Vascular disease can manifest as stenotic plaques or ectatic aneurysms. Human abdominal aortic aneurysms (AAA) comprise an inflammatory disease characterized by the predominance of T helper type 2 (Th2) cytokine expression. Leptin has been clearly demonstrated to play an important role in regulating Th0 cell to Th1. So, we hypothesize that leptin has a protective effect on aneurysm formation. In this study, we demonstrated that intraperitoneal injection of leptin attenuated Ang II-induced AAA formation in ApoE-/- mice with no effect on serum lipids and systolic blood pressure. To investigate the mechanisms involved, we found that leptin pretreatment exhibited decreased protein expression of matrix metalloproteinase 2 (MMP-2) and MMP-9 and increased transforming growth factor-beta 1 (TGF-beta 1). We also examined potential mechanism of leptin as a modulator of the immune response. Our results proved that pretreatment with leptin downregulated protein expression of Th2 cytokine IL-4 and mRNA levels of GATA-3, the key transcription factor for Th2 polarization, and upregulated Th1 cytokine INF-gamma and T-bet, the key transcription factor for Thl polarization. Taken together, leptin, with the effect of regulation of Th1/Th2 cytokines, may have therapeutic potential for the treatment of AAA. Leptin may constitute a novel therapeutic strategy to prevent AAA formation. (C) 2018 Elsevier Inc. All rights reserved.