Syndecan-1 Regulates Psoriasiform Dermatitis by Controlling Homeostasis of IL-17-Producing {gamma}{delta} T Cells [ALLERGY AND OTHER HYPERSENSITIVITIES]

IL-17 is a potent proinflammatory cytokine that drives pathogenesis of multiple autoimmune diseases, including psoriasis. A major source of pathogenic IL-17 is a subset of T cells (T17) that acquires the ability to produce IL-17 while developing in the thymus. The mechanisms that regulate homeostasis of T17 cells and their roles in psoriasis, however, are not fully understood. In this paper, we show that the heparan sulfate proteoglycan syndecan-1 (sdc1) plays a critical role in regulating homeostasis of T17 cells and modulating psoriasis-like skin inflammation in mice. sdc1 was predominantly expressed by T17 cells (but not IL-17^– T cells) in the thymus, lymph nodes, and dermis. sdc1 deficiency significantly and selectively increased the frequency and absolute numbers of T17 cells by mechanisms that included increased proliferation and decreased apoptosis. Adoptive transfer experiments ruled out a significant role of sdc1 expressed on nonhematopoietic cells in halting expansion and proliferation of sdc1-deficient T17 cells. When subjected to imiquimod-induced psoriasiform dermatitis, T17 cells in sdc1KO mice displayed heightened responses accompanied by significantly increased skin inflammation than their wild-type counterparts. Furthermore, transferred sdc1-deficient T cells caused more severe psoriasiform dermatitis than their sdc1-sufficient counterparts in TCR–β KO hosts. The results uncover a novel role for sdc1 in controlling homeostasis of T17 cells and moderating host responses to psoriasis-like inflammation.

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