Low-dose interleukin-2 in patients with bipolar depression: A phase 2 randomised double-blind placebo-controlled trial

Immune abnormalities including an insufficiency of regulatory T cells (Treg) and increased blood-based inflammatory markers have been observed in bipolar disorders (BD), particularly during depression. As Tregs are pivotal to control inflammation, Treg stimulation by low-dose IL-2 (IL-2_LD) could have a therapeutic impact on bipolar depression. We performed a randomized, double-blind, placebo-controlled (2 active: 1 placebo) proof-of-concept trial of add-on IL-2_LD in patients with bipolar depression. Patients received a placebo or IL-2_LD (1MIU) once a day for 5 days, and then once a week for 4 weeks starting on week 2. The primary objective was to demonstrate a biological Treg response to IL-2_LD assessed by fold increase in Treg percentage of CD4 + cells from baseline to day 5. Secondary objectives included safety assessment and mood improvement throughout the study period. This trial is registered with ClinicalTrials.gov, number NCT04133233. Fourteen patients with bipolar depression were included, with 4 receiving placebo and 10 IL-2_LD. Baseline clinical and biological characteristics were balanced between groups. The primary evaluation criterion was met, with IL-2_LD expanding 1.17 [95 % CI 1.01–1.34] vs 1.01 [95 % CI 0.90–1.12] (p = 0.0421) and activating Tregs. Secondary evaluation criteria were also met with significant improvements of depressive symptoms and global functioning from day-15 onwards in the IL-2_LD treated patients. The treatment was well-tolerated, with no serious adverse events related to treatment. This proof-of-concept trial shows that stimulating Tregs in patients with bipolar depression is safe and associated with clinical improvements. This supports a pathophysiological role of inflammation in BD and warrants pursuing the evaluation of IL-2_LD as an adjunct treatment of major mood disorders.