High-dose cholecalciferol supplementation significantly increases peripheral CD4⁺ Tregs in healthy adults without negatively affecting the frequency of other immune cells.

BACKGROUND Regulatory T cells (Tregs) play a central role in the maintenance of self-tolerance. Animal and in vitro studies suggest that vitamin D is involved in reducing the risk of autoimmunity by modulating Tregs. METHODS In a double-blind, placebo controlled study in 60 healthy volunteers, we assessed the effect of a 12-week high-dose oral cholecalciferol supplementation (140,000 IU/month) on the number and function of CD4(pos)CD25(high)FoxP3(pos)CD127(dim) Tregs. We also assessed the clinical safety of the supplementation and the effect on the frequency of other immune cells such as monocytes, dendritic cells, natural killer cells, natural killer T cells, B cells and subgroups of T cells. We also tested the in vitro effect of cholecalciferol on Tregs in human cell cultures. RESULTS By using FACS analysis, ex vivo suppressive co-cultures and apoptosis assays, we were able to show that a cholecalciferol supplementation leads to significantly increased numbers of peripheral Tregs in vivo. Tregs function and the frequency of other immune cells remained unchanged, and no clinically relevant safety concerns were found. The in vitro exposure of human peripheral blood mononuclear cells to cholecalciferol also supported our in vivo findings. CONCLUSIONS Our results indicate a substantial effect of a supplementation with inactive vitamin D on the immune system of healthy humans in vivo and provide a rationale for future studies to investigate the immunomodulatory effects of vitamin D in autoimmune diseases.