With the recent rise in obesity and related conditions in North America, atherosclerosis is likely to remain a major cause of morbidity and mortality for the coming decades. Understanding the underlying mechanisms for the onset of this disease may provide new strategies for delaying or preventing thrombotic complications associated with advanced atherosclerosis.
Atherosclerosis is initiated and accelerated by systemic risk factors such as elevated low density lipoprotein (LDL), hypertension, high blood sugar, and oxidation products from smoking. Although the entire vascular endothelium is exposed to these stimuli, atherosclerotic lesions develop preferentially at bifurcations, branch points, and inner curvatures of arteries, suggesting that local factors contribute to disease susceptibility. It is widely accepted that the complex patterns of blood flow in these regions expose the endothelium to “disturbed” hemodynamic forces (shear stress), and these may induce the expression of proinflammatory genes (1
). Our previous experiments in normocholesterolemic C57BL/6 mice focused on the transcription factor NF-κB and showed priming as well as a low level of activation of this signal transduction pathway in endothelial cells located in atherosclerosis-susceptible regions of the ascending aorta (4
). Consistent with these findings, the expression level of vascular cell adhesion molecule–1 (VCAM-1) was elevated in atherosclerosis-predisposed regions of the rabbit or mouse aorta, but was lower relative to expression induced by various inflammatory stimuli (4
). Recent gene profiling experiments with endothelial cells of the normal porcine aorta showed relative up-regulation of several other proinflammatory genes in the aortic arch, such as those of cytokines IL-1, IL-6, or the chemokine monocyte chemotactic protein–1 (MCP-1), as well as antioxidative genes, including glutathione peroxidase or microsomal glutathione S-transferase 2 (7
). In addition, changes in cell shape and proliferation or lipoprotein transport and retention in the intima were reported in atherosclerosis-susceptible regions (4
), underscoring the dramatic biological changes that accompany the low-grade expression of proinflammatory genes in these areas. Consistent with the up-regulation of chemokines and adhesion molecules in the lesion-prone areas, intimal leukocytes were found preferentially at several branch points of the normal rabbit aorta and in human carotid bifurcations, where accumulation of macrophages, dendritic cells, and T cells was reported (8
). However, the abundance of these leukocytes in atherosclerosis-predisposed versus -resistant regions and their relationship with atherosclerosis susceptibility have not been determined.
Genetic factors contribute to atherogenesis. In humans, it is well known that family history predicts cardiovascular events. Susceptibility to atherosclerosis also differs amongst inbred strains of mice. For instance, when fed an atherogenic diet, the susceptible C57BL/6 mice develop lesions in the aortic root (15
), and apolipoprotein E–deficient mice in the C57BL/6 background develop larger areas of lesions than in the resistant BALB/c or C3H/HeJ backgrounds (16
). Understanding the molecular basis for genetic susceptibility will likely lead to the identification of key elements that contribute to the development of atherosclerosis. Previous studies showed a correlation between endothelial cell responses to systemic risk factors and the genetic susceptibility of mice to atherosclerotic lesion formation (18
Mouse models have led to an understanding of key molecules involved in atherosclerotic lesion formation. They have demonstrated a critical role for proinflammatory genes, including various adhesion molecules and chemokines such as VCAM-1 and MCP-1, which mediate the accumulation of leukocytes in arterial lesions (20
). Our hypothesis is that arterial biology at lesion-predisposed regions of the normal intima recapitulates aspects of the inflammatory response in atherosclerosis, although at a lesser magnitude. The same molecules may mediate leukocyte recruitment and accumulation in the normal intima, and this inflammatory milieu may contribute to atherosclerotic lesion formation on introduction of atherosclerotic risk factors.
In this study, we demonstrated that in the normal intima, regions predisposed to atherosclerosis expressed relatively higher levels of several proinflammatory genes that have been implicated in atherogenesis and contained an abundance of dendritic cells. In contrast, macrophages and T cells were abundant throughout the entire adventitia (Adv). These data demonstrate for the first time that cellular composition and distribution are different in the intima and the Adv. The correlation between susceptibility to atherosclerosis and abundance of intimal (not adventitial) myeloid cells was not only topographic but also extended to strains with different genetic susceptibilities to atherosclerosis. We also demonstrated that bone marrow–derived monocytes are recruited to the normal aortic intima and showed that the accumulation of intimal CD68+ cells is dependent on the expression of VCAM-1. These data demonstrate low-grade chronic inflammation in atherosclerosis-predisposed regions of the normal arterial intima and suggest that mechanisms contributing to this process are analogous to those found in atherosclerosis.