, and cholesterol deposition through dyslipidemia. Toll-like receptors (TLRs) of the innate immune system have been closely linked to the development of atherosclerotic lesions. TLR7 recognizes viral or endogenous single-stranded RNA, which is released during vascular apoptosis and necrosis. The role of TLR7 in vascular disease remains controversial, and therefore, we sought to investigate the effects of TLR7 stimulation in mice.Intravenous injection of a ligand for TLR7 (R848) induced a significant pro-inflammatory cytokine response in mice. This was associated with impaired reendothelialization upon acute denudation of the carotid artery, as measured by Evan's blue staining, and increased numbers of circulating endothelial microparticles (EMPs) and circulating Sca1/Flk1 positive cells as a marker for increased endothelial damage. Chronic subcutaneous stimulation of TLR7 in apolipoprotein E-deficient (ApoE-/-) mice increased aortic production of reactive oxygen species (ROS), the number of circulating EMPs, and most importantly, augmented the formation of atherosclerotic plaque when compared with vehicle-treated animals.Systemic stimulation of TLR7 leads to impaired reendothelialization upon acute vascular injury and is associated with the production of pro-inflammatory cytokines and increased levels of circulating EMPs and Sca1/Flk1 positive cells. Importantly, ApoE-/- mice chronically treated with R848 displayed increased atherosclerotic plaque development and elevated levels of ROS in the aortic tissue. In addition, TLR7-activation-induced apoptosis and impaired migration in human coronary artery endothelial cells and showed significant upregulation of the signaling cascade of IL-1 receptor-associated kinase (IRAK) 2 and IRAK4. Our data highlight the importance of fully understanding the pathomechanisms involved in atherogenesis, and further studies are necessary to identify the ligand-specific effects of TLR7 for possible therapeutic targeting.