1α,25-dihydroxyvitamin D3 in Combination with Transforming Growth Factor-β Increases the Frequency of Foxp3⁺ Regulatory T Cells Through Preferential Expansion and Usage of Interleukin-2

A high prevalence of vitamin D insufficiency and deficiency exists worldwide, which is associated with an increased incidence and severity of a range of immune-mediated diseases. This has resulted in considerable interest in the immunodulatory functions of vitamin D. The active form of vitamin D, 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3], has been shown to increase the frequency of Foxp3(+) CD4(+) T regulatory (Treg) cells when present at high concentrations or under strong T-cell stimulation in culture. Supporting evidence exists in vivo for a positive association between serum 25(OH)D and Foxp3(+) Treg cell numbers in humans. The aim of this work was to identify the cytokine milieu required in vitro to promote Foxp3(+) Treg cells in cultures containing 1,25(OH)2D3 at more moderate concentrations (10(-7) M). Stimulation of human CD4(+) T cells with a combination of 1,25(OH)2D3 and transforming growth factor-β (TGF-β) greatly increased the frequency of Foxp3(+) Treg cells, which is proposed to result from the preferential expansion of Foxp3(+) Treg cells, as compared with the Foxp3(-) effector T cells, in culture. The differential effect on proliferation may result from enhanced availability and usage of interleukin-2 by the Foxp3(+) Treg cells compared with Foxp3(-) effector T cells. In summary, modulation of the cytokine environment to one high in TGF-β in the presence of 1,25(OH)2D3(10(-7) M) significantly increased Foxp3(+) Treg cell frequency. These data provide additional evidence for the important immunomodulatory properties of 1,25(OH)2D3 that exist and may help to control inflammatory diseases.