CCL17 was initially isolated from phytohemagglutinin-stimulated PBMCs. CCL17 is constitutively expressed in thymus and, upon activation, in several cell types. CCL17-mediated recruitment of Th2 cells and CLA+ CD4+ T cells and plays a key role in allergic diseases like atopic dermatitis, allergic asthma, allergic rhinitis, and allergic contact dermatitis. In addition, CCL17 has been detected in idiopathic pulmonary fibrosis. Also, CCL17 and CCL22 secreted by DCs seem to mediate the recruitment of Tregs to sites of inflammation in patients with chronic hepatitis. In addition, CCL17 is present in advanced human and mouse atherosclerosis, and CCL17+ DCs accumulate in atherosclerotic lesions. CCL17 expressed by DCs limit the expansion of Tregs. CCL17 is upregulated by a combination of cytokines (TNFa, IL-4, and IFNgamma) in bronchial epithelial cells; TGFbeta1, TNFalpha, and IFNgamma in the human keratinocyte cell line; and histamine and prostaglandin E2P in immature DC.