Abdominal aortic aneurysm is an inflammatory disease leading to destructive vascular remodeling and ultimately to lethal aortic rupture. Despite its frequent association with atherosclerosis, compelling studies have shown striking differences and potentially opposite roles of T-cell helper responses in aneurysm as compared with atherosclerosis, casting doubt on the relevance and suitability of T-cell–targeted therapies in this context.

Here, we show that selective depletion of T regulatory (Treg) cells using a CD25-specific monoclonal antibody significantly enhances the susceptibility of C57Bl/6 mice to angiotensin II–induced abdominal aortic aneurysm and promotes aortic rupture (n=25–44 mice/group). Similar results are observed in angiotensin II–treated Cd80^−/−/Cd86^−/− or Cd28^−/− mice with impaired Treg cell homeostasis (n=18–23 mice/group). Treg cell depletion is associated with increased immune cell activation and a blunted interleukin (IL)-10 anti-inflammatory response, suggesting an immunoinflammatory imbalance. Interestingly, Il-10^−/− mice (n=20 mice/group) show increased susceptibility to angiotensin II–induced abdominal aortic aneurysm and aortic rupture and are insensitive to Treg cell depletion. Finally, reconstitution of Cd28^−/− Treg-deficient mice with Treg cells (n=22 mice/group) restores a balance in the immunoinflammatory response, rescues the animals from increased susceptibility to aneurysm, and prevents aortic dissection.

These results identify a critical role for Treg cells and IL-10 in the control of aneurysm formation and its progression to rupture and suggest that therapies targeting Treg responses may be most suited to treat aneurysmal disease.