Prevalence of M4 macrophages within human coronary atherosclerotic plaques is associated with features of plaque instability

Abstract

Background: The platelet chemokine CXCL4 induces monocyte differentiation resulting in a macrophage phenotype called "M4", which co-expresses CD68, MMP7, and S100A8. We hypothesized that M4 macrophages are associated with plaque destabilization.

Methods: Atherosclerotic arteries were obtained from explanted hearts of patients with severe coronary artery disease (CAD, n = 32) and of patients with dilated cardiomyopathy and no or mild CAD (controls, n = 19). Coronary arteries were stained with H&E, and immuno-fluorescence was performed against CD68, MMP7, and S100A8.

Results: Both CD68(+) macrophages representing the entire macrophage population and MMP7(+)S100A8(+)CD68(+) M4 macrophages could be reproducibly identified within all arterial layers. The average proportion of the M4 macrophage phenotype amongst all CD68(+) macrophages was 31.7 ± 16.2%. The highest number of M4 macrophages was found in the adventitia, followed by the intima. CD68(+) and M4 macrophage numbers were significantly higher in patients with severe CAD. The presence of M4 macrophages within the intima and the media was significantly associated with plaque instability as determined by Stary class. Multivariate analysis showed a highly significant contribution of cardiovascular risk factors (P = 0.008) to plaque instability, while only trends were observed for age (P = 0.060) and intimal prevalence of M4 macrophages (P = 0.098).

Conclusions: We demonstrate for the first time that M4 macrophages can be reproducibly found in coronary artery plaques. The prevalence of M4 macrophages is associated with indexes of plaque instability, most likely representing a surrogate marker of inflammatory activity. These findings suggest a pathogenetic role of M4 macrophages in vulnerable atherosclerotic plaques.

Keywords: Atherosclerosis; Coronary artery disease; Macrophage; Unstable plaque.