IFN‐γ‐inducible expression of thymus and activation‐regulated chemokine/CCL17 and macrophage‐derived chemokine/CCL22 in epidermal keratinocytes and their roles in atopic dermatitis

Thymus and activation-regulated chemokine (TARC)/CCL17 and macrophage-derived chemokine (MDC)/CCL22 are a pair of CC chemokines known to selectively attract Th2 type memory T cells via CCR4. Here we examined circulating levels of TARC and MDC in patients with atopic dermatitis (AD) and control subjects by using plasma samples, which reflect blood contents of chemokines more accurately than serum samples. The plasma levels of TARC and MDC were significantly elevated in AD patients. These values also strongly correlated with disease severity and serum lactate dehydrogenase levels, and weakly correlated with serum total IgE levels and blood eosinophilia. Previous studies demonstrated TARC immunoreactivity in the epidermal layer of AD lesional skin and production of TARC by a human keratinocytic cell line HaCaT upon stimulation with IFN-g. Here we demonstrated MDC immunoreactivity in the epidermal layer of AD skin at levels stronger than that of TARC. Furthermore, primary epidermal keratinocytes expressed both TARC and MDC mRNA upon stimulation with IFN-g, but efficiently secreted only MDC. These results suggest a post-transcriptional regulation in TARC production. IFN-g also induced TARC and MDC mRNA in mouse skin. Collectively, both TARC and MDC play important roles in the local accumulation of Th2 cells in AD lesional skin. Production of Th2-attracting chemokines by epidermal keratinocytes upon treatment with IFN-g, which is also the potent inducer of Th1attracting chemokines, may underline the pivotal role of IFN-g in the chronic phase of AD where both Th1 and Th2 responses are mixed.