IL-21 has also been shown to be important for IL-17+ CD8+ T cell differentiation in mouse cells, either as part of a cytokine cocktail (TGF-β, IL-6, IL-1β, IL-2, IL-21, IL-23, α-IL-4 and α-IFN-γ mAb)  or in combination with TGF-β . Increased Il21 mRNA expression was observed in mouse CD8+ T cells cultured with TGF-β and IL-21, with TGF-β and IL-6, or with IL-21 alone . IL-21 production by IL-17+ CD8+ T cells may promote a positive feedback loop to expand IL-17+ CD8+ T cells further, an autocrine mechanism reported in IL-17+ CD4+ T cells , . Human stimulated IL-17+ CD8+ T cells from psoriatic lesions express IL-21 , however it remains to be established whether IL-21 is important for human IL-17+ CD8+ T cell differentiation.