A significant proportion of patients with ulcerative colitis (UC) have suboptimal responses to medical therapy. Expansion of regulatory T cells (Tregs), but not conventional T cells (Tcons), through the use of low doses (LD) of the T cell growth factor Interleukin-2 (IL-2, Proleukin®), has been a promising approach in other immune-mediated diseases and pre-clinical humanized UC mouse models. We performed a phase 1b/2a clinical trial in patients with moderate-to-severe UC to determine if subcutaneously (SC) administered LD IL-2 is safe and results in a biological response.