Plasmacytoid dendritic cells suppress Th2 responses induced by epicutaneous sensitization.

Epicutaneous (EC) sensitization with protein allergens is the most important sensitization route for atopic dermatitis. Plasmacytoid dendritic cells (pDCs) are characterized by massive secretion of interferon-alpha (IFN alpha). B6 mice are T helper type 1 (Th1)-prone and are representative of non-atopic humans, whereas BALB/c mice are Th2-prone and are representative of atopic humans. Here, we show that naive BALB/c mice contain a greater number of nonactivated pDCs in peripheral lymph nodes (LNs) than do naive B6 mice. Naive BALB/c mice also have more of the CD8 alpha(-) subset in LNs than naive B6 mice. Moreover, in vivo depletion of pDCs during EC sensitization results in enhanced Th2 responses in BALB/c mice, but not in B6 mice. Mechanistically, when BALB/c mice undergo EC sensitization, there is an increase in pDCs entering draining LNs. These cells exhibit modest activation including comparable costimulation expression but increased cytokine expression compared with those of naive mice. In vivo depletion of pDCs during EC sensitization significantly increases the activation of dermal dendritic cells (dDCs) suggesting a regulatory effect on these cells. To this end, a suppressive effect of pDCs on conventional dendritic cells was also demonstrated in vitro. Further, in vivo blockade of IFN alpha by an anti-IFNAR antibody (Ab) or in vivo reduction of IFN alpha production of pDCs by anti-siglec-H Ab both resulted in enhanced activation of dDCs. Collectively, our results demonstrate that pDCs suppress Th2 responses induced by EC sensitization via IFN alpha-mediated regulation of dDCs.