Vascular stiffening is defined as the reduced ability of the blood vessels to expand in response to an increase in blood pressure. Vascular stiffening is often not appreciated as a disease in and of itself but is important to recognize because it is considered a predictor of many cardiovascular disease states. Mechanisms of vascular stiffening remain largely unknown; however the immune system has been found to play major roles in cardiovascular disease and arterial remodeling. This dissertation therefore seeks to illustrate the role of the adaptive immune system in vascular stiffening. This has been done by modeling vascular stiffness in transgenic mice lacking an adaptive immune system as well as immunosuppression in normal mice using a novel method to stimulate regulatory T cells with a cytokine immune complex. We have found that inhibition of the immune system by the use of a genetic knockout (RAG 1 ⁻/⁻ mice) or suppression of an existing immune system with an IL-2/anti-IL-2 complex reduces the development of angiotensin II-induced vascular stiffening. This dissertation supports the role of the adaptive immune system, and particularly CD4⁺T cells, in the development of vascular stiffening as well as the protective roles of Tregs in the disease. It also highlights the use of the IL-2/anti-IL-2 complex as a new potential therapy for vascular stiffness. Therapeutics that suppress adaptive immune function may be beneficial in the treatment of vascular stiffening.