CCR5 deficiency is not protective in the early stages of atherogenesis in apoE knockout mice

Abstract

The accumulation of macrophages and T lymphocytes in vessel walls is a hallmark of atherogenesis. It has recently been demonstrated in mouse models of atherosclerosis that full disease potential is dependent on several regulators of leukocyte trafficking, including the chemokine monocyte chemotactic protein 1 (MCP-1) and the chemokine receptors CCR2 and CXCR2. A possible role for the chemokine receptor CCR5 in atherogenesis has been suggested by CCR5 expression on macrophages, T cells, coronary endothelial cells and aortic smooth muscle cells and by the presence of CCR5 ligands in atherosclerotic plaques. Moreover, individuals who are naturally deficient in CCR5 were reported to be at reduced risk for severe coronary artery disease (CAD) and early myocardial infarction (MI). To investigate whether CCR5 is pro-atherogenic in mice, we generated CCR5-deficient mice and crossed them with atherosclerosis-prone apoE-deficient mice. Although CCR5-deficient mice exhibit defects in induced macrophage trafficking, mean atherosclerotic lesion area did not differ significantly between apoE-deficient mice and apoE/CCR5-deficient mice after 16 weeks on a diet of normal chow. Ribonuclease protection assays (RPA) on RNA isolated from plaques from both apoE-deficient and apoE/CCR5-deficient animals showed strong signals for the macrophage marker F4/80 but no evidence for expression of prominent markers of T and B lymphocytes. These results indicate that the early stages of plaque formation in this model of lipid-mediated atherogenesis do not depend on CCR5.

Introduction

Atherosclerosis is a progressive vascular disease characterized by the early and persistent presence within arterial walls of macrophages, T lymphocytes and vascular dendritic cells [1], [2], [3], [4]. The increasing awareness that inflammatory mechanisms underlie atherogenesis has recently led to novel experimental strategies in mouse models to inhibit plaque initiation and progression. Interfering with macrophage and T cell adhesion to the endothelium resulted in large reductions in atherosclerotic fatty streak formation, an early event in plaque development [5], [6]. Also, atherosclerosis-prone mice with defective leukocyte trafficking because of disruptions in the genes for the chemokine monocyte chemotactic protein 1 (MCP-1) or the chemokine receptors CCR2 and CXCR2 have plaques that are smaller and less mature [7], [8], [9], [10]. CCR5 is another chemokine receptor that could potentially be involved in plaque development. CCR5 is expressed on macrophages, T cells, aortic smooth muscle cells and coronary endothelial cells [11], [12], [13], [14] and CCR5 ligands have been detected in plaques [15]. Furthermore, recent genetic screening studies showed that natural deficiency in CCR5 protects individuals from early myocardial infarction (MI) [16] and severe coronary artery disease (CAD) [17]. In the present report, we disrupted the mouse CCR5 gene to investigate whether the absence of CCR5 would have a measurable effect on early lesion formation in the apoE-deficient mouse model of hypercholesterolemia and spontaneous atherosclerotic plaque development [18].