Signaling through TLR7 enhances the immunosuppressive activity of murine CD4+CD25+ T regulatory cells

Abstract

In vitro and in vivo stimulation of the TLR7 signaling pathway of T regulatory cells causes increased IL-2 responsiveness and enhanced suppressor function.Although signaling through certain TLRs is known to modulate the function of T lymphocytes, the effect of TLR7 stimulation on CD4+CD25+ Treg cell activity has not yet been elucidated. In this study, we show that mouse CD4+CD25+ Treg cells express TLR7 mRNA and protein. We therefore used the TLR7 agonists imiquimod, gardiquimod, and single-stranded poly(U) to show that TLR7 stimulation enhanced the ability of murine Treg cells to suppress anti-CD3/anti-CD28 mAb-coated bead-stimulated proliferation of syngeneic CD4+CD25– Tresp cells. In contrast, imiquimod failed to enhance the suppressor function of Treg cells from mice deficient in the MyD88 adaptor protein involved in TLR7 and other TLR signal transduction. Imiquimod increased murine Treg cell-mediated suppression of Tresp cell proliferation induced by anti-TCRβ mAb in the presence of syngeneic BMDCs, and Treg cells from gardiquimod-treated mice exhibited enhanced in vitro suppressor function. Moreover, levels of Tresp cell-secreted IL-2 and IFN-γ were reduced further in the presence of Treg cells plus imiquimod in comparison with Treg cells alone. In addition, imiquimod treatment increased CD25 expression by Treg cells and caused exogenous IL-2 to enhance Treg cell suppressor function. Furthermore, combined treatment with imiquimod and IL-2 increased Foxp3 expression by Treg cells. Collectively, these findings suggest that TLR7 signaling enhanced the suppressor function of Treg cells by sensitizing Treg cells to IL-2-induced activation. We speculate that TLR7-stimulated enhancement of Treg cell suppressor function may modulate host T cell responses against ssRNA viruses.