Airway DCs play a crucial role in the pathogenesis of allergic asthma, and interfering with their function could constitute a novel form of therapy. The sphingosine 1–phosphate receptor agonist FTY720 is an oral immunosuppressant that retains lymphocytes in lymph nodes and spleen, thus preventing lymphocyte migration to inflammatory sites. The accompanying lymphopenia could be a serious side effect that would preclude the use of FTY720 as an antiasthmatic drug. Here we show in a murine asthma model that local application of FTY720 via inhalation prior to or during ongoing allergen challenge suppresses Th2-dependent eosinophilic airway inflammation and bronchial hyperresponsiveness without causing lymphopenia and T cell retention in the lymph nodes. Effectiveness of local treatment was achieved by inhibition of the migration of lung DCs to the mediastinal lymph nodes, which in turn inhibited the formation of allergen-specific Th2 cells in lymph nodes. Also, FTY720-treated DCs were intrinsically less potent in activating naive and effector Th2 cells due to a reduced capacity to form stable interactions with T cells and thus to form an immunological synapse. These data support the concept that targeting the function of airway DCs with locally acting drugs is a powerful new strategy in the treatment of asthma.

The neurotransmitter 5-hydroxytryptamine (5-HT), commonly known as serotonin, is released at peripheral sites from activated enterochromaffin cells, mast cells and platelets. In this study we analyzed the biological activity and intracellular signaling of 5-HT in human monocytes. By reverse transcription (RT) and PCR, messenger RNA (mRNA) expression of 5-HT receptor 1E (5-HTR_1E), 5-HTR_2A, 5-HTR₃, 5-HTR₄ and 5-HTR₇ could be revealed. Functional studies showed that 5-HT modulates the release of IL-1β, IL-6, IL-8/CXCL8, IL-12p40 and tumor necrosis factor-α (TNF-α), while it has no effect on the production of IL-18 and IFN-γ in LPS-stimulated human blood monocytes. Moreover, RT and PCR revealed that 5-HT modulated mRNA levels of IL-6 and IL-8/CXCL8, but did not influence mRNA levels of IL-1β and TNF-α. Pharmacological studies with isotype-selective receptor agonists allowed us to show that 5-HTR₃ subtype up-regulates the LPS-induced production of IL-1β, IL-6 and IL-8/CXCL8, while it was not involved in TNF-α and IL-12p40 secretion. Furthermore, activation of the G_s-coupled 5-HTR₄ and 5-HTR₇ subtypes increased intracellular cyclic AMP (cAMP) and secretion of IL-1β, IL-6, IL-12p40 and IL-8/CXCL8, while, on the contrary, it inhibited LPS-induced TNF-α release. Interestingly, 5-HTR₁ and 5-HTR₂ agonists did not modulate the LPS-induced cytokine production in human monocytes. Our results point to a new role for 5-HT in inflammation by modulating cytokine production in monocytes via activation of 5-HTR₃, 5-HTR₄ and 5-HTR₇ subtypes.

The neurotransmitter 5-hydroxytryptamine (5-HT), commonly known as serotonin, is stored at peripheral sites in mast cells and released from this peripheral source upon IgE cross-linking. In this study, we investigated the expression of serotoninergic receptors (5-HTR), the signaling pathway, and biological activity of 5-HT on human dendritic cells (DC), showing that immature and mature DC expressed mRNA for different serotoninergic receptors. Thereby, the mRNA of 5-HTR_1B, 5-HTR_1E, 5-HTR_2A, 5-HTR_2B, one splicing variant of the 5-HTR₃, 5-HTR₄, and 5-HTR₇ receptors were detected. Immature DC preferentially expressed mRNA for the heptahelical 5-HTR_1B, 5-HTR_1E, and 5-HTR_2B receptors, while mature DC mostly expressed 5-HTR₄ and 5-HTR₇. The mRNA expression level of the ligand-gated cation channel 5-HTR₃ and the heptahelical 5-HTR_2A did not significantly change during maturation. Isotype-selective receptor agonists allowed us to show that 5-HT stimulated 5-HTR₃-dependent Ca²⁺ influx in immature and mature DC. Moreover, we revealed that 5-HTR₁ and 5-HTR₂ receptor stimulation induced intracellular Ca²⁺ mobilization via G_i/o proteins in immature, but not mature, DC. Activation of 5-HTR₄ and 5-HTR₇ induced cAMP elevation in mature DC. Functional studies indicated that activation of 5-HTR₄ and 5-HTR₇ enhanced the release of the cytokines IL-1β and IL-8, while reducing the secretion of IL-12 and TNF-α in mature DC. In summary, our study shows that 5-HT stimulated, in a maturation-dependent manner, different signaling pathways in DC. These data point to a role for 5-HT in regulating the immune response at peripheral sites.

Sensitization to contact allergens requires activation of the innate immune system by endogenous danger signals. However, the mechanisms through which contact allergens activate innate signaling pathways are incompletely understood. In this study, we demonstrate that mice lacking the adenosine triphosphate (ATP) receptor P2X7 are resistant to contact hypersensitivity (CHS). P2X7-deficient dendritic cells fail to induce sensitization to contact allergens and do not release IL-1β in response to lipopolysaccharide (LPS) and ATP. These defects are restored by pretreatment with LPS and alum in an NLRP3- and ASC-dependent manner. Whereas pretreatment of wild-type mice with P2X7 antagonists, the ATP-degrading enzyme apyrase or IL-1 receptor antagonist, prevents CHS, IL-1β injection restores CHS in P2X7-deficient mice. Thus, P2X7 is a crucial receptor for extracellular ATP released in skin in response to contact allergens. The lack of P2X7 triggering prevents IL-1β release, which is an essential step in the sensitization process. Interference with P2X7 signaling may be a promising strategy for the prevention of allergic contact dermatitis.