Type 2 immunity is critical for defense against cutaneous infections, but also underlies the development of allergic skin diseases. We report the identification in normal murine dermis of an abundant, phenotypically unique group 2 innate lymphoid cell (ILC2) subset that depends on interleukin 7 (IL-7) and constitutively produces IL-13. Intravital multiphoton microscopy revealed that dermal ILC2 specifically interact with mast cells, whose function was suppressed by IL-13. Treatment of Rag1−/− mice with IL-2 resulted in the expansion of activated, IL-5-producing dermal ILC2, leading to spontaneous dermatitis characterized by eosinophil infiltrate and activated mast cells. Our data show that ILC2 exhibit both pro- and anti-inflammatory properties and uncover a novel interactive pathway between two innate immune cell populations implicated in type 2 immunity and allergic diseases.

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Background Mast cells have gained notoriety based on their detrimental contributions to IgE-mediated allergic disorders. Although mast cells express the vitamin D receptor (VDR), it is not clear to what extent 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), or its predominant inactive precursor metabolite in circulation, 25-hydroxyvitamin D3 (25OHD3), can influence IgE-mediated mast cell activation and passive cutaneous anaphylaxis (PCA) in vivo. Objective We sought to assess whether the vitamin D3 metabolites, 25OHD3 and 1α,25(OH)2D3, can repress IgE-dependent mast cell activation via mast cell-CYP27B1 and -vitamin D receptor activity. Methods We measured the extent of vitamin D3 suppression of IgE-mediated mast cell degranulation and mediator production in vitro, as well as the vitamin D3-induced curtailment of PCA responses in WBB6F1-KitW/W-v or C57BL/6J-KitW-sh/W-sh mice engrafted with mast cells that did or did not express VDR or CYP27B1. Results Here we show that mouse and human mast cells can convert 25OHD3 to 1α,25(OH)2D3 via 25-hydroxyvitamin D-1α–hydroxylase (CYP27B1) activity, and that both of these vitamin D3 metabolites suppressed IgE-induced mast cell-derived pro-inflammatory and vasodilatory mediator production in a VDR-dependent manner in vitro. Furthermore, epicutaneously applied vitamin D3 metabolites significantly reduced the magnitude of skin swelling associated IgE-mediated PCA reactions in vivo; a response that required functional mast cell-VDRs and mast cell-CYP27B1. Conclusion Taken together, our findings provide a mechanistic explanation for the anti-inflammatory effects of vitamin D3 on mast cell function by demonstrating that mast cells can actively metabolize 25OHD3 to dampen IgE-mediated mast cell activation in vitro and in vivo.