AIMS: Abdominal aortic aneurysm (AAA) is an age-associated disease characterized by chronic inflammation, vascular cell apoptosis and metalloproteinase-mediated extracellular matrix degradation. Despite considerable progress in identifying targets involved in these processes, therapeutic approaches aiming to reduce aneurysm growth and rupture are still scarce. Indoleamine 2-3 dioxygenase 1 (IDO) is the first and rate-limiting enzyme involved in the conversion of tryptophan (Trp) into kynurenine (Kyn) pathway. In this study, we investigated the role of IDO in two different models of AAA in mice. METHODS AND RESULTS: Mice with deficiencies in both low density receptor-deficient (Ldlr-/-) and IDO (Ldlr-/-Ido1-/-) were generated by cross-breeding Ido1-/- mice with Ldlr-/-mice. To induce aneurysm, these mice were infused with angiotensin II (Ang II) (1000 ng/min/kg) and fed with high fat diet (HFD) during 28 days. AAAs were present in almost all Ldlr-/- infused with AngII, but only in 50% of Ldlr-/-Ido1-/- mice. Immunohistochemistry at an early time point (day 7) revealed no changes in macrophage and T lymphocyte infiltration within the vessel wall, but showed reduced apoptosis, as assessed by TUNEL assay, and increased α-actin staining within the media of Ldlr-/-Ido1-/- mice, suggesting enhanced survival of vascular smooth muscle cells (VSMCs) in the absence of IDO. In another model of elastase-induced AAA in C57Bl/6 mice, IDO deficiency had no effect on aneurysm formation. CONCLUSION: Our study showed that the knockout of IDO prevented VSMC apoptosis in AngII -treated Ldlr-/- mice fed with HFD, suggesting a detrimental role of IDO in AAA formation and thus would be an important target for the treatment of aneurysm.