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The immune response of atopic individuals against allergens is characterized by increased levels of Th2 cytokines and chemokines. However, the way in which the cytokine/chemokine profile is matched to the type of invading allergen, and why these profiles sometimes derail and lead to disease, is not well understood. We recently demonstrated that pollen modulates dendritic cell (DC) function in a way that results in an enhanced capacity to initiate Th2 responses in vitro. Here, we examined the effects of aqueous birch pollen extracts (Bet.-APE) on chemokine receptor expression and chemokine production by human monocyte-derived DCs. Bet.-APE strongly induced expression and function of CXCR4 and reduced CCR1 and CCR5 expression on immature DCs. In addition, DC treatment with Bet.-APE significantly reduced LPS-induced production of CXCL10/IP-10, CCL5/RANTES; induced CCL22/macrophage-derived chemokine; and did not significantly change release of CCL17/thymus and activation-regulated chemokine. At a functional level, Bet.-APE increased the capacity of LPS-stimulated DCs to attract Th2 cells, whereas the capacity to recruit Th1 cells was reduced. Bet.-APE significantly and dose-dependently enhanced intracellular cAMP, suggesting that water-soluble factors from pollen grains bind a Gαs-protein-coupled receptor. E1-Phytoprostanes were identified to be one player in the Th2-polarizing potential of aqueous pollen extracts. In summary, our results demonstrate that pollen itself releases regulatory mediators which generate a Th2-promoting micromilieu with preferential recruitment of Th2 cells to the site of pollen exposure.

IL-12p70 is a key cytokine for the induction of Th1 immune responses. IL-12p70 production in myeloid cells is thought to be strictly controlled by T cell help. In this work we demonstrate that primary human monocytes can produce IL-12p70 in the absence of T cell help. We show that human monocytes express TLR4 and TLR8 but lack TLR3 and TLR7 even after preincubation with type I IFN. Simultaneous stimulation of TLR4 and TLR8 induced IL-12p70 in primary human monocytes. IL-12p70 production in peripheral blood myeloid dendritic cells required combined stimulation of TLR7/8 ligands together with TLR4 or with TLR3 ligands. In the presence of T cell-derived IL-4, but not IFN-γ, stimulation with TLR7/8 ligands was sufficient to stimulate IL-12p70 production. In monocytes, type I IFN was required but not sufficient to costimulate IL-12p70 induction by TLR8 ligation. Furthermore, TLR8 ligation inhibited LPS-induced IL-10 in monocytes, and LPS alone gained the ability to stimulate IL-12p70 in monocytes when the IL-10 receptor was blocked. Together, these results demonstrate that monocytes are licensed to synthesize IL-12p70 through type I IFN provided via the Toll/IL-1R domain-containing adaptor inducing IFN-β pathway and the inhibition of IL-10, both provided by combined stimulation with TLR4 and TLR8 ligands, triggering a potent Th1 response before T cell help is established.

Dendritic cells (DCs) can be segregated into various subsets based on phenotypic and functional differences. Whereas plasmacytoid DCs are known for their type I interferon (IFN) producing capacity, conventional (c) DCs are better known for their roles in T cell homeostasis and priming. Among cDCs the CD8α+ subset is especially efficient in producing IL-12p70 and the induction of immunity against various pathogens and cancer. Here, we reveal a new hallmark function of murine CD8α+ cDCs and their human BDCA3+ counterparts, namely the production of large amounts of IFN-lambda (IFN-λ, also termed IL-28/29) upon stimulation with poly IC. IFN-lambdas are potent immunomodulatory and antiviral cytokines. We demonstrate that the production of IFN-λ upon poly IC injection in vivo depends on hematopoietic cells and the presence of toll-like receptor (TLR)3, interferon regulatory factor (IRF)3, IRF7, IFN-IR, Fms-related tyrosine kinase 3 ligand (FL) and IRF8 but not on myeloid differentiation factor 88 (MyD88), Rig like helicases or lymphocytes. Furthermore, we show that both CD8α+ cDCs and plasmacytoid DCs produce large amounts of IFN-λ in response to HSV-1 or parapoxvirus. Thus, IFN-λ production in response to poly IC is a novel hallmark function of mouse CD8α+ cDCs and their human equivalents.