Atopic dermatitis often precedes the development of asthma, a phenomenon known as "atopic march". An important role of allergen sensitization developed through barrier-defective skin has been recognized in the onset of atopic march; however, the underlying mechanism remains poorly understood. In this study, we use an experimental atopic march mouse model, in which the sensitization to allergen is achieved through barrier-impaired skin, followed by allergen challenge in the airway. By using thymic stromal lymphopoietin (TSLP)(iep-/-) mice in which the cytokine TSLP is selectively and inducibly ablated in epidermal keratinocytes, we demonstrate that keratinocytic TSLP, the expression of which is induced by skin barrier impairment, is essential for generating skin allergic inflammation and allergen-induced T helper type 2 response, for developing sensitization to allergen, and for triggering a subsequent allergic asthma. Furthermore, using TSLP(over) mice in which overexpression of keratinocytic TSLP is induced by skin topical application of MC903 (a vitamin D3 analog) in a dose-dependent manner, we show that keratinocytic TSLP levels are correlated with skin sensitization strength and asthma severity. Taken together, our study uncovers a crucial role of keratinocytic TSLP in the "atopic march" by promoting allergen sensitization occurring in barrier-impaired skin, which ultimately leads to allergic asthma.

Thymic stromal lymphopoietin (TSLP) has emerged as a key initiator in Th2 immune responses, but the TSLP-driven immune cascade leading to Th2 initiation remains to be delineated. Here, by dissecting the cellular network triggered by mouse skin TSLP in vivo, we uncover that TSLP-promoted IL-4 induction in CD4 þ T cells in skin-draining lymph nodes is driven by an orchestrated 'DC-T-Baso-T' cascade, which represents a sequential cooperation of dendritic cells (DCs), CD4 þ T cells and basophils. Moreover, we reveal that TSLP-activated DCs prime naive CD4 þ T cells to produce IL-3 via OX40L signalling and demonstrate that the OX40L-IL-3 axis has a critical role in mediating basophil recruitment, CD4 þ T-cell expansion and Th2 priming. These findings thus add novel insights into the cellular network and signal axis underlying the initiation of Th2 immune responses.

Highlights d Mechanical skin injury promotes intestinal mast-cell expansion d Intestinal mast-cell expansion requires skin-derived IL-33 and gut-derived IL-25 d Intestinal mast-cell expansion requires ILC2 activation by IL-33 and IL-25 d ILC2-derived IL-4 and IL-13 directly cause intestinal mast-cell expansion