T cells are crucial mediators of the skin damage in psoriasis. We here show that interleukin-21 (IL-21), a T cell–derived cytokine, is highly expressed in the skin of individuals with psoriasis, stimulates human keratinocytes to proliferate and causes epidermal hyperplasia when injected intradermally into mice. In the human psoriasis xenograft mouse model, blockade of IL-21 activity resolves inflammation and reduces keratinocyte proliferation. Blocking IL-21 may represent a new therapeutic strategy in psoriasis.

DOI: 10.1038/nm.1995
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