Nuove molecole coinvolte nel differenziamento e nel reclutamento delle cellule dendritiche

In this thesis we explored the role of two molecules, activin A and chemerin, in dendritic cell biology. Dendritic cells (DC) represent an heterogeneous population of haematopoietic origin that are specialized in the capture, processing and presentation of antigens to naïve T cells. The interaction of DCs with T cells can lead to different forms of immune response, or to T-cell tolerance, depending on the type of DC and its activation state. Dendritic cell subtypes differ in location, membrane markers, migratory pathways and immunological functions. DCs are classically subdivided into two main categories: conventional dendritic cells (cDC) and plasmacytoid dendritic cells (pDC). Conventional DC already have a dendritic morphology in resting conditions and are located in lymphoid tissues or in peripheral tissues. Of particular interest are Langerhans cells (LCs), that are located in the epidermis where they act as sentinels. Plasmacytoid DC circulate through the blood and lymphoid tissues and only acquire the typical DC morphology after activation, which is accompanied by the release of type I interferons. Ιn vitro, DC can be generated from monocytes in presence of GM-CSF and IL-4 (Mo-DC). When monocytes are cultured in the presence of TGF-β in addition to GMCSF, IL-4, DC are generated displaying markers of LC (langerin and Birbeck granules). The first part of this thesis is centered on activin A, a TGF-β family member involved in skin morphogenesis and wound healing. We analysed activin A production by DC in vitro and we demonstrate that Mo-DC release abundant levels of activin A during the maturation process induced by Toll-like receptor (TLR) agonists, bacteria, TNF and CD40L. Activin A production by Mo-DC is selectively down-regulated by anti-inflammatory molecules such as dexamethasone or IL-10. We also report that activin A induces the differentiation of human monocytes into Langerhans cells in the absence of TGF-β. Activin A-induced LC (ActA-LC) are langerin+, E-cadherin+, CLA+, CCR6+ and Birbeck granules+ and possess typical APC functions. In human skin explants, intradermal injection of activin A increase the number of CD1a+ and Langerin+ cells in both the epidermis and dermis by promoting the differentiation of resident precursor cells. High levels of activin A are present in the upper epidermal layers and in the dermis of lichen planus biopsies in association with a marked infiltration of CD1a+ and Langerin+ cells. It is thus likely that activin A may have a prominent role in LC differentiation in lichen planus. In summary, this study presents a new model in which activin A induces the differentiation of circulating CD14+ cells into LC. Since activin A is abundantly produced during certain inflammatory conditions, we propose that this cytokine represents a new pathway, alternative to TGFβ, responsible for LC differentiation during inflammatory/autoimmune conditions. In the second part of this thesis we analyse the role of chemerin, a new chemotactic factor, in the recruitment of pDC into psoriatic skin. It’s known that psoriasis is characterized by the recruitment of pDC into the skin, but the molecules involved in pDC accumulation in psoriatic lesions are unknown. Chemerin is the only inflammatory chemotactic factor that is active on human blood pDC in vitro. It has been indeed demonstrated that pDC express functional chemerin receptors (namely ChemR23); in fact, pDC are able to respond to a chemerin gradient by migrating across an endothelial cell monolayer in transmigration assays. In this work we analyzed chemerin expression in the skin of psoriatic patients and we observed that prepsoriatic skin adjacent to active lesions and early lesions were characterized by a strong expression of chemerin in the dermis and by the presence of CD15+ neutrophils and CD123+/BDCA-2+/ChemR23+ pDC. Conversely, skin from chronic plaques showed low chemerin expression, segregation of neutrophils to epidermal microabscesses, and few pDC in the dermis. Chemerin expression was localized mainly in fibroblasts, so we examined chemerin production by fibroblasts from psoriatic skin or healty skin in vitro. Fibroblasts cultured from skin of psoriatic lesions expressed higher levels of chemerin messenger RNA and protein than fibroblasts from uninvolved psoriatic skin or healthy donors and promoted pDC migration in vitro in a chemerin-dependent manner. Therefore, chemerin expression specifically marks the early phases of evolving skin psoriatic lesions and is temporally strictly associated with pDC. These results support a role for the chemerin/ChemR23 axis in the early phases of psoriasis development. Altogether, the present results highlight the importance of activin A and chemerin in dendritic cell biology: activin A plays a role in the differentiation of Langerhans cells while chemerin is involved in the recruitment of pDC in skin psoriatic lesions. Further investigations on this topic may lead to development of strategies for prevention or early therapeutic intervention in psoriasis.