Thymus and activation regulated chemokine (TARC)/CCL17 and skin diseases.

Thymus and activation-regulated chemokine (TARC)/CCL17 is constitutively expressed in the thymus and is produced by dendritic cells (DC), endothelial cells, keratinocytes (KC) and fibroblasts. TARC is designated a Th2 type chemokine since it binds to CCR4. We review the pathogenic role of TARC in skin diseases such as atopic dermatitis (AD), bullous pemphigoid (BP) and mycosis fungoides (MF) focusing on epidermal KC and Langerhans cells (LC), which are epidermal DC. We have determined that serum TARC levels sharply reflect the disease activity of AD, which is thought to be a Th2-dominant inflammatory skin disease especially in the acute phase. Serum TARC levels are also related to the disease activity of BP, which is a blistering autoimmune skin disease, and MF, which is a cutaneous T-cell lymphoma, but very high serum TARC levels are only seen in a limited number of various other skin diseases. TARC may be a useful laboratory marker for the diagnosis of AD, especially cases which are moderate to severe, and for the evaluation of disease activity of AD. IL-4 and TGF-beta1 downregulate TNF-alpha and IFN-gamma induced TARC production in the human KC cell line, HaCaT cells, while IL-4 upregulates, and IFN-gamma downregulates TARC production by mouse LC. Because TARC and its receptor CCR4 are believed to play important roles in the pathogenesis of AD, BP and MF, TARC and CCR4 may be possible future targets for therapy of these diseases.