Chitinase 3‐like 1 drives allergic skin inflammation via Th2 immunity and M2 macrophage activation

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterized by defective skin barrier and Th2 immune responses. Chitinase 3‐like 1 (CHI3L1), also known as breast regression protein 39 (BRP‐39) in mice and human homologue YKL‐40, plays important roles in Th2 inflammation and allergen sensitization. CHI3L1 has been implicated in a variety of diseases including asthma characterized by inflammation, apoptosis and tissue remodelling, but its role in AD remains elusive.</jats:p></jats:sec><jats:sec><jats:title>Objective</jats:title><jats:p>The aim of this study was to investigate the role of CHI3L1 in the development and progression of AD.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>We investigated YKL‐40 levels in the serum and skin of AD patients by ELISA and immunofluorescence, respectively. Using a murine model of AD induced by ovalbumin (OVA), we investigated Th2 immune responses, M2 macrophage activation and skin barrier gene expression using wild‐type (WT) and BRP‐39 null mutant (BRP‐39<jats:sup>−/−</jats:sup>) mice.</jats:p><jats:p>YKL‐40 level was significantly increased in serum of AD patients. In addition, both mRNA and protein expression levels of BRP‐39 were higher in OVA‐sensitized WT mice than in control mice. OVA‐sensitized BRP‐39<jats:sup>−/−</jats:sup> mice showed decreased epidermal thickness, lower total serum IgE, Th2 cytokine levels and CD4<jats:sup>+</jats:sup> effector T cell populations than OVA‐sensitized WT mice. Induction of BRP‐39 was dominant in dermal macrophages. BRP‐39 deficiency was found to be involved in M2 macrophage activation. Consistently, the YKL‐40 level in the skin of AD patients was higher than in normal subjects and it was expressed in dermal macrophages. BRP‐39 deficiency attenuated dysregulation of skin barrier and tight junction genes.</jats:p></jats:sec><jats:sec><jats:title>Conclusions and Clinical Relevance</jats:title><jats:p>These findings demonstrate that CHI3L1 mediates the development of AD induced by OVA, affecting Th2 inflammation, M2 macrophage activation and skin barrier function.</jats:p></jats:sec>