TLR7-dependent eosinophil degranulation links psoriatic skin inflammation to small-intestinal inflammatory changes

Increased intestinal permeability can exacerbate psoriasis, a systemic inflammatory disease with complex pathogenesis. Using a mouse model of psoriasis elicited by the TLR7 ligand imiquimod, we found that psoriatic dermatitis was accompanied by small-intestinal inflammatory changes associated with eosinophil degranulation which led to impaired intestinal barrier integrity. Inflammatory responses in the skin and small intestine were accelerated in mice that are prone to eosinophil degranulation. Caco-2 human intestinal epithelial cells treated with media containing eosinophil granule proteins exhibited signs of inflammation and damage. Imiquimod-induced skin and intestinal inflammatory changes were attenuated in eosinophil-deficient mice, and this attenuation was counteracted by eosinophil transfer. Imiquimod levels and eosinophil distribution were positively correlated in the intestine. TLR7-deficient mice did not show intestinal eosinophil degranulation and exhibited attenuated skin and small-intestinal inflammation following imiquimod application. These results suggest a TLR7-dependent bidirectional skin-to-gut communication in psoriatic inflammation, and that intestinal inflammatory changes can accelerate psoriasis. At 8-10 weeks of age, the mice received a daily topical dose of 62.5 mg IMQ cream (5%) (Aldara™) or vehicle cream (Vaseline) on the shaved back and the right ear for 5-8 sequential days.