Mechanisms of food allergy elicited by cutaneous sensitization

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IgE-mediated food allergy is common in atopic dermatitis (AD), a disease characterized by increased skin permeability, intense scratching, cutaneous and systemic release of epithelial cytokines, and a Th2 dominated response to cutaneously encountered antigens. Epidemiologic studies implicate the skin as an important portal of sensitization in food allergy. Furthermore, in addition to developing a Th2-dominated response and AD-like skin inflammation, mice epicutaneously (EC) sensitized by application of ovalbumin (OVA), or peanut, to tape-stripped skin undergo IgE- and mast cell (MC)-dependent anaphylaxis in response to oral antigen challenge, providing a good model for food allergy in cutaneously sensitized subjects. Our preliminary data show that tape stripping (T/S) of mouse skin, a surrogate for scratching, causes expansion of gut MCs, increases intestinal permeability in a MC dependent manner, and promotes anaphylaxis to oral antigen challenge in passively sensitized WT mice, but not Itgb7-/- mice, which lack MCs in their gut, suggesting that gut MC expansion induced by mechanical skin injury promotes IgE-mediated food allergy. Tape stripping increased cutaneous expression of IL-33 and TSLP, but not IL-25, and selectively increased serum levels of IL-33. Gut MC expansion was intact in Tslpr-/- mice, but was severely reduced in Il1rl1-/- (ST2-/-) mice, and Il17rb-/- (IL-25R-/-) mice, suggesting that IL-25 from an non-cutaneous source could be important. T/S caused gut ILC2 expansion and upregulation of IL-4 and IL-13 expression. Gut MC expansion was intact in Rag2-/- mice, but was abolished in Rag2-/-γc-/- mice, Rora-/->wt chimeras, that are deficient in ILC2s, and Rag2-/- /Il4-/-/Il13-/- mice, suggesting a critical role for ILC2s and type 2 cytokines. Gut MC expansion was impaired in IgE deficient mice, but was restored by treatment with anti-CD25 mAb that depleted T regulatory (Treg) cells. Altogether, our results suggest that mechanical skin injury, such as that induced by scratching in AD, causes IL-33-, IL-25-, ILC2- and IL-4/IL-13- dependent MC expansion in the gut that acts in concert with IgE antibody and Th2 cytokines to promote a food allergy response to cutaneous antigen sensitization, whereas Treg cells inhibit gut MC expansion to limit this response. We propose to define the mechanisms by which mechanical skin injury drives gut MC expansion, thereby promoting food allergy. We will test the hypothesis that keratinocyte-derived IL-33 released following mechanical skin injury synergizes with gut epithelial cell-derived IL-25 to drive gut ILC2s to proliferate and secrete type 2 cytokines that cause gut MC expansion, and that Treg cells act directly on MCs, and indirectly on ILC2s, to constrain gut MC expansion caused by tape stripping. The relevance of these mechanistic analyses is supported by our observations that scratching upregulates cutaneous IL33 expression in humans, and intestinal MCs are expanded in AD patients with no overt food allergy, and by the presence of elevated serum IL-33 levels and increased intestinal permeability in patients with AD. Defining the mechanisms whereby mechanical skin injury results in gut MC expansion that promotes oral anaphylaxis has the potential to identify novel targets in the treatment of food allergy.