Fibronectin containing extra domain A (Fn-EDA) is an endogenous ligand of TLR4 (toll-like receptor 4) and is abundant in the extracellular matrix of advanced atherosclerotic lesions in human and mice. Irrespective of sex, deletion of Fn-EDA reduces early atherosclerosis in apolipoprotein E–deficient (Apoe^−/−) mice. However, the contribution of Fn-EDA in advanced atherosclerosis remains poorly characterized. We determined the contribution of Fn-EDA in advanced atherosclerotic lesions of aged (1-year-old) Apoe^−/− mice.

Plaque composition was determined in the innominate artery, a plaque instability site that is known to mimic several histological features of vulnerable human plaques. Female Apoe^−/−, Fn-EDA^−/−Apoe^−/−, TLR4^−/−Apoe^−/−, and Fn-EDA^−/−TLR4^−/−Apoe^−/− mice were fed a high-fat Western diet for 44 weeks. Fn-EDA^−/−Apoe^−/− mice exhibited reduced plaque size characterized by smaller necrotic cores, thick fibrous caps containing abundant vascular smooth muscle cells and collagen, reduced CD68/MMP9 (matrix metalloproteinase 9)-positive content, less accumulation of MMP-cleaved extracellular matrix aggrecan, and decreased vascular smooth muscle cell and macrophage apoptosis (P<0.05 versus Apoe^−/− mice). Together these findings suggest that Fn-EDA induces plaque destabilization. Deletion of TLR4 reduced histological features of plaque instability in Apoe^−/− mice but did not further reduce features of plaque destabilization in Fn-EDA^−/−Apoe^−/− mice, suggesting that TLR4 may contribute to Fn-EDA–induced plaque destabilization. Fn-EDA potentiated TLR4-dependent MMP9 expression in bone marrow–derived macrophages, suggesting that macrophage TLR4 may contribute to Fn-EDA–mediated plaque instability.

Fn-EDA induces histological features of plaque instability in established lesions of aged Apoe^−/− mice. The abundance of Fn-EDA in advanced atherosclerotic lesions may increase the risk of plaque destabilization.