Background: The vitamin D metabolite 1,25(OH)2D3 (VitD3) is a potent immunosuppressive drug and, among others, is used for topical treatment of psoriasis. A proposed mechanism of VitD3-mediated suppression is priming of dendritic cells (DCs) to induce regulatory T (Treg) cells. Objective: Currently, there is confusion about the phenotype of VitD3-induced Treg cells and the DC-derived molecules driving their development. We investigated Treg cell induction after VitD3 priming of 2 distinct skin DC subsets: Langerhans cells (LCs) and dermal dendritic cells (DDCs). Methods: LCs and DDCs primed with VitD3 were cocultured with allogeneic naive T cells. The phenotype and function of the DCs and induced T cells were analyzed. Results: Both VitD3-primed DC subtypes induced T cells with regulatory activity. Unexpectedly, whereas the Treg cell populations generated by VitD3-primed LCs were CD25hiCD127loforkhead box protein 3 (Foxp3)-positive cells, which meet the criteria of classical inducible Treg cells, the T cells developing in response to VitD3-primed DDCs were Foxp3-TR1 cells expressing IL-10. Inhibition experiments revealed that LC-derived TGF-β is a key factor in the induction of Foxp3+Treg cells, whereas DDC-derived IL-10 is important for the induction of IL-10+TR1 cells. Conclusion: Thus we report the novel finding that distinct but closely related DC subsets are differentially programmed by VitD3 to support development of either TGF-β-dependent Foxp3+Treg cells or IL-10-dependent IL-10+Treg cells. © 2011 American Academy of Allergy, Asthma & Immunology.
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