Interleukin 15 (IL-15) is a pro-inflammatory cytokine that modulates T cell recruitment and activation, independent of antigen. It has been detected in human atherosclerotic plaques and atherosclerotic plaques of apoE-/- mice. IL-15 regulates fractalkine (FKN)CX3CR1 chemokine signaling which is involved in atherogenesis and promotes SMC proliferation. We investigated the role of IL-15 in intimal thickening after arterial injury. Treatment of serum-stimulated SMC with IL-15 in vitro attenuated proliferation and suppressed CX3CRI and FKN mRNA expression. The role of endogenous IL-15 in vivo was investigated in injured carotid arteries of mice. Periadventitial arterial injury resulted in increased IL-15 expression in the media and neointima, paralleled by increased IL-15 receptor alpha expression. Blockade of endogenous IL-15 increased intimal thickening. FKN and CX3CR 1 expression increased after injury and were further augmented after IL-15 blockade. These data suggest that endogenous IL-15 attenuated intimal thickening after arterial injury. The potential mechanism of action is suppression of CX3CR1 signaling. (c) 2005 Elsevier Inc. All rights reserved.