TGF-β conditions intestinal T cells to express increased levels of miR-155, associated with down-regulation of IL-2 and itk mRNA

Transforming Growth Factor (TGF)-β, is an immunosuppressive cytokine that inhibits T cell activation. We hypothesized that TGF-β mediates its immuno-inhibitory effects by modulation of micro (mi)RNA-155. IL-2 and IFN-γ are down-regulated by TGF-β in activated CD4 peripheral blood T cells (PBT) and lamina propria T cells (LPT), but miR-155 is up-regulated 9-fold specifically in LPT. Consequently this study focuses on the role of TGF-β-enhanced miR-155 on LPT immune responses. TGF-β induces miR-155 in both freshly isolated and LPT lymphoblasts while other inducible miRNAs are not regulated by TGF-β. Using MAMI bioinformatics database we determined that inducible T cell kinase (itk) is a functional target of miR-155 that exhibits an inverse mRNA response to that of miR-155. To determine experimentally that miR-155 regulates itk, transfection experiments were performed that demonstrated miR-155 overexpression decreased itk and IL-2 mRNA, whereas antagonism of miR-155 restored both mRNAs in activated cells. These findings describe a TGF-β-dependent function for miR-155 in modulating cytokine and T cell immune responses in the gut.