Background: Atherosclerosis, the major cause of heart disease and stroke, is a lipid driven chronic inflammatory disease that is characterized by the continuous recruitment of inflammatory cells to the atherosclerotic plaque, a process orchestrated by chemokines. Abundant expression of the chemokine CCL18 (PARC) is observed in human atherosclerotic plaques, and especially in unstable regions. As the exact role of CCL18 in atherogenesis is currently unknown, we aimed to map the effects of focal and systemic CCL18 overexpression on plaque progression and stability. Furthermore, we identify CCR6 as a functional receptor for CCL18 that mediates CCL18 effects and activates intracellular signalling.
Methods: The study included in vivo analyses in patients with carotid atherosclerosis and in vitro experiments in cells involved in atherogenesis. For animals study, animals received CCL18 and were characterized the molecular mechanisms by histological (immunohistochemistry, immunofluorescence for TUNEL) and biochemical methods (Western blots, Real Time PCR, Ca2+ mobilization).
Results: We report the profound up-regulation of the expression of CCL18 in ruptured human atherosclerotic plaque, in particular by macrophages. Systemic gain of function in a mouse model of atherosclerosis indicated pro-atherogenic activity of CCL18; while plaque-targeted CCL18 overexpression was associated with increased Ca2+ mobilization in T cells. Importantly, we identify CCR6 as a functional receptor mediating CCL18 chemotaxis, intracellular Ca2+ mobilization and downstream signaling in human Jurkat T cells while in mouse CD4+ and CD8+ T cells these processes were abolished in the absence of CCR6. Subcutaneous CCL18 administration led to profound skin inflammation in WT but not CCR6-/- mice, characterized by edema, marked infiltration of neutrophils/macrophages and T cells, with a clear Th17 signature, compatible with the role of CCR6 as a migration receptor for Th17 cells.
Conclusions: Our studies identify CCR6 as bona fide receptor for CCL18, mediating chemotaxis of neutrophils, macrophages and Th17 cells, and augmenting inflammation and destabilization of the plaque. Our work sheds light on the mechanisms underlying CCL18-induced cardiovascular risk and may pave the way for targeted treatment and prevention of atherosclerosis.