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, 1 alpha, 25-dihydroxyvitamin D3 [1,25(OH)(2)D3], 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)(2)D3 at more moderate concentrations (10(-7) M). Stimulation of human CD4+ T cells with a combination of 1,25(OH)(2)D3 and transforming growth factor-beta (TGF-beta) greatly increased the frequency of Foxp(3+) Treg cells, which is proposed to result from the preferential expansion of Foxp(3+) Treg cells, as compared with the Foxp(3-) effector T cells, in culture. The differential effect on proliferation may result from enhanced availability and usage of interleukin-2 by the Foxp(3+) Treg cells compared with Foxp(3-) effector T cells. In summary, modulation of the cytokine environment to one high in TGF-beta in the presence of 1,25(OH)(2)D3 (10(-7) M) significantly increased Foxp(3+) Treg cell frequency. These data provide additional evidence for the important immunomodulatory properties of 1,25(OH)(2)D3 that exist and may help to control inflammatory diseases.