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Abstract: Oxidized low-density lipoprotein (ox-LDL) is an instrumental factor in atherogenesis, however, the effects of ox-LDL on the balance of Th17/Treg in acute coronary syndrome [ACS, including unstable angina (UA) and acute myocardial infarction (AMI)] is still unclear. CD4+CD25+ regulatory T (Treg) cells and Th17 cells, subsets of T-helper cells, play important roles in peripheral immunity and their imbalance leads to the development of tissue inflammation and autoimmune diseases. However, few studies have explored the effect of Th17/Treg balance in plaque destabilization and the onset of ACS. To explore the shift of Th17/Treg balance in ACS patients and the effect of ox-LDL on the balance, we examined the frequencies of Th17 and Treg cells, key transcription factors and relevant cytokines in patients with AMI, UA, stable angina (SA) and controls. We analysed the correlations of serum ox-LDL to Th17/Treg frequency, and the effects of ox-LDL on Th17/Treg cells in vitro. Our study demonstrated that ACS patients have shown a significant increase of Th17 frequency, RORγt expression and serum Interleukin 17 (IL-17), and a obvious decline of Treg frequency, Foxp3 expression, suppressive function, and serum IL-10. Serum ox-LDL positively correlated with the frequency of Th17 cells and negatively correlated with the frequency of Treg cells. In vitro incubation of peripheral blood mononuclear cells from controls with ox-LDL resulted in a significant reduction of Treg cells and a significant elevation of Th17 cells in a dose- and time-dependant manner. Treg and Th17 cells from ACS patients were significantly more susceptible to ox-LDL-mediated alterations. Th17/Treg numerical and functional imbalance exists in ACS patients, and ox-LDL has a direct effect on Th17/Treg imbalance which may contribute to the occurrence of ACS. [Copyright &y& Elsevier]