1 α ,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

Summary 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 α ,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 CD 4 + T cells with a combination of 1,25( OH ) 2 D3 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 ) 2 D3 (10 −7   m ) significantly increased Foxp3 + 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.