Chemokine CCL17 is expressed by dendritic cells in the CNS during experimental autoimmune encephalomyelitis and promotes pathogenesis of disease

The CC chemokine ligand 17 (CCL17) and its cognate CC chemokine receptor 4 (CCR4) are known to control leukocyte migration, maintenance of TH17 cells, and regulatory T cell (Treg) expansion in vivo. In this study we characterized the expression and functional role of CCL17 in the pathogenesis of experimental autoimmune encephalomyelitis (EAE). Using a CCL17/EGFP reporter mouse model, we could show that CCL17 expression in the CNS can be found in a subset of classical dendritic cells (DCs) that immigrate into the CNS during the effector phase of MOG-induced EAE. CCL17 deficient (CCL17−/−) mice exhibited an ameliorated disease course upon MOG-immunization, associated with reduced immigration of IL-17 producing CD4+ T cells and peripheral DCs into the CNS. CCL17−/− DCs further showed equivalent MHC class II and costimulatory molecule expression and an equivalent capacity to secrete IL-23 and induce myelin-reactive TH17 cells when compared to wildtype DCs. In contrast, their transmigration in an in vitro model of the blood-brain barrier was markedly impaired. In addition, peripheral Treg cells were enhanced in CCL17−/− mice at peak of disease pointing towards an immunoregulatory function of CCL17 in EAE. Our study identifies CCL17 as a unique modulator of EAE pathogenesis regulating DC trafficking as well as peripheral Treg cell expansion in EAE. Thus, CCL17 operates at distinct levels and on different cell subsets during immune response in EAE, a property harboring therapeutic potential for the treatment of CNS autoimmunity.