Transplantation of mesenchymal stem cells (MSCs) is beneficial in myocardial infarction and hind limb ischemia, but its ability to ameliorate atherosclerosis remains unknown. Here, the effects of MSCs on inhibiting endothelial dysfunction and atherosclerosis were investigated in human/mouse endothelial cells treated with oxidized low-density lipoprotein (oxLDL) and in apolipoprotein E-deficient (apoE2/2) mice fed a high-fat diet. Results with oxLDL inactivated the Akt/endothelial nitricoxide synthase (eNOS) pathway, induced eNOS degradation, and inhibited nitric oxide (NO) production in endothelial cells. Coculture with human MSCs reversed the effects of oxLDL on endothelial cells and restored Akt/eNOS activity, eNOS level, and NO production. Reduction of endotheliumdependent relaxation and subsequent plaque formation were developed in apoE2/2 mice fed a high-fat diet. Systemic infusion with mouse MSCs ameliorated endothelial dysfunction and plaque formation in high-fat diet-fed apoE2/2 mice. Interestingly, results with interleukin-8 (IL8)/macrophage inflammatory protein-2 (MIP-2) alone induced the similar effects of human/mouse MSCs on oxLDL-treated human/mouse endothelial cells. Neutralization antibodies (Abs) against IL8/MIP-2 also blocked the effects of human/mouse MSCs on oxLDL-treated human/mouse endothelial cells. Consistently, MIP-2 injection alone induced the similar effect of MSCs on the endothelial function in highfat diet-fed apoE2/2 mice. The improvement in endothelial dysfunction by mouse MSCs was also blocked when pretreating MSCs with anti-MIP-2 Abs. In conclusion, MSC transplantation improved endothelial function and plaque formation in high-fat diet-fed apoE2/2 mice. Activation of the Akt/eNOS pathway in endothelium by IL8/MIP-2 is involved in the protective effect ofMSCs. The study helps support the use and clarify the mechanism of MSCs for ameliorating atherosclerosis. S