Cholecalciferol supplementation improves suppressive capacity of regulatory T-cells in young patients with new-onset type 1 diabetes mellitus - A randomized clinical trial

•Suppressive capacity of Tregs decreases in the first year after T1D diagnosis in young patients.•Cholecalciferol supplementation improves suppressive capacity of Tregs in T1D patients.•Percentage of Tregs did not change after 12 months cholecalciferol supplementation in T1D.•Cholecalciferol supplementation stabilized insulin requirements.

It is unknown if cholecalciferol is able to modify defects in regulatory T cells (Tregs) in type 1 diabetes (T1D). In this randomized, double-blind, placebo controlled trial 30 young patients with new-onset T1D were assigned to cholecalciferol (70 IU/kg bodyweight/day) or placebo for 12 months. Tregs were determined by FACS-analysis and functional tests were assessed with ex vivo suppression co-cultures at months 0, 3, 6 and 12. Suppressive capacity of Tregs increased (p < 0.001) with cholecalciferol from baseline (− 1.59 ± 25.6%) to 3 (30.5 ± 39.4%), 6 (44.6 ± 23.8%) and 12 months (37.2 ± 25.0%) and change of suppression capacity from baseline to 12 months was significantly higher (p < 0.05) with cholecalciferol (22.2 ± 47.2%) than placebo (− 16.6 ± 21.1%). Serum calcium and parathormone stayed within normal range.This is the first study, which showed that cholecalciferol improved suppressor function of Tregs in patients with T1D and vitamin D could serve as one possible agent in the development of immunomodulatory combination therapies for T1D.