Transcriptomic Analysis Reveals That Intimal Monocytes Drive Inflammation In Early Atherogenesis

Hypercholesterolemia (HC) is a key risk factor for atherosclerosis, a chronic inflammatory disease that causes myocardial infarction and stroke. Although all regions of arteries are exposed to HC, atherosclerosis develops in discrete regions, such as the lesser curvature (LC) of the mouse aortic arch. In Ldlr -/- mice HC induces intimal myeloid cells to develop into the foam cells of nascent lesions. The purpose of this study is to characterize the transcriptomic changes occurring in the aortic intimal myeloid cells during the earliest stages of atherogenesis, and correlate them to cellular responses. Intimal cells were isolated from the LC of the ascending aortic arch of Ldlr -/- mice at 0, 5, 14, and 56 days of CRD feeding using en face enzymatic digestion and cell microisolation. Bulk RNA-seq and RT-qPCR revealed that genes associated with lipid-loaded macrophages (e.g., Abcg1 , Lgals3 ) were progressively elevated over time. Atherogenesis-associated inflammatory transcripts and pathways, such as leukocyte transendothelial migration, cell adhesion, and cytokine/chemokine signalling were significantly elevated after 14d of CRD, but not at 5d. Single cell transcriptomic analysis of the mouse aortic arch intima of Ldlr -/- mice revealed multiple sub-populations of myeloid cells. With our intima specific approach, we have uncovered a diverse landscape of intimal myeloid cells; including monocytes, macrophages, foam cells, DCs, and neutrophils. Atherogenesis-associated inflammatory transcripts were scarcely detected in macrophage and foam cell clusters, but were prominent in non-macrophage cell types. Ccr2 + intimal monocytes exhibited a hypercholesterolemia induced increase in inflammatory pathways within 5 days of CRD feeding. Furthermore, analysis of monocyte and macrophage clusters suggests that this inflammatory signature dissipates as recently recruited monocytes transition into a macrophage/foam cell fate. We are using Ccr2 + fate mapping models, to interrogate the hypercholesterolemia-induced transition of inflammatory monocytes within a growing lesion. We suggest that the atherogenesis-associated inflammation may be initiated by recently recruited monocytes, and may not originate directly from lipid loaded macrophages.