Microbe-Dependent Induction of IL-9 by CLA+ T Cells in Psoriasis and Relationship with IL-17A

IL-9 is present in psoriatic lesions and is produced by lymphocytes. However, it is not known whether this cytokine is induced by relevant pathogenic triggers of psoriasis, such as Streptococcus pyogenes . Here we addressed the production of IL-9 in response to various pathogens in a psoriatic ex vivo model. Extracts of S. pyogenes and Candida albicans triggered the production of IL-9 and also IL-17A and IFN-γ. This induction was dependent on the interaction between CLA + T cells and epidermal cells. Neutralization of IL-9 reduced S. pyogenes -induced IL-17A production by CLA + T cells but had no effect on IFN-γ production. Also, IL-9 increased the survival of circulating psoriatic CLA + T cells. Co-cultures from patients with guttate or plaque psoriasis with S. pyogenes produced similar amounts of IL-9. High cytokine responses in streptococcal-driven guttate patients paralleled peaks in Psoriasis Area Severity Index and anti-streptolysin O levels. Our results confirm that IL-9 promotes inflammation in psoriasis by up-regulating IL-17A production and support the clinical association of the immune response by streptococcal-sensitized CLA + T cells with this cytokine, especially in guttate psoriasis.