Genetic studies have demonstrated that loss-of-function mutations in NOD2 are associated with several chronic inflammatory disorders, including Crohn's disease [9,10], whereas gain-offunction mutations in NOD2 correlate with Blau syndrome [11], and early-onset sarcoidosis [12], underscoring a crucial role for NOD2 in immune homoeostasis. However, the data on such mutations in cardiovascular disease are inconclusive [13][14][15].The involvement of NOD2 in inflammatory cardiovascular diseases was initially postulated based on the observations that vascular endothelial cells express low basal levels of NOD2 [16,17] and that depletion of RIP2 in myeloid cells regulates the development of atherosclerosis by induction of TLR4-mediated macropinocytosis of LDL in macrophages [18]. Moreover, Kwon et al. reported that lack of NOD2 signaling increases neointimal formation in mice after vascular injury [19]. We recently showed that stimulation of NOD2 in human carotid plaque results in rapid activation of the prostaglandin E2 pathway and IL-1β signaling, the first evidence supporting the involvement of NOD2 in human atherosclerosis [20]. Besides the traditional risk factors such as hypercholesterolemia, hypertension, obesity, and smoking, chronic infection with Porphyromonas gingivalis has been shown to promote atherosclerosis. In this regard, innate immune surveillance mechanism may provide essential protection against atherosclerosis. A recent study on ApoE −/− mice has revealed an indispensable role for NOD2 in protecting the mice from oral infection by Porphyromonas gingivalis, thereby restraining infection-mediated oral bone resorption, aortic inflammation, and plaque development [21]. Together, these findings indicate that NOD2 is involved in multiple aspects of cardiovascular diseases. However, whether NOD2 plays a direct role in atherosclerotic plaque development is not yet proven. This study was designed to elucidate the function of NOD2 signaling in atherosclerosis. By studying MDP-induced NOD2 signaling in atherosclerosis-prone mice and NOD2-deficient chimeric mice, we show that NOD2 promotes vascular inflammation and contributes to the expansion of the lipid-rich necrotic core in hyperlipidemic mice. Deploying an ex vivo culture system of human atherosclerotic plaque, we also characterized NOD2 signaling mechanisms in human atherosclerosis. Fig. 1).
Results
ActivationDespite no significant alteration in body weight and plasma cholesterol levels (Table 1), MDP-treated mice exhibited a dramatic increase in atherosclerosis in the aortic root compared with the controls (Fig. 1A and B). In association with the greater lesion burden, MDP-treated mice experienced enhanced vascular inflammation, as evidenced by significant upregulation of VCAM-1 and IL-6, and a tendency of elevation in TNF-α, MCP-1, IFN-γ, and IL-10 (Fig. 1C). Further, MDP increased the infiltration of CD68-positive macrophages and levels of COX2, implying macrophage activation in the lesion ( Fig. 1C-E). In parallel, the MDP treatment also resulted in...
