Indirubin ameliorates imiquimod-induced psoriasis-like skin lesions in mice by inhibiting inflammatory responses mediated by IL-17A-producing γδ T cells

Objectives: Indirubin (IR) is a bisindole compound extracted from the leaves of Chinese herb Indigo Naturalis. Indigo Naturalis has been widely used in traditional Chinese medicine to treat inflammatory and autoimmune diseases. Psoriasis is a chronic immune-mediated inflammatory skin disease in which gamma delta T cells play an important role. This study aims to determine the immunoregulatory effects and the underlying mechanisms of Indirubin in psoriasis-related inflammatory responses. Methods: BALB/c mice with imiquimod (IMQ)-induced psoriasis-like dermatitis were treated with saline (Model), 1 mg/kg methotrexate (MTX) that serves as a positive control, or 12.5, 25 and 50 mg/kg Indirubin(IR) intragastrically. Keratinocytes proliferation, inflammatory cells infiltration, the expression of inflammatory cytokines and Jak/Stat pathway-related proteins in the skin lesion were examined. The abundance of gamma delta T cells in lymph nodes and spleen was determined by flow cytometry. The IL-17 expression and secretion, and the activation of Jak3/Stat3 pathways in in vitro cultured gamma delta T cell were tested. Results: Indirubin ameliorated keratinocyte proliferation, reduced the infiltration of CD3(+) T cells, IL-17 A-producing gamma delta T cells, and CD11b(+) neutrophils, inhibited the mRNA expression of Il1, Il6, Il23,Il17a and Il22, and the protein expression of Jak/Stat pathway-related molecules in the skin lesion. Indirubin also reduced the abundance of gamma delta T cell and CCR6(+) gamma delta T cells (the major IL-17 A producer) in spleen and lymph nodes. In cultured gamma delta T cells, Indirubin inhibited the mRNA expression of Il17a and Ifng, and the secretion of IL-17 A, while suppressed the activation of Jak3/Stat3 pathways. Conclusion: Indirubin alleviates IMQ-induced psoriasis-like dermatitis mainly through reducing the inflammatory responses mediated by IL-17 A-producing gamma delta T cells involving Jak3/Stat3 activation. Our results highlighted the novel mechanisms by which Indirubin ameliorates psoriasis-related inflammatory responses, supporting its therapeutic potential.