Figure 1: Dual activity of pDCs treated with respective low or high doses of CpG 1826 in vitro.

Splenic CD11c⁺ DCs from DBA/2 mice were fractionated according to mPDCA-1 or CD8 expression, pulsed with the P815AB peptide and transferred intravenously into recipient hosts to be assayed at 2 weeks for footpad reactivity to soluble peptide challenge. (a) Transferred pDCs cells were either untreated or treated overnight with 1 (pDC/L-CpG) or 10 (pDC/H-CpG) μg ml⁻¹ of ODN and were used in combination with a minority fraction (5%) of tolerogenic CD8⁺ DCs. (b) According to a similar design, after overnight exposure to 1 or 10 μg ml⁻¹ CpG, the pDCs were assayed for suppressive activity of immunogenic peptide presentation by the CD8⁻ DC fraction. In this setting, the suppressive potential conferred on pDCs by 10 μg ml⁻¹ CpG was ablated by silencing Ido1 expression or by pDC treatment with the competitive IDO inhibitor 1-methyl-tryptophan (1-MT). (c) The modified, non-CpG ODN 1826 (GpC ODN), but not the scrambled, nc sequence of the 1826 oligo (nc-ODN), displays dose-independent suppressive activity contingent on functional IDO1. The experimental design is the same as that in (b). The asterisks (*P<0.01 and **P<0.001; experimental versus control footpads; two-tailed paired t-test) indicate the occurrence of a positive skin test reaction as a result of unopposed presentation of the peptide by the pDCs made immunogenic by CpG or by the spontaneously immunogenic CD8⁻ DC fraction. Data are mean values±s.d. of three experiments.