M1 macrophages are an early feature of shear stress modulated vulnerable atherosclerotic plaques

Purpose: Atherosclerosis occurs in vascular sites subjected to complex flow, causing oscillatory shear stress (OSS) or low shear stress (LSS). Macrophages can be skewed towards a pro-inflammatory phenotype (called M1) or a regulatory one (broadly named M2) and may determine atherosclerotic plaque outcome. The aim of this study is to establish the polarisation of macrophages in different plaque phenotypes and shear stress conditions. Methods: A perivascular shear stress modifying cast was tied around a carotid artery for 6 or 9 weeks, mimicking LSS and OSS in ApoE-/- mice fed a high fat diet (HFD) from 17 weeks of age. We examined the expression of the pan-macrophage marker CD68 and polarisation markers iNOS (M1), CD206 (M2) and HO-1 (Mox subset) in the aortic root at 12, 20, 28, 35 and 53 weeks of age, and cast-treated carotid artery by Immunohistochemistry and Confocal Immunolocalisation Results: In the aortic root, most CD68+ cells stained positively for HO-1, particularly at 28 weeks (36.3±1.2% of lesion area; p<0.001) while iNOS and CD206 were expressed by few macrophages (max 5.6±2.3% and 4.2±1.1% at 20 weeks respectively). In stark contrast, most macrophages in early LSS-induced carotid artery lesions express iNOS (23.6±5.1% vs. 5.4±2.7%; p=0.006) while at 9 weeks, HO-1 expression becomes more abundant. Conclusions: The majority of macrophages in aortic root lesions express HO-1 (Mox phenotype). While in the carotid artery, M1 macrophages are significantly more prevalent in the low shear stress-modulated plaque. These differences suggest low shear stress promotes polarisation towards the M1 macrophage phenotype.