Inflammation within the atherosclerotic plaque is mediated by specific chemokines that recruit target inflammatory cells into the developing atheroma. In this study, we measured levels of 4 novel chemokines in a large, population-based sample, and correlated these levels with traditional atherosclerosis risk factors and with markers of subclinical coronary and peripheral atherosclerosis. Of the 4 chemokines measured, the most robust evidence for an association with atherosclerosis emerged with CCL23; this cytokine associated with a number of traditional risk factors as well as with CAC independent of these risk factors. We also found associations of CCL23 with atherosclerosis in the abdominal aorta in univariable analyses, but these were attenuated and no longer statistically significant after adjustment for traditional risk factors. To our knowledge, this is the first study evaluating plasma levels of CCL23 as a cardiac biomarker. In addition, we found that levels of CXCL1 associated with CAC in unadjusted analysis, but the association was of only borderline statistical significance after adjusting for traditional risk factors. We observed no association of CXCL2 and CCL11 with any of the atherosclerotic phenotypes.
Only limited data are available from studies measuring circulating CCL23 in humans. In patients with rheumatoid arthritis, levels of CCL23 have been shown to correlate with C-reactive protein, erythrocyte sedimentation rate, and rheumatoid factor (Inmaculada Rioja and others 2008
). This cytokine has not been well-studied in animal models of atherosclerosis, as no mouse homolog for this cytokine has yet been identified. Moreover, no reports have been published assessing its association with coronary risk factors or atherosclerosis in humans. Our findings of associations between plasma levels of CCL23 and CAC should prompt further exploration to identify a mouse homolog for this cytokine that would permit more in depth exploration of the role of this cytokine in atherosclerotic plaque development and progression. In humans, identification of genetic variants in the CCL23 gene that cause variation in circulating plasma levels would facilitate Mendelian Randomization experiments that could help to evaluate a potential causal role in atherosclerosis (Shah and de Lemos 2009
In contrast to CCL23, CXCL1 has been more extensively studied in atherosclerosis. One study reported higher circulating levels of CXCL1 among patients with unstable angina compared to patients with stable angina (Breland and others 2008
). Moreover, CXCL1 has been identified within human atherosclerotic plaques (Bechara and others 2007
; Breland and others 2008
). In vitro
studies suggest that CXCL1 promotes uptake of LDL by macrophages and secretion of matrix metalloproteinases from vascular smooth muscle cells (Breland and others 2008
). We observed a univariable association of CXCL1 with CAC that was attenuated and of borderline significance after multivariable adjustment for traditional risk factors. This finding suggests that if CXCL1 is involved in atherosclerosis development and progression, it may lie within the etiologic pathways linking risk factors with disease progression.
CXCL2 has similar activity as CXCL1, but much less is known about its role in atherosclerosis. In our analysis, we did not demonstrate an association between CXCL2 and CAC. The discordant associations between CXCL1 and CXCL2 with CAC and relatively modest correlations with each other suggest distinct roles of these 2 closely related GRO cytokines in atherosclerosis.
Interestingly, we found no association between CCL11 and any atherosclerosis phenotype. Prior studies have reported inconsistent associations of this eosinophil chemotactic protein with coronary heart disease. CCL11 has been shown to be up-regulated within the arterial wall of patients with atherosclerosis compared to normal controls (Haley and others 2000
; Economou and others 2001
). Levels of CCL11 have been associated with clinically significant coronary artery disease (based on a history of MI) (Ardigo and others 2007
) and with coronary angiographic evidence of atherosclerosis (Emanuele and others 2006
). In contrast, other investigators have not found an association between levels of CCL11 and atherosclerosis (Mosedale and others 2005
; Sheikine and others 2006
). Our findings within the largest and most ethnically diverse study population reported for CCL11 provide definitive null data with regard to the association between this cytokine and subclinical atherosclerosis. These null data should deter future epidemiologic research with this cytokine in the atherosclerotic process.
Potential role of CCL23 in atherosclerosis
CCL23 is a potent chemoattractant for monocytes, while having less activity for eosinophils and neutrophils (Forssmann and others 1997
). Several alternative splice variants of this chemokine have been identified that may have a role in human disease. CCL23 was first isolated from aortic endothelium (Patel and others 1997
), and we speculate that individuals with higher circulating levels of CCL23 may also have increased secretion of this protein from endothelial cells within the atherosclerotic plaque, although this has not yet been established. This chemokine pathway may operate in parallel or in synergy with other chemokine pathways recruiting monocytes into the vascular wall, including MCP-1 (CCL2), CCL4, and CCL5 (Charo and Ransohoff 2006
; Mestas and Ley 2008
; Zernecke and others 2008
). Our findings of a correlation between CCL23 and MCP-1 support this hypothesis. We also observed a robust association of CCL23 with sESAM, a junctional adhesion molecule, but only weak associations with sVCAM-1 and sICAM-1. CCL23 acts upon activated lymphocytes (Forssmann and others 1997
) as well, a finding that is reinforced by its strong correlation with LTβR, a receptor that binds activated lymphocytes and ensures efficient trafficking to sites of inflammation (Luther and others 2000
; Drayton and others 2003
; Browning and others 2005
). We observed modest positive correlations of CCL23 with LDL-C, triglycerides, and oxidized phospholipids and an inverse correlation with HDL-C; the role of oxidized lipids in stimulating production of CCL23 merits further exploration in cell culture experiments. In aggregate these findings suggest that CCL23 may work in concert with specific vascular inflammatory mechanisms and support further study of CCL23 in atherosclerosis.
Several important limitations merit comment. First, our study was an exploratory, cross-sectional study, and thus cannot provide clear insight into causation. Although our findings should be considered only hypothesis generating, they do provide an avenue for further exploration. A second important limitation is that the majority of subjects had undetectable levels of CXCL1 and CXCL2, which limited assessment of linear associations between levels of these cytokines and atherosclerosis. Because of this limitation, we could not evaluate differences below the detection limit that may potentially be of clinically importance. For CXCL1, in particular, additional study in higher risk populations and/or with more sensitive assays appears warranted, given the borderline-independent associations between this cytokine and atherosclerosis observed in the present study.