**3. Dendritic Cells**

Myeloid dendritic cells (mDC) are monocyte-derived antigen-presenting cells that are vital for initiating and regulating the adaptive immune response [20]. More specifically, mDCs accumulate in airways that are challenged by allergens and are involved in inducing and maintaining inflammatory responses by activating naïve T cells in the secondary lymphoid organs, implicating mDCs in potentially having a crucial role in allergic inflammation [20].

In a study that determined the effects of prostacyclin on dendritic cell function, ovalbumin-sensitized and challenged mice treated with inhaled iloprost in vivo had decreased expression of IL-4, IL-5, and IL-13 in restimulated lymph node cells compared to mice treated with the iloprost vehicle [11]. Iloprost treatment significantly decreased the number of eosinophils and lymphocytes in the BAL compartment and reduced peribronchial inflammation and goblet cell hyperplasia compared to vehicle treatment [11]. In vitro, activated iloprost-treated mouse dendritic cells that expressed CC chemokine receptor 7 (CCR7) had decreased chemotactic responsiveness to CC chemokine ligand 19 (CCL19) compared to vehicle-treatment, perhaps explaining the effect of iloprost treatment on the reduction of dendritic cell migration to the lymph nodes in vivo [11].

Another group reported that iloprost dose-dependently decreased TNF-α, IL-6, and IL-8 secretion by human mDCs compared to vehicle treatment [21]. A second group reported that, compared to vehicle treatment, iloprost decreased IL-6, IL-12, IL-23, and TNF-α, while it increased IL-10 in bone marrow dendritic cells (BMDCs) that were stimulated with ovalbumin and then intratracheally adoptively transferred into mice that were subsequently ovalbumin challenged through the airway [9]. Further, mouse CD4+ T cells that expressed a T cell receptor specific for an ovalbumin peptide had reduced production of IL-4, IL-5, and IL-13 when these cells were stimulated with DCs that had been cultured with ovalbumin BMDCs in the presence of iloprost compared to vehicle-treated BMDCs [11]. These results show promise for the use of prostacyclin analogs in attenuating adaptive allergic inflammatory responses.

The results of experiments detailing the effect of prostacyclin in downregulating proinflammatory cytokine production by mouse DCs are similar to those in which human DCs have been studied. Iloprost and treprostinil suppressed TNF-α expression by mDCs activated by the toll-like receptor (TLR) agonist, poly I:C. TNF-α is an important proinflammatory cytokine that recruits immune cells, regulates chemokine production, releases histamine, upregulates adhesion molecules, and is potentially involved in airway remodeling in dendritic cells [13,20]. Compared to vehicle treatment, iloprost-treated human mDCs also decreased the production of IFN-γ by CD4+ helper T cells, while iloprost enhanced the production of the immunosuppressive cytokine IL-10 [20]. These effects were seemingly modulated through the IP and PGE2 (EP) receptors but not PPARs [20]. Furthermore, the modulatory effects of treprostinil and iloprost in this study in which the cAMP pathway was activated were not completely IP-specific [20]. For instance, iloprost increased intracellular Ca2+ levels through EP1 receptor signaling and partly increased IL-10 levels while decreasing TNF-α via the EP1-Ca2+ pathway. In these studies, iloprost's suppressive effects on TNF-α in human mDCs were a result of MAPK-p38-ATF2 pathway signaling [20]. Thus, some prostacyclin analogs, particularly at higher concentrations, may have IP-independent effects by activating EP receptors. Additionally, the study also examined the in vitro effects of prostacyclin analogs on epigenetic regulation. Epigenetic regulation, observed by the activity of histone acetyltransferase and deacetylase, in this instance, regulates inflammatory gene expression [20]. In patients with asthma, there is an overexpression in inflammatory genes due to a decrease in histone deacetylase activity and an increase in histone acetyltransferase activity, and these changes in acetylation through epigenetic regulation also regulate the proliferation and differentiation of T lymphocytes [20,22]. In mDCs that were activated by poly I:C, iloprost downregulated H3K4 trimethylation of the TNFA gene promoter region and inhibited poly I: C-induced translocation of methyltransferases [20].
