IL-21R is essential for epicutaneous sensitization and allergic skin inflammation in humans and mice

Atopic dermatitis (AD) is a common allergic inflammatory skin disease caused by a combination of intense pruritus, scratching, and epicutaneous (e.c.) sensitization with allergens. To explore the roles of IL-21 and IL-21 receptor (IL-21R) in AD, we examined skin lesions from patients with AD and used a mouse model of allergic skin inflammation. IL-21 and IL-21R expression was upregulated in acute skin lesions of AD patients and in mouse skin subjected to tape stripping, a surrogate for scratching. The importance of this finding was highlighted by the fact that both rl21r(-/-) mice and WT mice treated with soluble IL-21R-IgG2aFc fusion protein failed to develop skin inflammation after e.c. sensitization of tape-stripped skin. Adoptively transferred OVA-specific WT CD4(+) T cells accumulated poorly in draining LNs (DLNs) of e.c. sensitized Il21r(-/-) mice. This was likely caused by both DC-intrinsic and nonintrinsic effects, because trafficking of skin DCs to DLNs was defective in Il21r(-/-) mice and, to a lesser extent, in WT mice reconstituted with Il21r(-/-) BM. More insight into this defect was provided by the observation that skin DCs from tape-stripped WT mice, but not Il21r(-/-) mice, upregulated CCR7 and migrated toward CCR7 ligands. Treatment of epidermal and dermal cells with IL-21 activated MMP2, which has been implicated in tracking of skin DCs. These results suggest an important role for IL-21R in the mobilization of skin DCs to DLNs and the subsequent allergic response to e.c. introduced antigen.