Macrophage small RNA export to HDL protects against inflammation and atherosclerosis

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Summary: Atherosclerosis is the underlying cause for ischemic cardiovascular disease (CVD), a pathology best characterized as uncontrolled inflammation in response to continual lipid deposition in the arterial wall. Despite the importance of inflammation in the progression of atherosclerosis, very few therapies are designed to target this process due to a general lack of knowledge of atherogenic stimuli outside of bioactive lipids. Previously, we have reported that lipoproteins, namely HDL and LDL, transport small non-coding RNAs (sRNA), and recently, we discovered that lipoproteins are highly-enriched with sRNAs derived from bacterial and fungal species in the microbiome and environment. Interestingly, these foreign, microbial sRNAs (msRNAs) are also highly-abundant in macrophages within the atherosclerotic lesions and activate sRNA-sensing toll-like receptors (TLR)7/8 in lesion macrophages. We posit that HDL accepts msRNAs from lesion macrophages, and thus, suppresses TLR7/8 activation and down-stream pro-inflammatory gene expression. Supporting this model, macrophages were found to readily export msRNAs to HDL and HDL treatments were found to block msRNA-induced activation. In CVD, HDL acquire reactive dicarbonyl modifications which are associated with HDL dysfunction. In preliminary studies, we found that reactive dicarbonyl modifications on HDL, e.g. isolevuglandins (IsoLG), impair HDL-msRNA export. Moreover, we found that IsoLG-modified HDL had increased macrophage uptake and retention and induced pro-inflammatory gene expression. Preliminary studies also found that treating mouse models with reactive scavengers to block IsoLG modifications decreased atherosclerosis. Therefore, we aimed to block msRNA activation of TLR7/8 using non-targeting locked-nucleic acids (ntLNA). Strikingly, we found that ntLNA treatments significantly reduced atherosclerotic lesion area by 30% over 4 weeks in Apoe-/- mice on western diet. In addition, single-cell RNA sequencing approaches demonstrated that ntLNA therapy repressed pro-inflammatory gene expression in lesion macrophages and reduced the number of pro-inflammatory macrophages in atherosclerotic lesions. Based on preliminary studies, we hypothesize HDL removes msRNA from lesion macrophages through retro-endocytosis, a process that is inhibited by IsoLG modifications on HDL. Furthermore, we posit that msRNA activation of endo-lysosome TLR7/8 in lesion macrophages can be readily targeted with ntLNAs to reduce atherosclerosis. To test this hypothesis, we aim to I.) Determine the mechanism(s) and consequences of macrophage msRNA export to HDL, the impact of HDL-msRNA export on macrophage activation, and define the HDL-mediated reverse sRNA transport (RsRT) pathway in vivo, II.) Define the impact of dicarbonyl modifications and msRNA cargo on HDL dysfunction in CVD, and III.) Target macrophage msRNA receptors to inhibit atherosclerosis progression and promote regression. This project will open entirely new fields of study for lipoproteins, extracellular sRNAs, host/non-host interactions, and inflammation, which have applicability to many other diseases, but is particularly critical for CVD.