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1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) is an immunomodulatory agent inducing dendritic cells (DCs) to become tolerogenic. To further understand its mechanisms of action, we have examined the effects of 1,25(OH)2D3 on tolerogenic properties of blood myeloid (M-DCs) and plasmacytoid (P-DCs) human DC subsets. Exposure of M-DCs to 1,25(OH)2D3 up-regulated production of CCL22, a chemokine attracting regulatory T cells, whereas production of CCL17, the other CCR4 ligand, was reduced. 1,25(OH)2D3 also decreased IL-12p75 production by M-DCs, as expected, and inhibited CCR7 expression. 1,25(OH)2D3 treatment markedly increased CD4+ suppressor T cell activity while decreasing the capacity of M-DCs to induce Th1 cell development. Surprisingly, 1,25(OH)2D3 did not exert any discernible effect on tolerogenic properties of P-DCs, and even their high production of IFN-α was not modulated. In particular, the intrinsically high capacity of P-DCs to induce CD4+ suppressor T cells was unaffected by 1,25(OH)2D3. Both DC subsets expressed similar levels of the vitamin D receptor, and its ligation by 1,25(OH)2D3 similarly activated the primary response gene cyp24. Interestingly, 1,25(OH)2D3 inhibited NF-κB p65 phosphorylation and nuclear translocation in M-DCs but not P-DCs, suggesting a mechanism for the inability of 1,25(OH)2D3 to modulate tolerogenic properties in P-DCs.

In the nonobese diabetic (NOD) mouse, pathogenic and suppressor CD4+ T cells can be distinguished by the constitutive expression of CD25. In this study, we demonstrate that the progression of autoimmune diabetes in NOD mice reflects modifications in both T cell subsets. CD4+CD25+ suppressor T cells from 8-, but not 16-wk-old NOD mice delayed the onset of diabetes transferred by 16-wk-old CD25-depleted spleen cells. These results were paralleled by the inhibition of alloantigen-induced proliferation of CD4+CD25− cells, indicating an age-dependent decrease in suppressive activity. In addition, CD4+CD25− pathogenic T cells became progressively less sensitive to immunoregulation by CD4+CD25+ T cells during diabetes development. CD4+CD25− T cells showed a higher proliferation and produced more IFN-γ, but less IL-4 and IL-10, whereas CD4+CD25+ T suppressor cells produced significantly lower levels of IL-10 in 16- compared with 8-wk-old NOD mice. Consistent with these findings, a higher frequency of Th1 cells was observed in the pancreas of 16-wk-old compared with 8-wk-old NOD mice. An increased percentage of CD4+CD25− T cells expressing CD54 was present in 16-wk-old and in diabetic NOD, but not in BALB/c mice. Costimulation via CD54 increased the proliferation of CD4+CD25− T cells from 16-, but not 8-wk-old NOD mice, and blocking CD54 prevented their proliferation, consistent with the role of CD54 in diabetes development. Thus, the pathogenesis of autoimmune diabetes in NOD mice is correlated with both an enhanced pathogenicity of CD4+CD25− T cells and a decreased suppressive activity of CD4+CD25+ T cells.

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